New Mexico Geological Society Annual Spring Meeting — Abstracts

Basins of the Rio Grande Rift (RGR) have been studied extensively, but relatively few studies have focused on changes in basin subsidence rates. This study has analyzed the tectonic subsidence of the RGR in ten locations: two in the southern San Luis Basin, four in the Española Basin, three in the Albuquerque Basin, and one in the Lemitar Mountains area. Six of the ten locations are wells and four are measured stratigraphic sections. We selected locations for which we had good subsurface age controls. The subsurface units were differentiated based on age control and lithologic differences. Parameters for these subsurface units (such as porosity depth coefficients, surface porosities, and bulk densities) were estimated using their lithologic descriptions and published estimates for similar lithologies.

We use the backstripping method to calculate tectonic subsidence. This is a technique that decompacts and successively removes sediment load driven subsidence using Airy isostasy. Interpretations need to consider that the backstripping method treats unconformities as periods of zero tectonic subsidence whereas in reality unconformities may coincide with times of uplift or possibly subsidence. With that caveat, we argue that these ten tectonic subsidence curves allow a comparison of subsidence rates along the rift on both inter- and intra-basin scales.

All but one of the subsidence curves with sufficient data follow a trend characterized by rapid tectonic subsidence rates of ~25-65 mm/Ka in the Miocene, followed by an unconformity, and slower tectonic subsidence in the Plio-Pleistocene. Post-unconformity rates of subsidence are ~10 mm/Ka between about 5 Ma and 2 Ma in all basins. This indicates the rift was still opening in Plio-Pleistocene time but much slower than in the Miocene. On well doesn’t follow these tectonic subsidence trends because it lies on a relatively deep half-graben developed on the immediate hanging wall of the active Parijtio Fault system, which has maintained high slip rates through the Pliocene. Our study confirms that the unconformity is a rift-wide feature. We also explore several literature models which our subsidence curves compliment that may explain the unconformity, including climate, tectonic processes and dynamic uplift.