Abstract
AbstractThis paper describes a new method for tracing paleo-shear zones of the continental crust by self-potential (SP) data inversion. The method falls within the deterministic inversion framework, and it is exclusively applicable for the interpretation of the SP anomalies measured along a profile over sheet-type structures such as conductive thin films of interconnected graphite precipitations formed on shear planes. The inverse method fits a residual SP anomaly by a single thin sheet and recovers the characteristic parameters (depth to the toph, extension in deptha, amplitude coefficientk, and amount and direction of dipθ) of the sheet. This method minimizes an objective functional in the space of the logarithmed and non-logarithmed model parameters (log(h), log(a), log(k), andθ) successively by the steepest descent (SD) and Gauss-Newton (GN) techniques in order to essentially maintain the stability and convergence of this inverse method. Prior to applying the method to real data, its accuracy, convergence, and stability are successfully verified on numerical examples with and without noise. The method is then applied to SP profiles from the German Continental Deep Drilling Program (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschla - KTB), Rittsteig, and Grossensees sites in Germany for tracing paleo-shear planes coated with graphitic deposits. The comparisons of geologic sections constructed in this paper (based on the proposed deterministic approach) against the existing published interpretations (obtained based on trial-and-error modeling) for the SP data of the KTB and Rittsteig sites have revealed that the deterministic approach suggests some new details that are of some geological significance. The findings of the proposed inverse scheme are supported by available drilling and other geophysical data. Furthermore, the real SP data of the Grossensees site have been interpreted (apparently for the first time ever) by the deterministic inverse scheme from which interpretive geologic cross sections are suggested. The computational efficiency, analysis of the numerical examples investigated, and comparisons of the real data inverted here have demonstrated that the developed deterministic approach is advantageous to the existing interpretation methods, and it is suitable for meaningful interpretation of SP data acquired elsewhere over graphitic occurrences on fault planes.
Highlights
Graphite precipitation on the planes of shear zones can play an important role in understanding the tectonic evolution of an area (e.g., Glover and Vine 1995; Haak 1989; Oohashia et al 2012)
The most important aspect for electrical conductivity is the secondary graphite, which is ubiquitous in the cataclastic shear zones, where it is always associated with iron sulfides and chlorite
A new method for the interpretation of the SP data measured over sheet-type structures has been developed
Summary
Graphite precipitation on the planes of shear zones can play an important role in understanding the tectonic evolution of an area (e.g., Glover and Vine 1995; Haak 1989; Oohashia et al 2012). The ELEKTB Group (1997) studied the electrical conductivity of the continental crust in the German Continental Deep Drilling Program (Kontinentales Tiefbohrprogramm der Bundesrepublik Deutschla - KTB) They concluded that the high electrical conductivities in the upper crust are primarily caused by graphite accumulations rather than by fluids and that these anomalous conductivities are related to shearing stress regimes. The most important aspect for electrical conductivity is the secondary graphite, which is ubiquitous in the cataclastic shear zones, where it is always associated with iron sulfides and chlorite This secondary graphite often forms a quasi-continuous (connective) and abundant coating along shear planes, and it is locally concentrated in millimeter-thick layers that constitute good electrical conductors over hundreds or thousands of meters in depth along the fault planes (Emmermann and Lauterjung 1997; Zulauf 1990)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
