Limbs Of Folds Research Articles

Geometrie kvarcitových těles v okolí Příchovic

Quarzite as a metasedimentary rock is usually figured in prolongated bands in geological maps. But muscovite quartzite from Krkonoše-Jizera Crystalline Unit in SE surrounding of Příchovice village forms irregular shapes in detailed geological maps (e.g., Mrázová – Štěpánek 2007, see Fig. 1), which do not allow simple interpretation of the geometry of quartzite bodies. To find correct 3D geometry of the bodies, we used standard methods of detailed mapping and documentation of outcrops as well as detailed structural analysis. Locations of terrain edges with quartzite outcrops have been indicated in the map (Fig. 3 –A, B, C). Generaly sub-horizontal to moderately dipping metamorphic foliation has been found. Foliation poles make wide girdle in the plot (Fig. 2) which indicates sub-cylindrical fold structure with horizontal axis in the NW–SE direction.Studied area can be geomorphologically divided into two different parts. The northeastern part of the territory is sub-horizontal plateau, while the southwestern part is generally inclined to the southwest. In the second part, the metamorphic foliation dips to the southwest under such angle, that it seems to be parallel to the inclination of the slope, see the old quarry (Fig. 3B) and SW part of Velká bílá skála (Fig. 3A). The last one show us bent outcrop respecting the change in slope inclination. Large shear bands were directly observed as a part of S-C structure. It indicates that SW limb of the fold was strained under simple shear condition with the drop SW kinematics (Fig. 4). Change in orientation of SW-dipping fold limb was produced by a ductile shear zone.Based on a combination of metamorphic foliation orientation of quartzite and field edges, we can interpret quartzite rocks as a part of one platy body, which forms the overall morphology of the landscape south of Příchovice. In the plateau, erosion stopped at subhorizontally oriented part of the quartzite layer, while denudation of the SW limb of the large fold predeterminates the inclined slope surface to the SW (Fig. 3D). Relics of thin overlying rocks remaining on quartzite in some places and erosion of the small creeks produce bizarre forms in geological map, which seems to be difficult to understand. This study has shown that such complex structure can be decrypted using the detailed structural analysis. In this case it was shown that quartzite forms a relatively simple body which was only slightly bent due to simple shear deformation.

Quantitative prediction of fracture distribution using geomechanical method within Kuqa Depression, Tarim Basin, NW China

Understanding and predicting the development and distribution of tectonic fractures in the super-deep tight sandstone reservoir are important for both gas exploration and exploitation activities in Kuqa Depression. We analyzed the characteristics of regional structural evolution and paleotectonic stress field setting. A reasonable geological model of the research area was built based on an interpretation of the geological structure, a test for rock mechanics, and experiment on acoustic emission. Thereafter, a three-dimensional paleotectonic stress field during the Late Himalayan movement was simulated by the finite element method. Composite rock failure criterion and geomechanical models between fracture density, aperture, strike and strain energy density were used to determine the quantitative development of fractures and predict zones that are prone to fracture development. Areas with well-developed tectonic fractures are mainly controlled by the tectonic stress field and elastic strain energy density, typically located in the fold limbs, fault zones and locations of changes in the orientation of fault. In turn, factors of faults, the lithology, depth and curvature control the development and distribution of tectonic fractures in the tight sandstone. The overall simulated results match well with core observation and FMI (Formation MicroScanner Image) results both in the whole geometry and fracture distribution. This evolutionary model of the structures present to us is a distinct and intuitive top-popup fold with low amplitude and relatively low-density fractures in the top, has only experienced pre-folding stage and strong folding stage, and has not yet been developed to vertical uplift stage.

Fracture zones constrained by neutral surfaces in a fault-related fold: Insights from the Kelasu tectonic zone, Kuqa Depression

Stress variation associated with folding is one of the controlling factors in the development of tectonic fractures, however, little attention has been paid to the influence of neutral surfaces during folding on fracture distribution in a fault-related fold. In this study, we take the Cretaceous Bashijiqike Formation in the Kuqa Depression as an example and analyze the distribution of tectonic fractures in fault-related folds by core observation and logging data analysis. Three fracture zones are identified in a fault-related fold: a tensile zone, a transition zone and a compressive zone, which may be constrained by two neutral surfaces of fold. Well correlation reveals that the tensile zone and the transition zone reach the maximum thickness at the fold hinge and get thinner in the fold limbs. A 2D viscoelastic stress field model of a fault-related fold was constructed to further investigate the mechanism of fracturing. Statistical and numerical analysis reveal that the tensile zone and the transition zone become thicker with decreasing interlimb angle. Stress variation associated with folding is the first level of control over the general pattern of fracture distribution while faulting is a secondary control over the development of local fractures in a fault-related fold.

Episodic gold mineralisation correlated with discrete structural events at Ballarat East, southeast Australia

New 40Ar/39Ar geochronological data suggest orogenic gold mineralisation at the Ballarat East deposit, southeast Australia, occurred in three main episodes at ca. 445–435Ma, ca. 420–415Ma and ca. 380–370Ma. The gold mineralisation is localised in muscovite-bearing quartz and quartz-carbonate veins hosted in the steep faults (70–90°), on limbs of tight and isoclinal folds in an Ordovician turbidite sequence, and within west-dipping (≤45°) faults, historically known as leather jacket lodes. Initiation of the ≤45° faults that are confined to fold culminations, begins at ca. 445Ma, with peak metamorphic conditions at 440Ma. The earliest vein sets (V1), were emplaced on limb thrusts at ca. 445–435Ma and are characterised by arsenopyrite-dominated quartz veins. These V1 veins parallel arsenopyrite-rich shale units, historically referred to as ‘indicator beds’. Both the steep and ≤45° faults were reactivated during fold amplification with deposition of the V2 auriferous veins at ca. 420–415Ma. A later set of auriferous veins (V3–V4) with ages of 380–370Ma, dominated by pyrite-sphalerite-galena-white-mica quartz-(V3) or carbonate-rich (V4) veins are predominantly associated with reactivation of the ≤45° west-dipping faults. This new geochronological data constrains the local kinematic history of the Ballarat East deposit and has regional implications. The V1–V2 vein development appears to be synchronous across the entire western section of the Lachlan Orogen, where previous studies have suggested that initial gold mineralisation was linked to orogenesis at ∼440Ma, asa result of metamorphic devolatilisation reactions in the lower crust. In contrast, a close spatial and temporal relationship exists between the felsic dykes and the mineralisation recognised in the V3–V4 veins. The deformation that accompanies V3–V4 vein development is attributed to small, localised events during east-west shortening, utilising pre-existing fold and fault structures. The origin of the fluids producing the V3–V4 veins may be metamorphic devolatilisation associated with widespread felsic magmatism that occurred at this time across central Victoria.

Flexural-slip during visco-elastic buckle folding

Flexural-slip is considered as an important mechanism during folding and a general conceptual and qualitative understanding has been provided by various field studies. However, quantitative evidence of the importance of the flexural-slip mechanism during fold evolution is sparse due to the lack of suitable strain markers. In this study, 2D finite element analysis is used to overcome these disadvantages and to simulate flexural-slip during visco-elastic buckle folding. Variations of single and multilayer layer fold configurations are investigated, showing that flexural-slip is most likely to occur in effective single layer buckle folds, where slip occurs between contacts of competent layers. Based on effective single layer buckle folds, the influence of the number of slip surfaces, the degree of mechanical coupling (based on the friction coefficient), and layer thickness, on the resulting slip distribution are investigated. The results are in agreement with the conceptual flexural-slip model and show that slip is initiated sequentially during the deformation history and is maximum along the central slip surface of the fold limb. The cumulative amount of slip increases as the number of slip surfaces is increased. For a lower degree of mechanical coupling increased slip results in different fold shapes, such as box folds, during buckling. In comparison with laboratory experiments, geometrical relationships and field observations, the numerical modeling results show similar slip magnitudes. It is concluded that flexural-slip should represent a significant contribution during buckle folding, affecting the resulting fold shape for increased amounts of slip.

Fault-related folds in the southern Pyrenees

ABSTRACT The fault-related folds observed in the south Pyrenean fold and thrust belt are hybrid structures with elements of detachment folding as well as elements of fault bend and fault propagation folding. Fanning growth strata in all the studied case studies indicate progressive limb rotation during deformation, mainly at their initial stages when fold growing mostly controlled the dispersal of syntectonic sediments as a result of higher uplift rates with respect to sedimentation rates. Break thrusts developed in the fold limb of the initial detachment folds to form fault propagation and fault bend geometries. Inversion folds developed at the rift margins. There the geometry of the folds as well as the brittle deformation patterns are controlled by the inherited fault systems, which are not symmetrical with respect to the fold axis. In the absence of previous structures, brittle patterns developed before or at the initial stages of folding by layer-parallel shortening preserving a symmetrical arrangement with respect to the fold axis.

Geomechanical Modeling of Stress and Fracture Distribution during Contractional Fault-Related Folding

Understanding and predicting the distribution of fractures in the deep tight sandstone reservoir are important for both gas exploration and exploitation activities in Kuqa Depression. We analyzed the characteristics of regional structural evolution and paleotectonic stress setting based on acoustic emission tests and structural feature analysis. Several suites of geomechanical models and experiments were developed to analyze how the geological factors influenced and controlled the development and distribution of fractures during folding. The multilayer model used elasto-plastic finite element method to capture the stress variations and slip along bedding surfaces, and allowed large deformation. The simulated results demonstrate that this novel Quasi-Binary Method coupling composite failure criterion and geomechanical model can effectively quantitatively predict the developed area of fracture parameters in fault-related folds. High-density regions of fractures are mainly located in the fold limbs during initial folding stage, then gradually migrate from forelimb to backlimb, from limbs to hinge, from deep to shallow along with the fold uplift. Among these factors, the fold uplift and slip displacement along fault have the most important influence on distributions of fractures and stress field, meanwhile the lithology and distance to fault have also has certain influences. When the uplift height exceeds approximately 55 percent of the total height of fold the facture density reaches a peak, which conforms to typical top-graben fold type with large amplitude and high-density factures in the top. The overall simulated results match well with core observation and FMI results both in the whole geometry and fracture distribution.

Investigation of Intrinsic and External Factors Contributing to the Occurrence of Coal Bumps in the Mining Area of Western Beijing, China

An investigation has been made to relate the occurrence of coal bumps to specific geological and mining conditions to the mining area of western Beijing. This investigation demonstrates that the high frequency of coal bumps in this area is due to four localized conditions, namely intrinsic coal properties, the presence of overturned strata and thrust faults, high in situ stress and the extraction of coal from island mining faces. Laboratory tests of coal samples indicated that the coals have a short duration of dynamic fracture, high bursting energy and high elastic strain energy, indicating that the coal is intrinsically prone to the occurrence of coal bumps. This investigation has also revealed that there are overturned strata and well-developed large- and medium-scale thrust faults in this area, and the presence of these structures results in plastic flow, severe discontinuities, rapid changes in overburden thickness and dipping of the coal seams. Well-developed secondary fold structures are also present in the axes and limbs of the primary folds. The instability of thrust faults, in combination with large-scale intrusion of igneous rocks, is closely associated with sudden roof breaking and induces sharp variations in electromagnetic radiation (EMR) and micro-seismic signals, which could be used to help predict coal bumps. In situ stress tests in the mining area demonstrate that the maximum and minimum principal stresses are nearly horizontal and that the intermediate principal stress is approximately vertical. The in situ stress level in the area is higher than the average in the Beijing area, North China and mainland China. In addition to the presence of overturned strata and thrust faults and high in situ stress levels, another external factor contributing to the frequency of coal bumps is coal extraction from island mining faces in this area. Island mining faces experience intermittent mining-induced abutment stress when a fault exists at one side of the island mining face due to reactivation of the fault, and this stress redistribution increases the likelihood of coal bumps during coal extraction from island mining faces.

Transtensional origin of multi-order cross-folds in a high-grade gneiss complex, southwestern Grenville Province: formation during post-peak gravitational collapse

The well-mapped western part of the Ottawa River Gneiss Complex (ORGC; new name), a large metamorphic core complex, hosts a system of gently plunging cross-folds of outcrop to regional scale situated in the ductile detachment zone between the lower grade cover and high-grade core of the complex. The cross-folds are buckle structures that deform the attenuated gneissic layering, plunge parallel to the regional elongation lineation, range from upright to recumbent, and exhibit distinctive hinge-parallel elongation lineations, all features of extension-dominated ductile transtension. Our L–S fabric data are consistent both with kinematic modelling that predicts progressive constrictional strain in the hinge zones of transtensional folds, and with dynamic modelling that predicts rotation of flattened fold limbs into moderately dipping attitudes. On the basis of petrologic data, we show that cross-folding postdated the peak Ottawan metamorphism and took place during retrogression and exhumation of the thrust-sheet stack. The cross-folds form an inclined system that is principally developed in retrograde melt-weakened amphibolite-facies rocks, with the transtensional origin implying that exhumation and retrogression of the high-grade core of the complex took place in an oblique extensional setting. A transtensional origin for the cross-folds removes the need to appeal to orogen-parallel regional shortening, an implausible requirement of previous interpretations, and is compatible with data indicating that much of the visible fabric and structure of amphibolite-facies domains of the ORGC developed during post-peak exhumation, retrogression, and gravitational collapse of the thrust-sheet stack.

Geological structural controls on stability of footwall slopes: an example from the Bowen Basin, Queensland

Abstract Surface mining of coal can involve extensive footwall slopes parallel to shallow to moderately dipping coal measures strata. Footwall failure mechanisms typically invoke bedding-parallel defects but also require either the existence of flatter structures, which cross-cut bedding, or require break-out through the rock mass to allow failure surfaces to emerge. Permian-aged Baralaba Coal Measures of the Bowen Basin, Queensland, are prospective for coal with extraction by open-pit methods. The Baralaba Coal Measures contains multiple seams within an interburden sequence comprising sandstone, siltstone, mudstone and carbonaceous variations. The coal measures sequence has been deformed into a complex pattern of NW-striking folds which has resulted in bedding dip ranging from 15° to 60°. Bedding has been classified as shallow (10–30°) to moderate (30–60°). Geotechnical investigations conducted to support coal extraction up to depths of 200 m suggest that structural controls strongly influence footwall slope design. For the purpose of footwall slope design, a distinction can be made between deposit areas of relatively simple structure (uniformly dipping bedding on fold limbs) and structurally complex areas (where layer-parallel shortening close to fold hinges has resulted in a system of low-angle thrusts and asymmetrical minor folds).

Multiple episodes of hematite mineralization indicated by U-Pb dating of iron-ore deposits, Marquette Range, Michigan, USA

Abstract High-grade hematite ore (so-called “hard ore”) was first mined in the Lake Superior region of North America in A.D. 1848. The development of ore deposit models from the region has strongly influenced research on the origin of this deposit type worldwide. The inability to date high-grade hematite ores has hindered the testing of competing genetic models. In the Marquette Range, northern Michigan (USA), high-grade hematite ore is hosted by the 1.9–1.85 Ga Negaunee Iron Formation and unconformably overlying Goodrich Quartzite. Field relations and new in situ U-Pb geochronology of monazite and xenotime intergrown with hematite from six mines across the Marquette Range, Michigan, indicate that the formation of high-grade hematite ore bodies involved up to three events. The presence of pebbles of hematite ore in basal conglomerate of the Goodrich Quartzite indicates that the first episode of iron-oxide mineralization occurred after deposition and deformation of the Negaunee Iron Formation, but before the ca. 1.85 Ga Sudbury impact. Monazite in iron-ore deposits within the hinges of Penokean-aged folds yields an age of ca. 1.8 Ga, which is interpreted to record a second phase of hematite enrichment. Monazite and xenotime in iron-ore deposits along the limbs of regional folds give ages of ca. 1.77 Ga and are interpreted to record a later episode of hematite upgrading. The ca. 1.8 Ga and ca. 1.77 Ga phosphate ages coincide with the timing of regional tectonic events, suggesting hematite mineralization was coeval with the Penokean orogeny and Yavapai-interval accretion. Our results support the proposal that the formation of high-grade hematite ore bodies is a multistage process linked to discrete orogenic events.

Tectonic evolution of a Laramide transverse structural zone: Sweetwater Arch, south central Wyoming

AbstractStructural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data record patterns of layer‐parallel shortening (LPS), vertical‐axis rotation, and regional fault‐fold evolution across the Sweetwater Arch, a major west to WNW trending, basement‐cored Laramide uplift in Wyoming. The southern arch flank is bounded by a WNW striking reverse fault zone that imbricated basement and cover rocks, the northern flank is bounded by a west striking fault zone with a component of strike‐slip and NW trending en echelon folds, and the eastern plunge transitions into an area of multiple‐trending faults and folds. Synorogenic strata record major arch uplift from Maastrichtian to Early Eocene time, followed by arch collapse. LPS, with development of systematic minor fault sets and AMS lineations, preceded large‐scale folding. LPS directions, estimated from both minor fault and AMS data, were oriented WSW along the northern flank, subparallel to Laramide regional shortening, but were refracted to the SSW along the southern flank, and to the west along the eastern arch plunge. Additional minor faults developed along steep fold limbs during continued shortening, with directions remaining SSW along the southern flank but becoming more variable along the eastern plunge where an increasingly heterogeneous stress field developed as additional faults were activated along basement heterogeneities. Vertical‐axis rotation was limited along the arch flanks, whereas the eastern plunge underwent counterclockwise rotation. Deflections in shortening directions were partly related to basement heterogeneities, including weak supracrustal belts on the arch flanks, a strong granitic core, and local reactivation of Precambrian shear zones.

Stress evolution during 3D single-layer visco-elastic buckle folding: Implications for the initiation of fractures

Buckle folds of sedimentary strata commonly feature a variety of different fracture sets. Some fracture sets including outer arc tensile fractures and inner arc shear fractures at the fold hinge zones are well understood by the extensional and compressional strain/stress pattern. However, other commonly observed fracture sets, including tensile fractures parallel to the fold axis, tensile fractures cutting through the limb, extensional faults at the fold hinge, and other shear fractures of various orientations in the fold limb, fail to be intuitively explained by the strain/stress regimes during the buckling process. To obtain a better understanding of the conditions for the initiation of the various fractures sets associated with single-layer cylindrical buckle folds, a 3D finite element modeling approach using a Maxwell visco-elastic rheology is utilized. The influences of three model parameters with significant influence on fracture initiation are considered: burial depth, viscosity, and permeability. It is concluded that these parameters are critical for the initiation of major fracture sets at the hinge zone with varying degrees. The numerical simulation results further show that the buckling process fails to explain most of the fracture sets occurring in the limb unless the process of erosional unloading as a post-fold phenomenon is considered. For fracture sets that only develop under unrealistic boundary conditions, the results demonstrate that their development is realistic for a perclinal fold geometry. In summary, a more thorough understanding of fractures sets associated with buckle folds is obtained based on the simulation of in-situ stress conditions during the structural development of buckle folds.

Transpressional structures and hydrocarbon potential along the Romanche Fracture Zone: a review

Abstract The Romanche Fracture Zone was originally a corridor of Aptian-age dextral transtensional rifting along the Equatorial Atlantic margins. Late Albian plate tectonic compression occurred due to a change in plate vectors, when the African and South American continents were still in contact across a 500 km-long section of the Romanche Fracture Zone. This dextral compression produced reactivation of the rift faults to produce asymmetric landward-vergent anticlines and thrusts that trend ENE to NE. Fold-axial planes dip seaward, parallel to the rift faults. Minor asymmetric anticlines were developed on the long seaward-dipping fold limbs and these have subvertical axial planes. The asymmetry of the minor folds is due to the southward stratal dip having been oblique to the horizontal maximum principal stress during the Albian inversion. The folds on the African margin were subsequently tightened by compression in Santonian and Oligo-Miocene times. Aptian-age ENE strike-slip faults were reactivated during the compression phases to produce broad positive flower structures up to 30 km wide that formed topographical ridges along the original strike-slip faults. The intervening and broader flat-bottomed synclines do not appear to be associated with rift faults. The folding and thrust faulting created seabed relief of 1–2 km at the end of the Albian; evidenced by the amount of subsequent erosion that removed the better-quality reservoirs in the upper Albian sequence from the major fold crests. Consequently, there has been a significant number of failed oil exploration wells drilled along the fold crests. The fold ridges would have diverted turbidite channels in the onlapping Cenomanian–Campanian sequence and these will be preferentially located on the landward side of the anticlinal crests. Late Cretaceous stratigraphic and structural traps located between the major anticlines have not yet been explored for hydrocarbons along the Romanche Fracture Zone margins.

Direct dating of folding events by 40Ar/39Ar analysis of synkinematic muscovite from flexural-slip planes

Timing of folding is usually dated indirectly, with limited isotopic dating studies reported in the literature. The present study investigated the timing of intracontinental, multi-stage folding in Upper Proterozoic sandstone, limestone, and marble near Beijing, North China, and adjacent regions. Detailed field investigations with microstructural, backscattered electron (BSE) images and electron microprobe analyses indicate that authigenic muscovite and sericite crystallized parallel to stretching lineations/striations or along thin flexural-slip surfaces, both developed during the complex deformation history of the study area, involving repeated compressional, extensional and strike-slip episodes. Muscovite/sericite separates from interlayer-slip surfaces along the limbs and from dilatant sites in the hinges of folded sandstones yield muscovite 40Ar/39Ar plateau ages of ∼158–159 Ma, whereas those from folded marble and limestone samples yield ages of 156 ± 1 Ma. Muscovite from thin flexural-slip planes on fold limbs and hinges yields ages within analytical error of ∼155–165 Ma. Further muscovite samples collected from extensionally folded limestone and strike-slip drag folds yield younger ages of 128–125 Ma with well-defined plateaus. To assess the potential influence of the detrital mica component of the host rock on the age data, two additional muscovite samples were investigated, one from a folded upper Proterozoic–Cambrian sandstone outside the Western Hills of Beijing and one from a folded sandstone sampled 20 cm from folding-related slip planes. Muscovite separates from these samples yield significantly older ages of 575 ± 2 Ma and 587 ± 2 Ma, suggesting that the timing of folding can be directly determined using the 40Ar/39Ar method. This approach enables the identification and dating of distinct deformation events that occur during multi-stage regional folding. 40Ar/39Ar dating can be used to constrain the timing of muscovite and sericite growth at moderate to low temperatures (<400 °C) during folding, yielding well-defined plateau ages and thereby the age of deformation in the upper crust.

Stratigraphic vs structural contacts in a late orogenic basin: the case of the Tertiary Piedmont Basin in the Sassello area (Ligurian Alps, Italy)

This geological map (1:10.000 scale) of the ‘Sassello Basin’ remnant covers an area of about 33.4 km2 of Liguria (NW Italy); it highlights the occurrence of two main types of contacts between the sediments of the Tertiary Piedmont Basin and the metamorphic substratum (Voltri Unit): (i) stratigraphic and (ii) structural (thrust or steeply dipping faults). (i) Stratigraphic contacts are represented by the main transgressive surface and the nonconformity between the metamorphic rocks of the substratum and the subaerial deposits. They are locally folded and occur along the steeply dipping short limbs of asymmetric folds related to the late-alpine/apennine tectonics. (ii) Structural contacts are related to the late-alpine/apennine tectonics (thrust faults) or (mostly) to Plio-Quaternary extensional/transtensional faulting.

Geomorphological evidence for late Quaternary tectonic deformation of the Cape Region, coastal west central Australia

A late Pleistocene (Marine Isotope Stage 5e) emergent marine sequence fringes the coastline of the Cape Region of coastal west central Australia and provides elevation and age control to characterize the locations and rates of crustal deformation. There is a systematic measurable change in relative paleo sea-level elevations across the Cape Region. High-precision leveling of modern and Pleistocene shoreline features indicates the minimum elevation range of MIS 5e shoreline features along the coast is 10.4m. This compares with the 2.5m elevation range for observed modern shoreline analogs. The lack of continuity of MIS 5e shoreline elevations along 300km of coastline demonstrates continuing tectonic deformation along coastal anticlines in the Cape Region. Topographic expression of MIS 5e features indicates tectonic uplift consistent with late Neogene to Quaternary deformation on the Cape Cuvier and Cape Range anticlines. Post-MIS 5e tectonic uplift rates are up to 0.054±0.035mm/yr at fold axial locations. Estimated subsidence rates are −0.013±0.034mm/yr on fold limbs. While the estimated vertical tectonic deformation is small and the rates are low, the geomorphological data also demonstrate tectonic activity, not stability.

25. Complete penoscrotal transposition and bilateral renal cystic dysplasia

Introduction Complete penoscrotal transposition is a rare male genital malformation where the scrotum is located entirely cephalad to the penis. This malformation is most commonly associated with congenital abnormalities involving the genitourinary system but anomalies of the gastrointestinal, cardiovascular and the central nervous systems have also been described. Case report We present a case of a twenty-four week old foetus with oligohydramnios, bilateral hydronephrosis, enlarged kidneys, thickened nuchal fold and abnormal limbs on antenatal ultrasound. The pregnancy proceeded to termination based on these features being incompatible with life. At autopsy, the foetus was found to have a complete penoscrotal transposition with hypospadias and urethral meatus stenosis. Both kidneys were markedly enlarged and cystic on macroscopic appearance. Microscopic examination confirmed bilateral renal cystic dysplasia with hydronephrosis. No other congenital abnormalities were identified. Discussion There are only a small number of cases of complete penoscrotal transposition reported in the literature. An association between genital malformations, including penoscrotal transposition, and distal deletions of the long arm of chromosome 13 have been reported, with this region encoding the EFNB2 gene. A brief review of the literature and genetic associations of penoscrotal transposition ensues.

Large‐scale mechanical buckle fold development and the initiation of tensile fractures

AbstractTensile failure associated with buckle folding is commonly associated to the distribution of outer arc extension but has also been observed on fold limbs. This study investigates whether tensile stresses and associated failure can be explained by the process of buckling under realistic in situ stress conditions. A 2‐D plane strain finite element modeling approach is used to study single‐layer buckle folds with a Maxwell viscoelastic rheology. A variety of material parameters are considered and their influence on the initiation of tensile stresses during the various stages of deformation is analyzed. It is concluded that the buckling process determines the strain distribution within the fold layer but is not solely responsible for the initiation of tensile stresses. The modeling results show that tensile stresses are most dependent on the permeability, viscosity, and overburden thickness. Low permeability (&lt;10−19 m2), high viscosity (≥1021 Pa s), and low overburden pressure can explain tensile failure at the fold hinge. Tensile stresses in the limb of the fold cannot (in general) be explained by buckling. Rather, it develops due to a combination of compression and erosional unloading. The modeling results show that erosion of high permeability rocks can explain the generation of tensile stresses at significant depths (∼2 km) both at the hinge of the fold and throughout the limb of the fold. This study shows that tensile stresses and associated failure within buckle folds is directly dependent on the distribution of material parameters but moreover to the strain history of the geologic system.

Revisiting the Hubbert–Rubey pore pressure model for overthrust faulting: Inferences from bedding-parallel detachment surfaces within Middle Devonian gas shale, the Appalachian Basin, USA

Both bedding-parallel slickensides and cleavage duplexes are forms of mesoscopic-scale detachment faulting populating black (Marcellus and Geneseo/Burket) and intervening gray (Mahantango) shales of the Middle Devonian, a section known for abnormal pore pressure below the Appalachian Plateau. The abundance and the orientation of slickensides and cleavage duplexes in the more organic-rich black shale relative to gray shale suggests that maturation-related abnormal pore pressure facilitates detachment, a mesoscopic manifestation of the Hubbert–Rubey pore pressure model for overthrust faulting. The former are discrete slip surfaces whereas the latter consists of nested, anastomosing slip surfaces, either cutting through bedding or on disrupted bedding surfaces stacked as mesoscopic versions of thrust duplexes. Cleavage duplexes are between a few cm and over 1 m thick with their hanging walls commonly transported toward the Appalachian foreland, regardless of local limb dip. Cleavage duplexes are most common near the stratigraphic maximum flooding surface, the organic-rich section most prone to develop maturation-related pore pressure in the Middle Devonian gas shales. Bedding-parallel slickensides are somewhat more evenly distributed in the black shale but also found in overlying gray shale. In both black and gray shales, slickensides are more abundant on the limbs of folds, an indication of pore-pressure-related flexural-slip folding. On the macroscopic scale, the Pine Mountain Block of the Southern Appalachian Mountains was enabled by a basal detachment cutting along the Upper Devonian Chattanooga black shale which has a thermal maturity sufficient for the generation of abnormal pore pressure. The Pine Mountain block is a large-scale overthrust showing little evidence of collapse of the hinterland side, a credible example of a pore-pressure-aided overthrust fault block of the type envisioned by the Hubbert–Rubey model.