THE MAIN BODY OF MONSON GNEISS, A MAJOR QUABOAGIAN (370-350 MA) NORTH-DIRECTED FOLD NAPPE: SECRETS OF ITS ROOT ZONE IN SOUTHERN MASSACHUSETTS

The Main Body of Monson Gneiss forms a large N-directed fold nappe carrying inverted gneiss exposed in several younger anticlines and synclines over inverted sequences of Ordovician Ammonoosuc Volcanics and Partridge, Silurian Clough Quartzite and Fitch, Silurian Rangeley, Perry Mountain, Francestown and Warner, and Lower Devonian Littleton. The Nappe top is marked by right-side up Ammonoosuc and Partridge in the overlying Greenwich Syncline, so the gneiss thickness in the Nappe core is ~6 km. The Nappe extends ~85 km from a root zone near the Connecticut line to an ~E-W trending anticlinal hinge near the New Hampshire line. The postulated location of this hinge in the air above the Keene Dome, Swanzey Lobe, may relate to the enormous "Beech Hill" E-directed backfold in the Monadnock Quadrangle.

The Prescott Syncline is key to geometry of the fold nappe, separating it from the autochthonous Bronson Hill sequences exposed in the Kempfield Anticline and Warwick Dome. A Prescott synclinal hinge, once postulated to occur just SE of Quabbin Hill, is now proved not to exist there. A distinct linear negative aeromagnetic anomaly follows the axial surface south into the Palmer Quadrangle, where it collides with another negative anomaly over the Greenwich Syncline. The N-S trending Quabbin Park Fault of Peterson (1992), located to truncate that fold in the original position, is now relocated to E as the Quabbin Park Shear Zone (QPSZ), truncating the Prescott Syncline at the new location, then extending S into Connecticut, en route slicing off a segment of Ammonoosuc Volcanics of the syncline's west limb in metamorphic zone IV. The Monson Quarry, type locality, lies well west of the QPSZ, in a direct S extension of the Kempfield Anticline belt that continues into Connecticut. The "Main Body" as described here, including the Greenwich Syncline, all east of the QPSZ, does not connect directly with S Connecticut, but is sheared out by the dextral Conant Brook Shear Zone to the E, and the QPSZ to the W. The new interpretation implies that the Monson Fold Nappe, with E-W extent of ~20 km in N Massachusetts, has a southern "root zone" in a narrow slot now reduced to 2.5 km in Palmer and as narrow as 0.4 km in S Monson. The QPSZ might be an E outlier of the Northfieldian (305-285 Ma) sinistral shear near the E edge of the Warwick Dome.