dynamical lagrangian remeshing dlr a new algorithm for solving large strain deformation problems and its application to fault propagation folding abstract we present a new algorithm to solve the problem of rock deformation the method is based on a classical lagrangian finite element solver in which the connectivity between the nodes of the numerical mesh is updated at the end of each time or deformation step the method which we term dynamic lagrangian remeshing dlr relies on the theory of delaunay triangulation and produces optimal finite element meshes in the sense that nodes are always connected to their set of nearest neighbour nodes because it is based on a lagrangian method in which the numerical grid is attached to material particles and advected with the deformation the dlr algorithm is ideally suited to track material boundaries and properly represent free surfaces in addition in comparison to classical lagrangian solvers the dlr method is not limited to small strain problems and may be used to model discontinuities such as faults and narrow shear zones we have used the dlr method to study the evolution of a crustal layer undergoing finite shortening driven by a velocity discontinuity along its base the numerical model predicts the formation of a large fold that is progressively cut by a fault originating at the velocity discontinuity and propagating through the crustal layer the model predictions provide support for the fault propagation folding model of suppe 1

seismic imaging of salt structures in the gulf of mexico in this article we discuss seismic imaging of salt structures in the gulf of mexico with emphasis on the recognition preservation and migration of salt signals we describe three salt imaging technologies

factors that control the angle of shear bands in geodynamic numerical models of brittle deformation abstract numerical models of brittle deformation on geological timescales typically use a pressure dependent mohr coulomb or drucker prager plastic flow law to simulate plastic failure despite its widespread usage in geodynamic models of lithospheric deformation however certain aspects of such plasticity models remain poorly understood one of the most prominent questions in this respect is what are the factors that control the angle of the resulting shear bands recent theoretical work suggest that both roscoe 45 coulomb angles 45 2 where is the angle of internal friction and arthur angles 45 4 where is the dilation angle as well as all intermediate angles are possible published numerical models however show a large range of shear band angles with some codes favoring arthur angles whereas others yield coulomb angles in order to understand what causes the differences between the various numerical models here i perform systematic numerical simulations of shear localization around an inclusion of given length scale both numerical element type geometrical and rheological viscoplastic versus viscoelastoplastic effects are studied results indicate that the main factor controlling shear band angle is the non dimensional ratio between the length scale of the heterogeneity d and the size of the numerical mesh x coulomb angles are observed only in cases where the inclusion is resolved well d x 5 10 and in which it is located sufficiently far from the boundary of the box in most other cases either arthur or roscoe orientations are observed if heterogeneities are one element in size coulomb angles are thus unlikely to develop irrespective of the employed numerical resolution whereas differences in element types and rheology do have consequences for the maximum obtainable strain rates inside the shear bands they only have a minor effect on shear band angles shear bands initiated from random noise or from interactions of shear bands with model boundaries or other shear bands result in stress heterogeneities with dimensionless length scales d x 1 2 such shear bands are thus expected to form roscoe or arthur orientations consistent with the findings in previous numerical models

2 6 ga high si rhyolites and granites in the kursk domain eastern sarmatia petrology and application for the archaean palaeocontinental correlations abstract a 2 61 ga intraplate magmatism took place locally within the kursk domain of the eastern sarmatia forming a volcanic plutonic association of rhyolites and granites the zircon lu hf and sm nd whole rocks isotope data indicate the crustal source heterogeneity of these neoarchaean intraplate granitoids their protoliths were dominated by palaeoarchaean metapelites and mesoarchaean ttg associations the neoarchaean rhyolites of the kursk domain are characterized by the high crystallization temperatures of c 900 c according to the data of mineral geothermobarometry and geochemistry of zircons tiq and tizr the magmatic chambers were located at a depth of no more than 7 km the silicic melt parental for these 2 61 ga old rhyolites and granites could be formed as a result of 20 dehydration melting of the metapelite ttg mixture in equilibrium with the ilm bt opx pl restite at a pressure of no more than 4 kbar the atamansky complex granites crystallized at deeper levels and contain an admixture of opx pl restite or cumulus component this caused their geochemical differences from rhyolites when these phases were removed during the melts migration to higher levels of the crust the atamansky granites are considered as intermediate chambers where differentiation of the initial silicic magmas took place with the formation of the residual rhyolitic melts the heat source could be a mafic plume magmatism known in the eastern sarmatia there in intraplate settings hot deep plume generated mafic magmas intruded into the crust forming shallow interim chambers the latter initiated melting of the felsic host rocks predominantly metapelites and ttg in the upper parts of the crust producing the silicic volcanic plutonic association the simultaneous 2 61 ga old granitoid intraplate magmatism in the eastern sarmatia and dharwar craton caused by the mafic heat source adds one more argument in favor of an assumption that the sarmatia pilbara kaapvaal and possibly dharwar cratons composed a single supercontinent vaalbara in the neoarchaean the dharwar craton perhaps was at the periphery of the vaalbara supercontinent next to the eastern sarmatia

detachment folding fold amplification and diapirism in thrust wedge experiments 1 the relations between detachment folding fold amplification and salt diapirism in contractional settings have been investigated by means of scaled analogue models the viscosity of the silicone layer simulating salt in nature and the shortening rates were combined in order to reproduce weak type 1 models and strong type 2 models decollements deformation patterns in the roof sequence exhibited two contrasting styles 1 outward propagation of detachment folding along the decollement ofp mode and 2 passive roof duplex prd mode in type 2 models detachment folding propagated away from the most external thrust in the floor sequence while in type 1 models long lived detachment folds almost invariably localized amplified above a floor thrust tip as a result of strain localization a silicone wall intruded occasionally into the crestal graben of detachment folds in type 1 and ofp models best fitting of transition models data points indicates nonlinear relations with regression curves close to the equilateral hyperbola equation for both ofp prd and amplified detachment folds box folds transitions a quantitative comparison of model results with nature has been attempted by plotting salt based fold and thrust belts data points on the scaled transition curves obtained from the modeling such a comparison relates shear stress products and ratios to the conditions favoring the amplification of detachment folds and the potential emplacement of ductile diapirs in their core by reducing the roof sequence strength pore fluid pressure b is inferred to shift the equilibrium of fold and thrust belts toward the field of ofp and diapirism

fault patterns within sediment layers overlying rising salt structures a numerical modelling approach abstract faulting of sediments overlying structures caused by salt movement is a widespread phenomenon and important for hydrocarbon production because the faults facilitate hydrocarbon migration to and from the reservoir or may compartmentalize the reservoir a major problem in mapping the intensity of deformation is that many faults and fractures are below the seismic resolution and that the seismic data may appear dimmed and distorted owing to escaping gas another major problem is that the salt displacement causing the deformation is often only reflected in the overlying fracture patterns and hence not directly observable this study uses a numerical spring slider model including vertical as well as horizontal movements of the substrata the salt the model demonstrates how the fracture patterns in the sediments above salt structures are controlled by the salt kinematics the modelling experiments show that concentric faults develop when only vertical salt movements are involved whereas radial faulting dominates when even small levels of horizontal movements are included a case study from the southern danish central graben illustrates that the fault structures in the cover sediments characterizes the salt movement the analysis and prediction of the systematics of these small scale faults which are too small to be recognized on seismic data is important when the fluid migration and hence hydrocarbon production from fields related to salt structures is to be optimized

zircon u pb ages and lu hf isotope systematics from late tectonic granites geita greenstone belt implications for crustal growth of the tanzania craton granite plutons that intruded south of lake victoria and north of the geita greenstone belt have geochemical characteristics similar to high k granites when compared to the late high k granites from the musoma mara region they have lower sio2 content and higher tio2 al2o3 cao na2o p2o5 mgo and feot they also show higher values for v sr zr ba and hf concentrations all samples display high total ree abundances ree 139 393 and weak to moderate eu depletion eu eu 0 25 0 77 their chondrite normalized pattern indicates that the light ree are moderately fractionated 3 03 lan smn 6 24 whereas the heavy ree are weakly fractionated 1 63 gdn ybn 2 80 the granite plutons are generally undeformed with 207pb 206pb ages between 2660 and 2620 ma and intruded after the main ductile deformation that affected the geita greenstone belt the 176hf 177hf ratios for zircons from all samples are identical within error suggesting evolution from an isotopically uniform reservoir the hf values plot along the chur evolution line indicating that the granite represents juvenile crustal melts derived from greenstone material available zircon ages from the tanzania craton suggest that crustal growth occurred in three distinct periods 2850 2800 ma 2770 2730 ma and 2700 2620 ma with the dominant period of crustal growth around 2700 ma zircon ages from the sukumaland and musoma mara greenstone belts indicate that the greenstone sequences formed during the same three periods of crustal growth

lithospheric scale folding numerical modelling and application to the himalayan syntaxes we describe the eastern and western himalayan syntaxes which are large scale antiforms situated at geodynamically similar locations and the metamorphic evolution of which is coeval in the india asia collisional history to understand the mechanical plausibility of the structural interpretation we present two dimensional finite element modelling of lithospheric folding the models reveal the coeval development of adjacent synformal basins analogous to the peshawar and kashmir basins on both sides of the western syntaxis similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large scale continental shortening and an efficient mountain building process

numerical analysis of how sedimentation and redistribution of surficial sediments affects salt diapirism abstract two dimensional finite element models are used to study how sedimentation and redistribution of sediments on the upper surface affects the development of subsurface salt diapirs a rising diapir creates a bulge flanked by topographic lows in a generally accumulating sedimentary pile we find that the rate at which this topography is flattened by erosion and redeposition controls the style of diapirism this is because the redistribution of material from topographic highs to flanking lows is equivalent to changing the effective forces acting on the salt redistributing a potential topography modulates diapiric growth rate the main effects of including surficial sediment redistribution in numerical models of diapirism are 1 1 diapirs grow 10 100 times faster 2 2 diapirs may rise above their level of neutral buoyancy and extrude 3 3 diapirs assume finger or stock like shapes rather than mushroom or balloon on string shapes 4 4 layers in the surrounding sediments remain nearly horizontal and only steepen sharply near the diapir in effect the rate of redistribution of surficial overburden strongly controls the mode of diapirism sediment redistribution referred to as erosion for brevity is modeled using a one dimensional diffusion equation we show the results of two different erosion rates infinitely slow no erosion and extremely fast which redistributes surficial sediments but does not remove them from the system we show that the shapes of model diapirs rising beneath surfaces subjected to rapid erosion simulate salt diapirs in the gulf of mexico columnar diapirs indicate rapid deposition on the shelf and plug like diapirs slow sedimentation on the abyssal plane diapirs rising beneath surfaces with negligible erosion have the mushroom shapes interpreted for salt diapirs in central iran

style and timing of salt tectonics in the dniepr donets basin ukraine implications for triggering and driving mechanisms of salt movement in sedimentary basins abstract the ukrainian dniepr donets basin ddb is a late palaeozoic intracratonic rift basin with sedimentary thicknesses up to 19 km displaying the effects of salt tectonics during its entire history of formation from late devonian rifting to the tertiary hundreds of concordant and discordant salt structures formed during this time it is demonstrated in this paper that the variety of styles of salt structure formation in the ddb provide important constraints on understanding the triggering and driving mechanisms of salt kinematics in sedimentary basins in general salt movement in the ddb began during the devonian syn rift phase of basin development and exerted controls on the later distribution of salt structures though the geometry of basement faults is not directly responsible for the regular spacing of salt structures post rift salt movements in the ddb occurred episodically episodes of salt movement were triggered by tectonic events specifically two extensional events during the carboniferous an extensional reactivation at the end of carboniferous earliest permian and a compressional event at the end of the cretaceous extensional events that induced salt movement were thick skinned i e basement involved in deformation rather than thin skinned most overburden deformation related to salt movements is ductile regardless of sedimentary bulk lithology and degree of diagenesis while the deformation of sedimentary cover in areas where salt is absent is mainly brittle this implies that the presence of salt changes the predominant mode of deformation of overlying sedimentary rocks episodes of salt movement lasted longer than the periods of active tectonics that initiated them buoyancy erosion and differential loading all played a role in driving halokinesis once tectonic forces had pushed the salt overburden system into disequilibrium among these factors erosion of overburden above growing salt structures acted as a key self renewing force for development of salt diapirs very high sedimentation rates related to high post rift tectonic subsidence rates particularly during the carboniferous were able to bury diapirs and to load salt bodies such that buoyancy erosion and differential loading forces eventually became insufficient to continue driving diapirism until the system was perturbed by an ensuing tectonic event in contrast some salt anticlines and diapirs developed continuously during the entire mesozoic because of much reduced tectonic subsidence rates and sedimentation supply during this time however a lower permian salt series and overhangs of buried diapirs played an important role in preventing overburden piercing and fracturing during the mesozoic and specifically during the late cretaceous salt diapirism phase

generation of metaluminous a type granites by low pressure melting of calc alkaline granitoids melting experiments on two calc alkaline rocks a tonalite and a granodiorite demonstrate that shallow dehydration melting of hornblende and biotite bearing granitoids generates metaluminous a type granitic melts profuse crystallization of calcic plagioclase orthopyroxene during low pressure p 4 kbar incongruent melting explains all of the major and trace element characteristics of a type granites e g their low al ca mg sr and eu contents and high ga al and k na ratios as well as their common association with anorthositic and mangeritic rocks the residual assemblage produced during melting of the same rocks in the deep crust p 8 kbar is dominated by clinopyroxene and as a consequence most of the chemical features diagnostic of a type granites are lost the high temperatures 900 c and low h 2 o contents 4 wt that are characteristic of a type magmas are adequately explained by the h 2 o poor nature of the granitoid source materials and require neither a granulitic nor a f rich source previous attempts to explain the origin of a type granites have focused on the nature of the source material these experiments show that pressure of melting is at least as important as source composition in engendering a type melts

numerical models of complex diapirs abstract numerically modelled diapirs that rise into overburdens with viscous rheology produce a large variety of shapes this work uses the finite element method to study the development of diapirs that rise towards a surface on which a diapir induced topography creeps flat or disperses erodes at different rates slow erosion leads to diapirs with mushroom shapes moderate erosion rate to wine glass diapirs and fast erosion to beer glass and column shaped diapirs the introduction of a low viscosity layer at the top of the overburden causes diapirs to develop into structures resembling a napoleon hat these spread lateral sheets

application of tracer injection tests to characterize rock matrix block size distribution and dispersivity in fractured aquifers the complexity of mass transfer processes between the mobile and immobile zones in geohydrologic settings and the limitations that currently exist in the characterization of contaminated sites demand the development of improved models in this work we present a model that describes the mass transfer in structured porous media this model considers divergent radial advective dispersive transport in fractures and diffusive mass transfer inside rock matrix blocks the heterogeneous nature of fractured formations is included with the integration of various distributions of rock matrix block sizes into the transport model breakthrough curves generated based on the developed model are analyzed to investigate the effects of the rate of injection dispersivity and the immobile to mobile porosity ratio on mass transfer between mobile and immobile zones it is shown that the developed model in conjunction with tracer data collected from a monitoring well can be used to estimate the dispersivity and fracture intensity results reveal that the dispersivity is independent of the rock matrix block size distribution for dispersion dominant transport in fractures these findings are used to develop a methodology to characterize rock matrix block size distribution in fractured aquifers and to estimate dispersivity based on a tracer test which will improve our decisions concerning the remediation of contaminated sites

geochemistry and petrogenesis of the mesoarchean granites from the canaa dos carajas area carajas province brazil implications for the origin of archean granites four mesoarchean 2 93 to 2 83 ga granite units which encompass the canaa dos carajas bom jesus cruzadao and serra dourada granites were recognized in the canaa dos carajas area of the archean carajas province the mesoarchean units are composed dominantly of biotite leucomonzogranites they are compared with the neoarchean planalto suite 2 73 ga which encompasses biotite hornblende monzogranites to syenogranites the canaa dos carajas bom jesus and the variety of the cruzadao granite with higher la yb n are geochemically more akin to the calc alkaline granites whereas the other varieties of the cruzadao granite are transitional between calc alkaline and alkaline granites the serra dourada granite has an ambiguous geochemical character with some features similar to those of calc alkaline granites and other peraluminous granites the planalto granites have ferroan character are similar geochemically to reduced a type granites and show a strong geochemical contrast with the mesoarchean studied granites the mesoarchean granites described in the canaa dos carajas area are geochemically distinct to those of the rio maria domain of the carajas province the canaa dos carajas and bom jesus granites are similar to the high ca granites whereas the cruzadao and serra dourada are more akin to the low cao granites of the yilgarn craton the geochemical characteristics of the mesoarchean studied granites approach those of the biotite granite group of dharwar but the latter are enriched in hfse and hree compared to the canaa dos carajas granites the neoarchean planalto suite granite has no counterpart in the mesoarchean rio maria domain of the carajas province neither in the yilgarn and dharwar cratons geochemical modeling suggests that partial melting of a source similar in composition to an archean basaltic andesite of the carajas province could give origin to the bom jesus and cruzadao granites in the case of the bom jesus granite the residue of melting contained in similar proportions plagioclase clinopyroxene amphibole and garnet with subordinate ilmenite the variations in la yb n and sr y of the cruzadao granite are controlled by changes in the residual melting phases garnet and amphibole are abundant in the residue of the variety with higher la yb n whereas in the rocks with moderate to lower la yb n garnet is absent magnetite appears in the residue and amphibole initially increases but is replaced by orthopyroxene in the rocks with the lowest la yb n in the canaa dos carajas and serra dourada granites garnet was probably an absent phase in the residue of melting and the influence of amphibole was also apparently limited a crustal environment and a variable pressure from 10 to 7 8 kbar are estimated for the generation of the granite magmas the bom jesus and cruzadao granites of carajas derived from basaltic andesite sources and not of ttg and their geochemical contrasts can be explained by variation in the pressure of melting the sources of archean granite magmas are more diversified and could be more mafic than generally admitted

seal bypass at the giant gjallar vent norwegian sea indications for a new phase of fluid venting at a 56 ma old fluid migration system highlights the giant gjallar vent is still active in terms of fluid migration and faulting the base pleistocene unconformity acts as a seal to upward fluid migration seal bypass in at least one location leads to a new phase of fluid venting the giant gjallar vent ggv located in the voring basin off mid norway is one of the largest 5 3 km vent systems in the north atlantic the vent represents a reactivated former hydrothermal system that formed at about 56 ma it is fed by two pipes of 440 m and 480 m diameter that extend from the lower eocene section up to the base pleistocene unconformity bpu previous studies based on 3d seismic data differ in their interpretations of the present activity of the ggv describing the system as buried and as reactivated in the upper pliocene we present a new interpretation of the ggv s reactivation using high resolution 2d seismic and parasound data despite the absence of geochemical and hydroacoustic indications for fluid escape into the water column the ggv appears to be active because of various seismic anomalies which we interpret to indicate the presence of free gas in the subsurface the anomalies are confined to the kai formation beneath the bpu and the overlying naust formation which are interpreted to act as a seal to upward fluid migration the seal is breached by focused fluid migration at one location where an up to 100 m wide chimney like anomaly extends from the bpu up to the seafloor we propose that further overpressure build up in response to sediment loading and continued gas ascent beneath the bpu will eventually lead to large scale seal bypass starting a new phase of venting at the ggv

rheological and tectonic modeling of salt provinces abstract technological improvements and a more rigorous research approach have led the petroleum industry to renew and expand its interest in physical modeling of faulting associated with halokinesis salt tectonics and the formation of structural hydrocarbon traps can be reproduced in models using viscous polymers for the substratum and frictional plastic granular materials for the overburden the fault patterns simulated in these dynamically scaled models are remarkably similar to those in depth converted seismic sections scale models are therefore useful for generating new concepts and for reconstructing the structural evolution of hydrocarbon traps although even unsealed models may help generating ideas the full transfer of quantitative data requires that models be dynamically scaled to their natural prototypes previously published scaling theory for salt tectonics has erroneously assumed fluid behaviour of the overburden this paper provides a sound scaling approach for geological deformations involving brittle overburdens on ductile substrata rheological data from overburden rocks and salt are compiled and compared to model rheologies using dimensions of laboratory analogs and other intrinsic material properties scaling approaches are compared and rules are formulated for achieving dynamic similarity in models of salt tectonics including those with synkinematic sedimentation

dimensions of the late cretaceous paleocene northeast atlantic rift derived from cenozoic subsidence abstract the distribution of cenozoic subsidence across northeast atlantic volcanic margins have been evaluated to define the width of the rift zone and magnitude of extensional deformation the subsidence profiles are corrected for the effects of lower crustal magmatic bodies emplaced during continental break up the dimensions of the bodies have been derived from the crustal velocity structure the width of the late cretaceous paleocene northeast atlantic rift zone was more than 300 km and the lithospheric extension factor increases gradually towards the line of continental separation a large number of high quality seismic reflection data tied to scientific and commercial wells reveals that the initiation of extensional deformation preceded continental separation by 18 m y on the voring margin off norway these results show that the northeast atlantic volcanic margins commonly considered as typical volcanic margins indeed have similar dimensions as non volcanic margins and as continental rifts thus these margins contrast significantly with previously suggested evolutionary models based on narrow rift zones and formation during rapid lithospheric failure the wide rift is compatible with volume of igneous rocks observed along these margins and with a thermal anomaly similar to that associated with production of northeast atlantic oceanic lithosphere

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

viscosity ratio estimates from natural rayleigh taylor instabilities the geometry of natural gravity driven overturns potentially contains information on viscosity ratios of the material involved irregularities at the interface between two superposed horizontal fluid layers tend to amplify and grow with time with a regular wavelength if the top iayer is more dense published formulas woidt for such rayleigh taylor r t instabilities have been used to construct graphs which can constrain the effective viscosity ratio at the onset of the overturn only the initial thicknesses of the two layers and the final wavelength of the structures need be known the theory is based on an infinite two layer system of newtonian fluids between rigid boundaries with no slip or free slip at contacts geological examples included are soft sediment dewatering on the scale of decimetres in northern norway an unusual instability between basalt and sand in australia of hundreds of metres salt diapirism of kilometres in germany and the persian gulf and granitic diapirism from northern sweden of tens of kilometres viscosity ratios in the order of 1 10 are found for soft sediments with the top layer as the more viscous ratios inferred from patterns of salt diapirs are very large with the overburden approximately 103 105 times more viscous than the salt the viscosities of granites are indicated as lower than their overburden and with ratios of 103

mechanics of active salt diapirism abstract an active diapir forcefully intrudes its overburden driven by diapir pressure that overcomes the resistance of the overburden strength possible causes for the driving pressure are differential loading of the source layer and a density contrast with the overburden possible resisting forces derive from the mass of the roof block and resistance to the faulting and folding that accommodate the intrusion for a typical density contrast between salt and a sedimentary overburden a simple force balance demonstrates that the diapir height must be more than two thirds to three quarters the thickness of the surrounding overburden to initiate substantial active diapirism narrow diapirs must be even taller physical and numerical modeling show that typical structures of active diapirism are a central crestal graben flanked by relatively unstrained flaps that rotate upward and outward curved reverse faults can separate the flaps from the regional overburden normal faults in the crestal graben propagate downward with new faults created farther outward as the roof arches over the rising diapir thin wide roofs develop multiple grabens separating a relatively flat roof from the rotating flaps in cases leading to emergence the piercing diapir evolves from a pointed crest to a progressively rounded and widening crest as the flaps rotate outward the numerical models show that overburden flexure contributes to early formation of active diapir structures but piercement does not continue to emergence unless the diapir is very wide or tall relative to the overburden thickness active diapirism becomes progressively more difficult for diapir crests ranging from a rectangle a round cornered rectangle a semicircle to a triangle with a pointed crest

numerical modelling of salt diapirism influence of the tectonic regime abstract in the context of sedimentary basin mechanics salt diapirism has usually been described as a phenomenon initiated and driven by gravity instabilities associated with buoyant forces or differential loading this result holds essentially for an approximation of linear rheologies of both salt and overburden this has been contested by recent analog models and analytical studies buoyancy alone cannot drive salt diapirism through a brittle overburden regional extension or compression could be a necessary mechanism likely to promote salt diapirism numerical models are presented here to describe the relative effects of gravity and lateral forces on the dynamics of salt diapirism the numerical scheme is based on a viscoplastic approximation of deformations and an updated lagrangian description of kinematics which allows a study of large non linear deformations of salt bodies and the evolution of material interfaces this scheme takes into account brittle and ductile deformations of sediments moreover it describes the initiation and evolution of shear bands which play a crucial role in basin mechanics and especially in the dynamics of salt diapirism we show why extension is the most propitious driving mechanism of diapirism necking and faulting induced by extension promote and drive salt diapirism in the case of a brittle overburden this result holds with or without buoyancy forces this demonstrates that buoyancy which alone is not sufficient to break a brittle overburden is not a necessary mechanism either buoyancy however enhances diapirism induced by lateral forces in contrast with extension salt diapirism during compression is strongly constrained by the rheology of the overburden a brittle mode of deformation favours faulting rather than buckling induced thrusting and thickening of the overburden inhibit massive diapirism and favour the confinement of small salt plugs finally the post extension stability of a mature diapir produced by extension is presented

a numerical approach for investigation of stress states induced by salt structures abstract the main oil and gas reserves around the world are found in regions containing evaporitic rocks owing to their low porosity and low permeability these rocks provide favorable conditions for hydrocarbon trapping increasing the probability of success in oil and gas exploration salt rocks exhibit time dependent strain behavior when submitted to deviatoric stresses this behavior is described by creep constitutive laws based on deformation mechanics one of the major challenges in modeling stress states in sedimentary basins in the presence of salt structures is to predict the magnitude of stress perturbations around salt bodies this work presents a geo mechanical approach to take into account the specific weight variation throughout the lithology and the effects of buoyancy in the resulting forces on the interface salt body formation modeling is driven by the fact that salt bodies cannot sustain deviatoric stresses numerical analyses using the finite element method under plane strain conditions provide the results for the comparison of the methodologies found in the literature with the one proposed by the authors two synthetic models with salt dome structures are investigated the proposed approach provided more realistic stress fields in the vicinity of salt structures

growth of correlated pore scale structures in sedimentary rocks a dynamical model this report explores statistical models and geochemical observations to determine new insights into the formation of sedimentary rock

geometry and growth kinematics of salt pillows in the southern north sea abstract low amplitude salt pillow structures can potentially reveal important details of the physical behaviour of salt and overburden from their growth histories this is because the initial characteristic wavelengths are likely to be preserved beds are more easily traceable across the tops of the structures and original salt thicknesses can be accurately estimated for these reasons a detailed analysis of depth converted seismic data was undertaken to gain insight into the formation growth and spatial distribution of salt pillows in the southern north sea basin around quadrant 44 this area contains a permian zechstein salt interval which has undergone widespread halokinesis to the pillow stage since triassic times but with the main movement during the tertiary perhaps due to inversion the wavelength of the pillows decreases northwards from over 24 to 1 km and estimated original salt thickness also decreases northwards from 1300 to 800 m and there is a strong positive relationship between salt thickness and pillow wavelength pillows appear to be spaced at a characteristic wavelength as they are not directly related to faults and the wavelength varies smoothly through the area however there is a strong alignment of some pillows parallel with the underlying nw se fault trend suggesting that their axis of elongation was controlled by faulting average vertical growth rates of the pillows vary from 40 m ma in the north to less than 10 m ma in the central part of the area measured over vertical distances up to 1250 m most of the growth occurred in the tertiary although a few pillows initiated in the triassic in the north of the area the northern pillows appear to have slowed or even stopped growing during the jurassic and cretaceous and then restarted in the tertiary assuming rayleigh taylor instability controlled the initial pillow wavelengths and using the calculated original salt overburden thicknesses ratios the viscosity ratio between the overburden and the salt can be estimated across the area and was found to vary from 6000 to less than 100 the pillows which moved earliest had the lowest inferred viscosity ratios which is probably because the sediments were less lithified at the time of movement

petrogenesis and emplacement of the ttg and k rich granites at the buzwagi gold mine northern tanzania implications for the timing of gold mineralization abstract the buzwagi gold mine found in the neoarchaean nzega greenstone belt of northern tanzania is underlain by neoarchaean mafic volcanic rocks which are intruded by a massive body of ultramafic rocks that are cross cut by grey colored ttg and pink microcline k rich granites geochemical alteration studies for the sheared and hydrothermally altered k granites show that the lfse and ree were significantly mobilized during the shearing and subsequent injection of hydrothermal fluid events whereas the hfse remained virtually unchanged the buzwagi mine ttg exhibit geochemical characteristics of other worldwide known ttg al2o3 15 0 wt na2o k2o ratios of 1 19 5 16 low concentration of heavy ree with y contents of 3 7 ppm and yb 0 3 0 5 ppm leading to high sr y ratios 61 152 and la yb 32 140 which are comparable to those of phanerozoic adakites the buzwagi mine ttg are characterized by strongly fractionated ree patterns la ybcn 23 100 with slightly negative to no eu anomalies eu eu 0 77 1 05 negative anomalies of nb ta and ti and end 2713 values of 1 19 to 1 77 these geochemical and isotopic characteristics are interpreted as formation of the ttg by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the late archaean subduction zone the buzwagi mine k rich granites differ from their ttg counterparts in having elevated concentrations of incompatible elements like k zr th hf and ree they however share similar nd isotopic compositions end 2674 ma 1 19 to 1 22 and some geochemical features including fractionated ree patterns la ybcn 13 204 with slightly negative to no eu anomalies eu eu 0 78 0 89 and negative anomalies of nb ta and ti these geochemical features and isotopic signatures have been interpreted as formation of the k granites by anhydrous partial melting of the ttg and greenstones the buzwagi mine k granites were emplaced at 2674 18 ma some 40 ma years after the emplacement of ttg at 2713 8 ma the timing of the buzwagi mine granitic magmatism falls within the peak crustal growth period in the northern part of the archaean tanzania craton gold mineralization at the buzwagi mine is hosted in quartz veins that are within strongly sheared and hydrothermally altered k granites in which silica and sericite alterations and sulphides mainly pyrite and chalcopyrite although chalcocite is also present are important the buzwagi gold deposit is thus a typical archaean orogenic deposit whose mineralization is hydrothermally and structurally controlled and the abundance of sulphides silica and sericite alteration assemblages suggests that the deposition conditions were reducing and acidic in nature owing to its epigenetic nature it can be inferred that the maximum age of deposition of gold at the buzwagi mine is 2674 ma which is more or less the same age inferred for the golden pride mine 2680 ma both of which are found in the nzega greenstone belt

implications of passive salt diapir kinematics for reservoir segmentation by radial and concentric faults abstract reservoirs associated with salt diapirs that are circular or elliptical in plan view can be segmented by radial faults concentric faults both or neither explanations for this variation are offered based on integration of fault kinematics published studies and seismic examples strata around diapirs can be divided into three structural domains with characteristic fault styles 1 roof zone between the present day depositional surface and crest of a buried inactive diapir this zone is characterised by doming radial faults and occasionally conic graben doming is usually caused by differential compaction and increases with depth 2 a steeply dipping sheath of strata surrounds the diapir out to a distance of less than one diapir radius recording near surface diapir growth in poorly lithified sediments these contain early gravity slumps and late concentric extensional faults regional layer bound polygonal fault systems form radial faults within approximately one diapir radius the only significant radial faulting mechanism below the roof zone 3 diapir root zone of strata whose structural architecture records the diapirs initiating mechanism typically rim synclines or graben that detach on the salt source layer concentric faults at radial distances of a kilometre or more have been found only in root zone rim synclines and sheared sheath strata these models and observations can assist seismic interpretation and structural model building in areas of poor geophysical imaging for instance below overhanging salt

gravitational and buckling instabilities of a rheologically layered structure implications for salt diapirism summary we study the rayleigh taylor instability of a structure consisting of a buoyant layer of viscous fluid overlain by a dense perfectly plastic layer which is represented by a strongly nonnewtonian fluid with the power law exponent tending to infinity the structure is subject to either horizontal extension or shortening and models rocksalt under a brittle overburden the growth rate and wavelength of the most unstable perturbation to the background pure shear flow are calculated and compared with those of models composed of two viscous layers or of two perfectly plastic layers the effects of the viscosity and thickness ratios and density contrasts between the two layers are assessed considering the viscosity of the buoyant layer to be much less than the effective viscosity of the overlying layer we obtain the following results i the instability pattern of the plastic viscous structure is similar to that of a plastic plastic structure ii the characteristic wavelength corresponding to the most unstable mode increases initially with the thickness ratio between the lower and upper layers but then decreases by a series of abrupt jumps iii the buckling instability induced by rapid horizontal extension or shortening overwhelms the gravitational instability and the growth rate of this instability depends linearly on the effective viscosity ratio we analyze the energy equation in order to develop an understanding of the mechanisms of instability as the system varies from a viscous fluid through a power law fluid to a perfectly plastic medium to test our analytical results we study models of diapirism in the great kavir iran we show that a small interdiapir spacing in the salt canopy province and a wide range of spacings in the salt pillow province of the region can be explained by the perfectly plastic sedimentary overburden and horizontal shortening

geochemistry of fine grained clastic sedimentary rocks of the neoproterozoic ikorongo group ne tanzania implications for provenance and source rock weathering abstract the neoproterozoic ikorongo group which lies unconformably on the late archaean nyanzian supergroup of the tanzania craton is comprised of conglomerates quartzites shales siltstones red sandstones with rare flagstones and gritstones and is regionally subdivided into four litho stratigraphic units namely the makobo kinenge sumuji and masati formations we report geochemical data for the mudrocks i e shales and siltstones from the ikorongo basin in an attempt to constrain their provenance and source rock weathering these mudrocks are compositionally similar to paas and ps indicating derivation from mixed mafic felsic sources however the siltstones show depletion in the transition elements cr ni cu sc and v and attest to a more felsic protolith than those for paas and ps the chemical index of alteration cia 52 82 reveal a moderately weathered protolith for the mudrocks the consistent ree patterns with lree enriched and hree depleted patterns la yb cn 7 3 38 3 coupled with negative eu anomalies eu eu 0 71 on average which characteristics are similar to the average paas and ps illustrate cratonic sources that formed by intra crustal differentiation geochemical considerations and palaeocurrent indications suggest that the provenance of the ikorongo group include high mg basaltic andesites dacites rhyolites and granitoids from the neoarchaean musoma mara greenstone belt to the north of the ikorongo basin mass balance calculations suggest relative contributions of 47 42 and 11 from granitoids high magnesium basaltic andesites and dacites respectively to the detritus that formed the shales corresponding contributions to the siltstones detritus are 53 43 and 4

salt flow and diapirism related to extension at crustal scale abstract the analysis of the structure and history of salt diapirs in large ancient evaporitic basins such as the southeastern north sea basin suggests that diapirism is controlled by crustal scale extension laboratory experiments on small scale models designed to simulate configurations comparable to the southeastern north sea basin show that diapirs which grow in a sedimentary cover during basement extension can develop close to basement normal faults or at a significant distance from them if the displacement on the basement faults is transfered horizontally along the salt layer for given values of salt viscosity strain rate and friction coefficient in brittle rocks the pattern of salt diapirism in a particular area depends on the ratio of cover thickness to salt thickness as well as the absolute thicknesses of cover and salt layers the experiments give criteria for identifying different types of diapirs and provide models of horizontal flow in a salt layer undergoing extension and diapir development some dynamic implications of the experiments are applied to the southeastern north sea basin

modeling stress evolution around a rising salt diapir abstract we model the evolution of a salt diapir during sedimentation and study how deposition and salt movement affect stresses close to the diapir we model the salt as a solid visco plastic material and the sediments as a poro elastoplastic material using a generalized modified cam clay model the salt flows because ongoing sedimentation increases the average density within the overburden sediments pressurizing the salt stresses rotate near a salt diapir such that the maximum principal stress is perpendicular to the contact with the salt the minimum principal stress is in the circumferential direction and drops near the salt the mean stress increases near the upper parts of the diapir leading to a porosity that is lower than predicted for uniaxial burial at the same depth we built this axisymmetric model within the large strain finite element program elfen our results highlight the fact that forward modeling can provide a detailed understanding of the stress history of mudrocks close to salt diapirs such an understanding is critical for predicting stress porosity and pore pressure in salt systems

neoarchean leucogranitoids of the kianta complex karelian province finland source characteristics and processes responsible for the observed heterogeneity abstract in this study we describe the geochemistry and discuss the genesis of neoarchean ca 2 7 ga leucogranitoids of the kianta complex within the karelian province of the fennoscandian shield the leucogranitoids occur extensively as dykes and intrusions of varying size and represent substantial recycling of older crust occurring as the last magmatic event before final cratonization of the karelian province geochemically the leucogranitoids resemble the leucosomes of migmatized ttgs both groups are compositionally heterogenous but are characterized by high silica contents mean sio2 73 4 wt and leucocratic appearance reflecting a paucity of mafic minerals mean mgo 0 37 wt mg 32 6 the majority of samples analyzed are weakly peraluminous mean a cnk 1 05 mineralogically the leucogranitoids form a series grading from trondhjemite to leucogranite which is observable as highly variable cao 0 07 3 84 wt mean 1 47 k2o 0 30 8 76 wt mean 3 53 and na2o 2 22 7 23 wt mean 4 49 we consider the local ttgs sensu lato which comprise both true ttgs k2o na2o 0 5 to be the predominant source material for the leucogranitoids however some samples overlap compositionally with local ttgs and could represent either partial melting of garnet amphibolites or alternatively products of nearly complete fusion of ttgs leucogranitoids and transitional ttgs can be differentiated to some extent on the basis of geochemistry but definitive discrimination requires demonstration of field relationships and or isotope dating the observed compositional scatter is interpreted to be a result of source heterogeneity retention of variable accessory minerals and accumulation processes melting is attributed to thickening of the crust in response to collisional tectonics and occurred dominantly under water fluxed conditions possible fluid sources are the sanukitoid and quartz diorite suites plus dehydration reactions and partial melting of the lower crust

thermally induced brittle deformation in oceanic lithosphere and the spacing of fracture zones brittle deformation of oceanic lithosphere due to thermal stress is explored with a numerical model with an emphasis on the spacing of fracture zones brittle deformation is represented by localized plastic strain within a material having an elasto visco plastic rheology with strain softening we show that crustal thickness creep strength and the rule governing plastic flow control the formation of cracks the spacing of primary crack decreases with crustal thickness as long as it is smaller than a threshold value creep strength shifts the threshold such that crust with strong creep strength develops primary cracks regardless of crustal thicknesses while only a thin crust can have primary cracks if its creep strength is low for a thin crust the spacing of primary cracks is inversely proportional to the creep strength suggesting that creep strength might independently contribute to the degree of brittle deformation through finite versus zero dilatation in plastic strain associated and non associated flow rule results in nearly vertical and v shaped cracks respectively changes in the tectonic environment of a ridge system can be reflected in variation in crustal thickness and thus related to brittle deformation the fracture zone free reykjanes ridge is known to have a uniformly thick crust the australian antarctic discordance has multiple fracture zones and thin crust these syntheses are consistent with enhanced brittle deformation of oceanic lithosphere when the crust is thin and vice versa

late neoarchean monzogranitic syenogranitic gneisses in the eastern hebei western liaoning province north china craton petrogenesis and implications for tectonic setting abstract the medium grained monzogranitic syenogranitic gneisses expose as roughly northeast southwest trending and cover 1500 km 2 in the southern part of the eastern hebei western liaoning province north china craton these plutonic gneisses have leucocratic features and weakly gneissic to massive structures and were metamorphosed under greenschist to lower amphibolite facies la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these monzogranitic syenogranitic gneisses were emplaced during 2527 2511 ma they are characterized by high contents of sio 2 71 64 78 08 and k 2 o 3 55 6 40 and low contents of cao 0 25 1 71 mgo 0 01 0 42 and na 2 o 2 29 4 96 according to their chondrite normalized ree patterns the monzogranitic syenogranitic gneisses can be subdivided into a high ree group and a low ree group studies of petrogenesis reveal that magmatic precursors of the high ree group samples were derived from partial melting of the regionally widespread porphyritic or medium grained dioritic quartz monzonitic granodioritic monzogranitic gneisses under low pressure conditions less than ca 10 kbar the magmatic precursors of the low ree group were formed by fractionated crystallization of the high ree group magma with titanite allanite epidote apatite and zircon as the major crystallization fractionation phases combined with previous investigations on the late neoarchean metavolcanic rocks and granitoid orthogneisses in the northwestern part of the eastern hebei western liaoning province the magmatic precursors of these monzogranitic syenogranitic gneisses were most likely formed in a back arc basin along the north of the eastern block north china craton

diapirism and topography summary a new numerical technique using markers and a deformable lagrangian mesh is used to study the deformation of the surface above a rising diapir this method allows modelling of a free surface boundary in a self consistent way the method makes it possible to investigate many different problems with non regular geometry the codes were verified through comparison with analytical solutions for the initial stages and with other numerical codes for mature stages our simulations led to the following conclusions 1 the growth rate of the diapir is more strongly influenced by the viscosity contrast than the wavelengths this is due to the strong growth rate difference during the initial stage 2 the maximum elevation above the diapir linearly increases with increasing wavelength and is approximately the same for different viscosity contrasts 3 the elevation will be considerably higher for a low viscosity diapir at a given depth than for an isoviscous diapir comparisons for different thicknesses of the buoyant layer showed that the highest topography is produced when the two layers have equal thickness we showed the dependence of the topographic behaviour on the parameter r the ratio of the density difference between two layers to the density of the upper layer it was found that topography behaves linearly up to r 0 15 it indicates that a posteriori estimation of topography for traditional calculations using the free slip boundary condition works well within this limit

major and trace element geochemistry and crustal evolution of archaean granodioritic rocks from eastern finland in the eastern part of central finland vast areas of granodioritic gneisses surround the ca 2 65 ga greenstone belts these granodioritic gneisses can be divided into two chronological and lithological units 1 grey gneisses 2 86 ga old with a typical archaean ttg tonalite trondhjemite granodiorite composition 2 augen gneisses 2 51 ga old with a granodioritic to granitic composition the sr and nd isotopic data preclude derivation of the grey gneisses from very much older continental material the highly fractionated ree distribution patterns la yb n 37 103 with few or no eu anomalies militate against a direct mantle origin the data suggest that the grey gneisses have most likely been derived by partial melting of short lived basaltic crustal material garnet bearing amphibolite it is estimated that the basaltic protoliths were separated from a mantle source no more than 150 ma before the melting such an early basaltic crust might have been completely destroyed or might have left some remnants which are now represented by mafic inclusions in the grey gneisses the augen gneisses have a relatively high isr value 0 705 which suggests an origin by recycling of pre existing crustal material grey gneisses their ree patterns are less fractionated than those of the grey gneisses and sometimes have negative eu anomalies using the sr isotopic and trace element data a model calculation suggests that the augen gneisses could have been derived by partial melting 50 of mixed protoliths with ttg greenstone belt volcanics in a proportion of 70 30 by volume followed by fractional crystallisation

numerical study of tectonic structure evolution in a multi layer evaporite sequence abstract field studies carried out in the klodawa salt structure kss poland reveal that within a carnallite halite kieserite sequence finger like structures developed during an early pre diapiric stage of the kss evolution we used 99 numerical simulations to model finger development and investigate mechanical properties of carnallite halite and kieserite during deformation models used inverse density stratification of a three layer set with varying 1 layer viscosities 2 thicknesses and 3 density difference ratios finger shapes in the models were categorized into single and multiwavelength structures among the single wavelength shapes bulb mushroom thumb and bloated i e without a regular shape structures were distinguished we show two transitions between the single and multiwavelength structures which correspond to the cases of either the lower or upper layer having viscosity more than an order of magnitude larger than the two other layers through comparison of finger shapes developed in the models and in the natural structures we estimate viscosities of 1017 pa s for halite

numerical modelling of asymmetric boudinage abstract asymmetric boudinage structures are commonly used as shear sense indicators but their development is incompletely understood this paper describes the influence of initial shape and kinematic parameters on the evolution of boudin trains using a numerical approach based on the finite difference code flac boudin trains are simulated as a series of competent objects embedded in a soft matrix subjected to general monoclinic ductile flow deformation of boudin trains includes heterogeneous stretching rotation of boudins and offset along the neck regions the sense of relative boudin offset is mainly influenced by the initial orientation of the interboudin plane in the boudinaging layer while kinematic vorticity number of the flow and the orientation of the boudin train with respect to the flow extensional eigenvector usually the shear zone boundary also play a role viscosity ratio and aspect ratio influence the magnitude of offset along the neck regions and the amount of rotation of boudins but not the sense of slip and rotation on the interboudin plane knowledge of the orientation of the interboudin plane itself is insufficient to use asymmetric boudins as independent shear sense indicators details of the boudin geometry such as the sense of deflection of marker horizons along the interboudin plane must be used for this purpose

towards a self consistent plate mantle model that includes elasticity simple benchmarks and application to basic modes of convection one of the difficulties with self consistent plate mantle models capturing multiple physical features such as elasticity non newtonian flow properties and temperature dependence is that the individual behaviours cannot be considered in isolation for instance if a viscous mantle convection model is generalized naively to include hypo elasticity then problems based on earth like rayleigh numbers exhibit almost insurmountable numerical stability issues due to spurious softening associated with the co rotational stress terms if a stress limiter is introduced in the form of a power law rheology or yield criterion these difficulties can be avoided in this paper a novel eulerian finite element formulation for visco elastic convection is presented and the implementation of the co rotational stress terms is addressed the salient dimensionless numbers of visco elastic plastic flows such as weissenberg deborah and bingham numbers are discussed in a separate section in the context of geodynamics we present an eulerian formulation for slow temperature dependent visco elastic plastic flows a consistent tangent incremental formulation of the governing equations is derived numerical and analytical solutions demonstrating the effect of visco elasticity co rotational terms are first discussed for simplified benchmark problems for flow around cylinders we identify parameter ranges of predominantly viscous and visco plastic and transient behavior the influence of locally high strain rates on the importance of elasticity and non newtonian effects is also discussed in this context for the case of simple shear we investigate in detail the effect of different co rotational stress rates and the effect of power law creep the results show that the effect of the co rotational terms is insignificant if realistic stress levels are considered e g deviatoric invariant smaller than 1 10 of the shear modulus say we also consider the basic convection modes of stagnant lid episodic resurfacing and mobile lid convection as applicable to a cooling planet the simulations show that elasticity does not have a significant effect on global parameters such as the nusselt number and the qualitative nature of the basic convection pattern our simple benchmarks show however also that elasticity plays a significant role for instabilities on the local scale of an individual subduction zone

some remarks on the origin of foredeeps abstract geological observations show that there are two main types of foredeep basins on the base of their subsidence rate and their relationship with the associated orogen or accretionary wedge high rates of subsidence up to 1600 m myr and a ratio lower than 1 between the area of the elevated belt and the area of the basin characterize the foredeeps associated with w dipping subduction low rates of subsidence up to 300 m myr and a ratio higher than 1 between the area of the orogen and the area of the basin characterize the foredeeps associated with e or ne dipping subductions this observation enables us to interpret the slow filling of foredeeps of the first type and the much faster filling of foredeeps of the second type moreover w dipping subductions have a steeper and faster monocline bounding the base of the deep foredeep in contrast to e or ne dipping subductions which have a shallower monocline this differentiation supports the notion of the eastward mantle flow with respect to the lithosphere detected in the hot spot reference frame in this view the foredeep depth in w dipping subductions is mainly controlled by the roll back of the subduction hinge pushed by the relative eastward mantle flow while foredeep depth in e or ne dipping subductions is instead mainly generated by the load of the thrust sheets and by the roll back of the subduction hinge due to the advancing upper plate contrasting the upward push of the mantle the shape of the foredeeps is regularly arcuate in case of w dipping subduction while is linear or following the shape of the inherited continental margin in case of e or ne dipping subduction fold development in the two opposite foredeeps is significantly different in w dipping subduction the folds are transported down in subduction while they are forming and consequently they are poorly eroded syntectonic sediments drape a preserved fold in the e ne dipping subduction folds and thrust sheets are instead uplifted and deeply eroded this is also predicted by the envelope of the crests in the two distinct accretionary wedges in the w dipping case the envelope may dip toward the subduction while it is rising toward the hinterland in the other cases this approach may explain the differences in terms of geometries amount and rates of subsidence between the foredeeps around the adriatic plate for instance the foredeep of the southern alps back thrust belt foredeep is deformed in the western part by the later apenninic foredeep development associated with the sinistral oblique ramp of the w dipping adriatic plate subduction the relationship in the frontal parts of a thrust belt between accretion and eustacy has to be analyzed in terms of when where and how fast thrusts propagation took place with respect to the randomly distributed sea level third order fluctuations and by the type of subduction which is connected to the foredeep and thrust belt or accretionary wedge therefore the stratigraphic patterns in foredeep basins are peculiar and casual successions that record the combination of regional tectonic evolution with independent wider scale sea level changes

physical modelling of overburden deformation around salt diapirs abstract salt diapirs produce highly complex deformation patterns in the surrounding overburden which are difficult to image seismically or model numerically to further the understanding of deformation around salt structures we have used physical models with brittle granular overburdens and simulated sedimentation accompanying diapiric rise rigid linear indentors produce a crestal horst above the indentor with two flanking graben large amounts of indentation produce predominantly reverse faults along the margins of the diapir and extensional faults over the crest in contrast hemispherical indentors produced inward dipping concentric reverse faults forefully intruded polymer diapirs produce a narrow crestal graben above the diapir crest downbuilt polymer diapirs produce extensional faults on the flanks of the diapirs which offset the diapir walls no clear secondary rim synclines were developed as polymer is withdrawn evenly from the whole of the polymer layer across the model strong and anisotropic layered overburden were simulated using cohesive clay and mica respectively detachments were formed along mica layers which suppressed faults with large throws cohesive layer deformation began with tensile fractures permitting rotation of fractured rafts around the sides of the diapir these progressively slip off the crest and create large gaps in the stratigraphy

geological interpretation and numerical modelling of salt movement in the barbastro balaguer anticline southern pyrenees abstract the barbastro balaguer salt anticline located in the south pyrenean foreland basin was characterised with seismic reflection profiles and well logs interpretation to be the result of a combination of thrusting buoyancy and differential loading mechanisms these mechanisms were responsible for the formation and the evolution of the structure from the late oligocene to the present seismic reflection interpretation indicates that alpine compressional tectonics played the major role in the early stages of the formation of the anticline a numerical model based on the finite difference involving thrusting buoyancy and differential loading mechanisms was developed and applied to the modelling of the development of the salt structure the results of our modelling suggest that buoyancy and differential loading have played an important role in the subsequent growth of the barbastro balaguer anticline

geochemistry and nd isotopic composition of potassic magmatism in the neoarchaean musoma mara greenstone belt northern tanzania abstract the neoarchaean musoma mara greenstone belt mmgb of northern tanzania is underlain in part by the 2649 ma post orogenic potassic rich granites which are the most abundant intrusive rocks in the belt the rocks are composed of plagioclase k feldspar quartz biotite sphene zircon hornblende chlorite they are characterized by high contents of sio 2 68 90 77 76 wt k 2 o 3 71 5 44 wt and low na 2 o 3 27 5 70 wt leading to low ratios of na 2 o k 2 o 0 63 1 02 the rocks are depleted in cao 0 22 2 41 wt as well as in sr 15 412 ppm cr 16 ppm and ni 5 ppm and their major element composition are similar to those of experimental melts derived from partial melting of tonalite on chondrite normalized ree patterns these rocks show fractionated patterns la yb cn 1 22 41 32 that are characterised by moderate to strongly negative eu anomalies eu eu 0 04 0 86 on primitive mantle normalized spidergrams these rocks are generally enriched in th u k and pb and depleted in ba sr nb ta and ti relative to adjacent elements the k rich granites have nd o at 2 649 ga values of 0 55 to 1 70 that compare well with those of associated volcanic rocks and ttg nd 0 44 to 2 66 which predate the emplacement of the k rich granitoids their mean crustal residence ages are 170 to 450 ma older than their emplacement ages the overall geochemical features of this suite of rocks together with evidence from experimental results are consistent with their generation by partial melting of relatively juvenile igneous rocks within the continental crust at pressures corresponding to depths