Faults and lineaments of the Western Quebec Seismic Zone, Quebec and Ontario

Abstract

This Open File contains the interpreted brittle lineaments and a compilation of mapped faults of an area bound by latitudes 44°N and 48°N and longitudes 73°W and 80°W. The area includes most of the Ottawa River watershed. It also covers most of eastern Ontario and the Laurentians, between Montréal (QC) and North Bay (ON). The study area includes the seismically active region known as the Western Quebec Seismic Zone (WQSZ). The WQSZ is an area where earthquakes as large as magnitude 6.2 have occurred in the past and where tens of smaller earthquakes are recorded yearly. Our study is an attempt to provide a homogeneous coverage of these brittle structures through an integration of visually interpreted lineaments and mapped faults. The possible relationships between these brittle faults and earthquakes will be examined later. Lineaments were observed mostly from the Digital Elevation Model (DEM) of the Canadian National Topographic Data Base (NTDB) at a scale of 1:250 000. The DEMs illuminated from two directions were used to first visually recognize lineaments and second, to georeference their surface expressions in a Geographic Information System (GIS). Since the final goal was to better map the brittle faults that could be reactivated in earthquakes, the clear ductile structures were not considered in this study and the more questionable ones re-evaluated subsequently against known geological information. Most of the region of interest is southeast of the Grenville Front, where all ductile structures are related to the Grenville orogeny (about 1 billion years ago). The recognition of brittle structures is based on the observation that they are essentially linear in plan view. On the other hand, the ductile structures are generally curved, enhance contact between different Grenvillian lithologies, or present a distinct structural pattern. With a few exceptions, only lineaments with a length greater than 5 km were included. After a first detection pass, lineaments at a more regional scale were drawn by interpolating between segments of lineaments. These interpolated segments corresponded to areas where the topography was subdued and where no conspicuous trace existed. The interpreted lineaments were then compared with the provincial geological maps of Quebec and Ontario, which often did not distinguish between brittle faults and ductile shear zones. Lineaments that coincided with diabase dykes were recognized by consulting geological maps and by examining the total magnetic field. Our final product is a 1:750,000 scale map that can be used in the future to better understand the seismotectonics of this region.