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Abstract

We investigated a segment of a 170 km long strike-slip fault zone belonging to the trench parallel Precordilleran Fault System in the Central Andes of northern Chile using structural geology, magnetotelluric (MT) and geochemical methods. A maximum fracture density of 3.3 m-1 at the main fault trace decreases to background within a zone of ~1 km width. Individual fault segments in this damage zone are steeply inclined with varying dip angle and direction. A 400 m wide coloured fluid alteration zone marks the fault core. The MT experiment with a dense site spacing of 100 m across the fault reveals a zone of enhanced conductivity about 300 m wide that coincides with the fault core. This shows that the 'conductive' width of the fault zone is 1/3 of the 'structural' width, which depends on deformation history. The ratio of fault width to fault length ranges from 2 x 10-3 to 6 x 10-3. The conducting zone dips steeply to the east, in agreement with most shear planes in the fault core. Geochemical analysis of the alteration zone rocks indicates a meteoric origin of fault zone fluids. We assume that water entering the ruptured zone of the fault core is most likely responsible for the conductivity enhancement. A second MT profile 4 km further south, with broader site spacing, again shows a good conductor located on the eastern side of the surface fault trace. The conducting zone reaches a maximum depth of 1500 m in both profiles. This suggests that permeability of the fault zone at greater depth is reduced due to healing and cementation of the fracture system. Two other conductive features that have no surface expression are imaged along the dense MT profile. These structures could indicate further fault branches of a wider fault zone.