a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

thermochronology and tectonics of the merida andes and the santander massif nw south america abstract new apatite u pb and multiphase 40ar 39ar data constrain the high to medium temperature 500 c 300 c thermal histories of igneous and metamorphic rocks exposed in the merida andes of venezuela and new apatite and zircon fission track data constrain the 500 c 60 c thermal histories of pre jurassic igneous and metamorphic rocks of the adjacent santander massif of colombia computed thermal history envelopes using apatite u pb dates and grain size information from an early palaeozoic granodiorite in the merida andes suggest that it cooled from 500 c to the generally accepted timing of amalgamation of pangaea along the ouachita marathon suture pre dates late permian to triassic cooling recorded in basement rocks of the merida andes by 30 ma and its effect on rocks preserved in north western south america is unknown we interpret late permian to triassic cooling in the merida andes to be driven by exhumation previous studies have suggested that a short phase of shortening and anatexis is recorded at 253 ma in the maya block which may have been adjacent to the basement rocks of the merida andes in the late permian the coeval onset of exhumation in the merida andes may be a result of increased coupling in the magmatic arc which was located along the western margin of pangaea triassic extension is documented in the central cordillera of colombia and ecuador between 240 ma and 215 ma although extension at this time has not been clearly identified in the merida andes or the santander massif permian to triassic cooling is not recorded in the structurally isolated caparo block in the southern merida andes suggesting that it may have constituted a distinct fault block in the triassic new fission track data from the santander massif suggest that it started exhuming at 40 ma during a period of accelerated convergence between the nazca farallon plate and the western margin of south america exhumation in the santander massif occurred diachronously since 18 ma in distinct fault blocks at rates of 0 5 1 km ma and may have been driven by east west compression as a result of the indentation of the panama choco terrane to western colombia

new caledonia obducted peridotite nappe offshore extent and implications for obduction and postobduction processes one of the largest ophiolitic peridotite masses in the world covers a quarter of the island of grande terre new caledonia the peridotite nappe was obducted during the eocene is weakly deformed and corresponds to the highest of a structurally simple pile of thrust nappes we present new marine seismic data that allows us to track the offshore continuation of the peridotite nappe along strike for a distance of more than 500 km south of new caledonia and to image its pre syn and post obduction sedimentary records offshore the peridotite nappe underlies a c 150 km wide and 2 km deep basin flat topped horsts of peridotite are clearly bounded by major normal faults in contrast faults are obscure onland to the east the peridotite nappe roots along the eastern margin of the felicite ridge new name a c 300 x 25 km dome shaped ridge which we interpret as being the southern extension of the hp lt metamorphic core complex observed in new caledonia two alternative tectonic models address the relative timing and relationships between peridotite nappe emplacement uplift of a metamorphic core complex and extensional tectonics these models provide new ideas for the understanding the formation of the eastern margin of the zealandia continent our results contribute to an understanding of how oceanic mantle is emplaced onto continental margins

the metamorphic sole of new caledonia ophiolite 40ar 39ar u pb and geochemical evidence for subduction inception at a spreading ridge amphibolite lenses that locally crop out below the serpentinite sole at the base of the ophiolite of new caledonia termed peridotite nappe recrystallized in the high temperature amphibolite facies and thus sharply contrast with blueschists and eclogites of the eocene metamorphic complex amphibolites mostly display the geochemical features of morb with a slight nb depletion and thus are similar to the youngest late paleocene eocene babb components of the allochthonous poya terrane thermochronological data from hornblende 40ar 39ar zircon and sphene u pb suggest that these mafic rocks recrystallized at 56ma using various geothermobarometers provides a rough estimate of peak recrystallization conditions of 0 5gpa at 800 950c the thermal gradient inferred from the metamorphic assemblage 60 ckm 1 geometrical relationships and geochemical similarity suggest that these mafic rocks belong to the oceanic crust of the lower plate of the subduction obduction system and recrystallized when they subducted below young and hot oceanic lithosphere they were detached from the down going plate and finally thrust onto unmetamorphosed poya terrane basalts this and the occurrence of slab melts at 53ma suggest that subduction inception occurred at or near to the spreading ridge of the south loyalty basin at 56ma 2012 american geophysical union all rights reserved

post eocene extensional tectonics in southern new caledonia sw pacific insights from onshore fault analysis and offshore seismic data ductile to brittle extensional deformation following thrusting of the peridotites nappe during the upper eocene has been shown to play a major role in the tertiary tectonic evolution of the northern part of the main island of new caledonia and its eastern and western margins in this study we provide new tectonic data from southern new caledonia that allow to better constrain the tectonic evolution of the southern part of the main island we present a kinematic analysis of faults and striations obtained mainly from exposures of sedimentary rocks in the region of noumea with complements from measurements made farther north at nepoui within post obduction middle miocene deposits we also present additional results of an interpretation of seismic lines from the lagoon south of the noumea peninsula which provide constraints on the current tectonic regime of southern new caledonia extensional faults in the noumea region have been studied within terranes of various ages including pre and syn obduction deposits and ophiolites hence we demonstrate that important extensional events have affected the southern part of the new caledonia block after the obduction of the peridotite nappe the direction of maximum extension is variable at the scale of the region both high angle and low angle normal faults are present and block rotation is observed at some localities this suggests that detachments accommodating significant displacements are cutting through the sedimentary pile the average final strain pattern of the region can be regarded as the results of a multidirectional flattening a hypothesis consistent with vertical uplift associated with regional extension these results are in good agreement with conclusions of earlier workers showing late extensional evolution of the ophiolites along

middle to late miocene extensional collapse of the north patagonian andes 41 30 42 s abstract the hinterland zone of the north patagonian andes between 41 and 43 s constitutes a poorly explored sector of the andes where no structural studies and scarce geochronological determinations have been carried out this paper focuses on two isolated volcanic sections hosted at the main andes in which field evidence indicates a common synextensional origin geochronological data establish that this volcanic event occurred diachronously at the innermost sector of the evolving fold and thrust belt during middle to late miocene times therefore the event was briefly coeval and postdated compressive tectonics recognized in the deformational front and correlative to the last phase of pluton emplacement of the north patagonian batholith local scale topographic swath profiles performed in this work reveal negative topographic anomalies where normal faults were recognized moreover regional swath profiles show not only a conspicuous depressed zone at the hinterland zone where the studied sections are located but also anomalously high altitudes at the foreland zone in addition calculated orogenic volumes increase in this sector of the fold and thrust belt which agrees with recent shortening estimations these topographic along strike variations in association with late miocene extension following the main compressive stage in the area are explained by a supercritical stage of the orogenic wedge that would have led to focused extension at the innermost sector of the fold and thrust belt

structural geology of the eastern nadaleen trend yukon territory canada implications for recently discovered sedimentary rock hosted gold abstract recently discovered gold rich mineral deposits in the eastern nadaleen trend of northeastern yukon territory occur in unmetamorphosed neoproterozoic carbonate and siliciclastic rocks and their location is partly controlled by structures regional deformation largely resulted from mid cretaceous nne vergent thin skinned fold thrust activity however structures in the eastern nadaleen trend are more complex and include strike slip and reverse faults and ssw w and e plunging folds local structures are cospatial with regional geologic features including 1 an e striking structural triangle zone defined by oppositely dipping reverse faults that verge toward the eastern nadaleen trend 2 a transition zone between paleozoic ogilvie platform rocks in the north to selwyn basin rocks in the south and 3 a change in regional deformation character from west to east whereby to the west rocks in the south are more deformed than rocks in the north whereas to the east rocks show no obvious change in deformation style the structural complexity within the eastern nadaleen trend probably resulted from these regional geological features and was further complicated by local competency contrasts between rock units the cospatial location of the regional geological features may have been caused by a pre existing w trending subsurface basement structure reverse faults bounding the e striking triangle zone may have acted as low permeability aquitards that directed mineralizing fluids up dip into reactive carbonate stratigraphy in the eastern nadaleen trend gold rich mineral deposits in the eastern nadaleen trend occur preferentially in ssw plunging anticlines and near faults both of which probably influenced the migration of mineralizing fluid

geologic and kinematic constraints on late cretaceous to mid eocene plate boundaries in the southwest pacific abstract starkly contrasting tectonic reconstructions have been proposed for the late cretaceous to mid eocene 85 45 ma evolution of the southwest pacific reflecting sparse and ambiguous data furthermore uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest pacific plate circuit it is agreed that the southwest pacific comprised three spreading ridges during this time in the southeast indian ocean tasman sea and amundsen sea however one and possibly two other plate boundaries also accommodated relative plate motions in the west antarctic rift system wars and between the lord howe rise lhr and pacific relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period and determine the time dependent evolution of the southwest pacific regional plate circuit a model of 1 west dipping subduction and basin opening to the east of the lhr from 85 55 ma and 2 initiation of northeast dipping subduction and basin closure east of new caledonia at 55 ma is supported west dipping subduction and basin opening were not driven by convergence as has previously been proposed our plate circuit analysis suggests that between at least 74 ma and subduction initiation at 55 ma there was little net relative motion between the pacific plate and lhr

seismic stratigraphic record of transition from mesozoic subduction to continental breakup in the zealandia sector of eastern gondwana abstract the southwest pacific between australia new zealand and new caledonia is a block of continental crust zealandia that moved away from australia and antarctica after a long period of subduction beneath eastern gondwana we use 100 000 line km of seismic reflection profiles to identify intra continental basins related to the gondwana active margin overlain with erosional unconformity by retrogradational strata we interpret this regional scale first order unconformity the eastern gondwana composite surface and seismic stratigraphic megasequence pattern to represent the transition from subduction to continental breakup and separation of eastern gondwana rocks that make up the lower seismic stratigraphic megasequence zealandia 3 permian to early cretaceous have been drilled near new zealand and can be correlated with murihiku supergroup rocks farther north on the lord howe rise we correlate this megasequence with triassic to jurassic rocks of the clarence moreton basin of eastern australia we reinterpret the fairway aotea basin to be underlain by a wedge of zealandia 3 deposits thrust in a retro arc like foreland basin that was active immediately prior to breakup the lower rift and retrogradational megasequence zealandia 2 late cretaceous to eocene we correlate with the pakawau and kapuni groups of taranaki basin new zealand and the formation a charbon and phtanites of new caledonia the boundary with the shallowest megasequence zealandia 1 late eocene to present is the eocene oligocene unconformity which is primarily overlain by pelagic carbonate rocks this regional unconformity likely represents the onset of the modern tonga kermadec subduction system

post obduction carbonate system development in new caledonia nepoui lower miocene for the rst time depositional models of lower miocene carbonate systems from new caledonia southwest paci c are proposed on the basis of a sedimentological and paleoenvironmental study of both cores and outcrops in the nepoui area two distinct stages of carbonate ramp development aquitanian lower nepoui and burdigalian upper nepoui carbonate systems separated by a phase of siliciclastic deltaic deposition are evidenced the postobduction marine transgression of the western new caledonian margin occurred at approximately 24 ma and is characterized by the development of an aggrading foraminiferal coralline algal scleractinian ramp system chapeau chinois limestone during the early aquitanian 24 23 ma a retrogradational event is evidenced at approximately 23 ma followed by the development of a shallowing upward carbonate unit operculina green sands and xuudhen limestone during the late aquitanian this unit is topped by a major erosional unconformity overlain by conglomeratic deposits pindai conglomerates and interpreted to record a signi cant uplift at around 21 19 ma during the burdigalian a marine transgression occurred at around 19 ma followed by the development of a low angle carbonate ramp or open platform nepu limestone up to the late burdigalian 19 17 ma in both aquitanian and burdigalian carbonate ramps extensive sea grass meadows are shown to have colonized the proximal ramp environments within the euphotic zone in the aquitanian carbonate ramp lower nepoui formation carbonate production within sea grass meadows is dominated by large benthic foraminifera together with red algae and sparse scleractinians mesophotic environments are characterized by large and at lepidocyclinids rhodoliths and platy corals whereas in deeper oligophotic settings signi cant carbonate producers consist mainly of large and at benthic foraminifera in the burdigalian carbonate ramp upper nepoui formation porcellaneous foraminifera thriving in sea grass meadows together with red algae and scattered coral colonies characterize the carbonate production in the euphotic zone antecedent topography is regarded as a major factor controlling the extension of carbonate systems at regional and local scale the thickness and development pattern of lower miocene deposits from nepoui are dominantly controlled by tectonic subsidence finally extensive sea grass development promoted the dominance of foralgal carbonate production within the euphotic zone

deep sea benthic foraminifera as proxies for palaeoclimatic fluctuations in the new caledonia basin over the last 140 000years abstract article history received 17 september 2012received in revised form 19 june 2013accepted 28 august 2013available online 4 september 2013keywords new caledonia basintasman seaquaternarysouthwest paci coceanforaminiferapalaeoenvironment the new caledonia basin ncb sw paci c a deep bathyal to abyssal basin bordered by the lord howe rise tothe west and the norfolk ridge to the east has received little scrutiny with regard to the taxonomic diversityofbenthicorganismsinitsquaternarytorecentdeepseasediments in2001 gravitycore4 gc4 wasextractedfrom the ncb and the uppermost 141 cm representing approximately the last 140 000 years was investigatedfor chemical isotopic carbonate non carbonate and trace element signatures in this study subsamples of thesame uppermost 141 cm of gc4 were picked and sorted for all foraminiferal taxa at selected intervals in gc4 allbenthicspecieswereidenti edandcountedformeasuresofrelativeabundance atotalof161speciesofben thic foraminifera were obtained from the core and 46 species occurred in suf cient numbers to investigatechangingpatternsandtrendsinbiodiversityandrelativeabundance changesfoundwithintheforaminiferaldis tribution down the core can be related to some oceanic and palaeoclimatic uctuations during the last 140 000 years within gc4 two distinct foraminiferal assemblages were detected using bray curtis clusteranalysis and multidimensional scaling mds analysis foraminiferal assemblage 1 occurs in lower bathyal toabyssal depths and was largely in uenced by high oxygen levels at the sediment water interface throughoutmarine isotopic stage mis 6 3 approximately 128 25 ka foraminiferal assemblage 2 dominated theupper bathyal shelf during mis 2 approximately 25 10 ka with surface waters characterised by high sea sur faceproductivity ssp andeutrophicconditions duringthetimecoveredbygc4 thereareinstanceswheretherelative abundanceofthe dominanttaxonepistominella exigua brady and other key taxadecreasesand oppor tunistic species of uvigerina adapted to lower oxygen levels increase in relative abundance indicating a shift ofconditions at the sediment water interface factors that in uence the foraminiferal assemblages include envi ronmentalsetting ssp oxygenlevelsatthesediment waterinterfaceandtransportationbyoceaniccurrentsys tems operating within the region 2013 elsevier b v all rights reserved

geodynamic evolution of the lithosphere and upper mantle beneath the alboran region of the western mediterranean constraints from travel time tomography an edited version of this paper was published by the american geophysical union copyright 2000 agu see also http www agu org pubs crossref 2000 2000jb900024 shtml http atlas geo cornell edu morocco publications calvert2000 htm

production of hybrid granitic magma at the advancing front of basaltic underplating inferences from the sesia magmatic system south western alps italy abstract the permian sesia magmatic system of the southwestern alps displays the plumbing system beneath a permian caldera including a deep crustal gabbroic complex upper crustal granite plutons and a bimodal volcanic field dominated by rhyolitic tuff filling the caldera isotopic compositions of the deep crustal gabbro overlap those of coeval andesitic basalts whereas granites define a distinct more radiogenic cluster sri 0 708 and 0 710 respectively afc computations starting from the best mafic candidate for a starting melt show that nd and sr isotopic compositions and trace elements of andesitic basalts may be modeled by reactive bulk assimilation of 30 of partially depleted crust and 15 30 gabbro fractionation trace elements of the deep crustal gabbro cumulates require a further 60 fractionation of the andesitic basalt and loss of 40 of silica rich residual melt the composition of the granite plutons is consistent with a mixture of relatively constant proportions of residual melt delivered from the gabbro and anatectic melt chemical and field evidence leads to a conceptual model which links the production of the two granitic components to the evolution of the mafic complex during the growth of the mafic complex progressive incorporation of packages of crustal rocks resulted in a roughly steady state rate of assimilation anatectic granite originates in the hot zone of melting crust located above the advancing mafic intrusion upward segregation of anatectic melts facilitates the assimilation of the partially depleted restite by stoping at each cycle of mafic intrusion and incorporation residual and anatectic melts are produced in roughly constant proportions because the amount of anatectic melt produced at the roof is a function of volume and latent heat of crystallization of the underplated mafic melt which in turn produces proportional amounts of hybrid gabbro cumulates and residual melt such a process can explain the restricted range in isotopic compositions of most rhyolitic and granitic rocks of the permo carboniferous province of europe and elsewhere

platform subsidence behind an active subduction zone numerical models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones1 4 this platform subsidence is predicted to cause an asymmetrical depression 500 1 000 km wide with a maximum depth of 2 km if filled with water direct evidence linking platform subsidence to mantle flow has remained elusive however here we offer evidence from western new zealand for a complete link in both space and time between subduction initiation and platform subsidence because of the broad wavelength 200 km of the observed 1 500 m subsidence it is unlikely that the subsidence is due to a flexural foredeep the observed subsidence does however match in both wavelength and amplitude that predicted by a model of subduction induced mantle flow the occurrence of platform subsidence has implications not only for understanding mantle convection but also for explaining geoid anomalies over active margins5 and patterns of continental submergence6 and sedimentation7 that could otherwise be interpreted as a eustatic sea level change8

deepwater sedimentation and cenozoic deformation in the southern new caledonia trough northern zealandia sw pacific abstract the new caledonia trough nct is a 2000 3000 m deep bathymetric feature that extends 2500 km from taranaki new zealand to the western margin of new caledonia northern zealandia sw pacific the underlying sedimentary basin originates from cretaceous extension but underwent a significant eocene tectonic event that shaped its present physiography we present an analysis of the basin based on multibeam data seismic profiles and rock samples collected on the tan1312 and tan1409 expeditions onboard r v tangaroa combined with legacy data we focus on the southern part of the basin where new data reveal a link between compressive deformation of paleocene strata and their potential reworking into the basin on the western basin side upper cretaceous to paleocene strata were deformed by local reverse faults and folds that created a sub circular bathymetric ridge and seabed exposure this folded unit seismic unit 2 is sharply overlain by a restricted interval imaged as chaotic high amplitude reflections that are interpreted as syntectonic mass transport deposits due to slope oversteepening seismic unit 1b this unit is stratigraphically below the main basin onlap surface and seismic mapping revealed that it rapidly thins away from the mouth of a present day submarine canyon imaged on the western slope of the basin and that is diverted by exposures of unit 2 deformed strata deformed and syntectonic intervals are in turn overlain by a flat lying unit seismic unit 1a we interpret as reflecting a passive basin fill our new data provide insight into an extensive deep water basin that is remote from terrigenous sediment sources and new constraints on its cenozoic tectonic history stratigraphic ages are constrained by seismic ties to taranaki basin petroleum wells and biostratigraphic dating of dredged samples this specific site has particular significance for understanding tectonic events in the southwest pacific indeed our observations show that deformation is younger than the paleocene and is envisaged to be a local expression of a widespread cenozoic compressive event called tecta tectonic event of the cenozoic in the tasman area which affected the region after the mesozoic rifting on a regional perspective this study provides new insights on the evolution of the submerged zealandia continent and associated geodynamic processes such as gondwana break up and initiation of the tonga kermadec subduction

the structure of the south fiji basin abstract new magnetic seismic and bathymetric data show that the south fiji basin is a structurally complex marginal basin a gap in the identifiable magnetic anomaly lineations exist over the central part of the basin and prevents the unequivocal linking of the anomaly lineations anomalies 7a to 12 associated with the ridgc ridge ridge triple junction in the north with an apparent single spreading centre of the same age in the south this gap which makes a detailed synthesis of the historical development of the basin difficult may arise from post spreading intraplate tectonics if symmetric spreading is assumed part of the oceanic lithosphere formed during the oligocene episode of seafloor spreading has subsequently been consumed presumably by subduction westward under three kings rise

location and migration of miocene quaternary volcanic arcs in the sw pacific region abstract we report new ar ar and u pb ages from ten rocks in the sw pacific region our results 1 establish a northward late miocene australian plate movement rate of 57 mm a for the lord howe hotspot chain 2 reinforce the previously established widespread nature of early miocene subduction related volcanism in onland and nearshore northern new zealand 3 indicate that leucotonalite xenoliths from raoul island are the products of quaternary kermadec arc magmatism rather than being cretaceous pliocene basement a synthesis of available sw pacific data emphasises that while there is a reasonably complete record of subduction related volcanism from at least 23 ma to the present day the process of back arc basin formation is highly episodic and asymmetric subduction related arcs stabilised along the taupo kermadec tonga arc from 2 ma to the present along the taranaki colville lau trend from 17 6 ma and along the three kings ridge northland plateau northland trend from 23 18 ma south of latitude 25 s back arc basin opening occurred during the early miocene and quaternary arc volcanic episodes but does not appear to have accompanied the 17 5 ma volcanism

incipient subduction at the contact with stretched continental crust the puysegur trench a seismic benioff zone and plate kinematics show puysegur trench south of new zealand transitioning to subduction because the local structure and its influence on subduction initiation is poorly understood we conducted a seismic survey with ocean bottom seismometers and multichannel seismic profiles our early results show that the overriding pacific plate beneath the solander basin is composed of block faulted and thinned continental crust and the inner trench wall of northern puysegur ridge is composed of folded and faulted sediment the megathrust interface has been imaged and shows 500 m of downgoing undisturbed sediments combining plate kinematic history with seismic velocity inferred density we show that the density difference across the plate boundary changed as oblique strike slip plate motion juxtaposed dense oceanic crust with thinned continental crust the density difference rapidly increased 18 to 15 ma coincident with subduction initiation suggesting that compositional differences have a large influence on subduction initiation

continental scale geographic change across zealandia during paleogene subduction initiation data from international ocean discovery program iodp expedition 371 reveal vertical movements of 1 3 km in northern zealandia during early cenozoic subduction initiation in the western pacific ocean lord howe rise rose from deep 1 km water to sea level and subsided back with peak uplift at 50 ma in the north and between 41 and 32 ma in the south the new caledonia trough subsided 2 3 km between 55 and 45 ma we suggest these elevation changes resulted from crust delamination and mantle flow that led to slab formation we propose a subduction resurrection model in which 1 a subduction rupture event activated lithospheric scale faults across a broad region during less than 5 m y and 2 tectonic forces evolved over a further 4 8 m y as subducted slabs grew in size and drove plate motion change such a subduction rupture event may have involved nucleation and lateral propagation of slip weakening rupture along an interconnected set of preexisting weaknesses adjacent to density anomalies

thermo mechanical controls on the mode of continental collision in the se carpathians romania abstract the carpathians orogenic system with its along arc variations in topography developed in the aftermath of continental collision is associated with unusual foredeep basins large scale strain and seismicity concentration and high velocity mantle bodies the east carpathians continental collision was non cylindrical leading to large scale variations in thrust nappe kinematics orogenic uplift patterns and foredeep subsidence controlled by the mechanics and geometry of the lower plate thermo mechanical modelling demonstrates that in this low rate convergence regime the subducted lithosphere had enough time to interact with the mantle to advance towards a thermal resettlement this is favored by the low degree of metamorphism mechanical weakness of the lower plate and the lack of active surface processes at the contact with and in the upper plate in contrast low buoyant thick lower crust and active surface processes keep the continuity of the slab intact and promote the development of typical foredeep basins the model explains in a self consistent manner the unusual geometry of the vrancea seismogenic slab in the bend zone of the romanian carpathians the model is also consistent with the presence of two high velocity bodies inferred from seismic tomography studies and explains the depth zonation of seismicity in the vrancea area differences between the northern part of east carpathians and the southeastern bend of the carpathians arc are largely controlled by lateral variations in crustal structure topography emplacement and surface processes along the arc mechanical heterogeneity of the carpathians subduction leads to the development of two end member modes of collision allowing a study of these states and their transition lithospheric configuration and tectonic topography appear to be prime factors controlling variations in slab behavior in the se carpathians at the terminal phase of continental convergence slab delamination roll back and depocenter migration appear to play a more limited role at shallow and lithospheric levels

crustal growth as revealed by integrated u pb and lu hf isotope analyses of detrital zircons from the ganjiang river southeastern china the ganjiang river one of eight major tributaries of the yangtze river located in the western hinterland of the cathaysia block se china has a length of 823 km and a drainage area of 82 809 km 2 whose detrital zircons provide a valuable means to trace sediment provenances of the river and explore the crustal growth and evolution of the cathaysia block in the current study 389 concordia zircon u pb age spots and rare earth element ree contents in combination with 201 lu hf isotope analyses have been determined oscillatory zoning high th u ratios and ree distribution patterns indicate that most detrital zircon grains are of magmatic origin the age can be further divided into seven groups 130 185 ma with a peak at 153 ma 7 217 379 ma with a peak at 224 ma 16 390 494 ma with a peak at 424 ma 37 500 698 ma with a peak at 624 ma 5 716 897 ma with a peak at 812 ma 10 902 1191 ma with a peak at 976 ma 13 and 2232 2614 ma with a peak at 2471 ma 5 the sources of almost all the zircon age groups can be found from the exposed rocks in particular yanshanian hercynian to indosinian pan african grenvillian and palaeoproterozoic archaean zircons can be mainly sourced from the northern guangdong southern jiangxi western fujian region while caledonian zircons come from southern and central jiangxi and jinningian zircons are from central and northern jiangxi most determined zircon grains exhibit negative e hf t values and t dm2 ages of 797 to 4016 ma with a wide peak at 1500 2100 ma and a keen peak at 1824 ma suggesting that most zircons are sourced from the reworked ancient crustal materials or crust mantle mixing the zircon hf model age cumulative probability diagram shows that rapid crustal growth took place at the palaeo to mesoproterozoic and that about 90 of the crust of the cathaysia block was formed before 1 5 ga

effective elastic thickness of zealandia and its implications for lithospheric deformation abstract zealandia was once located at the active gondwana margin and experienced complicated tectonic evolution from continental breakup to subduction and oblique slipping along the plate boundary here we investigate the lithospheric thermal and rheological properties via spatial variations in effective elastic thickness te using the topography sediment thickness and gravity data the fan wavelet coherence method is employed to recover te values the effects of dynamic topography and gravity in the subduction zone on the estimated te are considered and removed the final results reveal that the te variation pattern generally coincides well with the tectonic regimes relatively high te values up to 22 km are observed in north island and the southernmost part of south island while central and northern south island are dominated by smaller te values 15 km similar to those of the surrounding submerged plateaus we interpret the relatively high te values as indicating the combined lithospheric strength due to the contacting plates in the subduction zone to the north along the tonga kermadec subduction zone the lithosphere surprisingly exhibits anomalously high te values ranging from 22 to 36 km which contradicts the geological evidence of normal faults developed in the incoming pacific plate and the magmatism in the overlying australian plate that generally contribute to low lithospheric strength we speculate that the high te zone is related to the large bathymetry and gravity anomalies in the bending plate and controlled by non isostatic factors finally we find widespread low te values covering the entire arcs and oceanic basins in northern zealandia which may indicate that regional lithospheric thinning associated with delamination weakened the integrated strength of the lithosphere

tectonic history of northern new caledonia basin from deep offshore seismic reflection relation to late eocene obduction in new caledonia southwest pacific new high quality multichannel seismic reflection data from the western new caledonia offshore domain allow for the first time the direct continuous connection of seismic reflectors between the deep sea drilling project 208 drill hole on the lord howe rise and the new caledonia basin a novel seismic interpretation is hence proposed for the northern new caledonia basin stratigraphy which places the eocene oligocene unconformity deeper than previously thought and revisits the actual thickness of the pre oligocene sequences a causal link is proposed between the obduction of the south loyalty basin over new caledonia nc and the tectonic history of the northern new caledonia basin here it is suggested that as the south loyalty basin was being obducted during early oligocene times the nc basin subsided under the effect of the overloading and underthrusted to accommodate the compressional deformation which resulted in 1 the uplift of the northern fairway ridge and 2 the sinking of the western flank of new caledonia this event also had repercussions farther west with the incipient subsidence of the lord howe rise

wave equation reflection traveltime inversion with dynamic warping and full waveform inversion abstractin reflection seismology full waveform inversion fwi can generate high wavenumber subsurface velocity models but often suffers from an objective function with local minima caused mainly by the absence of low frequencies in seismograms these local minima cause cycle skipping when the low wavenumber component in the initial velocity model for fwi is far from the true model to avoid cycle skipping we discovered a new wave equation reflection traveltime inversion werti to update the low wavenumber component of the velocity model while using fwi to only update high wavenumber details of the model we implemented the low and high wavenumber inversions in an alternating way in werti we used dynamic image warping diw to estimate the time shifts between recorded data and synthetic data when compared with correlation based techniques often used in traveltime estimation diw can avoid cycle skipping and estimate the time shifts accurately even when shifts vary rapidly hence by minimizing tr

understanding sibumasu in the context of ribbon continents abstract well described ribbon continent collision belts and modern examples with 2d seismic coverage particularly zealandia provide analogues for the permo triassic development of sibumasu two key parameters for the ribbon continents are their width and how rifting developed which in turn impact the structural metamorphic igneous and stratigraphic development of ribbon continents in ways that make them significantly different from the collisional development of major continents key characteristics of the tectonics of sibumasu are 1 a tendency towards basin inversion during the late triassic and possibly early jurassic and mixed thick skinned and thin skinned style deformation 2 extensional collapse around 215 ma is an important feature of sibumasu particularly in northern thailand and probably other areas too 3 slab delamination and extension related decompression melting were probably important mechanisms for generating granites in addition to crustal thickening 4 metamorphism is relatively high temperature lower pressure

zircon u th pb hf isotopes of the basement rocks in northeastern cathaysia block south china implications for phanerozoic multiple metamorphic reworking of a paleoproterozoic terrane abstract the cathaysia block records a complex tectonic history the understanding of which is central to the debate on the evolution of the south china block and its position in various supercontinent assemblies here we investigate two key formations from this block the mayuan group in the northern fujian province northeastern cathaysia block and its equivalent the badu group in southwestern zhejiang province previous studies traced the paleoproterozoic records from the badu group whereas a neoproterozoic age was proposed for the mayuan group the rocks sampled in this study from both groups show similar mineral assemblages of garnet sillimanite biotite plagioclase quartz k feldspar muscovite graphite as well as high contents of sio 2 and al 2 o 3 typical of amphibolite to granulite facies metapelitic rocks zircon u pb data yield two discordia intercept ages of 1990 ma and 2450 ma from one sample and discordia intercept ages of 3 5 ga 2 5 ga 1 86 ga and 233 ma from another in the badu group zircons in two samples from the mayuan group yield intercept ages of 1859 ma and 249 ma in one sample and 2 6 ga 1 87 ga 257 ma and a weighted mean 206 pb 238 u age of 248 ma in the other the ca 1 86 1 87 ga and 230 250 ma ages are interpreted to represent the time of metamorphic reworking because zircon grains of these ages tend to have low th u ratios flat hree patterns and unzoned internal texture as revealed by cathodoluminescene cl images these results confirm that the badu group is a paleoproterozoic lithostratigraphic unit and also suggest that at least part if not all of the mayuan group is paleoproterozoic evidence for paleozoic metamorphic reworking that is considered to have affected the whole of cathaysia block is not revealed in this study in contrast our data clearly show obvious mesozoic metamorphic reworking at ca 230 250 ma zircon hf t values range from 19 to 11 with a peak at 5 5 and show t dm c hf ranging from 1 9 to 4 1 ga with a peak at ca 2 7 3 0 ga suggesting that a major crustal growth took place during this time this interpretation is consistent with the previously suggested crustal growth peaks of 2 7 ga and 2 9ga a synthesis of the reliable geochronological data gathered so far on phanerozoic metamorphic reworking of the northeastern cathaysia block reveals that the imprints of these tectonothermal events is differently distributed in the different zones rocks metamorphosed during the paleozoic tectonothermal event dominantly occur in the western zone whereas those reworked by high grade metamorphism during the mesozoic tectonothermal event mainly outcrop in the eastern zone our study alerts the previous notion of a uniform distribution of the reworked rocks by high grade metamorphism all across the northeastern cathaysia block and provides new insights on the evolution of the south china block

the west water formation hualapai plateau arizona usa as a calcrete paleosol sequence and its implications for the paleogene neogene evolution of the southwestern colorado plateau abstract analyses of stratigraphic sequences within the paleocanyons of the hualapai plateau arizona are important because these deposits offer the only evidence for the paleogene neogene geological history of the grand canyon area in this paper we focus on the origins and paleoenvironmental significance of the west water formation located within the milkweed and west water paleocanyons on the hualapai plateau we propose that the supposed limestone unit of the west water formation at and near its type section is not a limestone rather it is a 21 m thick valley calcrete overprinted by a 1 2 m thick pedogenic calcrete and subsequently dolomitized in its upper to middle sections with a superimposed 4 m thick red paleosol we also propose that this unit is not coeval in age or origin with the long point limestone on the coconino plateau and that the presence of a complex calcrete dolocrete paleosol alters previous interpretations associated with this unit evidence for a calcrete paleosol origin beyond the west water formation s lack of fossils includes its predominantly micritic calcite palygorskite composition its textures characteristic of valley and pedogenic calcrete deposits as exhibited by thin section sem and tem analyses and its association with a relatively thick overlying red paleosol that also contains abundant palygorskite stable carbon and oxygen isotope values from carbonate cements are also within the range expected of a near surface calcrete carbonate minerals within the calcrete dolocrete were precipitated at or near the water table in a valley setting due to evaporation and or co 2 degassing in a semi arid to arid environment of deposition high 87 sr 86 sr values within the calcrete were inherited from groundwater infiltrating through music mountain formation arkosic sediments which were derived from a precambrian source terrane to the south and southwest no absolute ages exist for the music mountain formation and west water formation in the milkweed and west water paleocanyons these sediments could range in age from the late cretaceous to the latest eocene earliest oligocene and may record the eocene oligocene transition in the grand canyon area

a cambrian mixed carbonate siliciclastic platform in sw gondwana evidence from the western sierras pampeanas argentina and implications for the early paleozoic paleogeography of the proto andean margin the western sierras pampeanas wsp of argentina record a protracted geological history from the mesoproterozoic assembly of the rodinia supercontinent to the early paleozoic tectonic evolution of sw gondwana two well known orogenies took place at the proto andean margin of gondwana in the cambrian and the ordovician i e the pampean 545 520 ma and famatinian 490 440 ma orogenies respectively between them an extensive continental platform was developed where mixed carbonate siliciclastic sedimentation occurred this platform was later involved in the famatinian orogeny when it underwent penetrative deformation and metamorphism the platform apparently extended from patagonia to northwestern argentina and the eastern sierras pampeanas and has probable equivalents in sw africa peru and bolivia the wsp record the outer deepest part of the platform where carbonates were deposited in addition to siliciclastic sediments detrital zircon u pb shrimp ages from clastic metasedimentary successions and sr isotope compositions of marbles from the wsp suggest depositional ages between ca 525 and 490 ma the detrital zircon age patterns further suggest that clastic sedimentation took place in two stages the first was sourced mainly from re working of the underlying neoproterozoic metasedimentary rocks and the uplifted core of the early cambrian pampean orogen without input from the paleoproterozoic rio de la plata craton sediments of the second stage resulted from the erosion of the still emerged pampean belt and the neoproterozoic brasiliano orogen in the ne with some contribution from the rio de la plata craton an important conclusion is that the wsp basement was already part of sw gondwana in the early cambrian and not part of the exotic precordillera cuyania terrane as was previously thought

terrane boundary reactivation barriers to lateral fault propagation and reactivated fabrics rifting across the median batholith zone great south basin new zealand prominent pre existing structural heterogeneities within the lithosphere may localise or partition deformation during tectonic events the ne trending great south basin offshore new zealand formed perpendicular to a series of underlying crustal terranes including the dominantly granitic median batholith zone which along with the boundaries between individual terranes exert a strong control on rift physiography and kinematics we find that the crustal to lithospheric scale southern terrane boundary of the median batholith zone is associated with a crustal scale shear zone that was reactivated during late cretaceous extension between zealandia and australia this reactivated terrane boundary is oriented at a high angle to the faults defining the great south basin we identify a large granitic laccolith along the southern margin of the median batholith expressed as sub horizontal packages of reflectivity and acoustically transparent areas on seismic reflection data the presence of this strong granitic body inhibits the lateral south westward propagation of ne trending faults which segment into a series of splays that rotate to align along the margin as they approach further we also identify two e w and ne sw oriented basement fabrics likely corresponding to prominent foliations which are exploited by small scale faults across the basin we show that different mechanisms of structural inheritance are able to operate simultaneously and somewhat independently within rift systems at different scales of observation the presence of structural heterogeneities across all scales need to be incorporated into our understanding of the structural evolution of complex rift systems

history of subduction and back arc extension in the central mediterranean summary geological and geophysical constraints to reconstruct the evolution of the central mediterranean subduction zone are presented geological observations such as upper plate stratigraphy hp lt metamorphic assemblages foredeep trench stratigraphy arc volcanism and the back arc extension process are used to define the infant stage of the subduction zone and its latest back arc phase based on this data set the time dependence of the amount of subducted material in comparison with the tomographic images of the upper mantle along two cross sections from the northern apennines and from calabria to the gulf of lyon can be derived further the reconstruction is used to unravel the main evolutionary trends of the subduction process results of this analysis indicate that 1 subduction in the central mediterranean is as old as 80 myr 2 the slab descended slowly into the mantle during the first 20 30 myr subduction speeds were probably less than 1 cm year x1 3 subduction accelerated afterwards producing arc volcanism and back arc extension and 4 the slab reached the 660 km transition zone after 60 70 myr this time dependent scenario where a slow initiation is followed by a roughly exponential increase in the subduction speed can be modelled by equating the viscous dissipation per unit length due to the bending of oceanic lithosphere to the rate of change of potential energy by slab pull finally the third stage is controlled by the interaction between the slab and the 660 km transition zone in the southern region this results in an important re shaping of the slab and intermittent pulses of back arc extension in the northern region the decrease in the trench retreat can be explained by the entrance of light continental material at the trench

the moho boundary above upper mantle peridotites or lower crustal eclogites a global review and new interpretations for passive margins abstract we have performed a global study of 2d crustal scale wide angle profiles across passive margins with regard to local elevations in the moho which could possibly be interpreted as indicative of lower crustal eclogites a total of 16 candidates have been found mainly in the north atlantic and around australia these cases make up c 6 only of the total profile length studied confirming the interpretation of the moho generally representing the top of the mantle the interpreted candidates for lower crustal eclogites indicate that there may be a link between eclogite bodies and continental suture zones in the barents sea off mid norway in the newfoundland appalachians and in the yilgarn craton western australia it is also possible that there is a genetic link between the formation of caledonian eclogites and the jan mayen fracture zone which is the only major fracture zone in the north atlantic several of the inferred lower crustal eclogite bodies are located close to lines of major changes in strain such as coastlines and shelf edges indicating that lower crustal eclogite bodies may be important in guiding the evolution of basin architecture interpreting the moho beneath the bedout high on the northwest shelf of australia as the top of a body of lower crustal eclogites may imply that the northern termination of the lambert shelf represents a paleo suture zone and that the western termination of the broome platform acted as a major transfer zone the significant increase in crustal thickness implied by the eclogite model has important implications for estimates of stretching history subsidence and hydrocarbon maturation modelling

crustal structure of the basin and ridge system west of new caledonia southwest pacific from wide angle and reflection seismic data 1 during the zoneco 11 marine geophysical survey september 2004 two deep reflection seismic profiles recorded by ocean bottom seismometers were acquired in the offshore domain west of new caledonia the northern profile crosses the new caledonia basin the fairway ridge the fairway basin and the lord howe rise the southern profile crosses the norfolk rise south of new caledonia the new caledonia basin the fairway ridge and basin and ends at the foot of lord howe rise on the northern profile the lord howe rise has a crustal thickness of 23 km and exhibits seismic velocities and velocity gradients characteristic of continental crust the crust thins to 12 15 km in the neighboring fairway basin which is interpreted to be of thinned continental origin based on the seismic velocities the crustal thickness of the fairway rise is 22 km and it is also interpreted to be of continental origin the new caledonian basin is underlain by crust of 10 km thickness which shows unusally high velocities between 7 0 and 7 4 uncharacteristic for either thinned continental or oceanic crust on the southern profile the norfolk rise is also found to be of continental nature here the new caledonia basin shows velocities crustal thickness and basement roughness characteristic of typical oceanic crust the crust in the fairway basin shows higher velocities than on the northern profile which could be caused by volcanic intrusions into the crust during extension a deep reflector in the upper mantle was imaged underneath the new caledonian basin on the northern profile

eocene arc continent collision in new caledonia and implications for regional southwest pacific tectonic evolution new caledonia preserves evidence that constrains models for the tectonic evolution of the southwest pacific region onland geology reflects four main tectonic phases 1 early mesozoic development of subduction related terranes and their accretion to the gondwana margin 2 cretaceous passive margin development and sea floor spreading during the gondwana breakup 3 foundering of an oceanic basin and the eocene arrival of thinned gondwana margin crust at a southwest facing subduction zone resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast and 4 detachment faulting during extensional collapse resulting in unroofing of metamorphic core complexes the last phase explains supposedly anomalous metamorphic gradients in the northeast of the island

subduction history of the tethyan region derived from seismic tomography and tectonic reconstructions 1 in the mantle underneath the tethyan suture zone large volumes of positive velocity anomalies have been imaged by seismic tomography and interpreted as the present day signature of subducted tethyan lithosphere we investigate the mesozoic cenozoic subduction history of the region by integrating independent information from mantle tomography and tectonic reconstructions three different subduction scenarios for the tethyan oceanic lithosphere representative for the available tectonic reconstructions are used to predict the present thermally anomalous volumes associated with the lithospheric surface subducted since the late mesozoic next these predicted thermal volumes and their expected positions are compared to the relevant anomalous volumes derived from seismic tomographic images in this analysis we include among others the possible effects of ridge subduction and slab detachment after the cenozoic continental collisions absolute plate motion and slab thickening in the mantle our preferred subduction model comprises the opening of large back arc oceanic basins within the eurasian margin the model points to slab thickening by a factor of 3 in the mantle in which case the estimated volumes allow for active oceanic spreading 1 2 5 cm yr in the tethyan lithosphere during convergence our results further indicate the occurrence of early oligocene slab detachment underneath the northern zagros suture zone followed by both westward and eastward propagation of the slab tear and diachronous eocene to miocene slab detachment below the eastern to western himalayas free sinking rates of the detached material of 2 cm yr in the lower mantle provide the best fit between the tomographic mantle structure and our tethyan subduction model

dynamics of subduction initiation with different evolutionary pathways changes of plate motion may have induced subduction initiation si but the tectonic history of si is different from one subduction zone to another izu bonin mariana ibm si accompanied by strong backarc spreading and voluminous eruption of boninites contrasts with the aleutians which shows neither using finite element models we explore visco elasto plastic parameters and driving boundary conditions for si evolution with an imposed velocity we find three different evolutionary modes of si continuous without backarc spreading continuous with backarc spreading and a segmented mode with an increase in the coefficient of friction and a decrease in the rate of plastic weakening the amount of convergence needed for si increases from 20 to 220 km while the mode changes from segmented to continuous with backarc spreading and eventually to continuous without backarc spreading if the imposed velocity boundary condition is replaced with an imposed stress the amount of convergence needed for si is reduced and backarc spreading does not occur these geodynamic models provide a basis for understanding the divergent geological pathways of si first ibm evolution is consistent with subduction of an old strong plate with an imposed velocity which founders causing intense backarc spreading and boninitic volcanism second the new hebrides si is in the segmented mode due to its weak plate strength third the puysegur si is in the continuous without backarc spreading mode with no associated volcanic activities fourth the aleutians si has neither trench rollback nor backarc spreading because the slab is regulated by constant ridge push forces

hinterland drainage closure and lake formation in response to middle eocene farallon slab removal nevada u s a abstract hinterland basins can accumulate high resolution archives of orogenic processes and continental climate but are challenging to reconstruct due to tectonic overprinting and the inherent complexity of their lithofacies assemblages the cordilleran hinterland of northeast nevada has been interpreted to have overlain a flattened farallon slab from the late cretaceous to eocene slab removal and advection of asthenospheric mantle beneath nevada have been invoked to explain a southwestward migrating wave of eocene to oligocene volcanism and proposed as a driver for topographic uplift however the timing of slab removal and possible subsequent delamination of north american lithospheric mantle can only ambiguously be related to the surface record subsequent neogene extension and basin filling has complicated the correlation and interpretation of strata that record these events here we apply single crystal sanidine 40ar 39ar geochronology to 26 ash beds in northeast nevada to reconstruct paleogene geographic and hydrologic evolution we use these ages and legacy geochronology to compare lithofacies and isotope proxy records of meteoric waters to regional tectonics and global climate and assess competing tectonic interpretations for lake basin formation lakes formed locally prior to ca 48 7 ma in northeast nevada coeval with foreland lakes of the green river formation the most expansive phase of lacustrine deposition resulted in onlap onto locally derived fluvial deposits and folded paleozoic bedrock and occurred between ca 43 4 and ca 40 8 ma elko formation strata exhibit a basin wide transition from fluvial lacustrine to fluctuating profundal lithofacies at ca 42 7 ma suggesting a shift towards regional hydrologic closure the stromatolitic upper elko formation is intercalated with ash fall tuffs and several partially welded to unwelded ignimbrites from increasingly proximal volcanism elko formation deposition ended by ca 40 4 ma 40ar 39ar ages for seven ash beds in the dead horse formation at copper basin in northern elko county indicate intermittent ash bed deposition between 45 2 ma and 38 6 ma and an episode of lacustrine deposition between 39 8 ma and 38 6 ma that post dates the main phase of lake elko d values of volcanic glass sampled from dated ash beds reflect changes in the hydrogen isotope compositions of local eocene waters and systematically vary by 80 102 according to their depositional environment the elko formation and overlying volcanic strata are overlain regionally by a pronounced unconformity of 20 m y in the copper basin area deposition continued locally into the oligocene in the hanging wall of a ductile detachment the geochronologic and isotopic framework presented here permits reanalysis of the pinon range carbonate proxy record that was previously interpreted to record both regional uplift and the middle eocene climatic optimum new data suggest instead that isotope values of hydration waters within the elko formation were strongly influenced by evaporation and a change from lacustrine to non lacustrine conditions can account for the 18 o shift that was interpreted to reflect regional uplift moreover the end of elko formation deposition predated the middle eocene climatic optimum we interpret the overall record of drainage ponding and paleovalley inundation progressively more evaporative lacustrine conditions increasingly proximal volcanism and subsequent prolonged unconformity to reflect the surface effects of progressive ne to sw removal of the farallon slab

a new look at the new caledonia basin an extension of the taranaki basin offshore north island new zealand abstract the new caledonia basin is a major bathymetric trough extending north westwards from taranaki and continuing past new caledonia this paper is concerned with the 250 km of the feature closest to new zealand and adjacent to the taranaki shelf the new caledonia basin appears to have formed as the surface expression of a failed rift system gravity models constrained by seismic reflection and refraction data indicate thinned crust below the basin the date of inception of rifting is not known but it must be pre cenozoic and may be pre cretaceous new seismic reflection data reveal tensional block faulting within the basin and on the flanks of the challenger plateau and the west norfolk ridge offset gravity anomalies and apparent compressional faulting observed on seismic data provide evidence for transverse possibly transform faulting accompanying and facilitating oblique extension more than 1 5 s twt of sediments were deposited during rifting this is equivalent to a thickness of up to 3 km they are likely to consist of cretaceous clastics and volcaniclastics deposited in fluvial and marginal marine environments terrigenous clays and mudstones probably cap the rift sequence post rift sedimentation commenced with thick authigenic limestone reflecting regional thermal subsidence and remoteness from a clastic source there is little record of tectonic activity until at least the late eocene and probably not until the early miocene when volcanic activity began seismic sections show domal uplift at the base of the challenger plateau and the formation of several large submarine volcanoes this activity may have been related to renewed rifting in the bellona trough an average thickness of approximately 0 5 km of plio pleistocene sediments onlap the margins of the basin channels and fans are evident on the seismic data and account for the distribution of a significant portion of the sediment eroded from the rising southern alps the fragmentation of this part of the gondwana margin resembles the evolution of the north east atlantic in particular the structure of the rockall trough closely resembles that of the new caledonia basin hydrocarbon source rocks may be present in the rift sequence which has been buried to a depth great enough for thermal maturation structural and stratigraphic traps may exist at a number of levels

rifting and subduction initiation history of the new caledonia trough southwest pacific constrained by process oriented gravity models summary the new caledonia trough is a bathymetric depression 200 300 km wide 2300 km long and 1 5 3 5 km deep between new caledonia and new zealand in and adjacent to the trough seismic stratigraphic units tied to wells include cretaceous rift sediments in faulted basins late cretaceous to eocene pelagic drape and 1 5 km thick oligocene to quaternary trough fill that was contemporaneous with tonga kermadec subduction a positive free air gravity anomaly of 30 mgal is spatially correlated with the axis of the trough we model the evolution of the new caledonia trough as a two stage process i trough formation in response to crustal thinning cretaceous and or eocene and ii post eocene trough fill sedimentation to best fit gravity data we find that the effective elastic thickness te of the lithosphere was low 5 10 km during phase i trough formation and high 20 40 km during phase ii sedimentation though we cannot rule out a fairly constant te of 10 km the inferred increase in te with time is consistent with thermal relaxation after cretaceous rifting but such a model is not in accord with all seismic stratigraphic interpretations if most of the new caledonia troughtopographywascreatedduringeoceneinceptionoftonga kermadecsubduction then our results place important constraints on the associated lower crustal detachment process and suggest that failure of the lithosphere did not allow elastic stresses to propagate regionally into the overriding plate we conclude that the gravity field places an important constraint on geodynamic models of tonga kermadec subduction initiation

stratigraphy of the southern norfolk ridge and the reinga basin a record of initiation of tonga kermadec northland subduction in the southwest pacific abstract seismic stratigraphic interpretations of a large new 2d seismic reflection dataset from the reinga basin region northwest of new zealand constrain the history of cretaceous fragmentation of gondwana and cenozoic initiation of tonga kermadec northland subduction the southern norfolk ridge system lay in a proximal location to plate boundaries that were active during both cretaceous and cenozoic events and persistent marine conditions led to a relatively complete record of sedimentation cretaceous extension was followed by regional subsidence and transgression two cenozoic contractional events are separated by substantial 1 km subsidence late eocene contraction led to reverse faulting and folding between new caledonia and the southern norfolk ridge system and topographic highs created in the north western sector were locally eroded by wave abrasion reinga basin was located at the southern tip of this event which had only limited impact in adjacent northern new zealand regional oligocene and miocene subsidence was contemporaneous with late oligocene and early miocene emplacement of nappes in northern new zealand and the onset of arc volcanism early miocene contraction in the reinga basin led to formation of the wanganella ridge these events can be related to calculated plate motions and are shown to be consistent with models of induced subduction nucleation that require c 150 km of convergence the first contraction is associated with regional inception of tonga kermadec subduction whereas oligocene to miocene events and the onset of hikurangi northland subduction were in response to a local change in plate boundary displacement rate and consequent linkage between subduction north of new zealand with alpine fault formation farther south

discussion of origins of hinterland basins to the rif betic cordillera and carpathians abstract the hinterland basins of the rif betic cordillera western alboran sea and the carpathians pannonian basin are in part composed of a thick in excess of 3 4 km thermal sag basin sequence the underlying crust has been thinned to 25 km in parts of the pannonian basin and to 15 km in parts of the alboran sea modern tectonic models for these areas envision tectonic collision and thickening of the crust followed by crustal thinning either by gravitational collapse back arc extension or a combination of the two contemporaneous with thrusting basin and range type extension has been envisioned for parts of these extensional collapse systems extensional features are common and they interact in a complex way with strike slip systems flexural cantilever modelling agrees with earlier mckenzie type pure shear modelling of the extensional systems in suggesting that extension alone cannot generate sufficient thermal decay to explain the great thickness of the sag basins associated with the alboran sea and pannonian basin consequently it is suggested that uplift of the asthenosphere accompanying subduction roll back is an important factor in the evolution of these basins roll back of the mantle lithosphere permits hot asthenospheric mantle to upwell and lie close to the base of the crust initially uplift and erosion of the crust overlying the asthenosphere occurs as the thermal perturbation cools the crust thermally subsides to produce a large sag basin

lithosphere delamination with foundering of lower crust and mantle caused permanent subsidence of new caledonia trough and transient uplift of lord howe rise during eocene and oligocene initiation of tonga kermadec subduction western pacific 1 we use seismic reflection and rock sample data to propose that the first order physiography of new caledonia trough and norfolk ridge formed in eocene and oligocene time and was associated with the onset of subduction and back arc spreading at the australia pacific plate boundary our tectonic model involves an initial cretaceous rift that is strongly modified by cenozoic subduction initiation hence we are able to explain 1 complex sedimentary basins of inferred mesozoic age 2 a prominent unconformity and onlap surface of middle eocene to early miocene age at the base of flat lying sediments beneath the axis of new caledonia trough 3 gently dipping variable thickness and locally deformed late cretaceous strata along the margins of the trough 4 platform morphology and unconformities on either side of the trough that indicate a phase of late eocene to early miocene uplift to near sea level followed by rapid oligocene and miocene subsidence of 1100 1800 m and 5 seismic reflection facies tied to boreholes that suggest absolute tectonic subsidence at the southern end of new caledonia trough by 1800 2200 m since eocene time the cenozoic part of the model involves delamination and subduction initiation followed by rapid foundering and rollback of the slab this created a deep 2 km enclosed oceanic trough 2000 km long and 200 300 km across in eocene and oligocene time as the lower crust detached with simultaneous uplift and local land development along basin flanks disruption of late cretaceous and paleogene strata was minimal during this cenozoic phase and involved only subtle tilting and local reverse faulting or folding basin formation was possible through the action of at least one detachment fault that allowed the lower crust to either be subducted into the mantle or exhumed eastward into norfolk basin we suggest that delamination of the lithosphere with possible mixing of the lower crust back into the mantle is more widespread than previously thought and may be commonly associated with subduction initiation such as cenozoic events in the mediterranean and western pacific

timing and thermal evolution of fold and thrust belt formation in the ultima esperanza district 51 s chile constraints from k ar dating and illite characterization k ar dating on llite in upper cretaceous low grade metamorphic pelites in the torres del paine area was used to set new time constraints on the development of the patagonian retroarc fold and thrust belt ftb caused by the subduction of the antarctic plate beneath the south american plate the combined use of illite crystallinity kubler index polytype quantification and k ar dating of illite fractions 0 2 2 and 2 6 m allowed to distinguish four distinct periods of illite growth based on their k ar ages and degree of regional metamorphism 1 early cenomanian 98 ma illite crystallization 2 widespread early campanian ca 80 ma diagenetic illite growth under anchizonal metamorphic conditions 3 a significant period of illite formation in the early paleocene ca 60 ma and 4 a late stage of illite growth in the early eocene 55 46 ma under epizonal conditions the earliest indication for the emergent ftb formation in the hinterland is documented in a metapelitic clast 14 9 2 m within the upper cretaceous cerro toro conglomerate which yields a k ar cooling age of 98 3 1 2 ma and an epizonal ki value of 0 24 2 after a certain period of geological quietness an interval of major thrusting and uplift occurred between ca 60 and 46 ma the east dipping rio nutria and rio rincon thrusts record the onset of thrust and fold activity which can be placed close to 60 ma they also mark the frontal thrust towards the less deformed magallanes foreland basin in the western part of the internal domain widespread fault and thrust activity of the frontal wedge and associated thermal overprint continued and is recorded until 46 ma by k ar illite cooling ages the flexural subsidence that is driven by the thrust sheet loading in the internal domain was responsible for the eastward migration of the foreland depocenter and the rapid increase of sedimentation rate along the monoclinal belt no miocene thrusting nor uplift event has been recorded by k ar illite dating in the study area

alignment between seafloor spreading directions and absolute plate motions through time this research was supported by sief project rp 04 174 s e w and r d m arc grant dp130101946 r d m and arc grant ih130200012 all authors we thank seth stein and an anonymous reviewer for comments joanne whittaker and sascha brune for earlier suggestions and john cannon and michael tetley for invaluable programming expertise results were derived using gplates boyden et al 2011 all data sets used are available on request figures were created using gmt wessel and smith 1998 and matplotlib hunter 2007

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

tectonic accretion and underplating of mafic terranes in the late eocene intraoceanic fore arc of new caledonia southwest pacific geodynamic implications this paper deals with the tectonic events that result in the accretion of mafic terranes in the fore arc region and a close juxtaposition of ultramafic rocks low grade and high grade mafic terranes in many collisional orogens the example is taken from new caledonia where tectonic accretion subduction underplating and obduction of mafic terranes took place during the late eocene in an intra oceanic forearc setting the late eocene tectonic complex comprised three major terranes an overlying ultramafic mainly harzburgitic allochthon named the ophiolitic nappe an intermediate mafic mainly basaltic off scraped melange composed of kilometre scale slices of oceanic upper crust called the poya terrane parts of which have been metamorphosed into an eclogite blueschist facies complex the pouebo terrane and a lower continental basement formed by the norkolk ridge terranes based upon exhaustive sampling of the mafic terranes and field surveys our tectonic micropaleontologic and geochemical data reveal that poya and pouebo terranes rocks originally formed within one single campanian to late paleocene oceanic basin floored by tholeiitic basalt associated with some minor seamount related intraplate alkali basalt the tholeiitic basalt displays a continuous range of compositions spanning between undepleted and depleted end members the former being volumetrically predominant the overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin the south loyalty basin which almost completely disappeared during eocene convergence the opening of this basin originally located to the east of the norfolk ridge was synchronous with that of tasman sea basin as a consequence of oceanward migration of the west dipping pacific subduction zone establishing the origin of the ultramafic ophiolitic nappe is beyond the scope of this paper however it appears to be genetically unrelated to the mafic poya and pouebo terranes although it was located in the late eocene fore arc the ophiolitic nappe and the corresponding oceanic lithosphere originated before the late cretaceous to the east of the south loyalty basin in a back arc setting or alternatively in a much older trapped basin for reasons that remain unclear a new east dipping subduction started in the eocene and consumed most of the south loyalty basin forming the intra oceanic loyalty arc due to a changing subduction regime underplating of the diahot terrane the mafic slices that now form the poya terrane were tectonically accreted in the loyalty fore arc region and remained under low pressure low temperature conditions possibly at the subsurface until the norfolk ridge reached the subduction zone diachronously this resulted in the final obduction of the fore arc area the two step obduction involved first the mafic complex forming the poya terrane and thereafter the lithospheric mantle that now forms the ophiolitic nappe in contrast pieces of the accretionnary complex were dragged down into the subduction zone underplated at depth ca 70 km and metamorphosed into high temperature eclogite to form the pouebo terrane metamorphics that display the same geochemical features as the poya terrane basalt a mid to late eocene syntectonic piggy back sedimentary basin the nepoui flysch basin mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore arc region due to the accretion and underplating of mafic ocean related and other material in contrast a slightly younger foreland basin located upon the norfolk ridge the priabonian bourail flysch basin received a massive input of detrital material derived from the norfolk ridge itself and a time increasing amount of mafic poya derived material that recorded the first step of obduction thereafter the bourail flysch was overthrust by the poya terrane and finally by the ophiolitic nappe at the same time buoyancy driven uplift and exhumation of the high pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone finally for a short time a new subduction started along the west coast of new caledonia and generated small amounts of active margin related magmas these events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre collision analogue of mafic ultramafic belts found in many collisional mountain ranges

intra orogenic extension driven by gravitational instability carpathian pannonian orogeny the extensional pannonian basin was formed in a few million years during miocene time synchronously with contraction in the surrounding alpine and carpathian orogens this system is characteristic of a class of extensional basins that form in the midst of active orogenic mountain forming belts the mechanism that causes this type of geological event is enigmatic but usually has been associated with subduction we examine a new hypothesis for intra orogenic extensional basin formation in which gravitational spreading of previously thickened crust triggers gravitational instability of the mantle lithosphere a basin is formed by lithospheric extension as shortening and lithospheric downwelling occur in the surrounding mountain belts this mechanism provides a mechanically self consistent explanation for all of the main structural features of the pannonian carpathian system and presents a plausible alternative to the popular view that subduction and slab rollback have driven the development of this basin

active tectonics of the western mediterranean geodetic evidence for rollback of a delaminated subcontinental lithospheric slab beneath the rif mountains morocco surface deformation in morocco derived from five years of global positioning system gps survey observations of a 22 station network four continuously recording gps cgps stations and four international gnss service igs stations in iberia indicates roughly southward motion 3 mm yr of the rif mountains morocco relative to stable africa motion of the rif is approximately normal to the direction of africa eurasia relative motion which is predominantly strike slip and results in shortening of the rif and subsequent crustal extension of the adjacent alboran sea region the sense and the n s asymmetry of the observed deformation i e no evidence for north directed shortening in the betic mountains north of the alboran sea cannot be easily explained in terms of crustal plate interactions suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western mediterranean the model that best fits the observations involves delamination and southward rollback of the african lithospheric mantle under the alboran and rif domains

magnetic anomalies and tectonic fabric of marginal basins north of new zealand detailed airborne magnetic studies conducted over the region of the s w pacific marginal basins extending from the solomon islands to new zealand suggest that three major phases of basin formation and island arc development have occurred in this region development of the tasman sea took place during the late cretaceous paleocene development of the basins to the east of the tasman sea occurred predominantly during the oligocene as well as during the upper miocene to recent the south fiji basin consisting of the kupe and minerva abyssal plains is marked by the presence of possibly two rrr triple junction spreading centers that were active between the times of anomalies 13 to 7 36 25 5 m y the kupe abyssal plain shows the presence of residual magnetic anomalies 7 to 13 of the eastern limb of the proposed spreading center the western limb appears to have been subducted beneath the present site of the three kings rise this seafloor spreading phase calculated half spreading rate of 35 mm yr was coincident with the overthrusting phase of the new caledonia ultramafic rocks during that period active volcanism along the then continuous solomons new hebrides fiji lau island arc was taking place magnetic anomalies from 1 to 4 0 8 m y b p are seen to extend along a clearly defined lineation pattern over the north fiji basin analysis of the magnetic anomalies suggests that during the upper miocene at about the time of magnetic anomaly 4 8 m y the solomons lau island arc commenced breaking up through the development of an active spreading center calculated half spreading rate of 35 mm yr located between the new hebrides and fiji portions of the island arc portions of the northeastern sector of the minerva abyssal plain were subducted beneath the north fiji basin during this period between 0 8 m y b p at the time of about anomaly 3 to 2 4 5 3 5 m y the development of the north fiji basin extended into the lau basin and havre trough thus separating the tonga and lau ridges as a result a distinct series of active spreading centers developed southward along the axis of the havre trough lau basin the havre trough is marked by present seismic and submarine volcanic activity residual magnetic anomalies 1 2 0 1 8 m y b p form a symmetrical pattern around the axis of the trough extending south from the lau basin to the active taupo white island volcanic zone of new zealand phases of development of the marginal basins described in this paper may be correlated with phases of changes in the direction and rate of pacific plate motion

widespread compression associated with eocene tonga kermadec subduction initiation eocene onset of subduction in the western pacific was accompanied by a global reorganization of tectonic plates and a change in pacific plate motion relative to hotspots during the period 52 43 ma we present seismic reflection and rock sample data from the tasman sea that demonstrate that there was a period of widespread eocene continental and oceanic compressional plate failure after 53 48 ma that lasted until at least 37 34 ma we call this the tectonic event of the cenozoic in the tasman area tecta its compressional nature is different from coeval tensile stresses and back arc opening after 50 ma in the izu bonin mariana region our observations imply that spatial and temporal patterns of stress evolution during western pacific eocene subduction initiation were more varied than previously recognized the evolving eocene geometry of plates and boundaries played an important role in determining regional differences in stress state

cenozoic structural history of the gippsland basin early oligocene onset for compressional tectonics in se australia abstract the gippsland basin contains some of the largest hydrocarbon accumulations in australia and has been in production since the 1920 s these hydrocarbons are trapped by large growth anticlines offshore in reservoirs of the cretaceous to eocene aged latrobe group despite the obvious importance of these growth anticlines the timing of their formation and the overall cenozoic tectonic history of the basin is not well understood here we present a detailed growth strata analysis of the faults and anticlines within the cenozoic sediments of the gippsland basin this indicates two major phases of tectonism in the basin 1 late cretaceous to eocene extension and 2 oligocene to holocene compression detailed analysis of the extensional phase indicates the development of numerous normal growth faults which display an overall reduction in the magnitude of extension from the late cretaceous to the eocene commonly terminating at the top of the latrobe group the shift to compressional tectonism occurred at approximately the eocene oligocene boundary 34 ma a major and widespread episode of compression then occurred with evidence of growth on anticlines and reverse faults beginning in the early oligocene this previously unrecognized early oligocene event produced significant growth 20 50 total growth of the major anticlines which host hydrocarbon accumulations the early oligocene event represents the first phase of the compressional tectonic regime that continues to the present day in se australia it appears likely that this oligocene event affected other basins in se australia and probably contributed to uplift of the eastern highlands the underlying tectonic cause for this oligocene compressional regime is enigmatic and may be related to far field tectonic processes a second pulse of compressional tectonism and anticline growth occurred during the mid miocene and is generally more significant in the onshore regions of the basin the youngest phase of compressional tectonism beginning in the late miocene 10 ma is also more intense in onshore regions and is marked by an unconformity that is widespread in se australia

anomalous uplift of the apennines and subsidence of the adriatic the result of active mantle flow 1 we consider the potential for interpreted mantle flow to generate anomalous topography across the north central mediterranean a seismic tomography section was translated into the temperature domain and used as input into a series of 2d thermo mechanical numerical models of crust mantle deformation the generated dynamic topography signal was then compared with a derived residual topography that showed a distinct residual low across the adriatic sea and easternmost apennines and a residual high to the west of the apennine range and into the tyrrhenian sea a good spatial correlation was found between the modeled dynamic effects of inferred underlying mantle structure and the observed residual topography the results of this study suggest that underlying mantle structure may play a role in supporting the dynamic anomalous surface topography along the investigated profile and within the north central mediterranean

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

lateral variation of aeromagnetic anomaly in south china and its tectonic implications we present a series of images from the aeromagnetic data observed in south china areas and reveal the detailed structures of the south china block by using lateral variation of the aeromagnetic anomalies we characterize individual geological blocks and suggest the deep boundaries of the blocks the yangtze block and jiangnan orogenic belt are characterized by low aeromagnetic anomalies whereas high values of the aeromagnetic anomaly are observed in the cathaysia block the boundary between the yangtze and cathaysia blocks which remains unclear to date can be plotted along the large magnetic gradients the high magnetic anomalies significantly relate to volcanic distributions in south china areas these aeromagnetic observations seem to be useful to investigate the intracontinental tectonic evolution in this region

origin of the new caledonian ophiolites based on a french australian seismic transect abstract the origin age and nature of the new caledonian ophiolites are still debated recently as part of the joint french australian seismic transect faust program the australian research vessel rig seismic recorded a series of multichannel deep seismic profiles between the new hebrides arc and the australian margin these profiles which ran south of new caledonia image the southern prolongation of the overthrusted ophiolites and allow a new interpretation of the eocene compressive tectonism along the new caledonia norfolk ridge a model involving abduction of the whole oceanic lithosphere of the loyalty basin is consistent with the age and origin of the ophiolite variations in tectonic style along strike in the north south trending part of the norfolk ridge have produced ophiolite exposures related to uplifted and partially overthrust upper mantle slivers within the southern part of the loyalty basin

the geologic record of deep episodic tremor and slip transient and episodic slow slip accommodates a great deal of tectonic strain and may be mechanically linked with locked regions of seismically hazardous faults best documented in subduction zones and associated with nonvolcanic tremor most proposed mechanisms for slow slip revolve around the transition between stable and unstable frictional sliding the dilemma is that slow slip is generated at a wide range of crustal depths including at pressure temperature conditions where frictional deformation mechanisms give way to predominantly viscous ones we present a model for how fracture and viscous flow within mid crustal shear zones can produce episodic creep transients our model for such transients stems from geological examples of shear zones that formed at temperatures and pressures of 500 c and 0 6 gpa during early orogenesis following late cretaceous subduction of a backarc ocean basin within these shear zones relatively strong lenses of metabasalt localized fluid filled fractures that were subsequently deformed by viscous flow in surrounding quartzofeldspathic gneiss the spatial and temporal characteristics of the modeled creep events are similar to those of slow slip events observed in modern subduction zones we therefore suggest that some episodic tremor and slip can originate through combined fracture and viscous flow across shear zones comprising mixtures of strong and weak materials

thermal evolution rate of exhumation and tectonic significance of metamorphic rocks from the floor of the alboran extensional basin western mediterranean high grade metamorphic rocks drilled at ocean drilling program site 976 in the alboran sea show a pt path characterized by decompression from about 1050 mpa 40 km depth to 350 mpa 13 km depth accompanied by an increase in temperature from about 550 50 c to 675 25 c the ar ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20 5 18 ma which coincides with the initiation of sedimentation in the alboran sea basin the alboran sea formed by miocene extension on the site of a late cretaceous to paleogene contractional orogen and extension coincided with thrusting in the peripheral parts of the betic rif arc which surrounds the basin on three sides thermal modeling of the pt path was carried out with the aim of constraining geodynamic models for the formation of the basin variables considered in the modeling included 1 the thickness and thermal gradient of the postorogenic lithosphere 2 the radiogenic heat production in the thickened crust 3 the time gap pause between the end of contractional tectonics and the start of extension 4 removal of lithospheric mantle below 125 75 or 62 5 km and 5 the rate of extension the only combinations of variables that produce modeled pt paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause to produce high temperatures in rocks initially at 40 km depth removal of lithosphere below 62 5 km to produce further heating during decompression extension by a factor of 3 in 6 m y to delay the attainment of the maximum temperature until the rocks reached shallow depths and final exhumation and cooling in about 3 3 m y to satisfy radiometric and petrological constraints this gives a maximum of about 9 m y for exhumation from 40 km depth to the surface lithospheric stretching in response to plate boundary forces such as trench rollback without removal of lithosphere cannot explain the late onset of heating and the high temperatures reached by these rocks removal of lithosphere at depths significantly greater than 62 5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature only a combination of significant postcollisional radiogenic heating then wholesale removal of lithospheric mantle below the orogenic crust followed by rapid stretching can explain the observed pt path these results appear to support some form of lithospheric delamination as the primary cause for the formation of the alboran sea basin

development patterns of an isolated oligo mesophotic carbonate buildup early miocene yadana field offshore myanmar abstract the development history of an oligo mesophotic early miocene isolated carbonate system 160 m in thickness forming the uppermost part of the oligo miocene yadana buildup northern andaman sea has been evidenced from the integration of sedimentological core studies from 4 wells cumulated core length 343 m well correlations seismic interpretation and analysis of the ecological requirements of the main skeletal components three types of carbonate factory operated on the top of the platform depending on water depth turbidity and nutrient level 1 a scleractinian factory developing under mesophotic conditions during periods of high particulate organic matter supplies 2 an echinodermal factory occupying dysphotic to aphotic area of the platform coevally with the scleractinian factory 3 a large benthic foraminiferal coralline algal factories prevailing under oligo mesophotic and oligo mesotrophic conditions the limited lateral changes in facies between wells together with the seismic expression of the yadana buildup suggest deposition on a flat topped shelf carbonate production and accumulation on the yadana platform was mainly controlled by light penetration nutrient content and hydrodynamic conditions scleractinian rich facies resulted from transport of coral pieces derived from mesophotic environments mounds and deposited in deeper low light mud rich environments in which lived abundant communities of suspension feeders such as ophiuroids changes in monsoonal intensity terrestrial runoff from the irrawaddy river upwelling currents and internal waves activity during the early miocene are likely responsible for significant variations in water turbidity and nutrient concentration in the andaman sea thus promoting the development of an oligo mesophotic incipiently drowned platform

kinematics and surface fracture pattern of the anaran basement fault zone in nw of the zagros fold thrust belt the preexisting north south trending basement faults and their reactivation played an important role during the evolution of the zagros fold thrust belt the anaran basement fault in the lurestan r

petrography and stable isotope geochemistry of oligocene miocene continental carbonates in south texas implications for paleoclimate and paleoenvironment near sea level abstract cenozoic sedimentary rocks in the southern texas gulf coastal plains contain abundant continental carbonates that are useful for reconstructing terrestrial paleoclimate and paleoenvironment in a region near sea level our field observations and thin section characterizations of the oligocene and miocene continental carbonates in south texas identified three types of pedogenic carbonates including rhizoliths carbonate nodules and platy horizons and two types of groundwater carbonates including carbonate cemented beds and carbonate concretions with distinctive macromorphologic and micromorphologic features based on preservations of authigenic microfabrics and variations of carbon and oxygen isotopic compositions we suggest these carbonates experienced minimal diagenesis and their stable isotopic compositions reflect paleoclimate and paleoenvironment in south texas our oligocene and miocene carbonate clumped isotope temperatures t 47 are 23 28 c slightly less than or comparable to the range of modern mean annual and mean warm season air temperature 21 27 c in the study area these t 47 values do not show any dependency on carbonate type or trends through time suggesting that groundwater carbonates were formed at shallow depths these data could indicate that air temperature in south texas was relatively stable since the early oligocene the reconstructed paleo surface water 18o values are similar to modern surface water which could indicate that meteoric water 18o values also remained stable since the early oligocene mean pedogenic carbonate 13c values increased 4 6 during the late miocene most likely reflecting an expansion of c4 grassland in south texas this study provides the first mid and late cenozoic continental records of paleoclimate and paleoecology in a low latitude near sea level region

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

linking active margin dynamics to overriding plate deformation synthesizing geophysical images with geological data from the norfolk basin the tonga kermadec subduction system in the southwest pacific preserves a series of crustal elements and sediments which have recorded subduction initiation rift and back arc basin formation the norfolk basin is the farthest landward of all back arc basins formed in the tonga kermadec region and may preserve the earliest record of subduction initiation regionally for the norfolk basin we use a set of multibeam bathymetry magnetic and seismic reflection and refraction data to constrain basin structure and the mode and timing of formation a structural interpretation reveals a two stage tectonic evolution 1 a convergent tectonic regime until 38 34 ma alternatively related to island arc collision or subduction initiation and 2 lithospheric extension after 34 ma these observations may help to constrain mechanical models that predict rapid extension following convergence of the overriding plate during subduction initiation or arc reversals

mesozoic history of the fairway aotea basin implications for the early stages of gondwana fragmentation the fairway ridge is a buried continental structure that separates the fairway basin from the new caledonia basin the proposed cretaceous age of the fairway basin has remained highly hypothetical to date deep offshore petroleum exploration wells revealed well dated mesozoic carbonaceous sedimentary rocks in the taranaki basin at the southern end of the aotea basin in this paper we use geophysical data to confirm the continuity of the 2000 km long fairway aotea basin connecting new caledonia to new zealand and prove its early late cretaceous age analysis of seismic reflection profiles together with newly compiled gravity and magnetic maps reveals late cretaceous ne sw trending lineaments projecting northeastward from major tasman sea fracture zones and the bellona trough which demonstrate that the opening of the fairway aotea basin predates the opening of the tasman sea this result combined with observations of the mesozoic regional geology suggests that the lord howe fairway and norfolk ridges are part of a remnant late early cretaceous continental arc which was fragmented into three pieces by the late early to early late cretaceous this event might be contemporaneous with a plate motion change between the gondwana and pacific plates and or the arrival of the hikurangi plateau in the subduction zone around 105 ma which caused the cessation of subduction along this plate boundary we interpret either of those two events as being possible trigger events for the post early cretaceous fragmentation of the eastern gondwana margin in a slab retreat process

the influence of inherited continental margin structures on the stress and strain fields of the south central taiwan fold and thrust belt in this paper we test whether or not structural and morphological features inherited from the eurasian continental margin are affecting the contemporary stress and strain fields in south central ta

petrogenesis of the early silurian dashuang high mg basalt andesite dacite in eastern south china origin from a palaeosubduction modified mantle the early paleozoic kwangsian event the first compressive orogeny after the rodinia assemblage in the south china block scb is characterized by the angular unconformity between the devonian and pre devonian sequences and widespread granitoids and small volume mafic rocks however the mechanism triggering this event is highly debated and the associated mantle source is poorly known our recent investigations identified the occurrence of early silurian high mg basalt andesite dacite in the dashuang area along the northwestern yunkai domain in the scb this study presents a set of new zircon u pb geochronological whole rock geochemical and sr nd isotopic data for these volcanic rocks they yield weighted mean 206 pb 238 u ages of c 430 443 ma similar to the age peaks of early middle paleozoic magmatism and metamorphism in the scb these volcanic rocks have high mgo cr and ni contents and mg numbers they also exhibit arc like geochemical affinities with crustal like sr nd isotopic compositions similar to the neoproterozoic high mg basalt andesite in the eastern scb such signatures combined with other geological observations suggest that the dashuang high mg rocks were derived from an ancient probably early neoproterozoic mantle wedge source the early paleozoic intra continental reactivation in response to late orogenic extension triggered the melting of this palaeosubduction modified mantle which led to the formation of these volcanic rocks

cavalli seamount northland plateau sw pacific ocean a miocene metamorphic core complex cavalli seamount 34 06 s 174 10 e is an irregular shaped flat topped seamount on the northland plateau sw pacific ocean in contrast to miocene lavas recovered from other seamounts in the area the rocks dredged from cavalli seamount consist of biotite schist with minor calc silicate bands mineralogy whole rock chemistry u pb dating of detrital zircons and sr and nd isotopic composition collectively indicate a siliciclastic carbonate protolith of latest cretaceous to paleogene stratigraphic age metamorphic index minerals are sillimanite andesine garnet ilmenite and k feldspar and indicate peak p t conditions of c 650 c and c 0 4 gpa u pb ar ar and fission track dating of minerals and micropalaeontological dating of associated limestones indicate rapid cooling in the interval 23 21 ma the schist contains chloritized and hematized slickenslides parallel to a penetrative lineation s c planes and conjugate microfractures and has limestone geopetal infillings parallel to foliation these features are consistent with the interpretation of cavalli seamount as part of the lower plate of an early miocene metamorphic core complex although other explanations are possible the core complex interpretation fits with regional early miocene events including crustal thickening as a result of allochthon emplacement in northern new zealand crustal thinning associated with rapid extension in the south fiji basin pacific trench rollback and abundant early miocene igneous activity

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

enigmatic formation of the norfolk basin sw pacific a plume influence on back arc extension 1 the norfolk basin is a small back arc basin in the sw pacific with an unknown age and origin for its formation moreover the process behind its formation as a back arc basin in a region of extensive volcanism has previously never been resolved we have conducted a detailed interpretation of recently acquired swath bathymetry gravity and magnetic data from the northern norfolk basin faust 2 survey together with existing data from the region to address the age and origin of the norfolk basin and propose a new model for basin evolution the norfolk basin is characterized by complex bathymetry which formed during three distinct tectonic phases 1 regions of elevated topography termed plateaus show characteristics of thickened oceanic crust formed during the cretaceous normal superchron in either a back arc basin environment or as a trapped piece of pacific phoenix oceanic crust 2 regions of deep bathymetry termed basins separate the cretaceous plateaus and show basement textured by seafloor spreading fabric of miocene age 3 widespread intraplate plume related volcanic activity post dating the formation of the plateaus but predating the formation of the basins a miocene model for the formation of the basin regions in the norfolk basin requires a 700 km landward jump in the location of back arc spreading even with continued subduction at the tonga kermadec trench and the seaward migration of the subduction hinge the cessation of spreading in the basin regions would have been followed by an eastward jump of back arc spreading by about 1000 km into the current lau basin we propose that the driving force for such an enormous landward jump in back arc spreading is active rifting driven by a mantle plume this scenario is supported by the extensive miocene intraplate volcanism observed in the norfolk basin and the elevated bathymetry of the entire region in this model mantle driven uplift and excess volcanism would have favored back arc rifting landward rather than seaward of the existing south fiji basin in an extensional regime on the over riding plate caused by the continued roll back of the subduction hinge this scenario is supported by the numerous miocene volcanic cones in the norfolk basin which are mainly located on the preexisting plateaus probably predating the formation of the basins as well as their plume melt signature the norfolk basin appears to be the first known example of a major back arc basin whose position relative to the slab was not controlled by subduction driven mantle flow and melting but by plume tectonics this requires a re evaluation of the traditional back arc basin theory in supra subduction zones

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

basement geology from three kings ridge to west norfolk ridge southwest pacific ocean evidence from petrology geochemistry and isotopic dating of dredge samples abstract we present new petrographie x ray fluorescence electron microprobe ar ar radiometric fission track and sr nd and pb isotopic data for igneous and metamorphic rocks from 18 dredges from a number of basins and ridges in the southwest pacific ocean the dredge samples can be divided into four groups 1 permian to cretaceous gabbroids granitoids and hornfelses from the west norfolk and wanganella ridges 2 mainly cretaceous mafic igneous rocks from the reinga ridge and vening meinesz escarpment 3 subduction related late oligocene and early miocene basalts and shoshonites from the three kings and southern norfolk ridges and 4 back arc basin and intraplate basalts of early miocene age in the south and north norfolk basins respectively and intraplate basalts of pliocene age in the reinga basin our dredge data combined with regional magnetic and gravity data provide much needed tests of earlier tectonic interpretations of the region correlatives of the permian brook street terrane and mesozoic median tectonic zone of onshore new zealand clearly extend along the west norfolk and wanganella ridges the three kings ridge was active at least in the 19 to 20 ma period and was probably generated above a west dipping subduction zone the norfolk basin was a back arc basin to this arc and had opened by 18 20 ma the oldest cenozoic volcanic rocks in the region are 26 ma subduction related volcanic breccias on the southern norfolk ridge they probably predate opening of the norfolk basin and associated eastward migration of the three kings ridge