A paleogeographic reconstruction of southeastern California and southwestern Arizona at 10 Ma has been made based on available geologic and geophysical data. Clockwise rotation of 39° has been reconstructed in the eastern Transverse Ranges, consistent with paleomagnetic data from late Miocene volcanic rocks, and with slip estimates for left‐lateral faults within the eastern Transverse Ranges and NW‐trending right‐lateral faults in the Mojave Desert. This domain of rotated rocks is bounded by the Pinto Mountain fault on the north. The model requires that the western part of the late Miocene Pinto Mountain fault was a thrust fault, which gained displacement to the west, because of the absence of evidence for rotation of the San Bernardino Mountains or for significant right slip faults within the San Bernardino Mountains. The Squaw Peak thrust system of Meisling and Weldon (1989) may be a western continuation of this fault system. The Sheep Hole fault bounds the rotating domain on the east. East of this fault an array of NW‐trending right slip faults and south‐trending extensional transfer zones has produced a basin and range physiography while accumulating up to 16 km of right slip. This maximum is significantly less than the 37.5 km of right slip required in this region by a recent reconstruction of the central Mojave Desert (Dokka and Travis, 1990a). Geologic relations along the southern boundary of the rotating domain are poorly known, but this boundary is interpreted to involve a series of curved strike slip faults and linking normal faults that accommodated noncoaxial extension. Quaternary movement on the Pinto Mountain and nearby faults is unrelated to the rotation of the eastern Transverse Ranges, and a hiatus during part of Pliocene time followed the deformation which produced the rotation. The reconstructed Clemens Well fault in the Orocopia Mountains, proposed as a major early Miocene strand of the San Andreas fault, projects eastward towards Arizona, where early Miocene rocks and structures are continuous across its trace, making large displacements on this structure unlikely. The model predicts a 14° clockwise rotation and 32% extension during late Miocene and early Pliocene time along a NW‐trending line parallel to the present trace of the San Andreas fault. Palinspastic reconstructions of the San Andreas system based on this proposed reconstruction may be significantly modified from current models.
The Harquahala and Buckskin mountains lie in the footwall of the Whipple‐Buckskin‐Bullard detachment system. In the Harquahala Mountains, Mesozoic fabric and structure are progressively more intensely overprinted by penetrative Tertiary deformation toward the northeastern pan of the range. Tertiary mylonitic deformation is recognized by the presence of deformed Miocene mafic dikes and characteristic textural features. Lineations in the mylonite trend 040°–060°, and megascopic kinematic indicators mostly indicate top‐to‐the‐northeast sense of shear. The crest of an antiform defined by Tertiary mylonitic fabric does not coincide with the topographic crest of the range. Reorientation of Mesozoic foliation in the vicinity of this antiform suggests that it originated as two shear zones, as opposed to a single zone that was then bent. The northeastern part of the Brown's Canyon granite acts as a large, low‐strain lozenge in the southeast limb of the foliation arch. Mylonitic foliation along the southeast side of the range, above this lozenge, is strongly oblique to the trend of the detachment fault, but the relative orientation is consistent with left‐oblique normal shear. Evidence of Tertiary plastic deformation is absent southwest of Sunset Pass. Minor low‐angle normal faults in the southwestern Harquahala Mountains and Little Harquahala Mountains dip northeast The low‐angle normal faults are cut by NW trending high‐angle, right‐oblique faults. In the central and southwestern Harquahala Mountains, 40Ar–39Ar age spectra from K‐feldspar, muscovite, and hornblende and total gas ages from biotite indicate Late Cretaceous to early Tertiary cooling to argon closure temperatures. Biotite and K‐feldspar from the area northeast of Sunset Pass record rapid early to middle Miocene cooling. Reconstruction of the original geometry of the detachment system, based on thermal differences indicated by contrasting cooling histories, and the orientation of an early Miocene dike swarm indicate that the initial dip of the detachment fault was most probably between 30° and 40°. Thus the Harquahala Mountains are a lilted block exposing of the order of a 10‐km section through the pre‐Tertiary crust. Heterogeneous Proterozoic gneiss, sparse Paleozoic and Mesozoic metasedimentary rocks, and an Oligocene plutonic complex are extensively overprinted by Tertiary mylonitic fabrics in the Buckskin Mountains. Hornblende 40Ar–39Ar cooling ages suggest that most of these rocks were below hornblende closure temperature by early Tertiary time, except in the vicinity of the Oligocène plutonic complex. Feldspar and biotite 40Ar–39Ar cooling ages suggest that the footwall of the Whipple detachment system experienced a more uniform cooling history in the Buckskin Mountains than in the Harquahala Mountains; cooling ages between about 13 and 20 Ma are recorded throughout the range with no consistent spatial pattern of ages. The Eagle Eye detachment fault, southeasternmost extension of the Whipple‐Buckskin‐Bullard fault system, becomes a transfer fault along ...
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.
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