Careful documentation of the Cenozoic geologic history of the Rio Grande rift in New Mexico reveals a complex sequence of events. At least two phases of extension have been identified. An early phase of extension began in the mid-Oligocene (about 30 Ma) and may have continued to the early Miocene (about 18 Ma). This phase of extension was characterized by local high-strain extension events (locally, 50-100%, regionally, 30-50%), low-angle faulting, and the development of broad, relatively shallow basins, all indicating an approximately NE-SW ¿25¿ extension direction, consistent with the regional stress field at that time. Extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism. A late phase of extension occurred primarily in the late and spatially associated with major magmatism. A late phase of extension occurred primarily in the late Miocene (10-5 Ma) with minor extension continuing to the present. It was characterized by apparently synchronous, high-angle faulting giving large vertical strains with relatively minor lateral strain (5-20%) which produced the modern Rio Grande rift morphology. Extension direction was approximately E-W, consistent with the contemporary regional stress field. Late phase graben or half-graben basins cut and often obscure early phase broad basins.

Early phase extensional style and basin formation indicate a ductile lithosphere, and this extension occurred during the climax of Paleogene magmatic activity in this zone. Late phase extensional style indicates a more brittle lithosphere, and this extension followed a middle Miocene lull in volcanism. Regional uplift of about 1 km appears to have accompanied late phase extension, and relatively minor volcanism has continued to the present. We have estimated geotherms and calculated lithospheric strength curves for the two phases of extension, using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure. A high geotherm was deduced for early phase extension, resulting in a shallow crustal brittle-ductile transition and negligible mantle strength. The lithosphere cooled after early phase extension, resulting in a deeper crustal brittle-ductile transition, and perhaps more significantly, a considerable zone of mantle strength immediately beneath the Moho. These results indicate that early phase extensional style was controlled by a crustal decollement near the brittle-ductile transition, which was prevented during late phase extension by significant strength in the uppermost mante. Late Cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling. However, this uplift does not appear to be restricted to the rift zone, and Pliocene to Recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust, perhaps unrelated to rifting. The complex interrelationship among regional an local preifting, synrifting, and postrifting events in the Rio Grande rift suggests that rifting, at least in this region, should not be considered in isolation of the other geologic events.