ABSTRACT: Structural Mapping and Analysis of a Madagascar Precambrian Shear Zone Using Enhanced Landsat Thematic Mapper Data

D. S. Kilmer, I. J. Duncan

Recently, the west coast of Madagascar has become a frontier region for petroleum exploration. Major structures in the Precambrian shield of Madagascar may have a strong control on the development of sedimentary basins, as has been documented in the Morondava basin. The 2.5-3.0+ Ga shield of Madagascar is an amphibolite- to granulite-grade metamorphic gneiss terrain, intruded by anorthosites and 550-Ma granites and pegmatites. Landsat Thematic Mapper data provides a cost-effective method for regional-scale structural mapping of this poorly known terrain. A five-component linear mixing model has been used to enhance the lithologic information in this six-band data. Lithologic component images thus produced utilize the full geologic spectral range of the data. A preliminary structural geologic map compiled from the component images has greater detail than existing maps at 1:100,000 scale, to which it has been compared. The Ankafotra-Saririaky shear zone has been identified as a north-northeast-trending, 15- to 20-km-wide region of appressed folds, attenuated layering, and subparallel faults on the western side of the shield. Two anorthosite massifs that occur within this shear zone have the structural characteristics of boudins in a ductile matrix. The shear deformed a preexisting terrain of poly-phase folding, characterized by tight folds and complex fold interference structures displayed by basins and domes on a scale of 10 km. Enhanced remote sensing data can be used to characterize the nature and mechanism of shear deformation in such zones.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990