The interplay of regional structure and emplacement mechanisms at the contact of the South Mountain Batholith, Nova Scotia: floor-down or wall-up?

Abstract

In southern Nova Scotia, the Devonian South Mountain Batholith was emplaced into metasedimentary rocks of the Cambro-Ordovician Meguma Group at ca. 370 Ma. The contact of the eastern end of the South Mountain Batholith transects at a high angle the trace of subhorizontal, upright Acadian (mid-late Devonian) folds formed in the Meguma Group. At two locations, where the contact is well exposed, there are contrasting structures in the country rocks adjacent to Acadian anticlinoria and synclinoria, respectively. Regional folds are affected by ductile deformation where anticlinoria abut the batholith but are undisturbed at the synclinoria. At the anticlinorial contacts, the metasedimentary bedding youngs towards the granite, and granite side-down shear resulted in a belt in which bedding is transposed to a new contact-parallel fabric. Deflection of linear structures that were initially horizontal in the Acadian folds (e.g., intersection lineations) illustrates the granite side-down shear. The reorientation of initially horizontal linear structures gradually diminishes as the contact is followed from the anticlinoria to the synclinoria, where the regional fold geometry is preserved right up to the contact, showing that there is no granite side-down shear in the synclinoria at the present level of erosion. Two models that potentially explain this variation in contact structure are discussed. In the first, it is explained as an artifact of emplacement of the batholith late in the growth of the Acadian folds, in which the horizontal, upright anticlinoria amplified and moved upward relative to the pluton. A shear zone was formed parallel to the contact along the thermally softened tip of the anticlinoria. The synclinoria remained fixed vertically and there was no differential movement between granite and country rock. Thus, regional structures and evidence for stoping are most widely preserved in the synclinoria, where they were not overprinted by the marginal shearing. The second model invokes floor-down emplacement of magma into folds of layered sediments with contrasting mechanical properties. The erosion surface within the synclinoria intersects slates of the Halifax Formation with mechanical properties that favour emplacement predominantly by dyking and stoping. Below the level of erosion, the stratigraphically underlying Goldenville Formation, having different mechanical properties than the Halifax, presumably is displaced downwards predominantly by ductile deformation (pure and simple shear). Within the anticlinoria, where the Goldenville Formation is exposed, the requirement of a level pluton floor necessitates that downward deflection is accompanied by relatively high ductile strains in the wall rock. A third possible model that combines features of the syntectonic and floor-down models is an obvious option.