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EVOLUTION OF THE SALTON TROUGH-GULF OF CALIFORNIA PART OF THE PACIFIC-NORTH AMERICA PLATE BOUNDARY: AN UPDATE
The northern Gulf basins, onland and offshore, have been imaged by Mexican researchers using PEMEX data. This reveals detailed basin geometry and development, including the role of low-angle normal faults within the marine basins, and inactive basins east of the Cerro Prieto fault. High heat flow along active basin-bounding faults constrains processes of crustal formation in the northern Gulf where true seafloor spreading is still absent.
Details of timing and amount of rifting remain controversial. Detailed studies of the N Gulf coastal regions indicate that at 12.5 Ma the area had low topographic relief, allowing deposition of a widespread subaerial ignimbrite, with faulting and basin formation starting at ~9-7 Ma. However, middle Miocene nannofossils from adjacent marine drill holes are interpreted as primary (Helenes et al 2009), suggesting that considerable pre-12 Ma lithospheric thinning should be found. Earlier estimates of ~270 km of net plate boundary slip within the N Gulf agree well with San Andreas system displacements and with a new correlation of evaporites from the Guaymas margin to Santa Rosalia in Baja California Sur, with offset of ~280 km since 7 Ma (Miller & Lizarralde 2013). Buried faults of inland Sonora may accommodate up to 100 km additional offset of ~12 Ma volcanic rocks (Vidal-Solano et al., in press). Geophysical estimates of total plate boundary slip are larger for the southern Gulf than for the northern Gulf.
Major changes in deformation at ~8 Ma in the southern Gulf and peninsula are interpreted from seismic evidence for basin development (Sutherland et al. 2012) and uplift (Brothers et al. 2012). Changes at 8-7 Ma along most of the modern Gulf may eventually be reconciled into a coherent kinematic model for plate boundary deformation linked to increased transtensional motion of the Pacific plate relative to North America at ~ 8 Ma. Entrance of seawater into the Gulf basins may not have been simultaneous along the entire length of the Gulf. The effect of ~5.5 Ma arrival of Colorado River water and sediment into the Northern Gulf still remains to be integrated into the history of plate margin development.