Extensional Fault-related Folding Research Articles

Structural inheritance controls crustal-scale extensional fault-related folding in the Exmouth and Dampier Sub-basins, North West Shelf, Australia

Structural inheritance controls crustal-scale extensional fault-related folding in the Exmouth and Dampier Sub-basins, North West Shelf, Australia

Extensional fault-related folding of the North West shelf, Western Australia

The occurrence of fault-related folds in extensional settings has been gaining increased recognition during the past 20 yr. Passive margins and rift basins often present intriguing structural complexities that are potential sites for the development of extensional fault–related folds. Fault-related folds are important anticlinal traps for hydrocarbon exploration, and understanding their evolution is critical for derisking the timing of trap formation and hydrocarbon generation. This study presents folds from three areas across the North West shelf (NWS) of Western Australia and aims to determine their relationship to associated extensional faults. A series of kinematic forward models are presented as well as detailed structural and stratigraphic interpretation of folding at the three locations. The models suggest each of the three examples have formed under extension that impacted the region during the Mesozoic. The modeling also highlights different characteristics of folding that are caused by fault geometry (ramp–flat) and a propagating fault tip (trishear) that can be used as diagnostic features to distinguish different fold mechanisms. Historical exploration across the NWS has targeted a relatively small number of these anticlinal traps, some of which have been successful (e.g., Marina-1 well, Petrel subbasin) and some have not (e.g., Candace-1, Sholl Island fault anticline). This paper suggests that the prospectivity of anticlinal structures should be more carefully assessed to determine whether or not they are related to extensional activity and hence likely to have formed prior to the bulk of the hydrocarbon charge. Similar structures undoubtedly exist in other basins that have experienced repeated extension.

Development of extensional fault and fold system: Insights from 3D seismic interpretation of the Enderby Terrace, NW Shelf of Australia

Abstract Extensional fault-related folding has been widely observed in rift systems that are characterised by basement-involved fault reactivation. On the Enderby Terrace, NW Shelf of Australia, oblique reactivation of a basement structure resulted in a depth-branched fault system that consists of six splay faults in the cover stratigraphy. Extensional fault-related folding is obvious and predominantly developed in the hangingwall of reactivated basement fault. The mechanically weak Locker Shale not only plays an important role in displacement accommodation at the basement fault tip but also exerts a strong impact on ‘flat’ development of the ‘ramp–flat–ramp’ fault, thereby controlling the location and geometry of the hangingwall anticline. The kinematic evolution of these structures can be determined based on the well-preserved stratigraphy of the hangingwall basins. Faults and folds deformation exhibits systematic lateral changes which provides clues for the kinematic evolution of the hangingwall folds, that is (i) hangingwall synclines and anticlines formed as a result of fault propagation and linkage, (ii) partial truncation of the anticline by adjacent fault segments due to lateral fault lengthening and interaction, and (iii) complete truncation of the anticline by a planar fault that results in the abandonment of the curved fault plane in the footwall. Results of this study are applicable to other extensional fault and fold systems of similar origin and provide models for partially preserved outcrop deformation and poorly imaged seismic data interpretation.

Extensional fault-related folding in the northwestern Red Sea, Egypt: segmented fault growth, fault linkages, corner folds and basin evolution

Abstract Segmented, planar, domino-style extensional fault arrays and their associated hanging wall fault-related folds form complex linked basins along the onshore margin of the northwestern Red Sea, Egypt. The extensional fault systems form half-graben basins with kilometre-scale, asymmetrical, doubly plunging longitudinal synclines and narrow, plunging transverse anticlines and synclines. The axial traces of the hanging wall longitudinal folds are curvilinear, sub-parallel to the half-graben Border faults, and bend or are offset at relay ramps and at fault linkage points. Transverse corner fold systems occur at the fault linkage points and fault jogs. The fold geometries, variations in fault displacement, and fault slip indicators indicate that the fold and fault systems are kinematically related and formed during the Late Oligocene–Miocene rifting of the northern Red Sea. The folds were controlled by vertical and lateral fault propagation and by the mechanical anisotropy of the pre-rift strata. The proposed model for these extensional folds is the initial formation of monoclinal flexures above reactivated blind basement faults. Increased displacement, propagation and segment linkage formed hanging wall longitudinal folds and transverse corner folds. The longitudinal folds grew progressively at the expense of the transverse folds and merged along-strike into long hanging wall synclinal basins.

3D geometry and kinematic evolution of extensional fault-related folds, NW Red Sea, Egypt

Abstract Fault-related folds are common structural features found at a variety of scales in extensional settings, and have been recognized in both outcrop and subsurface studies. However, the detailed geometry and origin of complex 3D folds adjacent to normal faults are poorly known, and, in some cases, are interpreted to be due to strike-slip tectonics and post-rift contraction. Here we examine the 3D geometry of seismic-scale folds in a rift margin – the Red Sea – and discuss the interrelationship between the growth of normal faults and the development of their related folds. Detailed field mapping of the NW Red Sea rift system has shown that the rift margin is dominated by two large extensional fault systems formed by a series of linked NNW-, north–south- and NNE-striking fault segments. These linked segments exhibit distinct zigzag fault patterns and combine to form a number of NNW-trending faults that dip NE with dominant hanging-wall stratal dips to the SW. Hanging-wall stratal dips define 3D extensional fault-related synclinal folds in pre- and early synrift strata. The hanging-wall synclines are kilometre-scale, gently doubly plunging, with curved axial surface traces orientated sub-parallel to the bounding faults. Field data demonstrated that these folds are formed by along-strike variations in fault displacements, and they form transverse synclines combined with hanging-wall extensional fault-propagation folds. The complex 3D geometry of the hanging-wall synclines is the result of the along-strike segment linkage. Adjacent to the bounding faults, the stratal dips are sub-parallel to the faults as a result of extensional fault-propagation folding controlled by highly anisotropic pre-rift strata. Palaeo-strain analyses of fault-slip data, together with analysis of the fold geometry, clearly indicate that the faulting and folding in the NW Red Sea are formed by pure NE–SW extension during the Late Oligocene–Miocene rifting, and that contraction or strike-slip tectonics need not be invoked.

Inverted intracontinental basin and vertical tectonics: The Saharan Atlas in Tunisia

The present-day architecture of the Saharan Atlas in Tunisia can be defined by two principal models: (1) The first model emphasizes a general SW–NE geological structure in the North forming successive and parallel bands (the Tellian zone, the diapir zone) and the central Atlas, which are cut by the southern Atlas ranges located within a NW–SE corridor. These zones are bordered to the East by the “North–South Axis”. (2) The second model defines the Tunisian Atlas in terms of an E–W strike-slip corridor, which initially controls the sedimentary facies distribution during the Meso-Cenozoïc, and which then generates elongate en echelon folds in the sedimentary cover by dextral shearing. In this study, we aim to show that the Saharan Atlas in Tunisia appears today as a triangular megablock, that we call the Tunisian Block (TB), bounded by three structural trends (N–S, SW–NE and NW–SE) belonging to the African strike-slip fault network: (1) The eastern boundary appears as a complex faulted and folded corridor limiting the folded zone of the central Atlas in the West and the depressed zone of the Sahel in the East: it corresponds to the “North–South Axis” as defined classically in the literature. (2) The southern boundary also corresponds to a faulted belt (Gafsa–Negrine-Tozeur corridor), which cuts off the continuation of the North–South axis southward into the Gabès region; it corresponds to the Southern Saharan Atlas, delimited by the Gafsa fault in the North and the Negrine-Tozeur fault in the South. (3) The northern boundary, trending SW–NE, appears rather in the form of a reverse tectonic bundle, facing SE or S (oblique convergence), whose major feature corresponds to the El Alia-Téboursouk fault. This northern boundary cuts across and delimits the N–S corridor towards the North, in such a way that its extension is limited at both extremities. Finally, the inner part of the TB actually corresponds to a mosaic of second-order blocks, each of which contains an arrangement of widely spaced SW–NE trending anticlines forming the main relief separated by vast plains very often occupied by sebkhas. The paleogeographic and structural evolution of this region during the Mesozoic and Palaeogene shows that the TB, along with its limits as defined here, developed an increasingly distinct identity at a very early stage, being characterized by an extensive and/or transtensive tectonic regime. Finally, the Tunisian Atlas Chain defines a triangular domain that owes its origin and particular character precisely because of the paleogeographic and structural history of this paleoblock. The boundaries of this paleoblock remain mobile, thus tectonically controlling the geometry and morphology of a typical intracontinental basin. The extension directions and the frequent changes of stress regime (or rotations) are related to the existence of two active basins: the strike-slip margin of the western branch of Tethys and the Mesogea oceanic basin, with tectonic activity becoming alternately dominant in one or other of the basins at different times. In this context, the Tunisian basin is characterized by rhythmic sedimentation, composed of a succession of filling sequences linked to the continuing tectonic instability of the sedimentary floor associated with two major crises: one at the end of the Aptian and the other at the end of the Ypresian. The vertical movements related to the extension and/or transtension of the blocks is accentuated by Triassic salt tectonics, giving rise to linear (salt axes) or point (salt domes) structures that lead to the formation of shoal zones during development of the basin, thus enhancing the vertical tectonics. The diapirism developed slowly and gradually from late Triassic through to Langhian times, leading to numerous sedimentary wedges on the flanks of the structures. The uprise of the diapirs exhibits three pauses corresponding to the end-Aptian, end-Ypresian and pre-Burdigalian. The vertical tectonics is characterized by abundant drape folds giving rise to an extensional fault-related folding and strike-slip/dip-slip faults creating frequent unconformities that are nevertheless always localized. Finally, the folded chain results from the structural inversion of this paleoblock from Tortonian times onward. We can only account for the various folds-axis directions in the context of an intracontinental chain where the pre-existing major vertical faults are able to develop on the surface as draped-folds in a transpressive regime by the local reorientation of stresses in crustal-scale faults. In detail, the structures produced by this vertical tectonic activity, which are profoundly controlled by inheritance, display a highly original style at very shallow levels in the crust.

Structural setting of Cretaceous pull-apart basins and Miocene extensional folds in the Quseir–Umm Gheig region, northwestern Red Sea, Egypt

We examine the evolution of the northwestern Red Sea, Egypt, by study of the Quseir–Umm Gheig subbasin. The subbasin records two main tectonic events. The first event is related to development of Late Cretaceous synclinal basins due to sinistral movement along the reactivated Najd fault system. Evidence for this includes: (1) the Cretaceous basins are concentrated mainly in the central Eastern Desert, which represents the main influence zone of the Najd fault system, (2) folds are not everywhere parallel to the faults and their axes are curvilinear, (3) the faults dislocated the axial plane of the synclines, (4) the Cretaceous basins occur in an en-echelon arrangement, (5) there is a difference of 20° between the orientation of the sinistral strike-slip shear zones and the associated en-echelon synclinal folds, (6) principal stress directions are delineated by subhorizontal σ1 and σ3 and subvertical σ2, (7) sheared conglomerate is detected in the Nubia Formation, (8) minor overturned folds and minor NE-vergent thrusts occur in the Duwi and Dakhla Formations, and (9) there is a predominance of NE-SW normal faults in Cretaceous–Eocene sequences. The second event is related to the sinistral movement along the NNE-SSW Aqaba–Dead Sea transform and dextral movement along Queih and Hamrawin shear zones. This movement was synchronous with northeast extension of the Red Sea. The structures developed during this movement include: (1) NW-trending extensional faults, (2) extensional fault-related folds in Miocene-Pliocene deposits, and (3) buckle folds in Pliocene and post-Pliocene sequences. Buckle folds were developed during NW compression associated with sinistral movement along NNE-SSW strike-slip faults. Gypsiferous shale-rich beds in Miocene-Pliocene rocks played the main role in development of fault-related folds and buckle folds in the Quseir–Umm Gheig subbasin.

Inversion tectonics, interference pattern and extensional fault-related folding in the Eastern Anti-Atlas, Morocco

The Anti-Atlas belt of Morocco is a Variscan chain which appears as a huge anticlinorium oriented NE-SW. In the internal part of this structure, the actual relief shows the basement cropping out as inliers (Pique 2001) or boutonnieres. The cover, a thick pile of Paleozoic sediments up to 12 km thick, is gently folded and of low grade metamorphism in its lower level. The lack of major decollement, of a deformation front or thrust-fault makes the Anti-Atlas an unusual type of belt, which does not fit with classic schemes. The Anti-Atlas has been considered to be a thick skinned fold belt, with the crystalline basement involved in the horizontal shortening and where the folding of the cover fits to a “buckle fold” mode (Burkhard et al. 2006). This structural style is determined by two key parameters: the total thickness of Paleozoic cover series and the relative abundance of shale vs. competent marker beds. The Eastern Anti-Atlas has particular features which are not found elsewhere in the Anti-Atlas. Its location at the intersection between the NW-SE Ougarta chain and the ENE-WSW main body of the Anti-Atlas, produces an egg-box interference pattern. The significance of minor folds and thrusts in the competent beds and their particular orientation is examined. The fact that the cover in the Eastern Anti-Atlas is only 6 km thick, which changes the shale vs competent beds ratio, influences its structural style. The feature that distinguishes the most Eastern Anti-Atlas is the presence of large E-W normal faults affecting the whole structure and creating extensional fault-related folding in the cover.

Transpressional inversion in an extensional transfer zone (the Saltacaballos fault, northern Spain)

This paper deals with an extensional transfer zone and its main structure, the Saltacaballos fault, developed during Early Cretaceous times, and obliquely inverted during the Alpine shortening. Geological mapping and structural analysis were carried out to determine the kinematic history of the deformation. The Saltacaballos ridge is the relay zone between two major and conjugate normal fault systems: the Bilbao and the La Granja-Samano fault systems. Three distinct phases in the structural evolution of this transfer zone can be identified. (1) During the initial stages of extensional deformation, shallow-water marine limestones were deposited on the relay ridge and deep-water marls on the flanking depocenters. At the same time, submarine landslides developed on paleoslope generated either by normal drag or by a fault-propagation fold related to the Saltacaballos normal fault. (2) With increasing displacement, normal faults antithetic to the Saltacaballos fault were developed probably in response to the collapse of its hanging-wall. (3) During the subsequent Alpine compressional stage, these normal faults were reactivated as dextral strike-slip faults as a result of transpressional inversion, whereas previous extensional fault-related folds were tightened. At the same time, some normal faults were overturned as a consequence of passive rotation in fold limbs.

Extensional fault-related folding, northwestern Red Sea, Egypt

The Duwi and Hamadat areas on the northwestern margin of the Red Sea rift system exhibit superb outcrop examples of kilometric scale, extensional fault-related folds. In this area, the Nakheil and Hamadat faults of the Rift border fault system consist of a series of WNW- and NW-trending segments that link through breached relay ramps. The fault hanging walls are characterised by a series of offset synclines with non-linear axial traces sub-parallel to the faults. The synclines are doubly plunging and, in the immediate hanging wall to the normal faults, the strata dip sub-parallel to the fault. These folds result from along-strike displacement variations on the individual fault segments together with extension-related fault-propagation folding as the faults propagated upwards through a highly anisotropic pre-rift sedimentary section. In the Duwi area, the axial traces of the synclines are offset and bend in the regions of relay ramps. In the Hamadat area, the synclines associated with individual fault segments are separated by transverse hanging-wall anticlines that formed where segment linkage occurred. Numerical simulations of extension-related fault-propagation folding using a trishear model produce geometries that closely match those of the Duwi and Hamadat folds. The dimensions of these hanging-wall synclines and their relationships to fault displacement variations indicate that they are formed by extensional fault-propagation folding rather than by frictional drag along the fault surface.

Abstract: Variation in Fan Delta Stacking Patterns Related to Synsedimentary Growth of Extensional Fault-related Folds, Miocene Suez Rift, Egypt

Abstract: Variation in Fan Delta Stacking Patterns Related to Synsedimentary Growth of Extensional Fault-related Folds, Miocene Suez Rift, Egypt

Structural Styles in Rift Basins: Interpretation Methodology and Examples from Southeast Asia: ABSTRACT

The authors present graphical solutions to the extensional fault-related folding equations of Xiao and Suppe (1992), simplifying the prediction of normal fault location or rollover geometry from subsurface data. These equations also predict the extent of bed thinning and elongation in hanging wall strata. They have derived new equations that relate change in fault slip across a fault bend to fault geometry. Applying these equations in seismic interpretation makes it easier to (1) construct balanced cross-sections, (2) account for the slip observed, and (3) determine the growth history of extensional fault-related folds. They have applied these concepts to several southeast Asian rift basins in Malaysia, Myanmar, Indonesia, and Thailand. These basins were formed by early Tertiary crustal extension, producing rollover structures in which sediment supply generally did not keep up with subsidence. These under-filled, internally drained depressions periodically contained lakes, providing the environment for deposition of organic-rich strata that ultimately became hydrocarbon source rock. Typically, the main basin bounding faults dip 35-55[degrees] near their upper terminations and flatten to become subhorizontal. Synthetic and antithetic secondary faults are usually present. Late compaction faulting often propagates upward from major extensional faults and may reactivate the upper portions of these faults. In manymore » basins, late compression produced inversion structures. By applying the concepts of extensional fault-related folding to these basins, they can (1) explain observed geometries, (2) predict poorly imaged geometries, (3) predict the location of source and reservoir facies, and (4) determine the timing of faulting relative to deposition of source and reservoir rocks.« less