northland ophiolite new zealand and implications for plate tectonic evolution of the southwest pacific the northland ophiolite and coeval ocean floor sedimentary rock form the northland allochthon obducted from the northeast onto new zealand in the late oligocene the ophiolite was probably emplaced as a single sheet and separated into individual massifs during subse quent movement of the allochthon chemically the bulk of the igneous rocks are normal mid ocean ridge basalts but the ophiolite also includes a younger suite of hornblende modal within plate alkalic rocks believed to represent seamounts two alternative models one involving a subduction flip the other involving continuous westward subduction are proposed to account for the obduction of the upper part of the northland ophiolite the oceanic crust from which the ophiolite originated was formed simultaneously with tasman sea spreading on the western rim of a once much larger south fiji plate assemblage

age and origin of orogenic gold mineralization in the otago schist belt south island new zealand constraints from lead isotope and 40ar 39ar dating studies the otago schist belt of southern new zealand hosts numerous orogenic gold deposits that formed in a range of structural and lithological settings during and after mesozoic metamorphism previous 40ar 39ar dating studies in the otago schist belt indicate formation ages of 106 to 101 ma for late postmetamorphic gold bearing quartz veins and shear zones samples of hydrothermal muscovite from several mineralized quartz vein systems were dated using the 40ar 39ar method muscovite from a gold bearing quartz stockwork vein at the macraes mine which is a relatively old mineralized system that formed during the latter stages of metamorphism yields a well defined plateau age of 135 1 0 7 ma fine grained muscovite from a quartz shear vein at macraes gives a rising age spectrum with a maximum age of 135 7 ma five 40ar 39ar dates from the scheelite bearing vein swarm in the glenorchy district are interpreted to indicate that mineralization occurred between 142 and 134 ma and that relatively slow cooling and or minor thermal overprints have disturbed the age spectra of some of the samples lead isotope compositions for a total of 42 sulfide samples from 17 separate mineralized zones in the otago schist belt are relatively radiogenic consistent with an entirely upper crustal source for the pb and presumably au the pb data define a less radiogenic and a more radiogenic cluster with no overlap lead isotope compositions from the sulfides from the macraes group of deposits form part of a less radiogenic cluster together with pb isotope compositions for sulfides from scheelite rich veins in the glenorchy camp lead isotope compositions for sulfides from younger postmetamorphic quartz veins and shear zones fall exclusively in the more radiogenic cluster the new 40ar 39ar dating results and pb isotope studies in conjunction with previously published work demonstrate that most orogenic gold mineralization within the otago schist belt formed during two discrete mineralizing events one at 142 to 135 ma and a later one at approximately 106 to 101 ma we interpret the two pulses of gold mineralization to reflect sequential extraction of ore forming components from the deeper part of the otago schist belt during short lived thermal events the younger event was probably associated with subduction of an actively spreading ridge and similar ridge subduction may have been involved in the older event

continuous metamorphic gradient documented by graphitization and k ar age southeast otago new zealand the chrystalls beach brighton coastal section southwest of dunedin new zealand exposes otago schist including the weakly metamorphosed triassic accretionary melange of the chrystalls beach complex and higher grade rocks of a largely felsic arc derived provenance melange zones contain pelagic sediments some of which are manganiferous and metabasites the grade of metamorphism increases progressively from southwest to northeast four mineral zones are recognized largely on the basis of mineral assemblages in psammitic and semipelitic rocks pumpellyite chlorite pumpellyite actinolite epidote actinolite and biotite d 002 data for carbonaceous material b 0 values for phengite and metamorphic phengite k ar ages are reported for 32 pelitic and semipelitic samples remarkably tight correlations are revealed between d 002 for progressively graphitized carbonaceous material k ar ages that range from 184 6 to 138 7 ma and linear distance along the section both the d 002 values and the k ar ages decrease progressively toward the northeast through the four mineral zones and with progressive textural changes all rocks cropping out in the section have undergone a coherent episode of progressive metamorphism associated with terrane collision and have not been disturbed by any later major displacements a convex trend for phengite b 0 values plotted against advancing graphitization suggests p t conditions during metamorphism as in the high pressure intermediate facies series the age of the metamorphic peak is inferred to be 175 155 ma middle to late jurassic with cooling to the mica closure temperature at 155 135 ma late jurassic to early cretaceous unloading denudation and cooling were accompanied by localized hydrothermal events the oldest covering strata were deposited at ca 100 ma and denudation was completed by ca 75 80 ma middle triassic radiolarian nodules in melange zones are dated as preceding the metamorphic peak by about 65 ma associated turbidites contain suspected late middle to early late triassic tube fossils

thermal history and exhumation of basement rocks from mesozoic to cenozoic subduction cycles central north island new zealand a new thermochronological study of the basement rocks of the central north island new zealand records thermal and exhumation histories related to two subduction cycles since the latest jurassic the basement comprises metasedimentary terranes accreted onto eastern gondwana during mesozoic subduction since the oligocene these terranes have been located on the hanging wall of the hikurangi subduction margin overriding the pacific plate results of zircon fission track 121 264 ma analysis yield detrital or slightly reset ages apatite fission track 19 8 122 ma and u th sm he 10 3 95 2 ma ages are fully reset results from inverse thermal history modeling of the data of individual and stacked samples imply the following 1 after their accretion the basement rocks were cooled and exhumed to shallow depths of the crust in the early cretaceous by 150 135 ma this was followed by an episode of reheating until 100 ma which we interpret to be the result of burial by sedimentation above the accretionary wedge during extensional deformation caused by continued underplating and thickening of the wedge from beneath 2 from 100 ma our data indicate thermotectonic quiescence until the oligocene 27 ma 3 during initiation of the hikurangi margin the fore arc proto axial ranges began to exhume during the late oligocene to early miocene 27 20 ma subduction triggered unroofing removed 1 2 2 8 km of overburden most of which was probably the less lithified passive margin sediments 4 along the western central north island the exhumational response to the hikurangi subduction was limited with a total magnitude of erosion 1 5 km in the cenozoic

a morphometric analysis of gullies scoured by post fire progressively bulked debris flows in southwest montana usa in the fall of 2001 an intense thunderstorm in southwest montana triggered many debris flows in the burned area of sleeping child creek in most instances the debris flows cut deep gullies into previously unchannelized colluvial hollows and deposited large volumes of sediment onto the valley floor the presence of rill networks above the gullies as well as the absence of landslide features indicate that the gullies were scoured by progressively bulked debris flows a process in which dilute surface runoff becomes increasingly more laden with sediment until it transforms into a debris flow in this contribution we present a morphometric analysis of six of the gullies to better understand this relatively understudied process we find that the locations of the rill heads and gully heads conform to slope area thresholds that are characteristic of erosion by overland flow our data also suggest that the volumes of the debris flows increase exponentially with normalized drainage area thus lending support to an assumption used in a recently proposed debris flow incision law finally the debris flow fans have been relatively unaltered since deposition suggesting that the valley may be currently aggrading while the hillslopes are being denuded

volcaniclastic debris flows at la fossa volcano vulcano island southern italy insights for erosion behaviour of loose pyroclastic material on steep slopes abstract this paper describes a qualitative model of erosion of loose volcaniclastic material on steep slopes of la fossa cone vulcano island italy that generates small volume debris flows the loose volcaniclastic cover rests on an almost impermeable substratum which comprises a fine grained hydromagmatic tuff morphologically three sectors were distinguished 1 a water supply zone slope 32 where rain water is enriched in mud and concentrated in the initiation zone 2 an initiation zone or source area sensu stricto slope with 30 where debris flows form and 3 a transport and deposition zone where debris flows deposited levees and terminal lobes slope 30 main episodes of erosion of loose volcaniclastic cover and debris flow generation occur in response to thunderstorms the erosion of tuff from superficial water added a significant amount of fines mud fraction to the initiating flows debris flow stoppage on steep slopes was interpreted as the result of a loss of mass estimated between 35 and 53 of total mass to form levees from field and laboratory data the density of debris flows was estimated at about 1950 kg m3 morphological data matrix texture mass number nm and clay content of debris flow deposits suggest that they were deposited by saturated granular slurries

a 2400 yr record of natural events and anthropogenic impacts in intercorrelated terrestrial and marine sediment cores waipaoa sedimentary system new zealand the waipaoa sedimentary system spans 100 km from terrestrial upland to continental rise alluvial buffering has little effect on sediment flux at the outlet of this mesoscale dispersal system and hinterland to margin transport is accomplished rapidly because of this synergy the floodplain and shelf depocenters are sensitive to changes in sediment production in the hinterland and natural and anthropogenically forced changes in sediment source dynamics that occur at several temporal and spatial scales leave distinctive signals in the stratigraphic record manifested as variations in sediment properties these signals appear in intercorrelated sediment cores from a headwater riparian storage area and the major terrestrial and marine repositories for sediment discharged during the past 2 4 k y the signals represent the landscape response to vegetation and land use change short term fluctuations in climate that affect surface properties and processes and extreme storms and subduction thrust earthquakes extreme storms are the minimum geomorphologically effective event preserved in the sediment records lower magnitude storms that are integral components of the prevailing hydrometeorological regime create high frequency fluctuations in sediment properties and collectively contribute to event sequences of 100 yr duration events and event sequences comprise a hierarchy of temporally sensitive phenomena the impacts of which are conditioned by frequency and magnitude by contrast vegetation disturbance is a spatially sensitive phenomenon that directly impacts sediment source areas and lowers the threshold of landscape sensitivity to erosion for this reason the taupo eruption of 1 718 ka and the piecemeal vegetation changes that occurred after the arrival of polynesian settlers also generated strong depositional signals after european colonization deforestation of the hinterland altered landscape sensitivity and precipitated the transition to an erosional regime that impacted sediment production and dispersal across the entire magnitude frequency spectrum of events regulating sediment delivery to and transport in stream channels no other perturbation had such a profound impact on the late holocene depositional record

channelization on plateaus with arbitrary shapes 1 the initiation of channels and the factors that control their spacing is one of the most fundamental problems in geomorphology here we present an analysis of channelization that is downstream driven so called because channelization is initiated at some point away from the divide where the froude critical flow is achieved by using two novel techniques the frozen time approach and the momentary stability concept we are able for the first time to investigate downstream driven channelization that occurs on a plateau with an evolving base profile the frozen time approach assumes that the time evolution of the base state can be ignored in the linearized equations whereas the momentary stability concept assumes that an unsteady base state becomes momentarily unstable to a disturbance when the growth of the disturbance is faster than the evolution of the base state the analysis shows that an arbitrary slope profile is momentarily unstable and incised by channels before it evolves into a steady base state if the topographic curvature at the location where the froude critical flow is achieved is sufficiently small as the curvature decreases further dominant channel spacing increases slightly in a purely erosional case while clear dominant channel spacing tends not to appear in an erosional depositional case it is also found from the analysis that in both cases the channel spacing is on the order of 1000 times the froude critical depth the results are compared with field observations and experiments and they show satisfactory agreement

post eruption sedimentation on the margin of a caldera lake taupo volcanic centre new zealand abstract a wide range of sedimentary environments developed in areas proximal to the vent following the emplacement of a voluminous unwelded and highly pumiceous ignimbrite at the close of the 1 8 ka eruption from taupo volcanic centre new zealand lithofacies indicate that sediments were deposited by debris and hyperconcentrated flows onto the new land surface and by dilute turbulent flows in a number of different environments early streams with shallow unstable channels larger braided rivers meandering rivers a delta temporary lakes and the beach and nearshore areas of the caldera lake sediments deposited by debris flows are dominantly pumiceous but are varied in texture structure and composition this variability is the product of differing clast support mechanisms source materials and depositional environments hyperconcentrated flows deposited sediments which are compositionally and texturally more diverse generally better sorted and with structures which include vague stratification isolated outsized clasts and lenses of material that is better sorted than the matrix the earliest fluvial deposits are also texturally and compositionally varied but are generally immature these sediments are dominated by planar tabular cross bedding plane lamination and possible antidune structures braided river deposits also show the last two structures but are dominated by trough cross bedding meandering river sediments are the most mature of all fluvial facies and have only lower flow regime structures deposits in temporary lakes and in nearshore parts of the caldera lake are characterized by extensive planar bedding and structures indicating deposition both from rapid lateral input and from suspension fallout in the caldera lake reworking by wave action additionally produced rippled sediments in the nearshore areas while concentrating denser components on the narrow beaches to produce submature lithic and crystal rich deposits whereas the higher sediment concentration flows merely redistributed the very poorly sorted source ignimbrite without much sorting the more dilute turbulent flows rapidly separated components according to grain size and density most of the lower density pumice sand and gravel was removed with glass shard sand and mud to low energy depocentres deposits in the higher energy environments thus have generally low mud proportions and show a concentration of the average to high density lithics and crystals which together had formed less than 10 percent of components in the ignimbrite source the sudden availability of large volumes of highly erodible material coupled with increased water yield and overland flow due to the destruction of the pre eruption forests and drainage networks ensured that this post eruption increase in the number and type of depocentres was accompanied by a sharp increase in the rate of sediment deposition to two orders of magnitude greater in some environments

reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents we use seismic reflection and refraction data to determine crustal structure to map a fore arc basin containing 12 km of sediment and to image the subduction thrust at 35 km depth seismic reflection megasequences within the basin are correlated with onshore geology megasequence x late cretaceous and paleogene marine passive margin sediments megasequence y a similar to 10 000 km 3 submarine landslide emplaced during subduction initiation at 22 ma and megasequence z a neogene subduction margin megasequence the moho lies at 17 km beneath the basin center and at 35 km at the southern margin beneath the western basin margin we interpret reflective units as deformed gondwana fore arc sediment that was thrust in cretaceous time over oceanic crust 7 km thick raukumara basin has normal faults at its western margin and is uplifted along its eastern and southern margins raukumara basin represents a rigid fore arc block 150 km long which contrasts with widespread faulting and large neogene vertical axis rotations farther south taper of the western edge of allochthonous unit y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward as proposed for the correlative onshore allochthon spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore arc crustal thickness the trench slope has many small extensional faults and lacks coherent internal reflections suggesting collapse of indurated rock rather than accretion of 1 km of sediment from the downgoing plate the lack of volcanic intrusion east of the active arc and stratigraphic evidence for the broadening of east cape ridge with time suggests net fore arc accretion since 22 ma we propose a cyclical fore arc kinematic rock moves down a subduction channel to near the base of the crust where underplating drives rock uplift oversteepens the trench slope and causes collapse toward the trench and subduction channel cyclical rock particle paths led to persistent trench slope subsidence during net accretion existing global estimates of fore arc loss are systematically too high because they assume vertical particle paths citation sutherland r et al 2009 reactivation of tectonics crustal underplating and uplift after 60 myr of passive subsidence raukumara basin hikurangi kermadec fore arc new zealand implications for global growth and recycling of continents tectonics 28 tc5017 doi 10 1029 2008tc002356

sandstone provenance and sediment dispersal in a complex tectonic setting taranaki basin new zealand abstract mineralogical geochemical and detrital zircon data from key reservoir intervals in taranaki basin are integrated with paleogeography and basement maps to provide a synthesis of changes in sediment provenance and sandstone composition through time results show that sandstone composition and quality in taranaki is strongly related to sediment provenance and basin history late cretaceous syn rift deposits display clear evidence for first cycle erosion from local basement rocks dominated by granites of the median batholith a ne sw trending shoreline belt developed through the paleocene which was associated with a gradual increase in sandstone maturity that peaked during the eocene passive margin phase sediment provenance continued to be primarily from granitoids with transport directions from the s sw to n ne progressive quartz enrichment is interpreted to be a result of cleaning through longshore drift longer transport distances and enhanced chemical weathering due to warmer paleoclimates we also invoke a component of eroded and recycled cretaceous paleocene sediment linked to an episode of uplift in the south a transition period ensued from the late eocene to early miocene associated with a change in basin tectonics from passive margin to active margin compositional and detrital zircon u pb data indicate that rocks to the east of taranaki basin became emergent with progressive erosion of cretaceous eocene cover rocks and underlying basement terranes minor southward propagating uplift of the eastern basin margin was initiated in the latest eocene and by the late oligocene provenance and sedimentation patterns had clearly changed with the earliest miocene strata sourced from eastern metasedimentary basement rocks the eastern province continued to be a major sediment source through the active margin phase but mixed with the re appearance of granite reworked sources and miocene volcaniclastic detritus these changes in sediment provenance through time are displayed on a set of eight sediment dispersal maps which can be used to help predict composition and hence reservoir quality in undrilled parts of the basin

post triassic multiple exhumation of the taihang mountains revealed via low t thermochronology implications for the paleo geomorphologic reconstruction of the north china craton abstract to better define the uplift and denudation process of the taihang mountains and its uplift subsidence coupling with the western bohai bay basin the zircon and apatite u th he thermochronology and apatite fission track thermochronology of samples from the taihang mountains trans north china orogen were examined in this study the weighted averages of single grain zircon and apatite u th he ages were 70 3 79 9 ma and 15 1 26 8 ma respectively the apatite fission track ages ranged from 41 8 ma to 51 9 ma with mean track lengths of 11 68 13 3 m three distinct episodes of cooling at 100 45 ma at 38 30 ma and after 20 ma revealed via joint inverse modeling were related to compression from the subduction of the kula plate during the late cretaceous the rapidly changing differential elevation and subsidence between the taihang mountains and the western bohai bay basin from the late eocene to early oligocene and the combined distant effects of the collision of the indian and eurasian plates and the rollback of the subducting pacific plate during late cenozoic respectively this paper presents the cooling histories of the shallow crust below 200 c in the taihang mountains and provides new insights into the geodynamic mechanism of the uplift events that occurred in the different areas of the north china craton understanding these events is very important to comprehend the differential uplift in the north china craton basin mountain coupling and its tectonic morphological evolution since the mesozoic

subduction platform subsidence and foreland thrust loading the late tertiary development of taranaki basin new zealand borehole seismic and gravity data are used to investigate deformation of continental lithosphere at a miocene collisional zone deformation is manifested in the three following principal forms a long wavelength 500 km platform subsidence ascribed to mantle convection flexural deformation on a scale of 100 200 km due to crustal thrusting at the eastern boundary of the taranaki basin and a ductile thickening evident on the deep seismic section of taranaki basin that occurs on a scale of 10 km evidence for flexural deformation principally comes from the deep seismic section that shows a 150 km wavelength bending of the moho down toward the major zone of thrusting within the taranaki fault zone paleowater depths however provide evidence for an initial early miocene regional subsidence that is too long in wavelength to be explained by flexure induced from thrust sheet loading instead we propose that this broad platform subsidence was driven by loading from a deep source probably subduction induced flow in the mantle by 22 19 ma 1 2 km of water existed over most of the area now occupied by south taranaki basin by 19 17 ma the water depth in the zone east of the taranaki basin the taranaki fault zone had been replaced by rock due to submarine thrusting and crustal thickening this build up of submarine topography in the taranaki fault zone constitutes part of the load 25 8 mpa that created and maintains south taranaki basin gravity data place further constraints on loading at the thrust front and point to an additional intracrustal loading equivalent to 15 7 mpa over a 50 km wide zone this intracrustal load is explained as being due to thick skinned thrusting bringing denser lower crustal rocks nearer to the surface in the thrust zone the complete load on the taranaki foreland is therefore in three parts the submarine topographic load the intracrustal load and the loading of infilling sediments

late cenozoic evolution and earthquake potential of an active listric thrust complex above the hikurangi subduction zone new zealand in the center of the frontal wedge of the hikurangi subduction zone new zealand mahia peninsula and its submarine continuation lachlan ridge are being uplifted and folded above an active landward dipping thrust fault complex that is 80 km long high quality marine seismic reflection profiles reveal complex deformation of a cretaceous to holocene sedimentary section and enable a detailed analysis of the stratigraphy structural evolution deformation rates and future earthquake potential the structural analysis is facilitated by uplifted marine terraces on mahia peninsula and by 14 submarine unconformities in the hanging wall sequence five of which are correlated across the eroded crest of lachlan ridge and into the footwall basin the ages of the unconformities are determined by seismic ties to an offshore exploration well onshore outcrops on the peninsula and seabed samples dated by pollen coccolith nannoflora and foraminifera biostratigraphy nine regional quaternary unconformities which developed in response to eustatic fluctuations in sea level and are not older than ca 1 ma are correlated with oxygen isotope stages in equatorial pacific ocean drilling project core 677 the ages of fault growth strata and progressive restorations of deformed stratigraphy indicate that lachlan ridge developed during three phases of deformation since subduction of the pacific plate commenced beneath the australian plate in the early miocene these include an initial phase of thrust faulting a subsequent phase dominated by extensional faulting and the current mainly pleistocene to holocene phase of structural inversion reactivated listric thrust faulting and folding early to middle miocene thrusts in the deeper core of the complex developed out of sequence by sequentially stepping up into the hanging wall section creating an imbricate fan with emerging thrust tips now buried within the forelimb basin in the middle miocene early pliocene listric extensional faults developed in the active thrust wedge possibly as a result of substantial relief on the subducted pacific plate and controlled the development of lachlan basin to the west of the ridge the principal active thrust is the lachlan fault a listric extensional detachment reactivated to accommodate thrust movement and consisting of at least three right stepping segments depth converted seismic profiles indicate that the fault dips westward at 15 20 6 8 km beneath the western flank of lachlan ridge and steepens to 55 70 or even steeper in the upper 1 2 km of section beneath the eastern flank syninversion pleistocene fault growth strata on both flanks of the associated anticline provide an exceptional record of progressive fold limb rotation resulting from the listric fault geometry a geometric analysis of the fault growth strata and deformed terraces was used to derive a maximum dip slip displacement rate of as fast as 3 0 6 5 mm yr the implied shortening rate of 2 6 6 3 mm yr represents 8 20 of the total 31 mm yr of orthogonal plate convergence across this part of the upper plate of the hikurangi subduction zone the top of the pre fault growth paleogene section reveals as much as 5 8 1 5 km of vertical separation in the north decreasing to 30 50 of this value in the south temporal 10 3 10 6 yr and spatial 10 3 10 4 m of strike length variations in vertical deformation rate have occurred during the past 1 m y maximum rates occurred in the holocene and middle quaternary a long term increase in vertical separation rate on all segments during the pleistocene largely reflects a change in thrust kinematics associated with structural inversion the relatively greater increase in uplift rate on the northern part of the fault during the past 1 m y could be related to the possibility that a subducted seamount lies 10 km beneath the peninsula estimates of earthquake source parameters incorporating paleoseismic uplift data from mahia peninsula indicate a potential moment magnitude of up to m w 7 6 8 0 for an earthquake that ruptures all three segments of the lachlan fault the average recurrence interval for such events is estimated to be 615 2333 yr which is consistent with a mean recurrence interval of 1062 yr for four late holocene earthquakes thus the uplift and folding of mahia peninsula and lachlan ridge results from coseismic displacements on a major listric thrust fault that ruptures the upper plate frequently in association with large magnitude earthquakes

the relationship between eruptive activity flank collapse and sea level at volcanic islands a long term 1 ma record offshore montserrat lesser antilles hole u1395b drilled southeast of montserrat during integrated ocean drilling program expedition 340 provides a long 1 ma and detailed record of eruptive and mass wasting events 130 discrete events this record can be used to explore the temporal evolution in volcanic activity and landslides at an arc volcano analysis of tephra fall and volcaniclastic turbidite deposits in the drill cores reveals three heightened periods of volcanic activity on the island of montserrat 930 ka to 900 ka 810 ka to 760 ka and 190 ka to 120 ka that coincide with periods of increased volcano instability and mass wasting the youngest of these periods marks the peak in activity at the soufriere hills volcano the largest flank collapse of this volcano 130 ka occurred towards the end of this period and two younger landslides also occurred during a period of relatively elevated volcanism these three landslides represent the only large 0 3 km3 flank collapses of the soufriere hills edifice and their timing also coincides with periods of rapid sea level rise 5 m ka available age data from other island arc volcanoes suggests a general correlation between the timing of large landslides and periods of rapid sea level rise but this is not observed for volcanoes in intra plate ocean settings we thus infer that rapid sea level rise may modulate the timing of collapse at island arc volcanoes but not in larger ocean island settings

microearthquakes in the tdp swarm turkey clustering in space and time summary the third occupation experiment tdp3 of recording sites above a persistent swarm of microearthquakes near the north anatolian fault with a larger seismic network and over a longer period of time confirms and refines previous observations with greater resolution the greater resolution in earthquake locations has revealed marked clustering in time and space many perhaps most of the earthquakes belong to clusters where successive earthquakes originate in a very small volume and have similar fault mechanisms such studies allow the progression of seismic activity of small earthquakes to be followed in some detail and may reveal features which are hidden in larger and more complex earthquake sequences

marine volcaniclastics of the hidden lake formation coniacian of james ross island antarctica an enigmatic element in the history of a back arc basin

motion and rigidity of the pacific plate and implications for plate boundary deformation 1 using up to 11 years of data from a global network of global positioning system gps stations including 12 stations well distributed across the pacific plate we derive present day euler vectors for the pacific plate more precisely than has previously been possible from space geodetic data after rejecting on statistical grounds the velocity of one station on each of the pacific and north american plates we find that the quality of fit of the horizontal velocities of 11 pacific plate pa stations to the best fitting pa euler vector is similar to the fit of 11 australian plate au velocities to the au euler vector and 20 better than the fit of nine north american plate na velocities to the na euler vector the velocities of stations on the pacific and australian plates each fit a rigid plate model with an rms residual of 0 4 mm yr while the north american velocities fit a rigid plate model with an rms velocity of 0 6 mm yr our best fitting pa au relative euler vector is located 170 km southeast of the nuvel 1a pole but is not significantly different at the 95 confidence level it is also close 70 km in position and 3 in rate to a pole derived from transform faults identified from satellite altimetry suggesting that the vector has not changed significantly over the past 3 myr our relative euler vector is also consistent with all known geological and geodetic evidence concerning the au pa plate boundary through new zealand the gps sites offshore of southern california are presently moving 4 5 1 mm yr relative to predicted pacific velocity with their residual velocities in approximately the opposite direction to pa na relative motion likewise the easternmost sites in south island new zealand are moving 3 1 mm yr relative to predicted pacific velocity with the residuals in approximately the opposite direction to pa au relative motion these velocity residuals are in the same sense as predicted by elastic strain accumulation on known plate boundary faults but are of a significantly higher magnitude in both southern california and new zealand implying that the plate boundary zones in both regions are wider than previously believed index terms 1206 geodesy and gravity crustal movements interplate 8155 1208 geodesy and gravity crustal movements intraplate 8110 1243 geodesy and gravity space geodetic surveys 8158 tectonophysics evolution of the earth plate motions present and recent 3040 8159 tectonophysics evolution of the earth rheology crust and lithosphere keywords pacific plate motion plate rigidity plate boundary deformation

slope stability models for rainfall induced lahars during long lasting eruptions abstract in this study we analyse the spatial distribution of potential lahar sources during long lasting eruptions using two slope stability models shalstab and trigrs the analysis is based on observed deposit grain size and thickness rainfall data and slope angle of the lahar events in the area surrounding the cordon caulle volcano chile that occurred during the 2011 eruption the main phase of the eruption 4 7 june was characterized by eruptive plumes from 7 to 11 km high which dispersed most of the tephra eastward toward argentina with a total estimated volume of about 1 km3 tephra fallout blanketed the region from ese to ene of the volcano with the thickness of the tephra fallout layers between approximately 1 m 15 km from vent and 0 06 m 240 km from the vent on 10 june 2011 a major lahar occurred close to the argentina chile border that reached the national road 231 28 km from the vent three field campaigns were undertaken to collect samples and data from tephra fallout deposits and triggering mechanisms in the lahar source area model input parameters were obtained from geotechnical tests and field measurements several metrics were used for model evaluation and best fit to the data were obtained for simulations considering non cohesive ash layers for shalstab and a cohesion of 0 5 kpa for trigrs both models are sensitive to the physical properties of the tephra fallout deposit and the hydraulic and materiel strength properties of the study area they both also show good agreement with field data but provide different information trigrs provides an estimate of the timing based on a storm event and location of a potentially unstable area while shalstab simulations result in landslide susceptibility classes based on critical rainfall value these outcomes provide fundamental insights into lahar triggering during long lasting volcanic eruptions and are crucial to the compilation of lahar hazard maps and emergency management plans in the south andes volcanic region

the geomorphological evolution of the paricutin cone and lava flows mexico 1943 1990 abstract erosion and sedimentation processes in the paricutin cone and adjacent lava fields have been evaluated 47 years after the eruption through field study of the morphological characteristics of the area previous studies indicate an initial stage of accelerated erosion decreasing quickly after the stripping of the ash layers and cessation of the volcanic activities in the cone and crater area revegetation is the main process after the first stage of stripping of fine ash the lava fields are the least affected areas by erosional processes with vegetation development in minor depressions crevices and along the edges of the lava flows the llanos or lava dammed plains are the most active physiographic units with continuous deposition of sediments they are completely covered by vegetation or under agricultural use incision of channels along the lava field edge promotes the integration of the drainage system but still most of the affected area drains into the lave field present erosion rates are higher than normal rates and a complete rehabilitation of the pyroclastic area may extend over decades filling of sediments and revegetation of the lava fields will develop in a longer period of several millennia

posteruption suspended sediment transport at mount st helens decadal scale relationships with landscape adjustments and river discharges received 11 december 2002 revised 30 september 2003 accepted 18 november 2003 published 17 january 2004 1 widespread landscape disturbance by the cataclysmic 1980 eruption of mount st helens abruptly increased sediment supply in surrounding watersheds the magnitude and duration of the redistribution of sediment deposited by the eruption as well as decades to centuries old sediment remobilized from storage have varied chiefly with the style of disturbance posteruption suspended sediment transport has been greater and more persistent from zones of channel disturbance than from zones of hillslope disturbance despite the severe landscape disturbances caused by the eruption relationships between discharge magnitudes and frequencies and suspended sediment transport have been remarkably consistent discharges smaller than mean annual flows generally have transported 5 but locally 15 of the annual suspended sediment loads and infrequent p 0 01 large floods have transported as much as 50 of the annual suspended sediment loads in a single day however moderate magnitude discharges those greater than mean annual flows but less than 2 year floods have transported the greatest amounts of sediment from all disturbance zones such discharges have transported on average 60 to 95 of the annual suspended sediment loads usually within cumulative periods of 1 3 weeks each year although small magnitude and largemagnitude discharges have locally and episodically transported considerable amounts of suspended sediment there has not been any notable change in the overall nature of the effective discharges moderate magnitude flows have been the predominant discharges responsible for transporting the majority of suspended sediment during 20 years of posteruption landscape adjustment index terms 1824 hydrology geomorphology 1625 1815 hydrology erosion and sedimentation 1860 hydrology runoff and streamflow keywords sediment yield suspended sediment transport mount st helens erosion volcano geomorphology

erosion calderas origins processes structural and climatic control the origin and development of erosion modified erosion transformed and erosion induced depressions in volcanic terrains are reviewed and systematized a proposed classification addressing terminology issues considers structural geomorphic and climatic factors that contribute to the topographic modification of summit or flank depressions on volcanoes breaching of a closed crater or caldera generated by volcanic or non volcanic processes results in an outlet valley under climates with up to 2000 2500 mm annual rainfall craters and calderas are commonly drained by a single outlet the outlet valley can maintain its dominant downcutting position because it quickly enlarges its drainage basin by capturing the area of the primary depression multi drained volcanic depressions can form if special factors e g high rate geological processes such as faulting or glaciation suppress fluvial erosion normal fluvial erosion modified volcanic depressions the circular rim of which is derived from the original rim are termed erosion craters or erosion calderas depending on the pre existing depression the resulting landform should be classed as an erosion induced volcanic depression if the degradation of a cluster of craters produces a single drained irregular shaped basin or if flank erosion results in a quasi closed depression under humid climates craters and calderas degrade at a faster rate mostly at subtropical and tropical ocean island and island arc volcanoes their erosion results in so called amphitheater valleys that develop under heavy rainfall 2500 mm year rainstorms and high elevation differences structural and lithological control and groundwater in ocean islands may in turn preform and guide development of high energy valleys through rockfalls landsliding mudflows and mass wasting given the intense erosion amphitheater valleys are able to breach a primary depression from several directions and degrade the summit region at a high rate occasionally amphitheater valleys may create summit depressions without a pre existing crater or caldera the resulting negative landforms which may drain in several directions and the primary origin of which is commonly unrecognizable should be included in erosion transformed volcanic depressions

post eruption erosion processes and plant recovery in the summit atrio of mt usu japan abstract the 1977 1978 eruptions of mt usu in japan provided a unique opportunity to examine recovery processes of erosional and vegetational patterns in the thickly tephra covered summit atrio surficial slope erosion in relation to revegetation was studied on the basis of field examination and aerial photo interpretation during the period 1980 1984 although heavy rilling and gullying brought about high sediment yield and impeded plant establishment they also had a positive effect on revegetation by allowing the survival and invasion of herbaceous plants such erosion processes thinned the tephra mantle and transported organic debris and seeds from upper somma walls to lower slopes and alluvial fans

thermal state of the taranaki basin new zealand the taranaki basin is an active margin basin that has been significantly affected by miocene subduction tectonics along the pacific australian plate boundary we have analyzed its present day thermal state using 354 bottom hole temperatures bhts from 115 wells distributed throughout the basin the measured temperatures were corrected using an exact solution to bullard s equation rather than the horner approximation thereby allowing for recovery dependence on well diameter and correction for some bhts at early time after circulation had ceased thermal conductivity measurements were completed on 256 samples from eight wells and matrix conductivities were determined for six end member lithologies by inversion formation conductivities are based on the conductivity and relative proportion of each end member component corrected bhts in situ thermal conductivity and estimates of sediment heat production were combined to compute the present day steady state heat flow the average heat flow is 60 mw m 2 but important geographic variations are present heat flow on the western platform is remarkably consistent at 53 60 mw m 2 attesting to its relative stability since the late cretaceous heat flow in the southern part of the basin is 65 70 mw m 2 due to as much as 3 km of late miocene erosion on the southern onshore and to the south of the peninsula heat flow is 50 3 mw m 2 possibly due to the heat sink effects of crustal thickening heat flow is highest at 74 mw m 2 on the northern peninsula adjacent to the taranaki volcanic zone suggesting a causal relationship between quaternary volcanism and high heat flow

ar ar dating of k feldspar in low grade metamorphic rocks example of an exhumed mesozoic accretionary wedge and forearc south island new zealand 1 new 40ar 39ar ages from detrital k feldspars and metamorphic white micas from the eastern province terranes of new zealand have been used to investigate the thermo tectonic history of different parts of an exhumed mesozoic forearc basin and accretionary wedge k feldspars from barely metamorphosed sedimentary host rocks mainly record detrital source area ages whereas those from zeolite and prehnite pumpellyite facies host rocks have textures and argon age spectra that indicate recrystallization during regional low temperature metamorphism the results contribute to a model that genetically links thermo tectonic events across the accretionary wedge and forearc basin elements of the convergent margin and into the median batholith arc probably by the early cretaceous and possibly by the middle jurassic thus even though multidiffusion domain mdd models cannot be used to make inference on cooling histories in such situations the k feldspar argon thermochronometer can provide useful information on the timing of geological events in sub greenschist facies rocks

an exceptionally widespread ignimbrite with implications for pyroclastic flow emplacement pumice rich pyroclastic flows leave deposits called ignimbrites1 which are common in the eruption records of arc and intracontinental volcanoes2 3 pyroclastic flows represent the most dangerous manifestation of volcanism4 because of their volumes rapid emplacement at high temperatures and capacity to extend 100 km from the source however maximum distances travelled and extents of areas destroyed by pyroclastic flows remain poorly known historic examples have been relatively small the largest reaching to only 35 km from the vent5 but prehistoric ignimbrites over 150 km across are known3 6 10 the largest ignimbrites are invariably partly buried or eroded and only minimum estimates of their size and hence of their destructive capacity can be made here we describe the l myr old kidnappers ignimbrite erupted from the taupo volcanic zone in new zealand which we have correlated for 385 km and which thus represents the most wide spread ignimbrite yet known we suggest that this wide distribution primarily reflects a coupling of high initial flow velocities with large mass fluxes and that favourable flow paths and emplacement of flows over a wet substrate may have aided their mobility

impacts of explosive volcanism on distal alluvial sedimentation examples from the pliocene holocene volcaniclastic successions of japan abstract volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation lahar processes after the cessation of eruptive activity although there are many studies dealing with the assessment and mitigation of volcanic hazards these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes however the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence and can ultimately result in severe hazards in downstream areas examination of the volcaniclastic successions of non marine pliocene holocene sedimentary basins in japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material typical volcaniclastic beds comprise centimetre to decimetre thick primary pyroclastic fall deposits overlain by metre to 10s of metres thick resedimented volcaniclastic deposits intercalated in sedimentary successions of non volcanic provenance the relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that 1 potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone and 2 the majority of resedimented material was likely derived from erosion of non welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes the nature distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of 1 proximity to ignimbrite but not directly with the distance to the eruptive centre 2 ignimbrite nature non welded or welded and volume 3 temporal changes in sediment flux from the source area 4 the physiography and drainage patterns of the source area and the receiving basin and any intervening areas and 5 the formation of ephemeral dam lakes and intra caldera lakes whose potential catastrophic failure can impact distal areas models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation

fluvial sediment transport and deposition following the 1991 eruption of mount pinatubo the 1991 eruption of mount pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows bedload sampling in the pasig potrero river one of the most heavily impacted rivers revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth fine grained bed dimensionless bedload transport rates in the pasig potrero river differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments the similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply in arid rivers this is attributed to a flashy hydrograph whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery this work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced d 2002 elsevier science b v all rights reserved

u pb dating of zircon in subsurface hydrothermally altered pyroclastic deposits and implications for subsidence in a magmatically active rift taupo volcanic zone new zealand abstract recognising and correlating hydrothermally altered rock units within buried volcanic sequences in the taupo volcanic zone tvz in new zealand is difficult this is because of broad similarities in the lithologies of many major ignimbrite units and the destruction by hydrothermal alteration of distinctive chemical and mineralogical characteristics however magmatic zircons are commonly present are highly resistant to hydrothermal alteration and yield crystallisation ages in intensely altered rocks crystallisation age spectra have been obtained by sims techniques shrimp rg on zircons extracted from cores from altered ignimbrites penetrated by drillholes at the waiotapu te kopia and orakei korako geothermal fields in the central tvz at waiotapu the thick up to 350 m densely welded waiotapu ignimbrite returned a zircon age spectrum with a probability density function pdf peak of 0 79 ma consistent with an eruption age from 40ar 39ar techniques of 0 71 0 06 1 s d ma three older ignimbrite sheets yielded age spectra that were consistent stratigraphically the shallowest of the three yielded sparse zircons that gave a pdf peak of 1 24 ma and it may correlate with the 1 18 0 02 ma ahuroa ignimbrite the middle sheet although 220 m thick yielded an age spectrum identical to that obtained from pumice in the widespread 1 21 0 04 ma ongatiti ignimbrite extending earlier estimates of the likely volume of this large deposit the deepest sheet has a spectrum consistent with an eruption age of 1 45 0 05 ma it has no surficial correlative but its likely coeruptive ash forms part of a concentrated group of primary or secondary tephra in sediments on the ocean floor east of new zealand and in sedimentary basins across the north island these three ignimbrites were previously correlated with either major ignimbrites exposed on the paeroa fault scarp 10 km to the west or the akatarewa ignimbrite that occurs in drillholes at te kopia and orakei korako but these correlations are disproved from our age data drillholes at te kopia tk and orakei korako ok reach but do not penetrate the base of one or more deposits collectively termed the akatarewa ignimbrite samples from 62 tk and 165 m ok below the top contact of this unit yielded closely similar age spectra and pdf maxima of 1 00 tk and 1 02 ma ok implying that the same deposit was penetrated at both fields this ignimbrite is not correlated with a specific surface equivalent several candidate deposits exist but all are areally restricted the akatarewa ignimbrite at tk and ok is however not the same deposit as those earlier correlated with it at waiotapu the age data are consistent with there being two zones of subsidence on both sides of a horst aligned with the three geothermal fields sampled the eastern zone is the reporoa basin which has high heat flow but little surface faulting and the western one is the axis of the taupo rift currently with low heat flow but abundant surface faulting overall subsidence rates at the three geothermal fields studied vary between 2 and

sediment yield following severe volcanic disturbance a two decade perspective from mount st helens explosive volcanic eruptions perturb water and sediment fluxes in watersheds consequently posteruption sediment yields can exceed preeruption yields by several orders of magnitude annual suspended sediment yields following the catastrophic 1980 mount st helens eruption were as much as 500 times greater than typical background level and they generally declined nonlinearly for more than a decade although sediment yields responded primarily to type and degree of disturbance streamflow fluctuations significantly affected sediment yield trends consecutive years 1995 1999 of above average discharge reversed the nonlinear decline and rejuvenated yields to average values measured within a few years of the eruption after 20 yr the average annual suspended sediment yield from the 1980 debris avalanche deposit remains 100 times 10 4 mg megagrams km 2 above typical background level 10 2 mg km 2 within five years of the eruption annual yields from valleys coated by lahar deposits roughly plateaued and average yields remain about 10 times 10 3 mg km 2 above background level yield from a basin devastated solely by a blast pyroclastic current diminished to background level within five years these data demonstrate longterm instability of eruption generated detritus and show that effective mitigation measures must remain functional for decades

modelling of erosion processes associated with rainfall triggered lahars following the 2011 cordon caulle eruption chile abstract in this work we investigate the sheet and rill erosion process as a possible lahar initiation mechanism in two catchments located in villa la angostura argentina southern andes in association with the 2011 eruption of cordon caulle chile a few days after the climactic phase of the eruption several lahar events occurred in villa la angostura where 15 cm of tephra mantled the slopes the physically based model lisem openlisem 2013a b was applied to two small catchments where sheet and rill erosion of tephra deposits was observed model input parameters related to vegetation soil surface and infiltration were obtained from field measurements geotechnical tests and the literature a total of 20 scenarios were simulated using a range of values of rainfall intensity and duration and hydraulic conductivity only 16 of these scenarios resulted in erosion results show that hydraulic conductivity ks and rainfall intensity significantly affect the runoff volume with the smallest ks i e the tephra deposits associated with the largest fraction of fine ash being associated with the largest eroded volumes and the largest discharges of water and sediment the scenarios with the highest hydraulic conductivity did not show erosion for any modelled rainfall intensity nonetheless we conclude that sheet and rill erosion were not the principal mechanism for rain triggered lahars associated with the cordon caulle eruption but mostly shallow landslide and similar erosion mechanisms contributed to lahars in villa la angostura in fact the eroded volumes calculated with lisem are significantly smaller than the volume calculated with shallow landslide models for the same catchments trigrs

surface height adjustments in pyroclastic flow deposits observed at unzen volcano by jers 1 sar interferometry abstract pyroclastic flows from the 1990 1995 eruption of unzen a dacitic volcano in the southwest of japan descended the mountain along a variety of routes causing widespread damage and loss of life interferograms constructed from jers 1 l band synthetic aperture radar sar images show a number of features related to these pyroclastic flows and their secondary effects the most useful interferogram in this respect is based on images acquired on 22 july 1993 and 1 december 1993 and shows the descent paths for pyroclastic flows occurring in four valley systems within this time window as zones of decorrelation caused by the repeated resurfacing the 22 july 1993 sar image was through considerable good fortune acquired only 2 6 days after a major pyroclastic flow had descended into the mizunashi valley so that in the absence of rainfall the fresh 2 m thick deposits were dry when first imaged the largest differential surface height changes observed in the interferogram represent height decreases in the vertical of 12 cm and significantly lie within a small region of the mizunashi valley which was resurfaced by the pyroclastic flow of 19 july 1993 but not subsequently within this small region radar coherence is higher maximum correlation value of 0 75 in a center valley site where ash but relatively few large boulders are present in a qualitative sense the new ash surfaces exhibit higher levels of radar coherence than the older pre 19 july deposits in other unzen valleys visited by pyroclastic flows smaller differential surface height decreases 4 cm are observed where the surface deposits were emplaced by events taking place between 1 3 months before the acquisition date of the 22 july 1993 image the extra 8 cm of surface height decrease observed in the case of the freshly laid mizunashi deposits must result from a deflationary mechanism or mechanisms operating in a spatially uniform manner in order for radar coherence to be maintained a number of possible causative mechanisms including hydroconsolidation initiated by the seepage of rainwater into a newly deposited ash sheet deflation on gas release basal erosion settlement and substrate relaxation are discussed here given the relatively low emplacement temperatures of the unzen ash 300 500 c it is unlikely that viscous deformation within the body of the fresh ash deposits is responsible for the subsidence comparison of the 22 july 1993 1 december 1993 interferogram with a landsat 4 thematic mapper image of unzen acquired on 10 january 1994 highlights a ribbon like zone of radar decorrelation running along the course of the mizunashi valley which marks the path of debris flows that occurred within the time interval between the two sar images used in the construction of the interferogram overall these results demonstrate the utility of interferometric sar derived surface height and coherence measurements for monitoring the pyroclastic and debris flow regimes of active volcanoes

evidence from zircon u pb age spectra for crustal structure and felsic magma genesis at taupo volcano new zealand u pb zircon age spectra from a metasedimentary xenolith and a young dacite provide evidence for rapid episodes of crustal melting and eruption at taupo volcano as well as unexpected crustal structure beneath the taupo volcanic zone ages from the xenolith in a 28 ka rhyolite dome match those from early cretaceous pahau terrane graywackes that crop out 75 km east of the volcano and were underthrust and partially melted during the penecontemporaneous accretion process boundaries between graywacke terranes in the north island are thus more complex than implied from map views in contrast ages from zircons in the 20 ka omega dacite match the nearby surficial graywacke of the jurassic kaweka terrane 55 of grains quaternary plutonic sources 39 and pahau terrane graywacke 6 five of 72 grains analyzed contain graywacke cores and pleistocene rims survival of euhedral xenocrystic zircons through the generation processes for omega dacite magma reflects wholesale melting of crustal protoliths including varieties not previously considered for petrogenetic modeling open system behavior and crustal melts being generated and erupted within 1 10 yr

acute sedimentation response to rainfall following the explosive phase of the 2008 2009 eruption of chaiten volcano chile the 10 day explosive phase at the start of the 2008 2009 eruption of chaiten volcano in southern chile 42 83 s 72 65 w blanketed the steep rain forest cloaked 77 km2 chaiten river drainage basin with 3 to 100 cm of tephra predominantly fine to extremely fine rhyolitic ash fell during the latter half of the explosive phase rain falling on this ash blanket within days of cessation of major explosive activity generated a hyperconcentrated flow lahar followed closely by a complex multi day muddy flood streamflow bordering on dilute hyperconcentrated flow sediment mobilized in this lahar flood event filled the chaiten river channel with up to 7 m of sediment buried the town of chaiten 10 km downstream of the volcano in up to 3 m of sediment and caused the lower 3 km of the channel to avulse through the town although neither the nature nor rate of the sedimentation response is unprecedented they are unusual in several ways 1 nearly 70 percent of the aggradation almost 5 m in the 50 70 m wide chaiten river channel was caused by a lahar triggered by an estimated 20 mm of rainfall over a span of about 24 h an additional 2 m of aggradation occurred in the next 24 36 h 2 direct damage to the town was accomplished by the sediment laden water flood phase of the lahar flood event not the lahar phase 3 the volume of sediment eroded from hillslopes and delivered to the chaiten river channel was at least 3 8 106 m3 roughly 15 40 of the minimum tephra volume that mantled the chaiten river drainage basin 4 the acute sedimentation response to rainfall appears to have been due to the thickness and fineness of the ash blanket inhibiting infiltration of rain and the steepness of the basin s hillslopes other possible factors such as the prior formation of an ash crust development of a hydrophobic surface layer or large scale destruction of rain intercepting vegetation did not play a role

unroofing of fore arc ranges along the hikurangi margin new zealand constraints from low temperature thermochronology abstract the axial ranges of north island new zealand parallel the hikurangi subduction margin they consist of uplifted and exhumed mesozoic meta sedimentary basement rocks of the overriding australian plate beneath which the pacific plate has been subducting since at least the late oligocene we investigate the unroofing histories of these fore arc mountains during the evolution of the hikurangi margin based on new and previous zircon and apatite fission track and apatite u th sm he data which we interpret based on inverse modelling the results suggest that the exhumation of rocks in the axial ranges initiated in the west and migrated trench wards towards the east onsets of accelerated exhumation in different parts of the ranges indicate significant eastwards thrusting on the margin parallel ngamatea fault before 20 17 ma and on the wellington mohaka fault before 10 7 ma the exhumation rate has varied significantly along strike since the late miocene lower in the central part of the axial ranges and significantly higher to the south since the late miocene the increasing exhumation rate from the central to southern axial ranges is consistent with the clockwise vertical axis rotation of eastern north island relative to the australian plate in the hikurangi margin although underplating of subducted material at the basal upper plate may have contributed to localised rock uplift e g in the raukumara range we suggest that the shortening of the fore arc upper plate was the chief driver of unroofing of the proto axial ranges

late cenozoic exhumation model of new zealand impacts from tectonics and climate abstract late cenozoic convergence between the australian and pacific plates has led to extensive crustal deformation and exhumation in new zealand over the past five decades the convergence associated exhumation history of the palaeozoic mesozoic new zealand basement has been studied using various thermochronometric techniques here we compile 992 published ages and invert them to estimate the temporal and spatial variation in exhumation rates since 25 ma which we then interpret with a plate tectonic reconstruction model the results reveal widespread increases in the exhumation rates at 6 4 ma a time correlated with accelerations in the relative plate motion the absolute increases in exhumation rates are however not uniformly distributed over new zealand since the plio pleistocene the inferred models suggest variable response of exhumation to global cooling in the non glaciated eastern north island the hikurangi subduction margin the million year average exhumation rates have remained relatively low 5 km myr have been observed in the collision zone of the central southern alps of south island where both tectonic deformation and precipitation rates are highest in fiordland the puysegur subduction margin despite the weak crustal deformation the exhumation rates have been high 0 5 3 km myr which we suggest is due to intensified glacial erosion our findings highlight the determining role of tectonics in setting up the regional exhumation pattern but both strong tectonic deformation and efficient erosion are required to maintain rapid exhumation rates the influence of climate on mountain erosion can only be distinguished in regions under slow to mediate exhumation

apatite u th he thermochronometry using a radiation damage accumulation and annealing model helium diffusion from apatite is a sensitive function of the volume fraction of radiation damage to the crystal a quantity that varies over the lifetime of the apatite using recently published laboratory data we develop and investigate a new kinetic model the radiation damage accumulation and annealing model rdaam that adopts the effective fission track density as a proxy for accumulated radiation damage this proxy incorporates creation of crystal damage proportional to production from u and th decay and the elimination of that damage governed by the kinetics of fission track annealing the rdaam is a version of the helium trapping model hetm shuster d l flowers r m and farley k a 2006 the influence of natural radiation damage on helium diffusion kinetics in apatite earth planet sci lett 249 148 161 calibrated by helium diffusion data in natural and partially annealed apatites the chief limitation of the hetm now addressed by rdaam is its use of he concentration as the radiation damage proxy for circumstances in which radiation damage and he are not accumulated and lost proportionately from the crystal by incorporating the rdaam into the hefty computer program we explore its implications for apatite u th he thermochronometry we show how u th he dates predicted from the model are sensitive to both effective u concentration eu and details of the temperature history the rdaam predicts an effective he closure temperature of 62 c for a 28 ppm eu apatite of 60 m radius that experienced a 10 c ma monotonic cooling rate this is 8 c lower than the 70 c effective closure temperature predicted using commonly assumed durango diffusion kinetics use of the rdaam is most important for accurate interpretation of u th he data for apatite suites that experienced moderate to slow monotonic cooling 1 0 1 c ma prolonged residence in the helium partial retention zone or a duration at temperatures appropriate for radiation damage accumulation followed by reheating and partial helium loss under common circumstances the rdaam predicts u th he dates that are older sometimes much older than corresponding fission track dates nonlinear positive correlations between apatite u th he date and eu in apatites subjected to the same temperature history are a diagnostic signature of the rdaam for many but not all thermal histories observed date eu correlations in four different localities can be explained with the rdaam using geologically reasonable thermal histories consistent with independent fission track datasets the existence of date eu correlations not only supports a radiation damage based kinetic model but can significantly limit the range of acceptable time temperature paths that account for the data in contrast these datasets are inexplicable using the durango diffusion model the rdaam helps reconcile enigmatic data in which apatite u th he dates are older than expected using the durango model when compared with thermal histories based on apatite fission track data or other geological constraints it also has the potential to explain at least some cases in which u th he dates are actually older than the corresponding fission track dates

natural age dispersion arising from the analysis of broken crystals part i theoretical basis and implications for the apatite u th he thermochronometer abstract over the last decade major progress has been made in developing both the theoretical and practical aspects of apatite u th he thermochronometry and it is now standard practice and generally seen as best practice to analyse single grain aliquots these individual prismatic crystals are often broken and are fragments of larger crystals that have broken during mineral separation along the weak basal cleavage in apatite this is clearly indicated by the common occurrence of only 1 or no clear crystal terminations present on separated apatite grains and evidence of freshly broken ends when grains are viewed using a scanning electron microscope this matters because if the 4 he distribution within the whole grain is not homogeneous because of partial loss due to thermal diffusion for example then the fragments will all yield ages different from each other and from the whole grain age here we use a numerical model with a finite cylinder geometry to approximate 4 he ingrowth and thermal diffusion within hexagonal prismatic apatite crystals this is used to quantify the amount and patterns of inherent natural age dispersion that arises from analysing broken crystals a series of systematic numerical experiments were conducted to explore and quantify the pattern and behaviour of this source of dispersion using a set of 5 simple thermal histories that represent a range of plausible geological scenarios in addition some more complex numerical experiments were run to investigate the pattern and behaviour of grain dispersion seen in several real data sets the results indicate that natural dispersion of a set of single fragment ages defined as the range divided by the mean arising from fragmentation alone varies from c 7 even for rapid c 10 c ma monotonic cooling to over 50 for protracted complex histories that cause significant diffusional loss of 4 he the magnitude of dispersion arising from fragmentation scales with the grain cylindrical radius and is of a similar magnitude to dispersion expected from differences in absolute grain size alone spherical equivalent radii of 40 150 m this source of dispersion is significant compared with typical analytical uncertainties on individual grain analyses c 6 and standard deviations on multiple grain analyses from a single sample c 10 20 where there is a significant difference in the u and th concentration of individual grains eu the effect of radiation damage accumulation on 4 he diffusivity assessed using the rdaam model of flowers et al 2009 is the primary cause of dispersion for samples that have experienced a protracted thermal history and can cause dispersion in excess of 100 for realistic ranges of eu concentration i e 5 100 ppm expected natural dispersion arising from the combined effects of reasonable variations in grain size radii 40 125 m eu concentration 5 150 ppm and fragmentation would typically exceed 100 for complex thermal histories in addition to adding a significant component of natural dispersion to analyses the effect of fragmentation also acts to decouple and corrupt expected correlations between grain ages and absolute grain size and to a lesser extent between grain age and effective uranium concentration eu considering fragmentation explicitly as a source of dispersion and analysing how the different sources of natural dispersion all interact with each other provides a quantitative framework for understanding patterns of dispersion that otherwise appear chaotic an important outcome of these numerical experiments is that they demonstrate that the pattern of age dispersion arising from fragmentation mimics the pattern of 4 he distribution within the whole grains thus providing an important source of information about the thermal history of the sample we suggest that if the primary focus of a study is to extract the thermal history information from u th he analyses then sampling and analytical strategies should aim to maximise the natural dispersion of grain ages not minimise it and should aim to analyse circa 20 30 grains from each sample the key observations and conclusions drawn here are directly applicable to other thermochronometers such as the apatite rutile and titanite u pb systems where the diffusion domain is approximated by the physical grain size

the immediate environmental effects of tephra emission the earth s history is punctuated by large explosive eruptions that eject large quantities of magma and silicate rock fragments into the atmosphere these tephra particles can sometimes be dispersed across millions of square kilometres or even entire continents the interaction of tephra with or in receiving environments may induce an array of physical chemical and biological effects the consequences for affected systems and any dependent communities may be chronic and localised in the event of frequent small eruptions while larger and rarer events may have acute regional scale impacts it is therefore necessary to document the range of possible impacts that tephra may induce in receiving environments and any resulting effects in interconnected systems we collate results from many studies to offer a detailed multidisciplinary and interdisciplinary review of the immediate post eruptive effects of tephra emission into the atmosphere onto vegetation soil or ice snow surfaces and in aquatic systems we further consider the repercussions that may be induced in the weeks to years afterwards in the atmosphere tephra can influence cloud properties and air chemistry by acting as ice nuclei in or by offering sites for heterogeneous reactions respectively tephra on vegetation causes physical damage and sustained coverage may elicit longer term physiological responses tephra deposits on soils may alter their capacity to exchange gas water and heat with the atmosphere or may have a specific chemical effect such as nutrient input or acidification on sensitive soils tephra deposition onto snow or ice may affect ablation rates rivers and lakes may experience turbidity increases and changes in their morphology as a result of fallout and prolonged months or years erosion from the tephra covered catchment in the first weeks after deposition tephra leaching may affect river chemistry the abundance and speciation of phytoplankton populations in lakes may be altered by tephra induced changes in water chemistry or sediment water nutrient cycling in the oceans tephra deposition may fertilise fe limited waters with potential impacts on the global carbon cycle embracing the full complexity of environmental effects caused by tephra fall demands a renewed investigative effort drawing on interdisciplinary field and laboratory studies combined with consideration of the interconnectivity of induced impacts within and between different receiving environments

tephra stratigraphy and eruptive volume of the may 2008 chaiten eruption chile on may 1st 2008 mount chaiten southern chile interrupted a long period of quiescence generating a sequence of explosive eruptions and causing the evacuation of chaiten town located a few kilometers south of the volcano the activity was characterized by several explosive events each associated with plumes which reached up to about 19 km above sea level the products were dispersed across a wide area with the finest ash reaching the atlantic coast of argentina our field observations in the proximal medial area 3 25 km from the vent indicate that the may 2008 tephra deposit consists of numerous layers most of which can be correlated with individual eruptive events these layers vary from extremely fine grained ash to layers of lapilli and blocks composed of both juvenile and lithic material here we describe the stratigraphy and physical characteristics of the may 2008 deposits and propose a reconstruction of the timing of the may 2008 events the deposits are mainly associated with the three main explosive phases which occurred on 1st 2nd may 3rd 5th may and 6th may with an estimated bulk tephra volume of 0 5 1 0 km3 integration of both exponential and power law fitting for the 6th may event represented by a layer composed mainly of lithic lapilli and blocks 2 mm an isopleth map was compiled from which a 19 km plume height was determined which is in good agreement with satellite observations

sediment routing in a small drainage basin in the blast zone at mount st helens washington u s a abstract a quantitative description of the routing of 400 10 3 t km 2 of sediment delivered to a 2 4 km 2 basin by the may 18 1980 eruption of mount st helens has been developed for the period may 1980 june 1981 on hillslopes this sediment resided in primary and secondary remobilized primary storage sites hillslope erosion processes redistributed sediment on hillslopes and delivered sediment to the channel system these processes included shallow sliding from slopes steeper than 0 70 sheet erosion and rill erosion the most important discrete sites of storage on hillslopes were colluvial wedges and sites upslope of logs or other organic debris most tephra delivered to the basin by the eruptions is likely to be preserved in the stratigraphic record twelve percent of the sediment delivered to the study area was transported to the channel system directly by the eruption and by hillslope erosion processes of this approximately half remained in storage in the channel system and the remainder was exported from the drainage basin important sites of alluvial sediment storage included alluvial fans sites related to logs channel beds and alluvial terraces

cretaceous termination of subduction at the zealandia margin of gondwana the view from the paleo trench abstract during the permian to cretaceous zealandia occupied a position on the proto pacific facing convergent margin of gondwana subduction on this margin ceased somewhere between 105 ma and perhaps 70 ma but the timing of this tectonic transition remains controversial resolution of this uncertainty is important for tectonic reconstructions of the southwest pacific and for global plate tectonic models here we revisit the problem by reference to new stratigraphic and geochemical data from the east coast basin of new zealand which occupied a position adjacent to and possibly superimposed on the relict cretaceous subduction trench at the time subduction ceased this basin is expected to preserve direct structural and stratigraphic evidence for or against late cretaceous subduction in western parts of the east coast basin the western sub belt a conspicuous unconformity separates undoubted accretionary prism rocks of the torlesse composite terrane from younger cretaceous cover units strata beneath and overlying this unconformity vary in age from place to place but abundant paleontological data and published ages of detrital zircons some reinterpreted herein indicate that exposed torlesse rocks are nowhere younger than 100 ma overlying zealandia megasequence cover strata are mostly younger than 110 ma between 110 and 85 ma the entire length of the western sub belt reveals complex stratigraphic architecture of relatively small scale depocentres subjected to alternating episodes of subsidence and local uplift and erosion there is widespread evidence for compression on the western sub belt over this period although the overall amount of shortening is likely to be relatively modest in contrast the eastern sub belt preserves a record of near continuous and apparently simple deposition over the same interval of time we see little and somewhat equivocal evidence for extension in either sub belt through the late cretaceous superimposed on this general pattern we observe discrete widespread or basin wide tectonic events at 95 ma and within the intervals 86 83 ma and 83 81 ma indicated by the presence of unconformities and sedimentary facies changes importantly all these events can be correlated with coeval events recorded elsewhere in zealandia suggesting that the east coast basin was subject to the same tectonic regime as the rest of zealandia and shared a common history during the mid to late cretaceous integrating these findings with data from elsewhere in zealandia we argue that there is diverse evidence to indicate that subduction finished along the new zealand segment of the gondwana margin by 100 ma that said the causes of on going late cretaceous compression in the east coast basin and in other parts of zealandia are not well constrained although several possible explanations are plausible there are few analogue abandoned subduction margins described in the literature however the situation in the south shetland trench shows strong similarities with patterns and complexities observed in the east coast basin

probabilistic analysis of rain triggered lahar initiation at tungurahua volcano semi continuous production of pyroclastic material by intermittent strombolian vulcanian and sub plinian eruptions at volcan tungurahua ecuador has created a persistent rain triggered lahar hazard during the 1999 present eruptive episode lahars threaten the city of banos which lies approximately 8 km from the crater as well as other villages and vital infrastructure situated in close proximity to the dense radial drainage network of the volcano this study analyses the initiation of rain triggered lahars and the influence of antecedent rainfall on this process in two northern instrumented drainages la pampa and the vazcun analysis of lahar triggering rainfall intensity and duration between march 2012 and june 2013 yields a power law relationship whilst receiver operating characteristic roc analysis indicates that peak rainfall intensity 10 30 and 60 min is the most effective single predictor of lahar occurrence the probability of a lahar exceeding a pre defined magnitude increases with peak rainfall intensity incorporation of antecedent rainfall 24 h and 3 5 and 7 days as a secondary variable significantly impacts lahar probabilities particularly during moderate high intensity rainfall events the resultant two and three dimensional lahar probability matrices are applied to rainfall data between 1st july and 31st december 2013 with the aim of predicting lahar occurrence composite lahar indicators comprised from the mean lahar probability estimates of individual matrices are shown to perform this task most effectively roc analysis indicates a probability 80 that these composite indicators will generate a higher estimated lahar probability for a randomly selected lahar event than a randomly selected non lahar event this method provides an average of 24 min of additional warning time compared with the current acoustic flow monitors afms used for lahar detection effectively doubling warning times for key downstream infrastructure in the two drainages ultimately this method of lahar analysis could be used to construct real time probabilistic rain triggered lahar forecasts as an aid to current lahar hazard mitigation techniques at any location with a significant rain triggered lahar hazard and a basic instrumental setup

the de ploey erosional susceptibility model for catchments es abstract this article is based on an unpublished manuscript by the late prof dr jan de ploey it focusses on the erosional susceptibility of catchments in terms of total energy input by the meteorological factors water and wind e s model the form and the universal character of the basic expression of the erosional susceptibility are explained applications of the model are illustrated for different processes such as landsliding and debris flows gullying creep and rill interrill combinations characteristic e s values exist for these processes also e s values vary in regions with differing pedo botanical characteristics and land use the difference between e s values for single storm events and longer periods are related to the magnitude frequency distribution of erosive activities and allow in many cases an assessment of the age of erosion phenomena and an estimation of the recurrence period of extreme single events

landward and seaward directed thrusting accompanying the onset of subduction beneath new zealand a short 5 m y episode of deformation accompanied the onset of the present phase of subduction beneath northern and eastern new zealand in the earliest miocene at many places along the continental margin the structures produced were dominated by subhorizontal thrust sheets with tens of kilometres of displacement thrusting was directed both landward with or without involvement of the upper part of the downgoing plate and seaward the structures are different in style from those typical of active trench slopes which are predominantly seaward directed imbricate thrusts and associated folds the structures show that delamination of the downgoing plate occurred at some places and suggest that a high degree of coupling between upper and lower plates occurred at others

fission track analysis of the late cenozoic vertical kinematics of continental pacific crust south island new zealand during the late cenozoic the pacific plate has been converging obliquely with the australia plate in south island new zealand a result of this convergence has been the growth of a major mountain range the southern alps at the leading edge of the pacific plate the results of fission track analysis of 140 samples from 13 transects across the alps reported here establish the late cenozoic vertical kinematics amount age and rate of rock uplift of the pacific crust underlying the alps the late cenozoic rock uplift of the pacific crust is asymmetrical with respect to the alpine fault being a maximum 19 km immediately east of the central part of the fault with lesser values at the eastern 3 km northern 10 km and southern 8 km extremities of the alps the age of the start of rock uplift varies spatially across the southern alps the earliest indications from fission track analysis being at 8 ma at the southern end of the alps decreasing to 5 ma at the northern end and 3 ma along the southeastern margin this age variation reflects the longer time over which the southern parts of the alps have been in collision the rate of propagation of rock uplift southeastward into the pacific plate has been 30 mm yr nearly 4 times the late cenozoic average rate of convergence normal to the plate boundary late cenozoic mean rock uplift rates range from a maximum of 2 8 mm yr at the alpine fault to a minimum of 1 0 mm yr in the east and have been sustained for periods of 3 8 m y accompanying denudation has exhumed amphibolite grade rocks immediately east of the alpine fault the rock uplift has been controlled by oblique slip displacement on the alpine fault a continental crustal section at least 19 km thick has been uplifted on the alpine fault comparison of the late cenozoic mean rock uplift rates with uplift rates derived from reset zircon data 2 10 mm yr near the alpine fault shows that uplift has accelerated over time but only significantly since 1 3 0 3 ma the amount of mesozoic uplift ranged from minimal amounts north of arthur s pass to 3 km near mount cook to 10 km in the south at lake wanaka

rate of sediment yield following small scale volcanic eruptions a quantitative assessment at the merapi and semeru stratovolcanoes java indonesia sediment yields were calculated on the anks of merapi and semeru volcanoes in java indonesia using two different methods during the rst year following the 22 november 1994 eruption of merapi a sediment yield in excess of 1 5 105 m3 km 2 yr 1 was calculated in the boyong river drainage basin based on the volumes of sediment that were trapped by ve check dams at semeru sediment discharges were assessed in the curah lengkong river from direct measurements on the lahars in motion and on the most signi cant stream ows the calculated rate of sediment yield during one year of data in 2000 was 2 7 105 m3 km 2 yr 1 sediment yields are dominated by rain triggered lahars which occur every rainy season in several drainage basins of merapi and semeru volcanoes mostly during the rainy season extending from october to april the return period of lahars carrying sediment in excess of 5 105 m3 is about one year in the curah lengkong river at semeru at merapi the volume of sediments transported by a lahar did not exceed 2 8 105 m3 in the boyong river during the rainy season 1994 95 on both volcanoes the sediments are derived from similar sources pyroclastic ow surges deposits rockfalls from the lava domes and old material from the riverbed and banks however daily explosions of vulcanian type at semeru provide a more continuous sediment supply than at merapi therefore sediment yields are larger at semeru copyright 2004 john wiley sons ltd

platform subsidence behind an active subduction zone numerical models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones1 4 this platform subsidence is predicted to cause an asymmetrical depression 500 1 000 km wide with a maximum depth of 2 km if filled with water direct evidence linking platform subsidence to mantle flow has remained elusive however here we offer evidence from western new zealand for a complete link in both space and time between subduction initiation and platform subsidence because of the broad wavelength 200 km of the observed 1 500 m subsidence it is unlikely that the subsidence is due to a flexural foredeep the observed subsidence does however match in both wavelength and amplitude that predicted by a model of subduction induced mantle flow the occurrence of platform subsidence has implications not only for understanding mantle convection but also for explaining geoid anomalies over active margins5 and patterns of continental submergence6 and sedimentation7 that could otherwise be interpreted as a eustatic sea level change8

rain triggered lahar susceptibility using a shallow landslide and surface erosion model abstract lahars are mass flows containing variable concentrations of water and volcanic debris that can cause catastrophic impacts to life livelihoods and infrastructure downstream from their volcanic origin accurate and quantitative information on lahar hazards are essential for reducing the impact of these events lahar hazard assessments often focus on the use of numeric or empirical models to describe flow behaviour and inundation areas which rely on historic lahar events and expert elicitation to define model inputs this results in qualitative or semi quantitative estimates of hazard that do not account for the mechanics of lahar initiation or in the case of rain triggered lahars the dependence of rainfall intensity and duration on initiation here we develop a method for calculating rain triggered lahar susceptibility defined as the occurrence probability of a particular lahar initial volume at a specific location the model relies on terrain and deposit characteristics and a probabilistic measure of rainfall in the form of rainfall intensity frequency duration relationships results for a case study of the october 28 1995 lahar at mangatoetoenui stream ruapehu volcano new zealand indicate lahar volume is controlled by a characteristic timescale relating the deposit depth h to the hydraulic diffusivity d 0 in the ratio h 2 d 0 the timescale describes the transmission of positive pore pressures within the deposit leading to shallow failure as a consequence of this timescale rainfall duration is the most important factor determining initial lahar sediment volume rainfall intensity plays a minor role controlling the volume of water in the lahar mixture this observation is consistent with power law relationships used to determine lahar triggering rainfall thresholds the rain triggered lahar susceptibility approach developed here is anticipated to improve probabilistic lahar hazard assessments by providing quantitative reproducible estimates of initial lahar volumes

extraordinary sediment delivery and rapid geomorphic response following the 2008 2009 eruption of chaiten volcano chile u s geological survey volcano science center sernageomin s programa de riesgos volcanicos conicyt fondecyt grants 1110609 1141064 11130671 conicyt fondap 15090013 vamos research centre

exhumation of basement cored uplifts example of the kyrgyz range quantified with apatite fission track thermochronology 1 the kyrgyz range the northernmost portion of the kyrgyzstan tien shan displays topographic evidence for lateral propagation of surface uplift and exhumation the highest most deeply dissected segment lies in the center of the range to the east topography and relief decrease and preserved remnants of a cretaceous regional erosion surface imply minimal amounts of bedrock exhumation the timing of exhumation of range segments defines the lateral propagation rate of the range bounding reverse fault and quantifies the time and erosion depth needed to transform a mountain range from a juvenile to a mature morphology new multicompositional apatite fission track aft data from three transects from the eastern kyrgyz range combined with published aft data demonstrate that the range has propagated over 110 km eastward over the last 7 11 myr on the basis of the thermal and topographic evolutionary history we present a model for a time varying exhumation rate driven by rock uplift and changes in erodability and the timescale of geomorphic adjustment to surface uplift easily eroded cenozoic sedimentary rocks overlying resistant basement control early rapid exhumation and exhibit slow surface uplift rates as increasing amounts of resistant basement are exposed exhumation rates decrease while surface uplift rates are sustained or increase thereby growing topography as the range becomes high enough to cause ice accumulation and to develop steep river valleys fluvial and glacial erosion becomes more powerful and exhumation rates once again increase independently determined range normal shortening rates also varied over time suggesting a feedback between erosional efficiency and shortening rate

geological constraints on the eruption of the jwaneng centre kimberlite pipe botswana abstract geological mapping has allowed constraints to be placed on the eruption mechanisms involved in the formation of the late permian early triassic jwaneng centre kimberlite pipe botswana twelve lithofacies and three lithofacies associations lfa 1 3 are recognised lfa 1 comprises massive to bedded volcaniclastic kimberlite and marginal shale breccias and outcrops over 65 of the surface area of the pipe it is characterised by a lithic population dominated by transvaal shale clasts lfa 1 grades into lfa 2 which comprises massive and bedded volcaniclastic kimberlite and volcaniclastic breccias and outcrops over 19 of the surface area of the pipe the lithic population of lfa 2 is dominated by contorted and fluidal outlined karoo age mudstones and siltstones lfa 3 comprises a wedge shaped unit in the north of the pipe and consists of a series of allochthonous megablocks of sedimentary and volcaniclastic strata the juvenile clast type and matrix mineral assemblages of the volcaniclastic deposits in the centre pipe differ from those in many other southern african kimberlite pipes various emplacement models for kimberlite pipes are discussed and evaluated in the light of the new geological data both a maar diatreme model and an explosive volatile driven eruption model could account for much of the geology of the centre pipe and distinguishing between the two models based on deposits alone is difficult there is strong circumstantial evidence for ambient conditions favourable to phreatomagmatism at the time of the eruption and the influence of external water may explain the differences between the jwaneng centre pipe and the class 1 kimberlites common across southern africa in terms of both the juvenile clasts and of the inter clast cement however low abundances of some types of lithic inclusions derived from major country rock units pose an unresolved problem for a classic maar diatreme model of pipe formation

late holocene changes in erosion patterns in a lacustrine environment landscape stabilization by volcanic activity versus human activity the most recent eruption of mount fuji japan the vei 5 hoei plinian eruption ce 1707 heavily impacted lake yamanaka a shallow lake located at the foot of mount fuji here we discuss the influence of the hoei eruption on the lacustrine sedimentation of lake yamanaka using high resolution geophysical and geochemical measurements on gravity cores hoei scoria fallout had two major impacts on lake yamanaka i reduction of the sedimentation rate from similar to 0 16 to similar to 0 09 cm year and ii the increase of in situ lake productivity sedimentation rates after the eruption were relatively low due to the thick scoria layer trapping underlying sediments in the catchment the lacustrine system took more than similar to 170 years to begin to recover from the hoei eruption most recently sedimentation recovery has been accelerated by changes in land use since the beginning of the twentieth century vegetated strips delimited cultivated parcels trapping sediment and minimizing anthropogenic impacts on the sedimentation rate over the last decade the decline of agriculture and the increase of other human activities has led to an increase in the sedimentation rate similar to 1 cm year this study highlights the effect of the grainsize of the volcanic ejecta on the sedimentation rate following a volcanic eruption coarse grained tephra are difficult to erode therefore their erosion and remobilization is largely limited to intense typhoons when porous scoria deposits are saturated by heavy rains moreover this study suggests that recent anthropogenic modifications of the catchment had a greater impact on the sedimentation rate than the hoei eruption

tracking crustal processes by ft thermochronology in a forearc high hikurangi margin new zealand involving cretaceous subduction termination and mid cenozoic subduction initiation abstract raukumara peninsula is a forearc high on the pacific margin of new zealand its basement comprises two sedimentary terranes that formed in subduction prisms during the late jurassic mid cretaceous marking the end of subduction along the pacific margin of eastern gondwanaland the cover includes allochthonous late cretaceous oligocene passive margin slope to basin facies emplaced into the forearc during the early miocene initiation of the current subduction regime as well as autochthonous neogene sediments apatite and zircon fission track ft data are reported and interpreted for the basement and cover sediments the detrital zircon ft ages constrain the maximum depositional ages of the low grade zeolite and pumpellyite facies basement pahau terrane is no older than 152 13 ma this zircon ft mode and older ones 198 8 ma 232 58 ma recording cooling denudation in the source areas probably including rakaia terrane waioeka terrane located oceanward of pahau terrane is no older than 125 2 ma being the age of volcanic derived zircons and supports early cretaceous dinoflagellate ages reported previously omaio petrofacies comprises the youngest part of the waioeka terrane and has a zircon ft age of 108 6 3 ma this indicates that basement sequences accumulated in the northeast contemporaneously with cover sequences koranga fm in the southwestern part of the peninsula these zircon ft ages together with the results of thermal history modelling of certain waioeka terrane samples are consistent with migration of a thrust system in a subduction prism environment during the late jurassic cretaceous until about 85 ma as proposed earlier when mesozoic subduction terminated apatite ft data indicate for all basement samples a phase of neogene heating and cooling associated with basin formation sediment accumulation and subsequent uplift and erosion resulting in almost complete removal of the neogene stratigraphic record over western parts of the peninsula forward modelling of the data imply rapid earliest miocene downwarping of the crust in bay of plenty coinciding with initiation of subduction of pacific plate followed by emplacement of a 1 5 2 0 km thick wedge of east coast allochthon from the northeast early miocene sediments accumulated in a foredeep along the front sw margin of the allochthon probably sourced from a block of pahau basement west of whakatane fault which was exhumed and denuded by 4 km contemporaneously during the late early to middle miocene the basin depocentre migrated further to the southwest while areas previously downwarped northeast were uplifted and eroded the depocentre migration and subsequent inversion is a pattern that affected most of north island and is related to mechanical or dynamical interaction of the two plates as the slab of pacific oceanic lithosphere was emplaced beneath north island this may be a general feature of young subduction zones

relations between postglacial fluvial incision rates and uplift rates in the north island new zealand 1 fluvial incision rates are increasingly being used as a proxy for rock uplift rates for inland settings we test the validity of this approach by comparing incision rates and rock uplift rates along the hikurangi margin new zealand postglacial fluvial incision rates were calculated by measuring the difference in altitude between the last glacial maximum lgm terrace tread 18 2 calendar ka and the present river bed for 10 major rivers catchment areas 1400 5900 km2 incision rates have thus been measured over a half glacial cycle they also represent average rates over the last 18 kyr during which time variations are likely due to processes such as knickpoint retreat rock uplift rates have been estimated from 1 altitude differences between the treads of pairs of fluvial fill terraces 55 and 18 calendar ka assumed to have formed at the same altitude i e rock uplift relative to the geoid 2 present day altitudes of late neogene paleoshoreline marker horizons 1 6 5 7 ma i e rock uplift relative to sea level and 3 present day altitudes of neogene mudstones for which maximum depth of burial has been calculated from porosity i e rock uplift relative to sea level these rock uplift rates overestimate tectonic uplift rates because they do not take into account isostatic compensation due to erosion mean incision rates range up to 11 7 mm yr 1 and mean rock uplift rates range up to 4 1 and 0 9 mm yr 1 for the three methods respectively we have made some qualitative and semiquantitative observations about nontectonic controls on postglacial incision rates and find that localized influence only is exerted by rainfall base level and bedrock lithology while negative or no correlations are shown with lgm fluvial fill thickness and stream power incision rates and uplift rates display a rough correlation with incision rates typically 1 5 5 times uplift rates this relation suggests that tectonic uplift is likely to exert an important control on fluvial incision in the hikurangi margin

morphological evolution of the north fork toutle river following the eruption of mount st helens washington abstract the north fork toutle river nftr has undergone extensive morphological changes following the catastrophic eruption of mount st helens washington in 1980 especially the upper reaches affected by a 2 5 km3 debris avalanche deposit caused by the eruption this paper reports analysis and interpretation of vertical adjustments to the thalweg long profile at some 33 km river reaches redeveloped on the debris avalanche deposit during the 30 year period since the eruption the results confirm that adjustments in the upper part of the study reaches have generally been led by degradation while that in the lower reaches have been led by aggradation with the middle reaches acting as a hinge zone trends of change in the thalweg long profile and bedslope reveal that channel gradients have decreased nonlinearly through time and with distance downstream from the volcano values of stream power have decreased with time commensurately owing to reductions in slope and channel widening while the bed has coarsened so that rates of erosion of the debris avalanche deposit in the upper nftr have slowed to the point that the long profile now perched and slightly steeper is relaxing toward a new equilibrium or graded condition thirty year relaxation paths for thalweg elevation were simulated at seven key cross sections using newly developed comprehensive rate law models based on nonlinear decay in rates of morphological response to perturbation the results indicate that both single and multistep rate law models can simulate the observed records consequently the rate law approach provides an effective method for studying and simulating morphological response of the fluvial system to a major instantaneous disturbance such as a volcanic eruption

thermochronology of the torlesse accretionary complex wellington region new zealand the torlesse complex comprises several mesozoic accretionary prism complexes together forming continental basement over large parts of new zealand this study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern north island that crosses the structural grain of the torlesse complex including its older and younger parts zircon fission track ft ages for the late triassic rakaia terrane which is the most inboard of the accretionary complexes are partially annealed some possibly reset and may indicate early cretaceous 134 10 ma cooling from maximum temperatures tmax probably related to imbrication of younger complexes of the pahau terrane numerical modeling of the zircon ft ages and published 40ara 39ar muscovite and biotite ages for the rakaia terrane suggest tmax values of 265 310 c and exhumation from depths of 10 12 km the rocks underlying the aorangi range and involving the youngest accretionary complex have experienced much lower tmax values of 210 and 110 c bracketed by reset apatite ft ages and detrital zircon ft ages the occurrence of a circa 100 ma component of zircon ft ages in both the weakly and highly indurated rocks beneath the aorangi range as well as in remnants of an overlying albian accretionary slope basin whatarangi formation imply multistorey accretion and incorporation of sediment into the youngest prism this circa 100 ma zircon ft age component also places a maximum age on the termination of mesozoic subduction beneath the new zealand region the occurrence of reset apatite ft ages across the whole of the wellington transect indicates that at least 4 km of exhumation occurred during the late miocene

pliocene exhumation and landscape evolution of central north island new zealand the role of the upper mantle 1 mudstone porosity analyses show that over 2 km of exhumation has occurred in central north island new zealand since 5 ma this combined with a coeval surface uplift of 500 m indicates a maximum 2 5 km of rock uplift in central north island the magnitude and wavelength 400 km of the rock uplift is similar to some topographic doming observed in continental areas associated with mantle upwelling yet central north island is recognized as a region of back arc extension marine terraces 100 150 km from the center of maximum exhumation show uplift rates of 0 2 0 3 mm yr 1 consistent with about 1 km of exhumation if averaged over 4 5 myr which compares well with mudstone porosity analyses seismic attenuation and slow pn velocities indicate that a thermal anomaly in the upper mantle is the source of this rock uplift the rock uplift may be explained by the replacement of a 100 km thick mantle lid by asthenosphere with a density 66 kg m 3 less than the lid about half this density contrast can be explained by thermal expansion and the rest is ascribed to partial melts in the upper mantle southward migration for the locus of maximum rock uplift is consistent with the zone of maximum incision erosion being 50 km south of the southernmost volcano such a migration is in keeping with other geological markers of volcanic and thermal phenomena and further strengthens the link of landscape evolution at active continental margins with processes in the mantle

quantitative analysis of climate versus human impact on sediment yield since the lateglacial the sarlieve palaeolake catchment france minimum rates of solid ssy and dissolved dsy sediment yield sy were evaluated in t km2 per yr from sediments stored in the sarlieve palaeolake french massif central for seven phases of the lateglacial and holocene up to the seventeenth century the catchment 29 km2 mainly formed of limestones and marls is located in an area rich in archaeological sites in the massif central the respective impacts of human activities and climate on sy were compared by quantification of human settlements through archaeological survey and palynological data during the lateglacial and early holocene up to about 7500 yr cal bp variations in ssy and dsy rates were mainly related to climate change with higher rates during colder periods younger dryas and preboreal and lower rates during warmer periods bolling allerod and boreal however cf1 tephra fallout induced a sharp increase in sy during the allerod during the middle and late holocene after 7500 yr cal bp ssy and dsy greatly increased by factors of 6 5 and 2 8 respectively particularly during the final neolithic at about 5300 yr cal bp when the climate became cooler and more humid after this date at least 75 of the ssy increase and more than 90 of the dsy increase resulted from human activities but ssy rates showed little variation during protohistoric and historic times up to the seventeenth century ssy and dsy rates and dsy ssy ratio indicate that catchment soils began to form during the lateglacial and preboreal thickened considerably during the boreal and atlantic finally thinning rejuvenation mainly as the result of human induced erosion during the sub boreal and sub atlantic increased mechanical erosion during the late holocene also induced an increase in chemical erosion

a new petrogenetic grid for low grade metabasites abstract we have used internally consistent thermodynamic data to present calculated phase equilibria for the system na2o cao mgo al2o3 sio2 h2o ncmash in the range 0 500 c and 0 1 10 kbar involving the phases anorthite glaucophane grossular heulandite jadeite laumontite lawsonite paragonite prehnite pumpellyite stilbite tremolite wairakite zoisite with excess albite clinochlore quartz and pure water average activity terms derived from published mineral chemical data were included for clinochlore glaucophane prehnite pumpellyite tremolite and zoisite the new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite prehniteactinolite prehnite pumpellyite pumpellyite actinolite blueschist and greenschist facies metabasites the stability fields of mineral assemblages containing prehnite pumpellyite epidote actinolite albite chlorite quartz were analysed in some detail using activity data calculated from five specific samples for example the prehnite actinolite facies covers a p t field ranging from about 220 to 320 c at pressures below 4 5 kbar the transition from the prehnite actinolite and pumpellyite actinolite to greenschist facies occurs at about 250 300 c at 1 3 kbar and at about 250 350 c at 3 8 kbar p t fields of individual facies overlap considerably due to variations in chemical composition

low temperature thermochronology and modeling strategies for multiple samples 1 vertical profiles abstract low temperature thermochronology is a powerful method for constraining the time temperature history of rocks and provides constraints on denudation chronologies landscape evolution and tectonic history of geological terrains we present a strategy for modelling thermal histories constrained by thermochronological data from multiple samples in vertical profiles the thermal history is specified to be similar in form for each sample and we include extra parameters to determine the offset temperature between the uppermost and lowermost samples we combine the likelihood from each sample to produce a joint likelihood for all samples together and using initially stochastic search then directed search methods we try to identify a maximum likelihood solution we also implement a test bayesian information criterion which allows us to assess whether we have overparameterised the thermal history and potentially introduced unwarranted complexity this test allows us to simplify the thermal history model without compromising the acceptable data fit subsequently we use a sampling approach to determine the uncertainty or resolution of the thermal history markov chain monte carlo is straightforward to implement and is used to produce joint and marginal probability distributions and from these we can infer credible intervals on the model parameters we demonstrate that combining the samples is preferable in that the final model is easier to interpret and generally has smaller uncertainties than the case where we model all samples independently we consider both synthetic and real data examples focusing on apatite fission track analysis but the general approach is applicable to other analytical methods such as u th he dating and 40ar 39ar analysis and in principle it is straightforward to combine these different data types into one joint thermal history model

sediment delivery after a wildfire we use a record of sedimentation in a small reservoir within the cerro grande burn area new mexico to document postfire delivery of ash other fine grained sediment carried in suspension within floods and coarse grained sediment transported as bedload over a five year period ash content of sediment layers is estimated using fallout 137 cs as a tracer and ash concentrations are shown to rapidly decrease through a series of moderate intensity convective storms in the first rainy season after the fire over 90 of the ash was delivered to the reservoir in the first year and ash concentrations in suspended sediment were negligible after the second year delivery of the remainder of the fine sediment also declined rapidly after the first year despite the occurrence of higher intensity storms in the second year fine sediment loads after five years remained significantly above prefire averages deposition of coarse grained sediment was irregular in time and was associated with transport by snowmelt runoff of sediment stored along the upstream channel during short duration summer floods coarse sediment delivery in the first four years was strongly correlated with snowmelt volume suggesting a transport limited system with abundant available sediment transport rates of coarse sediment declined in the fifth year consistent with a transition to a more stable channel as the accessible sediment supply was depleted and the channel bed coarsened maximum impacts from ash and other fine grained sediment therefore occurred soon after the fire whereas the downstream impacts from coarse grained sediment were attenuated by the more gradual process of bedload sediment transport

erosion of tephra from the 1980 eruption of mount st helens erosion of tephra on hillslopes north of mount st helens was measured at intervals of several months between 1980 and 1983 using large arrays of stakes rill erosion was initially greater than sheetwash but its importance decreased comparatively quickly both rill erosion and sheetwash decreased by one to two orders of magnitude over the three year period because a stable rill network developed and more permeable and less erodible substrates were created or exposed the decrease occurred prior to recovery of plants the causes for the rapid decline in erosion at mount st helens explain patterns of erosion following several other recent tephra eruptions basin wide annual rates of erosion were computed by mapping the surface cover the hillslope gradient and the texture and thickness of tephra and by using statistical relations developed between them and erosion rates annual erosion decreased from 26 mm between may 1980 and may 1981 to 1 8 mm between may 1982 and may 1983 projecting this rate of decline into the future indicated that only about one sixth of the tephra on hillslopes will be removed by water erosion before soil creep and other forms of mass wasting again dominate hillslope evolution

seismic stratigraphy of a buried volcanic arc northland new zealand and implications for neogene subduction abstract for over 6 million years in the early miocene the northland peninsula of new zealand s north island was the site of a volcanic arc comprising north westerly trending twin belts of marine and subaerial andesitic volcanoes approximately 50 volcanic edifices from major massifs to small cones have been newly identified on marine seismic profiles of the western belt which is largely buried the volcanoes were important point sources of sediment and the volcaniclastic deposits of each of the major and intermediate sized volcanoes form distinct seismic sequences the three main elements of the volcanic sequence are i a main body ii a sloping apron and iii a wide almost flat ring plain the downlap of the oldest discernible internal clinoform reflector relative to a known regional marker reflector underlying it indicates the approximate time of onset of activity whereas downlap of the top bounding reflector relative to those of other volcanic sequences provides a relative order of extinction sequence stratigraphic principles and super position determine relative stratigraphic hierarchy the volcanoes studied here were active only in the early miocene with few exceptions most were active by 22 ma and extinct by 16 ma with many extinct by 19 ma there was no pattern in their order of inception or extinction although a few became dominant burying or surrounding their neighbours at the south eastern end of the chain there is a rapid change over a short distance in both age and orientation from the northwest trending early miocene volcanism in the northland basin to north east trending middle and late miocene volcanism in the contiguous taranaki basin the northland volcanoes are interpreted to have been an arc facing the south westwards subducting south fiji basin whereas the taranaki volcanoes are interpreted to have been part of a separate pacific facing volcanic arc the lau colville ridge

extreme rainfall induced lahars and dike breaching 30 november 2006 mayon volcano philippines on 29 30 november 2006 heavy rains from supertyphoon durian remobilized volcanic debris on the southern and eastern slopes of mount mayon generating major lahars that caused severe loss of life and property in downstream communities the nearby legaspi city weather station recorded 495 8 mm of rainfall over 1 5 days at rates as high as 47 5 mm h far exceeding the initiation threshold for mayon lahars for about 18 h floods and lahars from the intense and prolonged rainfall overtopped river bends breaching six dikes through which they created new paths buried downstream communities in thick widespread deposits and caused most of the 1 266 fatalities in order to mitigate damage from future lahars the deposits were described and analyzed for clues to their generation and impact on structures and people post disaster maps were generated from raw aster and spot images using automated density slicing to characterize lahar deposits flooded areas croplands and urbanized areas fieldwork was undertaken to check the accuracy of the maps especially at the edges of the lahar deposits and to measure the deposit thicknesses the durian event was exceptional in terms of rainfall intensity but the dikes eventually failed because they were designed and built according to flood specifications not to withstand major lahars

the eisenhower range transantarctic mountains evaluation of qualitative interpretation concepts of thermochronological data abstract the transantarctic mountains tam were one of the first regions where apatite fission track aft thermochronology was applied routinely to study exhumation processes and long term landscape evolution pioneering publications from the region introduced or refined interpretation concepts of thermochronological data such as the break in slope in vertical age profiles as qualitative marker for the onset of accelerated rock cooling new aft data were compiled from vertical profiles in the eisenhower range northern tam and compared with published data samples originally examined by the population technique were re analyzed via the external detector technique aft ages increase from 32 2 ma at an elevation of 220 m to 175 14 ma at 2380 m geological evidence and thermal history modeling of the aft data require jurassic to late eocene reheating of the samples and an onset of cooling at 35 30 ma this requires the deposition of an 3 to 3 5 km thick sedimentary sequence on the granitic basement subsequent to jurassic ferrar magmatism at 180 ma the regression of paleotemperatures against sample altitudes infers a high jurassic geothermal gradient of 60 c km related to rifting processes and ferrar magmatism and a moderate cretaceous eocene geothermal gradient of 30 c km comparison of ages generated with population and external detector technique shows the importance of determining single grain ages for each sample even from granitic rocks of the same intrusion and thus strongly supports previous cases made for the determination of annealing kinetics and grain age evaluation age comparison additionally illustrates that samples above a break in slope record larger deviations between population and external detector ages than samples below a break in slope we demonstrate that the position and shape of a break in slope result from various factors such as the thermal history prior to final cooling maximum paleotemperatures cooling rate and geothermal gradient a break in slope does not straightly date the onset of final cooling and cannot substitute thermal history modeling therefore earlier studies from the tam and similar settings elsewhere need to be validated by combining thermal history modeling of thermochronological data and supplementary geological information

effects of forest land management on erosion and revegetation after the eruption of mount st helens the 1980 eruption of mount st helens covered soils with a tephra blanket and killed the forest tree cover in a 550 km2 area after the eruption rates of sheetwash and rill erosion and plant cover were measured on tephra covered hillslopes which had been subject to three land management practices grass seeding scarification and salvage logging on rapidly eroding hillslopes subject to grass seeding limited plant covers were established only after erosion had declined sharply logging of trees downed by the eruption and scarification of previously logged surfaces slowed erosion although the effect was small because erosion rates had already slowed substantially by the time these two practices were implemented the factors controlling erosion revegetation and their relative timing at mount st helens are similar to those following explosive volcanic eruptions elsewhere suggesting that grass seeding is not likely to be effective at slowing erosion following most tephra eruptions and that early mechanical disturbance could be an effective erosion control measure the results also indicate that even without deliberate conservation measures processes which mechanically disturb a surface layer of low hydraulic conductivity such as frost action or trampling can radically reduce runoff and erosion before revegetation has an important effect

surface morphology of caldera forming eruption deposits revealed by lidar mapping of crater lake national park oregon implications for deposition and surface modification abstract large explosive eruptions of silicic magma can produce widespread pumice fall extensive ignimbrite sheets and collapse calderas the surfaces of voluminous ignimbrites are rarely preserved or documented because most terrestrial examples are heavily vegetated or severely modified by post depositional processes much research addresses the internal sedimentary characteristics flow processes and depositional mechanisms of ignimbrites however surface features of ignimbrites are less well documented and understood except for comparatively small volume deposits of historical eruptions the 7700 calendar year b p climactic eruption of mount manama usa vented 50 km 3 of magma deposited first as rhyodacite pumice fall and then as a zoned rhyodacite to andesite ignimbrite as crater lake caldera collapsed lidar collected during summer 2010 reveals the remarkably well preserved surface of the manama ignimbrite and related deposits surrounding crater lake caldera in unprecedented detail despite forest cover the 1 m lateral and 4 cm vertical resolution lidar allows surface morphologies to be classified surface morphologies are created by internal depositional processes and can point to the processes at work when pyroclastic flows come to rest we describe nine surface features including furrow ridge sets and wedge shaped mounds in pumice fall eroded by high energy pyroclastic surges flow parallel ridges that record the passage of multiple pyroclastic flows perched benches of marginal deposits stranded by more mobile pyroclastic flow cores hummocks of dense clasts interpreted as lag deposit transverse ridges that mark the compression and imbrication of flows as they came to rest scarps indicating ignimbrite remobilization fields of closely spaced pits caused by phreatic explosions fractures and cracks due to extensional processes resulting from ignimbrite volume loss and stream channels eroded in the newly formed surface the nine morphologies presented here illustrate a dynamic depositional environment that varied spatially and with time during the eruption and show that multiple processes modified the ignimbrite after deposition both during and after the eruption

the mid cretaceous transition from basement to cover within sedimentary rocks in eastern new zealand evidence from detrital zircon age patterns detrital zircon u pb ages for 30 late jurassic and cretaceous sandstones from the eastern province of eastern new zealand combined with previously published geochronological and palaeontological data constrain the time of deposition in the pahau and waioeka terranes of the cretaceous accretionary margin of zealandia and their adjacent cover strata the zircon age patterns also constrain possible sediment source areas and mid cretaceous geodynamic models of the transition from basement accretionary wedge to passive margin cover successions pahau terrane deposition was mainly barremian to aptian but continued locally through to late albian time with major source areas in the adjacent kaweka and waipapa terranes and minor inputs from the inboard median batholith waioeka terrane deposition was mainly albian with distinctive and exclusive sediment sources principally from the median batholith but with minor inputs from the western province alternative tectonic models to deliver such exclusive median batholith and western province derived sediment to the mid cretaceous zealandia continental margin are 1 the creation of a rift depression across zealandia or 2 sinistral displacement of south zealandia with respect to north zealandia to expose western province rocks directly at the zealandia margin detrital zircon age patterns of cretaceous cover successions of the eastern province of eastern new zealand demonstrate purely local sources in the adjacent kaweka and waipapa terranes cretaceous zircon components show a decline in successions of late early cretaceous age and disappear by late late cretaceous time suggesting the abandonment or loss of access to both the median batholith and western province as sediment sources

environmental impact of the 1 8 ka taupo eruption new zealand landscape responses to a large scale explosive rhyolite eruption abstract large scale ignimbrite eruptions from rhyolitic caldera volcanoes can trigger geologically instantaneous changes in sedimentary systems over huge areas by either burying existing environments or overloading them with vast quantities of unconsolidated particulate material the post eruption readjustment of the landscape to such perturbations is one of the most dramatic processes in physical sedimentology exemplified here by the 1 8 ka taupo eruption in the central north island of new zealand this eruption generated voluminous fall deposits then climaxed with emplacement of a c 30 km 3 non welded ignimbrite over a near circular area of c 20 000 km 2 approximately 90 of the area but the headwaters of many of the north island s largest rivers were impacted by both the primary pyroclastic fall and flow material large scale post eruption remobilisation of this material coupled with the re establishment of fluvial systems occurred in a distinct sequence as recorded by the evolution of sedimentary facies in different sub environments following an initial period dominated by mass flows re establishment of fluvial systems began with the headward erosion of box canyons through the ponded ignimbrite deposits a process often associated with the break out of temporary lakes aggradational streams developed in these channels rapidly evolved from shallow ephemeral sediment laden outbursts associated with flash flood events to deeper permanent braided rivers before declining sediment yields led to retrenchment of single thread rivers and a return to pre eruption gradients and bedloads years to decades later typically the modern profile of many streams and rivers follow closely their pre eruption profiles and incision and erosion is overwhelmingly confined to the deposits of the eruption itself although the general remobilisation pattern is similar for all impacted river systems detailed studies of the waikato rangitaiki mohaka ngaruroro and whanganui catchments show that the relative timing and scale of each eruption response phase differs between each catchment these reflect differences in catchment physiography and hydrology and the volume and type of pyroclastic material deposited in each ultimately the landscape response reflects the relative spatial distributions of and the volumetric ratios between the volumes of pyroclastic debris water and accommodation space in the basin cf kataoka and manville this volume

spatial and temporal patterns in fluvial recovery following volcanic eruptions channel response to basin wide sediment loading at mount pinatubo philippines the june 1991 eruption of mount pinatubo philippines was one of the largest volcanic eruptions of the twentieth century emplacing 5 6 km 3 of pyroclastic fl ow material on the fl anks of the volcano the combination of abundant relatively fi ne grained easily erodible material and intense tropical rainfall led to numerous lahars immediately following the eruption even after major lahars ended sediment yields in some basins remained orders of magnitude above pre eruption levels using data collected from 1996 through 2003 we investigated fi ve basins that experienced varying amounts of sediment loading in the 1991 eruption from 1 to 33 of the basin area covered by valley fi lling pyroclasticfl ow deposits from measurements of fl ow and bed characteristics made through time we developed a general conceptual model for channel recovery following basin wide sediment loading initially fi ner grained sediment and pumice are mobilized preferentially through selective transport once the bed is coarse enough for gravel size clasts to interact with one another clast structures develop increasing form roughness and critical shear stress and inhibiting initial clast mobility as sediment inputs continue to decline the channel incises into valley bottom sediments progressively armoring through winnowing at pinatubo incision and armoring occur fi rst as dry season phenomena due to reduced sediment inputs eventually moving to year round low fl ow bed stability observed timing of the onset of in channel biological recovery suggests that reestablishment of channel stability helps catalyze aquatic ecosystem recovery

did the 73 ka toba super eruption have an enduring effect insights from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models abstract the potential impact of the 73 ka toba super eruption upon global and regional climate terrestrial ecosystems and prehistoric human populations remains unclear evidence from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models has provided some useful working hypotheses further progress requires a substantial improvement in the accuracy precision and resolution of the chronologies of each of the marine and terrestrial proxy records used to reconstruct the environmental impact of this extreme event

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

stratigraphic variations in lacustrine sediment gravity flow deposits intercalated in varved diatomite an example from the hiruzenbara formation okayama prefecture southwest japan abstract the hiruzenbara formation which is distributed within the hiruzenbara basin of southwest japan includes a section of varved diatomite representing ca 8000 y of deposition in a dammed lake formed during the middle pleistocene the varve on average ca 1 5 mm thick consists of light and dark green laminae sets and is predominately composed of planktonic diatoms several facies types of sediment gravity flow deposits are intercalated in the varved diatomite in this study we examined the sedimentary facies lateral changes and stratigraphic variations of these sediment gravity flow deposits in addition we also analyzed a time series of varve thickness fluctuations based on their sedimentary facies composition and diatom species sediment gravity flow deposits in the varved diatomite can be classified as flood generated or slope failure generated sedimentary facies analysis indicates that the slope failure deposits were deposited by slope failure within the lake since no significant segregation of rip up clasts is observed in the deposits a proximal depositional setting at the base of the lake slope is suggested the flood generated deposits have eroded lower contacts which indicate deposition by hyperpycnal flows in contrast deposits with no basal erosion indicate deposition by homopycnal or hypopycnal flows the thickness time series of the varve shows a thickening upward trend indicating an increase in diatom production or in diatom shells sizes running mean thicknesses show non stationary periodicities of 200 500 y the frequency of the deposit varies but is approximately 10 events per 400 y flood deposits in the middle part of the section are dominated by volcaniclastic silt this suggests that vegetation around the lake was reduced by ash fall resulting in frequent flood events during this interval the frequency and thickness of slope failure deposits increases in the lower part of the succession together with the occurrence of slump scars the frequency of the deposits in the upper part of the section is approximately 100 500 y similar to the postulated interval at activity of the nearby yamasaki fault system the results of this study improve our knowledge of paleoenvironmental changes in the study site and also provide new criteria for classifying sediment gravity flow deposits in lacustrine sedimentary cores

peak flow responses to landscape disturbances caused by the cataclysmic 1980 eruption of mount st helens washington years of discharge measurements that precede and follow the cataclysmic 1980 eruption of mount st helens washington provide an exceptional opportunity to examine the responses of peak fl ows to abrupt widespread devastating landscape disturbance multiple basins surrounding mount st helens 300 1300 km 2 drainage areas were variously disturbed by 1 a debris avalanche that buried 60 km 2 of valley 2 a lateral volcanic blast and associated pyroclastic fl ow that destroyed 550 km 2 of mature forest and blanketed the landscape with silt capped lithic tephra 3 debris fl ows that reamed riparian corridors and deposited tens to hundreds of centimeters of gravelly sand on valley fl oors and 4 a plinian tephra fall that blanketed areas proximal to the volcano with up to tens of centimeters of pumiceous silt sand and gravel the spatially complex disturbances produced a variety of potentially compensating effects that interacted with and infl uenced hydrological responses changes to water transfer on hillslopes and to fl storage and routing along channels both enhanced and retarded runoff rapid posteruption modifi cations of hillslope surface textures adjustments of channel networks and vegetation recovery in conjunction with the complex nature of the eruptive impacts and strong seasonal variability in regional climate hindered a consistent or persistent shift in peak discharges overall we detected a short lived 5 10 yr increase in the magnitudes of autumn and winter peak fl ows in general peak fl ows were larger and moderate to large fl ows q 2 yr were more substantively affected than predicted by early modeling efforts proportional increases in the magnitudes of both small and large fl ows in basins subject to severe channel disturbances but not in basins subject solely to hillslope disturbances suggest that eruptioninduced modifi cations to fleffi ciency along alluvial channels that have very mobile beds differentially affected fl ows of various magnitudes and likely played a prominent and additional role affecting the nature of the hydrological response

rainfall induced lahars in the belham valley montserrat west indies rain falling on loose volcanic debris over the soufriere hills volcano montserrat generates hazardous floods in the belham valley these rainfall induced lahars vary greatly in discharge and sediment concentration in space and time they differ from examples documented on other volcanoes in that 1 the eruption has been continuing since july 1995 generating repeated pulses of excess sediment 2 rainfall is the only significant trigger 3 the system is small with short distance to the sea and relatively low altitude at the catchment top repeat mapping and comparison with pre eruption data demonstrate significant geomorphological change with c 120 m shoreline progradation and c 0 4 m a 1 mean aggradation rate in the middle to lower valley the nature of the hazard and area of risk have changed as the valley has aggraded the channel widened and the runoff efficiency increased as a result of rilling and vegetation removal lahars in the belham valley correlate with days when 10 mm rain fell in 24 h with more events triggered in the late rainy season the flows are mainly newtonian but one non newtonian flow event has been demonstrated and is described in detail 20 march 2000 this flow is explained by direct volcanic ash input to the runoff

the 73 ka toba super eruption and its impact history of a debate abstract toba volcano in northern sumatra is located at the intersection of two major tectonic lineaments in one of the most seismically active regions in the world and its caldera is considered the largest quaternary caldera on earth the most recent toba explosive eruption at 73 ka was an order of magnitude larger than that of tambora in 1815 and has a volcanic explosivity index of 8 the exceptional magnitude of this super eruption and the widespread distribution of the younger toba tuff ytt in marine cores in the indian ocean the arabian sea and the south china sea have prompted sustained debate as to its global and regional impact on climate ecosystems and prehistoric human populations however despite climate modelling and appeals to a variety of proxy climatic data including geochemistry pollen analysis archaeology and stable isotopes the extent of the impact remains unclear in order to achieve better progress in the future greater care is needed to obtain higher resolution and more precise chronologies from the marine and terrestrial proxy records used to test whether or not the 73 ka toba super eruption had a significant environmental impact records from speleothems and ice cores have much to offer in this regard