internal geometry and origin of the hubat structural culmination oman mountains abstract the hubat structure is a doubly plunging antiform affecting an allochthonous succession of mesozoic sedimentary rocks the hawasina complex overlain by the haybi complex and the semail ophiolite it is cored by a well exposed imbricated nappe of the hamrat duru group detailed investigations of the hamrat duru group led to the recognition of a main phase of imbrication followed by a phase of meso and macroscopic folding with axes at a high angle to the strike of the imbricates parallel to their assumed transport direction thrust relationships displayed in this area indicate that the floor thrust of the semail ophiolite is younger than the emplacement of the lower units and therefore out of sequence this culmination may have originated either as a result of a post emplacement regional compressive event of tertiary age or during the emplacement of allochthons above an oblique ramp during the late cretaceous orogeny

a note on the impact factor of gondwana research with a view towards understanding the evolutionary history of the complex south indian shield several geological and geophysical studies have been carried out recent geophysical studies include magnetotelluric mt deep seismic sounding dss gravity magnetic and deep resistivity soundings drs in the present study mt results along 140 km andiyur turaiyur east west profile is presented the data are subjected to groom bailey decomposition and static shift correction before deriving a 2 d model the 2 d modeling results have shown that the upper crust up to about 15 km towards western part of the profile have exhibited high resistive character of about 40 000 ohm m as compared to the eastern part less than 5 000 ohm m the mid lower crust has shown a decrease in resistivity in western part of the profile the order of resistivity being 2 000 ohm m an anomalous steep conductive feature less than 100 ohm m is observed near sankari at mid lower crustal depths 20 km towards middle part of the profile this feature is spatially correlatable with the well known moyar bhavani shear zone mbsz the features obtained in the present study are consistent with earlier mt studies in this region and correlatable with other geophysical studies dss studies near the study region gave an evidence for differing crustal structure on either side of mbsz variation in geoelectric character along the profile both in the upper crust and mid lower crust indicate a block structure in the sgt with shear zones acting as boundaries the new evidence in the form of distinct geoelectric structure and also variation in seismic structure indicate a continent continent collision zone in this region and plays an important role for the gondwana reconstruction models of south indian shield

the northern margin of the grenville province in western labrador anatomy of an ancient orogenic front abstract in western labrador the northern margin of the grenville province known as the grenville front tectonic zone abuts against the superior foreland zone composed of an archean gneiss terrain and against the churchill foreland zone composed of weakly metamorphosed and deformed aphebian supracrustals in both terrains a similar tectonic style is developed in which aphebian rocks from within the grenville orogen have been thrust northwards over the adjacent foreland the grenville front is marked by mylonite phyllonite zones in the archean gneisses of the superior foreland whereas in the supracrustals it is characterized by cleavage folding grenvillian deformation involved 3 episodes of which two gave rise to northeast trending structural elements and are interpreted to be part of a single protracted event involving northerly translation of rocks from within the orogen towards the foreland a third deformation comprised large scale crossfolds and may be related to the formation of a major arcuate thrust salient spatially the northeast trending structural elements occur in the grenville front tectonic zone while the crossfolds dominate in the interior of the orogen a zone of interference structures related to the two fold trends occurs in the southern part of the grenville front tectonic zone the hudsonian front westerly limit of hudsonian deformation in the churchill province passes through the churchill foreland zone in the study area there grenvillian structures are superimposed on older hudsonian structures elsewhere in the area studied however all deformation and metamorphism of aphebian rocks is of grenvillian age metamorphic zones in pelitic rocks indicate a progressive increase in grade towards the interior of the grenville orogen the occurrence of the mineral pair kyanite k feldspar preceding the assemblage sillimanite k feldspar in the prograde sequence indicates that peak metamorphic conditions may have exceeded 8 kb and 700 c such elevated pressures during metamorphism may have been achieved by stacking of thrust slices in the grenville front tectonic zone both deformation and postorogenic uplift were diachronous in the grenville province deformation was initiated in the high grade core of the orogen and migrated northwards towards the margin whilst uplift occurred earlier in the margin than in the core

normal faulting sequence in the pumqu xainza rift constrained by insar and teleseismic body wave seismology normal faulting earthquakes play an important role in the deformation of continents and pose significant seismic hazard yet important questions remain about their mechanics we use insar and body wave seismology to compute dislocation models and centroid moment solutions for four normal faulting earthquakes mw 5 7 6 2 that occurred in the pumqu xainza rift pxr southern tibet a region where low angle normal faulting has previously been inferred we also use the fault locations and slip to investigate the correlation between earthquakes and surface topography and to calculate stress interactions between the earthquakes the insar and body wave models give consistent focal mechanisms except for the magnitude of the 1996 event which may be overestimated due to postseismic deformation in the long interval interferograms we calculate the static stress changes due to coseismic slip and find that the 1993 event was too distant to cause triggering of the later events but that the 1998 event pair occurred in regions of increased coulomb stress resulting from the 1996 event all the fault planes found here dip at 40 60 reinforcing the absence in observations for low angle normal faulting earthquakes dip 30 whose focal planes can be determined unambiguously the fault planes of the 1993 and 1996 events are not associated with any obvious surface geomorphology suggesting that sometimes it is unreliable to resolve the focal plane ambiguity by geomorphology even for mw 6 2 events furthermore these events occurred outside the center of the rift indicating that the active faulting is more distributed and over a length scale at least 25 50 km east west in extent rather than confined to the 20 km width seen in the current mapped faulting and topography these results suggest that seismic hazard in other extensional zones worldwide might also be more broadly distributed than suggested by geomorphology

discussion on carbon and oxygen isotope systematic of a paleoproterozoic cap carbonate sequence from the sausar group central india by s mohanty a barik s sarangi and a sarkar 2015 published in palaeogeography palaeoclimatology palaeoecology 417 195 209 abstract the recent proposition by mohanty et al 2015 that the lower part of sausar group of rocks in central india has a glacial origin and represents a major paleoproterozoic super glaciation event contradicts some well established geological and geochronological data on these rocks put forward by other workers in recent past the merits of this conclusion are discussed here

tectonic processes in a surge type glacier the 1982 1983 surge of variegated glacier involved the development growth and downglacier propagation of a velocity peak associated with rapid basal sliding facilitated by high subglacial water pressures passage of the velocity peak through the glacier was preceded by an episode of longitudinal shortening and followed by an episode of elongation the deformation history of the glacier ice was dependent upon location relative to the surge nucleus and the final position reached by the propagating velocity peak ice above the surge nucleus experienced continuous and cumulative elongation ice below the final position of the velocity peak experienced continuous and cumulative shortening ice between these two points experienced shortening followed by elongation and low cumulative strain the large scale pattern of ice structure development reflects these deformation histories surging is equivalent to thrust sheet emplacement by a combination of gravity gliding over a weakened basal layer and push from behind with the gravity driven motion of the surging part of the glacier providing the push which allows the surge front to propagate the relationships established between the deformation history of surging glaciers and the development of ice structures may facilitate the interpretation of structures in thrust sheets

the irish variscides a fold belt developed within a major surge zone an arcuate fold belt characterizes the irish variscides south of the dingle dungarvan line the folds are cross cut by numerous faults which tend to be sinistral in the west and dextral in the east clockwise transection of major fold axes by minor folds and by cleavage has been recorded in the east while anticlockwise transections are common in the west these and other features suggest that north south dextral and sinistral shearing occurred in the east and west respectively this is considered to be a consequence of the morphology of the munster basin which allowed the development of a surge zone in the centre of the fold belt the variscan sole thrust is believed to lie at the base of or within the old red sandstone however to the east thinning of the upper palaeozoic cover sequence forces the sole thrust to cut down laterally stratigraphic section into lower palaeozoic basement in the depocentre the sole thrust surged ahead by propagating along bedding surfaces or possibly a devonian carboniferous extensional detachment beneath the basin while to the east and offshore to the west its propagation was inhibited by the absence of easy decollement horizons within the basement consequently the tip line became strongly arcuate and the thrust developed into a surge zone the sole thrust propagated northwards by stick slip processes much of the deformation within the cover sequence occurred during sticking events particularly fold tightening and accommodation faulting

surge zones in the moine thrust zone of nw scotland abstract the caledonian thrust zones of assynt show several examples of large fault bounded structures surge zones up to 8 km 2 in extent which have moved further than adjacent rocks extensional faults can be traced into strike slip faults and then to contractional imbricate faults there are also zones of extensional and contractional flow as shown by strained bioturbation marks in the cambrian pipe rock several other low angle extensional fault zones have been recognized along the length of the moine thrust zone notably in the kinlochewe district recognition of these extensional faults and local surge zones has solved several local problems such as the lack of continuity of the glencoul thrust and the out of sequence character of some of the large low angle faults though the thrust propagation direction was generally from east to west in the transport direction several of the eastern faults have been reactivated later and locally cut down as extensional faults the so called moine thrust shows extensional fault movement at several localities along its length the extensional structures and the surge zones suggest that body forces have been important in driving the faults rather than just a push from the rear the moines and moine thrust zone were presumably driven to the wnw by gravity spreading and thinning of the main scottish caledonides

numerical models of alpine type cover nappes we investigate the dynamics of alpine cover nappes derived from basins with inherited geometries using a two dimensional finite element code model studies are based on a natural analogue the klippen nappe in the swiss alps which detached from the basement over a basal evaporite horizon and was transported onto the foreland in a thin skinned tectonic style finite element models are used to examine the influence of specific features on the structural style developing in the models experiments dealing with the influence of wedge geometry and mechanical properties show modifications in wavelength patterns and structures in detail but do not change the general trends the control that different layer thicknesses multilayered rocks and lateral heterogeneities in the underlying basement and within the sediments have on the style of deformation is investigated results suggest that the development of fold versus thrust structures in a cover nappe is controlled by the thickness of the weak detachment horizon and the relative thickness of individual layers thickness ratio n thickness of weak thickness of strong layer if n 1 folds with variable and irregular wavelengths form if 0 5 n 1 regularly spaced detachment folds develop imbricate thrusting is observed for values of n 0 5 lateral heterogeneities in both basement and rock formations cause stress concentrations control the geometry of the evolving folds and thrusts significantly and inhibit the growth of dominant wavelengths predicted by viscosity contrasts and thickness ratios

4 d evolution of rift systems insights from scaled physical models the four dimensional 4 d evolution of brittle fault systems in orthogonal oblique and offset rift systems has been simulated by scaled sandbox models using dry cohesionless fine grained quartz sand extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model the results of these analog model studies are compared with natural examples of rift fault systems rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction segmentation of the rift margin increases with increase in obliquity of the rift axis resulting in a consequent increase in displacement on intrarift fault systems offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone along strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones high relief extension parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults the second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low relief oblique slip zone or grabens these typically are found in highly oblique rift systems 45 degrees and above right steps in offset rift models

thrust tectonics thin skinned or thick skinned and the continuation of thrusts to deep in the crust abstract the paper analyses the geometry of thin skinned thrust zones where the thrusts shallow out at depth and of thicker skinned fault zones where much of the crust is involved and where the thrusts are frequently observed to become steeper downwards in the interiors of many orogenic belts the steep dip of faults is not original but due to the folding above lower decoupling zones the energy involved in the internal deformation of hanging wall rocks may prohibit many faults becoming more shallow upwards such shallowing upwards faults may occur in more ductile rocks to maintain compatibility between zones which have experienced different deformation intensities but displacements on the faults are unlikely to be large another mechanism for producing faults which steepen downwards is proposed for the major thrusts which form the southern margin to the himalayas these carry large thicknesses 30 to 100 km of crustal and upper mantle rocks to the south causing flexuring and isostatic depression of the indian plate the steeply dipping thrusts are not footwall ramps these may be some distance behind the steepened zone this thrust induced isostatic bending of the crust has important implications when considering regional seismic interpretations as well as thrust mechanics and kinematics

capturing magma intrusion and faulting processes during continental rupture seismicity of the dabbahu afar rift continental rupture models emphasize the role of faults in extensional strain accommodation extension by dyke intrusion is commonly overlooked a major rifting episode that began in 2005 september in the afar depression of ethiopia provides an opportunity to examine strain accommodation in a zone of incipient plate rupture earthquakes recorded on a temporary seismic array 2005 october to 2006 april direct observation of fault patterns and geodetic data document ongoing strain and continued dyke intrusion along the 60 km long dabbahu rift segment defined in earlier remote sensing studies epicentral locations lie along a 3 km wide 50 km long swath that curves into the se flank of dabbahu volcano a second strand continues to the north toward gab ho volcano considering the 8 m of opening in the september crisis we interpret the depth distribution of microseismicity as the dyke intrusion zone the dykes rise from 10 km to the near surface along the 60 km long length of the tectono magmatic segment focal mechanisms indicate slip along nnw striking normal faults perpendicular to the arabia nubia plate opening vector the seismicity insar continuous gps and structural patterns all suggest that magma injection from lower or subcrustal magma reservoirs continued at least 3 months after the main episode persistent earthquake swarms at two sites on dabbahu volcano coincide with areas of deformation identified in the insar data 1 an elliptical northwestward dipping zone of seismicity and subsidence interpreted as a magma conduit and 2 a more diffuse 8 km radius zone of shallow seismicity 2 km above a shadow zone interpreted as a magma chamber between 2 5 and 6 km subsurface insar and continuous gps data show uplift above a shallow source in zone 2 and uplift above the largely aseismic gab ho volcano the patterns of seismicity provide a 3 d perspective of magma feeding systems maintaining the along axis segmentation of this incipient seafloor spreading segment

thrust faults and back thrusts in madison range of southwestern montana foreland in the rocky mountain foreland of southwestern montana a zone of late cretaceous thrust faults named the hilgard fault system extends along the west side of the madison range from hebgen lake northward for about 50 mi 80 km the thrust faults are steep at their leading edges but flatten westward beneath the associated plates where they commonly dip 25 30 structural lows and highs are apparent beneath the beaver creek plate the major thrust sheet of the system and correlate with salients and reentrants of the plate the beaver creek plate consists primarily of archean metamorphic rocks but phanerozoic strata are preserved along the northern part of the plate s leading edge only the forward part of the plate is preserved in the madison range because c nozoic normal faults of the madison range fault system dropped much of the plate beneath the madison valley on the west the kirkwood plate lies east of and beneath the beaver creek plate and contains structures of the eastern part of the hilgard system the central segment of the leading edge of the kirkwood plate is not completely detached from underlying strata the leading edge of the northern part of the plate defined by the cache creek fault is flanked on the west by an associated anticline the southern end of the cache creek fault is an eastward dipping back thrust which abruptly steepens on the west adjacent to the anticline both the fault and the anticline are believed to have formed above a concealed detachment fault the southern part of the kirkwood plate displays structures interpreted to represent displacement transfer from the beaver creek plate back thrusts are associated with both thrust plates and played a key role in accommodating strain associated with folding the strata at the leading edges of the plates the back thrusts project upward from the main thrust fault beneath each plate they dip eastward at moderate angles in their lower segments but steepen at structurally higher levels they become overturned locally and dip westward in their uppermost segments the northern part of the study area contains the northwest trending buck creek reverse fault which dips steeply to the northeast movement on this structure ceased before the north trending thrust faults of the hilgard system were emplaced

thrust tectonics a personal view the structure of fold and thrust belts has been studied for just over one hundred years and has been the subject of a recent international conference at toulouse in may 1984 this article reviews some of the recent advances in thrust tectonics from a personal viewpoint in the light of the toulouse meeting key tools in correctly interpreting thrust systems are the use of balanced restorable cross sections and the resolution of three dimensional fault systems these analytical methods are now being widely developed and applied to numerous orogenic belts with many surprising results while our understanding of foreland thrust belts is now probably better than any other structural environment there is still great uncertainty as to the emplacement driving mechanisms of these belts future work will try to solve these and other outstanding problems and apply the new analytical methods to the interior parts of thrust belts in efforts to restore entire orogens

centrifuge modelling of fold thrust structures in a tripartite stratigraphic succession abstract analog models measuring 127 76 mm in plan were deformed at 2500 4000 g in a centrifuge scaled stratigraphic sequences were constructed of anisotropic multilayers with individual layers of plasticine and silicone putty as thin as 40 m the plasticine silicone putty multilayers are analogs for interbedded competent carbonates and clastics and incompetent pelites given the model ratios of acceleration 2500 g length 5 10 6 specific gravity 0 6 time 10 10 modelling of fold thrust tectonics emphasizes the influence of stratigraphic succession on structural evolution the models are constructed with a tripartite stratigraphic succession comprising basal and upper well laminated and incompetent units and a middle somewhat more isotropic and competent unit the models deform by three mechanisms layer parallel shortening folding and thrust faulting they reproduce a number of fold thrust relationships that have been observed in nature folds are typically periclinal in en echelon arrays folds and thrusts are arcuate in plan reflecting differential shortening fold attitudes grade from upright at high levels to overturned at deeper levels within a structural panel reflecting drag against the basal decollement fold axial surfaces and thrust faults are listric while competent units may be offset by localized displacement on thrust faults the discrete faults may die out both upwards and downwards into regions of ductile strain in less competent units thrust faults appear to follow staircase trajectories through the strata transecting incompetent units at shallow angles to bedding and competent units at steeper angles however the apparent staircase pattern results from propagation of a fault along a relatively straight trajectory through previously folded strata foreland verging thrusts are more common than back thrusts the latter have steeper dips the models suggest a mechanism of thrust ramp nucleation following detachment folding long wavelength buckling of a competent unit can initiate localized strain folding and layer parallel shear in an underlying incompetent unit beneath the anticlines of the competent unit thrust faults propagate up section from these high strain zones through the foreland dipping limbs of buckle folds in the competent unit this mechanism may explain the commonly observed spatial periodicity of thrust ramps the model results bear similarities to natural fold thrust belts in which the stratigraphic succession consists of three mechanical units for example the asiak foreland and bear creek hills fold thrust belts of the slave province northwest territories canada

analogue modelling of intraplate strike slip tectonics a review and new experimental results abstract intraplate strike slip zones commonly display intricate 3 d geometries with rapid changes in structural style along strike and with depth strike slip deformation typically results in complex vertical and horizontal sections that can be difficult to interpret coherently physical modelling of strike slip fault systems is a powerful and graphic tool to help in providing a unified picture of the evolution of strike slip zones with considerable spatial and temporal detail a large number of experimental studies have investigated different aspects of strike slip tectonics using materials such as dry sand wet clay or silicone the choice of analogue material and experimental design exerts a strong control on the structures that form in the model here we present a review of different experimental setups used to investigate intraplate strike slip tectonics from the classical riedel experiment to more sophisticated setups using brittle and viscous analogue materials we review our current understanding from models of distributed shear transtension transpression pull apart basins formed in releasing stepovers and popups formed in restraining stepovers in addition we present the results of two new experimental series that investigated 1 the effect of crustal weak zones on strike slip fault zone segmentation and 2 strike slip and transpressional reactivation of extensional basins

3 d deformation pattern analysis and evolution of the anisclo anticline southern pyrenees understanding the genetic relationships of folding related deformation structures is of importance for both academic and industrial purposes we analysed the three dimensional deformation pattern of the anisclo anticline in the southern pyrenees the specifically designed statistical method includes sequential steps of statistical and spatial data analysis and classification our results indicate that pressure solution cleavage frequency exhibits a spatial distribution that depends on the position within the fold conversely such a dependence does not occur in joints and veins consequently our data suggest that pressure solution cleavage is the most appropriate deformation structure for unravelling fault fold kinematics in carbonate multilayer in particular we found that the spacing of solution cleavages s is related to the corresponding bed thickness h this supports the use of h s value for deformation intensity quantification instead of spacing in the anisclo anticline analysis of h s indicates the occurrence of three main deformation panels whose restoration suggests that fold evolution progressed from layer parallel shortening to decollement folding up to thrust breakthrough and fault propagation folding comparison of h s values with finite strain data indicates that line length was not preserved during fold evolution this evidence has important implications for the kinematic interpretation of layer parallel shortening and cross section balancing

estimating source parameters from deformation data with an application to the march 1997 earthquake swarm off the izu peninsula japan we have applied two monte carlo optimization techniques simulated annealing and random cost to the inversion of deformation data for fault and magma chamber geometry these techniques involve an element of randomness that permits them to escape local minima and ultimately converge to the global minimum of misfit space we have tested the monte carlo algorithms on two synthetic data sets we have also compared them to one another in terms of their efficiency and reliability we have applied the bootstrap method to estimate confidence intervals for the source parameters including the correlations inherent in the data additionally we present methods that use the information from the bootstrapping procedure to visualize the correlations between the different model parameters we have applied these techniques to gps tilt and leveling data from the march 1997 earthquake swarm off of the izu peninsula japan using the two monte carlo algorithms we have inferred two sources a dike and a fault that fit the deformation data and the patterns of seismicity and that are consistent with the regional stress field

three dimensional numerical modeling of upper crustal extensional systems 1 we focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models to date few 3 d models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones we use an arbitrary lagrangian eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions plastic materials weaken with accumulating strain to localize deformation a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions controls on the geometry and spacing of three dimensional plastic shear zones are investigated the sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested the model results indicate the primary controls of strain dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction the amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other resulting in a trade off effect between the amount of offset between the initial grabens and the strain weakening ratio the three dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions 1 small offset grabens with a single relay zone 2 intermediate offset grabens with one or more secondary step over graben segments and 3 large offset grabens with limited to no significant segment interaction

current plate boundary deformation of the afar rift from a 3 d velocity field inversion of insar and gps extension faulting and magmatism are the main controls on the magnitude and localization of strain at mid ocean ridges however the temporal and spatial patterns of such processes are not clear since the strain distribution has not been resolved in the past at sufficient spatial resolution and over extended areas interferometric synthetic aperture radar insar and gps data with unprecedented resolution are now available to us from the afar rift of ethiopia here we use a velocity field method to combine insar and gps to form the first high resolution continuous three dimensional velocity field of afar we study an area that is 500 km wide and 700 km long covering three branches of the afar continental rift and their triple junctions our velocity field shows that plate spreading is currently achieved in afar in contrasting modes a transient postdiking deformation is focused at the dabbahu rift segment while in central afar spreading is distributed over several overlapping segments and southern afar exhibits an interdiking deformation pattern focused at the asal ghoubbet segment we find that current spreading rates at dabbahu following the 2005 2010 intrusions are up to 110 mm yr 6 times larger than the long term plate divergence a segment centered uplift of up to 80 mm yr also occurs indicating that magma flow is still a primary mechanism of deformation during postdiking on the other hand no vertical displacements are currently observed in central and southern afar suggesting lack of significant magmatic activity at shallow levels

source model for the mw 6 7 23 october 2002 nenana mountain earthquake alaska from insar 1 the 23 october 2002 nenana mountain earthquake mw 6 7 occurred on the denali fault alaska to the west of the mw 7 9 denali earthquake that ruptured the same fault 11 days later we used 6 interferograms constructed using radar images from the canadian radarsat 1 and european ers 2 satellites to determine the coseismic surface deformation and a source model data were acquired on ascending and descending satellite passes with incidence angles between 23 and 45 degrees and time intervals of 72 days or less modeling the event as dislocations in an elastic half space suggests that there was nearly 0 9 m of right lateral strike slip motion at depth on a near vertical fault and that the maximum slip in the top 4 km of crust was less than 0 2 m the nenana mountain earthquake increased the coulomb stress at the future hypocenter of the 3 november 2002 denali earthquake by 30 60 kpa

finite strain analysis in a transpressive regime variscan autochthon northeast portugal abstract we apply a method of estimating finite strain that can be widely used in armorican quartzites affected by transpressive regimes the method proposed uses data from skolithos which are quite common in these rocks the main assumption is that the orientation of the finite strain ellipsoid can be represented by the cleavage and the stretching lineation observed in the rocks this method was tested in a macroscopic fold in the moncorvo region northeast portugal

the afar depression transition between continental rifting and sea floor spreading abstract detailed geophysical and geological studies were undertaken in the 1970 s in the afar depression ethiopia in order to study the processes of continental break up occurring in this unique setting the afar is located between the ethiopian and somali plateaus and the red sea we have re evaluated the available seismic refraction and gravity data using modern interpretation techniques and incorporated new information derived from magnetic gravity seismicity and seismic refraction data the results of the re evaluation indicate that the afar depression is a transition zone between the continental rifts of kenya and the present ongoing sea floor spreading of the red sea and the gulf of aden in the depression itself the crust is greatly attenuated and is underlain by a low velocity 7 4 7 5 km s high temperature upper mantle material however it is still a continental crust which thins to the northeast and east towards the red sea and the gulf of aden it is not a newly formed oceanic crust as has been suggested in the past the trends of the areas of crustal thinning are indicated by the gravity anomalies the trend of the ethiopian rift reaches south afar where the trend of crustal thinning changes and becomes parallel to the red sea trend it is offset en echelon from the spreading area of the red sea which is marked by an alignment of gravity maxima the gulf of aden trend continues west towards the ethiopian rift the gravity trends coincide with areas of maximum crustal thinning and may thus suggest a possible triple junction in development

the 1994 sefidabeh eastern iran earthquakes revisited new evidence from satellite radar interferometry and carbonate dating about the growth of an active fold above a blind thrust fault in 1994 three shallow earthquakes of mw 6 occurred close together on blind thrusts near sefidabeh in eastern iran in an earlier study of the teleseismic waveforms the geomorphology and the faulting in the epicentral region it was suggested that these earthquakes were associated with the growth of a ridge above a blind thrust fault system whose activity could be detected by its effect on the surface drainage in this study we present a sar interferogram that precisely determines the location and amount of coseismic surface displacements showing that the earthquakes in the sefidabeh sequence probably occurred on en echelon fault segments associated with three stepping ridges we also present u th dates of 100 ka for lake deposits uplifted by the growing ridge from the cumulative dated uplift and knowledge of the surface displacements due to an earthquake sequence we estimate that 120 such events have occurred in the past 100 ka with an average recurrence interval of 830 yr and an average convergence rate of 1 5 mm yr 1 on the sefidabeh thrust each estimate has an uncertainty of a factor of two either way we argue that the sefidabeh fault originally formed by coalescence of many small fault segments and has grown in length at about 2 cm yr 1 in the past 100 ka though the coseismic surface deformation observed in the sar interferogram closely resembles folding the overall topography does not because of inherited topography associated with earlier geological deformation in spite of this the activity of the buried thrust fault can easily be detected by its effect on the surface drainage a significant lesson when interpreting landscapes that are not entirely due to the present day deformation

tectonic evolution of the satpura mountain belt a critical evaluation and implication on supercontinent assembly abstract the satpura mountain belt also referred as central indian tectonic zone in recent literature forms an important morphotectonic unit in the central part of india some of the recent workers have reported an orogenic event at 1000 900 ma termed sausar orogeny which led to amalgamation of the north indian block and the south indian block and formation of the satpura mountain belt in this model the stratigraphic relations of two important lithostratigraphic units on either side of the satpura mountain belt the sausar group in the south and the vindhyan supergroup on the north are suggested to be revised from previously held ideas critical analyses of available published work in the region to assess the status of the sausar group vis a vis the vindhyan supergroup was carried out it is found that the ideas proposed by the recent workers stem from an earlier interpretation that the sausar group has monocyclic evolution and the earliest fabric in the sausar group is marked by a schistosity with ew strike re mapping of the gneissic rocks and adjacent matasedimentary rocks of khawasa deolapar and kandri mansar areas revealed presence of gneissic rocks and granulites of two generations and of four phases of superposed deformations in the metasediments and gneisses the older gneisses and granulites constitute the basement over which the rocks of the sausar group were deposited and the younger gneisses developed by metamorphism and migmatisation of the rocks of the sausar group the latter types are found in the khawasa ramakona areas contrary to the belief of the recent workers that no volcanic activity is present in the sausar group volcanic rocks marked by amygdular basic flows and tuffs have been mapped from different parts of the sausar group migmatisation and metamorphism of these volcanic rocks of the sausar group have given rise to amphibolites and granulites in khawasa and ramakona areas therefore the use of fabric patterns in these areas to suggest that the granulite facies metamorphism in the ramakona katangi granulite domain was pre sausar in age is debatable available geochronological data of the satpura mountain belt and its eastward continuation into the chhotanagpur gneiss terrain indicate that the basement and cover rocks of these areas were subjected to multiple deformation and metamorphic episodes of similar style and nature the earliest deformation and metamorphism of the rocks of the sausar group and its equivalent rocks to the east took place at 2100 1900 ma the regional ew trend of the belt developed during the second deformation at 1800 1700 ma and again at 1600 1500 ma this deformation was accompanied by migmatisation and granulite facies metamorphism in the northern domain of the sausar belt and in the chhotanagpur gneiss region late phase low intensity deformations in the region were associated with thermal events at 1100 1000 ma and 900 800 ma the ew trending fabric referred as satpura orogenic trend in indian literature marks a major compressional tectonic event developed during the second deformation of the sausar group this has its counter part in western australia as the capricorn orogeny 1780 1830 ma the development of the satpura mountain belt during the grenvillian orogeny is ruled out from the synthesis of event stratigraphic data of the region and from its comparison with the western australian craton

internal waves of the second vertical mode in a stratified lake a time series of measurements of thermal structure and near bottom currents in a stratified lake revealed both first and second vertical mode internal seiching the transition into second mode seiching occurred after first mode waves usually generated by pulses of strong winds died down we describe the major features of the second mode seiche and discuss its effects on mixing in the lake below the thermocline on the basis of calculations of eddy diffusion coefficients there is only weak mixing in the interior of the hypolimnion scaling arguments suggest that mixing at the lake s edges due to motion of the isotherms is not significant despite the larger amplitudes of the second mode seiche the most important mixing region appears to be a turbulent bottom boundary layer driven by seiche currents however resuspension of bottom sediments did not occur probably due to a combination of sediment nature and low current speeds

vertical structure of currents in western lake constance simultaneous measurements of current velocity and density profiles at the sill of mainau western lake constance enabled the observation of the internal response of the lake and the calculation of gradient richardson number over the entire water column at the sampling site periodic changes of current direction corresponded with predicted periods of the two layer oscillation and could be confirmed with thermocline oscillation data in general the current profiles confirmed a two layer structure however an activating wind of speeds higher than 4 m s produced three layer profiles for a period of several hours at the sampling site when the velocities due to the two layer oscillation traversed zero at times the implied current shear produced supercritical values of gradient richardson number on a vertical scale of a few meters this occurred during winds in the epilimnion and after strong 4 m s winds in the lower thermocline and the hypolimnion in the 20 m above the lake bed topographical conditions helped the gradient richardson number to go supercritical sporadically across the thermocline no supercritical gradient richardson numbers could be confirmed at the measuring site a lower boundary for vertical mixing in the measuring site due to current shear could be evaluated

strains and folds within thrust sheets an analysis of the heilam sheet northwest scotland abstract the changes of shape and orientation of fossil worm burrows have been used to carry out a detailed strain analysis of the heilam thrust sheet in the moine thrust zone northwest scotland the strains have been factorised into components of layer parallel shear 1 layer normal shear 2 and layer parallel shortening or elongation variations in the 1 shear are largely lithologically controlled some strain is due to flexural processes as the thrusts and overlying beds change dip over thrust ramps or are tightened by later deformation while there may also be a component of drag which increases towards the overlying thrust this predating the shear due to flexure a contoured map of the values suggests that major folds within the thrust sheet developed as a result of stick or drag on the floor thrust prior to ramp climb this led to differential movement in the transport direction causing layer parallel shortening accompanied or followed by buckling of the layers a map showing layer normal shear strains 2 suggests that the heilam sheet forms a major surge zone the central part having moved further wnw than the northern and southern parts this has led to components of lateral shear along parts of the sheet especially in the north producing curvature of the imbricate faults

comparison of dike intrusions in an incipient seafloor spreading segment in afar ethiopia seismicity perspectives 1 oceanic crust is accreted through the emplacement of dikes at spreading ridges but the role of dike intrusion in plate boundary deformation during continental rupture remains poorly understood between 2005 and 2009 the 70 km long dabbahu manda hararo rift segment in ethiopia has experienced 14 large volume dike intrusions 9 of which were recorded on temporary seismic arrays a detailed comparison of the seismic characteristics of the seismically monitored dikes is presented with implications for dike intrusion processes and magmatic plumbing systems all of the migrating swarms of earthquakes started from a 80 of energy is released during the propagation phase with minimal seismic energy release after the dike propagation ceases we interpret that faulting and graben formation above the dikes occurs hours after the passage of the dike tip coincident with the onset of low frequency earthquakes dike lengths show no systematic reduction in length with time suggesting that topographic loading and stress barriers influence dike length as well as changes in tectonic stress the propagation velocities of all the dikes follow a decaying exponential northward propagating dikes had faster average velocities than those that propagated southward suggesting preconditioning by the 2005 megadike or ongoing heating from a subcrustal magma source north of the midsegment

fault related folding in the ramshorn peak area idaho wyoming thrust belt abstract the ramshorn peak area of the idaho wyoming thrust belt lies in the toe of the prospect thrust sheet along the eastern margin of the exposed part of the thrust belt the terrain is folded with axes trending n s and wavelengths ranging from 3 to 4 3 km thrusts occur exclusively along the eastern part of the map area where the toe of the prospect thrust sheet is thinnest the easternmost thrusts are backthrusts monoclinally folded rocks are thrust on less deformed rocks south of ramshorn peak this fold and fault complex is interpreted to have formed by thrusting over a large oblique and small forward step the oblique step is responsible for the formation of the monocline in the hanging wall of the thrust all faults and associated folds are rotated by subsequent buckle folding second and third order folds folds at the scale of the ramshorn peak fold and fault complex and smaller appear to be isolated features associated with faults fault related folds rather than buckle folds because they are not distributed throughout the map area these folds were probably initiated by translation and adhesive drag the early folding was terminated by large translation over a stepped thrust surface which caused additional folding as the hanging wall rocks conformed to the irregular shape of the footwall the rich model is utilized to explain the ramshorn peak complex because the fold is of monoclinal form and is an isolated feature rather than part of a buckle fold wave train

new tectonic divisions of the grenville province southeast canadian shield on the basis of geological geophysical and geochronological data the grenville province has been divided into three first order longitudinal belts the parautochthonous belt pb allochthonous polycyclic belt apb and allochthonous monocyclic belt amb these are set apart by three first order tectonic boundaries the grenville front gf allochthon boundary thrust abt and monocyclic belt boundary zone mbbz the belts are subdivided into terranes based on internal lithological character the gf separates the archean to proterozoic foreland northwest of the orogen from reworked equivalents to the southeast continuous at the scale of the orogen its main characteristic is that of a crustal scale contraction fault the pb although less clearly identified along the length of the orogen in most places represents upgraded and tectonically reworked rocks of the adjacent foreland the boundary between the pb and the apb to the southeast the abt is most clearly delineated in the eastern half of the province it is the locus of major crustal delamination along which high grade mostly middle proterozoic polycyclic terranes were tectonically transported northwest toward and onto the pb the amb comprises two separate areas underlain by the wakeham supergroup and what is currently known as the grenville supergroup respectively its basal contact the mbbz is a decollement zone of variable kinematic significance between older polycyclic rocks and tectonically overlying monocyclic rocks this first order zonation implies a tectonic polarity to the grenville province superimposed on which are second order features evident from contrasting tectonic styles and radiometric ages these characteristics are consistent with a diachronous or oblique collisional model for the grenville orogen

fault slip distribution of the 1999 mw 7 1 hector mine california earthquake estimated from satellite radar and gps measurements we use a combination of satellite radar and gps data to estimate the slip distribution of the 1999 m w 7 1 hector mine earthquake a right lateral strikeslip earthquake that occurred on a northwest southeast striking fault in the southern california mojave desert the data include synthetic aperture radar interferograms insar from both ascending and descending orbits radar amplitude image offset fields sario for both ascending and descending azimuth directions and campaign gps observations from 55 stations provided by agnew et al 2002 we model the fault with nine segments derived from the field mapped fault rupture the sario data and aftershock locations we first estimate the dip of each fault segment as well as a single constant strike slip component across each segment resulting in an average dip of 83 to the northeast and slip of up to 5 6 m then we fix the optimal fault segment dip discretize the fault segments into 1 5 km 1 5 km patches and solve for the variable slip distribution using a nonnegative least squares method that includes an appropriate degree of smoothing our preferred solution has both right lateral strike slip and reverse faulting the estimated geodetic moment is 5 93 10 19 n m m w 7 1 similar to seismological estimates indicating that there are insignificant interseismic and postseismic deformation signals in the data we find strike slip displacements of up to 6 0 m and reverse faulting of up to 1 6 m with the maximum slip located just northwest of the epicenter most of the slip is concentrated northwest and south of the epicenter little slip is found on the northeastern branch of the fault the sario data and our modeling indicate that the amount and extent of surface fault rupture were underestimated in the field

the palaeoenvironment and deformation of beaconites like burrows in the old red sandstone at gortnabinna sw ireland the gortnabinna area contains a number of well preserved deformed burrows within the fluviatile old red sandstone of s ireland detailed logging of the local succession enables subdivision into four facies facies 1 represents sheet flood deposits facies 2 ephemeral stream channels facies 3 marginal sheet flood deposits and facies 4 low energy settling of fines the burrows occur in facies 1 and 4 the sediments are now folded into a large upright westward plunging anticline the fold has a well developed axial planar cleavage and is extended along strike by listric normal faults the burrows in facies 1 were originally circular and orthogonal to bedding and reveal a n s layer parallel shortening strain of 2 1 followed by a shear strain of 1 48 during folding these burrows have a bimodal distribution of diameters with peaks in the ranges 50 80 and 110 170 mm the burrows of facies 4 are sub parallel to the bedding plane and are generally curved with a maximum width of 180 mm and lengths of up to 2 3 m these burrows display passive infill following abandonment the burrows are morphologically similar though distinct from beaconites barratti which has been described from elsewhere in the old red sandstone and are tentatively interpreted as lungfish burrows

thrust tectonics of s devon the tectonics of the area from start point to chudleigh sw england are shown to be dominated by northward verging thrust nappes two major thrusts are postulated the dartmouth antiform is considered to be a major thrust structure not a fold structure as previously interpreted the torquay limestones are allochthonous and part of a klippe of a major thrust nappe with a minimum displacement of 13 km northwards a progressive and diachronous sequence of deformation has been established with deformation starting in the s and moving n many of the folds and cleavages are probably related to the strains developed locally in thrust tips and may have no regional correlation more intense and complex deformation is found in the s compared to the n the diachroneity is also reflected in the k ar mineral ages in the s at start point southward verging backfolding is the last significant hercynian deformation phase folding the thrust nappes

geological constraints structural evolution and deep geology of the northwest scottish caledonides the front of the caledonian orogenic belt in nw scotland is marked by a zone of foreland thrusting which contains imbricates of lewisian basement proterozoic cover and cambrian shelf sediments faults within this moine thrust belt in most localities propagated in a foreland directed sequence therefore the earliest of these thrusts was the moine thrust which carried a basement assemblage of moine and lewisian rocks over the cambrian shelf the development of large duplex and imbricate structures in the footwall postadted movement on this thrust hence a restoration of these imbricates provides a minimum estimate of the eastern extent of the cambrian shelf no synsedimentary or prethrust extensional features are recognised within the cambrian succession which displays a remarkable stratigraphic and thickness consistency this implies a corresponding consistency in crustal structure beneath these cover rocks immediately prior to caledonian thrusting a balanced cross section constructed across the northern part of the moine thrust belt has given a restored width for the cambrian shelf of 54 km geophysical data suggest that the present foreland crustal thickness is 28 km if this is taken to be the crustal thickness beneath the cambrian shelf during caledonian thrusting a minimum cross sectional area of 1512 km of lewisian rocks must remain beneath the present outcrop of the moine thrust sheet this foreland basement wedge may have been imbricated subsequent to the development of the cambrian imbricates caledonian structures within the moine thrust sheet are interpreted as forming an imbricate stack with the moine thrust acting as a floor thrust the ductile imbricate thrusts possibly roofed into the naver slide late open folds which warp foliation slides and metamorphic isograds within the moine sheet may represent culminations in the foreland lewisian imbricates beneath the moine thrust these postulated culminations may have been sufficient to initiate gravity driven structures this may explain late movements on high level faults within the moine thrust belt

initiation of a proto transform fault prior to seafloor spreading transform faults are a fundamental tenet of plate tectonics connecting offset extensional segments of mid ocean ridges in ocean basins worldwide the current consensus is that oceanic transform faults initiate after the onset of seafloor spreading however this inference has been difficult to test given the lack of direct observations of transform fault formation here we integrate evidence from surface faults geodetic measurements local seismicity and numerical modelling of the subaerial afar continental rift and show that a proto transform fault is initiating during the final stages of continental breakup this is the first direct observation of proto transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms we demonstrate that they can initiate during late stage continental rifting earlier in the rifting cycle than previously thought future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading

shallow axial magma chamber at the slow spreading erta ale ridge thermal models predict that spreading velocity at mid ocean rifts should influence the geometry of the underlying magma chamber insar data from the slow spreading ethiopian rift identify a shallow elongated magma chamber a feature usually associated with fast spreading rifts beneath the erta ale segment implying that spreading velocity may not be so important after all

including strain data in balanced cross sections abstract almost all published balanced cross sections include cleaved and or strained rocks and may therefore violate the requirement that bed lengths and formation areas are preserved between undeformed and deformed states knowledge of distortions presence and type of cleavage and overprinting relations in poly deformed segments are necessary to define area changes related to volume loss strike elongation and poly deformation work in the southern and central appalachians has shown variation in structural complexity and changing deformation styles both along and across the thrust belt this necessitates separate treatment of individual sheets during balancing regions of little or no interstratal slip pinning points should be chosen for each thrust sheet and each fold strain in the sheet in external sheets where deformation intensity increases towards thrusts pin points should be back limb inflection points in intermediate and internal sheets where cleavage is approximately coeval with folding pin points should be at fold hinges where cleavage and strain are heterogeneously developed throughout a stratigraphic sequence a line length balance using massive relatively uncleaved beds e g knox group southern appalachians can be used as a basis for restoring the section unstraining of deformed layers given strain data and cleavage trajectories for individual thrust sheets provides line lengths and formation areas which can be fitted to the key bed of the undeformed section cleavages which partially or fully postdate folding indicate continued shortening in the thrust wedge and modify earlier pinning points in upper sheets these cleavages and poly deformed segments are difficult to restore as strain data are generally scarce in the metamorphosed internal thrust sheets

structural framework of deolapar area central india and its implications for proterozoic nappe tectonics abstract a controversial nappe like structure deolapar nappe in the intensely deformed and polymetamorphosed precambrian rocks of the sausar fold belt around the deolapar area of central india has remained the subject of a long standing unresolved debate while some workers interpreted it as the inverted limb of a large scale recumbent fold which has been thrusted onto the adjoining part of the supracrustal sequence sausar group others discounted the very existence of the nappe because of the lack of structural evidence in favour of the recumbent fold thrust structure recent structural study by the present authors around the deolapar area and also in the adjoining parts of the sausar fold belt has looked back into the controversy surrounding the deolapar nappe the study reveals that the deolapar nappe is constituted of an allochthonous block of metasedimentary rocks and crystalline gneiss migmatites placed over the metacalcareous rocks lohangi formation of the sausar group along a low angle thrust plane marked by a sheared fibrolite biotite schist the whole thrust sheet has been folded by later f2 upright folds however careful structural mapping confirmed that there is no evidence of any map scale f1 recumbent fold in the whole area although small scale thrust related reclined f1 folds are occasionally found also there is not enough field evidence suggesting regional inversion of stratigraphic succession so as to indicate the lower limb of a recumbent fold the deolapar nappe can therefore be best interpreted as a thrust nappe details of the existing controversy the new observations and a new tectonic model of the deolapar nappe is presented here

motion in the north iceland volcanic rift zone accommodated by bookshelf faulting along mid ocean ridges the extending crust is segmented1 on length scales of 10 1 000 km where rift segments are offset from one another motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis where segments overlap non transform offsets with a variety of geometries2 accommodate shear motions here we use micro seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north iceland between the rift segments we identify a series of faults that are aligned sub parallel to the orientation of the main rift these faults slip through left lateral strike slip motion yet movement between the overlapping rift segments is through right lateral motion together these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf faulting mechanism named after its resemblance to a tilting row of books on a shelf3 the faults probably reactivated existing crustal weaknesses such as dyke intrusions that were originally oriented parallel to the main rift and have since rotated about 15 clockwise reactivation of pre existing rift parallel weaknesses contrasts with typical mid ocean ridge transform faults and is an important illustration of a non transform offset accommodating shear motion between overlapping rift segments

geometry and evolution of the frontal part of the magallanes foreland thrust and fold belt vicuna area tierra del fuego southern chile the magallanes foreland thrust and fold belt is a thin skinned foreland thrust and fold belt of paleocene to oligocene age that deforms upper jurassic through tertiary volcanic volcaniclastic and siliciclastic strata of the magallanes basin southern andean cordillera chile this paper is a detailed description and analysis of the geology and structural evolution of the thrust front vicuna area of southern tierra del fuego reflection seismic and well data together with 1 50 000 scale geological mapping have been used in the analysis in the southern part of the vicuna area two different thrust systems have been found an upper imbricate fan that deforms upper jurassic and cretaceous strata and a younger lower duplex composed of cretaceous and probably upper jurassic rocks the imbricate fan is characterized by fault propagation folding in which listric thrust faults merge downward into a sole thrust that probably is located within the upper jurassic stratigraphy this fan system generates an intercutaneous wedge with a well developed triangle zone at its leading edge the sole thrust of the upper imbricates forms the roof thrust of the underlying duplex in the northern part of the vicuna area the syntectonic sedimentary wedge of the foredeep consists of late cretaceous through tertiary siliciclastics that have been deformed and uplifted by passive back thrusting at the triangle zone the structural style in the foreland region shows three main subhorizontal detachment levels located within the sedimentary wedge as a result of the progressive transfer of slip from the thrust belt to the foreland minor blind thrusts produce stacked pop up and triangle structures that result in complex geometries in the cores of anticlines a forward breaking sequence of thrusting is interpreted during deformation the active foredeep wedge migrated at least 10 km northward balanced geological cross sections indicate approximately 60 30 km shortening for this part of the magallanes thrust belt

constraints on the age of movement of the montsech and cotiella thrusts south central pyrenees spain palaeocene to oligocene thrusting in the pyrenees was synchronous with the development of the south pyrenean foreland basin collectively the tremp graus ainsa and jaca basins sedimentological and stratigraphical evidence from the foreland basin is used to constrain the age of movement of the montsech cotiella and external sierras thrusts movement on the emergent imbricate fan at the montsech front began in the early eocene illerdian during an early phase of thrusting that detached in the triassic evaporites in that region this phase of thrusting defined the tremp graus basin as a thrust sheet top basin following the early thrusting cuisian age tectonics in the tremp graus and ainsa basins was dominated by minor normal fault movements during flexural subsidence associated with thrust sheet loading growth of the boltana anticline to the west of the ainsa basin indicates that the ainsa basin had become detached as a thrust sheet top basin in the lutetian movement on the pena montanesa thrust and by inference on the cotiella thrust began in post lutetian times during the propagation of lower thrusts that involved pre triassic variscan basement thrusting in the external sierras caused folding and faulting of oligocene sediments indicating the continued southward propagation of basement involved thrusts in the upper eocene and oligocene many faults in the southern pyrenees have evidence of early normal and later reverse senses of movement indicating reactivation of normal faults during thrusting

thrust related strain gradients and thrusting mechanisms in a chevron folded sequence southeastern australia an e directed imbricate fan thrust system within a chevron folded monotonous quartz rich turbidite succession shows marked gradients in strain from the upper to lower parts of individual thrust sheets upper parts of thrust sheets show low to moderate strains xz strains 30 1 tight inclined chevron folds ila 30 with predominantly w dipping axial surfaces intense phyllitic cleavage with a strong down dip mineral stretching lineation and long curved tapering quartz pressure shadows on pyrite such differences in fold geometry the intensity of the cleavage and the geometry of quartz fibres in pressure shadows require thrust sheet emplacement to be synchronous with the internal deformation of sheets thrust sheets show a change from an approximately constant volume plane strain coaxial deformation in their upper parts to non coaxial high strain deformation with incremental i between those expected for pure shear and simple shear i e 0 i 5 45 in their basal parts observed geometries and intensities reflect partitioning of strain and deformation mechanisms within individual thrust sheet volumes both during the initiation and propagation of subsurface detachment faults and with their emplacement to shallower crustal levels strain softening related to chevron folding and more intense cleavage development within a 500 m basal zone has controlled the emplacement of these turbidite dominated thrust sheets transport and internal deformation of the thrust sheets did not involve gravitationally induced spreading but required either a push from the rear or some form of underthrusting

the nature of the crust beneath the afar triple junction evidence from receiver functions the afar depression is an ideal locale to study the role of extension and magmatism as rifting progresses to seafloor spreading here we present receiver function results from new and legacy experiments crustal thickness ranges from 45 km beneath the highlands to 16 km beneath an incipient oceanic spreading center in northern afar the crust beneath afar has a thickness of 20 26 km outside the currently active rift segments and thins northward it is bounded by thick crust beneath the highlands of the western plateau 40 km and southeastern plateau 35 km the western plateau shows vp vs ranging between 1 7 1 9 suggesting a mafic altered crust likely associated with cenozoic flood basalts or current magmatism the southeastern plateau shows vp vs more typical of silicic continental crust 1 78 for crustal thicknesses 2 0 can only be explained by significant amounts of magmatic intrusions in the lower crust this suggests that melt emplacement plays an important role in late stage rifting and melt in the lower crust likely feeds magmatic activity the crust between the location of the miocene red sea rift axis and the current rift axis is thinner 2 0 than beneath the eastern part of afar 26 km vp vs 1 9 this suggests that the eastern region contains less partial melt has undergone less stretching extension and has preserved a more continental crustal signature than west of the current rift axis the red sea rift axis appears to have migrated eastward through time to accommodate the migration of the afar triple junction

duplex geometry an example from the moine thrust belt abstract the geometry and microstructure of a small duplex formed in one bed from the moine thrust belt of northwest scotland is reported the structure is seen in oblique section within the cambrian pipe rock in an area of low strain a range of movement direction indicators are present in the structure an early grain shape fabric developed close to the roof thrust is taken as the best estimate of the overall movement direction towards 287 slickensides in the gouge developed on movement planes within the duplex show varied orientations on a given plane and are not considered useful indicators of thrust transport direction branch lines exposed converge and diverge suggesting little lateral continuity of the exposed structure the microstructures present within the structure indicate an increase in localised deformation and in cataclastic behavior as the duplex evolved early layer parallel shear is ubiquitous giving rise to an elongate grain shape fabric close to bedding surfaces in early formed horses a layer parallel oblate grain shape fabric which shows localised slip zones is overprinted by gouge formation later formed horses show only fracturing and gouge development this sequence is attributed to stick slip behavior in the propagation or displacement of the original fault now the floor thrust

reply to the discussion on carbon and oxygen isotope systematic of a paleoproterozoic cap carbonate sequence from the sausar group central india by anupam chattopadhyay 2015 published in palaeogeography palaeoclimatology palaeoecology 433 156 157 abstract the discussion on our paper on paleoproterozoic glaciation arises out of the failure of previous geological mapping in the sausar belt to identify many features which were reported earlier and to examine the outcrops critically for evidence of volcanism and glaciation in central india the details of these are given in this reply

strike slip tectonics during rift linkage the kinematics of rift segment linkage in magmatic rifts remain debated strain patterns from afar provide tests of current models of how segmented rifts grow in areas of incipient oceanic spreading here we present a combined analysis of seismicity interferometric synthetic aperture radar insar and gps derived strain rate maps to reveal that the plate boundary linkage between the red sea and gulf of aden rifts of afar is accommodated primarily by distributed extensional faulting large rotations about vertical axes predicted by bookshelf faulting models are not detected additionally models of stress changes and seismicity induced by recent dikes provide poor fits to the observed time space patterns of strike slip earthquakes instead we explain these features as resulting from rift perpendicular shearing at the tips of spreading rifts where extension terminates against less stretched lithosphere our results demonstrate that distributed extension drives rift perpendicular shearing achieving plate boundary linkage during incipient seafloor spreading

fold thrust structures where have all the buckles gone special publication title folding and fracturing of rocks 50 years of research since the seminal text book of j g ramsay we dedicate the paper to the memory of martin casey 1948 2008 who did much through good humored argument to ensure that buckling ideas were not lost to what he called the ramping club the thrust belt community the fold thrust research group has been funded by interoil oilsearch and santos we thank paul griffiths and anonymous referee for comments together with hermann lebit for scientific editing the views expressed here of course remain those of the authors

the strain and textural history of thin skinned tectonic zones examples from the assynt region of the moine thrust zone nw scotland abstract the moine thrust zone at assynt is a classical example of a foreland propagating thin skinned thrust zone ideal for an examination of thrust related folds deformation textures and strains and complex incremental strain history divergent transport directions oblique trending folds duplex zones and extensional strains normal to the main transport direction may all be explained in terms of thrust propagation leading to the development of oblique to lateral ramps the majority of thrusts cut up section from basement to cover in the transport direction but there is also localised extensional flow and thinning of the thrust sheets in northern assynt the thrust zone involves a wide vertical zone of sinistral shear within which forethrusts backthrusts and associated folds and cleavages are oblique to the general transport direction it is suggested that north of this shear the thrusts moved further probably under a thicker cover while to the south movement was more intermittent probably under a thinner cover this variation is probably due to a change in thrust geometry in the moines east of the moine thrust zone causing a variation in gravitational potential along the length of the moine thrust

predicting pore pressure in active fold thrust systems an empirical model for the deepwater sabah foldbelt abstract measurements related to mudrock shale and siltstone porosity such as acoustic velocity density or electrical resistivity have traditionally been used to predict pore pressures in extensional stress settings the underlying assumption is that burial and vertical effective stress ves which is the overburden minus the pore pressure controls the compaction of these rocks through porosity loss the dataset presented here compares ves and acoustic velocity of similar composition mudrocks in both an extensional and a compressional stress setting in the extensional stress environment the mudrocks follow a typical compaction trend with a porosity loss and increase in acoustic velocity that can be related to ves in an active fold thrust belt the compressive stresses further reduce the porosity and increase the acoustic velocity of the mudrocks first a layer parallel shortening compacts sediments beyond what is observed for the ves this additional compaction is further enhanced near thrust faults and in anticlinal forelimbs presumably due to additional shear stress in these areas the mudrocks located in folds that are buried by additional sedimentation do not compact again until the tectonic compaction is overridden by enough new burial after that the mudrocks follow the observed extensional setting compaction trend in the fold thrust belt the observed reduction in porosity by stresses other than burial leads to an under prediction of pore pressure using traditional methods to account for this we present a correction that can be applied to the acoustic velocity or porosity using two parameters a proximity to thrust faults and anticlinal forelimbs and b the amount of burial after fold formation with these corrections the extensional velocity ves compaction trend can be used to accurately predict pore pressure within the active fold thrust belt the correction is calibrated with well data and is empirical none the less it is a first step toward understanding the magnitudes of the compressive lateral stresses involved in generating the observed porosity loss within the fold thrust belt

tubular folds and sheath folds definitions and conceptual models for their development with examples from the grapesvare area northern sweden abstract two parameters are used in the classification of highly non cylindrical folds the hinge line angle and the ratio of the length of the cone axis to that of the long axis of the cross section of the structure at the position where the hinge angle was measured if in a non cylindrical fold a cross section can be selected so that the hinge angle w measured where the cross section cuts the cone is less than 90 and the length of the cone axis x is more than a quarter of the length of the long axis y of the cross section the fold is a sheath fold a tight sheath fold with w x y 1 can be termed a tubular fold a tubular fold is thus an extreme type of sheath fold while sheath folds with w 20 but x y 0 25 but tubular folds from the grapesvare area comply to the definition given above and they occur in a structural setting that is consistent with the models described

the thrust and shear zones of the moine thrust zone and the nw scottish caledonides the caledonian thrust belts of nw scotland exhibit many features typical of foreland fold and thrust belts the major structures propagated westwards towards the foreland the thin carapace of cambro ordovician sediments being accreted in imbricate and duplex zones beneath the major thrust sheets on a broad scale the rules of thrust geometry are obeyed but many local anomalies are found particularly in the assynt region in assynt several large fault bounded gravity spreading structures surge zones have been identified extensional faults have been traced into strike slip and then contractional faults there are zones of extensional and contractional imbricate faults and extensional and contractional ductile flow similar structures have been found along the length of the moine thrust belt suggesting that gravity spreading was an important mechanism during the later stages of movement within the main caledonian fold belt there are several major tectonic slides which are recognized by a local increase in intensity of deformation the majority are nw verging and were originally gently dipping in the highlands se of the great glen there are early major se verging recumbent folds associated with considerable crustal thickening reflected in high pressure 12kbar metamorphism many of the major slides may have formed by spreading away from this thickened crust the caledonian deformation and metamorphism occurred later in the moines and moine thrust zone than in the dalradian and grampian moines to the se of the great glen this may reflect different levels of deformation alternatively it may argue for a large strike slip displacement on the great glen fault juxtaposing crustal blocks which had experienced different ages of caledonian deformation and metamorphism the late movement on the moine thrust involved spreading from an area of thickened caledonian crust formed either by the stair step trajectory of the basal thrust to the moines and or by thickening due to intrusion of large masses of late caledonian granites

modal response of a deep stratified lake western lake constance the orthogonality relation of the internal wave equation has been derived and has been included in normalizing the particular modes under the assumption that observed currents in western lake constance all result from basin scale internal waves they can be decomposed into contributions of the particular modes in all cases of a response to a wind stress at first the velocity profiles indicate a quite pure first mode wave after strong winds a second mode wave contributes remarkably and consistently to the velocity profile later during the response during the zero transition of the first mode the observed velocity profile can be attributed mainly to a second mode wave

rift segment interaction in orthogonal and rotational extension experiments implications for the large scale development of rift systems abstract during extension of the continental lithosphere rift basins develop these are often initially offset and must interact and connect in order to create a continuous rift system that may ultimately achieve break up when simulating extensional tectonics and rift interaction structures analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation resulting in rotational extension and associated rift propagation and structural gradients here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings our modelling efforts show that rotational extension and orthogonal extension produce significantly different large scale structures rotational extension can cause important variations in rift maturity between rift segments delay rift interaction zone development and make rift segments propagate in opposite directions still local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting furthermore we find that various degrees of rift underlap produce three basic modes of rift linkage structures low underlap distance high angle experiments develop rift pass structures with increasing underlap distance ca 40 transfer zone basins develop high degrees of underlap 30 tend to result in en echelon sub basins our results match with data from previous modelling efforts and natural examples we furthermore propose a large scale tectonic scenario for the east african rift system based on rotational extension and associated rift propagation these insights may also be applicable when studying other large scale rift systems

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions

spatial structure of the dominant basin scale internal waves in lake kinneret field data and numerical simulations were used to investigate the effects of basin shape continuous stratification and rotation on the three dimensional structure of the dominant natural basin scale internal wave modes in lake kinneret a stratified lake large enough so that the earth s rotation influences the wave motion the structure of the modes was inferred from power spectral density of measured and simulated isotherm vertical displacements and from rotary power spectral density of isopycnal velocities obtained from numerical simulations the shape of the lake at the level of the thermocline in conjunction with the dispersion relationship determines the horizontal configuration of the natural modes the dominant response to wind forcing was an azimuthal and vertical mode 1 kelvin wave with a natural period of 22 6 h that propagated around the entire basin the second most dominant response was a vertical mode 1 wave with a natural period close to 10 5 h and composed of two counter rotating

two types of ductile deformation regional metamorphic belt abstract the geometry of plastic strains of regional metamorphic rocks is divided into two types one is the uniaxial extension strain prolate and the other the biaxial shortening strain oblate from the plane strain simple shear to the uniaxial shortening geometry the uniaxial extension geometry is found in deformation of metamorphic rocks of the sambagawa ryoke kamuikotan and franciscan belts while the biaxial type is seen in deformation of the shimanto the alpine the appalachian and the caledonian metamorphic rocks the classification of the metamorphic belts by means of strain geometry and strain path is independent of the metamorphic facies series the difference in strain geometry of metamorphic belts is responsible mainly for the tectonic stress regime in metamorphism it is suggested that the biaxial type belongs to the collision type orogeny and the uniaxial type to the cordilleran type orogeny

a genetic link between transform and hyper extended margins abstract the similarity between the geometry of the west african and south american coastlines is among one of the strongest natural observations supporting the plate tectonic paradigm however using classical plate tectonic approaches to model these conjugate transform margins results in a high degree of variability in palaeogeographic reconstructions using state of the art 3d coupled thermo mechanical numerical models we simulate for the first time crustal deformation at the onset of oceanisation along large offset oblique margins our models show that obliquity causes oceanic rift propagation to stall resulting in an apparent polyphased tectonic evolution and in some circumstances leads to the formation of hyper extended margins as a result conjugate margins located at the edge of future fracture zones are highly asymmetric from rifting to spreading with their lengths differing by a factor of 5 to 10 before the final phase of break up occurs accounting for this discrepancy should ameliorate future palaeogeographic reconstructions

40ar 39ar ages of precambrian manganese ore minerals from sweden india and morocco in order to test their chronometric potential 40ar 39ar stepheating and 4he analyses have been carried out on five manganese ore minerals of the hollandite cryptomelane series from three precambrian manganese deposits ultevis sweden sitapar india bachkoun morocco samples from the metamorphic occurrences ultevis and sitapar yielded ar ages of 1 8 ga and 0 95 ga interpreted as the age of postmetamorphic cooling hollandites ultevis and of an early k introducing alteration process subsequent to amphibolite facies metamorphism cryptomelanes sitapar both data are consistent with known chronologies of the svecokarelian and satpura orogenic cycles a date of 670 ma obtained for a hollandite from a volcanogenic vein deposit bachkoun however contrasts with published extrusion ages of 580 560 ma for the volcanic host rocks ouarzazate series probably due to incorporation of excess argon the use of the 40ar 39ar technique together with multiple isotope systematics made it nevertheless possible to establish a reasonable estimate of a mineralization age close to 580 ma measurement of fractional ar losses during vacuum step heating 500 1600 c although indicating good ar retentivities failed to define model diffusion parameters because of non linear arrhenius arrays helium diffusion results 200 1200 c indicated retention of radiogenic 4he by the samples corroborated by u he mineral dates between 0 96 and 0 31 ga potassium bearing manganese oxides are therefore able to retain argon possibly also 4he through geological times and may thus provide ages of ore forming processes and perhaps later cooling and alteration stages

a structural analysis of the moine thrust zone between loch eriboll and foinaven nw scotland abstract the northern part of the moine thrust zone as exposed around the valley of srath beag sutherland was developed by thrusts propagating in the tectonic transport direction deformation on any particular thrust surface evolved from dominantly ductile to dominantly brittle with time the foreland has been progressively accreted onto the overriding moine thrust sheet by duplex formation a process which has continuously folded the roof thrust and the rocks above its hanging wall fold culminations and depression can be related to lateral ramps which may give the rocks above the hanging wall a complex history of extensional and compressional strains normal to the transport direction folds within the thrust zone are laterally independent because they are controlled by short lived variations in deformation style on an evolving thrust footwall topography therefore there may be no correlation between structures across or along the thrust zone this variation limits the construction of balanced cross sections as structure cannot be projected onto particular section lines

thrust kinematics and transposition fabrics from a basal detachment zone eastern australia abstract emplacement of an upper crustal leading imbricate fan thrust belt in the lachlan fold belt of eastern australia was accomplished along a 0 5 1 km thick zone of heterogeneously deformed low grade phyllonite in pelitic rock continuous recrystallization and neocrystallization of mica in a zone of transposition layering has provided a weak zone at the base of a 100 km wide 150 km exposed length 10 km thick thrust system the basal deformation zone is characterized by a low moderately dipping strong intense transposition foliation enclosing elongate fault bounded slices up to 20 km long 5 km wide in map view of disrupted cambrian metavolcanics and upper ordovician black shales and slates these are derived from a structurally lower zone of duplexing or from the overturned limbs of anticlinorial structures the detachment zone is a 10 15 km wide zone of intense deformation showing a transition from open upright folds with weak cleavage to inclined tightisoclinal folds with strong axial surface cleavage the intensity of minor faults also increases into the zone leading imbricate fan thrust belts show maximum deformation effects along the basal detachment which forms the frontal or leading fault the leading imbricate geometry is due to emplacement of the basal detachment zone up the lowest and last formed imbricate thrust movement is along a relatively ductile low viscosity layer at the base where strain softening occurs with development of transposition layering this enables confined flow along the basal zone with transport and emplacement of the fold system and duplex zone to higher structural levels reaction enhanced ductility and grain boundary sliding may be important deformation mechanisms responsible for this flow localized polydeformation marked by mesofolds and crenulation cleavage reflects the interaction between thrust sheets and the movement on faults

structural evolution of gavilgarh tan shear zone central india a possible case of partitioned transpression during mesoproterozoic oblique collision within central indian tectonic zone abstract gavilgarh tan shear zone gtsz a map scale ductile shear zone occurring in the southern part of the central indian tectonic zone citz contains many important clues to the tectonics of this important proterozoic mobile belt of central india which is yet controversial and hotly debated data from structural mapping microstructural study and kinematic analyses of the gtsz have been presented in this paper which clearly implies that gtsz is mainly represented by a wrench dominated transpressional shear zone with sinistral shear sense analysis of lineation patterns shows that while horizontal stretching dominated within the ductile shear zone rocks on either side north and south of it stretched vertically with a transition zone of simultaneous horizontal and vertical stretching in between these two domains a general transpressional strain has thus been partitioned into wrench and pure shear dominated domains at large scale further strain partitioning also took place at the outcrop scale geochemistry of the granitoids occupying the shear zone and its surrounding area indicates continental collision set up which supports the structural model on the basis of earlier proposed tectonic models and limited geochronological data available of the southern part of citz it is suggested here that an oblique collision and final suturing between the northern bundelkhand craton and southern bastar craton terrains within the citz around 1100 ma have been partly accommodated by partitioned transpression along gtsz and its surrounding gneiss granitoid terrain more intensive geochronological constraint of the collision event from both northern and southern side are however lacking at present