(U-Th)/He ages have been measured on igneous apatites from the San Jacinto mountains, a high region at the junction of the Peninsular and Transverse Ranges, to investigate the potential of this technique for thermochronometry of slowly cooled rocks. Helium ages from 79 to 17 Ma are younger than ages obtained by other dating techniques, including apatite fission-track counting, and are consistent with laboratory experiments that indicate this system has a uniquely low closure temperature. Helium ages are strongly correlated with elevation and record the latest low-temperature thermal evolution of the range. They suggest relative tectonic quiescence in the latest Cretaceous through mid-Tertiary and provide no evidence for rapid unroofing of the block during this period. Nor do they obviously require a large degree of uplift associated with convergence between the Transverse and Peninsular ranges in the last few million years. Helium ages document modest westward tilting of the block (∼ 7°) and a significant vertical component of motion on the block's bounding faults after helium retention began. This work suggests that apatite helium ages record low-temperature tectonic and thermal histories that are not apparent from other dating techniques.

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