242-16 Preliminary Pressure-Temperature Constraints on Migmatization Within the Norumbega Fault System, South-Central Maine

Abstract:

Deformation within active strike-slip fault systems is well documented in the shallow crust, and restraining bends are recognized as important sites of strain localization. However, the processes accommodating deformation in the deep crust remain poorly understood because they are inaccessible in active plate-boundary settings. The Androscoggin Riverlands State Park in south-central Maine exposes mid-crustal rocks within a restraining bend on a splay of the Norumbega fault system. This provides an opportunity to investigate the role of metamorphism and partial melting in strain localization during deep-crustal transpressional deformation. To constrain the conditions under which deformation occurred, we integrate petrography and phase equilibria modeling to reconstruct the pressure–temperature (P–T) history of migmatitic rocks.

We present preliminary results from two metasedimentary migmatites (NMR-24-04 and NMR-24-06). Sample NMR-24-06 is a strongly restitic migmatite characterized by abundant biotite, garnet-bearing leucosomes, and textures consistent with biotite-dehydration melting. Phase equilibria modeling indicates peak metamorphic conditions of approximately 725–750 °C and 4–7 kbar, with melt stable at peak conditions. Sample NMR-24-04 records a similar low-pressure, high-temperature mineral assemblage consisting of garnet, biotite, sillimanite, plagioclase, quartz, and muscovite, with petrographic evidence for both muscovite-out and biotite-out melting reactions producing peritectic aluminosilicate and garnet, respectively. Preliminary phase equilibria modeling suggests subsequent near-isobaric cooling from peak conditions.

These preliminary results suggest these rocks record a clockwise, low-pressure, high-temperature P–T metamorphism accompanied by partial melting within the restraining bend. The inferred conditions are consistent with thermal input from Devonian gabbro emplacement and suggest that migmatization may have weakened the middle crust, promoting localization of deformation into adjacent high-strain shear zones. This work will be supplemented with outcrop- and thin section-scale structural analysis, traditional thermobarometry, and zircon/monazite U-Pb petrochronology to further constrain the pressure–temperature–time–deformation evolution of these rocks and provide further insight into the interplay between metamorphism, anatexis, and deformation in the deep crust of a strike-slip system. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 5, 2026

doi: 10.1130/abs/2026AM-20067

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