Background and Methodology

The Digne Shear Zone was defined by Beauchamp (2019) following a mapping survey conducted in the Cadieux Lake area in the summer of 2018. It is located at the contact between intrusive and volcanic rocks of the Dolent Formation, which is part of the Upper Eastmain Greenstone Belt (UEGB), to the south, and the Digne Pluton, to the north. It is described using geological observations collected by Beauchamp (2019) and Beauchamp et al. (2018), and using analysis of geophysical imagery (total magnetic field and vertical gradient) from the SIGÉOM and detailed magnetic maps by Wade et al., 2014.

Boundaries and Morphology

Stratigraphic Units Concerned

The only unit affected by the Digne Shear Zone is the Digne Pluton.

Structural Characteristics

❯ Main Fabrics

In the Structural Lexicon, the dominant fabric of a domain is, by definition, associated with the D_n phase. Earlier phases are referred to as D_n-1, D_n-2, etc. and later phases as D_n+1, D_n+2, etc. The D_n phase of a domain does not therefore necessarily correspond to that of neighbouring domains. The planar fabric (S_n), which is best expressed in the Digne Shear Zone, corresponds to regional fabric S₁. It predates the main regional fabric that is generally observed on outcrops in the Cadieux Lake and Bohier Island areas (S_n=S₂).

The main fabric of the Digne Shear Zone is stretch lineation L_n. Rocks of the Digne Pluton, which are in contact with the UEGB, form L>> S tectonites. Lineation is defined by very pronounced stretching of quartz and feldspar grains until bands are formed. A planar fabric is locally observed, especially as one moves northwards away from the Digne shear zone. The S_n planar fabric is weakly to moderately developed and is distinguished by the millimetric spacing of flattening planes. Phyllosilicates (biotite, chlorite and muscovite) are oriented in S_n foliation planes.

❯ Other Fabrics

Outcrop 18-AB-1085 clearly displays two fabrics. Foliation S_n is oriented E-W and dips moderately northwards. In thin sections, a NNE-SSW oriented crenulation cleavage S_n+1 folds the S_n fabric.

❯ Folds

As shown on the geological map of the Cadieux Lake area, the CSdig is folded by a succession of regional ENE-WSW folds (P_n+1) which axes plunge slightly to moderately towards the ENE. On outcrops, a few occurrences of P_n+1 folds having NNE-SSW axial planes parallel to crenulation S_n+1 are found.

❯ Crosscutting Relationships

The existence of very detailed magnetic maps of the Digne Shear Zone (Wade et al., 2014) has enabled mapping numerous brittle faults having steep dip. Two major families can be identified. A first family of NNW-SSE faults offsets shearing in a dextral manner. A second family of faults having sinistral movement has an orientation varying from NW-SE to WNW-ESE. These fractures also affect rocks of the Dolent Structural Domain (DSdol).

Geometric Parameters of Regional Faults Cutting the Digne Shear Zone:

❯ Kinematics



Kinematic indicators observed in the field reveal that the Digne Shear Zone was displaced in reverse movement. These kinematic indicators are:

• sigmoids and C/S fabrics indicating reverse movement in L-S tectonites of the Digne Pluton (outcrops 18-AB-1085, 18-AB-1027 and 18-AB-1033);
• fault striations indicating reverse movement (outcrop 18-AB-1085);
• protomylonitic textures (C/S fabrics) indicating apparent dextral movement (outcrop 18-LP-6198, 18-MP-5045 and 18-AB-1085).

Deformation Style

The CSdig is a straight to curvilinear, homogeneous and well-defined shear zone. It is characterized by the presence of L>>S tectonites. Deformation is mainly in the constriction domain. The very pronounced stretch lineation (313°/69°), which corresponds to the X-axis on the ellipsoid of the finished deformation (Flinn, 1965), indicates the main direction of tectonic transport of the rocks. This significant ductile deformation could possibly be caused by gravitational sinking of greenstones beneath granitoids of the Digne Pluton or by thrusting of the Digne Pluton over UEGB rocks.

Metamorphic Characteristics

Rocks of the Digne Pluton contain quartz-plagioclase-biotite ± chlorite ± muscovite ± garnet assemblages. The metamorphic facies reached along the CSdig cannot be determined. It is most probably similar to that reached in the UEGB, i.e. the amphibolite facies.

Alterations

Alterations observed along the CSdig are muscovitization, chloritization and albitization. The absence of outcrops in the deformation zone does not allow the distribution and intensity of alteration to be documented.

Geophysical Characteristics

The CSdig is defined by a straight to curvilinear lineament of low magnetic susceptibility. It is located directly north of the mafic-ultramafic sill of the Dolent Formation (nAdln3), which is characterized by very high magnetic susceptibility.

Chronological Markers

The Digne Shear Zone marks the contact between rocks of the Dolent Formation (2751 ±5 Ma; Davis, 2019), which represent the remains of a supracrustal cover, and Mesoarchean bedrock, the Digne Pluton (2814 ±7 Ma; Davis, 2019). Sinking of greenstones beneath the Digne Pluton, or thrusting of the Digne Pluton on the Dolent Formation, occurred after 2751 ±5 Ma (age of the Dolent Formation). This may have occurred during a deformation phase prior to regional N-S compression that generated the main fabric of the entire Eeyou Istchee James Bay region.

References

Publications Available Through Sigéom Examine

Other Publications

Suggested Citation

Contributors