Constraining the relative strengths of high-grade metamorphic rocks using foliation refraction angles: an example from the Northern New England Appalachians

Foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite-facies metaturbidites in the Presidential Range of the White Mountains, in eastern New Hampshire. An early-formed foliation, developed during km-scale nappe folding, consistently displays larger bedding-foliation angles in metapelitic units than metapsammitic units, suggesting that the metapelitic units had higher effective viscosities during this deformation. We collected bedding-foliation angle measurements from a combination of outcrops and locally-derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades. Our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers, which we attribute to the presence of large (up to 15 cm long), abundant (up to 30 vol.%) effectively rigid andalusite porphyroblasts in the metapelitic layers. We present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types, address the practical limitations we encountered, and suggest this method is an easy way of estimating the strength contrast between different rock types. Finally, we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis.