MAGMATIC AND TECTONIC INFLUENCES ON THE MARGINS OF THE BASIN AND RANGE PROVINCE, WESTERN USA

In the western US, the east and west margins of the Basin and Range province (BRP) differ in detail, but both have been greatly influenced by inherent crustal features (e.g. magmatic belts and ancient tectonic boundaries). An obvious present-day difference between the east and west margins is that extension dominates in the east, whereas transtension associated with dextral motion between the Pacific-North American plates prevails in the west. Nearly the entire west margin from southern California northward to the Sierra Nevada is controlled by Mesozoic batholithic belts. Similarly, a relatively unextended island occupies the central part of the BRP in central Nevada, where a large suite of Oligocene-Miocene calderas, associated with the ignimbrite flareup, indicates a belt of mid-Cenozoic batholiths. As surmised by others, a relative lack of preexisting weaknesses, compared to surrounding country rock, have presumably allowed the batholiths to resist extension.

In contrast, the east margin in Utah essentially follows the late Proterozoic miogeoclinal hinge-line, west of which thick sequences of late Proterozoic to Devonian passive-margin sediments accumulated. West of the hinge-line, subhorizontal discontinuities and weak, incompetent layers in the thick passive-margin sequences subsequently facilitated late Devonian to early Tertiary folding, thrusting, and crustal thickening. Gravitational collapse of the overthickened crust to the west of the hinge-line then drove large-magnitude extension in the mid- to late Tertiary. Thus, the east margin of the BRP in Utah parallels both the late Proterozoic miogeoclinal hinge-line and east edge of the Sevier-Laramide fold and thrust belts.

Where inherent tectonic boundaries or magmatic belts were oriented nearly orthogonal to ~E-W extension (Utah and California), relatively abrupt boundaries developed along the margins of the BRP. However, through much of Arizona, where the margin trends northwest oblique to late Cenozoic ~E-W extension, a broad transition zone separates the BRP and unextended Colorado Plateau. Curiously, it is at these latitudes where appreciable extension has been diffused eastward into the craton and reactivated Laramide structures along the Rio Grande rift and Trans-Pecos region of Texas.