Pop-Up and Tilt of the Mt. Givens Pluton, Central Sierra Nevada, California
Gilder, S A (1), McNulty, B A (2)
(1)Institut de Physique du Globe de Paris, Laboratoire de Pal\'{e}omagn\'{e}tisme, 4 place Jussieu, 75252 Paris, Cedex 05 France; (2) Earth Science Dept., California State Univ. Dominguez Hills, Carson, CA 90747 United States
Abstract:
The Mt. Givens pluton (MGP) is one of the largest and best exposed granites in the Sierra Nevada, California. Isotopic dates indicate very rapid cooling of the pluton- 400$\deg$C from 90 to 88 Ma. Cooling by 'passive' uplift and exhumation cannot explain the time-temperature paths of the MGP and neighboring plutons. These granite bodies were intruded at successively higher levels in the crust from west to east, yet today they reside at the same level. To help constrain how the MGP attained its relative position with respect to its surroundings, we demagnetized 277 paleomagnetic samples collected from widespread stations. Linear magnetic components were isolated in 265 samples. Remanent magnetization directions are held in titanium-free, multi-domain magnetite. Together they define an overall mean of D=1.9$\deg$, I=63.7$\deg$, $\alpha$95=3.0$\deg$. With respect to either neighboring coeval plutons or the North American reference, the MGP was significantly rotated 21$\deg$ clockwise but not significantly displaced latitudinally. The discordance is best resolved via a tectonic model of 10$\deg$ down-to-the-west tilting of the MGP along a horizontal axis that parallels the dominant regional fabric. Previous work on surrounding structures supports the tilt model in terms of timing, kinematics and amount of offset. Together with the paleomagnetic data, they suggest the MGP popped up and tilted along the Bench Canyon reverse fault to the east and the Courtright normal fault to the west. High dispersion of paleomagnetic directions found in our study and others from nearby plutons suggests that collecting samples over limited geographic areas tends to poorly define mean directions. Sampling strategies for paleomagnetic studies in plutons should concentrate on collecting numerous samples from widespread locations rather than densely sampling at a few sites and calculating the overall mean on the site level.
