THE MAHOGANY PEAKS FAULT, A LATE CRETACEOUS-PALEOCENE() NORMAL-FAULT IN THE HINTERLAND OF THE SEVIER OROGEN

The contact separating Ordovician rocks from the underlying lower part of the Raft River Mountains sequence, northwestern Utah, is reinterpr eted as a large-displacement low-angle normal fault, the Mahogany Peak s fault, that excised 4-5 km of structural section. High delta(13)C va lues identified in marble in the lower part of the Raft River Mountain s sequence suggest a Proterozoic, rather than Cambrian age. Metamorphi c conditions of hanging wall Ordovician and footwall Proterozoic strat a are upper greenschist and middle amphibolite facies, respectively, a nd quantitative geothermometry indicates a temperature discontinuity o f about 100 degrees C. A discordance in muscovite Ar-40/Ar-39 cooling ages between hanging wall and footwall strata in eastern exposures, an d the lack of a corresponding cooling age discordance in western expos ures, suggest a component of west dip for the fault. The juxtaposition of younger over older and colder over hotter rocks, the muscovite coo ling age discordance with older over younger, and top-to-the-west shea ring down-structure are consistent with an extensional origin. The age of faulting is bracketed between 90 and 47 Ma, and may be synchronous with footwall cooling at about 60-70 Ma. Recognition of the Mahogany Peaks fault, its extensional origin, and its probable latest Cretaceou s to Paleocene age provides further evidence that episodes of extensio n at mid-crustal levels in the hinterland of the Sevier orogenic belt were synchronous with protracted shortening in the foreland fold and t hrust belt, and that the Sevier orogen acted as a dynamic orogenic wed ge.