THE APPALACHIAN STRUCTURAL FRONT IN SOUTHERN QUEBEC: SEISMIC AND FIELD EVIDENCE FOR COMPLEX STRUCTURES AND A TRIANGLE ZONE AT THE EDGE OF THE FORELAND THRUST BELT

The 3D architecture and hydrocarbon potential of the Appalachian foreland thrust-and-fold belt remain poorly understood in southern Quebec, principally due to the rarity of exposures, regional bedrock mapping historically focussed toward biostratigraphy, and poor to average quality of existing seismic data. New interpretation of various reprocessed seismic reflection lines (e.g. MRNQ 2001, 2002, 2003 amongst others) across the St. Lawrence platform and Appalachian belt and complementary fieldwork show that the deformation style of the Appalachian structural front in southern Quebec is more complex than conventionally thought, and that the foreland thrust belt may be wider.

Seismic and field evidence show that the Appalachian structural front (not to be confused with Logan’s line) extends farther to the northwest than previously interpreted, and seems to be locally marked by a triangle zone induced by intracutaneous wedging along the distinctly steeper southeastern limb of the Chambly-Fortierville Syncline. To the north, the northwestern limb of the syncline is located above a footwall ramp of the main Appalachian décollement. This megascopic fold may thus be viewed as a an asymmetric foreland syncline that possibly has evolved laterally into a fault ramp syncline. The surface location and sub-surface geometry of Logan’s line are still, after reprocessing, a challenge to interpret. Along its footwall, however, the conspicuous Saint-Flavien slice is well-imaged. The latter is interpreted as a detached and folded sheet of platform sequence that was transported along a thrust fault and progressed over a footwall ramp induced by a major step in the rifted margin geometry. Such steps characteristically produce more complex structures in the overlying thrust sheets, such as thrust duplexes or imbricate fans, and exemplify the influence and control of the basement topography on the structural evolution of the thrust-and-fold belt.

Overall, this study provides a better 3D image of the Appalachian architecture in southern Quebec, especially along the foreland thrust-and-fold belt. It may also provide insights to identify or correlate similar structures that may extend southward in the Lake Champlain region and offshore in the Estuary and Gulf of St. Lawrence.