Shear-zone systems and melts: feedback relations and self-organization in orogenic belts

In orogenic belts, the common spatial and temporal association of granites with crustal-scale shear-zone systems suggests melt transfer from source to upper crust was the result of a feedback relation. In this relation, the presence of melt in the crust profoundly affects the rheology, and induces localization of strain within shear-zone systems. Consequently, melt is moved out of the source preferentially along high-strain zones, which helps the system to accommodate strain. Because actively deforming orogenic belts are non-equilibrium systems, they may generate dissipative structure by self-organization; we interpret crustal-scale shear-zone systems and their associated granites as the manifestation of this self-organization. The architecture and permeability structure are controlled by the type of shear-zone system (transcurrent, normal, reverse or oblique); this is the primary control on melt transfer in orogenic belts. During active deformation, movement of melt is by percolative flow and melt essentially is pumped through the system parallel to the maximum principal finite elongation direction. Ifa build-up of melt pressure occurs, melt-enhanced embrittlement enables tensile and dilatant shear fracturing, and transfer of melt is by channelized flow. We illustrate feedback relations between migmatites, crustal-scale shear-zone systems and granites using examples from the Cadomian belt of western France and the northern Appalachian orogen of the eastern U.S.A. In orogenic belts dominated by transcurrent shear, where the maximum principal finite elongation direction may have a shallow to subhorizontal plunge, granite arrested during ascent through the system commonly develops C-S fabrics. This suggests percolative flow is not effective in expelling melt from these systems; the resulting build-up of melt pressure enables fracturing and channelized transfer of melt, which crystallizes during persistent deformation (e.g. the St. Male migmatite belt, Cadomian belt), In contrast, in contractional orogenic belts dominated by oblique-reverse displacement, where the maximum principal finite elongation direction may have a steep to subvertical plunge, granite arrested during ascent through the system generally does not develop C-S fabrics. This suggests percolative melt flow during active deformation was effective in avoiding a build-up of melt pressure, probably because buoyancy forces helped melt flow up the maximum principal finite elongation direction. During waning deformation in these systems, however, rates of percolative melt flow decline in response to decreasing strain accommodation, and a build-up of melt pressure results. This enables fracturing and channelized transfer of melt through the shear-zone system; granites are late syntectonic (e.g. the Central Maine belt, northern Appalachian orogen). (C) 1998 Elsevier Science Ltd.