Kudos to Dugald Carmichael for his interesting and provocative synthesis of
potential interplays
between Earth rotation, tidal forces, and west-directed shear at the base
of the lithosphere. Presumably
a good test of these ideas would be to look at whether the asymmetries in
global tectonics that Dug cites hold not only today
but throughout Earth tectonic history insofar as we understand it. For
instance, assuming that opening of back-
arc basins is a signature of trench rollback, is it generally true through
history that offshore arcs have generally
developed above east-facing subduction zones whereas continental magmatic
arcs are most prevalent above west-
facing subduction zones? Does anyone feel qualified to address this issue?
A few moments thought suggests to me that
the proposed pattern holds through most of the Mesozoic and Cenozoic, but
what about the Paleozoic? For instance, a lot
of folks look to the western Pacific for modern-day analogs for the western
margin of North America during parts of the Paleozoic and Early Mesozoic.
Some would picture the opening and eventual closing of back-arc basins
above east-dipping
subduction zones in the mid-Paleozoic, for example. However, others
envision the collision of a far-traveled arc overlying an east-facing
subduction zone, which would presumably jive better with the hypothesis in
question. Can anyone cite any better examples of exceptions to the rule?
Given the slow time-scale at which plate tectonics evolves, can it
reasonably be argued that many of the contemporary plate tectonic patterns
Dug and others cite are simply artifacts of the plate geometry established
at the time of Pangean break-up? Just some musings that might prompt a
response . . .
Al McGrew
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