patterns of imbricate thrusting through the use of seismic reflection profiles satellite images and balanced kinematic models we describe patterns that help to identify imbricate structures and define the sequence of thrusting in fold and thrust belts measures of folding shear strains are employed to define break forward systems in which younger deeper faults refold overlying thrust sheets in contrast break backward sequences form a wide range of viable geometries that are not expected to conserve these folding strains thus measures of folding shear strains provide a means to define thrusting sequence we also explore map view patterns of imbricate thrusting by using three dimensional models and stereoscopic satellite images we present an example from the peruvian andes in which these map patterns are combined with seismic reflection profiles to define a complex imbricate system with components of both break forward and break backward thrusting these integrated methods provide new tools for understanding the deformational histories of fold and thrust belts including the definition of complex structural closures with opportunities for hydrocarbon reservoir duplication

shallow high resolution seismics and reprocessing of industry profiles in southern bavaria the molasse and the northern alpine front abstract the purpose of this seismic investigation in the upper bavarian miesbach area as part of the international transalp project was to study the tectonic contact between the autochthonous foreland molasse and the allochthonous folded molasse marking the tectonic front of the alpine orogen another specific target was the dip of the frontal emerging main thrust of the tectonic units helveticum ultrahelveticum and rhenodanubian flysch overriding the folded molasse twelve seismic profiles obtained from the hydrocarbon industry were reprocessed from the foreland molasse southward to the autochthonous molasse in the subsurface of the overthrust folded molasse conspicuous features such as steep normal faults at the molasse base s directed thickening of molasse sediments or sedimentary discordant base of upper marine molasse can be recognized shallow high resolution seismic measurements were conducted along two profiles across the tectonic contact between foreland molasse and folded molasse as well as along a profile across the frontal emerging main thrust of the helveticum ultrahelveticum and the rhenodanubian flysch geological structures could be identified in the top 300 500 ms two way traveltime interval which is hardly possible with the usual deep seismic method the method thus provides a bridge between deep reflection seismics and surface geology in contrast to the western bavarian molasse zone the tectonic boundary between the foreland molasse and the folded molasse in the investigated area is not characterized by a large blind thrust triangle zone but by a simple south dipping thrust plane adjacent to the s follow several steeply south dipping inverse molasse thrust slices and the miesbach syncline the inverse thrust slices are interpreted as the overturned and sheared northern limb of a fault propagation fold which linked the folded molasse to the foreland molasse during a final orogenic phase the main thrust of the helveticum ultrahelveticum and the rhenodanubian flysch are well imaged in the near surface interval of the high resolution reflection seismic data in contrast to previously published results these thrust planes show a gentle dip to the s from the surface down to at least 500 1000 m depth

duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

intraplate extension prior to continental extrusion along the ailao shan red river shear zone left lateral movement of the ailao shan red river shear zone lends support to the hypothesis of continental extrusion resulting from the collision of india with asia our new observations from northwestern yunnan china and northwestern vietnam on different sides of the shear zone demonstrate that the sinistral offset was 600 km according to correlations of permian triassic flood basalt successions and late paleogene highly potassic mafic magmas we conclude that the shear was propagating on the south china continental margin and does not correspond to a suture between south china and indochina furthermore the highly potassic magmas were emplaced from ca 40 to 30 ma before the shear movement which was caused by the late oligocene to early miocene ca 27 22 ma extrusion activity this suggests that a late eocene to early oligocene intraplate extension possibly induced by delamination of thickened continental lithosphere took place in northwestern yunnan or eastern tibet as a response to the india asia collision this extension and sea floor spreading of the south china sea that began ca 30 ma could have accounted for the initiation of the ailao shan red river shear zone

geometric and seismic interpretation of the perdido fold belt northwestern deep water gulf of mexico we interpret the perdido fold belt to be composed of detachment folds flanked by megascopic scale kink bands the two previous principal fold models for the perdido fold belt incorporated components of large scale faulting an imbricate fault bend fold model and a high angle reverse faulted detachment fold model we interpret the perdido fold belt to be fold dominated the belt consists of detachment folds overlying a ductile decollement layer the detachment fold crests are bounded by kink bands which are narrow zones of angularly folded strata the low reflectivity low signal to noise zones seen in seismic profiles across the perdido fold belt are poorly imaged sections of the folds that result from steeply dipping bedding in the kink bands the substantial width of these low reflectivity zones their subparallel edges in cross section and their orientation at a high angle to layering as well as their organized spacing and conjugate geometry support a kink band interpretation propagating reverse faults are unlikely in the folds because of the absence of backlimbs overlying the locations of hypothetical faults improved seismic imaging efforts have illuminated steeply dipping bedding that is not cut by faulting in many of the low reflectivity zones in the perdido fold belt

age of initiation of the india asia collision in the east central himalaya abstract we document the stratigraphy and provenance of the lower tertiary terrigenous sections in the zhepure shan region of the tethyan himalaya southern tibet using petrographic and geochemical whole rock and single grain techniques the cretaceous early tertiary shelf deposits of shallow marine carbonates and siliciclastics of the former indian passive margin near the western end of the zhepure shan are conformably overlain by lower tertiary clastic rocks sandstones in the jidula formation paleocene mostly contain monocrystalline quartz grains of cratonic origin in contrast significant amounts of immature framework grains with a distinct ophiolitic and volcanic arc influence are present in the youxia early eocene and shenkeza post early eocene formations major trace and rare earth element concentrations in both sandstones and shales complement the petrographic data and indicate that the source of the jidula formation consisted primarily of quartzose basement rocks probably of indian con

strike slip duplexes abstract strike slip fault systems often contain zones of steep imbricate faults geometrically similar to imbricate fans and duplexes in dip slip thrust and normal fault systems they are evident in map view rather than in vertical sections examples of duplexes are cited from both active and ancient systems and from theoretical and physical models duplexes may form at bends on strike slip faults by a process kinematically analogous to the sequential imbrication of ramps on dip slip faults however some may form and many may initiate as non sequential riedel fractures at fault offsets or on straight fault segments this process is more marked than in dip slip systems where primary anisotropy such as bedding exerts more control on fault geometry strike slip duplexes may be shunted along the fault system parallel to the regional slip vector however duplexes or individual horses will usually also move up or down perpendicular to the slip vector because of the unconstrained upper surface to the fault system these factors mean that no section through a strike slip system should be expected to area balance the faults of strike slip duplexes and imbricate fans may root in kinematically necessary low dip faults or may converge downwards and appear in vertical sections as flower structures

transpressive duplex and flower structure dent fault system nw england abstract revised mapping along the dent fault northwest england has improved the resolution of folds and faults formed during variscan late carboniferous sinistral transpression a nne trending east down monocline comprising the fell end syncline and taythes anticline was forced in carboniferous cover above a reactivated precursor to the dent fault within the lower palaeozoic basement the taythes anticline is periclinal due to interference with earlier acadian folds the steep limb of the monocline was eventually cut by the west dipping dent fault the hangingwall of the dent fault was dissected by sub vertical or east dipping faults together forming a positive flower structure in cross section and a contractional duplex in plan view the footwall to the dent fault preserves evidence of mostly dip slip displacements whereas strike slip was preferentially partitioned into the hangingwall faults this pattern of displacement partitioning may be typical of transpressive structures in general the faults of the taythes duplex formed in a restraining overlap zone between the dent fault and the rawthey fault to the west the orientations of the duplex faults were a response to kinematic boundary conditions rather than to the regional stress field directly kinematic constraints provided by the dent and neighbouring variscan faults yield a nnw sse regional shortening direction in this part of the variscan foreland

restoration of the cretaceous uplift of the harz mountains north germany evidence for the geometry of a thick skinned thrust reverse movement on the harz northern boundary fault was responsible for the late cretaceous uplift of the harz mountains in northern germany using the known geometry of the surface position and dip of the fault and a published cross section of the base permian horizon we show that it is possible to predict the probable shape of the fault at depth down to a detachment level we use the inclined shear method with constant heave and argue that a shear angle of 30 was most likely in this case the detachment level is at a depth of ca 25 km kinematic restoration of the harz mountains using this fault geometry does not produce a flat horizon rather it results in a ca 4 km depression airy heiskanen isostatic equilibrium adjustment of the harz mountains restores the base permian horizon to the horizontal as well as raising the moho to a depth of 32 km a typical value for northern germany restoration also causes a rotation of tectonic fabrics within the harz mountains of about 11 clockwise we show that this model geometry is very good fit to the interpreted dekorp basin 9601 deep seismic profile

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

structural evolution of the day nui con voi metamorphic complex implications on the development of the red river shear zone northern vietnam the day nui con voi dncv metamorphic complex in north vietnam is the southernmost high grade metamorphic zone along the nw se trending red river shear zone rrsz in indochina the rrsz was considered as a classical large scale continental strike slip fault that had played a significant role in the continental extrusion of southeast asia since the collision of india and eurasia earlier ideas determined the rrsz as a steep shear zone that penetrated the entire lithosphere both metamorphism and structures within rocks along the dncv metamorphic complex have been previously thought to be formed syn tectonically by left lateral shearing of the rrsz during the oligocene miocene continental escape tectonics however our meso and microstructural re examination of this region shows that these metamorphic rocks were formed during earlier tectonic episodes unrelated to strike slip shearing high angle to near orthogonal overprinting fabrics indicated that this region recorded three episodes of ductile deformation followed by brittle faulting events with different intensity spanning from the triassic to the tertiary d1 is preserved as nw se striking upright folds under garnet grade regional metamorphism during the triassic indosinian orogeny as south china block amalgamated with the indochina block the large scale horizontal d2 folds with a dominant top to n nw bottom to s sw sense of shear and sub horizontal fold axial planes suggest that the dncv metamorphic complex remained at midcrustal depths since the indosinian orogeny the youngest ductile deformation event d3 refolded d2 recumbent folds into a dome and uplifted the dncv as lower temperature fabrics s3 indicated steep mylonite zones with left lateral kinematic indicators and brittle faulting were developed on both limbs of the dome along the steep song hong and song chay faults during left lateral movement of the rrsz our new spatial temporal and kinematic correlations of metamorphic fabrics within the dncv metamorphic complex support the suggestion that the rrsz developed after regional metamorphism and remained purely a crustal fault the complicated deformation history recorded within the dncv metamorphic complex provides an alternative interpretation and suggests that crustal scale strike slip faults such as the rrsz need not root from the mantle

seismic interpretation and structural restoration of the heligoland glaciotectonic thrust fault complex implications for multiple deformation during pre elsterian to warthian ice advances into the southern north sea basin abstract despite a long history of research the locations of former ice margins in the north sea basin are still uncertain in this study we present new palaeogeographic reconstructions of pre elsterian and warthian ice margins in the southeastern north sea basin which were previously unknown the reconstructions are based on the integration of palaeo ice flow data derived from glaciotectonic thrusts tunnel valleys and mega scale glacial lineations we focus on a huge glaciotectonic thrust complex located about 10 km north of heligoland and 50 km west of the north frisian coast of schleswig holstein northern germany multi channel high resolution 2d seismic reflection data show a thrust fault complex in the upper 300 ms twt ca 240 m of seismic data this thrust fault complex consists of mainly neogene delta sediments covers an area of 350 km2 and forms part of a large belt of glaciotectonic complexes that stretches from offshore denmark via northern germany to poland the deformation front of the heligoland glaciotectonic complex trends approximately nne ssw the total length of the glaciotectonic thrust complex is approximately 15 km the thrust faults share a common detachment surface located at a depth of 250 300 ms twt 200 240 m below sea level the detachment surface most probably formed at a pronounced rheological boundary between upper miocene fine grained pro delta deposits and coarser grained delta front deposits although we cannot rule out that deep permafrost in the glacier foreland played a role for the location of this detachment surface restored cross sections reveal the shortening of the complex along the detachment to have been on average 23 ranging from ca 16 50 the determined ice movement direction from east southeast to southeast suggests deformation by an ice advance from the baltic region the chronospatial relationship of the thrust fault complex and adjacent northwest southeast to northeast southwest trending elsterian tunnel valleys implies a pre elsterian mis 16 age of the glaciotectonic complex however the age of these elsterian tunnel valleys is poorly constrained and the glaciotectonic complex of heligoland may also have been formed during an early elsterian ice advance into the southeastern north sea basin the glaciotectonic complex underwent further shortening and the elsterian tunnel valley fills that were incised into the glaciotectonic complex were partly deformed during the saalian drenthe and warthe 1 ice advances

simulation of foreland basin stratigraphy using a diffusion model of mountain belt uplift and erosion an example from the central alps switzerland foreland basin stratigraphy can be considered as the result of three interacting processes thrust deformation which builds the tectonic load sedimentary and erosional processes which redistribute that load and the flexural response of the lithosphere the resultant stratigraphy of foreland basins is commonly composed of a small number of shallowing and coarsening upward cycles bounded by regional unconformities to understand the development of these unconformities we present a simple model of these three processes coupling an evolving thrust wedge on a linear elastic plate with erosion and sedimentation defined by the diffusion equation applied to topography our model demonstrates the development of regional unconformities without recourse to either eustasy or complex viscoelastic models for the continental lithosphere the model describes the thrust wedge foreland basin system in terms of four parameters 1 the effective elastic thickness of the foreland plate te 2 the sediment transport coefficient k 3 the thrust wedge advance rate 4 the surface slope of the thrust wedge the model is applied to the oligocene miocene north alpine foreland basin nafb of eastern switzerland the stratigraphy of the nafb can be simplified into two large scale shallowing upward cycles separated by an unconformity at the base of the burdigalian 22 ma geological information is taken from the nafb to estimate suitable values for the parameters listed above assuming a linear elastic lithospheric rheology the te value is estimated at 10 5 km from decompacted sediment columns data to constrain the sequential development of the thrust wedge come from structural geology the early stages 40 24 ma of compression involved a relatively low angle thrust wedge with an advance rate of approximately 2 4 mm yr at about 24 ma the wedge advance slowed down and thickened by underplating crystalline basement of the foreland plate the value for the transport coefficient has been estimated from previous studies prior to attempting to simulate the broad scale geometry of the nafb the role of each parameter was assessed individually the values for te and k are held constant throughout the simulation of the nafb at 7 5 km and 500 m yr respectively the geometry of the base burdigalian unconformity is reproduced by variations in the parameters describing the thrust wedge the surface slope angle of the wedge is increased from 2 5 to 4 over 0 2 m y and the rate of thrust advance is decreased from 2 mm yr to 0 2 mm yr during the thickening event between 24 and 23 8 ma the rejuvenation of the internal parts of the thrust load causes backtilting of the foreland basin sediments and after a time lag erosion of the distal stratigraphy over the forebulge so simulating the base burdigalian unconformity

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

kinematics and ar 40 ar 39 geochronology of the gaoligong and chongshan shear systems western yunnan china implications for early oligocene tectonic extrusion of se asia the gaoligong and chongshan shear systems glss and csss in western yunnan china have similar tectonic significance to the ailaoshan red river shear system asrrss during the cenozoic tectonic development of the southeastern tibetan syntaxis to better understand their kinematics and the cenozoic tectonic evolution of se asia this paper presents new kinematic and 40 ar 39 ar geochronological data for these shear systems all the structural and microstructural evidence indicate that the glss is a dextral strike slip shear system while the csss is a sinistral strike slip shear system and both were developed under amphibolite to greenschist grade conditions the 40 ar 39 ar dating of synkinematic minerals revealed that the strike slip shearing on the glss and csss at least began at 32ma possibly coeval with the onset of other major shear systems in se asia the latestage shearing on the glss and csss is dated at 27 29ma by the biotite 40 ar 39 ar ages consistent with that of the wang chao shear zone wcsz but 10ma earlier than that of the asrrss the dextral gaoligong shear zone within the glss may have separated the india plate from the indochina block during early oligocene combined with other data in western yunnan we propose that the baoshan southern indochina block escaped faster southeastward along the csss to the east and the glss to the west than the northern indochina block along the asrrss accompanying with the obliquely northward motion of the india plate during early oligocene 28 36ma during 28 17ma the northern indochina block was rotationally extruded along the asrrss relative to the south china block as a result of continuously impinging of the india plate

structure kinematics and ages of transpression during strain partitioning in the chongshan shear zone western yunnan china abstract the chongshan shear zone extends from the eastern himalayan syntaxis to the lincang granitic pluton in yunnan province china the structure and kinematics show that the shear zone comprises mainly of mylonitic gneiss migmatite and schist with a dextral dominated strike slip motion in an n s trending northern segment and a sinistral strike slip shear in nw se trending middle and southern segments both were developed under a bulk regional scale sinistral transpression shrimp and la icpms u pb and 40 ar 39 ar dating reveal two tertiary magmatic events along the zone followed by younger sinistral strike slip shear the eocene magmatic event c 55 38 ma followed by metamorphism at c 36 ma happened before the strike slip motion the strike slip shear along the zone began c 32 ma which generated shear heating from 32 to 22 ma the 40 ar 39 ar ages of syn kinematic micas range from 19 to 14 ma and indicate that the strike slip shear continued to this time with coeval transpressional exhumation and uplift of the metamorphic rocks along the biluoxueshan chongshan chain the chongshan zone is thus a cenozoic shear zone which was contemporaneous with motion on the left lateral ailao shan red river shear zone and the right lateral gaoligong shear zone and should be important in accommodating the northwards movement of india during collision during oligocene to miocene times the continental block that was extruded between the ailao shan red river and gaoligong shear zones was dismembered into at least two major fragments by the chongshan shear zone

40ar 39ar dating of strike slip motion on the tan lu fault zone east china abstract ne trending sinistral ductile shear belts in the southern part of the tan lu fault zone east china are exposed along the eastern edge of the northern dabie belt and southern zhangbaling belt three hornblende three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40ar 39ar method in this study a hornblende plateau age of 143 3 ma is interpreted as representing the time of sinistral motion on the zhangbaling part of the tan lu fault zone seven biotite samples from the same part yield 40ar 39ar cooling ages from 137 to 125 ma a phengite plateau age of 138 8 ma from the eastern edge of the dabie belt is considered as being close to the time of deformation in the tan lu strike slip zone five biotite 40ar 39ar ages from 120 to 110 ma and one phengite age of 121 ma from the same area represent cooling ages after the sinistral faulting it is concluded on the basis of these 40ar 39ar ages that the sinistral ductile shear belts in the southern part of the tan lu fault zone were produced in earliest early cretaceous this sinistral motion on the tan lu fault zone is considered to be related to high speed highly oblique subduction of the pacific plate at this time

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

northwest trending middle cenozoic left lateral faults in southern yunnan china and their tectonic significance abstract the northwest striking jianshui qujiang and chuxiong faults and probably the zhongdian fault south china show evidence for different amounts of middle cenozoic pre pliocene and post early paleogene left lateral displacement that range from 6 to 25 km the age and orientation of the left lateral faults suggest that the faults are related to a regional deformational event associated with important left lateral shear on the ailao shan shear zone which lies 50 70 km to the south identification of these faults extends left lateral shear during this deformational event farther to the northeast than previously recognized unlike the ailao shan shear zone these faults do not appear to be part of a through going nw trending shear zone these faults are interpreted to be the northernmost part of a nw trending boundary whose main structure the left lateral ailao shan shear zone forms a broad zone of mylonitic rocks bounding a crustal fragment that rotated clockwise around the eastern himalayan syntaxis following middle cenozoic left lateral displacement only the jianshui and qujiang faults exhibited a reversal of displacement in post miocene time this reversal is interpreted to be the result of counterclockwise rotation within the broad n s striking left lateral xiaojiang fault zone none of the post miocene displacement on these two faults appears to be related to post miocene right lateral displacement on the red river fault system the chuxiong fault may have reversed its early left lateral displacement but evidence from surface geology for young and active right lateral displacement is not clear the zhongdian fault appears to have undergone only left lateral displacement some of which may be middle cenozoic in age and some post miocene in age active displacement on the zhongdian fault is interpreted to mark the eastern boundary for a small crustal fragment that rotates clockwise around the eastern himalayan syntaxis

exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone 1 the nordfjord sogn detachment zone nsdz is widely cited as one of the primary structures responsible for the exhumation of norwegian ultra high pressure uhp rocks here we review data from the considerable volume of research describing this shear zone and compile a strikeparallel cross section along the nsdz from the solund basin in the south to the soroyane uhp domain in the north this cross section highlights several previously unrecognized patterns revealing a shear zone with top to the west asymmetric fabrics that 1 initiated at amphibolite facies 2 overprints metamorphic breaks and tectonostratigraphic contacts and 3 has a gradational continuum of muscovite cooling ages these patterns constrain the kinematic evolution of the nsdz and suggest a new three step model for the exhumation of norwegian u hp rocks the initial stages of exhumation were characterized by the rise of crustal rocks from u hp depths to the base of the crust by buoyancydriven mechanisms not specified in this paper mantle exhumation was followed by top to thewest normal sense displacement within a broad noncoaxial ductile shear zone near the base of the crust that overprinted tectonostratigraphic contacts formed previously during mantle exhumation in the final stages of crustal exhumation top w brittleductile detachments soled into and partially excised this ductile shear zone dropping the devonian basins into contact with rocks of varying tectonostratigraphic levels this new interpretation of the nsdz is significant as it accounts for the extreme crustal excision observed in western norway using three sequentially overprinting structures active at different stages of uhp rock exhumation citation johnston s m b r hacker and t b andersen 2007 exhuming norwegian ultrahigh pressure rocks overprinting extensional structures and the role of the nordfjord sogn detachment zone tectonics 26 tc5001

age and origin of magmatism along the cenozoic red river shear belt china to decipher the geodynamic significance of cenozoic magmatism along the red river shear belt geochemical analyses u pb and rb sr dating and pb sr nd isotope tracing were undertaken zircon monazite titanite and a ti u oxide from foliated granitoid intrusions in the shear belt gneisses yield u pb emplacement ages of 33 1 0 2 2 31 9 0 3 25 8 0 2 and 24 7 0 2 ma and an age of 35 0 0 3 ma was obtained for the roughly 100 km long adjacent jinping phan si pang alkali granite together with our previous data the new ages suggest that magmatism and left lateral strike slip movements occurred coevally during latest eocene oligocene times from 33 to 22 ma the rb sr dating of muscovite and biotite from the northernmost gneisses indicates that cooling to 500 c occurred at 52 6 1 1 ma pre dating the onset of magmatism whereas further cooling to 300 c took place at 28 9 0 6 this shows that unroofing in the north took place almost 9 million years earlier than in the central gneiss segments of the shear zone geochemical data substantiate two types of magmas 1 amphibole bearing intrusions of alkaline trend which are derived from sources with isr 0 7065 0 7089 and i nd 3 7 to 6 6 2 leucogranitic layers and bodies having isr 0 7084 0 7354 and i nd 3 3 to 13 4 the former type of intrusion is found in both the gneisses and the adjacent unmetamorphosed cover rocks whereas leucogranites are restricted to the shear belt gneisses source signatures of the alkaline intrusions lie adjacent to the those of oib plotting at the lower end of the mantle array contamination of these melts by continental material seems to be very limited on the other hand the leucogranitic layers are essentially crustal derived but none of the them has country rock isotope signatures requiring melting of crust different from the actually exposed gneisses magma sources similar to those of ocean island basalt indicate magmatism to involve melting of light rare earth element and large ion lithophile element enriched mantle domains most likely present in the lithosphere underneath the region since lithospheric thickening or subduction can be ruled out to produce both types of magmas the presence of an important thermal anomaly is required which is coevally active with left lateral strike slip shear adiabatic decompression and melting within the rising anomaly is the most plausible mechanism to produce the mantle magmas which successively migrate through the crust to induce anatectic melting at 20 15 km crustal depth alkaline magmas largely dominate the volume of magmatism along the belt being continuously present in the shear zone for millions of years such lubrication potentially explains how very large amounts of displacement can be absorbed in surprisingly narrow shear zones such as the red river belt possibly also playing a role for where and when zones of plate scale lateral extrusion develop

experimental insights on the geometry and kinematics of fold and thrust belts above weak viscous evaporitic decollement abstract this paper discusses the differences in mechanical properties and kinematics between fold and thrust belts detaching on evaporitic decollements and those detaching on stronger detachments physical experiments are described that model shortening of a thick brittle cover overlying a weak viscous decollement to gain a better understanding of these differences we tested the influence of 1 the decollement layer thickness and 2 the presence of a deformable backstop on the hinterland side and of a decollement pinch out on the foreland side because of the very low shear strength of the viscous decollement folds and thrusts did not propagate according to a piggy back sequence but by centripetal back and forth propagation additional shortening was accommodated by growth of all existing structures fold symmetry and thrust vergence varied between experiments models confined between two rigid vertical endwalls always deformed by symmetric folding and thrusting in models having a deformable backstop and a foreland decollement pinch out forethrusting initially dominated folds were asymmetric and fault blocks rotated in models having a thick decollement layer folds kept growing asymmetrically diapirism also depended on the initial decollement geometry diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault related fold finally the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

lateral extrusion in transpression zones the importance of boundary conditions the homogeneous transpression strain model formulated by sanderson and marchini journal of structural geology 6 449 458 1984 has proved to be a useful tool in the analysis of complex three dimensional deformation patterns however some of the boundary conditions introduced in the mathematical model may be unrealistic and unnecessarily restrictive in this paper a strain matrix for unconfined transpression and transtension is derived in which material is allowed to move not only vertically but also horizontally in and out of the deforming zone parallel to its length lateral extrusion three end member plane strain components are defined wrench simple shear pure shear in xy lateral stretch and pure shear in yz vertical stretch these biaxial strains can be viewed as the apices of a strain triangle for unconfined transpression or transtension the edges of the triangle correspond to triaxial pure shear non coaxial biaxial lateral extrusion and the triaxial confined transpressional or transtensional strain of sanderson and marchini during unconfined transpression the orientation and in particular the geometry k value of the finite strain ellipsoid depends upon not only the amount of shortening across the zone and the amount of strike slip parallel to the zone but also upon the ratio of vertical to lateral stretch this can present serious difficulties when attempting to use finite strains to infer the direction and magnitude of zone boundary displacements examples of transpression zones in which there is evidence of a component of lateral extrusion are described from sw cyprus and central scotland these examples illustrate that antithetic strike slip shearing is a kinematic requirement of laterally unconfined transpression implying that synchronous shear sense indicators may give opposite senses of movement in shear zones specific geometric and mechanical boundary conditions together with internal fault zone rheologies may favour the lateral extrusion of material kinematic partitioning can occur to form fault bounded domains in which end member biaxial and or non coaxial strains are developed analysis of such domains can give a clearer understanding of regional scale triaxial deformations these findings illustrate the importance of establishing displacement boundary conditions when qualitatively or quantitatively analysing crustal deformation zones

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

tertiary diachronic extrusion and deformation of western indochina structural and 40ar 39ar evidence from nw thailand the wang chao and three pagodas fault zones cut the western part of the indochina block and run parallel to the red river fault evidence of intense ductile left lateral shear is found in the lansang gneisses which form a 5 km wide elongated core along the wang chao fault zone dating by 40 ar 39 ar shows that such deformation probably terminated around 30 5 ma the wang chao and three pagodas faults offset the north striking lower mesozoic metamorphic and magmatic belt of northern thailand 40 ar 39 ar results suggest that this belt suffered rapid cooling in the tertiary probably around 23 ma these results imply that the extrusion of the southwestern part of indochina occurred in the upper eocene lower oligocene it probably induced rifting in some basins of the gulf of thailand and in the malay and mekong basins in the oligo miocene the continuing penetration of india into asia culminated with the extrusion of all of indochina along the ailao shan red river fault this occurred concurrently with the onset of e w extension more to the south plotting in a geographical reference frame the diachronic time spans of movement on left lateral faults east and southeast of tibet implies that the northward movement of the indian indenter successively initiated new strike slip faults located farther and farther north along its path

the lancang river zone of southwestern yunnan china a questionable location for the active continental margin of paleotethys abstract in an attempt to constrain a late paleozoic tectono metamorphic event along the lancang river zone fourteen samples were processed for k ar dating on fine mineral fractions and detrital muscovites from this zone in southwestern yunnan china the samples include mica schists mylonites and gneisses from the proterozoic lancang group and phyllites from the western part of the simao basin in addition one ar ar analysis was performed on separated phengites from a blueschist of the central part of the lancang group the results reveal a considerable spread of ages the tectonic evolution of the zone is constrained by the new data which accentuate two temporally separate but spatially overlapping events i a late carboniferous high p low t metamorphism related to an east vergent late paleozoic thrust belt inverting a devonian to carboniferous marginal basin of the yangtze platform and ii an upper permian and triassic low p high t belt caused by a post orogenic stage of rifting with distinct petrological and geochemical similarities of the igneous rocks to the emeishan large igneous province these results imply that no active continental margin accounts for the subduction of the paleotethys main branch proposed to be recorded either along the lancang river or the changning menglian belt

metamorphism and tectonic evolution of the lancang paired metamorphic belts south western china the lancang metamorphic terrane consists of an eastern low p t belt and a western high p t belt divided by a n s trending fault protoliths of both units are mid late proterozoic basement and its cover the low p t belt includes the permian lincang batholith related amphibolite facies rocks of the damenglong and chongshan groups and permo triassic volcanic and volcaniclastic rocks most whole rock rb sr isochron and u pb zircon ages of the lincang batholith are in the range 290 279 and 254 212 ma respectively metamorphism of the low p t belt reaches upper amphibolite with local granulite facies 735 c at 5 kbar subsequently retrogressed at 450 500 c during post triassic time the high p t rocks grade from west to east from blueschist through transitional blueschist greenschist to epidote amphibolite facies estimated p t conditions follow the high p intermediate facies series up to about 550 600 c at which oligoclase is stable the 40ar 39ar plateau age of sodic amphibole in blueschist is 279 ma the paired metamorphic belts combined with the spatial and temporal distribution of other blueschist belts lead us to propose a tentative tectonic history of south east asia since the latest precambrian tectonic juxtaposition of paired belts with contrasting p t conditions perhaps during collision of the baoshan block with south east asia suggests that an intervening oceanic zone existed that has been removed the baoshan block is a microcontinent rifted from the northern periphery of gondwana successive collision and amalgamation of microcontinents from either gondwana or the panthalassan ocean resulted in rapid southward continental growth of c 500 km during the last 200 ma hence the lancang region in south east asia represents a suture zone between two contrasting microcontinents

variations in late cenozoic recent strike slip and oblique extensional geometries within indochina the influence of pre existing fabrics abstract from yunnan to northern thailand late cenozoic recent faults strike predominantly nne ssw n s to nne ssw and ne sw to ene wsw associated sedimentary basins are aligned ne sw to n s the regional fault patterns are commonly interpreted as strike slip dominated deformation throughout the area releasing bend and en echelon stepping patterns on faults bounding sedimentary basins indicate sinistral displacement on ne sw to ene wsw trending faults yet in the escape tectonics model left lateral displacement on the ne sw to ene wsw faults is thought to have occurred late in the miocene whilst earlier motion was dextral however in yunnan the ne sw shmax direction required for dextral motion on the n s sagaing nanting and gaoligong fault zones is consistent with sinistral motion on ene wsw striking faults which is still their sense of motion today in northern thailand the dextral sinistral switch model during the miocene is not tenable because the fang basin is of late oligocene pliocene age and requires similar age sinistral motion on the ene wsw mae chan fault in order to have opened in an alternative model northern thailand is interpreted to have evolved predominantly by oblique extension the golden triangle area marks a transition from transtensional deformation in the north to oblique extension in the south the activation of pre existing fabrics strongly affects both strike slip and extensional faults and has given rise to the similar extensional and strike slip fault patterns multiple episodes of basin inversion in northern thailand during the miocene require short term changes in stress pattern to produce the inferred changes in stress pattern it is suggested that stresses radiating out from the himalayan syntaxis exert a strong influence but were not the only important forces acting on the region

correlation of triassic stratigraphy between the simao and lampang phrae basins implications for the tectonopaleogeography of southeast asia abstract based on our extensive fieldwork in southwestern yunnan and northern thailand followed by detailed stratigraphic and paleontological studies we propose that the triassic simao basin in yunnan can be correlated with the triassic lampang phrae basin in thailand strata equivalent to those in the southern lancangjiang sub basin have not been identified in northern thailand we consider that during the triassic the simao and the lampang phrae basins belonged to the same tectonopaleogeographic unit the orogenic belt to the east of this unit includes the nan uttaradit and ailaoshan sutures the shan thai block in northern thailand can be divided from east to west into the sukhothai the inthanon and the shan terranes according to tectonopaleogeographic correlation our results support the idea that the sukhothai terrane including the lampang phrae basin belongs to the cathaysian domain and not to gondwana domain and that the geosuture corresponding to the changning menglian suture in yunnan must lie to the west of the sukhothai terrane in thailand

mechanical strength of the transition zone at the boundary between opal a and opal ct zones in siliceous rocks abstract in a massive siliceous mudstone at the site of the horonobe underground research laboratory in hokkaido japan a hard part having a very high unconfined compressive strength occurs at the top of siliceous mudstone opal ct zone at the boundary with the overlying diatomaceous mudstone opal a zone bulk chemical data does not indicate additional silica cementation or differences in mineralogy in the hard part compared with the remainder of the siliceous mudstone the hard part corresponds to a transition zone between opal a and opal ct where the two phases coexist sem images show that microcrystalline opal ct precipitates within pore spaces in the pre existing intricate diatom supported framework of the transition zone below this zone the framework is replaced by aggregations of relatively fine and equigranular opal ct the strength of the hard part probably reflects the strong framework cementation due to the coexistence of opal a and opal ct consequently the transition zone between opal a and opal ct is generally strong and brittle and if it tectonically deformed it could act as a fluid conduit

orogenic wedge growth during collision constraints on mechanics of a fossil wedge from its kinematic record rhenohercynian ftb central europe abstract the kinematic evolution of the rhenohercynian fold and thrust belt of the central european variscides can be reconstructed from incremental strain and palaeostress data synkinematic metamorphism geochronology synorogenic sediments and the geometry of shortening this data set allows to infer the mechanical development with respect to the mohr coloumb theory of critically tapered orogenic wedges growth of the rhenohercynian orogenic wedge was kinematically partitioned into a broad orogen normal moving and a narrow obliquely moving domain the ratio of the final widths of these domains varies from 1 6 to 2 3 this low ratio indicates strong basal coupling growth averages are 14 km ma of shortening and a strain rate of 6 10 16 s 1 the mode of mass transfer from lower to upper plate which controlled the accretion geometry and the particle paths evolved in the long term from dominantly basal to frontal accretion this evolution was accompanied by a decrease in taper from precollisional 18 28 to final 5 8 4 6 accretionary subsystems assemble the wedge each is characterised by a sequence of basal to frontal accretion both rooting in a common detachment the development of the subsystems is supported by the observation of stepwise migration of a foreland bulge ponding flysch subbasins these features imply shorter termed ca 2 ma cyclic mass transfer above a high friction detachment with episodes of basal accretion followed by episodes of a quickly forward propagating thrust front transfer of the basal detachment from ductile mid crustal to brittle conditions via a crustal ramp and particle flow across the ramp provoked large scale adjustment of wedge geometry internal wedge thickening is spatially and chronologically related to the formation of the ramp and was supported by surficial erosion of ca 3 kbar for keeping a critical taper three growth stages of the wedge can be reconstructed the high taper of the precollisional stage plots in the tectonically erosive field in a taper stability diagram then taper decreases to 10 after emplacement of a flysch nappe stack during the final stage of contraction an eastern and a western branch develop with geometries fluctuating around a critically stable state the two branches link with differences in large scale wedge architecture bulk contraction and detachment geometry the latter being controlled by the former basin geometry

biostratigraphy and palaeobiogeography of lower permian lower kungurian conodonts from the tak fa formation saraburi limestone thailand abstract lower permian lower kungurian conodonts are reported from the indochina block of southeast asia the fauna from the tak fa formation of the saraburi limestone group exposed in limestone hills nnw of khok samrong thailand includes sweetognathus subsymmetricus wang ritter and clark early forms and pseudosweetognathus costatus wang ritter and clark the co occurrence of these species indicates a lower permian age upper half of the lower kungurian for the sampled limestones representative of the mesogondolella siciliensis s subsymmetricus zone of south china sweetognathus and pseudosweetognathus are for the first time reported from the indochina terrane located in the palaeoequatorial belt in the kungurian pseudosweetognathus appears restricted to the kungurian of the south china and indochina terranes thus supporting palaeogeographic reconstructions that isolate these terranes within the palaeo tethys during the kungurian

structure and kinematics of an outcrop scale fold cored triangle zone triangle zones are widespread structural elements that link fold and thrust belts with their foreland basins we present a structural analysis of an outcrop scale 6 m 20 ft wide triangle zone that is exposed in the siliciclastic carboniferous strata of the harz mountains in northern germany the geometry of the triangle zone is critically compared with larger outcrop and seismic scale structures the external form of the triangle zone is the same as that proposed for larger examples with two bounding detachments that dip in opposing directions however the interior of the triangle zone is characterized by tight to isoclinal folds we demonstrate how the triangle zone probably evolved from a fault bend fold which accreted further folds behind it this is an alternative fold based interpretation for a structure that is commonly modeled as a duplex stack we present the resulting consequences for seismic interpretation and hydrocarbon reservoir evaluation

mechanics of triangle zones and passive roof duplexes implications of finite element models abstract finite element models of passive roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions friction on the roof fault cover strength and amount of overburden a foreland buttress is essential for backthrust displacement in the models this foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland with boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge incipient forethrust and backthrust ramps zones of high equivalent plastic strain cut through the cover in the models there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover because of pressure dependence both fault strength and material strength increase with increasing overburden therefore the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength in models with multiple imbricate faults the distribution of equivalent plastic strain adjacent to each fault is similar to that in single fault models with differences resulting from bending strains in the wedge and cover the strain distribution in the cover of the models is similar to that above natural passive roof duplexes characterized by layer parallel shortening above the tip of each horse in the duplex

structural evaluation of column height controls at a toe thrust discovery deep water niger delta industry discoveries to date in the deep water toe thrust play have limited column heights and lack any unequivocal thrust fault dependent hydrocarbon columns the causes of the limited success are controversial the main issue being whether the encountered columns are caused by leaking thrust faults or other causes the controversy is in large part caused by the presence of no seismic image zones that even on three dimensional 3 d prestack migrated data obscure the possible thrust cutoffs we use dipmeter seismic stratigraphic and fluid pressure data to construct an integrated geometrically and kinematically balanced cross section through a recent toe thrust discovery in which a prominent no seismic image zone on the southwest flank of the alpha structure appeared to be a thrust fault zone holding about 100 m 328 ft of thrust fault dependent oil column to constrain the structure within the unimaged forelimb dip panels and fold axial surfaces were constructed from dipmeter data recorded in both a vertical hole and a sidetrack well through the no seismic image zone stratigraphic tops from both wells were projected through the no seismic image zone using the dip panels and axial surfaces maintaining the observed stratigraphic thicknesses these data and analyses tightly constrain potential fault locations and offsets within the no data zone the resulting structural model shows that the no seismic image zone in the forelimb is not a thrust but instead an overturned limb without any fault offset of the pay section the results show that the trapped hydrocarbons are confined completely within the four way dip closure and that the trap s potential is not limited by the thrust faults seal capacities in addition top seal analyses show that top seal integrity is not a limiting factor for hydrocarbon column heights at the present time and has not been a limiting factor throughout the geological history of the structure taken together these observations indicate that the column heights are most likely controlled by access to charge instead of trap integrity consistent with an observed lack of thermogenic hydrocarbons in nearby four way dip closures these conclusions imply that where sufficient access to thermogenic charge is present there may be additional thrust fault dependent hydrocarbon columns

triangle zones geometry kinematics mechanics and the need for appreciation of uncertainties abstract triangle zones are important structures found in foreland fold and thrust belts all over the world and are commonly associated with tectonic wedging however tectonic wedging and consequently the formation of passive hinterland verging roof thrusts requires particular mechanic conditions such as pre fractured rocks syntectonic sedimentation or a specific stratigraphic layering with variable mechanically weak and rigid formations in this contribution models of triangle zones and hypotheses regarding their formation are reviewed our results show that the term triangle zone is often used in a contradictory sense and interpretations are affected by large uncertainties passive roof thrusts are not necessarily required in all cases and suggested models of triangle zones even if they are geometrically and kinematically viable are hampered by their mechanical implications with respect to a large number of published triangle zones and associated kinematic as well as mechanic models we present a new definition and classification scheme which is discussed and applied to natural examples worldwide we show that natural examples of triangle zones can be separated into two types 1 detachment dominated and 2 ramp dominated triangle zones both types imply particular mechanic conditions of involved detachments and regional dynamics regarding the large uncertainties associated with triangle zone interpretation we suggest to only use the term triangle zone in a very stringent manner and only if irrefutable evidences for stratigraphic repetitions within a duplex are given the results should then be validated with additional kinematic and mechanic considerations geometric and kinematic uncertainties as well as implications of the proposed model explaining observed triangular structures should be reflected in the nomenclature used

oblique plate motion and continental tectonics three dimensional deformation is necessarily associated with oblique plate convergence and commonly results in partitioning of deformation between contractional and transcurrent components along plate margins kinematic models of strike slip partitioning for transpression and transtension allow the exact relation among three critical parameters plate motion instantaneous strain axes and degree of strike slip partitioning to be calculated application to two end member tectonic environments characterized by a low south island new zealand and high central california degree of strike slip partitioning demonstrates a remarkable consistency among the three parameters suggesting that strike slip partitioned transpression is a valid model for deformation in these regions the extreme degree of strike slip partitioning in wrench dominated systems such as central california is tentatively associated with a fundamental misorientation of finite and instantaneous strain axes

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

the variscan belt in central europe main structures geodynamic implications open questions abstract the variscan belt of europe originated from the confrontation of northern europe and gondwana with intervening pre variscan blocks cadomian and older armorica tepla barrandean moravo silesian though a strike slip component cannot be excluded geological evidence suggests a subduction collision process newly discovered features include basaltic rocks with morb affinities high pressure metamorphic rocks trench deposits and large scale allochthonism which also affects the pre variscan basement and has often led to an inversion of the metamorphic isogrades large scale thrusting is evident at all crustal levels and is also observable by geophysical methods the structure of the variscan crust is best described as horizontal tectonic layering late ordovician through carboniferous convergence closed one rhenohercynian and one saxo thuringian basin to the north and at least one mediterranean basin to the south of the pre variscan blocks subduction occurred from the north as well as from the south down under the dorsal core region the exact timing and the driving mechanism of this bilateral activity remains a problem likewise uncertain are the exact limination of the pre variscan blocks and the existence and width of oceanic areas in the saxothuringian zone and in the south facing part of the moldanubian zone there is good evidence of early palaeozoic rifting and of late ordovician through carboniferous plate convergence palaeomagnetic and palaeoclimatic constraints however suggest only a limited extent of oceanic basins in these areas while they appear to allow a wider ocean at the rhenohercynian saxothuringian boundary which is relatively unobtrusive from the geological point of view research during the past decade has produced a wealth of information on the variscan fold belt in europe the resulting geodynamic models however are extremely diverging evidently it is not the time yet for definitive statements the present paper intends to review the state of knowledge to deduce such basic items of a geodynamic model as appear reasonably well documented and to point out the most urgent open questions stress will be laid on central europe which offers a complete cross section through the northern flank of the variscan belt and where we have carried out most of our studies during the last years

maximum effective moment criterion and the origin of low angle normal faults abstract the origin of the low angle normal faults of metamorphic core complexes has been debated for over two decades proponents of andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults a new theory named the maximum effective moment criterion is proposed here for their origin using an effective moment approach formulated as m eff fh where f is the force tangentially acting on the unit shear boundaries and h represents its arm the maximum value appears at angles of 54 7 with the 1 since extensional crenulation cleavages eccs and the contractional crenulation cleavages cccs occur in conjugate pairs with 110 angle between them it is suggested that they tend to be oriented in the directions of maximum effective moment the differential stress for formation of the ecc or ccc are less than that for fracturing the orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components the synthetic ecc set is much better developed than the antithetic set due to anisotropy upward propagation of the synthetic ecc set from mid crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle normal faults of metamorphic core complexes

40ar 39ar age pattern associated with differential uplift along the eastern highlands shear zone cape breton island canadian appalachians abstract the eastern highlands shear zone in cape breton island is a crustal scale thrust it is characterized by an amphibolite facies deformation zone 5 km wide formed deep in the crust that is overprinted by a greenschist facies mylonite zone 1 km wide that formed at a more shallow level hornblende 40ar 39ar plateau ages on the hanging wall decrease towards the centre of the shear zone in the older zone over 7 8 km from the centre the ages are between 565 and 545 ma in the younger zone within 4 5 km of the centre they are between 425 and 415 ma and in the transitional zone in between they decrease abruptly from 545 to 425 ma pressures of crystallization of plutons in the hanging wall based on the al in hornblende barometer and corresponding to depth of emplacement increase towards the centre of the shear zone and indicate a differential uplift of up to 28 km associated with movement along the shear zone the age pattern is interpreted to have resulted from the differential uplift the pressure data show that rocks exposed in the younger zone were buried deep in the crust and did not cool through the hornblende ar blocking temperature 500 c until differential uplift occurred the 40ar 39ar ages in the zone 425 415 ma thus date shear zone movement or the last stage of it in contrast rocks in the older zone were more shallowly buried before differential uplift and cooled through the blocking temperature soon after the emplacement of 565 555 ma plutons in the area long before shear zone movement the transitional zone corresponds to the ar partial retention zone before differential uplift the 40ar 39ar age pattern thus reflects a neoproterozoic to silurian cooling profile that was exposed as a result of differential uplift related to movement along the shear zone a similar k ar age pattern has been reported for the alpine fault in new zealand it is suggested that such isotopic age patterns can be used to help constrain the ages kinematics displacements and depth of penetration of shear zones

high temperature metamorphism in a major strike slip shear zone the ailao shan red river people s republic of china abstract petrographic and thermobarometric analysis provides constraints on thept path of the mylonitic gneisses of the left lateral ailao shan red river shear zone which has accommodated the lateral extrusion of indochina during the tertiary two different paragenesis p1 and p2 are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies microprobe analysis reveals that p1 garnets bear a chemical zonation from core to rim this zonation indicates a temperature increase during garnet growth conditions of formation of garnet rims p1b which are estimated using biotite garnet and plagioclase garnet thermobarometers are close to the granitic solidus 710 70 c and4 5 1 5 kbar p2 conditions are estimated to be approximately 500 c and

fault related folding a review of kinematic models and their application abstract folding that is directly related to fault activity is an important deformation feature that occurs all over the world in mountain belts accretionary wedges fold and thrust belts and intra plate settings in either strike slip compressional or extensional regimes due to their widespread occurrence knowledge about the development of these structures is important to a broad spectrum of geoscience sub disciplines such as structural geology seismology geomorphology petroleum geology and quaternary geology fault related folding has been analyzed intensively over the last 140 years for the sake of this review we divide the folds according to the way the faults and the folds form that is into detachment fault bend and fault propagation folds all fault related folds are caused by changes in fault parameters the simplest method to produce folds is to transport material along faults that have stepped flat ramp flat geometries fault bend fold alternatively the slip can decrease along the length of the fault and depending on whether the fault remains within a detachment layer or steps up through mechanical stratigraphy either a detachment fold or a fault propagation fold is formed respectively detachment folding was first investigated in the early 20th century whereas the full significance of fault propagation folds was recognized quite late in the 1980s seminal work on fault related folding was carried out in the 1930s but quantitative kinematic models have only been available in the last 30 years these models are extremely valuable because they allow a comprehensive understanding of the evolution of fault related folds and lead to more accurate predictions of the sub surface structure from the mid 1990s onwards numerical simulations have been used to identify how fault parameters such as dip and fault bend angle propagation to slip ratio and shape of the trishear zone influence the geometry of the related folding this is directly applicable to the analysis of the shape of anticlines produced however this does not mean that fold geometry is uniquely related to fault geometry on the contrary different kinematic approaches can lead to a similar fold shape

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

tectonic style and pressure distribution at the northern margin of the alps between lake constance and the river inn hydrocarbon exploration in the molasse basin and the alpine thrust belt has provided new information on the tectonic structure of the foreland margin of the overthrust belt two n s cross sections show the front of the folded molasse as a so called triangle zone in the area north of the hindelang 1 well bavaria and the sulzberg 1 well vorarlberg the triangle zone was described and defined byjones 1982 for the alberta foothills in canada

nested strike slip duplexes and other evidence for late cretaceous palaeogene transpressional tectonics before and during india eurasia collision in thailand myanmar and malaysia the late cretaceous palaeogene structural and tectonic events in the northern region of sundaland represent a poorly known but potentially important orogenic development in se asia extensive late cretaceous s type granites extending from malaysia through thailand to myanmar have long been used to infer an episode of crustal thickening an inference supported by the presence of late cretaceous eocene ophiolites in myanmar however structural evidence for a well developed associated fold and thrust belt has been lacking apatite and zircon fission track studies of thailand indicate a broad regional uplift from about 80 ma to 40 ma suggesting an episode of crustal thickening but the uplift amount is modest and not that expected for a major fold and thrust belt left lateral motion on major nw se trending strike slip fault zones mae ping and three pagodas faults in myanmar and thailand has been attributed to himalayan tibetan escape tectonics this paper demonstrates that the fault zones are considerably more complicated than just nw se trends they are a network of branching faults with important north south trends as well as nw se trends they form large scale nested strike slip duplex structures and affect a region over 1000 km long and up to 250 km wide this diffuse branching network of strike slip fault trends may represent a substantial late cretaceous palaeogene transpressional belt himalayan escape tectonics represent later increments of deformation with both oligocene sinistral and late oligocene recent dextral reactivation

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex