ION MICROPROBE δ18O-CONSTRAINTS ON FLUID MOBILITY AND THERMAL STRUCTURE DURING EARLY SLIP ON A LOW-ANGLE NORMAL FAULT, CHEMEHUEVI MOUNTAINS, SE CA

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.