DUCTILE DEFORMATION ZONES IN NEW ENGLAND'S CRYSTALLINE EXTERNAL MASSIFS(CEM) ALPINE ANALOGUES OR NOT

Recent studies of Alpine external massifs emphasize inheritance of Alpine ductile deformation zones (DDZ), from flaws in Hercynian basement reactivated in Alpine formation of the massifs (Mohn, et al, 2010, Nibouurel, et al 2021, Herwegh, et al, 2023). From comparison of Hercynian basement across many European CEM, they suggest orogen-wide reactivation at a crustal scale.

“Grenville” 1.4- 0.95 Ga polycyclic granulite gneisses cores CEM’s from the Hudson Highlands (HH), Berkshire (GM) to the Green Mountain (GM) in Vermont. Most ductile deformation zones (DDZ’s) formed during Upper Ordovician to earliest Silurian Taconian collision, under Barrovian lower greenschist to amphibolite overprinting, rehydration, and partial melting.

The 10 km -wide Ramapo- Canopus zone in the HH has evidence for reactivation. The Mesozoic Ramapo fault system overprinted late Ordovician greenschist facies ductile zones dated by dikes and plutons of the Cortlandt Intrusive suite at about 450Ma. Shawinigan DDZ’s define right- lateral transpression at about 1144 +- 7Ma based the age of the syntectonic Canopus pluton and dike system (Ratcliffe and Aleinikoff,2021). The sigmoidal deformation pattern ends eastward at a subvertical staurolite grade “retrogressive” transpressional DDZ. Numerous other a kyanite- fibrolite DDZ’s occur along basement cover contacts. Eastward DDZ’s become migmatitic. The Ottawan 1045+- 7 Ma Danbury augen granite (Walsh, et.al.2004) at the eastern edge of the massif is mylonitic internally. These DDZ’s are arched over the northeast end of the massif, splay upwards carrying cover rocks of the New Milford massif and basement slivers west and north. DDZ’s above are compatible with the grade of late Ordovician re-metamorphism across the massif. Similar progressions also occur across the BM where abundant traceable ductile deformation zones define limbs of fold nappes. DDZ’s in the GM are well defined but pass from core rock into cover without significant translation. Reactivation of “Grenville” basement faults did not play a role in formation of our external massifs which was dependent on thermal regime and rehydration contributed from metasediments beneath basal detachments. Lithologic contrasts notably major marble, calc silicate belts do localize DDZ’s.