reconciling heterogeneity in crystal zoning data an application of shared characteristic diagrams at chaos crags lassen volcanic center california isotope trace element and textural crystal zoning patterns record heterogeneity in magmatic systems not resolved by whole rock analyses these zoning data are used to infer crystal residence times magma mixing and other magmatic processes in many magmatic systems we present the shared characteristic diagram scd as an organizational framework for crystal zoning data that compares information from different phases and chemical tracers in a common framework an example from chaos crags in the cascade arc produces three main results 1 anorthite zoning profiles in plagioclase have fewer shared characteristics in mafic inclusions than in the host rhyodacite 2 single crystal 87sr 86sr data from previous studies tepley et al 1999 are consistent with more shared history between crystals than in anorthite profiles this difference reflects a more homogeneous distribution of 87sr 86sr than the intensive parameters controlling plagioclase composition 3 the chaos crags system exhibits a layer of heterogeneity in crystal populations that is not represented in whole rock analyses that indicate only simple binary mixing the inconsistency between 87sr 86sr and anorthite zoning data highlights decoupling between compositionally controlled and temperature water controlled zoning in plagioclase from chaos crags

influence of offset weak zones on the development of rift basins activation and abandonment during continental extension and breakup 1 we use numerical modelling to investigate reactivation of inherited offset weak zones owz in continental crust and mantle weak zones mwz to form offset rift basins during continental rifting and breakup offset rift basins are basins that are set off offset from the main rift locus of breakup weak zones embedded in a stiff layer are preferentially and rapidly reactivated whereas the same zones are either ignored or slowly reactivated when embedded in pliable layers here stiff implies a nonlinear flow law with a high stress exponent n 10 000 a plastic material and pliable means a low stress exponent n 2 5 as in ductile power law creep of rocks whether offset rift basins form during rifting of a composite lithosphere i e comprising stiff and pliable layers depends on the competition between necking instabilities that develop at the weak zones in the stiff layers and the coupling between the stiff and pliable layers stiff cratonic lithosphere results in early localization of the deformation at the mwz rapid necking and breakup without developing offset rift basins in contrast warm pliable lithosphere develops significant offset basins and has protracted rifting because the mwz is now embedded in a pliable layer we also investigate the influence of owz dip sedimentation and the sensitivity of reactivation to the distance from owz to the mwz and to the size of the mwz a tectonic rifting styles diagram is used to show that the model results agree with natural examples

kinematics of cenozoic extension on the south china sea continental margin and its implications for the tectonic evolution of the region abstract in the northern south china sea several large sedimentary basins developed during the cenozoic characterized by episodic rifting clockwise rotation of the rifts an eastward aging breakup unconformity and intensifying crustal extension to the east these features may be summarized by a scissors model the maximum rifting occurred north of the maximum thermal subsidence which was in turn north of the seafloor opening in contrast the nansha microcontinent south of the south china sea basin was dominated by compressional deformation during the cenozoic extension was of subordinate significance and manifested mainly by block faulting and tilting in the late early oligocene maximum tilting occurred in the southern part of the nansha microcontinent at distance from the deep sea basin of the south china sea the asymmetric development of extensional structures around the south china sea basin may be explained by the simple shear model proposed by wernicke 1981 the tectonic development of the south china sea was the consequence of various interactions of three major plates and may be summarized into three stages with dominating influences of respectively the retreat of the west pacific subduction zone in the late cretaceous the hard collision and impinging of india to tibet since the late eocene and the fast northward subduction of the indian ocean australian plate since the late early miocene

hyper extended rift systems in the xisha trough northwestern south china sea implications for extreme crustal thinning ahead of a propagating ocean abstract from the apex of the nw sub basin of the south china sea we present new long cable deep penetration seismic profiles and deepwater drillings together with corrected seismic interpretations from previous studies structural mapping demonstrates that the xisha trough is characterized by a hyper extended rift system that shows evidence of extremely thinned crust and rotated fault blocks a series of ne nee and w e striking listric normal faults along the xisha trough which merge at depth into moho reflection record strongly extensional deformation based on the absence of an acoustic basement beneath a wedge shaped rifted sequence we document three diapiric magma structures exiting the moho and penetrating into the extremely thinned continental crust which is covered by a cenozoic sequence for the xisha trough we can demonstrate based on seismic data and calculated fault slip rates the faults bounding the xisha trough were initially formed by distributed rifting and were reactivated and focused on faulting along the axis of the xisha trough however the peak faulting migrated along the margin strike from east to west and terminated at the apex of the xisha trough one of the key processes controlling the evolution of the xisha trough is depth dependent extension which may arise from differential lithospheric stretching by regional dynamic lower crustal mantle flow

geochemical and isotopic zoning patterns of plagioclase megacrysts in gabbroic dykes from the gardar province south greenland implications for crystallisation processes in anorthositic magmas chemical and sr isotopic zoning patterns in plagioclase megacrysts from gabbroic dykes in the gardar province can be used to elucidate magma chamber and emplacement processes the megacrysts occur either as single crystals or assembled as anorthosite xenoliths the size of the megacrysts varies from 1 cm to 1 m they consist of a large core with variable zonation an58 39 and a relatively small 600 m normally zoned rim an62 27 the contact between core and rim is sharp and marked by a sharp increase in anorthite content which can reach 11 mol an this gap is interpreted as having formed during dyke emplacement due to a sudden pressure release some of the megacryst cores show a fairly constant composition whereas others exhibit an unusual wavy oscillatory zoning which has not been reported elsewhere to our knowledge the oscillatory zoning has wavelengths of up to 2 500 m and a maximum amplitude of 7 mol an it is interpreted as reflecting movements of the crystals in the magma reservoir the sr isotopic composition of one crystal shows a radiogenic inner core 87sr 86sr i 0 7044 and a less radiogenic outer core 87sr 86sr i 0 7039 0 7036 the lack of a significant change between outer core and rim 87sr 86sr i 0 7037 is consistent with formation of the more an rich rim due to pressure release variations in the core may be related to movements of the crystal and or magma mixing a trace element profile across a megacryst shows a small increase in sr and small decreases in ba and la contents of the recalculated melt composition across the core rim boundary whereas p ce nd and eu remain constant melt compositional changes upon emplacement are therefore considered to be of minor importance constant ratios of incompatible trace elements in the megacryst cores indicate a dominant influence of a lower crustal source on trace element budgets

stratigraphy structure and potassic alteration of miocene volcanic rocks in the sleeping beauty area central mojave desert california the sleeping beauty area of the southeastern cady mountains central mojave desert contains a sequence of volcanic rocks 3 km thick which was erupted approximately 20 m y ago plate reconstructions indicate that the change from subduction to transform fault tectonics off the coast of california occurred about 20 m y ago as well and so these rocks contain a record of volcanism extensional faulting and potassic metasomatism during this time of tectonic transition the volcanic sequence comprises basalt to rhyolite flows and tuffs which are overlain by the widespread peach springs tuff and by terrestrial sediments bedding below the peach springs tuff generally dips 10 50 to the southwest and is cut by numerous steep northwest to north trending faults tilting predated deposition of the peach springs tuff bedding fault relationships indicate that deformation was not by tilted book geometry in its simplest form no significant low angle faults are exposed but such faults occur to the west and may underlie the area a major open southeast trending anticline in the area may be a drag fold related to the post late miocene right lateral north trending ludlow fault all units below the peach springs tuff were locally affected by severe potassic metasomatism which raised measured k2o contents to as high as 13 3 wt metasomatized rocks occur in irregular zones that follow northwest trending faults and are best developed around northwest trending breccia zones which have jasper matrices ba and mn prospects are invariably found in metasomatized rocks geologic and geochemical constraints indicate that metsomatism occurred at shallow depth 1 or 2 km and low temperature 150 c metasomatism occurred in at least two distinct pulses and apparently predated deposition of the peach springs tuff the k may have been derived from percolating closed basin brines or through hydrogen metasomatism of rocks deeper in the complex

numerical modeling of failed rifts in the northern south china sea margin implications for continental rifting and breakup abstract failed rifts record important information of continental extension and breakup process in the northern south china sea scs margin the tainan southern depression and the baiyun sag to the east are characterized with lower crust high velocity anomalies lchva and intracrust detachment faults whereas the xisha trough to the west develops on a larger scale with crust cutting normal faults and absence of lchva these contrasts indicate different rifting processes between the northeastern and northwestern scs 2d numerical modeling is performed to understand the formation mechanism of these failed rifts two types of mechanisms are proposed i syn rift competitive type and ii rift migration type with a half extension rate of 2 cm yr and 1 5 cm yr respectively in type i two rifts develop initially on the shoulders of the weak zone but they compete with each other during extension one rift becomes dominant to furnish the final breakup whereas the other one is abandoned the crust structure of this type fits the observations in the baiyun sag and the tainan southern depression however in type ii only one rift develops at the beginning the initial rifting center will migrate and the final continental breakup will occur at a place far from the initial rifting location in this type normal faults cut through whole crust and wide extensional margins will form such as observed in the xisha trough our results suggest that the depth dependent extension of the scs is strongly heterogeneous resulting primarily from varying extensional rates

seismic reflection evidence for a dangerous grounds miniplate no extrusion origin for the south china sea the collision of india and asia has caused large strike slip faults to form in east asia resulting in the extrusion of crustal blocks toward the southeast since the eocene as a result of the indentation of rigid india into asia it has been suggested that the south china sea opened as a result of relative motion between a rigid indochina sundaland block and china alternative models propose that rifting and seafloor spreading were driven by trench forces to the south we test these competing models by analysis of seismic reflection profiles across the boundary between sundaland and the southern rifted margin known as the dangerous grounds we show that the southern boundary of the dangerous grounds is a subduction zone that jammed in the middle miocene to the west the dangerous grounds are bounded by a strike slip zone also active until similar to 16 ma that becomes diffuse south of the now inactive south china sea seafloor spreading center we place the western edge of the dangerous grounds just to the east of the natuna arch lupar line the west baram line is confirmed as originating as a major strike slip fault within the dangerous grounds and is continuous with the red river fault zone because the dangerous grounds were independent of sundaland until similar to 6 ma its motion cannot have been constrained by motion of this block making extrusion impossible as a mechanism to rift the south china sea se motion by both the dangerous grounds and sundaland suggests subduction forces were the primary trigger for plate motions our reconstruction places a similar to 280 km upper limit on the motion on the red river fault and a similar to 1400 km width to the paleo south china sea

cenozoic denudation history of southern hainan island south china sea constraints from low temperature thermochronology abstract as a large block of continental crust hainan island located in the western south china sea forms a rift shoulder to the adjacent offshore yinggehai and qiongdongnan basins and lies in the footwall of their main boundary faults we present 11 apatite fission track aft data and 16 u th sm he ages to elucidate the island s cenozoic morphotectonic evolution our data indicate that much of southern hainan island experienced a well defined cooling episode commencing in late eocene oligocene time during this time most samples cooled from 110 c to 55 15 c at a rate of 4 0 c ma following which the cooling rate decreased to 2 5 c ma assuming a constant paleogeothermal gradient of 23 c km similar to that of the present day since oligocene time when most samples cooled into the base of the aft partial annealing zone the total eroded section is estimated at a minimum of 3 5 km about two thirds of which was removed during the oligocene rapid cooling phase this implies that elevation of southern hainan island has decreased since the oligocene and this is supported by independent paleobotanical evidence although a further cooling episode since 5 ma is also suggested by aft thermal history models cooling occurred from relatively low temperatures 45 50 c where annealing kinetics are not well understood so this cannot be rigorously assessed despite evidence for increased sedimentation in offshore basins at this time our data suggest that the no 1 fault of the adjacent offshore yinggehai basin to the west which is thought to be an offshore extension of the red river fault zone was mainly active from late eocene through much of the oligocene but declined rapidly after 22 ma the marked reduction of potential sediment supply rate from hainan island into neighboring basins after that time suggests that miocene to recent sedimentation in the yinggehai basin was mainly supplied by the red river

crystal melt separation and the development of isotopic heterogeneities in hybrid magmas if a magma is a hybrid of two or more isotopically distinct endmembers at least one of which is partially crystalline separation of melt and crystals after hybridization will lead to the development of isotopic heterogeneities in the magma as long as some of the preexisting crystalline material antecrysts retains any of its original isotopic composition this holds true whether the hybridization event is magma mixing as traditionally construed bulk assimilation or melt assimilation once a magma scale isotopic heterogeneity is formed by crystal melt separation it is essentially permanent persisting regardless of subsequent crystallization mixing or equilibration events the magnitude of the isotopic variability resulting from crystal melt separation can be as large as that resulting from differential contamination multiple isotopically distinct sources or in situ isotopic evolution in one model a redistribution of onethird of the antecryst cargo yielded a crystal enriched sample with 87 sr 86 sr of 0 7058 whereas the complementary crystal poor sample has 87 sr 86 sr of 0 7068 in other models crystal rich samples are enriched in radiogenic sr isotopic heterogeneities can be either continuous controlled by the modal distribution of crystals and melt or discontinuous when there is complete separation of crystals and liquid the first case may be exemplified by some isotopically zoned large volume rhyolites formed by the eruptive inversion of a modally zoned magma chamber in the latter case the isotopic composition of any for example interstitial liquid will be distinct from the isotopic composition of the bulk crystal fraction the separation of such an interstitial liquid may explain the presence of isotopically distinct late stage aplites in plutons crystal melt separation provides an additional option for the interpretation of isotopically zoned or heterogeneous magmas this option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation dominated processes non isotopic chemical heterogeneities can also develop in this fashion

the temporal and spatial distribution of volcanism in the south china sea region cenozoic magmatism in the south china sea the associated continental margins coastal south china and indochina is reviewed volcanism contemporaneous with rifting and sea floor spreading in the south china sea was very weak in the margins and adjacent areas there was only very limited magmatic activity most probably after the cessation of sea floor spreading the south china sea has non volcanic margins large basement relief caused by extension formed major basins and a continental oceanic transitional zone in the northern margin reflects high crustal rigidity during rifting and drifting the paucity of a magma supply and presence of rigid fault blocks are unfavorable to the mantle flow model previously proposed to account for the opening of the south china sea which was supposedly linked to the escape of indochina induced by indo eurasia collision c 2005 published by elsevier ltd

prolonged post rift magmatism on highly extended crust of divergent continental margins baiyun sag south china sea three dimensional 3d seismic borehole and geochemical data reveal a prolonged phase of post rift magmatism on highly extended crust of the baiyun sag south china sea two volcanic complexes are identified and described in the context of continental rifting and diachronous continental breakup of the south china sea biostratigraphic data from exploration wells by7 1 and by2 complemented by k ar datings from core samples confirm that magmatic activity in the baiyun sag occurred in two main stages 1 a first episode at the base of the miocene 23 8 ma and 2 a second episode occurring at the end of the early miocene 17 6 ma the relative location of volcanic complexes in the baiyun sag and their stratigraphic position reveals prolonged magmatism inboard of the ocean continent transition zone during continental breakup we suggest that magmatism in the baiyun sag reflects progressive continental breakup in the south china sea with the last volcanic episode marking the end of a breakup sequence representing the early post rift tectonic events associated with the continental breakup process seismic and borehole data from this breakup sequence records diachronous magma emplacement and complex changes in depositional environments during continental breakup c 2016 the authors published by elsevier b v

thrust tectonics along the north western continental margin of sabah borneo widely accepted plate tectonic models suggest that an inactive subduction zone lies along the north west continental margin of sabah in contrast interpretation of reflection seismic data acquired by bgr shows an autochthonous continental terrane comprising an oligocene to early miocene carbonate platform being progressively overthrust by an allochthonous rock complex progressive compression resulted in the development of four structural zones imbricated thrust sheets zone iii two thrust sheet systems one on top of the other zone iv a complex zone with multiphase deformation zone v and piercement ridges zone vi

crustal thinning mantle exhumation and serpentinization in the porcupine basin offshore ireland evidence from wide angle seismic data new wide angle seismic data were gathered along a 230 km long profile that runs east west across a deep structural feature in the porcupine basin offshore ireland known as the porcupine arch ocean bottom seismometers were deployed at 3 4 km intervals and seismic sources fired every 120 m along it prominent primary and secondary arrivals indicate that the continental crust is extremely thin locally less than 2 km across the basin centre the sedimentary succession is up to 12 km thick and comprises three distinctive seismic layers the two uppermost layers are interpreted as mostly a post rift succession of cretaceous and cenozoic strata the lowest layer thins rapidly towards the basin centre and is interpreted as a succession of predominantly jurassic synrift sediments a strong asymmetry in both the geometry of the crust and the sedimentary layers is probably related to a simple shear mode of extension and the subsidence that it induced crustal thinning is far greater than in the adjacent rockall basin and local exhumation of continental mantle lithosphere may have occurred in parts of the porcupine basin low p n velocities beneath the porcupine arch are compatible with larger amounts of mantle serpentinization than in the rockall basin

three dimensional numerical modeling of upper crustal extensional systems 1 we focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models to date few 3 d models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones we use an arbitrary lagrangian eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions plastic materials weaken with accumulating strain to localize deformation a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions controls on the geometry and spacing of three dimensional plastic shear zones are investigated the sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested the model results indicate the primary controls of strain dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction the amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other resulting in a trade off effect between the amount of offset between the initial grabens and the strain weakening ratio the three dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions 1 small offset grabens with a single relay zone 2 intermediate offset grabens with one or more secondary step over graben segments and 3 large offset grabens with limited to no significant segment interaction

continent ocean transition and voluminous magmatic underplating derived from p wave velocity modelling of the east greenland continental margin summary deep seismic refraction data were gathered across the entire east greenland rifted margin north of the jan mayen fracture zone between 72 n and 75 n in 2003 investigations of the deep crustal structure of this continental margin provide constraints on the formation of the margin and its structural evolution during and after late cretaceous early tertiary rifting and continental break up we present here the results along two profiles located in the prolongation of the godthab gulf and the kejser franz joseph fjord regional p wave velocity models were derived from forward traveltime modelling of land stations and ocean bottom hydrophone obh recordings for the first time long deep seismic sounding transects off east greenland provide a full insight into the crustal architecture of the transition from continental to oceanic crust a mean result is the identification of voluminous magmatic underplating which is wider and thicker than previously thought p wave velocities of the underplated material range between 7 1 and 7 4 km s 1 and the horizontal extents on the profiles are 225 and 190 km the maximum thickness of the underplated material is 15 16 km furthermore the p wave velocity models reveal a 120 130 km wide continent ocean transition zone cot based on an interpretation of the extent of cretaceous syn rift sediments mixed with basaltic intrusions and the lateral increase of velocities in the crustal layers excess magmatism must have been present during a long term rifting process accompanying the extension of the continental crust and giving rise to the voluminous magmatic underplating a consequence of our interpretation of the seismic refraction data is a likely rift propagation in the greenland sea from north to south additionally a comparison of p wave velocity models of the east greenland margin and voring margin reveals significantly asymmetric crustal architectures the voluminous magmatic underplating and asymmetrical conjugate margins formations are considered as a mirror of complex pre and syn rift processes

open system magma chamber evolution an energy constrained geochemical model incorporating the effects of concurrent eruption recharge variable assimilation and fractional crystallization ec e raχfc significant petrogenetic processes governing the geochemical evolution of magma bodies include magma recharge including formation of quenched inclusions or enclaves heating and concomitant partial melting of country rock with possible contamination of the evolving magma body assimilation and formation and separation of cumulates by f ractional crystallization rafc although the importance of modeling such open system magma chambers subject to energy conservation has been demonstrated the effects of concurrent removal of magma by eruption and or variable assimilation involving imperfect extraction of anatectic melt from wall rock have not been considered in this study we extend the ec rafc model to include the effects of eruption and variable amounts of assimilation ax this model called ec e 0racfc tracks the compositions trace elements and isotopes temperatures and masses of magma body liquid melt eruptive magma cumulates and enclaves within a composite magmatic system undergoing simultaneous eruption recharge assimilation and fractional crystallization the model is formulated as a set of 4 t i s coupled nonlinear differential equations where the number of trace elements radiogenic and stable isotope ratios modeled are t i and s respectively solution of the ece 0racfc equations provides values for the average temperature of wall rock ta mass of melt within the magma body mm masses of cumulates mct enclaves men and wall rock m o a and the masses of anatectic melt generated m a and assimilated cm a in addition t trace element concentrations and i s isotopic ratios in melt and eruptive magma cm em dm cumulates cct em dm enclaves cen er d o r and anatectic melt ca ea d o a as a function of magma temperature tm are also computed input parameters include the user defined equilibration temperature teq a factor describing the efficiency of addition of anatectic melt c from country rock to host magma the initial temperature and composition of pristine host melt tom c o m em d o m recharge melt t o r c o r er dor and wall rock t o a c o a ea d o a distribution coefficients dm dr da and their temperature dependences dhm dhr dha latent heats of transition melting or crystallization for wall rock dha pristine magma dhm and recharge magma dhr as well as the isobaric specific heat capacity of assimilant cp a pristine cp m and recharge cp r melts the magma recharge mass and eruptive magma mass functions mr tm andme tm respectively are specified a priori mr tm and me tm are modeled as either continuous or episodic step like processes melt productivity functions which prescribe the relationship between melt mass fraction and temperature are defined for end member bulk compositions characterizing the local geologic site ec e 0racfc has potential for addressing fundamental questions in igneous petrology such as what are intrusive to extrusive ratios i e for particular magmatic systems and how does this factor relate to rates of crustal growth how does i e vary temporally at single long lived magmatic centers what system characteristics are most profoundly influenced by eruption what is the quantitative relationship between recharge and assimilation in cases where the extraction efficiency can be shown to be less than unity what geologic criteria are important and

different expressions of rifting on the south china sea margins abstract rifting of continental margins is generally diachronous along the zones where continents break due to various factors including the boundary conditions which trigger the extensional forces but also the internal physical boundaries which are inherent to the composition and thus the geological history of the continental margin being opened quite recently in the tertiary in a scissor shape manner the south china sea scs offers an image of the rifting structures which varies along strike the basin margins the scs has a long history of extension which dates back from the late cretaceous and allows us to observe an early stretching on the northern margin onshore and offshore south china with large low angle faults which detach the mesozoic sediments either over triassic to early cretaceous granites or along the short limbs of broad folds affecting palaeozoic to early cretaceous series these early faults create narrow troughs filled with coarse polygenic conglomerate grading upward to coarse sandstone because these low angle faults reactivate older trends they vary in geometry according to the direction of the folds or the granite boundaries a later set of faults characterized by generally e w low and high angle normal faults was dominant during the eocene associated half graben basement deepened as the basins were filling with continental or very shallow marine sediments this subsequent direction is well expressed both in the north and the sw of the south china sea and often reactivated earlier detachments at places the intersection of these two fault sets resulting in extreme stretching with crustal boudinage and mantle exhumation such as in the phu khanh basin east of the vietnam fault a third direction of faults which rarely reactivates the detachments is ne sw and well developed near the oceanic crust in the southern and southwestern part of the basin this direction which intersects the previous ones was active although sea floor spreading was largely developed in the northern part and ended by the late miocene after the onset of the regional mid miocene unconformity known as mmu and dated around 15 5 ma latest miocene is marked by a regional basement drop and localized normal faults on the shelf closer to the coast the se margin of the south china sea does not show the extensional features as well as the northern margin detachments are common in the dangerous grounds and reed bank area and may occasionally lead to mantle exhumation the sedimentary environment on the extended crust remained shallow all along the rifting and a large part of the spreading until the late miocene when it suddenly deepened this period also corresponds to the cessation of the shortening of the nw borneo wedge in palawan sabah and sarawak we correlate the variation of margin structure and composition of the margin mainly the occurrence of granitic batholiths and mesozoic broad folds with the location of the detachments and major normal faults which condition the style of rifting the crustal boudinage and therefore the crustal thickness

isotopic responses to basaltic injections into silicic magma chambers a whole rock and microsampling study of macrorhythmic units in the pleasant bay layered gabbro diorite complex maine usa the pleasant bay layered gabbro diorite complex 420 ma formed via repeated injections of mafic magma into a felsic magma chamber it is dominated by repeating sequences macrorhythmic units with chilled gabbroic bases which may grade upward into medium grained gabbro diorite and granite each unit represents an injection of mafic magma into the chamber followed by differentiation increases in sri and decreases in endi with stratigraphic height indicate open system isotopic behaviour and exchange between the mafic and felsic magmas isotopic variations of whole rock samples in individual macrorhythmic units do not conform to bulk mixing or afc models between potential parental magmas sr isotopic studies of single feldspar crystals from one macrorhythmic unit indicate that exchange of crystals between the resident felsic magma and mafic influxes was important that some of the rocks contain feldspar xenocrysts and that the rocks are isotopically heterogeneous on an intercrystal scale xenocryst abundance increases with stratigraphic height suggesting that crystal exchange occurred in situ the lack of disequilibrium textures in the xenocrystic feldspar indicates the evolved macrorhythmic magma and resident silicic magma were of a similar composition and likely in thermal equilibrium at the time of crystal transfer mafic chilled margins are enriched in alkalis and isotopically evolved compared with mafic dikes representing the parental melts and suggest rapid in situ diffusional exchange following emplacement of individual mafic replenishments

seismic imaging of the transitional crust across the northeastern margin of the south china sea abstract crustal structure across the passive continental margin of the northeastern south china sea scs is presented based on a deep seismic survey cooperated between taiwan and china in august 2001 reflection data collected from a 48 hydrophone streamer and the vertical component of refraction reflection data recorded at 11 ocean bottom seismometers along a nw se profile are integrated to image the upper 1 6 2 4 km s lower 2 5 2 9 km s and compacted 3 4 5 km s sediment the upper 4 5 5 5 km s middle 5 5 6 5 km s and lower 6 5 7 5 km s crystalline crust successively the velocity model shows that the thickness 0 5 3 km and the basement of the compacted sediment are strongly varied due to intrusion of the magma and igneous rocks after seafloor spreading of the scs furthermore several volcanoes and igneous rocks in the upper middle crust 7 10 km thick and a high velocity layer 0 5 km thick in the lower crust of the model are identified as the ocean continent transition oct below the lower slope in the northeastern margin of the scs a thin continent nw of the oct and a thick oceanic crust se of the oct in the continental margin of the northeastern scs are also imaged but these transitional crusts cannot be classified as the oct due to their crustal thickness and the limited amount of the volcano the magma and the high velocity layer the extended continent next to the gravity low and a sag zone extended from the sw taiwan basin may have resulted from subduction of the eurasian plate beneath the manila trench whereas the thick oceanic crust may have been due to the excess volcanism and the late magmatic underplating in the oceanic crust after seafloor spreading of the scs

field and geochemical constraints on mafic felsic interactions and processes in high level arc magma chambers an example from the halfmoon pluton new zealand the 140 ma halfmoon pluton of stewart island new zealand provides direct evidence for a number of physico chemical processes that operate at depth within active arc settings it is characterized by a sequence of mingled mafic sheets and enclaves that are preserved within intermediate felsic host rocks way up structures and textures are consistent with a younging direction to the south and allow a south dipping magma chamber stratigraphy to be constructed the similarity in modal mineralogy geochemistry and isotopic signature between the various components and their proximity in space and time indicates that the system can be considered as a single composite pluton with discrete portions active at different times interpretation of mingling structures and textures in the field combined with detailed geochemical and geochronological analyses has allowed the identification of several physical and chemical processes operating within the chamber including 1 multiple mafic magma pulses that were extensively mingled and sometimes physically mixed at the exposed level and at depth 2 the development of mineral fabrics e g aligned plagioclase and hornblende crystals and alignment of mafic enclaves caused by processes of crystal accumulation magmatic flow and compaction 3 shortening of the chamber during crystallization as a result of magmatic loading 4 fractional crystallization of the intermediate felsic host magma that was interrupted periodically by mafic magma intrusion 5 incremental assembly of the pluton from the base up 6 limited chemical transfer between the mingled mafic sheets and enclaves and the surrounding intermediate felsic host rocks variations in morphology chemistry and texture of the mingled mafic rocks relative to their position within the magmatic stratigraphy indicate that processes operating within the chamber varied in both space and time mineral assemblages chemical characteristics primitive isotopic signatures and a lack of zircon inheritance indicate that the amphibole rich calc alkaline halfmoon pluton was emplaced into a juvenile arc setting and experienced no contamination with ancient crustal materials a model of pluton construction and evolution is presented that shows that the halfmoon pluton consisted of at least two adjacent magma pods that grew incrementally by episodic replenishments of mafic magma into a magma chamber of evolving intermediate felsic composition data are consistent with a model whereby hydrous amphibole rich basaltic magmas ponded at the crust mantle interface and episodically rose injected and mingled

reaction rim growth on olivine in silicic melts implications for magma mixing finely crystalline amphibole or pyroxene rims that form during reaction between silicic host melt and cognate olivine xenocrysts newly introduced during magma mixing events can provide information about the timing between mixing and volcanic eruptions we investigated rim growth experimentally by placing forsteritic olivine in rhyolitic and rhyodacitic melts for times between 25 and 622 h at 50 and 150 mpa h2o saturated at the ni nio buffer rims of orthopyroxene microlites formed from high silica rhyolite and rhyodacite melts at 885 c and 50 mpa and in the rhyolite at 150 mpa and 885 c rims of amphibole with lesser orthopyroxene formed in the rhyolite at 150 mpa and 800 c and in the rhyodacite at 150 mpa and 885 c irregular convolute olivine edges and mass balance between olivine melt and rim phases show that olivine partly dissolved at all conditions iron rich zones at the exteriors of olivines which increased in width parabolically with time show that fe mg interdiffusion occurring in olivines was not outpaced by olivine dissolution linear increases of the square of rim widths with time suggest that diffusion within the melt is the rate controlling process for olivine dissolution and rim growth rims grew one half to one order of magnitude faster when melt water contents were doubled unless conditions were far above the liquidus rim growth rate in rhyolite increases from 0 055 0 01 m2 h at 885 c and 50 mpa to 0 64 0 13 m2 h at 800 c and 150 mpa melt composition has a lesser effect on rim growth rates with growth rate increasing as melt sio2 content decreases pyroxene rims on olivines in andesite erupted from arenal volcano costa rica grew at a rate of 3 0 0 2 m2 h over an eleven year period this rate is faster than those of the experiments due to lower melt viscosity and higher temperatures and suggests that a magma mixing event preceded the start of the eruption by days

seismic evidence of hyper stretched crust and mantle exhumation offshore vietnam abstract we study the evolution of the eocene recent phu khanh basin opened during the rifting of the south china sea scs this sub basin formed when continental crust ruptured along the east vietnam boundary fault evbf at the western edge of the scs using high quality long streamer seismic lines we interpret structures that highlight the different phases of the scs rifting and processes related to crustal boudinage extreme crustal thinning and mantle uplift that sometimes places sediments in contact with the moho discontinuity mark the central part of the basin the mantle is shallowest there and marks the final rupture of the continental crust during an intense phase of mantle upwelling there a low angle detachment fault separates several crustal blocks from the moho the cylindrical axis of the moho rise is roughly parallel to the trend of the south china sea propagator above the mantle the upper and lower crusts form large crustal boudins the network of normal faults is dense in the upper crust and occasionally propagates into the lower crust however the lower crust is missing at some places the seismic facies above the moho rise is poorly stratified and might have been affected by a certain degree of metamorphism at the apex of mantle uplift there are frequent indications of fluid circulations including volcanic edifices and gas escapes features three stages of extension are clearly identifiable with ages of the two youngest constrained by well calibration the first and oldest rift sequence is situated between the tilted pre rift basement and the oligocene horizons 32 ma the second is delimited by the oligocene to the mid miocene 15 5 ma horizons and the third is bound by the mid miocene and the upper miocene before 10 5 ma horizons these three rift episodes formed in at least two extension directions the first n s and the second nw se the distinct mid miocene 15 5 ma horizon is tilted and the above layers show a diverging reflection these are in turn sealed by an erosional unconformity before 10 5 ma although tectonic activity appears diachronous from north to south we suggest that cessation of rifting did not occur before 12 10 5 ma this differs from models derived from magnetic anomalies observed in the south china sea 15 5 to 20 ma

north palawan block philippines its relation to asian mainland and role in evolution of south china sea the island of mindoro the northern part of palawan island and the reed bank area southwestern philippines together constitute a continental fragment the north palawan block lying within an island arc oceanic setting the permian to paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the philippine archipelago these rocks also suggest that the north palawan block once occupied a pre drift position contiguous with the south china mainland four prominent pre neogene regional unconformities are recognized both onshore and offshore the china mainland in taiwan and in the north palawan block the synchrony of these unconformities and the facies relation of the unconformity bounded sedimentary units strongly suggest a common pre neogene history for all these areas by contrast an important regional unconformity occurring at the top of the middle miocene throughout the palawan area is absent from the asian mainland extension in the south china sea basin since the mesozoic which has separated the north palawan block from the asian mainland has been approximately uniform from west to east however there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea floor spreading from the foregoing observations and using the most recent magnetic spreading anomaly data for the south china sea a suite of palinspastic reconstructions has been prepared which shows the evolution of the south china sea area from the late triassic to the pliocene the reconstructions illustrate 1 the convergent continental margin setting of the north palawan block during much of the jurassic and cretaceous 2 the late cretaceous inception of the philippine island arc system 3 the subsequent counter clockwise rotation of the arc system from the late eocene onward 4 the paleocene to middle miocene opening of the south china sea and 5 the early to middle miocene collision between the north palawan block and the palawan subduction system

thermal and rheological structures of the xisha trough south china sea the xisha trough located in the northwest of the south china sea scs mainly rifted 30 ma ago has been a failed rift since the cessation of the seafloor spreading of the nw subbasin based on the velocity depth model along profile obh 4 across the xisha trough a seven layer density depth model is used to estimate density structure for the profile the relationship between seismic velocity and radiogenic heat production is used to estimate the vertical distribution of heat sources in the lower crust the 2 d temperature field is calculated by applying a 2 d numerical solution of the heat conduction equation and the thermal lithosphere thickness is obtained from the basalt dry solidus bds the rheology of the profile is estimated on the basis of frictional failure in the brittle regime and power law steady state creep in the ductile regime rheological model is constructed for a three layer model involving a granitic upper crust a quartz diorite lower crust and an olivine upper mantle gravity modeling supports basically the velocity depth model the moho along profile obh 4 is of relatively high heat flow ranging from 46 to 60 mw m 2 and the moho beat flow is higher in the trough than on the flanks the depth of the thermal lithospheric lower boundary is about 54 km in the center deepens toward two sides and is about 75 km at the northern slope area and about 70 kin at the southern xisha zhongsha block rheological calculation indicates that the two thinnest ductile layers in the crust and the thickest brittle layer in the uppermost mantle lie in the central region showing that the xisha trough has been rheologically strengthened which are mainly due to later thermal relaxation in addition the strengthening in rheology during rifting was not the main factor in hampering the breakup of the xisha trough c 2002 elsevier science b v all rights reserved

melt generation at volcanic continental margins no need for a mantle plume melt generation in a rifting environment is studied using a dynamic 2 d finite element model the lithosphere is extended to large realistic thinning factors assuming a mantle temperature of 1333 c the focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations the timing of melt production late synrift and the amounts of melt are consistent with observations at volcanic margins the dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins

rifting and magmatism in the northeastern south china sea from wide angle tomography and seismic reflection imaging we present a new travel time tomography velocity model and seismic reflection images that delineate the rift architecture and magmatic features of the rifted margin in the northeastern south china sea these data reveal moderately stretched crust 25 km thick along the continental shelf and thin but laterally variable crustal thickness in the distal margin along the continental slope crust rapidly thins to 4 km in a basin characterized by tilted fault blocks that sole into a low angle detachment strain was localized to a degree within the highly stretched basin but failed to progress to breakup and seafloor spreading crust in the distal margin is 12 15 km thick few extensional structures are apparent in the distal margin but seismic velocities are suggestive of highly thinned and magmatically intruded continental crust the magmatic features we interpret include volcanic zones at the top of the basement that deform or disrupt overlying postrift strata sills intruded into the postrift sedimentary section and a high velocity 6 9 7 5 km s lower crustal layer that we take to be magmatic underplating or pervasive lower crustal intrusions these features primarily occur in the distal margin and may have been emplaced during postrift seafloor spreading the postrift magmatism may have been induced by convective removal of continental lithosphere following breakup and the onset of seafloor spreading in the south china sea

evidence for a thick oceanic crust adjacent to the norwegian margin the oceanic crust created during this first few million years of accretion in the norwegian greenland sea lies at an unusually shallow depth for its age has a smooth upper surface and in many places the results of multichannel seismic reflection profiling reveal that its upper layers comprise a remarkable sequence of arcuate seaward dipping reflectors these have been attributed to lava flows generated during a brief period of subaerial seafloor spreading we describe the results of inversions of digitally recorded sonobuoy measurements and two ship expanded spread profiles collected over the oceanic crust adjacent to the norwegian passive margin we find that the crust of the deep lofoten basin is indistinguishable from normal oceanic crust in thickness and structure closer to the margin we observe up to a four times expansion in thickness of layers with velocities equal to those of oceanic layer 2 while the layer 3 region retains approximately the same thickness the area over which the seaward dipping reflectors can be observed on reflection profiles corresponds to the region of greatest expansion in layer 2 thickness in the very oldest crust immediately adjacent to an escarpment that probably marks the continent ocean boundary we see evidence for a low velocity zone overlying an indistinct reflector that may mark the dyke lava interface in the thick crust comparing the structure of the thick crust to that of eastern iceland we find a strong resemblance especially in the expansion in thickness of material with layer 2 velocities these results support the suggestion that during the earliest stages of spreading extrusive volcanism at the ridge crest was unusually voluminous building a thick pile of lavas erupted from a subaerial spreading center

major unconformities termination of extension events and associated surfaces in the south china seas review and implications for tectonic development abstract the distribution of unconformities and end of cenozoic rifting events in the south china seas scs reflects both the modes of rift development and the effects of driving mechanisms continental rifting began in the eastern basins during the paleocene and propagated westwards to the vietnam basin margin in the late eocene continental breakup around 32 28 ma caused a regional reduction or cessation in extensional activity particularly affecting basins furthest from the spreading centre basins in the slope and deepwater area north of the spreading centre exhibit reduced fault activity until 21 20 ma propagation of oceanic crust westwards between 25 and 23 ma and termination of seafloor spreading sometime between 20 5 and 16 ma affected fault activity in the qiongdongnan and nam con song basins in the phu khanh basin and south in the dangerous grounds area extension continued until about 16 ma ending at the red unconformity the end of seafloor spreading around 20 5 ma reflects loss of extensional driving force as thinned continental crust entered the nw borneo subduction zone controversially a key component of the driving force maybe attributed to slab pull a transitional period of about 5 7 my between the onset of subduction of continental crust and final jamming of the subduction zone deep regional unconformity dru is inferred the last pulse of extension was focussed in the western scs and terminated around 10 5 ma detailed understanding of proto south china seas development remains uncertain and controversial

rifted margin architecture and crustal rheology reviewing iberia newfoundland central south atlantic and south china sea abstract crustal rheology controls the style of rifting and ultimately the architecture of rifted margins here we review the formation of three magma poor margin pairs iberia newfoundland the central segment of the south atlantic rift and the south china sea by integrating observational data into a numerical forward modelling framework we utilise a 2d version of the finite element code slim3d which includes nonlinear temperature and stress dependent elasto visco plastic rheology and is able to reproduce a wide range of rift related deformation processes such as flexure lower crustal flow and faulting extension in cold strong or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries hot weak or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system this simple recipe provides the standard framework to understand initial rift geometry however it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles which we reproduce with weak and strong crustal rheologies respectively exemplified by the iberia newfoundland conjugates and the central south atlantic we define three characteristic rift phases an initial phase of simultaneous faulting an intermediate phase of rift migration that involves sequential fault activity and finally the breakup phase crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins we illustrate that weak rheologies generally prolong the phase of simultaneous faulting while rift migration is enabled by initial fault asymmetry as well as relatively weak crust formation of the predominantly symmetric and wide margins of the south china sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup the weak crustal rheology of this region relates to the south china sea s pre rift history where plate convergence lead to crustal thickening and magmatic additions in a back arc regime shortly before the onset of rifting

a crustal structure profile across the northern continental margin of the south china sea an ocean bottom seismometer obs geophysical experiment was carried out across the northern continental margin of the south china sea two dimension kinematic ray tracing was used to construct synthetic records and to define the geological model along the obs profile according to the calculation and interpretation of refracted p wave phases the crustal thickness decreases from 22 km obs 1 in littoral area to 8 km obs 15 in deep sea the lower slope structured by volcanism changes sharply in moho depth from 22 to 9 km the volcanoes over the lower slope were markers of ocean continental transition zone a high velocity layer hvl with an average thickness of 4 km was found in the base of the lower crust over the shelf and slope the south china sea was originated by the passive rifting of south china continent and seafloor spreading however geological and geophysical data collected in the northern margin of the south china sea show a small amount of magma intruded into or erupted onto the crust during pull apart and breakup further analysis shows that the hvz in the base of the thinned continental crust was related to magmatism that occurred after the cessation of seafloor spreading of the south china sea magmatism during the crustal extension and breakup is very weak therefore the northern continental margin of the scs is classified as non volcanic passive margin the southern margin of the south china sea has a crust with a steady thickness of 18 24 km which is different from the nor them margin of the south china sea with a gradually thinned crust they constitute an asymmetrically conjugated pair of passive margins c 2001 elsevier science b v all rights reserved

deep penetration seismic soundings across the northern margin of the south china sea twenty reversed two ship expanding spread profiles esps with maximum source receiver offsets of 100 km were collected in three transects across the rifted northern margin of the south china sea source receiver offset versus two way travel time x t data were mapped into the intercept time versus ray parameter p domain and velocity depth solutions were obtained by a combination of sum inversion in the p domain and ray tracing in both the p and x t domains arrivals from the moho were detected on 17 of the esps the depths to moho determined for individual esp interpretations have reproducibilities of 0 1 km to 3 km in most cases the moho depth has been determined to within 1 5 km moho depths determined in this investigation represent a significant improvement over previous estimates of moho along the margin from gravity data variations in present day crustal thickness measured from top of prerift basement to moho are one measure of the amount and nature of the crustal thinning associated with the rifting of continental crust preceding the formation of the adjacent south china sea basin the esp interpretations reveal that across the eastern portion of the south china margin the crust appears to thin more or less continuously toward the continent ocean boundary in the west esp interpretations also show a general trend of seaward crustal thinning but in addition indicate at least two instances of focused localized crustal thinning crustal velocities and the relative proportion of upper crust vp 6 4 km s are used to identify areas of the south china margin with similar and contrasting crustal structures variations in these properties are believed to result primarily from contrasting prerift crustal structure across the margin however magmatic underplating during rifting depth dependent extension and pleistocene igneous intrusions may also have contributed to the variations in present crustal structure reliable information about variations in crustal thickness and velocity structure across and along the south china margin is an important prerequisite to understanding better the nature of the spatially variable rifting processes which dominated the formation of this margin

the lithosphere below the rockall trough wide angle seismic evidence for extensive serpentinisation abstract the longer range data from wide angle seismic reflection experiments along an axial and transverse profile determined the seismic properties of the shallow mantle lithosphere beneath the rockall trough in the north atlantic two subcrustal p and s wave reflections are observed the first defines a layer 3 10 km thick below the moho where p wave velocities vary from 7 5 to 7 8 km s v p v s ratios increase within this layer from 1 80 in the north to 1 83 in the south of the basin along the axial profile the second reflection from approximately 34 km depth identifies a layer 15 20 km thick with a vp velocity of approximately 8 1 km s and a v p v s ratio of 1 73 these values are typical for normal mantle peridotites both p and s wave velocities and v p v s ratios constrain the possible composition of the first layer which is interpreted as a zone of partially serpentinised peridotites below the moho about 15 volume alteration of the parent mantle peridotite is required to produce the observed seismic properties this degree of alteration accounts for a systematic deficit in total tectonic subsidence when compared with that predicted from the variation in bulk crustal stretching along the axis of the basin syntectonic cooling occurred during differential lithospheric stretching as the upper to mid crust became more extended over a narrower region than the mantle lithosphere this served to rheologically couple the lower crust to the mantle near the final stage of deformation as the primary brittle ductile transition zone in the crust migrated downwards into the mantle lithosphere the resultant fracturing generated the permeability necessary to facilitate the seawater circulation which hydrated the cold mantle

crystallization and resorption in plutonic plagioclase implications on the evolution of granodiorite magma gesiniec granodiorite strzelin crystalline massif sw poland abstract granodiorite from the gesiniec intrusion strzelin crystalline massif sw poland contains complexly zoned plagioclases five chemically and structurally distinct zones can be correlated among crystals cores 25 35 an inner mantles 40 45 an outer mantles 40 25 an resorption zones 35 50 an and rims 35 30 an good structural and chemical major and trace elements correlation of zones between crystals indicates that zonation was produced by changes in conditions of crystallization on a magma chamber scale plagioclase being the liquidus phase records a time span from the beginning of crystallization to emplacement and rapid cooling of granodiorite as thin dykes crystallization began with the formation of inner mantles the paucity and different sizes of inner mantles suggests slow crystallization in high temperature magma normally zoned inner mantles were formed under increasing undercooling compositional trends in mantles suggest closed system crystallization the major resorption zones were caused by injection of less evolved magma as indicated by the strontium increase in plagioclase the injection triggered a rapid rise of magma and plagioclase crystals facilitating mixing but also inducing fast kinetically controlled growth of complex multiple oscillatory zonation within resorption zones the ascent of magma caused decompression melting of plagioclase and produced melt inclusions within inner mantles the cores the decompression range is estimated at a minimum of 2 kbar emplacement of granodiorite as thin dykes allow rapid cooling and preservation of magmatic zonation in plagioclases melt inclusions crystallized completely during post magmatic cooling the zonation styles of plutonic plagioclase differ markedly from volcanic ones suggesting different magma evolution zones in plutonic plagioclase are well correlated indicating crystallization in quiescent magma where crystals accumulation and compositional magma stratification may occur crystals probably did not travel between different regimes resorption occurred but as single albeit complex episodes good correlation of zones in plutonic plagioclases allows a distinction between the main processes controlling zonation and superimposed kinetic effects

the continent ocean transition at the mid northern margin of the south china sea the northern margin of the south china sea scs has particular structural and stratigraphic characteristics that are somewhat different from those described in typical passive margin models the differences are attributable to poly phase tectonic movements and magmatic activity resulting from the interaction among the eurasian philippine sea and indo australian plates based on several crustal scale multi channel seismic reflection profiles and satellite gravity data across the northetn scs margin this paper analyzes the structures volcanoes and deep crust of the continent ocean transition zone cot at the mid northern margin of the scs to study the patterns and model of extension there

eruption triggered mixing of extra caldera basalt and rhyolite complexes along the east gallatin washburn fault zone yellowstone national park wy usa abstract though mixing and commingling of magmas is common mixing between rhyolite and basalt magmas is not commonly preserved in volcanic rocks the presence of at least four mixed magma complexes at yellowstone national park suggests that mingling is not due to random intersections of feeder dikes geochemical analyses also show that though these magmas appear to be commingled there is mixing between the two disparate end members our model combines previous work on the grizzly lake gardner river crystal spring and appolinaris spring mixed magma complexes with results from new analyses recent mixing experiments and regional structural geology coeval extensional tectonism as seen in the east gallatin washburn fault zone is also present in other areas of basalt and rhyolite mixing mingling e g iceland the central portions or core of the complexes contain increased concentrations of emulsion rock occasional basaltic pillows in a rhyolite matrix net veining and mixed magma with highly variable geochemistry sio 2 ranges from 50 to 78 wt phenocrysts have been transferred between mafic and felsic portions of the complexes and suggest that these mixed magmas did not have enough time or energy e g heat to thoroughly mix into complete hybrid intermediate magmas this implies that mixing occurred during eruption furthermore analyses at the micron scale suggest that zones of chaotic mixing between basalt and high silica rhyolites may be more complete than previously thought during mixing of high silica rhyolites and basalts with greater than 4 wt mgo the temperature of the rhyolitic magmas was approximately 850 c with a viscosity between 1 10 6 and 3 10 6 pas the basalt was approximately 1070 c with a viscosity of 2 10 2 to 9 10 3 pas prior to mixing mixing of these two extreme end members may have required decompression of the lower basaltic magma chamber during eruption of the overlying rhyolitic magma chamber into through structurally weakened zones of the upper crust the decompression induced mixing was facilitated by coeval extensional tectonics and structures in the norris mammoth corridor extra caldera rhyolites are some of the first volcanics following the collapse of the current caldera and also some of the youngest volcanics in the yellowstone plateau and require further understanding of volcanism in the yellowstone plateau

continent ocean transition on the voring plateau ne atlantic derived from densely sampled ocean bottom seismometer data 1 a 180 km long seismic wide angle profile was acquired across the voring plateau ne atlantic using a tuned air gun array and three component ocean bottom seismometers deployed with 5 km spacing the seismic p wave data have been modeled by ray tracing inversion and the model has been constrained by s wave and gravity modeling the data and modeling have allowed us to depict the crustal structure and nature of the continent ocean transition cot in a classical volcanic margin case the p wave velocity near the top of the main crustal layer is estimated to 6 0 km s landward of the 25 km wide cot the seaward increase to 6 5 km s in the cot is conformable with heavily intruded continental crust within this zone farther seaward the velocity increase to 6 9 km s in the same layer suggests the presence of oceanic crust the abundant magmatism landward of and within the cot is primarily observed as extrusives forming wedges of seaward dipping reflectors and mafic lower crustal intrusions underplating the maximum thickness of the oceanic crust is measured to 23 5 km at the seaward termination of the cot

crustal structure and extension mode in the northwestern margin of the south china sea combining multi channel seismic reflection and gravity modeling this study has investigated the crustal structure of the northwestern south china sea margin these data constrain a hyper extended crustal area bounded by basin bounding faults corresponding to an aborted rift below the xisha trough with a subparallel fossil ridge in the adjacent northwest sub basin the thinnest crust is located in the xisha trough where it is remnant lower crust with a thickness of less than 3 km gravity modeling also revealed a hyper extended crust across the xisha trough the postrift magmatism is well developed and more active in the xisha trough and farther southeast than on the northwestern continental margin of the south china sea and the magmatic intrusion extrusion was relatively active during the rifting of xisha trough and the northwest sub basin a narrow continent ocean transition zone with a width of 65 km bounded seaward by a volcanic buried seamount is characterized by crustal thinning rift depression low gravity anomaly and the termination of the break up unconformity seismic reflection the aborted rift near the continental margin means that there may be no obvious detachment fault like that in the iberia newfoundland type margin the symmetric rift extreme hyper extended continental crust and hotter mantle materials indicate that continental crust underwent stretching phase pure shear deformation thinning phase and breakup followed by onset of seafloor spreading and the mantle lithosphere may break up before crustal necking in the northwestern south china sea margin

melt extraction and accumulation from partially molten rocks current models for melt segregation and ascent are not adequate to accurately describe transport and accumulation in combination we propose that transport is discontinuous and in batches and that accumulation occurs by stepwise merging of batches a simple numerical model of jostling spheres that merge when they touch was used to represent stepwise accumulation and transport of batches by propagation of hydrofractures results of the numerical model indicate that such a system may quickly develop into a self organised critical soc state in this state the distribution of melt batch volumes can be described by a power law with an exponent m that lies between 2 3 and 1 once a self organised critical state is established the system is capable of discharging any additional melt without further change to itself deformation aids melt extraction efficiency as it increases the mobility of hydrofractures enhances accumulation and hence lowers the exponent m full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at very low melt fractions the chemical evolution of melt from source to emplacement level will be governed by the discontinuous mixing and mingling of batches each with different histories and possibly different sources if no subsequent homogenisation occurs in a magma chamber or the final emplacement structure the process can be identified by chemical heterogeneity of plutons and volcanic rocks

extreme crustal thinning in the south porcupine basin and the nature of the porcupine median high implications for the formation of non volcanic rifted margins the southern porcupine basin is characterized by axial stretching factors that are greater than six and typical of rifted margins as such the basin can be regarded as a natural laboratory to investigate the evolution and symmetry of rifting leading towards continental separation and breakup a bright reflection here named p cuts down to the west from the base of the sedimentary section is overlain by small fault blocks and appears to represent a detachment fault p may in part follow the top of partially serpentinized mantle this interpretation is consistent with gravity modelling and with numerical models of crustal embrittlement and mantle serpentinization during extension furthermore p closely resembles the s reflection west of iberia where such serpentinites are well documented although overall the basin remains symmetrical the consistent westward structural dip of the detachment implies that at high stretching factors extension became asymmetric farther south the porcupine median high appearing lens shaped in cross section overlies the tilted fault blocks and is onlapped by postrift sediment despite no evidence for synrift magmatism this high has previously been interpreted as a basaltic structure however it develops above the line of intersection of the crust mantle boundary with the p detachment and hence may be related to the spatial limit of serpentinization the median high may represent a serpentinite mud volcano or diapir we suggest that such structures produce the serpentinite breccias found within the rifted continent ocean transition of nonvolcanic margins

reprint of nature and significance of the west baram and tinjar lines nw borneo abstract a key consideration in tectonic models for se asia and opening of the south china sea is the role that the west baram and tinjar lines of nw borneo may have played in accommodating the motion of crustal blocks displaced from asia following india s collision there are few studies that focus on these lines using onshore geological studies and offshore seismic data to address the origin and tectonic significance of these this paper concludes that rather than a major transform boundary between luconia and the dangerous grounds the west baram line marks the boundary between domains of continental crust that underwent differential extension in the eocene the baram basin is underlain by hyperextended continental crust on the ne side of the baram line the strong contrast in the geological features across the tinjar and west baram lines likely reflects ancient differences in crustal rheology with luconia being the more rigid block although lack of significant strike slip faulting along the west baram line poses problems for tectonic models in which a wide proto south china sea is subducted beneath nw borneo intra plate deformation such as partial inversion of the dangerous grounds rift offers a potential mechanism to mass balance blocks displaced from asia with the reduced strike slip motion along the west baram line

pb isotopes during crustal melting and magma mingling a cautionary tale from the miki fjord macrodike central east greenland abstract pb isotopic data are presented for hybrid rocks formed by mingling between mantle derived tholeiitic magma of the eocene miki fjord macrodike east greenland and melt derived from the adjacent precambrian basement bulk mixing and afc processes between end members readily identified in the field fail to model the pb isotope systematics selective contamination during diffusional exchange which can explain the complex sr and nd isotope compositions of the hybrid rocks blichert toft et al 1992 cannot fully account for the variability of the pb isotopic data using the identified crustal end members the crustal anatectic end member although similar in sr and nd isotope composition has a markedly different pb isotopic composition than its source gneiss the differences are consistent with preferential incorporation of radiogenic pb from accessory phases such as metamict zircon or loosely bound pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma the crustal melt involved in magma interactions was therefore heterogeneous with respect to pb isotopes on a metre scale these results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end members are well constrained by field relations we show that the pb isotopic composition of the crustal contaminants and contamination trajectories for the miki fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic pb and sr lithologies resembling lewisian basement not exposed locally

deep seismic structure of the northeastern south china sea origin of a high velocity layer in the lower crust we present a 2 d seismic tomographic image of the crustal structure along the obs2012 profile which delineates the moho morphology and magmatic features of the northeastern south china sea margin the image was created by forward modeling rayinvr and traveltime tomographic inversion tomo2d overall the continental crust thins seaward from 27 km to 21 km within the continental shelf across the zhu i depression and dongsha rise with slight local thickening beneath the dongsha rise accompanying the increase in the moho depth the dongsha rise is also characterized by 4 7 km thick high velocity layer hvl 7 0 7 6 km s in the lower crust and exhibits a relatively high velocity 5 5 6 4 km s in the upper crust with a velocity gradient lower than those of the zhu i depression and tainan basin across the continental slope and continent ocean transition cot which contain the tainan basin the crust sharply thins from 20 km to 10 km seaward and a 2 3 km thick hvl is imaged in the lower crust we observed that volcanoes are located only within the cot but none exist in the continental shelf the dongsha rise exhibits a high magnetic anomaly zone and different geochemical characteristics from the cot based on those observations we conclude that the hvl underlying the cot is probably extension related resulting from the decompression melting in the cenozoic whereas the hvl beneath the dongsha rise is probably arc related and associated with the subduction of the paleo pacific plate these findings are inconsistent with those of some previous studies

the crustal structure of the rockall trough differential stretching without underplating the crustal structure along the axis of the rockall trough in the north atlantic has been studied along a 600 km refraction wide angle reflection transect containing three lines each 200 to 250 km long using explosives and ocean bottom seismometers one dimensional inversions of each section were made using the p method and forward modeling of the observed travel times in the next stage travel times and amplitudes were modeled using ray tracing techniques through two dimensional heterogeneous structures the results indicate that there are three sedimentary layers with velocities ranging from 2 km s to 4 5 km s the whole sedimentary section is up to 6 km thick and interpreted as late paleozoic to tertiary in age a two layer continental crust 5 to 7 km thick occurs along the length of the profile the upper crust 6 0 6 3 km s is circa 2 km thick and the lower crust 6 6 6 9 km s is circa 3 km thick a moho transition zone approximately 1 km thick lies at the base of the crust velocities in this transition zone increase from 6 9 km s up to 7 8 km s along the profile the underlying upper mantle has a laterally variable velocity between 7 6 and 7 8 km s unstretched crust onshore in ireland comprises a three layered crust with each layer approximately 10 km thick and a moho transition zone which is about 3 km thick the two upper layers in the onshore region are interpreted as corresponding to the upper crust in the rockall trough and indicate a stretching factor of 8 10 the velocity pattern in the lower crust in the rockall trough and under ireland are similar suggesting significantly less stretching 2 3 the differential stretching model is supported by the presence of the moho transition zone which is stretched by a similar factor to the lower crust the bulk stretching factor for the crust as a whole is in the range of 4 6 if this represents the lithospheric stretching factor significant underplating would be expected however if the stretching factor for the lower crust in the differential stretching model is more representative of overall lithospheric stretching little or no underplating is predicted the velocity patterns observed in the rockall trough indicate the absence of any significant underplating at the base of the crust such as that observed at the continental margin west of the hatton basin

shallow anatomy of a continent ocean transition zone in the northern south china sea from multichannel seismic data the cenozoic tectonic evolution of the south china sea is reflected in diverse tectonic processes including continental rifting seafloor spreading subduction terrane collision and strike slip fault movement a continent ocean transition zone in the northern south china sea caused by the lithospheric extension when the continent underwent break up rifting and later seafloor spreading is clearly imaged in the multi channel seismic data presented in this study the morphological units of the continent ocean transition zone are the rift depression the volcanic zone and tilted fault blocks the volcanic zone represents a highest extension zone within the continent ocean transition zone and is mainly distributed in the southern slope uplift zone along the northern passive margin of the south china sea the large listric normal faults bounding the dongsha rise and baiyun sag are evidenced in the seismic image the passive margin in the northern south china sea underwent the wide rift to narrow rift process in the transition zone as inferred from the relation between the surface heat flow and initial rifted crustal thickness the continent ocean transition zone in the continental margin of the south china sea is consistent with high heat flow zone average 90 mw m 2 observed in the previous heat flow measurements and at odp site 1148 and is manifested in a sharp change of the p wave velocity the rifted margin of the south china sea is a non typical magma poor passive margin or an intermediary form between the iberian type non volcanic and the greenland type volcanic margin compared to the world s typical passive margins c 2012 elsevier b v all rights reserved

evolution of the south china sea revised ages for breakup and seafloor spreading abstract the continental breakup which gave way to the formation of the oceanic south china sea scs basin began in the latest cretaceous in the northeastern scs and propagated in southern and western direction over a long period of time possibly more than 40 m y the seafloor spreading history of the south china sea has been interpreted in different ways in the past and the debate over the correct timing of the major tectonic events continues we review the different models that have been published and present a revised interpretation of seafloor spreading anomalies based on three datasets with documented high quality which cover all of the scs but the northernmost and southernmost parts we can precisely date the onset of seafloor spreading in the central part of the scs at 32 ma after a ridge jump at 25 ma spreading also began in the southwestern sub basin and spreading ended at 20 5 ma in the entire basin followed by a phase of magmatic seamount formation mainly along the abandoned spreading ridge spreading rates vary from 56 mm yr in the early stages to 72 mm yr after the ridge jump to 80 mm yr in the southwestern sub basin we find indications for a stepwise propagation of the seafloor spreading from northeast to southwest in segments bounded by major fracture zones seafloor spreading ended abruptly probably because the subduction zone along the eastern and southern boundary of the scs of which today the manila trench remains was blocked by collision with a continental fragment possibly the northern part of palawan or a part of the dangerous grounds

crustal structure across the xisha trough northwestern south china sea located at the northwestern part of the south china sea scs between the hainan and xisha paracel islands the xisha trough represents a failed rift in conjunction with the opening of the scs between 32 and 17 ma from west towards east within a scale of several hundred kilometers it presents all major stages of the rifting process and thus provides an ideal place to study the rifting process in great details in the autumn of 1996 a joint team of sino german scientists carried out a wide angle seismic experiment across the xisha trough with 10 ocean bottom hydrophones obh along a 237 km nnw sse oriented profile which was surveyed in 1987 with multi channel seismic mcs method by bgr of germany and soa of china favorable weather conditions and the powerful 4 x 12 1 air gun array rendered very good quality data with seismic signals observed at the offset of up to 110 km a detailed velocity depth model was obtained by using an interactive trial and error 2d ray tracing method interpretation of the nics data published by bgr provides very good geometrical constraints of the complex upper crustal structure which is characterized by fault blocks half horsts and half grabens filled with syn and post rift cenozoic sediments the velocity model in turn confirms the major structure outlined by the interpretation of the mcs data showing a varying sedimentary layer between i and 4 km of thickness and velocities between 1 7 and 4 5 km s the p wave velocity of 5 5 km s on the top of the crystalline basement is relatively low suggesting strong weathering within the crystalline crust the velocity increases downward continuously to 6 8 km s at the bottom of the crust without a clear differentiation in the middle crust showing clearly its continental nature even beneath the xisha trough the moho is marked by a sharp first order interface with a velocity of 8 0 8 1 km s at the uppermost mantle the moho depth is 15 km beneath the center of the trough and increases gradually to more than 25 kin towards north and south corresponding to an extreme thinning of the pre rift continental crust from more than 25 kin under the coast line to only 8 kin beneath the xisha trough the similar velocity structure of the continental nature on both sides of the xisha trough suggests a homogeneous pre rift continental setting the sharp moho and the lack of high velocity body hvb in the lower crust imply no magmatic underplating which is very different to interpretations across the eastern part of the continental margin in northern south china sea the intense faulting in the upper crust the strong but rather symmetrical crustal thinning centered at the xisha trough and the close neighborhood to the open scs nw subbasin suggest a pure shear rifting which failed most likely at the phase shortly before the continental breakup c 2001 elsevier science b v all rights reserved

isotopic evidence for multiple contributions to felsic magma chambers gouldsboro granite coastal maine abstract the gouldsboro granite forms part of the coastal maine magmatic province a region characterized by granitic plutons that are intimately linked temporally and petrogenetically with abundant co existing mafic magmas the pluton is complex and preserves a felsic magma chamber underlain by contemporaneous mafic magmas the transition between the two now preserved as a zone of chilled mafic sheets and pillows in granite mafic components have highly variably isotopic compositions as a result of contamination either at depth or following injection into the magma chamber intermediate dikes with identical isotopic compositions to more mafic dikes suggest that closed system fractionation may be occurring in deeper level chambers prior to injection to shallower levels the granitic portion of the pluton has the highest nd isotopic composition e nd 3 0 of plutons in the region whereas the mafic lithologies have nd isotopic compositions e nd 3 5 that are the lowest in the region and similar to the granite and suggestive of prolonged interactions and homogenization of the two components sr and nd isotopic data for felsic enclaves are inconsistent with previously suggested models of diffusional exchange between the contemporaneous mafic magmas and the host granite to explain highly variable alkali contents the felsic enclaves have relatively low nd isotopic compositions e nd 2 1 indicative of the involvement of a third lower e nd melt during granite petrogenesis perhaps represented by pristine granitic dikes contemporaneous with the nearby pleasant bay layered intrusion the dikes at pleasant bay and the felsic enclaves at gouldsboro likely represent remnants of the silicic magmas that originally fed and replenished the overlying granitic magma chambers the large isotopic and chemical contrasts between the enclaves and granitic dikes and granitic magmas may be in part a consequence of extended interactions between the granitic magmas and co existing mafic magmas by mixing mingling and diffusion alternatively the granitic magmas may represent an additional crustal source using granitic rocks such as these with abundant evidence for interactions with mafic magmas complicate their use in constraining crustal sources and tectonic settings fine grained dike rocks may provide more meaningful information but must be used with caution as these may also have experienced compositional changes during mafic felsic interactions

the continent ocean crustal transition across the southwest greenland margin this paper examines the complete crustal transition across the nonvolcanic southwest greenland continental margin of the labrador sea using wide angle and coincident vertical incidence seismic profiles six ocean bottom seismometers and a sonobuoy record p and s wave first and multiple arrivals from the crust and upper mantle which are analyzed by two dimensional dynamic ray tracing and one dimensional reflectivity modeling the resulting seismic velocity model requires that the preexisting 30 km thick continental crust is thinned abruptly to 3 km across the continental slope primarily by removal of the lower crust farther seaward the crust thickens to 6 km primarily through the addition of a high velocity 7 0 7 6 km s layer in the lower crust this lower crustal layer is 4 5 km thick extends for a horizontal distance of 80 km and is interpreted as partially serpentinized upper mantle it is overlain by a low velocity 4 0 5 0 km s upper layer which is interpreted as highly fractured upper continental crust our model suggests that seafloor spreading did not start until chrons 27 28 13 ma younger than previously suggested this interpretation is supported by two dimensional modeling of gravity and magnetic data along the refraction line our results are consistent with a simple shear mechanism for the initial rifting with the sw greenland margin as the upper plate however a full characterization of the rifting mechanism must await comparison with a seismic model for the conjugate margin east of labrador

symmetric and asymmetric lithospheric extension relative effects of frictional plastic and viscous strain softening 1 strain dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones we investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments parametric softening is specified as a linear decrease of the effective internal angle of friction the effective viscosity or both in the model rheologies the sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere results are classified according to the symmetry s or asymmetry a of the deformation of the upper and lower lithosphere during rifting strain softening is required for rifting asymmetry but is not always sufficient a range of model tectonic styles occurs including pure and simple shear modes with focused shear zones mode selection is mostly determined by the feedback between two primary controls the dominant rheology and the parametric strain softening mechanisms listed above softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology model results are consistent with the proposed primary controls and the factors that contribute to these controls in particular decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities

seismic structure in the northeastern south china sea s wave velocity and vp vs ratios derived from three component obs data a nearly 500 km long seismic profile with reflective and refractive wide angle ocean bottom seismometer obs data and multi channel seismic mcs data was acquired across the northeastern continental margin of the south china sea scs the s wave crustal structure and vp vs ratios have been obtained based on a previously published p wave model using the software raylnvr modeling of vertical and horizontal component obs data yields information on the seismic crustal velocities lithology and geophysical properties along the obs 2001 seismic profile s wave velocities in the model increase generally with depth but exhibit high spatial variability particularly from the shelf to the upper slope of the northeastern scs margin vp vs ratios also reveal significant lithological heterogeneity dongsha penghu uplift dpu is a tectonic zone with a thicker crust than adjacent areas and a high magnetic anomaly with a vp vs of 1 74 and a p wave velocity of 5 0 5 5 km s the dpu primarily consists of felsic volcanic rocks in the upper crust and is similar to the petrology of zhejiang fujian volcanic provinces which perhaps is associated with a mesozoic volcanic arc the ocean continent transition oct in the northeastern scs is characterized by a thinning continental crust volcanoes in the upper crust and a high velocity layer hvl in the lower crust the s wave velocity and vp vs ratio suggest that the hvl has a mafic composition that may originate from underplating of the igneous rocks beneath the passive rifted crust after the cessation of seafloor spreading c 2009 elsevier b v all rights reserved

the south china sea margins implications for rifting contrasts abstract implications regarding spatially complex continental rifting crustal extension and the subsequent evolution to seafloor spreading are re examined for the northern and southern rifted margins of the south china sea previous seismic studies have shown dramatic differences in the present day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south china although the total crustal extension is presumed to be the same along the margin and adjacent ocean basin the amount of continental crustal extension that occurred is much less along the east and central segments of the margin than along the western segment this difference was accommodated by the early formation of oceanic crust creating the present day south china sea basin adjacent to the eastern margin segment while continued extension of continental crust was sustained to the west using the observed cross sectional areas of extended continental crust derived from deep penetration seismics two end member models of varying rift zone widths and varying initial crustal thicknesses are qualitatively examined for three transects each model implies a time difference in the initiation of seafloor spreading inferred for different segments along the margin the two models examined predict that the oceanic crust of the south china sea basin toward the west did not begin forming until sometime between 6 12 my after its initial formation 32 ma toward the east these results are compatible with crustal age interpretations of marine magnetic anomalies assuming rifting symmetry with conjugate margin segments now residing along the southern portions of the south china sea basin implies that the total width of the zone of rifting in the west was greater than in the east by about a factor of two we suggest the most likely causes of the rifting differences were east west variations in the rheology of the pre rift crust and associated east west variations in the thermal structure of the pre rift lithosphere the calculated widths of rifted continental crust for the northern and southern margins when combined with the differential widths of seafloor generated during the seafloor spreading phase indicate the total crustal extension that occurred is about 1100 km and is remarkably consistent to within 10 for all three eastern central western segments examined

the tectonic evolution of the qiongdongnan basin in the northern margin of the south china sea abstract qiongdongnan basin is a cenozoic rift basin located on the northern passive continental margin of the south china sea due to a lack of geologic observations its evolution was not clear in the past however recently acquired 2 d seismic reflection data provide an opportunity to investigate its tectonic evolution it shows that the qiongdongnan basin comprises a main rift zone which is 50 100 km wide and more than 400 km long the main rift zone is arcuate in map view and its orientation changes from ene wsw in the west to nearly e w in the east it can be divided into three major segments the generally linear fault trace shown by many border faults in map view implies that the eastern and middle segments were controlled by faults reactivated from ne to ene trending and nearly e w trending pre existing fabrics respectively the western segment was controlled by a left lateral strike slip fault the fault patterns shown by the central and eastern segments indicate that the extension direction for the opening of the rift basin was dominantly nw se a semi quantitative analysis of the fault cut offs identifies three stages of rifting evolution 1 40 4 33 9 ma sparsely distributed ne trending faults formed mainly in the western and the central part of the study area 2 33 9 28 4 ma the main rift zone formed and the area influenced by faulting was extended into the eastern part of the study area and 3 28 4 20 4 ma the subsidence area was further enlarged but mainly extended into the flanking area of the main rift zone in addition estimates of extensional strain along nw se trending seismic profiles which cross the main rift zone vary between 15 and 39 km which are generally comparable to the sinistral displacement on the red river fault zone offshore implying that this fault zone in terms of sinistral motion terminated at a location near the southern end of the yinggehai basin finally these observations let us to favour a hybrid model for the opening of the south china sea and probably the qiongdongnan basin

ages and magnetic structures of the south china sea constrained by deep tow magnetic surveys and iodp expedition 349 combined analyses of deep tow magnetic anomalies and international ocean discovery program expedition 349 cores show that initial seafloor spreading started around 33 ma in the northeastern south china sea scs but varied slightly by 1 2 myr along the northern continent ocean boundary cob a southward ridge jump of approximate to 20 km occurred around 23 6 ma in the east subbasin this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the southwest subbasin which propagated for about 400 km southwestward from approximate to 23 6 to approximate to 21 5 ma the terminal age of seafloor spreading is approximate to 15 ma in the east subbasin and approximate to 16 ma in the southwest subbasin the full spreading rate in the east subbasin varied largely from approximate to 20 to approximate to 80 km myr but mostly decreased with time except for the period between approximate to 26 0 ma and the ridge jump approximate to 23 6 ma within which the rate was the fastest at approximate to 70 km myr on average the spreading rates are not correlated in most cases to magnetic anomaly amplitudes that reflect basement magnetization contrasts shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers in addition to large vertical intensity variations these complexities are caused by late stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0 5 which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization the primary contribution to magnetic anomalies of the scs is not in the top 100 m of the igneous basement

energy constrained open system magmatic processes 3 energy constrained recharge assimilation and fractional crystallization ec rafc 1 geochemical data for igneous rock suites provide conclusive evidence for the occurrence of open system processes within thermally and compositionally evolving magma bodies the most significant processes include magma recharge with possible enclave formation and magma mixing assimilation of anatectic melt derived from wallrock partial melting and formation of cumulates by fractional crystallization rafc in this study we extend the energetically constrained assimilation and fractional crystallization ec afc model spera and bohrson 2001 bohrson and spera 2001 to include the addition of compositionally and thermally distinct recharge melt during simultaneous assimilation and fractional crystallization energy constrained recharge assimilation and fractional crystallization ec rafc tracks the trace element and isotopic composition of melt cumulates and enclaves during simultaneous recharge assimilation and fractional crystallization ec rafc is formulated as a set of 3 t i s coupled nonlinear differential equations where the number of trace elements and radiogenic and stable isotope ratios modeled are t i and s respectively solution of the ec rafc equations provides values for the average wallrock temperature ta mass of melt within the magma body mm mass of cumulates mct and enclaves men mass of wallrock involved in the thermal interaction mao mass of anatectic melt assimilated m a concentration of t trace elements and i s isotopic ratios in melt cm cumulates cct enclaves cen and anatectic melt ca as a function of magma temperature tm input parameters include the equilibration temperature teq the initial temperature and composition of pristine melt tmo cmo emo recharge melt tro cro ero and wallrock tao cao eao temperature dependent trace element distribution coefficients dm dr da heats of transition for wallrock ha pristine melt hm and recharge melt hr and the isobaric specific heat capacity of assimilant cp a pristine melt cp m and recharge melt cp r the magma recharge mass function mr tm is specified a priori and defines how recharge magma is added to standing magma the present ec rafc simulator incorporates a weak coupling to major element mass balance and phase relations based on laboratory experiments or gibbs energy minimization e g ghiorso 1997 ec rafc can be applied to a variety of magmatic systems including volcanic suites that show evidence of magma mixing layered mafic intrusions and granitoid plutons predictions for masses as well as compositions of magmatic products are part of the ec rafc solution the systems approach provides an opportunity to quantitatively assess the roles of assimilation fractional crystallization and magma recharge in magma evolution using trace element and isotopic constraints together with energy conservation

seismic images of a collision zone offshore nw sabah borneo multichannel reflection seismic data from the southern south china sea refraction and gravity modelling were used to investigate the compressional sedimentary structures of the collision prone continental margin off nw borneo an elongated imbricate deepwater fan the toe thrust zone bounds the northwest borneo trough to the southeast the faults separating the individual imbricates cut through post early miocene sediments and curve down to a carbonate platform at the top of the subsiding continental dangerous grounds platform that forms the major detachment surface the age of deformation migrates outward toward the front of the wedge we propose crustal shortening mechanisms as the main reason for the formation of the imbricate fan at the location of the in the past defined lower tertiary thrust sheet tectonostratigraphic province a high velocity body was found but with a much smaller extend than the previously defined structure the high velocity structure may be interpreted either as carbonates that limit the transfer of seismic energy into the sedimentary layers beneath or as paleogene crocker sediments dissected by remnants of a proto south china sea oceanic crust that were overthrust onto a southward migrating attenuated continental block of the dangerous grounds during plate convergence

magma mixing in the genesis of the kalatongke dioritic intrusion implications for the tectonic switch from subduction to post collision chinese altay nw china abstract the kalatongke complex located in south chinese altay of the central asian orogenic belt caob consists of the 308 ma dioritic suite intruded by the 287 ma noritic suite the noritic suite is composed of olivine norite and hornblende norite and hosts sulfide mineralization the dioritic suite is composed of ferrodiorite diorite and quartz diorite ferrodiorites have 43 6 to 51 8 wt sio 2 8 2 to 12 1 wt fe 2 o 3 t 2 87 to 4 86 wt mgo with mg ranging from 42 to 53 they have highly variable al 2 o 3 17 4 20 7 wt na 2 o 2 78 4 50 k 2 o 0 58 3 70 sr 280 1300 ppm and ba 219 3100 ppm diorites and quartz diorites have 15 2 19 6 wt al 2 o 3 239 890 ppm sr and170 1100 ppm ba 11 2 25 5 ppm y and1 2 2 2 ppm yb all rocks have lree enriched ree patterns and do not display eu anomalies they have primitive mantle normalized trace element patterns enriched in large ion lithosphile elements lile e g ba and sr and depleted in ti nb and ta both ferrodiorites and diorites have relatively constant initial 87 sr 86 sr ratios of 0 7037 to 0 7040 but highly variable e nd t values of 4 5 to 11 6 whereas quartz diorites have higher initial 87 sr 86 sr ratios 0 7056 and lower e nd t values 0 5 plagioclase from ferrodiorites diorites and quartz diorites has similar initial 87 sr 86 sr ratios of 0 7015 to 0 7055 the dioritic suite formed from a mixed magma of an evolved mantle derived basaltic magma by partial melting of subduction modified mantle source in response to slab breakoff and a crustal derived granitic magma at 308 ma before mixing with granitic magmas basaltic magmas underwent fractional crystallization of olivine to form ferrodiorite the mixed magma also formed diorite and quartz diorite combining fractionation clinopyroxene amphibole and plagioclase the noritic suite formed from a mantle derived high mg magma in a post collisional environment both the dioritic and noritic suites in kalatongke suggest the switch from subduction to extension and the final collision between the junggar block and chinese altay at 308 ma

an arbitrary lagrangian eulerian formulation for creeping flows and its application in tectonic models summary this paper presents and analyses numerical techniques developed to investigate viscoplastic stokes flows within a model of lithospheric deformation in particular the techniques are related to a subduction model of compressional orogens the driving mechanism in the model corresponds to the near rigid convergence and subduction of one mantle lithosphere beneath another in plane strain and this boundary condition forces flow in an overlying viscoplastic model crust the numerical techniques use the arbitrary eulerian lagrangian formulation in which flows with free surfaces and large deformation are computed on an evolving eulerian finite element grid that conforms to the material domain a regridding algorithm allows the associated lagrangian motion and fields to be followed and in addition coupled back to the eulerian calculation of the flow mass flux boundary conditions are used so that the effects of erosion and deposition by surface processes and mass loss by subduction can be included in the model calculation the evolving model crustal layer is flexurally compensated using a general elastic beam formulation the applicability of the numerical techniques to problems ranging from accretionary wedges to crustal and lithospheric scale deformation is discussed simple flows a linear viscous subduction model a whirl flow and a quasi convection model are used to show that the mass conservation regridding and surface tracking errors are small the broader applicability of the modelling techniques is reviewed

crustal structure and inferred rifting processes in the northeast south china sea abstract taiger project deep penetration seismic reflection profiles acquired in the northeastern south china sea scs provide a detailed view of the crustal structure of a very wide rifted continental margin these profiles document a failed rift zone proximal to the shelf a zone of thicker crust 150 km from the shelf and gradually thinning crust toward the cob spanning a total distance of 250 300 km such an expanse of extended continental crust is not unique but it is uncommon for continental margins we use the high quality images from this data set to identify the styles of upper and lower crustal structure and how they have thinned in response to extension and in turn what rheological variations are predicted that allow for protracted crustal extension upper crustal thinning is greatest at the failed rift uc 7 5 but is limited farther seaward uc 1 2 we interpret that the lower crust has discordantly thinned from an original 15 17 km to possibly less than 2 3 km thick beneath the central thick crust zone and more distal areas this extreme lower crustal thinning indicates that it acted as a weak layer allowing decoupling between the upper crust and the mantle lithosphere the observed upper crustal thickness variations and implied rheology lower crustal flow are consistent with large scale boudinage of continental crust during protracted extension

a comparison between mid paleozoic new england usa and the modern western usa subduction of an oceanic ridge transform fault system abstract a detailed comparison is made between the mid paleozoic norumbega fault system nfs in maine usa and the san andreas fault system safs of coastal california usa and their tectonic settings the safs formed following subduction of an oceanic ridge transform system and the nfs is interpreted as having formed the same way the parallel evaluation of mid crustal processes associated with the nfs and surface processes associated with the safs gives a unique perspective on modern and inactive crustal scale strike slip fault systems the safs is separated from the cascadia subduction zone by the mendocino triple junction similarly the mid paleozoic nfs is interpreted as having been separated from a convergent zone to the southwest by the interpreted norumbega triple junction the safs cuts a subduction complex the franciscan complex that formed when the margin was still convergent no equivalent subduction complex associated with the nfs has been recognized perhaps because it was eroded away or displaced or because it never formed the safs experienced periods of transpression and transtension whereas the nfs shows transpressive structures only slab window magmatic rocks exist along the safs devonian to earliest carboniferous plutonic rocks along the nfs may in part be slab window magmatic rocks although the effects of the acadian and prior orogenies in the appalachians complicate their identification magmatic rocks along the safs are offset due to the high rate of displacement 20 40 mm yr magmatic rocks along the nfs show little offset consistent with the low rate of displacement 3 mm yr in the model transverse structures are associated with both the safs and nfs other mendocino type triple junctions may have existed in the past but may be difficult to recognize where a past crustal scale intracontinental strike slip fault system terminates on an along strike contemporaneous convergent system such a fault fault trench triple junction may be considered