Shear Fault-bend Fold Research Articles

An improved incremental shortening calculation method of the shear fault-bend fold

Shear fault-bend folds, a common feature of fault bend folds in foreland fold-and-thrust belts, originate in the early deformation stage. Building upon Suppe's geometric model of 'shear fault-bend folding,' we present refined calculation formulas for total shortening, incremental shortening, and the shear coefficient specific to these folds, as well as ideas to verify model parameters. The practicality and versatility of our refined approach are confirmed through case studies of the Pakuashan anticline in Taiwan and the Tugulu anticline in North Tian Shan. This improved incremental shortening calculation formula, conserves the bed length and eliminates errors of the widely used shortening calculation formula, is crucial for understanding deformation processes and assessing seismic risks shear fault bend folding. To obtain more precise incremental shortening calculations, it should further refine the geometric and kinematic models of shear fault bend folding in the future.

How do differences in interpreting seismic images affect estimates of geological slip rates?

Abstract. Uncertainties of geological structural geometry constructed based on seismic reflections can stem from data acquisition, processing, analysis, or interpretation. Uncertainties arising from structural interpretations and subsequent estimates of geological slip have been particularly less quantified and discussed. To illustrate the implications of interpretation uncertainties for seismic potential and structural evolution, I use an example of a shear fault-bend fold in the central Himalaya. I apply a simple solution from the kinematic model of shear fault-bend folding to resolve the geological input slip of given structure and then compare the result with a previous study to show how differences in structural interpretations could impact dependent conclusions. The findings show that only a little variance in interpretations owing to subjectivity or an unclear seismic image could yield geological slip rates differing by up to ∼ 10 mm yr−1, resulting in significantly different scenarios of seismic potential. To reduce unavoidable subjectivity, this study also suggests that the epistemic uncertainty in raw data should be included in interpretations and conclusions.

Changing in fold geometry from faulted detachment fold to fault-bend fold, a case study: The Zeloi Anticline in the Dezful Embayment, southwest of Iran

Determining the geometry variations in the different depths reduces the uncertainties of petroleum exploration in the Dezful Embayment. In this research, the effects of the major and minor detachment horizons on the folding style of the Zeloi oil field have been investigated using surface and subsurface data. Because of the impact of the detachment horizons in the different depths, the Zeloi Anticline shows a poly-harmonic folding from surface to depth. The activity of the Gachsaran evaporites decupled the structural styles of the surface and subsurface folds. The interaction of these evaporites and syn-tectonic sedimentations influences the kinematic evolution of the Zeloi Anticline. The signatures of the sub-ordinate detachment horizons (Pabdeh-Gurpi formations and Kalhur Member) can be seen as second order folds and swell at the Asmari level. The thickness changes of the intermediate detachment horizon are the main factor of geometry variations of the Asmari and Sarvak reservoirs. The Zeloi Anticline is symmetric and asymmetric faulted detachment fold in the central part and SE plunge, respectively. But the reduction of the thickness of the Garau to Dashtak formations to the NW plunge led to the formation of shear fault-bend fold. The ENE-WSW SHmax orientation, in most of the wells, is determined using the breakout and drilling-induced fractures although, in some of the wells, the NW-SE SHmax orientation is interpreted as a result of stress perturbation that made by fore and back thrust faults. The open fractures are classified into the pre-folding, syn-folding and present-day related fractures.

Three-dimensional geometrical and kinematic characteristics of boundary faults in Minfeng subsag Dongying Sag, Bohai Bay Basin

Abstract Under the constraint of high-precision 3D seismic data and well data, through 2D and 3D analysis, the geometrical and kinematic characteristics of the boundary faults of Minfeng subsag, Jiyang depression, Bohai Bay Basin were analyzed. According to the strike, dip and dip-angle, the boundary fault of the study area is divided into three segments, i.e. west, center and east. It is thought that the fault surface is a curved surface consisting of several inclined surfaces linked by axial plane (i.e. transversal axial plane and vertical axial plane), and a three-dimensional geometry model of the boundary fault was built. The Minfeng boundary fault surface is divided into 11 subzones by 6 transversal axial planes and 4 vertical axial planes. The kinematic characteristics of the Minfeng boundary fault were analyzed on the basis of the normal fault-bend folds theory, and the three balanced cross-sections in west, central and east segments are compared. It is concluded that the Minfeng boundary fault mainly experienced rotary motion in its early stage of development, and moved in the inclined shear fault-bend fold pattern in the late stage. There developed listric fault in the central segement, seat type normal faults in east and west segments of the Minfeng boundary fault. The activity intensity of west and central faults was affected by Shengbei fault, the rotation degree of Minfeng boundary fault weakens from west to east. The reverse degree of the Minfeng boundary fault in creased from west to east at the end of the Paleogene Dongying Formation deposition.

Fault displacement-distance relationships as indicators of contractional fault-related folding style

We present a method of using fault displacement-distance profiles to distinguish fault-bend, shear fault-bend, and fault-propagation folds, and use these insights to guide balanced and retrodeformable interpretations of these structures. We first describe the displacement profiles associated with different end-member fault-related folding models, then provide examples of structures that are consistent with these model-based predictions. Natural examples are imaged in high-resolution two- and three dimensional seismic reflection data sets from the Niger Delta, Sichuan Basin, Sierras Pampeanas, and Cascadia to record variations in displacement with distance updip along faults (termed displacement-distance profiles). Fault-bend folds exhibit constant displacement along fault segments and changes in displacement associated with bends in faults, shear fault-bend folds demonstrate an increase in displacement through the shearing interval, and fault-propagation folds exhibit decreasing displacement toward the fault tip. More complex structures are then investigated using this method, demonstrating that displacement-distance profiles can be used to provide insight into structures that involve multiple fault-related folding processes or have changed kinematic behavior over time. These interpretations are supported by comparison with the kinematics inferred from the geometry of growth strata overlying these structures. Collectively, these analyses illustrate that the displacement-distance approach can provide valuable insights into the styles of fault-related folding.

Insights into the mechanisms of fault-related folding provided by volumetric structural restorations using spatially varying mechanical constraints

We use a new, mechanically based volumetric structural restoration tool to investigate the mechanics of fault-related folding using natural examples imaged in three-dimensional (3-D) seismic data. The restoration technique is based on a finite element approach that simultaneously restores folding and faulting while allowing rock properties to spatially vary during restoration. We apply these techniques to two types of structures, detachment and shear fault-bend folds, where mechanical layering is a significant factor in their development. Our examples include a detachment anticline from the Caspian Sea and a shear fault-bend fold from the deep-water Niger Delta, both of which contain syntectonic growth horizons that help to constrain the restorations. Restorations of the detachment fold most closely match displacement fields specified in the kinematic forward models when materials are defined as incompressible and rigid, yet the variation of mechanical strength in restorations is perhaps more compatible with the growth of natural structures as recorded by syntectonic growth strata. This analysis shows that the restorations of the detachment fold favor a combination of both kink-band migration and limb rotation folding mechanisms. Numerical simulations of the growth shear fault-bend fold also closely match the displacement field prescribed by the kinematics of shear fault-bend fold models when weak basal units and bedding-plane slip surfaces, enabling flexural slip, are incorporated in the model. The results demonstrate that these techniques can be used to provide full 3-D restorations that closely match established two-dimensional kinematic theories, yet allow constraint of 3-D displacement fields and strain patterns in complex structures.

Earthquake‐by‐earthquake fold growth above the Puente Hills blind thrust fault, Los Angeles, California: Implications for fold kinematics and seismic hazard

Boreholes and high‐resolution seismic reflection data collected across the forelimb growth triangle above the central segment of the Puente Hills thrust fault (PHT) beneath Los Angeles, California, provide a detailed record of incremental fold growth during large earthquakes on this major blind thrust fault. These data document fold growth within a discrete kink band that narrows upward from ∼460 m at the base of the Quaternary section (200–250 m depth) to <150 m at 2.5 m depth, with most growth during the most recent folding event occurring within a zone only ∼60 m wide. These observations, coupled with evidence from petroleum industry seismic reflection data, demonstrate that most (>82% at 250 m depth) folding and uplift occur within discrete kink bands, thereby enabling us to develop a paleoseismic history of the underlying blind thrust fault. The borehole data reveal that the youngest part of the growth triangle in the uppermost 20 m comprises three stratigraphically discrete growth intervals marked by southward thickening sedimentary strata that are separated by intervals in which sediments do not change thickness across the site. We interpret the intervals of growth as occurring after the formation of now‐buried paleofold scarps during three large PHT earthquakes in the past 8 kyr. The intervening intervals of no growth record periods of structural quiescence and deposition at the regional, near‐horizontal stream gradient at the study site. Minimum uplift in each of the scarp‐forming events, which occurred at 0.2–2.2 ka (event Y), 3.0–6.3 ka (event X), and 6.6–8.1 ka (event W), ranged from ∼1.1 to ∼1.6 m, indicating minimum thrust displacements of ≥2.5 to 4.5 m. Such large displacements are consistent with the occurrence of large‐magnitude earthquakes (Mw> 7). Cumulative, minimum uplift in the past three events was 3.3 to 4.7 m, suggesting cumulative thrust displacement of ≥7 to 10.5 m. These values yield a minimum Holocene slip rate for the PHT of ≥0.9 to 1.6 mm/yr. The borehole and seismic reflection data demonstrate that dip within the kink band is acquired incrementally, such that older strata that have been deformed by more earthquakes dip more steeply than younger strata. Specifically, strata dip 0.4° at 4 m depth, 0.7° at 20 m depth, 8° at 90 m, 16° at 110 m, and 17° at 200 m. Moreover, structural restorations of the borehole data show that the locus of active folding (the anticlinal active axial surface) does not extend to the surface in exactly the same location from earthquake to earthquake. Rather, that the axial surfaces migrate from earthquake to earthquake, reflecting a component of fold growth by kink band migration. The incremental acquisition of bed dip in the growth triangle may reflect some combination of fold growth by limb rotation in addition to kink band migration, possibly through a component of trishear or shear fault bend folding. Alternatively, the component of limb rotation may result from curved hinge fault bend folding, and/or the mechanical response of loosely consolidated granular sediments in the shallow subsurface to folding at depth.

Deep-water Niger Delta fold and thrust belt modeled as a critical-taper wedge: The influence of elevated basal fluid pressure on structural styles

We use critical-taper wedge mechanics theory to show that the Niger Delta toe-thrust system deforms above a very weak basal detachment induced by high pore-fluid pressure. The Niger Delta exhibits similar rock properties but an anomalously low taper (sum of the bathymetric slope and dip of the basal detachment) compared with most orogenic fold belts. This low taper implies that the Niger Delta has a very weak basal detachment, which we interpret to reflect elevated pore-fluid pressure ( 0.90) within the Akata Formation, a prodelta marine shale that contains the basal detachment horizon. The weak basal detachment zone has a significant influence on the structural styles in the deep-water Niger Delta fold belts. The overpressured and, thereby, weak Akata shales ductilely deform within the cores of anticlines and in the hanging walls of toe-thrust structures, leading to the development of shear fault-bend folds and detachment anticlines that form the main structural trap types in the deep-water fold belts. Moreover, the low taper shape leads to the widespread development of backthrust zones, as well as the presence of large, relatively undeformed regions that separate the deep-water fold and thrust belts. This study expands the use of critical-taper wedge mechanics concepts to passive-margin settings, while documenting the influence of elevated basal fluid pressures on the structure and tectonics of the deep-water Niger Delta.

Structural styles in the deep-water fold and thrust belts of the Niger Delta

The deep-water Niger Delta includes two large fold and thrust belts, products of contraction caused by gravity-driven extension on the shelf that exhibit complex styles of thrusting. These contractional structures formed above multiple detachment levels in the overpressured shales of the Akata Formation. Using the patterns of growth sedimentation, fold shapes, fault-plane seismic reflections, and combined conventional and shear fault-bend folding theories, we describe and model the structural styles and kinematics of the fault-related folds and imbricate thrust systems that compose these belts. Individual fault-related folds, involving both forethrusts and backthrusts, are characterized by long planar backlimbs that dip less than the associated fault ramps, with upward shallowing of dips in growth strata above the backlimbs suggesting components of progressive limb rotation. Forelimbs are short compared to backlimbs, but growth strata show more consistent dips that suggest a component of folding by kink-band migration. Thus, we employ a combination of classic and shear fault-bend fold theories to describe these structures, including the influence of a weak basal detachment zone in the overpressured shales. We expand upon these theories to model the kinematics of imbricate thrust systems, which display a complex history of thrusting related to spatial and temporal variations in deposition across the delta. Regional patterns of folded growth strata are used to define break-forward, break-backward, and coeval thrusting involving single and multiple detachment levels. We define two main types of imbricate thrust systems: type I system with a single basal detachment level and type II imbricate system with multiple basal detachment levels, which cause massive structural thickening of the Akata Formation and refolding of shallow thrust sheets. Through the sequential restoration of two regional cross sections across these systems, we resolve the structural styles, the timing and sequences of thrusting, as well as the regional amounts of shortening, all of which have important implications for hydrocarbon maturation and charge in the deep-water Niger Delta.