A NEW LOOK AT LITHOSPHERIC STRUCTURE IN THE SOUTHERN ROCKY MOUNTAIN-RIO GRANDE RIFT REGION

The Rocky Mountain region has intrigued researchers and explorers ever since the gold rush days. These mountains are a tectonic puzzle because of their complex history and their distance from plate margins. The geological history of the region is relatively well-understood and recent additions of seismic and potential field data provide an ideal opportunity to revisit the evolution of its lithospheric structure. The Proterozoic assembly of Laurentia in this region is of great interest, but the break-up of Rodinia, the formation of the Ancestral Rocky Mountains, the Laramide orogeny, mid-Cenozoic magmatism, and late Cenozoic extension and uplift have obviously modified the lithosphere significantly. In many cases, the effects of these events have clear geophysical signatures that are generated in the upper crust. However, deeper signatures are more subtle and difficult to isolate from the effects of both older and younger features. Thus, it is important to integrate a broad spectrum of geological and geophysical data if we are to advance our understanding.

An early complexity of the evolution of this region is that the mafic lower crust and Moho likely formed, and reformed, in several stages. Proterozoic continental growth involved complex accretion events and arc activity similar to what has happened in the North American Cordillera in the Phanerozoic. At ca. 1.4 Ga, voluminous granitoid emplacement occurred, and 1.4 Ga lower crustal xenoliths and geophysical data indicate this event was accompanied by substantial mafic underplating. The subsequent break-up of Rodinia involved intraplate rifting that emplaced large Cambrian-aged mafic masses in the crust that are exposed in Colorado and Oklahoma today. The deformation that formed the Ancestral Rocky Mountains includes a massive inversion of these rift structures and is due to a plate collision in the late Paleozoic. The Laramide orogeny also produced considerable crustal scale deformation manifested by large basement uplifts and deep basins. Finally, late Cenozoic uplift and Rio Grande rift extension formed a series of basins that recent geophysical studies show are deep and complex. Recent geophysical studies also show that the deep signature of the Rio Grande rift is generally symmetrical with respective to its surface expression.