Active continental rifting in southern Baja California, Mexico: Implications for plate motion partitioning and the transition to seafloor spreading in the Gulf of California

Abstract

Three major earthquake series have occurred during the past 30 years on fault systems that are spatially and kinematically distinct from oceanic spreading centers and transform faults in the southern Gulf of California. Focal mechanism solutions for earthquakes of the April 1969 and June 1995 series indicate nearly pure normal displacement across NNW striking planes. This seismicity falls along different strands of a major system of normal faults that extends at least 300 km along strike and defines the western limit of the Gulf Extensional Province. The dominant normal faults control distribution of Neogene basins and have been active since middle to late Miocene time. Epicenters of the August 1969 earthquake series lie south of the Baja California peninsula and focal mechanism solutions indicate strike‐slip displacement. This seismic activity is likely related to active WNW striking faults in the continental borderland, which isolate Baja California as a microplate within the Pacific‐North American plate boundary. Therefore, modern Pacific‐North America plate motion is partitioned into at least three distinct fault systems: oceanic spreading centers and transforms in the Gulf of California, normal faults along the western margin of the Gulf of California, and faults along the Pacific margin of the Baja California peninsula. Kinematic partitioning of plate motion is required to accommodate the strongly three dimensional strain of transtensional rifting. Continental rifting and seafloor spreading have overlapped in time for the past 3.6 Myr., which also is likely attributed to the highly oblique kinematics of rifting.