NEO-ACADIAN DEFORMATION IN THE NEW ENGLAND APPALACHIANS DOCUMENTED BY NORTHWARD EXTRUSION OF THE CROYDON DOME IN SOUTHWESTERN NEW HAMPSHIRE

Gneiss domes cored by Proterozoic basement gneisses or Ordovician arc-related plutonic rocks form a north-south chain along the length of the New England Appalachians. The Ordovician plutonic rocks that core many of the domes are variably deformed and metamorphosed with predominant gneissic foliations preserved along the dome margins. The core rocks are mantled by Ordovician, Silurian, and Devonian metasedimentary and metavolcanic rocks that comprise the Bronson Hill Anticlinorium. The timing and mechanisms of gneiss dome emplacement in the New England Appalachians has not been completely clarified, but recent detailed mapping, structural analysis, and geochronology have improved structural models for gneiss dome emplacement.

The Croydon Dome in southwestern New Hampshire is an elongate gneiss dome emplaced within the Bronson Hill Anticlinorium. The eastern margin of the Croydon Dome is truncated by a Mesozoic normal fault and western margin is defined by a high-strain zone. The core of the Croydon Dome preserves undeformed Ordovician granodiorite that grades into gneissic fabric towards the margins of the dome. Augen gneiss within the dome and intensely strained metasedimentary and metavolcanic units define an approximately 250 m-wide high-strain zone along the western margin of the dome. Metasedimentary and metavolcanic units are truncated along the high-strain zone. Foliations that are generally parallel to the dome margin range from subvertical to moderately to steeply west-dipping. Lineations preserved in the augen gneiss are subhorizontal and trend N-S. Asymmetric augen and mica fish in the metasedimentary rocks indicate sinistral shear sense within the high-strain zone. The steep to subvertical foliation, subhorizontal lineation, and sinistral shear sense suggest northward extrusion of the Croydon Dome rocks.

In order to determine the timing of emplacement of the Croydon Dome, in situ U-Th-Pb monazite geochronology using the electron probe microanalyzer was carried out. These analyses indicate that monazites in the high-strain zone grew during Neo-Acadian deformation, ranging from Late Devonian to Early Mississippian.