STRAIN PARTITIONING IN A TRANSPRESSIVE OROGEN, CENTRAL COASTAL MAINE

Recent field, microstructural and chronological studies of Silurian and pre-Silurian rocks in the area between Waterville and Belfast, Maine, have identified a region of intense Devonian-age dextral shear strain that was focused in the older Liberty/Orrington (L-O) belt and adjacent younger rocks of the Fredericton Trough. This focused zone of dextral shear coincides with exposures of older gneisses, metavolcanics and metapelites that record a regional metamorphic high. Both the shear strain and metamorphic grade taper off to the northwest in the Central Maine Sequence (CMS) metapelites.

In Waterville, tight, upright, gently- to moderately-plunging folds are spatially associated with a dextral shear fabric. Limbs of these folds are moderately asymmetrically boudinaged, and carbonate veins filling the boudin necks contain curved fibers of calcite and have associated "flanking structures" along their margins that formed during dextral rotation of the veins about a steeply plunging axis. Further east, approaching the Hackmatack Pond Fault (HPF), pelitic slivers contain spiral garnet porphyroblasts. Evidence for the dextral shear deformation is pervasive from the HPF east to the Sennebec Pond Fault and includes strong asymmetric boudinage of competent layers and quartz veins, a strong vertical foliation with gently plunging mineral elongation lineations, and mylonites with strongly asymmetric fabrics. Upright folds seen in the CMS are preserved only locally in relatively competent units.

With the effects of contact metamorphism superficially removed, metamorphic grade jumps from garnet grade in the CMS rocks to sillimanite grade in the L-O rocks across the HPF, possibly indicating some later movement along this fault. Kyanite occurs locally along the east side of this fault, though it is commonly partially replaced by cordierite. This suggests that the kyanite was formed during a pre-Devonian metamorphism, or that metamorphic conditions in these rocks changed markedly during the Devonian. The above observations are consistent with Devonian-age transpression in south-central Maine.