Episodic,two-stage Neogene extension and short-term intervening compression in Western Turkey: field evidence from the Kiraz Basin and Bozdağ Horst

Western Anatolia (Turkey) is a region of widespread active N-S continental extension that forms the eastern part of the Aegean extensional province. The extension in the region is expressed by two distinct/different structural styles, separated by a short-term gap: (1) rapid exhumation of metamorphic core complexes along presently low-angle ductile-brittle normal faults commenced by the latest Oligocene-Early Miocene period, and; (2) late stretching of crust and, consequent graben evolution along Plio-Quaternary high-angle normal faults, cross-cutting the pre-existing low-angle normal faults. However, current understanding of the processes (tectonic quiescence vs N-S continental compression) operating during the short-time interval is incomplete. This paper therefore reports the results of recent field mapping and structural analysis from the NE of Küçük Menderes Graben—Kiraz Basin—that shed lights on the processes operating during this short-time interval. The data includes the thrusting of metamorphic rocks of the Menderes Massif over the Mio-Pliocene sediments along WNW-ESE-trending high-angle reverse fault and the development of compressional fabrics in the metamorphic rocks of the Menderes Massif. There, the metamorphic rocks display evidence for four distinct phases of deformation: (1) southfacing top-N ductile fabrics developed at relatively high-grade metamorphic conditions, possibly during the Eocene main Menderes metamorphism (amphibolite facies) associated with top-N thrust tectonics (D₁); (2) top-S and top-N ductile gentle-moderatley south-dipping extensional fabrics formed at relatively lower-grade metamorphic (possibly greenschist facies) conditions associated with the exhumation of Menderes Massif along presently low-angle normal fault plane that accompanied the first phase of extension (D₂); (3) moderately north-dipping top-S ductile-brittle fabrics, present configuration of which suggest a thrust-related compression (D₃); and (4) south-facing approximately E-W-trending brittle high-angle normal faults (D₄) that form the youngest structures in the region. It is interpreted that D₄ faults are time equivalent of graben-bounding major high-angle normal faults and they correspond to the second phase of extension in western Anatolia. The presence of thrust-related D₃ compressional fabrics suggests N-S compression during the time interval between the two phases of extension (D₂ and D₄). The results of the present study therefore support the episodic, two-stage extension model in western Anatolia and confirm that a short-time, intervening N-S compression separated the two distinct phases.     

Physical modelling of chemical compaction,overpressure development,hydraulic fracturing and thrust detachments in organic-rich source rock

Geological evidence for overpressure is common worldwide, especially in petroleum-rich sedimentary basins. As a result of an increasing emphasis on unconventional resources, new data are becoming available for source rocks. Abnormally high values of pore fluid pressure are especially common within mature source rock, probably as a result of chemical compaction and increases in volume during hydrocarbon generation. To investigate processes of chemical compaction, overpressure development and hydraulic fracturing, we have developed new techniques of physical modelling in a closed system. During the early stages of our work, we built and deformed models in a small rectangular box (40 × 40 × 10 cm), which rested on an electric flatbed heater; but more recently, in order to accommodate large amounts of horizontal shortening, we used a wider box (77 × 75 × 10 cm). Models consisted of horizontal layers of two materials: (1) a mixture of equal initial volumes of silica powder and beeswax micro-spheres, representing source rock, and (2) pure silica powder, representing overburden. By submerging these materials in water, we avoided the high surface tensions, which otherwise develop within pores containing both air and liquids. Also we were able to measure pore fluid pressure in a model well. During heating, the basal temperature of the model surpassed the melting point of beeswax (∼62 °C), reaching a maximum of 90 °C. To investigate tectonic contexts of compression or extension, we used a piston to apply horizontal displacements.In experiments where the piston was static, rapid melting led to vertical compaction of the source layer, under the weight of overburden, and to high fluid overpressure (lithostatic or greater). Cross-sections of the models, after cooling, revealed that molten wax had migrated through pore space and into open hydraulic fractures (sills). Most of these sills were horizontal and their roofs bulged upwards, as far as the free surface, presumably in response to internal overpressure and loss of strength of the mixture. We also found that sills were less numerous towards the sides of the box, presumably as a result of boundary effects. In other experiments, in which the piston moved inward, causing compression of the model, sills also formed. However, these were thicker than in static models and some of them were subject to folding or faulting. For experiments, in which we imposed some horizontal shortening, before the wax had started to melt, fore-thrusts and back-thrusts developed across all of the layers near the piston, producing a high-angle prism. In contrast, as soon as the wax melted, overpressure developed within the source layer and a basal detachment appeared beneath it. As a result, thin-skinned thrusts propagated further into the model, producing a low-angle prism. In some experiments, bodies of wax formed imbricate zones within the source layer.Thus, in these experiments, it was the transformation, from solid wax to liquid wax, which led to chemical compaction, overpressure development and hydraulic fracturing, all within a closed system. According to the measurements of overpressure, load transfer was the main mechanism, but volume changes also contributed, producing supra-lithostatic overpressure and therefore tensile failure of the mixture.     

Forces on a pitching plate: An experimental and numerical study

A flat plate in pitching motion is considered as a fundamental source of locomotion in the general context of marine propulsion. The experimental as well as numerical investigation is carried out at a relatively small Reynold number of 2000 based on the plate length c and the inflow velocity U_∞. The plate oscillates sinusoidally in pitch about its 1/3 − c axis and the peak to peak amplitude of motion is 20°. The reduced frequency of oscillation k = πfc/U_∞ is considered as a key parameter and it may vary between 1 and 5. The underlying fluid-structure problem is numerically solved using a compact finite-differences Navier–Stokes solution procedure and the numerical solution is compared with Particle Image Velocimetry (PIV) measurements of the flow field around the pitching foil experimental device mounted in a water-channel. A good agreement is found between the numerical and experimental results and the threshold oscillation frequency beyond which the wake exhibits a reverse von Kármán street pattern is determined. Above threshold, the mean velocity in the wake exhibits jet-like profiles with velocity excess, which is generally considered as the footprint of thrust production. The forces exerted on the plate are extracted from the numerical simulation results and it is shown, that reliable predictions for possible thrust production can be inferred from a conventional experimental control volume analysis, only when besides the wake's mean flow the contributions from the velocity fluctuation and the pressure term are taken into account.     

Geometry and evolution of fold-thrust structures at the boundaries between frictional and ductile detachments

Fold-thrust belts formed above a ductile detachment typically contain detachment folds, whereas those formed above frictional detachments contain fault-related fold complexes, such as imbricate thrust systems. Analog models, using silica sand to represent sediments and silicone gel to represent salt were conducted to study the fold geometry, fold-fault relations, and sequential development of structures formed in each setting and at the boundaries between the two settings. The results showed a relatively thinner wedge above a ductile detachment, so that the deformation front propagated farther forward than that above a frictional detachment. The thrust front connects across the two settings with a significant change in position and a resulting change in orientation. The geometry of the deformation front is strongly controlled by that of the detachment boundary, so that an oblique detachment boundary results in an oblique thrust front in the transition zone. Modifications in the taper geometry also result from the presence of a frictional belt behind a ductile belt, the width of the ductile detachment which limits the location of the deformation front, and the lateral propagation of thrust faults between the two regimes. The experimental models can be used to explain observed geometries in natural examples of fold-thrust belts marked by transitions between frictional and ductile detachments.     

Gas hydrate formation in compressional,extensional and un-faulted structural settings – Examples from New Zealand's Hikurangi margin

We investigate gas hydrate formation processes in compressional, extensional and un-faulted settings on New Zealand's Hikurangi margin using seismic reflection data. The compressional setting is characterized by a prominent subduction wedge thrust fault that terminates beneath the base of gas hydrate stability, as determined from a bottom-simulating reflection (BSR). The thrust is surrounded by steeply dipping strata that cross the BSR at a high angle. Above the BSR, these strata are associated with a high velocity anomaly that is likely indicative of relatively concentrated, and broadly distributed, gas hydrates. The un-faulted setting—sedimentary infill of a slope basin on the landward side of a prominent thrust ridge—is characterized by a strong BSR, a thick underlying free gas zone, and short positive polarity reflection segments that extend upward from the BSR. We interpret the short reflection segments as the manifestation of gas hydrates within relatively coarse-grained sediments. The extensional setting is a localized, shallow response to flexural bending of strata within an anticline. Gas has accumulated beneath the BSR in the apex of folding. A high-velocity zone directly above the BSR is probably mostly lithologically-derived, and only partly related to gas hydrates. Although each setting shows evidence for focused gas migration into the gas hydrate stability zone, we interpret that the compressional tectonic setting is most likely to contain concentrated gas hydrates over a broad region. Indeed, it is the only setting associated with a deep-reaching fault, meaning it is the most likely of the three settings to have thermogenic gas contributing to hydrate formation. Our results highlight the importance of anisotropic permeability in layered sediments and the role this plays in directing sub-surface fluid flow, and ultimately in the distribution of gas hydrate. Each of the three settings we describe would warrant further investigation in any future consideration of gas hydrates as an energy resource on the Hikurangi margin.     

Litho-tectonic sequence and their regional correlation along the Lohit and Dibang valleys,eastern Arunachal Pradesh

Geological mapping between Lohit and Dibang valleys of eastern Arunachal Pradesh reveals the existence of five thrust bound tectonostratigraphic units. In ascending structural order from SW to NE these are: (1) Sewak Group, (2) Lalpani Group, (3) Mayodiya Group, (4) Tidding Formation and (5) Lohit plutonic complex. This differs from previous mapping, because the three tectonostratigraphic units (Sewak, Lalpani and Mayodiya) were grouped under a single unit, referred as the Mishmi crystallines. The low-grade metamorphics of the Sewak Group occur at two different tectonic levels, one as persistent belt in the foothills and the other in the tectonic window beneath the high-grade metamorphics of the Mayodiya thrust sheet in the Higher Himalaya. The Tidding suture is the southeastern extension of the Indus-Tsangpo suture zone.     

Superposed lateral ramps in the Pell City thrust sheet, Appalachian thrust belt, Alabama

In the Appalachian thrust belt in Alabama, thrust sheets of Paleozoic strata generally strike northeastward and are imbricated northwestward; four transverse zones cross the regional strike of the thrust belt. The large-scale Pell City thrust sheet ends southwestward at an oblique lateral ramp within the Harpersville transverse zone, where the leading edge of the thrust sheet (the Pell City fault) curves abruptly 55° counterclockwise. The northwest-striking segment of the Pell City fault conforms to the geometry of an oblique lateral ramp in the footwall. Furthermore, the Pell City fault cuts up section in the hanging wall southwestward toward the transverse zone, indicating a hanging-wall lateral ramp emplaced over the footwall oblique lateral ramp.In the hanging wall adjacent to the northwest-trending segment of the Pell City fault, a pervasive train of upright, isoclinal folds (with 50% apparent shortening) trends N15°W, oblique to the regional translation direction. The fold train is limited to the southwestern part of the Pell City thrust sheet; farther northeast, the regional northeasterly strike prevails. The isoclinal folds in the hanging wall indicate contractional crowding perpendicular to the footwall oblique lateral ramp.     

河北兴隆复式叠瓦扇构造

河北省北部兴隍-平泉复向斜的西端发育了一种特殊类型的推覆构造,该推覆构造具有三重结构的特点,即由上叠瓦扇、下叠瓦扇和下伏系统组成。上叠瓦扇可以分为被分支断裂分割的太古字、长城系、蓟县系、青白口系和寒武-奥陶系5个逆冲岩席;各分支断裂上陡下缓,向下逐渐归并于F₁主逆冲断裂上。F₁断层下的石炭-二叠系也发育了一组叠瓦状逆冲断层,形成了与上叠瓦扇具有不同变形特征的下叠瓦扇。由于这一构造特殊的两套叠瓦扇结构,故笔者称其为复式叠瓦扇构造,这是一种新的推覆构造类型。     

藏南帕里至达吉地带的上地壳结构特征──REFTEK顺带广角地震观测结果分析

本文主要介绍１９９２年中美喜马拉雅和青藏高原深剖面与综合研究项目第一阶段（ＩＮＤＥＰＴＨＩ－１）广角地震观测资料利用Ｓｅｉｓ８１程序（Ｃｅｒｖｅｎｙ，１９８１）进行二维解释所获得的帕里一达吉地带的上地壳结构特征。主要成果为：（１）前寒武结晶基底之顶界表现为Ｒ１界面。据本项目地质调查，藏南拆离系（ＳＴＤＳ）在帕里以北约１０ｋｍ处出露，向北缓倾并向地下延伸。认为Ｒ１界面不仅是结晶基底之顶界的反映，而且ＳＴＤ可能沿着Ｒ１界面展布，也即Ｒ１界面同时是一条沉积盖层和基底之间的拆离层。Ｒ１界面埋深３±０．６—１１±０．６ｋｍ。（２）上述拆离层在萨马达一达吉之间，以Ｒ１界面之上的负速度梯度楔状体（ＬＶＬ）为特征。ＬＶＬ可能是ＳＴＤＳ活动时拖带下沉的中生代特提斯沉积，或可能是含水破碎带。（３）在结晶基底内部存在第二条拆离带（ＬＶＺ），表现为Ｔ２反射波组。ＬＶＺ在帕里埋深８．５±０．６ｋｍ，向北陡倾，至萨马达为２２．５±０．６ｋｍ深；再向北，倾角变缓，至达吉之下，达２７±０．６ｋｍ深。ＬＶＺ在帕里─萨马达之间，厚仅０．５ｋｍ；至达吉，增厚至５ｋｍ；也具楔形负速度梯度带性质；它可能是花岗岩局部融熔体之反映...更多。（４）在萨马达─达吉。     

Late Cenozoic deformation in the South Caspian region: effects of a rigid basement block within a collision zone

Active deformation in the South Caspian region demonstrates the enormous variation in kinematics and structural style generated where a rigid basement block lies within a collision zone. Rigid basement to the South Caspian Basin moves with a westward component relative both to stable Eurasia and Iran, and is beginning to subduct at its northern and western margins. This motion is oblique to the approximately north–south Arabia–Eurasia convergence, and causes oblique shortening to the south and northeast of the South Caspian Basin: thrusting in the Alborz and Kopet Dagh is accompanied by range-parallel strike–slip faults, which are respectively left- and right-lateral. There are also arcuate fold and thrust belts in the region, for two principal reasons. Firstly, weaker regions deform and wrap around the rigid block. This occurs at the curved transition zone between the Alborz and Talysh ranges, where thrust traces are concave towards the foreland. Secondly, a curved fold and thrust belt can link a deformation zone created by movement of the basement block to one created by the regional convergence: west-to-east thrusts in the eastern Talysh represent underthrusting of the South Caspian basement, but pass via an arcuate fan of fold trains into SSW-directed thrusts in the eastern Greater Caucasus, which accommodates part of the Arabia–Eurasia convergence. Each part of the South Caspian region contains one or more detachment levels, which vary dependent on the pre-Pliocene geology. Buckle folds in the South Caspian Basin are detached from older rocks on thick mid-Tertiary mudrocks, whereas thrust sheets in the eastern Greater Caucasus detach on Mesozoic horizons. In the future, the South Caspian basement may be largely eliminated by subduction, leading to a situation similar to Archaean greenstone belts of interthrust mafic and sedimentary slices surrounded by the roots of mountain ranges constructed from continental crust.