Crustal paleo-stress and permeability in a strike-slip setting: insights from the Southern Volcanic Zone (38-39°S), Chile

Abstract

This study addresses the interplay between strain/stress fields and geofluid migration in the Southern Volcanic Zone (SVZ). TheSVZ coexists with the margin-parallel Liquiñe-Ofqui Fault System (LOFS) and with NW-striking Andean Transverse Faults (ATF).To tackle the role of different fault-fracture systems on deformation distribution and magma/fluid transport was mapped thegeometry and kinematics of faults, veins, and dikes at various scales. Fault-slip data analysis allows calculating stress and strainfields from the full study area (regional scale) and fault zones representative of each fault system and a drill core from Tolhuacavolcano (local scale). Regional scale strain analysis shows kinematically heterogeneous faulting. Stress field analysis at regionalscale indicates a strike-slip dominated transpressional regime with N64E-trending σ1 and N30W-trending σ3. Deformation isfurther partitioned within the arc through NNE-striking dextral-reverse faults, NE-striking dextral-normal faults, and NW-strikingsinistral-reverse faults with normal slip activation. The regional tectonic regime controls the geometry of NE-striking dikes andvolcanic centers. NE-striking faults record local stress axes that are clockwise rotated with respect to the regional stress field andlocally with tensional stress field. NNE- and NE-striking faults are favorably oriented for reactivation under the regional stressfield and show poorly-developed damage zones. Conversely, sinistral-reverse NW-striking fault systems, misoriented under theregional stress field, show multiple fault cores, wider damage zones, dense veins networks. This fault zone records a NS-trendingtension axis that would be activated under fluid overpressure. The experimental permeability measurement in representative rockunits from the SVZ shows that the intact granodiorite sample has two orders of magnitude highest permeability (10-18 m2) thanintact volcanic rocks (10-20 m2). This could favor the existence of a fractured fluid reservoir in intrusive rocks, which is the mainlithology that composes the basement south of 38°S. Permeability increase with the fault activation from 4×10-20 m2 to 8×10-14m2 in crystalline tuff and from 1×10-18 to 3×10-14 in granodiorite. Conversely, permeability decreases with the effective pressureincrement and macro-fractures maintenance the permeability under high effective pressure (60 MPa).