fault royal holloway dynamics preprint / reprint...

Journal of the Geological Society 1996 153 375 3871

GEOLOGY DEPARTMENT

Royal HollowayUniversity of London

DYNAMICSFAULT

Royal Holloway University of London

PREPRINT / REPRINT

Strike-slip deformation in the Confi dence Hills, Southern Death Valley Fault Zone, Eastern California, USATim Dooley and Ken McClay

Fault Dynamics Research Group,Geology Department,Royal Holloway University of London,Egham, Surrey, TW20 0EX,United Kingdom.

This research was supported by the Fault Dynamics Project (spon-sored by Arco British Limited, Brasoil, U.K. Ltd., BP Exploration, Conoco (U.K.) Limited, Mobil North Sea Limited, and Sun Oil Britain). TD and KMcC also acknowledge funding from Sun Oil Britain. The authors express their gratitude to Bennie Troxel and Lauren Wright for their hospitality and for many hours of fruitful discussion. B. Murray is also thanked for discussions and cor-respondence on the geology of the Confi dence Hills. The US National Park Service at Furnace Creek, especially Tim Coonan, are thanked for their helpful advice. Landsat TM imagery was kindly provided by the Jet Propulsion Laboratory, Pasadena, Cali-fornia. The authors thank Dennis Brown, A.G. Sylvester, A. Aydin, A. Sarna-Wojicki, I. Cemen, L.Wright and M. Naylor for critical but constructive reviews of earlier versions of this manuscript.

The Confi dence Hills form a well exposed, com-posite, positive fl ower structure developed in Pliocene to Recent lacustrine and alluvial fan sediments. This structure has developed along the current trace of the southern Death Valley dextral strike-slip fault zone, California, USA. NW-SE-striking fault zones bound the Confi -dence Hills. In 3D, these fault segments are inferred to link at depth to a common basal fault system. The fl ower structure is formed by doubly plunging anticlines that roughly paral-lel the bounding fault segments. Fold develop-ment was aided by: (a) the presence of a basal salt deposit and numerous detachment horizons within the sedimentary pile; and (b) buttress-ing and uplift along and against the bounding oblique-slip reverse faults. Structural and pal-aeomagnetic evidence indicates that the anti-clines developed with axial surfaces parallel and sub-parallel to the trace of both fault zones. Folding appears to have initially devel-oped adjacent to and above the southeastern fault segment and then propagated outwards and to the northwest. The latest displacements along the current southern Death Valley fault zone are probably only in the orders of 100s of metres and as young as 0.9 Ma to Recent. This

young fault zone is highly segmented along the length of the fl oor of southern Death Valley; the high proportion of oblique-slip faulting con-sistent with the strike-slip zone being imma-ture. The Confi dence Hills structure displays close geometric similarities with other natural examples of positive fl ower structures and to the features found in scaled, analogue sandbox experiments of strike-slip faulting.

INTRODUCTION

The right-lateral southern Death Valley strike-slip fault zone (Fig. 1) forms the southwestern boundary of the Death Valley extensional sub-prov-ince, southeastern California (Wright and Troxel, 1970; Wright, 1976; Butler et al., 1988). This fault zone consists of a number of NW-SE striking seg-ments that are divided into two sub-zones - an eastern sub-zone and a western sub-zone (Fig. 1c) (Butler et al., 1988). Recent fault activity (1 Ma - present) has been recorded (Butler et al., 1988; this study) on the highly segmented eastern sub-zone in Plio-Pleistocene fl uvio-lacustrine sediments. The western subzone consists of older fault segments exposed on the northeastern fl anks of the Avawatz Mountains (Fig. 1c; Butler et al., 1988).

The Confi dence Hills are a low-lying range of hills (with elevations of up to 200m above the fl oor of Death Valley) along the northwestern extremity of the eastern subzone of the Southern Death Valley fault zone (Figs 1 and 2). The hills consist of well exposed, moderately incised, folded and faulted Plio-Pleistocene fl uvio-lacustrine strata and Pleis-tocene-Recent alluvial fans (Butler et al., 1988; Pluhar et al., 1992; Troxel et al., 1986) that form a narrow ‘positive fl ower structure’ along this fault zone. Quaternary alluvial fans that prograde into Death Valley from the Owlshead mountains to the west are disrupted by the Confi dence Hills (Figs 2 and 3).

‘Flower structures’ (Harding, 1974; 1985) are commonly cited as one of the key characteris-


Confi dence Hills Positive Flower Structure Tim Dooley & Ken McClay

2

Garlock Fault

Confidence Hills

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SDVFZ

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Avawatz Mountains

CONFIDENCEHILLS

OwlsheadMountains

Fig. 1c

Figure 1. Location (a) and summary map (b) of the Death Valley region. Summary geological map (c) of the southern Death Valley fault zone illustrating active and inactive sub-zones (data from the authors mapping and from Butler et al.,1988; Wright and Troxel, 1984; Wright, 1974). Abbreviations: FCFZ - Furnace Creek fault zone; GF - Garlock fault; MSF - Mule Springs fault; SAF - San Andreas fault; SDVFZ southern Death Valley fault zone.



3

tic structures found in strike-slip regimes. Their geometries and kinematics are, however, poorly understood and there are few detailed descriptions of fi eld examples of strike-slip fl ower structures. A ‘positive fl ower structure’, as defi ned by Hard-ing (1985), is a “shallow antiform displaced by the upward diverging strands of a wrench fault that have mostly reverse separations”. Positive fl ower structures developed along strike-slip fault systems have predominantly been documented from seis-mic-refl ection profi les (e.g. Harding, 1974, 1985; Bartholomew et al., 1993; Roussos and Triantafyl-los, 1991). Diffi culties in imaging high-angle reverse, oblique-slip and strike-slip faults on seis-mic profi les have generally precluded detailed analyses of these features, and only the broad structural architecture can be confi dently identi-fi ed. Analogue modelling has successfully repro-duced the structural styles of fl ower structures along strike-slip faults (Naylor et al., 1986; Richard et al., 1995; Richard and Cobbold, 1990; McClay and Dooley, 1994, 1995; Dooley, 1994), although the relative coarseness of the modelling materials (generally silica sand) has precluded detailed studies of their internal architecture. Detailed

description of surface examples of positive fl ower structures have been relatively few, the best docu-mented being those formed along the southern San Andreas fault zone in the vicinity of the Mecca Hills (Sylvester and Smith, 1976), the Transverse Ranges (e.g. Wilcox et al., 1973), and the Betic shear zone of southern Spain ( Montenat et al., 1990). All these examples have been clearly docu-mented as transpressive strike-slip fault zones.

This paper presents the results of a detailed structural analysis of a well exposed and geologi-cally recent, composite positive fl ower structure. Detailed 1:6,000 scale structural and sedimento-logical mapping of 62 km2 of the Confi dence Hills in southern Death Valley was undertaken using enlarged aerial photographs and Landsat TM imagery, together with measurement of strati-graphic sections, structural analysis and cross-sec-tion construction. The results of this analysis are compared with other examples of fl ower structures and with the results of scaled analogue models of strike-slip fault systems.

2 km

Cinder Hill

Owlshead Mountains

Black Mountains

Amargosa Wash

N

Figure 2. LANDSA TM image of the Confi dence Hills and immediate surrounds.



4

PREVIOUS WORK

Burchfi el and Stewart (1966) fi rst proposed a crustal scale ‘pull-apart’ model for the central Death Valley rhombochasm bounded by the Fur-nace-Creek fault zone to the north and by the southern Death Valley fault zone to the south (Fig. 1b). Since this model was fi rst proposed there has been considerable debate as to the applica-bility of this pull-apart interpretation, specifi cally regarding the amounts of displacement on the two bounding dextral strike-slip systems (e.g. Stewart 1967; 1983; Wright and Troxel, 1967; 1970; Butler et al., 1988; Serpa et al., 1988; Snow and Wer-nicke, 1989). Cemen et al. (1985) presented evi-dence for up to 50 km dextral displacement along the northern Death Valley/Furnace Creek whereas Snow and Wernicke (1989) documented up to 70 km dextral offset on this fault zone. Displace-ment estimates for the southern Death Valley fault zone have varied from 8 km (Wright and Troxel, 1967) to 80 km (Stewart, 1967). The estimate of Stewart (1967) was based on offset isopach trends of late Precambrian and Palaeozoic sedimentary rocks along the northern Death Valley fault zone and applying these values to the southern Death Valley fault zone. Wright and Troxel (1967) based theirs on offset formation contacts and isopach data from the southern Death Valley fault zone. Stewart (1983) partially resolved this confl ict by proposing that these fault zones are related but not connected and that activity began on the north-ern Death Valley fault zone prior to motion on the southern Death fault zone. Butler et al. (1988) proposed ≈ 35 km of dextral offset since the Mid-Miocene to ~ 1 Ma on the western subzone based on matching offset alluvial fan gravels. This value is on the same order of magnitude as the northern Death Valley fault zone. This western subzone was active from approximately 14 Ma to 1 Ma, becom-ing locked as the eastern termination of the Gar-lock fault, the Mule Springs fault, propagated and overrode this dextral system as a sinistral oblique-slip reverse fault (Fig. 1c; Butler et al., 1988).

Reconnaissance mapping of the Confi dence Hills (eastern sub-zone) was carried out by B.W. Troxel (unpublished data ) and aspects of the geology of selected parts of the area were docu-mented by Troxel and Butler (1986), Butler et al. (1988) and Wright and Troxel (1984). Prior to the research described in this paper a detailed map had not been made nor had a detailed structural analy-

sis been carried out. After our research began, a detailed sedimentological study of the Confi dence Hills was initiated by B. Murray, J. Kirschvink, K. Beratan and co-workers from the California Institute of Technology. This study was initiated because the sediments of the Confi dence Hills are possibly one of the best localities to coordinate palaeomagnetic reversals, tephrachronology, and argon-argon dating in order to locate the magnetic reversals in time. The preliminary fi ndings of this study are included in the text, where appropriate.

The Confi dence Hills present a unique exam-ple to study shallow level strike-slip tectonics in semi-consolidated sediments. We know of no other work that presents such an opportunity to study the detailed 3-dimensional geometries and kinematics of a positive fl ower structure. Further-more, this detailed study may be able to resolve some of the controversy concerning the amount of strike-slip displacement on portions of the south-ern Death Valley fault zone.

GEOLOGY OF THE CONFIDENCE HILLS, SOUTHERN DEATH VALLEY

The Confi dence Hills are approximately 19 km long from Cinder Hill in the northwest to their southeastern tip, and are up to 3.25 km wide (Figs 2 and 3). They consist of two overlapping and segmented dextral strike-slip fault systems (fault zones A and B, Fig 3), together with open to tight folds (amplitudes from 100 m to greater than 600 m and wavelengths from 200 m to greater than 2500 m) formed in Plio-Pleistocene to Recent lacustrine muds, silts, evaporites, alluvial sands and alluvial fanglomerates (Figs 2, 3 and 4). The folds plunge at shallow to moderate angles (6° to 22°) to both the NNW (301° to 335°) and to the ESE (099° to 130°) (Fig. 3). The structures are best exposed in the central and southern Confi -dence Hills (Figs 2, 3 and 8a) whereas to the north many exposures are mainly subcrop overlain by a thin veneer of gravels.

Stratigraphy

Three distinct lithostratigraphic sequences are exposed in the Confi dence Hills - a lower Plio-Pleistocene fl uvio-lacustrine sequence, a middle Pleistocene alluvial fan sequence, and an upper sequence of Quaternary - Recent alluvial fans (Fig. 5). In this paper we follow the suggestion of



5

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Figure 4. Cross-sections through the Confi dence Hills. Location of sections 1 to 6 are shown on Figure 3. Sections were constructed from 1: 6,000 scale maps and thus possess more detail than can be shown on Fig. 3



7

Beratan and Murray (1992) and name the lower sequence the Confi dence Hills formation (infor-mal stratigraphic name).

Sequence-1 - Confi dence Hills formation

Sequence-1 consists of up to 540 m of fl u-vio-lacustrine deposits (Fig. 5), subdivided into four lithostratigraphic units (LS 1-4, Fig. 5). The lowest unit (LS-1) consists of at least 20m of heavily veined, coarsely crystalline pink and white halite. Although poorly exposed this unit is rec-ognisable by its association with solution collapse and sink hole topographic features. Units LS 2-4

consist of approximately 520m of fl uvio-lacustrine deposits (Fig. 5). A broadly coarsening-upwards character is observed and LS-4 is capped by coarse quartzo-feldspathic sandstones and cobble con-glomerates of fl uvial/alluvial origin. Unit LS-3 (Fig. 5) is a distinctive 20 to 50 m thick marker unit of fi ne to coarsely crystalline, massive, banded to nodular gypsum with intercalated mudstones and siltstones. Unit LS-4 is both laterally and ver-tically variable in thickness. Ash bands within this LS-2 unit have given Upper Pliocene ages (2.0 Ma and 1.8 Ma, Fig. 5; Butler et al., 1988; Pluhar et al., 1992), and a tephra layer near the top of this sequence (Fig. 5) yielded an age range of 0.73 Ma to 1.14 Ma (Butler et al., 1988).

The contact between LS-4 and the overlying fanglomerate Sequence 2 varies from a distinct angular unconformity (Fig. 6a) that is folded in the southern Confi dence Hills (Fig. 3), to a dis-conformity in the central and northern Confi dence Hills (Fig. 3; sections 1 - 3 Fig. 4).

Sequence-2

Sequence 2 is subdivided into two patchily exposed units, Qg1 and Qg2 (Figs 3 and 5). Qg1 consists of tan-coloured, pebble to boulder con-glomerate with a predominant bimodal clast com-position of granite and basic volcanics in a coarse quartzo-feldspathic sandy matrix. Typical exposed thicknesses of Qg1 range from only a few meters to a maximum measured thickness of 120m (Fig. 5). On Shoreline Butte the basic volcanic clast content increases in these Qg1 conglomerates.

Qg2 consists of poorly bedded pebble to boul-der conglomerates of granitic and basaltic clasts together with fi ne to coarse quartzo-feldspathic sandstones. On Shoreline Butte these deposits contain abundant angular andesitic clasts probably sourced from the Shoreline Butte lavas (Fig. 2 and 3). Qg2 unconformably overlies and fl anks Qg1, and, in places, units of sequence 1 (Figs 2 and 3). Erosion and probable non-deposition make esti-mates of the thickness of this unit diffi cult.

Sequence-3

Sequence 3 includes a variety of Recent allu-vial fan deposits that overlie and fl ank all of the older strata in the study area. Three groups of allu-vial fans can be recognised based upon location,

Qg1 -

Fluvio-lacustrine deposits -gypsiferous mudstones, siltstones andsandstones,with interbedded coarse fluvial deposits.Maximum thickness 240m

Lacustrine mudstones, siltstones and gypsumbeds20 - 50m thickness

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Poorly exposed pink and white,massive halite~ 20 m exposed, base not observed

Ash bed 1.14-0.90 Ma. 1

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Huckleberry Ridge Ash ~ 2.0 Ma. 1,2

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Lensoid silts andsands

Figure 5. Summary stratigraphic section and ash bed ages of the Pliocene-Recent strata of the Confi dence Hills based upon detailed measurement of stratigraphic sections by the authors (1 - Butler et al., 1988; 2 - Pluhar et al., 1992; 3 - Beratan and Murray, 1992).



8

morphology and clast content. Two major alluvial fan systems drain the Owlshead and Black Moun-tains, and a third system drains the Confi dence Hills. The Amargosa Wash forms a distinct drain-age divide separating fans derived from the Black Hills to the northeast and fans derived from the Owlshead mountains and Confi dence Hills to the southwest (Fig. 2).

Volcanic rocks

The basaltic lava fl ows of Shoreline Butte

(Fig. 3) have been dated at 1.5 Ma (Wright and Troxel, 1984). Although the contact between the lavas and Sequence-1 strata is not exposed this age postdates the LS-2 unit. Angular basaltic clasts present in clastic wedges that interfi nger with LS-3 and LS-4 strata suggests that these lavas occur between LS-2 and LS-3 in the stratigraphic sequence (Figs 3 and 4). Horizontal shorelines cut into these lavas have been attributed to a lake that once occupied this part of southern Death Valley. The last high stand of this lake has been estimated as having occurred at approximately 70, 000 BP (Butler et al., 1988). An andesitic cinder cone,

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LS-3

LS-4

LS-2

Unconformity

Fault Zone A

Main Fold Closure

Folded Sequence-1 Strata

Tilted Qg1 and YoungerStrata

Trace of Fault Zone AStrike ~ 315°

Figure 6. a. Southern closure of the central anticline in Canyon-1 illustrating the unconformity between the Confi dence Hills Formation and overlying Qg1 deposits (left midground). View is to the southeast. Bedding dips around anticline is indicated by white arrows. Northeastern limb of fold is sub-vertical to overturned in Sequence 1 strata. Parasitic folds are well developed on this limb. b. View northwest along fault sub-zone A in the central domain. Canyon-1 runs to the east in the midground. Average strike is 315°. The dark material to the southwest of this fault zone is Qg1 and younger, blanketing fanglomerate. Sequence-1 strata are exposed to the northeast of this fault zone.



9

Cinder Hill, is present to the north of Shoreline Butte (Fig. 3) and has yielded aK/Ar age of 0.69 Ma. (Wright and Troxel, 1984).

Structure

The main structural features of the Confi -dence Hills are illustrated in Figure 3. The domi-nant structures are two NW-SE striking fault zones, A and B (Fig. 3 ). Fault zone A is highly seg-mented and extends along the entire southwestern boundary of the Confi dence Hills (Fig. 3). Fault zone B forms the northeastern boundary of the central and southern parts of the Confi dence Hills (Fig. 3). A 2.3 km wide zone of NW- and SE-plunging folds occurs between fault zones A and B (Fig. 3). The principal cross-sectional structural styles of the Confi dence Hills are shown in Figure 4.

Fault Geometries

The dominant fault structure of the Confi -dence Hills is fault zone A which consists of a single main NW-striking fault strand (average 315°) and associated segments in the central and southern Confi dence Hills (Fig. 3). This widens out into a 600-1000 m wide faulted zone to the north (Fig. 3, and cross-section 1, Fig. 4). Indi-vidual faults are generally poorly exposed and are usually characterised by erosional gullies (e.g. Fig. 6b) and rubble zones. Where observed in outcrop and where traced across the topography the fault surfaces are vertical to sub-vertical in orientation. Exposure of one fault surface revealed an inter-nal, 20 cm wide, sub-vertical gouge zone with a strike of 140° and an external bedding drag zone. Within this gouge zone numerous left-stepping, vertical, Riedel shears with a strike of 160-165° are present, indicative of a dextral fault system.

In the northern section of the central part of the map and in the northern Confi dence Hills, fault zone A is strongly segmented into en-echelon, left stepping fault arrays which produce buttressing, chaotic folding and disruption in the overlap zones (Fig. 3). The northernmost strand of fault zone A bounds the lavas of Shoreline Butte and can be traced to Cinder which is dextrally offset by 180-200m (Fig. 3). Ephemeral stream washes that cut across the Confi dence Hills (Figs 2 and 3) are defl ected dextrally by 200 m to 300 m as they cross the surface trace of fault zone A indicating

only limited recent fault movement . Three promi-nent faults are found at angles of 19° to 24° (clock-wise-oblique) to the main strand of fault zone A in the central Confi dence Hills(Fig. 3). Vertical uplift of as much as 200m across fault zone A is recorded in the central part of the map area (Fig. 3; cross-sections 2 - 4, Fig. 4).

Fault zone B is identifi ed both on Landsat TM images and aerial photographs as a distinct, ~ 8 km long, linear fault strand that bounds the south-eastern part of the Confi dence Hills (Fig. 3). Adja-cent to this fault zone the beds of sequence-1 are strongly rotated becoming sub-vertical and beds of sequence-2 are also tilted though less strongly than those of sequence 1 (Fig. 7).

The observed left-stepping fault arrays, clock-wise stream defl ections and the offset of Cinder Hill all indicate dextral strike-slip displacement along fault zone A. Folding, tilting, vertical stratigraphic separations also indicate a signifi cant amount of vertical uplift and buttressing across the fault zones.

Fold Geometries

The major folds in the Confi dence Hills are three anticlines that are sub-parallel to and bounded by the offset fault zones (Fig. 3 ). The fold styles vary from open, rounded folds to kink-like geometries with angular hinges and planar limbs. The major folds plunge both to the NW and to the SE (Fig. 3). Shortening estimates vary from 10-50%, with maximum shortening documented from the central anticline.

The northern Confi dence Hills consist of a fairly simple, broad, upright, anticlinal structure that is breached by strands of fault zone A (section 1, Fig. 4) whereas the central Confi dence Hills is formed by a 6 km long, NW-SE-striking, doubly plunging anticline (Fig. 3; cross-sections 2 - 4, Figs 4 and 6a). The strike of the axial surface of the central fold is less than 10° anticlockwise in an oblique orientation to fault zone A (Figs 3 and 6a). The northern closure of this fold plunges 15° to 335° and the southern closure plunges 10° to 130° (Fig. 3). From northwest to southeast this fold is fi rst upright, then overturned and becomes upright again southeast of Canyon-1 (Fig. 4, cross-sections 2-4).



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CANYON 1

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Best Fit19-301°

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48 Data

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Best Fit05-304°

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Best Fit15-313°

Sequence-1 Data

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Folded Unconformity -Disconformity

a

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Fault, increasinguncertainty

Fault Strike~ 315° Southern Dip Domain

34 Data

Best Fit13-122°

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Best Fit11-311°

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+

Figure 7. (a) Detailed structural map and stereoplots of the central and southern domains. (1) Poles to bed-ding plotted on southern hemisphere equal area stereographic projections for the three dip-domains that form the main anticline of the southern Confi dence Hills. (2) Poles to bedding plots of data from Sequence-1 and Qg1 strata for the syncline that separates the central and southern anticlines illustrating coaxial folding of these sequences. (b) Plot of fold axes in the central and southern Confi dence Hills.

Small scale folds with amplitudes of a few metres to a few tens of metres are developed along fault zone A. In Canyon 3 (Fig. 3) right-stepping en-échelon folds with amplitudes of 2 - 10 metres and mean plunges of 20° to 099 are oriented at approximately 45° to the strike of fault zone A, i.e. consistent with a dextral model. Parasitic folds to the central anticline are well developed in Canyon 1 (Fig. 3) with overturned northeastern limbs, interlimb angles of 45° - to 50°, and inclined axial surfaces that dip up to 60° to the west. Slicken-sides on bedding surfaces indicate a fl exural-slip fold mechanism.

The geology of the southern Confi dence Hills is shown in detail in Figure 7. The dominant struc-ture is a northwest-southeast striking, 4 km long, doubly plunging anticline (Figs 3 and 7a; cross-sections 5 and 6, Fig. 4). The geometry of this anticline changes from a broad anticline with sub-ordinate kinking and with shallowly to moder-ately dipping fl anks, to a box-fold geometry with

steeply dipping fl anks (cross-sections 5 and 6, Fig. 4).

Right-stepping parasitic fold arrays are found on the northeastern limb of the southern Confi -dence Hills anticline and are best developed in interbedded gypsum/anhydrite and siltstone strata (Fig. 3). The parasitic folds typically have axial surfaces that are sub-parallel to fault zones A and B, dip up to 60° to the southwest, and mean plunges of 24° to 320° (Fig. 7b). The parasitic folds die-out both up- and down-section and along plunge. The right stepping nature of these folds is consistent with a dextral system but their orienta-tion is problematic (see below).

In the region between cross-sections 3 and 5 (Fig. 3) the unconformity between Sequence-1 and Sequence-2 strata is well exposed (Fig. 8a). In Canyon-1 (Fig. 8a) the unconformity decapi-tates the hinge of the anticline in Sequence-1 strata (Figs 6a and 8a; cross-sections 3 and 4, Fig. 4). The unconformity is folded around the



11

central Confi dence Hills anticline and across the syncline between the central and southern Confi -dence Hills anticlines (sections 3 and 4, Fig. 4; Fig. 8a). Sequence 2 strata are, however, less folded than the Sequence 1 strata beneath the unconform-ity indicating that the underlying Sequence 1 units were folded prior to deposition of Sequence 2. The folds in these two non-parallel sequences have similar orientation but spatially distinct axial sur-faces and different fold plunges (section 4, Figure 4; Fig. 8a).

DISCUSSION

Detailed mapping and analysis of the Confi -dence Hills along the northern part of the NW-SE striking southern Death Valley fault zone has shown that the dominant structures are a series of anticlines sub-parallel to segmented strands of the fault zone. The anticlines are doubly plunging, are breached by faults, in places are overturned (cross-section 3, Fig. 4), and show signifi cant shortening and uplift of strata along the fault zone (Fig. 4). The fault surfaces are steep to vertical in orienta-tion. The structure of the Confi dence Hills resem-bles that of a positive fl ower structure (i.e. uplifts bounded by strike-slip faults with reverse separa-tion, cf. Harding, 1974, 1985) formed by doubly plunging ‘push-up’ anticlines aligned sub-parallel to, and along, the segmented strike-slip fault zone. No strike-slip deformation of this age, nor defor-mation associated with the eastern subzone, is doc-umented immediately to the NE and SW of the Confi dence Hills. The next segment of the eastern subzone is located some 5 km further to the south-east (Fig. 1; Butler et al., 1988).

Fault and Fold Kinematics

Regional tectonic considerations (cf. Stewart 1967; 1983; Wright and Troxel, 1967; 1970; Butler et al., 1988; Snow and Wernicke, 1989) indicate that the southern Death Valley fault zone is dextral. Dextral offset of Cinder Hill and clockwise stream wash defl ection in the Confi dence Hills (Figs 2 and 3) support this interpretation. In this dextral system the three oblique faults that fl ank fault zone A in the central Confi dence Hills (Fig. 3) can be interpreted to represent remnant Riedel shears (e.g. Naylor et al., 1986; Richard et al., 1995; McClay

and Dooley, 1995). This is further corroborated by the presence of right-stepping fold arrays in the northern and southern Confi dence Hills. In addition, buttressing and uplift of LS-4 strata is observed between left stepping fault segments on Shoreline Butte, i.e. a restraining stepover (e.g. Segall and Pollard, 1980; Fig. 3; cross-section 1, Fig. 4).

The folded unconformity surface between Sequences 1 and 2 (Fig. 7a) yields important infor-mation regarding the geometries and timing of folding in the southern and central sections of the Confi dence Hills. In the main anticline - syncline pair adjacent to fault zone A, strata above and below the unconformity are folded with a common axial surface orientation but with different plunges - 19° to 301° below the unconformity and 11° to 311° above the unconformity (Fig. 7a). To the northeast, however the two sequences are folded coaxially with a plunge of 6° to 310° (Fig. 7a), and possess a disconformable contact. These relation-ships indicate that folding adjacent to the present surface trace of fault A in the southern Confi -dence Hills began after deposition of Sequence 1 and before deposition of Sequence 2 which subse-quently eroded the crest of the anticline (Figs 6a, and 8a ; cross-sections 3 and 4, Fig. 4). The fold structure continued to grow and increase in ampli-tude and wavelength after deposition of Sequence 2. The axial surfaces of the folds in the strata above the unconformity are spatially offset from the axial surfaces of the folded strata below the unconformity surface as a result of folding non-parallel surfaces (c.f. Ramsay and Huber, 1987). These features also indicate that the folds in the Confi dence Hills grew at or near the surface with concommitant erosion and defl ection of alluvial fan drainages as slip accummulated on the fault system. Structural relationships also indicate fold growth prior to fault breaching (e.g. vertical uplift of the hinge region of the central anticline; cross-sections 2-4, Fig. 4). The problematic orientations of the right-stepping fold arrays on the northeast-ern fl ank of the southern Confi dence Hills (Fig. 8a) can be explained through localised transpres-sion resulting from a dominant component of ver-tical motion on fault zone B and concomitant uplift along fault A.

From the above it is clear that the major fold structures initially grew sub-parallel to the fault zone and were not rotated into their current atti-



12

b c NW SE1.0 sec

2.0 sec

SEA FLOOR

?Top Basement

Acoustic Basement 1 km

?Top Basement

Sea Floor

A T

A T

A T

T Motion out of pageA Motion into page

a

10 cm

A T

A T

A T

Baseplate

5 cm

ActiveZone

a

a'

a a'

Figure 8. (a) Plan view of experiment W88 illustrating deformation in the sandpack above a linear basement fault. Total displacement is 10 cm; (b) Cross-section through experiment W88 illustrating overburden deformation above a linear basement strike-slip fault illustrating push-up of the overlying sand-pack (from Dooley, 1994). Line of section indicated in a; (c) Line drawing of a seismic section across a positive fl ower structure from the Athos fault zone, North Aegean Trough, Greece (adapted from Roussos and Triantafyllos, 1991).

tude by progressive simple shear (e.g. as in Wilcox et al., 1973). This is further supported by the pre-liminary palaeomagnetic fi ndings of Pluhar et al. (1992) which indicate no signifi cant net tectonic rotation of the Confi dence Hills strata about ver-tical axes. Fault-parallel folding has also been described from active strands of the southern Death Valley fault zone further to the southeast by Troxel (1970). It is likely that the presence of a basal halite unit, numerous intra-formational detachment horizons, near surface conditions and the relatively poorly consolidated nature of the Confi dence Hills strata permitted folding to predominate at the early stages of deformation, prior to fault breaching.

Fault Displacements

Displacement determinations on this dextral strike-slip system are problematic due to a lack of piercing points (cf. Ramsay and Huber, 1987). 180 - 200 m right-lateral offset of the 0.69 Ma cinder cone at Cinder Hill is a minimum displace-ment for the northwestern end of fault zone A. This does not, however, preclude previous dis-placement along this section of the fault zone prior to 0.69 Ma. Stream defl ection in the central Con-fi dence Hills indicates a minimum of 200-300m dextral offset for this central section of fault zone A. It is therefore most likely that this northern end of the eastern sub-zone of the southern Death Valley fault zone has only undergone relatively



13

limited total displacement (100s of metres). Clast compositions in Qg1 and Qg2 strata are consist-ent with derivation from the Owlshead Mountains directly west of the Confi dence Hills and thus there are no anomalous sediments that require large lateral offsets to explain them (c.f. Butler et al., 1988). These low displacement estimates are also supported by the lack of tectonic rotation of the fold hinge lines and by the palaeomagnetic record (Pluhar et al., 1992).

Timing of Deformation

Stratigraphic and structural relationships in the south-central Confi dence Hills indicate that deformation along this northern part of the south-ern Death Valley fault zone began after deposition of the upper units of Sequnce 1 and prior to initial deposition of Qg1 fanglomerates - i.e. post 1.14 - 0.90 Ma (Figs 5 and 8a). Evidence presented above indicates that deformation was initially con-centrated adjacent to and above fault zone A and

then spread to the northeast. Offset of Cinder Hill along fault zone A indicates dextral strike-slip motion post-0.69 Ma. Recent activity on the southern Death Valley fault zone is indicated by the 1916 earthquake (Rogers et al., 1991).

Structural Model for the Confi dence Hills

Analogue models of deformation above linear basement faults carried out by Dooley (1994) illus-trate in-line uplifts bounded by oblique-reverse dextral strike-slip faults (Fig. 8a & b). Other stud-ies such as those by Naylor et al. (1986), Richard and Cobbold (1990) and Richard et al. (1995) typ-ically illustrate a push-up structure bounded by Riedel shears during the early stages of strike-slip fault zone evolution and at a later stage these struc-tures are bounded by anastomsoing fault zones cosisting of linked R-, P- and Y-shears (Fig. 8a, b). Natural examples of positive fl ower structures that have been imaged in seismic sections show simi-lar geometries (Fig. 8c; Harding, 1985; Roussos

Sequence-1form surface

Sequence-2form surface

INFERRED TIP-LINEFAULT ZONE B

FAULT ZONE A

FAULT ZONE A

DE

TAC

HE

DS

ED

IME

NTA

RY

CA

RA

PAC

E

CINDERHILL

BASAL FAULT SYSTEM

N

~ 16 km

≥ 3.0 km

Figure 9. 3D synoptic model of the Confi dence Hills, constructed from surface geology. For ease of presentation only two stratigraphic form surfaces are shown. Fault segments are projected above their present erosional level. Scales are approximate.



14

and Triantafyllos, 1991; Bartholomew et al., 1993) of upward splaying faults bounding shallow-level anticlinal folds. By analogy, the Confi dence Hills structures may be interpreted to have similar geo-metric forms and in particular the fault zones A and B may be inferred to link to a common basal fault system at depth (Fig. 9). Uplift in the analogue models is achieved by dilation of the sandpack, increasing the pore spaces between indi-vidual grains (Dooley, 1994). In the case of the Confi dence Hills this uplift was probably aided by halite mobility combined with fl exural slip along numerous intraformational detachment surfaces.

In the Confi dence Hills, there is signifi cant uplift along the entire range of hills (Fig. 4) and the anticlines have a detachment fold style that indicates probable fold detachment on the halite/evaporite LS-1 unit of Sequence 1. The stratig-raphy in the Confi dence Hills may be considered to be, mechanically, relatively weak consisting of poorly consolidated lacustrine sediments and evap-orites, together with poorly consolidated overly-ing alluvial fan sequences (Fig. 5) and as such favoured folding rather than faulting during the early stages of deformation. Harding (1985) states that positive fl ower structure formation is promoted by the presence of a thick, ductile sedimentary sec-tion, such as that present in the Confi dence Hills. Folding of the unconformity between Sequences 1 and 2 indicates the the dextral strike-slip faulting appears to have propagated outwards from, and northwestwards along, fault zone A. A synoptic model of the Confi dence Hills structure is shown in Figure 9 where fault zones A and B are inter-preted to merge with a common single basal fault zone at depth. A crudely derived depth to detach-ment estimate using Chamberlain’s (1910) Law indicates that this detachment surface lies some 3 km below sea-level (Fig. 9).

CONCLUSIONS

The Confi dence Hills is a 19 km long, high level, composite positive fl ower structure devel-oped in Pliocene to Recent lacustrine and alluvial fan sediments along the eastern sub-zone of the southern Death Valley dextral strike-slip fault zone. Two northwest - southeast striking fault zones, A and B, bound the Confi dence Hills. The south-eastern fault zone (A) extends for the entire length of the hills and bounds the southwestern side of the three anticlines that form the fl ower structure.

It is segmented at the northwestern end with dex-tral strike-slip displacement indicated by arrays of minor faults and en-echelon fold systems. Fault zone B bounds the northwestern edge of the central and southern Confi dence Hills. In 3D the faults zones and individual fault segments are inferred to link to a common basal fault zone at depth.

In the central and southern Confi dence Hills the positive fl ower structure is made up of doubly plunging anticlines that appear to detach on the basal halite/evaporite unit (LS-1) of Sequence 1 strata. Folding appears to have initially developed adjacent to and above fault zone A and then propa-gated outwards and to the northwest. Structural and palaeomagnetic evidence indicate that the anti-clines that form the positive fl ower structure devel-oped with axial surfaces parallel and sub-parallel to the major faults.

The structural style of the Confi dence Hills displays close geometric similarities to other nat-ural examples of positive fl ower structures and to the features found in scaled analogue sandbox experiments of strike-slip faulting. Highly com-plex deformation patterns and fl ower structures can develop early in the history of a strike-slip fault system that has only accumulated relatively small displacements, as a consequence of strike-slip and oblique displacements on a segmented fault system and between overstepping fault arrays.

Evidence presented in this study and that of Butler et al. (1988) indicate that the latest dis-placements on this eastern sub-zone of the south-ern Death Valley fault zone are probably only in the orders of 100s of metres and as young as 0.9 Ma to Recent. This fault zone is immature and consists of a number of non-linked segments (Fig. 1). Oblique-slip along these offset, segmented, fault systems has produced in-line uplifts such as the Confi dence Hills and those described by Troxel (1970), some 8 km to the southeast. How-ever, controversy still exists over the total amount of displacement along the extinct portions of the southern Death Valley fault zone. A simple pull-apart model for Death Valley requires that the northern and southern Death Valley faults are coeval and possess similar lateral displacements. Troxel (pers. comm., 1991) questions the 35 km+ lateral dispacement described by Butler et al. (1988) and advocates the low lateral displacement along the western subzone as described by Wright and Troxel (1967). If this is so, then a simple pull-



15

apart model for the current Death Valley rhombo-chasm is inadequate to explain the discrepancies in displacement along the boundary fault zones.

REFERENCES

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Beratan, K.K. & Murray, B. 1992. Stratigraphy and dep-ositional environments, Southern Confi dence Hills, Death Valley, California. Quarterly Journal of the San Bernardino County Museum Association, 39, 7-11.

Butler, P.R., Troxel, B.W. & Verosub, K.L. 1988. Late Cenozoic history and style of deformation along the southern Death Valley fault zone, California. Geological Society of America Bulletin, 100, 402-410.

Cemen, I., Wright, L.A. Drake, R.E. and Johnson, F.C., 1985, Cenozoic sedimentation and sequence of deforma-tional events at the southeastern end of the Furnace Creek strike-slip fault zone, Death Valley region, California, in Biddle, K.T. & Christie-Blick, N., eds., Strike-Slip Deforma-tion, Basin Formation, and Sedimentation: Society of Economic Mineralogists and Palaeontologists Special Publi-cation 37, p.127-141.

Chamberlain, R.T. 1910. The Appalachian folds of cen-tral Pennsylvania. Journal of Geology, 18, 228-251.

Dooley, T.P. 1994. Geometries and kinematics of strike-slip fault systems: - Insights from physical modelling and fi eld studies. PhD thesis, London University.

Harding, T.P.1974. Petroleum traps associated with wrench faults. American Association of Petroleum Geolo-gists Bulletin, 58, 1290-1304.

___________ 1985.Seismic characteristics and identifi ca-tion of negative fl ower structures, positive fl ower struc-tures, and positive structural inversion. American Association of Petroleum Geologists Bulletin, 69, 582-600.

McClay, K.R., & Dooley, T. 1995. Analogue models of pull-apart basins. Geology, 23, 711-714.

_________________________ 1994. 3-D strike-slip pull-apart basins – geometries determined from scaled analogue models. American Association of Petroleum Geol-ogists, Abstracts with Programs, 210.

Montenat, C., Masse, P., Coppier, G., & D’Estevou, P. 1990. The sequence of deformations in the Betic shear zone (S.E. Spain). Annales Tectonicœ, 4, 96-103.

Naylor, M.A., Mandl, G. and Sijpesteijn, C.H.K., 1986, Fault geometries in basement induced wrench faulting under different initial stress states; Journal of Structural Geology, 8, 737-752.

Pluhar, C.J., Holt, J.H., Kirschvink, J.L., Beratan, K.K. & Adams, R.W. 1992. Magnetostratigraphy of Plio-Pleos-tocene lake sediments in the Confi dence Hills of southern Death Valley, California. Quarterly Journal of the San Ber-nardino County Museum Association, 39, 12-20.

Ramsay, J.G. & Huber, M.I. 1987. The techniques of

modern structural geology. Volume 2: Folds and Fractures. Academic Press, London, 700pp.

Richard, P.D. & Cobbold, P.R. 1990. Experimental insights into partitioning of fault motions in continental convergent wrench zones. Annales Tectonicae, 4, 35-44.

Richard, P.D., Naylor,. M.A. & Koopman, A. 1995. Experimental models of strike-slip tectonics. Petroleum Geoscience, 1, 71-81.

Rogers, A.M., Harmsen, S.C., Corbett, E.J., Priestly, K. & dePolo, D. 1991. The seismicity of Nevada and some adjacent parts of the Great Basin. In: Slemmons, D.B., Eng-dahl, E.R., Zoback, M.D., & Blackwell, D.D. (eds) Neo-tectonics of North America Geological Society of America, DNAG Decade Map, 1, 153 - 184.

Roussos, N. & Triantafyllos, L. 1991. Structure of the central North Aegean Trough: an active strike-slip defor-mation zone. Basin Research, 3, 39-48.

Serpa, L., De Voogd, B., Wright, L., Willemin, J., Oliver, J., Hauser, E. & Troxel, B. 1988. Structure of the central Death Valley pull-apart basin and vicinity from COCORP profi les in the southern Great Basin. Geological Soci-ety of America Bulletin, 100, 1437-1450.

Snow, J.K. & Wernicke, B. 1989. Mesozoic backfold in the Death Valley extended terrane; New constraints on offset of the northern Death Valley-Furnace Creek fault zone, California and Nevada. Geological Society of America Bulletin, 101, 1351-1362.

Stewart, J.H. 1983. Extensional tectonics in the Death Valley area, California – Transport of the Panamint Range block 80 km northwestward. Geology, 11, 153-157.

__________ 1967. Possible large right-lateral displace-ment along fault and shear zones in Death Valley-Las Vegas area, California and Nevada. Geological Society of America Bulletin, 78, 131-142.

Sylvester, A.G., and Smith R.R. 1976. Tectonic transpres-sion and basement-controlled deformation in San Andreas fault zone, Salton Trough, California. American Association of Petroleum Geologists Bulletin, 60, 2081-2102.

Troxel, B.W. 1970. Anatomy of a fault zone, southern Death Valley area, California. Geological Society of Amer-ica Abstracts with Programs , 2, 154.

___________ 1986, Pleistocene and Holocene deforma-tion on a segment of the southern Death Valley fault zone, California. In: Troxel, B.W. (ed.) Quaternary Tectonics of Southern Death Valley, Field Guide. Friends of the Pleis-tocene , 13-16.

Troxel, B.W., Sarna-Wojcicki, A.M. & Meyer, C.E. 1986. Ages, correlations, and sources of three ash beds in deformed Pleistocene beds, Confi dence Hills, Death Valley, California, In: Troxel, B.W. (ed.) Quaternary Tectonics of Southern Death Valley, Field Guide. Friends of the Pleistocene,.29-30.

Troxel, B.W., & Butler, P.R. 1986. Multiple Quaternary deformation in the central part of the Confi dence Hills: an example of folding along a strike-slip fault zone. In: Troxel, B.W. (ed.) Quaternary Tectonics of Southern Death Valley, Field Guide. Friends of the Pleistocene,.25-29.

Wilcox, R.E., Harding, T.P. & Seely, D.R. 1973. Basic wrench tectonics. American Association of Petroleum Geologists Bulletin, 57, 74-96.

Wright, L.A. 1976. Late Cenozoic fault patterns and stress fi elds in the Great Basin and westward displacement



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of the Sierra Nevada block. Geology , 4, 489-494.____________ 1987. Overview of the role of strike-slip

and normal faulting in the Neogene history of the region northeast of Death Valley, California-Nevada. In: Ellis, M.A. (ed.) Late Cenozoic Evolution of the Southern Great Basin. Nevada Bureau of Mines and Geology Open File Report, 89-1, 1-14.

Wright, L.A. & Troxel, B.W. 1984. Geology of the north-ern half of the Confi dence Hills 15 minute quadrangle Death Valley region, eastern California: The area of the Amargosa Chaos. California Division of Mines and Geology Map Sheet 34.

________________________ 1967. Limitations on right-lateral strike-slip displacements. Death Valley and Fur-nace Creek fault zones, California. Geological Society of America Bulletin, 3, 933-950

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