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Recognizing detachment-mode seafloor spreading in the deep geological past

Maffione, Marco; Morris, Antony; Anderson, Mark W.
Fonte: Nature Publishing Group Publicador: Nature Publishing Group
Tipo: Artigo de Revista Científica
Publicado em 01/08/2013 EN
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36.47%
Large-offset oceanic detachment faults are a characteristic of slow- and ultraslow-spreading ridges, leading to the formation of oceanic core complexes (OCCs) that expose upper mantle and lower crustal rocks on the seafloor. The lithospheric extension accommodated by these structures is now recognized as a fundamentally distinct “detachment-mode” of seafloor spreading compared to classical magmatic accretion. Here we demonstrate a paleomagnetic methodology that allows unequivocal recognition of detachment-mode seafloor spreading in ancient ophiolites and apply this to a potential Jurassic detachment fault system in the Mirdita ophiolite (Albania). We show that footwall and hanging wall blocks either side of an inferred detachment have significantly different magnetizations that can only be explained by relative rotation during seafloor spreading. The style of rotation is shown to be identical to rolling hinge footwall rotation documented recently in OCCs in the Atlantic, confirming that detachment-mode spreading operated at least as far back as the Jurassic.

Insights into Contractional Fault-Related Folding Processes Based on Mechanical, Kinematic, and Empirical Studies

Hughes, Amanda
Fonte: Harvard University Publicador: Harvard University
Tipo: Thesis or Dissertation
EN_US
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36.56%
This dissertation investigates contractional fault-related folding, an important mechanism of deformation in the brittle crust, using a range of kinematic and mechanical models and data from natural structures. Fault-related folds are found in a wide range of tectonic settings, including mountain belts and accretionary prisms. There are several different classes of fault-related folds, including fault-bend, fault-propagation, shear-fault-bend, and detachment folds. They are distinguished by the geometric relationships between the fold and fault shape, which are driven by differences in the nature of fault and fold growth. The proper recognition of the folding style present in a natural structure, and the mechanical conditions that lead the development of these different styles, are the focus of this research. By taking advantage of recent increases in the availability of high-quality seismic reflection data and computational power, we seek to further develop the relationship between empirical observations of fault-related fold geometries and the kinematics and mechanics of how they form. In Chapter 1, we develop an independent means of determining the fault-related folding style of a natural structure through observation of the distribution of displacement along the fault. We derive expected displacements for kinematic models of end-member fault-related folding styles...

Experimental and seismological constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers

DeMartin, Brian J., 1976-
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 212 p.
ENG
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26.4%
Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded above an inferred magma reservoir along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the arcuate, subsurface structure of a long-lived, active detachment fault. This fault penetrates the entire oceanic crust and forms the high-permeability pathway necessary to sustain long-lived, high-temperature hydrothermal venting in this region. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock trigger grain boundary microfractures if stress intensities at the tips of naturally occurring flaws exceed a critical stress intensity factor.; (cont.) Experimental results coupled with geomechanical models indicate that pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide the high-permeability pathways necessary to sustain high-temperature fluid circulation...

Geometry and kinematics of the fold-thrust belt and structural evolution of the major Himalayan fault zones in the Darjeeling – Sikkim Himalaya, India

Bhattacharyya, Kathakali (1978 - ); Mitra, Gautam (1947 - )
Fonte: Universidade de Rochester Publicador: Universidade de Rochester
Tipo: Tese de Doutorado Formato: Number of Pages:xxv, 263 leaves
ENG
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36.37%
Thesis (Ph. D.)--University of Rochester. Dept. of Earth and Environmental Sciences, 2010.; The Darjeeling – Sikkim Himalaya lies in the eastern part of the Himalayan fold-thrust belt (FTB) in a zone of high arc-perpendicular convergence between the Indian and Eurasian plates. In this region two distinct faults form the Main Central thrust (MCT), the structurally higher MCT1 and the lower MCT2; both these faults have translated the Greater Himalayan hanging wall rocks farther towards the foreland than in the western Himalaya. The width of the sub-MCT Lesser Himalayan rocks progressively decreases from the western Himalaya to this part of the eastern Himalaya, and as a result, the width of the FTB is narrower in this region compared to the western Himalaya. Our structural analysis shows that in the Darjeeling – Sikkim Himalaya the sub-MCT Lesser Himalayan duplex is composed of two duplex systems and has a more complex geometry than in the rest of the Himalayan fold-thrust belt. The structurally higher Daling duplex is a hinterland-dipping duplex; the structurally lower Rangit duplex varies in geometry from a hinterland-dipping duplex in the north to an antiformal stack in the middle and a foreland-dipping duplex in the south. The MCT2 is the roof thrust of the Daling duplex and the Ramgarh thrust is the roof thrust of the Rangit duplex. In this region...

Microstructural and crystallographic-preferred orientation analysis of the Okanagan Valley fault system, British Columbia

Brown, Danielle
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN
Relevância na Pesquisa
36.31%
This study focuses on the structural evolution of the high-grade metamorphic infrastructure of the southeastern Canadian Cordillera exposed in a crustal section between Sicamous and Revelstoke, British Columbia. Of particular interest, in the Omineca morphogeological belt, is the Okanagan Valley fault system (OVfs), which consists of the west-dipping Okanagan Valley detachment fault and its sillimanite-grade footwall gneiss, part of the Shuswap metamorphic complex (SMC) and interpreted to be exhumed during early Cenozoic continental extension. Structural measurements and oriented samples were collected along a sixty-kilometre ENE-WSW oriented profile following the Trans-Canada Highway. Microstructural and quartz crystallographic-preferred orientation (CPO) analysis of the OVfs using standard microscope and Fabric Analyser (FA) techniques are used to characterize the sense of shear, dominant slip systems, ductile flow style, and temperature of deformation across the SMC. Coupled with textural mapping, FA techniques identify a variety of non-coaxial slip systems in the SMC, dominated by prism , rhomb , and prism slips. The recurrence of CPO fabrics moving towards prism slip suggests deformation temperatures greater than 600°C in the SMC. The temperature of deformation is consistent with observed sillimanite-grade metamorphism and quartz deformation regimes. Petrofabric analyses suggest a kinematic shear reversal from top-to-the-west shear dominated at the uppermost structural level of the OVfs to top-to-the-east shear dominated at the base...

Origin and structure of the Ceduna delta system, offshore South Australia.

MacDonald, Justin
Fonte: Universidade de Adelaide Publicador: Universidade de Adelaide
Tipo: Tese de Doutorado
Publicado em //2013
Relevância na Pesquisa
26.71%
This thesis consists of five papers, each of which complements the regional understanding of the Ceduna Delta System, offshore South Australia. Deltas commonly form linked systems of extension, on the delta top, and compression, in the delta toe. This link is promoted by the presence of a detachment layer of more ductile sediments at the base of the delta, which mechanically decouples deltaic sediments from those beneath it. These systems are often explained using Critical Taper Wedge mechanics, which is commonly applied to understand the rock strength properties and the role of pore fluid pressure at the base of the wedge (within the detachment layer) and within the wedge, in these linked systems of extension and compression. To better understand the effect of basal pore fluid pressure on delta—deepwater fold thrust belt systems, I present an in-depth application of Critical Taper Wedge mechanics to the data-rich deepwater Niger Delta Toe in West Africa, a modern day analogue for the ancient Ceduna Sub-basin (Paper 1). This application involved developing a new technique to measure key variables from seismic reflection data in the Niger Delta Toe and input them into a script to calculate the basal pore fluid pressure required to form the observed present-day geometries. With this new approach and increased understanding of the role of basal pore fluid pressure on delta wedge geometry it was possible to better understand how detachment variables control distribution of the lobes that comprise the Cretaceous-age Ceduna Delta. Regional structural mapping of the Ceduna Delta resulted in separation of the delta lobes based on tectonic style and also examining the linkages between the systems...

Structural evolution of metamorphic tectonites beneath the Silver Peak-Lone Mountain detachment fault, west-central Nevada

Kohler, Gretchen
Fonte: Universidade Rice Publicador: Universidade Rice
ENG
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46.56%
The Silver Peak Range of west-central Nevada reveals metamorphic tectonites below a low-angle fault deformed in a northwesterly-trending doubly-plunging anticline. The rock units in the region are divided into a lower plate, a Lower Paleozoic upper plate and an Oligocene and younger upper plate. The lower plate assemblage and Lower Paleozoic upper plate rocks share a common structural history, with the exception that peak metamorphic conditions in the lower plate reached lower amphibolite grade, whereas conditions in the upper plate never exceeded lower greenschist facies. Rocks of Oligocene and younger only experienced late-stage brittle deformation which warped the detachment fault into a doubly-plunging anticline. The cooling history of lower plate tectonites and structural evidence from the upper and lower plate rocks indicate an early history associated with Mesozoic thrusting, and a younger history of Miocene extension associated with displacement transfer between the Furnace Creek Fault and Walker Lane Belt.

CENTRIFUGE MODELLING STUDY OF CONTRASTING STRUCTURAL STYLES IN THE SALT RANGE AND THE POTWAR PLATEAU, PAKISTAN

FAISAL, SHAH
Fonte: Quens University Publicador: Quens University
Tipo: Tese de Doutorado
EN; EN
Relevância na Pesquisa
26.6%
The ENE-trending Himalayan fold-thrust belt in Pakistan exhibits contrasting deformation styles both along and across the strike. The centrifuge modelling technique has been used to investigate these variations in structural style. For the purpose of modelling, the Salt Range and Potwar Plateau (SR/PP) stratigraphy has been grouped into four mechanical units. From bottom to top these are the Salt Range Formation, carapace unit (Cambrian-Eocene platform sequences), Rawalpindi Group, and Siwalik Group. These stratigraphic units of alternating competence, composed of thin layers of plasticine modelling clay and silicone putty, rest on a rigid base plate that represents the crystalline basement of the Indian plate. The models are built at a linear scale ratio of ~10-6 (1mm=1km) and deformed in a centrifuge at 4000g. The models are subjected to horizontal shortening by collapse and lateral spreading of a “hinterland wedge” which simulates overriding by the Himalayan orogen (above the Main Boundary Thrust). The models of the central SR/PP show that the accretionary wedge develops a prominent culmination structure with fault-bend fold geometry over the frontal ramp, while the eastern SR/PP is more internally deformed by detachment folds...

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments; Marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

Williams, Clare Margaret
Fonte: Massachusetts Institute of Technology Publicador: Massachusetts Institute of Technology
Tipo: Tese de Doutorado Formato: 254 p.
ENG
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26.4%
The origin of symmetric alternating magnetic polarity stripes on the seafloor is investigated in two marine environments; along the ridge axis of the fast spreading East Pacific Rise (EPR) (90 25'-90 55'N) and at Kane Megamullion (KMM) (230 40'N), near the intersection of the slow-spreading Mid Atlantic Ridge with Kane Transform Fault. Marine magnetic anomalies and magnetic properties of seafloor samples are combined to characterize the magnetic source layer in both locations. The EPR study suggests that along-axis variations in the observed axial magnetic anomaly result from changing source layer thickness alone, consistent with observed changes in seismic Layer 2a. The extrusive basalts of the upper crust therefore constitute the magnetic source layer along the ridge axis and long term crustal accretion patterns are reflected in the appearance of the axial anomaly. At KMM the C2r.2r/C2An. In (- 2.581 Ma) polarity reversal boundary cuts through lower crust (gabbro) and upper mantle (serpentinized peridotites) rocks exposed by a detachment fault on the seafloor, indicating that these lithologies can systematically record a magnetic signal. Both lithologies have stable remanent magnetization, capable of contributing to the magnetic source layer. The geometry of the polarity boundary changes from the northern to the central regions of KMM and is believed to be related to changing lithology. In the northern region...

Estilos estructurales del Subandino Sur de Bolivia; Structural styles of the southern subandean zone of Bolivia

Rocha, Emilio
Fonte: Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires Publicador: Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires
Tipo: info:eu-repo/semantics/doctoralThesis; tesis doctoral; info:eu-repo/semantics/publishedVersion Formato: application/pdf
Publicado em //2013 SPA
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26.81%
El Subandino Sur de Bolivia es una típica faja plegada y corrida de lámina delgada, con una notable regularidad en la geometría de las estructuras. Esta es una característica típica de las fajas plegadas en las que no se involucra el basamento en la deformación. Sin embargo, cuando se analiza en detalle la geometría y evolución de la deformación del Subandino Sur, se verifica que existen numerosas desviaciones de dicha regularidad. En el presente trabajo se estudiaron los diferentes procesos que habrían condicionado el desarrollo de las estructuras del Subandino Sur, y se puso especial énfasis en el análisis de aquellos factores que podrían explicar las irregularidades que se identifican en la geometría y evolución de la deformación de las diferentes estructuras. Del análisis realizado se destaca la relevancia que tienen los niveles de despegue involucrados en la deformación. En particular, se analiza la importancia que habría tenido el nivel de despegue superior en el desarrollo de los diferentes estilos estructurales que se observan en el Subandino Sur. Este análisis se enfocó en el desarrollo de sobrepresiones, factor determinante para el funcionamiento del nivel de despegue en las pelitas devónicas de la Formación Los Monos. Para esto se realizaron una serie de modelos de sistemas petroleros 1D...

The West Andean Thrust, the San Ramón Fault, and the seismic hazard for Santiago, Chile

Lacassin, Robin; Vargas, Gabriel; Kausel, Edgar; Thiele Cartagena, Ricardo Augusto; Campos, Jaime; Rauld, Rodrigo; Armijo, Rolando
Fonte: Universidade do Chile Publicador: Universidade do Chile
Tipo: Artículo de revista
EN
Relevância na Pesquisa
26.41%
The importance of west verging structures at the western flank of the Andes, parallel to the subduction zone, appears currently minimized. This hampers our understanding of the Andes‐Altiplano, one of the most significant mountain belts on Earth. We analyze a key tectonic section of the Andes at latitude 33.5°S, where the belt is in an early stage of its evolution, with the aim of resolving the primary architecture of the orogen. We focus on the active fault propagation–fold system in the Andean cover behind the San Ramón Fault, which is critical for the seismic hazard in the city of Santiago and crucial to decipher the structure of the West Andean Thrust (WAT). The San Ramón Fault is a thrust ramp at the front of a basal detachment with average slip rate of ∼0.4 mm/yr. Young scarps at various scales imply plausible seismic events up to Mw 7.4. The WAT steps down eastward from the San Ramón Fault, crossing 12 km of Andean cover to root beneath the Frontal Cordillera basement anticline, a range ∼5 km high and >700 km long. We propose a first‐order tectonic model of the Andes involving an embryonic intracontinental subduction consistent with geological and geophysical observations. The stage of primary westward vergence with dominance of the WAT at 33.5°S is evolving into a doubly vergent configuration. A growth model for the WAT‐Altiplano similar to the Himalaya‐Tibet is deduced.Wesuggest that the intracontinental subduction at theWAT is amechanical substitute of a collision zone...

Crack formation on top of propagating reverse faults of the Chuculay Fault System, northern Chile: Insights from field data and numerical modelling

Cembrano, José; Espina, Jacob; Allmendinger, Richard; Gerbault, Muriel; Carrizo, Daniel; Martinod, Joseph; González, Gabriel
Fonte: PERGAMON-ELSEVIER SCIENCE Publicador: PERGAMON-ELSEVIER SCIENCE
Tipo: Artículo de revista
EN
Relevância na Pesquisa
26.37%
Reverse faults in northern Chile have formed 20-300 m high scarps that contain open fractures which occur in a zone of 20-1600 m wide. Two-dimensional numerical models were used to explore the geometrical and mechanical parameters needed to produce extension within a bulk contractional regime. All of the mechanical models show the same structure as the field: a concentration of cracks predominantly at the top, rather than on the forelimbs of the scarps. In the field case extension begins as soon as a discrete scarp forms; with progressive shortening the scarp height increases producing a broadening of the zone in extension. The numerical models show that this broadening stabilizes when a maximum in the scarp height is reached. To produce concentration of the extension on top of the scarps, the reverse fault needs to be weak (phi similar to 10 degrees). The models suggest that distribution of this extensional zone depends on the cross-sectional geometry of the fault and on the location of the detachment at depth. The main mechanism that produces extension on the top of the reverse scarp is stretching of the topographic surface by folding of the hanging wall at the tip of the fault zone.

The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen

Webb, Alexander G.; Yin, An; Harrison, T.M.; Celerier, Julien; Burgess, W.P.
Fonte: Association of Engineering Geologists Publicador: Association of Engineering Geologists
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.31%
The three Himalayan lithologic units, the Lesser Himalayan Sequence, the Greater Himalayan Crystalline complex, and the Tethyan Himalayan Sequence, have a specific structural correlation with the Main Central thrust and South Tibet detachment in the centr

North-eastward subduction followed by slab detachment to explain ophiolite obduction and Early Miocene volcanism in Northland, New Zealand

Schellart, Wouter
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
36.21%
Oligocene-Miocene models for northern New Zealand, involving south-westward subduction to explain Early Miocene Northland volcanism, do not fit within the regional Southwest Pacific tectonic framework. A new model is proposed, which comprises a north-east

Stratigraphic record of basin development within the San Andreas fault system: Late cenozoic fish Creek-Vallecito basin, Southern California

Dorsey, Rebecca J.; Housen, Bernard A.; Janecke, Susanne U.; Fanning, Christopher; Spears, Amy L.F.
Fonte: Association of Engineering Geologists Publicador: Association of Engineering Geologists
Tipo: Artigo de Revista Científica
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36.21%
The Fish Creek-Vallecito basin contains a 5.5-km-thick section of late Miocene to early Pleistocene sedimentary rocks exposed in the hanging wall of the West Salton detachment fault. These deposits preserve a high-fidelity record of late Cenozoic subsiden

Sedimentation Rates Test Models of Oceanic Detachment Faulting

Parnell-Turner, Ross; Cann, Johnson R.; Smith, Deborah K.; Schouten, Hans; Yoerger, Dana; Palmiotto, Camilla; Zheleznov, Alexei; Bai, Hailong
Fonte: Wiley Publicador: Wiley
Tipo: Article; accepted version
EN
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36.71%
This is the accepted manuscript version.The final version is available from Wiley at http://onlinelibrary.wiley.com/doi/10.1002/2014GL061555/full.; Long-lived detachment faults play an important role in the construction of new oceanic crust at slow-spreading mid-oceanic ridges. Although the corrugated surfaces of exposed low-angle faults demonstrate past slip, it is difficult to determine whether a given fault is currently active. If inactive, it is unclear when slip ceased. This judgment is crucial for tectonic reconstructions where detachment faults are present, and for models of plate spreading. We quantify variation in sediment thickness over two corrugated surfaces near 16.5?N at the Mid-Atlantic Ridge using near-bottom CHIRP data. We show that the distribution of sediment and tectonic features at one detachment fault is consistent with slip occurring today. In contrast, another corrugated surface 20 km to the south shows a sediment distribution suggesting that slip ceased ~150,000 years ago. Data presented here provide new evidence for active detachment faulting, and suggest along-axis variations in fault activity occur over tens of kilometers.; This work was supported by the National Science Foundation grant number OCE-1155650.

Development and evolution of detachment faulting along 50 km of the Mid-Atlantic Ridge near 16.5?N

Smith, Deborah K.; Schouten, Hans; Dick, Henry; Cann, Joe; Salters, Vincent; Marschall, Horst R.; Ji, Fuwu; Yoerger, Dana; Sanfilippo, Alessio; Parnell-Turner, Ross; Palmiotto, Camilla; Zheleznov, Alexei; Bai, Hailong; Junkin, Will; Urann, Ben; Dick, Spen
Fonte: Wiley on behalf of American Geophysical Union Publicador: Wiley on behalf of American Geophysical Union
Tipo: Article; accepted version
EN
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46.54%
This is the accepted manuscript. An edited version of this paper was published by AGU. Copyright 2014 American Geophysical Union.; A multifaceted study of the slow-spreading Mid-Atlantic Ridge (MAR) at 16.5?N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along ~50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, non-corrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal one other detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multi-scale corrugations are observed 2-3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6?) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low-angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless...

Kinematics of fault-related folding derived from a sandbox experiment

Bernard, Sylvain; Avouac, Jean-Philippe; Dominguez, Stéphane; Simoes, Martine
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 17/02/2007
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We analyze the kinematics of fault tip folding at the front of a fold-and-thrust wedge using a sandbox experiment. The analog model consists of sand layers intercalated with low-friction glass bead layers, deposited in a glass-sided experimental device and with a total thickness h = 4.8 cm. A computerized mobile backstop induces progressive horizontal shortening of the sand layers and therefore thrust fault propagation. Active deformation at the tip of the forward propagating basal décollement is monitored along the cross section with a high-resolution CCD camera, and the displacement field between pairs of images is measured from the optical flow technique. In the early stage, when cumulative shortening is less than about h/10, slip along the décollement tapers gradually to zero and the displacement gradient is absorbed by distributed deformation of the overlying medium. In this stage of detachment tip folding, horizontal displacements decrease linearly with distance toward the foreland. Vertical displacements reflect a nearly symmetrical mode of folding, with displacements varying linearly between relatively well defined axial surfaces. When the cumulative slip on the décollement exceeds about h/10, deformation tends to localize on a few discrete shear bands at the front of the system...

Detachment faulting and bimodal magmatism in the Palaeoproterozoic Willyama Supergroup, south-central Australia: keys to recognition of a multiply deformed Precambrian metamorphic core complex

Gibson, George M; Nutman, Allen
Fonte: Geological Society of London Publicador: Geological Society of London
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
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The multiply deformed (D1-D3) Palaeoproterozoic Willyama Supergroup in south-central Australia incorporates upper and lower c. 1700 Ma metasedimentary sequences with contrasting early tectonothermal histories that invite comparisons with the metamorphic core complexes and younger extensional orogens of western North America and Europe. A detachment surface of D1 age separating these two sequences has the deduced geometry of an extensional shear zone, juxtaposing rocks subjected to bimodal magmatism, sillimanite- to granulite-grade migmatization, and Na-Fe metasomatism against a less intensely metamorphosed upper plate lacking both migmatites and bimodal magmatism. Synextensional metamorphism took place under low-pressure-high-temperature conditions, producing regionally extensive andalusite- and sillimanite-bearing mineral assemblages before further high-grade metamorphism accompanying D2 recumbent folding and crustal thickening. D2 folding locally inverted the original D1 thermal structure so that sillimanite-grade lower plate rocks now lie structurally above andalusite-grade rocks of the upper plate, rendering recognition of the original detachment surface and associated thermal structure difficult. U-Pb dating of synextensional metabasites intruded into lower plate rocks just below the detachment surface indicate that extension and related bimodal magmatism peaked around 1690-1670 Ma...

Extensional detachment faulting and core-complex formation in the southern Barberton granite-greenstone terrain, South Africa: evidence for a 3.2Ga orogenic collapse

Kisters, Alexander F M; Stevens, Gary; Dziggel, Annika; Armstrong, Richard
Fonte: Elsevier Publicador: Elsevier
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.45%
The Barberton greenstone belt in South Africa is an Early- to Mid-Archaean, very low-grade metamorphic supracrustal belt that is bordered in the south by a mid- to lower crustal gneiss terrain. Detailed mapping of the contacts between the supracrustal and gneiss domains along the southern margin of the greenstone belt shows that the supracrustal rocks are separated from the high-grade metamorphic gneiss terrain by an extensional detachment that is situated at and close to the base of the belt. The extensional detachment is approximately 1-km wide and its location corresponds with the heterogeneous, mélange-like rocks of the Theespruit Formation. Within the detachment zone, two main strain regimes can be distinguished. Amphibolite-facies rocks at and below the granite-greenstone contacts are characterized by rodded gneisses and strongly lineated amphibolite-facies mylonites. The bulk constrictional deformation at these lower structural levels records, in a subhorizontal orientation, the vertical shortening and horizontal, NE-SW directed stretching of the mid-crustal rocks. The prolate, coaxial fabrics are overprinted by greenschist-facies mylonites at higher structural levels that cut progressively deeper into the underlying high-grade basement rocks. These mylonites have developed during non-coaxial strain and kinematic indicators consistently point to a top-to-the-NE sense of movement of the greenstone sequence with respect to the lower structural levels. This relationship between bulk coaxial NE-SW stretching of mid-crustal basement rocks and non-coaxial...