mission_creek_ · web viewthe mission creek and mill creek strands follow very different paths...

Kendrick and others, 1/8/13

Quaternary slip history of the Mill Creek strand of the San Andreas Fault in San Gorgonio Pass, southern California: implications for regional fault interaction

by

Kendrick, Matti and Mahan

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ABSTRACT

An alluvial complex within the Mission Creek drainage records fault-activity history

of the Mill Creek strand of the San Andreas Fault in the San Gorgonio Pass region. We

have investigated six surface remnants using detailed geologic mapping, morphometric

and stratigraphic analysis, geochronology of fills (IRSL) and pedogenic analysis. The

surfaces cap distinct alluvial fills having clasts that can be associated with sources in the

Mission Creek drainage in the nearby San Bernardino Mountains. We have distinguished

and correlated the disconnected surface remnants on the basis of morphometric analysis,

including their longitudinal profiles, surface texture and drainage density. The degree of

soil-profile development constrains the timing of initiation of movement along the Mill

Creek strand of the San Andreas Fault, when compared to other regional pedons that have

been independently dated. Pedogenic properties useful for age appraisals include soil-

development and rubification indices, and increase in clay volume. Correlation to

regional, independently dated soil profiles indicates that the oldest surfaces in the

Mission Creek drainage are significantly older than 500 ka, though this age range (>500

ka) that is particularly difficult to constrain with available geochronologic methods. IRSL

dates of 106 and 95 Ka from (respectively) 5 and 4 m beneath a younger pedon are

consistent with age estimates based on soil-profile development. The oldest surfaces in

this fan complex occur on deposits emplaced prior to movement along the Mill Creek

strand, and have been displaced approximately 8 km from their source across both the

Mill Creek and Mission Creek fault strands in San Gorgonio Pass. This is the same as a

total slip estimate for the Mill Creek strand based on the restoration of the displaced

drainage system Hell-For-Sure, North Fork Whitewater Rivers with South Fork

Whitewater River (7.1 – 8.7 km), as well as offset lithologic markers within the San

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Gabriel Mountains type crystalline bedrock (less than 10 km). The congruence of these

measurements suggests that no dextral motion occurred on the Mission Creek strand

since the deposition of these units. The youngest deposits within this complex

accumulated following cessation of movement along both the Mission Creek and Mill

Creek strands, and mark the time after which slip had transferred onto the Banning and

Garnet Hill strands. Displacement of the Mill Creek strand by the Pinto Mountain Fault

provides a short-term slip rate of 10 - 12.5 mm/yr for this fault, and an average longer-

term slip rate of approximately 2 to 2.5 mm/yr. Uplift of the Yucaipa Ridge block during

period of activity of the Mill Creek strand has been ~ 2.3 to 2.6 mm/yr. Between

approximately 500 to 100 ka, the Mill Creek strand had a slip rate of 18 to 22 mm/yr.

INTRODUCTION

Statement of problem

The San Gorgonio Pass region of southern California marks the junction between the

Mojave Desert and Coachella Valley segments of the San Andreas Fault (Fig. 1). The

segments have a left-stepping geometry, with the Mojave Desert segment stepped left

(west) about 15 km from the Coachella Valley segment (Matti and others, 1985, 1992;

Matti and Morton, 1993). The left step occurs in the San Gorgonio Pass region, where

complex structural relations have formed a structural knot in the San Andreas Fault.

Over the last million years or so, this knot has influenced the evolution of the entire plate-

boundary system in southern California, including (1) sequential development of multiple

San Andreas Fault strands in San Gorgonio Pass, (2) initiation of the dextral San Jacinto

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Fault west and south of the Pass region, and (3) integration of the Eastern California

Shear Zone into the Quaternary San Andreas Fault system.

For these reasons, the time-space history of the San Andreas Fault in San Gorgonio

Pass is critical not only to understanding the modern seismotectonic framework of the

southern San Andreas Fault system, but also to evaluating how various components of

that system have interacted since inception of the San Andreas Fault proper in Pliocene

time (~5 Ma). Previous workers have outlined key elements of this time-space history

(Allen, 1957; Matti and others, 1985, 1992; Matti and Morton, 1993; Yule and Sieh,

2003), and the modern San Andreas in San Gorgonio Pass region recently has been the

focus of considerable debate among seismologists concerned with fault-rupture scenarios

and attendant seismic risk (e.g., Yule, 2009, 2010). Despite this recent interest, little is

known about the Quaternary slip history for the San Andreas Fault in the San Gorgonio

Pass region, particularly with respect to other dextral faults like the San Jacinto Fault that

participate in the Quaternary strain budget within the southern San Andreas Fault system.

We address this issue by investigating the slip history for the Mill Creek strand of the San

Andreas Fault, using geologic relations that occur in the Mission Creek and Raywood

Flat areas of San Gorgonio Pass (Fig. 2).

San Andreas Fault Zone

Fault nomenclature

Nomenclature for the San Andreas Fault in southern California is as complex as the

fault’s geologic setting and movement history. Nowhere is this complexity greater than

in San Gorgonio Pass. Here, early investigators (Vaughan, 1922; Allen, 1957; Dibblee,

1964, 1968a, 1975) mapped multiple fault strands, but their significance and structural

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role within the overall San Andreas zone were not obvious. Later workers (Ehlig, 1977;

Farley, 1979; Matti and others, 1982, 1983) recognized that differences among

regionally-persistent crystalline-rock packages can be used to distinguish faults having

major right-slip displacement. These distinctions led researchers to formalize multiple

strands of the San Andreas Fault in the vicinity of San Gorgonio Pass (Fig. 2; Matti and

others, 1985, 1992; Matti and Morton, 1993). These strands developed sequentially (Fig.

3), and represent the full history of Pliocene-Quaternary slip on the San Andreas Fault in

southern California since its inception (Matti and Morton, 1993). Following Matti and

others (1992), in the San Gorgonio Pass region we refer to faults of the San Andreas zone

as strands, including the Mission Creek strand, Mill Creek strand, San Bernardino strand,

and Banning strand (fig. 2). Because these terms are cumbersome, for convenience we

commonly refer to them only by their specific names (e.g., Mill Creek strand, San

Bernardino strand); moreover, we use the terms “strand” and “fault” interchangeably,

without changing the intrinsic concept that each structure is a strand of the parent San

Andreas Fault.

Mission Creek and Mill Creek strands

In terms of total dextral slip, the Mission Creek strand is the main San Andreas strand

in southern California. In San Gorgonio Pass the Mission Creek strand has

approximately 90 km of dextral slip, and juxtaposes far-traveled crystalline rocks of San

Gabriel Mountains-type against crystalline rocks of Mojave Desert-type (Fig. 2; Matti

and others, 1992; Matti and Morton, 1993).

The Mill Creek strand lies north of, and subparallel to, the Mission Creek strand (Fig.

2). Although some workers attribute significant dextral slip to the fault (e.g., Dibblee,

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1968a, 1975, 1982a), and this pathway for fault motion is often used in geophysical and

computer modeling (e.g. Plesch, et al., 2007; Spinler et al., 2010), it probably has no

more than ~7-8 km of slip, displacing a narrow slice of Mojave Desert-type basement

northwestward away from the San Gorgonio Mountain block (Matti and others, 1983,

1985, 1992). This slice coincides with Yucaipa Ridge structural block of Spotila and

others (1998, 2001). In San Gorgonio Pass, neither the Mill Creek nor Mission Creek

strands has generated dextral slip in late Quaternary time, although the Mill Creek strand

is the most recently active of the two (Fig. 3; Matti and Morton, 1993).

San Bernardino, Banning, and Garnet Hill strands, and San Gorgonio Pass Fault zone

The San Bernardino strand of the San Andreas Fault along the southwestern base of

the San Bernardino Mountains and the Banning and Garnet Hill strands in the Coachella

Valley represent modern neotectonic strands of the San Andreas Fault in southern

California. These three faults approach the San Gorgonio Pass region from the northwest

and southeast, respectively (Figs. 1, 2), but as Allen (1957) demonstrated, none of them

can be mapped continuously through San Gorgonio Pass in a way that convincingly

demonstrates their geometric connections and kinematic interactions. Building on

Allen’s findings, Matti and others (1985; 1992; Matti and Morton, 1993) proposed that

relations among dextral faults in San Gorgonio Pass have been complicated and obscured

by a belt of contractional deformation, mapped as the San Gorgonio Pass Fault zone.

This fault zone intervenes between the Banning and Garnet Hill strands (to the southeast)

and the San Bernardino strand (to the northwest), and has obscured throughgoing

relations among the dextral faults.

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Focus of Investigation

This investigation focuses on two locations along the trace of the Mill Creek fault

strand, Upper Raywood Flat and the Mission Creek alluvial complex. Reconstruction of

drainage systems through Upper Raywood Flat allows for the determination of total slip

on the Mill Creek strand of the San Andreas Fault. Deposits within the Mission Creek

alluvial complex, located within the present Mission Creek drainage basin, constrain the

timing of movement of this fault strand. We use geologic mapping, geomorphic analysis,

pedogenesis, and optically stimulated luminescence (OSL) to reconstruct the geologic

history of this region over the past million years. The results of this research have

implications for the slip histories of both the Mill Creek strand of the San Andreas Fault

and the Pinto Mountain fault.

REGIONAL GEOLOGIC AND GEOMORPHIC SETTING

Crystalline basement rocks

Although Allen (1957) grouped crystalline rocks north and south of the Mission

Creek fault strand within his “San Gorgonio igneous and metamorphic complex”, he

noted that rocks south of the fault include distinctive lithologies not found to the north.

Later workers in San Gorgonio Pass (Ehlig, 1977; Farley, 1979; Matti and others, 1982,

1983) recognized that rocks south of the Mission Creek Fault are similar to those in the

San Gabriel Mountains, whereas those to the north are similar to rocks of the Mojave

Desert (Fig. 4). Coming from the other direction, Dillon (1975) correlated crystalline

rocks in San Gorgonio Pass with rocks in the Chocolate Mountains on the northeast side

of the Coachella Valley segment of the San Andreas Fault. Thus, contrasts in crystalline

rocks on either side of the Mission Creek Fault indicate that it is a major strand of the San

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Andreas, and correlates with the Mojave Desert segment of the San Andreas Fault north

of the San Gabriel Mountains (Fig. 4).

Geomorphic setting

The landscape of the San Gorgonio Pass region reflects the complexity of the fault

history. Spotila and others (1998) sought to understand this landscape by defining

structural blocks throughout the San Gorgonio Pass. In the region of this investigation,

they defined four structural blocks. To the north of the Mill Creek strand they identified

the San Gorgonio block. This block is characterized by high relief, and tilting to the

north, as constrained by the slope on an ancient geomorphic surface (Spotila and Sieh,

2000). This block is separated from the Santa Ana Valley block further to the north by

unnamed, high-angle faults, mapped by Sadler (1993). The San Gorgonio block has been

described as being deformed into an east-west elongate dome (Spotila et al., 1998, Sadler,

1993, Dibblee, 1964). The boundary between the San Gorgonio and Santa Ana Valley

blocks is not complete to the east, however, where the unnamed, high-angle faults do not

continue. Spotila and others define two blocks between the Mill Creek and Mission

Creek strands, which comprise a high, narrow ridgeline, shown as Yucaipa Ridge on

topographic maps. Wilson Creek block is at the west end of this ridge, and the east is

named the Yucaipa Ridge block (Spotila et al., 1998). These are separated by the

ancestral Wilson Creek strand, proposed by Matti and Morton (1993) to have been active

prior to the inception of movement on the Mission Creek strand (Fig. 3). The Morongo

block, as defined by Spotila and others, is located south of Yucaipa Ridge, and bound to

the north by the San Bernardino and Mission Creek strands, and to the south by the San

Gorgonio Pass Thrust Fault (Spotila et al., 1998).

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We follow the nomenclature and defined boundaries for the northern blocks of

Spotila and others. We define three blocks (Fig. 5) within the bounds of their Morongo

block, as follows. The Pisgah Peak block is bound on the north and east by the San

Bernardino strand and to the south by the San Gorgonio Pass Thrust Fault. The Kitching

Peak block is bound on the north by the Mission Creek strand, on the west by the San

Bernardino strand, and on the east by the Whitewater Fault. The Devil’s Garden block is

to the east of the Whitewater Fault, and is bound to the south by the Banning strand, and

to the north by the Mission Creek and Mill Creek strands. The eastern extent of this

block is masked by sediments of the Coachella Valley.

Helium thermochronology and fission track measurements suggest that these blocks

have experienced differing uplift histories (Spotila et al., 1998; Spotila et al., 2001;

Blythe et al., 2002). Yucaipa Ridge, including the Yucaipa Ridge and Wilson Creek

blocks, has experienced between 3-4 km of uplift in the most recent several million years,

and 1-2 km in the past 1.5 Ma (Spotila et al., 2001). The denudation rates that result from

this modeled uplift history agree well with the shorter-term denudation rates determined

using cosmogenic 10Be concentrations (Binnie et al., 2008). This uplift of Yucaipa Ridge

is significantly greater than that experienced by the blocks to the north and south.

METHODS

Geologic mapping for this investigation was conducted at a scale of 1:24,000 or

larger, mainly using traverse-mapping techniques. From the locality where a map unit or

its boundaries initially was identified or defined, it was extended to other areas through a

mapping process that includes direct outcrop observation and aerial-photographic

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interpretation. Map-unit boundaries (geologic contacts; in some cases faults) identified

along mapping traverses typically were extended laterally by using aerial photographs

and binoculars to project or extrapolate the boundary to its next identified occurrence

along a nearby traverse; faults that cut the units were extended laterally in the same

manner. Only rarely were individual geologic contacts or fault lines walked out to

determine their variability and character throughout the map area. For surficial units,

bounding contacts were plotted digitally from rectified and georeferenced 2005-vintage

NAIP aerial photographs that allow location accuracy equivalent to the accuracy standard

for the topographic-contour base.

Soils were described on selected surfaces within the Mission Creek alluvial complex,

after established methods (Schoeneberger et al., 2002; Birkeland, 1999). Laboratory

analysis was performed on collected samples, and included particle size determined by

pipette and wet-sieving methods. Profile development indices (Harden, 1982) and

rubification indices (Torrent, et al., 1980) were determined for each of these soil pedons,

as well as for three pedons previously described by McFadden (1982) within this region.

Within the Mission Creek alluvial complex, surface texture, including drainage

density, and post-depositional incision were determined using a series of lateral profiles,

constructed from a 10-m DEM. Long profiles were used to determine uplift and tilting.

Stream profiles were constructed and analyzed for discontinuities and knickpoints (for a

discussion see Wobus et al., 2006).

Samples for optical dating were taken by hammering a 5-cm interior diameter and 40

to 50 cm length of PVC tubing into profile walls. Optical ages were determined on the

fine-grained (4-11), polymineral fraction of the inner portion of the sample, using

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multiple aliquot techniques (Singhvi et al., 1982; Lang, 1994; Richardson et al., 1997;

Forman and Pierson, 2002) with infrared excitation. The fine-grained silt separate of

representative samples was isolated and dated using multiple aliquot techniques. To

determine dose rate, the U and Th content of sediment, assuming secular equilibrium in

the decay series, were determined by high-resolution gamma spectrometry (Ge detector)

at the USGS, Denver for all of the samples. The cosmic ray components of 0.11 to 0.09

mGy/yr (appropriate to depth and elevation) were included in the estimated dose rate

(Prescott and Hutton, 1994). These cosmic components were trivial and only accounted

for up to 1 to 2 percent of the dose rate. A measure of moisture content was determined

by laboratory measurement of complete saturation moisture content for each sample, then

half of the calculated total moisture was used as an estimate of average moisture over the

life of the sample.

RESULTS

Mission Creek and Mill Creek Strands of the San Andreas Fault

The Mission Creek and Mill Creek strands in the Mission Creek study area are

associated with an extensive zone of crushed and sheared crystalline rock. Discrete fault

planes are associated with this zone, in places occurring within it or bounding it to either

side or bounding it only on the northeast. None of these appears to break Holocene or

late Pleistocene surficial deposits. Southeastward into the Coachella Valley the broad

crush zone diverges valleyward from the mountain front, and here neither vintage aerial

photographs nor LiDAR data (Bevis and others, 2005) reveals evidence of faults in

alluvial deposits of any age (Matti and others, 1985, 1992).

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Our interpretation of fault patterns in the Mission Creek area differs in two ways from

that of Matti and others (1982, 1983):

Position of main San Andreas strand southeast of Whitewater River

Southeast of Whitewater River, two features influenced where Matti and others

(1982, 1983) positioned the main strand of the San Andreas Fault (Mission Creek strand;

Fig. 6): (1) the extensive crush zone developed in crystalline rocks of San Gabriel

Mountains-type and (2) the slice of volcanic-clast-bearing sedimentary rocks that Allen

(1957, pl. 1) correlated with his Deformed Gravels of Whitewater River. Our mapping of

these two features leads us to position Mission Creek strand differently than proposed by

Matti and others (1982, 1983).

Those workers (Matti and others, 1982, 1983) reasoned that the broad zone of

crushed and sheared crystalline rock represents pervasive distributed shear within the

movement zone of a major fault. Accordingly, they associated the entire crush zone with

the Mission Creek strand, rather than linking the strand to a specific mapped fault.

Earlier, Allen (1957) had mapped a fault between the crush zone and the volcanic-clast-

bearing sedimentary rocks; although he does not discuss this structure, he must not have

viewed it as important given his tentative correlation of the volcanic-clast-bearing unit

with the nearby Deformed Gravels of Whitewater River. Matti and others (1982)

interpreted this boundary as a depositional contact, implying that the volcanic-clast-

bearing sedimentary unit laps over the broad crush zone (Fig. 6). Whatever relationship

exists between the crush zone and this unit, the main San Andreas strand (Mission Creek

strand) must occur between it and basement rocks in the San Bernardino Mountains

directly to the northeast, because volcanic rocks do not occur there in drainages that have

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shed sediment across the San Andreas Fault into Cenozoic basins west of the fault.

Volcanic rocks are abundant in the Chocolate Mountains region, however (Dillon, 1975;

Crowe, 1978; Crowe and others, 1979), and these almost surely are the source for

volcanic clasts in the fault slice and in the Painted Hill Formation where Allen (1957)

mapped it south of the Mission Creek study area. Thus, the main strand of the San

Andreas Fault must occur between all outcrops of the Painted Hill Formation—including

those in the fault slice—and Mojave Desert-type basement. This leaves unresolved (1)

the origin and significance of the broad zone of crushed and sheared crystalline rock and

(2) the purported depositional boundary between this zone and the volcanic-clast-bearing

unit.

Divergence of Mission Creek and Mill Creek strands

The Mission Creek and Mill Creek strands follow very different paths through San

Gorgonio Pass proper, but where they project eastward into the Coachella Valley the two

faults basically are coextensive (Fig. 2). This requires that somewhere between the two

regions the two faults diverge. Matti and others (1982, 1983, 1985, 1992) indicate that

this occurs at the southeast end of the Yucaipa Ridge block, where the Middle and South

Forks of the Whitewater River join. There, three faults come together (Fig. 6):

(1) From its position along the northeast side of the volcanic-clast-bearing

sedimentary unit, the Mission Creek strand trends progressively more westerly and

projects beneath the active alluvial wash of the Whitewater River. From there it extends

along the Whitewater South Fork, where it (a) continues to separate rocks of San Gabriel

Mountains-type (to the south) from rocks of Mojave Desert-type (to the north) and (b)

has an anomalous east-west orientation.

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(2) The sinistral Pinto Mountain Fault approaches the Whitewater forks from the east,

projects beneath the Middle Fork wash, and obliquely intersects the Mission Creek Fault

(Allen, 1957, pl. 1).

(3) The Mill Creek Fault approaches the Pinto Mountain-Mission Creek intersection

from the west-northwest, but how it relates to this intersection is open to interpretation.

Allen (1957, pl. 1) shows three faults approaching the junction between the Middle and

South Whitewater Forks, and by implication these represent splays of his Mill Creek

Fault. However, on Allen’s map none of the three faults actually intersects the Pinto

Mountain or Mission Creek faults; more importantly, he does not indicate which of the

three is the main strand of the Mill Creek zone.

Matti and others (1982, 1983) refined Allen’s fault framework. They associated his

southwesternmost splay with the Mill Creek strand, and extended it completely across the

Yucaipa Ridge block where it appears to be truncated by the Pinto Mountain Fault (but

see an alternative rendering by Dibblee, 1964). This map solution resolves fault-

continuity questions raised by Allen’s mapping, but it has its own problems. As

interpreted by Matti and others (1982, 1983), the Mill Creek strand has a near-

perpendicular orientation to the Pinto Mountain and Mission Creek faults. This geometry

for the three faults is peculiar, considering that the Mill Creek strand ostensibly should

splay obliquely away from the abandoned Mission Creek crush zone as slip on the San

Andreas Fault jumped inboard to a new and younger fault to the northeast (Matti and

others, 1985, 1992; Matti and Morton, 1993).

To resolve this problem, Matti and others (1985, 1992, p. 13) proposed that the

junction between the two San Andreas strands has been modified by incursion of the

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Pinto Mountain Fault from the east. This caused a sinistral deflection of the Mill Creek

trace. We refine this proposal by suggesting that the Mill Creek Fault crush zone where

it crosses the Yucaipa Ridge block has not merely been deflected by the Pinto Mountain

Fault but has been truncated and displaced from a cross-fault counterpart concealed

beneath young alluvial wash of the Whitewater River Middle Fork (Fig. 6). This solution

explains why the Mill Creek strand just to the west on the Yucaipa Ridge block does not

actually meet the Mission Creek strand, and why the two faults are nearly perpendicular.

Mission Creek and Mill Creek strands in the Raywood Flat region

Further to the west, in the Raywood Flat area, the Mission Creek and Mill Creek

strands follow markedly different pathways (Fig. 7). The Mission Creek strand continues

to be associated with a broad zone of crushed and sheared crystalline rocks that clearly

separates San Gabriel Mountains-type crystalline rocks to the south from Mojave Desert-

type crystalline rocks of the Yucaipa Ridge block to the north (Fig. 2).

Mission Creek Fault strand

The Mission Creek Fault strand appears to have been long abandoned by the San

Andreas Fault. Its crush zone is buried by surficial deposits of all ages that extend from

upper Raywood Flat to lower Raywood Flat and from there eastward down the South

Fork Whitewater River (Fig.7). Along this entire extent the deposits occur in terraces

that stand well above modern canyon bottoms.

Two discrete faults associated with the Mission Creek crush zone probably are not

related to the Quaternary San Andreas Fault:

(1) Within the north part of the crush zone Matti and others (1983, 1985) mapped a

discrete fault trace that they interpret as a San Andreas strand older than the Mission

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Creek Fault. They correlated this older strand with the Wilson Creek Fault that occurs at

the northwest end of the Yucaipa Ridge block (Fig.4). The Wilson Creek Fault probably

is the oldest strand of the San Andreas Fault in the San Gorgonio Pass region (Fig. 3),

and therefore is not germane to Quaternary strain budgets within the San Andreas Fault

zone.

(2) Lower Raywood Flat is traversed by a degraded, north-facing fault scarp that

diagonals westward from the South Fork of Whitewater River to the San Gorgonio River

drainage basin. The scarp is developed in alluvial deposits correlative with those of

upper Raywood Flat, but tributary to those throughgoing axial deposits. Along the west

wall of the South Fork the fault juxtaposes the alluvial deposits against crushed and

sheared crystalline rock. Given its association with the Mission Creek crush zone, it is

tempting to relate the fault scarp to displacements on that fault. However, because the

scarp diagonals across the crush zone rather than paralleling it, and because the scarp

appears to trend toward Yucaipa Ridge rather than west and southwest along with the

Mission Creek crush zone, Matti and others (1983, 1985) attributed the landform to

younger tectonism that led to north-facing fault scarps throughout the San Gorgonio Pass

region (e.g., the north-facing scarp associated with the Mill Creek Fault at the Whitewater

Jumpoff; see Fig. 8). We accept this interpretation, and propose that the scarp reflects

tectonism related to the Pinto Mountain Fault, whose post-Mill Creek sinistral slip

probably fed westward along the crush zone of the Mission Creek strand. This sinistral

slip along the SW-NE trending Pinto Mountain fault would resolve as north-facing fault

scarps along the approximately E-W oriented Mission Creek strand.

Mill Creek Fault

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From its divergence with the Mission Creek strand southeast of Raywood Flat

(Fig.7), the Mill Creek strand trends west-northwest up the Middle Fork of Whitewater

River, where its trace mainly is concealed beneath young alluvial-wash deposits. At the

head of the Middle Fork the fault is spectacularly exposed in precipitous terrain of the

Whitewater Jumpoff (Fig. 8). There, a discrete fault trace we assign to the Mill Creek

strand dips steeply to the north and occurs within a wide zone of crushed and brecciated

crystalline rock. The fault juxtaposes dark-colored gneissose and granitoid rocks of the

Yucaipa Ridge block against light-colored pinkish granitoid rocks of the San Gorgonio

Mountain block. Displacement has produced a north-facing scarp that can be mapped for

a short distance west-northwest across the eastern lobe of the “Y-shaped” Raywood Flat

alluvial succession (Fig.7). The scarp landform does not persist, however, because either

(1) scarp-forming ground ruptures did not extend throughout the Raywood Flat area or

(2) the scarp landform has been obscured by younger degradational and aggradational

events. Except for this scarp, no other tectonogeomorphic features are associated with

the Mill Creek strand in the Raywood Flat area. We take this to mean that this strand of

the San Andreas Fault has not generated significant dextral slip in late Quaternary time

(Matti and others, 1985, 1992).

MISSION CREEK STUDY AREA


Crystalline rocks in the Mission Creek study area have been recognized as being

similar to those in the San Gabriel Mountains (Ehlig, 1977; Farley, 1979; Matti and

others, 1982, 1983). Figure 9 is a geologic map of the Mission Creek study area, based on

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Matti and others (1982) and on subsequent USGS geologic mapping (J.C. Matti, B.F.

Cox, and R.E. Powell, unpublished data, 1980-1986, 2009). These investigations

confirmed the basic geologic framework reported by Allen (1957), but elaborate and

refine his findings.

Cenozoic cover rocks

In the Mission Creek area, crystalline rocks of San Gabriel Mountains-type are

overlain nonconformably by Miocene nonmarine sedimentary rocks (Coachella

Fanglomerate of Vaughan, 1922; Fig. 9). These sedimentary rocks were investigated by

Peterson (1975), who reports that crystalline-rock clasts, contained within the unit, are

similar to bedrock in the southern Chocolate Mountains. This observation is consistent

with Dillon’s (1975) correlation of crystalline-basement lithologies between the

Chocolate Mountains and San Gorgonio Pass. Just south of the Mission Creek study area

the Coachella Fanglomerate is overlain unconformably by the uppermost Miocene marine

Imperial Formation (McDougall and others, 1999) and the nonmarine Painted Hill

Formation. Allen (1957) did not recognize these units in the Mission Creek study area;

however, in the northwest part of the area (Fig. 6) a restricted body of sedimentary rock

probably correlates with the Painted Hill, based on abundant volcanic clasts that typify

the formation elsewhere. Allen tentatively assigned these strata to his Deformed Gravels

of Whitewater River ([unit Qdgw]; see next section). This local occurrence is important

to our investigation because—if the strata represent the Painted Hill Formation—they

constrain the position of the main San Andreas strand.

Allen’s “Deformed gravels of Whitewater River” (unit Qdgw; Figs. 4, 9) is a unit of

moderately to steeply dipping sandy and conglomeratic rock that parallels the Mission

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Creek Fault in a narrow belt. It is poorly dated, but occupies a stratigraphic position

between (a) the underlying Miocene Coachella Fanglomerate and Pliocene Painted Hill

Formation and (b) overlying Quaternary alluvial deposits that unconformably lap over

tilted layers of the unit. Allen (1957) used these relations to assign a Quaternary age to

the deformed gravels unit. We concur: based on the likely upper age of the Painted Hill

Formation (late Pliocene or early Quaternary) and the lower age of oldest alluvial

deposits (~500-750 Ka), the Deformed gravels of Whitewater River probably are early to

middle Quaternary in age (~1.5 Ma to 750 Ka).

Clasts in the Deformed gravels of Whitewater River have ambiguous implications for

displacement on the San Andreas Fault because they include crystalline rocks of both San

Gabriel Mountains-type and Mojave Desert-type. The former are dominated by foliated,

mylonitic, and cataclastic granitoid rocks typical of the Vincent Thrust upper plate in the

San Gorgonio Pass region. They also include locally abundant Pelona Schist (albite-

actinolite-chlorite greenschist) of the Vincent Thrust lower plate. Today, Pelona Schist

crops out only in the westernmost part of San Gorgonio Pass (Fig. 4). Clasts of San

Gabriel Mountains-type are intermingled with clasts of Mojave Desert-type basement

rock, including texturally massive to foliated granitoids (monzogranite, granodiorite) and

various foliated and migmatitic granitoid and gneissose rock—all typical of drainages in

the eastern San Bernardino Mountains (Fig. 4; Dibblee, 1964, 1967; Matti and others,

1988). Absent from the unit are clasts of directly underlying basement rock (mainly

saussuritized quartz monzonite of Allen, 1957) and clasts demonstrably re-cycled out of

the underlying Miocene Coachella Fanglomerate.

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Quaternary surficial materials

In the Mission Creek study area Quaternary surficial materials occur in two different

modes: (1) younger materials (Holocene and late Pleistocene) that have accumulated

within the modern landscape, and (2) older materials (middle Pleistocene) that

accumulated on older landscapes that presumably have been re-arranged by

displacements on the San Andreas Fault. In the vicinity of Mission Creek proper both

modes form a nested series of alluvial deposits that are a focus of our investigations.

Alluvial Stratigraphy and Geomorphology of the nested alluvial complex

General statement

Where Mission Creek exits its bedrock canyon and crosses the San Andreas Fault, its

incised course bisects a landscape of surficial units that form a nested series of alluvial-

fan and alluvial-wash deposits (Fig. 9). The youngest of these is the active wash of

Mission Creek that has its headwaters in the San Bernardino Mountains and incises the

nested alluvial complex before flowing into Coachella Valley. The highest-standing

alluvial units are oldest, are the most dissected, and have progressively better developed

soil profiles; successively lower units are younger and buttress unconformably against the

older units, thus leading to the nested character of the alluvial succession (Fig. 9). This

sequence represents pulses of erosional downcutting followed by aggradational

backfilling. Each unit is a singular morphostratigraphic entity separable from others in

the succession, and each represents a significant volume of alluvial sediment presumably

sourced from one or more large drainage basins. Units older than Holocene all dip

beneath young alluvial fill of the Coachella Valley.

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Geologic ages for units in the Mission Creek nested alluvial complex are difficult to

determine. The units generally are too old for radiocarbon analysis, and frequently too

coarse grained for reliable luminescence dating. The oldest surfaces in this complex are

too old to be dated using luminescence or cosmogenic radionuclide (CRN) surface dating

techniques. All but the oldest deposits probably are younger than the Bruhnes-Matayama

reversal (780 ka), and in any event the materials are so coarse grained and sequences so

thin that magnetostratigraphic analysis is not promising.

Clemmens (2002) conducted a 10Be CRN investigation of boulders from the upper

surfaces of most of the alluvial units we recognize. In our view, the 10Be data reported by

Clemmens do not provide a consistent basis for assigning numerical ages to the

geomorphic surfaces or their underlying depositional fills (Clemmens, 2002; Clemmens

et al., 2003). Not only do ages for individual surfaces show considerable range, but mean

ages for the surfaces in some cases are out of sequence relative to their stratigraphic

position in the overall alluvial complex. Finally, the 10Be ages in general appear to be too

young relative to surface-age estimates based on pedogenic analysis (see below). To

evaluate Clemmens’ 10Be data, we conducted an analysis of 21Ne from surface boulders

(Kendrick et al., 2006; 2010). Although these ages seem to accord better with estimates

based on soils, the 21Ne data still do not provide a consistent basis for assigning numerical

ages to the geomorphic surfaces or their underlying depositional fills. To us, this means

that application of 10Be and 21Ne analyses to surface boulders is not helpful—at least to

the succession of geomorphic surfaces in the Mission Creek study area.

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Very old alluvial-fan deposits, unit 2

Qvof2 is the highest-standing deposit of the Mission Creek alluvial complex. This

unit represents a major fill event, with a thickness of at least 25 meters, although

thickness likely varies. This unit unconformably overlies various older units, including

(1) Deformed Gravels of Whitewater River, (2) Miocene Coachella Fanglomerate, and

(3) crystalline bedrock of San Gabriel Mountains- and Chocolate Mountains-type. This

unit is represented west of the modern Mission Creek by three remnants, which are

preserved at the edge of a broad surface underlain by Qvof3 (Fig. 10; see below).

Surfaces associated with these remnants are collectively referred to as A1. These

surfaces appear to have been truncated by a pair of unnamed faults. The gaps that formed

along these fault lines were eroded and broadened prior to the deposition of Qvof3. To

the east of the incised active channel, this unit is more continuous, and is associated with

a preserved surface, designated A2. Very well-developed soils are associated with surface

A2. These soils have strong red colors (2.5YR4/6), thick, abundant clay films, prismatic

structure and a 2 meter-thick silica cemented duripan (Appendix A; Table A1).

Using the degree of soil development to estimate an age of this surface is somewhat

problematic. The criteria we use throughout this study to determine approximate ages,

based on soils, include a profile development index, a rubification index, and the

weighted mean value of clay content, as compared to regional dated soil chronosequences

(Table 1). The profile development index is not designed to be used in a well-cemented

duripan, such as we have on this surface. The duripan imparts a buff or yellow color that

can mask the rubification that developed prior to cementation, so the rubification index is

not useful. Sampling the duripan to extract the clay would be problematic. So our

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estimates of age are only based on the pedogenic properties for the horizons above the

duripan, resulting in a significant underestimation, since the maximum development is

masked by the silica cementation. Using this limited part of the pedon, the profile

development values and the rubification indices both indicate that Surface A2 is greater

than 500 ka, as compared to the Cajon Pass chronosequence (McFadden and Weldon,

1987), and between 305 and 570 ka, as compared to the Merced chronosequence

(Harden, 1982, 1987; Table 1, Appendix A). However, the degree of development of the

duripan is much greater than that of regional duripans of roughly 500 ka. Such duripans

are preserved in surfaces within the San Timoteo Badland chronosequence (Kendrick and

Graham, 2002), just 50 km to the SW. Here the duripan is typified by widely-spaced

(averaging 50 cm), silica-rich seams, defining the tops of, and draping down the sides of,

large prismatic peds (pedogenic structural units). In contrast, the duripan associated with

surface A2 is nearly continuous, with much of the soil matrix engulfed in secondary silica

minerals.

Very old alluvial-fan deposits, unit 3

This unit is represented by large depositional bodies, separated by the incised Mission

Creek drainage (Fig. 10). On the upstream, western portion, these bodies are elongate,

oriented nearly east-west, and parallel to the Mission Creek strand. In this region,

deposition was confined in a corridor between escarpments along the Mission Creek fault

strand on the north, and the high-standing Miocene Coachella Fanglomerate on the south.

Adjacent to the Mission Creek fault strand escarpment are the Deformed Gravels of

Whitewater River, which comprise the high topography further towards the east in this

ancestral valley. On the downstream side of the active Mission Creek, Qvof3 deposits

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are preserved in a fan arrangement, where the deposition extended beyond, and was no

longer constrained by, the ridge of Coachella Fanglomerate bedrock. Like Qvof2, this

unit unconformably overlies the three older units, as well as a strath eroded into Qvof2,

and is more than 40 m thick, where exposed. The remnants of a once-continuous

geomorphic surface are associated with each of these depositional bodies, and are

designated B1 through B4. These surfaces are deeply incised and exhibit ridge and

ravine morphology, which is particularly pronounced on B1 and B2. The incision of

these surfaces is oriented east-west, and extends beyond the upstream edge of each

surface remnant, indicating that incision initiated when the surface was more extensive,

and before isolation of these surfaces. Incision into surfaces B1 and B2 is greater than

that of B3 and B4. This is likely a result of a stronger component of continued

downcutting into the upstream surfaces, as those surface drainages have experienced

greater headward erosion through capture by the active Mission Creek. Surface B2 is

vertically offset, which we interpret to be a result of displacement on the previously

mentioned unnamed pair of faults. The slope of surface B1 is aligned with that of the

uppermost part of surface B2 (Fig. 11), suggesting that the westernmost fault extends

across a large, unnamed tributary (in this study designated as Drainage 3) of the Mission

Creek and intersects B1 at its distal edge.

Soil development is characterized at two locations on this unit (Table 1). Pedon

MC01 is located on B2, and is characterized by strong 5YR colors and thick clay films in

a 48 cm thick Bt horizon (Table A1, Appendix A). Because of the location of this pedon

on a narrow remnant, we interpret that this soil has been eroded. Soil development was

described by McFadden (1982) at the edge of B3 as having 2.5YR colors, abundant thick

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clay films, and a sandy clay texture (Appendix A, Table A1). McFadden (1982)

classified this soil as S2, indicating that it was between 250 and 700 ka. We also

determine the age of this surface based on correlation of soil properties. For this surface,

the results of comparison to Merced and Cajon Pass chronosequences (Harden, 1987;

McFadden and Weldon, 1987) yields quite different results, depending on the criteria

used. In using the weighted mean value of clay content, this soil appears to be older than

500 ka, as compared to Cajon Pass, and older than 615 ka, when compared to Merced

(Table 1; Appendix A). However, using the profile development and rubification indices

generally suggests that this soil is greater than 305 ka and less than 570 ka, as compared

to Merced. In the case of the pedon developed on B3 (LDM 43), the rubification index

suggests an age of greater than 570 ka, though, as compared to Merced. These same

values, when compared to Cajon Pass, though, are intermediate to the values of the 55 ka

soil and the 500 ka soil in Cajon Pass. Again, the exception is the rubification index of

the LDM 43 pedon; this soil would then be estimated to be greater than 500 ka. Since we

recognize that the soil described on B2 is eroded, we weight the results of comparison

with Pedon LDM43, on B3, to infer that this soil is greater than 500 ka.

Old Alluvial Fan Deposits, unit 2

This unit (Qof2) was deposited as an alluvial fan deposit on a strath surface eroded

into Qvof3, and is at least 23 meters thick. The apex of the Qof2 fan points up the axial

wash of Mission Creek, and appears to be the depositional product of Mission Creek as it

incised into the older alluvial units, Qvof2 and Qvof3; it is inset and downstream of these

older units. Surfaces associated with this depositional unit are C1 through C3. These

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surfaces flank unnamed drainages 1 and 2b. The drainage pattern on these surfaces is

radial. The gullies head on the surface, rather than extending beyond the upstream edge.

There is a belt of no erosion on the upstream edge, where we interpret that flow is

dominated by sheetwash. This pattern suggests that these drainages resulted from

concentration of runoff that originates on each surface.

Pedogenesis associated with these surfaces displays moderate thick clay films and 7.5

to 5 YR hues in a 150 cm thick Bt horizon (Appendix A). Two pedons have been

described; LDM 38A by McFadden (1982), and MC03 in this investigation. Clay

accumulation reaches a maximum value of 19.81% at a depth of 11 to 44 cm in pedon

MC03.

The age of Qof2 is determined by two IRSL dates, sampled from depths of 4 and 5

meters below the surface yielded results of 95 ± 15 and 106 ± 15 ka, respectively (Table

2; Appendix B). Because the dose rates for these sediments showed very high

concentrations of Th and K, it would be expected that the luminescence centers would

become rapidly filled, and thus these age determinations are considered to be a minimum.

These IRSL ages agree well with age estimates based on the degree of soil development.

McFadden (1982) determined that this soil was an S3 in his classification, indicating a

broad age of between 70 – 250 ka. Correlation of pedogenic properties to those of the

Cajon Pass chronosequence (McFadden and Weldon, 1987) indicates that this soil is

intermediate to a soil dated at 55±12 ka and one determined to be 500±200 ka. The

methods of correlation include profile development indices, rubification indices, and the

weighted mean value of clay content.

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Old Alluvial Fan Deposit, unit 3

This unit (Qof3) occupies a fan-shaped area on the north side of the Mission Creek

axial wash, where it was deposited on a strath surface eroded into Qof2. The surface

associated with this deposit, D, is incised by a dense network of shallow drainages.

Similar to the drainage network associated with surfaces C1 and C2, this pattern suggests

post depositional erosion by flow originating on the surface. Pedon LDM 39A shows 7.5

YR colors, less frequent, thin to moderately thick clay films in a 59 cm thick Bt horizon

(McFadden, 1982). McFadden classified this soil as a S4, indicating that it is between 13

and 70 ka. Comparison of the profile development index of this soil to values from the

Cajon Pass chronosequence suggests that the soil is less than 67 ka; a similar comparison

to the profile development values of the Merced chronosequence suggests that the soil is

less than 103 ka. Using the rubification index as a basis for comparison, this soil would

be less than 43 ka, as compared to Cajon Pass, and greater than 103 ka, when compared

to the Merced chronosequence.

Long Profiles

The slopes of surface remnants A and B are parallel, with A just slightly above B

(Fig. 11). This suggests that there was not uplift or tilting in the interval between the

deposition of these two alluvial units, although a significant interval of time is

represented in the depth of erosion of surface A. Between the deposition of B and C,

regional tilting occurred, elevating the A and B surfaces up to the NW. This uplift

continued during the interval between the deposition of C and D. The surface remnant of

D is parallel to the local active stream, indicating that no observable tilt has occurred

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since the time of abandonment of surface D, although D is elevated above the current

drainage (Fig. 11).

Lithologic character

Stratigraphic units in the nested alluvial complex are generally similar lithologically.

Each unit consists mainly of coarse-grained sediment having gravel:sand particle-size

ratios ranging from 1:1 to 4:1. No single lithologic attribute distinguishes one unit from

another. However, within the complex itself the succession of units is obvious (Fig. 9),

and their mapping and correlation throughout the immediate Mission Creek area is fairly

straightforward (Fig. 4). Representative lithologic types include (1) cobble-pebble

gravel, (2) sandy cobble-pebble gravel, (3) gravelly sand, (4) cobble-boulder gravel, and

(5) slightly gravelly sand (rare).

Bedding characteristics.—Most outcrops afford only two-dimensional views of

stratification that suggest parallel bedding ranging in thickness from medium to

very thick; rare three-dimensional views indicate stratification lenticular over

distances of a few tens of meters to many hundreds of meters. Layer bases are

sharp, and commonly are channelate.

Depositional fabrics.—Depositional fabrics in gravelly layers range from clast-

supported to matrix-supported. Clast-supported gravelly layers commonly have

imbricated cobbles and pebbles; sandy interlayers are parallel laminated to cross

laminated. These materials are fluvial in origin. Matrix-supported fabrics are

texturally massive and consist of cobble- and boulder-size clasts surrounded by

sandy and pebbly matrix; these materials formed by sediment-gravity flow

processes, mainly as subaerial debris flows.

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Particle size, sorting and rounding.—Particle textures are quite variable. Particle

size ranges from boulders as large as 3 m to sand-size; pebble- and cobble-size

clasts are dominant, with boulders less common and sand-size particles occurring

either as matrix in clast-supported layers, as components of sandy gravel layers, or

as sand-rich layers between gravelly layers. Sorting typically is poor, given the

extreme range of particle sizes in most layers. Particle rounding is variable, with all

rounding classes represented; angular to subangular particles are common, but

subrounded to rounded particles occur.

Clast composition and weathering

The lithologic composition of pebbles, cobbles, and boulders in the nested alluvial

complex does not vary much from unit to unit. We base this conclusion not on statistical

analysis of clast counts from multiple observation stations but on numerous qualitative,

and limited quantitative observations obtained during the course of geologic mapping

(Matti and others, 1982; J.C. Matti, B.F. Cox, and R.E. Powell, unpublished data, 1980-

1986, 2009). Table 3 lists clast suites that we distil from these observations; Table 4 lists

clast-lithology frequencies from observations in units Qof2 and Qvof2.

Upper Raywood Flat Study Area

Upper Raywood Flat is an intermontane lowland situated in the north part of San

Gorgonio Pass (Fig. 7). The lowland occurs between the Mill Creek and Mission Creek

faults, and forms a conspicuous gap in the high-standing Yucaipa Ridge. This gap was

partly filled with Quaternary surficial deposits that have potential for (1) constraining the

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slip history of the Mill Creek and Mission Creek faults and (2) deciphering the uplift

history of the Yucaipa Ridge block.


Allen (1957) originally grouped crystalline rocks underlying the Raywood Flat area

within his “San Gorgonio igneous and metamorphic complex”—a terrane of plutonic and

metamorphic rocks underlying the entire San Gorgonio Pass region. Later workers

(Ehlig, 1977; Farley, 1979; Matti and others, 1983) demonstrated that crystalline rocks

beneath Upper Raywood Flat are more akin to Mojave Desert-type rocks north of the

Mill Creek Fault than to San Gabriel Mountains-type rocks south of the Mission Creek

Fault (Fig. 2). Thus, from a terrain-bounding perspective, the Mill Creek Fault is a minor

strand of the San Andreas Fault zone by comparison with the Mission Creek major

strand.

Beneath Upper Raywood Flat, no single lithology within the Mojave Desert-type

crystalline assemblage is distinctive. The rocks variably are textually massive to foliated

to migmatitic, and mainly are granodiorite and monzogranite with local gabbro-diorite

enclaves. Xenoliths and mappable screens of pre-batholithic metamorphic rock occur

locally (Farley, 1979; Matti and others, 1983). South of the Mission Creek Fault, San

Gabriel Mountains-type rocks are distinguished by (1) pervasive textural and

compositional foliation, (2) locally intense ductile and brittle-ductile deformation fabrics

(mylonite, cataclasite), (3) hornblende as a common mafic constituent, (4) pervasive

epidote mineralization, (5) common pegmatite and aplite dikes, and (6) distinctive rock

types such as Pelona Schist and Lowe intrusive complex (for regional summaries, see

Ehlig, 1982, and Joseph and others, 1982).

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Quaternary surficial materials

Surficial materials of the Upper Raywood Flat lowland accumulated in a gap in the

Yucaipa Ridge block (Fig. 7). In plan view the deposits form a “Y”-shaped pattern:

adjacent to the San Gorgonio Mountain block, two alluvial lobes separated by a spur of

crystalline rock converge southward into a single lobe that abuts against a ridge of San

Gabriel Mountains-type rocks before turning east down the South Fork of Whitewater

River. Here, the Upper Raywood Flat alluvial sequence depositionally overlaps the broad

crush zone of the Mission Creek Fault. By contrast, the two northern alluvial lobes are

faulted against the Gorgonio Mountain block by the Mill Creek Fault.

In the Upper Raywood Flat lowland, surficial deposits represent three stratigraphic

packages:

Axial deposits.—Much of the sediment was deposited by streamflows that followed

longitudinal axes of the Y-shaped drainage pattern (Figs. 4, 7). These dominantly are

gravelly deposits but include finer-grained materials. Clasts consist mainly of granitoid

rocks that are texturally massive to foliated; these range from granodiorite to

monzogranite, and include leucocratic muscovite-garnet monzogranite like that mapped

by Morton and others (1980) on the San Gorgonio Mountain block. Clasts of migmatitic

or gneissic rock are subordinate, and mafic granitoids are rare. Clasts typically range

from small boulders to small cobbles (Fig.12); in sandy layers, clasts range from small

cobbles to pebbles. Clasts typically are subrounded, but range from rounded to

subangular. Depositional fabrics include both clast-support and matrix-support,

indicating fluvial and sediment-gravity-flow processes, respectively. We have not

documented which depositional type is dominant, but in outcrops we examined, fluvial

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fabrics are common. The axial deposits are poorly dated, but they appear to be at least a

few hundred thousand years old based on the high-standing, deeply dissected character of

the deposits, and the well-developed argillic B horizons locally preserved on fill surfaces.

Tributary deposits.—Deposits attributable to throughgoing axial streamflows locally

are joined by fan-shaped sediment bodies that head on the Yucaipa Ridge block.

Deposits of the latter interfinger with those of the axial drainage, although we infer this

mainly from geomorphic relations. Where we examined them, tributary deposits are

compositionally more heterogeneous than axial deposits, reflecting both (1) the

heterogeneous composition of local Yucaipa Ridge source rocks and (2) compositional

winnowing by axial streamflows that selectively eliminated unstable foliated and

gneissose granitoid lithologies, thereby enriching these deposits in more stable granitoid

lithologies. In tributary deposits clast types include granitoid, gneissose, and migmatitic

rocks that typify crystalline rocks of the Yucaipa Ridge block.

Colluvial deposits.—Throughout the Raywood Flat area, axial and tributary deposits

have complex stratigraphic and geomorphic relations with colluvial deposits associated

with adjacent hillslopes. The colluvial materials complicate and obscure depositional and

contact relations between the major fills and their basement substrates. For example,

where the two prongs of the “Y”-shaped Raywood Flat axial regime abut against the

south flank of the San Gorgonio Mountain block, several generations of colluvial

sediment that mantle the block’s steep slopes have draped southward atop alluvial

sediment of the Raywood Flat lowland. This overlap makes it appear as though the two

deposit types (hillslope colluvium and valley-filling alluvium) originally formed a

depositional continuum, with the two interfingering throughout their history. However,

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the Mill Creek Fault separates the Raywood Flat deposits from the San Gorgonio

Mountain block, and colluvial deposits derived from that block overlap and obscure this

tectonic relationship. This example illustrates the camouflaging effect that young

colluvial deposits have on paleogeographic relations among Raywood Flat surficial

materials.

Geomorphic Setting - Upper Raywood Flat

To the north of the Mill Creek strand of the SAFZ there are several major drainages

that provide keys to decipher the sequence and timing of fault activity (Fig.5).

The North Fork Whitewater River drains the NE flank of San Gorgonio Mountain, the

highest peak in southern California, and flows into the Middle Fork Whitewater River at

the location of the Mission Creek strand of the SAF. It has a drainage area of 20.7 km2,

and basin relief of 2025 meters. This drainage system is being actively captured by

headward lengthening by the drainage currently occupying Raywood Flat, the South Fork

Whitewater River. The orientation of the upper portion of the North Fork Whitewater

drainage is influenced by the NW striking Lake Peak Fault. This fault has been active

during the Quaternary, as evidenced by displaced glacial deposits and pronounced

triangular facets along the NE side of the drainage.

Hell For Sure Canyon is parallel to, and to the east of, the North Fork Whitewater,

separated by a prominent ridgeline, Ten Thousand Foot Ridge. This drainage joins the

Middle Fork Whitewater River roughly 2 km to the east of the North Fork Whitewater

junction. It has a smaller area, 12.1 km2, and a range of elevation of 1770 m.

Just to the east of Hell For Sure drainage, the Mission Creek drainage heads on the

San Gorgonio block, at the western extent, and the Big Bear block to the east, and drains

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an area of approximately 87 km2. It flows through the Mission Creek alluvial complex of

this study, into the Coachella Valley, and integrates with the San Gorgonio River,

flowing towards the Gulf of California.

Upper Raywood Flat preserves the former course of drainages, flowing across the

Yucaipa Ridge block terrain, prior to disruption by uplift of the Yucaipa Ridge block by

fault activity. These two drainages joined at the location of Upper Raywood Flat. The

relative elevation of Upper Raywood Flat within the larger uplifted Yucaipa Ridge

indicates that the drainage was able to effectively downcut for a period of time before

being captured and rearranged. The depth of incision of this antecedent drainage valley

below the ridgeline of Yucaipa Ridge is approximately 400 meters. It is reasonable to

expect that the sharp-crested Yucaipa Ridge has been lowered by erosion, and so this

value is likely a minimum. The drainages that carved this valley had significant stream

power to keep pace with a rising landscape.

North Fork Whitewater and the Hell For Sure drainages, which currently flow to the

Whitewater River, are positioned to have flowed through, and formed this wind gap.

They are located 7.3 to 8.5 km to the SE, respectively, in agreement with the lithologic

offset of 8-10 km identified by previous workers (Matti and others, 1985). We assign an

uncertainty of 0.2 km to these measurements. The width of each of these two drainages

matches well with the widths of those in Raywood Flat (Fig. 13). The spacing between

these two drainage systems is slightly more than the spacing of the two lobes in Raywood

Flat, which would be expected, as they represent the upstream extension of the ‘Y’

shaped stream juncture. Aggradation within Raywood Flat, from both the flanks of

Yucaipa Ridge and from the San Gorgonio Mountain block has filled in this valley. In

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contrast, incision has continued in the Hell For Sure and North Fork Whitewater

drainages, as evidenced by the deep, ‘V’ shaped valley morphology, and lack of high

preserved terraces along these drainages.

This reconstruction indicates that there is approximately 930 25 meters elevation

difference between the western drainage in Raywood Flat and the North Fork Whitewater

River, and 1020 25 meters between the eastern drainage in Raywood Flat and the Hell

For Sure drainage. This suggests that Yucaipa Ridge has been uplifted by approximately

this amount during the time that lateral motion occurred on the Mill Creek strand.

Following the displacement of Upper Raywood Flat to its final position, headward

extension of the South Fork Whitewater River, which currently occupies this topographic

gap, has led to erosion in the flank of the San Gorgonio block. A reconstruction of

topography, by ‘filling in the canyons’ leads to a measurement of approximately 31.5 km

of material removed, over an area of 7 km2, since the final motion on the Mill Creek

strand. This approach to determine volume loss assumes a relatively smooth initial

condition, with no prior erosion to this escarpment. We would argue, however that the

measurement is valid in providing an estimate of denudation for two reasons. First, prior

to motion along the Mill Creek strand, the Yucaipa Ridge block was a part of the San

Gorgonio block, and formed a continuous landscape. Second, the uplift of the Yucaipa

Ridge block during the lateral offset of the Mill Creek strand, as discussed above, would

have resulted in higher topography to the south, and therefore the landscape to the north

would not likely experience significant incision. Binnie and others (2010) measured a

long-term denudation rate of 270 + 28 mm/yr at a nearby location on the San Gorgonio

block. Applying that rate of denudation to the headwaters of South Fork Whitewater

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leads to an estimate of 106 – 130 ka to remove the volume of material eroded from the

San Gorgonio block.

DISCUSSION

Drainage reconstruction- Implications for lateral fault slip

Prior to inception of slip on the Mill Creek strand, North Fork Whitewater and Hell

For Sure Rivers flowed through Upper Raywood Flat, and then on to the South Fork

Whitewater River. Mission Creek was positioned on a path to the northwest of

Walthier’s Landing, depositing Qvof2 in our alluvial complex (Fig. 14a). There is more

than one path that could have been used by flow from Mission Creek drainage to reach

the depositional area. We illustrate one route in this depiction; flow south along the west

side of the prominent bedrock island currently preserved near the juncture of South Fork

and Middle Forks, Whitewater River, followed by a sharp deflection towards the east,

along the southern edge of the island. Following deposition of unit Qvof2, the associated

surface was displaced by the pair of unnamed faults, discussed previously. Erosion

followed in the zones of weakness introduced by this faulting, and small valleys

separated the surface remnants, designated A1 (Fig. 10).

Deposition of Qvof3 followed, also preceding movement on the Mill Creek strand

(Fig. 14b). As currently preserved, Qvof3 is a more extensive deposit than Qvof2,

although it is not known to what extent Qvof2 was eroded prior to, and during the

deposition of Qvof3. Both of these units were deposited in a linear valley that was

parallel to the Mission Creek strand. We envision that this valley would have a modern

analog in Lone Pine Canyon, along the San Andreas Fault in the Cajon Pass region.

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Regional uplift and tilting of surfaces A and B followed, as evidenced in the long

profiles (Fig. 11). Extensive incision into these high surfaces occurred during this

interval. This resulted in the prominent east-west oriented ridge and ravine pattern on

these surfaces. The Mill Creek strand became active, and Mission Creek was moved into

a position where flow down an ancestral Whitewater Canyon was optimal (Fig. 14c).

The lower portion of Whitewater Canyon follows Whitewater fault, and was established

along this zone of weakness in the rock fabric. It should be noted, though, that at the

time that Mission Creek flowed in this canyon, it would not have been deeply incised, as

it is currently. High terraces are present throughout Whitewater Canyon, which may

have been deposited at this time. Dating of these is planned for future research.

After some lateral displacement along the Mill Creek Fault, Mission Creek was

blocked from easy access to Whitewater Canyon by the high ground just to the NW of

our Surface B1 (Fig. 10), and was positioned to incise into Qvof3 (Fig. 14d), by way of

stream reach 3 (Fig. 10).

It is difficult to determine the position of the upstream reach of Mission Creek when

the unit Qof2 was deposited. It is possible that it occurred at any of the positions

represented by figures 14d through 14f. We prefer the interpretation that this deposition

occurred at the position represented by figure 14f, when the Mill Creek strand had moved

the 8.5 km that represents the entire slip on this strand, though we recognize that this is

not a unique solution. This scenario is preferred because of the subsequent slip on the

Pinto Mountain Fault, as discussed below. Deposition of this unit following completion

of slip on the Mill Creek strand of the SAF (time represented by Fig. 14f) is consistent

with the amount of denudation experienced in the headwaters of the South Fork,

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Whitewater River, as discussed above. Additionally, this unit, Qof2, is tilted with respect

to unit Qof3 (Fig. 11), suggesting that some significant interval of time separated the

emplacement of these two units. After deposition of unit Qof2, flow for a time was along

the stream segment 2b, incising and isolating the two surface remnants C1 and C2 (Fig.

14f). The final history of this alluvial complex, as considered in this study, was the

emplacement and incision of unit Qof3 (Fig. 14g, h).

When the Mission Creek drainage was laterally positioned to flow against the

northern edge of B1, we hypothesize that it diverted to the east, carving the escarpment

that follows the trend of the Mission Creek strand. The final position of the Mission

Creek drainage was accomplished by capture towards the modern drainage, and

abandonment of the drainage segment 2b.

The 7.3 – 8.5 km of offset documented for the Mill Creek strand leads to a

determination of slip rate of between ~18 and 22 mm/yr for the interval of 500 – 100 ky

BP. This rate of slip is higher than those determined to the north and south of the San

Gorgonio Pass region. At the Plunge Creek location to the north, on the San Bernardino

strand, McGill and others (2013) establish a slip rate of 8.3-14.5 mm/yr for the past 35

ka, and 6.8-16.3 mm/yr for the past 10.5 ka. To the south, at the Biskra fan site, Behr and

others (2010) find 14 – 17 mm/yr slip for the past 45 to 54 ka. The differences in these

rates, may be attributed to the differing time intervals over which slip was measured.

Previous studies have suggested that the slip rates along the San Andreas Fault system

vary through time, with slip transfer to the San Jacinto Fault (Bennett et al., 2004).

Additionally, the Mill Creek strand represents an orientation that is aligned with the plate

boundary, and would be favorable for the efficient transfer of slip along the San Andreas

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Fault system. Three-dimensional modeling indicates an increased efficiency during this

time interval, and a modeled slip rate of 21 mm/yr, in agreement with our results (Cooke

and Dair, 2011).

Drainage Reconstruction – Implication for uplift of structural blocks

Spotila and others (1998, 2001) propose that the Yucaipa Ridge has been uplifted 3 –

4 km, based on very young apatite (U-Th)/He ages. Their thermochronologic models

suggest that the block experienced very rapid uplift, with rates as high as 7 mm/yr during

the interval 1.25 – 1.65 Ma. Following this rapid uplift, an additional 1 - 2 km uplift is

proposed during the interval 1.25 Ma to present, although the models do not constrain the

temporal distribution of this uplift within this interval of time. Their model further

suggests that uplift on the Yucaipa Ridge block exceeds that of the San Gorgonio block,

and that Yucaipa Ridge has been uplifted approximately 1 km higher than San Gorgonio

block.

The reconstruction of drainages in this study allows us to provide further evidence as

to the history of uplift of these two blocks. The deep incision of the drainage system of

Hell For Sure and North Fork Whitewater through Raywood Flat suggests that both the

Yucaipa Ridge and San Gorgonio blocks were being actively uplifted. This was likely in

response to oblique motion along the Mission Creek strand, as the strand became oriented

counter clockwise with respect to the plate motion vector. Following inception of the

Mill Creek strand, and lateral displacement and beheading of Upper Raywood Flat

drainage system, there has been approximately 975 70 m of uplift on the Yucaipa

Ridge block relative to the San Gorgonio block. We can use our best determination of

the age of surface B, greater than 500 ka, to infer that the differential uplift between the

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Yucaipa Ridge and San Gorgonio blocks occurred relatively recently. Since the Mill

Creek strand forms the structural boundary between these blocks, it is not unexpected that

the uplift differential occurred during the time of activity of this fault system. The uplift

recorded by this drainage reconstruction, combined with the duration of activity of the

Mill Creek fault strand leads to an uplift rate of ~ 2.3 – 2.6 mm/yr.

Mission Creek and Mill Creek strands

Prior to disruption by the Pinto Mountain Fault, the Mill Creek strand ostensibly was

a throughgoing structure that splayed northeast away from the older Mission Creek

strand, which, by this time, had been long abandoned by the San Andreas Fault (~ 500 ka

according to Matti and others, 1985, 1992; Matti and Morton, 1993). Whether the Mill

Creek strand initiated (1) by reactivation of the old Mission Creek trace or (2) by

pioneering a closely associated fault within the latter’s crush zone, the two strands

southeast of the Whitewater River forks would have been closely spaced and would have

trended southeasterly. However, west of the Whitewater River, where the Mission Creek

Fault crush zone gradually adopts a more westerly orientation, the zone would not have

been compatible with San Andreas slip vectors and efficient slip on the coextensive (but

younger) Mill Creek strand would have been progressively more difficult. Accordingly,

Matti and others (1985, 1992) reasoned that—at the confluence of the Whitewater River

forks—the Mill Creek strand stepped inboard of the Mission Creek crush zone. This new

trace would have been aligned better with San Andreas slip vectors, as suggested by its

regional orientation subparallel to the Coachella Valley and Mojave Desert segments of

the San Andreas (Figs. 1, 2). Indeed, three-dimensional modeling confirms the increase

in dextral efficiency during the time of activity of the Mill Creek strand (Cooke and Dair,

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2011). These regional considerations make sense, but they do not address what the

divergence zone between the Mill Creek and Mission Creek strands would have looked

like.

Our hypothesis that the Pinto Mountain Fault has truncated and sinistrally displaced

the Mill Creek Fault provides insight into this question. When 1 to 1.25 km of left-slip is

restored on the Pinto Mountain, the north-trending crush zone at the southeast end of the

Yucaipa Ridge block that we (and Matti and others, 1982, 1983) associate with the Mill

Creek Fault returns to a position close to the last observed occurrence of the strand

southeast of the Middle Fork of the Whitewater River. There, the Mill Creek and

Mission Creek faults either occupy the same trace or are closely spaced. Given this, and

given that their separate paths farther west in San Gorgonio Pass converge toward this

coextensive reach, then the point where the two faults originally would have diverged can

be constrained to a small area near where the Middle and South Whitewater Forks

currently join. Here, in order to acquire its west-northwest path along the Middle Fork of

the Whitewater River (Fig. 6), the Mill Creek strand would have had to splay abruptly

northward away from the Mission Creek strand. The original geometry of this

divergence is not obvious from current fault patterns. However, restoration of ~1 to 1.25

km of left-slip on the Pinto Mountain Fault returns the Mill Creek strand on the Yucaipa

Ridge block to a position where its north orientation makes sense. There, at the mouth of

the Middle Whitewater Fork, all that is needed to connect this trace of the fault with its

counterpart to the southeast is a short concealed segment that we infer between the two

bedrock traces. This inferred segment would have to be north-trending (1) in order to be

compatible with the north trend of the strand where it crosses Yucaipa Ridge, and (2) in

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order to efficiently jump the Mill Creek strand inboard to its path up the Whitewater

Middle Fork. Absent such a segment, we don’t see how otherwise to connect (a) the

west-northwest-oriented Mill Creek strand that extends through San Gorgonio Pass and

then westward for nearly 50 km along the base of the San Bernardino Mountains with (b)

the northwest-oriented Mill Creek/Mission Creek fault that extends for more than 100 km

southeast of the Whitewater forks (Fig. 2).

Our reconstruction of the Mill Creek strand across the restored Pinto Mountain Fault

has three consequences:

(1) Sinistral displacement on the Pinto Mountain Fault disrupted right-slip on the

Mill Creek Fault, effectively ending its role as a throughgoing strand of the San Andreas

Fault. We explore timing for these events in the next section.

(2) Sinistral offset of the truncated Mill Creek Fault by 1 to 1.25 km would have

significantly rearranged landscapes, especially the deeply entrenched canyon of the

Middle Fork of Whitewater River. For much of its length, that landform has a west-

northwest orientation. However, where the Middle Fork joins the South Fork, the

entrenched Middle Fork curves abruptly southward. If this abruptly curving segment

were part of the landscape prior to sinistral slip on the Pinto Mountain Fault, then

palinspastic reconstruction of that fault necessarily would reposition this landform east of

its current location. This would place the upstream portion of the Middle Fork

Whitewater River to Hell for Sure Drainage with the unnamed drainage, a tributary to the

Mission Creek drainage, designated on figure 6. It would align the North Fork

Whitewater River with the wind gap and the downstream portion of the Middle Fork

Whitewater.

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(3) In order for left-slip on the Pinto Mountain Fault to have truncated and offset

the Mill Creek Fault but not similarly affected the Mission Creek Fault, Pinto Mountain

slip must have fed into the Mission Creek crush zone that trends up the South Fork of

Whitewater River. Otherwise, the Mill Creek Fault could not have been displaced

(deflected sensu Matti and others, 1985, 1992) to its current position at the southeast end

of the Yucaipa Ridge block. This is consistent with the fact that the Pinto Mountain Fault

meets the Mission Creek crush zone at a low oblique angle, suggesting that (a) the two

faults physically merge and (b) allowing sinistral slip to feed into the Mission Creek

zone. This poses kinematic problems comparable to where other left-lateral strike slip

faults interact with the San Andreas (e.g., left-lateral faults of the Eastern Transverse

Ranges; Powell, 1993; Dickinson, 1997; Langenheim and Powell, 2009).

Pinto Mountain Fault

The Pinto Mountain Fault is a sinistral strike-slip fault that traverses the boundary

between the Mojave Desert and Transverse Ranges provinces. The fault is represented

by discontinuous scarps and bedrock traces that trend westward from the Twentynine

Palms area to Morongo Valley. The fault’s inception age is not agreed to, but Quaternary

displacements are well documented by deflected drainages, linear scarps, pressure ridges,

shutter ridges, closed depressions and springs (Bryant, 2000). Holocene fault movements

are documented by a palesoseismic investigation near Twentynine Palms (Cadena et al.,

2003, 2004). This location records five or six earthquakes that resulted in surface

rupture. The earliest event occurred between 11.1 – 14.1 ka, a possible later event

occurred at about 9.4 ka, and four events have occurred since 9.4 ka (Cadena et al.,

2004). Holocene displacements probably have occurred to the west, where discontinuous

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scarps in Quaternary alluvium occur between the communities of Joshua Tree and Yucca

Valley, but the stratigraphy and age of the alluvial deposits has not been documented

other than by general age estimates based on the development of soil properties and the

degree of desert pavement formation (Bryant, 2000; see the general geologic setting

presented by Nishikawa and others, 2003, based on unpublished geologic mapping by

J.C. Matti).

Information about the Pinto Mountain Fault in the vicinity of the Mission Creek study

area is known mainly from reconnaissance geologic mapping by Dibblee (1964, 1968b)

and Matti and others (1988) and from groundwater investigations (Bader and Moyle,

1958). Here, in the Morongo Valley area, the fault is concealed by Holocene alluvial

deposits but forms prominent scarps in older Quaternary deposits that probably are late

Pleistocene in age (Matti and others, 1988). Traced westward to the Whitewater River

the fault forms conspicuous crush zones in crystalline rocks and locally forms scarps in

Quaternary deposits. Precariously balanced rocks of undetermined age are located just

north of the fault between Morongo and Yucca Valleys (Brune and others, 2010),

suggesting that movement during the latest Holocene has been minimal.

The Pinto Mountain Fault zone as a whole has a reported maximum displacement of

between 16 km (Dibblee, 1975) and up to 19 km (Batcheller, 1978), determined from

offset Mesozoic and older crystalline bedrock units. Alternate hypotheses for the spatial

distribution of offsets include: (1) maximum slip is constant along the entire length of

the fault (Hopson, 1996); (2) a slip maximum occurs in the central region, diminishing as

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the fault becomes displaced vertically, north side up, to the west (Dibblee, 1968b); (3)

slip diminishes from west to east (Langenheim and Powell, 2009).

Prior slip-rate estimates range from 1 to 5.3 mm/yr, based on inferred ages of offset

units. Anderson (1979) used the total offset along the fault to estimate a slip rate of 1-2

mm/yr, based on an inferred inception of fault movement of 8 – 16 Ma. Langenheim and

Powell (2009) suggest that the inception of faulting on the sinistral faults of the Eastern

Transverse Ranges, of which the Pinto Mountain is the northernmost, occurred during the

late Miocene, likely younger than 6 Ma. This inception time would suggest long-term

slip-rates of 2.67 to 3.12 mm/yr.

Other workers used a 9.6 km offset measurement of a fanglomerate, estimated by

Dibblee (1975) to be Pleistocene. This leads to a long-term slip-rate estimate of 5.3

mm/yr, using the entire duration of the Pleistocene (Bird and Rosenstock, 1984). Other

studies interpreted the age of this offset landform to be Pleistocene to Miocene, resulting

in slip rate estimates of 0.3 to 5.3 mm/yr (Wesnousky, 1986; Peterson and Wesnousky,

1994). A Miocene age for this surface is inconsistent with a Miocene age of inception of

the fault system.

Our study suggests that, for a limited time in the recent geologic history, the slip rate

on the Pinto Mountain Fault was significantly higher than these previous determinations.

Between 1 and 1.25 km of slip along the Pinto Mountain Fault offset and disrupted the

Mill Creek strand following cessation of slip along that strand of the SAF. If the entire

slip along the Mill Creek strand was completed by the time of emplacement of deposits

associated with surface C1, with a preferred age estimate of ~ 100 ka, then slip rate on the

Pinto Mountain fault would be 10 - 12.5 mm/yr for this interval. If the emplacement of

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surface C1 occurred prior to complete slip on the Mill Creek strand (e.g., emplaced at

time indicated by Fig. 14e rather than that of Fig. 14f), then the slip rate on the Pinto

Mountain would be higher. We would propose that the rate of slip on the Pinto Mountain

Fault has been temporally variable. The Pinto Mountain Fault seems to have not been

active during the time of activity along the Mill Creek strand; had it been, it would have

complicated the trace of this very linear fault strand. If the Pinto Mountain Fault was

inactive at this time, then the slip rate for the past ~500 ka would average 2.5 mm/yr

along this fault.

CONCLUSIONS

Geologic mapping, geomorphic analysis and geochronological determinations allow

for an increased understanding of the evolution of the San Andreas Fault system through

the San Gorgonio Pass. The Mill Creek strand, which is frequently used for modeling of

active slip through this region, is found to have been active during the interval of

approximately 500 to 100 kyBP.

Erosion of the headwater region associated with the Upper Raywood Flat indicates

that approximately 106 – 130 ka is required to remove the volume of missing material,

using a nearby independent rate of erosion. This confirms the IRSL ages of 106 to 95 ka

for the cessation of movement of the Mill Creek strand.

Displacement of the Mill Creek strand, following abandonment of the strand, by the

Pinto Mountain Fault suggests that the Pinto Mountain fault has had variable slip rate

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over the past 500 ka, ranging from 2.0 – 2.5 mm/yr for the entire period of time, to ~10 –

12.5 mm/yr for the interval 100 ka to present.

The Yucaipa Ridge block has been uplifted 975 ± 70 m, relative to the San Gorgonio

block. This uplift occurred during the period of activity of the Mill Creek strand, and the

uplift rate is 2.3-2.6 mm/yr. The lateral slip rate on the Mill Creek strand during this

interval was 18-22 mm/yr.

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REFERENCES CITED

Allen, C.R., 1957, San Andreas fault zone in San Gorgonio Pass, southern California: Geological Society of America Bulletin, v. 68, p. 319-350.

Anderson, J.G., 1979, Estimating the seismicity from geologic structure, for seismic risk studies: Bulletin of the Seismological society of America, vol. 69, p. 135-158.

Bader, J.S., and Moyle, W.R., Jr., 1958, Data on water wells and springs in Morongo Valley and vicinity, San Bernardino and Riverside Counties, California: U.S. Geological Survey Open-File Report, 31 p.

Batcheller, J. III, 1978, Quaternary geology of the Mojave Desert-eastern Transverse Range boundary in the vicinity of Twentynine Palms, California: University of California, Los Angeles, unpublished M.S. thesis, 157 p.

Behr, W.M., Rood, D.H., Fletcher, K.E., Guzman, N., Finkel, R., Hanks, T.C., Hudnut, K.W., Kendrick, K.J., Platt, J.P., Sharp, W.D., Weldon, R.J., Yule, J.D., 2010, Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas Fault at Biskra Palms Oasis, Southern California: GSA Bulletin v. 122, no. 9-10, p. 1360-1377.

Bennett, R.A., Friedrich, A.M., and Furlong, K.P., 2004, Codependent histories of the San Andreas and San Jacinto fault zones from the inversion of fault displacement rates: Geology, v. 32, p. 961-964.

Bevis, M., Hudnut, K., Sanchez, R. , Toth, C. , Grejner-Brzezinska, D., Kendrick, E. , Caccamise, D.; Raleigh, D., Zhou, H., Shan, S., Shindle, W., Yong, A., Harvey, J., Borsa, A., Ayoub, F., Shrestha, R., Carter, B., Sartori, M., Phillips, D., and Coloma, F., 2005, The B4 Project: Scanning the San Andreas and San Jacinto Fault Zones, EOS, Transaction of the American Geophysical Union, Fall Meeting 2005, abstract #H34B-01.

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LISTING OF FIGURES AND TABLES

Figure 1. Index map showing the location of the San Gorgonio Pass within a regional

context. The San Gorgonio Pass is located between a step-over of the Mojave Desert

(MDS) and Coachella Valley (CVS) Segments of the San Andreas Fault.

Figure 2. Map showing location of Mission Creek and Raywood Flat study areas

(black boxes), relative to faults and basement-rock terranes in the region. Faults in red

are modern strands of the San Andreas Fault and the San Jacinto Fault; progressively

older strands of the San Andreas Fault include the Mill Creek strand (orange), Mission

Creek strand (Yellow), and Wilson Creek strand (blue). Magenta indicates the late

Miocene Banning fault. Paired black arrows indicate movement direction on strike-slip

faults; bar-and-ball symbol indicates down-thrown block of normal fault. Modified

slightly from Matti and others (1982) and Matti and Morton (1993). BPF, Beaumont

Plains Fault zone; CHH, Crafton Hills Horst and graben complex, GHF, Glen Helen

Fault; ICF, Icehouse Canyon Fault; MVF, Morongo Valley Fault; SCF San Antonio

Canyon Fault; SGPF, San Gorgonio Pass Fault zone; VT, Vincent Thrust.

Figure 3. Diagram illustrating displacement history of various faults in the greater

San Gorgonio Pass region, showing the timing of displacement (solid vertical lines) and

amount of right slip (in parentheses). From Matti and others, 1992; Matti and Morton,

1993. Note scale change at 1 Ma.

Figure 4. Geologic Map of the Mission Creek and Raywood Flat study areas.

Figure 5. Geomorphic summary of study area. Structural blocks include San

Gorgonio (SG), Yucaipa Ridge (YR), Pisgah Peak (PP), Kitching Peak (KP), and Devil’s

Garden (DG). Major drainages include North Fork Whitewater (NFWW), Hell For Sure

(HFS), Mission Creek (MC). The headwaters of the South Fork Whitewater (SFWW) are

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also delineated. Faults and fault strands are shown in red. White lines are the locations

of cross profiles in figure 13.

Figure 6. Google Earth image looking north to where the Mission Creek and Mill

Creek strands of the SAF intersect with the Pinto Mountain fault. This occurs at the

junction of three forks of Whitewater River, North Fork (NFWW), Middle Fork

(MFWW), and South Fork (SFWW). HFS, Hell-For-Sure Canyon, PH, volcanic-clast-

bearing sedimentary rocks that we correlate with the Painted Hill Formation; WR,

Whitewater River. Faults include: red, Mill Creek strand, San Andreas Fault; blue,

Mission Creek strand, San Andreas Fault; yellow, Pinto Mountain Fault. Line pattern

indicates crushed and sheared crystalline rocks of the Mission Creek fault zone.

Figure 7. Google Earth image looking north toward the alluviated Raywood Flat gap

in the Yucaipa Ridge block. Yellow fill represents surficial deposits; black dashed lines

indicate likely axes of longitudinal streams that deposited much of the materials that

underlie upper Raywood Flat (URF). Matti and others (1985, 1992) proposed that these

longitudinal streams headed onto the San Gorgonio block before drainage patterns were

disrupted by dextral slip on the Mill Creek strand of the San Andreas Fault. LRF, Lower

Raywood Flat; MCJ, Mill Creek Jump-off; SGM, San Gorgonio Mountain; MFWW,

Middle Fork Whitewater; SFWW, South Fork Whitewater; WWJ, Whitewater Jumpoff.

Faults include: red, Mill Creek strand, San Andreas Fault; blue, Mission Creek strand,

San Andreas Fault . Red arrow, orientation of figure 8.

Figure 8. View of Whitewater jump-off, showing contrasting lithologies across the

Mill Creek strand.

Figure 9. Geology of the surficial materials in the Mission Creek alluvial sequence,

located downstream from where Mission Creek exists the San Bernardino Mountains.

Younger units are progressively incised into older units, in the following order: very

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young wash deposits (Qvyw) incise Holocene through Pleistocene alluvial-fan deposits

(Qyf), that incise latest Pleistocene alluvial fan deposits (Qof3), that incise Late

Pleistocene fan deposits (Qof2). All of these units incise high-standing, well dissected

alluvial fan deposits (Qvof3, Qvof2) that have well-developed soil development,

indicating ages greater than 500 ka. Yellow dots, locations of soil descriptions; MC,

Miocene Coachella Conglomerate, including volcanic units (magenta color); Qdgw,

deformed gravels of Whitewater River of Allen (1957). MC, Mission Creek; WR,

Whitewater River. Faults include: red, Mill Creek strand, San Andreas Fault; blue,

Mission Creek strand, San Andreas Fault. Geology modified slightly from Matti and

others, 1982, 1983, 1985, 1992; Matti and Morton, 1993.

Figure 10. Geomorphology of Mission Creek alluvial complex.

Figure 11. Long profiles of the Mission Creek geomorphic surfaces.

Figure 12. Character of deposits of Raywood Flat.

Figure 13. Reconstruction of drainage- Hell For Sure and North Fork Whitewater

with Raywood Flat. Locations of cross profiles are from figure 5.

Figure 14. Reconstruction of slip on Mill Creek fault strand. W, Walthier’s Landing.

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Table 1. Summary of pedogenic properties of the surfaces preserved in the Mission

Creek study area.

Table 2. Luminescence results of Qof2.

Table 3. Representative clast suites in stratigraphic units of the Mission Creek nested

alluvial complex.

Table 4. Clast-lithology frequency from observations in unit Qof2 (stations 1-3) and

Qvof2 (station 4). Every clast larger than ~ 2 cm within a 2m x 2m grid was counted.

Clast size ranges from 2 cm to 30 cm, with boulders comprising less than 1%.

Appendix A – Soil development

Appendix B – Luminescence Dating

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mission_creek_ · web viewthe mission creek and mill creek strands follow very different paths...

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