tectonic evolution of a composite collision orogen an overview on the qinling tongbai hong an dabie sulu orogenic belt in central china abstract the formation of collisional orogens is a prominent feature in convergent plate margins it is generally a complex process involving multistage tectonism of compression and extension due to continental subduction and collision the paleozoic convergence between the south china block scb and the north china block ncb is associated with a series of tectonic processes such as oceanic subduction terrane accretion and continental collision resulting in the qinling tongbai hong an dabie sulu orogenic belt while the arc continent collision orogeny is significant during the paleozoic in the qinling tongbai hong an orogens of central china the continent continent collision orogeny is prominent during the early mesozoic in the dabie sulu orogens of east central china this article presents an overview of regional geology geochronology and geochemistry for the composite orogenic belt the qinling tongbai hong an orogens exhibit the early paleozoic hp uhp metamorphism the carboniferous hp metamorphism and the paleozoic arc type magmatism but the three tectonothermal events are absent in the dabie sulu orogens the triassic uhp metamorphism is prominent in the dabie sulu orogens but it is absent in the qinling tongbai orogens the hong an orogen records both the hp and uhp metamorphism of triassic age and collided continental margins contain both the juvenile and ancient crustal rocks so do in the qinling and tongbai orogens in contrast only ancient crustal rocks were involved in the uhp metamorphism in the dabie sulu orogenic belt without involvement of the juvenile arc crust on the other hand the deformed and low grade metamorphosed accretionary wedge was developed on the passive continental margin during subduction in the late permian to early triassic along the northern margin of the dabie sulu orogenic belt and it was developed on the passive oceanic margin during subduction in the early paleozoic along the northern margin of the qinling orogen three episodes of arc continent collision are suggested to occur during the paleozoic continental convergence between the scb and ncb the first episode of arc continent collision is caused by northward subduction of the north qinling unit beneath the erlangping unit resulting in uhp metamorphism at ca 480 490 ma and the accretion of the north qinling unit to the ncb the second episode of arc continent collision is caused by northward subduction of the prototethyan oceanic crust beneath an andes type continental arc leading to granulite facies metamorphism at ca 420 430 ma and the accretion of the shangdan arc terrane to the ncb and reworking of the north qinling erlangping and kuanping units the third episode of arc continent collision is caused by northward subduction of the paleotethyan oceanic crust resulting in the hp eclogite facies metamorphism at ca 310 ma in the hong an orogen and low p metamorphism in the qinling tongbai orogens as well as crustal accretion to the ncb the closure of backarc basins is also associated with the arc continent collision processes with the possible cause for granulite facies metamorphism the massive continental subduction of the scb beneath the ncb took place in the triassic with the final continent continent collision and uhp metamorphism at ca 225 240 ma therefore the qinling tongbai hong an dabie sulu orogenic belt records the development of plate tectonics from oceanic subduction and arc type magmatism to arc continent and continent continent collision

interaction between prerift salt and detachment faulting in hyperextended rift systems the example of the parentis and mauleon basins bay of biscay and western pyrenees prerift salt layers associated with extensional detachment faults exhuming mantle and deeper crustal rocks at the sea floor are observed in the parentis and arzacq mauleon basins located at the eastern termination of the bay of biscay how detachment faults interact with salt in hyperextended rift systems is yet little understood based on field observations and drill hole and seismic data we propose a new model to explain the interaction between salt tectonics and extensional detachment systems we demonstrate that the presence of a thick prerift salt layer in an area undergoing extreme crustal thinning can control the geometry and evolution of rift systems and obscure the rift related structures in the underlying basement during an initial stage of rifting prerift salt layers act as a decoupling horizon between sub and suprasalt units and hinder crustal detachment faults to cut through the salt layers and form breakaways at the sea floor as a consequence they sole out along the ductile salt layer and no subsalt material can be exhumed to the sea floor thus sub and suprasalt layers deform by different deformation modes which makes that detachment fault difficult to identify on seismic images in a later stage when salt has migrated and thinned out sub and suprasalt layers can locally couple and detachment faults can be exposed at the surface resulting in windows of exhumed basement surrounded by extensional allochthons formed by suprasalt sedimentary units

distribution of calanoid copepods across the mesoscale frontal zone of tropical subtropical convergence off mexico abstract the distribution of calanoid copepods was investigated in the convergence zone between the california current water and transitional tropical water in june 2010 to understand how this zooplankton community responds to mesoscale environmental structure hydrographic conditions and zooplankton profiles were measured along two transects which crossed a mesoscale front and adjacent structures visible in sea surface temperature and ocean color satellite images the hydrographic structure showed strong temperature 2 c and oxygen 50 mol kg 1 gradients in the frontal zone 90 km associated with a sinking of the thermocline and oxycline from 30 m depth in the transitional tropical water to 90 m depth in the california current water the front was marked by a salinity minimum

genesis of adakitic granitoids by partial melting of thickened lower crust and its implications for early crustal growth a case study from the huichizi pluton qinling orogen central china abstract adakitic rocks are often considered as a key to deciphering the genesis of archean ttgs and the early crustal growth granites from the huichizi pluton in the north qinling nq unit have high sr y and la yb n ratios similar to adakites their relatively high sio 2 k 2 o and na 2 o and very low mgo cr and ni contents are in the range of high sio 2 adakites and early archean ttgs and are compositionally similar to experimental melts derived from metabasalt sources new sims zircon u pb dating constrains the emplacement age of the huichizi pluton at 422 5 ma rock samples from the huichizi pluton have e nd t and zircon e hf t values similar to the neoproterozoic metabasalts in the nq unit in combination with their normal mantle like 18 o zir values these adakites are best explained by partial melting of the neoproterozoic mafic crustal root due to subduction of the shangdan ocean regional geological data suggest that the crust was probably thickened by a ca 490 ma arc collision process prior to the emplacement of the huichizi pluton our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust

zooplankton distribution and dynamics in a north pacific eddy of coastal origin ii mechanisms of eddy colonization by and retention of offshore species abstract mesoscale anticyclonic eddies form annually in late winter along the eastern margin of the subarctic north pacific eddies that originate off the southern tip of the queen charlotte islands near 52 n 132 w are called haida eddies during the subsequent 1 3 years they propagate westward into the alaska gyre enroute the eddies are colonized by zooplankton originating from the central british columbia continental shelf the continental slope and along slope boundary current and the oceanic alaska gyre eddies also gradually lose kinetic energy water properties and biota to the surrounding ocean in this paper we analyze zooplankton samples from haida eddies obtained in late winter early summer and autumn of 2000 and in early summer and autumn of 2001 and compare the within eddy zooplankton distributions abundances and community composition of the oceanic origin species to observations from the continental margin and alaska gyre source regions most between region comparisons were consistent with a hypothesis that the eddy zooplankton are a mixture intermediate in abundance and community composition between the bc continental margin and offshore alaska gyre source regions although usually closer to the alaska gyre however about 30 of the comparisons showed within eddy abundances higher than in either source region this outcome cannot arise from mixing alone aggregation and retention appear to be linked to vertical distribution behavior most of the successful taxa spend much of their time below the surface mixed layer this minimizes their exposure to wash out by three physical processes slow upwelling and surface divergence that accompanies weakening of the anticyclonic geostrophic currents rapid but intermittent flushing of the surface layer by ekman transport during strong wind events exchange across the eddy margin geostrophic streamlines caused by temporary displacement by wind driven inertial currents

origin of high potassium talc alkaline i type granitoids many i type granitoid magmas are generated through partial melting of older metaigneous rocks and the compositions of such melts are broadly calc alkaline and metaluminous these melts are granitic to tonalitic and result from thermal extremes in their lower crustal source regions data on the experimental partial melting of common crustal rocks suggest that high k i type granitoid magmas can be derived only from the partial melting of hydrous calc alkaline to high k calc alkaline mafic to intermediate metamorphic rocks in the crust because of their low k2o contents metabasaltic rocks of all kinds are unsuitable as sources and models that propose mixing of mantle derived basaltic magmas and crustal melts are also inadequate there is no requirement that i type calc alkaline magmatism be related in any way to subduction processes

neoproterozoic paleozoic and mesozoic granitoid magmatism in the qinling orogen china constraints on orogenic process abstract the qinling orogen is one of the main orogenic belts in asia and is characterized by multi stage orogenic processes and the development of voluminous magmatic intrusions the results of zircon u pb dating indicate that granitoid magmatism in the qinling orogen mainly occurred in four distinct periods the neoproterozoic 979 711 ma paleozoic 507 400 ma and early 252 185 ma and late 158 100 ma mesozoic the neoproterozoic granitic magmatism in the qinling orogen is represented by strongly deformed s type granites emplaced at 979 911 ma weakly deformed i type granites at 894 815 ma and a type granites at 759 711 ma they can be interpreted as the products of respectively syn collisional post collisional and extensional setting in response to the assembly and breakup of the rodinia supercontinent the paleozoic magmatism can be temporally classified into three stages of 507 470 ma 460 422 ma and 415 400 ma they were genetically related to the subduction of the shangdan ocean and subsequent collision of the southern north china block and the south qinling belt the 507 470 ma magmatism is spatially and temporally related to ultrahigh pressure metamorphism in the studied area the 460 422 ma magmatism with an extensive development in the north qinling belt is characterized by i type granitoids and originated from the lower crust with the involvement of mantle derived magma in a collisional setting the magmatism with the formation age of 415 400 ma only occurred in the middle part of the north qinling belt and is dominated by i type granitoid intrusions and probably formed in the late stage of a collisional setting early mesozoic magmatism in the study area occurred between 252 and 185 ma with the cluster in 225 200 ma it took place predominantly in the western part of the south qinling belt the 250 240 ma i type granitoids are of small volume and show high sr y ratios and may have been formed in a continental arc setting related to subduction of the mianlue ocean between the south qinling belt and the south china block voluminous late stage 225 185 ma magmatism evolved from early i type to later i a type granitoids associated with contemporaneous lamprophyres representative of a transition from syn to post collisional setting in response to the collision between the north china and the south china blocks late mesozoic 158 100 ma granitoids located in the southern margin of the north china block and the eastern part of the north qinling belt are characterized by i type i to a type and a type granitoids that were emplaced in a post orogenic or intraplate setting the first three of the four periods of magmatism were associated with three important orogenic processes and the last one with intracontinental process these suggest that the tectonic evolution of the qinling orogen is very complicated

mesoscale eddies in the northeastern pacific tropical subtropical transition zone statistical characterization from satellite altimetry 1 mesoscale eddies in the northeastern pacific tropical subtropical transition zone 16 n 30 n 130 w 102 w are analyzed using nearly 18 years of satellite altimetry and an automated eddy identification algorithm eddies that lasted more than 10 weeks are described based on the analysis of 465 anticyclonic and 529 cyclonic eddy trajectories we found three near coastal eddy prolific areas 1 punta eugenia 2 cabo san lucas and 3 cabo corrientes these three areas are located in places where the coastal morphology changes abruptly and strong surface current intensification occurs at some phase of the seasonal cycle although mesoscale eddies in these areas have been previously reported this study provides their first statistically supported characterization punta eugenia showed the highest eddy production with more cyclones generated followed by cabo corrientes also with more cyclones and cabo san lucas with more anticyclones cabo corrientes eddies showed the highest mean values in propagation speed swirling speed and eddy kinetic energy whereas punta eugenia eddies showed the lowest values cyclonic eddies increased their distance traveled and duration from south to north in contrast anticyclonic eddies increased from north to south in average anticyclones tend to travel faster than cyclones in all the subregions these long lived eddies were mainly nonlinear and therefore can redistribute coastal waters relatively far into the open ocean the peaks in the seasonal signal of eddy generation can be associated with the peaks in the strength of the offshore currents and or in the coastal upwelling index no clear relationship could be established between el nino events and eddy generation

formation of al rich titanite catisio4o caalsio4oh reaction rims on ilmenite in metamorphic rocks as a function of fh2o and fo2 abstract reaction rims of titanite on ilmenite are described in samples from four terranes of amphibolite facies metapelites and amphibolites namely the tamil nadu area southern india the val strona area of the ivrea verbano zone northern italy the bamble sector southern norway and the northwestern austroalpine otztal complex the titanite rims and hence the stability of titanite catisio4o and al oh titanite i e vuaganatite hypothetical end member caalsio4oh are discussed in the light of fh2o and fo2 buffered equilibria involving clinopyroxene amphibole biotite ilmenite magnetite and quartz in the systems cao feo fe2o3 tio2 sio2 h2o o2 cftsh and cao feo fe2o3 al2o3 sio2 h2o o2 cfash present in each of the examples textural evidence suggests that titanite reaction rims on ilmenite in rocks from tamil nadu val strona and the bamble sector originated most likely due to hydration reactions such as clinopyroxene ilmenite quartz h2o amphibole titanite and oxidation reactions such as amphibole ilmenite o2 titanite magnetite quartz h2o during amphibolite facies metamorphism or as in the case of the otztal complex during a subsequent greenschist facies overprint overstepping of these reactions requires fh2o and fo2 to be high for titanite formation which is also in accordance with equilibria involving al oh titanite this study shows that in addition to p t bulk rock composition and composition of the coexisting fluid fo2 and fh2o also play an important role in the formation of al bearing titanite during amphibolite and greenschist facies metamorphism

sequence stratigraphy in active extensional basins implications for the interpretation of ancient basin fills abstract systematic variations in sequences their component units and their stacking patterns exist within rift basins these variations can be related to displacement gradients associated with the large scale 12 50 km segmentation of normal fault zones which control accommodation sediment supply and basin physiography sediment supply is also strongly influenced by the nature of pre rift drainage networks and variations in bedrock lithology high rates of hangingwall subsidence close to the centre of normal fault segments may cancel out the effects of glacio eustatic sea level fall so that accommodation development is normally characterized by the continual addition of new space the resulting sequences lack type 1 sequence boundaries and lowstand systems tracts and stack into aggradational sequence sets the adjacent footwall is subject to uplift which may lead to subaerial exposure and incision generating a composite type 1 sequence boundary away from the fault zone and near segment boundaries slip rates are much lower and hence relative sea level change is dominated by eustasy here falls in relative sea level are important resulting in type 1 sequences sequence stacking patterns reflect not only eustasy and local fault controlled subsidence but also the interaction of these with adjacent uplifting footwalls and or regional uplift tectonic influence on sequence development will be more pronounced during greenhouse times than icehouse times

u pb zircon dating geochemical and sr nd hf isotopic compositions of motuo quartz monzonite implication for the genesis and diversity of the high ba sr granitoids in orogenic belt abstract early paleogene granitoids in southern lhasa subterrane have been widely investigated and many petrogenesis and geodynamic models have been proposed in the past few years however contemporaneous granitoids in the motuo tectono magmatic belt southeast lhasa terrane are still limitedly studied here we present the petrology zircon u pb geochronology whole rock geochemistry and sr nd hf isotope data of the damu and 52 k quartz monzonite in the motuo area la icp ms u pb zircon dating shows that they have magma crystallization ages of 49 and 69 ma respectively the damu quartz monzonite sio2 63 76 68 33 wt is high k calc alkaline k2o 2 54 4 02 wt with k2o na2o 0 59 1 09 and metaluminous to weakly peraluminous a cnk 0 99 1 07 the 52 k quartz monzonite sio2 61 12 66 12 wt shows slightly higher k2o contents 3 80 5 28 wt with k2o na2o 1 03 1 45 and metaluminous series a cnk 0 96 1 00 the analyzed samples are characterized by high ba 850 2573 ppm sr 534 986 ppm contents and fractionated ree patterns la yb n 22 72 and sm yb n 4 55 8 24 these geochemical features are comparable with those of high ba sr granite they display weakly evolved sr nd hf compositions whole rock 87sr 86sr 0 0 7068 to 0 7086 end t 4 20 to 3 41 and zircon ehf t 5 2 to 0 9 geochemical and sr nd hf isotopic data reflect that the damu and 52 k quartz monzonite represent residual magma from afc processes of lithospheric mantle derived mafic melts the over thickened lower crust in the eastern lhasa terrane had been delaminated during ca 83 70 ma which led to the replacement of ancient lithospheric mantle by the juvenile lithospheric mantle the juvenile mantle wedge in the study area was suspected to be metasomatized by melts that were derived from the foundering arc root rather than the subducted sediments thus the early paleogene high ba sr magmas from the se lhasa terrane may provide evidence for recycling of continental lithosphere by density destruction

a new conceptual model for the structural evolution of a regional salt detachment on the northeast scotian margin offshore eastern canada in this study we examine using seismic data in conjunction with numerical modeling a regional scale salt detachment and associated synkinematic sediments from the scotian margin offshore eastern canada this part of the scotian margin is characterized by an up to 4 5 km 2 8 mi thick and approximately 175 km 108 mi long synkinematic wedge of jurassic sediments with internal sigmoidal landward dipping reflectors the synkinematic wedge is laterally extensive encompassing an area of approximately 30 000 km2 11 600 mi2 and soles into an interpreted salt detachment the jurassic synkinematic wedge which is interpreted to have formed as an open ended allochthonous salt nappe was loaded by prograding sediments during the jurassic this loading squeezed the salt seaward and caused the overlying sediments to undergo extension and gravity spreading and gliding detaching on the salt sheet the open ended nappe model provides a mechanism for producing a large amount of extension with very little compensating contraction numerical model results indicate that high rates of extension detaching on even a thin salt layer can result in similar sigmoidal landward dipping strata based on the numerical modeling and seismic interpretation results we propose a new conceptual model for the jurassic paleogene structural evolution of the study area this model may also have implications for other passive margin salt basins with regional salt detachments

leaching and differential recrystallization of metamict zircon under experimental hydrothermal conditions we report results of hydrothermal experiments on four alluvial zircons from sri lanka which cover a wide range of radiation damage at 450 c and 1 3 kbar for 744 h with 2 m cacl2 solution as reactive fluid after the hydrothermal treatment the most metamict samples show micrometer thick reaction rims which surround apparently unreacted zircon as revealed by cathodoluminescence cl and nomarski differential interference contrast ndic images these rims have sharp curved and transgressive boundaries with unreacted zircon and are in some cases spread out along cracks the thickness of reaction rims increases with increasing cumulated dosage of the starting materials the reaction rims are strongly enriched in ca up to 7000 ppm and a water species and depleted in radiogenic pb zr and si as revealed by electron microprobe analyses a significant th loss from the reaction rims was detected in the case of the most metamict sample whereas u remained in the structure ft infrared spectrometry and x ray diffraction measurements revealed that the bulk run products were recrystallized using micro raman spectrometry we were able to demonstrate that differential recrystallization took place the reaction rims are strongly recrystallized whereas the unreacted grain interiors underwent only minor recrystallization recrystallization of the rims is accompanied by an enhancement of the integral cl intensity it is suggested that recrystallization in the reaction rims was catalyzed by water infiltration and ion exchange and prevented significant congruent zircon dissolution under the given experimental conditions previous zircon studies have shown that 1 a transgressive morphology 2 a reduced th u ratio and 3 an enhanced cl emission are also characteristics of rims in zircons from high grade metamorphic rocks based on these similarities between natural and experimentally produced rims it is suggested that leaching catalyzed recrystallization is an important alteration process in zircon under wet geological conditions and can account for many complex core rim structures found in natural zircons furthermore the strong enrichment of ca in the reaction rims supports previous assumptions that high ca concentrations in natural zircons are of secondary origin it is suggested that lower u pb concordia intercept ages obtained from single phase zircons with high ca contents date a leaching event

multiple zircon growth during fast exhumation of diamondiferous deeply subducted continental crust kokchetav massif kazakhstan diamondiferous rocks from the kokchetav massif kazakhstan represent deeply subducted continental crust in order to constrain the age of ultra high pressure uhp metamorphism and subsequent retrogression during exhumation zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study four different zircon domains were distinguished on the basis of transmitted light microscopy cathodoluminescence trace element contents and mineral inclusions mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions domain 1 consists of rounded cores and lacks evidence of uhp metamorphism domain 2 contains diamond coesite omphacite and titanian phengite inclusions providing evidence that it formed at uhp metamorphic conditions p 43 kbar t 950 c domain 3 is characterised by low pressure mineral inclusions such as garnet biotite and plagioclase which are common minerals in the granulite facies overprint of the gneisses p 10 kbar t 800 c this multi stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from zr saturated partial melts present in the gneisses domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet most probably at amphibolite facies conditions p 5 kbar t 600 c the metamorphic zircon domains were dated by shrimp ion microprobe and yielded ages of 527 5 528 8 and 526 5 ma for domains 2 3 and 4 respectively these indistinguishable ages provide evidence for a fast exhumation beyond the resolution of shrimp dating the mean age of all zircons formed between uhp metamorphic conditions and granulite facies metamorphism is 528 3 ma indicating that decompression took place in less than 6 ma hence the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1 8 cm year we propose a two stage exhumation model to explain the obtained p t t path fast exhumation on top of the subducted slab from depth 140 to 35 km was driven by buoyancy and facilitated by the presence of partial melts a period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision

compositional polarity of triassic granitoids in the qinling orogen china implication for termination of the northernmost paleo tethys abstract the qinling orogen was formed from the closure of the northernmost paleo tethys sea and the tectonic suturing of the yangtze and north china cratons the timing of this collision and the tectonic framework are debated the widely developed triassic granitoids in the western qinling orogen offer a key to understand the tectonic evolution of this region here we compile the geological geochemical and geochronological data of the triassic granitoids from the qingling orogen and conclude that the granitoids north of the mian lue suture were emplaced in an active continental margin related to the northward subduction of the mian lue oceanic plate during 248 200 ma the granitoids can be classified into i and s types with the former constituting the major variety northward from the mian lue fault an s type granite belt appears followed by and locally overlapping with an i type granite belt the i type granite belt can be subdivided into four sub belts from south to north with increasing contents of k2o k2o na2o sio2 th and u and the ratios of k2o na2o rb sr and 87sr 86sr i but decreasing contents of na2o al2o3 mg and ehf t except for the northernmost belt which occurs in the huaxiong block of the north china craton this geochemical polarity of i type granitoids and the zoned distribution of s and i types granitoids cannot be explained through continental collision orogeny including syn to post collision but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo tethys as represented by the mian lue ocean thus the termination of the northernmost paleo tethys and the onset of the continental collision between the yangtze and the north china plates are considered to have occurred at about 200 ma during the transition from triassic to jurassic our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the yangtze and the north china cratons

rapid transition from continental breakup to igneous oceanic crust in the south china sea continental breakup represents the successful process of rifting and thinning of the continental lithosphere leading to plate rupture and initiation of oceanic crust formation magmatism during breakup seems to follow a path of either excessive transient magmatism magma rich margins or of igneous starvation magma poor margins the latter type is characterized by extreme continental lithospheric extension and mantle exhumation prior to igneous oceanic crust formation discovery of magma poor margins has raised fundamental questions about the onset of ocean floor type magmatism and has guided interpretation of seismic data across many rifted margins including the highly extended northern south china sea margin here we report international ocean discovery program drilling data from the northern south china sea margin testing the magma poor margin model outside the north atlantic contrary to expectations results show initiation of mid ocean ridge basalt type magmatism during breakup with a narrow and rapid transition into igneous oceanic crust coring and seismic data suggest that fast lithospheric extension without mantle exhumation generated a margin structure between the two endmembers asthenospheric upwelling yielding mid ocean ridge basalt type magmatism from normal temperature mantle during final breakup is interpreted to reflect rapid rifting within thin pre rift lithosphere

geochemical sr nd and zircon u pb hf isotopic studies of late carboniferous magmatism in the west junggar xinjiang implications for ridge subduction abstract voluminous granitic intrusions are distributed in the west junggar nw china and they can be classified as the dioritic rocks charnockite and alkali feldspar granite groups the dioritic rocks sio 2 50 4 63 8 wt are calc alkaline and mg enriched average mgo 4 54 wt mg 0 39 0 64 with high sr y ratios average 21 2 weak negative eu average eu eu 0 80 and pronounced negative nb ta anomalies their sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7035 0 7042 e nd t 4 5 7 9 e hf t 14 1 14 5 show a depleted mantle like signature these features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials the charnockites sio 2 60 0 65 3 wt show transitional geochemical characteristics from calc alkaline to alkaline with weak negative eu average eu eu 0 75 but pronounced negative nb ta anomalies sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7037 0 7039 e nd t 5 2 8 0 e hf t 13 9 14 7 also indicate a depleted source suggesting melts from a hot juvenile lower crust alkali feldspar granites sio 2 70 0 78 4 wt are alkali and fe enriched and have distinct negative eu and nb ta anomalies average eu eu 0 26 low sr y ratios average 2 11 and depleted sr nd and zircon hf isotopic compositions 87 sr 86 sr i 0 7024 0 7045 e nd t 5 1 8 9 e hf t 13 7 14 2 these characteristics are also comparable with those of rocks derived from juvenile lower crust despite of the differences in petrology geochemistry and possibly different origins zircon ages indicate that these three groups of rocks were coevally emplaced at 305 ma a ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks as the slab window opened upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms partial melting of the subducting slab formed the dioritic rocks partial melting of the hot juvenile lower crust produced charnockite and alkali feldspar granite and partial melting in the mantle wedge generated mafic rocks in the region these results suggest that subduction was ongoing in the late carboniferous and thus support that the accretion and collision in the central asian orogenic belt took place in north xinjiang after 305 ma and possibly in the permian

dehydration melting and water saturated melting of basaltic and andesitic greenstones and amphibolites at 1 3 and 6 9 kb

tectonic evolution of the north qinling orogen from subduction to collision and exhumation evidence from zircons in metamorphic rocks of the qinling group abstract the north qinling block nqb is an important segment of the qinling orogen in central china here we report the results from sims geochronology and oxygen isotopes as well as la mc icpms hf isotopic analyses on zircon grains from a suite of metamorphic rocks felsic gneisses garnet plagioclase amphibolites and retrograde eclogite dikes in the qinling group of the nqb the age data show that these rocks underwent at least two episodes of metamorphism with the peak at 483 501 ma followed by 454 470 ma retrograde metamorphism these results are generally coeval with the periods of 500 480 ma for peak metamorphism and 460 420 ma for retrograde metamorphism previously obtained from the hp uhp metamorphic rocks of the nqb during the prograde and retrograde metamorphism widespread fluid and melt circulation within the block has been identified from the geochemical features of the metamorphic zircons the fluids that circulated in the felsic gneisses and retrograde eclogite dikes originated from the dehydration of altered oceanic basalts as inferred from the exceedingly low th u ratios positive ehf t 5 and extremely 18o 10 01 13 91 values in metamorphic zircons in contrast the melt involved in the formation of garnet plagioclase amphibolites appears to have been derived from continental sediments interlayered with the oceanic basalts since zircons crystallized during the peak and retrograde metamorphism show typical magmatic features with high u and th contents and th u ratios and enriched hf ehf t 5 42 to 0 18 and oxygen isotope composition 18o around 8 geochronological and geochemical features of the magmatic cores of the clear core rim textured zircons demonstrate that the protoliths of the gneisses were intermediate acid volcanic rocks erupted before neoproterozoic 800 ma which is further supported by the intrusion of basaltic magma of asthenospheric origin as represented by protoliths of retrograde eclogite dikes with the oldest magmatic zircon formed at 789 ma the protoliths of garnet plagioclase amphibolites appear to be altered oceanic basalts but had been significantly affected by the melt during the metamorphism combined with the previous studies the qinling group experienced overall subduction in the early paleozoic the nqb as represented by the qinling group was most likely a discrete micro block in the neoproterozoic and underwent deep subduction in the cambrian 483 501 ma and exhumation in ordovician 454 470 ma we propose that the nqb preserves a complete cycle of tectonic evolution of an orogen from an oceanic basin spreading and micro continent formation to deep subduction and exhumation

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

sea surface temperature gradients across blue whale and sea turtle foraging trajectories off the baja california peninsula mexico sea surface temperature sst fronts are integral to pelagic ecology in the north pacific ocean so it is necessary to understand their character and distribution and the way these features influence the behavior of endangered and highly migratory species here telemetry data from sixteen satellite tagged blue whales balaenoptera musculus and sea turtles caretta caretta chelonia mydas and lepidochelys olivacea are employed to characterize biologically relevant sst fronts off baja california sur high residence times are used to identify presumed foraging areas and sst gradients are calculated across advanced very high resolution radiometer avhrr images of these regions the resulting values are compared to classic definitions of sst fronts in the oceanographic literature we find subtle changes in surface temperature between 0 01 and 0 101c km across the foraging trajectories near the lowest end of the oceanographic scale between 0 03 and 0 31c km suggesting that edge detection algorithms using gradient thresholds 40 101c km may overlook pelagic habitats in tropical waters we use this information to sensitize our edge detection algorithm and to identify persistent concentrations of subtle sst fronts in the northeast pacific ocean between 2002 and 2004 the lower gradient threshold increases the number of fronts detected revealing more potential habitats in different places than we find with a higher gradient threshold this is the expected result but it confirms that pelagic habitat can be overlooked and that the temperature gradient parameter is an important one r 2006 elsevier ltd all rights reserved

mesozoic high ba sr granitoids from north china geochemical characteristics and geological implications mesozoic magmatism occurred extensively in the north china block ncb and the dabie sulu orogen dso post dating the north south china collision resulting in abundant intrusive and volcanic rocks ranging from basic to acidic compositions the intermediate acidic intrusive rocks can be grouped into two types namely high ba sr granitoids and low ba sr granitoids that both have distinct geochemical characteristics the high ba sr granitoids are similar in most of the incompatible trace element systematics to the associated basic rocks which probably originated from melting of subcontinental lithospheric mantle indicating significant mantle contributions to them geochemical similarities are observed between the basic rocks from the ncb and dso implying a regional scale magma generating mechanism and that mantle enrichment beneath the dso was independent from the triassic deep continental subduction in the region we therefore interpret that the mesozoic magmatism resulted from delamination of the ancient lithospheric mantle beneath the eastern part of north china

age and origin of high ba sr appinite granites at the northwestern margin of the tibet plateau implications for early paleozoic tectonic evolution of the western kunlun orogenic belt abstract the buya appinite granite is a typical high ba sr granite emplaced at the northern west kunlun orogenic belt along the northwestern margin of the tibetan plateau the granite is dated at ca 430 ma using the shrimp u pb zircon method it consists of alkaline feldspar granites with coeval appinite enclaves the granite possesses high sio2 69 77 72 69 k2o 4 44 5 10 and total alkalinity k2o na2o 8 80 9 92 sr 655 1100 ppm ba 1036 1433 ppm and lree and low hree and hfse contents and insignificant negative eu anomalies consequently the samples have very high sr y 74 141 and la yb n 37 96 ratios on the other hand they have low mgo or mg cr and ni contents and low radiogenic nd isotopes nd t 8 4 to 10 4 the high ba sr and other geochemical signatures of the granite also appear in the appinite enclaves except that the appinite enclaves have relatively higher abundances in these elements and higher nd t values 5 7 to 6 7 elemental and isotope compositions suggest that the appinites were derived from partial melting of an enriched lithospheric mantle source probably induced by upwelling of the asthenosphere due to the delamination of a subducted slab the granite was likely derived from partial melting of the mafic lower crust with residual garnet associated with involvement of minor lile enriched appinitic magma followed by crystal fractionation of hornblende biotite apatite and allanite in combination with previous investigations on the evolution of the western kunlun we suggest that the buya high ba sr plutons represent the end of an early paleozoic crust thickening event after a terrane accretion on southern tarim craton and the beginning of a post orogenic collapse phase in the paleozoic west kunlun orogenic belt

the western central asian orogenic belt a window to accretionary orogenesis and continental growth abstract the architecture of accretionary orogens is a key to understand continental growth here we present an overview of the orogenic components and their amalgamation in the western central asian orogenic belt caob the caob records the convergence and interactions among various types of orogenic components including the japan type mariana type and alaska aleutian type arc systems as well as the active marginal sequences of the siberia craton which incorporated wide accretionary complexes and accreted arcs and terranes during construction of the caob the kazakhstan arc chain was characterized by multiple subduction whereas the northern fringe of the tarim craton remained mostly as a passive margin the multiple convergence and accretions among these various orogenic components generated huge orogenic collages in the late paleozoic and even in the early triassic involving parallel amalgamation circum microcontinent amalgamation and oroclinal bending the preservation of trapped basins played a significant role in orogenesis with some parts of the oceanic plate being subducted and others behaving as rigid units the orogenesis in the caob was long lived lasting for more than 800 m y involving multiple subduction and long continuous accretion and featuring the complexity of accretionary orogenesis and continent growth

metamorphic growth and recrystallization of zircon distinction by simultaneous in situ analyses of trace elements u th pb and lu hf isotopes in zircons from eclogite facies rocks in the sulu orogen simultaneous in situ analyses of trace elements u th pb and lu hf isotopes were carried out on distinct domains of zircons in ultrahigh pressure uhp eclogite facies metamorphic rocks from the main hole of the chinese continental scientific drilling ccsd in the sulu orogen for the first time trace elements are directly linked to lu hf isotopes in metamorphic zircons with reference to their u pb dates this enables methodological integration to distinguish four types of metamorphic zircon solid state replacement and dissolution recrystallizations of protolith zircons and new growth from the aqueous fluid metamorphically grown zircons are characterized by concordant u pb ages for the metamorphism flat hree patterns typical of the garnet effect low contents of ree especially hree y nb ta and th u high contents of hf low lu gd n lu hf and th u 0 1 ratios and elevated 176hf 177hf ratios relative to solid state recrystallized zircons this suggests the effects of both garnet and fluid on the growth of metamorphic zircons in contrast metamorphic recrystallization has reset the u th pb isotope system of protolith zircons to different extents depending on the extents of fluid action during metamorphism solid state recrystallized zircons exhibit the lowest degrees of resetting and thus almost inherit all geochemical features from the protolith zircons which are characterized by discordant u pb ages close to or below the protolith age steep mree hree patterns typical of magmatic origin high contents of trace elements and their ratios and low 176hf 177hf ratios on the other hand dissolution recrystallized zircons show the highest degrees of reworking and thus have concordant or nearly concordant u pb ages for the metamorphism steep mree hree patterns lowered contents of trace elements such as ree th u y nb ta and ti relative to the protolith zircons and almost unchanged hf isotope ratios replacement recrystallized zircons display intermediate degrees of reworking and thus have their many features of elements and isotopes in between while the metamorphic growth in the presence of both garnet and fluid is characterized by both depletion of hree with flat pattern and the low contents of trace elements the metamorphic recrystallization in the presence of aqueous fluid is indicated by gradual decreases of mree to hree without the flat hree pattern therefore the simultaneous in situ analyses of metamorphic zircons have the advantage over single term analyses in making distinction between the new growth and the different types of recrystallization

cenozoic tectonic subsidence in deepwater sags in the pearl river mouth basin northern south china sea the cenozoic tectonic subsidence of the deepwater area in the pearl river mouth basin northern south china sea is studied by subsidence analysis via backstripping calculations based on data of newly interpreted sequence boundaries in the subsidence analysis local porosity depth parameters were estimated based on well data the paleo water depth parameters and lithology were estimated for all grid nodes based on well data and sedimentary system maps and ages of sequence boundaries were adjusted according to the international chronostratigraphic chart v2013 01 sensitivity analysis shows that these are essential to ensure the quality of the subsidence analysis in the slope areas maps of tectonic subsidence rates of 18 cenozoic sequences were constructed and spatial temporal variations of the tectonic subsidence were discussed the subsidence was restricted in the baiyun and liwan sags before 30 ma and extended to uplifts after 30 ma this indicates that the 30 ma unconformity is the breakup unconformity that separated the syn rift and post rift sequences four substages of post rift subsidence were identified by cyclic changes in the rate and pattern of tectonic subsidence at boundaries of 23 3 ma 17 2 ma and 11 9 ma respectively anomalous post rifting tectonic subsidence was significant and totaled to similar to 1200 m at the center of the baiyun sag and punctuated by rapid subsidence and uplift events the baiyun event was manifested in the study area by a short lived uplift event in 23 9 23 3 ma followed by a short lived rapid subsidence in 23 3 19 8 ma in the central study area this localized rapid subsidence caused northward jump and anti clockwise rotation of the shelf break two order teeterboard seesaw like subsidence was observed in the study area in the post rifting stage suggesting elastic deformations of the crust and lithosphere c 2014 elsevier b v all rights reserved

hydrographic and fish larvae distribution during the godzilla el nino 2015 2016 in the northern end of the shallow oxygen minimum zone of the eastern tropical pacific ocean based on hydrographic data and vertical distributions of tropical species of fish larvae diogenichthys laternatus vinciguerria lucetia bregmaceros bathymaster and auxis spp effects of godzilla el nino 2015 2016 in the shallow oxygen minimum zone off mexico were analyzed zooplankton samples were collected during four cruises before february 2010 and april 2012 and during june 2015 and march 2016 the warm event temporal series of sea surface temperature revealed that june 2015 was the warmest june of the last years conservative temperature was 2 c higher than normal in the surface mixed layer and the suboxic layer 4 4 mol kg reached as shallow as 100 m depth unexpected results were that larval abundances were relatively high during the warm event unlike zooplankton volumes which declined before the warm event v lucetia and auxis spp were more abundant in the surface mixed layer while b bathymaster and d laternatus dominated in the thermocline and shallow hypoxic layer 44 mol kg however during the event in june 2015 all species were most abundant in the surface mixed layer which implied that the species adapted to hypoxia had inverted their normal pattern of distribution possibly as consequence of the rise of the suboxic layer however further observations are required to confirm this generality results showed no dramatic change in the total larval abundance during the warm event nevertheless a differential response in their vertical distribution was evident in association with changes in the depth of the shallow hypoxic and suboxic layers this might indicate adaptability of tropical species to prolonged periods of warming in the oceans

episodic rifting and subsidence in the south china sea the south china sea experienced at least three stages of rifting and two intervening stages of sea floor spreading since the early cretaceous its evolution can be described by an episodic model of tectonism one of thermal cooling and subsidence pulsed by temporally and spatially confined heating events analysis of regional geologic and geophysical data suggests episodes of rifting and associated thermal activities initiated during the late cretaceous the late eocene and the late early miocene the rift system corresponding to the first episode trends northeast southwest whereas those of the second and third trend east west these two trends coincide with the orientations of the major tectonic lineations within the basin age estimates from heat flow and bathymetric data suggest the oceanic crust in the southwest subbasin is considerably older 55 ma than that in the northwest 35 36 ma or east 32 ma subbasins the episodic tectonic model is supported by basin subsidence analysis subsidence especially subsidence rate curves derived from the regional well data demonstrates that unlike a classic atlantic passive margin thermal subsidence in the south china sea was punctuated by rapid subsidence events which are chronologically consistent with the rifting episodes in terms of hydrocarbon potential the episodes of rifting and drifting would be conducive to the development of overprinted structures and the deposition of several discrete transgressive packages of source rocks and reservoirs separated by widespread unconformities the thermal maturity of sedimentary organic matter affected by episodic rifting and subsidence may be greater than expected on a purely passive margin of equivalent age that had not experienced repeated heating

quantification and restoration of extensional deformation along the western iberia and newfoundland rifted margins 1 many recent papers describe the structure of the iberia and newfoundland rifted margins however none of them propose kinematic restorations of the complete rift system to quantify the amount of extension necessary to exhume mantle and initiate seafloor spreading in our study we use two pairs of cross sections considered as conjugate lines one across the galicia bank flemish cap and the other across the southern iberia abyssal plain flemish pass both transects have been imaged by reflection and refraction seismic methods and have been drilled during ocean drilling program legs 103 149 173 and 210 drilling penetrated parts of the rift stratigraphy and the underlying basement the cross sections can therefore be considered as the best documented conjugate transects across present day hyperextended magma poor rifted margins the aim of this paper is threefold 1 provide a detailed description of the crustal architecture of the two conjugate sections 2 define the extensional structures and their ages and 3 quantify the amount of strain and strain rate accommodated along these lines this paper proposes a quantitative description of extension along the iberia newfoundland rift system and discusses the limitations and problems in quantifying extensional deformation along hyperextended rifted margins

tectonomagmatic evolution of the final stages of rifting along the deep conjugate australian antarctic magma poor rifted margins constraints from seismic observations the processes related to hyperextension exhumed mantle domains lithospheric breakup and formation of first unequivocal oceanic crust at magma poor rifted margins are yet poorly understood in this paper we try to bring new constraints and new ideas about these latest deformation stages by studying the most distal australian antarctic rifted margins we propose a new interpretation linking the sedimentary architectures to the nature and type of basement units including hyperextended crust exhumed mantle embryonic and steady state oceanic crusts one major implication of our study is that terms like prerift synrift and postrift cannot be used in such polyphase settings which also invalidates the concept of breakup unconformity integration and correlation of all available data particular seismic and potential field data allows us to propose a new model to explain the evolution of magma poor distal rifted margins involving multiple and complex detachment systems we propose that lithospheric breakup occurs after a phase of proto oceanic crust formation associated with a substantial magma supply first steady state oceanic crust may therefore not have been emplaced before 53 3 ma corresponding to magnetic anomaly c24 observations of magma amount and its distribution along the margins highlight a close magma fault relationship during the development of these margins

the northern boundary of the proto tethys ocean constraints from structural analysis and u pb zircon geochronology of the north qinling terrane abstract the north qinling terrane including the kuanping erlangping and qinling groups is located in the northern part of the qinling orogen here we present results from structural analyses of the north qinling terrane and its surrounding regions three episodes of deformation d 1 d 2 and d 3 are recognized the d 1 deformation is characterized by nne dipping and ssw dipping penetrative foliations s 1 the s 1 foliations are mainly marked by a strong preferred orientation of hornblende crystals and they were subsequently overprinted by the d 2 deformation the d 2 deformation is marked by tight asymmetric folds with nne dipping and ssw dipping axial plane foliations s 2 we propose that the d 1 and d 2 deformations mainly occurred during 440 400 ma after the peak ultra high pressure metamorphic event 520 480 ma and so these two deformation events could constrain the collision and exhumation processes of the north qinling terrane which probably indicates the closure of the kuanping ocean located between the north qinling terrane and the north china block in the early paleozoic these two deformations are overprinted by the d 3 deformation characterized by ene trending upright open folds two detrital zircon age spectra for the kuanping group with the youngest zircon of 640 ma and 510 ma respectively are different from those recorded for the north china block which may reveal the continuous southward subduction of the kuanping ocean during the neoproterozoic to early paleozoic interval we propose that the kuanping ocean marks the northern boundary of the proto tethys ocean in the study area

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

interactions between continental breakup dynamics and large scale delta system evolution insights from the cretaceous ceduna delta system bight basin southern australian margin 1 the interpretation of two regional seismic reflection profiles and the construction of a balanced cross section through the southern australian margin bight basin are designed to analyze the influence of the australia antarctica continental breakup process on the kinematic evolution of the cretaceous ceduna delta system the data show that the structural architecture of this delta system consists of two stacked delta systems the lower white pointer delta system late albian santonian is an unstable tectonic wedge regionally detached seaward above late albian ductile shales sequential restoration suggests that the overall gravitational sliding behavior of the white pointer delta wedge 45 km of seaward extension i e 27 is partially balanced by the tectonic denudation of the subcontinental mantle we are able to estimate the horizontal stretching rate of the mantle exhumation between 2 and 5 km ma 1 the associated uplift of the distal part of the margin and associated flexural subsidence in the proximal part of the basin are partially responsible for the decrease of the gravitational sliding of the white pointer delta system lithospheric failure occurs at 84 ma through the rapid exhumation of the mantle the upper hammerhead delta system late santonian maastrichtian forms a stable tectonic wedge developed during initial slow seafloor spreading and sag basin evolution of the australian side margin lateral variation of basin slope related to the geometry of the underlying white pointer delta wedge is associated with distal raft tectonic structures sustained by high sedimentation rates finally we propose a conceptual low angle detachment fault model for the evolution of the australian antarctic conjugate margins in which the antarctic margin corresponds to the upper plate and the australian margin to the lower plate

water masses and circulation in the tropical pacific off central mexico and surrounding areas abstractthe seasonal variations and the interactions of the water masses in the tropical pacific off central mexico tpcm and four surrounding areas were examined based on an extensive new hydrographic database the regional water masses were redefined in terms of absolute salinity sa and conservative temperature according to the thermodynamic equation of seawater 2010 teos 10 hydrographic data and the evaporation minus precipitation runoff balance were used to investigate the origin and seasonality of two salinity minima in the area the shallow 50 100 m salinity minimum originates with the california current system and becomes saltier as it extends southeastward and mixes with tropical subsurface waters while the surface salinity minimum extends farther north in the tpcm in summer and fall because of the northward advection of tropical surface waters the interactions between water masses allow a characterization of the seasonal pattern of circulation of the mexican coastal current mcc

how does the continental crust thin in a hyperextended rifted margin insights from the iberia margin the discovery of hyperthinned continental crust and exhumed mantle on present day deep water rifted margins leads to two fundamental questions 1 in detail how does the crust thin in extension and 2 what controls extreme crustal thinning and mantle exhumation reflection and refraction seismic lines across the iberian margin show decoupling levels in the crust cut by structures that eventually transfer deformation to mantle levels the region of decoupled extension appears to be more broadly distributed on the northern iberian margin and more localized on the southern margin based on drill hole data the transition from decoupled to coupled deformation occurred during tithonian time ca 145 ma this evolution from decoupled to coupled deformation may help explain the crustal architecture of the iberian margin an apparent delay of subsidence across the hyperextended coupled zone of the iberian margin may indicate that crustal thinning had to occur simultaneously with lithospheric necking and the advection of heat related to lithospheric mantle thinning

styles of extension offshore mid norway and implications for mechanisms of crustal thinning at passive margins 1 eocene magmatic breakup along the mid norway rifted margin was preceded by extreme jurassic cretaceous crustal thinning in a magma poor environment along the se borders of the rift top basement detachment faults with heaves on the order of 15 40 km evolved in at least two stages to become the boundaries between moderately thinned 20 30 km thick crust and a 100 200 km wide highly extended area with crustal thicknesses generally between 2 and 12 km under the present day more and voring basins in the footwalls of the basin flank detachments lower and middle crust was exhumed in extensional domes that became incised by a younger set of normal faults under the most highly thinned areas a more distal set of deep seated basin floor detachments incised and extended remnant crust and probably the upper mantle leaving as little as 5 km of continental crust to be preserved under thick synrift and postrift deposits we suggest that basin flank detachments such as the ones described above hold the potential to reduce the crustal thickness down to the critical value required for embrittlement of continental crust thus they prepare the ground for incision by the basin floor detachments which may become responsible for exhumation of deep crust or continental mantle if extension is allowed to proceed

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

the petrogenesis of early paleozoic high ba sr intrusions in the north qinling terrane china and tectonic implications abstract high ba sr hibasr intrusions provide important insights into early paleozoic magmatism and tectonic evolution of the qinling orogenic belt china here new zircon u pb ages whole rock major and trace element data and sr nd hf isotopic compositions are presented for three hibasr dioritic intrusions yangjiazhuang and tangzang quartz diorite intrusions and honghuapu diorite intrusion in the north qinling terrane the yangjiazhuang quartz diorites and honghuapu diorites are high k calc alkalic to shoshonitic rocks whereas the tangzang quartz diorites are na rich calc alkalic rocks la icp ms zircon u pb dating indicates that the k rich dioritic intrusions were formed at 439 438 ma and the na rich intrusion at 429 425 ma rocks of the three intrusions are metaluminous to weakly peraluminous and are characterized by high ba and sr contents of 792 3050 and 579 1385 ppm respectively with obvious enrichment in liles e g rb ba th and u and depletion in hfses e g nb ta and ti they exhibit relatively uniform sr and nd isotopic compositions with 87sr 86sr i ratios of 0 7032 0 7059 end t values of 0 65 to 2 78 and two stage nd model ages tdm2 of 1 23 1 39 ga indicative of an enriched mantle source the k rich intrusions exhibit minor nd hf isotopic decoupling ehf t 2 93 5 01 and have low nb u ratios 2 11 3 01 and mg numbers 48 5 52 1 suggesting that the yangjiazhuang quartz diorites and honghuapu diorites were formed through fractional crystallization of partial melts of enriched mantle metasomatized by terrigenous sediment derived fluids the na rich tangzang quartz diorites exhibit distinct ehf t values 5 72 9 35 and elemental contents e g high na2o and nb and highly variable rb y nb y th zr and nb zr ratios indicating that they formed through crustal assimilation and fractional crystallization of partial melts of enriched mantle previously fertilized by terrigenous sediment derived fluids and slab melts petrogenetic models for the hibasr dioritic rocks and late ordovician silurian igneous rocks of the western north qinling terrane indicate that the k rich hibasr intrusions formed in a slab subduction setting whereas the na rich intrusion was controlled by slab break off after continental collision together with granulite facies metamorphism in the region the emplacement of the hibasr dioritic rocks indicates that the closure of the shangdan ocean and initial collision between the north china craton and yangtze block occurred at ca 433 ma with subsequent slab break off triggering extensive silurian magmatism at ca 430 402 ma

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

successive detachment faults and mantle unroofing at magma poor rifted margins seismic depth images show that magma poor rifted margins between iberia and newfoundland are characterized by two different types of detachment that led to the unroofing of a broad expanse of mantle low angle detachments develop in serpentinites at the base of pre thinned crust and control further crustal thinning these detachments are cut by large offset faults rooting at a steep angle but with an exhumed slip surface and footwall flexurally rotated to a low angle during unroofing successive generations of this second type of detachment lead to the roughly symmetric unroofing of a broad expanse of mantle as new detachments repeatedly cut through the footwall of the preceding detachment leaving the abandoned root zone as landward dipping reflectors within exhumed mantle on both sides of the developing rift

the origin of high δ18o zircons marbles megacrysts and metamorphism the oxygen isotope ratios 18o of most igneous zircons range from 5 to 8 with 99 of published values from 1345 rocks below 10 metamorphic zircons from quartzite metapelite metabasite and eclogite record 18o values from 5 to 17 with 99 below 15 however zircons with anomalously high 18o up to 23 have been reported in detrital suites source rocks for these unusual zircons have not been identified we report data for zircons from sri lanka and myanmar that constrain a metamorphic petrogenesis for anomalously high 18o in zircon a suite of 28 large detrital zircon megacrysts from mogok myanmar analyzed by laser fluorination yields 18o from 9 4 to 25 5 the u pb standard cz3 a large detrital zircon megacryst from sri lanka yields 18o 15 4 0 1 2 se by ion microprobe a euhedral unzoned zircon in a thin section of sri lanka granulite facies calcite marble yields 18o 19 4 by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble small oxygen isotope fractionations between zircon and most minerals require a high 18o source for the high 18o zircons predicted equilibrium values of 18o calcite zircon 2 3 from 800 to 600 c show that metamorphic zircon crystallizing in a high 18o marble will have high 18o the high 18o zircons 15 from both sri lanka and mogok overlap the values of primary marine carbonates and marbles are known detrital gemstone sources in both localities the high 18o zircons are thus metamorphic the 15 25 zircon values are consistent with a marble origin in a rock dominated system i e low fluid external rock the lower 18o zircon values 9 15 are consistent with an origin in an external fluid dominated system such as skarn derived from marble although many non metasomatized marbles also fall in this range of 18o high 18o 15 and the absence of zoning can thus be used as a tracer to identify a marble source for high 18o detrital zircons this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks metamorphic zircon megacrysts have not been reported previously and appear to be associated with high grade marble identification of high 18o zircons can also aid geochronology studies that seek to date high grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble

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

post collisional late neoproterozoic high ba sr granitic magmatism from the dom feliciano belt and its cratonic foreland uruguay petrography geochemistry geochronology and tectonic implications abstract post collisional granitic magmatism with high barium strontium hibasr content and shoshonitic affinity has been recently recognized both in the southern section of the dom feliciano belt of uruguay and its cratonic foreland this group of granitic rocks has a distinctive age mineralogy chemistry and field characteristics new zircon u pb la icp ms data for three of the plutons confirm their late neoproterozoic age 634 7 3 sierra de los caracoles 604 3 0 ma cortez blanco and 597 3 6 ma guayabo similar ages were published previously for the solis de mataojo granitic complex 584 13 ma as well as las flores 586 2 7 ma and sobresaliente plutons 585 2 5 ma mineral assemblages of the studied quartz monzonites granodiorites and monzogranites comprise quartz orthoclase and microcline plagioclase ab 10 30 hornblende green biotite apatite titanite and allanite they plot predominantly in the high k calc alkaline field with the exception of a few that plot in the shoshonitic field characteristically they are relatively high in na 2 o normally 4 5 in acid varieties sio 2 65 decreasing to between 3 and 4 for more basic types k 2 o normally exceeds 3 5 but can be as low as 700 ppm sr 500 ppm and light rees alongside low nb ta and heavy rees the eu anomalies are negligible to slightly positive intermediate initial 87 sr 86 sr values 0 7077 to 0 7090 and very low initial epsilon nd values 15 8 to 19 3 at 600 ma with high nd tdm 2 2 2 8 ga suggest a recycling of ancient paleoproterozoic to late archaean sources late neoproterozoic granitoids of uruguay have been emplaced within a post collisional tectonic setting controlled by major shear zones and strike slip fault zones current field and analytical data suggest that these granitoids could have been mostly generated through the partial melting of an intermediate to lower mafic continental crust and or enriched crustal sources in the presence of garnet amphibole as stable residual mineral assemblage with a possible minor mantle component

oligocene evolution of the outermost continental margin in response to breakup and early spreading of the south china sea abstract preliminary results of the international ocean discovery program iodp expeditions 367 368 368x have confirmed the unique opening mechanism of the south china sea differentiating from that of the atlantic ocean however its detailed evolution history and controlling factors spanning from the early oligocene breakup to the early miocene steady spreading are still unclear here we conducted sedimentary geochemical and mineralogical analyses to a special sequence recovered at iodp expedition 367 site u1499 to reconstruct the oligocene tectono sedimentary evolution of the outermost margin our results indicate that a mesozoic arc related source primarily provided terrigenous clastics during rifting and early spreading periods in submarine canyon and ridge flank slope environments respectively late neoproterozoic to early paleozoic epimetamorphic fragments were enriched along with continental breakup and decreased upward in the overall retrograding sequence biogenous and hydrogenous contributions increased gradually and became dominant after the establishment of a stable pelagic deep basin in the earliest miocene provenance and environment features demonstrate that the south china sea was characterized by a special early spreading stage during which the outermost margin was relatively unstable and developed gravity flows to transport clastics to site u1499 two retrograding events occurred simultaneously with mid ocean ridge jumps at ca 27 0 ma and ca 23 0 ma implying the tectono sedimentary evolution of the distal margin was influenced by mantle activities

eclogite origin and timings in the north qinling terrane and their bearing on the amalgamation of the south and north china blocks the amalgamation of south scb and north china blocks ncb along the qinling dabie orogenic belt involved several stages of high pressure hp ultra high pressure uhp metamorphism the new discovery of uhp metamorphic rocks in the north qinling nq terrane can provide valuable information on this process however no precise age for the uhp metamorphism in the nq terrane has been documented yet and thus hinders deciphering of the evolution of the whole qinling dabie sulu orogenic belt this article reports an integrated study of u pb age trace element mineral inclusion and hf isotope composition of zircon from an eclogite a quartz vein and a schist in the nq terrane the zircon cores in the eclogite are characterized by oscillatory zoning or weak zoning high th u and 176lu 177hf ratios pronounced eu anomalies and steep heavy rare earth element hree patterns the zircon cores yield an age of 796 13 ma which is taken as the protolith formation age of the eclogite and implies that the nq terrane may belong to the scb before it collided with the ncb the hf t values vary from 11 3 to 3 2 and corresponding two stage hf model ages are 2402 to 1495 ma suggesting the protolith was derived from an enriched mantle in contrast the metamorphic zircon rims show no zoning or weak zoning very low th u and 176lu 177hf ratios insignificant eu anomalies and flat hree patterns they contain inclusions of garnet omphacite and phengite suggesting that the metamorphic zircon formed under eclogite facies metamorphic conditions and their weighted mean 206pb 238u age of 485 9 3 8 ma was interpreted to date the timing of the eclogite facies metamorphism zircon in the quartz vein is characterized by perfect euhedral habit some oscillatory zoning low th u ratios and variable hree contents it yields a weighted mean u pb age of 480 5 2 5 ma which registers the age of fluid activity during exhumation zircon in the schist is mostly detrital and u pb age peaks at c 1950 to 1850 1800 to 1600 1560 to 1460 and 1400 to 1260 ma with an oldest grain of 2517 ma also suggesting that the nq terrane may have an affinity to the scb accordingly the amalgamation between the scb and the ncb is a multistage process that spans c 300 myr which includes the formation of the erlangping intra oceanic arc zone onto the ncb before c 490 ma the c 485 ma crustal subduction and uhp metamorphism of the nq terrane the c 430 ma arc continent collision and granulite facies metamorphism the 420 to 400 ma extension and rifting in relation to the opening of the palaeo tethyan ocean the c 310 ma hp eclogite facies metamorphism of oceanic crust and associated continental basement and the final 250 to 220 ma continental subduction and hp uhp metamorphism

systematics of zircon crystallisation in the cretaceous separation point suite new zealand using u pb isotopes ree and ti geothermometry plutonic zircons from the cretaceous separation point suite sps were analysed by la icpms for u pb isotope ratios and trace element concentrations pooled 206pb 238u ages range from 112 to 124 ma cathodoluminescence imaging reveals minor inheritence and textural evidence of repeated dissolution and re precipitation of zircon core and rim spot analyses however document zircon growth during extended periods of time 2 myr protracted crystallisation histories for simple plutonic systems are inconsistent with generalised thermal constraints which predict cooling below the solidus within 1 myr consequently we conclude that the sps granitoids sampled in this study were not emplaced rapidly but incrementally over extended time periods zircon th u and zr hf ratios are positively correlated with crystallisation temperatures consistent with crystallisation from evolving melts however highly variable trace element concentrations along with temperature reversals are indicative of complex crystallisation histories involving continuous fractional crystallisation repeatedly punctuated by hotter more mafic magma recharge normalised abundances of the redox sensitive elements eu and ce in zircon vary systematically with degrees of whole rock differentiation pointing to evolutionary trends in magmatic oxidation states coupled with feldspar crystallisation

extension modes and breakup processes of the southeast china northwest palawan conjugate rifted margins abstract our understanding of continent ocean transition structures and magmatism in the absence of excessive magmatic additions has been guided by the observations and models developed at the magma poor iberia newfoundland conjugate margins recently these models have been challenged in the south china sea in light of new iodp expeditions 367 368 368x we have used an integrated analysis of high quality seismic reflection and gravity anomaly data calibrated against recent deep sea drilling results to investigate margin structure and tectono magmatic interplay during continental breakup and early seafloor spreading between the se china nw palawan conjugate margins the eocene oligocene south china sea rifting initiates in a heterogeneous and likely thermally un equilibrated lithosphere formed by the mesozoic yenshanian orogeny gravity and joint inversion methods confirm lateral variation of basement densities across the conjugate margins lithospheric and basement heterogeneities induced a rifting style characterized by a series of highly thinned rift basins associated with extensional faulting soling out at various crustal levels final rifting in late eocene triggered decompression melting forming mid ocean ridge type magmatism which emplaced within thinned continental crust as deep intrusions and shallow extrusive rocks this initial magmatic activity was concomitant with continued deformation of continental crust by extensional faulting integrated analysis of seismic reflection profiles and gravity anomaly data combined with deep sea boreholes accurately locate the continent ocean boundary we show that the initial igneous crust continentward of oceanic magnetic anomaly c10n is asymmetric in width and in morphology for the conjugate margins the wider and faulted newly accreted domain on the se china side indicates that magmatic accretion is associated with tectonic faulting during the formation of initial oceanic lithosphere we suggest that deformation was not symmetrically distributed between the conjugate margins during the initiation of seafloor spreading but evolved asymmetrically until the stabilisation of the spreading ocean ridge around c10n the analysis of the south china sea breakup reveals a transient interplay between faulting magmatic budget and extension rates during the formation of the continent ocean transition and initial emplacement of igneous crust

circulation at the entrance of the gulf of california from satellite altimeter and hydrographic observations 1 the surface circulation around the entrance to the gulf of california is described from satellite altimetry supported by 10 conductivity temperature depth ctd surveys the sea surface height calculated from the 1 4 world ocean database 2001 climatology plus the archiving validation and interpretation of satellite oceanographic altimeter data october 1992 to january 2008 were rms adjusted to the dynamic height calculated from ctd data the 27 0 kg m 3 isopycnal provided the optimum reference in the mean the surface circulation shows a branch of the california current heading landward toward the gulf of california entrance where it splits into two subbranches these are separated by a cyclonic circulation attached to the coast south of cabo corrientes this feature is produced by sverdrup dynamics and is the first observational indication that the mexican coastal current is generated locally by the wind stress curl as previously suggested by numerical models the global variance of the surface circulation can be separated into seasonal explained variance 35 interannual explained variance 35 and mesoscale explained variance 30 components the seasonal signal which shows the interplay of the poleward mexican coastal current and the equatorward branch of the california current can be explained by a long rossby wave model forced by the annual wind and by radiation from the coast the interannual component is dominated by the el nino southern oscillation which induces in the gulf entrance an anticyclonic cyclonic circulation during el nino la nina this circulation includes a poleward flowing branch during el nino parallel to the pacific coast of the baja california peninsula the mesoscale variability is caused by intense eddy activity

deepwater fold and thrust belt classification tectonics structure and hydrocarbon prospectivity a review abstract deepwater fold and thrust belts dwftbs are classified into near field stress driven type 1 systems confined to the sedimentary section and type 2 systems deformed by either far field stresses alone or mixed near and far field stresses dwftbs can occur at all stages of the wilson cycle up to early stage continent continent collision type 1 systems have either weak shale or salt detachments they occur predominantly on passive margins but can also be found in convergent related areas such as the mediterranean and n borneo examples include the niger and nile deltas the west coast of africa and the gulf of mexico type 2 systems are subdivided on a tectonic setting basis into continent convergence zones and active margin dwftbs continent convergence zones cover dwftbs developed during continent arc or continent continent collision and those in a deepwater intracontinental setting e g w sulawesi makassar straits active margins include accretionary prisms and transform margins the greatest variability in dwftb structural style occurs between salt and shale detachments and not between tectonic settings changes in fold amplitude and wavelength appear to be more related to thickness of the sedimentary section than to dwftb type in comparison with shale salt detachment dwftbs display a lower critical wedge taper more detachment folds long and episodic duration of deformation and more variation in vergence structures unique to salt include canopies and nappes accretionary prisms also standout from other dwftbs due to their relatively long continuous duration rapid offshore propagation of the thrust front and large amount of shortening in terms of petroleum systems many similar issues affect all dwftbs these include the oceanward decrease in heat flow offshore increase in age of mature source rock and causes of trap failure e g leaky oblique and frontal thrust faults breach of top seal by fluid pipes one major difference between type 1 and type 2 systems is reservoir rock high quality continent derived quartz rich sandstones are generally prevalent in type 1 systems more diagenetically reactive minerals derived from igneous and ophiolitic sources are commonly present in type 2 systems or many are simply poor in well developed turbidite sandstone units however some type 2 systems particularly those adjacent to active orogenic belts are partially sourced by high quality continent derived sandstones e g nw borneo s caspian sea columbus basin in some cases very high rates of deposition in accretionary prisms adjacent to orogenic belts coupled with uplift due to collision results in accretionary prism related fold belts that pass laterally from sub aerial to deepwater conditions e g s caspian sea indo burma ranges the six major hydrocarbon producing regions of dwftbs worldwide gulf of mexico niger delta nw borneo brazil west africa s caspian sea stand out as differing from most other dwftbs in certain fundamental ways particularly the very large volume of sediment deposited in the basins and or the great thickness and extent of salt or overpressured shale sdetachments

tectonosedimentary evolution of the deep iberia newfoundland margins evidence for a complex breakup history most of the conceptual ideas concerning sedimentary architecture and tectonic evolution of deep rifted margins are based on either intracontinental rift basins or proximal margins both of which underwent only small amounts of crustal thinning in this paper we investigate the tectonosedimentary and morphotectonic evolution related to continental breakup of the highly extended deep iberia newfoundland margins our results show that continental breakup is a complex process distributed in time and space on the basis of mapping of dated seismic units and borehole data we are able to identify two major phases of extension during a first phase dated as tithonian to barremian 145 128 ma deformation is related to exhumation of mantle rocks basins become younger oceanward and fault geometry changes from upward to downward concave resulting in complex sedimentary structures and basin geometries a second phase dated as latest aptian 112 ma overprints previously exhumed mantle and accreted juvenile oceanic crust over more than 200 km leading to the formation of basement highs the observed complex breakup history challenges classical concepts of rifting and leads to new interpretations for the tectonosedimentary evolution of deep rifted margins

contrasting styles of phanerozoic and precambrian continental collision abstract there are differences in the style of collisional orogens between the phanerozoic and the precambrian most notably the appearance of blueschists and ultrahigh pressure metamorphic uhpm rocks in the geological record since the late neoproterozoic whereas these rocks are absent from older orogens understanding collisional orogenesis in the context of present day values for ambient upper mantle temperature and radiogenic heat production provides a reference from which to extrapolate back to conditions in the precambrian to evaluate differences in the way phanerozoic and precambrian collisional orogens develop a series of experiments was run using a 2 d petrological thermomechanical numerical model in which the collision of spontaneously moving continental plates was simulated for values of ambient upper mantle temperature and radiogenic heat production increasing from those appropriate to the present day thus models of modern collisional orogens involving different modes of exhumation of uhpm rocks were extrapolated back to conditions appropriate for the precambrian based on these experiments an increase of the ambient upper mantle temperature to 80 100 k above the present day value leads to two distinct modes of collision that are different from the modern collision regime and for which the terms truncated hot collision regime strong mafic lower continental crust and two sided hot collision regime weak felsic lower continental crust are proposed some proterozoic orogens record post extension thickening to generate counter clockwise metamorphic p t paths followed by slow close to isobaric retrograde cooling such as occurred in the paleoproterozoic khondalite belt in the north china craton and the late mesoproterozoic early neoproterozoic eastern ghats province part of the eastern ghats belt of peninsular india these orogens have similarities with the truncated hot collision regime in the numerical models assuming subsequent shortening and thickening of the resulting hot lithosphere other proterozoic orogens are characterized by clockwise looping metamorphic p t paths and extensive granite magmatism derived from diverse crustal and subcontinental lithospheric mantle sources these orogens have similarities with the two sided hot collision regime in the numerical models both regimes are associated with shallow slab breakoff that precludes the formation of uhpm rocks the temperature of the ambient upper mantle where this transition in geodynamic regimes occurs corresponds broadly to the neoproterozoic era

mesoscale eddies in the southern gulf of california during summer characteristics and interaction with the wind stress 1 the hydrodynamic characteristics of mesoscale eddies of the southern gulf of california are described from observations made in august 2004 vertical profiles to 1000 m were made with a conductivity temperature depth ctd probe with dissolved oxygen and fluorescence sensors and a lowered acoustic doppler profiler the four sampled eddies were aligned along the gulf axis and had alternating sense of rotation the okubo weiss parameter was used to calculate the eddy core radii and it also revealed that the current field of almost all the sampled area was eddy dominated the mean radii of the eddy cores varied between 32 and 36 km maximum surface velocities were 0 4 0 5 m s the anticyclonic eddies were 500 700 m deep while the cyclones were 450 500 m deep the depression of the isolines associated with the anticyclonic eddies was 100 m at depths between 200 and 400 m however the thermocline pycnocline which spanned from 20 to 75 m was domed by 10 m the cyclonic eddies lifted the isolines by 70 m at depths above 500 m but the isolines were flat above 60 m this suggests that different dynamics operate in the layer above the strong and shallow pycnocline in particular the wind stress curl can affect circulation and pycnocline topography and therefore the patterns that appear in chlorophyll satellite images in our case the eddies were detectable in those images because of the chlorophyll enhanced streamers along their edges and for anticyclones probably because the chlorophyll maximum found at 40 50 m depth was domed following the pycnocline

the age of the potassic alkaline igneous rocks along the ailao shan red river shear zone implications for the onset age of left lateral shearing abstract the potassic alkaline igneous rocks along the ailao shan red river asrr shear zone range in composition from ultramafic to felsic and are characterized by similar trace element patterns and radiogenic isotope ratios their distribution and in some cases their structure and outcrop outline suggest a genetic relationship to the asrr shear zone zircons separated from 12 potassic alkaline bodies along the asrr zone have been dated by an in situ u pb method to temporally constrain the left lateral movements along the shear zone the ages of the alkaline bodies range from 34 0 to 36 3 ma these new zircon ages together with previous data suggest that the onset age of the left lateral movements along the asrr shear zone began at or slightly before 36 ma and that the movements lasted from 36 to 17 ma when uncertainties are taken into account the duration of motion on the asrr shear zone coincides with the opening of the south china sea implying a genetic relationship between these two events

early paleozoic tectonic evolution of the north qinling orogenic belt in central china insights on continental deep subduction and multiphase exhumation abstract the north qinling orogenic belt is an important component of qinling composite orogen in central china high pressure and ultra high pressure hp uhp rocks occur as lenses or layers surrounded by gneissic rocks in the northern central and southern qinling complex of the north qinling belt considered to have formed through deep continental subduction the magmatic protoliths of the hp uhp metamorphic rocks show formation ages ca 800 ma geochemical characteristics and pb nd isotopic compositions similar to those of the neoproterozoic igneous rocks of the south qinling belt continental materials of the south qinling belt were dragged down to mantle depths by the shangdan oceanic crust subducted and subjected to hp uhp metamorphism at ca 500 ma the erlangping and kuanping backarc basin formed in response to subduction of the shangdan ocean and might have developed into a limited small ocean basin at ca 500 ma the hp uhp rocks yielded retrograde metamorphic ages of ca 470 450 ma and ca 420 400 ma these ages are identical to the age of magmatic events in the north qinling hp uhp belt at 500 ma 450 ma and 420 ma related to deep subduction collision slab breakoff and crustal thinning during post collisional extension the dominant ca 500 400 ma ages of detrital zircons from the liuling group of the south qinling belt match well with those from the three stages of magmatic rocks and hp uhp rocks in the qinling complex this correlation suggests that the magmatic rocks and hp uhp metamorphic rocks in the north qinling belt initially exhumed to the surface eroded and were then deposited in the liuling basin in a post orogenic extensional setting during middle to late devonian new evidence suggests that the qinling complex is a tectonic complex rather than a uniform stratigraphic unit or a microcontinent as previously believed and is mainly composed of the exhumed hp uhp metamorphic rocks deep subduction exhumation related magmatic rocks and the early neoproterozoic granites together with the host rocks from the over riding plate at an active continental margin the early paleozoic tectonic history of the nqb includes oceanic slab subduction and formation of arc backarc spreading continental deep subduction arc continent collision break off and multi stage exhumation of the deep subducted slab as well as extension and thinning and associated erosion and deposition

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it

continentward dipping detachment fault system and asymmetric rift structure of the baiyun sag northern south china sea abstract the rift architecture and deep crustal structure of the distal margin at the mid northern margin of the south china sea have been previously investigated by using deep seismic reflection profiles however one fundamental recurring problem in the debate is the extensional fault system and rift structure of the hyperextended rift basins baiyun sag and liwan sag within the distal margin because of the limited amount of seismic data based on new 3d seismic survey data and 2d seismic reflection profiles we observe an array of fault blocks in the baiyun sag which were tilted towards the ocean by extensional faulting the extensional faults consistently dip towards the continent beneath the tilted fault blocks and extensional faults a low angle high amplitude and continuous reflection has been interpreted as the master detachment surface that controls the extension process during rifting the continentward dipping normal faults evolved in a sequence from south to north generating the asymmetric rift structure of the baiyun sag the baiyun sag is separated from the oceanic domain by a series of structural highs that were uplifted by magmatic activity in response to the continental breakup at 33 ma and a ridge jump to the south at 26 24 ma therefore we propose that magmatism played a significant role in the continental extension and final breakup in the south china sea

palaeozoic polymetamorphism in the north qinling orogenic belt central china insights from petrology and in situ titanite and zircon u pb geochronology abstract the qinling orogenic belt experienced multiple phases of orogenesis during the palaeozoic unraveling the timing and p t conditions of these events is the key to understanding the convergence processes between the south china and the north china blocks the songshugou complex located in the southern part of the north qinling orogenic belt has registered multistage metamorphism in palaeozoic and thus potentially provides insights into the tectonic evolution of the qinling orogenic belt in this study three metabasic rocks a garnet pyroxenite a garnet amphibolite and a gneissic amphibolite from the songshugou complex were selected for petrological study and zircon and titanite u pb dating our results show that the metabasic rocks experienced three metamorphic events during the palaeozoic the first metamorphic event m1 is characterized by high pressure conditions two zircon grains in equilibrium with garnet and in absence of plagioclase were recognized from the garnet pyroxenite sample they yielded ti in zircon temperatures of 660 851 c at 12 0 kbar and a weighted mean age of 498 15 ma providing the constraints on the temperature and timing of prograde or peak metamorphism m1 1 zircons that are inequilibrium with garnet from the garnet pyroxenite and the garnet amphibolite gave u pb ages of 494 9 ma and 484 4 ma and ti in zircon temperatures of 793 33 c and 738 18 c respectively thus these zircons were formed on the retrograde amphibolite facies conditions at 8 0 kbar m1 2 titanite inclusions were found in actinolite cores of zoned amphibole from the garnet amphibolite they yielded a u pb age of 470 ma and zr in titanite temperature of 676 23 c at pressure of 7 0 kbar suggesting that the amphibolite facies retrogression perhaps persisted to 470 ma weakly zoned zircons from the garnet amphibolite and inclusion free titanites from the garnet pyroxenite gave consistent u pb ages of 418 5 ma and 423 10 ma and ti in zircon temperature of 742 26 c and zr in titanite temperature of 764 18 c at 7 0 kbar respectively it is suggested that a heating event m2 is registered by a subsequent phase of amphibolite facies metamorphism the ilmenite bearing titanite crystals from the garnet pyroxenite yielded a u pb age of 352 4 ma recording a late thermal event m3 on the basis of combined petrological and geochronological results we propose a revised tectonic model for the north qinling orogeny in palaeozoic the high pressure granulites were formed by the northward subduction of the shangdan oceanic slab and the arc continent collision at ca 500 ma their exhumation happened at ca 494 484 ma as a result of slab breakoff subsequent amphibolite facies metamorphism dated at ca 440 420 ma are coeval with the widespread magmatism in the north qinling terrane which are likely caused by the reinitiation northward subducted of shangdan oceanic slab at ca 350 ma the north qinling terrane was likely affected by another thermal overprinting event

thick turbidite successions from supply dominated shelves during sea level highstand emphasis on the association between relative sea level lowstand and the formation of sandy deep water fans has tended to downplay the significance of high sediment supply and its potential to create deep water fans even during sea level highstands the lance fox hills lewis shelf margin in southern wyoming suggests that high supply was critical in causing the accretion of this moderately wide maastrichtian shelf margin at a minimum rate of 47 8 km m y and the generation of large sand rich fans during every shoreline regression across the shelf it is surprising that fans developed from shelf margin clinoforms that show systematically rising shelf edge trajectories proxy for rising relative sea level as well as from those that show flat trajectories stable to falling relative sea level however the latter producing more sediment bypass resulted in bigger and thicker fans whereas the former produced somewhat smaller and thinner fans we term the former highstand fans and suggest caution in using the lowstand model for high supply systems

the meta gabbroic complex of fushui in north qinling orogen a case of syn subduction mafic magmatism abstract sims zircon geochronology and oxygen isotopes as well as la icpms zircon lu hf isotopic analyses were carried out on a suite of rocks from the fushui complex to probe its origin and tectonic significance for the evolution of the qinling group in the north qinling orogenic belt the fushui complex comprises several phases of meta gabbros the dominant rock type is light colored meta gabbro followed by mica bearing and dark colored meta gabbros with abundant dark gabbroic enclaves with different sizes and shapes and occasional occurrence of mica bearing gabbroic pegmatite dikes age determination shows that the magmatic intrusion began at about 500 ma and surged at 490 480 ma with the latest zircon crystallized at about 476 ma this age spectrum is remarkably coeval with the peak metamorphism 500 480 ma of the surrounding qinling group metamorphic rocks dated by zircons from eclogites retrograded eclogites hp mafic granulites and uhp felsic gneisses representing syn subduction mafic magmatism these late cambrian to early ordovician zircons display a slightly enriched hf isotopic feature with a limited range in ehf t 5 to 2 and yield uniform tdm age 1 15 1 38 ga they also show a relatively small variation in 18o 6 69 0 44 2 8 75 0 34 2 much higher than the normal mantle value the isotopic feature suggests that the fushui complex was originally derived from enriched lithospheric mantle which was metasomatized by melts from ancient continental sediments and altered morb basalts the carboniferous zircons dominantly occurring as rims of early ordovician cores display very similar hf tdm ages 1 19 1 27 ga and oxygen isotopes as their counterparts corresponding to their recrystallization at 335 ma during amphibolite facies metamorphism when exhumed our study on the fushui complex together with the recently reported zircon u pb age data on the surrounding hp uhp metamorphic rocks demonstrates that the qinling group preserves a complete cycle of tectonic evolution in an orogenic belt from an oceanic basin spreading and mini continent formation to deep subduction of mini continent and multiple stage exhumations