geometric and kinematic models for detachment folds with fixed and variable detachment depths abstract detachment folds are defined by competent rock units and are cored by incompetent units deformed internally above a detachment horizon we have developed two geometric models to constrain possible geometries and kinematic paths for ideal detachment folds the models each independently relate fold geometry to shortening and to detachment depth model assumptions include plane strain constant competent bedlength constant cross sectional area chevron fold geometry and no bed parallel shear outside the fold detachment depth is constant in one model but may vary in the other thus allowing evaluation of the implications for fold geometry and kinematics of fixed vs variable detachment depth detachment folds formed above a detachment unit of constant thickness constant detachment depth must be initially symmetrical and cannot grow with fixed hinges fixed arc length or a self similar geometry detachment folds formed above a detachment unit of variable thickness variable detachment depth must also be initially symmetrical but any one fold geometry can have a range of possible initial and final detachment depths kinematic paths for folds with fixed hinges fixed arc lengths migrating hinges variable arc lengths and selfsimilar geometries are all possible if detachment depth varies the change in detachment depth during deformation can be determined using the variable detachment depth model if either initial or final detachment depth is known the models demonstrate a wider range of variability in the geometry and kinematics of ideal detachment folds particularly for the variable depth model than is the case for ideal fault bend and fault propagation folds this variability limits the usefulness of simple geometric models for reconstructing the geometry of natural detachment folds balancing cross sections over a sufficient area and evaluating strain may compensate for these limitations

pan african volcanism petrology and geochemistry of the dokhan volcanic suite in the northern nubian shield the late proterozoic dokhan volcanic suite 620 ma of the northern nubian shield is the product of late pan african volcanism the suite covers the entire spectrum from basalt to high silica rhyolite and occurs as two units a dark coloured unit containing basalt andesite dacite and a light coloured unit encompassing dacite rhyodacite rhyolite the latter unit is made up largely of ash flow tuffs and ign imbrites that are locally interstratified with basalt and andesite lava flows the suite forms a continuum in composition with a wide range of si0 2 48 77 wt cao 0 1 8 9 wt sr 81 906 ppm zr 85 340 ppm as well as most other elements and is moderately enriched in incompatible elements including rare earth elements ree the suite exhibits fractionated subparallel ree patterns that are similar overall to andean andesites and ignimbrites well defined major and trace element trends and fractionated ree profiles are consistent with a fractionated basalt to rhyolite calc alkaline magma series it is a typical calc alka line orogenic complex and exhibits mineralogical geochemical traits of arc related volcanism the suite neither resembles products of extensional nor transitional tectonic regimes as previously thought but was produced in a subduction related tectonic environment the mafic nature of the least evolved rocks of the suite along with its relatively low initial 87 sr 86 sr ratio 0 7039 are considered to indicate a mantle source a mantle derived basaltic magma fractionated with amphibole and plagioclase dominating the fractionating assemblage to produce the more felsic varieties as suggested by major and trace element fractionation modelling

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

kinematic models of basement cover interaction insights from the malargue fold and thrust belt mendoza argentina abstract we investigate through natural examples from the malargue fold and thrust belt the manner in which the basement structures interact with the cover in a thick skinned fold and thrust belt our work suggests that there is a broad spectrum of basement cover interaction mechanisms with two end members coupled and uncoupled interactions the kind of bonding between basement and cover is related to the propagation to slip ratio p s because high values of this ratio indicate rapid forward propagation of shortening into the cover while low values favour welding between basement and cover structures we analyse the factors controlling the kinematics of basement cover deformation and find that not only the strength of the cover its thickness and its mechanical anisotropy control the p s and basement cover bonding but also the strain rate the thickness of synorogenic strata and the presence of pre existing faults

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

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

geometry and kinematics of fault propagation folds with variable interlimb angle abstract several conceptual approaches have been proposed to account for the development of fault propagation folds whose geometry and kinematics depend on the amount of displacement along a basal decollement level the ramp angle and the slip to propagation ratio among these the variable interlimb angle model of mitra 1990 is able to explain open and close natural folds but its application is limited because the fold geometry and bed thickness evolution rely on imposed parameters that cannot be measured directly here we use the ramp and the interlimb angles as input data to develop a forward fold model that accounts for thickness variations in the forelimb the relationship between the fold amplitude and fold wavelength is subsequently applied to construct balanced geological cross sections from surface parameters only and to propose a kinematic restoration of the folding through time the model can catter for a wide variety of folds reconstruct the deep architecture of anticlines and deduce the kinematic evolution of the folding with time we consider three natural examples to validate the variable interlimb angle model along strike thickness variation in the forelimb of the turner valley anticline in the alberta foothills of canada precisely corresponds to the theoretical values proposed by our model reconstruction at depth of the alima anticline in the southern tunisian atlas implies that the decollement level is localised in the triassic liassic series as highlighted by seismic imaging the kinematic reconstruction of the ucero anticline in the spanish castilian mountains is also in agreement with the fold geometry derived from two cross sections the variable interlimb angle model predicts that the fault propagation fold can be symmetric normal asymmetric with a greater dip value in the forelimb than in the backlimb or reverse asymmetric with greater dip in the backlimb depending on the shortening amount

fold and thrust systems in mass transport deposits abstract improvements in seismic reflection data from gravity driven fold and thrust systems developed in offshore mass transport deposits mtds reveal a number of significant features relating to displacement along thrusts however the data are still limited by the resolution of the seismic method and are unable to provide detail of local fold and thrust processes investigation of exceptional gravity driven contractional structures forming part of mtds in lacustrine deposits of the dead sea basin enables us to present the first detailed outcrop analysis of fold and thrust systems cutting unlithified soft sediments we employ a range of established geometric techniques to our case study including dip isogons fault propagation fold charts and displacement distance diagrams previously developed for investigation of thrusts and folds in lithified rocks fault propagation folds in unlithified sediments display tighter interlimb angles compared to models developed for lithified sequences values of stretch which compares the relative thickness of equivalent hangingwall and footwall sequences measured along the fault plane may be as low as only 0 3 which is significantly less than the minimum 0 5 values reported from thrusts cutting lithified rocks and reflects the extreme variation in stratigraphic thickness around thrust related folds we suggest that the simple shear component of deformation in unlithified sediments may modify the forelimb thickness and interlimb angles to a greater extent than in lithified rocks the average spacing of thrust ramps and the thickness of the thrust sequence display an approximate 5 1 ratio across a range of scales in mtds in general thicker hangingwall and footwall sequences occur with larger thrust displacements although displacement patterns on thrusts cutting unlithified yet cohesive sediments are more variable than those in lithified rocks line length restoration of thrust systems in mtds reveals 42 shortening which reduces to 35 in overlying beds a 23 reduction in shortening by folding and thrusting along individual thrusts suggests that heterogeneous lateral compaction may increase by 10 towards the sediment surface thrust systems cutting unlithified sediments display distinct steps in cumulative displacement distance plots representing increased rates of slip along the floor thrust while displacement distance plots along individual thrusts also reveal horizontal steps relating to lithological variation competent units cut by thrust ramps may display the greatest displacement which then progressively reduces both upward and sometimes downward along the ramp this relationship demonstrates that ramps do not necessarily propagate upwards from the underlying flat as in some traditional models but rather initiate by offset of competent horizons in the hangingwall of the detachment critical taper angles in mtds may be an order of magnitude less than in accretionary complexes or lithified rocks overall thrusts cutting unlithified sediments in mtds display more variable displacement and more pronounced displacement gradients toward fault tips compared to thrusts cutting lithified sequences

geochemistry of gabbros and granitoids m and i types from the nubian shield of egypt roots of neoproterozoic intra oceanic island arc abstract the neoproterozoic intrusive rocks of the wadi ranga area nubian shield of egypt comprise gabbros and granitoids emplaced during oceanic island arc and post collision stages the plutonic rocks of the island arc stage include hornblende gabbros dabbah pluton trondhjemite abu ghalaga pluton and tonalites with subordinate quartz gabbro and quartz diorite reidi and abu ghusun plutons whereas the post collision intrusives include granodiorite and monzogranite helifi hamata pluton the gabbros and granitoids of the island arc stage are largely calcic low k rocks which have either tholeiitic gabbro and trondhjemite or transitional tholeiitic to calcalkaline nature tonalites on the other hand the granitoids of the post collision stage are medium to high k calcalkaline rocks all the investigated granitoids are metaluminous the spider diagrams with enrichment in lile and strong nb depletion and the almost flat to slightly lree depleted ree patterns of the gabbro and trondhjemite are similar to those of the wadi ranga low k tholeiitic basalts and silicic volcanics respectively suggesting that the gabbro and trondhjemite are the plutonic equivalents of the wadi ranga immature island arc extrusives and they were derived from mantle source at the early immature island arc stage similar to the trondhjemite the tonalites show lile enrichment and strong nb depletions on the morb normalized spider diagrams however the tonalites have ree patterns which are enriched in lree la yb 1 71 5 54 the derivation of the tonalites through fractionation of the same magma produced the trondhjemite seems unlikely therefore high degree partial melting of juvenile basaltic arc crust is favoured for the origin of tonalites during a late immature island arc stage the post collision granitoids show considerable enrichment in lile and to a lesser extent in hfse slight negative nb anomaly and strong negative p and ti anomalies relative to n morb their ree patterns are lree enriched la yb 5 19 with negative eu anomaly these characteristics are consistent with origin through lower degrees of partial melting of old basaltic arc crust and subsequent fractional crystallization the geochemical characteristics of the trondhjemite and tonalites and the granodiorite monzogranite classify them as m type and i type granitoids respectively the partly tholeiitic intrusives of the wadi ranga area south eastern desert have lower k2o rb and lree compared to the m type calcalkaline intrusives of the north eastern desert implying northwardly dipping subduction zone the geochemical similarities between the intrusives of neoproterozoic and phanerozoic oceanic island arcs imply that they share similar style of subduction which differs from that of the archaean the generation of high sio2 up to 74 5 wt low k2o 0 56 1 78 wt and slightly lree depleted trondhjemite in early immature oceanic island arc setting supports the arc origin of the primitive continental crust silicic magma production through partial melting of the early arc volcanic rocks during the evolution of the arc and the post collision stage drives the middle and upper oceanic arc crust towards a composition closer to that of the continental crust the present study indicates that the intra oceanic island arcs continued to play a role in the generation of the continental crust after the archaean

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

electromagnetic induction in a spherical earth with non uniform oceans and continents in electric contact with the underlying medium ii bimodal global geomagnetic sounding of the lithosphere summary the vertical electric currents flowing between the inhomogeneous surface layer and the conductive layers of the earth s mantle are substantially dependent upon the transverse resistivity of the earth s crust and upper mantle although these currents do not contribute straightforwardly to a surface magnetic field they change the vortex sub surface currents which contribute to it thus there exists an indirect influence of leakage currents upon the magnetic field this makes it possible to evaluate resistance of the high ohmic layers which could not be evaluated if the earth was laterally homogeneous and only the induction mode of the field was induced this paper presents the first results of the bimodal sounding based on s variation data an estimate of the lithosphere s transverse resistance t 3 x lo9 51 m2 has been obtained

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

earthquake slip estimation from the scarp geometry of himalayan frontal thrust western himalaya implications for seismic hazard assessment geometric and kinematic analyses of minor thrusts and folds which record earthquakes between 1200 ad and 1700 ad were performed for two trench sites rampur ghanda and ramnagar located across the himalayan frontal thrust hft in the western indian himalaya the present study aims to re evaluate the slip estimate of these two trench sites by establishing a link between scarp geometry displacements observed very close to the surface and slip at deeper levels as geometry of the active thrust beneath the scarp is unknown we develop a parametric study to understand the origin of the scarp surface and to estimate the influence of ramp dip the shortening estimates of rampur ghanda trench by line length budget and distance displacement d d method show values of 23 and 10 15 respectively the estimate inferred from the later method is less than the line length budget suggesting a small internal deformation ramnagar trench shows 12 shortening by line length budget and 10 25 by the d d method suggesting a large internal deformation a parametric study at the trenched fault zone of rampur ghanda shows a slip of 16 m beneath the trailing edge of the scarp and it is sufficient to raise a 8 m high scarp this implies that the rampur ghanda scarp is balanced with a single event with 7 8 m coseismic slip in the trenched fault zone at the toe of the scarp 8 15 mean deformation within the scarp and 16 m slip at depth along a 30 ramp for a pre 1400 earthquake event a 16 m slip is the most robust estimate of the maximum slip for a single event reported previously by trench studies along the hft in the western indian himalaya that occurred between 1200 ad and 1700 ad however the ramnagar trenched fault zone shows a slip of 23 m which is larger than both line length and d d methods it implies that a 13 m high scarp and 23 m slip beneath the rigid block may be ascribed to multiple events it is for the first time we report that in the south eastern extent of the western indian himalaya ramnagar scarp consists of minimum two events i pre 1400 ad and ii unknown old events of different lateral extents with overlapping ruptures if the more optimistic two seismic events scenario is followed the rupture length would be at least 260 km and would lead to an earthquake greater than mw 8 5

orogenic gold formation in an evolving decompressing hydrothermal system genesis of the samut gold deposit eastern desert egypt abstract samut is a vein type gold mineralization in the eastern desert of egypt which shares many geologic features with other orogenic deposits in this region the deposit has a long history of mining activity dating back to ancient times but so far has received little attention from the standpoint of geologic study here we present new field petrographic microthermometric and microanalytical data from the samut gold deposit to evaluate its genesis and fluid evolution vein quartz at samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time in some samples brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation this brittle style of deformation culminates with latest stage brittle opening and loss of confining pressure resulting in open space filling textures backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins py i py ii and py iii laser ablation icpms analysis of pyrites from mineralized quartz veins reveals that the earlier py i is low in arsenic and gold whereas py ii and py iii are enriched in both elements anhedral py ii is particularly gold rich and gives way to blocky euhedral py iii that is rich in arsenic but not as rich in gold compared to py ii fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of 300 c early in the hydrothermal system the calculated fluid pressures at homogenization of the aqueous carbonic inclusions furnish a wide range throughout the paragenesis perhaps on the order of 1 to 3 kbar the 34s values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of 0 6 to 4 9 overlapping with the broad range of 34s values calculated for ore fluids in orogenic gold deposits worldwide together these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn deformational vein formation and tending towards the late stages of vein growth prior to loss of confinement free milling gold appears to be mostly related to the later stages of brittle fracturing and re sealing of the veins during repeated fluid release as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions

upslope verging back thrusts developed during downslope directed slumping of mass transport deposits acknowledgements sm acknowledges the israel science foundation isf grant no 1436 14 and the ministry of national infrastructures energy and water resources grant 214 17 027 rw was supported by the israel science foundation isf grant no 1245 11

fault displacement distance relationships as indicators of contractional fault related folding style we present a method of using fault displacement distance profiles to distinguish fault bend shear fault bend and fault propagation folds and use these insights to guide balanced and retrodeformable interpretations of these structures we first describe the displacement profiles associated with different end member fault related folding models then provide examples of structures that are consistent with these model based predictions natural examples are imaged in high resolution two and three dimensional seismic reflection data sets from the niger delta sichuan basin sierras pampeanas and cascadia to record variations in displacement with distance updip along faults termed displacement distance profiles fault bend folds exhibit constant displacement along fault segments and changes in displacement associated with bends in faults shear fault bend folds demonstrate an increase in displacement through the shearing interval and fault propagation folds exhibit decreasing displacement toward the fault tip more complex structures are then investigated using this method demonstrating that displacement distance profiles can be used to provide insight into structures that involve multiple fault related folding processes or have changed kinematic behavior over time these interpretations are supported by comparison with the kinematics inferred from the geometry of growth strata overlying these structures collectively these analyses illustrate that the displacement distance approach can provide valuable insights into the styles of fault related folding

distinguishing thrust sequences in gravity driven fold and thrust belts abstract piggyback or foreland propagating thrust sequences where younger thrusts develop in the footwalls of existing thrusts are generally assumed to be the typical order of thrust development in most orogenic settings however overstep or break back sequences where later thrusts develop above and in the hangingwalls of earlier thrusts may potentially form during cessation of movement in gravity driven mass transport deposits mtds in this study we provide a detailed outcrop based analysis of such an overstep thrust sequence developed in an mtd in the southern dead sea basin evidence that may be used to discriminate overstep thrusting from piggyback thrust sequences within the gravity driven fold and thrust belt includes upright folds and forethrusts that are cut by younger overlying thrusts backthrusts form ideal markers that are also clearly offset and cut by overlying younger forethrusts portions of the basal detachment to the thrust system are folded and locally imbricated in footwall synclines below forethrust ramps and these geometries also support an overstep sequence however new short cut basal detachments develop below these synclines indicating that movement continued on the basal detachment rather than it being abandoned as in classic overstep sequences further evidence for synchronous thrusting where movement on more than one thrust occurs at the same time is provided by displacement patterns on sequences of thrust ramp imbricates that systematically increases downslope towards the toe of the mtd older thrusts that initiate downslope in the broadly overstep sequence continue to move and therefore accrue greater displacements during synchronous thrusting our study provides a template to help distinguish different thrust sequences in both orogenic settings and gravity driven surficial systems with displacement patterns potentially being imaged in seismic sections across offshore mtds

paleoseismic evidence for coseismic growth fold in the 1999 chichi earthquake and earlier earthquakes central taiwan abstract the 1999 chichi earthquake ruptured along previously unrecognized traces of the chelungpu fault because the traces were covered with thick bedded fluvial alluvial and colluvial deposits the earthquake created a 95 km long surface rupture and associated fault related fold scarps this study focused on the fault related fold at two locations where the fold scarp is characterized with trench excavation and shallow cored boring results the structural characteristics revealed by the two trench sites are consistent with a trishear fault propagation fold growth above a blind thrust several characteristics of the fold observed in the holocene deposits show smoothly rounded fold hinges unconformities onlapped structure and downward steepening of forelimb strata results from these structures suggest that the fold grows by progressive limb rotation of growth strata in sequential coseismic growth episodes the growth strata show several unconformable contacts as indicated by paleosoil horizons developed on event horizons based on the syntectonic sedimentary structure three events are revealed at the siangong temple site and two paleoearthquake events on the shijia site integration with the two trenches and the previous studies suggests the three paleoearthquake events occurred 300 430 710 800 and 1710 1900 yr b p these data on the two trench sites indicate that the average slip rate is 4 2 and 4 5 mm yr respectively

displacement distance methods in the analysis of fold thrust structures and linked fault systems a new method for analysing the internal strain in thrust sheets is described the displacement distance diagram can be used to quantify the relative shortening or relative stretch between the hangingwall and footwall beds of a thrust and is a useful way of analysing commonly occurring fold thrust structures the stretch is a function of the relative rates of thrust propagation and thrust slip the cumulative displacement distance diagram described here can be used to calculate the absolute shortening or extension when this is accommodated above a single decollement in a linked fault system this method offers a useful way of analysing duplex structures and large scale foreland thrust zones as well as extensional fault systems and can be used as an alternative to or in conjunction with balanced cross sections both methods can be used to quantify the variation in shortening above a single detachment zone this variation may be mechanically significant reflecting lithological control on thrust or normal fault growth

stretch in shear zones implications for section balancing abstract the stretch s of a line in a zone of general shear deformation is given by s wsin sin where the shear zone s final to initial width ratio is w and the line s inclination to the zone boundary is before deformation and after for the special cases of simple shear and pure shear s sin sin and s2 sin 2 sin 2 respectively fisher s and sorby s formulae may be combined in a form appropriate to general shear strain cot rcot where r is the axial ratio of the irrotational component of deformation and is the shear strain for simple shear and pure shear the equation reduces to cot cot and tan rtan respectively these relationships may be used to estimate strain in exposed faulted strata or to restrict the possible geometries of inferred fault ramps in balanced geological cross sections

spatiotemporal sampling of sq induced geomagnetic responses at leo satellite altitude for a radially conductive earth summary a new simulation tool is introduced for extracting earth conductivity information from geomagnetic satellites in low earth orbit leo such as champ swarm and potential follow on missions theoretical reconstruction of spherical harmonic spectra of global universal time ut geomagnetic field maps is analysed an idealized regular solar daily variation field is assumed along with its induced counterpart from a radially stratified earth solar quiet sq spectra can be reliably reconstructed from discrete time series of measurements sampled daily within a fixed ut window a single leo satellite should be in orbit for at least 1 yr under quiet time conditions to ensure accurate spectral reconstruction of an sq geomagnetic spectrum the length of the daily sampling window and random day to day variability in the sq source strength have only minor effects on spectral reconstruction the shape of the spectra and spectral ratios are independent of radial mantle conductivity further research employing 3 d forward modelling of induction in a laterally heterogeneous earth with more realistic and complete external source descriptions is advocated the new simulation tool should prove valuable to planners of future multisatellite geomagnetic missions as well as scientists interested in analysing and interpreting satellite induction signals

models of strain variation in nappes and thrust sheets a review abstract strain within nappes and thrust sheets can be modelled using simple boundary conditions an original rectangular prism of material resting on a thrust plane is considered to be subject to various pure and simple shear components operating parallel to the thrust deformation gradient tensors are determined for the models and solutions found for the principal strains a two dimensional model considers heterogeneous simple shear combined with a stretch in the direction of shear simple shear zones arise as a special case where 1 recumbent fold nappes characteristic of gravitational collapse can be modelled with 1 and thrust sheets with layer parallel shortening and folding with differential transport of nappes gives rise to additional wrench type shears solutions for the principal strains in this type of model are found and used to interpret side wall ramps and steep zones parallel to nappe movements by including a stretch in the transport direction these differential transport models produce prolate strains 1 and oblate strains finally a brief consideration of bending strains produced in a sheet passing over a ramp indicates reversals in the shear direction during the strain history these predict possible crenulation of early thrust fabrics at ramps

sense and non sense of shear in flanking structures with layer parallel shortening implications for fault related folds we investigated the effects of fault related folding mechanisms along a single fault that is pinned on both ends upsection and downsection using a numerical model we produced flanking structures under plane strain transtension for the whole range between simple shear general shear and pure shear with layer parallel shortening parallel and shear zone widening normal to the shear zone boundaries under these boundary conditions contractional flanking folds with thrusting kinematics are the structures that are most likely to form and s type flanking folds develop stable orientations comparison with natural examples reveals that contractional flanking structures occur from the outcrop scale within ductile shear zones where they can be used as kinematic indicators in special cases up to the mesoscopic scale within fold and thrust belts the fundamental differences of our model to existing fault related fold models like fault propagation folds fault bend folds or break thrust folds are 1 the fault does not necessarily maintain a stable orientation but may rotate during progressive development 2 the drag can change from reverse to normal along the fault 3 the displacement along the fault has its maximum in the centre of the fault and decreases in both directions downsection and upsection towards fixed fault tips

geochemical and sr nd pb isotopic data bearing on the origin of pan africangranitoids in the kid area southeast sinai egypt the kid plutonic suite in southeast sinai egypt is composed of 1 a gabbro diorite complex 2 quartz monzodiorite 3 granodiorite monzogranite association 4 syenogranite major and traceelement analyses and radiogenic isotope data rb sr sm nd and pb pb are presented for these rocks rb sr isochrons indicate ages of 581 11 ma 576 11 ma and 570 4 ma for the quartz monzodiorite granodiorite monzogranite association and syenogranite respectively the gabbro diorite complex is a layered intrusion varying in sio2 from 42 to 59 wt it shows a calc alkaline trend and the trace element characteristics of arc lavas subsequently this magma fractionated to give quartz monzodiorite andsyenogranite magmas the quartz monzodiorite has low initial 87 sr 86 sr ratios 0 703 and high nd values 4 4 0 3 with trace element characteristics of volcanic arc granites the granodiorite monzograniteassociation has higher initial sr ratio 0 7044 lower nd 1 5 2 7 and relatively high 207 pb 204 pb and 208 pb 204 pb significantly different from those of the older rocks isotopic data and ree modelling suggestthat it formed by partial melting of an early pan african amphibolitic lower crust the syenogranite isa typical highly fractionated post orogenic granite with a deep negative eu anomaly hfse and reeenrichment a 87 sr 86 sr initial ratio of 0 7033 nd of 4 5 0 5 and distinct variations in the 206 pb 204 pbratio the isotopic compositions of sr nd and pb indicate that the syenogranite and the quartzmonzodiorite are comagmatic increasing fluorine concentration from the quartz monzodiorite to thesyenogranite suggests that fluid fractionation involving fluorine complexing played an important role inthe genesis of the syenogranite this process led to the enrichment of hfse and hree to extensivefractional crystallization to changes in the u pb ratios and a distinct variation in 206 pb 204 pb

the distribution of layer parallel shortening fabrics in the appalachian foreland of new york and pennsylvania evidence for two non coaxial phases of the alleghanian orogeny

tectonic magmatic stages of shield evolution the pan african belt in northeastern egypt abstract the arabian nubian shield illustrates an example of plate tectonics during the pan african orogenic event ca 950 550 ma it is considered one of the most remarkable proterozoic shield areas yet known although there is now a general agreement that the shield has evolved by a series of magmatic arcs and terrain accretion the evolution of its northernmost part exposed in northeastern egypt remains controversial two contrasting tectonic models have been proposed for the evolution of this region regional crustal extension and magmatic arc regimes there is no convincing evidence of older archean sialic materials within the region although the region is flanked by archean crust the neoproterozoic crustal components exposed in northeastern egypt preserve a record for its evolution a large gabbro diorite tonalite complex gdt 881 ma emplaced during the early stage of the pan african orogeny is geochemically primitive exhibits a low initial 87 sr 86 sr ratio 0 704 and shows trace element characteristics of oceanic arc related lavas the gdt complex was produced by fractionation from a mantle derived tholeiitic magma formed within an island arc tectonic environment a synorogenic stage characterized by extensive volcanism that produced the dokhan volcanic suite dvs 620 ma forms a continuous basalt to rhyolitic ignimbrite calc alkaline magma series exhibiting features of arc related volcanism with ree profiles similar overall to andean andesites and ignimbrites the dokhan suite was produced in a continental arc setting a granodiorite adamellite leucogranite composite batholith 552 ma emplaced during the late stage of the pan african orogeny exhibits typical features of i type complexes trace element traits of volcanic arc granites and is also interpreted to have been formed in an andean type setting a trondhjemite pluton was also formed at this late orogenic stage by partial melting of gdt host rocks at depth cooling and relaxation of the newly formed pan african crust caused extensive fracturing which was followed by intrusion of ne sw and nnw sse trending dyke swarms at 493 ma this episode marks a fundamental inversion from orogenic compressional to anorogenic extensional processes around the neoproterozoic paleozoic boundary anorogenic magmatism was locally associated with such extension induced structures the mount gharib peralkaline granite 476 ma for example exhibits trace element traits of a type within plate granites and a high initial 87 sr 86 sr ratio 0 711 and was formed in a rift environment thus the region is characterized by lengthy episodes of discontinuous subduction 880 550 ma during which it evolved to form a primitive crust by early pan african island arcs and late pan african continental arcs this was followed by an even longer period of crustal extension 550 90 ma that produced localized anorogenic magmatism

mechanisms of thrust propagation some examples and implications for the analysis of overthrust terranes abstract evidence is presented for the existence of three fundamental mechanisms by which thrust sheets move the mechanisms are 1 failure of a stiff layer to form ramp flat geometry imbricated thrust sheets 2 detachment of a layer by folding decollement thrust sheets 3 differential layer parallel shortening lps thrust sheets the mechanisms are independent but may operate interactively to form families of hybrid thrust sheets since lps thrust sheets have not been previously described or documented data from the new york plateau deformed almost exclusively by differential lps is presented to demonstrate the physical characteristics of this thrust mechanism information from the plateau indicates that finite strain behavior closely reflects the geometric boundary conditions and is independent of temperature and depth of burial finite strain data are used to construct a set of iso strain maps in both the deformed and undeformed states the iso strain maps are in turn used to determine the displacement field for the thrust sheet the displacement field allows visualization of the effects of both lagrangian and eulerian transformations on an initially orthogonal grid utilizing sections from a number of overthrust belts it is shown that the three mechanisms occur universally although in various proportions lps is regionally developed throughout the central and northern appalachians the presence of the lps thrust sheets probably accounts for the failure of structural cross sections to bed length balance in these blind thrust terranes finally a series of examples drawn from the field as well as the literature are used to illustrate both the characteristics of hybrid thrust sheet families as well as to indicate how knowledge of their behavior can be utilized in developing strategies for the construction and balancing of structural cross sections

the influence of structural lithic units in fault related folds seminoe mountains wyoming u s a abstract analysis of basement involved folds in the seminoe mountains south central wyoming reveals that lithological contrasts within the sedimentary section created a mechanical anisotropy that influenced both fault geometry and the relative rates of fault propagation and fault slip two structural lithic units are identified a competent lower unit and an overlying incompetent unit the upper unit is made up of moderate ductility rocks that were thinned ahead of the hurt creek fault offset formations in the upper unit have small relative stretch r values and are interpreted to have represented an impediment to fault growth in contrast fault propagation was rapid relative to fault slip in the competent rocks of the lower structural lithic unit which have a correspondingly higher relative stretch the black canyon fault is oriented at a low angle to bedding in the lower structural lithic unit and is layer parallel near the base of the upper unit the red spring fault is a thin skinned thrust fault and is interpreted to be linked to the black canyon fault to generate a triangle zone geometry similar structures can be identified elsewhere in the rocky mountain foreland and this configuration may represent a previously unrecognized indicator of low angle basement faults changes in the deformation style between structural lithic units must be reflected in changing fold form thus preventing the direct application of geometric and kinematic models that predict a uniform fold profile