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Gas-rich hot springs throughout the Peruvian Andes contain a surprising contribution of mantle and crustal volatiles (CO 2 and N 2) despite being located along a volcanic gap associated with modern flat-slab subduction. Similar mantle and crustal volatile degassing is observed in springs from the backarc region of the northern Altiplano Plateau which experienced flat-slab subduction in the Oligocene. We constrain the sources of deeply-derived volatiles using C, N, O and H stable isotope analyses alongside previously reported helium isotope (3He/4He) and gas abundance data. δ18O and δ2H values of spring waters are consistent with regional meteoric water, with deviations towards higher values due to fluid mixing and high-temperature fluid-rock interaction. δ15N values of N 2 gas range from −0.5 to +4.4‰ (vs AIR), indicating contributions of mantle and crustal (sedimentary) nitrogen. δ13C values of dissolved inorganic carbon (DIC) and CO 2 gas from bubbling springs are used to model initial δ13C values before degassing, with resulting values ranging from −13.6 to −0.3‰ (vs VPDB) and an average value of −6.9‰. DIC concentrations range from 1 to 57 mmol/kg, of which carbonate dissolution accounts for only a small fraction (average of 15%). The remaining carbon (1 to 44 mmol/kg) is derived from a mixture of deep CO 2 sources with endmember δ13C values of −7‰ and −14‰ which, in light of relationships with 3He/4He ratios, are interpreted to be the subcontinental lithospheric mantle (SCLM) and metamorphic crustal carbon, respectively. These results are consistent with previous interpretations that slab-to-lithosphere fluid transfer is mobilizing volatiles from the SCLM to the continental crust above the flat slab and confirms that mantle CO 2 is migrating with mantle helium. Metamorphic CO 2 and N 2 are mobilized from the continental crust and entrained by mantle fluids on their way to the surface. In the backarc region, similar geochemical patterns suggest that volatiles are released by dehydration and/or partial melting of previously hydrated lithosphere. We estimate a CO 2 flux of ~3 × 108 mol yr−1 (34 t d−1) from 45 springs with available discharge values. If scaled to all reported thermal springs in Peru, then the total CO 2 emissions from Peruvian thermal springs may approach 2 × 109 mol yr−1 (203 t d−1), or ~ 0.2% of global emissions from subaerial volcanism. Regional CO 2 emissions may be 10 to 100 times greater when considering CO 2 diffusely lost along fault zones or temporarily dissolved near-surface aquifers. These results demonstrate that flat-slab subduction leads to an efficient and unexpected transfer of mantle and crustal volatiles to Earth's surface, and more generally, that deep volatile fluxes in subduction zones are not limited to active volcanism. • Mantle and crustal degassing observed in the volcanic gap above a modern flat slab. • CO 2 and N 2 in hot springs show isotopic evidence for mantle and crustal sources. • Point source CO 2 flux from Peruvian hot springs is estimated to be 2 × 109 mol yr−1. • Mantle volatiles are mobilized by metasomatism above the dehydrating Nazca slab. • Dehydration and/or melting facilitates degassing of modern backarc lithosphere. [ABSTRACT FROM AUTHOR]

• Mantle helium in Peruvian springs evidence for slab-to-lithosphere fluid transfer. • Flat slabs lead to hydration and metasomatism in overlying continental lithosphere. • Helium isotope ratios inform of fluid transit times through the crust. • Crustal structures and slab morphology influence flow rates through the crust. • Dehydration and/or melting of lithospheric mantle below northern Altiplano Plateau. The transfer of large volumes of fluid to the overriding lithosphere during flat-slab subduction should drastically alter the physical and chemical properties of continental margins. However, this process is poorly understood and without active magmatism, direct evidence for fluid transfer has remained elusive in modern systems. New helium isotope ratios (3He/4He) and gas species abundances in thermal springs above the Peruvian flat slab in South America demonstrate widespread transit of mantle-derived volatiles through thick (>45 km) continental crust. An unambiguous mantle signature is present in all 52 springs investigated over a 200,000 km2 region, with 3He/4He ratios ranging from 0.26 to 2.52 R C /R A (3He/4He ratio relative to the air ratio, R A , of 1.39 × 10 − 6 , corrected for air-derived helium). In the absence of recent magmatism, the observed 3He/4He ratios are best explained by slab-derived fluids mobilizing helium from the subcontinental lithospheric mantle (SCLM). As slab- and mantle-derived volatiles migrate through the continental crust, 3He/4He ratios are reduced from SCLM values (6.1 ± 2.1 R A) by ingrowth of radiogenic 4He and mixing with crustal fluids. The highest 3He/4He ratios in the flat slab region are observed above a probable tear in the Nazca Plate and along the Cordillera Blanca Detachment fault, implying that these features promote faster fluid transport through the crust. Our observations require a persistent flux of slab-derived fluids to move lithospheric-mantle-derived volatiles into and through the continental crust, providing evidence for active continental hydration in a modern flat-slab subduction system. Similar results from springs located in the backarc region to the south of the flat slab indicate that the mantle lithosphere below the northern Altiplano is experiencing dehydration and/or partial melting resulting from a mid-Cenozoic episode of flat-slab subduction. [ABSTRACT FROM AUTHOR]

The Cordillera Blanca detachment in the highest elevations of the Peruvian Andes has been accommodating synconvergent extension since the late Miocene. Stable isotope analysis of synkinematic mica from its exhumed footwall shear zone provides new constraints on deep meteoric-hydrothermal circulation during ductile deformation and regional paleoelevation. Muscovite and biotite that deformed and/or grew synkinematically in the shear zone have δ2H values of -131‰ to -58‰ and -149‰ to -98‰ (versus Vienna standard mean ocean water, VSMOW), respectively. The δ2H value difference between coexisting muscovite and biotite is consistent with equilibrium fractionation of the same fluid at the same temperature. Calculated δ2H values of water (-107‰ to -78‰) in equilibrium with these micas are indistinguishable from those of present-day, deeply circulated (9-11 km) hot spring waters emanating from the fault. Such low-δ2H fluids indicate circulation of meteoric water to the depths of the brittle-ductile transition that cannot be explained by other mechanisms. Average recharge paleoelevation for water entering the shear zone based on hydrogen isotopes was 3400 + 500/-700 m (1σ). This is near, but ~500 m below, the present-day mean elevation of the catchments feeding modern hot springs of 3965 ± 880 m, and ~700 m below the 4200 + 700/-900 m mean recharge elevation derived from δ2H values of modern surface and thermal water. The consistency between modern and ancient fault-assisted hydrothermal systems and elevation suggests that high topography, steep relief, and meteoric-hydrothermal circulation have persisted throughout the history of the Cordillera Blanca detachment system. [ABSTRACT FROM AUTHOR]

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 topdown-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. [ABSTRACT FROM AUTHOR]

The Cordillera Blanca shear zone (CBSZ) occupies the footwall of the Cordillera Blanca detachment fault, an active low-moderate angle detachment fault extending for ~200 km along the western margin of the Cordillera Blanca in central Peru. Recrystallized quartz paleopiezometry, two-feldspar thermometry on asymmetric strain-induced myrmekite, and structural analysis from the CBSZ provide evidence for shear zone evolution within a transient elevated geotherm during syn -convergent extension. Shear zone width varies by a factor of six along strike, with widths of up to ~400 m within the central segment of the shear zone. Microstructurally-derived strain rates, calculated from quartz paleopiezometry and two-feldspar thermometry results, are on the order of 10−14 to 10−12 s−1 and are broadly consistent with macrostructurally-derived strain rates of 10−14 to 10−13 s−1 calculated from shear zone thickness and previously determined exhumation rates. Within the context of existing thermochronological data, constraints on batholith emplacement, and previously determined offset along the Cordillera Blanca detachment, our data suggest that ductile deformation in the CBSZ occurred within a transient elevated geothermal gradient of at least ~50 °C/km and potentially as high as ~100 °C/km, which deviated from a ~30 °C/km gradient for an unspecified duration of time. We discuss the evolution of the Cordillera Blanca, where observed deformation fabrics are related to strain localization along an exhuming fault as well as thermal weakening from the interplay between faulting and magmatism. • New crustal strength profiles constructed for the Cordillera Blanca, Peru. • Ductile deformation occurred in elevated transient geothermal gradient. • Possible links between magmatism and detachment faulting are discussed. [ABSTRACT FROM AUTHOR]