AGE AND STYLE OF EMPLACEMENT OF BASEMENT UPLIFTS IN THE NORTHERN APPALACHIANS

Field evidence clearly shows that Middle Proterozoic basement of the Berkshire massif in Massachusetts and Connecticut was thrust over early Paleozoic shelf rocks of the Laurentian margin. Furthermore, it is widely accepted that thrusting occurred during the Ordovician Taconic orogeny and that the massif was internally shortened by numerous thrusts, even though the evidence for these assertions is limited. Samples from well-exposed fault zones on the west margin of the massif are spectacular mylonites with a strong, recrystallized, planar foliation and a steep strain gradient. Sense-of-shear indicators are mostly consistent with westward thrusting, although textures suggest local reactivation into east-dipping normal faults. No clear evidence exists for internal thrusting within the massif; it apparently behaved as a rigid block during uplift. Silurian sills near the eastern margin of the massif are distal arc-related rocks that we suggest intruded normal faults that juxtaposed Late Proterozoic pelitic schist with basement rocks of the massif.

Electron microprobe dating of monazite does not support a Taconic age for thrusting. Monazite from a highly deformed quartzite exposed in the hanging-wall of the Dry Hill fault gives an age of 392 +/- 14 Ma (1s.e., n=16). 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 grains in a graphitic schist from Benton Hill 200 m below a major thrust give a weighted average age of 436 +/- 8 Ma (n=21). Two localities from Yale Farm near the western frontal thrust of the Berkshire massif give weighted average monazite ages of 401 +/- 9 Ma (n=26) and 400 +/- 10 (n=40). These preliminary monazite ages from well-exposed fault zones suggest that motion on faults along the western margin of the massif occurred during the Devonian or Silurian. So far, dating of monazite provides no evidence that faults were active during the Ordovician Taconic orogeny. However, one sample, collected more than 0.5 km from any major mapped fault, does show two peaks in monazite ages at 470 and 520 Ma. This sample apparently preserves evidence for Early Ordovician metamorphism, but more data are needed to confirm this older age.