ONE DECADE OF INTERSEISMIC GPS OBSERVATIONS OVER THE PACIFIC-NORTH AMERICA PLATE BOUNDARY AT THE LATITUDE OF THE HEAD OF THE GULF OF CALIFORNIA

As part of a crustal motion study for Southern California and northern Baja California made by a combination of continuous GPS data from SCIGN (www.scign.org) and campaign surveys in northwest México from 1991 to 2001 (Prawirodirdjo, et. al., AGU 2001 Fall Meeting), 17 GPS site rates are estimated along a profile between northwestern Sonora desert, the head of the Gulf of California and Guadalupe Island in the Pacific Ocean. Solutions were derived in ITRF97 and computed with GAMIT/GLOBK software and global data from International GPS Service.

In order to look for the regional interplate crustal motion, the ITRF97 solution was rotated into a North America plate reference frame using the geological based NNR-NUVEL1-A (mean for the last 3 Myr) and the GPS based REVEL-2000 (mean for the last 10 yr), global plate models. The observed relative crustal motion between Puerto Peñasco in the northeastern Gulf of California and Guadalupe Island, a 12.5 Myr old spreading center traveling now on a “rigid” Pacific plate, is 49.6±1.1 mm/yr.

The GPS site rates along our profile agree with that two models in the azimuth of the predicted motion; but both disagree in the magnitude. I perform a better agreement with a “mixed” North America-Pacific plate boundary model: pole of rotation Lat=49.0°N, Long=284.7°E, and angular velocity w=0.746°/Myr. This present-day model (NAPA-CMJGG2002) gives a velocity of 49.3 mm/yr for the Alarcon seafloor spreading center along a strike of N53°W, that really, is that of the Pescaderos fault.

A observed GPS site in the northwestern gulf, near Puertecitos, has a present interseismic rate of 41.1±1.2 mm/yr on a N46.8°±1.7°W strike. If this slip, represent the secular constant crustal motion of the region, and the separation of conjugated margins between Puertecitos and Tiburon Island is 255±10 km (Oskin, et. al., Geology 2001); it can be obtained that the opening of the northern Gulf of California may have started 6.21±0.42 Myr before present.