APPLICATION OF EROSIONAL-RHEOLOGICAL COUPLED MECHANICAL MODELS TO ANCIENT OROGENS: AN EXAMPLE FROM THE NORUMBEGA SHEAR ZONE IN MAINE

Preliminary field mapping, and microstructural and petrologic analysis in south-central Maine suggests that pre-Silurian rocks of the Liberty/Orrington belt and Silurian rocks of the Fredericton belt record higher Devonian-age shear strains and peak metamorphic temperatures than adjacent rocks to the southeast, and that these strains and temperatures taper off into the Central Maine Basin Sequence to the northwest. The higher-strain rocks consist of more-competent metavolcanics and gneisses interlayered with less-competent metapelites that locally contain porphyroblasts of garnet, sillimanite, cordierite, andalusite, and some of the few occurrences of kyanite in Maine. We suggest that the Liberty/Orrington and Fredericton rocks may represent a zone of enhanced exhumation associated with Devonian dextral transpression within the long-lived orogen-parallel Norumbega shear zone.

Peak metamorphic temperatures of the Liberty/Orrington and Fredericton rocks, estimated at 625°C (± 50°C), are used to locate these rocks within a geographical reference frame using a three-dimensional mechanical model of an oblique orogen. Incorporating the evidence of enhanced exhumation as a boundary condition, together with the petrological information, the thermal evolution and physical trajectories of these rocks, relative to the adjacent terrains, can be calculated within this reference frame as the rocks are exhumed through the oblique orogen. In addition, the acquisition of the structural fabric by these rocks can be defined as the rock packets pass through the strain regimes characteristic of active transpression. Our geographically referenced petrological models are conditioned by surface observations from the active Southern Alps and Himalayan orogens and provide information on thermal and deformational histories of the Liberty/Orrington and Fredericton rocks during the Devonian orogeny.