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

➊

❷

➏

❸❹

❺

➊

➏

❸

❹

❺

200100

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

??

?

Folded Disconformity200100

0-100-200-300-400

Metres

?

?

?

?

??

??

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

0.5

0.0A T

A T

A T

Baseplate

5 cm

ActiveZone

10 cm

x

x'

a

b