LINKING THE STRUCTURAL HISTORY OF THE ROCKY POND SLICE TO THOSE OF THE NASHOBA TERRANE AND MERRIMACK BELT OF EASTERN MASSACHUSETTS

Detailed structural mapping was conducted to investigate potential relationships between the Rocky Pond Slice (RPS) and adjacent terranes in eastern MA. The Rocky Pond Slice (RPS) is a NE-trending fault-bounded block of ~35 km² that lies between low grade rocks of the SE Merrimack belt (MB) to the NW and high grade rocks of the Nashoba terrane (NT) to the SE. The RPS contains 3 metasedimentary units: the Boylston Schist in the northwest that was metamorphosed to greenschist facies in the NW to amphibolite-facies in the SE, the amphibolite-facies Sewall Hill Formation in the south and the greenschist-facies Vaughn Hill Formation in the east. Cm- to m-scale isoclinal folds in all these units are overprinted by map-scale shallowly to moderately NW-plunging upright folds. The Rocky Pond Granite intrudes the RPS in the north and is not folded. The Clinton-Newbury fault along the western boundary of the RPS is a moderately to steeply NW-dipping, ~100-150m wide fault zone, consisting of sheared phyllite in the north and gneiss and mylonite in the south. The Ball Hill shear zone along the eastern boundary of the RPS is composed of a series of 5~30m wide, moderately to steeply NW-dipping shear zones, within an ~1000m wide area that includes the previously defined Rattlesnake Hill fault and Ball Hill fault. It contains sheared chlorite-biotite schist in the NW and sillimanite-biotite schist in the SE. Based on shear sense indicators and mineral lineations, amphibolite-facies sinistral reverse shear was overprinted by greenschist-facies normal shear along the two shear zones bounding the RPS.

The NW part of NT shows isoclinal folds overprinted by NW-side-down asymmetric folds (related to ductile extrusion of the terrane). The MB has early isoclinal folds, locally overprinted by N-trending recumbent chevron folds. The moderately NW-dipping foliation and the isoclinal folds are common to all three domains. Based on the high metamorphic grade of the Boylston Schist and Sewall Hill Formation, the RPS is interpreted to be part of the NT, displaced along the Ball Hill shear zone. The NW-plunging folds in the RPS indicate minor NE-SW shortening and may have formed during the displacement along the Ball Hill shear zone.