I might as well jump in here and sink or swim. Perhaps your two schools of
thought are both correct to a degree. My observations working with fault
zones at different crustal levels in Vermont and western New Englalnd
(USA)is that there are important feedback mechanisms working in ductile
fault zones. For example, the development of mylonites in granitic
gneisses in Vermont is greatly facilitated by the breakdown of feldspar to
produce muscovite and quartz etc. If additional quartz is injected into
the systems we find muscovite rich schists with numerous quartz veins.
According to my colleague, Tracy Rushmer, the volume change is negative
(from reactants to products) no matter what reaction we write for the
breakdown of K spar. We interpret these reactions to produce significant
strain softened zones that greatly facilitate movement in the shear zones.
Thus the metamorphic reaction aids deformation. By the same token brittle
and ductile deformation reduces grain size and thus facilitates metamorphic
reactions. I suggest that we view the deformation and chemical reactions
in rocks as a dynamic system with feedback processes that reinforce or
even dampen the respective behavior. To say that one is more important than
the other is to deny their mutual importance in the strain history of the
crust.
Rolfe Stanley
Stanley Computer Center
Fletcher Extension
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