Miocene to Pleistocene sedimentation at ODP Site 133-823

More than 2000 turbidite, debris-flow, and slump deposits recovered at Site 823 record the history of the Queensland Trough since the middle Miocene and provide new insights about turbidites, debris flow, and slump deposits (herein termed gravity deposits). Changes in the composition and nature of gravity deposits through time can be related to tectonic movements, fluctuations in eustatic sea level, and sedimentological factors. The Queensland Trough is a long, relatively narrow, structural depression that formed as a result of Cretaceous to Tertiary rifting of the northeastern Australia continental margin. Thus, tectonics established the geometry of this marginal basin, and its steep slopes set the stage for repeated slope failures. Seismic data indicate that renewed faulting, subsidence, and associated tectonic tilting occurred during the early late Miocene (continuing into the early Pliocene), resulting in unstable slopes that were prone to slope failures and to generation of gravity deposits. Tectonic subsidence, together with a second-order eustatic highstand, resulted in platform drowning during the late Miocene. The composition of turbidites reflects their origin and provides insights about the nature of sedimentation on adjacent shelf areas. During relative highstands and times of platform drowning, planktonic foraminifers were reworked from slopes and/or drowned shelves and were redeposited in turbidites. During relative lowstands, quartz and other terrigenous sediment was shed into the basin. Quartzose turbidites and clay-rich hemipelagic muds also can record increased supply of terrigenous sediment from mainland Australia. Limestone fragments were eroded from carbonate platforms until the drowned platforms were buried under hemipelagic sediments following the late Miocene drowning event. Bioclastic grains and neritic foraminifers were reworked from neritic shelves during relative lowstands. During the late Pliocene (2.6 Ma), the increased abundance of bioclasts and quartz in turbidites signaled the shallowing and rejuvenation of the northeastern Australia continental shelf. However, a one-for-one relationship cannot be recognized between eustatic sea-level fluctuations and any single sedimentologic parameter. Perhaps, tectonism and sedimentological factors along the Queensland Trough played an equally important role in generating gravity deposits. Turbidites and other gravity deposits (such as those at Site 823) do not necessarily represent submarine fan deposits, particularly if they are composed of hemipelagic sediments reworked from drowned platforms and slopes. When shelves are drowned and terrigenous sediment is not directly supplied by nearby rivers/point sources, muddy terrigenous sediments blanket the entire slope and basin, rather than forming localized fans. Slope failures affect the entire slope, rather than localized submarine canyons. Slopes may become destabilized as a result of tectonic activity, inherent sediment weaknesses, and/or during relative sea-level lowstands. For this reason, sediment deposits in this setting reflect tectonic and eustatic events that caused slope instabilities, rather than migration of different submarine fan facies.

Tab. 1: Geochemical analysis and age dating of Kap Washington Group volcanites and related dykes

The bimodal, alkaline volcanic suite of the Kap Washington Group (KWG) at the northern coast of Greenland was investigated during the BGR CASE 2 expedition in 1994. Geochemical and Nd and Sr isotopic data are presented for basalts to rhyolites of the KWG and of related basaltic dykes cutting Lower Paleozoic sediments. In the evd(t) vs. (87Sr/86Sr)t diagram, the KWG basalts and rhyolites follow a common mixing trend with increasing crustal contamination from basic to acid volcanites. Assimilation of pre-existing crustal rocks during formation of the rhyolitic melt is documented by Nd model ages of 0.9-1.2 Ga and by different fractionation trends for the basalts and the rhyolites in the Y vs. Zr diagram. Petrographical and geochemical features indicate intra-plate volcanism which was active most probably during a continental rifting phase. A new Rb/Sr whole rock age on rhyolites of 64 ±3 Ma, corresponding to the result of LARSEN (1982), confirms that the volcanic activity lasted until the Cretaceous-Tertiary boundary. 40Ar139Ar dating on amphibol separates from a comendite yielded strongly disturbed age spectra with a minimum age of 37.7 ±0.3 Ma. This age is interpreted to date a hydrothermal overprint of the volcanic rocks related to compressive tectonics which led to the overthrust of basement rocks over the Kap Washington Group.

Lithogeochemical and isotope (Rb-Sr, Sm-Nd, Pb-Pb whole rock and Lu-Hf zircon) composition of samples from the Shackleton Range, East Antarctica

Three distinct, spatially separated crustal terranes have been recognised in the Shackleton Range, East Antarctica: the Southern, Eastern and Northern Terranes. Mafic gneisses from the Southern Terrane provide geochemical evidence for a within-plate, probably back-arc origin of their protoliths. A plume-distal ridge origin in an incipient ocean basin is the favoured interpretation for the emplacement site of these rocks at c. 1850 Ma, which, together with a few ocean island basalts, were subsequently incorporated into an accretionary continental arc/supra-subduction zone tectonic setting. Magmatic underplating resulted in partial melting of the lower crust, which caused high-temperature granulite-facies metamorphism in the Southern Terrane at c. 1710-1680 Ma. Mafic and felsic gneisses there are characterised by isotopically depleted, positive Nd and Hf initials and model ages between 2100 and 2000 Ma. They may be explained as juvenile additions to the crust towards the end of the Palaeoproterozoic. These juvenile rocks occur in a narrow, c. 150 km long E-W trending belt, inferred to trace a suture that is associated with a large Palaeoproterozoic accretionary orogenic system. The Southern Terrane contains many features that are similar to the Australo-Antarctic Mawson Continent and may be its furthermost extension into East Antarctica. The Eastern Terrane is characterised by metagranitoids that formed in a continental volcanic arc setting during a late Mesoproterozoic orogeny at c. 1060 Ma. Subsequently, the rocks experienced high-temperature metamorphism during Pan-African collisional tectonics at 600 Ma. Isotopically depleted zircon grains yielded Hf model ages of 1600-1400 Ma, which are identical to Nd model ages obtained from juvenile metagranitoids. Most likely, these rocks trace the suture related to the amalgamation of the Indo-Antarctic and West Gondwana continental blocks at ~600 Ma. The Eastern Terrane is interpreted as the southernmost extension of the Pan-African Mozambique/Maud Belt in East Antarctica and, based on Hf isotope data, may also represent a link to the Ellsworth-Whitmore Mountains block in West Antarctica and the Namaqua-Natal Province of southern Africa. Geochemical evidence indicates that the majority of the protoliths of the mafic gneisses in the Northern Terrane formed as oceanic island basalts in a within-plate setting. Subsequently the rocks were incorporated into a subduction zone environment and, finally, accreted to a continental margin during Pan-African collisional tectonics. Felsic gneisses there provide evidence for a within-plate and volcanic arc/collisional origin. Emplacement of granitoids occurred at c. 530 Ma and high-temperature, high-pressure metamorphism took place at 510-500 Ma. Enriched Hf and Nd initials and Palaeoproterozoic model ages for most samples indicate that no juvenile material was added to the crust of the Northern Terrane during the Pan-African Orogeny but recycling of older crust or mixing of crustal components of different age must have occurred. Isotopically depleted mafic gneisses, which are spatially associated with eclogite-facies pyroxenites, yielded late Mesoproterozoic Nd model ages. These rocks occur in a narrow, at least 100 km long, E-W trending belt that separates alkaline ocean island metabasalts and within-plate metagranitoids from volcanic arc metabasalts and volcanic arc/syn-collisional metagranitoids in the Northern Terrane. This belt is interpreted to trace the late Neoproterozoic/early Cambrian Pan-African collisional suture between the Australo-Antarctic and the combined Indo-Antarctic/West Gondwana continental blocks that formed during the final amalgamation of Gondwana.

Geochemistry of metamorphic rocks from the Mariana and Izu-Ogasawara forearcs

During Leg 125, two serpentinite seamounts were drilled in the Mariana and Izu-Ogasawara forearcs. Together with abundant serpentinized peridotites, low-grade metamorphic rocks were recovered from both seamounts. The metamorphic rocks obtained from Hole 778A on Conical Seamount on the Mariana forearc contain common blueschist facies minerals, lawsonite, aragonite, blue amphibole, and sodic pyroxene. Approximate metamorphic conditions of these rocks are 150° to 250° C and 5 to 6 kb. These rocks are considered to have been uplifted by diapirism of serpentinite from a deeper portion within the subduction zone. This discovery presents direct evidence that blueschist facies metamorphism actually takes place within a subduction zone and provides new insight about trench-forearc tectonics. The diagnostic mineral assemblage of the metamorphic rocks from Holes 783A and 784A on Torishima Forearc Seamount, in the Izu-Ogasawara region, is actinolite + prehnite + epidote, with a subassemblage of chlorite + quartz + albite + H2O, which is typical of low-pressure type, prehnite-actinolite facies of Liou et al. (1985). This metamorphism may represent ocean-floor metamorphism within trapped oceanic crust or in-situ metamorphism that occurred at depths beneath the island-arc.

Results of biological and geological bottom investigations carried out during Cruise 43 of R/V Akademik Kurchatov in the South Atlantic

The book is devoted to investigations of benthic fauna and geology of the Southern Atlantic Ocean. These works have been carried out in terms of exploring biological structure of the ocean and are of great importance for development of this fundamental problem. They are based on material collected during Cruise 43 of R/V Akademik Kurchatov in 1985-1986 and Cruise 43 of R/V Dmitry Mendeleev in 1989. Problems of quantitative distribution, group composition and trophic structure of benthos in the Southern Scotia Sea, along the east-west Transatlantic section along 31°30'S, and offshore Namibia in the area of the Benguela upwelling are under consideration in the book. Authors present new data on fauna of several groups of deep-sea bottom animals and their zoogeography. Much attention is paid to analysis of morphological structure of the Scotia Sea floor considered in terms of plate tectonics. Bottom sediments along the Transatlantic section and facial variation of sediments in the area of South Shetland Islands and of the continental margin of Namibia are under consideration.

Stress field of basaltic rocks of ODP Hole 127-794C

We carried out an experiment to estimate in-situ stresses at ODP Hole 794C (water depth: 2809 m) from the basaltic core samples by deformation rate analysis (DRA). Site 794 is located at the northern end of the Yamato Basin and 70 km west of the eastern Japan Sea intraplate or interplate convergent zone. Stress previously applied to a rock specimen is identified in the inelastic strain behavior of the specimen under uniaxial compression by the method used. Natural remanent magnetization of the sample was also measured to get a reference for the orientation of the horizontal stresses. The vertical, maximum, and minimum horizontal in-situ stresses estimated at a depth of 582 mbsf are 36.4, 43.1, and 31.2 MPa, respectively. The average of the largest and the least horizontal stresses is nearly equal in value to the vertical stress. This suggests that the site is in the stress field of the strike slip regime at the depth, while the stress field of the reverse fault regime has been estimated from the focal mechanism solutions of the earthquakes whose hypocenters are located near or on the convergent boundary. The directions of the largest and the least horizontal stress are estimated to be northeast-southwest and in northwest-southeast, respectively, in taking account of rotation tectonics of the Japan Sea since its formation. The directions of the largest and the least horizontal stresses are opposite to those determined from the earthquakes. These discrepancies of our results with those from earthquakes may be due mainly to the fact that the site is not in the convergent zone.