dynamic deformation of seguam island aleutian islands alaska 1993 2000 implications for magmatic and hydrothermal processes 1 the most recent volcanic eruption on seguam island was detected in december 1992 and lasted until july or august of 1993 a series of radar interferograms generated from imagery collected by the european space agency s ers 1 and ers 2 satellites are used here to map the spatial and temporal evolution of the deformation field from june of 1993 to september of 2000 this evolution consists of a coeruptive period of uplift surrounding the eruption two posteruptive subsidence signals centered within each of the two calderas on the island each of which lasted for 4 years after the end of the eruption and then uplift beneath the caldera on the eastern side of the island and the surrounding region from mid 1999 through the end of the time series in september 2000 the locations and magnitudes of the sources of the deformation are inferred from the interferometric synthetic aperture radar displacement data using an elastic half space earth model with multiple mogi point sources and a levenburg marquardt nonlinear inversion technique this inference reveals that the sources of subsidence are located within the upper 2 km of the earth s crust while the sources of uplift are located below 2 5 km the locations of dilatational sources are consistent with a model of the volcanic system where ascending magma accumulates below a density or structural boundary near 2 5 3 km depth the subsidence signals are most likely associated with cooling of shallow magmatic bodies emplaced during the 1992 1993 eruption however the possibility that they indicate a combination of magma cooling and depressurization of a hydrothermal system cannot be ruled out using the geodetic data alone implications for the long term evolution of the volcanic edifice are discussed

composition and structure of the central aleutian island arc from arc parallel wide angle seismic data new results from wide angle seismic data collected parallel to the central aleutian island arc require an intermediate to mafic composition for the middle crust and a mafic to ultramafic composition for the lower crust and yield lateral velocity variations that correspond to arc segmentation and trends in major element geochemistry the 3 d ray tracing 2 5 d inversion of this sparse wide angle data set which incorporates independent phase interpretations and new constraints on shallow velocity structure produces a faster and smoother result than a previously published velocity model middle crustal velocities of 6 5 7 3 km s over depths of 10 20 km indicate an andesitic to basaltic composition high lower crustal velocities of 7 3 7 7 km s over depths of 20 35 km are interpreted as ultramafic mafic cumulates and or garnet granulites the total crustal thickness is 35 37 km this result indicates that the aleutian island arc has higher velocities and thus more mafic compositions than average continental crust implying that significant modifications would be required for this arc to be a suitable building block for continental crust lateral variations in average crustal velocity below 10 km roughly correspond to trends in major element geochemistry of primitive mg 0 6 lavas the highest lower crustal velocities and presumably most mafic material are detected in the center of an arc segment between unmak and unalaska islands implying that arc segmentation exerts control over crustal composition

a petrologic re investigation of the adak volcanic center central aleutian arc alaska the adak volcanic center is located in the central part of the aleutian arc and consists of three main volcanic vents andrew bay volcano the oldest center has been mostly removed by erosion the next youngest vent mount adagdak was built in three major volcanic stages whereas mount moffett the largest volcanic edifice consists of a main cone and a parasitic cone each with several magmatic phases adak is unique compared to other modern aleutian volcanic centers in that it contains two xenolith suites conrad and kay 1984 debari et al 1987 one suite consisting predominantly of mafic xenoliths occurs on mount moffett whereas an assemblage of ultramafic and mafic xenoliths is found on mount adagdak lavas erupted at adak span the compositional range from 48 4 to 65 0 wt sio2 and are characterized by significant variations in al2o3 mgo sr ni and cr on harker diagrams this variability produces compositional trends with significant scatter the adak suite has total ree contents that vary from 32 to 154 ppm but do not correlate systematically with silica layb n ratios range from 2 41 to 21 72 with the majority of lavas between 2 41 and 6 06 on process identification diagrams the adak suite plots as steeply sloping trends that contrast with the horizontal patterns of most other aleutian centers measured isotopic ranges are large and nearly equal to those for the entire arc although they span similar silica ranges subtle geochemical and isotopic differences distinguish the different volcanic vents of adak on mount moffett a geochemically and isotopically distinct group of andesites 55 5 57 9 sio2 the mafic andesites occur on its ne flank these lavas have elevated mgo ni and cr but are depleted in al2o3 relative to other mount moffett andesites with similar silica they also have more heterogeneous ree abundances and isotopic ratios than most of the other andesites significant compositional differences exist between adak and the other volcanic centers of the central aleutian arc although these differences are characteristic of all geochemical systems they are greatest for major and rare earth elements and isotopic ratios the lack of coherent relationships on major and trace element harker diagrams the isotopic variability as well as the steeply sloping trends on ree process identification diagrams suggest that the adak volcanic suite was not formed predominantly by closed system crystal fractionation but must be the product of a complex open system process es the significant isotopic variability displayed by the suite suggests that contamination by an isotopically distinct contaminant must also have been an important petrologic component in the evolution of the suite ree data are also suggestive of a role for magma mixing such a complex petrologic evolution is consistent with an immature lithospheric plumbing system based on ree systematics the xenolith suites of adak cannot as previously proposed be related to the host lavas or the rest of the adak suite through crystal fractionation schemes rather they are probably accidental fragments derived from various depths along lithospheric conduits in light of their relation to xenolith bearing units the mafic andesites of adak presumably represent hybrid magmas formed during the interaction of ascending magmas with lithospheric wall rock they are therefore characteristic of immature volcanic centers and unlikely to be related directly to the magmatic processes responsible for the generation of primary arc magmas because of the close proximity of the vents and the subtle compositional differences between their lavas the adak volcanic center was probably supplied by a single deep lithospheric plumbing system that fed separate crustal magma chambers the absence of historic volcanic activity on adak suggests this plumbing system was abandoned before complete conduit development this decline in magmatism may reflect a re adjustment of volcano spacing within this part of the aleutian arc

event frequency and landscape stability on the floodplain systems of arid central australia abstract central australian floodplains contain landforms suggesting activity at three scales a set of large sand sheets sand threads and megaripple covered channels related to a few enormous floods the contemporary floodplain which consists of channels and levees floodouts unchannelled floodplains and floodbasins and erosion cell mosaics which result from local redistribution of sediment classical floodplain models do not fit these systems which behave more like low angle alluvial fans because they continue to accumulate sediment the sedimentary history of the central australian floodplains suggests that deposition in the last 10 000 years has been dominated by a few gigantic floods these events are not represented in the published slackwater deposit record but morphological evidence for them can be found in bedrock gorges as well as on floodplains since the last major flood channel levee systems have been growing slowly upwards and outwards and outwards from the mountain ranges away from these systems sediment supply is limited and the dominant process is local reworking this has intensified since the arrival of europeans 100 years ago and also as a result of increased flooding due to variations in rainfall

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

constraints on the composition of the aleutian arc lower crust from vp vs 1 determining the bulk composition of island arc lower crust is essential for distinguishing between competing models for arc magmatism and assessing the stability of arc lower crust we present new constraints on the composition of high p wave velocity vp 7 3 7 6 km s lower crust of the aleutian arc from best fitting average lower crustal vp vs ratio using sparse converted s waves from an along arc refraction profile we find a low vp vs of 1 7 1 75 using petrologic modeling we show that no single composition is likely to explain the combination of high vp and low vp vs our preferred explanation is a combination of clinopyroxenite 50 70 and alpha quartz bearing gabbros 30 50 this is consistent with aleutian xenoliths and lower crustal rocks in obducted arcs and implies that 30 40 of the full aleutian crust comprises ultramafic cumulates these results also suggest that small amounts of quartz can exert a strong influence on vp vs in arc crust

mercury spikes at the devonian carboniferous boundary in the eastern part of the rhenohercynian zone central europe and in the south china block abstract mercury hg chemostratigraphy is an efficient tool for tracing distal volcanism as suggested in many recent studies in our research we focused on hg and other trace element contents in two devonian carboniferous boundary sections from two different and spatially distant areas the lesni lom section czech republic was located in the south eastern part of the laurussia plate and the duli guangxi china section in the south china plate in both studied sections the highest hg hg fexs or hg al values occurred at the devonian carboniferous boundary in lesni lom organic matter remineralization was connected with origin of pyrite which sequestered a part of the hg content the decoupling of hg and toc within the duli section strata and the significant correlation with al and fe may suggest that hg is hosted primarily by clay particles derived from the landmass into the marine realm mercury was also incorporated into pyrites but the poor correlation between hg and ts total sulfur indicates a possible loss of s during oxygenation events in the lesni lom section and partly in the duli section fe and mn redox cycling may have played an important role in sediment hg content both sections were influenced by widespread hydrothermal and volcanic activity indicated by positive eu anomalies msi fe ti zr al2o3 and ti al2o3 values it remains to be clarified whether besides the interplay of local magmatic activity and redox oscillations there may have been some additional influence of the multi phase viluy large igneous province although its younger phase 364 4 1 7 ma slightly precedes the hg anomalies obtained

petrologic constraints on the spatial distribution of crustal magma chambers atka volcanic center central aleutian arc the atka volcanic center the largest modern central aleutian magmatic complex covers 360 km2 with an estimated eruptive volume of 200 km3 the oldest exposed rocks the old harbor series are a suite of basaltic andesite to dacite flows pyroclastic rocks and lahars of presumed tertiary age the youngest phase of activity on atka probably began 1 2 ma ago near the center of the present volcanic field and consists of basaltic and basaltic andesite flows which produced a large shield volcano atka volcano subsequent to this activity satellite vents formed along the margin of this structure collapse of the ancestral atka volcano produced a caldera 5 km in diameter and was accompanied by the eruption of a large dacitic flow big pink after caldera formation four major volcanic cones korovin mount kliuchef sarichef and konia grew around the margins of atka caldera this study focuses on the atka volcano korovin and mount kliuchef volcanic centers atka lavas range from basalt through dacite with all major volcanic vents erupting lavas spanning similar compositional ranges they are porphyritic with phenocrysts of plagioclase 25 36 olivine 0 4 3 6 or orthopyroxene 2 8 clinopyroxene 1 12 and a single fe ti oxide 0 2 hydrous phases are lacking even in the most evolved lavas two generations of plagioclase occur in the korovin suite but not in lavas from mount kliuchef korovin plagioclase also displays resorption surfaces and inclusion zones which are either absent in the phenocrysts of mount kliuchef lavas or very rare similarly clinopyroxene in korovin has reaction rims and abundant glass inclusions which are absent in the mount kliuchef lavas lavas erupted in the atka volcanic field vary from 47 to 66 sio2 although the complex is dominated volumetrically by basaltic lavas on major element harker diagrams mafic and intermediate atka volcano lavas form linear trends but the most siliceous lavas do not fall along these trends by contrast mount kliuchef lavas are compositionally bimodal whereas korovin samples range from 51 to 62 wt silica with intermediate lavas clustering around 55 sio2 incompatible trace element abundances of korovin lavas do not increase with silica content but those of atka volcano and mount kliuchef do on incompatible incompatible element diagrams the three suites behave very differently differences also exist between the ree characteristics of the three atka suites the petrographic and geochemical differences between the three suites suggest formation by markedly different crustal magmatic processes the characteristics of the atka volcano suite suggest formation of the basalt to dacite suite by crystal fractionation of an anhydrous phenocryst assemblage incompatible incompatible element trends suggest the mafic mount kliuchef lavas were also produced by crystal fractionation a conclusion supported by quantitative mass balance calculations the difference in slope between the atka volcano and mount kliuchef suites indicates however that there were significant differences in these fractionation schemes modeling as well as geochemical characteristics suggest that the mount kliuchef dacites are not the product of crystal fractionation they may represent 1 crustal melts 2 complete melts of siliceous lava produced during earlier magmatic stages or 3 residual liquids from the atka volcano magmatic system in contrast to these suites the korovin lavas could not have been formed by crystal fractionation rather the andesites are probably the result of mixing basaltic and dacitic magmas the atka data suggest 1 large volcanic centers may be supplied by a series of non communicating crustal magma chambers 2 local factors may be important in determining liquid lines of descent and 3 very different magmatic processes may occur in adjacent magma chambers if the complexity of the atka suite is a common feature of arc magmatic plumbing systems reconstructing differentiation processes requires analytical sample sets which are well constrained spatially and temporally

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

intra arc extension and magmatic evolution in the central aleutian arc alaska new geologic petrologic and sr isotope data from seguam island alaska reveal substantial variation in the composition of magmas of the central aleutian arc the data indicate that variable magmatic processes occur along the strike of the arc seguam is situated on extended arc crust and has erupted voluminous basalt and dacite in contrast to basalt from adjacent volcanic centers atop unextended crust which evolved by polybaric open system crystallization seguam lavas crystallized at shallow levels under closed system conditions seguam basalt therefore better reflects the character of its source materials than basalt erupted through unextended arc crust the authors conclude that along arc differences in the nature of crystallization were facilitated by changes in the thermal and density structure of the subarc lithosphere caused by intra arc extension

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

thresholds and the spatial variability of flood power during extreme floods abstract analysis of the effects of extreme events on overall landscape form has shifted geomorphic attention away from a focus on the magnitude and frequency relationship of discharge to a focus on flood power this paper analyzes the variation in flood power both at a station and downstream in a basin of differing lithologies and discusses the geologic and geomorphic controls on flood power design floods ranging from the 2 year discharge tothe 500 year discharge are routed trough a drainage basin and channel boundary shear stress and unit stream power are calculated using hec 2 along 21 cross valley transects because of the strong lithologic control on valley width and channel slope an irregular downstream pattern of flood power emerges which reflects primarily the pattern in valley width narrow valleys correspond to a greater percent of channel conveyance during large magnitude floods and increasing rates of depth increases thus maximizing flood power in these zones the opposite occurs in anomalously wide valleys where flood power is minimized a downstream spatial pattern emerges reflecting both local and general controls for a given probability event flood power varies at least threefold within the basin with a maximum occuring in a specific downstream section holding flood power constant produces an extremely wide range in flow probabilities in order to establish a link between critical flood power magnitude and flow frequency minimum flood powers are developed from an analysis of previous work on catastrophic flooding for humid alluvial channels these minimum thresholds tentatively correspond to 100 n m2 or 300 w m2 for shear stress and unit stream power respectively discharges required to attain these critical flood powers vary from events roughly twice the 100 year flood to a maximum of roughly 18 times the discharge of the 100 year flood

mid pleistocene basalt from the seguam volcanic center central aleutian arc alaska local lithospheric structures and source variability in the aleutian arc recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern aleutian arc and the sources and processes that produce the broad spectrum of aleutian basaltic lavas mid pleistocene basalt from the seguam island volcanic center in the central aleutian arc erupted through strongly extended arc crust contains 4 8 mgo is low in k2o 18 and is distinguished from aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements ree and low sr and pb content the ba la 47 72 and pb la 0 7 1 5 ratios are high incompatible element ratios are variable and ree patterns cross one another the 87sr 86sr ratio 0 70365 0 00003 is the highest value obtained from the aleutian islands also 206pb 204pb 18 91 0 02 207pb 204pb 15 60 0 02 and 208pb 204pb 38 51 0 04 ratios are among the most radiogenic values known whereas the 143nd 144nd ratio 0 51297 0 00003 is among the lowest measured low abundances of incompatible elements high ba la and pb la ratios uniform sr nd and pb isotope ratios and high eruptive temperatures 1160 c suggest that basalt erupted at seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics trace element abundances rule out an eclogite source and suggest instead a peridotite dominated source with 5 10 of a subducted sediment component this peridotite may reside in the mantle wedge or in the amlia fracture zone afz of the pacific plate that was subducted beneath seguam 1 ma in addition the afz possibly enhanced the subduction of terrigenous sediment into the source region beneath seguam and this extra terrigenous component may explain the ba la pb la and sr nd and pb isotope ratios the distinctive composition of seguam basalt reflects a complex set of physical conditions including fracture zone subduction and intraarc extension that affected both the source region and thermo chemical history of magma ascent local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the aleutian arc

mid pleistocene lavas from the seguam volcanic center central aleutian arc closed system fractional crystallization of a basalt to rhyodacite eruptive suite in contrast to adjacent volcanic centers of the modern central aleutian arc seguam island developed on strongly extended arc crust k ar dates indicate that mid pleistocene late pleistocene and holocene eruptive phases constitute seguam this study focuses on the petrology of the mid pleistocene 1 07 07 ma turf point formation tpf which is dominated by an unusual suite of porphyritic basalt and basaltic andesite lavas with subordinate phenocryst poor andesite to rhyodacite lavas increasing whole rock feo mgo from basalt to dacite the anhydrous plag ol cpx opx mt phenocryst assemblage groundmass pigeonite and the reaction ol liq opx preserved in the mafic lavas indicate a tholeiitic affinity thermometry and comparison to published phase equilibria suggests that most tpf basalts crystallized plag ol cpx mt at 1160 c between about 3 5 kb 1 2 h2o andesites crystallized plag cpx opx mt at 1000 c between 3 4 kb with 3 5 h2o and dacites crystallized plag cpx opx mt at 1000 c between 1 2 kb with 2 3 h2o all lavas crystallized at f o 2 close to the nno buffer mineral compositions and textures indicate equilibrium crystallization of the evolved lavas petrographic evidence of open system mixing or assimilation is rare mgo cao al2o3 cr ni and sr abundances decrease and k2o na2o rb ba zr and pb increase with increasing sio2 50 71 lree enrichment ce yb n 1 7 0 2 characterizes most tpf lavas total ree contents increase and eu anomalies become more negative with increasing sio2 relative to other aleutian volcanic centers tpf basalts and basaltic andesites have lower k2o na2o tio2 rb ba sr zr y and lree abundances 87sr 86sr ratios 0 70361 0 70375 and ratios of 206pb 204pb 18 88 18 97 207pb 204pb 15 58 15 62 208pb 204pb 38 46 38 55 are the highest measured for any suite of lavas in the oceanic portion of the aleutian arc conversely end values 5 8 to 6 7 are among the lowest from the aleutians sr nd and pb ratios are virtually constant from basalt through rhyodacite whereas detectable isotopic heterogenity is observed at most other aleutian volcanic centers major and trace element ree and sr nd and pb isotopic compositions are consistent with the basaltic andesitic andesitic dacitic and rhyodacitic liquids evolving from tpf basaltic magma via closed system fractional crystallization alone fractionation models suggest that removal of 80 wt cumulate 61 plag 17 cpx 12 opx 7 ol and 3 mt can produce 20 wt rhyodacitic residual liquid per unit mass of parental basaltic liquid petrologic and physical constraints favor segregation of small batches of basalt from a larger mid crustal reservoir trapped below a low density upper crustal lid in these small magma batches the degree of cooling crystallization and fractionation are functions of the initial mass of basaltic magma segregated the thermal state of the upper crust and the magnitude of extension tholeiitic magmas erupted at seguam evolved by substantially different mechanisms than did calc alkaline lavas erupted at the adjacent volcanic centers of kanaga and adak on unextended arc crust these variable differentiation mechanisms and liquid lines of descent reflect contrasting thermal and mechanical conditions imposed by the different tectonic environments in which these centers grew at seguam intra arc extension promoted eruption of voluminous basalt and its differentiates unmodified by interaction with lower crustal or upper mantle wallrocks

experiments on vertical basement fault reactivation below volcanoes scaled modeling has been conducted to study the role played by the reactivation of vertical basement faults in the destabilization process of overlying volcanoes results show that basement fault reactivation induces the formation of faults within the volcanic edifice these faults delimite a central block which is extruded from between the two undeformed lateral parts of the edifice collapse of this central block can occur when heterogeneous cones are used in experiments revealing that mechanical interfaces are of paramount importance in triggering rockslide avalanches collapse structures may display horseshoe shapes in map view and are then surprisingly similar to avalanche scars resulting from failure created by magmatic intrusions in natural volcanoes large scale destabilisation of half the surface of the cone may also occur in a single event depending on the position of the reactivated basement fault below the edifice it is emphasized that the process under consideration may occur on a dormant volcano as well provided that a fault is reactivated below it

lacustrine fluvial interactions in australia s riverine plains climatic forcing of fluvial systems has been a pre occupation of geomorphological studies in australia since the 1940s in the riverine plain southeastern australia the stable tectonic setting and absence of glaciation have combined to produce sediment loads that are amongst the lowest in the world surficial sediments and landforms exceed 140 000 yr in age and geomorphological change recorded in the fluvial fluvio lacustrine and aeolian features have provided a well studied record of quaternary environmental change over the last glacial cycle the region includes the willandra lakes whose distinctive lunette lakes preserve a history of water level variations and ecological change that is the cornerstone of australian quaternary chronostratigraphy the lunette sediments also contain an ancient record of human occupation that includes the earliest human fossils yet found on the australian continent to date the lake level and palaeochannel records in the lachlan willandra system have not been fully integrated making it difficult to establish the regional significance of hydrological change here we compare the willandra lakes environmental record with the morphology and location of fluvial systems in the lower lachlan an ancient channel belt of the lachlan willandra creek acted as the main feeder channel to willandra lakes before channel avulsion caused the lakes to dry out in the late pleistocene electromagnetic surveys geomorphological and sedimentary evidence are used to reconstruct the evolution of the first new channel belt following the avulsion single grain optical dating of floodplain sediments indicates that sedimentation in the new middle billabong palaeochannel had commenced before 18 4 1 1 ka a second avulsion shifted its upper reaches to the location of the present lachlan river by 16 2 0 9 ka the timing of these events is consistent with palaeohydrological records reconstructed from willandra lakes and with the record of palaeochannels on the lachlan river upstream willandra lakes shows high inflows during the last glacial maximum 22 ka but their subsequent drying between 20 5 ka and 19 ka was caused by river avulsion rather than regional aridity this case study highlights the benefits of combining fluvial with lacustrine archives to build complementary records of hydrological change in lowland riverine plains

flood channel morphology of a quiet river the lachlan downstream from cowra southeastern australia abstract detailed geomorphic mapping and sediment analyses of floodplain features were undertaken on a 30 km reach of the lachlan river in southeastern australia the reach is located in the transitional zone between the southern great dividing range and extensive alluvial plains to the west the channel resembles other fine grained low energy alluvial channels it is muddy narrow and deep and meanders irregularly it contains fine grained accreting bench deposits and in places is influenced by inherited fluvial forms both of which indicate long term channel stability by contrast the floodplain exhibits features that have been attributed elsewhere to catastrophic flood flows including elliptical scour scars swirl pits cobble and gravel deposits large chutes and chute bars zones of floodplain stripping flood levees and multiple flood channels sorting and texture of floodplain sediments generally increases with height above the bed although deposits are variable and poorly sorted indications of recent fluvial activity at several levels above the bed are confirmed by aerial photographs dating from 1954 although changes to the channel position have been small the alluvial facies model displays a compound floodplain in which a stable low energy channel is inset within a high energy flood dominated floodplain this interpretation is consistent with streamflow and documentary evidence of recurrent large floods since 1839 which have a return period of

stress fields of the overriding plate at convergent margins and beneath active volcanic arcs tectonic stress fields in the overriding plate at convergent plate margins are complex and vary on local to regional scales volcanic arcs are a common element of overriding plates stress fields in the volcanic arc region are related to deformation generated by subduction and to magma generation and ascent processes analysis of moment tensors of shallow and intermediate depth earthquakes in volcanic arcs indicates that the seismic strain field in the arc region of many convergent margins is subhorizontal extension oriented nearly perpendicular to the arc a process capable of generating such a globally consistent strain field is induced asthenospheric corner flow below the arc region

hydraulics and basin morphometry of the largest flash floods in the conterminous united states abstract the maximum rainfall runoff floods measured by indirect methods in small basins 0 39 370 km2 in the conterminous united states are examined this analysis identified twelve floods that were the largest ever measured these floods all occurred in semiarid to arid areas for eleven of the twelve largest rainfall runoff floods measured in small basins by the slope area method values of hydraulic depth hydraulic radius width depth ratio n value mean velocity velocity head coefficient froude number water surface energy and channel slopes shear stress and unit stream power are tabulated estimated composite n values weighted by subsection conveyance range from 0 028 to 0 048 with a mean of 0 038 mean velocities ranged from 3 47 to 9 97 m s 1 froude numbers ranged from 0 81 to 2 49 with 9 of 12 floods having values greater than 1 00 water surface energy and channel slopes vary considerably for each flood energy slope always was less than water surface slope by values of 1 104 channel slope was greater than energy slope in eight floods shear stresses ranged between 61 and 855 n m 2 and unit stream power from 212 to 8131 w m 1 floods in these small basins produced shear stresses and unit stream powers several hundred times greater than floods in large rivers floods on other small streams with smaller unit discharges produced greater shear stresses and stream powers this indicates that the force of a flood is controlled by the depth slope product not absolute discharge in the twelve watersheds studied basin relief ranged from 165 to 1280 m elongation ratios ranged from 0 55 to 0 80 the number of first order streams basin magnitude ranged from 10 to 4297 drainage density ranged from 4 1 to 10 9 km km 2 basin slope ranged from 0 0043 to 0 2486 relief ratio ranged from 0 0097 to 0 34 ruggedness number ranged from 0 69 to 7 17 and first order channel frequency ranged from 5 1 to 38 6 km 2 elongation ratios were larger and drainage density and first order channel frequency lower than other small flash flood prone basins in the united states these twelve basins have neither the most favorable morphometric characteristics which contribute to flash flood peaks nor did the storms causing these flash floods have the greatest short duration intensities maximum flood peaks originate from an optimal combination of basin morphology and physiography and storm intensity

downstream changes in dryland river channels the northern plains of arid central australia abstract many dryland rivers undergo marked downstream changes owing to factors such as infrequent floods flow transmission losses and typically few tributary inflows beyond the headwaters along the sandover bundey sandover bundey and woodforde rivers on the northern plains of arid central australia downstream channel changes are broadly similar in upland zones small rocky channels transporting sand and gravel gradually increase in size before entering piedmont zones where channels and narrow floodplains are confined by bedrock alluvial terraces or aeolian dunes in lower gradient lowland zones channels and floodplains remain confined and in the absence of tributary inflows channel cross sectional areas and discharges decrease downstream confining landforms are not present in floodout zones which results in splay formation increased floodplain widths and marked overall downstream decreases in cross sectional areas eventually channelised flow and bedload transport terminate although occasional large floods continue across extensive unchanneled alluvial surfaces termed floodouts these broad similarities apart downstream changes along the three rivers differ in detail the sandover is largely a single thread channel whereas many reaches of the sandover bundey and woodforde are anabranched on the small woodforde river downstream decreases in parameters such as cross sectional area and width are roughly linear on the larger sandover and sandover bundey downstream changes are more irregular particularly through the floodout zones where there are marked fluctuations in widths depths and bed slopes the irregular downstream changes typical of the lower reaches of these large rivers may be due to the reduced influence of vegetation on bankline stability and width adjustment relative to that of smaller rivers on the northern plains as in other drylands complex interactions between discharge sediment transport slope patterns of tributary drainage bank sediment type and vegetation result in variable patterns of downstream channel change

intra arc sedimentation in a low lying marginal arc eocene clarno formation central oregon abstract the clarno formation consists of andesitic volcaniclastic rocks interstratified with clayey paludal sediments and lava flows and cut locally by irregular hypabyssal stocks dikes and sills lateral lithofacies variations are pronounced and intrusive and extrusive volcanic rocks appear haphazardly emplaced throughout the formation a range of sedimentary environments is represented in the volcaniclastic sequences including near vent flow and breccia accumulations bouldery high gradient braided streams and relatively low gradient sandy tuff braidplains associated with paludal deposits we infer that the coarse grained volcaniclastic rocks of the clarno formation accumulated largely in volcanic flank and apron settings the formation s stratigraphy reflects an abundance of small volcanoes erupting into sedimentary lowlands and only a few scattered remnants of large central vent volcanoes are known the absence of systematic variation across a large outcrop belt argues against derivation of the succession from a line of volcanoes beyond the reaches of the present outcrop we infer that the arc was composed of small to medium size volcanoes arranged haphazardly across a broad area the sedimentary succession most probably accumulated in a series of intra arc depressions formed by diffuse block rotation driven by oblique subduction during the eocene