synthesis of salt tectonics in the southern north sea uk abstract a structural model encompassing the southern north sea basin west of the central graben has been developed this model consists of a rift system affecting the post salt section around the basin margin and a large area of detached compressional buckle folds within the basin this pattern is initially a response to gravity sliding of the post salt section on the salt within the basin during the late triassic to late jurassic a close relationship between the location and trend of the peripheral graben system and basement structures in the pre salt is noted pre jurassic extension across the peripheral graben systems was balanced by the sum of fault heaves at the pre salt rotliegend level and shortening across salt cored buckle folds in the post salt section salt pillows and swells passively infilled the cores of these gravity induced buckle folds cretaceous and tertiary inversion involved basin tilt and renewed movement on basin bounding basement faults notably reverse movements did not propagate from basement structures up into the peripheral graben systems the post salt sedimentary section experienced gravity spreading in response to inversion related uplift resulting in syn inversion extensional faulting in the sole pit high where the mesozoic section was thickest this extension combined with a loss of fault heave in the pre salt section was balanced by amplification of salt cored buckle folds in the centre of the basin in the context of the model described here salt pillows represent passive infill of thin skinned compressional buckle folds which later amplified during thick skinned basement shortening crestal collapse of such folds occurs via normal faulting accompanied by reactive diapirism such reactive diapirs establish conduits through which salt may leak leading to pillow deflation and ultimately conduit preservation as a salt wall flanked by rim synclines in areas where the buckle folds were emergent the salt structures described here are related to cover folds and faults which in turn reflect episodes of basin extension tilting and inversion hence individual salt structures can be said to be only remotely connected with regional intraplate stresses

the geology and mineralogy of the anuri kimberlite nunavut canada abstract the 613 6 ma anuri kimberlite is a pipelike body comprising two lobes with a combined surface area of approximately 4 5 ha the pipe is infilled with two contrasting rock types volcaniclastic kimberlite vk and less common hypabyssal kimberlite hk the hk is an archetypal kimberlite composed of macrocrysts of olivine spinel mica rare eclogitic garnet and clinopyroxene with microphenocrysts of olivine and groundmass spinel phlogopite apatite and perovskite in a serpentine calcite phlogopite matrix the ba enrichment of phlogopite the compositional trends of both primary spinel and phlogopite as well as the composition of the mantle derived xenocrysts are also characteristic of kimberlite the present day country rocks are granitoids however the incorporation of sedimentary xenoliths in the hk shows that the archean granitoid basement terrain at least locally was capped by younger proterozoic sediments at the time of emplacement the sediments have since been removed by erosion hk is confined to the deeper eastern parts of the anuri pipe it is suggested that the hk was emplaced prior to the dominant vk as a separate phase of kimberlite the hk must have ascended to high stratigraphic levels to allow incorporation of proterozoic sediments as xenoliths most of the anuri kimberlite is infilled with vk which is composed of variable proportions of juvenile lapilli discrete olivine macrocrysts country rock xenoliths and mantle derived xenocrysts it is proposed that the explosive breakthrough of a second batch of kimberlite magma formed the western lobe resulting in the excavation of the main pipe much of the resulting fragmented country rock material was deposited in extra crater deposits pyroclastic eruption s of kimberlite must have occurred to form the common juvenile lapilli present in the vks the vk is variable in nature and can be subdivided into four types volcaniclastic kimberlite breccia magmaclast rich volcaniclastic kimberlite breccia finer grained volcaniclastic kimberlite breccia and lithic rich volcaniclastic kimberlite breccia the variations between these subtypes reflect different depositional processes these processes are difficult to determine but could include primary pyroclastic deposition and or resedimentation there is some similarity between anuri and the lac de gras kimberlites with variable types of vk forming the dominant infill of small steep sided pipes excavated into crystalline archean basement and sedimentary cover

fault reactivation and structural inversion backarc and intraplate compressive deformations example of the eastern sunda shelf indonesia the tectonic evolution of the eastern sunda platform in indonesia is revealed by the study of seismic profiles and comprises three general tectonic periods during the cenozoic extension with graben infill from the paleogene to the early miocene a tectonically quiet period with only broad subsidence and a middle miocene recent period of folding inversion and thrusting these structures reveal changes in the intraplate stress field which was controlled by the changes in the boundary conditions of the plates collisions on the northern and eastern sides and interplate coupling southward along the java subduction zone inversions often occurred without reactivation of pre existing normal fault planes at the upper levels of these faults the factors controlling these deformations are discussed with special interest placed on the conditions for the potential reactivation of cohesionless normal faults inversion and reactivations were mostly controlled by the orientation of the pre existing fault planes in relation with the regional stress field and by the tectonic stress regime in a strike slip regime effective principal stresses 1 and 3 horizontal with 1 2 3 the optimum conditions for reactivation in isotropic rocks are pre existing faults that strike close to the potential two vertical conjugate failure planes the same mechanical properties as for pre existing fault prevail in such conditions strike slip movements and reverse strike slip movements and associated en echelon folds were generally observed in a compressive regime 1 and 2 horizontal with 1 2 3 typical steeply dipping normal faults are difficult to reactivate by slip movements we suggest that reverse slip movements occurred in the deeper levels of faults and were accommodated by flexure above the pre existing fault planes and the inversion of grabens the strongly inverted grabens are those whose orientation is perpendicular to the regional compression nevertheless with strong regional shortening large uplifts and thrust faults were observed with reactivation in the form of reverse or reverse strike slip movements along pre existing steeply dipping normal fault planes

upper permian zechstein supergroup carbonate evaporite platform palaeomorphology in the uk southern north sea abstract seismic interpretation geological mapping and depth conversion of the zechstein supergroup z2 cycle using high quality well calibrated three dimensional 3 d seismic has revealed the complex palaeomorphology of a deeply buried ancient carbonate shelf margin on the southwestern margin of the southern permian basin the new mapping shows that the margin comprises a series of corrugated embayments and promontories which extend up to 5 km 3 mi into the basin and locally have a palaeobathymetric relief of 200 m 650 ft well calibration across the margin demonstrates a strongly bipartite lateral thickness distribution between a high velocity anhydrite dominated shelf and a lower velocity halite dominated basin strong lateral and vertical velocity variations in the upper permian zechstein supergroup are known to have major impacts upon seismic imaging and depth conversion in the spb the resulting uncertainty remains one of the major challenges when interpreting and assessing the prospectivity of the underlying upper permian rotliegend group leman sandstone formation reservoirs in the uk southern north sea an understanding of the zechstein shelf edge s 3 d physiography and its velocity variation has implications for the delineation of traps containing the prospective reservoirs that lie below recognition of the complexity and effects of this shelf margin contributed to the recent juliet discovery whose position outside of the presently defined rotliegend group play fairway suggests that prospectivity is more extensive along the basin margin than previously thought as such the work provides a means by which to identify and delineate new structures and extend the life of this mature yet prolific gas province

structural evolution timing and tectonic style of the sole pit inversion abstract the nw se oriented sole pit high located in the western half of the southern north sea basin is the result of three different structural processes initiated during the late palaeozoic basin subsidence probably related to late hercynian pull apart movements led to continuous sedimentation from the permian to the middle jurassic mid jurassic doming of its eastern flank followed by erosion transtensional movements and differential subsidence until the early cretaceous reflect the mid and late kimmerian crustal distension and fragmentation of the north sea rift basinwide inversion occurred between the turonian and the campanian and was repeated in the oligocene resulting in a total uplift of some 1500 m the present structural style is represented by a complex network of transpressional and transtensional faults at permian rotliegend levels which are thought to represent the upward termination of a series of wrench induced flower structures with a right lateral sense of displacement these faults are detached by zechstein and triassic salt from more orderly transtensional faults systems at shallower levels in areas where salt is absent late kimmerian normal faults became reactivated in a reverse sense during inversion however the relationship of these shallow mappable tectonic components to the deeper pre permian structure of the sole pit high and to the underlying crustal configuration remains speculative

relic subduction in the southern north sea recognised from deep seismic reflection and gravity data abstract gravity models across the northern flank of the anglo brabant massif in the southern north sea constrained by interpretations of deep seismic reflection profiles suggest a structure in which the lower crust increases in thickness from south to north across a step in the moho to the south of this step a prominent reflector defines the top of a high density body in the middle crust which also has a strong magnetic signature by analogy with similar structures elsewhere the modelled structures are interpreted as having formed prior to and during the collision of armorica and baltica following southerly subduction of the tornquist ocean beneath the former in ordovician and silurian times it is hypothesised that the anglo brabant massif may have a compound origin in this collisional event and a period of northward directed subduction in the carboniferous

late cretaceous and cenozoic intra plate compressional deformations in the alpine foreland a geodynamic model abstract the mesozoic grabens and wrench induced troughs of western and central europe developed in response to triassic early cretaceous intra plate tensional stresses which affected the arctic north atlantic and the tethys borderlands during the rifting phase preceding the opening of the respective oceans this resulted in the fragmentation of the continental crust of the alpine foreland during the late cretaceous and tertiary alpine orogeny collision related compressive stresses exerted on the foreland induced the reactivation of these fracture systems causing the inversion of mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present alpine deformation front the structural style of these inverted basins and basement uplifts is indicative of compressional and transpressional deformations the total amount of crustal shortening associated with these deformations is unlikely to exceed a few tens of kilometres these displacements require a coupling between the foreland and the orogen at the alpine a subduction zones and within the foreland a decoupling at intra crustal levels between the crust and mantle and or at deeper lithospheric levels the example of the alpine foreland indicates that compressive stresses inducing major deformations can be transmitted from a collisional plate boundary through continental lithosphere over distances of up to 1300 km circumstantial evidence suggests that collision related compressive stresses not large enough to give rise to significant deformations can be transmitted through continental lithosphere over even greater distances

joint development in perturbed stress fields near faults abstract field evidence is presented for complex spatial and temporal perturbations of an otherwise systematic joint pattern around faults from well exposed faulted rock platforms joints propagating in perturbed stress fields will curve to follow the directions of the stress field trajectories a progressive change in joint direction is observed from unperturbed regions away from faults to strongly perturbed zones adjacent to faults this indicates that the joint pattern can reflect perturbations of the regional stress field around faults in the examples the stress field perturbations are probably due to points of high friction on the fault plane which concentrate stress and distort the stress field in the surrounding rock the corresponding joints converge at these points and are sub parallel to the fault along the remainder of the fault plane the possibility that a fault plane acts as a free surface contained within an elastic body is considered in this situation the fault plane induces a rotation of the principal stress axes to become either perpendicular or parallel to the fault the free surface model seems to explain the metre scale curvature of joints in the vicinity of existing joints but at the kilometre scale of a large fault plane the model becomes unrealistic unless the fault is open at the earth s surface two examples are investigated from the lias of great britain at nash point and robin hood s bay both comprise sub horizontal strata of relatively homogeneous lithology and bed thickness which provide striking examples of joints developed near faults

thermal and tectonic history of the east midlands shelf onshore uk and surrounding regions assessed by apatite fission track analysis apatite fission track analysis has been used to study the thermal and tectonic history of the east midlands shelf and surrounding areas of onshore uk apatites from outcrops of palaeozoic crystalline basement give fission track ages between 300 29 and 420 31 ma which are the oldest yet reported in the uk region and reflect the relative stability of this cratonic region mean track lengths in these samples are between 12 and 13 m namurian and triassic sandstones from outcrop give apatite fission track ages between c 100 and c 390 ma with mean track lengths generally between 10 and 12 5 m distributions of single grain ages and track lengths suggest that all samples have been annealed implying exposure to higher temperatures since deposition with maximum palaeotemperatures reached some time in the 50 80 ma range prior to the onset of cooling samples from five exploration wells show consistent patterns in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface to values in the 50 80 ma range at depths of c 1 2 km present temperatures of c 40 70 c showing that the samples studied have all been exposed to higher temperatures single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before c 60 ma subsequent cooling was protracted and the data are compatible with two phases in the early and mid tertiary palaeotemperature analysis suggests that palaeo geothermal gradients were similar to present values and that elevated palaeotemperatures were due to greater depth of burial with cooling due to subsequent uplift and erosion estimates of uplift and erosion vary from c 1 1 5 km on the east coast around the humber estuary to c 2 2 km in the east midlands and southern pennines compilation of palaeotemperature estimates from all samples analysed from the uk onshore region shows that mesozoic burial and tertiary uplift has affected a wide area and that uplift was due to broad scale regional warping with recognized inversion axes being areas where uplift and erosion were particularly pronounced or where younger sediments and or inverted basinal structures allow the identification of uplift and erosion the recognition of late cretaceous early tertiary unconformities across wide areas of northwest europe both onshore and offshore suggests that the regional uplift and erosion may have been far more widespread than previously recognized this has important implications for hydrocarbon exploration in northwest europe

cenozoic structural inversion from transtension to transpression in yingxiong range western qaidam basin new insights into strike slip superimposition controlled by altyn tagh and eastern kunlun faults abstract a cenozoic structural inversion event from transtension to transpression involving salt tectonics has been uncovered in the yingxiong range the western qaidam basin seismic reflection data show that there are two common structural styles in the yingxiong range 1 the positive flower structure 2 the thrust controlled fold at shallow depth and the positive inverted flower structure at deep levels which are separated by a salt layer in the upper xiaganchaigou formation the yingxiong range experienced a first stage of transtension in the eocene induced by the altyn tagh fault and a second stage of transpression from the early miocene to present jointly controlled by the altyn tagh and eastern kunlun faults the eocene transtension produced numerous nw striking right stepping en echelon transtensional normal faults or fractures in the yingxiong range at the same time evaporites and mudstone were deposited in the vicinity of these faults in the early miocene the eocene transtensional normal faults were reactivated in a reverse sense and the thrust controlled folds at shallow depth started to form simultaneously with transpression enhanced in the late cenozoic positive flower structures directly formed in places without evaporites the cenozoic transtension to transpression inversion of the yingxiong range is the result of strike slip superimposition controlled by the altyn tagh and eastern kunlun faults in time and space

inversion tectonics and magnetic fabrics in mesozoic basins of the western tethys a review abstract positive tectonic inversion of sedimentary basins has been recognized as one of the primary mechanisms of mountain building and intraplate deformation reconstructing the tectonic history of basins is relatively easy for the inversion stage but becomes more difficult for the basinal stage especially when strong deformation involving cleavage development is associated with the subsequent compressional tectonics since tectonic markers for the extensional episodes are not commonly well developed anisotropy of magnetic susceptibility ams has provided recently a tool for analyzing early stages in the evolution of sedimentary basins even in the absence of other outcrop scale mesostructures here we expose and discuss the applicability of magnetic fabrics by means of ams to different types of intra plate sedimentary basins in the western tethys region formed under extensional or transtensional regimes and which underwent different inversion styles total or partial inversion with or without cleavage development forming part of compressional thrust sheets etc owing to specific particular p t conditions and structural controls factors such as lithology magnetic mineralogy position within the sedimentary pile and deformation intensity are key to interpret the obtained magnetic fabrics in terms of tectonic evolution a basin classification is proposed according to inversion styles and magnetic fabrics where inversion did not involve cleavage development magnetic lineation is parallel to the stretching direction corresponding to the extensional stage the transition between non cleaved to inversion related cleaved units is marked by the switch of magnetic lineations from parallel to extension to parallel to the intersection lineation between cleavage and bedding these relationships are enhanced when extension and compression are roughly coaxial then favoring the clustering of axes of the magnetic ellipsoid even when extreme inversion occurs and the early extensional fabric is obliterated magnetic fabrics provide information about the interaction between preferred deformation directions associated with the main stages in basin evolution

salt as a 3d element in structural modeling example from the central european basin system abstract based on a well constrained 3d structural model of the central european basin system 3d modeling has been carried out to understand the regional scale regularities between movements of the upper permian zechstein salt and the major post permian tectonic events the 3d reconstruction of the paleo thickness of upper permian salt at the end of the permian period demonstrates that areas of thickened upper permian salt correlate with the present day distribution of the largest salt structures within the central european basin system this spatial correlation implies that the distribution of salt thickness at the end of deposition partially controlled the structural style of the basin during post permian phases of salt tectonics moreover the results of 3d salt redistribution for post permian times indicate that the initiation of salt movements took place in the triassic the triassic extensional events triggered several phases of salt movements within the coeval depocenters of the central european basin system such as the glueckstadt graben the horn graben the norwegian danish basin the fjerritslev trough and the himmerland graben and the polish basin during the late jurassic early cretaceous extensional transtensional tectonic event the strongest salt movement occurred within the central graben the lower saxony basin and the pompeckj block the intensity of the late jurassic early cretaceous phase of salt tectonics is essentially declining from the marginal areas of the central european basin system towards the horn graben the glueckstadt graben and adjacent areas the late early cretaceous early late cretaceous is characterized by tectonic quiescence without strong salt movements over the entire study area the next regional phase of salt movements was triggered by late cretaceous early cenozoic inversion involving almost the entire central european basin system in terms of renewed salt movements inversion related thick skinned salt tectonics is observed along the nw se trending elbe fault system and the teisseyre tornquist zone where strongest compressional deformations were localized on the contrary a thin skinned character of salt movements was prevailing within the largest part of the north german basin post inversion cenozoic subsidence was also accompanied by salt activity related either to diapiric rise due to regional shortening and or to local roughly e w directed extension the cenozoic phase of salt movements was especially prominent within the marginal troughs of the glueckstadt graben

structural evolution of inverted basins in the dutch offshore abstract the structural evolution of the dutch central north sea graben and the fault bounded broad fourteens basin is discussed graben formation started during the early triassic and culminated in strong differential subsidence during the late jurassic early cretaceous narrow fault bounded grabens and troughs and broad intervening highs were formed as a result of crustal distension and associated transtensional deformations the thickness of the late jurassic to early cretaceous fill of these basins may have exceeded 3000 m concurrent with the filling of these basins the bordering highs became deeply truncated during the late cretaceous and early tertiary compressive stresses resulted in inversion of these basins whereby movements along their bounding normal faults were reversed giving rise locally to overthrusting as for instance in the broad fourteens and west netherlands basins the timing of these compressional deformations has been palaeontologically dated and coincides with major orogenic events in the alps

the pyrenean inversion phase in northern belgium an example of a relaxation inversion the analysis of 2d seismic and well data provides new insights into the late eocene to earliest oligocene dynamics along the southern border of the north sea area belgium from the start of the priabonian onwards the northwestern part of the campine basin and the london brabant massif to its west experienced subsidence and developed into a shallow trough simultaneously several other southern north sea basins including the central and eastern part of the campine basin and the roer valley graben were inverted by what is generally referred to as the pyrenean inversion phase inversion caused broad flexural uplift and minor reverse fault movements the characteristics of inversions in the southern north sea basins are very similar to each other and to those described for a phase of intraplate stress relaxation the results of this study therefore suggest that the pyrenean inversion phase was triggered by a regional stress relaxation that started around the bartonian priabonian boundary and ended before the onset of the oligocene