POP-UP STRUCTURES AND THE FRACTURE PATTERN IN THE BALSAM LAKE AREA, SOUTHERN ONTARIO

An examination of topographic lineaments detectable in Landsat TM imag es and measurement of joints in outcrop in the Balsam Lake area reveal s a systematic bedrock fracture pattern with three principal sets orie nted 091-degrees, 027-degrees, and 152-degrees. The 027-degrees trend is parallel to a major aeromagnetic anomaly, the Niagara-Pickering Lin ear Zone (NPLZ), which underlies the Balsam Lake area and is thought t o mark the sub-Paleozoic continuation of the Proterozoic Central Metas edimentary Belt Boundary Zone (CMBBZ). Possible origins of the main jo int sets due to Acadian (091-degrees set), Alleghanian (152-degrees se t) and St. Lawrence rift system tectonics (091-degrees and 027-degrees sets) are discussed. En-echelon pop-up structures have a mean princip al trend of 118-degrees. They displace Lake Algonquin paleobeaches, su ggesting formation less than 12,500 years ago. The orientation of the pop-up structures is subnormal to the current in situ maximum horizont al stress direction, S(Hmax) (020-degrees), and is parallel to members of the 091-degrees joint set, indicating possible nucleation on favou rably-oriented pre-existing joints. These pop-ups are the only feature s in the Balsam Lake area with strong evidence for a neotectonic age.