Timing of ophiolite obduction in the Grampian orogen
David ChewJ. Stephen DalyT. MagnaLaurence PageChristopher L. KirklandMartin J. WhitehouseRebecca Lam
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Research Article| November 01, 2010 Timing of ophiolite obduction in the Grampian orogen David M. Chew; David M. Chew † 1Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland †E-mail: chewd@tcd.ie Search for other works by this author on: GSW Google Scholar J. Stephen Daly; J. Stephen Daly 2UCD School of Geological Sciences, University College Dublin, Dublin 4, Ireland Search for other works by this author on: GSW Google Scholar Tomas Magna; Tomas Magna 3Institute of Mineralogy and Geochemistry, University of Lausanne, Quartier UNIL-Dorigny, Bâtiment Anthropole, CH-1015 Lausanne, Switzerland4Institute of Mineralogy, University of Münster, Corrensstrasse 24, D-48149 Münster, Germany5Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic Search for other works by this author on: GSW Google Scholar Laurence M. Page; Laurence M. Page 6Department of Geology, GeoBiosphere Science Centre, Sölvegatan 12, 223 62 Lund University, Sweden Search for other works by this author on: GSW Google Scholar Christopher L. Kirkland; Christopher L. Kirkland 7Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Box 50 007, SE-104 05 Stockholm, Sweden8Geological Survey of Western Australia, 100 Plain St., East Perth WA 6004, Australia Search for other works by this author on: GSW Google Scholar Martin J. Whitehouse; Martin J. Whitehouse 7Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Box 50 007, SE-104 05 Stockholm, Sweden Search for other works by this author on: GSW Google Scholar Rebecca Lam Rebecca Lam 9MicroAnalysis Facility – Inco Innovation Centre, Memorial University, St. John's, NL A1C 5S7, Newfoundland, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information David M. Chew † 1Department of Geology, School of Natural Sciences, Trinity College Dublin, Dublin 2, Ireland J. Stephen Daly 2UCD School of Geological Sciences, University College Dublin, Dublin 4, Ireland Tomas Magna 3Institute of Mineralogy and Geochemistry, University of Lausanne, Quartier UNIL-Dorigny, Bâtiment Anthropole, CH-1015 Lausanne, Switzerland4Institute of Mineralogy, University of Münster, Corrensstrasse 24, D-48149 Münster, Germany5Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic Laurence M. Page 6Department of Geology, GeoBiosphere Science Centre, Sölvegatan 12, 223 62 Lund University, Sweden Christopher L. Kirkland 7Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Box 50 007, SE-104 05 Stockholm, Sweden8Geological Survey of Western Australia, 100 Plain St., East Perth WA 6004, Australia Martin J. Whitehouse 7Laboratory for Isotope Geology, Swedish Museum of Natural History, Stockholm, Box 50 007, SE-104 05 Stockholm, Sweden Rebecca Lam 9MicroAnalysis Facility – Inco Innovation Centre, Memorial University, St. John's, NL A1C 5S7, Newfoundland, Canada †E-mail: chewd@tcd.ie Publisher: Geological Society of America Received: 30 Jul 2009 Revision Received: 02 Nov 2009 Accepted: 17 Nov 2009 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2010 Geological Society of America GSA Bulletin (2010) 122 (11-12): 1787–1799. https://doi.org/10.1130/B30139.1 Article history Received: 30 Jul 2009 Revision Received: 02 Nov 2009 Accepted: 17 Nov 2009 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation David M. Chew, J. Stephen Daly, Tomas Magna, Laurence M. Page, Christopher L. Kirkland, Martin J. Whitehouse, Rebecca Lam; Timing of ophiolite obduction in the Grampian orogen. GSA Bulletin 2010;; 122 (11-12): 1787–1799. doi: https://doi.org/10.1130/B30139.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract This study addresses the timing and pressure-temperature (P-T) conditions of ophiolite obduction, one of the proposed causes of the ca. 470 Ma Grampian orogeny of Scotland and Ireland. This event gave rise to the main structural and metamorphic characteristics of the Grampian terrane—the type area for Barrovian metamorphism, the cause of which remains enigmatic despite a century of research. Zircons from the Highland Border ophiolite, Scotland, define a 499 ± 8 Ma U-Pb concordia age, which is interpreted as dating magmatism. Its metamorphism is dated by a 490 ± 4 Ma 40Ar-39Ar hornblende age, and a 488 ± 1 Ma 40Ar-39Ar muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580 °C relate to ophiolite obduction. Metamorphism of the Deerpark complex ophiolitic mélange (Irish correlative of the Highland Border ophiolite) is constrained by a 514 ± 3 Ma 40Ar-39Ar hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and 40Ar-39Ar muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580 °C for the mica schist constrain the conditions of ophiolite obduction. Metamorphic mineral ages from the Grampian terrane (Dalradian Supergroup) are substantially younger (ca. 475–465 Ma) than those from the ophiolites. If conductive heating in overthickened crust was the cause of Barrovian metamorphism, then collisional thickening must have started soon after ophiolite obduction at ca. 490 Ma in order to generate the ca. 470 Ma metamorphic peak in the Grampian terrane. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.Keywords:
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The pre-collisional tectonic evolution of the north Indian continental margin is best recorded in the few ophiolite complexes preserved, the largest of which occurs in the Spontang area of the Himalayas. Structural, sedimentological, palaeontological and geochemical work on the ophiolite and associated allochthonous thrust sheets has been carried out to constrain the timing and tectonic environment of ophiolite obduction. A distinct thrust sheet of accretionary complex rocks has been identified immediately underlying the ophiolite. Accreted units include thrust slices of tectonic melanges and alkaline basaltic lavas capped by limestones ranging from late Permian to late Cretaceous in age, interpreted as remnants of former seamounts. The accretionary complex formed above a north dipping intra-oceanic subduction zone during the Cretaceous, the Spontang ophiolite located in the hanging wall. Beneath the Photang thrust sheet, two further distinct, allochthonous thrust sheets of sedimentary melanges and continental slope deposits have been recognized. The structural relations of the allochthonous thrust sheets with the sediments of the north Indian margin have been mapped in detail and show clear evidence that obduction occurred in the late Cretaceous. At this time the Dras-Kohistan intra-oceanic arc had already collided with the southern Asian margin, over 1500 km to the north. Obduction of the Spontang ophiolite therefore records a separate tectonic episode in the Ladakh Himalaya.
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In the Lesser Caucasus and NE Anatolia, three domains are distinguished from south to north: (1) Gondwanian-derived continental terranes represented by the South Armenian Block (SAB) and the Tauride–Anatolide Platform (TAP), (2) scattered outcrops of Mesozoic ophiolites, obducted during the Upper Cretaceous times, marking the northern Neotethys suture, and (3) the Eurasian plate, represented by the Eastern Pontides and the Somkheto-Karabagh Arc. At several locations along the northern Neotethyan suture, slivers of preserved unmetamorphozed relics of now-disappeared Northern Neotethys oceanic domain (ophiolite bodies) are obducted over the northern edge of the passive SAB and TAP margins to the south. There is evidence for thrusting of the suture zone ophiolites towards the north; however, we ascribe this to retro-thrusting and accretion onto the active Eurasian margin during the latter stages of obduction. Geodynamic reconstructions of the Lesser Caucasus feature two north dipping subduction zones: (1) one under the Eurasian margin and (2) farther south, an intra-oceanic subduction leading to ophiolite emplacement above the northern margin of SAB. We extend our model for the Lesser Caucasus to NE Anatolia by proposing that the ophiolites of these zones originate from the same oceanic domain, emplaced during a common obduction event. This would correspond to the obduction of non-metamorphic oceanic domain along a lateral distance of more than 500 km and overthrust up to 80 km of passive continental margin. We infer that the missing volcanic arc, formed above the intra-oceanic subduction, was dragged under the obducting ophiolite through scaling by faulting and tectonic erosion. In this scenario part of the blueschists of Stepanavan, the garnet amphibolites of Amasia and the metamorphic arc complex of Erzincan correspond to this missing volcanic arc. Distal outcrops of this exceptional object were preserved from latter collision, concentrated along the suture zones.
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In his famous address to the Geological Society of America in 1957, H. H. Read concluded that ‘there are granites and granites’. This is equally true for ophiolites, slices of oceanic lithosphere produced by sea‐floor spreading and preserved by obduction during plate collision. Although they form in similar ways, it is clear that there are different types of ophiolite which originate under different conditions. Compared to the ‘classic’ ophiolites of Oman, many, such as those in the Alps, lack a sheeted dyke complex and were for a long time considered abnormal. Analogues for this type have now been found forming today and they occur when the rate of spreading is slow.
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Obduction of a dense oceanic lithosphere on top of a light continental lithosphere remains one of the oddest phenomenons in plate tectonics. In northern Oman, the emplacement of the Samail (or Oman) ophiolite took place during Upper Cretaceous. The Samail ophiolite is probably the best studied ophiolite in the world because of its large size and because it has been relatively well preserved from deformation after its emplacement. If many studies have been carried out on the ophiolite itself in order to better understand the structure of an oceanic crust, little is known on the processes leading to its emplacement on the top of the Arabian platform. The sedimentary series deposited on the Arabian platform during the emplacement of the ophiolite could help us to understand the processes involved during the obduction by providing tectono-stratigraphic constraints on the evolution of the platform affected by the obduction. Moreover the reconstitution of the palaeoenvironments and the identification of source areas would allow discussing the evolution of reliefs formed in such a unique context. However, these series, being located just below the allochthonous units, have been affected by deformation. They are grouped in one formation, the Muti Formation, consisting of various lithologies of which the age is uncertain. In consequence, sedimentological analyses require careful mapping of the sedimentary series and biostratigraphical determinations remain difficult. The first results of our sedimentological and stratigraphical investigations show (i) a diachronism of the onset of syn-tectonic sedimentation (chemostratigraphic and biostratigraphic data) and (ii) a high variation of the source areas within the basin. These data are used in order to present a new model of evolution of the basin formed during the emplacement of the Samail ophiolite on top of the Arabian Platform. Implications for the processes involved during the obduction are also discussed.
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Ophiolite obduction, the process by which part of the oceanic crust overlaps the continental margin, is challenging when it comes to the geodynamic reconstruction of lithospheric processes. The oceanic crust is, on average, denser than the upper continental lithosphere. This density difference makes the obduction of the oceanic crust difficult, if not impossible, when only buoyancy forces are considered. To overcome the difficulties posed by the negative buoyancy, the initial configuration of the oceanic basins must have specific thermal and geometric constraints. Here we present a systematic investigation of the geometrical/geodynamical parameters which control the ophiolite emplacement process. We used the LaMEM finite-difference code and acounted for petrologically consistent density structure of the oceanic and continental regions. Our study reveals which parameters are the most important during ophiolite emplacement and which are the most optimal geometries that favor ophiolite emplacement.Our current study focuses on “Tethyan” ophiolites which are characterized by relatively small inferred basin size and are commonly found in Mediterranean region. Based on a combination of various parameters, our study identified the most susceptible configurations for ophiolite obduction. Our models demonstrate, in agreement to geological data, that the obducted lithosphere must be young (<10Myr) and the length of the nature of Ocean-Continent-Transition (OCT) must be relatively sharp (length of initial OCT zone < 60 km) in order to achieve ophiolite obduction. In addition, our results show that the presence of a weak zone separating two parts of the oceanic lithosphere has a profound influence on the subduction initialization and final ophiolite obduction.
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Continental Margin
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Ophiolite obduction, the process during which part of the oceanic crust overlaps the continental margin, presents a challenge in geodynamic reconstructions of lithospheric processes. The difference in buoyancy between the dense oceanic crust and the relatively buoyant continental crust makes obduction of the oceanic crust difficult, if not impossible, if we only consider the buoyancy forces. The initial configuration of the oceanic basins must have specific thermal and geometric constraints to overcome the difficulties posed by the negative buoyancy. Here, we present a systematic investigation of the geometric and geodynamic parameters controlling the process of ophiolite emplacement. We show which parameters are the most important during ophiolite emplacement and the optimum geometries favouring this emplacement. We focus on ‘Tethyan’ ophiolites, which are characterized by a relatively small inferred basin size and are commonly found in the Mediterranean region. Based on a combination of parameters, we identify the configurations most susceptible to ophiolite obduction. Our models, in agreement with the geological data, show that to achieve ophiolite obduction, the obducted lithosphere must be young and the length of the ocean–continent transition zone must be relatively sharp. Supplementary material: Supplementary figures S1–S5, that are mentioned in the main text, are available at https://doi.org/10.6084/m9.figshare.c.6922526 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists
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Abstract Samail ophiolite emplacement has become a type-example for ophiolite obduction. Despite this, problems and controversy remain with respect to the age of high- P metamorphism, the vergence of structures in deformed rocks beneath the ophiolite, the suprasubduction character of the ophiolite and the P-T conditions of the metamorphic sole. The presence of major, regional-scale NE-facing isoclinal folds and SW- and west-dipping shear zones with top-to-the-NE shear sense in Arabian margin rocks beneath the Samail Ophiolite nappe requires that the margin was not simply passively overridden during obduction of the ophiolite. The development of these folds at 72–76 Ma is at the time the ophiolite is emplaced finally onto the margin (80–70 Ma), accompanied by development of a major shear zone in Saih Hatat (the upper plate-lower plate discontinuity described by earlier workers) at c. 82–80 Ma. Structural scenarios that incorporate these folds and shear zones include lateral escape from a rising buoyant crustal slice along the former subduction interface, back-folding (‘retrocharriage’) associated with major oceanwards-directed underthrusting, or simple underthrusting of the margin by the oceanic realm. Previous models involving craton-directed overthrusting with domal culminations related to deep-seated footwall and lateral ramps are more applicable to the Tertiary structure and Tertiary evolution of the mountain range. Oman ophiolite obduction clearly involves ocean-vergent thrusting within the continental margin platform to slope facies sequences.
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