Ashraf Emam

ASTER-based mapping of ophiolitic rocks: examples from the Allaqi–Heiani suture, SE Egypt

ABSTRACT The Allaqi–Heiani suture is an ~250 km-long ophiolite-associated fold-and-thrust belt in the South Eastern Desert of Egypt. It comprises imbricate thrust sheets of ophiolites (serpentinite, amphibolite, metagabbro, and metabasalt) and island-arc metavolcanic/metasedimentary rocks. Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)-based mafic–ultramafic indices are combined with band ratioing (BR) and relative absorption band-depth techniques to comprehensively map the ophiolitic rocks along the eastern part of the Allaqi–Heiani suture. The applied techniques are found efficient in delineating the ophiolitic rocks in the study area, despite their highly tectonized and variably carbonate-altered character. Identification and mapping of the exposed mafic–ultramafic rocks were possible using shortwave infrared (SWIR) band ratio 4/8 and thermal infrared (TIR) band ratio 12/14. The refined mafic index (MI) provides an efficient discrimination of the ophiolitic mafic rocks, while the mafic-ultramafic indices (M1 and M2) show high sensitivity to ophiolitic ultramafic units. The mafic–ultramafic indices enabled an efficient discrimination of mafic ophiolites from mafic island-arc rocks. Combining mafic indices and SWIR band ratio 4/8 allowed an enhanced delineation of the ophiolitic mafic–ultramafic rocks from other rock units in the area. Detailed mapping of the ophiolitic units in the ophiolitic terranes was obtained using the relative absorption band depth (RBD8) together with the MI and band ratio 4/8. This approach offers a quick method to discriminate among ophiolitic rock units and to map their structural contacts, i.e. in such hitherto poorly studied and hardly accessible areas.

Comprehensive Mapping of the Island Arc Rocks in the Marahiq Area, South Eastern Desert, Egypt

Remote sensing and geochemical data are used to discriminate between the different varieties of island arc assemblage covering Marahiq area, South Eastern Desert of Egypt. The Precambrian basement rocks exposed in the study area comprise ophiolite and island arc assemblages. The ophiolitic rocks include serpentinites and talc-carbonate, while the metavolcanics and metavolcanosedimentary rocks represent the island arc assemblage. Petrographically, the island arc units are differentiated in to metabasalt, meta-andesite, metadacite, meta-andesitic tuffs, chlorite talc schist and biotite chlorite schist. The geochemical data revealed that these metavolcanics and metavolcanosedimentary rocks are calc- alkaline to tholeiite in nature with low to medium-K, and likely represent an immature island arc. Image processing techniques applicable to the Enhanced Thematic Mapper (ETM+) data are used for mapping and delineating exposures of the different island arc units at the Marahiq area. Principal component analysis, minimum noise fraction and band rationing techniques provide efficient data for lithological mapping. New detailed map was generated as a result of the processed ETM+ images.