REGIONAL STRAIN ACCOMMODATION ON THE SOUTHEAST SIDE OF THE NORUMBEGA FAULT AND SHEAR ZONE SYSTEM, COASTAL MAINE

Regional strain accommodation on the SE side of the dextral strike-slip Norumbega Fault and Shear Zone System is evident over a broad area of coastal Maine as map-scale oblique-to-fault, upright, tight to isoclinal, F2 folds that show a progressive CW rotation to form large scale drag folds against the main fault. Subhorizontal layer-parallel and hinge-parallel elongation within the upright folds is expressed as stretching lineations and symmetric boudinage of competent layers within the upright fold limbs. Superimposed on this oblique-to-fault fold and elongation geometry are syntectonic granite dikes and quartz veins that follow an initially orthogonal-to-lineation emplacement model to form cross dikes and veins that cut the upright limb layers at high angles. Km-wide zones of high simple shear strain parallel to these upright fold limbs are recognized by the rotational deformation of initially-orthogonal dikes and veins to yield oblique-to-layer strings of quartz and granite boudins. Other kinematic indicators include asymmetric boudin pods and porphyroclasts, shear bands and asymmetric folds. Early deformation on the NW side of the main fault consists of SE-dipping layers, with E-plunging lineations and overthrust kinematic indicators and is truncated by a km wide zone of strongly-lineated gneisses plunging to the SW that mark the extensional main fault zone that runs through Inner Casco Bay. On the SE side of the main fault, oblique-to-fault upright folds with intervening limb shear zones dominate across eastern Casco Bay and are bound on the east by the km-wide Phippsburg dextral shear zone. This NNE-striking dextral shear zone gives way towards the east to a broad Mid-Coast Block of tight oblique-to-fault folds with crumpled cross intrusions that show no sign of rotational strain. The eastern boundary of this Mid-Coast Block is marked by the sinistral NS-striking shear zones of Pemaquid Point and eastern Muscongus Bay that involve granitic dikes related to the nearby Late Devonian Waldoboro intrusion. These regional strain characteristics are combined into a model of oblique-to-fault upright folding and fold tightening against a restraining bend in the Norumbega that eventually led to lateral crustal extrusion of a Mid-Coast block southward along opposing dextral/sinistral strike-slip shear zones.