Deep crustal thermal and mechanical processes that occur during orogenesis, and subsequent extensional collapse can range in duration and timing. New zircon laser ablation–inductively coupled plasma–mass spectrometry U-Pb data provide evidence for long-lived heating of the middle-to-lower crust exposed in the Black Mountains of Death Valley, California, USA. The data come from felsic intrusions and surrounding gneisses and schists in the footwalls of Miocene and younger detachment faults. By using depth profiling, we were able to identify Proterozoic, Miocene, and, most importantly, Cretaceous through Eocene age zones within individual zircons. Proterozoic and Miocene age zones have metamorphic Th/U ratios, whereas Cretaceous-Eocene ages have igneous signatures. In one case, igneous zircons from a felsic intrusion yielded only Late Cretaceous ages. These new ages from the Black Mountains, when combined with previously reported U-Pb ages from the region, show that deep crustal heating continued well past the cessation of upper crustal Sevier shortening in mid-Cretaceous time. Alternative hypotheses for the lower crustal heating include self-heating or small-scale convection in the mantle wedge.

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