The timing, magnitude, style, and kinematics of deformation in the Eastern California shear zone and Walker Lane belt are important in defining interactions between shear-dominated plate-boundary tectonics and extension-dominated plate-interior tectonics. Geologic studies of middle Miocene strata in a 50-km-long north-south belt along the west margin of the Wassuk Range, west-central Walker Lane belt, reveal a pattern of folding on subhorizontal axes associated with displacements on convex-upward faults, a pattern that greatly reduces estimates of extension. We report an array of previously unrecognized shortening structures, including east-west folds, reverse faults, and steep-axis folds that formed at high angle to, and synchronous with, regional extension in the Coal Valley portion of the Walker Lane belt west of the Wassuk Range. We also report new radiometric age data and field relationships that support a reinterpretation of the history, magnitude, and distribution of extensional deformation. Instead of extreme (150%–200%) extension at 15–13 Ma, the new data support moderate (perhaps 30%) extension beginning ca. 10 Ma, coincident with the development of an inboard component of plate-boundary transtensional deformation. Also, new age data from moderately tilted pre-extension volcanic rocks forming the lower part of the Tertiary stratigraphic section in the southern Singatse Range, west of the northern Wassuk Range, show a lack of the extreme extension reported for coeval strata in the central part of that range. Published tectonic models that assume that extreme middle Miocene extension was uniformly distributed from the central Singatse Range through the Coal Valley area and southward to the Mina Deflection are invalid, as are models of 100 km of westerly extensional strain migration from the Basin and Range into the Sierra Nevada. Our reinterpretation reflects a return to a Miocene history, developed almost four decades ago, of formation of a volcanic highland, followed by sedimentation in broad basins controlled partly by east-west structures, followed in turn by extensional deformation that formed the existing ranges and basins after ca. 10 Ma. Following these early studies, the history was revised based on geologic mapping and thermochronologic studies in the Gray Hills–Wassuk Range directly north of the Coal Valley area.
The published thermochronologic studies of a transect across the Wassuk Range show approximate invariance across 4 km of the central range. Previously, these data had been modeled as recording rigid, whole-block west tilting of ∼50° at ca. 15 Ma, and the approximate invariance resulted in geologically instantaneous uplift of ∼6 km. However, middle Miocene strata lack evidence for such rapid large uplift; that is, large volumes of proximal coarse clastic rocks are not found, and the strata do not exhibit a pattern of growth-fault fanning expected in the tilted fault-block model. The invariance is more consistent with arching during and following magmatism than with fault-related whole-block tilting. During main-phase extension, which began at ca. 10 Ma, the range was again flexed during uplift, similar to smaller structural blocks directly to the west.
How or whether the extension-normal shortening structures we describe accommodated plate-boundary strain is not clear. Northwest-striking dextral faults are not reported in the Coal Valley area or to the northwest, so there is no clear association between the shortening structures and strain accommodation at terminations, left bends, or stepovers of such faults. We speculate that the complex heterogeneous Miocene strain of the Coal Valley area records coupling of approximately east-west regional extension with extension-normal shortening. The shortening could record midcrustal flow, possibly responding to lateral gravity gradients.
Between 38°N and 39°N in the Coal Valley–Wassuk Range area of central western Nevada, a northwest-trending belt of complexly interrelated extension and lateral shear, first defined as the 700-km-long Walker Lane belt (Stewart, 1988) and more recently as the Eastern California shear zone–Walker Lane belt (ECSZ/WLB, Fig. 1), separates the block-faulted central Basin and Range from the massive west-tilted Sierra Nevada block (Fig. 2A). Coal Valley is located 16 km west of Hawthorne, Nevada, directly west of the Wassuk Range, a major northwest-trending physiographic feature with a length of almost 100 km and peaks as high as 3450 m. The Wassuk Range is widest (20 km) west of Hawthorne and narrows rather uniformly northward. The north part of the range is encircled by the Walker River system, which flows north along the west flank, bends around the north end, and flows southerly along the east flank where it flows into Walker Lake.
The Wassuk Range consists mostly of Mesozoic granitic and metamorphic rocks and is conspicuously asymmetric, with a well-defined fault-controlled east margin, adjacent to which are continuous basin-fill sediments, and a partially faulted west margin flanked by alternating bedrock highs containing common Tertiary volcanic rocks and basins containing diverse sedimentary rocks (Stewart and Carlson, 1976). To date, there has been no published interpretation of the possible tectonic significance of the alternations; we attempt that.
Prior to Cenozoic magmatism and tectonism, the broad area that was to become the Eastern California shear zone–Walker Lane belt, including the Singatse and Wassuk Ranges and Coal Valley area (Fig. 2B), as well as areas to the south and southeast through the Excelsior and White Mountains, was part of the early Mesozoic western continental margin that shared a tectonic and magmatic history with the Sierra Nevada block to the west (Dilles and Wright, 1988; Fliedner et al., 2000; Surpless et al., 2002; Ernst et al., 2003). Understanding the timing, style, and magnitude of the subsequent extensional breakup and right-lateral shear of this margin is critical to understanding how plate-boundary strain is partitioned inboard into the North America continent.
The Coal Valley area is located in the west part of the structurally heterogeneous Walker Lake structural block of Stewart (1988) (Fig. 2B), where major late Tertiary faults are mainly dip slip in the west part and mainly strike slip in the east part, a distribution that, according to Oldow (2003), also applies to strain distribution in the currently active strain field (Fig. 2A). Discrepancy currently exists as to the timing of the main episode of extensional deformation. On the basis of detailed geologic studies, Gilbert and Reynolds (1973) showed that the current system of normal-fault-bounded structural blocks, including the large Wassuk Range, did not form until late in the depositional history of the Coal Valley Formation (ca. 10 Ma) of the Wassuk Group. By contrast, recent thermochronologic studies (Surpless et al., 2002; Stockli et al., 2002) indicate that major (∼8 km) uplift of the Wassuk Range began at 15 Ma and led to steep (∼60°) west tilting of the entire range, deformation that they interpreted as recording major (>150%) extension. A noncontroversial youthful (Pliocene–Pleistocene) part of that deformation involves displacement on the east-dipping Wassuk Range fault at the east base of the range, displacement that resulted in ∼2.5 km of range uplift equivalent to ∼12° of west tilting, as well as sedimentation in the basin beneath the adjacent part of Walker Lake (Stockli et al., 2002). Resolving the discrepancy between the geologic and thermochronologic middle Miocene histories and assessing the validity of extreme extension and, if valid, its distribution are important to understanding whether, when, and how extensional deformation encroached westward from the Basin and Range, across the transition zone of the Walker Lane belt, into the Sierra Nevada.
We describe a complex array of previously unreported structures and strain gradients showing that the strain field is not only heterogeneous in the broadest sense, but it also includes a myriad of small-scale heterogeneities such as shallow-axis and steep-axis folds and folded faults that are atypical of an extensional or transtensional tectonic setting. They record tectonically significant extension-normal displacements. Our studies indicate that (1) most stratal tilting in the Coal Valley area and elsewhere along the west margin of the Wassuk Range is westward and occurred between ca. 10 and 7.6 Ma, ∼5 m.y. younger than deduced from published cross sections and modeling of thermochronologic data, and (2) tilting is more closely associated with fault-parallel flexing of fault blocks bounded by convex-upward faults than with the rigid tilting of those blocks between planar faults assumed in published tectonic models. The contrasting fault geometry results in potentially large differences in extension magnitude. Although the magnitude tends to be indeterminate owing to out-of-plane displacements associated with the north-south shortening strains, we estimate it is closer to 30% than 150%. A lack of mapped zones of structural accommodation between highly extended areas, such as those reported for the central Singatse Range and the Gray Hills, and less extended adjacent areas invites a reassessment of the tectonic model used to derive the large values (Dilles, 1992; Dumitru et al., 2000; Surpless et al., 2002; Stockli et al., 2002, 2003).
It is widely accepted that at 8–10 Ma, ∼30% of the dextral motion between the Pacific and North American plates (currently 15%–25%) began to be accommodated in the Eastern California shear zone–Walker Lane belt coeval with regional extension, but two contrasting pictures have emerged for the prior history of extension and shear. In one, dextral shear associated with plate-boundary deformation began ca. 22 Ma in what has been referred to as the ancestral Walker Lane (Oldow, 1992; Dilles and Gans, 1995; Oldow et al., 2001); mild regional extension began at ca. 25 Ma and, triggered by magmatism, became extreme and dominant from 15 to 13 Ma, after which the two strain elements have more or less coexisted. In the second, extension between 25 and 15 Ma was mild to nonexistent; if extreme extension occurred between 15 and 13 Ma, it was highly localized rather than regional; and widespread moderate extension (perhaps 30%) began ca. 10 Ma and continues to the present, possibly as an expression of plate-boundary transtensional deformation (Anderson and Berger, 2005).
Here, we summarize only the key points of the stratigraphy and lithology of Tertiary units that are critical to unraveling the Neogene tectonic history. Granitic, metavolcanic, and metasedimentary rocks of Mesozoic age form the basement of the region. These are not subdivided on the tectonic map (Fig. 3). Also, Figure 3 shows the main Tertiary volcanic and sedimentary units and none of the younger, flat-lying, Quaternary–Tertiary basin fill or surficial deposits, which cover extensive areas. For a detailed geologic map and accompanying descriptions of pre-Tertiary metamorphic and granitic rocks, ∼20 Tertiary stratigraphic units, and Quaternary surficial deposits and basalt in this area, the reader is referred to Gilbert and Reynolds (1973). The tectonic history of interest in the Coal Valley area for this paper ended with eruption of basalt and andesite flows, mapped extensively in the Pine Grove Hills (Fig. 1) and shown in the west part of the tectonic map (Fig. 3). Those subhorizontal flows overlie older tilted Tertiary rocks and yielded K/Ar ages ranging from 7.6 to 6.8 Ma (Gilbert and Reynolds, 1973).
The oldest Tertiary rocks recognized in the area are rhyolitic ash-flow tuffs that yielded 40Ar/39Ar ages of ca. 17 Ma (Table 1; Fig. 4; analytical data and related information are filed in GSA Data Repository1). Thus, the age range of interest is from ca. 17 to 7.4 Ma. The tectonic map (Fig. 3) shows two composite stratigraphic units within that age range. The younger unit, (K/Ar ages reported by Gilbert and Reynolds  are herein recalculated using current decay constants), the predominantly sedimentary Wassuk Group, is bracketed between ca. 12.7 and 7.4 Ma. Underlying the Wassuk Group, we map a composite mixture of volcanic, intrusive, and sedimentary rocks as the “older unit,” which ranges in age from 17 to 13 Ma. Locally, these rocks are subdivided into five separate units. They are generally correlative with predominantly andesitic and dacitic Miocene sequences that are widespread in the region (Gilbert et al., 1968; Bonham, 1969; Proffett, 1977, 1984; Silberman and Chesterman, 1972; Kleinhampl et al., 1975; Proffett and Dilles, 1984a, 1984b; Dilles and Gans, 1995), but the 17 Ma rhyolitic tuffs in the “older unit” do not correlate with any known regionally distributed tuffs.
The older unit in the Coal Valley area consists of mixed lithotypes forming disparate stratigraphic sections exhibiting abrupt lateral variation, possibly resulting from a combination of deposition in ancient erosional channels, localized volcanism, and accumulation in structurally controlled depocenters. The rocks were deposited between ca. 17 and 13 Ma (see following). Rather than attempt a detailed description of the stratigraphic contrasts between these disparate sections, we summarize their main aspects, emphasize variations between them, and assess whether they contain evidence for major local tectonism.
The older unit in the south part of the area, here informally referred to as the Aldrich Summit arch area (Figs. 3 and 5, section D–D′), is predominately andesite (ranging from basalt to dacite) flows and flow breccias. Most sections show an upward change from phenocryst-poor, weakly foliated andesite to conspicuously phenocrystic flow-foliated andesite and dacite. The unit is thickest (>500 m) and most massive against the basement rocks of the Wassuk Range in the eastern Aldrich Summit arch, where the oldest flow in the sequence yielded a 40Ar/39Ar age of ca. 17 Ma (5-214-6) (all sample numbers in parentheses refer to data in Table 1 and Figure 4). The oldest flows are underlain by andesite-clast breccia, indicating that older lavas existed in the region. The youngest flow in a section facing Coal Valley on the north yielded a 40Ar/39Ar age of 15.54 Ma (5-156-1), and facing Fletcher Valley on the south, an age of 15.65 Ma (5-214-20). These data suggest that this thick section is wholly older than 14.95–12.85 Ma andesitic/dacitic rocks in the north part of the Wassuk Range (Dilles and Gans, 1995). New age data from andesite/dacite flows and flow breccias in the southern Singatse Range (14.17 Ma, GR-S-4; 15.14 Ma, GR-S-6) also suggest ages slightly younger than the thick section in the Aldrich Summit arch.
The average of five K/Ar ages on an andesite dike and associated flow facing Fletcher Valley on the south margin of the Aldrich Summit arch is ca. 12.7 Ma (Gilbert and Reynolds, 1973). If the age of the “older unit” is broadened to include this young value, it conflicts with K/Ar ages on plagioclase/hornblende mineral pairs from tuffs in the overlying Wassuk Group, the oldest of which is 12.9 Ma (Gilbert and Reynolds, 1973). Although slightly arbitrary, we place the upper age boundary of the older unit at ca. 13 Ma and emphasize that the youngest age we obtained in an effort to establish its age range in the Aldrich Summit area was ca. 15.5 Ma (5-156-1 and 5-214-20), and in the Flying M extensional zone, the youngest age was 14.47–15.23 Ma (5-209-15). The thick andesitic volcanics in the Aldrich Summit arch are devoid of colluvial facies, providing no direct evidence for uplift and tilting of the adjacent Wassuk Range during their accumulation.
Stratigraphic details of lithologic changes in the older unit northward from the Aldrich Summit area are unavailable because, along a 3-km-long stretch of the west flank of the Wassuk Range, the older unit is absent, and Wassuk Group sedimentary rocks are in fault contact with basement rocks (Fig. 3; Gilbert and Reynolds, 1973). Somewhere along that stretch, the older unit changes drastically northward from the 500+ m thickness of andesitic flows of the Aldrich Summit area to a mixed lithologic assemblage that dips steeply westward. Underlying a thin, but possibly correlative, sequence of andesite flows in the Lapon Canyon area, from base to top, there are two cooling units of rhyolitic ash-flow tuff (as much as 350 m thick), yielding ages of 17.14 Ma (5-212-5) and 17.06 Ma (5-212-14), several flows of trachybasalt and olivine basalt (as much as 200 m thick), ca. 17.04 Ma in age (5-212-4A), and as much as 200 m of coarse sedimentary breccia containing common boulders and blocks of basement rocks, many of which are exotic to the area and were probably deposited in the channel of an ancient energetic east-west–trending stream (Gilbert and Reynolds, 1973). The overlying andesite is ∼150 m thick and potentially correlative with the andesite flows in the eastern Aldrich Summit arch noted already. Above the andesite, there are poorly exposed pre–Wassuk Group, fine-grained clastic sedimentary rocks of probable lacustrine origin. These strata dip steeply, are overturned locally, and are mostly concealed by a sheet of coarse young colluvium. Although continuous sections are not exposed, these beds appear to thicken abruptly southward from ∼150 m to an estimated 1100 m. If the Wassuk Range is assumed to have been abruptly uplifted and tilted uniformly as a rigid tilt block at ca. 15 Ma (Stockli et al., 2002), we might expect to find, during this time interval and in this range-margin setting, coarse colluvium recording that deformation, but only fine-grained sandstone and mudstone and minor andesite breccia are present.
In the north part of the area, west-dipping strata of the older unit are exposed above basement in several fault-bounded structural blocks we define as the Flying M extensional zone (Figs. 3 and 5 section C–C′). Stratigraphic sections differ from block to block, and all differ greatly from the sections in the Lapon Canyon area. Ash-flow tuff is not found, and in blocks 3, 5, and 6, the lowermost strata are coarse to very coarse conglomerate consisting mainly of basement-rock clasts. These beds probably record deposition in ancient stream channels. Coarse fluvial gravel of debris-flow origin consisting of andesite clasts is common in the upper part. An andesite flow intercalated low in the section in structural block 3 yielded a poorly constrained 40Ar/39Ar age of ca. 16.5 Ma (5-97-5) and in structural block 5, an even more poorly constrained age of ca. 16 Ma (5-160-5). A flow high in the section in block 6 yielded an age of 14.5–15.2 Ma (5-209-15). Even given the large uncertainty in these ages, they suggest that andesitic volcanism may have continued for a million years after it ceased in the Aldrich Summit area, consistent with even younger ages in the north part of the Wassuk Range. In any case, the underlying stream-channel deposits containing basement-rock clasts are older than ca. 16 Ma and therefore do not record the post–15 Ma uplift and tilting of the adjacent Wassuk Range interpreted from thermochronologic data (Stockli et al., 2002; Surpless et al., 2002). The andesite-clast debris-flow deposits could record that deformation, but the absence of basement-rock clasts restricts erosion to ∼1/8 of the paleodepth indicated by those data. Also, the Tertiary section in structural block 4 directly to the west (Fig. 5, C–C′) consists of thick, nonbedded, autoclastic andesitic breccia of probable primary volcanic origin, suggesting that the debris-flow deposits are of small volume and do not grade westward into finer-grained basin-medial clastic facies, as would be expected if deposition had recorded uplift and west tilting of the Wassuk Range.
The sedimentary rocks of the Wassuk Group are key factors in unraveling the tectonic significance of Miocene paleotopography. In ascending order, the group consists of the Aldrich Station, Coal Valley, and Morgan Ranch Formations (Axelrod, 1956; Gilbert and Reynolds, 1973). The Aldrich Station Formation (12.5–11 Ma) accumulated in a fluvial/lacustrine depositional environment and consists mainly of thin-bedded to laminated fine-grained siliciclastic mudstone, siltstone, diatomaceous shale, and sandstone with minor tuffaceous sandstone, pebbly sandstone, tuff, tuff breccia, and conglomerate. The Coal Valley Formation (11 Ma to ca. 9 Ma) is generally coarser grained than the Aldrich Station, but it likely accumulated in a similar environment and in a larger basin. It consists mainly of andesite-clast sandstone and conglomerate interbedded with mudstone, siltstone, and minor breccia and andesitic tuff. Near the margins of the Coal Valley basin, strata of both formations lap onto and bury paleotopographic features of approximate east-west trend developed on andesite flows in the older unit. Despite this paleotopography, clasts of underlying flows are sparse to absent, and clast compositions indicate derivation from distant sources, mainly to the south (Golia and Stewart, 1984). The overlying Morgan Ranch Formation accumulated as alluvial fans near active faults and consists mainly of a coarsening-upward sequence of interbedded coarse feldspathic sandstone and sharpstone gravel and breccia composed mostly of debris derived from directly adjacent Mesozoic plutonic and metasedimentary rocks.
More than 1 km of fluvial and lacustrine sediment accumulated in the Coal Valley basin before it began to be disintegrated by block faulting during deposition of the uppermost part of the Coal Valley Formation and the overlying Morgan Ranch Formation. The Morgan Ranch accumulated to thicknesses of 400–1225 m, and, although it has not been dated isotopically, it apparently accumulated between 8 or 9 Ma and ca. 7.6 Ma, i.e., the oldest age of mafic lavas that rest unconformably on it (Gilbert and Reynolds, 1973). From a maximum thickness of ∼2.5 km near the center of Coal Valley, the Wassuk Group thins to the north and south, reflecting onlap against structurally controlled paleotopography (Fig. 5, D–D′).
Description of structures is limited to those formed during the late Cenozoic, some of which are transverse to the general north and north-northwest major Basin and Range structural grain defined by Gilbert and Reynolds (1973) and Stewart (1988). We first describe the Coal Valley graben and associated structures, including its bounding faults (the Coal Valley fault on the west and the west Wassuk fault zone on the east), as well as some related structures such as the Coal Valley syncline in the south and the Morgan Ranch steep-axis hinge zone in the center. Next, we describe blocks of structurally high basement rock satellite to the main Wassuk Range block in the northeast (pop-up blocks, Fig. 3), followed by transverse structural arches against which the Coal Valley graben terminates on the south and north, the Aldrich Summit arch and Flying M extensional zone, respectively. Finally, we describe structures in the Gray Hills area north of the Coal Valley area (Figs. 1 and 2B), where a tectonic model of extreme extension was developed based on geologic mapping and thermochronologic data (Surpless et al., 2002; Stockli et al., 2002).
Coal Valley Graben and Associated Structures
The Coal Valley graben, the most conspicuous structure in the area, is expressed as a north-northeast–trending swatch of west-younging Wassuk Group sedimentary rocks bounded on the west against the Pine Grove Hills by the large-displacement Coal Valley fault and on the east against the Wassuk Range by a complex range-front fault of lesser displacement, the west Wassuk fault zone (Figs. 3 and 5, A–A′ and B–B′). The structural blocks adjacent to the graben on the east and west expose extensive tracts of basement rock that provide little stratigraphic control for assessing throw on the graben-bounding faults, and major lateral lithologic variation of pre–Wassuk Group Tertiary strata throughout the area also makes assessments of throw difficult. Wassuk Group strata in the central part of the graben dip uniformly west into the Coal Valley fault, giving the graben a relatively simple asymmetric form with at least 2 km of throw (Fig. 5, A–A′), but total throw is probably at least twice that. Also, the Aldrich Station and Coal Valley Formations thin to the north and south of the central Coal Valley graben, possibly recording kilometer-scale displacement gradients on the Coal Valley fault consistent with an east-west warp within the broad Wassuk Group basin (Fig. 5, D–D′). In the central part of the graben, westerly dips range from 35° to 60° in the east and from 10° to 45° in the west. This westward decrease in dip is abrupt and defines a west-facing monocline (Fig. 3). The monocline axis is more than 3 km east of the Coal Valley fault, an unlikely position for origin of the decrease in dip by fault drag (Fig. 6A).
The Coal Valley fault dips 25°–70°E, has a conspicuously sinuous trace, and is broken in the area northeast of Morgan Ranch by east- to northeast-striking faults, northeast of which it has a broad convex-southeast curvature. In the south, where it bends eastward around a conspicuous salient of basement rock, Wassuk Group rocks forming the hanging wall are folded, with the largest fold labeled the Coal Valley syncline (Fig. 3). As shown in Figure 7, this salient could reflect either the primary shape of the fault or the bending of a once-straight fault. We favor the latter origin based on the distribution of associated folds and fault-damaged rock. If the salient reflects the primary shape of the fault, dip slip on the northeast- and northwest-striking segments should have produced extension in the hanging-wall rocks directly adjacent to the salient (Fig. 7B), not contraction as indicated by the folds (Fig. 7A). Also, if basement rocks of the salient had been forced prow-like into the Wassuk Group strata of the Coal Valley graben, as suggested by Gilbert and Reynolds (1973), one would expect an anticline (Fig. 7C) instead of a syncline (Fig. 7A).
As the west-dipping Wassuk Group strata of the central Coal Valley graben are traced northward, they form a group of northeast-trending open folds, and their outcrop width narrows (Fig. 3). The narrowing of the width of the graben is largely a result of the bending eastward of the west-bounding Coal Valley fault, where it is broken into a highly irregular saw-toothed pattern by intersecting faults. We refer to the zone connecting the south limit of the folds in the Wassuk Group strata with the hinge in the bent and broken Coal Valley fault as the transverse Morgan Ranch steep-axis hinge zone (Fig. 3). Wassuk Group strata in that area are steep to overturned with complexly variable strikes that are either parallel or perpendicular to one or the other of the intersecting faults. The intersecting faults include two with reverse-sense displacement, which, together with the folding of strata in the hanging wall, indicate north-south structural crowding in this steep-axis hinge environment. Although only the Coal Valley formation is involved in the northeast-trending open folds, the Morgan Ranch Formation is fully involved in the structural crowding in the hinge, suggesting deformation during 9 or 8–7.4 Ma. Farther north, the Coal Valley fault resumes its north strike, giving its trace an additional strong curvature concave westward. Also, short, discontinuous Jurassic dikes, one dated at 158 Ma (5-159-23, Table 1), in the footwall directly beneath the concave-southwest curve in the Coal Valley fault strike approximately parallel to the curved fault, showing that the granitic footwall containing the dikes was flexed around a steep axis along with the Coal Valley fault. At the narrowest north part of the Coal Valley graben, we map a monoclinal flexure in rocks of the older unit (Fig. 3) and illustrate the flexure in Figure 8A as an eastward increase in dip from 20° to 45° in a single bed set. Thus, the west margin of the Wassuk Range block east of the Coal Valley graben is flexed on gentle axes as illustrated in sections A–A′, B–B′, and C–C′ (Fig. 5).
The east boundary of the Coal Valley graben is part of a geometrically complex zone of north- to northeast-striking normal faults, the west Wassuk fault zone (Fig. 3). The complex fault geometry imparts a highly irregular shape to the west margin of the Wassuk Range (Fig. 9). Much of the structural complexity results from major normal faults for which strikes are clockwise of the generally north-trending range. As noted by Gilbert and Reynolds (1973), nowhere along this structural margin do the Aldrich Station or Coal Valley Formations contain lithologic evidence that the Wassuk Range existed during their deposition. At the southeast margin of the graben, rocks of the Aldrich Station Formation strike parallel to the bounding fault with dips that are approximately parallel to the fault. Throw on that part of the fault could be small or even trivial, but elsewhere along the fault zone, those rocks and rocks of the older unit strike at moderate to high angles to the zone, requiring displacements of hundreds of meters. East of the north part of the graben, two large blocks of basement rock are structurally high relative to adjacent Miocene strata (labeled pop-up blocks, Fig. 3). The blocks are bounded by continuous, curved, sharply defined faults, adjacent to which are the uniformly west-tilted Miocene strata. The basement-rock blocks appear to have risen on the steep curved faults into previously tilted strata. Current summit elevations of these basement blocks are 500–700 m above the faulted contacts with adjacent Miocene rocks, and structural relief could be much greater, depending on erosion. Most, and possibly all, of the tilting postdates the Aldrich Station Formation, and the younger uplift of the blocks could have been coeval with the post-tilting bending of the Coal Valley fault.
Southwest of Lapon Canyon, strata of the Aldrich Station Formation and a thick sequence of pre–Wassuk Group clastic rocks dip steeply west, are overturned locally, and, as they are traced southward, bend sharply counterclockwise into the range-front fault, producing a steeply southwest-plunging anticline. Outcrop-scale reverse faults and associated folds and overturned beds in the north-trending limb suggest low-strain east-west shortening. Although overturning of beds in fault-propagation monoclines above steep normal faults is well known (Sharp et al., 2000), the steep-axis bending requires a separate explanation compatible with the overturning. If these structures are genetically and temporally related, south-directed displacement in a slightly transpressional setting is suggested (Fig. 10). Although this deformation is known only to postdate the Aldrich Station Formation, we suspect it to be coeval with similar structures of Morgan Ranch or younger age.
Transverse Flying M Extensional Zone
North of the Coal Valley graben, in the northernmost part of the mapped area (Fig. 3), six west-tilted, fault-bounded structural blocks (labeled 1–6, Fig. 5, section C–C′), in which pre-Tertiary granitic rocks are overlain by Tertiary rocks, define a transverse structurally high zone of apparently large extension. Block 6 is the west margin of the broad Wassuk Range structural block and includes the north end of the Coal Valley graben; block 5 is the footwall of the Coal Valley fault as noted already. Block 1 is the southern extreme of the Cambridge Hills. Blocks 3 and 4 appear to grow northward out of the large convex-east block 5, reflecting an apparent northward increase in extension (compare cross sections B–B′ and C–C′, Fig. 5). Mesozoic dikes dip 30°–35°E in the footwall of the Coal Valley fault, suggesting 60°–55°W tilting. Farther west, the overlying strata of the older unit also dip 60°W, but the dip in a single bed set decreases sharply westward to ∼20° (Fig. 8B), showing that basement and cover rocks are flexed in an approximately north-trending monocline similar to the west margin of the Wassuk Range (block 6, Fig. 5). The difference in dip suggests ∼45° west flexing of the footwall block beneath the Coal Valley fault. The fault there dips 25°–30°E, and beds of the Coal Valley Formation in the hanging wall dip only ∼20°W, suggesting that the footwall is flexed upward beneath a downward-steepening (convex-upward) Coal Valley fault in that area (Fig. 5, C–C′). Other blocks also contain extension-normal monoclines across which stratal dip approximately doubles abruptly from west to east (Figs. 3 and 5). Locally, the trace of the axial surface of these folds cuts gently across bedding, lending additional support to a postdepositional origin. West tilting defines the main episode of regional extension (Stockli et al., 2002), so the folding is synextensional. We have identified this type of folding over a north-south distance of 50 km along the west flank of the Wassuk Range. Although folds with this geometry could form by normal drag, such folds are likely to have axial planes located directly adjacent to the fault with bedding dragged approximately parallel to the fault (Fig. 8A). The folds west of the Wassuk Range show neither of these traits. Their axial planes are typically 300–500 m or more from the faults, and bedding cutoff angles are typically greater than 50°. We evaluate the significance of this folding for the timing and amount of extension and for choosing a tectonic model in the Discussion section.
Near the northeast corner of the map area in block 6 (Fig. 3), west-dipping beds of the Coal Valley Formation are interrupted by exposures of west-dipping structurally high rocks of the older unit. At the south margin of the northern patch of Wassuk Group strata, volcanogenic sandstone beds bend southwestward (clockwise) to approximately east-west strikes and are overturned locally. These small-scale contractional structures are similar to those along the west margin of the Coal Valley graben at the Morgan Ranch steep-axis hinge zone and also the clockwise bending in the south limb of the Coal Valley syncline. In all areas, the steep-axis folding may result from buttressing against older, structurally high Tertiary rocks. In any case, the folding and overturning suggest north-south structural shortening or crowding at the west margin of the Wassuk Range. Although these structures are known only to postdate the Coal Valley Formation, their similarity to structures along the west margin of the graben leads us to suspect they are largely of similar age.
In the west part of the Flying M extensional zone (west of the East Walker River), bedding in scattered exposures of clastic strata defines a gentle, open, east-trending syncline (Fig. 3). It is unclear whether or how this feature is related to the hinge zone. Its orientation is appropriate for an origin related to north-south structural shortening, but such an interpretation depends on whether the east-striking fault at its south margin is normal or reverse. At that fault, north-dipping debris-flow deposits consisting almost entirely of andesitic debris are downfaulted against pre-Tertiary granite. A fault with large throw is inferred because (1) the andesitic debris could not have been derived from and deposited against adjacent highlands consisting of basement rocks, and (2) nearby thick sections of rhyolitic ash-flow tuff and andesite flows are missing at the contact and are probably faulted out. The fault is not exposed, and its dip is unknown. If it is a normal fault, it is located along strike of reverse faults and associated folds in the hinge zone, and any temporal overlap of displacement on these disparate structures would require either abrupt lateral variation in strain or efficient partitioning and decoupling of strain between the two areas. These requirements do not exist if it is a reverse fault.
Transverse Aldrich Summit Arch
The Coal Valley graben terminates southward against a transverse structural arch we refer to as the Aldrich Summit arch (Fig. 3). It extends ∼15 km west from a conspicuous west-facing knee in the west margin of the Wassuk Range to the East Walker River, following a band of hills formed on Miocene andesite and older rocks. The arch is best defined structurally in its central part, where andesite flows dip moderately south toward a basin beneath Fletcher Valley and gently north toward the Coal Valley graben. The west end is not well defined, but an outcrop band of fault-bounded basement rock located along the projection of the arch axis is flanked on the north and south by andesite, giving the appearance of an arch. Also, along the north margin of Fletcher Valley, just beyond the southwest corner of Figure 3, strata of the Coal Valley Formation show upward-decreasing fanning (75°–35°) of south-southeast dip, suggesting that arching was active during deposition of the Coal Valley Formation.
The youngest flow beneath the Wassuk Group on the north flank of the arch, a dacite, yielded a 40Ar/39Ar age of ca. 15.4 Ma (5-156-1). The youngest flow on the south flank dips gently toward Fletcher Valley and yielded a 40Ar/39Ar age of ca. 15.6 Ma (5-214-20). As noted in the section on stratigraphy, an east-striking broad vertical andesite dike and associated subhorizontal flow in the west part of the arch yielded an average K/Ar age of 12.7 Ma, though this age is suspect because it is slightly younger than the oldest age on the overlying Wassuk Group rocks. Regardless of its precise age, the presence of an arch-parallel dike suggests connection of the arch to deep structure, as does confinement to the arch of the heat source that powered hydrothermal alteration, the distribution of which is shown in Figure 3. Control of the arch by pre-Cenozoic structure is possible, but we know of no direct evidence for such control.
Structures in the Gray Hills Area
Tilting of Miocene strata in the Gray Hills area, ∼20 km north of the Coal Valley area, forms the basis, along with thermochronology data from the adjacent Wassuk Range, for interpreting the extensional history and magnitude of the region (Surpless, 1999; Stockli et al., 2002; Surpless et al., 2002). We report here some previously unreported aspects of tilting and steep-axis bending of those strata critical to that history.
At the west base of the Wassuk Range athwart the westerly projection of a transect sampled for thermochronologic study by Surpless et al. (2002) (locality A, Fig. 11), a 150–200 m section of andesite of Lincoln Flat is underlain by 50 m of debris containing blocks of granite to 5 m and overlain by at least 400 m of Wassuk Group siliciclastic strata consisting mostly of lacustrine sandstone, siltstone, and thin-bedded to laminated shale. All units are conformable and dip vertical to 70°W, but in the fine-grained lacustrine Wassuk Group strata, dips decrease abruptly (within 200 m) to 35°W (marked by a monocline symbol in Fig. 11). Similarly, ∼4 km to the northwest (locality B, Fig. 11), dips in thin-bedded fine-grained Wassuk Group strata decrease westward from 56° overturned through vertical to horizontal over a 400 m cross-strike distance. In cross section, Surpless (1999) and Surpless et al. (2002) modeled the andesite of Lincoln Flat and Wassuk Group in this area as a fanning-upward synextensional succession in which dips decrease uniformly from ∼60° to 20° over a cross-strike distance of ∼2 km, which is clearly at variance with the actual dip distributions. In the Gray Hills structural block to the west, we note a similar discrepancy between the published depiction of a 2-km-wide fanning-upward Lincoln Flat andesite and Wassuk Group succession versus an actual westward decrease in dip of fluvial and debris-flow facies of the 15.08–14.39 Ma andesite of Lincoln Flat from 75° to horizontal over a cross-strike distance of only 350 m (locality C, Fig. 11). In the Buck Brush Spring block between the Gray Hills and Wassuk Range, abrupt up-section dip decreases are seen in along-strike blocks of 27.1 Ma ash-flow tuff and the much younger (<14.39 Ma) Wassuk Group strata (localities D, D′, Fig. 11), in clear violation of the progressive tilting history. Because these abrupt dip decreases in temporally disparate strata are similar to those we report here from locations directly adjacent to uplifted blocks of pre-Tertiary rocks in the Coal Valley area and previously from the northern Wassuk Range (Anderson and Berger, 2005), we assume a common origin as folds rather than rigid tilt blocks. Equally important, rather than the andesite of Lincoln Flat and Wassuk Group strata recording the deformation as a fanning-upward succession, the deformation postdates those strata.
Two localities in the Gray Hills area provide evidence of extension-normal shortening associated with steep-axis rotation. The Gray Hills structural block is bounded on the north by a WNW-striking fault, adjacent to which we identify strong steep-axis bending. As the fault is approached from the north, westerly dipping Wassuk Group strata rotate clockwise 90°, and their dip increases from 50° to 80°, consistent with right-sense drag in a transpressional setting, but south of the fault, steeply west-dipping older Tertiary strata rotate counterclockwise (locality E, Fig. 11). As drag features, these rotations would indicate a two-stage left- and right-slip history. We doubt such a history is valid because extension-normal shortening unrelated to fault drag is common in the region. Between the Wassuk Range and the Buck Brush Spring block, some displacement on a northerly striking right-slip fault appears to be absorbed by clockwise-rotated Wassuk Group strata (locality F, Fig. 11). Shortening recorded by these structures is consistent with the strain history in the Coal Valley area. The shortening conflicts with post–10 Ma northwest-southeast lengthening assumed for the Wassuk Range–White Mountains area by Stockli et al. (2003).
Here, we discuss the tectonic setting for three time intervals (17–13 Ma, 13–10 Ma, and 10–7.6 Ma), compare the overall strain style and history with other areas, present a tectonic model, and comment on the relation of the strain history to plate-boundary strain.
Tectonic Setting, Older Unit (17–13 Ma)
As noted previously herein, the older unit is known to be as young as 15 Ma but may be as young as 13 Ma. Varied depositional factors influencing the tectonic setting of the older unit include (1) explosive volcanism, possibly into east-west erosional channels in which coarse exotic-clast, nonvolcanic debris deposits were accumulating, (2) relatively passive eruption of flows and breccias of mafic to intermediate composition, possibly constructing small volcanic shields or tablelands, (3) fluvial debris flows that likely constructed alluvial fans or aprons adjacent to volcanic highlands, and (4) fine-grained sand and mud, probably deposited in a lacustrine basin. Because each of the lithotypes representing these depositional settings exhibits strong lateral north-south thickness variations, and because there is no obvious contrast in dip direction or magnitude throughout the section, the tectonic setting must allow for north-south lateral variations in lithology and thickness without much tilting. With the possible exception of the area west of Morgan Ranch, a general absence of sidewall colluvial facies seems to argue against deposition in east-west–oriented depocenters controlled by normal faults, accommodation faults, or transfer faults (Fig. 12).
Erosional development of generally westerly draining paleovalleys and deposition in them in the Coal Valley area are analogous to processes described by Henry et al. (2003) and Faulds et al. (2005) for large parts of the Walker Lane belt to the northwest. These processes contributed to thickness and lithologic variations in the older unit. Additionally, some of the lateral thickness and lithologic variations in the Coal Valley area probably reflect accumulation in east-west–trending syndepositional downwarps that varied in location with time (Fig. 5, D–D′). Such warping, if gentle, would produce stratal onlaps with subtle variations in dip direction and amount (Fig. 12C). We speculate that gentle warping on east-west trends occurred during the 17–13 Ma interval, and this reflects minor north-south contraction. The warping may have played a role in localizing paleovalleys.
Tectonic Setting, Aldrich Station and Coal Valley Formations (13–10 Ma)
The regional tectonic picture during the 13–10 Ma interval of Aldrich Station and Coal Valley deposition is one of uniform patterns of sedimentation in broad basins (Bonham, 1969; Stewart, 1992; Trexler et al., 2000; Henry and Perkins, 2001). Similar patterns are reported from elsewhere in the Basin and Range (Bohannon, 1984; Stewart, 1992; Wallace, 2004), showing that tectonic conditions in the Coal Valley area were common for that interval. The broad basins developed prior to similar basins along the east margin of the Sierra Nevada (Trexler and Cashman, 2005). Whether or not any of these basins represent major extension is controversial (Stewart et al., 2000). Detachment faults heralding large extension during basin development have not been mapped in the Coal Valley area, and, based on field study, we find no direct evidence for them to the north in the Gray Hills area mapped by Surpless (1999) (Fig. 11). Also, fanning-upward basin-fill assemblages that are potential indicators of large extension are not characteristic of sedimentation along the west margin of the Wassuk Range. Dilles and Gans (1995) reported such structures in the Wassuk Group sedimentary strata in the northern Wassuk Range, but they are invalidated on the basis of comprehensive bedding attitude data (Anderson and Berger, 2005). Dip distributions there and in the Gray Hills west of the central Wassuk Range are similar to those in the Coal Valley area and are generally inconsistent with large-extension growth-fault geometry (Figs. 3, 5, and 6A).
On the north flank of the Aldrich Summit arch, the Aldrich Station Formation is deposited against a north-northeast–facing paleosurface that Gilbert and Reynolds (1973) interpreted as the footwall of a steep west-northwest–striking basin-bounding normal fault active during deposition. Our study of that contact shows that locally derived colluvium that could record active fault displacement is thin or absent above the paleosurface. Except in the basal meter or so, gravel in the lower part of the north-thickening sedimentary sequence is devoid of locally derived andesite clasts, suggesting that the highland along the arch, instead of being flanked by a steep fault escarpment, was a moderate to gentle paleoslope that was passively inundated by sediments of the Aldrich Station Formation derived from distant sources. According to Golia and Stewart (1984), most of the clastic material came from southerly sources. If so, the materials were transported across a partially buried Aldrich Summit arch. Although fault control of the paleoslope is possible, total displacement would be very small because the fault is not mapped in along-strike older rocks (Gilbert and Reynolds, 1973). Also, little, if any, fault-related stratal tilting is evident (Fig. 3). More likely, the paleoslope reflects the north flank of the Aldrich Summit structural arch. As Wassuk Group sedimentation continued, it buried the paleoslope, marking a major expansion (to 1500 km2) of the depocenter (Fig. 1) in what must have been a rather passive tectonic setting analogous to other roughly coeval broad basins prior to their breakup by younger faulting (Bonham, 1969; Bohannon, 1984; Trexler et al., 2000; Wallace, 2004).
The southward thinning of the Aldrich Station Formation onto the paleoslope of the Aldrich Station arch is an exposed example of the type of onlap we speculate occurred as strata of the older unit thinned onto east-west paleotopographic highs. Additionally, the Aldrich Station and Coal Valley Formations thin northward onto an arch that subsequently became the Flying M extensional zone. In north-south cross section, the basin beneath Coal Valley is a downwarp between two structural arches, a large-scale example of the downwarps shown stylistically in the older unit (Fig. 5, D–D′). A protracted period dominated by east-west warps may have existed from 17 to ca. 10 Ma. We speculate that the warps reflect weak north-south shortening.
Tectonic Setting during and following Deposition of Upper Part of Wassuk Group (10–7.6 Ma)
The major Miocene deformation in the area, including uplift of range-scale blocks and breakup of the broad Wassuk Group basin, began late in the depositional history of the Coal Valley Formation (ca. 10 Ma) and continued through and outlasted deposition of the Morgan Ranch Formation (Gilbert and Reynolds, 1973). In the Coal Valley graben, deformation is recorded in range-front colluvial facies of the uppermost Coal Valley Formation and overlying Morgan Ranch Formation deposited against the steeply east-dipping Coal Valley fault. As in the northern Wassuk Range (Anderson and Berger, 2005), those deposits represent less than 25% of the volume of Wassuk Group strata. The west tilting defines an extensional tectonic setting, and a wide variety of ancillary structures complicate that setting. The most important of those ancillary structures in terms of extension magnitude are footwall flexures beneath convex-upward faults. Such flexing is not characteristic of a large-extension tectonism, in which faults decrease in dip downward and strain is accumulated across numerous tilted blocks. The Coal Valley graben and at least four of the blocks of the Flying M extensional zone contain an extension-normal monocline that formed during west tilting (A–A′ and C–C′, Fig. 5). This type of deformation produces footwalls that are tilted more steeply than adjacent hanging walls, a condition consistent with convex-upward fault geometry (Figs. 6A and 6B). If the faults curve to steep dips at depth, little, if any, of their displacement could accumulate large heave, as would be the case if the faults were listric and merged with a detachment fault. Also, small extension is more consistent with the abrupt across-strike stratigraphic variations than large extension, which, if restored, would place disparate stratigraphic sections in contact with one another. Other ancillary structures that complicate the extensional strain picture include (1) clockwise steep-axis bending of the Coal Valley fault and associated east-striking reverse faults in the hinge zone near Morgan Ranch, (2) open folds east of the clockwise-rotated block, (3) the Coal Valley syncline and adjacent footwall salient along the Coal Valley fault, and (4) a steep-axis fold along the east margin of the Coal Valley graben.
South of the Flying M extensional zone, the Coal Valley fault zone was deformed by clockwise steep-axis rotation (Anderson et al., 2000), as evidenced by (1) curvature of the fault trace, (2) fault-parallel curvature in the trends of Mesozoic dikes in the granitic footwall, (3) complex segmentation of the fault and associated structural crowding of hanging-wall sedimentary rocks in the knee of the bend near Morgan Ranch, (4) folding of hanging-wall Wassuk Group rocks in front of the rotated block north of the Morgan Ranch hinge zone, and (5) development of an apparent north-increasing extensional strain gradient behind the rotated block. Extension behind the rotated block is reflected in the series of west-tilted structural blocks comprising the Flying M extensional zone (Fig. 5, C–C′). Some of the block-bounding faults emerge northward out of the large clockwise-rotated basement-rock block, showing they developed synchronous with rotation (Fig. 3).
North of the Aldrich Summit arch, beds south of the axis of the Coal Valley syncline were apparently rotated clockwise as the fault zone and its granitic footwall were flexed eastward to form the adjacent salient. The clockwise rotation could reflect buttressing against the deeply rooted Aldrich Summit arch. Horizontal components of particle paths in the footwall and rotations in the hanging wall are sketched in Figure 7D. In support of this model, we note (1) the presence of at least three east-northeast–trending folds in Wassuk Group rocks between the salient and the arch and no folds north of the salient, (2) an anomalous thickness (to 200 m) of broken to pulverized granitic rock along the south margin of the salient, possibly recording a change in shape of the footwall during flexing, and (3) an analogous small-scale sharp clockwise bend in Coal Valley strata directly north of the transverse structural high in the easternmost part of the Flying M extensional zone. Coupling of footwall flexing and steep-axis hanging-wall rotation both reflect low-strain north-south contraction, possibly with a west-increasing strain gradient (Fig. 7D).
The steep-axis fold and localized overturned beds adjacent to the Wassuk Range southwest of Lapon Canyon also complicate the extensional strain picture, especially at the margin of a graben. The average strike of the range-front fault in that area is N30°E, clockwise of bedding strikes in the adjacent fine-grained lacustrine strata of the older unit and overlying Aldrich Station Formation. That fault is not exposed, so its attitude and slip sense are not known. We assume its dip to be >70° because it dips 75°W at Lapon Canyon. If the fault had a large component of left slip, the bending of previously tilted strata into it could reflect drag. However, the axis of the fold (Fig. 3) and the associated east-overturned beds (Fig. 10A) are located ∼1 km west of the fault, not against it, as would be expected if those structures were formed by drag. We suggest a genetic relation between the steep-axis bending and the pop-up–style uplift of the basement-rock blocks located directly north-northeast of it (Fig. 3). Perhaps space was created for the uplift of those blocks by generally south-directed displacement of the previously west-tilted Miocene strata. In this scenario, the fold represents shortening of the basin-fill strata associated with buttressing of south-directed displacement against the slightly askew faulted margin of the Wassuk Range block. In summary, the tectonic setting during the 10–7.6 Ma interval, as indicated by a wide assortment of ancillary structures in the Coal Valley area, reflects shortening at a high angle to coeval mild to moderate extension, in some cases by buttressing against transverse structures.
Comparisons with Other Areas
Here, we compare geologic relations in the Coal Valley area with those northward along the west flank of the Wassuk Range, followed by comparisons with other parts of the region.
The 40Ar/39Ar age data (Table 1) support a northward decrease in age of inception of major andesitic magmatism from ca. 17 Ma in southern Coal Valley (the Aldrich Summit area) to <15 or 14 Ma in the northern Wassuk Range (Dilles and Gans, 1995). Also, andesitic magmatism appears to have ended in the Aldrich Summit area (ca. 15.6 Ma) before it began in the north part of the range. Andesitic rocks in the northern Wassuk Range are approximately coeval with those in the Singatse Range to the west (Dilles and Gans, 1995; Anderson and Berger, 2005; this report). Controversy exists over the extent to which large-magnitude extension accompanied andesitic magmatism, and it centers on the extent to which andesitic rocks were tilted prior to deposition of Wassuk Group rocks (Anderson and Berger, 2005). From the northern Wassuk Range southward through the Coal Valley area and westward into the Singatse Range, the units are either conformable or in slight angular unconformity. In the Gray Hills area, cross sections show an average unconformity of 15° (Surpless, 1999), similar to the unconformity in the southern Singatse Range. At the west base of the Wassuk Range, west of the traverse sampled for thermochronologic study (Stockli et al., 2002), the units are conformable and approximately vertical. Extension, as deduced from tilting, probably accompanied andesitic magmatism but does not appear to have been major.
The youthful timing, anomalous styles, and inferred low amount of late Cenozoic extension we report here for the Coal Valley area compare well with the northernmost part of the Wassuk Range (Anderson and Berger, 2005) and with our reassessment of the structural development of the Gray Hills area. Detailed study of dip distributions in the northern Wassuk Range (Anderson and Berger, 2005) showed that stratal tilting was minor until late in the depositional history of the Coal Valley Formation (ca. 10 Ma) and that steep dips in Tertiary strata adjacent to blocks of uplifted basement rock resulted from flexing rather than rigid block tilting (Fig. 6C). Basalt from the Gray Hills area that yielded a 40Ar/39Ar age of 14.39 Ma was mapped as deposited above steeply tilted Wassuk Group strata, thus establishing an upper age limit for major tilting (Surpless, 1999). In contrast, our study shows the unconformity to be a local low-angle fault, above which the basalt and intercalated volcanogenic sedimentary rocks dip from 60° to 90°, thus allowing for a younger age for the major tilting event in that area. The present study of tilting in the Gray Hills area reveals patterns (Fig. 11) similar to those in the northern part of the range and in the Coal Valley area, including steep-axis bending.
Warping on east-west trends with little or no fault-related tilting during deposition of the older unit and most of the Wassuk Group could record small north-south contraction. The younger large- and small-scale examples of steep-axis bending and related (?) faulting and folding at all margins of the Coal Valley graben and in the Gray Hills area (Fig. 11) are more convincing evidence of north-south contraction than are the earlier arches and downwarps. Similar structures are reported elsewhere along the trend of the Walker Lane belt–Eastern California shear zone. Steep-axis bending and/or extension-parallel contractional folds have been reported from Warm Springs Valley 100 km north-northwest (Trexler et al., 2000), the northeastern Mojave Desert (Schermer et al., 1996; Guest et al., 2003), the central Mojave (Bartley et al., 1990), and the southwestern Mojave (Law et al., 2001), all of which are situated along the Eastern California shear zone as defined by Dokka (1992). Folds that intensify in number and magnitude at the margins of north-south–transported structural blocks are well documented in late Miocene strata in the Lake Mead area of the Walker Lane belt (Anderson et al., 1994; Anderson, 2003; Anderson and Beard, 2010).
Following construction of the volcanic highland, the broad basin that received sediments from the early and middle parts of the Wassuk Group formed. The structural breakup of the broad Wassuk Group Miocene basin in the Coal Valley area at ca. 10 Ma is similar to that in other parts of the Walker Lane belt (Stewart and Diamond, 1990; Stewart et al., 2000; Anderson and Berger, 2005). Our downward revision of the timing and amount of main-phase tilting and extension along the west margin of the Wassuk Range from ca. 15 Ma to ca. 10 Ma is inconsistent with the long-standing recognition of an outward migration of late Cenozoic deformation from the central Great Basin (Armstrong, 1968; Wernicke, 1992; Dilles et al., 1993) and especially with the westward migration of deformation from the Wassuk Range area of the western Basin and Range at ca. 15 Ma into the eastern Sierra Nevada at ca. 10 Ma (Oldow, 1992; Dilles and Gans, 1995; Henry and Perkins, 2001; Surpless et al., 2002). Instead, extension appears to have been coeval from the Wassuk Range westward to the Sierra Nevada (Anderson and Berger, 2005).
In the central Singatse Range (Fig. 1), 15–13.8 Ma andesitic flows are steeply west tilted and cut by two generations of large-displacement normal faults, producing an estimated 150% extension (Dilles and Gans, 1995; Proffett, 1977). In the southern Singatse Range, similar pre-extension andesitic flows for which we report similar ages of ca. 15–14 Ma (GR-S-4, GR-S-6, Table 1) are mildly faulted and tilted (30°–35°) and overlain by weakly tilted (15°–20°) Wassuk Group strata, providing a stark contrast in strain magnitude between the south and central parts of the range. Stockli et al. (2003) projected middle Miocene (15–10 Ma) extreme extension from Yerington in the central Singatse Range over an area of 10,000 km2 to the Mina Deflection, where right-lateral displacement is inferred to have accommodated unevenly distributed strain (Fig. 6E). However, geologic evidence for only small to moderate extension during that interval between the central Singatse Range and the Mina Deflection precludes such uniform distribution of extreme extension. Those authors also assumed that the Wassuk Range–Singatse Range area was subsequently (post–10 Ma) translated northwest in front of a northwest-southeast extensional zone between there and the White Mountains (Fig. 6E), seemingly at variance with evidence we present for north-south contraction during that interval.
Uplift and exhumation histories obtained from fission-track data are central to models of major extension starting at ca. 15 Ma, as well as existing models of westward migration of extensional deformation (Dilles and Gans, 1995; Surpless et al., 2002; Stockli et al., 2002). At the latitude of the Gray Hills, thermochronologic data were interpreted to indicate ∼50° of rigid west tilting of the Wassuk Range at ca. 15 Ma, during and following magmatism that produced volcanic and intrusive rocks of the andesite of Lincoln Flat. The strain path for the tilting was not defined, only an initial state of a block bounded by 65°E-dipping faults and a final state of the same block bounded by subhorizontal faults (Fig. 6D). For example, we have no understanding of (1) the amount of throw on the faults prior to their tilting to shallow dips, (2) the geometry of the initially steep faults below the fault-bounded block, (3) the current location of the block initially located east of the tilted block, or (4) the source of the mass, >6 km thick, inserted beneath the uplift. Eight apatite fission-track (AFT) ages spanning the 4-km-long central segment of the thermochron traverse reveal approximate invariance (Fig. 6D). Adherence to the rigid tilt block model results in ∼6 km of geologically instantaneous uplift. We find no geologic evidence along the west flank of the Wassuk Range for that event. We base this on general relations such as the (1) concordance or slight angular discordance between andesitic rocks and overlying Wassuk Group rocks, (2) absence of patterns of fanning-upward dips in early Wassuk Group strata, and (3) absence of large volumes of coarse, locally derived sediment in either the older unit or the Aldrich Station and lower Coal Valley Formations. Only a small volume of locally derived, ca. 15 Ma, coarse clastic strata is found in the older unit, and those strata could only record ∼1/8 of the paleodepth indicated in the rigid tilt-block model. There exists little justification for assuming 60° of uniform west tilt of the Wassuk Range, a relationship based on the assumption that cover rocks were deposited on and firmly coupled to basement rocks. Instead, much of the basement-cover contact is a fault (the west Wassuk fault zone), and common steep to overturned dips in cover rocks auger against the assumed 60° of uniform tilt. Also, the cover rocks adjacent to the Wassuk Range block and the smaller blocks to the west are folded on subhorizontal axes. Other structural features along the contact zone complicate the simple picture of structural coupling to a uniformly steeply tilted block, namely, the popped-up blocks of basement rock, the ragged nature of the western range margin, and the steep-axis folds.
Factors that might aid in reconciling inconsistencies between geologic and thermochronologic histories include: (1) Rather than having been rigidly tilted at 15 Ma, the Wassuk Range structural block was probably flexed, as indicated by the shape of the 110° isochron of that age in Figure 6D. (2) Miocene cover rocks may have been weakly tilted during this early event. (3) A steep geothermal gradient, possibly as high as 35 °C/km, may have resulted from heat advection during the north-migrating regional episode of 17–14 Ma andesitic magmatism and associated upflow of hydrothermal fluids. (4) Mild flexural uplift (∼3 km as illustrated in Fig. 6D) through a steep geothermal gradient from 16 to 14 Ma could have resulted from magmatic inflation rather than extensional faulting. (5) The geothermal gradient prior to uplift through the partial annealing zone from 16 to 14 Ma may not have been linear but instead was strongly kinked as a result of heat lost to adjacent basins by advective fluid flow from volcanic highlands. As noted by Dempster and Persano (2006), geotherms in active orogens typically have nonlinear shapes resulting from a combination of heat transfer to adjacent basins and topography. Similarly, the geologic and paleotopographic setting between 17 and 13 Ma was one of a weakly deformed volcanic highland extending westward into the Pine Nut Range (Fig. 2B) punctuated by transverse east-west arches and downwarps, some occupied by west-flowing drainages. Mild flexural uplift seems preferable to the geologically instantaneous major uplift and west tilting required in order for the rigid tilt-block model to satisfy the invariance in the AFT data. Also, coeval large volumes of basement-clast detritus were not carried down the western dip slope to an adjacent basin, but volcanic-clast debris deposits consistent with mild uplift from 16 to 14 Ma are found in both the Coal Valley and Gray Hills areas.
Several structures in the Coal Valley area are consistent with a regional northwest-striking, dextral shear system within which north-south contraction was significant (Fig. 13A). These include the north-south–striking segments of the Coal Valley fault, normal faults in the Flying M extensional zone, and easterly trending contractional structures such as the Coal Valley syncline and reverse faults in the Morgan Ranch hinge zone. However, stress conditions of a dextral shear system fail to explain (1) coeval (and probably cogenetic) northerly trending shallow-axis monoclines and steep-axis bends, (2) basement-rock blocks popped up into previously west-tilted Miocene strata, (3) the family of north-northeast folds in northern Coal Valley, and (4) the northeast-striking normal faults that control the ragged geometry of the western Wassuk Range. To integrate these diverse structures with graben formation, we envision coupled processes of approximately east-west extension or transtension and extension-normal shortening associated with midcrustal flow. Although absolute strain magnitudes are not known, our downward revision of extension magnitude is important in the context of coupled strains of potentially subequal magnitude. If extension were extreme, the shortening could be viewed as a minor anomaly in an otherwise plane-strain extensional system. This does not seem to be the case.
We envision the mid- to lower upper crust to have been preconditioned for flow by 17–14 Ma igneous activity with stress imparted to the upper-crustal blocks by basal traction. The coupled traction-extension model we envision differs from that presented by Hardyman and Oldow (1991) and Oldow (1992) for the same region. In their model, the slip direction on a decoupling structure defines the “global displacement field” into which slip on upper-plate faults of any orientation is fed. Tractions across the decoupling surface control all motions within the deforming upper-plate body, which sum to the “global displacement field.” In our model, extension-normal flow at depth, possibly responding to laterally variable gravity gradients and guided by laterally variable strength contrasts, transmits tractional drag to the overlying rock that was undergoing regional extension (Fig. 13B). The basal traction we postulate reflects motion orthogonal to, and superposed on, regional extension, but it does not require extension. Instead of reflecting the sum of the incremental strain, the traction-related strain could vary from 0% to 100% of the incremental strain.
Laterally variable shortening could reflect a response to laterally variable velocity gradients in a mid- to lower upper-crustal flow regime. For example, in the northern Coal Valley area, we assume tractional drag above a zone of partial decoupling beneath which there was laterally variable (east-increasing), extension-normal (south-directed) distributed shear or ductile flow resulting in coeval extension and steep-axis bending (down-directed arrows, Fig. 13B). We further speculate that the pattern of monoclinal flexing in the Flying M extensional zone reflects smaller-scale, laterally variable distributed shear or ductile flow of substrate that forced flexural uplift by differential (again, east-increasing) inflation of individual fault blocks (up-directed arrows, Fig. 13B). The popped-up basement-rock blocks to the southeast could reflect similar inflation that occurred within a relatively high-velocity south-translating zone along the west margin of the Wassuk Range. As noted previously, the steep-axis folding south of the popped-up blocks could be a genetically related consequence of such translation. Structural crowding, indicated by the abrupt clockwise bending and local overturning of the Coal Valley Formation at the north margin of the eastern Flying M extensional zone and of all Wassuk Group Formations in the hinge of the bent Coal Valley fault east of Morgan Ranch, represents small-scale north-south contractions consistent with the extension-normal basal traction model. The small scale of these structures is difficult to reconcile with regional (uniform?) far-field stress and suggests control by near-field strength contrasts.
Relation to Plate-Boundary Strain
Two main questions to be addressed concerning the possible plate-boundary significance of Miocene events in the Coal Valley area are: (1) How does the history relate to the 26–14 Ma tectonics of the ancestral Walker Lane, and (2) how does it relate to the post–10 Ma tectonically linked Walker Lane belt–Eastern California shear zone (Dokka and Travis, 1990)? Dilles and Gans (1995) suggested that dextral shear on northwest-striking faults of the 26–14 Ma ancestral Walker Lane at ∼39°N latitude reflects plate-boundary dextral shear strain that was guided inland into previously extended and weakened crust of western Nevada. This suggestion is at odds with the location of the north end of the growing plate-boundary transform more than 800 km to the south at that time (Atwater and Stock, 1998). Also, the Coal Valley area is located southwest of the margin of the ancestral Walker Lane, and it only contains Tertiary rocks capable of recording the last 3 m.y. of that deformation (17–14 Ma). Those rocks record west-flowing major streams followed by basin sedimentation and igneous activity of intermediate composition, all in a setting of apparently weak north-south contraction. Connection with dextral faulting is possible, but not apparent. Although northwest-striking dextral faults are not recognized in those rocks in the Coal Valley area, there is a remote possibility that the small-magnitude north-south shortening that appears to have controlled sedimentation patterns during the 17–14 Ma interval could reflect strain accommodation beyond the ends of such faults buried beneath the Pine Grove Flat–Cambridge Hills–East Walker Valley areas (Figs. 1 and 3).
Currently, deformation in the Walker Lane belt–Eastern California shear zone absorbs a significant component (∼25%) of shear strain between the Pacific and North America plates (Argus and Gordon, 1991; Oldow, 1992, 2003; Dixon et al., 2000). The major phase of fault-related tilting and north-south shortening along the west margin of the Wassuk Range occurred during a brief time window between ca. 10 and 7.6 Ma, possibly coinciding with formation of the Walker Lane belt–Eastern California shear zone as an inboard part of the active Pacific–North American transform-style plate-tectonic regime (Dokka and Travis, 1990; Dokka, 1992; Stewart, 1992; Oldow,1992; Oldow et al., 1994, 2001; Unruh et al., 2003). Recent studies place the inboard stepping at ca. 3 Ma (Stockli et al., 2003). If it occurred at 10 Ma, major deformation in the Coal Valley area appears to have ended by 7.6 Ma, leaving an opportunity to record only the early 2.6 m.y. of that history. The strain we observe in the 13–7.6 Ma interval, and especially the strong heterogeneous strain of the 10–7.6 Ma interval, lacks an obvious association with northwest-striking dextral shear. The northwest-striking faults in the Flying M extensional zone are unlikely candidates for a southerly distribution of the northwest dextral faults of the northern and central Wassuk Range (Dilles, 1992; McIntyre, 1990; Surpless, 1999; Stockli et al., 2002) because they have moderate to gentle dips, are not throughgoing structures, and appear to be mainly normal faults. Nevertheless, as with the 17–13 Ma interval, if northwest-striking dextral faults exist in areas to the northwest of Coal Valley and left slip were identified on the West Wassuk fault zone (Figs. 1 and 3), the steep-axis bending, associated folding, and strain gradients illustrated in Figure 13 could record terminations, stepovers, or bends of such faults. Because we are unaware of evidence for such strike slip, we pursue an alternative model.
The large domain in which the Coal Valley area is located has an anomalously northerly instantaneous motion as determined from geodetic data (Fig. 2A). The north-south shortening we highlight in this report is referenced only to bordering ranges or transverse structural highs. We recognize the possibility that absolute motions relative to the stable interior of North America could be northerly or southerly. North displacements of the ranges and transverse structural highs could be greater than those of the Coal Valley graben, for example. Northerly translations could be coupled to the major north-northwest translation of the Sierra Nevada block since 10 Ma (Snow and Wernicke, 2000), but again, such coupling would likely require dextral strike-slip faults for which we have no evidence.
Regardless of their absolute motion, if the north-south shortening structures and strain gradients in the Coal Valley area (Fig. 13) are not a direct response to plate-boundary shear, an alternative driving force such as lateral gravity gradients is likely. Perhaps the driving force was gravitational instability associated with features analogous to the current high heat flow, relatively low-viscosity lower lithosphere, and the shallow asthenosphere along the western margin of the Basin and Range (Malservisi et al., 2001; Sonder and Jones, 1999). Western Basin and Range lithosphere under these conditions would tend to spread outward, toward the Sierra Nevada, which might act as a backstop that constrains outward flow and forces flow parallel to the margin of the orogens, analogous to that suggested for the easternmost Basin and Range–Colorado Plateau boundary in Utah (Anderson and Barnhard, 1992; Wannamaker et al., 2001).
It is unlikely that crustal-scale boundary conditions could shape the small-scale velocity gradients we postulate. Instead, strain localization is likely to reflect local strength contrasts. It is also unlikely that the vertical components of the strain field are a direct response to regional gravitational collapse because paleobotanical data suggest the Wassuk Group strata are currently at about the elevation at which they were deposited (Wolfe et al., 1997). Basement-rock blocks, including the pop-up blocks and the main Wassuk Range, were relatively uplifted. No association of the vertical strain with plate-boundary strain is apparent, so we assume that lateral contrasts in subjacent flow caused differential uplift by localized subjacent inflation.
In the Coal Valley area, rocks ranging from 17 to ca. 10 Ma lack large volumes of locally derived clastic debris or persistent angular unconformities, providing little evidence for major local tectonism during that interval. Gentle warping and arching on generally east-west trends seem to have dominated. Consistent with previous geologic studies, we recognize no structural, stratigraphic, or lithologic evidence for either major extension or major uplift in the Wassuk Range area beginning at 15 Ma, as is inferred based on thermochronologic data.
The west-tilted Coal Valley graben and its basement-rock shoulders reflect extension beginning at ca. 10 Ma. This deformation was soon accompanied by north-south contraction evidenced by structures as diverse as (1) the east-trending Coal Valley syncline and a syncline northwest of Morgan Ranch, (2) steep tilting and reverse faulting in a steep-axis hinge zone east of Morgan Ranch, (3) steep-axis bending of the Coal Valley fault, and (4) bending and related (?) pop-up deformation along the east margin of the Coal Valley graben. The vertical-axis rotations are based on curvature of fault traces, dikes, and tilted beds, and are testable using paleomagnetic methods. This array of heterogeneous structures is not expected in a graben-dominated extensional tectonic setting and is best explained by localized superposed strain states that include regional east-west extension and localized north-south shortening. Whether the localized strains reflect near- or far-field stress conditions is unclear. They apparently lack association with northwest-striking right-slip faults, making direct association with plate-boundary strain difficult to assess.
We suggest that traction forces were applied to the base of the deforming mass by north-south displacement beneath a decoupling zone, possibly a continuation of displacement that accompanied earlier warping during the 17–10 Ma interval. Our basal traction model stems from a default position following failure to identify a suitable alternative capable of accommodating the complex strain picture. Lateral displacement gradients responding to contrasts in strength beneath the zone of partial decoupling are the likely cause of extreme lateral variations in strain magnitude, particularly apparent in fault-parallel folds, steep-axis bends, and popped-up blocks of basement rock at the west margin of the Wassuk Range (Fig. 13B). Collectively, the anomalous and heterogeneous structures illustrate that modeling of extensional fault blocks, such as the Wassuk Range basement block and its Miocene cover strata, as internally uniform rigid tilt blocks needs to be revised to include flexure and out-of-plane motions. No single cross section can fully describe displacements in such three-dimensional strain fields (Weil and Sussman, 2005). Out-of-plane motions and convex-upward faults make quantitative assessments of extension by extension-parallel reconstruction of little value. We suggest that the total extension could be reduced to as little as 30%, rather than the 200% derived from previously published models. The lateral flow we suggest could reflect a tendency for orogen-parallel spreading in thermally weak lithosphere. It could be driven by stored gravitational potential energy or by the backstop resistance to flow caused by the Sierra Nevada.
This research was funded through the Minerals Program of the U.S. Geological Survey. We are grateful to Larry Snee for early interest in and work on the geochronology, to Keith Howard, David John, and Mike Petronis for reviews of an early manuscript, and to Sue Priest for help with the illustrations. Technical reviews by Nathan Niemi led to many improvements for which we are grateful, and it is important to note that he disagrees with our revised interpretation of extension magnitude, especially that based on our reinterpretation of thermochron data.