Conductivity anomalies as markers of crustal reactivation - an example from the Damara Belt in Namibia
Abstract
Within the framework of an integrated geophysical and geological project we recorded broadband magnetotelluric and GDS data at more than 100 sites in NW Namibia. The MT work in 1998 concentrated on a regional study of the Damara Belt, while the emphasis in 1999 was a detailed study of the Waterberg Fault / Omaruru Lineament (WF/OL), a major fault zone. The regional, 2D electrical resistivity model and the induction arrow data exhibit three distictive zones: (i) a generally resistive upper crust which is typical for the granites and metasediments of the Damara belt, (ii) two interconnected subvertical conductors in upper to mid crustal level which correlate with major tectonic zone boundaries and (iii) a highly conductive middle to lower crust in the southern part of the profile. The geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone. The crustal shear zones were reactivated during the Cretaceous rifting and magmatism associated with the opening of the South Atlantic. Results from the work in 1999 indicate strong 3D effects in the vicinity of the WF/OL. 3D and 2D anisotropic modelling can reproduce most of these effects. The conductivity anomalies revealed in this area are (i) a conductive ring structure in the shallow crust north of the fault, (ii) an anisotropic region in the upper crust with high conductivity parallel to the WF/OL, (iii) anisotropy in the lower crust with a different but poorly constrained strike direction, and (iv) a shallow elongated conductor sub-normal to the WF/OL. The anisotropic region in the upper crust continues down to at least 14 km and suggests that the WF/OL is not a single fault but a wider fault zone.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11067W