Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

The largest earthquakes in northeastern United States and Southeastern Canada are along the seismically active St. Lawrence Fault Zone (SLFZ) named for the St. Lawrence River. Previously, it was believed that the St. Lawrence Fault Zone extends from the lower St. Lawrence River to Massena then strikes Northwest toward Ottawa. Recent geologic studies however suggest that it does not stop at Massena but continues southwest along the St. Lawrence Valley toward Chippewa Bay, Thousand Islands and into Lake Ontario. Seismic surveys in Lake Ontario indicate the existence of eight faults beneath Lake Ontario and submersible studies within the Rochester Basin of SE Lake Ontario suggest the Rochester Basin lies along the extension of SLFZ in Lake Ontario.

Chippewa Creek Valley lies in the St. Lawrence Valley midway between Massena and Lake Ontario. The Chippewa Creek fault is named after the linear Chippewa Creek valley which separates Precambrian granite on the southeast from Cambrian-Ordovician rocks on the northwest. The linearity of the valley and characteristics at two outcrop areas imply the Chippewa Creek fault is a normal fault with the southeast side having moved up relative to the northwest.

Fifty-eight magnetic measurements were made at 50 meter intervals across Chippewa Creek Valley. These magnetic measurements indicate a magnetic high over the valley which is interpreted as being due to uplift of magnetic basement to produce a normal fault dipping to the northwest. The magnetic measurements also show a series of magnetic lows of shallow origin possibly related to faults in the valley where faulting has destroyed the magnetization. The magnetic lows and steep gradients are located along prominent topographic lineaments. Closely spaced gravity measurements are being made at 50 meter intervals to determine if the faulting is expressed by gravity data as well.