pressure shadow

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pressure shadow The area in a regional metamorphic rock which is protected from deformation by the presence of a relatively rigid porphyroblast or porphyroclast. Randomly orientated quartz and/or chlorite concentrates in triangular regions next to the faces of the porphyroblast or porphyroclast which are themselves orientated perpendicular to the schistosity of the surrounding, deformed, metamorphic fabric. During metamorphism and deformation quartz and chlorite are dissolved from the region of high pressure, where the matrix is squeezed against the hard, unyielding porphyroblast or porphyroclast, and are redeposited in the no-stress shadow region on either side of the porphyroblast where the deformed fabric wraps around the crystal, so forming the pressure shadow.