cambrian ordovician depositional sequences in the middle east a perspective from turkey abstract in southern turkey taurus chain or taurides and southeastern turkey border folds of the arabian plate nearly complete cambrian to ordovician successions are preserved four major sedimentary sequences are defined according to the main transgressive events and the subsequent shelf progradations these sequences allow us to link the turkish mostly distal lower palaeozoic detritals to the more proximal arabian formations in spite of contrasting facies the lower palaeozoic development is outlined through four successive steps defined in southern turkey 1 initiation of a cratonic platform regime terreneuvian i e earliest cambrian depositional sequence 1 with fluviatile conglomerates and red bed sediments associated with volcanics 2 development of a stable marine platform late terreneuvian to early ordovician depositional sequences 2 and 3 including widespread but diachronous middle cambrian carbonates and subsequent storm dominated detritals 3 tectonic instability middle ordovician leading to a new palaeogeography on the gondwana platform with the differentiation of a sag basin bedinan qasim depocenter lower part of depositional sequence 4 and 4 glaciation latest ordovician upper part of depositional sequence 4 identified by glaciomarine deposits forming the hirnantian glacial record this thick 2000 m detrital succession records a first order long term subsidence driven cambrian to ordovician transgressive trend and includes second order sequences reflecting interactions between global eustasy and tectonic instability on a larger scale the identification of recurrent sequence stratigraphic features with similar trends in the cambrian ordovician successions on the arabian platform implies that a former location of the taurus domain cannot be situated in the present day eastern mediterranean area but should lie directly north of the arabian platform this location places southern turkey at the junction between the north african segment of the gondwana siliciclastic shelf to the west present day coordinates and the eastern arabian segment in good agreement with mixed faunal signatures from both areas primary control of the tectonostratigraphic development of southern turkey during the lower palaeozoic may be a forced evolution from arc platform to rift drift transitions cordilleran model that has migrated progressively eastwards from the avalonian domain along the northern gondwana margin to the arabian domain throughout the lower palaeozoic

history of the west african neoproterozoic ocean key to the geotectonic history of circum atlantic peri gondwana adrar souttouf massif moroccan sahara abstract bordered by the archaean basement of the reguibat shield to the e and s the adrar souttouf massif is located in the southern regions of the moroccan sahara and represents the northern part of the mauritanide belt the central areas of this massif comprise the dayet lawda and sebkha matallah units that are mainly composed of mafic rocks ten samples taken from these rocks yielded 531 zircon grains that were analysed with respect to their morphology u th pb and lu hf isotope composition additionally 155 apatite grains from six samples were also dated by the u th pb method mostly well rounded zircon grains commonly showed bright cathodoluminescent cl overgrowth domains and leaching zones indicating metamorphic overprint around unaffected oscillatory zoned core areas all samples but one yielded two significant zircon age populations 605 ma and 634 ma with some inherited grains in the range of 650 740 ma and very scarcely up to 1190 ma for six samples the hf isotope composition suggests a major contribution of neoproterozoic juvenile magmas from depleted mantle source the zircon hf data for the remaining samples points to a predominant recycling of older archaean to palaeoproterozoic crust based on the geochemical composition of the rocks an island arc setting is assumed at the periphery of the west african craton close to the cryogenian ediacaran boundary subsequent metamorphism during accretion and partial obduction onto the basement rocks took place at about 605 ma a minor variscan overprint could be demonstrated for only one sample this case study exemplifies the great potential of the widely occurring metamorphosed mafic and ultramafic rocks along the western margin of the west african craton for palaeogeographic and geodynamic reconstructions including the peri gondwanan terranes during the late neoproterozoic

atmospheric methane variability centennial scale signals in the last glacial period in order to understand atmospheric methane ch4 biogeochemistry now and in the future we must apprehend its natural variability without anthropogenic influence samples of ancient air trapped within ice cores provide the means to do this here we analyze the ultrahigh resolution ch4 record of the west antarctic ice sheet divide ice core 67 2 9 8 ka and find novel atmospheric ch4 variability at centennial time scales throughout the record this signal is characterized by recurrence intervals within a broad 80 500 year range but we find that age scale uncertainties complicate the possible isolation of any periodic frequency lower signal amplitudes in the last glacial relative to the holocene may be related to incongruent effects of firn based signal smoothing processes within interstadial and stadial periods the peak to peak signal amplitudes vary in proportion to the underlying millennial scale oscillations in ch4 concentration the relative amplitude change is constant we propose that the centennial ch4 signal is related to tropical climate variability that influences predominantly low latitude wetland ch4 emissions

supercontinents and superplume events distinguishing signals in the geologic record abstract simultaneous supercontinent and superplume events may reinforce or cancel signals preserved in the geologic record alternatively one signal may overwhelm the other for instance relatively low sea level during a 2 7 ga superplume supercontinent event may reflect direct hits of superplumes beneath the supercontinent although both supercontinent formation and superplume events occurred at 0 28 1 9 and 2 7 ga global warming at these times indicates the dominance of superplume events in controlling climate enhanced deposition of black shales correlates with superplume events and with supercontinent breakup carbon isotopes in seawater however show positive excursions during supercontinent breakup at 2 2 2 1 and 0 8 0 6 ga but show mixed signals or no signal during other supercontinent or superplume events probably due to negative feedbacks peaks in marine organism originations at 100 280 and 480 ma correlate with possible superplume events whereas an overall decrease in origination rate in the early paleozoic correlates with the growth of pangea and destruction of shallow marine environments increased production rates of juvenile crust correlate with formation of supercontinents and with superplume events there may be two types of superplume events catastrophic events which are short lived

u pb geochronological constraints on the triassic jurassic ayu complex southern mexico derivation from the western margin of pangea a abstract we present la icp ms laser ablation inductively coupled plasma mass spectrometry u pb detrital and igneous zircon data of poly deformed metamorphic and igneous rocks of the ayu area southern mexico these rocks were previously inferred to be part of the late paleozoic acatlan complex but new age data indicate that they formed in the mesozoic and should be placed in the newly designated ayu complex the ayu complex comprises polydeformed metasedimentary rocks chazumba lithodeme of a turbidite like protolith that are intercalated with boudinaged ortho amphibolites with transitional arc to morb tholeiitic geochemistry in the south the metasedimentary sequence is affected by a ca 171 ma partial melting which formed the magdalena migmatite migmatization was accompanied by 171 168 ma intrusions of granodioritic dioritic and granitic dikes and sheets as well as pegmatite bodies which are characterized by inherited zircon populations of ca 260 290 320 360 420 480 880 990 and 1080 1250 ma that are also found in the chazumba lithodeme u pb detrital zircon dating of seven metasedimentary samples from the migmatized and unmigmatized chazumba lithodeme yielded youngest detrital zircons and clusters of 192 198 214 250 266 and 291 ma and are interpreted to reflect the late triassic middle jurassic deposition of turbiditic rocks the transitional arc tholeiitic geochemistry of the chazumba amphibolites is consistent with turbidite sedimentation in a back arc environment along a rifted passive margin close to a contemporaneous magmatic arc inferred flattening of the subduction zone led to subduction erosion during the early middle jurassic and underthrusting of the chazumba lithodeme to depths equivalent to amphibolite facies metamorphism steepening of the subducting slab and diachronous rifting within the gulf of mexico contributed to extensional tectonics recorded on the mexican mainland and facilitated the tectonic exhumation of the chazumba lithodeme by normal faulting along the reactivated providencia shear zone during the middle late jurassic more generally the documentation of arc back arc assemblages in the ayu complex requires deposition adjacent to a subducting ocean and thus supports a pangea a reconstruction that was synchronous with the breakup of pangea

structural history of ophiolite obduction bay of islands newfoundland obduction related structures in the ophiolitic bay of islands complex west newfoundland are classified into shearing dominated and accretion dominated mapping of these structures is combined with other geological constraints and provides the basis for a reconstruction of the obduction history of the ophiolite the earliest detachment structures are shearing dominated and are preserved in the basal peridotites they indicate north directed thrusting detachment was facilitated by the shallow position of the lithosphere asthenosphere boundary below the recently extinct bay of islands spreading ridge detachment involved lithosphere of the southern side and immediate northern side of the ridge during the early stages of detachment the coastal complex an arc related unit located to the west of the bay of islands complex acted largely as a lateral ramp to obduction early accretion occurred in two tectonic situations 1 at the stage when the basal decollement plane intersected the boundary to the structurally and lithologically diverse coastal complex this resulted in accretion of blocks up to several hundred meters thick with a lithological make up not known from the bay of islands complex and 2 at the stage when the basal thrust plane intersected the oceanic crust deformation switched into the mafic and hydrated footwall lithologles and led to intermittent accretion of mafic oceanic crustal rocks now preserved in the metamorphic sole only the lewis hills massif located close to the coastal complex during detachment preserves evidence for both types of early accretion at a later stage of obduction localized removal of the basal sequence of the ophiolite was associated with accretion of a diverse assemblage of harzburgites amphibolites and greenschists

origin of the rheic ocean rifting along a neoproterozoic suture the rheic ocean is widely believed to have formed in the late cambrian early ordovician as a result of the drift of peri gondwanan terranes such as avalonia and carolina from the northern margin of gondwana and to have been consumed in the devonian carboniferous by continent continent collision during the formation of pangea other peri gondwanan terranes e g armorica ossa morena northwest iberia saxo thuringia moldanubia remained along the gondwanan margin at the time of rheic ocean formation differences in the neoproterozoic histories of these peri gondwanan terranes suggest the location of the rheic ocean rift may have been inherited from neoproterozoic lithospheric structures formed by the accretion and dispersal of peri gondwanan terranes along the northern gondwanan margin prior to rheic ocean opening avalonia and carolina have sm nd isotopic characteristics indicative of recycling of a juvenile ca 1 ga source and they were accreted to the northern gondwanan margin prior to voluminous late neoproterozoic arc magmatism in contrast sm nd isotopic characteristics of most other peri gondwanan terranes closely match those of eburnian basement suggesting they reflect recycling of ancient 2 ga west african crust the basements of terranes initially rifted from gondwana to form the rheic ocean were those that had previously accreted during neoproterozoic orogenesis suggesting the rift was located near the suture between the accreted terranes and cratonic northern gondwana opening of the rheic ocean coincided with the onset of subduction beneath the laurentian margin in its predecessor the iapetus ocean suggesting geodynamic linkages between the destruction of the iapetus ocean and the creation of the rheic ocean

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

evolution of the rheic ocean the rheic ocean which separated laurussia from gondwana following the closure of iapetus is arguably the most important ocean of the palaeozoic its suture extends from mexico to turkey and its closure produced the climactic variscan alleghanian ouachita orogeny that assembled the supercontinent pangaea following protracted cambrian rifting that represented a continuum from neoproterozoic orogenic processes the rheic ocean opened in the early ordovician with the separation of several neoproterozoic arc terranes from the continental margin of northern gondwana separation occurred along the line of a former neoproterozoic suture following the onset of subduction in the outboard iapetus ocean the timing of rift drift transition and drive for subsequent spreading was likely governed by slab pull accounting for the rapid rate 8 10 cm yr at which the rheic ocean widened during the ordovician the ocean broadened at the expense of iapetus and attained its greatest width 4000 km in the silurian by which time baltica had sutured to laurentia and the neoproterozoic arc terranes had accreted to laurussia closing iapetus in the process closure of the rheic ocean began in the devonian and was facilitated by northward subduction beneath southern baltica and southward subduction beneath northwest gondwana closure was largely complete by the mississippian as gondwana and laurussia sutured to build pangaea north africa colliding with southern europe to create the variscan orogen in the devonian carboniferous and west africa and south america suturing to north america to form the alleghanian and ouachita orogens respectively during the carboniferous permian the rheic ocean consequently plays a dominant role in the basement geology of southern europe in the appalachian ouachita orogeny of north america and in the palaeozoic sedimentary structural and tectonothermal record from middle america to the middle east with its closure the ocean brought about the assembly of pangaea and brought the palaeozoic era to an end

acatlan complex southern mexico record spanning the assembly and breakup of pangea new structural geochronological and geochemical data from the acatlan complex of southern mexico show that it preserves a complete history of pangea from assembly to breakup previously interpreted to be a vestige of the iapetus suture the acatlan complex records a history that can be sequentially linked to the rheic ocean the paleo pacific and the gulf of mexico this record is interpreted to reflect 1 the development of a rift passive margin on the southern flank of the rheic ocean in the cambrian ordovician 2 the formation of an extensional regime along the formerly active northern margin of gondwana throughout the ordovician 3 closure of the rheic ocean documented by subduction related eclogite facies metamorphism and exhumation during the late devonian mississippian 4 permian triassic convergent tectonics on the paleo pacific margin of pangea and 5 interaction with a jurassic mantle plume coeval with the opening of the gulf of mexico

u pb geochronology and petrology of the late paleozoic gil marquez pluton magmatism in the variscan suture zone southern iberia during continental collision and the amalgamation of pangea the origin of plutonic complexes that stitch suture zones developed during collision is not well understood in southern iberia the pulo du lobo suture zone pdlz is intruded by the syn to postcollisional gil marquez pluton gmp thought to be part of the sierra norte batholith u pb la icpms zircon data on various phases of the gmp yield from oldest to youngest 1 a 354 4 7 6 ma unfoliated gabbro 2 a 345 6 2 5 ma foliated intermediate phase 3 a 346 5 5 4 ma unfoliated porphyritic granite 4 a 335 1 2 8 ma unfoliated biotite granite this sequence is consistent with cross cutting relationships observed in the field the range in ages is consistent with interpretations that the gmp is part of the composite ca 350 308 ma snb inherited ages preserved in the gmp intermediate and felsic phases indicate that its magmas traversed through south portuguese zone and pdlz crust during emplacement the ca 345 ma emplacement of the late kinematic foliated intermediate phase constrains the age of late stage strike slip deformation within the pdlz and the lack of a foliation in the older gabbro indicates that is was not proximal to a shear zone neither at the time of emplacement nor during its subsequent history the unfoliated porphyritic granite and unfoliated biotite granite cut the foliation of the intermediate phase indicating emplacement during the waning stages of collision while the ca 335 ma biotite granite intrudes the santa ira flysch thereby providing a tight constraint for the latest stage of deformation in the pdlz

abrupt climate oscillations during the last deglaciation in central north america evidence from stable isotopes and a variety of proxies from two ontario lakes demonstrate that many of the late glacial to early holocene events that are well known from the north atlantic seaboard such as the gerzensee killarney oscillation also known as the intra allerod cold period younger dryas and preboreal oscillation also occurred in central north america these results thus imply that climatic forcing acted in the same manner in both regions and that atmospheric circulation played an important role in the propagation of these events

nature and origin of the volcanic ash beds near the permian triassic boundary in south china new data and their geological implications permian triassic boundary ptb volcanic ash beds are widely distributed in south china and were proposed to have a connection with the ptb mass extinction and the assemblage of pangea however their source and tectonic affinity have been highly debated we present zircon u pb ages trace element and hf isotopic data on three new found ptb volcanic ash beds in the western hubei area south china laser ablation inductively coupled plasma mass spectrometry u pb dating of zircons yields ages of 252 2 3 6 ma 251 6 4 9 ma and 250 4 2 4 ma for these three volcanic ash beds zircons of age c 240 270 ma zircons have negative t values 18 17 to 3 91 and mesoproterozoic palaeoproterozoic two stage hf model ages t 1 33 2 23 ga integrated with other ptb ash beds in south china zircon trace element signatures and hf isotopes indicate that they were likely sourced from intermediate to felsic volcanic centres along the simao indochina convergent continental margin the qinling convergent continental margin might be another possible source but needs further investigation our data support the model that strong convergent margin volcanism took place around south china during late permian early triassic time especially in the simao indochina active continental margin and possibly the qinling active continental margin these volcanisms overlap temporally with the ptb biocrisis triggered by the siberian large igneous province in addition our data argue that the south china craton and the simao indochina block had not been amalgamated with the main body of pangea by late permian early triassic time

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

two stage collision exploring the birth of pangea in the variscan terranes abstract the variscan suture exposed in nw iberia contains a stack of terranes including two allochthonous units with continental affinity and gondwanan provenance upper and basal units separated by an ophiolite belt where the most common units show protolith ages at c 395 ma recent lu hf zircon data obtained from these ophiolites indicate interaction between the gabbroic magmas and old continental crust hence the ophiolites could not have originated in a deep ocean basin associated with a mature mid ocean ridge or intraoceanic subduction the tectonothermal evolution of the continental terranes bounding the suture zone records two consecutive events of deep subduction the upper units record an initial high p ultra high p metamorphic event that occurred before 400 390 ma while the basal units were affected by a second high p low to intermediate t metamorphic event dated at c 370 ma continental subduction affected the most external margin of gondwana and developed in a setting of dextral convergence with laurussia development of the two high p events alternated with the opening of an ephemeral oceanic basin probably of pull apart type in early devonian times this ephemeral oceanic domain is suggested as the setting for the protoliths of the most common ophiolites involved in the variscan suture current ideas for the assembly of pangea advocate a single collisional event between gondwana and laurussia in the carboniferous however the new evidence from the allochthonous terranes of the variscan belt suggests a more complex scenario for the assembly of the supercontinent with an interaction between the colliding continental margins that started earlier and lasted longer than previously considered based on modern analogs of continental interaction the development of complex collisions as here suggested for gondwana and laurussia during the assembly of pangea could have been the norm rather than the exception throughout earth history

allochthonous terranes involved in the variscan suture of nw iberia a review of their origin and tectonothermal evolution abstract nw iberia includes a rather complete section of a variscan suture where different terranes with continental or oceanic affinities appear with clear structural relationships three groups of terranes namely upper ophiolitic and basal units and a frontal tectonic melange appear in galicia in cabo ortegal ordenes and malpica tui complexes they constitute a huge allochthonous pile thrust over the iberian parautochthonous and autochthonous domains which represent the section of the gondwanan margin that escaped continental subduction during the variscan cycle considering the allochthonous character of the nappe pile and the strong deformation associated to the variscan collision there are problems to identify the original tectonic setting of the terranes and thence it is difficult to reconstruct the paleogeographic context during the variscan and pre variscan times in detail key features to perform any model for the variscan convergence should consider the existence of two different high p metamorphic events dated at c 400 and 370 ma respectively separated in time for the generation of mafic ultramafic sequences at c 395 ma which constitute the most common ophiolites described in the variscan suture some dynamic models developed in nw iberia have important problems to explain the observed tectonothermal evolution especially the older high p metamorphic event and the exhumation of deeply subducted transitional type sections on the other hand the recently discovered participation of an older continental crust in the generation of different protoliths of the middle devonian ophiolites makes difficult the interpretation of these ophiolites in relation to open wide oceanic domains this paper describes the distribution structure lithologies geochronology and chemical composition of the terranes involved in the variscan suture of nw spain the scope of this description ranges from detailed regional aspects to the discussion of the development of the variscan orogen in the context of the assembly of pangea a two stage collisional model affecting a wide gondwanan platform may explain most of the evidences in nw iberia the generation of a long pull apart basin probably occurred after the first collision where the devonian ophiolites were formed these ophiolites have been in general described in the context of the rheic ocean but according to the new existing data they do not seem related to this ocean but rather they were formed after its closure

evidence for forearc seafloor spreading from the betts cove ophiolite newfoundland oceanic crust of boninitic affinity abstract the ordovician betts cove ophiolite of newfoundland has a well developed cumulate sequence in which is rooted a sheeted dyke complex that grades up into pillow lavas dominant chromite olivine orthopyroxene cumulate peridotites and orthopyroxenites have phase assemblages and mineral chemistries consistent with crystallization from boninitic magmas dykes and lavas have phenocrysts of olivine high cr al chromite orthopyroxene low tio2 clinopyroxene they have high sio2 and mgo contents and depleted u shaped trace element profiles indistinguishable from those of bonin islands boninites field data imply that cumulates dykes and lavas all are comagmatic while geochemical and mineralogical data indicate that all are of boninitic affinity since boninites are only found in forearcs this implies that the betts cove ophiolitic crust formed in a forearc since the entire oceanic crustal section at betts cove is of boninitic affinity then this implies that true seafloor spreading can initiate in forearc

the pangea conundrum geodynamic models for supercontinent assembly whereby the dispersing continental fragments of a supercontinent break up and migrate from geoid highs to reassemble at geoid lows fail to account for the amalgamation of pangea such models would predict that the oceans created by continental breakup in the early paleozoic e g iapetus rheic would have continued to expand as the continents migrated toward sites of mantle downwelling in the paleo pacific reassembling an extroverted supercontinent as this ocean closed instead pangea assembled as a result of the closure of the younger iapetus and rheic oceans geodynamic linkages between these three oceans preserved in the rock record suggest that the reversal in continental motion may have coincided with the ordovician emergence of a super plume that produced a geoid high in the paleo pacific if so the top down geodynamics used to account for the breakup and dispersal of a supercontinent at ca 600 540 ma may have been overpowered by bottom up geodynamics during the amalgamation of pangea

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

comment on chronostratigraphy and paleoclimatology of the lodeve basin france evidence for a pan tropical aridification event across the carboniferous permian boundary by michel l a tabor n j montanez i p schmitz m d davydov v i 2015 palaeogeography palaeoclimatology palaeoecology 430 118 131

the future of earth s oceans consequences of subduction initiation in the atlantic and implications for supercontinent formation subduction initiation is a cornerstone in the edifice of plate tectonics it marks the turning point of the earth s wilson cycles and ultimately the supercycles as well in this paper we explore the consequences of subduction zone invasion in the atlantic ocean following recent discoveries at the sw iberia margin we discuss a buoyancy argument based on the premise that old oceanic lithosphere is unstable for supporting large basins implying that it must be removed in subduction zones as a consequence we propose a new conceptual model in which both the pacific and the atlantic oceans close simultaneously leading to the termination of the present earth s supercycle and to the formation of a new supercontinent which we name aurica our new conceptual model also provides insights into supercontinent formation and destruction supercycles proposed for past geological times e g pangaea rodinia columbia kenorland

determining continuous basins across conjugate margins the east orphan porcupine and galicia interior basins of the southern north atlantic ocean abstract the east orphan porcupine and galicia interior basins are several of the most promising basins for hydrocarbon exploration along the rifted continental margins of the southern north atlantic ocean despite having formed at similar geological times the basins exhibit fundamental differences in symmetry crustal thickness sedimentary cover thickness and amount of extension interpretation of seismic reflection and well data was integrated with published 3d grids of depth to basement and moho proxy depth to interpret and restore select seismic reflection lines publicly available kinematic evolution models were also integrated to evaluate and compare the restored seismic reflection lines in a more global regional context interpretation of five seismo stratigraphic units and three tectonostratigraphic megasequences along the east orphan porcupine and galicia interior basins reveals similar seismic character for each unit and comparable tectonic history the structural restoration of the selected lines indicates that evolution sedimentary cover thickness faulting style crustal structure and kinematic evolution of the east orphan and porcupine basins differ significantly a variable and asymmetrical crustal structure is found in the east orphan basin contrasting with the elongated and symmetric porcupine and galicia interior basins rift domain maps of the three basins reveal that they are each underlain by hyperextended crust with possible exhumed mantle in the centre of the porcupine basin based on a holistic analysis of the results obtained the linkage between the east orphan and the porcupine basins seems implausible but rather a contemporaneous relationship is interpreted moreover a potential connection between the galicia interior basin and the porcupine basin during the early to late jurassic is proposed such scenarios imply an oblique and synchronous rifting evolution around the bay of biscay triple junction of the southern north atlantic

evidence for active subduction beneath gibraltar we report on a marine seismic survey that images an active accretionary wedge west of gibraltar ramp thrusts offset the seafloor and sole out to an east dipping decollement indicating ongoing westward vergent tectonic shortening new traveltime tomographic re sults image a slab of oceanic lithosphere descending from the atlantic domain of the gulf of cadiz passing through intermediate depth 60 120 km seismicity beneath the west ernmost alboran sea and merging with a region of deep focus earthquakes 600 660 km below granada spain together these new data provide compelling evidence for an active east dipping subduction zone

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

highly depleted oceanic lithosphere in the rheic ocean implications for paleozoic plate reconstructions the rheic ocean formed at ca 500 ma when several peri gondwanan terranes e g avalonia and carolinia drifted from the northern margin of gondwana and were consumed during the late carboniferous collision between laurussia and gondwana a key event in the formation of pangea several mafic complexes ranging in age from ca 400 330 ma preserve many of the lithotectonic and or chemical characteristics of ophiolites they are characterized by anomalously high end values that are typically either between or above the widely accepted model depleted mantle curves these data indicate derivation from a highly depleted hd mantle and imply that i the mantle source of these complexes displays time integrated depletion in nd relative to sm and ii depletion is the result of an earlier melting event in the mantle from which basalt was extracted the extent of mantle depletion indicates that this melting event occurred in the neoproterozoic possibly up to 500 million years before the rheic ocean formed if so the mantle lithosphere that gave rise to the rheic ocean mafic complexes must have been captured from an adjacent older oceanic tract the transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved possible mesozoic cenozoic analogues include the capture of the caribbean plate or the scotia plate from the pacific to the atlantic oceanic realm our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the rheic ocean was consumed by subduction during laurentia gondwana convergence

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

progressive magmatism and evolution of the variscan suture in southern iberia magmatic activity is an integral component of orogenic processes from arc magmatism during convergence to post collisional crustal melting southern iberia exposes a late paleozoic suture zone within pangea and where a crustal fragment of laurussia south portuguese zone is juxtaposed with parautochthonous gondwana ossa morena zone fault bounded oceanic metasedimentary rocks melanges and ophiolite complexes characterize the suture zone and are intruded by plutonic rocks and mafic dykes the generation and emplacement of these intrusive rocks and their relationship to development of the suture zone and the orogen are undetermined field evidence combined with u pb zircon geochronology reveals three main phases of plutonism a pre collisional unfoliated gabbroic phase emplaced at ca 354 ma crosscut by a syn tectonic ca 345 ma foliated granodiorite phase followed by a ca 335 ma granitic phase geochemical analyses major trace rare earth elements indicate that the gabbro exhibits a calc alkaline arc signature whereas the granodiorite and granite are typical of post collisional slab break off taken together these data demonstrate a protracted development of the orogen and support a complex late stage evolution broadly similar to the tectonics of the modern eastern mediterranean in this scenario the highly oblique closure of a small tract of oceanic lithosphere postdates the main collision event resulting in escape of parautochthonous and allochthonous terranes toward the re entrant

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