THE SUTURING OF NEW ENGLAND

A long standing conundrum of New England geology is the enigmatic relationship between the contemporaneous Barrovian metamorphism of Vermont, with its clockwise P-T paths culminating in maximum burial depths of ca 30-35 km (indicative of continental collision) and the Buchan metamorphism of New Hampshire, with counterclockwise P-T paths and regional high-T, low-P metamorphism (indicative of an extensional terrane). Thermal and tectonic considerations do not readily permit these distinctively different terranes to coexist in their present proximity.

We have examined a belt of Devonian Littleton Formation that crops out from northern New Hampshire to central Connecticut along the Bronson Hill anticlinorium in central New England, which we believe holds the answer to this paradox. The metamorphism in this unit is distinct from both overlying nappes and underlying schists and is characterized by nearly ubiquitous, large (several cm) staurolite crystals that postdate the dominant fabric (the “Big Staurolite nappe” or BSN).

Monazite from this unit is typically only weakly zoned with high Th cores, suggesting a single episode of growth. No multi-generation monazite has been observed. New electron probe dates on metamorphic monazite from this unit reveal no ages older than ca 340 Ma. Monazite ages from regions along the strike of the unit are: 280±10 from the Salmon Hole Brook syncline (northern NH); 300±10 from near Mascoma, NH (west-central NH); 320 ±10 from around Fall Mountain (west-central NH); 270±10 from the near Bolton, CT.

In our model, the BSN represents the trace of a mid-crustal shear zone that was responsible for the westward transport of the rocks of the Central Maine terrane onto the Bronson Hill anticlinorium, in juxtaposition with the Barrovian terrane of Vermont during the early stages of the Alleghanian orogeny. Transport took place over as much as tens of millions of years through the Carboniferous culminating in the Early to Mid Permian. Metamorphic pressures of 5-7 kbar suggest the shear zone was active at depths of ca 20 km depth. Rocks of the BSN are folded by the domes of the Bronson Hill anticlinorium, and it is suggested that dome formation is a consequence of thrust ramping of Oliverian plus Avalon basement onto the Laurentian margin.