Flexural Slip Research Articles

Co-seismic, geomorphic, and geologic fold growth associated with the 1978 Tabas-e-Golshan earthquake fault in eastern Iran

We describe the seismicity and late Cenozoic deformation associated with a blind thrust fault at Tabas-e-Golshan (hereafter referred to as Tabas), eastern Iran, which generated a devastating Mw 7.3 earthquake on the 16th September 1978. Measurements from a structural transect through the Sardar anticline segment indicate fault-propagation folding above a gently ~20° eastward-dipping blind thrust fault. The thrust flattens into a horizontal detachment at a depth of only ~2km. Tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes, with many of the slip surfaces showing fresh striations with a large component of right-lateral strike–slip. A steeply dipping fault zone showing almost pure strike–slip is also developed within the forelimb of the fold. Our field observations are consistent with the source parameters of the 1978 Tabas earthquake, and additional events in 1979 and 1980, which all involved slip on a shallowly-dipping thrust with a significant component of right-lateral slip. The surface of an alluvial fan, which is likely to have been abandoned at 8–10ka, has been folded as it crosses the Sardar anticline. The age constraints, combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth, provide an approximate rate of horizontal shortening of ~1.5mm/yr. Shortening at Tabas appears to result from transpressional bending at the north end of the Nayband strike–slip fault. A northward continuation of the Nayband Fault, which may be slipping at rates of >2mm/yr, is identified along the base of the Shotori Mountains ~10–20km east of the Tabas thrust. The range-front fault did not move in 1978 and constitutes an additional threat to local populations.

Handling natural complexity in three-dimensional geomechanical restoration, with application to the recent evolution of the outer fold and thrust belt, deep-water Niger Delta

Volumetric restoration can provide crucial insights into the structural evolution of three-dimensional (3-D) petroleum systems. A major limitation to its widespread application is the need to include complex architectures and realistic mechanics such as flexural slip. We apply an implicit approach that allows for, including unconformities, thin and/or pinched-out layers in the models but that cannot explicitly localize slip along horizons. To take advantage of this approach while accounting for flexural slip in 3-D restoration, we investigate new geomechanical properties. We consider flexural slip folding as a result of stacked rigid and thin weak layers, which can be modeled using transversely isotropic properties. We compare restorations of an anticline using transversely isotropic properties, isotropic properties, and a stack of rigid isotropic layers with nonfrictional slip between the layers. Our results show that transversely isotropic properties reasonably approximate flexural slip folding. We use these new tools to model the evolution of a complex system located in the Niger Delta toe. The system includes a detachment fold, a fault-bend fold, and a structural wedge formed in series. Growth stratigraphy and erosional surfaces delimit the kinematics of deformation. Regional erosive surfaces, 3-D gradients of fault slip, and vertical variations in mechanical strength motivated the use of our new restoration techniques. Restoring two growth units results not only in reinforcing the interpretation that the area is behaving as a deforming thrust sheet at critical taper, but also in highlighting coeval activity on both the hinterland structures and the toe of the thrust belt.

Method and technique of 3-D dynamic structural evolution modelling of fault basin

The important and difficult point in the 3-D basin structural evolution modelling is how to make the modelling conform to the real geological process. In this paper, the rectification of formation boundary deformation is implemented by 3-D flexural slip mechanism and shearing mechanism. In compaction correction, by taking account of the phase boundary and diagenetic effect on porosity, a formwork of the porosity-depth change characteristic is obtained by ‘plane divisional and longitudinal zoning’ method, and then the porosity data of grid cells in different geological times are calculated using compaction correction algorithm, and finally the decompaction correction calculation is finished based on volume-balance technique. Using this modelling method to simulate the structural evolution of Niuzhuang-Wangjiagang region in Dongying Sag, the 3-D dynamic evolution models of geologic body from the upper section of the 4th member of Shahejie formation to present are figured out and they are according with the process of the historical and spatial modality of geological structure development. It demonstrates that this method has certain practicality and can express the process of geological structure development effectively.

Development of extension-parallel corrugations in the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Extension-parallel detachment fault corrugations are one of the defining features of metamorphic core complexes, yet the origin of these corrugations remains disputed. Structural data from the Buckskin-Rawhide metamorphic core complex in west-central Arizona demonstrate that prominent NE-trending detachment fault corrugations are folds produced by extension-perpendicular (NW-SE) shortening during the middle to late stages of core complex extension (ca. 18–10 Ma). Kinematic data indicate that dominant NE-directed slip on the Buckskin-Rawhide detachment fault was locally overprinted by NW- and SE-directed slip associated with corrugation folding. The last motion recorded along several NW- and SE-dipping flanks of the detachment fault is reverse slip directed toward the corrugation antiform hinges. Orientation patterns of upper-plate bedding across the corrugations are compatible with folding about a NE-trending axis. Extension-perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding. Constriction and grain-scale y -axis shortening were most important during the later stages of mylonitization in the synextensional Swansea Plutonic Suite, which typically form L > S tectonites. Upright meter- and kilometer-scale lower plate folds parallel the detachment fault corrugations and are best developed in well-layered pre-Tertiary rocks. Most lower plate folding occurred by postmylonitic flexural slip that was coeval with detachment faulting. The total amount of NW-SE shortening across the lower plate is ∼10%, but the amount of NW-SE shortening recorded by the younger detachment fault is only ∼1%. The relatively late-stage development of corrugations in the Buckskin-Rawhide metamorphic core complex suggests that the constriction and corrugation folding developed primarily in response to crustal thinning and tectonic denudation.

Development of a deep karst system within a transpressional structure of the Dolomites in north-east Italy

The Piani Eterni karst system is one of the longest and deepest caves of Italy situated in the southern sector of the Dolomiti mountain range. The area where the cave was formed displays peculiar structural settings confined in a tectonic transpressive corridor between two regional thrusts (Belluno and Valsugana). During Miocene uplift of the range the inheritance of Mesozoic structures led to the formation of a deep and wide upward-branching flower (or palm tree) structure cutting the carbonate sequence and exposing the surrounding surface to karst processes after erosion. The relative lowering of the hydrologic base level, due both to the uplift of the area and then to the carving of deep glacial valleys in the Quaternary, allowed the formation of paleo-phreatic conduits at subsequently deeper levels, interconnected by vadose shafts and canyons.This work gives a detailed tectonic interpretation of the transpressive structure and picks out the tectonic features most favorable to the karst development. A detailed statistical analysis of the distribution and orientation of the karst conduits was performed using 31km of 3D surveys showing that the development of the cave was strictly guided by a few favorable surfaces of stratigraphic and tectonic origin. These features are known in the literature as inception horizons and tectonic inception features, respectively. Cave levels are usually related to lithologic favorable conditions associated with standings of the paleo-water table. Here we suggest that some tectonic surface geometries could have led to the opening of voids in the active tectonic phase leading to the formation of the original proto-conduit network. Different types of tectonic inception features identified in the cave were described in terms of geometry and kinematics. Tensional fractures, as well as fault plane undulations and flexural slip surfaces between beds, are described as the most favorable tectonic surfaces for the development of the conduits. Finally, we discuss why transpressional settings and related flower structures in soluble rocks can enhance the karst process allowing the formation of huge and deep karst systems.

Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events

The ∼60 km-long Sahel ridge west of Algiers (Tell Atlas, north Algeria) is considered as an ENE–WSW fault-propagation fold running along the Mediterranean coast and associated with a north-west dipping thrust. Its proximity with Algiers makes this structure a potential source of destructive earthquakes that could hit the capital city, as occurred in 1365 AD and 1716 AD. The first paleoseismologic investigation on the Sahel ridge was conducted in order to detect paleo-ruptures related to active faulting and to date them. From the first investigations in the area, a first trench was excavated across bending-moment normal faults induced by flexural slip folding in the hanging wall of the Sahel anticline thrust ramp. Paleoseismological analyses recognize eight rupture events affecting colluvial deposits. 14C dating indicates that these events are very young, six of them being younger than 778 AD. The first sedimentary record indicates two ruptures before 1211 AD, i.e. older than the first historical earthquake documented in the region. Three events have age ranges compatible with the 1365, 1673 and 1716 Algiers earthquakes, whereas three other ones depict very recent ages, i.e. younger than 1700 AD. Potential of these secondary extrados faults for determining paleoseismic events and thrust behaviour is discussed.

Folding of granite and Cretaceous exhumation associated with regional-scale flexural slip folding and ridge subduction, Kitakami zone, northeast Japan

The early Cretaceous granitic plutons intrude the Kitakami zone, northeast Japan, whose southern and northern regions consist of forearc basin and accretionary complex rocks, respectively. All country rock of the Kitakami zone exhibit prominent pressure-solution cleavage and associated folds formed during shortening with a small component of sinistral shear, whereas most plutons show only igneous textures. The Kesengawa granite and some other plutons, however, have foliations that cut the pluton boundaries and are continuous with those observed in surrounding sedimentary rocks. We document a tectonic fold with axial planar foliation in part of the Kesengawa granite. The metamorphic minerals associated with the contact aureole and country rocks of the Kesengawa and other deformed plutons indicate an increase in metamorphic grade toward the pluton showing that deformation of the pluton and country rock took place as the plutons cooled. The Kesengawa pluton and country rocks of the southern Kitakami zone are deformed into regional scale upright folds with parasitic asymmetric folds that verge toward regional anticlinal axes whereas regional scale folds in the northern Kitakami zone are overturned and verge to the east. The Kitakami basement is not bounded by normal or reverse faults, so the style of regional exhumation does not resemble the upright or inclined extrusion noted in other regions, nor is the exhumation associated with extensional doming. Instead, the vergence of parasitic folds toward the regional fold hinges indicates flexural slip deformation at least in the late stages of exhumation and exhumation occurred in the cores of regional scale anticlines. The regional shortening of accretionary prism, forearc basin, and older forearc basement was associated with intrusion of adakitic plutons that thermally weakened the forearc basin and enhanced the deformation and exhumation. The adakitic magmatism and forearc shortening resulted from subduction of the buoyant Izanagi–Kula ridge, a regional event known as the Oshima orogeny.

A NOTE ON FOLDING MECHANISMS IN THE CAPE FOLD BELT, SOUTH AFRICA

The relationship between folding and faulting in the Cape Fold Belt has been raised as an enigma. The mineral deformation mechanisms accommodating folding are integral to the relation between faults and folds. Here I discuss field and microstructural observations of folded rocks from the Laingsburg region in the Western Cape, and the underlying mineral deformation mechanisms accommodating strain in these rocks. In competent units, deformation was dominantly accomplished by flexural slip faulting and jointing; in relatively incompetent layers macroscale flow was accommodated by dissolution-precipitation creep and distributed cataclasis. Combined with previous studies suggesting deformation occurred at lowermost greenschist facies temperatures, these observations indicate that folding in the Cape Fold Belt occurred at temperatures and pressures within the normally frictional regime. Folding and thrust fault development therefore generally occurred concurrently, and partitioning between localised and distributed deformation was governed by factors such as fluid pressure conditions, strain rate, and relative viscosity.

Comparison of electrical image log with core in a fractured carbonate reservoir

Borehole image logs are kind of the modern logs which are frequently used to detect small-scale fractures intersecting well in fractured reservoirs. Electrical imaging tools like FMS (Formation MicroScanner) and FMI (Fullbore Formation MicroImager), electrically scan the borehole wall and deliver a pseudo-picture of the formation. Characteristics of fractures such as type, aperture and orientation can be determined by interpreting these logs.In this paper, we attempt to evaluate the ability of electrical image logs in detecting fractures and other geological features of a carbonate reservoir (Asmari) and explain the possible reasons of mismatch between the image log and core. To achieve this goal, we have compared the FMS of a well with the core. The results are verified with FMI and core data from seven nearby wells.Comparison of the FMS with the core revealed that FMS, as representative of electrical image logs, is robust in detecting open fracture and anhydrite; it is average in detecting bedding plane and flexural slip and is weak in detecting filled fracture, fault and compactional feature in carbonate reservoirs.

Challenges of structural diagenesis in foreland fold-and-thrust belts: A case study on paleofluid flow in the Canadian Rocky Mountains West of Calgary

Foreland fold-and-thrust belts are challenging because their structural complexity highly impacts fluid flow and diagenetic processes. Paleofluid flow in the Rocky Mountain fold-and-thrust belt west of Calgary was investigated following a structural diagenetic approach. Paleofluid flow or mass transfer mechanisms have been inferred based on vein morphology (completely filled, en echelon, slickenfibre, bed-parallel, open or micro-breccia veins), orientation and geochemistry (stable oxygen and carbon isotope and strontium isotopes). Bed-parallel veins probably developed by flexural slip during Laramide folding, and most completely filled and slickenfibre veins formed or were reactivated during the Laramide Orogeny. Both types of veins associated with the Laramide Orogeny precipitated under low fluid flux conditions, dominated by diffusional processes as inferred from stable isotopic signatures similar to the host rock. The open veins with meteoric calcite cement, marked by cathodoluminescence zonations and strongly depleted oxygen isotopic signature, point to advectional fluid flow processes in a post-Laramide extensional stress field. Some slickenfibre and completely filled veins that originally formed during the compressional phase of the Laramide Orogeny were probably reactivated at time of the open vein formation. The latter inference is based on the occurrence of the same cement in the open veins as in the centre of the slickenfibre and completely filled veins. In conclusion, there is no evidence for large-scale fluid flow during the Laramide Orogeny, but evidence points to post-Laramide large-scale fluid advection being prevalent. Challenges in this structural diagenetic study consist of (i) the lack of abundant vein–vein and vein-stylolite cross-cutting relationships in the outcrops studied, (ii) the difference between structural versus diagenetic concepts, (iii) the uncertainty of chronology and attribution of fracture sets and diagenetic cements to either a single or discrete episodes, and (iv) reactivation of veins.

Structural modeling based on sequential restoration of gravitational salt deformation in the Santos Basin (Brazil)

The structural restoration of two parallel cross-sections in the central portion of the Santos Basin enables a first understanding of existent 3D geological complexities. Santos Basin is one of the most proliferous basins along the South Atlantic Brazilian margin. Due to the halokinesis, geological structures present significant horizontal tectonic transport. The two geological cross-sections extend from the continental shelf to deep waters, in areas where salt tectonics is simple enough to be solved by 2D restoration. Such cross-sections display both extensional and compressional deformation. Paleobathymetry, isostatic regional compensation, salt volume control and overall aspects related to structural style were used to constrain basic boundary conditions. Several restoration algorithms, such as simple shear, flexural slip and free methods, were used to restore the sedimentary deformation, including salt gravity gliding. The results of the 2D restoration are consistent with five major sequences of sedimentary evolution: (1) the brittle pre-salt deformation, (2) the significant and fast salt deposition, (3) the initial post-salt deformation with predominant rafting tectonics, (4) the Late Cretaceous progradational deposition and coeval development of compressional minibasins, and (5) the Cenozoic sedimentary deposition, with less intense salt tectonics. A 1D subsidence analysis based on the 2D restored results is shown as a useful restoration control tool. The 1D results indicate that an initially proximal infill evolves towards distal regions under salt tectonics control. The 1D diagrams also record the history of the overburden movements through lateral depocenter migration in minibasins areas, submitted to large horizontal salt spreading. The results highlight an important isostatic movement during salt deposition, large but not enough to eliminate a needed depression to accommodate the thick evaporites. By quantifying the halokinetic lateral deformation through time, the results suggest less intensity of the phenomenon throughout the Paleogene, with minor impacts on the petroleum system in this period.

Saw-tooth structures and curved veins related to folds in the south-central Pyrenees (Spain)

Two generation of folds ( F 1 and F 2) and associated structures, developed in the Eocene turbidites of the south-central Pyrenees, are analyzed in this paper. F 1 folds are close, have sub-horizontal axes and southwards vergence. They have an associated cleavage S 1. Competent layers were folded by layer-parallel shortening, tangential longitudinal strain, some possible flexural flow and an obliquely superimposed homogeneous strain due mainly to simple shear. Flexural slip is also an important mechanism in the whole multilayer. F 2 folds are gentle and scarce; they fold the S 1 cleavage. Among the structures associated with F 1 folds, there are sets of veins with curved form in the competent layers. The displacement of each vein gave rise usually to a step in the layer boundary, so that a set of veins produces a structure that is named “saw-tooth structure”. The veins initiated as small faults that made flexural slip difficult and gave rise to a concentration of stress on the steps, leading to an opening of the fractures and a propagation of them along a curved path, as suggested by a simple mechanical model. This propagation agrees with finite element models developed by other authors.

Back-thrusting and overturning of the southern margin of the 1.85 Ga Sudbury Igneous Complex at the Garson mine, Sudbury, Ontario

The Garson deposit is one of several deformed magmatic Ni–Cu–platinum group element (PGE) deposits in the South Range of the 1.85Ga Sudbury structure. The deposits occur along the southeast limb of the folded Sudbury Igneous Complex (SIC), at the contact between the SIC basal norite and underlying Paleoproterozoic metabasalt and metasedimentary rocks of the Huronian Supergroup. At the Garson deposit inclusion-rich breccia and disseminated Ni–Cu–PGE sulfide ores are hosted by steeply south-dipping shear zones and splays that underwent two major ductile deformation events (D1 and D2). D1 is characterized by a steeply, south-dipping, S1 foliation and a down-dip L1 mineral stretching lineation defined by ferrotschermakite in metabasalt and by magnesiohornblende in norite. Coexisting ferrotschermakite and oligoclase in metabasalt indicate amphibolite facies conditions during D1. The shear zones formed along or near the SIC-Huronian contact during the 1.7–1.6Ga Mazatzal–Labradorian Orogeny. They formed as layer-parallel, north-dipping, north-over-south thrusts in response to flexural slip during buckling of the SIC. As the general transport direction was from south to north during the Mazatzal–Labradorian Orogeny, the D1 shear zones are back-thrusts with opposite transport direction. The thrusts imbricated the SIC, underlying Huronian rocks, and ore zones, and emplaced slivers of Huronian rocks and metabreccia into the overlying norite. In contrast to the D1 shear zones at Garson, the Thayer Lindsley and the regional South Range shear zones, which transgress the SIC-Huronian contact at a high angle, formed as moderately SE-dipping reverse shear zones as a result of localization of folding-induced strain near the hinge zone in order to accommodate further flattening and tightening of the SIC with progressive D1 shortening. Together with the southeast limb of the SIC, the D1 shear zones were overturned into their present steep southerly dips and were reactivated as south-over-north shear zones either later during the Mazatzal–Labradorian Orogeny or during the 1.5–1.45Ga Chieflakian event, which was coeval with accretion of juvenile Andean-style, calc–alkaline magmatic arcs along the entire southeastern margin of Laurentia. During this D2 reactivation event, S1 was transposed and locally preserved in crenulations bounded by a S2 chloritic shear foliation, which has a strong down-dip L2 mineral chlorite lineation and parallel L2 ductile slickenlines.

Along-strike variations and internal details of chevron-style, flexural-slip thrust-propagation folds within the southern Livingstone Range anticlinorium, a paleohydrocarbon reservoir in southern Alberta Foothills, Canada

The Livingstone Range anticlinorium marks a hanging-wall ramp across which the Livingstone thrust cuts up eastward approximately 1000 m (3280 ft) between regional decollements in the Devonian and the Jurassic strata. It is well exposed and provides actualistic models for exploration of analogous subsurface structures. More than 30 km (19 mi) of detailed mapping along strike reveals generally gradual changes, punctuated by abrupt terminations or offsets of structures at northeast- to east-trending tear faults, interpreted to be reactivated basement faults. Some folds plunge out at tear faults, forming domal culminations between them. Hinge zones of chevron-style, flexural-slip thrust-propagation folds display a distinctive pattern of ramp-flat thrusting comprising stacked detachment thrusts, each of which emerges from a different zone of interbed slip in the backlimb and deflects the hinge zone eastward. Each successively lower detachment thrust dies out in the hinge zone just below an overlying one. Displacements on the detachments, rotation of fold limbs, and interbed flexural slip were integrated kinematically. Thrust-propagation folding involved a form of cataclastic flow: individual blocks of rock delimited by faults, joints, and sheared bedding surfaces underwent minor relative displacements with little or no internal deformation, despite large translations and rotations of the thrust sheet. Pressure solution and vein formation were widespread but were minor components of the deformation. Complex fracturing within the anticlines is dominated by conspicuous widely spaced (150 m [490 ft]) transverse (east-northeast–striking) zones of intense fracturing that transect hinge zones and limbs. These permeable zones likely originated by reactivation of basement faults, and their significance may not be predictable by curvature analysis.

Plane slope failures in the Langhe region of Italy

Widespread plane slope failures have occurred in the hilly Langhe region (NW Italy) over the past two centuries and more recently in November 1994. The 1994 event caused the deaths of 20 people and major damage to properties. This paper is a description of the landslides that occurred in connection to flexural slip. The causes of the phenomena are discussed in detail, including the variation of the residual strength along the shear surface as a result of specific geological, chemical and mineralogical conditions. After examining the phenomena that contribute to create slide-prone slopes, the paper concentrates on the driving mechanisms applying to the case histories investigated.

Crystal bending, subgrain boundary development, and recrystallization in orthopyroxene during granulite-facies deformation

A prominent feature of a granulite-facies shear zone from the Hidaka Main Zone (Japan) is the folding of orthopyroxene (opx) porphyroclasts. Dislocation density estimated by transmission electron microscope (TEM) and chemical etching in homogeneously folded domains is too low to account for the amplitude of crystallographic bending, leading us to propose a model similar to ''flexural slip'' folding, where folded layers are micrometer-wide opx layers between thin planar clinopyroxene (cpx) exsolu- tions. Extension (compression) in the extrados (intrados) of the folded layer is accommodated by dislocations at the cpx-opx interfaces. Alternatively to distributed deforma- tion, crystal bending also localizes in grain boundaries (GBs), mostly oriented close to the (001) plane and with various misorientation angles but misorientation axes consistently close to the b-axis. For misorientation up to a few degrees, GBs were imaged as tilt walls composed of regularly spaced (100)(001) dislocations. For misorienta- tion angles of 7� , individual dislocations are no longer visible, but high-resolution TEM (HRTEM) observation showed the partial continuity of opx tetrahedral chains through the boundary. For 21� misorientation, the two adjacent crystals are completely separated by an incoherent boundary. In spite of these atomic-scale variations, all GBs share orientation and rotation axis, suggesting a continuous process of misorientation by symmetric incorporation of (100)(001) dislocations. In addition to the dominant GBs perpendicular to the (100) plane, boundaries at low angle with (100) planes are also present, incorporating disloca- tions with a component of Burgers vector along the a-axis. The two kinds of boundaries combine to delimit subgrains, which progressively rotate with respect to host grains around the b-axis, eventually leading to recrystallization of large porphyroclasts.

Antislope scarps, gravitational spreading, and tectonic faulting in the western Yakutat microplate, south coastal Alaska

Mountain ridges in the western Yakutat microplate are riddled with swarms of antislope scarps and troughs. These landforms were previously interpreted as gravity failures (sackungen), but if partly or wholly tectonic (flexural slip or bending moment faults), they represent part of the strain budget from ongoing plate collision. To determine scarp origin, we mapped landforms, bedrock structure, and trenched scarps at Kushtaka Mountain and south of Martin Lake. The Kushtaka scarps paralleled west-dipping coal and sandstone beds in the Tertiary Kultieth Formation, and were spaced 35–60 m apart with heights of 1–4 m. Structural and paleosol relationships in a 6-m-long, 1.3-m-deep trench indicate the scarp was produced by Holocene creep on a normal fault underlying the scarp. The Kushtaka scarps thus represent toppling-style slip on bedding-plane faults in the eastern limb of a syncline, as the fold “unfolds” due to gravitational spreading. The Martin Lake scarps are more complex and include downslope-facing landslide scarps, upslope-facing flexural toppling scarps, and an oblique-slip tectonic scarp. A 2-m-deep trench across the WNW-trending tectonic scarp exposed the underlying bedrock fault plane with slickensides raking only 17°–20°, indicating mainly left-lateral slip on a sinistral-normal fault. In contrast, swarms of ENE-trending antislope scarps showed normal-oblique (dextral) slip. The two scarp sets may form a conjugate pair that simultaneously accommodates left-lateral tectonic slip and NNE-directed gravitational spreading. Our results, plus those recently published by [Li et al. (2010)][1], show that most antislope scarps in the western Yakutat microplate are formed by normal slip on bedding-plane faults that dip into the mountain, and represent gravitational spreading expressed as toppling-style failure or “unfolding” of strata on fold limbs. Conversely, the sinistral-oblique slip fault at Martin Lake is one in a family of east-trending faults accommodating accretion of the Yakutat microplate into the cuspate syntaxis of the Alaskan plate margin. [1]: #ref-14

Evolution of fluid chemistry and fluid-flow pathways during folding and faulting: an example from Taemas, NSW, Australia

Abstract In the Taemas area, New South Wales, Australia, a swarm of hydrothermal calcite and quartz veins is hosted in upright, open to close folded limestones and shales. Overprinting relationships and vein geometries demonstrate that the vein swarm formed progressively during fold growth and associated reverse faulting. Textures preserved in veins reveal that veins formed via hundreds to thousands of individual dilation and mineral precipitation events. Bedding-parallel flexural slip during fold growth was associated with laminated vein development, and limb-parallel stretching during fold growth was associated with the formation of bedding-orthogonal extension veins. The presence of subhorizontal extension fractures and severely misoriented reverse faults imply that fluid pressures exceeded lithostatic levels, at least transiently, during the development of the vein swarm. Vein δ 18 O compositions increase upwards through the Murrumbidgee Group in response to a progressive reaction of an externally derived, upwards-flowing low-δ 18 O fluid (of probable meteoric origin) with host limestones. Vein δ 18 O and 87 Sr/ 86 Sr compositions vary spatially and temporally within the same outcrop, and within individual veins. These variations are inferred to be caused by the ascent of packages of fluid along constantly changing flow pathways caused by multiple permeability creation–destruction cycles associated with fault slip and fault sealing. Vein trace and rare earth element (REE) concentrations are more variable, probably reflecting rapid rock buffering along fluid pathways on length scales of less than 10 m. Our results indicate that fluid-flow pathways change dynamically during crustal shortening, with pathways switching between states of low and high permeability during episodic fault slip and associated fracture development. Supplementary material: Two appendices are available at http://www.geolsoc.org.uk/SUP18492 .

Kinematics of constant arc length folding for different fold shapes

Basic mathematical functions are applied for the two-dimensional geometrical and kinematical analysis of different fold shapes. Relationships between different fold parameters are established and related to the bulk shortening taking place during folding under upper crustal conditions. The bulk shortening taking place during constant arc length folding is mathematically related to the bulk shortening during homogenous pure shear using a particular aspect ratio, which is for folding the ratio of amplitude to half wavelength and for pure shear the ratio of vertical to horizontal length of the deformed, initially square body. The evolution of the fold aspect ratio with bulk shortening is similar for a wide range of fold shapes and indicates that the fold aspect ratio allows a good estimate of the bulk shortening. The change of the geometry of individual layers across a multilayer sequence in disharmonic folding indicates a specific kinematics of multilayer folding, referred to here as “wrap folding”, which does not require significant flexural slip nor flexural flow. The kinematic analysis indicates that there is a critical value for constant arc length folding between shortening values of 30–40% (depending on the fold geometry). For shortening values smaller than the critical value limb rotation and fold amplitude growth are dominating. For shortening larger than this value, faulting, boudinage and foliation development are likely the dominating deformation process during continued shortening. The kinematical analysis of constant arc length folding can be used for estimating the bulk shortening taking place during multilayer folding which is an important component of the deformation of crustal rocks during the early history of shortening. The bulk shortening is estimated for a natural, multilayer detachment fold and the shortening estimates based on the kinematic analysis are compared and supported by numerical finite element simulations of multilayer detachment folding in power-law materials.

Geomechanical modeling of an extensional fault-propagation fold: Big Brushy Canyon monocline, Sierra Del Carmen, Texas

Field structural data from the Big Brushy Canyon monocline developed in Cretaceous strata of west Texas are combined with nonlinear finite element modeling to help bridge the gap between geometric, kinematic, and mechanical analysis techniques for understanding the deformation history of reservoir-scale geologic structures. The massive Santa Elena Limestone is offset along a steep normal fault, and fault displacement is accommodated upward by the folding of the Buda Limestone and Boquillas Formation and the thinning in the intervening Del Rio Clay. Mesostructures within competent Buda Limestone beds are concentrated in the monocline limb instead of the hinge and include bed-perpendicular veins that accommodate bed-parallel extension and bedding-plane slip surfaces that offset the veins and accommodate flexural slip. Finite element models were constructed to reproduce the monocline geometry and deformation distribution as well as to assess the effect of material properties and boundary conditions on structural evolution. The initial model configuration replicated the assumed predeformational geometry, included frictional sliding surfaces to allow for bedding-parallel slip, and used a displacement boundary condition at the base of the Santa Elena footwall to simulate fault motion. Geometry and strain evolution were tracked so that (1) fold shape, (2) cumulative extension, and (3) layer-parallel shear strain could be compared to field observations. Iterative model runs successfully matched field data and revealed the importance of benchmarking the model results against monocline geometry, layer-parallel extensional strain, and bedding slip in the natural example. Our results illustrate the potential use of this modeling approach whereby calibration is performed using available data and is followed by strain measurement throughout the model domain to aid in prediction of subseismic faults and fractures. This geomechanical modeling approach provides a powerful tool for site-specific subsurface deformation prediction in hydrocarbon reservoirs that incorporates details of the local mechanical stratigraphy and structural setting.