Strain localization at brittle-ductile transition depths during Miocene magmatism and exhumation in the southern Basin and Range. (October 2022) - Zuza, Andrew V.
Abstract: Feldspar dominates the middle and lower continental crust. Models of crustal rheology depend on our knowledge of feldspar deformation mechanisms. To learn more about natural feldspar deformation, we conducted a multidisciplinary investigation of a brittle-ductile transition (BDT) hosted within a Cretaceous pluton in the Colorado River Extensional Corridor, southern Nevada. Here, a distinct BDT was observed at ∼10 km paleo-depth, where brittle faulting transitioned to discrete, localized ductile shearing. Field relationships confirm this BDT to have deformed and exhumed in the Miocene, temporally associated with the intrusion of two surrounding ca. 16 Ma plutons and subsequent footwall rotation in an east-directed normal-fault system. We established the overall structural framework, strain rates, and thermal histories through field traverses, aluminum-in-hornblende barometry to reconstruct paleo-depth, zircon (U–Th)/He thermochronology (ZHe) to constrain temperature-time histories, and 1D thermal models to further resolve deformation temperatures. Localized shear zones within the BDT formed ca. 16.2–15.5 Ma at temperatures of ∼500–600 °C, which continued deforming as they cooled both conductively and advectively to ∼200 °C by ca. 14 Ma, as constrained by ZHe dates. Strain was concentrated in fine-grained (7–10 μm) feldspar-rich ultramylonites that were interpreted to have primarily deformed via grain-size-sensitive diffusion creep. Grain-size reduction that allowed activation of diffusion creep likely resulted from brittle cataclasis, fluid-assisted fracturing and neocrystallization, and dislocation creep mechanisms, thus emphasizing the importance of these processes to localize deformation at relatively strong mid-crust conditions to facilitate the development of diffusion-creep shear zones. Grain-size piezometers suggest stresses of ∼50 MPa, which are significantly weaker than peak strength in quartz-rich BDTs that deform via dislocation creep. This integrated process allows relatively low viscosities (∼10 18 –10 19 Pa s) at lower crust temperatures and explains a coherent process of strain localization at near-BDT conditions in feldspar-dominated lithologies. We suggest that this naturally deformed feldspar shear zone was preserved due to the unique geologic history with fast heating and exhumation, which froze both brittle vs crystal-plastic structures to provide valuable insights into mechanisms of feldspar deformation. Highlights: Tilted pluton exhumed a Miocene brittle-ductile transition in the Basin and Range. BDT in feldspar-dominated lithology involves discrete ultramylonite shear zones. Cataclastic flow and dislocation creep primarily reduced grainsize to drive diffusion creep. Overprinting brittle-ductile deformation leads to weaker grainsize-sensitive deformation. Study shows natural example of strain focusing and localization at BDT depths.