BREACHING OF TRANSFORM FAULTS AND FLOODING OF PULL-APART BASINS TO INCREMENTALLY FORM THE EARLY GULF OF CALIFORNIA SEAWAY FROM ~8 TO 6.3 MA

It has long been known that a seaway formed during the proto-Gulf of California stage of the oblique–divergent Pacific-North America plate boundary, but the processes that formed the seaway are poorly known. Microfossils and volcanic units date the first marine incursions at ~8 Ma at the mouth of the Gulf, ~7 Ma in the central Gulf, and 6.3 – 6.5 Ma in the northern Gulf to Salton Trough. Our GIS-based maps of the plate boundary at 8, 7, and 6 Ma, combined with the Guaymas basin history from seismic data from other workers, show that the evolution of pull-apart basins produced an incremental seaway encroachment. The critical parameter is the late Miocene length of the strike-slip faults between pull-apart basins, which we can approximate. Fault length indirectly controls when high topography of footwall blocks pass by. Breaching can then occur upon juxtaposition of adjacent basin lows or along narrow transtensional zones. The marine incursion history for the Gulf was pre-determined by the long strike-slip faults that bound the Guaymas pull-apart basin and produced a 3-stage seaway evolution. (i) By ca. 8 Ma, a seaway had formed from the mouth of the Gulf to the Pescadero basin, based on our maps and marine strata in the Cabo and Tres Marias basins. There was a short barrier to the Farallon basin that suggests it was either a terrestrial basin, or if breaching occurred, there may be 8 Ma salt or initial marine deposits in the basin. This early southern seaway formed in a transtensional setting. (ii) At ca. 7 Ma, a series of marine incursions breached the transform barrier between the Farallon and Guaymas basins. Our maps show that periodic breaching occurred along a 100 – 150 km long strike-slip or transtensional zone and was the precursor to the Carmen basins. Repeated breaching events and the isolation of the Guaymas basin in a sub-tropical setting caused the formation of a 2-km-thick salt deposit imaged in seismic data, similar to the facies history of the Santa Rosalia basin on the western margin. The rhomb shape of the Guaymas salt basin suggests that it was either a composite pull-apart basin or a transtensional basin. (iii) By 6.3 ± 0.2 Ma, two major breaches occurred that formed a permanent seaway through the Guaymas basin and to the Salton Trough; for the first time a narrow (50 – 100 km wide) sea formed along the 1500-km-long Gulf of California.