Geometry of the allochthonous Daly Bay Complex, Northwest Territories: a model constrained by geology and a three-dimensional gravity interpretation

1995 ◽  
Vol 32 (9) ◽  
pp. 1292-1302
Author(s):  
Terence M. Gordon ◽  
Donald C. Lawton
Keyword(s):  
Three Dimensional ◽  
Outer Part ◽  
Complex Form ◽  
Granulite Facies ◽  
Ductile Deformation ◽  
Lower Grade ◽  
Crustal Material ◽  
Tectonic Event ◽  
Granulite Facies Metamorphism ◽  
Dimensional Gravity

The Daly Bay Complex is one of several metamorphic complexes making up the Aqxarneq gneisses north of Chesterfield Inlet in central District of Keewatin. Granulite-facies metamorphism (0.55 GPa, 750 °C) and ductile deformation have affected all of the rocks in the complex. A 1–15 km wide, inward-dipping, ductile shear zone forms the outer part of the complex and contains strongly deformed equivalents of rocks in the core. Mesoscopic structures and metamorphic mineralogy suggest the Daly Bay Complex was emplaced into the surrounding lower grade rocks by northward-directed thrusting. A three-dimensional gravity model, constrained by structural observations and 1091 surface density measurements, shows that the relatively dense rocks of the complex form a spoon-shaped structure with a long axis trending northwest–southeast. It is approximately 50 km by 120 km in lateral extent and reaches a maximum depth of about 9 km. The thin-skinned geometry of the Daly Bay Complex supports the notion that the crust in central Keewatin between the Daly Bay Complex and Baker Lake comprises a series of undulating imbricated gneiss sheets of middle and lower crustal material, which were juxtaposed by a major tectonic event sometime between 2.5 and 1.9 Ga. The interpreted basal décollement is comparable to seismic features in many orogens, and a predictable consequence of increased ductility with depth in the crust.

Accretion and evolution of an Archaean high-grade grey gneiss – amphibolite complex: the Fiskefjord area, southern West Greenland

1997 ◽  
pp. 1-115 ◽  
Author(s):  
Adam A. Garde
Keyword(s):  
Continental Crust ◽  
Granulite Facies ◽  
Ductile Deformation ◽  
Repeated Injection ◽  
High Grade ◽  
Volcanic Origin ◽  
Granulite Facies Metamorphism ◽  
West Greenland ◽  
Facies Metamorphism ◽  
Grey Gneiss

NOTE: This monograph was published in a former series of GEUS Bulletin. Please use the original series name when citing this series, for example: Garde, A. A. 1997: Accretion and evolution of an Archaean high-grade grey gneiss – amphibolite complex: the Fiskefjord area, southern West Greenland. Geology of Greenland Survey Bulletin 177, 115 pp. _______________ The Fiskefjord area in southern West Greenland, part of the Akia tectono-stratigraphic terrane, comprises a supracrustal association and two groups of grey quartzo-feldspathic orthogneises c. 3200 and 3000 Ma old. The supracrustal association forms layers and enclaves in grey gneiss and may comprise two or more age groups. Homogeneous amphibolite with MORB-like but LIL element enriched tholeiitic composition predominates; part, associated with cumulate noritic and dunitic rocks, represents fragments of layered complexes. Heterogeneous amphibolite of likely submarine volcanic origin, (basaltic) andesitic amphibolite, leucogabbro-anorthosite, and minor pelitic metasediment occur. Disruption by magmatic and tectonic events and geochemical alteration have obscured primary origin: the supracrustal association may represent oceanic crust. Grey orthogneiss of the tonalite-trondhjemite-granodiorite (TTG) association was generated during continental accretion at c. 3000 Ma, most likely by partial melting of wet and hot tholeiitic basaltic rocks subducted in a convergent plate setting. Most dioritic gneiss is c. 220 Ma older. A 3040 Ma dioritic to tonalitic phase, enriched in P2O5 , Ba, Sr, K, Pb, Rb and LREE, probably was derived from metasomatised mantle. Intense deformation and metamorphism accompanied the 3000 Ma magmatic accretion.Thrusts along amphibolite-orthogneiss contacts were succeeded by large recumbent isoclines, upright to overturned folds, and local domes with granitic cores. Syntectonic granulite facies metamorphism is thought to be due to heat accumulation by repeated injection of tonalitic magma. Strong ductile deformation produced steep linear belts before the thermal maximum ceased, whereby folds were reorientated into upright south-plunging isoclines. Two large TTG complexes were then emplaced, followed by granodiorite and granite. Post-kinematic diorite plugs with unusually high MgO, Cr and Ni, and low LIL and immobile incompatible element contents, terminated the 3000 Ma accretion. Hybrid border zones and orbicular textures suggest rapid crystallisation from superheated magma. The diorites most likely formed from ultramafic magma contaminated with continental crust. Widespread high-grade retrogression preserved a granulite facies core in the south-west; to the east the retrogressed gneiss grades into amphibolite facies gneiss not affected by granulite facies metamorphism and retrogression. LIL elements were depleted during granulite facies metamorphism and reintroduced during retrogression, probably transported in anatectic silicate melts and in fluids. Rb-Sr isotope data, and relationships between retrogression, high-strain zones and granite emplacement, show that retrogression took place shortly after the granulite facies metamorphism, before terrane assembly at c. 2720 Ma, probably by movement of melts and fluids into the upper, marginal zone of granulite facies rocks from deeper crust still being dehydrated. Retrogression during Late Archaean terrane assembly was in narrow reactivated zones of ductile deformation; in the Proterozoic it occurred with faulting and dyke emplacement.Geochemical data are presented for Early Proterozoic high-Mg and mafic dykes. A rare 2085 Ma microgranite dyke strongly enriched in incompatible trace elements was formed by partial anatexis of Archaean continental crust.  

Contractional uplift of deep crustal rocks along the Legs Lake shear zone, western Churchill Province, Canadian Shield

2003 ◽  
Vol 40 (8) ◽  
pp. 1085-1110 ◽  
Author(s):  
Kevin H Mahan ◽  
Michael L Williams ◽  
Julia A Baldwin
Keyword(s):  
High Pressure ◽  
Shear Zone ◽  
Granulite Facies ◽  
Low Pressure ◽  
Canadian Shield ◽  
Ionization Mass ◽  
Lower Grade ◽  
Granulite Facies Metamorphism ◽  
Multi Stage ◽  
High Pressure Granulite

The Legs Lake shear zone juxtaposes high-pressure (1.0+ GPa) granulite- and eclogite-facies rocks with low-pressure (~0.5 GPa) amphibolite- to granulite-facies rocks, and thus may represent an important crustal-scale, exhumation-related structure in the western Canadian Shield. Field mapping and structural and petrologic analysis document the deformation and metamorphic history of rocks within and adjacent to the shear zone. At least two important phases of deformation are recorded: (1) early oblique thrusting (D2) that placed high-grade rocks over lower grade rocks, and (2) more discrete and lower grade brittle–ductile normal faulting (D3) that may represent the later part of the exhumation history. The northwest-dipping shear zone consists of 5–8 km of mylonite in map view, which bounds the southeast margin of the East Athabasca mylonite triangle. High-pressure granulite-facies metamorphism (~750–850 °C, 1.0–1.2 GPa) occurred in the East Athabasca mylonite triangle at ca. 1900 Ma, prior to D2 juxtaposition with the adjacent low-pressure Hearne domain. Thermobarometry from Grt–Crd–Sil–Bt–Qtz metapelites in the Hearne domain suggests peak conditions reached 600–700 °C and 0.45–0.5 GPa, which are interpreted to have occurred late during D2. Published and preliminary U–Pb isotope dilution - thermal ionization mass spectrometry (ID-TIMS) zircon geochronology and electron microprobe monazite geochronology suggest that deformation in the Legs Lake shear zone coincided with the ca. 1830–1810 Ma terminal collision in the Trans-Hudson orogeny. Extensional faulting during D3 most likely occurred after ca. 1780 Ma. A multi-stage process of exhumation involving both thrust and normal-sense shearing, may serve as a model for the exhumation of other regionally extensive deep-crustal exposures.

scholarly journals The Qârusuk dykes: post-Nûk, pre-Qôrqut granitoid magmatism in the Godthåb region, southern West Greenland

1983 ◽  
Vol 112 ◽  
pp. 101-112
Author(s):  
V.R McGregor ◽  
D Bridgwater ◽  
A.P Nutman
Keyword(s):  
Granulite Facies ◽  
Ductile Deformation ◽  
Granitoid Magmatism ◽  
Granulite Facies Metamorphism ◽  
Regional Deformation ◽  
Mineral Assemblages ◽  
Similar Character ◽  
Granite Complex ◽  
Chemical Character ◽  
Shear Belt

The Qarusuk dykes are minor intrusions of granitic and trondhjemitic gneiss emplaced in the Godthåb region between 2700 and 2600 Ma ago. They post-date the Nuk gneisses and their subsequent regional deformation and metamorphism but pre-date the essentiaUy post-tectonic Qôrqut granite complex. They are associated in time and commonly in place with intense ductile deformation concentrated in distinct belts along which there was considerable movement of fluids accompanied by retrogression of earlier mineral assemblages. Outside the Godthåb district dykes of similar character post-date the regional c. 2800 Ma granulite facies metamorphism. The dykes are siliceous with low Fet + Mg0 and high Na20 + K20. Variation in Na2O/K2O is at least in part attributed to exchange between magma and percolating fluids. Non-magmatic controls are thought to affect trace element (including REE) and isotopic compositions. The overall chemical character is consistent with derivation by partial melting of sialic rocks associated with the influx of hydrous fluids associated with shear belt formation.

scholarly journals Relations between earthquake distributions, geological history, tectonics and rheology on the continents

2021 ◽  
Vol 379 (2193) ◽  
pp. 20190412 ◽  
Author(s):  
James Jackson ◽  
Dan McKenzie ◽  
Keith Priestley
Keyword(s):  
Earthquake Hazard ◽  
Granulite Facies ◽  
Crustal Material ◽  
Geological History ◽  
Granulite Facies Metamorphism ◽  
Earthquake Hazard Assessment ◽  
Continental Lower Crust ◽  
History Of ◽  
Basin Formation ◽  
Lithosphere Thickness

This paper is concerned with the distribution of earthquakes, particularly their depths, with the temperature of the material in which they occur, and with the significance of both for the rheology and deformation of the continental lithosphere. Earthquakes on faults are generated by the sudden release of elastic energy that accumulates during slow plate motions. The nonlinear high-temperature creep that localizes such energy accumulation is, in principle, well understood and can be described by rheological models. But the same is not true of seismogenic brittle failure, the main focus of this paper, and severely limits the insights that can be obtained by simulations derived from geodynamical modelling of lithosphere deformation. Through advances in seismic tomography, we can now make increasingly detailed maps of lithosphere thickness on the continents. The lateral variations are dramatic, with some places up to 300 km thick, and clearly relate to the geological history of the continents as well as their present-day deformation. Where the lithosphere thickness is about 120 km or less, continental earthquakes are generally confined to upper crustal material that is colder than about 350°C. Within thick lithosphere, and especially on its edges, the entire crust may be seismogenic, with earthquakes sometimes extending into the uppermost mantle if the Moho is colder than 600°C, but the continental mantle is generally aseismic. Earthquakes in the continental lower crust at 400–600°C require the crust to be anhydrous and so are a useful guide or proxy to both composition and strength. These patterns and correlations have important implications for the geological evolution of the continents. They can be seen to have influenced features as diverse as the location of post-collisional rifting; cratonic basin formation; the location, origin and timing of granulite-facies metamorphism; and the formation, longevity and strength of cratons. In addition, they have important consequences for earthquake hazard assessment in the slowly deforming edges and interiors of continental shields or platforms, where the large seismogenic thickness can host very large earthquakes. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record'.

scholarly journals Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

Solid Earth ◽  
2016 ◽  
Vol 7 (5) ◽  
pp. 1331-1347 ◽  
Author(s):  
Johann F. A. Diener ◽  
Åke Fagereng ◽  
Sukey A. J. Thomas
Keyword(s):  
Shear Zone ◽  
Active Faults ◽  
Fluid Pressure ◽  
Granulite Facies ◽  
External Source ◽  
Tectonic Event ◽  
Granulite Facies Metamorphism ◽  
Fluid Infiltration ◽  
Mylonite Zone ◽  
Tectonic Tremor

Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40 Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the P–T–fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the P–T conditions of deformation at 2.7–4.2 kbar and 450–480 °C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T–MH2O diagrams show that the mineral assemblages were fluid saturated and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localized in strongly dehydrated granulites, the fluid must have been derived from an external source, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of pervasive hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that, following initiation, fluid infiltration caused a positive feedback that allowed weakening and continued strain localization. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain-size reduction. Features such as tectonic tremor, which are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

40Ar/39Ar Dating of Paleoproterozoic Shear Zones in the Ellesmere-Devon Crystalline Terrane, Nunavut, Canadian Arctic

2021 ◽  
Author(s):  
Brandon Caswell ◽  
J.A. Gilotti ◽  
Laura E. Webb ◽  
William C. McClelland ◽  
Karolina Kośmińska ◽  
...  
Keyword(s):  
Regional Metamorphism ◽  
Shear Zones ◽  
Granulite Facies ◽  
Ellesmere Island ◽  
Ductile Deformation ◽  
Mobile Belt ◽  
Tectonic Zone ◽  
Granulite Facies Metamorphism ◽  
North West ◽  
Ductile Shearing

Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeast Ellesmere Island are deformed by m-scale, E-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the <sup>40</sup>Ar/<sup>39</sup>Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U-Pb dating of zircon. 40Ar/39Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have <sup>40</sup>Ar/<sup>39</sup>Ar ages that range from ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred between ≈1870–1840 Ma, or ≈90 m.y. after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeast Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.

scholarly journals Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

2016 ◽  
Author(s):  
Johann F. A. Diener ◽  
Åke Fagereng ◽  
Sukey A. J. Thomas
Keyword(s):  
Shear Zone ◽  
Active Faults ◽  
Fluid Pressure ◽  
Granulite Facies ◽  
External Source ◽  
Tectonic Event ◽  
Granulite Facies Metamorphism ◽  
Fluid Infiltration ◽  
Mylonite Zone ◽  
Tectonic Tremor

Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks-Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localised in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred c. 40 Ma after peak granulite facies metamorphism, during a discrete tectonic event, and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the P–T-fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures, and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the P–T conditions of deformation at 2.7–4.2 kbar and 450–480 °C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T–MH2O diagrams show that the mineral assemblages were fluid-saturated, and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localised in strongly dehydrated granulites, the most likely source of external fluid is meteoric, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid-bearing, the fluid-rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that the KMZ initiated by the reactivation of existing, favourably-oriented ductile structures, following which fluid infiltration caused a positive feedback that allowed weakening and continued strain localisation. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain size reduction. Features such as tectonic tremor, that are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

Application Of Electron Probe Analyses Of Minerals In Discussing The Relationship Between Ductile Deformation And Metamorphism

1990 ◽  
Vol 48 (2) ◽  
pp. 250-251
Author(s):  
Fan Guochuan ◽  
Sun Zhongshi
Keyword(s):  
Chemical Composition ◽  
Shear Deformation ◽  
Electron Probe ◽  
General Rule ◽  
Granulite Facies ◽  
Ductile Deformation ◽  
Higher Temperature ◽  
Ductile Shear ◽  
The Relationship ◽  
Ductile Shear Deformation

Under influence of ductile shear deformation, granulite facies mineral paragenesis underwent metamorphism and changes in chemical composition. The present paper discusses some changes in chemical composition of garnet in hypers thene_absent felsic gnesiss and of hypersthene in rock in early and late granulite facies undergone increasing ductile shear deformation .In garnet fetsic geniss, band structures were formed because of partial melting and resulted in zoning from massive⟶transitional⟶melanocrate zones in increasing deformed sequence. The electron-probe analyses for garnet in these zones are listed in table 1 . The Table shows that Mno, Cao contents in garnet decrease swiftly from slightly to intensely deformed zones.In slightly and moderately deformed zones, Mgo contents keep unchanged and Feo is slightly lower. In intensely deformed zone, Mgo contents increase, indicating a higher temperature. This is in accord with the general rule that Mgo contents in garnet increase with rising temperature.

scholarly journals Free partition functions and an averaged holographic duality

2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Nima Afkhami-Jeddi ◽  
Henry Cohn ◽  
Thomas Hartman ◽  
Amirhossein Tajdini
Keyword(s):  
Partition Function ◽  
Spectral Gap ◽  
Central Charge ◽  
Three Dimensional ◽  
Two Dimensions ◽  
Three Dimensions ◽  
Partition Functions ◽  
General Constraints ◽  
Dimensional Gravity ◽  
Holographic Duality

Abstract We study the torus partition functions of free bosonic CFTs in two dimensions. Integrating over Narain moduli defines an ensemble-averaged free CFT. We calculate the averaged partition function and show that it can be reinterpreted as a sum over topologies in three dimensions. This result leads us to conjecture that an averaged free CFT in two dimensions is holographically dual to an exotic theory of three-dimensional gravity with U(1)c×U(1)c symmetry and a composite boundary graviton. Additionally, for small central charge c, we obtain general constraints on the spectral gap of free CFTs using the spinning modular bootstrap, construct examples of Narain compactifications with a large gap, and find an analytic bootstrap functional corresponding to a single self-dual boson.

Sign in / Sign up

Export Citation Format

Share Document