sedimentary tectonics and subsidence modelling of the type upper cretaceous gosau basin northern calcareous alps austria the upper cretaceous gosau group of the austrian northern calcareous alps records sedimentation and subsidence after early cretaceous thrusting the gosau basin in the gosau russbach area is approximately 8 km wide and 10 km long it is filled by more than 1000 m alluvial shallow marine deposits of the lower gosau subgroup upper turonian lower campanian facies and thickness changes within short horizontal distances suggest synsedimentary basin margin faulting extensional structures within the triassic basement of the basin include fibrous gashes which indicate w nw e se directed extension of 6 18 w nw directed normal faults at the southeastern basin margin a prominent se directed normal fault with a minimum fault throw of approximately 500 m at the nw basin margin and dextral strike slip at the southwestern margin suggest a pull apart origin of the basin between nw se trending right overstepping dextral strike slip faults backstripped tectonic subsidence curves yield values of total subsidence of 700 m in approximately 6 ma forward computer modelling based on extensional models results in stretching factors between 1 09 1 15 local extension along strike slip faults is suggested as the main mechanism for basin formation

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

subduction kinematics and dynamic constraints the kinematics of subduction zones shows a variety of settings that can provide clues for dynamic understandings two reference frames are used here to describe the simple 2d kinematics of subduction zones in the first the upper plate is assumed fixed whereas in the second frame upper and lower plates move relative to the mantle relative to a fixed point in the upper plate u the transient subduction hinge h can converge diverge or be stationary similarly the lower plate l can converge diverge or be stationary the subduction rate vs is given by the velocity of the hinge h minus the velocity of the lower plate l vs vh vl the subduction rate 1 increases when h diverges and 2 decreases when h converges combining the different movements at least 14 kinematic settings can be distinguished along the subduction zones variable settings can coexist even along a single subduction zone as shown for the 5 different cases occurring along the apennines subduction zone apart from few exceptions the subduction hinge converges toward the upper plate more frequently along e or ne directed subduction zone whereas it mainly diverges from the upper plate along w directed subduction zones accompanying backarc extension before collision orogen growth occurs mostly at the expenses of the upper plate shortening along e ne directed subduction zones whereas the accretionary prism of w directed subduction zones increases at the expenses of the shallow layers of the lower plate the convergence shortening ratio is 1 along e or ne directed subduction zones whereas it is 1 along accretionary prisms of w directed subduction zones backarc spreading forms in two settings along the w directed subduction zones it is determined by the hinge divergence relative to the upper plate minus the volume of the accretionary prism or in case of scarce or no accretion minus the volume of the asthenospheric intrusion at the subduction hinge since the volume of the accretionary prism is proportional to the depth of the decollement plane the backarc rifting is inversely proportional to the depth of the decollement on the other hand along e or ne directed subduction zones few backarc basins form e g aegean andaman and can be explained by the velocity gradient within the hangingwall lithosphere separated into two plates when referring to the mantle the kinematics of subduction zones can be computed either in the deep or in the shallow hotspot reference frames the subduction hinge is mostly stationary being the slab anchored to the mantle along w directed subduction zones whereas it moves w or sw ward along e or ne directed subduction zones surprisingly along e or ne directed subduction zones the slab moves out of the mantle i e the slab slips relative to the mantle opposite to the subduction direction kinematically this subduction occurs because the upper plate overrides the lower plate pushing it down into the mantle as an example the hellenic slab moves out relative to the mantle i e sw ward opposite to its subduction direction both in the deep and shallow hotspot reference frames in the shallow hotspot reference frame upper and lower plates move westward relative to the mantle along all subduction zones this kinematic observation casts serious doubts on the slab negative buoyancy as the primary driving mechanism of subduction and plate motions w directed subduction zones rather provide about 2 3 times larger volumes of lithosphere re entering into the mantle and the slab is pushed down this opposite behavior is consistent with the down dip extension seismicity along e ne directed subduction zones and the frequent down dip compression along the w directed subduction zones subduction kinematics shows that plate velocity is not dictated by the rate of subduction along the w directed subduction zones the rate of subduction is rather controlled i by the hinge migration due to the slab interaction with the easterly trending horizontal mantle wind along the global tectonic mainstream ii by the far field plate velocities and iii by the value of negative buoyancy of the slab relative to the country mantle alternatively e ne nne directed subduction zones have rates of sinking chiefly determined i by the far field velocity of plates and ii by the value of negative buoyancy of the slab relative to the country mantle along this type of subduction the subduction hinge generally advances e ne ward toward the upper plate decreasing the subduction rate but it moves w sw ward relative to the mantle all this indicates that subduction zones have different origin as a function of their geographic polarity and the subduction process is more a passive feature rather than being the driving mechanism of plate motions a rotational component combined with mantle density and viscosity anisotropies seems more plausible for generating the global tuning in the asymmetry of subduction zones

the schneeberg normal fault zone normal faulting associated with cretaceous se directed extrusion in the eastern alps italy austria abstract the cretaceous eo alpine collisional event in the european eastern alps is generally accepted to induce w nw directed thrusting both in basement and in sedimentary cover units this study presents the first evidence of eo alpine w nw directed normal kinematics along the schneeberg normal fault zone which separates eo alpine high pressure rocks in a footwall position from pre alpine basement rocks in a hanging wall position new garnet sm nd data indicate that exhumation of the high pressure rocks along the normal fault zone started around 95 ma ago and continued up to low greenschist brittle conditions at 76 ma as indicated by a rb sr age from a low temperature mylonite the occurrence of pre alpine basement rocks both in the hanging wall and the footwall of eo alpine high pressure rocks suggests exhumation by extrusion processes despite the displacement or removal of parts of the lower portion of the high pressure unit by tertiary strike slip faults eo alpine top to ese thrusting as expected for the structurally lower part of an extruding wedge was found at and below the base of the eo alpine high pressure rocks a rb sr age of 77 ma from a greenschist facies mylonite in this thrust shear zone shows the contemporaneity of deformation at the base and the top of the wedge the tectonic transport direction within the extruding wedge was e se opposite to the w nw direction so far reported for the eo alpine event in the eastern alps the contemporaneity of opposite tectonic transport directions during continental subduction may be explained by a double vergent wedge model with a narrow zone of ductile flow where the high pressure rocks were exhumed

a revised subduction inception model to explain the late cretaceous double vergent orogen in the precollisional western tethys evidence from the northern apennines the meso cenozoic alpine belts of the mediterranean area are characterized by complex architectures result of a complex subduction and collision evolution that preserve also a legacy of the rifting related configuration of the continental margins the northern apennines is a segment of these belts originated during closure of the ligure piemontese ocean and collision between the europe and adria plates the different configuration of the adria and europe margins inherited from asymmetric rifting is recorded in the ligurian units that preserve incorporation into the subduction factory of fragments of the oceanic domain internal ligurian units and portions of the ocean continent transition zone octz toward adria external ligurian units we provide here unpublished data on the stratigraphy and sedimentology of these units together with a review of what is already established in literature both datasets combined testify that at 80 m a an accretionary prism was growing between the deposition basins of the two groups of units and fed both basins with clasts from the ocean realm the continental crust and the subcontinental mantle we propose that closure of the ligure piemontese ocean occurred through subduction that nucleated at the transition from the oceanic plate and the thinned adria margin and developed a double vergent prism by accreting oceanic material and continental extensional allochthons from the octz we believe the revised site of subduction initiation and the pre collisional architecture inherited from the rifting and spreading phases allow reconciling most of the discrepancies between the various interpretation proposed in literature for the pre collisional evolution of the apennines

investigation of longwall headgate stress distribution with an emphasis on pillar behavior abstract this study uses an extensive suite of borehole pressure cell bpc data from a us longwall mine to better characterize the global stress redistribution process during mining a new methodology employing the bieniawksi s strength gradient equation was used for converting the bpc measurements into rock pressure it was observed that the bieniawksi gradient equation underestimates coal strength for points located deeper into the pillar comparisons with previous empirical and analytical equations are also presented for different stages of longwall loading very low front abutment load in comparison to the side abutment load was observed along with a low abutment angle for this mine this abutment angle value was verified from stress measurements made within the solid coal face ahead of the advancing panel some other atypical trends were noted during data analyses such as a monotonic drop in stress from the panel edge within the coal face these trends were explained using an elastic flac3d model finally the load distribution in a pillar is presented in three dimensions using interpolated bar graphs for different locations of the longwall face

initiation of subduction in the alps continent or ocean the triggering event for the modern evolution of the european alps was the onset of subduction in the middle jurassic however there is some debate as to how this onset occurred as 1 oceanic subduction 2 the succession of an earlier tethys and a later penninic oceanic subduction or even 3 intracontinental subduction here we show that it is plausible that the continental lithosphere in the center of the adriatic plate was gravitationally unstable causing the onset of subduction inside the continent this gravitational instability was provided by thermal thickening of the mantle lithosphere in the wake of the permian extensional event and onset of subduction was facilitated by a strong rheological contrast inside the adriatic plate

eoalpine and mesoalpine tectonics in the southern alps according to structural and stratigraphic data eoalpine and mesoalpine tectonics can be clearly recognized in the southern alps and distinguished from the better known neoalpine deformation

yo yo subduction recorded by accessory minerals in the italian western alps high pressure metamorphic rocks form during subduction of earth s crust to mantle depths at convergent plate margins their exposure at the surface of earth today provides a record of the subduction zone process in general such metamorphic rocks record only a single cycle of subduction and exhumation yet tectonic models suggest that individual rock units should undergo multiple subduction exhumation cycles here we investigate the microstructure and chemical composition of metamorphic minerals in high pressure rocks exposed in the sesia zone in the italian western alps we find that the minerals white mica garnet allanite and zircon each exhibit multiple generations of mineral overgrowths in particular two generations of white mica with high silicon content indicative of formation at high pressure are separated by an overgrowth with low silicon content that formed during exhumation at low pressures furthermore the trace element signatures of distinct zones within zircon and allanite also reveal two episodes of high pressure metamorphism separated by a period of rapid exhumation we use uranium lead dating of zircon and allanite overgrowths to constrain the timing of this subduction exhumation subduction cycle to 79 65 myr ago we conclude that slices of the sesia zone crust experienced two cycles of burial to mantle depths in less than 20 million years the yo yo subduction occurred during oblique convergence between the african and european plates which involved a continental margin

kinematics of austro alpine cover nappes changing translation path due to transpression upper and middle austro alpine nappes of the eastern alps the structural uppermost subunits of the alps suffered two major superposed deformations d1 d2 during the eo alpine cretaceous orogeny d1 is characterized by nw to w directed thrusting and nappe internal rock flow d2 by large scale folding and imbrication toward the north and northeast structures strain metamorphic history microfabrics and stretching trajectories indicate that the superposition of d2 over d1 structures can be interpreted as a progressive deformation it is concluded that the emplacement of the nappes follows a curved translation path a model which emphasizes non coaxial deformation structures explains the detachment of the nappes the configuration of the separating thrust planes and the distribution of intense deformation zones the overall kinematic history of the austro alpine nappes fits a model in which strike slip shearing horizontal shortening and vertical lengthening act together continuously over a finite deformation zone transpressional model this model emphasizing a gradually changing strain regime eliminates the space problems encountered in models which consider simple south over north imbrication during the alpine orogeny the study stresses two interpretations 1 1 stretching lineations indicate nappe transport directions 2 2 transpression may be a common feature of orogenic belts

the alpine evolution of the southern alps around the giudicarie faults a late cretaceous to early eocene transfer zone abstract data supporting relevant late cretaceous early eocene sinistral displacement along the giudicarie fault zone and a minor neogene dextral displacement along the periadriatic lineament are discussed the pre adamello structural belt is present only in the internal lombardy zone located w of the adamello massif this belt is unknown in the dolomites and surrounding areas located to the e of the giudicarie lineament upper cretaceous early eocene thick syntectonic flysch deposits of lombardy and giudicarie are well preserved along the southern and eastern border of the pre adamello belt s vergent alpine orogen towards the e in the dolomites and in the carnic alps and external dinarides only incomplete remnants of flysch deposits aptian albian and turonian maastrichtian in age are present they can be considered as equivalent to those of lombardy and giudicarie formerly in connection to each other along the n giudicarie corridor to the s the syntectonic flysch deposits are laterally replaced by the calcareous red pelagites of the scaglia rossa and by the carbonate shelf deposits of the friuli to the e and bagnolo to the s carbonate platforms the different location in the southern structural accretion of the eastern and western opposite blocks the dolomites versus the pre adamello belt can be related to the cretaceous eocene convergence in this frame the n giudicarie fault has been considered as part of a former transfer zone which produced the sinistral lateral displacement of the southern alps front for an amount of some 50 km during the late eocene to early oligocene the transfer zone was mostly sealed by the paleogene adamello batholith oligocene to neogene compressional evolution inverted the n giudicarie fault into a backthrust of the austroalpine units over the south alpine chain

permian metamorphic event in the alps in addition to the well known variscan and alpine orogenic cycles a permian thermal event occurred in the european alps the geodynamic background of this event is not well understood here we suggest that the event is a reflection of lithospheric thinning accompanied by magmatic underplating causing partial melting of the lower crust and low pressure high temperature metamorphism the event was terminated by early triassic opening of the meliata ocean and was followed by sag stage subsidence during slow lithospheric cooling this last stage of the evolution allowed the onset of the mesozoic marine evolution with the orogen wide formation of triassic carbonate platforms well known from the dolomites and the northern calcareous alps the combined evidence for magmatism metamorphism and late subsidence identifies the permian event of the alps as an excellent example of underplating events suggested as a cause for low pressure high temperature terrains worldwide

divergence in subduction zones and exhumation of high pressure rocks eocene western alps abstract exhumation of high pressure rocks has long remained a controversial issue in the earth sciences in this article we analyze the tectono metamorphic stratigraphic and plate motion constraints from the western alps region providing new insights on exhumation mechanisms and tectonic evolution during the earliest orogenic stages eocene eclogites of the western alps form a 20 25 km wide belt on the upper plate side of the orogen eclogite belt exposed beneath extensional shear zones at the rear of a lower pressure accretionary wedge units of the eclogite belt show the youngest peak pressure assemblages within the subduction zone and experienced superfast tectonic exhumation since 45 40 ma the role of erosion was negligible during the whole of this stage eocene foreland basins remained starved and the massive arrival of axial belt detritus began well after exhumation was completed tectonic reconstructions based on fixed boundaries exhumation models e g channel flow and or implying fast erosion at the surface e g slab breakoff are thus not consistent with geological evidence in the lack of erosion exhumation through the overburden requires divergence within the subduction zone we demonstrate that this was not attained by rollback of the lower plate europe but was instead attained by nneward motion of the upper plate adria africa away from the western alps trench such motion induced localized extension within the weak portion of the upper plate at the rear of the accretionary wedge and allowed tectonic emplacement of the eclogite belt in the upper crust at rates much faster than subduction rates tectonic exhumation ceased in the oligocene when oblique divergence along the western alps traverse changed into oblique convergence the onset of slow erosional unroofing was synchronously recorded by pressure temperature paths in all major tectonic units of the western alps and by arrival of orogenic detritus in sedimentary basins this work demonstrates that divergence between upper plate and trench is a viable mechanism to exhume large and coherent eclogite units in continental subduction zones our exhumation model can be applied to other eclogite belts showing a similar exhumational record including the western gneiss region the dabie sulu and eastern papua new guinea

structure of the lithosphere beneath the eastern alps southern sector of the transalp transect abstract the interpretation of the seismic vibroseis and explosive transalp profiles has examined the upper crustal structures according to the near surface geological evidences and reconstructions which were extrapolated to depth only the southern sector of the transalp transect has been discussed in details but its relationship with the whole explored chain has been considered as well the seismic images indicate that pre collision and deep collision structures of the alps are not easily recognizable conversely good records of the neo alpine to present architecture were provided by the seismic sections two general interpretation models crocodile and extrusion have been sketched by the transalp working group 2002 both illustrate the continental collision producing strong mechanical interaction of the facing european and african margins as documented by giant lithosphere wedging processes arguments consistent with the extrusion model and with the indentation of adriatic southalpine lithosphere underneath the tauern window tw are according to the previous dss reconstructions the bouguer anomalies and the receiver functions seismological data the european moho descends regularly attaining a zone south of the periadriatic lineament pl the moho boundary and its geometry appear to be rather convincing from images of the seismic profile the tauern window intense uplift and exhumation is coherent with the strong compression regime which acted at depth thus originating the upward and lateral displacement of the mobile and ductile penninic masses according to the extrusion model the indentation of the penninic mobile masses within the colder and more rigid adriatic crust cannot be easily sustained wedging of the adriatic stiffened lower crust under high stresses and into the weaker penninic domain can be a more suitable hypothesis furthermore the intrusion of the european penninic crustal wedge underneath the dolomites upper crust is not supported by any significant uplifting of the dolomites the total average uplift of the dolomites during the neogene appears to be 6 7 times smaller than that recognized in the tw markedly the northward dip of the pl reaching a depth of approximately 20 km is proposed in our interpretation finally the adriatic upper crustal evolution points to the late post collision change in the tectonic grow up of the eastern alps orogenic chain the tectonic accretion of the northern frontal zone of the eastern and central alps was interrupted from the late miocene serravallian tortonian onward as documented by the molasse basin evolution on the contrary the structural nucleation along the s vergent tectonic belt of the eastern southern alps montello friuli thrust belt severely continued during the messinian and the plio pleistocene this structural evolution can be considered to be consistent with the deep under thrusting and wedge indentation of the adriatic lithosphere underneath the southern side of the eastern alps thrust and fold belt similarly the significance of the magmatic activity for the construction of the southern alps crust and for its mechanical and geological differentiation which qualified the evolution of the thrust and fold belt is highlighted starting with the permian triassic magmatism and progressing with the paleogene occurrences along the periadriatic lineament and in the venetian magmatic province lessini euganei hills

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

detrital fingerprints of fossil continental subduction zones axial belt provenance european alps abstract collision orogens such as the alps or the himalayas are generated by plate convergence culminating in attempted subduction of a thinned continental margin massive amounts of metamorphic rocks displaying high pressure parageneses are produced during such relatively brief tectonic events and then rapidly exhumed to form the axial backbone of the new orogen sediment composition provides a fundamental key to identify past events of continental subduction although coupled detrital geochronology techniques are needed to discriminate neometamorphic and paleometamorphic sources of detritus within the austroalpine cretaceous and penninic eocene axial belts of the alps we ideally distinguish three structural levels each characterized by diagnostic detrital fingerprints the shallow level chiefly consists of offscraped remnant ocean turbidites and unmetamorphosed continental margin sediments and mostly produces lithic to quartzolithic sedimentaclastic sands yielding very poor heavy mineral suites inc

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

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

the barremian and aptian stepwise development of the oceanic anoxic event 1a oae 1a crisis integrated benthic and planktic high resolution palaeoecology along the gorgo a cerbara stratotype section umbria marche basin italy abstract the barremian and aptian were times of global plate reconfiguration and profound changes in the ocean climate system culminating in the worldwide deposition of lower aptian black shale layers oae 1a based on high resolution lithostratigraphic and micropaleontological analyses precursor conditions and timing of the anoxia are here reconstructed along a 33 06 m thick section in the proposed barremian aptian boundary gssp stratotype at gorgo a cerbara umbria marche basin italy a non uniform history of benthic foraminiferal diversification is interrupted by the selli level oae 1a and by three turnover points for both benthic and planktic organisms each highlighted by prominent breaks in lithology and outcrop morphology the first two points at 1 1 myr and 20 50 kyr prior to the onset of the oae 1a correspond respectively to the nannoconid bathypelagic calcareous nannofossils decline and crisis events and separate a pelagic limestone chert claystone lithological cycles of the uppermost maiolica formation showing frequencies reminiscent of orbital eccentricity and containing an oligotrophic barremian benthic and planktic assemblage dominated by k selected nannoconids and rhizammina b greenish grey cherty marls of the lowermost marne a fucoidi formation associated to declining sedimentation rates and eutrophic early aptian assemblages dominated by radiolarians and lituolid benthic foraminifera and c anoxic radiolarites and shales of the carbonate free selli level high magnitude and frequency assemblage fluctuations occur between the nannoconid crisis 25 cm below the lowermost oae 1a black shale layer and the selli level base with suggestions of bottom water acidification at 800 kyr after the oae 1a end a third turnover point in the basal reddish member of the marne a fucoidi highlights middle aptian assemblages characterized by a bloom of early macroperforate planktic foraminifera hedbergella spp and appearances of new benthic species marking a shift towards better oxygenation oae 1a oxygen and pelagic carbonate factory crises are here suggested to have been step wise and linked to global warming conditions

u pb zircon age of the andaman ophiolite implications for the beginning of subduction beneath the andaman sumatra arc abstract the andaman ophiolites form the basement of the andaman islands which is a part of the outer forearc that links the indo burma accretionary complex to the north with the java sumatra trench arc system to the se upper mantle harzburgite and dunite are overlain by a cumulate peridotite gabbro complex high level intrusive rocks and a tholeiitic volcanic series the upper crust in the south andaman ophiolite shows also a prominent andesite dacite volcanic suite suggesting arc volcanism built onto ocean crust u pb zircon dating of a trondhjemitic rock from chiriya tapu in south andaman island using laser ablation inductively coupled mass spectrometry reveals an age of crustal formation of 95 2 ma the trondhjemites have geochemistry comparable with that of plagiogranites associated with ophiolite complexes and e nd values around 7 further confirm that they are derived from depleted mantle melts basaltic pillow lava and basaltic dykes that cut the trondhjemites have mid ocean ridge basalt like trace element geochemistry the new data show that the andaman volcanic arc was built on cenomanian ophiolite oceanic crust and that subduction was initiated at this time along tethys at least from cyprus through oman to the andaman islands

the alps in the cretaceous a doubly vergent pre collisional orogen current palaeotectonic models see the alps chiefly as a collisional orogen with the southalpine retrobelt developed only as a post collisional feature new structural and geochronological data show instead that the continental margin of africa adria was being structured as a doubly vergent orogen during subduction of alpine tethys in the cretaceous tens of million years before collision with europe subduction is envisaged here to have nucleated in the latest albian cenomanian within the attenuated adriatic margin most continental slivers exhumed as eclogites in the axial part of the cretaceous orogen contain gabbros and relics of ht lp metamorphic rocks indicating inheritance from permian to jurassic rifting cretaceous shortening in the southalpine retrobelt is indicated by several lines of evidence including orogen derived turbidites and syn sedimentary compressional activity since the cenomanian upper cretaceous pseudotachylytes dating major s severging thick skinned thrusts and widespread fold and thrust deformation cut by eocene plutons and dikes

a natural and controlled source seismic profile through the eastern alps transalp abstract the combined passive and active seismic transalp experiment produced an unprecedented high resolution crustal image of the eastern alps between munich and venice the european and adriatic mohos em and am respectively are clearly imaged with different seismic techniques near vertical incidence reflections and receiver functions rfs the european moho dips gently southward from 35 km beneath the northern foreland to a maximum depth of 55 km beneath the central part of the eastern alps whereas the adriatic moho is imaged primarily by receiver functions at a relatively constant depth of about 40 km in both data sets we have also detected first order alpine shear zones such as the helvetic detachment inntal fault and sub tauern ramp in the north apart from the valsugana thrust receiver functions in the southern part of the eastern alps have also observed a north dipping interface which may penetrate the entire adriatic crust adriatic crust interface aci deep crustal seismicity may be related to the aci we interpret the aci as the currently active retroshear zone in the doubly vergent alpine collisional belt

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

experimental constraints on phase relations in subducted continental crust synthesis piston cylinder experiments were carried out in the range 2 0 4 5 gpa and 680 1 050 c to investigate phase relations in subducted continental crust a model composition kcmash has been used because all major ultrahigh pressure uhp minerals of the whole range of rock types typical for continental crust can be reproduced within this system the combination of experimental results with phase petrologic constraints permits construction of a uhp petrogenetic grid the phase relations demonstrate that the most important uhp paragenesis consists of coesite kyanite phengite clinopyroxene and garnet in subducted continental crust below 700 c talc is stable instead of garnet as most of these minerals are also stable at much lower pressure and temperature conditions it is thus not easy to recognize uhp metamorphism in subducted crust a general feature however is the absence of feldspars at h2o saturated conditions plagioclase is never stable at uhp conditions but k feldspar can occur in h2o undersaturated rocks mineral compositions in the experiments are fully buffered by coexisting phases the si content of phengite and biotite increase with increasing pressure at 4 0 gpa 780 c biotite contains 3 28 si per formula unit which is most probably caused by solid solution of biotite with talc above 800 c the caal2sio6 component in clinopyroxene buffered with kyanite coesite and a mg phase increases with increasing temperature providing a tool to distinguish between cold and hot eclogites up to 10 ca eskolaite ca0 5 0 5alsi2o6 in clinopyroxene has been found at the highest temperature and pressure investigated 900 c 4 5 gpa garnet buffered with coesite kyanite and clinopyroxene displays an increase of grossular component with increasing pressure for a given temperature although the investigated system represents a simplification with respect to natural rocks it helps to constrain general features of subducted continental crust the observed phase relations and phase compositions demonstrate that at pressures 3 0 gpa and temperatures 800 c continental crust can retain significant amounts of h2o 1 wt whereas k free mafic or ultramafic rocks are dry at these conditions uhp parageneses are only preserved if the whole exhumation path is situated within the stability field of phengite i e if there is cooling during exhumation or if the whole exhumation occurred at t 700 c in contrast break down of phengite and concomitant partial melting in terranes that show isothermal decompression may lead to a complete recrystallization of the subducted crust during exhumation the density of uhp rocks can be estimated on the basis of the established phase relations pelitic rocks are likely to have a density close to mantle rocks 3 3 g cm3 because of significant amounts of dense garnet and kyanite whereas granitic rocks are less dense 3 0 g cm3 hence subducted average continental crust is most probably buoyant with respect to mantle rocks and tends to get exhumed as soon as it is detached from the down going slab electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00410 001 0336 3

the s reflector west of galicia spain evidence from prestack depth migration for detachment faulting during continental breakup the mechanism of continental extension and breakup and the role of detachment faults in these processes are currently the subject of intense debate one possible detachment fault is the s reflector imaged as an undulating in time locally discontinuous reflection on existing seismic profiles west of iberia here we present new images in depth of the four margin normal profiles across the west galicia rifted margin where the s reflector was originally defined and is best imaged it is shown that faults bounding wedge shaped units of late synrift sediments which hence were active during rifting immediately prior to breakup appear to detach at shallow levels onto the s reflector s itself appears as a continuous locally domal feature and does not generally appear offset the waveform of s is compatible with a reflection from a single interface such as a sharp tectonic boundary the depth sections show that s was active at 1 3 km below the seafloor during final rifting s is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of galicia furthermore the data provide constraints on the sense of shear of s analogy with detachment terranes the present synrift and structural dips of s and the identification of a breakaway to s imply that s accommodated top to the west shear toward the east of the profiles s becomes more complex possibly because of different phases of detachment faulting and the development of both incisement and excisement structures from the geometry of wedge shaped sedimentary units deposited during faulting above s it also appears that s was active at an angle of 20 or less and hence may be considered a genuine low angle normal fault

timing of high pressure metamorphism and exhumation of the eclogite type locality kupplerbrunn prickler halt saualpe south eastern austria constraints from correlations of the sm nd lu hf u pb and rb sr isotopic systems sm nd lu hf rb sr and sims u pb data are presented for meta gabbroic eclogites from the eclogite type locality hauy 1822 kupplerbrunn prickler halt and other areas of the saualpe se austria and pohorje mountains slovenia mg rich eclogites derived from early gabbroic cumulates are kyanite and zoisite rich whereas eclogites with lower mg contents contain clinozoisite kyanite calculated p t conditions at the final stages of high pressure metamorphism are 2 2 0 2 gpa at 630 740 c kyanite rich eclogites did not yield geologically meaningful sm nd ages due to incomplete nd isotope equilibration whereas sm nd multifraction garnet omphacite regression for a low mg eclogite from kupplerbrunn yields an age of 91 1 1 3 ma the sm nd age of 94 1 0 8 ma obtained from the fe rich core fraction of this garnet dates the initial stages of garnet growth zircon that also crystallized at eclogite facies conditions gives a weighted mean u pb sims age of 88 4 8 1 ma lu hf isotope analysis of a kyanite eclogite from kupplerbrunn yields 88 4 4 7 ma for the garnet omphacite pair two low mg eclogites from the gertrusk locality of the saualpe yield a multimineral sm nd age of 90 6 1 0 ma a low mg eclogite from the pohorje mountains 70 km to the se gives a garnet whole rock lu hf age of 93 3 2 8 ma these new age data and published sm nd ages of metasedimentary host rocks constrain the final stages of the eo alpine high pressure event in the saualpe pohorje part of the south easternmost austroalpine nappe system suggesting that garnet growth in the high pressure assemblages started at c 95 94 ma and ceased at c 90 88 ma probably at the final pressure peak zircon and amphibole crystallization was still possible during incipient isothermal decompression rapid exhumation of the high pressure rocks was induced by collision of the northern apulian plate with parts of the austroalpine microplate following jurassic closure of the permo triassic meliata back arc basin

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

subcrustal tectonic erosion in orogenic belts a model for the late cretaceous subsidence of the northern calcareous alps austria analysis of the upper cretaceous to eocene gosau group of the northern calcareous alps austria indicates rapid subsidence after a short interval of deformation uplift and erosion reconstructions of basement subsidence of 3000 m from the late turonian 89 ma onward show a distinct shift of the deformation and subsidence from the northwest to the southeast together with a tilting of the northern calcareous alps toward the north comparison with recent analogues suggests that subcrustal tectonic erosion due to the subduction of a topographic high of the south penninic ocean may be the cause for this subsidence pattern tectonic erosion along ancient plate margins can be recognized by deformation and widespread erosion followed by regional subsidence not related to extension or flexural loading

erratum to early permian pangea b to late permian pangea a earth planet sci lett 215 2003 379 394 a dipartimento di scienze della terra universita di milano via mangiagalli 34 20133 milan italy b lamont doherty earth observatory palisades ny 10964 usa c department of geological sciences rutgers university piscataway nj 08854 usa d dipartimento di scienze geologiche e geotecnologie universita di milano bicocca piazza della scienza 4 20126 milan italy e departement erdwissenschaften eth zentrum ch 8092 zu rich switzerland f department of earth sciences aarhus university aarhus denmark

early permian pangea b to late permian pangea a the pre drift wegenerian model of pangea is almost universally accepted but debate exists on its pre jurassic configuration since tedirving introd ucedpangea b by placing gond wana farther to the east by v3000 km with respect to laurasia on the basis of paleomagnetic data new paleomagnetic data from radiometrically dated early permian volcanic rocks from parts of adria that are tectonically coherent with africa gondwana integrated with published coeval data from gondwana and laurasia again only from igneous rocks fully support a pangea b configuration in the early permian the use of paleomagnetic data strictly from igneous rocks excludes artifacts from sedimentary inclination error as a contributing explanation for pangea b the ultimate option to reject pangea b is to abandon the geocentric axial dipole hypothesis by introducing a significant non dipole zonal octupole component in the late paleozoic time averaged geomagnetic field we demonstrate however by using a dataset consisting entirely of paleomagnetic directions with low inclinations from sampling sites confined to one hemisphere from gondwana as well as laurasia that the effects of a zonal octupole field contribution would not explain away the paleomagnetic evidence for pangea b in the early permian we therefore regard the paleomagnetic evidence for an early permian pangea b as robust the transformation from pangea b to pangea a took place during the permian because late permian paleomagnetic data allow a pangea a configuration we therefore review geological evidence from the literature in support of an intra pangea dextral megashear system the transformation occurred after the cooling of the variscan mega suture andlasted v20 myr in this interval the neotethys ocean opened between india arabia and the cimmerian microcontinents in the east while widespread lithospheric wrenching and magmatism took place in the west around the adriatic promontory the general distribution of plate boundaries and resulting driving forces are qualitatively consistent with a right lateral shear couple between gondwana and laurasia during the permian transcurrent plate boundaries associated with the pangea transformation reactivated variscan shear zones andwere subsequently exploitedby the opening of western neotethyan seaways in the jurassic

effect of a retreating subduction zone on deformation in simple regions of plate convergence plane strain two dimensional finite element models are used to calculate deformation patterns within model mountain belt systems where subduction zone retreat the process by which space is created between the subducting pro and overriding retro lithospheres either by motion of one or both is active the model uses coulomb plastic and thermally activated power law viscous rheologies overall deformation is determined by balancing the internal strength of the model layer against the sum of applied boundary stresses plus the gravitational stress induced by mass redistribution within the deforming model the effect of incorporating retreat in the subduction setting diffuses deformation as the moving retreating subduction zone deforms a larger cross sectional area of material for a shorter period of time than equivalent models without subduction zone retreat material is deformed while within the subduction zone but once the material has passed completely through the moving subduction zone it is passively appended onto the retrolithosphere and is no longer deformed thus relative to a convergent setting without subduction zone retreat the retreating convergent zone has an areally extensive zone of simple deformation a deformation factor is derived which characterizes the deformation shortening the material experienced within the subduction zone variable is the ratio of convergence rate rate at which material enters the subduction zone to subduction rate rate at which material is subducted variables and f the fraction of the layer that is subducted together determine the dominant style of deformation in the orogenic zone subduction zone retreat is shown to have significant and predictable effects on the model morphology e g basin or mountain formation and on the model geology e g its presence suggests lower topography lower metamorphic grade rocks at the surface and simpler deformational structures which agree with observations