Osozawa S.,Tohoku University |
Shinjo R.,University of Ryukyus |
Lo C.H.,National Taiwan University |
Jahn B.M.,National Taiwan University |
And 7 more authors.
Lithosphere | Year: 2012
New trace-element, radiogenic isotopic, and geochronologic data from the Troodos ophiolite, considered in concert with the large body of previously published data, give new insight into the tectonic history of this storied ophiolite, as well as demonstrating the variability of suprasubduction-zone ophiolites, and differences between them and commonly used modern analogs. Similar to earlier studies, we fi nd that island-arc tholeiite of the lower pillow lava sequence erupted fi rst, followed by boninite. We further divide boninitic rocks into boninite making up the upper pillow lava sequence, and depleted boninites that we consider late infi ll lavas. We obtained an Ar-Ar age from arc tholeiite of 90.6 ± 1.2 Ma, comparable to U-Pb ages from ophiolite plagiogranites. New biostratigraphic data indicate that most of the basal pelagic sedimentary rocks that conformably overlie the boninitic rocks are ca. 75 Ma. This suggests that voluminous eruption of boninitic rocks persisted until ca. 75 Ma. Limited eruption of boninitic lavas may have continued until 55.5 ± 0.9 Ma, based on the Ar-Ar age we obtained. The duration of arc magmatism at Troodos (at least 16 m.y., with some activity perhaps extending 35 m.y.) without the development of a mature arc edifi ce greatly exceeds that of other well-studied suprasubduction-zone ophiolites. We propose that Troodos was formed over a newly formed subduction zone, similar to many proposed models, but that the extended period of magmatism (boninitic) resulted from a prolonged period of ridge subduction. © 2012 Geological Society of America.
Schirmer W.,Heinrich Heine University Dusseldorf |
Weber J.,Sektion Geophysik |
Bachtadse V.,University College London |
Boudagher-Fadel M.,University College London |
And 4 more authors.
Central European Journal of Geosciences | Year: 2010
Southern Cyprus is situated within a mosaic terrane that has been fragmented between the northward drifting African and Arabian plates and the Eurasian plate. Enormous uplift of the earth mantle in the Tróodos Mountains is explained by two models. The subduction model explains subduction along the Cyprean arc to be the driving force for uplift whereas after the restraining bend model westward squeezing of Cyprus along strike-slip faulting is responsible for the enormous uplift at restraining bends. Since its emergence as an island in early Miocene times, landscape formation on Cyprus has been strongly controlled by this uplift. Until the Plio-Pleistocene, a strait belt separated the southern unroofed ophiolitic core region-the Tróodos Mountains-from the folded Kyrenia range to the north. This former sea basin, nowadays the Mesaoría Basin, is linked with the Tróodos Mountains by a dissected glacis with a thick cover of river deposits. The highest and oldest river deposits (Apalós Formation) were studied in the Vlokkariá hill southwest of Lefkosía. The 45.5 m thick Apalós Formation of Early Pleistocene age exhibits 24 sedimentary units (Fluviatile Series). Their magnetostratigraphical characters align with the Matuyama chron including the Olduvai and Jaramillo subchrons thus comprising about 1.15 Ma within the Early Pleistocene. This fluvial stack indicates a very flat and deeply lying river environment flowing from a slowly uplifting Tróodos hinterland. It happened during the end of Early Pleistocene when the enhanced Tróodos uplift started the dissection of the stacked river plain.
Bastow I.D.,University of Bristol |
Pilidou S.,Cyprus Geological Survey |
Kendall J.-M.,University of Bristol |
Stuart G.W.,University of Leeds
Geochemistry, Geophysics, Geosystems | Year: 2010
The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide-angle seismic data and complementary geophysical tools such as gravity analysis are not strongly sensitive to the geometry of subsurface melt intrusions. Studies of shear wave splitting in near-vertical SKS phases beneath the transitional Main Ethiopian Rift (MER) provide strong and consistent evidence for a rift-parallel fast anisotropic direction. However, it is difficult to discriminate between oriented melt pocket (OMP) and lattice preferred orientation (LPO) causes of anisotropy based on SKS study alone. The speeds of horizontally propagating Love (SH) and Rayleigh (SV) waves vary in similar fashions with azimuth for LPO- and OMP-induced anisotropy, but their relative change is distinctive for each mechanism. This diagnostic is exploited by studying the propagation of surface waves from a suite of azimuths across the MER. Anisotropy is roughly perpendicular to the absolute plate motion direction, thus ruling out anisotropy due to the slowly moving African Plate. Instead, three mechanisms for anisotropy act beneath the MER: periodic thin layering of seismically fast and slow material in the uppermost ∼10 km, OMP between ∼20-75 km depth, and olivine LPO in the upper mantle beneath. The results are explained best by a model in which low aspect ratio melt inclusions (dykes and veins) are being intruded into an extending plate during late stage breakup. The observations from Ethiopia join a growing body of evidence from rifts and passive margins worldwide that shows magma intrusion plays an important role in accommodating extension without marked crustal thinning. Copyright 2010 by the American Geophysical Union.
Ren L.,University of South Australia |
Ren L.,Hubei University |
Cohen D.R.,University of South Australia |
Rutherford N.F.,University of South Australia |
And 2 more authors.
Applied Geochemistry | Year: 2015
Rare earth elements (REEs) are used as indicators or proxies for a range of geological and mineralogical processes due to their unique geochemical characteristics. Total and aqua regia-extractable concentrations of REEs and 57 other elements have been determined for 5350 soil samples as part of the high sampling density Geochemical Atlas of Cyprus. The bedrock geology of Cyprus is dominated by the sequence of ultramafic to mafic units formed at a spreading ridge and subsequently obducted to form the Troodos Ophiolite (TO), and the surrounding carbonate-rich Circum-Troodos Sedimentary Sequence (CTSS) deposited in environments ranging from deep marine to sub-aerial. Total and aqua regia-extractable REE patterns are similar for each element and are largely controlled by parent lithology. Soil-to-rock REE ratios are generally elevated in the TO units (>4 for LREEs and 1.5-3 for HREEs) due to loss of more mobile elements during weathering but are close to 1 in the CTSS units. HREEs are more elevated than LREEs in soils derived from TO units with upper continental crust-normalised patterns indicating the main source to be pyroxenes and zircon. Trace element trends indicate REEs in the CTSS were largely derived from detrital material shedding off the TO and deposited in progressively shallowing basins under largely anoxic conditions (absence of Ce4+ anomalies), with a minor contribution from seawater via adsorption onto secondary Fe+Mn oxides or co-precipitation with carbonates. Heavy mineral-associated elements such as Zr and Th display a relative consistent ratio in the CTSS soils. Peak HREE concentrations occur in the mafic cumulates and intrusives where the median LaUCC/YbUCC is ~0.12, whereas in CTSS units the LREEs are more elevated with a median LaUCC/YbUCC ~0.7. Due to the strong lithogeochemical controls, soil REE spatial patterns reflect even subtle mineralogical variations within the various TO units, the location of major transform faults and other structures, and areas that have been affected by hydrothermal alteration. © 2015 Elsevier Ltd.
Cohen D.R.,University of New South Wales |
Rutherford N.F.,University of New South Wales |
Morisseau E.,Cyprus Geological Survey |
Zissimos A.M.,Cyprus Geological Survey
Science of the Total Environment | Year: 2012
The soil geochemical atlas of Cyprus is a recent addition to the series of national to continental-scale geochemical mapping programmes implemented over the last two decades for environmental and resource applications. The study has been conducted at the high sampling density of 1 site per 1km2, with multi-element and multi-method analysis performed on samples of top soil (0-25cm) and sub soil (50-75cm) from a grid of over 5350 sites across a major portion of Cyprus. Major and most trace elements display sharp concentration changes across the main geological boundaries but a high degree of spatial continuity and consistency of values within those boundaries. Some elements display one to two orders of magnitude difference in median concentrations between the soils developed over ultramafic or mafic units and those developed over sedimentary rocks or alluvial units. The ratio of aqua regia-extractable to total metal contents provides an indication of the general mineralogical host for a number of trace elements. The majority of soils are near-neutral to alkaline with the small proportion of areas with soil pH<5 largely restricted to the major Cu deposits. There is strong correlation between top soil and sub soil geochemical values. Where the concentrations of some elements (including Pb, Hg and Sn) are indicative of contamination, the values are typically higher in the top soil samples in these areas. Variations in the concentration of elements with strong redox controls on mobility are linked to changes in sedimentary environment between deep and shallow marine conditions. Some element patterns can be related to the effects of urbanisation and sulphide mining operations; however the dominant control on soil geochemistry is the parent geology and regolith forming processes. The atlas demonstrates the effectiveness of high-density sampling in mapping local to regional-scale features of the geochemical landscape. © 2012 Elsevier B.V.