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

Abstract

Hot spring geochemistry from the Cordillera Blanca and Cordillera Huayhuash, Peru, reveal the influence of crustal‐scale structures on geothermal fluid circulation in an amagmatic region located above a flat‐slab subduction zone. To test the influence of contrasting modes of faulting in these regions, springs were targeted along the Cordillera Blanca detachment fault, within its hanging wall, in the footwall of the detachment, and in the Cordillera Huayhuash. Hot springs along the Cordillera Blanca detachment fault zone are associated with recent extension and normal faulting, and those in its footwall and the Cordillera Huayhuash are located in the Marañon fold and thrust belt where compressional structures dominate. Springs along and in the hanging wall of the Cordillera Blanca detachment fault yield brackish‐saline, alkaline‐chloride waters, with oxygen, hydrogen, carbon, and chlorine stable isotope values that suggest mixing between meteoric groundwater and saline brine affected by high water‐rock interaction. Geothermometry reservoir temperature estimates (RTEs) of 91–226°C indicate maximum flow path depths of 8.7 or 11 km, depending on geothermal gradient, associated with the Cordillera Blanca detachment fault. In contrast, springs in the footwall and in the Cordillera Huayhuash exhibit a wide range of water types with an isotopic affinity to meteoric water, suggesting a greater influence from shallow groundwater and less water‐rock interaction. For these springs, RTEs of 40–98°C correspond to much shallower circulation (1.6–4 km). Results indicate that the Cordillera Blanca detachment system accommodates significantly deeper circulation of crustal fluids compared to other regional compressional structures.

Connections Between Hydrothermal System Geochemistry and Microbiology: Traversing Tectonic Boundaries in the South-Central Peruvian Andes

“…6), albeit at shallower depths and cooler temperatures. However, FSS do contain some volatiles (e.g., CO2, H2), which could be associated with deep-seated decarbonation reactions and possibly mobilized by the migration of slab-derived fluids, which are consistent with observations elsewhere along the Peruvian flat slab (Newell et al, 2015;Scott et al, 2020).…”

“…Thermal spring geochemistry is controlled by the depth of fluid circulation, the types of rocks encountered along fluid flow paths, water-rock interactions along these pathways, and the degree of mixing with shallow meteoric groundwater (e.g., Fournier, 1989;Arnold et al, 2017). These factors are closely tied to the overall tectono-magmatic setting, and spring chemical and isotopic composition (e.g., He, C, O, S) can inform the relative contribution from magmatic, crustal, and meteoric sources (Hilton, 1996;van Soest et al, 1998;de Hoog et al, 2001;Hilton et al, 2002;Zimmer et al, 2004;Newell et al, 2008;Sano et al, 2009;Scott et al, 2020). Given the linkages between spring chemistry and microbiology it is likely that the types of microorganisms inhabiting hot springs may also be influenced by tectonic setting.…”

“…Two reports encompassing the transition from flat-slab subduction to the active backarc of the Altiplano Plateau provide the preliminary geochemical framework for this study (Steinmüller and Huamaní Huaccán, 1999;Huamaní Huaccán, 2001). In addition to INGEMMET reports, geochemical studies on springs in Peru have focused on magmatically active hydrothermal systems (e.g., Pajuelo et al, 2020) and hot springs of the Cordillera Blanca and Cordillera Huayhuash regions (Newell et al, 2015;Scott et al, 2020), providing key data for comparison.…”

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

“…Reconstructed shear zone waters are isotopically indistinguishable from modern hot springs (δ 2 H H 2 O = −116‰ to −74‰, mean = −99‰ ± 12‰) and surface water (δ 2 H H 2 O = −124‰ to −88‰, mean = −106‰ ± 7‰) from the Cordillera Blanca (Fig. 4; Mark and McKenzie, 2007;Newell et al, 2015;Scott et al, 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). These flow paths approach the modern brittle-ductile transition zone (BDTZ) at ∼10 km (Deverchère et al, 1989).…”