on the trace of the iapetus suture in ireland and britain the iapetus suture in ireland and britain is that line which separates caledonian rocks of the laurentian and avalonian continents the suture is cryptic nowhere is there an exposed fault zone containing ophiolite remnants blue schist melanges or trench deposits instead a suture line may be traced with varying degrees of confidence through a series of faults that traverse the iapetus suture zone which contains two or more tectonostratigraphic terranes several data sets are utilized to constrain this trace the distribution of faunal provinces caused by oceanic separation can be used most reliably to define early ordovician terranes fauna1 intermingling in the mid ordovician reduces provinciality and the confidence of terrane identification using faunal data the late ordovician and particularly silurian tectonostratigraphic histories of terranes either side of the suture suggest that amalgamation of the terranes elimination of the iapetus ocean and development of the suture zone had begun by the ashgill therefore the resolution of the suture trace becomes unreliable where ordovician rocks do not crop out because silurian turbidite fans of laurentian provenance may have dispersed across a significant width of avalonian crust similarly the precision of structural identification of the suture zone by correlating the suture with a major fold facing confrontation may be weakened by late overthrusting tectonic interleaving of crustal flakes in the zone is supported by geophysical evidence which suggests that some major boundaries in the lower crust do not always coincide with their inferred projections in outcrop tectonic and sedimentary mixing of crustal blocks and sediments in the zone is further reflected by nd and pb isotopic patterns the rb sr patterns of granites record a stronger distinction between north and south of the suture despite these difficulties we depict a possible suture trace in ireland that departs significantly from the traditionally placed line by following a ne sw aligned trace through the slane fault and the navan tipperary lineament and thence through a dog leg to pass north of the dingle peninsula

joint inversion of 2d resistivity and seismic travel time data to image saltwater intrusion over karstic areas the simultaneous inversion of multiple geophysical data types has been proven to be a powerful tool to both improve subsurface imaging and help in the interpretation process the main goal of this paper was to develop joint inversion strategies to provide improved resistivity and seismic velocity images for delineating saline water zones in karstic geological formations the cross gradient constraint approach was adopted to jointly invert resistivity and seismic first arrival data the basic idea of this approach is to quantitatively estimate the structural similarity between resistivity and seismic velocity models using the cross product of their gradients and to achieve a unified geological model which satisfies both data sets initially synthetic data were employed to help develop a joint inversion strategy to be used over such complex geological structures the proposed strategy uses a weighting factor for the cross gradient constraints and separate damping factors for the resistivity and seismic data this strategy was applied successfully on field data from the karstic region of stilos crete greece

structure of the dingle peninsula sw ireland evidence for the nature and timing of caledonian acadian and variscan tectonics the palaeozoic rocks of the dingle peninsula provide a record of the evolution of the caledonides acadides and variscides the succession ranges from early ordovician deep water sediments through silurian shallow marine to non marine sediments and volcanic rocks to an old red sandstone ors succession topped by carboniferous marine shales comparison of structural styles in the unconformity bounded groups together with a detailed analysis of fault zones allows the tectonic history to be deduced the older rocks record caledonian processes on the margin of avalonia during early ordovician time and convergence then soft collision with laurentia during silurian time the dingle basin was developed during the late silurian early devonian transtension in the iapetus suture zone and was inverted in the latest emsian acadian orogenic episode post dingle group ors groups in the north of the peninsula were deposited in a syn rift footwall block to the main munster basin the acadian transpressional and munster basin extensional structures were reactivated or overprinted in the variscan deformation such that acadian folds are transected by variscan cleavage in both plan and vertical views after iapetus closure changes in the tectonic regime are believed to be a result of adjustments in the geometry of subduction of the rheic ocean

is taiwan the result of arc continent or arc arc collision abstract conventionally it has been accepted that the formation of taiwan results from the collision of the luzon arc with the eurasian continental margin we suggest that taiwan results from the collision of the luzon arc with the former ryukyu subduction zone before the collision the latter extended a few hundred kilometres southwest of its present day termination in the early to middle miocene the subduction became inactive in its southwest portion and the manila trench migrated northeastward giving rise to the formation of the luzon arc the collision started in the late miocene by closing the oceanic domain located between the luzon arc and the former ryukyu arc because the luzon arc moved northwestward with respect to the former ne sw trending ryukyu arc the oblique collision first resulted in the indentation on the south ryukyu arc west of 123 5 e and the narrowing of the corresponding portion of the okinawa trough prior to the uplift of taiwan today the hsuehshan and backbone ranges represent the uplifted portion of the backarc basin and the longitudinal valley corresponds to the suture zone between the former ryukyu and the luzon arcs in this model we propose to link although they are not of the same age two truncated backarc basins the active okinawa trough to the northeast and the former active tainan backarc basin to the southwest the former backarc basin extended from southwest of taiwan to southern japan and could have been initiated near the paleo location of taiwan as early as the eocene oligocene the result of the collision is the truncation of the ancient backarc basin associated with the shortening deformation and uplift of the ancient outer arc and related backarc region which provides the fundamental mechanism of the taiwan mountain building this model is in agreement with a counterclockwise rotation of the portion of taiwan located west of the coastal range belonging to the luzon arc and a clockwise rotation of the south ryukyu arc earthquake focal mechanisms geodetic observations and the distribution of the petrologic and structural features are also consistent with the proposed arc arc collision model

silurian turbidite provenance and the closure of iapetus subduction of iapetus crust during the caledonian orogeny has been seen by some authors as terminating at the end of ordovician times and by others as continuing through the silurian we present petrological and chemical data which support a coeval arc origin for detritus in the lettergesh formation silurian western ireland the arc based on continental crust lay to the north of the present outcrops of this formation along the laurentian margin of iapetus implying active oceanic subduction in the silurian this subduction may have been followed by some continental underplating around the middle of the wenlock giving rise to a successor basin in ireland and scotland

implications for caledonian plate tectonic models of chemical data from volcanic rocks of the british old red sandstone volcanic rocks associated with the continental sediments known as the old red sandstone outcrop widely in northern britain and form a dominantly calc alkaline suite comparable to those developed on modern continental margins rocks rich in ni and cr are unusually abundant and are thought to have incompatible element concentrations little different from those of primary magmas such rocks show pronounced spatial chemical variation with concentrations of sr ba k light rare earth elements and p 2 o 5 and the ratio la y showing up to a six fold increase in a northwestward traverse across scotland in more evolved rocks a similar increase is shown by sr ba and la y but regional comparisons are made difficult by the varying effects of fractional crystallization the chemistry of all old red sandstone volcanic rocks n of the southern uplands fault is consistent with the former existence of a subduction zone that changed in strike from ene in ireland and southern scotland to n in the north sea this orientation in conjunction with almost e w plate motion may explain the great contrast in end silurian deformation styles between britain and scandinavia old red sandstone volcanic and plutonic rocks s of the southern uplands fault are thought to be younger and are perhaps unrelated to subduction activity

opening mode of the okinawa trough paleomagnetic evidence from the south ryukyu arc more than 250 samples of volcanics and sedimentary rocks were collected from the south ryukyu arc for paleomagnetic study and k ar dating stable primary components of magnetization were isolated from 18 sites after thermal demagnetization the mean paleomagnetic direction for 15 eocene volcanic sites d 30 1 i 40 3 95 10 3 indicates a direction which is deflected 18 6 11 6 clockwise from the expected field direction calculated from the 40 ma pole of eurasia the clockwise deflection is observed also in the dike rock d 144 0 i 32 1 the age of which is determined to be 9 6 0 8 ma by the conventional k ar dating method these results indicate that with respect to eurasia the southern part of the ryukyu arc has rotated clockwise 19 during the past 10 m y we attribute this rotation to the back arc opening of the southern part of the okinawa trough which occurred by means of the wedge mode later than 10 ma

high resolution crosswell imaging of a west texas carbonate reservoir part 1 project summary and interpretation a carbon dioxide flood pilot is being conducted in a section of chevron s mcelroy field in crane county west texas prior to co2 injection two high frequency crosswell seismic profiles were recorded to investigate the use of seismic profiling for high resolution reservoir delineation and co2 monitoring these preinjection profiles provide the baseline for time lapse monitoring profile 1 was recorded between an injector well and an offset observation well at a nominal well to well distance of 184 ft 56 m profile 2 was recorded between a producing well and the observation well at a nominal distance of 600 ft 183 m the combination of traveltime tomography and stacked cdp reflection amplitudes demonstrates how high frequency crosswell seismic data can be used to image both large and small scale heterogeneity between wells transmission traveltime tomography is used to image the large scale velocity variations cdp reflection imaging is then used to image smaller scale impedance heterogeneities the

opening of the grenada back arc basin and evolution of the caribbean plate during the mesozoic and early paleogene abstract geological and geophysical data indicate that the grenada basin was presumably created during the paleocene by sundering of a proto eastern caribbean arc into a remnant arc to the west aves swell and an active arc to the east lesser antilles ridge grenada basin spreading is thought to have been penecontemporaneous with the creation of the yucatan basin located at the opposite side of the caribbean sea i suggest that a continuous mesozoic caribbean arc m c a including the greater antilles the aves lesser antilles system and the aruba blanquilla chain netherland venezuelan antilles was initiated in the pacific probably about 130 120 ma ago its arrival in front of and its subsequent motion inside the central atlantic tethyan seaway caused the opening of both yucatan and grenada basins which occurred at the two initial points of contact with the north and south american cratons in contrast to the style of many other island arcs this back arc spreading event occurred only once in the long history of the m c a the lesser antilles appear to be the oldest currently active intra oceanic island arc

exceedance probability distributions of steady conduit flow in karst aquifers discharge exceedance probabilities are calculated for a simple model karst aquifer composed of a few multilevel conduits with recharge from a single sinking stream with an exponential flow exceedance distribution it is assumed that outflow instantaneously matches inflow so that the conduit volume is constant but discharge is governed by the head in a volumeless shaft at the top end of the system it is shown that small single conduit aquifers will frequently overflow at the surface during floods and the exceedance probability of flow through the aquifer and over the surface can be defined as a function of the inflow distribution and the form of the aquifer systems with multiple conduits will overflow less frequently but each conduit will exhibit a flow distribution characteristic of its form and position in the vertical hierarchy comparison of these findings with actual flow data from a conduit aquifer shows that the approach is valid although imprecise the model is unlikely to be applied directly as it requires unusually detailed data however it provides for the first time an indication of typical flow durations for surface overflows and individual conduits in a karst aquifer contrasts in flow duration will have a profound influence on solutional and sedimentary processes in the karst system

nonlinear 3 d traveltime inversion of crosshole data with an application in the area of the middle ural mountains this paper deals with the problem of nonlinear seismic velocity estimation from first arrival traveltimes obtained from crosshole and downhole experiments in three dimensions a standard tomographic procedure is applied based on the representation of the crosshole area into a number of cells which have an initial slowness assigned for the forward modeling the raypath matrix is computed using the revisited ray bending method supplemented by an approximate computation of the first fresnel zone at each point of the ray hence using physical and not only mathematical rays since 3 d ray tracing is incorporated the inversion technique is nonlinear velocity images are obtained by a constrained least squares inversion scheme using both damping and smoothing factors the appropriate choice of these factors is defined by the use of appropriate criteria such as the l curve the tomographic approach is improved by incorporating a priori information about the media to be imaged into our inversion scheme th

k ar age and geochemistry of the sw japan paleogene cauldron cluster implications for eocene oligocene thermo tectonic reactivation abstract systematic k ar dating and geochemical analyses of paleogene cauldrons in the sanin belt of sw japan have been made to explore the relationship between the timing of their formation and the paleogene subduction history of sw japan documented in the shimanto accretionary complex we also examine the magma sources and tectonics beneath the backarc region of sw japan at the eastern plate boundary of eurasia fifty eight new k ar ages and 19 previously reported radiometric age data show that the cauldrons formed during middle eocene to early oligocene time 43 30 ma following a period of magmatic hiatus from 52 to 43 ma the hiatus coincides with absence of an accretionary prism in the shimanto belt resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench as a common tectono thermal event within a subduction system the cauldrons are composed of medium k calc alkaline basalts to rhyolites and their plutonic equivalents these rocks are characterized by lower concentrations of large ion lithophile elements lile including k2o ba rb th u and li lower la yb n ratios lower initial sr isotopic ratios 0 7037 0 7052 and higher end t values 0 5 to 3 5 relative to late cretaceous to early paleogene equivalents there are clear trends from enriched to depleted signatures with decreasing age from the late cretaceous to the paleogene the same isotopic shift is also confirmed in lower crust derived xenoliths and is interpreted as mobilization of pre existing enriched lithospheric mantle by upwelling depleted asthenosphere relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the sw japan arc trench system during the eocene to oligocene newly identified late eocene low silica adakites and high mg andesites in the sanin belt and early eocene a type granites in the sw korea peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate the backarc region was an extensional tectonic setting and some paleogene rift basins and sanin belt cauldrons occur in linear arrays the eocene oligocene sanin se korea continental arc lies on the ne extension of the east china sea basin the initial stage of which probably formed by continental arc rifting this rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab in response to decreased convergence rate between the pacific and eurasian plates

collisional model for rapid fore arc block rotations arc curvature and episodic back arc rifting in subduction settings 1 we review geophysical and geological data from 23 active and ancient plate boundaries to document a compelling spatial and temporal relationship between collision plate boundary curvature rapid tectonic rotations and the occurrence of back arc rifting our observations support a conceptual model where the change from subduction to collision causes rapid fore arc rotation when viewed in a reference frame relative to the bounding tectonic plates leading to marked plate boundary curvature our global compilation reveals that most active back arc rifts are associated with rapidly rotating fore arcs and nearby collisions thus we propose that collision and rapid fore arc rotations play a major role in the evolution and kinematics of back arc basins we conduct numerical modeling to better understand the physical processes behind these observed rapid tectonic rotations at convergent margins our results suggest that the presence of an indentor or choke point in the subduction system e g collision can generate rapid rotation of the fore arc about a nearby pole and lead to back arc rifting the rate of fore arc rotation and back arc rifting depends on the incoming indentor velocity and can be greatly enhanced by slab rollback and the presence of a low viscosity back arc where viscosity of the back arc is low fore arc rotation dominates where back arc viscosity is high the formation of strike slip faults and tectonic escape dominates our observational and model derived results illustrate that shallow crustal forces produced by the entry of buoyant features into subduction zones are a fundamental mechanism for the generation of fore arc block rotations plate boundary curvature back arc rift evolution and tectonic escape in subduction systems rollback of the subducting slab within the mantle will certainly enhance the processes we describe but it is not the only driving force

back arc rifting in the okinawa trough abstract geological and geophysical data reveal that the okinawa trough shows incipient continental rifting and crustal separation started from about 2 ma the early extensional movements in the trough are probably of miocene age in addition to the miocene phase two main periods of extension are recognized a pleistocene phase between 1 9 and 0 5 ma and the present day phase during the stage short central rifts central grabens were formed the opening however may have occurred only in the southern part of the trough basin having an average half spreading rate of 2 cm yr 1 since early pleistocene time producing its present width of several tens of kilometres these activities were well represented by igneous intrusions sedimentary facies and sedimentary structures in and around the okinawa trough the width of the zone affected by back arc extension defined as greater okinawa trough is larger than the present okinawa trough whose width is 200 250 km the present form of the greater okinawa trough started to form at the same time as that of the okinawa trough

back arc extension in the okinawa trough the okinawa trough lying to the east of china is a back arc basin formed by extension within continental lithosphere behind the ryukyu trench arc system middle to late miocene uplift associated with normal faulting of the initially adjacent ryukyu nonvolcanic arc and the taiwan sinzi folded belt corresponds to the first rifting phase the timing of rifting is supported by the presence of marine sediments of corresponding age drilled in the northern okinawa trough the rifting occurred after a major early miocene change in the motion of the philippine plate with respect to eurasia and ceased during the pliocene a second rifting phase started about 2 m y ago at the plio pleistocene boundary and has continued until the present time it has proceeded to a more advanced stage in the middle and southern okinawa trough than it has farther north detailed bathymetric sea beam seismic reflection and magnetics data collected during the pop 1 cruise of the r v jean charcot reveal the principal features of the extensional processes the back arc spreading phase started very recently in the southern and middle okinawa trough as exemplified by several en echelon and in some cases overlapping active central graben oriented n70 e n80 e some of these depressions are intruded by volcanic ridges of fresh back arc basalt with associated large magnetic anomalies transform faults between these en echelon active rifts are not obvious we suggest that the major part of the southern okinawa trough is underlain by a thinned continental crust and that except for the system of en echelon rifts of the southern okinawa trough the back arc basin oceanic domain is limited to a width of a few tens of kilometers or less in the axial portion of the trough the system of axial back arc volcanic ridges that occur in the rifts ends at the latitude of okinawa island whereas active volcanoes in the ryukyu arc occur only north of okinawa island we refer to this transition between active arc and back arc volcanism as the volcanic arc rift migration phenomenon vamp globally back arc volcanism propagated from the southern okinawa trough to the okinawa vamp area rifting continues to occur in the northern okinawa trough but is not yet accompanied by associated volcanism the okinawa vamp area is characterized by a series of parallel basaltic ridges oriented n75 e with associated linear magnetic anomalies characteristic of dyke intrusions we suggest that the formation of the back arc oceanic domain took place along the axial back arc extensional zone trending n75 e and that this zone presently ends at the southern extremity of the active volcanic chain the initial phase of formation of back arc basin oceanic crust is non steady state and is characterized by the lack of a developed fracture zone pattern the termination of the vamp area in the direction of the volcanic zone of the arc is consistent with the suggestion of molnar and atwater that the volcanic arc is a fundamental line of weakness which determines where initial back arc spreading occurs apparently back arc extension initially occurred within the continental lithosphere located westward of the ryukyu arc along its whole length but the subsequent back arc volcanism was initiated in the southernmost portion of the region and then moved northward this migration was accompanied by the shutting down of volcanic activity along the abandoned portions of the arc it is this transfer of volcanism that we call the vamp process thus arc and back arc basin volcanism seem to be associated in such a manner that spreading tends to migrate simultaneously with a cessation in volcanic activity along the arc this interplay of arc and back arc activity is probably linked to changes in the parameters of plate convergence since the plate motion in the phillippine sea is oblique to the trench at least in the southern part of the okinawa trough we suggest that the oblique resisting force applied to the edge of the overriding plate engenders the development of en echelon extensional features within and behind the arc the motion of the ryukyu platelet with respect to eurasia is consequently an extensional strike slip motion trending roughly parallel to the okinawa trough

high alkalinity due to sulfate reduction in the clam hydrothermal field okinawa trough abstract high alkalinity 1 8 mmol kg 1 ph 4 7 5 3 and ammonium 5 mmol kg 1 in hydrothermal fluids from the jade and clam sites in the mid okinawa trough back arc basin demonstrate organic matter decomposition during high temperature fluid sediment interaction the endmember fluid from the jade site 320 c black smoker which is free of mg and so4 has similar chemical characteristics as those at the escanaba trough one of the representative sediment hosted ridge systems in the eastern pacific on the other hand fluid samples from the clam site 100 c show remarkably high alkalinity 10 3 mmol kg 1 comparable to the highest alkalinity of 10 6 mmol kg 1 guaymas basin yet observed in hydrothermal fluids and significantly higher 34s of so4 25 than ambient seawater 21 it is suggested that sulfate reduction plays a dominant role in decomposing organic matter to increase alkalinity in the clam site variation of the 34s values of h2s from 0 2 to 3 0 in the clam fluids taken from different vents suggests that the clam site should consist of multiple local hydrothermal systems in different evolutionary stages with regard to sulfate reduction

helium and carbon geochemistry of hydrothermal fluids from the mid okinawa trough back arc basin southwest of japan abstract hydrothermal systems located in the okinawa trough back arc basin are considered to be related to rifting activities in the continental margin hydrothermal fluids sampled from two sites jade and clam are significantly enriched in gas species compared to those from mid oceanic ridges a high temperature fluid 320 c from the jade site shows a co 2 concentration of 200 mmol kg 1 which is more than one order of magnitude higher than that of sediment starved ridge crest hydrothermal systems both helium r r a 6 1 6 5 and carbon 13 c co2 5 0 to 4 7 pdb isotopic compositions are typical of island arc magmatic gases also the abundance ratio of co 2 3 he 3 10 10 is within the range of values found in island arcs chemical and isotopic composition of the jade fluid are considered to indicate that both co 2 and helium are derived from a magmatic source beneath the hydrothermal system without significant contamination contrary to these species ch 4 is attributed to a thermogenic origin based on its isotopic composition 13 c ch 4 41 to 36 pdb and high concentration 7 1 mmol kg 1 which values are similar to those observed in sediment hosted hydrothermal systems the clam fluids show gas geochemistry much more affected by fluid sediment interaction lower 3 he 4 he ratios r r a 3 7 3 8 and higher co 2 3 he ratios 10 11 may reflect involvement of gas species introduced during secondary fluid circulation within the sedimentary pile magmatic activity is the main contributor of gas species to the hydrothermal fluids in both the jade and clam sites although fluid sediment interaction modifies their composition the relatively high abundance of co 2 in the mid okinawa trough hydrothermal systems reflects the geochemical nature of island arc magma

volcano tectonic interactions during rapid plate boundary evolution in the kyushu region sw japan evolution of the local plate tectonic and volcanic system relationship at kyushu island is defined by major changes in tectonics and volcanic style at ca 15 10 6 and 2 ma plate reconstructions presented here suggest that prior to 15 ma the pacific plate subduction dominated kyushu tectonics from 15 to 6 ma the evolving relative plate motions shifted the triple junction between the pacific plate philippine sea plate and southwest japan northwards so that the philippine sea plate was subducted beneath kyushu we suggest that a lack of subduction related volcanism from 10 to 6 ma is due to shallow subduction of the young shikoku basin lithosphere by 6 5 ma changes in the philippine sea plate motion led to more rapid nearly trench normal subduction of the eocene west philippine basin crust beneath kyushu this model is supported by an increase in arc like geochemistry of lavas since ca 6 5 ma subduction of fluid rich features such as the kyushu palau ridge introduced large volumes of fluids into the kyushu arc system leading to voluminous volcanism across kyushu focused particularly in areas where the ridge subduction occurs in tandem with local extensional tectonics key issues such as the timing of izu arc collision with central japan and the history of motion of the philippine sea plate are reassessed here resulting in a model that favors izu arc central japan collision at ca 8 6 ma rather than the more widely accepted date of ca 15 ma

heat flow in the southwestern okinawa trough abstract lying to the southeast of the east china sea continental shelf the okinawa trough is believed to have been formed by crustal spreading behind the ryukyu island arc it has the highest mean heat flow among the pacific marginal basins the mean of 18 reliable measurements is3 34 2 75hfu the next highest mean heat flow for a marginal basin is2 53 1 60hfu for the fiji plateau the high mean value for the okinawa trough may be partially attributable to a sampling bias because most measurements were made in or near the well sedimented central rift or graben the reported values range from 0 36 to 10 49 hfu with low values probably resulting from hydrothermal circulation in an area of large relief the high heat flow suggests that the age of the okinawa trough may be as young as 2 ma such an estimate is in gross agreement with the inferred plio pleistocene age for the drifting in the okinawa trough the opening of the okinawa trough may continue onto taiwan but the relation between the opening of the trough and the occurrence of a nearby onshore geothermal resource area remains unsolved

the okinawa trough genesis of a back arc basin developing along a continental margin abstract the okinawa trough is the only example of a marginal back arc basin type at a young stage of evolution that developed along a continental margin the first phase of crustal stretching and subsidence occurred during late miocene early pliocene times a second major phase occurred after the latest pliocene or early pleistocene with en echelon rift axes along the axes of the okinawa trough oblique to the general trend of the arc and trench crustal separation and active spreading has occurred in the central and southern okinawa trough but the north okinawa trough is only in a rifting stage the development of the basin has several correlations with the general tectonism of the arc system 1 1 there was a synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the fore arc terrace 2 2 back arc rifting was initiated in the volcanic arc and propagated along it during the neogene remnants of a volcanic arc are found on the continental side of the basin the beginning of the back arc spreading observed in the central and southern okinawa trough correspond to a relative minimum in the activity of the volcanic arc 3 3 the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events such a collision lateral back arc opening model could explain the initiation of the opening of back arc basins but additional phenomena such as thermal convection retreating trench model or anchored slab model are indispensable to maintain extension in the back arc basin

microearthquakes and faulting in the southern okinawa trough abstract southern okinawa trough represents an early stage of back arc rifting and is characterized by normal faulting and microearthquakes earthquake distribution and deep structure of fault was investigated to clarify active rifting in the southern okinawa trough where two parallel grabens are located a network of ocean bottom seismometers obss that displayed the hypocenters of 105 earthquakes were observed for a period of 4 days in southern graben sg most of the microearthquakes occurred in a cluster about 7 km wide which on a cross section striking n45 e dips 48 to the southwest relocated hypocenters which are recorded by a local seismic network show scattered distribution around the southern graben there are no remarkable surface faults in the southern graben on the other hand the recalculation of hypocenter locations of 1996 earthquakes swarm recorded by a local seismic network suggests that the swarm is associated with normal faulting on the southern side of northern graben ng thus the undeveloped southern graben is located to the south of the developed northern graben southward migration of rifting which may be caused by migration of volcanism could thus be occurring in the southern okinawa trough the extension rate computed for the southern okinawa trough from the fault model of the northern graben is 4 6 cm year which is 59 102 of the extension rate gps measurements this result indicates that the majority of extensional deformation is concentrated within the center of the northern graben in the okinawa trough

geochemical character of silurian volcanism in sw ireland the dunquin group mid silurian of dingle southwest ireland is a sequence of basaltic to rhyolitic lavas and pyroclastic rocks interbedded with shallow to non marine sediments that records one of only four known outbreaks of silurian volcanicity south of the iapetus suture in the british isles new geochemical petrographic and field evidence indicate that the volcanic sequences exposed in the dunquin inlier and the islands of beginish and inishvickillane could have formed by fractionational crystallization in a layered repeatedly replenished basaltic magma chamber the site of which is probably coincident with an intense aeromagnetic anomaly to the southwest of inishvickillane the volcanic suite is subalkaline transitional between calc alkaline and tholeiitic and is non bimodal high lile hfse ratios and high lree concentrations suggest that the parent basaltic magmas were generated at a destructive continental margin although subduction may have ceased before eruption

systematic variation in chemistry and nd sr isotopes across a caledonian calc alkaline volcanic arc implications for source materials abstract sr and nd isotope compositions and major and trace element chemistry are reported for 42 primitive calc alkaline lavas from the siluro devonian scottish old red sandstone northwestward increases in concentrations of sr ba k p and lree associated with northwest subduction of iapetus ocean crust 1 are not accompanied by change in rb th or ti group elements these relationships show that the spatial variation cannot be simply the result of change in the degree of partial melting with depth initial 143 nd 144 nd expressed as e nd at 410 ma shows a wide range from 6 4 to 3 7 and in contrast to e sr decreases markedly to the northwest samples from the midland valley show a strong anticorrelation between e nd and e sr with the correlation band offset to more radiogenic compositions than that defined by modern oceanic volcanic rocks the mantle array such an offset is common in modern volcanic arc magmas but cannot be explained by addition to the overlying mantle of a single component enriched in radiogenic sr derived from subducted lithosphere southwest highland lavas lie in a completely separate field on an e sr e nd diagram most fall within the mantle array but samples with highest concentrations of sr etc are offset to less radiogenic compositions consideration of the chemical and isotopic relationships with respect to several models of crustal contamination shows that the additional lree in the northwest cannot be derived from a crustal source the data imply the existence beneath scotland of a large region of mantle with time integrated lree enrichment parts of which may have anomalous nd sr isotopic compositions less radiogenic than the mantle array midland valley lavas are probably derived from a long term lree depleted mantle source modified by interaction with subducted oceanic lithosphere the coexistence of two mantle sources with differing histories is best explained by vertical stratification with deeper more lree enriched layers accessible only to magmas formed at great depth in a subduction zone the spatial variation probably in part reflects this mantle layering and in part the operation of some mantle zone refining process that permits efficient concentration of incompatible elements from large regions of mantle

opening of the okinawa basin and collision in taiwan a retreating trench model with lateral anchoring using a two dimensional finite element model with an elasto plastic behavior we show that the opening of the okinawa basin behind the ryukyu trench since about 6 ma can be explained by a retreating trench model with lateral anchoring due to the collision in taiwan we assume that a suction force is applied to the edge of the overriding plate this force corresponds to the difference between the mean lithospheric pressure in the overriding lithosphere and the normal pressure applied to its edge it results in an outward motion of the edge of the continental margin retreating trench and in the opening of the okinawa basin if on the contrary no suction force is applied the collision in taiwan does not result in any extension in the okinawa area on both extremities of the ryukyu trench this outward motion is locked by the collision in taiwan and the buoyant subduction of the palau kyushu ridge it results in the arcuate shape of the ryukyu arc in order to explain the actual amount of extension in the okinawa basin it is however necessary to take into account the presence of the miocene volcanic arc which results in a weakened lithosphere in our model this weak zone is simulated by a lower yield condition finally the dissymmetry of the ryukyu arc and okinawa basin can be explained by a lateral variation of the suction force related to the variation of the length of the subducting slab thus of the slab pull force

development of the porcupine seabight use of magnetic data to show the direct relationship between early oceanic and continental structures the porcupine seabight is a broad composite rectilinear embayment in the margin of the irish continental shelf which seems to be floored at least in the southern part by oceanic crust there is a geometrically straightforward relationship between areas of crustal thinning rifting and the position of probable 9oceanic crust9 it is proposed that an important crustal unit here termed the porcupine segment includes the porcupine bank and the western end of the slyne ridge interpretation of a new magnetic map indicates that opening in the porcupine seabight appears to have taken place along a set of transform faults which 9root9 in an older dextral shear fault set oriented at about 130 three major zones along the longitudinal axis of the seabight relate to the geometry of magnetic stripes in the subjacent basement and can be used in conjunction with opening related to a distant pole of rotation to explain the overall present physical configuration of the basin the porcupine seabight probably opened no earlier than about 170 ma following the establishment of a jurassic volcanic province in the vicinity of the southern seabight elbow

japan sea opening history and mechanism a synthesis the respective tectonic effects of back arc spreading and continental collision in asia are considered either as two independent processes or as closely interrelated extrusion tectonics assumes that the opening of the south china sea and the left lateral motion along the red river fault are geometrically linked in a pull apart manner this model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated in the case of the japan sea we can show without ambiguity that back arc opening was controlled by large intracontinental strike slip faults which can be easily understood as effects of the india asia collision far from the indenter the japan sea opened in the early miocene in a broad pull apart zone between two major dextral strike slip shear zones the first one extends from north sakhalin to central japan along 2000 km it has accommodated about 400 km of finite displacement deformation along it varies from dextral transpression in the north to dextral transtension in the south the second is between korea and sw japan and has accommodated a smaller displacement of about 200 km the extensional domain in between lies in the back arc region of japan distributed stretching of the arc crust resulted in the formation of most of the japan sea while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the japan basin the first oceanic crust was formed in a small triangle based on the eastern shear zone and spreading propagated westward inside the pull apart region timing of oceanic crust formation of formation of the dextral shear zones and of block rotation in between as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history this evolution is discussed by comparison to other manifestations of the arc and back arc activity such as the history of sedimentation and volcanism the paper then suggests that the collision of india can have tectonic consequences as far north as japan and sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion

high resolution crosswell imaging of a west texas carbonate reservoir using crosswell data collected at a depth of about 3000 ft 900 m in west texas carbonates one of the first well to well reflection images of an oil reservoir was produced the p and s brute stack reflection images created after wavefield separation tied the sonic logs and exhibited a vertical resolution that was comparable to well log resolution both brute stacks demonstrated continuity of several reflectors known to be continuous from log control and also imaged an angular unconformity that was not detected in log correlations or in surface seismic profiling the brute stacks particularly the s wave reflection image also exhibited imaging artifacts we found that multichannel wavefield separation filters that attenuated interfering wavemodes were a critical component in producing high resolution reflection images in this study the most important elements for an effective wavefield separation were the time alignment of seismic arrivals prior to filter application and the implementation of wavefield separation filters in multiple domains particularly in common offset domain the effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals in this study 2 5 ft 0 76 m vertical sampling intervals for both source and receiver were used whereas most previous crosswell data sets were collected with much coarser sampling intervals resulting in spatial aliasing and limiting the utility of the data for reflection processing the wavefield separation techniques employed in this study used data volumes and associated filtering operations that were several orders of magnitude larger than those encountered in conventional vsp data analysis

inorganic and biogenic as sulfide precipitation at seafloor hydrothermal fields highlights as sulfides are minor minerals at volcanic arc and back arc hydrothermal settings there are two modes of as sulfide precipitation inorganic and biogenic the most stable as sulfide at low t seafloor hydrothermal conditions is orpiment biogenic type of orpiment represents completely mineralized fungal hyphae fungal orpiment precipitation is a mechanism of bio sequestration of hydrothermal as we investigated as sulfides orpiment as2s3 and realgar as4s4 from four seafloor hydrothermal fields three from back arc and one from mid ocean ridge settings our studies suggest two different modes of as sulfide precipitation at the seafloor hydrothermal sites inorganic and biogenic the eh ph modeling shows that the most stable as sulfide at low temperature t 100 c seafloor hydrothermal conditions is orpiment whereas realgar appears as a stable as sulfide phase at t 25 c and under a narrow range of eh ph values inorganically precipitated realgar forms euhedral crystals whereas the orpiment is mostly colloform the biogenic type of orpiment no realgar of this type was found represents completely mineralized fungal hyphae the proposed scenario of biogenic orpiment formation assumes that crack dwelling fungal filaments in the sub seafloor were killed by an as rich hydrothermal fluid moving along the same crack network the fungal organic matter served as a geochemical trap for hydrothermal as which reacted with s leached from the basement or reduced from seawater sulfate and mineralized the fungal hyphae as as2s3 this process of fungal orpiment precipitation is a mechanism of bio sequestration of hydrothermal as and another mode of soft tissue fossilization not described so far

the southwest ryukyu arc is a migrating microplate forearc sliver abstract we propose a new interpretation for the tectonism of the southwest ryukyu arc based on inland stress field analyses and the interpretation of seismic reflection profiles the recent stress field in the southwest ryukyu arc is under tension the axial direction of which is parallel to the trench axis but an e w dextral slip shear zone is developing at its western extremity yonaguni and westwards to taiwan these features are closely related to the degree of the oblique subduction of the philippine sea plate and the strength of coupling between it and southwest ryukyu arc we regard the southwest ryukyu arc as a migrating microplate the neotectonics of the southwest ryukyu arc are attributed to the westward migration of the microplate which is bounded by the miyako depression to the east and by taiwan to the west the microplate obducting onto the eurasian plate at taiwan started to migrate westward at about 4 ma when the relative motion between two plates philippine sea and eurasian changed direction from s n to se nw in region

three dimensional shallow structure from high frequency ambient noise tomography new results for the mygdonia basin euroseistest area northern greece we examine the use of ambient noise cross correlation tomography for shallow site characterization using a modified two step approach initially we extract rayleigh wave traveltimes from correlation traces of vertical component seismic recordings from a local network installed in mygdonia basin northern greece the obtained rayleigh wave traveltimes show significant spatial variability as well as distance and frequency dependence due to the 3 d structure of the area dispersion and anelastic attenuation effects the traveltime data sets are inverted through a surface wave tomography approach to determine group velocity maps for each frequency the proposed tomographic inversion involves the use of approximate fresnel volumes and interfrequency smoothing constraints to stabilize the results in the last step we determine a final 3 d velocity model using a node based monte carlo 1 d dispersion curve inversion the reliability of the final 3 d velocity model is examined by spatial and depth resolution analysis as well as by inversion for different model parameterizations the obtained results are in very good agreement with previous findings from seismic and other geophysical methods the new 3 d vs model provides additional structural constraints for the shallow sediments and bedrock structure of the northern mygdonia basin up to the depth of 200 250 m present work results suggest that the migration of ambient tomography techniques from large scales tens or hundreds of km to local scales few hundred meters is possible but cannot be used as a black box technique for 3 d modeling and detailed geotechnical site characterization

subduction related shoshonitic and ultrapotassic magmatism a study of siluro ordovician syenites from the scottish caledonides syenites are important or predominant components of several plutonic complexes emplaced between 456 and 415 ma along the nw margin of the caledonian orogenic belt adjacent to the lewisian foreland in w and nw scotland although there are in detail chemical differences between the syenites from each centre they form a well defined compositional group overall ratios amongst their trace elements especially very high values of la nb are quite different from those trachytes and syenites formed by fractional crystallisation of ocean island basalts and their continental equivalents emplaced in regions of anorogenic crustal tension instead the scottish caledonian syenites closely resemble chemically the fractional crystallisation residua of potassic subduction related magmas such as the shoshonitic series a comendite minor intrusion from a swarm associated with the loch borralan and loch ailsh syenitic complexes is remarkably similar in composition to recent obsidian from the shoshonitic volcano of lipari in the aeolian arc published sr and pb isotopic ratios preclude a significant component of either upper proterozoic moine schists or lower crust granulite facies archaean lewisian or proterozoic grenvillian gneisses in all these syenites except in local syenitic facies of the glenelg ratagain complex fractional crystallisation appears to be the mechanism by which the liquids which formed these syenites evolved from basic parental magmas the phases involved in this process may have included plagioclase alkali feldspar pyroxene amphibole biotite garnet fe ti oxide sphene allanite apatite zircon and zirconolite and therefore all the ratios amongst even the so called incompatible elements may have changed during the evolution of the leucocratic magmas nevertheless a detailed study of the glen dessarry complex shows that the changes are insufficient to disguise the geochemical nature of the parental magmas these appear to be picritic shoshonite mgo 15 ni 400ppm la yb 20 la nb 5 for the glen dessarry loch borralan and loch ailsh syenites and picritic ultrapotassic magmas mgo 15 ni 400 ppm la yb 60 la nb 7 such as would crystallise to minettes for the loch loyal and glenelg ratagain syenites mafic shoshonites were erupted amongst the 410 ma lome lavas of this region and also occur as widespread approximately contemporaneous volcanic feeder plugs minettes of similar age are also common as dykes in nw scotland numerous large tonalite granite complexes with minor diorites and gabbros were emplaced in w and nw scotland between 435 and 400 ma these are generally acknowledged to be mixtures of magmas from crustal and mantle sources their more mafic members show compositional features such as very high ba and sr which group them with the rare syenites and the lorne lavas of the same region into a distinctive geochemical province within which shoshonitic and allied magmas were the mantle derived component two models are presented to account for the generation of strongly potassic subduction related magmas several hundred km behind a nw directed subduction zone or during the period immediately following continental collision

geochemical constraints on the origin of bimodal magmatism at the okinawa trough an incipient back arc basin abstract the magmatism at the axial zone of the middle okinawa trough a young continental back arc basin comprises a bimodal basaltic rhyolitic suite accompanied by minor intermediate rocks we report major and trace element and sr nd isotopie data for the intermediate to silicic suites to provide constraints on their petrogenesis the rhyolites recovered as lava and pumice fall into three geochemical groups type 1 2 and 3 rhyolites type 1 rhyolites have 87sr 86sr 0 7040 0 7042 and 143nd 144nd 0 5128 0 5129 identical to those of associated basalts and are characterized by highly fractionated ree patterns petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt in contrast type 2 rhyolites and andesite have slightly higher 87sr 86sr 0 7044 0 7047 but similar 143nd 144nd 0 5128 compared to those of the basalts the compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization afc processes of the basalt magma quantitative analysis suggests assimilation fractional crystallization ma mc ratios of 0 05 hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present we emphasize that mechanical extension in this part of the okinawa trough involves gabbro ic lower crust that resulted from fractionation of mantle derived basaltic magmas type 3 rhyolite occurs only as pumice which makes its derivation questionable this rhyolite has major and trace element compositions and sr nd isotopie ratios which suggests that it may be derived from volcanic activity on the southern ryukyu volcanic front and arrived in the okinawa trough by drifting on the kuroshio current

geomorphological significance of the palaeodrainage network on a karst plateau the una korana plateau dinaric karst croatia abstract karst plateaus often have a complex geological and geomorphological history it is widely accepted that their development requires a long period of karst denudation this study reconstructs the palaeodrainage network of a karst plateau analyses its properties and establishes its geomorphological significance the main purpose of this research was to deepen our understanding of a key stage in the evolution of karst plateaus the transition from a fluvial land surface to one dominated by karst surface processes the study was conducted on a large part of the una korana plateau the largest plateau in the dinaric karst the majority of the plateau is made of carbonate rocks of triassic jurassic and cretaceous age that set the conditions for the development of the karst we have reconstructed the palaeodrainage network based on 1 25 000 topographic maps with 10 m contours the transition of the surface drainage network to the underground karst drainage network is still in progress so depending on the degree of karstification the drainage network was divided into three categories active dry and relict it was found that 90 5 of the pre existing drainage network has undergone some degree of karstification the active surface drainage network gradually shifted to a dry network then to a relict network the surface drainage network is gradually replaced by a dense network of dolines today the flat and karstified inter fluvial area is drained underground towards the main watercourses and these drain the entire region over the surface towards the pannonian basin this is the largest known karst palaeodrainage network in the dinaric karst that has been reconstructed in this way

crustal structure of the middle urals based on reprocessing of russian seismic reflection data summary reprocessed russian seismic reflection data combined with existing russian geological maps and seismic interpretations provide the basis for a revised tectonic interpretation of the upper crust in the middle urals the main uralian fault muf the principal suture of the urals separating the east european craton to the west from accreted terranes to the east is found to penetrate to depths of at least 18 km and perhaps much deeper with a uniform eastward dip of 35 after migration as the muf appears to truncate reflections in both the foreland and hinterland of the urals it is arguably one of the youngest structural features of the orogen basement rocks of the east european platform underthrust the suture and comprise the deep crust a minimum of 30 km east of the surface expression of the muf the tagil volcanic arc complex greater than 1000 km in length along strike comprises at least the upper 10 km of the crust east of the muf this terrane shallows eastwards where it appears to be truncated by the west dipping serov mauk fault based on correlation with surface geology the hinterland of the urals east of the muf is characterized by a previously unrecognized zone of discontinuous mid crustal reflectivity at depths of 14 16 km that is discordant with upper crustal reflections this mid crustal zone of reflectivity is interpreted to represent either a major subhorizontal structural detachment or a zone of magmatic intrusion

structural characteristics of the northern okinawa trough and adjacent areas from regional seismic reflection data geologic and tectonic implications abstract analysis of regional multi channel seismic data from the northern okinawa trough and adjacent shelf provides some important constraints on the structural development of the area the sedimentary strata in the northern east china sea shelf basin separated from the okinawa trough by the taiwan sinzi belt are affected by the miocene compressional tectonism and truncated by the late miocene unconformity in contrast those in the western margin of the northern okinawa trough are cut by numerous normal faults and the late miocene horizon forms a conformable surface this suggests that the taiwan sinzi belt acted as a buttress for the northern okinawa trough against the compressional tectonism our data also reveal the ho basin in the western margin of the northern okinawa trough previously known only from proprietary industry data the longwan ridge lying between the ho basin and the northern okinawa trough may be the youngest of the relict arcs or buried ridges in the area the abrupt along strike change in the fault polarity near the northern margin of the depocenter of the ho basin suggests a nw trending left lateral transfer zone or fault the western boundary of the northern okinawa trough is characterized by the gently deepening seafloor whereas that of the southern okinawa trough is marked by steep border faults suggesting more rapid subsidence in the south this together with the well developed symmetric axial faults in the south and the greater width and the lack of well developed axial zone in the north may suggest focused rifting extension in the southern okinawa trough and diffuse rifting extension in the northern okinawa trough we postulate that the diffuse rifting extension in the northern okinawa trough is due to the tectonic perturbation caused by the convergence or subduction of the topographically high and buoyant amami plateau at the northern ryukyu arc

island arc tectonics abstract tectonic stress states in arc backarc systems have been examined on simplified model structures corresponding to the chilean mariana and intermediate arc types two sources of tectonic stress are assumed the stress due to lateral density variations and the stress transmitted from the subducting oceanic plate to the forearc the average vertical stress zz of the former type has been calculated for each isostatically compensated model lithospheric plate using the difference from a reference structure from the sign of xx zz one can estimate whether the tectonic situation is extentional or compressional if we assume that xx is caused by mechanical coupling between the subducting oceanic slab and is laterally constant in one arc backarc system we can estimate at least the upper or lower boundary of the mechanical coupling for the chilean type arcs where the truly great interplate earthquakes occur and no backarc spreading takes place the transmitted average stress must be larger than 660 bar for the mariana type arcs where no large interplate earthquakes are recorded and spreading takes place in the backarc region xx should be smaller than 100 bar in any arc backarc system the most extentional feature is expected at the highest topography of the volcanic arc once spreading has started the viscosity under the backarc region drops dramatically and the spreading will continue in the backarc even if its topography is subdued in comparison with the now separated main and remnant arcs

is the southern uplands of scotland really an accretionary prism we argue that there is little which demands the accretionary prism model for the widespread silurian rocks of the southern uplands of scotland and ireland with new data from ireland we reinterpret these rocks as the turbidite fill of a successor basin that formed after the subduction of iapetus oceanic crust finally stopped at the end of the ordovician the structure of the basin is a sinistral transcurrent imbricate stack the result of oblique terminal collision of cadomia with laurasia at the end of the silurian

the lower silurian osmundsberg k bentonite part ii mineralogy geochemistry chemostratigraphy and tectonomagmatic significance the lower silurian osmundsberg k bentonite is a widespread ash bed that occurs throughout baltoscandia and parts of northern europe this paper describes its characteristics at its type locality in the province of dalarna sweden it contains mineralogical and chemical characteristics that permit its regional correlation in sections elsewhere in sweden as well as norway estonia denmark and great britain the 2 m clay fraction of the osmundsberg bed contains abundant kaolinite in addition to randomly ordered ro illite smectite i s modelling of the x ray diffraction tracings showed the i s consists of 18 illite and 82 smectite the high smectite and kaolinite content is indicative of a history with minimal burial temperatures analytical data from both pristine melt inclusions in primary quartz grains as well as whole rock samples can be used to constrain both the parental magma composition and the probable tectonic set ting of the source volcanoes the parental ash was dacitic to rhyolitic in composition and originated in a tec tonically active collision margin setting whole rock chemical fingerprinting of coeval beds elsewhere in baltoscandia produced a pronounced clustering of these samples in the osmundsberg field of the discriminant analysis diagram this together with well constrained biostratigraphic and lithostratigraphic data provides the basis for regional correlation and supports the conclusion that the osmundsberg k bentonite is one of the most extensive fallout ash beds in the early phanerozoic the source volcano probably lay to the west of baltica as part of the subduction complex associated with the closure of iapetus

weathering geomorphic work and karst landscape evolution in the cave city groundwater basin mammoth cave kentucky abstract following the pioneering work of wolman and miller wolman m g miller j p 1960 magnitude and frequency of forces in geomorphic processes j geol 68 54 74 in evaluation of geomorphic work and the frequencies and magnitudes of forces that drive it a large number of quantitative studies have focused on the evolution of fluvial systems and transport of clastic sediment less attention has been given to understanding frequencies and magnitudes of processes in rock weathering including investigation of rates at which solutes are removed from landscapes under various flow distributions as an analog to wolman and miller s wolman m g miller j p 1960 magnitude and frequency of forces in geomorphic processes j geol 68 54 74 concept of geomorphic work in this work we use 1 year of high resolution flow and chemical data to examine the work done in landscape evolution within and at the outlet of kentucky s cave city basin a well developed karst landscape aquifer system that drains about 25 km2 we consider both removal of solutes contributing to landscape denudation based on calcium mass flux as well as predicted dissolution rates of the conduit walls at the outlet of this basin based on limestone dissolution kinetics intense short duration events dominate storms that filled the logsdon river conduit occurred

geochemistry of holocene volcanic rocks associated with ridge subduction in baja california mexico although the geochemistry of magmatism associated with the subduction of oceanic lithosphere is well understood1 6 the geochemical signature of lavas produced shortly after subduction of an oceanic spreading centre has not been characterized the baja california peninsula in mexico provides an ideal scenario to study the latter process geophysical models indicate that in response to oblique collision of the ancestral east pacific rise epr with the western seaboard of north america at 29 myr subduction of the farallon plate was succeeded by the progressive southwards development of a transform fault system7 9 at 12 5 myr a considerable length of the epr was simultaneously subducted beneath baja thus terminating subduction processes along this segment10 by 3 5 myr the locus of transcurrent faulting had switched from the west of baja to within the developing gulf of california coupling baja to the pacific plate11 13 we present here data for holocene volcanic rocks from baja california norte which show that post ridge subduction volcanism has a distinctive geochemistry these results have important consequences for the interpretation of ancient orogenic belts

plate tearing in the northwestern corner of the subducting philippine sea plate the philippine sea plate phs simultaneously subducts northwestward and collides eastward with the eurasian plate eu in northeast taiwan these two tectonic events induce high seismic activity which makes northeastern taiwan one of the most seismically active zones in the world to understand the mechanical processes at work we used existing geophysical data and the aftershocks recorded following a recent large strike slip event occurring within the phs oceanic crust during this event a nw se trending left lateral sub parallel to the phs eu convergence vector was active as a consequence of the collision subduction plate geometry we show that the lithosphere of the northwestern corner of the phs has been torn in a nw se orientation this tectonic feature is associated with an abrupt tectonic stress boundary and could generate large intra plate earthquakes

tectonics of the westernmost gulf of aden and the gulf of tadjoura from submersible observations the gulf of tadjoura separating the danakil and somalia plates1 represents the western extension of the sheba ridge between arabia and somalia in the gulf of aden fig 1 it has been assumed that a simple plate tectonic scheme could be applied to the area with ne sw transform faults offsetting roughly east west zones of accretion represented by the major bathymetrie troughs1 8 fig 1c more recently this boundary has been interpreted as a westward propagating rift with its present day tip on land in the region of the asal lake9 10 here we present data from dives with the submersible cyana which show that there have been two episodes of extensional faulting an older inactive phase parallel to the general ene wsw direction of the gulf and a younger fault set along discontinuous zones striking 140 there is no surface evidence for any transform fault zone in the area covered and the incipient spreading segments whose geometry implies extension on 050 do not define a plate boundary the faulting observed is at odds with that predicted by orthodox plate tectonics

devonian intra oceanic arc rift sedimentation facies development in the gamilaroi terrane new england orogen eastern australia abstract silurian devonian rocks within the gamilaroi terrane part of the new england orogen of eastern australia provide a well preserved on land example of the development of an intra oceanic arc rift ten sedimentary facies represent different phases in the evolution of the gamilaroi rift and are typified by predominantly volcaniclastic conglomerates and mudstones with minor sandstones all of which were deposited by debris flows and turbidity currents subordinate facies include limestones crystal rich volcaniclastic sandstones volcanic breccias and olistostromes the facies are distributed throughout a succession of six lithostratigraphic units each of which represents a distinct phase in the history of the arc rift felsic volcanic and volcaniclastic rocks at the base of the section represent part of the original arc and are overlain by volcaniclastic sandstones and mudstones deposited within an arc basin lower devonian emsian limestones underwent rapid subsidence concurrent with an influx of volcaniclastic sediments indicating initiation of rift related faulting a thick conglomerate succession was deposited rapidly on a debris apron system adjacent to the steep sided rift walls overlying tuffaceous mudstones indicate a decrease in clastic input later in the history of the rift basin thick intervals of massive and pillowed basalts at the top of the succession indicate a major period of volcanism within the rift oceanic island arc rift sequences are characterised by highly variable sedimentary facies distribution facies distribution over the 400 km extent of the gamilaroi terrane is also highly variable modern arc rift zones are divided into a series of smaller separate elongate basins providing an explanation for the high facies variability observed in the field

seismic imaging of reflectors in the sg4 borehole middle urals russia the upper crust in the middle urals is known to be highly reflective however it is not clear whether the observed reflections originate from lithological boundaries or from fault zones the sg4 borehole located in the tagil synform of the middle urals drilled to 5354 m as of august 1995 presents one opportunity to study the source of this reflectivity the hole has penetrated several different lithologies as well as numerous tectonic or fracture zones the upper 5070 m consist of island arc rocks which have presumably been thrust on top of basalts to allow correlations between available surface seismic studies and the borehole observations vertical and offset seismic profile data were acquired in the borehole over the interval 520 m to 3940 m two shot points were used a near offset one at 135 m from the wellhead and a far offset one at 1845 m from the wellhead after processing the borehole seismic data show numerous reflecting horizons many reflections correlate with major fracture zones while others have no clear correlation with fracture zones nor lithological contrasts the interval between approximately 4450 m and 5100 m which is imaged below the bottom of the survey shows a layered reflective pattern that correlates with the lower part of a flysch unit which ends at 5070 m the basalts below appear to be relatively transparent a prominent east dipping reflector on a w e cdp line about 700 m north of the wellhead is not clearly imaged on the borehole seismic however in a pilot hole which deviates a few hundred metres in the northerly direction from the main hole a marked low velocity zone is observed at the corresponding depth for this reflector indicating its source to be from a fault zone that the fault zone is not clearly observed on the borehole seismic data is attributed to 3d effects

foreland basin development and tectonics on the northwest margin of eastern avalonia the early palaeozoic convergence of avalonia and laurentia created a foreland basin at the suture zone of the former lapetus ocean sedimentological and stratigraphic evidence of shallowing and contemporaneous shortening suggests that the southern part of the basin the windermere group became detached from its basement in the late ludlow and began to invert the detachment beneath the basin rooted into a northwest dipping mid crustal thrust system contemporaneous uplift to the north of the late silurian basin involved shortening of the avalonian foreland basement by thrusting basin inversion occurred ahead of a southeastward advancing mountain front we postulate a foreland southeast prograding sequence of thrusting through the ludlow in the lake district the basin continued to migrate onto the avalonian foreland through the early devonian ahead of an advancing orogenic wedge finally coming to a stop in the emsian

accelerated soil erosion in a karst area the burren western ireland abstract the burren is a plateau karst on the west coast of ireland it is characterized by thin soils patchy vegetation and large areas of bare rock supposedly a legacy of glacial erosion archeological and palynological evidence suggest that the area was well populated and forested in prehistoric times though now it is a marginal area agriculturally investigation of paleosols and of karren forms on ancient structures support the idea of an extensive cover of mineral soil removed by forest clearance initiated erosion over a relatively short period of time

active structures as deduced from geomorphic features a case in hsinchu area northwestern taiwan abstract two previously documented active faults in northwestern taiwan the hsinchu and hsincheng faults are evaluated for their earthquake hazard potential by using a digital elevation model dem aerial photos and field mapping both of these thrust faults contain active anticlines in their hanging walls based on folding of young alluvial deposits mapping of the hsinchu fault suggests the presence of numerous scarps oriented sub parallel to the fault trace that was previously defined by projection of subsurface geology we interpret the geometry of the surface scarps to be mainly produced by the thrust component but in part by a lateral component of fault slip the scarps associated with the hsincheng fault system are less complex and offset a flight of terraces including the modern flood plain a back thrust exposed 1 2 km east of the hsincheng fault is interpreted as linked with the main thrust surface deformation is only apparent south of the touchien river and becomes less obvious across another wrench fault system which is linked to hsinchu fault in the west and extends southeastward parallel to the touchien river the wrench fault system mentioned above not only offsets the hsinchu fault but also divides the hsincheng fault into segments the supporting evidence found for this includes pressure ridges and pull apart sags two associated hanging wall anticlines are both recently active based on deformed geomorphic surfaces two major ne striking thrusts and one nw wrench fault system are identified reflecting the compression stress is partitioned in this area which might be resulted from recently influenced extension tectonics i e okinawa back arc rifting

snowdon basalts and the cessation of caledonian subduction by the longvillian basalts of the bedded pyroclastic formation snowdon volcanic group north wales of longvillian caradoc age are heterogeneous in trace element composition forming a mixing trend between ocean island basalt oib type and magmatic arc basalt the two end member compositions were partial melts of two distinct upper mantle sources the oib end member is similar to icelandic oib and is the only known example of basalt of this type in the ordovician silurian welsh basin since ocean island basalt magma type is thought never to erupt above well established subduction zones the eruption of the longvillian oib provides a minimum age for the end of subduction of iapetus oceanic lithosphere below the welsh basin a model for the evolution of the mantle below the welsh basin is proposed which accounts for the geochemistry of magmas erupted from the tremadoc to the early silurian

okinawa trough genesis structure and evolution of a backarc basin developed in a continent abstract the okinawa marginal basin was opened by crustal extension into the asian continent north of the taiwan collision zone it is located behind the ryukyu trench subduction zone and the ryukyu active volcanic arc if we except the andaman sea the okinawa trough is the only example of marginal backarc basin type opened into a continent at an early stage of evolution active rifting and spreading can be observed synthesis of siesmic reflection seismic refraction drilling dredging and geological field data has resulted in interpretative geological cross sections and a structural map of the ryukyu okinawa area the main conclusions of the reconstruction of this backarc basin volcanic arc evolution are 1 backarc rifting was initiated in the volcanic arc and propagated along it during the neogene it is still active at both ends of the basin remnants of volcanic arc are found on the continental side of the basin 2 there was synchronism between opening and subsidence of the okinawa trough and tilting and subsidence of the forearc terrace the late miocene erosional surface is now 4000 m below sea level in the forearc terrace above the trench slope retreat and subsidence of the ryukyu trench line relative to the asian continental plate could be one of the causes of tilting of the forearc and extension in the backarc area 3 a major phase of crustal spreading occurred in pliocene times 1 9 my ago in the south and central okinawa trough 4 en echelon rifting and spreading structures of the central axes of the okinawa trough are oblique to the general trend of the arc and trench the ryukyu arc sub plate cannot be considered as a rigid plate rotation of 45 to 50 of the southern ryukyu arc since the late miocene is inferred the timing and kinematic evolution of the taiwan collision and the south okinawa trough opening suggest a connection between these two events the indentation process due to the collision of the north luzon arc with the china margin could have provoked lateral extrusion clockwise rotation 45 to 50 according to palaeomagnetic data and buckling of the south ryukyu non volcanic arc tension in the weak crustal zone constituted by the south ryukyu volcanic arc and opening of the south okinawa trough similar lateral extrusions rotations buckling and tensional gaps have been observed in indentation experiments additional phenomena such as thermal convection retreating trench model or anchored slab model could maintain extension in the backarc basin such a hypothetical collision lateral backarc opening model could explain the initiation of opening of backarc basins such as the mariana trough bonin trough parece vela shikoku basin and sea of japan a new late cenozoic palaeogeographic evolution model of the philippine sea plate and surrounding areas is proposed

seismic structure of the crust and uppermost mantle in the incipient stage of back arc rifting northernmost okinawa trough 1 in 1999 a marine seismic structure survey was conducted in the northernmost okinawa trough outcrops of the acoustic basement through the sedimentary layers are seen in the obtained structures velocities of lower layer in upper crust are varying laterally from 5 7 to 6 2 km s and thickness of 6 0 km s layer ranges from 3 to 9 km the lower crust has uniform thickness of about 15 km the pn velocity is 7 7 7 8 km s and the total crustal thickness is about 25 km including sedimentary layers the structure is in a first approximation similar to that of island arcs however the large heterogeneity of upper crust can be interpreted as a result of crustal thinning the low pn velocity and the outcrops of the acoustic basement relate to igneous activity in this area these results suggest that the northernmost part of the okinawa trough is in the incipient stage of a back arc rifting

depth dependency of stress ratios during the sedimentation of nw gyeongsang basin cretaceous southeast korea estimate of stress parameters and timing of tectonic episodes abstract the natural stress state in the lithosphere consists of the vertical load and poisson ratio and then additional horizontal compression and extension denoted by h and t respectively are assumed to be superimposed upon this gravitational stress field the resulting stress state is composed of the maximum medium and minimum stresses denoted by 1 2 and 3 respectively the stress ratio is given as 2 3 1 3 a linear relation is found between or 1 and the vertical load in wrench faulting and extensional stress regimes respectively the slope and intercept of the linear relation result in the additional horizontal stresses and level of average paleo surface respectively stress ratio is also determinable by the stress tensor inversion of fault slip data the earliest tectonic event t 1 event in the cretaceous gyeongsang basin consists of coexisting e w compression and n s extensional faulting episodes plots of or 1 against the burial depth or vertical load display several linear trends two clusters in extensional episodes and two or three clusters in wrench faulting episodes because h is assumed to be null or negligible in the extensional regime t is determinable from the slopes of two linear clusters as being 2 5 to 4 0 mpa in wrench faulting episodes the values of h are given to be 61 6 101 4 mpa by applying determined additional horizontal extensions determined levels of average paleo surfaces and those of syndepostional structures illustrate that more than five wrench faulting or extensional episodes have occurred during the t 1 event whose active age consequently ranges from the barremian to the coniacian this supports that the coexisting coaxial faulting episodes with the same extension may correspond to the alternation of wrench faulting and extensional episodes

two point ray tracing in general 3d media1 abstract the modelling of realistic 3d geology and wave propagation phenomena is an important aspect of exploration geophysics ongoing research on a new model representation in terms of strongly reflecting interfaces described by surface patches and on two point i e source receiver non zero offset ray tracing in such models is presented examples are used to demonstrate how this model representation facilitates the description of geological unconformities such as pinched out layers overhangs and reverse faults neither pseudo layers nor transparent or zero impedance extensions are needed for surfaces that do not naturally cover the geological window under study these are therefore no longer layered models and consequently they require new ray tracing techniques which are described and exemplified

monitoring interseismic activity on the ilan plain ne taiwan using small baseline ps insar gps and leveling measurements partitioning from arc continent collision and backarc extension

tomographic inversion of local earthquake data from the hengill grensdalur central volcano complex iceland parameters is generally low however a quantitative analysis of resolution provides an unambiguous assessment of well resolved volumes within the better resolved regions of the model the averaging volumes are 1 2 km and 2 4 km in vertical and horizontal extent respectively results of tomographic inversion image three distinct bodies of anomalously high velocity two of these extend from near the surface to a depth of about 3 km these high velocity volumes are located directly beneath the surface expressions of the extinct grensdalur volcano and the extinct husmuli basalt shield the third high velocity structure occurs in the depth range of 3 4 km but does not extend to the surface these three high velocity bodies are interpreted to be solidified magmatic intrusions relatively low velocities underlay limited portions of the trace of the present accretionary axis and a low velocity body is imaged in the roots of the active hengill volcano the volume of lower velocities located beneath the surface expression of the hengill volcano is interpreted to be a region of partial melt

extensional collapse of the northern taiwan mountain belt as an active collision zone between the luzon arc and the china continental margin the taiwan mountain belt particularly its south central part is undergoing crustal shortening and rapid uplift the northern part of the orogen is however subjected to crustal stretching and rifting as a result of flipping of subduction from the northwest facing luzon arc system to the south facing ryukyu arc system taiwan and its neighboring southern ryukyu used to be part of the rifted china continental margin before the collision started in late miocene time engineered by the northwest movement of the philippine sea plate in the past 5 m y the north south trending luzon arc obliquely overrode the northeast southwest trending continental margin and tectonized it into a collision orogen that progressively enlarged and migrated from northeast to southwest following the southwest propagating collision the north dipping philippine sea plate which was subducting beneath the ryukyu arc also extended westward and caused flipping of subduction in the northern part of the collision orogen in consequence the orogen lost the compressive support by the colliding plates and became subjected to lithospheric stretching induced by the trench suction the collision orogen collapsed as a result of crustal thinning and its central part foundered into a deep submerged basin a whole spectrum of active tectonic scenarios ranging from orogenic buildup by collision to orogenic collapse by rifting and subsidence can be readily observed in the taiwan ryukyu area and provides a vivid example for the process of orogenic collapse

sediment entrainment and transport in fluviokarst systems the primary geomorphic process active in the development of karst stream systems is generally regarded as bedrock dissolution however physical erosional processes may also be an important geomorphic agent in karst development the objectives of this study were to determine the sediment transport threshold in two fluviokarst streams the source of the sediment was internal to the karst basins the approach used was to calculate basal and critical shear stresses from streams in two karst systems to determine if and how frequently storm induced flows are capable of mobilizing stream sediment the fluviokarst systems investigated as a part of this research are capable of transporting 50 85 of their stream substrates during bankfull discharge conditions based on the discharge and precipitation frequency stream flows capable of entraining d50 and d85 particles occur at intervals of 2 4 and 11 7 months 0 98 yr respectively thus the sediment transport threshold in fluviokarst streams is exceeded by relatively common discharge events q 2004 elsevier b v all rights reserved

detailed volcanic and tectonic morphology of nakadomari hill in the southern okinawa trough abstract nakadomari hill is an unexplored edifice in the southern okinawa trough back arc basin where crustal extension by continental rifting occurs and few submarine eruptions associated with back arc volcanism have been mapped in detail high resolution bathymetric mapping with an autonomous underwater vehicle auv reveals that nakadomari hill is a submarine volcanic complex with recent lava flow eruptions the high resolution 2 m bathymetry shows striking contrast between rugged terrain in the central area and the adjacent smooth terrain the cone shaped features in the rugged terrain are covered by ropey fabric with high acoustic reflectivity and are devoid of tectonic deformation thus they are interpreted to be relatively younger lava domes the cone shaped features in the smooth terrain show low reflectivity and are dissected by ene wsw trending tectonic lineaments suggesting they were formed during older volcanism nakadomari hill developed through multi stage eruptions before and after back arc rifting and faulting the ropey and curtain folded morphology with high acoustic reflectivity are interpreted to be a field of lava flows and they are relatively young the area considered to be lava flows is estimated to be over 13 km2 showing that eruptions in the southern okinawa trough have been extensive an active volcanic system is supported by indicators of high temperature hydrothermal venting hydrothermal plumes with concurrent sharp temperature increases of up to 0 17 c were present even at the vehicle altitude of 100 m above the bottom the alignment of volcanic cone shaped features at nakadomari hill is in the nnw sse direction which is clearly different from the orientation of local rifting ene wsw at a larger scale the volcanic edifices west of kume island from the arc to back arc are also aligned roughly in the nnw sse direction and they are sub parallel to a major tectonic structure in the arc the kerama gap the agreement of the local volcanic and tectonic trends implies that magmatic focusing may have taken place along the tectonic structures of the kerama gap and in turn caused the extensive back arc volcanism in this area

the hydraulics of conduit flow in carbonate aquifers abstract analysis of the morphology of dissolution bedform assemblages and hydraulically transported sediments found within conduits in carbonate aquifers in north america and the british isles has allowed the hydraulic conditions under which conduit flows occur to be established mean values of flow velocity boundary shear stress conduit reynolds number conduit froude number boundary friction factor boundary roughness and flow power have been calculated the values obtained are in agreement with other evidence in the conduits and are comparable to those obtained from other similar hydraulic systems

deep structure of southern ireland a new geological synthesis using birps deep reflection profiling the birps deep seismic reflection data of wire west of ireland 1 1b from galway bay to the celtic sea are integrated with geological and other geophysical data to produce a synthesis of the fundamental tectonic character of the crust in and around southern ireland the iapetus suture forms a zone of north dipping reflectors which projects up between the shannon estuary and the dingle peninsula and is overlain by a late caledonian reflector with a shallow south dip across the iapetus suture zone the crust thickens southward and the lower crust becomes less reflective it is proposed that the lower palaeozoic volcano sedimentary rocks which form the leinster massif extend southwestward beneath the variscan province and at least 30 km offshore to the wire lines we argue that the weak south dipping reflectors in the upper and middle crust south of the iapetus suture represent an early caledonian fault system linked to a mid crustal detachment which formed during the south dipping subduction event recorded onshore by the ordovician volcano sedimentary successions these faults were selectively reactivated causing the predominant ene trend of younger regional tectonic features in and around southern ireland the three onshore upper palaeozoic basins close east of the wire lines we point out that the variscan front is only a map line across the british isles with no consistent tectonic character thus the south dipping reflector below the north margin of the north celtic sea basin on swat 4 generally referred to as the variscan front is correlated by us with the northern boundary of the rosslare complex thus making it the northern tectonic boundary of the caledonian basement block known as the rosslare monian terrane furthermore the north dipping south irish sea lineament on winch 4 can be correlated with the menai straits lineament and could thus be the southern boundary of this terrane the south dipping reflector on swat 2 also previously called the variscan front is correlated with the johnston benton fault system in southwest wales

flow models for back arc spreading abstract a secondary flow model for back arc spreading is developed in this paper that shows some of the characteristics of observed back arc spreading back arc spreading has formed marginal seas around the west and southwest rim of the pacific the episodic spreading and different directions of opening are not completely understood however there does appear to be a limited lifetime

discussion on no exploits back arc basin in the iapetus suture zone of ireland journal of the geological society london 172 740 747 mcconnell et al 2015 claimed that the absence of middle ordovician back arc volcanic rocks in the bellewstown terrane in the irish caledonides implies that the tetagouche exploits back arc basin never existed in ireland the emphasis on the absence of evidence surprised us because absence of evidence is not evidence of absence however if they are correct their interpretation has major implications for the tectonic processes responsible for the final closure of iapetus

local efficiency in fluvial systems lessons from icicle bend development of fluvial systems is often described and modeled in terms of principles related to maxima minima or optima of various hydraulic or energy parameters that can generally be encompassed by a principle of efficiency selection more efficient flow routes tend to be preferentially selected and enhanced however efficiency selection is highly localized and the cumulative effects of these local events may or may not produce more efficient pathways at a broader scale this is illustrated by the case of icicle bend on shawnee run a limestone bedrock stream in central kentucky field evidence indicates that a paleochannel was abandoned during downcutting of the stream and the relocation was analyzed using a flow partitioning model the bend represents abandonment of a steeper straighter more efficient channel at the reach scale in favor of a longer currently less steep and less efficient flow path this apparently occurred owing to capture of shawnee run flow by a subsurface karst flow path that was subsequently exhumed the development of icicle bend illustrates the local nature of efficiency selection and the role of historical contingency in geomorphic evolution

simultaneous inversion for hypocenters and lateral velocity variations an iterative solution with a layered model an iterative inversion technique has been developed that uses the direct p and s wave arrival times from local earthquakes to compute simultaneously a three dimensional velocity structure and relocated hypocenters crustal structure is modeled by subdividing flat layers into rectangular blocks an interpolation function is used to smoothly vary velocities between blocks allowing ray trace calculations of travel times in a three dimensional medium tests using synthetic data from known models show that solutions are reasonably independent of block size and spatial distribution but are sensitive to the choice of layer thicknesses application of the technique to observed earthquake data from north central utah shows the following 1 lateral velocity variations in the crust as large as 7 occur over 30 km distances 2 earthquake epicenters computed with the three dimensional velocity structure were shifted an average of 3 0 km from locations determined assuming homogeneous flat layered models and 3 the laterally varying velocity structure correlates with anomalous variations in the local gravity and aeromagnetic fields suggesting that the new velocity information can be valuable in acquiring a better understanding of crustal structure

no exploits back arc basin in the iapetus suture zone of ireland a controversial aspect of the closure history of the iapetus ocean concerns the existence of the exploits basin in ireland and britain the exploits tetagouche back arc basin of the canadian appalachians opened during the middle ordovician as the popelogan victoria volcanic arc migrated northwards from the ganderian margin the bellewstown terrane within the iapetus suture zone of ireland lies between the ganderian avalonian leinster terrane and the laurentian grangegeeth terrane an early ordovician celtic shelly fauna is hosted in volcanogenic breccia that we demonstrate is stratigraphically overlain by shales with no volcanic horizons the lower 24 m of shale are unfossiliferous but shale above this stratigraphic level has yielded a new graptolite fauna consistent with the upper part of the artus biozone darriwilian c 464 ma u pb zircon dating of the volcanic horizon yields an age of c 474 ma the shale therefore appears to represent c 10 myr of deposition and a significant gap in volcanism this together with a sandbian anglo welsh fauna suggests that the bellewstown terrane remained in a peri gondwanan position throughout the ordovician hence exploits back arc basin opening and ganderian arc migration did not occur in the irish sector of iapetus supplementary materials the table of u pb zircon data cl images of zircon grains concordia plot of all u pb zircon analyses and additional drawings and notes on the graptolite fauna are available at www geolsoc org uk sup18858

earthquake doublet in an active shear zone southwest japan constraints from geophysical and geochemical findings abstract in 1997 the kagoshima earthquake doublet consisting of two closely associated mw 6 strike slip events five km and 48 days apart has occurred in southwest japan the location is where an e w trending discontinuity along 32 n latitude on southern kyushu island is clearly defined in gps velocities indicating the presence of a highly active left lateral shear zone however there have not been any obvious indications of active faulting at the surface prior to the earthquake doublet which could be associated with this shear zone three dimensional inversion of magnetotelluric sounding data obtained in the source region of the earthquake doublet reveals a near vertical conductive zone with a width of 20 km extending down to the base of the crust and perhaps into the upper mantle toward the okinawa trough the prominent conductor corresponds to the western part of the active shear zone elevated 3 he 4 he ratios in groundwaters sampled from hot spring and drinking water wells suggest the emission of mantle derived helium from the seismic source region the geophysical and geochemical observations are significant indications that the invasion of mantle fluids into the crust driven by upwelling asthenosphere from the okinawa trough triggers the notable left lateral shearing in the zone in the present day subduction system in addition the existence of aqueous fluids in and below the seismogenic layer could change the strength of the zones and alter the local stress regime resulting in the occurrence of the 1997 earthquake doublet

the iapetus suture in the british isles the position of the iapetus ocean suture can be traced between ordovician fossil localities in the southern uplands and the lake district which contain respectively distinct north american and european faunas the southern uplands contains north american caradoc faunas in the northern belt and is considered to have been accreted onto the laurentian north american margin between the llandeilo and the end of the wenlock deep seismic reflection profiling shows that a surface probably parallel to the suture zone dips down to the northwest from the solway line in eastern ireland the surface trace of the suture coincides with the navan fault which separates the longford down massif from ordovician sediments containing european faunas the navan silvermines fault may represent the surface expression of the suture but it is masked by wenlock turbidites which appear to have crossed over the plate boundary northward subduction of continental crust below the margin of laurentia during the late silurian may explain the influx of turbidite fans from the north or north west onto depressed crust to the south of the suture termination of subduction coincides with the widespread acadian orogeny in the emsian at 395 5 ma

reply to discussion on no exploits back arc basin in the iapetus suture zone of ireland journal of the geological society london 172 740 747 we thank van staal winchester 2017 for their discussion of our paper mcconnell et al 2015 and their concise summary of the evolution of the popelogan victoria arc tetagouche exploits basin and ganderian margin in the northern appalachians they make the case that we have failed to recognize the same tectonic elements and geological history in ireland they propose that we in concluding there was no exploits basin equivalent in ireland have an absence of evidence rather than evidence of absence we have documented a stratigraphical sequence that provides evidence of a 10 myr gap in volcanism at bellewstown c 474 464 ma at a time when the tetagouche exploits basin was actively opening further to the west along the orogen we have placed that observation in the regional context of a middle late ordovician stratigraphical and faunal linkage to the leinster terrane parkes harper 1996 where volcanism demonstrates continued subduction under a north facing volcanic arc on the ganderian margin mcconnell 2000 we concede that the bellewstown succession if taken in isolation could represent deposits on the passive margin of the tetagouche exploits basin but the darriwilian sandbian volcanic rocks of north leinster are too voluminous too young and geochemically incompatible with representing remnants of the passive margin of a tetagouche exploits basin aged rifted arc basin

a polygenetic palaeosol from the silurian wenlock of southwest ireland a problematic palaeosol unit caps a thick basalt at the top of the clogher head formation dunquin group late wenlock silurian in the dingle peninsula southwest ireland the main part of the unit consists of an altered calc alkaline basalt with a fine clay assemblage of chlorite corrensite and smectite probably resulting from weathering during wenlock times the uppermost part of the unit consists of detrital silicates which were admixed by bioturbation and well defined burrow fills are present the underlying weathered basalt was possibly truncated before the detrital horizon was deposited and finally the whole unit underwent a phase of surface water gleying a catenary relationship existed locally with the pseudo gley developed lower in the landscape while contemporaneous basalts in topographically higher better drained areas largely escaped the effects of gleying these now exhibit reddened tops the complex history and the possiblity of the incompleteness of the profile makes accurate assessment of the soil climate function difficult however the occurrence of calcrete bearing palaeosols in the overlying mill cove formation supports the view that the basalt was weathered under a sub humid or semi arid climate considering the alteration of the dunquin group to actinolite prehnite grade the preservation of the smectite is highly unusual there is no evidence for its origin due to more recent weathering or interstratal neoformation it is thus interpreted as a relict product of the original weathering phase

the newer granite problem a geotectonic view the siluro devonian suite of granitic plutons in the british caledonides known as the newer granites together with their associated extrusive rocks represent one of the most extensively researched examples of calc alkaline magmatism apparently related to orogeny although recent chemical studies have credibly interpreted some of the scottish intrusions and volcanic rocks as part of a continental margin magmatic arc generated by the subduction of lapetus oceanic lithosphere beneath laurentia insurmountable problems of distribution and timing arise when attempts are made to relate the magmatic activity as a whole to a traditional two plate collision model for the orogeny newer granite magmatism is here discussed in the context of more mobilistic models for the post grampian evolution of the british caledonides which involve e w closure between laurentia and baltica terminated by collision in the silurian followed by the northward accretion of gondwana derived terranes in the early devonian the former produced the main caledonian tectonometamorphism of the northern highlands of scotland the latter the late caledonian deformation of the slate belts in the paratectonic caledonides these models imply much more complex convergence geometries which can in principle account for the whole newer granite suite as a series of subduction generated magmatic arcs overlapping in space and time the model proposed involves three late caledonian magmatic arcs in addition to the ordovician borrowdale arc which is not considered in this paper one is related to laurentia baltica convergence with westward subduction beneath the scottish sector of the laurentian margin in the ordovician and early silurian which generated the early members of the newer granite suite in the highlands a second is related to northward silurian early devonian subduction at the solway line which generated the younger newer granites and volcanic rocks north of the highland border and a third related to northward accretion of the armorican terrane in early devonian time produced intrusive and extrusive magmatism as far south as southeast ireland and the english midlands

methane seepage inferred from pore water geochemistry in shallow sediments in the western slope of the mid okinawa trough abstract widely distributed cold seeps are crucial sources of carbon to the seawater whereas anaerobic oxidation of methane aom and precipitation of authigenic carbonate might change the compositions of the methane rich fluids and thus reduce the outputs of dissolved carbon to seawater in this work we analyze the pore water compositions of four gravity cores with high methane concentration abnormalities in pore water or in overlying seawater in the western slope of the mid okinawa trough for the northern research area active weak methane seepage through the seafloor is identified in the vicinity of mud volcanoes sites c01 and c10 and strong emissions of gas and fluids may occur in the central mud volcanoes in a submarine canyon at the south c23 site exhibits rarely methane seepage while high rate transportation of methane from deep sediments and associated aom are identified at site c25 where small vertical faults and gas bearing layers are developed at the surrounding aom at site c25 is indicated by 1 quasi linearly decrease of sulfate concentrations with depths 2 intense increases of dissolved inorganic carbon dic concentrations and 3 significantly negative 13c values vary from 7 2 to 26 3 of dic organoclastic degradation in the shallow sediments is not obviously recognized in the study area because of the low concentrations of nh4 as well as negligible sulfate consumption at three of the four sites our data reveal that the cold systems are one of the dic sources the fluxes are ca 11 2 mmol m2 yr to the seawater in the okinawa trough which can be tracked by high depletion of 13c 13cdic ranges from 7 2 to 26 3 vpdb

stratigraphical and structural relationships of the dingle group silurian county kerry ireland previous views on the relationship between the thick continental deposits of the dingle group and the older fossiliferous silurian rocks of the dunquin group are briefly reviewed detailed field evidence is presented showing that in the coastal sections of coosgorrib coosglass and coosshaun there is faulting between the two groups the same situation obtains at the northern end of the great blasket island fortunately inland exposures are sufficient to show that there is stratigraphical continuity from the lower ludfordian ludlow beds of the croaghmarhin formation at the top of the dunquin group into the purple siltstones of the dingle group the dingle group is followed uncomformably by upper old red sandstone essentially upper devonian

a deductive model of karst evolution based on hydrological probability combination of a conduit flow law with the exceedance probability of stream discharge allows the estimation of the exceedance probability of water level in a simple conduit aquifer the probability of water levels higher than available relief is interpreted as the probability of surface as opposed to underground runoff very high probability of surface runoff implies a fluvial environment whereas very low probabilities define a mature karst or holokarst intermediate probabilities identify fluviokarst overflow probability depends on available relief mean discharge and especially conduit radius growth rate of the underground conduit depends on saturation deficit which thus controls the rate of evolution of the fluvial landscape through fluviokarst to holokarst however variations of discharge and sedimentation through time can cause dramatic reversion of karst drainage into less mature states landscapes experiencing such periodic rejuvenation will have a confused morphology a functional definition of landscape may be more objective and pertinent than arbitrary interpretation of form

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

a rigid block rotation model for the gps derived velocity field along the ryukyu arc abstract using a gps derived velocity field we try to estimate the kinematic features of the ryukyu arc and discuss its implications to the spreading of the okinawa trough the gps derived strain rate field implies that internal deformation within the ryukyu arc is not significant therefore we adopt a rigid block rotation model for the motion of the ryukyu arc the velocity field in the ryukyu arc is best explained by a model with three blocks rather than one with only one block the estimated block rotations predict the spreading along the okinawa trough the predicted spreading is slow 10 mm per year in the north and fast 50 mm per year in the southwest the direction of the predicted spreading is normal to the okinawa trough n170 e n180 e in the southwest while in the north the direction of the spreading is oblique to the okinawa trough n150 e n180 e