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Quaternary Exhumation of the Verdugo Mountains? Constraints from (U-Th)/He Ages and Geomorphology
Jeanette C. Arkle
Advisor: Dr. Phillip A. Armstrong
Quaternary Exhumation of the Verdugo Mountains? Constraints from (U-Th)/He Ages and Geomorphology
Jeanette C. Arkle
Advisor: Dr. Phillip A. Armstrong
Undergraduate Thesis
Bachelor of Science in Geology
Department of Geological Sciences California State University, Fullerton
May 2008
Table of Contents Abstract . . . . . . . . . 1 Introduction . . . . . . . . . 2 Geologic Background . . . . . . . 3 Methods . . . . . . . . . 4 Low-temperature Thermochronology Methods . . . 4 Tectonic Geomorphology Methods . . . . . 6 Results and Interpretations . . . . . . . 8 AHe age Results . . . . . . . 8 AHe age Interpretations . . . . . . 11 Geomorphology Results . . . . . . 12 Geomorphology Interpretations . . . . . 14 Discussion . . . . . . . . . 16 Conclusions . . . . . . . . . 19 Acknowledgments . . . . . . . . 21 References Cited . . . . . . . . 22 Figures . . . . . . . . . 24 Appendix A . . . . . . . . . 37 Grain Dimension and Morphology Data Appendix B . . . . . . . . . 48
Individual Apatite Grain Descriptions and Photographs
Appendix C . . . . . . . . . 58 Apatite (U-Th)/He Data Appendix D . . . . . . . . . 61 Geomorphic Analysis
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Abstract
The Verdugo Mountains are bound to the south by the Verdugo blind-thrust and to the
north by the Sierra Madre thrust of the San Gabriel Mountains. This is a key location to
assess the rates of movement on a fault located beneath a major metropolitan area.
Evidence from low-temperature thermochronometry, geomorphic properties, and
stratigraphic sequences suggest that the VMB was recently (late Pliocene-Quaternary)
uplifted and exhumed. Apatite (U-Th)/He ages (AHe), from samples collected along
transects across the 950-m-relief VMB antiform, range from 13±4 to 78±2 Ma.
Elevation-projected AHe ages suggest approximately 2 km of erosion since 2.0 to 0.5 Ma
at an exhumation rate of 1 to 4 mm/yr, respectively. Geomorphic analyses support and
are consistent with young and rapid uplift suggested by the AHe ages. This block is a part
of the complex fault system that generally youngs southward away from the San Gabriel
Mountains and suggests that the range-front fault system has stepped southward into the
basin so that the basin-bounding fault may now be the Verdugo Fault.
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Introduction
The Verdugo Mountain block (VMB) (Verdugo Hills) is situated in the Los
Angeles basin and abuts the Transverse Ranges to the south (Figure 1). The VMB is
bound to the north by the Sierra Madre fault and the Verdugo fault to the south. The
structural geology and tectonic evolution of the Los Angeles basin is well documented,
but is very complex. Ingersoll and Rumelhart (1999) model the evolution of the Los
Angeles basin in three stages that include: (1) transrotation (18-12 Ma), (2) transtension
(12-6 Ma), and (3) Transpression (6-0 Ma). They interpret that exhumation of Mesozoic
middle-crustal rocks were a result of extension along detachment faults due to
transrotation. The activation of the San Andreas Fault occurred during transpression,
which formed positive inversion structures such as the Puente Hills, San Jose Hills, and
the Verdugo Hills.
The San Gabriel Mountains are a major block of the Transverse Ranges in
Southern California and have relatively well-constrained exhumation rates based on
apatite (U-Th)/He and fission-track data (Blythe et al., 2002). Meigs et al. (2003)
estimated uplift and exhumation of the VMB by measuring stratigraphic thickness across
structural highs and lows; they coupled these data with the San Gabriel Mountains (U-
Th)/He data from Blythe et al. (2000) to conclude that the VMB experienced uplift of
2.5-4.0 km and exhumation of ~1.5-2 km at rates of 1.1 km/Myr and 0.9 km/Myr,
respectively. These estimates assume that the Saugus Formation is complete and
continuous over the VMB. However, no quantitative data have been published for the
VMB to positively constrain exhumation. In this study, apatite (U-Th)/He ages (AHe)
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and geomorphic data are used to better constrain timing and rates of exhumation in a
region associated with the major fault systems in California.
Geologic Background
The VMB is expressed as an east-west trending antiform that is ~15 km long and
~6 km wide, has a maximum elevation of 953 m, and is composed mainly of late
Mesozoic quartz monzonite-granodiorite (Dibblee 1989; 1991; 2001) (Figure 2). The
Verdugo fault is a blind thrust that dips north under Miocene strata and is associated with
mid-crustal decollement (Fuis, 2001), which is part of a generally southward-propagating
zone of contractional deformation related to the “Big Bend” region of the San Andreas
Fault (Figure 3) (Luyendyk, 1991; Yeats, 1981). The structure between the hanging wall
of the Verdugo fault to the footwall of the Sierra Madre fault is characterized by an
anticline-syncline pair that form the Verdugo Mountains and Merrick syncline (Figure 4)
(Meigs et al., 2003). The nonmarine Saugus Formation was deposited from about 2.3 to
0.5 Ma and is exposed in the Merrick syncline and within the footwall of the Verdugo
fault (Levi and Yeats 1993; Meigs et al., 2003). The Verdugo fault is completely covered
by sediments, thus it is unknown how active it has been in the Quaternary.
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Methods
Low-temperature Thermochronology Methods
Low-temperature thermochronology has proven to be a powerful tool for
determining thermal histories in the upper 1-6 km of the crust (e.g., Armstrong et al.,
2003; Blythe et al., 2000; Ehlers and Farley, 2003; Green et al., 1989). Specifically,
apatite (U-Th)/He ages may be used to constrain the cooling history of rocks in the upper
1.5-3 km of the crust (Farley, 2000), which allows us to measure uplift and exhumation
due to tectonic or denudation processes based on the accumulation of radioactive isotopes
(Wolf et al., 1996). The spontaneous decay of 235U, 238U, 232Th, and 147Sm, produces α
particles (4He) and when at temperatures of less than 70-75 ˚C, the isotopes behave as in
a closed-system and He daughter products are partially retained in the apatite crystals.
Conversely, at higher temperatures, isotopes behave as in an open-system and He
daughter products are diffused out of the crystal. At temperatures above 85 ˚C there is
complete diffusive loss of He and the AHe age is reset to zero (Wolf et al., 1998). The
retention of He is thermally sensitive, thus the switch from diffusive loss to retention is
gradual within an apatite crystal, and occurs over an interval of ~40-70 ˚C and He is only
partially retained.
Depths that correspond to these temperatures are referred to as the partial
retention zone (PRZ) (Farley, 2000). Assuming a normal geothermal gradient (in
southern California it is ~30 ˚C/km (Wright, 1991)) and a surface temperature of 15 ˚C,
the PRZ corresponds to crustal depths of ~1.5-2.5 km. The specific temperature at which
an apatite crystal retains all He daughter products, which reflects its apparent age, is
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dependant on cooling rate, grain size, and chemical composition. This temperature is
referred to as the closure temperature (Tc) (Ehlers and Farley, 2003). For typical crystal
sizes and compositions, and at rapid monotonic cooling rates (greater then 10 oC/m.y.),
the closure temperature for apatite is ~70 ˚C (Farley, 2000). Thus, AHe ages record a
cooling history from which exhumation rates can be calculated (Farley, 2000).
Apatite is an accessory mineral usually found in medium to coarse-grained
granodiorites or tonalities. After using normal apatite separation techniques, the
following criteria are used to pick individual crystals for dating: (1) the grain should be
greater than 80 µm in diameter to ensure there are substantial amounts of He to work
with and to limit the possibility of α ejection; (2) grains are euhedral, to reduce the error
for imbalanced (or parentless) isotopes; (3) grains should be inclusion free to reduce the
possibility of high-U phases, such as zircon, that would supply excess uranium and
thorium and yield an erroneously high He date (Ehlers and Farley, 2003; Wolf et al.,
1996). Original grain sizes and geometries are measured and AHe ages are multiplied by
a factor of 1.2 to 1.5 to correct for α ejection (Figure 5) (Ehlers and Farley, 2003).
To extract He, individual apatite crystals are heated with a laser to ~900 ˚C, which
degasses the He that is collected and analyzed in a mass spectrometer (House et al., 2000).
A second reheating will reveal inclusions within an apatite crystal if He is re-extracted; in
these cases the sample is omitted. Grains are dissolved in nitric acid and sent through an
inductively coupled plasma mass spectrometer (ICP-MS) where U, Th, and Sm content
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are measured. The apparent age (or AHe age) reflects the amount of He present at time t
and is determined by the equation from Wolf et al. (1998):
4He=8 238U (e λ238t - 1) + 7 (238U/137.88) (e λ235t - 1) + 6 232Th (e λ232t -1)
where:
4He, 238U, and 232Th = the measured present-day amount of these isotopes
λ238, λ235, and λ232 = the decay constants for each isotope
t = the Helium age
Tectonic Geomorphology Methods
The field of tectonic geomorphology provides quantitative methods for evaluating
relative ages of topographic features and for assessing the mechanisms and rates of
geomorphic processes (Burbank and Anderson, 2001). Tectonics and climate are the two
primary Earth processes that tend to shape topography. Inherently, tectonic activity
begets specific geomorphic features that can be associated with a particular type of fault
and to some extent the rates and/or timing of activity. Folds, like the Verdugo anticline,
are geomorphic features that often form in response to buried faults, such as the blind
thrust faults (Montgomery and Brandon, 2002). The morphologic expression of the VMB
shows distinct spatial variation of denudation and topographic dissection. These features
are quantified and compared with AHe ages to make general interpretations of rates of
exhumation across the VMB.
Elevation profiles that trend N-S and E-W were extracted from 10m Digital
Elevation models (DEM) that provided a measure to qualify general slope morphologies
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and to compare the varying regions of the VMB. The gradient for of the individual slope
faces was calculated (change in elevation over the change in distance) along four
transects and a mean value was derived. The gradient values were used to calculate the
angle between the horizontal plane and the surface of the mountain side. This calculation
yields individual slope angles, which were averaged.
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Results and Interpretations
AHe age Results
A suite of nine samples collected along transects across the 950-m-relief VMB
antiform are identify as late Mesozoic quartz monzonite-granodiorite and quartz diorite,
with rock ages of 90 and 120 m.y. (Dibblee, 1989; 1991; 2001), respectively. The
measured AHe data for individual grains from each of the samples are summarized and
given in Table 1. The raw AHe ages are multiplied by a factor of 0.70 to 0.86 (the Ft) to
correct for α ejection (Ehlers and Farley, 2003). The FT values are based on
measurements of original grain size and grain geometry (see Appendix A for these
measurements). Cracks and chipped grains provide conduits where He may diffuse out of
the crystal and thus, is not accounted for in the gas collection process. The FT correction
increases AHe ages slightly (by 2-4 M.y.) to a more accurate corrected AHe age that
accounts for these defects.
The V1 –V8 apatite (U-Th)/He ages were determined from 2-4 grains from the
corrected AHe ages for each sample (Figure 6). Uncertainties are reported as the standard
error (at 1σ). Problem grains were excluded based on three criteria: (1) helium was re-
extracted; (2) the AHe age was older than the rock age; and (3) the age was an outlier (i.e.
the crystal contained an inclusion(s)). The grains V1c and V1d both re-emitted He upon a
second heating and the latter was not apatite. These grain ages were therefore excluded
from sample V1 and the mean AHe age is 13±4 Ma. Grains from sample V2 show a large
variation in AHe ages (~27 m.y.) however, none of these grains violate the given criteria
for a “bad” grain and were all used for a mean AHe age of 56±6 Ma. All four grains
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from sample V3 have AHe ages that are close in range (~ 6 m.y.); however, He was re-
extracted from samples V3a and V3c and thus, they were not included in the mean
sample AHe age of 13±3 Ma. Sample V4 contained two grains, V4c and V4d, with AHe
ages that are older than the rock, 135 and 196 Ma, respectively. These grains were not
included in the mean AHe age for V4. Grains V4a and V4b contained very low uranium
concentrations that may have attributed to a younger AHe age respective to the other
grains in this sample. Regardless, the AHe age was calculated with these two grains and
is 78±2 Ma. Sample V5 contained four good grains that average to an AHe age of 32±3
Ma. Sample V6 contained three grains that are within age range of one another; however,
grain V6c had a re-emission of He upon a second heating and grain V6a is an outlier, thus
both were omitted. The mean AHe age for sample V6 is 17±1 Ma. A possible inclusion
was identified in grain V7d during the final inspection of the grains. This grain was not
processed and the mean AHe age, 36±7 Ma, was calculated from the three other grains.
Grain V8d has an AHe age of 40 m.y., which is about 21 m.y. older than the other three
grains in sample V8. This grain is considered an outlier and was omitted. Grains V8a-
V8c have AHe ages that are close in range (~3 m.y.) and have a mean AHe age of 17±1
Ma. Sample V9 contains four grains that have AHe ages that are within 4 m.y. of one
another. The mean AHe age for this sample is 16±1 Ma.
These corrected mean AHe ages are plotted with respect to elevation (Figure 7)
and located on a three dimensional digital elevation model of the VMB (Figure 8). These
data range from 13±4 to 78±3 Ma and show increasing age with increasing elevation with
the exception of sample V2. Sample V2 has an AHe age of 56±6 Ma and is located about
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400 m above sea level. Three other samples from this study (V1, V3, and V9) were
collected at approximately the same elevation and have AHe ages ~ 40 m.y. younger than
sample V2. The relatively older age could be considered an outlier; however, we suggest
that the AHe age reflects the location of sample V2. Sample V2 was collected on the far
west flank of the VMB and lies to the west of a small fault that separates it from the other
eight samples. This side of the block may not have had the same uplift and exhumation
history as the east side of the VMB where the majority of the samples were collected.
Since sample V2 lies in a different structural domain we have omitted it from the general
trends that will be discussed in out interpretations.
AHe ages at the highest elevations from samples near the summit (~910 m) to
lower elevations (~766 m) decrease substantially from ~78±3 to 17± 1 Ma, respectively,
with a slight decrease in elevation (Figure 8). These data suggest that during this period
these rocks were stalled and/or very slowly being exhumed as they traveled through the
PRZ. Samples V6, V8 and V9 show little age variation at ~17 Ma with a sharp decline in
elevation starting at ~770 to ~380m. This implies that at ~17 Ma cooling was initiated (or
the first phase of rapid cooling). At similar elevations, but slightly below ~380m, AHe
ages deviate from this mean age to ~4 m.y. younger. These samples (V1 and V3) have
AHe ages of ~13 Ma and are the lowest elevation samples collected (~350m). These data
allude to a shallowing trend in age versus elevation which suggests the initiation of slow
cooling; however there is the possibility that these data show a natural age range.
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AHe age Interpretations
Distinct breaks in slope of the elevation-age data show two possible, Miocene to
recent, cooling histories: (1) a single phase of slow/steady cooling since the Miocene (13-
17 Ma); and (2) two punctuated periods of rapid cooling with the first occurring during
the Miocene (~17 Ma), and the second occurring subsequent. In order to evaluate when
this later cooling event may have occurred, predicted AHe ages were plotted at lower
elevations following different data trends that reflect two possible cooling histories
(Figure 9).
Interpretation (A) (Figure 9) is constrained by the work from Tsutsumi and Yeats
(1999) and Meigs et al. (2003). Their work shows that Miocene-Quaternary strata project
over the Verdugo Fault to the footwall (Figure 4) and implies that exhumation of the
VMB was concurrent with and/or began after deposition of the Saugus Formation.
Considering this stratigraphic control, samples were projected to the ages of base and the
top of the Saugus Formation, 2.3 to 0.5 Ma, respectively. This interpretation (A) suggests
that granitic rocks slowly cooled prior to ~17 Ma and a punctuated period of rapid
cooling occurred for about 4 My during the Miocene. This initial period of rapid
exhumation occurring ~17 Ma may be associated with the start of transrotation of the Los
Angeles basin, which Ingersoll and Rumelhart (1999) suggest occurring between 18-12
Ma. The break in slope around 13-17 Ma suggests slow cooling to between 2.3 and 0.5
Ma, when the second phase of rapid cooling and exhumation ensued. Initiation of rapid
exhumation at these time periods accounts for ~2 km of erosion and VMB exhumation
rates of between 1 and 4 mm/yr.
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Interpretation B (Figure 9) treats the lowest elevation samples as being essentially
the same age as indicated by the overlapping uncertainties and disregards the break in
slope at 16 Ma at the lowest elevations. Predicted AHe ages are plotted along a best fit
line between the lowest samples (V1 and V3) and the upper break in slope at 17 Ma (V8).
This trend in projected data suggests that granitic rocks were stalled in the shallow crust
slowly cooling prior to the early Miocene (~17 Ma), and since the late Miocene, ca. 13-
17 Ma, they have been rapidly cooling. These data suggest that at these ages and
corresponding elevations denudation of about 1.8 km started 13-17 Ma at an exhumation
rate of ~0.1 mm/yr.
Geomorphology Results
Mean relief and mean slope angles are calculated from topographic profiles of the
VMB 10-meter digital elevation model or DEM (Figure 10). An east-west trending
topographic profile (A-A’) was constructed over the VMB that extends approximately 16
km (Figure 11). This topographic profile was constructed generally along the ridge line
that separates the north and south faces of the range, indicated by the cross-section line.
The cross-section A-A’ was measured from east to west, reaching a minimum elevation
of about 300 m on both sides, and a maximum elevation near the peak of about 940 m.
This topographic profile shows a steeper gradient on the east (107 m/km) than the west
side (74 m/km) (Figure 11).
Three north-south topographic profiles (B-B’, C-C’ and D-D’) reveal that the
south side of the range has a steeper mean gradient (248 m/km) than the north facing side
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(152 m/km) (Figure 11). The asymmetry between the north and south sides of the VMB
is typical of fault-propagation folds where the steeper dipping side is the forelimb (the
south face) and the more shallow dipping side lies over the decollement zone (the north
face) (Keller and Pinter, 2002). These topographic profiles were constructed over the
inner-fluves that generally follow along the indicated cross-section lines. The cross-
section B-B’ was measured from south to north, reaching minimum elevations of about
250 m and 380 m, respectively, and a maximum elevation of about 870 m. This was the
widest of the three cross-sections, extending approximately 5400 m. The cross-section C-
C’ was measured from south to north, reaching minimum elevations of about 470 m and
500 m, respectively, and a maximum elevation near the peak of about 920 m. The cross-
section D-D’ was measured from south to north, reaching minimum elevations of about
340 m and 375 m, respectively, and a maximum elevation of about 590 m. The VMB
anticline displays a flat ramp geometry that is common of thrust-folded orogens, which is
most likely responsible for a higher elevated basin on the north side of the range.
The gradient values were used to calculate the mean slope angle for the east and
west flank as well as the north and south faces of the VMB. Mean slope angles are often
used in lieu of slope gradients and are useful when comparing slopes angles to well-
developed geomorphic models (e.g. Blythe et al., 2000). It is generally accepted that the
magnitude and rates of erosion are slope dependant and can therefore, be determined by
slope angle (Burbank and Anderson, 2001).
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Mean slope angles calculated on the east and west flanks are 6.1˚ and 4.3˚,
respectively. The mean slope angle difference from east to west is ~2˚. These data
suggest the steeper east flank has been uplifted and exhumed more than the west flank,
which perpetuates higher rates of denudation. Increased denudation on the east slope
may be responsible for steeper slope angles on the east flank of the range compared to the
west flank.
The mean slope angles calculated on the north and south face of the VMB are 8.7˚,
14.0˚, respectively. The south face of VMB has a considerably larger mean slope angle (a
difference of 5.3˚) than the north. These data are consistant with well-developed models
that show that the magnitude of erosion is higher on the forelimb of a fold compared to
the backlimb (Burbank and Anderson, 2001). These slope angles may indicate that rates
of denudation and exhumation on the south side are higher than the north side.
Geomorphology Interpretations
In regions of tectonically active mountain blocks indicators of geologically recent
uplift events, are associated with steep relief (Blythe et al., 2000; Burbank and Anderson,
2001). It is generally recognized that overall mountain front gradients increase when they
are uplifted. Consequently, rates of downslope transport increases as the threshold slope
angle for failure is approached (Burbank and Anderson, 2001). Numerous factors affect
slope angle, however as slope angles increase mass wasting processes increase and result
in highly dissected landforms (Blythe et al., 2000; Montgomery and Brandon, 2002).
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The mean north (152 m/km) and south (248 m/km) gradient values are indicators
of steep topography which may be attributed to relatively recent uplift. The calculated
mean slope angles on the south (14.0˚) and west (6.1˚) flanks indicate high rates of
denudation that have resulted in moderate-highly dissected topography in these areas,
which also indicates recent uplift. Furthermore, these data imply that the south and
southeast flank of the VMB, adjacent to the Verdugo fault, has been uplifted more rapidly
than the north and northwest flank. These geomorphic features are consistent with
younger AHe ages on the south and southeast flanks and in combination suggest that
uplift and exhumation has been relatively rapid and recent at these locations.
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Discussion
Apatite (U-Th)/He ages from this study are consistent with rapid exhumation of
the Verdugo Mountains above the Verdugo Fault. Geomorphic analyses and AHe ages
imply that the south side of the VMB, adjacent to the Verdugo Fault, have been uplifted
and exhumed more rapidly than farther north into the hanging wall; thus, this antiform is
propagating to the southeast. Of the two interpretations outlined earlier in the AHe
section, the Quaternary rapid exhumation interpretation (A) is preferred based on regional
stratigraphic constraints and geomorphic indicators. The Saugus Formation is exposed in
the Merrick syncline and within the footwall of the Verdugo fault (Figure 4), which was
deposited from about 2.3 to 0.5 Ma, implying that exhumation of the VMB was
concurrent with and/or began after deposition of the Saugus Formation (Levi and Yeats,
1993; Meigs et al., 2003). Moderate-highly dissected and steep slopes characterize the
south and southeast flanks of the VMB and are consistent with AHe ages, which may
indicate relatively recent uplift as well. This rapid exhumation interpretation relies on
whether or not some or all of the Plio-Pleistocene Saugus Formation (2.5 – 0.5 Ma) was
deposited prior to uplift (similar to Meigs et al., 2003 interpretation). More samples,
however, at lower elevations are needed to confirm this interpretation.
Alternatively, it is quite possible that the VMB has a simpler and slower one
phase exhumation history, as suggested by interpretation (B). An important observation
needs to be addressed with regards to the relationship between the stratigraphic
information and the thermochronology data. This must be considered since the initiation
of exhumation that we infer relies heavily on the local stratigraphy; in-particularly, the
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Saugus Formation. The Saugus Formation has important implications for the timing of
exhumation; however, there is a possible inconsistent relationship between the location of
the cross-section and the complementing AHe age where it overlies. The cross-section,
used in this study, is constructed through the western flank of the range and projects north
through the Merrick syncline. The only sample located in this area, sample V2, has an
AHe age of 56±6 Ma. We suggest this age anomaly is a product of being located in a
distinct structural boundary that is not associated with the other AHe ages. If the west
side of the range has in fact experienced a different exhumational history than the east
side, it is possible that these stratigraphic sequences in the interpreted cross-section do
not project over the eastern flank of the VMB. If so, perhaps the VMB was a topographic
high during the Quaternary so that Saugus Formation could not have been deposited
overtop.
In contrast, it is possible that the rate of lateral propagation of the fold might have
been greater than vertical slip rates along the fault. If this were the case, then the Saugus
Formation could have been deposited over the area of the present day VMB and since
eroded away. A later transition from predominant strike-slip motion to a more dominate
thrust motion would promote vertical growth and increase denudation, concentrated over
the main decollement. This could be the reason why we see a structural boundary with an
older AHe age on the west flank and accelerated exhumation rates on the eastern flank.
These interpretations show that we have not yet fully constrained the exhumation history
of the VMB; however, we suggest that ca. 17 M.a. is a maximum bound for the initiation
of exhumation for this range.
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The youngest AHe ages in Verdugo Mountains (this study) are within the range of
the ages for the western San Gabriel block of the San Gabriel Mountains (Blythe et al.,
2000), but are considerably younger than the AHe ages from the Tujunga block located
just north of the Verdugo Mountains (Figure 12). Data from Blythe et al. (2000) show
apatite FT and apatite He ages that range from 16±2 to 63±6 Ma in the western San
Gabriel block. They suggest that slow cooling lasted from ca. 40 – 15 Ma for this block.
AHe ages in the VMB show similar middle Miocene cooling that indicates a relatively
static thermal history from ca. 78 to 17 Ma. Further to the west and just north of the
VMB, the Tujunga block yielded older AFT and AHe ages ranging from ca. 47 to 60 Ma
and 33 to 42 Ma (Blythe et al., 2000; 2002), respectively. Blythe et al. (2002) suggest that
these older ages represent more recent exhumation (ca. ~7 Ma) when the Sierra Madre
fault transitioned from a strike-slip to a more dominate thrust and increased exhumation
rates along the San Gabriel strand in the Tujunga region. AFT and AHe ages from the
Sierra Madre block range from ca. 12 to 13 Ma and 3 to 7 Ma (Blythe et al., 2000; 2002),
respectively. Blythe et al., (2000; 2002) suggest that these data represent a slower
cooling period from ca. 13 to 3 Ma preceded by the onset of rapid exhumation from ~3
Ma to the present. It may be reasonable to consider that the VMB had a similar slow
cooling phase followed by a recent and rapid cooling phase, as suggested by
interpretation (A) in this study (from ~12 to 2 Ma).
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Conclusions
A suite of quantitative data consisting of apatite (U-Th)/He ages have been obtained for
the Verdugo Hills of Southern California. In this study, apatite helium ages and
geomorphic data were used to better constrain the timing and rates of exhumation of the
VMB within the Los Angeles Basin. Stratigraphic controls and well-constrained
exhumation rates of the surrounding inversions (based on apatite (U-Th)/He and fission-
track data from Blythe et al., (2000; 2002)) have been used to place constraints and make
interpretations of the exhumational history of the VMB. Our major conclusions are:
(1) The earliest phase of rapid cooling seen in the VMB began ~17 Ma. This cooling may
be associated with transrotation within the L.A. basin from 18-12 Ma.
(2) These data from this study show two possible cooling histories from about 12 Ma to
the present. Interpretations (A) and (B) have been identified as rapid exhumation of ~ 4
mm/yr since ~2 Ma, and slow, steady exhumation of ~ 0.1 mm/yr since ~17 Ma.
Interpretation (A) is the preferred interpretation due to stratigraphic (the Saugus
Formation) and geomorphic controls from this study. Furthermore, data and regional
kinematic interpretations from other studies support a rapid exhumation interpretation.
(3) Interpretation (A) - Rapid exhumation: The VMB shows a similar exhumational
history to that of adjacent orogens in the L.A. basin. For example, during transtension
about 12-6 Ma, the Sierra Madre block experienced slow cooling. As the L.A. basin
transitioned to a transpressional region about 6-0 Ma strain was accommodated for along
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range-front thrusts. This may be reflected by the on-set of rapid cooling since ~3 Ma in
the western region of the Sierra Madre block. The projected AHe ages in this study seem
to mirror these data from surrounding regional studies; however, more data from the
VMB are needed to positively confirm this interpretation.
(4) Interpretation (B) – Slow and steady exhumation: It is possible that the VMB has
exhumed slowly since 17 Ma and sat as a topographic high as the Saugus Formation was
being deposited from 2.3 to 0.5 Ma. Younger AHe ages in the VMB compared to the
Tujunga block suggest that strain imparted along regional decollements within the L.A.
basin may have been partially accommodated for more recently by the Verdugo thrust
(Tsutsumi et al., 2001).
(5) The relationship between the Tujunga, Sierra Madre and Verdugo blocks, regardless
of which interpretation is favored, suggests that the range-front fault system has stepped
southward into the basin in the Verdugo area and that the basin-bounding fault may now
be the Verdugo Fault. Farther east, the basin-bounding fault is the Sierra Madre thrust
and the main phase of deformation has not stepped southward in that area.
(6) Based on the conclusion above, the two very different interpretations presented may
be considered non-unique and thus better constrained with more AHe ages and possibly
AFT data from the VMB.
Arkle 21
Acknowledgments
I would like to thank: Dr. Phillip Armstrong for his continued guidance, insight, and
support. His dedication and motivation that he presents to students to achieve is
incredible and contagious; Dr. Kenneth Farley and Lindsey Hedges (California Institute
of Technology) for sample processing, providing (U-Th)/He data and valuable
instruction; and Brian Kohl for his aid with graphics. This project was funded by a
CSUF undergraduate research grant and a scholarship from the Victor Valley Gem and
Mineral Club.
Arkle 22
References
Armstrong, P. A., Ehlers, T. A., Chapman, D. S., Farley, K. A., and Kamp, P. J. J., 2003, Exhumation of the Central Wasatch Mountains, 1: Patterns and timing deduced from Low-temperature Thermochronometry data: Journal of Geophysical Research, v. 108, p. 2172.
Blythe, A. E., Burbank, D. W., Farley, K. A., and Fielding, E. J., 2000, Structural and
topographic evolution of the central Transverse Ranges, California, from apatite fission-track, (U-Th)/He and digital elevation model analyses: Basin Research, v. 12, p. 97-114.
Blythe, A. E., House, M. A., and Spotila, J. A., 2002, Low-temperature
thermochronology of the San Gabriel and San Bernardino Mountains, southern California: Constraining structural evolution: Geology, p. 231-250.
Burbank, D. W., and Anderson, R. S., 2001, Tectonic Geomorphology: Oxford,
Blackwell Science, 274 p. Dibblee, T. W., Jr., 1989, Geologic map of the Pasadena Quadrangle quadrangle: Dibblee
Geological Foundation; Map DF-23, scale 1:24,000. Dibblee, T. W., Jr., 1991, Geologic map of the Sunland and Burbank (North 1/2)
quadrangles: Dibblee Geological Foundation; Map DF-32, scale 1:24,000. Dibblee, T. W., Jr., 2001, Geologic map of the Hollywood and Burbank (South 1/2)
quadrangles: Dibblee Geological Foundation; Map DF-30, scale 1:24,000. Ehlers, T. A., and Farley, K. A., 2003, Apatite (U-Th)/He thermochronometry: Methods
and applications to problems in tectonic and surface processes: Earth and Planetary Science Letters, v. 206, p. 1-14.
Farley, K. A., 2000, Helium diffusion from apatite: General behavior as illustrated by
Durango fluorapatite: Journal of Geophysical Research, v. 105, p. 2909-2914. Fuis, G. S., Ryberg, T., Godfrey, N. J., Okaya, D. A., and Murphy, J. M., 2001, Crustal
structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California: Geology, v. 29, p. 15-18.
Green, P. F., Duddy, I. R., Laslett, G. M., Hegarty, K. A., Gleadow, A. J. W., and
Lovering, J. F., 1989, Thermal annealing of fission tracks in apatite 4: quantitative modeling techniques and extension to geological timescales: Chemical Geology (Isotope Geoscience Section), v. 79, p. 155-182.
Arkle 23
House, M. A., Farley, K. A., and Stockli, D., 2000, Helium chronometry of apatite and titanite using Nd-YAG laser heating: Earth and Planetary Science Letters, v. 183, p. 365-368.
Keller, E. A., and Pinter, N., 2002, Active Tectonics: Earthquakes, Uplift, and
Landscape: Upper Saddle River, New Jersey, Prentice Hall, 362 p. Levi, S., and Yeats, R. S., 1993, Paleomagnetic Constraints on the Initiation of Uplift on
the Santa Susana Fault, Western Transverse Ranges, California: Tectonics, v. 12, p. 688-702.
Luyendyk, B. P., 1991, A model for Neogene crustal rotations, transtension, and
transpression in southern California: Geological Society of America Bulletin, v. 103, p. 1528-1536.
Meigs, A., Yule, D., Blythe, A., and Burbank, D., 2003, Implications of distributed
crustal deformation for exhumation in a portion of a transpressional plate boundary, Western Transverse Ranges, Southern California: Quaternary International, v. 101-102, p. 169-177.
Montgomery, D. R., and Brandon, M. T., 2002, Topographic controls on erosion rates in
tectonically active mountain ranges: Earth and Planetary Science Letters, v. 201, p. 481-489.
Tsutsumi, H., and Yeats, R. S., 1999, Tectonic setting of the 1971 Sylmar and 1994
Northridge earthquakes in the San Fernando Valley, California: GSA Bulletin, p. 1232-1249.
Tsutsumi, H., Yeats, R. S., and Huftile, G. J., 2001, Late Cenozoic tectonics of the
northern Los Angeles fault system, California: GSA Bulletin, v. 113, p. 454-468. Wolf, R. A., Farley, K. A., and Kass, D. M., 1998, Modeling of the temperature
sensitivity of the apatite (U-Th)/He thermochronometer: Chemical Geology, v. 148, p. 105-114.
Wolf, R. A., Farley, K. A., and Silver, L. T., 1996, Helium diffusion and low-temperature
thermochronometry of apatite: Geochimica et Cosmochimica Acta, v. 60, p. 4231-4240.
Wright, T. L., 1991, Structural geology and tectonic evolution of the Los Angeles basin:
in Biddle, K. T., ed., Active Margin Basins, American Association of Petroleum Geologists, p. 35-134.
Yeats, R. S., 1981, Quaternary flake tectonics of the California Transverse Ranges:
Geology, v. 9, p. 16-20. Yeats, R. S., 2004, Tectonics of the San Gabriel Basin and surroundings, southern
California: GSA Bulletin, v. 116, p. 1158-1182.
Arkle 24
Figures: Verdugo Mountain Block
Lowlands
Upper Cretaceous -Cenozoic rocks
Santa Monica Formation
PR-SM basement
San Gabriel basement
119 W34 N
0 10 20 30
KILOMETERS
118 W
118 W
119 W
Ocean
PacificDume Fault
? ?
Malibu Coast Fault
P.V.Hills
Palos Verdes F.
Newport-Inglew
ood Fault
Los
Angeles
Basin CH
Puente Hills Thrust
Whittier Fault
Puente
HillsSAR
Santa Ana
Mountains
Chino F. PerrisBlock
Elsinore F.
Chino
Basin
JurupaMts.
San Jose Hills
SJF
San GabrielValley
WCF
SierraMadre Fault
Hollywood
SMF
SanFernando
Valley
F.
Verdugo F.
Verdugo Mts.
Santa Monica Mountains
Simi HillsSimi F.
SantaSusana
OakRidge Fault
Ventura
Basin
SanCayetano
F.
TopatopaMountains
Canton
Fault
Ridge Basin
San Gabriel F.
Soledad Basin
F. SFF
Sierra PelonaSanta Ynez Fault
SanGabriel
Mountains
San Gabriel Fault
VCF
Punchbowl
San
Andreas
Fault geology notshown
Fault
Cucamonga F.
SAC
F
SJcF
Study Area
S
N
Figure 1: Generlized geologic map of the central Transverse Ranges, San Gabriel Valley, Los Angeles Basin, and surrounding regions. Faults shown in heavy lines, dashed where covered or blind. Abbreviations: CH – Coyote Hills; PV Hills – Palos Verde Hills; RF – Raymond fault; SACF – San Antonio Canyon fault; SAR – Santa Ana River; SFF – San Fernando fault; SJF – Sand Jose fault; SJcF – San Jacinto fault; SMF – Santa Monica fault; VCF – Vasquez Creek fault; WCF – Walnut Creek fault. Regional map modified from Yeats (2004). Bold dotted line, S-N, shows location of cross section in Figure 3.
Arkle 25
N
S
Figure 2: Simplified geologic map of the study area. Faults are shown in heavy black lines and include: the Verdugo (Vf), Sierra Madre (SMt), San Gabriel (SGf), Raymond (Rf), Lakeview (Lt), Santa Susana (SSf). Other key features include: the Western San Gabriel Mountains (WSG), the San Fernando Valley (SFv), the Big Tujunga Wash (BTw), and the Merrick syncline (Ms). Figure from Meigs et al. (2003).
Arkle 26
SaugusFormation
SaugusFormation
SN
STUDY AREA
NS
Figure 3: Simplified crustal section across the Verdugo and San Gabriel Mountains showing approximate mid-crustal detachment depth. Location is from Figure 1 and geology is from Figure 2. Faults are shown in heavy black lines and include: the Verdugo (Vf), San Gabriel (SGf), and the San Andreas (Saf). Figure from Meigs et al. (2003).
Figure 4: Generalized cross section across the Verdugo Mountains. Faults are shown in heavy black lines and include the Verdugo (Vf), Sierra Madre (SMt), San Gabriel (SGf), and Lakeview (Lt). Key folds include the Verdugo anticline (projected strata) and the Merrick syncline (Ms). Base of the Saugus Formation (dark) is 2.3 Ma and the top is 0.5 Ma. Cross section modified from Meigs et al. (2003). A
rkle 27
Figure 5: From Ehlers and Farley, 2003. Diagram showing how α ejection effects the helium age of an apatite. When helium is produced kinetic energy allows it to travel through the crystal lattice, and it will remain within a diameter of ~20 µm to the parent. Isotopes that are located near an edge of a crystal have the potential to eject daughter products. The prism cross section must be measured and then multiplied by an appropriate correction factor in order to yield a more precise age.
Arkle 28
Table 1: Summary of (U-Th)/He Data Arkle 29
Sample Number
Location UTM
Elevation (m)
Raw AHe Age (Ma)
Ft Correction
Corrected AHe Age (Ma) Notes
V1(a):laser 379826 E 350 6.43 0.70 9.18 Very low uraniumV1(b):laser 3785922 N 12.76 0.72 17.79 Very low uraniumV1(c):laser 21.98 0.72 30.58 Re-emmsionV1(d):laser 3.65 0.65 5.60 Not apatiteMean AHe Age 13±4
V2(a):laser 376081 E 404 35.41 0.74 47.68V2(b):laser 3787518 N 56.38 0.79 71.48V2(c):laser 32.94 0.73 44.90V2(d):laser 47.80 0.78 60.86Mean AHe Age 56±6
V3(a):laser 385866 E 371 9.96 0.77 12.92 Re-emmsionV3(b):laser 3782421 N 9.17 0.84 10.89V3(c):laser 7.31 0.70 10.41 Re-emmsionV3(d):laser 12.14 0.74 16.47Mean AHe Age 13±3
V4(a):laser 381817 E 910 63.82 0.79 80.92 Very low uraniumV4(b):laser 3786946 N 59.19 0.79 75.11 Very low uraniumV4(c):laser 108.88 0.80 135.33 Older than rockV4(d):laser 155.31 0.79 196.57 Older than rockMean AHe Age 78±3
V5(a):laser 383188 E 749 27.43 0.76 35.86V5(b):laser 3786882 N 14.29 0.69 20.59V5(c):laser 27.64 0.76 36.51V5(d):laser 28.26 0.78 36.02Mean AHe Age 32±4
V6(a):laser 384342 E 504 28.36 0.73 38.59 Outlier AHe ageV6(b):laser 3786723 N 15.37 0.84 18.24V6(c):laser 11.11 0.78 14.22 Re-emmsionV6(d):laser 12.55 0.78 16.17Mean AHe Age 17±1
V7(a):laser 383315 E 812 16.67 0.78 21.44V7(b):laser 3785524 N 37.32 0.77 48.41V7(c):laser 29.30 0.77 38.15Mean AHe Age 36±8
V8(a):laser 383927 E 766 15.20 0.78 19.36V8(b):laser 3784888 N 13.42 0.84 16.07V8(c):laser 14.46 0.86 16.73V8(d):laser 31.40 0.78 40.40 Outlier AHe ageMean AHe Age 17±1
V9(a):laser 385358 E 383 10.50 0.76 13.73V9(b):laser 3784073 N 13.47 0.79 16.95V9(c):laser 11.55 0.71 16.17V9(d):laser 13.56 0.79 17.24Mean AHe Age 16±1*Location UTM coordinates: Datum is NAD 84, UTM zone 11* Mean AHe age: see text for the mean AHe age calculation. Error is given as the standard error.
Explanation for single apatite grains
Inclusion ?Re-extract
Older than the rock
Apatite grains used
0
20
40
60
80
100
V7-1 V7-2 V7-3
Sample V7 Helium Ages
(U-T
h)H
eAg
e(M
a)
Grain Number
Average: 36.0 ± 7.9
0
20
40
60
80
100
V8-1 V8-2 V8-3 V8-4
Sample V8 Helium Ages
( U-T
h)H
eAg
e(M
a)
Grain Number
Average: 17.4 ± 1.0
0
20
40
60
80
100
V9-1 V9-2 V9-3 V9-4
Sample V9 Helium Ages
(U-T
h )H
eAg
e( M
a)
Grain Number
Averge: 16.0 ± 0.8
0
20
40
60
80
100
V3-1 V3-2 V3-3 V3-4
Sample V3 Helium Ages
(U- T
h)H
eAg
e(M
a)
Grain Number
Average: 13.7 ± 2.8
0
20
40
60
80
100
V2-1 V2-2 V2-3 V2-4
Sample V2 Helium Ages
( U-T
h)H
eAg
e(M
a )
Grain Number
Average: 56.2 ± 6.1
40
60
80
100
Sample V1 Helium Ages
Grain Number
Average: 13.5 ± 4.3
(U-T
h)H
eA
ge(M
a)
0
20
V1-1 V1-2 V1-3 V1-4
0
20
40
60
80
100
V6-1 V6-2 V6-3 V6-4
Sample V6 Helium Ages
(U-T
h)H
eAg
e(M
a)
Grain Number
Average: 17.2 ± 1.0
0
20
40
60
80
100
V5-1 V5-2 V5-3 V5-4
Sample V5 Helium Ages
(U-T
h)H
eAg
e( M
a)
Grain Number
Average: 32.2 ± 3.9
0
50
100
150
200
V4-1 V4-2 V4-3 V4-4
Sample V4 Helium Ages
(U-T
h)H
eAg
e(M
a)
Grain Number
Average: 78.0 ± 2.9
Arkle 30
Individual Apatite Grain He Ages
Figure 6: Individual apatite grain He ages for samples V1-V9. Mean AHe ages given and uncertainties (shaded region) within the chosen suite of a sample are given as the standard error (at 1σ).
0
200
400
600
800
1000
0 10 20 30 40 50 60 70 80
V4
V1V2
V3
V5
V6
V7V8
V9
Sample from a different structural domain
(U-Th)/He Age (Ma)
Helium Ages vs. ElevationVerdugo Mountains, Southern California
Ele
vatio
n (m
)
Figure 7: Plot of average (U-Th)/He ages verses elevation for all samples. Error bars are reported as one standard error.
Arkle 31
Figure 8: Three dimensional digital elevation model of the Verdugo Mountains showing sample locations and faults as heavy black lines.
Arkle 32
Interpretation B: Slow Exhumation
T~ 45ºCTop of the Present-Day PRZ
Tc~ 60ºCEffective Closure Temperature
Base of the Present-Day PRZT~ 75ºC
-2000
-1500
-1000
-500
0
500
1000
0 5 10 15 20 25 30 35 40
Ele
vatio
n (m
)
AHe Age (Ma)
V5
V6
V7V8
V9V1
V3
EF
G
G1G2
~1.8 km of erosion in 13 – 17 My
Interpretation A: Rapid Exhumation
Tc~ 72ºCEffective Closure Temperature
Base of the Present-Day PRZT~ 75ºC
T~ 45ºCTop of the Present-Day PRZ
-2000
-1500
-1000
-500
0
500
1000
0 5 10 15 20 25 30 35 40
Ele
vatio
n (m
)
AHe Age (Ma)
V5
V6
V7V8
V9V1
V3
AB
C
D
~2 km of erosion in 0.5 – 2 My
D1
D2
Figure 9: Verdugo AHe ages (solid diamonds) and predicted AHe ages (open diamonds). Predicted age trends reflect two possible cooling histories. Interpretation A: (A) At highest elevations, ages decrease substantially with a slight decrease in elevation (granitic rocks slowly cooled). (B) Break in slope at 17 Ma – little age variation with decrease in elevation (first phase of rapid cooling). (C) Break in slope at 16 Ma – ages decrease with slight decrease in elevation (slow cooling). (D) Predicted AHe ages, 2.3 (D1) and 0.5 (D2) Ma (the base and top of the Saugus Formation) and break in slope (second phase of recent and rapid cooling). Interpretation B: (E) At highest elevations, ages decrease substantially with a slight decrease in elevation (granitic rocks slowly cooled). (F) Break in slope at 17 Ma – little age variation with decrease in elevation (cooling). (G) Cooling since ~13 Ma. Projected cooling from 13 (G2) – 17 (G1) Ma to present. Note that Interpretation A shows much more rapid cooling in the Quaternary than interpretation B.
Arkle 33
118º 15’
34º 15’
Figure 10: A 10-meter digital elevation model of the Verdugo Mountian block showing topographic profile locations and sample locations.
Arkle 34
B’B
800
600
400
0 1000 2000 3000 4000 5000
Ele
vatio
n (m
)
Distance (m)
South North
300
500
700
900
0 5000 10000 15000Distance (m)
Elev
atio
n (m
)
A A’EastWest
900
500
600
700
800
0 1000 2000 3000 4000
Ele
vatio
n (m
)
Distance (m)
SouthC C’
North
350
400
450
500
550
0 500 1000 1500 2000 2500
Ele
vatio
n (m
)
Distance (m)
SouthD’D
North
Arkle 35
Figure 11: Topographic profiles measured from the VMB 10-meter digital elevation model. Note vertical exaggeration varies for each profile.
Western San Gabiriel Block
SAf
VERDUGO BLOCKAvg. Helium Age: 30 Ma
Range: 13 - 78 MaN = 9
TUJUNGA BLOCKAvg. Helium Age: 36 Ma
Range: 33 - 42 MaN = 3
SIERRA MADRE BLOCKAvg. Helium Age: 6 Ma
Range: 3 - 7.6 MaN = 5
WESTERN SAN GABRIEL BLOCKAvg. Helium Age: 35 Ma
Range: 23 - 42 MaN = 3
MT. BALDY BLOCKAvg. Helium Age: 7 Ma
Range: 5.1 - 9 MaN = 2
SMtVf
Figure 12: Three-dimensional elevation model with apatite helium ages of individual blocks within the San Gabriel Mountains. Interpreted block boundaries are dashed lines and faults shown as heavy solid lines. San Gabriel block data is from Blythe et al. (2000). Boundary lines are redrawn and AHe ages are averaged. Red dashed line shows the VMB and AHe results from this study.
Arkle 36
Arkle 37
Appendix A
Grain Dimension and Morphology Data This section contains original laboratory data sheets that have been scanned and a summary of these data.
Arkle 38Grain Dimensions and Morphology
Apatite Lab Data Sheets
Sample # MorphologyLength
(microns)Width
(microns)JAVM06-1A N,P 165.84 88.67JAVM06-1B N,P 176.15 95.58JAVM06-1C N,I 205.38 94.4JAVM06-1D SP,I 162.65 79.55
JAVM06-2A N,N 178.67 104.17JAVM06-2B N,I 233.52 127.89JAVM06-2C N,N 160.73 100.4JAVM06-2D N,P 174.78 136.78
JAVM06-3A N,P 160.32 127.77JAVM06-3B N,P 281.07 176.69JAVM06-3C N,P 170.79 90.55JAVM06-3D N,SP 215.58 100.12
JAVM06-4A N,I 211.44 131.75JAVM06-4B N,P 291.01 132.21JAVM06-4C SP,I 152.75 139.48
JAVM06-5A N,N 237.85 112.35JAVM06-5B N,P 181.9 86.4JAVM06-5C N,P 160.86 120.68JAVM06-5D N,N 258.93 122.71
JAVM06-6A N,P 132.81 105.98JAVM06-6B N,P 205.36 191.05JAVM06-6C SP,I 162.87 126.94JAVM06-6D N,P 203.22 125.18
JAVM06-7A N,N 219.76 122.73JAVM06-7B N,P 238.19 115.08JAVM06-7D P,P 208.75 116.6
JAVM06-8A N,SP 230.56 125.94JAVM06-8B N,SP 380.2 160.98JAVM06-8C N,SP 308.75 208.16JAVM06-8D N,P 164.95 130.42
JAVM06-9A N,P 203.38 119.16JAVM06-9B N,N 303.12 129JAVM06-9C N,P 292 88.09JAVM06-9D SP,I 249.05 123.11
Average: 214.21 122.25Range: 132-380 79-208
*MorphologyN = NormalP = ParallelSP = SubparallelI = Irregular
Arkle 48
Appendix B
Individual Apatite Grain Descriptions and Photographs
Photographs have been resized maintaining the original grain ratios. Note that the scale for individual apatite grains can be determined from the grain dimensions and morphology laboratory data sheets.
JAVM06-1A
GRAIN DESCRIPTIONJAVM06-1A: Picked, measured and packed. No photo.JAVM06-1B: Euhedral, 1 terminated end, clean.JAVM06-1C: Euhedral, 1 terminated end, clean. Cracks and possilbe inclusions.JAVM06-1D: Euhedral, 2 broken ends.
Individual Apatite Grain DescriptionsReflected Light Refracted Light
JAVM06-1A
JAVM06-1B JAVM06-1B
JAVM06-1C JAVM06-1C
JAVM06-1D JAVM06-1D
No Photo No Photo
Arkle 49
JAVM06-2A
GRAIN DESCRIPTIONJAVM06-2A: Euhedral, 2 terminated ends, very clean.JAVM06-2B: Euhedral, 2 terminated ends, very clean.JAVM06-2C: Euhedral, 1 terminated end, 1 clear broken end, very clean.JAVM06-2D: Euhedral, 1 terminated end, 1 clear broken end, clean.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-2A
JAVM06-2B JAVM06-2B
JAVM06-2C JAVM06-2C
JAVM06-2D JAVM06-2D
Arkle 50
JAVM06-3A
GRAIN DESCRIPTIONJAVM06-3A: Euhedral, 1 terminated end, semi-clean.JAVM06-3B: Euhedral, 2 terminated ends, semi-clean.JAVM06-3C: Euhedral, 1 terminated end, clean.JAVM06-3D: Euhedral, 1 terminated end, semi-clean.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-3A
JAVM06-3B JAVM06-3B
JAVM06-3C JAVM06-3C
JAVM06-3D JAVM06-3D
No Photo
Arkle 51
JAVM06-4A
GRAIN DESCRIPTIONJAVM06-4A: Euhedral, 2 terminated ends, semi-clean. Surficial coloration on ends; reflectinos?JAVM06-4B: Euhedral, 2 terminated ends, semi-clean.JAVM06-4C: Euhedral, 1 terminated end, clean.JAVM06-4D: Do not use.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-4A
JAVM06-4B JAVM06-4B
JAVM06-4C JAVM06-4C
JAVM06-4D JAVM06-4D
Arkle 52
JAVM06-5A
GRAIN DESCRIPTIONJAVM06-5A: Euhedral, 1 terminated end, very clean.JAVM06-5B: Euhedral, 1 terminated end, clean.JAVM06-5C: Subhedral, 1 terminated end, clean.JAVM06-5D: Euhedral, 2 terminated ends, very clean.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-5A
JAVM06-5B JAVM06-5B
JAVM06-5C JAVM06-5C
JAVM06-5D JAVM06-5D
Arkle 53
JAVM06-6A
GRAIN DESCRIPTIONJAVM06-6A: Subhedral, 1 terminated end, semi-clean.JAVM06-6B: Euhedral, 1 terminated end, dirty. Potential inclusion; cloudy surface.JAVM06-6C: Euhedral, 2 broken ends, clean.JAVM06-6D: Euhedral, 1 terminated end, clean.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-6A
JAVM06-6B JAVM06-6B
JAVM06-6C JAVM06-6C
JAVM06-6D JAVM06-6D
100 µm 100 µm
100 µm 100 µm
100 µm 100 µm
100 µm 100 µm
Arkle 54
JAVM06-7A
GRAIN DESCRIPTIONJAVM06-7A: Subhedral, 1 terminated end, semi-clean.JAVM06-7B: Euhedral, 1 terminated end, clean. Potential inclusions along edge - reflections?JAVM06-7C: Subhedral, 1 terminated end, clean.JAVM06-7D: Do not use. Potential inclusinos; probably crack in grain.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-7A
JAVM06-7B JAVM06-7B
JAVM06-7C JAVM06-7C
JAVM06-7D JAVM06-7D
Arkle 55
JAVM06-8A
GRAIN DESCRIPTIONJAVM06-8A: Euhedral, 1 terminated end, clean. Big crack.JAVM06-8B: *Zircon? Euhedral, 1 terminated end, clean. JAVM06-8C: Euhedral, 1 terminated end, semi-clean. Inclusions or surfical feature?JAVM06-8D: Euhedral, 1 terminated end, clean. Excellent.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-8A
JAVM06-8B JAVM06-8B
JAVM06-8C JAVM06-8C
JAVM06-8D JAVM06-8D
Arkle 56
JAVM06-9A
GRAIN DESCRIPTIONJAVM06-9A: Euhedral, 1 terminated end, clean.JAVM06-9B: Euhedral, 2 terminated ends, clean. JAVM06-9C: Euhedral, 2 terminated ends, very clean. Long prismatic crystal.JAVM06-9D: Subhedral, 1 terminated end, very cracked. Irregular edges.
Individual Apatite Grain DescriptionsRefracted Light Reflected Light
JAVM06-9A
JAVM06-9B JAVM06-9B
JAVM06-9C JAVM06-9C
JAVM06-9D JAVM06-9D
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Appendix C
Apatite (U-Th)/He Data
These data are from the California Institute of Technology. The “Ref” numbers correspond to the assigned Verdugo individual grains and samples sited in this paper.
Apatite (U-Th)/He Data from the California Institute of Technology
Ref Raw Ma Corr Ma U ppm Th/U Th ppm He nmol/g mass ug Ft Correction
Crystal width (um)
Crystal length (um) RE? #grains
J1(a):laser 6.43 9.18 2.70 2.64 7.12 0.15 2.63 0.70 44.34 248.76 0.00 1.00 V. low U.J1(b):laser 12.76 17.79 2.35 3.47 8.14 0.30 3.24 0.72 47.79 264.23 0.00 1.00 V. low U.J1(c) :laser 21.98 30.58 2.75 3.83 10.53 0.63 3.69 0.72 47.20 308.07 1.00 1.00 V. low U.J1(d):laser 3.65 5.60 0.11 36.85 3.93 0.02 2.07 0.65 39.78 243.98 1.00 1.00 NOT APATITE?
J2(a):laser 35.41 47.68 17.25 1.49 25.72 4.49 3.90 0.74 52.09 268.01 0.00 1.00J2(b):laser 56.38 71.48 6.01 1.68 10.10 2.58 7.63 0.79 63.70 350.28 0.00 1.00J2(c):laser 32.94 44.90 10.73 1.03 11.00 2.39 3.26 0.73 50.20 241.10 0.00 1.00J2(d):laser 47.80 60.86 10.01 4.01 40.17 5.07 6.58 0.78 68.39 262.17 0.00 1.00
J3(a):laser 9.96 12.92 3.32 3.52 11.71 0.33 5.27 0.77 63.89 240.48 1.00 1.00J3(b):laser 9.17 10.89 23.91 1.45 34.55 1.60 17.67 0.84 88.35 421.61 0.00 1.00J3(c):laser 7.31 10.41 4.74 4.31 20.42 0.38 2.82 0.70 45.28 256.19 1.00 1.00J3(d):laser 12.14 16.47 23.44 2.35 55.11 2.40 4.35 0.74 50.06 323.37 0.00 1.00
J4(a):laser 63.82 80.92 2.54 2.55 6.49 1.42 7.39 0.79 65.88 317.16 0.00 1.00 V. low U.J4(b):laser 59.19 75.11 2.65 2.80 7.40 1.42 7.39 0.79 65.88 317.16 0.00 1.00 V. low U.J4(c):laser 108.88 135.33 20.68 0.86 17.78 14.85 10.24 0.80 66.11 436.52 0.00 1.00J4(d):laser 155.31 196.57 5.65 1.93 10.93 7.03 5.98 0.79 69.74 229.13 0.00 1.00
J5(a):laser 27.43 35.86 21.30 1.92 40.84 4.62 5.99 0.76 56.18 353.78 0.00 1.00J5(b):laser 14.29 20.59 4.62 3.96 18.26 0.69 2.73 0.69 43.20 272.85 0.00 1.00J5(c):laser 27.64 36.51 4.20 6.42 26.94 1.59 4.72 0.76 60.34 241.29 0.00 1.00J5(d):laser 28.26 36.02 9.00 1.63 14.64 1.91 7.85 0.78 61.36 388.40 0.00 1.00
J6(a):laser 28.36 38.59 5.84 1.39 8.09 1.20 3.00 0.73 52.99 199.22 0.00 1.00J6(b):laser 15.37 18.24 3.42 1.54 5.28 0.39 15.09 0.84 95.53 308.04 0.00 1.00J6(c):laser 11.11 14.22 3.88 1.62 6.30 0.32 6.41 0.78 62.59 304.83 1.00 1.00J6(d):laser 12.55 16.17 7.53 1.58 11.87 0.70 5.28 0.78 63.47 244.31 0.00 1.00
J7(a):laser 16.67 21.44 32.64 2.42 78.93 4.64 6.67 0.78 61.37 329.64 0.00 1.00
J7(b):laser 37.32 48.41 13.88 1.72 23.90 3.97 6.35 0.77 57.54 357.29 0.00 1.00J7(c):laser 29.30 38.15 25.73 2.02 51.91 6.05 5.71 0.77 58.30 313.13 0.00 1.00
J8(a):laser 15.20 19.36 13.63 1.74 23.69 1.59 7.36 0.78 62.97 345.84 0.00 1.00J8(b):laser 13.42 16.07 13.39 1.55 20.70 1.33 19.84 0.84 80.49 570.30 0.00 1.00J8(c):laser 14.46 16.73 16.17 1.21 19.62 1.63 26.88 0.86 104.08 462.23 0.00 1.00J8(d):laser 31.40 40.40 12.87 2.98 38.32 3.74 5.65 0.78 65.21 247.43 0.00 1.00
J9(a):laser 10.50 13.73 8.70 4.78 41.54 1.05 5.81 0.76 59.58 305.07 0.00 1.00J9(b):laser 13.47 16.95 10.33 2.10 21.72 1.13 10.16 0.79 64.50 454.68 0.00 1.00J9(c):laser 11.55 16.17 12.70 2.91 36.97 1.34 4.56 0.71 44.05 438.00 0.00 1.00J9(d):laser 13.56 17.24 16.90 0.94 15.93 1.52 7.60 0.79 61.56 373.58 0.00 1.00
Raw Ma: This column shows the apatite He age (in millions of years) before the Ft correction is accounted for. See text for discussion.Corr Ma: This column shows the corrected apatite He age (in millions of years) that is used for analysis. See text for discussion.RE: The number 1.00 in this column indicates a re-extract. See text for discussion.
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Appendix D
Geomorphic Analysis
Geomorphic Analysis
Slope Face Cross Section X final X initial
Dist. Change X
(m)Y Final Y Initial Elev Change Y (m) Graident
(m/km)Slope
Angle ( )
East-West A-A'West 8500 0 8500 940 300 640 75.3 4.3East 16000 10000 6000 940 300 640 106.7 6.1
East to West Graident Difference (m/km): 31.4 Difference ( ): 1.8
South Face B-B' 2500 0 2500 860 260 600 240.0 13.5C-C' 1600 0 1600 900 500 400 250.0 14.0D-D' 900 0 900 570 340 230 255.6 14.3
Average South Face Graident (m/km): 248.5 Mean Slope ( ): 13.9
North Face B-B' 5400 2500 2900 860 375 485 167.2 9.5C-C' 4200 1800 2400 910 500 410 170.8 9.7D-D' 2700 950 1750 560 350 210 120.0 6.8
Average North Face Graident (m/km): 152.7 Mean Slope ( ): 8.7
North and South Face Difference (m/km): 95.8 Difference ( ): 5.3
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