The Tectonic Evolution of the Gulf of California

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The major boundary zone between the Pacific and North America plates continues southward from the San Andreas fault system into the Gulf of California. Across this boundary, extension and strike-slip faulting is causing the Baja California peninsula to separate away from mainland Mexico, thereby opening the Gulf of California. This is one example of how a small fragment of crust (Baja California) may get transferred from one plate (North America) to another plate (Pacific), a process that has happened many other times in geologic history.

Our research has focused on trying to understand the early history of this rift system, using a variety of approaches: 1) geological field study of faults within the Gulf Extensional Province (surrounding the Gulf of California) to document geometries, timing, and amounts of fault motion within this region; 2) volcanic stratigraphy of the Puertecitos Volcanic Province, a major Miocene-Pliocene rhyolite field that was dismembered by phases of extensional and strike-slip faulting related to the evolution of the Gulf of California; 3) argon geochronology to establish ages of the various volcanic units; 4) paleomagnetic work, to establish magnetic reversal stratigraphy of both volcanic sequences and marine sequences, and to look for tectonic rotation, about vertical axes, during the faulting; 5) processing of Landsat-TM and ASTER satellite imagery, to distinguish the main volcanic packages based on their spectral reflectances, and to identify active faults in the region. Results of this work so far indicate that the westernmost fault systems have been active more than 6 million years, that is, before the time that is commonly assumed that major plate motion in the Gulf really began. The eastern fault systems have evolved through time, with high percentages of early (between 6 Ma and 3 Ma) extension, and rather low percentages of post-3 Ma extension. We have identified a major discontinuity called the "Matomi Accommodation Zone" south of which the deformation may be younger (Pliocene), due to a jump of the Gulf spreading centers. We have found new tie points in late Miocene rocks on both sides of the Gulf of California (Upper Delfin Basin segment), including on the two major Gulf islands, which allow us to constrain the total amount of slip in this segment of the Gulf since late Miocene time.

Our present research focus is focusing on the Tuff of San Felipe, an ignimbrite that is about 12.5 Ma that was erupted before the Gulf of California opened, and crops out now on both sides of the Gulf (in Baja California and in Sonora). Recent posters about this work can be found here:

Collaborators: Dr. Arturo Martin-Barajas and colleagues at the Centro de Investigacion Cientifica y Educacion Superior de Ensenada (CICESE), Ensenada, Mexico; Dr Francisco Paz-Moreno and Dr. Jesus Vidal-Solano at the Universidad de Sonora.

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