THE EXHUMATION OF A HIGH-METAMORPHIC GRADE TERRANE WITHIN A TRANSPRESSIONAL OROGEN: CONSTRAINTS FROM NUMERICAL MODELING IN CENTRAL COASTAL MAINE

The geology of central coastal Maine consists of northeast-trending terranes separated by steeply-dipping faults of varying age and temperature. Preliminary field mapping and microstructural and petrologic analysis suggests that pre-Silurian rocks of the Liberty-Orrington (L-O) 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 to the northwest. The higher-strain rocks consist of metavolcanics and gneisses interlayered with metapelites that locally contain porphyroblasts of garnet, sillimanite, cordierite, andalusite, and some of the few occurrences of kyanite in Maine. Strain in the L-O belt is characterized by early convergent structures such as tight folds preserved in competent rocks, followed by dextral strike-slip deformation consisting of heterogeneous zones of mylonite and highly strained rock that largely obscure prior structures.

Three-dimensional numerical modeling of transpressive zones has identified the depth-dependent separation of zones that concentrate lateral strain from those on which convergent strain is concentrated. In addition, metamorphic differences between these two zones constrain the pattern and relative rates of exhumation during transpression. We have identified candidates for these shear zones within present the field area, and suggest that the Liberty-Orrington belt and some adjacent rocks may represent a zone of enhanced exhumation associated with Devonian dextral transpression within a long-lived orogen-parallel fault system.

We use models developed for erosional-rheological coupling within the currently active Himalaya orogen and Southern Alps of New Zealand to evaluate the possibility that this orogen-parallel fault system was a zone of enhanced exhumation during Devonian orogeny and that exhumation was linked to its development as a large-scale transpressional boundary.