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

modes of continental lithospheric extension experimental verification of strain localization processes abstract three modes of continental lithospheric extension narrow rift wide rift and core complex were studied by means of analogue experiments the experimental method is based on models which simulate an idealized lithospheric strength profile and are scaled for gravity various styles of lithospheric extension were induced by systematic variation of brittle upper crustal thicknesses narrow rifting occurs during stable lithospheric extension when the resistance of the brittle upper crust dominates the deformation process wide rifting results from unstable lithospheric extension when the brittle upper crust is thin and ductile flow in the lower crust allows expansion of the deformation zone periodic instabilities with two wavelengths dominate wide rifting core complex mode extension is transitional between unstable and stable lithospheric extension it begins as a distributed zone of unstable extension due to thin upper brittle crust but lower crustal flow induced by the gravity force causes a relocalization of strain into narrow zones transitions between the three modes of extension are characterized in terms of competition between the strength of lithosphere and gravity forces as well as between stable and unstable deformation processes

tectonics and topography for a lithosphere containing density heterogeneities the purpose of this paper is to clarify the dynamic role of lithospheric density heterogeneities in particular with respect to mountain building and other processes of intraplate deformation density anomalies within or just beneath the lithosphere constitute major sources for tectonic stress fields the product of their magnitude by their depth is shown to characterize their ability to induce deformation this rule of the density moment directly yields the lithospheric thickening or thinning rate when applied to structures of large lateral extent for anomalies of lateral extent that is small in comparison with their depth the deformation is vertically inhomogeneous and has been computed with the help of simple physical models of a stratified viscous newtonian lithosphere the analytical treatment is based on fourier transform continent continent collision thickens not only the crust but the entire lithosphere the cold root underlying a mountain chain induces strong regional compressive stresses able to sustain the mountain bulding process without further help from forces transmitted from far away thus the continental lithosphere is in a somewhat metastable mechanical state adiabatic i e rapid thickening or thinning tends to grow further once initiated tectonic phases of strong compression correspond to the climax of such instabilities the response of models with cold lithospheric roots of various intensities has been computed both in two and three dimensions they yield velocity distributions and stress fields instructive comparisons are made with earthquake focal mechanisms and in situ stress measurements in the alpine and appalachian regions in the presence of lateral variations of the mechanical properties of the lithosphere the tectonic style is not only function of the local topography and of the nature of its compensation deformations in neighbouring provinces are coupled as shown by 3 dimensional models for example thickening sustained by a cold lithospheric root may generate extension in peripheral zones of weakness these last results illustrate the point that the computation of regional tectonic stresses requires the knowledge of the density anomalies within the lithosphere on the one hand and of geometrical constraints related to lateral mechanical heterogeneities on the other

viscoelastic relaxation in a heterogeneous earth following the 2004 sumatra andaman earthquake abstract consideration of the three dimensional heterogeneity of mantle rheology allows models of viscoelastic relaxation following the 2004 sumatra andaman earthquake to simultaneously fit both the observed far field and near field postseismic deformation we use horizontal and vertical campaign and continuous gps observations from the andaman nicobar and sumatran forearc islands mainland sumatra thailand the malay peninsula the indian ocean and southern india spanning the first five years of postseismic deformation the postseismic relaxation models consider contributions from the 2004 mw 9 2 sumatra andaman the 2005 mw 8 7 nias and 2007 mw 8 4 bengkulu earthquakes far field motions to the east of the ruptures are equally well fit by homogeneous or laterally variable earth models however only models with contrasting rheology across the subducting slab a ten times higher mantle viscosity under the indian ocean lithosphere than the backarc mantle can also produce the observed enduring postseismic uplift along the forearc and lack of far field transient displacements in southern india while postseismic uplift of forearc stations can also be produced by rapid and enduring down dip afterslip the inferred rheology structure is consistent with the distribution of mantle temperature inferred from seismic tomography

structural features in a brittle ductile wax model of continental extension structural features produced during the rifting of continents depend on the layered rheological properties of the crust and lithosphere and in particular on the presence of any transitions between brittle and ductile behaviour1 here we use a wax model to explore the gross structural response of continental lithosphere under pure shear extension in the presence of a continuous brittle ductile transition the wax models were deformed under various boundary conditions to reflect a variety of different regions most notably the basin and range province of north america our experiments show the development of listric normal faults structures common to regions of continental extension we also observe the formation of distributed and discrete rifting and intrusion and occlusion of the upper brittle layer by the ductile lower layer the factor controlling deformation style in each case appears to be the relative thickness of the brittle and ductile layers although a relatively high rate of strain generally promotes discrete rifting

the mohorovicic discontinuity beneath the continental crust an overview of seismic constraints abstract the seismic signature of the moho from which geologic and tectonic evolution hypotheses are derived is to a large degree a result of the seismic methodology which has been used to obtain the image seismic data of different types passive source earthquake broad band recordings and controlled source seismic refraction densely recorded wide angle deep seismic reflection and normal incidence reflection using vibroseistm explosives or airguns have contributed to the description of the moho as a relatively complex transition zone of critical importance for the quality and resolution of the seismic image are the acquisition parameters used in the imaging experiments a variety of signatures have been obtained for the moho at different scales generally dependent upon bandwidth of the seismic source this variety prevents the development of a single universally applicable interpretation in this way source frequency content and source and sensor spacing determine the vertical and lateral resolution of the images respectively in most cases the different seismic probes provide complementary data that gives a fuller picture of the physical structure of the moho and its relationship to a petrologic crust mantle transition in regional seismic studies carried out using passive source recordings the moho is a relatively well defined structure with marked lateral continuity the characteristics of this boundary change depending on the geology and tectonic evolution of the targeted area refraction and wide angle studies suggest the moho to be often a relatively sharp velocity contrast whereas the moho in coincident high quality seismic reflection images is often seen as the abrupt downward decrease in seismic reflectivity the origin of the moho and its relation to the crust mantle boundary is probably better constrained by careful analysis of its internal details which can be complex and geographically varied unlike the oceanic moho which is formed in a relatively simple well understood process the continental moho can be subject to an extensive variety of tectonic processes making overarching conclusions about the continental moho difficult speaking very broadly 1 in orogenic belts still undergoing compression and active continental volcanic arcs the moho evolves with the mountain belt 2 in collapsed phanerozoic orogenic belts the moho under the collapse structure was formed during the collapse often by a combination of processes 3 in regions having experienced widespread basaltic volcanism the moho can result from underplated basalt and basaltic residuum in precambrian terranes the moho may be as ancient as the formation of the crust in others precambrian tectonic and magmatic processes have reset it we note that seismic reflection data in phanerosoic orogens as well as from precambrian cratonic terranes often show thrust type structures extending as deep as the moho and suggest that even where crust and mantle xenoliths provide similar age of formation dates the crust may be semi allochothonous

crustal structures revealed from a deep seismic reflection profile across the solonker suture zone of the central asian orogenic belt northern china an integrated interpretation abstract the solonker suture zone is one of the most important tectonic boundaries in the southeastern part of the central asian orogenic belt caob an 630 km long reflection seismic profile across this suture was recently completed by the chinese sinoprobe project the processed seismic data show clear crustal structures and provide new constraints on the tectonic and crustal evolution models the moho is delineated as a relatively flat boundary between a strongly reflective lower crust and a transparent mantle at a depth of 40 45 km 14 5 s two way travel time which is in agreement with the refraction data recorded along the same profile in a broad view the profile images an orogen that appears bivergent with and approximately centered on the solonker suture zone the southern portion of this profile is dominated by a crustal scale cratonward propagating fold and thrust system that formed during the late permian and triassic through collision and subsequent convergence in a post collisional stage the major thrust faults are truncated by mesozoic granitoid plutons in the upper crust and by the moho at the base of the crust this geometry suggests that the moho was formed after the thrusting event the northern portion of the profile although partially obliterated by post collisional magmatic bodies shows major south dipping folding and thrusting bands of layered reflectors immediately overlying the moho are interpreted as basaltic sills derived from the mantle episodic mafic underplating may have occurred in this region giving rise to post collisional magmatic events and renewal of the moho a few mantle reflectors are also visible the overall geometry of these mantle reflectors supports the tectonic models that the southern orogen manchurides experienced south directed subduction and the northern orogen altaids underwent north directed subduction prior to collision along the solonker suture zone

extension with lateral material accommodation active vs passive rifting abstract in many rifted areas of the world differential thinning is observed between upper and lower crust or between crust and lithospheric mantle a kinematic model based on a finite element algorithm is presented which allows thermal modelling of differential stretching in order to avoid space problems the velocity field is defined under the assumption that the total amount of extension is the same for every lithospheric layer several combinations of far field velocity and local extensional velocities in the rift area have been used to simulate basin formation under different conditions of active far field velocity smaller than extensional velocities and passive rifting consequences on elevation surface heat flow and lithospheric yield strength are investigated far field velocities lower than local extensional velocities produce narrow basins bordered by high shoulders whereas a wide basin without shoulders is obtained when the far field velocity is similar to the maximum extensional velocity a detailed study is presented for the first velocity distribution when the detachment is located in the lower crust a thickening of the crust occurs below the rift shoulders which continue to rise during the post rift phase lithospheric strength below the shoulders increases strongly during rifting due to cooling of the lithospheric caused by the down welling of material in this area in contrast post rift thermal relaxation leads to a rapid decrease of strength resulting eventually in a minimum located at the rift shoulders

centrifuge modelling of the influence of crustal fabrics on the development of transfer zones insights into the mechanics of continental rifting architecture abstract centrifuge analogue experiments are used to model the reactivation of pre existing crustal fabrics during extension the models reproduced a weakness zone in the lower crust whose geometry was varied in order to investigate its role in controlling the architecture of rift segments and related transfer zones the typical rift system geometry was characterised by two offset rift segments connected by a major transfer zone in which boundary faults were oblique to the extension vector and displayed a significant transcurrent component of movement the transfer zone was also characterised by cross basin faults with both trend and strike slip component of movement opposite to that displayed by the master faults typically different structural patterns were obtained by changing the offset angle between the rift segments supporting that the structural pattern at transfer zones is strongly influenced by the orientation of pre existing discontinuities with respect to the stretching vector in the models the aspect ratio ratio of length vs width of the transfer zone shows a positive correlation with the offset angle i e the more the inherited fabric is parallel to the extension direction the longer and narrower the transfer zones in case of staircase offset of the rift segments 90 the structural pattern was characterised by two isolated rift depressions linked by a narrow transfer zone in which border faults with alternating polarity overlapped prominent rise of the ductile lower crust was also observed at the transfer zone many of these geometrical features display striking similarities with natural rift systems the results of the current experiments provide useful insights into the mechanics of continental rift architecture supporting that rift propagation width and along axis segmentation may be strongly controlled by the reactivation of pre existing pervasive crustal fabrics

the role of transfer structures on volcanic activity at campi flegrei southern italy abstract the tyrrhenian margin of central italy underwent extension during pliocene and quaternary extension occurred mainly through nw se normal faults bordering a sequence of plio quaternary basins these basins are offset by coeval ne sw faults which show strike slip and normal motions and have been interpreted as transfer faults plio quaternary volcanic activity along the margin occurred along a nw se belt systematically in correspondence with ne sw transverse systems the campi flegrei volcanic district cfvd on the southern tyrrhenian margin consists of an active ne sw volcanic ridge developed along ne sw fractures we performed a structural field analysis with analogue and mechanical models to investigate the role of transverse structures upon volcanism at campi flegrei field analysis at campi flegrei recognized ne sw and to a lesser extent coeval nw se active fractures analogue experiments have simulated the development of transfer fault systems in brittle extensional domains the experiments show that subvertical transfer faults connect offset adjacent normal faults dipping 60 the mechanical model is based on the stress equations in uniaxial lithostatic conditions and absence of regional stresses it shows how pre existing subvertical fractures require the smallest magmatic pressures to be penetrated for a given magmatic pressure subvertical fractures might be penetrated more deeply tapping more easily primitive magmas these results suggest that the cfvd is located along a ne sw transfer zone connecting nw se regional normal faults volcanic activity along such ne sw trend would be induced by the subvertical dip of the transfer faults the subvertical dip of transfer faults also suggests an explanation for the emission of the more primitive products along ne sw systems at campi flegrei these considerations find a wider application on the remaining volcanic districts of the margin located in the same overall structural setting

evidence for motion between nubia and somalia along the southwest indian ridge the east african rift marks the northern boundary of the nubian west african and somalian east african plates and has formed by horizontal stretching due to the separation of these plates1 south of 20 s any expression of deformation or seismicity due to the relative motion of these two distinct plates vanishes although the boundary must continue until it intersects another plate boundary the nearest such boundary is that of the antarctic plate marked by the southwest indian ridge but previous analyses of plate motion data have indicated no significant difference between nubia antarctica and somalia antarctica motion2 3 here we show using a large compilation of plate motion data that nubia antarctica motion does differ from somalia antarctica motion and we determine a relative angular velocity of the two plates that has compact confidence limits our analysis places the pole of rotation near to the southern limit of african seismicity implying that the southern part of the nubian somalian plate boundary is a diffuse zone of convergence up to 2 mm yr 1 whereas up to 6 mm yr 1 of separation is accommodated across the east african rift about half the separation rate of the slowest mid ocean ridge

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

cenozoic thermal mechanical and tectonic evolution of the rio grande rift careful documentation of the cenozoic geologic history of the rio grande rift in new mexico reveals a complex sequence of events at least two phases of extension have been identified an early phase of extension began in the mid oligocene about 30 ma and may have continued to the early miocene about 18 ma this phase of extension was characterized by local high strain extension events locally 50 100 regionally 30 50 low angle faulting and the development of broad relatively shallow basins all indicating an approximately ne sw 25 extension direction consistent with the regional stress field at that time extension events were not synchronous during early phase extension and were often temporally and spatially associated with major magmatism a late phase of extension occurred primarily in the late miocene 10 5 ma with minor extension continuing to the present it was characterized by apparently synchronous high angle faulting giving large vertical strains with relatively minor lateral strain 5 20 which produced the modern rio grande rift morphology extension direction was approximately e w consistent with the contemporary regional stress field late phase graben or half graben basins cut and often obscure early phase broad basins early phase extensional style and basin formation indicate a ductile lithosphere and this extension occurred during the climax of paleogene magmatic activity in this zone late phase extensional style indicates a more brittle lithosphere and this extension followed a middle miocene lull in volcanism regional uplift of about 1 km appears to have accompanied late phase extension and relatively minor volcanism has continued to the present we have estimated geotherms and calculated lithospheric strength curves for the two phases of extension using geologic data to constrain earlier events and geophysical data to constrain the modern geotherm and crustal structure a high geotherm was deduced for early phase extension resulting in a shallow crustal brittle ductile transition and negligible mantle strength the lithosphere cooled after early phase extension resulting in a deeper crustal brittle ductile transition and perhaps more significantly a considerable zone of mantle strength immediately beneath the moho these results indicate that early phase extensional style was controlled by a crustal decollement near the brittle ductile transition which was prevented during late phase extension by significant strength in the uppermost mantle late cenozoic uplift of the rift zone cannot be explained by crustal thinning during extension and geotherm evolution predicted from simple cooling however this uplift does not appear to be restricted to the rift zone and pliocene to recent volcanism and heat flow data suggest that uplift may be caused by magmatic thickening of the crust perhaps unrelated to rifting the complex interrelationship among regional and local prerifting synrifting and postrifting events in the rio grande rift suggests that rifting at least in this region should not be considered in isolation of other geologic events

structural characteristics of the yilan yitong and dunhua mishan faults as northern extensions of the tancheng lujiang fault zone new deep seismic reflection results abstract the tancheng lujiang fault zone tlfz can be subdivided into three segments that exhibit sharp contrasts in their deep structures a deep seismic reflection profile length 600 km across the north part of the tlfz which provides new constraints on the structural styles of the northern tlfz was recently completed by the chinese sinoprobe project here the tlfz branches into the yilan yitong fault yyf to the west and the dunhua mishan fault dmf to the east the yyf developed as an internal fault in the songnen zhangguangcai massif while the dmf serves as the tectonic boundary between the nadanhada terrane and the khanka massif both faults developed large scale flower structures with that of the yyf being negative and that of the dmf being positive with reverse faults the moho in the profile is at a depth of 25 39 km and is offset by the faults the north part of the tlfz extends into the upper mantle as thin shear zones with the reflectors truncated in the middle lower crust this feature differs from most crustal scale strike slip faults that distribute over a discrete shear zone in the lower crust such as the san andreas fault

rheological and physical characteristics of crustal scaled materials for centrifuge analogue modelling abstract we characterize a set of analogue materials used for centrifuge analogue modelling simulating deformation at different levels in the crust simultaneously specifically we improve the rheological characterization in the linear viscoelastic region of materials for the lower and middle crust and cohesive synthetic sands without petroleum binding agents for the upper crust viscoelastic materials used in centrifuge analogue modelling demonstrate complex dynamic behaviour so viscosity alone is insufficient to determine if a material will be an effective analogue two series of experiments were conducted using an oscillating bi conical plate rheometer to measure the storage and loss moduli and complex viscosities of several modelling clays and silicone putties tested materials exhibited viscoelastic and shear thinning behaviour the silicone putties and some modelling clays demonstrated viscous dominant behaviour and reached newtonian plateaus at strain rates 2 s 1 while other modelling clays demonstrated elastic dominant power law relationships based on these results the elastic dominant modelling clay is recommended as an analogue for basement cratons inherently cohesive synthetic sands produce fine detailed fault and fracture patterns and developed thrust strike slip and extensional faults in simple centrifuge test models these synthetic sands are recommended as analogues for the brittle upper crust these new results increase the accuracy of scaling analogue models to prototype additionally with the characterization of three new materials we propose a complete lithospheric profile of analogue materials for centrifuge modelling allowing future studies to replicate a broader range of crustal deformation behaviours

modelling the extension of heterogeneous hot lithosphere abstract the consequences of weak heterogeneities in the extension of soft and hot lithosphere without significant previous crustal thickening has been analysed in a series of centrifuge models the experiments examined the effects of i the location of heterogeneities in the ductile crust and or in the lithospheric mantle and ii their orientation perpendicular or oblique to the direction of bulk extension the observed deformation patterns are all relevant to the so called wide rifting mode of extension weak zones located in the ductile crust exert a more pronounced influence on localisation of deformation in the brittle layer than those located in the lithospheric mantle the former localise faulting in the brittle crust whereas the latter tend to distribute faulting over a wider area this latter behaviour depends in turn upon the decoupling provided by the ductile crust localised thinning in the brittle crust is accompanied by ductile doming of both crust and mantle domains of maximum thinning in the brittle crust and ductile crust and mantle are in opposition lateral differences in brittle crust thinning are accommodated by lateral flow in the ductile crust and mantle this contrasts with cold and strong lithospheres whose high strength sub moho mantle triggers a necking instability at the lithosphere scale this also differs from the extension of thickened hot and soft lithospheres whose ductile crust is thick enough to give birth to metamorphic core complexes thus for the given lithospheric rheology the models have relevance to backarc type extensional systems such as the aegean and the tyrrhenian domains

sand box modelling of basement controlled transfer zones in extensional domains the interaction modalities of transfer zones connecting rift segments may be influenced by several factors amongst these the location and architecture of transfer zones in narrow rifts has been repeatedly associated with the presence of inherited basement anisotropies sand box models were made to investigate how the orientation geometry and kinematics of transfer zones depend upon pre existing basement anisotropies analogue models reproduced offset rift segments linked by transfer zones bordered by arcuate normal faults strike slip faults are present inside the transfer zone provided that angle between the extension direction and the axis of the transfer zone is less than 50 narrower transfer zones striking oblique to the extension direction occur for angle between the direction perpendicular to the extension direction and the direction of the basement anisotropy less than 90 wider transfer zones subparallel to the extensional direction occur for 90 increasing the overstep induces narrower transfer zones striking subparallel to the extension direction similar geometrical and kinematical patterns have been found at transfer zones in narrow rifts the comparison between experimental and natural data shows how the geometry kinematics and orientation of natural transfer zones depends upon the trend of inherited anisotropies transfer zones along inherited basement structures set at lower angles to the extension direction and display lower aspect ratios than transfer zones where inherited anisotropies are absent

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

mechanics of continental rift architecture continental rifts are long narrow features bounded on one or both sides by normal faults along which several kilometers of displacement has occurred resulting in deep rift valleys that form sedimentary basins continental rifts all share the same geometrical features they have widths that are characteristic of the region in which they form they are predominantly asymmetric and they are segmented along axes at a length scale that is also characteristic of the region we describe a very simple mechanical model to explain these three features the rift initiates with the formation of a conjugate set of normal faults that intersect at the center of the seismogenic layer one of these becomes the boundary fault system the other the failed conjugate fault being offset by the former becomes locked early in the rifting history hence an asymmetric graben is formed which is the basic building block of rifts the failed conjugate fault has distinctive properties that allow it to be easily distinguished from other faults examples of these faults from the tanganyika and malawi rifts show excellent agreement with predictions about their positions with respect to the boundary fault system and their net displacement and slip history the width of the asymmetric graben is a simple function of seismogenic thickness the net displacement on the boundary fault system is limited by the resistive friction and the flexural restoring force we calculate this limiting boundary fault system displacement and using the displacement length scaling law for normal faults suggest that this controls the along axis segmentation length both maximum fault slip and segmentation length are functions of the seismogenic thickness and the effective elastic thickness of the crust these calculations agree very well with observations of segmentation from the african rifts and the maximum length of range fronting normal faults in nevada

continental rifting at pre existing lithospheric weaknesses continental rifting is the poorly understood thermomechanical process by which continents break up and new ocean basins form two dominant styles of continental rifting are recognized1 crevice style rifts are characterized by initial graben formation local uplift of the graben shoulders and little or no volcanism arch volcanic rifts exhibit an initial doming of the crust extensive volcanism and late stage graben formation the prevailing expla nation for the two styles is that crevice rifts form in response to regional horizontal stresses principally derived from the inter action of the lithospheric plates along their edges whereas arch volcanic rifts are thought to be the result of mantle convection currents which upwell directly beneath the rift axis2 we propose an alternative working model of continental rifting in which both styles initiate in response to regional horizontal stresses the different surface expressions of the two styles are explained in terms of differences in the mode of failure at different kinds of pre existing weaknesses in the continental lithosphere rather than differences in the nature of the driving forces involved

a cambrian mixed carbonate siliciclastic platform in sw gondwana evidence from the western sierras pampeanas argentina and implications for the early paleozoic paleogeography of the proto andean margin the western sierras pampeanas wsp of argentina record a protracted geological history from the mesoproterozoic assembly of the rodinia supercontinent to the early paleozoic tectonic evolution of sw gondwana two well known orogenies took place at the proto andean margin of gondwana in the cambrian and the ordovician i e the pampean 545 520 ma and famatinian 490 440 ma orogenies respectively between them an extensive continental platform was developed where mixed carbonate siliciclastic sedimentation occurred this platform was later involved in the famatinian orogeny when it underwent penetrative deformation and metamorphism the platform apparently extended from patagonia to northwestern argentina and the eastern sierras pampeanas and has probable equivalents in sw africa peru and bolivia the wsp record the outer deepest part of the platform where carbonates were deposited in addition to siliciclastic sediments detrital zircon u pb shrimp ages from clastic metasedimentary successions and sr isotope compositions of marbles from the wsp suggest depositional ages between ca 525 and 490 ma the detrital zircon age patterns further suggest that clastic sedimentation took place in two stages the first was sourced mainly from re working of the underlying neoproterozoic metasedimentary rocks and the uplifted core of the early cambrian pampean orogen without input from the paleoproterozoic rio de la plata craton sediments of the second stage resulted from the erosion of the still emerged pampean belt and the neoproterozoic brasiliano orogen in the ne with some contribution from the rio de la plata craton an important conclusion is that the wsp basement was already part of sw gondwana in the early cambrian and not part of the exotic precordillera cuyania terrane as was previously thought

magma induced strain localization in centrifuge models of transfer zones abstract scaled centrifuge experiments have been used to investigate the dynamic relations between deformation and magma distribution in rift related transfer zones the physical models were built using suitable analogue materials such as sand to represent the brittle upper crust various kinds of silicone mixtures to simulate the lower crust and upper mantle and glycerol to reproduce magma models simulated the development of transfer zones across pre existing glycerol reservoirs placed at the base of the analogue continental crust in plan view different geometries dimensions and positions of subcrustal reservoirs were reproduced in three different sets of experiments to compare results models were also performed without magma simulating glycerol set 1 experiments incorporating a narrow rectangular glycerol reservoir show that the low viscosity material is able to localise deformation into the overlying crust giving rise to discrete transfer zones this concentrated surface deformation corresponds at depth to major magma accumulation set 2 experiments with an initial wide squared glycerol reservoir show instead that deformation is distributed across the whole model surface corresponding at depth to relatively minor magma accumulation set 3 experiments explored various positions of a small squared reservoir that invariably localised faulting in the overlying analogue brittle crust at the onset of model deformation the overall model behaviour suggests that magma distribution at depth can effectively control the strain distribution in the overlying crust and the deformative pattern of transfer zones strain distribution in turn may control magma emplacement as localized deformation would favour major accumulation of magma at transfer zones coupled to a strong thermal weakening of the country rocks this process may ultimately lead to a positive feedback interaction between magma and deformation

successive orthogonal and oblique extension episodes in a rift zone laboratory experiments with application to the ethiopian rift small scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique type 1 and from oblique to orthogonal type 2 in both the type 1 and type 2 models faults produced during the first stage strongly control fault development during the second stage in type 1 models the oblique faults developed during the second oblique phase are confined within a first phase graben whereas in type 2 models the oblique faults produced during the first phase continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks type 1 models are compared with the structural setting of the ethiopian rift the evolution of the rift is related to a recent extensional event whose principal direction of stretching trends at around 50 to preexisting major normal faults type 1 laboratory models are fairly comparable to the northern sector of the ethiopian rift referred to here as mer they account for both the development of the en echelon oblique faults of the wonji fault belt and the sinistral shear gradient running parallel to the eastern border of the mer which formed during an oblique rifting extension the statistical analysis of the whole ethiopian rift fault pattern by reference to the experimental data allows the determination of a n100 n110 mean direction of stretching

continental rift architecture and patterns of magma migration a dynamic analysis based on centrifuge models 1 small scale centrifuge models were used to investigate the role of continental rift structure in controlling patterns of magma migration and emplacement experiments considered the reactivation of weakness zones in the lower crust and the presence of magma at moho depths results suggest that surface deformation which reflects the weakness zone geometry exerts a major control on patterns of magma migration in the case of a single rift segment the experimental lower crust and magma were both transferred in an extension parallel direction toward the rift flanks this lateral migration reflected the dominance of far field stresses over extension induced buoyancy forces local pressure gradients favored the raise of experimental magma in correspondence of marginal grabens the lateral migration gave rise to major accumulations below the footwall of major boundary faults providing the magma source able to feed off axis volcanoes in nature as inferred for the main ethiopian rift in the case of two offset rift segments a major transfer zone developed this transfer zone was characterized by prominent experimental lower crust doming and strong magma accumulation dynamic analysis showed that the transfer zone development caused a strong pressure difference in a rift parallel direction which dominated over the far field thinning owing to this pressure gradient almost all the underplated experimental magma collected below the lower crust dome suggesting a rift parallel extension orthogonal migration this process has a direct relevance for the localization of magmatic activity at transfer zones in natural continental rifts such as in the western branch of the east african rift system

controls of lithospheric structure and plate kinematics on rift architecture and evolution an experimental modeling of the baikal rift 1 analog models investigate the evolution and architecture of the baikal rift in relation to the rheology of the extending lithosphere and rift kinematics the models focus on the development of the narrow deep and asymmetric basins composing lake baikal and reproduce the extension between the strong siberian craton and the weaker sayan baikal belt model results suggest that the presence of a near vertical weak suture separating the cratonic keel from the mobile belt represents the more convenient rheological configuration leading to a narrow rift characterized by prominent vertical motions and deep depressions these depressions are typically asymmetric and model results suggest that this asymmetry is a consequence of lateral variations in lithospheric rheology which is in turn related to both the variation in thickness of the strong mantle and more importantly the variation in the brittle ductile transition depth between the craton and the belt a significant shallowing of the brittle ductile transition in the crust passing from the craton to the belt is required to fit the asymmetric architecture of the baikal basins with a master fault on the craton side and a monocline with no significant faulting on the belt side analysis of the model deformation pattern suggests that the overall architecture of the basins hosting lake baikal is best fitted for a n140 e directed extension similar to the current gps derived motion and compatible with the stress field inferred on the basis of fault and focal mechanism data this kinematics along with the shape of the siberian craton exerted the major control on the plan view fault architecture and its along axis variations

recent volcano tectonic activity of the ririba rift and the evolution of rifting in south ethiopia abstract the relationships between volcanic activity and tectonics at the southernmost termination of the main ethiopian rift mer east africa still represent a debated problem in the mer evolution new constraints on the timing evolution and characteristics of the poorly documented volcanic activity of the dilo and mega volcanic fields vf near the kenya ethiopia border are here presented and discussed the new data delineate the occurrence of two distinct groups of volcanic rocks 1 pliocene subalkaline basalts observed only in the dilo vf forming a lava basement faulted during a significant rifting phase 2 quaternary alkaline basalts occurring in the two volcanic fields as pyroclastic products and lava flows issued from monogenetic edifices and covering the rift related faults 40ar 39ar dating constrains the emplacement time of the large basal lava plateau to 3 7 ma whereas the youngest volcanic activity characterising the two areas dates back to 134 ka dilo vf to as recent as the holocene mega vf volcanic activity developed along tectonic lineaments independent from those of the rift no direct relations are observed between the pliocene roughly n s trending major boundary faults of the ririba rift and the ne sw oriented structural trend characteristic of the quaternary volcanic activity we speculate that this change in structural trend may be the expression of 1 inherited crustal structures affecting the distribution of the recent volcanic vents and 2 a local stress field controlled by differences in crustal thickness following a major episode of reorganization of extensional structures in the region due to rift propagation and abandonment

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

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

the role of gravitational potential energy in active deformation in the southwestern united states in orogenic belts intrinsic buoyancy and extrinsic boundary forces act on the continental lithosphere determining the relative importance of these forces is fundamental to understanding the dynamics of continental deformation in the western united states estimates of gravitational potential energy show that given available constraints on rheology buoyancy forces are sufficient to produce the wide variety of rates and styles of observed deformation including absence of deformation this underscores the importance of gravitational potential energy for deformation in orogenic belts

physical characteristics and evolutionary trends of continental rifts rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension they are widespread and occur in a variety of tectonic settings and range up to 2 600 m y in age the object of this review is to highlight characteristic features of modern and ancient rifts to emphasize differences and similarities in order to help characterize evolutionary trends to identify physical conditions favorable for initiation as well as termination of rifting and to provide constraints for future modeling studies of rifting rifts are characterized on the basis of their structural geomorphic magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts and it is concluded that while simple models can give valuable insight into specific processes of rifting individual rifts can rarely if ever be characterized by well defined trends predicted by these models more data are required to clearly define evolutionary trends and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories

are buoyancy forces important during the formation of rifted margins summary profiles of crustal thickness across rifted continental margins are examined in an attempt to understand the key observations and controlling parameters crustal stretching factor profiles from rifted continental margins supplemented by isochron data for early seafloor spreading have been used to determine a correlation between strain rate e and stretching factor despite the different methods assumptions and data sources our e relationship for rifted margins is consistent with that observed by newman and white for intracontinental rift basins the e relationship we derive is also consistent with the dynamic models of newman and white which include thermorheological strain hardening and strain softening but omit crustal buoyancy forces generated by lateral crustal thickness variations whilst crustal buoyancy forces are not included in the above dynamic models the e data do not necessarily preclude their importance simple numerical models of buoyancy force evolution show that for the first 30 myr after rifting the thermally derived buoyancy forces within the lithosphere that assist extension are larger than the crustal buoyancy forces that oppose extension this rift push force acts as a positive feedback mechanism is of the order of 3 10 12 nm 1 and dominates over the opposing crustal buoyancy forces immediately after rifting it is therefore clear that the delocalising effects of the crustal buoyancy force are dominant over a restricted range of conditions namely at low strain rate and at long times after rifting histograms of the lateral pressure gradients derived from crustal thinning factors along rifted margins show a dominant peak at 0 05 0 3 kpa m 1 and a significant secondary peak at 0 8 0 3 kpa m 1 the lower lateral pressure gradient peak corresponds to thinned parts of the continental crust which is adjacent to unstretched continental crust and may define the edge of a zone of thermal strain softening independent observations show that narrow margins are associated with rapid strain rates and are consistent with thermal strain softening predicted by thermorheological models however the dominant near zero pressure gradient peak is consistent with the operation of crustal buoyancy force processes during rifting which attempt to remove variations in crustal thickness

crustal structure of the main ethiopian rift from gravity data 3 dimensional modeling abstract a three dimensional interpretation of the newly compiled bouguer anomaly map of the main ethiopian rift is presented a high resolution 3 d model constrained with the seismic results reveals a possible crustal thickness and density distribution beneath the graben the bouguer anomalies along the axial portion of the rift floor as deduced from the results of the regional and residual separation are mainly caused by the deep seated structures the inferred zone of intrusion which is the main subject of the present study coincides with the maximum gravity anomaly of the rift floor the intrusion is displaced at several sectors along the east west direction and the two major displacements coincide with the locations of the major rift offsets on the surface because of the asthenospheric uplift the crust under the main ethiopian rift is slightly thinned the zone of crustal thinning 31 km coincides with the location of the intrusion beneath the rift floor and the maximum of which is attained in the northern and central sectors of the graben the trend of the crustal thinning zone which is from south to north is the same as the one obtained in the afar depression the southeastern and western plateaus on the other hand show by far the largest crustal thickness in the region 38 51 km in contrast to the afar depression where the crust is partly oceanized the thickness and density of the crust suggest that the main ethiopian rift is underlain by a purely continental crust the deep and relatively large nature of the intrusion leads to the conclusion that a large scale asthenospheric upwelling might be responsible for the thinning of the crust and subsequent rifting of the graben

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

evidence for intracontinental thrust related exhumation of the ultra high pressure rocks in china coesite and diamond bearing ultra high pressure metamorphic rocks produced during the triassic continental collision between the sino korean and yangtze plates are offset by about 530 km along the sinistral tanlu fault in eastern china the tanlu fault terminates abruptly south of the ultra high pressure metamorphic terrain in dabie shan and does not extend into the foreland fold belt along the yangtze river this suggests that in this region the mesozoic major sinistral displacement along the tanlu fault was transferred to southward directed thrusting which led to the exhumation of the ultra high pressure metamorphic rocks formed earlier postcollisional himalayan type intracontinental thrusting and concomitant erosion appear to be the main agents in the exhumation of the very high pressure metamorphic rocks in china

dynamics of magma emplacement in centrifuge models of continental extension with implications for flank volcanism a set of experimental small scale models of continental extension investigated the emplacement of orthogonal and oblique magma chambers initially underplated at the base of the crust the models are driven by a centrifugal body force which simulates the role of gravity in nature the models represent crustal conditions analogous to relatively mature continental rifts and consider both symmetric and asymmetric extension the experimental results suggest that magma emplacement is controlled by the interactions between tectonics and rheology of the crustal layers in particular extension is mainly accommodated by lateral flow and ductile doming in the viscous layer simulating the lower crust and by listric normal faults in the overlying brittle layer associated with the ductile domal uplift stretching of the continental crust induces a reactive migration of viscous layers into the footwall of major normal faults where magma accumulation at the core of the domes takes place this behavior suggests that widespread magmatism is expected to localize in the footwall of major normal faults a situation that is often observed in core complex structures although the models presented here are intended to simulate magma emplacement during extension of a two layer brittle ductile system lateral flow and migration of magma initially underplated at the base of the crust beneath narrow rifts e g the ethiopian rift may also provide a similar mechanism and a possible explanation for the occurrence of important volcanoes on the plateaus flanking the rift zones in the suggested model such volcanoes are related to large basaltic magma reservoirs located at the base of the crust to accommodate the space vacated by lateral flow and thinning of the ductile crust the similarity in structure of the models with natural examples of continental extension modes such as rift systems and core complexes may suggest a close similarity of dynamic processes

on the relationship between surface uplift and gravitational extension this paper highlights the relationship between isostatically supported surface elevation and the lateral buoyancy forces of an orogen the former is a linear function the latter is a quadratic function of the thickness of the lithosphere thus surface elevation and lateral buoyancy forces may have differing evolutions in the development of an orogen indeed during the evolution of collisional orogens the qualitative relationship between these two parameters may reverse so that only in the early orogenic evolution topographically high regions exert a net lateral buoyancy force on topographically low regions later in the evolution this may change to a counterintuitive scenario in which there is a net lateral buoyancy force from the topographically lower regions toward the topographically higher regions to illustrate this fact we use a simple one dimensional kinematic model for an orogen which undergoes simultaneous thickening due to convergence and erosion at the surface it can be shown that for reasonable erosion and thickening rates surface elevation increases through time until a steady state is reached while the lateral buoyancy force changes from positive to negative giving rise to the inward collapse of an orogen in the field this may be recognized as the occurrence of two successive coaxial convergent deformation events present day examples may be the formation of back arc basins where extension in the low lying hinterland of an andean type mountain belt is caused by a gravitational compressive force exerted by the hinterland onto a belt of significantly higher surface elevation but possibly of lower potential energy

squeeze box in a centrifuge abstract a squeeze box which operates in a centrifuge is presented in the experimental set up the centrifugal body force mimics the role of gravity in nature and at the same time drives a hydraulic press which simulates lateral forces the rate of lateral shortening is controlled by varying the loads placed on the hydraulic press utilization of a squeeze box in a centrifuge enhances the range of scaling and enables the use of coherent model materials without violating similitude requirements in the traditional squeeze box which operates under normal gravity similitude requirements severely limit the choice of model materials as a test of the apparatus simple experiments were carried out comparing models shortened in a centrifuge with similar models shortened under normal gravity the realistic form of decollement structures e g folds faults and diapirs shortened in a centrifuge in these tests as well as in other tests which will be reported later show the potential of this new technique for rigorous modelling of geodynamic systems which evolve under the influence of both gravity and a push from the rear the set up can also be used as a simple shear box in a centrifuge after a slight adjustment

rheologic control of buoyancy driven extension of the rio grande rift extensional deformation in the western united states particularly in the basin and range has frequently been ascribed to buoyancy forces arising from horizontal density contrasts the rio grande rift is subject to buoyancy forces 2 3 times larger than those in the tectonically active northern basin and range suggesting that it should be extending considerably faster than the northern basin and range yet paradoxically is extending more slowly using a thin viscous sheet model we test the hypothesis that regions surrounding the rio grande rift which have rheologically stronger lithosphere resist deformation and reduce the rate of extensional deformation in the rio grande rift to values consistent with observations we find that these regions must have lithospheric strength at least twice that of the rio grande rift this modest contrast in lithospheric strength is consistent with independent estimates derived from observations of heat flow seismicity and crustal thickness

insights into the effects of oblique extension on continental rift interaction from 3d analogue and numerical models abstract continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity these rift segments arise from rift initiation at non aligned crustal heterogeneities and need to interact to develop a full scale rift system here we test the effects of 1 oblique extension and 2 initial heterogeneity seed offset on continental rift interaction with the use of an improved analogue model set up x ray computer tomography ct techniques are used to analyse the 3d models through time and the results are compared with additional numerical models and natural examples the experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike slip dominated systems towards the strike slip domain we also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface initial soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal rather than linking at depth the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough orthogonal extension promotes the formation of intra rift horsts which may provide hydrocarbon traps in nature

volcanic and structural evolution of the okataina volcanic centre dominantly silicic volcanism associated with the taupo rift new zealand article the okataina volcanic centre ovc contains the northeasternmost caldera complex in onshore taupo volcanic zone tvz new zealand sited largely within the taupo rift pyroclastic fall deposits and ignimbrites deposited in the bay of plenty coast area between 420 and 625 ka were probably erupted from ovc and these provide a maximum age for the centre the earliest ignimbrite for which there is good evidence for eruption from ovc is the c 550 ka quartz biotite ignimbrite exposed to the west of lake okataina and to se of ovc this ignimbrite probably correlates with one of the early ignimbrites found in the kawerau geothermal wells and is large enough to have been accompanied by caldera collapse it was followed by extensive rhyolitic explosive eruptions of the murupara subgroup culminating with eruption of the matahina ignimbrite at c 325 ka this c 160 km 3 magma volume eruption was accompanied by caldera collapse to form the southern part of the present day okataina caldera complex a long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of ovc before eruption of the 100 km

inward migration of faulting during continental rifting effects of pre existing lithospheric structure and extension rate abstract lithospheric scale analogue models are used to analyse the parameters controlling the typical evolution of deformation during continental narrow rifting characterized by early activation of large boundary faults and basin subsidence followed by localization of tectonic activity in internal faults at the rift axis integration of current and previous experiments shows that the evolution of deformation in particular the amount of extension needed for the abandonment of boundary faults and migration of deformation to in rift faults is dependent on at least five boundary conditions i thickness of brittle layers including syn rift sediments ii thickness of ductile layers iii extension rate iv width of the weak zone localizing extension and v rift obliquity with respect to the extension direction an increase in the amount of extension corresponding to the inward migration of faulting i e a longer phase of slip on boundary faults is observed for a an increase in the thickness of both brittle and ductile crustal layers and syn rift sediment accumulation b a decrease in extension rate and width of the weak zone and c a decrease in rift obliquity a unified account of these correlations is presented based on the hypothesis that fault migration occurs when boundary faults can no longer accommodate the imposed bulk extension leading to time space variations of internal strain and strain rate and consequently stress in the ductile layers which overcome the total resistance of brittle layers to thoroughgoing faulting

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

redistribution of the lithosphere deformation by the emplacement of underplated mafic bodies implications for microcontinent formation abstract sedimentary basin migration and microcontinent formation could be the result of deformation redistribution in which the emplacement of an underplated mafic body upmb may play an important role in this study a 2d finite element model is used to examine the redistribution of the deformation that can be triggered by a upmb it is shown that three modes of deformation redistribution can exist 1 a shift completed mode where the deformation is completely redistributed into the newly weakened region 2 a transition mode where the deformation is redistributed but extension is accommodated by thinning in both regions 3 a shift failed mode where the deformation is not redistributed whether or not the deformation is redistributed depends on the configuration of the upmb and on the initial rheological heterogeneity in the initially deformed region however it becomes difficult for any upmb to initiate the redistribution of the deformation once the stretching factor in the first deformed region exceeds a critical value the microcontinent formations observed in the indian ocean in the norwegian greenland sea in the red sea afar region and in the western mediterranean are particularly discussed in relation to the transition mode providing some important geodynamic implications

basin inversion and contractional reactivation of inherited normal faults a review based on previous and new experimental models abstract compressional inversion of former extensional sedimentary basins is a particularly common phenomenon given that passive margins share the same destiny of being subsequently incorporated into fold and thrust belts during basin inversion pre existing faults may concentrate stress and localise future thrust ramps or may be reactivated recording reversal of movement from extension to reverse this process has attracted attention largely because of its economic implications related to petroleum and ore deposit prospects and seismic hazard assessment since more than two decades analogue modelling has been used to indentify and or test a number of parameters relevant for the inversion process this paper aims therefore to offer a reasoned review of the analogue modelling work done on the subject coupling such previous results with those of a new experimental series inspired to the northern apennines stratigraphy and evolution past models have explored the role of main factors governing fault reactivation susceptibility such as fluid pressure fault weakness fault steepness angle of shortening and sediment loading the new models have addressed the role played by geometry and strength of a basal ductile layer during inversion two sets of sand silicone models were first extended orthogonally at different velocities to produce dissimilar pre inversion internal geometries and then shortened coaxially at different velocities to vary the brittle ductile coupling the experimental results confirm the selectiveness of fault inversion and reveal strong similarities with structural styles of tectonic inversion reported from the northern apennines and various areas worldwide the preferred geological model inferred from this modelling involves contribution of 1 syn inversion rotation of pre existing faults to shallower dip and 2 rotation of principal stress axes which could explain the invariable reactivation of only one of the two oppositely dipping graben bounding faults these factors would thus interact with the other well established parameters controlling the fault reactivation process

gravity driven crustal shortening in failed rifts it is proposed that gravity driven crustal shortening caused by the potential energy of marginal uplifts adjacent to thinned crust in a rift zone occurred immediately after rifting ceased in both the gulf of suez and north sea rifts compressional stresses of the order of 1 2 mpa would have been generated which were relieved by finite contractional strains of the order of 2 5 shortening of this magnitude has been observed as post rift contractional structures in the western sinai gulf of suez and the viking graben north sea these observations are compatible with a physical model for stresses generated in the crust due to lateral variations in crustal thickness this model helps to explain some apparently anomalous structures within rifts such as tight hanging wall folds some aspects of fault block rotation footwall uplift and tilting and also has important implications for the formation of unconformities and the redistribution of sediments in rifts the phenomenon of gravity driven crustal shortening in failed rifts may have occurred in other major intra cratonic rifts

rift segment interaction in orthogonal and rotational extension experiments implications for the large scale development of rift systems abstract during extension of the continental lithosphere rift basins develop these are often initially offset and must interact and connect in order to create a continuous rift system that may ultimately achieve break up when simulating extensional tectonics and rift interaction structures analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation resulting in rotational extension and associated rift propagation and structural gradients here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings our modelling efforts show that rotational extension and orthogonal extension produce significantly different large scale structures rotational extension can cause important variations in rift maturity between rift segments delay rift interaction zone development and make rift segments propagate in opposite directions still local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting furthermore we find that various degrees of rift underlap produce three basic modes of rift linkage structures low underlap distance high angle experiments develop rift pass structures with increasing underlap distance ca 40 transfer zone basins develop high degrees of underlap 30 tend to result in en echelon sub basins our results match with data from previous modelling efforts and natural examples we furthermore propose a large scale tectonic scenario for the east african rift system based on rotational extension and associated rift propagation these insights may also be applicable when studying other large scale rift systems

dynamic processes controlling evolution of rifted basins abstract the extension of the lithosphere controlling the development of rifted basins is driven by a combination of plate boundary forces frictional forces exerted on the base of the lithosphere by the convecting asthenosphere and deviatoric tensional stresses developing over upwelling branches of the asthenospheric convection system although mantle plumes are not a primary driving force of rifting they play an important secondary role by weakening the lithosphere and by controlling the level of rift related volcanic activity a distinction between active and passive rifting is only conditionally justified the extension of the lithosphere depending on its rate and magnitude and the potential temperature of the asthenosphere can cause by adiabatic decompression partial melting of the lower lithosphere and upper asthenosphere in rift systems the level and timing of volcanic activity is highly variable the lack of volcanic activity implies passive rifting an initial passive rifting stage can be followed by a more active one during which magmatism plays an increasingly important role magmatic destabilization of the moho may account for the frequently observed discrepancy between upper and lower crustal extension factors combined with evidence for thermal thinning of the mantle lithosphere this suggest that the volume of the lithosphere is not necessarily preserved during rifting as advocated by conventional stretching models the structural style of rifts is controlled by the rheological structure of the lithosphere the availability of crustal discontinuities that can be tensionally reactivated the mode orthogonal or oblique and amount of extension and the lithological composition of pre and syn rift sediments simple shear extension prevails in rifts that subparallel the structural grain of the basement pure shear extension is typical for rifts cross cutting the basement grain pre existing crustal and mantle lithospheric discontinuities contribute to the localization of rift systems the duration of the rifting stage of extensional basins is highly variable stress field changes can cause abrupt termination of rifting in major rift systems progressive strain concentration on the zone of future crustal separation entails abandonment of lateral rifts depending on constraints on lateral block movements crustal separation can be achieved after as little as 9 my and as much as 280 my of rifting activity syn rift basin subsidence is controlled by isostatic adjustment of the crust to mechanical stretching of the lithosphere its magmatic inflation and thermal attenuation of the mantle lithosphere post rift basin subsidence is governed by thermal reequilibration of the lithosphere asthenosphere system deep seated thermal anomalies related to syn rift pull up of the asthenosphere lithosphere boundary have decayed after 60 my by about 65 and after 180 my by about 95 the magnitude of post rift subsidence is a function of the rift induced thermal anomaly and crustal density changes the potential temperature of the asthenosphere and initial water depths intraplate stresses can have an overprinting effect on post rift subsidence stretching factors derived from post rift subsidence analyses must be corrected for such effects

modeling heterogeneous stretching during episodic or steady rifting of the continental lithosphere the geometry of brittle ductile extension and modes of normal faulting during steady or episodic rifting of the continental lithosphere are investigated by analogue modeling using the centrifuge technique we use a frictional material to simulate the brittle behavior and various nonlinear bouncing putties to represent the ductile rheology of the stratified lithosphere steady stretching of the continental lithosphere supports rapid differential subsidence and flexure of the brittle crust with formation of marginal grabens and a panel of domino style fault blocks in contrast episodic stretching of the lithosphere progressing in a series of pulses separated by longer periods of quiescence supports formation of spaced grabens and horsts with less subsidence this is due to a counterflow of ductile material from the margins to the central rift zone we show here that the isostatic adjustment in the lithosphere can be related to the steady or episodic mode of heterogeneous stretching we apply the model results to the southern red sea for which the inferred opening history involved continuous or stepwise extension

continental rifting parallel to ancient collisional belts an effect of the mechanical anisotropy of the lithospheric mantle analysis of major rift systems suggests that the preexisting structure of the lithosphere is a key parameter in the rifting process rift propagation is not random but tends to follow the trend of the orogenic fabric of the plates systematically reactivating ancient lithospheric structures continental rifts often display a clear component of strike slip deformation in particular in the early rifting stage moreover although the close temporal and spatial association between flood basalt eruption and continental breakup suggests that mantle plumes play an important role in the rifting process there is a paradox between the pinpoint thermal and stress perturbation generated by an upwelling mantle plume and the planar geometry of rifts these observations suggest that the deformation of the lithosphere especially during rifting is controlled by its preexisting structure on the other hand 1 the plasticity anisotropy of olivine single crystal and aggregates 2 the strong crystallographic orientation of olivine observed in mantle xenoliths and lherzolite massifs and 3 seismic anisotropy data which require a tectonic fabric in the upper mantle coherent over large areas suggest that preservation within the lithospheric mantle of a lattice preferred orientation lpo of olivine crystals may induce a large scale mechanical anisotropy of the lithospheric mantle we use a polycrystal plasticity model to investigate the effect of a preexisting mantle fabric on the continental breakup process we assess the deformation of an anisotropic continental lithosphere in response to an axi symmetric tensional stress field produced by an upwelling mantle plume by calculating the deformation of textured olivine polycrystals representative of the lithospheric mantle at different positions above a plume head model results show that a lpo induced mechanical anisotropy of the lithospheric mantle may result in directional softening leading to heterogeneous deformation during continental rifting this mechanical anisotropy may induce strain localisation in domains where extensional stress is oblique 30 55 to the preexisting mantle fabric this directional softening associated with olivine lpo frozen in the lithospheric mantle may also guide the propagation of the initial instability that will follow the preexisting structural trend the preexisting mantle fabric also controls the deformation regime imposing a strong strike slip shear component a lpoinduced mechanical anisotropy may therefore explain the systematic reactivation of ancient collisional belts during rifting structural inheritance the plume rift paradox and the onset of transtension within continental rifts fl 2001 elsevier science b v all rights reserved

the influence of a stratified rheology on the flexural response of the lithosphere to un loading by extensional faulting summary we present a two layered finite diverence model for the flexural response of the lithosphere to extensional faulting the model allows for three modes of flexure 1 fully coupled with the upper crust and mantle welded together by the lower crust 2 fully decoupled with the upper crust and mantle behaving as independent layers and 3 partly decoupled signifying that the response of the upper crust to small wavelength loads is superimposed on the response of the entire lithosphere to long wavelength loads which of these modes of flexure is to be expected depends on the rheology and especially the thermal state of the lithosphere coupled behaviour is related to a cold and strong lithosphere the baikal rift zone provides a typical example for this mode of flexure a fully decoupled lithosphere is an exceptional case related to anomalous high temperatures in the lower crust and is observed in the basin and range province the most common case is a partly decoupled lithosphere with the degree of decoupling depending on the thickness and viscosity of the lower crust this is inferred for example for the bay of biscay margin

post rift compressional reactivation potential of passive margins and extensional basins poly phase deformation of a compressional nature is a common feature in the post rift evolution of passive margins and rifts the compressional mode of deformation in these sedimentary basins originally formed by extension in an intraplate setting is characterized by a spectrum of spatial wavelengths spanning several tens of kilometres up to several hundreds of kilometres the actual mode of compressional deformation appears to be strongly affected by the rheological structure of the underlying lithosphere the level of the regional intraplate stress field and the geometry of the rifted basin configuration prior to late stage compressional reactivation the interplay of plumes and intraplate compressional deformation can lead to temporal transitions from basin inversion to lithospheric folding these modes of deformation lead to substantial differential vertical motions late stage anomalies in subsidence and uplift patterns the development of innovative combinations of numerical and analogue modelling techniques is the key to differentiating different modes of compressional deformation of passive margins and extensional basins the geological society of london 2008

asymmetric rifts due to asymmetric mohos an experimental approach large scale asymmetries in structures and in the uplift width and volcanism of pairs of conjugate passive margins have been documented in various parts of the world these asymmetries reflect the dominant mode of extension preceding the break up of the continental lithosphere of the many parameters involved the pre rift crustal structure has been shown to play a fundamental role in controlling the process of continental rifting in this work we have used analogue models to show how initial asymmetric moho configurations are prone to produce asymmetric rifting with prominent asymmetries in the amount of extension accommodated by boundary faults in the patterns of lithospheric thinning asthenospheric upwelling and in the trajectories of magma migration after continental break up these deformative patterns are expected to lead to the development of a pair of asymmetric passive margins with one narrow strongly volcanic margin and a wider less volcanic or with areas of concentrated volcanic activity counterpart

tectonics of the northern canadian cordillera imaged using modern magnetotelluric analysis abstract magnetotelluric data from lithoprobe snorcle corridor 2 in western canada crossing the rocky mountain trench tintina fault between 56 and 60 n have been analyzed using modern mt methods to provide new insights into the tectonics of the north american cordillera and ancestral north america resistivity images of the crust were obtained using two independent sets of unconstrained 2 d inversions and constrained 2 d inversions incorporating seismic constraints structure to the west of the tintina fault includes uniform moderate mid to lower crustal resistivity in a seismically defined westward tapering wedge of ancestral north american rocks to the east between the tintina fault and the liard line a transfer zone in asymmetric neoproterozoic rifting the wedge includes a major synformal conductor that parallels seismic reflections the large lateral changes in resistivity can be explained by transition between different blocks of asymmetric rifting the synformal conductor is attributed to graphitic rocks from the aida formation of the mesoproterozoic muskwa assemblage that were re distributed in a sag basin on the lower plate margin in the blackwater block spatially complex upper crustal resistivity in the cordillera can be interpreted in terms of lithological variations and alteration processes an east dipping conductor in stikinia is attributed to east dipping jurassic and relatively high conductivity beneath the eskay rift may be explained by crustal fertilization a conductor in the cache creek terrane is attributed to listwanite alteration of ultramafic bodies on the king salmon fault and high conductivity in the slide mountain terrane within the sylvester allochthon to localized carbonaceous alteration a series of conductors above north american basement is attributed to deformation and metamorphic processes associated with mesozoic thrust faulting to the west of the tintina fault the source material for these conductors is likely from the stikinia terrane and to the east it is likely from the muskwa assemblage