Rio Grande Rift- an Overview

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Journal Article: Rio Grande Rift- an Overview

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. (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. (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.The P-velocity is abnormally low (6.4-6.5 km s_1) in the lower half of the crust beneath the rift, suggesting high crustal temperatures. However, associated seismic and volcanologic data indicate the sub-rift lower crust is not dominated by a massive composite mafic intrusion such as is sometimes inferred for the East African rifts. Seismic and magnetotelluric data suggest the presence of a thin (< 1 km) sill-like contemporary midcrustal magma body which may perhaps extend intermittently along much of the length of the rift. Seismic and structural studies indicate a dominant horizontal fabric in the upper and middle crust. The brittle-ductile transition is at depths _15 km except for the major volcanic fields, where it rises to 2-3 km.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.

K.H. Olsen, W.S. Baldridge and J.F. Callender

Published Journal 
Tectonophysics, 1987

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K.H. Olsen,W.S. Baldridge,J.F. Callender. 1987. Rio Grande Rift- an Overview. Tectonophysics. 143(1-3):119-139.