Deformation Conditions During Syn-Convergent Extension Along the Cordillera Blanca Shear Zone, Peru

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 \u3e400 °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

Mantle-Derived Helium in Hot Springs of the Cordillera Blanca, Peru: Implications for Mantle-to-Crust Fluid Transfer in a Flat-Slab Subduction Setting

Fault-controlled hot springs in the Cordillera Blanca, Peru provide geochemical evidence of mantle-derived fluids in a modern flat-slab subduction setting. The Cordillera Blanca is an ~200km-long mountain range that contains the highest peaks in the Peruvian Andes, located in an amagmatic reach of the Andean arc. The Cordillera Blanca detachment defines the southwestern edge of the range and records a progression of top-down-to-the-west ductile shear to brittle normal faulting since ~5Ma. Hot springs, recording temperatures up to 78°C, issue along this fault zone and are CO2-rich, near neutral, alkaline-chloride to alkaline-carbonate waters, with elevated trace metal contents including arsenic (≤11ppm). Water δ18OSMOW (-14.2 to -4.9‰) and δDSMOW (-106.2 to -74.3‰), trends in elemental chemistry, and cation geothermometry collectively demonstrate mixing of hot (200-260°C) saline fluid with cold meteoric water along the fault. Helium isotope ratios (3He/4He) for dissolved gases in the waters range from 0.62 to 1.98 RA (where RA=air 3He/4He), indicating the presence of up to 25% mantle-derived helium. Given the long duration since, and large distance to active magmatism in the region, and the possible presence of a tear in the flat slab south of the Cordillera Blanca, we suggest that mantle helium may originate from asthenosphere entering the slab tear, or from the continental mantle-lithosphere, mobilized by metasomatic fluids derived from slab dehydration. © 2015 Elsevier B.V

Rongbuk Re-Visited: Geochronology of Leucogranites in the Footwall of the South Tibetan Detachment System, Everest Region, Southern Tibet

The Hermit\u27s Gorge area adjacent to the Rongbuk Glacier on the north side of Mount Everest is a critical location to establish the timing and duration of movement along the ductile strand of the South Tibetan Detachment system (STDS), a low-angle, north-dipping normal fault that bounds the upper part of the Greater Himalayan Sequence. Monazite from four leucogranite samples in the Hermit\u27s Gorge that bracket the timing of ductile fabric development has been dated using both U/Th–Pb ID-TIMS and LA-MC-ICPMS. Results suggest that the earliest, ~ 16.4 Ma, leucogranite sills have been folded and deformed along with the host sillimanite gneisses and calc-silicates whereas structurally higher sills and dykes that post-date fabric development are slightly younger, all within uncertainty of one another at 15.6 to 15.4 Ma. Field relations combined with age data constrain ductile fabric formation associated with movement along this strand of the STDS as being on-going at 16.4 Ma but had ceased prior to 15.6 Ma, while brittle faulting along the STDS is younger than 15.4 Ma. Combined with data from the Everest massif and surrounding region, ages of granite crystallization and ductile shearing propagated up-structural section and northward with time

Wager sample list: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Full list of Wager's specimens examined, with geographical coordinate

Mineral analyses: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Mineral compositions used for thermobarometr

Mineral analyses: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Mineral compositions used for thermobarometr

Wager sample list: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Full list of Wager's specimens examined, with geographical coordinate

Average PT results: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Results of THERMOCALC average P–T and average T calculation

Average PT results: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Results of THERMOCALC average P–T and average T calculation

Wager sample list: Structural and thermal evolution of the South Tibetan Detachment shear zone in the Mt Everest region, from the 1933 sample collection of L. R. Wager

Full list of Wager's specimens examined, with geographical coordinate