DOCUMENTATION OF THE TRANSITION BETWEEN CONTRASTING STRUCTURAL ZONES IN WESTERN MAINE, USA: TESTING A MODEL

In western Maine (Devonian Acadian orogen), high-T strain partitioning is recorded by contrasting structural zones that alternate at the km scale across strike. These zones are defined by previous research based upon the correlation between the shape, orientation, and intensity of metamorphic mineral fabrics as they correspond with the geometry of folds of pelite-psammite interlayers and their hinge line attitudes. Regionally, the structural system is defined as steeply-dipping zones of apparent flattening strain (AFZs) that anastomose around intervening zones of apparent constrictional strain (ACZs), both with fabrics defined by the same minerals at the same metamorphic grade. Previous work shows a transition zone between the two types of zones, rather than cross-cutting, to suggest the two zones developed synchronously. An oblique section across a transition zone is found along the Swift River, NW of Coos Canyon (Byron, ME). In the canyon, rocks show typical AFZ fabrics that are consistently oriented in a NNE trend, defined by a penetrative sub-vertical mica foliation and steeply NNE-plunging acicular mica lineation (L - S fabric). Compositional layers range in thickness from 0.5 cm to 5.5 cm. Up to 25% of these rocks is composed of St, Grt, Bt and And porphyroblasts (decreasing abundance). In contrast, rocks found ~ 1 km to the NNW show a penetrative, moderately NE-plunging mica lineation but a weakly developed, variable foliation (L > S fabric). Traverses to the NNW (from Coos Canyon) show the fabrics become systematically variable over ~ 50 m, ending where foliations are not strongly developed, and the rock shows a consistent L > S fabric. Compositional layers are increasingly thick, ranging from 2 to > 10 cm to suggest decreased recorded flattening. Also, the percentage of porphyroblasts decrease to < 10%. Shapes of the strain ellipsoids defined by matrix minerals along this traverse shows that the transition zone is ~50 m thick across strike. Over this distance the strain ellipsoids show a consistent change from strongly plane-strain to prolate, with only small variations in the attitude of the maximum principal finite strain axis. The transition zone rocks are defined in the field as showing metamorphic mineral fabrics transitional between AFZ and ACZ types consistent with a model that the two main zones were coeval.