Structural characterization of late Silurian normal faults in the Quebec Appalachians: Implications for sedimentary basin formation and Laurentian margin exhumation during the Salinic orogeny

Abstract

In the Quebec Appalachians, the St-Joseph fault is a major normal fault that mostly juxtaposes metamorphic remnants of the Laurentian margin in its footwall and the adjacent oceanic domain accreted during the Ordovician Taconian orogeny in the hanging wall. It locally merges into and overprints the Baie Verte-Brompton line (BBL). The existence of late Silurian extensional deformation is well known in the Appalachians, but the associated structures, like the St-Joseph fault, and their regional distribution have not previously been precisely described and characterized. We present a detailed description of the macroscopic fabrics and extensional fault structures observed along the St-Joseph fault for hundreds of kilometers in the Quebec Appalachians. A petrographic and numerical microstructural analysis is presented as a test to quantitatively evaluate the metamorphic and structural contrasts between the footwall and hanging wall sequences, as well as the shear strain and fault offset related to the St-Joseph fault. The main movement along this SE-dipping fault is normal and characterized by brittle-ductile fabrics and fault surfaces marked in places by tectonic slices of serpentinite and/or altered and sheared peridotite. Besides, a strong textural contrast between the rock units of its hanging wall and footwall imply a fault throw of several kilometers (4–8 km). The inferred timing of the maximum, normal-sense displacement activity along this structure is Late Silurian-Early Devonian and interpreted as genetically related to the sedimentation of the Gaspé Belt and exhumation of the Laurentian margin. It is attributed to a late Salinic tectonic period of large-scale crustal extension of ca. 10 Ma lasting while the internal parts of the orogen (Maritimes and New-Brunswick) were already experiencing collision with Avalonia. The down-dip normal fault movement features are overprinted by consistent curvilinear and low-plunging slickenlines indicating that fault movement has evolved into late dextral strike-slip. This suggests that during the early Devonian, crustal extension progressively transitioned into a compressional regime due to the approaching Avalonia, which was accommodated by a transtensionnal phase with a progressive rotation of fault movement toward a dextral component.