Reverse drag revisited: Why footwall deformation may be the key to inferring listric fault geometry

Although reverse drag, the down warping of hanging wall strata toward a normal fault, is widely accepted as an indicator of listric fault geometry, previous studies have shown that similar folding may form in response to slip on faults of finite vertical extent with listric or planar geometry. In this study we therefore seek more general criteria for inferring subsurface fault geometry from observations of near-surface deformation by directly comparing patterns of displacement, stress, and strain around planar and listric faults, as predicted by elastic boundary element models. In agreement with previous work, we find that models with finite planar, planar-detached, and listric-detached faults all develop hanging wall reverse-drag folds. All of these model geometries also predict a region of tension and elevated maximum Coulomb stress in the hanging wall suggesting that the distribution and orientation of near-surface joints and secondary faults may also be of limited utility in predicting subsurface fault geometry. The most notable difference between the three models, however, is the magnitude of footwall uplift. Footwall uplift decreases slightly with introduction of a detachment and more significantly with the addition of a listric fault shape. A parametric investigation of faults with constant slip ranging from nearly planar to strongly listric over depths from 1 to 15 km reveals that footwall fold width is sensitive to fault geometry while hanging wall fold width largely reflects fault depth. Application of a graphical approach based on these results as well as more complete inverse modeling illustrates how patterns of combined hanging wall and footwall deformation may be used to constrain subsurface fault geometry.     

Relay structures in a Lower Permian basement-involved extension system,East Greenland

Extensional relay structures are described as offset listric faults having the same subhorizontal detachment in depth. The extension along one fault is transferred or relayed across a relay ramp defined between the tip-lines of the offset faults. The relay ramp interconnects the hangingwall and the footwall during deformation. An example of a crystalline basement-involved symmetrical relay system is described from the Lower Permian in the Karstryggen area, East Greenland.     

Fault-slip, bed-length and area variations in experimental rollover anticlines over listric normal faults: influence in extension and depth to detachment estimations

To check the behaviour of physical experiments of simple rollover anticlines over listric normal faults, several fault-slip, bed-length and area parameters are measured for different beds in two stages of evolution of two physical experiments. Given a certain amount of extension, the heave and dip of the displacement vary for different beds, whereas the throw is approximately constant for small amounts of extension and the displacement remains constant for high amounts of extension. Neither bed lengths nor areas beneath beds remain constant with increasing extension. A number of techniques allow amounts of extension and detachment depth to be estimated given one or more marker horizons, the portion of the fault between hanging-wall and footwall cut off points, the depth to detachment and/or the shear angle. These techniques are applied to the physical experiments of listric normal faults analysed. Since many of these techniques rely on parameters measured on the physical experiments such as fault slip, bed length and area, the influence of these parameters on the different magnitudes of extension and detachment depths estimated using different techniques is discussed and the accuracy of the results is compared.     

Variable-heave models of deformation above listric normal faults: the importance of area conservation

In recent years work on extensional fault-controlled structures has illustrated the relevance of spatial heave variations along faults in producing structures such as hangingwall synclines. In this note I mention two models which have been described: in one, the fault heave varies and vertical marker lines remain vertical (the ‘modified Chevron model’), whilst in the other, markers perpendicular to the fault remain perpendicular during movement (the ‘slip-line model’). I show that both these models produce large area changes and cannot be used quantitatively unless these spurious effects are removed. I give here the derivation of models which satisfy the same general criteria but conserve area. Some aspects of the amended models are detailed. Both the modified Chevron model and slip-line model predict strains which increase without limit away from the fault plane, and in the slip-line model infinite strains may be predicted in geologically reasonable scenarios.     

控制中国东部中-新生代盆地的铲形断裂构造

根据断裂延伸长度、切割深度、断裂的活动强度和断裂产生的前后序次及其对盆地的控制作用不同,分为基底断裂和同生断裂。 基底断裂控制着盆地或者盆地内二级构造单元的边界。同生断裂明显地控制了盆内的沉积作用,包括岩相、厚度变化等。 在中国东部中、新生代盆地形成和发展过程中,上述两类断裂不仅分布广,而且数量多。在形态特征上,一般断层面弯曲朝上,并随深度增加而变缓,都具有正滑性质,所以统称为铲形断裂。     

Listric versus planar normal fault geometry: an example from the Eisenstadt-Sopron Basin (E Austria)

In a gravel pit at the eastern margin of the Eisenstadt-Sopron Basin, a satellite of Vienna Basin (Austria), Neogene sediments are exposed in the hanging wall of a major normal fault. The anticlinal structure and associated conjugated secondary normal faults were previously interpreted as a rollover anticline above a listric normal fault. The spatial orientation and distribution of sedimentary horizons and crosscutting faults were mapped in detail on a laser scan of the outcrop wall. Subsequently, in order to assess the 3D distribution and geometry of this fault system, a series of parallel ground penetrating radar (GPR) profiles were recorded behind the outcrop wall. Both outcrop and GPR data were compiled in a 3D structural model, providing the basis for a kinematic reconstruction of the fault plane using balanced cross-section techniques. However, the kinematic reconstruction results in a geologically meaningless normal fault cutting down- and up-section. Additionally, no evidence for a weak layer serving as ductile detachment horizon (i.e. salt or clay horizon) can be identified in stratigraphic profiles. Instead, the observed deflection of stratigraphic horizons may be caused by a displacement gradient along a planar master fault, with a maximum displacement in the fault centre, decreasing towards the fault tips. Accordingly, the observed deflection of markers in the hanging wall—and in a nearby location in the footwall of the normal fault—is interpreted as large-scale fault drag along a planar fault that records a displacement gradient, instead of a rollover anticline related to a listric fault.     

The terminology of structures in thrust belts

A review of structures and geometric relationships recognized in thrust belts is presented. A thrust is defined as any contractional fault, a corollary being that thrusts must cut up-section in their transport direction. ‘Flats’ are those portions of a thrust surface which were parallel to an arbitrary datum surface at the time of displacement and ‘ramps’ are those portions of thrusts which cut across datum surfaces. Ramps are classified on the basis of their orientation relative to the thrust transport direction and whether they are cut offs in the hangingwall or footwall of the thrust. Lateral variations in the form of staircase trajectories are joined by oblique or lateral ramps which have a component of strike-slip movement.An array of thrusts which diverge in their transport direction may form by either of two propagation models. These are termed ‘piggy-back’ propagation, which is foreland-directed, and ‘overstep’ propagation which is opposed to the thrust transport direction. An array of thrust surfaces is termed an ‘imbricate stack’ and should these surfaces anastamose upwards a ‘duplex’ will result; the fault-bounded blocks are termed ‘horses’. A duplex is bounded by a higher, ‘roof’ thrust and a lower, ‘floor’ thrust. The intersection of any two thrust planes is termed a ‘branch line’.Thrusts can be classified on the basis of their relationship to asymmetric fold limbs which they cut. A further classification arises from whether a particular thrust lies in the hangingwall or footwall of another one.The movement of thrust sheets over corrugated surfaces, or the local development of thrust structures beneath, will fold higher thrust sheets. These folds are termed ‘culminations’ and their limbs are termed ‘culmination walls’. Accommodation of this folding may require movement on surfaces within the hangingwall of the active thrust. These accommodation surfaces are termed ‘hangingwall detachments’ and they need not root down into the active thrust. This category of detachment includes dip-slip ‘hangingwall drop faults’ which are developed by differential uplift of duplex roofs, and ‘out-of-the-syncline’ thrusts which develop from overtightened fold hinges. Back thrusts, as well as forming as hangingwall detachments, may also form due to layer-parallel shortening above a sticking thrust or by rotation of the hangingwall above a ramp.     

Hanging-wall deformation and gas-migration associated to a major salt detaching fault in the Norwegian Danish Basin

The central and southern regions of the North Sea Basin are characterized by mobile Zechstein Salt and represent an excellent area to study listric normal fault development related to halokinesis. The North Sea Basin–in spite of being a mature explored hydrocarbon producing basin-contains a generally not fully understood hydrocarbon plumbing system. Faults and associated fluid migration routes represent critical elements of the plumbing system. Here we present a combined 3D seismic mapping of a listric fault and detailed seismic attribute analysis of shallow gas anomalies near the fault implying a close and complex relationship between a major detaching fault, hangingwall deformation and fluid migration and entrapment. The fault and associated antithetic and secondary synthetic faults is controlled by the growth of the underlying salt structure. Three separate segments of the fault are recognized showing distinct and varying characteristics of structural maturity and fluid migration. The studied fault and shallow gas anomalies thus represent an example of a complex faulted shallow fluid migration system developing in relation to an underlying salt structure which could be relevant for understanding similar salt, fault and fluid migration systems around the world.     

煤层断裂模式及形成机制的研究

本文以山西省汾西矿区崔家淘井田断层构造的展布规律、断层面构造岩特征民层滑动擦痕为依据，分析断层发育的力学机制衣成生欠，应用赤平投影图矿区两条主干断层，得出 区三向主支应力轴方向的认识，提出一种在华北地区有代表性煤矿区展布模式芝对断层的识别和预测具有指导意义。     

Listric normal fault profiles: calculation using bed-length balance and fault displacement

A bed-length balance technique is presented to calculate listric normal fault trajectories using roll-over geometry and fault displacements. The technique can also be applied to subsidiary listric normal fault fans above a major fault to calculate the major listric fault geometry.     

The Gubbio normal fault (Central Italy): geometry, displacement distribution and tectonic evolution

Normal faults within orogenic belts can be pre-, syn- or post-orogenic features. We studied the Gubbio normal fault (central Italy), which is an example of a pre-orogenic fault reactivated in a post-orogenic stage. The Gubbio Fault is a 22-km-long fault bordering a Quaternary basin and part of an active faults system in the Umbria–Marche region (Central Italy). The interpretation of a set of seismic profiles enables us to reconstruct the fault geometry in detail and to measure displacement and throw distributions along the fault strike. Seismic data indicate that the Gubbio Fault represents an example of multiple reactivation: at least a portion of the fault was active in the Miocene and only a part of the total displacement was achieved in the Quaternary. The reconstruction of the fault geometry at depth shows that the fault is characterised by listric geometry. The fault is also characterised by a bend along strike and structure contours show that this geometry is maintained at depth. As the fault is commonly addressed as presently active, the maximum fault dimensions are correlated to the maximum expected earthquake, and the presence of the fault bend is discussed as a possible barrier to seismic ruptures propagation.     

GUI based inversion code for automatic quantification of strike limited listric fault sources and regional gravity background from observed Bouguer gravity anomalies

A method coupled with a GUI based code in JAVA is developed in the space domain to simultaneously estimate the structures of strike limited listric fault sources and regional gravity background from a set of observed Bouguer gravity anomalies. The density contrast within the hanging wall of the structure is assumed to be varying continuously with depth based on a parabolic equation. The limiting surface of the fault plane is described with an exponential function. This method is automatic in the sense that it initializes both parameters of a strike limited listric fault source and regional gravity background from a set of observed Bouguer gravity anomalies and improves them iteratively until the modeled gravity anomalies fit the observed anomalies within the specified convergence criteria. The advantage of the code is that besides generating output in both ASCII and graphical forms it displays the animated versions of (i) the changes in model geometry, (ii) variation of each model parameter and misfit with iteration number, (iii) improvements in modeled gravity anomalies, and (iv) variation of density contrast with depth. The applicability of the code is demonstrated on both synthetic and real field gravity anomalies. In case of synthetic example pseudorandom noise is added to the residual gravity anomalies of the structure prior to inversion. The noisy anomalies are then inverted for the unknown parameters presuming (i) an ideal listric fault structure bounded by an exponential limiting surface with perfect flat top and bottom surfaces, (ii) non-ideal structure with uneven top and bottom surfaces with imperfect exponential limiting surface. Further, the robustness of the algorithm is exemplified by adding both regional gravity background and pseudorandom noise to the anomalies of the structure before inversion. In all cases, the interpreted parameters of the structure closely mimic the assumed parameters. The interpretation of gravity anomalies across the master fault of the Chintalpudi sub-basin in India has yielded information that is consistent with both DSS results and drilling information. The highlight of the code is that it can be used to interpret the gravity anomalies of listric fault sources even when the profile along which the interpretation is intended fails to bisect the strike length of the structure.     

Footwall geometry and the rheology of thrust sheets

The inter-relationships between the exact footwall geometry and the rheology of thrust sheets are investigated. Deviations in the thrust fault surface from an ideal plane will induce a local heterogeneous deformation. The resulting deformation processes depend upon the rate of thrust sheet displacement, the geometry of the feature causing heterogeneous flow, the deformation conditions and the lithologies involved. Two classes of features are particularly important in causing heterogeneous deformation in thrust sheets. The first features are small perturbations on bedding planes which may be inherited sedimentary structures or produced during layer-parallel shortening; the second class of features are ramps, where the thrust sheet climbs up the stratigraphic section. Displacement over these features causes repeated, cyclic straining in the hanging-wall during movement. The strain rates associated with deformation at perturbations, ramps of different geometries and different displacement rates are estimated and used to discuss the influence of footwall geometry on the structural evolution of a thrust sheet. Particular attention is given to the range of fault rocks and deformation microstructures preserved after movement over a footwall with a complex geometry. Perturbations are suggested to be important in the localization of ramps, either because they create ‘sticking points’ near the fault tip during propagation or because they induce eventual failure in the hanging-wall after the movement over a number of these features raises the accumulated damage to a critical level. Analysis of the influence of the exact geometry of ramps on deformation processes during displacement leads to two important conclusions. Firstly, the exact geometry of ramps (i.e. the maximum dip angle and the straining distance from a flat to this maximum angle) may be used to estimate a maximum displacement rate of the thrust sheet. Secondly, the listric geometry of ramps may be an equilibrium shape adjusted to the displacement rate and the rheology of the hanging-wall. Adjustments towards the final geometry may involve the generation of shortcuts on either hanging- or footwall which reduce the imposed deformation rate in the hanging-wall during displacement.     

A method and a GUI based JAVA code for interactive gravity modeling of strike limited listric fault sources with arbitrary density-depth variations

A method coupled with a GUI based computer program, FRGMLSTRK, coded in JAVA, has been developed to interactively model the gravity anomalies of strike-limited listric fault sources where the detached hanging wall of the structure consists of several geologic formations that have different densities and thicknesses. The program is simple and user friendly in the sense that it allows interactive model construction and modification, the display of fault geometry, depth and the densities of various sub-surface formations, and real-time computation of the gravity anomalies arising from the model. The non-planar fault planes are analytically constructed by fitting a polynomial function of arbitrary but specific degree to a set of points selected on the fault plane by means of a few mouse clicks in the structure panel of the graphical layout. Further, input parameters pertaining to depths and densities of formations can be specified by means of mouse clicks in respective panels. Subsequent changes in these parameters can be realized by simple drag and drop mouse operations. The modeled gravity anomalies are automatically updated and displayed whenever changes are made to: (i) the geometry of the fault plane; (ii) depths of density interfaces; and (iii) densities of formations. These changes can be made either independently or in combination. In addition to displaying the results in graphical form, the code also generates the output in ASCII format. The applicability of the code is demonstrated with both a simple synthetic model and real-world gravity anomalies from the margin of the Chintalpudi sub-basin in India.     

Active normal faulting during the 1997 seismic sequence in Colfiorito,Umbria: Did slip propagate to the surface?

In order to determine whether slip during an earthquake on the 26th September 1997 propagated to the surface, structural data have been collected along a bedrock fault scarp in Umbria, Italy. These collected data are used to investigate the relationship between the throw associated with a debated surface rupture (observed as a pale unweathered stripe at the base of the bedrock fault scarp) and the strike, dip and slip-vector. Previous studies have suggested that the surface rupture was produced either by primary surface slip or secondary compaction of hangingwall sediments. Some authors favour the latter because sparse surface fault dip measurements do not match nodal plane dips at depth. It is demonstrated herein that the strike, dip and height of the surface rupture, represented by a pale unweathered stripe at the base of the bedrock scarp, shows a systematic relationship with respect to the geometry and kinematics of faulting in the bedrock. The strike and dip co-vary and the throw is greatest where the strike is oblique to the slip-vector azimuth where the highest dip values are recorded. This implies that the throw values vary to accommodate spatial variation in the strike and dip of the fault across fault plane corrugations, a feature that is predicted by theory describing conservation of strain along faults, but not by compaction. Furthermore, published earthquake locations and reported fault dips are consistent with the analysed surface scarps when natural variation for surface dips and uncertainty for nodal plane dips at depth are taken into account. This implies that the fresh stripe is indeed a primary coseismic surface rupture whose slip is connected to the seismogenic fault at depth. We discuss how this knowledge of the locations and geometry of the active faults can be used as an input for seismic hazard assessment.     

渤海湾盆地辽西低凸起传递斜坡构造特征与潜山油气聚集

传递斜坡是调节区域构造应变平衡的一类重要的构造样式。根据新的三维地震资料解释和断层位移-距离关系以及相干体切片分析,探讨了渤海湾盆地辽西低凸起传递斜坡的特征,并简要分析传递斜坡与潜山油气聚集的关系。地震剖面解释表明,辽西低凸起潜山带的传递斜坡连接辽西1号断层的下盘和辽西2号断层的上盘;在数据体相干切片上,辽西1号和2号断层呈清晰的侧接形态;而位移-距离关系显示,辽西1号断层的位移沿着断层走向自南向北逐渐传递到辽西2号断层,断层位移在传递斜坡内部基本保持守恒。传递斜坡对潜山油气聚集的意义主要表现在对储层形成和改造方面,优质储层发育是JZ25-1S潜山大油气田的重要成藏要素。     

Quasistatic propagation of a normal fault: A fracture mechanics model

We have carried out boundary element calculations to simulate quasistatic propagation of a normal fault in the earth's crust under a horizontal tensile loading. Byerlee's frictional law is employed to describe the mechanical behavior of the fault surface. We hypothesize that in order for a normal fault to grow quasistatically, the mixed-mode effective shear stress intensity factor must exceed a threshold value (fracture toughness), a crustal material property. We suggest that the fault grows in a direction of local maximum shear stress. The direction of fault propagation thus depends on the ratio of tensile and shear stress intensity factors. A listric normal fault is likely to form in crustal material with a small shear fracture toughness. A listric normal fault is also more likely to form in crustal material with a high degree of plasticity.The propagation trajectory of an incrementally growing normal fault is examined. As the normal fault extends to a greater depth, the shear stress intensity factor drops, owing to an increase in fault surface friction. The equilibrium depth to which a normal fault will grow is controlled by the far field loading and the fracture mechanical property of the crustal material. The decrease of shear stress intensity with fault length also stabilizes the fault growth.     

黔东北燕山期构造变形特征及对华南"大塘坡式"锰矿勘查的影响——以普觉锰矿床犁式正断层为例

黔东北地区大地构造位置位于扬子板块西南部,自元古宙以来经历了多期次构造变形。不同期次的构造叠加对区内早期形成的沉积型矿产的埋藏和保存产生了重要影响。文章针对黔东北地区燕山期构造变形,采用中小尺度构造解析的方法开展露头尺度的构造样式、叠加关系剖析研究,同时,对研究区广泛发育的犁式正断层及其对南华纪"大塘坡式"锰矿的找矿勘查进行深入探讨。现有勘查成果认为犁式正断层为燕山早期—燕山晚期应力体制转换产物,该类断层倾角随深度增加而渐趋于缓,断距和断层错断区域伴随深度增加而加大,并伴生少量次级陡倾角分支断裂。冷水溪犁式断裂及其分支断裂将贵州松桃普觉超大型锰矿床体切割为多个独立单元,造成锰矿层不连续,且在矿区内形成了大小规模不等的断层错断区域。同时,由于冷水溪断层上盘下降,上盘锰矿体的埋藏深度增加,断层错断区域亦伴随深度加深而加大。对冷水溪犁式断层及其对锰矿体之间关系的分析,有利于深入研究整个黔湘渝地区"大塘坡式"锰矿的埋藏与保存特征,对助推区域上深部锰矿勘查,合理规避勘探风险具有重要的现实意义。     

Numerical modelling of depositional sequences in half-graben rift basins

ABSTRACT A three‐dimensional numerical model of sediment transport and deposition in coarse‐grained deltas is used to investigate the controls on depositional sequence variability in marine half‐graben extensional basins subject to eustatic sea‐level change. Using rates of sea‐level change, sediment supply and fault slip reported from active rift basins, the evolution of deltas located in three contrasting structural settings is documented: (1) footwall‐sourced deltas in high‐subsidence locations near the centre of a fault segment; (2) deltas fed by large drainage catchments at fault tips; and (3) deltas sourced from drainage catchments on the hangingwall dip slope. Differences in the three‐dimensional form and internal stratigraphy of the deltas result from variations in tilting of the hangingwall and the impact of border fault slip rates on accommodation development. Because subsidence rates near the centre of fault segments are greater than all but the fastest eustatic falls, footwall‐sourced deltas lack sequence boundaries and are characterized by stacked highstand systems tracts. High subsidence and steep bathymetry adjacent to the fault result in limited progradation. In contrast, the lower subsidence rate settings of the fault‐tip and hangingwall dip‐slope deltas mean that they are subject to relative sea‐level fall and associated fluvial incision and forced regression. Low gradients and tectonic tilting of the hangingwall influence the geometry of these deltas, with fault‐tip deltas preferentially prograding axially along the fault, creating elongate delta lobes. In contrast, broad, sheet‐like delta lobes characterize the hangingwall dip‐slope deltas. The model results suggest that different systems tracts may be coeval over length scales of several kilometres and that key stratal surfaces defining and subdividing depositional sequences may only be of local extent. Furthermore, the results highlight pitfalls in sequence‐stratigraphic interpretation and problems in interpreting controlling processes from the preserved stratigraphic product.     

High precision analysis of an embryonic extensional fault-related fold using 3D orthorectified virtual outcrops: The viewpoint importance in structural geology

Image-based 3D modeling has recently opened the way to the use of virtual outcrop models in geology. An intriguing application of this method involves the production of orthorectified images of outcrops using almost any user-defined point of view, so that photorealistic cross-sections suitable for numerous geological purposes and measurements can be easily generated. These purposes include the accurate quantitative analysis of fault-fold relationships starting from imperfectly oriented and partly inaccessible real outcrops. We applied the method of image-based 3D modeling and orthorectification to a case study from the northern Apennines, Italy, where an incipient extensional fault affecting well-layered limestones is exposed on a 10-m-high barely accessible cliff. Through a few simple steps, we constructed a high-quality image-based 3D model of the outcrop. In the model, we made a series of measurements including fault and bedding attitudes, which allowed us to derive the bedding-fault intersection direction. We then used this direction as viewpoint to obtain a distortion-free photorealistic cross-section, on which we measured bed dips and thicknesses as well as fault stratigraphic separations. These measurements allowed us to identify a slight difference (i.e. only 0.5°) between the hangingwall and footwall cutoff angles. We show that the hangingwall strain required to compensate the upward-decreasing displacement of the fault was accommodated by this 0.5° rotation (i.e. folding) and coeval 0.8% thickening of strata in the hangingwall relatively to footwall strata. This evidence is consistent with trishear fault-propagation folding. Our results emphasize the viewpoint importance in structural geology and therefore the potential of using orthorectified virtual outcrops.