Grimes, MacDonald Present at GSA Rocky Mountain Meeting

Shear band (cataclasite) where the green bands mark the upper and lower contact of a ~3 cm-thick zone of deformation. -Collected from the Chemehuevi Mountains

Ohio University Geological Sciences researchers presented May 2014 at the Joint Rocky Mountain/Cordilleran Section Meeting of the Geological Society of America. They are part of an international team working in the Chemehuevi Mountains in Southeast California. See their work on Tumblr.

Oxygen Isotope Constraints on the Early Slip History of the Mohave Wash Fault, Chemehuevi Mountains, Southeast California

Ohio University’s Cody MacDonald, a graduate student, and Dr. Craig Grimes, along with co-authors Barbara John  and Justin Scott LaForge (University of Wyoming), , Rüdiger Kilian, Renee Heilbronner (Basel University, Switzerland), Holger Stünitz (Universitetet i Tromsø, Norway), and John W. Valley and Michael J. Spicuzza (University of Wisconsin-Madison), presented a paper on Oxygen Isotope Constraints on the Early Slip History of the Mohave Wash Fault, Chemehuevi Mountains, Southeast California.

Abstract: Fluids are likely significant during the life-cycle of low-angle normal faults (LANFs), 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 key site to evaluate this question. Sampling was focused along 15 km down dip along the MWF, which slipped 1-2 km during the Miocene before being denuded passively to the surface by extension preferentially localized on the higher-level Chemehuevi Detachment Fault. To evaluate fluid-rock interactions during initiation of this fault system, δ¹⁸O values of quartz and epidote were measured by CO₂-laser fluorination.

The MWF cuts Cretaceous granitic rocks and Precambrian gneiss. The 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 observed in cataclasites. The shifts to lower δ¹⁸O (mineral) are explained by interaction with heated, low δ¹⁸O fluids (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 replicated by samples collected near one another, and 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 generally increase in the paleodip direction. The temperature difference across the footwall estimated from Δ¹⁸O_(Qtz-Ep) versus Ar/Ar closure temperatures either indicate the mineralization occurred prior to Ar/Ar closure or reflect localized upwelling of hot, deep-seated fluids during slip along the MWF. Calculated δ¹⁸O_H2O in equilibrium with mineral pairs decreases with lower temperature, consistent with influx of progressively lower δ¹⁸O fluids with decreasing temperature and depth.

Initiation and Strain Localization Along a Low-Angle Normal Fault in Crystalline Basement

LaForge, John, Grimes, MacDonald, Heilbronner, Kilian, Stünitz, and Erin Campbell-Stone (University of Wyoming) presented a paper on Initiation and Strain Localization Along a Low-Angle Normal Fault in Crystalline Basement.

Abstract: Exposures of the Miocene Chemehuevi detachment fault system (SE CA) provide an opportunity to study the initiation and incipient slip on low-angle normal faults (LANFs) active near the base of the seismogenic zone (5-15 km paleodepth). The regional fault system formed at ≤20° dip in heterogeneous gneissic and granitoid rocks, with ambient footwall temperatures from 200 to >400°C. The fault system is characterized by three stacked, anastomosing low-angle normal faults; one of these (the Chemehuevi detachment – CDF) preferentially localized ≥ 18 km of NE directed slip rendering the deepest fault, the Mohave Wash Fault (MWF), inactive after <2 km of slip. At outcrop scale, damage zones to each fault are planar, but at map scale both the MWF and CDF are corrugated parallel to slip.

Detailed macro- to microstructural studies of fault rocks associated with the MWF, sampled over 15 km down dip, provide insight into strain localization at initiation. Throughout its exposure, the MWF is characterized by a principal slip zone of chloritic cataclasite ≤2 m thick, with an asymmetric damage zone tens of meters thick localized in the hanging wall. At structurally shallow levels (T 200-250° C; 6-8 km paleodepth at initiation), the MWF cuts isotropic granitic rocks; cohesive cataclasites and semi-brittle phyllonites show subgrain development in quartz, with fractured plagioclase locally hosting deformation twins and kinking. Five kilometers down dip (T 300-350° C), crystal plastic deformation intensifies but is still overprinted by cataclasis; syntectonic dikes show crystal plastic deformation with minimal brittle overprint. Rare pseudotachylite is present within meters of the principal slip zone. At the structurally deepest exposures of the fault (T ≥ 400°C; 12-15 km paleodepth), the MWF juxtaposes Cretaceous granitoids against gneissic basement hosting dikes with a well-developed mylonitic lineation parallel to the extension direction. Here, mylonitic dikes and shear bands cutting the gneiss display microstructures indicative of subgrain rotation recrystallization, cut locally by zones of fine cataclasite hosting reworked mylonitic clasts.

The distribution of preserved increasingly ‘hot’ deformation mechanisms down dip (SW to NE) implies slip on the MWF was maintained at a dip of ≤20° throughout the seismogenic zone.