TECTONIC IMPLICATIONS OF GRAVITY ANOMALIES IN THE NORTHERN GULF OF CALIFORNIA AND SALTON TROUGH RIFT SYSTEMS

The crust of the northern Gulf of California and the Salton Trough trans-tensional rift systems exhibits a transition from continental to oceanic domains. The sparse seismic data available indicate regional variations in the seismic velocity of the upper mantle. The broad features in the gravity maps of the northern and southern segments these rift systems appear to result from both density variations within the upper mantle and variations in crustal thickness.

Our new gravity map using 40,000 gravity measurements, compiled from various surveys on both sides of the international border, clearly displays the gravity anomalies caused by these deep structures beneath the Delta of the Colorado River and the northern Gulf of California. Two new preliminary crustal models, that cross west to east in the Cerro Prieto and Wagner pull-apart basins are each characterized by strong contrasts in the gravitational field (-50 and –10 mGal, respectively), in spite of having similar thicknesses of sediment (~6 km). These modeled gravity profiles are constrained by deep well data, seismic reflection profiles, and crustal thicknesses derived from seismic refraction studies and by spectral analysis of aeromagnetic anomalies. Both the Cerro Prieto and Wagner basin sections show crustal thicknesses of about 14 km and upwelling of hot upper mantle material, suggesting a transitional crust that has not been completely rifted.

Previous interpretations of regional gravimetric profiles across the northern and southern parts of this rift system indicate the presence of an upper mantle of low density (3.1 g.cm^-3 and low seismic velocity (7.2 km/sec). In the Salton Trough this anomalous mantle body is ~130 km wide and 10 km thick. In the northern Gulf of California this low density body shortens to ~30 km and thins to ~6 km. This suggests that normal upper mantle is shallower in the northern Gulf compared to the Cerro Prieto and Salton Trough areas. As more seismic data become available it will be possible to constrain and invert the gravity data in terms of the density distribution within the low-velocity zone.