Abstract
We compare recent magnetotelluric investigations of four large fault
systems: (i) the actively deforming, ocean-continent interplate San
Andreas Fault (SAF); (ii) the actively deforming, continent-continent
interplate Dead Sea Transform (DST); (iii) the currently inactive,
trench-linked intraplate West Fault (WF) in northern Chile; and (iv)
the Waterberg Fault/Omaruru Lineament (WF/OL) in Namibia, a fossilized
intraplate shear zone formed during early Proterozoic continental
collision. These fault zones show both similarities and marked differences
in their electrical subsurface structure. The central segment of
the SAF is characterized by a zone of high conductivity extending
to a depth of several kilometres and attributed to fluids within
a highly fractured damage zone. The WF exhibits a less pronounced
but similar fault-zone conductor (FZC) that can be explained by meteoric
waters entering the fault zone. The DST appears different as it shows
a distinct lack of a FZC and seems to act primarily as an impermeable
barrier to cross-fault fluid transport. Differences in the electrical
structure of these faults within the upper crust may be linked to
the degree of deformation localization within the fault zone. At
the DST, with no observable fault-zone conductor, strain may have
been localized for a considerable time span along a narrow, metre-scale
damage zone with a sustained strength difference between the shear
plane and the surrounding host rock. In the case of the SAF, a positive
correlation of conductance and fault activity is observed, with more
active fault segments associated with wider, deeper and more conductive
fault-zone anomalies. Fault-zone conductors, however, do not uniquely
identify specific architectural or hydrological units of a fault.
A more comprehensive whole-fault picture for the brittle crust can
be developed in combination with seismicity and structural information.
Giving a window into lower-crustal shear zones, the fossil WF/OL
in Namibia is imaged as a subvertical, 14 km-deep, 10 km-wide zone
of high and anisotropic conductivity. The present level of exhumation
suggests that the WF/OL penetrated the entire crust as a relatively
narrow shear zone. Contrary to the fluid-driven conductivity anomalies
of active faults, the anomaly here is attributed to graphitic enrichment
along former shear planes. Once created, graphite is stable over
very long time spans and thus fault/shear zones may remain conductive
long after activity ceases.
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