ASSESSMENT OF (U-TH) HE THERMOCHRONOMETRY - THE LOW-TEMPERATURE HISTORY OF THE SAN-JACINTO MOUNTAINS, CALIFORNIA/

(U-Th)/He ages have been measured on igneous apatites from the San Jac into mountains, a high region at the junction of the Peninsular and Tr ansverse Ranges, to investigate the potential of this technique for th ermochronometry of slowly cooled rocks. Helium ages from 79 to 17 Ma a re younger than ages obtained by other dating techniques, including ap atite fission-track counting, and are consistent with laboratory exper iments that indicate this system has a uniquely low closure temperatur e, Helium ages are strongly correlated with elevation and record the l atest low-temperature thermal evolution of the range, They suggest rel ative tectonic quiescence in the latest Cretaceous through mid-Tertiar y and provide no evidence for rapid unroofing of the block during this period, Nor do they obviously require a large degree of uplift associ ated with convergence between the Transverse and Peninsular ranges in the last few million Sears, Helium ages document modest westward tilti ng of the block (similar to 7 degrees) and a significant vertical comp onent of motion on the block's bounding faults after helium retention began, This work suggests that apatite helium ages record low-temperat ure tectonic and thermal histories that are not apparent from other da ting techniques.