Background and Methodology

The Gabriel Fault (FAgab) has been interpreted by Simard et al. (2013) in the Kuujjuaq area as a thrust fault that marks the boundary between their Gabriel and Baie aux Feuilles domains, to the west and east, respectively. These divisions were not retained by Lafrance et al. (2020) in their synthesis of the Southeastern Churchill Province (SECP). These authors therefore integrated the FAgab into the Baleine Lithotectonic Domain.

The information available on the FAgab is limited as it is rarely exposed on the surface and the work was carried out at scale 1:250 000.

 

Boundaries and Morphology

Stratigraphic Units Concerned

The lithodemic units affected by the FAgab are the False, Ralleau, Akiasirviup, Curot and Tasialuk suites as well as the Kaslac and Ungava complexes.

 

Structural Characteristics

 

❯ Main Fabrics

The FAgab’s main fabric is a secondary tectonometamorphic mineral foliation plane S_n. This fabric is commonly folded due to subsequent deformation. Its orientation is undulating but overall NNW-SSE. As with other thrust faults (Lac Hérodier, Lac Olmstead, Pingiajjulik, Pointe Reef, Rachel and Lac Turcotte) in the western part of the SECP, the FAgab would be associated with an early thrust phase and then folded during a subsequent deformation phase (Clark and Wares, 2004).

Although the fault zone does not outcrop much, decimetric to metric layers of protomylonite and mylonite have been locally observed along the FAgab. These more deformed layers display a protomylonite or mylonite planar fabric. No lineation was measured along the fault. The more deformed zones are characterized by the presence of feldspar porphyroclasts, discontinuous quartz bands and subtle C/S fabrics.

❯ Other Fabrics

Not observed.

 

❯ Folds

Clark and Wares (2004) attribute the development of large open NW-SE folds having shallow plunges to the SE to a late dextral strike-slip movement along large thrust faults associated with the New Quebec Orogen. Simard et al. (2013) report that areas on either side of the FAgab are likely to be affected by folds overturned to recumbent towards the SW.

 

❯ Crosscutting Relationships

The FAgab meets a set of E-W deformation zones in the northern part of the Saffray Lake area (Lafrance et al., 2014). These deformation zones correspond to a major regional magnetic discontinuity that coincides with significant lithological and structural changes within the Baleine Domain (Lafrance et al., 2014, 2020). To the north of this E-W discontinuity, Archean gneiss of the Ungava Complex constitute the dominant lithology. These display a subhorizontal or shallowly dipping NW-SE to NNE-SSW foliation (Simard et al., 2013). To the south, the large younger Archean plutonic masses of the Saffray Suite and the cover of migmatized metasedimentary rocks are predominant and display an abrupt E-W foliation (Lafrance et al., 2014). According to these authors, it is possible that this regional E-W discontinuity is Archean and continues into the Superior Craton to the west, where it marks the boundary between the Douglas Harbour and Utsalik domains to the north and south, respectively.

 

❯ Kinematics

Several deformed outcrops were observed in the vicinity of the FAgab, but the movement along it could not be determined with certainty. However, kinematic indicators, mainly feldspar porphyroclasts, suggest dextral movement.

Deformation Style

The FAgab is interpreted by Simard et al. (2013) as belonging to the SW-verging thrust fault system of the NW part of the SECP. This main movement would have been followed by late dextral strike-slip.

 

Metamorphic Characteristics

The FAgab generally coincides with the metamorphic isograde associated with the disappearance of muscovite between the Akiasirviup and False suites (Charette et al., 2016). Parageneses observed in the False Suite are quartz-plagioclase-biotite ± garnet ± sillimanite ± hornblende (Lafrance et al., 2020). In the north of the Kuujjuaq area (Simard et al., 2013), the False Suite does not outcrop much and metasedimentary rocks of the Akiasirviup Suite, metamorphosed to the amphibolite facies, are in direct contact with granulite facies metamorphic rocks of the Kaslac Complex.

 

Alterations

Not observed.

 

Geophysical Characteristics

The FAgab corresponds to a relatively well-defined lineament on the regional aeromagnetic map.

 

Chronological Markers

The major collision event in the western part of the SECP corresponds to the edification of the New Quebec Orogen, which occurred between 1.82 Ga and 1.77 Ga (Wardle et al., 2002). Transpressive stresses developed during this oblique collision resulted in the formation of thrust and strike-slip faults (Clark and Wares, 2004). Faults in the western part of the SECP, including the Gabriel Fault, would have acted as thrust faults at the beginning of the orogenic phase, and would have been transformed into dextral strike-slip faults towards the end of the orogenic phase (Clark and Wares, 2004). Recent dating indicates burial of metasedimentary rocks in the Rachel-Laporte Lithotectonic Domain – located west of the Baleine Domain – under conditions of average crustal thickness at ~1804 Ma (Godet et al., 2020). This episode of thrusting and crustal thickening appears to predate the final dextral strike-slip movement, which would have occurred between 1793 Ma and 1783 Ma (Machado et al., 1989; Clark and Wares, 2008).

 

References

Publications Available Through Sigéom Examine

Other Publications

Suggested Citation

Contributors

First publication

Isabelle Lafrance, P. Geo., M.Sc. isabelle.lafrance@mern.gouv.qc.ca; Marc-Antoine Vanier, Jr. Eng., M.Sc. marc-antoine.vanier@mern.gouv.qc.ca, (redaction); Ghyslain Roy, P. Geo. (coordination); Claude Dion, Eng., M.Sc. (critical review); Simon Auclair, P. Geo., M.Sc. (editing); André Tremblay (HTML editing); Céline Dupuis, P. Geo., Ph.D. (english version)