Abstract

The Rio Grande rift encompasses uplifts of the southern Rocky Mountains and their southern extension as well as axial fault blocks. The rift widens irregularly southward from a narrow horst in Colorado into a broad collapsed vault, characterized by grabens, in southern New Mexico. Whether manifested by horsts or grabens, primarily extensional strain is involved which increases in magnitude southward. Extensional faulting along the rift occurred in Neogene to Quaternary time, but the rift follows an axis of Laramide, Pennsylvanian, and possibly earlier uplifts. Gravity gradients due to the low density of graben fill delineate major faults of the rift system, which show a gridded or en echelon pattern over distances of tens of kilometres. Aeromagnetic data show these faults to be aligned with basement structural grain. Zigzags hundreds of kilometres long in the trend of the rift may also be related to basement grain. Basement trends in the Colorado Plateau to the west seem to differ in direction from those in the High Plains to the east. Seismic data also show that the rift occurs in an area of transition between anomalous crustal and upper mantle structure typical of the Cordillera and crustal structure typical of the High Plains. Deep seismic data are sparse within the rift, but high heat flow, high elevation, high electrical conductivity, and both residual positive (shallow source) and negative (deep source) gravity anomalies suggest the presence of symmetrical anomalous crustal and upper-mantle structure along the axis of the rift. In the Socorro area, where the rift has been studied intensively, available data indicate relatively low compressional velocity, rapid Holocene uplift, and the presence of magma within the crust. In view of geomorphic evidence for widespread Holocene faulting, the seismicity of the rift is surprisingly low.

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