paleomagnetism of jurassic to miocene sediments from the apenninic carbonate platform southern apennines italy evidence for a 60 counterclockwise miocene rotation abstract the tectonic evolution of the apennine belt southern tyrrhenian sea system is addressed through a paleomagnetic study of lias to langhian sediments from the apenninic carbonate platform southern apennines italy reliable paleomagnetic data gathered from 21 sites document a regional scale post langhian 80 counterclockwise ccw rotation since previous studies of the plio pleistocene clays spread over the orogen had shown a 20 ccw rotation we conclude that the southern apennines rotated by 60 during middle late miocene our data provide evidence that the southeastward drift of calabrian block and synchronous spreading of the southern tyrrhenian sea induced saloon door like deformation of the southern apennines and sicily which underwent similar magnitude although opposite in sign orogenic rotations a paleomagnetically derived paleogeographic reconstruction shows that at 15 ma late langhian the alpine apennine belt collided with a nne oriented carbonate platform corridor surrounded by oceanic basins we speculate that both the end of the corsica sardinia rotation and the eastward jump of the locus of back arc extension from the liguro provencal to the tyrrhenian sea may have been consequences of this event

cenozoic magmatism of the valencia trough western mediterranean relationship between structural evolution and volcanism abstract the cenozoic magmatism of the valencia trough is characterized by the existence of two volcanic cycles the first cycle early miocene middle miocene comprises calc alkaline rocks which have been recognized in the central to eastern offshore areas and in mallorca the second cycle middle miocene to recent is represented mainly by poorly differentiated alkaline rocks which are predominantly distributed along the iberian margin of the valencia trough in accordance with the inferred structural evolution of the valencia trough petrological and geochemical data as well as volcanological evidence we suggest that two different geodynamic environments existed during these magmatic cycles the early to middle miocene calc alkaline volcanism developed under compressive tectonic conditions and seems to be associated with the northwestern dipping subduction of the african plate in contrast the middle miocene to recent alkaline volcanism is associated with extensional tectonics and can be explained within the framework of a low volcanicity rift model three zones catalan valencian and columbretes each located at different parts of the rift structure can be distinguished in the middle miocene to recent volcanism this intraplate volcanism results from partial melting due to extension driven decompression with the largest degrees of melting in the centre of the valencia trough columbretes volcanic zone in the catalan volcanic zone a progressive increase in the extension rate from the first to the last volcanic episode has occurred favouring the interaction of two mantle sources

estimates of modern arc parallel strain rates in fore arcs deflections of slip vectors of interplate thrust earthquakes from expected directions are used to estimate arc parallel strain rates within the overriding plates at the world9s major convergent plate margins arc parallel extension strain rates between 10 8 yr and 10 7 yr and significant at 2 standard deviations are observed in the fore arcs of the aegean aleutian mariana sumatran southern kuriles new hebrides scotia and southern central american lat 8 to 12 n subduction zones and the himalayas northern chile lat 17 to 31 s and central america lat 11 to 18 n show arc parallel compression available geologic and geodetic estimates of fore arc slip and strain rates agree within a factor of two with slip vector estimates arc parallel strain in fore arcs is rapid enough to produce geologically significant effects such as unroofing of high grade metamorphic rocks and disruption of transported fore arc terranes fore arcs deform even where convergence is perpendicular to curved margins demonstrating that head on subduction can produce a three dimensional strain field

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

evolution of retreating subduction boundaries formed during continental collision retreating subduction boundaries formed where the rate of subduction exceeds the rate of overall plate convergence appear to be commonly developed features within regions of early or incomplete continent continent collision they are characterized by regional extension within the overriding plate and at their leading edge by thin skinned arcuate thrust belts that are concave towards the overriding plate as is illustrated by examples from the mediterranean region the formation of retreating subduction boundaries is intimately related to the process of continental collision during the early stages of collision retreating subduction boundaries are commonly formed by lateral ejection from zones of crustal shortening along the main collision boundary retreating plate boundaries can also form before the main collision and the associated thrust belts emplaced as precollisional accretionary assemblages because the driving mechanism for retreating subduction boundaries appears to be gravity acting on a dense subducted slab slab pull subduction usually ceases when and only when thick buoyant continental crust enters the subduction zone thus differences in the evolution and duration of retreating subduction systems can be largely attributed to the size and configuration of the deep water regions available to be subducted in some cases retreating subduction boundaries may escape into the open ocean where they form nearly isolated local tectonic systems in these systems the rate of subduction is approximately compensated by the rate of upper plate extension and migration of the system across the oceanic region may be very rapid for example the horseshoe seamounts located about 800 km offshore in the eastern north atlantic may be the active expression of an east dipping westwardly migrating retreating subduction boundary that has evolved from the betic cordillera rif system active in miocene time and may now be progressing across the atlantic at approximately 50 mm yr an analogous situation may be represented by the scotia arc system a westward dipping retreating subduction system located between the south american and antarctic plates which may have escaped into the south atlantic ocean from a zone of crustal shortening in the andes and is now progressing across the atlantic at a rate of about 80 mm yr

an early cretaceous extinct spreading center in the northern natal valley abstract we have identified an extinct e w spreading center in the northern natal valley on the basis of magnetic anomalies which was active from chron m11 133 ma to 125 3 ma just before chron m2 124 ma in the early cretaceous seafloor spreading in the northern natal valley accounts for approximately 170 km of north south motion between the mozambique ridge and africa this extension resolves the predicted overlap of the continental central and southern mozambique ridge and antarctica in the chron m2 to m11 reconstructions from mesozoic finite rotation parameters for africa and antarctica in addition the magnetic data reveal that the mozambique ridge was an independent microplate from at least 133 to 125 ma the northern natal valley extinct spreading center connects to the spreading center separating the mozambique basin and the riiser larsen sea to the east it follows that the northern mozambique ridge was either formed after the emplacement of the surrounding oceanic crust or it is the product of a very robust spreading center to the west the extinct spreading center connects to the spreading center separating the southern natal valley and georgia basin via a transform fault prior to chron m11 there is still a problem with the overlap of mozambique ridge if it is assumed to be fixed with respect to either the african or antarctic plates some of the overlap can be accounted for by jurassic deformation of the mozambique ridge mozambique basin and dronning maud land it appears though that the mozambique ridge was an independent microplate from the breakup of gondwana 160 ma until it became part of the african plate 125 ma

numerical modeling of simultaneous extension and compression the valencia trough western mediterranean a two dimesional kinematic model which incorporates lateral accommodation of extension and differential stretching is used to investigate the geodynamic evolution of the valencia trough this neogene basin located between the iberian peninsula and the balearic promontory underwent two rifting episodes the first one being coeval with compression model predictions are compared with observations of tectonic subsidence bouguer gravity anomaly surface heat flow and the present day crustal structure the best fitting model assumes three stages in the evolution of the basin during the first stage late oligocene early miocene extension is mainly restricted to the central and northwestern parts of the basin whereas its southeastern margin is locked by the alpine deformation front the material expelled from the axial area is partly accommodated under the balearic promontory producing compression and crustal thickening during the second rifting stage middle miocene extension affects a much broader area including the balearic promontory and the algero provencal basin the third stage late miocene to recent corresponds to a postrift thermal relaxation phase our results put further constraints on the geodynamic evolution of the area

timing of rotational motion of apuseni mountains romania paleomagnetic data from tertiary magmatic rocks paleomagnetic results are reported from 8 sites in eocene to oligocene magmatic rocks and 27 sites in middle miocene to upper miocene volcanic rocks of the apuseni mountains following af demagnetization 29 sites were selected for tectonic interpretation these results together with previously published data imply a northward displacement of the area studied between the late cretaceous and oligocene followed by a large clockwise rotation prior to the middle miocene after which no significant movement has occurred these results agree with other data from the carpatho pannonian domain showing a period of great mobility during the early miocene

a preliminary paleomagnetic pole for mid cretaceous rocks from tobago further evidence for large clockwise rotations in the caribbean south american plate boundary zone abstract the island of tobago occupies the eastern end of the central igneous belt of the caribbean south american plate boundary zone volcaniclastic sediment of the albian 100 ma tobago volcanic group and dikes of similar age within it were sampled in two homoclinal sections with different attitudes the mean of virtual geomagnetic poles for 12 sites 25 4 n 24 1 e a95 4 2 is well defined pre tilting and apparently reliable yet is far removed from a similar age reference pole for south america five other paleomagnetic studies of cretaceous rocks from widely separated localities farther west in the plate boundary zone yield pole positions that are remarkably similar to the tobago pole poles obtained from the guajira peninsula of colombia the islands of aruba and bonaire and the caribbean mountains of venezuela are among those that agree with the tobago result the paleolatitudes for study areas within the plate boundary zone are consistent with an origin on or near south america yet the poles throughout the zone are rotated roughly 90 dextral relative motion between the caribbean and south american plates was probably responsible for the rotation

oppositely directed pairs of propagating rifts in back arc basins double saloon door seafloor spreading during subduction rollback 1 when a continent breaks up into two plates which then separate from each other about a rotation pole it can be shown that if initial movement is taken up by lithospheric extension asthenospheric breakthrough and oceanic accretion propagate toward the pole of rotation such a propagating rift model is then applied to an embryonic centrally located rift which evolves into two rifts propagating in opposite directions the resultant rhombic shape of the modeled basin initially underlain entirely by thinned continental crust is very similar to the oligocene to burdigalian back arc evolution of the valencia trough and the liguro provencal basin in the western mediterranean existing well and seismic stratigraphic data confirm that a rift did initiate in the gulf of lion and propagated southwest into the valencia trough similarly seismic refraction gravity and heat flow data demonstrate that maximum extension within the valencia trough liguro provencal basin occurred in an axial position close to the north balearic fracture zone the same model of oppositely propagating rifts when applied to the burdigalian langhian episode of back arc oceanic accretion within the liguro provencal and algerian basins predicts a number of features which are borne out by existing geological and geophysical particularly magnetic data these include the orientation of subparallel magnetic anomalies presumed to be seafloor spreading isochrons in both basins concave to the west fracture zones southwest of the north balearic fracture zone and concave to the east fracture zones to its northeast a spherical triangular area of nw oriented seafloor spreading isochrons southwest of sardinia the greater nw extension of the central youngest magnetic anomaly within this triangular area in agreement with the model predicted northwestward propagation of a rift in this zone successively more central younger magnetic anomalies abutting thinned continental crust nearer to the pole of rotation in the liguro provencal basin the latter feature demonstrates that a rift also propagated northeast in the liguro provencal basin at least in its oceanic accretion phase of development an adaptation of an existing model for subduction slab detachment occurring along the north african margin in the late burdigalian langhian proposes propagation in opposite directions of the slab tear the resultant rhombic slab detachment is closely associated in space and time with the rhombic form of the algerian liguro provencal basins suggesting a cause and effect relationship

tepee shaped agglutinated microbialites an example from a famennian carbonate platform on the lennard shelf northern canning basin western australia distinctive metre scale antiformal structures are well developed in a famennian carbonate platform in the chedda cliffs area of the lennard shelf reef complexes the structures are distinguished by chevron shaped crests and thickened cores and contain abundant non skeletal allochems ooids pisoids peloids and intraclasts of silt to pebble size and variably developed laminations and fenestrae the internal morphology and pervasive occurrence of fenestral clotted and wavy laminated fabrics suggest that these structures are microbial mounds composed of agglutinated stromatolites and thrombolites microbial fabrics most probably originated through sediment trapping and binding by microbial mats with early lithification involving microbial calcification and cementation of trapped sediment the facies and stratigraphic context of the mounds support a shallow subtidal transitional backreef to reef flat setting however alone these mounds do not provide unequivocal environmental information other large antiformal structures in famennian platforms on the lennard shelf previously described as tepee structures show morphological similarities to the chedda cliffs mounds which suggests that these other structures may also be microbial mounds the presence of microbial mounds in platform successions further highlights the importance of microbial communities in the lennard shelf reef complexes

relics of a complex triple junction in the weddell sea embayment antarctica abstract interpretation of an airborne magnetic data compilation containing a key new survey together with re tracked satellite gravity data from the weddell sea embayment wse west antarctica suggests rift rift rift triple junction formation at the onset of gondwana breakup in the early middle jurassic a complex system of northwest southeast rifts was active contemporaneously with an east west trending rift this rift activity led to northward separation of the falkland plateau and formation of the weddell sea by sea floor spreading atypically the jurassic passive margin of gondwana shows evidence for coeval extension in two directions and a large volume of interpreted magmatic material this is consistent with initial doming above a mantle plume and we suggest that this resulted in the formation of a triple junction magnetic anomalies indicate a series of faults perpendicular to igneous intrusions and extrusions with outlines that range in shape from lozenges to parallel ridges they show remarkably good spatial correlation with free air gravity anomalies even in areas of sea ice we base a structural elements map and timing sequence for the events in the wse during early gondwana breakup on anomaly cross cutting relationships

creation and destruction of weddell sea floor in the jurassic new paleomagnetic data invite reassessment of the breakup history of parts of the pacific margin of gondwana due to the mesozoic opening of the southern ocean basins these data indicate clockwise rotation of the antarctic peninsula relative to east antarctica between 175 and 155 ma that would have created as much as 1000 km of pre anomaly m25 ocean floor in the weddell sea basin the paleomagnetic data also suggest that between 155 and 130 ma the antarctic peninsula rotated counterclockwise relative to east antarctica rotation that along with the southward motion of east antarctica would have caused subduction of the newly created weddell sea ocean floor beneath the southern antarctic peninsula east vergent folding and thrusting of jurassic back arc basin rocks along the southeastern antarctic peninsula known as the palmer land event probably resulted from deformation along this newly created convergent margin the presence of jurassic weddell sea floor would have isolated eastern gondwana east antarctica australia and india from the antarctic peninsula and western gondwana africa and south america encouraging increased marine faunal connections between the pacific and weddell sea mozambique somali ocean basins during the 175 to 130 ma interval after 130 ma the antarctic peninsula would have been connected to east antarctica via the other west antarctic terranes thereby facilitating land migrations between eastern and western gondwana

quaternary evolution of the corinth rift and its implications for the late cenozoic evolution of the aegean summary we present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active aegean region we focus first on the 130 km long corinth rift an asymmetric graben where a flight of terraces of marine origin are uplifted we show that the edges of the terraces lie in the footwall of the normal fault bounding the corinth rift and correspond to sea level highstands of laic pleistocene age using a detailed analysis of aerial and spot imagery supported by field observations we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault the elevation of the terraces inner edges was estimated at 172 sites with an error of 5m this data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level to date the deformation we correlate the corinth terraces with late pleistocene oxygen isotope stages of high sea level stands and with global sea level fluctuations using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault we show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000 all the terraces are fitted for fault slip increasing in proportion to terrace age and the component of regional uplift is found to be less than 0 3 mm yr 1 the best fits give a slip rate of 11 3 mm yr 1 on the main rift bounding fault over the last 350 kyr other geological and morphologic information allows us to estimate the total age of the main fault 1 ma and to examine the mechanical evolution of the corinth rift the minimum observed sediment thickness in the gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6 7 mm yr 1 40 per cent less than estimated with modelling even this slip rate is nearly 10 times higher than for comparable features in most of the aegean and elsewhere in the world at a larger scale the spacing and asymmetry of the rift systems in the aegean suggest strain localization in the upper mantle with slow extension starting 15 myr ago or earlier the more recent 1 myr rapid phase of rifting in corinth partly reactivated this earlier phase of extension the younger faulting in corinth appears to result from its present location in the inhomogeneous stress field process zone of the south westward propagating tip of the southern branch of the north anatolian fault we extend these relations to propose a mechanical model for the late cenozoic evolution of the aegean as the arabia europe collision progressed in eastern turkey it caused anatolia to move to the west and the north anatolian fault to propagate into the aegean where the early slow extension started to be modified about 5 ma ago the process of propagation dramatically increased the activity of some but not all of the earlier rifts the model we present is compatible with tectonic observations as well as with the seismicity the palaeomagnetic rotations and the displacement field now observed with gps and slr

paleomagnetic evidence for late miocene counterclockwise rotation of north coast carbonate sequence puerto rico a paleomagnetic study of the essentially undeformed middle tertiary carbonate sequence along the north coast of puerto rico reveals statistically significant pre pliocene discordance of characteristic component directions against those expected from cratonic north america for much of the section despite generally weak to moderately weak magnetic intensities confirmation of the magnetization as primary in origin comes from the presence of two distinct components of magnetization intrasite bipolarity and or the reproducibility of measurements the mean geographic direction for the upper oligocene to middle miocene strata is 335 2 degree 32 9 degree and the corrected mean paleomagnetic pole is 207 6 degree 66 5 degree n 3 alpha 95 4 3 degree this suggests a counter clockwise ccw block rotation of puerto rico and its microplate of 24 5 degrees plus minus 5 8 degrees during the late miocene using a width of 250 km for the northern caribbean plate boundary zone ncpbz between the north american plate and caribbean plate the mean left lateral displacement implied is 1 8 to 2 4 cm yr which agrees fairly well with published relative motion rates for the two plates average rotation rate for 50 ma to 20 ma was 0 7 degree my but perhaps as great as 4 degree my in the miocene resolution of mean paleolatitude indicates northward motion of a degree or lessmore during the period of rotation causes of this short lived rotation may include 1 tectonic escape from the inhibiting presence of the bahama banks and beata ridge during eastward motion of puerto rico along the sinistral transpressive puerto rico trench and muertos trough fault systems or 2 changes in relative plate motions of the caribbean and north american plate during the late miocene less

variations in the degree of crustal extension during formation of a back arc basin abstract ophiolite complexes in southern chile represent the remnants of the mafic portion of the floor of a cretaceous back arc basin which widened markedly from north to south over a length of 600 km detailed field and geochemical studies of ophiolites in the northern sarmiento complex and southern tortuga complex extremities of the originally wedge shaped back arc basin floor indicate significant north south differences in the mode of emplacement of basaltic magmas into the pre existing continental crust during the formation of the basin in the northern narrow extremity of the original basin mafic melts intruded into the continental crust over a diffuse zone causing extensive remobilization and reconstitution of the sialic continental crust in the southern wider part of the original basin mafic magmas appear to have been emplaced at a localized oceanic type spreading centre the observed north south variations resulted in formation of back arc floor with crustal characteristics ranging from intermediate between continental and oceanic to typically oceanic these variations are interpreted as representing different stages of evolution of a back arc basin which formed due to a subtle interplay between subduction induced back arc mantle convection and the release of stress across the convergent plate boundary possibly related to ridge subduction prior to the release of stress heat transferred from mantle diapirs to the base of crust caused widespread silicic volcanism in south america with the release of stress mantle derived melts erupted to the surface along structural pathways resulting in extensive basaltic volcanism in a linear belt behind the island arc and the cessation of silicic volcanism initially basaltic magmas intruded the continental crust over a diffuse region causing reconstitution of sialic crustal rocks progressive localization of the zone of intrusion of mafic magmas from the mantle eventually resulted in the development of an oceanic type spreading centre observations in southern chile and elsewhere suggest that variability in horizontal stress across a convergent plate boundary may be the overriding factor in determining the regional response of continental crust to subduction induced back arc convection and hence the mechanism of emplacement into the crust of mafic mantle melts the various lithologies observed in southern chile could also be expected to form during the opening phase of major ocean basins and to currently underlie atlantic type continental margins

high resolution teleseismic body wave tomography beneath se romania i implications for three dimensional versus one dimensional crustal correction strategies with a new crustal velocity model summary the calixto carpathian arc lithosphere x tomography experiment offers a dense high quality data set to study the lithospheric asthenospheric system underneath se romania an earthquake prone region in se europe to increase the image resolution of structures in the uppermost mantle the application of crustal traveltime corrections by a priori information before the teleseismic traveltime inversion has become a well accepted procedure for such a correction we present a regional 3 d crustal seismic velocity model that serves as the basis for a high resolution teleseismic tomography forthcoming paper by martin et al our 3 d crustal model is based on recent research in the region we collect new results from two long range seismic refraction lines 3 d refraction tomography and teleseismic ps conversions adding previously published models of the sediment distribution conrad and moho depths as well as crustal seismic p wave velocities we compile a 3 d crustal model for se romania this 3 d model does not contain shallow small scale heterogeneities 10 km but it reflects the large scale structures such as variations in sediment thickness average seismic velocities and 3 d moho depth it is well suited for the correction of teleseismic traveltime residuals a prerequisite for a high resolution teleseismic tomography study for example traveltime delays of up to 1 3 s are caused by the almost 20 km thick layer of sediments in the focsani basin such delays are comparable to or larger in size than the expected upper mantle traveltime residuals we study the significance of 3 d crustal traveltime corrections relative to 1 d station corrections and show that the complex basin structures in se romania require a 3 d approach to reduce the smearing of crustal anomalies into the mantle by modelling synthetic mantle structures with a slab as it is expected for se romania we also investigate how to adapt the inversion strategy if crustal corrections are applied significant improvements are found by including the already corrected crustal layers in the inversion procedure thereby enabling the inversion algorithm to project still remaining uncertainties in the less resolved upper crustal layers however the fixing of the upper layers during the inversion due to the a priori knowledge of the crustal velocity anomalies clearly leads to smearing of uncorrected anomalies that are possibly located close to the crust mantle boundary

geodynamic evolution of the antarctic peninsula during mesozoic times and its bearing on weddell sea history this review of the tectonic evolution of the antarctic peninsula during mesozoic times highlights four main events 1 late triassic late jurassic extension 2 late jurassic early cretaceous dextral transpression 3 early cretaceous extension and 4 mid cretaceous compression magmatism was virtually continuous during much of this period with the exception of possible breaks in the known record in early jurassic and late jurassic early cretaceous times the second of these breaks corresponded to the first compressional event there was no apparent hiatus in the magmatic record during the mid cretaceous compressional event although there was a significant change in the pattern of sedimentation in the larsen basin on the eastern margin of the weddell sea at about that time the tectonic evolution of the peninsula is compared to and puts some constraints on existing weddell sea models the late triassic late jurassic arc extension correlates with initial rifting in the weddell sea region during sinistral motion between east and west gondwana however there is no known record of large scale pre mid jurassic transcurrent deformation in the antarctic peninsula that would have been consistent with rotation of west antarctic crustal blocks in the initial rifting period the peninsula wide late jurassic early cretaceous compressional event may correlate with the major change in gondwana plate motions from e w to n s african reference frame separation of east and west gondwana this change probably resulted in formation of an ocean continent boundary along the northern margin of the weddell sea embayment and initial seafloor spreading geological data do not seem to support subduction of southwestern proto weddell sea oceanic lithosphere beneath the eastern margin of the peninsula at that time cretaceous arc extension was coeval with the initial seafloor spreading phase in the weddell sea mid cretaceous arc compression linked to a global increase in ocean floor spreading rates and a superplume event correlates with a change from ne sw to nw se spreading in the weddell sea

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

gondwana breakup via double saloon door rifting and seafloor spreading in a backarc basin during subduction rollback abstract a model has been developed where two arc parallel rifts propagate in opposite directions from an initial central location during backarc seafloor spreading and subduction rollback the resultant geometry causes pairs of terranes to simultaneously rotate clockwise and counterclockwise like the motion of double saloon doors about their hinges as movement proceeds and the two terranes rotate a gap begins to extend between them where a third rift initiates and propagates in the opposite direction to subduction rollback observations from the oligocene to recent western mediterranean the miocene to recent carpathians the miocene to recent aegean and the oligocene to recent caribbean point to a two stage process initially pairs of terranes comprising a pre existing retro arc fold thrust belt and magmatic arc rotate about poles and accrete to adjacent continents terrane docking reduces the width of the subduction zone leading to a second phase during which subduction to strike slip transitions initiate the clockwise rotated terrane is caught up in a dextral strike slip zone whereas the counterclockwise rotated terrane is entrained in a sinistral strike slip fault system the likely driving force is a pair of rotational torques caused by slab sinking and rollback of a curved subduction hingeline by analogy with the above model a revised five stage early jurassic to early cretaceous gondwana dispersal model is proposed in which three plates always separate about a single triple rift or triple junction in the weddell sea area seven features are considered diagnostic of double saloon door rifting and seafloor spreading i earliest movement involves clockwise and counterclockwise rotations of the falkland islands block and the ellsworth whitmore terrane respectively ii terranes comprise areas of a pre existing retro arc fold thrust belt the permo triassic gondwanide orogeny attached to an accretionary wedge magmatic arc the falklands islands block is initially attached to southern patagonia west antarctic peninsula while the ellsworth whitmore terrane is combined with the thurston island block iii paleogeographies demonstrate rifting and extension in a backarc environment relative to a pacific margin subduction zone accretionary wedge where simultaneous crustal shortening occurs iv a ridge jump towards the subduction zone from east of the falkland islands to the rocas verdes basin evinces subduction rollback v this ridge jump combined with backarc extension isolated an area of thicker continental crust the falkland islands block vi well documented ew oriented seafloor spreading anomalies in the weddell sea are perpendicular to the subduction zone and propagate in the opposite direction to rollback vii the dextral strike slip gastre and sub parallel faults form one boundary of the gondwana subduction rollback whereas the other boundary may be formed by inferred sinistral strike slip motion between a combined thurston island ellsworth whitmore terrane and marie byrd land east antarctica

investigation on solid liquid transition of soft mud under steady and oscillatory shear loads abstract an experimental study on the rheological properties of soft mud under steady and oscillatory shear was conducted the phenomenon of solid liquid transition is clarified by steady state and dynamic tests with an increase of shear load magnitude the rheological property of soft mud could be classified into three stages solid stage solid liquid transition stage and liquid stage this transition is found to be related to changes of the particle network structure during shear transitions between the three stages could be defined by two yield stresses stated as static yield stress and fluidic yield stress the first yield stress corresponds to the transition from solid stage to solid liquid transition stage associated with breakup of the initial particle network structure the second yield stress indicates the complete breakup of the aggregate beyond the fluidic yield stress the flow units becomes individual particles and the soft mud behaves like a viscous fluid to establish a relation between the rheological behavior of mud and its measurable quantities density and oscillatory frequency samples of different density 1107 1546 kg m3 were tested under various load combinations results indicate that both the static and fluidic yield stresses show exponential growth with density for samples of medium density 1356 1168 kg m3 there will be a sudden loss of strength associated with a decrease of stress at the beginning of the transition stage oscillatory frequency 0 05 2 05 hz has a neglectable influence at the solid stage but plays an important role at the other two stages moreover the nonlinear responses of soft mud under large amplitude oscillatory shear were also studied from the stress waveform and lissajous pattern by the fourier transformation method results show that nonlinearity dramatically increases at the transition stage

a revised fit of south america and south central africa abstract a new set of parameters for the total plate tectonic reconstruction of south america and south central africa is presented euler pole 46 75 n 32 65 w rotation angle 56 40 this fit is constrained by at least three pre drift tectonic features crossing from one continent to the other 1 the geophysically defined eastern and western boundaries of the submarine jurassic outeniqua basin south africa and the falkland plateau basin 2 the late precambrian transcurrent fault and mylonite belts of pernambuco brazil and foumban west africa and 3 the triassic northern tectonic front of the cape fold belt and the major morphological feature on the falkland plateau with which it is closely lined up isotopic ages of falkland plateau gneisses correspond to cape pluton and cape fold belt ages suggesting their palaeoposition was within the realm of the cape fold belt in addition the bathymetrically and geophysically defined northeastern apex of the falkland plateau fits into the re entrant angle defined on the south african margin by the steep southeast facing sheared agulhas margin and the southern face of the tugela cone simultaneously known precambrian outcrops in northeastern brazil and in the gulf of benin area of west africa are juxtaposed rather than overlapped reconstructions producing a closer fit of these cratonic areas are considered untenable

mid jurassic age for the botany bay group implications for weddell sea basin creation and southern hemisphere biostratigraphy new u pb zircon ion microprobe ages from the alluvial conglomerates and flood plain sediments of the botany bay group demonstrate that sedimentation occurred at c 167 ma coeval with rift related silicic volcanism in the northern antarctic peninsula in contrast rift related volcanism and sedimentation in the southern antarctic peninsula latady basin occurred at c 183 ma the new data indicate that syn rift sedimentation and volcanism was diachronous from south to north consistent with early opening of the weddell sea embayment by anti clockwise rotation of the antarctic peninsula in the mid jurassic a definitive date for the botany bay group floras has important implications for southern hemisphere biostratigraphic correlations

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

aeromagnetic data provide new insights on the volcanism and tectonics of vulcano island and offshore areas southern tyrrhenian sea italy 1 the active vulcano island southern tyrrhenian sea represents the southernmost portion of a nw se elongated volcanic ridge that includes also lipari and salina islands the ridge is affected by a regional nw se to n s striking fault system the elaboration and analysis of data from three high resolution aeromagnetic surveys carried out between 1999 and 2004 on vulcano and offshore allow us to recognize high intensity magnetic anomalies related to volcanic centers conduits or shallow intrusions previously unreported offshore submarine vents have been also recognized some of them may correspond with source areas of outcropping exotic pyroclastics on vulcano the spatial analysis of the recognized magnetic anomalies and volcanic structures shows that they are preferably aligned along the strikes of the main regional faults that affect the volcanic ridge submarine volcanic conduits revealed by the aeromagnetic survey might represent potential sources for future submarine effusive or explosive activity

when did the falklands rotate abstract similarities in the styles and relative timings of tectonic events in the outeniqua basin south africa and the north falkland basin suggest that basin formation in both regions may have preceded rotation of the falklands microplate contrary to previous models for the break up of gondwana which suggest jurassic rotation the data implies valanginian rotation contemporaneous with the first recorded motion on the agulhas falkland fracture zone and south atlantic rifting the data also suggests that the formation of the falkland plateau basin may also be a cretaceous event as opposed to the previously assumed jurassic age such a model is consistent with new offshore seismic evidence while the inconclusive nature of the supportive evidence for jurassic rotation does not exclude later rotation as a possibility

crustal structure of the filchner ronne shelf and coats land antarctica from gravity and magnetic data implications for the breakup of gondwana understanding the tectonic evolution of the weddell sea embayment provides valuable insights on the breakup of gondwana existing geodynamic models for this region differ mainly in the nature of the crust beneath the filchner ronne shelf and the movement of the ellsworth whitmore mountains ewm crustal block new gravity data presented here constrain the area and timing of this ewm movement from its prebreakup position to its present day position the crustal thickness estimates of 29 5 km and 27 4 km derived from power spectral analysis as well as forward gravity modeling make the existence of oceanic crust beneath the filchner ronne shelf unlikely a new crustal unit consisting of precambrian crust has been identified with gravity and magnetic data and extends 175 km off the coast of coats land this crustal unit conflicts with the late mesozoic movement of the ewm in this area the effective elastic thickness of t e 35 5 km has been estimated from coherence spectra for the ronne shelf together with the crustal thickness of 27 km from refraction seismic studies and comparison with theoretical models the thermal age is estimated to be between 165 and 230 ma for the ronne shelf this age marks the last significant tectonic event in this area and is in good agreement with the breakup of gondwana it further makes later movements of microplates like the ewm in this region unlikely thus a filchner block consisting of continental crust should be considered in any geodynamic model of the early weddell sea evolution

on the post 25 ma geodynamic evolution of the western mediterranean during the neogene and quaternary western mediterranean geodynamics were dominated by the eastward migration of the apenninic arc and associated back arc basins the migration was controlled by retreat of the apenninic slab and was associated with boudinage of the lithosphere in the back arc area palaeo reconstruction of the kinematics of the arc suggests about 775 km of migration from the late oligocene to present along a transect from the gulf of lions to calabria a maximum of 135 km of n s converge occurred between africa and europe during the same time span the western mediterranean was thus mainly shaped by the migration of the slab related to west directed subduction it is hypothesized that minor n s convergence deformed the arc but was not the cause of its formation 1998 elsevier science b v all rights reserved