Fabric development as the key for forming ductile shear zones and enabling plate tectonics

Lithospheric deformation on Earth is localized under both brittle and ductile deformation conditions. As high-temperature ductile rheologies are fundamentally strain-rate hardening, the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature. In this contribution, I develop a localization potential that quantifies the weakening associated with these changes. The localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions. I provide analytical expressions for the localization potential associated with a temperature increase, grain size reduction, an increase in water fugacity, melt content, or the abundance of a weak mineral phase. I show that these processes cannot localize deformation from a mantle convection scale (10³ km) to a ductile shear zone scale (1 km). To achieve this, is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers. This process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non-linear rheology. Micas, melt, and fine-grained aggregates – unless dry rheologies are used – have the necessary characteristics. As none of these phases is expected to be present in the dry lithosphere of Venus, this concept can explain why Venus, unlike the Earth, does not display a global network of plate boundaries. The diffuse plate boundary in the Central Indian Ocean may be as yet non-localized because serpentinization has not reached the ductile levels of the lithosphere.     

Fractal approach in modelling of earthquakes

A model for an earthquake simulation is proposed with the use of a fractal approach. Multiple generation and coalescence of shear faults in stochastic brittle media (modelled as a 2d lattice) are considered to be a source of seismicity. Dynamics of local failure events are governed by accumulation of shear defects, described in terms of continuum damage mechanics. Fractal tree structure is used as an analogue for a stress redistribution process. Energy release, caused by the non-uniform failure, is studied for a non-conservative case. Effect of various types of rocks' properties stochasticity on energy release dynamics is analysed with a utilization of multifractal formalism. The latter is shown to be an additional method for seismicity characterization.     

Rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones

Emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures, conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones. Much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock-mechanics-based yield criteria for the inelastic deformations at the top and base. At mid-plate depths, stresses are likely to be supported elastically because bending strains and elastic stresses are low. The collisional tectonic regime, however, is far more complex because very large permanent strains are sustained at mid-plate depths and this requires us to include the broad transition between brittle and ductile flow. Moreover, important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid-plate depths.Two specific contributions of laboratory rock rheology research are considered in this paper. First, the high-temperature steady-state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks. Rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere. The thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm T_c above which (at geologic strain rates) the high-temperature ductile strength falls below some arbitrary strength isobar (e.g., 100 MPa). For olivine T_c is about 700°–800°C but for other crustal silicates, T_c may be as low as 400°–600°C, suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the Moho, as suggested by a number of workers on independent grounds. Put another way, the Moho is a rheological discontinuity. A second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones. Ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant-strain-rate tests or as an accelerating-creep-rate stage at constant differential stress. A number of physical mechanisms have been identified that may be responsible for ductile faulting, including the onset of dynamic recrystallization, phase changes, hydrothermal alteration and hydrolytic weakening. Microscopic evidence for these processes as well as larger-scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere.     

Micromechanics modelling for stress–strain behaviour of brittle rocks

A micromechanics model for stress–strain behaviour of brittle rocks has been developed. Microcracking is the mechanism of the non-linear deformation behaviour for brittle rocks in the pre-peak stage. The non-linear behaviour in this stage is simulated by considering the local axial splitting of microcracks. The relationships between the compressive stresses, the growth of microcracks, and the fracture-induced deformation are analytically established. In the post-peak stage the shear faulting predominates the process of deformation, which is simulated by a damage model. This micromechanics model is helpful in understanding the failure process in brittle rocks. The model can be used to simulate the complete stress–strain behaviour of rock. The model simulations are consistent with experimental results.     

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.     

Cumulative Benioff strain-release, modified Omori's law and transient behaviour of rocks

Irreversible thermodynamic theories with internal state variables can be used to derive a general constitutive law for both transient and steady-state behaviours of rocks. This constitutive law can represent the concepts of damage and damage evolution in either the fibre-bundle model or continuum damage mechanics. We have previously proposed an empirically based constitutive law for both the transient and steady-state behaviours of rocks ultimately derived from laboratory experimental data. We show here that this law is concordant with the general constitutive law derived from irreversible thermodynamic theories, and that the relaxation modulus has a temporal power–law that depends on a structural fractal property of rocks. Our constitutive law predicts forms for the cumulative Benioff strain-release for precursory seismic activations and the modified Omori's laws of aftershocks, both aspects of the temporal fractal properties of seismicity. These seismic properties can also be derived by the fibre-bundle model or continuum damage mechanics. Our model suggests that these time-scale invariant processes of seismicity may be regulated by the fractal structures of crustal rocks.     

Quasi-adiabatic shear zones in the lithosphere: numerical and experimental approaches

Continental breakup, or compressive lithosphere scale faulting, requires a physical mechanism for wholesale faulting of the lithosphere. We compared numerical and experimental models for the nucleation of quasi-adiabatic shear bands in polyvinylchloride (PVC) with those in an idealized viscoelastoplastic mantle with olivine rheology. In both materials fault nucleation is caused by elastic stress concentration on pre-existing imperfections, with localized yielding confined to its vicinity. Faulting occurs rapidly after the initial elastic energy in the system is charged sufficiently to cause wholesale yielding. Propagation of the fault, monitored by looking at the dissipation of plastic energy, reveals migration of a sharp, thermal-mechanical “crack”- like instability, which appears in the temperature field as a slightly diffused signal. The initial temperature rise in the crack is subtle but increases suddenly when the plate is severed. This autocatalytic behavior has also been described in ductile polymers, which can be used as mechanical analogues. We suggest that elastoplastic coupling in quasi-adiabatic shear banding is a key for fast (< 1 Ma) nucleation of shear zones. These nonlinear phenomena will be illustrated for both experimental and numerical results by nine movies     

Numerical modelling of the late stage of subduction zone transference after an accretion event

The resumption of subduction of an oceanic plate at a contact with a docked island (continental) margin is modelled numerically in 2D. The mechanical properties of the crustal and mantle rock masses are treated with visco‐plastic rheologies where the viscosities are described by a dynamic power law phenomenologically accounting for the transient texture evolution of the deforming rock masses. The study is limited to the extreme case of a fluid weakened lithosphere characterized by a uniform effective yield stress. At a constant push of 8 cm a^?1, the shear zones split the oceanic slab into blocks at the contact with the island. At a moderate yield stress (σ_Y < 80 MPa) and normal relaxation rate of viscosity, subduction resumes in less than ca 1 Ma. High σ_Y = 100 MPa leads to the island edge bending. A detached block of the island is subducted after the plunge of the oceanic plate reverses.     

珠江口盆地白云凹陷新生代构造演化动力学

白云凹陷构造演化史的研究对在白云凹陷开展油气勘探和深水沉积研究具有重要的意义。通过对断裂与沉积结构平面和剖面特点的分析,结合岩浆活动特点,文中提出白云凹陷是一个复式地堑,推测这种结构特点与凹陷下地壳的强烈韧性减薄和颈缩变形有关,表现为热岩石圈的伸展。其发育机制推测与白云凹陷位于构造转换带上有关,特殊的构造位置使白云凹陷成为强烈构造变形区,岩石圈地壳强烈减薄,伴随伸展过程和地幔上涌,脆性地壳或上地幔中部分熔融物质的出现导致岩石圈强度的急剧降低,在区域伸展应力场下以韧性流变方式减薄。岩浆在构造转换带下聚集并发育主岩浆房,由于白云凹陷南北边缘没有发育正断裂系统,岩浆主要沿垂直伸展的方向运移,因此在珠琼运动一幕和二幕南南东向伸展应力作用下,岩浆向白云凹陷的东部和西部运移至北西向基底深大断裂处,那里由于北西向断裂表现为左行张剪性质而成为压力较低的地区,从而成为岩浆上涌和侵位的地方。在岩浆聚集的地区,活动岩浆体附近的脆性变形被分散的韧性变形所取代,因此在凹陷的东北和西南两个角上,发育了张性和张剪性小断裂群,由于热岩石圈弹性较差,白云凹陷长期持续沉降。白云凹陷的断裂活动和沉积演化史还受到南海海盆扩张活动的影响。     

Structural softening of the lithosphere

Structural softening is a decrease in the amount of stress needed to deform the lithosphere at a particular rate because of its structural reorganization while all true rheological properties remain constant. Structural softening is fundamentally different than material softening, where the decrease in stress is generated by a change in rheological properties with progressive deformation, such as grain size reduction resulting from large shearing strain. We study structural softening generated by folding of the crust-mantle boundary, which is a structural instability that inevitably develops during compression of the mechanically layered lithosphere. For ductile rheologies, the stress decrease represents a decrease of the effective lithospheric viscosity, which is proportional to the ratio of stress to lithospheric shortening strain rate. We present analytical and numerical results quantifying the decrease in stress and effective viscosity that occur during shortening at a constant rate. The decrease in effective viscosity can be up to 10-fold.     

岩体损伤问题

从微观及宏观两方面对岩体损伤的意义及性质作了阐述,介绍了连续损伤力学中有关损伤变量的定义和量度,给出了韧性、脆性、疲劳与蠕变状态下的损伤演变方程。文内除了列举几种反映岩体软化及节理裂隙性质的损伤模型外,还介绍了细观与统计损伤模型。最后简要叙述了几种损伤测量方法,包括超声衰减技术、声发射以及现代光学技术。     

甘孜—理塘断裂带北段构造特征及其演化过程

甘孜-理塘断裂带是义敦造山带与雅江褶皱带的分界断裂。该带由韧性又脆性冲断层、平移断层,以及各种岩块、构造岩片等组合而成。其演化历史主要经历了晚三叠世洋壳的俯冲、晚三叠世末期弧-陆碰撞、陆内会聚和喜马拉雅期断陷的复杂演化过程。     

Rheological stratification of the Indian continental lithosphere: Role of diffusion creep

Dynamic recrystallization and reduction in grain-size at large strains, e.g. in shear zones, leads to rheological weakening of the lithosphere and facilitates intense ductile deformation. In the present work, we include this effect into the rheological models of the Indian continental lithosphere to analyse its role in modifying the rheological structure and strength of the Indian lithosphere. The results computed by using quartz and felspar rheologies for the upper and lower crust, respectively, and grain-size dependent olivine rheology for the upper mantle, indicate an increase in the ductility of the mantle lithosphere.     

Fault-valve behaviour in optimally oriented shear zones: an example at the Revenge gold mine, Kambalda, Western Australia

Quartz vein systems developed in and adjacent to shear zones host major gold deposits in the Kambalda region of the Norseman–Wiluna greenstone belt. At the Revenge Mine, two groups of mineralised reverse shear zones formed as conjugate, near-optimally oriented sets during ESE subhorizontal shortening adjacent to a major transpressional shear system. The shear zones developed at temperatures of about 400°C in a transitional brittle–ductile regime. Deformation was associated with high fluid fluxes and involved fault-valve behaviour at transiently near-lithostatic fluid pressures. During progressive evolution of the shear system, early brittle and ductile deformation was overprinted by predominantly brittle deformation. Brittle shear failure was associated with fault dilation and the formation of fault-fill veins, particularly at fault bends and jogs. A transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses. The formation of subhorizontal stylolites, locally subvertical extension veins and minor normal faults in association with thrust faulting, indicates episodic or transient reorientation of the near-field maximum principal stress from a subhorizontal to a near-vertical attitude during some fault-valve cycles. Local stress re-orientation is interpreted as resulting from near-total shear stress release and overshoot during some rupture events. Previously described fault-valve systems have formed predominantly in severely misoriented faults. The shear systems at Revenge Mine indicate that fault-valve action, and associated fluctuations in shear stress and fluid pressure, can influence the mechanical behaviour of optimally-oriented faults.     

On formation mechanisms of deep sedimentary basins: Is there enough evidence for eclogitization?

This is a critical comment on the model of basin formation by eclogitization of mafic crust suggested by E. Artyushkov. The eclogitization model bears uncertainties in average parameters (thickness, density, pressure) of lithospheric mantle, crust, and sediments, which may bias the estimates of subsidence magnitude. Main pitfalls, however, lie in high-pressure petrology: The lithostatic pressure is insufficient for eclogite to form in the lower crust beneath deep basins. It is shown that linear extrapolation of laboratory data on the gabbro-to-eclogite transition onto the field of relatively low pressures and temperatures in the lithosphere is incorrect. The hypothesized role of hot mantle fluids in the gabbro-eclogite transformation appears doubtful in terms of both petrology and kinetics of metamorphic reactions. Eclogite volumes in none of well known eclogitic sites agree with those required for eclogitization-driven subsidence. Artyushkov’s criticism of the extension basin formation model is not quite just. There are recent models of a two-layer lithosphere that imply a possibility of brittle and ductile deformation at different crust rheologies. The models we refer to predict most of extension to occur in mantle lithosphere rather than in the crust, this extension being able to produce deep continental basins.     

新世纪构造地质学两大支柱理论: 最大有效力矩准则与变位形分解

传统构造地质学用摩尔-库伦准则和贝克尔的应变椭球体理念分别解释地壳中的脆性断层和塑性变形,将变形局部化的韧性剪切带形成解释为平行应变椭球体的圆切面,却无法解释变形局部化的共轭剪切带稳定夹角~110°面对应缩短方向。变形局部化是独立于脆性和塑性变形外的变形领域,受最大有效力矩准则控制。20世纪末提出的变位形分解理念,摆脱连续介质力学的束缚,合理地说明广泛存在的走滑断层平行俯冲带或逆冲断层带。非均匀变形和非连续介质力学理念的建立,为地质学与力学的结合开辟了新的前景。文章试用上述两理念概略分析中国和邻区中新生代构造格局,以期引发讨论。      

壳-幔动力学与活化构造(地洼)理论

壳-幔动力学是地球内部物理学和大地构造演化的重要研究方向之一。本文从地球物理角度出发,以物理概念和数学描述相结合的定量方式,对陈国达院士生前所创建的活化构造(地洼)理论研究中的某些地球深部动力学问题进行了较系统的综合评述和探讨。主要论题包括岩石圈的性质与物理学、地幔流变学、重力与均衡理论、地球的温度和热传递,诸如热传导、物质的物理运动所引起的热传输、地球内部的热对流及地幔柱的形成和作用等。作者特别强调了构造演化的定量分析问题,如热时间常数、热应力与其它力源、水平运动与垂直运动的关系,以及地壳断裂作用。岩石圈的构造作用与演化是与深部热运动有关的水平 (压缩和扩张)应力和由地壳厚度差异所导致的垂直应力差的共同结果。热应力的构造意义主要表现为短时间尺度的脆性断裂或柔性应变松弛过程。局部对流机制对活化构造(地洼)研究值得重视。     

Derivation and evaluation of a mechanical model for sheet intrusions

A two-dimensional mechanical model for sheet intrusions based on the elastic deformation around a pressurized elliptical hole is derived. After evaluating the effects of irregularities in contact shape, possible magma pressure distributions, regional boundary conditions, host rock properties, and interference from adjacent intrusions the model can be applied with confidence to understand many features associated with dike or sill emplacement. Three mechanisms for sheet intrusion propagation are extension fracturing, brittle faulting, and ductile faulting. This deformation is concentrated at the intrusion termination, the site of large principal stress differences and large stress gradients. In both brittle and ductile host rock the magma pressure needs only barely to exceed the regional stress acting normal to the intrusion's length for propagation to occur. Propagation direction is controlled by the regional stress orientation, interference from adjacent intrusions, planar discontinuities, and changes in host rock properties. The classic method of distinguishing dilational from non-dilational intrusions may not work because contacts are not planar or parallel and displacements are not everywhere perpendicular to the intrusion's length. Equations are derived for calculating the magma pressure for intrusions in relatively simple tectonic settings.     

The influence of different rheological parameters on the surface deformation and stress field of the Aegean–Anatolian region

The Aegean–Anatolian region is characterised by an inhomogeneous deformation pattern with high strain rates and a high seismicity both at the boundaries and in the plate interior. This pattern is controlled by the geometry and rheology of the structural units involved and their tectonic setting. A numerical analysis with a finite-element model of the region is used to quantify the influence of different rheological parameters. Viscoelastic material behaviour is implemented for the mantle lithosphere, whereas the crust is modelled with an elastic–plastic rheology. The variation of the inelastic material properties (viscosity and plastic strength) quantifies the influence of these material parameters on the deformation, stress, and strain patterns. Comparison of the modelled results with geodetic and geophysical observations reveals that the viscosity of the mantle lithosphere is the key to explaining the inhomogeneous deformation pattern. The best-fit model yields a viscosity of 10²⁰ Pa s beneath Anatolia, whereas adjacent regions have viscosities between 10²¹ and 10²³ Pa s. The model also explains the intra-plate seismicity and the stress field as well as its partitioning into regions with strike-slip and normal faulting. The final model is in good agreement with seismological, geodetic, and geological observations. Local deviations can be tracked down to small-scale structures, which are not included in the model.