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 (∼10¹⁸–10¹⁹ 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.

长石在大陆地壳中部和下部占主导地位。地壳流变模型取决于我们对长石变形机制的了解。为了了解更多关于天然长石变形的信息，我们对位于内华达州南部科罗拉多河延伸走廊的白垩纪岩体中的脆韧转变 (BDT) 进行了多学科调查。在这里，在约 10 公里的古深度观察到一个明显的 BDT，其中脆性断层转变为离散的、局部的韧性剪切。现场关系证实，这个 BDT 在中新世已经变形和挖掘出来，在时间上与两个周围 ca 的入侵有关。东向正断层系统中的 16 Ma 岩体和随后的下盘旋转。我们通过场遍历建立了整体结构框架、应变率和热历史，角闪石铝气压法重建古深度，锆石 (U-Th)/He 热年代学 (ZHe) 以约束温度-时间历程，以及一维热模型以进一步解析变形温度。BDT内的局部剪切带形成了约。16.2-15.5 Ma 在约 500-600 °C 的温度下，当它们传导和平流冷却到约 200 °C 时继续变形。14 Ma，受 ZHe 日期限制。应变集中在细粒（7-10 μm）富含长石的超糜棱岩中，这些超糜棱岩被解释为主要通过粒度敏感的扩散蠕变变形。允许扩散蠕变激活的晶粒尺寸减小可能是由脆性断裂、流体辅助压裂和新结晶以及位错蠕变机制引起的，因此强调了这些过程在相对较强的中地壳条件下局部变形以促进扩散蠕变剪切带的发展的重要性。晶粒大小的压力计表明应力约为 50 MPa，这明显弱于通过位错蠕变变形的富含石英的 BDT 的峰值强度。这种集成过程允许相对较低的粘度（~10¹⁸ –10 ¹⁹  Pa s) 在较低地壳温度下，并解释了在长石为主的岩性中近 BDT 条件下应变局部化的连贯过程。我们认为，由于具有快速加热和折返的独特地质历史，这种自然变形的长石剪切带得以保留，这冻结了脆性和晶体塑性结构，从而为长石变形机制提供了有价值的见解。