The Trans Mojave-Sierran shear zone and its role in Early Miocene collapse of southwestern North America

Abstract

The relative motion between the Pacific and North American plates in early Miocene time was not parallel to the overall NW strike of the transform, but was instead oblique and transtensional. It has recently been proposed that in response to this divergence, the western edge of the North American plate east of the transform gravitationally collapsed and moved 100-150 km to the southwest ($50~176The region of collapse covered an area of nearly 106 km 2 and included what is now southern California, southwestern Arizona, and northwestern Mexico. A major structure facilitating collapse between 21 and 18 Ma was the Trans Mojave-Sierran shear zone (TMSSZ). This east-west shear zone linked the classic detachment fault terranes and metamorphic core complexes of the Mojave desert, southeastern California, southern Arizona, and Sonora, Mexico, to the transtensional plate boundary.To more fully understand the nature and kinematic significance of the TMSSZ and its role in facilitating early Miocene fragmentation of the North American plate, a palinspastic reconstruction of the Mojave desert was performed to remove the disruptive effects of the TMSSZ and younger tectonic events. Features formed just before movements along the TMSSZ were used as markers to assess the TMSSZ deformation. Our analysis indicates that TMSSZ deformation was distributed across a c. 90 km wide band; restoration of markers to their original positions implies that >80 km of dextral shear occurred along the TMSSZ. First-order dextral shear deformation within the TMSSZ is expressed internally by clockwise vertical axis rotations of large areas that were facilitated by second-order zones of sinistral shear that separated the blocks. These second-order sinistral zones apparently exploited older transfer zones of the 24-21 Ma Mojave Extensional Belt.

Cenozoic evolution of the Mojave block of southern California

“…A modest rotation of 25Њ clockwise is consistent with most models of strike-slip deformation in the Mojave block (see Schermer et al, 1996, for discussion). Schermer et al found no evidence for counterclockwise rotations near left-lateral faults that accommodate rotation within the larger subblock, as predicted by the Dokka et al (1998) hypothesis.…”

“…However, such studies in the Mojave Desert region have produced a bewildering variation in paleomagnetic declinations, with studies in neighboring areas commonly giving contradictory results, and studies in the same area yielding both clockwise and counterclockwise declination anomalies over brief stratigraphic intervals (e.g., Valentine et al, 1993;Dokka et al, 1998).…”

“…Bidirectional declination anomalies occur in some areas. Valentine et al (1993) and Dokka et al (1998) argued that such anomalies represent vertical-axis rotations caused by drag along, or decoupled rotation between, faults that are inferred to separate subblocks with different anomalies. This interpretation is possible, but structural and stratigraphic complexities render it debatable and current data are inadequate to test it.…”

“…For example, many of the units sampled by these authors have been affected by intense noncylindrical folding, which makes standard structural corrections inadequate Walker et al, 1990). The Kramer Hills study reported by Dokka et al (1998), for example, was performed in rocks that were strongly folded, locally isoclinally, in the late Cenozoic ( Fig. 4; Linn et al, this volume, Chapter 10).…”

Vertical strain partitioning in hot Variscan crust: Syn-convergence escape of the Pyrenees in the Iberian-Armorican syntax

“…Under brittle conditions, strain is partitioned between strike-slip faults and normal or reverse faults for transtension and transpression, respectively (Tikoff and Teyssier, 1994;Allen et al, 1998;Dokka et al, 1998). Under plastic or ductile regime, strain may be partitioned among shear zones with variable orientations and slip, on the one hand, and lower strain domains of homogeneous strain, on another hand (Gapais et al, 1987;Holdsworth and Strachan, 1991;Holdsworth et al, 2002).…”