CONTINENTAL RIFT STRUCTURES AND DYNAMICS WITH REFERENCE TO TELESEISMIC STUDIES OF THE RIO-GRANDE AND EAST-AFRICAN RIFTS

Telescismic travel time residuals measured on 600-1000 km seismic arrays across the Rio Grande and East African rifts provide evidence for upper mantle low velocity zones beneath each rift. We propose that these zones arise from asthenosphere replacing the base of the lithosphere. Refraction studies show thinning of the crust below the rift axes. Regional uplift, which extends for hundreds of kilometers on either side of the axes, is isostatically compensated by negative density contrast at depth. Since the refraction results argue against compensation by crustal thickening, compensation within the upper mantle is suggested. Temperature effects alone do not explain the magnitudes of the reduced velocities. Instead, generation of a few percent of partial melt is the most probable source of the reduction. A passive extensional mechanism is investigated for the Rio Grande rift in which hot asthenosphere intrudes into the lithosphere, raising its temperature and generating moderate melting. It requires less than 1% extension to explain the melting; in contrast, a prodigious 26% would be required to raise the temperature from a pre-rifting continental geotherm to the solidus. Geological estimates based on palinspastic reconstruction find that extension is less than 10% over the uplift zone. A passive mechanism is possible if the pre-rifting lower lithosphere geotherm lies closer to the solidus than the geotherm of a stable continent. This hypothesis is supported by the observed high heat flow in the Rio Grande rift uplift region which, given the long time constant for thermal diffusion, reflects elevated temperatures at depth well before the onset of rifting. The elevated asthenosphere boundary, found teleseismically, lies directly beneath both rifts favoring a "pure shear" over the "simple shear" model of rifting. We propose a model for continental rifting in which the asthenosphere intrudes into dike-like structures in the lithosphere beneath the rift zone. This explains the observed upwarp of the Moho beneath the rift as asthenosphere intrudes the upper mantle and base of the crust. The forces associated with wedging apart of the upper mantle give rise to tensional and compressional stresses in the crust which form the graben and uplifted shoulders, respectively. Mantle xenoliths indicate brittle diking occurs in the upper mantle to depths of 70 km. Furthermore, evidence of asthenospheric intrusion from Neodymium ratios supports the role of asthenospheric advection in the rifting process.