MODES OF FLOW IN THE OROGENIC INFRASTRUCTURE: THE EXAMPLE OF THE CENTRAL GNEISS

A tectonic model for the Central Gneiss Belt (CGB) has Phase 1 orogenic thickening

followed by Phase 2 infrastructural flow of ductile nappes detached from a

stiff superstructure. The CGB cross section comprises a stack of lower crustal

nappes interlayered during Phase 2 with sheets of ductile mid-crust. The

regional map pattern shows resistant lumps derived from lower crustal nappes

containing relict Phase 1 structures, enveloped by sinuous Phase 2 gneiss &

km-scale transport-direction parallel folds formed from either ductile

nappe-carapaces or mid-crustal gneiss.

Whereas most gneisses are compositionally similar granitoids, their structural

fate & mode of flow in Phase 2 is dependant on differences in rheology

determined by one or more factors. For example the inherited pre-orogenic state

may be crucial, thus rocks not metamorphosed before the Grenville orogeny were

fertile with respect to development of abundant ductility-enhancing leucosome

in Phase 1 & 2. In contrast, infertile lithologies were melted in an earlier

orogeny or attained granulite facies in Phase 1 & thus, in Phase 2, formed

resistant lumps or competent members in folded multilayers. Formation of a

ductile carapace of transposed Phase 1 gneiss around some of these resisters

was accomplished by addition of water mediated by pegmatite intrusion. The

pegmatites may have originated from the leucosome-rich fertile lithologies and

given that the latter themselves needed water added to be melted & that the

majority of Phase 2 structures had the same requirement, an ultimately

exogenous origin is postulated for this fluid. Another important mode of Phase

2 is buckle-&-shear deformation which was responsible for large & small

transport direction folds. This mode was dependant on the presence of layering

anisotropy developed during Phase 1 through homogenous deformation of

compositionally variable protolith or by interlayering of fertile- (incompetent

when migmatised) & competent infertile gneiss layers.