Interlayer material transport during layer-normal shortening. Part II. Boudinage, pinch-and-swell and migmatite at Søndre Strømfjord Airport, West Greenland

Abstract

Slow deformation of rock at metamorphic conditions involves an interplay of ductile flow, fracturing and mass-transfer. Boudinage and pinch-and-swell structures from a Precambrian terrane in West Greenland illustrate this point. Macroscopic fractures between separating boudins are expected to be 0.1 mm or less in width. It is in such narrow openings, simultaneously dilating and being filled, that deposition from an oversaturated pore fluid leads to macroscopic differentiation.

The microscopic process thought to underly macroscopic mass-transfer is as follows. Pore fluid occurring throughout the deforming rock has a pressure lower than the normal stresses existing between grains. It will therefore be oversaturated compared to a hydrostatic equilibrium situation in which all solid-solid contacts experience normal stresses equal to the pore pressure. Local solution and deposition cause local rearrangements in the pore network and contribute to local ductile flow. Only heterogeneous deposition—in separate, mechanically controlled and dynamically reestablished groupings and alignments of sites—gives rise to macroscopic differentiation products.

The macroscopic theory developed in Part I and the microscopic view presented here are combined in an interpretation of some common features of boudinage and pinch-and-swell structures.

At high metamorphic grade the rocks are melted to some small degree. The principles governing differentiation are not affected by changing the pore fluid from a hydrous phase to melt. Veins in anatectic terranes grow as relatively competent deposits from a highly incompetent melt permeating the deforming rock.