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 31 毫秒
1.
The Mont Blanc massif is one of a chain of basement culminations which crop out along the external French Alps. Its southwestern margin is interpreted as being a major thrust belt which propagated in a piggy-back sequence towards the foreland. These imbricates have developed in the footwall of the high-level Valais thrust. The depth to the floor thrust and shortening within imbricates above this thrust are estimated by a series of partially balanced cross-sections drawn between the ‘synclinal median’ and the Valais thrust. These sections restore to a pre-thrust length of at least 50 km, probably exceeding 100 km, above a floor thrust never deeper than 1 km below the sub-Triassic unconformity. All this thrust displacement is transferred via a series of lateral branch lines onto the Mont Blanc thrust in the Chamonix area. A corollary of this is that the Aiguilles Rouges and the main part of the Mont Blanc massif were separated by probably as much as 100 km prior to Alpine thrusting. Such large shortening estimates imply a hitherto unsuspected Dauphinois stratigraphic consistency in both thickness and lithology.To achieve a balance a restored crustal cross-section must show an equal length of both lower and upper crust. Thus a high-level basal detachment which floors large thrust displacements must overlie a long, undeformed lower crustal wedge. A restored section 100 km long requires such a lower crustal wedge to exist beneath the entire Alpine internal zones. Perrier & Vialon's crustal velocity profile through the western Alps is reinterpreted in these terms. The Ivrea body is considered to be a portion of an external lower crustal wedge which has been uplifted by thrusts after most of the displacement on the external thrust belt.  相似文献   

2.
The northeastern portion of the Mont Blanc massif in western Switzerland is predominantly comprised of the granitic rocks of the Mont Blanc intrusive suite and the Mont Blanc basement gneisses. Within these metamorphic rocks are a variety of sub-economic Fe skarns. The mineral assemblages and fluid inclusions from these rocks have been used to derive age, pressure, temperature and fluid composition constraints for two Variscan events. Metamorphic hornblendes within the assemblages from the basement amphibolites and iron skarns have been dated using 40Ar/39Ar, and indicate that these metamorphic events have a minimum age of approximately 334 Ma. Garnet-hornblende-plagioclase thermobarometry and stable isotope data obtained from the basement amphibolites are consistent with metamorphic temperatures in the range 515 to 580 °C, and pressures ranging from 5 to 8 kbar. Garnet-hornblende-magnetite thermobarometry and fluid inclusion studies indicate that the iron skarns formed at slightly lower temperatures, ranging from 400 to 500 °C in the presence of saline fluids at formational pressures similar to those experienced by the basement amphibolites. Late Paleozoic minimum uplift rates and geothermal gradients calculated using these data and the presence of Ladinien ichnofossils are on the order of 0.32 mm/year and 20 °C/km respectively. These uplift rates and geothermal gradients differ from those obtained from the neighbouring Aiguilles Rouges massif and indicate that these two massifs experienced different metamorphic conditions during the Carboniferous and Permian periods. During the early to late Carboniferous period the relative depths of the two massifs were reversed with the Aiguilles Rouges being initially unroofed at a much greater rate than the Mont Blanc, but experiencing relatively slower uplift rates near the termination of the Variscan orogeny. Received: 23 April 1996 / Accepted: 27 August 1996  相似文献   

3.
The structure of the northwest external French Alps, around the internal Belledonne and Mont Blanc massifs in the Beaufortain area, can be explained by foreland (WNW) propagating thrust tectonics. The internal Belledonne massif is interpreted as having developed by imbrication of an originally thin ‘skin’ of basement with a Mesozoic cover. Above this massif all thrusts tend to climb lateral sections to the south to converge with the frontal Pennine thrust: the southern termination of the Mont Blanc massif is merely a spectacular example of such a lateral ramp.  相似文献   

4.
The Helvetic nappes in Switzerland consist of sediments, which have been sheared off and thrust over the crystalline basement of the European passive continental margin during Alpine orogeny. Their basal shear zones usually root above the external crystalline massifs. However, the mechanisms that initiated the shear zones and the associated nappe formation are still debated. We perform two-dimensional numerical simulations of the shearing of linear viscous fluids above a linear viscous fluid with considerably higher viscosity (quasi-undeformable). The boundary between the fluid, mimicking the sediments, and the quasi-undeformable fluid, mimicking the basement, exhibits geometrical perturbations, mimicking half-grabens. These geometrical perturbations can trigger significant strain localization and the formation of shear zones within the linear viscous fluid although no rheological softening mechanism is active. This kinematic, ductile strain localization is caused by the half-grabens and the viscosity ratio between basement and sediments. The viscosity ratio has a strong control on the kinematics of strain localization, whereas the depth of the half-grabens has a weak control. For sediment viscosities in the order of 1021 Pas and typical half-graben geometries of 5 km depth and 25 km width the localization generates (a) low-angle shear zones at the basement-sediment interface, but also entirely within the sediments, (b) horizontal transport >10 km associated with the shear zones, (c) shear zones with thickness in the order of 100 m, (d) an ordered stacking of model nappes and (e) shear zones that root above the basement. The results suggest that tectonic inheritance in the form of half-grabens and associated kinematic strain localization could have been the triggering mechanism for Helvetic nappe formation, and not rheological softening mechanisms, which might, however, have subsequently further intensified shear localization significantly.  相似文献   

5.
Spatial control for in situ dating of mineral phases in fine‐grained rocks is a significant challenge in geochronology, and the precision of microsampling is a crucial factor in obtaining accurate results. In this study, a new microsampling approach to Rb–Sr geochronology has been applied to greenschist facies mylonitic shear zones in the Mont Blanc‐Aiguille Rouges region of the western European Alps. Using a laser‐ablation system for microsampling by laser cutting followed by conventional TIMS Rb–Sr isotopic analysis of μg‐sized samples provides an improved workflow for texturally controlled, quasi in situ dating of mineral phases. The automated cutting process minimizes material loss and the risk of handling errors, while facilitating sampling of complex shapes of almost any size, a significant improvement over earlier microscope‐mounted microdrills. The new Rb–Sr white mica–calcite ages of between 27 and 30 Ma indicate Oligocene deformation in Alpine shear zones from two specific areas in the Mont Blanc‐Aiguilles Rouges region.  相似文献   

6.
The tectonic evolution of the Mont Blanc range with regard to its cooling and exhumation history has been discussed and debated over many years and is still controversial. Recently, several low-temperature thermochronology studies have determined the cooling history of the massif in considerable detail and various tectonic models proposed to explain the young and fast exhumation signal established from these studies. Here we present detailed field data from the wider Mont Blanc area and assess possible exhumation processes in terms of these field constraints. Our observations indicate that none of the major faults or shear zones around the Mont Blanc massif (i.e. Mont Blanc shear zone, Mont Blanc back-thrust, Penninic thrust) was active in Late Neogene times and that young exhumation is therefore not controlled by movements along these structures. We demonstrate that the position of Mont Blanc in the bend of the western Alps plays an important role in its tectonic history and that simple 2D models are insufficient to explain its evolution. Interference between NW–SE compression and orogen-parallel extension along the Rhône-Simplon fault system resulted in a complex regional structural pattern, with strike-slip movements on both sides of the Mont Blanc massif. Young brittle faults are predominantly strike slip without significant vertical offset. The young (<2 Ma) rapid exhumation of Mont Blanc is more broadly distributed and cannot be directly linked to discrete faults bounding the massif. The mechanisms driving this recent accelerated exhumation must similarly be of broader scale.  相似文献   

7.
Field evidence around the southwestern termination of the Mont Blanc basement massif casts doubt on Butler's interpretation of the massif as a relatively thin thrust-sheet extending over the imbricated Dauphind cover. A thick-skinned model of basement faulting, analogous to Laramide faulting in the western U.S.A., seems more appropriate for the area.  相似文献   

8.
The structural analysis and the 3D modelling of Stephanian granites of the Pelvoux Massif characterize an emplacement along sinistral NW–SE- and dextral NE–SW-trending shear zones in the Pelvoux and in the Aiguilles Rouges–Mont Blanc Massifs, respectively. This Carboniferous shear system is consistent with a north–south extension direction known in the whole Variscan belt at this time. To cite this article: P. Strzerzynski et al., C. R. Geoscience 337 (2005).  相似文献   

9.
Petrological and structural investigations in metapelites and amphibolites were undertaken to unravel the Variscan tectonic and metamorphic evolution of the southwestern Aiguilles Rouges massif. The geometry of planar and linear structures indicates a transpressive deformation regime with a change from predominantly subhorizontal to subvertical movements during its evolution. In metapelite samples from gneiss areas, garnet zonation trends have been correlated with chemical variations in plagioclase and biotite by microstructural observations. Applied conventional cation-exchange and net-transfer geothermobarometers yielded anti-clockwise P–T-deformation paths. Clockwise P–T-deformation paths resulted from (Na, Ca)-amphibole zonations of amphibolite samples from a different lithotectonic unit. Notable differences in P–T conditions and the general shape of P–T paths for the gneisses and amphibolites indicate that the units do not represent a continuous metamorphic zonation and underwent no common tectono-metamorphic evolution throughout the entire deformation. The combination of P–T-deformation paths and structural investigations revealed first-stage thrust tectonics followed by a transpressive deformation which was responsible for the uplift of the southwestern Aiguilles Rouges massif.  相似文献   

10.
The hydrothermal system of Saint-Gervais-les-Bains, France is located in a south western low-elevation point of the Aiguilles Rouges crystalline Massif. The crystalline rocks are not directly outcropping in the studied area but certainly exist beyond 300 m depth. Uprising waters are pumped from two different aquifers below the Quaternary deposits of the Bon Nant Valley. In the Lower Trias-Permian aquifer crossed by De Mey boreholes (27–36 °C), the ascending Na-SO4 and high-Cl thermal water from the basement (4.8 g/L) is mostly mixed by a Ca-SO4 and low-Cl cold water circulating in the autochthonous cover of the Aiguilles Rouges Basement. The origin of the saline thermal water probably results from infiltration and circulation in the basement until it reaches deep thrust faults with leaching of residual brines or fluid inclusions at depth (Cl/Br molar ratio lower than 655). The dissolution of Triassic halite (Cl/Br > 1000) is not possible at Saint-Gervais-les-Bains because the Triassic cold waters have a low-Cl concentration (< 20 mg/L). Water–rock interactions occur during the upflow via north–south strike-slip faults in the basement and later on in the autochthonous cover. For the De Mey Est borehole, gypsum dissolution is occurring with cationic exchanges involving Na, as well as low-temperature Mg dissolution from dolomite in the Triassic formations. The aquifer of imbricated structures (Upper-Middle Trias) crossed by the Lépinay well (39 °C) contains thermal waters, which are strongly mixed with a low-Cl water, where gypsum dissolution also occurs. The infiltration area for the thermal end-member is in the range 1700–2100 m, close to the Lavey-les-Bains hydrothermal system corresponding to the Aiguilles Rouges Massif. For the Ca-SO4 and low-Cl end-member, the infiltration area is lower (1100–1300 m) showing circulation from the Mont Joly Massif. The geothermometry method indicates a reservoir temperature of probably up to 65 °C but not exceeding 100 °C.  相似文献   

11.
The Helvetic nappes of western Switzerland are discussed as an example of an arcuate foreland fold- and thrust belt in which active fold-axis parallel stretching occurred. Fold-axis parallel extension is recorded by:

1. (1) Incremental strain data from pressure shadow fibres. The significance of pressure shadow fibres for the determination of the deformation history of a region is discussed. Pressure shadows are used to quantify the amount of, and to describe the distribution of fold-axis parallel extension occurring in the Helvetic nappes.

2. (2) The extension directions of conjugate systems of en échelon veins. It is shown that an analysis of the geometry of conjugate vein systems can reveal a regional deformation pattern. The relative age of the conjugate en échelon vein systems in the Helvetic deformation history can be assessed, the geometry of the conjugate sets relative to the local anisotropy plane is described, and the significance of the preferred orientation of their extension directions is discussed.

3. (3) Fold-axis parallel sections. A comparison of the regional distribution of the fold-axis parallel strain with the shape of the Helvetic nappes in fold-axis parallel sections shows that the fold-axis parallel strain cannot be related to the footwall topography of the nappes.

It is concluded that the fold-axis parallel extension in the Helvetic nappes was induced by a change of direction of overthrust shear. This change occurred late in the deformation history and was superposed on the already formed nappes. The changing direction of overthrust shear is the expression of an overall anticlockwise rotation going on in the overthrusting Alpine nappe pile, relative to the European plate, a rotation which lead to the arcuate shape of the Western Alps.  相似文献   


12.
The U-Pb SHRIMP age determinations of zircons from the Habach terrane (Tauern Window, Austria) reveal a complex evolution of this basement unit, which is exposed in the Penninic domain of the Alpine orogen. The oldest components are found in zircons of a metamorphosed granitoid clast, of a migmatitic leucosome, and of a meta-rhyolitic (Variscan) tuff which bear cores of Archean age. The U-Pb ages of discordant zircon cores of the same rocks range between 540 and 520 Ma. It is assumed that the latter zircons were originally also of Archean origin and suffered severe lead loss, whilst being incorporated into Early-Cambrian volcanic arc magmas. The provenance region of the Archean (2.64-2.06 Ga) zircons is assumed to be a terrane of Gondwana affinity: i.e., the West African craton (Hoggar Shield, Reguibat Shield). The Caledonian metamorphism left a pervasive structural imprint in amphibolite facies on rocks of the Habach terrane; it is postdated by discordant zircons of a migmatitic leucosome at <440 Ma (presumably ca. 420 Ma). Alpine and Variscan upper greenschist- to amphibolite-facies conditions caused partial lead loss in zircons of a muscovite gneiss ('white schist') only, where extensive fluid flow and brittle deformation due to its position near a nappe-sole thrust enhanced the grains' susceptibility to isotopic disturbance. The Habach terrane - an active continental margin with ensialic back-arc development - showed subduction-induced magmatic activity approx. between 550 and 507 Ma. Back-arc diorites and arc basalts were intruded by ultramafic sills and subsequently by small patches of mantle-dominated unaltered and (in the vicinity of a major tungsten deposit) altered granitoids. Fore-arc (shales) and back-arc (greywackes, cherts) basin sediments as well as arc and back-arc magmatites were not only nappe-stacked by the Caledonian compressional regime closing the presumably narrow oceanic back-arc basin and squeezing mafic to ultramafic cumulates out of high-level magma chambers (496-482 Ma). It also induced uplift and erosion of deeply rooted crystalline complexes and triggered the development of a successor basin filled with predominantly clastic greywacke-arkosic sediments. The study demonstrates that the basement rocks exposed in the Habach terrane might be the 'missing link' between similar units of the more westerly positioned External domain (i.e., Aar, Aiguilles Rouges, Mont Blanc) and the Austroalpine domain to the east (Oetztal, Silvretta).  相似文献   

13.
  相似文献   

14.
We performed a gravity survey in the Mont-Blanc and Aiguilles Rouges ranges in order to improve the gravity data coverage of the Alpine crystalline external ranges, and to constrain the deep geometry of a crustal scale thrust. Preliminary results allow us to propose a geometry for the deep structure of the ranges, taking into account a major reverse fault that bounds the Mont-Blanc range: the Mont-Blanc shear zone. To cite this article: F. Masson et al., C. R. Geoscience 334 (2002) 1011–1019.  相似文献   

15.
The Morcles microgranite is located in the N–E termination of the Aiguilles Rouges massif (External Crystalline Massifs, Switzerland). It outcrops as dykes, a few meters to 150 m in thickness, intruding the Aiguilles Rouges polymetamorphic basement, and presents variation of texture from granophyric to rhyolitic. We present here for the first time, in situ U–Pb zircon dating of the Morcles microgranite/rhyolite based on laser-ablation—inductively coupled plasma—mass spectrometry (LA-ICP-MS) data. Results indicate late Variscan emplacement ages at ~303 and ~309–312 Ma, a major Caledonian inherited component age at ~445–460 Ma, and secondary inherited ages ranging from Pan-African (550–1000 Ma) to Paleoproterozoic (2.3 Ga). Geochronological and geochemical data indicate that the Morcles microgranite/rhyolite shares a common origin with the higher (or “H”) facies of the neighbouring Vallorcine granitic intrusion. This close affinity is further corroborated by the geographical alignment of both intrusive bodies on either side of the Rhone Valley. The fine-grained texture of the microgranite groundmass and the rhyolite indicates a very rapid cooling rate and emplacement close to the surface, suggesting that the Morcles microgranite/rhyolite may constitute the shallow-level counterpart of the Vallorcine granite. The mineralogical assemblages observed in the Morcles microgranite/rhyolite support the idea of high-temperature melting conditions provided by underplating of mantle-derived magmas during the Carboniferous extension of the Variscan cordillera.  相似文献   

16.
The Helvetic nappes in the Swiss Alps form a classic fold-and-thrust belt related to overall NNW-directed transport. In western Switzerland, the plunge of nappe fold axes and the regional distribution of units define a broad depression, the Rawil depression, between the culminations of Aiguilles Rouge massif to the SW and Aar massif to the NE. A compilation of data from the literature shows that, in addition to thrusts related to nappe stacking, the Rawil depression is cross-cut by four sets of brittle faults: (1) NE-SW striking normal faults, (2) NW-SE striking normal faults and joints, (3) ENE-WSW striking and (4) WNW-ESE striking normal plus dextral oblique-slip faults. Fault set 1 was probably initiated during sedimentation and reactivated during nappe stacking, whereas the other fault sets formed after emplacement of the Helvetic nappes. We studied in detail two well-exposed parallel fault zones from set 4, the Rezli fault zones (RFZ) in the Wildhorn Nappe. They are SW-dipping oblique-slip faults with a total displacement across the two fault zones of ~200 m vertically and ~680 m horizontally. The fault zones crosscut four different lithologies: limestone, intercalated marl and limestone, marl and sandstone. The internal architecture of the RFZ strongly depends on the lithology in which they developed. In the limestones, they consist of extension veins, stylolites, cataclasites and cemented gouge, in the intercalated marls and limestones of shear zones, brittle fractures and chaotic folds, in the marls of anastomosing shear zones, pressure solution seams and veins and in the sandstones of coarse breccia, brittle faults and extension veins. Sharp, discrete fault planes within the broader fault zones cross-cut all lithologies. Fossil fault zones in the Rezli area can act as a model for studying processes still occurring at deeper levels in this seismically active region.  相似文献   

17.
An integrated interpretation of seismicity, fault plane solutions and deep seismic reflection data suggests that the NE–SW to NW–SE trending Rhone–Simplon fault zone and the gently S-dipping basal Penninic thrust separate fundamentally different stress regimes in the western Swiss Alps. North of the Rhone-Simplon fault zone, strike-slip earthquakes on steep-dipping faults within the Helvetic nappes are a consequence of regional NW–SE compression and NE–SW extension. To the south, vertical maximum stress and N–S extension are responsible for normal mechanism earthquakes that occur entirely within the Penninic nappes above the basal Penninic thrust. Such normal faulting likely results from extension associated with southward movements (collapse) of the Penninic nappes and/or continued uplift and relative northward displacements of the underlying Alpine massifs. Geological mapping and fission-track dating suggest that the two distinct stress regimes have controlled tectonism in the western Swiss Alps since at least the Neogene.  相似文献   

18.
柯坪塔格推覆构造几何学、运动学及其构造演化   总被引：29，自引：1，他引：29  
大量野外构造地质调查和深部构造解释表明柯坪塔格推覆构造由多组倒转复式背斜、复式箱状背斜构成的推覆体及其前缘逆冲断裂组成 ,由寒武系—第四系组成的推覆体由北向南逆—斜冲 ,平面上构成向南凸出的弧形推覆构造 ;普昌断裂由各不相连的逆冲斜冲断裂段组成 ,而不是完整的一条走滑断层 ,各推覆体前缘逆冲断裂与各推覆体的普昌断裂段共同构成统一的前缘逆冲斜冲逆冲断裂和推覆构造系统 ;普昌断裂段以西的推覆体具有向东抬升、向西倾覆的鼻状构造特征 ,普昌断裂段以东的推覆体具有向西抬升、向东倾覆的鼻状构造特征 ,普昌基底隆起带是巴楚隆起隐伏在柯坪塔格推覆构造之下的部分。各推覆体前缘断裂在深部均归并于统一的寒武系底部的滑脱面 ,其南浅北深 ,东浅西深 (普昌隆起带以西 )或西浅东深 (普昌隆起带以东 ) (6 10km ) ,埋深较大区发育多组滑脱面。柯坪塔格推覆构造的形成时期为晚第四纪 ,为现今活动的推覆构造系统。文中认为各推覆体向南西的倾覆端基底滑脱面和中新生界内部的滑脱面没有贯通 ,是未来 6级以上地震的发震构造部位。  相似文献   

19.
The different segments of the tectonic boundary between external (European) and internal (Penninic) units in the Western Alps, the so-called Penninic Front (PF), formed at different times and according to different kinematic scenarios. During a first episode (Eocene), the PF corresponds to a transpressive suture zone between Penninic and European units. North- to NNW-trending stretching lineations, found along internal nappe contacts within the Penninic units, are related to this episode. This subduction zone was sealed by the Priabonian flysch of the Aiguilles d'Arves, a detrital trench formation that formed during the final stages of subduction. During a second episode, starting in mid-Oligocene times, the PF, imaged along the ECORS-CROP profile, acted as a WNW-directed thrust. This thrust, the Roselend Thrust (RT), only partially coincides with the PF. South of Moûtiers, the RT propagates into the Dauphinois units, carrying the former Eocene PF (including the Priabonian flysch) passively in its hangingwall. South of the Pelvoux massif the RT finds its continuation along the "Briançonnais Front", an out-of-sequence thrust behind the Embrunais-Ubaye nappes. On a larger scale, our findings indicate oblique (sinistral) collision within the future Western Alps during the Eocene, followed by westward indentation of the Adriatic block.  相似文献   

20.
鄂尔多斯西缘北段大型陆缘逆冲推覆体系   总被引：5，自引：0，他引：5       下载免费PDF全文
鄂尔多斯西缘北段是一个自中生代末以来形成的、结晶基底和早古生代大陆边缘沉积盖层同时卷入的巨型陆缘逆冲推覆构造体系。根据区域地层发育、变形岩石属性、冲断层几何学以及它们与联冲断层的关系,大体上可以分为不同形成阶段的3个冲断层构造组合,包括9个次级构造单元(B1—B7,BN,BS)。受冲断层运动自西向东的一致推进,整体呈现一个局部被近东西走向联冲断层切错、向东凸出的弧形:前端为陆缘褶皱冲断带;中部表现为一系列"原地"或"异地"推覆体和冲断席,发育低角度滑脱层和双冲构造;后部又被最晚期的冲断体叠置。侏罗-白垩纪为逆冲推覆构造的主要发展阶段,经历了3期主要的冲断层作用。第Ⅰ期发生在侏罗纪末,沿阿拉善—华北两类不同性质结晶基底之间的主滑脱面发生大规模冲断层作用,形成桌子山—岗德尔山褶皱冲断带。第Ⅱ期冲断层作用的持续位移,形成了具有上、下两个构造层的石嘴山—尖山大型异地推覆体,主滑脱面为石炭纪煤系地层,其中发育典型的双冲构造。新生代(距今65Ma)以来,印度—欧亚板块挤压碰撞和青藏高原早期向北推挤,加剧了鄂尔多斯西缘逆冲推覆构造的进一步发育,第Ⅲ期冲断层作用在东部陆缘褶皱冲断带形成了苏海图反冲构造的同时,在西部将异地推覆体下部的奥陶系再次推至地表。第Ⅰ期和第Ⅱ期冲断层作用累计位移幅度可能达到60~80km,第Ⅲ期冲断层作用的位移幅度为8km。相邻冲断席之间位移矢量的差异,通过近东西走向的联冲断层得到了调整。  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