M. Nakada, CONVECTIVE COUPLING BETWEEN DUCTILE LOWER CRUST AND UPPER-MANTLE, ANDITS TECTONIC IMPLICATIONS, Geophysical journal international, 118(3), 1994, pp. 579-603
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.