Continental Breakup, Mantle Flow and Thermal Anomalies

• Main Authors: Cheng, Wei-Ying, 程暐瀅
• Other Authors: Kuo Ban-Yuan, Ma Kuo-Fong
• Format: Others
• Language: zh-TW
• Published: 1998
• Online Access: http://ndltd.ncl.edu.tw/handle/93520393051621133154

碩士 === 國立中央大學 === 地球物理研究所 === 86 === Continental rifting induces a far different mantle flow structure from oceanicplate spreading. Evidence from heat flow measurements, seismic tomography,and petrogenesis of diamond inclusion indicates that the Proterozoic to Archeancratonic lithosphere can be 200-300 km thick, being effectively much moreviscous than the oceanic lithosphere. We examine how continental/oceanicplates together shape the mantle flow and generate thermal anomalies. Theconventional propagator matrix method in sphere is modified to allow for a thick"continental" lid implemented on oceanic plates. In a pilot experiment, twohemispheric "oceanic" plates spread along a model ridge, causing a block of"continent" to break up. Continental rifting sucks converging flow fromsideways, which descends and induces a thermal minimum on the oceanicspreading center. A moving continent is usually trailed by a downgoing "stern"flow which also contributes to cooling of the mantle in the wake. Our riftingmodel is employed to explain the presence of low partial melting and theanomalously cold upper mantle underlying the Equator Mid-Atlantic Ridge thathave been documented by geochemical analysis. There are old cratons on bothSouth American and African continents, which are roughly aligned with thetemperature minimum on ridge in the rifting direction. To simulate the riftingscenario, we adopt simple geometry for plates, cratons and transform faults, andcalculate the thermal evolution of rifting for 120 MY with a 2 cm/yr half rate.The simple thermal model predicts a cold Equatorial ridge segment and analong-ridge thermal gradient although it couldn*t exactly mimic the observation in magnitude.