fault confidence hills positive flower ...geode.colorado.edu/~structure/teaching_geol4721/6...lines...
TRANSCRIPT
TROXELCANYON
N
5
6° - 310°
42
36
8030
14
30
14
2832
22
23
70
38
8573
26
24
87
50
34
60
3024
60
22
40
32
2422
20
26
26
80
32
BestFit19-301°
Northern Dip Domain
48 Data
+
Central Dip Domain
BestFit05-304°
51 Data
+
BestFit15-313°
Sequence-1 Data
30 Data
+
Sequence 2
Sequence 1
Bedding Form Lines
Folded Unconformity -Disconformity
Folded Unconformity
1 km
1
2
Folded Disconformity
Fault, increasinguncertainty
Fault Strike~ 315° Southern Dip Domain
34 Data
BestFit13-122°
+
BestFit11-311°
Qg1 Data
22 Data
+
© FAULT DYNAMICS PROJECT 1997
❐ INTRODUCTION
The right-lateral southern Death Valley strike-slip fault zone (SDVFZ; Fig. 1) formsthe southwestern boundary of the Death Valley extensional sub-province, California(Wright & Troxel, 1967; Butler et al., 1988; Dooley & McClay, 1996). This fault zoneconsists of a number of NW-SE striking segments that are divided into two sub-zones - east and west (Fig. 1c; Butler et al., 1988). The western subzone consistsof now deformed fault segments exposed on the northeastern flanks of the AvawatzMountains, that were active from 14 Ma to ca. 1 Ma (Fig. 1c; Butler et al., 1988).Recent fault activity (1 Ma - present) has been recorded on the highly segmentedeastern sub-zone in Plio-Pleistocene fluvio-lacustrine sediments (Butler et al., 1988;Dooley & McClay, 1996). The Confidence Hills are a low-lying range of hills (withelevations of up to 200m above the floor of Death Valley), composed of well-exposedPlio-Pleistocene fluvio-lacustrine strata and Pleistocene-Recent alluvial fan deposits(Figs 4 & 5). These hills are situated along the northwestern extremity of the easternsubzone of the SDVFZ, forming a narrow 'positive flower structure' along this faultzone (Figs 1, 2 & 3).
❐ STRATIGRAPHY
❐ Sequence 1 (Confidence Hills Formation - Beratan & Murray, 1992) consists ofup to 540 m of Plio-Pleistocene fluvio-lacustrine deposits, subdivided into fourlithostratigraphic units (Fig. 4; Fig. 5a). A broadly coarsening-upwards character isobserved and LS-4 is capped by coarse quartzo-feldspathic sandstones and cobbleconglomerates of fluvial/alluvial origin (Fig. 4).
❐ Sequence 2 consists of two, patchily exposed, units – Qg1 and Qg2 (Figs 4 &5b). Qg1 and Qg2 consist of tan-coloured, pebble to boulder conglomerate with apredominant bimodal clast composition of granite and basic volcanics that have asource area directly west of the Confidence Hills. These deposits unconformablyto disconformably overlie Sequence-1 deposits with a spectacular erosive, angularunconformity exposed in Troxel Canyon (Fig. 8c).
❐ Basaltic lavas comprise much of Shoreline Butte (Fig. 6). These have been datedat 1.5 Ma (Butler et al., 1988). Cinder Hill comprises an andesitic cinder cone witha K/Ar age of 0.69 Ma (Butler et al., 1988).
❐ STRUCTURE
❐ The structural geometry of the Confidence Hills consists of two, NW-SE striking,dextral-oblique fault segments (A & B) and an associated, NW-SE striking, anticlinorium(Figs 6, 7 & 8).
❐ Fault Geometries
– Field studies and LANDSAT TM interpretation reveal that the two fault segmentsare sub-vertical features with average strikes of 315° (Figs 6, 7 & 8a, b).
– Dextral offset of Cinder Hill (180-200m; Figs 2 & 6), en-echelon fault arrays,left stepping fault arrays that produce localised pop-up structures, and, dextraldeflection of ephemeral streams that cut across the trace of these fault zones allindicate dextral kinematics.
– Cross-section reconstruction indicates vertical uplift (northeast side-up) ofup to 200m across fault zone A (Fig. 6).
– Combining the above reveals these fault segments to be dextral, oblique-slip,fault zones with displacements of only 100s metres.
❐ Fold Geometries
– The northern Confidence Hills consists of a simple, broad, upright anticlinalstructure that is breached by strands of fault zone A (Fig. 6).
– The central anticline is approximately 6 km long, with a variable attitude fromNW to SE (Figs 3, 6, 7 & 8c). Parasitic folds to this structure are well developed,commonly with interlimb angles of 45-50° and axial surfaces that dip up to 60° SW(Fig. 8d). Slickensides on bedding surfaces indicate a flexural-slip fold mechanism.
– The southern Confidence Hills consists of a 4km long, NW-SE striking, doublyplunging anticline (Fig. 7). Right stepping parasitic folds are well developed on thelimbs of this fold.
– The contact between Sequences 1 and 2 varies from angular unconformityto disconformity (Figs 7 & 8c). The folds in these two non-parallel sequences havea similar orientation but spatially distinct axial surfaces and different fold plunges(Figs 6, 7 & 8c). These structures have not been rotated, indicative of low strains.This is in agreement with the low lateral offset estimates on the bounding fault zones.
❐ SUMMARY
❐ The Confidence Hills is a 19km long, high-level, composite, positive flowerstructure developed in Pliocene to Recent fluvio-lacustrine and alluvial fan sediments.
❐ By comparison with analogue model results and seismic examples of similarfeatures, the bounding fault zones are inferred to link at depth to a common basaldetachment (Figs 9 & 10).
❐ Uplift along the Confidence Hills was probably aided by halite mobility combinedwith flexural slip along numerous intraformational detachment surfaces. Thestratigraphy of the Confidence Hills may be considered to be mechanically weak andas such favoured folding rather than faulting in the early stages of deformation, priorto fault breaching. Harding (1985) states that positive flower structure formation ispromoted by the presence of a ductile sedimentary section.
❐ Low lateral offset estimates and the high proportion of oblique-slip kinematicsin the Confidence Hills are consistent with the modern SDVFZ being immature.
❐ REFERENCES
Beratan, K.K. & Murray, B. 1992. Stratigraphy and depositional environments, SouthernConfidence Hills, Death Valley, California. Quarterly Journal of the San Bernardino CountyMuseum Association, 39, 7-11.Butler, P.R., Troxel, B.W. & Verosub, K.L. 1988. Late Cenozoic history and style ofdeformation along the southern Death Valley fault zone, California. Geological Society ofAmerica Bulletin, 100, 402-410.Dooley, T.P., & McClay, K.R., 1996. Strike-slip deformation in the Confidence Hills, southernDeath Valley fault zone, eastern California, USA. Journal of the Geological Society ofLondon, 153, p. 375-387.Harding, T.P., 1985. Seismic characteristics and identification of negative flower structures,positive flower structures, and positive structural inversion. American Association ofPetroleum Geologists Bulletin, 69, 582-600.Wright, L.A. & Troxel, B.W., 1967. Limitations on right-lateral strike-slip displacements.Death Valley and Furnace Creek fault zones, California. Geological Society of AmericaBulletin, 3, 933-950.Roussos, N. & Triantafyllos, L., 1991. Structure of the central North Aegean Trough: anactive strike-slip fault zone. Basin Research, 3, 39-48.
CONFIDENCE HILLS POSITIVE FLOWER STRUCTURE, SOUTHERN DEATH VALLEYTim Dooley & Ken McClay
Fault Dynamics Research Group, Geology Department,Royal Holloway University of London, Egham, Surrey TW20 OEX, UK
Figure 1. Regional and local setting of the Confidence Hills (modifiedfrom Butler et al., 1988). Abbreviations: FCFZ, Furnace Creek faultzone; GF, Garlock fault; MSF, Mule Springs fault; SAF, San Andreasfault; SDVFZ, southern Death Valley fault zone.
LOCATION STRATIGRAPHY STRUCTURE OF THE CONFIDENCE HILLS SUMMARY
Figure 5. (a) Fine grained lacustrine sediments of marker horizon LS-3;(b) Pebble to boulder conglomerates of Qg-1. Photograph from Wright
Canyon, immediately west of fault zone A (see Fig. 6).
Figure 4. Stratigraphy of the Confidence Hills. Ash bed data fromButler et al. (1988) and Beraton & Murray (1992).
Figure 3. View northwest along the Confidence Hills from Troxel Canyon.
Figure 2. Landsat TM image of the Confidence Hills.
Figure 6. Geological map and cross-sections of the Confidence Hills based on 1:6,000mapping by the authors.
Figure 8. (a) View NW along fault zone A. Photograph taken from just south of Troxel Canyon. (b) Fault zoneA in the northern Confidence Hills. Note the less eroded nature of the fault zone compared with that observedin the central and southern Confidence Hills. (c) Southern closure of the central anticline in Troxel Canyonillustrating the unconformity between the Confidence Hills Formation and the overlying Qg1 deposits. View isto the southeast. Note the presence of tight, overturned, parasitic folds. (d) Southwesterly-vergent, tight, parasiticsyncline from the northeastern flank of the central anticline. View is to the southeast.
Figure 10. Plan view (a), section view (b) and 3D relief map (c) of analogue model llustratingdeformation in the overburden above a linear basement fault. Total dextral displacementof 10 cm. (d) Seismic section, and line drawing (e), through a positive flower structure fromthe Athos fault zone, North Aegean Trough (modified from Roussos & Triantafyllos, 1991).
GEOLOGY DEPARTMENT
Royal HollowayUniversity of London
DYNAMICSFAULT
Royal Holloway University of London
Sand Dunes, Stovepipe Wells, Northern Death Valley
N
60
5
23
85
4024
1436
42
30
30
462
30
30
13° - 122°
38
6° -310°
1410
8
2454
TroxelCanyon
22
10
34
40
36
32
70
80
12
16
3630
10
24
58
16
9
2
32
52
22
6058
70
10° -130°
Map Scale2 km
LS 4 lacustrine andalluvial deposits
LS 3 gypsumhorizons
LS 2 lacustrine muds,silts and gypsum
LS 1 halite
1.5 Ma. Shoreline ButteLavas
Qg1 deformedfanglomerates
Qg2, younger tiltedalluvial fanglomerate
AndesiticCinder Cones
Anticline, with plunge
Syncline, with plunge
Lines of cross-section
WrightCanyon
Fault segment
Fault, inferred
28
30
87
60
32
50
24
30
CinderHill
Shoreline Butte
Northern Confidence Hills
Southern Confidence Hills
Recent Alluvial Fans
Recent Alluvial Fans
Recent Alluvial Fans
Recent Alluvial Fans
22° - 099°
KEY
15°- 335°5
19°
14°- 323°
Folded Unconformity
LEGEND
unconformity
unconformity - disconformity
Central Confidence Hills
Inferred Tip-PointFault Zone B
FAULTZONE B
FAULTZONE A
FAULTZONE A
38
80
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0-100-200-300-400
SW NE
AmargosaRiver
200100
0-100-200-300-400
Folded Unconformity
AmargosaRiver
200100
0-100-200-300-400
Metres
AmargosaRiver
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Folded Disconformity200100
0-100-200-300-400
Metres
?
?
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??
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LS-3LS-2
Qg1
MainClosure
LS-4 StrataQg1
Fault Zone A
LS-2 StrataQg1
a
b
a
c
b
d
Figure 7. Detailedst ructura l map andstereoplots of the southernConfidence Hills. (1) Poles tobedding plotted on southernhemisphere equal area stereographicprojections for the three dip domains thatcomprise the southern anticline. (2) Polesto bedding plots of data from Sequence-1 andQg1 strata for the syncline that separates the centraland southern anticlines illustrating coaxial folding ofthese sequences.
Figure 9. 3D synopticmodel of the Confidence
Hills, constructed from surfacegeology. For ease of presentation
only two stratigraphic form surfacesare included. Fault segments are
projected above their present erosionlevels. Scales are approximate.
INFERRED TIP-LINEFAULT ZONE B
FAULT ZONE A
FAULT ZONE A
DETACHEDSEDIMENTARYCARAPACE
BASAL FAULT SYSTEM
N
~ 16 km
≥ 3.0 km
CINDERHILL
Sequence-1form surface
Sequence-2form surface
Qg1 -
Fluvio-lacustrine deposits -gypsiferous mudstones, siltstonesand sandstones,with interbedded coarse fluvialdeposits.Maximum thickness 240m
Lacustrine mudstones, siltstones andgypsum beds20 - 50m thickness
VVVVVVVVVV
VVVVVVVVVV
VVVVVVVVVV
VVVVVVVVVV
VVVVVVVVVV
VVVVVVVVVV
VVVVVVVVVV
Pleistocene and Quaternaryfanglomerate depositsMaximum thickness 120m
LS 2 - Lacustrine mudstones, siltstones, andsandstones.Gypsum beds common. Sparsefluvial sandstones.Maximum thickness 230m
Poorly exposed pink and white,massive halite~ 20 m exposed, base not observed
Ash bed 1.14-0.90 Ma.
LS 4 -
LS 3 -
LS 1 -
Ash bed 1.8 Ma.
M S SS C
Erosional Unconformity - Disconformity
Seq
uen
ce 1
- C
on
fid
ence
Hill
s F
orm
atio
nS
equ
ence
2
Recent gravels and alluvial fan deposits
Seq
uen
ce 3
Tilted Unconformity
Qg2 - Tilted alluvial fan deposits, 0-20 m
Scour
Coarsening upwards beds
Piedmont surface
Fining upwards beds
Isolated cobbles or boulders
Cross-bedding
VVVVVV VVVVVVVVVV VVVV
Pebble stringers
Channel scour fill
Planar bedding
Gypsum beds
Wavey lamination
Lensoid silts and sands
Huckleberry Ridge Ash ~ 2.0 Ma.
2 km
Cinder Hill
Owlshead Mountains
Black Mountains
Amargosa Wash
N
N
N
Garlock Fault
Confidence Hills
c
a b
Eastern Subzone,SDVFZ
Inactive WesternSubzone, SDVFZ
STUDYAREA
10 km
CALIFORNIA
MOJAVEDESERT
SAF
GF
OwlsheadMountains
AvawatzMountains
Precambrian - Tertiaryrocks
Plio-Pleistocene rocksassociated with SDVFZ
Quaternary alluvium
10 km
Eastern Subzone, SDVFZ
MSF
FuneralMountains
BlackMountains
DE
AT
HV
AL
LE
Y
PanamintMountains
SDVFZ
FCFZ
CONFIDENCEHILLS
c
A T
A TA T
A TA T
A T
SECTION SCALE1 km
H = V
200100
0-100-200-300-400
➊
❷
(?)SALT CORE
?
Metres
A T ?
NE
A T
200100
0-100-200-300-400
SW
?
A T
?
A T
Acoustic Basement
Sea FloorSEA FLOOR
Acoustic Basement1 km
1.0
2.0
TW
T (
sec)
A T
A T
A T
1 km
A T
A T
A T
T Motion out of pageA Motion into page
d e
x x'c
Ver
tica
l Rel
ief A
bo
ve T
op
San
dp
ack
(cm
)
1.0
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0.0A T
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A T
Baseplate
5 cm
ActiveZone
10 cm
x
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a
b