The PIs propose to measure and interpret U-Pb crystallization ages of a varied suite of volcanic and plutonic rocks recovered from water depths of 300-3500m at several hundred sites on the submerged and previously unsampled rifted continental crust that underlies most of the southern Gulf of California. Many samples were collected along traverses up fault scarps and across volcanic features by a remotely operated vehicle; 50 dredge hauls also recovered igneous rock. All relevant samples have been petrographically described and geochemically analyzed. These samples record a long and complex magmatic history in this region, which in the past 20 Myr has changed from a volcanic arc to a subaerial intra-continental rift, been flooded by a marine incursion, and developed a chain of axial basins growing by seafloor spreading. Radiometric dates are needed to define this history, and to test hypotheses that up till now have been solely derived from still-subaerial outcrops at the margins of the rift. Existing dating show striking discordance between the age estimates, hinting at interesting cooling histories with likely tectonic implications. Targeted zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronologic analyses and modeling will constrain thermal histories, thereby contributing to understanding rift tectonics including continental uplift and subsidence in the gulf rift.
The work will very likely deepen our understanding of rifting history of the southern Gulf of California. Funding will also provide research opportunities for graduate students, foster collaboration with Mexican scientists and the results will be used in ongoing education and outreach programs, such as SDSU's K-8 project.