CENOZOIC CRUSTAL PROCESSES IN SOUTHWEST UTAH; INTERACTION OF MAGMATISM AND DEFORMATION ALONG A LONG-LIVED BOUNDARY ZONE

Southwest Utah occupies a pivotal long-lived deformational boundary separating the Cenozoic Basin and Range Province (west) from the stable, high-standing Colorado Plateau (east). This area served as a key crustal boundary during late Proterozoic extension (miogeoclinal hinge-line), late Paleozoic to early Tertiary shortening, and mid-Cenozoic extension. Short-lived (<1.5 m.y.) but voluminous magmatism occurred in the region during the Miocene, likely related to sublithospheric Farallon slab activity. Between 22 and 20 Ma, about a dozen large, shallow level laccoliths were preferentially emplaced laterally along a pre-existing structure (a Sevier thrust fault). A growing body of evidence suggests the laccoliths were fed from one or more batholiths which may have exploited deeper seated discontinuities (i.e., basement cored thrusts). Today, the laccoliths are largely undeformed, whereas surrounding Miocene volcanic rocks (north and west) are highly faulted and exhibit substantial vertical axis rotations. We suggest that large scale, plutonic stitching of an ancient crustal discontinuity caused subsequent Cenozoic deformation to migrate around the locus of laccolith emplacement. The Cenozoic geologic history of this area may serve as a snap-shot of how magmatism and deformation interact in stabilizing ancient crust. The concept of tectonic heredity suggests that pre-existing crustal heterogeneities greatly influence the geometry and style of subsequent crustal deformation (i.e., fault reactivation). However, being zones of weakness, discontinuities may also tend to channelize magmas. In southwest Utah, magma emplacement may have served to anneal this major zone of weakness in the crust. An example of this process occurring on a continental scale would be the pervasive magmatic ‘stitching' of province boundaries in the Proterozoic U.S. mid-continent (Whitmeyer and Karlstrom, 2004), which resulted in rapid stabilization of continental lithosphere.