TAKING THE PULSE OF THE SAHARAN METACRATON

We have used seismic shear wave velocity distribution and inversion modeling to image lithospheric characteristics of the Saharan Metacraton in northern Africa to a depth of 250 km. The Saharan Metacraton (the term metacraton refers to a craton that has been re-mobilized during an orogenic event but that is still recognizable dominantly through its rheological, geochronological and isotopic characteristics) is a lithopsheric entity which extends from the Neoproterozoic Arabian-Nubian Shield in the east to the Neoproterozoic-Paleoproterozoic Tuareg Shield to the west. In the south, the metacraton is bounded by the Neoproterozoic Oubanguides Orogenic Belt. Surface geology, geochronological and isotopic data all suggest that the Saharan Metacraton – although significantly different from the orgogenic belts surrounding it – has been re-mobilized during Neoproterozoic time. This re-mobilization was in the form of opening and closing of restricted basins (Red Sea-type), green-schist facies metamorphism, and widespread granitic emplacement. Seismic tomography and inversion modeling have shown the following: (1) The Saharan Metacraton is characterized by a cold continental root that can be imaged to a depth of 250 km. This root is centered around Uweinat at the Sudanese/Egyptian/Libyan boarder, the only place where Archean-Paleoproterozoic rocks are exposed within the Saharan Metacraton. Continental root under the Saharan Metacraton is not as strongly defined as those of African cratons, especially the West African Craton; (2) Seismic inversion modeling, assuming both Archon and Proton compositions, gave geotherm variation between 35 and 50 mW/m2. The low geotherm values are found under the interior of the metacraton whereas the high geotherm values tend to concentrate around the rim of the metacraton. These geotherm values are midway between geotherms under cratons (Archons) and those of orogenic belts (Tectons). We attribute the current state of the Saharan Metacraton to mild but regional sub-lithospheric mantle delamination that might have accompanied Neoproterozoic collisions along the margins of the metacraton.