comparison of dynamic flow models for the central aleutian and tonga kermadec subduction zones using three dimensional 3 d instantaneous dynamic models of the central aleutian subduction zone we show that a low viscosity low density deltarho 10 kg m3 region in the wedge is needed to match observations of topography the geoid and the stress state in the slab and overriding plate previous models of the tonga kermadec subduction zone also require a low viscosity low density deltarho 20 kg m3 region in the wedge a low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down dip tension to down dip compression if the low viscosity region is large as in the tonga kermadec models then the decoupling may also cause the overriding plate to go into extension these are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

the development of magmatism along the cameroon volcanic line evidence from teleseismic receiver functions 1 the cameroon volcanic line cvl in west africa is a chain of cenozoic volcanism with no clear age progression the reasons for its existence are unclear and the nature of its magmatic plumbing system is poorly understood specifically whether or not the cvl crust presently contains melt and or mafic intrusions as is often observed at hot spots and rifts elsewhere is presently unknown to address this issue we present a receiver function study of crustal structure using earthquakes recorded by the cameroon broadband seismic experiment in regions of the cvl unaffected by cretaceous extension associated with the breakup of gondwana e g the garoua rift vp vs ratios are markedly low network average 1 74 compared to hot spots elsewhere providing no evidence for either melt or cooled mafic crustal intrusions due to cvl magmatism the character of p to s conversions from beneath the cvl also indicates that lower crustal intrusions often termed underplate are not present beneath the region our observations thus corroborate earlier petrological studies that show cvl alkaline magmas fractionate in the mantle not the crust prior to eruption hypotheses for the formation of the cvl should not include markedly elevated upper mantle potential temperatures or large volumes of partial melt both of which can explain observations at hot spots and rifts worldwide the protracted yet sporadic development of small volume alkali melts beneath the cvl may instead be explained better by lower melt volume mechanisms such as shear zone reactivation or lithospheric delamination

composition and tectonic evolution of the chinese continental crust constrained by poisson s ratio article i nfo we have measured both p and s wave velocities vp and vs and poisson s ratios of 60 typical ultrahigh pressure uhp metamorphic rock samples from the chinese continental scientific drilling ccsd main and pre pilot holes and surface outcrops in the sulu dabie orogenic belt at hydrostatic confining pressures up to 850 mpa the experimental results together with those compiled in handbook of seismic properties of minerals rocks and ores ji s c wang q xia b 2002 handbook of seismic properties of minerals rocks and ores polytechnic international press montreal 630 pp reveal that except for monomineralic rocks such as quartzite serpentinite anorthosite limestone and marble the rest of the rock types have poisson s ratios falling along an upward convex curve determined from the correlations between elastic moduli and density poisson s ratios increase with density as the lithology changes from granite felsic gneiss and schist through diorite syenite intermediate gneiss and metasediment to gabbro diabase amphibolite and mafic gneiss and then decrease as the rocks become ultramafic in composition eclogite has a higher density but a lower poisson s ratio than peridotite the results were applied to constrain the crustal composition and tectonic evolution of the chinese continental crust based on crustal thickness h and poisson s ratio from 248 broadband seismic stations measured using teleseismic receiver function techniques the north china yangtze south china and northeast china blocks and songpan ganzi terrane are dominated by low b0 26 and moderate 0 26 b0 28 values n70 suggesting the dominance of felsic composition in the crust the lhasa terrane qiangtang terrane and indochina block are characterized by high proportions 33 42 of measurements with very high values 0 30 and h is found for the south china block northeast block lhasa block qiangtang terrane and indochina block indicating either tectonic thickening of the felsic upper and middle crust by folding and thrusting or the removal of mafic layers from the lower crust into the upper mantle by delamination

possible orogeny parallel lower crustal flow and thickening in the central andes the relatively low elevation and thick crust in the altiplano in comparison to the higher elevation but thinner crust in the puna plateau together with geophysical data suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the altiplano excess density in the altiplano mantle could create differential horizontal stress in the order of 25 mpa between both lithospheric columns numerical models accounting for pressure and temperature dependent rheology show that such stress can induce horizontal ductile flow in the lower crust from the puna towards the altiplano with a minimum viscosity of 1019 pa s this flow reaches 1 cm year displacing more than 50 km of material within 5 ma if the lower crust viscosity is smaller the amount of orogeny parallel lower crustal flow can be even greater such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the altiplano and in the puna because of the strength of the elastic brittle upper crust this channel flow does not necessitate large amounts of surface deformation except vertical uplift making it difficult to detect from the geology

geology geochemistry and tectonic settings of the molybdenum deposits in south china a review abstract south china block scb is the area including the yangtze craton and the huanan orogen where scattered precambrian terranes are usually regarded as segments of cathaysia land it is the third most important molybdenum metallogenic province in china next to the qinling dabie area and northeast china containing 29 mo only or mo dominated 9 w mo sn bi and 8 cu mo deposits these 46 deposits are located mainly in 1 the lower yangtze river belt of the northeastern yangtze craton 2 the northern jiangnan orogenic belt that is generally considered a meso neoproterozoic magmatic arc complex accreted onto the southeastern margin of the yangtze craton 3 the wuyi yunkai orogenic belt characterized by local exposures of proterozoic metamorphic terranes and the more widespread sinian uppermost proterozoic to triassic sedimentary sequences and 4 the southeast coastal volcanic belt characterized by yanshanian andesitic to felsic volcanic rocks their genetic types are dominated by porphyry and skarn mineral systems with only a few quartz vein systems the orebodies form veins lens cylindrical shapes pipes or irregular in shape usually controlled by faults at various scales and volcanic subvolcanic complexes the host rocks are variable in lithologies including granites porphyries volcanic breccias and tuffs clastic sediments and carbonate rocks but the high grade orebodies are usually hosted in carbonate shale sequences hydrothermal mineralization processes can be generally divided into four stages from early to late they are 1 k feldspar quartz veins or veinlets 2 quartz molybdenite stockworks 3 quartz polymetallic sulfide stockworks and 4 quartz carbonate fluorite veinlets fluid rock interaction as exemplified by wallrock alteration evolved from k silicate alteration k feldspar quartz mica through phyllic quartz sericite chlorite epidote to propylitic or argillic alteration with skarn alteration typically occurring in skarn type mineral systems hydrothermal mineral assemblages vary between two end members namely the dry system formed by co2 rich fluids and marked by quartz k feldspar fluorite carbonate and epidote and the wet system mainly originated from co2 poor fluids and composed of biotite sericite and chlorite the ore forming fluids are magmatic in origin and show high temperature and high salinity the melt fluid systems forming cu mo deposits are more oxidizing than those forming the w mo or mo deposits as suggested by accessory minerals in granitoids and daughter minerals in fluid inclusions the cu mo deposits are related to the i type granitic rocks adakite like whereas the w mo and mo only systems are related to granitic rocks of s or a types although all of them show high k contents available isotope ages show that the mo and mo bearing deposits were predominantly formed in the early yanshanian orogeny 170 134 ma followed by the late yanshanian orogeny 110 92 ma and the caledonian orogeny 450 410 ma the caledonian mo mineralization has been observed only in the wuyi yunkai orogenic belt and related to the collision between the yangtze craton and the pre devonian huanan orogen or terranes separated from the cathaysia land linked to the assembly of the gondwana supercontinent the early yanshanian mineralization affected the entire huanan orogen and the eastern yangtze craton and resulted from the syn to post collisional tectonism following the closure of eastern paleo tethys the late yanshanian mo deposits mainly occur in the southeast coastal volcanic belt and the southeastern margin of the wuyi yunkai orogenic belt and are related to the westward subduction of the paleo pacific plate the skarn type mineral systems generally show lower re contents than the porphyry type deposits in a same tectonic unit suggesting that carbonate host rocks have lower re contents than the causative porphyries the re contents in molybdenites from porphyry or porphyry skarn cu mo systems are 50 ppm mainly 100 ppm suggesting a source significantly contributed by the mantle whereas the re contents in molybdenites from the mo only or w mo dominated deposits are

regional tectonic setting of hong kong implications of new gravity models hong kong lies on the se margin of the cathaysia block and straddles the ne trending lianhuashan fault zone one of the dominant structural features of se china isotope signatures of the mesozoic granites have indicated that the zone overlies a major crustal discontinuity the proposed gravity model of hong kong consists of a heterogeneous upper crust underlain by middle to lower crust composed of a felsic archaean segment approximately 25 km wide flanked by more mafic proterozoic crust the southern boundary of the felsic segment dips steeply to the north whereas the northern boundary is subvertical euler gravity anomalies define the fundamental faults in the upper crust and many of these can be correlated with faults that have been mapped at surface the middle to lower crustal discontinuities are considered to be associated with a major shear zone within the cathaysia block that has similar dimensions to transcratonic structures in other precambrian shield areas the geological development of hong kong and neighbouring se china has largely been controlled by periodic reactivation of this deep crustal shear zone since the late neoproterozoic

plate boundary forces at subduction zones and trench arc compression abstract viscoelastic finite element modelling has been used to study the state of stress in the overriding and subducting plates meeting at a subduction zone the subduction fault is included using the dual node technique it is demonstrated that substantial horizontal deviatoric compressive stress occurs in the trench arc region as a result of the downpull of the dense slab and the associated surface depression including the trench and other downflexing of the plates this may be masked at the trench by bending stress it is the lack of significant shearing stress along an unlocked subduction fault in the presence of this compressive stress that gives rise to the slab pull and trench suction plate boundary forces slab pull and trench suction were found to be of comparable magnitude within the range 1 0 to 4 0 10 12 n m in models studied with vertical subduction and there are indications that this may also apply when the slab dips at 45 as a result of viscous flow induced by rollback when the slab dips beneath the arc backarc region it is shown that horizontal deviatoric compression can occur in this region contemporaneous with plate interior tension produced by trench suction this suggests that backarc tension associated with marianas type trench arch systems may be related to the nearly vertical slab whereas backarc compression in the chilean type may result from the low dip and small downpull of the slab it is also shown that successive locking and unlocking of the subduction fault may give rise to large variations of stress in plate interiors

state of stress in slabs as a function of large scale plate kinematics the state of stress of slabs subducting worldwide as revealed by seismicity is extremely variable both with depth and between different subduction zones although in principle slab pull should enhance downdip extension in the slab the reconstructed stress fields for intermediate depths between 100 and 300 km range from downdip compression to downdip extension using 2 d viscoelastic plane strain models we investigate the dependency of the stress field of slabs on geometry dip of the slab and kinematics velocity of convergence between upper and lower plates and their absolute velocity with respect to the underlying mantle of subduction zones we conclude that although the state of stress in slabs is also controlled by other processes downdip compression in the subducting slab is enhanced by mantle flow opposing the direction of the dip of the slab whereas downdip extension is favored by mantle flow in the same direction of the slab dip i e sustaining it these predictions are in agreement with available geophysical observations although exceptions to this simple pattern are observed worldwide in addition if the slab is decoupled from the upper plate convergence between upper and lower plates induces a downdip compressional component of stress within the slab decreasing the magnitude of extension in models characterized by mantle flow sustaining the slab and increasing compression in models with mantle flow opposing subduction however these are second order variations when compared to the control exerted by absolute plate kinematics and by the magnitude of slab pull sensitivity analysis of rheological parameters allows us to conclude that these results are generally consistent although low values of viscosity of the lithospheric mantle render this prediction less stable

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

crustal structure and magmatic evolution in the pearl river delta of the cathaysia block new constraints from receiver function modeling abstract we utilized receiver functions to study the crustal structure beneath the pearl river delta in the southwestern part of the cathaysia block which was characterized by widespread tectono magmatism during the late mesozoic combination of waveform modeling and h k stacking was used to derive crustal thickness vp vs ratio and average p wave velocity the results show that the crustal thickness ranges from 26 km in the center of the pearl river delta to 32 km in the periphery the vp vs ratio is between 1 70 and 1 74 the average p wave velocity ranges from 6 4 to 6 8 km s and shows higher value beneath the islands along the lianhuashan fault zone a weakly positive correlation between the crustal thickness and vp vs ratio is observed in southwestern pearl river delta indicating depth dependent extension of the continental crust that is ductile lower crust was stretched to a much higher degree in comparison with the brittle felsic upper crust the crust in the northeastern pearl river delta may be more basaltic owing to underplating caused by the subduction of the paleo pacific plate and thus a relatively high vp vs ratio is observed there the overall low crustal vp vs ratio and the relatively higher crustal p wave velocities beneath the lianhuashan fault zone may imply a felsic intermediate composition and high metamorphic grade of the crust along the fault the lianhuashan fault zone may provide a pathway for rapid magmatic transportation from the underplating beneath the crust to the surface

crustal thickness variations in the aegean region and implications for the extension of continental crust 1 we installed 5 broadband and 45 short period temporary seismic stations distributed partly as a dense 100 km long n s linear array and partly as a regional network throughout the menderes massif of western turkey in order to study crust upper mantle structure and seismicity in this study we have combined teleseismic waveform data from these stations with data from several permanent seismic stations to determine crustal thickness variations in the aegean region receiver function studies at seven broadband stations using the h stacking method have yielded crustal thicknesses and vp vs ratios over a broad region of the aegean a more detailed crustal image was obtained in the central menderes massif where we applied common conversion point stacking to receiver functions obtained from the n s linear array the results show a general trend of westward crustal thinning from 36 km in central anatolia to 28 30 km in the central menderes massif to 25 km beneath the aegean sea the results also indicate that crustal thinning in the aegean is not uniform in the n s extensional direction the crust is thinner in the central menderes massif 28 30 km of crustal thicknesses and the cycladic massif 25 26 km than in surrounding regions where crustal thicknesses are 32 34 km the long lived elevated moho under the metamorphic core complexes suggests that the lower crust in the aegean region is at least 3 times more viscous than that in the basin and range province where the moho is much flatter

ductile extensional shear zones in the lower crust of a passive margin we describe and interpret an unpublished set of ion geophysical seismic reflection profile showing strong organized seismic reflectors at the base of the continental crust of the uruguayan volcanic rifted margin we distinguish two main groups of reflectors in the lowermost continental crust a first group at depths ranging from 32 km below the continent to 16 km in the continent ocean transition comprises reflectors continuous over tens of kilometers peculiarly visible near the mantle crust boundary a second group of reflectors dipping toward the ese oceanward is widely distributed in the lower crust these reflectors are slightly curved and tend to merge and become sub parallel with the first group of reflectors together they draw the pattern of thick shallow dipping top to the continent shear zones affecting the lower continental crust such sense of shear is also consistent with the continentward dip of the normal faults that control the deposition of the thick syn tectonic volcanic formations sdr a major portion of the continental crust behaved in a ductile manner and recorded a component of top to the continent penetrative simple shear during rifting indicative of a lateral movement between the upper crust and the mantle

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

volcanic plutonic connections in a tilted nested caldera complex in hong kong 1 exceptional exposures of four precisely dated middle jurassic to early cretaceous silicic volcanic centers and their plutonic equivalents in hong kong have provided an excellent opportunity to examine close connections in time and space between magma chambers and their overlying calderas here we describe a 14 km crustal section through a collapsed caldera in southeastern hong kong where the intracaldera fill suggests that the magmatic discharge was of supereruption scale the main subvolcanic components that link a magma chamber with surface are revealed by well established field relationships supplemented by high precision geochronology whole rock geochemistry and geophysical data exposures and outcrop patterns reveal kilometer scale caldera subsidence and evidence of the simultaneous evacuation of hundreds of cubic kilometers of high silica rhyolite magma through dike like conduits from a shallow subcrustal reservoir the resultant volcanotectonic depression within which is preserved a single cooling unit of massively columnar jointed densely welded tuff high island tuff is interpreted to form part of a larger tilted early cretaceous nested caldera complex the high island eruption signaled the end of a 24 myr period of voluminous pulsed middle jurassic to early cretaceous silicic magmatism in the hong kong region characterized by four discrete ignimbrite flare ups

phosphorus phosphorites and skeletal evolution at the precambrian cambrian boundary clustering of sedimentary phosphate deposits around the precambrian cambrian boundary is related to a global phosphogenic event this event probably resulted from a complex interplay of factors and had a profound effect on the biota with high concentrations of phosphorus in the photic zone producing an increase in the total shallow marine biomass faunal diversification the precipitation of skeletal structures and explosive evolution

patterns of deep seismicity reflect buoyancy stresses due to phase transitions thermal perturbation of mantle phase relations in subduction zones gives rise to significant buoyancy anomalies finite element modeling of stresses arising from these anomalies reveals transition from principal tension to compression near 400 km depth down dip compression over 400 690 km peaking at 550 km and transition to rapidly fading tension below 690 km such features even when complicated by olivine metastability are consistent with observed patterns of deep seismicity that such a simple model neglecting all effects other than buoyancy anomalies due to temperature and to thermal perturbation of olivine phase relations successfully generates so many observed features of deep seismicity suggests that these buoyancy anomalies are significant contributors to the stress field in subducting slabs it also suggests that the depth distribution of deep seismicity may largely reflect the state of stress in the slab rather than simply a particular mechanism of stress release

nd and sr isotope geochemistry of plutonic rocks from hong kong implications for granite petrogenesis regional structure and crustal evolution abstract combined nd and sr isotope data allow the plutonic rocks of hong kong to be grouped into three ne trending domains those of zone 1 in the northwestern part of the territory have nd t 87 sr 86 sr 0 0 71 the plutonic rocks in the central zone 2 exhibit a broad spectrum of isotope characteristics with nd t values of 4 2 to 7 0 tdm between 1 30 and 1 51 ga and 87 sr 86 sr 0 ratios in the range 0 7061 0 7102 granites of zone 3 in the southeastern part of the territory display relatively homogeneous isotope signatures with nd t between 5 5 and 6 5 and 87 sr 86 sr 0 ratios in the range 0 7067 0 7109 tdm model ages are between 1 39 and 1 47 ga the contrasting isotope signatures between zone 1 granites and those in zones 2 and 3 suggest there are inherent differences in the source regions and point to the existence of a basement terrane boundary in this part of southeast china the presence of a deep crustal discontinuity transecting hong kong would also account for the distribution and compositional variation of the granites and the dominant northeast structural control on dyke emplacement the variation in isotope signatures and tdm model ages observed in the hong kong rocks are consistent with a model involving the progressive interaction of mantle derived melts with two distinct crustal sources during a transition from compressional to extensional tectonics the boundary between these two crustal sources is marked by zone 2 in which isotope characteristics are more variable and a stronger mantle influence is preserved

tectonics of south china key to understanding west pacific geology abstract south china is not a post caledonian platform but a composite of orogenic belts this late proterozoic mesozoic orogen is a collage of three continental fragments the three blocks are yangzi huanan and dongnanya and they are separated by the banxi nanpanjiang formerly xianggangzhe and gunanhai suture zones yangzi was separated from gondwanaland during late precambrian when an open ocean called banxi was present between the two continents tectonic processes at an active margin during sinian and early paleozoic time led to the genesis of an accretionary wedge complex the banxi melange and the huanan flysch nappes on the northern margin of huanan which was then the northern margin of the gondwana continent reorganization of plates during the devonian suspended subduction at the huanan active margin huanan was uplifted and unconformably overlain by transgressive deposits of devonian and or carboniferous age a remnant ocean the nanpanjiang sea still existed between yangzi and huanan and deep water sedimentation continued both at the southern margin of yangzi and at the northwestern margin of huanan the latter again became an active margin during late paleozoic when permian and triassic flysch sediments were deposited in foredeeps ahead of advancing nappes huanan and yangzi collided during the triassic and resulted in the deformation of the passive margin of the latter to form the yangzi deformed belt huanan was separated from a more southerly continent dongnanya in the devonian by seafloor spreading which created a late paleozoic and early mesozoic ocean gunanhai ocean a continuously deposited sequence ranging in age from devonian to triassic was laid down on the southern passive margin of huanan on the other side of the ocean the dongnanya permian strata include glacial marine deposits of gondwanaland affinity dongnanya became separated from gondwanaland during the late permian when it marched northward to be reunited with huanan the huanan passive margin sequence was deformed by folding and overthrusting after the late mesozoic collision of huanan and dongnanya scattered outcrops of the suture melange resulting from this collision occur at a few localities in coastal fujian dongnanya in our model was the mesozoic continent of southeast asia peripheral to mainland asia this may or may not have been the eastern continuation of the microcontinent sibumasu siam thailand burma malaysia sumatra the collision of dongnanya and asia gave rise not only to the gunanhai melange of coastal fujian but also to the ophiolite melanges parts of which are present in taiwan tailuko the philippines and western malaysia

constraining velocity and density contrasts across the crust mantle boundary with receiver function amplitudes summary this paper investigates the ability of p wave receiver functions to constrain both the velocity and density contrasts across the moho expressions for receiver function amplitudes corresponding to a layer over a half space are extended to media with depth dependent properties by explicitly accounting for free surface reflection coefficients forward numerical computations show that receiver function amplitudes become frequency dependent for depth dependent structures and that for a given frequency wavelengths associated to ps converted phases are larger than wavelengths associated to multiply reverberated phases when consistent measurements of the amplitudes of ps phase and multiples can be obtained the extended expressions suffice to account for the observed amplitudes receiver function amplitudes are sensitive to both velocity and density contrasts across the moho and a two step grid search procedure is proposed to recover the contrasts from their measurement the near surface velocity is recovered in the first step from the amplitude of the direct p wave and then used in the second step to work out the relative density and s wave velocity contrast from the amplitude of the secondary phases examples from central spain pab and the indian shield hyb are discussed and demonstrate that receiver function amplitudes can constrain contrasts across the moho accurately enough to be utilized in geological interpretation

crustal structure in an onshore offshore transitional zone near hong kong northern south china sea to study the crustal structure beneath the onshore offshore transitional zone a wide angle onshore offshore seismic experiment was carried out in northern south china sea near hong kong using large volume airgun sources at sea and seismic stations on land the crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the dangan islands based on the characteristics of pg and multiple pg and the crustal structure beneath the sedimentary layer is relatively simple the moho depth is about 25 28 km along the profile and the p wave velocity increases gradually with depth the velocities in the upper crust range from 5 5 to 6 4 km s while that in the lower crust is 6 4 6 9 km s it also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75 90 km distance which suggests that the littoral fault zone lfz exists beneath the onshore offshore transitional zone the magnetism anomalies bouguer gravity anomalies and active seismic zone along the coastline imply the lfz is a main tectonic fault in the onshore offshore area combined with two previously published profiles in the continental south china l g profile and in the northern margin of south china sea obs1993 respectively we constructed a land sea super cross section about 1000 km long the results show the onshore offshore transitional zone is a border separating the unstretched and the stretched continental crust the low velocity layer lvl in the middle crust was imaged along l g profile however the high velocity layer hvl in the lower crust was detected along obs 1993 by analyzing the mechanisms of the lvl in the middle crust and hvl in the base of crust we believe the crustal structures had distinctly different attributes in the continental south china and in the northern scs which indicates that the lfz could be the boundary fault between them c 2009 elsevier ltd all rights reserved

anisotropy beneath a highly extended continental rift we have employed shear wave splitting techniques to image anisotropy beneath the d entrecasteaux islands in southeastern papua new guinea our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid ocean ridge tip we image the transition from continental to oceanic extension a dense shear wave splitting data set from a 2010 to 2011 passive source seismic deployment is analyzed using single and multichannel methods splitting delay times of 1 1 5 s are observed and fast axes of anisotropy trending n s parallel to rifting direction predominate the results this trend is linked to lattice preferred orientation of olivine primarily in the shallow convecting mantle driven by up to 200 km of n s continental extension ahead of the westward propagating woodlark rift this pattern differs from several other continental rifts that evince rift strike parallel fast axes and is evident despite the complex recent tectonic history we contend that across most of this rift the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid ocean ridge like mantle fabric stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region

new and revised set of cretaceous paleomagnetic poles from hong kong implications for the development of southeast china a paleomagnetic study of upper cretaceous 80 ma dikes from hong kong was conducted with the aim of better understanding the late cretaceous cenozoic kinematic history of southeast china twelve out of seventeen sites collected from four dikes revealed primary trms and yield a late cretaceous paleopole at 69 38n 211 28e where a95 8 98 and k 107 a synthesis of south china block cretaceous poles indicates that coastal southeast china which is bounded by the lianhuashan fault zone to the northwest and the changlenanao fault zone to the southeast has moved relative to inland central south china the discordant cretaceous paleopoles from the south china block suggest a moderate net clockwise rotation of the coastal southeast china relative to central south china since the late cretaceous this clockwise rotation is probably the result of micro blocks along the south china margin experiencing vertical axis rotations in response to the penetrative dextral shear along the se china margin during oblique subduction of the pacific plate beneath south china an alternative interpretation involves regional extension that displaced a raft block southeastwards of the margin generating apparent paleomagnetic records with no real block rotations q 2004 elsevier ltd all rights reserved