Pull-apart basins at releasing bends of the sinistral Late Jurassic Mojave-Sonora fault system
Thomas H. Anderson, Jonathan A. Nourse, 2005. "Pull-apart basins at releasing bends of the sinistral Late Jurassic Mojave-Sonora fault system", The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, Thomas H. Anderson, Jonathan A. Nourse, James W. McKee, Maureen B. Steiner
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A 200–500-km-wide belt along the southwestern margin of cratonic North America is pervaded by northwest- and east-trending faults that flank basins containing thick deposits of locally derived conglomerate and sedimentary breccia. These deposits that crop out mainly in the northern part of mainland Mexico, or southern parts of Arizona and New Mexico are unconformable at their bases, have similar Upper Jurassic and/or Lower Cretaceous stratigraphic ages, and commonly preserve volcanic components in the lower parts of upward-fining sections. We argue that these basins share a common structural origin, based on: (1) the presence of faults, locally preserved, that generally define the basin margins, (2) similar basal units comprised of coarse conglomeratic strata derived from adjacent basement, and (3) locally preserved syntectonic relationships to bounding faults. Fault orientations, and our observation that the faults (and their associated basins) extend south to the inferred trace of the Late Jurassic Mojave-Sonora megashear, suggest that the basins formed in response to transtension associated with sinistral movement along the megashear. Northwest-striking left-lateral strike-slip faults that terminate at east-striking normal faults define releasing left steps at which crustal pull-apart structures formed. These faults, plus a less-developed set of northeast-striking right-lateral faults, appear to comprise a cogenetic system that is kinematically linked with the Mojave-Sonora megashear; that is, the maximum principal stress trends east and the plane containing maximum sinistral shear stress strikes northwesterly.
Late Jurassic faults northeast of the Mojave-Sonora megashear controlled the regional distribution of the pull-apart basins and influenced the orientation and style of many younger structures and intrusions. Most Late Jurassic faults were modified during subsequent episodes of deformation. N60°E-directed contraction during the Late Cretaceous (Laramide) orogeny reactivated older east-striking normal faults as sinistral strike-slip faults; northwest-striking sinistral faults were reactivated as steep reverse faults. Some stratigraphically low units were thrust across basin margins as a result of inversion. Many of the pull-apart basins encompass outcrops of Late Jurassic igneous rocks and/or mineralized Laramide or Tertiary plutons. Some northwesterly faults appear to have influenced the position of breakaway zones for early Miocene detachment faults. Despite the common and locally strong structural and magmatic overprinting, remnants of the Late Jurassic faults are recognizable.