DISPLACEMENT ESTIMATES FOR EXTERNAL BASEMENT MASSIFS IN THE NORTHERN APPALACHIANS: RECONSTRUCTING THE RIFTED LAURENTIAN MARGIN

The distribution of Grenvillian massifs in the Appalachians provides a vital constraint on the shape of the Laurentian margin following rifting of Rodinia. Reconstruction of the continental margin, however, requires accurate restoration of the massifs to their pre-deformation positions. Ratcliffe and Hatch (1979) estimated internal shortening in the Berkshire massif in MA to be 40 km, and transport along the western frontal thrust to be 21 km, which together require displacement of 61 km. We suggest this value is an overestimate.

Felsic sills in Mesoproterozoic gneisses of the Berkshire massif were interpreted as syntectonic intrusives into Taconic thrusts, and their distribution coincides with many mapped faults. However, new U-Pb SHRIMP zircon ages (997 +/- 10, 1004 +/- 19, and 1003 +/- 8 Ma) from three sills disprove this interpretation; they formed during late Grenvillian orogenesis.

Also, electron microprobe dating of monazite from fault-zone samples does not support a Taconic age for thrusting. The western frontal thrust and klippen of the massif contain mylonites with steep strain gradients. Monazite from a deformed quartzite exposed in the Dry Hill fault gives an age of 392 +/- 14 Ma. Quartz-rich schist from Umpachene Falls contains monazite grains with multiple age populations that peak at 530, 435, 380, and 290 Ma. Monazite in a schist from Benton Hill below a major thrust gives a weighted average age of 436 +/- 8 Ma. Two localities from the western frontal thrust in Norfolk, CT give weighted average monazite ages of 401 +/- 9 Ma and 400 +/- 10 Ma. Thus, thrusting of the massif occurred in the Silurian and Devonian.

Careful examination of mapped faults revealed no evidence for internal thrusting in the massif; it apparently behaved as a rigid block. We suggest that the Berkshire massif was emplaced during the Salinic or Acadian orogeny as a rigid intracrustal wedge with moderate displacement. The leading edge of the massif-wedge is commonly an overturned fold defined by Neoproterozoic and Cambrian units, locally isolated as klippen. The distance between the eastern-most footwall rock and the western-most klippe requires no more than 18 km of displacement. This lower displacement estimate suggests that the New York Promontory of Thomas (2006) is a less prominent irregularity in the Laurentian margin.