Modes of thickening of analogue weak lithospheres

Several compressional contexts, such as those involving juvenile or thickened crust, are expected to be associated with rather hot lithospheres whose mechanical behaviour remains poorly documented. In this paper, we present a series of analogue models dedicated to compression of lithospheres characterized by a thin upper brittle crust overlying a weak ductile crust and a ductile sub-Moho mantle. The models show that (1) deformation is controlled by the ductile layers that undergo distributed thickening, (2) thrust systems are limited to the upper brittle crust, (3) thrusting induces burial and stacking of upper crust pop-downs. The overall deformation patterns can be basically interpreted in terms of pop-down thrusting of the brittle crust and pure-shear type ductile flow of crust and mantle. Moreover, the models show that the sinking of supracrustal units does not require inverse density profiles but can be simply driven by compression. Model deformation patterns are consistent with those shared by many ancient belts, including not only Archaean granite–greenstone belts, but also more generally Paleoproterozoic ones. They provide also insights on deformation modes that may characterize modern thickened and abnormally hot domains like High Plateaus.     

Modelling the extension of heterogeneous hot lithosphere

The consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models. The experiments examined the effects of i) the location of heterogeneities in the ductile crust and/or in the lithospheric mantle, and ii) their orientation, perpendicular or oblique to the direction of bulk extension. The observed deformation patterns are all relevant to the so-called “wide rifting” mode of extension. Weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle: the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area. This latter behaviour depends in turn upon the decoupling provided by the ductile crust. Localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle. Domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition. Lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle. This contrasts with “cold and strong” lithospheres whose high strength sub-Moho mantle triggers a necking instability at the lithosphere-scale. This also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes. Thus, for the given lithospheric rheology, the models have relevance to backarc type extensional systems, such as the Aegean and the Tyrrhenian domains.     

Mountain building and exhumation processes through time: inferences from nature and models

Orogenic belts involving initially hot lithospheres, as exemplified by accretionary-type orogens, commonly show distributed deformation and retrograde PT paths with a concomitant decrease in pressure and temperature. Paths may track geotherms, indicating thermal equilibrium, consistent with slow strain and exhumation rates, limited strain localization, and consequently limited topographical gradients and distributed erosion. Such patterns are more common in Precambrian times than in younger periods of the Earth history. In contrast, orogens involving initially stiff lithospheres show exhumation PT paths that track isothermal decompression reflecting high strain rates along major shear zones, a feature typical of Phanerozoic collision belts. Field evidence, analogue and numerical models emphasize that strain localization has first-order consequences for the tectonic evolution of orogenic zones including structure, metamorphism, exhumation processes, topography, erosion and sedimentation modes.     

内蒙古大青山高级变质岩韧性剪切带及其流变机制

大青山高级变质岩不仅记录华北克拉通早期大陆形成演化历史,也保留了中下部地壳岩石流变信息,它们经历了下部地壳构造层次高角闪岩相-麻粒岩相条件变质变形、深熔作用改造,形成了复杂构造样式和构造要素组合.韧性剪切带是高级变质岩中主要构造形迹,控制着早前寒武纪高级变质岩主体构造格架.依据野外地质产状、变形特征与构造要素叠加改造关系,韧性剪切带划分为早期近水平顺层伸展型和晚期陡倾韧性剪切带.近水平顺层伸展韧性剪切带呈残留状保留在后期变形改造较弱部位上,主要沿着不同地质单元或者岩性层界面上发育,是在伸展变形体制下形成的.晚期陡倾韧性剪切带呈近东西方向展布,规模较大,叠加和改造早期构造形迹,形成于晚期造山挤压构造环境中,以左行滑移为主.这两种韧性剪切带都形成于地壳中深部构造层次高角闪岩相-麻粒岩相条件下,变形机制主要为熔体增强颗粒边界扩散和颗粒流动,使岩石发生大规模的塑性流动.在宏观上形成了不对称流动组构、条纹条带构造、熔融线理、层内流动褶皱等构造形迹,在微观上矿物晶体没有发生明显塑性变形,均匀消光,晶体为三边平衡结构,与静态变质结构相似,形成了地壳深部构造层次上变质构造岩-构造片麻岩.      

Interaction of metamorphism, deformation and exhumation in large convergent orogens

Coupled thermal‐mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr^?1, and a slope‐dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m^?3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m^?3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 10¹⁹ Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro‐side of the system, and is driven by a depth‐dependent pressure gradient that is facilitated by the development of a temperature‐dependent low viscosity horizon in the mid‐crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post‐convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust‐related exhumation on the flanks. Velocities in the pro‐side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro‐side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal‐sense offset at its upper boundary, reverse‐sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where P_max precedes T_max, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.     

利用航磁、重力资料研究川滇地区大陆变形特征

利用航磁、重力资料对川滇地区大陆变形特征进行了研究。航磁异常揭示了研究区内的基底性质及其横向差异,研究区内的强烈地震主要集中在航磁异常突变带上,基底性质横向差异有利于应力的相对集中,成为地震孕育和发生的有利部位。康定—绵阳一带刚性基底的阻挡造成了青藏高原下地壳物质向东的塑性流动被迫转向南东—南南东方向,进而造成了川滇菱形块体内广泛的地壳增厚,布格重力异常等值线呈向南东伸出的舌状是其地壳增厚作用的直观反映。川滇地区的大陆变形特征既存在广泛的地壳增厚现象,同时在下地壳塑性流动的驱动下,中、上地壳破碎成大小不一的块体,变形特征表现出整体刚性,因而不能简单地套用“地壳增厚”或“大陆逃逸”模式。     

Folding by cataclastic flow at shallow crustal levels in the Canyon Range,Sevier orogenic belt,west-central Utah

Folds form by ductile deformation typically involving continuous flow. In the elastico-frictional regime, such deformation may be accomplished by cataclastic flow involving collective movement on a population of fractures and zones. The Canyon Range (CR) syncline, part of the CR thrust sheet in west-central Utah, developed in this regime. The CR syncline is composed of thick-bedded quartzite units with a small material contrast between layers, limiting limb rotation by flexural slip alone. Thus, fracture populations developed to accommodate fold tightening by limb rotation and thinning, and the formation of transverse zones across the fold. Several generations of fracture and deformation zone (DZ) networks are recognized from mesoscopic and microscopic evidence, and can be related to stages of folding. The net result of the large number of distributed fractures and deformation zones is a continuous deformation that is homogeneous at the scale of the outcrop. All these lines of evidence suggest that large-scale cataclastic flow accommodated folding by allowing rigid mesoscopic blocks to slide along bounding DZs.Along its length, the CR syncline consists of several segments bounded by transverse zones with different mechanisms accommodating fold tightening in adjacent segments. In one segment, fold tightening progressed by limb rotation, and then out-of-the-core thrusting. In contrast, fold tightening in the adjoining segments occurred by rotation and thinning of one limb and possible hinge migration, with the steeply dipping to overturned limb showing progressive thinning of units on a megascopic scale and progressive increase in the thickness and density of deformation zones at all scales.     

Aeromagnetic signature of an exhumed double dome system in the SW Grenville Province (Canada)

Analysis of aeromagnetic data in the Grenville Province reveals the presence of two regional‐scale unmapped structural domes (the Morin and Mékinac‐Taureau domes) with an oval‐shaped magnetic pattern bounded by regional‐scale shear zones and a geometry that is similar to that produced in crustal flow models under extension, which predict two upright domes of foliation (double dome) separated by a steep shear zone. The Mékinac‐Taureau dome, a metamorphic core complex, and the Morin dome may have been exhumed by channel flow. Exhumation occurred by a combination of thrust, normal‐sense and wrench shear zones. The preservation of paragneisses in the Morin dome suggests that it underwent a lesser degree of exhumation than did the Mékinac‐Taureau dome. This study shows how the integration of local field information with magnetic data in a regional tectonic setting can reveal and delineate concealed gneiss domes and highlights a role for strike‐slip tectonics in the creation of regional structures involving the exhumation of deep crust.     

Some remarks on high-temperature—low-pressure metamorphism in convergent orogens

Many high-temperature–low-pressure (high- T –low- P ) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric-scale strains must greatly outlast the deformation. Consequently, high- T –low- P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, _c, and the lithosphere, ₁ respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high- T –low- P metamorphism during crustal thickening (with _c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (f_c= f₁) for the same initial lithospheric configuration.     

Numerical modelling of orogenic processes and gold mineralisation in the southeastern part of the Yilgarn Craton,Western Australia

We use numerical modelling codes to simulate aspects of some current hypotheses for the origin of gold deposits and hydrothermal systems in the Yilgarn Craton of Western Australia. In particular, we investigate conceptual models advocating vertically continuous hydrothermal systems as well as those invoking extensive lateral flow and possible links with advection of heat by late orogenic granitic magmatism. Numerical models of part of the Eastern Goldfields Province and Southern Cross Province have been built with FLAC3D, to simulate crustal‐scale coupled interaction between deformation and fluid flow. These illustrate the potential for fluid focusing and mixing in shear zones, including downflow of meteoric water, lateral fluid flow driven by topographic elevation and upwards flow of fluids derived from melting and metamorphism in the deep crust. In some cases, downflow also occurs within the middle crust, at depths where fluid influx might trigger melting if the geothermal gradient were appropriate. The models indicate that tectonic wedging within a layered crust and diverging thrust systems that generate ‘pop‐up’ wedges may be important in facilitating efficient fluid upflow and downflow during uplift, while topographic elevation related to asymmetric thrust migration and loading tends to promote lateral fluid flow. However, the effect of topography appears more important than the precise depth or location of the site of fluid production in the deep crust. The effects of thermal convection and fluid‐fluid interaction have also been numerically modelled for a simplified section across the Kalgoorlie Terrane. Modelling under both hydrostatic and lithostatically overpressured pore‐pressure gradients has effectively delineated domains of convective fluid flow within the middle and upper crust, and has identified two generic sites that are favourable for fluid mixing, notably hangingwall and footwall environments in major shear zones, such as the Bardoc Shear, and in broad antiforms, such as the Goongarrie ‐ Mt Pleasant Antiform. The thermal effect of small plutons embedded in a regional metamorphic regime can cause significant lateral displacement of fluid convection patterns, over distances greater than pluton diameter, as well as more proximal effects on precipitation and dissolution of mineral species. However, these results are highly dependent on the pore‐pressure gradient and the permeability structure of the crust, and require magmatic and metamorphic fluid generation to be precisely timed with respect to deformation, thus reinforcing the dynamic feedback between deformation, magmatism and fluid production and migration.     

中国大陆现今构造应变率场及其动力学成因研究

通过分析中国大陆地壳运动GPS速度场得到现今构造应变率场。结果显示在印度板块北向推挤作用下 ,青藏高原内部及其邻域形变场并不局限于少数大型走滑断裂 ,而是在大范围内广泛分布 ,各地区构造运动驱动机制也可能各有不同。藏南地区主应变率场呈均衡的约 2× 10 -8a-1南北向挤压和东西向拉张 ,显示印度板块下插造成的地壳增厚和岩石圈拆离可能形成上地壳与上地幔间形变解耦 ,地壳内部在南北向挤压及重力场作用下产生东向塑性流驱使上地壳产生东西向拉张。西藏中部羌塘地区主应变率场显示均衡的约 2× 10 -8a-1北北东向挤压和北西西向拉张 ,反映本地区一系列走向北东和北西的共轭剪切断裂的活动 ,可能源于南北向挤压和软流层内东向塑性流的驱动。柴达木盆地及周边地区主应变率场呈约 2× 10 -8a-1北东向压缩和约 (0 1)× 10 -8a-1北西向拉张 ,表明地壳增厚造成的地壳温度上升可能还不足以造成上下地壳的充分解耦 ,南北向的消减还未能有效地转换成东西向的拉张 ,形变以褶皱和逆冲断裂运动为主。当今青藏高原形变场的形成应是构造运动从南到北阶段性发展过程中地壳与上地幔介质性质差异造成驱动机制不同的结果。     

龙门山陆内俯冲带形成机制探讨

本文分析了龙门山陆内俯冲带两侧岩石圈的强度结构特征及在侧向力作用下所发生的变形过程。盆地岩石圈中高强度层厚而紧凑,显示了较好的整体高强度性;造山带岩石圈上地壳具高强度,其下为低强度层。在侧向挤压力的作用下,变形主要发生于造山带一侧,最可能的变形方式是其脆性上地壳出现倾向后陆的逆冲断层,盆地岩石圈沿此断层俯冲,挤压其下部的低强度层,使之发生韧性增厚变形。     

The Hospitalet gneiss dome (Pyrenees) revisited: lateral flow during Variscan transpression in the middle crust

A new structural and kinematic study of the Hospitalet dome (Pyrenees) is presented. This dome corresponds to the eastern half of an EW‐trending antiformal structure made of an orthogneissic core intruded by granitoids, partly covered by Upper Proterozoic to Lower Ordovician metapelites. Its Variscan evolution can be split into four stages: (i) development of a strong high temperature pervasive deformation associated with subhorizontal foliations and lineations, and with non‐coaxial top‐to‐the‐east kinematics; (ii) formation of a south‐verging overturned megafold; (iii) emplacement of calc‐alkaline granitoids; and (iv) formation of mylonitic bands on the southern border of the dome, with reverse dextral kinematics. The flat lying pervasive high‐T deformation is interpreted as a large lateral flow developed in a dextral transpressive regime inducing an important uncoupling between the middle and upper crusts. The next stages happened in a progressive deformation in the same transpressive regime.     

Decoupling between the middle and upper crust during transpression-related lateral flow: Variscan evolution of the Aston gneiss dome (Pyrenees, France)

We present a structural, AMS, microstructural and kinematic study of the Aston gneiss dome (French Pyrenees), which consists of a core made up of orthogneiss and paragneiss intruded by numerous sills of Carboniferous peraluminous granite. The orthogneiss corresponds to a former Ordovician granitic laccolith. Four Variscan events have been evidenced in this gneiss dome: (i) D1 deformation observed only as relics in the orthogneisses and their country-rocks located above the sillimanite isograd, and characterized by a NS to NE–SW non coaxial stretch associated to top to the south motions (NS convergence); (ii) D2-a deformation observed in the orthogneisses and their country-rocks, mainly migmatitic paragneisses, located below the sillimanite isograd and in the peraluminous granites whatever their structural level, and characterized by an EW to N120°E stretch associated to a top to the east flat shearing (lateral flow in the hot middle crust in a transpressive regime); (iii) D2-b deformation characterized by EW-trending megafolds corresponding to the domes in the middle crust and by EW-trending tight folds with subvertical axial planes in the metasedimentary upper crust; (iv) subvertical medium-temperature mylonitic bands developed by the end of the transpression.The Aston massif is a good example of decoupling between a cold upper crust and a hotter middle crust overheated by a thermal event originated in the upper mantle. This decoupling allowed the lateral flow of the migmatitic middle crust along a direction at high angle with respect to the more or less NS-trending direction of convergence. We suggest that the HT-LP metamorphism developed before the formation of the domes during D2-a, coevally with the emplacement of numerous sills of peraluminous granite, whereas the emplacement of the large calc-alkaline plutons in the upper crust occurred by the end of D2-b. Our data invalidate the previous geodynamical models based on either early or late extensional regime to explain the development of the HT-LP metamorphism. This new interpretation of the dynamics of the Variscan crust of the Pyrenees is consistent with recent studies conducted in Archaean and Palaeoproterozoic hot continental crusts having undergone oblique convergence, and characterized by a competition between vertical thickening and lateral flow induced by the important rheological contrast between two thermally different levels.     

Crustal anisotropy from Moho converted Ps wave splitting analysis and geodynamic implications beneath the eastern margin of Tibet and surrounding regions

In addition to crustal thickening, distinctly different mechanisms have been suggested to accommodate the huge convergences caused by the continental collision between India and Eurasia. As the transition zone between the two grand tectonic domains of Asia, the Tethys and the Pacific, east Tibet and its surrounding regions are the ideal places to study continental deformation. Pervasive rock deformation may produce anisotropy on the scale of seismic wavelengths; thus, seismic anisotropy provides insight into the deformation of the crust and mantle beneath tectonically active domains. In this study, we calculated receiver function pairs of radial- and transverse-components at 98 stations located in Sichuan and Yunnan provinces, China. We selected 7423 pairs with high signal-to-noise ratio (SNR) and unambiguous Moho converted Ps phases (Pms) to measure the Pms splitting owing to the crustal anisotropy. Both the crustal thickness and the average crustal Vp/Vs ratio were calculated simultaneously by the H–k stacking method. The geodynamic implications were also investigated in relation to surface geological features, GPS velocities, absolute plate motion (APM), SKS/SKKS splitting, and other seismological observations. In addition to the fast polarization directions (FPDs) of the crustal anisotropy, we observed a conspicuous sharper clockwise rotation around the eastern Himalayan syntaxis than was revealed by GPS velocities. The distributed FPDs within and near the main active fault zones also favored the directions parallel to the faults. This implied that the deformation of a continuous medium revealed by GPS motions is a proxy for the deformation of the brittle shallow crust only, while the main active faults and the deep crustal interiors both play important roles in the deep deformation. Our results suggest that the deformation between the crust and upper mantle within the northernmost section of the Indochina block is decoupled due to the large difference in the directions between the observations related to the crust (GPS and crustal anisotropy) and mantle (APM and mantle anisotropy). Focusing on the transition zone between the plateau and the South China and Indochina blocks, we suggest that the motion of the Central Yunnan sub-block is a southeastward extrusion by way of tectonic escape. There is less deformation in the deep crust and the motion is controlled by the active boundary faults of the Ailaoshan–Red River shear zone to the west and the Xianshuihe–Xiaojiang fault to the east; the lower crustal flow within the plateau southeastward reached the Lijiang–Xiaojinhe fault, but further south it was obstructed by the Central Yunnan sub-block.     

Metamorphism and Tectonics in Southern Madagascar: An Overview

The scope of this paper is to briefly summarize the general tectonic pattern and the metamorphic evolution of the continental crust from southern Madagascar. After a presentation of the main geophysical features of the Malagasy crust, the brittle and the ductile strain patterns are established at a crustal scale. The Pan-African metamorphic zonation is discussed and interpreted through a tectonic model related to a transpressive regime.     

Emplacement mechanisms of late-orogenic granites: structural and geochemical evidence from southern Finland

The country rock in southern Finland formed mainly during the Svecofennian orogeny ca. 1.9 Ga ago. The middle and lower crust was partially melted 1.83 Ga ago due to crustal thickening and subsequent extension. During this event, S-type migmatites and granites were formed along a 100×500 km zone. This Late Svecofennian Granite–Migmatite zone (LSGM zone) is a large crustal segment characterised by roughly E–W trending sub-horizontal migmatites and granites. Combined ductile E–W shear movements and NNW–SSE compressional movements defined a transpressional tectonic regime during the emplacement. Partial melts that moved through the crust pooled as granite sheets or froze as migmatites. Major transpressive shear zones border the LSGM zone, which forms a tectonic and metamorphic zone that crosscuts the earlier Svecofennian granitoids. Based on field observations and geochemical data from two sets of outcrops, we show that the great volumes of late-orogenic granites and migmatites in southern Finland were transported and emplaced as small chemically variable batches, possibly extracted from different protoliths. These melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust. In the Nagu area, the melt batches were trapped under a roof-layer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and E–W trending fold axes. The sheets of microcline granite are, in places, strongly sheared; the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syn-tectonic movements of the layers as well as a syn-tectonic mechanism for the late-magmatic fractionation. Depending on the degree of crystallisation of the individual melt batches during shearing at different intensities, the granites have slightly different appearances. Some sheared zones show a cumulate-like trace element geochemistry, indicating that melt fractions were expelled from the system, producing layers of deformation enhanced fractionated granites and cumulate layers. Our interpretation is that the Nagu area shows shear-assisted fractionation mechanisms in granitic melts, and that similar processes are responsible for the fractionation trends seen in the sub-horizontal sheeted granites in Hämeenlinna at higher levels in the crust.     

Lower crust indentation or horizontal ductile flow during continental collision?

Conditions for indentation and channelised flow are investigated with two-dimensional thermomechanical models of Alpine-type continental collision. The models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide, coherent with several geological reconstructions. We study in particular the role of lower crustal strength in developing peculiar geometries after 20 Ma of shortening at 1 cm/year. Crustal layers produce geometries of imbricate layers, which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid-like indentation:

– Channelised lateral flow develops when the lateral lower crust has a viscosity less than 10²¹ Pa s, exhibiting velocities opposite to the direction of convergence. This mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle. It is also associated with a topographic plateau that equilibrates with a wide (about 200 km) but quasi-constant crustal root about 50 km deep.

– In contrast, indentation occurs with lateral lower crust layers that have a viscosity greater than about 10²³ Pa s, producing significant shortening and thickening of the central crust. In this case topography develops steep and narrow (around 100 km wide), associated with a thickened crust exceeding 60 km depth. A crustal-scale pop-up forms bounded by subvertical shear zones that root into the mantle lithosphere.

关于华北克拉通燕山期岩石圈减薄的机制与过程的讨论:是拆沉,还是热侵蚀和化学交代?

关于华北克拉通燕山期岩石圈减薄作用,主要有两种模型:(1)岩石圈拆沉;(2)热侵蚀和/或化学交代。文中主要从岩浆活动与构造变形两个途径,通过(1)燕山带造山幕和结构要素组合以及造山过程的p-T-t轨迹;(2)收缩构造变形、火成岩构造组合和下地壳岩石捕虏体3个独立证据提出陆壳的构造加厚;(3)火成岩成因的壳幔相互作用模型和热模拟等,试图讨论华北地区克拉通有浮力的岩石圈如何转变为密度大的岩石圈,随之发生拆沉作用,而不是热侵蚀或化学交代机制使岩石圈地幔改造为EMI印记实现的减薄作用。大量对流的软流圈物质注入克拉通是诱发陆壳发生局部熔融所必需的条件。底侵玄武质岩浆的加热并弱化先前的冷和强的克拉通地壳,创造一个流变学条件,以使收缩构造变形和陆壳加厚成为可能。陆壳最下部和岩石圈地幔中形成的大量玄武质岩石,在构造加厚作用下,相转变为榴辉岩,致使原先有浮力的岩石圈转变为密度大的岩石圈,随之发生拆沉作用。     

The role of viscous heating in Barrovian metamorphism of collisional orogens: thermomechanical models and application to the Lepontine Dome in the Central Alps

Thermal models for Barrovian metamorphism driven by doubling the thickness of the radiogenic crust typically meet difficulty in accounting for the observed peak metamorphic temperature conditions. This difficulty suggests that there is an additional component in the thermal budget of many collisional orogens. Theoretical and geological considerations suggest that viscous heating is a cumulative process that may explain the heat deficit in collision orogens. The results of 2D numerical modelling of continental collision involving subduction of the lithospheric mantle demonstrate that geologically plausible stresses and strain rates may result in orogen‐scale viscous heat production of 0.1 to >1 μW m^?3, which is comparable to or even exceeds bulk radiogenic heat production within the crust. Thermally induced buoyancy is responsible for crustal upwelling in large domes with metamorphic temperatures up to 200 °C higher than regional background temperatures. Heat is mostly generated within the uppermost mantle, because of large stresses in the highly viscous rocks deforming there. This thermal energy may be transferred to the overlying crust either in the form of enhanced heat flow, or through magmatism that brings heat into the crust advectively. The amplitude of orogenic heating varies with time, with both the amplitude and time‐span depending strongly on the coupling between heat production, viscosity and collision strain rate. It is argued that geologically relevant figures are applicable to metamorphic domes such as the Lepontine Dome in the Central Alps. We conclude that deformation‐generated viscous dissipation is an important heat source during collisional orogeny and that high metamorphic temperatures as in Barrovian type metamorphism are inherent to deforming crustal regions.