Ductile High-strain Zone Research Articles

The Patterson Lake corridor of Saskatchewan, Canada: defining crystalline rocks in a deep-seated structure that hosts a giant, high-grade Proterozoic unconformity uranium system

The Patterson Lake corridor in the Athabasca Basin region of Saskatchewan, Canada, hosts a large-scale uranium system with two major deposits already delineated. The corridor developed in crystalline rocks of the SW Rae Province, which host all of the known uranium endowment. Orthogneisses along with voluminous pegmatites are the hosts of the uranium mineralization. These rocks, however, underwent significant open-system metasomatic–hydrothermal modification. Principal amongst these alterations is early and pervasive quartz flooding that resulted in the development of widespread secondary quartzites and associated rock types. These secondary quartzites and their altered host rocks suffered ductile deformation, typically focused at silicification fronts. Late carbonatite dykes exploited the associated shear zones. Semi-brittle deformation zones nucleated near the previously developed ductile high-strain zones. Graphite and associated iron-sulfides precipitated in a semi-brittle structural regime. These graphitized zones provided the necessary structural architecture to focus the uranium system, which exploited the conduit hundreds of millions of years later at c. 1.425 Ga. Host rocks of the Patterson Lake corridor prove that metasedimentary rocks are not a requirement for the development of giant Proterozoic unconformity uranium deposits. Crustal-scale fault zones with access to the mantle (i.e. carbonatites) should be considered a key parameter in the exploration model for Proterozoic unconformity uranium deposits. Given the similarity of the mineral assemblages in the crystalline basement rocks of the main exploration corridor in the eastern Athabasca Basin region, it is likely that a similar, cryptic geological boundary focused the giant uranium system in that region. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Chapter 11 Reworking of older (1.8 Ga) continental crust by Mesoproterozoic (1.5–1.4 Ga) orogeny, Blekinge–Bornholm orogen, southeastern Sweden

Abstract The Blekinge–Bornholm orogen in southeastern Sweden consists of calc-alkaline to alkali–calcic intrusive rocks, rhyolites and dacites (1.8 Ga) that were structurally reworked under amphibolite facies conditions, affected by migmatization at mid-crustal levels at c. 1.44 Ga and intruded at c. 1.47–1.43 Ga by ferroan alkali–calcic plutons. This Mesoproterozoic orogen is bordered westwards by the Sveconorwegian orogen and northwards, along the boundary with well-preserved 1.8 Ga magmatic rocks in the Svecokarelian orogen, by a stitching c. 1.45 Ga pluton and steeply dipping ductile zones with a south-side-up, dip-slip shear component. A variably developed gneissic fabric (S 1 ) dips gently to moderately northwards and is affected by asymmetrical F 2 folds with a southerly vergence. Ductile high-strain zones with top-to-the south shear sense are suggested to correspond at depth to anomalously reflective zones along seismic profile BABEL line A. Open folding of the gneissosity around gently, north-plunging fold axes (F 3 ) completed the ductile deformational evolution. Uncertainty remains about the timing of the amphibolite facies ductile fabric and the D 2 folding, which is either late-stage Svecokarelian ( c. 1.77–1.75 Ga) or Hallandian ( c. 1.47–1.43 Ga). Non-collisional, accretionary orogenic systems are suggested to have operated during both time periods, radical reorganization of the subduction trend accompanying the Mesoproterozoic event.

Preliminary investigation of a major high-strain zone in the Caledonian Highlands, southern New Brunswick

A major ductile high-strain zone up to 5 km in width can be traced for at least 70 km diagonally across the Avalonian Caledonia terrane of southern New Brunswick. A study of the northeastern part of this zone from the Prosser Mountain area in the northwest to the Point Wolfe River area west of Fundy National Park shows that both the ca. 630-620 Ma Broad River Group and associated plutons and the 560-550 Ma Coldbrook Group contain similar structural elements, related to a largely shared deformational history. Some of this history is apparent also in the 560–550 Ma plutonic rocks. A pervasive foliation (S1) lies parallel to bedding (S0), and although evidently composite (S0-1) in the Broad River Group, this fabric is very heterogeneous in the younger Coldbrook Group, where low strain enclaves are widespread. No folds have been seen of an F1 generation, and no reversals of facing or vergence are apparent. A mineral lineation (L1m) is locally prominent. The plutonic rocks have early fabrics, including a foliation (S1) producing augen-gneiss with a prominent L-tectonite (L1m). S1 also includes a schistosity associated with the growth of white mica and breakdown of feldspar. Geometry suggests that S1 in the granites is related to S0-1 in the supracrustal rocks, and L1m in both units shares a common orientation. S1 and S0-1 are crenulated by a strong second cleavage (S2) axial planar to folds (F2), the large-scale expression of which is an asymmetric synform containing a belt of Coldbrook Group rocks. Kinematic indicators suggest an overall top-to-the-SE motion along thrusts that stack units of Broad River Group, Coldbrook Group, and plutonic rocks. Fabric development in the plutonic rocks implies a history of exhumation beginning under hot, anhydrous conditions, followed by hydration during retrogression as plutonic rocks were tectonically emplaced into this crustal stack. The age of these tectonic events is not yet well constrained, but could be as young as Carboniferous.
 
 RÉSUMÉ
 
 Il est possible de retracer une importante zone de forte contrainte ductile ayant jusqu’à cinq kilomètres de largeur sur une distance de 70 kilomètres en diagonale à travers le terrane avalonien de Caledonia, dans le Sud du Nouveau-Brunswick. Une étude de la partie nord-est de cette zone à partir du secteur du mont Prosser, dans le nord-ouest, jusqu’au secteur de la rivière Pointe Wolfe, à l’ouest du parc national du Canada Fundy, révèle que le groupe d’environ 630 à 620 Ma de la rivière Broad, les plutons connexes et le groupe de 560 à 550 Ma de Coldbrook abritent des éléments structuraux similaires, apparentés à des déformations passées largement partagées. Une certaine partie de ce passé est également apparente dans les roches plutoniques de 560 à 550 Ma. Une foliation intense (S1) se manifeste parallèlement à la stratification (S0) et, même si cette fabrique est nettement composite (S0-1) dans le groupe de la rivière Broad, elle est très hétérogène dans le groupe plus récent de Coldbrook, où les enclaves de faible contrainte sont répandues. On n’a observé aucun pli d’une production F1 et aucune inversion du regard ni de la vergence n’est apparente. Une linéation minérale (L1m), définie par des agrégats de biotite, est localement bien visible. Les roches plutoniques possèdent des fabriques qui se sont constituées pendant et peu après la cristallisation, notamment une foliation (S1) produisant du gneiss oeillé avec L-tectonite (L1m) en évidence. S1 à l’intérieur des roches plutoniques comporte en outre une schistosité associée à la croissance de mica blanc et à la décomposition de feldspath. La géométrie permet de supposer que S1 dans les granites est apparentée à S0-1 dans les roches supracrustales et que la linéation minérale (L1m) dans les deux unités partage une orientation commune. S1 et S0-1 sont crénelés par une seconde schistosité (S2) prononcée, de plan axial par rapport aux plis (F2), dont l’expression à grande échelle est une synforme asymétrique renfermant une ceinture de roches du groupe de Coldbrook. Les indicateurs cinématiques des structures F2 permettent de supposer un mouvement général du sommet vers le sud-est le long des chevauchements qui empilent les unités du groupe de la rivière Broad, du groupe de Coldbrook et des roches plutoniques. Le développement de la fabrique dans les roches plutoniques plus âgées suppose une exhumation passée ayant commencé dans des conditions très chaudes et anhydres pendant et peu après la cristallisation, vers 620 Ma, suivie par une hydratation pendant la rétrogression au moment où les roches plutoniques se sont tectoniquement mises en place à l’intérieur de cet éperon crustal. L’âge des événements tectoniques tardifs n’est pas encore bien circonscrit, mais ils pourraient remonter au Carbonifère. [Traduit par la redaction]

Gold Mineralization at the Anomaly A Deposit, Clarence Stream Area, Southwestern New Brunswick: Distal Deposits of a Syn-Deformational Intrusion-Related Gold System?

Gold-bearing mineralization at Anomaly A occurs in polydeformed Ordovician turbidites of the Kendall Mountain Formation of the St. Croix belt in southwestern New Brunswick, and is hosted by WSW-trending, shallow-dipping, brittle–ductile high-strain zones associated with the latest stage of the second of four regional deformation events. Gold-bearing syn-deformational stockwork, massive, and multiply brecciated quartz-sulfide veined zones form irregular pods and sheets up to several meters thick, which contain arsenopyrite, pyrrhotite, pyrite, stibnite, and a variety of rare minerals that are gold-bearing. Mineralized zones are enveloped by locally auriferous, weakly to intensely altered wall rock. Vein paragenesis includes: (1) early (pre- to syn-D 2 ) quartz-chlorite veins that are largely the product of migration of quartz by pressure solution; (2) crosscutting, multiphase, vuggy quartz-sulfide veins that contain most of the gold and are interpreted to be structurally controlled; (3) late, crosscutting, nondeformed, non-gold-bearing veins containing laumontite, chlorite, muscovite, quartz, fluorite, and base metal sulfides. Carbonate-poor alteration associated with zones of gold mineralization is lithology-specific, and appears to reflect overprinting of multiple events. Major elements removed during alteration include Si, Na, Mn, and Mg. The minor and trace elements removed include Co, Li, Sc, and possibly Sr, Ni, and Zn. Minor and trace elements added include Au, Ag, As, Sb, S, and possibly F, Cu, Pb, and Zr. On the basis of proximity, time constraints, and a similar ore mineral assemblage, gold mineralization at Anomaly A is interpreted to be a distal equivalent of the intrusion-related gold mineralization at the nearby Clarence Stream Main zone deposit.

Deep crustal and local rheological controls on the siting and reactivation of fault and shear zones, northeastern Newfoundland

The siting of a c. 25 km wide, transpressive, high-strain zone at the eastern margin of the Gander Zone of NE Newfoundland corresponds to the trace of a fundamental contact between two Gondwanan basement blocks displaced sinistrally relative to one another during Silurian orogenesis. Changes in plate motion during the Devonian led to kinematic reversal and reactivation of the shear zone and, at high structural levels, the development of a major brittle-ductile fault system. At a local scale within the Silurian ductile high-strain zone, the focus of deformation and shifts in siting of shear were closely related to magma presence. We consider that granite magmas exploited shear zones within the crust to aid ascent, and in doing so enhanced local deformation. Cessation of magma supply and/or cooling of magmas within conduits caused deformation to relocate elsewhere. NE Newfoundland hence provides an example of how fault/shear zone siting and reactivation may be controlled by regional-scale pre-existing basement configuration and more localised processes affecting rheology, notably plutonism.

Constraints on the timing of crustal imbrication in the central Trans-Hudson Orogen from single zircon 207Pb/206Pb ages of granitoid rocks from the Pelican thrust zone, Saskatchewan

The Pelican thrust is a major ductile high-strain zone in the Reindeer Zone, Trans-Hudson Orogen, northern Saskatchewan. It is interpreted as the main sole thrust separating stacked juvenile Paleoproterozoic allochthons and underlying Archean microcontinental crust in this central part of the orogen. Exposed nonmylonitic rocks in the footwall of the thrust consist of the Sahli monzocharnockite and the smaller, more highly retrograded MacMillan Point granite. Protomylonitic to ultramylonitic gneisses in the thrust zone derive from a variety of prethrust protoliths. A footwall "internal suite" mainly comprises quartzofeldspathic orthogneisses ("Q" gneisses) and high-grade migmatitic paragneisses. Hanging-wall "external suite" mylonitic gneisses include feldspar-porphyroclastic hornblendic grey gneisses probably derived from arc plutons, and laminated amphibolites derived from volcanic rocks. The overlying allochthon mainly comprises protoliths equivalent to those of the porphyroclastic orthogneisses and laminated amphibolites, together with interfolded and overlying Paleoproterozoic paragneisses of the Kisseynew domain. The Sahli monzoeharnockite yields 207Pb/206Pb zircon and whole-rock Rb–Sr ages of ca. 2500 Ma, and the "Q" gneisses give 207Pb/206Pb zircon ages of up to ca 2900 Ma, implying that most of the internal suite (footwall) mylonite protoliths are Archean. In contrast, external suite (hanging wall) porphyroclastic orthogneisses yield ca. 1880–1840 Ma 207Pb/206Pb zircon ages. Main, peak-metamorphic displacement on the Pelican thrust is interpreted to have occurred mainly between 1840 and 1820 Ma, as indicated by 207Pb/206Pb zircon ages from small, highly deformed synthrusting granite–pegmatite neosomal bodies in the thrust zone. Undeformed postcollisional granites and pegmatites were emplaced~1789 Ma. Total duration from arc development to completion of arc–continent collision was ~100 Ma. The Pelican thrust zone may be similar in significance and style to younger, major, ocean closure related thrusts such as the Frontal Pennine thrust of the western Alps and the Main Mantle, Main Boundary, and Main Central thrusts of the Himalayas. As for the Pelican thrust, these displace oceanic rocks over older basement.

Structures within greenschist facies Alpine Schist, central Southern Alps, New Zealand

Abstract Ductile structures in the greenschist facies rocks of the Alpine Schist that lie in the headwaters of the Whataroa, Callery, and Balfour valleys formed during Mesozoic deformation, similar to those in the Otago Schist to the south. A general westward increase in metamorphic grade is locally disrupted and repeated by several late metamorphic faults. The first appearance of biotite occurs at tectonic boundaries in the Callery and Balfour sections. Metagreywackes are juxtaposed against a greenschist‐metapelite‐metachert sequence by a synmetamorphic ductile high strain zone in the lower Balfour valley. A prominent quartz rodding lineation in the high strain zone defines a stretching direction that plunges about 30° southwest. Structures in the high strain zone are locally cut by quartz (±biotite) veins, which have been weakly deformed. Fluid inclusions in quartz veins indicate that the veins formed about 8–10 km deep and that uplift was initially nearly isothermal. Pervasive foliation that formed dur...

The interaction between oblique and layer-parallel shear in high-strain zones: Observations and experiments

The Scandinavian Caledonides consist of a rigid basement, a weak décollement zone, and a series of crystalline nappes. The basement is cut by oblique, west-dipping extensional shear zones, associated with west-verging folds in the overlying décollement zone. The oblique shear zones may, theoretically, have formed after, prior to, or during post-contractional, top-to-the-west shear movements. Physical modelling of this geometrical/mechanical situation shows that each of the three deformation histories results in a characteristic structure. If the oblique, extensional shear zones postdate the layer-parallel (décollement) shear, a normal-type, oblique shear zone structure occurs. If the layer-parallel shear occurs after the development of oblique shear zones, the resulting structure is characterized by a low-strain region above the oblique shear zone. Some folds typically develop above this “protected” region. However, simultaneous oblique and layer-parallel shear provides abundant asymmetric folds with gently dipping axial planes and sub-horizontal, shear zone-parallel fold axes in the region above the oblique shear zone. A neutral surface may be encountered above, and parallel to the oblique shear zone, separating a lower, thin zone of layer-parallel extension from the main, contractional region. The differences between the three types can be used to interpret field observations of this type of structures. The large-scale, post-contractional structures in the Scandinavian Caledonides are consistent with layer-parallel shear related to back-movement of nappes, synchronous with oblique shear zones in the basement. Similar geometries are found as meso- and micro-scale structures in ductile high-strain zones, where oblique shear bands or shear zones affect more competent layers within the tectonites. Also, these structures compare well with those produced experimentally by simultaneous layer-parallel and oblique shear, and are consistent with the general assumption that oblique shear bands form contemporaneous with the general shear in shear zones.

A unified view of craton evolution motivated by recent deep seismic reflection and refraction results

An attempt is made to integrate recent continental seismic reflection, refraction, and geologic observations into a unified scheme of craton evolution. Nascent continental crust of basaltic bulk composition is produced in island arcs. As with oceanic crust, Moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below. The petrologic crust/mantle boundary, lying ∼25 per cent deeper in the lithosphere, has no seismic expression, and corresponds to a depositional/magmatic contact between cumulate ultramafic rocks above and residual mantle (ultramafic tectonite) below. Both contacts are presumably extensively disrupted by magmatic injection. Island arcs are amalgamated into continents at collision zones. Delamination of the lower mafic/ultramafic portion of the crust along with the mantle lithosphere during ‘hard’ collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition. Subsequent extensional collapse of the overthickened crust together with thermal re-equilibration of the lithosphere results in ‘youthful’ stable continental crust, which is ∼30 km thick with its top surface awash at sea level and has the bulk composition of andesite. This crust characteristically exhibits extensional features (half grabens) in the upper portion, and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination. The Moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high-strain zone or intrusive contact separating mafic/intermediate composition material above from fertile mantle (lherzolite) below. Subsequent ‘cratonization’ is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of Myr (as manifest by the ubiquitous overlapping dike swarms of the Precambrian shields). This results in net long-term thickening of the crust by underplating, shifts the bulk crust back toward mafic composition, periodically produces a new deeper Moho and petrologic crust/mantle boundary that do not coincide, and through repeated dike injection disrupts pre-existing laminated reflectors. The first magmatic injection event ‘sweats out’ the light melting fraction in the pre-existing andesitic crust, producing voluminous granitic magmatism typified by the great Proterozoic anorogenic granite/rhyolite provinces. Subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences. The cumulative result is shield-type crust in which intermediate crustal levels are exposed over wide areas, crustal thickness commonly exceeds 40 km, average crustal velocity is somewhat higher than in younger stable crust, and the crust is commonly complexly reflective/diffractive throughout. Two consequences of this model are that exposed ‘lower’ crustal sections in cratonic regions likely do not expose the lower crust, and continental growth rate estimates based on the ‘Freeboard Argument’ are likely to be grossly in error.