Cenozoic normal fault mosaics bounded beneath by a basal fault in the Basin and Range Province (BRP) have traditionally been described either in terms of large-scale surficial gravity sliding or by some form of in situ lower plate accommodation. I suggest here that these areas may be an extensional analogue to thin-skinned compressional orogens, a process which may even dominate active BRP tectonics.

Low-angle normal faults in the Basin and Range Province: nappe tectonics in an extending orogen

https://stuff.mit.edu/afs/athena.mit.edu/course/12/12.115/www/lister%201989%20the%20origin%20of%20metamorphic%20core%20complexes%20and%20detachment%20faults%20formed%20during%20tertiary%20continental%20extension%20in%20the%20northern%20colorado%20river%20region.PDF

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Modes of extensional tectonics

Cordilleran metamorphic core complexes form a belt of uplifted metamorphic rock that extends from southern Canada to northwestern Mexico just west of, or astride, the foreland thrust belt of the North American Cordillera. During the past several years the age and tectonic significance of the core complexes have been a topic of considerable controversy. Some geologists view the complexes as an uplifted erogenic core zone that formed behind the thrust belts mainly during Mesozoic regional compression. An opposing view is that they are mainly Tertiary in age and of extensional origin. We support a model that unifies these seemingly inconsistent views by suggesting that Mesozoic crustal telescoping resulted in an overthickened plateaulike crustal welt along the Cordilleran hinterland. During Cenozoic time this gravitationally unstable mass spread laterally, resulting in deep-seated crustal extension. The extension was aided by a thermal pulse of Cenozoic magmatism that reduced crustal viscosity and by a lowering of intraplate convergent stress fields due to changing plate kinematics. The chief advantage of the model is that it reconciles opposing views as to the age and tectonic significance of the complexes and places them in a more comprehensible setting amid Mesozoic-Cenozoic Cordilleran thermotectonic history.

Cordilleran metamorphic core complexes: Cenozoic extensional relics of Mesozoic compression

Summary Extensional tectonism produces characteristic half-graben/tilt-block systems whose facies mosaics are influenced by tectonically induced slopes resulting from hanging wall downtilting and footwall uplift. The characteristic asymmetrical subsidence vectors that therefore develop across the graben also exert a fundamental control upon facies distributions. A number of predictive tectono-sedimentary facies models are presented in which these various influences are explored. Alluvial fans and cones react to tilting by becoming segmented, those in the hanging wall showing down-dip hanging wall off-lap and those sourced in the footwall showing progradation from the apex. Lake and coastal waters react instantly to tilting, causing transgression and seiche-induced erosion. Axial through-flowing river channels and delta lobes tend to migrate or avulse towards the axis of maximum subsidence but may be constrained by the toes of footwall-sourced fans. Peat accumulation or soil development are accentuated up the hanging wall dip slope away from the locus of maximum deposition. In coastal areas, fan deltas sourced in the footwall pass offshore into small submarine fans whilst axial fans issue from delta fronts where individual fan lobes may migrate under fault control. In carbonate provinces the footwall scarp may become a bypass margin whilst the hanging wall dip slope may undergo a ramp-to-rimmed shelf evolution with time.

Sedimentary models for extensional tilt-block/half-graben basins

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.

Thin Skin Distension in Tertiary Rocks of Southeastern Nevada

Implications of selected subsurface data on the structural form and evolution of some basins in the northern Basin and Range province, Nevada and Utah

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.

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

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

Two normal fault systems are present in Nevada Test Site and Nellis Air Force Range. The earlier system consists of two sets that strike northeast and northwest, and the later system, a single set, strikes north. In the southern Kawich and Belted Ranges the Belted Range Tuff, dated at 13 to 14 m.y. and faulted only by the north-trending faults, laps onto older volcanic rocks that are broken by both fault systems. The older faults probably started to form during or shortly after the extrusion of the oldest welded tuff dated at 26.5 m.y. (age dates by R. W. Kistler, written commun., 1964), as is evidenced by essentially the same fault densities and displacements in preTertiary rocks as in Tertiary rocks. The north-trending faults first began to appear after the deposition of the Fraction Breccia, dated at 17.8 my, and the present mountain ranges were well defined prior to extrusion of the Thirsty Canyon Tuff dated at 7 m.y. In the southern part of the Bombing and Gunnery Range near the Las . Vegas Valley shear zone, north-trending faults that are inferred to be the same age as those to the north change strike gradually from north to northeast as the valley is approached from the north. This rotation, interpreted as being caused by drag during later right-lateral movement rather than a change in the stress field, suggests that much of the displacement along the shear zone has taken place within the last 17 my

R. Ernest Anderson

Papers

Thin Skin Distension in Tertiary Rocks of Southeastern Nevada

Implications of selected subsurface data on the structural form and evolution of some basins in the northern Basin and Range province, Nevada and Utah

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

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

Age of Basin and Range Normal Faults in Nevada Test Site and Nellis Air Force Range, Nevada