Structural evolution of the Appalachian front in southern Québec: implications for oil and gas exploration
Stephan Séjourné, Michel Malo (INRS-ETE) and Martine M. Savard (GSC-Q)

The Appalachian orogenic front represents a major target for hydrocarbons in southern Québec. In order to refine strategies already developed by the industry, the orogenic front between Drummondville and the Vermont border was the subject of a multidisciplinary geoscientific study designed to better understand its tectonic and diagenetic evolution (with emphasis on the carbonate slices) and to place this evolution in a regional context.

The overall architecture of the orogenic front – the autochthonous and parautochthonous domains – was elucidated by field observations and seismic interpretations. In the autochthonous domain, deformation appears restricted to pre- to early syn-orogenic normal faults in the platform, whereas the overlying flysches were affected by folds and reverse faults, such that the western edge of the front extends westward beneath the surface. The parautochthonous domain consists of imbricated shales and carbonate slices in a duplex geometry dipping towards the hinterland. The dominant structural style within the slices is characterized by thrust and reverse faults, minor backthrust faults, asymmetric folds, strike-slip faults and oblique ramps, and syn- to late-imbrication normal faults. The structural complexity and diversity are strongly dependent on lithology (thickness of the beds and clay content), and on the initial geometry of the basin. Finally, a major network of strike-slip faults parallel to the orogenic front developed in a late phase.

The Saint-Dominique slice, observable at the surface, was used as an example to illustrate the nature and evolution of fluids involved in the deformation of the carbonate slices. The diagenetic analysis (petrography, fluid inclusions, geochemistry) of structural cements reveals that the imbrication post-dated the first phase of liquid hydrocarbon generation and migration. The imbrication began with burial (to approx. 6 km), and the tectonic uplift of the slice was accompanied by a gradual decrease in temperature of the system, from about 245°C to 150°C. External fluids, of basinal and metamorphic origin, circulated during deformation. A second phase of maturation and migration followed imbrication, and the source of the hydrocarbons was likely in the footwall of the slice. Finally, the development of strike-slip faults parallel to the orogenic front promoted the mixing of residual waters with a magmatically-derived fluid related to Mesozoic intrusions. The development of these structures probably took place at shallow depth (about 2.5 km).