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This paper focusses on obtaining a better understanding of the subsurface geology of the Chibougamau area, in the northeast of the Abitibi greenstone belt (Superior craton), using geophysical data collected along a 128 km long traverse with a rough southwest–northeast orientation. We have constructed two-dimensional (2D) models of the study area that are consistent with newly collected gravity data and high-resolution magnetic data sets. The initial models were constrained at depth by an interpretation of a new seismic section and at surface by the bedrock geology and known geometry of lithological units. The attributes of the model were constrained using petrophysical measurements so that the final model is compatible with all available geological and geophysical data. The potential-field data modelling resolved the geometry of plutons and magnetic bodies that are transparent on seismic sections. The new model is consistent with the known structural geology, such as open folding, and provides an improvement in estimating the size, shape, and depth of the Barlow and Chibougamau plutons. The Chibougamau pluton is known to be associated with Cu–Au magmatic-hydrothermal mineralisation and, as the volume and geometry of intrusive bodies is paramount to the exploration of such mineralisation, the modelling presented here provides a scientific foundation to exploration models focused on such mineralisation. [ABSTRACT FROM AUTHOR]

The Piché Structural Complex (PSC) extends over 150 km within the Cadillac – Larder Lake Fault Zone (CLLFZ), a gold-endowed, east-trending, and high-strain corridor located along the southern edge of the Archean Abitibi Subprovince. The PSC consists of discontinuous units of volcanic rocks (<1 km thick) that host multiple gold deposits. It is spatially associated with molasse-type Timiskaming sedimentary basins. This study describes and interprets the origin of structures and lithologies within the poorly understood PSC to unravel the tectonic evolution of the CLLFZ. Field mapping, chemical analyses, as well as interpretations of cross-sections from drill-hole data, were used to interpret the geometry and structure of the PSC. The PSC is subdivided into six homogeneous fault-bounded segments or slivers. These slivers consist mostly of ultramafic to intermediate volcanic rocks and include some felsic volcanic flows and intrusions. Volcanic facies, chemical compositions, and isotopic ages confirm that these slivers are derived from the early volcanic units of the southern Abitibi greenstone belt, which are located north of the CLLFZ. Cross-cutting relationships between volcanic rocks of the PSC and the Timiskaming-aged intrusions suggest that the slivers were inserted into the CLLFZ during the early stages of the accretion-related deformation (<2686 Ma) and prior to Timiskaming sedimentation and ductile deformation (>2676 Ma). The abundant ultramafic rocks located within the CLLFZ may have focused strain, thereby facilitating the nucleation of the fault as well as the displacements along this crustal-scale structure. [ABSTRACT FROM AUTHOR]

The geological history and evolution of the Dharwar craton from ca. 3.5-2.5 Ga is reviewed and briefly compared with a second craton, Kaapvaal, to allow some speculation on the nature of global tectonic regimes in this period. The Dharwar craton is divided into western (WDC) and eastern (EDC) parts (separated possibly by the Closepet Granite Batholith), based on lithological differences and inferred metamorphic and magmatic genetic events. A tentative evolution of the WDC encompasses an early, ca. 3.5 Ga protocrust possibly forming the basement to the ca. 3.35-3.2 Ga Sargur Group greenstone belts. The latter are interpreted as having formed through accretion of plume-related ocean plateaux. The approximately coeval Peninsular Gneiss Complex (PGC) was possibly sourced from beneath plateau remnants, and resulted in high-grade metamorphism of Sargur Group belts at ca. 3.13-2.96 Ga. At about 2.9-2.6 Ga, the Dharwar Supergroup formed, comprising lower Bababudan (largely braided fluvial and subaerial volcanic deposits) and upper Chitradurga (marine mixed clastic and chemical sedimentary rocks and subaqueous volcanics) groups. This supergroup is preserved in younger greenstone belts with two distinct magmatic events, at 2.7-2.6 and 2.58-2.54 Ga, the latter approximately coincident with ca. 2.6-2.5 Ga granitic magmatism which essentially completed cratonization in the WDC. The EDC comprises 2.7-2.55 Ga tonalite-trondhjemite-granodiorite (TTG) gneisses and migmatites, approximately coeval greenstone belts (dominated by volcanic lithologies), with minor inferred remnants of ca. 3.38-3.0 Ga crust, and voluminous 2.56-2.5 Ga granitoid intrusions (including the Closepet Batholith). An east-to-west accretion of EDC island arcs (or of an assembled arc - granitic terrane) onto the WDC is debated, with a postulate that the Closepet Granite accreted earlier onto the WDC as part of a 'central Dharwar' terrane. A final voluminous granitic cratonization event is envisaged to have affected the entire, assembled Dharwar craton at ca. 2.5 Ga. When Dharwar evolution is compared with that of Kaapvaal, while possibly global magmatic events and freeboard-eustatic changes at ca. 2.7-2.5 Ga may be identified on both, the much earlier cratonization (by ca. 3.1 Ga) of Kaapvaal contrasts strongly with the ca. 2.5 Ga stabilization of Dharwar. From comparing only two cratons, it appears that genetic and chronologic relationships between mantle thermal and plate tectonic processes were complex on the Archaean Earth. The sizes of the Kaapvaal and Dharwar cratons might have been too limited yet to support effective thermal blanketing and thus accommodate Wilson Cycle onset. However, tectonically driven accretion and amalgamation appear to have predominated on both evolving cratons. [ABSTRACT FROM AUTHOR]

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In western Superior province, the North Caribou terrane (NCT) constitutes a Mesoarchean proto-continent heavily overprinted by Neoarchean magmatism and deformation resulting from the western Superior Province accretion. Locally, along the southern margin of the NCT, Mesoarchean (~3.0 Ga) rift sequences are preserved. These sequences are of key importance to our understanding of the early tectonic evolution of continental crust. The Wallace Lake greenstone belt is located at the southern margin of the NCT and includes the Wallace Lake assemblage, the Big Island assemblage, the Siderock Lake assemblage, and the French Man Bay assemblage. The Wallace Lake assemblage exposes one of the best-preserved Mesoarchean rift sequences along the southern margin of the NCT. The volcano-sedimentary assemblage (3.0–2.92 Ga) exposes arkoses derived from the uplift of a tonalite basement in a subaqueous environment, capped by carbonate and iron formation. Mafic to ultramafic volcanic rocks exhibiting crustal contamination and derived from plume magmatism cap this rift sequence. The Wallace Lake assemblage exhibits D1 Mesoarchean deformation. The Big Island assemblage comprises mafic volcanic rocks of oceanic affinity that were docked to the Wallace Lake assemblage along northwest-trending D2 shear zones. The timing of volcanism and docking of the Big Island assemblage remain uncertain. The Siderock Lake and French Man Bay assemblages were deposited in strike-slip basins related to D3 and D4 stages of movement of the transcurrent Wanipigow fault (<2.709 Ga). Regionally, the Wallace Lake assemblage correlates with the Lewis–Story Rift assemblage observed in Lake Winnipeg, whereas the Big Island assemblage appears to correlate with the Black Island assemblage observed in the Lake Winnipeg area. Thus, the North Caribou terrane appears to preserve vestiges of a Mesoarchean rifted succession together with overlying Neoarchean allochthonous, juvenile, volcanic successions over a considerable distance along its present-day southern margin. [ABSTRACT FROM AUTHOR]

Western Superior Lithoprobe seismic-reflection line 1 exhibits a broad region of northward-dipping reflectors in the Uchi subprovince, which gives way to southward-dipping reflectors farther north in the Berens River sub province. Mafic metavolcanic rocks across the region of northward-dipping reflectors exhibit a decline in metamorphic pressure, from pressures of 6 kbar (1 kbar = 100 MPa) in the south to only 2 kbar 80 km to the north. This indicates that the southern edge of the Uchi subprovince has undergone significantly more unroofing than regions farther north. The differential unroofing is not consistent with a doubly vergent thrusting origin for the northward- and southward-dipping reflector pattern. It could result from a crustal-scale synform, of which the region of northward-dipping reflectors would make up the southern limb. Metamorphic pressures from samples off the seismic line, however, provide only limited support for a regional synform, and suggest that much of the pressure variation may result from deformation associated with motion on late faults that are widespread in the western Superior Province. These faults occur in a WNW-striking set with dextral offsets and an ENE-striking set with sinistral offsets. They could result from north–south compression and east–west extension, provided the faults have rotated towards the east–west direction during deformation. Regional tilting and (or) jostling of crustal blocks is attributed to deformation associated with the fault rotation. Motion on the faults and the associated deformation of intervening fault blocks may be important contributors to the present crustal architecture of the western Superior Province, including the surface distribution and form of the greenstone belts. [ABSTRACT FROM AUTHOR]

Evidence for both vertical and horizontal movement is well preserved in the Cross Lake greenstone belt in the northwestern Superior Province. The vertical movement components are concentrated in high-strain zones along pluton–greenstone contacts and are characterized by pluton-side-up or greenstone-side-down movement, and the geometry, kinematics, and strain distribution are consistent with a vertical tectonic model involving diapirism and sagduction. The horizontal components are concentrated in major east-southeast-trending dextral high-strain zones and in subordinate northeast-trending sinistral, antithetic high-strain zones and can be readily explained by a horizontal tectonic model involving dextral transpression. Results of a detailed structural analysis indicate that the vertical and horizontal tectonism were more or less synchronous, and there was a transition from dominantly vertical tectonism at the early stages to dominantly horizontal tectonism at the late stages. The Cross Lake Group, consisting of Timiskaming-type sedimentary rocks, was deposited in a synclinal keel between granitoid domes associated with vertical tectonism. It is suggested that synchronous vertical and horizontal tectonism was a common process in the Neoarchean and might represent a transition from dominant vertical tectonism in the Mesoarchean (and Paleoarchean?) to dominant horizontal tectonism in the Proterozoic and Phanerozoic. [ABSTRACT FROM AUTHOR]

The western Superior Province sustained rapid crustal growth in the interval 2.72–2.68 Ga through amalgamation of microcontinental crustal blocks and juvenile oceanic terranes. Recent field, isotopic, and geophysical surveys provide insight on the nature, timing, and scale of this accretionary growth. However, few places offer the rich tectono-stratigraphic and structural detail with which to establish accretion of oceanic and continental blocks as does the Savant–Sturgeon area. Here, 3.4–2.8 Ga continental crust of the Winnipeg River terrane is juxtaposed with 2.775–2.718 Ga juvenile oceanic rocks of the western Wabigoon terrane across a 2.85–2.75 Ga, southwest-facing, continental margin sequence. The continental margin was reactivated at ~2.715 Ga with the establishment of an arc, recorded by 2.715–2.70 Ga tonalite and associated intermediate volcanic rocks. This magmatic activity is interpreted to reflect north- and east-dipping subduction that led to consumption of a small tract of oceanic crust between the Winnipeg River and western Wabigoon terranes, ultimately leading to their amalgamation after 2.703 Ga. The telescoped fore arc also includes continental-derived turbiditic wacke, siltstone, and iron formation (Warclub assemblage) that are in tectonic contact with diverse oceanic rocks of the western Wabigoon terrane. Collision is bracketed between 2.703 Ga (the maximum age of marine fore arc deposits) and ~2.696 Ga (the minimum age of a late-tectonic pluton). Effects include thrust stacking and the development of shallow-plunging folds and bedding-parallel fabrics (D1), overprinted by steeply plunging inclined folds, steep foliations, and shear zones (D2). Collectively, these structures have penetratively reworked the suture between the ancient fore-arc and oceanic rocks in the Savant–Sturgeon area.Back to Table of Contents [ABSTRACT FROM AUTHOR]

The Grenville Province and other parts of the Canadian Shield contain major (>100 km long) high-strain zones, also called shear belts or ductile shear zones, that are hosted by heterogeneously deformed gneisses and schists. In well-exposed segments of three nontabular zones whose dip angle is known locally, at the erosion level and (or) in the shallow subsurface, we investigate the tangential shear strain (better called the tangential unit shear or TUS) without assuming that mineral-shape lineations, common varieties of stretching lineations, are effectively parallel to the local TUS direction. Employing a graphic technique that copes with the geometric conditions of general triaxial strain, we approximate the actual direction and find the sense of local TUS in parts of (i) the Parry Sound shear zone, Grenville Province; (ii) the South Range shear zone, Southern Province; and (iii) the Uchi – English River subprovince boundary zone, Superior Province. Information thus obtained for individual high-strain zones in Ontario confirms the validity of published hypotheses: (i) 1020–970 Ma, normal-sense distributed shearing in the Grenvillian thrust stack; (ii) northwest- directed thrusting of Huronian rocks over Archean basement; and (iii) north-directed thrusting of English River metasediments and associated migmatites onto the Uchi granite–greenstone terrain, under peak metamorphic conditions. [ABSTRACT FROM AUTHOR]

This study investigates the geochemistry and tectonic setting of the 2.66 Ga Banting Group, the younger sequence of volcanic rocks in the Yellowknife greenstone belt, and its relationship to older tholeiitic volcanic rocks of the 2.72–2.70 Ga Kam Group. The Banting Group includes a much higher proportion of felsic volcanic and volcaniclastic rocks than the Kam Group, but mafic to intermediate volcanic rocks are common. Banting basalts are tholeiitic and are melts of Archean depleted upper mantle, as are basalts of the Kam Group. In contrast, Banting dacites and rhyolites have much lower heavy rare earth element abundances and generally have higher initial [sup 143] Nd/[sup 144] Nd than Kam felsic rocks. The chemistry of the felsic rocks provides a geochemical signature to distinguish rocks of Kam versus Banting age where complex structures have obscured the stratigraphy. Whereas Kam felsic rocks evolved from mafic parents by assimilation – fractional crystallization processes, Banting felsic rocks have compositions similar to Archean tonalite–trondhjemite–dacite suites, as well as modern adakites, and appear to be melts of juvenile, garnet-bearing, hydrated mafic crust, possibly underplated Kam basalts. The nearby 2.66 Ga felsic complex at Clan Lake mimics the geochemical systematics of the Banting Group, and thus Banting-like rocks may reflect a regional crustal melting event at this time.La présente recherche étudie l'environnement géochimique et tectonique du Groupe de Banting, 2,66 Ga, la plus jeune séquence de roches volcaniques dans la ceinture de roches vertes de Yellowknife, et ses relations avec d'autres roches volcaniques tholéiitiques de Groupe de Kam, 2,72 à 2,70 Ga. Le Groupe de Banting comprend une proportion beaucoup plus élevée de roches volcaniques felsiques et volcanoclastiques que le Groupe de Kam, mais les roches volcaniques mafiques à intermédiaires sont fréquentes. Les basaltes de Banting sont tholéiitiques et résultent de la fusion de la croûte archéenne supérieure appauvrie, tout comme les basaltes du Groupe de Kam. Par contre, les dacites et les rhyolites de Banting présentent beaucoup moins d'éléments de terres rares lourds et le rapport [sup 143] Nd/[sup 144] Nd initial est généralement supérieur à celui des roches felsiques du Groupe de Kam. La chimie des roches felsiques fournit une signature géochimique qui permet de distinguer les roches de l'âge du Groupe de Kam de celles de l'âge du Groupe de Banting là où des structures complexes masquent la stratigraphie. Alors que les roches felsiques de Kam ont évolué de parents mafiques par un procédé d'assimilation – de cristallisation fractionnée, les roches de Banting ont des compositions semblables à celles des suites tonalite–trondhjémite–dacite (Archéen) et à celles des adakites modernes et semblent résulter de la fusion d'une croûte mafique, hydratée, juvénile et grenatifère, possiblement des basaltes de Kam qui seraient sous la plaque. Le complexe felsique de Clan Lake, 2,66 Ga, à proximité, imite la systématique géochimique du Groupe de Banting; les roches de type Banting pourraient donc être le reflet d'une fusion régionale de la croûte à cette époque.[Traduit par la Rédaction] [ABSTRACT FROM AUTHOR]