R. Ernest Anderson

Thin Skin Distension in Tertiary Rocks of Southeastern Nevada: Reply

Volcanic rocks of late Tertiary age, aggregating about 17,000 ft, accumulated on a surface of low relief cut on Precambrian rocks in the Basin and Range province south of Lake Mead, in Nevada and Arizona. They consist mostly of lava and flow breccia of intermediate composition with minor ash-flow tuff, bedded tuff, and lava of rhyolitic composition. The last of three main phases of volcanism was accompanied by widespread epizonal plutonism and intense faulting. All or parts of six similarly but separately fault-deformed structural units are recognized in a 92-sq-mi mapped area. The structural units are highly distended by a system of closely spaced north-to northwest-striking shingling normal faults (many of which are low angle) that displace younger over older rocks in a west to west-southwest direction. Cumulative amounts of distension approximate the breadth of the structural units and are as much as 20,000 ft, whereas cumulative vertical displacements are much less and in some places are minimal. The structural units are floored at or near the present level of exposure by complex low-angle zones of detachment or decollement into which the numerous shingling normal faults merge. Where the units abut along their strike, they are separated by complex zones of transcurrent faults that appear to merge with the detachment structures and thus mark the ultimate limits of the structural units. Displacement on the detachment structures has the same sense as, but in some places is much greater than, that of the cumulative offset on the shingling faults, thus indicating low-angle movement of the structural units as platelike or lobate masses. These relationships indicate remarkably thin-skinned, large-scale, fault-related tectonism of a type which is present in a broad belt south of Lake Mead and in numerous other areas in the Basin and Range province. The best exposed structural units exhibit a serial eastward progression from broad areas of steeply dipping strata, low-angle faults, and deep denudation to gently dipping strata, high-angle faults, and little denudation. Reverse-drag flexing, a volume-compensating mechanism for movement on concave-upward faults, is inferred to have produced the gentle to moderate dips of the strata, whereas the nearly vertical dips in the western parts of the units probably resulted from a combination of reverse-drag flexing and rotation related to uplift. Evidence of compression-related folding is absent. The extreme distension is viewed as a surficial feature of a crustal belt that was subjected to a brief episode of tensional rifting. Rifting at subjacent levels along the belt was compensated for by emplacement of plutons. The surficial rocks were stretched and thinned over the plutons.

Neogene paleostress changes in the Basin and Range: A case study at Hoover Dam, Nevada-Arizona

At Hoover Dam, 40 km southeast of Las Vegas, Nevada, well-exposed, highly faulted Miocene rocks provide an excellent opportunity to study the paleostress history of a very small area within a region where previous geologic studies indicate clockwise rotation of paleostress and a nearness to a major strike-slip fault-zone boundary. Within <0.5 km3 of rock, the sense of slip was determined on almost 500 separate faults. The fault-slip data show internal consistency with respect to lithology, size of faults, and location within the small area. With respect to fault slip, however, the data represent an in-homogeneous mixture of primarily strike-slip and dip-slip motions. From this mixture, it is possible to resolve two distinct stress fields with directions of extension that differ by ∼60°. Each stress field corresponds to a mixture of strike-slip and dip-slip faults, and thus the orientations of σ 1 and σ 2 are not tightly constrained. If, however, the data are first separated into strike-slip and dip-slip faulting modes and subsequently searched for sub-populations that correspond to contrasting paleostress orientations, the computations yield tightly constrained tensors that illustrate two distinct stress fields with subhorizontal σ 3 axes that trend N50°E and N75°W. The σ 1 and σ 2 axes permutate in two vertical planes that strike N40°W and N15°E, because σ 1 and σ 2 are close in value, relative to σ 3. These relationships suggest that strike-slip and dip-slip faulting belong to the same tectonic regime. Qualitative observations of polyphase slip, fault-fault offsets, and fault-bedding geometric relationships, when evaluated in the context of changes in σ 3 orientation and permutations of σ 1 and σ 2, provide a basis for a two-stage, late Cenozoic structural evolution at Hoover Dam. These stages are (1) strike-slip faulting (partly pre-tilt) and dip-slip faulting and associated stratal tilting and (2) mostly post-tilt, complexly interrelated strike-slip, oblique-slip, and dip-slip faulting. The qualitative evaluations indicate that strike-slip and dip-slip faulting alternated in time during the second stage of deformation and may have done so during the first. These mixed-mode movements probably represent stress oscillations in time and space rather than discrete stress reorganizations. In contrast, the two different orientations of σ 3 either represent a major clockwise rotation of the stress field or a major counterclockwise rotation of the rocks during the faulting history. Each alternative is consistent with regional geologic relationships, and the choice of which is correct cannot be made within the small area that was studied.

Significance of K-Ar Ages of Tertiary Rocks from the Lake Mead Region, Nevada-Arizona

K-Ar ages for 43 igneous and tuffaceous rocks of the Lake Mead region, Nevada-Arizona, are reasonably consistent with mapped stratigraphic and structural relations. They serve to establish age ranges for episodes of Miocene and Pliocene volcanism, plutonism, and tectonism in the region. The late Tertiary geology of the northwestern part of the region contrasts sharply with that of the southeastern part. The northwestern part was characterized by active sedimentation in local basins followed by intense disruption by normal and transcurrent faults. The southeastern part was characterized by brief episodes of overlapping volcanism, plutonism, extreme tensional rifting, followed by severe local uplift. Although diverse in style, the tectonism of the two areas appears to have been largely synchronous. The synchronous character of the brief igneous and tectonic processes suggests strong genetic ties between them. The southeastern part of the region may be a volcanic rift zone that originated as a zone of tension near the end of a major transcurrent fault—the Las Vegas shear zone.

A methodology for probabilistic fault displacement hazard analysis (PFDHA)

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

Roles of plutonism, midcrustal flow, tectonic rafting, and horizontal collapse in shaping the Miocene strain field of the Lake Mead area, Nevada and Arizona

The Lake Mead area contains a well-exposed record of highly contrasting Neogene structural development that includes major magmatism and extension in its southern part and large-magnitude lateral translations (to 65 km) of amagmatic structural blocks in its northern part. These highly contrasting parts are joined at the Hamblin Bay fault, a major crustal boundary that forms the south margin of the Lake Mead fault system and the north margin of the lower Colorado River extensional corridor. Intrusive rocks adjacent to this crustal boundary on the south record major crustal rifting normal to an 350° axis. They also display steep-axis disharmonic folds that formed by post-magmatic extension-related horizontal collapse of highly weakened crust. The rocks directly north of the crustal boundary not only display large lateral translations, but they also are cut by fault systems suggestive of diverse paleostress conditions and formed into diversely oriented coeval folds collectively producing a complex Neogene deformation field that defies interpretation in terms of a uniform system of remote stresses. Because structures forming this complex deformation field range from outcrop scale to 65 km of lateral displacement and can not be uniformly partitioned according to relative age, we prefer tectonic models based on a protracted uniform dynamic process or super-posed dynamic processes to models based on time-varying states of stress. We suggest that the large lateral displacements and extreme structural complexity record synextensional rafting of structural blocks atop a flowing undermass and concomitant contraction of the zone of flowage. Structural blocks north of the main zone of flowage (the Great Basin sector of the Basin and Range) moved southward and occluded to the block south of the zone (the Colorado River extensional corridor) as a large west-ward widening wedge of upper crust was carried to the west–southwest toward California on the flowing undermass. Where occlusion is almost complete in the northern Black Mountains, north–south shortening is estimated at 55 km. The narrow zone of strong rifting, plutonism, crustal softening, and horizontal collapse that developed at the north margin of the Colorado River extensional corridor is interpreted to result from contact with the zone of crustal flowage.

Holocene faulting in the western Grand Canyon, Arizona: Discussion and reply

No Abstract Available. The article discussed was published in the Bulletin , v. 88, p. 1619–1622.

Timing, magnitude, and style of Miocene deformation, west-central Walker Lane belt, Nevada

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.

Discussion and reply: Variation in displacement along strike of the South Virgin−White Hills detachment fault: Perspective from the northern White Hills, northwestern ArizonaDiscussion

[Duebendorfer and Sharp (1998)][1] interpreted three faults in the Gold Butte−White Hills area, Arizona, as forming a 55-km-long detachment fault, the South Virgin−White Hills detachment fault. They suggested that the fault might represent a breakaway for extensional allochthons located to the