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Rio Grande rift: An overview

1987; Elsevier BV; Volume: 143; Issue: 1-3 Linguagem: Inglês

10.1016/0040-1951(87)90083-7

1879-3266

Kenneth H. Olsen, W. Scott Baldridge, J. F. Callender,

Geochemistry and Geologic Mapping

The Rio Grande rift of the southwestern United States is one of the world's principal continental rift systems. It extends as a series of asymmetrical grabens from central Colorado, through New Mexico, to Presidio, Texas, and Chihuahua, Mexico—a distance of more than 1000 km. Although the Rio Grande rift is closely related in timing and structural style to the contiguous Basin and Range extensional province, the two can be distinguished by a variety of geological and geophysical signatures. Rifts (both oceanic and continental) can be defined as elongate depressions overlying places where the entire lithosphere has ruptured in extension. The lithosphere of the Rio Grande rift conforms to this definition, in that: (1) the crust is moderately thinned—Moho depths range from about 45 km under the flanks to about 33 km beneath the rift axis. (2) anomalously low Pn velocities (7.6–7.8 km s−1) beneath the rift and a long wavelength gravity low suggest that the asthenosphere is in contact with the base of the crust. Structural development of the rift occurred mainly during two time intervals: the early phase beginning at −30 Ma. and lasting 10–12 m.y., and the late phase extending from −10 to 3 Ma. The early phase involved extensive low-angle normal faulting throughout the rift region which was subsequently offset by high-angle normal faulting during the later deformational event. Volcanism of the Rio Grande rift is minor compared to some other continental rifts. Most of the volcanism is basaltic and occurred less than about 5 m.y. ago. Compositions range from alkalic to tholeiitic, with no unique spatial or temporal pattern. Magmas were probably derived from a variety of depths, indicating an unintegrated heat source with only local melting. Basaltic andesites and related calc-alkaline rocks erupted in the southern rift between about 30 and 18 m.y. ago were not uniquely related to the rifting process. Rather, the thermal pulse which generated these magmas was part of the previous, subduction-related event. Our interpretation of existing data concerning the evolution of the Rio Grande rift does not fit either simple active or passive "end-member" models. In particular, there is no compelling evidence for a major thermal event in the mantle uniquely associated with rifting. Yet heat—inherited from the immediately-preceding deformational regime—was certainly a critical factor in, and was probably a necessary condition for, rifting.