Optical-Radar-DEM Remote Sensing Data Integration for Geological Mapping in the Afar Depression, Ethiopia

The advantages of integrating optical (Landsat Enhanced Thematic Mapper Plus (ETM+) and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER)) and radar (Shuttle Imaging Radar (SIR) - C, X-band Synthetic Aperture Radar (SAR) and RADARSAT-1) remote sensing data, and digital elevation models (DEMs) (Shuttle Radar Topography Mission (SRTM)) for geological mapping in arid regions such as the Afar Depression in Ethiopia are demonstrated. The Afar Depression in NE Africa is a natural laboratory for studying processes of sea-floor spreading and the transition from rifting to true sea-floor spreading. It is ideal for geological remote sensing because of its vastness, remoteness and inaccessibility together with almost continuous exposure, and lack of vegetation and soil cover. Optical-radar-DEM remote sensing data integration is used for: (1) Distinguishing spatial and temporal distribution of individual lava flows in the Quaternary Erta \u27Ale Volcanic Range in the northern part of the Afar Depression, by integrating band-ratios of ASTER thermal infrared (TIR) data with Landsat ETM+ visible and near infrared (VNIR) and SIR-C/X-SAR L-band (λ = 24 cm) data with horizontally transmitted and horizontally received (HH) polarization. (2) Visualizing and interpreting extensional imbrication fans that constitute part of the Dobe Graben in the central part of the Afar Depression by integrating Landsat ETM+ VNIR data with RADARSAT C-band (λ = 6 cm) data with HH polarization and SRTM DEMs. These imbrication fans were developed as layer-parallel gravitational slip of the border fault hanging-wall towards the graben center. (3) Mapping morphologically defined structures in rhyolite flows exposed on the flanks of the Tendaho Rift by merging ASTER VNIR and short wave infrared (SWIR) with RADARSAT C-band data with HH polarization. The Tendaho Rift constitutes part of the Tendaho-Gobaad Discontinuity that separates the southern and the central eastern parts of the Afar Depression. Optical-radar-DEM data integration proved to be an effective approach for aiding geological mapping and structural analysis in arid regions such as the Afar Depression

The Nubian Swell

We use the name Nubian Swell to refer to a complex, east-west trending structural high in southern Egypt and northern Sudan. This 500 km wide zone of uplifted Neoproterozoic crystalline basement and Paleozoic sediments and parallel troughs extend westward for more than 800 km from the flanks of the Red Sea Hills. The Nile in this region is called the Cataract Nile and is in a youthful stage, particularly in northern Sudan where it is incised in the Neoproterozoic crystalline basement. The northern Cataract Nile flows through the rapids of the Batn el Hajar or \u27Belly of Stones\u27 region, characterized by structurally controlled 90⁰ turns, frequent bifurcation and disruption by several cataracts, and near-absence of floodplains. Orbital imaging radar has advanced our understanding of the Nubian Swell, through the discovery and mapping of paleochannels and faults that indicate tectonic uplift during Cenozoic time. Earthquakes in southern Egypt during the early 1980s provide evidence that portions of the Nubian Swell are still tectonically active, with recent seismic activity concentrated where E-W trending structures intersect N-S trending structures of the Aswan corridor. We conclude that the Nubian Swell is an important tectonic feature of North Africa, with episodic but continuing uplift

Neoproterozoic Deformation in the Northeastern Part of the Saharan Metacraton, Northern Sudan

The northeastern part of the Saharan Metacraton is dominated by medium to high-grade gneisses and migmatites, disrupted by belts of low-grade volcano-sedimentary sequences representing arc assemblages and highly dismembered ophiolites, and intruded by A-type granitoids. This part of the Saharan Metacraton is affected by three Neoproterozoic deformation events: (1) Emplacement of S- to SE-verging nappes of ophiolites and passive margin sediments. The most prominent of these nappes is the Atmur-Delgo fold and thrust belt which extends westward from the eastern margin of the Saharan Metacraton to just east of the Third Cataract Nile. This belt is interpreted as manifesting the closure of a restricted oceanic basin between the Bayuda and Halfa Terranes at ~700-650 Ma. (2) Development of N- to NE-trending folds as a result of E-W shortening that accompanied NW-SE oblique collision between the Saharan Metacraton and the Arabian-Nubian Shield at ~650-590 Ma. This event, although dominantly localized along the Keraf Suture at the boundary between the Saharan Metacraton and the Arabian-Nubian Shield, also resulted in deformation within the northeastern part of the Saharan Metacraton. N- to NE-trending folds occur either as broad belts (Wadi Halfa Fold Belt) or narrow and discrete zones (Third Cataract Shear Zone). Thrust stacking and folding thickened the Saharan Metacraton lithosphere as shown by numerous gneissic domes, such as the Delgo Dome, in the northern part of the Bayuda Terrane. Crustal thickening also generated A-type granitoids that intrude the northeastern part of the Saharan Metacraton. E-W shortening culminated in the development of ~590-550 Ma, N- to NNW-trending sinistral strike-slip shear zones, sometimes coinciding with the N-trending fold belts as in the case of the Third Cataract Shear Zone. (3) Thickening of the Saharan Metacraton lithosphere triggered orogenic collapse with N-trending moderate-angle normal-slip faults developed after 550 Ma. Neoproterozoic deformation and igneous activity in the northeastern part of the Saharan Metacraton are interpreted as due to processes that involve collision, lithospheric mantle delamination, and regional extension