CONVECTIVE COUPLING BETWEEN DUCTILE LOWER CRUST AND UPPER-MANTLE, ANDITS TECTONIC IMPLICATIONS

Authors
Citation
M. Nakada, CONVECTIVE COUPLING BETWEEN DUCTILE LOWER CRUST AND UPPER-MANTLE, ANDITS TECTONIC IMPLICATIONS, Geophysical journal international, 118(3), 1994, pp. 579-603
Citations number
46
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0956540X
Volume
118
Issue
3
Year of publication
1994
Pages
579 - 603
Database
ISI
SICI code
0956-540X(1994)118:3<579:CCBDLC>2.0.ZU;2-6
Abstract
The dynamic response of the crust and upper mantle to asthenospheric c onvective flow related to hot mantle diapiric upwelling is numerically evaluated in order to examine the implications for geological phenome na such as rifting, back-are spreading, sedimentary basin and mountain building. For island are areas such as the Japanese Islands and for r idge subducting areas such as the Basin and Range region, where both l ower crust and upper mantle may behave as ductile layers in some evolu tionary stages, the mechanical coupling between convection in the two layers may play an important role in the mantle dynamics related to th ese geological phenomena. Significant surface response to mantle diapi ric upwelling occurs even when the whole crust effectively works as an elastic layer. The magnitude of surface uplift predicted for this cru stal theological state depends on the thickness of the lithosphere and on the spatial scale of the mantle diapir. However, the effective vis cosity of the lower crust may decrease with increasing duration of dia piric upwelling. If the effective viscosities of the lower crust and a sthenosphere are 10(19)-10(21)Pa s, the asthenospheric convective flow induced by a mantle diapiric upwelling with a radius about 50 km, a s cale of feature which has been seismically observed in the upper mantl e beneath the Japanese Islands, may squeeze out the lower crustal mate rial laterally. This would be followed by maximum surface subsidence o f about 300-500 m and by upward migration of the Moho discontinuity by up to 3-5 km on a geological time-scale of 5-50 Ma. Thus, the couplin g between the convection in the lower crust and upper mantle is mechan ical, and the lower crustal material is dragged along by the shear str esses operating at the base of the lower crust. In this process, the s pace originally occupied by lower crustal material is replaced by mant le material, leading to thinning of the lower crust. Thus, in some geo logical situations having a hot mantle plume beneath the crust, a chan ge of tectonic style can be expected as a result of a change of the st ate of convective coupling between the lower crust and upper mantle. G eological phenomena associated with this convective coupling may also depend on the geometry of the region surrounding the mantle diapiric u pwelling. For example, the subducted slab in island are areas will ope rate as a barrier to the lateral movement of lower crustal material. T hus, accumulation of lower crustal material, referred to as lower crus tal megalith, may occur at the edge of the overriding plate. The overp ressure caused by the accumulation of this megalith may be a crucial f actor in determining the coupling state between the subducted slab and island are, and may play an important role in and/or work as a trigge r in the evolutionary histories of the island are and related back-are spreading.