DIGITAL TOPOGRAPHIC AND GEOMORPHIC MAPPING ALONG THE TIBESTI LINEAMENT IN THE TIBESTI MASSIF, NORTHERN CHAD, AFRICA

Previous research has documented the NW-SE Tibesti Lineament, a 200-300 km wide fault swarm that extends 6000 km from Kenya to southwestern Algeria. The type locality for the Tibesti Lineament is the Tibesti Massif, an isolated, roughly 100,000 km² swell that rises some 2 km above the Sahara Desert floor in northern Chad. An estimated 30,000 km² of the massif is comprised of Cenozoic volcanic rocks which rest unconformably on Cambro-Ordovician sedimentary rocks along the massif flanks, while sparse exposures of Nubian Sandstone (probable Cretaceous age) directly overlie Precambrian metamorphic rocks in the central part of the swell.

Geological studies of Tibesti Massif have been limited due to its location in the middle of the Sahara Desert and episodes of political unrest. For these reasons, we utilized Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data in an attempt to better understand the relationship between volcanism and tectonism in relation to the Tibesti Lineament within the massif. Forty-two 60 X 60 km ASTER DEMs were mosaicked and used to create a variety of derivative digital maps used for topographic profiles, knickpoint analysis, and fracture pattern trends which were plotted on Rose Diagrams. ASTER short-wavelength infrared (SWIR) and thermal infrared (TIR) data were used to map lithologies where needed for geomorphic or structural interpretation.

Based on our results, the Tibesti Lineament can be classified into sectors within the massif based on topographic profiles and knickpoint analysis. From the southeast, a 400-600 m escarpment grades into a volcanic-filled (up to 1500 m) half-graben in the central part of the massif. The northwestern part of the lineament is an elevated fissure-fed volcanic ridge that extends about 75 km beyond the massif. Normal faulting can explain the southeastern and central sectors of the lineament associated with the formation of the massif. The absence of Cambro-Ordovician sedimentary rocks overlying Precambrian units on the downthrown side of the normal fault suggests that fault inversion occurred.