Patrizia Mariani

The enigmatic Chad lineament revisited with global gravity and gravity-gradient fields

Abstract The crustal structure of northern Africa is puzzling, large areas being of difficult access and concealed by the Sahara. The new global gravity models are of unprecedented precision and spatial resolution and offer a new possibility to reveal the structure of the lithosphere beneath the Sahara. The gravity gradients correlate better than gravity with geological features such as rifts, fold belts and magmatic deposits and intrusions. They are an ideal tool to follow geological units (e.g. basement units) below a stratigraphic layer of varying density (e.g. sediments). We focus on the Chad lineament, a 1300 km arcuate feature located between the west and central African rift system. The gravity fields show differences between the lineament and the west and central African rift system. Along the centre of the lineament high-density rocks must be present, which relate to either magmatic or metamorphic rocks. This is very different to the lineaments of the western and central-west African rift system which are filled with sediments. Considering present models of rifting and the absence of topography, the lineament cannot be coeval to the west and central African rift system and is most likely older. We suggest that the lineament is a structural element of the Saharan Metacraton.

Detecting the Elevated Crust to Mantle Section in the Kohistan-Ladakh Arc, Himalaya, from GOCE Observations

The Kohistan Ladakh area in northeastern Pakistan is an exposed top to bottom crustal section, implying that high density rocks of the lower crust are at the surface. The new GOCE satellite observations improve the gravity field in this remote area, giving a new dataset for geophysical interpretation. We use the new data to determine the crustal thickness variations and to define the geometry of the overturned crustal columns constituting the base of the former island arc. For the first time the entire extension of the arc is traced with the help of the gravity field observed by GOCE. The entire arc generates a positive gravity signal up to 180 mGal, limited by two geological boundaries, the Main Karakorum Thrust at north, and the Main Mantle Thrust at south. The Main Karakorum thrust marks the transition from the Indian to the Eurasian plate. The crustal thickness varies here between 40 and 70 km. The three geologic units that define the Kohistan arc, the South Plutonic Complex, the Chilas and Gilgit Complexes, occupy the upper crust, with depths increasing northwards between 14 and 44 km. There are not enough constraints to model the eastern part of the arc, the Ladakh, but the similarity of the gravity signal suggests that the thickness of the upper dense crustal units is similar.