SUBSIDENCE HISTORY OF THE RIO GRANDE RIFT AND IMPLICATIONS FOR RIO GRANDE DEVELOPMENT (Invited Presentation)

Wells and stratigraphic sections are used to describe the mid-Miocene to present tectonic subsidence history of the Rio Grande rift. Tectonic subsidence was quite fast until about 8 Ma, and was followed by a late Miocene-early Pliocene unconformity that developed mainly along the flanks of most rift basins. The age of its associated lacuna is spatially variable but falls within 8-3 Ma (mostly 7-5 Ma). Tectonic subsidence rates either remained similar or decreased after the Mio-Pliocene unconformity. We explain the spatial distribution and timing of the unconformity by dynamic mantle uplift. We then investigate how such a change in tectonic subsidence rate (and thus creation of accommodation space) affects the endorheic (internally drained) to exorheic (open) drainage change of the rift zone, forming the Rio Grande. Our tectonic-landscape evolution models show that slow rift-opening rates, a slowing-down of rift opening, or increase of headwater topography (e.g., upstream epeirogenic uplift), are tectonic situations that can cause a transition from an endorheic to an exorheic drainage state in a rift basin. Our results also show that wet climate conditions lead to a permanent exorheism that persists regardless of rift opening rates. In semi-arid climates, endorheic conditions are favored, and may last for the duration of rifting except for when rift opening is very slow. In the Rio Grande rift, the endorheic-exorheic transition may have occurred as a result of slow rift-opening and formation of headwater topography.