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Abstract

A geochemical study has been carried out along three profiles (A,B and C) crossing the West Fault strike slip system in Northern Chile. The analysis includes whole-rock X-ray flurescence analysis of major and minor elements, stable isotope analysis and rare-earth element (REE) determinations from veins and surrounding host rocks and thermometric measurements of fluid inclusions. The geochemical data were used to describe the mechanical and chemical effects of fluids on faulting processes. For profile B, the geochemical investigations were combined with a magnetotelluric study for the subsurface electrical conductivity structure along the same profile. The results document considerable differences of fluid-rock interactions between the selected profiles. Within the area of profile A and C fluid activities have not led to exchange reactions between fluids and rocks and to changes in whole rock and mineral composition of fault rocks relative to their undeformed host rock. Investigations of stable isotopes indicate infiltration of predominantly descending (meteoric) and ascending hydrothermal (subcrustal?) fluids. In the case of profile B fluid-enhanced weakening mechanisms are dominant. Dissolution and solution transfer led to the formation of an up to 300m wide alteration zone. The thickness of the alteration zone corresponds with the width of a conductivity anomaly suggesting a fluid induced conductivity contrast. The depth extent of the fault zone conductor is reliably resolved down to 1500m. The geochemical results suggest that fluid entered the fault zone at least two times, descending from different sources. However, we have no indications for warm fluids derived from a deep source. Rather, fault rock alteration took place due to the infiltration of low temperature meteoric water. All geochemical data of profile B indicate an open fluid system. The extreme differences in fluid-rock interaction between the selected 3 profiles may reflect a difference in fundamental faulting processes along strike. We suggest that the observed variations represent different stages of fault evolution.