242-18 Pressure-Temperature Conditions of a Fossil Brittle-Ductile Transition in the Great Common Fault Zone, Norumbega Fault System, Maine, USA

Abstract:

Evaluating the pressure-dependent brittle behavior and temperature-dependent ductile rheology are key to understanding earthquake processes and crustal rheology and thus require constraints on the pressure and temperature (P-T) conditions. The Great Common fault zone, associated with the regionally extensive Norumbega Fault System, is a world-class exposure of an exhumed deep seismogenic zone localized in the Rye Complex, a distributed ductile shear zone developed in polymetamorphosed rocks. Field observations of mylonitized pseudotachylyte suggest the current exposure represents a brittle-ductile transition (BDT), but the exact conditions of this transition remain poorly constrained. We use petrography, electron probe microanalysis (EPMA), electron backscatter diffraction (EBSD) on quartz, and phase equilibria modeling to quantitatively constrain the P-T conditions of metamorphism, distributed ductile deformation and localized brittle-ductile cycling.

We petrographically distinguish textures and mineral assemblages corresponding to prograde (M0), peak (M1), ductile fabric development (M2), and retrograde (M3) metamorphic events in eight samples across the fault zone. Using pseudosections constructed in Perple_X, we match observed equilibrium mineral assemblages with phase stability fields and measured mineral compositions with isopleths to obtain P-T estimates for each episode. 

We interpret the garnet cores, large biotite and muscovite grains, resorbed staurolite, and andalusite clots to represent M0 heating from 450–550 ºC at 2.5–3 kbar. Sillimanite wrapping around andalusite and in the foliation places peak (M1) conditions in the sillimanite zone. The foliation is defined by elongate biotite, muscovite, and sillimanite, which are offset by C’ shear bands, and in chemical equilibrium with euhedral garnets. Foliation parallel quartz veins exhibit subgrain rotation dynamic recrystallization and grain boundary migration. Quartz c-axis pole figures show centered CPOs recording prism <a> slip, which corresponds to amphibolite facies temperatures. Preliminary constraints on M2 conditions are 575-625 ºC and 3.25-5.25 kbar. The path from M2 through brittle-ductile cycling to M3 is challenging to constrain. M1 and M2 are variably overprinted by incomplete retrograde (M3) chlorite. Preliminary chlorite thermobarometry records temperatures between 370-400 ºC.

Taken together, preliminary findings suggest peak metamorphism reached sillimanite zone conditions, distributed ductile deformation occurred at near peak conditions and mid-crustal depth, and brittle-ductile cycling occurred following M2 into M3. These P-T constraints indicate cyclic brittle-ductile deformation was occurring at temperatures hotter than but depths comparable to the conditions of modern seismicity along active strike-slip faults.

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

doi: 10.1130/abs/2026AM-19632

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