A common and puzzling feature of migmatite terranes is the presence of synkinematic leucosomes oriented perpendicular to the maximum principal compressive stress axis, σ1, particularly leucosomes oriented parallel to the axial plane of folds. These are parallel to planes of compaction, but experiments predict that leucosomes should form in dilational bands oriented instead at small angles to σ1. This discrepancy suggests that our understanding of melt extraction is incomplete. We propose that leucosomes at high angles to σ1 are results of ductile compaction instabilities that arise in compressive environments as a result of compaction of porous media with nonlinear rheology and interstitial fluids. These instabilities form at high angles to σ1 and their spacing can be regular, controlled by the compaction length.

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