It is proposed that the regional pattern of early Miocene lowâ and highâangle normal faulting and dike swarm emplacement in the western and central Mojave Desert is the result of regional extension developed within a roughly east trending zone, named here the âMojave Extensional Beltâ. This zone of extension, although now disrupted by late Cenozoic rightâslip faulting, can be traced from the intersection of the San Andreas and Garlock faults to the eastern Mojave Desert near Bristol Lake and the Granite Mountains fault. Restoration of postâOligocene movement along the San Andreas fault system places the now buried western part of the Mojave Extensional Belt opposite a similar ageâextended area of west central California. Surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the Mojave Extensional Belt was lowâangle, normal sense, simple shear (detachment faulting). Strain within the Mojave Extensional Belt is partitioned between four domains (Edwards, Waterman, Daggett, and Bullion terranes) that each consist of one or more halfâgrabens. Each halfâgraben is composed of tilted, normal faultâbounded blocks that lie above a rooted, lowâangle, brittleâductile normal sense shear zone. Tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping (âŒ80°) planar to slightly curviplanar normal faults. At least one generation of highâangle faults postdates the formation of the detachments. Differential extension between the domains was accommodated by strikeâslip faults (transfer zones). Similarly, the lack of early Miocene extensional elements north of the western segment of the Garlock fault suggests that the Garlock may have been active in the early Miocene and served as the northern transfer boundary of the Mojave Extensional Belt. The Mojave Extensional Belt initially began to open about 22 m.y. ago, with the major phase of extension occurring between âŒ22 and 20 Ma. Local, highâangle, normal faults cut older detachments and were active between âŒ20 and 17 Ma. Kinematic indicators suggest that the western part of the Mojave Extensional Belt opened in a NEâSW direction, whereas the eastern portion extended in an ENEâWSW direction (presentâday reference frame). Rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the Mojave Extensional Belt was originally oriented âŒNâS and was rotated as much as 50° (clockwise) between âŒ20 and 18 Ma (Early Miocene).
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