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Origin, HP/LT metamorphism and cooling of ophiolitic mélanges in southern Evia (NW Cyclades), Greece (2000)

Katzir, Y. ; Avigad, * D. ; Matthews, A. ; [et al.]

Oxford, UK : Blackwell Science Inc

Journal of metamorphic geology 18 (2000), S. 0

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ISSN: 1525-1314

Source: Blackwell Publishing Journal Backfiles 1879-2005

Topics: Geosciences

Notes: Basic and ultrabasic blocks within ophiolitic mélanges of the Cycladic Blueschist Unit in southern Evia provide a detailed insight into its ocean floor igneous and hydrothermal evolution, as well as the regional poly-metamorphism occurring during Alpine orogenesis. The upper structural levels (Mt. Ochi exposures) are dominated by metamorphosed wehrlites, gabbros and highly light rare earth element (LREE)-enriched pillow basalts, whereas the underlying Tsaki mélange consists of basic protoliths with much less fractionated REE patterns as well as mantle harzburgites. Most of the metabasites show Nb anomalies, indicative of derivation from a subduction-affected mantle. The igneous bodies were juxtaposed and incorporated into the enclosing sedimentary sequences prior to high-pressure/low-temperature (HP/LT) metamorphism (M1). Glaucophane, epidote, sodic clinopyroxene and high-Si phengite constitute the Eocene M1 assemblage, which is estimated to have formed at 〉11 kbar and 400–450 °C. High δ18O values of M1 minerals in Ochi metagabbros indicate that the formation of the high-pressure assemblage was controlled by infiltration of fluids from the dehydrating host sediments. Cooling during decompression is indicated by an overprinting (M2, Early Miocene) pumpellyite–actinolite facies assemblage in metabasic rocks, calculated to have developed at P〈8 kbar and T 〈350 °C. Possible mechanisms for such cooling include: exhumation from shallower burial levels relative to the eclogites of the NW Cyclades, accretion of colder rocks from below and extensional unroofing by low-angle normal faults and detachments. The occurrence of sodic augite in the M2 assemblage of Tsaki metagabbros indicates that rocks at the base of the Blueschist Unit cooled faster or longer than their higher level Ochi counterparts. This suggests that differential cooling of the blueschists was enhanced by the underthrusting of colder rock units.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1046/j.1525-1314.2000.00281.x

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Fluid-rock interaction and thermal evolution during thrusting of an Alpine metamorphic complex (Tinos island, Greece) (1999)

Matthews, A. ; Lieberman, J. ; Avigad, D. ; [et al.]

Springer

Contributions to mineralogy and petrology 135 (1999), S. 212-224

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract This study examines the fluid-rock interaction and thermal evolution along a thrust that juxtaposes calcite-rich marbles of high P-T metamorphic unit of the Attic-Cycladic Massif (Greece) on top of a lower-grade dolomite marble unit. The Tertiary thrust represents a major phase of tectonic movement related to the decompression of the Alpine orogen in the Hellenides. The stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions. The depletion is most pronounced in calcite, but is almost entirely missing in coexisting dolomite. The isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water-rich H2O-CO2-CH4 fluid [X C (=X CO 2+X CH 4)〈0.05] at water-rock ratios on the order of 0.1 to 0.5 by weight. The fluid-induced calcite recrystallization is viewed as an important rheological control during thrusting. The temperature evolution of the footwall, hanging wall and mylonitic tectonic contact was determined by calcite-dolomite solvus thermometry. Histograms of calcite-dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate. Conversely, the hanging wall marble unit was cooled during the thrusting. The calcite-dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units, and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050507

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A thin orogenic wedge upon thick foreland lithosphere and the missing foreland basin (1998)

Garfunkel, Z. ; Greiling, R. O.

Springer

Geologische Rundschau 87 (1998), S. 314-325

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ISSN: 0016-7835

Keywords: Key words Collisional orogen ; Concave orogenic wedge ; Lithospheric flexure ; Modelling tectonic loads ; Proforeland basin

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Along the Caledonian front in central Scandinavia, the expected peripheral or pro-foreland basin is neither physically present nor are there any significant traces in the sedimentary record. In order to explain and quantify this situation, the authors assess the major geometric and mechanical constraints on the Caledonian orogenic wedge and model the orogenic load and its influence on the foreland lithosphere of Baltica. Geologic and geophysical data show a strong foreland lithosphere with a flexural parameter (α) of approximately 100 km. The shape of the orogenic wedge and its critical taper angle are dependent mainly on basal friction and wedge strength. In the external part organic-rich black shales provide a low-friction horizon both at the basal detachment surface and within the wedge itself. The more internal part of the wedge is composed of metamorphic and crystalline rocks, which cooled and strengthened prior to thrusting. As a result, the external part of the wedge had a lower strength and a smaller critical taper angle than its internal part, so the orogenic load is upward concave. Modelling of the effect of such a load on the Baltica lithosphere shows a very small depression in front of the load (2 km). The flexural depression produced by the main part of the orogenic load is filled up by the thickening thrust-and-fold belt, so that there is little space left for a foreland basin. These results imply that the missing foreland basin in front of the central Scandinavian Caledonides is not due to subsequent erosion, but is a primary feature.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s005310050212

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Constraints on the structural development of Afar imposed by the kinematics of the major surrounding plates (2007)

Garfunkel, Z. ; Beyth, M.

In: Geological Society Special Publication 259: 23-42.

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Publication Date: 2007-10-08

Description: The development of the Afar rift-rift-rift triple junction is analysed from the view-point of the Nubia, Arabia and Somali plate kinematics. A variety of constraints allow definition of a range of kinematic models that approximate well to the plate motions with a resolution of a couple to tens of kilometres. Rigid plate kinematics probably cannot resolve smaller motions. The size and location of the new area that opened between the major plates is inferred from plate kinematics and this provides a framework in which to assess the structural development that accommodates plate separation. The development of the Afar region was complicated by the presence of microplates the Danakil and Aisha blocks which results in a complex plate boundary geometry. This led to local deformation that does not directly reflect the divergence of the major plates, e.g. rotations of microplates and of minor blocks about vertical axes and strike-slip faulting. The opening of new area was accommodated by various crustal growth and accretion mechanisms, e.g. building of thick new igneous crust, normal seafloor spreading, and/or crustal stretching. Thus, plate motions by themselves do not determine the development of the plate boundaries, as this is strongly influenced by other factors such as lateral variations of the rates of magma supply (e.g. away from, and over, the Afar plume). The plate boundaries changed e.g. the Gulf of Aden spreading centre propagated westward c. 2 Ma ago and normal seafloor spreading began along portions of the Red Sea axis since c. 5 Ma ago while there were no resolvable changes in the plate motions. Such changes therefore signify a reorganization of the way in which plate divergence and addition of new area is accommodated: diffuse extension may give way to separation along a narrow spreading centre, or new plate boundaries may form at the expense of other boundaries that became inactive.

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The geodynamic evolution of the Alpine orogen in the Cyclades (Aegean Sea, Greece): insights from diverse origins and modes of emplacement of ultramafic rocks (2007)

Katzir, Y. ; Garfunkel, Z. ; Avigad, D. ; [et al.]

In: Geological Society Special Publication 291: 17-40.

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Publication Date: 2007-12-12

Description: The Alpine orogen in the Cyclades, wherein both high-pressure metamorphic rocks and ultramafic rocks co-occur, is a key area in studying the emplacement of mantle rocks into the crust. Within the Cyclades three distinct ultramafic associations occur: (1) HPLT ophiolitic melanges of the Cycladic Blueschist Unit (CBU) on Evia and Syros; (2) meta-peridotites associated with migmatized leucogneisses on Naxos, which represent the deepest exposed levels of the CBU; (3) a greenschist-facies metamorphosed dismembered ophiolite juxtaposed on top of the CBU by an extensional detachment on Tinos. Most of the Cycladic ultramafic rocks were serpentinized prior to Alpine metamorphism, suggesting denudation prior to reburial. The Naxos meta-peridotites preserve, however, relict mantle assemblage and mantle-like oxygen isotope ratios, and thus indicate direct emplacement from the mantle into an underthrust continent during collision and HP metamorphism (M1). Thus conditions for M1 in the Naxos leucogneiss core are constrained by ultramafic assemblages to 550650 {degrees}C and [≥]14 kbar. Mafic blocks of the ophiolitic melanges in the NW Cyclades span a wide range of chemical compositions indicating derivation from variable oceanic settings and sequential events of alteration and metasomatism. Given the comparable geochemical heterogeneity in the Syros and Evian melange intervals, the garnet-bearing meta-basites of the Syros melange record higher M1 temperatures (450500 {degrees}C) than the garnet-free epidote blueschists of the Evian melanges (400430 {degrees}C). It follows that going southeastwards from Evia progressively deeper (i.e. hotter) levels of the subducted plate are exposed. Correspondingly, temperatures of the M2 overprint also increase from pumpellyite-bearing assemblages on southern Evia, through greenschists on Syros to upper-amphibolite, sillimanite-bearing gneisses on Naxos. The diverse PT paths of the CBU form an array wherein the deeper a rock sequence is buried, the hotter' is its exhumation path. Such a pattern is predicted by thermal modelling of tectonically thickened crust unroofed by either erosion or uniform extension.

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Origin and nature of the rapid late Tertiary filling of the Levant Basin (2011)

Steinberg, J. ; Gvirtzman, Z. ; Folkman, Y. ; Garfunkel, Z.

Geological Society of America (GSA)

In: Geology 39: 355-358.

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Publication Date: 2011-04-01

Description: The recent huge gas discovery in Early Miocene siliciclastics, offshore Israel, highlighted a new world-class gas province, and has subsequently sparked great interest in the Oligocene-Miocene deep-water sediments of the Levant Basin. Based on a dense network of seismic data, we present two fundamental observations. First, more than half of its sedimentary column accumulated within a mere [~]15% of the basin's life span, i.e. an [~]6-km-thick section accumulated within 37 m.y. (post-Middle Eocene). Second, this young section first accumulated in the deep basin and only then did large amounts of sediments amass along the eastern Levant continental margin. These fundamental observations allude to two feasible source-to-sink scenarios: (1) the thick late Tertiary section is composed of large amounts of terrigenous material that originated in Arabia and was transported via the Israeli margin, and (2) the terrigenous material originated in Africa and was transported northward via the region that eventually evolved into the Nile River cone. Our data emphasize the difficulties of the first scenario and suggest that Africa was the dominant sediment source. This suggestion requires the submarine transport of sediments from offshore North Africa to have been northeastward, and thus raises new questions regarding ocean circulation at a time in which the neo-Tethys was closing and the marine connection to the Mesopotamian Basin gradually ceased.

Print ISSN: 0091-7613

Electronic ISSN: 1943-2682

Topics: Geosciences