duplex style and triangle zone formation insights from physical modeling duplexes are a common feature in thrust belts at many scales their geometries vary significantly from antiformal stacks with significant forethrusting in the cover e g southern pyrennes spain to triangle zones e g foreland canadian rockies to low displacement individually spaced ramp anticlines e g sub andean thrust belt bolivia we present a series of physical experiments demonstrating that the strength of the decollements relative to that of the intervening and overlying rock layers plays a significant role in controlling the duplex style the models comprise brittle layers made of dry quartz sand and decollements made of two types of viscous silicone polymers the strength of the decollements in the models is a function of the shortening rate applied to the model the relative strength of the decollements and surrounding rocks affects the development of active or passive roof duplexes triangle zones it also affects the amount of translation of individual thrust blocks and the spacing of thrust ramps which in turn determine if a duplex evolves into an antiformal stack or into individually spaced ramp anticlines model results indicate that specific associations of structural features form systematically under similar rheological and boundary conditions the presence of relatively strong decollements promotes local underthrusting of the cover individual ramp anticlines internal deformation of thrust sheets low early layer parallel shortening and sequential towards the foreland propagation of structures weak decollements promote forethrusting of the cover antiformal stacks coeval growth of structures and low internal strain with the exception of significant early layer parallel shortening no underthrusting at a regional scale occurred in any model

geometry and evolution of a fault bend fold mount bertha anticline mount bertha anticline is a major thrust associated fold in the rocky mountains of western canada well displayed in a 900 m thick middle paleozoic carbonate sequence that forms the core of the fold this carbonate sequence may be divided into three mechanical stratigraphic units of roughly similar thicknesses a lower massive dolostone a middle layered dolostone and an upper micritic limestone substantial variations in structural geometry occur along strike these variations correlate with changes in the thickness of the lower dolostone unit in the core of the fold that result from an oblique hanging wall ramp a mode i fault bend fold develops along most of the exposed strike length of mount bertha anticline this configuration corresponds to regions where a full thickness of the lower dolostone unit occurs in the core of the fold where the lower dolostone unit is thin or absent in the hanging wall of the mount bertha thrust the configuration of mount bertha anticline is dominated by second order fault propagation and detachment folds developed in the upper two mechanical stratigraphic units along the length of the mount bertha structure the limestone unit has been progressively attenuated and or faulted in the lower forelimb region and overridden by the advancing thrust sheet the fold geometry of the central part of mount bertha anticline compares favorably with geometric models of fault bend fold structures that allow forelimb thinning except for parts of the fold affected by the tectonic removal of rock from the lower forelimb existing geometric models of fault bend folds do not consider this process a companion fold within the hanging wall of the mount bertha thrust cranswick anticline is interpreted to be a mode i fault bend fold that has been significantly modified by second order fault propagation folding during the waning phases of movement along the mount bertha thrust

source facies and oil families of the malay basin malaysia the malay basin consists of a number of separate petroleum systems driven exclusively by nonmarine source rocks these systems range from lower oligocene to middle miocene and show a progression from lacustrine dominated source facies in the lower oligocene to lower miocene section to coastal plain delta plain coal related sources in the lower to middle miocene section two lacustrine sources are recognized in the older section and multiple source reservoir pairs are recognized in the younger coaly section the lacustrine sources can be recognized using well log analysis combined with detailed core and sidewall core sampling chemically they are characterized by low pristane phytane ratios low oleanane contents and a general absence of resin derived terpanes these sources have tocs in the 1 0 4 0 range and hydrogen indices of up to 750 in contrast the coal related sources are chemically distinct with pristane phytane ratios of up to 8 very high oleanane contents and often abundant resinous compounds all these sources are generally overmature in the basin center and immature toward the basin margin the oils sourced from all sources in the malay basin are generally low in sulfur and of very high economic value detailed biomarker analysis of the oils in the malay basin has allowed the recognitionmore of families associated with the above sources and demonstrated that oil migration has been largely strata parallel with little cross stratal mixing of families less

assessment of earthquake hazard in the tokyo area japan abstract the probability of occurrence of an inland earthquake originating beneath the tokyo area is evaluated for a 10 year period from 1991 on the basis of an earthquake catalogue containing some 40 earthquakes which caused some damage to edo now tokyo over a period of 400 years the parameters of the weibull and lognormal probability distributions applied to recurrence interval statistics are estimated with these parameters it is concluded that an earthquake directly beneath the capital the magnitude of which is equal to or greater than 6 0 occurs with a probability of 0 40 for the period in question when the magnitude threshold is assumed as 6 4 and 7 0 the probabilities become 0 17 and 0 049 respectively these probability evaluations are extended to those for the seismic intensity at typical sites in the area although no accurate evaluation is possible because the exact focal depths of historical earthquakes are not known on the assumption that these earthquakes occur near the upper surface of the down going philippine sea plate however a typical depth of 30 km or so is suggested should an earthquake having a mean magnitude of 6 4 say and at a mean location of past earthquakes occur the probability of seismic intensity at typical sites in the capital area exceeding 5 on the japan meteorological agency jma scale would amount to 0 17 the possibility of having a highly damaging earthquake of intensity 6 on the jma scale also cannot be ruled out judging from earthquake disasters in the past although the probability does become lower to summarize it may be said that the probability of japan s capital area being hit by a damaging earthquake in the foreseeable future is not particularly low

new ams dates stratigraphic correlations and decadal climatic cycles for the past 4 ka at lake turkana kenya abstract new ams radiocarbon dates and stratigraphic correlations of 7 piston cores recovered from lake turkana east africa show that the frequency of cyclic variability is greater than previously reported for a single piston core almost 60 new ams dates of various carbonate fractions indicate that large 150 m ostracode carapaces yield the most accurate chronology for the sediment record due to the input of old fine grained carbonate stratigraphic correlations among the cores by carbonate abundance and magnetic susceptibility profiles provide supporting evidence for the ostracode based chronology this chronology refines earlier estimates of sediment accumulation that impacts the recurrence interval of lamination deposition from one lamination every two years to one lamination per year in two north basin cores the record of carbonate content within the north basin of the lake reflect its dilution with terrigenous sediments input from the omo river but direct paleoclimatic interpretations are complicated by lateral migrations of the delta channels with changing lake levels time series analysis of both individual cores and a combined carbonate profile show significant spectral peaks at 1000 76 32 22 18 6 and 11 yr the latter three suggest an important link between the sediments accumulating in lake turkana and global cycles of climatic variability

earthquakes recurrence and interoccurrence times the purpose of this paper is to discuss the statistical distributions of recurrence times of earthquakes recurrence times are the time intervals between successive earthquakes at a specified location on a specified fault although a number of statistical distributions have been proposed for recurrence times we argue in favor of the weibull distribution the weibull distribution is the only distribution that has a scale invariant hazard function we consider three sets of characteristic earthquakes on the san andreas fault 1 the parkfield earthquakes 2 the sequence of earthquakes identified by paleoseismic studies at the wrightwood site and 3 an example of a sequence of micro repeating earthquakes at a site near san juan bautista in each case we make a comparison with the applicable weibull distribution the number of earthquakes in each of these sequences is too small to make definitive conclusions to overcome this difficulty we consider a sequence of earthquakes obtained from a one million year virtual california simulation of san andreas earthquakes very good agreement with a weibull distribution is found we also obtain recurrence statistics for two other model studies the first is a modified forest fire model and the second is a slider block model in both cases good agreements with weibull distributions are obtained our conclusion is that the weibull distribution is the preferred distribution for estimating the risk of future earthquakes on the san andreas fault and elsewhere

interpreting the geologic map expression of contractional fault related fold terminations lateral oblique ramps versus displacement gradients abstract fault related folds in contractional settings do not extend indefinitely but rather commonly terminate as plunging anticlines near the tips of the genetically related fault geologists often attribute the formation of such terminations to loss of slip on the underlying fault i e a displacement gradient to changes in fault geometry in which the fault laterally changes stratigraphic position along strike i e a lateral oblique ramp or to some combination of these mechanisms discerning between these formative mechanisms solely through the interpretation of the geologic map expression of the termination can be difficult because of the subjective nature of the criteria used e g lateral oblique ramps often are interpreted at terminations where folds plunge at steep angles where hanging wall cutoff lines trend at high angles to fault strike where stratigraphic contacts trend at high angles to fault strike etc we created pseudo three dimensional model terminations of individual fault related folds using both displacement gradients and lateral oblique ramps to determine if unique characteristics in their map expressions exist we show that map patterns of folds produced by a displacement gradient along a thrust fault of constant geometry are similar to map patterns of folds produced by constant slip on a thrust fault with a lateral oblique ramp specifically our modeling results suggest that 1 simple fault bend folds that plunge less than 20 and simple fault propagation folds that plunge less than 50 at their terminations can be created by both displacement gradients and lateral oblique ramps angles greater than these values suggest that a lateral oblique ramp may be involved in forming the termination 2 hanging wall cutoff lines at terminations that trend at angles less than about 35 to fault strike for simple fault bend and fault propagation folds also may be produced by both displacement gradients and lateral oblique ramps higher angles likely indicate the presence of a lateral oblique ramp at the termination 3 the angle at which stratigraphic contacts trend to the fault strike cannot be used to uniquely identify displacement gradients or lateral oblique ramps for simple fault related folds 4 stratigraphic separation diagrams can indicate the presence of ramps in the thrust sheet but do not uniquely differentiate between frontal ramps with displacement gradients and lateral oblique ramps and 5 actual changes in fault orientation after topographic influences have been taken into account by definition indicate a lateral oblique ramp in reality most natural fault related fold terminations probably share components of both displacement gradients and lateral oblique ramps with each structure possessing contributions from each mechanism

paleoclimate of the past 4000 years at lake turkana kenya based on the isotopic composition of authigenic calcite abstract authigenic calcite was analyzed for 13 c and 18 o in a 12 m long core from lake turkana that spans the last 4200 yr the 18 o record is interpreted to reflect lake level history and indicates high stands prior to 4000 yr b p from 1800 to 2000 yr b p and within the last century this coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for lake turkana based on 14 c dates of exposed shoreline deposits the 13 c curve roughly parallels the 18 o curve and may reflect the influence of the omo river plume both of the isotope records are strongly cyclic with periodicites common to both at 80 120 150 and 225 yr other somewhat longer periodicites are nearly equal in the two records and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same turkana core

early permian 90 clockwise rotation of the maures esterel corsica sardinia block confirmed by new palaeomagnetic data and followed by a triassic 60 clockwise rotation

influence of layering and boundary conditions on fault bend and fault propagation folding the influence of heterogeneous layering and boundary conditions on the structural development of fault bend and fault propagation folds has been investigated through petrographic study of nonscaled rock models the models are deformed in a triaxial rock deformation apparatus at room temperature and a 50 mpa confining pressure the models consist of a single layer of sandstone containing a saw cut ramp that is inclined 20 to the layering and an overlying intact thinly layered unit that is composed of limestone interlayered with lead or mica analysis of the fold thrust structures generated in sequentially shortened models with different loading conditions and layer types suggests that the mode of fold thrust interaction activated upon shortening will depend on fault zone drag bending and shearing resistance of the hanging wall shear strength of layer interfaces and loading conditions for the models these parameters may be expressed as a strength ratio describing the resistance to foreland translation relative to the resistance to internal deformation of the thrust system low strength ratios favor fault bend folding high strength ratios favor internal shortening of the sheet isotropic and thick relative to ramp height units above a propagating thrust tip will shorten primarily by faulting whereas thinly layered anisotropic units will shorten by fault propagation folding during both modes of fold thrust interaction the dips of the fold limbs increase interlimb angles decrease and imbricate faults form in the hanging wall or footwall with shortening in one model suite the imbrication is associated with a transition from fault bend folding to fault propagation folding and produces a highly asymmetric ramp anticline similar to a second mode fault bend fold or to a transported fault propagation fold the model data suggest that fault propagation folding in heterogeneously layered rock occurs by the discontinuous formation growth and linkage of faults below the growing fold amplification of the fault propagation fold is affected by the amount of slip transferred out of the deforming region imbrication and buckling the changes in the mode of fold thrust interaction and modifications in the local geometry and strain distribution that occur during shortening result from slip hardening on faults or from rotation or strain induced variations in the strength of the layers

environmental control on concretion forming processes examples from paleozoic terrigenous sediments of the north gondwana margin armorican massif middle ordovician and middle devonian and sw sardinia late ordovician abstract concretions of various compositions are common in the paleozoic terrigenous successions of the north gondwana margin this study focuses on phosphatic p and siliceous si concretions present in some successions of the armorican massif nw france and sw sardinia w italy it shows that they consist of mudstones fine to very fine grained sandstones or shellbeds with a more or less abundant p cement and form a continuum between a phosphatic end member and a siliceous biogenic end member the p 2 o 5 contents are ranging from 0 26 to 21 5 and are related to apatite the sio 2 contents vary from 25 to 82 and are linked both to a terrigenous phase and to a biogenic silica phase concretions showing the lower p contents p 2 o 5 2 al 2 o 3 5 comparison with the surrounding sediments shows that all the concretions are enriched in chlorite and in middle rare earth elements la s gd s 0 12 0 72 and some of them in y up to 974 ppm rare earth elements more than 300 ppm and sr 260 880 ppm the concretions with highest biogenic silica concentrations are contained in the outer shelf sediments whereas the other concretions are present from the proximal part of the inner shelf to the outer shelf a genetic model in two stages is proposed during early diagenesis the dissolution of shells and degradation of organic matter progressively enrich the pore water in dissolved si ca and p when the suboxic zone is reached p precipitation begins leading to the formation of protoconcretions in shallow environments the relative permeability of sediments and the winnowing or reworking of the upper few centimetres by bottom currents allow for suboxic conditions to be maintained leading to p rich concretion formation in deeper environments the anoxic zone is reached more rapidly thereby preventing extensive phosphogenesis nevertheless in the protoconcretions the early p cement preserves pore spaces from compaction in the presence of biogenic siliceous particles the fluids are enriched in dissolved silica and diffuse towards the protoconcretions silica precipitation can thus occur later in the intergranular spaces

depositional setting and origin of berthierine oolitic ironstones in the lower miocene terengganu shale tenggol arch offshore peninsular malaysia abstract berthierine oolitic ironstones have been found in the subsurface lower miocene strata of the tenggol arch area offshore peninsular malaysia the ironstones occur within a 30 to 50 m thick laterally extensive unit of very finely laminated to massive dark green mudstone known as the terengganu shale this shale unit was deposited during a regional marine transgression over the tenggol arch which is a fault bounded pre tertiary basement high that separates the cenozoic intracratonic malay and penyu basins at the southwestern margin of the ancestral south china sea the oolitic ironstone beds up to 45 cm thick have sharp erosional bases and are composed of mainly berthierine ooids and pisoids some terrigenous quartz feldspar clasts and locally phosphatic ooids thicker ironstone beds occurring in the lower part of the shale unit show gradation from granule sandstone to oolitic ironstone sedimentological evidence suggests that the oolitic ironstones were deposited as offshore shelf storm layers the close association of terrigenous clastic grains with the berthierine ooids suggests that these ooids were derived from lateritic soils on land transformation of the lateritic soil ooids into berthierine ooids possibly via an intermediate odinite phase took place under reducing early diagenetic conditions like many other ironstone deposits described in the literature the terengganu ironstones were deposited during a period of sea level highstand coincident with a marked reduction in terrigenous coarse grained sediment influx palynological evidence further suggests that deposition of the terengganu shale took place during a dry climatic phase which may have caused the reduction of terrigenous influx into the basin and promoted ironstone accumulation phosphatic ooids are found in the ironstones deposited near the inferred early miocene shelf edge and they are texturally similar to phosphatic ooids described from modern continental margins they were probably formed during early diagenesis near the sediment water interface under anoxic bottom conditions and then were subsequently reworked and mixed with the berthierine ooids the occurrence of phosphatic ooids near the paleo shelf edge suggests that the anoxic bottom conditions favorable for phosphate deposition may have been related to upwelling at the paleo shelf margin

an automatic processing system for broadcasting earthquake alarms in japan most of the damage caused by large earthquakes is concentrated within areas less than about 100 km from the focal regions we have developed an earthquake alarm system that determines earthquake parameters within a few seconds of the p wave s arrival at the closest station and then transmits the earthquake information before the s wave arrival in areas of possible serious earthquake damage since an earthquake alarm system requires the determination of reliable earthquake parameters as quickly as possible it is unreasonable to wait until waveform data from numerous stations have been collected for analysis for this reason we have developed a novel method of determining the hypocentral location by using the arrival times for only a few stations as well as the lack of p wave arrivals at other stations at a given time the use of not yet arrived data makes it possible not only to determine reliable hypocenter parameters within a few seconds but also to detect erroneous arrival time readings and remove them automatically since the available waveform data increases with time the system was designed to redetermine earthquake parameters every second our method was deployed in a real time system starting in july 2002 the real time system locates 10 20 events per day which includes a few felt earthquakes occurring in and around japan it was shown from the waveform data for about 100 felt earthquakes that almost all the events except those far from the network could be located within a few seconds when most of seismic energy has not even arrived at some stations close by we recently started widespread broadcasting of earthquake information in real time by using a satellite transmission system

paleohydrology of lake turkana and its influence on the nile river system abstract the detailed paleohydrological record of lake turkana the largest lake in the eastern branch of the east african rift is necessary for determining the connectivity between adjacent watersheds in tropical africa the migration of both the intertropical convergence zone itcz and the congo air boundary cab constrains rainfall amount and duration for most of east africa lake turkana in northern kenya is the world s largest desert lake and experiences two itcz associated rainy seasons annually with cumulative rainfall of 200 mm yr evidence from new continuous high fidelity sediment core records and high resolution chirp seismic reflection data suggests that lake turkana received enough rainfall during the african humid period ahp to fill the lake to its sill 100 m above current lake level and spilled over into the white nile river system an atmospheric configuration with an eastward shifted cab over the turkana region and the northern kenya rift is invoked as an additional source of rainfall for the catchment this configuration began abruptly at 11 ka and lasted until 5 ka when lake turkana became a closed basin and was cut off from the white nile prior to the ahp lake turkana experienced at least two desiccation events following the last glacial maximum at 18 5 and 17 ka when the lake was at least 100 m lower than modern as evidenced by basin wide high amplitude reflections and 14 c dated shallow water facies in sediment cores lake level fluctuations generally follow trends in mean solar insolation however the onset of lake level extremes are much more abrupt than rates of insolation change lake turkana s location in relation to atmospheric convergence and dynamic rainfall patterns makes its susceptible to extreme climate change over relatively short timescales

crustal thickening and deformation sequence in the footwall to the suture of the variscan belt of northwest spain abstract the sequence of formation and propagation of structures as well as the evolution of crustal thickening in the variscan belt of northwest spain are proposed in this paper the foreland of this mountain belt shows thin skinned tectonics a polyphase deformation with associated regional metamorphism is recognized in the hinterland zones two important culminations the narcea and lugo antiforms are present in an e w cross section through the footwall to the suture of this orogen such culminations are derived from the stacking of several thrust systems generated during two successive periods the sequence of thrust emplacement helps to clarify the formation of different types of structures at the same time in different places in the orogen

facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration examples from lakes turkana and tanganyika abstract two african lakes turkana and tanganyika illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins they demonstrate contrasts between sediments deposited in a semi arid moderately deep mainly volcanic basin turkana and a semi humid extremely deep basin without significant volcanism tanganyika turkana deposits are dominated by rapidly accumulating organic poor terrigenous muds coarse clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers profundal coarse clastic deposition is rare sediments contain little biogenic components except in the sediment starved south basin sediments are also poor in cartonates tanganyika deposits are dominated by organic rich biogenic muds steep fault escarpments and uniform slopes along shelf platform and rift axis lake margins promote the efficient transport of sands into profundal settings total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief backslopping a limited drainage basin area and presettling in upstream lake kivu sediment ponding along margins is rare littoral carbonates are common a consequence of low net clastic sedimentation rates many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief volcanic activity and climate between the two lakes the results of this study suggest that lake deposits formed in tanganyika type rift basins are more appropriate exploration target for hydrocarbon exploration than those from turkana type basins

acoustic stratigraphy of bear lake utah idaho late quaternary sedimentation patterns in a simple half graben a277 kmnetworkofhigh resolutionseismic reflectionprofiles supplementedwithasidescan sonarmosaicofthelakefloor was collected in bear lake utah idaho in order to explore the sedimentary framework of the lake s paleoclimate record the acoustic stratigraphy is tied to a 120 m deep continuously cored drill hole in the lake based on the age model for the drill core the oldest continuously mapped acoustic reflector in the data set has an age of about 100 ka although older sediments were locally imaged the acoustic stratigraphy of the sediments below the lake indicates that the basin developed primarily as a simple half graben with a steep normal fault margin on the east and a flexural margin on the west as expected for a basin controlled by a listric master fault seismic reflections steepen and diverge toward the fault bounding eastward thickening sediment wedges secondary normal faults west of the master fault were imaged beneath the lake and many of these faults show progressively increasing offset with depth and age several faults cut the youngest sediments in the lake as well as the modern lake floor the relative simplicity of the sedimentary sequence is interrupted in the northwestern part of the basin by a unit that is interpreted as a large 410 km paleodelta of the bear river the delta overlies a horizon with an age of about 97 ka outcrops at the lake floor and is onlapped by much of the uppermost sequence of lake sediments a feature interpreted as a wave cut bench occurs in many places on the western side of the lake the base of this bench occurs at a depth 22 24 m similar to that 20 25 m of the distal surface of the paleodelta pinch outs of sedimentary units are common in relatively shallow water on the gentle western margin of the basin and little holocene sediment has accumulated in water depths of less than 30 m on the steep eastern margin of the basin sediments commonly onlap the hanging wall of the east bear lake fault however no major erosional or depositional features suggestive of shoreline processes were observed on acoustic profiles in water deeper than about 20 25 m d 2005 elsevier b v all rights reserved

climatic and tectonic effects on sedimentation in a rift valley lake evidence from high resolution seismic profiles lake turkana kenya approximately 900 km of 28 khz echo sounding and 770 km of 1 khz seismic reflection profiles were obtained from lake turkana in northern kenya between march and december 1984 the profiles penetrate as much as 60 m below the lake floor representing perhaps the last few tens of thousands of years they show that the predominant pattern of sedimentation is one of simple and rapid basin infilling the profiles suggest an abundance of gas probably methane in the holocene sediments particularly near the major rivers and in the deepest basins where sedimentation rates are relatively high high angle growth faults characteristic of extensional regimes are common and indicate that faulting is active today particularly in the central basin of the lake a major erosional unconformity indicates that lake level was about 60 m lower than present lake level probably just prior to 10 000 yr b p when unusually arid conditions prevailed throughout much of east africa

internal structure and kinematics of variscan thrust sheets in the valley of the trubia river cantabrian zone nw spain regional tectonic implications the variscan belt in western europe shows an arcuate geometry that is usually named ibero armorican arc the nucleus of this arc known as the asturian arc comprises the cantabrian zone which is a foreland fold and thrust belt the trubia river area is located in the inflexion zone of the asturian arc which is a strategic structural position for unraveling the geometry and kinematics of the variscan thrust sheets and related folds geological mapping construction of stratigraphic and structural cross sections analysis of kinematic indicators and estimate of shortening for each cross section have been carried out this area consists of two major antiform synform pairs related to two imbricate thrust systems these folds are asymmetric tight and their axial traces follow the trend of the asturian arc they have been interpreted as fault propagation folds the emplacement directions measured in the trubia river area change from north to south and converge towards the core of the asturian arc the minimum shortening estimated ranges between 16 4 and 17 6 km which corresponds to 56 9 and 59 4 the complex cross cutting relationships between folds and thrusts suggest that in general the different structural units followed a forward breaking sequence of emplacement with some breaching and a few out of sequence thrusts the analysis of the transport vectors together with the disposition of the fold axes and post thrusting faults that deform the thrust stack are evidence of a late deformation event that is partially or totally responsible for the arcuate form of the asturian arc the timing of the asturian arc amount of shortening and sequence of emplacement of the structures are in accordance with previous regional studies of the cantabrian zone

the development of a single cleavage in an area of repeated folding abstract structural investigations in the variscan fold interference pattern of sw sardinia italy document an intriguing example of the relationship between cleavage folding and finite strain although palaeozoic sediments were affected by two major folding events both under identical low grade metamorphic conditions only a single penetrative cleavage formed which is not unambiguously correlated with either of these events cleavage orientation changes smoothly along sigmoidally curving trajectories throughout the interference pattern thus the same cleavage can be axial planar to either the first or the second phase folds or transect both when curving from one set into the other the singularity of the cleavage is supported by microscopic studies which show no evidence of a pre existing or overprinting tectonic cleavage finite strain determinations and numerous field observations demonstrate that cleavage always parallels the principal plane of finite strain even though local strain varies according to the relative position in the fold interference pattern similar strain patterns were achieved by forward modelling of superposed buckle folds based on our findings we propose i that the finite strain pattern is due to the superposition of the heterogeneous strain fields associated with the individual folding events and ii that cleavage reflects the cumulative deformations that effected the rock this has consequences for the regional interpretation of cleavage in our study area and elsewhere and for the distinction between deformation phases in general

the ibero armorican arc a collision effect against an irregular continent the ibero armorican arc is the main variscan macrostructure in western europe although it was recognized in the 1920s its genesis is still debatable and the proposed models for its generation questionable however even if some doubts persist it seems that most agree on the general geodynamic evolution of this virgation in both branches in the middle to late devonian iberia was deformed by a sinistral transpressive regime while in the northern branch thrusting events were predominant in the carboniferous a dextral transpression begins to predominate in the armorican branch while the southern branch was deformed by southward thrusting in an attempt to correlate these events we propose that during the late devonian a cantabrian indentor moved northward producing the oblique closure of the southern part of the rheic ocean and an almost orthogonal closure in central europe in the carboniferous the collision with the irregular margin of laurasia induced a rotation of the indentor the intracontinental deformation was then achieved by dextral transpression in the northern branch and thrusting in the southern one

from thickening to extension in the variscan belt kinematic evidence from sardinia italy the variscan nappe stack of se sardinia originated as a result of several stages of nappe imbrication during the lower carboniferous phases of the variscan orogeny the crustal shortening caused regional ssw and w directed thrusting greenschist facies metamorphism and open to isoclinal polyphase folding the final stage of shortening produced large scale antiforms and synforms post collisional deformation resulted in inversion of earlier thrusts as normal faults development of low angle normal faults and refolding of earlier foliation and thrust planes by asymmetric folds with subhorizontal axial planes facing directions of these latest folds are directed horizontally outward from the hinge zones of main antiforms suggesting that they cannot be regarded as parasitic folds of the latest thickening phase but instead are the consequence of vertical shortening during gravitational collapse of dome like km scale antiforms leading to denudation of antiformal culminations

climate change in the turkana basin as deduced from a 4000 year long δo18 record abstract a model of the oxygen isotopic composition of lake turkana waters sheds insight into the paleoclimatic interpretation of a 18 o profile of inorganic calcite from a core taken from the lake the model indicates that high frequency fluctuations seen in the isotopic record are probably due to shifts in lake level while the long term increase of 1 4 seen from 4 to 2 kyr bp cannot be due solely to lake level change it must be due to changes in other climatic parameters such as the isotopic composition of the inflow these findings suggests that the abrupt change observed in lake turkana and many other african lakes at 4 ka was the precursor to a 2000 year long adjustment in the climate over the ethiopian plateau the isotopic record from lake turkana agrees with other paleoclimate records from the basin including the diatom stratigraphy and radiocarbon dates on exposed shoreline deposits

pollen and isotopic records in late holocene sediments from lake turkana kenya abstract a 12 m piston core lt 84 8p taken in the northern basin of lake turkana has been dated by 6 ams radiocarbon dates on large ostracods and analysed for its pollen content during the past 2500 years the pollen assemblages were dominated by chenopodiaceae amaranthaceae cheno am and gramineae representing the local pollen signal of the surrounding vegetation under arid conditions percentage fluctuations of cheno am are interpreted as reflecting lake level variations in good correspondance with oxygen isotope from the same core a rise in lake level is recorded at about 1600 yr b p preceded by a relative lowstand during much of the period from 2500 to 1600 yr b p the top of the core is estimated to be about 600 yr old and shows a shift of unknown duration towards a more humid condition the high lake level documented in this paper roughly coincides with that reconstructed from other cores in the lake based on diatom and carbonate analyses

foreland and hinterland verging structures in fold and thrust belt an example from the variscan foreland of sardinia in the variscan foreland of sw sardinia western mediterranean sea close to the leading edge of the nappe zone nappe emplacement caused folding and repetition of stratigraphic successions km scale offset of stratigraphic boundaries and an extensive brittle ductile shear zone thrusts assumed a significant role accommodating a progressive change of shortening direction and forming complicated thrust triangle zones during thrust emplacement of the nappes strong penetrative deformation affected rocks beneath the basal thrust of the nappe stack and produced coeval structures with both foreland directed and hinterland directed backthrusting shear sense cross cutting and overprinting relationships clearly show that the shortening direction changed progressively from n s to e w producing in sequence 1 e w trending open folds contemporaneous with early nappe emplacement in the nearby nappe zone 2 recumbent quasi isoclinal folds with axial plane foliation and widespread top towards the sw penetrative shearing 3 n s trending folds with axial plane foliation contemporaneous with late nappe emplacement 4 backthrusts and related asymmetrical folds developed during the final stages of shortening postdating foreland verging structures structures at 3 and 4 occurred during the same tectonic transport top towards the e of the nappe zone over the foreland the several generations of folds thrusts and foliations with different orientations developed result in a complex finite structural architecture not completely explicable by the theoretical model proposed up to date

determination of earthquake early warning parameters τ c and pd for southern california summary we explore a practical approach to earthquake early warning in southern california by determining a ground motion period parameter c and a high pass filtered displacement amplitude parameter pd from the initial 3 s of the p waveforms recorded at the southern california seismic network stations for earthquakes with m 4 0 at a given site we estimate the magnitude of an event from c and the peak ground motion velocity pgv from pd the incoming threecomponent signals are recursively converted to ground acceleration velocity and displacement the displacements are recursively filtered with a one way butterworth high pass filter with a cut off frequency of 0 075 hz and a p wave trigger is constantly monitored when a trigger occurs c and pd are computed we found the relationship between c and magnitude m for southern california and between pd and pgv for both southern california and taiwan these two relationships can be used to detect the occurrence of a major earthquake and provide onsite warning in the area around the station where onset of strong ground motion is expected within seconds after the arrival of the p wave when the station density is high the methods can be applied to multistation data to increase the robustness of onsite early warning and to add the regional warning approach in an ideal situation such warnings would be available within 10 s of the origin time of a large earthquake whose subsequent ground motion may last for tens of seconds

east african mid holocene wet dry transition recorded in palaeo shorelines of lake turkana northern kenya rift abstract the wet early to mid holocene of tropical africa with its enhanced monsoon ended with an abrupt shift toward drier conditions and was ultimately replaced by a drier climate that has persisted until the present day the forcing mechanisms the timing and the spatial extent of this major climatic transition are not well understood and remain the subject of ongoing research we have used a detailed palaeo shoreline record from lake turkana kenya to decipher and characterise this marked climatic transition in east africa we present a high precision survey of well preserved palaeo shorelines new radiocarbon ages from shoreline deposits and oxygen isotope measurements on freshwater mollusk shells to elucidate the holocene moisture history from former lake water levels in this climatically sensitive region in combination with previously published data our study shows that during the early holocene the water level in lake turkana was high and the lake overflowed temporarily into the white nile drainage system during the mid holocene 5270 300 cal yr bp however the lake water level fell by 50 m coeval with major episodes of aridity on the african continent a comparison between palaeo hydrological and archaeological data from the turkana basin suggests that the mid holocene climatic transition was associated with fundamental changes in prehistoric cultures highlighting the significance of natural climate variability and associated periods of protracted drought as major environmental stress factors affecting human occupation in the east african rift system

lacustrine to fluvial floodplain deposition in the eocene bridger formation abstract the depositional systems and sedimentology of lithofacies sequences in the bridger formation unit b are described a lithofacies association is determined and includes from base to top limestone claystone thin bedded sandstone and siltstone and a cross bedded sandstone facies the limestone facies is generally composed of a dense resistant limestone that may contain fossil plant impressions tufa and or stromatolites gastropods bivalves and other fossils the claystone facies contains abundant fossil turtles gar crocodiles gastropods and plants desiccation cracks plant roots and soil features are lacking extremely abundant turtles may occur in this facies sometimes associated with abundant coalified plant fragments the thin bedded sandstone and siltstone facies is composed of silty clay to medium sand at a few localities it grades laterally into medium to course grained sandstone channels up to 3 m thick fossil turtles are present but are much less abundant than in the claystone facies the cross bedded sandstone facies is composed of thick laterally extensive channel sandstones associated laterally with silty claystone beds the lithofacies association is interpreted as a shallowing upward sequence it started with a basal limestone facies deposited in a widespread shallow carbonate precipitating lake the lake was rapidly filled by episodic volcaniclastic deposition delivered via air fall and or prograding fluvial deltaic systems deltaic deposition may have been similar to the infilling of the northern portion of lake turkana in africa by the omo river delta the claystone facies is interpreted as a short lived fluvial lacustrine system that prograded out over the limestone facies and or was deposited as air fall ash into the entire lake the sharp upper contact of the limestone facies with the claystone facies suggests that an abrupt paleoenvironmental change may have taken place with a large influx of volcaniclastics an organic rich unit the organic turtle bed within the black mountain turtle layer a claystone facies contains abundant plant material and turtles and is interpreted as representing an episodic depositional event probably storm related as volcaniclastic deposition continued to dominate the system the thin bedded sandstone and siltstone facies sequence was deposited it is interpreted as distributary and crevasse splay sheet sands uniformly deposited laterally correlative over large areas this depositional system was ultimately replaced by a well established fluvial floodplain environment with large fluvial channels representing major meandering rivers development of the succeeding lake system occurred when siliciclastic deposition significantly slowed relative to the rate of basin subsidence deposition in the bridger formation unit b suggests a delicate balance between a closely interacting lacustrine and fluvial system although fluvial processes and clastic sedimentation dominated depositional patterns lacustrine depositional regimes frequently returned

impact of fault parameter uncertainties on earthquake recurrence probability examined by monte carlo simulation an example in central taiwan abstract the region around taiwan is well known for its unique geological setting which has resulted in a high seismicity the central geological survey taiwan cgst has launched a series of investigation on characterizing the active faults on the island the cgst revealed that there are 33 active faults and their fault parameters are suggested in the latest publication in 2010 however the uncertainties of return period and earthquake magnitude are not provided the main objective of the paper is to study the impact of unspecified parameter uncertainties on recurrence earthquake probabilities in addition a numerical approach involving the use of monte carlo simulation was proposed to estimate the exceedance probabilities in which the parameter uncertainties were taken into account the meishan fault in central taiwan with the best estimate return period of 162 years and earthquake magnitude of 7 1 was used as an example the analyses verified the impact of parameter uncertainties on the exceedance probability in fault induced earthquake recurrence when uncertainty is high the exceedance probability associated with large magnitudes is high it was also observed that the probabilistic analysis in this study can generate a realistic continuous distribution of exceedance probabilities that cannot be obtained by a deterministic approach the best estimate distribution of exceedance probability was established by enveloping the curves estimated by both deterministic and probabilistic approaches accordingly the best estimate exceedance probability for a 7 5 magnitude earthquake recurring in 2011 2060 induced by the meishan fault in central taiwan was estimated as 11

geochemical evolution of closed basin lakes general model and application to lakes qinghai and turkana in contrast to most previous models for the evolution of closed basin lakes we present an integrated model which considers various water budget patterns clay regradation so4 reduction and subbottom leakage in addition to the classical equilibrium approach of mineral precipitation the model was applied to lakes qinghai and turkana which significantly differ in the lithologies of their drainage areas but are representative of the carbonate rich sedimentary rock province of the tibet qinghai plateau and the silicate rock province of eastern africa both lakes are now topographically closed but to some degree hydrologically open subbottom leakage major results of the mode calculations show that the lithology controls the ultimate brine which is of na k cl type for qinghai lake and of na hco3 cl type for lake turkana so4 reduction delays the onset of sulfatic mineral precipitation and favours the formation of na carbonates such as trona at the expense of calcite clay mineral regradation plays an important role before the saturation of sulfatic or chlorine minerals is reached in particular magnesite formation may be in competition with mg bearing clay minerals finally simulations with various hydrological scenarios have shown that the modern hydrochemistry of both lakes cannot be reproduced by simply evaporating inflow water but reflects long term accumulation and evolution of solutes by continuous inflow over several thousand years the diversity of lake water composition within a uniform lithological province can thus be largely ascribed to varying hydrological conditions

modelling earthquake rupture rates in fault systems for seismic hazard assessment the eastern betics shear zone abstract earthquake surface fault ruptures can show very complex geometries and involve different faults simultaneously consequently modern fault based probabilistic seismic hazard assessments psha need to account for such complexities in order to achieve more realistic modellings that treat fault systems as a whole and consider the occurrence of earthquake ruptures as aleatory uncertainties we use sherifs a recent approach of modelling annual rates of complex multi fault ruptures to obtain system level magnitude frequency distributions mfds for the eastern betics shear zone ebsz spain considering four fault rupture hypotheses we then analyze the consistency of each scenario based on data from the earthquake catalogue and paleoseismic studies the definition of the different rupture hypotheses was discussed within the frame of fault2sha esc working group and critical fault input data is extracted from previous published studies the four rupture hypotheses are defined as incremental scenarios based on fault geometry and kinematics with lengths varying from minimal fault sections to a rupture of nearly the whole system the results suggest that multi fault ruptures involving lengths up to single to several whole faults are consistent with the annual rates from both the instrumental catalogue and paleoseismic record the method does not allow to completely discard any hypothesis but it allows to weight the different models in a logic tree for seismic hazard assessment the approach is revealed as a practical tool for obtaining fault system mfds and as a useful tool for highlighting limitations and uncertainties in geological and paleoseismic data to be assessed this study aims to constitute a step forward in the consideration of complex multi fault ruptures for future seismic hazard assessments in the region

change of nappe transport direction during the variscan collisional evolution of central southern sardinia italy abstract the collisional variscan early carboniferous evolution of the greenschist facies metamorphic basement of sardinia is characterized by three deformation phases and two 90 reorientations of shortening direction during the first phase of deformation n s shortening occurred with south facing isoclinal folding south directed nappe emplacement and mylonitization in central and southeastern sardinia and large scale e w striking upright folds in southwestern sardinia emplacement of the barbagia meana sardo gerrei and riu gruppa units took place during this event this deformation phase was followed by e w shortening with w directed emplacement of the sarrabus and arburese units along the villasalto thrust and upright to e dipping large scale n s striking upright folds in southwestern sardinia during the third deformation event the shortening direction again rotated 90 and n s shortening produced kilometer scale antiforms and synforms the most prominent features in the metamorphic basement of sardinia the limbs of the kilometer scale antiforms and synforms are vertically shortened during gravitational collapse with contemporaneous faulting and conjugate shear zone development presence of large scale e w oriented antiforms was therefore critical for the formation of late variscan extensional features such as e w striking low angle normal faulting

petroleum geology of cretaceous tertiary rift basins in niger chad and central african republic this overview of the petroleum geology of rift basins in niger chad and central african republic car is based on exploration work by exxon and partners in the years 1969 1991 the work included 50 000 km of modern reflection seismic 53 exploration wells 1 000 000 km2 of aeromagnetic coverage and about 10 500 km of gravity profiles the results outline ten cretaceous and tertiary rift basins which constitute a major part of the west and central african rift system wcars the rift basins derive from a multiphased geologic history dating from the pan african approximately 750 550 ma to the holocene main rifting was probably caused by the stretching and subsidence of african crustal blocks during the breakup of gondwana in the early cretaceous and by r activated plate movements in the early tertiary wcars in the study area is divided into the west african rift subsystem was and the central african rift subsystem cas was basins in niger and chad are chiefly extensional and are filled by up to 13 000 m of lower cretaceous to holocene continental and marine clastics the basins contain five oil 19 43 api and two oil and gas accumulations in upper cretaceous and eocene sandstone reservoirs the hydrocarbons are sourced and sealed by upper cretaceous and eocene marine and lacustrine shales the most common structural styles and hydrocarbon traps usually are associated with normal fault blocks cas rift basins in chad and car are extensional and transtensional and are filled by up to 7500 m of chiefly lower cretaceous continental clastics the basins contain eight oil 15 39 api and one oil and gas discovery in lower and upper cretaceous sandstone reservoirs the hydrocarbons are sourced by lower cretaceous shales and sealed by interbedded lacustrine and flood plain shales structural styles range from simple fault blocks through complex flower structures the main hydrocarbon traps are in contractional anticlines geological conditions favor the discovery of potentially commercial volumes of oil in wcars basins of niger chad and car

upper permian lacustrine organic facies evolution southern junggar basin nw china abstract the jingjingzigou lucaogou and hongyanchi formations of the southern junggar basin together comprise over 1000 m of organic rich lacustrine facies they record an overall evolution from relatively shallow evaporative lakes to freshwater lakes associated with fluvial systems the jingjingzigou formation is characterized by low toc and hi and biomarker features such as abundant carotane consistent with a specialized saline or hypersaline biota biomarker distributions in jingjingzigou formation extracts most closely resemble oils from the giant karamay oilfield the lucaogou formation represents one of the richest and thickest lacustrine source rock intervals in the world yet it contradicts conventional lacustrine source rock models in at least two important aspects first deposition occurred at middle paleolatitudes 39 43 n rather than in the tropics second limited nutrient supply in a drainage basin dominated by intermediate volcanic rocks appears to have caused low to moderate primary productivities stable salinity stratification and low inorganic sedimentation rates in a deep lake nonetheless resulted in deposits with up to 20 toc and hi near 800 the overlying hongyanchi formation has 1 5 toc but low hi and was deposited in freshwater oxic to sub oxic lakes

subsurface seepage of seawater across a barrier a source of water and salt to peripheral salt basins many of the great phanerozoic salt basins share a common paleogeography in which a deep lake or sea is separated from the ocean by a narrow barrier examples include the aptian evaporites of the south atlantic rift basins and the messinian salt deposits of the mediterranean sea and the north caspian depression marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins we test an alternate hypothesis that subsurface seepage of seawater through the barrier was a significant source of water influencing both water level and water composition our model permits flux of water and salt into and out of the basin via 1 seepage through the barrier 2 evaporation from the water surface 3 rainfall and or rivers and 4 incorporation into basin sediments our investigation focuses on three settings a modern analog in the gregory rift of northern kenya the messinian mediterranean basin and the early cretaceous aptian salt basins of the south atlantic we test our model through a numerical simulation applying reasonable values for the dimensions of the barrier differences in water level and hydraulic conductivities to calculate flow rates through the barrier modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant depending on the climate size of the peripheral basin and hydraulic conductivity of the barrier the flux of salt is significant over the full range of modeled hydraulic conductivities producing brines in the peripheral lake and or sea within 0 5 m y in most cases this demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions and that marine transgressions across the barrier are not necessary to explain the apparent high salinities

mechanics of triangle zones and passive roof duplexes implications of finite element models abstract finite element models of passive roof duplexes simulate the evolution and distribution of fault slip and deformation as functions of boundary conditions friction on the roof fault cover strength and amount of overburden a foreland buttress is essential for backthrust displacement in the models this foreland buttress is analogous to a pin line between the cover and footwall forelandward of the triangle zone in a balanced cross section and could be caused by a stratigraphic pinchout of the upper detachment or a strong cover in the foreland with boundary conditions that prevent the cover from slipping toward the hinterland relative to the wedge incipient forethrust and backthrust ramps zones of high equivalent plastic strain cut through the cover in the models there is more backthrust displacement with a lower friction coefficient on the upper detachment and with a stronger cover because of pressure dependence both fault strength and material strength increase with increasing overburden therefore the effect of overburden on backthrust displacement is determined by the nature of the pressure dependencies for fault friction and material strength in models with multiple imbricate faults the distribution of equivalent plastic strain adjacent to each fault is similar to that in single fault models with differences resulting from bending strains in the wedge and cover the strain distribution in the cover of the models is similar to that above natural passive roof duplexes characterized by layer parallel shortening above the tip of each horse in the duplex

estimation of paleostress orientation within deformation zones between two mobile plates the orientation of the principal stress axes within deformation zones between two mobile plates is modeled here analytically using a thin plate theory the simple analytical approach helps to explain why plates cease to move after collision orogenic periods last only several tens of million years because the stress associated with a particular constant driving force causing a constant strain rate is no longer able to maintain a significant horizontal displacement in contrast the uplift rate increases rapidly as the horizontal velocity decreases and this may explain why the termination of orogenic epochs are usually heralded by the rapid deposition of thick sequences of immature sedimentary rocks or flysch the analytical model also elaborates the relationship between homogeneous bulk deformations driven by a constant stress orientation and those due to a fixed displacement direction of physical boundaries of the deformation zone two major kinematic possibilities are considered 1 plates converging either orthogonally or obliquely making the deformation zone an analogue for orogenic collision and 2 plates diverging either orthogonally or obliquely so that the deformation zone is dynamically similar to initiating rift basins the theoretical investigation led to the formulation of the following rules deformation zones between converging plates have the major axis of bulk deviatoric stress coinciding with the bisector of the acute angle between the relative plate velocity vector and the normal to the deformation zone boundary in case of extension within a deformation or rift zone separating diverging plates the bisector will outline the minor axis of the bulk deviatoric stress the deformation tensor of the analytical model yields a new method for estimating the orientation of paleostress in natural examples here applied to the deformed wall rock of the moroccan border fault the marker used is a competent sequence of devonian sandstone and limestone asymmetrically folded adjacent to the dextral border fault the steeply plunging z folds of the marker beds suggest that the principal deviatoric paleostress 1 was oriented 37 44 to the fault trace the age of the moroccan border fault is poorly constrained and may be variscan or younger the 1 orientation implies a major component of simple shear parallel to the strike slip fault and a minor component of extension perpendicular to the fault trace the implied crustal movement is compatible with the modern tectonics of the eurasian african collision zone where part of the differential motion between the eurasian and african plates is accommodated by strike slip faults

extreme value distribution of earthquake magnitude abstract probability distribution of maximum earthquake magnitude is first derived for an unspecified probability distribution of earthquake magnitude a model for energy release of large earthquakes similar to that of adler lomnitz and lomnitz is introduced from which the probability distribution of earthquake magnitude is obtained an extensive set of world data for shallow earthquakes covering the period from 1904 to 1980 is used to determine the parameters of the probability distribution of maximum earthquake magnitude because of the special form of probability distribution of earthquake magnitude a simple iterative scheme is devised to facilitate the estimation of these parameters by the method of least squares the agreement between the empirical and derived probability distributions of maximum earthquake magnitude is excellent

late pleistocene holocene rise and collapse of lake suguta northern kenya rift abstract the late pleistocene to middle holocene african humid period ahp was characterized by dramatic hydrologic fluctuations in the tropics a better knowledge of the timing spatial extent and magnitude of these hydrological fluctuations is essential to decipher the climate forcing mechanisms that controlled them the suguta valley 2 n northern kenya rift has recorded extreme environmental changes during the ahp extensive outcrops of lacustrine sediments ubiquitous wave cut notches shorelines and broad terrace treads along the valley margins are the vestiges of lake suguta which once filled an 80 km long and 20 km wide volcano tectonic depression lake suguta was deep between 16 5 and 8 5 cal ka bp during its maximum highstand it attained a water depth of ca 300 m a surface area of ca 2150 km2 and a volume of ca 390 km3 the spatial distribution of lake sediments the elevation of palaeo shorelines and other geomorphic evidences suggest that palaeo lake suguta had an overflow towards the turkana basin to the north after 8 5 cal ka bp lake suguta abruptly disappeared a comparison of the lake suguta water level curve with other reconstructed water levels from the northern part of the east african rift system shows that local insolation which is dominated by precessional cycles may have controlled the timing of lake highstands in this region our data show that changes of lake levels close to the equator seem to be driven by fluctuations of spring insolation while fluctuations north of the equator are apparently related to variations in summer insolation however since these inferred timings of lake level changes are mostly based on the radiocarbon dating of carbonate shells which may have been affected by a local age reservoir alternative dating methods are needed to support this regional synthesis between 12 7 and 11 8 cal ka bp approximately during the northern hemisphere high latitude younger dryas the water level of lake suguta fell by ca 50 m suggesting that remote influences also affected local hydrology

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs

three dimensional geometry of large scale fault propagation folds in the cantabrian zone nw iberian peninsula abstract in the western part of the cantabrian zone a transition occurs from a southern zone mainly dominated by thrusts to a northern zone where folds predominate detailed structural analysis within the transitional zone reveals that large scale fault propagation folds constitute the structural framework of this area the topography together with variations in plunge of the fold axes allows us to visualize their three dimensional geometry they exhibit a complex internal structure due to thrusting before during and after major folding thrusts are the structures that accommodate most of the shortening in deep levels whereas the shortening is accommodated stratigraphically upward by major fault propagation folds imbricate thrust systems and minor structures in general during the initial stages of fold amplification the backlimb of the antiforms thickened whereas during the latter stages their forelimbs thinned after folding the major folds were translated and rotated due to emplacement of younger thrusts located in the foreland although these natural large scale fault propagation folds are more complex than many theoretical models proposed up to date certain specific structural features observed in the field examples can be explained by comparing them with theoretical models

initiation and reactivation of faults during movement over a thrust fault ramp numerical mechanical models abstract with numerical continuum models we investigate the effects of fault geometry fault friction material properties and anisotropy on the initiation and reactivation of faults in the hanging wall of a thrust fault ramp the models use an elastic plastic frictional dilatant cohesion softening material in which deformation may localize as shear bands with a sharp lower fault bend backthrust shear bands propagate up from the concentration of differential stress at the fault bend with a rounded lower fault bend bedding parallel shear bands develop above the fault bend in the center of the layer where differential stress is highest friction on the ramp enhances the development of backthrusts and causes them to initiate at shallower dips increased overburden decreases the amount of localization within shear bands and the spacing between them shear bands that develop at the lower fault bend are weaker than the surrounding material and are thus potential sites of reactivation during subsequent deformation because of this potential reactivation the style of deformation at the upper fault bend depends on the deformation that accumulates at the lower fault bend at the upper fault bend backthrusts are reactivated as extension faults and are crosscut by more steeply dipping extension faults bedding parallel shear bands are reactivated at the upper fault bend with top to the hinterland sense of shear low angle extension faults are listric into the reactivated bedding parallel shear bands producing hinterland verging extensional duplexes above the upper flat

lithofacies characteristics of diatreme deposits examples from a basaltic volcanic field of sw sardinia italy abstract a deeply eroded diatreme field consisting in several decametric sized vertical mainly clastic volcanic bodies of basaltic composition is described for the first time in the variscan basement of sw sardinia the recognition and description of four different lithofacies in these diatremes allowed discussion of the role of the different processes which control magma eruption and conduit infilling and making general inferences about diatremes the studied diatremes have a cross sectional shape from elliptical to sub triangular and are slightly elongated nearly parallel to the main foliation of the intruded meta sedimentary rocks foliation of host rocks is locally reoriented or folded close to the contact with the diatremes suggesting that magma possibly rose to the surface through fissures oriented nearly parallel to host rock foliation textural features of the volcanic bodies show many analogies with kimberlitic diatremes despite the difference in petrography and composition juvenile lapilli are mainly made by ghosts of mafic phenocrysts olivine and clinopyroxene set in a groundmass formed by plagioclase microlites immersed in a cryptocrystalline chlorite rich matrix the four lithofacies were described mainly based on the shape and physical features of the clasts and textural anisotropy a globular juvenile rich lapilli tuff facies gjlt an angular juvenile rich lapilli tuff facies ajlt a lithic rich lapilli tuff facies lirlt and a coherent lava like facies coh all the clastic lithofacies are generally well sorted and typically lack a fine grained matrix juvenile fragments are lapilli sized and from equant to oblate in axial ratio and from rounded globular to very angular in shape conversely lithic clasts are largely variable in shape and size and are mainly represented by basement derived clasts the absence of bedding the scarcity of the coherent facies and the dominance of clast supported structureless volcaniclastic facies suggest that the outcropping portion of these volcanic bodies represents the lower diatreme zone the presence of diffuse welding and the globular shapes of some juvenile fragments together with their vesicularity suggest that magma fragmentation was mainly driven by magmatic gas exsolution occurring at a deeper level respect to classical basaltic explosive activity textural features facies association and facies architecture of the studied deposits are suggestive of an important affinity with kimberlitic and other ultramafic diatremes

depositional subenvironments in an arid closed basin the wilkins peak member of the green river formation eocene wyoming u s a the wilkins peak member of the green river formation is a non marine dolomitic deposit that formed in an arid hydrologically closed basin a transect from the margin to the centre of the wilkins peak basin was examined in outcrop exposures in the south eastern corner of the basin and was supplemented with published core descriptions the rocks were divided into seven facies based upon lithology layering style and sedimentary fabrics 1 af facies quartzose conglomerates and sandstones representing sandy alluvial fans coming from the uinta mountains 2 sf facies quartzose peloidal dolomitic sandstones which represent sheet flood deposits extending basinward from the alluvial fans 3 dm facies irregularly laminated dolomitic mudstones with abundant mudcracks which represent sheet flood deposits on a subaerially exposed mudflat 4 el facies dolomitic mudstones with regular lamination and fewer but larger mudcracks representing sporadically exposed lake margins or shallow ponds 5 pl facies finely laminated oil shales and dolomitic mudstones with rare mudcracks which represent shallow perennial lakes 6 sm facies trona or halite deposits with dolomitic mudstone partings or dolomitic mudstones and sandstones disrupted by evaporites of varied morphology these represent a central salt pan and the surrounding saline mudflat 7 as facies arkosic sandstones siltstones and mudstones which represent braided river and more basinal deltaic plain deposits the five dolomitic facies form poorly defined cycles representing the intersection of sheet floods with a lake which transgressed and regressed in response to climatic variations there are at least forty 3 4 m thick cycles in the wilkins peak and one large scale cycle comprising the whole member which mimics the facies changes of the smaller cycles this is compatible with the transitions of the wilkins peak member to the tipton and laney members as climatic responses and not shifts in drainage the sheet flood sandstones actively aggrading subaerial mudflats and bedded evaporites of the wilkins peak are apparently unique within the green river formation they reflect the unusual provenance and hydrology of the wilkins peak and suggest chemical and sedimentary models established for the other green river members will not be applicable

calcite twins their geometry appearance and significance as stress strain markers and indicators of tectonic regime a review twin lamellae are a widespread feature in calcites of any type and origin twinning in calcite is an important intracrystalline deformation mechanism with low critical resolved shear stress and virtually no temperature dependence in contrast to most slip systems in rock forming minerals twinning therefore permits intracrystalline deformation at very low temperature and very little confining pressure twinning alone however cannot lead to large strains because only one independent slip system is effectively available twins lead to discontinuous shearing on the grain scale which gives rise to considerable strain incompatibilities at grain boundaries twinning consequently leads to strain hardening detailed observations of the geometry of twin lamellae show that incompatibilities at grain boundaries are eliminated by pressure solution on the grain scale at low temperature and dynamic recovery processes at higher temperatures twins are found in various tectonic environments and record very small strains in rocks deformed under very little cover different twinning strain and twinning stress methods and palaeopiezometers are critically reviewed all methods are limited to strains of less than 15 shortening or extension by twinning twinning nevertheless is an important although not directly measurable contribution to the total strain in highly strained calcite rocks at least up to middle greenschist facies under which conditions it is the most effective contributor to the formation of strong lattice preferred orientations in calcite tectonites the appearance of calcite twin lamellae changes systematically as a function of deformation temperature micro twins and straight narrow 1 5 m but fewer twins are developed above approximately 200 c curved twins twinned twins and completely twinned grains indicate the progressive importance of other slip systems and larger intracrystalline strains are possible at ca 250 c and above ancient straight twin lamellae are modified into irregular geometries by recrystallization and grain boundary migration the appearance of calcite twins is proposed as an approximate but rapid and easy to use geothermometer in low temperature environments

geometry and kinematics of vertical axis rotations in fold and thrust belts abstract vertical axis rotations indicated by palaeomagnetic study are a common and important component of deformation in fold and thrust belts the kinematics of rotating thrust sheets may be best described as a combination of a far field rotation or translation in a planar geometry and a near field or local rotation so that the instantaneous rotation axis lies outside the body of the sheet this simplifies the thrust sheet kinematics significantly as it means that all the points on the sheet have a component of forward motion and the bounding fault is a thrust over all its area the consequences of a simple thrust sheet rotation for the interpretation of fault lineation data are investigated the lineations recorded in the footwall and the hangingwall are likely to be different the hangingwall lineations being skewed towards the direction of rotation the principal mechanism for local thrust sheet rotation is differential shortening either on folds or thrusts or by a combination of these structures the forward propagation of thrusting towards the foreland can accumulate large 120 rotations on the uppermost sheets these rotational thrust sheets cannot extend indefinitely along strike instead they terminate against lateral ramps or transfer faults sidewall duplexes may form as a consequence of rotation over a lateral ramp examples of rotational geometries from the subbetic zone of southern spain are described on a larger scale obliquely convergent orogenic wedges can evolve by rotation of faults with an initial thrust geometry into progressively greater components of strike slip motion reliable palaeomagnetic data are not always available for fold and thrust belts so independent evidence for rotational deformation is of great importance fold and thrust geometries particularly along strike variability in fold geometries highly variable axial traces of folds evidence for near synchronous refolding and the deviation of regional trends may all be a consequence of thrust sheet rotation detailed mapping and particularly the identification of the position of footwall and hangingwall cut offs can also provide strong evidence for rotation

controls of sedimentation in the malawi rift valley central africa abstract the malawi rift is the southermost classic rift valley trough in the cenozoic african rift valley system much of its floor is flooded by lake malawi which is approximately 570 km long 60 km across and up to 500 m deep this rift basin is a potentially useful analogue of many economically important ancient sedimentary troughs rift valley structures are extremely important in controlling both the distribution of sediments in the basin as a whole and also details of sedimentation on a local scale on the basin wide scale an important association between the type of rift boundary structure and amount of rift floor subsidence is recognised this association appears to apply to several section of the african rift system pre rift topography is major control on the position of major rivers and their deltas or turbidite fans along the basin margin the area of pre rift drainage network captured in the course of rift movements is a significant factor in the hydrologic budget of the basin climate is however the dominant control on the hydrologic budget and at present the basin is flooded to overflow level prevailing winds are important in beach ridge and back beach aeolian dune development climatic conditions produce thermal stratification of the lake with an oxic epilimnion down to 250 m depth overlying an anoxic hypolimnion the oxic bottom waters in the shallower parts of the lake appear to promote iron precipitation in the sediments