Processes controlling the development of the Rio Grande Rift at long timescales

This manuscript highlights several processes that may have been influential in the development of the Rio Grande rift by reviewing important results from recent studies. In the Albuquerque Basin, low-angle normal faults exist in several locations, but are discontinuously preserved and generally offset and rotated by high-angle normal faults. The Sandia and Sierra Ladrones rift flank uplifts, which are the highest elevation rift flanks on opposite sides of the Albuquerque Basin, both have low-angle normal faults, have maximum extension in the Albuquerque Basin, show fault dips that increase from the rift margin towards the axis of the basin, and show fault ages that young towards the center of the basin. Thermochronologic data suggest that both of these rift flanks were exhumed at nearly the same time, 10-25 Ma. These observations suggest a rolling-hinge mechanism for the formation of low-angle normal faults in the Rio Grande rift, where isostatic uplift appears to be a dominant process in regions of maximum extension. This process can therefore dramatically affect basin and rift-flank geometry as rifting progresses and suggests that the Sandia and Ladron uplifts are moderate-extension analogs to core complexes. To further understand extensional processes and timing of extension within the Rio Grande rift, apatite fission-track (AFT) and apatite (U-Th)/He thermochronologic methods were used to produce thermal history models from Rio Grande rift flank uplifts in Colorado and New Mexico. These models indicate that extension along the majority of the length of the rift was synchronous from 10-25 Ma. Existing geodynamic models for rift formation such as collapse of high topography or reduction of far field stresses due to growth of the San Andreas transform, or reactivation of older weaknesses, may not adequately explain the simultaneous 10-25 Ma opening of the Rio Grande rift from Colorado to Texas, nor explicitly link rifting to prior events such as the ignimbrite flare-up and Laramide orogeny. A model is therefore favored that involves Laramide flexure of the downgoing Farallon plate at the eastern Rocky Mountain front, delamination of sections of the Farallon Plate beneath the San Juan and Mogollon Datil volcanic fields to initiate and explain migrations of volcanism in the ignimbrite flare up, then a “big break” and foundering of the Farallon plate beneath the Rio Grande rift at ca. 25-30 Ma. This event focused asthenospheric upwelling along a north-south trend, weakening the overlying North American lithosphere and facilitating E-W extension from Colorado to southern New Mexico. 195