Popup field in Lake Ontario south of Toronto, Canada: Indicators of late glacial and postglacial strain

A field of stress-release bedrock structural features occurs on the floor of western Lake Ontario south of Toronto, Canada. These features were investigated using side- scan and multibeam sonars, high-resolution seismic profiling, and submersible dive observations. The study region was mostly stripped of its glacial drift in late glacial time, and the region has since accumulated only a relatively thin, discontinuous cover (1-2 m) of lacustrine sediment. The stress-release features affect the flat to gently dipping interbedded shales and calcareous siltstones of the Upper Ordovician Georgian Bay Formation. The features consist of sub-lakefloor buckles, about 50-100 m wide with structural relief of 5+ m, and surface bedrock popups, 10-15 m wide with a general relief of 1-2 m. Deeper bedrock faults are possibly associated with some of the sub- lakefloor buckles. Trends of the popups and buckles can be grouped into six modes from 7.5° to 347.5°. Abutting and sediment onlap relationships suggest that the popups formed throughout late and postglacial time following the Last Glacial Maximum ∼20, 000 yr ago. The earliest set of popups is estimated to have formed before 9500 B.P.; they trend WNW, collinear with isobases of glacial rebound, and do not parallel major geophysical or structural linear zones in the region. These and other factors suggest that this set developed in response to glacial rebound-induced stress. Later popups form an irregular pattern with several orientations of axes, suggesting that the horizontal principal stress vectors were of similar magnitude. The decrease of rebound strain with time and clockwise rotation of modern contours of basin tilting relative to glacial lake isobases suggest that popups today are likely a response to reduced glacial stress combined with far-field tectonic stress.