GEOLOGIC STRUCTURES OF PRECAMBRIAN ROCKS OF SRI LANKA : FOR THE BASIS OF COMPARATIVE STUDIES WITH ANTARCTICA

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Recent geochronological studies (GERMAN-SRI LANKAN CONSORTIUM : Geol. Survey Dept. Sri Lanka, Sp. Pub. IGCP236,26,1987; KRONER et al. : J. Geol., 95,775,1987; MILISENDA et al. : J. Geol., 96,608,1988) indicate that there are fundamental differences in the ages of original rocks between the Highland Group and the Vijayan Complex. The comparative structural study of the Sri Lankan Precambrian rocks, therefore, is now an urgent necessity. Based on the structural data from southwest, central and southeast Sri Lanka, the following results have been obtained :i) Rocks of the Highland and Southwest Groups suffered rootless-isoclinal folds (D_1), overturned-recumbent folds (D_2), upright folds (D_3), and ductile fault and gentle folds (D_4). ii) D_1 structures are well developed as small folds throughout gneisses and charnockites of both Highland and Southwest Groups. Fold hinges generally plunge NNW or SSE, paralleling the major structures, but sometimes distinctly inclined from them.iii) D_2 and D_3 structures are major fold structures throughout the study areas with their hinges generally trending NNW-SSE. Most of lineations are parallel to hinges of the major folds. Axial plane schistosity is generally associated with D_3 structures. iv) Rocks of the Highland and Southwest Groups have similar structural characteristics and structural histories.v) Rocks in arenas (doubly plunging synforms) in the area around Kandy have similar structural histories to the Highland Group rocks outside the arenas regarding D_3 and D_4 structures; D_3 structures of rocks in arenas, however, are upright tight-isoclinal folds, and related lineations are steep and are inclined from hinges of the major structures. vi) Pre-D_3 structures of rocks in the arenas are the earlier isoclinal and rootless folds with their hinges discordant to the major structures and later veinings discordant from the original s-surfaces. They all differ from D_1-D_2 structures of rocks outside the arenas. vii) Rocks of the East Vijayan Complex have common structures with those of the Highland Group rocks regarding D_3 and D_4 structures. Pre-D_3 structures are complex and need to be studied in more detail.
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Yoshida Masaru
Department Of Gastroenterology Kobe University Hospital

GEOLOGIC STRUCTURES OF PRECAMBRIAN ROCKS OF SRI LANKA : FOR THE BASIS OF COMPARATIVE STUDIES WITH ANTARCTICA

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