LOWER CRUSTAL FLOW IN AN EXTENSIONAL SETTING - CONSTRAINTS FROM THE HALLORAN HILLS REGION, EASTERN MOJAVE DESERT, CALIFORNIA

Postextensional uplift and tilting of Quaternary lake sediments in the Halloran Hills, eastern Mojave desert, California, suggest that regio nal-scale flow within the lower crust is occurring in response to tect onic loads created during upper crustal extension in late Miocene time . The concept that significant flow within the lower crust may occur f or millions of years after the cessation of extension indicates that t opographic gradients can be modified by regional-scale flow within the lower crust and that direct constraints on lower crustal flow can be obtained by quantitative analysis of rates of postextensional surface tilting. An analytic solution, developed in this paper, combines visco us flow in the lower crust, flexural bending within the midcrust, and isostasy into an invertible linear differential equation that describe s the topographic response to upper crustal extension. Assuming that t here was no topographic relief across the Halloran Hills region prior to extension, inversion of modem topographic data indicates that crust al thinning above the Halloran Hills detachment increases westward wit h a listric geometry and that little crustal thinning has occurred eas t of the western flank of Clark Mountain, in good agreement with the k nown geometry of the fault and the location of its breakaway zone. Inc orporation of Quaternary tilting data indicates that the modem viscosi ty beneath the Halloran Hills (assuming a 10-km lower crustal channel) is a maximum of 10(19) Pa s and that the viscosity of the lower crust has decreased by at least 1 to 2 orders of magnitude since 8 Ma. This corresponds to a temperature increase of at least 75-degrees to 100-d egrees-C at the Moho. In our opinion, the most likely source of this t emperature increase is a regional-scale thermal event within the under lying mantle and diffusion of heat upward into the lower crust. If con -ect, these results have important implications for the way in which c rustal extension is linked to mantle heating within the Basin and Rang e Province.