Abstract

The experimental technique of centrifuge modelling has been used to elucidate patterns of total strain and progressive deformation associated with troughs that form by gravitational subsidence of a dense cover into a less dense basement. Model structures are analogous to synclines found between mantled gneiss domes, and the models provide data with which to test the hypothesis that Archean greenstone belts are products of vertical gravity tectonics.Experimental results include the following. (1) Strain within the main body of subsiding belts is dominated by strong horizontal contraction and vertical extension. In natural structures this strain pattern would produce vertical foliation and steeply plunging mineral lineation. (2) Within cover units near trough margins the flattening plane trends parallel with or at a low angle to the cover–basement interface at all levels in the trough. (3) Patterns of strain within subsiding troughs, at least as expressed in surface deformation, reflect the geometry of the cover–basement interface at depth. Flow within the cover units converges towards points of maximum trough subsidence to produce local vertical constrictive strain. Deformation above trough saddles is characterized by vertical flattening strain. (4) Horizontal shortening within a subsiding, stratified cover sequence produces early folds with horizontal axes and steep axial surfaces. Where the stratified sequence forms a relatively low-density unit within the cover, folds and thrusts verge consistently away from the trough axis. Continued subsidence and horizontal contraction tend to rotate initially shallowly dipping structures into steeply dipping attitudes.

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