Sensitivity of shear zones in orogenic wedges to surface processes and strain softening

We investigate the internal deformation of orogenic wedges growing by frontal accretion with a two-dimensional numerical model. Our models are limited to crustal deformation and assume a horizontal detachment as observed for various natural orogens (e.g. Alaska and Costa Rica). The model wedges develop as a result of convergence of a brittle sediment layer in front of a strong backstop. We find that our reference model develops in-sequence forward-thrusts which propagate upward from the basal detachment. For this reference model we investigate the sensitivity of shear zone activity to surface processes and strain softening. Model results show that diffusive or slope dependent erosion enhances material transport across the wedge and slows down forward propagation of the deformation front. Frictional strain softening focuses deformation into narrow shear zones and enhances displacement along them. This has also been postulated for natural thrusts such as the Glarus thrust in the Swiss Alps and the Moine thrust in the Scottish Caledonides. A second series of models investigates the effects of regularly spaced weak inclusions within the sediment layer which simulate remnants of previous deformation phases. These inclusions facilitate and focus internal deformation, influence the thrust dip and thrust vergence and enable thrust reactivation in the internal part of the wedge. Our results show that inactive thrusts in the internal part of the wedges may be reactivated in models with diffusive surface processes, strain softening or weak inclusions. Thrust reactivation occurs as models seek to maintain their critical taper angle. First order characteristics of our numerical models agree well with natural orogenic wedges and results from other numerical and analogue models.     

Benchmarking analogue models of brittle thrust wedges

We performed a quantitative comparison of brittle thrust wedge experiments to evaluate the variability among analogue models and to appraise the reproducibility and limits of model interpretation. Fifteen analogue modeling laboratories participated in this benchmark initiative. Each laboratory received a shipment of the same type of quartz and corundum sand and all laboratories adhered to a stringent model building protocol and used the same type of foil to cover base and sidewalls of the sandbox. Sieve structure, sifting height, filling rate, and details on off-scraping of excess sand followed prescribed procedures.Our analogue benchmark shows that even for simple plane-strain experiments with prescribed stringent model construction techniques, quantitative model results show variability, most notably for surface slope, thrust spacing and number of forward and backthrusts. One of the sources of the variability in model results is related to slight variations in how sand is deposited in the sandbox. Small changes in sifting height, sifting rate, and scraping will result in slightly heterogeneous material bulk densities, which will affect the mechanical properties of the sand, and will result in lateral and vertical differences in peak and boundary friction angles, as well as cohesion values once the model is constructed. Initial variations in basal friction are inferred to play the most important role in causing model variability.Our comparison shows that the human factor plays a decisive role, and even when one modeler repeats the same experiment, quantitative model results still show variability. Our observations highlight the limits of up-scaling quantitative analogue model results to nature or for making comparisons with numerical models. The frictional behavior of sand is highly sensitive to small variations in material state or experimental set-up, and hence, it will remain difficult to scale quantitative results such as number of thrusts, thrust spacing, and pop-up width from model to nature.     

Topography as a major factor in the development of arcuate thrust belts: insights from sandbox experiments

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.     

Critical taper behaviour and out‐of‐sequence thrusting on orogenic wedges – an example of the Eastern Alpine Molasse Basin

We demonstrate that increasing erosion during the kinematic evolution of a thrust wedge will lead to out‐of‐sequence thrusting as a result of backwards critical taper movement. In‐sequence thrusting in the Subalpine German Molasse Basin built a critical‐tapered foreland Coulomb thrust wedge. Later, out‐of‐sequence thrusts dissected all but the frontal duplex stacks. The footwall/hangingwall relation visible on seismic data proves the out‐of‐sequence nature of the latest thrusting stage. Establishing a stable drainage system leads to increased erosion in elevated areas of the thrust wedge, resulting in flattening of the critical wedge. In order to keep its predefined angle, the critical wedge repositions and the tip of the taper moves towards the hinterland. Thus, thrusting will also reposition and move towards the hinterland.     

Revisiting the orogenic energy balance in the western Taiwan orogen with weak faults

Orogens at convergent margins must meet the energetic requirements necessary to lift rocks against gravity, allow for frictional sliding along basal detachments and accommodate internal deformation processes. The combination of critical taper and kinematic wedge theories predicts the partitioning between these energy sinks as a function of both fault and crustal strengths. Integrating contemporary estimates of both crustal pore fluid pressures and the coefficient of friction on major faults, we find that work associated with internal deformation processes is the dominant energy sink in the western Taiwan orogenic wedge. These processes consume 54% of the total work budget, while the dissipation rates associated with frictional sliding on the basal detachment and lifting rocks against gravity are lower, requiring only 11% and 35% respectively. These results suggest a mechanical dichotomy in orogenic wedges where the faulting dominates the kinematic deformation budget, but internal distributed deformation processes dominate the energy budget.     

The Siwaliks of western Nepal: II. Mechanics of the thrust wedge

Comparison between numerical models and structural data is used for a better understanding of the evolution of the Siwalik thrust belt of western Nepal. The numerical model involves discontinuities within a critical wedge model, a kinematic forward model of serial cross sections, and a linear diffusion algorithm to simulate erosion and sedimentation. In western Nepal, large Piggy-back basins (Duns) are located above thick thrust sheets that involve more than 5500 m of the Neogene Siwalik Group, whereas Piggy-back basin sedimentation is less developed above thinner thrust sheets (4300 m thick). Numerical model results suggest that thrust sheet thickness and extension of wedge-top basins are both related to an increase of the basal décollement dip beneath the duns. The West Dang Transfer zone (WDTZ) is a N–NE trending tectonic lineament that limits the westward extent of the large Piggy-back basins of mid-western Nepal and is linked to a thickening of the Himalayan wedge eastward. The WDTZ also affects the seismotectonics pattern, the geometry of the thrust front, the lateral extent of Lesser Himalayan thrust sheets, and the subsidence of the foreland basin during middle Siwalik sedimentation. Numerical models suggest that the individualisation of the Piggy-back basins at the transition between the middle Siwalik and upper Siwaliks followed the deposition of the middle Siwaliks that induced a geometry of the foreland basin close to the critical taper. As WDTZ induces an E–W thickning of the Himalayan wedge, it could also induce a northward shift of the leading edge of the ductile deformation above the basal detachment in Greater Himalayas of far-western Nepal. Field data locally suggest episodic out-off-sequence thrusting in the frontal thrust belt of western Nepal, whereas numerical results suggests that episodic out-off sequence reactivation could be a general characteristic of the Himalayan wedge evolution often hidden by erosion.     

西昆仑山前东段新生代褶皱冲断带及其锥形楔机制

西昆仑山前东段的和田-柯克亚构造带与塔里木盆地内的麻扎塔格构造带相距为200 km,两者近似平行,遥相对应,麻扎塔格构造带是和田-柯克亚构造带的前沿,山前挤压变形量沿古近系阿尔塔什组膏泥岩向北传递约200 km到麻扎塔格,形成往北东逆冲的断层传播褶皱,在盆地腹部表现为一条显著的北西向地形隆起。因此,整个西昆仑山前东段新生代褶皱冲断带宽约为270 km,长约为220 km,由和田-柯克亚构造带、麦盖提斜坡和麻扎塔格构造带组成。中部近200 km的麦盖提斜坡内没有明显构造变形,且整个褶皱冲断带的锥形楔角度远小于活动造山带的临界锥形角,说明古近系底部阿尔塔什组膏泥岩以及新生代沉积体的形态是这个褶皱冲断带锥形楔形成的主控原因,膏泥岩控制了锥形楔的顶面坡度α基底构造形态及上覆沉积楔形体则控制了锥形楔的底部滑脱层倾角β。     

Effect of Rheology and Thickness Variation of Basal Detachment on Fold-and-Thrust Belts: Numerical SimulationEI北大核心CSCD

Detachment is the prime factor affecting the fold-and-thrust belts. In order to investigate the effect of rheology and thickness variation of basal detachment on fold-and-thrust belts, numerical simulations are carried out by using two dimensional plane strain mechanical models with elastoplastic or viscoelastoplastic material behavior solved with the finite difference software FLAC. Universal mudstone detachment has frictional property. Such fold-and-thrust belts are simulated with elastoplastic constitutive model. In contrast, salt detachment has creep property, so viscoelastoplastic constitutive models are employed for these fold-and-thrust belts. The results show that rheology and thickness of basal detachment influence the fold-and-thrust belts greatly. When the detachment is mudstone, the typical structural style of imbricate fan will develop, also the propagating forward piggy-back thrust sequence; however, when the detachment is salt, structural style will show Jura-type consisted of a series of anticlines and synclines, and out-of-sequence thrusts will appear. Varieties of thickness of basal mudstone detachment will change eventual surface slope of the fold-and-thrust belts, but have little effect on the structural styles and thrust sequence. For salt detachment, the effect is significant, no matter where the thickest detachment distributes, most strongly deformation and the highest surface will be located there. Thickening of salt detachment combined with slight thinning of overburden strata from hinterland to foreland will result in uncommon structural style and thrust sequence: back thrusts and backward propagating. These differences are mainly due to the much smaller shear strength of salt than that of mudstone. © 2017, Science Press. All right reserved.     

Intrinsic versus extrinsic variability of analogue sand-box experiments – Insights from statistical analysis of repeated accretionary sand wedge experiments

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.     

底部滑脱层流变性和厚度变化对褶皱冲断带影响的数值模拟

滑脱层是影响褶皱冲断带的第一因素。为了探讨底部滑脱层流变性和厚度变化对褶皱冲断带的影响,本文通过二维有限差分软件FLAC开展了数值模拟研究。常见的泥岩滑脱层具摩擦性质,此类冲断带采用弹塑性模型;而盐岩滑脱层具蠕变性质,此类冲断带则采用黏弹塑性模型。结果表明,底部滑脱层流变性以及厚度对冲断带的变形影响很大。当滑脱层为泥岩时,发育典型的叠瓦状构造样式,变形序列为背驮式向前扩展的有序序列。而当滑脱层为盐岩时,发育背、向斜相间排列的侏罗山式构造样式,并以无序序列演化。底部滑脱层厚度的变化,对泥岩滑脱层的冲断带而言,主要改变其表面坡角,对构造样式和演化序列影响不大;但对盐岩滑脱层的冲断带影响很大,不管滑脱层最厚的地方分布在后陆还是前陆,变形都会最强烈,地表凸起最高。从后陆向前陆加厚的盐岩滑脱层结合由后陆向前陆略为减薄的上覆地层可产生罕见的后冲、后展式演化序列。这些变形的差异主要是由于盐岩比泥岩具有小得多的剪切强度所致。     

First-order kinematics of wedge-scale active Himalayan deformation: Insights from Darjiling–Sikkim–Tibet (DaSiT) wedge

The Indian Plate has collided with the Eurasian Plate along an arcuate boundary over the last 55–60 million years defining the Himalayan Mountain belt. The geometry of the collision boundary is wedge-shaped; the base of this wedge is defined by a decollement named the Main Himalayan Thrust (MHT). In the Darjiling–Sikkim–Tibet (DaSiT) Himalayan wedge, a crustal-scale fault-bend fold (Kangmar Anticline) and the Lesser Himalayan Duplex (LHD) are dominant structures that have built taper and controlled the foreland-ward propagation of the thrust sheets. A frontal physiographic half-window has eroded through the Main Central Thrust (MCT) sheet to expose the LHD in the DaSiT wedge. Preliminary data suggest that active tectonics and seismicity in the DaSiT wedge may be concentrated in the half-window; this suggests that LHD may be an active structure. High-precision Global Positioning System measurements in the DaSiT wedge suggest that a minimum of 12 mm/yr convergence is being accommodated in the Darjiling–Sikkim Himalaya out of which ∼4 mm/yr convergence is being taken up in the LHD. Given that decollement earthquakes with minimum internal deformation in a deforming wedge occur when it attains critical taper, continued deformation within the DaSiT wedge and the lack of great decollement earthquakes indicate that the DaSiT Himalayan wedge is presently sub-critical and in the process of building taper. The sub-critical nature of the DaSiT wedge is probably the result of low topographic and decollement slopes, weaker rocks and pronounced erosion in the frontal part of the wedge.     

Interaction of surface erosion and sequential thrust progression: Implications on exhumation processes

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 (μ_b = 0.46). In contrast, the process is quite effective in wedges with low basal friction (μ_b =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.     

Deterministic chaos in frictional wedges revealed by convergence analysis

A triangular wedge, composed of a frictional material such as sand, and accreting additional material at its front, is the classical prototype for accretionary wedges and fold‐and‐thrust belts. A simplified method is proposed to capture the internal deformation of this structure resulting from a large number of faulting events during compression. The method combines the application of the kinematic approach of limit analysis to predict the optimum thrust‐fold and a set of geometrical rules to update the geometry accordingly, at each increment of shortening. It is shown that the structure topography remains approximately planar with a slope predicted by the critical Coulomb wedge theory. Failure by faulting occurs anywhere within the wedge at criticality, and its exact position is sensitive to topographic perturbations resulting from the deformation history. The convergence analysis in terms of the shortening increments and of the topography discretization reveals that the timing and the position of a single faulting event cannot be predicted. The convergence is achieved nevertheless in terms of the statistics of the distribution of the faulting events throughout the structure and during the entire deformation history. It is these two convergence properties that are presented to justify the claim that these compressed frictional wedges are imperfection sensitive, chaotic systems. Copyright © 2013 John Wiley & Sons, Ltd.     

Magma chambers and localization of deformation during thrusting

Terra Nova, 22, 390–395, 2010 Abstract We present the results of coupled analogue and numerical models that provide new insights into the relationships between volcanoes and thrusts. The effects of both upper‐crustal magma chambers and the load of volcanoes on the geometry of thrust systems were investigated. Analogue modelling points to a strong influence exerted by a magma chamber on thrust geometry, which, as suggested by the numerical models used to rationalize these results, is related to the stress redistribution around the weak heterogeneity. The low‐viscosity body below a volcanic edifice localizes compressional deformation and causes a curvature of the thrusts towards the magma chamber, opposite to the direction of tectonic transport. In these conditions, the volcanic load has a negligible effect on the structural geometry. These results are in contrast with those of previous studies, where intrusions or the load of major volcanoes generated a curvature of the thrusts away from volcanic edifices in the direction of tectonic transport.     

对逆冲断层一些普遍观念的质疑──以美国西部科迪勒拉及中国华北的野外研究为例(英文)

在过去的25年里,由于许多原因,作为最常见、分布也最广泛的地质构造形迹之一,逆冲断层成为倍受关注的科学研究主题。文中指出,关于逆冲断层及其几何学特征的许多普遍认识（或观念）,并不像以往文献中所阐述的那样简单。其中之一的"薄皮"冲断构造是受地层控制的,极少有或者没有结晶基底物的卷入。文中主张,"薄皮"一词只有逆冲板片的几何学形态含义,而不应包含地层意义,并列举了一些完全由结晶岩石所构成的薄皮逆冲构造的例子来说明这一主张。近来,逆冲双重构造成为构造文献中的热点。关于逆冲双重构造的成因,引用得最多的是1982年Boyer和Elliot在其重要论文"逆冲断层系统"中所作的解释。他们认为,双重道冲构造是通过在冲断坡底部发生下盘破裂。新生断裂不断向前扩展并进入先存断层下盘的一系列变形过程中逐渐形成的。根据Boyer和Elliot提出的这种变形过程,将形成一个具有平面状顶板断层的边冲双重构造,这个顶板断层只在活动断坡的顶部是主动向前扩展的。依笔者之见,在实际的构造变形当中,是不可能具备形成平顶过冲双重构造的地质条件的。而能对平顶过冲双重构造形成作出最好解释的是反序（out－of－sequence,OOS）边冲断层的发育,即断层向着主冲断层的后方发展,在先存道冲构造的上部?     

Modelling the geometry of Coulomb thrust wedges

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 l_r ∼ S_r/ϱ_ra_r ≤ 10⁻⁶. This was constrained by comparing geometry of cohesive sand wedges shortened at different body force per unit mass.     

库车坳陷西段盐构造二维平衡恢复与复原构造剖面分析

天山南麓库车坳陷西部沉积古近系膏盐岩,盐层下伏中生界发现万亿立方米储量天然气田,是我国西气东输重要气源。库车坳陷盐下是油气勘探关注目标,目前研究认识不能满足勘探需求,尤其是盐构造形成机制存在争议,盐构造平衡恢复是研究难点。本文应用二维构造平衡剖面技术,综合地震剖面、钻井和地表地质资料,选取库车坳陷西段四条剖面,通过StructureSolver公司开发的构造恢复软件(简称SS),开展平衡恢复工作,分层恢复盐上层、膏盐层、盐下层,复原挤压变形前地震剖面。研究结果揭示库车坳陷西段发育二期盐构造:渐新世-中新世发育刺穿型盐丘,上新世-第四纪发育挤压型盐构造。挤压作用下早期盐丘演变为盐席、盐墙,盐上层发育盐逆冲断层和盐背斜,盐下发育叠瓦状逆冲断层和挤压构造楔。库车坳陷西段盐层上覆冲积扇沉积(重力)差异负载是发育盐丘的原因,盐丘分布于山前冲积扇前端和却勒冲积扇南侧。库车坳陷盐下发育逆冲构造楔,应用临界楔理论研究构造楔顶面坡度与底部滑脱层角度变化,盐下构造楔顶面坡度突变,指示底部滑脱层角度发生变化,推断古生代基底正断层卷入逆冲构造楔。     

Particle motions in a physical model of shallow angle thrust faulting

A physical model of thrust faulting has been constructed out of elastic foam rubber. The thrust wedge has an angle of 25°., and is forced from the back by a steel-hydraulic system of effectively infinite rigidity, analogous to the force exerted by tectonic plates. The observed particle motions show many features different from those commonly assumed in dislocation models of subduction thrusts. Interface waves associated with fault opening propagate along the thrust plane (from the back) and temporarily decouple the overlying hanging wall plate from the foot wall. Because of the geometry of the reflecting fault boundary, and the free surface (also reflecting), energy is trapped in the wedge and, as a consequence, the particle motions and energy in the hanging wall are much greater than in the foot wall. The interface wave and the energy trapped in the upper plate propagate up the wedge and break out at the toe of the thrust with a spectacular increase in motions. If this model is analogous to the real earth, it suggests that some common assumptions in dislocation modeling may be incorrect. The model may explain apparent missing energy radiated to teleseismic distances, the anomalously low number of empirical Green’s functions needed to model teleseismicp waves, and the evidence of intense shaking on the hanging wall toe of some thrust fault earthquakes. The results support the suggestion that interface waves may explain the lack of high frictional heat generation from thrust faults, and may explain the ‘paradox of large overthrusts’. The results also suggest that the seismic hazard of great subduction thrust earthquakes and continental shallow angle thrust faults might in some cases be seriously underestimated.     

Influence of syntectonic sedimentation on thrust geometry. Field examples from the Iberian Chain (Spain) and analogue modelling

Steep thrusts are usually interpreted as oblique-slip thrusts or inverted normal faults. However, recent analogical and numerical models have emphasised the influence of surface mass-transfer phenomena on the structural evolution of compressive systems. This research points to sedimentation and erosion during deformation as an additional explanation for the origin of steeply dipping thrusts. The present study uses both field observations and analogue modelling to attempt to isolate critical parameters of syntectonic sedimentation that might control the geometry of the upper part of thrust systems.A field study of thrust systems bounding two compressive intermountain Tertiary basins of the Iberian Chain is first briefly presented. We describe the surface geometry of thrusts surrounding the Montalbán Basin and the Alto Tajo Syncline at the vicinity of deposits of Oligocene–Early Miocene alluvial fans at the footwall of faults. Field observations suggest that synthrusting sedimentation should influence the structure of thrusts. Indeed, the faults are steeper and splitted at the edge of the syntectonic deposits.Effects of sedimentation rate on footwall of thrusts, and of its change along fault strike are further investigated on two-layer brittle-ductile analogue models submitted to compression and syntectonic sediment supply. Two series of experiments were made corresponding to two end-members of depositional geometries. In the first series, the sedimentation was homogeneously distributed on both sides of the relief developed above the thrust front. In the second series, deposits were localised on a particular area of the footwall of thrust front. In all experiments, the sedimentation rate controls the number and the dip of faults. For low sedimentation rates, a single low-angle thrust develops; whereas for high sedimentation rates, a series of steeper dipping thrust is observed. In experiments with changing sedimentation rate along fault strike, the thrust geometry varies behind the areas with the thickest sediment pile.     

Extrusion vs. accretion at the frictional–viscous décollement transition in experimental thrust wedges: the role of convergence velocity

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