SEGMENTATION OF THE RIO GRANDE ALLUVIAL SURFACE AND EVOLUTION OF THE RIO GRANDE RIFT

This research describes new implications for rift tectonic history that is derived from segmentation of a broad geomorphic surface produced by deposition of the Rio Grande during the Pleistocene. The southern end of the Rio Grande Rift was a depocenter throughout the latest Tertiary and Early Pleistocene. Near El Paso, Texas, the Rio Grande shifted back and forth across the southern end of the rift and created a gently south-sloping constructional surface between 3.8 and about 670,000 years ago when the present valley began to be excavated. Post depositional faults have segmented this surface, creating a series of plateaus at different elevations. Analysis of offsets of this surface reveal a pattern of progressive confinement as the Rio Grande was restricted by tectonism progressively to fewer basins.

Important results are that the fault movement along the Franklin Mountains has uplifted the center of the range higher than the northern and southern ends, the present-day topography is mimicked by uplifted Pliocene terraces and the range is the topographic expression of a composite fault segment. Faulting since the Mid Pleistocene has uplifted the Franklin Mountains by 10 m and dropped the Hueco Basin by at least 40 m.

Mid and late Pleistocene offset is greater on faults closer to uplifts in the Franklin, Hueco and Potrillo mountains. There has been a progressive widening of the Hueco and Mesilla Basins by footwall fault activation.

The current course of the Rio Grande through the Franklin Mountains is controlled by a previously unknown graben that trends northwest across pre-existing north-south trending structures. This graben has little relief and is defined by offset terraces. The northwest trend of this Mid-late Pleistocene graben is consistent with historic earthquake activity, which has occurred on northwest-trending faults.