sandstone matrix olistostrome deposited on intra subduction complex serpentinite franciscan complex western marin county california abstract detailed field mapping in the franciscan complex in western marin county california offers insight into the origins and development of melanges as well as accretionary complex architecture from structurally high to low the study area units comprise a structurally high sandstone matrix olistostrome serpentinite a structurally intermediate sandstone matrix olistostrome early cretaceous nicasio reservoir terrane greenstone and coherent cretaceous greywacke all exhibiting prehnite pumpellyite burial metamorphism the structurally high sandstone matrix olistostrome includes volcanic blocks up to 700 m in diameter and chert blocks up to 100 m in diameter whereas the structurally intermediate olistostrome unit comprises a lithic rich sandstone matrix enclosing volcanic blocks 1 20 m in diameter the youngest detrital zircons in the high olistostrome and the coherent greywacke have u pb ages of 20 m y older than the inferred depositional ages of the units themselves suggesting that the bulk of the detritus comprising these units was derived by exposure and sedimentary recycling of significantly older previously accreted sediments the matrix of both olistostrome units exhibits minimal strain with weak local development of pressure solution cleavage these findings demonstrate introduction of franciscan exotic blocks and development of franciscan block in matrix fabrics through sedimentary sliding and support sedimentary interpretations for more highly strained shale and serpentinite matrix melanges additionally these data suggest that sandstone matrix olistostromes melange may constitute a larger part of the franciscan and other accretionary complexes than previously believed the structurally high olistostrome preserves a depositional contact on serpentinite verifying an intra subduction complex origin for the serpentinite the serpentinite lacks block in matrix structure and likely represents a tectonically emplaced unit derived from the subducting plate as a coherent nappe rather than a sedimentary serpentinite deposit multi kilometer scale west vergent overturned folds with subhorizontal fold axes deform the nappes of the field area and a later generation of open northeast trending folds overprints the earlier folds

simulation of run off from the greenland ice sheet for planning hydro electric power ilulissat jakobshavn west greenland simulations of run off from the greenland ice sheet were made as part of a feasibility study for provision of hydroelectric power for liulissatfjakobshavn west greenland the aims were to see if the available short series of run off measurements are typical of those under present climatic conditions and to assess possible changes in run off likely to be caused by gross changes in drainage pattern on the ice sheet specific run off was calculated from climatological data whilst run off volumes were calculated by integrating specific run off over the area of the ice sheet there have been substantial year to year variations in run off but the 6 year measurement period is reasonabl y re prese ntati ve of present climatic conditions run off could be reduced by 21 as a result of changes in hydrauli c conditions on the ice sheet without this having a significant effect on the economy of the planned hydro electric power station

using stable isotopes as natural tracers to delineate hydrological drainage basins on the greenland ice sheet margin abstract stable isotope studies of surface ice and water samples collected in the ablation zone of the greenland ice sheet margin at paakitsup akliarusersua 40 km northeast of ilulissat jakobshavn west greenland show that there is a large system atic spatial variation of the 18 o 16 o ratio 18 o over the ablation area 18 o can therefore be used as a natural tracer to locate the origin of the melt water runoff a simple model is set up for simulating the 18 o variation in the melt water discharge from the area simulations are made with a drainage basin obtained by modelling the subglacial drainage conditions which requires costly and time consuming mapping of the subglacial topography the simulated values agree well with observed values of water samples from three pro glacial lakes which receive the melt water discharge this suggests that the stable isotope method can be used to give an initial estimate of melt water drainage basin boundaries on the greenland ice sheet margin

the geochemistry and age of ophiolitic strata of the xinglongshan group implications for the amalgamation of the central qilian belt abstract the xinglongshan group comprises a metabasalt and metasedimentary rock quartz mica schist assemblage representative of low grade metamorphic basement strata in the central qilian belt metabasaltic compositions include morb like tholeiites and alkaline basalts of axial seamount or ocean island origin depending on assumed origin the latter yield sm nd model ages between 824 and 490 ma whole rock rb sr and sm nd systematics of acid leached metasedimentary rock samples define a late cambrian isochron of 513 17 ma with initial an 87 sr 86 sr ratio of 0 70930 0 00050 which is interpreted to mark the timing of basin closure high abundances of trace elements cr ni sc associated with ferromagnesian minerals relative to elements th zr associated with resistant minerals precludes derivation of the sediment assemblage solely from gneisses of the central qilian belt rather the nd sr isotope systematics of the metasedimentary rock samples indicate mixing between a mafic or arc component and an ancient felsic crustal composition th sc abundances suggest the felsic component was not the supracruastal sequences of the north china block isotopic and geochemical data thus suggest low grade metamorphic basement strata in the central qilian belt are of likely neoproterozoic age assembly of the belt took place during the early stages of the caledonian orogenic cycle before closure of ocean basins in the north qilian belt and that the central qilian belt likely bears an allochthonous relationship to north china

geochemistry and u pb zircon ages of metamorphic volcanic rocks of the paleoproterozoic luliang complex and constraints on the evolution of the trans north china orogen north china craton abstract the paleoproterozoic luliang complex is situated in the central part of the western margin of the trans north china orogen and consists of volcanic rocks sedimentary rocks and paleoproterozoic granitoid intrusions the volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist to amphibolite facies these metamorphosed volcanic rocks are dominated by basalts to basaltic andesites the parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5 to 30 partial melting of spinel lherzolites to spinel garnet lherzolites which had been enriched by the subduction melts mafic magma experienced fractional crystallization and crustal assimilation u pb zircon dating on two metamorphosed volcanic rocks from the yejishan and luliang groups reveals that they formed at 2210 13 ma and 2213 47 ma respectively and were metamorphosed at 1832 ma this suggests that the metamorphosed volcanic rocks in the yejishan and luliang groups formed synchronously in the paleoproterozoic these new ages integrated with recently reported u pb zircon ages for the jiehekou group and paleoproterozoic granitoids suggest that all of the lithological assemblages of the luliang complex formed and were metamorphosed in the paleoproterozoic not in the neoarchean petrological geochronological and geochemical data suggest that the geodynamic evolution of the paleoproterozoic luliang complex was involved in the development of a magmatic arc system at an active continental margin generating widespread arc related magmatism at 2 2 ga the luliang complex then underwent intense deformation and metamorphism and was incorporated into the trans north china orogen during the 1 88 1 83 ga collisional event which was followed by post collision extension at 1 80 ga

geochemical discrimination of metabasalt rocks of the fan karategin transitional blueschist greenschist belt south tianshan tajikistan seamount volcanism and accretionary tectonics the fan karategin metamorphic belt south tianshan tajikistan is regarded to be an ancient subduction accretionary complex and is composed of three tectonostratigraphic units which display lithologies consistent with different tectonic settings the mafic schists which make up the major part of the older unit of the belt contain both alkali and tholeiitic metabasalts on the basis of rare earth and other immobile element characteristics the alkali metabasalts are akin to within plate ocean island basalts whereas the tholeiitic metabasalts resemble e type morb the association is interpreted to have been formed on seamount like structures under a within plate plume bedded cherts and marbles in the unit are regarded as ancient pelagic sediments and carbonate caps developed upon basaltic seamounts respectively dismemberment of the seamount related basalts and pelagic sediments and the high p low t prograde metamorphism of the unit rocks up to transitional blueschist greenschist facies was the result of paleoseamount submergence into a subduction zone this unit is tectonically overlain by arc derived metavolcanic unit and a disrupted mainly clastic unit of upper ordovician lower silurian age metavolcanic and metasedimentary rocks of the two upper units have geochemical characteristics compatible with subduction related origin the lithological assemblages of the individual units and their juxtaposition suggest an origin involving collision accretionary processes the fan karategin belt is a subduction accretionary complex which formed during subduction of oceanic crust under a volcanic arc and was subjected to tectonic juxtaposition and imbrication of seamount deep sea trench and volcanic arc sequences

mega fold interference patterns in the beishan orogen nw china created by change in plate configuration during permo triassic termination of the altaids abstract kilometer size fold interference patterns in the beishan orogenic collage boc in the southernmost altaids formed by fold superimposition in fossiliferous permian sedimentary rocks first phase f 1 upright and almost north trending folds were refolded by e to ene trending f 2 folds whose axial planes and axes are vertical or subvertical from east to west there is a regional change in style of interference patterns from lobate cuspate to crescent to mushroom shape this variation is accompanied by a westward decrease in the f 2 interlimb angle and related to a higher percentage of coarse grained clastic rocks suggesting a dependence of the f 2 deformation on lithology axial planar slaty cleavages are well developed in f 1 and poorly developed in f 2 folds the superposed folds mainly underwent flexural slip and flexural flow folding to give rise to the lobate cuspate pattern and to the crescent pattern caused by flattening and flexural flow folding where the sediments were unconsolidated and enriched in fluids the two folding events are interpreted to result from a major change in plate configuration that caused the inversion of an inter arc basin during the final amalgamation of the boc in the latest permian to early to mid triassic the two folding events bracketed between a maximum detrital zircon age of

formation of taconic melanges and broken formations in the hamburg klippe central appalachian orogenic belt eastern pennsylvania abstract the hamburg klippe of the central appalachian orogenic belt exposed in eastern pennsylvania displays a complex record of poly phase melange and broken formation development in a convergent margin setting it includes an imbricate stack of tectonic slices which consist of upper cambrian to upper ordovician deep water and continental slope sedimentary rocks emplaced by gravity sliding onto the laurentian passive margin during deposition of the upper ordovician martinsburg formation based on their internal structure and stratigraphy the block matrix ratios and relations and the inferred tectonic settings of origin we have differentiated the following melanges and broken formations whose evolutionary stages coincide with specific deformational phases in a complete orogenic cycle from subduction to collision i sedimentary broken formations without exotic blocks formed by in situ and local down slope remobilization during the early stages of closure of the octoraro sea basin in the early ordovician ii two types of sedimentary melanges with exotic blocks formed at the front of the advancing accretionary wedge during the middle ordovician subduction of the laurentian continental margin beneath a microcontinent magmatic arc tectonic assembly iii layer parallel extensional broken formation and a diapiric melange formed in the outer trench and at the toe of the accretionary wedge respectively during the early late ordovician iv precursory olistostromes formed during the late ordovician collisional episodes as the hamburg klippe was emplaced in the martinsburg formation on the downgoing laurentian continental margin and v contractional deformation related broken formations formed at the base of main thrust faults overprinting the previously formed melanges and broken formations this sequential development of different melange types in the central appalachians was strongly controlled by the degree of consolidation of the layered strata and their rheological differences the structural level of melange formation within the accretionary wedge and the kinematics of deformational processes

the application of zircon cathodoluminescence imaging th u pb chemistry and u pb ages in interpreting discrete magmatic and high grade metamorphic events in the north china craton at the archean proterozoic boundary abstract two distinct late neoarchean to early paleoproterozoic magmatic events are recognized at the northern margin of the eastern block of the north china craton based on interpretation of zircon structure cathodoluminescence signature th u pb chemistry and age the first event is represented by emplacement of the chaihe trondhjemite and xianjinchang tonalite with magmatic zircon core ages ranging from 2559 2553 7 ma respectively these rocks experienced a subsequent high grade metamorphic event constrained by partial recrystallization of primary magmatic zircon and growth of new zircon rims between 2517 and 2490 ma a second magmatic episode generated by partial melting of crustal rocks during this metamorphic event is represented by the hongshilazi syenogranite with an age of 2502 11 ma the short time interval between initial magmatism and high grade regional metamorphism signifies they were related to the same major tectonothermal event when considered along with evidence of a similar sequence of events at this time 800 km to the south in eastern hebei and the previously established 2500 ma age of granulite facies metamorphism in mafic rocks across the craton it is proposed that a large mantle plume developed beneath the north china craton at the close of the archean

mappable features of melanges derived from ocean plate stratigraphy in the jurassic accretionary complexes of mino and chichibu terranes in southwest japan abstract the melanges of the jurassic accretionary complexes of japan have very complicated structures which were formed by the progressive disruption and mixing of ocean plate stratigraphy ops during various stages of accretionary processes along the ancient arc trench system the ops is composed of permian basalt limestone chert permian triassic boundary claystone early triassic early jurassic chert early jurassic earliest cretaceous siliceous shale and jurassic to early cretaceous turbidite in ascending order the ops was detached from the oceanic plate along a decollement which developed within the p t boundary claystone during the offscraping and underplating processes disruption and mixing first occurred along the decollement then along successively developed out of sequence thrusts neighboring rocks along the faults are mixed to become various sized clasts and matrices of melanges mappable slabs in the melanges were survivors from tectonic disruption of the accreted ops on the surface of accretionary wedge a part of the ops was collapse by submarine sliding and mud diaprisim to form chaotic sediments on the top of the ops thus the full extent of accretionary complex was analyzed by age stage development of melange formation in the melanges some of the large tectonic slabs in the melanges kept the original order of the ops even though they were deformed in various degrees this suggests that the ops was tectonically stacked during the offscraping process and was gradually disrupted along the decollement and out of sequence thrusts during the accretionary processes that form the melange structures the melanges structures mentioned above are shown in the geological maps at the scale of 1 50 000 recently published by the geological survey of japan these geological maps clearly show the melange formation processes during ocean plate subduction along the mesozoic asian continental margin

role of river incision in enhancing deformation many active compressional belts contain examples of transverse rivers intersecting anticlines at their highest structural and topographic position this unusual association between rivers and doubly plunging anticlines at relatively small scales is usually explained by some form of antecedence or superposition in this article i suggest an alternative explanation i e that river erosion unloads the crust and that this process causes local deformation to be enhanced in the vicinity of rivers i show the plausibility of this hypothesis with the aid of a coupled three dimensional mechanical surface process mathematical model the conclusion is that river incision can have a major influence on deformation of the surrounding crust if incision occurs at the same time that the crust is deforming plastically in response to regional compression in this case incision amplifies background deformation at a relatively small scale leading to the formation of noncylindrical folds with culminations coinciding with river incision in the absence of regional deformation the response of the crust to river incision is small and of long wavelength because it is governed by flexural isostasy of relatively rigid elastic crust thus whether rivers exert an influence on local deformation depends critically on the timing between river incision and regional deformation this point may explain why in some cases rivers appear to have had no significant influence on local deformation whereas in other cases they have

paleogene metamorphism of an accretionary flysch terrane eastern gulf of alaska reconnaissance field studies have identified a regional metamorphic complex at least 200 km long and 25 to 50 km wide in the eastern chugach mountains southern alaska the metamorphic complex was developed within the cretaceous valdez group a flysch sequence that was accreted to the continental margin during the late cretaceous or early tertiary the metamorphic complex shows the progressive development from subgreenschist and green schist facies slate phyllite and semischist that is regionally characteristic of the valdez group to biotite plagioclase quartz schist muscovite cordierite andalusite staurolite garnet and sillimanite in an intermediate zone and amphibolite facies muscovite biotite plagioclase quartz schist and gneiss sillimanite and garnet in a higher grade central zone k feldspar is characteristically absent in the amphibolite facies schists but it is present in migmatitic rocks that are common to the central zone the mineral assemblages and petrographic relations indicate that metamorphism took place under low pressure intermediate facies series conditions partial melting took place at the highest grade of metamorphism melting produced granodioritic magmas in migma titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex the timing of metamorphism as indicated by field relations and k ar data is about 50 to 65 m y ago and overlaps or is coeval with the emplacement throughout the gulf of alaska region of early tertiary plutons that are thought to be anatectic in origin the distribution of these plutons suggests that a high temperature low pressure metamorphic belt about 1 800 km long developed along the leading edge of the continental margin in late paleocene to early eocene time

u pb zircon age geochemical and sr nd isotopic data as constraints on the petrogenesis and emplacement time of andesites from gerze southern qiangtang block northern tibet abstract mesozoic post collisional andesites occur in the gerze area within the qiangtang terrane of northern tibet china geochronological geochemical and whole rock sr nd isotopic analyses were performed on this suite of volcanic rocks to characterize their petrogenesis laser ablation inductively coupled plasma mass spectrometry u pb zircon analyses yielded consistent ages ranging from 122 4 0 4 ma to 124 4 0 4 ma for two andesite samples jc01 and gz01 the andesites belong to the alkaline and sub alkaline magma series in terms of k2o na2o contents 4 7 10 wt and belong to the high k calc alkaline and calc alkaline series on the basis of their high k2o contents 1 0 3 3 wt the volcanic rocks are further characterized by the enrichment of light rare earth elements la yb n 7 4 11 5 and large ion lithophile elements e g rb slightly negative eu anomalies eu eu 0 79 0 94 and negative anomalies in high field strength elements e g nb ta and ti relative to primitive mantle the samples show slightly elevated 87sr 86sr i values that range from 0 7049 to 0 7057 and low end t values of 3 4 to 0 7 these results suggest that the rocks were derived from a compositionally heterogeneous mantle source and that their parent magmas are basalt magma the parent mafic magmas may have undergone fractional crystallization of olivine pyroxene hornblende and to a lesser extent plagioclase during ascent through the thickened crustal pile with minor or no crustal contamination before the finally erupting as andesites

hinge migration as a mechanism of superimposed folding abstract paraffin wax analogue modelling of superimposed folding shows how early fold shape and orientation determine fold axis orientation type of interference pattern and mechanism of superposed folding during two successive deformations the behaviour depends on the angle between the first and second compression directions i when they are perpendicular to each other classic superposed folding with either domes and basins or folds with vertical axes develop depending on whether the earlier folds were open or closed ii when the two compression directions are oblique the earlier folds rotate the degree of rotation depends on the angle between the first folds and the later compression direction and is proportional to the amount of strain the orientations of the new folds are close to those of the earlier open folds the final deformation pattern consists of domains where the orientations of the axes depend on whether the early folds were previously either open or closed many early folds are reused to become later folds by the mechanism of hinge migration these results are consistent with the pattern of superimposed folding observed in the devoluy structure of the french alps they show that simply by studying the directions of fold axes in areas of superposed folding it is not possible to define compression directions moreover there is the possibility that superposed folding may go unnoticed if hinge migration has occurred

crescent like large scale structures in the external zones of the western gibraltar arc betic rif orogenic wedge the external wedge of the gibraltar arc orogenic system western mediterranean is a natural case of a fold and thrust salient although at the scale of the arc a swing of the structural trend can be observed the presence of closed crescent like structures some of them more than 25 km long and scattered over an area of 3500 km 2 characterize the gibraltar arc external wedge this feature makes it very different from a classical arc shaped fold and thrust belt detailed structural maps of representative crescent like structures are presented synorogenic sediments reveal that they probably formed during a short time interval most probably during a single shortening event the crescent like structures are localized in front of a recess formed by the internal zones that acted as backstop this backstop shape and a viscous substrate essentially triassic evaporites below the fold and thrust external wedge constrain the experimental setting of an analogue model in which we were able to reproduce structures that developed originally with a highly non cylindrical shape during a single straight convergence moreover the tectonic transport directions along the thrusts are broadly comparable in the natural case and the model

formation of a metamorphic complex along an obliquely convergent margin structural and thermochronological evolution of the chugach metamorphic complex southern alaska this study documents the structural and metamorphic evolution of middle to lower crust along an oblique convergent curved continental margin during a time span of 60 myr our study documents the importance of variable obliquity during convergence which led to the development of overprinting fabrics and distinct exhumation histories along strike of the margin we present structural analyses 40ar 39ar rb sr and zircon fission track ages from middle to lower crust exposed along the southern alaskan margin in the chugach metamorphic complex together with the metamorphic history and additional geochronology from the literature we derive the following tectonic evolution for this area accretion of sediments during dextrally oblique convergence led to strain partitioned d 1 structures and greenschist facies metamorphism prior to circa 55 ma at 55 51 ma a margin parallel stretching phase with vertical thinning d 2 affected the margin and led to andalusite sillimanite grade metamorphism and the onset of partial melting a switch back to dextral transpression d 3 shortly after d 2 led to rapid cooling of the western and central parts of the complex associated with exhumation of parts of the core of the complex until circa 46 ma the southeastern part of the complex cooled and exhumed regularly and slowly until circa 5 ma due to its highly oblique orientation relative to the convergence direction an increase in cooling and exhumation occurred after circa 5 ma in the entire southeastern part of the complex associated with the neogene collision of the yakutat terrane

the amphibolite facies metamorphosed mafic rocks from the maxianshan area qilian block nw china a record of early neoproterozoic arc magmatism abstract the amphibolite facies metamorphosed mafic rocks exposed in the maxianshan area qilian block nw china intruded granitic gneisses of the maxianshan group in the field they are seen to inject into and capture fragments of the surrounding granitic gneisses their rock types range from amphibole rich amphibolite to plagioclase rich amphibolite their whole rock major element compositions suggest that their protoliths were subalkalic gabbroic rocks the rare earth element patterns show slight light rare earth element enrichment with a very small europium negative anomaly the amphibolites exhibit enrichment in large ion lithophile elements and negative anomalies of titanium strontium niobium and tantalum on the basis of their spider diagrams the shrimp u pb ages of the zircon grains are 919 10 ma for the amphibole rich amphibolite and 905 6 ma for the plagioclase rich amphibolite indicating the ages of protolith formation the e nd 910 ma values are in the range of 1 6 and 2 4 consistent with generation in a continental arc tectonic discriminations by whole rock trace element compositions suggest that they originated in a continental arc environment the gabbroic protoliths were metamorphosed under amphibolite facies conditions the maxianshan amphibolite facies metamorphosed mafic rocks could be correlated with the mafic ultramafic rocks from marginal areas of the yangtze block this correlation implies that the qilian block and marginal areas of the yangtze block were active continental margins in the early neoproterozoic and provides evidence for a strong tectonic affinity of the qilian block with the south china block and even for a unified qilian south china block at that time it is inferred that the qilian block carrying maxianshan mafic rocks underwent amphibolite facies metamorphism after it was rifted away from the south china block in the late neoproterozoic as a result of the breakup of the rodinia supercontinent and collided with the alaxa block in the early paleozoic

fold interference structures the influence of layer competence contrast using the geometrical properties of curved surfaces it can be predicted that the geometries of natural refolded folds should differ from the well known forms resulting from superposed shear folds if the sequence undergoing folding contains competent horizons the simple dome and basin patterns expected from kinematic considerations are incompatible with constant bed length folding isometric deformation isometric deformation requires that a flat sheet folds to give a surface consisting of points of zero gaussian curvature like for example the surfaces of cylindrical and conical folds we suggest that the coaptation folds recently recognized by stauffer 1988 are topologically necessary crumples on the flanks of domes and basins rather than bends at the intersection of two oblique intersecting folds and should be known as curvature accommodation folds

boninite like volcanic rocks in the 3 7 3 8 ga isua greenstone belt west greenland geochemical evidence for intra oceanic subduction zone processes in the early earth abstract the 3 7 3 8 ga isua greenstone belt of southwest greenland is characterized by variably metamorphosed metasomatised and deformed lithotectonic successions of volcanic and sedimentary rocks the voluminous mafic volcanic rocks are composed primarily of pillow basalts intercalated with ultramafic units the sedimentary rocks consist mainly of banded iron formation cherts conglomerates and siliciclastic turbidites the least altered metavolcanic amphibolites the garbenschiefer unit from the central tectonic domain of the isua greenstone belt are characterized by high mg number 0 60 0 80 mgo 7 18 wt al2o3 14 20 wt ni 60 645 ppm and cr 60 1920 ppm contents but low tio2 0 20 0 40 wt zr 12 30 ppm y 6 14 ppm and rare earth element ree concentrations these compositional features collectively represent a coherent mafic to ultramafic suite chondrite normalized ree patterns are concave upward on the primitive mantle normalized extended trace element diagrams they are characterized by relative depletion of nb but with an enrichment of zr relative to neighboring rees alteration deformation and crustal contamination can be ruled out as the cause of the distinct and coherent composition the average initial end value of these metavolcanic rocks is 2 collectively these geochemical characteristics are comparable to those of phanerozoic boninites given the observation that in the tertiary boninites are exclusively associated with intra oceanic subduction environments e g izu bonin mariana subduction system this suggests that intra oceanic subduction zone like geodynamic processes were operating as early as 3 7 3 8 ga

geochronological and geochemical evidence of amphibolite from the hualong group northwest china implication for the early paleozoic accretionary tectonics of the central qilian belt abstract the hualong group located in the central qilian belt northwest china consists mainly of schist amphibolite quartzite and marble ranging from greenschist to amphibolite facies metamorphism on the basis of the medium grade metamorphism the group has been considered to comprise proterozoic basement rocks in this study geochemical sr nd isotopic and zircon u pb geochronological analyses were performed on lentoid amphibolites from the hualong group to characterize their age petrogensis and tectonic setting uranium lead zircon dating of amphibolite revealed a formation age of 456 2 ma and a metamorphic age of 440 1 ma major trace and rare earth element data indicate that the amphibolites are predominantly basaltic andesitic to andesitic rocks with island arc affinities the trace element patterns show enrichment in large ion lithophile elements and depletion in high field strength elements relative to the n morb which confirm their island arc signatures obviously enriched light rees la yb n 2 5 16 9 to chondrite normalized ree patterns further support this interpretation the e nd t values for the amphibolites range from 4 6 to 2 1 indicating subducted sediments as a larger endmember in the source geochemical data for these rocks suggest an island arc setting and the rocks were derived from the depleted mantle that was enriched by melts of subducted sediments in an active continental margin setting at ca 456 ma together with regional evidence it suggests that the hualong group is an accretionary complex that was incorporated into the central qilian belt during the 440 400 ma orogenic event

continuity of the north qilian and north qinling orogenic belts central orogenic system of china evidence from newly discovered paleozoic adakitic rocks abstract adakitic intrusive rocks of 430 450 ma were discovered in the north qilian orogenic belt the western section of the central orogenic system cos in china these adakitic rocks were lower crust melts rather than slab melts as indicated by their crustal ce pb nb u ti eu and nd sm ratios and radiogenically enriched 87sr 86sr i of 0 7053 0 7066 and end t of 0 9 to 1 7 while they are all characterized by low yb 65 and la yb n 13 7 ratios these adakitic rocks are classified into the low mgo ni cr and high mgo ni cr groups the low mgo samples were derived from partial melting of thickened lower crust whereas the high mgo samples were melts from delaminated lower crust which subsequently interacted with mantle peridotite upon ascent adakitic rocks from the adjacent north qinling orogenic belt also originated from thickened lower crust at 430 ma in addition the north qilian and north qinling orogenic belts both consist of lithological assemblages varying from subduction accretionary complexes at south to central arc assemblages which include adakitic rocks then to backarc phases at north such a sequence reflects northward subduction of the qilian and qinling oceans in these two orogenic belts the occurrence of adakitic rocks of common origin and ages together with the similarities in tectonic configurations and lithological assemblages are considered to be the evidence for the continuity between eastern qilian and western qinling forming a 1000 km early paleozoic orogenic belt in such a tectonic configuration the qilian and qinling oceans that subducted from south possibly represent parts of the large proto tethyan ocean this inference is supported by the coexistence of early paleozoic coral and trilobite specimens from asia america and australia in the north qilian orogenic belt post 400 ma volcanic rocks occur in the north qinling orogenic belt but are absent in the north qilian orogenic belt indicating that these two orogenic belts underwent distinct evolution history after the closure of the proto tethyan ocean 420 ma

effect of glaciers on annual run off johan dahl land south greenland on compare le ruissellement de 2 bassins dont l un contient des glaciers avec la precipitation et l ablation mesuree dans le bassin glaciaire les variations saisonnieres du ruissellement pour les 2 bassins sont similaires tandis pue le ruissellement du bassin glaciaire presente des variations annee apres annee plus petites on montre sur un modele statistique qu il s agit du resultat d une correlation negative entre l ablation et la precipitation

hualong complex south qilian terrane u pb and lu hf constraints on neoproterozoic micro continental fragments accreted to the northern proto tethyan margin abstract the neoproterozoic hualong complex dominates the south qilian terrane it contains mafic to felsic magmatic rocks cherts and turbiditic sediments some of which have been metamorphosed to amphibolite grade and is traditionally regarded as a pre cambrian micro continental block rifted from the south china plate new u pb geochronology of morphologically complex zircons sheds light on the history of the complex garnet bearing amphibolite and quartzite have detrital zircon u pb main peak ages of 1 47 1 78 ga with youngest u pb ages of 967 ma and 964 ma they are intruded by neoproterozoic orthogneisses that crystallized between ca 850 and 940 ma psammitic paragneisses are dominated by 940 780 ma detrital zircon populations with a youngest u pb weighted mean age of 721 3 ma and a main peak at 906 ma metamorphic overgrowths on older zircons record amphibolite facies metamorphism of the hualong complex and correspond to the emplacement of the younger plutonic rocks during the early paleozoic northward subduction of proto tethyan oceanic lithosphere 426 ma and 455 ma detrital zircon populations that originated from synkinematic granitoids dominate quartz mica schist negative to positive hf t values for both 940 850 ma granitic orthogneisses and intruding 460 410 ma granites suggest a crustal mantle mixture source hf t dm1 model ages and inherited zircon u pb ages are the same as those for garnet bearing amphibolites and psammitic gneiss indicating that late magmatic rocks were formed by melting of the accreted neoproterozoic rocks

neoproterozoic to early cambrian franciscan type melanges in the tepla barrandian unit bohemian massif evidence of modern style accretionary processes along the cadomian active margin of gondwana abstract melanges have been reported from a variety of modern tectonic settings and are a hallmark of the franciscan type subduction accretion complexes in the precambrian however well documented examples of melanges become extremely scarce this paper describes in detail neoproterozoic to early cambrian melanges recently recognized in the cadomian basement of the tepla barrandian unit bohemian massif the melanges are of sedimentary and tectonic origin and include voluminous successions of graywackes interpreted to represent mass transport deposits presumably along the upper slope of an accretionary wedge the clast compositions indicate multiple episodes of mixing of terrigenous arc derived and deep water pelagic material moreover abundant bodies of meta basalts interpreted as dismembered seamounts or pillow volcanoes were emplaced tectonically into various structural levels of this sedimentary melange by two different mechanisms at an upper level the irregular to elongated bodies of basalts were offscraped only weakly strained and incorporated in the sedimentary matrix that exhibits steep cleavage indicating horizontal pure shear shortening and oblate strain in contrast at a lower level disc shaped and closely spaced basalt bodies are ductilely sheared and reworked into a flat lying prolate to plane strain fabric suggesting horizontal ductile flow presumably along the base of the accretionary wedge during flat slab oceanic subduction our new u pb ages also reveal that the melange formation continued until the early cambrian

loading paths to joint propagation during a tectonic cycle an example from the appalachian plateau u s a abstract based on the timing of joint propagation during the history of burial lithification deformation and denudation of clastic rocks within sedimentary basins four types of joints may be distinguished tectonic hydraulic unloading and release tectonic and hydraulic joints form at depth prior to uplift in response to abnormal fluid pressures whereas unloading and release joints form near the surface in response to thermal elastic contraction accompanying erosion and uplift tectonic joints are distinguished from hydraulic joints in that tectonic compaction is a mechanism for achieving abnormal pore pressures leading to the propagation of the former whereas compaction by overburden loading leads to the abnormal pore pressures in the latter case the orientation of unloading joints is controlled by either a residual or contemporary tectonic stress whereas the orientation of release joints is controlled by a rock fabric examples of some of these joints are found within the devonian catskill delta of the appalachian plateau new york during the alleghanian orogeny tectonic joints cross fold joints formed under abnormal pore pressure as indicated by the observation that joints propagated in the siltstones before they developed in shales and by the cross cutting relationships of folds cleavage and joints this sequence is compatible with oil company hydraulic fracture data which show that the least principal stress within sandstone layers is less than that in the intercalated shale layers plumose structures indicate that the joints within siltstones propagated as discontinuous rupture events each of which affected less than a meter of bed length the discontinuous rupturing is compatible with models for natural hydraulic fracturing release joints strike joints post date the alleghanian orogeny as indicated by abutting relationships within the deeper parts of the devonian clastic section unloading joints are orthogonal to the contemporary tectonic stress field

evidences for a noachian hesperian orogeny in mars a number of tectonic structures have been located at the thaumasia plateau daedalia planum and aonia terra mars they include isolated folds with axial traces up to 200 km long trains of tightly folded structures tens of km long and thrusts their size and geometry are similar to those on earth and the direction of compression seems to have varied amply with time suggesting a complex tectonic evolution crater counts on the deformed terrain point to noachian to early hesperian ages on the basis of the geometry and geological relations of these structures we propose that they form part of an old martian orogen the thaumasia aonia orogen which embraced not only the thaumasia plateau but areas of daedalia planum aonia terra and nereidum montes as well a regional coherent layering is previous to the deformation and could represent the trace of even older stresses on the martian lithosphere

modes of superposed buckling in single layers controlled by initial tightness of early folds in a series of experiments with soft test models superposed buckling folds were produced in a competent layer resting on a slab of incompetent painter s putty the angle between the hinge lines of the first set of cylindrical folds f1 and the direction of the second compression p2 was varied in the different experiments the experiments showed that with an increase in the initial tightness of f1 there was a transition from one mode of superposed buckling to another when the interlimb angle of f1 was very large the superposed deformation gave rise to a dome and basin pattern the first mode of superposed buckling the second mode with small f2 folds riding over larger f1 folds developed when the initial interlimb angle ranged roughly between 135 and 90 the third mode was observed when the f1 interlimb angle was less than about 90 but the fold was not very tight in the third mode a set of non plane non cylindrical folds developed however the sinuous hinge line of the non cylindrical structure was newly created by replacing the old f1 hinge when f1 was very tight or isoclinal the fourth mode of superposed buckling led to the development of non plane non cylindrical folds without concomitant hinge replacement

early paleozoic to devonian multiple accretionary model for the qilian shan nw china abstract the qilian shan located in the northern tibetan plateau of nw china has an excellent record of early to middle paleozoic subduction accretion that resulted from convergence between the alax and qaidam blocks but there is no consensus about its detailed tectonic history this paper summarizes its tectonic divisions and discusses its tectonic evolution from the cambrian to devonian the belt has the following tectonic divisions in the far north the southern passive margin of the alax block is juxtaposed against an early cambrian to ordovician marianan type intra oceanic arc north qilian the north qilian arc is separated from a japanese type arc central qilian to the south by a high pressure metamorphic belt composed mostly of ophiolitic slices and oceanic crusts the central qilian is bounded to the south by a wide melange zone south qilian consisting of ophiolitic slices and continental margin sequences farther to the south lies the oulongbuluk microcontinent that is separated from the qaidam block and farther south by an ultrahigh pressure metamorphic belt tectonostratigraphic analysis together with geochemical geochronological and geophysical data indicates a complex evolution by subduction accretion processes from the cambrian to the devonian before final amalgamation and docking to the northern alax block this model solves the long lasting discussion on the polarity of subduction in paleozoic time this multiple subduction accretion history sheds light on the continuity of paleozoic sutures along the qilian shan and the nature of the altyn tagh fault and thus contributes to an improved understanding of the tectonic architecture of central asia

spatial coincidence of rapid inferred erosion with young metamorphic massifs in the himalayas a spatially distributed rate of erosion index ei based on models of bedrock river incision documents a strong spatial correspondence between areas of high erosion potential and young metamorphic massifs as well as structural highs throughout the himalayas the ei is derived from slopes and drainage areas calculated from a hydrologically cor rected digital elevation model gtopo30 combined with precipitation data iiasa to generate synthetic annual stream discharges these variables drive three generalized pro cess models to produce ei maps that while differing in detail provide an internally con sistent spatially continuous index of large scale erosion rates the large spatial variation in potential erosion rates in the himalayas suggested by the ei patterns contrasts with the uniform convergence of the indian subcontinent if these ei gradients persist through time they support the emerging view of a positive feedback between localized rapid ero sion and upward advection of lower crust

erosion controlled geometry of buckle fold interference refolding structures in the aliaga area iberian chain spain show how erosion can exert a control on the geometry of buckle fold interference early miocene ene trending buckle folds are superposed on a large nnw sse anticline eocene oligocene north of aliaga the earlier hinge zone in the competent limestone unit controlling buckling urgon facies lower cretaceous constitutes a mechanical obstacle to refolding which induces development of essentially conical type 2 interference to the south erosional removal of the hinge zone at that competent unit allowed the near vertical eastern limb to behave as independent layers being refolded into cylindrical near vertical axis folds showing a snake like map pattern lower miocene conglomerates containing pebbles of jurassic origin whose source area was the core of the earlier nnw sse trending anticline indicate that this jurassic core had been effectively exhumed before refolding occurred in this way snake like folds constitute a new case of erosion controlled tectonic structure

the influence of initial fold geometry on type 1 and type 2 interference patterns an experimental approach abstract experiments on superposed folding were performed in a plane strain pure shear rig using paraffin wax as an analogue for rocks a series of pre formed cylindrical folds were refolded with the compression direction acting parallel to the initial fold hinge direction the intermediate axis perpendicular to the first fold axial plane and the extension direction parallel to the first fold axial plane and perpendicular to its axis type 1 interference geometry roundness tightness and amplitude were varied to investigate the influence of first fold geometry on the interference patterns experimental folding of an initially planar layer oriented parallel to the first fold envelope provides a reference geometry for comparison the results suggest that hinge roundness has less influence on the interference pattern than other factors close initial folds refold into type 2 interference patterns which become even more pronounced as the tightness increases open folds produce type 1 interference patterns folds with the same interlimb angle and the same roundness produce different interference patterns depending on relative amplitude folds with relatively large amplitude are refolded into type 2 patterns whereas folds with small amplitude give clear dome and basin structures in a second set of experiments the compression direction also acted parallel to the first fold hinge but the orientations of the intermediate and extension axes were interchanged the interference patterns obtained are very similar to those in type 1 interference geometry suggesting that the major factor in determining whether type 1 or type 2 interference patterns develop is the initial fold geometry and not the kinematics of the second deformation

erosional control on the structural evolution of a transpressional thrust complex on the alpine fault new zealand abstract the waikukupa thrust is a 4 km long oblique thrust segment of the new zealand alpine fault that has developed over the last 65 000 years emplacing mylonite and cataclasite over fluvioglacial gravel during the last 20 000 years or so a 2 km long out of sequence imbricate the hare mare thrust has formed on the east side of the waikukupa river valley the average rate of slip on the two thrusts is estimated at 22 30 mm year analysis of the fault structure in terms of critical wedge theory is consistent with imbrication resulting from a reduction in wedge taper below the critical value due to rapid river erosion the internal structure of the waikukupa thrust sheet consists of reverse faults and angular folds forming duplex like systems associated with minor strike slip faults within the limits of uncertainty the structures are consistent with strain accumulating during internal transpressional shear of the thrust sheet while the taper of the latter was subcritical prior to the transfer of fault displacement to the hare mare imbricate the thrust complex forms part of a serrated range front fault system consisting of oblique thrust sections linked by strike slip faults we introduce the term serial partitioning for this type of partitioned transpressional fault system its development is linked to the erosional processes at the range front an alternative system consisting of parallel thrust and strike slip faults we here term parallel partitioning while angle of obliquity is an important parameter in determining which partitioning model develops we suggest that erosion rate is also an influential factor

multiple phases of deformation in the southern helanshan tectonic belt northern china abstract the southern helanshan tectonic belt shtb is surrounded by the alxa massif the ordos basin and the tibetan plateau and its phanerozoic structures records crucial phanerozoic deformation characteristics for studying the multi phased intracontinental deformation processes among these neighboring units combined with regional geology our structural analysis reveals that the shtb has experienced five phases of shortening deformation during the phanerozoic the first phase d1 is represented by e w trending overturned folds in the cambrian and ordovician strata these folds resulted from two episodes of n s shortening during the late cambrian early ordovician and late ordovician early devonian respectively which were attributed to the closure of the qilian ocean the second phase d2 is characterized by e w trending open folds involving the upper devonian strata which was the products of n s shortening caused by the assembly of the qilian ocean and the north china craton ncc at the end of devonian the third phase d3 is indicated by some wnw trending jura type folds in the carboniferous strata formed by nne ssw shortening during the late triassic early jurassic related to the collision between the ncc and the south china block the fourth phase d4 is inferred by nne trending shortening structures in the jurassic and its underlying strata generated by wnw ese shortening in the late jurassic echoing the paleo pacific subduction the fifth stage d5 is characterized by the ne sw shortening in the late miocene leading to the formation of many nw se striking thrust faults in the miocene and its underlying strata because the outward expansion of the tibetan plateau

landscape dissection isostatic uplift and the morphologic development of orogens we examine the morphologic factors that determine the isostatic response to landscape dissection in orogenic terrains in order to quantify the relative contributions of active tectonic uplift and passive isostatic uplift in generating mountain topography we demonstrate that although peaks many times the elevation of an assumed initial plateau can theoretically be generated only moderate amounts of peak uplift can be explained by isostatic rebound in real terrains analysis of topographic data from the european alps and other orogens indicates that isostatic uplift in response to dissection can account for about one fourth of the elevation of the highest peaks if local isostatic compensation is assumed and less if the lithosphere has significant flexural rigidity we conclude that although isostatic uplift can be significant in orogens high peaks are predominantly a consequence of tectonic processes in convergent settings

in situ u pb zircon dating using laser ablation multi ion counting icp ms high resolution in situ u pb zircon geochronology on zoned grains can obtain isotope signatures from multiple growth or thermal events we present a method using laser ablation multicollector inductively coupled plasma mass spectrometry la mc icp ms to overcome complications associated with intricately zoned zircon crystals through in situ sampling of zircon volumes as small as 12 m 25 m and 40 m in diameter by about 10 m in depth high precision u pb age of a series of zircon standard covering a wide age range of 30 to 1 065 ma was acquired using la mc icp ms the precision of measured pb u ratios in homogeneous zircon is about 2 2 resulting in routinely achieved precision of u pb ages obtained by external calibration of 1 2 or better all masses of interest can be simultaneously recorded with a multi ion counting system mic operating in static mode and the short ablation required to achieve such precision results in spatial resolution that is superior to comparable u pb zircon analyses by single collector icp ms the resulting present u pb age for five zircon reference samples and two geological samples show an excellent agreement with the previously reported id tims or shrimp data

a cold early palaeozoic subduction zone in the north qilian mountains nw china petrological and u pb geochronological constraints the north qilian high pressure hp low temperature lt metamorphic belt is composed mainly of blueschists eclogites and greenschist facies rocks it formed within an early palaeozoic accretionary wedge associated with the subduction of the oceanic crust and is considered to be one of the best preserved hp lt metamorphic belts in china here we report new lawsonite bearing eclogites and eclogitic rocks enclosed within epidote blueschists in the north qilian mountains five samples contain unaltered lawsonite coexisting with omphacite and phengite as inclusions in garnet indicating eclogite facies garnet growth and lawsonite pseudomorphs were observed in garnet from an additional 11 eclogites and eclogitic rocks peak pressure conditions estimated from lawsonite omphacite phengite garnet assemblages were 2 1 2 4 gpa at temperatures of 420 510 c in or near the stability field of lawsonite eclogite and implying formation under an apparent geothermal gradient of 6 8 c km 1 consistent with metamorphism in a cold subduction zone shrimp u pb dating of zircon from two lawsonite bearing eclogitic metabasites yields ages of 489 7 ma and 477 16 ma respectively cl images and mineral inclusions in zircon grains indicate that these ages reflect an eclogite facies metamorphism an age of 502 16 ma is recorded in igneous cores of zircon grains from one lawsonite pseudomorph bearing eclogite which is in agreement with the formation age of early ordovician for some ophiolite sequences in the north qilian mountains and may be associated with a period of oceanic crust formation the petrological and chronological data demonstrate the existence of a cold early palaeozoic subduction zone in the north qilian mountains

superposed buckle folding in the eastern iberian chain spain abstract the aliaga area eastern iberian chain shows large scale examples of buckle superposition developed during tertiary folding in most cases ene trending folds overprint earlier nnw sse trending ones the resulting structures are mapped analysed and genetically classified by comparison with analogue models described by several authors the following types are found standard type 1 1a dome and basin structure 1b unequal wavelength overprinted folds modified type 1 1c t shaped joined folds 1d t shaped abutting folds 1e l shaped folds 1f snake like folds standard type 2 2a non cylindrical buckling of earlier axial surfaces involving hinge replacement different superposed sets of flexural slip striations record successive folding episodes in snake like folds and hinge replacement in the case of type 2a superpositions types 1 and 2 apparently develop where the earlier folds have interlimb angles over and below 90 respectively which fits the results of analogue modelling and theoretical analysis by previous authors types 1b and 1d are associated with higher w1 w2 wavelength ratios than types 1a and 1c other controlling factors are viscosity contrast and erosion processes specifically erosion of competent limestone beds in the hinge zone of a nnw sse trending anticline allowed the near vertical eastern limb to be refolded into snake like folds