Geology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2774
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Item Age, composition, and origin of the lower continental crust, northern Tanzania(2008) Mansur, Adam T.; Rudnick, Roberta L; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Granulite xenoliths from the East African rift in Tanzania sample the lower continental crust beneath both the stable Tanzanian craton and the adjacent, polymetamorphic Mozambique belt. Protoliths of surface rocks in both terranes formed 2.8 to 2.6 Ga ago, but only the Mozambique belt underwent significant post-Archean metamorphism. Most granulite xenoliths represent mafic magmas formed in a convergent margin at ca. 2660 Ma that were subsequently metamorphosed at lower crustal conditions. However, a small number of xenoliths are more similar to intraplate magmas, suggesting that at least two episodes of lower crustal growth occurred during the Archean. No post-Archean additions to the crust were detected beneath either terrane, but xenoliths from the Mozambique belt record high P-T conditions, suggesting equilibration at the base of orogenically thickened crust. By contrast, xenoliths from beneath the craton equilibrated under conditions typical of unthickened continental crust.Item Kinematics and Deformation History of the Cross Lake Greenstone Belt(2004-12-17) Dai, Tianhuan; Jiang, Dazhi; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Greenstone belts are a common feature of Archean terrains. However, the tectonic environment for Archean earth remains in hot debate: did vertical or horizontal crustal movement dominate the Archean Eon? Small-scale structural analysis is applied to a late Archean greenstone belt in the Superior Province of the Canadian Shield in order to test the two end member hypotheses. Detailed structural analysis reveals that the Cross Lake greenstone belt has undergone three major events of deformation. The early event of ESE-WNW convergence and crustal thickening initiated folding and produced northeast-trending shear zones. The configuration of the northwestern Cross Lake area is largely due to this event. This was followed by the juxtaposition of the Nelson River - Pipestone Lake high-strain zone. The last event was the juxtaposition of the Eves Rapids Complex with the development of a major northeast-trending fault, which overprints all previous deformation. The strain geometry and structural features suggest that the first event of crustal thickening likely involved vertical tectonics induced by gravitational instability rather than transpression.