| Title | Polyphase Deformation and Structural Controls on Hydrothermal Fluid Flow: Insights from an Exposed Outcrop of the Andean Transverse Faults, Southern Andes (39ºS) |
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| Authors | Tomás ROQUER, Gloria ARANCIBIA, E. Andrés VELOSO, Julie ROWLAND, Eduardo MOLINA, Josefa SEPULVEDA, Jorge CREMPIEN, Diego MORATA |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | hydrothermal fluid flow; strike-slip tectonics; stress field; Liquiñe-Ofqui Fault System |
| Abstract | Fault-related fracture networks control the hydrologic behavior of the brittle lithosphere by acting as conduits or barriers for different crustal fluids (e.g. hydrothermal fluid flow). Generally, fracture networks present a complex tectonic behavior, which can vary according to the governing tectonic conditions and subsequent fault-driven fluid flow. In this work, we explore the role of polyphase deformation and associated hydromechanical controls on crustal fluid flow. We selected the Coñaripe area of the Southern Volcanic Zone of the Andes (SVZ) (39ºS) as a case study to evaluate the role of polyphase tectonics in structural permeability. In the northern termination of the Liquiñe-Ofqui Fault System (LOFS) (38ºS), recent studies suggest that two groups of faults exert a first-order control on the development of shallow fault-related fracture networks: the LOFS and the Andean Transverse Faults (ATF). The LOFS is an intra-arc active fault system composed of NNE-striking dextral and dextral-reverse faults. The ATF include a group of NW to WNW-striking faults, apparently older than the LOFS. During the interseismic phase of the subduction cycle, the LOFS is favorably oriented for fluid flow, in contrast to the ATF, which are oriented at high angle to the inferred direction of the principal stress, 1. The ATF record sporadic and/or local kinematic switches, likely related to the short-term stress conditions of the co-postseismic phase of the subduction seismic cycle, where an increase in fluid pressure is inferred. We conducted a structural study in an exposed 4-km-long transect corresponding to an exposure of the ATF. We collected fault-slip and vein data, and inverted stress solutions using the Multiple Inverse Method (n=75) and the GArcmB software (n=38). Calculated principal stresses cluster in two tectonic solutions consistent with polyphase deformation as defined from field observations. The first solution is compatible with long-term stress conditions and likely related to hydrothermal fluid flow, whereas the second solution is consistent with anomalous local or short-term hydromechanical behavior reported in different traces of the ATF. These results confirm that the ATF record a complex hydrostructural behavior, where sporadic release of hydrothermal fluids takes place at specific conditions during distinct phases of fault zone deformation history. |