seismic reflection imaging of the juan de fuca plate from ridge to trench new constraints on the distribution of faulting and evolution of the crust prior to subduction we present prestack time migrated multichannel seismic images along two cross plate transects from the juan de fuca jdf ridge to the cascadia deformation front df offshore oregon and washington from which we characterize crustal structure distribution and extent of faults across the plate interior as the crust ages and near the df in response to subduction bending within the plate interior we observe numerous small offset faults in the sediment section beginning 50 70 km from the ridge axis with sparse fault plane reflections confined to the upper crust plate bending due to sediment loading and subduction initiates at 120 150 km and 65 80 km seaward of the df respectively and is accompanied by increase in sediment fault offsets and enhancement of deeper fault plane reflectivity most bend faulting deformation occurs within 40 km from the df on the oregon transect bright fault plane reflections that extend through the crust and 6 7 km into the mantle are observed if attributed to serpentinization 0 12 0 92 wt water within the uppermost 6 km of the mantle is estimated on the washington transect bending faults are confined to the sediment section and upper middle crust the regional difference in subduction bend faulting and potential hydration of the jdf plate is inconsistent with the spatial distribution of intermediate depth intraslab seismicity at cascadia a series of distinctive ridgeward dipping 20 40 lower crustal reflections are imaged in 6 8 ma crust along both transects and are interpreted as ductile shear zones formed within the ridge s accretionary zone in response to temporal variations in mantle upwelling possibly associated with previously recognized plate reorganizations at 8 5 ma and 5 9 ma

deep seismic reflection images of the wharton basin oceanic crust and uppermost mantle offshore northern sumatra relation with active and past deformation 1 we present deep seismic reflection images along two profiles collected in 2006 in the wharton basin offshore northern sumatra the main profile is located subparallel to the sumatran trench at a distance of 32 66 km faulting of the entire sedimentary section strike slip deformation sometimes accompanied by a dip slip component is imaged over two fracture zones of the extinct wharton spreading center that prior studies have shown to be reactivated as left lateral faults the western fracture zone is associated with a wide region of strong basement topography a difference in crustal thickness of 1 5 km and an age offset of 9 ma the epicenters of the 11 april 2012 mw 8 6 great strike slip earthquake its mw 7 2 foreshock and mw 8 2 aftershock align along this major structure 100 km south of the profile intersection our high quality long offset seismic reflection data also reveal bright dipping reflections extending down to a maximum of 24 km into the oceanic mantle 37 km below sea level apparent dips are mostly 25 35 corresponding to 30 55 along either n s to nne ssw or e w to wnw ese directions which encompass the directions of plate fabric and nodal planes of the mw 8 6 event we suggest that these enigmatic reflections arise from presently inactive dip slip fault planes reaching for the deepest ones to the base of the brittle layer possible origins include extension related to plate bending or an episode of now inactive thrust type deformation reactivating paleonormal faults similar to that taking place in the central indian basin

the crustal domains of the alboran basin western mediterranean the alboran basin in the westernmost mediterranean hosts the orogenic boundary between the iberian and african plates although numerous geophysical studies of crustal structure onshore iberia have been carried out during the last decade the crustal structure of the alboran basin has comparatively been poorly studied we analyze crustal scale images of a grid of new and reprocessed multichannel seismic profiles showing the tectonic structure and variations in the reflective character of the crust of the basin the nature of the distinct domains has been ground truthed using available basement samples from drilling and dredging our results reveal four different crustal types domains of the alboran basin a a thin continental crust underneath the west alboran and malaga basins which transitions to b a magmatic arc crust in the central part of the alboran sea and the east alboran basin c the north african continental crust containing the pytheas and habibas sub basins and d the oceanic crust in the transition towards the algero balearic basin the alboran basin crust is configured in a fore arc basin west alboran and malaga basins a magmatic arc central and east alboran and a back arc system in the easternmost part of the east alboran basin and mainly algero balearic basin the north african continental crust is influenced by arc related magmatism along its edge and was probably affected by strike slip tectonics during westward migration of the miocene subduction system the distribution of active tectonic structures in the current compressional setting generally corresponds to boundaries between domains possibly representing inherited lithospheric scale weak structures

decomposing of mixed pattern of arsenic using fractal model in gangdese belt tibet china abstract decomposing mixed geochemical patterns is a challenge in geochemical exploration and environmental assessment in this paper the spectrum area technique s a is used to decompose a mixed pattern of arsenic in gangdese belt based on stream sediment data s a is a multifractal model based on power law relationships between area of the set consisting of wave numbers with spectral energy density above s a s on the 2d frequency domain the original spatial distribution map of arsenic obtained by inverse distance weighted idw shows a mixed pattern due to superposition of different geological processes or events and is converted into the frequency domain by means of fourier transformation two components including power spectrum density and phases are obtained the spectrum energy density s and the area a enclosed by the above threshold spectrum energy density is plotted on a log log scale two cutoff values determined by three straight lines define three filters which decompose the original map of arsenic into background anomalous and high frequency noise components the areas with high anomaly of arsenic mainly are located surrounding known cu deposits indicating that arsenic anomalies may be related to cu mineralization

birth of a volcanic margin off argentina south atlantic 1 based on a dense grid of multichannel reflection seismic lines we study the evolution of a volcanic rifted margin segment off argentina the segment under study is located between 44 s and 41 s we describe in detail the facies of the extrusive basaltic complexes across the margin these include single and multiple inner seaward dipping reflectors sdrs with varying architecture along the margin outer highs and outer sdr wedges a detailed interpretation of a transitional zone between the inner and outer sdr wedges is presented multiple inner sdrs are concentrated at the southern part of the margin segment while in the north only one steeply dipping wedge is present the dip of the inner sdr wedges changes along the margin and we propose subsidence by loading as main cause the transitional zone between the inner and outer sdr wedges becomes narrower toward the north by correlating with magnetic data we conclude that the emplacement of the 30 100 km wide multiple inner sdrs occurred episodically we propose an injection center which migrated in a seaward direction with proceeding extension as the origin for the multiple inner sdrs a scissor like opening of the margin segment resulted in different amounts of stretching along strike of the margin segment and is likely the cause for the varying distribution of the extrusive basaltic complexes the varying extension rates within the margin segment contribute to decreasing volumes of melts generated in a northward direction within this rift segment

zircon u pb geochronology and hf isotopes of major lithologies from the jiaodong terrane implications for the crustal evolution of the eastern block of the north china craton abstract as an important component of the western shandong complex in the eastern block of the north china craton the yishui terrane consists of neoarchean high grade supracrustal rocks granitoid gneisses and charnockites la icp ms zircon u pb dating and hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the neoarchean crustal evolution of the eastern block of the north china craton new zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2 54 2 53 ga and 2 57 2 55 ga respectively and the charnockites were emplaced contemporaneously around 2 56 2 53 ga single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at 2 50 ga suggesting that the yishui terrane experienced a high grade metamorphic event at the end of the neoarchean detrital zircons from pelitic gneisses yield 207pb 206pb ages of 2 89 2 65 ga with a major age peak at 2 53 ga and a subordinate age peak at 2 70 ga implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from 2 53 ga rocks in the yishui terrane and 2 70 ga rocks in the adjacent luxi granite greenstone terrane hf isotopic compositions show that the neoarchean magmatic zircons have positive ehf t values ranging from 1 4 to 7 8 and depleted mantle model ages of 2 92 2 60 ga with a prominent peak at 2 8 2 7 ga suggesting that the neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the yishui area combined with previous data from the adjacent luxi granite greenstone terrane and other complexes in the eastern block it is concluded that the neoarchean was an important period of crust accretion in the eastern block of the north china craton

structures within the oceanic crust of the central south china sea basin and their implications for oceanic accretionary processes internal structures in mature oceanic crust can elucidate understanding of the processes and mechanism of crustal accretion in this study we present two multi channel seismic mcs transects across the northern flank of the south china sea basin to reveal the internal structures related to cenozoic tectono magmatic processes during seafloor spreading bright reflectors within the oceanic crust including the moho upper crustal reflectors and lower crustal reflectors are clearly imaged in these two transects the moho reflection displays varied character in continuity shape and amplitude from the continental slope area to the abyssal basin and becomes absent in the central part of the basin where abundant seamounts and seamount chains formed after the cessation of seafloor spreading dipping reflectors are distinct in most parts of the mcs data but generally confined to the lower crust above the moho reflection these lower crustal reflectors merge downward into the moho without offsetting it probably arising from shear zones between the crust and mantle characterized by interstitial melt although we cannot exclude other possibilities such as brittle faulting or magmatic layering in the local area a notable feature of these lower crustal reflector events is their opposite inclinations we suggest the two groups of conjugate lower crustal reflector events observed between magnetic anomalies c11 and c8 were associated with two unusual accretionary processes arising from plate reorganizations with southward ridge jumps c 2018 the author s published by elsevier b v

the building of an archean microcontinent evidence from the north china craton abstract continents on the early earth are considered to have been built through the accretion of microterranes oceanic arcs and plateaus the north china craton ncc envisaged in recent models as a collage of several microblocks which were amalgamated along multiple zones of ocean closure during the late neoarchean provide a typical case to investigate the origin and amalgamation of microcontinents through convergent margin processes here we report a suite of magmatic rocks developed at the periphery of one of these microblocks the jiaoliao block that forms part of the composite eastern block of the ncc we integrate our new data with those from various parts of this microblock to elucidate the mechanism of continent building in the archean we present petrological geochemical and zircon u pb geochronological and lu hf isotopic data from the magmatic suite that belongs to the yishui complex geochemically the felsic units of the suite straddle from monzonite through granodiorite to granite with dominantly metaluminous affinity magnesian composition and arc related features the metagranites ttg gneisses and charnockites are characterized by negative nb ta anomalies and positive k and pb anomalies the diorites and gabbros display negative anomalies of th u nb ta and zr hf and positive anomalies at ba pb and sm with negative eu anomalies and minor positive ce anomalies attesting to arc related features in the tectonic discrimination diagrams the rocks plot in the volcanic arc field indicating arc related origin in subduction setting zircon grains from all the rocks display core rim texture with the cores showing magmatic crystallization and the narrow structureless rims corresponding to metamorphic overgrowth the magmatic zircons from the metagranites show upper intercept ages or 207 pb 206 pb weighted mean ages of 2505 29 ma and 2569 20 ma to 2513 27 ma those from the ttg gneisses show 2535 17 ma to 2546 39 ma from charnockites display 2543 20 ma 2555 15 ma and diorite and gabbro show 2587 15 ma and 2516 13 ma respectively the zircon rim ages of 2472 23 ma 2457 35 ma 2545 30 ma and 2511 35 ma suggest the timing of metamorphism ca 2 55 2 45 ga magmatic zircons with slightly older ages of ca 2 73 ga 2 64 ga also occur suggesting multiple magmatic pulses the lu hf isotopic data show positive ehf t values ranging from 0 2 to 5 7 for metagranites with hf model ages of 2602 2815 ma t dm and 2658 3002 ma t dm c whereas for ttg gneisses the positive ehf t values are up to 6 5 and display dominant mesoarchean hf model ages with limited early neoarchean hf model ages charnockite samples show positive ehf t values 2 3 5 7 and display the hf model ages ranging from 2601 ma to 2772 ma t dm and 2658 ma to 2904 ma t dm c diorite and gabbro also show positive ehf t 2 3 6 9 and yield hf model ages of 2625 2788 ma t dm and 2647 ma to 2903 ma t dm c the hf isotopic data indicate that the magmas were derived from neoarchean mesoarchean juvenile sources integrating our data with those from the entire jiaoliao microblock reveals vestiges of hadean crust involved in building the eoarchean nucleus of this microblock vigorous convergent margin processes ranging from mesoarchean to late neoarchean with multiple pules of arc magmatism associated with subduction tectonics led to further growth of continental crust culminating in paired high temperature and high pressure metamorphism during late neoarchean early paleoproterozoic transition

seismic attenuation in upper ocean crust at hole 504b seismic attenuation and its relationship to borehole stratigraphy in the upper 1 8 km of ocean basement at hole 504b are determined from analysis of vertical seismic profile vsp data vsp data provide unambiguous measurements of seismic amplitude decay along a vertical propagation path through the crust and ancillary borehole measurements enable detailed modeling of the relative contributions from geometrical spreading scattering and intrinsic loss mechanisms to this decay about 60 of the total observed amplitude decay occurs in the pillow basalt section and is due mostly to geometrical spreading and scattering from impedance contrasts the remaining amplitude decrease is concentrated in two layers at 500 650 and 800 900 meters below seafloor mbsf 225 375 and 525 625 m below basement across which amplitude rapidly decays and the frequency characteristics of the downgoing wave field are significantly and permanently modified attenuation in these layers is not due to scattering but rather to an intrinsic mechanism that can be characterized by q of 10 and 8 respectively it is likely that the q structure of both of these intervals is formed with the crust near the ridge and thus related to fundamental ocean crust forming processes the shallow interval coincides with a change in alteration mineralogy deposited by late stage fluid flow and may separate lower lavas that were emplaced within the rift zone from upper lavas that were emplaced by off axis flow through large lava tubes intrinsic attenuation in the deeper horizon is probably due to an increase in porosity and cracking associated with either intracrustal deformation or subhorizontal faulting there is negligible attenuation of seismic frequencies in the dikes below 1000 mbsf 725 m subbasement

compressional tectonic inversion of the algero balearic basin latemost miocene to present oblique convergence at the palomares margin western mediterranean interpretation of new multichannel seismic reflection profiles indicates that the palomares margin was formed by crustal scale extension and coeval magmatic accretion during middle to late miocene opening of the algero balearic basin the margin formed at the transition between thinned continental crust intruded by arc volcanism and back arc oceanic crust deformation produced during the later positive inversion of the margin offshore and onshore is partitioned between n50 e striking reverse faults and associated folds like the sierra cabrera and abubacer anticlines and n10 20 e sinistral strike slip faults like palomares and terreros faults parametric subbottom profiles and multibeam bathymetry offshore structural analysis available gps geodetic displacement data and earthquake focal mechanisms jointly indicate that tectonic inversion of the palomares margin is currently active the palomares margin shows a structural pattern comparable to the north maghrebian margins where africa eurasia plate convergence is accommodated by ne sw reverse faults nnw sse sinistral faults and wnw ese dextral ones contractive structures at this margin contribute to the general inversion of the western mediterranean since 7 ma coeval to inversion at the algerian margin shortening at the alboran ridge and al idrisi faults occurred later since 5 ma indicating a westward propagation of the compressional inversion of the western mediterranean

the 3 d geometry of detachment faulting at mid ocean ridges seismic images of cretaceous slow spreading crust from the eastern central atlantic provide new constraints on the process of seafloor spreading and the importance of detachment faulting the seismic depth sections image detachment faults that appear to have exhumed footwall massifs of similar geometry to massifs of plutonic and mantle rocks mapped at the present mid atlantic ridge the detachments are consistent with the structure and microearthquakes of the mid atlantic ridge at 26 n with the footwall rotation inferred from paleomagnetic data and with numerical modeling of oceanic detachments other seismically imaged detachments have similar dimensions and geometry but are covered by a layer of small fault blocks the detachment types differ in whether or not the fault locks up in the subsurface probably controlled by the fault strength the elastic thickness and whether the exhumed footwall is partly covered by basalts toward the segment middle decreasing mantle serpentinization decreasing elastic thickness and thicker median valley basalts all increase the likelihood that the fault locks up and a new fault propagates upwards from the still active root zone transferring a slice of the hanging wall to the footwall to be rafted with the footwall out of the median valley as a result an oceanic detachment fault exhuming the footwall at a segment end to form an oceanic core complex may disappear laterally beneath rafted blocks detachment faulting may be more widespread at slow spreading ridges than interpreted from seafloor mapping

geophysical evidence for hydration of the crust and mantle of the nazca plate during bending at the north chile trench water transported in subducting oceanic plates plays a key role in a number of phenomena including intraslab seismicity and arc magmatism however the locus of plate hydration and water distribution in crust and mantle of plates entering subduction zones is debated we present evidence for anomalously low seismic velocities and densities of the crust and upper mantle of the nazca plate at the north chile trench crustal seismic velocities at the trench are lower than velocities of mature fast spreading crust and even lower than velocities of highly extended slow spreading crust in addition the nazca plate at the north chile trench may contain an 20 km thick upper mantle layer with 17 serpentine which implies 2 5 wt water these results document pervasive rock alteration by water percolation linked to bending related extensional faulting

subsurface structure of the petit spot volcanoes on the northwestern pacific plate 1 a seismic reflection survey was conducted in the northwestern pacific to investigate subsurface structure of small volcanoes considered to be formed by the newly discovered petit spot intra plate volcanism in contrast with an acoustically transparent sedimentary layer in the ambient northwestern pacific sedimentary layers are acoustically opaque with interbedded strong reflections beneath the volcanoes these reflections are possibly caused by inhomogeneous structure due to intrusion of central vent subhorizontal strong reflections at base of the sedimentary layer identified in the vicinity of volcanoes are probably reflections from sills of dense lavas flowing out within the sedimentary layer these volcanoes could be monogenetic volcanoes produced by small amount of magma intrusion 1 km3 in volume although such small volcanoes have never been discovered unless through a high resolution bathymetric mapping the invisible intra plate volcanism affects successive evolution modification of the crustal architecture of old age oceanic plates

origin of dipping structures in fast spreading oceanic lower crust offshore alaska imaged by multichannel seismic data abstract multi channel seismic mcs reflection profiles across the pacific plate south of the alaska peninsula reveal the internal structure of mature oceanic crust 48 56 ma formed at fast to intermediate spreading rates during and after a major plate re organization oceanic crust formed at fast spreading rates half spreading rate 74 mm yr has smoother basement topography thinner sediment cover with less faulting and an igneous section that is at least 1 km thicker than crust formed at intermediate spreading rates half spreading rate 28 34 mm yr mcs data across fast spreading oceanic crust formed during plate re organization contain abundant bright reflections mostly confined to the lower crust above a highly reflective moho transition zone which has a reflection coefficient rc of 0 1 the lower crustal events dip predominantly toward the paleo ridge axis at 10 30 reflections are also imaged in the uppermost mantle which primarily dip away from the ridge at 10 25 the opposite direction to those observed in the lower crust dipping events in both the lower crust and upper mantle are absent on profiles acquired across the oceanic crust formed at intermediate spreading rates emplaced after plate re organization where a moho reflection is weak or absent our preferred interpretation is that the imaged lower crustal dipping reflections within the fast spread crust arise from shear zones that form near the spreading center in the region characterized by interstitial melt the abundance and reflection amplitude strength of these events rc 0 15 can be explained by a combination of solidified melt that was segregated within the shear structures mylonitization of the shear zones and crystal alignment all of which can result in anisotropy and constructive signal interference formation of shear zones with this geometry requires differential motion between the crust and upper mantle where the upper mantle moves away from the ridge faster than the crust active asthenospheric upwelling is one possible explanation for these conditions the other possible interpretation is that lower crustal reflections are caused by magmatic mafic ultramafic layering associated with accretion from a central mid crustal magma chamber considering that the lower crustal dipping events have only been imaged in regions that have experienced plate re organizations associated with ridge jumps or rift propagation we speculate that locally enhanced mantle flow associated with these settings may lead to differential motion between the crust and the uppermost mantle and therefore to shearing in the ductile lower crust or alternatively that plate reorganization could produce magmatic pulses which may lead to mafic ultramafic banding

seismological evidence of mantle flow driving plate motions at a palaeo spreading centre at oceanic spreading centres it is unclear whether plate motions drag the underlying mantle or mantle flow pulls the overlying plates seismic imaging of a former speading centre in the pacific plate reveals strong seismic anisotropy that was generated by mantle flowing at speeds greater than plate motions implying that mantle flow pulled this part of the plate

gold and pathfinder elements in ferricrete gold deposits of the yilgarn craton of western australia a review with new concepts abstract secondary mineral deposits have played an important role in the global mineral resource economy for over 50 years with lateritic au al fe and ni deposits having a significant input to global metal production and reserves in the yilgarn craton of western australia a deeply weathered mantle is commonly capped with 2 10 m of lateritic residuum residual lateritic gravels and duricrust and or ferricrete fe oxide cemented sediment which formed under seasonally humid tropical and sub tropical climates during the cenozoic the principal constituents of these units are goethite hematite maghemite kaolinite and quartz they are commonly overlain by younger 2 10 m thick transported cover deposited under later semi arid conditions both ferricrete and lateritic residuum may host exploitable secondary gold deposits typically small

tectonic evolution of the jurassic cretaceous great valley forearc california implications for the franciscan thrust wedge hypothesis interpretation of seismic reflection data and restoration of depositional geometries of cretaceous forearc basin strata in the northwest great valley of california provide important controls on structural reconstructions of the western margin of the sacramento valley and northern coast ranges monoclinal eastward dips of great valley group strata and fault systems striking northwest southeast which are features proposed as evidence for a west dipping blind great valley franciscan sole thrust and related backthrusts instead are expressions of bedding geometry that resulted from folding of the paskenta and related synsedimentary normal faults depositional onlap and a major structural stratigraphic discontinuity the discontinuity separates east dipping aptian and younger great valley group strata from beds of lower great valley group and coast range ophiolite that were deformed and erosionally or structurally truncated by mid cretaceous time dip divergence imaged between the supracrop and subcrop of the discontinuity is not unique to the ancient great valley forearc but is also observed in modern forearc basins advocates of the franciscan thrust wedge model have also proposed that west dipping shingled patterns of seismic events imaged beneath the sacramento valley are imbricate thrust slices of the great valley group this hypothesis however is incompatible with borehole potential field and seismic refraction data that characterize the sacramento valley basement as ophiolitic seaward dipping reflections in the ophiolitic basement of the sacramento valley are analogous to layering developed in the oceanic crust of volcanic rifted margins or generated along midocean ridges thus late stage tectonic mechanisms are not required to interpret a forearc that owes much of its present day bedding architecture to processes coeval with deposition thickening of the great valley group stratigraphic section valanginian turonian in the hanging walls of the paskenta elder creek and cold fork fault zones combined with attenuation or complete omission of preextensional units including the coast range ophiolite and geometric evidence based on seismic reconstructions suggest that these faults are jurassic cretaceous normal faults that developed in a submarine setting down structure views of the great valley outcrop belt simplify otherwise complex map relations and portray the paskenta and related faults as half graben bounding faults that accommodated significant northwestward tectonic transport of hanging wall rocks it is significant that these faults sole into the coast range fault an enigmatic forearc structure that juxtaposes rocks of the franciscan complex blueschists with rocks of the coast range ophiolite and great valley group that have sustained only zeolite grade metamorphism discovery of jurassic cretaceous crustal scale extension in the great valley forearc suggests that a significant part of coast range fault related attenuation developed early in the history of the subduction complex

mapping singularities with stream sediment geochemical data for prediction of undiscovered mineral deposits in gejiu yunnan province china abstract this paper proposes the concept of singularity for characterizing hydrothermal mineralization processes a singular physical process may result in anomalous amounts of energy release or material accumulation within a narrow spatial temporal interval mineralization can be considered as a type of singular process due to large amounts of material accumulation and element enrichment a spatial singularity mapping technique is developed based on the local singularity involved in the context of multifractal theory it assembles geochemical map at different scales and calculates an index indicating the scaling characteristics of enrichment and depletion of geochemical concentration at multiple scales this index was used for quantifying spatial singularity of mineralization associated geochemical anomalies a case study for the gejiu tin polymetallic mineral district yunnan province china demonstrates that the concepts of singularity and the singularity mapping technique are applicable and useful for delineating anomalies caused by mineralization and for predicting the locations of undiscovered mineral deposits

shrimp u pb zircon geochronology of the liaoji granitoids constraints on the evolution of the paleoproterozoic jiao liao ji belt in the eastern block of the north china craton abstract the paleoproterozoic jiao liao ji belt lies at the eastern margin of the eastern block of the north china craton and is composed mainly of the liaohe group and the liaoji granitoids the liaoji granitoids are divisible into pre tectonic magnetite and hornblende biotite monzogranitic gneisses and post tectonic or anorogenic porphyritic monzogranites and alkaline syenites shrimp u pb zircon geochronology combined with cathodoluminescence cl imaging has enabled for resolution of magmatic and metamorphic events that can be directed towards understanding the history of the paleoproterozoic jiao liao ji belt shrimp u pb zircon analyses reveal that the pre tectonic magnetite monzogranitic gneisses were emplaced in the period 2176 2166 ma and metamorphosed at 1914 ma and the pre tectonic hornblende biotite monzogranitic gneisses were emplaced at 2150 2143 ma these data demonstrate that the pre tectonic monzogranitic gneisses of the liaoji granitoids were not younger than the deposition of the liaohe group as previously considered since the latter contains a large amount of 2100 2000 ma detrital zircons in contrast the pre tectonic monzogranitic gneisses of the liaoji granitoids or similar aged granitoids may have been an important component of the provenance for the liaohe group shrimp u pb zircon analyses also reveal that the post tectonic porphyritic monzogranites and granites in the jiao liao ji belt were emplaced at 1875 10 and 1856 31 ma respectively simultaneously with the emplacement of the alkaline syenites that was dated at 1857 20 and 1843 23 ma similar post tecotnic or anorogenic granites are not only limited to the jiao liao ji belt but are also found in the north and south korea suggesting that the 1 85 ga post tectonic or anorogenic granitic magmatic event may have occurred in both the korean peninsula and the eastern block of the north china craton the shrimp zircon ages of this study combined with lithological structural metamorphic and geochemical data is consistent with a rift closure model which suggests that the archean complexes on the northern and southern sides of the jiao liao ji belt were originally situated on a single continental block that underwent paleoproterozoic rifting associated with the formation of the liaohe group and liaoji granitoids and closed upon itself at 1 9 ga

accurate measurements of residual topography from the oceanic realm in the oceans our understanding of plate subsidence as a function of age permits residual depth anomalies to be identified and mapped these anomalies may reflect dynamic topography and could be an important means for constraining convective circulation of the sublithospheric mantle here we analyze a global database of seismic reflection and wide angle profiles from heavily sedimented oceanic crust which abuts continental lithosphere at 449 locations we calculated water loaded subsidence compared it with a reference age depth relationship and determined residual depth we then combined these spot measurements of residual depth with observations from mid oceanic ridges and from selected ship track bathymetry to construct a global map of residual depth our results suggest that the amplitude of residual depth varies by up to 1 km with wavelengths of order 103 km we compare our residual depths with free air gravity and seismic tomographic anomalies our results show that residual depths correlate with long wavelength gravity anomalies in contrast correlations between residual depths and vertically averaged shear velocity anomalies within the upper and or the lower mantle are weaker the largest discrepancies occur at short 1000 km wavelengths these combined observations suggest that residual depth anomalies could be generate by density variations within a thin 102 km low viscosity layer beneath the lithosphere our global compilation should play a significant role in helping to refine predictive geodynamical models

seismic structure of 15 ma oceanic crust formed at an ultrafast spreading east pacific rise evidence for kilometer scale fracturing from dipping reflectors 1 a grid of seismic reflection data from 15 ma cocos plate offers an unprecedented view of the structure of mature oceanic crust formed at an ultrafast spreading 200 mm yr full rate east pacific rise epr the data contain an unexpected quantity of bright reflectors throughout the crust generally 5 km in length significant reflection dip angles 10 30 are seen only on the isochron profiles and several reflectors have a confirmed isochron dip orientation certain upper crustal reflectors ucrs appear to form a single network with isochron dip the most prominent example apparent reflection coefficient 0 2 projects toward a 70 m disruption in the igneous seafloor and waveform analysis demonstrates that the reflector is likely the product of a low velocity zone on the order of tens of meters thick our observations suggest that the isochron dipping ucrs represent small slip kilometer scale faults fractures and none are primarily related to the seismic 2 3 boundary or to the sheeted dike gabbro transition lower crustal reflectors lcrs have no preferred orientation or regular pattern on the 30 km scale of this survey certain lcrs may be similar to the ucrs in origin while others may originate from near axis ductile flow lithologic banding off axis magmatism or some combination thereof overall reflectors in this area are abundant and complex consistent with previous off axis studies but at odds with observations of young crust 5 ma near the epr this dichotomy may be resolved by a delay in reflector formation through some combination of postaccretionary tectonics magmatism and hydrothermal alteration

laterally confined volcanic successions lcvs recording rift jumps during the formation of magma rich margins abstract seaward dipping reflectors sdrs are a characteristic feature of magma rich margins and represent the generation of large volumes of flood basalts at the point of continental breakup a number of recent studies provide new insights into the emplacement and tilting of sdrs and conclude that the majority of sdrs are contained within new magmatic crust that has a close affinity to oceanic crust however the process by which these initial magmatic systems evolve into a fully established spreading centre remains poorly understood several characteristic features of magma rich margins may be explained by the occurrence of rift jumps during sdr emplacement yet the cause and prevalence of such rift jumps remain unknown here we constrain the 3d geometry of the continent ocean transition in the orange basin offshore south africa this allows us to test if where and why such rift jumps occur our results demonstrate an order of along strike segmentation previously unobserved in these settings we demonstrate that the sdr belt is disrupted by the occurrence of a volcanic stratigraphic package defined as the laterally confined volcanic succession lcvs not previously identified on a rifted margin we interpret this as a magmatic spreading centre that was abandoned by a subsequent rift jump identification of lcvss is important for two reasons first we argue that the lcvs formed via the same process as sdrs and hence provides a unique example of sdr geometry prior to their separation onto conjugate plates second as we can map out the 3d geometry of the lcvs and sdrs we propose that rift jumps during magma rich margin formation may be fundamental to the establishment of a laterally continuous incipient spreading centre

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

extremely thin crust in the indian ocean possibly resulting from plume ridge interaction the thickness of the crust created at ocean spreading centres depends on the spreading rate and melt production in the mantle it is 5 8 km for a crust formed at slow and fast spreading centres and 2 4 km at ultra slow spreading centres away from hotspots and mantle anomalies the crust is generally thin at the fracture zones and thick beneath hotspots and large igneous provinces here we present results for the crust generated at the fast wharton spreading centre 55 58 ma ago using seismic reflection and refraction data we find that the crust over a 200 km segment of the wharton basin is only 3 5 4 5 km thick the thinnest crust ever observed in a fast spreading environment a thin crust could be produced by the presence of depleted and or cold mantle numerical simulations and recent laboratory experiments studying the impact of a hot plume under a lithosphere show that a curtain of weak cold downwellings of depleted mantle material is likely to develop around the edges of the hot plume pond because of a strongly temperature dependent viscosity of lithospheric material the hotter therefore less viscous bottom of the lithosphere can be mobilized by an impinging plume if sampled by a spreading centre the locally cold and depleted mantle should result in low production of melt we suggest that the observed thin crust in the wharton basin is likely to have been formed by the interaction between the kerguelen mantle plume and the wharton spreading centre 55 ma ago

structural evolution of preexisting oceanic crust through intraplate igneous activities in the marcus wake seamount chain multichannel seismic reflection studies and seismic refraction surveys with ocean bottom seismographs in the marcus wake seamount chain in the northwestern pacific ocean reveal p wave velocity structures of hot spot origin seamounts and adjacent oceanic crust inside the seamounts are central high velocity 6 5 km s structures extending nearly to the top that may indicate intrusive cores thick sediment layers up to 4 km with p wave velocities of 4 5 km s have accumulated on seafloor that predates seamount formation downward crustal thickening of up to 2 km was documented beneath a large seamount cluster but thickening was not confirmed below a small seamount cluster volume ratios of an intrusive core to a seamount body are 15 20 indicating that most of the supplied magma was consumed in forming the thick sedimentary and volcaniclastic layer constituting the seamount flanks underplating and downward crustal thickening may tend to occur when second or later intrusive cores are formed in a seamount p wave velocities in the lowest crust and in the uppermost mantle below the seamount chain are 0 1 0 2 km s higher and 0 3 0 5 km s lower respectively than velocities below oceanic crust we explain this difference as a result of sill like intrusion of magma into the lower crust and uppermost mantle reflected waves observed at offsets 200 km are from mantle reflectors at depths of 30 45 km and 55 70 km the shallower reflectors may indicate structures formed by intraplate igneous activities and the deeper reflectors may correspond to the lithosphere asthenosphere boundary

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

multifractal idw interpolation and fractal filtering method in environmental studies an application on regional stream sediments of italy campania region in recent years environmental geochemical mapping has assumed an increasing relevance and separation of background values to evaluate pollution is probably even more critical than the separation between background and anomalies in mineral prospecting studies the recognition of background values assumes particular relevance as a function of national environmental legislation which fixes intervention limits for some elements such as the harmful ones e g as cd hg pb in this paper a recently developed multifractal idw interpolation method and a fractal filtering technique are applied to separate natural background and anthropogenic values for the compilation of environmental geochemical mapping from stream sediment samples of campania region italy where no mineralization occurs to discuss the application of these recently developed techniques the elements pb and u were selected because they show two completely different situations the high pb values being mostly of anthropogenic origin and high u values being mostly of geogenic origin the new fractal filtering method works well in both extreme situations

crustal types and tertiary tectonic evolution of the alboran sea western mediterranean multichannel seismic reflection images across the transition between the east alboran and the algero balearic basins show how crustal thickness decreases from about 5 s two way traveltime twtt 15 km thick in the west east alboran basin to 2 s twtt typical of oceanic crust 6 km thick in the east algero balearic basin we have differentiated three different crustal domains in this transition mainly on the basis of crustal thickness and seismic signature boundaries between the three crustal domains are transitional and lack evidence for major faults tilted blocks related to extension are very scarce and all sampled basement outcrops are volcanic suggesting a strong relationship between magmatism and crustal structure stratigraphic correlation of lithoseismic units with sedimentary units of southeastern betic basins indicates that sediments onlap igneous basement approximately at 12 ma in the eastern area and at 8 ma in the western area linking seismic crustal structure with magmatic geochemical evidence suggests that the three differentiated crustal domains may represent from west to east thin continental crust modified by arc magmatism magmatic arc crust and oceanic crust middle to late miocene arc and oceanic crust formation in the east alboran and algero balearic basins respectively occurred during westward migration of the gibraltar accretionary wedge and shortening in the betic rif foreland basins arc magmatism and associated backarc oceanic crust formation were related to early to middle miocene subduction and rollback of the flysch trough oceanic basement subduction of this narrow slab beneath the alboran basin was coeval with collision of the alboran domain with the iberian and african passive margins and subsequent subcontinental lithosphere edge delamination along the betic rif margins

fractals in petrology abstract a fractal power law distribution is the only statistical distribution applicable to a scale invariant process one of the major problems in petrology is the distribution of trace elements in the earth s crust extreme values of this distribution result in ore deposits there is accumulating observational evidence that tonnage grade statistics of ore deposits are often fractal power law rayleigh distillation and chromatographic models can explain power law fractal distributions for the extreme values of trace element concentrations an alternative fractal approach to problems in petrology is to use self affine fractals the standard approach is to take a fourier transform of a continuous signal an example would be the concentration of a mineral along a linear track if the fourier coefficients scale as a power law of the wavelength the distribution is a self affine fractal many well logs give this result multifractal analyses can also be applied to problems in petrology

growth and reworking of the early precambrian continental crust in the north china craton constraints from zircon hf isotopes abstract we synthesize more than 2600 hf isotope data on the archean paleoproterozoic zircons from the north china craton ncc recalculation of the data based on single stage and two stage hf model ages of the eastern block of the ncc shows peak ages of 3902 13 ma and 3978 18 ma respectively and also small peaks at 3 5 4 0 ga the majority of zircon ehf t values are positive suggesting the possibility of the crust and the mantle differentiation at ca 3 9 4 0 ga in the eastern block of the ncc most magmatic zircons from the whole of ncc have their hf model age range of 2 4 2 9 ga and the single stage model ages is cluster at 2698 4 ma whereas the two stage model ages concentrate at 2714 5 ma implying that the protoliths were juvenile crustal rocks the most prominent peak at 2 7 ga indicates that this period marks the most important stage of the crust mantle differentiation and crust formation of the ncc the widespread 2 5 ga rocks in the ncc and the absence of the 2 5 ga peaks in hf model ages are consistent with the partial melting and reworking of the juvenile rocks at 2 5 ga furthermore the 2 5 1 7 ga zircon hf isotope features are also related to the reworking of the crustal rocks our results from the integration of a large database suggest that the eastern block and the trans north china orogen have undergone similar crust mantle differentiation and magmatism leading to the conclusion that the essential cratonization of the north china took place at the end of neoarchean

reflective oceanic crust formed at a fast spreading center in the pacific we present new seismic images of cretaceous crust formed at a fast spreading center in the pacific the high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast spreading centers are not seismically detectable subhorizontal reflections can be traced at 600 800 ms two way time below the top of basement for tens of kilometres suggesting the presence of a widespread seismic boundary possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes lower crustal reflections dipping dominantly toward the paleo spreading center may represent mafic ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections

a comparison study of the c a and s a models with singularity analysis to identify geochemical anomalies in covered areas abstract fractal multifractal modeling of geochemical data is an interesting topic in the field of applied geochemistry identification of weak anomalies for mineral exploration in covered areas is one of the most challenging tasks for utilization of geochemical data in this study three fractal models consisting of the concentration area c a spectrum area s a and singularity index models were applied to identify geochemical anomalies in the covered area located in the chaobuleng fe polymetallic district inner mongolia china the results show that 1 the grassland cover weakens the concentrations of geochemical elements 2 the c a model has a limitation to identify weak anomalies in covered areas 3 the s a model is a powerful tool to decompose mixed geochemical patterns into a geochemical anomaly map and a varied geochemical background map but suffers edge effects in an irregular shaped study area and 4 the singularity index is a useful tool to identify weak geochemical anomalies

enhanced mantle upwelling melting caused segment propagation oceanic core complex die off and the death of a transform fault the mid atlantic ridge at 21 5 n crustal structure provides the key to understand the interplay of magmatism and tectonism while oceanic crust is constructed at mid ocean ridges mor at slow spreading rates magmatic processes dominate central areas of mor segments whereas segment ends are highly tectonised the tammar segment at the mid atlantic ridge mar between 21 25 n and 22 n is a magmatically active segment at 4 5 ma this segment started to propagate south causing the termination of the transform fault at 21 40 n this stopped long lived detachment faulting and caused the migration of the ridge offset to the south here a segment centre with a high magmatic budget has replaced a transform fault region with limited magma supply we present results from seismic refraction profiles that mapped the crustal structure across the ridge crest of the tammar segment seismic data yield crustal structure changes at the segment centre as a function of melt supply seismic layer 3 underwent profound changes in thickness and became rapidly thicker 5 ma this correlates with the observed bull s eye gravimetric anomaly in that region our observations support a temporal change from thick lithosphere with oceanic core complex formation and transform faulting to thin lithosphere with focused mantle upwelling and segment growth temporal changes in crustal construction are connected to variations in the underlying mantle we propose there is a link between the neighbouring segments at a larger scale within the asthenosphere to form a long highly magmatically active macro segment here called the tammar kane macrosegment

the mohorovicic discontinuity in ocean basins some observations from seismic data abstract since the late 1970s studies of the oceanic crust using airgun sources and towed hydrophone arrays have been conducted in a wide range of ocean basin settings including mid ocean ridges and old oceanic crust the very earliest studies were performed at the fast spreading east pacific rise epr and revealed a very distinct almost continuous vertical incidence reflection event at a depth corresponding to the crust mantle transition as inferred from seismic refraction studies this suggested that the transition was quite sharp in comparison to the source wavelength that moho was observed very close to or even exactly beneath the ridge crest implied that it was formed at essentially zero age since then many experiments using progressively improving airgun arrays and streamer systems have expanded these observations here we review the literature presenting studies of normal oceanic crust produced at mid ocean ridges with the objective of assessing the age of formation of moho and the nature of variability of moho signature in multi channel seismic data moho is observed as a consistent feature for all spreading rates but appears quite variable being very distinct in some areas complex in form in others and absent in many regions as much as 40 although fast spread crust is associated with the strongest simplest and most laterally continuous moho images we see significant variability at almost all spreading rates and ages where moho is absent from vertical incidence data this cannot be fully explained by the effect of scattering or attenuation in the crustal section above moho can be observed at zero age in only a small number of areas at or near overlapping spreading centers on the epr after three decades of marine seismic studies many fundamental questions remain concerning the nature of the moho that will require targeted experiments to solve

slice of intraoceanic arc insights from the first multichannel seismic reflection profile across the south sandwich island arc we present the first multichannel seismic mcs reflection line that crosses the south sandwich trench south sandwich island arc and east scotia sea backarc basin the line is used in combination with earthquake catalogue data to interpret the strain distribution across the sandwich plate and the relationship of forearc structures to processes operating at the trench the mcs data reveal a 1 2 km high fault scarp associated with a 20 km wide arcward tilted block in the mid forearc these features indicate large scale gravitational collapse and earthquake data are consistent with trench normal extension at shallow depth in this area there is however little evidence of distributed extension within the interior of the sandwich plate the mcs data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench backarc magnetic data mid forearc extension and the small size of the frontal wedge are all consistent with long term and ongoing subduction erosion earthquake data suggest that this erosion is taking place in an environment of low interplate stress

seismic structure of the central tyrrhenian basin geophysical constraints on the nature of the main crustal domains in this work we investigate the crustal and tectonic structures of the central tyrrhenian back arc basin combining refraction and wide angle reflection seismic was gravity and multichannel seismic mcs reflection data acquired during the medoc mediterraneo occidental 2010 survey along a transect crossing the entire basin from sardinia to campania at 40 n the results presented include a 450 km long 2 d p wave velocity model obtained by the traveltime inversion of the was data a coincident density model and a mcs poststack time migrated profile we interpret three basement domains with different petrological affinity along the transect based on the comparison of velocity and velocity derived density models with existing compilations for continental crust oceanic crust and exhumed mantle the first domain includes the continental crust of sardinia and the conjugate campania margin in the sardinia margin extension has thinned the crust from 20 km under the coastline to 13 km 60 km seaward similarly the campania margin is also affected by strong extensional deformation the second domain under the cornaglia terrace and its conjugate campania terrace appears to be oceanic in nature however it shows differences with respect to the reference atlantic oceanic crust and agrees with that generated in back arc oceanic settings the velocities depth relationships and lack of moho reflections in seismic records of the third domain i e the magnaghi and vavilov basins support a basement fundamentally made of mantle rocks the large seamounts of the third domain e g vavilov are underlain by 10 20 km wide relatively low velocity anomalies interpreted as magmatic bodies locally intruding the mantle

identification of geochemical anomaly and gold potential mapping in the sonakhan greenstone belt central india an integrated concentration area fractal and fuzzy ahp approach abstract the present work combines concentration area c a fractal model with the fuzzy analytical hierarchy process fahp on a gis platform for mapping gold potential in the sonakhan greenstone belt india a set of stream sediment geochemical data obtained over a part of the study area was used to delineate potential regions of gold mineralisation in order to develop a suitable predictive model the gold exploration target was taken as target dependent variable and concentration of pathfinder elements of gold such as au as ag hg sb and se in the stream sediments were used as predictors independent variables the c a fractal model was applied to the geochemical data of each element to decompose anomaly and background components of the spatial dispersion of element concentration in the study area ahp combined with fuzzy set theory fahp was used to determine the priority or the weight of each evidential geochemical anomaly map the gold exploration targets are delineated by multiplying the weights with the respective fuzzy normalised geochemical anomaly map of the gold pathfinder elements and integrating the weighted geochemical evidential layers using fuzzy gamma operator the reliability of the outcome was assured by the coincidence of known gold mineralisation with the potential regions delineated in the final gold potential map the areas with high potential are good targets for detailed exploration of gold in the region the results highlight that a application c a fractal model can effectively separate geochemical anomalies as it considers the spatial variation of the data unlike the conventional statistical methods which account only the frequency of the data b the fahp allows more flexibility in judgements of multiple decision makers in a group and reduces the inconsistency of the result which is essentially required in the knowledge driven predictive mapping of mineral prospectivity