Abstract
This thesis describes a distortion analysis and
modeling of magnetotelluric (MT) and deep geomagnetic sounding (GDS) data of
the Southern Central Andes. The data was collected during several field
campaigns between 1995 and 1999, within the framework of the German
Collaborative Research Programme SFB267 ''Deformation Processes in the Andes''.
The measurements were carried out in the forearc and magmatic arc regions of
the subduction zone, covering an area about 200 km long (W-E) and 60 km wide
(N-S).
Three dimensional (3-D) electrical conductivity
models of the crust are required to explain the data measured, which are found
to be strongly affected by current channeling and magnetic distortion effects,
manifested in impedance phases above 90°. Understanding the source of the
distortion has been possible by applying to the MT data a method of current
channelling analysis developed in this thesis, based on the boundary conditions
that the electric field fulfils at a conductivity contrast. This analysis allows
a qualitative recognition of the main 3-D high conductivity anomalies, thus
serving as a priori information for the construction of the 3-D forward
models.
The method developed here for recognizing
current channeling and hence the existence of elongated conductors in the crust
has been seen useful to apply in shear zones, especially when the strike of the
faults is different from that of a regional conductivity structure.
Two mega-fault systems oriented sub-parallel to
the coast line (~N-S), composed of strike slip faults associated with the
oblique subduction of the Nazca plate underneath the South American plate, are
identified as highly conductive zones.
Salinary and/or ore fluids circulating in the
fractures of a brittle crust is a plausible explanation for the conductivity
enhancement detected in the Atacama and Precordillera fault system, given also
the low geotherms (<300°C), the low seismic attenuations and the evidence of
crustal seismicity.
The clearly higher conductivity values to the
north of latitude 21°S than to the south at depths of 10-30 km beneath the
Precordillera fault system are suggested as being due to a distinct
concentration of fluids which are produced by metamorphic reactions that may
have evolved under different P-T conditions from north to south, considering
the different age of volcanic activity known at these latitudes (21°S). An
additional explanation is that in the north, where magmatism is older (>25
Ma) than in the south (<10 Ma), the crust might be more fractured due to a cooler
and more brittle regime than in the south, allowing the fluids to be better
interconnected and hence the electrical conductivity to be enhanced.
In the Altiplano high-plateau a high
conductivity zone (HCZ; <2 Wm) is encountered below 20 km depth extending to the west to
the beginning of the Western Cordillera (the Recent magmatic arc), and is
interpreted as partial melting. The HCZ strikes NNW-SSE, similar to the bending
of the volcanic arc.
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