K. R. McClay

Physical models of thrust wedges

Scaled sandbox models have been used to simulate the growth and sequential development of critical thrust wedges in isotropic cohesionless and anisotropic cohesionless materials. Variations in the initial thickness of the layered sequence, the friction of the basal detachment, and the anisotropy of the layered system have been systematically investigated. Imbricate fans of dominantly foreland-vergent thrust systems are developed similar to those found in accretionary prisms and in foreland fold and thrust belts. Critical taper wedges close to theoretically predicted geometries are developed for intermediate values of basal friction (µb = 0.47 whereas for the lower value of basal friction low-taper wedges are formed with tapers less than predicted by theory. Supra-critical wedges are formed when the basal friction equals or is greater than the coefficient of friction in the wedge and the wedge has a high taper closer to the angle of rest for the modelling material. The spacing/thickness ratio of foreland-vergent thrusts increases as the layer thickness increases. The spacing of thrust faults increases with increased basal friction. Higher basal friction or anisotropy within the layered systems favours displacement along foreland-vergent thrusts and suppresses backthrusts.

Analogue modelling of orthogonal and oblique rifting

Abstract The three-dimensional progressive development of orthogonal and oblique rift systems has been simulated using scaled, analogue sandbox models. The orientation, α, of the zone of rifting (where α = angle between the rift axis and the extension direction) was varied systematically from α = 90° (orthogonal rifting) to α = 75°, 60° and 45° (oblique rifting). Completed models were impregnated and sectioned both vertically and horizontally. Orthogonal rift models are characterized by long, straight rift border faults and intra-rift faults perpendicular to the extension direction. Oblique rift models are characterized by shorter, segmented rift border faults and intrarift faults. In the oblique models the rift border faults formed en echelon arrays parallel to the inderlying zone of basement stretching, whereas the intra-rift faults formed at a high angle to the extension vector. Increased extension in the moderately oblique and oblique rift systems (α = 60° and 45°) caused the intra-rift faults to rotate towards parallelism with the rift border faults. No strike-slip or oblique-slip transfer faults were found in any experiments. Rift border faults and intra-rift faults in the models are segmented along-strike with relay ramp structures formed between overlapping fault tips. Along the strike of the rift variations in half-graben polarities and offsets of depocentres are accomplished by accommodation zones formed by interlocking conjugate fault arrays. The experimental fault patterns compare well with the fault architectures found in those modern or relatively young rift systems where well defined, basement controlled strike-slip or oblique-slip transfer faults are absent. An accommodation zone model consisting of interlocking conjugate extension faults is proposed for the along-strike switching of half-graben asymmetries and offsetting of depocentres in rift systems. The fault geometries in the analogue models can be used as templates for recognition of fault styles in both orthogonal and in oblique rift environments.

Extensional fault systems in sedimentary basins: a review of analogue model studies☆

Abstract Scaled analogue models have provided graphic new insights into the progressive development of extensional fault systems. This paper reviews recent results of an on-going analogue modelling programme and compares the structural styles in the models with examples of natural extensional fault systems. Four fundamental model styles, scaled to simulate deformation in brittle sedimentary rocks in the upper 10 km of the crust, are reviewed. Extension above a basal detachment which undergoes stretching over a limited are produces an asymmetric rift graben bounded by planar faults and in which the internal deformation is accommodated by rotational domino faults. These experimental results are directly comparable to structures found in intracontinental rifts. Extension above a basal detechment which undergoes extension under the whole model and which has no constraints at the ends produces domino-style fault arrays in the pre-rift sequence and listric growth faults in the syn-rift sequence. These results compare with highly extended rift basins and terranes such as the Basin and Range Province, USA. Extension above a simple listric fault produces a characteristic hangingwall roll-over and crestal collapse graben system. At high values of extension, superposed crestal collapse grabens occur and a fan of listric growth faults is developed. These structures bear striking similarities to growth faults in progradational delta systems. Ramp-flat listric extensional fault systems using sand-mica models produce a characteristic family of structures with an upper roll-over crestal collapse system, a ramp syncline and reverse fault/fold zone and a lower roll-over crestal collapse system. An example of a ramp-flat fault system from the Gulf Coast, USA, is presented as a comparison with the analogue model results. Preliminary results of three-dimensional extensional model experiments are presented to illustrate the complexities in such fault systems. In all of the experimental models, it is found that the geometry of the underlying detachment system exerts fundamental control on structures produced in the hangingwall. The analogue models illustrated in this review provide valid geometric and kinematic templates for the progressive evolution of extensional fault systems in sedimentary basins.

4-D evolution of rift systems: Insights from scaled physical models

The four dimensional (4-D) evolution of brittle fault systems in orthogonal, oblique, and offset rift systems has been simulated by scaled sandbox models using dry, cohesionless, fine-grained quartz sand. Extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model. The results of these analog model studies are compared with natural examples of rift fault systems. Rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction. Segmentation of the rift margin increases with increase in obliquity of the rift axis, resulting in a consequent increase in displacement on intrarift fault systems. Offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone. Along-strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones. High-relief, extension-parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems. Changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults. The second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low-relief oblique-slip zone or grabens. These typically are found in highly oblique rift systems (<45 degrees) and above right steps in offset rift models.

Experiments on basin inversion above reactivated domino faults

Abstract Results are presented from scaled sand box experiments which investigate the structural inversion of half-graben basins generated by the contractional reactivation of extensional domino fault systems. The overburden was deformed above a series of rigid basement fault blocks that were extended in a domino manner. The models were constructed from either (1) sand with mica interlayers (anisotropic behaviour), (2) pure sand (isotropic behaviour), or (3) a sand-clay mixture (behaviour with increased competence). During the extensional phase of deformation, sand or alternating sand/mica layers were deposited in teh half-grabens associated with individual domino faults to simulate a syn-extensional stratigraphic sequence together with a component of overall subsidence. After termination of the extensional phase, sand layers were added to represent a post-rift passive subsidence stratigraphy. Subsequent contraction of the experimental configuration resulted in progressive reactivation of the domino fault system which was accompanied by back-rotation of both the footwall and hanging wall in each of the individual fault blocks. This produced null points along the profile section of the fault planes which separated net contraction above from net extension below. Continued shortening of the system led to further contraction being accommodated along footwall shortcut faults. It is suggested that these shortcuts developed in response to (1) the accommodation of space problems generated by increased bed length in the graben acquired during extension and (2) by more efficient low angle fault trajectories in keeping with Coulomb failure during horizontal to subhorizontal compression during inversion. In the anisotropic model, folding in the footwall partially suppressed the initiation of footwall shortcut faults. Footwall shortcut faulting did not occur in experiments where the dips of the original extensional faults were decreased before the inversion. In all inversion experiments the contractional deformation associated with the shortening of the half-graben was concentrated in the footwall to each domino basin-bounding fault. A control experiment in which an undeformed sand pack was deposited above an extended series of basement fault blocks revealed that the early extensional architecture formed in the other experiments had a profound influence on the resultant inversion geometries. The results show clear similarities to interpreted examples of natural basin inversion structures associated with reactivated domino fault systems, as well as to inferred strike-slip induced flower structures.

Extensional fault segmentation and linkages, Bonaparte Basin, outer North West Shelf, Australia

A detailed structural analysis of the Zone of Cooperation A-2 (ZOCA-2) three-dimensional (3-D) seismic survey on the outer northwestern edge of the Sahul Platform, northern Bonaparte Basin, outer North West Shelf, Australia, has identified three major populations of extensional faults. From oldest to youngest, these are (1) Jurassic north-south–trending extensional synrift faults, (2) Jurassic–Cretaceous east-west– to east-northeast–west-southwest–trending extensional faults, and (3) Neogene to present-day northeast-southwest–striking, right-stepping en-echelon faults. Seismic attribute analyses combined with fault-displacement analyses have illustrated the initial segmentation of all of the fault systems and characterized both the horizontal and vertical linkages formed by soft-linkage relay structures. The strongly segmented Neogene to present-day northeast-southwest–striking faults are interpreted to be a result of Neogene reactivation of underlying zones of weakness generated by preexisting east-northeast–trending Jurassic to Early Cretaceous faults. The Neogene to present-day northwest-southeast–directed extension was oblique to the underlying zones of weakness and thus formed characteristic strongly segmented en-echelon fault arrays. The Jurassic to Early Cretaceous extensional faults together with the overlying northeast-southwest–striking Neogene to present-day fault systems form a pseudoconjugate fault system separated by a Cretaceous sequence that acted as a decoupling horizon. Within this Cretaceous interval, which is characterized by polygonal fault systems, vertically segmented, isolated, and overlapping extensional fault arrays form a zone of soft linkage between the underlying Jurassic to Early Cretaceous rift faults and the overlying Neogene to present-day fault systems. This study shows that extension oblique to preexisting deeper fault systems produces en-echelon segmented extensional faults in the overlying sequences. Such en-echelon segmentation does not indicate major strike-slip deformation. The results of this research have implications for understanding the distribution, segmentation, linkages, and ages of extensional faults in many other rift basins as well as for the northern Bonaparte Basin. In addition, the complex 3-D linkage patterns shown in this study have significant implications for understanding trap geometries and fault-seal characteristics in other extensionally faulted basins.

Salt tectonics above complex basement extensional fault systems: results from analogue modelling

Physical model studies of salt tectonics related to thick-skinned extension have been largely confined to 2Dstructures. An experimental programme, utilizing silicone polymer and silica sand/ceramic beads as ductile and brittle analogues respectively, was designed to investigate the 3D relationships between salt diapirs and intersecting basement fault systems seen in the Central Graben, UK North Sea. During extension, intersection points in the basement fault system generate complex single, two-way or three-way, flap structures in the overburden, localizing deformation and footwall diapiric activity on the rift margins. Flap structures and associated diapirs are located adjacent to, but diagonally inboard of, the basement intersection points and consist of single, or multiple, convex-to-the-hanging-wall fault segments that are gradually breached with increasing extension. Major diapirs that attain passive status accommodate much of the continued basement extension through down building processes. Intra-basin horst systems develop inwardly dipping graben, cored by a major salt wall, during initial extension. Axial flow along this structure feeds growing diapirs. Grounding of the brittle overburden results in source cut-off and deflation of the salt wall due to continued expansion by extensional faulting. During subsequent inversion major diapirs are reactivated, exhibiting rapid active rise through the overburden, suggesting that buoyancy forces associated with diapirs play a major role in their reaction to subsequent tectonic stresses. Basin margins with major corner point systems develop highly segmented inversion fronts related to the localized nature of brittle structuring during extension. Diapir crests and entrained salt bodies are nucleation sites for the development of brittle reverse faults. Basin margins with minor corner systems commonly demonstrate reactivation of the basin margin fault system, producing a linked inversion front. The models display close similarities to structures seen in the UK Central North Sea and other salt basins, and have important implications for basement control on diapir locations during post-salt basin extension.

4D evolution of segmented strike-slip fault systems: applications to NW Europe

Scaled, physical sandbox modelling has been used to simulate the progressive deformation of segmented strike-slip fault systems at both releasing (pull-apart basins) and restraining oversteps (‘pop-up’ structures) developed in a weak sedimentary cover above rigid basement. Pull-apart basin evolution is characterized by the initial development of a spindle graben bounded by oblique-extensional faults that cross the ‘basement’ overstep, and subsequent lengthening of the basin as displacement on the master faults increases. The pull-apart basins are bounded by terraced oblique slip extensional sidewall fault systems that link the laterally offset principal displacement zones (PDZ). The sidewall faults show changes in kinematics from dominantly dip-slip extension in their central sections to oblique slip and strike-slip at either end where they merge with the PDZ. The pull-apart basin models are flat-bottomed in the centre of the basin and become asymmetric at either end where the sidewall fault systems join the PDZ. Cross-basin fault systems, which are characteristic of all models, cut the floor of the pull-apart basins and link the offset PDZs. Deformation above restraining offsets is characterized by the formation of rhombic to lozenge-shaped uplifts or ‘pop-ups’ – structures bounded by oblique-slip reverse faults that fan outwards from the PDZ. With increased displacement on the master faults the rhombic-uplifts grow in amplitude. The upper surfaces of the ‘pop-up’ structures are dissected by both synthetic and antithetic strike- and oblique-slip faults. Distinctive asymmetric positive flower structures characterize the lateral margins of the ‘pop-up’ structures. The analogue models are compared with natural examples of strike-slip pull-apart and ‘pop-up’ fault systems from NW Europe. The models show many strong similarities in structural geometry and stratal architectures with many natural examples, although it must be emphasized that similarities between structures of purely extensional origin (multiphase) and those associated with inversion do exist. Therefore, these analogue models of complex strike-slip structures may provide useful templates for seismic interpretation in petroleum basins provided there is a demonstrable link between strike-slip motion on basement structures and deformation in the cover.

Structural analysis and kinematic evolution of the inverted central South Celtic Sea Basin

Abstract The central South Celtic Sea Basin shows excellent examples of tectonics and sedimentation relationships as evidenced by detailed geological interpretation of a local grid of seismic profiles. These relationships together with balanced⧹restored geological cross-sections and block diagrams have been used to unravel the post-Variscan evolution of this part of the Celtic Sea basin. Three sedimentary mega-sequences separated by two erosional surfaces have been recognized overlying a deformed Palaeozoic basement in this area. (1) Permo?-Triassic-Jurassic syn-rift mega-sequence. Sedimentation of the lowest part of this succession is strongly controlled by extensional faults, whereas the rest of the sequence indicates much less fault control. Lateral extent of these rocks is also controlled by the disposition of the Palaeozoic basement which appears as an elevation in the Pembrokeshire Ridge area, where no sedimentation occurs. (2) Cretaceous post-rift mega-sequence. (3) Tertiary post-rift mega-sequence. The Cretaceous and Tertiary successions are more extensive than Permo?–Triassic–Jurassic sediments covering the central South Celtic Sea Basin and the Pembrokeshire Ridge. The structural framework displayed by the syn-rift and post-rift sequences is very different. Thus, the general structure beneath the Jurassic–Cretaceous (Late Cimmerian) unconformity consists of a WSW–ENE trending fault system and related folds. This resulted from an extensional phase, which led to a sub-basin compartmentalization of the area, and a subsequent inversional event, which caused significant reverse movement along previous extensional structures. Early Cretaceous erosion was responsible for an important and unequal denudation. The timing of the switch between extension and inversion cannot be determined due to Early Cretaceous erosion. Overlying the Jurassic–Cretaceous unconformity, Cretaceous sediments are slightly deformed by broad anticlines formed during Early Tertiary mainly as a result of tightening of previous folds. The Tertiary sequence shows a subhorizontal disposition responsible for the present-day, smooth bathymetric relief of the South Celtic Sea Basin.

Structure of the Zagros fold and thrust belt in the Kurdistan Region, northern Iraq

The Subandean Basins of South America extending from Trinidad to Tierra del Fuego have been the object of intensive exploratory activities (Fig. 1). The largest amount of hydrocarbons discovered during the last 30 years in these basins was found in complex structural terrains. A total of 59 Billion Barrels of Oil Equivalent (BBOE) have been discovered in areas affected by compressional tectonics. Of these basins, the largest discoveries are in the Furrial Trend of Venezuela (24 BBOE), followed by the Chaco area in Bolivia and Argentina (13 BBOE), the Llanos Foothills of Colombia (4.4 BBOE), and the Madre de Dios Basin of Peru (4.2 BBOE).