INTRACRUSTAL WEDGING AND EMPLACEMENT OF EXTERNAL BASEMENT MASSIFS IN THE NORTHERN APPALACHIANS

Mesoproterozoic gneisses and unconformably overlying Neoproterozoic to Cambrian clastic rocks of the Berkshire massif in MA and CT were thrust westward over early Paleozoic carbonate rocks of the Laurentian margin. Although, it is widely accepted that displacement on the frontal thrust occurred during the Ordovician Taconic orogeny, evidence for the age of thrusting is limited. We used zircon SHRIMP methods to date five sills previously interpreted as syn-tectonic intrusives into Taconic thrusts. Two of these samples are from the eastern margin of the massif, and give Silurian crystallization ages of 432 +/- 3 Ma and 434 +/- 5 Ma. Igneous zircon rims from three sills located in the interior and western margin of the massif yield SHRIMP ages of 997 +/- 10 Ma, 1004 +/- 19 Ma, and 1003 +/- 8 Ma. Thus, none of the sills formed during the Ordovician Taconic orogeny.

The western frontal thrust of the massif contains mylonites with steep strain gradients. Electron microprobe dating of monazite from fault-zone samples does not support a Taconic age for thrusting. Monazite from a deformed quartzite exposed in the hanging-wall of the Dry Hill fault gives an age of 392 +/- 14 Ma. Quartz-rich schist from a fault zone at 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 200 m below a major thrust give a weighted average age of 436 +/- 8 Ma. Two localities from Yale Farm near the western frontal thrust of the Berkshire massif give weighted average monazite ages of 401 +/- 9 Ma and 400 +/- 10 Ma.

We found no evidence for internal thrusting within the massif; it apparently behaved as a rigid block during uplift. Further, we argue against thrusting along the eastern margin of the massif, where Neoproterozoic schists identical to rocks in Taconic thrust sheets overlie basement gneisses. We suggest that the Berkshire massif was emplaced during the Salinic or Acadian orogeny as a rigid intracrustal wedge. The leading edge of the massif-wedge is commonly an overturned fold, and it closely followed the boundary between the Taconic thrust sheets and the carbonate shelf rocks. Wedging was favored by the mechanical contrast between the rigid basement gneisses and clastic rocks of the massif compared to mica-rich schists of the Taconic and the carbonate rocks of the shelf sequence.