CHARACTERIZATION OF THE EASTERN BORDER FAULT IN THE DURHAM, CT, VICINITY

The Connecticut Valley Eastern Border Fault marks the separation between late Triassic flood basalts and alluvial fan sediments filling the rift basin, and the metamorphic rocks of the Eastern Highlands. The geomorphic expression of the EBT in the vicinity of Durham, CT, is a wide alluvial valley with several tributary offshoots. Proximity to the fault from the west has always been inferred from the size of clasts in the Mesozoic-age fanglomerates at the margins of the graben. There is no consensus as to the exact position, number and nature of the fault(s) in this area. Previous literature has identified only one exposed fault cut along Route 77, which has so degraded over time that the contact is no longer discernible. When the cut was fresh, exposures indicated a zone of fault gouge and a fault plane dip of approximately 55 degrees. In the study area, from Lake Quonipaug 3.5 km north to the juncture of Routes 77 and 79, the fault is characterized by a 7m-wide silicified zone in the Collins Hill Schist on the fault’s upthrown side. Here, a series of quartz veins have two dominant orientations, demonstrating a northeasterly shift in the trace of the fault. In the northern part of the study area, quartz veins, including a single vein 1.8m thick extending for 37m, show azimuths of 200 - 210 degrees, dipping 19 degrees SW; to the south, a second set oriented 185 degrees, dipping 24 degrees SW is present. Further evidence of the fault(s) position lies to the south where a series of sag ponds and the linear N-S trending Lake Quonipaug are apparently established over the fault. To delineate the actual trace of the fault(s) and to characterize the fault zone itself, cesium magnetometer surveys across the projected fault trace were run. Pronounced differences in magnetic signatures between the Eastern Highland metamorphics and the Connecticut Valley redbeds allow us to more precisely map the position of the graben’s border fault.