Attachment between brittle and ductile crust at wrenching plate boundaries

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Using strain modeling we analyze the ductile de- formation beneath rigid upper crustal blocks that rotate and translate in wrench zones. We define the zone of deformation as an attachment zone and assume strain continuity between the wrench shearing in the ductile crust and the horizontal shearing induced by the rotation/translation of rigid blocks. For reasonable amounts of rotation/translation of rigid blocks and reasonable thicknesses of attachment zones, the orienta- tion and shape of the finite strain ellipsoids within attach- ment zones are calculated, and the orientation of planar and linear fabrics are predicted. Attachment zones beneath rotat- ing blocks should display radiating foliation and concentric lineations; if rotation of rigid blocks is driven from below, de- formation in the attachment zone is dominated by the wrench component, altering significantly the radiating and concen- tric patterns of foliation and lineation. Attachment zones be- neath translating blocks display a wide range of foliation ori- entations, with steep foliations below the central part of rigid blocks, and gently dipping foliations toward the margins. Be- low the upper crustal strike-slip faults, foliation is shallowly dipping, strain intensity is maximum, and there is an abrupt reversal of sense of shear across a presumed discontinuity. The linear belt of greenschist-grade metamorphic rocks in Trinidad's Northern Ranges and eastern Venezuela's Paria Peninsula is a candidate for an exhumed attachment zone de- veloped beneath translating upper crustal blocks during Neo- gene highly oblique convergence between the Caribbean and South America plates.