Master of Science (MS), Ohio University, 2015, Geological Sciences (Arts and Sciences)

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). 480 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 ~25% of the shear zones record heterogeneous δ18O(mineral) 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 temperature, assuming rims equilibrated) vs. vertical position are resolved. Δ18O(Qtz-Ep) generally increases (= decreasing temperature) over ~30–100 m vertical transects from the footwall into the d (open full item for complete abstract)

Committee: Craig Grimes Ph.D. (Advisor); Greg Nadon Ph.D. (Committee Member); Damian Nance Ph.D. (Committee Member) Subjects: Geology

Master of Science (MS), Ohio University, 2014, Geological Sciences (Arts and Sciences)

Fluids are likely significant during the life-cycle of low-angle normal faults (LANFs) as well as other fault systems, but the role of those fluids and their source at fault initiation are unclear. The Mohave Wash Fault (MWF), a LANF situated within the Chemehuevi Mountains core complex (SE CA), offers a well exposed site to evaluate this question. The MWF slipped 1-2 km during the Miocene before being denuded passively to the surface by extension localized on the higher-level Chemehuevi Detachment Fault. To evaluate fluid-rock interactions during the early slip on this fault system, δ18O values of whole rocks, quartz, and epidote were measured by CO2-laser fluorination and interpreted along with field and microscopic observations of fault rocks from this area. The MWF damage zone is variable in thickness and characterized by cracked granitic rocks hosting mineralized fractures, cohesive cataclasites, thin foliated shear zones, and rare pseudotachylite. δ18O of quartz hosted by undeformed granite ranges from 9.0-10.3‰, defining predeformation values. Foliated shear zones and quartz veins extend to lower δ18OQtz from 10.1-6.1‰, while cataclasites record the lowest δ18OQtz values down to 1.1‰. The δ18O values of epidote (from all types) ranges from 5.3‰ to - 0.4‰; the lowest values are generally in cataclasites. The shifts to lower δ18O are explained by interaction with heated, low δ18O fluids from an external source (evolved meteoric fluids or basin brines). Apparent temperatures from stable isotope thermometry on coexisting quartz and epidote (from 0.5 cc of rock) from the footwall are typically 50- 150°C higher than ambient footwall temperatures at 23 Ma (fault initiation) determined using 40Ar/39Ar closure temperatures (John and Foster, 1993). Temperatures defined by both methods increase in the paleodip direction. The temperature difference across the footwall estimated from Δ18O(Qtz-Ep) versus Ar/Ar closure temperatures either indicates the mineralization (open full item for complete abstract)

Committee: Craig Grimes PhD (Advisor); Damian Nance PhD (Committee Member); David Kidder PhD (Committee Member) Subjects: Geochemistry; Geology