2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 318-4
Presentation Time: 9:00 AM-6:30 PM


BROWN, James E.1, GRIMES, Craig B.1, JOHN, Barbara E.2, KITAJIMA, Kouki3 and VALLEY, John W.4, (1)Department of Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, OH 45701, (2)Geology and Geophysics, University of Wyoming, Laramie, WY 82071, (3)Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, (4)WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215, W. Dayton St, Madison, WI 53706, jb517613@ohio.edu

The Mohave Wash fault (MWF), a low angle normal fault (~2 km of slip) initiated near the brittle-ductile transition in crystalline rocks, is associated with the regionally developed Chemehuevi detachment system. To address the role of water on initiation and early slip, δ18O of quartz/epidote pairs from thin shear zones and vein-fill were analyzed in situ using a 10 μm ion microprobe spot (precision ±0.3‰, 2 SD). 503 analyses were made on 317 grains in 23 samples collected from three vertical transects from the footwall and through the damage zone, distributed over 17 km down-dip. Quartz from undeformed hosts defines pre-faulting δ18O = 9.0 – 10.4‰ VSMOW. δ18O values decrease within damage zone microstructures down to -1.0‰ for quartz and -5.3‰ for epidote. Such low δ18O values at the structurally deepest exposures are interpreted to reflect influx of surface-derived fluids to depths of >10 km.

Syn- and post-deformation mineralization in ~30% of the shear zones record heterogeneous δ18O(min) on the scale of <100 mm2. Inter- and intra-crystalline variability in δ18O is greatest in the damage zone. Host clasts are often preserved, but textural relations also signify heterogeneity in new mineral growth within discrete shear zones. Of 123 grains analyzed with multiple spots, 36% are zoned in δ18O; single-grain gradients reach 8.7‰ (over 500 μm) for quartz and 2.1‰ (over 300 μm) for epidote. Differences in Δ18O(qtz-ep) from adjacent rims over <100 mm2 range from 0.2 – 8.0‰ (in damage zone) and 0.6 – 2.2‰ (below damage zone). Large variability in measured Δ18O(qtz-ep)is consistent with variable oxygen isotope exchange, and sub mm-scale heterogeneities in permeability.

Despite the intrasample-variability, overall trends in Δ18O(qtz-ep) from rims on adjacent grains (and thus T, assuming rims equilibrated) vs. vertical position are resolved. Δ18O(qtz-ep) generally increases (= decreasing T) over ~30-100 m vertical transects from the footwall into the damage zone at structurally-deep exposures, consistent with footwall refrigeration. T defined at shallow exposures is relatively high, and implies significant heat transfer up the fault. These results are interpreted to reflect surface-derived fluid infiltration at the onset of slip followed by fluid recirculation likely driven by syntectonic dike emplacement.