CONFIRMATION OF MESOZOIC TECTONIC ADJUSTMENTS TO PALEOZOIC TERRANES IN CENTRAL NEW ENGLAND

Apatite fission-track (AFT) ages for 24 samples from eastern Massachusetts confirm previously proposed gradients and discontinuities adjacent to and across faults bounding Paleozoic thermo-tectonic terranes. These AFT ages suggest both the Mesozoic reactivation of Paleozoic faults and tilting of Paleozoic terranes. In a W-E traverse from the Bronson Hill terrane across the Central Maine terrane, AFT ages increase from ~103 Ma near Erving to ~140 Ma at Gardner and then decrease again to ~106 Ma at Sterling which is located NW of the Wekepeke Fault, (WpF). The 140 Ma AFT age from Gardner is near the northern end of a NE-trending zone of 130-140 Ma AFT ages extending from Hampden near the Connecticut border to Winchendon near the New Hampshire border. The ~40 m.y. age gradient east and west of this NE-trending region indicates that exhumation occurred at < 100 Ma or middle Cretaceous time. Across the adjacent Merrimack and Putnam-Nashoba terranes, the majority of the AFT ages are ~160 Ma (Late Jurassic) showing a > 50 m.y. age discontinuity across the WpF. An additional SE traverse across the Avalon terrane from Wayland to Sherborn yielded AFT ages ranging ~170 Ma to 150 Ma indicating Middle to Late Jurassic exhumation and consistent with the previously determined AFT age range of 178 Ma to 145 Ma. Two AFT ages from near Medfield showed a slight increase to ~160 Ma to 180 Ma.

The AFT age gradients across the Central Maine and Avalon terranes suggest extension accommodated by a set of synthetic listric normal faults occurred during the Cretaceous. Tilting of the Paleozoic terranes is also indicated by the age gradients in which block rotation had a down to the northwest and up to the southeast motion. Across the Central Maine terrane, extension at <100 Ma produced a trough of older AFT ages. The Wekepeke Fault was reactivated as a synthetic listric normal fault and the Eastern Border fault of the Hartford basin as an antithetic normal fault in the Late Cretaceous to create this age pattern. In the Avalon terrane, the timing of fault movement along the Bloody Bluff and tilting was earlier, < 140 Ma. Through Paleozoic normal fault reactivation, Jurassic and Cretaceous extension formed a series of half-grabens displaced by long listric normal faults which are probably joined by a major detachment fault that extends farther east, into the Gulf of Maine.