Thermal regime of the southern Basin and Range Province: 2. Implications of heat flow for regional extension and metamorphic core complexes

Abstract

The average heat flow for the presently quiescent southern Basin and Range Province (82±3 mW m−2) is only marginally lower than that for the northern Basin and Range (92±9 mW m−2), even though the former has been relatively inactive over the past 10–15 m.y. This result is consistent with a broad range of simple homogeneous models for province‐wide extension; the high heat flow (roughly equivalent to doubling the stable mantle contribution) can develop quickly from nonconductive 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 (β = 2) in the Basin and Range is consistent with the observed heat flow but not required by it. Extension models involving lithosphere thinning by delamination (and not solely by stretching), and/or magmatic additions, can provide the observed heat flow with less extension. These models are easier to reconcile with the buoyancy requirements of the present high elevation. Loss of buoyancy required by 100% extension poses special problems; either the preextensional terrain 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 be important in this context), trapped melt or lighter phases in the mantle, or a locally hot plume‐fed upper asthenosphere. Superimposed on the regional scale extension are zones where intense local extension has removed much of the upper crust to form metamorphic core complexes. Two thermal observations might bear on their origin: (1) Relatively low 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 μW m−3) compared to the other crystalline rocks in the region (2.1±.2 μW m−3). These observations are consistent with the belief that radioactivity decreases downward in the crust and that MCCs are sites of massive unroofing. In fact, the measured heat flow low could be explained by the removal of 10–20 km of upper crustal radioactivity provided the opposing effects of transient heating from unroofing were short‐lived. According to an idealized model, this would be the case if the compensating mass flow occurred in the lower crust, not in the mantle. In this sense the regional thermal results provide some support for the lower crustal return flow hypothesis for metamorphic core complexes.