TIMING AND SIGNIFICANCE OF DEXTRAL TRANSPRESSION IN CENTRAL MAINE: IT'S EARLY AND IMPORTANT

Field observations support the idea that a well-established dextral transpressive episode of deformation in central Maine began in the early Devonian, and that it represents a continuum of strain from early contractional deformation through to late, localized transcurrent shearing along the Norumbega shear zone. Both coaxial-dominant (D₁-D₂) and non-coaxial-dominant (D₃) deformation has long been recognized in central Maine, but the significance, timing, and interpretation of the relationship between the two varies widely in the literature. Recent studies reiterate the two regional interpretations for this area: 1) early contractional and later transpressive deformation are separated by a significant time gap, and 2) deformation was effectively continuous from the contractional phase through early Devonian dextral transpression. Key structures developed in low-metamorphic-grade limestone and phyllite in central Maine support the later scenario. Northeast-trending foliation associated with D₂ folds is reactivated by D₃ cleavage associated with the non-coaxial deformation of calcite veins. The northeast-trending composite fabric and calcite veins are then deformed by northerly-trending, locally developed Z-folds and axial-plane parallel cleavage, traditionally associated with regional D₃. The D₂- D₃ composite fabric is cut by a 399 Ma dike, and is included in peak-metamorphic biotite, whereas the north-trending D₃ cleavage deforms the dike and locally controls biotite orientation. The above relationships suggest that dextral transpression at least began very soon after contractional deformation, and is perhaps better described as the central deformation phase in regional, long-lived, strain-partitioned oblique convergence.