CRUSTAL STRUCTURE, GRAVITY-ANOMALIES AND HEAT-FLOW IN THE SOUTHERN RIO-GRANDE RIFT AND THEIR RELATIONSHIP TO EXTENSIONAL TECTONICS

The Rio Grande rift is a Neogene structure which has undergone a two-phase history of extension and volcanism. Significant amounts of new data are available in the southern Rio Grande rift region which is the area of this study. The first phase of extension (15-30 Ma) was of the order of 30-50% and was directed NE-SW. The second phase (3-10 Ma) involved extension of about 10% which was directed E-W. Crustal structure in the region of the rift can be mapped using recent seismic and gravity data and the observed thinning associated with the rift is consistent with extension of about 10%. However, heat flow data in the southern rift cannot be explained simply in terms of extension. Both the heat flow and uplift history of the area require crustal thickening possibly by intrusion in the lower crust, a process not directly related to extension. © 1990.