<P> The southernmost Rio Grande Rift broadens, becoming more complex in structure and physiographically indistinguishable from the adjoining Basin and Range province . </P> <P> In cross-section, the geometry of the basins within the rift are asymmetrical half - grabens, with major fault boundaries on one side and a downward hinge on the other . Which side of the basin has the major fault or the hinge alternates along the rift . The alternation between these half - grabens occurs along transfer faults, which trend across the rift to connect the major basin - bounding faults and occur between basins or, in places, within basins . The Precambrian basement changes relief sharply in this area, from 8,700 m below sea level at the bottom of the Albuquerque basin to 3,300 m above sea level in the nearby Sandia Mountains, which flanks the Albuquerque basin to the east . Flanking mountains are generally taller along the east side of the rift (although some of this relief may be Laramide in origin). The thickness of the crust increases to the north beneath the rift, where it may be as much as 5 km thicker than it is in the south . The crustal thickness underneath the rift is on average 30--35 km, thinner by 10--15 km than the Colorado Plateau on the west and the Great Plains to the east . </P> <P> The rift is presently nearly tectonically quiescent, but significant deformation and faulting with offsets of many kilometers were responsible for its formation starting about 35 Ma . The largest - scale manifestation of rifting involves a pure - shear rifting mechanism, in which both sides of the rift pull apart evenly and slowly, with the lower crust and upper mantle (the lithosphere) stretching like taffy . This extension is associated with very low seismic velocities in the upper mantle above approximately 400 km depth associated with relatively hot mantle and low degrees of partial melting . This intrusion of the asthenosphere into the lithosphere and continental crust is thought to be responsible for nearly all of the volcanism associated with the Rio Grande rift . </P> <P> The sedimentary fill of the basins consists largely of alluvial fan and mafic volcanic flows . The most alkalic lavas erupted outside the rift . The sediments that were deposited during rifting are commonly known as the Santa Fe Group . This group contains sandstones, conglomerates, and volcanics . Aeolian deposits are also present in some basins . </P>

What is currently happening in the rio grande rift zone