the tertiary structural and thermal evolution of the central alps compressional and extensional structures in an orogenic belt the western alpine arc has been created during the cretaceous and the tertiary orogenies the interference patterns of the tertiary structures suggest their formation during continental collision of the european and the adriatic plates with an accompanying anticlockwise rotation of the adriatic indenter extensional structures are mainly related to ductile deformation by simple shear these structures developed at a deep tectonic level in granitic crustal rocks at depths in excess of 10 km in the early palaeogene period of the tertiary orogeny the main tertiary nappe emplacement resulted from a nw thrusting of the austroalpine penninic and helvetic nappes heating of the deep zone of the upper cretaceous and tertiary nappe stack by geothermal heat flow is responsible for the tertiary regional metamorphism reaching amphibolite facies conditions in the lepontine gneiss dome geothermal gradient 25 c km the tertiary thrusting occurred mainly during prograde metamorphic conditions with creation of a penetrative nw se oriented stretching lineation xi finite extension parallel to the direction of simple shear earliest cooling after the culmination of the tertiary metamorphism some 38 ma ago is recorded by the cooling curves of the monte rosa and mischabel nappes to the west and the suretta nappe to the east of the lepontine gneiss dome the onset of dextral transpression with a strong extension parallel to the mountain belt and the oldest s vergent backfolding took place some 35 to 30 ma ago during retrograde amphibolite facies conditions and before the intrusion of the oligocene dikes north of the periadriatic line the main updoming of the lepontine gneiss dome started some 32 30 ma ago with the intrusion of the bergell tonalites and granodiorites concomitant with s vergent backfolding and backthrusting and dextral strike slip movements along the tonale and canavese lines argand s insubric phase subsequently the center of main updoming migrated slowly to the west reaching the simplon region some 20 ma ago this was contemporaneous with the westward migration of the adriatic indenter between 20 ma and the present the western aar massif toce culmination was the center of strong uplift the youngest s vergent backfolds the glishorn anticline and the berisal syncline fold the 12 ma rb sr biotite isochron and are cut by the 11 ma old rhone simplon line the discrete rhone simplon line represents a late retrograde manifestation in the preexisting ductile simplon shear zone this fault zone is still active today the oligocene neogene dextral transpression and extension in the simplon area were concurrent with thrusting to the northwest of the helvetic nappes the prealpes 35 15 ma and with the jura thin skinned thrust 11 3 ma it was also contemporaneous with thrusting to the south of the bergamasc 35 5 ma and milan thrusts 16 5 ma the oligocene neogene dextral transpression in the central alps is of the same age as the thrusting to the west and southwest in the external french part and to the east in the internal italian part of the alpine arc of the western alps

the mount hay block central australia another puzzle piece for paleo mesoproterozoic tectonic history abstract the 100 km 2 mount hay block arunta region central australia is a 15 km thick crustal cross section comprised of granulite facies tectonites containing a tectonothermal record from ca 1803 to ca 1552 ma as demonstrated by the combination of prior work and new monazite geochronology and thermobarometry igneous and sedimentary protoliths formed ca 1803 1798 ma during 1810 1790 ma stafford tanami time between ca 1790 and 1740 ma nearly continuous magmatism in the aileron province and possibly ages of recrystallized zircon and monazite in the mount hay block record subduction along the southern margin of the proterozoic australia mawson continent geologic mapping and zircon and monazite geochronology demonstrate that the mount hay block was penetratively deformed between ca 1720 ma and 1700 ma during 1735 1690 ma strangways time this includes the 8 km thick mount hay sheath fold which records ne sw subhorizontal shear once paleozoic tilting of the mount hay block is removed while still in the deeper crust 798 33 c 7 6 0 7 kbar fabrics along the northern edge of the mount hay block were transposed by the cross cutting 7 km thick capricorn ridge shear zone at ca 1551 7 5 5 ma during chewings time once restored the capricorn ridge shear zone records ne sw extension a jump in strain rate while at high temperatures 725 c initiated severe localization accommodated initially by pseudotachylite formation and cataclasis then mylonitization during fe rich fluid flux shearing may have continued to pressure temperature conditions as low as

exhumation of the dora maira ultrahigh pressure unit by buoyant uprise within a low viscosity mantle oblique slip shear zone the brossasco isasca subunit biu of the dora maira massif is currently the only known continental crustal ultrahighpressure uhp unit in the western alps the peak pressure temperature conditions are 3 5 4 5 gpa 730 c exhumation from 3 5 gpa to 1 gpa occurred within 2 2 1 8 ma but the exhumation mechanism is incompletely understood we present a conceptual model for the buoyancy driven exhumation of the biu inside a low viscosity dense mantle shear zone weakened by increased strain rates due to simultaneous strike slip and subduction oblique slip of the european plate two dimensional thermo mechanical models simulate such a buoyant uprise of an ellipse inside an inclined layer simulations i show the feasibility of the conceptual model ii fit the pressure temperature time record and iii constrain effective viscosities the model is compatible with the i small volume of continental crustal uhp rock in the western alps ii minor erosion during exhumation and iii strike slip deformation during the exhumation period

early mesozoic tectonics of the south china block insights from the xuefengshan intracontinental orogen abstract intracontinental orogens remain less understood than accretionary or collisional orogens that are related to plate margin interactions in the center of the south china block the xuefengshan belt provides a well exposed example of such an intracontinental orogen of early mesozoic age detailed field tectonic observations indicate that the xuefengshan belt can be divided into a western outer zone characterized by km scale box fold structures and an eastern zone separated from the western outer zone by the se dipping main xuefengshan thrust in the eastern zone nw verging folds coeval with a pervasive slaty cleavage and a nw se trending lineation are the dominant structures from west to east the dip of the cleavage surface exhibits a fan like pattern the bulk architecture of the xuefengshan belt results from polyphase deformation d 1 is characterized by a top to the nw ductile shearing d 2 corresponds to se directed back thrusting and folding d 3 consists of upright folds with vertical cleavage and lineation at depth a high strain zone characterized by greenschist facies metamorphic rocks and a top to the nw ductile shearing corresponds to a ductile decollement zone that accommodated the deformation of the neoproterozoic to early triassic sedimentary series kinematic compatibility suggests that the synmetamorphic ductile shearing was coeval with the d 1 event in the sedimentary cover the xuefengshan belt is interpreted as an early mesozoic intracontinental orogen which possibly originated from the se directed continental subduction of a piece of the south china block in response to northwestwards subduction of the pacific plate

partitioning of deformation within a subduction channel during exhumation of high pressure rocks a case study from the western alps the metamorphic belt of the western alps was subjected to widespread extensional tectonism at the end of the eocene ca 45 35 ma extension was accommodated by hinterland directed movements along gently inclined extensional shear zones which facilitated rapid exhumation of high pressure and ultra high pressure rocks this deformation resulted in a normal metamorphic sequence extension in the inner parts of the western alps was coeval with shortening at the front of the belt foreland directed thrusts which took place during decompression and emplaced higher grade metamorphic units over lower grade metamorphic rocks thus forming an inverse metamorphic sequence two mechanisms for this extensional episode are discussed 1 collapse of an overthickened lithosphere and 2 internal readjustments within the orogenic wedge due to subduction channel dynamics we favour the latter mechanism because it can account for the development of the observed inverse and normal metamorphic sequences along foreland directed thrusts and hinterland directed detachments respectively this hypothesis is supported by published structural metamorphic and geochronological data from four geological transects through the western alps this study also emphasizes the importance of post shearing deformation e g horizontal buckling versus vertical flattening which can modify the distribution of hinterland and foreland directed shear zones in orogenic belts

geodynamic models of crustal scale episodic tectonic accretion and underplating in subduction zones we investigate the dynamics of crustal scale episodic accretion at convergent margins using a finite element model of subduction the numerical model incorporates effects from flexure subduction loading and large scale variations in material properties the model is used to assess the influence of these parameters on the discrete transfer of units from the lower to the upper plate during subduction the primary controls investigated are spatial variations in density material strength and thickness of material layers these controls correspond to natural examples where episodic accretion results from the thermal structure of subducting lithosphere giving rise to areas of thermally activated ductile creep attempted subduction of small crustal blocks density frictional and or ductile strength variations interaction between well attached basement blocks and overlying detachable sediments and spatial changes in thickness of sediment and crust in the experiments accretion of small continental terranes within a model subduction zone can cause crustal scale fold nappes and shear zones to develop with accompanying tectonic underplating and or frontal accretion the model results may provide insight into real cases of episodic accretion for example the emplacement of the upper penninic nappe stack of the central alps

nappe geometry in the western swiss alps abstract detailed geological mapping during the last 20 years in the western swiss alps has shown clearly that most of the lower basement nappes are fold nappes possessing normal and inverted limbs moreover their cores are made of strongly deformed gneisses indicating that important ductile strain took place during the formation of the fold nappes it is therefore probably wrong to imagine deep basement nappes as rigid slices as often actually claimed especially when interpreting seismic profiles true brittle type thrust nappes involving basement rocks only occur in the internal and upper parts of the belt cover nappes on the contrary are in most parts of the alpine belt thrust sheets following more or less the rules of thin skinned tectonics many basement fold nappes lost part of their sedimentary cover during or just before their formation by decollement along ductile horizons the result is that many cover thrust nappes in the external part of the alps are directly related to their original basement fold nappes

aeromagnetic study of the hengshan wutai fuping region unraveling a crustal profile of the paleoproterozoic trans north china orogen abstract an integrated crustal profile of the intervening trans north china orogen tnco is one of the key issues to understanding the tectonic evolution of the north china craton however the existing geological studies focus only on the surface mapping based petrological geochemical and structural analysis but lack subsurface geophysical evidence and thus make the crustal profile interpretations ambiguous in contrast the current geophysical data covers a very large scale lithospheric mantle and fails to image the detailed structural pattern of the orogenic crust to achieve this goal we present high resolution aeromagnetic data for the hengshan wutai fuping region the largest exposure of the central tnco the reduced to pole magnetic anomaly map firstly verifies the regional tectonic subdivision that the high grade metamorphic terranes i e hengshan and fuping complexes are consistent with high magnetic responses and long wavelength anomalies intervened by a low grade terrane wutai complex characterized by low magnetic responses and short wavelength anomalies 3d euler deconvolution reveals that the tendencies of the clustered solutions show large consistence with the major structural pattern of the region which is characterized by a fan shaped doubly vergent orogenic wedge upward continuation further shows that the northwest part of the orogen yields a thicker crust and is most likely located closer to the paleosubduction zone the new aeromagnetic data combined with structural petrological and metamorphic data indicate that an eastward dipping subduction zone was most possibly active before the collision of the western and eastern blocks leading to the formation of the tnco and the final amalgamation of the north china craton

incomplete blueschist re crytallization in high grade metamorphics from the sesia lanzo unit vasario sparone subunit western alps a case history of metastability abstract the vasario sparone vs subunit is part of the sesia lanzo unit western alps and represents a thin and discontinuous slice of pre alpine deep continental crust it is tectonically interposed between the eclogitic mica schists and the gneiss minuti subunits metastable relics and a sequence of incomplete transformations controlled by a local equilibrium reaction mechanism are preserved in the vs subunit the vs rocks record a pre alpine high grade of metamorphism t 780 50 c p 6 kbar which is comparable to the metamorphism dated around 500 ma in the ivrea zone during the alpine evolution the vs rocks sustained an early blueschist stage t 300 400 c p 8 10 kbar of probable cretaceous age and a later low t low p stage of tertiary age the blueschist transformations are governed by slow reaction kinetics by the heterogeneous distribution of pervasive deformations and by syn metamorphic fluid phase in particular transient kyanite formed in the early stages of the blueschist re crystallization in response to a process controlled by fluid deficient conditions and by internal buffering of a h 2 o at low values

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

compression versus extension in the exhumation of the dora maira coesite bearing unit western alps italy abstract several recent papers suggest that uplift and exhumation of high pressure low temperature hp lt metamorphic rocks are due to crustal extension coeval with and or post dating the contraction responsible for the metamorphism a detailed survey of the coesite bearing unit of the southern dora maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra high pressure uhp rocks this uhp unit consists of pre alpine basement rocks which equilibrated at a depth of 100 km probably 120 100 ma ago it is overlain by two units with a similar lithology and eoalpine metamorphic age and high p t gradients which equilibrated at 60 and 50 km respectively this eoalpine basement complex is intercalated between blueschist facies units the whole pile was affected by a mesoalpine greenschist facies mylonitic deformation during which the early contacts were reactivated as low angle shallow dipping shear zones extension is documented only during the mesoalpine collisional period and is referred to the gravity collapse of a thickened orogenic wedge with an exhumation rate of about 1 mm yr the most important part of the exhumation of the uhp rocks occurred earlier at a higher rate close to 4 mm yr a forced return flow or extrusion tectonics of imbricate slices in an on going subduction setting seems appropriate to account for this early stage the fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed

mechanical model for subduction collision tectonics of alpine type compressional orogens alpine type orogens are characterized by three distinct convergent tectonic phases subduction with deformation that has primarily single vergence a transition from subduction to collision and continental collision with double vergence although the cenozoic history of the european alps has additional complexities a mechanical explanation for these three phases would provide the necessary crustal scale framework in which to develop an understanding of the smaller scale processes we present results from a simple numerical model which explain the mechanics of these three phases as a consequence of the changing buoyancy of the lithosphere subducted beneath the orogen the development and exhumation of a subduction complex suture zone and basement nappe stack piemont suture penninic nappes the presence of a crustal scale back fold and thrust insubric line and uplift of basement on the pro european side of the orogen external basement massifs may be explained as a simple consequence of changing dynamics during the transition from subduction to collision

the tectonic and rheological evolution of an attenuated cross section of the continental crust ivrea crustal section southern alps northwestern italy and southern switzerland the tectonic and rheological evolution of the southern alpine continental crust is reconstructed from structural petrological and radiometric studies in the ivrea and strona ceneri basement units the deep crust of the southern alps acquired its present compositional and metamorphic zonation during paleozoic magmatism and amphibolite to granulite facies regional metamorphism inferred strength contrasts between lower crustal and upper mantle rocks in the ivrea zone are low at the high temperatures of regional metamorphism late paleozoic transtension and basic to intermediate magmatism in all crustal levels preceded extensional faulting associated with the formation of a passive continental margin during early mesozoic time extensional uplift and cooling of the basement section is coupled with crustal scale trends of increasing rheological stratification grain size reduction and strain localization mylonitic shear zones in the lower crust and a broad zone of noncoaxial shear at the base of the intermediate crust in the ivrea zone are inferred to have controlled the high strain rheology of the attenuating southern alpine crust the deep crust in the thinned ivrea crustal cross section transferred noncoaxial strain from the upper mantle to the intermediate and upper crust final uplift and exposure of the deep southern alpine crust occurred during late cretaceous to tertiary alpine thrusting

the maggia nappe an extruding sheath fold basement nappe in the lepontine gneiss dome of the central alps the lepontine gneiss dome represents a unique region of the central alps where oligocene miocene amphibolite facies grade rocks and fold nappes of the deepest tectonic level of the alpine orogenic belt are exposed in a tectonic window the cenozoic structures of the maggia nappe reveals a giant tens of kilometre scale tubular fold structure that cross cut through the surrounding lower penninic nappes from its root situated in the southern steep belt of the alps near bellinzona the mesozoic sedimentary cover of the maggia nappe is typical for the helvetic stratigraphic domain the age of formation of the lower penninic fold nappes by ductile detachment of the upper european crust during its underthrusting below the higher penninic and austroalpine orogenic lid and adriatic indenter was estimated between 40 and 30 ma maximal pressures of 8 9 kbars and temperatures of 600 700 c were attended during and after the nappe emplacement some 30 22 ma ago the maggia and surrounding nappes are crosscut by the isograds of the barrovian regional metamorphism

a geodynamic framework for interpreting crustal scale seismic reflectivity patterns in compressional orogens summary 2 d plane strain finite element models are used to calculate crustal deformation patterns within model compressional orogens and the results are compared with natural examples the fundamental driving mechanism for model deformation is provided by asymmetric detachment and underthrusting of mantle lithosphere with deformation being rooted at the stress singularity where the mantle detaches deformation steps up into the overlying crust and spreads laterally according to the rhcological properties assumed for the crust the crust is modelled using plastic frictional and viscous rheologies and incorporates the effects of compositional layering and variable geothermal gradients overall deformation is determined by balancing the internal strength of the crust against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming orogen the amount of convergence in the models is limited to 150 km or less and so the results are not strictly comparable with larger orogens involving greater convergence nevertheless larger orogens seem to evolve through stages that may be understood in terms of the models provided that consideration is confined to smaller scale components that are limited in time and space the continuum deformation calculated by the finite element models is qualitatively converted to equivalent pattcrns of discrete faults and shear zones by associating these discrete features with the areas of highest cumulative strain using the results of sandbox experiments as guides significant styles of deformation include 1 coupling between step up shears and subhorizontal detachments at one or more levels 2 localization of upper crustal deformation by subduction of the lower crust and 3 pinning of deformation by surface denudation a partial catalogue of such model styles is presented in which the strength of basal coupling between crust and mantle the number of rhcological layers within the crust the thickness of subducted lower crust and the extent of surface denudation are varied systematically compressional orogens with a wide range of ages are characterized by crustal scale seismic reflectivity similar to patterns of fault shear zones represented in this catalogue of styles this similarity suggests that crustalscale seismic reflection data can be used to constrain the dynamic processes involved in compressional orogcnesis

fluid migration through thrust faults in the helvetic nappes western swiss alps the 18o 13c and 87sr 86sr values of calcite and organic matter were measured for carbonate mylonites from numerous thrusts in the helvetic alps carbonate mylonites in most of the thrusts retain essentially unaltered protolith 18o and 13c values consistent with there having been little to no advection of isotopically distinct fluid through these faults only carbonate mylonites from the basal thrusts of the largest nappes have 18o and or 13c values that differ from those of their protoliths the zone of isotopic alteration exchange is confined to c 10 to 20 meters of these fault contacts we propose the fluids that migrated through these faults contained variable amounts of organically derived carbon and radiogenic strontium and were probably derived from dewatering of the sedimentary rocks and prograde metamorphic reactions in the nappes root zones apart from the basal thrusts of the largest nappes that behaved as narrow laterally extensive conduits for fluids there is little isotopic evidence that large quantities of fluids passed through most of the carbonate hosted thrusts in the helvetic alps

late neoproterozoic to permian tectonic evolution of the quebec appalachians canada abstract due to the unique location of the quebec appalachians in a re entrant of the paleo north american margin and to the fact that the area largely escaped late paleozoic deformation this segment of the orogen preserves evidence of the early orogenic stages that are partly obscured elsewhere despite diachronicity both along and across strike of the orogen five tectonic events of regional significance are recognized 1 the earliest event is mainly recorded by neoproterozoic to early ordovician normal faults associated with extension of the laurentian craton and the establishment of a passive continental margin 2 the second event the ordovician taconian orogeny is marked by foreland directed deformation and corresponds to the principal deformational and metamorphic events recorded by the laurentian margin 3 the third event early salinic is early silurian it is restricted to the southern part of the quebec appalachians and consists of hinterland se directed thrusts folds and retrograde metamorphism 4 the fourth event late salinic is recorded by regional scale late silurian to early devonian normal or transtensional faults and is associated with the formation of the connecticut valley gaspe trough and 5 the last event the devonian acadian orogeny affects with variable intensity all the pre carboniferous lithologies this tectonic synthesis implies that the subdivision of the peri laurentian oceanic tracts proposed in other segments of the appalachians should be applied with caution in quebec and that the structural style the number of oceanic domains and possibly the vergence of subduction change along the strike of the orogen in particular the similarities of the thetford mines and boil mountain ophiolitic complexes and sedimentary covers and the lack of evidence for a major intervening suture are consistent with the maximum allochtony hypothesis i e the assumption that the southern quebec ophiolites are rooted east of the chain lake massif a geodynamic model involving the taconian obduction of a large ophiolitic nappe s the docking of a peri gondwanan gander medial new england terrane in early salinic the formation of large sedimentary basins connecticut valley gaspe and merrimack troughs due to trench rollback and or delamination during the late salinic event and collision at the outer margin of composite laurentia during the acadian orogeny can account for the protracted structural and metamorphic history of the quebec appalachians

exhumation of the sanbagawa blueschist belt sw japan by lateral flow and extrusion evidence from structural kinematics and retrograde p t t paths

the western alps from the jurassic to oligocene spatio temporal constraints and evolutionary reconstructions abstract despite extensive research in the last 150 years the regional tectonic reconstruction of the western alps has remained controversial the curved orogenic belt consists of several ribbon like continental terranes sesia austroalpine internal crystalline massifs brianconnais which are separated by two or more ophiolitic sutures piemonte valais antrona lanzo canavese high pressure hp metamorphism of each terrane occurred during distinct orogenic episodes at 65 ma in the sesia austroalpine at 45 ma in the piemonte zone and at 35 ma in the internal crystalline massifs it is suggested that these events reflect individual accretionary episodes which together with kinematic indicators and the speed and direction of plate motions provide constraints for the discussed reconstruction model the model involves a prolonged orogenic history that took place during relative convergence of europe and adria here considered as a promontory of the african plate the first accretionary event involved the sesia austroalpine terrane final closure of the piemonte ocean occurred during the eocene 45 ma and involved ultra high pressure uhp metamorphism of the piemonte oceanic crust incorporation of the brianconnais terrane in the accretionary wedge occurred thereafter possibly during or after subduction of the valais ocean in the late eocene 45 35 ma this subduction was terminated at ca 35 ma when the internal crystalline massifs i e the assumed internal parts of the brianconnais terrane were buried into great depths and underwent hp and uhp metamorphism

from rifting to passive margin the examples of the red sea central atlantic and alpine tethys abstract evolution of the red sea gulf of suez and the central atlantic rift systems shows that an initial transtensive rifting phase affecting a broad area around the future zone of crustal separation was followed by a pre oceanic rifting phase during which extensional strain was concentrated on the axial rift zone this caused lateral graben systems to become inactive and they evolved into rift rim basins the transtensive phase of diffuse crustal extension is recognized in many intra continental rifts if controlling stress systems relax these rifts abort and develop into palaeorifts if controlling stress systems persist transtensive rift systems can enter the pre oceanic rifting stage during which the rift zone narrows and becomes asymmetric as a consequence of simple shear deformation at shallow crustal levels and pure shear deformation at lower crustal and mantle lithospheric levels preceding crustal separation extensional denudation of the lithospheric mantle is possible progressive lithospheric attenuation entails updoming of the asthenosphere and thermal doming of the rift shoulders their uplift provides a major clastic source for the rift basins and the lateral rift rim basins their stratigraphic record provides a sensitive tool for dating the rift shoulder uplift asymmetric rifting leads to the formation of asymmetric continental margins corresponding in a simple shear model to an upper plate and a conjugate lower plate margin as seen in the central atlantic passive margins of the united states and morocco this rifting model can be successfully applied to the analysis of the alpine tethys palaeo margins such as rif and the western alps

kinematic vorticity and strain rate patterns associated with ductile extrusion in the chelmos shear zone external hellenides greece abstract kinematic vorticity strain rate finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal scale shear zone the chelmos shear zone csz of the external hellenides the csz has a curved shape which is marked by a frontal flat and a steep zone the spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone the pure shear component of deformation decreases from back to front and from top to bottom the ductile thinning normal to the shear zone s boundaries is higher in the steep zone and decreases westwards quartz grain size and strain rate in the csz is distributed into two approximately homogeneous domains the higher strain rate is recorded in the steep zone and the base of the frontal flat zone with lower rates in the upper parts of the frontal flat zone based on mesoscopic structural data and the spatial distribution of the shear strain rate the existence of a west dipping back flow zone in the boarder zone between these two domains is inferred the overall pattern of deformation in the csz implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone ductile deformation of rocks probably started close to simple shear on a planar low angle shear zone progressively became penetrative and more general shear occurred as the csz steepened up this progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal flat zone solid state extrusion in the csz was also associated with a heterogeneous and progressive increase in strain rate

tectonic overpressure in weak crustal scale shear zones and implications for the exhumation of high pressure rocks 1 a two dimensional numerical simulation of lithospheric shortening shows the formation of a stable crustal scale shear zone due to viscous heating the shear zone thickness is controlled by thermomechanical coupling that is resolved numerically inside the shear zone away from the shear zone lithospheric deformation is dominated by pure shear and tectonic overpressure i e pressure larger than the lithostatic pressure is proportional to the deviatoric stress inside the shear zone deformation is dominated by simple shear and the deviatoric stress decreases due to thermal weakening of the viscosity to maintain a constant horizontal total stress across the weak shear zone i e horizontal force balance the pressure in the shear zone increases to compensate the decrease of the deviatoric stress tectonic overpressure in the weak shear zone can be significantly larger than the deviatoric stress at the same location implications for the geodynamic history of tectonic nappes including high pressure ultrahigh pressure rocks are discussed

interactions between tectonics erosion and sedimentation during the recent evolution of the alpine orogen analogue modeling insights 1 on the basis of a section across the northwestern alpine wedge and foreland basin analogue modeling is used to investigate the impact of surface processes on the orogenic evolution the basis model takes into account both structural and lithological heritages of the wedge during shortening erosion and sedimentation are performed to maintain a critical wedge frontal accretion leads to the development of a foreland thrust belt underplating leads to the formation of an antiformal nappe stack in the internal zones important volumes of analogue materials are eroded out of the geological record which in the case of the alps suggests that the original lengths and volumes may be underestimated the foreland basin evolves differently depending on the amounts of erosion sedimentation its evolution and internal structuring is governed by the wedge mechanics thought to be the main controlling mechanism in the development of the molasse basin in a feedback interaction with surface processes

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

retroward extrusion of high pressure rocks an example from the hellenides pelion blueschist nappe nw aegean current tectonic models interpret the hellenides as a unidirectional sw vergent orogenic belt new micro structural and amphibole chemistry analyses show instead that the exhumation of the pelion blueschist nappe pbn of the internal hellenides was achieved by retroward ene directed ductile extrusion which opposes the principal proward orogenic vergence retroward extrusion occurred between two sub parallel major ductile shear zones basal thrust and upper detachment with opposite shear senses which operated simultaneously under blueschist to greenschist facies conditions during the late eocene oligocene because the pbn is tectonically sandwiched between pelagonian basement rocks we suggest that the pbn protolith was accumulated during the late cretaceous in an incipient backarc basin developed above the ne dipping subduction of the pindos ocean beneath the pelagonian microcontinent subsequent apulian pelagonian collision triggered basin inversion and the proward dipping intracontinental subduction that caused the early middle eocene blueschist facies metamorphism followed by the retroward extrusion of the pbn this article is protected by copyright all rights reserved

experimental crystal growth in glass inclusions the possibilities and limits of the method glass inclusions trapped in bytownite phenocrysts from the ardoukoba eruption djibouti republic 7 november 1978 are used as an experimental environment to follow the evolution of crystallization and residual liquids in a magma with tholeiitic affinities the results are compared with those obtained from abyssal tholeiites by walker et al 1979 in near equilibrium crystallization conditions simultaneous crystallization of olivine and pyroxene is obtained in the enclosed silicate liquid as well as plagioclase as overgrowths on the cavity walls oxides only appear at lower temperatures the olivine is homogeneous and rather rare 5 in weight it forms at a temperature between 1 186 c 3 fo85 and 1 126 c fo66 the distribution coefficient of mg between the liquid and the olivine varies regularly with the temperature until titanomagnetite appears the augite is much more abundant 20 in weight of the original trapped melt and is characterized by a composition that varies from the centre to the edges of the crystals wo 43 5 36 2 en 43 8 41 7 fs 12 7 22 1 composition shows an unsatisfactory correlation with temperature and at most an fs enrichment may be noted with temperature decrease the composition of the plagioclase deposited on the cavity walls varies linearly with temperature from an 73 3 at 1 186 c to an 56 5 at 1 135 c in the case of contemporaneous crystallization of pl cpx ol and ox if experiments are carried out by undercooling as much as 150 c in relation to crystallization of the ferromagnesian minerals the composition of the plagioclase changes from an 78 8 at 1 160 c to an 52 8 at 1 025 c in the case of the pl ol cpx and ox crystallization the included liquids evolve from tholeiitic basalts to ferrobasalts in the same way as the lavas studied in the asal rift demange et al 1980 the advantages and limitations of the method are discussed

a simple thermo mechanical shear model applied to the morcles fold nappe western alps abstract we present a one dimensional shear zone model with a power law flow law and a temperature dependent viscosity the analytical solution for the velocity profile across the shear zone depends only on the single dimensionless parameter depends on the activation energy of the applied flow law the temperature at the base of the shear zone and the temperature difference across it the solution can describe three types of shear deformation 1 homogeneous simple shear for 1 and 3 heterogeneous simple shear potentially generating fold nappes for 5 10 we perform a systematic parameter search and determine the parameters that provide the best fit between a sheared model line and a reference line representing the first order geometry of the frontal part of the morcles nappe we also apply a more elaborated pressure driven model to test whether published flow laws for calcite and realistic pressure gradients provide realistic velocities and strain rates for the morcles nappe the value of applicable to the morcles nappe is estimated with three independent methods 1 applying reported values for temperatures and activation energy to the analytical formula for 2 fitting the first order geometry of the morcles fold nappe and 3 comparing results of a velocity driven i e kinematic boundary condition and pressure driven i e pressure source term and free surface shear zone model all three methods yield consistent estimates for between 3 and 15 our results show that previously published kinematic models which explain the tectonic evolution of the morcles fold nappe within a crustal shear zone are physically feasible the results further suggest that viscous heterogeneous simple shear played a dominant role during the formation of the morcles fold nappe

tectonics of the middle triassic intracontinental xuefengshan belt south china new insights from structural and chronological constraints on the basal decollement zone in orogenic belts a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover in the early mesozoic intracontinental xuefengshan belt of south china such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late orogenic granitic plutons our detailed multi scale structural analysis documents a synmetamorphic ductile deformation in the basal decollement the neoproterozoic pelite and sandstone and the intruding early paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss respectively the metamorphic foliation contains a nw se stretching lineation associated with top to the nw kinematic indicators the ductile shearing of these high strained rocks can be correlated with nw verging folds and thrusts recognized in the neoproterozoic to early triassic sedimentary cover monazite u th pbtot chemical dating and zircon sims u pb dating provide age constraints of the ductile shearing between 243 and 226 ma and late orogenic granite emplacement around 235 215 ma in agreement with recent geochronological data these new results show that the xuefengshan belt is an early mesozoic orogen dominated by the nw directed shearing and thrusting at the southeastern boundary of the xuefengshan belt the chenzhou linwu fault separates the early mesozoic domain to the nw from the early paleozoic domain to the se the tectonic architecture of this belt was possibly originated from the continental underthrusting to the se of the south china block in response to northwest directed subduction of the paleo pacific plate

structural geometry lower crustal magmatic underplating and lithospheric stretching in the ivrea verbano zone northern italy abstract the ivrea verbano zone is believed to represent an up ended cross section through the lower continental crust as it existed at the end of the hercynian orogeny in the southern alps structurally oriented geological mapping has been carried out in the central and northern parts of the ivrea verbano zone a region some 30 km along strike the geology of the southern half of the zone is dominated by a large basic ultrabasic complex the mafic formation which is in contact with a strongly banded metasedimentary metabasic sequence to the north particular attention was paid to a the structural relationship between the rocks of the mafic formation and their envelope of high grade metasedimentary and metabasic rocks b the geometrical configuration throughout the ivrea verbano zone of high temperature shear zones which accommodated post hercynian crustal extension and c the geometry of late low temperature faulting the effects of which have been removed in order to produce a restoration of the structure as it existed during the post hercynian extensional phase the intrusion of large volumes of basic magma ca 50 of the outcrop area to form the rocks of the mafic formation appears to be coeval with the onset of extension ca 280 ma the main basic body has a laccolithic form which was originally more than 10 km thick overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot immediately subsolidus or partially molten body incorporating its envelope of hot metasediments into a large originally recumbent fold a geometrical association with a high temperature low angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion the ivrea verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating which may aid in the interpretation of present day deep seismic profiles it also demonstrates the geometry of a network of conjugate high temperature low angle shear zones in a well layered lower crustal section

kinematic analysis of rock flow and deformation temperature of the sirjan thrust sheet zagros orogen iran microstructural finite strain and vorticity analyses of quartz rich mylonites were used in order to investigate kinematics of rock flow and deformation temperature in the sirjan thrust sheet exposed in a structural window within the sanandaj sirjan high pressure low temperature hp lt metamorphic belt that forms part of the hinterland of the zagros orogenic belt of iran a dominant top to the sw sense of shear in the study area is indicated by several shear sense indicators such as asymmetric boudins rotated porphyroclasts mica fish and s c fabrics quantitative analyses reveal approximately plane strain deformation conditions with r xz values ranging from 2 5 to 4 3 and increasing towards the sirjan thrust opening angles of quartz c axis fabrics and recrystallization regimes suggest deformation temperatures vary from 430 to 625 50 c in the hanging wall rocks oblique grain shape and quartz c axis fabrics were used to estimate the degree of non coaxiality during deformation the obtained vorticity profile indicates a down section increase in kinematic vorticity number w m from 0 6 to 0 89 this range of vorticity numbers confirms contributions of both simple 41 68 and pure shear 32 59 deformation components the structural characteristics of the study area ultimately were controlled by oblique motion of the afro arabian plate relative to the iranian plate

fluid migration above a subducted slab constraints on amount pathways and major element mobility from partially overprinted eclogite facies rocks sesia zone western alps

lithospheric scale structures from the perspective of analogue continental collision abstract analogue models were employed to investigate continental collision addressing the roles of 1 a suture zone separating different crustal blocks 2 mid crustal weak layers and 3 mantle strengths these models confirmed that low amplitude lithospheric and crustal buckling is the primary response to shortening with a wavelength mainly controlled by the strong upper crust and or lithospheric mantle with increasing shortening bivergent thrusts formed across the brittle layers equivalent to upper crust at inflection points of the buckles while ductile layers lower crust and mantle continued being folded the resulting geometries displayed pop ups and pop downs above the boundaries sutures between blocks further shortening led to wider thrust bounded deformation zones belts in front of the stronger blocks identified as effective indenters during shortening upward extrusion into belts allowed exhumation of deep material weak layers enhanced strain localisation these modelling results have similarity in large scale structures of modern orogens and are relevant to the understanding of natural collisional processes

mechanical model for the tectonics of doubly vergent compressional orogens a mechanical model of crustal shortening and deformation driven by the relative convergence of rigid underlying mantle plates explains many features of convergent orogens results based on numerical models and supported by sandbox models show that a coulomb crustal layer subject to basal velocity boundary conditions corresponding to asymmetric detachment and subduction of the underlying mantle passes through three stages of orogenic growth 1 block uplift bounded by step up shear zones 2 development of a low taper wedge over the underthrusting mantle plate and 3 development of a low taperwedge overlying the overthrusting mantle plate and verging in the opposite direction when modified by isostasy basal viscous flow surface erosion and denudation and sedimentation the resultant model orogens exhibit a variety of styles with characteristics in common with small rapidly denuded orogens large orogens with plateaus and extensional characteristics and active subduction margins with doubly vergent accretionary wedges and deformed fore arc basins

stable isotope geochemistry and formation mechanisms of quartz veins extreme paleoaltitudes of the central alps in the neogene quartzveins ranging in size from less than 50 cm length and 5 cm width to greater than 10 m in length and 5 m in width are found throughout the central swiss alps in some cases the veins are completely filled with milky quartz while in others sometimes spectacular void filling quartz crystals are found the style of vein filling and size is controlled by host rock composition and deformation history temperatures of vein formation estimated using stable isotope thermometry and mineral equilibria cover a range of 450 c down to 150 c vein formation started at 18 to 20 ma and continued for over 10 my the oxygen isotope values of quartz veins range from 10 to 20 permil and in almost all cases are equal to those of the hosting lithology the strongly rock buffered veins imply a low fluid rock ratio and minimal fluid flow in order to explain massive nearly monomineralic quartz formation without exceptionally large fluid fluxes a mechanism of differential pressure and silica diffusion combined with pressure solution is proposed for early vein formation fluid inclusions and hydrous minerals in late formed veins have extremely low d values consistent with meteoric water infiltration the change from rock buffered static fluid to infiltration from above can be explained in terms of changes in the large scale deformation style occurring between 20 and 15 ma the rapid cooling of the central alps identified in previous studies may be explained in part by infiltration of cold meteoric waters along fracture systems down to depths of 10 km or more an average water flux of 0 15 cm3 cm 2yr 1 entering the rock and reemerging heated by 40 c is sufficient to cool rock at 10 km depth by 100 c in 5 million years the very negative d values of

slab break offs in the alpine subduction zone after the onset of plate collision in the alps at 32 34 ma the deep structure of the orogen is inferred to have changed dramatically european plate break offs in various places of the alpine arc as well as a possible reversal of subduction polarity in the eastern alps have been proposed we review different high resolution tomographic studies of the upper mantle and combine shear and body wave models to assess the most reliable geometries of the slabs several hypotheses for the tectonic evolution are presented and tested against the tomographic model interpretations and constraints from geologic and geodetic observations we favor the interpretation of a recent european slab break off under the western alps in the eastern alps we review three published scenarios for the subduction structure and propose a fourth one to reconcile the results from tomography and geology we suggest that the fast slab anomalies are mainly due to european subduction adriatic subduction plays no or only a minor role along the tauern window sections possibly increasing towards the dinarides the apparent northward dip of the slab under the eastern alps may be caused by imaging a combination of adriatic slab from the dinaric subduction system and a deeper lying european one as well as by an overturned retreating european slab

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

burial rates during prograde metamorphism of an ultra high pressure terrane an example from lago di cignana western alps italy abstract estimation of burial rates and duration of prograde metamorphism of ultra high pressure uhp rocks t 590 630 c p 2 7 2 9 gpa of the zermatt saas ophiolite from lago di cignana italy may be made through combined lu hf and sm nd garnet geochronology in conjunction with petrologic estimates of the prograde p t path we report a lu hf garnet omphacite whole rock isochron age of 48 8 2 1 ma from the uhp locality at lago di cignana which stands in contrast to the sm nd age of 40 6 2 6 ma amato et al earth planet sci lett 171 1999 425 438 obtained from the same sample and mineral material the sm nd and lu hf ages as well as other ages determined on metamorphic garnet zircon and white mica amato et al earth planet sci lett 171 1999 425 438 mayer et al eur union geosci 10 1999 abstr 809 rubatto et al contrib mineral petrol 132 1998 269 287 dal piaz et al int j earth sci 90 2001 668 684 from lago di cignana and elsewhere in the zermatt saas ophiolite lie within a range of 50 38 ma which we interpret to encompass the duration of prograde metamorphism and possibly the duration of garnet growth the difference in measured sm nd and lu hf ages from cignana can be accounted for by expected core to rim variations in lu hf sm and nd the measured yttrium content in garnet which may be a proxy for lu is highest in garnet core and lowest in the mineral rim generally following a profile that is predicted by rayleigh fractionation preferential enrichment of lu in the core produces a lu hf age that is weighted toward the older garnet core sm nd ages as predicted by rayleigh fractionation of sm and nd during garnet growth however reflect later grown garnet as compared to lu hf ages the difference in sm nd and lu hf ages from a single sample should therefore be a minimum estimate for the duration of garnet growth and prograde metamorphism so long as sm nd and lu hf blocking temperatures were not exceeded for a long period of time based on the 12 myr duration of prograde garnet growth estimated in this study burial rates for rocks at lago di cignana were on the order of 0 23 0 47 cm yr these values correlate with continuous shortening rates of 0 4 1 4 cm yr between the european plate and the african adriatic promontory between 50 and 38 ma which is on the order of that calculated for plate velocities from plate reconstructions suggesting that the zermatt saas ophiolite may have remained well coupled to the down going slab to uhp conditions

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

evolutionary model for alpine corsica mechanism for ophiolite emplacement and exhumation of high pressure rocks a tectonic model of alpine corsica is proposed based on geological studies its evolution starts after the jurassic with intraoceanic subduction followed by mid cretaceous subduction of the european continental margin under the oceanic segment of the adriatic plate after subduction of the continental crust to a depth of 150 km slices of crustal material are buoyantly uplifted together with high pressure oceanic rocks ophiolites and schistes lustres high pressure low temperature continental gneiss units overthrust the outer segments of the european crust while producing a normal sense motion along the upper surface of the rising crustal body during the eocene the closure of the remnant ligurian oceanic basin separating the proto corsican belt and adria resulted in a second orogenic phase with the emplacement of unmetamorphosed ophiolitic nappes which overthrust the previously exhumed and eroded hp belt this corsican model suggests an original evolutionary path for orogenic belts when continental collision is preceded by intraoceanic subduction

linking the northern alps with their foreland the latest exhumation history resolved by low temperature thermochronology the evolution of the central alpine deformation front subalpine molasse and its undeformed foreland is recently debated because of their role for deciphering the late orogenic evolution of the alps its latest exhumation history is poorly understood due to the lack of late miocene to pliocene sediments we constrain the late miocene to pliocene history of this transitional zone with apatite fission track and u th he data we used laser ablation inductively coupled mass spectrometry for apatite fission track dating and compare this method with previously published and unpublished external detector method fission track data two investigated sections across tectonic slices show that the subalpine molasse was tectonically active after the onset of folding of the jura mountains this is much younger than hitherto assumed thrusting occurred at 10 8 6 5 ma and potentially thereafter this is contemporaneous with reported exhumation of the external crystalline massifs in the central alps the jura mountains and the subalpine molasse used the same detachments as the external crystalline massifs and are therefore kinematically coupled estimates on the amount of shortening and thrust displacement corroborate this idea we argue that the tectonic signal is related to active shortening during the late stage of orogenesis

styles of crustal deformation in compressional orogens caused by subduction of the underlying lithosphere abstract crustal scale deformation is calculated for models in which the driving mechanism corresponds to the asymmetric detachment and underthrusting of the underlying mantle lithosphere the plane strain finite element model results provide indications of the styles of deformation to be expected in small compressional orogens in particular these styles occur where shortening of the mantle lithosphere is achieved by the nearly rigid convergence between lithospheric mantles and the subduction of one mantle beneath the other the crust is modelled using coulomb plastic frictional and thermally activated power law viscous rheologies and the effects of compositional layering and variable geothermal gradients are included results are presented for a range of models in which the strength of the coupling between the model crust and its basal boundary surface denudation partial and total subduction of the crust and compositional layering are examined the results show the development of inclined step up shear zones which are a consequence of conjugate thrusting in regions of coulomb controlled rheology these zones link to sub horizontal shear zones which occur where composition and temperature render the crust viscously weak the model strain fields are interpreted in terms of deformation on discrete planes and the seismic reflectivity fabric that may be associated with this discrete deformation finally we ask whether similar seismic reflectivity fabric can be recognized in observations from small compressional orogens

passive margin uplift around the north atlantic region and its role in northern hemisphere late cenozoic glaciation tectonic climatic models of late cenozoic global cooling emphasize the importance of middle latitude uplifts e g tibetan plateau and the american west but ignore widespread tectonic events on the margins of the north atlantic ocean pleistocene glaciations after 2 5 ma are characterized by circum north atlantic continental ice sheets that formed by the coalescence of perennial snow fields on extensive plateau surfaces in eastern canada northwest britain and scandinavia plateaus record cenozoic uplift of peneplains in response to semisynchronous magmatic underplating and thermal buoyancy of rifted continental margins high standing plateaus are very sensitive to small reductions in summer temperature as late cenozoic climate cooling proceeded driven by uplift in regions external to the north atlantic region elevated plateaus became sites for extensive snow fields and ultimately ice sheets circum atlantic uplift took place in the key latitudinal belt that is most sensitive to orbitally forced changes in solar irradiation this together with albedo effects from large snow fields could have amplified the relatively weak milankovitch signal

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

the influence of backstop dip and convergence velocity in the growth of viscous doubly vergent orogenic wedges insights from thermomechanical laboratory experiments abstract we investigate the effects of convergence velocity and backstop geometry on the evolution of experimental viscous doubly vergent orogenic wedges experiments are performed in a thermomechanical shortening box using the newtonian properties of 52 54 en type paraffin as analogue to the natural strength distribution in the continental crust a velocity discontinuity is imposed at the subduction slot where the base plate is pulled below a rigid backstop the mobile base plate dips 5 two different backstop dips 60 and 30 are used in combination with constant convergence velocities of 2 and 10 cm h 1 results point out that backstop geometry exerts a first order control on the evolution of doubly vergent experimental wedges steeply dipping backstops cause the stationary uplift of the wedge axial region above the subduction slot whereas shallow dipping backstops allow its retroward translation in concomitance with a more pronounced deformation in the retrowedge convergence velocity controls the uplift in the wedge axial region and the amount of the outward migration of the deformation front in both the prowedge and retrowedge regions for fast convergence velocity and shallow dipping backstop deformation in the retrowedge is favoured an increase in the backstop dip and or a decrease in the convergence velocity favour deformation in the prowedge both prowedge and retrowedge tend to grow in self similar conditions with an aspect ratio depending upon the backstop dip and convergence velocity these results may have important consequences for the evolution and deformation style of ductile dominated orogenic wedges suggesting that location of the axial region and deformation progression in natural orogens can be strongly controlled by boundary and kinematic conditions

precollisional multistage exhumation of subducted continental crust the sesia zone western alps 1 the sesia zone within the tertiary arc of the western alps is a relic of the subducted part of the adriatic continental margin along the se border of the tethyan ocean the sesia zone comprises three basement nappes which individuated during late cretaceous 65 80 ma subduction to different depths at high pressure hp blueschist eclogite facies conditions peak pressures of 1 0 1 2 1 0 1 5 and 1 5 2 0 gpa the thrusts bounding these nappes developed where the crust was previously thinned during jurassic rifting crustal scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing hp rocks initial exhumation of the internal part of the accreted margin involved thrusting d1 and transpressional shearing d2 along a subvertical e w trending mylonitic shear zone under retrograde blueschist to greenschist facies conditions this exhumation was nearly isothermal to a depth of about 25 km where the basement nappes were juxtaposed subsequent exhumation of these nappes to a common depth of about 15 20 km occurred in the footwall of a greenschist facies top se extensional shear zone d3 preserved in some of the highest mountain peaks of the sesia zone new rb sr mineral ages constrain d2 to have occurred at about 60 65 ma and d3 at about 45 55 ma thus top se extensional exhumation was broadly coeval with eocene se directed subduction of the liguro piemont oceanic lithosphere beneath the adriatic margin slow cooling and erosional denudation of the sesia zone from 45 to 30 ma occurred in the hanging wall of the gressoney extensional shear zone d4 which itself contributed to the exhumation of eocene hp and ultra hp oceanic rocks in its footwall by 30 ma hp rocks of the sesia zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments oligo miocene insubric backfolding and thrusting d5 only exhumed northeastern parts of the sesia zone where hp metamorphism is absent or was overprinted by tertiary temperature dominated metamorphism most exhumation of continental hp rocks in the sesia zone therefore preceded tertiary alpine collision and coincided with late cretaceous to early tertiary subduction of the adriatic and tethyan lithosphere the transition from d2 to d3 in the sesia zone is interpreted to mark a change from high stress oblique se directed subduction and accretion of the distal adriatic continental margin to nw retreating low stress subduction of the liguro piemont oceanic lithosphere

transition from basement to cover a geometric model abstract a new simple geometric model is proposed that explains the possible relationships between superficial thrust sheets and deeper fold nappes it demonstrates that the simultaneous formation of these two types of structures at different levels of the crust is kinematically and geometrically possible the model is based on the difference of rheological behaviour between a lower tectonic unit that will suffer mainly a ductile deformation along a shallow dipping shear zone and an upper and rigid tectonic unit that will deform mostly by bedding parallel slip at constant bed length the difference in rheologic behaviour implies that the horizontal shortening that can be produced within the two units is unequal and therefore leads to a relative movement between them this movement takes place within a detachment level of variable thickness the most efficient layers to enable detachment will consist of ductile sediments like evaporites shales or flysch type sediments recent geophysical data have confirmed the view that compressional mountain belts are mainly the result of continental lithosphere subduction the deformation is essentially localized in the upper part of the down going continental crust this fundamental basally driven mechanism constrains the geometry and kinematics of the proposed model examples from the alps show that it can be applied to many cases where cover thrust sheets are directly related to basement fold nappes

thermal modeling of extensional tectonics application to pressure temperature time histories of metamorphic rocks one and two dimensional finite difference models are used to generate theoretical pressure temperature time ptt paths for rocks uplifted from deep and intermediate crustal levels during extension via ductile stretching of the entire lithosphere pure shear and via movement of crustal blocks along rooted large scale low angle normal faults simple shear the temperature time paths of rocks uplifted by these pure shear and simple shear mechanisms have similar morphologies although rocks from pure shear settings reach their final depth at higher temperatures than do rocks uplifted by simple shear and experience a greater amount of posttectonic cooling the depth temperature paths of rocks uplifted from deep crustal levels 30 km depth via pure shear extension are often characterized by a period of nearly isothermal high dp dt uplift during the early stages of extension a modified pure shear extension model that includes enhanced heating in the lower lithosphere during extension generally produces ptt paths characterized by a more protracted period of nearly isothermal uplift in contrast simple shear uplift along low angle normal faults produces almost linear depth temperature paths with moderate dp dt and isothermal uplift is not observed for a given initial geotherm the single factor controlling the ptt paths of rocks unroofed below gently dipping normal faults that dip less than 30 is the rate at which unroofing takes place unroofing rate is defined as the rate of uplift relative to the surface and the syntectonic ptt paths are insensitive to fault dip or horizontal displacement rate current geothermometric and geobarometric techniques yield typical uncertainties in real pressure temperature and temperature time data of 1 kbar and 50 c thus it will be difficult to distinguish between these various extensional mechanisms on the basis of ptt data derived from a single sample or from a single structural level although use of data from samples collected from a broad range of structural levels may improve resolution ultimately the best prospect for distinguishing between simple shear and pure shear mechanisms through petrologic work lies in improving the precision of metamorphic pressure and temperature measurements to 0 5 kbar and 25 c

kinematics of ductile shearing from outcrop to crustal scale in the monte rosa nappe western alps significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the mesozoic cover quartzites and marbles of the monte rosa nappe in the inner part of the western alps we summarize here the results of a detailed microstructural study at several scales of observation in the northern part of the nappe and discuss the kinematic significance and compatibility of such results most of the observed deformations can be accounted for by progressive wnw vergent shearing during and after the late eocene within a ductile shear zone of crustal scale the displacement of the hanging wall may have been in excess of several tens of kilometers however some ese vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the monte rosa nappe

pre collisional high pressure metamorphism and nappe tectonics at active continental margins a numerical simulation the evolution of a subduction channel and orogenic wedge is simulated in 2d for an active continental margin with p t paths being displayed for selected markers in our simulation subduction erosion affects the active margin and a structural pattern develops within a few tens of millions of years with four zones from the trench into the forearc i an accretionary complex of low grade metamorphic sedimentary material ii a wedge of nappes with alternating upper and lower crustal provenance and minor interleaving of oceanic or hydrated mantle material iii a megascale melange composed of high pressure hp and ultra high pressure uhp metamorphic rocks extruded from the subduction channel and iv the upward tilted frontal part of the remaining lid the p t paths and time scales correspond to those typically recorded in orogenic belts the simulation shows that hp uhp metamorphism of continental crust does not necessarily indicate collision but that the material can be derived from the active margin by subduction erosion and extruded from the subduction channel beneath the forearc during ongoing subduction

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

mesoproterozoic plate tectonics a collisional model for the grenville aged orogenic belt in the llano uplift central texas the llano uplift of central texas united states exposes the core of a mesoproterozoic orogenic belt that formed along the southern margin of laurentia during grenville time a new collisional model is proposed that reconciles differences in structural stacking apparent tectonic transport and deformation conditions between the eastern and western portions of the uplift and explains uplift and exhumation of high pressure eclogitic rocks emplacement of ophiolitic rocks and subsequent late stage to postcollisional plutonism our model proposes that subduction with southward polarity resulted in collision of an exotic arc with laurentia emplacement of ophiolitic rocks and telescoping of the intervening basinal sediments followed by overriding of the arc and margin of laurentia by a southern continent with transport toward laurentia the model further proposes that convergence led to subduction of the laurentian margin resulting in high pressure metamorphism but buoyancy forces due to subduction of continental crust under the southern continent resulted in uplift and retrotransport away from laurentia in a manner similar to that proposed for the alpine orogeny slab breakoff resulted in upwelling of the asthenosphere leading to intrusion of juvenile granitic plutons subduction along strike caused continued contraction that waned with time the eastern uplift records continent arc continent collision whereas the western uplift records continent continent collision the two regions also expose different crustal levels in the orogen the striking similarity with phanerozoic orogens including emplacement of ophiolites and formation of high pressure rocks implies that plate tectonic processes including subduction were active prior to the neoproterozoic

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga

post rift compressional reactivation potential of passive margins and extensional basins poly phase deformation of a compressional nature is a common feature in the post rift evolution of passive margins and rifts the compressional mode of deformation in these sedimentary basins originally formed by extension in an intraplate setting is characterized by a spectrum of spatial wavelengths spanning several tens of kilometres up to several hundreds of kilometres the actual mode of compressional deformation appears to be strongly affected by the rheological structure of the underlying lithosphere the level of the regional intraplate stress field and the geometry of the rifted basin configuration prior to late stage compressional reactivation the interplay of plumes and intraplate compressional deformation can lead to temporal transitions from basin inversion to lithospheric folding these modes of deformation lead to substantial differential vertical motions late stage anomalies in subsidence and uplift patterns the development of innovative combinations of numerical and analogue modelling techniques is the key to differentiating different modes of compressional deformation of passive margins and extensional basins the geological society of london 2008