DIFFERENTIAL EXHUMATION DURING ALLEGHANIAN TRANSPRESSION, MEGUMA TERRANE, SW NOVA SCOTIA

Kinematic and quasi-mechanical models of transpression generally invoke vertical extrusion in response to zone-normal shortening. If so, opposite-sense kinematic indicators should be observed on the margins of these shear zones, and higher-grade rocks should occupy their central regions. In SW Nova Scotia two coeval Alleghanian shear zones display features compatible with these predictions. Accretion of the Meguma terrane to the Appalachian orogen during the Acadian orogeny produced trains of upright folds (D1, ca. 390 Ma), followed by peak metamorphism (M1, ca. 380 Ma). In the study area, the Acadian fold belt was reworked by steeply dipping Alleghanian shear zones (D2, M2, ca. 320 Ma) that overprinted the limbs of the Yarmouth syncline. On the NW limb, the Cranberry Point shear zone (CPSZ) is a 4-5 km wide zone of pervasive D2 greenschist-facies ductile strain. Kinematic indicators switch polarity across the zone, implying differential upward movement of the shear zone interior (extrusion) during D2 triclinic transpression. Extrusion was accompanied by differential exhumation of M1 metamorphic assemblages, which range from staurolite-andalusite zone (T=550°C; P=4 kb) in the central part of the CPSZ to biotite zone along its margins. On the SE limb, the Chebogue Point shear zone (CBPSZ) has NW-side-up kinematic indicators on its SE margin, and coincides with a second linear M1 culmination cored by staurolite-andalusite grade rocks (T=575-600°C; P=4.2 kb). Both shear zones developed in dominantly pelitic rocks of the Halifax and White Rock Formations; they are flanked on the NW and SE by psammites of the Goldenville Formation and separated by metavolcanic rocks of the White Rock Formation. These observations suggest that variable shortening during D2 transpression led to extrusion of less competent lithologies on the syncline limbs, resulting in differential exhumation of pre-existing metamorphic assemblages.