9: Models of an extending lithosphere and heat flow in the Basin and Range province
Arthur H. Lachenbruch, J. H. Sass, 1978. "9: Models of an extending lithosphere and heat flow in the Basin and Range province", Cenozoic Tectonics and Regional Geophysics of the Western Cordillera, Robert B. Smith, Gordon P. Eaton
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Reduced heat flow in the Basin and Range province is characteristically greater by 50% to 100% than that in stable regions; in the hotter subprovinces like the Battle Mountain High, it is greater by 300%. Evidence for distributed tectonic extension and magmatism throughout the province suggests that much of the anomalous heat is transferred from the asthenosphere by convection in the lithosphere, in the solid state by stretching, and in the magmatic state by intrusion. Simple steady-state thermomechanical models of these processes yield relations among reduced heat flow, asthenosphere flux, lithosphere thickness, extension rate, and basalt production by the asthenosphere. Thermal effects in an extending lithosphere lead to decreased estimates of temperature and increased estimates of lithosphere thickness in the Basin and Range province. Moderate extension rates can account for high heat flow in the province without calling on anomalous conductive flux from the asthenosphere. The heat and mass budgets of bimodal volcanic centers suggest that they occur at points where the lithosphere is pulling apart rapidly, drawing up basalt to fill the void. Intrusion can probably facilitate lithosphere extension at low stress levels either by brittle “hydrofracturing” by basaltic dikes or by warming and thinning caused by basaltic underplating. Whether lithosphere extension occurs in the distributed mode or in the plate-tectonic mode might depend largely upon whether the lateral divergence of mass can be supplied by asthenosphere basalt, or whether it must be supplied by the ascent of very viscous ultramafic material which requires wide conduits separated by large distances. For a range of plausible models of distributed extension, the anomalous heat flow increases roughly 1 HFU (10−6 cal cm−2·s−1) for every 1% to 2%/m.y. increase in extension rate; the relation suggests extension rates in the Great Basin consistent with estimates from structural evidence. It also suggests much more rapid local extension in the hotter subprovinces, an inference supported by limited evidence from other sources.