validating novel boundary conditions for three dimensional mechanics based restoration an extensional sandbox model example geomechanical restoration methods are dependent on boundary conditions to ensure geological consistency of the restored model in terms of geometry and strain classical restoration boundary conditions such as flattening a datum horizon may lead to inconsistent displacement and strain fields we restore a laboratory structural sandbox model with known deformation history to develop guidelines for definition of boundary conditions that produce improved results from geomechanical restorations the sandbox model has a basal silicone layer includes synkinematic deposition and is characterized by structures analogous to those found in suprasalt extensional environments the deformed geometry is interpreted from three dimensional tomography imaging and a time series of cross section tomography images provides a benchmark to quantify restoration error and inform boundary conditions we confirm that imposing a lateral displacement equal and opposite to far field tectonic shortening or extension provides a more accurate restoration however the amount of displacement may not be known in real cases we therefore test several established methods using only the unrestored geometries to assess the amount of shortening that should be used to guide geomechanical restorations an accurate estimation is provided by the area depth method and potentially by a dilatation analysis additionally novel fault compliance boundary conditions produce improved results in the vicinity of crossing and branching faults application of similar methods should produce improved restoration of natural geologic structures

practical and efficient three dimensional structural restoration using an adaptation of the geochron model we document a novel approach to balanced three dimensional structural restoration based on an adaptation of the geochron model conventionally the geochron model defines a transformation of a geological model to a flattened space u v t with paleogeographic coordinates defined by the horizontal axes u v and geologic time on the vertical axis t in our new balanced structural restoration scheme the complete stratigraphy is restored using a transformation constrained only by the datum horizon scaling the vertical t axis to depth in a manner that preserves volume or layer thickness results in a geometric restoration that approximately minimizes strain globally this restoration provides a geometrically plausible representation of the geologic structure at the time when the datum horizon was deposited restoration is independent of mechanical rock properties and is thus most applicable to regions in which mechanical rock properties are approximately homogeneous restoration kinematics may be constrained by growth strata if present we validate the method with kinematic forward models and a laboratory sandbox model and apply it to two natural examples to demonstrate its capabilities for model validation and palinspastic restoration we identify four criteria for assessing the validity of a structural model using the results of restoration 1 anomalous fault throw 2 timing of fault activity 3 fault compliance and 4 restoration strain analysis of the sandbox results and limitations of volume conservation derived from uncertainties in compaction states suggest accuracy of the method to be in the 5 20 range

role of compaction in listric shape of growth normal faults growth normal faults propagate syndepositionally at the depositional surface and their dips flatten with depth as the surrounding sediments compact as described by the equation tan thgr tan thgr0 1 phgr0 1 phgr where phgr0 and phgr are the initial and final porosities of the surrounding sediments and thgr0 and thgr are the initial and final fault dips after compaction this equation is tested on several southern louisiana growth normal faults whose shapes are known from well penetrations and whose porosity depth relationships for sand and shale are derived from well logging the initial surface fault dip thgr0 67 and the initial sand and shale porosities phgr0ss 39 3 and phgr0sh 68 respectively were determined by an inversion technique on an extensive data set from one fault once these initial conditions were determined the shapes of other faults were computed accurately based on knowledge o the sand shale stratigraphy from single well logs the compaction equation allows accurate determination of the proportion of fault flattening caused by compaction and other processes for example one fault flattens from an initial dip of 67 to a dip of 38 at 5 km depth 23 of the flattening is caused by compaction and 6 is caused by footwall folding finally in pure shale the compaction induced flattening of a growth fault is as much as 19 in the first 500 m of burial and reduces to about 1 km below this depth in pure sand the flattening is approximately 1 km throughout

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

structural analysis of seismically mapped horizons using the developable surface model a new method for the analysis of folding of seismically mapped horizons is described based on a model of developable surfaces the local geometrical properties are determined by analyzing the variation of dip and strike along linear strips on the surface by considering strips in different directions a plunge line the approximation to the generatrix of a developable fold is identified as the direction associated with the least variation of surface attitude the map pattern obtained by analyzing the plunge and trend of plunge lines across an area allows the identification of domains where folding accords with a developable geometry such domains are recognized from straight plunge lines defining convergent or parallel patterns deviations from these patterns correspond to regions of structural complexity associated with ductile or brittle straining of the horizon we suggest that plunge line analysis may offer a useful technique for automatic fault recognition

a new technique for 3 d flexural slip restoration abstract a new flexural slip structural restoration technique for three dimensional 3 d digital models has been developed this technique utilises a slip method that preserves volume in 3 d line length in a given unfolding direction of a specified surface and of layers parallel to this surface and orthogonal bed thickness these constraints enable the restoration of 3 d fault propagation fault bend and detachment folds the 3 d model is comprised of objects such as interpreted horizon and or fault surfaces that are created from irregular triangulated meshes for a given model a parallel sinuous slip system is calculated from the geometry of a specified template surface and from a fixed pin surface that passes through all vertices of the triangulated meshes in the specified folding direction the entire slip system then is folded to a new shape which is defined by a geometric surface that can be curved or planar in doing so all vertices within the system are transformed to their new locations to generate a newly folded 3 d model we demonstrate the 3 d restoration technique by using a case study of an evaporite cored contractional fold in the nw german basin our restorations depict the 3 d sequential growth of the fold from 146 ma through late mesozoic time and show that the shortening direction was towards nne with the main contractional phase initiating during the late cretaceous

geometrical modeling of fault related folds a pseudo three dimensional approach abstract area balanced cross sections of ramp anticlines and tip line folds in thin skinned fold thrust belts can be quantitatively constructed using plane strain fault bend and fault propagation fold models cross sections alone however are somewhat inadequate for interpreting along strike changes in thrust related fold geometry complex interplay between along strike changes in fault slip and fault shape determines a range of geometric configurations which are difficult to visualize without the aid of quantitative three dimensional models we extend conventional two dimensional folding theories into the third dimension by allowing for continuous variations in fault slip and fault shape along strike geometric equations relating map view angles between fold axial surface traces are derived for a case assuming a uniform fault geometry and an along strike displacement gradient the resulting folds possess no unique fold axis but rather are characterized by multiple fold hingelines each corresponding to bends in the fault for typical displacement gradients observed in the field analysis of the models suggests that discernible differences exist between map patterns of fault bend folds and fault propagation folds whereas little difference exists between map patterns of similar and parallel fault bend folds map view angles between axial surface traces for parallel fault bend folds and fault propagation folds are a function of both ramp cutoff angle and fault slip whereas these same angles for similar fault bend folds are solely a manifestation of fault slip examples assuming uniform shortening and continuously varying fault geometry along strike are also presented fault geometry is varied along strike to simulate oblique and lateral ramps depending upon model type and fault shape parameter modified resulting folds exhibit either cylindrical or non cylindrical fold geometries closure is generated by a decrease in fold amplitude where there is a decrease in ramp cutoff angle along strike or where a fault laterally cuts up section in either instance closure develops without varying shortening along strike model map patterns resemble actual thrust related fold geometries and can be compared to maps of real structures to help predict subsurface geology

p wave tomography of the western united states insight into the yellowstone hotspot and the juan de fuca slab abstract we used 190 947 high quality p wave arrival times from 8421 local earthquakes and 1 098 022 precise travel time residuals from 6470 teleseismic events recorded by the earthscope usarray transportable array to determine a detailed three dimensional p wave velocity model of the crust and mantle down to 1000 km depth under the western united states us our tomography revealed strong heterogeneities in the crust and upper mantle under the western us prominent high velocity anomalies are imaged beneath idaho batholith central colorado plateau cascadian subduction zone stable north american craton transverse ranges and southern sierra nevada prominent low velocity anomalies are imaged at depths of 0 200 km beneath snake river plain which may represent a small scale convection beneath the western us the low velocity structure deviates variably from a narrow vertical plume conduit extending down to 1000 km depth suggesting that the yellowstone hotspot may have a lower mantle origin the juan de fuca slab is imaged as a dipping high velocity anomaly under the western us the slab geometry and its subducted depth vary in the north south direction in the southern parts the slab may have subducted down to 600 km depth a slab hole is revealed beneath oregon which shows up as a low velocity anomaly at depths of 100 to 300 km the formation of the slab hole may be related to the newberry magmatism the removal of flat subducted farallon slab may have triggered the vigorous magmatism in the basin and range and southern part of rocky mountains and also resulted in the uplift of the colorado plateau and rocky mountains

an automatic method for determining three dimensional normal fault geometries we describe a general kinematic method for automatically determining the three dimensional 3 d geometry of normal faults at depth from the 3 d geometry of deformed hanging wall horizons the method should be applicable to normal faults which do not change shape as deformation proceeds it is a generalization of two dimensional models and assumes that the hanging wall deforms by arbitrarily inclined bulk simple shear deformation in three dimensions is parameterized by the three euler angles which describe the horizontal slip vector the rake angle and the inclination of shear planes as in two dimensions differential compaction can be included by assuming that the axis of shortening is parallel to the direction of shear three dimensional fault geometry and the three euler angles can all be automatically determined provided the geometries of two or more hanging wall horizons are known here the method is tested on both synthetic data and on 3 d sand box models synthetic modeling shows that unique solutions can be found and more importantly that the horizontal slip vector can be constrained solutions obtained from sand box models are also encouraging in most cases the method can predict automatically the correct direction of extension and a 3 d fault geometry close to the fault geometry used hence arbitrarily inclined bulk simple shear appears to be a good approximation of hanging wall deformation in three dimensions this general method will allow 3 d palinspastic analysis to be undertaken on 3 d seismic reflection data sets

enabling 3d geomechanical restoration of strike and oblique slip faults using geological constraints with applications to the deep water niger delta abstract we present a new approach of using local constraints on fault slip to perform three dimensional geomechanical restorations geomechanical restoration has been performed previously on extensional and contractional systems yet attempts to restore strike and oblique slip fault systems have generally failed to recover viable fault slip patterns the use of local measures of slip as constraints in the restoration overcomes this difficulty and enables restorations of complex strike and oblique slip systems to explore this approach we develop a synthetic restraining bend system to evaluate different ways that local slip constraints can be applied our restorations show that classical boundary conditions fail to reproduce the fault offset and strain pattern in contrast adding piercing points and or properly constraining lateral walls enables restoration of the structure and resolves the correct pattern of slip along the faults we then restore a complex system of tear faults in the deepwater niger delta basin we use channel offsets imaged by the seismic data to define local fault slip constraints balancing these constraints equally on both sides of the major faults yields the most consistent restoration outcomes this approach resolves reasonable slip styles on the complex set of thrust normal and strike slip faults in the structure this suggests that limited geologic fault slip constraints can be effectively incorporated in geomechanical restorations yielding accurate restoration kinematics and thereby forecasting faults slip patterns within the structures

control of syntectonic erosion and sedimentation on kinematic evolution of a multidecollement fold and thrust zone analogue modeling of folding in the southern subandean of bolivia abstract several analogue modeling studies have been conducted during the past fifteen years with the aim to discuss the effects of sedimentation and erosion on foreland fold and thrust belt among which a few have analyzed these processes at kilometric scale malavieille et al 1993 nalpas et al 1999 barrier et al 2002 pichot and nalpas 2009 the influence of syn deformation sedimentation and erosion on the structural evolution of fftb has been clearly demonstrated here we propose to go further in this approach by the study of a more complex system with a double decollement level the natural study case is the bolivian sub andean thrust and fold belt which present all the required criteria such as the double decollement level a set of analogue models performed under a ct scan have been used to test the influence of several parameters on a fold and thrust belt system among which i the spatial variation of the sediment input ii the spatial variation of the erosion rate iii the relative distribution of sedimentation between foreland and hinterland these experiments led to the following observations 1 the upper decollement level acts as a decoupling level in case of increased sedimentation rate it results in the verticalization of the shallower part above the upper decollement level while the deeper parts are not impacted 2 similarly the increase of the erosion rate involves the uplift of the deeper part below the upper decollement level whereas the shallower parts are not impacted 3 a high sedimentation rate in the foreland involves a fault and fold vergence reversal followed by a back thrusting of the shallower part 4 a high sedimentation rate in the hinterland favours thrust development toward the foreland in the shallower parts

three dimensional seismic velocity structure of the northwestern united states 1 we present a high resolution three dimensional p wave tomography model for the northwestern united states major features of the model include clear well resolved images of the juan de fuca gorda slab to depths of at least 500 km reduced velocities beneath newberry volcano and north central oregon increased velocities beneath the idaho batholith and a deep slab like eastward dipping feature of increased velocities beneath nevada we find no evidence for a mantle plume in this region and demonstrate that suggested disruption of the juan de fuca slab beneath oregon in the vicinity of newberry volcano is an inversion artifact we interpret the primary features of this new model within the framework of post laramide tectonomagmatism and the continuing tectonic evolution of the cascadia subduction zone a model explaining the regional tectonomagmatic history consistent with the imaged seismic velocity structure is one driven by mantle flow responding to the evolving subduction system

the fate of the juan de fuca plate implications for a yellowstone plume head beneath the pacific northwest the juan de fuca plate a remnant of the farallon plate continues subducting beneath the north american continent to the east of the cascadia subduction zone lies the yellowstone hotspot track the origins of this track can be traced back to the voluminous basaltic outpourings in the columbia plateau around 17 ma if these basalts are the result of a large melting anomaly rising through the mantle to the base of the north american continent such as a mantle plume head the anomaly would need to punch through the subducting juan de fuca slab here we use teleseismic body wave travel time tomography to investigate the fate of the subducted slab and its possible interaction with a plume head our dataset is derived from the oregon array for teleseismic study oats deployment in oregon and all other available seismic data in this region during the same period in our jdf07 models we image the subducted juan de fuca plate in the mantle east of the cascades beneath oregon where the slab has not been imaged before to a depth of 400 km but no deeper the slab dips similar to 50 degrees e and has a thickness of similar to 75 km immediately beneath the slab we image a low velocity layer with a similar geometry to the slab and extending down to at least similar to 575 km depth in the v model the total length of the high velocity slab is similar to 660 km about 180 km longer than the estimated length of slab subducted since 17 ma assuming similar slab geometry to today this 180 km length of slab would reach similar to 60 km depth comparable to the thickness of continental lithosphere we propose that the absence of the slab below 400 km today is due to the arrival of the yellowstone plume head similar to 17 ma which destroyed the juan de fuca slab at depths greater than the thickness of the continental lithosphere given this scenario the low velocity anomaly beneath the slab is likely the remnant plume head material which has been pulled down by traction with the subducting plate the amplitude of the observed low velocity anomaly is comparable with that expected for plume head material 100 300 degrees c hotter than the surrounding asthenosphere c 2007 elsevier b v all rights reserved

cenozoic tectonics of western north america controlled by evolving width of farallon slab the basin and range province of western north america exemplified by the alternating mountain ridge and valley landscapes across nearly the entire u s state of nevada started to form 50 million years ago through a series of extensions of the continental crust prior to that massive mountain building collisions at the boundary between north america and the pacific ocean formed a subduction zone and compressed the continent by combining observations of global subduction zone velocities with numerical modeling schellart et al p 316 1 demonstrate that the thinning of the subducting portion of the oceanic plate controlled how and when the transition from compression to extension occurred even today where the much smaller remnant oceanic plate continues to slowly subduct below north america in the pacific northwest the width of the slab and not its age controls the velocity of subduction indeed this relation may explain the dynamics of other modern subduction zones from south america to japan 1 lookup doi 10 1126 science 1190366

mantle structure beneath the western edge of the colorado plateau 1 teleseismic traveltime data are inverted for mantle vp and vs variations beneath a 1400 km long line of broadband seismometers extending from eastern new mexico to western utah the model spans 600 km beneath the moho with resolution of 50 km inversions show a sharp large magnitude velocity contrast across the colorado plateau great basin transition extending 200 km below the crust also imaged is a fast anomaly 300 to 600 km beneath the nw portion of the array very slow velocities beneath the great basin imply partial melting and or anomalously wet mantle we propose that the sharp contrast in mantle velocities across the western edge of the plateau corresponds to differential lithospheric modification during and following farallon subduction across a boundary defining the western extent of unmodified proterozoic mantle lithosphere the deep fast anomaly corresponds to thickened farallon plate or detached continental lithosphere at transition zone depths

compatibility constraints on folded and faulted strata and calculation of total displacement using computational restoration unfold program abstract a balanced surface method is proposed that allows one to test the reliability of the interpretation of the structural geometry of folded and faulted strata it also estimates both the finite total displacement field linked to the folding and faulting processes and the finite displacement field linked to the folding the method describes a thin competent folded and faulted sedimentary layer using rigid triangular elements their sizes depending on the curvature of the surface the elements are laid flat and are automatically fit to form a horizontal surface which represents the initial state of the layer the degree of compatibility given by different indicators tests the reliability of the geometric fitting of the layer if folding and faulting occur without bed stretching or if this change is known and introduced as a parameter in the code a plausible interpretation can be perfectly retrodeformed just as a folded and torn sheet of paper may be smoothed with an iron the method has been applied to two natural examples in oil field regions using three or two dimensional depth migrated seismic data the main results reveal in general that the petroleum company s interpretations of the data were nonoptimal a careful reinterpretation of the seismic data was necessary to obtain balanced folded and faulted surfaces the estimation of the finite displacement fields revealed the compatibility between fold and fault deformation and also the strike slip movement or rotation associated with the deformation

mantle provinces under north america from multifrequency p wave tomography this is a survey of mantle provinces large scale seismic anomalies under north america from the surface down to 1500 1800 km depth the underlying p velocity model was obtained by multifrequency tomography a waveform based method that systematically measures and models the frequency dependence of teleseismic body waves a novel kind of three dimensional rendering technique is used to make the considerable structural complexities under north america accessible in the transition zone and below the north american mantle is dominated by seismically fast provinces which represent distinct subduction episodes of the farallon plate i attempt to date and interpret the various slab fragments by reconciling their present positions with paleotrench locations from plate tectonic reconstructions and with major geologic surface episodes differences in vertical sinking velocity have led to large vertical offsets across adjacent coeval slabs some of the mantle provinces have not been discussed much previously including 1 a seismically slow blanket overlying the oldest farallon subduction along the eastern continental margin 2 a transition zone slab coeval with the laramide orogeny ca 80 60 myr which i discuss in analogy to the stagnant slab subduction style commonly found in the western pacific today 3 the lower mantle root of present day cascadia subduction which may have started out as intraoceanic subduction 4 a lower mantle slab under arizona and new mexico the last material to subduct before strike slip motion developed along the san andreas boundary and 5 two narrow plate tears thousands of kilometers long one of which is the subducted conjugate of the mendocino fracture zone

rayleigh wave phase velocities small scale convection and azimuthal anisotropy beneath southern california 1 we use rayleigh waves to invert for shear velocities in the upper mantle beneath southern california a one dimensional shear velocity model reveals a pronounced low velocity zone lvz from 90 to 210 km the pattern of velocity anomalies indicates that there is active small scale convection in the asthenosphere and that the dominant form of convection is three dimensional 3 d lithospheric drips and asthenospheric upwellings rather than 2 d sheets or slabs several of the features that we observe have been previously detected by body wave tomography these anomalies have been interpreted as delaminated lithosphere and consequent upwelling of the asthenosphere beneath the eastern edge of the southern sierra nevada and walker lane region sinking lithosphere beneath the southern central valley upwelling beneath the salton trough and downwelling beneath the transverse ranges our new observations provide better constraints on the lateral and vertical extent of these anomalies in addition we detect two previously undetected features a high velocity anomaly beneath the northern peninsular range and a low velocity anomaly beneath the northeastern mojave block we also estimate the azimuthal anisotropy from rayleigh wave data the strength is 1 7 at periods shorter than 100 s and decreases to below 1 at longer periods the fast direction is nearly e w the anisotropic layer is more than 300 km thick the e w fast directions in the lithosphere and sublithosphere mantle may be caused by distinct deformation mechanisms pure shear in the lithosphere due to n s tectonic shortening and simple shear in sublithosphere mantle due to mantle flow

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

excess area and depth to detachment a plot of the excess area in cross section vs the depth to a reference level for several stratigraphic horizons in a compressional fold produces a curve that gives the depth to detachment the curve is a straight line for detachment folds the slope of the line is the displacement on the lower detachment and the depth intercept where excess area zero is the depth to detachment for a fault bend fold the result is a line that abruptly changes slope at the position corresponding to the upper detachment the difference in slope between the two parts of the line is the displacement on the upper detachment added layer parallel shear produces a curve on the graph tested on detachment folds from the jura mountains the method accurately predicts the depth to detachment he tip top field anticline in the wyoming thrust belt is found to be interpretable as either a detachment anticline with a basal detachment close to the top of the basement or as a fault bend fold with a basal detachment at the base of the triassic the excess area diagram is useful for testing a cross section for balance determining the structural style detachment anticline vs fault bend fold and for examining alternatives in the construction of cross sections

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

segmentation of the farallon slab abstract recent tomography images reveal a complex 3d mantle structure beneath western united states with feature morphology varying rapidly with depth by assimilating plate motion history paleo age of sea floor and paleo geography of plate boundaries in a 3 d numerical model we simulate the farallon juan de fuca subduction during the past 40 ma we find that the highly segmented upper mantle structure of western u s is a direct result of the farallon subduction we show that the tilted horseshoe shaped fast seismic anomaly beneath nevada and utah at 300 600 km depth range is in fact a segment of curled slab subducted since 15 ma and the shallower linear slab beneath the cascades is younger than 5 ma the distinct morphology between these two parts of the subduction system indicates the strong influence of the fast trench rollback since 20 ma the northward migrating jf pa na triple junction and the toroidal flow around slab edges the observed mantle structures are used to constrain the rheology of the upper mantle through matching the shape depth and location of modeled subducted slab segments the inferred viscosity for the asthenosphere is 5 10 19 pa s and those for the transition zone and lower mantle are 1 5 10 21 pa s and 2 10 22 pa s respectively the slab is found to be about 2 orders of magnitude stronger than the ambient mantle above 410 km depth but of similar order of magnitude viscosity in the transition zone

simultaneous inversion of local and teleseismic data for the crust and mantle structure of southern california we determined a detailed three dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern california using local and teleseismic data simultaneously we used 131 372 p wave arrival times from 6437 local earthquakes and 12 134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the caltech us geological survey southern california seismic network compared with previous local and teleseismic studies the station coverage is considerably improved and the number of data used is greatly increased in the present study this together with the local and teleseismic joint inversion approach results in a unified three dimensional 3 d p wave velocity model of the crust and mantle in southern california with a higher resolution than the previous models the result obtained shows very heterogeneous structures in the crust and upper mantle in southern california shallow crustal structures correlate well with surface geological features sedimentary basins such as the los angeles basin ventura basin and santa maria basin are imaged well as low velocities and batholiths such as the peninsular ranges and san gabriel mountains are imaged as high velocities the velocity in the crust is generally low in the mojave desert the crustal velocities change abruptly across major faults such as the san andreas san jacinto elsinore and garlock faults in the upper mantle three major anomalies are found beneath the transverse ranges salton trough and southern sierra nevada the transverse ranges feature appears as a curtain like east trending and north dipping high velocity anomaly with a thickness of approximately 50 km and extends most deeply at its eastern end to about 250 km the salton trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth the southern sierra nevada feature is a slab like high velocity anomaly which extends to about 250 km the mantle velocity is generally low in the upper 100 200 km beneath the volcanic areas in southern california the seismic velocity anomalies in the mantle beneath southern california are considered to result from small scale mantle convection

influence of erosion and sedimentation on strike slip fault systems insights from analogue models abstract we describe 18 experiments on the formation of strike slip fault systems in sand all models were in a rectangular box a piston imparted strike slip motion along a basal cut in some experiments uplifted areas underwent erosion in others all areas were subject to sedimentation in experiments without erosion or sedimentation first to develop were r faults at 16 to the basal cut at later stages p faults and y faults took over in section faults splayed upward forming flower structures the splays had reverse components of slip this was due to dilation which reached 7 within fault splays in experiments with erosion but no sedimentation faults were less steep and accumulated greater amounts of reverse slip in experiments with erosion and sedimentation some faults propagated through their syn kinematic cover others became buried and inactive whilst yet others were exposed by erosion therefore the average fault dip increased significantly in experiments with sedimentation but no erosion early faults propagated whereas others became buried flower structures in nature have similar features in areas of sedimentation fault splays with gentle dips die out at depth whereas steeper faults penetrate higher in areas of erosion strike slip systems exhibit large amounts of reverse slip on steep bounding faults

pn tomography of the western united states using usarray 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states which will enable increased resolution for studies of the lithospheric and deeper structure of the north american continent here we analyze pn arrival times from the transportable stations of usarray to resolve crustal thickness and uppermost mantle structure we use 123 008 pn picks from april 2004 to october 2009 as measured by the array network facility at epicentral distances from 180 to 1450 km these picks are derived from 778 stations at 70 km spacing and 7903 earthquakes and quarry blasts applying the classic time term method we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the moho we also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2 azimuthal variation our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details we obtain an average upper mantle velocity of 7 93 km s with higher velocities beneath eastern washington and northern idaho and lower velocities near the california mexico border the sierra nevada the northern coastal california region and the greater yellowstone area we observe large anisotropic anomalies in southern california as well as in the snake river plain area these results should complement other seismic studies e g body and surface wave tomography and shear wave splitting to provide information about composition temperature and tectonic processes in the western united states

insights into the mechanisms of fault related folding provided by volumetric structural restorations using spatially varying mechanical constraints we use a new mechanically based volumetric structural restoration tool to investigate the mechanics of fault related folding using natural examples imaged in three dimensional 3 d seismic data the restoration technique is based on a finite element approach that simultaneously restores folding and faulting while allowing rock properties to spatially vary during restoration we apply these techniques to two types of structures detachment and shear fault bend folds where mechanical layering is a significant factor in their development our examples include a detachment anticline from the caspian sea and a shear fault bend fold from the deep water niger delta both of which contain syntectonic growth horizons that help to constrain the restorations restorations of the detachment fold most closely match displacement fields specified in the kinematic forward models when materials are defined as incompressible and rigid yet the variation of mechanical strength in restorations is perhaps more compatible with the growth of natural structures as recorded by syntectonic growth strata this analysis shows that the restorations of the detachment fold favor a combination of both kink band migration and limb rotation folding mechanisms numerical simulations of the growth shear fault bend fold also closely match the displacement field prescribed by the kinematics of shear fault bend fold models when weak basal units and bedding plane slip surfaces enabling flexural slip are incorporated in the model the results demonstrate that these techniques can be used to provide full 3 d restorations that closely match established two dimensional kinematic theories yet allow constraint of 3 d displacement fields and strain patterns in complex structures

chronologic modeling of faulted and fractured reservoirs using geomechanically based restoration technique and industry applications we have developed a geomechanically based restoration method to model reservoir deformation the approach founded on the finite element method simulates the physical behavior of the rock mass and considers heterogeneous material properties bedding slip and the mechanical interaction of faults to demonstrate the method s potential we analyze the deformation and fault growth in the hanging wall of a synsedimentary listric normal fault from a sand box model which provides an analog for evaluating complex faulted reservoirs the numerical model results are then analyzed to investigate the chronology of faulting the numerical model corresponds well to the physical model and provides additional insights about reservoir evolution and deformation the approach is also tested on a natural example of folding using outcrop data to study contractional deformation these examples illustrate how undetected faults and fractures reservoir compartmentalization hydrocarbon migration pathways and hydrocarbon traps can be understood in the context of tectonic processes and how this understanding can be exploited in decision making and reducing risk we conclude that the geomechanically based restoration of faulted and fractured reservoirs has significant potential for industry applications compared to common geometric restoration techniques which lack a mechanical basis

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

geodynamics of the yellowstone hotspot and mantle plume seismic and gps imaging kinematics and mantle flow abstract integration of geophysical and geological data show that the yellowstone hotspot resulted from a mantle plume interacting with the overriding north america plate a process that has highly modified continental lithosphere by magmatic and tectonic processes and produced the 16 17 ma 700 km long yellowstone snake river plain ysrp silicic volcanic system accessibility of the ysrp allowed large scale geophysical projects to seismically image the hotspot and evaluate its kinematic properties using geodetic measurements seismic tomography reveals a crustal magma reservoir of 8 to 15 melt 6 km to 16 km deep beneath the yellowstone caldera an upper mantle low p wave velocity body extends vertically from 80 km to 250 km beneath yellowstone but the anomalous body tilts 60 wnw and extends to 660 km depth into the mantle transition zone we interpret this conduit shaped low velocity body as a plume of up to 3 5 vp and 5 5 vs perturbation that corresponds to a 1 2 partial melt models of whole mantle convection reveal eastward upper mantle flow beneath yellowstone at relatively high rates of 5 cm yr that deflects the ascending plume into its west tilted geometry a geodynamic model of the yellowstone plume constrained by vp and vs velocities and attenuation parameters suggests low excess temperatures of up to 120 k corresponding to a maximum 2 5 melt and a small buoyancy flux of 0 25 mg s i e properties of a cool weak plume the buoyancy flux is many times smaller than for oceanic plumes nonetheless plume buoyancy has produced a 400 km wide 500 m high topographic swell centered on the yellowstone plateau contemporary deformation derived from gps measurements reveals sw extension of 2 3 mm yr across the yellowstone plateau one fourth of the total basin range opening rate which we consider to be part of basin range intraplate extension locally decadal episodes of subsidence and uplift averaging 2 cm yr characterize the 80 year yellowstone caldera monitored history and are modeled as hydrothermal magmatic sources moreover a recent episode 2004 2009 of accelerated uplift of the yellowstone caldera at rates up to 7 cm yr has been modeled as resulting from magmatic recharge of a 10 km deep sill at the top of the crustal magma reservoir regionally gravitational potential energy of the yellowstone swell drives the lithosphere southwest and downhill from the yellowstone plateau 400 km where it coalesces with basin range province wide westward extension based on the geometry and its assumed 660 km depth we extrapolate the plume source southwest to its original location at 17 ma and 600 km southwest and 200 km north of the ysrp importantly this location is beneath the southern part of the columbia plateau flood basalt field of the same age and implies that the yellowstone mantle plume may be the common source for both of these large volcanic fields our time progression model suggests that the original plume head rose vertically behind the juan de fuca plate but at 12 ma it lost the protection of the subducting plate from eastward mantle flow and encountered cooler thicker continental lithosphere becoming entrained in eastward upper mantle flow these results reveal that yellowstone plume plate processes have had a profound effect on late cenozoic geologic evolution and topography of a large part of the western u s

destruction mechanism of the north china craton insight from p and s wave mantle tomography abstract the eastern part of the north china craton ncc was reactivated and its lithosphere was destructed during the mesozoic to cenozoic several destruction models have been proposed by geochemical and geologic studies in this work we determined detailed p and s wave velocity and poisson s ratio structures under the ncc which show different structural patterns in different blocks of the ncc the present results provide important new constrains on the ncc evolution mechanism integrating geochemical and geologic observations and our tomographic results we consider that deep subduction of the pacific slab under east asia caused local or regional scale lithospheric delamination that first took place along some special locations such as the trans north china orogen dabie sulu and yanshan orogenic belts in early mesozoic or paleoproterozoic as a result asthenospheric upwelling in the big mantle wedge above the stagnant pacific slab led to widespread magmatism in and around the eastern part of the ncc hence the thermal erosion and or chemical metasomatism might be a main dynamic mechanism for the lithospheric evolution of eastern ncc during the mesozoic to cenozoic we suggest that the tanlu fault zone and trans north china orogen may have acted as two main conduits for the asthenospheric upwelling which have played an important role in the mesozoic cenozoic destruction of the pre existing archean lithospheric mantle

detailed three dimensional shear wave velocity structure of the northwestern united states from rayleigh wave tomography abstract since the mid miocene the northwestern united states has experienced extensive flood basalt volcanism followed by the formation of two time progressive tracks of silicic volcanism the yellowstone snake river plains ysrp and the high lava plains hlp the ysrp track progresses towards the northeast parallel to north american plate motion and has therefore often been attributed to a deep mantle plume source however the hlp track progresses to the northwest over the same time frame in a direction not consistent with any regional plate motion the causes of the mid miocene flood basalts and the tracks of the ysrp and hlp are a matter of ongoing debate we present results of rayleigh wave phase velocity inversions and inversions for 3 d shear wave velocity structure of the northwestern united states using data collected from the high lava plains seismic experiment and the earthscope usarray transportable array ta the large number of stations used in these inversions allows us to show an unprecedented level of detail in the seismic velocity structures of this tectonically complex area our velocity images indicate that low s wave velocities in the uppermost mantle do not well match the track of hlp volcanism while at the surface the newberry caldera appears to anchor the nw end of the hlp hotspot track the seismic results show that it lies in a separate north south trending low velocity band just east of the cascades that is distinct from the main hlp trace the ultra low s wave velocities beneath the ysrp track extend locally to at least 175 km depth and are by far the most prominent seismic anomalies in the region along axis the ysrp hotspot track is characterized by a discrete low velocity channel in the upper mantle that shallows narrows and intensifies to the northeast but then deepens rapidly to the north beneath yellowstone the shallowing of the low velocity anomaly to the northeast is consistent with a stationary heat source beneath a moving plate coupled with basin and range extension and heating

uncertainties and implications of the late cretaceous and tertiary position of north america relative to the farallon kula and pacific plates we present updated global plate reconstructions and calculated uncertainties of the pacific kula and farallon vancouver plates relative to north america for selected times since 68 ma improved magnetic data from the indian ocean decrease the uncertainties in the global plate circuit approach these uncertainties are now considerably smaller than those inherent in equivalent reconstructions based on the assumption of fixed hotspots major differences between these results and those of others are due to our use of more detailed africa north america reconstructions separate vancouver and farallon plate reconstructions and the assumption of a rigid antarctica plate during cenozoic time the uncertainties in the relative positions of the pacific and north america plates since the time of anomaly 7 26 ma range up to 100 km in position or from 1 to 3 m y in time if the mendocino triple junction initiated at about 28 5 ma its position would have been at 31 3 n 130 km relative to fixed north america unacceptable overlap of oceanic crust of the pacific plate with continental crust of western north america in the anomaly 10 30 ma reconstruction is a minimum of 340 200 km along an azimuth of n60 e and may be accounted for by basin and range extension pacific north america displacement in the past 20 ma is found to be considerably less than that calculated by fixed hotspot reconstructions farallon vancouver north america convergence velocity decreased greatly between the times of anomalies 24 and 21 56 to 50 ma prior to the 43 ma age of the hawaiian emperor bend and the often quoted 40 ma end of the laramide orogeny a change in direction of farallon north america convergence occurred sometime between 50 and 42 ma and also may not correlate with the time of the hawaiian emperor bend the lack of data from subducted parts of the farallon and kula plates permits many possibilities regarding the position of the kula farallon ridge the age of subducted crust or the position of oceanic plateaus during the laramide orogeny leaving open the question of the relationship between plate tectonic scenarios and tectonic style during laramide time displacements of points on the various oceanic plates along the west coast of an arbitrarily fixed north america during the interval between anomalies 30 31 and 18 68 to 42 ma are found to be pacific plate 1700 200 km northward farallon plate 3200 400 km northeastward vancouver plate 3000 400 km northeastward kula plate if attached to the pacific plate after a24 time 2500 400 km northward

seismic image and origin of the changbai intraplate volcano in east asia role of big mantle wedge above the stagnant pacific slab abstract we determined p wave tomography of the crust and upper mantle under the changbai volcanic area in northeast asia by using 289 318 arrival time data from 4802 local earthquakes and 9599 teleseismic events a prominent low velocity anomaly is clearly imaged in the crust and upper mantle down to about 410 km depth beneath the changbai volcano a broad high velocity anomaly is revealed in the mantle transition zone and deep earthquakes occurred actively there suggesting that the subducting pacific slab is stagnating in the mantle transition zone under east asia our results suggest that the formation of the changbai and other intraplate volcanoes in northeast asia is related to the upwelling of hot and wet asthenospheric materials in the big mantle wedge above the stagnant pacific slab hence the active intraplate volcanoes in northeast asia are not hotspots related to deep mantle plumes but are caused by plate tectonic processes in the upper mantle

seismic data processing in vertically inhomogeneous ti media the first and most important step in processing data in transversely isotropic ti media for which velocities vary with depth is parameter estimation the multilayer normal moveout nmo equation for a dipping reflector provides the basis for extending the ti velocity analysis of alkhalifah and tsvankin to vertically inhomogeneous media this nmo equation is based on a root mean square rms average of interval nmo velocities that correspond to a single ray parameter that of the dipping event therefore interval nmo velocities including the normal moveout velocity for horizontal events vnmo 0 can be extracted from the stacking velocities using a dix type differentiation procedure on the other hand which is a key combination of thomsen s parameters that time related processing relies on is extracted from the interval nmo velocities using a homogeneous inversion within each layer time migration like dip moveout depends on the same two parameters in vertically inhomogeneous media namely vnmo

an inverted continental moho and serpentinization of the forearc mantle volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back arc region but the forearc mantle in the corner of the wedge is thought to be significantly cooler here we explore the structure of the mantle wedge in the southern cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers we find very low shear wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an inverted continental moho which reverts to normal polarity seaward of the cascade arc this observation provides compelling evidence for a highly hydrated and serpentinized forearc region consistent with thermal and petrological models of the forearc mantle wedge this serpentinized material is thought to have low strength and may therefore control the down dip rupture limit of great thrust earthquakes as well as the nature of large scale flow in the mantle wedge

multibend fault bend folding this paper shows how slightly complex but angular fault shapes with only a few bends lead to broadly curved and or highly complex folds a common perception exists that simple angular fold geometries are a necessary consequence of the normal assumptions of fault bend fold theory namely angular fault bends straight faults and flexural slip to the contrary it is shown here that exact application of these low level assumptions produces great complexity from simple fault shapes this complexity is caused by the combination of 1 generation of new axial surfaces by displacement of hangingwall cut offs past successive fault bends and 2 fragmentation of axial surfaces by mutual interference the result is entwined arrays of fold axial surfaces that produce quasi curved and or complex folds from discrete angular fault bends fold complexity grows in a highly non linear way in the most extreme case the number of fold axial surface segments grows as the fourth power of the number of fault bends this paper presents examples of the immense variety of complex fold geometries that are thus generated 0 1997 elsevier science ltd all rights reserved

deep structure of southern california abstract we used 214 210 p wave arrival times from 7536 local earthquakes and 16 470 travel time residuals from 332 teleseismic events recorded by the southern california seismic network to determine a detailed three dimensional 3d p wave velocity structure of the crust and mantle down to 600 km depth beneath southern california in this study we have taken into account the moho topography under this region determined by a previous receiver function study we found that the undulations of the moho discontinuity affect considerably the tomographic images of the lower crust and uppermost mantle when the moho topography is taken into account ray paths and travel times can be computed more accurately the tomographic images show a very heterogeneous structure in the crust and upper mantle under southern california the velocity structure in the shallow depth correlates well with the surface geological features three major anomalies in the upper mantle are revealed clearly beneath the southern sierra nevada salton trough and transverse ranges compared with the previous result both the shape and size of the three anomalies show some differences our result revealed two prominent low velocity low v anomalies above the high velocity high v anomalies in the upper mantle under southern sierra nevada and the transverse ranges we consider that the upper mantle anomaly under the transverse ranges was formed through convergence and sinking of the entire subcrustal lithosphere vertical motions and removal of the dense crust root caused the high v anomaly under the southern sierra nevada the prominent low v anomalies above the two high v anomalies were either caused by the crustal materials pulled down to the uppermost mantle or by upwelling of partial melt in asthenosphere when the sinking of lithosphere occurs the salton trough low is the response to the lithospheric extension when the pacific plate was rifted away from the north american plate