a subducting seamount beneath the nankai accretionary prism off shikoku southwestern japan a multi channel seismic mcs reflection survey was conducted to study the structure of the nankai convergent margin off shikoku island in july 1997 based on reflection characteristics we could identify three major seismic reflection units i e units a b and c the mcs data as well as swath bathymetric data reveal a buried circular seamount subducting beneath the nankai accretionary prism the subducting seamount is responsible for deformation of the accretionary wedge resulting in a compressed uplifted sediment knoll and many steep escarpments with ne sw strike a possible thrust fault was identified on the seaward flank of the seamount indicating compressive deformation a tectonic model for subduction of the seamount is presented based on the interpretation of the mcs data

subduction of the daiichi kashima seamount in the japan trench lallemand s culotta r and von huene r 1989 subduction of the daiichi kashimi seamount in the japan trench in j p cadet and s uyeda editors subduction zones the kaiko project tectonophysics 160 231 247 in 1984 1985 the kaiko consortium collected seabeam single channel seismic and submersible sampling data in the vicinity of the daiichi kashima seamount and the southern japan trench we performed a prestack migration of a shell multichannel seismic profile that crosses this area and examined it in the light of this unusually diverse kaiko dataset unlike the frontal structure of the northern japan trench where mass wasting appears to be the dominant tectonic process the margin in front of the daiiclr kashima shows indentation imbrication uplift and erosion emplacement of the front one third of the seamount beneath the margin front occurs without accretion we conclude that the daiichi kashima seamount exemplifies an intermediate stage between the initial collision and subduction of a seamount at a continental margin

remagnetization of the coast range ophiolite and lower part of the great valley sequence in northern california and southwest oregon overprinted magnetizations have been found at four localities in the middle jurassic coast range ophiolite and the overlying upper jurassic and lower cretaceous great valley sequence in northern california and at one locality in the partially correlative lower cretaceous days creek formation in southwest oregon at del puerto canyon on the east side of the diablo range a pilot study of the upper jurassic lotta creek formation gives in situ paleomagnetic directions grouped around the present geomagnetic field suggesting magnetic overprinting long after deposition and folding we suggest that the loss of original magnetization could be the result of long burial at about 7 km depth followed by late tertiary uplift the possibility of chemical remagnetization however can not be excluded paleomagnetic data from sandstones from the lower cretaceous great valley sequence in the wilbur springs area suggest cenozoic remagnetization that could be related to pliocene and pleistocene volcanic and hydrothermal activity in the area the results from calcareous concretions in the wilbur springs area from 21 sites from basalts and overlying sedimentary rocks of the great valley sequence at stonyford and from several sites in various rock types in the coast range ophiolite and great valley sequence near paskenta suggest unblocking of magnetization during the long period of burial at depth of 7 km or more and remagnetization during tectonic uplift in the late cretaceous or tertiary chemical remagnetization however can not be excluded the paleomagnetic data from six sites in lower cretaceous sedimentary rocks of the days creek formation in southwest oregon which overlies a dismembered ophiolite fail the fold test in situ paleomagnetic directions group near the expected tertiary field directions these sedimentary rocks may have been remagnetized during a major eocene tectonic event known to have occurred in this region the paleomagnetic results confirm that the coast range ophiolite and the seemingly little deformed great valley sequence in northern california as well as correlative rocks in southwest oregon have had a complex tectonic history once the nature and timing of these events are better understood the timing of remagnetization may be further constrained which in turn could give further insight into the nature of the tectonic events

tectonic evolution of a continental collision zone a thermomechanical numerical model we model evolution of a continent continent collision and draw some parallels with the tectonic evolution of the himalaya we use a large scale visco plasto elastic thermomechanical model that has a free upper surface accounts for erosion and deposition and allows for all modes of lithospheric deformation for quartz olivine rheology and 60 mm yr convergence rate the continental subduction is stable and the model predicts three distinct phases during the phase 1 120 km or 6 of shortening deformation is characterized by back thrusting around the suture zone some amount of delaminated lower crust accumulates at depth during phase 2 120 km 420 km or 6 22 of shortening this crustal root is exhumed medium to high grade rocks along a newly formed major thrust fault this stage bears similarities with the period of coeval activity of the main central thrust and of the south tibetan detachment between 20 16 myr ago during phase 3 420 km or 22 of shortening the crust is scraped off from the mantle lithosphere and is incorporated into large crustal wedge deformation is localized around frontal thrust faults this kinematics should produce only low to medium grade exhumation this stage might be compared with the tectonics that has prevailed in the himalaya over the last 15 myr allowing for the formation of the lesser himalaya the experiment is conducted at constant convergence rate which implies increasing compressive force considering that this force is constant in nature this result may be equivalent to a slowing down of the convergence rate as was observed during the india asia collision

geology petrology and tectonic setting of the late jurassic ophiolite in hokkaido japan abstract the gokurakudaira formation which has a n s zonal distribution within a latest jurassic greenstone belt in hokkaido island japan constitutes the uppermost ultramafic mafic unit of the horokanai ophiolite the following three hypotheses for the origin of the ophiolite have been proposed 1 a mid oceanic ridge 2 an oceanic plateau and 3 an island arc the gokurakudaira formation can be subdivided into four zones extending nnw to sse from east zone i to west zone iv based on lithofacies and areal distribution zones i and iii consist of aphyric tholeiite resembling back arc basin basalt babb while zone ii is characterized by the coexistence of babb like tholeiite along with high mg andesite zone iv has a different lithology from the other zones and is composed mainly of picrite and thick sedimentary sequences of island arc tholeiite iat type andesitic subaqueous pyroclastic deposits and terrigenous sediments these stratigraphic and petrological characteristics of the gokurakudaira formation cannot be explained by the oceanic plateau or mid oceanic ridge models but they could correspond to the marginal sea model as in the lau basin therefore we conclude that the horokanai ophiolite was formed in the late jurassic in a marginal basin above a supra subduction zone on the margin of the asian continent

the geology of north and central malaita solomon islands the thickest and most accessible part of the world s largest ontong java ocean plateau

tectonic implications of diverse igneous blocks in franciscan melange northern california and southwestern oregon more than 50 of the igneous blocks from melanges of the franciscan subduction complex in northernmost california and from the equivalent dothan formation of southwestern oregon are more silica rich than basalt and some contain supra subduction zone geochemical signatures such as elevated th ta ratios in contrast blocks from more southerly parts of the franciscan melange belt are more commonly mid ocean ridge and ocean island seamount basalts the data indicate a north south variation in the block population due to differences in source terrane or in the processes that provided the blocks and transported them into the melange or both we suggest that in the northern franciscan a relatively greater fraction of igneous melange blocks was emplaced by block slumping from the forearc crust into the trench than was the case to the south

crustal structure and tectonics of the hidaka collision zone hokkaido japan revealed by vibroseis seismic reflection and gravity surveys abstract this study is the first integrated geological and geophysical investigation of the hidaka collision zone in southern central hokkaido japan which shows complex collision tectonics with a westward vergence the hidaka collision zone consists of the idon nappu belt ib the poroshiri ophiolite belt pob and the hidaka metamorphic belt hmb with the hidaka belt from west to east the pob metamorphosed ophiolites is overthrust by the hmb steeply eastward dipping palaeo arc crust along the hidaka main thrust hmt and in turn thrusts over the idon nappu belt melanges along the hidaka western thrust hwt seismic reflection and gravity surveys along a 20 km long traverse across the southern hidaka mountains revealed hitherto unknown crustal structures of the collision zone such as listric thrusts back thrusts frontal thrust and fold structures and duplex structures the main findings are as follows 1 the hmt which dips steeply at the surface is a listric fault dipping gently at a depth of 7 km beneath the eastern end of the hmb and cutting across the lithological boundaries and schistosity of the hidaka metamorphic rocks 2 a second reflector is detected 1 km below the hmt reflector the intervening part between these two reflectors is inferred to be the pob which is only little exposed at the surface this inference is supported by the high positive bouguer anomalies along the hidaka mountains 3 the shallow portion of the ib at the front of the collision zone has a number of nne dipping reflectors indicative of imbricated fold and thrust structures 4 subhorizontal reflectors at a depth of 14 km are recognized intermittently at both sides of the seismic profile these reflectors may correspond to the velocity boundary 5 9 6 6 km s previously obtained from seismic refraction profiling in the northern hidaka mountains 5 these crustal structures as well as the back thrust found in the eastern end of the traverse represent characteristics of collisional tectonics resulting from the two collisional events since the early tertiary

the coal mine roof rating cmrr a decade of experience abstract the coal mine roof rating cmrr was developed 10 years ago to fill the gap between geologic characterization and engineering design it combines many years of geologic studies in underground coal mines with worldwide experience with rock mass classification systems like other classification systems the cmrr begins with the premise that the structural competence of mine roof rock is determined primarily by the discontinuities that weaken the rock fabric however the cmrr is specifically designed for bedded coal measure rock since its introduction the cmrr has been incorporated into many aspects of mine planning including longwall pillar design roof support selection feasibility studies extended cut evaluation and others it has also become truly international with involvement in mine designs and funded research projects in south africa canada and australia this paper discusses the sources used in the development of the cmrr describes the cmrr data collection and calculation procedures and briefly presents a number of practical mining applications in which the cmrr has played a prominent role

processes of seamount subduction in the mariana and izu bonin trenches abstract morphology of oceanic plate seamounts near the mariana and izu bonin trenches in the western pacific indicates that the response of these seamounts to subduction related tectonic processes is a function of two factors seamount size and the degree of fracturing of the surrounding sea floor as the oceanic plate bends prior to subduction seamounts smaller than about 40 km in diameter are fractured on the same scale as or to a greater degree than the surrounding sea floor larger seamounts 40 100 km in diameter show less evidence of fracturing prior to subduction since there is no evidence of accretion of large amounts of plate material onto the mariana fore arc it is probable that plate seamounts are subducted at least partially intact the subduction of plate seamounts beneath the mariana arc may be responsible for some areas of uplift on the outer mariana fore arc broad areas of uplift on the mariana and bonin fore arcs are associated in some cases with apparent subduction of ridges of large greater than 100 km diameter seamounts the large seamounts at the western end of the michelson ridge appear to be breaking up into a series of blocks separated by en echelon fractures that are oriented oblique to the trench axis and lie in line with the convergence direction of the pacific plate such large seamounts may interrupt normal subduction processes this suggestion is consistent with the occurrence of gaps in benioff zone seismicity where chains of such large seamounts abut the trenches it appears likely that these large seamounts are also eventually subducted although they may first undergo significant fracturing and reorientation

accretion and exhumation structures formed by deeply subducted seamounts in the kamuikotan high pressure temperature zone hokkaido japan 1 the lower cretaceous iwashimizu complex in the southern kamuikotan zone japan was formed by accretion of seamounts which subducted to the lawsonite albite to blueschist facies the complex is a pile of four nappe units each unit comprises a duplex with horses of seamount volcanics sedimentary rocks and ramp shear zones suggesting a thin skinned peeling of subducted seamounts each unit bounded fault is regarded as the floor thrust of an overlying duplex and the roof thrust of an underlying duplex reflecting a downward progression of underplating accretion the structurally upper pirashuke unit was metamorphosed at higher pressure than the structurally lower units this inverted arrangement of metamorphic grade can be explained by shallowing of progressively younging subducted slab structurally upper units were exhumed synchronously with growth of the structurally lower units this synaccretionary exhumation can be explained by repetition of undercritical duplex forming compression and overcritical extensional unroofing during seamounts subduction

magmatic origin of low t mafic blueschist and greenstone blocks from the franciscan melange san simeon california abstract the franciscan melange exposure near san simeon contains abundant greenstone and minor blueschist blocks that were tectonically boudinaged while encased in the shale matrix tectonic deformation of the blueschists is evident from variable amounts of cataclastic flow along their margins and in pinched tails major trace and rare earth elemental analyses indicate that blueschist and greenstone blocks in this area of the melange were derived from sources with morb like composition along with some having trace element and ree patterns similar to oib compositions most blocks are low lree basalts that probably formed in an open ocean ridge setting but some blocks have high lree contents similar to off axis seamounts linear trends of incompatible elements for both blueschist and greenstone blocks indicate that both lithologies were probably derived from a similar variably fractionated tholeiitic magma blueschist blocks with sodic amphibole lawsonite epidote were pervasively recrystallized at 300 350 c and foliated during ductile deformation that included folding their protolith can only be identified as mafic a few blocks contain very small amounts of metasedimentary materials indicating some were probably seafloor basalts but some may have been diabase or even gabbro where interlayered sediment was present the mafic protolith was enriched in k rb and na greenstones on the other hand contain abundant pseudomorphic evidence of magmatic textures alteration to albite chlorite and pumpellyite at temperatures of 100 200 c is intense especially in cataclastic margins and pinched tails some of the basaltic greenstones have attached radiolarian chert and a few have relict diabasic textures the mafic blueschists and greenstones in the melange near san simeon are probably fragments from the uppermost part of the farallon plate the blueschists may be mostly mafic slabs uprooted from the subducting crust underplated to the base of the north american plate and later detached and carried back towards the surface in upwelling melange the greenstones are most likely fragments detached from seamounts subducted during times of melange upwelling

crustal volumes of the continents and of oceanic and continental submarine plateaus global topographic data and the assumption of airy isostasy have been used to estimate the crustal volumes of the continents and the oceanic and continental submarine plateaus the calculated crustal volumes are 7182 x lo6 km3 for the continents 242 x lo6 km3 for continental submarine plateaus and 369 x lo6 km3 for oceanic plateaus the falkland plateau and the lord howe rise are the two largest continental submarine plateaus with volumes of 48 x lo6 km3 and 47 x lo6 km3 respectively total continental crustal volume is 7581 x lo6 km3 including the volume of continental sediments on the ocean floor 160 x lo6 km3 in good agreement with previous estimates continental submarine plateaus on the seafloor comprise 3 2 of the total continental crustal volume the largest oceanic plateaus in order of decreasing size are the ontong java plateau the kerguelen plateau the caribbean the chagos laccadive ridge the ninetyeast ridge and the mid pacific mountains together they comprise 54 of the total anomalous crustal volume in oceanic plateaus an upper bound to the continental crust addition rate by the accretion of oceanic plateaus is 3 7 km3 yr a value that assumes accretion of all oceanic plateaus with a total volume of 4 9 of the continental crustal volume on a 100 myr time scale even if a substantial fraction of the crustal volume in oceanic plateaus is subducted accretion of oceanic plateaus could make a contribution to continental growth since the upper bound to the addition rate exceeds recent estimates of the island arc addition rate subduction of continental submarine plateaus with the oceanic lithosphere on a 100 myr time scale gives an upper bound to the continental crustal subtraction rate of 2 4 km3 yr much larger than recent estimates of crustal subtraction by subduction of seafloor sediments effective subduction of all oceanic plateaus implies equally effective subduction of continental submarine plateaus a potentially important way to recycle continental crust back into the mantle may be the break off of small fragments from the continents entrapment of the continental fragments in the seafloor and subduction of the fragments with the oceanic lithosphere this process may be occurring in the mediterranean for corsica and sardinia

tectonics of formation translation and dispersal of the coast range ophiolite of california data from the coast range ophiolite and its tectonic outliers in the northern california coast ranges suggest that the lower part of the ophiolite formed 169 to 163 ma in a forearc or back arc setting at equatorial latitudes beginning about 156 ma and continuing until 145 ma arc magmatism was superimposed on the ophiolite and concurrently a transform developed along the arc axis or in the back arc area rapid northward translation of this rifted active magmatic arc to middle latitudes culminated in its accretion to the california margin of north america at about 145 ma this late jurassic episode of translation arc magmatism and accretion coincided with the nevadan orogeny and a proposed major plate reorganization in the eastern pacific basin the high rate of poleward motion necessary to translate the coast range ophiolite to middle latitudes during this time implies that the ophiolite traveled north on a fast moving plate of the eastern pacific basin here termed plate x plate x probably was driven by a cryptic ridge east northeast of the pacific farallon izanagi ridge triple junction structural relations indicate that following late jurassic time parts of the coast range ophiolite were displaced from the west side of the great valley province and incorporated into the central belt of the franciscan complex along steep dipping to low angle reverse faults having dominant components of dextral shear a northwest trending eastern zone of these right laterally displaced outliers shows strong affinities to the main coast range ophiolite of the northwestern sacramento valley the elder creek terrane in that the outliers include ophiolitic breccias of oxfordian to kimmeridgian age a southwestern zone of outliers lacks ophiolitic breccia and instead includes latest oxfordian or kimmeridgian to tithonian arc derived volcanic rocks like those found in the del puerto and stanley mountain terranes of the main ophiolite whereas outliers of the northeastern outlier zone are right laterally displaced no more than 260 km from the western side of the sacramento valley outliers of the southwestern zone are displaced a minimum of 169 to 249 km this displacement occurred between about 60 and 52 ma ophiolitic rocks in the decatur terrane of western washington that have recently been correlated with the coast range ophiolite and the great valley sequence of california were apparently displaced at least 950 to 1200 km from the west side of the great valley between early tertiary and early cretaceous time derived rates of northward translation for the ophiolite outliers in california are in the range of 1 to 4 cm yr rates for the decatur terrane are in the range of 2 5 to 4 cm yr if translation was initiated 90 ma but as much as 11 9 to 15 cm yr if it was not initiated until 60 ma the lower rates for the decatur terrane are consistent with the rates derived for the california outliers and with the northward component of relative motion between the farallon and north american plates from 90 to 50 ma the higher rates require northward transport on the kula plate or on a fast moving microplate the higher translation rates derived for the decatur terrane are also consistent with paleomagnetically determined rates for some limestones of the central belt this may indicate that outliers of the coast range ophiolite dispersed in the central belt of the franciscan complex record only part of the total displacement which occurred along the late cretaceous to early tertiary western margin of north america

the submarine volcanic succession of the basal complex of fuerteventura canary islands a model of submarine growth and emergence of tectonic volcanic islands three lithostratigraphic units have been distinguished in the volcanic succession of the basal complex of fuerteventura island these units are from bottom to top the submarine volcanic group the transitional volcanic group and the subaerial volcanic group these three groups record the submarine growth and emergence of the island the volcanism is represented by ultra alkaline and strongly alkaline igneous series the igneous activity was due to the presence of an anomalous zone in the sublithospheric mantle the low density of which also caused uplift of the mesozoic oceanic crust two extensional phases and an intervening contractional phase developed coeval to the generation of the volcanic succession the submarine volcanic group was deposited in the hanging wall basin of a large listric extensional detachment directed toward the ssw the transitional volcanic group was syntectonic with respect to a late inversion of the listric detachment finally the subaerial volcanic group resulted from a second episode of wnw extension this study of the evolution of the basal complex of fuerteventura serves as the basis for a tectonic model of submarine growth and emergence of volcanic islands

factors predictive of roof instability in addition to the existing cmrr criteria at two case study coal mines abstract this work uses two underground longwall coal mines located in the western us mines a and b as case studies to investigate which parameters are indicative of roof fall potential and whether the coal mine roof rating cmrr is predictive of stability at these mines data were collected at 30 sites in each mine these data sets included the cmrr a record of the roof stability and a series of non cmrr parameters thought to also be potentially indicative of roof stability but which are not included in the cmrr these data were then statistically analyzed for correlation between cmrr and roof stability the cmrr was found to be moderately effective at mine a but not effective at mine b at mine a faulting was found to be the primary control on roof stability topographic curvature slope angle and depth of cover were found to have reasonable predictive capability for assessment of roof stability category at mine b based on a logistic regression analysis these differences in roof stability controls are likely due at least in part to the unusual topography above mine b with differential erosion resulting in a landscape of flat plateaus and sharp river valleys it is likely that these sudden changes in slope and topography lead to in situ stress rotation and the development of shear stresses near the excavation at mine b this combined with a lack of major discontinuities such as slickensides which are central to the cmrr system explains why the cmrr is much less effective at predicting roof stability at mine b compared to mine a

plumes in central panthalassa deductions from accreted oceanic fragments in japan accretion of a large number of seamounts or fragments of huge oceanic plateaus is recorded in the geology of japan major accretion occurred twice a late early carboniferous seamount chain accreted in latest permian to middle jurassic time and a late jurassic oceanic plateau accreted in the early cretaceous the late jurassic plateau now in northern japan and sakhalin is here named the sorachi plateau and a tithonian age for it is well documented on the basis of microfossils from pelagic cherts and limestones petrochemical characteristics of basalts suggest two kinds of sources depleted mantle under oceanic lithosphere and enriched mantle related to plume upwelling all the accreted fragments in japan can be traced back to the central part of the panthalassa by using paleomagnetic data and plate trajectory analysis the timing of generation of the accreted seamount chain and the plateau is consistent with a time of global change their place of origin in the mid pan thalassa retains a positive residual geoid anomaly and slow vp in the lower mantle which suggest a past upwelling of hot material from the core mantle boundary

the stonyford volcanic complex a forearc seamount in the northern california coast ranges jurassic age volcanic rocks of the stonyford volcanic complex sfvc comprise three distinct petrological groups based on their whole rock geochemistry 1 oceanic tholeiites 2 transitional alkali basalts and glasses 3 high al low ti tholeiites major and trace element and sr nd pb isotopic data indicate that the oceanic tholeiites formed as low degree partial melts of normal mid ocean ridge basalt n morb source asthenosphere similar in isotope composition to the east pacific rise today the alkalic lavas were derived from an enriched source similar to that of e morb the high al low ti lavas resemble second stage melts of a depleted morb source asthenosphere that formed by melting spinel lherzolite at low pressures trace element systematics of the high al low ti basalts show the influence of an enriched component which overprints generally depleted trace element characteristics tectonic discrimination diagrams show that the oceanic tholeiite and alkali suites are similar to present day basalts generated at mid oceanic ridges the high al low ti suite resembles primitive arc basalts with an enriched alkali basalt like overprint isotopic data show the influence of recycled components in all three suites the sfvc was constructed on a substrate of normal coast range ophiolite in an extensional forearc setting the close juxtaposition of the morb like olivine tholeiites with alkali and high al low ti basalts suggests derivation from a hybrid mantle source region that included morb source asthenosphere enriched oceanic asthenosphere and the depleted supra subduction zone mantle wedge we propose that the sfvc formed in response to collision of a mid ocean ridge spreading center with the coast range ophiolite subduction zone formation of a slab window beneath the forearc during collision allowed the influx of ridge derived magmas or the mantle source of these magmas continued melting of the previously depleted mantle wedge above the now defunct subduction zone produced strongly depleted high al low ti basalts that were partially fertilized with enriched alkali basalt type melts and slab derived fluids

metamorphic evolution of the kamuikotan high pressure and low temperature metamorphic rocks in central hokkaido japan the kamuikotan complex in the sorachi yezo belt located in central hokkaido japan has been extensively subjected to subduction related high pressure and low temperature metamorphism on the basis of geological petrological and geochronological data the coherent metamorphic rocks are divided into six units as follows susunai horokanai harushinai biei pankehoronai and shizunai based on their metamorphic mineral sequences the six units can be classified into the following three baric types high pressure 1 hp1 high pressure 2 hp2 and high pressure intermediate hi the horokanai and shizunai units belong to the hp1 type the harushinai and biei units belong to the hp2 type and the susunai and pankehoronai units belong to the hi type metabasites of the hp1 type are defined by the association of lawsonite na amphibole or epidote na amphibole glaucophane or crossite the hp2 type is characterized by the assemblage of pumpellyite na pyroxene chlorite and the representative low variance asssemblages are lawsonite pumpellyite na pyroxene chlorite and pumpellyite na pyroxene na amphibole chlorite in the hi type the most common low variance assemblage is pumpellyite epidote actinolite chlorite these results together with the previously published k ar and 40ar 39ar ages suggest that the kamuikotan metamorphic rocks gradually decreased in pressure during the cretaceous this tendency indicates an increase in the kamuikotan subduction geotherm during early cretaceous to early eocene time the increase in the paleogeotherm may result from the change of subduction rates and subducting slab ages

coulomb theory applied to accretionary and nonaccretionary wedges possible causes for tectonic erosion and or frontal accretion based on observations from both modem convergent margins and sandbox modeling we examine the possible conditions favoring frontal accretion and or frontal and basal tectonic erosion mean characteristic parameters t t and are used to discuss the mechanical stability of 28 transects across the frontal part of convergent margins where the coulomb theory is applicable natural observations reveal that typical accretionary wedges are characterized by low tapers with smooth surface slope and subducting plate low convergence rates and thick trench sediment while nonaccretionary wedges display large tapers with irregular surface slopes and rough subducting plate high convergence rates and almost no trench fill sandbox experiments were performed to illustrate the effects of seamounts ridges in the subduction zone on the deformation of an accretionary wedge these experiments show that a wedge of sand is first trapped and pushed in front of the seamount which acts as a moving bulldozer this is followed by a tunnelling effect of the subducting seamount through the frontal wedge material which results in considerable sand reworking at an advanced subduction stage the d6collement jumps back from a high level in the wedge to its former basal position we conclude that a high trench sedimentation rate relative to the convergence rate leads to frontal accretion in contrast several conditions may favor tectonic erosion of the upper plate first oceanic features such as grabens seamounts or ridges may trap upper plate material during their subduction process second destabilization of the upper plate material by internal fluid overpressuring causing hydrofracturing is probably another important mechanism

evaluating the influence of aseismic ridge subduction and accretion on detrital modes of forearc sandstone an example from the kronotsky peninsula in the kamchatka forearc abstract the kronotsky peninsula in the forearc region of the kamchatka magmatic arc lies on trend with the emperor seamount chain situated on the currently subducting pacific tectonic plate detrital modes of volcaniclastic sandstone interbedded with mafic eocene basement rocks and within the overlying sedimentary sequence provide insight into the late cenozoic geologic history of this area eocene and basal miocene sandstones are primarily composed of variably altered mafic volcanic debris their detrital modes are similar to those of emperor seamount sandstones and hawaiian beach sands although aspects of the stratigraphy and volcaniclastic sand composition are consistent with a seamount setting there is no physical evidence for an accretion event and the suggested eocene age for this unit makes an emperor seamount origin unlikely a seamount origin cannot be ruled out for older kronotsky basement complexes however a miocene lull in kronotsky volcanism was followed by rapid basin subsidence and influx of arc derived turbidites from the west detrital modes of these sandstones are typical of a moderately evolved continental or micro continental arc an anomalously high proportion of sedimentary lithic fragments is the only possible compositional fingerprint attributable to seamount or ridge subduction

deformation mechanism paths for oceanic rocks during subduction and accretion the mesozoic forearc of alexander island antarctica the lemay group of alexander island antarctica is a mesozoic accretionary prism that contains slivers of ocean floor and ocean island material accreted under a range of conditions and depths it provides a rare opportunity to compare the deformation mechanism paths between oceanic and trench fill lithologies during subduction and accretion by offscraping or underplating ocean floor slivers consist of a bedded basalt volcaniclastite chert rock association overlain by trench fill sedimentary rocks the oceanic lithologies initially deformed by cataclasis combined with particulate flow in the volcaniclastic rocks at shallow levels the clastic trench fill sedimentary rocks deformed by independent particulate flow which changed to cataclastic flow at depth at those depths crystal plasticity affected the cherts and jaspers at the deepest levels achieved in the subduction zone crystal plasticity was the dominant deformation mechanism in the clastic sedimentary rocks whilst cataclasis continued in the lavas deformation was synchronous with metamorphism up to transitional greenschist blueschist facies all lithologies were affected by pressure solution after accretion with the formation of a bedding parallel or sub parallel cleavage the deformation mechanism paths indicate that 1 there is a crystal plastic deformation field at deep levels in the subduction zone and 2 that the base of the pressure solution field is concordant with the decollement a frontally accreted ocean island is exposed in the lully foothills and consists of a basalt volcaniclastite tuff rock association deformation is represented only by a patchily distributed bedding sub parallel pressure solution cleavage in the tuffs and volcaniclastites and is probably related to loading metamorphism does not exceed prehnite pumpellyite facies the accreted ocean island indicates that such features can be incorporated into accretionary prisms at shallow levels without significant segmentation

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

deformation of accretionary wedges in response to seamount subduction insights from sandbox experiments sandbox experiments using a two dimensional and a three dimensional approach are used to study the deformation of margins in response to seamount subduction successive mechanisms of deformation are activated during the subduction of conical seamounts first reactivation of the frontal thrusts and compaction of the accretionary wedge is observed then back thrusting and conjugate strike slip faulting develops above the leading slope of the subducted seamount the basal d6collement is deflected upward in the wake of the subducting high and a large shadow zone develops behind the seamount trailing slope consequently frontal accretion is inhibited and part of the frontal margin is dragged into the subduction zone when the main d6collement returns to its basal level in the wake of the seamount the margin records a rapid subsidence and a new accretionary wedge develops closing the margin reentrant the sediments underthrusted in the wake of the seamount into the shadow zone are underplated beneath the rear part of the accretionary wedge substantial shortening and thickening of the deformable seaward termination of the upper plate basement associated with basal erosion is observed seamount subduction induces significant material transfer within the accretionary wedge favors large tectonic erosion of the frontal margin and thickening of the rear part of the margin the subduction and underplating of relatively undeformed water ladden sediments associated with fluid expulsion along the fractures affecting the margin could modify the fluid pressure along the basal ddcollement consequently significant variations of the effective basal friction and local mechanical coupling between the two plates could be expected around the subducting seamount

diverse sources for igneous blocks in franciscan melanges california coast ranges igneous blocks in franciscan melanges are of three chemical petrologic types 1 tholeiitic basalts of both arc and spreading center origin with depletions in light relative to heavy rare earth elements 3 tio 2 1 high y zr and y ti ratios and relict augites that generally have low al and ti and well defined iron enrichment trends 2 basalts of probable seamount origin with marked enrichments in light relative to heavy rare earth elements 5 tio 2 1 lower y zr and y ti than 1 and ti al rich augites showing little if any iron enrichment trends and 3 hypabyssal intrusives having sio 2 52 tio 2 1 flat or only slightly fractionated rare earth abundance patterns and diopsidic augites that are very low in ti and al and show no iron enrichment trends all of the blocks are metamorphosed most are undeformed pumpellyite bearing greenstones and a few contain sodic amphibole lawsonite sodic pyroxene the melanges are probably olistostromal in origin deriving the

the effect of rock mass gradual deterioration on the mechanism of post mining subsidence over shallow abandoned coal mines in this study the effect of gradual deterioration on the long term stability of shallow abandoned room and pillar coal mines is investigated the conventional tributary area method is adopted and modified to take into account the rate of deterioration on the lifetime of pillars moreover a simple quantitative approach is proposed to incorporate the effects of deterioration in numerical modelling the suggested methods are then used to assess the stability of the shallow abandoned room and pillar coal mines around dolphingstone village uk results of this study indicate that the gradual deterioration plays a significant role in the long term stability of shallow abandoned coal mines the stand up time of a room and pillar mining panel is also estimated based on the modified q system a good agreement is observed between the results of numerical modelling and the rock mass classification system

overview on the application of geophysics in coal mining abstract since their introduction to the coal mining industries of the united kingdom and west germany in the 1970s geophysical methods are now utilised in coal mining around the world the range of applications in both surface and underground mining is extensive applications include coal seam mapping and geological fault detection lithological mapping geotechnical evaluation assessment of the rock mass response to mining detection of voids location of trapped miners and guidance of drills and mining equipment the range of techniques that can be employed is also extensive and includes geophysical borehole logging the potential field methods seismic reflection 2d and 3d resistivity electromagnetics and microseismic monitoring using active and passive sources this paper discusses the major applications and the geophysical methods that can be applied it also discusses future trends and suggests that the future motives for applying geophysics will not only include the current motivations of mine safety and productivity but will also include an increased emphasis on environmental management the monitoring of sequestration activities and the provision of sensors to enable autonomous mining

noncohesive critical coulomb wedges an exact solution active fold and thrust belts or submarine accretionary complexes can be modeled as critically tapered wedges of material on the verge of coulomb failure everywhere overlying a basal decollement where frictional sliding is occurring ignoring cohesion the four strength parameters needed to describe a critical coulomb wedge are its internal and basal coefficients of friction and b and its internal and basal hubbert rubey fluid pressure ratios and b an exact relation between surface slope and basal dip of a noncohesive critical wedge with uniform properties is derived the state of stress within such a wedge has the same orientation everywhere and is constant if is and vice versa a coefficient of internal friction 1 1 is consistent with the known surface slope basal dip and pore fluid pressures in the active fold and thrust belt of western taiwan assuming that byerlee s law b 0 85 is valid on the base the wide variety of tectonic styles observed to occur along convergent margins including subduction erosion active accretion subduction without accretion and even extension and normal faulting may be controlled by relatively small spatial or temporal variations in either b or b

geotechnical evaluation of coal deposits based on the geophysical strata rating abstract in exploration programs for coal mining geophysical logging of boreholes is mainly undertaken to reveal seam locations and the basic lithological section through the introduction of the geophysical strata rating gsr this paper develops new geotechnical applications for geophysical logs the gsr is an empirical rating scheme based on p wave velocity data from sonic logs and estimates of the clay content and porosity derived from natural gamma and density logs in coal measure strata gsr values range between 0 and 100 with values of given that geophysical logging data are obtained at closely spaced intervals within boreholes and that coal measure strata are usually laterally persistent between boreholes geophysical results such as the clay content and the gsr can be modelled in two and three dimensions this allows geotechnical information to be viewed in a geological context examples are provided of the log analysis and modelling geotechnical applications in risk identification hazard assessment and design optimisation are also discussed these applications exist in both underground and open cut mining

a new coal mine floor rating system and its application to assess the potential of floor heave abstract several underground coal mines have reported floor heave associated with the buckling mechanism however studies on the coal mine floor especially the buckling failure mechanism in the floor and relevant analysis techniques are still insufficient this paper introduces a new floor classification system the coal mine floor rating based on the underlying failure mechanisms the new floor mass rating system quantifies the stability of floor strata by considering two main factors uniaxial compressive strength and discontinuity spacing of the floor units the coal mine floor rating is incorporated into an empirical method the floor heave index developed by statistical analysis of a database obtained from actual floor failure cases the floor heave and non floor heave cases are determined depending on the coal mine floor rating and the horizontal stress rating a proxy that represents the magnitude of horizontal stress while obtaining underground floor data is challenging the new floor classification system and the empirical method help assess the potential of significant floor heave on development in underground coal mine roadways for new mining projects or future workings

the effect of seamount subduction on seismic coupling it has long been speculated that the subduction of seamounts may have profound mechanical consequences particularly on the generation of large subduction zone earthquakes we consider this problem as a special case of the seismic coupling model of scholz and campos the model predicts that the subduction of a large seamount will increase the normal stress across the subduction interface and hence will enhance seismic coupling in the case of coupled arcs this increased coupling will result in a large increase in the recurrence intervals of earthquakes such events will therefore appear rare in the instrumental record as is observed for decoupled arcs the subduction of a large seamount should cause them to become locally coupled three of the latter localities have been found with satellite altimetry data two on the tonga kermadec trench and one on the izu bonin trench the diagnostic feature is a shallowing or obstruction of the trench axis adjacent to a chain or group of large seamounts on the subducting plate in each case the small to moderate plate interface seismicity typical of decoupled seismic zones is almost completely absent in a 100 200 km arc segment indicating that the subduction zone is locally locked as predicted by the model the occurrence of large interplate earthquakes at both tonga sites confirms that they are seismically coupled

shape analysis of pacific seamounts abstract shape statistics have been compiled from 85 profiles of well surveyed pacific seamounts in the height range 140 3800 m a flat topped cone was fit to each seamount s cross sectional profile maintaining the slopes of the sides as closely as possible on each profile a basal width d b a summit width d t and a maximum height h were measured the height to basal radius ratio is h is estimated by the ratio 2hd b and flatness f by the ratio d t d b slope angle arctan e is estimated from e 2h d b d t summit height and basal radius are found to be highly correlated r 0 93 the 85 point sample mean of the height to basal radius ratio is h 0 21 0 08 implying that a seamount s summit height is typically one fifth its basal radius despite the high correlation individual points show some scatter and there may be groupings into different morphological types for example all but one of the seamounts with summit heights above 1000 m have values of h that are larger than the sample mean the 85 point sample mean of flatness is f 0 31 0 18 data points show a large scatter with values of f varying between 0 a pointy cone and 0 69 a flat topped cone a histogram representation of flatness however indicates that certain values of f may be more common than others the histogram shows a bimodal distribution with maxima occurring at values of f in the ranges 0 10 0 20 and 0 35 0 50 moreover there is some evidence that the mean flatness decreases with summit height so that the preferred shape of a large sized seamount may be a pointy cone slope angle has an 85 point sample mean of 18 6 individual values of vary between 5 and 36 in addition to having a lower than average mean flatness seamounts with heights above 2600 m also have a lower than average mean slope angle 15 to determine which variables account for most of the observed variation in the seamount shapes a multivariate principal component analysis was performed on the data using five shape variables summit height basal radius summit radius flatness and slope the analysis indicates that most of the variation is described by two variables flatness and summit height

the abrupt termination of the younger dryas climate event previous studies on two deep greenland ice cores have shown that a long series of climate oscillations characterized the late weichselian glaciation in the north atlantic region1 and that the last glacial cold period the younger dryas ended abruptly 10 700 years ago2 here we further focus on this epoch defining event and present detailed heavy isotope and dust concentration profiles which suggest that in less than 20 years the climate in the north atlantic region turned into a milder and less stormy regime as a consequence of a rapid retreat of the sea ice cover a warming of 7 c in south greenland was completed in about 50 years

geology of tok island korea eruptive and depositional processes of a shoaling to emergent island volcano detailed mapping of tok island located in the middle of the east sea sea of japan along with lithofacies analysis and k ar age determinations reveal that the island is of early to late pliocene age and comprises eight rock units trachyte i unit p i unit p ii trachyandesite 2 7 0 1 ma unit p iii trachyte ii 2 7 0 1 ma trachyte iii 2 5 0 1 ma and dikes in ascending stratigraphic order trachyte i is a mixture of coherent trachytic lavas and breccias that are interpreted to be subaqueous lavas and related hyaloclastites unit p i comprises massive and inversely graded basaltic breccias which resulted from subaerial gain flows and subaqueous debris flows a basalt clast from the unit derived from below trachyte i has an age of 4 6 0 4 ma unit p ii is composed of graded and stratified lapilli tuffs with the characteristics of proximal pyroclastic surge deposits the trachyandesite is a massive subaerial lava ponded in a volcano tectonic depression probably a summit crater a pyroclastic sequence containing flattened scoria clasts unit p iii and a small volume subaerial lava trachyte ii occur above the trachyandesite suggesting resumption of pyroclastic activity and lava effusion afterwards shallow intrusion of magma occurred producing trachyte iii and trachyte dikes

the pliocene seamount series of la palma canary islands a pliocene submarine series of alkali basaltic pillow lavas hyaloclastites and breccias a a sheeted dike swarm b and a basal suite of gabbro and ultramafic rocks c from la palma canary islands is interpreted as a cross section through an uplifted seamount this series has been tilted to its present orientation of 50 230 plunge and azimuth probably by upwarping due to intrusions in the central portion of the island the basal plutonic complex c also includes intrusives coeval with up to 2000 m of younger subaerial alkali basaltic lavas unconformably overlying the submarine series the plutonic suite c is overlain abruptly by more than 1800 m of sills b 0 4 1 m thick on average with minor screens of lavas and breccias extrusives a form a 1750 m thick sequence of pillow lavas breccias and hyaloclastites the clastic rocks increase in abundance upward and are of four main types 1 breccias consisting of partly broken pillows formed nearly in situ 2 heterolithologic pillow fragment breccias 3 hyaloclastites composed dominantly of highly vesicular lapilli and ash sized shards the latter thought to have formed by near surface explosive eruptions and been subsequently transported downslope by mass flows 2 and 3 being interpreted to have been resedimented and 4 pillow scoria breccias from the upper 700 m of the extrusive section consisting of amoeboidal highly vesicular pillows and lava stringers and local bombs probably formed by cracking and bleeding of gas rich expanding pillow lava and some shallow submarine subaerial lava fountaining the extrusive series is chemically and mineralogically crudely zoned with the most differentiated rocks metatrachytes and mugearites at the base and most picritic lavas occurring near the top of the series subsequent to emplacement the entire extrusive and intrusive series has been hydrothermally altered the lower part to greenschist and the upper part to smectite zeolite facies mineral assemblages the la palma succession combined with evidence from surface studies of seamounts suggests that seamounts are formed by intrusive and extrusive processes in approximately equal portions the nature of eruptive clastic and depositional mechanisms changes drastically during growth of a seamount if the critical depth for major magmatic degassing is surpassed and especially if magmatic explosive processes can occur at very shallow water depth the critical depth depending on magma and thus volatile composition changes in slopes of a seamount influence depositional processes based on these factors at least three major depositional sites develop as a seamount grows summit flank and apron facies nonexplosive extrusive processes prevail in the deep water stage dominantly producing pillow lavas 75 these consist of individual pillow volcanoes up to 200 m high with large pillows near the base and decreasing pillow size toward the top of a volcano pillow breccias and pillow fragment breccias comprise approximately 20 of this facies the deep water flank and apron facies are characterized by debris flow deposits with possibjy rather dense matrix material the shallow water shoaling stage is reached when the seamount top reaches the critical depth for drastic increase in exsolution of magmatic volatiles about 800 m for the alkali basaltic seamount of la palma resulting in formation of mainly clastic rocks 70 in situ pillow rind breccias and scoriaceous amoeboidal breccias and pillows are formed in the summit regions of the seamount by repeated expansion and leaking of frothy pillow lava possibly by lava fountaining resedimented heterolithologic pillow fragment breccias lapilli breccias and hyaloclastites are deposited on the flanks and aprons of the seamount pillow lavas comprise 30 of these deposits

sedimentologic and geochemical evidence for middle ordovician near trench volcanism in the central appalachian orogen the jonestown volcanic rocks are a minor but important component of the greenwich slice of the hamburg klippe of eastern pennsylvania sedimentary rocks of the greenwich slice are arranged in faultbounded coarsening upward sequences of pelagic deposits early to middle ordovician overlain by hemipelagic and turbidite deposits middle ordovician petrographic and geochemical characteristics of the sedimentary rocks indicate derivation from continental rather than volcanic sources the coarsening upward sequences of the greenwich slice are inferred to record advance of a site on oceanic lithosphere toward and ultimately into a trench by middle ordovician time rocks of the jonestown volcanic suite were extruded onto and intruded into the inferred middle ordovician near trench and trench axis mudstone and sandstone immobile element concentrations of the jonestown suite are typical of mid oceanic ridge basalts and along with the lack of associated shallow water and or volcaniclastic sedimentary rocks a

in situ estimation of roof rock strength using sonic logging sonic travel time logging of exploration boreholes is routinely used in australia to obtain estimates of coal mine roof rock strength because sonic velocity logs are relatively inexpensive and easy to obtain during exploration the technique has provided australian underground coal mines with an abundance of rock strength data for use in all aspects of ground control design however the technique depends upon reliable correlations between the uniaxial compressive strength ucs and the sonic velocity this paper describes research recently conducted by niosh aimed at developing a correlation for use by the u s mining industry from two coreholes in illinois two from pennsylvania and one each from colorado western kentucky and southern west virginia sonic velocity logs were compared with ucs values derived from point load tests for a broad range of coal measure rock types for the entire data set the relationship between ucs and sonic travel time is expressed by an exponential equation relating the ucs in psi to the travel time of the p wave in s ft the coefficient of determination or r squared for this equation is 0 72 indicating that a relatively high reliability can be achieved with this technique the strength estimates obtained from the correlation equation may be used to help design roof support systems the paper also addresses the steps that are necessary to ensure that high quality sonic logs are obtained for use in estimating ucs

the snow mountain volcanic complex an on land seamount in the franciscan terrain california the snow mountain volcanic complex in the franciscan terrain of northern california is a 1 1 2 to 2 km thick sequence of altered submarine pillow lavas pillow breccias diabase and interbedded sediments forming a klippe that structurally overlies nearby rocks of the franciscan assemblage the coast range ophiolite and great valley sequence all of the snow mountain rocks bear a weak overprint of blue schist facies metamorphism the volcanic rocks were deposited on top of 200 300 m of interbedded turbiditic greywacke shale and radiolarian chert which are interpreted as an interfingering abyssal fan and pelagic sequence the volcanic pile is therefore supracrustal and is not an ophiolitic fragment formed at a mid ocean spreading center rather it is probably the remnant of a former seamount the lavas are mostly titanium rich transitional basalts minor associated rocks include rhyolite and peralkaline rhyolite relict clinopyroxenes in the basaltic rocks are all titanium aluminum rich augites thes

transition from circular to stellate forms of submarine volcanoes large volcanic islands and guyots have stellate forms that reflect the relief of radiating volcanic rift zones multiple volcanic centers and embayments due to giant flank failures small mid ocean ridge volcanoes in contrast are commonly subcircular in plan view and show only embryonic rift zones in order to characterize the transition between these two end members the morphology of 141 seamounts and guyots was studied using the shape of the depth contour at half the height of each edifice irregularity was characterized by measuring perimeter distance elongation and moment of inertia of the contours assuming an ideal edifice is circular the analysis reveals a general transition over 2 4 km edifice height best transition estimate 3 km while some large edifices 4 5 km high show no major embayments or ridges suggesting considerable variation in the effectiveness of mechanisms that cause flank instability and growth of rift zones the various origins of the transition are discussed and the upper limit of magma chambers many of which lie above the basement of the larger edifices is proposed to affect the morphologic complexity via a number of mechanisms and is an important factor affecting the mode of growth the origins of the truncated cone shape of mid ocean ridge volcanoes are also discussed of the eruption mechanisms that have been proposed to explain their flat summits the most likely mechanisms involve eruption from small ephemeral magma bodies lying within the low density upper oceanic crust the discussion includes speculations on factors affecting the depths of magma chambers beneath oceanic volcanoes supporting table is available via web browser or via anonymous ftp from ftp kosmos agu org directory append username anonymous password guest subdirectories in the ftp site are arranged by paper number information on searching and submitting electronic supplements is found at http www agu org pubs csupp about html

jasper seamount structure seafloor seismic refraction tomography the velocity structure of jasper seamount was modeled using one and three dimensional inversions of p wave travel times the results represent the first detailed seismic images of a submerged intraplate volcano two seismic refraction experiments were completed on jasper seamount incorporating ocean bottom seismometers and navigated seafloor shots the p wave travel times were first used to compute a one dimensional velocity profile which served as a starting model for a three dimensional tomographic inversion the seamount p velocities are significantly slower than those observed in typical oceanic crust at equivalent subbasement depths this suggests that jasper seamount is constructed predominantly of extrusive lavas with high average porosity the velocity models confirm morphological predictions jasper seamount is a shield volcano with rift zone development high seismic velocities were detected beneath the large radial ridges while low velocities characterize the shallow summit and flanks comparisons between p velocity models of jasper seamount and the island of hawaii reveal that these two shield volcanoes are not structurally proportional jasper seamount is far smaller than hawaii yet both volcanoes exhibit an outer extrusive layer of similar thickness this suggests that seamount size influences the intrusive extrusive proportions density equilibrium between melt and country rock may explain this behavior

mining induced fault reactivation associated with the main conveyor belt roadway and safety of the barapukuria coal mine in bangladesh constraints from bem simulations fault reactivation during underground mining is a critical problem in coal mines worldwide this paper investigates the mining induced reactivation of faults associated with the main conveyor belt roadway cbr of the barapukuria coal mine in bangladesh the stress characteristics and deformation around the faults were investigated by boundary element method bem numerical modeling the model consists of a simple geometry with two faults fb and fb1 near the cbr and the surrounding rock strata a mohr coulomb failure criterion with bulk rock properties is applied to analyze the stability and safety around the fault zones as well as for the entire mining operation the simulation results illustrate that the mining induced redistribution of stresses causes significant deformation within and around the two faults the horizontal and vertical stresses influence the faults and higher stresses are concentrated near the ends of the two faults higher vertical tensional stress is prominent at the upper end of fault fb high deviatoric stress values that concentrated at the ends of faults fb and fb1 indicate the tendency towards block failure around the fault zones the deviatoric stress patterns imply that the reinforcement strength to support the roof of the roadway should be greater than 55 mpa along the fault core zone and should be more than 20 mpa adjacent to the damage zone of the fault failure trajectories that extend towards the roof and left side of fault fb indicate that mining induced reactivation of faults is not sufficient to generate water inflow into the mine however if movement of strata occurs along the fault planes due to regional earthquakes and if the faults intersect the overlying lower dupi tila aquiclude then liquefaction could occur along the fault zones and enhance water inflow into the mine the study also reveals that the hydraulic gradient and the general direction of groundwater flow are almost at right angles with the trends of faults fb and fb1 which could act as barriers to groundwater flow into the mines

irregular glacial interstadials recorded in a new greenland ice core the greenland ice sheet offers the most favourable conditions in the northern hemisphere for obtaining high resolution continuous time series of climate related parameters profiles of 18o 16o ratio along three previous deep greenland ice cores1 3 seemed to reveal irregular but well defined episodes of relatively mild climate conditions interstadials during the mid and late parts of the last glaciation but there has been some doubt as to whether the shifts in oxygen isotope ratio were genuine representations of changes in climate rather than artefacts due to disturbed stratification here we present results from a new deep ice core drilled at the summit of the greenland ice sheet where the depositional environ ment and the flow pattern of the ice are close to ideal for core recovery and analysis the results reproduce the previous findings to such a degree that the existence of the interstadial episodes can no longer be in doubt according to a preliminary timescale based on stratigraphic studies the interstadials lasted from 500 to 2 000 years and their irregular occurrence suggests complexity in the behaviour of the north atlantic ocean circulation

geochemical evidence for a subducted infant arc in franciscan high grade metamorphic tectonic blocks some of the evidence of tectonic and chemical processes that occur in subduction zones is preserved in blueschists and eclogites as validated by the geochemical data presented from high grade metamorphic tectonic blocks from the franciscan complex in central california combining major elements compatible and incompatible trace element concentrations and ratios and sr nd pb isotopic data we conclude that the protoliths of these high grade franciscan blocks were arc lavas with no continental crust derived components although the protoliths underwent high grade metamorphism at the inception of subduction the tectonic affi nity of the protoliths can be distinguished by our geochemical study our geochemical data are signifi cant in that 1 the protoliths of these high grade blocks have a different mode of origin than the protoliths of the lower grade franciscan basalts that were incorporated later in the subduction complex as the latter generally are considered to be oceanic island basalts oib and derived from mid oceanicridge basalt morb and 2 the protoliths of the high grade blocks were formed over a pre franciscan subduction zone based on the geochemical evidence for an arc origin and the available geochronological data we suggest that protoliths of the highgrade tectonic blocks of the franciscan complex and the coast range ophiolite formed in the same infant arc setting initiation of franciscan subduction in this arc crust led to the formation of the high grade blocks beneath the young coast range ophiolite continued subduction resulted in subduction of older oceanic crust of morb and oib origin some of which was scraped off to form lower grade metabasites within the franciscan complex

coal pillar extraction at deep cover with special reference to indian coalfields majority of the developed indian coal seams are lying under strong and massive overlying strata depth of cover of these coal seams standing on pillars varies from 50 to 500 m present pillar extraction depillaring practises of the country have predominantly adopted intermediate mechanisation along with few fully mechanised depillaring faces most of these faces are operating at shallow cover but the existing techno economical conditions of the industry are attracting a fully mechanised depillaring system for deep seated coal pillars which is likely to grow in near future here the circumstances of bord and pillar mining are to face difficult surroundings mainly due to rock mass characteristics and stress conditions of the deep horizon although these two parameters are difficult to be pre assessed accurately a couple of recent experiences of depillaring at deep cover have shown their influence over efficiency and safety of the depillaring a review of pillar extraction practises of other coal producing countries under varying geo mining conditions showed importance of these parameters for the depillaring providing a brief review of underground coal pillar extraction scenario this paper presents geo technical challenges encountered during caving of massive roof strata at two deep indian depillaring faces these two trials experienced problems of a roof instability b barrier pillar instability and c pillar spalling floor heaving in absence of an effective hard roof management approach these problems could simply be dealt up to some extent by improvement in competency of the natural support

the kamuikotan zone in hokkaido japan tectonic mixing of high pressure and low pressure metamorphic rocks in the kamuikotan zone central hokkaido japan two distinct types of metamorphic rocks are tectonically mixed up along with a great quantity of ultramafic rocks one type consists of high pressure metamorphic rocks and the other of low pressure ones the high pressure metamorphic rocks are divided into two categories 1 prograde greenschist to glaucophaneschist facies rocks derived from mudstone sandstone limestone a variety of basic rocks such as pillow and massive lavas hyaloclastite and tuff and radiolarian valanginian to hauterivian chert among which the basic rocks and the chert and occasionally the sandstone occur as incoherent blocks or inclusions enveloped by mudstone 2 retrograde amphibolites with minor metachert and glaucophane calcite rock which are tectonic or exotic blocks enclosed within prograde mudstone or serpentinite or separated from these prograde rocks by faults the k ar ages of the prograde metamorphic rocks 72 107 and 116 ma on phengitic muscovites are younger than those of the retrograde rocks 109 132 135 and 145 ma on muscovites and 120 ma on hornblende the low pressure metamorphic rocks consist of the mafic members of an ophiolite sequence with a capping of radiolarian tithonian chert with the metamorphic grade ranging from the zeolite facies through the greenschist partly actinolite calcic plagioclase facies to the amphibolite partly hornblende granulite facies the low pressure metamorphism has a number of similarities with that described for ocean floor metamorphism the tectonic evolution of such a mixed up zone is discussed in relation to mesozoic plate motion

subducting seamounts and deformation of overriding forearc wedges around japan abstract two subducting seamounts under inner trench slopes have been identified around japan on the basis of magnetic anomalies morphology and geological structure the first one is located under the foot of the inner trench slope at the junction between the japan trench and the kuril trench another one occurs beneath the slope slightly seaward of the tosabae the basement high at the trench slope break along the nankai trough off shikoku the magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i e a swell landward and a depression seaward the seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions which suggests that the subducting seamounts have kept roughly their original shapes the morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward deformation of the forearc wedge by the subducting seamount extends to the forearc basin the seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes two other examples of deformed inner trench slopes around japan which can be explained by subduction of topographic highs are presented one example is a depression on the foot of the inner trench slope northeast of the junction between the kyushu palau ridge and the nankai trough another one is an area of complex morphology of the inner trench slope along the japan trench around the daiichi kashima seamount

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro

influence of fracture induced weakening on coal mine gateroad stability abstract the ground stability of gateroads is a major concern in underground coal mines especially when the surrounding strata are weak and fractured this paper presents a novel numerical modelling technique for gateroad stability analysis based on a case study conducted at zhaogu no 2 mine china considering the occurrence of fracturing and its weakening effect on the stiffness of the rock mass a tension weakening model is implemented into flac3d whereby the stiffness of the rock mass is progressively decreased with the failure state a relationship between the intensity of fractures and the residual strength of the rockmass is built in the numerical model a parametric study of the tension weakening model is carried out and the results are compared to elastic perfectly plastic and strain softening models it is shown that the tension weakening model exhibits noticeable effects on ground deformation and rock support loading which are observed from field measurements thus the tension weakening model offers a more realistic simulation of the gateroad behavior than elastic plastic and strain softening models the proposed model can be utilized for other applications involving rock reinforcement of mine openings under similar geotechnical conditions

a vendian cambrian island arc system of the siberian continent in gorny altai russia central asia an extended vendian cambrian island arc system similar to the izu bonin mariana type is described in the gorny altai terrane at the margin of the siberian continent three different tectonic stages in the terrane are recognized 1 a set of ensimatic active margins including subducted oceanic crust of the paleo asian ocean the uimen lebed primitive island arc oceanic islands and seamounts the set of rocks is assumed to be formed in the vendian 2 a more evolved island arc comprising calc alkaline volcanics and granites a fore arc trough in middle late cambrian time was filled with disrupted products of pre middle cambrian accretionary wedges and island arcs 3 collision of the more evolved island arc with the siberian continent folding metamorphism and intrusion of granites occurred in late cambrian early ordovician time in the late paleozoic the above mentioned caledonian accretion collision structure of the siberian continent was broken by large scale strike slip faults into several segments this resulted in the formation of a typical mosaic block structure

large scale melange formation due to seamount subduction an example from the mesozoic accretionary complex in central japan deformation of forearc wedges due to seamount subduction in modern trenches is related to the origin of mesozoic melanges in central japan and a hypothetical model for large scale melange formation due to seamount subduction is proposed recent studies of seamount subduction in modern trenches have revealed that seamounts pass under forearc wedges retaining their approximate original shape but the forearc wedges themselves are swelled in front of subducting seamounts and indented behind them this strongly suggests that forearc wedges are widely disrupted and transformed into large scale melanges that include seamount fragments the mesozoic accretionary complex in the mino terrane of central japan is comprised of large scale melanges including seamount fragments and coherent tectonically stacked sedimentary successions of chert siliceous shale and turbidite in ascending order melanges are widespread in a belt 20 to 40 km wide and about 100 km long and seamount fragments of permian basalts and ree

texture mineralogy and rock strength in horizontal stress related coal mine roof falls abstract geologic structures can represent planes of preferential weakness that by dismembering the roof beam may contribute to the failure of roof spans however beam deflection and roof failure also occur in rocks where no visible geologic discontinuities are present this suggests that roof failure may depend on rock strength which in turn depends on intrinsic textural properties inherent to the rock in this study rock samples were collected from horizontal stress related roof fall material in coal mines for petrographic characterization and compressive strength testing brittle stress failure prone rock types include thinly interlaminated siltstone and shale and black shale that had been lightly recrystallized samples exhibit a narrow range of density values between approximately 2 5 3 0 g cm3 but exhibit a wide range of unconfined compressive strength values between approximately 20 70 mpa results of laboratory observations suggest that for samples of coal mine immediate roof shale compressive strength is not well correlated with density grain size sutured grain boundaries or quartz content these results for shale are generally at odds with the results of similar studies for sandstone the great variability of strength texture and mineralogy documented in these samples may be an indication of their complexity and the need for specialized methodology in the study of shale strength

pillow lavas as protoliths for eclogites evidence from a late precambrian cambrian continental margin seve nappes scandinavian caledonides pillow lavas containing an eclogitic mineral assemblage locally with glaucophane crossite are described from the tsakkok lens within the seve nappes north central scandinavian caledonides critical primary relationships to the host metasedimentary rocks which are dominated by marble and quartz garnet phengite schist are preserved it is argued that these clastic rocks and the ongoing igneous activity in the tsakkok lens are late precambrian to cambrian in age and that these rocks were deposited emplaced along the outermost part of a continental margin related to the continent baltica

early cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the franciscan subduction complex 1 during subduction at the franciscan trench beginning at 170 160 ma and continuing to the present marine sedimentary and lesser volcanic rocks have been underthrust accreted and metamorphosed to form the franciscan accretionary wedge the south fork mountain schist sfms forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the franciscan new u pb ages of detrital zircons and 40ar 39ar ages of metamorphic white micas indicate that the sfms accreted surprisingly late in the time span of franciscan subduction probably at ca 123 ma at least parts of the valentine spring formation and yolla bolly terrane accreted soon thereafter therefore accretion of the sfms apparently marks a fundamental transition from virtually nonaccretionary behavior to strongly accretionary behavior in the franciscan forearc about 75 of modern forearcs are nonaccretionary so recognition of an early nonaccretionary period is consistent with modern examples the franciscan mafic high grade exotic blocks and related very small slabs which yield ca 132 to 169 ma metamorphic ages are the only well documented rock record of the early 40 m y long nonaccretionary period and represent very sparse accretion during that interval the transition into an accretionary regime at ca 123 ma was essentially synchronous with the end of the late jurassic early cretaceous magmatic lull in the sierra nevada magmatic arc with the waning of early cretaceous faulting and warping within the great valley forearc basin and possibly with changes in motion vectors of the pacific and farallon plates

significance of klamath rocks between the franciscan complex and coast range ophiolite northern california small fault bounded slabs of low grade prehnite pumpellyite bearing slate metagraywacke and greenstone occur between the coast range ophiolite and south fork mountain schist for at least 60 km south of the klamath mountains northern california the metagraywacke slabs differ from typical franciscan complex metagraywacke to the west by the absence of blueschist facies minerals and the abundance of quartz and chert clasts and they differ from sandstone and mudstone of the great valley sequence to the east by the presence of a penetrative cleavage and low grade metamorphism the metasedimentary rock in the slabs is petrographically identical to the jurassic galice formation which occurs along the western and southern margins of the klamath mountains upward drag along a west dipping reverse fault best explains the present structural position of the slabs and is consistent with available geological and geophysical data these data imply that the coast range ophiolite structurally overlies sierran klamath basement at least locally

metabasalts as sources of metals in orogenic gold deposits although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits the mobility of au and related elements as sb se and hg from these rocks during alteration and metamorphism is poorly constrained we investigate the effects of increasing metamorphic grade on the concentrations of au and related elements in a suite of metabasaltic rocks from the otago and alpine schists new zealand the metabasaltic rocks in the otago and alpine schists are of morb and wpb affinity and are interpreted to be fragments accreted from subducting oceanic crust gold concentrations are systematically lower in the higher metamorphic grade rocks average au concentrations vary little between sub greenschist 0 9 0 5 ppb and upper greenschist facies 1 0 0 5 ppb but decrease significantly in amphibolite facies samples 0 21 0 07 ppb the amount of au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in otago arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element the concentrations of sb and hg decrease between sub greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered morb protoliths and therefore unaltered oceanic crust cannot be a net source of sb and hg in a metamorphic environment the concentrations of au as sb and hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism we suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit

origins and variations of fluoride in greenland precipitation summit greenland firn and ice core sections have been analyzed to determine the fluoride background level of high northern latitude precipitation and to investigate its temporal variations over various time periods a continuous profile covering the last century and a discontinuous study of the last 27 000 years spanning the entire holocene time period 0 to 11 500 years b p and the late part of the last glaciation have been achieved this first detailed study of the fluoride content of polar ice help us obtain a better understanding of the atmospheric cycle of inorganic fluoride in remote atmospheres first our data suggest that blown dust is among the nonvolcanic sources that contributed to the natural fluoride background level of this high latitude precipitation primary sea salt emissions represent an insignificant contribution our study also indicates that high latitude biomass burning debris which sometimes reaches the greenland ice cap represents an additional weak source of fluoride for greenland precipitation second this work provides useful information for evaluating the impact of the volcanic activity on the fluoride budget of these high northern latitude regions indeed our data demonstrate that although the natural background fluoride content of greenland snow is very often sporadically disturbed by volcanic emissions from iceland numerous hekla eruptions and the 1783 laki eruption for instance located relatively close to the greenland ice cap large explosive eruptions tambora in 1815 for instance only weakly disturb this background level our data suggest that even in the case of significant input within the stratosphere fluoride is rapidly scavenged probably along with ash particles finally our study of recent snow layers suggests that man made sources mainly coal burning represent an important contribution which has dominated this high northern hemisphere fluoride budget for the three last decades furthermore over the last 10 years a possible influence of the stratospheric reservoir builded up from the chlorofluorocarbons degradation cannot be ruled out and in the future this growing contribution may represent a significant part of the anthropogenic fluoride input compared to the input derived from coal burning

thrust type subduction zone earthquakes and seamount asperities a physical model for seismic rupture a thrust type subduction zone earthquake of m sub w 7 6 ruptures an area of approximately 6 000 km sup 2 has a seismic slip of approximately 1 m and is nucleated by the rupture of an asperity approximately 25km across a model for thrust type subduction zone seismicity is proposed in which basaltic seamounts jammed against the base of the overriding plate act as strong asperities that rupture by stick slip faulting a m sub w 7 6 event would correspond to the near basal rupture of a approximately 2 km tall seamount the base of the seamount is surrounded by a low shear strength layer composed of subducting sediment that also deforms between seismic events by distributed strain viscous flow planar faults form in this layer as the seismic rupture propagates out of the seamount at speeds of kilometers per second the faults in the shear zone are disrupted after the event by aseismic slow viscous flow of the subducting sediment layer consequently the extent of fault rupture varies for different earthquakes nucleated at the same seamount asperity because new fault surfaces form in the surrounding subducting sediment layer during each fast seismic rupture

subducted sedimentary serpentinite melanges record of multiple burial exhumation cycles and subduction erosion abstract serpentinite matrix melanges give insight into large scale convergent plate margin processes particularly because of the derivation of the serpentinite from oceanic mantle similar to shale matrix melanges a field geologist may easily recognize the sedimentary origins of little deformed serpentinite matrix melanges but melanges within accretionary prisms have undergone significant deformation and recrystallization of matrix serpentinite melanges of the franciscan subduction complex of california have a seemingly intact and foliated matrix such exposures contrast sharply with the granular undeformed sedimentary serpentinite melanges of the coeval great valley group gvg forearc basin deposits that depositionally overlie coast range ophiolite that structurally overlies the franciscan nonetheless franciscan serpentinite melanges display evidence of sedimentary origins including sedimentary breccia composed of exotic block material tolay ridge sedimentary serpentinite breccia panoche pass road basal serpentinite conglomerate with exotic clasts sunol regional wilderness and serpentinite sandstones and conglomerates including a basal conglomerate overlying coherent metagraywacke tiburon peninsula these examples record two burial exhumation cycles to blueschist facies depths in addition a melange breccia in the panoche pass area may have components that record three burial exhumation cycles to blueschist or greater depth exhumation rates for various cycles ranged from about 1 2 to 10 mm year the tiburon peninsula serpentinite melange occupies the structurally highest horizon in the franciscan of the san francisco bay area and regional field relationships indicate deposition at ca 100 ma apparently about 65 ma of subduction erosion non accretion followed initiation of franciscan subduction in this region the oldest franciscan serpentinite melanges are at least 35 ma younger than sedimentary serpentinites of the gvg subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench evidence of multiple burial exhumation cycles indicates reworking of subduction complex material consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages

accreted oceanic materials in japan abstract the phanerozoic circum pacific orogenic belts contain numerous ocean derived materials accreted through plate converging processes japanese islands in particular display various kinds of oceanic materials of different origins including fragments of seamounts oceanic reef limestone morb like rocks and oceanic mantle and pelagic sediments the compilation of these rocks in many subduction complexes of late permian to the present led to following conclusions accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust i e layer 1 and layer 2 many fragments of seamount with alkali basalt 600 hot spot seamount 26 oceanic reef limestone 291 morb like basalt 200 and numerous cherts more than 1000 are recognized as ancient oceanic materials accreted to the japanese islands however gabbros and mantle materials of layer 3 and lower parts of the oceanic lithosphere scarcely occur in subduction accretion complexes except for a few examples of back arc basin or fore arc origin accretion occurs episodically in southwest japan oceanic materials were accreted intermittently in 1 a end permian 2 b middle late jurassic 3 c late cretaceous times 4 d at ca 50 ma and 5 e in miocene times while in northeast japan and hokkaido this occurred in b middle late jurassic c late cretaceous and f early cretaceous times in contrast to the general belief on accretion of younger oceanic plates the majority of japanese subduction accretion complexes were formed during the subduction of plates up to 160 ma old the accretionary events in end permian and middle late jurassic times coincide with northward collision of ancient island arcs oceanic rises or seamount chains of hot spot origin with the asian continent accretion relevant to subduction of older plates may be controlled by the collision subduction process of these topographic reliefs on an oceanic plate in addition the chronological coincidence with the continent collision amalgamation between the sino korean and siberian platforms and between the sino korean and yangtze blocks also implies collision induced voluminous supply of elastics from back arc regions and its contribution to the formation of huge accretionary complexes accreted fragments of ancient seamounts are much smaller than the average size of modern seamounts this implies that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels with respect to the metamorphic grades for japanese subduction complexes oceanic materials have less than 1 vol in the zeolite facies 15 20 in the prehnite pumpellyite metagraywacke facies and ca 30 in the greenschist glaucophane schist facies and albite epidote amphibolite fades this relationship indicates that the major process for landward accretion of oceanic materials is not off scraping or sedimentary mixing at the trench but underplating subcretion at much deeper levels of a subduction zone

subduction of a late cretaceous seamount of the louisville ridge at the tonga trench a model of normal and accelerated tectonic erosion the hotspot generated louisville ridge is a 4000 km chain of seamounts typically 2 2 5 km high and 10 40 km in diameter and an underlying crustal swell 1 5 km high and 100 km wide trending nnw across the southwestern pacific the northwest end of the ridge collides with the north trending tonga trench 26 s which just north of that point is exceptionally deep 10 8 km and lacks both a turbidite wedge and a bordering accretionary complex the collision zone is moving rapidly southward multichannel seismic reflection data in the collision zone show a west dipping subsurface platform 2 3 km beneath the lower western trench slope which is interpreted as the flat summit of a subducted guyot motuku of the louisville chain projected eastward the summit plain passes 1 2 km above the trench floor dredging of the nearby inner trench wall recovered uppermost cretaceous maestrichtian oceanic pelagic sediments interpreted to be fragments of the sedimentary mantle of a subducted louisville seamount the principal effects of hotspot ridge collision with a sediment starved trench are 1 the impacting seamounts are subducted rather than accreted and 2 although some seamount rocks are temporarily accreted the inner trench wall is tectonically eroded arcward at rates possibly as high as 50 km m y accelerated tectonic erosion is related to 1 fracturing shearing and general weakening of arc substrate rocks as they are lifted by the swell penetrated by impacting seamounts and left to collapse as the ridge moves away 2 a more effective removal of weakened rock in underthrusting grabens which are larger in the crustal swell 3 a possible elevation of the subduction decollement to account for the removal of as much as 30 000 km of material from a 400 km sector of the trench in 1 million years and 4 a reduction in supply of arc derived debris resulting from the gap in arc volcanism accompanying subduction of the ridge normal tectonic erosion in the tonga trench is apparently minor and we conclude that the bulk of the 37 000 km of material which fills subducting grabens each million years is arc derived volcanic and pelagic sediment

nature and tectonic setting of accreted basalts from the mino terrane central japan the mino tectono stratigraphic terrane of central japan is dominated by thrust deformed jurassic terrigenous clastic sediments thought to have formed as a subduction accretion complex at an active margin however in the central mino terrane basic volcanic rocks together with stratigraphically overlying permian shallow water platform and slope carbonates and radiolarian cherts okumino group are present as tectonic intercalations these permian units have previously been interpreted as representing a large oceanic seamount type body formed in a distal oceanic setting in addition basaltic volcanic and volcaniclastic rocks of unknown age and affinities are also found as tectonic slivers within the adjacent jurassic trench clastic sequences neo formation new petrological and geochemical investigations of all these basalts show that two main compositional groups are present i alkali basalts mid permian which are highly enriched in incompatible trace elements and the light ree group 1 ii tholeiitic basalts mainly lower permian which are moderately enriched in incompatible trace elements and light ree relative to n type morb group 2 these characteristics suggest derivation from an enriched mantle source such as a mantle plume the compositions of neo formation basalts closely resemble those of the okumino group suggesting that they were probably derived from the latter by both tectonic and sedimentary processes geochemical comparison with modern oceanic plume influenced basalts notably using la sm n sm yb n ratios suggests that the older group 2 basalts are more akin to volcanics erupted along a constructive plate boundary p or e type morbs than to tholeiitic basalts of intraplate oceanic islands oib the sedimentary evidence for bathymetric elevation further indicates that the setting for this volcanism was most likely a spreading axis centered oceanic plateau or ridge e g iceland ninety east ridge the younger alkali basalts probably formed in an off axis position either due to normal lateral spreading processes or perhaps more complex spreading centre plume interactions accretion of this plateau onto the southern margin of the asian landmass in the mid late jurassic apparently involved significant subduction erosion of the plateau evidenced by reworking of the permian units into adjacent trench clastic sequences

oblique subduction and collision forearc tectonics of the kuril arc the tectonics of the southwestern kuril arc are a result of the oblique subduction of the pacific plate at the kuril trench in association with forearc sliver migration caused by the oblique subduction collision tectonics occur at the leading margin of the sliver and tensional tectonics take place at the tapering margin as a result of the collision a deep crustal section of island arc is observed at the leading margin of the forearc sliver tectonics of the kuril arc related to oblique subduction are different from those of the western sunda arc where backarc spreading occurs at the leading margin this difference is due to margin morphology of the oblique subduction zone