A Unified Theory for the Early Miocene History of Southwestern North America and Implications for the Origin of Detachment Faults, Metamorphic Core Complexes, Oroclinal Folding, and the San Andreas Fault System

Roy K. Dokka

A model is proposed that relates early Miocene plate interactions to the development of the classic extensional terranes and metamorphic core complexes of southeastern California, the Mojave Desert, and Arizona, as well as oroclinal folding of the southern Sierra Nevada-central Mojave Desert region and the San Andreas fault system. Plate reconstructions indicate that early Miocene Pacific-North American relative motion was not parallel to the transform, but was instead oblique and transtensional. It is proposed that the western edge of the North American plate collapsed to the southwest in response to this westward migration. Collapse was oriented nearly perpendicular to the plate boundary, suggesting that gravity was the driving force. The spatial and temporal correlatio of the collapsed area with the "slab gap" in the subducted Farallon plate suggests that heating by upwelling asthenosphere or related magmatism may have controlled initial localization of the collapse of the North American plate. According to the model, the northern and eastern edges of the collapsed region underwent extension between 22 and 20 Ma. Whereas extension in southeastern California and Arizona during this interval was coaxial with the overall S60°W collapse direction, the kinematics along the northern boundary (Mojave Extensional Belt) was the result of the combined effects of the SW collapse and the independent ~NW translation of the region to the north; collectively these motions resulted in ~N-S extension of the Mojave Extensional Belt. Collapse continued between 20 a d 16 Ma with major extension persisting in southeastern California and Arizona; extension along the northern boundary waned, however, due to the apparent cessation of NW motion of the region to the north. Thus, the kinematics of the northern boundary changed to dextral shear and minor extension (Trans Mojave-Sierran shear zone). This ~E-W zone of mainly right shear oroclinally folded the southern tail of the Sierra Nevada and the central Mojave Desert; rocks within this 70-100 km-wide band have been rotated on average 40-51° (clockwise) about vertical axes. Finally, as collapse moved the edge of the North American plate progressively west across the locus of transform shear, that portion became increasingly subject to the motion of the Pacific plate. Over time, the edge of orth America was fragmented and truncated by strike-slip faults of the San Andreas system and transferred to the Pacific plate.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995