RECONSTRUCTING THE GULF OF CALIFORNIA-SALTON TROUGH OBLIQUE PLATE BOUNDARY WITH GIS MAPS SINCE 12 MA

We present GIS-based reconstructions for the Pacific-North America (PAC-NAM) oblique plate boundary. Our maps track PAC-NAM deformation since 12 Ma, at 1-2 Myr intervals, and use existing data to close basins, restore slip on faults, and restore rotated blocks. We use modern-day GPS motion between the Baja California microplate (BCM) and North America (NAM) (from R. Malservisi) for the BCM and intervening tectonic blocks. Extrapolation of these rates back to 7 Ma produces maps that agree well with many aspects of the southern and northern Gulf. GPS-based rates in the north result in ~320 km of BCM-NAM motion since 7 Ma, and ~300 km since 6.5 Ma. These rates agree with geologic tie points across the northern Gulf and with the latest geologic evidence along a transect of dextral offsets from the northern BCM to coastal Sonora, east of Tiburon Island. In the southern Gulf, the 7 Ma model results in ~340 km of BCM-NAM offset at San Jose del Cabo and ~330 km at Loreto. Cross-Gulf geologic tie points are lacking in the southern Gulf, but the 7 Ma map fits many documented geologic aspects. At 7 Ma, we model an enclosed Guaymas basin similar to the salt basin observed in seismic data and on the conjugate Santa Rosalia coastline. At ~7 Ma, Guaymas basin was isolated from the northern Gulf basins, and from the Farallon basin to the south, then the north end of a ~100 km-wide seaway. This incipient seaway was co-located with a longer, nascent transtensional belt that developed from the northern end of the Maria Magdalena rise to the Salton Tough. In order to match the full 51 mm/yr PAC-NAM budget, we add 21 km of dextral offset west of the BCM in the south and 35 km in the north since 7 Ma. The proto-gulf maps from 7 to 12 Ma are much less constrained. PAC-NAM motion requires ~610 km total offset since 12 Ma. This suggests that there must be 250-300 km of dextral offset outside the Gulf axis prior to 7 Ma, distributed in the borderland west of the BCM and/or in mainland Mexico. Our preferred model accommodates ~200 km proto-gulf motion west of the BCM. This preferred model shows ~400 km BCM-NAM offset in the south, and ~360 km offset in the north since 12 Ma; the north-south discrepancy is due to variable Euler pole distance along plate boundary strike. Maximum allowable offset is ~450 km in the south and ~400 km in the north, as any further offset produces unacceptable overlap of crust.