首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 828 毫秒
1.
Jacobsoniidae is a small but perplexing beetle family, with unknown phylogenetic relationships to other polyphagan Coleoptera. To date, only a single fossil jacobsoniid has been described, from Eocene Baltic amber (~ 40 Ma). Here, we push back the oldest definitive record of Jacobsoniidae by approximately 60 million years with a new fossil species recovered from mid-Cretaceous (~ 99 Ma) Burmese amber from Myanmar. Remarkably, exploration of the fossil's morphology with confocal laser scanning microscopy revealed that it belongs to an extant genus, Derolathrus. The similarity of the new taxon, Derolathrus abyssus n. sp., to modern congeners provides a striking example of morphological stability over deep evolutionary time—a possible outcome of long-term persistence of mesic microhabitats, a hypothesis we argue is supported by a variety of other Recent, litter-inhabiting arthropod taxa now known to be largely unchanged since the Mesozoic. Many such examples belong to the Staphylinoidea—a hyperdiverse beetle superfamily that dominates contemporary mesic habitats, and with which Jacobsoniidae may have a close phylogenetic relationship.This published work has been registered in ZooBank, http://zoobank.org/urn:lsid:zoobank.org:pub:7C65ACAF-456E-4301-BDD7-0A801768EEB9.  相似文献   

2.
A single metapelitic sample from the Verbaard locality, near Messina was investigated in order to construct a P–T path and moreover, highlight pertinent contradictions in the current P–T database. Interpretations based on P–T pseudosections, garnet isopleth thermobarometry and mineral mode/isopleth modelling indicate that the mineral assemblages, textures and zonations developed in the metapelite formed along a single clockwise P–T path. The metamorphic evolution is characterized by an early high-pressure phase at 10–11 kbar/800 °C, followed by a simultaneous pressure decrease and temperature increase to ∼8/850 °C and subsequent retrogression via decompression-cooling to 4–5 kbar at T < 650 °C. Growth zoning in garnet provides evidence for an earlier, prograde history, however, as potential melt-loss was not accounted for this must be deemed speculative. The results of this study agree entirely with that of [Zeh, A., Klemd, R., Buhlmann, S., Barton, J.M. 2004. Pro- and retrograde P–T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa); constraints from quantitative phase diagrams and geotectonic implications. Journal of Metamorphic Geology 22, 79–95], who adopted a similar approach to thermobarometry i.e. pseudosections. The results are, however, inconsistent with recent publications that argue for a twofold, metamorphic history defined by two decompression-cooling paths (DC1 ∼2.6 Ga and DC2 ∼2.0 Ga) that are separated by an isobaric heating path (∼2.0 Ga). The disparity in the results obtained from different workers can be explained by an examination of the thermobarometric methods employed. The methodology employed to derive the twofold, polymetamorphic P–T path appears to be erroneous. At present, the most reliable and robust method for determining P–T paths is the pseudosection approach to thermobarometry. Future modelling of Limpopo Belt granulites should adopt this strategy and ensure potential melt-loss is taken into account. Alternatively, this potential problem can be avoided altogether by investigating rocks of mafic composition.  相似文献   

3.
《Comptes Rendus Geoscience》2014,346(1-2):20-27
Subfossil tree trunks deposits are common in large rivers, but their status as a source for dating alluvial sequences and palaeoenvironmental studies is still discussed. Particularly their origin and the process(es) of deposition as well as a possible remobilization were pointed as a limit to their use to document river alluvial changes. In this work we report the discovery of the largest subfossil trunks deposits in the Garonne valley. These new data are compared to the previous ones. A set of 17 tree trunks and more than 300 smaller wood fragments were collected. The xylologic study shows the prevalence of Quercus and a single occurrence of Ulmus. These two hardwood species are commonly associated with riparian forest. The 14C dating carried out on seven trunks and a single branch of Quercus on the outermost identified growth rings, indicates age ranging from 8400–8000 cal. BP for the oldest fragment (bough) to 4300–4000 cal. BP for the most recent tree trunk. Radiocarbon ages of the trunks are aggregated into two main periods: 5300–5600 cal. BP (four trunks) and 4300–4000 cal. BP (three trunks). The radiocarbon (charcoal) dating of the top of the alluvial sequence overlaying the trunks gives an age between 1965–1820 and 1570–1810 cal. BP, i.e. between the 2nd and the 5th c. AD. In addition, the discovery of two unpublished subfossil tree trunks deposits in Finhan are reported (six trunks). At the light of these results, we discuss previously proposed models for the Garonne floodplain building.  相似文献   

4.
Surface sediments from the Gulf of Cádiz (GoC) were analyzed by alkaline CuO oxidation, in order to estimate the contribution of terrigenous organic matter (TOM) to the inner continental shelf of the southwest Iberian Peninsula. The parallel analysis of sediment samples from the two most important rivers draining to this coastal area (i.e. Guadiana River and Tinto–Odiel fluvial system) provided fundamental information regarding local terrestrial sources. Relatively constant intensive lignin parameters (S:V = 1.0 ± 0.1 and C:V = 0.22 ± 0.04) and high values of the lignin phenol vegetation index (LPVI = 155 ± 43) indicated that non-woody angiosperm tissues constitute the dominant component of vascular plant material reaching the shelf sediments. The NW to SE decreasing isotopic (13C) and molecular (Λ8) signatures found among the sediments, coinciding with the Guadiana delivery plume, suggest that this river is the main terrestrial source in the inner GoC shelf. Slightly elevated values of degradation indicative ratios ([Ad:Al]V = 0.41 ± 0.10; [Ad:Al])S = 0.34 ± 0.07; [3,5-Bd:V] = 0.14 ± 0.05; P:[V + S] = 0.24 ± 0.09) suggested the alteration state of the shelf sediments. The two fold higher ratios of the river sediments (Guadiana: [Ad:Al]V = 0.82 ± 0.08; [Ad:Al]S = 0.84 ± 0.03; Tinto–Odiel: [Ad:Al]V = 0.86 ± 0.12; [Ad:Al]S = 0.83 ± 0.013) and the increasing degradation trend observed outward in the shelf, lead us to consider preferential sorption processes, instead of in situ diagenesis, to affect the degradation signature of the shelf sediments. Preferentially solubilized degraded OM is more likely to be sorbed and stabilized prior to transport to the marine system, showing an apparently more advanced degradation state. The use of the 3,5-Bd:V ratio in conjunction with (Ad:Al)V revealed a composition continuum of the sedimentary OM ranging from fresh plant materials to highly altered soil humic constituents. Elemental and molecular analyses show a land to sea gradient by a NW to SE decrease of the terrestrial influence, accounting for larger terrestrial inputs (TOM: 71–98%) in those sediments near the Guadiana mouth, and predominantly autochthonous composition (TOM: 42–50%) in those located offshore. This work utilizes lignin derived biomarkers to determine the contribution of terrigenous OM delivered to this poorly described coastal area from regional rivers. Within a context of increasing international efforts to better understand the global C cycling, this study illustrates the relevance of using the alkaline CuO oxidation approach to evaluate C budgets and continental influence in river dominated ocean margins.  相似文献   

5.
The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO2 concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO2 varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO3 and Fe(CO3)33−, with formation constants log*β°1,1,1 = 10.76 ± 0.38 and log β°1,0,3 = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO2 and at T = (25 ± 1) °C, as log*Ks,0 = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO2-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers.  相似文献   

6.
Our study provides detailed information on the Lateglacial landscape and vegetation development of Tibet. Based on a suite of geomorphological and palynological proxy data from the Nianbaoyeze Shan on the eastern margin of the Tibetan Plateau (33°N/101°E, 3300–4500 m asl.), we reconstruct the current state as a function of climate history and the longevity of human influence. Study results constrain several major phases of aeolian sedimentation between 50–15 ka and various glacier advances during the Late Pleistocene, the Holocene and the Little Ice Age. Increased aeolian deposition was primarily associated with periods of more extensive glacial ice extent. Fluvial and alluvial sediment pulses document an increase of erosion starting at 3926 ± 79 cal yr B.P., coinciding with cooling (Neoglacial) and a growing anthropo-zoogenic influence. Evidence for periglacial mass movements indicate that the late Holocene cooling started at around 2000 cal yr B.P., demonstrating increased surface activity under the combined effects of human influence and climate deterioration. The onset of peat growth generally depended on local conditions that include relief, meso-climate and in more recent times also on soil compaction due to animal trampling. We distinguish three initiation periods of peat growth: 12,700–10,400 cal yr B.P. for flat basins inside last glacial terminal moraines; 7000–5000 cal yr B.P. for the main valley floors; and 3000–1000 cal yr B.P. for the higher terrace surfaces.The Holocene vegetation history started with an open landscape dominated by pioneer shrubs along braided rivers (<10,600–9800 cal yr B.P.), followed by the spreading of conifers (Picea, Juniperus, Abies) and Betula-trees accompanied by a successive closing of the vegetation cover by Poaceae, Cyperaceae and herbs (9800–8300 cal yr B.P.). First signs of nomadic presence appear as early as 7200 cal yr B.P., when temperatures were up to 2 °C warmer than today. Forest remained very patchy with strong local contrasts. During the following cooling phase (5900–2750 cal yr B.P.) the natural vegetation was transformed by nomadic grazing to Bistorta-rich Kobresia pygmaea-pastures. Modern nomadic migration routes were established at least 2200 years ago. Overgrazing and trampling led to the shrinking of Bistorta and the spreading of annual weeds. Short-lived cold events (8000, 6200, 3500 cal yr B.P.) impacted on the vegetation only temporarily.As the transformation of the natural Poaceae-rich vegetation into Kobresia-pastures modified the influence of the Tibetan Plateau (“hot plate”) on the monsoon system, our data even point to an early start of a nomadic (!) Anthropocene nearly 6000 years ago. Against the background of a very long grazing history, modern Tibet must be seen as a cultural landscape.  相似文献   

7.
Arguments over the extinction of Pleistocene megafauna have become particularly polarised in recent years. Causes for the extinctions are widely debated with climate change, human hunting and/or habitat modification, or a combination of those factors, being the dominant hypotheses. However, a lack of a spatially constrained chronology for many megafauna renders most hypotheses difficult to test. Here, we present several new U/Th dates for a series of previously undated, megafauna-bearing localities from southeastern Queensland, Australia. The sites were previously used to argue for or against various megafauna extinction hypotheses, and are the type localities for two now-extinct Pleistocene marsupials (including the giant koala, Phascolarctos stirtoni). The new dating allows the deposits to be placed in a spatially- and temporally constrained context relevant to the understanding of Australian megafaunal extinctions. The results indicate that The Joint (Texas Caves) megafaunal assemblage is middle Pleistocene or older (>292 ky); the Cement Mills (Gore) megafaunal assemblage is late Pleistocene or older (>53 ky); and the Russenden Cave Bone Chamber (Texas Caves) megafaunal assemblage is late Pleistocene (～55 ky). Importantly, the new results broadly show that the sites date prior to the hypothesised megafaunal extinction ‘window’ (i.e., ～30–50 ky), and therefore, cannot be used to argue exclusively for or against human/climate change extinction models, without first exploring their palaeoecological significance on wider temporal and spatial scales.  相似文献   

8.
We report 6 K–Ar ages and paleomagnetic data from 28 sites collected in Jurassic, Lower Cretaceous and Paleocene rocks of the Santa Marta massif, to test previous hypothesis of rotations and translations of this massif, whose rock assemblage differs from other basement-cored ranges adjacent to the Guyana margin. Three magnetic components were identified in this study. A first component has a direction parallel to the present magnetic field and was uncovered in all units (D = 352, I = 25.6, k = 57.35, a95 = 5.3, N = 12). A second component was isolated in Cretaceous limestone and Jurassic volcaniclastic rocks (D = 8.8, I = 8.3, k = 24.71, a95 = 13.7, N = 6), and it was interpreted as of Early Cretaceous age. In Jurassic sites with this component, Early Cretaceous K–Ar ages obtained from this and previous studies are interpreted as reset ages. The third component was uncovered in eight sites of Jurassic volcaniclastic rocks, and its direction indicates negative shallow to moderate inclinations and northeastward declinations. K–Ar ages in these sites are of Early (196.5 ± 4.9 Ma) to early Late Jurassic age (156.6 ± 8.9 Ma). Due to local structural complexity and too few Cretaceous outcrops to perform a reliable unconformity test, we only used two sites with (1) K–Ar ages, (2) less structural complexity, and (3) reliable structural data for Jurassic and Cretaceous rocks. The mean direction of the Jurassic component is (D = 20.4, I = −18.2, k = 46.9, a95 = 5.1, n = 18 specimens from two sites). These paleomagnetic data support previous models of northward along-margin translations of Grenvillian-cored massifs. Additionally, clockwise vertical-axis rotation of this massif, with respect to the stable craton, is also documented; the sense of rotation is similar to that proposed for the Perija Range and other ranges of the southern Caribbean margin. More data is needed to confirm the magnitudes of rotations and translations.  相似文献   

9.
A recently recognized molybdenum (Mo) metallogenic belt is present within and adjacent to the northern part of the North China Craton (NCC). More than 20 Mo deposits are present in the belt, including the Sadaigoumen and Dacaoping porphyry deposits located in the Fengning region of the northern part of Hebei Province. The Sadaigoumen deposit has a Re–Os molybdenite weighted mean age of 236.5 ± 2.2 Ma (MSWD = 1.4, n = 6), which is more reliable than existing dates and is interpreted as the precise age for formation of the deposit. The Dacaoping Mo deposit is about 100 million years younger, with a Re–Os molybdenite isochron age of 140.1 ± 3.4 Ma (2σ, MSWD = 0.26, n = 5), which is within error of the weighted mean age of 139.4 ± 0.9 Ma. The ages of the two deposits show that there are at least two episodes of Mo porphyry formation in the Fengning region. In combination with the regional geological evolution of this part of the craton margin, we propose that the Triassic Mo event at Sadaigoumen is associated with a collisional event during the closure of the ancient Asian Ocean, whereas the Early Cretaceous Mo event at Dacaoping is associated with lithospheric thinning of the NCC.  相似文献   

10.
Widespread Cenozoic sediments in and around the Tibetan Plateau (TP) are thought to have played an important role in explaining the process of the India-Asia collision as well as its interactions with global and regional paleoclimate. However, high-resolution temporal frameworks of sedimentary sequences and controls on geological and climatic events are still absent. To study the abovementioned issues, we investigate the Oligocene-Miocene lacustrine sequences (the Dingqinghu Formation) of the Lunpola Basin, central TP. In this work, cyclostratigraphic analyses are conducted with gamma ray log and pollen data to establish a high resolution temporal framework ranging from ca. 25.4 to 18.0 Ma for the sections. Along these sections, sediment accumulation rates are calculated with orbital signals to monitor clastic input of the lake basin; elemental, palynological, and isotopic data are summarized to depict the paleoclimate and paleoelevation evolution of this drainage system. Integrating all these clues together, we sort out a chronological list of events including lake basin, tectonics, and paleoclimate: regional uplift took place at 23.7 Ma; simultaneously, a distinct lake-basin transition characterized by accelerated sediment accumulation rate is recognized; about 0.2 Ma later at 23.5 Ma, catchment scale drought occurred and maintained to the end of the sections. Our results demonstrate that paleoclimate did not impose decisive influence on the late Oligocene-early Miocene evolution of the lake basin; instead, regional uplift and its associated accelerated exhumation of the source area resulted in the lake-basin transition and paleoclimatic drought. After reviewing the Oligocene-Miocene sedimentary records distributed in and around the TP, we argue that the 23.7 Ma geological event of the Lunpola Basin is probably not a single case but a regional effect of a dramatic tectonic transition of the plateau.  相似文献   

11.
Epithermal gold (Au) deposits result from the combination of a sustained flux of metal-rich fluids and an efficient precipitation mechanism. Earthquakes may trigger gold precipitation by rapid loss of fluid pressure but their efficiency and time-integrated contribution to gold endowment are poorly constrained. In order to quantify the feedbacks between earthquake-driven fracturing and gold precipitation in the shallow crust, we studied the gold-rich fluids in the active Tolhuaca geothermal system, located in the highly seismic Southern Andes of Chile. We combined temperature measurements in the deep wells with fluid inclusion data, geochemical analyses of borehole fluids and numerical simulations of coupled heat and fluid flow to reconstruct the physical and chemical evolution of the hydrothermal reservoir. The effect of seismic perturbations on fluid parameters was constrained using a thermo-mechanical piston model that simulates the suction pump mechanism occurring in dilational jogs. Furthermore, we evaluated the impact of fluid parameters on gold precipitation by calculating the solubility of gold in pressure (P)–enthalpy (H) space. The reconstructed fluid conditions at Tolhuaca indicate that single-phase convective fluids feeding the hydrothermal reservoir reach the two-phase boundary with a high gold budget (~ 1–5 ppb) at saturated liquid pressures between 20 and 100 bar (210 °C < Tsat < 310 °C). We show that if hydrothermal fluids reach this optimal threshold for gold precipitation at a temperature near 250 °C, small adiabatic pressure drops (~ 10 bar) triggered by transient fault-rupture can produce precipitation of 95% of the dissolved gold. Our results at the active Tolhuaca geothermal system indicate that subtle, externally-forced perturbations – equivalent to low magnitude earthquakes (Mw < 2) of a hydrothermal reservoir under optimal conditions – may significantly enhance gold precipitation rates in the shallow crust and lead to overall increases in metal endowment over time.  相似文献   

12.
《Comptes Rendus Geoscience》2018,350(8):476-486
In this study, the model H(i) = 109.6103 + C1 × F1(i) + C2 × F2(i) + … + C33 × F33(i) obtained from depth modelling based on 33 recent benthic foraminifer species distribution, has been applied to the fossil benthic foraminifers from the borehole GDEC-4-2 drilled at a water depth of 491 m, in the East-Corsica basin, covering the last 550,000 years. The obtained variations of the paleo-depths show a medium correlation with the oscillations of the relative sea level and also with the fluctuations of the oxygen isotopic ratio (δ18O G. bulloides and δ18O C. pachyderma–C. wuellerstorfi). This newly developed transfer function is accompanied by an error margin of ± 86 m, suggesting that this model will probably be more suitable for a time scale of the order of a million years where sea level variations are recorded with larger amplitudes. Without considering these problems related to amplitudes, it also turns out that the “eustatic” signal of the microfauna is accompanied by a “trophic” signal, which should not to be neglected, especially at a millennial scale time resolution. Thus, the application of this method would require taking into account the bottom trophic effects strongly controlling the distribution of benthic foraminifer assemblages.  相似文献   

13.
《Gondwana Research》2014,25(2):585-613
The Belomorian eclogite province was repeatedly affected by multiple deformation episodes and metamorphism under moderate to high pressure. Within the Gridino area, high pressure processes developed in a continental crust of tonalite–trondhjemite–granodiorite (TTG) affinity that contains mafic pods and dykes, in which products of these processes are most clearly evident. New petrological, geochemical and geochronological data on mafic and felsic rocks, including PT-estimates, mineral chemistry, bulk rock chemistries, REE composition of the rocks and zircons and U–Pb and Lu–Hf geochronology presented in the paper make it possible to reproduce the magmatic and high-grade metamorphic evolution in the study area. In the framework of the extremely long-lasting geologic history recorded in the Belomorian province (3–1.7 Ga), new geochronological data enabled us to define the succession of events that includes mafic dyke emplacement between 2.87 and 2.82 Ga and eclogite facies metamorphism of the mafic dykes between ~ 2.82 and ~ 2.72 Ga (most probably in the time span of 2.79–2.73 Ga). The clockwise PT path of the Gridino association crosses the granulite- and amphibolite-facies PT fields during the time period of 2.72 Ga to 2.64 Ga. A special aspect of this work concerns the superposed subisobaric heating (thermal impact) with an increase in the temperature to granulite facies conditions at 2.4 Ga. Later amphibolite facies metamorphism occurred at 2.0–1.9 Ga. Our detailed geochronological and petrological studies reveal a complicated Mesoarchaean–Palaeoproterozoic history that involved deep subduction of the continental crust and a succession of plume-related events.  相似文献   

14.
The Linzizong Group (64–44 Ma) of the Lhasa Terrane in Tibet is critically positioned for establishing the paleoposition of the southern leading edge of the Asian continent during Paleogene times and constraining onset of the India–Asia collision. Here we report paleomagnetic results from a collection comprising 384 drill-core samples from 34 sites embracing all three formations of this group. Comprehensive demagnetization and field tests isolate characteristic remanent magnetizations (ChRM) summarized by overall tilt-corrected formation-mean directions of D = 183.6°, I = −12.4° (α95 = 8.1°) for the Dianzhong (64–60 Ma), D = 1.0°, I = 18.1° (α95 = 8.1°) for the Nianbo (60–50 Ma), and D = 12.4°, I = 23.2° (α95 = 7.3°) for the Pana (50–44 Ma). Fold tests are positive in each formation suggesting a pre-folding origin and we interpret the magnetizations as quasi-primary and acquired at, or slightly later than, formation of the Linzizong Group. Revised Paleogene paleopoles with Ar–Ar age constraints for the Lhasa Terrane indicate that onset of the India–Asia collision occurred no later than ∼60.5 ± 1.5 Ma at a low paleolatitude of ∼10°N. Analysis of 60 site-mean observations from a range of studies of the Pana Formation in the higher part of the succession highlight a large dispersion of ChRM directions; a number of possible causes are suggested but further study of this formation over a wider area is required to resolve this issue.  相似文献   

15.
Three models were examined to predict C aromaticity (fa) of biochars based on either their elemental composition (C, H, N and O) or fixed C (FC) content. Values of fa from solid state 13C nuclear magnetic resonance (NMR) analysis with Bloch-decay (BD) or direct polarisation (DP) techniques, concentrations of total C, H, N, and organic O, and contents of FC of 60 biochars were either compiled from the literature (dataset 1, n = 52) or generated in this study (dataset 2, n = 8). Models were first calibrated with dataset 1 and then validated with dataset 2. All models were able to fit dataset 1 when atomic H to C ratio (H/C) < 1 (except two ash rich biochars) and to estimate fa of HF treated biochars (H/C < 1). Model 1, which was based on values of H/C only and calibrated with a root mean square of error (RMSE) of 0.04 fa-unit (n = 41), could predict the experimental data with a RMSE = 0.02 fa-unit (n = 6). Model 2, which was based on biochar elemental composition data, showed the most accurate prediction, with a RMSE of 0.03 fa-unit (n = 41) for the calibration data, and of 0.02 fa-unit (n = 6, H/C < 1) for the validation data. Model 3, which was based on contents of FC and C, and modified with a correction factor of 0.96, displayed the highest RMSE (0.06 fa-unit, n = 19) among the three models. Models 1 and 2 did not work properly for samples having either an H/C ratio > 1, high concentrations of carbonate or high inorganic H. These models need to be further tested with a wider range of biochars before they can be recommended for classification of biochar stability.  相似文献   

16.
The seismological study of recent seismic crises near Oleron Island confirms the coexistence of an extensional deformation and a transtensive regime in the Atlantic margin of France, which is different from the general western European stress field corresponding to a strike-slip regime. We argue that the switch of the principal stress axes σ1/σ2 in a NW–SE vertical plane is linked with the existence of crustal heterogeneities. Events of magnitude larger than 5 sometimes occur along the Atlantic margin of France, such as the 7 September 1972 (ML = 5.2) earthquake near Oleron island and the 30 September 2002 (ML = 5.7) Hennebont event in Brittany. To test the mechanism of local strain localization, we model the deformation of the hypocentral area of the Hennebont earthquake using a 3D thermo-mechanical finite element code. We conclude that the occurrence of moderate earthquakes located in limited parts of the Hercynian shear zones (as the often reactivated swarms near Oleron) could be due to local reactivation of pre-existing faults. These sporadic seismic ruptures are favoured by stress concentration due to rheological heterogeneities.  相似文献   

17.
《Gondwana Research》2016,29(4):1482-1499
The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. 40Ar/39Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.  相似文献   

18.
During the Pleistocene a fauna composed of large (biomass > 44 kg) and giant mammals (biomass > 1000 kg) that are usually associated with open environments lived in the Brazilian Intertropical Region. We present here new information concerning the paleoecology and chronology of some species of this megafauna. Carbon isotope analyses were performed for a better understanding of the paleoecology of the species Eremotherium laurillardi (Lund, 1842), Notiomastodon platensis (Ameghino, 1888) and Toxodon platensis (Owen, 1849). The δ13C data allow attributing a generalist diet to these species, which varied according to the kind of habitat in which they lived. In more open habitats all species were grazers; in mixed habitats E. laurillardi and T. platensis were mixed feeders, and N. platensis was grazer; and in more closed habitats all species were mixed feeders.  相似文献   

19.
Acyclic isoprenoids were among the earliest lipids on Earth and today > 30,000 isoprenoid-derived compounds are known, testifying to the remarkable diversification in isoprenoid biosynthetic pathways over time. Many of the isoprenoids have proven to be useful biomarkers in geochemical studies and evidence from sedimentary studies has the potential to provide a timeline for the evolution of different types of isoprenoid biosynthesis. A single green microalgal species termed Botryococcus braunii has been recognised as a major contributor of organic matter to sediments as old as the Precambrian. Modern studies of the taxonomy of B. braunii using molecular biology techniques have shown that the major clades identified from 18S rRNA gene sequences correspond to races A, B, L and S, which are defined on the basis of their hydrocarbon composition. The biosynthetic pathways by which C30–C37 botryococcenes are produced by the B race have now been shown to be due to a duplication of the squalene synthase gene, followed by subsequent changes to the genes, such that one pathway leads to the production of the C30 botryococcene and the other to squalene. Both products are then methylated to produce botryococcenes and methylated squalenes having higher carbon numbers. The mode of biosynthesis of lycopadiene in race L is unclear, but may involve coupling of two C20 phytyl diphosphates. From an examination of the geological record of botryococcenes and lycopadiene it seems likely that these pathways probably evolved early in the Eocene (ca. 55 Ma) and thus are more recently evolved than the genes for highly branched isoprenoid (HBI) alkenes first produced by diatoms about 92 Ma ago. Botryococcane produced from botryococcenes and a monoaromatic hydrocarbon produced from lycopadiene-related lipids, presumably under anoxic conditions, show promise as age diagnostic biomarkers. In view of these results, it seems likely that the Botryococcus species recorded in sediments predating the Eocene lacked the ability to produce botryococcenes or lycopadiene, but nonetheless still contained polymeric non-isoprenoid alkyl chains in the form of an algaenan which on preservation gave rise to a multitude of organic rich rocks.  相似文献   

20.
We synthesize more than 2600 Hf isotope data on the Archean-Paleoproterozoic zircons from the North China Craton (NCC). Recalculation of the data based on single stage and two-stage Hf model ages of the Eastern Block of the NCC shows peak ages of 3902 ± 13 Ma and 3978 ± 18 Ma, respectively, and also small peaks at 3.5–4.0 Ga. The majority of zircon εHf(t) values are positive, suggesting the possibility of the crust and the mantle differentiation at ca. 3.9–4.0 Ga in the Eastern Block of the NCC. Most magmatic zircons from the whole of NCC have their Hf model age range of 2.4–2.9 Ga, and the single stage model ages is cluster at 2698 ± 4 Ma, whereas the two-stage model ages concentrate at 2714 ± 5 Ma, implying that the protoliths were juvenile crustal rocks. The most prominent peak at 2.7 Ga indicates that this period marks the most important stage of the crust-mantle differentiation and crust formation of the NCC. The widespread 2.5 Ga rocks in the NCC and the absence of the 2.5 Ga peaks in Hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2.5 Ga. Furthermore, the 2.5–1.7 Ga zircon Hf isotope features are also related to the reworking of the crustal rocks. Our results from the integration of a large database suggest that the Eastern Block and the Trans-North China Orogen have undergone similar crust-mantle differentiation and magmatism, leading to the conclusion that the essential cratonization of the North China took place at the end of Neoarchean.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号