The damage and geochemical signature of a crustal scale strike-slip fault zone

Abstract

Fluid-flow migration in the upper crust is strongly controlled by fracturenetwork permeability and connectivity within fault zones, which can lead to fluid-rock chemicalinteraction represented as mineral precipitation in mesh veins and/or mineralogical changes (alteration)of the host rock. While the dimensions of fault damage zones defined by fracture intensity is beginningto be better understood, how such dimensions compare to the size of alteration zones is less wellknown. Here, we show quantitative structural and chemical analyses as a function of distance from acrustal-scale strike-slip fault in the Atacama Fault System, Northern Chile, to compare fault damagezone characteristics with its geochemical signature.The Jorgillo Fault (JF) is a ca. 18 km long NNW striking strike-slip fault cutting Mesozoic rocks withsinistral displacement of ca. 4 km. In the study area, the JF cuts through orthogranulitic and gabbroicrocks at the west (JFW) and the east side (JFE), respectively. A 200 m fault perpendicular transectwas mapped and sampled for structural and XRF analyses of the core, damage zone and protolith.The core zone consists of a ca. 1 m wide cataclasite zone bounded by two fault gouge zones ca. 40cm. The damage zone width defined by fracture density is ca. 50 m wide each side of the core.The damage zone in JFW is characterized by NW-striking subvertical 2 cm wide cataclastic rocksand NE-striking milimetric open fractures. In JFE, 1-20 mm wide chlorite, quartz-epidote and quartzcalciteveins, cut the gabbro. Microfracture analysis in JFW reveal mm-widecataclasitic/ultracataclasitic bands with clasts of protolith and chlorite orientated subparallel to the JFin the matrix, calcite veins in a T-fractures orientation, and minor polidirectional chlorite veins. In JFE,chlorite filled conjugate fractures with syntaxial growth textures and evidence for dilational fracturingprocesses are seen. Closest to the core, calcite veins crosscut chlorite veins.Whole-rock XRF analyses show Al and Ca content decrease with increasing Si, whereas Naincreases towards the core. This can be interpreted as compositional changes of plagioclase to albiterichones due to chloritic-propylitic alteration. In the damage zone, LOI increases towards the corebut decreases inside of it. This is explained by H2O-rich clays and gypsum in the fault core boundaryrepresented as fault gouge zones whereas in the cataclastic core zone, the decrease in LOI isexplained by epidote.Our results show the JF had an evolving permeability structure where a cataclasite-rich core is formedat an early stage, and then a gouge-bounded core is developed which acted as a barrier to fluid fromeast to west of the fault.