PhD thesis,
Combined structural and magnetotelluric investigation across the West Fault Zone in nothern Chile
Universität Potsdam, Potsdam, Germany, (2002)
Abstract
The characterisation of the internal architecture of large-scale fault
zones is usually restricted to the outcrop-based investigation of
fault-related structural damage on the Earth's surface. A method
to obtain information on the downward continuation of a fault is
to image the subsurface electrical conductivity structure.This work
deals with such a combined investigation of a segment of the West
Fault, which itself is a part of the more than 2000 km long trench-linked
Precordilleran Fault System in the northern Chilean Andes. Activity
on the fault system lasted from Eocene to Quaternary times. In the
working area (22 deg 04'S, 68 deg 53'W), the West Fault exhibits
a clearly defined surface trace with a constant strike over many
tens of kilometers. Outcrop condition and morphology of the study
area allow ideally for a combination of structural geology investigation
and magnetotelluric (MT) / geomagnetic depth sounding (GDS) experiments.
The aim was to achieve an understanding of the correlation of the
two methods and to obtain a comprehensive view of the West Fault's
internal architecture.Fault-related brittle damage elements (minor
faults and slip-surfaces with or without striation) record prevalent
strike-slip deformation on subvertically oriented shear planes. Dextral
and sinistral slip events occurred within the fault zone and indicate
reactivation of the fault system. Youngest deformation increments
mapped in the working area are extensional and the findings suggest
a different orientation of the extension axes on either side of the
fault. Damage element density increases with approach to the fault
trace and marks an approximately 1000 m wide damage zone around the
fault. A region of profound alteration and comminution of rocks,
about 400 m wide, is centered in the damage zone. Damage elements
in this central part are predominantly dipping steeply towards the
east (70-80 deg).Within the same study area, the electrical conductivity
image of the subsurface was measured along a 4 km long MT/GDS profile.
This main profile trends perpendicular to the West Fault trace. The
MT stations of the central 2 km were 100 m apart from each other.
A second profile with 300 m site spacing and 9 recording sites crosses
the fault a few kilometers away from the main study area. Data were
recorded in the frequency range from 1000 Hz to 0.001 Hz with four
real time instruments S.P.A.M. MkIII.The GDS data reveal the fault
zone for both profiles at frequencies above 1 Hz. Induction arrows
indicate a zone of enhanced conductivity several hundred meters wide,
that aligns along the WF strike and lies mainly on the eastern side
of the surface trace. A dimensionality analysis of the MT data justifies
a two dimensional model approximation of the data for the frequency
range from 1000 Hz to 0.1 Hz. For this frequency range a regional
geoelectric strike parallel to the West Fault trace could be recovered.
The data subset allows for a resolution of the conductivity structure
of the uppermost crust down to at least 5 km.Modelling of the MT
data is based on an inversion algorithm developed by Mackie et al.
(1997). The features of the resulting resistivity models are tested
for their robustness using empirical sensitivity studies. This involves
variation of the properties (geometry, conductivity) of the anomalies,
the subsequent calculation of forward or constrained inversion models
and check for consistency of the obtained model results with the
data. A fault zone conductor is resolved on both MT profiles. The
zones of enhanced conductivity are located to the east of the West
Fault surface trace. On the dense MT profile, the conductive zone
is confined to a width of about 300 m and the anomaly exhibits a
steep dip towards the east (about 70 deg). Modelling implies that
the conductivity increase reaches to a depth of at least 1100 m and
indicates a depth extent of less than 2000 m. Further conductive
features are imaged but their geometry is less well constrained.The
fault zone conductors of both MT profiles coincide in position with
the alteration zone. For the dense profile, the dip of the conductive
anomaly and the dip of the damage elements of the central part of
the fault zone correlate. This suggests that the electrical conductivity
enhancement is causally related to a mesh of minor faults and fractures,
which is a likely pathway for fluids. The interconnected rock-porosity
that is necessary to explain the observed conductivity enhancement
by means of fluids is estimated on the basis of the salinity of several
ground water samples (Archie's Law). The deeper the source of the
water sample, the more saline it is due to longer exposure to fluid-rock
interaction and the lower is the fluid's resistivity. A rock porosity
in the range of 0.8\% - 4\% would be required at a depth of 200 m.
That indicates that fluids penetrating the damaged fault zone from
close to the surface are sufficient to explain the conductivity anomalies.
This is as well supported by the preserved geochemical signature
of rock samples in the alteration zone. Late stage alteration processes
were active in a low temperature regime (<95C) and the involvement
of ascending brines from greater depth is not indicated. The limited
depth extent of the fault zone conductors is a likely result of sealing
and cementation of the fault fracture mesh due to dissolution and
precipitation of minerals at greater depth and increased temperature.Comparison
of the results of the apparently inactive West Fault with published
studies on the electrical conductivity structure of the currently
active San Andreas Fault, suggests that the depth extent and conductivity
of the fault zone conductor may be correlated to fault activity.
Ongoing deformation will keep the fault/fracture mesh permeable for
fluids and impede cementation and sealing of fluid pathways. urn:nbn:de:kobv:517-0000569
