皖南江南陆内造山带的基本特征与中生代造山过程

皖南地区的江南隆起带 ,在震旦纪—中三叠世与周边一样处于被动大陆边缘海相环境。印支—早燕山期 ,该带成为陆内造山带 ,其中发育了一系列近东西向、向北逆冲的逆冲—推覆构造 ,使基底岩系相互叠置、强烈隆升。江南陆内造山带属板内叠置山系 ,不具阿尔卑斯式远程推覆体。地球物理资料表明 ,该陆内造山带下的地壳和岩石圈曾显著加厚。江南陆内造山带形成于北部华北与扬子板块发生陆—陆碰撞、南部华南板块向北推挤的区域动力学背景下     

青藏高原羌塘盆地南部古近纪逆冲推覆构造系统

西藏羌塘地块南部古近纪发育肖茶卡-双湖逆冲推覆构造、多玛-其香错逆冲推覆构造、赛布错-扎加藏布逆冲推覆构造,构成古近纪大型逆冲推覆构造系统。沿逆冲推覆构造的前锋断层,二叠系白云岩与大理岩化灰岩、三叠系砂岩与页岩、侏罗系碎屑岩与碳酸盐岩和三叠纪—侏罗纪蛇绿岩自北向南逆冲推覆于古近纪红色砂砾岩之上,形成规模不等的构造岩片与飞来峰。羌塘盆地南部主要的逆冲断层和下伏的褶皱红层被中新世湖相沉积地层角度不整合覆盖,表明逆冲推覆构造运动自中新世以来基本停止活动。羌塘盆地南部古近纪逆冲推覆构造运动在近南北方向产生的最小位移为90km,指示新生代早期上地壳缩短率约为47%。古近纪逆冲推覆构造对羌塘盆地油气资源具有重要影响。     

赣中峡江地区推滑覆构造特征及成因机制

通过对赣中峡江地区1：5万新干图组区域地质调查资料的综合研究。作者首次在该区提出了泥盆纪地层为外来体的观点，认为是印支晚期-燕山早期泥盆系由NW向SE推覆，燕山晚期再向SE伸展滑脱。江西北部和南部均有EW，NNE推，滑覆构造存在的报道，资料较多，但对赣中很少见有报道，此次峡江地区推，滑覆构造的发现和确定。对认识赣南与赣北NNE向推，滑覆构造的联系乃至全区整体认识，在地质构造上有着重要的意义，本文以详实的野外第一性资料，着重阐述了其推，滑覆构造特征，并分析了成因机制。     

The upper Hawasina nappes in the central Oman Mountains: stratigraphy,palinspastics and sequence of nappe emplacement

Abstract

In the Oman mountains, a succession of sedimentary decollement nappes, the Hawasina nappes, is sandwiched between the Samail ophiolite nappe and its underlying melange and the “autochthonous” sequences of the Arabian platform. The sediments of the Hawasina nappes document the Mesozoic evolution of the northeastern Arabian continental margin and the adjacent Tethys Ocean. In earlier paleogeographic reconstructions, based on simple telescoping of the tectonic units, the upper Hawasina nappes represent the distal part and the lower nappes the proximal part of the margin. New stratigraphic data suggest a revision of the paleogeography and a more complex model for nappe emplacement in the central Oman mountains. The lower Hawasina nappes with their Jurassic and Cretaceous base of slope and basin sediments (Hamrat Duru, Wahrah) form the original cover of part of the upper Hawasina nappes. In the latter (Al Ayn, Haliw), Triassic pelagic sediments, locally overlain by massive sandstone successions are preserved. Complete Mesozoic sequences with pelagic Cenomanian sediments as youngest dated elements are found in the highest Hawasina units (Al Aridh and Oman Exotics). The stratigraphic data indicate polyphase thrusting in the central Oman mountains. Downward propagation of thrusting in front of the Samail is responsible for cutting the original stratigraphie sequence into a number of thrust-sheets, involving successively older and more external formations. This kind of thrust propagation eventually leads to the observed superposition of originally lower stratigraphie units onto their original cover. Regional deformation of the nappe contacts in post-nappe culminations (J. Akhdar, Saih Hatat) is related to ramp-flat-systems in the Arabian foreland.     

The nappe structure of the central Northern Calcareous Alps and its disintegration during Miocene tectonic extrusion—a contribution to understanding the orogenic evolution of the Eastern Alps

The classical concept of the nappe structure of the central Northern Calcareous Alps (NCA) is in contradiction with modern stratigraphic, structural, metamorphic and geochronological data. We first perform a palinspastic restoration for the time before Miocene lateral tectonic extrusion, which shows good continuity of structures, facies and diagenetic/metamorphic zones. We present a new nappe concept, in which the Tirolic unit practically takes the whole area of the central NCA and is divided into three subunits (nappes): Lower and Upper Tirolic subunit, separated by the Upper Jurassic Trattberg Thrust, and the metamorphic Ultra-Tirolic unit. The Hallstatt (Iuvavic) nappe(s) formed the highest unit, but were completely destroyed by erosion after nappe stacking. Remnants of the Hallstatt nappes are only represented by components of up to 1 km in size in Middle/Upper Jurassic radiolaritic wildflysch sediments ("Hallstatt Mélange" belonging to the Tirolic unit). Destruction of the continental margin started in Middle to Upper Jurassic time and prograded from the oceanic side towards the shelf. The original substratum of the external nappes (Bavaric units) of the NCA was largely the Austroalpine crystalline basement, of the internal nappes (Tirolic units) the weakly metamorphosed Palaeozoic sequences (Greywacke Zone and equivalents). Eocene movements caused limited internal deformation in the Tirolic unit.     

Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P–T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5–3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10–20 Myr (after 400–600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P–T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.