Spatial variation in strain localization along the Cordillera Blanca Shear Zone (CBSZ), Peru
Cameron Hughes, University of Tennessee-Knoxville
Micah Jessup, University of Tennessee-Knoxville
Colin Shaw, Montana State University
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The CBSZ is situated in the footwall of an active moderate- to low-angle WSW-dipping detachment fault above the Peruvian flat-slab segment of the Andean margin. Deformation within the CBSZ is associated with exhumation of a granodiorite batholith across the brittle-ductile-transition. Tectonites preserving a strain gradient within the CBSZ lend insight into processes of strain localization in polyphase granitic shear zones. We describe the spatial variation of strain localization along the shear zone through a comparison of tectonites from three transects (northern, central, and southern) across the stra¬¬in gradient. This study applied microstructural analysis, recrystallized quartz paleopiezometry, microtextural analysis of quartz crystallographic preferred orientations (CPOs), and two-feldspar thermometry of asymmetric strain-induced myrmekite to estimate deformation temperature and differential stress conditions recorded during strain localization along the CBSZ.
The three observed transects preserve similar records of strain localization regardless of shear zone thickness, which ranges from ~50 m in the south to ~450 m in the north. A general relationship of increasing differential stress with decreasing temperature is observed in all transects, with recorded stresses ranging from ~15 to ~90 MPa and temperatures from <280 to >630 degrees C. Deformation temperature and differential stress estimates show only a weak correlation to structural depth. Highest stresses occur in the central transect ~100-150 m structurally below the detachment surface near samples where pseudotachylite overprints ductile deformation fabrics. Within each transect, quartz CPOs from a wide range of structural depths dominantly record prism <a> slip, with varying lesser contributions of rhomb <a> or basal <a> slip. Prism [c] slip is observed in structurally deeper positions of the northern and southern transects. These data suggest pervasive deformation at moderate- to high temperatures, which are consistent with quartz recrystallization microstructures and deformation T >400 degrees C recorded by asymmetric strain-induced myrmekite.