Geophysical and structural signatures of syntectonic batholith construction: the South Mountain Batholith, Meguma Terrane, Nova Scotia

The Late Devonian South Mountain Batholith is a very large (7000 km²) composite peraluminous granitoid complex situated within the Meguma Terrane of the northern Appalachians. It is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380–370 Ma during the Acadian Orogeny, i.e. during the collision of Gondwana with the eastern margin of North America. A significant geophysical and geological database makes the South Mountain Batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen. Gravity models reveal the plutons have flat or gently dipping floors at approximately 7.0 km depth and aspect ratios >6:1. They are underlain by deeper (>10 km) elongate northeast–southwest-trending roots that may indicate magma feeder zones. Dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale. The very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement; space was mostly created by vertical displacements of country rocks. The data are consistent with a laccolithic model for syntectonic batholith assembly. The laccolithic plutons may have been emplaced at the base of the Meguma Supergroup metasedimentary rocks, suggesting a maximum thickness of approximately 7.0 km for the supracrustal rocks in the Meguma Terrane.