Major geologic structures between Lordsburg, New Mexico, and Tucson, Arizona
— Harald D. Drewes and C. H. Thorman

The region of southwestern New Mexico and southeastern Arizona traversed by the joint field trip of the New Mexico and Arizona Geological Societies in 1978 is noted for its abundant and varied structural features. Gilluly's (1956) description of part of the Dragoon and Mule mountains shows many of these features and clearly implies a complicated tectonic history. In recent decades numerous field studies have been made of local areas or of selected structural features of southeastern Arizona and adjacent areas; from these studies varied and seemingly conflicting interpretations have been offered. Field review of past mapping and much new mapping during the last 16 years have provided the basis for a tectonic synthesis of southeastern Arizona (Drewes, 1976, 1978a, 1978b). This synthesis is being extended by us into New Mexico, where Corbitt and Woodward (1970) have also presented tectonic interpretations of Laramide deformation.

 

In this article we will summarize the main geologic features along the excursion route. Emphasis will be on description or field interpretation of age, geometry and direction of movement of faults and folds. This emphasis is in keeping with the main objectives of the trip: to illustrate representative features of a broad variety of geologic topics rather than presenting evidence in support of one or another hypothesis covering a single regional topic. With this background, the trip participants may be better prepared to participate in such informal discussions on regional geologic interpretations as may arise.

 

The structural complexities of the region reflect the follow-ing conditions: (a) Deformation has occurred repeatedly, first during the Precambrian Mazatzal revolution, and again in the Triassic, the Late Jurassic or Early Cretaceous, the Late Cretaceous to Paleocene Cordilleran orogeny, and the mid-Tertiary; some activity has continued to historic time. (b) Some structures were formed in response to regional stresses; others developed under local conditions. (c) Segments of many faults were reactivated, some of them repeatedly and with diverse directions of movement. Hereafter, such faults will be referred to as complex faults. Under these conditions it is no wonder that some seemingly conflicting interpretations occur. Indeed, the only structural generalization that may be valid is that it is unlikely that the rocks were ever structurally isotropic in the time span represented by the geologic record, except on a very local scale.

 

A brief review of the geologic setting of the region indicates the probable significance of the major kinds of structures. About 1,600 m.y. ago the region was covered by sedimentary and some volcanic rocks. Later these rocks were deeply buried; many of them were metamorphosed and massively invaded by granite stocks and batholiths before being uplifted and peneplaned. During late Precambrian and Paleozoic time the region bordered a craton and fluctuated between gentle uplifts and downwarps. Crustal disturbances and magmatic activity were renewed during the Triassic and extended into the Early Cretaceous, followed by a quiescent time during which an arm of the sea briefly invaded the region from the south. During Late Cretaceous and Paleocene time the region was subjected to compressive stress like that occurring throughout the Cordilleran orogenic belt, resulting in folds and faults. Finally, from Oligocene time onward the extensional conditions of the Basin and Range province created the characteristic landscape of fault-block mountains separated by sediment-filled basins.