ALBERT

Beschreibung: Terra Nova, 23, 195–205, 2011 Abstract The Tian Shan range offers a natural laboratory to study orogenic processes. Most of the previous studies focused on either the Palaeozoic evolution of the range or its Cenozoic intracontinental evolution linked with the India–Asia collision. In this study, it is shown that detailed field investigations on the relationship between sedimentary cover and basement constrain the Mesozoic evolution of the northern Tian Shan. Sedimentological observations argue for a limited transport distance for Lower and Uppermost Jurassic deposits. Geological sections presented in this paper show that, in preserved locations, Triassic to Jurassic sedimentary series present a continuous onlap type sedimentary unconformity on the top of the basement. At different scales, observations clearly evidence the existence of major palaeorelief during the Mesozoic. According to the present study, the topography of the current Tian Shan range and the associated movements along its northern front structures cannot be considered as the consequence of Cenozoic reactivation alone.

Beschreibung: The Ediacaran–Cambrian transition was one of the most critical intervals in Earth history. During this interval, widespread chert was precipitated, commonly as a stratal wedge in carbonates, along the southern margin of the Yangtze Platform, South China. The chert wedge passes into a full chert succession further basinward to the south-east. Four lithotypes of chert are identified across the marginal zone in western Hunan: mounded, vein, brecciated and bedded chert. The mounded chert is characterized by irregular to digitiform internal fabrics, generally with abundant original vesicles and/or channels that mostly are lined by botryoidal chalcedony cements with minor quartz and barite crystals. The host chert (or matrix) of these mounds is dominated by amorphous cryptocrystalline silica, commonly disseminated with pyrite. The vein chert, with minor quartz locally, generally cross-cuts the overlying dolostone and chert horizons and terminates under the mounded and/or bedded chert bodies. The brecciated chert commonly occurs as splayed ‘intrusions’ or funnel-shaped wedges and cross-cuts the topmost dolostones. The bedded chert, the most common type, generally is thin to medium-bedded and laminated locally; it is composed of amorphous silica with minor amounts of black lumps. Microthermometry of fluid inclusions from vein and void-lining minerals (mainly quartz and barite) revealed homogenization temperatures from 120 to 180°C for the trapped primary fluids. Compositionally, these chert deposits generally are pure, with SiO 2  〉 92 wt%, and only minor Fe 2 O 3 and Al 2 O 3 contents, most of which show positive Europium anomalies in rare earth element patterns, especially for the mounded chert. All these data suggest that the marginal zone chert deposits resulted from a low-temperature, silica-rich hydrothermal system, in which the mounded chert was precipitated around the releasing vents, i.e. as silica chimneys. The vein and splayed brecciated chert, however, was formed along the syndepositional fault/fracture conduits that linked downward, while the bedded chert was precipitated in the quieter water column from the fallout of hydrothermal plumes onto the sea floor. These petrological and geochemical data provide compelling evidence and a new clue to the understanding of the extensive silica precipitation; rapid tectono-depositional and oceanic changes during the Ediacaran–Cambrian transition in South China.

Beschreibung: Detrital zircons from Cretaceous micaschist, Late Eocene-Earliest Oligocene sandstone and Early Oligocene siltstone of the Western Alps fall into three main separable age clusters at 610-540 Ma, 490-430 Ma, and 340-280 Ma that correspond to the Cadomian (Neoproterozoic), Ordovician and Variscan (Carboniferous) events widespread in Western and Central Europe. Hf isotopic results indicate that these three magmatic and tectonic episodes did not give rise to significant production of juvenile crust. A distinguishable group of Triassic zircons, around 250-200 Ma which is considered to derive from the Southern Alps, has been detected in the Early Oligocene "Schistes à Blocs" formation and the Brianconnais “Flysch Noir”. In contrast, this age group is absent in Late Eocene-Earliest Oligocene sandstones. In agreement with sedimentological studies, our results show that the main source areas of the Eocene sandstone were probably located in the European continent. The arrival of detritus from the Internal Zone occurred in Early Oligocene, coeval with the tectonic rotation from northwestward to westward in the propagation of allochthonous units. Based on previous studies and our new data, we argue that the the Brianconnais Zone was likely a paleorelief since the Middle Eocene that accounts for the lack of detritus from the Adriatic units. Contemporary sediments were accumulated in the foredeep of the Adriatic Plate. From Oligocene time onwards, the blockage was cut through after a regional uplifting, and thus the Internal Zone started to provide detritus into the western flexural basins.

Beschreibung: This work firstly presents field structural analysis, Anisotropy of Magnetic Susceptibility (AMS) measurements, kinematic and microstructural studies on the Neoproterozoic Pengguan complex located in the middle segment of the Longmenshan thrust belt (LMTB), NE Tibet. These investigations indicate that the Pengguan complex is a heterogeneous unit with a ductilely deformed NW domain and an undeformed SE domain, rather than a single homogeneous body as previously thought. The NW part of the Pengguan complex is constrained by top-to-the-NW shearing along its NW boundary and top-to-the-SE shearing along its SE boundary, where it imbricates and overrides the SE domain. Two orogen-perpendicular gravity models not only support the imbricated shape of the Pengguan complex, but also reveal an imbrication of high density material hidden below the Paleozoic rocks to the west of the LMTB. Regionally, this suggests a basement-slice-imbricated structure that developed along the margin of the Yangtze Block, as shown by the regional gravity anomaly map, together with the published nearby seismic profile and the distribution of orogen-parallel Neoproterozoic complexes. Integrating the previously published ages of the NW-normal faulting and of the SE-directed thrusting, the locally fast exhumation rate, and the lithological characteristics of the sediments in the LMTB front, we interpret the basement-slice-imbricated structure as the result of SE-ward thrusting of the basement slices during the Late Jurassic-Early Cretaceous. This architecture makes a significant contribution to the crustal thickening of the LMTB during the Mesozoic, and therefore the Cenozoic thickening of the Longmenshan belt might be less important than often suggested.

Beschreibung: The question of lateral and/or vertical continuity of subducted slabs in active orogens is a hot topic partly due to poorly resolved tomographic data. The complex slab structure beneath the Alpine region is only partly resolved by available geophysical data, leaving many geological and geodynamical issues widely open. Based upon a finite-frequency kernel method, we present a new high-resolution tomography model using P-wave data from 527 broadband seismic stations, both from permanent networks and temporary experiments. This model provides an improved image of the slab structure in the Alpine region, and fundamental pin-points for the analysis of Cenozoic magmatism, (U)HP metamorphism and Alpine topography. Our results document the lateral continuity of the European slab from the Western to the Central Alps, and the down-dip slab continuity beneath the Central Alps, ruling out the hypothesis of slab breakoff to explain Cenozoic Alpine magmatism. A low velocity anomaly is observed in the upper mantle beneath the core of the Western Alps, pointing to dynamic topography effects. A NE-dipping Adriatic slab, consistent with Dinaric subduction, is possibly observed beneath the Eastern Alps, whereas the laterally continuous Adriatic slab of the Northern Apennines shows major gaps at the boundary with the Southern Apennines, and becomes near vertical in the Alps-Apennines transition zone. Tear faults accommodating opposite-dipping subductions during Alpine convergence may represent reactivated lithospheric faults inherited from Tethyan extension. Our results suggest that the interpretations of previous tomography results that include successive slab breakoffs along the Alpine-Zagros-Himalaya orogenic belt might be proficiently reconsidered.