DEFINING A BRITTLE-DUCTILE TRANSITION ACROSS AN EXHUMED CRUSTAL SECTION: GEOLOGIC MAPPING AND MICROSTRUCTURES ACROSS THE IRETEBA PLUTON IN THE SOUTHERN BASIN AND RANGE

The Late Cretaceous garnet-bearing two-mica Ireteba Pluton, located in the southern Basin and Range, was tilted and exhumed due to extension in the Colorado River Extensional Corridor in the middle Miocene. Previous mapping shows that the pluton's western part developed brittle deformation while the eastern part developed ductile deformation. The transition is marked as a Miocene Brittle-Ductile Transition (BDT), located at ~8.5 km depth relative to the Miocene paleo-surface marked by coeval overlying volcanics and knowledge of regional crustal tilting. We use outcrop observations and microstructures to define the spatial range, strain characteristics, and deformational temperature across this BDT. From structurally shallow to deep levels, we observe the following changes. At paleodepths above 7-8 km, we observe fractures in the pluton mostly free of ductile deformation except for local discrete ductile shear zones and remnant magmatic fabrics. Quartz and feldspar show cold-moderate-temperature dynamic recrystallization (~300-450°C). At paleodepths of ~8-9 km, we observe more pervasive narrow zones with strongly developed ductile lineation and foliation and mm-scale to 10 cm- scale ductile shear zones with intermittent weakly deformed zones. Samples across a ~3 m shear zone at this depth document a decrease in feldspar porphyroclast sizes from 2.5 mm to 0.6 mm, and quartz and feldspar show recrystallization mechanisms indicative of hotter temperatures (~450 - >550°C). At paleodepths >10 km, we observe more strongly lineated, diffuse ductile fabrics and shear zones, with hotter deformation textures (~>550°C), including pervasive myrmekitic textures. Based on our observations, the deformation pattern in the Ireteba pluton overall shows a temperature-increasing trend toward depth, and local strain heterogeneity exists. We suggest that the BDT can be defined by a ~1-2-km-wide zone characterized by the reduction of feldspar porphyroclast sizes and more pervasive narrow shear zones, and no sharp boundaries of the BDT are observed.