defining a 3 dimensional trishear parameter space to understand the temporal evolution of fault propagation folds abstract the application of trishear in which deformation occurs in a triangular zone in front of a propagating fault tip is often used to understand fault related folding a key element of trishear in comparison to kink band methods is that non uniform deformation within the triangle zone allows the layer thickness and length to change during deformation by varying three controlling parameters independently trishear propagation slip ratio trishear apical angle and fault dip we construct a three dimensional parameter space to demonstrate the variability of resultant geometry feasible with trishear we plot published natural examples in this parameter space and identify two clusters and show that the most applicable typical trishear propagation slip ratio is 2 to 3 while the trishear apical angle varies from 30 to 100 we propose that these findings can help estimate the best fit parameters for natural structures we then consider the temporal evolution of specific geometric examples and factors that increase the complexity of trishear including 1 fault dip changes and 2 pre existing faults to illustrate the applicability of the parameter space and complex trishear models to natural examples we apply our results to a sub surface example from the qaidam basin in northern tibetan plateau

precessional climate cyclicity in late cretaceous early tertiary marine sediments a high resolution chronometer of cretaceous tertiary boundary events we report well dated late cretaceous and early tertiary precessional climatic cycles recorded by rhythmic carbonate maxima and minima in south atlantic deep sea sites spectral analyses of digitized sediment color a suitable carbonate proxy show prominent regularities in the spacing marl carbonate beds magnetostratigraphic dating over a number of magnetic chrons constrains the duration of the cycles which can be detected over at least 20 myr of sedimentation at 7 coring locations their mean absolute period of23 5 4 4kyr agrees closely with the predicted late cretaceous precessional period of 20 8 kyr because they can be matched to a physical forcing mechanism with a known repeat time the cycles offer a new high resolution tool to measure rates of climate change before and after the cretaceous tertiary k t boundary from counts of carbonate cycles we derive the position of the k t boundary within c29r at 350 kyr after the base of the reversal the constancy of cycle thickness linearly related to sedimentation rate and amplitude up to the boundary clay does not give evidence for climate instability preceding the boundary orbital chronometry records a step function decrease in sediment accumulation rate at the cretaceous tertiary boundary that is consistent with a geologically instantaneous event

true three dimensional trishear a kinematic model for strike slip and oblique slip deformation most structural kinematic models are inherently two dimensional even several recent three dimensional models are pseudo three dimensional in that they consist of a series of parallel two dimensional cross sections lack of a true three dimensional formulation hampers our abilities to simulate three dimensional structures such as oblique and strike slip faulting and displacement gradients perpendicular to the slip vector the mathematical formulation of trishear deformation using incompressibility of flow is well suited to a solution in three dimensions we derive one plausible velocity field for true three dimensional flow in a triangular shear zone this formulation allows us to simulate the deformation in oblique slip deformation zones as well as flower structures associated with strike slip fault zones the strain distribution in flower structures combined with some simple mechanical assumptions suggests that faults in these zones would have a helicoidal geometry the results of the kinematic model compare well to well described structures in the colorado plateau andaman sea and death valley as well as to data from analogue experiments

geometric analysis of fold development in overthrust terranes abstract fault bend folding fault propagation folding and detachment or decollement folding are three distinct scenarios for fold thrust interaction in overthrust terranes simple kink hinge models are used to determine the geometric associations implicit in each scenario bedding maintains constant thickness in the models except in the forelimb of the fold the forelimb is allowed to thicken or thin without limit the models address individual folds and the calculated fold geometries are balanced structures each mode of fold thrust interaction has a distinct set of geometric relationships final fold geometry is adequate in itself to discern many fault bend folds this is not the case for fault propagation and detachment folds these two fold forms have very similar geometric relationships some knowledge of the nature of the underlying thrust or decollement zone is usually needed to distinguish between them the geometry of a fold is altered in a predictable fashion by transport through an upper ramp hinge and by fault parallel shearing of the structure the shearing results in a tighter fold whereas transport through the ramp hinge produces a broader fold the viability of the geometric analysis technique is demonstrated through its application to a pair of detachment folds from the canadian cordillera the geometric analysis is also used to evaluate cross sections through subsurface structures in an example from the turner valley oil field the analysis indicates how the interpretation should be altered so as to balance the cross section the analysis reveals hidden assumptions and specific inconsistencies in structural interpretations

polymetamorphism in the schirmacher hills granulites east antarctica implications for tectonothermal reworking of an isobarically cooled deep continental crust abstract the precambrian basement of the schirmacher hills queen maud land east antarctica has evolved through multiple episodes of deformation and metamorphism the rocks have suffered at least five phases of deformation the imprint of the early deformation d 1 is preserved in some mafic isolated enclaves the second and the third deformations d 2 and d 3 are the dominant deformations of this area and produced isoclinal folds with transposition of earlier cleavages the later deformations d 4 and d 5 produced two sets of open upright folds detailed mineralogical textural mineral chemical studies and geothermobarometry on khondalite leptynite as well as different varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions followed by an amphibolite facies overprint m 1 is broadly coeval with d 1 only in mafic granulite enclaves within enderbitic gneiss and took place at ca 10 kbar 900 c the mafic magma parental to the enclaves probably crystallized at 11 2 kbar following post peak near isobaric cooling the mafic granulites were transported to shallower levels by the enderbitic magma m 2 recorded in all the lithologies occurred at ca 8 kbar 800 850 c and synchronous with d 2 post peak m 2 evolution of the rocks was characterized by a pressure temperature drop of 2 kbar and 200 c respectively and textures indicative of both cooling and decompression are preserved in different rocks the relative timing of the two however cannot be worked out m 3 synchronous with d 3 took place at 6 kbar 600 650 c and evolved hydrous fluid flux correlation with available structural and geochronological data shows that both m 1 and m 2 could be of grenvillian event m 3 could well be pan african age

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

basin modeling meets rift analysis a numerical modeling study from the jeanne d arc basin offshore newfoundland canada abstract the jeanne d arc basin on the grand banks offshore newfoundland is a confined failed rift basin its initial development and burial history were controlled by crustal stretching and thinning this study provides a detailed analysis of the heat flow tectonic subsidence stretching factor maturation and temperature history based on 44 1d models and one full 4d space and time model in basin models the heat flow history is usually calculated from vitrinite reflectance vrr data however this causes uncertainties and reflects only the periods of sedimentation which where penetrated by wells therefore other methods should be used to reconstruct the early heat flow history not recorded by sediments in this study a discontinuous pure shear stretching model including radioactive heat production reflecting the timing and magnitude of stretching factors of crust and lithospheric mantle was applied to a 4d numerical model for the jeanne d arc basin it is the first time that this new technique is shown on a real case scenario and it gives new insights into the regional geodynamics results show that to understand the thermal evolution of the jeanne d arc sedimentary basin completely it is crucial to consider the triassic rift system this first rift generated the structural framework of the basin where most of the sediments were deposited the second extension in the cretaceous represents most likely an ultra slow extension phase with a heat impulse too weak to leave any thermal record this study demonstrates that the entire evolution of the jeanne d arc basin can be reconstructed assuming just one single triassic thermal rift additionally it shows the effects of a lateral heat transfer on the determination of stretching factors and thus on tectonic subsidence and the resulting heat flow history

three dimensional imaging of active structures using earthquake aftershocks the northridge thrust california abstract aftershocks of the 1994 northridge earthquake m 6 8 allow us to image the structure beneath the san fernando valley in northwestern los angeles in three dimensions we combine aftershocks and geological data to build an image of the three dimensional 3 d geometry of the previously unrecognized north vergent northridge blind thrust to a depth of 21 km the most striking feature of the imaged fault is mega corrugations oriented parallel to the mean aftershock slip vector with most of the 1994 slip confined to west of the largest corrugation we also image the partially overlying south vergent san fernando thrust which broke to the surface in a complex rupture in 1971 m 7 1 both thrusts produce fault related folding because of either fault propagation or fault bends however this deeper folding is masked by overlying complex deformation in the cover which is one reason why the northridge thrust was not identified until it ruptured in 1994 we use trishear fold modeling based on our 3 d fault geometry to evaluate possible folding due to slip on the northridge thrust as well as its interaction with the overlapping san fernando thrust and with shallow structures in the cover this example illustrates the importance of earthquake data to structural geology and the value of its 3 d integration with surface and near surface geological data

three dimensional segmentation and different rupture behavior during the 2012 emilia seismic sequence northern italy describing the slip behavior of an active fault system is central to understanding the seismic potential of seismogenic areas different elements control the nature and the extent of the coseismic and post seismic ruptures including the geometry of faults the nature of faulted rocks and the stress changes caused by the mainshocks in may june 2012 a severe seismic sequence struck a portion of the po plain northern italy where a thick blanket of plio quaternary sediments hides a number of seismogenic sources corresponding to the external thrust systems of the northern apennines we used deep seismic reflection data to reconstruct the geometry of the faults responsible for the sequence these faults exhibit significant non planarity due to tectono stratigraphic heterogeneities inherited from a complex pre thrusting extensional tectonic phase a comparison of the fault parameters derived from our geological analysis and the evidence supplied by seismological aftershock distributions and geodetic data insar allowed to identify the causative fault segments of the two mainshocks we then modeled the coulomb stress changes produced by two mainshocks to analyze on and off fault seismicity discrepancies between the magnitude of the earthquakes and the extent of active faults suggest that the mainshocks did not rupture the entire thrust planes we contend that seismogenic ruptures were confined in the mesozoic carbonates and were stopped by lithological changes and or mechanical complexities of the fault planes both along dip and along strike our findings highlight that along the active structures of the po plain slip tends to be seismogenic where faults are located in mesozoic carbonate rocks

a unified kinematic model for the evolution of detachment folds abstract detachment folds represent a major structural element in a number of fold belts they are common in the jura mountains the zagros fold belt the central appalachian fold belt the wyoming fold belt the brooks range the parry islands fold belt and parts of the subandean belt these structures form in stratigraphic packages with high competency contrasts among units the competent upper units exhibit parallel fold geometries whereas the weak lower unit displays disharmonic folding and significant penetrative deformation two distinct geometric types disharmonic detachment folds and lift off folds have been recognized however these structures commonly represent different stages in the progressive evolution of detachment folds the structures first form by symmetric or asymmetric folding with the fold wavelength controlled by the thickness of the dominant units volumetric constraints require sinking of units in the synclines and movement of the ductile unit from the synclines to the anticlines continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration initially limb rotation occurs primarily by flexural slip folding but in the late stages of deformation the rotation may involve significant internal deformation of units between locked hinges the folds eventually assume tight isoclinal geometries resembling lift off folds variations in the geometry of detachment fold geometry such as fold asymmetry significant faulting and fold associated with multiple detachments are related to variations in the mechanical stratigraphy and pre existing structure

reappraisal of anoxia and organic richness with emphasis on cretaceous of north atlantic accumulation of anoxic sediments in the north atlantic during barremian through turonian time early to middle cretaceous was controlled mainly by local conditions although global climatic and oceanographic factors played a supporting role evidence does not support the popular hypothesis developed over the past few years of global or oceanwide anoxia data which contradict this hypothesis are based on 1 a reevaluation of criteria for identifying sediments that accumulated in anoxic water 2 a critical reappraisal of reported worldwide occurrences of anoxic sediments of these ages and 3 an examination of the spatial and temporal distribution of anoxic sediments in the north atlantic and elsewhere a model is proposed in which the earliest anoxic sediments of cretaceous age in the north atlantic were deposited in deep restricted basins by middle cenomanian time about 97 m y ago sluggish circulation had led to a gradual expansion of the oxygen minimum layer permitting deposition of anoxic sediment in nonbasinal settings as well expansion of the oxygen minimum layer caused the calcite compensation depth to rise promoting oxidation of organic carbon and causing contraction of the oxygen minimum layer development of anoxia was thus self damping anoxia effectively disappeared by early senonian time 88 m y ago when improved circulation created oceans more like those of the present some evidence indicates that enhanced biologic productivity and upwelling may have been local factors in fostering anoxia but the early and middle cretaceous were generally not highly productive times

how do horizontal frictional discontinuities affect reverse fault propagation folding abstract the development of new reverse faults and related folds is strongly controlled by the mechanical characteristics of the host rocks in this study we analyze the impact of a specific kind of anisotropy i e thin mechanical and frictional discontinuities in affecting the development of reverse faults and of the associated folds using physical scaled models we perform analog modeling introducing one or two initially horizontal thin discontinuities above an initially blind fault dipping at 30 in one case and 45 in another and then compare the results with those obtained from a fully isotropic model the experimental results show that the occurrence of thin discontinuities affects both the development and the propagation of new faults and the shape of the associated folds new faults 1 accelerate or decelerate their propagation depending on the location of the tips with respect to the discontinuities 2 cross the discontinuities at a characteristic angle 90 and 3 produce folds with different shapes resulting not only from the dip of the new faults but also from their non linear propagation history our results may have direct impact on future kinematic models especially those aimed to reconstruct the tectonic history of faults that developed in layered rocks or in regions affected by pre existing faults

bedding plane slip in initial stages of fault related folding abstract frictional slip along bedding planes contributes to fault related folding of layered rocks we use numerical experiments to investigate the deformation of frictional bedding planes near dipping faults under layerparallel contraction and extension within the numerical experiments contraction boundary conditions produce asymmetric anticlines and extension produces asymmetric synclines the fold shape may be used to infer dip of the underlying fault in situations where the fault may not be observable additionally sense of slip along bedding planes may indicate proximity to the fault tip under uniform remote tectonic strain fault slip induces deformation in both the hangingwall and the footwall at depths as shallow as 1 km there is no significant difference between fold amplitudes in the hangingwall and the footwall this result is contrary to many kinematic models currently in use kinematic models of fault related folds commonly include the development of flat ramp and flat ramp flat fault geometries which may be attributed to initial ramp thrusting and later flat development our mechanical models show that fault flats may be produced from fault ramps due to slip along frictional bedding planes near the thrust fault tips our numerical experiments also evaluate joint initiation joints perpendicular to bedding are promoted in extensional environments we compare the results of the mechanical model to kinematic models of fault propagation folds and conclude that mechanical models offer important insights to better understand the folding process

mechanical stratigraphy and the transition from trishear to kink band fault propagation fold forms above blind basement thrust faults a discrete element study abstract we employ a discrete element technique to investigate the influence of mechanical stratigraphy on the transition from trishear to kink band fault propagation fold forms above a blind basement thrust fault we find that a classic trishear fold consisting of a broad upwards widening monocline with limb dips increasing towards the fault tip occurs where the cover stratigraphy is weak and has no strong mechanical layering in contrast we find that marked bedding parallel mechanical layering in the cover promotes much narrower kink like fold geometries with more constant limb widths and dips we discuss the modelling results in terms of their implications for structural interpretation and the application of kinematic models of fault related folding in contractional settings

high resolution cyclostratigraphic analysis from magnetic susceptibility in a lower kimmeridgian upper jurassic marl limestone succession la meouge vocontian basin france abstract high resolution magnetic susceptibility ms analysis was carried out on a lower kimmeridgian alternating marl limestone succession of pelagic origin that crops out at la meouge vocontian basin southeastern france the aim of the study was to characterize the strong dm scale sedimentary cyclicity of the succession at a very high resolution and to analyze the cycles for evidence of astronomical forcing from marl to limestone ms varies progressively and closely tracks the highest frequency cyclicity corresponding to the basic marl limestone couplets long term wavelength cycling modulates the high frequency cyclicity couplets and appears to be controlled by clay content spectral analysis of the ms record reveals the presence of the complete suite of orbital frequencies in the precession obliquity and eccentricity 95 128 ka and 405 ka bands with very high amplitude of the precession index cycles originating from dm scale couplets 405 ka eccentricity cycles are very pronounced in the ms maxima of the marl members of the couplets suggesting eccentricity driven detrital input to the basin 405 ka orbital tuning of the ms maxima further sharpens all of the orbital frequencies present in the succession these results are similar to those of previous studies at la meouge that used carbonate content observed in field our results are also in accordance with cyclostratigraphic studies in spain and canada that report dominant precession index forcing by contrast in the kimmeridge clay dorset uk obliquity forcing dominates cyclic sedimentation with weaker influence from the precession index ammonite zone duration estimates are made by counting the interpreted precession cycles and provide an ultra high resolution assessment of geologic time in sum this study demonstrates the power of the ms as a proxy in characterizing the high resolution cyclostratigraphy of mesozoic sections particularly in alternating marl limestone successions and for high resolution correlation and astronomical calibration of the geologic time scale

cyclicity in the egret member kimmeridgian oil source rock jeanne d arc basin offshore eastern canada the origin of the observed variability in lithology total organic carbon and petrophysical properties of the egret member source rock interval in the jeanne d arc basin offshore eastern canada is examined detailed analyses of petrophysical logs indicate that the egret member is cyclic three different orders of cycles recognizable on well logs occur in the egret member the large and medium scale cycles are correlatable among wells the estimated cyclicities of the large and medium scale cycles fall into the milankovitch like cycle bands suggesting that the depositional cycles are the product of orbitally forced sea level and climatic changes and implying that egret member deposition lasted at least 1 65 million years using chronological information drawn from sedimentary cycles both spatial and vertical temporal variations in sedimentation rate are examined for the member a correlation exists between low sedimentation rates and high total organic carbon values in this basin it is most probable that the organic rich layers were deposited during times of high sea level stands and warmer periods

cyclostratigraphy orbital tuning and inferred productivity for the type kimmeridge clay late jurassic southern england three independently measured variables magnetic susceptibility photoelectric factor and total gamma ray obtained from throughout the type kimmeridge clay fm in dorset southern england were used to identify regular metre scale sedimentary cycles spectral analysis demonstrates that for long stratigraphical intervals the cycles are expressed as large amplitude cycles of 1 87 4 05 m wavelength and smaller amplitude cycles of around half that wavelength these cycles are interpreted to record orbital obliquity and precession respectively the much larger amplitude of the inferred obliquity cycles compared with the precession cycles may indicate a high latitude climatic forcing transferred to lower latitudes via sea level variations orbital tuning indicates that the early kimmeridgian sensu anglico lasted at least 3 6 ma 95 longer wavelength cycles and the late kimmeridgian at least 3 9 ma 103 longer wavelength cycles the first detailed productivity estimates for the kimmeridge clay fm on a cycle by cycle calculation indicate that average productivity of the type kimmeridge clay 220 g m 2 a 1 was less than the average productivity on modern continental shelves the high average organic carbon content of the type kimmeridge clay 3 8 total organic carbon cannot be attributed to high average productivity however the average organic carbon content is consistent with low siliciclastic mineral dilution of organic matter and or elevated preservation linked to reduced bottom water oxygenation

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

propagation history of the osaka wan blind thrust japan from trishear modeling abstract mapping the nucleation and 3d fault tip growth of the active osaka wan blind thrust provides an opportunity to asses how reactivated thrusts build slip from preexisting faults and the threat they pose as sources of large earthquakes analysis of folded growth strata based on 2d trishear inverse modeling allows a range of best fit models of the evolution of slip and propagation of the fault to be defined the depth of the fault tip at 1200 ka varies between 1 5 4 5 km suggesting the fault grew upward from high in the crust and that it is reactivated from its onset at 1500 ka the fault grew rapidly along strike in 300 ky and upwards with a p s ratio of 2 5 3 0 but variable fault slip in space and time shallower depths of the fault tip at initiation and thinner basin fill correlates with slower propagation with time contradicting models that argue for sediments as inhibitors of fault growth results also suggest the displacement profile of the currently active thrust is offset from its predecessor assuming shallower depths to the original fault correlate with greater displacement in its prior history these results suggest reactivated faults may accrue slip differently than newly developed ones based on the history of upward fault propagation

numerical modeling of trishear fault propagation folding in contrast to kink band migration modeling methods trishear numerical models produce fault propagation folds with smooth profiles and rounded hinges modeled fold hinges tighten and converge downward within a triangular zone of distributed deformation which is focused on the fault tip such features have been reported from field studies and are also seen in analogue models of compressional deformation however apart from its initial application to laramide folds little quantitative work has been undertaken on trishear fault propagation folding in other settings in addition no study has been undertaken into the growth strata which might be associated with such structures this paper uses an equivalent velocity description of the geometric model of trishear together with models of erosion and sedimentation to investigate trishear fault propagation folding of both pregrowth and growth strata the trishear model is generalized to include a variety of fault propagation to slip ratios and fault propagation from a flat decollement the models show continuous rotation of the forelimb with the characteristic development of cumulative wedges within growth strata when total slip on a structure is high the model predicts overturned pregrowth and growth strata during the initial stages of deformation beds in the forelimb thicken but later thin when they become steep or overturned the effect of variations in fault propagation to slip ratios on two dimensional finite strain in the models is assessed by the use of initially circular strain markers high fault propagation to slip p s ratios lead to narrow zones of high finite strain while lower p s ratios lead to more ductile deformation and broader zones of high strain in all cases hanging wall anticlines and footwall synclines originate as early ductile folds which are later cut by the propagating fault modeled structures are compared with natural examples

a quadshear kinematic model for folding and clay smear development in fault zones abstract a new kinematic model based on the trishear model has been developed to simulate shearing and folding of a ductile interval sandwiched between two brittle layers as faults propagate from the brittle layers into the ductile interval this quadshear model can be used to study fault zone evolution and the development of clay smears which often form when a stratigraphic sequence including ductile clay rich beds is sheared into a developing fault zone the model also gives the critical clay smear factor ccsf the ratio of fault displacement to bed thickness at which a clay smear becomes discontinuous and hence a potential leak point the main control on clay smear development is the position of the clay rich beds relative to brittle fracture prone layers clay rich beds near the centre of a ductile interval can shear to form much longer continuous clay smears than clay rich beds adjacent to brittle layers fault propagation rates are also a strong control on clay smear development slow propagating faults form longer continuous clay smears than fast propagating faults however the width of the shear zone the dip of the fault and the presence of releasing or restraining bends in the fault are minor controls on clay smear development

hemipelagic shelf sedimentation and climatic cycles the basal jurassic blue lias of south britain abstract the sedimentology cyclicity and hiatuses of the blue lias formation basal jurassic were investigated at five localities in south britain most carbonate mud which is the dominant carbonate component has been neomorphosed to microspar however carbonate mud was apparently supplied by coccoliths in zooplankton faecal pellets suggesting hemipelagic sedimentation clay is assumed to have been supplied by rivers the three sediment types involved reflect cyclic changes in bottom water oxygenation levels combined with changes in the clay to carbonate mud ratio walsh power spectra have been applied for the first time to measured sections in order to investigate the control of cyclicity this application is analogous to palaeoclimatological studies of pleistocene oxygen isotope ice volume records the spectra show that 1 or 2 sedimentary cycles of constant thickness occur in four out of five sections each of which represents several million years the maximum possible duration of these cycles is too short for their generation by sea level changes their regularity stability and order of duration tens of thousands of years are consistent with the hypothesis of sedimentary cyclicity controlled by climatic change itself forced by orbital changes in insolation milankovitch theory the duration of both cycles recognised is less than 93 000 years 93 ka and may record changes in orbital precession 21 ka and obliquity 41 ka assuming that one ammonite zone represents 1 ma comparison of the longest possible with the implied durations of the cycles suggests that the sections are at best 20 40 complete loss of sediment may have occurred during winter storms or hurricanes a model relating the sedimentary cycles to changes in the volume of runoff is proposed

a paleoceanographic approach to the kimmeridge clay formation the upper jurassic lower cretaceous kimmeridge clay formation kcf is northern europe s premier source rock and can be understood using a new but relatively simple oceanographic model this explains or accommodates most current observations about the kcf and its depositional environment and draws upon paleogeographic paleoclimatic geochemical sedimentological and paleontological evidence

fault propagation ductile strain and displacement distance relationships abstract displacement distance analysis of fault related fold structures can reveal important information about the distribution of fault displacement and in some cases the history of fault propagation this method is commonly used to 1 determine the ratio of displacement rate to propagation rate s p ratio 2 predict the location of unconstrained fault tips 3 determine the location of fault nucleation points and 4 evaluate structural interpretations although the displacement distance approach represents a potentially useful method for gaining information about displacement parallel fault propagation histories of natural fault related fold structures the theoretical relationship between s p and relative stretch er that forms the basis of this method remains essentially untested because displacement rates and propagation rates are generally unknown for natural structures these relationships can only be objectively tested by analysing model structures with known geometries strain distributions and fault propagation histories displacement distance analysis of different fault related fold models reveals that 1 displacement distance relationships are highly dependent on the nature of the hanging wall strain 2 relative stretch is not related to s p in any general way 3 fault tip locations cannot always be predicted by linear extrapolation of displacement distance trajectories 4 displacement maxima are not necessarily indicative of fault nucleation points in all cases and 5 although calculation of s p ratios from observed relative stretch magnitudes may not be appropriate the use of displacement distance relationships can nonetheless be useful in comparing structures and improving structural interpretations

depositional mechanisms and organic matter in mowry shale cretaceous wyoming four lithofacies which accumulated under different depositional conditions can be distinguished in the lower cretaceous mowry shale of wyoming type and amount of organic matter in these lithofacies are governed largely by sediment transport mechanisms nearshore silt and mud containing terrestrial organic matter were deposited in a prodelta environment by tractive processes and from suspension these sediments along with terrestrial organic matter were redistributed by waning bottom flows over a dysaerobic slope pelagically deposited muds containing marine derived organic matter accumulated between flow events muds farthest offshore accumulated in dysaerobic and anaerobic water by pelagic settling in a region unaffected by bottom currents these muds contain predom nantly marine derived organic matter petroleum source potential increases in a southeastward direction across west and central wyoming this trend results from differential input of marine and terrestrial organic matter clastic dilution and postdepositional biodegradation

fault propagation folds above thrusts with constant dip abstract previous models of fault propagation folds have been extended to include the case where folding initiates above a thrust ramp of constant dip the model may be useful in analyses of fold thrust structures in thin and thick skinned compressional terrains it may be most applicable to fault propagation folds that develop as a fault propagates from a thick massive brittle unit into a thinly layered anisotropic unit

a quantitative study of lower cretaceous cyclic sequences from the atlantic ocean and the vocontian basin se france milankovitch type periodicity was identified within lower cretaceous cyclic sequences from the western central atlantic ocean and the vocontian basin southeast france through application of the walsh spectral method to detailed stratigraphic sections the repetitious bedding in these sequences appears to be the product of cyclic climatic changes controlled by the orbital forcing obliquity seems to be the most important and persistent orbital element responsible for cyclic sedimentation whereas precession is less important upper berriasian and valanginian strata from the atlantic ocean contain indications of eccentricity induced periodicity and of a nonlinear response of the climatic system variations in orbitally tuned pelagic sedimentation rates in the atlantic ocean correlate with the changes in the character of cycles and with the occurrence of calcareous turbidites therefore this suite of sedimentary features are responding to evolving paleoceanographic conditions and possible relative sea level fluctuations the orbitally tuned durations of the valanginian and hauterivian stages in the vocontian basin are a minimum of 5 9 and 5 3 m y respectively

the kinematics of break thrust folds abstract the kinematics of asymmetric open to close often overturned thrust related folds are commonly explained with fault propagation or detachment fold models field observations of thrust related folds exposed in tennessee virginia wyoming and montana that exhibit this geometry indicate that models of fault propagation and detachment folds do not adequately describe the kinematics of these structures existing models of fault propagation and detachment folding employ migrating kink shaped hinges and relate fold geometry entirely to fault geometry slip and to the thickness of the basal detachment layers the models exclude the relation between fold shape and amplitude and they do not predict a correlation between competent layer thickness and fold wavelength as expected from buckling theory thrust related folds examined in this study exhibit unique distributions of fabrics in the hinges and forelimbs which demonstrate a lack of hinge migration furthermore the ratios of dominant member thickness to fold wavelenght agree with those expected for folds controlled by a single layer and they are within the expected range for folds with multiple evenly spaced harmonically deforming layers our analyses suggest these folds evolved from an initial brief stage of sinusoidal buckling to a later stage of fixed hinge kinking and thrusting thrust related folds which exhibit a geometry and mesofabric distribution suggesting this type of evolution are defined as break thrust folds

a balanced cross section of the 1994 northridge earthquake southern california the northridge earthquake of 17 january 19941 was the latest in a series of very damaging thrust fault generated earthquakes to strike california following the san fernando2 1971 coalinga3 1983 and whittier narrows4 5 1987 events like the last two of these the northridge event occurred along a fault that did not reach the surface and which had not been detected by traditional seismic hazard methods6 7 balanced cross sections8 9 which flatten and remove the crustal deformation can be used to identify and quantify the seismic hazard posed by thrust faults here we present a balanced cross section through the northridge portion of the transverse ranges fold and thrust belt10 which shows that the earthquake occurred on what we call the pico thrust a cross section of this sort constructed before the earthquake would have revealed the fault although it would not have predicted the earthquake cross sectional modelling of the pico thrust yields an average slip rate of 1 4 1 7 mm yr 1 and a recurrence interval of northridge sized mw 6 7 earthquakes every 1 500 1 800 years we show that the pico thrust is the back thrust to the 170 km elysian park thrust3 4 which underlies some of the most densely urbanized portions of the los angeles basin

double edge fault propagation folding geometry and kinematics abstract fault propagation folding is a common folding mechanism in thrust and fold belts and accretionary prisms several geometrical models relating the fold shape to the ramp shape have been proposed in all these models ramps always emanate from a basal fault and propagate upwards we have developed a new kinematic and geometric model of fault propagation folding named double edge fault propagation folding the model simulates folding at thrust ramps as a function of their nucleation site and propagation history within the folded multilayer the fold shape depends on the initial length and location of the ramp its dip and the s p ratio i e incremental ramp slip versus propagation of both the upper and lower ramp tips this solution increases the geometrical flexibility of fault propagation folding reducing for example the direct dependence between the backlimb dip and the ramp dip as double edge fault propagation folding is characterised by a backlimb panel not necessary parallel to the ramp non parallelism between the ramp and the backlimb is commonly observed in thrust related anticlines within fold and thrust belts and accretionary prisms the excess layer parallel shear imposed by the development of double edge fault propagation folding can be easily accommodated by discrete faulting and or penetrative deformation the dependence of the fold shape on the fault behaviour provides a tool for including the role of mechanical stratigraphy and environmental conditions of deformation into kinematic models natural examples of anticlines that could be modelled by double edge fault propagation are presented

wavelet analysis of well logging data from oil source rock egret member offshore eastern canada wavelet analysis is a sensitive method for automatically detecting and distinguishing abrupt discontinuities i e faults unconformities cyclicity and gradual changes in sedimentation rate by transforming depth related sedimentary signals i e gamma rays into wavelengths at distinct depth intervals we used wavelet analysis for evaluation of the spatio temporal distribution of oil source rocks and for estimating accumulation rates in a sedimentary basin having high resolution the method was applied to 16 gamma ray logs from the jurassic egret member an oil source rock succession 55 m to 227 m in thickness offshore eastern canada dominant gamma ray cycles having wavelengths varying from 2 8 m western margin of the basin to 24 m eastern part of the basin have been detected by wavelet analysis the coincidence of the ratio of predominant gamma ray cycles with the ratio of milankovitch spectra about 400 100 40 20 k y suggests that climatic cycles are an important factor controlling sedimentary cyclicity in the egret member dominant wavelengths likely represent 100 k y eccentricity giving accumulation times of 1 9 m y for stratigraphically complete sections having 19 successive 2 8 m gamma ray cycles and giving accumulation times of 600 k y for incomplete successions having only 6 cycles up to four discontinuities occur in gamma ray log cyclicity and separate the egret member into subunits we interpret the discontinuities as unconformities or faults and as related to sediments having low petroleum potential the stratigraphic completeness of the egret member is correlated to total mass of organic carbon and decreasing thickness of non source rock intervals having correlation coefficients of r 0 8 and r 0 76 respectively

hemipelagic sediment accumulation rates in the south china sea related to late quaternary sea level changes sediments of 13 piston cores from opposite continental slopes of the south china sea off southern china and sabah northern borneo were analyzed by sedimentological methods and dated by oxygen isotope stratigraphy sediments mostly consist of hemipelagic clay with 20 carbonate off sabah and 40 off china we calculated terrigenous and carbonate accumulation rates for up to 11 time slices from the holocene to oxygen isotope stage 6 terrigenous accumulation rates generally increase with water depth and reach a maximum at the middle slope off sabah and at the lower continental slope off china during glacial and interglacial times this distribution pattern did not markedly change despite an increase of accumulation rates for glacial periods by a factor of 2 to 5 compared to interglacial periods rates are negatively correlated with positions of sea level which controls the partition of fluviatile terrigenous material for deposition on shelf slope and abyssal plain carbonate accumulation rates are higher off china by a factor of 2 compared to sabah probably indicating higher calcareous plankton productivity

critical state finite element models of contractional fault related folding part 2 mechanical analysis abstract finite element modeling of contractional fault related folding with critical state mechanics concepts reveals fundamentally different behaviors at two levels depending on 1 mechanical stratigraphy and 2 initial fault geometry deviatoric plastic strain rate and strain patterns show that strong layers like normally consolidated sandstone or overconsolidated shale develop strain localization in form of narrow plastic shear bands soft layers like normally consolidated shale fail to develop localizations and respond by diffuse distributed plastic strain inter layered overburdens display both deformation modes models with listric thrust fault seeds develop basement cored anticlines in the hangingwall that are characterized by locally tensile stress regimes within overall compressive regional stress fields the anticline crests of these models show evidence for strain softening by contrast anticlines emerging in detached fault related fold models are subject to locally compressive stress regimes consistent with the applied far field shortening these anticline crests strain harden during deformation stresses resulting from the finite element models deviate from stress estimates based on integrated densities the discrepancy is small for basement involved models but significant for detached style models suggesting that stress predictions based on integrated densities may be inaccurate for contractional systems first order predictions of plastic deformation suggest that footwall synclines in basement involved fold styles may host reservoirs with relatively poor quality owing to horizontal and vertical compaction anticlines found in detached deformation styles are bounded by compactive domains whereas the crestal regions are less affected by volumetric plastic deformation thus producing relatively better reservoir quality

evolution of fault efficiency at restraining bends within wet kaolin analog experiments abstract restraining bends are regions of mechanical inefficiency along strike slip faults where faults evolve to improve efficiency analog experiments using wet kaolin examine the evolution of a variety of restraining bends restraining bends with 15 bends continue to slip while systems with greater bends develop new faults the new faults which flank the uplifted region accommodate right lateral slip when parallel to the plate movement and oblique slip motion when parallel to the restraining segment within the wet kaolin strain is partitioned into fault slip and off fault deformation such as distributed shear and uplift the wet kaolin produces restraining bend deformation patterns fault sequence and mechanical efficiency similar to dry sand experiments and natural restraining bends the propagation of new faults in the wet kaolin improves the mechanical efficiency of the fault system by increasing the ratio of fault slip to off fault deformation local inefficiencies such as linkage of faults via a sharp kink do not affect the overall increase in efficiency of the linked fault system furthermore wider restraining bend stepovers have lower mechanical efficiency than close stepovers but the difference in mechanical efficiency decreases as faults grow and link up around the restraining bends this study demonstrates that restraining bend fault systems evolve toward greater mechanical efficiency

rheologic testing of wet kaolin reveals frictional and bi viscous behavior typical of crustal materials 1 new rheological data for wet kaolin support its use in analog table top experiments that simulate deformation of the earth s crust creep tests at small strain reveal that wet kaolin 62 66 water by mass exhibits both elastic and viscous deformation characteristic of a burger s material when sheared to failure the shear strength is relatively insensitive to the strain rate the shear strength appears sensitive to the amount of initial compaction within the rheometer which may indicate a normal stress dependency typical of frictional materials stepped velocity tests at large strain demonstrate velocity weakening rate and state frictional behavior after yielding because the wet kaolin exhibits deformation characteristic of both frictional materials and bi viscous materials this material is well suited to simulate a variety of crustal deformational processes in scaled analog experiments

predicted and measured petrophysical and geochemical characteristics of the egret member oil source rock jeanne d arc basin offshore eastern canada abstract the log r wireline log technique has been used to predict high vertical resolution geochemical profiles for 17 well penetrations of the upper jurassic egret member source rock of the jeanne d arc basin offshore newfoundland the predictions are compared with rock eval data obtained from the same interval high resolution comparison of the computed total organic carbon toc curves with gamma ray logs implies that the source rock layers in the egret member are mainly shale claystone and the non source rock layers are limestone marl to the south and siltstone sandstone to the north these comparisons also indicate that in the mature area some coarse grained non source rock layers are filled with hydrocarbons implying primary migration of oil gas using the average computed toc value the thickness of the egret member and the average formation density the mass of organic carbon was calculated at 19 well locations and regional variations examined areas with a great mass of organic carbon were identified the present mass of organic carbon is primarily related to the amount of organic matter being deposited and preserved but the expulsion of oil gas may also exert a control using 1 wt as the lower limit for a source rock it was observed that the percentage of non source rock in the egret member is generally higher in immature wells lom 7 than in mature wells probably as a result of hydrocarbon migration expulsion after maturation

fault bend folding as an end member solution of double edge fault propagation folding fault bend folding is the most commonly used kinematic mechanism to interpret the architecture and evolution of thrust related anticlines in thrust wedges however its basic requirement of an instantaneous propagation of the entire fault before hangingwall deformation limits its kinematic effectiveness to overcome this limitation we used the interdependence between fold shape and fault slip vs propagation rate s p ratio implemented in double edge faultpropagation folding we show that very small s p values produce fault propagation anticlines that when transported

why blind thrust faults do not propagate to the earth s surface numerical modeling of coseismic deformation associated with thrust related anticlines high fault tip stress concentrations are associated with coseismic slip on blind thrust faults and suggest that these structures should readily propagate to the earth s surface seismic profiles of blind thrust related earthquakes reveal diffuse zones of aftershocks surrounding the fault tip which are attributed to inelastic deformation such as flexural slip or extensional fracturing the complex interaction between blind thrust faults and secondary structures may control the evolution of blind thrust systems the influence of bedding plane slip on fault propagation is simulated with numerical models using the boundary element method we use two parameters to estimate the tendency for thrust fault propagation 1 the mode ii stress intensity factor and 2 the maximum coulomb stress near the fault tip calculations from both analyses suggest that shallow thrust faults may exhibit an increased tendency to propagate as a result of interaction with the earth s surface and slip along bedding planes above the fault tip and a decreased tendency to propagate due to slip along bedding planes at or below the fault tip our results demonstrate that the magnitude and style of inelastic deformation in active fault systems control fault propagation

anoxia vs productivity what controls the formation of organic carbon rich sediments and sedimentary rocks discussion the prevailing explanation for the origin of organic rich sediments and rocks invokes deposition under conditions of anoxia however recent research suggests that high primary production and not water column anoxia provides the first order control on the accumulation of organic rich facies in the modern oceans oxygen minima do not appear to have any direct effect on carbon accumulation in continental margin or marginal sea environments sediments accumulating in the modern black sea the type euxinic basin are not particularly enriched in organic matter despite the presence of an anoxic water column although a sapropel containing extremely high carbon concentrations was deposited during the holocene at a time when the basin was oxic results of a recently published co pled ocean atmosphere model indicate that during the cretaceous thermohaline and surface circulation in the oceans was similar to or more intense than modern conditions despite the overall equable climate such conditions confound the idea that circulation in the cretaceous atlantic for example was punctuated by oceanic anoxic events brought about by more sluggish circulation sporadic temporal and spatial increases in primary production reflecting changes in the behavior and or state of the ocean atmosphere system constitute a more tenable explanation for the occurrence of modern and quaternary carbon rich sediments and cretaceous black shales consequently the fundamental control on the accumulation of carbon rich facies in the oceans and marginal seas is not the presence or absence of anoxia

diagenesis of the kimmeridge clay formation onshore uk study of sedimentology carbonate geochemistry and clay mineralogy of four laterally continuous beds from the outcrop of the kimmeridge clay formation indicates strong lateral variations in the depositional and early diagenetic processes reflecting the basin and swell topography of the depositional basin the mudstones from the swell environment with low sedimentation and subsidence rates are shelly and carbonate rich with thin poorly developed oil shales they were deposited under dysaerobic conditions with consequent poor organic matter preservation early diagenetic processes were limited to the sulphate reduction and methanogenesis zones with the formation of discrete horizons of concretions the shallow burial depths attained are reflected by the immaturity of the organic matter in the mudstones although ordered illite smectites are present sediments from other locations comprise black organic rich mudstones the thick beds and good organic matter preservation indicating deposition under high sedimentation and subsidence rates with anoxic bottom waters in a basinal environment these indurated mudstones contain a few shell fragments and much pyrite indicating intense reaction in the sulphate reduction zone carbonates were precipitated in the methanogenesis and decarboxylation zones as ferroan calcites and dolomites the much greater burial depths are indicated by the higher maturity of the organic matter and by the ordered nature of the illite smectites and the loss of kaolinite

the marl limestone rhythmites from the lower kimmeridgian platynota zone of the central prebetic and their relationship with variations in orbital parameters abstract the origin of the rhythmic marl limestone bedding recognized in lower kimmeridgian platynota zone succesions of the central prebetic southern spain is interpreted a detailed analysis was carried out on the basis of field observations and mathematical procedures of two sections characterized by the variable alternation of marly and limestone horizons the study primarily reveals signs of a possible secondary imprint in the stratification however we conclude that this imprint cannot have been sufficiently important to cause the recorded stratification or significantly alter the existing primary bedding the mathematical procedure applied to the thicknesses of the beds reveals the existence of different orders of cyclicity that are not recognizable in the field we were able to date these cycles by detailed biostratigraphy and the application of fourier spectral analysis the close relationship between the timing calculated for the marl limestone rhythms recognized in the lower kimmeridgian platynota zone successions and the periodicity assumed for milankovitch cycles has been shown for the first time in the prebetic zone and leads us to accept the influence of orbital variations on sedimentation in the sector that was studied we have also been able to recognize the influence of local factors which determined differences in the depositional organization in the sections studied

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

dip range for intracontinental reverse fault ruptures truth not stranger than friction abstract histograms of fault dips have been compiled for moderate to large m 5 5 reverse slip intracontinental earthquakes with the slip vector raking 90 30 in the fault plane the principal data set is restricted to earthquakes where the fault plane in the focal mechanism can be unambiguously distinguished from the auxilliary plane the reverse fault dips are bracketed within the range 12 1 the observed dip range is consistent with reactivation of faults possessing rock friction coefficients within byerlee9s 1978 range 0 85 s 0 6 undergoing frictional lockup at dips approaching 60 the broad 25 to 35 peak may arise from progressive domino steepening of imbricate reverse faults above the optimal dip for reactivation in regions undergoing bulk shortening paucity of very low angle thrusts implies that it is generally the steeper ramps within ramp flat assemblages that fail in moderate to large earthquakes the subsidiary peak at 45 to 55 likely results from compressional reactivation of former normal faults in areas undergoing tectonic inversion requiring some degree of fluid overpressuring the results are consistent with previous studies on the dip range for active normal faults that again demonstrate frictional lockup at reactivation angles approaching 60 together these analyses suggest that byerlee friction coefficients apply to faults with displacements of up to a few kilometers

low productivity and slow deep water circulation in mid cretaceous oceans paleofertility estimates are derived from organic carbon accumulation rates determined in nonbioturbated mid cretaceous deep sea deposits using organic carbon preservation factors observed in laminated sediments from anoxic holocene environments the estimated mid cretaceous primary production rates are as much as an order of magnitude lower than those measured in today s oceans an evaluation of dissolved oxygen budgets using a simple oceanic circulation model indicates very low deep water renewal rates in mid cretaceous oceans 28 references 2 tables 4 figures

recurring patterns of total organic carbon and source rock quality within a sequence stratigraphic framework sequence stratigraphy has been combined with total organic carbon toc analysis from well logs core and cuttings to develop a model of toc accumulation in marine source rocks routine well profiling of toc in marine source rocks has revealed that recurrent patterns in the vertical distribution of toc occur that can be explained with sequence stratigraphic concepts the model assumes that the principle controls on organic carbon accumulation are 1 the presence of low oxygen conditions at the sediment water interface and 2 sedimentation rate under these circumstances toc will vary inversely with terrigenous clastic influx if accommodation is high clastic influx is low in distal locations and the resulting weight percent toc in basinal sediments is increased th maximum toc in a vertical profile through a single marine sequence probably correlates to the maximum flooding surface toc decreases above this surface due to increasing sediment dilution during highstand progradation and decreases below it with the higher sedimentation rate of the older transgressive systems tract commensurate with toc increase towards the maximum flooding surface the organic matter type becomes more marine and therefore increasingly oil prone several marine source rock examples are presented as well as a lacustrine example which also conforms to the marine model

estimation of fault propagation distance from fold shape implications for earthquake hazard assessment a numerical grid search using the trishear kinematic model can be used to extract both slip and the distance that a fault tip line has propagated during growth of a fault propagation fold the propagation distance defines the initial position of the tip line at the onset of slip in the santa fe springs anticline of the los angeles basin we show that the tip line of the underlying puente hills thrust fault initiated at the same position as the 1987 magnitude 6 0 whittier narrows earthquake

fault related folding in sandbox analogue models of thrust wedges abstract the nucleation and growth of thrust fault related folds has been simulated using high resolution sandbox coulomb wedge type analogue models thrust related anticlines grew progressively from initial layerparallel shortening to decollement folding thrust tip folding and eventually thrust ramp folding thrust ramps nucleated in the central sector of the overturned forelimb of decollement folds as a consequence of limb thinning thrust tip folding occurred during outward migration of the ramp tips the fault propagation rate was greater at the lower ramp tip which quickly joined the basal thrust flat while folding continued beyond the upper tip the development of the analogue model thrust fault related folds by progressive changes in kinematics from decollement folding to thrust tip folding and finally thrust ramp folding compares well with analyses of naturally occurring thrust fault related folds

mechanical models of fault propagation folds and comparison to the trishear kinematic model abstract faults propagating through the earth generate a wave of deformation ahead of their tip lines we have modeled this process to understand the relationship between fold geometry and fault propagation using finite element modeling fem we investigate the response of incompressible frictionless and frictional materials and a compressible frictional material with associated flow to vertical and dipping faults whose tip lines propagate at rates 3 3 5 times their slip rates the fold geometries finite strain and velocity fields in models with incompressible materials are very similar to those produced by the trishear kinematic model even though the latter uses a purely ad hoc linear velocity field furthermore when the trishear grid search is applied to the final geometry of the mechanical folds the best fit kinematic models have approximately the same propagation to slip ratio as was used in the fem experiments however when the compressible frictional material is used the mechanical models exhibit a main triangular shear band in front of the tip line and a conjugate shear band in the fold backlimb both migrating with the propagating tip line the conjugate shear band antithetic to the fault produces a gentle anticlinal back limb even though there is not a bend in the fault

pyrite formation in bioturbated clays from the jurassic of britain the dunans clay and upper oxford clay of britain show the importance of bioturbating organisms in controlling the formation of pyrite in the surface layers of normal shales these clays are characterized by low pyrite sulphur content 1 3 low organic carbon content 2 and light 34s values generally lighter than 29 these values are consistent with the formation of pyrite close to the surface where contact with the overlying seawater is maintained by the bioturbating fauna low inputs of organic carbon and its consumption by both aerobic and anaerobic bacteria limits the formation of pyrite it is thought that particularly large apparent fractionations between pyrite and jurassic seawater up to 58 are the result of incorporation of isotopically light sulphate from extensive reoxidation of sulphide

slip rates of blind thrusts in slow deforming areas examples from the po plain italy abstract we calculate plio pleistocene slip rates on the blind thrusts of the outer northern apennines fronts that are the potential sources of highly damaging earthquakes as shown by the m w 6 1 6 0 2012 emilia romagna seismic sequence slip rates are a key parameter for understanding the seismogenic potential of active fault systems and assessing the seismic hazard they pose however they are difficult to calculate in slow deforming areas like the po plain where faulting and folding is mostly blind to overcome this we developed a workflow which included the preparation of a homogeneous regional dataset of geological and geophysical subsurface information rich in plio pleistocene data we then constructed 3d geological models around selected individual structures to decompact the clastic units and restore the slip on the fault planes the back stripping of the differential compaction eliminates unwanted overestimation of the slip rates due to compaction induced differential subsidence finally to restore the displacement we used different methods according to the deformation style i e fault parallel flow for faulted horizons trishear and elastic dislocation modeling for fault propagation folds the result of our study is the compilation of a slip rate database integrating former published values with 28 new values covering a time interval from the pliocene to the present it contains data on 14 individual blind thrusts including the mirandola thrust seismogenic source of the 29 may 2012 m w 6 0 earthquake our study highlights that the investigated thrusts were active with rates ranging between 0 1 and 1 0 mm yr during the last 1 81 myr the mirandola thrust slipped at 0 86 0 38 mm yr during the last 0 4 myr these rates calculated with a homogeneous methodology through the entire po plain can be charged entirely to the thrust activity and not to secondary effects like the differential compaction of sediments across the structures

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

experimental models of extensional forced folds we have used single layer and multilayer clay models to study the development of forced folds above normal faults our modeling results show that the deformation patterns associated with extensional forced folding depend on the dip of the underlying normal fault and the presence of layer parallel detachments in single layer clay models extensional forced folds are upward widening monoclines anticlinal axial surfaces dip in the same direction as underlying master normal faults and synclinal axial surfaces dip in the opposite direction of master normal faults most secondary faults are upward steepening normal faults if master normal faults are steeply dipping however many secondary normal faults become high angle reverse faults at shallow depths the propagation and linkage of secondary faults into through going normal faults terminates the development of extensional forced folds more folding occurs prior to fault linkage if the master normal fault is steeply dipping rather than gently dipping most dipping beds and secondary faults are preserved in the hanging walls of the through oing normal faults in multilayer clay models with layer parallel detachments extensional forced folds are also upward widening monoclines slip on the lowest detachment laterally transfers extension induced by normal faulting and forced folding from the master normal fault to the detachment edge slip on overlying detachments accommodates minor thickness changes associated with upward widening of the fold secondary faults include low angle normal faults near the anticlinal axial surface minor thrust faults near the synclinal axial surface and high angle normal faults above the detachment edge the model predicted deformation patterns are similar to those of extensional forced folds from the gulf of suez and offshore norway this similarity suggests that our modeling results apply to extensional forced folds and can provide guidelines for interpreting field well and seismic data

kinematic evolution of thrusts wedge and erratic line length balancing insights from deformed sandbox models kinematic evolution of fold thrust structures has been investigated by analogue models that include syntectonic sedimentation different decollement dips and basement thicknesses produced different wedge geometries and propagating characteristics a model with one decollement level was characterized by a closely spaced thrust system during early stages of shortening as compared to the late stages the frequency of fault nucleation was rapid during the early stages of deformation conversely the frequency of fault nucleation was low and thrust spacing was significantly wider in a model with two decollement levels individual faults became locked at steep dips and deformation stepped forward as a new fault nucleated in sequence in front of the older locked structure once the thrust system was established up to 27 overall shortening an overlying bed was introduced to simulate syntectonic deformation model sand wedge did not grow self similarly but rather its length and height increased episodically with deformation restoration of deformed models show that layer parallel shortening accommodated for approximately half of the total model shortening across the multilayers calculated error in apparent layer shortening from the restored layers revealed a direct relation with depth of the layers in the models the experimental results are comparable to a natural example from the northern apennines fold and thrust belts