Maximum sustainable fluid pressure within a strike-slip tectonic setting: preliminary results from the Andean Transverse Faults, Southern Volcanic Zone of the Andes (39ºS)

Abstract

Geological faults and fracture networks control fluid flow in the upper crust. Maximum sustainable fluid pressure is defined by extension fracturing under low differential stress, and it depends on mechanical properties of the host rock, tectonic state of stress (principal stresses and related stress tensor shape ratio φ) and the existence of pre-existing fractures. Maximum sustainable fluid pressure can be defined in terms of the pore fluid factor λ=PF/σV, where PF is the absolute fluid pressure and σV is the lithostatic pressure. There are several proxies for estimating bulk fluid pressures during failure – e.g. physical models based on fracture mechanics or dimensionless estimations based on fault-fracture geometry. In this work, we explore the control that crustal deformation plays on maximum sustainable fluid pressure using a combined approach. We selected the Calafquén area of the Southern Volcanic Zone of the Andes (39ºS) as a case study, where recent studies suggest that two groups of faults control the development of shallow, fault-related fracture networks: the Liquiñe-Ofqui Fault System (LOFS) and the Andean Transverse Faults (ATF). The LOFS is an active fault system composed of NNE-striking dextral and dextral-reverse faults, whereas the ATF include a group of sinistral and sinistral-reverse NW to WNW-striking faults. We conducted a structural study in a 4-km-long transect corresponding to an exposure of the ATF. Collected fault-slip (n=75) and vein orientation (n=38) data were inverted. Preliminary results suggest that hydrothermal fluid flow is likely enhanced by local stress conditions under uniaxial compression (φ=0.02-0.10), with EW-trending, steeply dipping σ1-axis and a NE-trending, moderately dipping σ3-axis. Maximum sustainable fluid pressure estimations are ca. λ>0.6. The combined tectono-hydro-mechanical conditions argue that ATF hydrothermal storage capacity critically depends on the existence of non-Andersonian fractures, which control crustal strength and limit subsequent structurally-assisted fluid flow.