MAGMA CHAMBERS IN RIFTS

In continental rifts, lower than normal velocities and resistivities at mid-crustal and sub-crustal depths suggest a low degree of partial melting at these levels that could be a general feature of these structures. On the other hand, geophysical detection of significant accumulations of magma beneath or within the crust of continental rifts is rare. Two rift areas where a number of geophysical observations indicate the presence of relatively large quantities of magma are the Rio Grande rift in central New Mexico and Yellowstone National Park. In the former area, a thin nearly horizontal lens of magma with a minimum areal extent of 1700 km2 has been seismically detected at midcrustal levels. Historical surface uplift and the spatial and temporal distribution of seismic activity above the magma chamber indicate contemporaneous inflation. The shape of the magma body, as well as the composition of recent extrusives in the rift, suggests the chamber was formed by the injection of basalt. Significant accumulations of magma have been postulated beneath the Yellowstone caldera on the basis of (1) extraordinarily large heat-flow (900 to 1800 mW/m2), (2) gravity and magnetic lows, (3) shallow seismicity, (U) attenuation of P and S waves, and (5) traveltime delays for teleseismic events.