STRUCTURAL CONTROL OF THE NEOPROTEROZOIC SAWAWIN BANDED IRON FORMATION, ARABIAN SHIELD, SAUDI ARABIA FROM REMOTE SENSING AND FIELD STUDIES

We have used the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) remote sensing data and field studies to understand the spatial distribution of the Neoproterozoic Sawawin banded iron formation (BIF) which occurs in the northwestern part of the Arabian Shield in Saudi Arabia occupying part of the Midyan terrain. The BIF is confined to the NW-trending Silasia formation which is dominated by felsic tuffs. The Silasia formation structurally overlies the Ghawjah formation which is dominated by andesitic lavas and mafic and intermediate pyroclastic rocks. These rocks are intruded by syn-tectonic Sawawin batholith and other syn- and post-tectonic plutons. Digital image processing and interpretation allow us to distinguish between different rock types as well as mapping the BIF which is made up of the spectrally distinctive iron oxides which constitute about 50% of the total mineralogy. ASTER data which have three bands in the visible and near infrared (VNIR), six bands in the short wave infrared (SWIR) and 5 bands in the thermal infrared (TIR) portion of the electromagnetic spectrum can be effectively used for geological and structural mapping especially in arid regions such as the Arabian Shield in Saudi Arabia. Spatial distribution of the BIF in the ASTER images suggest that the BIF is folded along NW-trending axes. Examination of map-scale and mesoscopic structures indicated the presence of two distinctive structural styles. The early structures are in the form of NE- and SW-verging folds sometimes related to box folding. The folds were developed in close association to strongly SW-verging thrusts that can be clearly observed at the outcrop scale where they are found deforming the well developed layering of the Silasia formation and the BIF. We relate these structures to a SW-directed thrusting that resulted in the emplacement of the Silasia formation including the BIF over the Ghawjah formation. It is not clear when this thrusting event took place but it might be older than 640 Ma. Fold and thrust structures are deformed by steep NW-trending shear zones where a sinistral strike-slip sense of shearing can be observed. We relate these structures to the effect of the NW-trending Najd Fault System which was developed between 640 and 560 Ma as the last phase in the evolution of the Arabian-Shield.