DISPLACEMENT HISTORY OF THE GRANTHAM FAULT IN SOUTHWEST NEW HAMPSHIRE CONSTRAINED BY APATITE FISSION-TRACK AGES

Twelve new AFT ages from samples of Paleozoic rocks collected across the steeply east-dipping Grantham fault in southwestern New Hampshire provide new constraints on both the timing and sense of displacement across this structure. Map data support normal motion on the fault which may have locally continued into the Cenozoic. This is in contrast to published apatite fission-track (AFT) data for the brittle faults of New England that typically yield Jurassic to Cretaceous cooling ages and indicate little offset after that time.

Samples were taken from diverse lithologies including the Ordovician Oliverian Plutonic Suite in the Croydon and Unity plutons, the metamorphosed cover rocks of the Silurian-Devonian Clough, Fitch and Littleton Formations, and the Devonian Bethlehem Gneiss. On the west side of the Grantham fault (footwall - FW), AFT ages (n=3) range from 148±27 Ma for the southern Unity pluton to 102±12 Ma for the Croydon pluton in the north. On the east side of the fault, AFT ages (n=9) range from 140±29 Ma to 58±8 Ma with 6 ages <115 Ma. However, AFT ages for Bethlehem Gneiss and Clough Quartzite were older in the vicinity of North Newport (140±29 Ma) and decreased slightly to 120±19 Ma to the north near Croydon Four Corners.

The discontinuity in AFT ages across the Grantham fault suggests motion during the Early Cretaceous (<111 Ma) to possibly as young as 58 Ma (Paleocene), the youngest AFT age determined in New England. This sample was collected immediately east of the fault and its seemingly anomalously young age may be due to processes related to local fault dynamics. The Early Cretaceous AFT age offset agrees with age offsets determined previously for the Norumbega fault system and the Northey Hill fault – both of which represent Mesozoic reactivation of older Paleozoic faults. However, with only one sample from the west side of the fault in the north, it is difficult to draw significant conclusions from the data. Track length measurements and modeling are in progress to estimate the rate of cooling below 100°C across the fault. The track length models may help explain the cooling history of the anomalously young 58 Ma AFT age.