paleomagnetism of the silurian volcanism at almaden southern spain the silurian volcanism of the almaden mining district southern spain has been the object of a very detailed paleomagnetic study the 175 cores 25 sites sampled all along the eastern termination of the almaden syncline were subjected to magnetic susceptibility measurements thermomagnetic cycles progressive alternating field and thermal demagnetizations as well as microscopic observations the magnetic mineralogy of these volcanics flows is dominated by magnetite in at least two very distinct forms including small cubic distributed grains and large elongated crystals the observed anisotropy of magnetic susceptibility varies according to the relative abundance of the two magnetite phases showing a complete inversion of the susceptibility tensors this anisotropy has been related to the emplacement of the flows on a gentle slope dipping 10 toward the southwest implying that their volcanic source was located to the northeast the paleomagnetic analysis has revealed two characteristic components of magnetization the first direction d 109 i 11 95 9 pole position at 11 s 69 e dp 5 dm 9 yields a negative fold test and is interpreted as a late paleozoic partial overprint its declination is 46 away of the expected late paleozoic field direction which confirms that rotations are still occurring after the main folding phases of the variscan orogeny the second direction d 62 i 36 95 14 pole position at 8 n 118 e dp 9 dm 13 is prefolding as evident from the significant clustering when applying appropriate tectonic corrections and we interprete it as the original magnetization of silurian age the compilation of all paleozoic paleomagnetic results from spain reveals that the iberian meseta has been moving along with gondwana at high drift rates throughout ordovician and silurian times

kinematic constraints on thrust belt curvature from syndeformational magnetizations in the lagos del valle syncline in the cantabrian arc spain a detailed paleomagnetic study in a single first order refolded syncline in the western hinge zone of the variscan foreland fold and thrust belt of northern spain was carried out to constrain the tectonic deformation of the cantabrian arc results show a complex pattern of structural deformation and acquisition of at least two secondary syntectonic magnetizations on the basis of local and regional fold tests we document an early permian b component that postdates earlier f1 westphalian and stephanian thrusting and folding and a late carboniferous to earliest permian c component that is synfolding acquired during fl the b component only resides in the carbonates and is carried by magnetite the c component is carried by magnetite in the gray carbonates and by hematite in the red beds both b and c components reveal significant vertical axis rotations that indicate that an originally cylindrical lagos del valle syncline was refolded f2 about a vertical axis after acquisition of the b component magnetization the pattern of rotations suggests that the entire cantabrian arc may have originated as a much more linear fold and thrust belt which was later modified by significant oroclinal bending with relative rotations totaling at least 120 the temporal distribution of remagnetization and deformation also suggests a protracted variscan orogeny in the area lasting from at least the late carboniferous into the permian

tectonothermal evolution of the badajoz cordoba shear zone sw iberia characteristics and 40ar 39ar mineral age constraints field characteristics indicate that the badajoz cordoba shear zone has had a long polyphase tectonothermal evolution 40ar 39ar cooling ages recorded within central ductile sectors of the badajoz cordoba shear zone range from ca 370 to 360 ma amphibole and 340 to 330 ma muscovite these are interpreted to date post metamorphic cooling through contrasting temperatures required for intracrystalline retention of argon following a regionally significant variscan tectonothermal overprint these results combined with field relationships indicate that the badajoz cordoba shear zone experienced at least 15 km of uplift relative to adjacent areas during late paleozoic sinistral transpression the data also indicate that individual structural units within the shear zone experienced contrasting uplift histories both in total amount and rate the 40ar 39ar cooling ages indicate complete variscan rejuvenation of all older intracrystalline argon systems within central sectors of the badajoz cordoba shear zone

late cenozoic tectonism collapse caldera and plateau formation in the central andes abstract the evolution of andean volcanism including the formation of late miocene to recent collapse calderas on the puna plateau is generally interpreted in terms of the kinematic framework of the nazca and south american plates we present evidence that caldera dynamics and associated ignimbrite volcanism are genetically linked to the activity of first order nw se striking zones of left lateral transtension on the local and regional scales consequently ages of collapse calderas indicate activity of these fault zones which initiated at about 10 ma on the puna plateau the onset of such faulting points to a change in the deformation regime from dominantly vertical thickening to orogen parallel stretching upon reaching maximum crustal thickness and critical surface elevation horizontal magma sheets that formed at mid crustal level possibly due to heat advection by volume increase of asthenospheric mantle below thickened crust were tapped by sub vertical faults this accounts well for the observed tectono magmatic phenomena at surface it follows that formation of collapse calderas and eruption of voluminous ignimbrites appear to be related to the mechanical evolution of the andean plateau rather than to changes in the geometry of the wadati benioff zone or plate boundary kinematics

strain and rotation rate from gps in tibet anatolia and the altiplano 1 deformation measured by regional gps networks in continental plateaus reflects the geologic and tectonic variability of the plateaus for two collisional plateaus tibet and anatolia and one noncollisional the altiplano we analyze the regional strain and rotation rate by inverting gps velocities to calculate the full two dimensional velocity gradient tensor to test the method we use gridded velocities determined from an elastic block model for the eastern mediterranean middle east region and show that to a first order the deformation calculated directly from the gps vectors provides an accurate description of regional deformation patterns principal shortening and extension rate axes vertical axis rotation and two dimensional 2 d volume strain dilatation are very consistent with long term geological features over large areas indicating that the gps velocity fields reflect processes responsible for the recent geologic evolution of the plateaus differences between geological and gps descriptions of deformation can be attributed either to gps networks that are too sparse to capture local interseismic deformation or to permanent deformation that accrues during strong earthquakes the altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2 d dilatation everywhere vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau in contrast the collisional plateaus have large regions of quasi rigid body rotation bounded by strike slip faults with the opposite rotation sense from the rotating blocks tibet and anatolia are the mirror images of each other both have regions of positive dilatation on the outboard sides of the rotating blocks positive dilatation in the aegean correlates with a region of crustal thinning whereas that in eastern tibet and yunnan province in china is associated with an area of vertical uplift rollback of the hellenic trench clearly facilitates the rotation of anatolia rollback of the sumatra burma trench probably also enables rotation about the eastern syntaxis of tibet

strain displacement and rotation associated with the formation of curvature in fold belts the example of the jura arc abstract a new simplified genetic classification scheme for arcuate fold thrust belts is proposed based on total strain patterns and displacement vector fields we distinguish three extreme end member models 1 oroclines pure bending of an initially straight belt 2 piedmont glacier with divergent transport directions and 3 primary arcs a simple geometric model set up for the simulation of strain patterns in primary arcs with uniform transport direction demonstrates that divergent strain trajectories and rotations of passive marker lines do not require any divergence in displacement directions these often quoted arguments are insufficient for the identification of oroclinal bending or piedmont glacier type of arc formation only three dimensional restorations of an arc provide the critical information about displacement directions in their absence arc parallel stretches and rotations in comparison with total strains provide the most useful criteria for the distinction of arc formation modes as an example the jura fold thrust belt of the external alps is discussed a large set of strain data includes total shortening estimates based on balanced cross sections local strain axes orientations from the inversion of fault populations homberg c 1996 unpublished phd thesis universite de paris vi france tectonic stylolites and micro strains from twinning in sparry calcite strain trajectories maximum shortening direction computed from these data define a strongly divergent fan with a 90 opening a complete displacement vector field for the entire jura has been determined from balanced cross sections augmented with three dimensional block mosaic restorations philippe y 1995 unpublished phd thesis universite de chambery france displacement vectors diverge by about 40 markedly less than strain trajectories the non parallelism between strain trajectories and transport directions indicates that considerable wrenching deformation did occur in both limbs of the jura arc paleomagnetically determined clockwise rotations of 0 13 from ten sites kempf o et al terra nova 10 6 10 behind the right hand half of the jura arc and two sites with a combined 23 anticlockwise rotation behind the left hand half of the arc are and additional argument in favor of such a wrenching deformation we conclude that the jura arc formed as a primary arc with a minor component of piedmont glacier type divergence in transport directions

anatomy of an escape tectonic zone western irian jaya indonesia 1 the western fold and thrust belt of new guinea in irian jaya is presently a complex boundary dominated by the bird s head block escape from the collision between the australian plate and the remnants of volcanic belts carried by the pacific plate the escape rate given by geodetic measurements is of the order of 7 cm yr and movement is accommodated by a broad shear zone we analyze this shear pattern using fault slip analyses performed in the field from 1992 to 1996 combined with focal mechanism inversions moment tensors summations and surface trace of structures inferred from radar and multispectral satellite images time control of the deformation is attained by isotope dating of the recent syntectonic intrusives the geometry of the macro and microstructures occurred in two stages from the early pliocene to the present the first stage 5 to 2 ma is marked by flat and ramps structures guided by n60 e lateral ramps associated with a n60 e cleavage affecting the whole of western irian jaya the second stage 2 ma to present shows the collapse of the western fold and thrust belt and the escape of the bird s head and the lengguru belts along n60 e transtensile faults in the latter stage the strike slip offset is distributed on the n60 e schistosity zone along which some fracture planes are reactivated as left lateral transtensile faults shallow earthquakes moment tensors have been inverted for stress and summed to get strain rates to define contrasting structural provinces the spatial variations in both stress and strain fields from earthquakes and microtectonics show that 1 they are consistent and are assumed to be coeval 2 they reveal that oblique convergence is partitioned and 3 they are influenced by the existence of a free boundary we see no significant rotation of stress axes laterally along the escape zone instead stresses change according to the different orientations of basement structures and thus undergo rapid spatial variations or principal axes permutations eastward in front of the australian craton the oblique convergence is partly distributed on reverse and strike slip faults westward along the nw part of the craton we infer the seram trench is a free boundary which allows material to escape these results show that the evolution of the escape zone depends on the importance of the geometry of the former margin of australia e g tilted blocks which controls the style deformation relative to the regional stress and the boundary conditions which control the variation of the state of stress through time

origin of structural arcs of the carpathian balkan region burtman vs 1986 origin of structural arcs of the pat an balkan region in l p zonenshain editor tectonics of the eurasian fold belts tectonophysics 127 245 260 the late cretaceous reconstruction shows that the inner zones of the west east and south carpathians were adjacent to the dinarides and hellenides at that time forming together with them a linear fold system with bilateral vergence this fold system had a northwest strike in paleogene early miocene times the general deformation of the given part of the alpine belt took place it was caused by rotation of its asia minor balkan part relative to the more northern part of the belt this rotation 30 40 counter clockwise was accompanied by a shortening of the considered segment of the alpine belt by 30 hence exfoliation of the carpathian pontide and dinaric tauride branches of the alpine belt from each other and their disharmonious plastic deformation resulted in the formation of the aegean and tauride structural arcs and the carpathian structural loop during formation of the carpathian loop the gemeric tatric tectonic block moved northwards along the pemrinic oceanic substratum and rotated over 90 counter clockwise relative to the eastern alps

topography and subduction geometry in the central andes clues to the mechanics of a noncollisional orogen the central andean orogen between 12 and 32 s latitude exhibits a high degree of spatial order principally an extraordinary bilateral symmetry that is common to the earth s surface the underlying wadati benioff zone and the nazca south america plate kinematics which has been stable since the mid tertiary this spatial order must reflect the physical mechanisms of mountain building in this noncollisional orogen the shapes of the topography and subduction zone can be reduced to symmetric and antisymmeric components relative to any vertical symmetry plane the particular plane which minimizes the antisymmetry and maximizes the symmetry is well resolved and is essentially coincident with the stable euler equator of nazca south america relative motion since the mid tertiary that the topography subduction geometry and persistent mid tertiary plate kinematics share common spatial and geometric elements suggests that the distribution of topography in this orogen depends strongly on the dynamics of subduction other factors that might affect the topography and underlying tectonics such as climate and inherited structural fabric which have different spatial characteristics must be of less significance at a continental scale furthermore the small components of asymmetry among the various elements of the orogen appear to be mutually related in a simple way it is possible that this coupled asymmetry is associated with a late tertiary change in plate kinematics these observations suggest that there is a close connection between plate tectonics and the form of the earth s surface in this noncollisional setting it follows that the distribution of topography near convergent plate boundaries may provide a powerful constraint for understanding the dynamics of subduction

conical folding in the core of an orocline a geometric analysis from the cantabrian arc variscan belt of nw iberia abstract the cantabrian arc of the variscan belt has been recently defined as a true orocline constraining kinematics and deformation timing this curved sector of the orogenic belt is characterized by two different fold sets 1 one runs parallel to the outcrops of the main thrusts and describes a horseshoe shape concave towards the east and 2 another is radial to the arc a detailed geometric study of the fold interference patterns in the cantabrian arc revealed the conical nature of the folds belonging to the radial set these conical folds developed with different geometrical characteristics semi apical angles and axis attitudes depending on the initial orientation and geometry of the folded surfaces they are interpreted to result from a vertical axis rotation during oroclinal buckling of the variscan belt in nw iberia this study of conical folds in the cantabrian arc highlights that conical folds in curved orogenic arcs are a powerful tool for establishing the sequence of tectonic events because interference patterns due to vertical axis secondary differential rotations provide unique geometrical characteristics observed in the cantabrian arc that can be extrapolated to other oroclines

tectonic evolution of arcuate mountain belts on top of a retreating subduction slab the example of the calabrian arc 1 in this paper new paleomagnetic results from the calabrian arc are presented together with a critical review of all paleomagnetic data collected in the last decades in southern italy our study is focused on the upper miocene to middle pleistocene deposits of the crati extensional basin a sector of the arc where an abrupt change in the sense of paleomagnetic rotations is observed paleomagnetic data indicate that the crati basin underwent a uniform clockwise cw rotation of about 15 20 in its central and southern part whereas the northern sector is organized in small scale fault bounded blocks which rotated independently we interpret this pattern of deformation as the evidence of the complex nature of this area which represents the boundary between two domains characterized by opposite rotations the southern apennines which rotated counterclockwise and the calabria and sicily which rotated cw integrating these new paleomagnetic data with paleomagnetic data from southern italy we reconstruct the history of paleomagnetic rotations through time paleomagnetic rotations highlight the peculiarity of the formation of the calabrian arc curvature and imply that either an oroclinal bending model or a progressive arc model cannot be simply applied to the calabrian arc formation we describe a realistic tectonic geodynamic model where the progressive curvature of the calabrian arc is framed within the space time evolution of the ionian subduction system

strain variation in thrust sheets across the sevier fold and thrust belt idaho utah wyoming implications for section restoration and wedge taper evolution abstract although strain in fold and thrust belts is typically inhomogeneous on the largest scale strain within thrust sheets shows regular patterns the strain which is a quantitative measure of the deformation of any thrust sheet is independent of the translation of the sheet the internal deformation can be represented by a deformation profile the change in shape during thrusting of a fault perpendicular line which can then be used to unstrain the sheet during restoration in cross section balancing deformation profiles in well studied thrust sheets show regular geometric patterns and compatibility constraints can be used to minimize the data needed to define the profiles in other sheets from the geometry of deformation profiles and direct field observations the most important component of internal deformation of thrust sheets in many fold and thrust belts is early layer parallel shortening lps this can be removed in a straightforward manner during restoration and significantly affects the geometry i e taper of the restored section original basin wedge taper may have important implications for the mechanical evolution of fold and thrust belts thus changes in original basin taper resulting from strain removal may affect significantly the way in which we interpret the evolution of a fold and thrust belt in an area

influence of trench width on subduction hinge retreat rates in 3 d models of slab rollback subduction of tectonic plates limited in lateral extent and with a free trailing tail i e free subduction is modeled in a three dimensional 3 d geometry the models use a nonlinear viscoplastic rheology for the subducting plate and exhibit a wide range of behaviors depending on such plate characteristics as strength width and thickness we investigate the time evolution of this progressive rollback subduction measure the accompanying return flow in the upper mantle and quantify the plate kinematics due to the 3 d geometry flow is allowed to accompany slab rollback around the lateral edges of the slab the toroidal component as opposed to 2 d geometry where material is forced to flow underneath the slab tip the poloidal component a simple force balance is provided which relates the speed of backward trench migration to the resistive forces of generating flow and weakening the plate our results indicate most of the gravitational energy of the system i e the negative buoyancy of the subducting slab is converted into a toroidal flow 69 a much smaller amount goes into weakening the plate 18 and the remaining amount goes into driving flow parallel to displacement of the slab 13 for the trench widths w we investigate 1500 km a maximum trench retreat rate occurs for trenches 600 km wide which is attributed to the interaction between a plate of finite width and the induced flow which has a lengthscale in the horizontal direction these numerical results quantitatively agree with comparable 3 d laboratory experiments using analogue models with a purely viscous plate material schellart 2004a 2004b including correlations between increasing retreat rate with increasing plate thickness trench width for maximum retreat rate 500 km and estimated amount of slab buoyancy used to drive rollback induced flow 70 several implications for plate tectonics on earth result from these models such as rollback subduction providing a physical mechanism for ephemeral slab graveyards situated above the more viscous lower mantle and endothermic phase transition prior to a flushing event into the lower mantle mantle avalanche

remagnetizations and postfolding oroclinal rotations in the cantabrian asturian arc northern spain devonian carbonates in the cantabrian arc reveal characteristic magnetizations with coherent shallow upward inclinations the magnetizations appear to be carried by magnetite within site directions are very well grouped but site mean declinations range from easterly to south southwesterly in in situ as well as tilt corrected coordinates as has also been observed in previous studies of other formations in the arc the widely varying declinations of all studies roughly correlate with the overall structural trends of the arc and suggest that the sites underwent rotations in a process that involved folding about vertical axes and tightening of arc upon tilt correction the inclinations of our study on the other hand become scattered and it is concluded that the magnetizations were acquired after late carboniferous folding about horizontal axes the oroclinal rotations therefore also must have occurred after the late carboniferous folding phase the previous paleomagnetic results had been interpreted mostly as primary magnetizations residing in hematite however inclination only fold tilt tests applied to these results suggest that many if not all of the directions were acquired during the earlier stages of the late carboniferous folding thus all paleomagnetic results from the cantabrian arc appear to be remagnetizations but the ages of the remagnetizations vary from pre to synfolding for the mostly hematitic formations to postfolding for the devonian carbonates the reversed polarity inclinations of the hematite bearing formations have mean values ranging from 25 to 5 whereas the carbonates have a mean inclination of 8 on the basis of inclinations predicted for the area from results from stable europe the pyrenees and the iberian meseta the ages of these remagnetizations can be inferred to range from about 320 ma to 260 ma because all the remagnetizations reveal rotated declination patterns the oroclinal rotations occurred well after the main phase of hercynian deformation 320 280 ma while the timing of the rotations is unconstrained at the younger end they must have occurred during or after the permian best estimate is less than 260 ma which is much later than anticipated from other geological considerations

diachronous variscan tectonothermal activity in the nw iberian massif evidence from 40ar 39ar dating of regional fabrics abstract multigrain concentrates of hornblende and muscovite together with whole rock slate phyllite samples have been dated 27 analyses using 40ar 39ar incremental release methods along a systematic traverse across the various lithotectonic structural elements which comprise northwestern sectors of the variscan iberian massif hornblende concentrates from amphibolites in the allochthonous ordenes complex yield plateau isotope correlation ages of 425 ma and 377 ma muscovite concentrates and whole rock slate phyllite from this and the cabo ortegal complex yield plateau ages which range from 367 ma to 295 ma analyses of similar material from the relative autochthon yield plateau ages between 359 ma and 275 ma muscovite concentrates from three late to post kinematic granitic stocks yield plateau ages between 309 ma and 274 ma at least seven of the 40ar 39ar analyses from metamorphic rocks record variable thermal rejuvenation of intracrystalline argon systems associated with emplacement of proximal granitic stocks the remaining analyses may be used to constrain the local age of various variscan tectonothermal events the oldest fabric ages are recorded in allochthonous units whereas the youngest fabric ages occur along the boundary between internal and external zones middle devonian ages are recorded in the allochthon and suggest a chronological continuity with deformational events in the relative autochthon where variscan deformation initiated in the upper devonian and diachronously prograded eastward the first deformational events recorded in the limit within the internal and external zones occured ca 20 25 ma later lower namurian variscan deformation systematically prograded diachronously eastward across the orogen as new crustal material was added along the front of the developing orogenic wedge however the entire orogen remained tectonically active with different structural features forming at different times and at different places an average propagation rate of ca 5 km m y is suggested by consideration of a 20 25 ma difference in correlative fabric ages and present separations

massive hydrothermal dolomites in the southwestern cantabrian zone spain and their relation to the late variscan evolution abstract large scale massive burial dolomitization affected upper carboniferous carbonates of the southwestern variscan cantabrian zone spain replacive and void filling dolomites formed and were post dated by calcite cementation the process of dolomitization was due to the circulation of hypersaline and hydrothermal marine derived brines controlled by rock anisotropies the dolomites probably formed in early permian time during post thrusting orocline formation in an extensional setting lithospheric delamination induced increased heat flow and allowed thermal convection of the fluids the dolomites from the cantabrian zone are excellently exposed and represent an interesting outcrop analogue for hydrocarbon reservoirs elsewhere in comparable geotectonic settings of the european variscan orogen large masses of similar dolomites could have formed if mg rich brines were available

kinematics of retreating subduction along the carpathian arc romania kinematic axes and resultant vectors of displacement along the carpathian arc and the apuseni mountains aid in the reconstruction of retreating subduction along the carpathians the recent vertical position of the subducted slab below the vrancea area of the eastern carpathians represents the final rollback stage of a small fragment of oceanic lithosphere formerly situated between the moesian and east european plates the ages and locations of the eruption centers of the andesitic volcanic chain along the carpathian arc in the overriding plate as well as the thrust directions are used as markers to reconstruct the rollback area of the subducted slab between the moesian and east european plates

arc continent collision and orocline formation closing of the central american seaway closure of the central american seaway was a local tectonic event with potentially global biotic and environmental repercussions we report geochronological six u pb la icp ms zircon ages and geochemical 19 xrf and icp ms analyses data from the isthmus of panama that allow definition of a distinctive succession of plateau sequences to subduction related protoarc to arc volcaniclastic rocks intruded by late cretaceous to middle eocene intermediate plutonic rocks 67 6 1 4 ma to 41 1 0 7 ma paleomagnetic analyses 24 sites 192 cores in this same belt reveal large counterclockwise vertical axis rotations 70 9 6 7 and moderate clockwise rotations between 40 4 1 and 56 2 11 1 on either side of an east west trending fault at the apex of the isthmus rio gatun fault consistent with isthmus curvature an oligocene miocene arc crosscuts the older deformed and segmented arc sequences and shows no significant vertical axis rotation or deformation there are three main stages of deformation 1 left lateral strike slip offset of the arc 100 km and counterclockwise vertical axis rotation of western arc segments between 38 and 28 ma 2 clockwise rotation of central arc segments between 28 and 25 ma and 3 orocline tightening after 25 ma when this reconstruction is placed in a global plate tectonic framework and published exhumation data is added the central american seaway disappears at 15 ma suggesting that by the time of northern hemisphere glaciation deep water circulation had long been severed in central america

belt bending driven by lateral bending of subducting lithospheric slab geophysical evidences from the northern apennines italy abstract many of the mountain belts displaying a curved shape are oroclines i e are produced after progressive bending of an originally straight fold and thrust belt the bending process was previously explained as a consequence of several possible events taking place in the crustal orogenic wedge such as occurrence of obstacles non coaxial deformation and movements on wrench faults recent paleomagnetic results from the northern apenninic arc document that this belt is properly an orocline and results from late messinian early pliocene bending of a messinian straight belt foredeep system tomographic images in turn show the presence of a high velocity body interpreted as subducted slab in the upper mantle beneath the northern apennines between 35 and 670 km depth down to 100 km this body displays an arcuate shape which closely mirrors the geological outlines while it appears to be straight and parallel to the messinian pre rotated belt at depth we explore here the possibility that the arcuate shape of the northern apennines is a consequence closely following in time of much deeper processes than previously suggested i e the lateral bending of the subducting adriatic plate

a model of displacement and strain for arc shaped mountain belts applied to the jura arc abstract a plan view geometric model for simple parallel differential displacements is presented as an analogue for models of arcuate mountain belt formation we use the model to predict strain patterns produced by parallel displacement in front of a rigid versus deformable indenter a rigid indenter is simulated by an irregular quadrilateral displaced a constant amount along its hinterland boundary a deformable indenter is simulated by an irregular quadrilateral displaced along a hinterland boundary that is allowed to rotate some simple test cases show that the deformable indenter model leads to a pattern of strain very similar to that encountered in arcuate mountain belts short axes orientations are most deviated from the transport direction along the lateral edges of the model with minimum displacement and shortening and vary across the central domain mirroring strain features with variable orientations from mountain belts such as fold trends and minor deformation features such as horizontal stylolite peaks the rigid indenter model also generates short axes deviated from the transport direction but showing far less variation displacement strain relationships from the jura mountains switzerland and france are quantified based upon a simplified version of the finite displacement field for the jura fold thrust belt of philippe y 1995 rampes laterales et zones de transfer dans le chaines plissees unpublished phd thesis universite de savoie we find that the model short axis orientation pattern is very similar over at least the eastern and central jura to the stylolite patterns from the region whilst the long axes closely match the fold axes trends the model suggests that the jura mountains could have formed as a result of a progressive deformation with uniform transport in a general northwest direction

oroclines thick and thin an orocline is a thrust belt or orogen that is curved in map view due to it having been bent or buckled about a vertical axis of rotation two distinct types of oroclines are recognized progressive and secondary progressive oroclines are restricted to the scale of a thrust sheet to thrust belt are thin skinned and develop during thrust sheet emplacement secondary oroclines are larger occurring at the scale of an orogen and are plate scale features that affect crust and lithospheric mantle unlike progressive oroclines which develop during initial orogenesis and in response to the same orogen perpendicular stress responsible for thrust sheet emplacement secondary oroclines are extra orogenic developing after initial orogenesis and in response to an orogen parallel principal compressive stress that is oriented at a high angle to the stress responsible for orogen development we present case studies of the wyoming salient a progressive orocline that characterizes the sevier thrust belt of the western united states and the coupled cantabrian and central iberian oroclines which are linked secondary oroclines affecting the variscan orogen of iberia the vertical axis rotations involved in progressive and secondary orocline formation are most readily quantified through paleomagnetic analysis detailed three dimensional palinspastic restoration that incorporates translation rotation and strain can distinguish the role if any of primary curvature in progressive oroclines the use of tectonic vectors such as paleocurrent directions provides a means of recognizing and characterizing the initial geometry of secondary oroclines because secondary oroclines involve the entire lithosphere detailed studies of coeval metamorphism and magmatism provide a means of constraining the fate of the mantle lithosphere during oroclinal buckling

counterclockwise rotation of late eocene oligocene fore arc deposits in southern peru and its significance for oroclinal bending in the central andes the results of a paleomagnetic study along the fore arc of southern peru 15 18 s and northern chile 18 19 s are reported from middle to late miocene ignimbrites 7 sites late oligocene to early miocene ignimbrites 72 sites paleogene sediments 20 sites and mesozoic and paleocene volcanics and intrusions 31 sites comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations ranging from 5 2 11 3 clockwise to 55 6 7 0 counterclockwise spatially the magnitude of counterclockwise rotations increases northward from 0 within the chilean fore arc south of 18 30 s to 45 north of 16 30 s in southern peru paleomagnetic rotations recorded in paleogene red beds decrease from late eocene to late oligocene whereas miocene ignimbrites display no evidence of rotation these new results confirm that the rotations recorded in the fore arc of southern peru were acquired at least before 15 ma and probably before 25 ma and thus prior to the late neogene shortening of the sub andes the onset of major andean shortening in the eastern cordillera during the latest eocene earliest oligocene is interpreted to have triggered the bending of the peruvian fore arc the region of the peruvian fore arc with the largest rotations appears to be the fore arc counterpart of the abancay deflection a remarkable ne sw offset in the axis of the eastern cordillera induced by a major regional preorogenic structure we underline that the abancay deflection should be seen as the northwestern boundary and therefore as a key element of the bolivian orocline

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

structure and cenozoic kinematics of the eastern cordillera southern bolivia 21 s 1 the eastern cordillera is a main morphotectonic unit of the central andes in southern bolivia it is predominantly composed of ordovician sedimentary rocks as thick as 10 km revision of the ordovician stratigraphy and new structural fieldwork resulted in detailed balanced cross sections improved estimates of crustal shortening and a new kinematic model for the eastern cordillera new estimates of total shortening in the eastern cordillera are up to 95 km 41 44 higher than previous estimates and sufficient to thicken the andean crust to its present state between the eastern foreland and the magmatic arc after initial tertiary uplift at 58 ma first thrusts developed in the middle eocene to early oligocene 40 30 ma exhuming the central part of the eastern cordillera tectonic shortening culminated in late oligocene and early miocene time 25 20 ma when west facing thrust systems developed in the central and western part of the eastern cordillera thrusting terminated in the western part by the end of the early miocene and middle miocene tectonic activity was restricted to minor strike slip faulting in the central part thrusting remained active until the late miocene by 10 ma shortening in the eastern cordillera essentially ceased kinematic fault analyses indicate clockwise rotation of the shortening direction from nne ssw or ne sw in eocene early oligocene time over e w to wnw ese or even nw se in late oligocene to middle miocene time nevertheless wnw ese directed shortening and subvertical extension predominate sinistral oblique motion on many thrusts probably reflects plan view flexural slip in the clockwise rotating southern limb of the bolivian orocline it is proposed that space problems in the oroclinal hinge slowed the rotation there and caused decoupling of the still rotating south limb along ne trending transfer zones

orocline timing through joint analysis insights from the ibero armorican arc abstract the timing and kinematics of oroclinal bending in the core of the ibero armorican arc iaa has recently been constrained using paleomagnetic data from the cantabrian zone in northern iberia this study analyzes the joint patterns present in rock units deposited pre syn and post oroclinal bending systematic changes in the orientations of tensional joint sets in superimposed stratigraphic units are interpreted to record the progressive stages of oroclinal bending in the core of the iaa time constrains for joint set development are constrained by the known ages of the bounding unconformities that limit the studied stratigraphic units joint azimuth variability in the pre orocline rocks neoproterozoic to pre upper carboniferous is comparable to the present arc curvature of the orocline about 180 the joints in the syn orocline rocks upper pennsylvanian or stephanian 304 to 299 ma show a lower azimuthal variability that is comparable to about 50 70 of the total curvature seen in pre orocline rocks finally post orocline rocks permian contain joints that have uniform azimuths for each set across the entirety of the present day arc together these spatially and temporally distinct joint sets suggest that rotations in the cantabrian zone took place in the upper pennsylvanian during a ca 10 ma time period which agrees well with previous paleomagnetic arguments the data also provides supporting evidence for oroclinal bending by rotation around vertical axes of an initially linear or nearly linear orogenic belt and finally these data highlight the potential power in using tectonic joint sets for constraining thrust belt kinematics in curved orogenic systems when unconformity bounded stratigraphic sequences are present that are coeval with orocline development

low p metamorphism following a barrovian type evolution complex tectonic controls for a common transition as deduced in the mondonedo thrust sheet nw iberian massif the mondonedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low and medium p metamorphic domains this unit underwent an initial medium p event during the initial stages of variscan convergence related to crustal thickening subsequently the thrust sheet evolved to a low p baric type of metamorphism related to syn convergence thinning and exhumation its footwall cropping out in two tectonic windows registered a different evolution with a low p history that evolved from low to high t under a high geothermal gradient several different p t paths of the mondon edo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area following the initial crustal thickening two main syn convergence extensional shear zones developed one of them occurs in the hangingwall whereas the other affects the footwall unit both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet and their activity is viewed as a consequence of the need for gravitational re equilibration within the orogenic wedge the most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low p metamorphism is essentially a late phenomenon and is linked to late orogenic tectonic activity in the mondon edo thrust sheet our conclusions indicate that low p metamorphism may also develop during convergence and that this may occur in at least two cases one is tectonic denudation of an allochthonous unit during its emplacement and the other thinning and extension at the footwall unit of an advancing thrust sheet as a consequence the low p evolution may show different characteristics in different units of an orogenic nappe pile

mechanics and rates of tidal inlet migration modeling and application to natural examples tidal inlets on barrier coasts can migrate alongshore hundreds of meters per year often presenting great management and engineering challenges here we perform model experiments with migrating tidal inlets in delft3d swan to investigate the mechanics and rates of inlet migration model experiments with obliquely approaching waves suggest that tidal inlet migration occurs due to three mechanisms 1 littoral sediment deposition along the updrift inlet bank 2 wave driven sediment transport preferentially eroding the downdrift bank of the inlet and 3 flood tide driven flow preferentially cutting along the downdrift inlet bank because it is less obstructed by flood tidal delta deposits to quantify tidal inlet migration we propose and apply a simple mass balance framework of sediment fluxes around inlets that includes alongshore sediment bypassing and flood tidal delta deposition in model experiments both updrift littoral sediment and the eroded downdrift inlet bank are sediment sources to the growing updrift barrier and the flood tidal delta such that tidal inlets can be net sink of up to 150 of the littoral sediment flux our mass balance framework demonstrates how with flood tidal deltas acting as a littoral sediment sink migrating tidal inlets can drive erosion of the downdrift barrier beach parameterizing model experiments we propose a predictive model of tidal inlet migration rates based upon the relative momentum flux of the inlet jet and the alongshore radiation stress we then compare these predicted migration rates to 22 natural tidal inlets along the u s east coast and find good agreement

tectonic delamination and subcrustal imbrication of the precambrian lithosphere in northwestern canada mapped by lithoprobe seismic reflection profiling of the precambrian lithosphere in northwestern canada has produced images of delamination structures tectonic wedges at crustal scale to about 30 km depth and for the first time at lithospheric scale to about 100 km depth and has further delineated subcrustal structures that were likely caused by imbrication of subducted lithosphere coupled with observations of similar structures in thrust and fold belts delamination and lower plate imbrication are persistent features at many scales

the kinematic history of the central andean fold thrust belt bolivia implications for building a high plateau this paper presents a model for the kinematic evolution of the central andean plateau based on balanced cross sections across the bolivian andes the proposed model links the formation of the andean plateau to the development of the andean fold thrust belt through the creation and propagation of two large basement mega thrusts support for large basement involved thrust sheets is found in significant steps in both the topography and the exposed structural elevation of the andean fold thrust belt the structurally highest basement thrust raised folds and faults in predominantly lower paleozoic rocks of the eastern cordillera with respect to tertiary rocks in the broad internally drained basin of the altiplano to the west and east verging folds and faults in upper paleozoic rocks of the interandean zone to the east the interandean zone was in turn raised both structurally and topographically with respect to the frontal folds and faults of the fold thrust belt the subandean zone by a second structurally lower basement thrust sheet thus these two megathrusts divide the andean fold thrust belt into four areas of markedly different structural elevations the eastern cordillera can be further subdivided into two zones of west and east vergent folds and thrusts shortening accommodated by the fold thrust belt can be divided among these tectono structural zones and linked to shortening accommodated by the inferred basement megathrusts the proposed kinematic model suggests that the eastward propagation of the structurally highest basement thrust fed 105 km of slip into the eastern cordillera along east vergent and west vergent faults this structure also fed 90 km of eastward slip into the interandean zone the initiation and eastward propagation of a lower basement thrust structurally elevated the interandean zone with respect to the foreland while feeding 65 km of slip into the subandean zone out of sequence basement thrusting to the west is proposed to have elevated the western edge of the plateau and accommodated 40 km of shortening within the altiplano total cumulative shortening within the cover rocks of the andean fold thrust belt 300 330 km can be balanced by an equivalent amount of shortening along two basement megathrusts to the first order the eastern margin of the central andean plateau defined by the 3 km topographic contour is contiguous with the leading edge of the upper basement megathrust this relationship between the basement highs and the physiographic boundaries of the andean plateau suggests that extensive megathrust sheets involving strong rocks such as crystalline basement or quartzite play an important role in the formation of the central andean plateau and a similar link between megathrust sheets and plateaus may be found in other orogens

rapid late miocene rise of the bolivian altiplano evidence for removal of mantle lithosphere abstract the oxygen isotopic compositions of carbonates deposited in the northern altiplano demonstrate a rapid change in late miocene time which we attribute to an increase in elevation rainfall samples over an elevation transect adjacent to the altiplano of northern bolivia display a systematic decrease in mean annual 18 o values and define a local 18 o vs altitude gradient r gonfiantini m a roche j c olivry j c fontes g m zuppi the altitude effect on the isotopic composition of tropical rains chem geol 181 2001 147 167 which we apply to paleo meteoric water values determined from carbonates age constraints from magnetostragraphy and paleoelevation estimates suggest surface uplift of 2 5 to 3 5 km occurred between 10 3 ma and 6 8 0 4 ma when the altiplano rose to its current elevation only the removal of dense eclogitic lower crust and mantle lithosphere can generate a change of this magnitude and rapidity this change coincides with an 30 decrease in the rate of convergence between the south america and nazca plates and the propagation of deformation into the eastern subandean zone which we infer to have resulted from the increase in the force per unit length applied by the elevated andean plateau to the surrounding lowlands

kinematic constraints on buckling a lithospheric scale orocline along the northern margin of gondwana a geologic synthesis abstract the paleozoic variscan orogeny was a large scale collisional event involving amalgamation of multiple continents and micro continents existing data suggests oroclinal buckling of an originally near linear convergent margin during the last stages of variscan deformation in the late paleozoic closure of the rheic ocean resulted in e w shortening present day coordinates in the carboniferous producing a near linear n s trending east verging belt subsequent n s shortening near the carb permian boundary resulted in oroclinal buckling this late stage orogenic event remains an enigmatic part of final pangea amalgamation the present day arc curvature of the variscan has inspired many tectonic models with little agreement between them while there is general consensus that two separate phases of deformation occurred various models consider that curvature was caused by dextral transpression around a gondwana indentor strike slip wrench tectonics or a change in tectonic transport direction due to changing stress fields more recent models explain the curvature as an orocline with potentially two opposite facing bends caused by secondary rotations deciphering the kinematic history of curved orogens is difficult and requires establishment of two deformation phases an initial compressive phase that forms a relatively linear belt and a second phase that causes vertical axis rotation of the orogenic limbs historically the most robust technique to accurately quantify vertical axis rotation in curved orogens is paleomagnetic analysis but recently other types of data including fracture geochemical petrologic paleo current and calcite twin data have been used to corroborate secondary buckling a review of existing and new variscan data from iberia is presented that argues for secondary buckling of an originally linear orogenic system together these data constrain oroclinal buckling of the cantabrian orocline to have occurred in about 10 ma during the latest carboniferous which agrees well with recent geodynamical models and structural data that relate oroclinal buckling with lithospheric delamination in the variscan

diachronous post orogenic magmatism within a developing orocline in iberia european variscides 1 u pb zircon crystallization ages of 52 late variscan granitoid intrusions from nw iberia 19 from new data 33 from previous studies constrain the lithospheric evolution of this realm of the variscan belt of western europe and allow assessment of the relationship between oroclinal development and magmatism in late carboniferous early permian times the u pb ages in conjunction with a range of geological observations are consistent with the following sequence of events i oroclinal bending starts at 310 305 ma producing lithospheric thinning and asthenospheric upwelling in the outer arc of the orocline accompanied by production of mantle and lower crustal melts ii between 305 and 300 ma melting continues under the outer arc of the orocline central iberian zone of the iberian variscan belt and mid crustal melting is initiated coevally the lithospheric root beneath the inner arc of the orocline thickened due to progressive arc closure iii between 300 and 292 ma foundering of the lithospheric root followed by melting in the lithospheric mantle and the lower crust beneath the inner arc due to upwelling of asthenospheric mantle iv cooling of the lithosphere between 292 and 286 ma resulting in a drastic attenuation of lower crustal high temperature melting by 285 ma the thermal engine generated by orocline driven lithospheric thinning delamination had cooled down beyond its capability to produce significant amounts of mantle or crustal melts the model proposed explains the genesis of voluminous amounts of granitoid magmas in post orogenic conditions and suggests that oroclines and similar post orogenic granitoids common constituents of numerous orogenic belts may be similarly related elsewhere

quantifying vertical axis rotation in curved orogens correlating multiple data sets with a refined weighted least squares strike test 1 map scale curvature is a fundamental feature of most contractional orogenic belts and is central to understanding the kinematic and dynamic evolution of mountain systems paleomagnetic analysis combined with detailed structural studies is the most robust means of quantifying vertical axis rotations that produce curvature over a range of temporal and spatial scales this paper explores how vertical axis rotations can best be evaluated for multiple data sets by applying a weighted least squares method to the classic strike test this refined method provides measures of best fit slope confidence interval and goodness of fit between map scale structural trend paleomagnetic rotations and deformation fabric orientations structural trend is estimated by averaging fold axial trace formation contact and bed strike data over kilometer scale areas paleomagnetic and deformation fabric site mean orientations and measurement uncertainties are estimated using vector and bootstrap statistics weighting factors are estimated from combined measurement uncertainty structural noise related to small scale block rotation and stress strain refraction and variations in restoration paths the number of sites needed to obtain a significant confidence interval in strike test slope is a function of combined uncertainty in paleomagnetic or deformation fabric directions and the total range in structural trend around a curved orogen improved estimates of strike test slope and rotation thus require systematic sampling with a wide distribution of sites evaluation of appropriate weighting factors and statistical analysis a case study is presented that highlights application of this refined method to paleomagnetic and deformation fabric data sets from the wyoming salient of the sevier thrust belt paleomagnetic data yield a strike test slope of 0 76 0 11 indicating that the wyoming salient is a progressive arc with 3 4 secondary curvature related to vertical axis rotation synchronous with large scale thrusting and 1 4 initial curvature finite strain anisotropy of magnetic susceptibility and mesoscopic structural orientations which are related to early layer parallel shortening all yield strike test slopes of 0 9 0 1 comparing these slopes with paleomagnetic results indicates that deformation fabrics had initial curvature and thus cannot be used alone to accurately estimate rotations by integrating systematic paleomagnetic and structural data using statistical analysis curvature models for the wyoming salient are closely constrained

geological constraints on the paleozoic tectonic evolution of tectonostratigraphic terranes in the iberian massif abstract the presence of oceanic paleozoic sequences in the iberian variscan fold belt is used to identify and separate suspected exotic terranes from a reference iberian autochthon terrane which constitutes the greater part of it in this paper the relevant data characterizing the pre accretionary evolution of the paleozoic iberian terranes are presented as well as the available evidence concerning the accretionary process itself emphasis is placed on the analysis of the sedimentary igneous and structural expression of the major events which affected the iberian terranes before during and after their amalgamation the main conclusions of such an analysis may be summarized as follows 1 a recognition of rift and passive margin stages in all the iberian massif terranes during the early paleozoic 2 b late paleozoic accretion of the exotic terranes to the iberian autochthon via a long and complex process variscan orogeny which involved initial subduction underneath the latter and eventual obduction followed by oblique collision of a promontory the iberian autochthon northwestern margin with a northwestern continental block this latter process could account for the large scale crustal imbrication shown by the northern branch of the orogen coeval with predominant strike slip faulting in the southern branch as well as with the formation of the typical arcuate shape of this orogenic belt major present uncertainties relate to the palinspastic provenance of continental suspect terranes and their correlation with similar units in other circum atlantic paleozoic orogens a discussion and speculations on this subject are dealt with in the final section of this paper

paleomagnetic study of thrust sheet rotation during foreland impingement in the wyoming idaho overthrust belt upper jurassic and lower cretaceous strata in the darby and absaroka thrust sheets were sampled in order to paleomagnetically evaluate thrust sheet rotation in the wyoming idaho overthrust belt no significant rotations were found lack of rotation in these thrust sheets suggests that previously reported rotation of the prospect thrust sheet was not transmitted to the older darby and absaroka thrust sheets indicating that the prospect thrust sheet only experienced local rotations in response to collision with the teton gros ventre uplift the upper jurassic stump formation possesses a magnetization that passes the fold test and gives a paleomagnetic pole at 64 n 170 e k 18 a95 15 in good agreement with other published upper jurassic poles for cratonic north america samples from all lower cretaceous sites are believed to possess a synfolding remagnetization of chemical origin

vertical axis rotation in the bolivian orocline south america 1 paleomagnetic analysis of cretaceous and cenozoic rocks the results of a paleomagnetic study are reported for 45 new sites or localities in the central andes of bolivia and northern chile 17 s 23 s south america in intrusive volcanic and sedimentary rocks ranging in age between 7 and 80 ma these outcrop across the width of the northern and southern limbs of the bolivian orocline from the sub andean zone in the east to the volcanic arc in the west for most localities a mean primary magnetization has been identified which is assumed to reflect a geocentric axial dipole field a comparison of directions of magnetizations with those predicted by the relevant paleomagnetic reference poles for stable south america reveals declination anomalies which are interpreted in terms of the rotation about a local vertical axis of rigid crustal blocks hundreds of meters to tens of kilometers across relative to stable south america these data when combined with previous paleomagnetic studies of upper cretaceous and cenozoic rocks show a pattern of vertical axis rotations which defines three regional domains extending across the width of the bolivian andes the differences between the various domains are best seen in rocks in the age range 20 to 80 ma younger rocks acquired a primary magnetization at later stages in the rotation history for rocks older than 20 ma crustal blocks in domain 1 on the northern flank of the bolivian orocline have undergone an anticlockwise rotation about a vertical axis in the range 8 to 33 with a geometric mean of 22 blocks from domain 2 immediately south of the main oroclinal bend have rotated between 19 anticlockwise and 40 clockwise with a geometric mean of 8 clockwise even farther south in domain 3b blocks have rotated between 3 and 57 clockwise with a geometric mean of 37 a localised area domain 3b in which blocks have undergone 32 clockwise rotation in the last 65 myr lies within the southeastern part of domain 1 the northern boundary of domain 1 and the southern boundary of domain 3a are as yet not well defined the amount and timing of cenozoic vertical axis rotation vary across the width of the andes and are interpreted to record both regional bending of both limbs of the bolivian orocline and small block rotation in zones of distributed sinistral or dextral shear during three stages in the cenozoic 1 45 to 25 ma 2 25 to 10 ma 3 10 ma to 0 bending of the bolivian andes during the last 25 myr was accommodated relative to stable south america by gradients of shortening along the eastern margin of the andes particularly in the sub andean zone

growth of the central andean plateau by tectonic segmentation is controlled by the gradient in crustal shortening abstract crustal deformation orocline formation and growth of the central andean plateau as well as the mutual relations of these processes with climate are a matter of international debate earthquake focal mechanism fault kinematic paleomagnetic and cross section balancing data are paramount for understanding the morphotectonic evolution of this noncollisional orogen but collectively await an explanation in terms of a comprehensive kinematic model we offer such a model that approximates simple shear in plan view and that predicts the kinematics of first order fault zones in the southern central andes the model indicates that segmentation of the upper crust into rhomb shaped domains confined by transpressive ranges resulted from the decrease in crustal shortening with distance to the central bend of the andes an anastomosing network of kinematically coupled reactivated orogen parallel and newly formed ne trending deformation zones encompassing components of sinistral and dextral strike slip res

oroclines of the variscan orogen of iberia paleocurrent analysis and paleogeographic implications abstract coupled structural and paleomagnetic analyses have shown that the northern iberian bend of the variscan orogen referred to as the cantabrian orocline developed by vertical axis rotation of an originally linear orogen however palinspastic restoration of the orocline has proven difficult owing to 1 an unusually great orogenic width of over 700 km and 2 exposure of shallow water strata of the gondwanan margin in the northern and southern portions of the orogen we present paleocurrent data from lower ordovician shallow marine clastic sedimentary rocks across the variscan of northern and central iberia collected to constrain palinspastic restoration of the orogen paleocurrent data were collected from over 50 sites and include cross bed foresets ripple crests and casts as well as rare ball and pillow structures syn sedimentary slump folds and incised channels paleocurrent directions fan around the cantabrian orocline are consistently oriented at a high angle to structural strike and yield a consistent offshore direction outward from the oroclinal core similarly changes in structural strike and paleocurrent direction across central iberia imply the presence of a second more southerly orocline the central iberian orocline that is continuous with but convex in the opposite direction of the cantabrian orocline together the cantabrian and central iberian oroclines define an s shaped pair of continental scale buckle folds palinspastic restoration of the oroclines yields a linear continental margin 1500 km long characterized by consistent offshore paleoflow to the west defining a westerly oceanic domain presumably the rheic ocean and an easterly landward direction presumably gondwana recognition of the southern orocline explains the unusual width of the orogen the geometry of aeromagnetic anomalies attributable to variscan rocks and is consistent with available structural data paleomagnetic declination data and the distribution of correlative paleozoic and older rock sequences including shallow water strata of the gondwanan margin the s geometry of the oroclines is similar to the z geometry of the alaskan oroclines of the north america cordillera suggesting that as in the cordillera the iberian oroclines may record a significant margin parallel translation event

new paleomagnetic data from the northern argentine puna central andes rotation pattern reanalyzed 1 along the central andes a pattern of vertical axis rotations carp has been paleomagnetically identified these rotations which are counterclockwise north of arica and clockwise to the south have been detected in mesozoic and cenozoic rocks cropping out in the andean forearc and the altiplano puna different models have been proposed to explain the geodynamic evolution of the central andes but the origin of these rotations remains controversial such models involve oroclinal bending and or local block rotations this lack of agreement is partly due to the scarce paleomagnetic database particularly for the argentine puna here we report new paleomagnetic data obtained in the northern argentine puna from 57 sites drilled from miocene volcaniclastic rocks and 12 sites from the cretaceous eocene salta group it was determined that 1 the morro blanco area did not undergo significant vertical axis rotation since at least the middle miocene 2 the loma blanca mine area underwent a clockwise vertical axis rotation of 10 later than the late miocene and 3 the coranzuli area underwent a clockwise vertical axis rotation of 40 later than the cretaceous eocene the central andes database was analyzed and some features of the carp were determined these features were explained considering domino style models for the forearc and a local block rotation model in response to penetrative shear for the altiplano puna penetrative shear would occur along a high electrical conductivity zone however the possibility of the existence of an oroclinal bending of 5 cannot be discarded

kinematics of orocline tightening in the core of an arc paleomagnetic analysis of the ponga unit cantabrian arc northern spain 1 paleomagnetic and structural analyses of the western european variscan belt wevb suggest that the most viable kinematic model for variscan deformation in northern iberia is oroclinal bending of an originally linear belt in a two stage tectonic history this history represents two regional compression phases e w in the late carboniferous and n s in the permian both in present day coordinates which resulted in the refolding about steeply plunging axes of initially north south trending thrusts and folds in the hinge zone and oroclinal tightening due to vertical axis rotation of the belt s limbs however the orocline model has yet to be critically tested in the wevb s core this study reports new paleomagnetic rock magnetic and structural data from the inner core of the wevb in order to test opposing kinematic models for the well documented fault and fold interference structures formed by late stage variscan deformation and to better understand the overall development of the wevb arc map scale structural features within the wevb core have a highly sinuous geometry characterized by transverse and thrust parallel fold systems formed by fault bend folding over footwall ramps the intersections of these two fold systems produce steeply plunging interference folds which are best exposed in the ponga thrust unit a total of 67 paleomagnetic sites were collected in the barcaliente formation of the ponga unit with emphasis placed on detailed spatial coverage of individual structural domains a multicomponent paleomagnetic remanence was measured which is interpreted to be composed of two components a low unblocking temperature recent viscous magnetization and a high unblocking temperature ancient magnetization that was acquired in the latest stephanian to early permian after initial d1 thrusting and folding bur prior to major secondary rotation rock magnetic experiments show that the characteristic remanence magnetization is carried by secondary authigenic pseudosingle domain magnetite these paleomagnetic results are used to determine the kinematics and geometry of postmagnetization deformation by restoring in situ magnetic vectors back to a defined reference direction on the basis of the restored ponga unit geometry the steeply plunging interference folds found in the wevb core are best described by a combination of secondary buckling of frontal ramp parallel hanging wall anticlines and modification of d1 lateral oblique ramps as frontal ramps during late stage north south shortening this combination of structural modification of the inner core of the wevb was necessary to accommodate oroclinal bending during the final amalgamation of pangea in the late paleozoic

the armorica microplate fact or fiction critical review of the concept and contradictory palaeobiogeographical data abstract the problem of an armorica microplate detached from gondwana and having had during part of the palaeozoic an independent latitudinal evolution is reconsidered in terms of a critical review of the palaeomagnetic data that were the very roots of this concept and through an alternative approach based on palaeoclimatic and palaeobiogeographical data palaeomagnetic data for the silurian and the devonian of the south european regions supposedly constituting the armorica microplate remain rare and ambiguous those from gondwana are more numerous but contradictory enough to give rise to diverging models regarding the latitudinal evolution of this continent consequently the reality of an armorica microplate cannot be considered as established on the contrary lithological indicators of palaeoclimate and palaeobiogeographical data are in total harmony and indicate that in actual fact the southern european regions remained permanently closely connected with gondwana of which they composed the northern margin although repeatedly maintained for more than 20 years the concept of an armorica microplate can thus be considered a fiction this conclusion should lead to the dismissal of geodynamical models proposed for the variscan belt to which this concept was integrated and are contradicted by inescapable palaeobiogeographical constraints

active deformation in the bolivian andes south america a method is described for determining the horizontal field of velocity and velocity gradients in the bolivian andes in the vicinity of the bolivian orocline using the following data 1 space geodetic measurements of crustal motions 2 the style distribution and rate of late miocene quaternary folding and faulting and 3 paleomagnetic measurements of rigid body rotations about a vertical axis these data were analyzed in a network of 26 triangles which spanned the bolivian andes by solving the velocity gradient compatibility equations between adjacent triangles subject to the input data constraints this yielded 120 140 linear equations to constrain 108 unknowns making the problem moderately overdetermined a series of experiments were carried out with different combinations of the input data to determine the best fit in the least squares sense field of quaternary horizontal velocities and velocity gradients the azimuths of velocity vectors in the high andes are subparallel to both the relative plate convergence vector and the symmetry axis of the bolivian orocline with magnitudes relative to stable south america in the range 7 17 mm yr 1 accommodating up to 30 of the relative plate convergence however the azimuths of velocities along the range front are nearly orthogonal to the range with a component of distributed range parallel dextral or sinistral shear within the eastern cordillera and altiplano the horizontal velocity field requires bending about a vertical axis of both limbs of the bolivian orocline the field of horizontal dilatation assuming homogeneous deformation of the entire crustal thickness requires vertical velocities of material points up to 2 5 mm yr in the sub andean zone decreasing to 0 1 mm yr farther east in the eastern cordillera and altiplano the direction of maximum gradients of buoyancy stress determined from the field of gradients of appropriate combinations of velocity gradients using a thin sheet model of viscous flow are broadly parallel to the direction of maximum topographic gradients in the eastern cordillera providing support for the idea that the bulk behaviour of the bolivian lithosphere is fluid like a comparison of these gradients with buoyancy stress gradients calculated from crustal thickness contrasts suggest that the bulk lithospheric viscosity is in the range 0 5 1 0 1022 pa s gradients of velocity gradients along the eastern margin of the bolivian andes can be interpreted in terms of marked gradients of viscosity with viscosity increasing toward the brazilan shield in the cratonic core of south america buoyancy stresses calculated from crustal thickness contrasts calibrated with the crustal thicknesses at the changeovers from thrust to strike slip or normal faulting suggest that the average lithosphericscale deviatoric stresses are in the range 7 15 mpa

paleomagnetic evidence for a neogene two phase counterclockwise tectonic rotation in the northern apennines italy paleomagnetic directions have been determined for a new collection of early oligocene and late miocene pliocene epiligurian clastic sediments from the frontal portions of the northern apennines these results are combined with cenozoic data from the literature to evaluate whether rotations of units in this region are related to the oligomiocene corsica sardinia rotation and or to younger phases of deformation of the apennine chain when corsica sardinia moved counterclockwise ov the coast of france the ligurian units located at its front were presumably pushed eastward and rotated counterclockwise above a main boundary thrust onto the adria africa margin we propose that about 24 out of a total of 52 of rotation observed in the epiligurian units can be related to the oligo miocene motion of the corsica sardinia block in partial agreement with previous conclusions and the remaining 28 to the pliocene tectonic phase at the apennine chain front which may have re activated thrust planes in the adria africa succession below the ligurian wedge 2000 elsevier science b v all rights reserved

40ar 39ar dating of the beja gabbro timing of the accretion of southern portugal the sup 40 ar sup 39 ar dating of the amphibole from the beja gabbro southern portugal yields a plateau age at 336 4 plus minus 0 8 ma 2 sigma level the corresponding calculated isotopic closure temperature is around 800c the comparison of this temperature with the magnetic blocking temperature approximately 570c allows an estimation of a probable thermoremanent acquisition age for the characteristic magnetization component of the beja gabbro between 335ma and 315ma assuming cooling rates between 10c ma and 100c ma these results combined wtih paleomagnetic results from the beja gabbro and late paleozoic rocks from southern portugal perroud et al 1985 suggest that the southermost part of spain and portugal was separated from northern iberia in early carboniferous times and was accreted to europe during the late carboniferous

origin of the uinta recess sevier fold thrust belt utah influence of basin architecture on fold thrust belt geometry abstract structural trends in the sevier fold thrust belt define a pronounced concave to the foreland map view curve the uinta recess in north central utah this recess separates two convex to the foreland curves the provo salient on the south and the wyoming salient on the north the two limbs of the recess comprise transverse zones fault zones at a high angle to the regional trend of the orogen that border the flanks of the east west trending uinta cottonwood arch our structural analysis indicates that the transverse zones formed during the sevier orogeny and that they differ markedly from each other in structural style the charleston transverse zone ctz on the south side of the arch initiated as a complex sinistral strike slip fault system that defines the abrupt northern boundary of the provo salient the mount raymond transverse zone mrtz on the north side of the arch represents the region in which the southeast verging southern limb of the gently curving wyoming salient was tilted northwards during the laramide phase of uplift of the uinta cottonwood arch in effect the mrtz represents an oblique cross section through a thrust belt the contrasting architecture of these transverse zones demonstrates how pre deformation basin geometry influences the geometry of a fold thrust belt analysis of isopach maps indicates that at the time the sevier fold thrust belt formed the area just north of the present site of the uinta cottonwood arch was a basement high with a gently dipping north flank and a steeply dipping south flank thus predeformational sediment thickened abruptly to the south of the high and thickened gradually to the north of the high as illustrated by sandbox models the distance that a fold thrust belt propagates into the foreland depends on the thickness of the sedimentary layer being deformed so the shape of the salient mimics the longitudinal cross sectional shape of the sedimentary basin where basins taper gradually along strike the thrust belt curves gently but where basins taper abruptly along strike the thrust belt curves so tightly that it disarticulates and becomes bounded laterally by a strike slip accommodation zone the geometry of the uinta recess provides a field example of this concept differential movement of sevier thrusts led to formation of gradually curving thrusts on the north side of the high because of the gradual slope of the high s north flank but led to the along strike disarticulation of thrusts on the south side of the high because of the steep slope of the high s south flank in effect therefore thrust belt map view geometry provides insight into predeformational basin geometry

late cenozoic tectonism and landscape development in the foreland of the andes northern sierras pampeanas 26 28 s argentina the argentine sierras pampeanas are reverse fault bounded mountain blocks of precambrian to paleozoic basement rocks in the foreland of the central andes uplift in the northernmost sierras pampeanas fault blocks of sierra de quilmes sierra cumbres calchaquies and sierra aconquija started about 7 ma and became pronounced between 4 and 3 4 ma the movements culminated after 2 9 ma when the conformable mio pliocene santa maria group was overthrusted faulted and folded in the course of principal basement uplift these movements created climate and base level conditions that resulted in the formation of five pediment levels in the deformed basin strata between 2 5 and 0 3 ma tectonically induced base level changes in the piedmont resulted in three tectonism related pediment surfaces between 2 5 and 0 6 ma which attest to the neotectonic activity in the andean foreland the chronology of young tectonic uplift contradicts models in which early uplift between 20 and 13 ma is due to the interaction between high topography on the subducting nazca plate and the south american craton in conjunction with the principal uplift of the adjacent puna plateau the northernmost sierras pampeanas are better explained by e w compressional stresses superposed on a thinned lithosphere and inherited zones of structural weakness that facilitated uplift

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

post middle oligocene origin of paleomagnetic rotations in upper permian to lower jurassic rocks from northern and southern peru abstract we report paleomagnetic data from 28 sites of upper permian to lower jurassic strata from northern and southern peru in northern peru 6 s a stable magnetic component from six permo triassic sites passes fold and reversal tests the overall mean pole agrees well with late permian to triassic poles from cratonal south america suggesting this part of peru has experienced neither significant rotation nor latitudinal transport since the permo triassic in southern peru 13 to 16 s thermal demagnetization isolates stable magnetic components in 16 of 22 upper permian to lower jurassic sites collected along the transition between the altiplano and the eastern cordillera these 16 sites are rotated 14 to 147 counterclockwise and pass an inclinations only fold test within the same structural zone three other permo triassic sites as well as 10 paleocene sites also show important counterclockwise rotations roperch and carlier j geophys res 97 1992 17233 17249 butler et al geology 23 1995 799 802 the large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component no correlation exists between rotation amount and rock age suggesting the rotations are post paleocene in age because the rotations occur along the fringe of the eastern cordillera they were likely produced during its structural formation hence from the late oligocene to present sinistral shear acting in the northern part of the bolivian orocline appears much more pronounced than that north of the abancay deflection which likely arises from differences in convergence obliquity

bending the bolivian orocline in real time global positioning system gps data from the central andes record vertical axis rotations that are consistently counterclockwise in peru and bolivia north of the bend in the mountain belt and clockwise to the south in southern bolivia argentina and chile these geologically instantaneous rotations have the same sense as rotations that have accrued over millions of years and are recorded by paleomagnetic and geologic indicators the change in sign of the rotation at both decadal and million year time scales occurs across the axis of topographic symmetry that defines the bolivian orocline when extrapolated to a common time interval the magnitudes of rotation from geologic features and from gps are surprisingly similar given that a significant part of the instantaneous deformation field is probably elastic and due to interseismic locking of the plate boundary some of the interseismic deformation field must reflect permanent deformation and or some of the current elastic deformation will be converted to upper plate permanent deformation over time rather than be recovered by elastic rebound during interplate earthquakes we suggest that the spatial patterns of the elastic and the permanent modes of bending are similar because they are driven by the same stress field

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

bending of the bolivian orocline and growth of the central andean plateau paleomagnetic and structural constraints from the eastern cordillera 22 24 s nw argentina 1 we report new paleomagnetic and structural data from late cretaceous to mio pliocene continental sandy silty sedimentary rocks from the eastern cordillera central andes here n s to nne ssw ridges hosting paleozoic basement and upper cretaceous continental red beds overthrust thick adjacent cenozoic basins pretilting and likely primary reliable directions gathered at 15 sites document 45 9 9 4 30 1 23 9 and 15 4 19 3 clockwise cw rotations with respect to south america occurring after the late cretaceous 80 ma oligo miocene 20 30 ma and late miocene pliocene 5 10 ma respectively conversely four upper cretaceous sites from the walls of a n s left lateral strike slip fault yavi abra pampa fault yield a null rotation about 20 km to the west flower structures and subvertical syntectonic strata dated at 14 26 0 19 ma are exposed along the subparallel abra moreta left lateral strike slip fault relying on data from the literature on the period when deformation began we suggest that since eo oligocene times 30 40 ma the eastern cordillera has undergone a regional cw rotation of 40 50 synchronous with crustal shortening and large scale bending of the andean salient the cw rotation is possibly still active today as documented by regional gps data from the andes since 15 ma ago the activity of n s left lateral strike slip faults induced counterclockwise rotations along the fault zone locally annulling the regional cw rotation in agreement with a previous model we speculate that mid miocene strike slip activity accommodated the progressive southward spreading of the altiplano puna high altitude plateau laterally migrating from the overthickened crustal region of the salient apex

consistency of geologic and geodetic displacements during andean orogenesis displacement and convergence rates inferred from gps and marine magnetic data suggest that this trend may be continuing at present hence an increasing fraction of convergence is being absorbed by mountain building this change may reflect increased plate coupling due to decreased sediment supply younger subducting lithosphere or increased normal stress at the interface from the effects of uplift index terms 8102 tectonophysics continental contract orogenic belts 8157 evolution of the earth plate motions past 3040 8158 evolution of the earth plate motions present and recent 3040 8099 structural geology general or miscellaneous

coal basins in the cantabrian mountains northwestern spain abstract the cantabrian mountains containing about 70 of the total coals of the country and 95 of the anthracitic and bituminous resources is the most important coal mining district of spain coal bearing successions are late carboniferous in age and their deposition took place in a syn orogenic context during the hercynian activity mainly in three different time successive locations foreland basins intrathrust basins and intramontane fault controlled basins foreland deposition occurred in strongly asymmetrical basins located ahead of thrust sheets units intrathrust basins resulted from folding and tightening of these units during or after their emplacement intramontane and fault controlled basins were developed along lineaments with strike slip movements and as a result of fold and fault reactivation peat mires in foreland and intrathrust coal basins mainly occupied broad areas on abandoned delta and fan delta lobes coals in intramontane and fault controlled basins were related with alluvial fan and lacustrine environments coal ranks vary from high volatile bituminous coals to anthracite depending on the structural location of the coalfields hydrothermal activity seems to be the most important factor in coal evolution

neogene paleomagnetism and oroclinal bending of the central andes of bolivia geophysical and tectonic evidence suggests that the principal vertical axis bending of the bolivian orocline is a late cenozoic event related to subduction and andean uplift to further test this hypothesis paleomagnetic data are presented from miocene basins within the altiplano and cordillera oriental of bolivia located in the presumed southern limb of the orocline to factor out potential vgp data samples were collected from sedimentary sections spanning at least 0 5 m y new paleomagnetic data are presented for two sites i e gerdas 21 s latitude 16 15 ma and quehua 20 s latitude 13 7 ma in conjunction with previously published paleomagnetic results from middle to late tertiary rocks from bolivia and peru spanning some 16 of latitude 6 22 s a consistent pattern of oroclinal development is as follows significant counterclockwise rotation about 15 20 in the northern limb negligible rotation in the axis at the latitude of the arica deflection and significant clockwise rotation about 15 20 in the southern limb although these paleomagnetic data could represent independent mesoscale 102 to 104 km2 block rotations the preferred interpretation is that they confirm the hypothesis that a significant portion about 30 40 of the oroclinal bending occurred during the late cenozoic using the unit mean paleomagnetic declinations in the well dated sections minimum rates of oroclinal bending during the late cenozoic are calculated to range from 0 6 m y to 1 7 m y the remaining 15 20 of observed curvature of the orocline probably resulted from an earlier phase of andean deformation or may represent the shape of the ancestral andes

structure of the mount raymond transverse zone at the southern end of the wyoming salient sevier fold thrust belt utah abstract the mount raymond transverse zone mrtz forms the east west trending boundary between the wyoming salient of the sevier fold thrust belt and the uinta cottonwood arch in north central utah major faults in the zone dip 40 to 45 north our structural analysis indicates that the mrtz contains both contractional and extensional structures the contractional structures thrusts and related folds initially formed as part of a southeast verging northeast trending thrust system this system gradually curved and merged to the north with the east verging absaroka thrust system in the apex of the wyoming salient the contrast in trend between structures in the mrtz and those in the apex of the wyoming salient reflects the initial curvature of the salient this curve formed because the stratigraphic sequence involved in thrusting thinned gradually southward toward the axis of a proto uinta arch the present east west trend of the mrtz developed during laramide uplift of the unita cottonwood arch an event which tilted the faults into their present position cenozoic crustal extension subsequently reactivated segments of the mrtz in sum the evolution of the mrtz illustrates how post thrusting processes can affect the map view geometry of thrust belts to create transverse zones

rapid uplift of the altiplano revealed through 13c 18o bonds in paleosol carbonates the elevation of earth s surface is among the most difficult environmental variables to reconstruct from the geological record here we describe an approach to paleoaltimetry based on independent and simultaneous determinations of soil temperatures and the oxygen isotope compositions of soil waters constrained by measurements of abundances of 13 c 18 o bonds in soil carbonates we use this approach to show that the altiplano plateau in the bolivian andes rose at an average rate of 1 03 0 12 millimeters per year between 10 3 and 6 7 million years ago this rate is consistent with the removal of dense lower crust and or lithospheric mantle as the cause of elevation gain

modeling the behavior of the continental mantle lithosphere during plate convergence in studying orogenic processes the mechanism of viscous rayleigh taylor type removal of gravitationally unstable lithosphere is often invoked to explain the behavior of the mantle lithosphere using numerical models we consider this mechanism and explore alternate styles of deep lithospheric deformation during tectonic convergence the numerical experiments incorporate a mix of viscous and plastic rheologies to model the mechanical evolution of the lithosphere asthenosphere system our results suggest that there are a number of deformational modes of the model mantle lithosphere 1 a dripping or rayleigh taylor type instability 2 an asymmetric underthrusting or subduction 3 symmetric ablative plate consumption 4 slab breakoff the failure and detachment of the strong lithosphere and 5 mixed modes with combinations of these processes the development of the modes is controlled by the rate of convergence associated with the background tectonic regime the density field and the rheology of the mantle lithosphere it is important to determine whether these modes occur in the earth beneath collisional orogens

interaction of basement uplift and thin skinned thrusting moxa arch and the western overthrust belt wyoming a hypothesis immediately north of big piney in western wyoming the moxa arch is modeled as emplaced during the late cretaceous along an east dipping low angle thrust moxa thrust that has precambrian basement and paleozoic and younger cover rocks in the hanging wall the west verging moxa thrust cut up section from the basement cover contact and flattened to the west along a detachment in lower triassic rocks thaynes detachment during motion on the moxa thrust the leading edge of the hanging wall wedged westward along the thaynes detachment peeling back triassic and younger rocks and thrusting them relatively eastward along the ancestral prospect thrust the 5 3 km of westward movement along the moxa thrust was matched by eastward movement along the west dipping ancestral prospect thrust the ancestral prospect thrust moved an additional 5 1 km during the late paleocene when thrust belt deformation progressed eastward to the moxa arch it appears that in the snider basin area the prospect thrust does not share a ramp with the darby thrust and that the emplacement of the moxa arch and prospect thrust determined the position of the later and more westerly darby thrust

strike slip faulting thrusting and related basins in the cenozoic evolution of the southern branch of the bolivian orocline abstract the evolution of the cenozoic deformation of the cordillera oriental and of the contact zone with the altiplano in southern bolivia is well documented in the tupiza nazareno and estarca basins the tectonic evolution started at about 29 ma the period between 29 and about 22 21 ma is marked by development of a pull apart basin related to n s trending left lateral strike slip faulting during this period initial deposition consisted of andesitic lavas and detrital sediments breccias matrix supported red conglomerates sands and silts of the catati and tupiza formation deposited in alluvial fan and flood plain environments from 21 to 20 ma the tectonic setting evolved to n s oriented thrusts which had a dextral component this event caused the tupiza basin to evolve into a full ramp basin and triggered the development of two piggy back basins nazareno and estarca both basins record detrital deposition nazareno formation in an alluvial fan environment at 10 9 ma deformation in this area stopped allowing peneplanation during which time the san juan de oro erosional surface was formed and the fluvial conglomerates of the oploca formation were deposited this segment of the andes was then tectonically transported to the east while uplift due to thrusting continued in the eastern andean front and in the subandean zone

space geodetic observations of nazca south america convergence across the central andes space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic nazca and continental south american plates in peru and bolivia roughly half of the overall convergence about 30 to 40 millimeters per year accumulated on the locked plate interface and can be released in future earthquakes about 10 to 15 millimeters per year of crustal shortening occurred inland at the sub andean foreland fold and thrust belt indicating that the andes are continuing to build little 5 to 10 millimeters per year along trench motion of coastal forearc slivers was observed despite the oblique convergence

bounds on the viscosity coefficient of continental lithosphere from removal of mantle lithosphere beneath the altiplano and eastern cordillera 1 the rapid rise of the central andean plateau between 10 and 6 8 ma implies that mantle lithosphere including eclogitized lower crust was removed from beneath the region in that time interval we infer from that removal that the average viscosity coefficient of mantle lithosphere was quite low when removal occurred using scaling laws for the growth of perturbations to the thickness of a dense layer over an inviscid substratum rayleigh taylor instability we place bounds on the average viscosity coefficient for central andean lithosphere when compared with laboratory measurements of flow laws for olivine and eclogite the allowed range of viscosity coefficients yields bounds on the temperature of 500 800 c at the moho beneath this region and suggests that mean stresses across mantle lithosphere during continental deformation are less than 50 mpa this range of temperature is comparable with if a slightly lower than we might expect for lithosphere approximately doubled in thickness and not yet equilibrated with the doubled crustal radioactivity the mean deviatoric stress is comparable to that associated with stresses that drive plates and hence shows that lithospheric material is not too strong to prevent removal of its mantle part

variscan accretionary complex of northwest iberia terrane correlation and succession of tectonothermal events the allochthonous terranes of northwest iberia can be correlated with specific paleogeographic realms of the continental masses and intervening oceans involved in the variscan collision assuming that the existing ophiolites represent the suture formed by the closure of the rheic ocean the units in the footwall to the suture correspond to the outer edge of the gondwana continental margin and the units in the hanging wall are interpreted as fragments of the conjugate margin represented by the meguma terrane this correlation establishes a precise link between circum atlantic terranes and makes it possible to draw a relatively simple scenario of the successive tectonothermal events recorded following the amalgamation of avalon to laurentia the remaining outboard terranes underwent a progressive accretion to this continent that ended with the collision between laurentia and gondwana

paleomagnetism and the orocline hypothesis eldredge s bachtadse v and van der voo r 1985 paleomagnetism and the orocline hypothesis in n l carter and s uyeda editors collision tectonics deformation of continental lithosphere tectonophysics 119 153 179 oroclines were originally defined by carey as curved mountain belts which initially were straight or at feast straighter than they are today in the last few years the definition has been broadened to include any curved mountain belt regardless of its original shape since the occurrence of oroclinal bending is best recorded in the change of declination as a function of tectonic setting paleomagnetic and structural data from six potential oroclines have been compiled and analyzed to determine the amount of rotation displayed by the change of paleomagnetic declination relative to the change in strike of the fold belt the arcuate belts investigated are the sicilian calabr n arc and the umbrian arc of italy the swiss portion of the jura mountains the central portion of the appalachian mountains from pennsylvania to virginia u s a the wyoming idaho overthrust belt of western north america and the hercynides of western and central europe the jura mountains and the pennsylvania virginia portion of the appalachians fail to show significant oroclinal bending the wyoming idaho belt shows a combination of rotated possibly oroclinai and unrotated thrust sheets in the sicilian calabrian arc significant oroclinat bending caused by the impingement of the calabria peloritani nappes in the late tertiary can be demonstrated while the umbrian arc of similar age in the northern apennines also shows oroclinal bending on a smaller scale hercynian europe the only belt included in which deformation of basement rocks can be demonstrated shows oroclinal bending at least so as well as a marked original curvature 70 in its western part common to all the orociines studied in this paper is the probable impingement of a rigid block or continental margin during the orogeny causing subsequent deformation and bending of the fold belt

paleogeography and andean structural geometry northwest argentina neogene tectonic segmentation of the andean cordillera in northwestern argentina is a function of both the dip of the subducted plate and of major paleogeographic structures and sedimentary basins vergence fault dip and lateral continuity of neogene structures can be related to the character and pre existing geometries and structures of the materials in which they form south of 27 s the thick skinned sierras pampeanas which occur mostly over a flat segment of the subducted nazca plate formed where little sedimentary cover overlay crystalline basement north of 24 s the thin skinned subandean belt overlying an inclined slab segment formed within a thick elongate eastward tapering trough of paleozoic and mesozoic sediments that may represent a complex of pre andean foreland basin deposits the transitional region between 24 27 s the santa barbara system and cordillera oriental is characterized paleogeographically by complex cross trending structures and irregular basin geometries and the neogene structures are equally complex and variable the paleogeographic features of nw argentina are a product of compressional tectonism during much of the paleozoic that resulted in an orogenic belt oriented obliquely to the modern andes this earlier orogen was disrupted by several zones of transverse structures and by cretaceous extensional or pull apart basins these features clearly influence the geometry of the superposed andean structures more intriguing however is the possibility that preexisting structures of the continental lithosphere may in some way help to modify the geometry of the subducted plate

lacustrine molluscan carbonates an interspecific approach toward the understanding of palaeoenvironmental conditions during the holsteinian interglacial mis 11 using δ18o and δ13c abstract stable isotope geochemistry is a well known tool for the investigation of palaeoclimate the application of this methodology to molluscan carbonate materials has increasingly receiving more attention in recent decades stimulating the development of numerous environmental reconstructions among the different habitats and shell species investigated so far the freshwater are the ones that have received least attention the scarcity of systematic studies on lacustrine and riverine molluscan species considerably limits the use of these remains to reconstruct the environmental conditions of inland palaeontological and archaeological sequences the present study focuses on bithynia tentaculata a freshwater species abundant in the european quaternary fossil record the aim of the research is to validate the use of oxygen 18o and carbon stable isotopes 13c of this species as palaeoenvironmental proxies for this purpose shells and opercula from stratigraphic sequences of three palaeolakes in eastern poland formed during the holsteinian interglacial mis 11 are analysed the intercorrelation between b tentaculata and two other molluscan species together with good agreement with the pollen record reveal that 18o and 13c are suitable indicators of water temperature and humidity levels for lacustrine environments these results pave the way for using b tentaculata in further palaeoclimatic reconstructions

strain fabric associated with the meade thrust sheet implications for cross section balancing abstract emplacement of the meade thrust sheet southeastern idaho u s a was accompanied by the development of a strain fabric and significant footwall deformation characteristics of the strain fabric are based on strain estimates using pressure solution cleavage and deformed fossils within the jurassic twin creek formation the strain fabric formed during early layer parallel shortening at the thrust front and accommodates from 10 to 35 shortening within the incipient thrust sheet footwall strata were later folded above a detachment at the base of the twin creek formation and the early strain fabric was passively rotated the structural development of this area consisting of 1 strain fabric development 2 folding of the footwall and 3 the truncation of footwall structures by imbricates of the meade thrust are placed in a relative time frame to aid in section reconstruction the passive reorientation of the strain fabric and the truncation and incorporation of footwall structures into the thrust sheet are used as kinematic and geometric constraints for cross section balancing we present a sequential retrodeformation of the meade thrust sheet in which strain is removed from the cross section at the appropriate geometric stages in the deformation these sections illustrate some methods by which strain data can be incorporated into cross sections and also indicate the difficulty involved in balancing cross sections of internally deformed thrust sheets interpretation of these sections suggests that a significant amount of footwall deformation accompanied the emplacement of the meade thrust sheet and that footwall deformation is strongly controlled by the local stratigraphic package the sections also illustrate that the earliest deformation within a thrust sheet may be unrelated to motion on the underlying incipient thrust and that the trace of a thrust fault which cuts previously deformed strata will not restore to a typical ramp and flat geometry

cordillera formation mantle dynamics and the wilson cycle we propose that the andes cordillera formed by a collision between the trenchward moving south american plate and the nazca slab and subslab mantle resistance to trenchward motion in the form of horizontal compressive normal stress deforms south america9s leading edge the advancing continent also perturbs the subslab mantle flow field as mantle is displaced this flow field detected via shear wave splitting includes trench parallel flow away from a central stagnation point and corner flow around the northern and southern ends of the continent the corresponding spatial variation in applied stress accounts for the large scale characteristics of andean deformation buckling at the stagnation point maximum normal stress forming the central andes and bolivian orocline and corner flow associated with the eastward motion of the caribbean and scotia plates the high stresses required to form and maintain the andes imply that south america9s trenchward motion is primarily driven by deep mantle flow coupled to its base no other driving force of sufficient magnitude is available similarities between north and south american plate motion history and large scale cordillera structure indicate that the western north american laramide rockies formed in a similar manner and that the north american plate is also driven westward by deep mantle flow we infer that atlantic spreading itself is similarly driven by contrast slab rollback in a closing ocean basin opens marginal basins at convergent margins as in the western pacific we suggest that there is a causal link between the opening and closing of ocean basins and the formation of cordilleras and marginal basins cordillera formation occurs at the leading continental margin during the spreading phase of the wilson cycle whereas marginal basins form during the closing phase

conical folds and apparent rotations in paleomagnetism a case study in the southern pyrenees abstract the so called apparent rotation was defined as the angular deviation between a local paleomagnetic direction after the standard bedding correction and their corresponding paleomagnetic reference j geophys res 85 1980 3659 in this paper we make a theoretical exploration on this concept and we conclude that depending on the number sequence orientation and magnitude of the deformation axes that have affected to the rock volume the apparent rotation may be the addition of a vertical axis rotation plus a spurious rotation the later is an error whose origin is the inappropriate application of the bedding correction during the restoration which does not fit the reverse sequence of deformations then apparent rot s l spurious rot vertical axis rot conical folds are complex geometries that cannot be restored by using the bedding correction however appearance of apparent and spurious rotations has not been studied even though the presence of this kind of folds is very common in fold and thrust belts in this paper we show a way to restore these structures and its associated paleomagnetic data by means of forward modelling on a stereographic projection the modelling has to be based on a good characterization of the geometry fold axis orientation and understanding of the kinematics of the fold general modelling has also allowed us to predict the apparent rotation in conical synclines its magnitude depends on the semiapical angle and on the degree of development of the fold the sense of the rotation clockwise or counter clockwise will depend on the sense of rotation the fold axis the western external sierras provide an excellent case study of apparent rotations due to the presence of a conical fold in the footwall ebro foreland basin of the south pyrenean sole thrust in addition a vertical axis clockwise rotation up to 47 32 in average has been detected in the hagingwall an apparent rotation up to 28 20 in average is observed in the footwall of the structure when a simple bedding correction is used this deviation does not fit with the expected ebro basin direction reference and is caused by the effect of the riglos conical syncline developed by the flexure of the foot wall ramp of the south pyrenean sole thrust the forward modelling carried out considering the geometry and kinematics non significant rotations in the autochthonous foot wall of this structure predicts very well the paleomagnetic observations in the field geographic coordinates with angular departures of only 5 in average the only application of the bedding correction would introduce errors spurious rotations up to 21 12 in average related to the conical geometry that would not allow the differentiation of these distinct structural units

u pb zircon and monazite geochronology of variscan magmatism related to syn convergence extension in central northern portugal abstract the viseu area is located in the central iberian zone of the iberian variscan belt and hosts numerous post thickening collision related granitoids intruded into upper and middle crustal levels the present paper reports high precision u pb zircon and monazite ages for four plutons of the viseu area the syn kinematic granitoids of maceira 314 5 ma casal vasco 311 1 ma and junqueira 307 8 0 7 ma and the late kinematic biotite monzogranites of cota 306 9 ma this points to a synchronous emplacement of the different syn kinematic plutons shortly followed by the intrusion of the late kinematic granites and shows that the upper carboniferous plutonism occurred within a short time span of ca 10 million years the ascent of granite magmas took place after an extensional tectonic event d 2 and is coeval with dextral and sinistral crustal scale transcurrent shearing d 3 field and petrographical evidence suggest a narrow time span between peak t metamorphic conditions and the intrusion of granitic melts which implies very fast uplift rates accommodated through active tectonic exhumation magma compositions evolve through time reflecting an increasing involvement of mid crustal sources and the underplating effect of an upwelling asthenospheric mantle at the base of a thinning and stretching continental crust

self subduction of the pangaean global plate one of the most striking and rare occurrences in the earth s history is the amalgamation of most of the continental lithosphere into one supercontinent the most recent supercontinent pangaea lasted from 320 to 200 million years ago here we show that after the continental collisions that led to the formation of pangaea plate convergence continued in a large wedge shaped oceanic tract we suggest that plate strain at the periphery of the supercontinent eventually resulted in self subduction of the pangaean global plate when the ocean margin of the continent subducted beneath the continental edge at the other end of the same plate our scenario results in a stress regime within pangaea that explains the development of a large fold structure near the apex of the palaeotethys ocean extensive lower crustal heating and continental magmatism at the core of the continent as well as the development of radially arranged continental rifts in more peripheral regions of the plate

oroclinal bending distributed thrust and strike slip faulting and the accommodation of arabia eurasia convergence in ne iran since the oligocene regional shortening is accommodated across ne iran in response to the collision of arabia with eurasia we examine how n s shortening is achieved on major thrust systems bounding the eastern branch of the alborz east of 57 e sabzevar and kuh e sorkh mountain ranges which lie south of the kopeh dagh mountains in ne iran although these ranges have experienced relatively few large earthquakes over the last 50 yr they have been subject to a number of devastating historical events at neyshabur esfarayen and sabzevar a significant change in the tectonics of the eastern alborz occurs directly south of the central kopeh dagh near 57 e to the east shortening occurs on major thrust faults which bound the southern margin of the range resulting in significant crustal thickening and forming peaks up to 3000 m high active shortening dies out eastward into afghanistan which is thought to belong to stable eurasia the rate of shortening across thrust faults bounding the south side of the eastern alborz north of neyshabur is determined using optically stimulated luminescence dating of displaced river deposits and is likely to be 0 4 1 7 mm yr 1 shortening across the sabzevar range 150 km west of neyshabur has previously been determined at 0 4 0 6 mm yr 1 although reassessment of the rate here suggests it may be as high as 1 mm yr 1 migration of thrust faulting into foreland basins is common across ne iran especially in the esfarayen region near 57 e where the northward deflection of the east alborz range reaches a maximum of 200 20 km from its presumed linear e w strike at the beginning of the oligocene west of 57 e the tectonics of the alborz are affected by the westward motion of the south caspian region which results in the partitioning of shortening onto separate thrust and left lateral strike slip faults north and south of the range at the longitude of 59 e published gps velocities indicate that 50 per cent of the overall shortening across ne iran is accommodated in the kopeh dagh the remaining 50 per cent regional shortening must therefore be accommodated south of the kopeh dagh in the eastern alborz and kuh e sorkh ranges assuming present day rates of slip and the fault kinematics are representative of the late cenozoic deformation in ne iran the total 200 20 km n s shortening across the eastern alborz and kopeh dagh mountains since the beginning of uplift of the kopeh dagh basin would be accommodated in 30 8 ma although this extrapolation may be inappropriate over such a long timescale the age is nevertheless consistent with geological estimates of post early to middle oligocene 30 ma for the onset of kopeh dagh uplift

a non cylindrical model for the northwest iberian allochthonous terranes and their equivalents in the hercynian belt of western europe abstract a case is made for a non cylindrical development of the european hercynian belt far travelled allochthonous terranes in the belt consist of a mixture of units including catazonal units ophiolites and units derived from the continental margin of gondwana they often depict a metamorphic history of intermediate to high pressure type and caledonian to eo hercynian age and are thought to have been put together in the early stages of the orogeny through a process partly obductive directed onto the continental margin of gondwana considerations upon the disposition of the terranes their relationships with the paleogeography of the gondwana realm and the structures in them suggest that their emplacement took place in an oblique fashion at a small angle to the margin of the continent the proposed allochthon could be related to the development of an island arc or a wedge like subplate in the southern part of the baltica plate later convergence between gondwana and laurussia involving transcurrent mostly dextral movements combined with tectonic escape phenomena would account for the present complexity of the belt and for the narrow strip the allochthonous terranes delineate in westernmost europe

paleomagnetic considerations of the development of the pennsylvania salient in the central appalachians abstract late paleozoic remagnetizations of central appalachian clastic and carbonate rocks of ordovician to devonian age constrain the structural evolution of the pennsylvania salient new paleomagnetic results from 26 sites located on nine folds combined with previously published results from 60 sites yield a mean direction for the northeastern limb of the salient at 172 1 k 49 and a mean direction for sites southwest of the salient at 173 1 k 70 these two directions are not statistically different from each other and are not significantly rotated from the expected permian paleomagnetic direction for north america individual fold test results show that this secondary magnetization is synchronous with or even older than folding across the region these paleomagnetic results suggest that curvature of fold axes around the salient is an original feature of fold growth and not the result of post orogenic oroclinal bending in contrast analysis of the primary magnetization in the silurian bloomsburg formation shows that directions from the two limbs of the salient are significantly different new results from twenty sites yield a mean direction for the northeastern limb at 4 40 k 31 and for the southwestern limb at 338 8 k 23 combining these results with previously published directions from the bloomsburg formation yields a mean direction for the northeastern limb at 1 34 k 32 and for the southeastern limb at 343 34 k 30 the difference in declination does not appear to be the result of internal deformation of the rocks or acquisition of magnetization during a period of rapid apparent polar wander rather these directions indicate that the northeastern limb has been rotated clockwise 18 9 relative to the southwestern limb the sense and amount of this rotation is consistent with the curvature of the orogen around the salient however within each limb of the salient the primary component declinations do not directly correlate with the orientation of the first order fold axes collectively therefore these paleomagnetic results suggest that the structural evolution of the salient included pre orogenic rotations or rotations that occurred early in the alleghanian orogeny followed by later remagnetization and folding of the central appalachian foreland

neogene and quaternary rollback evolution of the tyrrhenian sea the apennines and the sicilian maghrebides reconstruction of the evolution of the tyrrhenian sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 ma it involved two episodes of back arc extension which were induced by the rollback of a west dipping subducting slab the first period of extension 10 6 ma was prominent in the northern tyrrhenian sea and in the western part of the southern tyrrhenian sea the second period of extension mainly affected the southern tyrrhenian sea began in the latest messinian 6 5 ma and has been accompanied by subduction rollback at rates of 60 100 km myr 1 slab reconstruction combined with paleomagnetic and paleogeographic constraints indicates that in the central apennines the latest messinian 6 5 ma arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike slip faults these processes resulted in the formation of a narrow subducting slab beneath the ionian sea that has undergone faster subduction rollback and induced extreme rates of back arc extension

controls on the geometry of fold thrust belt salients salients convex to the foreland curves in the map traces of orogens typically involve fold thrust belts formed along the foreland margin of orogens an examination of the map view shape degree of asymmetry dimensions and structural trend line patterns i e the variation in fold trend and fault strike along the length of the belt of 20 fold thrust belt salients worldwide illustrates relationships between salient geometry and the geologic setting in which the salient formed in order to understand these relationships we used simple sandbox models to simulate the formation of three of the several types of salients specifically we examined indenter controlled salients the geometry of which reflects interaction of a fold thrust belt with a hinterland indenter basin controlled salients the geometry of which reflects lateral variations in the thickness of predeformational basin fill and detachment controlled salients the geometry of which reflects lateral variations in the strength of a detachment surface our models demonstrate that thrusts in these three types of salients initiate with a curved shape and that rocks within thrust sheets on the limbs of salients can undergo a vertical axis rotation in response to simple shear without requiring major rotation of the fault trace furthermore trend line patterns distinguish basin controlled from indenter controlled salients specifically structural trend lines in basin controlled salients converge at the end points of the curve while in indenter controlled salients they diverge at the end points of the curve the asymmetry of a salient correlates with the asymmetry of the predeformational basin and or of the indenter and with the convergence direction thus our results explain geometric characteristics of individual salients and address controversies concerning paleomagnetic data in salients they also may help prioritize petroleum exploration targets in salients foreland basin oil fields concentrate at the apex of basin controlled salients because the geologic factors that generate the salient also contribute to localizing oil fields

paleomagnetic evidence for neogene tectonic rotations in the northern apennines italy abstract paleomagnetic analysis was carried out in the northern apennines on eocene to pliocene epiligurian units five early miocene and two middle miocene sites yielded dual polarity site mean directions which show signs of clustering after correction for bedding tilt these likely primary magnetizations in conjunction with data from the literature give an overall mean late oligocene middle miocene paleomagnetic pole which shows a large and significant counterclockwise rotation of 52 8 with respect to the africa reference paleopoles or a similar amount of rotation with respect to the coeval europe reference paleopole however this paleopole falls close to the roughly coeval paleopole for corsica sardinia which is here calculated by averaging data from the literature three additional early miocene sites from an area west of parma affected by pliocene tectonics yielded site mean directions which pass the fold test and are rotated counterclockwise by a lesser amount than the rest of the miocene sites most of the remaining sites bear paleomagnetic directions acquired after tilting during a recent phase of remagnetization we suggest that the large scale rotation observed in the northern apennines was associated with the motion of the corsica sardinia block within the general context of the africa europe relative motions a compilation of published data from the central apennines also shows a differential rotation of the northern relative to the southern umbria belt which occurred after the motion of corsica sardinia and may have been due to pivoting of the northern umbria belt against a deep seated lineament during the non rotational opening of the tyrrhenian sea

paleogene building of the bolivian orocline tectonic restoration of the central andes in 2 d map view using available information on the magnitude and age of tectonic shortening as well as paleomagnetically determined tectonic rotations we have run a series of 2 d map view restorations of the central andes neogene shortening in the foreland belt induced only slight orogenic curvature of the central andes the constraints on the ages of the large observed fore arc rotations average of 37 counterclockwise in southern peru and 29 clockwise in northern chile indicate that the bolivian orocline formed during the eocene oligocene as a consequence of differential shortening focused in the eastern cordillera to minimize local block rotations along the fore arc the restoration that best fits the central andean rotation pattern requires about 400 km of total paleogene plus neogene shortening near the arica bend this value corresponds to the upper bound of estimates of maximum horizontal shortening for the central andes along strike variations in horizontal shortening in the back arc induced bending of the continental margin block rotations with fore arc along strike extension and or orogen parallel transport of upper crustal material toward the symmetry axis of the orocline