横跨喜马拉雅造山带的构造运动转换与变形分配

喜马拉雅造山带包含喜马拉雅弧和东、西构造结3个基本部分,它们是大陆碰撞后印度板块继续向北移动,并向西藏高原下俯冲产生的构造变形系统.该系统的重要地质特征之一,是同时存在多种不同样式、不同或相反性质的地壳变形,例如地壳南北向缩短与东西向伸展,高原隆起与山间盆地下沉,与造山带走向大致平行的向北倾斜或向南倾斜的逆断层,东西向...     

喜马拉雅东构造结周边地区主要断裂现今运动特征与数值模拟研究

本研究通过对东构造结及其周边地区主要断裂进行野外考察,通过GPS观测数据和地质、地球物理资料的综合分析,建立三维有限元模型;运用数值模拟方法对东构造结周边地区主要断裂现今运动特征进行模拟研究,取得一些初步的认识:(1)东构造结北侧和东侧地块总体上围绕构造结发生顺时针旋转,右旋走滑的东南边界断裂不是嘉黎断裂,可能是阿帕龙...     

Build‐up and depositional dynamics of an arc front volcaniclastic complex: the Miocene Tepoztlán Formation (Transmexican Volcanic Belt,Central Mexico)

Volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on Earth. Ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment. The Early Miocene Tepoztlán Formation at the southern edge of the Transmexican Volcanic Belt belongs to the few remnants of this ancestral magmatic arc, and therefore is thought to represent an example of the initial phase of evolution of the Transmexican Volcanic Belt. Based on geological mapping, detailed logging of lithostratigraphic sections, palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility, mapping of two‐dimensional panels from outcrop to field scale, and geochronological data in an area of ca 1000 km², three periods in the evolution of the Tepoztlán Formation were distinguished, which lasted around 4 Myr and are representative of a volcanic cycle (edifice growth phases followed by collapse) in a magmatic arc setting. The volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons, similar to volcanic ring plains, around at least three different stratovolcanoes. These volcanoes resulted from various eruptions separated by repose periods. During the first phase of the evolution of the Tepoztlán Formation (22·8 to 22·2 Ma), deposition was dominated by fluvial sediments in a braided river setting. Pyroclastic material from small, andesitic–dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes, resulting in sediments ranging from cross‐bedded sand to an aggradational series of river gravels. The second phase (22·2 to 21·3 Ma) was characterized by periods of strong volcanic activity, resulting in voluminous accumulations of lava and tuff, which temporarily overloaded and buried the original fluvial system with its detritus. Continuous build‐up of at least three major volcanic centres further accentuated the topography and, in the third phase (21·3 to 18·8 Ma), mass flow processes, represented by an increase of debris flow deposits, became dominant, marking a period of edifice destruction and flank failures.     

The P–T–X(fluid) evolution of meta‐anorthosites in the Eastern Granulites,Tanzania

Meta‐anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up of clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within the feldspar‐rich matrix (70–90% plagioclase), mafic domains with metamorphic corona textures were used for P–T calculations. Central parts of these textures constitute high‐Al clinopyroxene – which is a common magmatic mineral in anorthosites – and is therefore assumed to be a magmatic relict. The clinopyroxene rims have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxene is surrounded by amphibole and scapolite suggesting that a mixed CO₂–H₂O fluid was present during their formation. Thermobarometric calculations give the following conditions for the metamorphic peak of the individual meta‐anorthosite bodies: Mwega: 11–13 kbar, 850–900 °C; Pare Mountains: 12–14 kbar, 850–900 °C; Uluguru Mountains: 12–14 kbar, 850–900 °C. The P–T evolution of these bodies was modelled using pseudosections. The amount and composition of the metamorphic fluid and <0.5 mol.% fluid in the bulk composition is sufficient to produce fluid‐saturated assemblages at 10 kbar and 800 °C. Pseudosection analysis shows that the corona textures most likely formed under fluid undersaturated conditions or close to the boundary of fluid saturation. The stabilities of garnet and amphibole are dependent on the amount of fluid present during their formation. Mode isopleths of these minerals change their geometry drastically between fluid‐saturated and fluid‐undersaturated assemblages. The garnet coronae developed during isobaric cooling following the metamorphic peak. The cooling segment is followed by decompression as indicated by the growth of amphibole and plagioclase. The estimated of the metamorphic fluid is ～0.3–0.5. Although the meta‐anorthosites have different formation ages (Archean and Proterozoic) they experienced the same Pan‐African metamorphic overprint with a retrograde isobaric cooling path. Similar P–T evolutionary paths are known from the hosting granulites. The presented data are best explained by a tectonic model of hot fold nappes that brought the different aged anorthosites and surrounding rocks together in the deep crust followed by an isobaric cooling history.     

Mega-fold interference patterns in the Beishan orogen (NW China) created by change in plate configuration during Permo-Triassic termination of the Altaids

Kilometer-size fold interference patterns in the Beishan Orogenic Collage (BOC) in the southernmost Altaids formed by fold superimposition in fossiliferous Permian sedimentary rocks. First-phase (F₁), upright and almost north-trending folds, were refolded by E- to ENE-trending F₂ folds, whose axial planes and axes are vertical or subvertical. From east to west there is a regional change in style of interference patterns from lobate–cuspate-, to crescent- to mushroom-shape. This variation is accompanied by a westward decrease in the F₂ interlimb angle and related to a higher percentage of coarse-grained clastic rocks, suggesting a dependence of the F₂ deformation on lithology. Axial planar slaty cleavages are well developed in F₁ and poorly developed in F₂ folds. The superposed folds mainly underwent flexural-slip and flexural flow folding to give rise to the lobate–cuspate pattern, and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids. The two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter-arc basin during the final amalgamation of the BOC in the latest Permian to early to mid-Triassic. The two folding events bracketed between a maximum detrital zircon age of <273 Ma, and the youngest age of an intruded dyke at 219.0 ± 1.2 Ma suggest rapid plate reconfiguration related to final amalgamation of the Altaids orogen.     

喜马拉雅造山带造山模式探讨

喜马拉雅是典型的碰撞型造山带,造山带结构构造复杂,可大致划分为以逆冲推覆构造为主的南喜马拉雅造山带和以各种伸展性构造为主的北喜马拉雅造山带,造山带内各类构造均发生过多期变形,且发生过多次缩短与伸展的构造反转,大喜马拉雅结晶杂岩系(GHC)内变形、岩浆及变质作用证明造山过程中存在渠道流作用。据此,本文提出一种由印度-欧亚大陆汇聚速率控制的多阶段造山模式:两大陆汇聚速度快时,青藏高原内形成南北向裂谷系(NSTR),喜马拉雅内经历造山过程,并在造山带中、下地壳形成作为底部拆离层的塑性层,汇聚速率慢时,青藏高原内形成共轭走滑断裂,喜马拉雅造山带内的塑性层发生松弛和重力扩散,形成渠道流,导致藏南拆离系(STDS)的启动、GHC的挤出和北喜马拉雅片麻岩穹窿(NHGD)的形成。上述的增厚与松弛均是在挤压体制下形成的,构造的反转是因挤压速率变化而产生的结构调节作用。     

Geochemistry and geochronology of the Rathjen Gneiss: Implications for the early tectonic evolution of the Delamerian Orogen

The Rathjen Gneiss is the oldest and structurally most complex of the granitic intrusives in the southern Adelaide Fold‐Thrust Belt and therefore provides an important constraint on the timing of the Delamerian Orogen. Zircons in the Rathjen Gneiss show a complex growth history, reflecting inheritance, magmatic crystallisation and metamorphism. Both single zircon evaporation (‘Kober’ technique) and SHRIMP analysis yield best estimates of igneous crystallisation of 514 ± 5 Ma, substantially older than other known felsic intrusive ages in the southern Adelaide Fold‐Thrust Belt. This age places an older limit on the start of the Delamerian metamorphism and is compatible with known stratigraphic constraints suggesting the Early Cambrian Kanmantoo Group was deposited, buried and heated in less than 20 million years. High‐U overgrowths on zircons were formed during subsequent metamorphism and yield a ²⁰⁶Pb/²³⁸U age of 503 ± 7 Ma. The Delamerian Orogeny lasted no more than 35 million years. The emplacement of the Rathjen Gneiss as a pre‐ or early syntectonic granite is emphasised by its geochemical characteristics, which show affiliations with within‐plate or anorogenic granites. In contrast, younger syntectonic granites in the southern Adelaide Fold‐Thrust Belt have geochemical characteristics more typical of granites in convergent orogens. The Early Ordovician post‐tectonic granites then mark a return to anorogenic compositions. The sensitivity of granite chemistry to changes in tectonic processes is remarkable and clearly reflects changes in the contribution of crust and mantle sources.     

Sulfur isotope distribution in Late Silurian volcanic‐hosted massive sulfide deposits of the Hill End Trough,eastern Lachlan Fold Belt,New South Wales

Volcanic‐hosted massive sulfide (VHMS) deposits of the eastern Lachlan Fold Belt of New South Wales represent a VHMS district of major importance. Despite the metallogenic importance of this terrane, few data have been published for sulfur isotope distribution in the deposits, with the exception of previously published studies on Captains Flat and Woodlawn (Captains Flat‐Goulburn Trough) and Sunny Corner (Hill End Trough). Here is presented 105 new sulfur isotope analyses and collation of a further 92 analyses from unpublished sources on an additional 12 of the VHMS systems in the Hill End Trough. Measured δ³⁴S values range from ‐7.4% to 38.3%, mainly for massive and stockwork mineralisation. Sulfur isotope signatures for polymetallic sulfide mineralisation from the Lewis Ponds, Mt Bulga, Belara and Accost deposits (group 1) are all very similar and vary from ‐1.7% to 5.9%. Ore‐forming fluids for these deposits were likely to have been reducing, with sulfur derived largely from a magmatic source, either as a direct magmatic contribution accompanying felsic volcanism or indirectly through dissolution and recycling of rock sulfide in host volcanic sequences. Sulfur isotope signatures for sulfide mineralisation from the Calula, Commonwealth, Cordillera and Kempfield deposits, Peelwood mine and Sunny Corner (group 2) are similar and have average δ³⁴S values ranging from 5.4% to 8.1%. These deposits appear to have formed from ore fluids that were more oxidising than group 1 deposits, representing a mixed contribution of sulfur derived from partial reduction of seawater sulfate, in addition to sulfur from other sources. The δ³⁴S values for massive sulfides from the John Fardy deposit are the highest in the present study and have a range of 11.9–14.5%, suggesting a greater component of sulfur of seawater origin compared to other VHMS deposits in the Hill End Trough. For barite the sulfur isotope composition for samples from the Commonwealth, Stringers and Kempfield deposits ranges from 12.6% to 38.3%. More than 75% of barite samples have a sulfur isotope composition between 23.4 and 30.6%, close to the previously published estimates of the composition of seawater sulfate during Late Silurian to earliest Devonian times, providing supporting evidence that these deposits formed concurrently with the Late Silurian volcanic event. Sulfur isotope distribution appears to be independent of the host rock unit, although there appears to be a relation linking the sulfur isotope composition of different deposits to defined centres of felsic volcanism. The Mt Bulga, Lewis Ponds and Accost systems are close to coherent felsic volcanic rocks and/or intrusions and have sulfur isotope signatures with a stronger magmatic affinity than group 2 deposits. By contrast, group 2 deposits (including John Fardy) are characterised by ³⁴S‐enrichment and a lesser magmatic signature, are generally confined to clastic units and reworked volcanogenic sediments with lesser coherent volcanics in the local stratigraphy, and are interpreted to have formed distal from the magmatic source. An exception is the Belara deposit, which is hosted by reworked felsic volcanic rocks and has a more pronounced magmatic sulfur isotope signature.     

Cyclic sedimentation in the Permian coal measures of New South Wales

Statistical analysis of borehole sections through the Illawarra and Newcastle Coal Measures of the Sydney Basin shows that cyclic sedimentation is present. The composite sequence for the Southern Coalfield (Illawarra Coal Measures) is (in ascending order): sandstone—sandstone/siltstone alternations—shale—coal, whereas that for the Newcastle Coalfield is: shale—sandstone/siltstone alternations—sandstone, often conglomeratic, or conglomerate—sandstone/siltstone alternations—shale —coal.

The environment of deposition is discussed. It is suggested that in the Southern Coalfield cyclicity is due to sedimentational processes inherent in the deltaic and alluvial conditions envisaged during Permian times. Periodic influxes of glacial meltwaters, although not essential, are not ruled out.

In the Newcastle Coalfield, however, the composite sequence does not match easily the ideal cycles expected in deltaic and/or alluvial regimes. Contemporary volcanism and tectonism complicated matters and lack of sedimentological details makes it impossible at present to give preference to any one mechanism of cycle formation.     

Spores from a carboniferous section in the hunter valley,New South Wales

In this initial systematic study of Carboniferous spores from New South Wales, Australia, fifteen species (all but one of them new) are formally described and are distributed among eight established genera and two new genera (Rattiganispora, a distally annulate trilete form, and Psomospora, an inaperturate or proximally hilate form). The species were selected as being the most characteristic and distinctive forms found in the Italia Road Formation at its well‐exposed type section in the Hunter Valley, east‐central New South Wales. The formation is a cyclical non‐marine unit, over 300 metres (1,000 ft) thick, consisting of lithic arenites together with carbonaceous shales, claystones, and siltstones; its age is regarded as West‐phalian‐Stephanian. The microfiora is compared with those known from sediments of similar age elsewhere and its place in the Australian Palaeozoic palynostratigraphic record is discussed.

New specific institutions are as follows: Punctatisporites lucidulus, P. sub‐tritus, Verrucosisporites aspratilis, V. italiaensis, Raistrickia accincta, R. radiosa, Reticulatisporites asperidictyus, R. magnidictyus, Foveosporites pellucidus, Rattiganispora apiculata (type species), Kraeuselisporites kuttungensis, Grandispora maculosa, Psomospora detecta (type species), and Wilsonites australiensis.