boninitic and tholeiitic dykes in the lewis hills mantle section of the bay of islands ophiolite implications for magmatism adjacent to a fracture zone in a back arc spreading environment a detailed integrated field petrographic and geochemical study of the springers hill area of the bay of islands ophiolite exposed in the lewis hills was undertaken to explain the anomalously high abundance of veins and dykes of chromitite orthopyroxenite and clinopyroxenite and their associated dunites hosted by a refractory harzburgite dunite mixture a geodynamic situation is presented which is constrained by previous studies requiring formation of the springers hill mantle section at a ridge fracture zone intersection and the whole of the bay of islands ophiolite within a back arc spreading environment the veins and dykes formed during magmatism at the ridge fracture zone intersection and along the fracture zone as progressively hotter more fertile richer in clinopyroxene asthenosphere ascended and was channelled up and along the fracture zone wall shallow melting of refractory harzburgite in the presence of subduction derived hydrous fluids produced light rare earth element lree enric

reconstruction of the geometry of accretion during formation of the bay of islands ophiolite complex two types of highly allochthonous ophiolitic terrains crop out adjacent to one another in the bay of islands region of southwestern newfoundland canada the first called the coastal complex cc is exposed as a long linear belt along the west coast it consists of a highly deformed ophiolitic assemblage interpreted as oceanic crust and upper mantle with a previous transform fault tectonic history the second called the bay of islands complex boic crops out to the east and consists of a series of four relatively undeformed ophiolitic massifs interpreted as oceanic crust and upper mantle with a more normal seafloor spreading tectonic history an autochthonous relationship can be demonstrated between the two types of ophiolitic terrains in the southernmost of these massifs indicating that each evolved along a single ridge transform system the internal structure of both the boic and the cc is consistent with the above interpretations the ne sw structural trends in the cc transform are oriented approximately parallel to the sea floor spreading direction inferred from the average orientation of the sheeted diabase dikes within the boic one way chilling statistics from the sheeted diabase dike unit growth directions of dendritic crystals along inclined comb layers in the plutonic section and the sense of shear along the cc transform all indicate that the boic was accreted on the ne side of a nw se trending spreading center present geographic reference frame juxtaposition of the boic with the ne sw trending cc occurred at a ridge transform intersection the transform exhibited a left lateral offset magma chamber profiles have been identified in the layered gabbroic sections of the boic by using the patterns outlined by the attitudes of fine scale igneous layering these patterns depict arcuate structures which appear to define roughly a cross sectional view of a systematically oriented half cylinder the long axis of which is oriented parallel with the plane of the sheeted dikes and major lithologic unit contacts thus we believe that a large cylindrical shaped crustal level subaxial magma chamber extended in a direction parallel with the ridge axial plane at the time of formation of the bolc the cc marks the abrupt truncation of this long linear magma chamber petrologic constraints demonstrate that the ultramafic and transition zone cumulates which form the base of the plutonic section crystallized at a range of high to intermediate pressures possibly along a long narrow conduit extending to 30 km depth beneath the crustal level magma chamber after formation these cumulates were deformed at high temperatures in the zone of mantle upwelling and divergence then tectonically transported to crustal or near crustal levels to form the basement upon which the remainder of the ophiolite stratigraphy was accreted a geometric and tectonic model is presented for the accretion process responsible for formation of the bay of islands ophiolite

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

initiation of subduction zones along transform and accreting plate boundaries triple junction evolution and forearc spreading centres implications for ophiolitic geology and obduction

the pipestone pond complex central newfoundland complex magmatism in an eastern dunnage zone ophiolite ophiolitic rocks are preserved in both the notre dame and exploits subzones in the dunnage zone of the newfoundland appalachians ophiolites in the exploits subzone are generally less well preserved and exposed than their notre dame subzone counterparts and consequently have received less attention in the literature the pipestone pond complex is an exploits subzone ophiolitic sequence which outcrops on the western side of a structural window through the exploits subzone into the underlying gander zone it includes a basal harzburgite which passes upwards into a cumulate pyroxenite and gabbro sequence and thence into isotropic gabbro intruded by pegmatitic gabbro diabase and plagiogranite there is no sheeted dyke unit pillow lava occurs at the top of the sequence but is not observed to be in stratigraphic contact with the intrusive rocks the ophiolitic rocks are structurally disrupted and no single cross section traverses the complete ophiolitic stratigraphy although the stratigraphic sequence of

arc continent collisions sediment recycling and the maintenance of the continental crust author posting geological society of london 2009 this is the author s version of the work it is posted here by permission of geological society of london for personal use not for redistribution the definitive version was published in geological society london special publications 318 2009 75 103 doi 10 1144 sp318 3

mode of mixture of oceanic fragments and terrigenous trench fill in an accretionary complex example from southern sakhalin abstract four modes of mixture of oceanic materials such as cherts limestones and basalts and terrigenous trench fill sediments are recognized in the cretaceous accretionary complexes in southern sakhalin far eastern pacific margin of the u s s r the first mode is a thin skinned stacking of pelagic bedded cherts with black shale due to duplex formation in association with shallow underplating or off scraping the second mode is a mixture of the uppermost part of the oceanic crust with terrigenous trench fill sediments due to olistostrome formation reconstruction of the original sequence in ascending order suggests that the basement is pillow basalt with overlying pelagic sediments of chert and limestone olistostrome caused by collapse of oceanic crust due to its bending before encountering a trench and finally trenchfill sediments of turbidite this sequence is imbricated by thrusting which appears to have occurred at the time of underplating the third mode is a tectonic mixing of oceanic fragments and terrigenous sediments due to shear which is related to underthrusting of the oceanic plate a reduction in the supply of trench fill sediments causes copious accretion of the upper part of oceanic crust the fourth mode is in situ basalt volcanism at the trench where thick baked margins of terrigenous shale are formed around the injected basalts this mode appears to represent a ridge trench encounter these four different modes of mixing seem to be usual in ancient accretionary complexes in orogenic belts

the elastic properties of the lithosphere beneath scotian basin abstract to assess the possibility that the north atlantic ocean may subduct at scotian basin east of canada we investigate the present compensation state of this deep basin a fourier domain analysis of the bathymetry depth to basement and observed gravity anomalies over the oceanic area east of nova scotia indicates that the basin is not isostatically compensated moreover the analysis emphasizes that in addition to the sediments density perturbations exist beneath the basin the load produced by the sediments and these density perturbations must have been supported by the lithosphere we simulate the flexure of the lithosphere under this load by that of a thin elastic plate overlying an inviscid interior it is shown that a plate with a uniform rigidity does not adequately represent the lithosphere beneath the basin as well as the oceanic lithosphere far from the basin rather the rigidity of the lithosphere directly beneath the basin is about one to two orders of magnitude smaller than elsewhere we relate this weakening to the thermal blanketing effects of the thick sediments and the fact that the lithosphere has a temperature dependent rheology we suggest that this weak zone would have a controlling effect on the reactivation of normal faults at the hinge zone of the basin that were formed during the break up of africa and north america and were locked in the early stages after the break up the weak zone would facilitate reactivation of the faults if tensional stresses were produced by possible reorientation of the spreading direction of the north atlantic ocean in the future the reactivation of the faults would create a free boundary condition at the hinge zone allowing further bending of the lithosphere beneath the basin and juxtaposition of this lithosphere to the mantle beneath the continent this may provide a favorable situation for initiation of slow subduction due to subsequent compressional forces

inversion of synthetic geodetic data for dip slip faults clues to the effects of lateral heterogeneities and data distribution in geological environments typical of the apennines italy

the ordovician evolution of the south mayo trough western ireland the south mayo trough a broad synclinorium containing a 10 km sequence of early ordovician turbidites passing up into shallow water sandstones is interpreted as a forearc basin with an ophiolitic basement that formed a backstop to an accretionary prism on the northern edge of a north facing arc the arc collided with the laurentian margin in early llanvirn times to deform the dalradian supergroup rifted margin clastic sequence prior to the late llandovery sinistral transcurrent faulting transposed the connemara dalradian terrane to its present position on the outboard ocean ward side of the south mayo trough the south mayo trough was fed principally by arc and ophiolite derived elastics during the arenig with an increasing metamorphic component during the llanvirn the late ordovician transpressional docking of connemara provided the southerly source for high grade metamorphic clasts in the deny veeny formation

a parallochthonous group of sedimentary rocks unconformable overlying the bay of islands ophiolite complex north arm mountain newfoundland this paper reports and documents the existence on the north arm mountain massif of a parallochthonous group of sedimentary rocks that overlie the bay of islands ophiolite complex with marked angular unconformity some of these rocks are those previously regarded as oceanic sediments lying conformably on top of the igneous rocks of the ophiolite complex and some were mapped as part of the igneous rocks themselves the name crabb brook group is proposed for this sedimentary sequence and three new formation names are also put forward massive sedimentary breccias directly overlie and contain clasts from all units of the ophiolite complex evidence of intense predepositional weathering of the clasts and the underlying bedrock is common bedded shales and their disrupted equivalents in olistostromes containing sedimentary and igneous blocks overlie the breccias two fossil localities within the shales yield acritarchs of llanvirnian age and inarticulate brachiopods probably not older than this a small remnan

the moravka meteorite fall 1 description of the events and determination of the fireball trajectory and orbit from video records the moravka czech republic meteorite fall occurred on may 6 2000 11 52 ut during the daytime six h56 ordinary chondrites with a total mass of 1 4 kg were recovered the corresponding fireball was witnessed by thousands of people and also videotaped by 3 casual witnesses sonic booms were recorded by 16 seismic stations in the czech republic and poland and by one infrasonic station in germany a total of 2 5 of the fireball eyewitnesses reported electrophonic sounds satellites in earth orbit detected part of the fireball light curve in this first paper from a series of 4 papers devoted to the morvka meteorite fall we describe the circumstances of the fall and determine the fireball trajectory and orbit from calibrated video records moravka becomes one of only 6 meteorites with a known orbit the slope of the trajectory was 20 4 to the horizontal the initial velocity was 22 5 km s and the terminal height of the fireball was 21 km the semimajor axis of the orbit was 1 85 au the perihelion distance was 0 982 au and the inclination was 32 2 the fireball reached an absolute visual magnitude of 20 at a height of 33 km

structural study of intra oceanic thrusting in the klamath mountains northern california implications on accretion geometry the kinematics of the basal intra oceanic thrust of the trinity preston peak seiad and josephine ultramafic bodies of the klamath mountains are deduced from microstructural and mineral preferred orientation studies when corrected of possible postthrusting rotations these kinematics are thought to reflect the paleoconvergence geometry of the pacific plate relative to the north american continent at the time of intra oceanic thrusting we first detail the kinematics of thrusting for each studied massif these results are then integrated in a plate tectonics reconstruction and a model of continuous ne sw convergence for mesozoic times is proposed

geochemical evidence for sundering of the west mariana arc in miocene ash from the parece vela basin abstract glass and mineral fragments from discrete volcanic ash layers were sampled from dsdp ipod site 450 in the parece vela basin philippine sea and analyzed by electron microprobe the ashes are interpreted as eruptive products of the adjacent west mariana arc system between 25 and 14 ma b p and have compositions between basaltic andesite and rhyolite and rarely boninite continuous chemical trends appear to reflect mixing of mafic and silicic magmas discontinuous trends between these end members are relatively few and are consistent with liquid lines produced by fractional crystallization andesitic tephra become progressively richer in mgo and cao through the middle miocene while boninite appears towards the end of the sequence between 14 and 15 ma b p coeval rhyolitic glasses become richer in k 2 o and na 2 o with maximum concentrations at about 15 ma b p chronologic changes in fractionation type and composition of parent magmas are interpreted to reflect the subaerial volcanic evolution of the west mariana arc the appearance of boninite is believed to signal early stages of arc sundering and corresponds temporally with regional uplift of the sea floor above the carbonate compensation depth precursor to a new pulse of back arc spreading

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

ophiolite accretion in the lachlan orogen southeastern australia slivers of dismembered cambrian ophiolites are preserved in major fault zones of the turbidite dominated western and central lachlan orogen of southeastern australia geometrical chronological and metamorphic constraints indicate that these slivers did not undergo classic tethyan style obduction but were incorporated into evolving turbidite wedges as offscraped slices imbricated fault slivers or duplexes and as blocks in melange the processes involved include decoupling of the oceanic crustal stratigraphy predominantly above the moho formation and metamorphism of serpentinite talc matrix melange up to blueschist conditions and accretion via underplating or offscraping during underthrusting these processes have produced major large scale fault zones within the chevron folded turbidites that show marked along strike variation in structural complexity that relate to magnitude of displacement the fault zones may have formed preferentially where topographic highs occurred in the oceanic crust promoting fold and fault nucleation within irregularities in the layering these fault zones have predominantly brittle character due to their formation under a low geothermal gradient duplex like structures in the fault zones and the inferred development through buckling of then faulting of the oceanic lithosphere have analogues in modern oceanic environments e g indian ocean and young subduction systems e g sulu sea southeast asia they are interpreted to have formed due to convergence in a non collisional intra oceanic setting during closure of a cambrian backarc basin

constraints on plate tectonics initiation from scaling laws for single cell convection abstract the earth is the only planet known to have plate tectonics while other planets are covered with a stagnant lid on the earth the initiation of subduction which is thought to be the fundamental process for plate tectonics initiation is caused not only by the negative buoyancy of the lithosphere but also by the forces from plate motions however for planets which do not have plate tectonics the very first episode of lithospheric failure has to be caused by forces other than plate motions sublithospheric convection has been proposed as a possible mechanism that provides lithospheric instability through inducing stresses in the lithosphere and lithospheric failure can occur when the yield stress is below a critical value we test the applicability of scaling laws for the critical yield stress obtained in single cell convection simulations to strongly time dependent multi cell systems we show that with an appropriate choice of characteristic aspect ratio for the convective system the scaling laws from single cell simulations can be used to evaluate the conditions on the terrestrial planets in the inner solar system for plate tectonics to exist in agreement with previous studies the estimated values for critical yield stress and coefficient of friction are much lower than the expected values for the earth s lithosphere

early proterozoic oceanic crust in the northern colorado front range implications for crustal growth and initiation of basement faults 1 early proterozoic oceanic crust exposed in a shear zone in the northern colorado front range provides insight into the tectonic evolution of the colorado province tectonically intermingled mylonitic amphibolite locally pillowed metagabbro clinopyroxenite metachert and melange are preserved within the buckhorn creek shear zone west of fort collins whole rock geochemical data support formation of the igneous protoliths in an oceanic environment major and trace element data overlap samples from modern oceanic ridges and are distinct from modern arc trends metagabbros and clinopyroxenites preserve a subhorizontal lineation defined by igneous pyroxenes core and mantle microstructures indicate initial deformation at temperatures 900 c secondary amphiboles define both subhorizontal and subvertical lineations kinematic analysis indicates that the rocks experienced e w strike slip deformation prior to and during amphibolite facies metamorphism followed by n s shortening 40ar 39ar data from secondary hornblende yield age steps as old as 1 9 ga based on the chemical data and kinematic evidence for strike slip deformation we propose that the buckhorn creek rocks originated at an oceanic ridge transform intersection the high t pyroxene fabrics formed during ocean floor deformation prior to 1 7 ga regional deformation of the colorado province the buckhorn creek fragment likely represents original oceanic basement for 1 8 1 7 ga arc magmatism and sedimentation that ultimately formed the colorado province continental crust reactivation of original oceanic fracture zones may be responsible for the network of regularly spaced northeast trending shear zones in colorado once formed subvertical transform faults are likely to survive the transition from oceanic to continental crust and persist as long lived crustal discontinuities

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

reconstructing plate boundaries in the jurassic neo tethys from the east and west vardar ophiolites greece and serbia jurassic subduction initiation in the neo tethys ocean eventually led to the collision of the adria africa and eurasia continents and the formation of an 6 000 km long alpine orogen spanning from iberia to iran reconstructing the location and geometry of the plate boundaries of the now disappeared neo tethys during the initial moments of its closure is instrumental to perform more realistic plate reconstructions of this region of ancient ocean basins in general and on the process of subduction initiation neo tethyan relics are preserved in an ophiolite belt distributed above the dinaric hellenic fold thrust belt here we provide the first quantitative constraints on the geometry of the spreading ridges and trenches active in the jurassic neo tethys using a paleomagnetically based net tectonic rotation analysis of sheeted dykes and dykes from the west and east vardar ophiolites of serbia maljen and ibar and greece othris pindos vourinos and guevgueli based on our results and existing geological evidence we show that initial middle jurassic 175 ma closure of the western neo tethys was accommodated at a n s trending west dipping subduction zone initiated near and parallel to the spreading ridge the west vardar ophiolites formed in the forearc parallel to this new trench simultaneously the east vardar ophiolites formed above a second n s to nw se trending subduction zone located close to the european passive margin we tentatively propose that this second subduction zone had been active since at least the middle triassic simultaneously accommodating the closure of the paleo tethys and the back arc opening of neo tethys

the subduction plate interface rock record and mechanical coupling from long to short timescales abstract short and long term processes at or close to the subduction plate interface e g mineral transformations fluid release seismicity and more generally deformation might be more closely related than previously thought increasing evidence from the fossil rock record suggests that some episodes of their long geological evolution match or are close to timescales of the seismic cycle this contribution uses rocks recovered episodically from subduction zones together with insights from thermomechanical modelling to provide a new dynamic vision of the nature structure and properties of the plate interface and to bridge the gap between the mechanical behavior of active subduction zones e g coupling inferred from geophysical monitoring and fossil ones e g coupling required to detach and recover subducted slab fragments based on critical observations and an exhaustive compilation of worldwide subducted oceanic units for which the presence near the plate interface rock types pressure temperature t p gradients thickness and timing of detachment can be assessed the present study demonstrates how long term mechanical coupling exerts a key control on detachment from the slab and potential rock recovery critical assessment of rock t p characteristics indicates that these fragments can indeed be used as natural probes and provide reliable information on subduction interface dynamics down to 2 8 gpa rock clusters are identified at depths of 30 55 60 and 80 km with some differences between rock types data also reveal a first order evolution with subduction cooling in the first 5 myr which is interpreted as reflecting a systematic trend from strong to weak mechanical coupling after which subduction is lubricated and mostly inhibits rock recovery this contribution places bounds on the plate interface constitution regular thickness

dynamics of intraoceanic subduction initiation 1 oceanic detachment fault inversion and the formation of supra subduction zone ophiolites subduction initiation is a critical link in the plate tectonic cycle intraoceanic subduction zones can form along transform faults and fracture zones but how subduction nucleates parallel to mid ocean ridges as in e g the neotethys ocean during the jurassic remains a matter of debate in recent years extensional detachment faults have been widely documented adjacent to slow spreading and ultraslow spreading ridges where they cut across the oceanic lithosphere these structures are extremely weak due to widespread occurrence of serpentine and talc resulting from hydrothermal alteration and can therefore effectively localize deformation here we show geochemical tectonic and paleomagnetic evidence from the jurassic ophiolites of albania and greece for a subduction zone formed in the western neotethys parallel to a spreading ridge along an oceanic detachment fault with 2 d numerical modeling exploring the evolution of a detachment ridge system experiencing compression we show that serpentinized detachments are always weaker than spreading ridges we conclude that owing to their extreme weakness oceanic detachments can effectively localize deformation under perpendicular far field forcing providing ideal conditions to nucleate new subduction zones parallel and close to or at spreading ridges direct implication of this is that resumed magmatic activity in the forearc during subduction initiation can yield widespread accretion of suprasubduction zone ophiolites at or close to the paleoridge our new model casts the enigmatic origin of regionally extensive ophiolite belts in a novel geodynamic context and calls for future research on three dimensional modeling of subduction initiation and how upper plate extension is associated with that

high pressure micro inclusions and development of an inverted metamorphic gradient in the santiago schists ordenes complex nw iberian massif spain evidence of subduction and syncollisional decompression the santiago schists are located in the basal unit of the ordenes complex one of the allochthonous complexes outcropping in the inner part of the hercynian belt in the north west of the iberian peninsula their tectonothermal evolution is characterized by the development of an eo hercynian metamorphic episode c 374 ma of high p low to intermediate t the mineral assemblage of the high p episode is preserved as a very thin si s1 foliation included in albite porphyroblasts being composed of albite garnet i white mica 1 chlorite 1 epidote quartz rutile ilmenite the equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 10 c and 14 7 0 7 kbar probably minimum pressure the later evolution syn d2 of the schists defines a decompressive and slightly prograde p t path which reached its thermal peak at c 525 10 c and 7 kbar decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient so that the zones of garnet ii biotite with an upper subzone with chloritoid and staurolite developed from bottom to top of the formation the estimated p t path for the santiago schists suggests that the basal unit probably a fragment of the gondwana continental margin was uplifted immediately after its subduction at the beginning of the hercynian orogeny it also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension probably related to the continued subduction of the gondwana continental margin and the contemporaneous development of compensatory extension above it the inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin when it came into contact with the upper parts of the schists along a detachment probably of extensional character the general metamorphic evolution of the santiago schists with the development of high p assemblages with garnet prior to decompressive and prograde parageneses with biotite is unusual in the context of the european hercynian belt and shows a close similarity to the tectonothermal evolution of several high p low to intermediate t circum pacific belts

the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland the mesoarchean ca 3075 ma ivisaartoq greenstone belt contains well preserved primary magmatic structures such as pillow lavas volcanic breccias and clinopyroxene cumulate layers picrites despite the isoclinal folding and amphibolite facies metamorphism the belt also includes variably deformed gabbroic to dioritic dykes and sills actinolite schists and serpentinites the ivisaartoq rocks underwent at least two stages of post magmatic metamorphic alteration including seafloor hydrothermal alteration and syn to post tectonic calc silicate metasomatism between 3075 and 2961 ma these alteration processes resulted in the mobilization of many major and trace elements the trace element characteristics of the least altered rocks are consistent with a supra subduction zone geodynamic setting and shallow mantle sources on the basis of geological similarities between the ivisaartoq greenstone belt and phanerozoic forearc ophiolites and intra oceanic island arcs we suggest that the ivisaartoq greenstone belt represents a relic of dismembered mesoarchean supra subduction zone oceanic crust this crust might originally have been composed of a lower layer of leucogabbros and anorthosites and an upper layer of pillow lavas picritic flows gabbroic to dioritic dykes and sills and dunitic to wehrlitic sills the sm nd and u pb isotope systems have been disturbed in strongly altered actinolite schists in addition the u pb isotope system in pillow basalts appears to have been partially open during seafloor hydrothermal alteration gabbros and diorites have the least disturbed pb isotopic compositions in contrast the sm nd isotope system appears to have remained relatively undisturbed in picrites pillow lavas gabbros and diorites as a group picrites have more depleted initial nd isotopic signatures end 4 23 to 4 97 than pillow lavas gabbros and diorites end 0 30 to 3 04 consistent with a variably depleted heterogeneous mantle source in some areas gabbros include up to 15 cm long white inclusions xenoliths these inclusions are composed primarily 90 of ca rich plagioclase and are interpreted as anorthositic cumulates brought to the surface by upwelling gabbroic magmas the anorthositic cumulates have significantly higher initial end 4 8 to 6 0 values than the surrounding gabbroic matrix 2 3 to 2 8 consistent with different mantle sources for the two rock types

structure of the sheeted dike complex of the samail ophiolite near ibra oman the sheeted dike complex is a regionally mappable geologic unit with a consistent position in the samail ophiolite stratigraphy discontinuous exposures of the complex in the ibra area of the southeast oman mountains provide structural data that allow reconstruction of a paleo spreading ridge axis of 347 for the region and suggests a maximum spreading width of 275 km for the ophiolite contact relations within the well exposed al ahmadi section indicate that the complex was formed by multiple parallel intrusion over a narrow zone at the paleo ridge axis split dikes are abundant facing directions indicate a spreading ridge southwest of the present outcrop area in conflict with other spreading axis direction indicators

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

dynamics of intraoceanic subduction initiation 2 suprasubduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions analyzing subduction initiation is key for understanding the coupling between plate tectonics and the underlying mantle here we focus on suprasubduction zone ssz ophiolites and how their formation links to intraoceanic subduction initiation in an absolute plate motion frame ssz ophiolites form the majority of exposed oceanic lithosphere fragments and are widely recognized to have formed during intraoceanic subduction initiation structural petrological geochemical and plate kinematic constraints on their kinematic evolution show that ssz crust forms at fore arc spreading centers at the expense of a mantle wedge thereby flattening the nascent slab this leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during ssz crust crystallization former spreading centers are preserved in forearcs when subduction initiates along transform faults or off ridge oceanic detachments we show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone upon inception of slab pull due to e g eclogitization the sole is separated from the slab remains welded to the thinned overriding plate lithosphere and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge we propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust our study highlights how far field geodynamic processes and absolute plate motions may force intraoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

basalts erupted along the tongan fore arc during subduction initiation evidence from geochronology of dredged rocks from the tonga fore arc and trench a wide variety of different rock types were dredged from the tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 boomerang voyage 40ar 39ar whole rock and u pb zircon dating suggest that these fore arc rocks were erupted episodically from the cretaceous to the pliocene 102 to 2 ma the geochemistry suggests that mor type basalts and dolerites were erupted in the cretaceous that island arc tholeiites were erupted in the eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time the ages generally become younger northward suggesting that fore arc crust was created in the south at around 48 52 ma and was extended northward between 35 and 28 ma between 9 and 15 ma and continuing to the present day the episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the bonin marianas arc the bonin marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 ma at the beginning of subduction above a rapidly foundering west dipping slab instead a model where the 52 ma basalts that are presently in a fore arc position were created in the arc back arc transition behind the 57 35 ma loyalty three kings arc and placed into a fore arc setting after arc reversal following the start of collision with new caledonia is proposed for the oldest rocks in tonga this is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time

petrogenesis of a tholeiite boninite sequence from ayios mamas troodos ophiolite evidence for splitting of a volcanic arc lavas and intrusives from ayios mamas at the western end of the limassol forest complex south of the arakapas fault belt troodos comprise three groups a a basal sequence of pillow lavas intruded by dikes and sills with the phenocryst assemblage plag cpx mt ol b a middle sequence of pillows and massive flows cpx plag ol and c an upper sequence mostly of pillows and breccia ol opx cpx sp the lower group is tholeiitic and the upper two groups boninitic group c whole rock and glass mg numbers exceed 72 and range up to 78 with corresponding olivine compositions of fo92 they are strongly depleted in high field strength elements and on the basis of cr y variation and major element mass balances are interpreted to reflect partial melting of a basalt depleted cpx harzburgite source group a tholeiites in contrast are interpreted to derive from a more fertile lherzolite source magmaphile and compatible trace element variation is best modelled after an open magma system involving periodic replenishment tapping and fractionation although neither tholeiites nor boninites represent steady state compositions the boninites belong to a cao rich sio2 poor variant resembling those from guam but contrasting with those from bonin and cape vogel morb normalized enrichment of low field strength elements k sr ba and rb in all groups indicates subduction derived source contamination it is proposed that the ayios mamas section reflects splitting of a volcanic arc prior to back arc spreading in an environment analogous to that of the present day andaman sea

subduction origin on early earth a hypothesis i propose a hypothesis for the origin of subduction on early earth that is directly coupled with mantle upwelling and the formation of mafi c crust the hypothesis invokes spatial and temporal overlap of both endogenic and exogenic processes broad quasi cylindrical mantle upwelling and large bolide impact respectively leading to subduction and spreading two signature processes of modern plate tectonics the spatial and temporal intersection of these processes could have occurred variably across early earth and thus subduction could have begun at different locations and times globally the hypothesis postulates that the ability of a terrestrial planet to evolve plate tectonics results from a balance between the strength of its lithosphere and the size bolide it can attract and survive both factors to a fi rst order are a function of planet size

near ridge initiation of intraoceanic subduction effects of inheritance in 3d numerical models of the wilson cycle abstract despite its global significance much of the wilson cycle which infers kinematic inversion from plate divergence to convergence remains conjectural in particular for what concerns subduction initiation we present a high resolution 3d thermomechanical numerical model of the wilson cycle evolving from continental rifting through breakup and oceanic spreading to convergence and self consistent subduction initiation in the models intra oceanic subduction initiates off ridge and is an intrinsically three dimensional process its location is controlled by convergence induced ridge swelling and the subduction geometry is controlled by three main factors 1 the inherited compositional and thermal heterogeneity of the ridge and oceanic lithosphere from rifting and seafloor spreading 2 the curvature obliquity of the ridge with respect to the convergence direction and 3 the duration of transition from plate divergence to convergence the modelled mechanisms are consistent with geological records for example in the oman subduction obduction system and are undervalued elements of the wilson cycle

a tensile flexural model for the initiation of subduction we argue that subduction may be initiated at passive continental margins without shortening the lithosphere overcoming the lithosphere s high compressive strength requires special circumstances and these make it difficult to explain the nearly complete recycling of old sea floor instead we present a model that predicts tensile decoupling of the continental and oceanic lithosphere passive rifting and foundering of the seafloor beneath material welling up in the rift this occurs because the lithosphere in a new ocean basin establishes mechanical continuity with the continent at a depth comparable to mid ocean ridges later subsidence at the margin is therefore inhibited by flexure which implies shear stresses that promote fault slip and tensile stresses necessary to balance the component of the plate s weight directed down the margin slope we show that this tension can more than offset ridge push in our model an important additional tension arises from basal shear tractions resisting the plate s motion away from the mid ocean ridge although these tractions cannot be evaluated with confidence slip on a high angle fault decouples the oceanic and continental lithosphere when shear stresses arising from flexure and the applied tension exceed the lithosphere s shear strength under these loads a passive rift then forms allowing a mantle column to rise to the height of mid ocean ridges over 3 km above the old seafloor and flow onto the surface in a gravity current this load flexes the plate downwards which enhances the flow and lets the old oceanic lithosphere founder this model is consistent with the presence of oceanic material in continental forearcs and the youth of ophiolites when they are obducted as both might be explained by rifting a margin and underthrusting the juvenile crust formed there boninites in ophiolite complexes and tectonites at their bases show depleted and hydrated geochemistries consistent with the melting of rifted mantle lithosphere that receives volatiles from foundering oceanic crust

earthquake focal mechanism and oceanic thrust in easter microplate analogy with oman ophiolite previous work has suggested compression and thrust faulting along the northern boundary of the easter microplate due to its motion relative to the nazca plate inversion of the p and sh seismic waves related to a recent ms 7 1 earthquake located along the northern boundary of the easter microplate indicates a thin sliver of crust thrusting at a shallow angle towards the south giving support to this interpretation the oman ophiolite where similar thrusts have been mapped has been recently interpreted as being derived from similar microplate environment boudier et al 1997 this interpretation 1 provides possible tests to document the existence of oceanic thrusts near microplates and 2 suggests a new solution to the problem of how many ophiolites could have obtained oceanic detachment faults which will favour obduction

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

musings in tectonics i outline and discuss my career in the context of the history of structural geology and tectonics the progressive developments that led to plate tectonics the people who have encouraged and influ

development of concepts concerning the genesis and emplacement of tethyan ophiolites in the eastern mediterranean and oman regions this article uses evidence mainly from the eastern mediterranean and oman to help understand the origin and emplacement of ophiolites field studies in these regions emphasize the diversity of ophiolite related sedimentary and tectonic processes topics to be discussed here include the sedimentary covers of ophiolites their metamorphic soles related melanges and emplaced continental margin slope rise units modes of ophiolite emplacement will also be considered and tectonic reconstructions that have a direct bearing on the origin and emplacement of ophiolites the writer argues that there is compelling evidence that many ophiolites formed by spreading above intraoceanic subduction zones evidence from their intact sedimentary covers however indicates that such subduction was only incipient in most cases as directly associated volcanic arcs or back arc basins are rare metamorphic soles record fragments of oceanic lithosphere derived from settings ranging from open ocean to continental slope rise settings underlying melanges record the subduction of most of the ocean lithosphere formed at spreading ridges continental margin units beneath ophiolites demonstrate that these ophiolites were formed in ocean basins that were mainly initiated by continental rifting in the eastern mediterranean and oman regions ophiolite emplacement was commonly achieved by trench margin collision although strike slip transpression and large scale accretion along subducting margins played a role in some cases taking account of the mesozoic palaeotectonic setting of the eastern mediterranean region it is widely believed that ophiolites of two main ages mid jurassic and late cretaceous were formed in several small oceanic basins separated by rifted microcontinents the history of tethys in pre mesozoic time remains more open to question lastly giant subduction related ophiolites as in oman cyprus or the balkans may not be forming anywhere in the world ocean today but may instead relate to rare events in earth history when regional to global plate boundaries were drastically reorganised

thin ophiolites of north island new zealand abstract the subsurface geometry of ophiolite bodies which is indicative of their structural style and hence obduction history is commonly inferred from stratigraphie and structural data however new gravity data reported here illustrate how under favourable circumstances detailed subsurface forms can be modelled ophiolites in northern new zealand are shown to be particularly thin sheets

archaean crustal growth processes in southern west greenland and the southern superior province geodynamic and magmatic constraints abstract eo to mesoarchaean greenstone belts e g 3800 3700 ma isua c 3075 ma ivisaartoq 3071 ma qussuk occur within orthogneisses of the southern west greenland craton greenstone belts are composed mainly of metavolcanic rocks with minor ultramafic and sedimentary schists compositionally volcanic rocks are dominantly tholeiitic basalts boninites and picrites with minor intermediate to felsic volcanic rocks these greenstone belts appear to have formed in convergent margin geodynamic settings detailed field observations contrasting ages and metamorphic and structural histories suggest that this craton was assembled in several accretionary tectonothermal events involving accretion of arcs back arcs forearcs and continental fragments by horizontal tectonics the superior province of canada was also built by the amalgamation of oceanic and continental fragments ranging in age from 3700 to 2650 ma during five discrete tectonothermal events over 40 ma between 2720 and 2680 ma the neoarchaean 2750 2670 ma wawa greenstone belts are composed of tectonically juxtaposed fragments of oceanic plateaux oceanic island arcs back arcs and siliciclastic trench turbidites following juxtaposition these diverse lithologies were collectively intruded by syn to post kinematic granitoids with subduction zone geochemical signatures oceanic island arc lavas are easily distinguished from oceanic plateau counterparts because they possess positively fractionated rare earth element la sm cn 1 and gd yb cn 1 and high field strength element depleted nb th pm pm c 1000 km scale subduction accretion complex that formed along an intra oceanic convergent plate margin during trenchward migration of the magmatic arc axis several first order geological observations on archaean greenstone belts of sw greenland and the superior province suggest that phanerozoic style plate tectonic models can provide an elegant explanation for their structural lithological metamorphic and geochemical characteristics

synchronous formation of the metamorphic sole and igneous crust of the semail ophiolite new constraints on the tectonic evolution during ophiolite formation from high precision u pb zircon geochronology abstract the semail oman united arab emirates and other tethyan type ophiolites are underlain by a sole consisting of greenschist to granulite facies metamorphic rocks as preserved remnants of the underthrust plate sole exposures can be used to better understand the formation and obduction of ophiolites early models envisioned that the metamorphic sole of the semail ophiolite formed as a result of thrusting of the hot ophiolite lithosphere over adjacent oceanic crust during initial emplacement however calculated pressures from granulite facies mineral assemblages in the sole suggest the metamorphic rocks formed at 35 km depth and are too high to be explained by the currently preserved thickness of ophiolite crust and mantle up to 15 20 km we have used high precision u pb zircon dating to study the formation and evolution of the metamorphic sole at two well studied localities our previous research and new results show that the ophiolite crust formed from 96 12 95 50 ma our new dates from the sumeini and wadi tayin sole localities indicate peak metamorphism at 96 16 and 94 82 ma 0 022 to 0 035 ma respectively the dates from the sumeini sole locality show for the first time that the metamorphic rocks formed either prior to or during formation of the ophiolite crust and were later juxtaposed with the base of the ophiolite these data combined with existing geochemical constraints are best explained by formation of the ophiolite in a supra subduction zone setting with metamorphism of the sole rocks occurring in a subducted slab the 1 3 ma difference between the wadi tayin and sumeini dates indicates that in contrast to current models the highest grade rocks at different sole localities underwent metamorphism and may have returned up the subduction channel at different times

palaeomagnetic insights into the evolution of neotethyan oceanic crust in the eastern mediterranean abstract a synopsis of palaeomagnetic data from three late cretaceous eastern mediter ranean tethyan ophiolites troodos hatay and baer bassit and their sedimentary coversequences is presented these data provide valuable insights into the role of regional andlocal scale tectonic rotations in the geodynamic evolution of neotethyan oceanic crust thegeologically earliest phases of tectonic rotation are documented in the troodos ophiolite where rotations around both subvertical and subhorizontal axes are readily related to thedevelopment of the spreading fabrics and structures during crustal genesis subsequent c 74 anticlockwise intra oceanic rotation of a troodos microplate has been quantified throughanalysis of the in situ sedimentary cover of the troodos ophiolite results indicate that bulkanticlockwise rotation began soon after the cessation of spreading and ended by the end ofthe eocene with c 50 60 of microplate rotation being over by the maastrichtian the timeat which ophiolite thrust sheets were emplaced onto the arabian continental margin to theeast of troodos recent results from the emplaced structurally dismembered baer bassitophiolite indicate extreme anticlockwise rotations of ophiolitic thrust sheets varying on akilometre scale new data from the post emplacement sedimentary cover confirm that onlya small component of these rotations is due to post emplacement tectonism baer bassitrepresents the leading edge of the emplaced ophiolitic sheet new data from the morecoherent section preserved in the hatay ophiolite to the north demonstrate significantanticlockwise rotation this is equivalent to the rotation of the most northerly part of thebaer bassit units to the south and is of the same sense and magnitude as the pre maastrichtian phase of microplate rotation documented in the troodos this suggests acommon intra oceanic origin for the majority of the troodos and hatay rotations and asignificant component of the more variable rotations observed in baer bassit overall therefore the data support a model involving 1 intra oceanic rotation of a coherent regionof crust within the southern neotethyan basin this rotated unit is more areally extensivethan has previously been inferred from consideration of data from the troodos ophiolitealone 2 emplacement of part of the rotated unit onto the arabian platform 3 subsequentlocalized post emplacement modification related to the development of the current plateconfiguration

kinematics of late cretaceous subduction initiation in the neo tethys ocean reconstructed from ophiolites of turkey cyprus and syria formation of new subduction zones represents one of the cornerstones of plate tectonics yet both the kinematics and geodynamics governing this process remain enigmatic a major subduction initiation event occurred in the late cretaceous within the neo tethys ocean between gondwana and eurasia suprasubduction zone ophiolites i e emerged fragments of ancient oceanic lithosphere formed at suprasubduction spreading centers were generated during this subduction event and are today distributed in the eastern mediterranean region along three e w trending ophiolitic belts several models have been proposed to explain the formation of these ophiolites and the evolution of the associated intra neo tethyan subduction zone here we present new paleospreading directions from six upper cretaceous ophiolites of turkey cyprus and syria calculated by using new and published paleomagnetic data from sheeted dyke complexes our results show that nne ssw subduction zones were formed within the neo tethys during the late cretaceous which we propose were part of a major step shaped subduction system composed of nne ssw and wnw ese segments we infer that this subduction system developed within old triassic lithosphere along fracture zones and perpendicular weakness zones since the neo tethyan spreading ridge formed during gondwana fragmentation would have already been subducted at the pontides subduction zone by the late cretaceous our new results provide an alternative kinematic model of cretaceous neo tethyan subduction initiation and call for future research on the mechanisms of subduction inception within old and cold lithosphere and the formation of metamorphic soles below suprasubduction zone ophiolites in the absence of nearby spreading ridges

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

episodic formation of neotethyan ophiolites tibetan plateau snapshots of abrupt global plate reorganizations during major episodes of supercontinent breakup abstract neotethyan ophiolites along the bangong nujiang and yarlung tsangpo sutures tibetan plateau consist of mantle peridotite cumulates isotropic gabbro diabase plagiogranite and basaltic lava they comprise morb iab oib and boninitic rocks and mostly belong to supra subduction zone ssz type compositionally different rocks crystallized synchronously over a distance of 2000 km along the bangong nujiang and yarlung tsangpo sutures during the short periods of 190 182 and 132 120 ma respectively resembling the izu bonin mariana ibm proto arc the early jurassic bangong nujiang ophiolites formed in forearc settings during subduction initiation while the middle jurassic early cretaceous bangong nujiang ophiolites and the early cretaceous yarlung tsangpo ophiolites formed in forearc settings at pre existing subduction zones crystallization of the neotethyan ssz ophiolites from the eastern mediterranean middle east tibetan plateau to southeast asia and formation of the associated metamorphic rocks temporally coincided with major supercontinent breakup events 1 initial breakup of pangea in central atlantic and westernmost tethys 190 ma 2 separation of west and east gondwana 170 160 ma and initial opening of piemont ligurian tethys 175 164 ma 3 initial breakups of east and west gondwana 135 125 ma and 4 final breakup of gondwana 104 96 ma far field forces associated with abrupt global plate reorganizations may have triggered subduction initiation or transient interplate coupling decoupling at pre existing subduction zones leading to episodic formation of the neotethyan ssz ophiolites and rapid exhumation of metamorphic rocks in subduction zones the formation events of neotethyan ssz ophiolites and associated metamorphic rocks represent snapshots of global plate reorganizations during the major episodes of supercontinent breakup

tectonics and geology of spreading ridge subduction at the chile triple junction a synthesis of results from leg 141 of the ocean drilling program an active oceanic spreading ridge is being subducted beneath the south american continent at the chile triple junction this process has played a major part in the evolution of most of the continental margins that border the pacific ocean basin a combination of high resolution swath bathymetric maps seismic reflection profiles and drillhole and core data from five sites drilled during ocean drilling program odp leg 141 provide important data that define the tectonic structural and stratigraphic effects of this modern example of spreading ridge subduction a change from subduction accretion to subduction erosion occurs along strike of the south american forearc this change is prominently expressed by normal faulting forearc subsidence oversteepening of topographic slopes and intensive sedimentary mass wasting overprinted on older signatures of sediment accretion overthrusting and uplift processes in the forearc data from drill sites north of the triple junction sites 859 861 show that after an important phase of forearc building in the early to late pliocene subduction accretion had ceased in the late pliocene since that time sediment on the downgoing oceanic nazca plate has been subducted site 863 was drilled into the forearc in the immediate vicinity of the triple junction above the subducted spreading ridge axis here thick and intensely folded and faulted trench slope sediments of pleistocene age are currently involved in the frontal deformation of the forearc early faults with thrust and reverse kinematics are overprinted by later normal faults the chile triple junction is also the site of apparent ophiolite emplacement into the south american forearc drilling at site 862 on the taitao ridge revealed an offshore volcanic sequence of plio pleistocene age associated with the taitao fracture zone adjacent to exposures of the pliocene aged taitao ophiolite onshore despite the large scale loss of material from the forearc at the triple junction ophiolite emplacement produces a large topographic promontory in the forearc immediately after ridge subduction and represents the first stage of forearc rebuilding

initiation of subduction by small scale convection 1 the possibility of initiation of subduction by sublithospheric small scale convection is investigated with the help of fowler s theory numerical constraints on the lithospheric stresses generated by convection and direct numerical simulations of subduction the derived scaling laws show that the critical yield stress for initiation of subduction by small scale convection is of the order of 30 mpa this value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone this implies that small scale convection can play an important role in the initiation of subduction on the present day earth this also implies that small scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early earth and other terrestrial planets the critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the earth since the critical yield stress depends only weakly on planetary parameters water seems to be the key condition required for plate tectonics to occur on a terrestrial planet

a deep seismic reflection transect across the irish caledonides new reflection profiles recorded to 18s c 60km along a 600 km north south transect west of ireland wire lines provide a cross section of the irish caledonides from lewisian basement in the north to the variscan fold and thrust belt in the south and constrain the offshore extrapolations of major caledonide structures we interpret the profiles as showing the great glen fault and associated ne trending strike slip faults as near vertical structures which cut the whole crust and the uppermost mantle beneath the surface trace of the fair head clew bay line we image a north dipping zone which soles into horizontal layers just above the reflection moho the north dipping zone separates south dipping reflections above and to the north from north dipping reflections below and to the south from the fair head clew bay line as far south as the iapetus suture the lower crust contains north dipping packages of reflectors possibly related to acadian accretionary underplating the iapetus suture zone can be identified as a zone more than 50 km wide dipping north at 30 from 5 km depth offshore dingle peninsula to sole into the moho at c 30 km depth west of the shannon estuary in the upper crust the shallow part of the iapetus suture zone appears to be transported to the north by late caledonian retrocharriage or by variscan thrusting the iapetus suture does not penetrate through the present day reflection moho but there are scattered reflectors in the mantle not continuous with crustal reflectors perhaps relics of a subducted slab we recognize lower crustal reflectivity and the reflection moho along almost all the wire profiles regionally the travel time to the reflection moho varies from 9 s c 28 km offshore donegal northwest of ireland to 12 s c 35 km offshore kerry and cork southwest of ireland apparent moho offsets rapid lateral changes in the travel time to the reflection moho north of donegal and north of mayo are probably due to the great glen fault and associated faults the lower crustal reflectivity varies considerably in thickness and prevalent dip direction along the wire profiles in donegal bay this reflectivity is observed at depths less than 5 km c 1 5s whereas west of kerry prominent reflectivity is only observed beneath 6s c 17 km though the age of crustal reflectors cannot in general be determined beneath central ireland from donegal to dingle the lower crustal reflectors mimic in dip direction the structural vergence of the exposed dalradian and early palaeozoic rocks so that the lower crustal reflectivity patterns and probably much of the crust must be caledonian in age

comparative evolution of the iapetus and rheic oceans a north america perspective abstract differences in the origin and evolution of the iapetan and rheic oceans profoundly affected the development of the ouachitan appalachian variscan orogen the iapetus ocean was initiated in the late neoproterozoic early cambrian by the separation of large continental landmasses laurentia baltica gondwana whereas the rheic ocean originated in the late cambrian early ordovician as the result of the separation of several ribbon continents collectively termed peri gondwanan terranes from the northern margin of gondwana compared to the classic passive margin developed along the laurentian margin of iapetus passive margin successions to the rheic ocean are more limited in extent during convergence both margins of the iapetus ocean developed western pacific type arc back arc systems whereas subduction of the rheic ocean was primarily directed beneath laurentia and was andean in style closure of iapetus involved a number of arc back arc accretionary events and had occurred by the middle silurian as a result of laurentia baltica collision to form laurussia and the accretion of the peri gondwanan terranes rheic ocean closure occurred in the carboniferous by way of continent continent collision between laurussia and gondwana with gondwana on the lower plate within iapetus the formation of ophiolites occurred early in the orogenic cycle and was followed soon after by obduction hp lt metamorphism and foreland directed thrusting that is intimately related to accretionary events these features dominate the northern appalachian orogen where ht lp metamorphism is attributed to post accretionary slab break off events in contrast hp lt metamorphism within the rheic ocean reflects a type subduction during laurussia gondwana collision and is rapidly succeeded by ht lp metamorphism attributed to orogenic collapse ophiolites are rare and were formed either very early early ordovician or very late devonian carboniferous in the evolution of the ocean the early ordovician ophiolites were not obducted until the carboniferous at least 100 million years after their protoliths were formed rheic ocean closure resulted in the formation of large scale bivergent structures within the orogen with symmetry changing on either side of the rheic suture comparison of the evolution of the iapetus and rheic oceans suggests that differences in rift configuration passive margin development and subduction zone geometry within the two oceans profoundly influenced the subsequent style of orogenesis within the ouachita appalachian variscan orogen

paleomagnetic study of the ordovician table head group port au port peninsula newfoundland a paleomagnetic study of the ordovician table head group in the port au port peninsula of western newfoundland reveals a simple two component magnetization history comprising a reversely magnetized stable southeasterly remanence with a shallow to moderate inclination and an unstable present day overprint pole positions for the stable component both with and without tectonic tilt correction correspond with the early to middle ordovician pole positions for north america suggesting this remanence is early although the nature of the geomagnetic field in the ordovician is not well known the polarity observed is consistent with that reported from other mid ordovician studies and appears to reflect a predominance of reverse polarity for this time interval evidence of significant rotation of any of the sites studied is absent indicating that the continental margin in this region acted in an integral rather than a fragmented fashion during deformation previously published and new but preliminary resul