the lilliput effect in the aftermath of the end permian extinction event early triassic animal body fossils and trace fossils are small relative to those in older and younger intervals size decreases sharply through the end permian extinction event and permian triassic boundary and the smallest sizes are encountered in the parvus and isarcica zones of the earliest induan animals appearing within these two zones are also exceedingly small compared to younger congenerics and conspecifics temporary dramatic size decrease of surviving taxa in the immediate aftermath of the extinction event is an example of the lilliput effect coined by urbanek a 1993 biotic crises in the history of upper silurian graptoloids a palaeobiological model historical biology 7 29 50 body size increases somewhat from the carinata zone mid induan but remains depressed for the duration of the early triassic and pre extinction sizes are not commonly recorded until at least the middle triassic marine and terrestrial faunas appear to be similarly affected the lilliput effect and longer term size reduction could be the result of several factors environmental parameters such as marine anoxia due to low atmospheric concentrations of oxygen at this time coupled with sluggish ocean circulation in a greenhouse world and food shortage are the likely proximal causes for the early triassic lilliput effect no single cause can explain all the observations and a combination of factors are likely to be involved

san jacinto intrusive complex 2 geochemistry rocks from three large 100 2 km tonalitic intrusions exposed in the san jacinto mountains of southern california show a restricted compositional range of between 63 and 68 wt sio 2 for all but volumetrically minor felsic differentiates with si0 2 70 wt all rocks with less than 65 5 wt sio 2 show linear element element covariation felsic differentiates have characteristics higher sio 2 k 2o rb ba u higher and variable rare earth elements consistent with derivation by in situ fractionation rocks with between 65 5 and 70 wt sio 2 have intermediate characteristics and are interpreted as derived from liquids formed by mixing primitive liquids with fractionated liquids within an intermittently recharged continuously solidifying magma chamber mafic inclusions extend the compositional trends of the mafic tonalites to 55 wt sio 2 the chemical variations of both inclusions and more mafic tonalites are interpreted as resulting from processes acting before injection of their parental liquids into the observed crustal magma chambers effects of chamber processes are minor for all but the most felsic rocks the major effect of recharge is to buffer the thermal and chemical properties of liquids within the magma chambers yielding large volumes of relatively homogeneous tonalite for those elements where the bulk distribution coefficient is between about 0 5 and 2 concurrent recharge and solidification produces rocks that closely approximate the composition of the added liquids estimated rayleigh numbers for these liquids are high 10 10 implying convection throughout much of the solidification history of each chamber existence of trace element variations within analyzed rocks imply that convection was not totally efficient at homogenizing the various batches of liquid added to each chamber

empirical relationships among seismic velocity effective pressure porosity and clay content in sandstone we use a multivariate analysis to investigate the influence of effective pressure pe porosity and clay content c on the compressional velocity vp and shear velocity vs of sandstones laboratory measurements on water saturated samples of 64 different sandstones provide a large data set that was analyzed statistically for each sample relationships between effective pressure and vp and vs have been determined all samples were well fit by relationships that have an exponential increase in velocity at low pe tapering to a linear increase with pe for pe greater than 0 2 kbar there are differences in the pressure dependences of velocity for different rocks particularly at very low pressures however the differences cannot be attributed to or c for the combined set of measurements from all samples the best fitting formulations are vp 5 77 6 94 1 73c 0 446 pe e 16 7pe and vs 3 70 4 94 1 57c 0 361 pe e 16 7pe while this is admittedly a very simplified parameterization it is remarkable how well

interseismic stress accumulation at the locked zone of nankai trough seismogenic fault off kii peninsula abstract a primary source of stress variation prior to great subduction zone earthquakes is the tectonic loading due to plate convergence we have calculated the elastic stress around the locked zone accumulated during an interseismic period numerically using a 2d plane strain finite element model in the nankai trough off kii peninsula and examined its dependence on elastic properties and some boundary conditions for the modeling we used the geometry of subducting plate interface determined previously through seismic surveys and the physical properties that were obtained through iodp scientific drilling offshore kii peninsula to construct a physically realistic model a dislocation along the plate interface is calculated from a 5 m displacement imposed on one side of the subducting plate with the locked zone defined at a depth of 10 to 25 km along the plate interface calculated shear stress is maximized at the downdip edge of the locked zone decreases exponentially seaward and is almost zero near the updip edge maximum shear stress is primarily proportional to the young s modulus of the overlying section comparing the maximum shear stress with the average stress drop for subduction earthquakes 3 4 mpa the most appropriate young s modulus and poisson s ratio ranges are 50 70 gpa and 0 3 respectively which are in good agreement with previous estimations from observed vp vs the stress accumulated during one seismic cycle can be reasonably explained by a linear elastic model with an appropriately sized locked region we found that the envelope of simulated horizontal displacement on the surface agrees well with gps velocity data which also supports this inference disturbance near the updip edge is no larger than 0 1 mpa of stress or 0 1 radian of tilt which are difficult to detect with current borehole measurement techniques and likely complex borehole conditions

high palaeolatitude hodh mauritania recovery of graptolite faunas after the hirnantian end ordovician extinction event marine shales directly overlying lower hirnantian uppermost ordovician glacially related sediments in mauritania northwest africa have produced a rich graptolite fauna spanning the ordovician silurian boundary in an area of high palaeolatitude the lowermost transgressive sandy shales are barren of graptolites but overlying shales show a sudden appearance of a diverse fauna indicative of the terminal ordovician persculptus zone suggesting that with improving conditions colonisation by a relatively cold tolerant fauna was possible this fauna is replaced by a low diversity assemblage dominated by long ranging taxa probably representing the basal silurian acuminatus and atavus zones with the extinction of the persculptus zone fauna conditions were still hostile to warm water silurian graptolites and a normalograptus fauna was again established a sudden influx of fairly diverse taxa marks the base of the acinaces zone and the establishment of a typical lower silurian fauna with the establishment of warmer water conditions

high angle reverse faulting associated with the 1946 nankaido earthquake abstract analysis of vertical crustal deformation data in the southwestern part of shikoku southwest japan suggests that the nankaido earthquake of 1946 m w 8 1 which is a principal interplate thrust earthquake was accompanied by subsidiary faulting on a splay fault adjacent to the coast of shikoku discarding crustal movement resulting from the main thrusting of the nankaido earthquake local leveling data are explained by slip on a simple rectangular thrust fault located just offshore of shikoku although it is difficult to constrain the fault location a possible result is a high angle thrust dipping landward at an angle of about 70 with a dislocation of about 1 5 m and source dimensions of 30 13 km along strike and dip respectively this result indicates that the fault may be one of the steeply dipping subsidiary faults branching from the main low angle thrust as was the case in the alaska earthquake of 1964 although several lines of evidence suggest that this faulting occurred as slow aseismic slip its discrimination from the main seismic event is extremely difficult this kind of high angle thrusting just offshore of the coast would play an important role for the formation of the marine terraces during the late quaternary period

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

the early rhuddanian survival interval in the lower silurian of the oslo region a third pulse of the end ordovician extinction abstract ordovician silurian boundary layers with rhuddanian strata are exposed as a long continuously fossiliferous sequence in the solvik formation in the asker area central oslo region norway brachiopods belonging to benthic assemblage 5 are preserved in the lower parts of the formation this level is investigated for the presence of a survival interval after the last end ordovician extinction event the criteria for a survival interval include taxa that are dwarfed long ranging eurytopic often opportunistic as well as assemblages that show low density and diversity four species isorthis prima leangella scissa dicoelosia osloensis and eoplectodonta duplicata were collected and measured the three former were dwarfed compared to a younger aeronian fauna belonging to the same benthic assemblage detailed investigations showed a statistically significant two step reduction in size for all but d osloensis the first interval immediately above the last end ordovician extinction event displays dwarfed brachiopods but diversity is high and the number of long ranging and eurytopic species is low due to the presence of globally and locally relict ordovician species the second interval the upper parts of lower rhuddanian shows all the characteristics of a survival interval in which the sizes are statistically significantly smaller than both those in the interval below and those in the aeronian interval above no lithological change within lower rhuddanian strata could be linked to the pronounced reduction in size the second step in size reduction may be related to global occurrence of anoxia in the deep oceans with a pulse of anoxic water pushed onto the shelf at that time it is coeval with a short negative 13 c excursion found in some locales signifying a brief period of global warming the results suggest a protracted extinction event through parts of the early rhuddanian and a third and final extinction event followed by a clear survival interval

out of sequence thrust faults developed in the coseismic slip zone of the 1946 nankai earthquake mw 8 2 off shikoku southwest japan a multi channel seismic mcs reflection survey was conducted to study the subsurface structure of the coseismic slip zone of the 1946 nankai earthquake mw 8 2 off shikoku island in 1997 we could identify a splay fault system consisting of several sigmoid out of sequence thrust ost faults dipping landward with slope 10 25 on the poststack depth migrated mcs profiles most of the osts are apparently developed from the subducting oceanic basement to the seafloor in the forearc region cutting both underthrust sediments and the overriding accretionary prism in addition most of the osts are within the coseismic zone of the 1946 nankai earthquake and the interseismic locked zone the osts are considered to be related to large interplate earthquakes including the 1946 nankai earthquake these osts may be responsible for tsunami generation following deformation of forearc accretionary wedge

possible splay fault slip during the 1946 nankai earthquake this paper considers seafloor deformation associated with the 1946 nankai earthquake which occurred in the nankai trough subduction zone off the coast of southwest japan we note that recent high resolution determinations of the slip distribution assumed to be confined to the plate boundary result in a pronounced maximum in seafloor displacement midway between the coast of shikoku and the trough axis we show that this feature could be due to slip on a splay fault rather than the plate boundary such a model for the western part of the slip distribution is able to account for the main features of the tsunamigenic seafloor deformation obtained in previous studies with a maximum slip consistent with the plate convergent rate and history of earthquake recurrence at the nankai trough

root distribution and seasonal water status in weathered granitic bedrock under chaparral soils in mountainous terrain are often thin and unable to store sufficient water to support existing vegetation through dry seasons this observation has led to speculation about the role of bedrock in supporting plant growth in natural ecosystems since weathered bedrocks often have appreciable porosity and like soil can store and transmit water this study within a chaparral ecosystem in southern california was designed to determine the extent of rooting within weathered granitic bedrock and to measure the relative contributions of soil and weathered bedrock to water use by chaparral shrubs adenostorna fasciculatum hook and am arctostaphylos glandulosa eastw and ceanothus greggii a gray the rooting pattern was mapped from the wall of a trench excavated into the weathered bedrock water contents of soil weathered bedrock profiles were measured at one to four week intervals for two years using a neutron probe chaparral roots penetrate deeply 4 m into the weathered bedrock and are largely confined to joint traces during summer dry seasons the shrubs extracted 39 4 cm of water from a 2 9 m thick zone of weathered bedrock accessing nearly ten times as much water as from the 0 35 m thick soil typic xerorthents although it is commonly neglected in ecological inventories and analyses weathered bedrock can be an essential ecosystem component particularly where soils are thin and seasonal drought occurs

carbon isotope excursion near the wenlock ludlow silurian boundary in the anglo welsh area oxygen and carbon isotope analyses have been performed at close sampling intervals on rocks from several exposures that show the boundary between the wenlock and ludlow series in the classic silurian areas of wales the welsh borderland and the west midlands of england our data indicate a monotonic decline in 13 c from positive to negative values across the boundary between the much wenlock limestone formation wenlock series and the lower elton formation ludlow series and correlatives in all sites investigated a slightly earlier negative excursion is also present in the upper wenlock strata of two of these localities both negative 13 c excursions appear to be contemporaneous with episodes of increased ocean ventilation and eustatic sea level lowstand postulated by kemp at least one and possibly both of the late wenlock carbon isotope depletions can also be related to the decline in pelagic graptolite diversity the distribution of the carbon isotope depletion in both carbonate platform and off shelf facies implies that this trend is unlikely to have been caused by local diagenetic alteration isotope chemostratigraphy may also provide an extra correlation tool that can be integrated with traditional biostratigraphic methods for silurian strata

three dimensional morphologies of metasedimentary and mafic enclaves from ascutney mountain vermont two types of enclaves are found in the main syenite unit of the cretaceous igneous complex at ascutney mountain vermont metasedimentary xenoliths consist of fine grained hornfels with bands and lenses of coarser grained quartz mafic enclaves of igneous origin contain phenocrysts in an aphanitic groundmass we have reconstructed the three dimensional shapes of a metasedimentary xenolith and 15 mafic enclaves from serial sections created by sawing blocks of syenite containing enclaves into parallel slabs enclave outlines on each side of the slabs were digitized and computer software was used to reconstruct three dimensional images results are presented for three typical mafic enclaves and for both the hornfels and quartz portions of the metasedimentary enclave stereographic drawings show that the metasedimentary and the mafic enclaves have complex shapes in three dimensions with branches lobes and concavities syenite from the host magma penetrated the xenolith along hornfels quartz boundaries producing complementary concave convex surfaces adjacent mafic enclaves commonly have complementary surfaces and are not observed to impinge on each other syenite also penetrated both the metasedimentary and mafic enclaves along finger like projections syenite infiltration into the metasedimentary xenolith apparently occurred by a combination of mechanical and chemical processes controlled by the presence of compositionally favorable boundaries a similar model is suggested for the mafic enclaves in which syenite infiltration occurred preferentially along quartz feldspar grain boundaries or involved larger scale compositional heterogeneities within the mafic enclaves three dimensional views of enclaves provide information complementary to chemical and petrologic studies of assimilation

initial shear wave particle motions and stress constraints at the anza seismic network summary we use focal plane solutions to constrain principal stress directions in the vicinity of six anza network stations which show evidence for shallow shear wave anisotropy in the vicinity of the anza seismic gap region of the san jacinto fault faulting near all stations is consistent with approximately n s maximum compressive stress five of these stations show nearly n s initial particle motion alignment consistent with the anisotropy stress relationship expected for stress aligned microcracks however one station knw has a well defined preferred initial shear wave polarization direction of n40 w although this polarization direction differs dramatically from the local maximum compressive stress direction it is consistent with the anisotropy expected due to a local alignment of anisotropic bedrock minerals particularly biotite thus anisotropy observed at this station most likely reflects a fixed palaeostrain alignment of anisotropic minerals and or microcracks and does not require a dependence on the current stress field

can the lilliput effect be detected in the brachiopod faunas of south china following the terminal ordovician mass extinction abstract in the immediate aftermath of global extinctions organisms were normally much smaller than those prior to these events this lilliput effect can be subdivided into two types 1 a specific type following the original definition of the effect which targets species level taxa associated with inhospitable environments and 2 a more general type related to the reactions of higher rank taxa above the species level the body sizes of brachiopods from south china through the ordovician and silurian transition late katian hirnantian and earliest rhuddanian are compared at generic superfamilial ordinal and class levels the results indicate that the body sizes of the taxa of lower rank e g genus level are highly variable within these different intervals the type of evidence for the lilliput effect through the end ordovician mass extinction is thus quite different from that of the end permian mass extinction probably reflecting differences in the intensity of these two major bioevents however the relationships between the contrasting trends in body size change of some taxa of higher rank e g at the ordinal level and the relative dominance of these taxa in the latest ordovician and earliest silurian suggest that the brachiopods of the two major ordovician groups the strophomenoids and orthoids adopted different survival strategies during and immediately after the crisis from those of the pentamerides and rhynchonellides that were common in silurian assemblages

slip history of the 1994 sanriku haruka oki japan earthquake deduced from strong motion data abstract we analyzed the seismic waves of the 1994 sanriku haruka oki earthquake m w 7 7 which occurred in the aftershock area of the 1968 tokachi oki earthquake m w 8 2 applying a multiple time window inversion scheme to near source strong motion data we obtained a detailed spatiotemporal rupture process and compared it with that of the tokachi oki earthquake the fault geometry is constructed based on the aftershock distribution the obtained rupture model is consistent with the cmt solution even for a non double couple component the total seismic moment is 4 0 10 20 n m large slips are concentrated in three asperities the first asperity centers about 40 km south and 50 km west from the hypocenter with a maximum slip of 4 4 m the second one centers about 60 km west from the hypocenter with a maximum slip of 2 2 m and the third one lies about 110 km west from the hypocenter with a maximum slip of 2 6 m the obtained moment rate and the duration on the first and second asperities are lower and much longer than those on the third asperity respectively the first asperity does not overlap with an area of large slip during the tokachi oki earthquake but the second or third seem to overlap with or be adjacent to the asperity of the tokachi oki earthquake our inversion result also shows an abrupt change of the rupture velocity from 1 8 to 3 0 km sec at the central part of the fault plane a difference of the seismic coupling between the oceanic and the continental lithospheres at the trenchward side and at the landward side of the 143 e meridian seems to affect the rupture process of this earthquake

a fault model of the 1946 nankaido earthquake derived from tsunami data abstract a fault model of the 1946 nankaido earthquake m 8 2 is determined by the use of tsunami records of uwajima shimotsu and hososhima which were located within or near the area of major coseismic crustal deformation synthetic tsunamis computed for various fault models are matched with the observed tsunamis to determine the fault parameters a low angle thrust model slightly revised from a previous model by ando is consistent with the observed tsunamis the duration of faulting is constrained as less than 10 min based upon the tsunami the fault is divided into an eastern and a western segment corresponding to areas associated with and without aftershocks respectively the fault area and dislocation for the western segment are 150 70 km2 and 6 m and those for the eastern segment are 150 70 km2 and 3 m respectively the total seismic moment is 4 7 1028 dyn cm significantly smaller than that obtained from a geodetic model by fitch and scholz but still larger than that of the seismic model by kanamori the discrepancy in seismic moment between the seismic and the present models ran2 could be interpreted in terms of a slow dislocation on the fault but this interpretation does not match the seismic intensity distribution and damage pattern and the slow slip model for the nankaido earthquake is rejected the discrepancy between the two seismic moments is considered insignificant within error involved in data and modeling assumptions if the revised geodetic model ran2 is modified the seismic moment required to explain the observed tsunamis would be reduced further by 30 if we consider the uncertainties involved in the fault model of kanamori and the fault finiteness effect affecting the amplitude of seismic waves the seismic moment required to interpret the seismic wave data could be increased possibly being more than twice that of kanamori thus the two seismic moments from the different data sets could be close to each other within allowable tolerance this implies that the rise time of the nankaido earthquake was short enough to generate short period seismic waves from both the western and the eastern fault segments

detailed plate boundary structure off northeast japan coast in 1997 a seismic experiment using an airgun array and ocean bottom seismographs obss was performed in the forearc region of the northeast japan nej arc the objectives of this experiment were to clarify whole of the velocity structure around the forearc region of nej arc including a detailed plate boundary structure and the heterogeneous structure in this paper we estimated the heterogeneous velocity structure around the forearc region of northeast japan by applying 2 d ray tracing and travel time inversion to the airgun obs data the depth where the island arc moho comes into contact with the subducting oceanic crust is about 20 km we suggest the existence of an oceanic layerl overlying the oceanic crust subducting down to at least near 18 km by comparing observed waveforms with these calculated using the reflectivity method

depth distribution of coseismic slip along the nankai trough japan from joint inversion of geodetic and tsunami data coseismic slip distribution on the fault plane particularly in the downdip direction associated with large subduction earthquakes can be estimated by joint inversion of geodetic and tsunami data two large earthquakes the 1944 tonankai earthquake mw 8 1 and the 1946 nankaido earthquake mw 8 3 occurred on the nankai trough southwestern japan where the philippine sea plate is subducting beneath the eurasian plate the source areas of these events extended over both land and ocean coseismic crustal movements on land were measured by leveling while those in ocean were recorded as tsunami waveforms on tide gauges the coseismic slip distribution inverted from these data shows that the slip on the shallower part of the fault plane is comparable to that on the deeper parts this indicates that large coseismic slip can occur beneath an accretionary wedge where current seismicity is low the result has implications for other subduction zones having a similar tectonic environment such as for the pacific northwest region of the united states although the possibility of a large earthquake there is still debated should a large subduction earthquake occur in this region coseismic slip on the shallow part could be large and the potential for large tsunamis is high

directivity analysis of the december 28 1994 sanriku oki earthquake mw 7 7 japan on december 28 1994 a segment along the northern japan trench failed in a mw 7 7 underthrusting earthquake the sanriku oki earthquake less than 30 years earlier the great tokachi oki underthrusting earthquake of may 16 1968 mw 8 2 occurred in the same region we examine the rupture process of the recent event using teleseismic body and surface wave data and compare our results to the rupture characteristics of the tokachi oki earthquake as estimated by various authors in previous studies we use an empirical green function technique to obtain relative source time functions spanning a large range of the directivity parameter the strong azimuthal dependence of the durations of these source time functions indicates a significant unilateral component of rupture in a down dip direction west northwest the main rupture initiated about 35 km southwest of the epicenter approximately 20 seconds after the onset of a low energy precursor the main tokachi oki rupture was also preceded by low moment release however it is unclear whether the low moment release preceding both earthquakes represents part of the rupture process or a distinct foreshock the main rupture areas asperities of the sanriku oki and the tokachi oki earthquakes do not significantly overlap

bathymetric and isotopic evidence for a short lived late ordovician glaciation in a greenhouse period the end ordovician glaciation is distinct among phanerozoic glaciations in that co 2 levels were generally high yet major continental ice sheets accumulated on the gondwana supercontinent new oxygen isotopic data indicate substantial changes in sea water temperatures and ice volume coinciding with glacio eustatic changes in sea level reflecting the growth and decay of the gondwana ice cap major glaciation was apparently confined to the hirnantian and was 0 5 1 m y long rather than the 35 m y of earlier estimates carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep water circulation and then back again we believe that the changes in the carbon cycle effected a reduction in p co 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world

deepwater high resolution expanding spread and split spread seismic profiles in the nankai trough in deep water the source receiver offsets that are required for accurate velocity determinations cannot be achieved with single ship multichannel seismic methods two ships one equipped with a multichannel receiving array and the other with a seismic source have previously been employed to acquire common midpoint expanding spread profiles principally to determine deep crustal velocity structures we extend this method to higher resolution in space and time to determine the velocities of sedimentary layers in deep water offshore japan in the nankai trough this high resolution two ship data acquisition method used a 13 1 l water gun source array a 1 6 km 96 channel receiving array with 0 0166 km active group and shore based navigation analysis of the data was performed in the p domain by successive downward continuation of the plane wave data interactive velocity analysis methods for both one dimensional and two dimensional earth models are described for both common source receiver and common midpoint profiles results in one of two areas surveyed show a low velocity zone below the subduction decollement which is consistent with models of low wedge taper high pore fluid pressure and reflection polarity reversal described previously by other researchers the velocity profiles show the expected landward increase in velocity assumed to be due to lateral strain and porosity decrease but the effect is small only slightly greater than would be expected in an area of no lateral strain

paleothermal structure of the shimanto accretionary prism shikoku japan role of an out of sequence thrust vitrinite reflectance study on the cretaceous shimanto accretionary prism of southwest japan shows that the vitrinite reflectance varies from 0 6 to 3 0 corresponding to the estimated maximum paleotemperature variation of 90 210 c the thermal structure shows a sharp discontinuity across an out of sequence thrust separating it into two thermal blocks in each block vitrinite reflectance values increase systematically toward the prism younging direction and the thermal structure is clearly overprinted on the tectonic imbrication of turbidite and melange units this provided us with a set of data to calculate the geometry and displacement of the out of sequence thrust the fission track zircon partial annealing ages indicate that this thrust was active about 43 ma probably in response to the accretion of eocene rocks this study shows that initial thickening of the accretionary prism in an order of several kilometres was mainly controlled by out of sequence thrust

shear thickening dilatancy in crystal rich flows abstract rheological models of molten rock are mostly based on behaviours in which viscosity is constant with respect to strain rate e g newtonian bingham plastic or viscosity decreases with increasing strain rate pseudo plastic or shear thinning rheological models in which viscosity increases with increasing strain rate shear thickening or dilatant models have rarely been used yet such behaviour is common in particulate systems whereas magmas with low crystal content may be appropriately modelled by the former behaviours greater crystal content may lead to particle interactions resulting in shear thickening behaviour the effect is illustrated using a powdered rhyolitic rock mixed with water which exhibits extreme shear thickening behaviour caused by particles moving apart as they move past each other the expansion of the grain fabric during flow is opposed by the applied loads such that dilatancy is invariably associated with shear thickening rheology crystal rich lava may exhibit shear thickening dilatancy a proposition supported by observations of glass rich microscopic shear zones disrupting crystal fabrics plutonic crystal mush may also exhibit shear thickening dilatancy which can explain synplutonic intrusion involving brittle fracture followed by ductile disruption in general strain rate is an important contributor to the transition from ductile to brittle behaviour in crystal rich igneous rocks

tomography of southern california via bayesian joint inversion of rayleigh wave ellipticity and phase velocity from ambient noise cross correlations the data used in this work are archived and distributed by the southern california earthquake data center scedc http scedc caltech edu research tools waveform html and the iris earthquake data center https ds iris edu ds nodes dmc both the scec community velocity models used in this work cvms 4 26 and cvmh 15 1 0 are available from the southern california earthquake center https github com sceccode ucvmc this study was supported by the southern california earthquake center award 17195 the national science foundation grants cybersees 1442665 and ear 162061 and the king abdullah university of science and technology award ocrf 2014 crg3 2300 we also thank the reviewers of this paper an anonymous individual and jan dettmer for their valuable insight and constructive reviews of the manuscript the 3 d shear wave velocity model will be available from iris data services products earth model collaboration https ds iris edu ds products emc

the late ordovician mass extinction abstract near the end of the late ordovician in the first of five mass extinctions in the phanerozoic about 85 of marine species died the cause was a brief glacial interval that produced two pulses of extinction the first pulse was at the beginning of the glaciation when sea level decline drained epicontinental seaways produced a harsh climate in low and mid latitudes and initiated active deep oceanic currents that aerated the deep oceans and brought nutrients and possibly toxic material up from oceanic depths following that initial pulse of extinction surviving faunas adapted to the new ecologic setting the glaciation ended suddenly and as sea level rose the climate moderated and oceanic circulation stagnated another pulse of extinction occurred the second extinction marked the end of a long interval of ecologic stasis an ecologic evolutionary unit recovery from the event took several million years but the resulting fauna had ecologic patterns similar to the fauna that had become e

western nankai trough seismogenic zone results from a wide angle ocean bottom seismic survey the nankai trough southwestern japan is recognized as a vigorous seismogenic zone with well studied historic earthquakes this paper presents results of a wide angle ocean bottom seismographs obs study at the western nankai trough seismogenic zone the obs data used were acquired on a profile 250 km long across the presumed coseismic slip zone of the 1946 nankaido earthquake ms 8 2 the main purpose of the seismic study is to obtain an entire crustal cross section of the seismogenic zone for the 1946 earthquake the crustal model is characterized by a gentle sloping of subducting oceanic crust and thick overlying sedimentary wedge p wave seismic velocities of the subducting oceanic crust show normal oceanic crustal velocities vp 5 0 5 6 km s and 6 6 6 8 km s in oceanic layers 2 and 3 respectively the maximum thickness of the sedimentary wedge is 9 km at 70 km from the trough axis with vp 3 4 4 6 km s in the deeper part the subducting oceanic crust traced down to 25 km depth shows that the subduction angle becomes steeper landward 3 2 and 7 2 at 0 50 km and 50 100 km from the trough axis respectively the oceanic crust is smooth to the hypocenter zone down to 40 km depth beneath shikoku island our crustal model shows that the downdip limit of the coseismic slip area does not extend to the deep end of the oceanic crust island arc crust contact zone even though there is large uncertainty about the seaward limit of the coseismic slip zone the crustal model clearly indicates that the updip limit of the coseismic slip zone extends beneath the young accretionary prism

dwarfism of irregular echinoids echinocorys from poland during the campanian maastrichtian boundary event abstract the echinoid genus echinocorys is one of the dominant irregular echinoid in the cretaceous of poland our analysis of the body size trends of these echinoids shows that they underwent dwarfism in the early maastrichtian the reduction in body size in these echinoids was likely an ecological response to the major climate perturbations connected with the campanian maastrichtian boundary event cmbe that was characterized by a long lasting negative carbon isotope excursion and cooling this is consistent with the fact that food limitations and cold temperatures are among the major factors decreasing growth rates in modern echinoids so far in the case of echinoids the so called lilliput effect has been only recognized in the early danian early paleogene heart urchins atelostomata thus our study shows that the reduction in the body size in echinoids was a more common biotic response to environmental stress than previously thought

recovery of benthic communities following the toarcian oceanic anoxic event in the cleveland basin uk during the toarcian oceanic anoxic event oae considerable environmental changes occurred that were associated with global warming perturbations to the c cycle and ocean deoxygenation which resulted in a mass extinction of marine fauna recovery of the biota after the event was protracted and has to date undergone limited study however understanding the patterns and processes of recovery are critical to anticipating ecosystem responses to the environmental changes predicted for the near future results showed that increases in benthic diversity and the re establishment of the toarcian infauna was gradual and followed the changing redox conditions pioneering infauna such as dacryomya ovum that dominated the seafloor after the event in the cleveland basin yorkshire uk can modify the physico chemical environment and thus facilitate ecological succession after disturbance the length of d ovum increased 8 mm throughout the bifrons zone and these body size changes were linked with total organic carbon toc content suggesting a link to primary productivity although only at intermediate levels of deoxygenation major changes in the phytoplankton and so food supply seem to have driven changes in bivalve body size across trophic guilds both during and after the event in yorkshire and on the mid to lower shelf in spain and france respectively primary productivity collapse seems then to have been a major driver of biotic change throughout the toarcian event as it was during the permian triassic triassic jurassic and cretaceous tertiary mass extinctions further investigation of both the palaeontological and geochemical changes that occurred within early successional toarcian infaunal communities are required to more fully understand the pattern of recovery after the oae

construction solidification and internal differentiation of a large felsic arc pluton cathedral peak granodiorite sierra nevada batholith the tuolumne batholith tb sierra nevada batholith usa is an archetypal large zoned arc intrusion c 1200 km 2 previous work proposed that compositional zonation observed in the tb was produced in situ by inward differentiation of a large magma chamber and or large scale intrachamber magma mixing recent geochronology shows that the tb was intruded over 8 9 ma making single pulse fractionation or mixing in a magma chamber of tb dimensions unlikely we examine processes responsible for compositional variation in the cathedral peak granodiorite which is the largest mapped unit of the tb new field geochemical and geochronological work along a roughly contact perpendicular 5 km transect indicates 1 magmatic foliation is steeply dipping 60 2 field evidence for repeated separation of crystals from melt and local magma mixing is observed 3 u pb zircon ages at opposing ends of the transect are indistinguishable within error c 87 5 ma 4 bulk composition varies only modestly but trace elements show variable degrees of scatter 5 end t and 87 sr 86 sr i have small variation compared with that in the whole tb geochemical and isotopic data are compatible with fractionation of major silicates and accessory minerals however the geochemical spatial variation minor isotopic variation and field evidence suggest that fractionation was highly disorganized and also involved mixing with new input magma and remobilization of crystal mush as the pluton solidified our observations are consistent with the construction of a large and dynamic magma system within the last c 1 ma of tb growth

bursting the bubble of ballooning plutons a return to nested diapirs emplaced by multiple processes a popular model for the emplacement of roughly spherical plutons is that of ballooning or in situ inflation of a magma chamber in a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent while the hotter tail of magma continues to rise and expand the already crystallized outer margin this expansion forms a concentric gneissic to mylonitic foliation and flattening type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures our reexamination of three supposedly ballooned plutons ardara ireland cannibal creek australia papoose flat california and evaluation of published descriptions of many others indicate that this model is largely incorrect deflections of country rock structures strains and porphyroblast matrix relationships indicate that only minor to moderate expansion usually 30 or less occurred during emplacement that other emplacement mechanisms must have occurred and that regional deformation continued during and after emplacement internal structures indicate that when magma chamber expansion did occur it did so by flow of magma that magmatic foliations and lineations formed late in the magma chamber evolution that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion and that only minor internal subsolidus deformation results from emplacement this study indicates that many plutons previously interpreted as post tectonic ballooning plutons are better viewed as syntectonic nested diapirs emplaced by a variety of country rock material transfer processes this nested diapir model implies that magma ascent may occur by rise of large magma batches instead of transport in dikes followed by ballooning that magma chamber dynamics differ from that in the ballooning model and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt

magma mixing in granitic rocks of the central sierra nevada california abstract the el capitan alaskite exposed in the north american wall yosemite national park was intruded by two sets of mafic dikes that interacted thermally and chemically with the host alaskite comparisons of petrographic and compositional data for these dikes and alaskite with published data for sierra nevada plutons lead us to suggest that mafic magmas were important in the generation of the sierra nevada batholith specifically we conclude that 1 intrusion of mafic magmas in the lower crust caused partial melting and generation of alaskite rhyolitic magmas 2 interaction between the mafic and felsic magmas lead to the observed linear variation diagrams for major elements 3 most mafic inclusions in sierra nevada plutons represent chilled pillows of mafic magmas related by fractional crystallization and granitoid assimilation that dissolve into their felsic host and contaminate it to intermediate granodioritic compositions 4 vesiculation of hydrous mafic magma upon chilling may allow buoyant mafic inclusions and their disaggregation products to collect beneath a pluton s domed ceiling causing the zoning mafic margins to felsic core that these plutons exhibit

structure of the nankai trough accretionary zone from multichannel seismic reflection data new multichannel seismic reflection data collected over the nankai trough image the accretionary complex in two areas the international program of ocean drilling leg 87 transect area western area and the region of upcoming ocean drilling program leg 131 eastern area the incoming shikoku basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites the basin section is overlain by a trench wedge that is 12 16 km wide and 350 750 m thick at the thrust front accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge the zone in the western area is 4 5 km wide and is characterized by kink folds the zone in the eastern area is only 2 5 km wide and does not exhibit such folds the frontal thrusts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section the overthrusting sediments form fault bend folds over these ramps thrust spacing at the toe of the slope is 1 5 2 5 km the second thrust cuts up from an inferred decollement within the shikoku basin sedimentary section in the eastern area a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6 3 km seaward of the thrust front and underlies the entire protothrust zone this reflector continues with reversed polarity under the accretionary complex and is at the level of the basal decollement the underlying basin pelagic section is apparently thrust undisturbed beneath the accretionary prism the reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface this decrease in velocity and density may indicate that the decollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism the reflections from the first and second thrusts are also reversed polarity possibly indicating that they also are pathways of fluid expulsion the critical wedge taper of the western area is greater than that of the eastern area an observation that is consistent with the existence of an overpressured decollement in the eastern area

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

environmental controls on marine ecosystem recovery following mass extinctions with an example from the early triassic abstract the recovery of marine ecosystems following a mass extinction event involves an extended interval of increasing biotic diversity and ecosystem complexity the pace of recovery may be controlled by intrinsic ecosystem or extrinsic environmental factors here we present an analysis of changes in marine conditions following the end permian mass extinction with the objective of evaluating the role of environmental factors in the protracted 5 myr long recovery of marine ecosystems during the early triassic specifically our study examines changes in weathering productivity and redox proxies in three sections in south china chaohu daxiakou and zuodeng and one in northern india mud our results reveal 1 recurrent environmental perturbations during the early triassic 2 a general pattern of high terrestrial weathering rates and more intensely reducing marine redox conditions during the early griesbachian late griesbachian mid smithian and more weakly the mid spathian 3 increases in marine productivity during the aforementioned intervals except for the early griesbachian and 4 stronger and more temporally discrete intervals of environmental change in deepwater sections chaohu and daxiakou relative to shallow and intermediate sections zuodeng and mud our analysis reveals a close relationship between episodes of marine environmental deterioration and a slowing or reversal of ecosystem recovery based on metrics of biodiversity within community alpha diversity infaunal burrowing and ecosystem tiering we infer that the pattern and pace of marine ecosystem recovery was strongly modulated by recurrent environmental perturbations during the early triassic these perturbations were associated with elevated weathering and productivity fluxes implying that nutrient and energy flows were key influences on recovery more regular secular variation in deepwater relative to shallow water environmental conditions implies that perturbations originated at depth i e within the oceanic thermocline and influenced the ocean surface layer irregularly finally we compared patterns of environmental disturbance and ecosystem recovery following the other four big five phanerozoic mass extinctions to evaluate whether commonalities exist in general the pace of ecosystem recovery depends on the degree of stability of the post crisis marine environment

coseismic slip resolution along a plate boundary megathrust the nankai trough southwest japan geodetic survey measurements are used to estimate the coseismic slip distribution in the 1944 tonankai mw 8 1 and 1946 nankaido mw 8 3 earthquakes and to assess quantitatively the degree to which this slip is resolved on the plate boundary megathrust data used include 798 angle changes from triangulation surveys 328 leveling section differences and 5 coseismic tidal gage offsets many of the nominally coseismic triangulation data span 50 years nearly half the earthquake cycle and correction for interseismic deformation using post 1950 observations is applied microseismicity is used to define the configuration of the plate boundary interface and approximate it with a continuous multisegment fault model because the onshore geodetic data have very limited resolving power for offshore fault segments offshore coseismic slip was constrained by satake s 1993 estimation based on tsunami data the majority of the coseismic slip occurs between 15 and 25 km depth although resolution declines toward the trench axis it is sufficiently good to define two distinct high slip regions one off southeastern shikoku island 11 m maximum and the other offshore of kii peninsula 3 m maximum the slip magnitude off southeastern shikoku coupled with the plate convergence rate would imply an recurrence interval of about 270 years much longer than the average repeat time of 120 years for historical great earthquakes on the nankai trough however the maximum coseismic slip is sensitive to the assumed fault geometry and slippage on trough parallel splay faults could significantly decrease the maximum slip to about 6 m

magma mixing and the production of compositional variation within granite suites evidence from the granites of southeastern australia granite suites are groups of plutons possessing characteristic features that are a result of their derivation from source material of a specific composition variation within suites has been ascribed to a variety of processes magma mixing or mingling is a popular hypothesis generally proposed in terms of blending between a crustal melt and mafic material from the mantle that caused that melting when the compositions of pain of suites from the bega batholith of southeastern australia are compared any differences seen at either end of the range in composition are also seen at the other limit so that both the most mafic and mostfelsic rocks show similar relative abundances of particular elements similar relationships are seen for other granites in the region these observations are not consistent with largescale magma mixing or mingling and although those processes may operate on a small scale they cannot have been responsible for the major compositional variations likewise assimilation of country rocks had no significant role in producing variation in the granites of southeastern australia the production of variation by differential separation of melt from residual solid source material or restite must be favoured for many of the granite suites of this region

the end ordovician mass extinction a single pulse event abstract the end ordovician mass extinction eome is widely interpreted as consisting of two pulses associated with the onset and demise of the gondwana glaciation respectively with the second pulse eradicating the distinctive glacially related hirnantian benthic biota hbb a global review of occurrence data of latest ordovician benthic marine organisms reveals that virtually all warm water benthic assemblages previously assigned to the hbb comprise two distinct and clearly postglacial faunas both younger middle and late hirnantian respectively than the cool water hirnantia fauna latest katian to early hirnantian the newly recognised three transitional benthic faunas i e tbfs 1 3 can be closely tied to graptolite conodont and chitinozoan biozonations the hirnantian isotope carbon excursion hice and the glaciation thereby providing an integrated much higher resolution timescale for understanding the tempo and nature of the eome at this finer resolution we postulate a more profound impact of the first pulse of the eome than hitherto envisaged as evidenced by opportunistic expansion of the hirnantia fauna globally and the complete absence of metazoan reefs in its immediate aftermath we also argue based on high quality data from well documented benthic groups in south china i e brachiopods tabulate and rugose corals trilobites and sponges that the magnitude of the second pulse of the eome caused by the deglaciation has been overestimated because the two postglacial faunas i e tbfs 2 3 were part of a subsequent recovery phase of marine ecosystems rather than contributing to biodiversity decline thus it is more plausible to reinterpret the eome as a single pulse rapid event that was followed by a prolonged initial recovery intermittently impeded by climatic shocks through the hirnantian prior to the onset of a progressive reestablishment of marine ecosystems during the early silurian rhuddanian and aeronian associated with an overall amelioration of climatic conditions

lilliput effect in freshwater ostracods during the permian triassic extinction abstract the lilliput effect following the permian triassic mass extinction and its aftermath has been documented in a variety of marine animal groups but it is less known in terrestrial and freshwater invertebrates here we present new investigations of the size variations of terrestrial ostracods of the genus darwinula based on fossil records from a permian triassic section on the northern limb of the dalongkou anticline section in northwest china quantitative analyses reveal that ostracod test sizes decreased sharply through the terrestrial permian triassic mass extinction interval the lilliput effect in terrestrial ostracods is characterized by the extinction of large taxa and the rise of small sized and elongate new forms coupled with the dramatic loss of conchostracans charophytes and the blooming of lycopod spores the size decrease in terrestrial ostracods following the biotic crisis through the permian triassic interval was probably triggered by several interacting events including global warming anoxia and enhanced sediment input following acid rain and wildfire

chemical and isotopic evidence for major mass transfer between mafic enclaves and felsic magma abstract in both the strontian and criffell zoned granitoid plutons certain chemical elements notably k rb ca sr and pb display a correlation between their concentration in microdiorite enclaves and their concentration in the immediately enclosing host granitic rock this appears to represent the effects of post incorporation mixing and exchange between the enclave and its host magma enclave initial 87 sr 86 sr ratios are indistinguishable from those of the host while enclave initial 143 nd 144 nd ratios are systematically more radiogenic establishing that these enclaves are not melt segregations nor restite the isotopic data are in agreement with the chemical data in indicating that sr is almost entirely derived from the host despite clear evidence of major mass transfer of alkali metals alkali earths and transition metals a comparison of the concentrations and ratios of hfse and ree and nd isotopic ratios with those of a number of potential starting protoliths allows the enclave precursors to be positively discriminated in this regard enclave hfse and ree compositions may allow for models of granitoid production by the injection of hot mantle derived magmas into the crust to be tested

seismic structure of western end of the nankai trough seismogenic zone 1 in 1998 a seismic experiment along a 320 km long profile using an air gun array and ocean bottom seismographs obss was collected in the on shore offshore forearc region of the western nankai trough seismogenic zone southwestern japan this area is outside the coseismic rupture area of the 1946 nankaido earthquake ms 8 2 the main objectives of this paper are to clarify the p wave and the poisson s ratio profile around the forearc region of southwestern japan and estimate differences between this area and the nankaido coseismic rupture area the p wave velocity model estimated by two dimensional 2 d ray tracing and travel time inversion indicates three units with distinctive velocity patterns the lens shaped unit has a maximum thickness of 5 km and velocity of 3 4 4 0 km s the underlying unit has velocity of 5 4 5 6 km s and its p wave velocity increases on land vp 6 2 6 4 km s the relative homogeneous unit with p wave velocity of 6 6 6 9 km s exists just beneath secondary unit with p velocity of 5 4 5 6 km s the angle of subduction is 7 we also obtained a poisson s ratio of the crustal structure of the forearc region the poisson s ratio of the southern part of secondary unit is larger than that of the northern part we conclude that these data indicate clay rich metamorphic rocks plus oceanic basaltic rocks rather than high porosity materials comprise the majority of the layer we suggest that the region of high poisson s ratio probably corresponds to underplating and the metamorphism of the clay rich sediments and possibly the oceanic basalt just below the decollement

high precision slip distribution of the 1944 tonankai earthquake inferred from tsunami waveforms possible slip on a splay fault abstract inversion of tsunami waveforms is a well established technique for estimating the slip distributions of subduction zone earthquakes with some of the most detailed results having been obtained for earthquakes in the nankai trough sw japan the present study although it uses a method and tsunami waveform data set almost identical to previous study aims to improve on previous work by using a more precise specification of initial conditions for the calculation of tsunami green s functions specifically we incorporated four improvements in the present study 1 we used a realistic plate model based only on seismic survey results and assumed it to be the fault plane of the 1944 tonankai earthquake 2 the smallest subfaults consistent with the long wavelength approximation were used in the tsunami inversion analysis 3 we included the effect of horizontal displacement of the ocean bottom on tsunami generation and 4 we performed a checkerboard resolution test as obtained in previous studies a zone of high slip 2 0 m was resolved off the shima peninsula however the more precise calculation of tsunami green s functions has revealed additional detail that was not evident in previous studies which we demonstrate is resolvable and correlates with the position of known faults in the accretionary prism while there was little or no slip near the trench axis in the eastern part of the rupture zone there was up to 1 5 m of slip resolved within 30 km of the trough axis in the western part along the coast of the kii peninsula this troughward slip zone coincides with the position of a large splay fault mapped in multichannel reflection surveys furthermore it is also clear that the upper edge of the enshu fault off shima and atsumi peninsulas is consistent with the up dip limit of slip in the eastern part of our model we tested the possibility that slip occurred on the former splay fault instead of on the plate interface during the 1944 tonankai earthquake and find that slip on this splay fault is also consistent with the data although we cannot distinguish whether slip was dominant on the splay fault or on the plate interface we further suggest that the position of the enshu fault may be determined by the subduction of topographic highs and that such faults may have an important influence on the up dip rupture limit of the 1944 tonankai and potentially other subduction zone earthquakes

a deep strong reflector in the nankai accretionary wedge from multichannel seismic data implications for underplating and interseismic shear stress release 1 we present the results of structural and stratigraphic interpretations for the multichannel seismic profiles to reveal the structure of the nankai accretionary wedge off shikoku island on the basis of reflection characteristics we identify three major seismic units that correspond to a unit consisting of both oligocene miocene accretionary prism and the sedimentary cover unit a a unit of underthrust miocene pliocene hemipelagic sediments unit b and a unit of subducting pre miocene oceanic crustal rocks unit c according to relative strength of the topmost reflector of unit b the nankai accretionary wedge can be divided into three different structural zones along the cross strike direction apparent decollement reflector zone low amplitude reflector lar zone and deep strong reflector dsr zone despite the velocity uncertainty involved in the depth conversion and therefore thickness estimation error the decollement step down and the possible duplex structures in the lar and dsr zones support that the landward thickness variation of unit b is caused by the structural thickening such as the underplating process structural features and reverse polarity suggest that the dsr as an upper interface of the underplated layer unit b corresponds to a shear plane or a fault zone with high pore fluid pressures we found that most of the dsr was observed in a region which has not suffered from the 1946 coseismic dislocation even though it was locked during the interseismic period the dsr as a shear plane or a fault zone constrains a regional variety of the coseismic dislocation around the updip limit of the nankai seismogenic zone considering all of the dsr characteristics we propose a hypothesis for the dsr s development and role the dsr may play an important role as a possible releaser of the shear stress energy stored at the plate boundary around the updip limit of the nankai seismogenic zone a good correlation between the theoretical vertical displacement and seafloor topographic feature support the possible slip along the dsr as a shear plane

tracking chemical alteration in magmatic zircon using rare earth element abundances abstract trace elements in magmatic zircon provide a wealth of petrogenetic information about host magmas however they are susceptible to alteration through post magmatic interaction with hydrothermal fluids trace element analyses can also be biased by the inadvertent inclusion of exotic materials such as mineral or glass inclusions in analyzed volumes of zircon in order to screen out samples with altered chemical signatures zircons with high flat light rare earth element lree patterns are typically considered to be altered however visual selection of such patterns is qualitative and does not address ambiguous cases the light rare earth element index lree i dy nd dy sm provides an approach for quantitative screening for aqueous alteration and contamination of zircon by exotic materials and was used to assess secondary processes in the greenschist facies jack hills detrital zircon suite however in addition to aqueous alteration the lree i is also sensitive to melt compositional evolution and its applicability to alteration in settings other than the jack hills quartzite is thus far undetermined we investigate igneous zircon populations from a variety of geologic settings that show evidence for alteration by contact metamorphism during magma intrusion and deuteric fluid interactions during pluton crystallization in suites with a high proportion of texturally altered zircons low lree i values are common and this parameter correlates well with other contamination indicators based on the observed secondary phases deposited in the zircons during fluid flow filtering zircon trace element compositions based on the lree i appears to remove the majority of chemically altered zircon in many cases revealing previously obscured magmatic signals

shear wave reflectivity anisotropies poisson s ratios and densities of a southern appalachian paleozoic sedimentary sequence abstract the physical properties of a sequence of paleozoic sedimentary rocks have been examined in detail with an emphasis on laboratory measurements of density shear wave velocity shear wave splitting and vp vs ratios seismic properties of 147 cores from 49 rock samples collected from the thorn hill sedimentary sequence of eastern tennessee are examined in terms of implications for future seismic studies in the southern appalachians the shear wave velocities of these rocks are strongly influenced by the relatively high shear wave velocity of quartz shear wave velocity anisotropy is present in most of the lithologic groups it is highest in the shales while being almost insignificant in the dolostones the related phenomenon of shear wave splitting occurs to some degree in all of the lithologies studied and at high pressures originates from mineral orientation splitting due to preferred mineral orientation is most prominent in the shales but also is significant in some of the carbonates compressional to shear velocity vp vs ratios of approximately 1 82 dolostones and 1 95 limestones effectively characterize the carbonates while other lithologies display wider ranges of vp vs primarily due to the influence of accessory minerals such as quartz densities of the sample suite range from 2 34 g cm3 shale to 2 86 g cm3 dolostone using average data for each lithologic group to calculate normal incidence reflection coefficients significant differences in the shear and compressional reflectivity of hypothetical interfaces are found normal incidence shear and compressional wave synthetic seismograms approximating the subsurface reflectivity of the entire thorn hill section indicate that three zones of high amplitude reflections would be seen on reflection records obtained over this 3327 m thick sequence overall the shear and compressional synthetics are similar in appearance differences are seen at some interfaces in the mississippian devonian interval which are more reflective to shear waves and in the ordovician martinsburg formation which appears more reflective to compressional waves

coupled sr o isotope variations as an indicator of source heterogeneity for the northern peninsular ranges batholith primary 18o values for tonalitic rocks from the san jacinto intrusive complex range from 9 0 to 10 6 initial87sr 86sr for the same samples varies from 0 7058 to 0 7076 rocks with low 18o tend to have higher initial87sr 86sr the majority of samples have a limited range of 18o values 10 0 10 4 that is anticorrelated with colour index this variation is compatible with crystallization of rocks with varying mineral abundances from an homogeneous for oxygen reservoir more mafic rocks show a large range of 18o values reflecting original variations in source material compositions the combined sr o isotope data demonstrate that three or more distinct components were involved in the genesis of these rocks these three components are also seen in batholithic rocks from the mojave block and sierra nevada to the north these data are interpreted as indicating a 0 35 oxygen atom basis contribution to the san jacinto rocks from an old continental lithosphere source

mingling in mafic magma chambers replenished by light felsic inputs fluid dynamical experiments abstract we investigate by means of laboratory analogues the fluid dynamics of magma hybridisation caused by the intrusion of felsic magma into a mafic magma chamber the input felsic magma is modelled by a cold low density fluid and the resident mafic magma by a layer of fluid wax close to its solidification point the cold rising input drives convection and solidification in the resident fluid and the solidification causes significant perturbations to an otherwise simple input flow greatly enhancing mixing and mingling between the two model magmas the dimensionless parameters controlling the evolution of the system are determined and different morphologies of the solid mass fit into clearly defined fields in the parameter space the most important implication of the experiments is that the freezing of mafic chambers due to felsic intrusion may give rise to intense hybridization of large volumes of magma this potentially important mechanism for hybridization has to date received little attention in the literature

interplate coupling in southwest japan deduced from inversion analysis of gps data abstract recently the geographical survey institute of japan completed the installation of a gps continuous observation network in japan which has enabled us to investigate real time crustal movements in this study we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest japan using horizontal and vertical deformation rates which were observed at 247 gps observation stations during the period from april 6 1996 to march 20 1998 for this purpose we carried out an inversion analysis of geodetic data incorporating akaike s bayesian information criterion abic as a result strong interplate coupling was found off shikoku and kumanonada regions which corresponds well with the fault regions of the 1946 nankai m 8 1 and the 1944 tonankai m 8 0 earthquakes respectively we also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the shikoku and kii peninsula the recurrence time of great trench type earthquakes was roughly estimated as 107 years which is consistent with previous research the direction of relative plate motion is oriented n53 w which is close to the direction predicted from the plate motion model on the other hand a large forward slip was found in the hyuganada region off southeast of kyushu since the coseismic displacements associated with the two 1996 hyuganada earthquakes m 6 6 m 6 6 are removed from the gps data this suggests that after slip occurred near the source region and or that kyushu moves southeastward stationarily due to other tectonic forces

mafic enclaves in the wilson ridge pluton northwestern arizona implications for the generation of a calc alkaline intermediate pluton in an extensional environment the wilson ridge pluton is an epizonal calc alkaline pluton that formed about 13 5 ma during a period of mid miocene extension faulting and erosional dissection provide a cross section of the pluton the apex of the pluton in the boulder wash area nevada is composed of hypabyssal quartz monzonite and dacite the base of the pluton is 20 km to the south where quartz monzodiorite monzodiorite and diorite are in low angle intrusive contact with precambrian basement the pluton was separated from cogenetic volcanic rocks in the river mountains by movement along the saddle island detachment fault at about 13 4 ma the river mountains now lie 20 km to the west of the pluton the wilson ridge pluton is composed of the teakettle pass suite consisting of foliated monzodiorite and quartz monzodiorite and unfoliated quartz monzonite and the older horsethief canyon diorite rocks of the pluton contain 2 4 modal percent sphene intermediate rocks of the teakettle pass suite contain abundant basaltic and diorite enclaves basaltic enclaves are lensoidal and pillow like and commonly have crenulate and fine grained margins enclaves are chemically similar to mafic dikes of the wilson ridge pluton and to cogenetic alkali basalt flows in the river mountains they probably represent blobs of mafic liquid that commingled and mechanically mixed with felsic magma to produce the intermediate rocks of the pluton basaltic enclaves commonly occur as inclusion rich zones that represent synplutonic mafic dikes that were injected into a quartz monzonite host mafic magma was entrained and mechanically broken down by magmatic flow shear a continuum in shape exists from enclaves that are bulbous and ellipsoidal to those that are thin tabular mafic selvages and schlieren and ultimately to the mafic component in foliated quartz monzodiorite and monzodiorite diorite enclaves have angular contacts with host rocks and are interpreted as xenoliths field evidence and major and trace element models suggest that the intermediate rocks of the pluton were produced by the commingling of a large volume of mafic magma with a smaller volume of felsic magma a mafic felsic ratio of about 70 30 as well as fractional crystallization similar open system processes may be responsible for the production of calc alkaline intermediate rocks in other parts of the great basin

san jacinto intrusive complex 1 geology and mineral chemistry and a model for intermittent recharge of tonalitic magma chambers geological mapping within the san jacinto mountains of southern california has delineated three major and numerous minor plutons of the cretaceous peninsular ranges batholith early minor intrusives emplaced into quartz rich metasedimentary sequences span the compositional range olivine gabbro to granite the three large to 250 km2 plutons span a limited compositional range between mafic tonalite color index ci 15 and k feldspar poor granodiorite ci 10 all units are composed of plagioclase an30 40 50 55 quartz 20 30 k feldspar 1 8 biotite 10 15 hornblende 0 5 titanite 0 2 and accessory zircon apatite allanite and ilmenite variations in mineral abundances are geographically systematic only within the youngest major mass unit iii which grades from marginal mafic tonalite to central k feldsparpoor granodiorite mineral foliations and banding schlieren and inclusion orientation within each unit usually parallel the nearest contact alignment of foliations and apparent flow sorting and scour features are interpreted as reflecting flow patterns within each chamber mafic synplutonic dykes of quartz diorite and tonalite intruded the tonalites and were broken up to form extensive inclusion trains dyke tonalite relations are interpreted as showing that 1 magma adjacent to the pluton wall had considerable yield strength 2 magmatic flow adjacent to pluton walls was capable of moving material some distance up to kilometers to form the inclusion trains and 3 the dykes represent conduits through which a considerable amount of liquid was added to the inflating magma chambers mineral compositions throughout the major plutons are relatively uniform mean plagioclase composition ranges from an40 in the most mafic tonalites to an30 in the most felsic granodiorites the total microprobe observed range is an44 to an25 and to an47 in a mafic inclusion mg mg fe mn of biotite and hornblende drop similarly from 0 44 to 0 36 reflecting changes in rock mg mg fe the sole oxide mineral is almost pure ilmenite the limited ranges of mineral compositions imply considerable stability of physico chemical conditions during the crystallization of each pluton the field petrographic and mineral chemical data are interpreted in terms of an intermittently recharged continuously solidifying magma chamber

petrology of the chilliwack batholith north cascades washington generation of calc alkaline granitoids by melting of mafic lower crust with variable water fugacity calc alkaline granitoid rocks of the oligocene pliocene chilliwack batholith north cascades range from quartz diorites to granites 57 78 sio2 and are coeval with small gabbroic stocks modeling of major element trace element and isotopic data for granitoid and mafic rocks suggests that 1 the granitoids were derived from amphibolitic lower crust having ree rare earth element and sr nd isotopic characteristics of the exposed gabbros 2 lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting the main effect of fh 2 o variation is to change the relative proportions of plagioclase and amphibole in the residuum the ree data for intermediate granitoids quartz diorite granodiorite eu eu 0 84 0 50 are modeled by melting with fh 2 o 1 kbar leaving a plagioclase pyroxene residuum in contrast data for leucocratic granitoids leuco granodiorites and granites eu eu 1 0 0 54 require residual amphibole in the source and are modeled by melting with fh 2 o 2 3 kbar consistent with this model isotopic data for the granitoids show no systematic variation with rock type 87sr 86sri 0 7033 0 7043 end 0 3 3 to 5 5 and overlap significantly with data for the gabbroic rocks 87sr 86sri 0 7034 0 7040 end 0 3 3 to 6 9 the fh 2 o variations during melting may reflect additions of h2o to the lower crust from crystallizing basaltic magmas having a range of h2o contents chillwack gabbros document the existence of such basalts one dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large scale melting of amphibolitic lower crust provided that ambient wallrock temperatures exceed 800 c based on lithologic and geochemical similarities this model may be applicable to other cordilleran batholiths

on the dynamics of magma mixing by reintrusion implications for pluton assembly processes many plutons have formed by repeated intrusion with complex internal contacts characterized by sharp and diause kinematic and compositional domains we performed numerical experiments of mixing following magma chamber recharge that explicitly considers the fluid dynamics of multiphase mixtures in the limit of zero diausivity of intensive scalar quantities and low stokes number mingling by fluid instability is equivalent to deformation and persistent fluid structures are kinematic attractors three distinct regimes are exemplified and can be described by their multiphase reynolds or grashof number for a reynolds number greater than about 100 an internal intrusive contact will collapse by internal wave breaking and chaotic magma mingling and mixing yield a nearly chamber wide stratification this flow may scour mushy regions at the walls and widely distribute previously crystallized material for reynolds numbers from 10 to 100 the internal wave does not break and the stratification occupies less of the chamber leaving islands of unmixed material for reynolds numbers around 1 internal slumping and folding can occur and is the most likely to be preserved by a mineral fabric internal sub vertical contacts require a high absolute viscosity in the resident magma representing a time break between episodes of reintrusion the shape of perched mafic domains captures the progress of sinking from higher levels into regions of increasing strength and crystallinity 7 2000 elsevier science ltd all rights reserved