Magnetic anomalies associated with salt tectonism,deep structure and regional tectonics in the Maritimes Basin,Atlantic Canada

The structure and tectonic evolution of an evaporite basin are investigated in this case study, which combines the interpretation of magnetic data with the more commonly applied seismic reflection and gravity methods. The Maritimes Basin contains up to 18 km of Upper Palaeozoic sedimentary rocks resting on the basement of the Acadian orogeny. Carboniferous rocks are intensely deformed to the southeast of the Magdalen Islands as a result of deformation of evaporites of the Viséan Windsor Group. Short‐wavelength (<5 km) magnetic lineations define NNE‐ and ENE‐trending linear belts, coincident with the mapped pattern of salt structures. Magnetic models show that these lineations can be explained by the infill of subsidence troughs by high‐susceptibility sediment and/or the presence of basaltic rocks, similar to those uplifted and exposed on the Magdalen Islands. Additional shallow, magnetic sources are interpreted to result from alteration mineralization in salt‐impregnated, iron‐rich sedimentary rocks, brecciated during salt mobilization. Magnetic susceptibility measurements of samples from the Pugwash mine confirm the presence of higher susceptibility carnallite‐rich veins within salt units. Salt tectonism and basin development were influenced by the structure of the base group, the deepest regionally continuous seismic reflections (ca. 5–11 km), associated with an unconformity at the base of the Windsor Group, sampled at the Cap Rouge well. Salt structural evolution, formation of the magnetic lineations and geometry of the base group are associated with regional dextral transpression during basin development (late Carboniferous) and/or Alleghanian Orogeny (late Carboniferous to Permian). In this and similar studies, the effective use of magnetics is dependent upon the presence of rocks of high magnetic susceptibility in contrast to the low‐susceptibility salt bodies. In the absence of high‐susceptibility rocks, magnetic lows over the salt structures may be modelled, similar to commonly applied gravity techniques, to derive the internal structure and geometry.     

Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition,salt wall growth and supra‐salt stratigraphic architecture

The northern Paradox Basin evolved during the Late Pennsylvanian–Permian as an immobile foreland basin, the result of flexural subsidence in the footwall of the growing Uncompahgre Ancestral Rocky Mountain thick‐skinned uplift. During the Atokan‐Desmoinesian (～313–306 Ma) fluctuating glacio‐eustatic sea levels deposited an ～2500 m thick sequence of evaporites (Paradox Formation) in the foreland basin, interfingering with coarse clastics in the foredeep and carbonates around the basin margins. The cyclic deposition of the evaporites produced a repetitive sequence of primarily halite, with minor clastics, organic shales and anhydrite. Sediment loading of the evaporites subsequently produced a series of salt walls and minibasins, through the process of passive diapirism or downbuilding. Faults at the top Mississippian level localised the development of linear salt walls (up to 4500 m high) along a NW–SE trend. A crosscutting NE–SW structural trend was also important in controlling the evaporite facies and the abrupt termination of the salt walls. Seismic, well and field data define the proximal Cutler Group (Permian) as a basinward prograding sequence derived from the growing Uncompahgre uplift that drove salt basinwards (towards the southwest), triggering the growth of the salt walls. Sequential structural restorations indicate that the most proximal salt walls evolved earlier than the more distal ones. The successive development of salt‐withdrawal minibasins associated with each growing salt wall implies that parts of the Cutler Group in one minibasin may have no chronostratigraphic equivalent in other minibasins. Localised changes in along‐strike salt wall growth and evolution were critical in the development of facies and thickness variations in the late Pennsylvanian to Triassic stratigraphic sequences in the flanking minibasins. Salt was probably at or very close to the surface during the downbuilding process leading to localised thinning, deposition of diapir‐derived detritus and rapid facies changes in sequences adjacent to the salt wall structures.     

Rates of sediment deposition in a hypersaline lake in the northern Great Plains,western Canada

Remarkably high rates of sedimentation (up to nearly 60 kg m⁻² yr⁻¹) are documented for offshore areas of Freefight Lake, a deep, hypersaline, meromictic lake in the northern Great Plains of Canada. Most material in the offshore sediment traps consists of soluble and suparingly soluble endogenic salts. Deposition of allogenic sediment is minor. The observed high sedimentation rates are in response to several mechanism: (i) freeseout precipitation of salts from the mixolimnion; (ii) redistribution of shallow water endogenic precipitates by turbidity flows and interflow processes; and (iii) precipitation of salts from within the monimolimnion Because the bottom water mass of the lake is essentially isothermal and isochemical, large scale temperature induced remobilization of these salts at or near the sediment-water interface is unlikely. Similarly, the morphology of the basin suggests that resuspension of bottom sediment by waves in the offshore areas of the basin is negligible. Although the sedimentation rates observed in Freefight Lake are extraordinarily high and considerably greater than other modern lacustrine sedimentation rates documented in the literature to date, the observed rates are entirely compatible with suggested accumulation rates of ancient evaporite sequences. Palliser Tringle Global Change Contribution Number 6.     

The stratigraphic and structural evolution of the Dzereg Basin, western Mongolia: clastic sedimentation, transpressional faulting and basin destruction in an intraplate, intracontinental setting

The Dzereg Basin is an actively evolving intracontinental basin in the Altai region of western Mongolia. The basin is sandwiched between two transpressional ranges, which occur at the termination zones of two regional‐scale dextral strike‐slip fault systems. The basin contains distinct Upper Mesozoic and Cenozoic stratigraphic sequences that are separated by an angular unconformity, which represents a regionally correlative peneplanation surface. Mesozoic strata are characterized by northwest and south–southeast‐derived thick clast‐supported conglomerates (Jurassic) overlain by fine‐grained lacustrine and alluvial deposits containing few fluvial channels (Cretaceous). Cenozoic deposits consist of dominantly alluvial fan and fluvial sediments shed from adjacent mountain ranges during the Oligocene–Holocene. The basin is still receiving sediment today, but is actively deforming and closing. Outwardly propagating thrust faults bound the ranges, whereas within the basin, active folding and thrusting occurs within two marginal deforming belts. Consequently, active fan deposition has shifted towards the basin centre with time, and previously deposited sediment has been uplifted, eroded and redeposited, leading to complex facies architecture. The geometry of folds and faults within the basin and the distribution of Mesozoic sediments suggest that the basin formed as a series of extensional half‐grabens in the Jurassic–Cretaceous which have been transpressionally reactivated by normal fault inversion in the Tertiary. Other clastic basins in the region may therefore also be inherited Mesozoic depocentres. The Dzereg Basin is a world class laboratory for studying competing processes of uplift, deformation, erosion, sedimentation and depocentre migration in an actively forming intracontinental transpressional basin.     

Inferring denudation variations from the sediment record; an example of the last glacial cycle record of the Golo Basin and watershed,East Corsica,western Mediterranean sea

Geophysical data and sampling of the Golo Basin (East Corsica margin) provide the opportunity to study mass balance in a single drainage system over the last 130 kyr, by comparing deposited sediments in the sink and the maximum eroded volume in the source using total denudation proxies. Evaluation of the solid sediments deposited offshore and careful integration of uncertainties from the age model and physical properties allow us to constrain three periods of sedimentation during the last climatic cycle. The peak of sedimentation initiated during Marine Isotopic Stage (MIS) 3 (ca. 45 ka) and lasted until late in MIS 2 (ca. 18 ka). This correlates with Mediterranean Sea palaeoclimatic records and the glaciation in high altitude Corsica. The yield of solid sediment into the Golo Basin drops in the observed present day Mediterranean basins (gauging stations), whereas the palaeo‐denudation estimate derived from the sediments over the last glacial period is one to ten times higher than that predicted using cosmogenic or thermochronometer estimates of exhumation. The catchment‐wide denudation rate calculated from deposited solid sediment ranges from 47 to 219 mm kyr^?1, which is higher than the estimate from palaeosurface ablation in the proximal part of the source (9–140 mm kyr^?1) and lower than the distal, narrow, incised channel of the Golo River (160–475 mm kyr^?1). This mismatch raises questions about the investigation of denudation at millennial‐time scale (kyr) and at higher integrating times (Myr) as a reliable tool for determining the effect of climate change on mountain building and on sedimentary basin models.     

Strike-slip tectonics and development of the Tertiary Queen Charlotte Basin, offshore western Canada: evidence from seismic reflection data

Interpretation of seismic reflection data have led to a new model of the development of the Queen Charlotte Basin. New multi-channel data collected in 1988 and an extensive network of unpublished older single- and multi-channel profiles from industry image a complex network of sub-basins. Structural styles vary along the axis of the basin from broadly spaced mainly N-trending sub-basins in Queen Charlotte Sound, to closely spaced NW-trending sub-basins in Hecate Strait, to an E-W en echelon belt of sub-basins in Dixon Entrance. Transtensional tectonics dominated in the Miocene and transpression dominated in the Pliocene except in Queen Charlotte Sound. The data we present prove that the origin of the basin is extensional and its most recent deformation is compressive. Evidence for the strike-slip origin of tectonism includes along-axis variations in structures, simultaneous extension and compression in adjacent sub-basins, lack of correlations across faults, and mixed normal and reverse faults within structures. We infer that the Pacific-North America plate boundary has been west of the Queen Charlotte Islands since the Miocene when relative plate motions have been dominantly strike-slip. The formation and development of the Queen Charlotte Basin is the result of distributed shear; by which a small percentage of the plate motion has been taken up in a network of faults across the continental margin. As this region of crust deforms it interacts with neighbouring rigid crust resulting in extension dominating in the south of the basin and compression in the north. Continental crust adjacent to some transform plate boundaries can be sheared over a wide region; the network of basins in southwestern California is a good analogue for the Queen Charlotte Basin.     

The interplay between deformation,erosion and sedimentation in the deep‐water Mexican Ridges foldbelt,western Gulf of Mexico basin

We analyse a regional 2D seismic section of the Mexican Ridges foldbelt (MRFB), western Gulf of Mexico, and construct excess‐area diagrams for each of the structures comprising the foldbelt to estimate shortening, the onset of folding and the degradation of the folded seafloor. From the chronostratigraphy, we derive rates of tectonic and superficial mass transport and illustrate how they change across the MRFB. The resulting tectonic transport in the MRFB is 11.8 km forming a train of twelve buckle folds above a detachment at a depth of ca. 6 s of two‐way travel time, with an average strain of ca. 10%. The fold train grew at a mean uplift rate of ca. 0.21 mm year^?1. Cross‐sectional balancing demonstrates that shortening balances the down‐slip motion of the Quetzalcoatl extensional system (QES), suggesting that horizontal compaction, volume loss and other penetrative deformation mechanisms are negligible. By assuming steady‐state denudation, we are able to distinguish sediments derived locally from sediments transported from distant sources. The constant of mass diffusivity, a parameter controlling the degradation rate, is ca. 0.42 m² year ^?1, which is characteristic of rapid, episodic, superficial mass movements. The combined sedimentation rate from both, local and distal sources is ca. 0.23 mm year ^?1. Those values are not constant; structures proximal to the continental shelf are rising rapidly and are being degraded more intensely than those in the distal part of the MRFB, where sedimentation outweighs tectonic uplift. Our results indicate deformation initiated up to 3 Myr earlier than estimated from stacking patterns. Moreover, we find deformation started synchronously during the Late Miocene throughout the MRFB and not in two episodes as the stacking relations suggest. The discrepancy can be explained by a delay in the sedimentary response to folding. During early fold growth, nearly constant thickness strata are deposited before a progressive unconformity and other converging geometries develop. The development of growth strata is fast in the folds near the QES, which are being uplifted rapidly and degraded vigorously. Under these conditions, the stratigraphic relations give only a broad estimate of the pretectonic/syntectonic limit when compared to the excess‐area method. On the other hand, the development of growth strata took twice as much time for folds near the abyssal plain, which are being uplifted at a slower rate and where degradation is less intense. Consequently, the delay takes more time, and the use of stratigraphic relations introduces an even more pronounced bias towards younger ages in the identification of the onset of folding.     

东江流域近50年径流系数时空变化特征

基于东江流域1964-2012 年降雨、蒸发和径流等气象水文资料,采用线性倾向估计、M-K趋势检验及变化幅度和变化弹性等多种方法,结合ArcGIS自相关等空间统计分析技术,分析了东江流域径流系数的时空变化特征及其对降雨、蒸发和地表植被等影响因素的空间对应关系。结果表明：近50 年来,东江流域年径流系数均呈上升趋势,且随着流域空间尺度的增大而增加,总体形成“南北低、中间高”的空间分布格局;径流系数变化幅度及变化弹性在20 世纪90年代之后显著增加。径流系数变化幅度热度指数高值区大致位于东源及其以北地区。相较于蒸发和地表植被覆盖,流域降雨变化趋势与径流系数及其变动幅度热度指数的空间分布格局具有较好的一致性。本文可为识别变化环境下流域来水过程变异提供科学参考依据。     

Tectonic implications of transtensional supradetachment basin development in an extension‐parallel transfer zone: the Kocaçay Basin,western Anatolia,Turkey

The Kocaçay Basin (KÇB) is a key area in western Anatolia – a well‐known extended terrane where regional segmentation has received limited attention – for investigating strike‐slip faults kinematically linked to detachment faults. In this paper, we present results of an integrated sedimentologic, stratigraphic, and structural study of Miocene alluvial fan/fan‐delta/lacustrine deposits that accumulated in the KÇB, a NE‐trending basin with connections to the Menderes Metamorphic Core Complex (MCC). We mapped and evaluated most of the key faults in the KÇB, many for the first time, and recognised different deformation events in the study area near the E margin of the MCC. We also present field evidence for kinematic connections between low‐angle normal and strike‐slip faults which were developed in an intermittently active basement‐involved transfer zone in western Anatolia. We find that the KÇB contains a detailed record of Miocene transtensional sedimentation and volcanism that accompanied exhumation of the MCC. Structural data reveal that the basin was initially formed by transtension (D1 phase) and subsequently uplifted and deformed, probably as a result of early Pliocene wrench‐ to extension‐dominated deformation (D2 phase) overprinted by Plio‐Quaternary extensional tectonics (D3 phase). These results are consistent with progressive deformation wherein the axis of maximum extension remained in the horizontal plane but the intermediate and maximum shortening axes switched position in the vertical plane. Combining our results with published studies, we propose a new working hypothesis that the KÇB was a transtensional supradetachment basin during the Miocene. The hypothesis could provide new insights into intermittently active extension‐parallel zone of weakness in western Anatolia.These results also suggest that the termination of low‐angle normal fault systems within an extension parallel transfer zone may have resulted in a transtensional depressions which are different from classical supradetachment basins with respect to the sedimentation and deformational pattern of the basin infills.     

拉萨河流域高山水热分布观测结果分析

利用架设在念青唐古拉山南坡9个海拔高度(4300～5500m)的自动气象站一年(2006年8月1日-2007年7月31日)的实测数据,对山坡1.5m高度的气温和季风期(6-9月)降水随海拔梯度和时间的变化进行了分析.表明4300～4950m存在一个逆温带,逆温时间自10月至翌年4月.年逆温频率为11.5%(42天).4300～5500m年平均气温直减率为0.61℃/100m;念青唐古拉山南坡季风期各月最大降水带都在海拔5100m.最大降水高度以下,山坡降水量递增率为4～7mm/100m,最大降水高度以上,降水递减率数值上为降水递增率的1.6～2.3倍.7月和8月降水量占季风期总降水量比例大于6月和9月.降水月内分配山坡上部总体较山坡下部均匀.降水主要发生在4:00-10:00以外的时间段,而大一中雨(3～14mm/h)主要发生在18:00-22:00.山坡强降水段相对集中在4650～5100m海拔高度.     

The synrift phase of the early Domeyko Basin (Triassic,northern Chile): Sedimentary,volcanic, and tectonic interplay in the evolution of an ancient subduction‐related rift basin

The geodynamic setting along the SW Gondwana margin during its early breakup (Triassic) remains poorly understood. Recent models calling for an uninterrupted subduction since Late Palaeozoic only slightly consider the geotectonic significance of coeval basins. The Domeyko Basin initiated as a rift basin during the Triassic being filled by sedimentary and volcanic deposits. Stratigraphic, sedimentological, and geochronological analyses are presented in order to determine the tectonostratigraphic evolution of this basin and to propose a tectonic model suitable for other SW Gondwana‐margin rift basins. The Domeyko Basin recorded two synrift stages. The Synrift I (~240–225 Ma) initiated the Sierra Exploradora sub‐basin, whereas the Synrift II (~217–200 Ma) reactivated this sub‐basin and originated small depocentres grouped in the Sierra de Varas sub‐basin. During the rift evolution, the sedimentary systems developed were largely controlled by the interplay between tectonics and volcanism through the accommodation/sediment supply ratio (A/S). High‐volcaniclastic depocentres record a net dominance of the syn‐eruptive period lacking rift‐climax sequences, whereas low‐volcaniclastic depocentres of the Sierra de Varas sub‐basin developed a complete rift cycle during the Synrift II stage. The architecture of the Domeyko Basin suggests a transtensional kinematic where N‐S master faults interacted with ~NW‐SE basement structures producing highly asymmetric releasing bends. We suggest that the early Domeyko Basin was a continental subduction‐related rift basin likely developed under an oblique convergence in a back‐arc setting. Subduction would have acted as a primary driving mechanism for the extension along the Gondwanan margin, unlike inland rift basins. Slab‐induced dynamic can strongly influence the tectonostratigraphic evolution of subduction‐related rift basins through controls in the localization and style of magmatism and faulting, settling the interplay between tectonics, volcanism, and sedimentation during the rifting.     

白令海、楚科奇海、加拿大海盆沉积物重金属潜在生态风险评价

利用电感耦合等离子体质谱仪(ICP-MS仪)测定北极亚北极海区的白令海、楚科奇海、加拿大海盆海区沉积物中8种重金属元素的浓度,并对8种重金属元素的地球化学特征、空间分布特征、可能来源进行了研究。研究发现,Cr、Zn、Co、Ni、Cu、Pb、Bi有很强的相关性,说明其具有相似的来源,且空间分布的特征相似,均为沉积物底质中的浓度在研究区内呈现出在白令海和楚科奇海较低,在加拿大海较高的特征;而Cd与其他重金属空间分布差异明显。应用Hakanson潜在生态危害指数法与内梅罗综合污染指数法对各重金属的潜在风险进行了评价,其中,用Hakanson潜在生态危害指数法求得北极整体生态风险性指数值达到43.01,处于轻微生态风险状态,Co重金属元素风险最大。研究结果表明,两种评价结果得出类似规律,白令海生态安全性最高,楚科奇海大部分地区生态安全性较高,加拿大海盆的生态安全性最差。在研究区内整体上呈现出由高纬度向低纬度生态安全逐渐增高,西部生态风险性大于东部的趋势。     

闽江口潮汐盐沼湿地土壤碳氮磷的空间变化

以闽江口区最大的鳝鱼滩潮汐盐沼湿地为研究对象,选择远、近潮沟区2个不同潮水水淹地段设置样线,采集3种优势植物芦苇(Phragmites australis)、咸草(Cyperus malac-censisvar.bervifolius)和藨草(Scirpus triqueter)下18个土壤剖面分层样品,分析土壤中DOC、TN和TP等含量和储量的空间分布规律及土壤物理特征对其的影响。结果表明:远、近潮沟区3种植物下土壤表层(0~10 cm)TN浓度范围为0.24~1.91g kg-1,TP为0.21~1.34 g kg-1,DOC为13.68~93.73 mg kg-1,无论是近潮沟区还是远潮沟区,芦苇和咸草下土壤DOC、TN和TP含量十分接近,且均大于藨草下土壤DOC、TN含量,藨草下土壤TN和TP含量和储量在近潮沟区均明显大于远潮沟区,而芦苇和咸草下土壤TN和TP含量在远、近潮沟区差距不大;土壤DOC、TN和TP含量与土壤粒径、容重和含水量呈显著相关,盐度对土壤中NH4+-N和NO3--N含量影响显著,与NH4+-N为正相关关系,与NO3--N为负相关关系。     

A seismological study of normal faulting in the Demirci, Alaşehir and Gediz earthquakes of 1969–70 in western Turkey: implications for the nature and geometry of deformation in the continental crust

Summary. In this study, seismological techniques are combined with surface observations to investigate the faulting associated with three large earthquakes in western Turkey. All involved normal faulting that nucleated at 6–10 km depth with dips in the range 30–50°. The two largest earthquakes, at Alaşehir (1969.3.28) and Gediz (1970.3.28), were clearly multiple events and their seismograms indicate that at least two discrete subevents were involved in producing the observed surface faulting. In addition, their seismograms contain later, longer-period signals that are likely to represent source, not structure or propagation, complexities. These later signals can be modelled by subevents with long time functions on almost flat detachment-type faults.
As a result of these observations, we propose a model for the deformation of the lower crust, in which brittle failure of the top part occurs when high strain rates are imposed during an earthquake that ruptures right through the upper, brittle crust. Under these special circumstances, seismic motion occurs on discrete faults in the lower crust, which otherwise normally deforms by distributed creep. In the case of the normal faults studied here, motion in the uppermost lower crust takes place on shallow dipping faults that are downward continuations of the steeper faults that break to the surface. The faults thus have an overall listric geometry, flattening into a weak zone below the brittle layer at a depth that is probably dependent on the termperature gradient. This interpretation explains why detachment-type mechanisms are not seen in first motion fault plane solutions of normal faulting earthquakes, and suggests an origin for the Metamorphic Core Complexes seen in the Basin and Range Province, which probably represent flat lower crustal faults, analogous to those postulated at Alaşehir and Gediz, that have been uplifted to the surface.     

Temporal and spatial variations of δ^18O in precipitation of the Yarlung Zangbo River Basin

This paper reveals the temporal and spatial variations of stable isotope in precipitation of the Yarlung Zangbo River Basin based on the variations of δ^18O in precipitation at four stations （Lhaze, Nugesha, Yangcun and Nuxia） in 2005. The results show that δ^18O of precipitation has distinct seasonal changes in the Yarlung Zangbo River Basin. The higher value of δ^18O occurs in spring prior to monsoon precipitation, and the lower value occurs during monsoon precipitation. From the spatial variations, with the altitude-effect and rainout process during moisture transport along the Yarlung Zangbo River Valley, δ^18O of precipitation is gradually depleted. Thus, δ^18O of precipitation decreases gradually from the downstream to the upstream, and the lapse rate of δ^18O in precipitation is approximately 0.34‰/100m and 0.7%J‰/100km for the two reasons. During monsoon precipitation, spatial variation of δ^18O in precipitation is dominated by the amount effect in the large scale synoptic condition.     

Structural style and evolution of a salt‐influenced rift basin margin; the impact of variations in salt composition and the role of polyphase extension

Because salt can decouple sub‐ and supra‐salt deformation, the structural style and evolution of salt‐influenced rifts differs from those developed in megoscopically homogenous and brittle crust. Our understanding of the structural style and evolution of salt‐influenced rifts comes from scaled physical models, or subsurface‐based studies that have utilised moderate‐quality 2D seismic reflection data. Relatively few studies have used high‐quality 3D seismic reflection data, constrained by borehole data, to explicitly focus on the role that along‐strike displacement variations on sub‐salt fault systems, or changes in salt composition and thickness, play in controlling the four‐dimensional evolution of supra‐salt structural styles. In this study, we use 3D seismic reflection and borehole data from the Sele High Fault System (SHFS), offshore Norway to determine how rift‐related relief controlled the thickness and lithology of an Upper Permian salt‐bearing layer (Zechstein Supergroup), and how the associated variations in the mechanical properties of this unit influenced the degree of coupling between sub‐ and supra‐salt deformation during subsequent extension. Seismic and borehole data indicate that the Zechstein Supergroup is thin, carbonate‐dominated and immobile at the footwall apex, but thick, halite‐dominated and relatively mobile in high accommodation areas, such as near the lateral fault tips and in the immediate hangingwall of the fault system. We infer that these variations reflect bathymetric changes related to either syn‐depositional (i.e. Late Permian) growth of the SHFS or underfilled, fault scarp‐related relief inherited from a preceding (i.e. Early Permian) rift phase. After a period of tectonic quiescence in the Early Triassic, regional extension during the Late Triassic triggered halokinesis and growth of a fault‐parallel salt wall, which was followed by mild extension in the Jurassic and forced folding of Triassic overburden above the fault systems upper tip. During the Early Cretaceous, basement‐involved extension resulted in noncoaxial tilting of the footwall, and the development of an supra‐salt normal fault array, which was restricted to footwall areas underlain by relatively thick mobile salt; in contrast, at the footwall apex, no deformation occurred because salt was thin and immobile. The results of our study demonstrate close coupling between tectonics, salt deposition and the style of overburden deformation for >180 Myr of the rift history. Furthermore, we show that rift basin tectono‐stratigraphic models based on relatively megascopically homogeneous and brittle crust do not appropriately describe the range of structural styles that occur in salt‐influenced rifts.     

Tectonics and sedimentation in the hangingwall of a major extensional detachment: the Devonian Kvamshesten Basin, western Norway

The Middle Devonian Kvamshesten Basin in western Norway is a late-orogenic basin situated in the hangingwall of the regional extensional Nordfjord–Sogn Detachment Zone. The basin is folded into a syncline with the axis subparallel to the ductile lineations in the detachment zone. The structural and stratigraphic development of the Kvamshesten Basin indicates that the basin history is more complex than hitherto recognized. The parallelism stated by previous workers between mylonitic lineation below the basin and intrabasinal fold axes is only partly reflected in the configuration of sedimentary units and in the time-relations between deposits on opposing basin margins. The basin shows a pronounced asymmetry in the organization and timing of sedimentary facies units. The present northern basin margin was characterized by bypass or erosion at the earliest stage of basin formation, but was subsequently onlapped and eventually overlain by fanglomerates and sandstones organized in well-defined coarsening-upwards successions. The oldest and thickest depositional units are situated along the present southern basin margin. This as well as onlap relations towards basement at low stratigraphic level indicates a significant component of southwards tilt of the basin floor during the earliest stages of deposition. The inferred south-eastwards tilt was most likely produced by north-westwards extension during early stages of basin formation. Synsedimentary intrabasinal faults show that at high stratigraphic levels, the basin was extending in an E–W as well as a N–S direction. Thus, the basin records an anticlockwise rotation of the syndepositional strain field. In addition, our observations indicate that shortening normal to the extension direction cannot have been both syndepositional and continuous, as suggested by previous authors. Through most of its history, the basin was controlled by a listric, ramp-flat low-angle fault that developed into a scoop shape or was flanked by transfer faults. The basin-controlling fault was rooted in the extensional mylonite zone. Sedimentation was accompanied by formation of a NE- to N-trending extensional rollover fold pair, evidenced by thickness variations in the marginal fan complexes, onlap relations towards basement and the fanning wedge geometry displayed by the Devonian strata. Further E–W extension was accompanied by N–S shortening, resulting in extension-parallel folds and thrusts that mainly post-date the preserved basin stratigraphy. During shortening, conjugate extensional faults were rotated to steeper dips on the flanks of a basin-wide syncline and re-activated as strike-slip faults. The present scoop-shaped, low-angle Dalsfjord fault cross-cut the folded basin and juxtaposed it against the extensional mylonites in the footwall of the Nordfjord–Sogn detachment. Much of this juxtaposition may post-date sedimentation in the preserved parts of the basin. Basinal asymmetry as well as variations in this asymmetry on a regional scale may be explained by the Kvamshesten and other Devonian basins in western Norway developing in a strain regime affected by large-scale sinistral strike-slip subparallel to the Caledonian orogen.     

Constraining recycled detritus in quartz‐rich sandstones: Insights from a multi‐proxy provenance study of the Mid‐Carboniferous,Clare Basin,western Ireland

Quartz‐rich sandstones can be produced through multiple sedimentary processes, potentially acting in combination, such as extensive sedimentary recycling or intense chemical weathering. Determining the provenance of such sedimentary rocks can be challenging due to low amounts of accessory minerals, the fact that the primary mineralogy may have been altered during transport, storage or burial and difficulties in the recognition of polycyclic components. This study uses zircon and apatite U‐Pb geochronology, apatite trace elements, zircon‐tourmaline‐rutile indices and petrographic observations to investigate the sedimentary history of mineralogically mature mid‐Carboniferous sandstones of the Tullig Cyclothem, Clare Basin, western Ireland. The provenance data show that the sandstones have been dominantly and ultimately sourced from three basement terranes: older Laurentian‐ associated rocks (ca. 900–2500 Ma) which lay to the north of the basin, peri‐Gondwanan terranes (ca. 500–700 Ma) to the south and igneous intrusive rocks associated with the Caledonian Orogenic Cycle (ca. 380–500 Ma). However, the multi‐proxy approach also helps constrain the sedimentary history and suggests that not all grain populations were derived directly from their original source. Grains with a Laurentian or a Caledonian affinity have likely been recycled through Devonian basins to the south. Grains with a peri‐Gondwanan affinity appear to be first cycle and are potentially derived from south/southwest of the basin. Taken as a whole, these data are consistent with input into the basin from the south and southwest, with the reworking of older sedimentary rocks, rather than intensive first‐cycle chemical weathering, likely explaining the compositional maturity of the sandstones. This study highlights the need for a multi‐proxy provenance approach to constrain sedimentary recycling, particularly in compositionally mature sandstones, as the use of zircon geochronology alone would have led to erroneous provenance interpretations. Zircon, together with U‐Pb geochronology from more labile phases such as apatite, can help distinguish first‐cycle versus polycyclic detritus.