Research of the Conductive Structure of Crust and the Upper Mantle beneath the South-Central Tibetan Plateau

With the super-wide band magnetotelluric sounding data of the Jilong (吉隆)-Cuoqin (措勤) profile (named line 800) which was completed in 2001 and the Dingri (定日)-Cuomai (措迈) profile (named line 900) which was completed in 2004,we obtained the strike direction of each MT station by strike analysis,then traced profiles that were perpendicular to the main strike direction,and finally obtained the resistivity model of each profile by nonlinear conjugate gradients (NLCG) inversion. With these two models,we described the resistivity structure features of the crust and the upper mantle of the center-southern Tibetan plateau and its relationship with Yalung Tsangpo suture: the upper crust of the research area is a resistive layer with resistivity value range of 200-3 000 ?·m. The depth of its bottom surface is about 15-20 km generally,but the bottom surface of resistive layer is deeper in the middle of these two profiles. At line 900,it is about 30 km deep,and even at line 800,it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15-45 km,and a conductive layer is beneath it with resistivity even less than 5 ?·m. This conductive layer is composed of individual conductive bodies,and at the south of the Yalung Tsangpo suture,the conductive bodies are smaller with thickness about 10 km and lean to the north slightly. However,at the north of the Yalung Tsangpo suture,the conductive bodies are larger with thickness about 30 km and also lean to the north slightly. Relatively,the conductive bodies of line 900 are thinner than those of line 800,and the depth of the bottom surface of line 900 is also shallower. At last,after analyzing the effect factors to the resistivity of rocks,it was concluded that the very conductive layer was caused by partial melt or connective water in rocks. It suggests that the middle and lower crust of the center-southern Tibetan plateau is very thick,hot,flabby,and waxy.     

The Othris Ophiolite, Greece: A snapshot of subduction initiation at a mid-ocean ridge

The mantle section of the Tethyan-type Othris Ophiolite, Greece, records tectono-magmatic processes characteristic of both mid-ocean ridges and supra-subduction zones. The Othris Ophiolite is a remnant of the Jurassic Neotethys Ocean, which existed between Eurasia and Gondwanaland. Othris peridotites range from fertile plagioclase lherzolites to depleted harzburgites. Abundances of Al₂O₃ and CaO show well-defined inverse linear correlations with MgO, suggesting that the Othris peridotites formed as residua from variable degrees of partial melting.

Peridotites from the Fournos Kaïtsa and western Katáchloron sub-massifs are similar to abyssal peridotites and can be explained by a multistage model with some melting in the garnet stability field followed by moderate degrees of anhydrous near-fractional melting in the spinel stability field. In contrast, the peridotites from the Metalleio, Eretria, and eastern Katáchloron sub-massifs, and the Vourinos ophiolite are highly depleted and have extremely low concentrations of Al₂O₃ and heavy rare earth elements. These peridotites have enriched light REE contents compared to the middle REE. These residua are best modelled by hydrous melting due to a flux of slab-derived fluid to the mantle wedge during melting.

The occurrence of both styles of melting regimes within close spatial and temporal association in the same ophiolite is explained by intra-oceanic thrusting and forced subduction initiation at (or near) a mid-ocean ridge. Thus, the Othris Ophiolite, and probably Tethyan-type ophiolites in general, represent a transient phase of plate tectonic reorganisation rather than quasi-steady state plate tectonics.     

Neogene stratigraphy and the sedimentary and oceanographic development of the NW European Atlantic margin

A regional correlation of Neogene stratigraphy has been attempted along and across the NW European Atlantic continental margin, between Mid-Norway and SW Ireland. Two unconformity-bounded successions are recognised. These are referred to as the lower and upper Neogene successions, and have been dated as Miocene–early Pliocene and early Pliocene–Holocene, respectively, in age. Their development is interpreted to reflect plate-wide, tectonically driven changes in the sedimentary, oceanographic and latterly climatic evolution of the NE Atlantic region. The lower Neogene succession mainly preserves a record of deep-water sedimentation that indicates an expansion of contourite sediment drifts above submarine unconformities, within this succession, on both sides of the eastern Greenland–Scotland Ridge from the mid-Miocene. This is interpreted to record enhanced deep-water exchange through the Faroe Conduit (deepest part of the Southern Gateway), and can be linked to compressive inversion of the Wyville–Thomson Ridge Complex. Thus, a pervasive, interconnected Arctic–North Atlantic deep-water circulation system is a Neogene phenomenon. The upper Neogene succession records a regional change, at about 4 Ma, in the patterns of contourite sedimentation (submarine erosion, new depocentres) coeval with the onset of rapid seaward-progradation of the continental margin by up to 100 km. This build-out of the shelf and slope is inferred to record a marked increase in sediment supply in response to uplift and tilting of the continental margin. Associated changes in deep-water circulation may be part of an Atlantic-wide reorganisation of ocean bottom currents. Glacial sediments form a major component of the prograding shelf margin (shelf-slope) sediment wedges, but stratigraphic data indicate that the onset of progradation pre-dates significant high-latitude glaciation by at least 1 Ma, and expansive Northern Hemisphere glaciation by at least 3 Ma.     

Cenozoic alongslope processes and sedimentation on the NW European Atlantic margin

Based on studies of sediment accumulations deposited from-and erode by-alongslope flowing ocean currents on the European continental margin from Porcupine (Ireland) to Lofoten (Norway), the evolution of the Cenozoic paleocirculation was reconstructed as part of the STRATAGEM project. There is evidence of ocean current-controlled erosion and deposition in the Rockall Trough, in the Faeroe-Shetland Channel and on the Vøring Plateau since the late Eocene, although the circulation pattern remains ambiguous. The late Palaeogene flow in the Rockall Trough was almost probably driven by southerly-derived Tethyan Outflow Water. The extent and strength of any northerly-derived flow is uncertain. From the early Neogene (early-mid-Miocene), there was a massive regional expansion of contourite drift development both in the North Atlantic and in the Norwegian-Greenland Sea. This was most probably related to the development of the Faroe Conduit, the opening of the Fram Strait and the general subsidence of the Greenland-Scotland Ridge. These may have combined to cause a considerable acceleration in the exchange and overflow of deep waters between the Arctic and Atlantic Oceans. An early late Neogene (late early Pliocene) regional erosional event has been ascribed to a vigorous pulse of bottom-current activity, most probably the result of a global reorganisation of ocean currents associated with the closure of the Central American Seaway. During the late Neogene, contourites and sediment drifts developed in deep-water basins, between units of glacigenic sediments as well as infill of several paleo-slide scars. These sediments were derived from areas of bottom-current erosion as well as from the development of Plio-Pleistocene prograding sediment wedges, incorporating the extensive sediment supply derived from shelf-wide ice sheets. Presently a profound winnowing prevails along the shelf and upper slope due to the inflowing currents of Atlantic water. Depocentres of sediments derived from the winnowing are located (locally) in lower slope embayments and in slide scars.     

Hydration-induced climb dissociation of dislocations in naturally deformed mantle olivine

Widely dissociated dislocations have been observed in mantle olivine from the Erro-Tobbio peridotite in N.W. Italy. Analysis of diffraction contrast in transmission electron microscopy (TEM) indicates that the dissociation reaction involves the climb dissociation on (001) and {021} planes of b=[001] unit dislocations into partial dislocations with Burgers vectors approximately equal to 1/x 011. In the most extreme case a unit dislocation dissociates into four partials which bound three planar defects. The unusually wide dissociation and the greater extent of dissociation in olivine from amphibole-bearing rocks suggests that the dissociation is related to hydration. The occurrence of fluid inclusions along the dislocations confirms that the partials and planar defects are saturated with volatiles. Analysis of possible planar defect structures in Fo₉₀ shows that; (i) the most likely partial Burgers vectors are b=0 3/11 1/4; (ii) two of the planar defects are cation-deficient and can be stabilised by segregation of H⁺ to produce (Mg, Fe) (OH)₂ layers which are iso-structural with the OH-rich interlayer of the humite group minerals; (iii) the central planar defect is formed by removing a stoichiometric olivine (002) layer so does not produce any local chemical changes. The climb dissociation provides a possible mechanism for the transformation of olivine to a humite group mineral. OH-rich interlayers may nucleate on dislocations and extend into the crystal by climb resulting in a gradual increase of (Mg, Fe) (OH)₂ content. The only addition of material required is hydrogen which can rapidly diffuse into olivine. If the dissociation is stable and occurs at high temperatures and pressures it may significantly influence the nature and kinetics of deformation mechanisms and the olivine — spinel shear transformation mechanism in hydrated olivine.     

Lateglacial ice-cap dynamics in NW Scotland: evidence from the fjords of the Summer Isles region

The seaboard of western Scotland is a classic fjord landscape formed by glaciation over at least the last 0.5 Ma. We examine the glacial geology preserved in the fjords (or sea lochs) of the Summer Isles region of NW Scotland using high-resolution seismic data, multibeam swath bathymetry, seabed sediment cores, digital terrain models, aerial photographs, and field investigations. Detailed analyses include seismic facies and lithofacies interpretations; sedimentological and palaeoenvironmental analyses; and radiocarbon dating of selected microfauna. Our results indicate that the Pleistocene sediments of the Summer Isles region, on- and offshore, can be subdivided into several lithostratigraphic formations on the basis of seismic character, geomorphology and sedimentology. These are: subglacial tills; ice-distal and glacimarine facies; ice-proximal and ice-contact facies; moraine assemblages; and Holocene basin fill. The submarine landscape is also notable for its large-scale mass-movement events – the result of glaciodynamic, paraglacial or seismotectonic processes. Radiocarbon dating of marine shells indicate that deglaciation of this part of NW Scotland was ongoing between 14 and 13 ka BP – during the Lateglacial Interstadial (Greenland Interstadial 1) – consistent with cosmogenic surface-exposure ages from previous studies. A sequence of numerous seafloor moraine ridges charts oscillatory retreat of the last ice sheet from a buoyant calving margin in The Minch to a firmly grounded margin amongst the Summer Isles in the early part of Lateglacial Interstadial (GI-1) (pre-14 ka BP). Subsequent, punctuated, frontal retreat of the ice mass occurred in the following ～1000 years, during which time ice-cap outlet glaciers became topographically confined and restricted to the fjords. A late-stage readvance of glaciers into the inner fjords occurred soon after 13 ka BP, which calls into question the accepted limits of ice extent during the Younger Dryas Stadial (Greenland Stadial 1). We examine the wider implications of our chronostratigraphic model, discussing the implications for British Ice Sheet deglaciation, Lateglacial climate change, and the style and rates of fjord sedimentation.     

Sedimentology and palaeoenvironment of the southeast Sicilian Tertiary platform carbonates

A thick succession of shelf and shelf-slope carbonates is developed in southeastern Sicily. Sedimentological aspects of the Oligocene to Tortonian sequence, including distribution and palaeoenvironmental factors, are considered. A new formation and four new members are proposed in the upper part of the succession.The region comprises a western half, typically exhibiting the principal deeper-water planktonic facies including marls, and an eastern area displaying typically inner-shelf carbonates and an attenuated succession. Coralline algal limestones and calcarenites are more typical of this latter region but there are also some volcanic intercalations. An attempt is made to relate the Hyblean succession to that of the Maltese Islands which lie at the southern end of this Tertiary carbonate platform.