Abstract
Large-scale strike-slip fault zones are often imaged as electrically
conductive structures in the brittle crust. However, the relationship
of conductivity and internal architecture of the fault zone remains
largely unclear. This paper presents results of a study designed
to compare the record of structural deformation across a fault zone
with its electrical conductivity image. Two high-resolution magnetotelluric
profiles trend perpendicularly across the West fault, a branch of
the Precordilleran fault system in northern Chile. The magnetotelluric
and geomagnetic response functions in the frequency range from 1000
Hz to 0.1 Hz clearly image a fault zone conductor (FZC) about 350
m wide and 1500 m deep, trending along the surface trace of the fault.
The position of the FZC and its geometric properties (width, dip)
correlate with a region of intense fluid alteration and the orientation
of fault-related damage elements (minor faults, fractures). According
to estimates of fluid salinity and rock porosity, the conductivity
anomaly results from meteoric water penetrating into a permeable
zone. This suggests that the increase in electrical conductivity
is causally related to a mesh of minor faults and fractures, acting
as a pathway for fluids. The FZC reflects the central and most fractured
part of the damage zone. In view of similar published magnetotelluric
studies, we infer a dependency of a fault's state of activity and
the characteristics of the FZC such as its conductance, width, and
depth extent. Ongoing deformation is the controlling factor for maintaining
a permeable and electrically conductive fracture network.
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