oxygen isotope evidence for hydrothermal alteration within a quaternary stratovolcano lassen volcanic national park california brokeoff volcano a quaternary stratocone located in the lassen volcanic center in northern california has been deeply eroded exposing a 10 km2 meteoric hydrothermal alteration zone at the core of the volcano portions of the former volcanic edifice are sufficiently well preserved that an unusual opportunity exists wherein the alteration pattern can be correlated with the position of the volcanic cone the 18o analyses of more than 100 whole rock samples consisting primarily of andesitic lavas vary from 9 8 to 0 6 per mil the highest 18o values occur in bleached solfatarically altered rocks that have interacted with low ph fumarolic hot springs associated with the present day hydrothermal system low 18o values are found in propylitically altered rocks that underwent isotopic exchange with meteoric hydrothermal fluids at elevated temperatures mostly during the stratovolcanic stage 650 400ka of the hydrothermal system but probably continuing today at depth electron microprobe analyses of secondary layer silicate minerals in strongly propylitized samples 18o 5 0 revealed the presence of discrete chlorite suggesting that temperatures up to 200 to 250 c were attained in the shallow levels of the system two zones of pervasive meteoric hydrothermal alteration defined by concentric 18o contours that are probably interconnected at depth are located within the original topographic edifice of the volcano the most intensely altered rocks within these equant zones of alteration define nnw trends that coincide with stream valleys and with regional structural patterns a comparison of the characteristics of the 18o depleted zone at brokeoff with those of more deeply eroded volcanic centers such as the comstock lode mining district criss and champion 1991 permits the construction of composite 18o cross sections through a hypothetical intact stratovolcano at both brokeoff and comstock hydrothermal fluids were strongly focused into plumelike zones of intense 18o depletion at comstock these low 18o plumes are associated with faults although major fault displacements are not observed at brokeoff the topographic and alteration patterns are consistent with the presence of a linear array of faults that acted as conduits for fluid flow up into the shallow levels of the volcano

rapid circulation of warm subtropical waters in a major glacial fjord in east greenland the recent rapid increase in mass loss from the greenland ice sheet is primarily attributed to an acceleration of outlet glaciers oceanographic data obtained in summer 2008 show that subtropical waters that reside year round in the shelf ocean off greenland continuously enter a large glacial fjord in east greenland and contribute to melting at the glacier terminus the recent rapid increase in mass loss from the greenland ice sheet1 2 is primarily attributed to an acceleration of outlet glaciers3 4 5 one possible cause of this acceleration is increased melting at the ice ocean interface6 7 driven by the synchronous warming8 9 10 of subtropical waters offshore of greenland however because of the lack of observations from greenland s glacial fjords and our limited understanding of their dynamics this hypothesis is largely untested here we present oceanographic data collected in sermilik fjord east greenland by ship in summer 2008 and from moorings our data reveal the presence of subtropical waters throughout the fjord these waters are continuously replenished through a wind driven exchange with the shelf where they are present all year the temperature and renewal of these waters indicate that they currently cause enhanced submarine melting at the glacier terminus key controls on the melting rate are the volume and properties of the subtropical waters on the shelf and the patterns of along shore winds suggesting that the glaciers acceleration has been triggered by a combination of atmospheric and oceanic changes our measurements provide evidence for a rapid advective pathway for the transmission of oceanic variability to the ice sheet margins

subaqueous melting of store glacier west greenland from three dimensional high resolution numerical modeling and ocean observations we present three dimensional high resolution simulations of ice melting at the calving face of store glacier a tidewater glacier in west greenland using the massachusetts institute of technology general circulation model we compare the simulated ice melt with an estimate derived from oceanographic data the simulations show turbulent upwelling and spreading of the freshwater laden plume along the ice face and the vigorous melting of ice at rates of meters per day the simulated august 2010 melt rate of 2 0 0 3 m d is within uncertainties of the melt rate of 3 0 1 0 m d calculated from oceanographic data melting is greatest at depth above the subglacial channels causing glacier undercutting melt rates increase proportionally to thermal forcing raised to the power of 1 2 1 6 and to subglacial water flux raised to the power of 0 5 0 9 therefore in a warmer climate store glacier melting by ocean may increase from both increased ocean temperature and subglacial discharge key points simulated melt agrees with estimates from a hydrographic survey of the glacier with more runoff glaciers will melt faster at constant ocean temperature 2013 american geophysical union all rights reserved

molecular records of microbialites following the end permian mass extinction in chongyang hubei province south china abstract by using gas chromatograph gc and gc mass spectrometry a series of biomarkers were identified in the aliphatic fractions of extracts from microbialites following the end permian faunal mass extinction in chongyang section hubei province south china the dominance of lower molecular weight n alkanes in the samples from the section suggests that algae and bacteria were the dominant contributors to the organic fraction preserved in this marine section with bacteria particularly enhanced within the stromatolite the end permian biotic crisis coincided with a short term suppression of autotrophs indicated by the ratio of pristane pr and phytane ph to c 17 and c 18 n alkanes and proliferation of heterotrophs shown by the ratio of branched to normal c 17 alkanes the biotic crisis also may have been associated with a transient oxygenation event in the shallow marine indicated by elevated pr ph ratios and lowered c 27 18 h 22 29 30 trisnorneohopane ts to c 27 17 h 22 29 30 trisnorhopane tm and gammacerane to c 31 homohopane c 31 hp ratios subsequently three short term episodes of intensified anoxia or enhanced salinity at chongyang are recorded by decreases in pr ph ratios in conjunction with lithological boundaries in the section

uplift of the sierra nevada of california gravity and most seismic interpretations agree that the sierra nevada is at present isostatically compensated by a crustal root the most reasonable timing for emplacement of the root was during mesozoic batholithic intrusions this is difficult to reconcile with the evidence that major uplift of the mountains occurred in the past 10 m y a simple quantitative model for flexural isostasy of an elastic plate before and after breaking resolves this problem and explains the tilt of the sierra nevada block and variations in topography along the range we postulate that cooling and elastic strengthening of the mesozoic magmatic arc prevented its reaching local isostatic equilibrium during erosion in the cenozoic thus an overcompensated residual mountain range was held down elastically until basin and range extension broke the elastic plate along the owens valley and allowed rapid uplift the moho density contrast that best models the amplitude of both late cenozoic uplift and prior topography is slightly larger than seismic and gravity models suggest this implies that up to 20 of the uplift may be driven by density contrasts in the upper mantle the idea of overcompensated erosional remnants and their release upon lithospheric faulting has a more general application to anorogenic uplift of mountain blocks

the effect of meltwater plumes on the melting of a vertical glacier face abstractfreshwater produced by the surface melting of ice sheets is commonly discharged into ocean fjords from the bottom of deep fjord terminating glaciers the discharge of the freshwater forms upwelling plumes in front of the glacier calving face this study simulates the meltwater plumes emanated into an unstratified environment using a nonhydrostatic ocean model with an unstructured mesh and subgrid scale mixing calibrated by comparison to established plume theory the presence of an ice face reduces the entrainment of seawater into the meltwater plumes so the plumes remain attached to the ice front in contrast to previous simple models ice melting increases with height above the discharge also in contrast to some simple models and the authors speculate that this overcutting may contribute to the tendency of icebergs to topple inwards toward the ice face upon calving the overall melt rate is found to increase with discharge flux only up to a critical value which depends on the channel size

estimating ocean heat transports and submarine melt rates in sermilik fjord greenland using lowered acoustic doppler current profiler ladcp velocity profiles submarine melting at the ice ocean interface is a significant term in the mass balance of marine terminating outlet glaciers however obtaining direct measurements of the submarine melt rate or the ocean heat transport towards the glacier that drives this melting has been difficult due to the scarcity of observations as well as the complexity of oceanic flows here we present a method that uses synoptic velocity and temperature profiles but accounts for the dominant mode of velocity variability to obtain representative heat transport estimates we apply this method to the sermilik fjord helheim glacier system in southeastern greenland using lowered acoustic doppler current profiler ladcp and hydrographic data collected in summer 2009 we find a mean heat transport towards the glacier of 29 10 9 w implying a submarine melt rate at the glacier face of 650 m a 1 t he resulting adjusted velocity profile is indicative of a multilayer residual circulation where the meltwater mixture flows out of the fjord at the surface and at the stratification maximum

rapid submarine melting of the calving faces of west greenland glaciers widespread glacier acceleration has been observed in greenland in the past few years oceanographic observations taken in summer 2008 show that ocean waters melted a substantial fraction of ice along the calving fronts of three west greenland glaciers indicating that submarine melting has a profound influence on grounding line stability widespread glacier acceleration has been observed in greenland in the past few years1 2 3 4 associated with the thinning of the lower reaches of the glaciers as they terminate in the ocean5 6 7 these glaciers thin both at the surface from warm air temperatures and along their submerged faces in contact with warm ocean waters8 little is known about the rates of submarine melting9 10 11 and how they may affect glacier dynamics here we present measurements of ocean currents temperature and salinity near the calving fronts of the eqip sermia kangilerngata sermia sermeq kujatdleq and sermeq avangnardleq glaciers in central west greenland as well as ice front bathymetry and geographical positions we calculate water mass and heat budgets that reveal summer submarine melt rates ranging from 0 7 0 2 to 3 9 0 8 m d 1 these rates of submarine melting are two orders of magnitude larger than surface melt rates but comparable to rates of iceberg discharge we conclude that ocean waters melt a considerable but highly variable fraction of the calving fronts of glaciers before they disintegrate into icebergs and suggest that submarine melting must have a profound influence on grounding line stability and ice flow dynamics

post 12 m y rotation of southwest washington paleomagnetic field directions from the basalt of pack sack lookout are compared to those from the pomona member of the saddle mountains basalt of the columbia river basalt group the pomona crops out over a wide region on the columbia plateau east of the cascade range and the basalt of pack sack lookout crops out well to the west of the cascades about 30 to 60 km east of the washington coast our paleomagnetic results support existing geologic and geochemical evidence that both these units are part of a single 12 m y old flow that erupted in western idaho and flowed to the washington coast the magnetic data further indicate that southwest washington has undergone clockwise rotation with respect to the columbia plateau and stable north america the data suggest that both a large scale regional rotation of 16 and locally complex small scale rotations exist the two being present to different degrees in the eastern and western regions of southwest washington the cascade range appears to coincide with the tectonic boundary separating rotated and unrotated regions of washington state rotation of southwest washington appears to have been associated with the rotation of large pieces of western oregon and southern washington possibly as a result of either a ridge push force from the basin and range province or shear along the pacific north america plate boundary

mafic magmatism and associated tectonism of the central high cascade range oregon volcanism in the central high cascade range has been dominated since late miocene time by the construction of a mafic platform of coalescent shield volcanoes within an intra arc graben that developed in response to extensional plate tectonics graben development along the cascade crest was possibly activated by failure of crust that had become thermally weakened during previous episodes of calc alkaline magmatism lithospheric extension along the central cascade arc is attributed to a decrease in the convergence rate of the farallon north american plate system since early tertiary time geochemical variations compiled from new and existing data are used to delineate four major catagories of basalts sio2 47 53 wt and basaltic andesites sio2 53 60 wt 1 early high cascade high alumina olivine tholeiitic haot basalts 2 normal high cascade haot basalts 3 mount washington mw type basaltic andesites and 4 north sister ns type basaltic andesites compositional variations among the four types possibly reflect source differences although much of the variation especially in basaltic andesites might be related to open system evolution of primary magmas early high cascade basalts erupted during late miocene to early pliocene time display depleted light rare earth element ree and other large ion lithophile element llle abundances similar to those observed in mid ocean ridge baslts morb later basalts erupted since the pliocene have variably enriched lile and a wide range of ree fractionation e g la yb ratios and chemically resemble ocean island basalts oib pliocene to holocene basaltic andesites exhibit ranges in total ree zr hf and sc abundances that are similar to those in normal basalts yet the abundances of these elements in ns types are overall lower than the average basalt contents fractionation among the relative abundances of ree zr hf and sc between ns and mw types are not appreciable compared to normal basalts both types of basaltic andesite exhibit uniformally elevated abundances of mobile lile cs rb k sr ba and th and marginally depleted abundances of high field strength elements hfse ta and nb typical of magmas erupted in magmatic arcs chemical signatures in central high cascade basalts imply that the mantle beneath the central cascades has been subjected to variable influences of morb and oib like components the implication of oceanic mantle beneath the central cascades is consistent with the existence of the columbia embayment into which a segment of oceanic lithosphere was compressed and thickened relatively high mobile lile and somewhat depleted hfse in the basaltic andesites considered to be secondary effects to oib morb mantle evolution are believed to believed to result from interaction of aqueous subduction derived fluids with evolving magmas or their sources in lower crustal or upper mantle regions the preponderance of haot basalts that exhibit within plate chemical signatures argues for less input relative to typical calc alkaline magmatic arcs of aqueous fluids due to subduction processes the petrologic signicance of a subduction zone beneath the cascade arc may be restricted to the tectonic overprinting of an extensional system caused by a reduction in convergence rate on a waning of calc alkaline magmatism physical models of the high cascade subduction system are presented to explain the apparent chemical decoupling between calc alkaline and tholeiitic magmatism

21st century evolution of greenland outlet glacier velocities earlier observations on several of greenland s outlet glaciers starting near the turn of the 21st century indicated rapid annual scale and large 100 increases in glacier velocity combining data from several satellites we produce a decade long 2000 to 2010 record documenting the ongoing velocity evolution of nearly all 200 of greenland s major outlet glaciers revealing complex spatial and temporal patterns changes on fast flow marine terminating glaciers contrast with steady velocities on ice shelf terminating glaciers and slow speeds on land terminating glaciers regionally glaciers in the northwest accelerated steadily with more variability in the southeast and relatively steady flow elsewhere intraregional variability shows a complex response to regional and local forcing observed acceleration indicates that sea level rise from greenland may fall well below proposed upper bounds

convection driven melting near the grounding lines of ice shelves and tidewater glaciers subglacial meltwater draining along the bed of fast flowing marine terminating glaciers emerges at the groundingline wheretheiceeithergoesafloattoformaniceshelforterminates inacalvingface theinputof freshwater to the ocean provides a source of buoyancy and drives convective motion alongside the ice ocean interface this process is modeled using the theory of buoyant plumes that has previously been applied to the study of the larger scale circulation beneath ice shelves the plume grows through entrainment of ocean waters and the heat brought into the plume as a result drives melting at the ice ocean interface the equations are nondimensionalized by using scales appropriate for the region where the subglacial drainage rather than the subsequent addition of meltwater supplies the majority of the buoyancy forcing it is found that the melt rate within this region can be approximated reasonably well by a function that is linear in ocean temperature has a cube root dependence on the flux of subglacial meltwater and has a complex dependency on the slope of the ice ocean interface the model is used to investigate variability in melting induced by changes in both ocean temperature and subglacial discharge for a number of realistic examples of ice shelves and tidewater glaciers the results show how warming ocean waters and increasing subglacial drainage both generate increases in melting near the grounding line

diverse discrete mantle derived batches of basalt erupted along a short normal fault zone the poison lake chain southernmost cascades the poison lake chain consists of small monogenetic calc alkaline basaltic volcanoes located east of the cascade arc axis 30 km ene of lassen peak in northeastern california this chain consists of 39 distinguishable units in a 14 km long and 2 km wide zone trending nnw parallel to nearby quaternary normal faults the 39 units fall into nine coherent groups based on stratigraphy field characteristics petrography and major element compositions petrographic differences among groups are expressed by different amounts and proportions of phenocrysts mgo sio 2 k 2 o sio 2 and tio 2 sio 2 variation diagrams illustrate clear differences in compatible and incompatible elements among the groups variation of k 2 o tio 2 and k 2 o p 2 o 5 with mgo indicates that most of the basalts of the poison lake chain cannot be related by crystal fractionation at different pressures and that compositions have not been affected significantly by incorporation of low degree silicic crustal melt or interaction with sialic crust limited trace element and whole rock isotopic data also suggest little if any incorporation of upper crustal material and that compositional variation among groups primarily reflects source compositional differences precise 40 ar 39 ar determinations show that the lavas were erupted between 100 and 110 ka the migration of paleomagnetic remanent directions over 30 suggests that the entire poison lake chain could represent three short lived episodes of volcanism within a period as brief as 500 yr the diverse geologic petrographic chemical paleomagnetic and age data indicate that each of the nine groups represents a small discrete magma batch generated in the mantle and stored briefly in the lower crust a nnw normal fault zone provided episodic conduits that allowed rapid ascent of these batches to the surface where they erupted as distinct volcanic groups each aligned along a segment of the poison lake chain compositional diversity of these primitive magmas argues against widespread long lived ponding of uniform basalt magma at the base of the crust in this region and against interaction with a zone of melting assimilation storage and homogenization mash in the lower crust

modeling turbulent subglacial meltwater plumes implications for fjord scale buoyancy driven circulation abstractfjord scale circulation forced by rising turbulent plumes of subglacial meltwater has been identified as one possible mechanism of oceanic heat transfer to marine terminating outlet glaciers this study uses buoyant plume theory and a nonhydrostatic three dimensional ocean ice model of a typical outlet glacier fjord in west greenland to investigate the sensitivity of meltwater plume dynamics and fjord scale circulation to subglacial discharge rates ambient stratification turbulent diffusivity and subglacial conduit geometry the terminal level of a rising plume depends on the cumulative turbulent entrainment and ambient stratification plumes with large vertical velocities penetrate to the free surface near the ice face however midcolumn stratification maxima create a barrier that can trap plumes at depth as they flow downstream subglacial discharge is varied from 1 750 m3 s 1 large discharges result in plumes with positive temperature and salinity anomalies in the upper water column for

the 1936 1945 1947 and 1950 earthquake sequences near lassen peak california three vigorous earthquake sequences occurred near lassen peak in 1936 between 1945 and 1947 and in 1950 the latter two sequences included mainshocks of magnitude 5 0 and 5 5 respectively and thousands of smaller events no comparable earthquake sequences have occurred near lassen peak since 1950 the epicentral area lies within 20 km of the southern boundary of lassen volcanic national park in a northwest striking seismic zone that extends from lake tahoe to the vicinity of mount shasta in comparing their time history and magnitude distribution with other earthquake sequences that have occurred in regions of cenozoic volcanism within and east of the cascade range and the sierra nevada we find that the lassen earthquake sequences show similar characteristics to two earthquake sequences that occurred on basin and range faults near herlong california and klamath falls oregon we interpret this similarity as evidence that the lassen earthquakes were caused by basin and range extension and may have occurred on one or more basin and range faults in the lassen region however the limitations of the data do not allow other possible sources such as magmatic injection to be ruled out the most important implication of the lassen earthquake sequences is that earthquakes of m 5 or greater may occur in the lassen region perhaps quite close to lassen peak or other volcanoes the record of holocene volcanism and fault displacements in the region indicates that earthquake sequences driven by either tectonic or magmatic processes may occur near lassen peak and any significant earthquake sequence should be carefully monitored to assess its nature

tectonic setting of the southern cascade range as interpreted from its magnetic and gravity fields we have compiled and analyzed aeromagnetic data from the southern cascade range and compared them with residual gravity data from the same region in order to investigate regional aspects of these young volcanic rocks and of basement structures beneath them various constant level aeromagnetic surveys were mathematically continued upward to 4 571 m and numerically mosaicked into a single compilation extending from lat 40 10 n to lat 44 20 n these data were reduced to the pole upward continued an additional 10 km and compared with a magnetic topographic model and with residual gravity data upward continued to the same level several intriguing regional features are suggested by these data 1 the trinity ophiolite complex that is exposed west of mount shasta probably dips at a shallow angle to the east and continues in the subsurface at least 10 km east of mount shasta 2 mount shasta lassen peak and medicine lake volcanoes are located in a widespread magnetic low possibly caused by an upwarp of the curie temperature isotherm 3 crater lake caldera is located at the intersection of various linear anomalies interpreted to be related to structure in basement rocks below the cascade range 4 three sisters volcanoes and newberry crater are connected to each other by an arcuate magnetic source 5 the high cascades from lat 40 10 n to at least lat 44 30 n are marked by a residual gravity low which includes the three sisters volcanoes mount shasta medicine lake volcano mount mcloughlin and crater lake we believe this gravity feature represents a major structural depression beneath the high cascades 6 except for newberry crater every major volcano of the study area is located on the perimeter of a local gravitational low we suggest that the gravity lows reflect subsidence of low density volcanic material relative to denser country rock and that the major volcanoes have developed over structures at the perimeters of their respective depressions

emptying filling boxes the fluid mechanics of natural ventilation this paper describes the fluid mechanics of the natural ventilation of a space connected to a large body of stationary ambient fluid the flows are driven by buoyancy differences between the interior and exterior fluids connections with the ambient fluid are high level and low level openings two main forms of ventilation are identified mixing ventilation and displacement ventilation mixing ventilation occurs when the incoming ambient fluid mixes with the fluid within the space as is the case if dense fluid enters through a high level inlet in this case vertical stratification is weak displacement ventilation occurs when dense fluid enters at low levels and displaces the lighter fluid within the space out through high level openings a strong stable stratification develops in this case and there is little mixing between the incoming fluid and that in the interior both of these modes of ventilation are studied theoretically and the results are compared with laboratory experiments transient draining flows which occur when a space initially contains fluid of a density different from the ambient are examined the presence of internal sources of buoyancy allows steady states to be established and the effects of point line and vertically distributed sources are studied these steady states are extensions of filling box models with the addition of continuous exchange of fluid with the environment outside the space a major result of this work is that the form of the stratification within the space depends on the entrainment caused by the convective elements plumes produced by the buoyancy sources but is independent of the strength of the sources the strength of the stratification and the magnitudes of the velocities do however depend on the source strength the effects of opening size s and configurations are determined and criteria for producing a particular stratification within the space are established applications of this work to the ventilation of buildings are presented

anisotropic structure beneath central java from local earthquake tomography 1 in this study we present the new tomographic code anita which provides 3 d anisotropic p and isotropic s velocity distribution based on p and s traveltimes from local seismicity for the p anisotropic model we determine four parameters for each parameterization cell this represents an orthorhombic anisotropy with one predefined direction oriented vertically three of the parameters describe slowness variations along three horizontal orientations with azimuths of 0 60 and 120 and one is a perturbation along the vertical axis the nonlinear iterative inversion procedure is similar to that used in the lotos code we have implemented this algorithm for the updated data set of central java part of which was previously used for the isotropic inversion it was obtained that the crustal and uppermost mantle velocity structure beneath central java is strongly anisotropic with 7 10 of maximal difference between slow and fast velocity in different directions in the forearc area between southern coast and volcanoes the structure of both isotropic and anisotropic structure is strongly heterogeneous variety of anisotropy orientations and highly contrasted velocity patterns can be explained by a complex block structure of the crust beneath volcanoes we observe faster velocities in vertical direction which is probably an indicator for vertically oriented structures channels dykes in the crust beneath the middle part of central java north to merapi and lawu volcanoes we observe a large and very intense anomaly with a velocity decrease of up to 30 and 35 for p and s models respectively inside this anomaly e w orientation of fast velocity takes place probably caused by regional extension stress regime in a vertical section we observe faster horizontal velocities inside this anomaly that might be explained by layering of sediments and or penetration of quasi horizontal lenses with molten magma in the mantle trench parallel anisotropy is observed throughout the study area such anisotropy in the slab entrained corner flow may be due to presence of b type olivine having predominant axis parallel to the shear direction which appears in conditions of high water or and melting content

ancient and modern subduction zone contributions to the mantle sources of lavas from the lassen region of california inferred from lu hf isotopic systematics hafnium isotopic compositions have been determined on a suite of introduction calc alkaline and high alumina olivine tholeiitic lavas from the the geochemical and isotopic variability of mantle wedge lassen region of california and are used in conjunction with peridotites beneath continental volcanic arcs is poorly previously published mineralogical geochemical and isotopic data constrained this is a result of the fact that most of what to constrain their petrogenesis positive correlation between hf is known about the geochemistry of the mantle wedge is values and geochemical indices of the modern subduction component inferred from the composition of basaltic arc lavas which indicates that the isotopic compositions of the calc alkaline lavas although derived from the wedge are affected by variable record addition of radiogenic hf from the subducted slab however contributions from the subducting slab and continental the addition of the modern subduction component increases the hf crust e g gill 1981 tatsumi et al 1986 hawkesworth values of most calc alkaline lavas by 0 5 units over estimates of et al 1993 1994 despite these limitations there is non subduction enriched peridotites of the mantle wedge the lu hf significant evidence to suggest that mantle wedge periisotopic systematics of the lassen lavas suggest that the calc alkaline dotites are heterogeneous in composition and mineralogy magmas have equilibrated with garnet at some point in their history for example it has been postulated that the compositions whereas the tholeiitic magmas have not these observations require of mantle peridotites beneath the cascade arc vary from the two lava types to be derived from different sources the isotopic compositions that are similar to sources for mid ocean variability of the lassen lavas cannot be produced by mixing mantle ridge basalt morb hawaiian ocean island basalt sources inferred to be present in the eastern central pacific and oib and high alumina olivine tholeiites haot from western usa with a modern subduction component instead the the basin and range province hughes 1990 leeman isotopic variability is consistent with mixing of a depleted mantle et al 1990 bacon et al 1997 borg et al 1997 clynne source a more fertile mantle source enriched by an ancient subduction borg 1997 the restite mineralogy of the source component and a modern subduction component regions for cascade magmas is poorly constrained as well for example experiments conducted by baker et al 1994 found that calc alkaline basalts from the mt shasta region of california did not have garnet on their liquidus and reiners et al 2000 were able to model

late miocene and early pliocene basaltic rocks and their implications for crustal structure northeastern california and south central oregon a dominantly basaltic late miocene and early pliocene about 5 to 10 m y old lava field lies directly east of the cascade range on the northwesternmost edge of the basin and range province more than 12 400 km 2 of modoc county california and klamath and lake counties oregon termed the devils garden is underlain by basalt with a total volume of about 850 km 3 the basalt of devils garden is diktytaxitic olivine tholeiite characterized by high al content low k rb and cs content and high k rb and k cs ratios in these respects it resembles mid oceanic ridge basalt morb although it differs in other respects such as high concentration of ba and sr low k ba and higher 87 sr 86 sr 0 7036 0 7039 ratios chemical characteristics indicate that little or no contamination by sialic crustal material has occurred the basalts originated in the upper mantle and were erupted through crust thinned by tectonic extension behind the cascade range volcanic arc

distribution and compositions of magmatic inclusions in the mount helen dome lassen volcanic center california insights into magma chamber processes abstract variations in spatial abundances compositions and textures of undercooled magmatic inclusions were determined in a glaciated pleistocene lava dome mt helen 0 6 km3 at the lassen volcanic center lvc southernmost cascades spatial variations were determined by point counting at 86 locations separated by 100 m on the dome major and trace element compositions of host rocks and inclusions at 12 locations along the flow length of the dome were obtained important results include the following 1 inclusion abundances range from 3 19 vol with the highest values generally located along the little eroded northwestern margin and flow front of the dome 2 host rock compositions are markedly uniform across the dome 65 4 0 4 wt sio2 indicating that the degree of inclusion disaggregation was uniform despite large spatial variations in inclusion abundances 3 inclusion sizes range from a maximum of 1 m across to mm sized crystal clots of phenocrysts plus adhering ca rich plagioclase microphenocrysts 4 inclusions have variable macroscopic textures indicating that partial undercooling both prior to and following entrapment in cooler dacitic host magma were important processes 5 inclusions are variably fractionated magmas with large variations in ni 79 11 ppm and cr 87 7 ppm contents that are lower than presumed mantle derived melts furthermore large ranges in incompatible trace elements indicate that inclusion compositions also reflect deep processes involving either melting of variable mantle source rocks or assimilation fractional crystallization 6 inclusions are variably mixed magmas 56 61 wt sio2 that contain up to 50 host dacitic magma 7 correlations between ni and cr contents in hosts and inclusions from individual outcrops indicate that the effect of inclusion disaggregation and magma mingling on host dacitic magma was local e g these features are interpreted to reflect protracted recharge of diverse composition mafic magmas into the base of a shallow magma reservoir containing a lower layer of compositionally evolving hybrid mafic magma and an upper layer of rhyodacitic magma complex interactions among magmas in the upper and lower layers resulted in formation of mafic inclusions by both pre and post entrapment undercooling mechanisms followed by buoyant rise and accumulation near the roof of the reservoir the main stage of inclusion disaggregation probably occurred in the conduit during eruptive ascent and largely ceased upon surface eruption we infer that endogenous growth concentrated inclusions along the margins and top of the dome as more inclusion poor dacite was progressively tapped from deeper regions of the reservoir

different magnitudes of projected subsurface ocean warming around greenland and antarctica accurate projections of global sea level rise require information of future ocean warming in the vicinity of the large ice sheets an analysis of 19 climate model projections suggests that subsurface ocean warming near both polar ice sheets will be substantial with the potential to lead to significant increases in ice mass loss

does calving matter evidence for significant submarine melt abstract during the summer in the northeast pacific ocean the alaska coastal current sweeps water with temperatures in excess of 12 c past the mouths of glacierized fjords and bays the extent to which these warm waters affect the mass balance of alaskan tidewater glaciers is uncertain here we report hydrographic measurements made within icy bay alaska and calculate rates of submarine melt at yahtse glacier a tidewater glacier terminating in icy bay we find strongly stratified water properties consistent with estuarine circulation and evidence that warm gulf of alaska water reaches the head of 40 km long icy bay largely unaltered a 10 20 m layer of cold fresh glacially modified water overlies warm saline water the saline water is observed to reach up to 10 4 c within 1 5 km of the terminus of yahtse glacier by quantifying the heat and salt deficit within the glacially modified water we place bounds on the rate of submarine melt the submarine melt rate is estimated at 9 m d 1 at least half the rate at which ice flows into the terminus region and can plausibly account for all of the submarine terminus mass loss our measurements suggest that summer and fall subaerial calving is a direct response to thermal undercutting of the terminus further demonstrating the critical role of the ocean in modulating tidewater glacier dynamics

seismic refraction study of upper crustal structure in the lassen peak area northern california in september 1982 the u s geological survey recorded a 125 km long refraction profile through the lassen peak area of the cascade range this profile extended northwestward to the klamath mountains and southeastward to the sierra nevada foothills interpretation of the refraction data determined the upper crustal structure in this volcanic region of northern california near surface velocities 1 5 5 55 km s are consistent with a thickness of 2 4 km for the cascade volcanic rocks between 0 5 km above and 1 8 km below sea level velocities are about 4 7 5 55 km s the highest velocities are modeled below the devils kitchen terminal geyser area possibly indicating hydrothermal alteration of the volcanic rocks the absence of low velocities less than about 3 4 5 km s deeper than 1 km above sea level suggests that the cretaceous sedimentary rocks exposed west of lassen peak do not underlie the volcano southeast of lassen peak at 1 8 km below sea level a 0 3 km thick layer with a velocity of 6 1 km s overlies a lower velocity 5 7 5 8 km s 2 9 km thick layer because these modeled velocities do not change across the boundary between the sierra nevada foothills and the cascade range sierran basement could continue under the cascade volcanic rocks as far northwest as lassen peak the velocity model does not indicate the presence of a magma chamber larger than 3 km in diameter within the top 8 km of the crust

re os isotopic systematics of primitive lavas from the lassen region of the cascade arc california abstract rhenium osmium isotopic systematics of primitive calc alkaline lavas from the lassen region appear to be controlled by mantle wedge processes lavas with a large proportion of slab component have relatively low re and os abundances and have radiogenic os and mid ocean ridge basalt like sr and pb isotopic compositions lavas with a small proportion of slab component have higher re and os elemental abundances and display mantle like os sr nd and pb isotopic compositions assimilation with fractional crystallization can only generate the re os systematics of the lassen lavas from a common parent if the distribution coefficient for re in sulfide is 40 1100 times higher than most published estimates and if most incompatible element abundances decrease during differentiation high re os ratios in mid ocean ridge basalts makes subducted oceanic crust a potential source of radiogenic os in volcanic arcs the slab beneath the southernmost cascades is estimated to have 187os 188os ratios as high as 1 4 mixing between a slab component and mantle wedge peridotite can generate the os isotopic systematics of the lassen lavas provided the slab component has a sr os ratio of 7 5 105 and os abundances that are 100 600 times higher than mid ocean ridge basalts for this model to be correct os must be readily mobilized and concentrated in the slab component perhaps as a result of high water and hcl fugacities in this subduction environment another possible mechanism to account for the correlation between the magnitude of the subduction geochemical signature and os isotopic composition involves increasing the stability of an os bearing phase in mantle wedge peridotites as a result of fluxing with the slab component melting of such a source could yield low os magmas that are more susceptible to crustal contamination and hence have more radiogenic os isotopic compositions than magmas derived from sources with a smaller contribution from the slab thus the addition of the slab component to the mantle wedge appears to result in either the direct or indirect addition of radiogenic os to arc magmas

rapid submarine melting driven by subglacial discharge leconte glacier alaska 1 we show that subglacial freshwater discharge is the principal process driving high rates of submarine melting at tidewater glaciers this buoyant discharge draws in warm seawater entraining it in a turbulent upwelling flow along the submarine face that melts glacier ice to capture the effects of subglacial discharge on submarine melting we conducted 4 days of hydrographic transects during late summer 2012 at leconte glacier alaska a major rainstorm allowed us to document the influence of large changes in subglacial discharge we found strong submarine melt fluxes that increased from 9 1 1 0 to 16 8 1 3 m d 1 ice face equivalent frontal ablation as a result of the rainstorm with projected continued global warming and increased glacial runoff our results highlight the direct impact that increases in subglacial discharge will have on tidewater outlet systems these effects must be considered when modeling glacier response to future warming and increased runoff

multiple plagioclase crystal populations identified by crystal size distribution and in situ chemical data implications for timescales of magma chamber processes associated with the 1915 eruption of lassen peak ca products of the 1915 lassen peak eruption reveal evidence for a magma recharge magma mixing event that may have catalyzed the eruption and from which four compositional members were identified light dacite black dacite andesitic inclusion and dark andesite crystal size distribution textural and in situ chemical major and trace element and sr isotope data for plagioclase from these compositional products define three crystal populations that have distinct origins phenocrysts long axis40 5 mm that typically have core an contents between 34 and 36 mol microphenocrysts long axis between 0 1 and 0 5 mm that have core an contents of 66 69 and microlites long axis50 1mm with variable an core contents from 64 to 52 phenocrysts are interpreted to form in an isolated dacitic magma chamber that experienced slow cooling based on textural compositional and isotopic data for the magma represented by the dacitic component magma recharge was not an important process until just prior to the 1915 eruption average residence times for phenocrysts are in the range of centuries to millennia microphenocrysts formed in a hybrid layer that resulted from mixing between endmember reservoir dacite and recharge magma of basaltic andesite composition high thermal contrast between the two end member magmas led to relatively high degrees of undercooling which resulted in faster crystal growth rates and acicular and swallowtail crystal habits some plagioclase phenocrysts from the dacitic chamber were incorporated into the hybrid layer and underwent dissolution precipitation seen in both crystal textures and rim compositions average microphenocryst residence times are of the order of months microlites may have formed in response to decompression and or syn eruptive degassing as magma ascended from the chamber through the volcanic conduit chemical distinctions in plagioclase microlite an contents reveal that melt of the dark andesite was more mafic than the melt of the other three compositions we suggest that mixing of an intruding basaltic andesite and reservoir dacite before magma began ascending in the conduit allowed formation of a compositionally distinct microlite population melt in the other three products was more evolved because it had undergone differentiation during the months following initial mixing as a consequence melt and microlites among these three products have similar compositions the results of this study highlight the integrated use of crystal size distribution textural and in situ chemical data in identifying distinct crystal populations and linking these populations to the thermal and chemical characteristics of complex magma bodies

oceanic heat delivery via kangerdlugssuaq fjord to the south east greenland ice sheet acceleration of the greenland ice sheet gris tidewater outlet glaciers has increased the ice sheet s contribution to global sea level rise over the last two decades coincident increases in atmospheric temperatures around greenland explain some of the increased ice loss but warm atlantic origin water aw is increasingly recognized as contributing to the accelerating ice mass loss particularly via the outlet glaciers of south east se greenland however there remains a lack of understanding of the variability in heat content of the water masses found to the east of greenland and how this heat is communicated to the outlet glaciers of the gris here a new analysis is presented of ocean gris interaction in which the oceanic heat flux toward the ice sheet in kangerdlugssuaq fjord 0 26 tw is an order of magnitude greater than that reported for the other major outlet glacier of se greenland helheim heat delivered by aw to the calving front of kangerdlugssuaq is equivalent to 10 m d 1 melt i e 30 60 of the ice flow speed and thus is highly significant during the observational campaign in september 2010 warm polar surface water psww melted a substantial volume of ice within the fjord equivalent to 25 of the volume melted by aw alone satellite derived sea surface temperatures show large interannual variability in psww over the 20 year period 1991 2011 anomalously warm psww was observed within the fjord prior to the well documented major ice front retreats of may 2004 and november 2010

regional velocity structure in northern california from inversion of scattered seismic surface waves seismic surface waves recorded by the berkeley digital seismic network have been analyzed in order to constrain three dimensional lateral heterogeneity of the upper mantle under northern california a total of 2164 seismograms from 173 teleseismic events were windowed for the fundamental mode rayleigh wave followed by estimation of complex amplitude spectra over the period range 16 to 100 s using a multiple taper method since rayleigh waves at shorter periods particularly below 35 s suffer from serious multipathing or non plane wave arrivals these amplitude spectra have been interpreted as the product of wavefront distortion along the teleseismic propagation path and seismic structure beneath the network the amplitude spectra are first modeled in terms of non plane incoming wavefields and structural phase velocity perturbations period by period after corrections for moho and surface topography the phase velocity maps are inverted for three dimensional shear velocity perturbations s down to a depth of 200 km the s maps are in good agreement with the results of body studies over a broad spatial scale the dominant signals are associated with the thermal effects of the active gorda and fossil farallon subducted slab stretching from mount shasta through the western sierran foothills to the southern great valley and asthenospheric upwelling beneath the northern coast ranges the southern sierra nevada range is characterized by fast s down to 50 km and slow velocities between 60 and 120 km depth in agreement with independent inferences of a cold crust and warm upper mantle which may provide the buoyancy forces necessary to support the elevation of the range

correlation of the rockland ash bed a 400 000 year old stratigraphic marker in northern california and western nevada and implications for middle pleistocene paleogeography of central california abstract outcrops of an ash bed at several localities in northern california and western nevada belong to a single air fall ash layer the informally named rockland ash bed dated at about 400 000 yr b p the informal rockland pumice tuff breccia a thick coarse compound tephra deposit southwest of lassen peak in northeastern california is the near source equivalent of the rockland ash bed relations between initial thickness of the rockland ash bed and distances to eruptive source suggest that the eruption was at least as great as that of the mazama ash from crater lake oregon identification of the rockland tephra allows temporal correlation of associated middle pleistocene strata of diverse facies in separate depositional basins specifically marine littoral estuarine and fluvial strata of the hookton and type merced formations correlate with fluvial strata of the santa clara formation and unnamed alluvium of willits valley and the hollister area in northwestern and west central california and with lacustrine beds of mohawk valley fluvial deposits of the red bluff formation of the eastern sacramento valley and fluvial and glaciofluvial deposits of fales hot spring carson city and washoe valley areas in northeastern california and western nevada stratigraphic relations of the rockland ash bed and older tephra layers in the great valley and near san francisco suggest that the southern great valley emerged above sea level about 2 my ago that its southerly outlet to the ocean was closed sometime after about 2 my ago and that drainage from the great valley to the ocean was established near the present northerly outlet in the vicinity of san francisco bay about 0 6 my ago

three dimensional geometry of the gorda plate beneath northern california the isostatic residual gravity field over northern california displays a gravity gradient interpreted to reflect the south edge of the gorda plate where it is subducted eastward beneath the north american plate the locus of points of maximum slope defines a line trending s60 e from a coastal point approximately 20 km south of cape mendocino a point where the buried plate boundary is inferred from magnetic and seismicity data southeast from the coast for a distance of 120 km the gravity anomaly parallels the strike of the blanco fracture zone and the present direction of relative motion between the pacific and northern gorda plates calculations from the form of the anomaly yield depth estimates that fit an east southeast plunge of approximately 9 for the top of the gorda south edge the sense of the anomaly higher gravity to the south supports the hypothesis that a window developed in the subducted slab east of the san andreas fault and south of the gorda plate south of the gorda boundary the base of the north american plate is thus in contact with hot material from the asthenosphere that invaded the window because the overlying north american plate has been moving relatively south across the gorda boundary the north american plate beneath the coast ranges east of the san andreas fault in central california may be decoupled from the underlying material at a depth slightly deeper than the depth to the top of the boundary at the time the north american plate passed over it

modeling the impact of glacial runoff on fjord circulation and submarine melt rate using a new subgrid scale parameterization for glacial plumes the injection at depth of ice sheet runoff into fjords may be an important control on the frontal melt rate of tidewater glaciers here we develop a new parameterization for ice marginal plumes within the massachusetts institute of technology general circulation model mitgcm allowing three dimensional simulation of large 500 km2 glacial fjords on annual or longer time scales we find that for an idealized fjord without shelf driven circulation subglacial runoff produces a thin strong and warm down fjord current in the upper part of the water column balanced by a thick and slow up fjord current at greater depth although submarine melt rates increase with runoff due to higher melt rates where the plume is in contact with the ice front we find that annual submarine melt rate across the ice front is relatively insensitive to variability in annual runoff better knowledge of the spatial distribution of runoff controls on melt rate in those areas not directly in contact with plumes and feedback mechanisms linking submarine melting and iceberg calving are necessary to more fully understand the sensitivity of glacier mass balance to runoff driven fjord circulation

observation and parameterization of ablation at the base of ronne ice shelf antarctica parameterizations of turbulent transfer through the oceanic boundary layer beneath an ice shelf are tested using direct measurements of basal ablation observations were made in the southwestern part of ronne ice shelf about 500 km from open water the mean basal ablation rate was measured over a month long and a year long period using phase sensitive radar to record the thinning of the ice shelf ocean temperatures were observed within about 25 m of the ice shelf base over the period of the radar observations while the tidally dominated ocean currents were estimated from tidal analysis of collocated current observations from an earlier period ablation rates derived using these ocean data and a number of bulk parameterizations of turbulent transfer within the boundary layer are compared with the direct measurements the ablation rates derived using a parameterization that explicitly includes the impact of ocean currents on the turbulent transfer of heat and salt match the observations to within 40 with suitable tuning of the drag coefficient the mismatch can be reduced below the level of the observational errors equally good agreement can be obtained with two slightly simpler current dependent parameterizations that use constant turbulent transfer coefficients andtheoptimalvaluesforthecoefficientsatthis particularlocationon ronneice shelfaregiven

modeling thermodynamic ice ocean interactions at the base of an ice shelf abstract models of ocean circulation beneath ice shelves are driven primarily by the heat and freshwater fluxes that are associated with phase changes at the ice ocean boundary their behavior is therefore closely linked to the mathematical description of the interaction between ice and ocean that is included in the code an hierarchy of formulations that could be used to describe this interaction is presented the main difference between them is the treatment of turbulent transfer within the oceanic boundary layer the computed response to various levels of thermal driving and turbulent agitation in the mixed layer is discussed as is the effect of various treatments of the conductive heat flux into the ice shelf the performance of the different formulations that have been used in models of sub ice shelf circulation is assessed in comparison with observations of the turbulent heat flux beneath sea ice formulations that include an explicit parameterization of the oceanic boundary layer give results that

stratigraphic lithologic and major element geochemical constraints on magmatic evolution at lassen volcanic center california the lassen volcanic center is the most recent of several long lived volcanic centers in the southernmost cascade range these centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism the evolution of the lassen volcanic center is described in three stages stages i and ii comprise the brokeoff volcano and 80 km3 andesitic stratocone active from 600 to 400 ka brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism stage iii comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km3 volcanism during stage iii was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas stage iii began at 400 ka with a rhyolitic caldera forming pyroclastic eruption and chemically related lavas additional sequences of dacite erupted between 250 200 ka and 100 0 ka hybrid andesites erupted adjacent to the silicic dome field between 300 and 0 ka porphyritic andesite and dacite with high al2o3 low tio2 medium k2o and feo mgo ratios of 1 5 2 0 are the most abundant rock types in the lassen volcanic center however the single most voluminous unit is sparsely phyric rhyolite pumice in general the lavas of lassen volcanic center form a single coherent trend on major element variation diagrams and in pseudo quaternary phase space consistant with an origin either by fractional crystallization or magma mixing in detail however the lack of systematic temporal change in silica and subtly crossing trends indicate a complex origin a variety of statistically successful fractional crystallization models can be constructed that derive brokeoff andesites from regional magmas an important conclusion of the modeling is that if fractional crystallization is the process responsible for generation of brokeoff andesite then the parent magma must be low to medium k in geochemical affinity in order to explain the variation in k2o however although major element variation can be modeled by fractional crystallization petrographic and stratigraphic evidence indicates that magma mixing is an important but subtle process in brokeoff lavas and suggests that lavas evolved in small independent batches lavas erupted during stage iii while predominantly silicic range from 53 to 75 sio2 disequilibrium mineral assemblages in the stage iii lavas indicate that they are not directly derived from brokeoff andesite by fractional crystallization mixing of silicic magma with regional mafic magma and disaggregation of andesitic quenched magmatic inclusions play dominant roles in the compositional diversity of stage iii lavas

a kinematic model for the gorda deformation zone as a diffuse southern boundary of the juan de fuca plate several lines of evidence indicate that the space problem caused by southeastward motion of the juan de fuca plate toward the east west mendocino transform is resolved by distributed deformation of the oceanic crust north of the transform a kinematic model for this deformation based on the assumption of nondivergent flow conservation of area within the area known as the gorda plate and on the assumption of no subduction along the mendocino transform successfully matches the isochron pattern inferred from magnetic anomalies the model assumes strain rate decreasing to zero approaching the rigid juan de fuca plate with no sharp boundary or separate rigid area that would imply a discrete gorda plate this interpretation allows magnetic anomalies from south of the blanco fracture zone to be used to constrain the kinematics of the juan de fuca plate these anomalies imply a substantial change in the distance to the rotation pole at roughly 3 ma in addition to the clockwise shift in motion direction at 5 ma

tectonomagmatic evolution of cenozoic extension in the north american cordillera the spatial and temporal distributions of cenozoic extension and magmatism in the cordillera suggest that the onset of major crustal extension at a particular latitude was confined to a relatively narrow belt 100 km pre extension and followed the onset of intermediate and silicic magmatism by no more than a few million years extension began in early eocene time in southern british columbia northern washington idaho and montana farther s extension began at about the eocene oligocene boundary in the great basin and slightly later in the mojave sonora desert region the intervening area at the latitude of las vegas remained quiescent until mid miocene time compositional and isotopic characteristics of most pre miocene magmas are consistent with their containing major components of melted continental crust in mid miocene time two major changes occurred widening of the area of extension and the widespread appearance of basaltic magmas the area affected by extension from southwestern montana to the lake mead region widened to several hundred kilometres by this time extension in southern british columbia northern washington and northern montana had ceased probably before the end of the eocene and extension s of lake mead except in the gulf of california had waned regions affected by the broader belt of extension during late miocene pliocene and quaternary time experienced basaltic magmatism which began along a central rift zone in the northern part of the region and which had within a few million years spread to include most of the region later basaltic activity has tended to concentrate in restricted zones especially near the margins of the extended area we recognize a correlation between the amount of earlier crustal thickening and cenozoic extension and between the length of time after shortening but before extension and the degree to which a given region was intruded by late cretaceous plutons the localization of extension in areas of previous crustal thickening and the dependence of the timing of extension on the thermal state of the overthickened crust is consistent with a simple thermal mechanical model developed in a companion paper sonder et al this raises the possibility that stresses inherent in the north american plate dominated over plate interaction forces as controls of the cenozoic tectonomagmatic evolution of the north american cordillera especially in its earlier stages

evidence for a 180 km long subducted slab beneath northern california we present new seismic data for subduction of a 180 km long segment of the gorda plate beneath cape mendocino in northern california the inclined seismic zone dips 10 to 11 in the direction s68 e for a length of 120 km to depths of 30 to 35 km then steepens in dip to 25 and plunges to depths of at least 60 km p wave focal mechanisms of two earthquakes at depths of 30 and 58 km are consistent with downdip tension and imply that these events resulted from stresses within the slab and not from gorda north american plate interaction

seasonal dynamic thinning at helheim glacier a b s t r a c t we investigate three annual mass balance cycles on helheim glacier in south east greenland using tandem x interferometric digital elevation models dems bedrock gps measurements and ice velocity from feature tracking the dems exhibit seasonal surface elevation cycles at elevations up to 800 m a s l with amplitudes of up to 19 m from a maximum in july to a minimum in october or november concentrated on the fast flowing areas of the glacier indicating that the elevation changes have a mostly dynamic origin by modelling the detrended bedrock loading unloading signal we estimate a mean density for the loss of 671 70 kg m 3 and calculate that total water equivalent volume loss from the active part of the glacier surface flow speeds 1m day 1 ranges from 0 5 km 3 in 2011 to 1 6 km 3

modeling glacial meltwater plume dynamics and sedimentation in high latitude fjords 1 a model sedplume has been developed to simulate marine sediment deposited by glacial meltwater plumes emerging from tidewater glaciers turbid meltwater emerging from beneath a glacier into a fjord rises as a buoyant forced plume due to density contrasts with the ambient fjord water sedplume assumes that meltwater discharge flows at a constant rate for long enough periods that the plume reaches a steady state entrainment of ambient fluid into the turbulent plume is assumed to occur at a rate proportional to the local velocity of the plume plume motion is considered in two dimensions one horizontal dimension perpendicular to the glacier front and the vertical dimension an integral model is formulated for the conservation equations of volume momentum buoyancy and sediment flux along the path of a turbulent plume injected into stably stratified ambient fluid sedimentation occurs from the plume when the radial component of the sediment fall velocity exceeds the entrainment velocity when the plume reaches the surface it is treated as a radially spreading surface gravity current for which exact solutions exist for the sediment deposition rate flocculation of silt and clay particles is modeled using empirical measurements of particle settling velocities in fjords to adjust the settling velocity of fine grained sediments sedplume has been applied to mcbride inlet alaska a temperate glaciated fjord where the majority of sedimentation originates from meltwater sources sedplume produces rates and patterns of sedimentation in good agreement with observations with calculated peak ice proximal annual sedimentation rates of approximately 22 m yr 1

late cenozoic volcanism subduction and extension in the lassen region of california southern cascade range hundreds of short lived small to moderate volume mostly mafic volcanoes occur throughout the lassen region of ne california and surround five longer lived large volume intermediate to silicic volcanic centers younger than 3 ma volcanic rocks older than 7 ma are scarce in the lassen region we identify 537 volcanic vents younger than 7 ma and we classify these into five age intervals and five compositional categories based on sio2 content maps of vents by age and composition illustrate regionally representative volcanic trends by 2 ma the eastern limit of voicanism had contracted westward toward the late quaternary arc late quaternary volcanism is concentrated around and north of the silicic lassen volcanic center the belt of most recent volcanism 25 0 ka has been active since at least 2 ma most mafic volcanism is cakalkaline basalt and basaltic andesite however lesser volume of low potassium olivine tholeiite lkot a geochemically distinctive basalt type found in the northern basin and range province also has erupted throughout the lassen segment of the cascade arc since the pliocene thus models of the mantle source and tectonic control of lkot magmatism should be applicable both within and behind the subduction related arc normal faults and linear groups of vents are evidence of widespread crustal extension throughout most of the lassen region nnw alignments of these features indicate nnw orientation of maximum horizontal stress ene extension which is similar to the stress regime in the adjacent northwestern basin and range and northern sierra nevada provinces the large long lived volcanic centers developed just west of a zone of closely spaced nnw trending normal faults within that zone of faulting pervasive ene extension has precluded growth of large long lived crustal magma systems we interpret the western limit of the zone of nnw trending normal faults as the western boundary of the basin and range province where it overlaps the lassen segment of the cascade arc in our view the lassen volcanic region occurs above the subducting gorda north plate but also lies within a broad zone of distributed extension that occurs in the north american lithosphere east and southeast of the present cascadia subduction zone an episode of ene extension that began in the late miocene in the northwestern basin and range province appears to have triggered widespread late miocene to quaternary mafic volcanism in the lassen region the scarcity of volcanic rocks older than 7 ma suggests that a more compressive lithospheric stress regime prior to the late miocene extensional episode may have suppressed volcanism even though subduction probably was occurring beneath the lassen region

heat sources for glacial melt in a sub arctic fjord godthabsfjord in contact with the greenland ice sheet 1 recent warming of subpolar mode water off greenland has been suggested to accelerate the mass loss from tidal outlet glaciers of the greenland ice sheet we present a comprehensive analysis of water masses dynamics and interannual hydrographic variability in godthabsfjord a sill fjord in contact with tidal outlet glaciers on the west coast of greenland through seasonal observations we recognize an intermediate baroclinic circulation mode driven by tidal currents and an associated important local heat source for the fjord during summer this results in significant warming and freshening of the intermediate layer of the main fjord and the increase in heat content is equivalent to melting of 2 1 km3 of glacial ice this is comparable to 8 km3 glacial ice discharge estimated from the kangiata nun ta sermia calving front per year during winter the external heat source in the west greenland current enters the fjord as intermittent inflows of either cold 2 c dense water in pulses of 1 to 3 months duration four distinct circulation modes are observed in the fjord of which all can contribute to glacial ice melt an important aspect of the ice distribution in the fjord is that only a minor fraction is exported out of the fjord

areal distribution and age of low k high alumina olivine tholeiite magmatism in the northwestern great basin field petrographic chemical and geochronologic information has led to the identification and characterization of a widespread low k high alumina olivine tholeiite haot magma type in the northwestern great basin this basalt covers at least 22 000 km 2 and is estimated to represent a total volume of at least 650 km 3 the time period over which haot lavas were erupted extends from late miocene to holocene 10 5 0 m y b p this interval overlaps with the timing of snake river cascade and northwestern basin and range volcanism but distinguishes haot from the main pulse of columbia river volcanism 15 m y b p furthermore three major pulses of haot magmatism are suggested from the geochronology of this study 0 to 2 5 m y b p 3 5 to 6 m y b p and 7 to 10 m y b p the distinctive holocrystalline nonporphyritic and diktytaxitic texture the low incompatible element concentrations and the high mgo feo of haot serve to distinguish this basalt from other basalts of the northwestern united states the low incompatible element signature accentuates the similarities between haot mid ocean ridge basalts and back arc basin basalts these similarities combined with the haot chronology support the idea that the processes giving rise to extensional tectonism and haot magmatism in the northwestern great basin are similar to those acting in active back arc spreading regions

effect of near terminus subglacial hydrology on tidewater glacier submarine melt rates submarine melting of greenlandic tidewater glacier termini is proposed as a possible mechanism driving their recent thinning and retreat we use a general circulation model mitgcm to simulate water circulation driven by subglacial discharge at the terminus of an idealized tidewater glacier we vary the spatial distribution of subglacial discharge emerging at the grounding line of the glacier and examine the effect on submarine melt volume and distribution we find that subglacial hydrology exerts an important control on submarine melting under certain conditions a distributed system can induce a factor 5 more melt than a channelized system with plumes from a single channel inducing melt over only a localized area subglacial hydrology also controls the spatial distribution of melt which has the potential to control terminus morphology and calving style our results highlight the need to constrain near terminus subglacial hydrology at tidewater glaciers if we are to represent ocean forcing accurately

stratigraphic sedimentologic and petrologic record of late miocene subsidence of the central oregon high cascades pliocene to holocene volcanics of the central oregon high cascades largely confined to an intra arc graben bury older volcanic centers early high cascade volcanism is recorded by late miocene volcanics and sediments of the deschutes formation that accumulated in a fluvial basin east of the cascades aggradation in the adjacent fluvial basin occurred episodically when pyroclastic debris choked streams deschutes basin aggradation commenced near 7 4 ma and was synchronous with extension influenced cascade magmatism dominated by basalts and basaltic andesites initial subsidence of the cascades occurred at about 5 4 ma in response to plate margin processes structural isolation of the basin from ignimbrites and volcanism induced sedimentation resulted in widespread development of superimposed paleosols in the basin subsequent mafic lavas erupted near the eastern margin of the initial depression were truncated by east stepping down to the west faults at about 5 3 ma

acceleration of jakobshavn isbrae triggered by warm subsurface ocean waters observations over past decades show a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning in 1997 this switch is associated with a doubling in glacier velocity hydrographic data show a concurrent sudden increase in subsurface ocean temperatures along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were triggered by the arrival of relatively warm water originating from the irminger sea observations over the past decades show a rapid acceleration of several outlet glaciers in greenland and antarctica1 one of the largest changes is a sudden switch of jakobshavn isbrae a large outlet glacier feeding a deep ocean fjord on greenland s west coast from slow thickening to rapid thinning2 in 1997 associated with a doubling in glacier velocity3 suggested explanations for the speed up of jakobshavn isbrae include increased lubrication of the ice bedrock interface as more meltwater has drained to the glacier bed during recent warmer summers4 and weakening and break up of the floating ice tongue that buttressed the glacier5 here we present hydrographic data that show a sudden increase in subsurface ocean temperature in 1997 along the entire west coast of greenland suggesting that the changes in jakobshavn isbrae were instead triggered by the arrival of relatively warm water originating from the irminger sea near iceland we trace these oceanic changes back to changes in the atmospheric circulation in the north atlantic region we conclude that the prediction of future rapid dynamic responses of other outlet glaciers to climate change will require an improved understanding of the effect of changes in regional ocean and atmosphere circulation on the delivery of warm subsurface waters to the periphery of the ice sheets

subsurface structure of a maar diatreme and associated tuff ring from a high resolution geophysical survey rattlesnake crater arizona abstract geophysical survey techniques including gravity magnetics and ground penetrating radar were utilized to study the diatreme and tuff ring at rattlesnake crater a maar in the san francisco volcanic field of northern arizona significant magnetic anomalies 1600 nt and a positive gravity anomaly 1 4 mgal are associated with the maar joint modeling of magnetic and gravity data indicate that the diatreme that underlies rattlesnake crater has volume of 0 8 1 km 3 and extends to at least 800 m depth the modeled diatreme comprises at least two zones of variable density and magnetization including a low density highly magnetized unit near the center of the diatreme interpreted to be a pyroclastic unit emplaced at sufficiently high temperature and containing sufficient juvenile fraction to acquire thermal remanent magnetization magnetic anomalies and ground penetrating radar gpr imaging demonstrate that the bedded pyroclastic deposits of the tuff ring also carry high magnetization likely produced by energetic emplacement of hot pyroclastic density currents gpr profiles on the tuff ring reveal long 100 m wavelength undulations in bedding planes elsewhere comparable bedforms have been interpreted as base surge deposits inflated by air entrainment from eruption column collapse interpretation of these geophysical data suggests that rattlesnake crater produced highly energetic phreatomagmatic activity that gave way to less explosive activity as the eruption progressed the positive gravity anomaly associated with the maar crater is interpreted to be caused by coherent bodies within the diatreme and possibly lava ponding on the crater floor these dense magnetized bodies have excess mass of 2 4 10 10 kg and occupy approximately 5 of the diatreme by volume magnetic anomalies on the crater floor are elongate nw se suggesting that the eruption may have been triggered by the interaction of ascending magma with water in fractures of this orientation gpr imaging of the tuff ring also suggests that substantial land slip may have occurred on the western rim perhaps causing part of the tuff ring to collapse into the crater strong radar reflections indicative of well developed weathering horizons are present as well the techniques employed at rattlesnake crater demonstrate the value of combining multiple geophysical techniques in areas where exposures are limited and invasive exploration is not an option

geochemistry petrogenesis and tectonic implications of central high cascade mafic platform lavas petrographic and multi element geochemical data for a 37 sample suite representing oregon9s central high cascade mafic platform allow the distinction of basalts 2 from two types of basaltic andesites with associated high silica basaltic andesites 53 62 wt sio 2 all units yield fractionated ree and incompatible element enriched patterns characteristic of continental tholeiitic magmas mount washington mw type basaltic andesites are distinguished from north sister ns types by relatively higher total ree and other lile abundances although lithologic similarities prevail trace element signatures of both types do not support their derivation from magmas represented by basalts but represeint near primary melts from regions apart from the sources of primary basalt magmas normal basalts are modeled with 14 incongruent partial melting and concomitant olivine fractionation of spinel iherzolite upper mantle in the presence of water rich fluids derived from a subducted oceanic slab basaltic andesites are modeled as 6 to 12 partial melting of fluid enriched spinel iherzolite accompanied by olivine clinopyroxene plagioclase fractionation unusual units including one with ground mass biotite and rare quartz xenocrysts probably resulted from divergent processes of crustal assimilation crystal fractionation or melting from alternate sources early pleistocene eruptions were largely diktytaxitic basalt outpourings in response to crustal readjustments and subsidence which occurred prior to 4 5 m y ago the transformation to extensional tectonics abruptly reduced the amount of andesitic and silicic volcanism and allowed tapping of deeper mafic magma source regions chemical and petrographic data support the concept of juxtaposed tectonomagmatic types such that extension related basalts erupted within the realm of calc alkaline basaltic andesites and their silicic derivatives

oxygen isotope geochemistry of the lassen volcanic center california resolving crustal and mantle contributions to continental arc magmatism this study reports oxygen isotope ratios determined by laser fluorination of mineral separates mainly plagioclase from basaltic andesitic to rhyolitic composition volcanic rocks erupted from the lassen volcanic center lvc northern california plagioclase separates from nearly all rocks have o values 6 1 8 4o higher than expected for production of the magmas by partial melting of little evolved basaltic lavas erupted in the arc front and back arc regions of the southernmost cascades during the late cenozoic most lvc magmas must therefore contain high o crustal material in this regard the o values of the volcanic rocks show strong spatial patterns particularly for young rhyodacitic rocks that best represent unmodified partial melts of the continental crust rhyodacitic magmas erupted from vents located within 3 5 km of the inferred center of the lvc have consistently lower o values average 6 3o 0 1o at given sio2 contents relative to rocks erupted from distal vents 47 0 km average 7 1o 0 1o further magmas erupted from vents situated at transitional distances have intermediate values and span a larger range average 6 8o 0 2o basaltic andesitic to andesitic composition rocks show similar spatial variations although as a group the o values of these rocks are more variable and extend to higher values than the rhyodacitic rocks these features are interpreted to reflect assimilation of heterogeneous lower continental crust by mafic magmas followed by mixing or mingling with silicic magmas formed by partial melting of initially high o continental crust 9 0o increasingly hybridized by lower o 6 0o mantle derived basaltic magmas toward the center of the system mixing calculations using estimated endmember source o values imply that lvc magmas contain on a molar oxygen basis approximately 42 to 4 isotopically heavy continental crust with proportions declining in a broadly regular fashion toward the center of the lvc conversely the o values of the rhyodacitic rocks suggest that the continental crust in the melt generation zones beneath the lvc has been substantially modified by intrusion of mantlederived basaltic magmas with the degree of hybridization ranging on a molar oxygen basis from approximately 60 at distances up to 12 km from the center of the system to 97 directly beneath the focus region these results demonstrate on a relatively small scale the strong influence that intrusion of mantle derived mafic magmas can have on modifying the composition of pre existing continental crust in regions of melt production given this result similar but larger scale regional trends in magma compositions may reflect an analogous but more extensive process wherein the continental crust becomes progressively hybridized beneath frontal arc localities as a result of protracted intrusion of subduction related basaltic magmas

scalings for submarine melting at tidewater glaciers from buoyant plume theory abstractrapid dynamic changes at the margins of the greenland ice sheet synchronous with ocean warming have raised concern that tidewater glaciers can respond sensitively to ocean forcing understanding of the processes encompassing ocean forcing nevertheless remains embryonic the authors use buoyant plume theory to study the dynamics of proglacial discharge plumes arising from the emergence of subglacial discharge into a fjord at the grounding line of a tidewater glacier deriving scalings for the induced submarine melting focusing on the parameter space relevant for high discharge tidewater glaciers the authors suggest that in an unstratified fjord the often quoted relationship between total submarine melt volume and subglacial discharge raised to the power is appropriate regardless of plume geometry provided discharge lies below a critical value in these cases it is then possible to formulate a simple equation estimating total submarine melt volume as a function of discharge fjord temperature

seasonal variability of submarine melt rate and circulation in an east greenland fjord 1 the circulation in a glacial fjord driven by a large tidewater glacier is investigated using a nonhydrostatic ocean general circulation model with a melt rate parameterization at the vertical glacier front the model configuration and water properties are based on data collected in sermilik fjord near helheim glacier a major greenland outlet glacier the approximately two layer stratification of the fjord s ambient waters causes the meltwater plume at the glacier front to drive a double cell circulation with two distinct outflows one at the free surface and one at the layers interface in summer the discharge of surface runoff at the base of the glacier subglacial discharge causes the circulation to be much more vigorous and associated with a larger melt rate than in winter the simulated double cell circulation is consistent in both seasons with observations from sermilik fjord seasonal differences are also present in the vertical structure of the melt rate which is maximum at the base of the glacier in summer and at the layers interface in winter simulated submarine melt rates are strongly sensitive to the amount of subglacial discharge to changes in water temperature and to the height of the layers they are also consistent with those inferred from simplified one dimensional models based on the theory of buoyant plumes our results also indicate that to correctly represent the dynamics of the meltwater plume care must be taken in the choice of viscosity and diffusivity values in the model

externally forced fluctuations in ocean temperature at greenland glaciers in non summer months the factors that control the submarine melt rate at greenland s glaciers are uncertain and largely inferred from brief summer surveys in the fjords where glaciers terminate continuous records of water properties and velocity for the months september to may from two large greenland fjords reveal strong variability on 3 to 10 day timescales as a result of pulses of water that are propagated from the shelf ocean

a one dimensional model of ice shelf ocean interaction large scale oceanic circulation beneath antarctic ice shelves is driven by the thenmohaline differences which result from mass and energy exchange at the ice ocean interface dense saline waters are drawn underneath the ice shelves and emerge cooled and diluted as plumes of ice shelf water a simple one dimensional model of this process has been developed in which the ice shelf water plume is treated as a turbulent gravity current initiated at the inland margin by a flow of fresh meltwater emerging from beneath the grounded ice subsequent evolution of the plume as it ascends along an ice shelf base of specified geometry can be simulated the model has been applied to a flow line on ronne ice shelf antarctica to explain the observed distribution and rate of basal melting and freezing calculations indicate that the present mean melt rate of 0 6 m yr 1 would increase to 2 6 m yr 1 if the underlying water were to warm by 0 6 c this would not only lead to significant thinning of the ice shelf but could also cause a profound change in ocean circulation on the open continental shelf

impact of periodic intermediary flows on submarine melting of a greenland glacier the submarine melting of a vertical glacier front induced by an intermediary circulation forced by periodic density variations at the mouth of a fjord is investigated using a nonhydrostatic ocean general circulation model and idealized laboratory experiments the idealized configurations broadly match that of sermilik fjord southeast greenland a largely two layers system characterized by strong seasonal variability of subglacial discharge consistent with observations the numerical results suggest that the intermediary circulation is an effective mechanism for the advection of shelf anomalies inside the fjord in the numerical simulations the advection mechanism is a density intrusion with a velocity which is an order of magnitude larger than the velocities associated with a glacier driven circulation in summer submarine melting is mostly influenced by the discharge of surface runoff at the base of the glacier and the intermediary circulation induces small changes in submarine melting in winter on the other hand submarine melting depends only on the water properties and velocity distribution at the glacier front hence the properties of the waters advected by the intermediary circulation to the glacier front are found to be the primary control of the submarine melting when the density of the intrusion is intermediate between those found in the fjord s two layers there is a significant reduction in submarine melting on the other hand when the density is close to that of the bottom layer only a slight reduction in submarine melting is observed the numerical results compare favorably to idealized laboratory experiments with a similar setup

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