Dr. Tyler Grambling
Visiting CC

Topic: Fingerprinting fluid-assisted reactions in mid-crustal shear zones: Implications for rock strength and paleoaltimeters

Academic host: Kevin Mahan

Abstract: Interaction between meteoric water and silicates is an important contributor to strain-softening reactions in quartzofeldspathic crust and a useful archive of fluid sources during faulting. Stable isotopes of oxygen and hydrogen are commonly used to detect the presence and source of fluids migrating through and reacting with crystal-plastic shear zones. While both elements trace water-rock reactions, the relative abundance of oxygen in the crust relative to hydrogen leads to informative variations in the atomic water/rock ratio required to fully equilibrate d²H, d¹⁸O, and D'¹⁷O values in deforming minerals. Additionally, cost-effective analysis of d²H values is limited to H-bearing phases such as micas and amphiboles, whereas d¹⁸O and D'¹⁷O can readily be measured in most phases. While these challenges potentially limit the utility of each isotope system, they can be capitalized on to clearly identify fluid-assisted metamorphic reactions. d²H values from muscovite and biotite are presented alongside d¹⁸O and D'¹⁷O from both mica phases, quartz, and feldspar from multiple samples across nine depth transects through the Miocene-Pliocene Cordillera Blanca shear zone in the Peruvian Andes. Commonly, each isotope system records different atomic water/rock ratios within the same sample, informing the robustness of each isotopic tracer in the presence of volumetrically low water concentrations.  While d²H values clearly preserve a compositional record of the fluid source and most accurately inform regional paleoelevation during deformation, d¹⁸O and D'¹⁷O record mixing of primary magmatic oxygen reservoirs with meteorically derived fluids. Intrasample variability of d¹⁸O and D'¹⁷O in different phases clearly identifies the metamorphic breakdown of feldspar to quartz and muscovite during migration of meteoric fluids within the deforming rock. This pattern is confirmed in through the difference in water/rock ratio recorded in muscovite and biotite. Breakdown of feldspar to wet quartz and muscovite drives strain softening through grain-area reduction of rigid feldspar and development of interconnected micaceous domains that are occasionally significant enough to drive a shift from a quartz-dominated rheological framework to a deformation regime dictated by continuous slip along basal cleavage planes of micas.