<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 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>
what is currently happening in the rio grande rift zone
what is currently happening in the rio grande rift zone
nearly tectonically quiescent
nearly tectonically quiescent