The Role of Low-Angle Normal Faulting in Active Tectonics of the Northern Owens Valley, California

The role of low-angle normal faulting in active tectonics of the northern Owens Valley, California

Fred M. Phillips and Lisa Majkowski DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCE, NEW MEXICO TECH, SOCORRO, NEW MEXICO 87801, USA

ABSTRACT

The Owens Valley of eastern California is an extensional graben. The mechanics of extension have traditionally been explained by means of high-angle normal faulting. However, this mechanism appears to be inconsistent with both the accepted tectonic structures of associated basins and with the expected kinematics of regional extension. We have therefore reexamined several lines of evidence that bear on the fault structures bounding the northern Owens Valley. Examination of fault-outcrop geometry indicates that valley-bounding fault planes dip between 26° and ~90°. Measurement of numerous fault planes that dip between 25° and 35° demonstrates that low-angle faulting must play an important role in the extensional process. We examined the alluvial fan/drainage basin area ratio of alluvial fans along the west slope of the White Mountains. These vary between ~1.00 and 0.05. The larger area ratios are associated with low-angle mountain-front faults, and the smaller ratios are associated with high-angle faults. The Bishop tuff, both in outcrop and in subcrop, shows obvious anticlinal rollovers as the tuff sheet approaches the bounding faults, which may indicate listric faulting geometry. Relocated earthquake hypocenter data deﬁ ne a west-dipping band of seismicity at 4–7 km depth beneath the Owens Valley. Fault-plane solutions for these events permit low-angle westward-directed slip. These observations indicate that the traditional high-angle normal faulting model is inadequate. More plausible alternative structures include low-angle planar normal bounding faults and faulting controlled by either east-dipping or west-dipping master detachment faults.

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INTRODUCTION the surfi cial neotectonic features there rooted Finally, seismic hazard analyses also require in low-angle structures similar to the Miocene assumptions regarding fault geometry. A bet- The Great Basin constitutes a continental- ones to the east, or is the geometry of faulting ter understanding of fault geometry may help scale topographic and hydrologic feature cre- instead “steep and fairly penetrating,” as postu- to improve hazard estimates, and such insights ated by extensional tectonics. Extension began lated by Wernicke et al. (1988, p. 1751)? may be transferable to other portions of the during the Miocene in the central Great Basin, A second reason to study the mechanics of Basin and Range Province. but at present, most of the tectonic activity is extension in this area is to investigate a discrep- concentrated on the western and eastern mar- ancy between contemporary geodetic estimates STUDY AREA gins of the region (Wernicke, 1992). The west- of displacement rate and rates inferred over geo- ern margin is particularly active. The mechanics logical time scales based on fault investigations. The Owens Valley defi nes the western mar- of extension in this region are of interest for sev- The geodetically based rates are quite high. gin of the Great Basin and the eastern margin eral reasons. One reason is to help understand Geodetic measurements indicate that at the of the Sierra Nevada block (Fig. 1). The valley the continuity, or lack thereof, between tectonic latitude of the northern Owens Valley, approxi- consists of a trough-like structure, 140 km in mechanisms in the highly extended terrain of the mately two-thirds of the total extension between length and 25–10 km in width, striking 30° to Las Vegas–Death Valley area (in considerable the North American craton and the Pacifi c plate 20° west of north. Along most of its length, the part accomplished during the Miocene) and the is concentrated in a zone of only 70 km between western side of the valley is formed by the dra- more limited Pliocene–Quaternary extension the Sierra Nevada crest and Fish Lake Valley matic eastern escarpment of the Sierra Nevada, directly east of the Sierra Nevada block. Due to (Dixon et al., 2000). However, in a comparison with total relief varying from 1700 to 2700 m. a long interval of extension and erosion, deep- of geodetic and geological displacement rates The eastern side is formed by the White and seated low-angle structures are well exposed in, using a kinematic block model, the modeled Inyo Mountains, which are more gentle than for example, the Las Vegas area, Death Valley, rates based on geodetic data are two to fi ve the Sierra Nevada and which have crests gener- and Panamint Valley (Wernicke et al., 1988). times larger than the ones based on fi eld stud- ally rising 1500–1000 m above the valley fl oor, In contrast, along the highly active southwest- ies of faults (McCaffrey, 2005). The fault-based but which extend up to 2700 m above the val- ern boundary of the Great Basin, the struc- rates necessarily involve assumptions regard- ley fl oor in the northern portion of the White tures facilitating extension are generally buried ing fault geometry, and thus elucidation of fault Mountains. This generally north-south–ori- beneath subsiding and aggrading basins. Are structures may shed light on this discrepancy. ented graben-like structure is typical of valleys

Editor’s note: This article is part of a special issue devoted to the GSA Field Forum titled Structure and Neotectonic Evolution of Northern Owens Valley and the Volcanic Tableland, California, convened by David A. Ferrill, Southwest Research Institute, Alan P. Morris, Southwest Research Institute, and Nancye H. Dawers, Tulane University. More papers on this subject will follow in subsequent issues, and these will be collected online at http://lithosphere.gsapubs.org/ (click on Themed Issues).

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118°45′0″W 118°20′0″W

Montgomery Creek

Morris Creek White Marble Creek Mountains

Study Queen Dicks Canyon area

Falls Canyon

Rock Creek Middle Canyon

Birch Creek

Hammil Valley Lone Tree Creek

Milner Creek

Sabies Canyon

Straight Canyon Chalfant Valley Piute Creek 37°30′0″N Wheeler Crest 37°30′0″N Coldwater Canyon RVF FSF Volcanic Tableland Gunter Creek

Pine Creek Silver Canyon Mount Tom Round Legend Valley Faults Mountain down Bishop WMFZ Oblique Basin Mtn Strike slip Poleta Canyon Valley down Undifferentiated

Drainage basins

Alluvial fans Basin Depth (km) 0–0.05

0.05–0.2

0.2–0.5 Mount Humphreys 0.5–1 N Black Canyon Coyote Warp 1–2

2–3 3–4 OVRO 4–5

5–6

6–7 Big Pine 7–21.28 OVFZ

37°5′0″N 37°5′0″N

118°45′0″W 118°20′0″W Figure 1. Tectonic geology of the northern Owens Valley. Faults are from Bryant (2005) and from mapping by the authors. RVF—Round Valley fault, WMFZ—White Mountain fault zone, FSF—Fish Slough fault, OVFZ—Owens Valley fault zone. OVRO—Owens Valley Radio Observatory. Color contours are depth to basement from gravity-data inversion by Saltus and Jachens (1995). Yellow arrows represent the direction of fault-plane dips obtained from three-point solutions on scarps or other fault-related geomorphic features (see Table 1); the numbers in the arrows are the dip value in degrees. Additional numbers in italics are dips obtained from inclinometer measurements on fault-plane exposures. For faults with steep dip solutions, double- headed arrows are shown. Inasmuch as the dip orientation estimates are not robust, the preferred dip direction is indicated by a black triangle on the end of one of the arrows. Arrows along the White Mountains fault zone have been shifted east of the point of measurement to avoid obscuring other data. The red circles represent the fan/drainage basin area ratios of alluvial fans along the western side of the White Mountains (see Table 2 for data). Selected fans and associated drainage basins are highlighted.

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in the Great Basin to the east. The mechanics continental boundary to the west changed to a faulted continuation of this valley fl oor, indicat- of tectonics in the northern Owens Valley since subduction zone. Throughout the Mesozoic, arc ing that Miocene faulting may have been toward 760 ka can be traced by evaluation of the defor- volcanism dominated, and coalescence of plu- the eastern side of the present Owens Valley. At mation of the Bishop tuff, created by the erup- tons under that arc created the Sierra Nevada that time, extension was directed approximately tion of Long Valley caldera. Batholith. Today, the Inyo and White Moun- east-west (Wernicke and Snow, 1998; McQuar- The Owens Valley is clearly a tectonically tains that bound the Owens Valley on the east rie and Wernicke, 2005), and thus normal fault- produced feature, and its structural characteris- are dominated by Paleozoic sedimentary rock, ing was initiated along an approximately north- tics have largely been a matter of consensus for while the Sierra Nevada range to the west has south strike. the past 140 yr. It is bounded on the west side by mostly been eroded down to the granitic rocks Middle to late Neogene cooling ages in the the Sierra Nevada frontal fault and on the east of the batholith (Bateman and Wahrhaftig, Owens Valley region exhibit a distinctly bimodal by the White Mountains fault zone and the Inyo 1966; Saleeby, 1999). distribution, with one mode close to 12 Ma and Mountains fault zone (Fig. 1). These are normal Subsequent to the Late Cretaceous, the the other close to 3 Ma (Surpless et al., 2002; faults, with the exception of the northern por- Sierra Nevada entered a passive phase (Bate- Stockli et al., 2003; Maheo et al., 2004). This is tion of the White Mountains fault zone, which man and Wahrhaftig, 1966; Wakabayashi and paralleled by a similar bimodal distribution of exhibits oblique slip. The Owens Valley fault Sawyer, 2001). Thermochronologic data over basaltic volcanism, with groupings between 12 zone runs down the center of most of the valley the Cenozoic (Clark et al., 2005) and modern and 9 Ma and 4 and 3 Ma (Moore and Dodge, and is predominantly a right-lateral strike-slip cosmogenic nuclide data (Riebe et al., 2001) 1980; Manley et al., 2000; Phillips et al., 2011). fault. The earliest investigators (Gilbert, 1883, both indicate that the uplands of the southern Phillips (2008) and Jayko (2009) have reviewed 1884; Le Conte, 1901; Lee, 1906; Knopf, 1918) Sierra have eroded downward at an average evidence indicating relative tectonic quiescence considered the Owens Valley to be a graben, by rate of ~40 m/m.y. This material was removed between these episodes, and this is supported which they meant a crustal block that has been from the highlands of the range and deposited by similar evidence from portions of the Sierra lowered (in a relative sense) along high-angle in the San Joaquin Valley, on the toe of the Nevada to the north of the Owens Valley (Gil- normal faults that bound mountain ranges on Sierra Nevada block. This unloading of the bert et al., 1968; Henry and Perkins, 2001; Surp- either side (i.e., horsts). Although the realiza- eastern part of the block and loading onto the less et al., 2002). tion that the Owens Valley fault zone is a strike- western portion produced an isostatic response The 4 to 3 Ma eruptive pulse has been linked slip fault has somewhat complicated this model of very gradual westward tilting of the block to delamination of the dense eclogitic root of (Beanland and Clark, 1994), high-angle normal (Small and Anderson, 1995). The net result the Sierra Nevada and its subsequent convec- block faulting has continued to be accepted as was presumably a slow decrease in the average tion downward into the mantle (Wernicke et al., the mechanism for the formation of the Owens elevation of the range. 1996; Manley et al., 2000; Farmer et al., 2002; Valley as a topographic feature (Matthes, 1937; The middle Neogene paleotopography of Zandt et al., 2004). The delamination event Pakiser et al., 1964; Bateman, 1965; Bateman the area now forming the Owens Valley has not has, in turn, been implicated as the initiator of and Wahrhaftig, 1966; Hollett et al., 1991; been well established. Extensive basalt fl ows a major increase in the rate of extension at that Berry, 1997; Le et al., 2007). that have been dated to ca. 12 Ma rest on bed- time in the area immediately east of the Sierra The faulting of the Owens Valley is one com- rock surfaces along the crest of the White Moun- Nevada (Farmer et al., 2002; Jones et al., 2004). ponent of the regional-scale extension of the tains and the Inyo Mountains (Krauskopf, 1971; A wide range of evidence indicates that the rate western Great Basin. This extension was initi- Jayko, 2004, 2009), close to the crest of the of tilting of the Sierra Nevada increased mark- ated during the middle to late Miocene and has Sierra Nevada west of Bishop, and in the foot- edly at about the same time (Le Conte, 1886; continued episodically to the present (Jones et hills of the Owens Valley (Phillips et al., 2011). Lindgren, 1911; Huber, 1981; Unruh, 1991; al., 2004; Phillips, 2008; Jayko, 2009). The locus At ca. 12 Ma, the northern White Mountains Stock et al., 2005). of extension has progressively migrated from underwent a major cooling episode (Stockli There is good reason to think that the origin east to west. As a result, the extensional basins et al., 2003), and similar cooling ages are also of the modern Owens Valley dates to this late exhibit decreasing maturity westward, with found near Mount Whitney (Maheo et al., 2004). Pliocene interval. Wernicke and Snow (1998), the fl oor of Death Valley now below sea level, Phillips (2008) evaluated this information in the Jones et al. (2004), and Phillips (2008) have that of Panamint Valley at ~500 m, and Owens context of additional paleoenvironmental data reviewed evidence that points to initiation at Valley at ~1300 m. Death Valley and Panamint and concluded that, prior to 12 Ma, the area that time of the current regime of relatively Valley are generally accepted to have extended between the crests of the Sierra Nevada and rapid extension. This regional evidence can be along a combination of strike-slip and low-angle White/Inyo Mountains was probably a broad supported by local data. The geomorphology of detachment faults (Burchfi el et al., 1987; Hamil- plateau-like summit region of subdued topog- the Sierra Nevada front along Owens Valley is ton, 1988; Burchfi el et al., 1995). Due to the rel- raphy between 2000 and 3000 m elevation. An consistent with formation of the range front in atively advanced stage of extension, low-angle initial westward step of Basin and Range exten- a single tectonic pulse, which is ongoing. Aside normal faults are clearly exposed in outcrop near sion between 12 and 11 Ma (Henry and Perkins, from the high ridge crests mentioned already as the bottoms of these basins. 2001; Surpless et al., 2002) probably produced a possible remnant of Miocene tectonism, the a relatively narrow and shallow proto–Owens front of the range along the northern Owens Val- NEOGENE AND QUATERNARY Valley. Long, relatively fl at ridge crests extend- ley generally presents a relatively smooth and TECTONISM IN THE OWENS VALLEY AREA ing eastward from the southern Sierra Nevada uniform profi le. Phillips et al. (2011) obtained crest, such as Lone Pine Peak, may be remnants a 3.4 Ma 40Ar-39Ar date from basaltic tephra During the Paleozoic, the area now compris- of the fl oor of this valley (Matthes, 1937), sug- contained in glacial fi ll of a doubly beheaded ing the Owens Valley was in a marginal marine gesting a valley depth of 750–500 m (Phillips paleovalley near the summit of Mt. Humphreys setting that produced thick sequences of sedi- et al., 2011). Matthes (1937) speculated that (west of Bishop; Fig. 1). The glacial valley mentary rock. Starting in the late Paleozoic, the the Alabama Hills were a subsequently down- was thus present during the 4 to 3 Ma volcanic

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pulse. The paleovalley is evidently a relic of the In summary, prior to the middle Miocene, Nevada block and the entire southwestern por- gentle Miocene–early Pliocene upland land- the area now occupied by the Owens Valley tion of the Great Basin, and the azimuth of 312° scape. The paleovalley is currently beheaded to was probably a broad, low-relief summit upland from Dixon et al. (2000) is probably preferable. the east by erosion encroaching westward due stretching between the current crests of the If this displacement rate (4.1 ± 0.8 mm yr–1 to down drop of the Owens Valley along the Sierra Nevada and White/Inyo Mountains, and toward 312°) is decomposed into components Sierra Nevada frontal fault. The preservation of perhaps further east. An initial pulse of east- parallel to the Owens Valley fault zone and the Pliocene tephra and till in the paleovalley west extension in the interval 12–9 Ma pro- perpendicular to it, the parallel component is implies that, at the time of the eruption, a low- duced approximately north-south normal faults, 3.7 ± 0.8 mm yr–1 toward 340° and the perpen- relief upland landscape containing the valley probably the ancestor of the current White dicular component is 1.5 ± 0.3 mm yr–1 toward extended to the west and had not yet been dis- Mountains fault zone. The throw across these 250°. For comparison, the displacement rate rupted by normal displacement along the Sierra faults was probably in most places limited to parallel to the Owens Valley fault zone was Nevada frontal fault. Initiation of signifi cant less than 1 or 2 km. After an interval of relative estimated, using an elastic half-space model movement on the Sierra Nevada frontal fault quiescence, extension was renewed starting at incorporating terrestrial geodetic data, to be 7 in the northern Owens Valley thus presumably ca. 3.5 Ma, but with a northwest-southeast ori- ± 1 mm yr–1 (Savage and Lisowski, 1995). Also postdates 3.4 Ma. entation. This Pliocene episode of extension has using an elastic half-space model, in this case The sedimentation history of the Owens been linked to delamination of the eclogitic root incorporating GPS data, Dixon et al. (2000) Valley also supports initiation of major down- of the Sierra Nevada. This extension has contin- obtained a fault-parallel displacement rate of faulting at this time. The Bishop tuff has been ued to the present day, resulting in the formation 6 ± 2 mm yr–1. Using a viscoelastic coupling mapped in the subsurface within the fi ll of the of the modern Owens Valley. The change in ori- model specifi cally parameterized to account northern Owens Valley (Bateman, 1965; Hol- entation required preexisting Miocene faults to for the 1872 Owens Valley earthquake, Dixon lett et al., 1991). For example, beneath the town accommodate transtensive stresses. et al. (2003) obtained a displacement rate of of Bishop, the tuff is encountered at a depth of 2.1 ± 0.7 mm yr–1. Our empirical estimate lies 200 m. The total thickness of fi ll at this location Modern Strain Regime between those from the elastic half-space and is ~950 m, based on gravity modeling (Saltus viscoelastic coupling models. and Jachens, 1995). The age of the Bishop tuff is Numerous geodetic campaigns utilizing very well established at 760 ka (Sarna-Wojcicki global positioning system (GPS) technology Strain Regime Based on Geological Data et al., 2000). These values imply a sedimenta- have quantifi ed the modern displacement rates tion rate of ~260 m/m.y. If the sedimentation in the Owens Valley region. Based on evaluation Numerous studies in the Owens Valley area rate has remained approximately constant, the of a large GPS data set, Dixon et al. (2000) esti- have employed paleoseismic and longer-term total thickness of ~950 m of fi ll would take mated that the Sierra Nevada block is moving geological methods to estimate fault displace- ~3.6 m.y. to accumulate. Similar calculations 13 ± 1.2 mm yr–1 toward 312°, relative to sta- ment rates over periods ranging up to several at other locations in the northern Owens Valley ble North America. This can be compared to a million years. For the problem of determining yield estimated basin-fi ll ages ranging from 3.5 high-quality displacement measurement of 10.6 a displacement rate between the White/Inyo to 6 Ma. The assumption of constant deposition ± 0.1 mm yr–1 toward 310° at the Owens Valley Mountains block and the Sierra Nevada, the task rate is clearly only an approximation. It seems Radio-Astronomy Observatory (OVRO) using is considerably complicated by the partitioning likely that, if deposition rates were not constant, very long baseline array interferometry (data of strain among the Sierra Nevada frontal fault, they were probably larger in the early stages of compiled by Dixon et al., 2000). The OVRO the Owens Valley fault zone, and the White/ basin fi lling, rendering the estimated fi ll ages site, however, is in the fl oor of the Owens Valley, Inyo Mountains fault zone (Le et al., 2007). maxima. Although the analysis is only semi- and its displacement should differ from that of In addition to these major fault zones, there quantitative, the results appear consistent with the Sierra block by the amount of displacement exist numerous minor faults, and additional basin initiation at ca. 3.5 Ma. Pinter and Keller across the Sierra Nevada frontal fault. Dixon et unmapped faults may exist beneath the young (1995) obtained a similar result from an inde- al. (2000), indeed, found that a large propor- alluvium of the Owens Valley fl oor and beneath pendent approach involving basin tilting. tion of the strain across the Great Basin at this the bed of Owens Lake, which expanded far up One important difference between the Mio- latitude was absorbed in the region between the the Owens Valley in the late Quaternary and cene and Pliocene episodes of extension is that Sierra Nevada block and Death Valley. may have obliterated evidence for older fault at ca. 8 Ma, the direction of extension shifted In this paper, we analyze mechanisms of displacements (Bacon and Pezzopane, 2007). from approximately east-west to northwest- deformation across the northern Owens Valley, These prior studies are too numerous to evalu- southeast (Wernicke and Snow, 1998; McQuar- which refl ects displacement between the Sierra ate here, but Bacon and Pezzopane (2007) and rie and Wernicke, 2005). However, the normal Nevada and White/Inyo Mountains blocks. One Le et al. (2007) have recently provided critical component of extension was accommodated approach to estimating this displacement rate reviews. Le et al. (2007) employed displace- mostly along north-south faults, resulting in a is to compute the differences between stations ment estimates based on their own data and a transtensive strain regime. This was likely due in the two mountain blocks from the compila- large number of prior studies to construct dis- to reactivation of preexisting north-south faults tion of GPS-measured displacement rate vectors placement-vector diagrams across the southern formed during the Miocene, but it could also in Oldow (2003). Six GPS station velocities in Owens Valley. These resulted in estimates of have resulted from the formation of new north- the southern Sierra Nevada block were aver- displacement azimuths ranging from 306° to south faults, the orientations of which were con- aged and subtracted from the average of fi ve 331° and displacement rates ranging from 2.0 trolled either by the edge of the Sierra Nevada station velocities in the White/Inyo Mountains to 3.2 mm yr–1. The range of azimuth overlaps Batholith or by the Owens Valley fault zone, a block. The difference was 4.1 ± 0.8 mm yr–1 the GPS-based estimate of 312°. The range of feature that dates back to at least the Cretaceous directed toward 305° ± 6°. This direction is the rates also overlaps the GPS-based estimate of (Bartley et al., 2007). same, within uncertainties, as that of the Sierra 4.1 ± 0.8 mm yr–1, but the GPS value is clearly

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at the upper end of their range. However, their reasons that have not yet been explained. A third of points picked from fault scarps that disrupt displacement-rate estimate does not include hypothesis is that the traditional interpretation topographic features. As fault orientations movement on the southern Inyo Mountains fault that the Owens Valley is bounded by high-angle become steep, the fault-dip estimates become (Bacon et al., 2005) nor displacements on likely normal faults is not valid and that both it and the less robust, unless the fault runs across large unmapped or concealed faults in the valley fl oor basins to the east extended along moderate- to vertical topographic features, and so for these (Bacon and Pezzopane, 2007). Given a mod- low-angle normal faults. we report only limiting minimum dip estimates est amount of additional displacement on these The principal objective of this study is there- and fault strike rather than direction of dip. faults, the geological and GPS-based estimates fore to evaluate structural interpretations of The results are given in Table 1 and illus- of the total displacement vector between the faulting in the Owens Valley area in the context trated in Figure 1. The data show a wide range White/Inyo and Sierra Ranges would appear to of tectonic forcing over the past ~4 m.y., in order of dip estimates, even along individual faults. be in reasonable agreement. to determine the fault geometries that are most Along the Round Valley fault, for example, dips consistent with both the inferred tectonic history are lowest in the center of the fault, at ~31°. Objectives and the neotectonic evidence from the area. We They steepen toward the south and especially have employed direct observations on fault out- the north, where, at the north end of Wheeler Two factors, in particular, motivate a reex- crops, tectonic geomorphology, deformation of Crest, the fault becomes near-vertical. The amination of the mechanics of faulting in the the Bishop tuff surface, and three-dimensional Round Valley fault has previously been con- northern Owens Valley. The fi rst is structural. visualization of earthquake hypocenters to eval- sidered to be characterized by only normal dis- The Owens Valley is a relatively shallow gra- uate alternative interpretations. placement (Bryant, 1984, 2005; dePolo et al., ben. Only small portions of the basin exceed 1993), but we found evidence of at least 150 m 3 km of basin fi ll. A large proportion, espe- EVIDENCE FOR NORMAL FAULT of dextral displacement of shutter ridges and cially of the southern Owens Valley, contains GEOMETRY streams (Fig. 2) between Swall Meadow and <1.5 km of fi ll (Saltus and Jachens, 1995). the north end of Red Mountain west of Rock Given an average topographic difference of Direct Evidence from Fault Exposures Creek. The increase in fault dip correlates with 2.7 km between the crest of the Sierra Nevada increase in elevation of the fault outcrop while and the valley fl oor, this indicates typical As described in the previous sections, most moving from the center of the fault on the face total vertical displacement of ~4 km. This is geological researchers in the northern Owens of Mount Tom toward the northern and southern surprisingly shallow if the Owens Valley has Valley area have accepted high-angle normal ends of the fault. This suggests that the dip of been the locus of strong east-west extension faulting as the mechanism for relative displace- the exposed portion of the fault is related to the (~1.5 mm yr–1) over the past 3.5 m.y. If the ment of the Owens Valley fl oor and the bound- amount of displacement across that portion of bounding faults dip at 60°, 8–10 km of verti- ing mountain ranges. However, actual data on the fault (i.e., decreasing from the center of the cal displacement would be produced. This fault geometry are sparse. We have therefore fault toward its tips). This, in turn, suggests that discrepancy between observed and calculated attempted to fi nd localities where direct mea- the fault may have a listric geometry. displacement can, of course, be reconciled by surement of fault-plane orientation on bedrock Fault-exposure dips on the White Mountains calling on temporal variations in strain rate outcrops can be performed. Such sites, however, fault zone are more complex. At the south- or orientation, but it nevertheless raises the are scarce, so we have supplemented them with ern end of the study area (“southern section”), question of whether other structural confi gura- orientations determined from three-point solu- where fault displacement is apparently only tions might produce better agreement between tions on the horizontal and vertical coordinates normal (Kirby et al., 2006), the dip is steep observed and inferred vertical displacement. A second motivation is provided by comparison with the observed fault structure in the TABLE 1. LOCATIONS, DIPS, AND DIP AZIMUTHS OF FAULT PLANES BOUNDING THE OWENS VALLEY extensional basins to the east. Cumulative exten- Fault zone Locality Location (UTM) Dip Azimuth of Dip sion much larger than in the Owens Valley, com- (°) (°) Easting Northing bined with lower rates of basin sedimentation, has revealed that the opening of Death Valley Round Valley Basin Mountain 354624 4129654 35 94 fault zone Mount Tom 356591 4135034 26 87 and Panamint Valley was largely accomplished Little McGee 354598 4128030 52 104 along west-dipping low-angle faults (Burchfi el Tinemaha Creek 375546 4103266 50 47 et al., 1987; Wernicke et al., 1988; Hayman et al., Lower Wells Meadow 354883 4145110 46 101 2003). Similar mechanisms can be less directly Pine Creek 354002 4140772 32 25 inferred for Eureka Valley and Deep Spring Val- Swall Meadow 353195 4153659 87 31 ley (Peltzer and Rosen, 1995; Lee et al., 2001). White Mountain Rock Creek 377441 4179547 46 222 If Owens Valley is instead opening along high- fault zone Queen Dicks Canyon 376311 4181254 37 234 angle faults, the difference should be explicable Montgomery Canyon 374572 4190002 36 283 Middle Creek 379803 4174798 39 256 within a regional tectonic framework. One pos- Lone Tree Creek 382778 4165017 41 249 sibility is that Owens Valley and the basins to Straight Canyon 381380 4157481 >70 253 the east all initially opened on high-angle faults, Coldwater Canyon 383401 4147780 34 260 but that the older eastern ones have subsequently Silver Canyon 383487 4140506 >60 253 rotated to lower dips (Proffett, 1977; Wernicke Between Silver and 384407 4137838 >60 267 Poleta Canyon and Axen, 1988). A second is that the Owens Black Canyon 386822 4127581 >60 260 Valley and the basins to the east actually do have fundamentally different styles of tectonics, for Fish Slough fault Fish Slough 375841 4144894 58 285

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Figure 2. Google Earth images illustrating displacement on the northern portion of the Round Valley fault. (A) Shutter ridges to the west of Rock Creek. The blue lines highlight the crests of the shutter ridges; offset is ~150 m. The UTM coordinates of the fault in the center of the image are E347300, N4156440, and the image orientation is toward 220°. (B) Offset alluvial fan north of Swall Meadow. The blue lines highlight the crest of the fan; offset is 85 m. The UTM coordinates of the fault in the center of the image are E352900, N4153950, and the image orientation is toward 230°.

(>60°). Toward the north, the pattern of faulting fault. The measured dip angles decrease from been a subject of investigation for many years becomes more complex, with numerous fault 45° at Rock Creek to ~35° north of Queen Dicks (Bull, 1964; Denny, 1965; Hooke, 1968; Lecce, strands and overall dextral-oblique slip. Slip Canyon. The dips we obtained south of Rock 1990; Gordon and Heller, 1993; Ritter et al., partitioning between fault strands is not readily Creek are similar to those measured by Stockli 1995; Whipple and Trayler, 1996). After a com- apparent. In this section, all measured dips were et al. (2003), but those north of this point are sig- prehensive evaluation of previous studies and quite steep (>60°). Starting at about Gunter nifi cantly less. One possible explanation is that an analysis of factors affecting fan morphology, Creek (Fig. 1), a pattern of slip partitioning Stockli et al. (2003) measured kinematic indi- Whipple and Trayler (1996, p. 358) concluded begins to develop, with strike-slip displacement cators on small-displacement faults subsidiary “in any tectonic setting where subsidence is apparently focused on linear faults that run par- to the main range-bounding fault, whereas our nonuniform, relative fan sizes are largely con- allel to the mountain front on the upper portions hillslope-scale measurements were on the main trolled by the spatial distribution of subsidence of the alluvial fans and dip-slip concentrated on fault. Displacement may be partitioned, with rates and bear little direct relation to the physical discontinuous fault segments at the mountain oblique slip focused on steeper faults behind or characteristics of the source area.” Employing a front to the east. This pattern grows more pro- in front of the range front and dip slip on a shal- simplifi ed model for steady-state geomorphic nounced toward the north. In this portion of the lower range-front fault. conditions (e.g., Equation 16 of Whipple and fault, the strike-slip strands appear to be essen- The most important fi nding from the fault Traylor), the relation can be expressed as: tially vertical in orientation, while limited mea- outcrop study is that signifi cant portions of the surements on the mountain-front normal seg- valley-bounding faults show low-angle (<30°) ε ments appear to indicate westward dips of ~45°. dips or dips close to low angle. Low-angle fault- A = A , (1) f z d We refer to this section as the “central section.” ing clearly plays at least some role in the tecton- p Between Birch and Falls Canyons, the ics of the Owens Valley. ε White Mountain fault zone transitions to a third where Af is the area of the fan, is the average regime. The strike-slip strand to the west dies Alluvial-Fan Size volumetric erosion rate of the drainage basin 3 –2 –1 out, while the zone of mountain-front dip-slip (L L T ), zp is the rate of basin subsidence –1 faulting dissolves into several separate strands The surface area of alluvial fans is a func- (L T ), and Ad is the drainage basin area. Divid- that step eastward up the face of the range while tion of catchment drainage area, average ero- ing through by drainage basin area gives: maintaining a dip of ~45° (Stockli et al., 2003). sion rate, and the production of accommodation Farther northward (“northern section”), as Rock space within the depositing basin. The relation A ε Creek is approached, this zone of normal faults between the tectonic subsidence rate of a basin f = . (2) Az coalesces into a single, normal, mountain-front and the area of the fans within the basin has dp

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Assuming that erosion rates are similar in drain- to the point where a single fault strand along tain front. However, there are some signifi cant ages along a single range front, the fan area ratio the mountain front transitions southward into discrepancies. For example, the greatest basin is inversely proportional to the basin subsidence multiple strands stepping back into the range. In depth and steepest descent of the bedrock from rate. Lecce (1991) has proposed that there is the zone of distributed mountain-slope faulting the mountain front is between Milner and Piute signifi cant lithologic control on fan area in the (between Falls and Milner Canyons), the broad, Canyons, but the fan/basin ratios are only mod- White Mountains, but Whipple and Trayler shallow piedmont to the west of the mountain erate in this area. Conversely, the broad, shallow (1996) argued that this conclusion is an artifact front narrows to a small shelf that falls off west- bench between Piute and Gunter Canyons does of lithologic and tectonic boundaries coinciding. ward to a deep basin (3–4 km depth). not correspond to especially large fan/drain- We have in Table 2 compiled data on drain- South of Milner Creek (where fan/drainage ratios. The similarity of fan/drainage ratios age areas, fan areas, and fan/drainage area ratios age area ratios drop to ~0.2), the narrow shelf between these two sections may indicate that the for the entire west face of the White Mountains. pinches out and in the subsurface the bedrock deep basin to the west of Milner/Piute Canyons The geographical distribution of the fan/drain- drops directly from the range front into a deep is a product of a past tectonic regime and is no age ratios is shown in Figure 1. Across the (5–6 km depth) basin. In this section, the range- longer subsiding at an unusually large rate. study area, the fan/drainage area ratio varies by front faulting is partitioned into strike-slip dis- Another discrepancy arises from compari- a factor of 20, which is surprisingly large. The placement (often with minor antithetic vertical son of tectonic geomorphology of the mountain ratios can be divided into three spatial group- displacement) on the upper alluvial fans west of front with the fan/drainage ratios. The southern ings. From Morris Creek to Rock Creek, near the mountain front and normal faulting along the mountain front between Silver and Black Can- the northern end of the range, the ratios are close mountain front. Farther south (approximately yons does not appear highly active. The moun- to one. From Falls Canyon to Milner Creek, in at Piute Canyon), the deep basin to the west is tain front is sinuous and does not exhibit pro- the central section, the ratios vary from 0.5 to replaced by a wide, shallow shelf. However, this nounced facets. The fl oors of canyons draining 0.2, with most in the range 0.5–0.35. South of transition does not appear to be refl ected in the the range are somewhat “U” shaped. Fan heads, Milner Creek, the ratios decrease and average fan/drainage ratios. however, are not entrenched. This mountain 0.2 between Sabies and Silver Canyons, then Finally, south of approximately Gunter Can- front would probably fall in the range of tec- decrease further to values less than 0.1 for the yon, the broad shelf begins to slope downward tonic activity classes 2 (active) and 3 (slow) of three southernmost canyons. into the very deep (~5 km) basin of the main Bull (2007). In contrast, the northern section of As previously noted by Whipple and Tray- Owens Valley, and the fan/drainage area ratios the White Mountains is characterized by a linear ler (1996), these groupings are associated with drop to values as low as 0.06. In this section, the mountain front, planar, very steep interfl uves, structural features. They also correspond to the clear slip partitioning is replaced by complex steep V-shaped canyons, and no entrenchment. fault-style divisions we described previously. dextral-oblique motion on multiple fault strands. It would probably fall in tectonic activity class The area of large fan/drainage ratios is located Assuming that the fan/drainage area ratio is 1A or 1B (maximal activity). However, this where thin fi ll (<0.5 km) forms a veneer over principally controlled by basin subsidence rate maximally active mountain front has very high an extensive pediment (Fig. 1). The rather (Whipple and Trayler, 1996), the spatial distri- fan/drainage ratios, while the moderate-activity abrupt transition from ratios approximately bution of the ratios appears to at least loosely southern section has exceptionally low ratios. equal to one to those less than 0.5 corresponds correspond to basin depth west of the moun- At least part of this discrepancy may be explained by the fault-geometry data. The southern section of the White Mountains fault zone is characterized by very steep (>60° and TABLE 2. BASIN AREAS, FAN AREAS, AND FAN/DRAINAGE AREA RATIOS OF probably >70°) fault planes. The northern sec- SELECTED DRAINAGES IN THE WHITE MOUNTAINS tion has low dips (~35°). For a unit of horizon- Drainage Basin area Fan area Fan/drainage area ratio tal extension, the steep dip will produce two (km2) (km2) or more units of vertical displacement. For the Morris Creek 12.2 12.1 1.00 same extension, the shallow-dip northern faults Montgomery Creek 12.7 16.6 1.31 Marble Creek 14.7 17.1 1.16 will produce only ~0.5 units of vertical displace- Queen Dicks 8.6 6.2 0.72 ment. A highly active mountain front is thus not Rock Creek 9.3 8.9 0.96 necessarily incompatible with only a moderate Falls Canyon 6.9 2.4 0.35 subsidence rate. We note that the transition from Pellisier Creek 12.7 4.5 0.35 very large fan/drainage ratios (~1) to moderate Middle Canyon 6.6 1.2 0.18 ratios (<0.5) coincides closely with the transi- Birch Creek 21.8 4.2 0.19 Willow Creek 13.4 6.8 0.51 tion from ~35° to ~45° mountain-front faults Cottonwood Creek 12.3 4.7 0.38 between Rock Creek and Falls Canyon. Lone Tree Creek 16.3 4.7 0.29 This observation regarding fault-plane geom- Jeffrey Mine Canyon 7.6 3.4 0.44 etry may also help to explain an apparent incon- Millner Creek 34.1 16.5 0.48 sistency between low unroofi ng rates inferred Sabies Canyon 11.8 3.4 0.28 from thermochronology and the high activity Straight Canyon 10.4 3.7 0.36 Sacramento Canyon 23.6 3.1 0.13 of the northern segment of the White Mountain Piute Creek 22.2 4.3 0.19 front. The U/Th-He cooling ages of Stockli et Coldwater Canyon 31.4 6.4 0.20 al. (2003) imply denudation rates on the order Silver Canyon 55.1 4.9 0.09 of 0.3 mm/yr, or less, and yet, as described pre- Poleta/Redding Canyons 21.4 1.3 0.06 viously, evidence from tectonic geomorphology Black Canyon 30.5 2.3 0.07 indicates a highly active mountain front. If the

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mountain-front fault in this segment dips at a relatively low angle, it could maintain a high rate of displacement (high tectonic activity class) with only a moderate or low unroofi ng rate. A second important factor is clearly basin A sedimentation rate. For the northern section of the White Mountains fault zone, the only signifi cant source of sediment is the White B B′ Mountains themselves. In contrast, for the southern section, the Owens River transports ′ in large volumes of sediment from the Sierra C Nevada to the north and west. Sedimentation C has apparently nearly kept pace with basin subsidence, resulting in a relatively stable base level and thus only a moderately active mountain front. The canyons along this section are partially backfi lled with alluvial and colluvial deposits dating to 2.5–2.0 Ma (Bate- man, 1965; Lueddecke et al., 1998; Kirby et al., 2006). These deposits are found starting at Legend only ~100 m above the fl oor of the Owens Val- Cross-section A,B ley, indicating that over the past 2.5 m.y., the Cross-section C Bishop tuff contours valley fl oor has fl uctuated between minor sub- Faults A A′ sidence and aggradation relative to the moun- Bishop tuff (elevation in meters) 980–1025 tain block. The large sediment supply from the 1025–1070 1070–1115 Owens River may thus explain the additional 1115–1160 decrease in the fan/drainage ratio from ~0.2 1160–1205 1205–1250 north of Silver Canyon, where Owens River 1250–1295 sediment is not available, to 0.10–0.05 south 1295–1340 of Silver Canyon.

Deformation of the Bishop Tuff km

The Bishop tuff, erupted at 758.9 ± 1.8 ka (Sarna-Wojcicki et al., 2000), provides an excel- lent stratigraphic marker across the Owens Val- Figure 3. Topographic map with elevation contours of the base of the Bishop tuff superimposed. Tuff ley. As noted by previous investigators (Bateman, elevation data are from Bateman (1965). 1965; Pinter and Keller, 1995), the tuff forms a broad, asymmetric arch, highest toward the western side of the valley and downwarped toward at the time of the eruption of the Bishop tuff opposite Coldwater Canyon to 260 m oppo- the edges. On both the west (Round Valley) and (760 ka) was similar to the present, total down- site Black Canyon. Given the 760 ka age of east (Hammil Valley) side of the valley, the tuff warping has been at least 200 m and possibly as the tuff, this implies an increase in the burial sheet is not simply buried beneath alluvial fans much as 300 m in some locations. The Bishop rate from 0.16 mm yr–1 at Coldwater Canyon prograding from the adjacent mountains, but tuff is found in outcrop on the footwall side of to 0.33 mm yr–1 at Black Canyon. The fan/ rather it curves smoothly downward in the direc- the fault (Bateman, 1965; Kirby et al., 2006). At drainage area ratio at Coldwater Canyon is tion of the facing mountains. When well logs are the mouth of Silver Canyon, Kirby et al. (2006) 0.20. Given the burial rate and the fan/drain- used to contour the base of the tuff beneath the documented 400–430 m of vertical separation age ratio at Coldwater Creek and the burial rate valley (Fig. 3), the structure is even more appar- between such outcrops and the buried Bishop at Black Canyon, Equation 2 predicts that at ent. On the east side of the valley (the only one tuff at the mountain front. At the southern end Black Canyon, the fan/drainage ratio should where the well log data were contoured), the tuff of the area, where the subcrop of the Bishop tuff be 0.10; the actual value is 0.07, corroborating has been warped smoothly downward to the east has been mapped (Fig. 4B), the marker shows the use of fan/drainage area ratio as a proxy for with a vertical displacement of 100–150 m over ~260 m of eastward tilting across the valley that subsidence rate. The northward decrease in the ~3 km horizontal distance (Figs. 4A and 4B). is not refl ected in the northern profi le. This tilt- subsidence rate may be due to a portion of the Wells were lacking in immediate proximity to ing demonstrates that the Coyote Warp and the normal component of displacement stepping the fault zone, and vertical displacement may Owens Valley fl oor are being deformed uni- westward to the Fish Slough fault. be even more there. Across the same interval, formly, with consistent downwarping toward the Subsurface data on the structure of the Bishop the present topography slopes upward from the White Mountains fault zone. tuff are not available on the western side of the Owens River toward the fault at the base of the Along the eastern edge of the mapped valley adjacent to the Round Valley fault, so there White Mountains, with a total elevation differ- portion of the Bishop tuff subcrop, the depth we measured profi les up the moraines that lie ence of 100–150 m. Assuming the topography to the tuff increases smoothly from ~120 m on top of the Pine Creek alluvial fan (Fig. 4C).

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1500 Land surface 1300 A A′ 1100 Base of Bishop tuff Fish Slough fault 900 700

500 White Mountain fault zone 300 100 Elevation (m) -100 -300 A -500 0 2000 4000 6000 8000 10,000 12,000 Distance east (m) 1600 ′ 1500 B B White Mountain fault zone 1400 Land surface 1300 1200 1100 Base of Bishop tuff 1000

Elevation (m) 900 800 Bedrock 700 B 600 0 2000 4000 6000 8000 10,000 12,000 14,000 Distance east (m)

2200

C C′

2000 Right-lateral moraine crest

1800

Left-lateral moraine crest Elevation (m)

1600 Canyon bottom

C Round Valley fault 1400 0 1000 2000 3000 4000 5000

Distance northeast (m)

Figure 4. (A) Cross-section A-A′ west of Piute Creek, illustrating rollover of Bishop tuff as the White Mountains fault zone is approached. (B) Cross-section B-B′ west of Poleta Canyon showing eastward tilting of the Coyote Warp/Owens Valley fl oor block. Bedrock elevation estimate is from Saltus and Jachens (1995). (C) Cross-section C-C′ east of Pine Creek. Quadratic functions (dashed, in black) have been fi t to the eastern portions of the right-lateral moraine and the valley fl oor profi les. Right- and left- lateral labels on the moraines refer to their position relative to the direction of glacier fl ow and not to tectonic displacements.

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The canyon bottom and the two lateral moraines 4 km from the range-bounding faults toward of the White Mountains, but most of those not show similar quasi-linear slopes upward from which they plunge (for example, at Hammil associated with the Chalfant Valley fault form Round Valley except within 1.0–1.5 km of the Valley). This scale of deformation may sup- a band of seismicity dipping westward from a Round Valley fault. At this distance, they begin port an interpretation that these features result depth of ~1 km under Chalfant Valley to ~8 km to roll over. At the fault trace, the valley bottom from geometrically induced deformation over under the Sierra Crest (10° to 15° dip). This is ~45 m lower than the projected profi le, and the listric range-bounding faults. At a minimum, the band of seismicity is suggestive of activity on a moraine crests are ~100 m lower. deformation of the tuff is consistent with defor- low-angle fault. Such anticlinal rollovers have tradition- mation over listric faults. To further test this hypothesis, we exam- ally been considered to result from simple ined the dips of events in the latitude interval volumetric compensation in the hanging wall Seismic Evidence (37.40°N–37.50°N) that had fault-plane solu- during progressive fl exure over a curved (i.e., tions. Figure 6 illustrates the dips of events listric) fault surface (Hamblin, 1965; Yamada Due in part to concerns regarding vol- with reported azimuths between 200° and 300° and McClay, 2003; Twiss and Moores, 2007). canic activity in the Long Valley caldera, a (i.e., west-directed slip that would be consistent There appears to be little doubt that this sim- relatively dense network of seismographs has with motion on a west-dipping fault plane). A ple geometrical mechanism can produce pro- been installed in the study area since the early large proportion of these events have calculated nounced anticlinal rollover in the hanging wall. 1980s. The operation of this network has coin- dips <15°, and an even larger proportion have However, recent numerical and analog model- cided with a high level of seismic activity in the dips <30°. The combination of a plane of sub- ing has shown that similar hanging-wall defor- northern portion of the study area. Combined horizontal seismicity with a large number of mation can be produced by mechanical stresses with recent advances in hypocenter relocation low-angle fault-plane solutions on that plane during slip on planar faults (Grasemann et al., (Waldhauser and Schaff, 2008), this database suggests that the seismicity may be produced by 2005; Resor, 2008). has enabled high-resolution imaging of seismic a low-angle west-dipping detachment at depths At this point, the signifi cance of the pro- structures beneath the Volcanic Tableland. How- of 4–7 km beneath the Owens Valley. nounced rollovers observed in the Bishop tuff is ever, relatively low seismic activity north of the uncertain. One possible distinction between the Volcanic Tableland has not allowed similarly Discussion stress-induced rollovers on planar faults and the refi ned imaging in that area. geometrically forced ones on listric faults may Hypocenters related to the 1984–1986 epi- In this section, we present several types of be the scale of deformation. The stress-induced sode of unusual activity in the region form a evidence that bear on the geometry of the faults rollovers depend on mechanical properties of distinctive pattern. Figure 5 is a cross section bounding the northern Owens Valley. These the rock that are limited in terms of the magni- illustrating relocated hypocenters of events types of evidence include geometrical projec- tude and spatial extent of deformation that can between 1980 and 2008 in the latitude interval tions from fault-trace mapping, the spatial be induced. Geometrically caused deformation, 37.46°N–37.50°N (under the center of the Vol- distribution of alluvial fan/drainage basin area however, depends only on the scale of curvature canic Tableland). Hypocenters of events belong- ratios, distributions of earthquake hypocenters, of the underlying fault, which can be over dis- ing to three brief swarms in 1984 and 1986 are deformation of the Bishop tuff, and land-surface tances of many kilometers. The observed roll- highlighted. Many of the hypocenters are asso- profi les adjacent to faults. All of these types of over anticlines in the Bishop tuff extend up to ciated with the Chalfant Valley fault to the west evidence indicate that many, but not all, of the

8 CVCCVFVF

12 Depth (km)

20 118.9 118.8 118.7 118.6 118.5 118.4 118.3

Longitude (°W) Figure 5. Relocated hypocenter locations for seismic events between 1980 and 2008 in the latitude band 37.46°N to 37.50°N (gray dots), from Wald- hauser and Schaff (2008). Clusters of events on 26–28 November 1984 (red dots), 24 July 1986 (pink dots), and 28 August–5 September 1986 (dark red dots) are highlighted. Possible fault plane is indicated between the blue arrows. CVF—Chalfant Valley fault.

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Depth (km) All hypocenters 90–60 16 60–35 35–15 15–0 20 118.9 118.8 118.7 118.6 118.5 118.4 118.3 Longitude (°W) Figure 6. Fault-plane solution dips for seismic events between 37.4°N and 37.5°N for the time interval 1 January 1984–14 April 1987. Only solutions with possible azimuths between 200° and 300° (western quadrant) are shown. Events with large uncertainty in the solution (maximum half width of 90% conﬁ dence range of dip >25°) have been culled. Gray dots in the background are relocated hypocenters of all earthquakes between 1980 and 2008 from Waldhauser and Schaff (2008).

bounding faults exhibit moderate- to low-angle With regard to the mechanisms of fault The second interpretation is one in which dips. In general, the high-angle faults appear to rotation we cite herein, domino-style tilting the east-west extension across the valley has be characterized by lateral or oblique slip and is quite unlikely to have infl uenced the faults been accommodated on a shallow, east-dipping the low-angle ones by dip slip. we have described. The rationale is that we detachment that originates along the Sierra To what extent do these moderate- to low- have mapped faults that defi ne the bound- Nevada frontal fault (here represented by the angle fault planes accommodate the active tec- ary between the Owens Valley terrain and the Round Valley fault). The primary motivating tonics of the Owens Valley? The mechanical mountain blocks on either side. Domino-style factors for this interpretation are that it permits feasibility of low-angle normal faulting is still faulting can affect only subsidiary faults within ~5 km of east-west extension without requir- controversial (Collettini and Sibson, 2001). It is the hanging wall. On the other hand, given the ing excessive vertical displacement and that the well known that in extensional settings, faults that magnitude of tectonic denudation at the mar- east-dipping Sierra Nevada frontal fault forms were originally active as high-angle features can gins of the Owens Valley, we consider it likely the western boundary of extension along the be rotated to much shallower dips (Morton and that footwall rebound is to some extent rotating entire Owens Valley, with the exception of the Black, 1975). Two mechanisms for such rotation the bounding faults we have mapped toward Coyote Warp. Monastero et al. (2002) inter- have been commonly invoked: domino-style tilt- shallower dips. These faults are nevertheless preted refl ection seismology data from the ing (Proffett, 1977) and rolling-hinge footwall clearly still active. Indian Wells Valley, south of the Owens Valley, deformation (Wernicke and Axen, 1988). These to show a listric, east-dipping low-angle fault mechanisms could transform formerly active ALTERNATIVE INTERPRETATIONS originating at the Sierra Nevada frontal fault. high-angle faults bounding the Owens Valley This interpretation is consistent with that struc- into inactive low-angle ones that are not charac- In Figure 7, we illustrate three alternative ture. Along the east side of the southern Owens teristic of the current tectonic regime. interpretations for the tectonic structures respon- Valley, the range front of the Inyo Mountains is We argue that contemporary extension of the sible for the formation of the Owens Valley. The relatively inactive, which argues against tectonic Owens Valley is indeed being accommodated fi rst is traditional high-angle normal faulting. structures originating from that side of the val- on the low- to moderate-angle structures we This interpretation is supported by the measure- ley as the primary mechanism for extension. have described herein. The fundamental basis ment of steep dips along some of the margins The third interpretation accommodates for our argument is that the evidence we cite in of the valley (particularly, the northern part of extension on a west-dipping detachment. Like support of low- to moderate-angle fault geom- Wheeler Crest and the White Mountains fault the second interpretation, it allows signifi cant etries is almost entirely derived from active tec- zone south of Straight Canyon). However, it is extension without undue vertical displacement. tonic features. Most importantly, the fault-trace not supported by the measurement of low fault It is consistent with the inferences, based on seis- maps from which the fault-plane angles are dips (>35°) in outcrop over a considerable por- mic data, that indicate a west-dipping plane of calculated are based on visible fault scarps and tion of the margins of the valley. As described seismic activity beneath the valley. Finally, it is other geomorphic indications of active faulting. previously, this interpretation also overestimates similar in orientation to the well-evidenced west- Similarly, the alluvial fans used in our analysis the total vertical displacement of the valley fl oor ward-dipping detachments along which Death are all active fans. The hypocenter locations if the modern strain regime is representative of and Panamint Valleys opened. As previously were measured within the past 40 yr. the long-term average. suggested by Wesnousky and Jones (1994), the

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Figure 7. Alternative interpretations of the tectonic regime of the northern Owens Valley. The structures are superimposed on a topographic slice from 37.42°N to B 37.46°N and 118.3°W to 118.9°W (E-W distance 58 km), viewed from the southeast (i.e., Sierra Nevada on the left). There is no vertical exaggeration. Dots indicate the positions of all hypocenters from 1980 to 2008 that have been relocated by Waldhauser and Schaff (2008). Hypocenters on the leading edge (37.42°N–37.43°N) are highlighted in red to assist in three- dimensional perspective. Mapped faults are shown in yellow. The tan-colored surface below the land 10 km surface is the bedrock topography, from gravity inversion by Saltus and Jachens (1995). Fault conﬁ gu- rations are intended to illustrate generalized conceptual models 10 km and should not be taken as spe- ciﬁ c interpretations. (A) Simple, high-angle faulting. (B) Interpreta- tion with shallow structures con- C trolled by a west-dipping detachment fault. (C) Interpretation with shallow structures controlled by an east-dipping detachment fault.

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low-angle faults in Death, Panamint, and Owens these events include many that are consistent the Round Valley fault from apparently a simple Valley may all sole into a common detachment. with westward dips <15°. This distribution of low-angle normal fault (near Pine Creek) to a Both of the low-angle faulting interpreta- hypocenters can be interpreted as events along high-angle dextral-oblique fault at the north end tions address the vertical displacement discrep- a westward-dipping detachment fault. of Wheeler Crest is particularly interesting. It ancy associated with the high-angle model. Hor- We note that an interpretation of active tec- echoes similar variations along the strike of the izontal motion at 4.1 mm yr–1 for 3.4 m.y. would tonics that accommodates east-west extension Death Valley and Panamint Valley fault systems produce 14 km of horizontal displacement. If mostly along low-angle faults helps to resolve (Cichanski, 2000; Walker et al., 2005) and may the motion were on a detachment dipping 15° the discrepancy between geodetically based and imply similar underlying tectonics. In any case, at depth, ~4 km of vertical displacement would geologically based estimates of displacement our observations indicate that low-angle normal be produced, which is in agreement with the rates (McCaffrey, 2005). Most geological fault faulting probably plays an important role along observed vertical separation. studies in the area have assumed fault-plane dips at least portions of the valley margins. Our data of ~60°. If the actual dips are <30°, horizontal cannot unequivocally address two corollary CONCLUSIONS displacement rates are increased by a factor of questions: (1) are the low-angle faults planar or ~4. This brings them into much better agree- listric, and (2) are the low-angle faults simply The east-west component of regional trans- ment with the geodetic rates. components of a complex but essentially local tension across the Owens Valley has tradition- Direct fi eld observations (triangulation of tectonic structure or are they manifestations of ally been thought to be accommodated on high- fault planes, alluvial fans with high fan/drainage an underlying detachment that is a controlling angle valley-bounding faults. The topography area ratios under highly active mountain fronts) structure in relation to regional transtension? of the valley and at least some observations argue strongly against the fi rst of the alterna- With regard to the fi rst question, we note that on exposed fault planes seem to support this tive interpretations presented in the discussion a somewhat modifi ed version of the fi rst inter- model. However, this fault geometry appears to section: simple high-angle normal faulting. In pretation (high-angle normal faulting) involving be inconsistent with that of nearby, tectonically contrast, these features are expected from low- moderate-angle planar faults might still be con- associated basins and is also inconsistent with angle faulting. The low-angle interpretations are sistent with our observations. Active slip along outcrop observations of fault dips as low as 26° further supported by indirect evidence such as planar moderate- to low-angle normal faults has bounding the valley. These factors encourage large-scale rollover features and interpretation been demonstrated elsewhere in the Great Basin reconsideration of the traditional model. of seismic events. Finally, the low-angle inter- (Abbott et al., 2001; Louie and Pullammanap- Calculation of fan/drainage area ratios for pretations allow extrapolation of current strain palli, 2007). If this interpretation is correct, it canyons on the west side of the White Mountains rates over the known period of transtension to could allow local tectonics to be explained with- shows a very large range of variation, by a factor explain the observed vertical displacement of out recourse to regional detachments. Subsur- of ~20. Paradoxically, the highest ratios are at the base of the Owens Valley, while the high- face fault geometry is diffi cult to demonstrate the north end of the range, where the mountain angle interpretation does not. We tend to favor without seismic-refl ection data, and this may be front appears to be highly active, and the low- the third interpretation (westward-dipping the only defi nitive way of addressing this ques- est ratios are at the south, where the range front detachment) over the second because it is sup- tion. Such data are very expensive to obtain; the appears much less active. However, these varia- ported by the regional pattern and by seismic only present indication of the possible results is tions in fan/drainage ratio are correlated with data, but a clear choice between interpretations from one of the few seismic surveys across the variations in the dip angle and style of normal/ is not possible without further evidence. Sierra front, in Indian Wells Valley, which did oblique faulting along the mountain front. Fault- With regard to the three hypotheses pre- identify a listric east-dipping detachment fault plane dip appears to decrease northward, and as sented under “objectives,” our data clearly do (Monastero et al., 2002). it does, the fan/drainage ratio increases. The not support the fi rst one: that Owens Valley and The relocated earthquake hypocenter data set association between these parameters supports the basins to the east all initially opened on high- presents the most convincing case for answer- the idea that low-angle faulting is important angle faults, but that the older eastern ones have ing the second question in the affi rmative. If the along at least parts of the valley margin. subsequently rotated to lower dips, inasmuch as planar alignment of hypocenters does image a The Bishop tuff is an important stratigraphic the active normal tectonics of the Owens Valley detachment, it is attractive in a regional tectonic marker in both the surface and subsurface envi- appear to be on moderate- to low-angle faults. context to speculate that it extends north and ronments. In localities near the faulted valley The same observations argue against the second south under the Owens Valley and that it keys margin, where the distal portions of alluvial hypothesis that high-angle faulting character- into a regional detachment at 5–10 km depth fans begin to slope upward toward the moun- ized only the Owens Valley. These observations, that integrates displacement between Death Val- tains (and probably also did so at the time the however, do support the third hypothesis, that ley and the Sierra Nevada. In summary, our data tuff was erupted), the exposed surface of the the mechanisms of extension in all the basins indicate that any tectonic interpretation explain- tuff now exhibits notable anticlinal rollovers. are similar but that the Owens Valley is at an ing extensional opening of the Owens Valley The cross-strike width of the rollovers is in the earlier stage of extension. should include some component of low-angle range of 1–4 km. 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