localisation of normal faults in multilayer sequences abstract existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong and brittle layers and are later linked in weak and ductile layers we use the discrete element method for modelling the growth of a normal fault in a brittle ductile multilayer sequence the modelling reveals that faults in brittle ductile sequences at low confining pressure and high strength contrast localise first as mode i fractures in the brittle layers low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers the initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers faults localise therefore as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults the modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared with weak layers where the difference in propagation distance is most likely related to strength and or ductility contrast layer dependent variations in fault localisation rates generate fringed rather than smooth fault tip lines in multilayers

structure of tertiary and pleistocene normal faults in iceland normal faults were measured in three selected areas of the late tertiary and pleistocene lava pile in iceland the arithmetic mean throw of 315 faults is 8 2 m the minimum being 0 5 m and the maximum 150 m about 93 of the faults have throws of less than 20 m the mean strike of the faults is n31 e but the mode is n45 e the faults dip from 42 to 89 nearly 80 dip between 65 and 79 and the mean dip is 73 fault breccia is common and varies in thickness from 0 05 m to 2 m there is a positive linear relation between breccia thickness and throw the linear correlation coefficient being 0 78 it is proposed that the normal faults in iceland form by two basic mechanisms one is the development of faults from sets of inclined joints in the lava pile the initially vertical columnar joints in the lavas gradually decrease in dip with depth in the crust because the regional dip of the lavas increases by about 1 for every 150 170 m depth the inclined joints form sets of en echelon cracks that may link up through tension cracks that propagate from near the ends of the joints the other mechanism involves formation and growth of normal faults by coalescence of smaller tension fractures the great majority of fractures within the active rift zone are tension fractures but when they attain the length of several hundred meters they normally change into normal faults it is concluded that normal fault development in iceland commonly starts at depths of about 1 km in the crust

evolution of fault zones in carbonates with mechanical stratigraphy insights from scale models using layered cohesive powder abstract we present analogue models of the formation of dilatant normal faults and fractures in carbonate fault zones using cohesive hemihydrate powder caso 4 h 2 o the evolution of these dilatant fault zones involves a range of processes such as fragmentation gravity driven breccia transport and the formation of dilatant jogs to allow scaling to natural prototypes extensive material characterisation was done this showed that tensile strength and cohesion depend on the state of compaction whereas the friction angle remains approximately constant in our models tensile strength of the hemihydrate increases with depth from 9 to 50 pa while cohesion increases from 40 to 250 pa we studied homogeneous and layered material sequences using sand as a relatively weak layer and hemihydrate graphite mixtures as a slightly stronger layer deformation was analyzed by time lapse photography and particle image velocimetry piv to calculate the evolution of the displacement field with piv the initial predominantly elastic deformation and progressive localization of deformation are observed in detail we observed near vertical opening mode fractures near the surface with increasing depth dilational shear faults were dominant with releasing jogs forming at fault dip variations a transition to non dilatant shear faults was observed near the bottom of the model in models with mechanical stratigraphy fault zones are more complex the inferred stress states and strengths in different parts of the model agree with the observed transitions in the mode of deformation

textures of paleozoic chert and novaculite in the ouachita mountains of arkansas and oklahoma and their geological significance scanning electron micrographs of cherts and novaculites from the ouachita mountain fold belt of arkansas and oklahoma show a sequential range in textures from cryptocrystalline anhedral quartz 1 m in diameter in the nonmetamorphosed chert and novaculite to coarse polygonal triple point euhedral quartz 100 m or more in diameter where the cherts have been thermally metamorphosed the coarsest being in xenoliths the textures have been correlated with similar texture and crystal sizes in the chert from a contact metamorphic aureole on the isle of skye scotland where classic metamorphic mineral assemblages from talc through tremolite diopside and forsterite grades have been identified texture of chert thus can be used as an indicator of elevated temperature mean apparent crystal diameters measured of the quartz in the arkansas novaculite and associated cherts were plotted on a map of the ouachita mountain fold belt extending from little rock arkansas to near broken bow oklahoma the regional trend in texture parallels the structural core trend and in addition is strongly overprinted by localized areas of coarser crystallinity near little rock magnet cove and potash sulphur springs arkansas where igneous intrusions have occurred and near broken bow oklahoma temperature estimates from studies of fluid inclusions stable isotope ratios mineral chemical phase relationships in associated rocks and novaculite as xenoliths suggest that maximum temperatures as high as 760 c may have been reached by portions of the novaculite other examples of triple point texture in chert collected from distant localities which range in age from precambrian to tertiary show that polygonal triple point texture in chert is world wide in occurrence it follows that small samples of chert novaculite can yield evidence of a history of elevated temperature such evidence may be used to estimate maturation or degradation of hydrocarbons in the rocks and to furnish clues during exploration for thermally related metallic and nonmetallic minerals

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

structural profiles of ouachita mountains western arkansas the ouachita mountains of oklahoma and arkansas are the largest exposure of the pennsylvanian age orogen rimming the southern margin of north america the exposure consists of a thick carboniferous flysch sequence overlying a thin early paleozoic deep water sequence and is generally interpreted to have been deformed in a south dipping subduction zone two balanced cross sections 40 km apart of the ouachita mountains in western arkansas are presented here illustrating the regional structural style major features of the cross sections include from north to south 1 triangle zones along the northern border of the frontal thrust zone produced by imbrication at depth 2 large scale 10 km wavelength fault propagation folds in the frontal thrust zone formed prima ily above normal faults that offset the basement and act as buttresses at depth 3 a late stage basement uplift along the reactivated johns valley normal fault system resulting in the antiformal structure of the benton uplift and backthrusts in the northern benton uplift and 4 small scale 1 3 km heavily faulted folds in the early paleozoic deep water rocks exposed in the benton uplift greenschist metamorphism in these rocks is attributed to the estimated 13 km of carboniferous overburden which was later eroded reconstructions of the late paleozoic continental margin are made from the two cross sections the reconstructed shelf to slope transition is interpreted to underlie the southern flank of the benton uplift using modern analogs for the across strike width of the shelf to slope transition a minimum regional shortening estimate of 30 50 110 155 km is obtained for deep water rocks currently exposed in the benton uplift

two dimensional distinct element modeling of the structure and growth of normal faults in multilayer sequences 2 impact of confining pressure and strength contrast on fault zone geometry and growth 1 the growth of normal faults in periodically layered sequences with varying strength contrast and at varying confining pressure is modeled using the distinct element method the normal faulting models are composed of strong layers bonded particles and weak layers nonbonded particles that are deformed using a predefined fault at the base of the sequence the model results suggest that faults in sequences with high strength contrast at low confining pressure are highly segmented because of different types of failure extension versus shear failure in the different layers the degree of segmentation decreases as the strength contrast decreases and confining pressure increases faults at low confining pressure localize as extension mode i fractures within the strong layers and are later linked via shallow dipping faults in the weak ones this leads to initial staircase geometries that with increasing displacement cause space problems that are later resolved by splaying and segmentation as confining pressure increases the modeled faults show a transition from extension to hybrid to shear fracture and an associated decrease in fault refraction with a consequent decrease in fault surface irregularities therefore the mode of fracture which is active in the strong layers of a mechanical multilayer at a particular confining pressure exerts an important control on the final fault geometry

new information on shanshanosaurus huoyanshanensis a juvenile tyrannosaurid theropoda dinosauria from the late cretaceous of china shanshanosaurus huoyanshanensis from the subashi formation upper cretaceous of xinjiang in northwestern china has long been thought of as a distinctive genus of small theropod although usually assigned to its own family it has generally been included in the tyrannosaurid subfamily aublysodontinae in recent years restudy and description of the only known specimen reveal that it is not a small species but is a juvenile tyrannosaurine possibly tarbosaurus with a total estimated length of 2 3 m it is the smallest tyrannosaurid skeleton known shanshanosaurus provides the best information available on ontogenetic changes in these enormous carnivores and reveals that young tyrannosaurids had long low skulls huge pubic boots and well developed limb joints evidence suggesting that young tyrannosaurs had relatively longer forelimbs than the adults is not supported by shanshanosaurus

structural evolution of calderas insights from two dimensional discrete element simulations the manner in which calderas develop is a key question in volcanology and has important implications for associated volcanogenic risk and geothermal and ore exploitation to better understand the structural evolution of calderas i use a discrete element model of a frictional cover undergoing piston like subsidence at its base simulating magma chamber deflation and cover collapse these novel simulations capture not only the initiation of cal deras but also much of the complexity of faulting during their later development in all models both normal and reverse faults accommodate deeper subsidence at higher structural levels curved outward dipping reverse faults are consistently the first structures to develop subsequent caldera growth is mainly the result of movement on vertical or steeply inward dipping normal faults this may be followed by a later phase of lateral collapse on shallower normal faults the roof aspect ratio r cover thickness piston width is seen to be an important factor calderas with low aspect ratios r 1 exhibit a coherent central subsiding block while those with higher aspect ratios have a complexly faulted internal structure

a new carnosaur dinosauria theropoda from the jurassic of xinjiang people s republic of china in 1987 a sino canadian expedition known as the dinosaur project china canada alberta ex terra discovered a large theropod skeleton in the upper jurassic shishugou formation of the junggar

discrete element modeling of extensional fault related monocline formation abstract the interplay of faulting and folding has long been recognized in extensional systems and numerous investigations have documented the importance of mechanical layering on development of normal fault related monoclines in this study we use discrete element modeling to explore the impact of mechanical layering and fault geometry on normal fault related folding for a well exposed field example in southwestern iceland the model honors the mechanically stratified character of the deformed sequence and replicates the monocline geometry by simulating displacement and upward propagation of a buried fault a close match between model geometry and field observations was obtained using a refracted fault geometry and a cover sequence with alternating 40 m thick relatively strong and 10 m thick relatively weak layers initial deformation is accommodated by folding of mechanically weaker intervals and fracturing of mechanically stronger layers and maximum monocline width is developed early as overall fault displacement increases throughgoing fracture connections along the footwall side of the monocline develop leaving the monocline limb attached to the hanging wall consistent with field observations model results suggest that significant near surface and subsurface fracture porosity is developed in strong layers reproducing the complexity of the natural normal fault related monocline in iceland requires incorporating strongly contrasting mechanical properties within a layered cover sequence

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

magmatic and tectonic history of iceland s western rift zone at lake thingvallavatn high rates of holocene sedimentation in a lake spanning the thingvellir rift zone of western iceland provide an unusual opportunity to study the interaction of tectonic and magmatic processes on timescales of thousands of years lake thingvallavatn is oriented sw ne parallel to the trend of normal faults and fissures which extend northeast from hengill a central volcano system thingvallavatn s lake sediments provide a continuous high fidelity record of tectono magmatic processes chirp sub bottom profiler and sidescan records together with sediment core information enabled the lake stratigraphy to be constrained well since the emplacement of a post glacial lava at 9 1 0 3 ka this lava together with three younger horizons enabled detailed study of the main tectonic and magmatic events a major hengill volcanic event which controlled the development of the present day lake morphology occurred at 1 9 ka with the eruption of a scoria cone within the lake sandey during this event the nesjahraun lava was erupted into the southern part of the lake the sandey scoria cone was formed and major faulting and subsidence occurred in the northern part of the lake resulting in the formation of an asymmetrical rift within the southern part of the lake a deformed sequence of sediments of age 2 9 1 5 ka between undeformed younger and older sediments indicate that liquefaction phenomena are associated with the emplacement of the nesjahraun lava analysis of fault displacement reveals that the total throw summed over all faults across the width of the rift zone is approximately constant 110 130 m along the long axis of the rift we estimate an extension rate on the faults of 3 3 8 2 mm yr 1 since 9 1 ka assuming fault dips of 60 75 which represents 17 43 of the total plate boundary extension estimated from global plate motion inversion we speculate that the remaining extension must either be taken up elsewhere in iceland for example in the eastern rift zone or along the south island seismic zone or that extension estimated over the last 9kyr underestimates the long term extension rate due to incomplete sampling of the episodic magmatic component

mechanical and geometric controls on the structural evolution of pit crater and caldera subsidence 1 pit craters and calderas are volcanic depressions produced by subsidence of a magma reservoir roof to identify how geometric and mechanical factors may influence the structural evolution of this subsidence we used two dimensional distinct element method numerical models the reservoir host rock was represented as an assemblage of bonded circular particles that interact according to elastic frictional laws varying particle and bond properties produced a range of bulk material properties characteristic of natural rock masses fracturing results when bonds break once their shear or tensile strength is exceeded the magma reservoir was represented as a region of nonbonded low friction particles withdrawal of magma was simulated by incrementally reducing the area of the reservoir particles resultant gravity driven failure and subsidence of the reservoir roof were explicitly replicated interaction of the roof s strength young s modulus thickness diameter ratio t d and the reservoir s shape yields a variety of model structures and subsidence styles in conceptual terms four end member subsidence styles developed 1 central sagging favored by low strength and low t d 2 central snapping favored by high strength low t d and a sill like reservoir shape 3 single central block favored by low to intermediate strength high young s modulus and intermediate t d and 4 multiple central blocks favored by high strength low young s modulus and high t d most model realizations incorporated some combination of each style however the models provide a geomechanical framework for understanding natural pit crater or caldera structures as at nindiri nicaragua fernandina galapagos dolomieu la reunion and miyakejima japan

extensional fault propagation folds mechanical models and observations from the modoc plateau northeastern california extensional fault propagation folds are produced by warping of material ahead of a growing normal fault we use field observations of surface breaking normal faults in the modoc plateau california in conjunction with three dimensional linear elastic boundary element computer models to address the mechanical controls on extensional fault propagation folds detailed topographic and structural maps of the near tip regions of several kilometer long faults reveal throw decreases rapidly toward the tip suggesting elliptical slip distributions fault parallel monoclinal folds are present beyond the tip with axes offset into the hanging wall and open fissures are common in the tip region parametric tests with the model show that the amplitude and form of folds at the fault terminations are influenced by slip distribution fault dip elastic properties of the rock and the presence or absence of a blind portion of the fault models reproduce the form of observed folds to first order and suggest that the faults both isolated and in relay zones are probably blind for some lateral extent beyond the surface break

architecture of normal faults in the rift zone of central north iceland this work examines the architecture and propagation of normal faults that accommodate extension in the rift zone of central north iceland it is based on a structural study of portions of three grabens their normal faults and dilational fractures in basaltic lavas of the fremri namur and dyngjufjol volcanic systems we analyzed the shape throw and dilation along faults in order to infer their possible evolution the shape of throw profiles departs from elliptical varying along each fault with throw being lower at the fault and segment tips propagation of fault segments is accommodated by dilational fractures nucleating at their respective tips segments link by lateral propagation of one or both tips or by the development of intervening linking segments two orders of fault segmentation indicate that the present throw accumulated by many increments as the fault scarps changed their geometries faults have large dilational components weakly correlated with fault throw and length

the growth of orogenic belts and the role of crustal heterogeneities in decollement tectonics a series of finite element models of contractional deformation are presented that examine the role preexisting mechanical heterogeneities within the lithosphere play in controlling the partitioning of stress and strain during orogenesis the models focus particularly on orogens that develop near previously rifted continental margins although many of the observations are relevant to other fold and thrust belts the models indicate that orogen evolution is strongly influenced by crustal architecture during the first 150 to 200 km of shortening major decollements develop at midcrustal and lower crustal levels partitioning strain into upper crustal lower crustal and mantle strain domains the magnitude and spatial distribution of strain within each domain are nearly uniform but differ significantly between domains the decollements extend throughout the entire width of the orogen providing a successively increasing degree of decoupling between deformation in the shallow crust and deeper levels the midcrustal decollement terminates beneath the leading edge of the thrust front but the deep crustal decollement extends more than 500 km beyond the foreland this allows up to 5 10 strain to develop well in front of the orogenic belt in the interior of the orogen deformation extends through all levels of the crust beginning during the earliest stages of contraction deformation propagates continentward with increasing shortening with the foreland regions undergoing the most intense shortening in the shallow crust the loci of maximum total shortening within the upper and lower crust are laterally offset with the thickest crust lying continentward of the greatest elevations and the site of most intense surface deformation this pattern of crustal thickness variation is a primary feature of orogenesis in the models and is not related to postorogenic collapse after 150 to 200 km of shortening sufficient topography develops to cause the balance between rock strength gravitational forces and shear stress on the basal detachment to dominate the style of deformation preexisting mechanical heterogeneities thereafter exert little influence on orogenic processes

the internal structure of fault zones in basaltic sequences abstract normal faults in basalt along mid ocean ridges and continental rifts play a major role in the formation of topography advection of fluids and the dynamics of biosphere however almost nothing is known of the internal structure of volcanic growth faults at depth where many key processes take place in this paper we present the results of scaled model experiments validated against outcrop studies of a volcanic growth fault system on hawai i the analogue model uses cohesive hemihydrate powder with carefully characterized physical properties a cohesion of 62 pa friction angle of 0 71 and tensile strength of 33 pa imply a length scaling ratio of 1 5000 50 000 between the model and the upper part of extending oceanic crust with a basalt column above a normal fault time lapse imagery high resolution particle image velocimetry and field observations show ground flexure in an initial elastic stage before subvertical mode i fissures propagate from the surface downwards some of the mode i fractures become inactive while others develop into massively dilatant normal faults the dominant processes within the faults are brecciation block rotation and gravitational transport of fragments along the fault maintaining meter sized cavities to a depth of several hundred meters even when these volcanic growth faults are resurfaced a pervasive dilatancy persist within the system if circulating seawater provides a sufficient oxygen supply the large cavities in normal faults at mid ocean ridges could harbour large ocean floor life forms to several hundred meters depth

dependence of the flexural rigidity of the continental lithosphere on rheology and temperature while the first order mechanical properties of oceanic and continental lithosphere are well defined and explain the observed increase in lithospheric flexural strength with thermal age the temporal variation of continental flexural rigidity following loading and its dependence on lithospheric composition and temperature structure is only just beginning to be appreciated we describe here the results of a quantitative theological model in which laboratory determined rock deformation data and transienttemperature distributions resulting from intra plate basin formation within continental interiors have been integrated continental flexural rigidity is shown to be controlled by crustal thickness lithospheric thickness through the temperature structure and the interaction of these factors during basin formation for thermally young lithosphere the flexural rigidity is dominated by the quartzo feldspathic theology of the crust while for the thermally older lithosphere it is dominated by the olivine rheology of the mantle flexural data through thermo mechanical models of lithospheric deformation provide a powerful constraint on both the transient and steady state temperature structure of the lithosphere

gravity constraints on lithosphere flexure and the structure of the late paleozoic ouachita orogen in arkansas and oklahoma south central north america spectral analysis of bouguer gravity anomalies in western central arkansas and eastern oklahoma indicates that the thickness of the crust in the ouachita fold and thrust belt increases from 38 km in the western ouachitas to 44 km in the eastern ouachitas the change in crustal thickness occurs near the western end of the broken bow uplift and coincides with an abrupt decrease in the flexural rigidity of the lithosphere from 1 8 1024 n m in the western ouachitas to 5 0 1023 n m in the eastern ouachitas the flexural rigidity in the western ouachitas is similar to values determined in the appalachian fold and thrust belt and coincides with the depth of the 450 c isotherm predicted by conductive cooling models for the thermal evolution of the early paleozoic southern laurentian rifted continental margin the thick crust in the eastern ouachitas results in lithosphere that is anomalously weak for rifted continental crust of this age the thicker crust is attributed to an eastward transition from a rift segment to a transform segment of the paleozoic continental margin a layered density model derived from the gravity data shows that strata interpreted to be deformed ouachita facies rocks are thickest in the eastern ouachitas and are consistent with a greater amount of shortening in the central thrust belt in arkansas as compared to oklahoma the opposite relationship is observed in the frontal ouachita province where shortening appears greater in oklahoma the cross strike changes in the locus of shortening crustal thickness flexural rigidity and the inferred transition from rift to transform segments of the early paleozoic continental margin all coincide with the location of a previously hypothesized zone of diffuse rightlateral shear located at the western end of the benton uplift flexural modeling indicates that the load required to produce the observed bouguer gravity low in the arkoma foreland basin trends parallel to the benton and broken bow uplifts but is located 114 to 276 km farther south in the western ouachitas the position of the load coincides with the northern edge of the sabine uplift and is interpreted to mark the southern extent of ouachita facies rocks that were emplaced on the laurentian continental margin and or attached remnant oceanic crust

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

paleozoic continent ocean transition in the ouachita mountains imaged from passcal wide angle seismic reflection refraction data a wide angle reflection reflection experiment sponsored by the program for array seismic studies of the continental lithosphere passcal was conducted in the ouachita mountains area of southwestern arkansas and northwestern louisiana this experimental employed 400 state of the art seismic recorders and overlapped the southern one third of the cocorp consortium for continental reflection profiling deep seismic reflection profile in the area a wide variety of data processing and interpretation techniques was employed to derive an earth model from these data the model depicts a preserved early paleozoic continental margin buried beneath allochthonous paleozoic strata and younger sedimentary rocks the southern part of the model indicates the presence of oceanic or highly extended continental crust overlain by about 15 km of mostly paleozoic sedimentary rock these results are consistent with little if any shortening of crystalline continental crust during the ouachita orogeny

cover deformation above steep basement normal faults insights from 2d discrete element modeling abstract a discrete element model is used to investigate progressive cover deformation above a steep 70 basement normal fault the cover materials are homogenous with frictional material behavior in the model shown here both normal and reverse faults in the cover accommodate displacement on the underlying basement fault the earliest faults are curved reverse faults which propagate upwards from the basement fault tip into the proto hanging wall these are replaced progressively towards the footwall by subvertical to steep normal faults and finally by a normal fault which dips at an angle predicted by mohr coulomb theory thus most early secondary structures are located in the hanging wall of the final through going fault this structural evolution produces an asymmetric triangular zone of deformation above the basement fault tip which superficially resembles that associated with trishear however its progressive development is quite different results also emphasize that the occurrence of reverse faults in extensional settings is not diagnostic of inversion

early paleozoic transform margin structure beneath the mississippi coastal plain southeast united states a geophysical transect across the central gulf of mexico coastal plain shows that the early paleozoic continental margin of southern laurentia is preserved in a nearly pristine state beneath younger strata that were emplaced during the late paleozoic ouachita orogeny and formation of the modern gulf of mexico coastal plain the thickness of the crystalline crust decreases abruptly across the margin over a distance of 50 km from 35 km beneath the black warrior foreland basin to 10 km beneath the ouachita fold and thrust belt this abrupt decrease in crustal thickness is similar to modern transform margins but very different from most rifted margins which display much more gradual transitions in crustal thickness the geophysical data indicate an absence of synrift intrusive and volcanic rocks underplated mafic rocks at the base of the crust and abnormally thick oceanic crust adjacent to the margin the lack of these features is also characteristic of modern transform margins combined with transects across the margin farther west the data confirm previous suggestions that the central gulf of mexico coastal plain overlies an 800 km long transform segment of the late proterozoic early paleozoic southern laurentian continental margin that extends continuously from western arkansas to southeast alabama

cocorp seismic reflection profiling in the ouachita mountains of western arkansas geometry and geologic interpretation cocorp seismic reflection profiling in the ouachita mountains of western arkansas indicates that 1 carboniferous foreland basin deposits within the arkoma basin thicken dramatically toward the south reaching an aggregate thickness in excess of 12 km 4 5 s two way travel time beneath the southern edge of the frontal thrust zone 2 evidence for northward directed low angle thrusting within this clastic sequence is prominent with a probable decollement surface lying at or near the contact with underlying early paleozoic shelf carbonates 3 beneath the benton uplift a sequence of discontinuous events defines a broad antiform cresting at approximately 7 km 2 8 s this structure appears to mimic the anticlinorial shape of the benton uplift indicated by the surface outcrop pattern 4 beneath the southern ouachitas south dipping stratified events are observed to depths in excess of 14 km 5 0 s 5 deeper in the section a prominent gently north dipping reflection occurs at approximately 22 km depth 7 6 s beneath the northern coastal plain southern ouachitas extrapolation of data along strike in the ouachita belt and consideration of large scale structure observed in other collisional orogenic belts suggest that the reflection events observed beneath the benton uplift represent early paleozoic shelf carbonates correlative with those that floor the arkoma basin to the north this interpretation requires that the early paleozoic deep water sediments exposed in the core of the ouachitas be allochthonous and also implies significant basement uplift beneath the core zone to the south the prominent package of layered reflections occurring beneath the southern ouachitas and northern coastal plain indicates that a significant portion of the crust in that region is composed of imbricate sedimentary and or metasedimentary strata at relatively shallow levels these strata are correlative with the carboniferous flysch cropping out in the southern ouachitas and at somewhat deeper levels they are correlative with the early to middle paleozoic deep water sediments exposed on the benton uplift

spatial variations in te in the southern appalachians eastern united states various studies in the oceans have shown that the flexural rigidity or equivalently effective elastic thickness te of the lithosphere is determined by the load and plate age the results of studies in the continents have however been more controversial determinations of te made using spectral studies and based on the bouguer anomaly coherence technique and surface and subsurface loading suggest that north america has a high te core 100 km which is flanked by lower values in contrast studies based on the free air admittance suggest that te of north america is 25 km and of the order of the seismogenic layer thickness it has been proposed that this discrepancy results from the fact that estimates based only on bouguer coherence may be biased upward due to topographical erosion which introduces noise especially at short wavelengths in order to address this question further we have used a maximum entropy based coherence method to determine the wavelength relationship between gravity and topography in the southern appalachians a region where the bouguer coherence free air admittance and forward modeling techniques have already been applied our studies reveal a variable te structure with a mean te of 51 km and values which have a range 20 to 100 km the mapped te fabric has a distinct ne sw trend which appears to follow the tectonic elements of the southern appalachians in particular the foreland is generally associated with higher te values than the flanking orogenic belt correlations at smaller scales are difficult however to establish the te fabric does not reflect the complex terrains that make up highly deformed regions within the orogenic belt our spectrally determined te estimates are in close agreement with ones based on forward modeling they are a factor of 4 higher however than results previously based on free air admittance we attribute this to the fact that we have used the bouguer coherence technique which accounts for both surface and subsurface loading our results suggest that while other factors such as erosion may upwardly bias te in some regions buried loads satisfactorily explain the spectral estimates in the southern appalachians irrespective of whether they are based on bouguer coherence or free air admittance there is solid geological evidence in the southern appalachians which is readily attributable to the presence of subsurface loads this is a strong argument we believe to suggest that te in the continents is high and can have values which range from 20 to 100 km

dem simulation of normal faults in cohesive materials abstract we performed discrete element method dem simulations of the formation of normal faults in a brittle cohesive material above a basement fault focusing on the formation of open fractures and cavities in areas of low normal stress i e near the surface of the model the simulations produce a wide range of fundamental structures similar to those observed in nature and in analogue models comparisons with these sandbox experiments show that the fundamental structures generated are similar between the analogue and the numerical models in a parameter study of the material properties and the model geometry we have investigated the effect of these parameters far beyond what is possible in analogue models over a range of parameters we show that the evolving structural style is robust i e all the models develop similar structures in a similar sequence results show a systematic dependence of some structural features such as location an style of faults on the simulation parameters in particular we observe that the maximum depth of the open fractures varies linearly with the cohesion suggesting that we can predict the depth of open cavities formed by tensile failure in natural fault systems if the material properties are known

relative scale and the strength and deformability of rock masses abstract the strength and deformation of rocks depend strongly on the degree of fracturing which can be assessed in the field and related systematically to these properties appropriate mohr envelopes obtained from the rock mass rating rmr classification system and the hoek brown criterion for outcrops and other large scale exposures of fractured rocks show that rock mass cohesive strength tensile strength and unconfined compressive strength can be reduced by as much as a factor often relative to values for the unfractured material the rock mass deformation modulus is also reduced relative to young s modulus a cook book example illustrates the use of rmr in field applications the smaller values of rock mass strength and deformability imply that there is a particular scale of observation whose identification is critical to applying laboratory measurements and associated failure criteria to geologic structures

changes in structural style of normal faults due to failure mode transition first results from excavated scale models abstract the effects of failure mode transition from tensile to shear on structural style and fault zone architecture have long been recognized but are not well studied in 3d although the two modes are both common in the upper crust of earth and terrestrial planets and are associated with large differences in transport properties we present a simple method to study this in physical scale models of normal faults using a cohesive powder embedded in cohesionless sand by varying the overburden thickness the failure mode changes from tensile to hybrid and finally to shear hardening and excavating the cohesive layer allows post mortem investigation of 3d structures at high resolution we recognize two end member structural domains that differ strongly in their attributes in the tensile domain faults are strongly dilatant with steep open fissures and sharp changes in strike at segment boundaries and branch points in the shear domain fault dips are shallower and fault planes develop striations map view fault traces undulate with smaller changes in strike at branches these attributes may be recognized in subsurface fault maps and could provide a way to better predict fault zone structure in the subsurface

half graben structures balanced models of extensional fault bend folds one common structural style in extensional regimes is a half graben bounded on one side by a master normal fault and on the other side by a domain of beds dipping toward the master fault this geometry is modeled as being caused by a bend in the master fault the hanging wall beds dipping toward the master fault are bounded by axial surfaces formed as the hanging wall moves past the bend resulting in an extensional fault bend fold the footwall beds remain undeformed and unrotated the major assumptions used in the derivation are that the geometry is area balanced and that the axial surfaces in the hanging wall have dips equal and opposite to the dip of the master fault above the bend important consequences of the model include a relationship between master fault dip above and below a bend and the amount of hanging wall dip the horizontal width of the dipping beds in the hanging wall is twice the heave on the master fault above the bend the asymmetry of the axial surfaces in the hanging wall requires that the rotated beds be strained hanging wall beds dipping toward the master fault are predicted for a fault bend wherein the dip decreases downward hanging wall beds dipping away from the master fault are predicted for a bend wherein the dip increases downward the model fits measurements taken from a published ramp and flat clay model by e cloos and is used to develop a balanced and restorable cross section of the schell creek fault a basin and range structure in nevada

geochemistry of mississippian tuffs from the ouachita mountains and implications for the tectonics of the ouachita orogen oklahoma and arkansas the ouachita orogeny was the result of plate convergence at the southern margin of the north american continent although the nature of the converging southern plate and the direction of subduction remain uncertain the presence of areally extensive tuff layers interbedded with shale in the mississippian stanley group of the ouachita mountains oklahoma and arkansas provides the potential to define the tectonic environment of volcanism from the geochemistry of the tuffs and thereby delimit the subduction configuration the tuffs contain relic primary magmatic quartz plagioclase and alkali feldspar and range from crystal to vitric rich mineralogical sorting and diagenetic effects have caused chemical variability within individual tuff units but overall the tuffs have retained their primary igneous geochemical characteristics the beavers bend and hatton tuffs are geochemically very similar and are more evolved higher sio2 rb th ree lower sr ba than the mud creek tuff the stratigraphically equivalent sabine rhyolite is geochemically distinct from the tuffs having less fractionated rare earth element ree patterns and different trace element ratios both the ouachita tuffs and sabine rhyolite have the geochemical characteristics of subduction related magmas for example strong depletion of nb and ta relative to other incompatible trace elements consideration of trace element systematics in the tuffs compared to those of modern high silica volcanic rocks from different subduction related tectonic settings suggests a continental arc origin and implies southward subduction beneath a southern continent during carboniferous ocean basin closure

dike induced deformation and martian graben systems abstract the tharsis region of mars is characterized by large volcanic and tectonic centers with distinct sets of graben systems many of the radially oriented grabens have been inferred to form in response to intrusion of magmatic dikes this interpretation is based primarily upon early physical and numerical boundary element models that were developed originally to understand surface deformation associated with dike emplacement on earth in this study we constructed and analyzed two dimensional discrete element models to test the hypothesis of shallow dike emplacement and widening as a primary mechanism for the production of grabens on mars in particular our models are designed to explore the extent to which a widening subsurface dike in the absence of regional extension or pre existing faults will induce near surface graben formation the use of discrete element models allows for the permanent deformation and material heterogeneity to be captured as opposed to boundary element models that are limited by an assumption of homogeneous elastic behavior our analyses consider both homogeneous materials as well as mechanical stratification the results indicate that forcible widening of a dike alone is unlikely to produce grabens at the surface furthermore estimates of subsurface dike widths and emplacement depths based solely on measurements of surface graben widths may significantly over or underestimate actual parameters finally the model results suggest that the ability to distinguish between fault induced or dike induced graben formation mechanisms may not be possible unless the subsurface mechanical stratigraphy can be well characterized

lithospheric structure of the south central united states recent seismic data in the ouachita mountains area and the gulf of mexico make it possible to construct a lithospheric scale cross section transect from the midcontinent region to the gulf the authors constructed a transect in the form of a gravity model but it incorporates all available seismic drill hole and geologic data as constraints the thrust sheets of the ouachita orogenic belt appear as a thin veneer covering the southern part of the arkoma basin and the preserved paleozoic continental margin mesozoic rifting is evident in three areas 1 southern arkansas and northern louisiana where extension was minor 2 the vicinity of the texas louisiana coastline where modification of the lithosphere and subsidence were considerable and 3 the deep gulf of mexico where rifting was successful a significant variation in the average density of the mantle which could delineate the north american craton as a lithospheric feature was detected near the paleozoic continental margin

compensation of paleozoic orogens a comparison of the urals to the appalachians abstract the urals and appalachians are two paleozoic mountain belts with similar geologic and morphologic features we combine geophysical bouguer gravity and deep seismic sounding and geological observations from the ural mountains to test whether the surficial similarities of these paleozoic orogens extend to include deep lithospheric structure and compensation mechanism we find the combined data suggest support of the mountains by a stiff continental slab which is depressed by an effective subcrustal load in addition to the topography as has been recently proposed for the appalachians our preferred model which fits the geophysical data and predicts the presence and shape of the ural foredeep to the west of the mountains requires eastward directed underthrusting of the russian platform behaving as an elastic plate more than 50 km thick it is unlikely that an elastic plate this stiff could have resulted from conductive cooling of a thermal plate only 125 km thick the weight of the existing mountains on such an elastic plate is insufficient to produce the deflection of the moho inferred from seismic and gravity data an additional load on the elastic plate may be provided by a buried mass of similar origin to surface outcrops of dense mafic and ultramafic rocks in the eastern urals alternatively the load may represent the effect of a shallowing of the moho east of the mountains which is a remnant of a buried passive continental margin seismic data and bouguer gravity anomalies suggest that the effect of the subsurface mass anomaly is at least as great as the present day topography and that the russian plate flexurally underthrusts the urals for more than 100 km we speculate that similar effective subsurface loads may exist at younger continental orogens but are obscured by the topographic load in less eroded mountain belts

introduction to the special issue commemorating the 30th anniversary of the royal tyrrell museum of palaeontology with a summary of the museum s early history and its research contributions located just west of drumheller in southern alberta canada the royal tyrrell museum of palaeontology rtmp is nestled among outcrops of the upper cretaceous horseshoe canyon formation exposed along the red deer river valley fig 1 since it officially opened on 25 september 19851 the museum has become one of alberta s must see tourist attractions a significant contributor to the local economy and earned an international reputation for the quality of its palaeontological collections and the research conducted by its scientists the rtmp is unique within canada in being the only government supported museum dedicated to palaeontology and it is one of the few such institutions in the world fig 1 the royal tyrrell museum of palaeontology a front of museum and main entrance circa 2010 looking westwards and with horizontal beds of the upper cretaceous horseshoe canyon formation exposed in the walls of the red deer river valley in the distance b aerial photograph oblique angle and looking to the north northeast the museum sits on top of the number 7 coal seam sensu gibson 1977 and at the base of the horsethief member of the horseshoe canyon formation d a eberth pers comm 2015 that stratigraphic position correlates with an age of about 71 5 ma i e late campanian and lies within the upper part of the edmontosaurus regalis pachyrhinosaurus canadensis dinosaur assemblage zone see eberth et al 2013 fig 6 c the iconic running theropod logo that until recently graced the exterior of the museum below a life sized tyrannosaurid sculpture by brian cooley one of many such sculptures by cooley that have been featured at the museum since its opening in 1985 and with the education wing opened in 2003 as the atco tyrrell learning centre in the background d the cretaceous alberta life sized diorama opened in 2007 an example of a rtmp exhibit that relies heavily on discoveries and interpretations by museum researchers in this case work done in the albertosaurus bonebed and at other localities throughout the horseshoe canyon formation all photographs courtesy of the royal tyrrell museum of palaeontology this special issue of the canadian journal of earth sciences presents a collection of papers commemorating the rtmp s 30th or pearl anniversary our rationale for doing a 30th anniversary volume was two fold first it is an opportunity to showcase the breadth of research conducted by current and former museum scientists as well as by some colleagues having a history of collaboration with the rtmp the 13 invited papers included in this volume are a mix of review and original research papers all thematically focused on the geology and palaeontology of alberta to varying degrees each contribution is the result of work supported by the rtmp and relies on specimens housed in the museum s collections second the 30th anniversary is an opportunity to reflect on the rtmp s history and its research program as is typical for most institutions the rtmp has done little to formally document its own history this year likely will be the last major milestone for which the original generation of museum staff are still working and available to share their recollections about the museum fieldwork and research already deaths and departures have winnowed away part of the original

discrete element modelling of the influence of cover strength on basement involved fault propagation folding abstract a discrete element model is used to investigate the influence of sedimentary cover strength on the development of basement involved fault propagation folds we find that uniformly weak cover best promotes the development of classical trishear like fault related folds showing marked anticlinal thinning and synclinal thickening with cover dips increasing downwards towards the fault tip uniformly strong cover results in more rounded fold forms with only minor hinge thickening thinning and significant basement fault propagation into the sedimentary cover heterogeneous layered cover sequences with marked differences in strength promote the development of more complex and variable fold forms with a close juxtaposition of brittle and macroscopically ductile features which diverge from the predictions of simple kinematic models in these structures the upper layers are often poor indicators of deeper structure in addition we find that in layered cover sequences fault propagation into the cover is a complex process and is strongly buffered by the weaker cover units

discrete element modeling of martian pit crater formation in response to extensional fracturing and dilational normal faulting 1 pit craters circular to elliptical depressions that lack a raised rim or ejecta deposits are common on the surface of mars similar structures are also found on earth venus the moon and smaller planetary bodies including some asteroids while it is generally accepted that these pits form in response to material drainage into a subsurface void space the primary mechanism s responsible for creating the void is a subject of debate previously proposed mechanisms include collapse into lave tubes dike injection extensional fracturing and dilational normal faulting in this study we employ two dimensional discrete element models to assess both extensional fracturing and dilational normal faulting as mechanisms for forming pit craters we also examine the effect of mechanical stratigraphy alternating strong and weak layers and variation in regolith thickness on pit morphology our simulations indicate that both extensional fracturing and dilational normal faulting are viable mechanisms both mechanisms lead to generally convex steepening downward slope profiles extensional fracturing results in generally symmetric pits whereas dilational normal faulting produces strongly asymmetric geometries pit width is established early whereas pit depth increases later in the deformation history inclusion of mechanical stratigraphy results in wider and deeper pits particularly for the dilational normal faulting and the presence of strong near surface layers leads to pits with distinct edges as observed on mars the modeling results suggest that a thicker regolith leads to wider but shallower pits that are less distinct and may be more difficult to detect in areas of thick regolith

structure and evolution of the central gulf of mexico continental margin and coastal plain southeast united states as in many continental margins mesozoic rifting in the gulf of mexico trended subparallel to older tectonic structures created by opening and closure of a prior ocean basin unlike the situation in most other margins however extension in the gulf of mexico did not involve widespread reactivation of these older tectonic features this difference may be related to the unusual facts that 1 the antecedent passive continental margin was a transform rather than a rift margin and 2 the prior orogeny did not substantially thicken the crust in order to characterize the interaction between mesozoic rifting and prior tectonic events two geophysical transects were constructed across the central gulf of mexico coastal plain and continental shelf the transects show that the early paleozoic passive continental margin buried beneath the late paleozoic ouachita orogen was little affected by compressional tectonism or mesozoic extension during opening of the gulf of mexico the thickness of the crust on this ancient margin decreases from 35 km to 10 km over a distance of only 50 km consistent with previous interpretations that this was a transform margin the ouachita fold and thrust belt is shown to be a thin skinned doubly vergent orogen that formed above a well preserved southward dipping subduction system relict cambrian oceanic crust may exist beneath the southern part of the orogen but its presence is not required by the data the thin crust and shallow mantle in the ouachita suture created a zone of relatively high strength that acted as a barrier to mesozoic extension which is restricted to regions south of the orogen extension was initially distributed over a broad area that included the mississippi interior salt basin and regions south of the wiggins arch extension became progressively more focused south of the wiggins arch with time culminating in the onset of seafloor spreading beneath the modern continental rise mesozoic extension factors are 1 4 1 9 beneath the mississippi interior salt basin 2 9 immediately south of the wiggins arch and 4 adjacent to the oldest oceanic crust in the central gulf of mexico

flexural behavior of the continental lithosphere in italy constraints imposed by gravity and deflection data a procedure is developed that systematically matches observations of plate flexure and gravity anomalies at simple convergent boundaries to the bestfitting two dimensional flexural geometry obtained by a thin elastic sheet of specified elastic strength the procedure allows analytical solution for three independent variables one of which is the initial preflexure elevation of the reference surface and two of which are related to the vertical forces and bending moments that act on the plate application to four profiles constructed across the apennine convergent system in italy shows that the observed depth of the basal pliocene surface which marks the base of the foredeep basin and the observed bouguer gravity anomalies can be fit by a two dimensional elastic plate model with an effective elastic thickness of about 20 km on three profiles and about 10 km on the fourth profile the initial elevation prior to flexure is determined to have been about 1000 m below sealevel on all profiles large bending moments and or vertical shear forces are required to act at the effective plate ends in order to maintain the observed deflection moments between 0 5 1017 and 3 1017 n and shear forces between 3 1012 and 3 1012 n m gravity modelling indicates that on the two northernmost profiles these terminal forces can not be supplied by a static mass at crustal depths and suggests that on all four profiles the terminal forces are supplied by anomalously dense material present at depths of 75 150 km beneath and west of the west coast of italy this dense material probably corresponds to westward subducted lithosphere of the adriatic plate extending from the northern apennines to the southern apennines and calabria this study suggests that in subduction systems like the apennines that are characterized by backarc extension forces acting on subducted lithosphere at depth can be expressed at the surface in the geometry of the foredeep basin that forms adjacent to the subduction zone

propagation of normal faults to the surface in basalt koae fault system hawaii normal fault scarps of the koae fault system on kilauea volcano consistently display locally breached monoclines underlain by prominent cavities deep gaping fissures on the footwall finer fissures on the hanging wall and buckles at the scarp base elastic analyses reveal that this assemblage forms as a fault propagates up towards the surface rather than down from it models of a planar blind normal fault with a dip exceeding 60 yield a monocline with a tensile stress concentration at the surface where gaping fissures occur a stronger subsurface tensile stress concentration near the blind fault tip where cavities occur and a compressive stress concentration at the surface where buckles occur the footwall fissures grow down from the tensile stress concentration at the surface and link with a fault as its scarp grows in contrast the cavities initiate at depth near the fault tip and propagate with it up towards the surface the hanging wall fissures apparently open in response to slip on late forming blind antithetic faults near the surface stoped blocks derived from footwall fissure walls help prop the footwall fissures open as a normal fault breaches the surface the fissures cavities and scarp rubble provide highly conductive hydraulic pathways

allometric growth in tyrannosaurids dinosauria theropoda from the upper cretaceous of north america and asia many tyrannosaurid skeletons have been collected in canada the united states and mongolia these fossils tend to represent mature individuals but juveniles are also known skeletons of five gene

continental boundaries of the jalisco block and their influence in the pliocene quaternary kinematics of western mexico extensional faulting observed in southwestern mexico has been related to the incipient rifting of the jalisco block from the mexican mainland since the pliocene on the basis of new structural and geophysical data we propose that 1 the continental boundaries of the jalisco block are ancient structures reactivated since the pliocene at a low 1 mm yr rate of deformation and 2 pliocene quaternary extensional faulting at the edges of jalisco block is a basement controlled intraplate deformation related to plate boundary forces rather than to active continental rifting the jalisco block boundaries first developed in response to the uplift of the puerto vallarta batholith in pre neogene time and underwent a complex contractile deformation before the pliocene during pliocene quaternary times north northeast extension reactivated the northern boundary forming the tepic zacoalco rift whereas east southeast extension formed the northern colima rift south of the colima volcano active extension is found only west of the so called southern colima rift and partly reactivates old northeast trending basement faults the parallelism between the subducted rivera cocos plate boundary zone and the eastern neotectonic boundary of the jalisco block supports east southeastward motion of the southern mexican blocks induced by the differential motion and oblique subduction of the cocos and rivera plates on the other hand we envisage pliocene quaternary extension along the northern boundary as an upper plate response to the low convergence rate and the steep subduction angle of the rivera plate

propagation of blind normal faults to the surface in basaltic sequences insights from 2d discrete element modelling abstract a discrete element model is used to investigate the progressive deformation of a thin basaltic cover overlying a pre existing blind normal fault as it propagates to the surface the cover materials representing basalt are homogeneous strong and display elastic brittle material behaviour cover deformation is seen to evolve through a series of distinct stages initial displacement on the underlying fault produces a very gentle monoclinal flexure with continued displacement open fractures develop at the monocline surface and propagate downwards whilst the deeper fault propagates upwards simultaneously a series of fractures in the future hanging wall of the main fault develop in the upper part of the cover the monoclinal flexure is then cut by these structures producing a surficial fault and fracture bounded wedge finally a prominent surface fracture and the upward propagating fault link cutting the entire cover sequence this fault is dilatant in the upper c 100 m of the cover has a significant surface aperture and forms a prominent fault scarp many of the key model results are strikingly similar to those seen in natural settings and emphasise that the occurrence of dilatant faults open fractures and cavities caves in extensional settings is not necessarily restricted to the very shallow section but can extend to several hundred metres depth therefore the results have implications for permeability and fluid flow in such settings comparison is also made with a weak cover experiment using granular materials with no cohesion or tensile strength similar to the dry sand used in many analogue modelling studies

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

thermal maturity of carboniferous strata ouachita mountains the ouachita mountains a relatively untested potential hydrocarbon province contain a thick paleozoic section of apparently favorable source beds reservoir beds and trap configurations to estimate the thermal maturity of these strata vitrinite reflectance was measured on 89 samples collected mostly from carboniferous rocks from throughout the ouachita outcrop area thermal maturity contours generally parallel structural grain in the western two thirds of the ouachitas reflectance values in oil range from 3 0 in the core area in oklahoma and from about 1 0 to 3 0 in analogous positions in arkansas older strata on upthrown sides of thrust faults are more thermally mature than younger strata on downthrown sides in the eastern one third of the outcrop belt thermal maturities are significantly higher 2 0 5 0 ro and thermal maturity contours cut obliquely across structural grain strata on either side of thrust faults display identical levels of maturity in the western two thirds of the ouachitas sedimentary and tectonic burial have been the major influences of thermal maturation although a mesozoic thermal overprint has increased levels of maturity in arkansas throughout this area strata are within the oil and wet gas windows the anomalous levels and patterns of thermal maturity in the eastern third of the ouachitas are probably the result of a thermal overprint related to mesozoic rifting and intrusive activity in the mississippi embayment this thermal overprint has elevated the maturity of strata in most of this area above the level of hydrocarbon preservation although maturities within the dry gas window are retained locally

low angle detachment geometry of the late precambrian cambrian appalachian ouachita rifted margin of southeastern north america the late precambrian cambrian rifted margin of southeastern north america is framed by northeast striking rift segments offset by northwest striking transform faults palinspastically restored structural profiles thickness and composition of synrift rocks and thickness of postrift strata vary along the trace of the rifted margin the distribution of these variables suggests application of low angle detachment models of continental rifting including specific recognition of upper plate margins lower plate margins transform faults that offste the rift and transform faults that bound domains of oppositely dipping low angle detachments the trace of the mississippi valley rough creek rome intracratonic fault system suggests incipient rift segments offset by a transfer fault system

kinematic modelling of extensional fault propagation folding abstract many studies have shown that in extensional basins discrete faulting at depth is commonly linked to more distributed deformation in particular folding at higher levels such extensional fault propagation folds are particularly common where there is a distinct mechanical contrast between faulted basement and sedimentary cover outcrop and analogue modelling studies indicate that such folds form as upward widening zones of distributed deformation monoclines above discrete faults at depth with increasing displacement strain the folds are cut by faults as they propagate upwards into the cover to date however there has been little investigation into the kinematics of linked basement faulting and extensional fault propagation folding here we present a two dimensional kinematic model of linked basement faulting and fault propagation folding which is based upon trishear the model allows investigation of the influence of shear zone geometry and the rate of fault propagation upon the style of folds and the strains associated with them the evolution of linked basement faulting and folding predicted by the model is compared in detail to that observed in an analogue model the kinematic model reproduces well many of the features seen both in the analogue model and reported from outcrop and seismic studies

dyke emplacement in a layered and faulted rift zone abstract in rift zone eruptions magma is normally transported to the surface through dykes most injected dykes however do not make it to the surface but rather become arrested at certain depths in the rift zone surface deformation studies are routinely used to infer the depths to the tips as well as the geometries of arrested dykes most such studies model the rift zone as a homogeneous isotropic elastic half space all rift zones however contain normal faults and are composed of rock layers with different mechanical properties the numerical models presented here indicate that when the upper tip of a vertically propagating dyke reaches the same crustal level as the bottom tips of the boundary faults of an active graben the faults close as the tip continues its propagation up into the graben the overpressured dyke encourages reverse slip on the boundary faults the present numerical models also indicate that mechanical layering has strong effects on dyke emplacement and the surface deformation associated with arrested dykes in rift zones in particular soft low young s modulus layers and weak contacts at shallow depths contribute to offset arrest and aperture thickness variations of the dykes and suppress the dyke induced surface stresses and associated brittle deformation when the magmatic overpressure of a dyke is the only loading the dyke tends to be thicker inside soft layers than inside stiff high young s modulus layers soft layers and weak contacts discontinuities common at shallow depths in rift zones suppress dyke tip tensile stresses thereby encouraging dyke arrest and may result in little if any surface stresses and brittle deformation being induced by an arrested dyke weak contacts also encourage transfer of the dyke induced surface tensile stresses to the regions above the lateral ends of the contacts then the tensile stresses at the rift zone surface in a large area above the dyke itself become small or negligible all the tensile stress occur above the lateral ends of the contact which may be many kilometres from the dyke tip for a dyke arrested in a layered rift zone straightforward inversion of surface deformation data may therefore yield geometric results that have little relation to the actual geometry of the arrested dyke the results of the numerical models presented here are generally supported by field observations of dykes in rift zones