Showing papers on "Oceanic plateau published in 1998"

Oceanic plateau formation: a cause of mass extinction and black shale deposition around the Cenomanian-Turonian boundary?

Abstract: The Cenomanian–Turonian boundary (90.4 Ma) represents a major period of worldwide environmental disturbance. The physical manifestations of this are: elevated atmospheric and oceanic temperatures; a significant sea-level transgression; and a period of widespread anoxia, leading to the formation of oceanic black shales, and the extinction of 26% of all genera. Elevated δ 13 C values and enrichment of trace elements in Cenomanian–Turonian boundary sediments, combined with a reduction in 87 Sr/ 86 Sr, also imply a severe environmental perturbation. At this time oceanic crustal production rates reached their highest level of the last 100 million years. This was principally caused by extensive melting of hot mantle plumes at the base of the oceanic lithosphere, and the development of vast areas (up to 1×10 6 km 2 ) of thickened oceanic crust in the Pacific and Indian Oceans. The anomalous volcanism associated with the formation of these oceanic plateaux may have been responsible for the environmental disturbances c. 90 Ma. These eruptions would also have resulted in the emission of large quantities of CO 2 into the atmosphere, leading to global warming. Additionally, the emission of SO 2 , H 2 S, CO 2 and halogens into the oceans would have made seawater more acidic resulting in the dissolution of carbonate, and further release of CO 2 This run-away greenhouse effect was probably put into reverse, by the decline of the anomalous volcanic activity, and by increased (CO 2 -driven) productivity in oceanic surface waters,leading to increased organic carbon burial, black shale deposition, anoxia and mass extinction in the ocean basins

Tectonic setting and terrane accretion of the Archean Zimbabwe craton

Abstract: The Archean Zimbabwe craton is made of a number of distinct tectonostratigraphic terranes assembled by plate tectonic processes. The central Tokwe terrane consists of 3.5–2.95 Ga gneissic rocks and structurally complex inliers of possibly older greenstone belts. These are overlain unconformably by a 2.9–2.8 Ga assemblage of mafic and felsic volcanic rocks and conglomerates, and a separate 3.0–2.7 Ga southeastward thickening platform sequence of sandstone, shale, and limestone. 2.7 Ga greenstone belts form two distinctly different domains flanking the central terrane. Northwest of the ancient gneissic terrane, ca. 2.7 Ga greenstone belts comprise a series of calc-alkaline lavas and intercalated sedimentary rocks intruded by syn-volcanic plutons. Southeast of the ancient gneissic complex, 2.7 Ga greenstone belts consist of thick piles of tholeiitic basalts overlying ultramafic lavas, resting allochthonously over the shallow-water platform sequence and older gneissic terrane. This division of the Zimbabwe craton is interpreted to show that the central Tokwe terrane had a continental magmatic arc built on its northwestern edge, as its southeastern margin rifted from another fragment, forming the Sea of Umtali. A passive-margin sedimentary wedge formed on the rifted southeastern edge of this ancient continent, and prograded onto the craton during sedimentary and tectonic loading of the craton margin. The southeastern greenstone belts formed as thick oceanic crust (oceanic plateau) in this back-arc basin, and were later obducted on to the rift and passive margin sequence as the Sea of Umtali closed ca. 2.7 Ga. This was followed by intrusion of granitic plutons of the Chilimanzi suite ca. 2.6 Ga in a tectonic regime of intracontinental strike-slip faulting, representing a response to the Zimbabwe-Kaapvaal continent-continent collision. Crustal and lithospheric thickening during intrusion of these late granites may have played a role in stabilizing the Zimbabwe craton and forming the lithospheric root.

Petrographie et geochimie des unites magmatiques de la Cordillere occidentale d'Equateur (0 degrees 30'); implications tectoniques

Abstract: The coast and western Cordillera of Ecuador are made of accreted oceanic terranes. separated from the continental margin by a suture zone containing tectonic slices of mafic rocks. The western Cordillera contains three distinct magmatic units. Ultramafic and mafic cumulates from the suture zone (San Juan slice) represent likely the plutonic roots of oceanic plateau basalts. The mafic cumulates are LREE-depleted and Ta and Pb enriched (primitive mantle). Their Nd and Pb isotopic compositions suggest that they derived from an enriched OIB type mantle source. Pre-Coniacian arc-tholeiites present flat REE patterns, low Pb and Th contents, and high epsilon Nd (T = 100 Ma) (+7.5 to +7.9) which are indicative of their derivation from a mantle source. These arc-tholeiites developed likely in an intra-oceanic setting. The Eocene calc-alkaline lavas differ from the arc-tholeiites because they are LREE-enriched and have lower epsilon Nd (T = 50 Ma) ratios. Their high Pb and Th contents are probably related to crustal assimilation during the magmas ascent. Their Pb isotopic compositions support involvement of subducted pelagic sediments in their genesis. These lavas represent likely the remnants of a continental calc-alkaline magmatic arc. The continental-arc setting of the Eocene lavas demonstrates that these volcanic rocks postdate the accretion of the western Cordillera, upon which they rest unconformably. Therefore, the accretion of the western Cordillera may have occurred in late Palaeocene times, as for part of the oceanic terranes of coastal Ecuador. Nevertheless, the occurrence of a collisional event during late Santonian-early Campanian times is strongly suggested by: (i) the arrival of detrital quartz on oceanic series of the western Cordillera by Campanian-Maastrichtian times. (ii) a regional unconformity locally dated as early Campanian, (iii) the arc-jump observed on coastal Ecuador in Santonian or Campanian times, and finally (iv) a thermal event recognised in the eastern Cordillera around 85-80 Ma.

Paleo-Tethys evolution recorded in the Changning-Menglian Belt, western Yunnan, China

Abstract: As a trunk part of the Paleo-Tethyan archipelago ocean, the Changning-Menglian belt of western Yunnan, China, presents varied volcano-sedimentary records ranging in time from Early Devonian through Middle-Late Triassic. The typical pelagic sediments represented by radiolarites can not only signify a continuously evolved ocean but also constitute a complicated basin comprising different tectono-sedimentary units. It is likely that oceanic plateaus of diverse origins played a key role in constructing the pattern of eastern Tethys. The opening and closing of the Yunnan Tethys are discussed in the paper as well.

A Complex History for the Caribbean Oceanic Plateau: Petrology, Geochemistry and Geochronology of the Beata Ridge

Abstract: The Caribbean province is an area where the thickness of oceanic crust is between 8 and more than 20 kin, well in excess that of 'normal' oceanic crust. This has led to the conclusion, supported by geochemical studies and tectonic reconstructions, that much of the Caribbean plate formed as an oceanic plateau in the eastern Pacific. Most authors accept that the Caribbean plateau formed through melting of the Galapagos plume head. Through eastward movement of the Farallon plate, part of this plateau was entrained between the two American plates while the southern part collided with northern South America. At least two major phases have been recognized in the plateau building. The first, volumetrically most important event was around 90-88 Ma, and the second around 76 Ma. New petrological, geochemical and geochronological data from the Beata topographic ridge, south of Hispaniola, highlight the complexi ty of the Caribbean plateau construction and may require a revision of this model. The Beata ridge is an area of particularly thick crust (up to 20 km) between the Dominican and Haitian sub-basins. The ridge is 2000 to 5000 m deep and is bounded to the west by a normal fault. In this study we focus on the magmatic samples recovered by submersible during the Nautica-Beata cruise. The ridge is mainly composed by gabbros and dolerites, with minor diorite and rare pillow basalt. The gabbros and dolerites contain euhedral plagioclase (An54_62) and clinopyroxene (Wo37_4o En44_47 Fs13 19) grains with a variable but minor amount of olivine and/or orthopyroxene relicts, ilmenite and acicular apatite. The presence of orthopyroxene and the early crystallisation of plagioclase indicate a tholeiitic affinity. Textures that vary significantly, from coarse-grained gabbros to very fine-grained dolerites, reflect differences in cooling rates and suggest a subsurface, hypabyssal environment. The rare basaltic flows contain phenocrysts of weathered olivine, plagioclase (Anso-61) and clinopyroxene (Wo44-47 En40-44 Fs12-16) in a matrix of plagioclase microlites (An5o-55), clinopyroxene microcrysts (Wo45-46 En37-4o Fs15-16) and a glassy, vesicular mesostasis. The early crystallisation of olivine, an absence of orthopyroxene and the late crystallisation of plagioclase suggest an alkaline affinity. The major element compositions of gabbros and dolerites plot on simple trends that correspond to fractional crystallisation of olivine, clinopyroxene and plagioclase (MgO 5-11%; A1203 12-17%; CaO 8-15%). Trace-element ratios are close to chondritic [(Nb/Zr)N 0.85-1.1 ] and rare-earth-element patterns are almost flat [(La/Yb)N 0.63-1.02]. The source was isotopically depleted [~Na +7.4 to +9.5]. The basalts have higher trace-element ratios and enriched rare earth element patterns [(Nb/Zr)N 3.45; (La/Yb)N 28-30]. Their source was less depleted than that of the gabbros and dolerites [eNa +5]. Several samples were dated by the 39Ar/4~ incremental heating method, either on whole rocks or separated plagioclases. Six samples have ages between 84 to 70 Ma and these correspond with previous dates within the province. But others are surprisingly young, with ages at 62, 56 and 50 Ma. There is no apparent order to the ages: all are found in different rock types and from one end of the ridge to the other. Within the younger age group, consistent results were obtained for both whole rocks and plagioclases. In contrast, within the older age group, whole-rocks and plagioclase gave conflicting ages. For this reason, we consider that the young ages are not due to thermal resetting but probably represent a true magmatic event. In contrast, the ages for older

The creation and preservation of Archaean continental crust

Abstract: Thirty years ago Armstrong (1968) proposed that the crust of the Earth was formed by 4.0 Ga and since this time steady-state recycling has maintained it at a constant volume at the surface of the Earth. In his model, the small proportion of old rocks preserved at the surface today were simply remnants of crust that survived the recycling process. More recent models of crustal growth (i.e. All~gre and Rousseau, 1984; McCulloch and Bennett, 1994) invoke the gradual growth of continental crust, punctuated by a series of peaks in production. The latter models imply a relatively simple process of progressive depletion of the upper mantle and growth of crust, in which, by the end of the Al'chaean (2,5 Ga), only 50-60% of the crust had been formed. The input parameters to the models are the radiogenic isotopes of Sr, Nd and Pb, and, to a lesser extent constraints from U/Pb zircon distribution. The Pb and U balance of crust, OIB and MORB provide important constraints on the amount of sediment that is recycled into the mantle, and, based on current fluxes, most models cannot accommodate >10% of recycled sediment. In recent years improvements in analytical techniques have made it possible to obtain reliable estimates of key trace element ratios in Archaean basaltic and ultramafic material (i.e. Jochum et al., 1991) and these authors argue that the sub-chondritic ratios of Nb/U and the depletion in epsilon-Nd in the early Earth are consistent with extraction of a proto-crust, since destroyed. In recent years several authors have stressed the role of accreted large igneous provinces, rather than arc accretion in crustal growth (Kusky and Kidd, 1992, Abouchami et al., 1990). These arguments were in large part based on the identification of the giant mafic-ultramafic basaltic provinces in the ocean basins (Coffin and Eldholm, 1994), and the detection of large tracts of geochemically juvenile material with oceanic plateau characteristics in ancient greenstone sequences.