Modal mineralogy of the continental crust and implications for fault-zone rheology: Data mining the Southern Sierra Nevada exhumed crustal section

Alex E. Morelan, & Michael E. Oskin

Published August 15, 2018, SCEC Contribution #8753, 2018 SCEC Annual Meeting Poster #149

We examine the exhumed section from the southern Sierra Nevada to constrain modal mineralogy of batholithic crust as a function of depth, from 6 km to 35 km, for inclusion into the Community Rheology Model. We combine mineralogy data collected by Ross (1987) and paleobarometry data compiled by Chapman et al. (2012) to quantify mineral content with depth and across an east-west compositional gradient. Modal mineralogy of approximately 1500 samples from across the southern Sierra Nevada batholith are reported in the form of small, hand-drawn maps of each pluton body with corresponding handwritten tables listing the modal mineralogy of each sample. In order to mine this dataset, we georeferenced 62 maps of individual plutonic bodies to locate the samples and digitized the associated data tables. We predict paleo-depth for each sample by interpolating paleobarometric data using a kriging algorithm. By integrating these datasets we have a clearer picture of how modal mineralogy changes with depth in the crust over areas of intact batholithic crust, including the Peninsular Ranges batholith where crossed by the Elsinore and San Jacinto faults. We confirm that mineralogy of the batholith changes little over most of its depth extent, consisting of 54 to 59% feldspar, primarily plagioclase (averages reported over 5 km intervals). Rheologically weak quartz and biotite are present throughout the crust, at 21-24% and 9-11% average abundance above 30 km, declining to 14% and 7%, respectively, averaged from 30 to 35 km depth. Rheologically strong hornblende increases in abundance with depth, from 7% to 22%. This unique dataset will be used to test bulk and shear-zone models for the crustal strength.

Citation
Morelan, A. E., & Oskin, M. E. (2018, 08). Modal mineralogy of the continental crust and implications for fault-zone rheology: Data mining the Southern Sierra Nevada exhumed crustal section. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
SCEC Community Models (CXM)