THE MIOCENE-PLIOCENE UNCONFORMITY IN THE RIO GRANDE RIFT

Tectonic subsidence curves are calculated for the northern and central basins of the Rio Grande rift. They reveal rapid subsidence until ~8 Ma followed by an unconformity or slow-down of subsidence. Tectonic subsidence resumes again ~5 Ma. This Late-Miocene-Early Pliocene unconformity was previously described in several basins of the Rio Grande rift; this study shows that this unconformity is more widespread than previously thought, and that it is present also outside of the rift zone. The age of its associated lacuna is spatially variable but falls within 8–3 Ma (mostly 7–5 Ma) and thus is synchronous with eastward tilting of the western Great Plains. We explain the spatial distribution and timing of the unconformity, as well as eastward tilting of the western Great Plains, by dynamic mantle uplift. This explanation is supported by geoid-to-elevation ratio analyses that suggest that topography of the northern Rio Grande rift region is compensated by a component of mantle-driven dynamic uplift.