Abstract

Plate reconstructions indicate that early Miocene Pacific–North American relative motion was not parallel to the transform, but was instead oblique and transtensional. We propose that the western edge of the North American plate east of the transform gravitationally collapsed to the southwest in response to this divergence. The northern and eastern edges of the collapsed region underwent extension between 24 and 20 Ma. Whereas extension in southeastern California and southern Arizona during this interval was coaxial with the overall S50°–60°W collapse direction, the extension direction along the northern oundary (Mojave extensional belt) was north-south and the result of the combined effects of the southwestward collapse and the independent northwestward translation of the region to the north. Collapse continued between 20 and 16 Ma, and extension persisted in southeastern California and southern Arizona; extension along the northern boundary waned owing to the apparent cessation of motion of the region to the north. Thus, the kinematics of the northern boundary changed to dextral shear and extension along what is here named the “Trans–Mojave-Sierran shear zone.” This east-west zone of mainly right shear oroclinally folded the southern tail of the Sierra Nevada and the central Mojave Desert; rocks within this 83–110-km-wide zone were rotated (Approx.)40°–60° (clockwise) about vertical axes. Finally, as collapse moved the North American plate progressively west across the locus of transform shear, its western edge became increasingly subject to the motion of the Pacific plate. Over time, the edge of North America was truncated by faults of the San Andreas system and transferred to the Pacific plate.

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