Newfoundland Ophiolites and the Geology of the Oceanic Layer
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Ophiolites, recognized in most of the world's orogenic belts, are generally interpreted to be oceanic crust and upper mantle (lithosphere) fragments that have been incorporated into continental margins at consuming plate boundaries. We suggest that the mechanism for ophiolite emplacement is the same in both the Alpine and Andean‐type orogenes. In both geological settings, obduction of oceanic lithosphere onto the continental lithosphere is caused by the convergence of light, buoyant bodies such as oceanic plateaus, continental slivers, island arcs, or old hot spot traces. For example, the Troodos ophiolite complex, previously interpreted by some workers as resulting from continental collision, may have been emplaced by the collision of Cyprus with the Eratosthenes Plateau embedded in the oceanic eastern Mediterranean crust. On the other hand, the Upper Jurassic Coast Range Ophiolites of California, previously interpreted as resulting from typical oceanic subduction, may be the result of a continuous injection of thick nonsubductable packages of light, continentally derived sedimentary rocks, seamounts, and plateaus into the subduction zones. Many other ophiolite complexes may be similarly related to accreted terranes.
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We have compiled a composite oceanic crustal section for strontium, oxygen, and sulfur isotope tracers of seawater interaction, including new oxygen isotope data for a 1-km-thick section of lower oceanic crust, and in this paper, we summarize the main similarities and differences between hydrothermal alteration in ophiolites and that in oceanic crust. Ophiolitic crust is consistently more intensely recrystallized, and isotope tracers exhibit greater shifts in ophiolites than in situ oceanic crust. These differences require greater time-integrated fluid fluxes through the ophiolites (by a factor of ∼3-6), as well as penetration of larger fluid fluxes to greater depths in some ophiolites. The reasons for this fundamental difference must be related in some way to differences in tectonic setting, primary chemical compositions, or heat sources, but the exact reasons remain problematic. The greater fluid fluxes and tracer exchange in ophiolites point out the need for caution when applying data from ophiolites to the global mid-ocean ridge system and estimation of the effects on oceanic and crustal chemistry. Despite significant differences between hydrothermal effects in ophiolites and oceanic crust, many important similarities exist, and the ophiolite analogy remains invaluable to understand the structure of oceanic crust and processes at mid-ocean ridges. A need remains, however, for additional and more continuous sections through ocean and ophiolitic crust in different tectonic settings, in particular through the critical lithologic transitions that coincide with changes in fluid circulation and alteration.
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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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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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Petrological constitution of the oceanic lithosphere is discussed in comparison with the origin and derivation of ophiolitic suites. Recent ODP results for Hess Deep, equatorial Pacific, indicate the petrological similarity of deep-seated oceanic rocks of a fast spreading ridge system with those of some ophiolites, such as Samail ophiolite, Oman. Mantle peridotite from Hess Deep is harzburgite with chromian spinel of Cr# from 0.5 to 0.6. Dunite is common around gabbro-troctolite intrusions. Chromian spinel is sometimes concentrated within dunite and troctolite. Interaction of high-pressure MORB with harzburgite may be more pervasive in the fast spreading ridge than in the slow spreading one.Lithospheric slice as an ophiolite complex has a much more complicated history than the present-day oceanic lithosphere. Some petrological discrepancy, therefore, is expected between ophiolite and oceanic lithosphere. Arc-related rocks in the ophiolite may have been formed at a relatively later stage of arc-like setting which is inevitable for the oceanic lithosphere to be obducted onto continental margins. Highly refractory rocks with Cr-rich spinel (Cr# > 0.7), which are commonly found in ophiolites and have not been found in the present-day ocean floors, were formed at arc-related settings or could be rarely present in the oceanic setting. It is noteworthy that all of the constituent rocks of ophiolites, and even those from ocean floor, were not always formed at the same tectonic setting.
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Forearc
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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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