AGE SIMILARITIES BETWEEN LATE CRETACEOUS GRANITOIDS IN THE PALM SPRINGS-MECCA REGION (SOUTHERN CALIFORNIA) AND THE ONES IN COASTAL SONORA (NW MEXICO): TIES ACROSS THE GULF OF CALIFORNIA-SAN ANDREAS SHEAR SYSTEM

At the base of the eastern flank of the Santa Rosa Mountains (SRM), in the region between Palm Springs and Mecca, in southern California, there are abundant outcrops of granitoids. Some of them have been previously dated and have crystallization ages between ~93 Ma (La Posta age granitoids), followed by a younger pulse of magmatism at ~86–83 Ma (Laramide granitoids). This later pulse has been considered the last granitic pulse recorded in the region.

New U-Pb zircon LA-ICPMS determinations, performed on a small suite of granitoids collected as part of an educational field trip along the eastern flank of the SRM, allowed us to reproduce the previously proposed age groupings, including La Posta granitoids (~98–93 Ma) and the Laramide granitoids (~85–82 Ma). However, we were able to date two new samples with surprising ages at ~80 Ma and four samples with also unsuspected Laramide ages between ~76.5–74.6 Ma. It is important to note that these younger granitoids contain abundant zircon inheritance with ages at ~85 and ~80 Ma, indicating that those previous lithologies had significant participation in the newer magma genesis event.

The eastern part of the SRM should restore against the Gulf of Santa Clara-Puerto Peñasco region in coastal Sonora in NW Mexico when accounting for at least ~300 km of Late Miocene latitudinal displacement along the Gulf of California-San Andreas dextral strike-slip system. Only one true La Posta granitoid age has been found in northwestern Sonora (~96 Ma; in the vicinity of San Luis Rio Colorado). However, SRM granitoids are very similar in age to the ones from coastal Sonora when considering the presence of abundant ~86.6–81.1 Ma (n = 11) granitoids, as well as granitoids with ages ranging between ~76.3–71.1 Ma (n = 20). Laramide granitoids in Sonora become younger towards the continent, recording eastward arc migration up to the Mid-Eocene.

Lastly, Laramide granites in the SRM present relatively slow cooling histories, from crystallization to final exposure, in contrast to older Peninsular Ranges granitoids to the west (Miggins et al., 2014). This peculiarity seems to be shared by Laramide granitoids from coastal Sonora, where preliminary thermal histories also indicate very slow cooling. Future, more detailed thermochronological work is required to test these preliminary ideas.

UNAM-PAPIIT-IN111718