| Summary: | 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.
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