Three‐dimensional seismic characterization of a complex sediment drift in the South China Sea: Evidence for unsteady flow regime

This study describes a previously unobserved reflection seismic configuration comprising a honeycomb planform and a repeated erosion/infill cross‐section, based on high‐resolution three‐dimensional/two‐dimensional seismic data and bathymetric data. The honeycomb structures cover an area of more than 5000 km² and are developed within the Late Miocene to recent deep‐water sediments of the north‐western South China Sea. Linear erosional troughs up to 10 km long and 1 km wide are widely developed in this area, are intimately related to the particular seismic configuration and interpreted to represent a new type of sediment drift that is caused by unsteady bottom current regimes operating since the Late Miocene. The unsteady bottom current regimes are suggested to be triggered by irregular seabed morphologies. Considerable sea‐floor topography was generated as a direct result of tectonic movements in the area since the Late Miocene, and this topography then influenced the pathways of strong bottom currents. This study highlights that: (i) an unsteady bottom current regime can be laterally extensive and persist for millions of years; (ii) structurally controlled sea‐floor relief plays an important role in controlling the depositional pattern; and (iii) the bottom currents were active since the Late Miocene, flowing from the south‐east through the Xisha–Guangle Gateway and crossing the honeycomb structure zone. This study documents a new style of drift and will help to improve current knowledge of palaeoceanography and understanding of the South China Sea deep‐water circulation which is at present still poorly understood.     

How turbidity current frequency and character varies down a fjord‐delta system: Combining direct monitoring,deposits and seismic data

Submarine turbidity currents are one of the most important processes for moving sediment across our planet; they are hazardous to offshore infrastructure, deposit petroleum reservoirs worldwide, and may record tsunamigenic landslides. However, there are few studies that have monitored these submarine flows in action, and even fewer studies that have combined direct monitoring with longer‐term records from core and seismic data of deposits. This article provides one of the most complete studies yet of a turbidity current system. The aim here is to understand what controls changes in flow frequency and character along the turbidite system. The study area is a 12 km long delta‐fed fjord (Howe Sound) in British Columbia, Canada. Over 100 often powerful (up to 2 to 3 m sec^?1) events occur each year in the highly‐active proximal channels, which extend for 1 to 2 km from the delta lip. About half of these events reach the lobes at the channel mouths. However, flow frequency decreases rapidly once these initially sand‐rich flows become unconfined, and only one to five flows run out across the mid‐slope each year. Many of these sand‐rich, channelized, delta‐sourced flows therefore dissipated over a few hundred metres, once unconfined, rather than eroding and igniting. Upflow migrating bedforms indicate that supercritical flow dominated in the proximal channels and lobes, and also across the unconfined mid‐slope. These supercritical flows deposited thick sand beds in proximal channels and lobes, but thinner and finer beds on the unconfined mid‐slope. The distal flat basin records far larger volume and more hazardous events that have a recurrence interval of ca 100 years. This study shows how sand‐rich delta‐fed flows dissipate rapidly once they become unconfined, that supercritical flows dominate in both confined and unconfined settings, and how a second type of more hazardous, and much less frequent event is linked to a different scale of margin failure.     

Widespread overspill from a saline density‐current channel and its interaction with topography on the south‐west Black Sea shelf

Seaward of the Bosphorus Strait, the south‐west Black Sea shelf is dominated by the world's largest channel network maintained by a quasi‐continuous saline (ca 35 → 31 psu) underflow. Calculations indicate that >85% of the initial discharge of ca 10⁴ m³s^?1 spills overbank before the shelf edge. This paper documents interaction of the overspill with sea bed topography using multibeam bathymetry, echo‐sounder images of the water column, conductivity–temperature–depth profiles and sediment cores. Overbank spill is widespread, particularly through crevasse channels and on the middle shelf where confinement by channel banks is negligible. Towards the outer shelf, the wind‐driven Rim Current advects mud along the shelf, contributing to levée successions and deposition on stoss sides of elongate transverse ridges. Echo‐sounder profiles reveal metre‐scale eddies over megaflutes, and breaking lee waves and internal hydraulic jumps over ridges. Megaflutes reach 600 m long and 7 m deep, yet form where the underflow, outside the flute, is no thicker than ca 2 to 5 m. Two types of elongate seaward‐facing ridges are recognized. Type 1 ridges, 2 to 5 m high, consist of bivalve‐rich muddy sand in low‐angle (3·5° to 6°) cross‐sets created by the underflow. Type 2 ridges, ca 5 m high, have crests up to 2 km long and a buried wedge‐shaped foundation (the ‘ridge‐core’) comprised of facies similar to Type 1 ridges. These ridge‐cores are blanketed on the landward side by stratified muds, and are capped by obliquely oriented ribs supporting a diverse benthic community. This facies distribution is interpreted to result from stoss‐side and lee‐side velocity and turbulence fluctuations induced by internal hydraulic jumps and breaking lee waves in overspilling portions of the underflow. Experimental results published by W.H. Snyder and co‐workers effectively explain ridge evolution and flow across the ridges, and therefore can be applied with confidence to less easily studied deep‐marine settings swept by turbidity currents.     

Morphological evolution of a well‐constrained,subaerial–subaqueous source to sink system: Wabush Lake

Since 1964, the Iron Ore Company of Canada has deposited iron tailings resulting from mining operations into Wabush Lake, Western Labrador, Canada. This activity has resulted in a continuous hyperpycnal flow flowing over the lake floor. Bathymetric surveys were carried out between 1999 and 2008 as part of the overall environmental Iron Ore Company of Canada plans to maintain safe disposal strategies of tailings into the lake. Here, this dataset is used to study the evolution of different channels present in Wabush Lake and their link with knickpoints found at the head of most of these channels. The channels show notable evolution from one survey to the next. Some knickpoints and channels may be followed throughout more than one survey; the evolution of these knickpoints is the process responsible for the creation of erosional channels. The context in which they appear, as well as the evolution of some of the subaqueous channels is analysed. Cyclic steps and sediment waves are also found in the lake. These rhythmic bedforms, also created by the passage of turbidity currents, are analysed. Sediment waves found in Wabush Lake show values for both wavelength and wave height that are approximately one order of magnitude smaller than most of those found in the marine environment.     

Anatomy of a submarine pyroclastic flow and associated turbidity current: July 2003 dome collapse, Soufrière Hills volcano, Montserrat, West Indies

The 12 to 13 July 2003 andesite lava dome collapse at the Soufrière Hills volcano, Montserrat, provides the first opportunity to document comprehensively both the sub‐aerial and submarine sequence of events for an eruption. Numerous pyroclastic flows entered the ocean during the collapse, depositing approximately 90% of the total material into the submarine environment. During peak collapse conditions, as the main flow penetrated the air–ocean interface, phreatic explosions were observed and a surge cloud decoupled from the main flow body to travel 2 to 3 km over the ocean surface before settling. The bulk of the flow was submerged and rapidly mixed with sea water forming a water‐saturated mass flow. Efficient sorting and physical differentiation occurred within the flow before initial deposition at 500 m water depth. The coarsest components (～60% of the total volume) were deposited proximally from a dense granular flow, while the finer components (～40%) were efficiently elutriated into the overlying part of the flow, which evolved into a far‐reaching turbidity current.     

Bed‐thickness and grain‐size trends in a small‐scale proglacial channel–levée system; the Carboniferous Jejenes Formation,Western Argentina: implications for turbidity current flow processes

Preserved in Quebrada de las Lajas, near San Juan, Argentina, is an ancient subaqueous proglacial sedimentary succession that includes a small‐scale (ca 50 m thick and ca 200 m wide) channel–levée system with excellent exposure of the channel axis and levée sediments. Coeval deposition of both the channel axis and the levées can be demonstrated clearly by lateral correlation of individual beds. The channel axis consists predominantly of a disorganized, pebble to boulder conglomerate with a poorly sorted matrix. The channel axis varies from 10 to 20 m wide and has a total amalgamated thickness of around 50 m. Beds fine gradationally away from the cobble–boulder conglomerates of the channel axis within a few metres, transitioning to well‐organized pebble to cobble conglomerates and sandstones of the channel margin. Within 60 m outboard of the channel axis in both directions, perpendicular to the trend of the channel axis, the mean grain size of the beds in the levées is silt to fine‐grained sand. Deposits in this channel–levée system are the product of both debris flows (channel axis) and co‐genetic turbidity currents (channel margins and levées). Bed thicknesses in the levées increase for up to 10 to 25 m away from the channel axis, beyond which bed thicknesses decrease with increasing distance. The positions of the bed thickness maxima define the levée crests, and the thinning beds constitute the outer levée slopes. From these relationships it is clear that the levée crest migrated both away from and toward the channel axis, and varied in height above the channel axis from 4 to 5 m (undecompacted), whereas the height of the levée crest relative to the distal levée varied from 4·5 to 10 m, indicating that the channel was at times super‐elevated relative to the distal levée. Bed thickness decay on the outside of the levée crest can be described quite well with a power‐law function (R² = 0·85), whereas the thickness decay from the levée crest toward the channel axis follows a linear function (R² = 0·78). Grain‐size changes are quite predictable from the channel margin outward, and follow logarithmic (R² = 0·77) or power‐law (R² = 0·72) decay curves, either of which fit the data quite well. This study demonstrates that, in at least this case: (i) levée thickness trends can be directly related to channel‐flow processes; (ii) individual bed thickness changes may control overall levée geometry; and (iii) levée and channel deposits can be coeval.     

东非鲁伍马盆地深水X气藏海底扇储层构型研究:重力流—底流交互作用的指示意义*

底流在陆缘深水环境下广泛存在,可对深水沉积过程及砂体分布产生重要影响。前人对重力流与底流的交互作用机制及沉积产物开展了大量研究,但目前有关底流改造型的海底扇储层构型模式仍然研究不够深入。东非鲁伍马盆地是当前重力流—底流交互作用研究的热点地区,文中以其代表性的下始新统海底扇水道体系为例,综合岩心、测井及三维地震资料开展储层构型精细表征,建立重力流—底流交互作用下的海底扇水道体系构型模式。研究表明,目标水道体系内部发育水道、溢岸及朵叶3种构型要素,其中水道可分为水道复合体、单一水道及其内部不同级次的构型单元。底流对细粒物质的搬运可形成非对称的溢岸沉积,导致水道复合体之间呈逆底流侧向迁移叠置样式,其间泥岩隔层容易保存; 单一水道之间呈顺水道纵向迁移或逆底流侧向迁移样式,其中纵向迁移部位水道切叠连通,而侧向迁移部位容易保存泥质侧向隔挡体。受重力流沉积演化的影响,单一水道内部充填由砂泥交互型逐渐演化为富砂型,且在水道弯曲段的轴部砂体最为发育。     

Fluvial and submarine morphodynamics of laminar and near‐laminar flows: a synthesis

The interaction of flow with an erodible bed in alluvial rivers and deep‐sea channels gives rise to a wide range of self‐formed morphologies, including channels, ripples, dunes, antidunes, alternate bars, multiple‐row bars, meandering and braiding. As the flow is invariably turbulent in field manifestations of these morphologies, there has been a tendency to assume that turbulence is necessary for them to form. While turbulence undoubtedly has an important influence when it is present, it is not necessary for any of these features. Indeed, all of these features can be formed by the morphodynamic interaction of purely laminar or nearly laminar flow with an erodible bed. This paper provides a survey and synthesis of a wide range of laminar or near‐laminar flow analogues of morphologies observed in the field. Laminar‐flow analogues of turbulent‐flow morphologies cannot and should not be expected to satisfy dynamic similarity in terms of all relevant dimensionless parameters. What is of more significance is the convergence of the underlying physics. It is illustrated in this paper that many existing theoretical frameworks for the explanation of turbulent‐flow morphodynamics require only relatively minor modification in order to adapt them to laminar flows.     

Flow processes and sedimentation in unidirectionally migrating deep‐water channels: From a three‐dimensional seismic perspective

Three‐dimensional seismic data were used to infer how bottom currents control unidirectional channel migration. Bottom currents flowing towards the steep bank would deflect the upper part of sediment gravity flows at an orientation of 1° to 11° to the steep bank, yielding a helical flow circulation consisting of a faster near‐surface flow towards the steep bank and a slower basal return flow towards the gentle bank. This helical flow model is evidenced by the occurrence of bigger, muddier (suggested by low‐amplitude seismic reflections) lateral accretion deposits and gentle channel wall with downlap terminations on the gentle bank and by smaller, sandier (indicated by high‐amplitude seismic reflectors) channel fills and steep channel walls with truncation terminations on the steep bank. This helical flow circulation promotes asymmetrical depositional patterns with dipping accretion sets restricted to the gentle bank, which restricts the development of sinuosity and yields unidirectional channel migration. These results aid in obtaining a complete picture of flow processes and sedimentation in submarine channels.     

湖相重力流水道沉积特征与沉积模式:以鄂尔多斯盆地瑶曲铁路桥剖面三叠系延长组为例*

陆相深水重力流水道的类型细分及其沉积模式是制约其油气勘探开发的重要因素,但研究程度低。通过对鄂尔多斯盆地南缘瑶曲铁路桥剖面三叠系延长组实测、水道形态参数统计及岩相、粒度等分析,开展了湖相重力流水道的沉积特征、沉积过程及沉积模式研究。结果表明:(1)研究区内可识别出4期复合水道,主要为洪水重力流成因。根据其内部单一水道及单砂体形态特征、岩相组成,将其细分为沉积型和过渡型两类。(2)剖面下部2期复合水道为沉积型,以悬浮载荷成因岩相为主,常见块状净细砂岩、薄层泥岩岩相组合和鲍马序列岩相组合;上部2期复合水道为过渡型,岩相以底床载荷与悬浮载荷共存为特征,自下而上以交错层理细砂岩或叠瓦状泥砾细砂岩与含泥砾/泥岩撕裂屑块状细砂岩、平行层理粗粉砂岩及薄层泥岩的岩相组合为特征。(3)结合单一水道规模及其相互关系,建立了区内过渡、沉积型重力流水道的半定量沉积模式。过渡型水道内部侵蚀与沉积作用共存,单一水道宽度小、宽厚比低,呈透镜状,水道间切割性强,砂体横向稳定性较低,表现出不定向叠加、侧向拼接样式;沉积型水道内部由沉积作用主导,单一水道宽度较大、宽厚比较高,呈似板状—透镜状,砂体横向稳定性较高,表现出稳定的垂向加积样式。     

Facies architecture of individual basin‐plain turbidites: Comparison with existing models and implications for flow processes

Current understanding of submarine sediment density flows is based heavily on their deposits, because such flows are notoriously difficult to monitor directly. However, it is rarely possible to trace the facies architecture of individual deposits over significant distances. Instead, bed‐scale facies models that infer the architecture of ‘typical’ deposits encapsulate current understanding of depositional processes and flow evolution. In this study, the distribution of facies in 12 individual beds has been documented along downstream transects over distances in excess of 100 km. These deposits were emplaced in relatively flat basin‐plain settings in the Miocene Marnoso Arenacea Formation, north‐east Italy and the late Quaternary Agadir Basin, offshore Morocco. Statistical analysis shows that the most common series of vertical facies transitions broadly resembles established facies models. However, mapping of individual beds shows that they commonly deviate from generalized models in several important ways that include: (i) the abundance of parallel laminated sand, suggesting deposition of this facies from both high‐density and low‐density turbidity current; (ii) three distinctly different types of grain‐size break, suggesting waxing flow, erosional hiatuses and bypass of silty sediment; (iii) the presence of mud‐rich debrites demonstrating hybrid flow deposition; and (iv) dune‐scale cross‐lamination in fine‐medium grained sandstones. Submarine sediment density flows in basin‐plain settings flow over relatively simple topography. Yet, their deposits record complex flow events, involving transformation between different flow types, rather than the simple waning surges often associated with the distal parts of turbidite systems.     

Metamorphic P–T profile and P–T path discontinuity across the far‐eastern Nepal Himalaya: investigation of channel flow models

Thermobarometric data and compositional zoning of garnet show the discontinuities of both metamorphic pressure conditions at peak‐T and P–T paths across the Main Central Thrust (MCT), which juxtaposes the high‐grade Higher Himalayan Crystalline Sequences (HHCS) over the low‐grade Lesser Himalaya Sequences (LHS) in far‐eastern Nepal. Maximum recorded pressure conditions occur just above the MCT (～11 kbar), and decrease southward to ～6 kbar in the garnet zone and northward to ～7 kbar in the kyanite ± staurolite zone. The inferred nearly isothermal loading path for the LHS in the staurolite zone may have resulted from the underthrusting of the LHS beneath the HHCS. In contrast, the increasing temperature path during both loading and decompression (i.e. clockwise path) from the lowermost HHCS in the staurolite to kyanite ± staurolite transitional zone indicates that the rocks were fairly rapidly buried and exhumed. Exhumation of the lowermost HHCS from deeper crustal depths than the flanking regions, recording a high field pressure gradient (～1.2–1.6 kbar km^?1) near the MCT, is perhaps caused by ductile extrusion along the MCT, not the emplacement along a single thrust, resulting in the P–T path discontinuities. These observations are consistent with the overall scheme of the model of channel flow, in which the outward flowing ‘HHCS’ and inward flowing ‘LHS’ are juxtaposed against each other and are rapidly extruded together along the ‘MCT’. A rapid exhumation by channel flow in this area is also suggested by a nearly isothermal decompression path inferred from cordierite corona surrounding garnet in gneiss of the upper HHCS. However, peak metamorphic temperatures show a progressive increase of temperature structurally upward (～570–740 °C) near the MCT and roughly isothermal conditions (～710–810 °C) in the upper structural levels of the HHCS. The observed field temperature gradient is much lower than those predicted in channel flow models. However, the discrepancy could be resolved by taking into account heat advection by melt and/or fluid migration, as these can produce low or nearly no field temperature gradient in the exhumed midcrust, as observed in nature.     

Channelized debris‐flow deposits and their impact on turbidity currents: The Puchkirchen axial channel belt in the Austrian Molasse Basin

Deposits of submarine debris flows can build up substantial topography on the sea floor. The resulting sea floor morphology can strongly influence the pathways of and deposition from subsequent turbidity currents. Map views of sea floor morphology are available for parts of the modern sea floor and from high‐resolution seismic‐reflection data. However, these data sets usually lack lithological information. In contrast, outcrops provide cross‐sectional and lateral stratigraphic details of deep‐water strata with superb lithological control but provide little information on sea floor morphology. Here, a methodology is presented that extracts fundamental lithological information from sediment core and well logs with a novel calibration between core, well‐logs and seismic attributes within a large submarine axial channel belt in the Tertiary Molasse foreland basin, Austria. This channel belt was the course of multiple debris‐flow and turbidity current events, and the fill consists of interbedded layers deposited by both of these processes. Using the core‐well‐seismic calibration, three‐dimensional lithofacies proportion volumes were created. These volumes enable the interpretation of the three‐dimensional distribution of the important lithofacies and thus the investigation of sea floor morphology produced by debris‐flow events and its impact on succeeding turbidite deposition. These results show that the distribution of debris‐flow deposits follows a relatively regular pattern of levées and lobes. When subsequent high‐density turbidity currents encountered this mounded debris‐flow topography, they slowed and deposited a portion of their sandy high‐density loads just upstream of morphological highs. Understanding the depositional patterns of debris flows is key to understanding and predicting the location and character of associated sandstone accumulations. This detailed model of the filling style and the resulting stratigraphic architecture of a debris‐flow dominated deep‐marine depositional system can be used as an analogue for similar modern and ancient systems.     

Structural kinematics,metamorphic P–T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south‐central Tibet: implication for a revised channel flow

A specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ～749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ～4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.     

Mud dispersal across a Cretaceous prodelta: Storm‐generated,wave‐enhanced sediment gravity flows inferred from mudstone microtexture and microfacies

Determining sediment transport direction in ancient mudrocks is difficult. In order to determine both process and direction of mud transport, a portion of a well‐mapped Cretaceous delta system was studied. Oriented samples from outcrop represent prodelta environments from ca 10 to 120 km offshore. Oriented thin sections of mudstone, cut in three planes, allowed bed microstructure and palaeoflow directions to be determined. Clay mineral platelets are packaged in equant, face‐face aggregates 2 to 5 μm in diameter that have a random orientation; these aggregates may have formed through flocculation in fluid mud. Cohesive mud was eroded by storms to make intraclastic aggregates 5 to 20 μm in diameter. Mudstone beds are millimetre‐scale, and four microfacies are recognized: Well‐sorted siltstone forms millimetre‐scale combined‐flow ripples overlying scoured surfaces; deposition was from turbulent combined flow. Silt‐streaked claystone comprises parallel, sub‐millimetre laminae of siliceous silt and clay aggregates sorted by shear in the boundary layer beneath a wave‐supported gravity flow of fluid mud. Silty claystone comprises fine siliceous silt grains floating in a matrix of clay and was deposited by vertical settling as fluid mud gelled under minimal current shear. Homogeneous clay‐rich mudstone has little silt and may represent late‐stage settling of fluid mud, or settling from wave‐dissipated fluid mud. It is difficult or impossible to correlate millimetre‐scale beds between thin sections from the same sample, spaced only ca 20 mm apart, due to lateral facies change and localized scour and fill. Combined‐flow ripples in siltstone show strong preferred migration directly down the regional prodelta slope, estimated at ca 1 : 1000. Ripple migration was effected by drag exerted by an overlying layer of downslope‐flowing, wave‐supported fluid mud. In the upper part of the studied section, centimetre‐scale interbeds of very fine to fine‐grained sandstone show wave ripple crests trending shore normal, whereas combined‐flow ripples migrated obliquely alongshore and offshore. Storm winds blowing from the north‐east drove shore‐oblique geostrophic sand transport whereas simultaneously, wave‐supported flows of fluid mud travelled downslope under the influence of gravity. Effective wave base for sand, estimated at ca 40 m, intersected the prodelta surface ca 80 km offshore whereas wave base for mud was at ca 70 m and lay ca 120 km offshore. Small‐scale bioturbation of mud beds co‐occurs with interbedded sandstone but stratigraphically lower, sand‐free mudstone has few or no signs of benthic fauna. It is likely that a combination of soupground substrate, frequent storm emplacement of fluid mud, low nutrient availability and possibly reduced bottom‐water oxygen content collectively inhibited benthic fauna in the distal prodelta.     

Non‐normality and induced plastic anisotropy under fractional plastic flow rule: a numerical study

In this paper, an implementation of fractional plastic flow rule in the framework of implicit and explicit procedures is under consideration. The fractional plastic flow rule is obtained from a generalisation of the classical plastic flow rule utilising fractional calculus. The key feature of this new concept is that in general, the non‐associative flow is obtained without necessity of additional potential assumption. If needed, the model can cover the anisotropy induced by plastic deformation. Illustrative examples showing the unusual flexibility of this model are also presented. Copyright © 2015 John Wiley & Sons, Ltd.     

龙门山后山震旦系—古生界变形变质作用:松潘-甘孜造山带中生代伸展垮塌下的中地壳韧性流壳层

中生代早期造山作用使松潘-甘孜地区地壳厚度加厚到约50~60km,因而随即经历了大规模区域性地壳伸展和减薄作用,然而迄今为止,对伸展和减薄事件的形成和发育机制还缺少深入了解。通过对龙门山前陆逆冲带腹陆地区,特别是其中发育的变质核杂岩及伸展变质穹隆体的详细构造解析,发现震旦系—古生界中普遍发育各种形式的顺层韧性流变构造,如韧性剪切带、透入性顺层面理及矿物拉伸线理、糜棱岩化及绿片岩相—低角闪岩相变质作用,并在龙门山北、中和南段造成大规模和不同程度的地层构造缺失或减薄;韧性流变构造流变方向在龙门山北段指向南或SSE,中、南段则指向SE;对志留系茂县群变质作用温压条件进行估算,其温度变化范围为265~405℃,压力变化范围为0.31~0.48GPa,代表了中地壳韧性流壳层(middle crustal ductile channel flow)的形成条件;前人用39Ar/40Ar和SHRIMP锆石U-Pb等方法对这一套区域动力变质岩石变质年龄的时代限定为190~150Ma,与中生代早期造山后板内伸展减薄事件相匹配。因此表明造山作用加厚地壳在中地壳层次以大规模韧性流变变形和变质作用对地壳厚度进行了调整,相对于上地壳层次变形和变质作用而言,中地壳韧性流壳层是松潘-甘孜造山带伸展和减薄的主要原因。在区域上如果消除新生代松潘-甘孜高原加厚和相对上扬子地块逆时针旋转的影响,中生代韧性流壳层流变方向总体为SSE或向南,因此代表南秦岭造山作用后的板内演化阶段,并且是造成松潘-甘孜造山带伸展垮塌的主要原因。     

A three‐dimensional numerical model of sediment transport,erosion and deposition within a network of channel belts,floodplain and hill slope: extrinsic and intrinsic controls on floodplain dynamics and alluvial architecture

A three‐dimensional numerical model of sediment transport, erosion and deposition within a network of channel belts and associated floodplain is described. Sediment and water supply are defined at the upstream entry point, and base level is defined at the downstream edge of the model. Sediment and water are transported through a network of channels according to the diffusion equation, and each channel has a channel belt with a width that increases in time. The network of channels evolves as a result of channel bifurcation and abandonment (avulsion). The timing and location of channel bifurcation is controlled stochastically as a function of the cross‐valley slope of the floodplain adjacent to the channel belt relative to the down‐valley slope, and of annual flood discharge. A bifurcation develops into an avulsion when the discharge of one of the distributaries falls below a threshold value. The floodplain aggradation rate decreases with distance from the nearest active channel belt. Channel‐belt degradation results in floodplain incision. Extrinsic (extrabasinal, allogenic) and intrinsic (intrabasinal, autogenic) controls on floodplain dynamics and alluvial architecture were modelled, and sequence stratigraphy models were assessed. Input parameters were chosen based on data from the Rhine–Meuse delta. To examine how the model responds to extrinsic controls, the model was run under conditions of changing base level and increasing sediment supply. Rises and falls in base level and increases in sediment supply occurred over 10 000 years. Rising base level caused a wave of aggradation to move up‐valley, until aggradation occurred over the entire valley. Frequency of bifurcations and avulsions increased with rate of base‐level rise and aggradation rate. Channel‐belt width varied with water discharge and the lifespan of the channel belt. Wide, connected channel belts (and high channel‐deposit proportion) occurred around the upstream inflow point because of their high discharge and longevity. Less connected, smaller channel belts occurred further down‐valley. Such alluvial behaviour and architecture is also found in the Rhine–Meuse delta. During base‐level fall, valley erosion occurred, and the incised valley contained a single wide channel belt. During subsequent base‐level rise, a wave of aggradation moved up‐valley, filling the incised valley. Bifurcation and avulsion sites progressively moved upstream. Relatively thin, narrow channel belts bordered and cut into the valley fill. These results differ substantially from existing sequence stratigraphy models. The increase in sediment supply from upstream resulted in an alluvial fan. Most bifurcations and avulsions occurred at the fan apex (nodal avulsion), and channel belts were the widest and the thickest here (giving high channel‐deposit proportion) due to their high discharge and longevity. The width and thickness of channel belts decreased down‐valley due to decreased discharge, longevity and aggradation rate. This behaviour occurs in modern alluvial fans. Intrinsic controls also affect floodplain dynamics and alluvial architecture. Variation of aggradation rate, bifurcation frequency and number of coexisting channel belts occurred over periods of 500 to 2000 years, compared with 10 000 years for extrinsic controls. This variation is partly related to local aggradation and degradation of channel belts around bifurcation points. Channel belts were preferentially clustered near floodplain margins, because of low floodplain aggradation rate and topography there.     

Using white mica 40Ar/39Ar data as a tracer for fluid flow and permeability under high‐P conditions: Tauern Window,Eastern Alps

New single‐grain‐fusion muscovite and paragonite ⁴⁰Ar/³⁹Ar data from eclogite and blueschist units exposed in the Tauern Window, Eastern Alps yield a range of apparent ages from 90 to 23 Ma. These apparent ages are generally older than expected for ⁴⁰Ar/³⁹Ar cooling ages, given constraints from other geochronological systems such as Rb–Sr and U–Pb. Numerical Ar‐in‐muscovite diffusion models for Tauern Window nappe P–T paths in an open system suggest that ⁴⁰Ar/³⁹Ar ages should lie between 29 and 24 Ma, and that they should constrain cooling and decompression following the post‐high pressure Barrovian overprint. The measured ranges of apparent ⁴⁰Ar/³⁹Ar dates suggest that the assumption of open system behaviour is not valid for this region. The local and/or regional generation of fluid during exhumation promoted pervasive recrystallization of high pressure lithologies throughout the Tauern Window to greenschist and amphibolite facies assemblages. The old apparent ⁴⁰Ar/³⁹Ar white mica dates in all lithologies are therefore interpreted as being due to inefficient removal of grain boundary Ar by the grain boundary fluids during the Barrovian overprint, due to high Ar concentrations or limited connectivity or both. This caused spatially (mm‐scale) and temporally variable fluxes of Ar out of, and probably into, white mica in both metasedimentary and metabasic lithologies.