EVIDENCE OF EARLY POSTGLACIAL PALEOEARTHQUAKES FROM LAKES IN NORTHEASTERN ONTARIO-WESTERN QUEBEC, CANADA

Glacial Lake Ojibway was a large glacial lake in northeastern Ontario-western Quebec that persisted roughly between 10.57-8.47 ka cal BP, and experienced high rates of glaciolacustrine sedimentation. Mass transport deposits (MTDs) buried at differing stratigraphic levels within the glaciolacustrine sediments are ubiquitous throughout the region. Detailed mapping of the sub-bottom deposits at Dasserat, Duparquet, and Dufresnoy lakes, located 24 to 38 km apart, near Rouyn-Noranda, Quebec, produced 26 MTD event horizons of which 15 contain three or more MTDs. High-resolution, relative dating control from varves indicates: that the event horizons span a period of ~450 years (between about 9.4-8.95 ka cal BP); that one set of identically-aged event horizons is common to each lake; and another set is common to two lakes. The 15 event horizons containing three or more MTDs are interpreted to be evidence of 11 paleoearthquakes of magnitude about M_w5 or larger. The confidence of each interpretation is weighted from low to high, reflecting the relative strength of the MTD signature in each event horizon (moderate (three to five MTDs) or strong (more than six)), and evidence of common- or similar-aged MTD signatures at two or more distinctly-separate locations. The event horizon with the most numerous and widespread MTDs in each lake occurred in the identical varve year 1483 (~9.1 ka cal BP). These event horizons are part of a regional MTD signature present in other lakes and in subaerial exposures across at least 220 km of northeastern Ontario and western Quebec. This signature is interpreted to be best explained by a paleoearthquake of ~M_w7.3. The interpreted paleoearthquakes occurred at a time when early postglacial uplift had diminished moderately, but was still high, compared to mid-Holocene and late Holocene rates. The record, thus, is likely representative of a period of elevated, early postglacial, seismicity associated with rapid crustal unloading.