THE OPENING OF THE GULF OF CALIFORNIA TROUGH WITHIN THE NORTH AMERICAN CORDILLERA

The Gulf fills most of a 1400km-long structural depression that began opening in the Middle Miocene along the landward side of Cretaceous batholiths, parallel to the continental margin but 200km inland. By the end of the Miocene it had assumed its present role as the main plate boundary zone between the Pacific and North American plates, and had evolved into a belt of strike-slip shearing and localized seafloor spreading, linked in the Mouth of the Gulf region to the East Pacific Rise (EPR) zone of crustal accretion by spreading centers that developed rapidly from rifts across a coastal batholith. Recent marine surveys and sampling in and around the Gulf itself have clarified the nature, timing and geologic consequences of the regional tectonic history, and the processes and preexisting structures that controlled it.

Definition of the Gulf depression by rifting of cordilleran lithosphere began in earnest soon after the adjacent Californian margin changed from subduction to a zone of oblique plate divergence. This was not a sudden process induced by entry of the EPR axis into a Californian subduction zone and southward migration of a "Rivera triple junction" along the margin. Instead, by 14.5Ma subduction of the EPR ceased along the entire margin as its east-flank plate broke up into several slow-moving microplates, which after several Myr were episodically captured by the Pacific plate. The main event that brought the Pacific and North American plates in contact along an obliquely extensional margin was the 11.5Ma capture of the 850km-long Guadalupe and Magdalena microplates. Interplate strain was largely partitioned between a continental transform fault system along the west coast, linking the San Andreas system to a Pacific-Rivera risecrest at lat.23N, and the inland "proto-Gulf" zone of margin-orthogonal rifting, which mainly followed the young weak lithosphere of an Early Miocene arc. At ~8Ma the Pacific-Rivera risecrest was replaced by one further east, and a strike-slip connection from the San Andreas system through the proto-Gulf region may have begun. By 6Ma, spreading centers producing magnetically lineated oceanic crust were established in the Mouth, and the complex Plio-Pleistocene history of changing patterns of pull-apart basins, spreading centers, and transform + non-transform shear zones within the Gulf soon followed.