NEOACADIAN NORTHWARD EXTRUSION OF THE CROYDON DOME WITHIN THE BRONSON HILL ANTICLIORIUM 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 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 geochronology and structural analysis has improved structural models for gneiss dome emplacement.

The Croydon Dome in southwestern New Hampshire is an elongate gneiss dome that has a high-strain zone along the western margin and the Mesozoic Grantham Fault along the eastern margin. The dome is surrounded by Bronson Hill rocks and the Bethlehem Gneiss. U-Pb zircon ages, using LA-ICP-MS indicate the Bethlehem Gneiss crystallized at ca. 410 ± 4.8 Ma. The core of the Croydon Dome preserves undeformed Ordovician granodiorite that grades into gneissic fabric toward 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 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. In situ U-Th-Pb monazite geochronology, using the electron probe microanalyzer, record monazite growth within the high-strain zone ranging from ca. 370–350 Ma. In addition, the northern margin of the dome preserves moderately plunging N-trending lineations and top-to-the-S shear sense. The N-trending lineations overprint regional NW-SE lineations. Sinistral shear sense along the western margin, top-to-the-S shear sense along the northern margin, U-Th-Pb monazite ages suggest northward extrusion of the Croydon Dome rocks during Neoacadian deformation.