EARLY CRETACEOUS DISPLACEMENT ON THE GEORGES MILLS FAULT, LAKE SUNAPEE, NEW HAMPSHIRE BASED ON APATITE FISSION-TRACK ANALYSIS

Four samples of Paleozoic crystalline rocks were collected in a transect across the Georges Mills fault at the northern end of Lake Sunapee in southwestern New Hampshire to determine the timing and magnitude of the inferred faulting using apatite fission-track (AFT) analysis. The units sampled were the Silurian-Devonian Littleton Formation, the Devonian Bethlehem Gneiss and the 2-mica granite of the Sunapee pluton.

The Georges Mills fault is a prominent lineament defined by the arms at the north end of Lake Sunapee and by the drainage pattern beyond the lakeshore (Allen, 2012). The fault does not crop out anywhere. However, it creates an apparent horizontal offset of ~3 km in the metasediments of the Sunapee Septum, which separates the Mount Clough and Cardigan plutons. Assuming the fault is vertical and perpendicular to the strike of the septum, and that the dip of the metasediments is ~ 60°, the apparent 3 km horizontal offset might be the result of a 5 km vertical offset. In addition, a significant vertical displacement would explain the difference in metamorphic grade of the metasediments on opposite sides of the fault, migmatites on the northeast side and lower-sillimanite zone rocks on the southwest side. The northern shore of Lake Sunapee also has steep topography which is reminiscent of a fault scarp.

Two samples from the northeast side of the Georges Mills fault (footwall – FW) yielded AFT ages of 92 ± 11 and 114 ±14 Ma. From the southwestern side of the fault (hanging wall – HW), two samples yielded older AFT ages of 126 ± 20 and 132 ± 20 Ma. The discontinuity in these AFT ages suggests Early Cretaceous displacement on the Georges Mills fault in a normal sense of motion at <~100 Ma. The timing of this faulting is comparable to the timing of reactivation previously determined for Paleozoic faults such as the Norumbega Fault zone, Maine, the Northey Hill fault in northwestern New Hampshire, and preliminary results for Mesozoic displacement on the Grantham fault in southwestern New Hampshire. The magnitude of this faulting is yet to be determined through track length measurements and thermal history models which will help delineate the cooling history of both sides of the fault below 100°C.