TIMING OF DETACHMENT FAULTING IN THE BULLFROG HILLS AND BARE MOUNTAINAREA, SOUTHWEST NEVADA - INFERENCES FROM AR-40 AR-39, K-AR, U-PB, ANDFISSION-TRACK THERMOCHRONOLOGY/

Crustal extension in the Bullfrog Hills and Bare Mountain area of sout hwest Nevada is associated with movement along a regional detachment f ault. Normal faulting in the upper plate and rapid cooling (denudation ) of the lower plate were coeval with Miocene silicic volcanism and wi th west-northwest transport along the detachment fault. A west-northwe st progression of tilting along upper plate normal faults is indicated by ages of the volcanic rocks in relation to angular unconformities. Near the breakaway, tilting in the upper plate occurred between 12.7 a nd 11.6 Ma, continued less strongly past 10.7 Ma, and was over by 8.2 Ma. Ten to 20 km west of the breakaway, tilting occurred between 10.7 and 10.33 Ma, continued less strongly after 10.33 Ma, and was over by 8.1 Ma. The cooling histories of the lower plate metamorphic rocks wer e determined by thermochronologic dating methods: K-Ar and Ar-40/(39)A on muscovite, biotite, and hornblende, Ar-40/(39)A on K-feldspar, U-P b on apatite, zircon, and sphene, and fission track on apatite, zircon , and sphene. Lower plate rocks 10 km west of the breakaway cooled slo wly from Early Cretaceous lower-amphibolite facies conditions through 350+/-50 degrees to 300+/-50 degrees C between 57 and 38 Ma, then cool ed rapidly from 205+/-50 degrees to 120+/-5O degrees C between 12.6+/- 1.6 and 11.1+/-1.9 Ma. Lower plate rocks 20 km west of the breakaway c ooled slowly from Early Cretaceous upper-amphibolite facies conditions through 500+/-50 degrees C at 78-67 Ma, passed through 350+/-50 degre es to 300+/-50 degrees C between 16.3+/-0.4 and 10.5+/-0.3 Ma, then co oled rapidly from 285+/-50 degrees to 120+/-50 degrees C between 10.2 and 8.6 Ma. Upper plate tilting and rapid cooling (denudation) of the lower plate occurred simultaneously in the respective areas. The early slow-cooling part of the lower plate thermal histories was probably r elated to erosion at the Earth's surface, which stripped off about 9 k m of material in 50 to 100 m.y. The results indicate an initial fault dip greater than or equal to 30 degrees and a 12 mm yr(-1) west-northw est migration of the locus of rapid tilting in the upper plate.