Strike-Slip Deformation in the Confidence Hills, Southern Death Valley Fault Zone, Eastern California, USA

Journal of the Geological Society, London, Vol. 153, 1996. pp. 375-387. 9 figs. Printed in Northern Ireland

Strike-slip deformation in the Confidence Hills, southern Death Valley fault zone, eastern California, USA

TIM P. DOOLEY & KEN R. McCLAY Fault Dynamics Project, Royal Holloway, University of London, Egham, Surrey W20OEX, UK

Abstract: The Confidence Hills form a well exposed, composite, positive flower structure developed in Pliocene to Recentlacustrine and alluvial fan sediments. The structurehas developed along the current trace of the southern Death Valley dextral strike-slip fault zone. California, USA. NW-SE- striking fault zones bound the Confidence Hills. In 3D, these fault segments are inferred to link at depth to a common basal fault system. The flower structure is formed by doubly plunging anticlines that roughly parallel the bounding fault segments. Fold development was aided by: (a) the presence of a basal salt deposit and numerous detachment horizons within the sedimentary pile and (b) buttressing and uplift along and against the bounding oblique-slip reverse faults. Structural and palaeomagnetic evidenceindicates that the anticlines developed with axial surfaces parallel and sub-parallel to the trace of both fault zones. Folding appearsto have initially developedadjacent toand abovethe 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 hundreds of metres and as young as 0.9 Ma to Recent. This young fault zone is highly segmented along the length of the floor of southern Death Valley: the high proportion of oblique-slip faulting consistent with thestrike-slip zone being immature. The Confidence Hills structure displays close geometric similarities with other naturalexamples of positive flower structures andto thefeatures found in scaled. analogue sandbox experiments of strike-slip faulting.

Keywords: Death Valley Fault, strike-slip faults. folds, 3D models.

The right-lateral southern DeathValley strike-slip fault zone flower structuresdeveloped along strike-slip fault systems (Fig. 1)forms the southwestern boundary of theDeath havepredominantly been documented from seismic- Valley extensionalsub-province, southeastern California reflectionprofiles (e.g. Harding 1974,1985; Roussos & (Wright & Troxel 1970; Wright 1976; Butler et al. 1988). Triantafyllos 1991; Bartholomew et al. 1993). Difficulties in Thisfault zone consists of a number of NW-SE-striking imaging high-angle reverse, oblique-slip and strike-slip faults segments that can be divided into two sub-zones: an eastern on seismic profiles have generally precludeddetailed sub-zoneand a westernsub-zone (Fig. lc) (Butler et al. analyses of thesefeatures, and only thebroad structural 1988). Recentfault activity(1Ma-Present) hasbeen architecturecan be confidently identified. Analogue recorded(Butler et al. 1988; thisstudy) on the highly modelling has successfully reproducedthe structural styles segmentedeastern sub-zone in Plio-Pleistocene fluvio- of flower structuresalong strike-slipfaults (Naylor et al. lacustrine sediments. The western subzone consists of older 1986; Richard & Cobbold 1990; Dooley 1994; McClay & fault segmentsexposed on the northeastern flanks of the Dooley 1994, 1995: Richard et al. 1995),although the Avawatz Mountains (Fig. lc: Butler et al. 1988). relativecoarseness of the modelling materials(generally The Confidence Hills are a low-lying range of hills (with silica sand) has precluded detailed studies of their internal elevations of up to 200 m above the floor of Death Valley) architecture.Detailed descriptions of surfaceexamples of along the northwestern extremity of the eastern subzone of positive flower structures have been relatively few, the best theSouthern Death Valley fault zone (Figs 1 and 2). The documented being thoseformed along the southern San hills consist of well-exposed, moderately incised, folded and Andreas fault zone in the vicinityof the Mecca Hills faultedPlio-Pleistocene fluvio-lacustrine strataand (Sylvester & Smith1976), theTransverse Ranges (e.g. Pleistocene-Recent alluvial fans (Troxel et al. 1986: Butler Wilcox et al. 1973), and the Betic shear zone of southern et al. 1988: Pluhar et al. 1992) that form a narrow ‘positive Spain (Montenat et al. 1990). All these examples have been flower structure’along the fault zone. Quaternary alluvial clearly documented as transpressive strike-slip fault zones. fansthat prograde into Death Valley fromthe Owlshead This paper presents the results of a detailedstructural mountains to the west are disrupted by the Confidence Hills analysis of a well exposed and geologically recent, composite (Figs 2 and 3). positive flower structure.Detailed 1:6000 scalestructural ‘Flower structures’(Harding 1974,1985) are commonly and sedimentological mapping of 62 km2 of the Confidence cited as one of the key characteristic structures of strike-slip Hillsin southernDeath Valley was undertaken using regimes. Theirgeometries and kinematics are, however, enlargedaerial photographs and Landsat TM imagery, poorly understood and there are few detailed descriptions of together with measurement of stratigraphicsections, field examples of strike-slip flower structures. A ‘positive structuralanalysis and cross-section construction. The flower structure’, as defined by Harding (1985), is a ‘shallow results of this analysis are compared with other examples of antiformdisplaced by the upward diverging strands of a flower structuresand with the results of scaledanalogue wrench fault that have mostly reverse separations’. Positive models of strike-slip fault systems. 37s

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CALIFORNIA

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

Previous work systems (e.g. Stewart 1967, 1983;Wright & Troxel 1967, 1970; Butler et al. 1988; Serpa et al. 1988; Snow & Wernicke Burchfiel & Stewart(1966) first proposeda crustal scale 1989). Cemen et al. (1985)presented evidence for up to 'pull-apart'model for the central Death Valley rhom- 50 km dextraldisplacement along thenorthern Death bochasm bounded by the Furnace Creek fault zone to the Valley/FurnaceCreek whereas Snow C% Wernicke (1989) north and by the southern Death Valley fault zone to the documented up to 70 km dextral offset on this fault zone. south (Fig. lb). Since this model wasfirst proposedthere Displacement estimates for the southern Death Valley fault has been considerable debate as to the applicability of this zonehave varied from 8 km (Wright & Troxel1967) to pull-apart interpretation, specifically regarding the amounts 80 km (Stewart 1967). The estimate of Stewart (1967) was of displacement on the twobounding dextral strike-slip basedon offset isopach trends of latePrecambrian and

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Fig. 2. Aerial photograph of the Confidence Hills and immediate surrounds.

Palaeozoicsedimentary rocks along the northern Death researchbegan, a detailed sedimentological study of the Valley fault zone and applying these values to the southern Confidence Hills was initiated by B. Murray, J. Kirschvink, Death Valley fault zone. Wright & Troxel (1967) based their K. Beratan and co-workers from the California Institute of estimate on offset formation contacts and isopach data from Technology. This study was initiated because the sediments thesouthern Death Valleyfault zone. Stewart (1983) of the Confidence Hills are possibly one of the best localities partially resolved this conflict by proposing that these fault to coordinatepalaeomagnetic reversals, tephrachronology, zones are related but not connected, and that activity began andargon-argon dating in orderto locate the magnetic on the northern Death Valley fault zone prior to motion on reversalsthrough time. The preliminaryfindings of this the southern Death fault zone. Butleret d.(1988) proposed study are included in the text, where appropriate. c. 35km of dextral offset betweenthe Mid-Miocene and The Confidence Hills presenta unique opportunity to c. 1 Ma on the western subzone. This value was based on study shallow level strike-slip tectonics in semi-consolidated matching offset alluvial fan gravels and is of the same order sediments. We know of no other work that documents the of magnitude as the northern Death Valley fault zone. The detailedthree-dimensional geometries and kinematics of a western subzone was active from approximately 14 Ma to 1 positiveflower structure.Furthermore, this detailedstudy Ma,becoming locked as the eastern termination of the may be able to resolve some of the controversy concerning Garlockfault, the Mule Springs fault, propagated and theamount of strike-slip displacement onportions of the overrode this dextralsystem assinistral a oblique-slip southern Death Valley fault zone. reverse fault (Fig. lc; Butler et al. 1988). Reconnaissancemapping of the Confidence Hills Geology of the Confidence Hills, southern Death (easternsub-zone) was carried out by B.W. Troxeland aspects of the geology of selectedparts of thearea were Valley documented by Troxel & Butler (198h), Butler et al. (1988) The Confidence Hills areapproximately 19 km longfrom and Wright & Troxel (1984). Prior to the research described Cinder Hill in the northwest to their southeastern tip, and in this paper a detailed map had not been made nor had a are up to 3.25 km wide (Figs 2 and 3). They consist of two detailedstructural analysisbeen carried out. Afterour overlapping and segmented dextral strike-slip fault systems

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FAULT ZONE A

Fig. 3. Geological map of the Confidence Hills based upon 1:6OOO scale mapping by the authors.

(fault zones A and B,Fig. 3), together with open to tight with solutioncollapse and sink hole topographicfeatures. folds(amplitudes from 100 m togreater than 600m and Units LS 2-4 consist of approximately 520m of fluvio- wavelengths from 200 m to greater than 2500 m) formed in lacustrinedeposits (Fig. 5). Abroadly coarsening-upwards Plio-Pleistocene to Recent lacustrine muds, silts, evaporites, character is observed and LS 4 is capped by coarse quartzo- alluvial sands and alluvial fanglomerates (Figs 2, 3 and 4). feldspathicsandstones andcobble conglomerates of The folds plunge at shallow to moderateangles (6" to 22") to fluvial/alluvial origin. Unit LS 3 (Fig. 5) is a distinctive both the NNW (301" to 335") and to the ESE (099" to 130") 20-50111 thickmarker unit of fine to coarsely crystalline, (Fig. 3). The structures are best exposed in the central and massive, banded to nodular gypsum with intercalated mud- southern Confidence Hills (Figs 2, 3 and sa) whereas to the stones and siltstones. Unit LS 4 is both laterally and verti- north they are mainly overlain by a thin veneer of gravels. cally variable in thickness. Ash bands within this LS 2 unit have given Upper Pliocene ages (2.0 Ma and 1.8 Ma, Fig. 5; Stratigaphy Butler et al. 1988; Pluhar et al. 1992),and a tephra layer near the topof this sequence (Fig. 5) yielded an age range of Three distinct lithostratigraphicsequences are exposed in 0.73-1.14 Ma (Butler et al. 1988). the Confidence Hills: lowera Plio-Pleistocene fluvio- The contactbetween LS andthe 4 overlying lacustrinesequence, middlea Pleistocene alluvial fan fanglomerateSequence 2 varies from a distinct angular sequence,and an upper sequence of Quaternary-Recent unconformity (Fig. 6a)that is folded in thesouthern alluvial fans (Fig. 5). In this paper we follow the suggestion Confidence Hills (Fig. 3), to a disconformity in the central of Beratan & Murray (1992) and name the lower sequence and northern Confidence Hills (Fig. 3; sections 1-3 Fig. 4). the ConfidenceHills formation (informal stratigraphic name). Sequence 2. Sequence 2 is subdivided into twopatchily Sequence 1: Confidence Hills formation. Sequence 1 consists exposed units, Qgl and Qg2 (Figs 3 and 5). Qgl consists of of up to 540 m of fluvio-lacustrinedeposits (Fig. S), sub- tan-coloured,pebble to boulderconglomerate with apre- divided intofour lithostratigraphic units (LS 1-4,Fig. 5). dominantbimodal clast composition of graniteand basic The lowest unit (LS 1) consists of at least 20m of heavily volcanics in a coarse quartzo-feldspathic sandy matrix. Typi- veined, coarsely crystalline pink and white halite. Although cal exposedthicknesses of Qgl rangefrom only a few poorlyexposed this unit is recognizable by itsassociation meters to a maximum measured thickness of 120 m (Fig. 5).

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I' ' Metres II NE

Metres 2 SW 200 1

NE 0rnyt gravels and alluvial

Qg2, younger tilted alluvial fanglomerate

Qgl deformed 0fanglornerates LS 4 lacustrine and alluvial D deoosits SW NE 4 Metres ~ - Folded Dlsconiormity 0LS 3 gypsumhorizons 200 Sh oreline Butte Lavas 100 Butte Shoreline 0 LS 2 lacustrine deposits -100 -200 LS 1 halite -300 a -400 400 m Horizontal = Vertical NE

Fig. 4. Cross-sections through the Con- fidcnce Hills. Location of sections 1 to 6 are shown on Fig. 3. Sections were constructed from 1: 6000 scale maps and thus possess more detail than can be shown on Fig. 3.

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Twomajor alluvial fan systems drainthe Owlshead and Black Mountains, and a third system drains the Confidence Hills. The Amargosa Wash forms a distinct drainage divide separating fans derived from the Black Hills to the northeast andfans derived from the Owlsheadmountains and Con- fidence Hills to the southwest (Fig. 2).

Volcanic rucks. The basaltic lavaflows of ShorelineButte (Fig. 3) have been dated at 1.5 Ma (Wright & Troxel 1984). Although thecontact between the lavas andSequence-l ...... strata is notexposed, this age postdates the LS 2 unit...... Ash bed 1.144.90 Ma. Angular basaltic clasts present in clastic wedges that inter- .-...... ::...... --, fingerwith LS 3 and LS 4 strata suggest thatthese lavas ...... occur between LS 2 and LS 3 in the stratigraphic sequence LS 4 - Fluvto-lacustrine deposits - gypsiferous mudslones. siltstones and (Figs 3 and 4).Horizontal shorelines cut into these lavas sandstones, have been attributed to a lake that once occupied this part wlth interbedded coarse fluvlal deposlts Maxlmum thickness 240m of southern Death Valley. The last high stand of this lake l:::\-::;/...... hasbeen estimated as having occurredat approximately 70 000 BP (Butler er al. 1988). Anandesitic cinder cone, LS 3 - Lacustrine mudstones, siltstones and gypsum Cinder Hill, is present to the north of Shoreline Butte (Fig. beds 20 - 50m thickness 3) and has yielded a K/Ar age of 0.69 Ma (Wright & Troxel 1984).

Ash bed 1.8 Ma. Structure The mainstructural features of the Confidence Hills are Huckleberry RidgeAsh - 2.0 Ma. t2 illustrated inFig. 3. Thedominant structures are two LS 2 - Lacustrine mudstones. SiltstoneS. and NW-SE-striking fault zones, A and B (Fig. 3 ). Fault zone sandstones. Gypsum beds common. Sparse A highlyis segmented andextends along the entire lluvial sandstones. southwesternboundary of the Confidence Hills(Fig. 3). Maximum thickness 230m~~ Fault zone B forms the northeastern boundary of the central and southern parts of the Confidence Hills(Fig. 3). A 2.3 LS 1 - Poorly exposed pink and white, massive hallte km wide zone of NW- and SE-plunging foldsoccurs between 111 I - 20 m exposed, base not observed fault zones A and B (Fig. 3). The principal structural styles MSS C of the Confidence Hills as they appear in cross-section, are shown in Fig 4. Scour ...... Pebble stnngers

___.(.... Coarseningupwards beds Channel scour flll Fault geumetries. The dominant fault structure of the Con- 0.09-0Piedmont surface -Planar beddlng AAAA fidence Hills is fault zone A which consists of a single main ..... bedsupwards Fining AAAAAA Gypsumbeds NW-striking fault strand (average 315") and associated seg- .S% cobblesIsolated or boulders Wavey lamination ments in the central and southern Confidence Hills (Fig. 3). ,,S,,, Cross-bedding Lensoid slits and sands This widens into a 600-1000m wide faultedzone to the Fig. 5. Summary stratigraphic section and ash bed ages of the north (Fig. 3, and cross-section 1, Fig. 4). Individual faults Pliocene-Recent strata of the Confidence Hills based upon detailed are generally poorly exposed and are usually characterised measurement of stratigraphic sections by the authors (' Butler et d. by erosional gullies (e.g. Fig. 6b) and rubble zones. Where 1988: ' Pluhar et 01. lYY2: Beratan & Murray. 1992). observed in outcropand where traced across the topog- raphy, the fault surfaces are vertical to sub-vertical in orien- On Shoreline Butte the basic volcanic clast content increases tation. Exposure of one fault surface revealed an internal,20 in these Qgl conglomerates. cm wide, sub-vertical gouge zone with a strike of 140" and Qg2 consists of poorly beddedpebble to boulder an externalbedding drag zone. Within this gouge zone conglomerates of graniticand basaltic clasts together with numerous left-stepping, vertical, Riedel shears with a strike of 160-165" are present, indicative of a dextral fault system. fine to coarse quartzo-feldspathic sandstones. On Shoreline Buttethese deposits contain abundant angular andesitic In thenorthern section of thecentral part of the map clasts probably sourced from the Shoreline Butte lavas (Figs and in thenorthern Confidence Hills, faultzone A is 2 and 3). Qg2 unconformably overlies and flanks Qgl, and, stronglysegmented intoen-echelon, left steppingfault in places,units of sequence 1 (Figs 2 and 3).Erosion and arrays which producebuttressing, chaotic folding and probable non-deposition make estimates of the thickness of disruption in the overlap zones (Fig. 3). The northernmost this unit difficult. strand of fault zone A bounds the lavas of Shoreline Butte and can be traced to Cinder Hill which is dextrally offset by Sequence 3. Sequence 3 includes a variety of Recent alluvial 180-200m (Fig. 3). Ephemeralstream washes- thatcui fan deposits that overlie and flank all of the older strata in acrossthe Confidence Hills (Figs 2 and3) are deflected the study area. Three groups of alluvial fans can be recog- dextrally by 200-300mas they cross the surfacetrace of nised,based upon location, morphology and clast content. fault zone A indicating only limited recent fault movement.

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Fig. 6. (a) Southern closure of the central anticline in Canyon 1 illustrating the unconformity between the Confidence Hills Formation and overlying Qgl deposits (left midground). View is to the southeast. Bedding dips around the 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 Qgl and younger, blanketing fanglomerate. Sequence-l strata are exposed to the northeast of this fault zone.

Threeprominent faults are found at angles of 19" to 24" The observed left-stepping fault arrays, clockwise stream (clockwise-oblique) to the main strand of fault zone A in the deflections and the offset of Cinder Hill all indicate dextral central Confidence Hills (Fig. 3). Vertical uplift of as much strike-slip displacement along fault zone A. Folding, tilting, as 200 m across fault zone A is recorded in the central part vertical stratigraphicseparations also indicate a significant of the map area (Fig. 3; cross-sections 2-4, Fig. 4). amount of vertical uplift andbuttressing across thefault Fault zone B is identified both on Landsat TM images zones. andaerial photographs as a distinct, c. 8 kmlong, linear faultstrand that bounds the southeastern part of the Fold Geometries. The majorfolds in the Confidence Hills ConfidenceHills (Fig. 3). Adjacent to this faultzone the are three anticlines that are sub-parallel to and bounded by beds of sequence 1 are strongly rotated becoming the offset fault zones (Fig. 3 ). The fold styles vary from sub-vertical and beds of sequence 2 are also tilted though open,rounded folds to kink-likegeometries with angular less strongly than those of sequence 1 (Fig. 7). hinges and planar limbs. The major folds plunge both to the

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Fig. 7. (a) Detailed structural map and stereoplots of the central and southern domains. (1) Poles to bedding plotted on southern hemisphere equal area stereographic projections for the three dip-domainsthat form the main anticline of the southern Confidence Hills. (2) Poles to bedding plots of data from Sequence-l and Qgl 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 Confidence Hills.

NW and to the SE (Fig. 3). Shortening estimates vary from west.Slickensides onbedding surfaces indicate a flexural- 10-50%,with maximum shorteningdocumented from the slip fold mechanism. central anticline. The geology of the southern Confidence Hills is shown in The northern Confidence Hills consist of a fairly simple, detail inFig. 7. Thedominant structure is aNW-SE broad,upright, anticlinal structurethat is breached by striking, 4 km long, doubly plunging anticline (Figs3 and 7a; strands of faultzone A (section 1, Fig. 4) whereasthe cross-sections 5 and 6, Fig. 4). The geometry of this anticline centralarea is formed by a6 km long,NW-SE-striking, changes from a broad anticlinewith subordinate kinking and doubly plunging anticline (Fig. 3; cross-sections 2-4, Figs 4 with shallowly to moderately dipping flanks, to a box-fold and 6a). The strike of the axial surface of the central fold is geometry with steeply dipping flanks (cross-sections5 and 6, less than 10" anticlockwise in an oblique orientation to fault Fig. 4). zone A (Figs 3 and 6a). The northern closure of this fold Right-steppingparasitic fold arrays are found on the plunges 15" to 335" and the southern closure plunges 10" to northeastern limb of the southern Confidence Hills anticline 130" (Fig. 3). From northwest to southeast this fold is first andare best developed in interbeddedgypsum/anhydrite upright,then overturned and becomes upright again andsiltstone strata (Fig. 3). The parasiticfolds typically southeast of Canyon-l (Fig. 4, cross-sections 2-4). have axial surfaces that are sub-parallel to fault zones A and Small scale folds with amplitudes of a few metres to a B, dip up to 60" to the southwest, and mean plunges of 24" few tens of metresare developed along fault zone A. In to 320" (Fig. 7b). The parasitic folds die-out both up- and Canyon 3 (Fig. 3) right-steppingen-echelon folds with down-section and along plunge. The right stepping nature of amplitudes of 2-10m and mean plunges of 20" to 099 are thesefolds is consistent with adextral system but their oriented at approximately 45" to the strike of fault zone A, orientation is problematic (see below). i.e. consistent with adextral model.Parasitic folds tothe In the region between cross-sections 3 and 5 (Fig. 3) the centralanticline are well developed in Canyon 1 (Fig. 3), unconformity between Sequence 1 and Sequence 2 strata is with overturnednortheastern limbs,interlimb angles of well exposed (Fig. 8a).InCanyon 1 (Fig. 8a)the 45-50", and inclined axial surfaces that dip up to 60" to the unconformitydecapitates the hinge of the anticline in

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@ Motion into page @ Motion out of page Fig. 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 flower structure from the Athos fault zone. North Aegean Trough, Greece (adapted from Roussos & Triantafyllos 1991).

Sequence1 strata (Figs 6a and8a; cross-sections 3 and 4, a series of anticlines sub-parallelto segmented strands of the Fig. 4). The unconformity is folded aroundthe central fault zone. The anticlines are doubly plunging, are breached Confidence Hills anticline and across the syncline between by faults, in places are overturned (cross-section 3, Fig. 4), thecentral and southern Confidence Hills anticlines and show significant shortening and uplift of strata along the (sections 3 and 4, Fig. 4: Fig. sa).Sequence 2 strata are, fault zone (Fig. 4). The fault surfaces are steep tovertical in however, less folded than the Sequence 1 strata beneath the orientation. The structureof the Confidence Hills is that of a unconformityindicating thatthe underlyingSequence 1 positive flower structure (i.e. uplifts bounded by strike-slip unitswere folded prior to deposition of Sequence 2. The faultswith reverse separation, cf. Harding 1974,1985) folds in thesetwo non-parallel sequences have similar formed by doublyplunging ‘push-up’ anticlines aligned orientation but spatially distinct axial surfaces and different sub-parallelto, and along, the segmented strike-slip fault fold plunges (section 4. Fig. 4 Fig. Sa). zone.No strike-slip deformation of this age,nor deformationassociated with theeastern subzone, is Discussion documentedimmediately tothe NE and SW of the Detailed mapping and analysis of the Confidence Hills along Confidence Hills. The next segment of the eastern subzone thenorthern part of the NW-SE-striking southernDeath is located some 5 km further to the southeast (Fig. 1; Butler Valley fault zone has shown that the dominant structures are et al. 1988).

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Fault and fold kinematics poorlyconsolidated nature of the Confidence Hills strata permittedfolding to predominateat the early stages of Regionaltectonic considerations (cf. Stewart1967, 1983; deformation, prior to fault breaching. Wright & Troxel 1967; Butler et al. 1988; Snow & Wernicke 1989) indicate that the southern Death Valley fault zone is dextral. Dextral offset of Cinder Hill and clockwise stream wash deflection in the ConfidenceHills (Figs 2and 3) Fault displacements support this interpretation. In this dextral system the three Displacementdeterminations on this dextral strike-slip obliquefaults that flank fault zone A in thecentral system are problematic due to a lack of piercing points (cf. ConfidenceHills (Fig. 3)can be interpretedto represent Ramsay & Huber 1987). 180-200 m right-lateral offset of remnant Riedel shears (e.g. Naylor etal. 1986;McClay & the 0.69 Ma cindercone at Cinder Hill is aminimum Dooley1995; Richard al.et 1995). This is further displacement for the northwestern end of fault zone A. This corroborated by the presence of right-stepping fold arrays in doesnot, however,preclude previous displacement along thenorthern and southern Confidence Hills. In addition, this section of thefault zone prior to 0.69Ma. Stream buttressing and uplift of LS 4 strata is observed between left deflection in thecentral Confidence Hills indicatesa stepping fault segments on Shoreline Butte, i.e. a restraining minimum of 200-300 m dextral offset for this central section stepover (e.g. Segall & Pollard 1980; Fig. 3; cross-section 1, of fault zone A. It is therefore most likely that this northern Fig. 4). end of the eastern sub-zone of the southern Death Valley Thefolded unconformitybetween Sequences 1 and 2 fault zonehas only undergone relatively limitedtotal (Fig. 7a)yields importantinformation regarding the displacement(hundreds of metres).Clast compositions in geometries and timing of folding in the southern and central Qgl and Qg2 strata are consistent with derivation from the sections of the Confidence Hills. In the mainanticline- Owlshead Mountains directly west of the Confidence Hills synclinepair adjacent to faultzone A, strata above and andthus there areno anomalous sediments that require below the unconformity arefolded with acommon axial large lateral offsets to explain them (cf. Butler et al. 1988). surfaceorientation but with differentplunges: 19" to 301" These low displacement estimates are also supported by the below the unconformityand 11" to 311" abovethe lack of tectonic rotation of the fold hinge lines and by the unconformity (Fig. 7a). To the northeast, however the two palaeomagnetic record (Pluhar et al. 1992). sequences are folded coaxially with a plunge of 6" to 310" (Fig. 7a),and possess adisconformable contact. These relationshipsindicate that folding adjacent to the present surfacetrace of fault A in thesouthern Confidence Hills Timing of deformation began after deposition of Sequence 1 and before deposition Stratigraphic and structural relationshipsin the south-central of Sequence 2 which subsequently eroded the crest of the Confidence Hills indicatethat deformation along this anticline (Figs 6a, and Sa ; cross-sections3 and 4,Fig. 4). northern part of the southern Death Valley fault zone began The fold structurecontinued to grow and increase in after deposition of the upper units of Sequnce 1 and prior to amplitudeand wavelength after deposition of Sequence 2. initial deposition of Qgl fanglomerates, i.e. post 1.14- The axial surfaces of the folds in thestrata above the 0.90 Ma (Figs 5 and sa). Evidence presented above indicates unconformity are spatially offset from the axial surfaces of that deformation was initially concentrated adjacent to and the folded strata below the unconformity surface as a result above fault zone A and then spread to the northeast. Offset of folding non-parallel surfaces (cf. Ramsay & Huber 1987). of Cinder Hill along fault zone A indicates dextral strike-slip These features also indicate that the folds in the Confidence motion post-0.69 Ma. Recent activity on the southern Death Hills grew at or near the surface with concommitant erosion Valley fault zone is indicated by the 1916 earthquake and deflection of alluvial fan drainages as slip accummulated (Rogers et al. 1991). onthe fault system.Structural relationships also indicate foldgrowth prior to faultbreaching (e.g. vertical uplift of the hinge region of the central anticline; cross-sections 2-4, Fig. 4). The problematic orientations of theright-stepping Structural model for the Confidence Hills foldarrays on the northeastern flank of thesouthern Analoguemodels of deformationabove linear basement Confidence Hills (Fig. sa) canbeexplained through faults carried out by Dooley (1994) illustrate in-line uplifts localisedtranspression resulting from dominanta com- bounded by oblique-reverse dextral strike-slip faults (Fig. 8a ponent of vertical motion on fault zone B and concomitant & b). Other studies such as those by Naylor etal. (1986), A. uplift along fault Richard & Cobbold (1990) andRichard et al. (1995) From the above it is clear that the major fold structures typicallyillustrate a push-up structure bounded by Riedel initially grewsub-parallel tothe fault zoneand were not shearsduring the early stages of strike-slip fault zone rotatedinto their current attitude by progressivesimple evolution and at a later stage these structures are bounded shear (e.g. as inWilcox er al. 1973).This is further by anastamosing fault zones cosisting of linked R-, P- and supported by the preliminarypalaeomagnetic findings of Y-shears (Fig. Sa, b).Natural examples of positiveflower Pluhar et al. (1992) which indicate no significant net tectonic structures that have been imaged in seismic sections show rotation of the Confidence Hills strata about vertical axes. similargeometries (Fig. 8c; Harding 1985; Roussos & Fault-parallelfolding has also been described from active Triantafyllos 1991; Bartholomew et al. 1993) of upward- strands of the southern Death Valley fault zone further to splaying faults boundingshallow-level anticlinal folds. By the southeast by Troxel (1970). It is likely that the presence analogy, the Confidence Hills structures may be interpreted of abasal halite unit, numerous intra-formational detach- to have similar geometric forms and in particular the fault menthorizons, near surface conditions and the relatively zones A and B may be inferred to link to a common basal

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C:INDER HILL

FA Uf .7

BASAL FAULT SYSTEM

Fig. 9. 3D synoptic model of the Confidence 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.

fault system at depth (Fig. 9). Uplift in the analogue models indicates that this detachment surface lies some 3 km below is achieved by dilation of the sandpack, increasing the pore sea-level (Fig. 9). spaces between individual grains (Dooley 1994). In the case of the Confidence Hills, this uplift was probably aided by halite mobility combined with flexural slip along numerous Conclusions intraformational detachment surfaces. The Confidence Hills is a 19 km long, high level, composite, In the Confidence Hills, there is significant uplift along positive flower structuredeveloped in Plioceneto Recent the entire range of hills (Fig. 4) and the anticlines have a lacustrineand alluvial fan sedimentsalong the eastern stylethat indicates probable fold detachment on the sub-zone of thesouthern Death Valleydextral strike-slip halite/evaporite LS 1 unit of Sequence 1. The stratigraphy in faultzone. Two NW-SE-striking fault zones, A and B, the Confidence Hills may be considered to be, mechanically, boundthe Confidence Hills. Thesoutheastern fault zone relatively weak consisting of poorly consolidated lacustrine (A) extends for the entire length of the hills and bounds the sediments and evaporites, together with poorly consolidated southwesternside of thethree anticlines that form the overlying alluvial fan sequences (Fig. 5) and as such flower structure. It is segmentedat the northwestern end favoured folding rather than faulting during the early stages with dextral strike-slip displacement indicated by arrays of of deformation.Harding (1985) statesthat positive flower minor faultsand en-echelon fold systems.Fault zone B structure formation is promoted by the presence of a thick, bounds the northwestern edge of the central and southern ductilesedimentary section, such as thatpresent in the Confidence Hills. In 3D the faults zones and individual fault Confidence Hills. Folding of theunconformity between segments are inferred to link to a common basal fault zone Sequences 1 and 2 indicatesthe the dextral strike-slip at depth. faultingappears to havepropagated outwards from, and In the central and southern Confidence Hills the positive northwestwardsalong, fault zone A. A synopticmodel of flower structure is made up of doublyplunging anticlines the Confidence Hills structure is shown in Fig. 9 where fault that appear to detach on the basal halite/evaporite unit (LS zones A and B areinterpreted to merge with acommon 1) of Sequence 1 strata. Folding appearsto have initially single basal fault zone at depth. A crudely derived depth to developedadjacent to and above fault zone A andthen detachmentestimate using Chamberlain’s(1910) Law propagated outwards and to the northwest. Structural and

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palaeomagneticevidence indicate that the anticlines that and style of deformationalong the southern Death Valley fault zone, formthe positiveflower structuredeveloped with axial California. Geological Society of America Bulletin. 100. 402-410. CEWEN,I.. WRICHT.L.A. DRAKE,R.E. AND JOHNSON.F.C. 1985. Cenozoic surfaces parallel and sub-parallel to the major faults. sedimentation and sequence of deformational events at the southeastern The structural style of the Confidence Hills displays close end of theFurnace Creek strike-slip fault zone. Death Valley region, geometric similarities to other natural examples of positive California. In: BIDDLE.K.T. & CHRISTIE-BLICK,N. (eds) Strike-Slip flower structuresand tothe features found in scaled Deformation, Basin Formation, and Sedimentation. Society of Economic Mineralogists and Paleontologists Special Publications, 37. p. 127-141. analogue sandbox experiments of strike-slip faulting. Highly CHAMBERLAIN.R.T. 1910. TheAppalachian folds of central Pennsylvania. complexdeformation patterns and flower structures can Journal of Geology. 18. 228-251. develop early in the history of a strike-slip fault system that DOOLEY.T.P. 1994. Geometries and kinematic.7of strike-slip fault hasonly accumulated relatively small displacements, as a .systems:-1nsight.s from phy.sica1 modelling and field strrdies. PhD thesis, London University. consequence of strike-slip andoblique displacements on a HARDINC,T.P. 1974. Petroleum traps associated with wrench faults. American segmentedfault system and between overstepping fault Association of Petroleum Geo1ogist.s Bulletin, 58. 1290-1304. arrays. - 1985. Seismic characteristicsand identification of negative flower Evidence presented in this study and that of Butler et al. structures. positive flower structures,and positive structural inversion. (1988) indicate that the latest displacements on this eastern American Association of Petroleum Geologists Bulletin, 69, 582-600. MCCLAY,K.R. & DOOLEY,T. 1994. 3-Dstrike-slip pull-apart basins - sub-zone of thesouthern Death Valley fault zoneare geometriesdetermined from scaled analogue models. American probably only in the orders of hundreds of metres and as Association of Petroleum Geologists. Abstracts with Programs. 210. young as 0.9 Ma to Recent. This fault zone is immature and - & - 1995. Analoguemodels of pull-apart basins. Geologv. 23. consists of anumber of non-linkedsegments (Fig. 1). 711-714. MONTENAT, C..MASSE. P.. COPPIER,G. & D'ESTEVOII.P. 1990. The sequence Oblique-slip along these offset, segmented, fault systems has of deformations in the Betic shear zone (SE. Spain). Annules Tectonicae. produced in-line uplifts suchas the Confidence Hills and 4, 96-103. thosedescribed by Troxel(1970), some 8 km tothe NAYLOR.MA., MANUL,G. & SIJPESTEIJN.C.H.K. 1986. Fault geometries in southeast.However, controversy still exists overthe total basementinduced wrench faultingunder different initial stressstates. Journal of Structural Geology. 8, 737-752. amount of displacementalong the extinct portions of the PLIIHAR,C.J.. HOLT,J.H., KIRSCHVINK. J.L., BERATAN.K.K. & ADAMS.R.W. southernDeath Valleyfault zone. A simplepull-apart 1992. Magnetostratigraphy of Plio-Pleostocene lake sediments in the modelfor Death Valley requires that the northern and Confidence Hills of southern Death Valley, California. Quarterly Journal southern Death Valley faults are coeval and possess similar of the San Bernardino County Museum Association. 39. 12-20. RAMSAY.J.G. & HUBER,M.I. 1987. The technrques of modern strrrctrrral lateraldisplacements. Troxel (pers. comm. 1991) questions geology. Volume 2: Folds and Fracrures. Academic Press, London. the 35 km + lateral dispacement described by Butler et al. RICHARD.P.D. & CORBOLI).P.R. 1990. Experimental insights into partitioning (1988) and advocates the low lateral displacement along the of faultmotions in continentalconvergent wrench zones. Annales western subzone as described by Wright & Troxel (1967). If Tectonicae, 4. 35-44. -, NAYLOR. MA.& KOOPMAN. A. 1995. Experimental models of strike-slip this is so, thena simple pull-apart model for the current tectonics. Petroleum Geoscience. 1. 71-81. Death Valleyrhombochasm is inadequateto explain the ROGERS,A.M., HARMSEN. S.C., CORRETT.E.J.. PRIEWLY.K. & DEPOLO.D. discrepancies in displacementalong the boundary fault 1991. The seismicity of Nevadaand some adjacent parts of theGreat zones. Basin. In: SLEMMONS.D.B., ENGDAHL,E.R.. ZOBACK, M.D. & BLACKWELL.D.D. (eds) Neotectonm of North America. Geological Society of America. DNAG Map. 1, 153-184. Thisresearch was supported by theFault Dynamics Project Ro~~ssos,N. & TRIANTAFYLLOS. L. 1991. Structure of thecentral North (sponsored by Arco British Limited, Brasoil, UK Ltd, BP Aegean Trough: an active strike-slip deformation zone. Basin Reseurch. Exploration, Conoco (UK) Limited, Mobil North Sea Limited, and 3, 39-48. Sun Oil Britain). T.D. and K.McC. also acknowledge funding from SECALL,P. & POLLARD. D.D.1980. Mechanics of discontinuous faults.Journal of Geophysical Research, 85, 4337-4350. Sun OilBritain. The authors expresstheir gratitude to B. Troxel SERPA.L., DE VOOC;~.B., WRICHT.L.. WILLEMIN,J., OLIVER. J.. HAUSER.E. and L. Wright for their hospitality and for many hours of fruitful & TROXEL.B. 1988. Structure of thecentral Death Valley pull-apart discussion. B. Murray is also thanked for discussions and basin and vicinity from COCORP profiles in the southern Great Basin. correspondence on the geology of the Confidence Hills. The US Geological Society of America Bulletm. 100. 1437-1450. National Park Service at Furnace Creek. especially T. Coonan, are SNOW.J.K. & WERNICKE,B. 1989. Mesozoic backfold in theDeath Valley thanked for their helpful advice. Landsat TM imagery was kindly extendedterrane; New constraints on offset of thenorthern Death Valley-Furnace Creekfault zone. California andNevada. Geological provided by the Jet Propulsion Laboratory, Pasadena,California. Society of America Bulletin. 101. 1351-1362. Theauthors thank D. Brown, A.G. Sylvester, A. Aydin, A. SrEwART. J.H. 1967. Possible large right-lateral displacement along fault and Sarna-Wojicki, I. Cemen, L.Wdghtand M. Naylor for critical but shearzones in Death Valley-Las Vegas area. California and Nevada. constructive reviews of earlier versions of this manuscript. Geological Society of America Bulletin. 78. 131-142. Correspondence to T. Dooley (e-mail: [email protected]). - 1983. Extensionaltectonics in theDeath Valley area. California- Transport of the Panamint Range block 80 km northwestward. Geology, 11, 153-157. References SYLVESTER.A.G. & SMITH R.R. 1976. Tectonic transpression and basement-controlleddeformation in San Andreas fault zone.Salton BARTHOLOMEW,I.D.. PETERS, J.M. & POWELL.C.M. 1993. Regional structural Trough.California. American Association of Petroleum Geologists evolution of the North Sea: oblique slip and the reactivation of basement Bullerin. 60, 2081-2102. lineaments. In: PARKER,J.R. (ed.) Petrolenm Geology of Northwest TROXEL,B.W. 1970. Anatomy of a fault zone, southern Death Valley area. Europe: Proceeding.s of the 4th Conference. Geological Society. London. California. Geological Society of America Abstracts with Programs. 2, 1109-1122. 154.

BERATAN,K.K. & MORRAY,1992.B. Stratigraphyand depositional ~ 1986. Pleistoceneand Holocene deformation on a segmentof the environments,Southern Confidence Hills. Death Valley, California. southernDeath Valley fault zone.California. In: TROXEL.B.W. (ed.) Quarterly Journal of the San Bernardino County Museum Association. 39, Quaternary Tectonics of Southern Death Valley. Field Guide. Friends of 7-11. thc Pleistocene, 13-16.

BURCHFIEL.B.C. & SIEWAK'I.J.H. "Pull-apart" origin of the central segment ~ & BUTLER.P.R. 1986. Multiple Quaternary deformation in the central of Death Valley. California. Geological Society of America Bulletin, 77, part of the Confidence Hills: an example of folding along astrike-slip 439-442. fault zone. In: TROXEL,B.W. (ed.) Quaternary Tectonics of Southern BUTLER. P.R., TROXEL,B.W. & VEROSIIB, K.L.1988. Late Cenozoic history Death Vallev, Field Guide. Friends of the Pleistocene. 25-29.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/153/3/375/4901080/gsjgs.153.3.0375.pdf by guest on 02 October 2021 STRIKE-SLIPDEFORMATION,CONFIDENCEHILLS 387

-, SAwA-WomrKI.A.M. & MEYEK.C.E. 1986. Ages.correlations. and Neogene history of the region northeast of Death Valley, California- sources of three ash beds in deformed Pleistocene beds. Confidence Hills, Nevada. In: ELLIS, M.A. (ed.) Lure Cenozoic Evolution of the Southern Death Valley. California. In: TKOXEL.B.W. (ed.) Qlraternury Tect<~nic.s(If Greut Basin. Nevada Bureau of Mines and Geology Open File Report, Southern Deuth Vulley. Field Guide. Friends of thePleistocene, 29- 89-1. 1-14.

30. ~ & TROXEL,B.W. 1967. Limitat~ons on right-lateralstrike-slip WILCOX.R.E.. HARDINC.T.P. & SEELY.D.R. 1973. Basic wrench tectonics. displacements. Death Valley and Furnace Creek fault zones. California. Anlericrrn Associotion of Petrolrrrnl Geologim Blrllerrn. 51. 74-96. Geological Society of Amrricu Bulletin. 3. 933-950 WKIGHT,L.A. 1976. Late Cenozoic fault patterns and stress fields in the Great __, 1984. Geology of the northern half of the Confidence Hills 15 Basin and westward displacement of the Sierra Nevada block. Geologv. minute quadrangle Death Vulleyregion. eastern California: The area of 4,489-494. the AmargosaChaos. California Division of Mines andGeology Map - 1987. Overview of therole of strike-slipand normal faulting in the Sheet, 34.

Received 6 March 1995; revised typescript accepted 27 October 1995. Scientific editing by Alex Maltman.

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