Structural interpretation and tectonic evolution of a part of the Najd Shear Zone (Saudi Arabia) using Landsat thematic-mapper data

Abstract

Digital-Landsat thematic-mapper data were used to extrapolate structural features associated with the Ajjaj Shear Zone from well characterized to less well known areas. The satellite images were enhanced using a linear-mixing model, with the spectra of known rock types (measured from the images) as endmember components. The approach uses information for all six spectral bands. The effects of desert varnish/coating in modulating the spectral response of rock exposures are specifically accounted by using of an empirically derived coat component. Comparison between the lithologic component images, produced from the unmixing procedure, and published geologic maps demonstrate that these images can be effectively used to map brittle faults, ductile shear zones, and certain spectrally distinct lithologies. By combining interpretation of the enhanced images with available geologic data, it has been shown in this study that the Ajjaj Shear Zone is dominantly a ductile structure, characterized by the continuity of rotated layers across the shear zone, with a minimum sinistral displacement of 25 to 30 km. The northern boundary of this shear zone is dominantly ductile, whereas the southern boundary consists of a continuous brittle displacement surface which juxtaposes contrasting lithologies. Several brittle faults within and crosscutting the ductile shear zone had previously been mapped as Najd faults. These faults are not strike-slip in character and clearly postdate Najd deformation. In the middle of the area studied the Ajjaj Shear Zone changes orientation from WNW to NNW. This change corresponds spatially to the intersection of the shear zone with a N-S oriented structure, here named the Hanabiq Shear Zone. The Hanabiq Shear Zone is one of a set of broadly N-S trending dextral shear zones which crosscut the Arabian Shield. Proponents of the rigid-indentor model for the origin of the Najd fault system have suggested that these faults form a conjugate set to the NW trending sinistral Najd faults. Our study presents clear evidence that the Ajjaj Shear Zone postdates the Hanabiq Shear Zone. This supports recent criticisms of the rigid-indentor model as applied to the Najd. The tectonic significance of the Najd fault system remains as intriguing problem.