influence of a pre existing basement weakness on normal fault growth during oblique extension insights from discrete element modeling abstract we use discrete element modeling to investigate three dimensional fault geometry and the three dimensional evolution of a fault network that develops above a 60 dipping planar pre existing weakness striking 60 relative to the extension direction the evolution of the fault network comprises three stages i reactivation of pre existing structure and nucleation of new faults 0 10 extension ii radial propagation and interaction between reactivated structure and new faults 15 20 extension and iii linkage between reactivated structure and adjacent faults 20 25 extension during the first stage the pre existing structure mostly reactivates forming a long and under displaced fault new faults are mainly extension perpendicular and dip at 60 during the second stage saw tooth fringes grow upwards from the upper tip of the reactivated structure which becomes the major fault and influence the density and orientation of surrounding faults during the third stage the reactivated structure links laterally and vertically with adjacent faults creating non planar fault geometries following linkage the reactivated structure enhances the displacement of linked faults along branch lines our study demonstrates that pre existing weak faults can be reactivated propagating upwards in an irregular saw tooth pattern and affecting fault density orientation dip and displacement and providing the nucleation site of new faults

basement structure and its influence on the structural configuration of the northern north sea rift the northern north sea rift basin developed on a heterogeneous crust comprising structures inherited from the caledonian orogeny and devonian post orogenic extension integrating two dimensional regional seismic reflection data and information from basement wells we investigate the pre rift structural configuration in the northern north sea rift three seismic facies have been defined below the base rift surface 1 relatively low amplitude and low frequency reflections interpreted as pre caledonian metasediments caledonian nappes and or devonian clastic sediments 2 packages of high amplitude dipping reflections 500 ms thick interpreted as basement shear zones and 3 medium amplitude and high frequency reflections interpreted as less sheared crystalline basement of proterozoic and paleozoic caledonian origin some zones of seismic facies 2 can be linked to onshore devonian shear zones whereas others are restricted to the offshore rift area interpreted offshore shear zones dip s ese and wnw in contrast to w to nw dipping shear zones onshore w norway our results indicate that devonian strain and ductile deformation was distributed throughout the caledonian orogenic belt from central south norway to the shetland platform most of the devonian basins related to this extension are however removed by erosion during subsequent exhumation basement shear zones reactivated during the rifting and locally control the location and geometry of rift depocenters e g in the stord and east shetland basins pre rift structures with present day dips 15 were reactivated although some of the basement shear zones are displaced by rift faults regardless of their orientation relative to rift extension direction

influence of pre existing pervasive fabrics on fault patterns during orthogonal and oblique rifting an experimental approach abstract pervasive mechanical anisotropy e g metamorphic foliation in the pre rift basement rocks often appears to control the orientation of new faults in natural rift systems and thus tend to influence formation of fault bounded sedimentary basins however the extent to which this influence works in nature has remained debatable and contrasting ideas exist regarding the issue of structural inheritance in natural rifts like the east african rift system earlier experimental works have investigated different kinematic aspects of faulting in simulated orthogonal and oblique rift systems including the role of a crustal scale pre existing zone of weakness on continent scale rifting these experiments have tested the influence of discrete pre existing weak zones e g pre existing faults or an ancient orogenic belt on localization of subsequent rifting but have not considered the control of pervasive fabric anisotropy e g metamorphic foliation etc in pre rift basement rocks on the fault patterns during rifting here we present a series of simple roughly scaled analog model experiments where pervasive strength anisotropy in the basement was created by brushing plaster of paris on a viscous pitch substratum this way a transverse anisotropy of tensile strength was generated in the plaster of paris layer due to thickness variation across the brush marks the basement was overlain by a dry sand pack simulating weak sedimentary rocks overlying a foliated crystalline basement orthogonal and oblique extension of the models at various angles and with differently oriented strength anisotropy exhibited the control of the anisotropy on the orientations of new faults in the sand layer in general anisotropy could significantly control the fault pattern only when it was at a high angle 45 with respect to the maximum instantaneous horizontal stretching direction e h1 otherwise they largely disregarded the anisotropy and formed orthogonal to e h1 faults never formed orthogonal to the displacement vector d in oblique rifting unless the fault controlling anisotropy was perpendicular to d faults strongly following the anisotropy were much longer than those not following that trend faults oblique to the anisotropy were short segmented and tortuous in nature and mainly grew by segment linkage the experimental results support many field observations made in the east african rift system and are likely to be applicable to other rift systems like gondwana basins of india where field data on such structural inheritance are meager

basement reactivation in the development of rift basins an example of reactivated caledonide structures in the west orkney basin the west orkney basin is a poorly studied palaeozoic and mesozoic rift basin that overlies a highly seismic reflective caledonian orogenic basement terrane this study presents a novel approach of using 2d seismic data to map single intra basement reflection packages and their relationship to overlying normal faults in the west orkney basin it is demonstrated that the rift architecture of the west orkney basin is more linear with a prominence of relay zones relative to previous interpretations and that rift structures form complex discordant and concordant relationships with pre rift caledonide basement structures restoration of basement fabrics to their pre extensional geometry indicates that the reactivation of basement structures as normal faults has occurred only where the pre extensional dip of basement structures is greater than 30 the relatively high density of relay zones mapped in the west orkney basin is proposed to be the result of the rift system forming a partially exploitative relationship with basement structure where extension has been accommodated between segments that have or have not reactivated basement fabrics

late mesozoic development of the southern qinling dabieshan foreland fold thrust belt central china and its role in continent continent collision abstract the southern qinling dabieshan foreland fold thrust belt is a key region in which to investigate the continental continental collision between north china and south china we provide structural evidence of a diachronous dextral strike slip thrusting deformation from the middle triassic in the east to the late late triassic in the west which indicates a northwestward oblique subduction and suturing of the south china plate under the north china qinling dabieshan plate we also identify a later phase of intracontinental deformation that included an orthogonal intracontinental collision and south vergent thrusting during the early and middle jurassic indentation of south china into the qinling dabieshan orogen and arc shaped extrusions of the southern qinling dabieshan foreland fold thrust belt from the late jurassic to the cretaceous our results reveal that the long term intracontinental collision rotated clockwise in a northwest southeast vergent direction in the late triassic to northeast east southwest west vergent in the late cretaceous this collision may have been driven by a north vergent subduction of the meso tethys ocean in the south west vergent subduction of the izanagi plate in the east and south vergent compression of the eurasia plate in the north

influence of crustal heterogeneity on normal fault dimensions and evolution southern south africa extensional system abstract the tectonic configuration of southern south africa is dominated by a continental scale mesozoic extensional system superimposed upon a significant and well constrained palaeozoic lithospheric scale heterogeneity through integrating onshore structural analysis with offshore subsurface studies it is possible to evaluate the applicability of established normal fault growth models in a heterogeneous crustal setting at a basin scale for the first time the 480 km long mesozoic extensional system comprises a number of fault arrays that vary in length from 78 to 230 km coupled with displacements of up to 16 km the fault arrays are amongst the longest and largest displacement of high angle normal faults dips of 45 60 documented in continental lithosphere although comprised of co linear segments rather than the en echelon segment configuration typical of many other extensional systems this atypical geometry is considered to be a consequence of the high degree of structural inheritance between the underlying paleozoic cape fold belt and the subsequent extensional system furthermore it is proposed that the overall displacement length dimensions of the extensional faults were inherited from the underlying compressional faults the establishment of a seismic stratigraphic framework for the pletmos and gamtoos offshore basins reveals that the faults established their long lengths 160 and 90 km respectively within 6 myr of rift initiation prior to accruing their substantial displacement 16 km furthermore there is no evidence for the development of intra basin faults the southern south africa extensional system presents an end member case of structural inheritance where extensional structures are parallel to and reactivate underlying compressional structures in this study structural inheritance is considered to have a significant effect on mechanisms of fault growth the pre existing structures not only result in the rapid establishment of fault lengths of up to 160 km but additionally to strain being localised onto the pre existing fabric to such an extent that no intra basin faults evolve and strain is accommodated entirely on the bounding faults

an unusual triangle zone in the external northern alpine foreland switzerland structural inheritance kinematics and implications for the development of the adjacent jura fold and thrust belt abstract triangle zones represent typical structural elements of thin skinned foreland fold and thrust belts here we report the results of an in depth structural analysis of a rather unusual triangle zone at the front of the easternmost jura fold and thrust belt in the otherwise only very mildly deformed alpine foreland of northern switzerland the investigation is based on the interpretation of recently reprocessed and depth migrated 2d reflection seismic sections classical bed length and area cross section balancing methods were used to validate the interpretation and unravel the tectonic evolution of the triangle zone according to our interpretation the analyzed triangle zone formed along the baden irchel herdern lineament bih lineament a regional paleozoic normal fault that shows evidence of cenozoic reactivation the triangle zone is composed of one major foreland directed thrust rooting in triassic evaporites and a back thrust splaying from it in the middle jurassic opalinus clay pointing to the importance of secondary detachments steeply dipping secondary reverse faults next to the triangle zone suggest reactivation of pre existing normal faults the formation of the thrust triangle is considered to relate to thin skinned foreland deformation in late miocene time strain estimations of the thrust triangle along strike show a laterally very uniform amount of shortening which is in contrast to the southward adjacent jura fold and thrust belt we interpret this constant shortening to represent the maximum contractional strain attainable by the specific geometry of the bih triangle zone at this point the complex structure became mechanically ineffective and further shortening led to the formation of new contractional structures in its hinterland this kinematic hypothesis suggests an early stage formation of the bih triangle zone followed by back stepping of the deformation front as such it challenges the classical view of a purely forward breaking sequence for the jura fold and thrust belt in the northwestern foreland of the alps

intermediate and deep earthquakes in spain recent improvements in the seismological networks on the ibero maghrebian region have permitted estimation of hypocentral location and focal mechanisms for earthquakes which occurred at south spain alboran sea and northern morocco of deep and intermediate depth with magnitudes between 3 5 and 4 5 intermediate depth shocks range from 60 to 100 km with greater concentration located between granada and malaga fault plane solutions of 5 intermediate shocks have been determined they present a vertical plane in ne sw or e w direction seismic moments of about 1015 nm and dimensions of about 1 km have been determined from digital records of spanish stations p wave forms are complex this may be explained by the crustal structure near the station discontinuities in the upper mantle and inhomogeneities near the source deep activity at about 650 km has only 3 shocks since 1954 1954 1973 1990 shocks are located at a very small region fault plane solutions show a consistent direction of the pressure axis dipping 45 in e direction for the 1990 shock seismic moment is 1016 nm and dimensions 2 6 km thep waves are of simpler form with a single pulse the intermediate and deep activities are not connected and no activity has been detected between 100 and 650 km the intermediate shocks may be explained in terms of a recent subduction from africa under iberia in se direction the very deep activity must be related to a sunk detached block of lithospheric material still sufficiently cold and rigid to generate earthquakes

the widespread occurrence of low angle normal faults in a rift setting review of examples from thailand and implications for their origin and evolution abstract at least 31 low angle 1 km cenozoic normal faults are identified on seismic reflection data onshore and offshore thailand although some faults have been moderately rotated to lower angles pre rotation dips still indicate formation at low angles about 25 and 35 the dominant east dip of low initial fault dips suggest that pre existing fabrics controlled the fault dip direction in the mergui basin patches of gently dipping basement reflections on seismic reflection data support this observation well data points to a pre rift basement with a widespread component of slaty phyllitic and chlorite schist lithologies that are associated with significant strength anisotropy the subduction zone setting of se asia with a high fluid flux into the crust associated with high levels of co2 has parallels with the setting for lanfs in italy where it is suggested high pore fluid pressures played an important role in lanf reactivation in cross section most lanfs curve from a high angle in sedimentary section to planar or stepped in basement until they reach the brittle ductile transition zone in a few cases lanfs have a convex up geometry some long segmented faults are composed of mixed regions of high and low angle segments the local mix of high and low angle faults indicates that the principal stresses probably were vertical and horizontal during fault formation any rotation of stresses would have to be local not regional lanfs in the mergui basin have very high displacement gradients as low as 2 1 indicating the same fault segments some 20 30 km long have repeatedly slipped while displacements can decline dramatically at key discontinuities the lanf segments are inferred to be very weak and probably slipped mostly aseismically while adjacent lower displacement high angle segments were probably seismically active hence in areas of mixed high and low angle fault segments seismicity may erroneously suggest that high angle normal faults are the dominant fault type the presence of such a high concentration of low and high displacement lanfs in a non metamorphic core complex setting demonstrates that lanfs can form initially at a low angle that isostatic rotation of initially high angle faults to a low angle is not necessary or appropriate to explain the faults and that low angle normal faults are not just features associated with orogenic collapse

geometry styles and quantification of inversion structures in the jiyang depression bohai bay basin eastern china abstract inversion structures of jiyang depression evolved during mesozoic and cenozoic are classified into positive and negative inversion geometry styles the geometry styles of the reactivated faults in the study area are attributed to their multi phase history which includes at least four phases of compressive deformation and subsequent subsidence caused by extensional reactivation of faults since mesozoic in the case of negative inversion style the negative inversion ratios cannot be calculated by the same method of calculating the degree of positive inversion thus a new method of quantifying the negative inversion ratio is presented in this paper negative inversion ratio is defined as the ratio of extensional displacement to contractional displacement measured along the fault this method provides a useful complement to conventional estimation of the degree of inversion and adapts to any negative inversion faults based on this method we obtained the negative inversion ratios of some faults in the jiyang depression which indicates a decrease in deformational degree from east to west in the whole region

cenozoic structural deformation and dynamic processes of the bohai bay basin province china abstract cenozoic structures in the bohai bay basin province can be subdivided into eleven extensional systems and three strike slip systems the extensional systems consist of normal faults and transfer faults the normal faults predominantly trend nne and ne and their attitudes vary in different tectonic settings paleogene rifting sub basins were developed in the hanging walls of the normal faults that were most likely growth faults neogene quaternary sequences were deposited in both the rifting sub basins and horsts to form a unified basin province the extensional systems were overprinted by three nne trending right lateral strike slip systems fault zones although the principal displacement zones pdz of the strike slip fault zones are developed only in the basement and lower basin sequences in some cross sections the structural deformation characteristics of the upper basin sequences also indicate that they are basement involved right lateral strike slip fault zones according to the relationships between faults and sedimentary sequences the extensional systems were mainly developed from the middle paleocene to the late oligocene whereas the strike slip systems were mainly developed from the oligocene to the miocene strike slip deformation was intensified as extensional deformation was weakened extensional deformation was derived from horizontal tension induced by upwelling of hot mantle material whereas strike slip deformation was probably related to a regional stress field induced by plate movement

strain migration during multiphase extension observations from the northern north sea many rifts develop through multiphase extension it can be difficult however to determine how strain is distributed during reactivation because structural and stratigraphic evidence associated with earlier rifting is often deeply buried using 2 d and 3 d seismic reflection and borehole data from the northern north sea we examine the style magnitude and timing of reactivation of a preexisting permian triassic rift phase 1 fault array during a subsequent period of middle jurassic to early cretaceous rift phase 2 extension we show that rift phase 2 led to the formation of new n s striking faults close to the north viking graben but did not initially reactivate preexisting rift phase 1 structures on the horda platform we suggest that at the beginning of rift phase 2 strain was focused in a zone of thermally weakened lithosphere associated with the middle jurassic north sea thermal dome rather than reactivating extant faults diachronous reactivation of the permian triassic fault network eventually occurred with those faults located closer to the middle jurassic to early cretaceous rift axis reactivating earlier than those toward the eastern margin this diachroneity may have been related to flexural down bending as strain became focused within the north viking graben and or the shifting of the locus of rifting from the north sea to the proto north atlantic our study shows that the geometry and evolution of multiphase rifts is not only controlled by the orientation of the underlying fault network but also by the thermal and rheological evolution of the lithosphere and variations in the regional stress field

applicability of thin or thick skinned structural models in a region of multiple inversion episodes southern south africa abstract the deformation of the cape fold belt has been attributed to repeated structural reactivation of a mega detachment from the late proterozoic to the mesozoic 650 65 ma through the integration of onshore cross sections with observations from the offshore mesozoic extensional system this study evaluates the applicability of the mega detachment model regional scale cross sections through the permian triassic cape fold belt reveal that it comprises two main structural domains a northern domain dominated by northward verging and asymmetric folds and a southern domain comprising a series of approximately 8 km wavelength box folds the genesis of these box folds is attributed to motion on underlying high angle 45 reverse faults this variation between north and south in the fold belt is reflected by a similar variation in extensional geometry of the mesozoic normal faults as revealed by subsurface data the normal faults demonstrate a progressive increase in dip from 24 in the north to 60 in the south features commonly attributed to thin and thick skinned tectonic models are observed in both domains therefore it is not appropriate to describe the observed deformation as one of the two end members in addition the structures are inferred to have undergone at least two stages of inversion irrespective of dip this is not predicted by either end member model

mesozoic tectonic evolution of the daba shan thrust belt in the southern qinling orogen central china constraints from surface geology and reflection seismology the daba shan thrust belt is located along the southern margin of the qinling orogen that separates the north china block in the north from the south china block in the south despite decades of research the total magnitude of shortening accommodated by continent continent convergence across the qinling orogen after triassic ocean closure between north and south china remains poorly constrained the lack of knowledge on the shortening magnitude in turn limits our ability to test a wide array of tectonic models for the development of the qinling orogen and thus the convergence history between north and south china in order to address this issue we construct a balanced cross section and develop a new kinematic model for the evolution of the daba shan thrust belt this work was accomplished by integrating 1 surface geologic mapping 2 detailed kinematic analysis of key structures 3 existing geochronologic and thermochronological data and 4 a recently obtained lithospheric scale seismic reflection profile restoration of the cross section indicates that the minimum shortening strain increases northward from 10 in the foreland to 45 in the thrust belt interior the estimated amount of upper crustal shortening across the daba shan thrust belt is 130 km which is sufficient to allow the inferred mafic lower crust of the subducted south china lithosphere to have experienced eclogite phase transition thus our work supports that the development of the daba shan thrust belt may have been driven by slab pull of the subducted mafic lower crust at the leading edge of the down plunging south china continental lithosphere

monotonic and cyclic liquefaction of very loose sands with high silt content the results from an experimental study on sands with high nonplastic silt content are presented drained and undrained triaxial compression tests undrained cyclic triaxial tests and drained undrained instability tests were performed on specimens of loose nevada sand with 40 silt content the behavior was observed to be somewhat different from previously published tests with sands at lower silt content the greater silt content appears to provide a more volumetrically contractive response throughout the entire stress strain curve however some aspects of the response were similar to sands with lower silt content monotonic undrained tests indicated reverse behavior i e static liquefaction occurred at low confining pressures and increasing dilatant volume change tendency was observed with increasing confining pressure analyzing the results using the concepts of steady state resulted in a unique steady state line only when undrained tests were sheared from the same isotropic compression line when

the role of pre existing precambrian structures in rift evolution the albertine and rhino grabens uganda abstract we integrated shuttle radar topography mission srtm digital elevation models dem airborne magnetic radiometric and three dimensional full tensor gravity gradiometry 3d ftg data to investigate the role of precambrian structures in the evolution of the largely amagmatic miocene recent aged albertine and rhino grabens in uganda these grabens represent the northern segment of the western branch of the east african rift system ears the two ne trending grabens are connected by a right stepping transfer zone and they extend within the archean paleoproterozoic northeast congo block which represents the northeastern extension of the congo craton our results show the following and highlight the importance of pre existing structures in the evolution of continental rift systems 1 the ne extent of the albertine full graben is controlled by ne trending precambrian fabric and the graben terminates at its northeastern end when it encounters a multiply folded precambrian basement terrain with poorly developed nw trending structural grain additionally the northeastern termination of the albertine graben coincides with the presence of nw trending right stepping high density bodies within the precambrian terrain 2 the transfer zone between the albertine and rhino grabens is controlled by ne trending precambrian structures which might have facilitated the development of relay ramp faults 3 strain localization within the better developed southeastern border fault of the rhino half graben is facilitated by the presence of precambrian structures better aligned in a ne direction in the southeastern part of the basin compared to its northwestern part 4 further to the northeast the rhino graben is segmented and transitions into a narrower ene trending half graben with a better developed border fault on its northwestern side this segmentation coincides with the presence of n trending precambrian structures 5 the rhino graben terminates farther northeast against the nw trending precambrian aswa shear zone a prominent structure with complex but generally nw trending fabric

rapid spatiotemporal variations in rift structure during development of the corinth rift central greece the corinth rift central greece enables analysis of early rift development as it is young 5 ma and highly active and its full history is recorded at high resolution by sedimentary systems a complete compilation of marine geophysical data complemented by onshore data is used to develop a high resolution chronostratigraphy and detailed fault history for the offshore corinth rift integrating interpretations and reconciling previous discrepancies rift migration and localization of deformation have been significant within the rift since inception over the last circa 2 myr the rift transitioned from a spatially complex rift to a uniform asymmetric rift but this transition did not occur synchronously along strike isochore maps at circa 100 kyr intervals illustrate a change in fault polarity within the short interval circa 620 340 ka characterized by progressive transfer of activity from major south dipping faults to north dipping faults and southward migration of discrete depocenters at 30 m kyr since circa 340 ka there has been localization and linkage of the dominant north dipping border fault system along the southern rift margin demonstrated by lateral growth of discrete depocenters at 40 m kyr a single central depocenter formed by circa 130 ka indicating full fault linkage these results indicate that rift localization is progressive not instantaneous and can be synchronous once a rift border fault system is established this study illustrates that development processes within young rifts occur at 100 kyr timescales including rapid changes in rift symmetry and growth and linkage of major rift faults

reactivation of intrabasement structures during rifting a case study from offshore southern norway abstract pre existing structures within crystalline basement may exert a significant influence over the evolution of rifts however the exact manner in which these structures reactivate and thus their degree of influence over the overlying rift is poorly understood using borehole constrained 2d and 3d seismic reflection data from offshore southern norway we identify and constrain the three dimensional geometry of a series of enigmatic intrabasement reflections through 1d waveform modelling and 3d mapping of these reflection packages we correlate them to the onshore caledonian thrust belt and devonian shear zones based on the seismic stratigraphic architecture of the post basement succession we identify several phases of reactivation of the intrabasement structures associated with multiple tectonic events reactivation preferentially occurs along relatively thick c 1 km relatively steeply dipping c 30 structures with three main styles of interactions observed between them and overlying faults i faults exploiting intrabasement weaknesses represented by intra shear zone mylonites ii faults that initiate within the hangingwall of the shear zones inheriting their orientation and merging with said structure at depth or iii faults that initiate independently from and cross cut intrabasement structures we demonstrate that large scale discrete shear zones act as a long lived structural template for fault initiation during multiple phases of rifting

implications of structural inheritance in oblique rift zones for basin compartmentalization nkhata basin malawi rift ears abstract the cenozoic east african rift system ears is an exceptional example of active continental extension providing opportunities for furthering our understanding of hydrocarbon plays within rifts it is divided into structurally distinct western and eastern branches the western branch comprises deep rift basins separated by transfer zones commonly localised onto pre existing structures offering good regional scale hydrocarbon traps at a basin scale local discrete inherited structures might also play an important role on fault localisation and hydrocarbon distribution here we consider the evolution of the central basin of the malawi rift in particular the influence of pre existing structural fabrics integrating basin scale multichannel 2d and high resolution seismic datasets we constrain the border mlowe nkhata fault system mnf to the west of the basin and smaller mbamba fault mf to the east and document their evolution intra basin structures define a series of horsts which initiated as convergent transfers along the basin axis the horsts are offset along a ne sw striking transfer fault parallel to and along strike of the onshore karoo permo triassic ruhuhu graben discrete pre existing structures probably determined its location and oriented obliquely to the extension orientation it accommodated predominantly strike slip deformation with more slowly accrued dip slip to the north of this transfer fault the overall basin architecture is asymmetric thickening to the west throughout while to the south an initially symmetric graben architecture became increasingly asymmetric in sediment distribution as strain localised onto the western mnf the presence of the axial horst increasingly focussed sediment supply to the west as the transfer fault increased its displacement so this axial supply was interrupted effectively starving the south east while ponding sediments between the western horst margin and the transfer fault this asymmetric bathymetry and partitioned sedimentation continues to the present day overprinting the early basin symmetry and configuration sediments deposited earlier become increasingly dissected and fault juxtapositions changed at a small 10 100 m scale the observed influence of basin scale transfer faults on sediment dispersal and fault compartmentalization due to pre existing structures oblique to the extension orientation is relevant to analogous exploration settings

spatio temporal evolution of strain accumulation derived from multi scale observations of late jurassic rifting in the northern north sea a critical test of models for lithospheric extension abstract we integrate observations of lithospheric extension over a wide range of spatial and temporal scales within the northern north sea basin and critically review the extent to which existing theories of lithospheric deformation can account for these observations data obtained through a prolonged period of hydrocarbon exploration and production has yielded a dense and diverse data set over the entire viking graben and its flanking platform areas these data show how syn rift accommodation within the basin varied in space and time with sub kilometer scale spatial resolution and a temporal resolution of 2 3 myr regional interpretations of 2d seismic reflection refraction and gravity data for this area have also been published and provide an image of total basin wide stretching for the entire crust these image data are combined with published strain rate inversion results obtained from tectonic subsidence patterns to constrain the spatio temporal evolution of strain accumulation throughout the lithosphere during the 40 myr 170 130 ma period of late jurassic extension across this basin for the first 25 30 myr strain localisation dominated basin development with strain rates at the eventual rift axis increasing while strain rates over the flanking areas declined as strain rates across the whole basin were consistently very low 16 s l thermally induced strength loss cannot explain this phenomenon the strain localisation is manifest in the near surface by a systematic migration of fault activity the pattern and timing of this migration are inconsistent with flexural bending stresses exerting an underlying control especially when estimates of flexural rigidity for this area are considered the best explanation for what is observed in this time period is a coupling between near surface strain localisation driven by brittle or plastic failure and the evolving thermal structure of the lithosphere we demonstrate this process using a continuum mechanics model for normal fault growth that incorporates the strain rate dependence of frictional strength observed in laboratory studies during the final 10 myr of basin formation strain accumulation was focused within the axis and strain rates declined rapidly replacement of weak crust by stronger mantle material plus crustal buoyancy forces can adequately explain this decline

a continuum mechanics model for normal faulting using a strain rate softening rheology implications for thermal and rheological controls on continental and oceanic rifting abstract we use a strain rate dependent visco plastic rheology in a finite element model to investigate the effects of crustal rheology and thermal structure on the development of normal faults in 2 d extending lithosphere strain rate softening in the brittle regime is used to simulate the rate dependence of frictional strength observed in laboratory studies results of numerical experiments show that efficient strain rate softening can result in localized zones of high strain rate which develop in response to the rheology and boundary conditions and are not imposed a priori we argue that these zones of localized shearing are analogous to faults and use the calculated deformation field to predict the preferred location of fault formation for a range of thermal and rheologic conditions when no regional temperature gradient is imposed deformation is predicted to be distributed between several sets of conjugate normal faults however in the presence of a horizontally varying temperature field faulting is calculated to focus where the lithosphere is thinnest forming rift like topography model results predict that when crustal thickness is sufficiently small that no ductile layer is present in the lower crust deformation is mantle dominated and rift half width is controlled primarily by the vertical geothermal gradient in contrast when crustal thickness is large enough that the stress accumulation in the upper crust becomes much greater than the stress accumulation in the upper mantle deformation becomes crust dominated and the calculated rift half width is a function of both crustal rheology and the vertical geothermal gradient this implies that deformation in the brittle upper crust does not behave independently of a strong upper mantle even in situations where ductile flow occurs in the lower crust

the missing piece of the south atlantic jigsaw when continental break up ignores crustal heterogeneity crustal heterogeneity is considered to play a critical role in the position of continental break up yet this can only be demonstrated when a fully constrained pre break up configuration of both conjugate margins is achievable limitations in our understanding of the pre break up crustal structure in the offshore region of many margins preclude this in the southern south atlantic which is an archetypal conjugate margin this can be achieved because of the high confidence in plate reconstruction prior to addressing the role of crustal heterogeneity two questions have to be addressed first what is the location of the regionally extensive gondwanan orogeny that remains enigmatic in the orange basin offshore south africa and second although it has been established that the argentinian colorado rift basin has an east west trend perpendicular to the orange basin and atlantic spreading where is the western continuation of this east west trend we present here a revised structural model for the southern south atlantic by identifying the south african fold belt offshore the fold belt trend changes from north south to east west offshore and correlates directly with the restored colorado basin the colorado orange rifts form a tripartite system with the namibian gariep belt which we call the garies triple junction all three rift branches were active during the break up of gondwana but during the atlantic rift phase the colorado basin failed while the other two branches continued to rift defining the present day location of the south atlantic in addressing these two outstanding questions this study challenges the premise that crustal heterogeneity controls the position of continental break up because seafloor spreading demonstrably cross cuts the pre existing crustal heterogeneity furthermore we highlight the importance of differentiating between early rift evolution and subsequent rifting that occurs immediately prior to seafloor spreading

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

normal fault growth displacement localisation and the evolution of normal fault populations the hammam faraun fault block suez rift egypt abstract fault segment linkage migration of the locus of fault activity and displacement localisation were important processes controlling the late oligocene recent evolution of the normal fault population of the hammam faraun fault block suez rift initial fault activity was distributed across the fault block on fault segments that had attained their final length within 1 2 my of rifting these initial segments then either grew by increasing displacement and linked to form longer segmented fault zones or died during a rift initiation phase that lasted 6 8 my following this rift initiation phase displacement became localised onto 25 km long border fault zones bounding the fault block and many of the early high displacement intra block fault zones died following displacement localisation onto the major faults bounding the fault block the locus of maximum displacement continued to migrate with post middle miocene displacement focused on the western margin of the fault block this migration of fault activity between major crustal scale normal faults can be viewed in terms of strain localisation at the rift scale the results from this study question conventional fault growth models based on final displacement distributions and highlight the sequential nature of faulting on major normal faults bounding domino style tilted fault blocks

cenozoic rifting and volcanism in eastern china a mantle dynamic link to the indo asian collision abstract the indo asian continental collision is known to have had a great impact on crustal deformation in south central asia but its effects on the sublithospheric mantle remain uncertain studies of seismic anisotropy and volcanism have suggested that the collision may have driven significant lateral mantle flow under the asian continent similar to the observed lateral extrusion of asian crustal blocks here we present supporting evidence from p wave travel time seismic tomography and numerical modeling the tomography shows continuous low velocity asthenospheric mantle structures extending from the tibetan plateau to eastern china consistent with the notion of a collision driven lateral mantle extrusion numerical simulations suggest that at the presence of a low viscosity asthenosphere continued mass injection under the indo asian collision zone over the past 50 my could have driven significant lateral extrusion of the asthenospheric mantle leading to diffuse asthenospheric upwelling rifting and widespread cenozoic volcanism in eastern china

the effect of multiple pre existing weaknesses on formation and evolution of faults in extended sandbox models abstract high resolution three dimensional seismic data show that fault distribution and evolution in rift basins can be very complicated and inconsistent with the classic model of fault formation and propagation fault development in the presence of multiple pre existing weaknesses is still not clearly studied trying to understand the role of multiple pre existing weaknesses in sand models two series of experiments were prepared in the first series of models the mechanical properties of pre existing weaknesses were the same but their orientations were different whereas in the second series of models both mechanical properties and orientations were different the basic characteristics of fault formation and evolution in a rift basin revealed by the models are as follows 1 weakness reactivated faults faults that formed directly along and above the pre existing weaknesses and weakness related faults faults which are not located above the pre existing weaknesses but have similar orientation form much earlier than weakness unrelated faults the order of formation of different weakness reactivated faults is determined by the relative reactivation tendency factors fa of the weaknesses 2 weakness reactivated faults and weakness related faults develop mainly along or parallel to pre existing weaknesses in contrast weakness unrelated faults develop nearly perpendicular to the extension direction 3 a complicated fault system can form during a single extension phase in the presence of multiple pre existing weaknesses and 4 the mode of fault formation and propagation and the increase in length and number of faults have close ties to the nature of pre existing weaknesses fault displacement and length do not have a power law relationship for the majority of faults and there is no unique relationship for different kinds of faults in addition to providing new insights to the formation of complex fault systems the experiment results presented here could be reasonably explained using the reactivation tendency theory they can also be applied to the complicated fault system in rift basin