STRAIN PARTITIONING DURING TRANSPRESSIONAL DEFORMATION: EVIDENCE FROM BOUDIN PARTINGS, QUARTZ VEINS AND GRANITE INTRUSIONS IN NORUMBEGA SHEARING

Host rock fabrics across mid-coast Maine promoted strain partitioning during dextral transpression related to the orogen-parallel Norumbega fault/shear zone system. Regional non-coaxial deformation due to oblique convergence was accommodated by simple shear and pure shear components parallel to this preexisting fabric. Upright F2 fold limbs with a subhorizontal hinge-parallel stretching lineation defined the local shear component geometry. Lineation-parallel elongation was expressed as symmetric competent-layer and foliation boudinage with initial partings that range from thin veinlets to tapered quartz-filled puckers, blocky partitions, larger-scale quartz veins and granite dike intrusions. The abundance of veins and intrusions that utilized this initially-orthogonal emplacement geometry suggests that this is an important mechanism for fluid transport and magma emplacement during transpression in the mid to upper crust. Modification of these initially-orthogonal structures by fabric-parallel simple shear led to the progressive rotation of initial structures, antithetic slip along the veins induced by block rotation, elongation into strings of quartz and granite boudins (oblique to flow layers) that yield asymmetric tails with continued separation and shear. Minimum strain estimates utilized beta angles between reoriented structures and the fabric for gamma shear strains; line lengths of folded veins and intrusions compared to outcrop lengths for layer-normal shortening and surface areas of boudin pods for the elongation that accompanied shear. The main inner Casco Bay zones have higher dextral shear strains (~10-50) and elongations (~315-2360%) and are marked by gently SW-plunging lineations in steeply SE-dipping flow layers. The mid-coast area on the SE side of the main zone is dominated by upright folds oblique to the fault trace with hinge-parallel elongation (60-300%) and layer-normal shortening (30-95%) that produced tightly-folded veins and intrusions. Localized higher shear strain zones are dextral in northeastern Casco Bay but sinistral in the Muscongus Bay area 40 km to the east. The intervening mid-coast block is interpreted to have extruded southward out of Norumbega convergence during regional shearing accommodated by flanking zones of opposing shear sense.