Tectonomagmatic evolution of Cenozoic extension in the North American Cordillera

Abstract

The spatial and temporal distributions of Cenozoic extension and magmatism in the Cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt (< 100 km, pre-extension) and followed the onset of intermediate and silicic magmatism by no more than a few million years. Extension began in early Eocene time in southern British Columbia, northern Washington, Idaho and Montana. Farther S, extension began at about the Eocene-Oligocene boundary in the Great Basin and slightly later in the Mojave-Sonora Desert region. The intervening area, at the latitude of Las Vegas, remained quiescent until mid-Miocene time. Compositional and isotopic characteristics of most pre-Miocene magmas are consistent with their containing major components of melted continental crust. In mid-Miocene time, two major changes occurred: widening of the area of extension and the widespread appearance of basaltic magmas. The area affected by extension, from southwestern Montana to the Lake Mead region, widened to several hundred kilometres. By this time extension in southern British Columbia, northern Washington and northern Montana had ceased (probably before the end of the Eocene), and extension S of Lake Mead (except in the Gulf of California) had waned. Regions affected by the broader belt of extension during late Miocene, Pliocene, and Quaternary time experienced basaltic magmatism, which began along a central rift zone in the northern part of the region, and which had within a few million years spread to include most of the region; later basaltic activity has tended to concentrate in restricted zones, especially near the margins of the extended area. We recognize a correlation between the amount of earlier crustal thickening and Cenozoic extension, and between the length of time after shortening but before extension and the degree to which a given region was intruded by Late Cretaceous plutons. The localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal-mechanical model developed in a companion paper (Sonder et al.). This raises the possibility that stresses inherent in the North American Plate dominated over plate-interaction forces as controls of the Cenozoic tectonomagmatic evolution of the North American Cordillera, especially in its earlier stages.