tectonic and structural evolution of gough volcano a volcanological model abstract morphostructural stratigraphic and tectonic data indicate that the evolution of gough volcano is similar to other oceanic intraplate volcanoes is older than 1 ma and is related to a transform fault at least six evolutionary stages can be distinguished within two major magmatostructural periods dominated by basaltic and trachytic magmas respectively the basaltic shield volcano is characterized by a curved elongated shape in plan and a rift zone with a high density of dykes combined with a radial intrusive system the latter is interpreted as being fed by a magma chamber some 4 km below the surface the activity of the volcano became more centralized at the end of the basaltic period and its slopes became steeper this corresponds to the development of a shallower and narrower central conduit in the edifice the basaltic period was terminated by formation of a shield caldera related to the 4 km deep magma chamber the term shield caldera is used for a collapse structure that is postmagmatic large in comparison with the diameter of the volcano and delimited by normal faults that do not show a closed circular pattern but rather a series of arcs in contrast summit calderas are defined as smaller circular shaped centrally situated synmagmatic features related to a central shallow column during the basaltic period landslides were generated on the flanks of the edifice as a result of slope stability factors which are not easy to determine at present and dynamic factors among which the intrusion of magma along a curved zone certainly played a major role the trachytic period is characterized by comparatively rare pyroclastic deposits and a large volume of thick flows extruded from domes these extrusions as well as plugs formed from vertical cylindrical columns of magma rising from shallow individual magma pockets fed by the main reservoir

elemental compositions and monazite age patterns of core sediments in the changjiang delta implications for sediment provenance and development history of the changjiang river core from a continuous borehole in the changjiang delta to a depth of 318 7 m dated back to the pliocene 3 58 ma and was selected for geochemical measurements and determinations of th u pb ages of monazite in order to investigate the changing sediment provenance and development history of the changjiang river geochemical proxies including fractionation parameters of rare earth elements cerium and europium anomalies and elemental ratios cr th nb co and th co suggest that the pliocene and quaternary sediments have remarkably different provenances six peak ages of monazite grains dated at 25 50 200 200 400 400 550 800 1000 and 1800 2000 ma are consistent with the main tectonic and magmatic events in the yangtze craton the data imply that the pliocene sediments were mostly derived from proximal and more silicic sources whereas the quaternary sediments were sourced from distal and more basic provenances including the emeishan basalt province in the upper changjiang valley we propose that during the pliocene the paleo changjiang or its eastern equivalent was a locally small river draining today s lower changjiang valley whereas during the early pleistocene not later than 1 18 ma it changed its drainage pattern and developed into a large river that originated from the eastern tibetan plateau this time matches well with many previous studies based upon geomorphologic geographic and tectonic observations in the jinshajiang valley and the three gorges

effects of h2o on elastic wave velocities in ultrabasic rocks at 900 c under 1 gpa simultaneous measurements of compressional and shear wave velocities vp and vs in five ultrabasic rocks with hydrous minerals were carried out from room temperature to 900 c under 1gpa in a closed system both vp and vs in all five rocks decreased markedly near 600 c and or 800 c following the dehydration of hydrous minerals in the rocks considering velocities only in the anhydrous matrix which were estimated from the volumetric ratio of forsterite and enstatite in each sample the effects of water produced by the dehydration reactions were estimated the ratio of v at 900 c under 1gpa is represented by the following equation v 1 0 0027 w to both p and s wave where v is the velocity in the rock including the extricated water w kg m 3 and is the velocity in the sample excluding the extricated water the low velocity layer lies between the descending lithosphere and the overlying asthenosphere beneath the japanese island arcs and the ratio of vp drop for the low velocity layer to that of the underlying descending lithosphere is approximately 5 if the observed velocity drop is wholly attributed to the free water produced by dehydration of serpentine minerals in the descending slab the required volumetric ratio of serpentine minerals in the descending slab would be 5 10

thermal structure beneath tohoku northeast japan combined with geophysical observations and petrological considerations we have studied the thermal models of the tohoku northeast japan subduction zone the temperature of the deeper part of the mantle wedge is constrained by the petrological requirements that demand a high temperature region 1400 c beneath the volcanic zone to satisfy this condition namely to explain the arc volcanism in the tohoku subduction zone a high temperature upper mantle beneath the japan sea possibly 1400 c at the depth shallower than 100 km is required heat flow distribution which constrains the temperature structure at shallower depth shows that low heat flow values ranging from 20 to 50 mw m2 are distributed between the trench and aseismic front and consistently high heat flow values 80 120 mw m2 prevail in the area landward of the volcanic front including the japan sea recent heat flow measurements indicate that the transition from low to high heat flow occurs between the aseismic front and volcanic front this heat flow distribution restricts the form of the flow in the mantle wedge and shear stress acting at the plate boundary a rigid non moving triangular region bounded by the subducting slab and the vertical plane beneath the aseismic front is necessary to explain the low heat flow zone between the trench and aseismic front shear stress of about 500 to 1000 bar is supposed to act at the interface between the subducting slab and rigid triangular mantle we also suggest that to explain the peak of heat flow near the volcanic front 120 mw m2 magma migration rate of about 0 05 cm3 cm 2 year 1 is necessary a comparison of the thermal structure of the tohoku subduction zone with that of the central south america which was preliminary calculated by the similar scheme shows that the temperature in the mantle wedge of the central south america is probably lower than that of the tohoku subduction zone this difference may result in that of the nature of volcanic rocks

new fossil evidence and diet analysis of gigantopithecus blacki and its distribution and extinction in south china the present paper reports the recently discovered fossil teeth of early pleistocene gigantopithecus blacki and associated mammalian fauna from baeryan cave bijie county in guizhou province and also reviews briefly the known fossil sites of gigantopithecus in south china of pleistocene in early pleistocene gigantopithecus had a wider distribution but withdrew southward in middle pleistocene to a limited area mainly in south china and it disappeared in the late pleistocene according to the present fossil records diet and habitat analysis from carbon isotope evidence is used to investigate the reasons for the extinction of gigantopithecus which fed on a pure c3 diet and lived in a forest habitat it was clearly different from early hominins in south and east africa such as australopithecus africanus paranthropus robustus and paranthropus boisei which had c4 diets the extinction of gigantopithecus was also related to the great changes of climate and environment of the pleistocene especially the last one million years during which homo became more and more prosperous and exerted great pressure on g blacki

weathering rates of sandstone in a semi arid environment hunter valley australia the rate of sandstone weathering in the semi arid climate of the hunter valley new south wales australia has been estimated from observations of gravestone weathering in the area the gravestone data points to two distinct stages in the weathering process the first stage covering the first century of exposure is characterised by a relatively low recession rate of 0 5 mm 100 years this is followed by a second stage in which the rate of weathering increases sharply to ca 2 5 mm 100 years the non linear nature of the weathering trends over time suggests that during the first century of exposure structural changes took place within the sandstone material which lay the foundation for accelerated weathering after further exposure laboratory trials were also conducted to identify the effectiveness of different weathering processes in the decay of sandstone in this region of the four processes examined only the freeze thaw cycle produced a significant degree of mass loss and is therefore most likely a strong contributor to the weathering of sandstone in this region

anelasticity and thermal structure of the oceanic upper mantle temperature calibration with heat flow data the thermal structure of the oceanic upper mantle is examined on the basis of seismic attenuation surface heat flow and mid ocean ridge basalt morb composition the comparison of seismic q with laboratory q determined at sufficiently high pressure and temperature allows us to constrain temperature in the earth since the inferred absolute temperature relies on knowledge of the frequency dependence of q if any we calibrate an unknown frequency effect on q by comparing the derived temperatures with geotherms calculated from surface heat flow and morb composition this comparison yields a correction factor found to be small for the geotherm derived from laboratory ultrasonic q examples of thermal structure are presented for the iceland plateau and the pacific ocean under the iceland plateau we expect partial melt for the asthenosphere only in the vicinity of the spreading ridge in asthenosphere older than 5 m y of the iceland plateau and older than 40 m y of the pacific ocean temperatures are below the dry peridotite solidus and no partial melt is expected these results are consistent with the thermal structure derived from a comparison of seismic and laboratory velocities it is shown that there is a good agreement between temperatures determined from seismic attenuation heat flow and morb chemistry since the laboratory measurements were made on a dry peridotite and we employ a dry peridotite solidus for determining the geotherm the consistency in temperatures derived from three independent data strongly suggests that the upper mantle remote from subduction zones is essentially dry it appears that the spreading process in mid ocean ridge causes asthenosphere material to ascend from greater depths where it was at higher temperature this gives rise to pressure release melting

seismic structure of the northeastern japan convergent margin a synthesis many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern japan arc and its relationship to seismic and volcanic activity spatial distributions of the depths to the conrad and the moho discontinuities estimated from shallow earthquake data and seismic explosion data show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the japan sea and the pacific ocean the pn velocity has a lateral variation it is as low as 7 5 km s beneath the land area while that beneath the japan sea and the pacific ocean is 8 0 8 2 km s it changes abruptly at the transition zones which are located along the coastlines precise structure and location of the subducted pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate the pacific plate is composed of a thin 5 km low velocity upper layer and a thick high velocity lower layer its total thickness being 80 90 km the upper plane seismicity of the double seismic zone is confined to the thin low velocity upper layer which probably corresponds to the subducted former oceanic crust the lower plane seismicity lies at the middle of the high velocity lower layer and the lower half of the plate below it is incapable of generating earthquakes the shallower portion of the upper surface of the plate beneath the pacific ocean along which major seismicity with low angle thrust faultings is actually occurring is also located by seismic observations on land and in the sea the pacific plate subducts at an extremely low angle of 5 for the first 25 km depth and then the dip steepens rather abruptly to 30 normal fault type events at the top of the plate have not been detected in the portion where the downward bending is the largest but have been ditected near the trench axis where it is rather small tomographic inversions for seismic velocity structure clearly delineate the inclined high velocity pacific plate with a thickness of 80 90 km and low velocity zones in the crust and the mantle wedge beneath active volcanoes seismic attenuation tomography also shows similar zones of low q value beneath active volcanoes although its spatial resolution is much lower the low velocity zones with 2 6 velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100 150 km in the mantle wedge and are approximately parallel to the dip of the underlying pacific plate these low velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the earth s surface corresponding to a portion of the subduction induced secondary mantle wedge flow

dating plio pleistocene glacial sediments using the cosmic ray produced radionuclides 10be and 26al we use the cosmic ray produced radionuclides 26al and 10be to date plio pleistocene glacial sediment sequences these two nuclides are produced in quartz at a fixed ratio but have different decay constants if a sample is exposed at the surface for a time and then buried by overburden and thus removed from the cosmic ray flux the 26al 10be ratio is related to the duration of burial we first attempted to date pre wisconsinan tills by measuring 26al and 10be in fluvial sediments beneath them and applying the method of burial dating which previous authors have used to date river sediment carried into caves this method however requires simplifying assumptions about the 26al and 10be concentrations in the sediment at the time of burial we show that these assumptions are not valid for river sediment in glaciated regions 26al and 10be analyses of such sediment do not provide accurate ages for these tills although they do yield limiting ages in some cases we overcome this difficulty by instead measuring 26al and 10be in quartz from paleosols that are buried by tills we use a more general mathematical approach to determine the initial nuclide concentrations in the paleosol at the time it was buried as well as the duration of burial this technique provides a widely applicable improvement on other means of dating plio pleistocene terrestrial glacial sediments as well as a framework for applying cosmogenic nuclide dating techniques in complicated stratigraphic settings we apply it to pre wisconsinan glacial sediment sequences in southwest minnesota and eastern south dakota pre wisconsinan tills underlying the minnesota river valley were deposited 0 5 to 1 5 ma and tills beneath the prairie coteau in eastern south dakota and adjacent minnesota were deposited 1 to 2 ma

cosmogenic nuclide burial ages and provenance of the xigeda paleo lake implications for evolution of the middle yangtze river abstract hundred meter thick lacustrine sediments are widespread along the middle yangtze river the distribution of these sediments suggests that an event blocked the river forming a lake stretching 160 km from east to west and 110 km north to south with a depth of 500 m using the cosmogenic nuclides 10 be and 26 al we have dated the time of burial of the lacustrine sediments and fluvial gravels beneath the sediments our results indicate a deposition age for these sediments between 1 34 and 1 58 ma studies of zircon u pb age distributions within lacustrine sediments and fluvial sands related to the paleo lake formation show diverse provenances from the upper yangtze river and yalong river a major tributary of the yangtze river these deposits contain geologic fingerprints that allow identification of the source region of the lacustrine deposits and fluvial sands our results support the reversal of the middle yangtze river before the paleo lake formation we hypothesize that the geologic event that dammed the originally southward flow was the lateral movement of chenghai fault in the dali fault system initiated in early quaternary water eventually cut through the spillway and flowed to the east

migration of fluid phases and genesis of basalt magmas in subduction zones dehydration and hydration reactions in both the downgoing lithosphere and the overlying mantle wedge have been examined in order to understand the role of h2o in the production of magmas at convergent plate boundaries the subduction of oceanic lithosphere occurring with increasing pressures and rising temperatures causes liberation of h2o from the slab amphibole which can be stable to the highest pt conditions among hydrous phases in the slab decomposes at around 90 km depth it follows that the subducted lithosphere is essentially anhydrous beneath volcanic arcs lying more than 110 km above the slab and that the supply of slab derived h2o is not a direct trigger for the production of arc magmas instead the h2o released from downgoing lithosphere reacts with the forearc mantle wedge to crystallize hydrous minerals serpentine talc amphibole chlorite and phlogopite this hydrated peridotite is dragged downward on the slab toward higher pt regions and releases h2o to shallower potential magma source regions in the mantle wedge combining experimental data on the stability of serpentine and talc with the thermal structure in the mantle wedge it is concluded that those minerals decompose beneath the forearc region on the other hand high pt experimental and thermodynamic data suggest that dehydration of amphibole and chlorite in the downdragged hydrated peridotite can take place just beneath a volcanic front phlogopite in the peridotite decomposes to release h2o at a deeper level about 200 km h2o liberated from the hydrated peridotite causes partial melting of overlying mantle wedge peridotites along with the migration of h2o through the above processes subduction components especially large ion lithophile elements can be overprinted on the magma source region which governs the geochemical characteristics of arc magmas

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

estimation of thermal structure in the mantle wedge of northeastern japan from seismic attenuation data 1 we estimated thermal structure in the mantle wedge of ne japan by applying the experimental results of olivine dominated rocks to seismic attenuation data obtained temperatures at a 40 km depth are 1000 1130 c and 960 1090 c beneath the volcanic front and the back arc side respectively we cannot obtain the areas with a temperature higher than 1400 c in the mantle wedge which have been inferred from petrological studies this is perhaps due to the localization of such anomalous areas the obtained temperature is higher than that of the wet solidus of peridotite in the greater part of the back arc side which suggests that the addition of aqueous fluids plays an important role in generating melts in the mantle wedge

three dimensional structure of vp vs and vp vs beneath northeastern japan implications for arc magmatism and fluids we estimated both p v p and s wave velocity v s structures beneath northeastern japan by applying a tomographic method to 169 712 p and 103 993 s wave arrival time data from 4338 local events the average value of v p v s ratio is 1 69 in the upper crust 1 75 in the lower crust and 1 77 in the uppermost mantle these differences in v p v s ratio may mainly reflect the lithological variations with depth low v p low v s and high v p v s zones are extensively distributed along the volcanic front in the uppermost mantle and are downward to the back arc side in the mantle wedge here the low v p low v s and high v p v s values are interpreted because of partial melting materials which imply the presence of a vast amount of melt in the uppermost mantle in the lower crust low v p low v s and high v p v s zones are not continuously visible along the volcanic front but are confined to individual volcanic areas which suggests that melting of lower crustal materials occurs just beneath each active volcano in contrast the upper crust beneath active volcanoes exhibits low v p low v s and low v p v s these results suggest that the partial melting zones under northeastern japan spread out from the uppermost mantle along the volcanic front up to the midcrust right beneath active volcanoes the low v p low v s and low v p v s features in the upper crust suggest the existence of h 2 o rather than melt right beneath active volcanoes

tectonics of marion and prince edward volcanoes indian ocean result of regional control and edifice dynamics abstract a structural analysis of marion and prince edward islands shows two kinds of tectonic control 1 1 regional tectonics the parallelism between a common structural trend on these two islands characterized by a n20 e and e w association the morphology of the underlying submarine plateau the structure of the sea floor and the fact that both islands display the same rhythm of activity all suggest that these volcanoes all resulted from the reactivation of oceanic lithosphere paleostructures 2 2 edifice tectonics the marion cone is better suited for a study of edifice dynamics than prince edward volcano of which about 80 has subsided below sea level marion is composed of a main volcanic center and a rift zone on which a second volcanic center developed it is proposed that the weight of the edifice and the geometry of the feeding and intrusive system emanating from small relatively deep magma chambers are responsible for the peculiar dynamic properties of the marion edifice the latter are the absence of a caldera a low rate of activity and numerous flank landslides a three fold classification of the stability of volcanoes based on structural patterns is proposed

ultrasonic measurements of vp and qp relaxation spectrum of complex modulus on basalt melts abstract ultrasonic compressional wave velocity v p and quality factor q p have been measured in alkali basalt olivine basalt and basic andesite melts in the frequency range of 3 4 22 mhz and in the temperature range of 1100 1400 c velocity and attenuation of the melts depend on frequency and temperature showing that there are relaxation mechanisms in the melts complex moduli are calculated from the ultrasonic data the results fit well a complex modulus of arrhenius temperature dependence with log normal gaussian distribution in relaxation times of attenuation the analysis yields average relaxation time its activation energy relaxed modulus unrelaxed modulus and width of gaussian distribution in relaxation times relaxed modulus is smaller 17 5 gpa for basic andesite melt of high silica and high alumina contents than for the other two basalt melts 18 1 18 4 gpa the most probable relaxation times decrease from 3 10 10 s for basic andesite to 10 11 s for alkali basalt at 1400 c activation energies of attenuation ranging from 270 to 340 kj mol 1 in the three melts are highest in basic andesite longitudinal viscosity values and their temperature dependences are also calculated from v p and q p data the volume viscosity values are estimated from the data using the shear viscosity values longitudinal volume and shear viscosities and their activation energies are highest in the basic andesite melt of the most polymerized structure

hot fingers in the mantle wedge new insights into magma genesis in subduction zones abstract quaternary volcanoes in the northeast japan arc can be grouped into 10 volcanic clusters striking transverse to the arc these have an average width of 50 km and are separated by parallel gaps 30 75 km wide this clustering of volcanic centres topographic profiles low velocity regions in the mantle wedge and local negative bouguer gravity anomalies along the japan sea side of the volcanic arc are closely correlated all these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined 50 km wide fingers each of the 10 fingers recognised extends from deep mantle 150 km below the back arc region towards the shallower mantle 50 km beneath the volcanic front quaternary volcanoes are built immediately above the hot mantle fingers the volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust accompanied by quaternary volcanic activity at the surface although volcanic activity is rare along the japan sea coast tomographic results show that hot low velocity mantle fingers exist within the mantle wedge the negative bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers these have not yet been erupted but have accumulated at the moho discontinuity

cosmogenic burial ages reveal sediment reservoir dynamics along the yellow river china burial dating using cosmogenic al 26 and be 10 is an emerging technique for establishing chronologies of fluvial deposits ranging in age from ca 0 5 ma to 5 ma the determination of burial age however requires an assumption that sediment is buried with a known initial al 26 be 10 ratio using a sequence of well dated fluvial terraces along the yellow river in lanzhou china we demonstrate that this assumption may be violated when upstream sediment sources include neogene sedimentary basins samples of fluvial gravel from six terraces yield burial ages ranging from ca 0 83 ma to 2 81 ma comparison of these results to independent ages determined from magneto stratigraphic and loess paleosol studies demonstrates a high degree of correspondence between ages in the lower three terraces in the higher terraces however burial ages are systematically too old implying a previous burial history these results can be exploited to place quantitative constraints on the provenance of sediment in the terrace deposits our results reveal that rapid incision and excavation of late neogene sedimentary basins ca 1 7 ma led to injection of fluvial sediment with low initial al 26 be 10 ratios dilution of this source through time occurred as incision progressed into upstream bedrock ranges our results suggest that cosmogenic nuclides can illuminate the timing of long term storage and remobilization of sediment in continental scale river systems

rock uplift rates in south africa from isochron burial dating of fluvial and marine terraces the high plateau of southern africa is considered to be either uplifting due to mantle driven dynamic topography or to have been stable since mesozoic rifting to address this debate we determined rock uplift in south africa from the long term incision rate of the sundays river near port elizabeth and from an uplifted marine terrace near durban we dated the terraces with cosmogenic 26 al and 10 be using both isochron and simple burial dating methods we find that the sundays river has incised at 16 1 1 3 m m y for the past 4 m y and the marine terrace yields a rock uplift rate of 9 4 2 2 m m y these results are inconsistent with rapid neogene uplift

distinct s wave reflector in the midcrust beneath nikko shirane volcano in the northeastern japan arc distinct s waves reflected from a midcrustal seismic velocity discontinuity are detected beneath nikko shirane volcano in the southernmost part of the northeastern japan arc a detailed travel time analysis of the reflected s waves by using data acquired through a dense seismic network temporarily set up in this region shows that this unusual s wave reflector is distributed over an area of 15 15 km2 at depths ranging from 8 to 15 km the reflector has a conical shape becoming shallow toward the summit of nikko shirane volcano observed amplitude spectral ratios of reflected s waves to direct s waves show that the reflector body has a strong velocity contrast to the surrounding medium and its thickness is of the order of 100 m at most the reflector body is approximated by two thin layers probably filled with partially molten materials cutoff depth for shallow seismicity in this area is 3 5 km above the reflector and becomes shallow toward nikko shirane volcano nearly parallel to the reflector the depth to brittle ductile transition zone seems to be prescribed by the existence of the reflector body which is perhaps a thin magma body

the valle del bove mount etna its origin and relation to the stratigraphy and structure of the volcano abstract the valle del bove is a major horseshoe shaped depression some 5 km in diameter and over 1000 m deep carved into the eastern flank of mount etna volcano the origin of the valle del bove has been a subject of debate for over a century but recent models relate it to the collapse of the ancient trifoglietto ii centre of etna either by caldera collapse or phreatomagmatic explosive activity careful morphological analysis shows the valle del bove to have formed after the post trifoglietto ii ellittico and leone centres and therefore to be considerably younger than the trifoglietto ii centre it has also been argued that the pyroclastic deposits that occur on the flanks of the volcano in close spatial relationship with the valle del bove were produced by the same event these pyroclastics are divided on the basis of age and composition into two groups the upper and lower tephra deposits the stratigraphy petrology and radiocarbon date of the lower tephra deposits indicate a trifoglietto ii age at least 20 000 years before the formation of the valle del bove the upper tephra deposits have a strong phreatomagmatic character and are closest in age to the valle del bove there is no evidence however to link them with its formation morphological study shows the valle del bove to consist of a series of coalescing scallops indicative of formation by landsliding the material removed by slope failure and subsequent fluvial transport followed a valley to the coast where it forms a fan shaped accumulation the chiancone consisting of volcaniclastics of fluvial and mudflow origin it is argued that the valle del bove did not result from simple caldera collapse but formed from a series of collapses caused by gravitational slope failure the chiancone is considered to be the product of this collapse

the ninole basalt implications for the structural evolution of mauna loa volcano hawaii lava flows of the ninole basalt the oldest rocks exposed on the south side of the island of hawaii provide age and compositional constraints on the evolution of mauna loa volcano and the southeastward age progression of hawaiian volcanism although the tholeiitic ninole basalt differs from historic lavas of mauna loa volcano in most major element contents e g variably lower k na si higher al fe ti ca ree and other relatively immobile minor elements are similar to historic and prehistoric mauna loa lavas and the present major element differences are mainly due to incipient weathering in the tropical environment new k ar whole rock ages from relatively fresh roadcut samples suggest that the age of the ninole basalt is approximately 0 1 0 2 ma although resolution is poor because of low contents of k and radiogenic ar originally considered the remnants of a separate volcano the ninole hills are here interpreted as faulted remnants of the old south flank of mauna loa deep canyons in the ninole hills eroded after massive landslide failure of flanks of the southwest rift zone have been preserved from burial by younger lava due to westward migration of the rift zone landslide induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread basaltic ash that overlies the ninole basalt subaerial presence of the ninole basalt documents that the southern part of hawaii island had grown to much of its present size above sea level by 0 1 0 2 ma and places significant limits on subsequent enlargement of the south flank of mauna loa

volcano structural evolution of piton des neiges reunion island indian ocean four volcano structural stages have accompanied the building of piton des neiges 1 emergent growth stage of the island the major eruptive system is a rift zone trending n 120 associated with dextral strike slip faults trending n 30 and en echelon extensional fissures trending n 70 breccias and lava tubes produced by aerial and phreatomagmatic activity are injected with outward dipping dike swarms along ring fractures suggesting a mechanism analogous to cauldron subsidence 2 shield building stages of growth are related to fissures along the main rift zone and three minor rifts trending n 160 n 45 and n 10 the summit of the basaltic shield volcano is stretched and collapsed in a graben like caldera depression along normal and antithetic faults 3 differentiated lavas are erupted during two stages separated by the opening of a new caldera corresponding to an explosive activity a silicic cone sheet system and a collapse structure 4 younger volcanic activity restricted to the inside caldera has presumably emptied the underlying magma reservoir building a central volcano collapsed along ring internal dip fractures

tomographic inversion for p wave velocity structure beneath the northeastern japan arc using local and teleseismic data we have determined three dimensional p wave velocity structure up to a depth of 296 km beneath northern tohoku honshu japan by inverting arrival times of local and teleseismic events simultaneously velocities in the upper crustal layers correlate well with the surface geology and bouguer gravity anomalies low velocity zones are ubiquitous in the crust and the uppermost mantle beneath the quarternary volcanic areas a well known feature of a pronounced low velocity mantle wedge underlain by the dipping high velocity high v pacific plate is generally recovered over the study area strong heterogeneity within the high velocity pacific plate is suggested by the presence of a pronounced low velocity anomaly confined to the depths of 130 200 km the decreased seismicity of intermediate focus events may be correlated with the low velocity zone a two dimensional inversion scheme was invoked to determine stably the average mantle structure perpendicular to the island arc in the depths of 50 150 km the inclined high v zone has a thickness of 80 100 km with its top coinciding with the upper plane of the double planed deep seismic zone below the depth of 200 km the high v zone is shifted toward the bottom of the 80 km thick reference slab in the intermediate depth layer 150 200 km the high v zone is not clearly visible within the 80 km thick reference slab

ultrasonic velocity and attenuation measurements on basalt melts to 1500 c role of composition and structure in the viscoelastic properties viscoelastic properties of basalt melts of a wide range of composition are determined from ultrasonic compressional wave velocity vp and quality factor qp measurements in the frequency range of 3 4 22 mhz and the temperature range of 1150 1500 c the basalt samples are grouped into two types low magnesia group a containing 5 9 wt mgo and high magnesia group b containing 18 23 wt mgo the vp values of the melts determined at 1400 c are in the range 2 53 2 64 km s for group a basalts and 2 79 2 86 km s for group b basalts the relaxed low frequency velocity vr is a principally linear function of the melt density least squares fits of the data give velocity density relations vr km s 0 691 0 712 g cm3 for group a basalts and vr 0 328 0 907 for group b basalts at 1400 c the mean molecular weights m are 65 0 0 3 and 58 2 0 5 g mol for groups a and b basalts respectively suggesting that m is also a principal factor determining the melt velocity the relaxed modulus mr of the basalt melts range from 16 5 to 22 8 gpa and is also a linear function of the melt density the values of longitudinal l and volume v viscosities are calculated from the measured vp and qp and the available shear viscosity s data the viscosities and related activation energies decrease almost linearly as nbo t nonbridging oxygens per tetrahedrally coordinated cation increases

petrology of tholeiitic lavas from tanna island new hebrides importance of cumulative processes in island arc magmatism tanna one of the southernmost islands of the new hebrides volcanic arc is made of late pliocene to recent island arc tholeiitic basalts and andesites with sio2 contents ranging from 45 to 57 these lavas are highly porphyritic 30 50 in volume phenocrysts of plagioclase are the most abundant together with olivine and clinopyroxene the groundmass contain plagioclase augite olivine magnetite and glass pigeonite tridymite sanidine and rarely biotite may also occur the olivines and clinopyroxenes show an iron enrichment from the cores of phenocrysts to their rims and the groundmass crystals but their compositional variations are not correlated with the mg fe ratio of bulk host rocks the most fe rich compositions being found in mg rich lavas plagioclase compositions range from an95 to an60 in the basalts and an60 to an50 in the andesites but within each group they are not correlated with sio2 or na2o contents of host lavas consequently the bulk major element compositions of tanna volcanic rocks cannot be considered as primarily controlled by crystal separation from successive liquids the oxyde sio2 variations diagrams and the modal compositions and mineral chemistry show that crystal accumulation is the predominant mechanism accounting for bulk rock compositions however this does not exclude fractional crystallization the variation of the calculated groundmass mineralogy strongly suggest the occurrence of crystal removal mainly clinopyroxene and magnetite

deep structure of arc volcanoes as inferred from seismic observations possible evidence for deep seated magmatic activity beneath the northeastern japan arc has been obtained from micro earthquake observations tomographic inversions for p wave velocity clearly delineate low velocity zones which are inclined to the west and continuously distributed from the uppermost mantle to the upper crust beneath active volcanoes exceptionally deep 22 40 km micro earthquakes are found at 10 locations around the low velocity zones beneath active volcanoes all these events have extremely low predominant frequencies suggesting a close relation to the magmatic activity in this depth range at shallower depths 8 15 km distinct reflectors of s waves from shallow events are found at five locations in or around the low velocity zones their locations and reflection coefficients suggest that the reflectors are very thin magma bodies existing in the mid crust these observations shed some light on the state of magma at depths beneath volcanic arcs

relationships between fluvial evolution and karstification related to climatic tectonic and eustatic forcing in temperate regions this paper reviews the diversity of relationships between river evolution and karstogenesis it also underlines the fundamental role of numerical dating methods e g cosmogenic nuclides applied to sedimentary sequences in tiered cave passages as they have provided new insights into these complex interactions although karst terrain is widespread worldwide we focus on european karst catchments where the sedimentary records are especially well preserved we review the recent dating of fluvial sediments and speleothems to examine the timing of karstification incision and deposition in cave levels the most complete alluvial records occur in tectonically uplifted high mountains where some of the oldest sediment fills date to the miocene evidence indicates that not only uplift but also climatic conditions and fluvial dynamics e g knickpoint retreat increased channel flow and or sediment load and stream piracies can play a major role in speleogenesis and geomorphological evolution in evaporite rocks speleogenesis is characterized by rapid dissolution and subsidence in european catchments gypsum cave development largely occurred during cold climate periods while limestone caves formed during warm interglacial or interstadial phases our synthesis is used to propose four models of fluvial and karst evolution and highlight perspectives for further research

new evidence for the incision history of the liuchong river southwest china from cosmogenic 26al 10be burial ages in cave sediments cosmogenic nuclides be 10 and al 26 have been analyzed for sediments from the multilevel riverside caves along the liuchong river at the southeastern margin of the tibetan plateau the liuchong river is the northern origin of wujiang river which passes through the northwestern guizhou plateau and cuts down hundreds of meters into the bedrock leaving behind an abundance of multilevel caves the measured al 26 be 10 ratios produced the apparent burial ages in range of 0 49 2 85 ma taking into account of geomorphic and geological backgrounds the dashi cave located at the highest level along the liuchong river system formed around 0 75 ma ago which probably suggests the initial formation age of the modern wujiang river the resulted incision rate of similar to 480 m ma in guizhou in the last 0 75 ma is slightly higher than those in adjacent areas this feature implies an intensive downcutting history of the liuchong river during the quaternary which might be primarily caused by the uplift of tibetan plateau cut through of the three gorges and soluble carbonate bedrock c 2013 elsevier ltd all rights reserved

quaternary downcutting rate of the new river virginia measured from differential decay of cosmogenic 26al and 10be in cave deposited alluvium the concentrations of the cosmogenic radionuclides sup 26 al and sup 10 be in quartz can be used to date sediment burial here we use sup 26 al and sup 10 be in cave deposited river sediment to infer the time of sediment emplacement sediment burial dates from a vertical sequence of caves along the new river constrain its quaternary downcutting rate to 27 3 4 5 m m y and may provide evidence of regional tectonic tilt 32 refs 3 figs 1 tab

a 900 k y record of strath terrace formation during glacial interglacial transitions in northwest china the timing of the development of strath terraces with respect to climatic variability remains equivocal previous studies attribute strath terrace formation to glacial or interglacial climates or to variations in sediment and water fluxes that cause lateral erosion followed by vertical incision a chronology of strath terrace formation spanning similar to900 k y has been generated on the basis of loess paleosol couplets and paleomagnetic thermoluminescence and radiocarbon dating of strath terraces in the qilian shan of northeastern tibet repetitive stratigraphic and geomorphic patterns on each terrace indicate that they formed during glacial interglacial transitions long term bedrock incision rates and inferred rock uplift rates appear steady and unrelated to strath formation over the past 900 k y

early pleistocene incision of the san juan river utah dated with 26al and 10be cosmogenic 26al and 10be in alluvial gravel on a strath 150 m above the san juan river utah reveal a depositional age of batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document 1 36 0 20 0 15 end document ma this gravel is correlative with a series of terraces that grade to glen canyon on the colorado river indicating a similar age for incision there the calculated incision rate 110 14 m m y is somewhat slower than that of the colorado river in the eastern grand canyon and suggests active steepening of the colorado river the cosmogenic nuclides also indicate rapid erosion in the sediment source area and are consistent with alluviation due to enhanced pleistocene erosion in the san juan mountains

submarine geology of the hilina slump and morpho structural evolution of kilauea volcano hawaii abstract marine geophysical data including sea beam bathymetry hawaii mr1 sidescan and seismic reflection profiles along with recent robot submersible observations and samples were acquired over the offshore continuation of the mobile kilauea volcano south flank this slope comprises the three active hot spot volcanoes mauna loa kilauea and loihi seamount and is the locus of the hawaiian hot spot the south flank is the site of frequent low intensity seismicity as well as episodic large magnitude earthquakes its sub aerial portion creeps seaward at a rate of approximately 10 cm year the hilina slump is the only large submarine landslide in the hawaiian archipelago thought to be active and this study is one of the first to more highly resolve submarine slide features there the slump is classified into four distinct zones from nearshore to the island s base estimates of size based on these data indicate a slumped area of 2100 km 2 and a volume of 10 000 12 000 km 3 equivalent to about 10 of the entire island edifice the overall picture gained from these data sets is one of mass wasting of the neovolcanic terrain as it builds upward and seaward though reinforcement by young and pre hawaii seamounts adjacent to the pedestal is apparent extensive lava delta deposits are formed by hyaloclastites and detritus from recent lava flows into the sea these deposits dominate the upper submarine slope offshore of kilauea with pillow breccia revealed at mid depths along the lower flanks massive outcrops of volcanically derived sedimentary rocks were found underlying kilauea thus necessitating a rethinking of previous models of volcanic island development the morphologic and structural evolutionary model for kilauea volcano and the hilina slump proposed here attempts to incorporate this revelation a hazard assessment for the hilina slump is presented where it is suggested that displacement of the south flank to date has been restrained by a still developing northeast lateral submarine boundary when it does fully mature the south flank may be more subject to land slips triggered by large long duration earthquakes and thus kilauea may undergo more frequent episodes of failure with increased displacements

petrology of arc volcanic rocks and their origin by mantle diapirs abstract calc alkalic chemical trends characteristic of arc volcanic rocks mainly result from three mechanisms which act additively 1 fractional crystallization involving separation of titanomagnetite 2 selective concentration of plagioclase phenocrysts and selective depletion of titanomagnetite phenocryst compared with the actually fractionated proportion and 3 mixing of magmas on continuous fractionation trends the association of calc alkalic and tholeiitic trends in a single composite volcano may not represent different fractional crystallization processes or different chemistries of primary magmas but the calc alkalic chemical trend can be considered as a mixing trend resulting from mixing of various magmas on associated tholeiitic chemical trends chemical variations of most arc volcanic rocks including calc alkalic ones can accordingly be essentially accounted for by the low pressure fractional crystallization of phenocrystic phases from primary basaltic magmas crystallization sequences of arc magmas which are strongly dependent on water content in magmas are deduced from the phenocryst assemblages the crystallization sequence changes laterally across arc suggesting increasing water contents in magmas toward the back arc side as is also seen for other incompatible elements such as k and rb systematic differences in the characteristic crystallization sequence are also observed among arcs roughly correlating with the crustal thickness water content in magma like other incompatible elements tends to increase with increasing crustal thickness the variation of incompatible elements including water roughly represents that of the degree of partial melting of the upper mantle which is broadly controlled by the crustal thickness the variation of water content indicates that arc magmas are not saturated with water during differentiation to late differentiates such as dacite or rhyolite this strongly constrains the maximum water contents in primary basaltic magma at most 2 5 wt this value suggests that magma generation beneath arcs is dependent on dry solidus of peridotite diapiric uprise of the hot deeper mantle and associated adiabatic decompression would be necessary for mantle peridotite to attain the temperature as high as dry solidus diapirs that begin to rise from the subduction zone may stop at or near the crust mantle boundary because of the surrounding density change and their degree of partial melting is roughly controlled by their stopped depth assuming their similar temperature across arc variation is also explained by the stopped depth of diapirs but is not controlled by crustal thickness

three dimensional q structure beneath the kanto tokai district japan the q structure of the upper mantle beneath the kanto tokai district central japan where the eurasian pacific and philippine sea plates are converging is investigated from seismic p and s wave spectral ratios by means of a three dimensional 3 d inversion method it is found that the subducting philippine sea and pacific plates are characterized by high q zones a high q zone also exists between the philippine sea plate and the pacific plate to the east of the volcanic front on the other hand a low q zone in the uppermost mantle extends westward horizontally from the volcanic front under the philippine sea plate and appears to exist over the wedge mantle beneath the western tokai district a very low q value obtained here is roughly equal to the average qvalue in the low velocity layer derived from surface wave and free oscillation data a low q region that suggests a magma chamber under active volcanoes is not detected in the crust

dating of pliocene colorado river sediments implications for cosmogenic burial dating and the evolution of the lower colorado river we applied cosmogenic 26 al 10 be burial dating to sedimentary deposits of the ancestral colorado river we compared cosmogenic burial ages of sediments to the age of an independently well dated overlying basalt flow at one site and also applied cosmogenic burial dating to sediments with less precise independent age constraints all dated gravels yielded old ages that suggest several episodes of sediment burial over the past 5 3 m y comparison of burial ages to the overlying 4 4 ma basalt yielded good agreement and suggests that under the most favorable conditions cosmogenic burial dating can extend back 4 5 m y in contrast results from other sites with more broadly independent age constraints highlight the complexities inherent in burial dating these complexities arise from unknown and complicated burial histories insufficient shielding postburial production of cosmogenic isotopes by muons and unknown initial 26 al 10 be ratios nevertheless and in spite of the large range of burial ages and large uncertainties we identify samples that provide reasonable burial age constraints on the depositional history of sediment along the lower ancestral colorado river these samples suggest possible sediment deposition and burial at ca 5 3 4 7 and 3 6 ma our calculated basinwide erosion rate for sediment transported by the modern colorado river 187 mm k y 1 is higher than the modern erosion rates inferred from the historic sediment load 80 100 mm k y 1 in contrast basinwide paleo erosion rates calculated from pliocene sediments are all under 40 mm k y 1 the comparatively lower denudation rates calculated for the pliocene sediment samples are surprising given that the sampled time intervals include significant pliocene aggradation and may include much incision of the grand canyon and its tributaries this conflict may arise from extensive storage of sediment along the route of the colorado river slower paleobedrock erosion or the inclusion of sediments that were derived preferentially from higher elevations in the watershed

three dimensional seismic attenuation structure beneath the japanese islands and its tectonic and thermal implications a three dimensional 3 d seismic attenuation structure beneath the japanese islands is determined by inversion of seismic intensity data the result clearly reveals a remarkable contrast in the attenuation structure and reflects the lateral variation of thermal structure beneath the japanese islands for the inversion the method developed by hashida and shimazaki is used the intensity which is assumed to be a measure of the maximum acceleration of the s wave at the observation point is inverted to estimate the attenuation structure and the acceleration at the seismic source the data consist of about 15 000 intensity readings for about 800 earthquakes reported by the japan meteorological agency between 1951 and 1983 by combining 3 d structures of eight districts in japan 3 d attenuation maps consisting of three layers are constructed to a depth of 90 km the obtained structure reveals the following features first a distinct difference exists in the crustal structure between ne and sw japan in ne japan low q high attenuation areas on the continental side of the arc are dominant whereas some high q low attenuation areas exist on the pacific side of the arc in sw japan except for the kyushu district high q areas are dominant and small low q areas exist mainly on the pacific coast these crustal features generally continue down to the uppermost mantle second the obtained crustal structure shows that attenuation is age dependent low qareas correspond to the distribution of quaternary volcanic materials and high q areas correspond to the geological provinces where pre cenozoic rocks are distributed since the thermal structure strongly depends on the crustal age this feature suggests that the attenuation structure reflects the thermal structure third in the upper mantle low q loci are found which correspond to the distribution of active and other quaternary volcanoes forming the arc volcanic chain it should be noted that even on the continental side of the volcanic front these loci do not exist beneath areas where no volcano exists this suggests that the upper mantle attenuation is controlled by the existence of upwelling diapirs which are considered to lie beneath a volcano or a group of volcanoes fourth high q zones in the upper mantle exist on the pacific side of the volcanic front along the kurile northeast honshu arc and along ryukyu arc the high q mantle lying above the sinking slab seems to reflect a cooling effect of the subducting cold oceanic plates the thickness of the lithosphere may reach 60 km or more in such outer arcs although the thickness has been considered to be about 30 km or so in sw japan where the high q uppermost mantle also lies a thick lithosphere of about 60 km may exist possible thermal cross sections are proposed for ne and sw japan based on the obtained 3 d attenuation structure and other geophysical considerations in the uppermost mantle of ne japan a higher temperature is expected than in sw japan the thermal structures reflect the difference of tectonic settings in the two regions such as the subducting plate overriding lithosphere and volcanic activity

rates of erosion and topographic evolution of the sierra nevada california inferred from cosmogenic26al and10be concentrations concentrations of cosmogenic 26 al and 10 be in cave sediments and bedrock surfaces combined with studies of landscape morphology elucidate the topographic history of the southern sierra nevada over the past 5 ma caves dated by 26 al 10 be in buried sediments reveal that river incision rates were moderate to slow between c 5 and 3 ma 0 07 mm a 1 accelerated between 3 and 1 5 ma c 0 3 mm a 1 and then have subsequently become much slower c 0 02 mm a 1 although the onset of accelerated incision coincides in time with both postulated pliocene tectonism and pronounced global climate change we argue that it primarily represents the response to a discrete tectonic event between 3 and 5 ma dated cave positions reveal that prior to 3 ma river canyons displayed up to 1 6 km of local relief suggesting that pliocene rock uplift elevated pre existing topography renewed incision beginning c 3 ma deepened canyons by up to 400 m creating narrow inner gorges tributary streams exhibit strong convexities indicating that the transient erosional response to pliocene uplift has not yet propagated into upland surfaces concentrations of 26 al and 10 be in bare bedrock show that upland surfaces are eroding at slow rates of c 0 01 mm a 1 over the past c 3 ma upland surfaces eroded slowly while adjacent rivers incised rapidly increasing local relief although relief production probably drove at least modest crestal uplift considerable prepliocene relief and low spatially averaged erosion rates suggest that climatically driven rock uplift is not sufficient to explain all uplift implied by tilted markers at the western edge of the range despite the recent pulse of erosion spatially averaged erosion rates are low and have probably acted to preserve the broad topographic form of the sierra nevada throughout much of the late cenozoic copyright 2005 john wiley sons ltd

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

nd and sr isotopic study of volcanic rocks from japan two older granitic rocks and some selected quaternary volcanic rocks from the japanese islands were analyzed in a reconnaissance study for the purpose of examining the relationships between nd and sr isotopic abundances and the megatectonic structure around the japanese islands model ages of 0 9 ae were determined by the nd and sr methods on a paleozoic gneiss which confirms that a relatively ancient acidic basement exists in the japanese islands the nd and sr isotopic data show that the cretaceous granodiorite is the result of partial melting of older crust the nd of tholeiitic rocks from the izu arc gives e nd ranging from 8 3 to 9 3 and with the corresponding e sr from 14 5 to 18 5 these results are identical to those found for the mariana arc these values are distinctly lower than typical morb by around 1 2 eu this difference in e nd between arcs and morb is attributed to the contribution of oceanic sediments to the partial melts produced during subduction of oceanic crust the hakone volcano is clearly confirmed as belonging to an oceanic source by nd isotopic results e sr e nd values of the volcanics from a section along the fossa magna show a clear indication that they are a blend of oceanic mantle material and continental crustal material the crustal component clearly increases in going from south to north volcanics across the northeast japan arc also show a distinct correlation of e sr e nd related to the position relative to the active subduction zone but with the opposite trend these relationships of the present isotopic pattern and the zonal arrangement relative to the subduction zone suggest the former existence of a local spreading center in the japan sea in general there appear to be regular isotopic relationships between the izu mariana oceanic island arc and the continental island arc of japan which indicates that partially melted or assimilated older continental basement is admixed with young rising oceanic arc magmas

the distribution of basaltic volcanism on tenerife canary islands implications on the origin and dynamics of the rift systems abstract one of the most characteristic features of volcanic islands is the existence of rift zones defined commonly as orientated eruptive fissures or parallel rows of elongate cinder cones and dyke swarms occasionally these rifts can appear at the birth of the volcanic island and persist until the last episodes of its constructions controlling the form and structure of the island e g azores islands in the case of tenerife canary islands it is possible to observe two rift zones santiago del teide and dorsal rifts running nw se and ene wsw marked by parallel rows of aligned cones and eruptive fissures additionally at the southern part of the island southern volcanic zone basaltic volcanism is characterized by scattered vents and apparently non coherently orientated eruptive fissures some authors relate the existence of the latter volcanism to a n s running rift zone that defines the third branch of a three armed rift system in the island in the present paper we first investigate the tectonic controls on the distribution of basaltic volcanism at the southern volcanic zone and their relation with the nw se and ene wsw rifts the numerical results obtained suggest that basaltic volcanism of the southern part of tenerife can be easily explained as the result of a extensional stress field derived from the combined effects of the nw se and ene wsw rifts as a second objective we have also investigated the origin of the santiago del teide and dorsal rift zones and their role on the formation of the original shield volcano and the subsequent evolution of the whole island our numerical results contrast with previously published explanations on the origin of the tenerife rifts that included fracturing due to volcano spreading or to deformation of the volcano due to magma intrusion we consider that volcanic activity in tenerife began throughout fissural volcanism along these structures that were already present in the oceanic basement progressively accumulating the basaltic series that gave rise to the construction of the composite shield volcano

lower pliocene hominid remains from sterkfontein cosmogenic aluminum 26 and beryllium 10 burial dates of low lying fossiliferous breccia in the caves at sterkfontein south africa show that associated hominid fossils accumulated in the lower pliocene these dates indicate that the skeleton stw 573 and newly discovered specimens from jacovec cavern have much the same age approximately 4 million years these specimens are thus of an age similar to australopithecus anamensis from east africa

dating offset fans along the mojave section of the san andreas fault using cosmogenic 26al and 10be analysis of cosmogenic 1 0 be and 2 6 al in samples collected from exposed boulders n 20 and from buried sediment n 3 from offset fans along the san andreas fault near little rock california yielded ages ranging from 16 to 413 ka which increase with dis lance from their source at the mouth of little rock creek in order to determine the age of the relatively younger fans the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established cosmogenic nuclide inheritance values that range between 8 5 x 10 3 and 196 10 3 atoms 1 0 be g 1 quartz were determined by measuring the concentrations and ratios of 1 0 be and 2 6 al in boulders n 10 and fine sediment n 7 at the outlet of the present active stream boulder erosion rate ranging between 17 and 160 mm k y 1 was estimated by measuring 1 0 be and 2 6 al concentrations in nearby bedrock outcrops n 8 since the boulders on the fans represent the most resistant rocks in this environment we used the lowest rate for the age calculations monte carlo simulations were used to determine ages of 16 5 and 29 7 ka for the two younger fan surfaces older fans older than 100 ka were dated by analyzing 1 0 be and 2 6 al concentrations in buried sand samples the ages of the three oldest fans range between 227 242 and 413 185 ka although fan age determinations are accompanied by large uncertainties the results of this study show a clear trend of increasing fan ages with increasing distance from the source near little rock creek and provide a long term slip rate along this section of the san andreas fault slip rate along the mojave section of the san andreas fault for the past 413 k y can be determined in several ways the average slip rate calculated from the individual fan ages is 4 2 0 9 cm yr 1 a linear regression through the data points implies a slip rate of 3 7 1 0 cm yr 1 a most probable slip rate of 3 0 1 0 cm yr 1 is determined by using a 2 test these rates suggest that the average slip along the mojave section of the san andreas fault has been relatively constant over this time period the slip rate along the mojave section of the san andreas fault determined in this study agrees well with previous slip rate calculations for the quaternary

abrupt glacial valley incision at 0 8 ma dated from cave deposits in switzerland glacial erosion dramatically alters mountain landscapes but the pace at which glaciers carve a previously fluvial landscape remains poorly defined because long term valley incision rates are difficult to measure here we reconstruct the lowering history of the aare valley switzerland over the past 4 m y by dating cave sediments with cosmogenic 26al and 10be incision accelerated from 120 m m y to 1200 m m y at 0 8 1 0 ma at least 1 m y after the onset of local glaciation rapid incision may have been triggered by lowering of the equilibrium line altitude at the mid pleistocene climate transition

an isochron method for cosmogenic nuclide dating of buried soils and sediments we describe an improved method for dating buried paleosols using measurements of the cosmic ray produced radionuclides 10be and 26al in quartz grains and apply it to a sequence of intercalated tills and paleosols in central missouri usa that record plio pleistocene advances of the laurentide ice sheet a buried paleosol implies a period of surface exposure and nuclide accumulation followed by burial and a halt to nuclide production if the paleosol is formed in a sedimentary unit such as till this unit may also have been emplaced with unknown 26al and 10be concentrations inherited from past surface exposure if the inherited nuclide concentrations are the same at all depths in the soil as is true for well mixed sediments such as till then the 26al and 10be concentrations at different depths in the paleosol will show a linear relationship the slope of this line depends on the duration of burial of the paleosol but not on the inherited nuclide concentrations or on the sample depths thus one can date strata overlying buried paleosols by measuring 26al and 10be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron we focus on applying this approach to till paleosol sequences but the basic idea of forming an 26al 10be burial isochron with a set of samples that share the same burial age but differ in other aspects of their exposure history applies to other stratigraphic settings as well the method yields ages for four tills in missouri that are stratigraphically consistent agree with paleomagnetic age constraints and show that ice advanced into missouri near 1 25 ma near 0 8 ma and twice between ca 0 4 and 0 2 ma

geomorphic inheritance and the development of tower karst four genetic types of tower karst are identified the most typical consisting of residual hills protruding from a planed carbonate surface veneered by alluvium two extreme possibilities for tower karst evolution are recognized a direct development that is independent of any previous morphology and a sequential development that depends explicitly on the morphological characteristics of a previous cockpit karst phase examples of both are described although the latter appears the more common south china tower karst is examined in this light palaeomagnetic analysis of cave deposits in a tower near guilin indicate the feature to be a time transgressive landform four principal controls of tower development are discussed and a model of tower evolution is presented

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

initial incision of the jinshan gorge of the yellow river china constrained by terrestrial in situ cosmogenic nuclides chronology abstract how post burial terrestrial in situ cosmogenic nuclides tcn production is build up in slow burial condition remains uncertain in this study we dated high fluvial terrace sediments that overlain by loess with a slow deposition rate that is based on the assumption of steady accumulation rate via 26al 10be burial dating method for sample in deep and rapid buried condition prior to slow overlying post burial production shows low component 4 6 in measured concentration however post burial production occupied 39 79 of measured 26al and 10be concentrations which suggests approximately 0 2 0 3 ma underestimation in simple burial dating mode if post burial production is neglected corrected by post burial production we yielded burial ages of two top terraces ca 1 7 2 5 ma and 1 5 ma respectively below the tangxian planation surface along the jinshan gorge of the yellow river implying that the jinshan gorge initially incised at ca 1 7 2 5 ma corresponding well to regional tectonic activities such as extension in the fenwei graben accelerative uplift of the tangxian planation surface vigorous dissection of north china and significant tectonic deformation in northeastern tibetan plateau the slow burial mode is probably a potential method for plateau paleo elevation estimation taking advantage of height sensitivity of tcn production rate if overlying loess magnetostratigraphic age is known

partial melt below iceland a combined interpretation of seismic and conductivity data a combined interpretation of published seismic and electric data is carried out to impose constraints on the state of melting below central iceland these data indicate high electrical conductivity and low vp velocity regionally below 6 km s for the crust mantle transition region while the mantle below shows lower conductivity but still anomalous low velocity the velocity data are treated by utilizing gravity data to quantify the elastic modulus decrease due to partial melt a comprehensive set of theoretical models on elasticity anelasticity and electrical conductivity of partially molten rock is applied to this data set the geometry the fraction and the interconnection of the melt phase are varied systematically the conductivity models are based on published laboratory data on conductivity of solid and melt phases of rocks anisotropy and possible unexplored conductivity mechanisms near the melting temperature are not considered here by means of the melt models the data can be explained most easily as follows in a depth range roughly between 20 and 50 km only a small percentage of melt 2 at the most 5 if melt pockets are also present occurs in thin films possibly with a distribution of aspect ratios and a low degree of interconnection if in the crust mantle transition region zones of low p wave velocity 6 km s are correlated with those of high conductivity 0 08 s m melt fractions between 10 and 20 are likely and the degree of interconnection should be high thin films can be excluded as a dominant melt geometry in that region apart from improved constraints on melt geometry the results are in good agreement with published models based only on conductivity data however they disagree with published models based entirely on seismic data the discrepancy is now resolved by the combined interpretation

late cenozoic uplift of southeastern tibet the age of surface uplift in southeastern tibet is currently unknown but the initiation of major river incision can be used as a proxy for the timing of initial uplift the topographically high eastern plateau and gently dipping southeastern plateau margin are mantled by an elevated low relief relict landscape that formed at a time of slow erosion at low elevation and low tectonic uplift rates prior to uplift of the eastern tibetan plateau thermochronology from deep river gorges that are cut into the relict landscape shows slow cooling between ca 100 and ca 10 20 ma and a change to rapid cooling after ca 13 ma with initiation of rapid river incision at 0 25 0 5 mm yr between 9 and 13 ma a rapid increase in mean elevation of eastern tibet beginning at this time supports tectonic climate models that correlate the lateral eastern expansion of high topography in tibet with the late miocene intensification of the indian and east asian monsoons

preliminary study of quaternary tectono climatic cycles in china major cycle ii this started in the middle part of the aggradation of early pleistocene lacustrine and fluviolacustrine beds nihewan yuanmou and qiangtant formations and ceased with the deposition of the top part of these strata during this cycle the second glacial stage corresponding to the shizishan glacial stage of the east kunlun mountains is essentially indicated by sporepollen assemblages and periglacial phenomena reflecting a cold climate until the second interglacial stage or the late part of it intense tectonic movements also named nihewan yuanmou and qiangtang movements occurred simultaneously all over the country and interrupted the development of early pleistocene lakes the timing is shown by magnetostratigraphy and began near the gauss matuyama boundary and ended after the olduvia event major cycle i this began about 3 4 ma ago when global climate became colder and continued to about 2 4 ma bp corresponding more or less with the entire gauss epoch the moraines and the fluvioglacial or periglacial sediments representing the first glaciation rest mostly unconformably on bedrock including pliocene sequences with partial conformity in places overlying fluviolacustrine beds are generally in overlapping or transitional relationship with them for example it can be seen that the sammen movement marking the end of major cycle i led to a break in the sedimentation in the ancient sanmen lake of the sanmenxia area on yellow river cao et al 1983 several boundaries between the various cyclothems stratigraphically and geomorphologically using magnetostratigraphy and climatostratigraphy the boundaries between three major cycles can be established as follows

crustal melting in the lower parts of island arcs an example from the bremanger granitoid complex west norwegian caledonides the 440 5 ma bremanger granitoid complex bgc west norwegian caledonides comprises tonalites granodiorites and minor granites the bgc is coeval with a nearby subduction related gabbronoritic dioritic intrusion the 443 4 ma gasoy intrusion and an ophiolite complex generated in a back arc basin the 443 3 ma solund stavfjord ophiolite complex the bgc has a restricted isotopic variation nd440ma 0 04 to 2 21 87sr 86sr440 ma 0 7045 to 0 7054 indicating a substantial mantle component but it is enriched in crustal material relative to the primitive parts of the gabbronoritic rocks of the gasoy intrusion nd440 ma 1 67 87sr 86sr440 ma 0 7048 the chondrite normalised ree patterns are fractionated la yb n 8 to 16 smooth or with negative or positive eu anomalies geochemical modelling suggests that the bgc does not represent residual melts produced by fractional crystallisation of basic magma but is compatible with partial melting of mafic source rocks with various amounts of h2o residual garnet is suggested by hree depletion indicating a minimum pressure of 9 10 kbar during melting

young uplift in the non glaciated parts of the eastern alps abstract we report the first incision rates derived from burial ages of cave sediments from the mur river catchment at the eastern margin of the eastern alps at the transition zone between the alpine orogen and the pannonian basin this river passes through the paleozoic of graz a region of karstifiable rocks called the central styrian karst this river dissects the study area in a north south direction and has left behind an abundance of caves these caves can be grouped into several distinct levels according to their elevation above the present fluvial base level age estimates of abandoned cave levels are constrained by dating fluvial sediments washed into caves during the waning stages of speleogenesis with the terrestrial cosmogenic nuclide method these ages and the elevations of the cave levels relative to the current valley floor are used to infer a very complex history of 4 million years of water table position influenced by the entrenchment and aggradation of the mur river we observe rather low rates of bedrock incision over the last 4 ma in the order of 0 1 mm y with an e folding decrease in this trend to lower rates at younger times we relate this incision history to a tectonic setting where an increase of drainage area of the mur river due to stream piracy in late miocene to pliocene times is linked to surface uplift the later decrease in valley lowering rates is attributed to the rise of the base level related to aggradation of sediments within the valley sediment transport through the valley from the upstream section of the mur river limited the erosional potential of the river to a transport limited state at the later stages of the incision history

late pliocene volcanic island growth and flood basalt like lava emplacement in the hyblean mountains se sicily 1 since 200 ma the hyblean mountains southeastern sicily have been the site of recurrent mainly submarine mafic volcanism in the upper pliocene large volumes of tholeiitic products were erupted in a shallow marine environment leading to the growth of volcanic islands the deposits of this tholeiitic episode can be grouped into two main facies associations 1 pillow lavas and hyaloclastite tuffs erupted and deposited entirely under water and 2 submarine pillow breccias fed from subaerial lava flows entering the ocean and subaerial lavas which are distinguished into a coastal lava platform facies and an inland facies lava flows into the ocean led to the growth of lava deltas consisting of a basal flow foot breccia and an overlying sequence of p hoehoe flow units the latter rarely exceeding 0 5 m in thickness in outcrop this facies association shows evidence for significant instability of the lava deltas during growth indicating very similar processes to those observed during the current eruption of k lauea volcano hawaii away from the coast of the growing island the inland facies is characterized by thicker up to several meters p hoehoe flow units showing evidence for sheet flow inflation and limited outcrops of scoria and or bomb deposits evidence for lava transport through tubes exists in the form of tumuli and multiple vesicle layers drained tubes are absent all tholeiitic products were emitted in a single instantaneous voluminous event 120 km3 consistent with very high effusion rates similar to those of flood basalts or the 1783 skaftar fires iceland eruption

deep structure of japan subduction zone as derived from local regional and teleseismic events we have determined a detailed three dimensional p wave velocity structure of the japan subduction zone to 500 km depth by inverting local regional and teleseismic data simultaneously we used 45 318 p wave arrivals from 1241 shallow and deep earthquakes which occurred in and around the japan islands the arrival times are recorded by the japan university seismic network which covers the entire japan islands densely and uniformly we also used 4211 travel time residuals from 100 teleseismic events which are read from seismograms recorded by seismic stations in northeastern japan in comparison with the previous results obtained from only local and regional events the present result for the area around the lower plate boundary and the mantle below the plate is determined more reliably because of the addition of 7035 data from 100 teleseismic events and 41 very deep earthquakes in the crust and uppermost mantle low velocity zones are clearly visible beneath active volcanoes in the mantle wedge the low velocity zones generally parallel with the slab and exist continuously to a depth of about 200 km which is consistent with the petrological geochemical and geodynamic studies we consider that the existence of volcanism related low velocity anomalies in the mantle wedge is a general seismological characteristic of subduction zones in light of all the available tomographic results for many subduction zones in the world the pacific slab beneath japan is imaged more clearly than in previous studies as a high velocity zone with a thickness of 80 90 km and a p wave velocity 4 6 higher than the normal mantle lower velocity anomalies are found in the mantle below the slab

attenuation structure beneath the volcanic front in northeastern japan from broad band seismograms abstract anelastic structure in the asthenosphere beneath the volcanic front in northeastern japan arc is estimated by using the spectral amplitude ratio data of p and s waves from about 100 events which occurred in the subducting pacific slab below japan these earthquakes occurred within a 90 km radius centered about the station sawauchi swu with focal depths ranging from 60 to 200 km waveforms were recorded by the carnegie broad band three component seismograph and were corrected for instrument responses crustal reverberations corner frequencies and superimposed noise ray paths and travel times of p and s waves are calculated using a three dimensional velocity model zhao d hasegawa a horiuchi s 1992 j geophys res 97 19909 19928 we find a low q region qs 70 extending down to 55 km depth from the lower crust beneath the volcanic front using q temperature laboratory results sato h sacks i s murase t muncill g fukushima h 1989 j geophys res 94 10647 10661 this implies a temperature of about 130 c higher than the eastern forearc region and about 30 c higher than the western backarc region in good agreement with the tomographic results of zhao et al zhao d hasegawa a horiuchi s 1992 j geophys res 97 19909 19928 this suggests that low velocities in the crust and uppermost mantle beneath swu may be explained by a subsolidus temperature increase without partial melting

three dimensional mapping of magma source and transport regions from seismic data the mantle wedge beneath northeastern japan we investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern japan the comparison of the seismic tomography with laboratory velocity data on a partially molten mantle rock yields estimates of melting zones in three dimensions we employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature melting and magma rich zones derived from the velocity structure coincide with observed low q zones the results of the three dimensional mapping indicate that the source of magma in island arc is diapir like melting patches localized within the low velocity zones of the mantle wedge extensive volcanic activity along the volcanic front is due to the presence of vast magma rich zones just beneath the moho those melting zones in the uppermost mantle may in turn cause melting of lower crustal materials and produce felsic magma melt appears to stay at and beneath the moho where crystallization fractionation may proceed melt exists at greater depths in the back arc region which may correlate with across arc variations of chemical compositions of the volcanic rocks observed in northeastern japan we suggest that magma migration in the ductile lower crust may cause low frequency microearthquakes and magma penetration into the brittle upper crust may produce mid crustal s wave reflectors

geological constraints on cave development in the plateau gorge karst of south china wulong chongqing abstract the houping tiankeng cluster is a part of the south china karst unesco world natural heritage site within the distinctive wulong plateau gorge karst 200 km of cave passages have been documented to date this paper focuses on detailed tectonic and morphological research on the luo shui kong cave enriched with u series dating of speleothems and complemented by morphometric analysis of the san wang dong and er wang dong caves all of these caves exhibit three regional levels of cave development 1 1040 1020 m a s l 2 900 840 m a s l and 3 740 660 m a s l the houping tiankeng area is a carbonate rock sequence several hundred meters thick overlain by the less soluble lower ordovician strata limiting recharge points to faults exposing underlying easily soluble formations this leads to the domination of concentrated high volume inflow and thus results in caves of large volume in the plateau gorge karst shafts connecting the surface with cave passages located underneath formed along faults changing the hydrogeological pattern through karst water capture and remodeling of existing conduits albeit mainly by increasing their overall dimensions rather than by deepening them the most favorable structures for cave level development are two sets of joints conjugated with gently inclined bedding since these joints are characterized by a small vertical extent downward development is limited hence most of the passages are wide but not deep canyons and typical of a water table cave pattern places where the fault plane is eroded from the surface and where at the same time an underneath cave chamber ceiling expands upwards are particularly predisposed to the formation of a tiankeng

magma reservoir subsidence mechanics theoretical summary and application to kilauea volcano hawaii an analytic model has been developed for the prediction of the three dimensional deformation field generated by the withdrawal of magma from a sill like storage compartment during an intrusion or eruption cycle the model is based on the work of berry and sales 1961 1962 and predicts the vertical displacement components over the areal plane model parameters are the depth of burial h the intrusion half width a the intrusion half length b the thickness of the magmatic interior at the moment of melt withdrawal tm and the planform aspect ratio a b the products of the model include areal deformation maps systematic variation in model parameters within the context of kilauea volcano hawaii have revealed that circular and elliptical deformation patterns result from the collapse of draining rectilinear intrusions at depth moreover the geometric parameters of a storage compartment may interact in complex ways to produce similar deformation patterns the model has been applied to kilauea volcano for three periods of pronounced summit subsidence 1 1921 1927 bracketing the steamblast eruptive phases of 1924 2 june 1972 to december 1972 and 3 december 1972 to may 1973 application of the model requires the simultaneous optimization of five predicted deformation features with respect to field measurements and the derivative deformation maps 1 the vertical displacement maxima 2 the vertical displacement gradients over the areal plane 3 the lateral extent of the deformation field 4 the aspect ratio of the subsidence pattern and 5 the strike of the major axis of the deformation field the constrained geometries and volumes of the inferred collapsed storage cavities for each period are 1 1921 1927 depth 3 km a 1500 m b 4500 m tm 20 m v 540 106 m3 2 june 1972 to december 1972 depth 3 3 km a 600 m b 2000 m tm 1 m v 4 8 106 m3 and 3 december 1972 to may 1973 depth 2 2 km a 500 m b 1612 m tm 1 m v 3 2 106 m3 for 2 and 3 calculated magmatic thicknesses tm happen to be in the range 3 48 0 15 m of measurements for sill like bodies in deeply dissected hawaiian shield volcanoes the fits obtained between calculated and observed deformation patterns allow quantification of the location overall dimensions orientation and volume of the discrete still molten interior of sill like compartments from which magma is tapped during eruption or intrusion

quantitative analysis of the inclined low velocity zone in the mantle wedge of northeastern japan a systematic change of melt filled pore shapes with depth and its implications for melt migration abstract travel time tomography beneath the northeastern japan arc reveals that an inclined low velocity zone exists in the mantle wedge sub parallel to the down dip direction of the slab this zone distributed continuously along the arc as a single inclined sheet has been considered as the main source of arc magma a quantitative interpretation of both p and s wave velocity structures precisely determined from the recent travel time tomography is done in terms of thermal heterogeneity and fluid content the combined analysis of both p and s wave velocity structures makes it possible to independently derive the information on the shape and volume fraction of the fluid filled pores the reference p and s wave velocities representing the host rock velocities at reference temperature t 0 are estimated from the tomographic data while an alternative method based on high temperature and high pressure elasticity of upper mantle minerals is used to confirm the validity of the present method the calculated velocity anomalies are corrected for the thermal effect using the three dimensional thermal structure estimated from the p wave attenuation data showing that the observed low velocity anomalies cannot be explained by the thermal effect alone the remaining velocity anomalies are explained by the existence of melt filled pores and effective aspect ratio and volume fraction of the pores are determined the results show a systematic change in melt filled pore shapes with depth suggesting the existence of 3 6 vol melts as grain boundary tubules at a depth of 90 km 0 04 0 05 vol melts as thin cracks or dikes with aspect ratio of 0 001 at a depth of 65 km and 1 2 vol melts as cracks or dikes with aspect ratio of 0 02 0 04 at a depth of 40 km

olivine flotation in mantle melt abstract molten komatiite and peridotite have been compressed in an octahedral multi anvil device up to 10 gpa densities of the melts were measured at pressure intervals in the range 7 to 10 gpa by observing sinking and floating san carlos olivines and synthetic forsterite marker spheres the multi anvil results for komatiite when combined with piston cylinder measurements done at 4 to 6 gpa and a calculated reference density at 105 pa yield a birch murnaghan isothermal bulk modulus of k1900c 26 gpa and pressure derivativek 4 25 the pressure of neutral buoyancy for olivine in komatiite is confirmed to be near 8 gpa as predicted in earlier work olivine flotation in the experimental komatiite commences at a pressure close to where the liquidus phase changes from olivine to denser garnet leading to the possibility of density driven crystal sorting during fractionation molten peridotite klb 1 shows an isothermal compression 2000 c of 0 065 g cm 3 gpa 1 in the interval 105 pa to 8 2 gpa the olivine liquid peridotite density crossover is predicted to lie between 9 and 11 gpa indicating that olivine flotation can operate at depths of 300 500 km in a molten peridotitic mantle

magmatic processes under arcs and formation of the volcanic front temperature structure and stress under arcs are simulated in a two dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge results of the calculations show three important features with respect to magmatic processes under arcs 1 temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction the temperature structure of the slab and that of the back arc region this indicates that physical conditions such as temperature and pressure are similar under various arcs it is thus inferred that primary magmas generated under various arcs should have similar chemical compositions if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same 2 calculated deviatoric stress magnitude is relatively large more than a few tens of megapascals in the partially molten mantle cracks may open under high differential stress and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds 3 the deviatoric stress values in the region over the partially molten mantle are relatively large and the direction of the principal stress changes horizontally the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore arc region it is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magma fracturing mechanism the propagation direction of magma filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress the calculated stress is highly compressive horizontally on the trench side while it becomes tensile on the back arc side the location of this stress transition coincides with that of the volcanic front the location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma filled cracks under relatively high differential stress field

quaternary sediment in the yichang area implications for the formation of the three gorges of the yangtze river sediment data from the yichang area in the jianghan basin of hubei province in china suggest deposition in a lacustrine environment prior to 0 75 ma b p followed by incision of the yangtze river the earliest quaternary yunchi formation accumulated in an alluvial fan to fan delta environment the subsequent shanxiyao formation was deposited in an environment that changed from fan delta to lacustrine the distribution of sedimentary facies suggests the presence of a lake in the yichang area prior to 0 75 ma b p the lack of sediments contemporaneous with the yunchi and shanxiyao formations in other areas of the jianghan basin suggests that this ancient lake was limited to the yichang area this lake predates the present yangtze river in the yichang area and the jianghan basin provenance studies of gravels in the yunchi and shanxiyao formations as well as gravels in terraces and the channel of the yangtze river indicate a variety of sediment sources but suggest that no material from the area west of the three gorges had been carried into the yichang area prior to 0 75 ma b p the yangtze river cut through three gorges area only after 0 75 ma b p

interpretation of geoid anomalies in the contact zone between the east european craton and the palaeozoic platform ii modelling of density in the lithospheric mantle summary we present analysis of lateral variations of density in the upper mantle in the area of contact of the precambrian east european craton eec and the palaeozoic platform pp in poland obtained by analysis of the gravimetric geoid undulations a precise 3 d density model of the crust in the study area down to a depth of 50 km discussed in the first part of this paper majdanski et al in press did not explain all features of the observed geoid this suggests that these features can be due to density inhomogeneities in the upper mantle to estimate them we performed inversion of a residual between the observed geoid and undulations caused by the 3 d density distribution in the crust basing on the assumption of local isostatic compensation and pratt hayford isostasy model the density distribution in the upper mantle was parametrized as a 40 km thick layer located above the assumed compensation depth of 140 km and subdivided into irregular blocks the boundaries of the blocks were defined according to boundaries of major tectonic units in the study area and position and shape of the most pronounced anomalies in the residual geoid a series of sensitivity tests calculated for such density heterogeneities in the upper mantle showed that they can produce geoid undulations of the order of several metres the density values in each unit were taken as model parameters for the inversion procedure and inverse problem was solved using global optimization with constraints the density variations in the upper mantle in the final model correlate well with the surface heat flow this suggests that these variations can be due to diversity in mantle temperature the trans european suture zone tesz which is a major suture separating the eec from the pp is not observed as a distinct unit in the mantle instead our study suggests continuation of the lithosphere of the eec beneath the pp and confirms subdivision of the tesz into terranes with distinctly different evolution a simple calculation shows that condition of local isostatic equilibrium is almost achieved at the assumed compensation depth with the exception of several small scale units however different loads and mass deficiencies in the lithosphere are compensated at different depth levels this proves that the lithosphere is alternation of mechanically strong and weak layers both in the crust and upper mantle and that different loads can be compensated at different depth however the total compensation of all lithospheric loads occurs in the upper mantle

surface fed dikes the origin of some unusual dikes along the hilina fault zone kilauea volcano hawaii the origin of dike like bodies along the hilina fault scarp on the south flank of kilauea volcano hawaii has been the subject of recent controversy some geologists favour an origin by intrusion of magma from below others favour intrusion of lava derived from above lava derived from fluid surface flows which poured down open cracks in order to distinguish between deep versus surface sources for the bodies a suite of dike and other samples were analyzed for s h2o and cl all surface flows are degassed whereas known dikes are volatile rich samples of the hilina dikes and dikes from the ninole formation mauna loa are degassed indicating that these dikes were surface fed formed by magma which had been de volatized by surface transport a model is presented whereby the hilina dikes form in talus and lava cones that drape the hilina fault scarp seismic activity during eruption may have played an important role in the formation of the hilina dikes similar dikes in the ninole formation probably formed in a similar environment

simultaneous estimation of attenuation structure source parameters and site response spectra application to the northeastern part of honshu japan abstract we have developed an inversion method for estimating q structure by the use of a large number of waveform data from earthquakes we divide an area to be analyzed into small blocks and simultaneously estimate the q value in each block source parameters for each event and site response spectrum for each station the source spectrum is assumed to be expressed by the 2 model and values of corner frequencies and low frequency levels of source spectra are estimated amplitudes are calculated by taking the orientation of focal mechanism solutions into account a trade off among estimated parameters is tested by numerical experiments using three sets of artificial data in a case when artificial data include error of 15 in the orientation of the p axis of focal mechanism solutions the inverted result shows that the original checkerboard pattern of q structure is well reconstructed although estimated q 1 values are about 10 higher than those of the original one the result without the corrections for radiation effect shows that the checkerboard pattern is only reconstructed in the blocks having a sufficient number of ray paths we applied this inversion method to a real data set in the northeastern part of honshu japan estimated q p structure shows that low q p regions are distributed beneath the volcanic front and they move towards the west with increasing depth previous studies of three dimensional seismic velocity structure in this area show that p wave low velocity regions exist in the crust beneath the volcanic front and they are inclined towards the west in the mantle wedge the low q p regions estimated in this study coincide in general with the p wave low velocity regions

a new chronology for the age of appalachian erosional surfaces determined by cosmogenic nuclides in cave sediments the relative chronology of landscape evolution across the unglaciated appalachian plateaus of kentucky and tennessee is well documented for more than a century geomorphologists have carefully mapped and correlated upland erosional surfaces inset by wide valley straths and smaller terraces constraining the timing of river incision into the appalachian uplands was difficult in the past due to unsuitable dating methods and poorly preserved surface materials today burial dating using the differential decay of cosmogenic 26al and 10be in clastic cave sediments reveals more than five million years of landscape evolution preserved underground multilevel caves linked hydrologically to the incision history of the cumberland river contain in situ sediments equivalent to fluvial deposits found scattered across the eastern highland rim erosional surface cave sediments correlate with 1 thick lafayette type gravels on the eastern highland rim deposited between c 5 7 and c 3 5 ma 2 initial incision of the cumberland river into the eastern highland rim after c 3 5 ma 3 formation of the parker strath between c 3 5 ma and c 2 0 ma 4 incision into the parker strath at c 2 ma 5 formation of a major terrace between c 2 0 ma and c 1 5 ma 6 shorter cycles of accelerated incision and base level stability beginning at c 1 5 ma and 7 regional aggradation at c 0 85 ma initial incision into the appalachian uplands is interpreted as a response to eustasy at 3 2 3 1 ma incision of the parker strath is interpreted as a response to eustasy at 2 5 2 4 ma a third incision event at c 1 5 ma corresponds with glacial reorganization of the ohio river basin widespread aggradation of cave passages at c 0 85 ma is interpreted as the beginning of intense glacial interglacial cycling associated with global climate change copyright 2006 john wiley sons ltd

denudational response to surface uplift in east tibet evidence from apatite fission track thermochronology east tibet preserves broad areas of low relief that occur at elevations 3000 m and are dissected by major strike slip faults that have been repeatedly reactivated since the late mesozoic apatite fission track samples from these low relief high elevation surfaces indicate cretaceous cooling ca 90 ma fast rock uplift of the 110 c isotherm and a fossilized partial annealing zone began in the plateau interior of east tibet in the miocene low denudation rates in the plateau interior during the cenozoic which are constrained by thermal history modeling and a consistency in ages from apatite fission tracks suggest that dip slip displacement on faults has been minor some localized areas have experienced the effects of structurally enhanced rock uplift and denudation or fault related hydrothermal reheating cooling ages in eocene sediments deposited on the older plateau surface suggest that rapid denudation occurred from the miocene to holocene other evidence of significant cenozoic denudation comes from river valleys in particular the yalong river valley where incision was initiated in the oligocene to miocene 28 12 ma estimated mean denudation in east tibet during the cenozoic was 1 2 km in the low relief high elevation plateau interior and at least 5 km at the high relief plateau margin in the longmen shan the region south of the longmen shan and the sichuan basin was not uplifted with the main tibetan plateau in the miocene but it has undergone enhanced denudation since the eocene due to an orographic effect most of the denudation in the east tibetan plateau interior has occurred in major river valleys incision in the major rivers of the longman shan occurred simultaneously and has kept pace with surface uplift that began in the middle late miocene 12 ma with acceleration of denudation rates in the late miocene 5 ma

pliocene pleistocene incision of the green river kentucky determined from radioactive decay of cosmogenic 26al and 10be in mammoth cave sediments cosmogenic 26al and 10be in sediments washed into mammoth cave kentucky record the history of 3 5 m y of water table position governed by incision and aggradation of the green river a tributary of the ohio river upper levels of the cave formed during a period of slow river incision and were later filled with sediment due to river aggradation at 2 3 2 4 ma a brief surge of river incision ca 2 ma was followed by river stability and cave passage formation at a lower level rapid incision through 15 m of bedrock ca 1 5 ma was prompted by repositioning of the ohio river to its present course along an ice sheet margin renewed incision ca 1 2 ma and aggradation at 0 7 0 8 ma correlate with major ice advances in the ohio river basin measurements of 26al and 10be also indicate that sandstone capped uplands have maintained slow erosion rates of 2 7 m m y for the past 3 5 m y despite accelerated pleistocene river incision rates of 30 m m y

location and configuration of magma bodies beneath long valley california determined from anomalous earthquake signals recordings of 281 small earthquakes which occurred recently near the southern edge of long valley caldera at depths from 1 to 15 km are used to map the subsurface geometry of shear wave attenuating bodies in the caldera signatures of these events recorded northwest north northeast and east of long valley with ray paths through the caldera are often anomalous in that s wave arrivals have very low amplitudes and high frequency p and s wave energy is missing for the same station event combinations the volcanic and geothermal history of the region suggests that ray paths for these anomalous signals have passed through magma which absorbed much of the s wave energy and also attenuated the higher frequencies with over 1200 normal and anomalous ray paths through the caldera we have located two apparently massive magma bodies in the central and northwest caldera as well as two more dispersed magma areas in the southern caldera and beneath crowley lake the central magma body extends from 4 5 to at least 13 km beneath the surface and the northwest body is as shallow as 5 5 km these two magma bodies are probably connected below about 8 km secondary arrivals seen on select seismograms are tentatively interpreted as reverberations inside the central magma chamber and are consistent with the interpreted lateral extent of the magma the northwest and central magma bodies lie beneath the northwest and southeast corners of the caldera s resurgent dome and probably connect beneath the dome s center also the shallower parts of these magma bodies and the southern anomalous area lie beneath the surface expression of the medial graben faults suggesting that structural features seen at the surface of the valley may be related to magma location at depth

comparison of u th paleomagnetism and cosmogenic burial methods for dating caves implications for landscape evolution studies caves are useful in landscape evolution studies because they often mark the level of previous water tables and when dated yield incision rates dating caves is problematic however because their ages are only constrained by the oldest deposits contained within which may be far younger than the cave itself we dated cave deposits in the sierra nevada using u th dating of speleothems paleomagnetic dating of fine sediment and cosmogenic 26 al 10 be burial dating of coarse sediment the sampled caves formed sequentially as the water table lowered providing an important stratigraphic test for the dating methods large discrepancies between deposit ages from similar cave levels demonstrate that even when accurately determined deposit ages can seriously underestimate the timing of cave development drip type speleothems are most prone to this minimum age bias because they can accumulate long after caves form and because the u th method is limited to 400 ka paleomagnetic dating requires correlation with the global reversal chronology and is hindered by a lack of continuous stratigraphy the fine sediment analyzed for paleomagnetism is also highly susceptible to remobilization and deposition in cave passages well above base level cosmogenic 26 al 10 be dates bedload material deposited when caves were at or very near river level and can date material as old as 5 ma in the sierra nevada speleothem u th ages and sediment burial ages from the same cave levels differ by as much as an order of magnitude these results suggest speleothem ages alone may significantly underestimate cave ages

climate driven formation of fluvial terraces across the tibetan plateau since 200 kya a review abstract the causes of terrace formation in active orogenic belts have long been studied here we present a synthesis of 147 available papers on the ages of quaternary terraces within the tibetan plateau and along the surrounding orogenic belts more than 1500 ages were collected to establish a database for exploring the causes of fluvial terrace formation because the age data were collected from different settings ages younger than 200 ka were grouped into three regional climate systems the westerlies indian summer monsoon and east asian summer monsoon we further examined the relationship between terrace formation and climate change as well as the contribution of tectonic uplift our results indicate that the intervals of fluvial terrace formation are synchronous with transitions in global climate from cooler to warmer conditions stadials to interstadials we propose that the formation of fluvial terraces across the tibetan plateau has been controlled mainly by cyclical climate changes although there is not a one to one correspondence