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

Highlights

• Late Silurian normal fault throw is estimated at 2–4 km.

• Normal faults are genetically related to the formation of the Gaspé Belt sedimentary basin.

• Normal faults are attributed to a late Salinic tectonic period (ca.420-410 Ma) of large-scale crustal extension.

• Change in kinematic of St-Joseph normal fault is related to a change in kinematics of the plate tectonics.

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.

Introduction

Syn- and post-orogenic extension is usually associated with large crustal-scale shear zones in the middle crust and normal faults in the upper crust, and to the exposure of crustal metamorphic rocks in the footwall (Armstrong 1982; Crittenden et al., 1980; Davis and Coney 1979). These features are frequently intimately linked with the formation of sedimentary basins in the hanging wall (Jolivet and Goffé 2000; Wemicke 1992). However, the presence of crustal extension in orogenic belts can be difficult to prove since the exhumation of lower crustal metamorphic rocks (which may be associated with extension) can also occur during contractional events. It is therefore important to accurately document the nature and chronology of potential extensional features in order to determine if they are actually caused by crustal-scale extension. The exhumation of the high-grade core of the Montagne Noire of the Central Massif (France), for example, has been historically attributed to extensional and/or transtensional deformation (e.g. Brun and van den Driessche 1994; Echtler and Malavieille 1990; Rey et al., 2011 among others); it is only recently that it has been shown as the result of lower crust contraction and the formation of a pull-apart basin in the upper crust (Roger et al., 2015). The occurrence of sedimentary basins directly on the hanging wall of normal faults of regional extent is one of the best pieces of evidence for crustal-scale extension. However, in order to validate such event of extension and to link its upper crustal consequences to deep processes, it is essential to demonstrate the consistency, in space and time, between metamorphic rocks exhumation, normal faulting and the age and location of sedimentary basins (e.g. Jolivet and Goffé 2000; Roger et al., 2015; Wemicke 1992).

In the Northern Appalachians, a period of lithospheric extension is believed to have started in the late Silurian, following the compressive Late Ordovician to Silurian Salinic orogeny (Cawood et al., 1994; Cawood et al., 1995; Dunning et al., 1990; van Staal et al., 1998). Silurian extensional deformation has been suggested for several decades in the Québec Appalachians (Bourque et al., 2000; Castonguay et al., 2001; Pinet et al., 1996; Tremblay and Castonguay 2002) but the tectonic mechanisms linking that extension to the crustal shortening, as documented in the more internal domains of the Appalachian orogen (in Maine (USA) and Atlantic Canada; (van Staal and Barr 2012), are still unclear.

In the Québec Appalachians, crustal extension is thought to be responsible for the initiation of the deposition of the Gaspé Belt (Bourque et al., 2000; Perrot et al., 2018; Tremblay and Pinet 2005), a major Upper Ordovician to Middle Devonian sedimentary sequence (with sparse volcanic units) that extends for c. 1500 km along-strike from southern Québec and adjacent New England to Gaspé Peninsula, and is mainly preserved within the Connecticut Valley-Gaspé (CVG) trough (Bourque et al., 2000; Perrot et al., 2018; Perrot et al., 2020; Tremblay and Pinet 2005, 2016). In Gaspé Peninsula, the stratigraphy of the Gaspé Belt is notably marked by an extensive reef belt developed in the footwall of syn-depositional normal faults (Bourque et al., 2000). In southern Quebec, this extension event corresponds to the exhumation of the Laurentian margin (Humber zone; see Tremblay and Pinet 2016) and is marked by normal-sense faulting along both the St-Joseph (Pinet et al., 1996) and the La Guadeloupe faults (Tremblay et al., 1989); the latter being interpreted as an Acadian inverted reverse fault (Tremblay and Castonguay 2002). Despite the fact that exhumed rocks of the Laurentian margin and sedimentary rocks of the Gaspé Belt have been extensively studied over the years (Bourque et al., 2000; Bourque et al., 1995; Castonguay et al., 2007; Castonguay et al., 2001; Castonguay and Tremblay 2003; Perrot et al., 2018; Perrot et al., 2020), Silurian normal faults, such as the St-Joseph fault, and their regional distribution have not been fully-characterized, except for a single structural transect along the Chaudière River (Pinet et al., 1996). This contribution presents a detailed description of macroscopic and microscopic structures observed in several open pit asbestos mines and quarries distributed over several hundreds of kilometers along the strike of the St-Joseph fault. We also present a semi-quantitative evaluation of finite strain and fault offset of that structure in order to better define the nature and importance of the extension. We review the time and space parameters regarding exhumation of the internal Humber zone, the main period of normal faulting and the deposition of the Gaspé Belt to, finally, better extrapolate the geodynamic setting of formation and discuss the kinematic evolution of the fault, which we relate to plate tectonics of the Northern Appalachians.