Rio Grande rift--Problems and perspectives
— W. Scott Baldridge, Kenneth H. Olsen, and Jonathan F. Callender

Our intent in this paper is to present new ideas concerning the Rio Grande rift which we hope are at least provocative, if not correct, and to emphasize important unanswered questions which will be fruitful areas for continued research. Our approach is to evaluate, relate, and interpret existing data derived from a number of disciplines, rather than primarily to present new data. We hope thereby to achieve a synthesis of sorts—a new and broad perspective on the Rio Grande rift. This perspective should help guide future observations by directing attention toward the more fundamental questions regarding rifting processes. In this paper we use the Basaltic Volcanism Study Project (1981, p. 838) definition of rifts as elongate depressions overlying places where the entire thickness of the lithosphere has ruptured in extension.

Among the topics and ideas that we address are: (1) the regional extent of the Rio Grande rift, (2) the structure of the crust and upper mantle, (3) whether the evidence for an "axial dike" (i.e., a composite mafic intrusion) in the lower crust is compelling, (4) the nature of faulting and extension in the crust, and (5) the structural and magmatic development of the rift. These issues provide important constraints on the nature of thermal and tectonic processes involved in formation of the Rio Grande rift.

The Rio Grande rift is a region where the lithosphere is being permanently altered through thinning and (probably less importantly) intrusion of mafic magmas. The rift is the culmination of a long and complex geologic history. Initiation of rifting probably resulted from plate-boundary interactions along the west coast of North America (e.g. , Lucas and Ingersoll, 1981). However, the presence of a major thermal event associated with the immediately preceding subduction regime may have been a necessary condition for rifting.