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1.
We report quantitative results from three brittle thrust wedge experiments, comparing numerical results directly with each other and with corresponding analogue results. We first test whether the participating codes reproduce predictions from analytical critical taper theory. Eleven codes pass the stable wedge test, showing negligible internal deformation and maintaining the initial surface slope upon horizontal translation over a frictional interface. Eight codes participated in the unstable wedge test that examines the evolution of a wedge by thrust formation from a subcritical state to the critical taper geometry. The critical taper is recovered, but the models show two deformation modes characterised by either mainly forward dipping thrusts or a series of thrust pop-ups. We speculate that the two modes are caused by differences in effective basal boundary friction related to different algorithms for modelling boundary friction. The third experiment examines stacking of forward thrusts that are translated upward along a backward thrust. The results of the seven codes that run this experiment show variability in deformation style, number of thrusts, thrust dip angles and surface slope. Overall, our experiments show that numerical models run with different numerical techniques can successfully simulate laboratory brittle thrust wedge models at the cm-scale. In more detail, however, we find that it is challenging to reproduce sandbox-type setups numerically, because of frictional boundary conditions and velocity discontinuities. We recommend that future numerical-analogue comparisons use simple boundary conditions and that the numerical Earth Science community defines a plasticity test to resolve the variability in model shear zones. 相似文献
2.
Analogue models are not perfectly reproducible even under controlled boundary conditions which make their interpretation and application not always straight forward. As any scientific experiment they include some random component which can be influenced both by intrinsic (inherent processes) and extrinsic (boundary conditions, material properties) sources. In order to help in the assessment of analogue model results, we discriminate and quantify the intrinsic versus extrinsic variability of results from “sandbox” models of accretionary wedges that were repeated in a controlled environment. The extrinsic source of variability, i.e. the parameter varied is the nature of the décollement (material, friction and thickness). Experiment observables include geometric properties of the faults (lifetime, spacing, dip) as well as wedge geometry (height, slope, length).For each variable we calculated the coefficient of variance (CV) and quantified the variability as a symmetric distribution (Normal, Laplacian) or asymmetric distribution (Gamma) using a Chi squared test (χ2). Observables like fault dip/back thrust dip (CV = 0.6–0.7/0.2–0.6) are less variable and decrease in magnitude with decreasing basal friction. Variables that are time dependent like fault lifetime (CV = 0.19–0.56) and fault spacing (CV = 0.12 – 0.36) have a higher CV consequently affecting the variability of wedge slope (CV = 0.12–0.33). These observables also increase in magnitude with increasing basal friction. As the mechanical complexity of the evolving wedge increases over time so does the CV and asymmetry of the distribution. In addition, we confirm the repeatability of experiments using an ANOVA test. Through the statistical analysis of results from repeated experiments we present a tool to quantify variability and an alternative method to gaining better insights into the dynamic mechanics of deformation in analogue sand wedges. 相似文献
3.
In many cases, thrust wedges accreted at shallow crustal levels show an across‐strike rheological variability along the basal décollement, notably from brittle to ductile behaviour. In this paper, we illustrate the results of sandbox analogue modelling research devoted to studying the influence of convergence velocity on wedge architecture when laterally juxtaposed frictional and viscous materials occur along the basal décollement of accreting thrust wedges. Our results show that slow convergence favours a near symmetrical distribution of thrust vergence within wedge sectors accreted above viscous décollement material, whereas fast convergence favours vergence asymmetry. In particular, at fast convergence rates the hinterlandward extrusion of viscous décollement material at the toe of the frictional wedge is favoured and contributed to accommodate a significant amount of the total contraction. Terra Nova, 18, 241–247, 2006 相似文献
4.
不同变形速率下前陆褶皱冲断带的发育特征——基于砂箱物理模拟实验研究 总被引：1，自引：0，他引：1
构造模拟实验是研究和模拟自然界地质构造现象、变形特征、成因机制和动力学过程的一种物理实验方法。本文基于砂箱构造物理模型高、中、低速7组不同单向挤压速度的构造模拟实验,揭示不同变形速率下砂箱物质变形的几何学、运动学及其演化特征,探讨不同变形速度(尤其是不同量级速度)对前陆褶皱冲断构造变形的重要性。高速单向挤压变形过程中(0.4~0.1mm/s),砂箱模型中石英砂体后缘构造加积强烈,砂体变形主要以楔形体向前扩展变形为主,其构造样式主要表现为前展式叠瓦冲断构造。中速挤压变形过程中(0.05~0.005mm/s),砂体后缘加积相对较弱,构造变形样式主要为砂箱楔形体前缘先形成(前展式为主)逆冲断层,当达到临界楔形体后,反冲断层发育并与前展式逆冲断层构成冲起构造,构造样式上表现为叠瓦构造与冲起构造的无序叠加。低速挤压变形过程中(0.002mm/s),砂箱物质构造样式以典型冲起带为主。由于砂箱模拟过程的时效性,即如何在最有效时间内获得最全面而复杂的演变过程,通过本次系列实验,我们建议将此有效挤压速度设定为0.05~0.005mm/s范围内,可以先后清晰而全面的获得高速和低速挤压下砂体的变形过程和构造样式。 相似文献
5.
从反转构造的砂箱模型看柴达木早-中侏罗世盆地的构造性质 总被引：7，自引：0，他引：7
设计了基底均匀伸展半地堑正反转、基底均匀伸展书斜式正反转和对称地堑正反转等3个砂箱实验模型,着重研究后期构造反转对前期正断层构造的叠加与改造作用.单侧挤压实验结果表明,正反转过程中,挤压变形主要通过形成新的低角度逆冲断层实现,前期正断层多数可被保存下来,仅倾角不同程度地变陡.前期正断层全部反转为逆断层的可能性极小,仅在强烈逆冲变形带,逆冲断层才有可能完全取代前期的正断层.根据实验结果并结合柴达木盆地的实际资料,认为柴达木盆地早-中侏罗世时期为挤压性质的盆地. 相似文献
6.
This paper investigates the evolution of thrust wedges with concomitant surface erosion, and its bearing on the exhumation
processes in orogenic belts. We performed sandbox experiments, simulating syn-orogenic erosion on forelandward sloping surfaces
(∼4°). Experiments show that the erosion process has a significant control on the progression of frontal thrusts. In case
of no-erosion condition, wedges with high basal friction develop frontal thrusts with strongly increasing spacing. In contrast,
for the same basal friction the thrusts show uniform spacing as the wedge development involves concomitant surface erosion.
On the other hand, the erosion promotes reactivation of hinterland thrusts in wedges with low basal friction. We show that
erosion-assisted thrust reactivation is the principal mechanism for exhumation of deeper level materials in orogens. Efficiency
of this mechanism is largely controlled by basal friction. The exhumation of deeper level materials is limited, and occurs
within a narrow, sub-vertical zone in the extreme hinterland when the basal friction is high (μb = 0.46). In contrast, the process is quite effective in wedges with low basal friction (μb =0.36), resulting in exhumation along gently dipping foreland-vergent thrusts as well as along thrusts, subsequently rotated
into steep attitude. The zone of exhumation also shifts in the foreland direction in the course of horizontal movement. Consequently,
deeper level materials cover a large area of the elevated part of the wedge. 相似文献
7.
冲起构造广泛发育于挤压逆冲构造体系和走滑构造体系,具有极其重要的油气勘探价值,因而备受关注。砂箱模拟模型已经被证明是强大的可视化工具,用来模拟不同岩层中复杂的构造现象。基于相似初始砂箱构造模型条件下不同挤压速率(0.3 mm/s、0.1 mm/s、0.005 mm/s)变形过程,揭示出褶皱冲断带发育过程中的典型两类冲起构造：叠加冲起构造和单一冲起构造,它们对于褶皱冲断带演化过程及其油气勘探具有明显不同的重要性。砂箱物理模拟实验揭示楔形体扩展变形序列对前陆褶皱冲断带冲起构造发育类别具有重要控制作用,即褶皱冲断带前缘以前展式扩展变形为主,主要发育单一冲起构造;后缘以后展式扩展变形为主,主要发育叠加冲起构造。 相似文献
8.
Mohammad Irfan Ahmad A. K. Dubey Giovanni Toscani Lorenzo Bonini Silvio Seno 《International Journal of Earth Sciences》2014,103(1):329-347
Kinematic evolution of fold-thrust structures has been investigated by analogue models that include syntectonic sedimentation. Different decollement dips and basement thicknesses produced different wedge geometries and propagating characteristics. A model with one decollement level was characterized by a closely spaced thrust system during early stages of shortening as compared to the late stages. The frequency of fault nucleation was rapid during the early stages of deformation. Conversely, the frequency of fault nucleation was low and thrust spacing was significantly wider in a model with two decollement levels. Individual faults became locked at steep dips and deformation stepped forward as a new fault nucleated in-sequence in front of the older locked structure. Once the thrust system was established up to 27 % overall shortening, an overlying bed was introduced to simulate syntectonic deformation. Model sand wedge did not grow self similarly but rather its length and height increased episodically with deformation. Restoration of deformed models show that layer parallel shortening accommodated for approximately half of the total model shortening across the multilayers. Calculated error in apparent layer shortening from the restored layers revealed a direct relation with depth of the layers in the models. The experimental results are comparable to a natural example from the Northern Apennines fold-and-thrust belts. 相似文献
9.
《Journal of Structural Geology》1988,10(8):847-859
Internal geometry and detachment of Coulomb thrust wedges is examined in a combined gravity and pressure force field under various boundary conditions and Coulomb parameters. Frontally buttressed sand wedges accreted with narrow tapers above soles with negligible friction. In these models, serial imbricates stepped-up in conjugate sets which operated synchronously or serially. With increases in sole friction, model Coulomb wedges accreted in piggyback style with enhanced taper but narrower spacing of imbricates. In this mode, flat-topped box-fold anticlines nucleated at the frontal tip of a sole thrust prior to the fore-limb failing as a ramp. The initial ramp-thrusts subsequently rotated and steepened to acquire concave-upwards listric geometry. When cohesion was a significant component of strength, and the sand grains interlocked thoroughly, ramp climb during piggyback thrusting occurred through the generation of a stack of backthrusts. The taper of cohesionless sand wedges, which lack any length scale, was consistent with the critical cohesionless Coulomb wedge taper. In contrast, the taper of experimental cohesive sand wedges, which are scale-dependent, was consistent with the theoretical cohesive Coulomb narrow taper, only when the cohesive length scale was lr ∼ Sr/ϱrar ≤ 10−6. This was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass. 相似文献
10.
地壳浅层次褶皱冲断带普遍受控于具耦合互馈特性的多种机制或边界条件,从而具有长期的构造演化过程和复杂的构造特征。自19世纪初以来,基于地质构造过程自相似性和"无理有效性"的砂箱物理模型,为解译褶皱冲断带的演化过程及其动力学机制等提供了独立有效的手段。本文主要基于国内外地壳浅表褶皱冲断带构造变形过程为研究对象的构造砂箱物理模型研究结果与进展进行综述性报道,以期为同行提供参考与借鉴。基于与地壳浅表地层具相似流变学特性的砂箱物理模拟研究,揭示出地壳浅层次构造变形过程普遍自相似性生长过程,符合库伦临界楔理论。褶皱冲断带挤压变形过程中,基底特性(基底几何学、有效摩擦角、基底耦合性和流体超压)、变形物质特性(空间几何学、能干层、流变学和非均一性)、动力学机制(砂箱几何边界、汇聚速率和汇聚方向)、浅表作用(剥蚀和沉降)等对于地壳浅表构造变形过程具有明显的控制作用。构造砂箱物理模型在解译不同构造变形过程与机制的研究中发挥着越来越突出的作用。 相似文献
11.
桩基负摩阻力时间效应试验研究 总被引：2，自引：0，他引：2
由于黏土固结缓慢,桩基负摩阻力存在明显的时间效应,然而目前相关研究仍显不足。设计实施了能实现桩顶加载及较大超载值的单桩及双桩负摩阻力模型试验,桩周土采用砂土和软黏土夹层,测定了模型桩身应力、桩顶位移以及土体分层沉降随固结时间的变化。试验结果显示,沉降、负摩阻力具有明显的时间效应。土表超载作用下土体沉降带动桩沉降,桩与土体的沉降均表现出早期快、后期慢的趋势。试验加载初期,黏土夹层处的负摩阻力略小于砂层,但随土体固结而增长,其基本变化规律与沉降相同。因桩端砂土层沉降稳定迅速,中性点随桩身沉降增长略呈上移趋势。此外,相同荷载作用下桩间距较小的双桩,由于下拉力较小,其沉降较小。试验条件下,3D桩间距的负摩阻力群桩效应系数在0.71～0.77之间,6D桩间距时不存在负摩阻力群桩效应。 相似文献
12.
The continuity of clay-rich fault gouge has a large effect on fluid transmissibility of faults in sand–clay sequences, but clay gouge continuity and composition in 3D are not well known. We report observations of 3D clay smear continuity in water-saturated sandbox experiments where the sheared clay layers were excavated after deformation. The experiments build on existing work on the evolution of clay gouge in similar 2D experiments where interpretations were made in profile view.We used well-known model materials (“Benchmark” sand and uncemented kaolinite–sand mixtures) that were further characterized using standardized geotechnical tests and triaxial compression experiments at effective pressures corresponding to the sandbox experiments. Results show a nonlinear failure envelope of the sand, in agreement with existing models. Unconfined compression experiments with the clay show cohesion around 50 Pa and brittle behavior.A sheared, ductile clay layer embedded in sand above a 70° dipping basement fault reveals a complex, natural-looking clay gouge architecture with relay ramps, breached relays and fault lenses. The clay gouge shows clear variations in composition and thickness and becomes locally discontinuous at throw-thickness ratios above 7, in contrast to our earlier 2D observations where discontinuous clay-gouge only formed in cemented clay layers. In addition to tectonic telescoping in the relays, the thin, continuous parts of the clay gouge were transformed from an initial pure clay by mechanical mixing of sand and clay.We also discuss the applicability of these results to the evolution of normal fault zones and deformation bands in sand–clay sequences at effective pressures below the onset of cataclasis and conclude that in fault zones a higher degree of internal segmentation reduces the probability of the formation of discontinuities. 相似文献
13.
准确地确定岩土设计参数统计特征值诸如均值、标准差是岩土工程可靠度分析与设计的重要前提。在满足岩土设计参数统计特征值计算精度条件下,文中提出了岩土工程最小勘探数据量的确定方法,定义了相对误差和相对变异性指标衡量岩土设计参数统计特征值计算准确性。系统地分析了静力触探试验数据量对砂土有效内摩擦角统计特征值计算精度的影响,并且根据相对误差和相对变异性指标确定了静力触探最小勘探数据量。研究结果表明,由静力触探试验间接估计砂土有效内摩擦角时均值相对误差较低,砂土有效内摩擦角相对变异性指标随静力触探试验数据量的增加而降低,即由认知不足引起的不确定性占总变异性的比值随静力触探试验数据量的增加而减小;当砂土有效内摩擦角容许相对变异性指标小于0.2时砂土有效内摩擦角在最大变异（COV=20%）与最小变异性（COV=5%）范围内,满足预定要求所需的最小静力触探试验数据量为10～100;若容许相对变异性指标小于0.3,所需的最小静力触探试验数据量为5～43。此外,间接估计岩土设计参数时经验回归模型不确定性对最小勘探数据量有显著影响。静力触探试验最小勘探数据量随经验回归模型不确定性的增大而增加,在确定岩土设计参数统计特征值时应尽量广泛收集勘探数据并选择精度较高的计算模型。 相似文献
14.
Gbor Windhoffer Gbor Bada Dick Nieuwland Gza Wrum Ferenc Horvth Sierd Cloetingh 《Tectonophysics》2005,410(1-4):389-415
We present the results of a thrust fault reactivation study that has been carried out using analogue (sandbox) and numerical modelling techniques. The basement of the Pannonian basin is built up of Cretaceous nappe piles. Reactivation of these compressional structures and connected weakness zones is one of the prime agents governing Miocene formation and Quaternary deformation of the basin system. However, reactivation on thrust fault planes (average dip of ca. 30°) in normal or transtensional stress regimes is a problematic process in terms of rock mechanics. The aim of the investigation was to analyse how the different stress regimes (extension or strike-slip), and the geometrical as well as the mechanical parameters (dip and strike of the faults, frictional coefficients) effect the reactivation potential of pre-existing faults.
Results of analogue modelling predict that thrust fault reactivation under pure extension is possible for fault dip angle larger than 45° with normal friction value (sand on sand) of the fault plane. By making the fault plane weaker, reactivation is possible down to 35° dip angle. These values are confirmed by the results of numerical modelling. Reactivation in transtensional manner can occur in a broad range of fault dip angle (from 35° to 20°) and strike angle (from 30° to 5° with respect to the direction of compression) when keeping the maximum horizontal stress magnitude approximately three times bigger than the vertical or the minimum horizontal stress values.
Our research focussed on two selected study areas in the Pannonian basin system: the Danube basin and the Derecske trough in its western and eastern part, respectively. Their Miocene tectonic evolution and their fault reactivation pattern show considerable differences. The dominance of pure extension in the Danube basin vs. strike-slip faulting (transtension) in the Derecske trough is interpreted as a consequence of their different geodynamic position in the evolving Pannonian basin system. In addition, orientation of the pre-existing thrust fault systems with respect to the Early to Middle Miocene paleostress fields had a major influence on reactivation kinematics.
As part of the collapsing east Alpine orogen, the area of the Danube basin was characterised by elevated topography and increased crustal thickness during the onset of rifting in the Pannonian basin. Consequently, an excess of gravitational potential energy resulted in extension (σv > σH) during Early Miocene basin formation. By the time topography and related crustal thickness variation relaxed (Middle Miocene), the stress field had rotated and the minimum horizontal stress axes (σh) became perpendicular to the main strike of the thrusts. The high topography and the rotation of σh could induce nearly pure extension (dip-slip faulting) along the pre-existing low-angle thrusts. On the contrary, the Derecske trough was situated near the Carpathian subduction belt, with lower crustal thickness and no pronounced topography. This resulted in much lower σv value than in the Danube basin. Moreover, the proximity of the retreating subduction slab provided low values of σh and the oblique orientation of the paleostress fields with respect to the master faults of the trough. This led to the dominance of strike-slip faulting in combination with extension and basin subsidence (transtension). 相似文献
15.
进行砂土的单桩和2×2群桩的压、拔试验,测定桩身轴力、桩顶荷载、位移以及桩端阻力。试验研究结果表明,群桩的抗压群桩效应系数 >1,最优桩间距为4D(D为直径),对应的 约为1.2;桩端阻力群桩效应系数 大于桩侧摩阻力群桩效应系数 ,其最优桩间距为5D,对应的 值约为1.3;桩侧摩阻力群桩效应系数在3D时约为1.2,但随桩间距的增大而减小。与前人理论分析的结果不同,由于砂土的挤密效应,试验测得抗拔群桩效应系数也大于1,最优桩间距在4D～5D之间,系数约为1.2。桩间距为7D时压、拔群桩效应系数均趋近于1,可基本忽略群桩效应。 相似文献
16.
Topography as a major factor in the development of arcuate thrust belts: insights from sandbox experiments 总被引：2，自引：0，他引：2
We have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts. In experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston, the geometry of the developing thrust wedge was highly sensitive to variations in surface topography. In the absence of erosion and sedimentation, the surface slope tended to become uniform, as predicted by the theory of critical taper. Under these conditions, the wedge propagated by sequential accretion of new thrust slices. In contrast, where erosion or sedimentation caused the topographic profile to become irregular, thrusts developed out of sequence. For example, erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland. Where initial topography was irregular in plan view, accreting thrusts tended to be arcuate. They were convex towards the foreland, around an initially high area; concave towards the foreland, around an initially low area. Initial plateaux tended to behave rigidly, while arcuate thrust slices accreted to them. Thrust motions were radial with respect to each plateau. Within transfer zones to each side, fault blocks rotated about vertical axes and thrust motions were oblique-slip. At late stages of deformation, the surface slope of the thrust wedge tended towards a uniform value. Initial mountains of conical shape (representing volcanoes) also escaped deformation, except at depth, where they detached. Arcuate thrust slices accreted to front and back. Where a developing thrust wedge was subject to local incision, accreting thrust slices dipped towards surrounding areas of high topography, forming Vs across valleys.Arcuate structural patterns are to be found around the three highest plateaux on Earth (Tibet, Pamirs and Altiplano) and around the Tromen volcanic ridge in the Neuquén Basin of northern Patagonia. We infer that these areas behaved in quasi-rigid fashion, protected as they were by their high topography. 相似文献
17.
Mechanical stratigraphy as a factor controlling the development of a sandbox transfer zone: a three-dimensional analysis 总被引：3，自引：0，他引：3
In thrust belts, fold–fault terminations are common features of the structural architecture and can pose complicated problems to unravel, in particular when two or more terminations are in close proximity. Such terminations usually reflect pre-existing attributes. Amongst the many factors, lateral variations in the mechanical stratigraphy can control along-strike geometry and kinematics of fault-related folds.A displacement transfer zone was produced in a compressional sandbox model by means of two adjacent, mechanically different stratigraphic domains. The experiment allowed two discrete chains to develop in the different domains, so that a complex structural setting occurred in the connecting area. Periclinal folds, oblique thrust fronts and oblique ramps developed in the resulting transfer zone. The interaction between periclines in the transfer zone produced lateral culminations in the folded structures. The analysis of displacement across the structural domains revealed that a significant loss of slip along the faults occurred in the relay zone. In this area, imbricate faulting was partially replaced by layer-parallel shortening. A linear relationship appears to exist between the bed length of the thrust sheet and the related fault slip. 相似文献
18.
Role of the backstop-to-cover thickness ratio on vergence partitioning in experimental thrust wedges
The ratio between the thickness of the sedimentary sequences offscraped from the subducting plate ( H c ) and the average thickness of the backstop ( H b ) is expected to significantly influence vergence partitioning in thrust wedges. A series of simple sandbox experiments was performed to study the role of the H c / H b ratio in model thrust wedges. The results obtained show that thrust wedges with H c / H b values lower than 1 are essentially pro-ward verging, with minor retro-ward faulting; H c / H b values greater than 1 imply an increase of vergence partitioning. When a threshold value is exceeded, fully partitioned, doubly vergent thrust wedges develop. Comparison with natural submarine thrust wedges suggests that these experimental results may provide useful insights on the evolution of structural architecture at convergent margins. 相似文献
19.
对双圆（DOT）盾构隧道周围土体采用椭圆形非等量径向位移模式,在镜像法基本原理上,推导土体损失引起的附加应力计算公式,研究双圆盾构正面附加推力、双圆盾构机与土体之间的摩擦力以及土体损失在平行既有隧道上引起的总的附加荷载的分布规律。研究结果表明,附加荷载的变化规律与双圆盾构机和邻近平行隧道的相对位置密切相关,是一个三维问题;随着双圆盾构机开挖面通过前后,附加荷载由压力变为拉力;土体损失是引起相邻隧道上附加荷载的主要因素,其次分别是盾壳摩擦力和正面附加推力;已建隧道受到的附加荷载变化规律与其和双圆盾构机的净距 S 密切相关,随着 S 的减 小,附加荷载急剧增大;随着平行既有隧道轴线埋深的变化,在轴线位置处,双圆盾构机对平行既有隧道的附加荷载影响最大。 相似文献
20.
本文设计完成了三种双滑脱层模型的物理模拟实验,并运用粒子成像测速(PIV,Particle Image Velocimetry)技术计算出实验过程中各阶段模型剖面上的速度场分布,进而对褶皱冲断带的运动学过程和变形机制进行讨论。实验结果表明,双滑脱层模型中,基底滑脱层控制了整体的构造样式,浅部滑脱层决定局部的浅层构造。笔者等将实验结果与龙门山褶皱冲断带南段双滑脱体系构造进行了比较,验证了这一结论。PIV分析显示,逆冲断层的产生经历一个平行层缩短的变形过程。该过程在塑性层上、下具有明显差异,塑性层上的变形传递得更快更远。当缩短进行到一定阶段,断层开始发育,发生初始破裂,断层下盘的变形消失,应变集中在断面上,断层上盘沿断面同步逆冲。 相似文献