Widespread abiotic methane in chromitites

Recurring discoveries of abiotic methane in gas seeps and springs in ophiolites and peridotite massifs worldwide raised the question of where, in which rocks, methane was generated. Answers will impact the theories on life origin related to serpentinization of ultramafic rocks, and the origin of methane on rocky planets. Here we document, through molecular and isotopic analyses of gas liberated by rock crushing, that among the several mafic and ultramafic rocks composing classic ophiolites in Greece, i.e., serpentinite, peridotite, chromitite, gabbro, rodingite and basalt, only chromitites, characterized by high concentrations of chromium and ruthenium, host considerable amounts of 13C-enriched methane, hydrogen and heavier hydrocarbons with inverse isotopic trend, which is typical of abiotic gas origin. Raman analyses are consistent with methane being occluded in widespread microfractures and porous serpentine- or chlorite-filled veins. Chromium and ruthenium may be key metal catalysts for methane production via Sabatier reaction. Chromitites may represent source rocks of abiotic methane on Earth and, potentially, on Mars

Spring and surface water quality of the Cyprus ophiolites

A survey of surface, spring and borehole waters associated with the ophiolite rocks of Cyprus shows five broad water types (1) Mg-HCO3, (2) Na-SO4-Cl-HCO3, (3) Na-Ca-Cl-SO4-OH-CO3, (4) Na-Cl-SO4 and (5) Ca-SO4. The waters represent a progression in chemical reactivity from surface waters that evolve within a groundwater setting due to hydrolysis of the basic/ultrabasic rock as modified by CO2-weathering. An increase in salinity is also observed which is due to mixing with a saline end-member (modified sea-water) and dissolution of gypsum/anhydrite. In some cases, the waters have pH values greater than 11. Such high values are associated with low temperature serpentinisation reactions. The system is a net sink for CO2. This feature is related not only to the hydrolysis of the primary minerals in the rock, but also to CaCO3 or Ca-Mg-CO3 solubility controls. Under hyperalkaline conditions, virtually all the carbon dioxide is lost from the water due to the sufficiently high calcium levels and carbonate buffering is then insignificant. Calcium sulphate solubility controls may also be operative when calcium and sulphate concentrations are particularly high

The ophiolite-related Mersin Melange, southern Turkey: its role in the tectonic–sedimentary setting of Tethys in the Eastern Mediterranean region

WOS: 000222988100001The Mersin Melange underlies the intact Mersin Ophiolite and its metamorphic sole to the south of the Mesozoic Tauride Carbonate Platform in southern Turkey The Melange varies from chaotic melange to broken formation, in which some stratigraphic continuity can be recognized. Based on study of the broken formation, four lithological associations are recognized: (1) shallow-water platform association, dominated by Upper Palaeozoic-Lower Cretaceous neritic carbonates; (2) rift-related volcanogenic- terrigenous-pelagic association, mainly Upper Triassic andesitic-acidic volcanogenic rocks, siliciclastic gravity flows, basinal carbonates and radiolarites; (3) within-plate-type basalt radiolarite-pelagic limestone association, interpreted as Upper Jurassic-Lower Cretaceous seamounts with associated radiolarian sediments and Upper Cretaceous pelagic carbonates; (4) ophiolite-derived association, including fragments of the Upper Cretaceous Mersin Ophiolite and its metamorphic sole. Locally, the ophiolitic melange includes granite that yielded a K/Ar radiometric age of 375.7 +/- 10.5 Ma (Late Devonian). This granite appears to be subduction influenced based on 'immobile' element composition. The Mersin Melange documents the following history: (1) Triassic rifting of the Tauride continent; (2) Jurassic-Cretaceous passive margin subsidence; (3) oceanic seamount genesis; (4) Cretaceous supra-subduction zone ophiolite genesis; (5) Late Cretaceous intra-oceanic convergence/metamorphic sole formation, and (6) latest Cretaceous emplacement onto the Tauride microcontinent and related backthrusting. Regional comparisons show that the restored Mersin Melange is similar to the Beysehir-Hoyran Nappes further northwest and a northerly origin best fits the regional geological picture. These remnants of a North-Neotethys (Inner Tauride Ocean) were formed and emplaced to the north of the Tauride Carbonate Platform. They are dissimilar to melanges and related units in northern Syria, western Cyprus and southwestern Turkey, which are interpreted as remnants of a South-Neotethys. Early high-temperature ductile transport lineations within amphibolites of the metamorphic sole of the Mersin ophiolite are generally orientated E-W, possibly resulting from vertical-axis rotation of the ophiolite while still in an oceanic setting. By contrast, the commonly northward-facing later stage brittle structures are explained by backthrusting of the ophiolite and melange related to exhumation of the partially subducted northern leading edge of the Tauride continent

Ediacaran Obduction of a Fore-Arc Ophiolite in SW Iberia: A Turning Point in the Evolving Geodynamic Setting of Peri- Gondwana

The Calzadilla Ophiolite is an ensemble of mafic and ultramafic rocks that represents the transition between lower crust and upper mantle of a Cadomian (peri-Gondwanan) fore arc. Mapping and structural analysis of the ophiolite demonstrates that it was obducted in latest Ediacaran times, because the Ediacaran-Early Cambrian sedimentary series (Malcocinado Formation) discordantly covers it. The ophiolite and emplacement-related structures are affected by Variscan deformation (Devonian-Carboniferous), which includes SW verging overturned folds (D1) and thrusts (D2), upright folds (D3), extensional faults (D4), and later faults (D5). These phases of deformation are explained in the context of Variscan tectonics as the result of the progressive collision between Gondwana and Laurussia. Qualitative unstraining of Variscan deformation reveals the primary geometry of Ediacaran-Cambrian structures and uncovers the generation of east verging thrusts as responsible for the primary obduction of the Calzadilla Ophiolite. Restoration of planar and linear structures associated with this event indicates an Ediacaran, east directed obduction of the ophiolite, that is, emplacement of the Cadomian fore arc onto inner sections of the northern margin of Gondwana. According to regional data, the obduction separates two extension-dominated stages in the tectonic evolution of the African margin of northern Gondwana preserved in southern Europe. Preobduction extension brought about the onset and widening of fore-arc and back-arc basins in the external part of the continent, while postobduction extension facilitated the formation of extensional migmatitic domes, an oceanward migration of back-arc spreading centers across peri-Gondwana, and the eventual opening of a major basin such as the Rheic Ocean

Geodynamic setting and origin of the Oman/UAE ophiolite

The ~500km-long mid-Cretaceous Semail nappe of the Sultanate of Oman and UAE (henceforth referred to as the Oman ophiolite) is the largest and best-preserved ophiolite complex known. It is of particular importance because it is generally believed to have an internal structure and composition closely comparable to that of crust formed at the present-day East Pacific Rise (EPR), making it our only known on-land analogue for ocean lithosphere formed at a fast spreading rate. On the basis of this assumption Oman has long played a pivotal role in guiding our conceptual understanding of fast-spreading ridge processes, as modern fast-spread ocean crust is largely inaccessible

Accretionary Tectonics of the North American Cordillera

Continental geology stands on the threshold of a change that is likely to be as fundamental as plate-tectonic theory was for marine geology. Ongoing seismic-reflection investigations into the deep crustal structure of North America are verifying that orogenic zones are underlain by low-angle faults of regional extent (Brown et al 1981). The growing body of regional field relations is likewise delineating numerous orogenic sutures that bound discrete crustal fragments. Paleomagnetic and paleobiogeographic studies are revealing major latitudinal shifts and rotations within and between suture-bounded fragments, particularly within the North American Cordillera. Such interdisciplinary studies are leading to a consensus that the Cordillera has been built by progressive tectonic addition of crustal fragments along the continent edge in Mesozoic and early Cenozoic time. Such crustal growth is referred to as accretionary tectonics. In this paper, we review some of the important concepts in accretionary tectonics, discuss the nature of the materials accreted between central Alaska and southern California in Jurassic and Cretaceous time, and consider the general relations between Cordilleran accretion and the movement of lithospheric plates. The concept of continents growing by peripheral accretion through geologic time has long been a topic of great interest. With the advent of plate tectonics a number of different mechanisms for crustal accretion have arisen, along with mechanisms for crustal attrition. Accretion mechanisms include the growth of imbricated sedimentary prisms along inner-trench walls, slicing off of submarine topographic irregularities within subducting plates, and collision of continents and volcanic arcs by ocean-basin closure. Tectonic attrition mechanisms include rifting, transform faulting, and strike-slip or underthrust removal of inner-trench wall materials coincident with or in place of accretionary prism growth. Growth of intraorogenic ocean basins by seafloor spreading is an additional important mechanism for creating accretionary materials as well as displacing crustal fragments. An important implication of plate kinematic theory is the likelihood for accretionary and attritionary mechanics to operate in series both in time and space along continental margins. Since attrition by nature leaves little material evidence of having operated, one of the major problems confronting Cordilleran geologists lies in the recognition of such attrition within the ancient record, particularly when interspersed with accretionary events. The spectrum of accretion and attrition mechanisms viewed at cm yr^-1 plate-transport rates over time scales of 100 m.y. leads one to suspect a highly mobile history for continental-margin orogens. The serial arrangement of subducting, transform, and rifting links along the modern Cordillera plate-juncture system and both serial and parallel arrangements in the western Pacific systems show the complex interplay of such mechanisms through space. Similar arrangements overprinted through time are suggested by the rock assemblages and structural patterns within the Cordillera, which presently resemble a collage of crustal fragments (Davis et al 1978). Recognition of the structural state of this collage geologic field mapping and geophysical investigations will bring about a new level of understanding in the growth of continental crust, and the reading of stratigraphic records within the fragments and future palinspastic restorations will lead to a new level of understanding in paleogeography and Earth history. The first problem to be considered is the recognition of native North American crust from exotic fragments that have been accreted to its edge