Deformation conditions during syn-convergent extension along the Cordillera Blanca shear zone, Peru

Abstract

Strain localization across the brittle-ductile transition is a fundamental process in accommodating tectonic movement in the mid-crust. The tectonically active Cordillera Blanca shear zone (CBSZ), a ∼200-km-long normal-sense shear zone situated within the footwall of a discrete syn-convergent extensional fault in the Peruvian Andes, is an excellent field laboratory to explore this transition. Field and microscopic observations indicate consistent top-down-to-the-southwest sense of shear and a sequence of tectonites ranging from undeformed granodiorite through mylonite and ultimately fault breccia along the detachment.Using microstructural analysis, two-feldspar and Ti-in-quartz (TitaniQ) thermometry, recrystallized quartz paleopiezometry, and analysis of quartz crystallographic preferred orientations, we evaluate the deformation conditions and mechanisms in quartz and feldspar across the CBSZ. Deformation temperatures derived from asymmetric strain-induced myrmekite in a subset of tectonite samples are 410 ± 30 to 470 ± 36 °C, consistent with TitaniQ temperatures of 450 ± 60 to 490 ± 33 °C and temperatures >400 °C estimated from microstructural criteria. Brittle fabrics overprint ductile fabrics within ∼150 m of the detachment that indicate that deformation continued to lower-temperature (∼280–400 °C) and/or higher-strain-rate conditions prior to the onset of pervasive brittle deformation. Initial deformation occurred via high-temperature fracturing and dissolution-precipitation in feldspar. Continued subsolidus deformation resulted in either layering of mylonites into monophase quartz and fine-grained polyphase domains oriented subparallel to macroscopic foliation or the interconnection of recrystallized quartz networks oriented obliquely to macroscopic foliation. The transition to quartz-controlled rheology occurred at temperatures near ∼500 °C and at a differential stress of ∼16.5 MPa. Deformation within the CBSZ occurred predominantly above ∼400 °C and at stresses up to ∼71.4 MPa prior to the onset of brittle deformation.

Structural Controls on Crustal Fluid Circulation and Hot Spring Geochemistry Above a Flat‐Slab Subduction Zone, Peru

“…Furthermore, the calculated depths are consistent with the two styles of faulting in the area. The Cordillera Blanca detachment system is extensional with penetration depths at least 10 km (Giovanni et al, 2010; Hughes et al, 2019; McNulty & Farber, 2002). In contrast, most of the MFTB structures are more shallowly penetrating and sole out into décollements within 5 km (e.g., Scherrenberg et al, 2016), consistent with thin‐skinned fold and thrust belt deformation.…”

“…The detachment is well exposed near the mouth of glacially cut valleys (quebradas) along the western slopes of the Cordillera Blanca. North of Huaraz, the structure is characterized by an extensional shear zone with a gradient from mylonitic fabrics to brittle overprinting toward to the detachment surface (Giovanni et al, 2010; Hughes et al, 2019). South of Huaraz the mylonitic zones are rare to absent, and the fault breaks into segments, terminating ~80 km to the SE (Figure 1).…”

Natural and Experimental Constraints on a Flow Law for Dislocation‐Dominated Creep in Wet Quartz

“…We do not use naturally constrained strain rate and deformation data from the RGD (Dunlap et al., 1997; Hirth et al., 2001), the Ailao Shan–Red River shear zone (ARSZ; Boutonnet et al., 2013; Sassier et al., 2009), nor the Cordillera Blanca shear zone (CBSZ; Hughes et al., 2019, 2020). In the RGD, Hirth et al.…”

“…We do not use naturally constrained strain rate and deformation data from the RGD (Dunlap et al, 1997;Hirth et al, 2001), the Ailao Shan-Red River shear zone (ARSZ; Boutonnet et al, 2013;Sassier et al, 2009), nor the Cordillera Blanca shear zone (CBSZ; Hughes et al, 2019Hughes et al, , 2020. In the RGD, Hirth et al (2001) constrained strain rates within ∼1.5 orders of magnitude (1 × 10 −15 to 5 × 10 −14 s −1 ) and the differential stress to between 60 and 100 MPa (although dynamically recrystallized grain sizes reported by Dunlap et al (1997) suggest that lower stresses were present).…”

Miocene to modern hydrothermal circulation and high topography during synconvergent extension in the Cordillera Blanca, Peru

“…Others have suggested that the meteoric water in shear zones originates from unconformities or is brought to depth during polyphase deformation prior to shear zone formation (Raimondo et al, 2013;Bons and Gomez-Rivas, 2020). The granite forming the CBD footwall was intruded before and up to fault initiation, and the system has a monophasic deformation and exhumation history that precludes these mechanisms as the origin of low-δ 2 H fluids (Margirier et al, 2016;Hughes et al, 2019). Additionally, an active meteorichydrothermal system permits circulation of groundwater along the CBD and bedrock fractures to depths of 9-11 km ( Newell et al, 2015;Scott et al, 2020).…”