release fault a variety of cross fault in linked extensional fault systems in the sergipe alagoas basin ne brazil abstract two types of cross faults are herein recognized transfer faults and the newly termed release faults transfer faults form where cross faults connect distinct normal faults and horizontal displacements predominate over vertical ones in contrast release faults form where cross faults associated with individual normal faults die out within the hangingwall before connecting to other normal faults and have predominantly vertical displacements release faults are geometrically required to accommodate variable displacements along the strike of a normal fault thus they form to release the bending stresses in the hangingwall and do not cut normal fault planes nor detachment surfaces at depth release faults have maximum throws adjacent to normal faults and may be nearly perpendicular or obliquely oriented to the strike of the latter such geometry appears not to depend upon pre existing weaknesses but such variable orientation to normal faults is an inherent property of release faults release faults commonly appear as simple normal faults in seismic sections without implying extension along the strike of rift and basins three dimensional strain deformation occurs in the hangingwall only between the terminations of an individual normal fault but regionally release faulting is associated with plane strain deformation in linked extensional fault systems

fault reactivation control on normal fault growth an experimental study field studies frequently emphasize how fault reactivation is involved in the deformation of the upper crust however this phenomenon is generally neglected except in inversion models in analogue and numerical models performed to study fault network growth using sand silicon analogue models we show how pre existing discontinuities can control the geometry and evolution of a younger fault network the models show that the reactivation of pre existing discontinuities and their orientation control i the evolution of the main fault orientation distribution through time ii the geometry of relay fault zones iii the geometry of small scale faulting and iv the geometry and location of fault controlled basins and depocenters these results are in good agreement with natural fault networks observed in both the gulf of suez and lake tanganyika they demonstrate that heterogeneities such as pre existing faults should be included in models designed to understand the behavior and the tectonic evolution of sedimentary basins

high resolution fault displacement mapping from three dimensional seismic data evidence for dip linkage during fault growth abstract detailed mapping using high resolution three dimensional seismic data has revealed a number of sub horizontal anomalies in the distribution of vertical displacement throw on the planes of growth faults in the gulf of mexico recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled because they represent local decreases in throw which are confined to small discrete parts of the fault planes the distribution of the anomalies is inconsistent with model displacement fields of quasi elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip line propagation an alternative model is proposed whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction dip linkage overlap and linkage of fault tips in the dip direction results in relay structures that are sub parallel to fault strike and therefore displacement minima that are sub horizontal on normal and thrust faults however since they are orthogonal to the main slip direction on the fault these structures have a low preservation potential and are therefore unlikely to be well resolved on cross sectional seismic profiles

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

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

normal fault development during two phases of non coaxial extension an experimental study we use scaled experimental analog models to study the effect of a pre existing fault fabric on fault development during extension in our models a homogeneous layer of wet clay undergoes two noncoaxial phases of extension whose directions differ by up to 45 the normal faults that develop during the first phase create a pronounced fault fabric that influences normal fault development during the second phase in all models the pre existing faults are reactivated during the second phase of extension their sense of slip depends on the angle between the first and second phase extension directions specifically the component of dip slip relative to strike slip decreases as the angle between the first and second phase extension directions increases new normal faults also form during the second phase of extension in all models the number of new fault segments increases as the angle between the first and second phase extension directions increases the orientations of the new normal fault segments are both orthogonal and oblique to the second phase extension direction indicating that both the secondphase extension direction and the pre existing fault fabric control the orientation of new fault segments some of the new normal faults cut and offset the pre existing faults whereas others terminate against them producing complex fault patterns and interactions the modeling results explain fault interactions observed in the jeanne d arc rift basin of offshore newfoundland canada and the reactivation of abyssal hill normal faults at outer highs near subduction zones

morphometric analysis of the submarine arc volcano monowai tofua kermadec arc to decipher tectono magmatic interactions morphometric analysis of multibeam bathymetry and backscatter data is applied to monowai a submarine volcano of the active tofua kermadec arc to map and document the structure and evolution of the volcanic centre low rates of erosion and sedimentation and pervasive tectonic and magmatic processes allow quantification through detailed structural analysis and measurement of deformation the slope aspect curvature rugosity and hydrology flow tools of arcgis provide a robust structural interpretation and the development of a model of monowai evolution a nested caldera structure with a volume of 31 km3 and a stratovolcano of 18 km3 dominate the magmatic constructs the outer caldera is elongate along 125 and the inner caldera along 135 numerous parasitic cones and fissure ridges are also observed oriented at 039 and 041 respectively northeast trending faults with a regional average strike of 031 are widespread within this part of the backarc forming a nascent rift graben to the west of the monowai caldera complex the distribution of throw varies spatially reaching a maximum total along rift of 320 m and across rift of 120 m with greater throw values measured in the west elongation directions of the two nested calderas are near perpendicular to the trends of faults and fissure ridges the inner caldera is more orthogonal to the magmatic constructs fissure ridges and aligned vent cones and the outer caldera is approximately orthogonal to the regional fault fabric suggesting a strong interaction between magmatic and tectonic processes and the directions of the horizontal principal stress we present a detailed morphometric analysis of these relationships and the data are used to interpret the spatial and temporal evolution of the tectono magmatic system at monowai and classify the type of rifting as transtensional similar analysis is possible elsewhere in the kermadec backarc and within other regions of submarine volcanism

early stage evolution of growth faults 3d seismic insights from the levant basin eastern mediterranean the geometry and kinematic evolution of small growth faults were analysed from a high resolution 3d seismic dataset located at the margins of the levant basin in the eastern mediterranean the 3d geometry segmentation history and throw distribution of one particular fault was reconstructed to evaluate and illustrate the changes in dimension and displacement distribution that occurred during the transition from purely blind propagation to propagation at the free surface the fault is considered to have grown by blind radial propagation of three main segments that hard linked prior to surface interaction the fault subsequently reached the seabed and continued to accrue displacement as a syn sedimentary fault most of the fault surface area formed during the blind propagation phase but most of the displacement was added during the syn sedimentary phase of the growth history with little increase in surface area the interaction of the fault with the free surface led to a change in the position of the point of maximum displacement as well as modifying the vertical throw distribution the amount of displacement added after this transition from blind fault to growth fault is discussed with respect to existing fault growth models

noise and target strength degradation accompanying shallow buried explosions ground shock from shallow buried explosions is commonly accompanied by acoustic noise analysis of records from a series of 10 ton chemical detonations in gulf coast sediments provides a basis for the first quantitative description of such noise during a cratering event the test series included shots at depths of 3 12 m leading to craters about 20 m in radius most of the records studied were taken within the eventual crater they include stress velocity and acceleration oscillations arising from reflections off the free surface or subsurface layers are evident in stress records high frequency oscillations attributed to reflections or scattering from smaller scale inhomogeneities are particularly striking in the acceleration records where peak accelerations were 10 100 km s2 oscillations with frequencies of 5 25 khz are obvious they exhibit exponential decay with the dominant frequency decreasing with time approximate values of q estimated from these accelerograms are 30 20 agreeing with q for similar materials derived from seismic techniques since seismic q s include both internal dissipation and scattering losses whereas explosive q s include internal dissipation and radiative losses the magnitude of scattering and radiative losses must be comparable the latter alone correspond to an effective q of about 300 so that internal dissipation dominates q in these poorly consolidated sediments acoustic fluidization may have occurred for a couple of milliseconds in the top 1 or 2 m of soil within the eventual crater by this we mean that the material was liable to flow if the threshold shear stress were exceeded this result is inferred from ground shock stress duration acoustic amplitude decay and initial overburden stress fluidization of such limited duration and extent would have no effect on crater growth just as would be expected for such a low yield event

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

origin of contrasting devonian supradetachment basin types in the scandinavian caledonides supradetachment basins that formed during the devonian extensional collapse of the scandinavian caledonides have been explained as hanging wall basins formed along the listric breakaway zone of a major detachment fault system this model involves significant rotation of bedding as the detachment flattens to an approximately horizontal orientation and explains the large stratigraphic thicknesses of east dipping layers overlying in tectonic contact the top to the west nordfjord sogn detachment zone nsdz however it fails to explain one of the basins the hasteinen basin where east dipping devonian strata rest unconformably on a metamorphic substrate that forms part of the upper plate of the detachment system based on detailed field mapping we present a model where the hasteinen forms as a ramp basin that develops on the upper plate above a major west facing ramp in the nsdz we test both of these models by forward modeling using 2dmove two dimensional kinematic modeling software the results show how two different types of supradetachment basins can coexist in a subhorizontal detachment zone in an area that underwent many tens of kilometers of lateral crustal extension

oblique rifting and sequential faulting in the jurassic development of the northern north sea abstract jurassic structural development of the northern north sea has characteristics of oblique rifting this is interpreted to result from an obliquity between jurassic nw se to wnw ese directed extension and a preexisting n s trending zone of deformation related to permo triassic rifting and thinning of the crust the zone of maximum jurassic extension i e the viking and sogn grabens defines a narrow 25 40 km depression which trends diagonally nne ssw across a broader 130 150 km permo triassic basin the jurassic rift zone consists of a system of ene chelon rift segments with changing symmetry along strike bounded by faults oblique to the overall graben envelope sequential jurassic fault development as well as the interference between n s and ne sw striking faults are explained within the context of oblique rifting the early late bajocian oxfordian rift stage was dominated by reactivation of n s striking major faults whereas a number of coeval but smaller faults have a ne sw orientation ne sw striking faults show increasing importance during the jurassic stretching phase and major faults defining this orientation accommodated most of the kimmeridgian volgian extension faults striking nesw are observed to cross cut earlier n s trending faults they interfere with n s trending faults to define faultblocks with rhombohedral shape and they are associated with partial collapse of crestal areas of n s oriented fault blocks to form structural terraces

fabric anisotropy controls faulting in the continental crust the influence of pre existing structural anisotropy on faulting in the continents is best tested in recently exhumed crust e g nanga parbat massif nw himalayas where earlier brittle structures have been annealed the kinematics of young faults formed in a single continuing tectonic regime nnw compression are distinctly different depending upon the orientation of the early ductile foliations around them faulting is subparallel and statistically simple where foliation is moderately dipping but highly complex where foliation is steeply dipping thus structural anisotropy does control faulting in the continental crust a result with important implications for seismogenesis fluid flow and basin evolution

sm nd and zircon u pb ages from garnet bearing eclogites ne oman constraints on high p metamorphism poly deformed and poly metamorphosed glaucophane eclogite mega boudins beneath the samail ophiolite oman record an early subduction related high p metamorphism as well as subsequent overprinting deformation and metamorphism related to exhumation previously published rb sr ages of 78 ma and 40ar 39ar ages of 82 79 ma record the major ne directed shearing event that partially exhumed the eclogites to a shallower crustal level new sm nd garnet garnet leachate whole rock isochron data from garnet bearing eclogite assemblages in the as sifah subwindow in ne oman are 110 9 ma dg02 87d 5 point isochron and 109 13 ma dg02 86e 3 point isochron on the basis of microfabric and field structural relationships these ages are interpreted to reflect the timing of prograde peak high p metamorphism in the rocks structurally beneath the samail ophiolite this metamorphism clearly predates the age of formation of the obducted samail oceanic lithosphere 97 94 ma as well as the subsequent obduction onto the margin 80 70 ma a u pb shrimp zircon age from small 200 m in length zircons with herring bone textured zoning from dg02 87d indicate that rapid zircon growth associated with high si phengites occurred at 82 1 ma zircon growth is possibly related to liberation of zr on garnet breakdown during decompression metamorphism under high p conditions with exhumation these data require that crustal stacking models attendant with ophiolite obduction are inappropriate to explain the oman high p metamorphism

tectonics of the arabian margin associated with the formation and exhumation of high pressure rocks sultanate of oman continental crustal rocks now structurally beneath the allochthonous samail ophiolite underwent blueschist to eclogite facies metamorphism prior to the emplacement of the oman ophiolite onto the arabian margin the recognition of a major low angle fault within this polydeformed and polymeta morphosed sequence of metacarbonates metabasites quartzites and quartz mica schists greatly simplifies the interpretation of the structure and metamorphic zonation within the saih hatat window ne oman carpholite bearing upper plate rocks consist of pre permian to jurassic rocks that have been folded into large recumbent nappe structures which exhibit a marked increase in deformation intensity toward the boundary with the lower plate the lower plate rocks have higher peak pressure and temperature assemblages they are exposed in two windows separated by jabal abu daud although more intensely deformed the lower plate rocks are recognizable as metamorphosed continental platform sediments fold axes are parallel to the regional nne ssw lineation sense of shear indicators yield a transport direction of south over north in the lower plate opposite to the sense of motion inferred for the emplacement of the ophiolite lower plate eclogite facies metabasalts are only preserved in kilometer scale megaboudins found in the easternmost window at as sifah these metamorphic assemblages along with their remnant east west fabrics define the existence of a short lived arabian platform directed nascent subduction zone the stretching lineation elsewhere within the upper and lower plates the hatat schist the basement and the metamorphic sole of the ophiolite is consistently nne ssw suggesting that the exhumation of high pressure metamorphic rocks of saih hatat is related to the ophiolite obduction exhumation of the high pressure rocks was accompanied by intense deformation involving regional scale fold nappes in a convergent margin setting the geotherm remained suppressed for a period 30 myr greater than the thermal relaxation time of the crust the geometric and thermal constraints from oman may be applicable to the general problem of the formation and preservation of high pressure low temperature rocks

postcollisional extension of the caledonide orogen in scandinavia structural expressions and tectonic significance the scandinavian caledonides are much more affected by postcontractional devonian extension than previously thought the extensional deformation is not restricted to the internal parts of the orogen but can be traced for more than 200 km from the west coast of norway toward the foreland it involved significant reactivation of the basal thrust zone as a low angle extensional detachment whic consequently contains abundant top to the north west sense of shear indicators the extensional deformation involved substantial backsliding of the orogenic wedge eduction of the baltic shield together with stretching of the basement by west dipping extensional shear zones after 30 km of backsliding the extensional detachment splayed to penetrate the overlying orogenic edifice in the coastal region in southern norway which led to inactivation of the main portion of the basal thrust zone as an extensional detachment

displacement loss on growth faults due to sediment compaction compaction models of normal growth faults combined with displacement data from a natural growth fault cape egmont fault new zealand are used to quantify the loss of displacement arising from compaction in circumstances where growth faults are blanketed and buried by compacting sediment in both footwall and hangingwall comparison of imposed tectonic displacements with the preserved displacements indicates that although the thicknesses of sand shale sequences may decrease by up to c 55 due to compaction associated losses in displacement on growth faults are typically 70 and post faulting burial is large i e km scale

the growth of non colinear normal fault systems what can we learn from 3d seismic reflection data abstract many rift zones exhibit a range of fault orientations rather than simple colinear faults that strike orthogonal to the inferred least principal stress the formation of non colinear fault sets has implications for assessing rift zone kinematics as well as determining palaeo stress state in extensional basins using 3d seismic reflection data we deduce the likely mechanisms responsible for the formation of a population of non colinear faults in the maloy slope area of the northern north sea three basement displacing fault populations exist on the maloy slope i large 1 km throw n s striking faults ii smaller

comment on structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains by m p searle et al

unconformities related to the jurassic cretaceous synrift post rift transition of the northern north sea in the jurassic cretaceous north sea basin the synrift sequence is separated from the post rift sequence by the base cretaceous or late cimmerian unconformity the unconformity covers almost the entire basin has a distinct character in seismic reflection data and wireline logs and hence is easily identified and correlated making it the most important marker horizon in the area the unconformity displays great local complexity in many localities and great variability on a regional scale from one locality to another thus the unconformity is classified as a nonconformity a disconformity and an angular unconformity we suggest that these variations basically reflect different structural position within the basin so that the short wavelength variation is dominated by local structural development e g the rotational history of a fault block whereas the long wavelength variation reflects basin scale tectonic thermal and isostatic processes the merging of this unconformity with younger erosional surfaces its complex configuration and polychronous character makes the general term base cretaceous unconformity inadequate thus the term northern north sea unconformity complex is used here

structural and stratigraphic controls on the origin and tectonic history of a subducted continental margin oman eclogites and blueschists exposed in saih hatat oman record the subduction and exhumation of continental crustal material beneath the cretaceous semail ophiolite during ophiolite obduction the eclogite bearing lower plate originally part of oman s distal continental margin is exposed in two tectonic windows through the less metamorphosed upper plate by a previously mapped low angle high strain decollement structure a major tectonic break currently poorly exposed records the juxtaposition of the highest pressure eclogites and garnet blueschists against lower pressure epidote blueschists the subsequent exhumation of the entire lower plate to mid crustal levels is marked by a pervasive shearing event associated with a regional greenschist facies overprint the decollement truncates structures and the metamorphic field gradient in the lower plate but does not significantly truncate structures or stratigraphy in the upper plate it is not responsible for the exhumation of the high pressure rocks to mid crustal levels most of the displacement across this structure was accommodated during continuing convergence after the subduction system had ceased to be active and post ophiolite emplacement onto the platform carbonate sequences a revised tectonic model is presented which accounts for the structural geochronological and metamorphic observations

origin and significance of intra basement seismic reflections offshore western norway we use 3d seismic data to image a series of enigmatic sw dipping reflection packets within pre mesozoic crystalline basement offshore western norway based on their low angle dip and complex reflection wave train our preferred interpretation is that the reflection packets are the seismic expression of mylonitic zones generated by nappe emplacement during the caledonian orogeny late jurassic faults truncate and offset these reflection packets by several hundred metres suggesting that these faults did not exploit pre existing basement weaknesses our observations suggest that older basement fabrics may not always play a significant role in determining the geometry of later fault systems supplementary materials details of the time depth relationships from wells that have been used to depth convert interpretations s1 a supplementary figure showing an uninterpreted version of the seismic profile presented in fig 2 s2 and a throw v distance plot for fault f1 at the structural levels of the basement reflection and intra basement reflection 1 are available at www geolsoc org uk sup18683

exhumation of high pressure rocks in northeastern oman the exhumation of high p rocks in the northeast saih hatat window oman mountains formed regional nappes associated with greenschist facies metamorphism exhumation resulted in the juxtaposition of two plates with different structural style stratigraphy and metamorphic history a major crustal discontinuity separates the two plates with lower metamorphic grade rocks in the hanging wall lower plate units were exhumed during strong top to the northeast shearing resulting in the formation of regional closures that evolved by folding and transposition of the high pressure fabrics while the lithostatic pressure was decreasing upper plate units were later exhumed with the juxtaposition of the two plates truncation of lower plate closures and the formation of upper plate nappes that are attenuated and sheathlike toward the upper and lower plate boundary in some high p terranes apparent increases in metamorphic grade down structural section and low grade c type shear bands are often attributed to extensional exhumation in oman however these exhumation features formed during crustal scale contraction at a convergent margin the low geothermal gradient regimes afforded by convergent margins are critical for the generation and preservation of high p low t rocks

analogue modelling of multiphase rift systems abstract analogue models of multiphase rift systems show that structures produced by the first phase of extension exertsignificant control on the geometries of the later phase extensional faults two experimental series series 1 and series 2 used different modelling media subjected to two phases of extension series 1 models used a simple homogeneous sandpack whereas series 2 models used a sand polymer multilayer in each series kinematic scenarios included orthogonal extension followed by oblique extension or vice versa extension directions for oblique rift kinematics varied from 75 to 30 to the axis of rifting in all models where oblique rifting superseded earlier orthogonal rift structures the second deformation phase produced segmented fault systems and reorientation of fault tips into high angles with respect to the extension vector higher angles of obliquity resulted in increased fault segmentation orthogonal extension superposed on earlier oblique rifting leads to coalescence of segmented faults although remnants of the inherited initial rift structure remained even for low angles of obliquity the sand polymer multilayer models produced more complex fault patterns than those in the simple homogeneous sandpack showing the possible complexities that can be generated during polyphase rifting persistent first phase rift faults caused boundary fault segmentation and salients and embayments along the boundary faults during the second deformation phase the natural occurrence of similar structural styles particularly in map view may indicate that previously unrecognised multiphase rifting has occurred in these natural examples

tectonic inheritance and continental rift architecture numerical and analogue models of the east african rift system 1 the western branch of the east african rift is composed of an arcuate succession of elongate asymmetric basins which differ in terms of interaction geometry fault architecture and kinematics and patterns of uplift subsidence and erosion sedimentation the basins are located within proterozoic mobile belts at the edge of the strong tanzanian craton surface geology suggests that the geometry of these weak zones is an important parameter in controlling rift development and architecture although other processes have been proposed in this study we use lithosphere scale numerical models and crustal scale analogue experiments to shed light on the relations between preexisting structures and rift architecture results illustrate that on a regional scale rift localization within the mobile belts at the curved craton s western border results in an arcuate rift system which implies that under a constant extensional stress field part of the western branch experienced orthogonal extension and part oblique extension largest depocenters are predicted to form mostly orthogonal to the extension direction and smaller depocenters will form along the oblique parts of the rift the varying extension direction along the rift zone furthermore results in lengthwise varying rift asymmetry segmentation characteristics and border fault architecture trend length and kinematics analogue models predict that discrete upper crustal fabrics may influence the location of accommodation zones and control the architecture of extension related faults at a local scale models support that fabric reactivation is responsible for the oblique slip kinematics on faults and for the development of z shaped or arcuate normal faults typically documented in nature

development of the north viking graben inferences from laboratory modelling abstract the north viking graben is a classic sedimentary basin in the north sea that has been thoroughly studied over the last decade despite the amount and quality of seismic and well data now available the processes of extension remain unclear and the relationship between sedimentation and deformation is still a matter of debate we present the results of small scale laboratory modelling the models are made with sand and silicone putty to simulate brittle and ductile behaviour on a crustal scale the experiments were designed to study the effects of a the degree of coupling between brittle and ductile layers b the obliquity of extension and c the reactivation of inherited crustal scale structures the results are used to discuss the symmetry of extensional processes on crustal and lithospheric scales as well as the relationship between sedimentation and deformation it is proposed that the asymmetry of faulting in the upper crust and the nearly symmetric thinning of the whole crust are not mutually exclusive a model involving nearly pure shear on the lithospheric scale is used to explain the development of an asymmetrical basin at shallow level in addition this model accounts for the nearly horizontal stratification of lower cretaceous sediments the basal cretaceous unconformity which caps many of the normal faults and the localization of deformation to the west of the graben during the lower cretaceous

transition from subduction to exhumation related fabrics in glaucophane bearing eclogites oman evidence from relative fabric chronology and 40ar 39ar ages abstract controversy over the age of peak metamorphism and therefore the tectonic evolution of the arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane bearing eclogites from the saih hatat window beneath the allochthonous samail ophiolite in ne oman these eclogites contain relicts of earlier fabrics structures and metamorphic assemblages and provide a record of change from subduction to exhumation the eclogites are part of a mafic layer that was disrupted into boudins up to 0 5 km in length within a lower plate shear zone as sifah shear zone the megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ene trending lineation and sheathlike isoclines enveloped by the flat lying schistosity the boudin bearing layer is isoclinally folded with calc schist mafic schist and quartz mica schist where the regional folds have axes parallel to the ne trending stretching lineation a type folds textural evidence suggests multiple growth events for garnet and clinopyroxene requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins the surrounding calc schist and quartz mica schist are both intensely deformed with transposition foliation containing an ne trending lineation in phengite and asymmetric shear indicators such as c type shear bands and asymmetric pressure shadows around garnets that give top to the ne sense of shear although consistent ene trending lineations in all the boudins suggest that they have largely acted as passive nonrotating rigid bodies the presence of ne vergent asymmetric mesofolds extensive dynamic recrystallisation multiple generations of phengites and a range of 40ar 39ar apparent ages within the megaboudins suggest however that they have not acted entirely passively during the later deformation phengites isolated from the high p low t fabrics show groupings in 40ar 39ar apparent ages interpreted as distinct metamorphic cooling intervals at 140 135 120 98 and 92 80 ma microstructural relations suggest that age groupings younger than 100 ma reflect phengite growth during exhumation with the top to the ne shearing the older ages 120 110 ma from fabrics that give top to the s shear sense may reflect growth during the subduction phase the combination of groupings of apparent argon ages older than the crystallisation age of the samail ophiolite the suggestion of different geothermal gradients and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite the glaucophane eclogites are interpreted as having formed at different times under varying pressure temperature conditions during underthrusting with variations in the rate of underthrusting allowing thermal equilibration and or rapid cooling at different crustal levels

new high precision p t estimates for oman blueschists implications for obduction nappe stacking and exhumation processes oman blueschists and eclogites lie below the obduction nappe of the semail ophiolite in one of the key areas on earth for the study of plate convergence here new metamorphic and tectonic constraints are provided for the central yet poorly constrained hulw unit sandwiched between the low grade units 10 kbar 300 c and the as sifah eclogites pmax 23 kbar tmax 600 c tweequ multi equilibrium thermobarometry using both compositional mapping and spot analyses and raman spectroscopy of carbonaceous material yield a high precision p t path for the hulw and as sheikh units and reveal that they shared a common p t history in four stages i a pressure decrease from 10 12 kbar 250 300 c to 7 9 kbar 300 350 c ii almost isobaric heating at 8 10 kbar from 300 350 c to 450 500 c iii a pressure decrease at moderate temperatures 450 500 c and iv isobaric cooling at 5 6 kbar from 450 500 to 300 c no significant pressure or temperature gap is observed across the upper plate lower plate discontinuity to the north and west of the hulw unit the combination of tectonic and p t data constrains the stacking chronology of the three main metamorphic units comprising the saih hatat window i e the ruwi quryat the hulw as sheikh and the diqdah as sifah units these results strengthen the view that the tectonic and metamorphic data are conveniently accounted for by a simple n vergent continental subduction of the passive arabian margin below the obduction nappe along a cold p t gradient

tectonic setting origin and obduction of the oman ophiolite the semail ophiolite in the oman mountains is the world9s largest and best preserved thrust sheet of oceanic crust and upper mantle 10 000 km 2 550 km long 150 km wide it was emplaced onto the arabian continental margin during late cretaceous time the ophiolite originated 96 94 ma at a spreading center above a northeast dipping subduction zone associated with initiation of immature island arc tholeiitic lavas lasail arc at the highest levels of the ophiolite simultaneous underthrusting of triassic and jurassic mid oceanic ridge basalt and alkalic volcanic rocks beneath 12 km of upper mantle depleted harzburgites produced garnet clinopyroxene amphibolites formed at temperatures of 850 c dated as 95 93 ma subduction cannot have been initiated at a mid oceanic ridge otherwise the protolith of the amphibolites in the metamorphic sole would be the same age and composition as the ophiolite volcanic rocks above in the northern part of the oman mountains in the bani hamid area united arab emirates 870 m of granulite facies rocks enstatite spinel diopside quartzites garnet diopside wollastonite calc silicate marbles clinopyroxene bearing amphibolites were formed at temperatures similar to those of the garnet diopside amphibolites of the oman sole 800 850 c but at slightly higher pressures as much as 9 kbar they are interpreted as deeper level metamorphosed continental margin sedimentary rocks exhumed by out of sequence thrusting placing granulites over mantle sequence harzburgites during the later stages of obduction subduction of the arabian continental crust beneath the obducting semail ophiolite to 78 90 km depth has been proven by thermobarometry of the as sifah eclogites to 20 23 kbar in the eastern sector in the united arab emirates the subducted continental crust began to partially melt producing unusual biotite muscovite garnet tourmaline cordierite andalusite bearing granites that intrude the uppermost mantle sequence harzburgites and lowermost crustal sequence cumulate gabbros of the ophiolite we suggest that the entire leading northeast edge of the arabian plate was subducted beneath the ophiolite during the final stages of obduction leading to eclogitization of the crustal rocks higher temperatures and pressures in the united arab emirates sector possibly due to a thicker or double thickness ophiolite section led to blueschist amphibolite and granulite facies conditions in the metamorphic sole and crustal melting in the subophiolite basement produced leucocratic granites that intruded up as dikes through the obducted ophiolite a model for ophiolite obduction is presented which accounts for all the structural and metamorphic conditions reported from the oman mountains

land offshore tectonic links in western norway and the northern north sea aeromagnetic and marine gravity data sets for the northern north sea and western norway have been interpreted in terms of land to offshore tectonic links the data allow identification of linking structures both in crystalline basement and overlying sedimentary rocks and consequently facilitate a more comprehensive interpretation of regional structural features and trends with the help of bathymetric data in the inshore region major fault structures can be traced between western norway and the offshore region west of the oygarden fault zone in this way spatial and chronological links have been established between tectonic events recognized on land and the development of the north sea basin an interpretation of basement geology is proposed for the region between western norway and the uer terrace this is complemented with a depth to top crystalline basement map covering the north sea basin between the uer terrace and the east shetland basin two regional nw se lineaments were recognized offshore western norway and given the names marflo lineament and troll lineament the marflo lineament is interpreted to be a caledonian or older zone of weakness that accommodated development of a 20 30 km offset in the jurassic viking graben axis the lineament can be traced into the north atlantic where it parallels oceanic fracture zones and transform faults the troll lineament relates to a deep geological discontinuity crossing the uer and lomre terraces a grouping of earthquake epicentres along part of the lineament suggests that the discontinuity is tectonically active

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

slip distributions on intersecting normal faults abstract slip distributions on normal faults often are asymmetric and display multiple local maxima numerically computed slip distributions from elastic models indicate that such irregular slip distributions can be caused by mechanical interaction between intersecting faults that produce local perturbations of the stress field resolved on the faults a three dimensional boundary element model based on the displacement discontinuity method has been used to analyze the mechanical interaction of faults that form y or t shaped intersections slip distributions are asymmetric with a steeper slip gradient toward the line of intersection multiple slip maxima occur depending on the angle between the faults but generally they are not located along the intersections nor at the fault centers examples of intersecting normal faults taken from oil reservoir seismic surveys and sandbox experiments have been used to confirm the application of the theoretical results despite considerable simplifications in terms of geometry and boundary conditions the computed slip distributions capture the characteristics of the observed slip distributions

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 case of obduction related high pressure low temperature metamorphism in upper crustal nappes arabian continental margin oman p t paths and kinematic interpretation in the southeastern oman mountains the saih hatat tectonic window reveals arabian continental material beneath the oceanic sensu lato nappes i e the hawasina sedimentary units and the huge sumail ophiolite the northeastern internal half of the window suffered an intense deformation associated with a high pressure low temperature hp lt metamorphism approximately dated at 80 75 ma according to the available stratigraphie data after the deformation due to the absence of collision the obduction inherited structural and metamorphic pattern was virtually unmodified a metamorphic map is established based on mineral assemblages in metapelites and metabasites low grade assemblages occur in the highest units muscat nappes quryat unit characteristic minerals are femg carpholite lawsonite associated with kaolinite or pyrophyllite and locally sudoite in the metapelites while lawsonite and blue amphiboles are found in the metabasites high grade rocks outcrop in the deepest and easternmost units as sifah antiform and are typified by an epidote glaucophane bearing eclogite assemblage intermediate units hulw show assemblages of intermediate grade chloritoid schists and glaucophane bearing metabasites higher grade assemblages such as talc chloritoid were not found inverted gradients are observed locally in the muscat nappes pyrophyllitebearing units above kaolinite quartz bearing units and these are ascribed to late metamorphic thrust displacements which are demonstrated by microstructures a complex p t evolution is fairly well documented by the mineral assemblages prograde evolution in the subophiolitic continental rocks began with a rather hot gradient of about 30 35 c km with chloritoid growth in the higher units thereafter pressure increased with the temperature remaining constant or even decreasing chloritoid replaced by kaolinite oxides and carpholite growth in synkinematic conditions the peak p t conditions are close to 8 kbar 270 c for the muscat nappes and 11 kbar 400 c for the as sifah unit the corresponding surface related gradient is low 10 15 c km during the retrograde evolution pressure and temperature generally decreased concomitantly but in some of the upper units temperature remained constant or increased slightly for some time at which point sudoite growth from carpholite took place the metamorphic pattern and mineral evolution are interpreted in the framework of the late cretaceous obduction the already cold oceanic lithosphere was thrust upon a relatively hot thinned passive margin with coeval basaltic volcanism pressure progressively increased in the passive margin due to the wedge shape of the advancing ophiolite the maximum thickness of which was about 25 km in the as sifah area in this area the overburden was also attributable to the accumulation of continental material up to 5 km thick by means of a complex set of synmetamorphic often conjugated folds and thrusts the basal peridotites of the obducted slab were probably not hotter than 300 c during its thrusting upon the continental margin the temperature of the tectonic pile below the ophiolite remained low because it was thrust above more external and still colder continental areas late thrusting in the same southerly direction controlled the retrograde evolution together with erosional and gravity driven tectonic unloading

mechanisms of inheritance of rift faulting in the western branch of the east african rift tanzania the western branch of the east african rift system is commonly cited as a result of phanerozoic reactivation of the paleoproterozoic ubendian belt in western tanzania geological evidence is provided to show that prominent mechanical anisotropies successively appeared during proterozoic evolution of the precambrian basement and that their different reactivation behavior contributed to the phanerozoic rift pattern the ubende belt 1950 1850 ma is a nw oriented amphibolite facies ductile lateral shear belt in which older 2100 2025 ma and complex granulite facies terranes are included along trend retrograde multiphase sinistral strike slip mylonites developed along the nw oriented ductile shear belt they reflect persistent proterozoic wrench fault reactivation of the latter shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the proterozoic wrench fault reactivation neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone which appears as an elongate and nw trending positive flower structure locally enhanced by late proterozoic contraction phanerozoic rifting is demonstrated by others to occur in three distinct episodes during which the complex rift segment formed upon the multiphase proterozoic wrench fault zone the evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered the proterozoic prerift fabrics correspond with a dextral transpressional and ductile deformational pattern which became selectively reactivated by sinistral transpressional ductile brittle mylonites proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults according to the position of these mylonites in the center or in the external parts of their nw oriented neoproterozoic transpression they reactivate as complex and multiphase rift faults or as normal and recent faults respectively the paleoproterozoic nw oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy its reactivation in phanerozoic stress fields is likely dextral oblique transtension considered as a leading mechanism of the pluriphase and nw oriented deep rift basins

structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains abstract 170 m y of relatively stable passive margin sedimentation along the northern continental margin of arabia was abruptly terminated during the cenomanian turonian 95 ma when the oman continental margin collapsed and subsided rapidly aruma basin to accommodate obduction of the semail ophiolite complex and underlying thrust sheets haybi and hawasina complexes in the oman mountains the ophiolite was emplaced at least 200 km over the passive margin probably over 450 km in total from ne to sw during the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite bearing rocks 6 8 kbar blueschist 12 15 kbar and eclogite ca 20 kbar facies metamorphism formed in a ductile deforming ne dipping subduction zone along the northeastern margin of the saih hatat culmination in ne oman the mesozoic shelf carbonates are deformed by recumbent folding on all scales the present day structure is dominated by four sw vergent thrust slices in carpholite grade rocks at higher structural levels overlying lower units of crossite bearing blueschist hulw unit garnet glaucophane chloritoid blueschists and garnet clinopyroxene phengite glaucophane eclogites 15 20 kbar the largest shear zone separates 7 8 kbar hulw unit blueschists above from 12 15 kbar retrogressed eclogites and ca 20 kbar eclogites of the as sifah unit below further inboard sw a major shear zone separates the lower plate hulw blueschist facies rocks from the upper plate sedimentary rocks upper plate lower plate discontinuity which also show carpholite grade hp metamorphism towards the ne restoration of the entire saih hatat continental margin shows that the upper plate lower plate discontinuity cuts down section to the sw as far as the proterozoic hatat schists ne directed extensional crenulation schistosity and nne oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with sw directed exhumation of footwall hp rocks ne facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds with shortening in the upper plate balanced by the subduction of the lower plate consistent with a ne directed subduction of the continental margin rocks beneath the sw obducting ophiolite haybi and hawasina thrust sheets recent suggestions of a nascent sw directed subduction beneath the oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of saih hatat two main phases of tectonic uplift of saih hatat and jebel akhdar are firstly the latest cretaceous ramp culmination associated with foreland propagating thrusts in the sw and backfolds in the ne and secondly post late eocene oligocene uplift by ca 2000 m

high resolution record of tectonic and sedimentary processes in growth strata growth strata are used to determine the kinematics of synsedimentary structures such as faults classical methods of analysis such as thickness versus throw plot consider that the available space created by fault slip in the hanging wall of faults is instantaneously filled up by sediments this has lead many previous works to identify a cyclic activity for growth faults here we perform a careful analysis of the variation of strata thicknesses on each side of a very well documented normal growth fault in the niger delta we show that these thickness variations are induced by the alternation of sedimentary processes during continuous fault slip suspended load processes induce either uniform or slightly variable thickness of a large majority of mudstone layers bedload processes result in a preferential thickening of sand layers in the hanging wall these high quality data thus provide strong grounds for doubting the polycyclic growth diagnosed for some faults at the scale of sedimentary cycles and supports the notion that fault displacement rates can be very well behaved our study emphasizes the important conclusion that stable fault growth and related displacement rates can appear to be punctuated when viewed at the scale of sedimentary cycles it follows that care should be taken when attempting to derive displacement rates on temporal scales equivalent to those of alternating sedimentological cycles

temporal and spatial development of a gravity driven normal fault array middle upper jurassic south viking graben northern north sea abstract three dimensional seismic and well data from the south viking graben northern north sea basin is used to investigate the temporal and spatial development of a gravity driven normal fault array above an evaporite rich detachment two moderate throw 500 900 m middle to upper jurassic normal faults the gudrun and brynhild faults are developed within the study area both faults die out laterally and tip out upwards at different structural levels within the syn rift succession both faults terminate downwards into late permian evaporites zechstein group and do not offset pre evaporite basement units this thin skinned fault array developed in response to westwards tilting of the hangingwall of the south viking graben during late jurassic rifting and consequent westward gliding and extensional break up of units above the mechanically weak evaporite horizon isochron mapping and well based correlation of middle to upper jurassic syn rift units allow constraints to be placed on the temporal evolution of the fault array several stages of structural development are observed which document i a period of relatively minor early i e pre rift halokinesis ii variable spatial activity on individual faults within the array and iii the progressive upslope migration of active faulting within the array as a whole the progressive upslope migration of fault activity is interpreted to reflect progressive unbuttressing and extensional faulting of upslope post evaporite units the overall structural style and kinematic evolution identified here shares many characteristics with both rift raft tectonics documented in other rifts developed above an evaporitic sub stratum and raft tectonics described from passive margin basins containing thick mobile salt or shale intervals this style of fault array evolution differs from that observed in rifts lacking mobile layers at depth and highlights the importance of these units in the structural development of rifts

interaction of permo triassic and jurassic extensional fault blocks during the development of the northern north sea the main structural elements resulting from late permian early triassic extension have been resolved across the total width of the northern north sea the northern segment of the basin north of 61 n bounded to the west by the hutton fault and to the east by the norwegian mainland was occupied by four westerly tilted half grabens 25 35 km in width prior to jurassic extension the central segment of the basin 60 20 61 n exhibits a full graben beneath the jurassic horda platform flanked by half grabens tilted towards the basin margins the change in permo triassic fault block configuration defining structural domains is largely constrained by a basement discontinuity which runs diagonally ne sw across the basin maximum jurassic extension is represented by several centres of subsidence aligned nne ssw across a n s oriented and broader permo triassic basin jurassic extension was concentrated within the viking graben proper south of 60 n which in this area is represented by single westerly tilted en echelon half graben segments 20 25 km wide the area affected by jurassic extension becomes progressively wider to the north again governed by the basement discontinuity eventually encompassing the total width of the basin c 200 km north of 61 n the central segment of the northern north sea displays the greatest contrast in permo triassic and jurassic structural styles and basin geometry jurassic stretching and faulting was negligible to the east horda platform precisely the area which had been the site of maximum permo triassic stretching and fault activity it has generally been assumed that jurassic fault configuration along the eastern margin of the central segment of the viking graben to a large extent reflects the reactivation of major permo triassic faults this study demonstrates the interference of permo triassic and jurassic basin geometries in this area due to major faults of the two generations dipping in opposite directions as a consequence the e tilted oseberg fault block of jurassic origin and c 20km in width is underlain by a wider c 35 km and w tilted permo triassic fault block

planar detaching faults in the southern horn graben danish north sea the danish horn graben is located in the south eastern north sea and suffered fault controlled differential subsidence during the triassic within the horn graben several faults cut the triassic succession but detach at or close to the top pre zechstein surface an e w striking trend of planar detaching faults is mapped using 2 d seismic sections and the displacement on the faults is analysed with respect to timing and spatial distribution of the displacement the spatial distribution of the displacement is analysed using the fault analysis projection system badley earth sciences whereas the timing is analysed by reconstructing the seismic sections and thus obtaining the spatial distribution of displacement at different times the occurrence of the detaching planar faults appears to be controlled by the difference in basal friction across the pinch out line of the mobile zechstein sediments a dipping detachment surface is necessary to create a driving mechanism but is of minor importance in initiating the faulting the timing of the faulting is controlled by a general change in tectonic pattern enabling a n s extension of the sediments covering the top pre zechstein surface

exhumation of ultrahigh pressure rocks beneath the hornelen segment of the nordfjord sogn detachment zone western norway the nordfjord sogn detachment zone of western norway represents an archetype for crustal scale normal faults that are typically cited as one of the primary mechanisms responsible for the exhumation of ultrahighpressure uhp terranes in this paper we investigate the role of normal sense shear zones with respect to uhp exhumation using structural geology thermobarometry and geochronology of the hornelen segment of the nordfjord sogn detachment zone the hornelen segment of the zone is a 2 6 km thick top w shear zone primarily developed within amphibolite grade allochthonous rocks that juxtaposes the uhp rocks of the western gneiss complex in its footwall with lower grade allochthons and carboniferous devonian basins in its hanging wall new thermobarometry and sm nd garnet geochronology show that these top w fabrics were initiated at lower crustal depths of 30 40 km between 410 ma and 400 ma structural geology and quartz petrofabrics indicate that top w shear was initially relatively evenly distributed across the shear zone and then overprinted by discrete ductile brittle detachment faults at slower strain rates during progressive deformation and exhumation these results require a three stage model for uhp exhumation in which normal sense shear zones exhumed uhp rocks from the base of the crust along initially broad ductile shear zones that were progressively overprinted by discrete ductile brittle structures

structure and metamorphism of blueschist eclogite facies rocks from the northeastern oman mountains the northern part of the saih hatat window oman shows high pressure metamorphic rocks derived from shelf sediments and pre premian continental basement and is atypical of sub ophiolite metamorphism elsewhere the high pressure rocks are divided into structural units originally bounded by foreland propagating thrusts formed during ophiolite obduction although now many contacts are backthrusts normal faults or extensional shear zones metamorphic breaks exist across many unit boundaries the deepest unit as sifah has eclogite facies assemblages in metabasites and metapelites which record evolution of p t conditions along a clockwise path culminating al 23 2 5kbar 540 75 c contrasting markedly with overlying units 5 10 kbar 200 500 c although separated by 40 ar 39 ar apparent ages our tectonic model relates all the high p units to a single convergent event in the late cretaceous the as sifah eclogites were exhumed in two stages i tectonic emplacement against other units at c 20 25 km depth and ii exhumation of the entire high p zone by culmination collapse after obduction

quantitative analysis of triassic extension in the northern viking graben it has been recognized for over a decade that large displacement pre jurassic faults are present in the northern viking graben part of the north sea rift these faults define a series of major fault blocks below the more obvious jurassic rift basin we attempt here to quantify the amount of extension associated with this older rift event which is probably of early triassic age quantitative modelling of the triassic rift and the succeeding period of thermal subsidence has been undertaken using a combination of flexural backstripping and flexural cantilever forward modelling these techniques suggest that triassic extension across the horda platform norwegian sector reached c 40 s 1 4 the consequences of this extension were deposition of a thick 3 km triassic upper jurassic syn rift and post rift sequence prior to renewed but minor extension in the late jurassic earliest cretaceous the thickness of the viking group reservoirs in the troll area appears to have been almost entirely controlled by sediment loading during post triassic thermal subsidence jurassic extension on the horda platform was in the uk sector of the viking graben triassic structures are less obvious than those below the horda platform because of jurassic overprinting they are however still present average triassic extension across the east shetland basin was c 15 comparable with the magnitude of jurassic extension in the same area we believe that the tern eider and cormorant fault blocks with proven shallow basement comprised a large eroded horst during the early triassic uplifted in the footwalls of major faults flanking the magnus and statfjord half graben during the triassic the magnus half graben was contiguous with the unst basin now situated in the western footwall of the younger jurassic basin the presence of the unst basin suggests that triassic extension occurred across the area that is now the northern shetland platform continuing into the west shetland area although the more obvious structures in the viking graben are jurassic in age the earlier triassic event was equally as important in controlling the structural and stratigraphic history of the basin

re orientation of the extension direction and pure extensional faulting at oblique rift margins comparison between the main ethiopian rift and laboratory experiments in this study we draw on a unique combination of well resolved fault slip data and earthquake focal mechanisms to constrain spatial variations in style of faulting in the obliquely extending main ethiopian rift east africa these data show that both boundary and internal faults oblique and orthogonal to the plate divergence pd respectively exhibit almost pure dip slip motion and indicate significant local deflection in orientation of the extension direction at rift margins scaled analogue models closely replicate the multidisciplinary observations from the rift and suggest that the process is controlled by the presence of a deep seated pre existing weakness oblique to the direction of pd that is able to cause a local rotation in the orientation of the extension direction at rift margins minor counterclockwise block rotations are required to accommodate the difference in slip direction along the different fault systems as supported by existing and new palaeomagnetic data from the rift

patterns of basement structure and reactivation along the ne atlantic margin a lineament pattern on the ne atlantic margin is discussed illustrated by gravity and magnetic images in the norwegian sea and reviewed in the context of onshore field evidence while most possible fault trends exist three major sets predominate a ne sw left stepping lineament set defines the gross geometry of the margin while interposing northerly trends impose a rhomboidal geometry at a variety of scales the margin is segmented by nw se transfer zones sometimes involving significant offsets the principal trends are primarily a function of mesozoic cenozoic plate wide extensional stress fields certain proterozoic and caledonian lineaments were however opportunistically reactivated according to the extension direction caledonian ne sw orogen oblique shears typified by the more trondelag fault zone were reactivated via jurassic strike slip or oblique slip and were further exploited during cretaceous early cenozoic extensional episodes leading to continental break up jurassic e w extension may also have reactivated n s faults existing in the basement or generated in duplex systems between the ne sw shears precambrian and caledonian basement lineaments striking at a low angle to the extension direction probably predisposed the formation of major transfer zones

geometry and significance of internal windows and regional isoclinal folds in northeast saih hatat sultanate of oman saih hatat contains tiered sets of low angle structural breaks that reflect a complex stacking of sheets which requires multiple upward movement of units within an unstable orogenic wedge in a convergent setting early recumbent folds within the saih hatat window are subparallel to but cut by a major low angle contractional fault decollement that separates lower plate high pressure rocks some eclogites exposed in two windows hulw and as sifah windows from high pressure upper plate rocks containing carpholite and lawsonite parts of this decollement had been mapped or speculated upon to explain the variation in metamorphic grade across saih hatat upper plate fold nappes are controlled by the mapped decollement surface below the decollement sheath like lower plate closures formed independently in an intense zone of top to the northeast shearing eclogites occur within mafic boudins at the structurally lowest level of the retrograde lower plate shear zone this zone was responsible for the partial exhumation of the lower plate very early in the observed structural history and has a normal movement sense the two lower plate windows are essentially foliation domes reflecting fold interference between regional scale sheath like recumbent isoclines and more open upright northwest north and northeast trending fold sets the early upper plate fold nappes reflect a major top to the north or northeast movement of the rocks away from the oman margin at the same time the ophiolite was being emplaced over the top in the opposite direction top to the southwest or south south directed faulting and shearing is superimposed on the early recumbent folds in the mountain range mapped relationships and structural observations agree with bureau de recherches geologiques et minieres mapping that recognised fold nappes in this part of the oman mountains all of the mapped structures are associated with exhumation of the high pressure rocks making reconstruction of the margin during high pressure metamorphism problematic

3d seismic analysis of the structure and evolution of a salt influenced normal fault zone a test of competing fault growth models abstract in this paper we determine the structure and evolution of a normal fault system by applying qualitative and quantitative fault analysis techniques to a 3d seismic reflection dataset from the suez rift egypt our analysis indicates that the october fault zone is composed of two fault systems that are locally decoupled across a salt bearing interval of late miocene messinian age the sub salt system offsets pre rift crystalline basement and was active during the late oligocene early middle miocene it is composed of four planar nw se striking segments that are hard linked by n s striking segments and up to 2 km of displacement occurs at top basement suggesting that this fault system nucleated at or more likely below this structural level the supra salt system was active during the pliocene holocene and is composed of four nw se striking listric fault segments which are soft linked by unbreached relay zones segments in the supra salt fault system nucleated within pliocene strata and have maximum throws of up to 482 m locally the segments of the supra salt fault system breach the messinian salt to hard link downwards with the underlying sub salt fault system thus forming the upper part of a fault zone composed of i a single amalgamated fault system below the salt and ii a fault system composed of multiple soft linked segments above the salt analysis of throw distance t x and throw depth t z plots for the supra salt fault system isopach maps of the associated growth strata and backstripping of intervening relay zones indicates that these faults rapidly established their lengths during the early stages of their slip history the fault tips were then effectively pinned and the faults accumulated displacement via predominantly downward propagation we interpret that the october fault zone had the following evolutionary trend i growth of the sub salt fault system during the oligocene to early middle miocene ii cessation of activity on the sub salt fault system during the middle miocene to early pliocene iii stretching of the sub and supra salt intervals during pliocene regional extension which resulted in mild reactivation of the sub salt fault system and nucleation of the segmented supra salt fault system which at this time was geometrically decoupled from the sub salt fault system and iv pliocene to holocene growth of the supra salt fault system by downwards vertical tip line propagation which resulted in downward breaching of the salt and dip linkage with the sub salt fault system the structure of the october fault zone and the rapid establishment of supra salt fault lengths are compatible with the predictions of the coherent fault model although we note that individual segments in the supra salt array grew in accordance with the isolated fault model our study thereby indicates that both coherent and isolated fault models may be applicable to the growth of kilometre scale basin bounding faults furthermore we highlight the role that fault reactivation and dip linkage in mechanically layered sequences can play in controlling the three dimensional geometry of normal faults

how do the properties of a pre existing normal fault population influence fault development during a subsequent phase of extension abstract we use scaled experimental analog models to investigate how the properties of a population of pre existing normal faults influence fault development during a subsequent phase of extension in the models a homogeneous layer of wet clay undergoes two phases of extension whose extension directions differ by 45 to vary the properties of the first phase fault population we vary the magnitude of the first phase extension as the magnitude of the first phase extension increases the number average and maximum length and average and maximum displacement of the first phase normal faults increase for a poorly developed first phase fault population new normal faults which strike perpendicular to the second phase extension direction form during the second phase for a well developed first phase fault population many first phase normal faults are reactivated as oblique slip faults during the second phase new normal faults also form these second phase normal faults are shorter and have displacement maxima adjacent to the reactivated first phase faults they are less likely to cut the pre existing first phase faults than second phase normal faults that form in models with a less developed first phase fault population in all models pre existing faults serve as nucleation sites for new faults in models with a well developed first phase fault population the pre existing faults also act as obstacles to the propagation of the second phase normal faults fault geometries in the models vary considerably parallel zig zag or intersecting fault geometries develop depending on whether the first phase fault population is poorly moderately or well developed respectively

structural analysis of the smeaheia fault block a potential co2 storage site northern horda platform north sea abstract smeaheia a prominent fault block located on the horda platform northern north sea is identified as a potential subsurface co2 storage site we utilise the gn1101 3d seismic survey to generate a high resolution subsurface geomodel to inform the structural style and evolution of the fault block to investigate geological controls on proposed co2 storage and provide a geometric framework as a basis for future analyses two basement involved first order north south trending fault systems the vette fault zone vfz and the oygarden fault complex ofc bound the 15 km wide fault block the vfz bifurcates down section where it is hard linked with two separate basement structures a phenomenon we term as dual rooted apart from activity during the permo triassic rift phase 1 and the late jurassic early cretaceous rift phase 2 we present evidence that rifting in this part of the north sea continued into the late cretaceous with minor reactivation in the palaeocene eocene two segments of the vfz interacted and linked at a relay ramp during rift phase 2 second order thin skinned faults show basement affinity and developed during rift phase 2 in two distinct pulses a population of polygonal faults intersects the overburden and developed during the eocene to middle miocene we have revised the areal extent of two structural closures that define the smeaheia fault block alpha vfz footwall and beta ofc hanging wall which consist of upper jurassic viking group target formations simplified cross fault juxtaposition analysis of the vfz and second order intra block faults are presented and inform pressure communication pathways between the smeaheia and tusse fault block as well as reservoir integrity and compartmentalisation the geomodel further identifies important geological controls on co2 storage in the fault block including a thinning caprock above the alpha structure and identification of hard linkage between deep tectonic faults and shallow polygonal faults