TEMPORAL CONSTRAINTS AND CAUSES FOR EARLY ANCESTRAL RIO GRANDE INTEGRATION, SOUTH-CENTRAL NEW MEXICO

It has previously been interpreted that at ~5 Ma the ancestral Rio Grande, the axial river of the Rio Grande rift, fluvially integrated 250 km through previously endorheic basins between Socorro (130 km south of Albuquerque) and Las Cruces in southern New Mexico. We use new sedimentologic, ⁴⁰Ar/³⁹Ar, and paleomagnetic data to revise the timing and stratigraphic details of this important event.

­­­Our investigation focuses on outcrops near the towns of Socorro and Truth or Consequences (TorC), the latter located in the northern Palomas basin 120 km south of Socorro. Both localities feature terminal playa muds interfingering northwards with fluvial-deltaic sand tongues (FDST), overlain by axial-fluvial sand. Our data indicate that FDST prograded southwards across the Socorro-La Jencia basins between 10 and 7.5 Ma. Detrital sanidine results of earliest axial-fluvial sand in the Socorro area and FDST in the TorC area strongly suggest the ancestral Rio Grande integrated through the Socorro basin and ended in a terminal playa in the Palomas basin during 7.4-6.9 Ma. Magnetostratigraphic and ⁴⁰Ar/³⁹Ar data indicate a through-going river at TorC and possible integration of the entire Palomas basin at 6.4-6.0 Ma.

During 10-8.5 Ma, a west-sloping, large alluvial fan complex was located just south of the terminal playa in the Socorro basin, creating a paleotopographic high (sill). An extensive 8.5 Ma basalt flowed west down this fan, then turned north into the playa. Gravel composition and paleoflow changes below vs. above this basalt indicate a basin reorganization (ca. 8.5-7 Ma) probably by onset of a third generation of high-angle rift faults near Socorro. This tectonic change created a paleotopographic low and basin floor in the southeast Socorro basin, allowing the ancestral Rio Grande to extend past the former sill into the next rift basin to the south. We argue this change in the locus of faulting was critical for fluvial integration through the Socorro basin. Fluvial integration was likely facilitated by increased sediment flux rates in the ancestral Rio Grande due to 8-6 Ma reduction of basin subsidence rates and unconformity development in upstream rift basins, perhaps related to mantle-driven uplift, which promoted sediment overfilling and spillover of the Socorro basin and other successive, downstream terminal basins.