CONSTRAINTS ON THE HISTORY OF THE LATE CENOZOIC PACIFIC-NORTH AMERICAN PLATE BOUNDARY FROM MARINE MAGNETIC ANOMALIES AND GLOBAL PLATE CIRCUITS

Round-the-world plate tectonic reconstructions, when combined with offshore California seafloor patterns of magnetic anomalies, supply strong constraints on the origin and development of the San Andreas plate boundary, its deformation budget, and the configurations of subducted slabs beneath it.

Circuit solutions provide tight estimates of the total deformation across western North America for four different times (2.6 Ma, 11.1 Ma, 19.7 Ma, and 33.2 Ma) and less precise estimates for many more times. Because of possible unknown displacements along the continent-ocean join, these estimates provide only upper limits for the on-land deformation budget, but they appear to approximate the actual deformation values for times younger than 20 Ma. The on-land deformation budget totals include both near-coast offsets on the San Andreas and Gulf of California systems and interior deformations on the Walker-Lane and Eastern California Shear Zone and in the Basin and Range provinces of the U.S. and Mexico. Interior deformations in the coast-parallel direction are particularly tricky since they imply substantial extensions/shortenings at the edges of the region. Motions of the Great Valley-Sierra Nevada block define the partitioning of deformation for central and northern California and cause the plate boundary region to extend far north of the Mendocino triple junction. Details of the transfer of Baja California to the Pacific plate dictates the late Miocene tectonic history of Southern California.

Offshore magnetic anomaly patterns help describe the breakup events that occurred during the demise of the Farallon plate and the origin of Pacific-North American plate contact. In most cases, the spreading center at the edge of the Pacific plate never reached the subduction zone, but rather, it slowed and stalled offshore because the main subducting plate broke away at depth, abandoning a surface microplate. These breakups and microplates, in turn, dictated the locations and shapes of resulting “slab windows” in the subducting plate and drove and localized patterns of volcanism and other events on the overriding plate.