UPPER-MANTLE SEISMIC VELOCITY STRUCTURE BENEATH TANZANIA, EAST-AFRICA- IMPLICATIONS FOR THE STABILITY OF CRATONIC LITHOSPHERE

The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structur e beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseism ic P and S travel times indicate that high-velocity lithosphere beneat h the Tanzania Craton extends to a depth of at least 200 km and possib ly to 300 or 350 km. Based on the thickness of mantle lithosphere bene ath Archean cratons elsewhere, it appears that the mantle lithosphere of the, Tanzania Craton has not been extensively disrupted by the Ceno zoic tectonism in east Africa, thus corroborating the assertion of cra tonic stability in the model for deep continental structure. The prese nce of thick, high-velocity structure beneath the Tanzania Craton impl ies relatively low temperatures within the cratonic mantle lithosphere , consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east A frican rifts that extend to depths below 400 km. Our models show a she ar velocity contrast between the cratonic lithosphere and the uppermos t mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about ha lf of this velocity contrast to the depleted composition of the craton ic keel and the other half to thermal alteration of upper mantle benea th the rifts. Low-density structures that may be required to provide b uoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300-350 km.