Background and Methodology

The « Moonbase Deformation Zone » was identified by Ryan (1990) in the Kogaluk River area of Labrador. Initially, it was not described in Quebec and its NW extension was derived from aeromagnetic maps (Wardle et al., 1990, 2002). The NW portion of the Moonbase Zone was mapped by Lafrance et al. (2015, 2016) and renamed the Moonbase Shear Zone (ZCmob).

The structural and textural analysis of the ZCmob was the subject of a bachelor’s thesis (Côté-Roberge, 2015) and a master’s thesis (Vanier et al., 2017, 2018).

Boundaries and Morphology

Stratigraphic Units Concerned

Marking the boundary between two lithotectonic domains, the ZCmob distorts units near the contact between the Mistinibi-Raude and Falcoz domains. In this way, it distorts the Kangiqsualujjuaq, Siimitalik and Fougeraye, and Pelland Suite in the Falcoz Domain, in addition to the Mistinibi Complex in the Mistinibi-Raude Domain. This shear zone also affects fabrics of certain units of the George Lithotectonic Domain, such as the Saint-Sauveur, Tunulic and Guesnier complexes, and De Pas Granitic Suite.

Structural Characteristics

❯ Main Fabrics

The ZCmob is characterized by well-developed mylonitic foliation or banding (S_n), as well as quartz rods (L_n) or bands, and porphyroclasts. Several decimetric to metric mylonite zones have C/S fabrics. At the junction with the ZCblm to the east, orthomylonites composed of nearly 50% of highly recrystallized matrix occur locally. Bands of broken granite containing K-feldspar or polymineralic porphyroclasts are also observed.

Textures associated with the main fabric include crenulation, undulatory extinction of feldspar crystals, microfractures in grains, dismemberment of microliths and grinding. Within microliths, evidence of dynamic recrystallization is observed such as wedged grain edges and, more locally, grain edge migration.

In its western section, the ZCmob transposes the ZCrge fabric. Within a kilometric strip, the ZCrge NNW-SSE fabric is reoriented to an NW-SE axis. This influence of the ZCmob on ZCrge fabrics is also observed on the vertical magnetic gradient map.

❯ Other Fabrics

Does not apply.

❯ Folds

Tight to isoclinal folds are commonly observed in ZCmob mylonitic zones. Their axial plane and fold axis are parallel to the main fabric (S_n and L_n respectively). The formation of these folds (P_n) therefore appears to be associated with the same transpression stress regime producing ZCmob mylonites.

❯ Cross-Cutting Relationships

Not observed.

❯ Kinematics

The main apparent movement of the ZCmob is sinistral. This is inferred by C/S shapes produced in the structural grain as observed on the aeromagnetic map, as well as kinematic indicators observed in outcrop and thin section. Although kinematic indicators indicate a generally sinistral apparent movement, dextral kinematic indicators are also observed. Sigmoid-shaped macroscopic and microscopic porphyroclasts are the most common kinematic indicators. Sigmoids are phi or delta type. Grain rotations, mica fishes and pressure shadows are also observed.

Deformation Style

The intensity of deformation within the ZCmob varies from protomylonites to localized zones of ultramylonites. The ZCmob is a sinistral shear zone that may be part of a conjugate deformation system with the ZCrge (Vanier et al., 2018). The angle between these two shear zones as well as their respective suggest that they belong to the same stress system. The geometry of this system is also consistent with the NE-SW orientation of extension fractures interpreted from geophysical lineaments locally occupied by dykes (Vanier et al., 2018). Although structural lineaments suggest that these two shear zones are synchronous, the concomitant deformation along these structures is, however, still to be demonstrated by geochronological data.

Metamorphic Characteristics

Metamorphic minerals are oriented in the mylonitic fabric and are affected by grain reduction or dynamic recrystallization in intensively deformed zones.

Alterations

Late alteration is observed locally on outcrops as hematite and epidote associated with fractures. This alteration and brittle deformation are superimposed on the mylonitic deformation.

Geophysical Characteristics

The magnetic pattern of the ZCmob is characterized by a succession of NW-SE to NNW-SSElinear magnetic ridges, where ridges of high magnetic susceptibility dominate. Within the George Lithotectonic Domain, the magnetic pattern varies from areas of high magnetic susceptibility to areas of low magnetic susceptibility, depending on lithologies crossed. Interpretation of aeromagnetic surveys determines the sinistral regional movement of shearing.

Chronological Markers

A very limited number of chronological markers are available to constrain the age of the ZCmob deformation. A crystallization age of 1861.7 ±4.7 Ma was obtained from U-Pb analyses on zircons in protomylonitic granodiorite (David, personal communication, 2018). Lafrance et al. (2016) interpret its emplacement as syntectonic, which implies that the shear zone was already active at that time.

Late granites of the De pas Granitic Suite (unit pPdep4) are not affected by the ZCmob mylonitic deformation. Their crystallization age estimated between 1820 Ma and 1805 Ma represents the minimum age for this deformation.

References

Publications available through Sigéom Examine

Other publications