THERMAL REGIME OF THE SOUTHERN BASIN AND RANGE PROVINCE .2. IMPLICATIONS OF HEAT-FLOW FOR REGIONAL EXTENSION AND METAMORPHIC CORE COMPLEXES

The average heat flow for the presently quiescent southern Basin and R ange Province (82 +/- 3 mW m(-2)) is only marginally lower than that f or the northern Basin and Range (92 +/- 9 mW m(-2)), even though the f ormer has been relatively inactive over the past 10-15 m.y. This resul t is consistent with a broad range of simple homogeneous models for pr ovince-wide extension; the high heat flow (roughly equivalent to doubl ing the stable mantle contribution) can develop quickly from nonconduc tive processes (extension, magmatism, delamination), but the decay is a conductive process that might have little effect in the first 10-15 m.y. of quiescence. The frequently cited province-wide estimate of 100 % extension (beta = 2) in the Basin and Range is consistent with the o bserved heat flow but not required by it. Extension models involving l ithosphere thinning by delamination (and not solely by stretching), an d/or magmatic additions, can provide the observed heat flow with less extension. These models are easier to reconcile with the buoyancy requ irements of the present high elevation. Loss of buoyancy required by 1 00% extension poses special problems; either the preextensional terrai n was as high as any on Earth today, or a large compensating source of buoyancy developed during the extension process. Possible candidates are magmatic contributions to the crust (which must be very large to b e important in this context), trapped melt or lighter phases in the ma ntle, or a locally hot plume-fed upper asthenosphere. Superimposed on the regional scale extension are zones where intense local extension h as removed much of the upper crust to form metamorphic core complexes. Two thermal observations might bear on their origin: (1) Relatively l ow heat flow (67 +/- 4 mW m(-2)) is associated with the main trend of metamorphic core complexes (MCCs) in Arizona, and (2) the outcropping rocks in the core complexes have a low radioactive heat production (1. 3 +/- .3 mu W m(-3)) compared to the other crystalline rocks in the re gion (2.1 +/- .2 mu W m(-3)). These observations are consistent with t he belief that radioactivity decreases downward in the crust and that MCCs are sites of massive unroofing. In fact, the measured heat flow l ow could be explained by the removal of 10-20 km of upper crustal radi oactivity provided the opposing effects of transient heating from unro ofing were short-lived. According to an idealized model, this would be the case if the compensating mass flow occurred in the lower crust, n ot in the mantle. In this sense the regional thermal results provide s ome support for the lower crustal return flow hypothesis for metamorph ic core complexes.