Effects of tectonics and lithology on long profiles of 16 rivers of the southern Central Massif border between the Aude and the Orb (France)

The analysis of longitudinal profiles of river channels and terraces in the southern Central Massif border, between the Aude and the Orb, allows the detection of anomalies caused by lithology and/or tectonic distortions. The rivers which have abnormally high slope and non-lithological knickzones indicate the main uplifted zones: the Montagne Noire and the Saint-Chinian ridge. A geomorphological and sedimentological analysis of detrital deposits was carried out as a basis for correlating the different formations, reconstructing the palaeodrainage and finding the main uplift and fluvial incision stages. During the Miocene, uplift remains limited as it is shown by the correlative fine deposits in the Languedocian piedmont. The Messinian incision (5.7–5.3 Ma) does not cross the Saint-Chinian ridge. On the other hand, fluvial incision becomes widespread in the Montagne Noire during the Upper Pliocene (3.4–2 Ma) when coarse deposits overlie either the Pliocene clay in the Orb palaeovalley or the Messinian conglomerates at the Cesse outlet. An Upper Pliocene uplift of the Montagne Noire and of the Saint-Chinian ridge is the cause of this incision and also of the diversion of the Cesse towards the Aude. Where the uplift rate was higher than incision rate, knickzones have developed like in the Avant-Monts south-side. The knickzones of lithological origin maintain a strong vertical stability during all the river incision stages. On the other hand, those of tectonic origin or base level lowering record upstream migration and their rate of retreat is controlled by the river discharge. As incision occurs only during the cold/temperate transition periods during the Quaternary, upward erosion slowly migrates (15 km since the Upper Pliocene, on the Orb) and so does not reach the riverheads.     

Massif Central (France): new constraints on the geodynamical evolution from teleseismic tomography

The Massif Central, the most significant geomorphological unit of the Hercynian belt in France, is characterized by graben structures which are part of the European Cenozoic Rift System (ECRIS) and also by distinct volcanic episodes, the most recent dated at 20 Ma to 4000 years BP. In order to study the lithosphere-asthenosphere system beneath this volcanic area, we performed a teleseismic field experiment.
During a six-month period, a joint French-German team operated a network of 79 mobile short-period seismic stations in addition to the 14 permanent stations. Inversion of P -wave traveltime residuals of teleseismic events recorded by this dense array yielded a detailed image of the 3-D velocity structure beneath the Massif Central down to 180 km depth. The upper 60 km of the lithosphere displays strong lateral heterogeneities and shows a remarkable correlation between the volcanic provinces and the negative velocity perturbations. The 3-D model reveals two channels of low velocities, interpreted as the remaining thermal signature of magma ascent following large lithospheric fractures inherited from Hercynian time and reactivated during Oligocene times. The teleseismic inversion model yields no indication of a low-velocity zone in the mantle associated with the graben structures proper. The observation of smaller velocity perturbations and a change in the shape of the velocity pattern in the 60–100 km depth range indicates a smooth transition from the lithosphere to the asthenosphere, thus giving an idea of the lithosphere thickness. A broad volume of low velocities having a diameter of about 200 km from 100 km depth to the bottom of the model is present beneath the Massif Central. This body is likely to be the source responsible for the volcanism. It could be interpreted as the top of a plume-type structure which is now in its cooling phase.     

AMS study of the Pont-de-Montvert–Borne porphyritic granite pluton (French Massif Central) and its tectonic implications

Thermomagnetic experiments, isothermal remanent magnetization (IRM), hysteresis loops, X-ray reflection analyses, optic microscopic observations, bulk magnetic susceptibility and natural remanent magnetization (NRM) show that the susceptibility signal of 56 sites in the Pont-de-Montvert–Borne pluton is derived mostly from biotite with a very small proportion of ferromagnetic material. Low anisotropy degree ( P parameter) and consistent AMS orientations among monzonite, enclaves and aplitic dykes indicate that the AMS was acquired during pluton emplacement in the subsolidus phase. Magnetic fabrics demonstrate that linear deformation is prominent in the area where the granite is in direct contact with micaschists, and that planar deformation becomes more important in the area where the pluton is surrounded by non-porphyritic peraluminous granites. AMS measurements also show that shallow plunging E–W-stretching lineations are the dominant structure over most of the studied area. Foliation orientations follow the pluton contour in the western part, indicating that the Pont-de-Montvert–Borne pluton is probably rooted in the west and extruded towards the east. These AMS results agree to some extent with fabric inferred from mineral preferred orientation. The AMS data support an E–W extensional tectonic setting during the Pont-de-Montvert–Borne pluton emplacement, which resulted from the Late Carboniferous crustal thinning of the Hercynian orogeny.     

The influence of upper-crust lithology on topographic development in the central Coast Ranges of California

A fundamental geological tenet is that as landscapes evolve over graded to geologic time, geologic structures control patterns of topographic distribution in mountainous areas such that terrain underlain by competent rock will be higher than terrain underlain by incompetent rock. This paper shows that in active orogens where markedly weak and markedly strong rocks are juxtaposed along contacts that parallel regional structures, relatively high topography can form where strain is localized in the weak rock. Such a relationship is illustrated by the topography of the central Coast Ranges between the Pacific coastline and the San Andreas fault zone (SAFZ), and along the length of the Gabilan Mesa (the “Gabilan Mesa segment” of the central Coast Ranges). Within the Gabilan Mesa segment, the granitic upper crust of the Salinian terrane is in contact with the accretionary-prism mélange upper crust of the Nacimiento terrane along the inactive Nacimiento fault zone. A prominent topographic lineament is present along most of this lithologic boundary, approximately 50 to 65 km southwest of the SAFZ, with the higher topography formed in the mélange on the southwest side of the Nacimiento fault.This paper investigates factors influencing the pattern of topographic development in the Gabilan Mesa segment of the central Coast Ranges by correlating shortening magnitude with the upper-crust compositions of the Salinian and Nacimiento terranes. The fluvial geomorphology of two valleys in the Gabilan Mesa, which is within the Salinian terrane, and alluvial geochronology based on optically-stimulated luminescence (OSL) age estimates, reveal that the magnitude of shortening accommodated by down-to-the-southwest tilting of the mesa since 400 ka is less than 1 to 2 m. Our results, combined with those of previous studies, indicate that at least 63% to 78% of late-Cenozoic, northeast-southwest directed, upper-crustal shortening across the Gabilan Mesa segment has been accommodated within the Nacimiento terrane. This is significant because perpendicular to orogenic strike the Nacimiento terrane constitutes less than ¼ of the distance between the coast and the SAFZ, and the other ¾ (or greater) of the distance between the coast and the SAFZ is underlain by the granitic upper crust of the Salinian terrane. We propose that strain and mountain building are localized within the Nacimiento terrane because it consists predominantly of the relatively weak Franciscan Complex mélange, and because the upper crust of the Salinian terrane is composed of relatively strong granitic rocks. Our hypothesis is supported by the distribution of post-seismic surface uplift associated with the 2003, 6.5 M_W San Simeon earthquake, which mimics the topography of the southwestern part of the Gabilan Mesa segment of the central Coast Ranges.     

Large rockslides in the Southern Central Andes of Chile (32–34.5°S): Tectonic control and significance for Quaternary landscape evolution

The distribution and age of large (> 0.1 km²) Pliocene to recent rockslides in the Chilean Cordillera Principal (32–34.5 S), the Southern Central Andes, has been analyzed to determine the rockslide triggering mechanisms and impact on regional landscape evolution. Most of the rockslides appear in the western Cordillera Principal and cluster along major geological structures. Variographic analyses show spatial correlation between rockslides, geological structures and shallow seismicity. A relative chronosequence was calibrated with existing ¹⁴C and ⁴⁰Ar/³⁹Ar dates and new cosmogenic nuclide exposure ages for selected rockslides. Rockslide-induced sediment yield was estimated with empirical relations for rockslide area distributions. Throughout the Quaternary, rockslides have delivered sediment to streams at rates equivalent to denudation rates of 0.10 ±0.06 mm a^− 1, while estimates using short term (20 a) seismicity records are 0.3_− 0.2^+ 0.6 mm a^− 1. The estimates of sediment transfer and the spatial distribution of rockslides reflect a landscape in which tectonic and geological controls on denudation are more significant than climate.     

The dynamics and characteristics of aeolian dust in dryland Central Asia: possible impacts on human exposure and respiratory health in the Aral Sea basin

The drying up of the Aral Sea has been described as one of the most staggering environmental disasters of the twentieth century. Over the last 40 years over 36 000 km² of the former seabed have been exposed, creating a potentially significant aeolian dust source. It is widely believed, but little researched, that increased dust storm activity in the region has had a major impact on human health. In this paper we report the findings of a study into the link between dust exposure and respiratory health amongst children in the Autonomous Republic of Karakalpakstan, Uzbekistan located on the southern shore of the Aral Sea. The findings indicate that dust deposition rates across the region are high, with sites located near the former shore of the sea being the worst affected. For these northerly regions the former bed of the Aral Sea is the most likely source of dust. The situation for the rest of the country appears to be far more complex. In these regions it appears that local sources and more distant sources to the south and south-west represent significant sediment providers, particularly in the early summer. Provisional analysis of the respiratory health data suggests that children living in the north of the country, where aeolian dust deposition rates are greater, show a lower frequency of respiratory problems. This inverse relationship requires further investigation, but highlights the complexities of environmental and human health inter-relationships.     

基于大数据分析的城市旅游地网络结构特征及其演化模式——以新浪微博签到数据为例

流动性表征了旅游地域系统要素间的内在关系,深刻影响着旅游地域网络结构的演变过程和方向。借助新浪微博爬虫技术,探讨了城市旅游地的网络结构特征,测度了旅游地节点的对外联系强度,提炼了复杂网络结构的演化模式。结果表明：① 从旅游地的等级结构来看,形成了以北京和上海为核心的全国性中心城市,核心城市凭借自身的旅游集聚和吸引功能,扩大了对周边城市的旅游影响,网络化的复杂程度逐步加深。② 从旅游地节点的层级结构来看,复杂化网络形态愈加明显,旅游地节点呈现明显的点状式分布态势,以北京、上海、广州为核心的旅游地域网络结构基本形成。③ 通过对比网络演化的过程表明,单中心发育模式由在一定时期内占据主导,逐渐转向以北京、上海、广州为核心的多地域、多中心的发展模式,核心城市的主导地位更加突显。与此同时,以成都和重庆为中心的区域性双核心空间结构发育模式表现突出,形成了以多中心发育模式和区域性双核心结构模式共存的局面。以上研究无疑为新流动性范式下重新认知和揭示复杂旅游网络体系特征及其对节点、网络、结构、空间的互动关系的深刻解读,提供了一次有益的理论尝试。