Subprovince accretion tectonics in the south-central Superior Province

Development of tectonic subprovinces as shear-bounded granite–greenstone and sediment-dominated terranes during the late Archaean is reviewed and interpreted from relationships between portions of the Wabigoon, Wawa, and Quetico subprovinces.Greenstone-dominated subprovinces (Wabigoon and Wawa) are complex successions of tholeiites, 2.76–2.70 Ga calc-alkaline volcanic centres, and derived sediments. Supracrustal rocks aggregated on a scale of tens of kilometres, forming homoclines, locally upright folded, intruded by granitoids, exhibiting variable fabric trends and strains, and cut by transcurrent shear zones. Small-scale (10–100 km) accretion juxtaposed these varied supracrustal sequences, which were engulfed granitoid magmas, to form greenstone belts.Sediment-dominated subprovinces (Quetico) are metamorphosed wacke sequences deposited during and after the volcanic climax in the period 2.70–2.69 Ga. Overthrust imbrication at both the Wabigoon–Quetico and the Quetico–Wawa contacts occurred along north-dipping shears, now vertical. Continued right-lateral convergence at subprovince margins induced progressive shortening within the Quetico Subprovince, producing a regional planar fabric. Abukuma–style metamorphism, migmatite formation, and S-type granite intrusions occurred during the period 2.67–2.65 Ga.Greenstone-belt developments, terminated during large-scale (100–1000 km) late neo-Archæan accretion, are preserved within elongate, batholith-dominated terranes separated by metasedimentary migmatite belts. Geochronological, lithotectonic, and metamorphic patterns on a scale of hundreds of kilometres are permissive of an accretionary model of greenstone terrane coalescence in which formation of long-lived, complex volcanic arcs and a complementary fore-arc accretionary prism culminated in large-scale accretion and the formation of stable continental crust.

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Effective rheology of a two-phase subduction shear zone: insights from numerical simple shear experiments and implications for subduction zone interfaces

10.5194/egusphere-egu21-10396 ◽

2021 ◽

Author(s):

Paraskevi Io Ioannidi ◽

Laetitia Le Pourhiet ◽

Philippe Agard ◽

Samuel Angiboust ◽

Onno Oncken

Keyword(s):

Simple Shear ◽

Large Scale ◽

Confining Pressure ◽

Shear Zones ◽

Dislocation Creep ◽

Small Scale ◽

Two Phase ◽

Weak Phase ◽

Pure Phases ◽

Geodynamic Models

Exhumed subduction shear zones often exhibit block-in-matrix structures comprising strong clasts within a weak matrix (m&#233;langes). Inspired by such observations, we create synthetic models with different proportions of strong clasts and compare them to natural m&#233;lange outcrops. We use 2D Finite Element visco-plastic numerical simulations in simple shear kinematic conditions and we determine the effective rheology of a m&#233;lange with basaltic blocks embedded within a wet quartzitic matrix. Our models and their structures are scale-independent; this allows for upscaling published field geometries to km-scale models, compatible with large-scale far-field observations. By varying confining pressure, temperature and strain rate we evaluate effective rheological estimates for a natural subduction interface. Deformation and strain localization are affected by the block-in-matrix ratio. In models where both materials deform viscously, the effective dislocation creep parameters (A, n, and Q) vary between the values of the strong and the weak phase. Approaching the frictional-viscous transition, the m&#233;lange bulk rheology is effectively viscous creep but in the small scale parts of the blocks are frictional, leading to higher stresses. This results in an effective value of the stress exponent, n, greater than that of both pure phases, as well as an effective viscosity lower than the weak phase. Our effective rheology parameters may be used in large scale geodynamic models, as a proxy for a heterogeneous subduction interface, if an appropriate evolution law for the block concentration of a m&#233;lange is given.

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Interaction between Crustal-Scale Darcy and Hydrofracture Fluid Transport: A Numerical Study

Geofluids ◽

10.1155/2020/8891801 ◽

2020 ◽

Vol 2020 ◽

pp. 1-14

Author(s):

Tamara de Riese ◽

Paul D. Bons ◽

Enrique Gomez-Rivas ◽

Till Sachau

Keyword(s):

Fluid Flow ◽

Numerical Model ◽

Transport Mechanism ◽

Large Scale ◽

Fluid Transport ◽

Numerical Study ◽

Ballistic Transport ◽

Accretionary Prism ◽

Small Scale ◽

Porous Flow

Crustal-scale fluid flow can be regarded as a bimodal transport mechanism. At low hydraulic head gradients, fluid flow through rock porosity is slow and can be described as diffusional. Structures such as hydraulic breccias and hydrothermal veins both form when fluid velocities and pressures are high, which can be achieved by localized fluid transport in space and time, via hydrofractures. Hydrofracture propagation and simultaneous fluid flow can be regarded as a “ballistic” transport mechanism, which is activated when transport by diffusion alone is insufficient to release the local fluid overpressure. The activation of a ballistic system locally reduces the driving force, through allowing the escape of fluid. We use a numerical model to investigate the properties of the two transport modes in general and the transition between them in particular. We developed a numerical model in order to study patterns that result from bimodal transport. When hydrofractures are activated due to low permeability relative to fluid flux, many hydrofractures form that do not extend through the whole system. These abundant hydrofractures follow a power-law size distribution. A Hurst factor of ~0.9 indicates that the system self-organizes. The abundant small-scale hydrofractures organize the formation of large-scale hydrofractures that ascend through the whole system and drain fluids in large bursts. As the relative contribution of porous flow increases, escaping fluid bursts become less frequent, but more regular in time and larger in volume. We propose that metamorphic rocks with abundant veins, such as in the Kodiak accretionary prism (Alaska) and Otago schists (New Zealand), represent regions with abundant hydrofractures near the fluid source, while hydrothermal breccias are formed by the large fluid bursts that can ascend the crust to shallower levels.

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Synsedimentary submarine slope failure and tectonic deformation in deep-water carbonates, Cow Head Group, western Newfoundland

Canadian Journal of Earth Sciences ◽

10.1139/e86-050 ◽

1986 ◽

Vol 23 (4) ◽

pp. 476-490 ◽

Cited By ~ 20

Author(s):

Mario Coniglio

Keyword(s):

Large Scale ◽

Slope Failure ◽

Shear Zones ◽

Small Scale ◽

Tectonic Deformation ◽

Head Group ◽

Shallow Subsurface ◽

Deformation Structures ◽

Lenticular Bedding ◽

Basin Morphology

The Cow Head Group, interpreted as a southeast-dipping base-of-slope carbonate apron, contains intraformational truncation surfaces and slide masses. Synsedimentary shear zones are formed (1) below intraformational truncation surfaces; (2) in the basal parts of slide masses; and (3) in the shallow subsurface because of downslope creep. Shear zones are characterized by a variety of synsedimentary deformation structures. Limestones are subject to folding, brecciation, rotation of fragmented beds, and the development of fitted-lenticular bedding. In the interbedded shales, there is both disruption of fine laminations and small-scale isoclinal folding and faulting. Outcrops characterized by these features and the lack of truncation surfaces or slide masses may reflect minor downslope creep. The presence of truncation surfaces, slide masses, and shear zones indicates deposition on an unstable sloping surface.The recognition of intraformational truncation surfaces and slide masses usually requires extensive strike exposure, which when lacking, (e.g., drill cores), limits the potential of these large-scale features as useful indicators of slope deposition. In the Cow Head Group, the recognition and proper interpretation of the common, small-scale deformation structures of synsedimentary shear zones provides evidence for slope deposition that is independent of other sedimentologic, stratigraphic, and regional data.In some parts of the Cow Head Group, "wrinkled" limestones characterized by a prominent dome-and-basin morphology reflect layer-parallel shortening related to tectonic deformation. The deformation of these limestones was previously considered to be synsedimentary, but their association with late-diagenetic precipitates and tectonic stylolites, in conjunction with their continuity and regularity, distinguishes these folds from those produced during synsedimentary deformation.

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The Neogene–Recent Hatay Graben, South Central Turkey: graben formation in a setting of oblique extension (transtension) related to post-collisional tectonic escape

Geological Magazine ◽

10.1017/s0016756808005013 ◽

2008 ◽

Vol 145 (6) ◽

pp. 800-821 ◽

Cited By ~ 31

Author(s):

SARAH J. BOULTON ◽

ALASTAIR H. F. ROBERTSON

Keyword(s):

Large Scale ◽

Upper Cretaceous ◽

Structural Data ◽

Strike Slip ◽

Stress Axis ◽

Normal Faults ◽

Small Scale ◽

South Central ◽

Collision Zone ◽

Tectonic Escape

AbstractStructural data and a regional tectonic interpretation are given for the NE–SW-trending Hatay Graben, southern Turkey, within the collision zone of the African (Arabian) and Eurasian (Anatolian) plates. Regional GPS and seismicity data are used to shed light on the recent tectonic development of the Hatay Graben. Faults within Upper Cretaceous to Quaternary sediments are categorized as of first-, second- and third-order type, depending on their scale, location and character. Normal, oblique and strike-slip faults predominate throughout the area.The flanks of the graben are dominated by normal faults, mainly striking parallel to the graben, that is, 045–225°. In contrast, the graben axis exhibits strike-slip faults, trending 100–200°, together with normal faults striking 040–060° and 150–190° (a subset strikes 110–130°). Similarly orientated normal faults occur throughout Upper Cretaceous to Pliocene sediments, whereas strike-slip faults are mostly within Pliocene sediments near the graben axis. Stress inversion of slickenline data from mostly Pliocene sediments at ten suitable locations (all near the graben axis) show that σ3 directions (minimum stress axis ≈ extension direction) are uniform in the northeast of the graben but orientated at a high angle to the graben margins. More variable σ3 directions in the southwest may reflect local block rotations. During Miocene times, the Arabian and Anatolian plates collided, forming a foreland basin associated with flexurally controlled normal faulting. During the Late Miocene there was a transition from extension to transtension (oblique extension). The neotectonic Hatay Graben formed during the Plio-Quaternary in a transtensional setting. In the light of modern and ancient comparisons, it is suggested that contemporaneous strain was compartmentalized into large-scale normal faults on the graben margins and mainly small-scale strike-slip faults near the graben axis. Overall, the graben reflects Plio-Quaternary westward tectonic escape from a collision zone towards the east to a pre- or syn-collisional zone to the west in the Mediterranean Sea.

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Gravitational sliding or tectonic thrusting?: Examples and field recognition in the Miura-Boso subduction zone prism

10.1130/2021.2552(10) ◽

2021 ◽

Author(s):

Yujiro Ogawa ◽

Shin’ichi Mori

Keyword(s):

Large Scale ◽

Accretionary Prism ◽

Small Scale ◽

Tectonic Deformation ◽

Detachment Faults ◽

Thrust Belts ◽

Fold And Thrust Belt ◽

Fold Belts ◽

Geologic Record ◽

Accretionary Prisms

ABSTRACT Discrimination between gravity slides and tectonic fold-and-thrust belts in the geologic record has long been a challenge, as both have similar layer shortening structures resulting from single bed duplication by thrust faults of outcrop to map scales. Outcrops on uplifted benches within the Miocene to Pliocene Misaki accretionary unit of Miura-Boso accretionary prism, Miura Peninsula, central Japan, preserve good examples of various types of bedding duplication and duplex structures with multiple styles of folds. These provide a foundation for discussion of the processes, mechanisms, and tectonic implications of structure formation in shallow parts of accretionary prisms. Careful observation of 2-D or 3-D and time dimensions of attitudes allows discrimination between formative processes. The structures of gravitational slide origin develop under semi-lithified conditions existing before the sediments are incorporated into the prism at the shallow surfaces of the outward, or on the inward slopes of the trench. They are constrained within the intraformational horizons above bedding-parallel detachment faults and are unconformably covered with the superjacent beds, or are intruded by diapiric, sedimentary sill or dike intrusions associated with liquefaction or fluidization under ductile conditions. The directions of vergence are variable. On the other hand, layer shortening structure formed by tectonic deformation within the accretionary prism are characterized by more constant styles and attitudes, and by strong shear features with cataclastic textures. In these structures, the fault surfaces are oblique to the bedding, and the beds are systematically duplicated (i.e., lacking random styles of slump folds), and they are commonly associated with fault-propagation folds. Gravitational slide bodies may be further deformed at deeper levels in the prism by tectonism. Such deformed rocks with both processes constitute the whole accretionary prism at depth, and later may be deformed, exhumed to shallow levels, and exposed at the surface of the trench slope, where they may experience further deformation. These observations are not only applicable in time and space to large-scale thrust-and-fold belts of accretionary prism orogens, but to small-scale examples. If we know the total 3-D geometry of geologic bodies, including the time and scale of deformational stages, we can discriminate between gravitational slide and tectonic formation of each fold-and-thrust belt at the various scales of occurrence.

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Deformation at the southern boundary of the late Archaean Atâ tonalite and the extent of Proterozoic reworking of the Disko terrane, West Greenland

Geological Survey of Denmark and Greenland Bulletin ◽

10.34194/ggub.v181.5123 ◽

1999 ◽

pp. 155-169 ◽

Cited By ~ 4

Author(s):

John Grocott ◽

Steven C. Davies

Keyword(s):

Large Scale ◽

Hanging Wall ◽

Shear Zones ◽

High Grade ◽

Southern Boundary ◽

Late Archaean ◽

West Greenland ◽

North West ◽

North East ◽

The North

NOTE: This article was published in a former series of GEUS Bulletin. Please use the original series name when citing this article, for example: Grocott, J., & Davies, S. C. (1999). Deformation at the southern boundary of the late Archaean Atâ tonalite and the extent of Proterozoic reworking of the Disko terrane, West Greenland. Geology of Greenland Survey Bulletin, 181, 155-169. https://doi.org/10.34194/ggub.v181.5123 _______________ The c. 2800 Ma old Atâ tonalite in the area north-east of Disko Bugt, West Greenland has largely escaped both Archaean and Proterozoic regional deformation and metamorphism. At its southern margin the tonalite is in contact with migmatitic quartz-feldspar-biotite gneiss and to the south both are progressively deformed in a high-grade gneiss terrain. The main deformation in the high grade gneisses involved hanging wall north-west displacements on a system of low-angle ductile shear zones that structurally underlie the Atâ tonalite. This shear zone system is folded by a large-scale, steeply inclined and north-west-trending antiform deﬁned by the change in dip of planar fabrics. Minor folds related to the antiform are present and there is some evidence that folding was synkinematic with emplacement of a suite of c. 1750 Ma old ultramaﬁc lamprophyre dykes. In much of the north-east Disko Bugt area it remains difﬁcult to separate Archaean from Proterozoic structures and hence the extent of the Archaean terrane that has escaped intense Proterozoic reworking remains uncertain.

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Using Convection-Allowing Ensembles to Understand the Predictability of an Extreme Rainfall Event

Monthly Weather Review ◽

10.1175/mwr-d-16-0083.1 ◽

2016 ◽

Vol 144 (10) ◽

pp. 3651-3676 ◽

Cited By ~ 21

Author(s):

Erik R. Nielsen ◽

Russ S. Schumacher

Keyword(s):

Large Scale ◽

Numerical Models ◽

Regional Scale ◽

Extreme Rainfall ◽

Rainfall Event ◽

Small Scale ◽

Extreme Rainfall Event ◽

Error Growth ◽

Ensemble Spread ◽

South Central

This research uses convection-allowing ensemble forecasts to address aspects of the predictability of an extreme rainfall event that occurred in south-central Texas on 25 May 2013, which was poorly predicted by operational and experimental numerical models and caused a flash flood in San Antonio that resulted in three fatalities. Most members of the ensemble had large errors in the location and magnitude of the heavy rainfall, but one member approximately reproduced the observed rainfall distribution. On a regional scale a flow-dependent diurnal cycle in ensemble spread growth is observed with large growth associated with afternoon convection, but the growth rate then reduced after convection dissipates the next morning rather than continuing to grow. Experiments that vary the magnitude of the perturbations to the initial and lateral boundary conditions reveal flow dependencies on the scales responsible for the ensemble growth and the degree to which practical (i.e., deficiencies in observing systems and numerical models) and intrinsic predictability limits (i.e., moist convective dynamic error growth) affect a particular convective event. Specifically, it was found that large-scale atmospheric forcing tends to dominate the ensemble spread evolution, but small-scale error growth can be of near-equal importance in certain convective scenarios where interaction across scales is prevalent and essential to the local precipitation processes. In a similar manner, aspects of the “upscale error growth” and “downscale error cascade” conceptual models are seen in the experiments, but neither completely explains the spread characteristics seen in the simulations.

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Strain localisation in mechanically Layered Rocks, insights from numerical modelling

Solid Earth Discussions ◽

10.5194/sed-4-1165-2012 ◽

2012 ◽

Vol 4 (2) ◽

pp. 1165-1204 ◽

Cited By ~ 1

Author(s):

L. Le Pourhiet ◽

B. Huet ◽

P. Agard ◽

L. Labrousse ◽

L. Jolivet ◽

...

Keyword(s):

Large Scale ◽

Shear Bands ◽

Structural Diversity ◽

Shear Zones ◽

Small Scale ◽

Lithostatic Pressure ◽

Shear Direction ◽

Strain Localisation ◽

Initial Anisotropy ◽

Micro Structures

Abstract. Small scale deformation in stratified rocks displays a large diversity of micro-structures, from the microscopic scale to the scale of orogens. We have designed a series of fully dynamic numerical simulations aimed at assessing which parameters control this structural diversity and which underlying mechanisms lead to strain localisation. The influence of stratification orientation on the occurrence and mode of strain localisation is tested by varying the initial dip of inherited layering versus the large scale imposed simple shear. The detailed study of the models indicates that (1) the results are length-scale independent, (2) the new shear zones are always compatible with the kinematics imposed at the boundary (3) micro-structures formed encompass the full diversity of micro-structures observed in the field and chiefly depend on the direction of the initial anisotropy versus shear direction, (4) depending on the orientation of the anisotropy, the layers may deform along subtractive or additive shear bands, (5) the deformation in anisotropic media results in non-lithostatic pressure values that are on the order of the deviatoric stress in the strong layers and (6) the introduction of brittle rheology is necessary to form localised shear bands in the ductile regime.

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Multi-scale spatial distribution of K, Th and U in an Archaean potassic granite: a case study from the Heerenveen batholith, Barberton Granite-Greenstone Terrain, South Africa

South African Journal of Geology ◽

10.25131/sajg.124.0005 ◽

2021 ◽

Author(s):

J-F. Moyen ◽

M. Cuney ◽

D. Baratoux ◽

P. Sardini ◽

S. Carrouée

Keyword(s):

Large Scale ◽

Gamma Ray ◽

Shear Zones ◽

Small Scale ◽

Multi Scale ◽

Gamma Ray Spectrometer ◽

Rock Types ◽

Large Scale Integration ◽

Scale Integration

Abstract We describe the multi-scale distribution of K, Th and U in the ca. 3.1 Ga Heerenveen batholith of the Barberton Granite-Greenstone Terrain. Data were obtained with a combination of tools, including a portable gamma-ray spectrometer from the scale of the whole batholith to the scale of outcrops, and autoradiography for the thin section scale. U is concentrated preferentially in minor phases in the border shear zones of the batholith and, within these shear zones, in late pegmatites as well as fractures. The processes responsible for the concentration of U in the Heerenveen batholith is discussed in terms of magmatism, hydrothermalism (redistribution of U in fissures associated with magmato-hydrothermal fluids), and supergene alteration. The statistical properties of K, Th and U concentrations are different. K shows spatial correlation over large distance, largely mirroring mappable rock types, with increased variability at larger scales. In contrast, U is dominated by small-scale variations (“nugget effect”) and its variability is, averaged and smoothed by large-scale integration. Spatial and statistical features thus offer useful and complementary insights on petrogenetic and metallogenic processes in granitoids in addition to standard approaches (petrography, geochemistry).

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Deformation structure and peak-metamorphic temperature in Kodiak accretionary complex, Alaska

10.5194/egusphere-egu21-9856 ◽

2021 ◽

Author(s):

Kristijan Rajic ◽

Hugues Raimbourg ◽

Vincent Famin ◽

Donald Fisher ◽

Kristin Morell

Keyword(s):

Simple Shear ◽

Large Scale ◽

Thermal Structure ◽

Hanging Wall ◽

Accretionary Prism ◽

Shear Zones ◽

Wall Material ◽

Foot Wall ◽

Essential Information ◽

Short Period

The Kodiak archipelago (Southwest Alaska) represents a well exposed paleo-accretionary prism with its modern equivalent to the modern Alaskan Trench further southeast. The complex consists of metasedimentary and magmatic rocks, whose age span from the Triassic-Jurassic units on the northwestern side of the archipelago towards the Miocene units on the southeast. The complex dominantly consists of trench sediments, in which the sedimentary stratification is still visible. In addition, two tectonic m&#233;langes, composed of lenses of metabasites embedded in sheared metasediments, are intercalated between the coherent formations. We carried out an extensive field survey to describe the kinematics and temperature conditions of deformation across the whole subduction complex.M&#233;lange terrains are characterized by subduction-related deformation in the form of a pervasive network of top-to-the-trench shear zones. In contrast, we observed wider range of deformation geometries in coherent units: The Kodiak Landward belt is characterized by top-to-the-trench simple shear. In the Kodiak Central belt, strain geometry varies spatially from dominant top-to-the-trench simple shear to horizontal extension evidenced by conjugate sets of extensional shear bands. Further to southeast, the Kodiak Seaward belt and the Ghost Rocks Formation are characterized by horizontal shortening with conjugate thrust faults and symmetric folds. Post-Paleocene deformation includes strike-slip faulting in the southeastern part as well as in the Kodiak granite, which was previously described as completely undeformed. The main tectonic contact in the area is the Uganik Thrust, delimiting the Uyak Complex and the Kodiak Formation. The thrust consists of a meter-thick mylonitic zone of the hanging wall material (Uyak Complex), with significantly deformed foot wall (Kodiak Formation). Finally, extension can be observed in the Narrow Cape Formation, unconformably overlying the Ghost Rocks m&#233;lange in the SE margin of the belt. Such extension predates the very recent-to-present deformation, characterized by normal faulting and block tilting within the SE margin.Preliminary results of Raman spectroscopy of carbonaceous material (RSCM) provide essential information as to the large-scale thermal structure of the accretionary prism. In the investigated profile, running from southeastern margin towards the northwest, the temperature does not increase monotonically towards the inner part of the wedge. Indeed, the highest temperatures (>300 &#8451;) are found within the central part of the complex, in very thick turbiditic series accreted in a short period of time in the Paleocene. The thermal gap at the unconformity between the Ghost Rocks and Narrow Cape formations indicates fast uplift after accretion, followed by erosion, subsidence and sedimentation of Narrow Cape sediments. On the other side, no thermal gap is found around the Uganik Thrust like described at other OOST thrusts, which suggests that its activity predates exposure to the peak temperature.&#160;

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