Stephen J. Reynolds

Structural evolution of the Whipple and South mountains shear zones, southwestern United States.

The Whipple and South mountains of the southwestern United States have undergone a strikingly similar sequence of deformations. In both ranges, gently dipping mylonitic fabrics have been overprinted by successively more brittle structures associated with a low-angle detachment fault. Kinematic indicators reveal that the mylonitic rocks, brittle structures, and detachment faults are kinematically coordinated and were all formed by top-to-the-northeast shear. The structural evolution of both areas can be explained in terms of major, shallow-dipping shear zones that accommodated Tertiary crustal extension. We suggest that detachment faults and associated zones of brecciation, cataclasis, and seismic slip were originally continuous downdip along the low-angle shear zones into mylonitic gneisses formed below or near the ductile-brittle transition. As the mylonites were drawn out from beneath the brittlely extending upper plate, they were progressively uplifted above the ductile-brittle transition and were overprinted by successively more brittle structures.

Structural aspects of fluid-rock interactions in detachment zones

Metamorphic core complexes and associated detachment faults of the North American Cordillera represent gently dipping, normal-displacement shear zones (detachment zones) along which hot, deeper levels of the crust were transported upward and outward from underneath a brittlely distended upper plate. Structures that formed during the ductile-to-brittle evolution of detachment zones can be used to reconstruct the relative magnitudes of fluid pressure and deviatoric stress at different levels within the shear zones. Mylonitization, which occurred along deeper segments of detachment zones below the brittle-ductile transition, was locally accompanied by tensile failure, indicative of low deviatoric stress and high fluid pressure. This condition persisted into the earliest phases of brittle deformation, after which shear fractures formed due to both a reduction in fluid pressure and an increase in deviatoric stress. Brittle deformation of upper-plate rocks occurred largely under conditions of low fluid pressure. Structural and geochemical data suggest that normal displacement on detachment zones results in establishment of two fluid systems: (1) an upper-plate system driven by convection and dominated by meteoric and connate fluids at near-hydrostatic pressures and (2) a system within deeper levels of the shear zone, where fluids are largely derived from igneous sources and fluid migration is aided by dilatancy pumping. The late phases of normal displacement on detachment zones structurally juxtaposed rocks affected by the two fluid systems and locally caused the shear-zone rocks to be overprinted by mineralization related to the upper-plate fluid system.

Denudation of metamorphic core complexes and the reconstruction of the transition zone, West central Arizona: Constraints from apatite fission track thermochronology

Apatite fission track thermochronologic results from transects across the Basin and Range and Transition Zone provinces in west central Arizona provide constraints on the denudational history and structural framework of the region. Apatite fission track ages decrease from ∼21 to 14 Ma in the Harcuvar Mountains and from ∼16 to 13 Ma in the Buckskin-Rawhide Mountains in the slip direction (SW - NE) of detachment faults in the lower plates of the metamorphic core complexes. Mean lengths of confined fission tracks from the core complexes are all >14 μm, indicating that the apparent apatite ages record rapid cooling through the apatite partial annealing zone ( 40°C/m.y. for lower plate rocks. Apparent apatite ages in the Transition Zone province generally increase from ∼25 Ma to ∼100 Ma away from the Basin and Range province. This trend of increasing apatite age is disrupted by faulting as many as seven times at the fronts of major mountain ranges and within valleys between the Weaver Mountains and the Colorado Plateau. Gradients of apatite fission track age and confined track length with elevation in the mountain ranges and in the Phillips-Kirkland drill hole reveal parts of denuded Mesozoic and Cenozoic apatite partial annealing zones. These paleopartial annealing zone profiles provide a reference datum for preextension reconstructions of fault blocks. The reconstructions indicate that the major faults in the Transition Zone province have relative displacements of >1 km and that offset on them occurred mostly after 25 Ma. These data also indicate that most of the rocks now exposed in the Transition Zone of west central Arizona were not exposed until Miocene time.

Tectonics of Mid-Tertiary Extension along a transect through west central Arizona

Large-magnitude Miocene extension in west central Arizona occurred primarily along three imbricate, northeast dipping normal faults. The structurally highest of these faults, the gently dipping Buckskin-Rawhide detachment fault, accommodated approximately 66 km of crustal extension, whereas the two structurally lower faults accommodated a total of about 20 km extension. Due to this large-magnitude extension, an area at the Earths surface that was 10 to 20 km wide is now over a 100 km wide, and crystalline rocks with mid-Tertiary mylonitic fabrics, uncovered by detachment faulting, are exposed over roughly 2000 km² in the Harcuvar metamorphic core complex. Most of the upper plate of the Buckskin-Rawhide detachment fault was largely undeformed by internal extension; only the thin, tapered end of the upper plate was highly extended. During extension the lower plate must have flexed to conform to the listric underside of the upper plate and to have flattened to its present subhorizontal form as it was uncovered. Grooves on the underside of the upper plate were apparently imposed on the pliable lower plate as it was denuded, forming extension-parallel folds in the lower plate. Low flexural strength characterized the lower plate during denudation, and a highly mobile, low-viscosity deeper crust must have effectively decoupled the upper crust from the mantle lithosphere.

Evidence for large-scale transport on the Bullard detachment fault, west-central Arizona

The Bullard detachment fault is a gently to steeply dipping normal fault that flanks the Harcuvar and Harquahala mountains of the Basin and Range province in west-central Arizona. The stratigraphy of upper-plate Miocene conglomerates and the regional distribution of upper- and lower-plate pre-Tertiary units indicate that upper-plate rocks were displaced about 50 km to the northeast with respect to the lower plate during middle to late Tertiary time. Normal slip of this magnitude on the regionally northeast-dipping Bullard fault indicates that deep-seated Tertiary-Cretaceous(?) mylonitic gneisses and Mesozoic thrust faults of the lower plate were drawn out from beneath Precambrian rocks along the margin of the Transition Zone of central Arizona during middle to late Tertiary crustal extension. The Transition Zone was, therefore, affected by deep-seated tectonism in both Mesozoic and Tertiary time.

Folding of mylonitic zones in Cordilleran metamorphic core complexes: Evidence from near the mylonitic front

Metamorphic core complexes of North America are asymmetric when viewed perpendicular to the direction of tectonic transport. On one side, mylonitic fabrics and the overlying detachment fault dip gently in the direction of upper-plate transport, and the mylonitic rocks have a down-dip sense of shear. On the opposite side, the mylonitic zone rolls over to a dip opposed to the direction of upper-plate transport, diverges downward from the detachment fault, and dies out upward across a mylonitic front. New kinematic data show that the backdipping mylonitic zone in most complexes is a downfolded continuation of the main mylonitic zone. Folding of the mylonitic zone was accompanied by the formation of antithetic shear zones and occurred during the late stages of mylonitization.

The role of visualization in learning from computer‐based images

Among the sciences, the practice of geology is especially visual. To assess the role of spatial ability in learning geology, we designed an experiment using: (1) web‐based versions of spatial visualization tests, (2) a geospatial test, and (3) multimedia instructional modules built around QuickTime Virtual Reality movies. Students in control and experimental sections were administered measures of spatial orientation and visualization, as well as a content‐based geospatial examination. All subjects improved significantly in their scores on spatial visualization and the geospatial examination. There was no change in their scores on spatial orientation. A three‐way analysis of variance, with the geospatial examination as the dependent variable, revealed significant main effects favoring the experimental group and a significant interaction between treatment and gender. These results demonstrate that spatial ability can be improved through instruction, that learning of geological content will improve as a result, and that differences in performance between the genders can be eliminated.

Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre-Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large-magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

K-metasomatism and detachment-related mineralization, Harcuvar Mountains, Arizona

The Bullard detachment fault, a regional low-angle normal fault exposed in the Harcuvar Mountains of west-central Arizona, separates lower-plate mylonitic rocks and chloritic breccia from upper-plate volcanic and sedimentary rocks. Areally extensive K-metasomatism has converted upper-plate mafic flows and felsic ash-flow tuffs into rocks with 8 to 12 wt. % K 2 O, 2 O, and a simple K-feldspar-hematite-quartz mineralogy. The secondary K-feldspar is very pure (Or 95 to 99.5), monoclinic, and structurally similar to orthoclase. Differences in δ 18 O values between secondary K-feldspar replacing sanidine phenocrysts (9‰ to 11‰) and K-feldspar replacing groundmass (11‰ to 14‰) in the tuffs imply differential O-exchange with migrating fluids. Whole-rock δ 18 O values for tuffs (10‰ to 14‰) and mafic flows (6‰ to 9‰) do not, therefore, represent primary igneous values. The rocks apparently became K-metasomatized and 18 O-enriched while interacting with low- to moderate-temperature, neutral to alkaline, oxidizing brines that accumulated in an extensional basin above the detachment fault. Cu-Au-Ag mineralization is concentrated in faults and fissures in metasomatized mafic flows above the detachment fault. Fluid-inclusion studies show that mineralizing fluids had minimum temperatures of 290 to 330 °C along the detachment fault, 240 to 290 °C in mafic flows above the detachment fault, and 100 to 130 °C in barite-calcite-Mn-oxide veins farther from the detachment fault. The predominant mineralizing fluids near the detachment fault and in the mafic flows were saline brines with 13 to 17 equivalent wt.% NaCI More dilute brines with 6 to 12 equivalent wt. % NaCI formed the barite-calcite-Mn-oxide veins. Inferred δ 18 O values of the mineralizing fluids range from +3‰ for high-temperature quartz-sulfide veins to -5‰ for lower-temperature barite-calcite-Mn-oxide veins. The high salinities, oxygen isotope compositions, and geologic setting indicate that the mineralizing fluids were basinal brines. The mineralizing fluids apparently evolved from early deep-level, reducing basin brines to a later stage marked by the influx of higher-level, oxidizing basin brines. Relatively minor amounts of less evolved, lower- 18 O meteoric water entered the system during the very late stages of mineralization. Paragenetic relations and geochemical and isotopic data indicate that mineralization was superimposed on previously K-metasomatized rocks. Mineralization and K-metasomatism may be indirectly linked, however, because both occurred during detachment faulting and both involved basinal brines. Specifically, K-metasomatism of upper-plate units liberated elements, such as Cu, Pb, Zn, Mn, Sr, and Na, that were incorporated into the mineralizing basin brines. Multiple fluid regimes during detachment faulting are indicated, because basin-brine-dominated mineralization overprinted lower-plate mylonitic rocks and breccia that had probably previously equilibrated with igneous or metamorphic fluids at deeper levels of the detachment system.

Origin and Paleogeography of an Immense, Nonmarine Miocene Salt Deposit in the Basin and Range (Western USA)

The Hualapai basin, northwestern Arizona, contains one of the thickest known, nonmarine halite deposits in a continental rift. The basin is a large half-graben in the hanging wall of a listric normal fault along the eastern margin of the Basin and Range province. Seismic reflection and drill-hole data indicate that the little-deformed halite is 2.5 km thick in the central part of the basin, approaches