Age and tectonostratigraphic significance of the Upper Carboniferous series in the basement of the Andean Frontal Cordillera: Geodynamic implications     

P. Busquets  F. Colombo  N. Heredia  N. Sole de Porta  L.R. Rodriguez Fernandez  J. Alvarez Marron   《Tectonophysics》2005,399(1-4):181

The age and tectonosedimentary environment of the Palaeozoic sediments on the Frontal Cordillera is not well known and earlier studies have been unable to satisfactorily explain the geological history of the basement of the Andes.In the vicinity of the old Castaño Viejo mine crop out various levels of partially metamorphosed microbialite limestones, which alternate with thin marly–lutitic interstrata. These levels contain abundant palynomorph remains, which allow the series to be dated as Silurian–Devonian. These data, together with the presence of warm climate fossils, lend support to the hypothesis of a major allochtony of the Chilenia Terrane (of which the Frontal Cordillera formed part), relative to the Cuyania Terrane (which included the Precordillera), prior to their amalgamation.Upper Carboniferous palynomorphs found during this study occur in association with resedimented palynomorphs and chitinozoa, of possible Devonian age. This demonstrates the equivalence of both fossiliferous series and their location within the upper part of the Upper Carboniferous Agua Negra Fm. The Silurian–Devonian elements, deformed during a phase prior to the Gondwanic orogeny, were eroded and transported to the foreland basin during the Upper Carboniferous.The palynomorph associations found in all samples correspond to the Ancistrospora palynological zone and to the Raistrickia densa–Convolutispora muriornata Biozone, which are indicative of Upper Carboniferous times. Characteristic forms such as Ancistrospora verrucosa and C. muriornata, both indicative of an Upper Carboniferous age, were found in samples from the Castaño Viejo area.Earlier interpretations of the Frontal Cordillera attributing the sedimentation to a palaeo-latitude at some distance from Gondwana, were based on the presence of Silurian–Devonian hot water stromatolithic limestones. Our results suggest that Cuyania and Chilenia were not necessarily separated by a great distance before their amalgamation. This in turn means that a large ocean was not necessarily consumed in the process.  相似文献   

Age and significance of core complex formation in a very curved orogen: Evidence from fission track studies in the South Carpathians (Romania)   总被引：1，自引：1，他引：1  

B. Fügenschuh  S.M. Schmid 《Tectonophysics》2005,404(1-2):33-53

Twenty-four new zircon and apatite fission track ages from the Getic and Danubian nappes in the South Carpathians are discussed in the light of a compilation of published fission track data. A total of 101 fission track ages indicates that the Getic nappes are generally characterized by Cretaceous zircon and apatite fission track ages, indicating cooling to near-surface temperatures of these units immediately following Late Cretaceous orogeny.The age distribution of the Danubian nappes, presently outcropping in the Danubian window below the Getic nappes, depends on the position with respect to the Cerna-Jiu fault. Eocene and Oligocene zircon and apatite central ages from the part of the Danubian core complex situated southeast of this fault monitor mid-Tertiary tectonic exhumation in the footwall of the Getic detachment, while zircon fission track data from northwest of this fault indicate that slow cooling started during the Latest Cretaceous. The change from extension (Getic detachment) to strike-slip dominated tectonics along the curved Cerna-Jiu fault allowed for further exhumation on the concave side of this strike-slip fault, while exhumation ceased on the convex side. The available fission track data consistently indicate that the change to fast cooling associated with tectonic denudation by core complex formation did not occur before Late Eocene times, i.e. long after the cessation of Late Cretaceous thrusting.Core complex formation in the Danubian window is related to a larger-scale scenario that is characterized by the NNW-directed translation, followed by a 90° clockwise rotation of the Tisza-Dacia “block” due to roll-back of the Carpathian embayment. This led to a complex pattern of strain partitioning within the Tisza-Dacia “block” adjacent to the western tip of the rigid Moesian platform. Our results suggest that the invasion of these southernmost parts of Tisza-Dacia started before the Late Eocene, i.e. significantly before the onset of Miocene-age rollback and associated extension in the Pannonian basin.  相似文献   

Flexural and gravity modelling of the Mérida Andes and Barinas–Apure Basin, Western Venezuela     

Luis Chacín  María I. Jcome  Carlos Izarra 《Tectonophysics》2005,405(1-4):155-167

The kinematic evolution of the Barinas–Apure Basin and the southern Mérida Andes from Lower Miocene to the Present is numerically modelled using flexural isostatic theory and geophysical and geological data. Two published regional transects are used to build up a reference section, which is then used to constrain important parameters (e.g. shortenings and sedimentary thicknesses) for the flexural modelling. To control the location of the main fault system in the flexural model earthquake information is also used. The estimated flexural elastic thickness of the South American lithosphere beneath the Barinas–Apure Basin and the Mérida Andes Range is 25 km. The value for the final total shortening is 60 km. The flexural isostatic model shows that the Andean uplift has caused the South American lithosphere subsidence and the development of the Barinas–Apure Basin.In addition, gravity modelling was used to understand deep crustal features that could not be predicted by flexural theory. Consequently, the best-fit flexural model is used to build a gravity model across the Mérida Andes and the Barinas–Apure Basin preserving the best-controlled structural features from the flexural modelling (e.g. basin wavelength and depth) and slightly changing the main bodies density values and deep crustal structures. The final gravity model is intended to be representative of the major features affecting the gravity field in the study area. The predicted morphology in the lower crustal level of the final gravity model favours the hypothesis of a present delamination or megathrust of the Maracaibo crust over the South American Shield. This process would use the Conrad discontinuity as a main detachment surface within an incipient NW dipping continental subduction.  相似文献   

Upper mantle structure of the South American continent and neighboring oceans from surface wave tomography     

Maggy Heintz  Eric Debayle  Alain Vauchez   《Tectonophysics》2005,406(1-2):115-139

We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle.The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km).Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP).  相似文献   

Intrusion and its implication for subsidence: A case from the Baiyun Sag, on the northern margin of the South China Sea   总被引：12，自引：0，他引：12  

Xiaobin Shi  Evgenii Burov  Sylvie Leroy  Xuelin Qiu  Bin Xia 《Tectonophysics》2005,407(1-2):117-134

We develop a series of simple numerical models to explain the anomalous subsidence and deposition phenomena on the northern continental margin of the South China Sea, in particular in the Baiyun Sag. The results suggest that a short-period high rate deposition of around 17 Ma is related to a rapid subsidence event, which may be due to episodic emplacement of a dense intrusion. Necking and gravity models indicate that in the basement of the Baiyun Sag, there is a dense zone that is 100–200 kg/m³ more dense than the surrounding country rock. Considering its high magnetic intensity and regional igneous activity, the dense zone is thought to be related to a phase of basalt intrusion that may have taken place around 17 Ma. Thermal and subsidence models indicate that a hot denser intrusion can cause significant subsidence immediately after the intrusion. The subsidence rate then slows down with cooling, thus becoming consistent with the observed subsidence curves at around 17 Ma. The results also indicate that the lithospheric strength under the Baiyun Sag is negligible, and that the high-velocity layer in the lowermost crust may be not an original part of the pre-rift crust. Instead, it is thought to be underplated intrusion emplaced at around 30 Ma when the continental margin broke up.  相似文献   

C-isotope Composition of Carbonates from Indravati Basin, India: Implications for Regional Stratigraphic Correlation     

Anil Maheshwari  A.N. Sial  Rajeeva Guhey  V.P. Ferreira 《Gondwana Research》2005,8(4):603-610

The Indravati Basin represents an unmetamorphosed and unfossiliferous sequence and shows a broad similarity in lithological association with other Purana Basins of Central India. The carbonates belonging to Indravati Basin have on average, moderately positive δ13 C values (up to +4.5). Numerous successions reported from other parts of world, show similar δ13 C values and contain evidence to suggest that the latest Mesoproterozoic to early Neoproterozoic period may represent an isotopically recognizable interval globally. The elevated δ13 C values of Indravati Basin are also correlatable with other Purana Basins in India including Upper Vindhyan Supergroup.  相似文献   

Near-vertical seismic reflection image using a novel acquisition technique across the Vrancea Zone and Foscani Basin, south-eastern Carpathians (Romania)     

I. Panea  R. Stephenson  C. Knapp  V. Mocanu  G. Drijkoningen  L. Matenco  J. Knapp  K. Prodehl 《Tectonophysics》2005,410(1-4):293-309

The DACIA PLAN (Danube and Carpathian Integrated Action on Process in the Lithosphere and Neotectonics) deep seismic sounding survey was performed in August–September 2001 in south-eastern Romania, at the same time as the regional deep refraction seismic survey VRANCEA 2001. The main goal of the experiment was to obtain new information on the deep structure of the external Carpathians nappes and the architecture of Tertiary/Quaternary basins developed within and adjacent to the seismically-active Vrancea zone, including the Focsani Basin. The seismic reflection line had a WNW–ESE orientation, running from internal East Carpathians units, across the mountainous south-eastern Carpathians, and the foreland Focsani Basin towards the Danube Delta. There were 131 shot points along the profile, with about 1 km spacing, and data were recorded with stand-alone RefTek-125s (also known as “Texans”), supplied by the University Texas at El Paso and the PASSCAL Institute. The entire line was recorded in three deployments, using about 340 receivers in the first deployment and 640 receivers in each of the other two deployments. The resulting deep seismic reflection stacks, processed to 20 s along the entire profile and to 10 s in the eastern Focsani Basin, are presented here. The regional architecture of the latter, interpreted in the context of abundant independent constraint from exploration seismic and subsurface data, is well imaged. Image quality within and beneath the thrust belt is of much poorer quality. Nevertheless, there is good evidence to suggest that a thick (10 km) sedimentary basin having the structure of a graben and of indeterminate age underlies the westernmost part of the Focsani Basin, in the depth range 10–25 km. Most of the crustal depth seismicity observed in the Vrancea zone (as opposed to the more intense upper mantle seismicity) appears to be associated with this sedimentary basin. The sedimentary successions within this basin and other horizons visible further to the west, beneath the Carpathian nappes, suggest that the geometry of the Neogene and recent uplift observed in the Vrancea zone, likely coupled with contemporaneous rapid subsidence in the foreland, is detached from deeper levels of the crust at about 10 km depth. The Moho lies at a depth of about 40 km along the profile, its poor expression in the reflection stack being strengthened by independent estimates from the refraction data. Given the apparent thickness of the (meta)sedimentary supracrustal units, the crystalline crust beneath this area is quite thin (< 20 km) supporting the hypothesis that there may have been delamination of (lower) continental crust in this area involved in the evolution of the seismic Vrancea zone.  相似文献   

Land degradation in the source region of the Yellow River, northeast Qinghai-Xizang Plateau: classification and evaluation   总被引：6，自引：0，他引：6  

Jianmin Feng  Tao Wang  Shanzhong Qi  Changwei Xie 《Environmental Geology》2005,47(4):459-466

Land degradation imposes a great threat to the world. It is not merely an environmental issue, but also a social and economic problem. Land desertification is among the main aspects of environment changes in the source region of the Yellow River. Previous studies focused on water resource utilization and soil erosion, but land degradation in the source region of the Yellow River even the whole Qinghai-Xizang Plateau received little attention. Based on the data obtained by field investigation and TM satellite images of 2000, this study provides the classification and evaluation information of the land degradation in the source region of the Yellow River. There are six types of land degradation in this region: water erosion in the northern mountains around the Gonghe Basin, sandy desertification in the Gonghe Basin and Upland Plain Area, aridization in the lower reaches, salinization in the Gonghe Basin, vegetation degradation in the intramontance basin and freezing and thawing erosion in the high mountains. The total degraded area is 34,429.6 km², making up 37.5% of the land in the study area. Finally, land degradation in the source region of the Yellow River was evaluated according to changes in the physical structure and chemical component of soils, land productivity, secondary soil salt and water conditions.  相似文献   

Hydrography of the eastern Arabian Sea during summer monsoon 2002     

D. Shankar  S. S. C. Shenoi  R. K. Nayak  P. N. Vinayachandran  G. Nampoothiri  A. M. Almeida  G. S. Michael  M. R. Ramesh Kumar  D. Sundar  O. P. Sreejith 《Journal of Earth System Science》2005,114(5):459-474

Hydrographic observations in the eastern Arabian Sea (EAS) during summer monsoon 2002 (during the first phase of the Arabian Sea Monsoon Experiment (ARMEX)) include two approximately fortnight-long CTD time series. A barrier layer was observed occasionally during the two time series. These ephemeral barrier layers were caused byin situ rainfall, and by advection of low-salinity (high-salinity) waters at the surface (below the surface mixed layer). These barrier layers were advected away from the source region by the West India Coastal Current and had no discernible effect on the sea surface temperature. The three high-salinity water masses, the Arabian Sea High Salinity Water (ASHSW), Persian Gulf Water (PGW), and Red Sea Water (RSW), and the Arabian Sea Salinity Minimum also exhibited intermittency: they appeared and disappeared during the time series. The concentration of the ASHSW, PGW, and RSW decreased equatorward, and that of the RSW also decreased offshore. The observations suggest that the RSW is advected equatorward along the continental slope off the Indian west coast.  相似文献   

应用遥感方法研究黄河三角洲地表蒸发及其与下垫面关系   总被引：10，自引：1，他引：10  

崔亚莉  徐映雪  邵景力  张长春 《地学前缘》2005,12(Z1):159-165

文中主要应用遥感方法计算了黄河三角洲地表蒸发量及其地表特征参数。地表特征参数及其合理组合揭示出黄河三角洲下垫面的基本特征:农田植被指数和天然植被的植被指数有不同的变化规律,下垫面覆盖度低,裸地较多,地表较湿润,蒸发量较大。蒸发量时空分布主要受下垫面条件控制,滨海裸地和受人类活动影响较大的农田等地蒸发量较大,年际平均蒸发量在570~860 mm之间。  相似文献