a petrologic re investigation of the adak volcanic center central aleutian arc alaska the adak volcanic center is located in the central part of the aleutian arc and consists of three main volcanic vents andrew bay volcano the oldest center has been mostly removed by erosion the next youngest vent mount adagdak was built in three major volcanic stages whereas mount moffett the largest volcanic edifice consists of a main cone and a parasitic cone each with several magmatic phases adak is unique compared to other modern aleutian volcanic centers in that it contains two xenolith suites conrad and kay 1984 debari et al 1987 one suite consisting predominantly of mafic xenoliths occurs on mount moffett whereas an assemblage of ultramafic and mafic xenoliths is found on mount adagdak lavas erupted at adak span the compositional range from 48 4 to 65 0 wt sio2 and are characterized by significant variations in al2o3 mgo sr ni and cr on harker diagrams this variability produces compositional trends with significant scatter the adak suite has total ree contents that vary from 32 to 154 ppm but do not correlate systematically with silica layb n ratios range from 2 41 to 21 72 with the majority of lavas between 2 41 and 6 06 on process identification diagrams the adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other aleutian centers measured isotopic ranges are large and nearly equal to those for the entire arc although they span similar silica ranges subtle geochemical and isotopic differences distinguish the different volcanic vents of adak on mount moffett a geochemically and isotopically distinct group of andesites 55 5 57 9 sio2 the mafic andesites occur on its ne flank these lavas have elevated mgo ni and cr but are depleted in al2o3 relative to other mount moffett andesites with similar silica they also have more heterogeneous ree abundances and isotopic ratios than most of the other andesites significant compositional differences exist between adak and the other volcanic centers of the central aleutian arc although these differences are characteristic of all geochemical systems they are greatest for major and rare earth elements and isotopic ratios the lack of coherent relationships on major and trace element harker diagrams the isotopic variability as well as the steeply sloping trends on ree process identification diagrams suggest that the adak volcanic suite was not formed predominantly by closed system crystal fractionation but must be the product of a complex open system process es the significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite ree data are also suggestive of a role for magma mixing such a complex petrologic evolution is consistent with an immature lithospheric plumbing system based on ree systematics the xenolith suites of adak cannot as previously proposed be related to the host lavas or the rest of the adak suite through crystal fractionation schemes rather they are probably accidental fragments derived from various depths along lithospheric conduits in light of their relation to xenolith bearing units the mafic andesites of adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock they are therefore characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas because of the close proximity of the vents and the subtle compositional differences between their lavas the adak volcanic center was probably supplied by a single deep lithospheric plumbing system that fed separate crustal magma chambers the absence of historic volcanic activity on adak suggests this plumbing system was abandoned before complete conduit development this decline in magmatism may reflect a re adjustment of volcano spacing within this part of the aleutian arc

tectonic history of sumba island indonesia since the late cretaceous and its rapid escape into the forearc in the miocene abstract the island of sumba lies within the sunda banda forearc and contains volcanic plutonic and volcaniclastic rocks that record a history of arc volcanism extending from late cretaceous 80 ma or earlier to oligocene 31 ma time during the late cretaceous sumba formed part of a great indonesian volcanic arc system near southeastern eurasia between late cretaceous and early miocene time sumba remained part of that arc system which ceased to be volcanically active by 31 ma during the late miocene 18 ma sumba as part of the relict arc system was situated near the present site of alor and wetar at 16 ma sumba was torn from the relict arc and began to move at a velocity of 50 mm yr in a west south westerly direction volcanism along the modern banda arc soon followed by 7 ma sumba finally came to rest after having moved 450 km into the forearc we interpret the rapid motion of sumba into the forearc as a discrete block to have been a response to the collision between australia and indonesia in the area of timor the savu basin possibly originated as an arc crestal rift which then opened behind sumba as the latter moved westward over the past 16 my more than 3 km of sediment have been deposited in the savu basin by erosion from the surrounding islands sumba stopped moving to the west south west at 7 ma after reaching approximately its present location since then the island has ceased to be a discrete tectonic fragment and has behaved as an integral part of the forearc during the past 7 my compressional tectonics within the forearc has led to elevation of the rocks exposed on sumba by as much as 4 km

h2o transport and release in subduction zones experimental constraints on basaltic and andesitic systems phase relationships in natural andesitic and synthetic basaltic systems were experimentally investigated from 2 2 to 7 7 gpa and 550 c to 950 c in the presence of an aqueous fluid in order to determine the stability of hydrous phases in natural subducted crustal material and to constrain reactions resulting in the release of water from subduction zones to the mantle wedge water reservoirs in subducted oceanic crust at depths exceeding the amphibole stability field 70 80 km are lawsonite 11 wt h2o mg chloritoid 8 wt talc 5 wt and zoisite clinozoisite 2 wt in basaltic rocks and lawsonite zoisite clinozoisite phengite 4 wt and staurolite 2 wt in andesitic compositions the thermal stability of lawsonite at 6 0 gpa extends to 800 c and 870 c in basaltic and andesitic compositions respectively at pressures above amphibole out 2 3 2 5 gpa lawsonite reacts through continuous reactions with steep positive dp dt slopes to zoisite clinozoisite until 3 0 3 2 gpa and at higher pressures to more than 7 7 gpa to assemblages containing garnet clinopyroxene and garnet clinopyroxene kyanite in basaltic and andesitic compositions respectively on the contrary the breakdown of zoisite clinozoisite is mainly pressure sensitive phengite represents the hydrous phase with the largest stability field encountered in this study in andesite phengite is stable to more than 7 7 gpa and more than 920 c talc and staurolite contribute in minor amounts to the water balance in basaltic and andesitic rock compositions a model for water release from the subducted slab is developed combining thermal models for subduction zones with the experimentally determined phase relationships up to 1 wt and 2 wt h2o in basaltic and andesitic rocks respectively can be stored to depths beyond 200 km in cold subduction zones mainly by lawsonite and phengite dehydration rates are high until amphibole out and relatively low at greater depths the amphibole out reactions are found to release a significant amount of water in a depth interval of several kilometers however they do not represent a discrete pulse of fluid and do not completely dehydrate the descending slab fluid release at depths greater than 200 km through phengite and progressive lawsonite breakdown would hydrate the overlying mantle causing the generation of amphibole or phlogopite peridotite at higher geothermal gradients epidote zoisite contributes to fluid flux to the mantle wedge at 100 120 km depth the extensive stability field of phengite may greatly enhance the role of sediments and the small amount of potassium in mafic compositions for the fluid budget in subduction zones at increasing depth

geochemical and strontium isotopic characteristics of parental aleutian arc magmas evidence from the basaltic lavas of atka eighteen flows from a basal stratigraphic sequence on the aleutian island of atka were analyzed for major elements trace elements and initial 87sr 86sr ratios petrographically these lavas contain abundant plagioclase 24 45 and lesser amounts of olivine 7 magnetite and clinopyroxene phenocrysts compositionally the lavas are high alumina 20wt basalts 48 51 wt sio2 with low tio2 1 and mgo 5 within the section compositional variations for all major elements are quite small while mgo content correlates with olivine phenocryst contents no such relationship exists between the other oxides and phenocryst content these lavas are characterized by 8 10 ppm rb high sr 610 669 ppm 308 348 ppm ba and very constant zr 23 29 ppm and sc 23 29 ppm abundances ni and cr display extremely large compositional ranges 12 118 ppm and 12 213 ppm respectively no correlation exists between trace element concentrations and phenocryst contents strontium isotopic ratios show a small but significant range 0 70314 0 70345 and are slightly elevated with respect to typical morb no systematic correlation between stratigraphic position and petrography or geochemistry is evident ree abundances measured on six samples are lree enriched la yb n 2 20 2 81 and display similar chondrite normalized patterns one sample has a slight positive eu anomaly but the other lavas do not compared to other aleutian basalts of similar silica content these lavas are less lree enriched and have lower overall abundances the geochemical characteristics of these basalts suggest they represent true liquid compositions despite their highly porphyritic nature published phase relations indicate fractionation of a more mgo rich magma could not have produced these lavas the high al2o3 and low mgo and compatible element abundances suggest a predominantly oceanic crustal source for parental high alumina basalts

pseudo static lateral earth pressures on broken back retaining walls displacement of retaining walls during earthquakes causes damage to the structures founded on their backfill the displacement of the wall can be reduced by decreasing the lateral earth pressure applied on its back this can be achieved in a broken back wall as the size of the failure wedge formed behind the wall is reduced therefore the calcula tion of lateral earth pressures is essential in assessing the safety of and designing broken back retaining walls in this study a series of reduced scale shaking table model experiments were performed on broken back quay walls composed of concrete blocks with two different rear face shapes in comparison with a vertical back wall earth pressures increased at the upper forward i e seaward leaning rear face segments of the wall whereas they decreased at lower backward i e landward leaning elevations because of the wide application of the pseudo static method of mononobe okabe in engi neering practice and design codes lateral earth pressures have also been estimated using this approach the comparison between the measured lateral earth pressures and those calculated using the mononobe okabe method shows fairly good agreement in predicting the overall distribution of lateral active earth pressure during and after the shaking resume le deplacement des murs de soutenement durant un seisme cause des dommages aux structures ayant des fonda tions installees sur le remblai de ces murs de soutenement le deplacement du mur peut etre reduit en diminuant la pres sion laterale des terres appliquees au dos du mur cette diminution peut etre accomplie par l utilisation d un mur a dos brise puisque la zone de fracture formee derriere le mur est reduite ainsi il devient essentiel de calculer les pressions la terales des terres pour evaluer la securiteet pour concevoir des murs de soutenement a dos brise dans cette etude des es sais al echelle reduite a l aide d un modele de table asecousses sont effectues sur des murs a dos brisede quais faits de blocs de beton avec deux formes de dos differentes comparativement aun mur ados vertical la pression des terres aug mente dans les sections de la face arriere du mur orientees vers le devant et le haut c a d vers la mer tandis que la pression des terres diminue dans les sections orientees vers le derriere et vers le bas c a d vers le sol les pressions late rales des terres sont aussi estimees al aide de l approche pseudo statique de mononobe okabe puisque cette approche est largement utilisee dans l ingenierie et la conception la comparaison entre les pressions laterales des terres mesurees et celles calculees a l aide de la methode mononobe okabe montre la concordance des resultats apredire la distribution glo bale des pressions laterales actives des terres pendant et apres les secousses mots cles methode mononobe okabe mur de soutenement pression laterale des terres seisme essai sur table a secous ses mur a dos brise traduit par la redaction

open shear fractures at depth in anticlines insights from the kuqa foreland thrust belt tarim basin shear fractures that close under compressive stress regime were described by previous workers here we show the existence of hinge parallel shear opening mode fractures at a certain depth within an anticline structure through core observations formation micro imager fmi log interpretations and production data studied reservoir is a tight sandstone body of cretaceous age which is emplaced in ks2 anticline with depth ranging between 6000 and 8100 m we explore the mechanism of hinge parallel shear fractures by stereographic technique and backtilting as well as show field evidences for opening displacements these shear fractures mainly consist of vertical tension set moderate dipping shear set steeply dipping shear set and sub horizontal shear set crosscutting relationships and surface textures suggest that although the moderate dipping shear set is older and maybe sealed by cements it may have been opened by local stress due to outer arc extension through folding calcite bridges with rhombohedral structures may support fracture walls helping them to remain open in case of compressive stress our work documents shear fracture openings in the anticline at depth and fluid flow in subsurface reservoirs

cenozoic deformation of the tarim basin and surrounding ranges xinjiang china a regional overview abstract with its central position between the tibetan plateau and the tian shan range the tarim basin is a key element of the cenozoic asian orogenic system however a comprehensive regional study and more particularly the quantification of shortening through this basin and its margins are still needed to understand its role in the cenozoic deformation of asia from a compilation of previous works together with an extensive dataset of satellite field seismic and well data we provide a tectonic map of the cenozoic structures and four balanced geological transects of the tarim basin and its surrounding ranges based on this map and these cross sections we characterize the cenozoic deformation of the original proterozoic tarim block from structural restorations and crustal budgets we also quantify the compressive component of this deformation most of the cenozoic compressive deformation from 94 to 100 is concentrated in the ranges along the block margins to the west up to 78 23 km and 54 24 18 km of crustal shortening are accommodated across the compressive western kunlun and southwestern tian shan ranges while to the east up to 38 6 18 km and 15 20 15 km are accommodated across the transpressive altyn tagh and southeastern tian shan ranges a non negligible amount of compressive deformation up to 6 is also accommodated within the tarim basin by large basement cored uplifts with a vergence synthetic to the deformation of the tibetan plateau edge to the west the bachu uplift absorbs 5 km of the total crustal shortening of the western kunlun thrust system while to the south the tanan uplift accumulates 0 6 km of the altyn tagh strike slip system structural inheritance has a major influence on the cenozoic deformation since 33 3 to 100 of the total shortening is accommodated by reactivated basement structures inherited from the protero paleozoic history of the tarim block finally we argue that the basement cored uplifts in the centre of the basin imply a deformation transfer from the tibetan plateau to the tian shan above a deep crustal decollement decoupling the deforming crust from an underlying rigid mantle

magmatic evolution of semisopochnoi island alaska trace element and isotopic constraints semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas sio 2 50 65 zr 64 250 ppm la 10 60x chondrites whose bulk rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite trace element data however are not consistent with crystal fractionation operating alone instead a variety of plots involving incompatible elements or ratios among them demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace element variations and must have been operative as well we are unable to distinguish between a silicic end member represented by residual dacitic liquid like one of the analyzed lavas therefore implying magma mixing and one consisting of pre existing silicic crustal rocks therefore implying assimilation variations in sr and nd isotopic compositions are very limited 143 nd 144 nd 0 51225 0

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

petrologic constraints on the spatial distribution of crustal magma chambers atka volcanic center central aleutian arc the atka volcanic center the largest modern central aleutian magmatic complex covers 360 km2 with an estimated eruptive volume of 200 km3 the oldest exposed rocks the old harbor series are a suite of basaltic andesite to dacite flows pyroclastic rocks and lahars of presumed tertiary age the youngest phase of activity on atka probably began 1 2 ma ago near the center of the present volcanic field and consists of basaltic and basaltic andesite flows which produced a large shield volcano atka volcano subsequent to this activity satellite vents formed along the margin of this structure collapse of the ancestral atka volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow big pink after caldera formation four major volcanic cones korovin mount kliuchef sarichef and konia grew around the margins of atka caldera this study focuses on the atka volcano korovin and mount kliuchef volcanic centers atka lavas range from basalt through dacite with all major volcanic vents erupting lavas spanning similar compositional ranges they are porphyritic with phenocrysts of plagioclase 25 36 olivine 0 4 3 6 or orthopyroxene 2 8 clinopyroxene 1 12 and a single fe ti oxide 0 2 hydrous phases are lacking even in the most evolved lavas two generations of plagioclase occur in the korovin suite but not in lavas from mount kliuchef korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of mount kliuchef lavas or very rare similarly clinopyroxene in korovin has reaction rims and abundant glass inclusions which are absent in the mount kliuchef lavas lavas erupted in the atka volcanic field vary from 47 to 66 sio2 although the complex is dominated volumetrically by basaltic lavas on major element harker diagrams mafic and intermediate atka volcano lavas form linear trends but the most siliceous lavas do not fall along these trends by contrast mount kliuchef lavas are compositionally bimodal whereas korovin samples range from 51 to 62 wt silica with intermediate lavas clustering around 55 sio2 incompatible trace element abundances of korovin lavas do not increase with silica content but those of atka volcano and mount kliuchef do on incompatible incompatible element diagrams the three suites behave very differently differences also exist between the ree characteristics of the three atka suites the petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes the characteristics of the atka volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage incompatible incompatible element trends suggest the mafic mount kliuchef lavas were also produced by crystal fractionation a conclusion supported by quantitative mass balance calculations the difference in slope between the atka volcano and mount kliuchef suites indicates however that there were significant differences in these fractionation schemes modeling as well as geochemical characteristics suggest that the mount kliuchef dacites are not the product of crystal fractionation they may represent 1 crustal melts 2 complete melts of siliceous lava produced during earlier magmatic stages or 3 residual liquids from the atka volcano magmatic system in contrast to these suites the korovin lavas could not have been formed by crystal fractionation rather the andesites are probably the result of mixing basaltic and dacitic magmas the atka data suggest 1 large volcanic centers may be supplied by a series of non communicating crustal magma chambers 2 local factors may be important in determining liquid lines of descent and 3 very different magmatic processes may occur in adjacent magma chambers if the complexity of the atka suite is a common feature of arc magmatic plumbing systems reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally

modeling of crystal size distributions csds in sills abstract a new model for the generation of crystal size distributions csds in igneous rocks is presented here synthetic or numerically simulated csds are generated with a growth rate that is proportional to the amount of precipitating solids and inversely related to the second moment of the csd total surface area and with the log nucleation rate i vs log cooling rate relationship of cashman cashman k v 1993 relationship between plagioclase crystallization and cooling rate in basaltic melts contrib mineral petrol v 113 pp 126 142 for crystal nucleation the resultant csds resemble those observed in natural rocks in the new model growth rate is constrained by a mass balance and crystal population systematics it is not calculated as a function of cooling rate or undercooling the development of the numerical model was motivated in part by the failure of analytical modeling of crystal populations based solely on cooling rate to generate csds similar to those observed naturally the new model is used to create a suite of csds from various positions within a sill cooling and solidification of the sill are calculated numerically the model reproduces many features observed in the csds of natural rocks such as linear csds in plots of ln population density vs crystal size d shaped mean crystal size profiles and decreasing csd intercept and slope magnitude i e slope with distance from the sill wallrock contact and the csd intercept vs slope relationship the model suggests the use of inversion to more accurately determine residence time from a natural csd

evidence for fractionation of quaternary basalts on st paul island alaska with implications for the development of shallow magma chambers beneath bering sea volcanoes abstract st paul island is the youngest volcanic center in the bearing sea basalt province we have undertaken a field petrographic and geochemical study of select st paul volcanic rocks in order to better understand their differentiation specifically to test the hypothesis that magmas erupted from individual bering sea basaltic volcanoes are not related by shallow level processes such as crystal fractionation petrographically all of the st paul volcanic rocks are olivine plagioclase and clinopyroxene phyric textural features and modal contents of olivine phenocrysts however vary widely throughout the spectrum of basalt compositions although differing in size and abundance olivine phenocrysts in all rock compositions are euhedral and commonly skeletal suggesting rapid growth during ascent or eruptive quenching none however display reaction textures with surrounding groundmass liquid compositionally the st paul volcanic rocks are basalts and tephritic basalts and all have high contents of normative nepheline 8 to 16 concentrations of many major and incompatible trace elements display no clear correlations with bulk rock sio2 and mgo contents or modal abundances of phenocrysts suggesting that much of the compositional diversity of these magmas reflects variable mantle sources and degrees of partial melting similarly chondrite normalized ree patterns show variable degrees of light ree enrichment lan 70 90 that do not correlate with bulk rock mg numbers in contrast concentrations of compatible trace elements ni cr and co are positively correlated with mgo contents and modal percentages of olivine phenocrysts maximum forsterite contents of olivine phenocryst cores in most st paul rocks decrease with decreasing bulk rock mg number and are similar to the calculated equilibrium range this is evidence that the high mg numbers are magmatic and do not result from olivine accumulation instead major and compatible trace element mass balance calculations support derivation of the low mg number lavas from the high mg number lavas mainly by olivine fractionation which in turn implies that st paul magmas may have temporarily resided in crustal magma chambers prior to eruption

magmatism of the westernmost komandorsky segment of the aleutian island arc abstract four magmatic associations are distinguished in the westernmost komandorsky segment of the aleutian island arc basalt plagiorhyolite 45 28 ma trachybasalt teschenite 27 16 ma diorite granitoid 12 5 8 5 ma and dacite rhyolite 0 7 0 ma the latter association is represented by the lavas of a recently discovered active submarine volcano piip volcano about 40 km north of the komandorsky islands and 700 km west of the quaternary center on buldir is detailed petrogeochemical data including ree and 143 nd 144 nd and 87 sr 86 sr isotopic ratios show that the parental melts of the komandorsky is were of mantle origin and their sources remained isotopically homogeneous during the whole geological history of the aleutian arc 10be concentrations in lavas of the piip volcano 2 7 106 at g indicate subduction related incorporation of young pelagic sediments in their magmatic sources or contamination of ascending melts by non solidified sedimentary material directly beneath the volcano the komandorsky segment of the aleutian arc evolved from a depleted arc tholeiitic series through a markedly alkaline series to later calc alkaline andesites and dacites and finally to late stage calc alkaline dacites and rhyodacites erupted inboard of the older volcanic centers

the influence of water on the petrogenesis of subductionrelated igneous rocks the presence of dissolved water can significantly change the sequence in which different minerals crystallize from a silicate magma1 3 this will alter the compositional path followed by residual liquids the liquid line of descent and modify the types and compositions of crystals that accumulate at the site of cooling the sharp decrease in the solubility of water in silicate melts with decreasing pressure makes direct measurements of pre eruptive concentrations in magmas difficult because much of the dissolved water boils off before eruption here we report experimental results on the crystallization of basalts that contain substantial dissolved water up to 6 wt and show how the results can be used to infer the amount of dissolved h2o involved in the petrogenesis of lavas that preserve a record of their liquid lines of descent or the accumulated minerals left behind after solidification the presence of abundant magmatic water is a signature of subduction zone volcanism4 and can be used to associate a suite of magmas or igneous cumulates with a convergent margin back arc or interarc tectonic setting

assimilation of crustal material by basaltic magma strontium isotopic and trace element data from the edgecumbe volcanic field se alaska the edgecumbe volcanics which range from basalt through rhyodacite have sr contents between 125 and 370 p p m rb contents of 1 to 70 p p m ba ranging from 50 550 p p m and initial l7sr sr ratios between 0 70291 and 0 70404 no simple correlation exists between these components and silica the highest sr sr values occur in a group of intermediate lavas 55 60 wt per cent sio2 while the rhyodacites have initial ratios between 0 7035 and 0 7038 with increasing silica sr increases to a maximum in the andesites and then steadily decreases ba and rb increase over the same compositional range the highest l7sr 86sr lavas have major and trace element concentrations which depart from trends defined by most of the lavas the variation in strontium isotopic compositions suggests interaction between parental basaltic magma and crustal material attempts to model the assimilation process using fixed end member assimilation and assimilation fractional crystallization models have failed to produce the observed chemical trends because the parental basaltic liquid underwent little fractionation the variability in hybrid lavas is attributed to variation in contaminant composition initial melts were low in cao al2oj mgo and sr and enriched in sio2 k2o na2o rb and ba as melting progressed melts became enriched in the more refractory components because hybrid strontium isotopic composition is a function of sr concentration as well as isotopic compositions the sr content of the assimilant strongly influences resultant isotopic systematics the development of the assimilants suggests plagioclase was a residual phase during early melting this model of crustal assimilation represents one end member in the spectrum of processes responsible for the generation of continental volcanic suites

numerical simulation and shaking table test of geotextile bag retaining wall structure this study analyzed the dynamic behavior of flexible retaining walls constructed with geotextile bags the behavior of geotextile bag retaining wall system was simulated using a large scale shaking table in which 1 g n of one directional dynamic load was applied the dynamic behavior of the retaining wall was analyzed with simulated ground consisting of a geotextile bag in which 0 154 0 44 g n of ground acceleration was applied using historic and artificial seismic waves by el centro and hachinohe dynamic characteristics of the geotextile bag retaining wall structure in response to acceleration displacement and earth pressure were also analyzed based on results of large scale shaking table test and numerical analysis with the finite element method results showed that the dynamic active earth pressure increased with increasing ground acceleration at an action point of 0 6 h in addition the higher the point of passive positive earth pressure and the lower the increment of active positive earth pressure the more gradual the slope of the geotextile bag retaining wall

a test of a quartz eclogite source for parental aleutian magmas a mass balance approach the observation that magmatic conduits evolve with time has provided a new framework within which to interpret aleutian compositional and isotopic data accordingly a two stage physical model of aleutian magma genesis has been developed and used to model quantitatively geochemical and isotopic mass balance relations parental magmas approximated by high alumina basalts are presumed to be generated by partial melting of eclogite and subducted sediment the incorporation of sediment produces elevated parental magma sr and pb isotopic ratios but lowered epsilon nd relative to morb if these magmas ascend along well established conduits their isotopic characteristics are little changed during ascent in immature conduits interaction with oceanic lithosphere produces significant isotopic variability according to this model the isotopic and geochemical characteristics of parental magmas are fixed by mixing eclogite and sediment melts as a test we have quantitatively examined the mass balance rel

compositional variation and provenance of late cenozoic distal tephra beds fairbanks area alaska abstract thin fine grained discontinuous tephra beds from interior alaska can be divided into two compositional groups by their mineralogy and major and trace element glass geochemistry the source vents of these beds are unknown but are believed to be in the aleutian arc alaskan peninsula and wrangell volcanic field type i tephra has evolved trace element profiles while type ii is primitive type ii contains abundant hydrous minerals that are absent or rare in type i trace elements were used to compare the fairbanks tephra beds to volcanic rocks in the potential source areas there are very few published glass trace element analyses from these regions this necessitates comparing whole rock with glass analyses for this reason incompatible elements particularly rare earth elements are most useful whole rock analyses and petrography of lavas throughout the aleutian arc alaskan peninsula and the wrangell volcanic field are similar to type i tephra lavas and intrusive rocks similar to type ii tephra are rare and occur at one volcano in the alaskan peninsula and at three volcanoes in the wrangell volcanic field

seismic interpretation of the kelasu triangle zone in the southern tian shan foothills northwestern china seismic reflection data show that the kelasu structure a frontal structure of the south tian shan foothills thrust belt is a triangle zone with the upper detachment following an overpressured lower tertiary anhydrite zone and the lower detachment zone probably in the lower mesozoic seismic reflectors the south verging thrusts emerging at the surface are the small roof structures rooted in the upper detachment although the upper detachment was a relatively lubricated surface the passively folded formations show strong resistance to the blind wedging underneath particularly in the anticlines during the triangle zone development as a result of the massive shear forces created by the opposing movement of the two tectonostratigraphic units the top part of the thrust sheets immediately under the crest of the frontal anticline was ripped off in the form of backthrusting and brecciation the large backthrust fault blocks and breccias were retained in the anticlines whereas the underlying duplex continued wedging toward the foreland in the early formed duplex structures in the hinterland the tips of the thrust sheets were completely shredded leaving sharp angular contacts between the thrust sheets and the upper detachment surface different types of fractures and present day geostress stress of the earth measured along the wellbore interpreted from the borehole image logs suggest a complex deformation history and reversal of geostress the kelar gas reservoirs are located within the backthrust fault blocks under the frontal anticline the current model reduces the previous concerns about potential damage to the anhydrite seal by the surface thrusts because the thrusts occurred entirely above the seal more importantly because the reservoir rocks are repeated many times in the duplex thrust sheets below the continuous anhydrite seal any anticlinal structure of the upper detachment surface is a potential prospect

tectonic controls on tholeiitic and calc alkaline magmatism in the aleutian arc the tectonic position of aleutian arc volcanic centers and their magmatic differentiation trends calc alkaline or tholeiitic appear to correlate from 160 w to 175 e the volcanoes form four major arc segments that coincide with earthquake aftershock zones and major geographic features on both the upper and lower plates the tholeiitic volcanoes are large primarily basaltic centers that occur between or at the end of segments where magmas can more easily reach the surface and undergo shallow closed system differentiation the calc alkaline volcanoes are smaller more andesitic centers that occur in the middle of segments where transit through the upper plate is apparently more difficult differentiation is deeper and the intrusive to extrusive ratios are higher than in the tholeiitic centers the chemistry of the least differentiated basalts at both types of centers is similar and suggests a common parent magma probably derived from mantle peridotite conditions for the derivation of the two trends through crystal fractionation support the proposed model the tholeiitic magmas show characteristics i e no hydrous phenocrysts fe enrichment trend parallel ree patterns vitrophyric and esites and dacites consistent with low pressure high temperature crystallization in large shallow magma chambers the calc alkaline magmas show characteristics i e some hydrous phenocrysts no fe enrichment trend nonparallel rare earth patterns porphyritic lavas consistent with higher pressure and lower temperature of crystallization than the tholeiitic series tertiary plutons also show both calc alkaline and tholeiitic trends and appear to be chemically similar to the quaternary volcanoes the larger plutons are calc alkaline and probably represent the extensive 1002 km magma chambers of the small calc alkaline volcanoes the small shallow tholeiitic plutons complement the large tholeiitic volcanoes and reflect the greater percent of extrusive rocks in the tholeiitic series

alkaline and calc alkaline lavas near los volcanes jalisco mexico geochemical diversity and its significance in volcanic arcs a remarkably diverse suite of lavas erupted during the late pliocene at the volcanic front of the western mexican volcanic belt near the town of los volcanes jalisco this region is much closer to the middle america trench than the main axis of quaternary andesite dacite stratovolcanoes and volcanism occurred in a complex tectonic regime involving both subduction of the young rivera plate and transverse crustal extension of the jalisco structural block the variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the mexican volcanic belt other alkaline varieties intermediate between these extremes include absarokites trachybasalts and trachyandesites phlogopite amphibole and apatite are common phenocryst phases whole rock compositions show a wide range of alkali contents e g k2o of 1 0 8 6 wt and typically contain 5 wt mgo mgo ni v and cr show little systematic decrease with increasing sio2 suggesting that these lavas have evolved from primitive mantle derived magmas with a wide range of sio2 contents strong enrichments in incompatible trace elements are observed in all of the lavas sr 700 5100 ppm ba 470 4800 ce 22 325 ppm zr 90 700 ppm as is the relative enrichment of large ion lithophile ba sr rb and light rare earth elements la ce over the high field strength elements ti zr which is typical of magmas in volcanic arcs this enrichment pattern suggests that these magmas come from source regions which contain incompatible element rich phases such as phlogopite amphibole and apatite the petrological and geochemical features of the lavas which occur in the los volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins the complex tectonic regime in western mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub arc mantle

seismic earth pressure on retaining structures a simple kinematic method to predict the seismic earth pressure against retaining structures is developed the fundamental solution to the free field seismic response considering nonlinear plastic behavior of soil is included in the retaining wall analysis for the first time perturbation to the free field response caused by soil structure interaction effects for different types of wall movement is considered results from this kinematic method are compared with those obtained from finite element analysis and observed from laboratory shaking table tests performed on model retaining walls

influence of syntectonic sedimentation and decollement rheology on the geometry and evolution of orogenic wedges analog modeling of the kuqa fold and thrust belt nw china

some thermal problems associated with magma migration abstract the mechanisms by which magma migrates from the point in the earth s interior where melting occurs to the earth s surface are poorly understood in this paper several aspects of this problem are examined magma can migrate upward due to its differential buoyancy on the scale of crystalline grains or as large dispairs magma transport is an effective means of heat transport magma transport at a rate of 0 15 cm yr is equivalent to a heat flow of 10 6 cal cm 2 s if magma encounters country rock witha lower melting point the original magma is likely to solidify while melting the country rock this would be an effective mechanism of purging silicic rocks and incomparable elements from the lower crust under some circumstances magma must penetrate up to 100 km or more of cold lithospheric rock in order for this magma to reach the surface without solidification a heated path must be provided the heating of this path requires the solidification of some magma it is estimated that magma penetrates the lithosphere in about 5000 years and that the crack is lined by several hundred meters of frozen basalt

3d structural modeling and restoration of the apennine maghrebian chain in sicily application for non cylindrical fold and thrust belts abstract thirteen seismic reflection profiles and field observations have been used to build a three dimensional watertight geological model of the sub surface architecture of the trapanese carbonate platform unit of the apennine maghrebian chain in the mt kumeta and mt rocca busambra nw sicily italy either a single step or a two step thrust fault deformation model is acceptable on the basis of seismic interpretation but the integration of 3d model reconstruction and 3d geomechanical restoration validates only the scenario with a single stage of deformation the 3d model highlights along strike variations of the structural style for the trapanese unit where pre existing discontinuities e g inherited mesozoic normal faults played an important role in the preferential occurrence of hinterland verging structures in detail backthrusts n verging are dissected by tear faults along the mt kumeta structure whereas an imbricate backthrust system evolves into thrust along the mt rocca busambra structure shortening estimate indicates low amount of internal deformation affecting the carbonate platform unit

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

aleutian lead isotopic data additional evidence for the evolution of lithospheric plumbing systems abstract lead isotopic ratios and concentrations have been measured in lavas from the aleutian volcanic centers of adak 12 and atka 12 whereas lavas from atka have very small pb isotopic ranges 206 pb 204 pb 18 78 18 86 207 pb 204 pb 15 55 15 62 208 pb 204 pb 38 31 38 52 those from adak have ranges nearly twice as large 206 pb 204 pb 18 34 18 86 207 pb 204 pb 15 47 15 63 208 pb 204 pb 37 81 38 56 on lead lead isotopic diagrams atka data form tight clusters whereas those from adaak define a well developed linear trend extending from the atka field toward less radiogenic values at a given 206 pb 204 pb the lavas from both islands plot in the morb trend for 208 pb 204 pb but atka lavas plot above the morb 207 pb 204 pb 206pb 204pb trend and those from adak fall both within and above the trend lead contents in lavas from atka increase four fold 5 21 ppm over the compositional range of the volcanic suite 49 68 sio2 in contrast actak lavas 49 65 have concentration levels of 0 6 13 ppm and display no simple correlation with sio2 the lead isotopic data alone can be explained by three different processes model 1 assigns lead isotopic differences to original magma source heterogeneity according to model 2 the isotopic ratios of a primary non radiogenic component from the mantle are elevated by the addition of an isotopically enriched slab derived component in contrast model 3 assumes a primary radiogenic magma produced by melting of the slab is contaminated by a non radiogenic lithospheric component during conduit formation because these models all adequately explain the lead isotopic data supplementary geologic petrographic geochemical and isotopic data must be used to select the most likely model careful consideration of the evidence suggests model 3 best explains our extensive lead isotopic data as well as other characteristics of aleutian lavas our study suggests detailed isotopic studies of individual volcanic centers arc or otherwise can be extremely useful in understanding the complex processes of magma generation extraction ascent and evolution

eruptive history and petrology of mount drum volcano wrangell mountains alaska mount drum is one of the youngest volcanoes in the subduction related wrangell volcanic field 80x200 km of southcentral alaska it lies at the northwest end of a series of large andesite dominated shield volcanoes that show a northwesterly progression of age from 26 ma near the alaska yukon border to about 0 2 ma at mount drum the volcano was constructed between 750 and 250 ka during at least two cycles of cone building and ring dome emplacement and was partially destroyed by violent explosive activity probably after 250 ka cone lavas range from basaltic andesite to dacite in composition ring domes are dacite to rhyolite the last constructional activity occured in the vicinity of snider peak on the south flank of the volcano where extensive dacite flows and a dacite dome erupted at about 250 ka the climactic explosive eruption that destroyed the top and a part of the south flank of the volcano produced more than 7 km3 of proximal hot and cold avalanche deposits and distal mudflows the mount drum rocks have medium k calc alkaline affinities and are generally plagioclase phyric silica contents range from 55 8 to 74 0 wt with a compositional gap between 66 8 and 72 8 wt all the rocks are enriched in alkali elements and depleted in ta relative to the lree typical of volcanic arc rocks but have higher mgo contents at a given sio2 than typical orogenic medium k andesites strontium isotope ratios vary from 0 70292 to 0 70353 the compositional range of mount drum lavas is best explained by a combination of diverse parental magmas magma mixing and fractionation the small but significant range in 87sr 86sr ratios in the basaltic andesites and the wide range of incompatible element ratios exhibited by the basaltic andesites and andesites suggests the presence of compositionally diverse parent magmas the lavas show abundant petrographic evidence of magma mixing such as bimodal phenocryst size resorbed phenocrysts reaction rims and disequilibrium mineral assemblages in addition some dacites and andesites contain mg and ni rich olivines and or have high mgo cr ni co and sc contents that are not in equilibrium with the host rock and indicate mixing between basalt or cumulate material and more evolved magmas incompatible element variations suggest that fractionation is responsible for some of the compositional range between basaltic andesite and dacite but the rhyolites have k ba th and rb contents that are too low for the magmas to be generated by fractionation of the intermediate rocks limited sr isotope data support the possibility that the rhyolites may be partial melts of underlying volcanic rocks

structural features of northern tarim basin implications for regional tectonics and petroleum traps the rhombus shaped tarim basin in northwestern china is controlled mainly by two left lateral strike slip systems the northeast trending altun fault zone along its southeastern side and the northeast trending aheqi fault zone along its northwestern side in this paper we discuss the northern tarim basin s structural features which include three main tectonic units the kalpin uplift the kuqa depression and the north tarim uplift along the northern margin of the tarim basin structural mapping in the kalpin uplift shows that a series of imbricated thrust sheets have been overprinted by strike slip faulting the amount of strike slip displacement is estimated to be 148 km by restoration of strike slip structures in the uplift the kuqa depression is a mesozoic cenozoic foredeep depression with well developed flat ramp structures and fault related folds the baicheng basin a quaternary pull apart basin developed at the center of the kuqa depression subsurface structures in the north tarim uplift can be divided into the mesozoic cenozoic and the paleozoic lithotectonic sequences in seismic profiles the paleozoic litho tectonic sequence exhibits the interference of earlier left lateral and later right lateral strike slip structures many normal faults in the mesozoic cenozoic litho tectonic sequence form the negative flower structures in the north tarim uplift these structures commonly directly overlie the positive flower structures in the paleozoic litho tectonic sequence the interference regions of the northwest trending and northeast trending folds in the paleozoic tectonic sequence have been identified to have the best trap structures our structural analysis indicates that the tarim basin is a transpressional foreland basin rejuvenated during the cenozoic

fracture systems and petrophysical properties of tight sandstone undergoing regional folding a case study of the cretaceous reservoirs in the kuqa foreland thrust belt tarim basin abstract the kuqa foreland thrust belt of the tarim basin hosts a prolific hydrocarbon system the main reservoirs are characterized by tight sandstone with low porosity and ultra low permeability buried presently at more than 5000 m depth natural fracture systems have a significant impact on reservoir performance and it may form paths for fluid flow in tight sandstone reservoirs and therefore plays an essential role in controlling high production rates with the aim of analyzing the distribution of favorable reservoir we studied subsurface fracture patterns and petrophysical properties of the bashijiqike formation with data collected from dabei structure and keshen structure we document the presence of four fracture systems whose distribution is related to far field stress thrusting and folding in the keshen anticlines higher fracture intensities are observed in the limbs with principal east west hinge parallel fracture system direction the hinge parallel fracture system includes a moderate dipping fracture set and a vertical fracture set crosscutting relationships stress field inversion and fracture surface textures suggest that the moderate dipping fracture set consists of shear fractures and can be explained by intra layer shear caused by thrusting the vertical fracture set may be formed due to the local outer arc extension during folding the oblique fracture system shows scattering in strike and is supposed to be related to a shear event of locally left lateral strike slip whereas in the dabei anticlines the major system is hinge perpendicular fracture system nw se showing a trending of increasing fracture intensity from limbs to hinge the hinge perpendicular fractures system in the dabei anticlines consist of shear fractures and tension fractures stress inversion result shows it corresponds to northwest southeast orientation compression stage besides we analyzed petrophysical properties include porosity permeability young s module poisson ratio and compressive strength of the bashijiqike formation concerning their structural position within folds it indicates that the highest porosity and permeability are associated with dissolution and fracturing the compressive strength of different structural positions also restrict the variations of mechanical properties of rocks the best reservoir quality is suggested in the hinge zone and areas close to thrust the findings have given maybe helpful for providing favorable reservoir targets for prospecting in the compressional basin

the origin of anorthosites and related rocks from the lofoten islands northern norway ii calculation of parental liquid compositions for anorthosites clinopyroxene orthopyroxene and plagioclase in the proterozoic introduction lofoten anorthosites are inferred to be in equilibrium with ferrodioritic proterozoic massif type anorthosites are huge intrusive melts on the basis of literature partition coefficients for these phases bodies that contain hundreds to thousands of km of in equilibrium with basaltic melts ferrodiorites and high al gabbros nearly pure 90 plagioclase cumulates of intermediate occur as fine grained dykes in spatial association with the 1 8 composition an50 10 ashwal 1993 and references ga lofoten anorthosites they are linked to each other by an fetherein most proterozoic anorthosites are not associated enrichment fractionation trend typical of tholeiites and they are with significant amounts of mafic or ultramafic cumulates characterized by al2o3 contents of 14 18 5 wt and xfe they contain plagioclase that is relatively sodic usually feo feo mgo between 0 5 and 0 8 the association around an50 and this makes them distinct from other of high al gabbros ferrodiorites and anorthosites is common in anorthositic rocks archean anorthosites lunar anormany proterozoic anorthosites we infer that the high al gabbros thosites anorthosites in layered intrusions which contain to ferrodiorites represent the parental liquids from which these more calcic plagioclase proterozoic anorthosite comproterozoic anorthosites formed the lofoten rocks show fractionation plexes contain anorthosite sensu stricto 90 plagioclase trends that are similar to those of high al gabbros and ferrodiorites as well as variable amounts of leuco gabbroic troctolitic from other proterozoic anorthosite complexes we interpret these trends and noritic rocks which respectively may contain up to in terms of incorporation of intermediate composition plagioclase into 35 clinopyroxene olivine or orthopyroxene a typical tholeiitic magma with subsequent fractionation the first the proterozoic anorthosites of the lofoten islands in process produces positive eu anomalies and the high al2o3 contents northern norway heier 1960 green jorde 1971 of the melts and explains the lack of negative eu anomalies in romey 1971 griffin et al 1978 wade 1985 are most of the ferrodiorites from lofoten and laramie associated with large amounts of temporally related intrusive mangerites charnockites and gabbros markl et al 1998 there are two different anorthosite intrusions present one is a relatively calcic complex which occurs on the island of flakstadoy flakstadoy basic complex fbc and the other is a relatively sodic complex which

isotope and trace element geochemistry of augustine volcano alaska implications for magmatic evolution k e johnson r s harmon j m richardson s moorbath and d f strong department of geology 1112 turlington hall university of florida gainesville fl 32611 usa us army research office p o box 12211 research triangle park nc 27709 usa geoscience branch ontario geological survey willet green miller centre sudbury ont pse 6b5 canada department of earth sciences university of oxford parks road oxford oxi 3pr uk centre for earth and ocean research university of victoria victoria b c v8w 3p6 canada

along strike structural variation in a salt influenced fold and thrust belt analysis of the kuqa depression abstract the kuqa depression is associated with complex salt structures variation in fold geometries and subsalt fault activities were investigated using parallel cross sections to reveal detailed along strike variations within the keshen 5 segment and their controlling factors along strike variations were identified in both suprasalt and subsalt units linkages such as the location of a transfer zone were apparent while the distribution of variations above and below the salt layer were decoupled the results indicate that the pre existing salt wall is the key factor driving along strike variations because it acts as a weak zone that influences deformation propagation suprasalt structural variation results in different sediment depths which have a significant impact on subsalt fault activity in the eastern segment fault activity decreases from the hinterland to the basin in contrast owing to the pre existing salt wall and deep sedimentation near the hinterland in the western segment the fault beneath the salt wall is characterized by maximum displacement this study reveals the connection between suprasalt and subsalt deformation and the role of that a salt diapir plays in along strike structural evolution variations for the keshen 5 segment of the kuqa depression these findings are relevant for understanding along strike deformation propagation and structural variation in other fold and thrust systems

silurian clastic sediments in the north qilian shan nw china chemical and isotopic constraints on their forearc provenance with implications for the paleozoic evolution of the tibetan plateau abstract the north qilian shan is an accretion collision orogenic belt in the northeastern margin of the tibetan plateau that connects the north china plate to the north with the tethyan orogenic assemblage to the south its early paleozoic temporal spatial tectonic evolution has been extensively studied but the nature and tectonic setting of its silurian deposition is poorly understood our new petrological analysis suggests that the clast composition of fan conglomerates is complex in the danbanshan and zhongdabanshan sections abundant lenticular conglomerate beds contain mainly subrounded rounded metamorphic clasts whereas the tongziba and haichaoba sections contain volcanic chert limestone ultramafic and granitoid clasts however in the sunan area clast types are relatively simple dominated by basalt and granitoids and at tongziba gabbro and basalt clasts are subrounded and angular indicating two different types of provenance ultramafic limestone and abundant chert clasts in the haichaoba and tonghe conglomerates indicate an ophiolitic source whereas subrounded rounded quartzite and granitoid gneiss clasts were probably derived from the central qilian basement geochemical data from the volcanic and granitoid clasts suggest an arc related source and their zircon u pb ages of 515 429 ma suggest that the arc magmatism did not end in the mid silurian sandstone petrology and detrital modes suggest that the silurian sediments were deposited in a forearc basin paleogeographic reconstructions inferred from nnw nne paleocurrent data indicate that an island arc was present off the southern margin of the silurian basin and that southward subduction of oceanic crust between north china and the qilian shan was active or continued in the silurian

development and growth of basement involved structural wedges in the northwestern qaidam basin china structural wedges contain two connected fault segments a fore thrust and back thrust that bound a triangular or wedge shaped fault block coeval displacements on both faults drive the wedge into surrounding rock causing distinctive patterns of uplift and folding we identified a series of thick skinned structural wedges in northwestern qaidam basin china where syntectonic strata record deformation timing and kinematics the qaidam basin is located at the northern margin of the tibetan plateau and contains more than 100 large anticlinal structures many of which exhibit characteristics of structural wedges we interpreted and modeled these structures using seismic reflection data well logs surface geological exposures remote sensing images and digital elevation data these wedge structures are distinguished by having elevated and deformed strata in the footwalls of the back thrusts and synclinal folds that extend upward from the wedge tips with the constraints of fold shape growth strata structural relief and shallow fault geometry we developed a series of forward models that describe the geometry and kinematic evolution of these wedge structures the analysis suggests that some of the wedge structures reactivate preexisting normal faults our results provide a better understanding of how to identify thick skinned wedge systems and model their geometry and kinematic evolution which has implications for studying the tectonic history of the basin understanding the geometry and kinematics of the wedge structures is also crucial for petroleum exploration because both hanging wall and footwall traps constitute an important component of current exploration targets

a sub salt structural model of the kelasu structure in the kuqa foreland basin northwest china abstract the kuqa foreland basin adjacent to the south tianshan mountains is a major hydrocarbon accumulation basin in western china the kelasu structural belt is the focus for hydrocarbon exploration in the basin due to the presence of ramp related anticline traps and a thick salt seal the model of the kelasu sub salt structure is still contentious because of the structural complexity and poor seismic imaging below the salt layer the area depth strain ads method is applied to the southern part of the kelasu fault a regional fault that cuts basement rocks the ads results are consistent with the seismic data which indicate that both thin skinned thrusting and basement involved deformation occur within the kelasu structure with the kelasu fault acting as the boundary between the two regions of contrasting deformation the ads results also suggest that the depth of the lower detachment of the thin skinned thrust belt is 9 5 10 km which may correspond to the base of the triassic the kelasu structure has undergone approximately 8 15 10 76 km of horizontal shortening in the east and 16 34 km in the west of the structure

triangle zones geometry kinematics mechanics and the need for appreciation of uncertainties abstract triangle zones are important structures found in foreland fold and thrust belts all over the world and are commonly associated with tectonic wedging however tectonic wedging and consequently the formation of passive hinterland verging roof thrusts requires particular mechanic conditions such as pre fractured rocks syntectonic sedimentation or a specific stratigraphic layering with variable mechanically weak and rigid formations in this contribution models of triangle zones and hypotheses regarding their formation are reviewed our results show that the term triangle zone is often used in a contradictory sense and interpretations are affected by large uncertainties passive roof thrusts are not necessarily required in all cases and suggested models of triangle zones even if they are geometrically and kinematically viable are hampered by their mechanical implications with respect to a large number of published triangle zones and associated kinematic as well as mechanic models we present a new definition and classification scheme which is discussed and applied to natural examples worldwide we show that natural examples of triangle zones can be separated into two types 1 detachment dominated and 2 ramp dominated triangle zones both types imply particular mechanic conditions of involved detachments and regional dynamics regarding the large uncertainties associated with triangle zone interpretation we suggest to only use the term triangle zone in a very stringent manner and only if irrefutable evidences for stratigraphic repetitions within a duplex are given the results should then be validated with additional kinematic and mechanic considerations geometric and kinematic uncertainties as well as implications of the proposed model explaining observed triangular structures should be reflected in the nomenclature used

coefficient of earth pressure at rest for sands subjected to vibration an oedometer instrumented to measure horizontal pressure was used to examine the behavior of the coefficient of earth pressure at rest ko of clean sands subjected to vertical vibration reconstit

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

on the na2o content of convergent zone high alumina basalts abstract high alumina basalts in the convergent zone are classified into high medium and low na type the high na high alumina basalts occur in arcs without low alkali tholeiites where younger and warmer oceanic lithospheres are subducting the low na high alumina basalts occur in arcs where on the contrary older and cooler plates are descending and occur also in arcs with extensional tectonics even though younger lithospheres are underthrusting a good correlation is observed between the na2o content of high alumina basalts and the age of the subducting lithosphere this suggests that generation of high alumina basalt magma is controlled by the nature of the descending plate

earth pressures against rigid retaining walls this paper outlines the results of unique shaking table experiments to determine neutral and active static and dynamic stresses and the points of application of corresponding total thrust generated by granular soils against rigid retaining walls and compares the experimental data with the well known coulomb and mononobe okabe equations a new criterion is proposed for identification of the wall deformation level at which the active state of stress develops this criterion states that active stress develops when the angle of friction between the wall and the backfill soil reaches its maximum value based on the above criterion experimental results show that the backfill soil enters the active state at much lower wall displacement levels than generally believed an equation is proposed for calculating the displacement necessary to initiate active static or dynamic stress this equation reveals that the displacement necessary to initiate active state increases with the height of the wall and decreases with backfill soil strength

strontium isotopic and selected trace element variations between two aleutian volcanic centers adak and atka implications for the development of arc volcanic plumbing systems major and trace element concentrations and initial 87sr 86sr ratios of lavas from the aleutian volcanic centers of adak and atka have been used to study the evolution of their respective lithospheric plumbing systems the centers are within 150 km of one another and show similar overall silica ranges 47 67 but adak 40 km3 is smaller than atka 200 km3 adak s lavas are chemically and isotopically heterogeneous 87sr 86sr 0 70285 0 70330 and two units contain lithospheric xenoliths the lavas of the much larger atka on the other hand have much less variability in major and trace elements as well as 87sr 86sr 0 70320 0 70345 we suggest that these characteristics are a measure of the relative maturity and cleanliness of the lithospheric plumbing systems that supply magma to these centers because aleutian volcanic centers often remain fixed for relatively long periods of time 5 m y once established magmatic passageways are repeatedly used young plumbing systems are relatively cool and contain large amounts of wallrock contaminant and ascending magmas undergo contamination as well as concurrent crystallization and fractionation with time however heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas in effect the plumbing system matures the chemical heterogeneity displayed by young dirty systems like adak reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation thus it is the petrologic data of mature clean systems like atka that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution

phase relations of a high mg basalt from the aleutian island arc implications for primary island arc basalts and high al basalts many volcanic centers in the aleutian islands have erupted lavas that range in composition from high mg basalt mgo 9 wt to more fractionated and voluminous high al basalts and basaltic andesites the petrogenetic relationships between these rock types and the composition of primary magmas has been vigorously debated the phase relations of a typical high mg basalt from the makushin volcanic field on unalaska island provide important constraints on petrogenetic models results of one atmosphere and moderate to high pressure 5 20 kb anhydrous experiments are similar to results obtained from primitive morb at low pressures olivine is the liquidus phase joined by plagioclase and clinopyroxene at progressively lower temperatures clinopyroxene is the second phase to crystallize at pressures greater than 5 kb and replaces olivine on the liquidus at approximately 10 kb above 10 kb the liquidus pyroxene is aluminous augite and orthopyroxene is the second phase to crystallize glasses in equilibrium with olivine and clinopyroxene at intermediate pressure 5 to 10 kb are similar in composition to high al basalt plagioclase is not involved and most likely does not become a liquidus phase until the liquid has evolved significantly although our studies do not confirm the primary nature of high mg basalts they do support a model in which high al basalts are generated by moderate amounts of crystal fractionation from more primitive high mg mg fe lower al2o3 basaltic magmas near the arc crust mantle boundary

seismic passive earth pressures in soils seismic passive earth pressure coefficients were computed by the method of limit equilibrium using a pseudostatic approach for seismic forces composite curved rupture surfaces were considered in the analysis while earlier studies using this type of analysis were mainly for sands seismic passive earth pressure coefficients were obtained in the present study considering the effects of cohesion surcharge and own weight the minimum seismic passive force was obtained by adding the individual minimum values of these components and the validity of the principle of superposition was examined other parameters considered in the analysis were wall batter angle ground surface slope soil friction angle wall friction angle wall adhesion to soil cohesion ratio and horizontal and vertical seismic accelerations the seismic earth pressure coefficients were found to be highly sensitive to the seismic acceleration coefficients both in the horizontal and vertical directions results of the study are presented in the form of figures and tables comparisons of the proposed method with available theories in the seismic case are also presented

crystal size distributions and scaling laws in the quantification of igneous textures this study demonstrates the value of the combined use of scaling of the chemical evolution of cooling magmas have investigated the detailed relationships between temanalysis and crystal size distribution csd measurements in the interpretation of igneous textures and outlines applications of these perature phase abundance and phase composition the reaction series bowen 1922 and the cipw norm cross methods to other fundamentally important problems theoretical calculations and measurements of natural samples are used to et al 1903 have been supplemented by modern thermodynamic models that predict the compositions and abundcharacterize the relationship between igneous texture and cooling history the total number nt and mean length l of crystals ances of minerals in equilibrium with melt of a given composition and temperature e g ariskin et al 1993 in a sample are correlated through a scaling relationship of the form n t l this proportionality depends on the mineralogy ghiorso sack 1995 although there are a few areas of uncertainty the chemistry of igneous petrogenesis is of the rock and so a modal normalization factor c is introduced the csd slope s and intercept ln n are uniquely related to fairly well understood but chemical models cannot address the central probeach other through the equation ln n 4ln s ln c where s l this overall relationship allows the texture of a rock lem of igneous crystallization which is the physical process of generating a set of crystals from magma two to be related to the local duration of cooling through an arbitrary crystal growth model quantification of the link between the texture magmas with the same composition will normally produce the same minerals in the same proportions however of a rock and its cooling history makes it possible to predict spatial variations in texture we show an example of this method using depending on their thermal histories these two magmas can solidify into rocks with drastically different physical the sudbury igneous complex canada additional applications include relating the textures of volcanic rocks to spatial and temporal appearances e g basalt and gabbro the set of minerals is the same but what distinguishes them is the abundance variations in the magma chamber and extracting kinetic parameters from suites of comagmatic rocks and size of the constituent crystals the fine grained rocks in the chilled margins of most basic intrusions cooled rapidly and the coarse grained rocks in the interiors of plutons cooled slowly this has

mass balance calculations with end member compositional variability applications to petrologic problems although end member compositional variability is quite common most quantitative mass balance procedures cannot accommodate this variability in a systematic manner by rearranging the traditional mass balance relations a series of equations can be derived to account for such heterogeneity while this approach can be applied ton component systems the results are difficult to represent graphically accordingly the procedure is most useful for two and three component systems for two components the concentration of an element in a mixture is cpm chmx 1 1x c1 and the isotopic signature is epmc pm ehmc hmx 1 1x e1c 1 where pm hm and 1 refer to parental magma hybrid magma and component 1 respectively x is the proportion of parental magma c is concentration is isotopic ratio and c the concentration of the denominator isotope in the particular isotopic ratio of interest these equations describe straight lines termed isoproportional or ip lines of fixed mixing proportion in the cpm c1 concentration or epmc pm e1c 1 planes only those ip lines intersecting the rectangle defined by observed end member compositions represent viable mixing proportions for three component systems the mass balance equations are y chm c2 x cpm c2 c1 c2 and y c hmehm c 2e2 x c pmepm c 2e2 c 1e1 c 2e2 where y is now the proportion of component 1 and the proportion of component 2 z is simply 1 x y using major trace and rare earth element as well as isotopic data a region of the x y plane representing possible mixing combinations can be defined due to the compositional variability of most magmatic end members this new mass balance procedure should be applicable to a diverse range of petrologic problems this procedure has been applied to three different petrologic processes aleutian parental magma genesis assimilation contamination and crust mantle differentiation

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex