A migration model of magmatism explaining a ridge subduction, and its details on a statistical analysis of the granite ages in Cretaceous Southwest Japan
O. Kinoshita, A migration model of magmatism explaining a ridge subduction, and its details on a statistical analysis of the granite ages in Cretaceous Southwest Japan, ISL ARC, 8(2), 1999, pp. 181-189
A migration model of magmatism based on the granite ages in Southwest Japan
is proposed to explain the ridge subduction beneath the Eurasia continent
as the cause of the along-are and across-are youngings of the granite ages
and the very high activity of the magmatism in the Cretaceous. For the cons
truction of the magmatic model, the localities of the granite age samples a
re denoted by the cartesian coordinates X and Y, which are measured along a
nd normal to the Median Tectonic Line (MTL), respectively, and their ages a
re set corresponding to the coordinate Z vertical to the X-Y plane. The age
trend is then formulated by a regression plane of Z on X and Y, which incl
ines in both directions along and normal to the MTL, and approximates the a
ges with the very high multiple correlation coefficient 0.91. Evaluating th
e magmatic trend by such a method, various characteristics of the activitie
s can be taken easily; for example, the isochronous line of the magmatism,
which is an intersection of the regression plane and an arbitrary horizonta
l plane, is found to extend landward obliquely across the continental margi
n. The migrating rate of the isochronous line along the MTL is also taken t
o be 2.8 cm/year as a reciprocal of the inclination of the along-are youngi
ng. The isochronous line is speculated to be the out-cropped manifestation
of the subcrustal linear heat source. Such a migrating linear heat source i
s probably due to the subduction of an active ridge, the Kula (or Izanagi)-
Pacific ridge in the Cretaceous. The migration model of magmatism harmonize
s very well with the plates and the ridge motions in the East Asia area dur
ing the late Mesozoic. The ridge subduction is one of the important phenome
na that explain the unusually active are magmatism and the migrating slab w
indow; it is important to grasp dynamically the geological messages issued
from the system.