ground motion modeling of the 1906 san francisco earthquake part ii ground motion estimates for the 1906 earthquake and scenario events abstract we estimate the ground motions produced by the 1906 san francisco earthquake making use of the recently developed song et al 2008 source model that combines the available geodetic and seismic observations and recently constructed 3d geologic and seismic velocity models our estimates of the ground motions for the 1906 earthquake are consistent across five ground motion modeling groups employing different wave propagation codes and simulation domains the simulations successfully reproduce the main features of the boatwright and bundock 2005 shakemap but tend to over predict the intensity of shaking by 0 1 0 5 modified mercalli intensity mmi units velocity waveforms at sites throughout the san francisco bay area exhibit characteristics consistent with rupture directivity local geologic conditions e g sedimentary basins and the large size of the event e g durations of strong shaking lasting tens of seconds we also compute ground motions for seven hypothetical scenarios rupturing the same extent of the northern san andreas fault considering three additional hypocenters and an additional random distribution of slip rupture directivity exerts the strongest influence on the variations in shaking although sedimentary basins do consistently contribute to the response in some locations such as santa rosa livermore and san jose these scenarios suggest that future large earthquakes on the northern san andreas fault may subject the current san francisco bay urban area to stronger shaking than a repeat of the 1906 earthquake ruptures propagating southward towards san francisco appear to expose more of the urban area to a given intensity level than do ruptures propagating northward

litho1 0 an updated crust and lithospheric model of the earth we present the litho1 0 model which is a 1 tessellated model of the crust and uppermost mantle of the earth extending into the upper mantle to include the lithospheric lid and underlying asthenosphere the model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers such as water ice sediments crystalline crust lithospheric lid and asthenosphere litho1 0 is created by constructing an appropriate starting model and perturbing it to fit high resolution surface wave dispersion maps love and rayleigh group and phase over a wide frequency band 5 40 mhz we examine and discuss the model with respect to key lithospheric parameters such as average crustal velocity crustal thickness upper mantle velocity and lithospheric thickness we then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency it appears that litho1 0 represents a reasonable starting model of the earth s shallow structure crust and uppermost mantle for the purposes in which these models are used such as traveltime tomography or in efforts to create a 3 d reference earth model the model matches surface wave dispersion over a frequency band wider than the band used in the inversion there are several avenues for improving the model in the future by including attenuation and anisotropy as well as making use of surface waves at higher frequency

regional seismic wave propagation lg and sn and pn attenuation in the arabian plate and surrounding regions an edited version of this paper was published by blackwell publishing copyright 2004 blackwell publishing see also http www blackwell synergy com doi abs 10 1111 j 1365 246x 2004 02246 x http atlas geo cornell edu middleeastnorthafrica publications al damegh2004 htm

paleomagnetism and 40ar 39ar chronology of lavas from meseta del lago buenos aires patagonia 1 as part of a larger project investigating paleosecular variation in south america 36 paleomagnetic and geochronology sites were sampled on meseta del lago buenos aires 47 s 289 e 200 km east of the active plate boundary in southern patagonia basaltic lavas sampled range from late miocene to late pleistocene yet most of the individual lavas are younger than 3 3 ma thirty eight isochron ages determined via 40ar 39ar incremental heating experiments frame a detailed stratigraphy the isochron ages range from 66 9 4 1 ka for scoria cones and a youthful lava flow in the rio pinturas valley to 10 12 1 35 ma for thick plateau forming lavas exposed along the se edge of the meseta the last 3 3 myr is characterized by seven episodes of volcanism at ca 3 2 3 0 ma 2 4 ma 1 7 ma 1 35 ma 1 0 ma 750 ka 430 330 ka and 110 ka the bulk of lavas forming the surface of the meseta erupted in the last 1 7 my all sites have stable magnetization and after step demagnetization using either thermal or alternating field techniques yield characteristic directions held by magnetite and or titanomagnetite ten sites have distinct transitional directions defined by pole latitudes 55 and associated ages indicate possible connections to known reversals within the matuyama chron including the onset of the jaramillo subchron 1 016 0 01 ma the cobb mountain subchron 1 25 0 03 ma the ontong java 1 event 1 37 0 03 ma and the termination of the olduvai subchron 1 72 0 02 ma remaining sites are divided into normal 14 sites and reversed polarity 12 sites at 95 confidence mean normal and reversed directions overlap the mean direction for 26 sites is i 63 0 d 3 4 95 5 4 which is indistinguishable from the expected geocentric axial dipole paleosecular variation measured by the dispersion of virtual geomagnetic poles about the rotation axis is higher than expected at 20 0 model g using data for the past 5 myr predicts a dispersion of 17 this discrepancy may be due to true dispersion of the field in southern south america or it may be an artifact of inadequate sampling

subduction of the south chile active spreading ridge a 17 ma to 3 ma magmatic record in central patagonia western edge of meseta del lago buenos aires argentina abstract the chile triple junction is a natural laboratory to study the interactions between magmatism and tectonics during the subduction of an active spreading ridge beneath a continent the mlba plateau meseta del lago buenos aires is one of the neogene alkali basaltic plateaus located in the back arc region of the andean cordillera at the latitude of the current chile triple junction the genesis of mlba can be related with successive opening of slabs windows beneath patagonia within the subducting nazca plate itself and between the nazca and antarctic plates detailed 40ar 39ar dating and geochemical analysis of bimodal magmatism from the western flank of the mlba show major changes in the back arc magmatism which occurred between 14 5 ma and 12 5 ma with the transition from calc alkaline lavas cerro plomo to alkaline lavas mlba in relation with slab window opening in a second step at 4 3 ma alkaline felsic intrusions were emplaced in the western flank of the mlba coevally with the mlba basalts with which they are genetically related these late oib like alkaline to transitional basalts were generated by partial melting of the subslab asthenosphere of the subducting nazca plate during the opening of the south chile spreading ridge related slab window these basalts differentiated with small amounts of assimilation in shallow magma chambers emplaced along transtensional to extensional zones the close association of bimodal magmatism with extensional tectonic features in the western mlba is a strong support to the model of patagonian collapse event proposed to have taken place between 5 and 3 ma as a consequence of the presence of the asthenospheric window scr 1 segment of south chile ridge below the mlba area

rift jump process in northern iceland since 10 ma from 40ar 39ar geochronology due to the westward american eurasian plate boundary migration the rift zone location in northern iceland imposed by the stable icelandic hotspot has jumped eastward the present day north volcanic zone of iceland is thus shifted about 100 km to the east with respect to the kolbeinsey ridge a miocene paleo rift location was proposed along the hunafloi skagi axis unconformities inside the lava pile on both sides of the present day rift as a result of the new rift formation have been used to date the rift jump but timing is still controversial whether extinction of the paleo rift was synchronous with initiation of the new rift or the two rifts were active during some time period is unknown using the 40ar 39ar radiochronology method 37 dykes were dated along a 350 km profile parallel to the divergent plate motion direction i e ese wnw crossing the present rift the unconformities and the inferred paleo rift our results reveal that the paleo rift axis was not located along the hunafloi skagi axis but 60 km to the east this previously unknown paleo rift is named the skagafjordur paleo rift it remained active until approximately 3 ma the activity of the present north volcanic zone started about 8 8 5 ma intruding 9 9 5 myr rocks located at least 10 km east of the skagafjordur paleo rift axis consequently the two rifts were simultaneously active for 5 5 5 myr during this time period their accretion rates were nearly equal but with asymmetrical opening the largest rates occurring on external flanks these results are coherent in the icelandic context and permit a first reconstruction of the evolution of northern iceland since 10 ma

relative motion of the nazca farallon and south american plates since late cretaceous time by combining reconstructions of the south american and african plates the african and antarctic plates the antarctic and pacific plates and the pacific and nazca plates we calculated the relative positions and history of convergence of the nazca and south american plates despite variations in convergence rates along the andes periods of rapid convergence averaging more than 100 mm a between the times of anomalies 21 49 5 ma and 18 42 ma and since anomaly 7 26 ma coincide with two phases of relatively intense tectonic activity in the peruvian andes known as the late eocene incaic and mio pliocene quechua phases the periods of relatively slow convergence 50 to 55 30 mm a at the latitude of peru and less farther south between the times of anomalies 30 31 68 5 ma and 21 and between those of anomalies 13 36 ma and 7 correlate with periods during which tectonic activity was relatively quiescent thus these reconstructions provide quantitative evidence for a correlation of the intensity of tectonic activity in the overriding plate at subduction zones with variations in the convergence rate

bimodal back arc alkaline magmatism after ridge subduction pliocene felsic rocks from central patagonia 47 s abstract volumetrically minor microsyenites alkali microgranite and related trachytic dykes intrude early pliocene oib like alkali basaltic and basanitic flows of the meseta del lago buenos aires in central patagonia 47 s 71 30 w and occur together with scarce trachytic lava flows whole rock k ar ages between 3 98 and 3 08 ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post plateau basaltic sequence that they intrude during a bimodal mafic felsic magmatic episode devoid of intermediate compositions chemically these rocks have a1 type granitoid affinities and are characterized by high silica and alkali contents 60 68 wt sio2 8 7 10 8 wt na2o k2o major and trace elements patterns evidencing evolution by low pressure fractional crystallization and sr and nd isotopic signatures similar to those of coeval basalts 87sr 86sr o 0 70488 0 70571 143nd 144nd o 0 512603 0 512645 nevertheless some of them have the most radiogenic sr values ever reported for a magmatic rock in the meseta and even in the whole neogene patagonian plateau lavas province 87sr 86sr o 0 70556 0 70571 143nd 144nd o 0 512603 0 512608 in addition very high contents of strongly incompatible elements in the most evolved rocks together with sr isotopic ratios higher than those of coeval basalts suggest the occurrence of open system magmatic processes continuous fractional crystallization from a primitive basaltic source similar to post plateau coeval basalts towards alkali granites combined with small rates of assimilation of host jurassic tuffs afc in a shallow magmatic reservoir best explains the geochemical and petrographic features of the felsic rocks therefore a1 type magmatic rocks can be generated by open system crystallization of deep asthenospheric melts in back arc tectonic settings in central patagonia these 3 4 ma old alkaline intrusions occur aligned along a n160 170 trending lineament the zeballos fault zone stacking the morphotectonic front of one segment of the patagonian cordillera intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area related to major regional tectonics occurring in possible relation with the collision of one segment of the chile spreading ridge with the trench

deglacial environmental changes on isla de los estados 54 4 s southeastern tierra del fuego the island of isla de los estados is situated at 54 5 degrees s 64 degrees w east of argentinian tierra del fuego and is located in a sensitive geographic position in relation to the zonal circulation between antarctica and south america its terrestrial records of the last deglaciation recording atmospheric conditions but within an oceanic setting can help to clarify changes of regional circulation patterns both atmospheric and marine here we present geochemical analyses from 16 10 ka cal bp of a peat core from lago galvarne bog at the northern coast of the island and a lake sediment core from laguna cascada 3 kin further south the data comprise tc tn loss on ignition analyses and continuous xrf scanning on both cores as well as age depth modeling based on ams c 14 dating deglaciation and onset of peat formation in the coastal areas began before 16 ka cal bp followed by a rapid glacial retreat and the start of lacustrine sedimentation further inland data suggest initially windy conditions with permafrost succeeded by gradually warmer and wetter conditions until ca 14 5 ka cal bp the warming trend slows down until ca 13 5 ka cal bp followed by arid conditions culminating around 12 8 ka cal bp our data suggest fairly warm conditions and the establishment of denser peat and forest vegetation ca 10 6 ka cal bp contemporaneous with the onset of the antarctic thermal optimum this indicates large scale shifts in the placement of zonal flow and the westerlies at the beginning of the holocene c 2008 elsevier ltd all rights reserved

the march 11 2002 masafi united arab emirates earthquake insights into the seismotectonics of the northern oman mountains a moderate m approx 5 earthquake struck the northeastern united arab emirates uae and northern oman on march 11 2002 the event was felt over a wide area of the northern emirates and was accompanied by smaller felt events before and after the march 11 main shock the event was large enough to be detected and located by global networks at teleseismic distances we estimated focal mechanism and depth from broadband complete regional waveform modeling we report a normal mechanism with a slight right lateral strike slip component consistent with the large scale tectonics the normal component suggests relaxation of obducted crust of the semail ophilite specifically the khor fakkan block while the right lateral strike slip component of the mechanism is consistent with shear across the oman line felt earthquakes are rare in the region however no regional seismic network exists in the uae to determine local seismicity this event offers a unique opportunity to study the active tectonics of the region as well as inform future studies of seismic hazard in the uae and northern oman

finite difference simulations of seismic scattering implications for the propagation of short period seismic waves in the crust and models of crustal heterogeneity synthetic seismograms produced by the finite difference method are used to study the scattering of elastic and acoustic waves in two dimensional media with random spatial variations in seismic velocity the results of this study provide important insights about the propagation of short period 5 the self similar medium is characterized by a scattering q that is constant with frequency whereas theory predicts that the apparent q in an exponential medium is proportional to frequency these alternative models of crustal heterogeneity can thus be tested by improved measurements of the frequency dependence of crustal q at frequencies greater than about 1 hz assuming that scattering is responsible for most of the attenuation at these frequencies measurements of the time decay of the synthetic coda waves clearly show that the single scattering model of coda decay is not appropriate in the presence of moderate amounts of scattering attenuation scattering q 200 in these cases q values derived from the coda decay rate using the single scattering theory do not correspond to the transmission q of the medium the cross correlation of synthetic waveforms observed for an array of receivers along the free surface is observed to be dependent on the correlation distance of the medium the self similar random medium proposed here for the crust produces waveform variations at high frequencies 15 30 hz similar to those reported for actual small scale seismic arrays with apertures of hundreds of meters

late quaternary palaeoenvironmental change in western staaten island 54 5 s 64 w fuegian archipelago abstract late glacial holocene environmental conditions were interpreted in western isla de los estados staaten island from geomorphological and palynological analysis the geomorphological data from caleta lacroix 54 50 s 64 40 w indicate the presence of a fossil dune field that suggest exposition of a larger land surface and stronger wind intensity predominantly from sw and w probably during late glacial times when sea level was lower than today deglaciation and onset of peat formation in the western coastal area began prior to 12 600 cal bp in response to warmer conditions the pollen data indicates initial treeless herbaceous and paludal vegetation with scarce empetrum ericaceae type heaths and scrubs as a result of plant invasion and short term succession vegetal communities spreading over the shoreline areas under locally more humid conditions the vegetation between 10 300 and 8300 cal bp included dwarf shrub heaths scrubs cushion plants and grasses with scattered trees under warmer and drier climate conditions than today after 8300 cal bp more humid conditions allowed the expansion of an open nothofagus forest associated with dwarf shrub heath communities it was followed at 6700 cal bp by a gradual closed forest development in association with drimys winteri and shrub and herb vegetation indicative of subantarctic evergreen forest magellanic moorland vegetation transition under cold and wet conditions after 5500 cal bp the rate of evergreen beech forest greatly increased with the development of almost pure subantarctic evergreen forest communities these vegetation changes accompanied a modification of the climate toward colder and wetter conditions after 2700 cal bp the closed forest was replaced by an open nothofagus forest indicative of warm and dry conditions the minimum of nothofagus pollen registered between 1000 and 500 cal bp may correspond to the medieval climate anomaly mca period all these vegetation changes are in turn related to the positioning and intensity of the southern westerlies wind belt sea ice antarctic extent and changes in the sea level

palaeoenvironmental conditions during the middle holocene at isla de los estados staaten island tierra del fuego 54 s argentina and their influence on the possibilities for human exploration abstract the middle holocene is a particular time period in which there are uneven environmental dynamics all along the southern portion of the continent whereas in the puna extreme aridity and high temperatures the so called hypsithermal period have been recorded a more humid and rainy phase dominated southern tierra del fuego the present paper explores the palaeoenvironmental situation of isla de los estados staaten island 54 38 54 55 s 63 48 64 46 w and its relationship with the archaeological record left behind by ancient canoeing people the considered period is characterized by exceptionally windy conditions the palaeoenvironmental information could be used as an explanation for the absence of cultural remains and also for the scarce archaeological record for peninsula mitre at the beginning of the middle holocene the diatom record shows a large amount of aulacoseira sp that could indicate either windy conditions or higher water levels at the site core the more distinctive changes in the diatom assemblages occurred at ca 5000 cal b p suggesting a shallower water level and more acidic conditions by 3000 cal b p the diatom species are strongly linked to peat and swampy environments however during the late holocene milder conditions might have favored seasonal travelling to the outer islands of the fuegian archipelago though no definitive or permanent settlements have been found

a reappraisal of earthquake focal mechanisms and active shortening in the zagros mountains of iran summary the zagros mountains of sw iran are one of the most seismically active intra continental fold and thrust belts on earth and an important element in the active tectonics of the middle east surface faulting associated with earthquakes is extremely rare and so most information about the active faulting comes from earthquakes we use long period teleseismic p and sh body waves to determine the orientation and depth of faulting in 16 new earthquakes and then evaluate and synthesize all the available teleseismic data on earthquake source parameters in the zagros we use this information to investigate the style and distribution of active faulting in the zagros and how it contributes to the n s shortening of the arabia eurasia collision when the data are ranked in quality and carefully evaluated simple patterns are seen that are not apparent when routine catalogue data are taken at face value an important change in the fault configuration occurs along strike of the belt in the nw overall convergence is oblique to the trend of the belt and the surface anticlines and is achieved by a spatial separation partitioning of the orthogonal strike slip and shortening components on separate parallel fault systems by contrast in the se overall convergence is orthogonal to the regional strike and achieved purely by thrusting in the central zagros between these two structural regimes deformation involves parallel strike slip faults that rotate about vertical axes allowing extension along the strike of the belt the overall configuration is similar to that seen in other curved shortening belts such as the himalaya and the java sumatra trench all the zagros earthquakes we have been able to check have centroids shallower than 20 km and are confined to the upper crust many of the larger earthquakes are likely to occur in the basement beneath the sedimentary cover which is active even beneath areas of known shallow structural decollement such as the dezful embayment the dominant style of shortening is high angle reverse faulting with dips 30 though some lower angle thrusting occurs in places active thrust and reverse faulting is relatively confined to the lower topography on the sw edge of the belt today and only strike slip faulting affects the higher topography profound vertical changes in structural and stratigraphic level indicate that a similar style of deformation was once active across the width of the simple folded belt but has progressively migrated sw over the last 5 ma there is no evidence for a seismically active structural decollement such as a low angle thrust beneath the zagros nor is there any seismic evidence for active subduction either beneath the zagros or beneath central iran instead the arabian margin seems to have shortened by distributed thickening of the basement only in the syntaxis of the oman line at the se end of the zagros is there any evidence for a low angle thrust of regional extent here earthquakes continue 50 km north of the zagros thrust line the geological suture between the arabian margin and central iran reaching depths of 30 km and may represent thrusting of arabian basement beneath central iran to this extent

igneous history of the andean cordillera and patagonian plateau around latitude 46 degrees s from the middle jurassic onwards persistent igneous activity in the southern andes around 46 s was controlled by easterly dipping subduction along the pacific margin cogenetic plutonic rocks belonging to the patagonian batholith and calc alkaline volcanics ranging from basaltic andesites to rhyolitic tuffs and ignimbrites are the principal products erosion of the primary volcanics has led at various times to the development of thick volcaniclastic sequences for example in the cretaceous lower tertiary divisadero formation the coyhaique region marks the northerly extension of a narrow back arc basin in which the marine neocomian successions accumulated volcaniclastics from the island arc which presumably lay to the west are intercalated with the sediments although the marine basin was short lived a mildly extensional back arc regime may have existed through much of mesozoic recent times widespread basalt rhyolite volcanism on the eastern side of the cordillera seems to have been associated with this tectonic environment remnants of the patagonian basalt plateau at latitude 45 47 s extend from the argentine chile frontier to lago colhue huapi four principal age and compositional groups have been distinguished in the lavas i the oldest which are about 80 ma occur in sections at senguerr and morro negro they are almost exclusively tholeiitic but show some calc alkaline affinities and resemble in other respects basalts from marginal basins ii the second group 57 43 ma occur in the lower part of the chile chico section with a compositional spread from olivine tholeiites through alkali basalts to one occurrence of a basanite iii the upper part of the main plateau sequence where the flows are in the range 25 9 ma are dominantly of alkali basalt composition iv post plateau flows from small cinder cones on the surface of the plateau range in age from ca 4 ma to 0 2 ma or less they are mostly highly undersaturated basanites with occasional leucite basanites enriched in incompatible elements a few of the earlier tholeiites with calc alkali traits may have been closely associated with subduction or marginal basin processes the younger lavas are more alkalic intraplate types generated in the remote back arc extensional zone

climatic and ecologic changes during miocene surface uplift in the southern patagonian andes abstract the up to 4 km high southern patagonian andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies and cause one of the most drastic orographic rain shadows on earth geologic data imply that this climatic pattern has been established or significantly enhanced during miocene surface uplift of this andean segment we report evidence for important climatic and ecologic changes in the eastern foreland of the patagonian andes that appear to be the result of this uplift to provide constraints on miocene plant ecosystems and precipitation in the eastern leeward foreland of the patagonian andes we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a 500 m thick section of the continental santa cruz formation the age of these deposits was constrained by ar ar dating of intercalated tuffs which range from 22 to 14 ma at 16 5 ma the 13 c values increase by 3 the 18 o values decrease by 2 and the scatter in the oxygen isotope data increases significantly we interpret these changes as the consequence of 1 km surface uplift in this andean segment from the 18 o values and increased aridity to its east from the 13 c values and the increased scatter in the 18 o values sediments overlying the santa cruz formation are very limited in extent and volume and dominated by coarse conglomerates related to pleistocene and older glaciations it thus seems that by 14 ma deposition in the eastern foreland of the southern patagonian andes had essentially ceased as the result of rain shadow formation

ground motion modeling in the eastern caucasus in this study we analyzed the performance of a preliminary three dimensional 3d velocity model of the eastern caucasus covering most of the azerbaijan the model was developed in support to long period ground motion simulations and seismic hazard assessment from regional earthquakes in azerbaijan the model s performance was investigated by simulating ground motion from the damaging mw 5 9 2012 zaqatala earthquake which was well recorded throughout the region by broadband seismic instruments in our simulations we use a parallelized finite difference method of fourth order accuracy the comparison between the simulated and recorded ground motion velocity in the modeled period range of 3 20 s shows that in general the 3d velocity model performs well areas in which the model needs improvements are located mainly in the central part of the kura basin and in the caspian sea coastal areas comparisons of simulated ground motion using our 3d velocity model and corresponding 1d regional velocity model were used to locate areas with strong 3d wave propagation effects in areas with complex underground structure the 1d model fails to produce the observed ground motion amplitude and duration and spatial extend of ground motion amplification caused by wave propagation effects

southern patagonian plateau basalts and deformation backarc testimony of ridge collisions abstract the distribution and volume of tertiary patagonian plateau basalts and the evolution of the patagonian fold and thrust belt between 46 and 49 s appear to be closely tied with ridge trench interactions along the continental margin to the west eocene plateau basalts south of 43 s are associated with a gap in the eocene arc and can be related to a proposed collision of the aluk farallon ridge against the trench at this time eruptions of late miocene to recent plateau basalts between 46 and 49 s can be related to time transgressive slab windows in the underlying mantle generated by the collision of segments of the chile ridge at 10 14 ma and 6 ma the principal deformation in the patagonian fold and thrust belt predates the eruption of the basalts and also appears to be related with late miocene ridge collision comparison of plateau basalts related to the collision of the three distinct ridge segments suggests that the volume of basalt erupted increases with the length of the collided ridge segment almost all eocene to recent plateau basalts between 46 and 49 s have oib like ocean island basalt trace element signatures showing that the mantle source region changed little with time plateau basalts with more arc like signatures appear to be contaminated by continental crust relative melting percentages inferred from the trace element chemistry of the plateau basalts correlate with volumes of erupted basalt implying spatial and temporal changes in temperatures in the mantle source that can be correlated with slab windows

pliocene extensional tectonics in the eastern central patagonian cordillera geochronological constraints and new field evidence recent field work and review of radiometric data obtained from neogene lavas and plutonic rocks exposed in the eastern central patagonian cordillera 46 48os which overlie subducted segments of the south chile ridge suggest important late miocene to pleistocene morphological changes in relation to base level variations and or tectonic events we present new field observations from a region south of lago general carrerabuenos aires between the main cordillera and the meseta del lago buenos aires demonstrating that normal faulting controlled valley incisions and occurred during lava emplacement at 5 4 ma and after 3 ma we also show that the 12 ma basaltic flows of the meseta del lago buenos aires 2000 m a s l have been subjected to deep incision with younger lavas dated at 1 2 ma partially filling the valleys these incisions are thought to reflect progressive eastward tilting of the entire meseta our new observations together with additional features from central patagonia strongly suggest that tectonic events led to a regional widespread morphological change after 5 ma the coincidence in time and space between the subduction of segments of the south chile ridge at 6 and 3 ma causing opening of a slab window and strong base level variations in the studied area suggests a cause and effect relationship in central patagonia compressional tectonics ended well before extensional events reported here causes of uplift and further extension are probably completely disconnected the uplift is purely tectonic in origin and occurred prior to the subduction of the south chile ridge extension should be a consequence of this subduction

evidence in cerro pampa volcanic rocks for slab melting prior to ridge trench collision in southern south america late miocene approx 12 ma hornblende bearing andesitic to dacitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage wasysym pagestyle empty declaremathsizes 10 9 7 6 begin document mathrm 63 to 68 sio 2 1 2 to 1 9 k 2 o end document adakite flows at the small cerro pampa center in patagonia 47 55 s 71 25 have some of the clearest slab melt geochemical signatures yet seen in a phanerozoic center on continental crust these magmas formed in response to melting of the hot thin slab that was subducting beneath south america prior to the collision of the chile rise at approx 6 ma or at approx 10 ma their n morb like 87 sr 86 sr 0 70285 0 70309 and low 206 pb 204 pb 18 44 18 59 ratios show that they could have been generated by

late holocene palaeoclimatic records from lake sediments on james ross island antarctica sediment profiles from several lakes ponds in the northern ice free area of james ross island antarctica were the subject of a multi disciplinary palaeoenvironmental investigation the results obtained from lithological geomagnetic geochemical and diatom analyses and the frequency of branchinecta eggs were evaluated with multivariate statistics and provide a fairly detailed picture of climate change during the last 5000 years new radiocarbon dates are combined with previously published 14c dates so as to date the palaeoenvironmental and palaeohydrological changes identified by the stratigraphic studies in combination with the findings of glacial geological studies our data suggest that the brandy bay glacier began to recede rapidly just before 5000 yr b p due to calving and moisture starvation caused by cold and arid conditions in one of the ponds studied gypsum began to precipitate as a consequence of these conditions when the influence of glacial meltwater ceased this hypothesis was tested by hydrochemical modeling at approximately 4200 yr b p the climate became more humid and warmer and the pond supported a fairly diverse biota which included a variety of diatom species and for example branchinecta gaini a crustacean that feeds on benthic cyanobacterial mats the brandy bay glacier probably advanced during this humid period but the advance was interrupted by subsequent arid conditions which started at 3000 yr b p during this arid phase glaciers became completely absent from the study area as a result formerly large lakes fed by glacial meltwater transformed into small enclosed brackish water bodies that contained b gaini the arid and fairly cold conditions lasted for 1500 2000 years until approximately 1200 yr b p when the aquatic systems expanded and their salinity was lowered as a result of increased snow accumulation and glacial expansion the increased deposition of minerogenic matter caused by glacial erosion and meltwater inflow probably caused the disappearance of branchinecta on james ross island we regard this most recent phase as a period of increased humidity and warmth compared to the previous period although the conditions appear to be less warm and humid than those which prevailed during the climatic optimum between 4200 and 3000 yr b p in spite of certain correlation problems we tentatively correlate our results to other studies of holocene environments in antarctica and from that it can be suggested that the climatic optimum was of circumpolar significance therefore the climatic oscillations recorded in sediments on james ross island may reflect oscillations in the anticyclonic stormtracks and the strength of the high pressure cell over the inland ice sheet

propagation characteristics of short period sn and lg in the middle east abstract we present propagation characteristics of the short period regional seismic phases sn and lg for the middle east digital waveform data from the iranian long period array ilpa and the global seismic network gsn stations abkt alibek turkmenistan anto ankara turkey keg kottamya egypt and kiv kislovodsk georgia were used in this study three component rotated seismograms were bandpass filtered 0 5 to 5 0 hz and examined for the presence and strength amplitude of sn and lg relative to the pn and pg phases the short period sn and lg waveforms show great variability sn propagates efficiently for paths crossing the mediterranean black and caspian seas as well as for paths passing north of the greater caucasus alborz and kopet dag mountains generally sn is not observed or propagates inefficiently for paths with segments beneath the turkish plateau and northern iranian plateau consistent with a previous study by kadinsky cade et al 1981 this study finds that the zone of inefficient sn propagation coincides with a zone of low pn velocities pn tomography lg is consistently not observed for paths crossing the mediterranean black and caspian seas lg propagates efficiently for paths crossing the northern arabian shield generally lg propagates efficiently for paths contained within the turkish iranian plateau paths in the middle east greater than about 2000 km consistently show no or little lg energy the correspondence of zones of inefficient sn propagation and low pn velocities suggests that high temperatures and partial melt lie at shallow depths in the upper mantle beneath the turkish plateau and northern iranian plateau the presence of water presumably due to subduction of hydrated oceanic crust lowers the solidus temperature and enhances partial melt lg is blocked by oceanic crustal structures as well as by structural and topographic variations associated with tectonic boundaries

40ar 39ar and k ar chronology of pleistocene glaciations in patagonia during the pleistocene east of lago buenos aires argentina at 46 5 s at least 19 terminal moraines were deposited as piedmont glaciers from the patagonian ice cap advanced onto the semi arid high plains adjacent to the southern andes exceptional preservation of these deposits offers a rare opportunity to document ice cap fluctuations during the last 1 2 m y 4 0 ar 3 9 ar incremental heating and unspiked k ar experiments on four basaltic lava flows interbedded with the moraines provide a chronologic framework for the entire glacial sequence the 4 0 ar 3 9 ar isochron ages of three lavas that overlie till 90 km east of the cordillera at lago buenos aires and another 120 km from the andes along rio gallegos at 51 8 s that underlies till strongly suggest that the ice cap reached its greatest eastward extent ca 1100 ka at least six moraines were deposited within the 256 k y period bracketed by basaltic eruptions at 1016 10 ka and 760 14 ka similarly six younger more proximal moraines were deposited during an 651 k y period bracketed by an underlying 760 14 ka basalt and the 109 3 ka cerro volcan basalt flow that buried all six moraines coupled with in situ cosmogenic surface exposure ages of moraine boulders the 109 ka age of cerro volcan implies that moraines deposited during the penultimate local glaciation correspond to marine oxygen isotope stage 6 further westward toward lago buenos aires six additional moraines younger than the cerro volcan basalt flow occur surface exposure dating of boulders on these moraines combined with the 1 4 c age of overlying varved lacustrine sediment indicates deposition during the last glacial maximum lgm 23 16 ka although antarctic dust records signal an important patagonian glaciation at 60 40 ka moraines corresponding to marine oxygen isotope stage 4 are not preserved at lago buenos aires apparently these were overrun and obliterated by the younger ice advance at 23 ka notwithstanding the overall pattern of glaciation in patagonia is one of diminishing eastward extent of ice during successive glacial advances over the past 1 m y we hypothesize that tectonically driven uplift of the patagonian andes which began in the pliocene yet continued into the quaternary in part due to subduction of the chile rise spreading center during the past 2 m y maximized the ice accumulation area and ice extent by 1 1 ma subsequent deep glacial erosion has reduced the accumulation area resulting in less extensive glaciers over time

timing and origin of deformation along the patagonian fold and thrust belt the andean orogeny in the patagonian cordillera of southern south america reflects the consequences of the mesozoic and cenozoic subduction of an oceanic plate beneath the south american continental margin the geological evolution of the region has been influenced by the eocene collision and subduction of the farallon aluk ridge and the miocene recent subduction of the chile ridge another aspect of plate interaction during this period was two intervals of rapid plate convergence one at 50 42 ma and the other at 25 10 ma between the south american and the oceanic plates it has been proposed that the collision of the chile ridge with the trench was responsible for the development at least in part of the patagonian fold and thrust belt this belt extends for more than 1000 km along the eastern foothills of the southern andes between 46 and 54 s along the southwestern rim of the austral basin the interpretation of a link between subduction of the ridge and formation of the fold and thrust belt is based on assumed time coincidences between contractional tectonism and the collision of ridge segments during middle and late miocene times the main tertiary contractional events in the patagonian fold and thrust belt took place during latest cretaceous palaeocene eocene and during miocene times although the timing of deformation is still poorly constrained the evidence currently available suggests that there is little or no relationship between the timing of the fold and thrust belt and the collision of ridge segments most if not all of the contractional tectonism pre dated the latest episodes of ridge collision collision of a ridge crest with the continental margin has been active for the past 14 to 15 million years contrary to the suggestion of a relationship between ridge subduction and compression the main result of this collision has been fast uplift and extensional tectonism the initiation of the patagonian fold and thrust belt in latest cretaceous or early tertiary times coincided with a fundamental change in the tectonic evolution of the austral basin throughout the cretaceous most of this basin subsided as a broad backarc continental shelf only in latest cretaceous times and coinciding with the initiation of the fold and thrust belt the basin underwent a transition to a retro arc foreland basin this change to an asymmetrically subsiding foreland basin with an associated foreland fold and thrust belt was related to uplift of the andean orogenic belt in the west

late quaternary glaciation history of isla de los estados southeasternmost south america isla de los estados is a mountainous island southeast of tierra del fuego in southernmost south america its central and eastern parts have an alpine topography transected by u shaped valleys small partly over deepened fjords and a multitude of abandoned cirques all associated with extensive former local glaciations traces of glacial erosion generally reach 400 450 m a s l and above that trimline a distinct sharp edged nunatak derived landscape is present the westernmost part of the island has a lower more subdued topography reflecting its softer geology but possibly also over running and erosion by mainland derived ice streams the present study concentrated on glacigenic sediment sequences exposed along coastal erosional cliffs a combination of osl and c 14 datings show that these sediments mostly date from the latest wisconsinan weichselian glacial cycle i e from the last ca 100 ka with the oldest glaciolacustrine deposits possibly as old as 90 80 ka the upper parts of overlying tills with associated lateral and terminal moraines from glaciers that expanded onto an eustatically exposed dry shelf north of the island date from the last global glacial maximum lgm radiocarbon ages of peat and lake sediments indicate that deglaciation began 17 16 cal ka bp c 2010 university of washington published by elsevier inc all rights reserved

a morphotectonic analysis of central patagonian cordillera negative inversion of the andean belt over a buried spreading center 1 we use digital elevation models and satellite imagery data completed with field observations to develop an integrated morphotectonic analysis of the anomalous topography of the central cordillera of patagonia between 46 s and 48 s in a region overlying subducted segments of the chile spreading ridge this region shows a number of peculiar features not expected in a normal subduction related belt located away from a buried spreading center first it is characterized by overall short wavelength topography and highly contrasted reliefs typical of deeply incised young mountain belts but surprisingly also by an average very low elevation of 900 m second the cordillera lacks central high reliefs instead there is a central depressed domain with average low slope gradients including the deepest point of the study area 300 m below sea level in the bottom of lake general carrera buenos aires this depressed area is flanked by western and eastern regions of higher elevation having opposite general slope dips to the east the central patagonian cordillera is now separated from the flat plains of the pampean domain representing its former piedmont and several elevated basaltic plateaus mesetas by 160 n 180 n trending morphotectonic corridors and the western border scarps of the mesetas these scarps correspond to neotectonic features controlled by recent vertical faults mostly active after 3 ma remnants of a former peneplain the avellanos surface are widespread within the entire cordilleran region from geological evidence this peneplain formed between 15 and 4 ma and was connected originally with the mesetas domain and now is dissected by these corridors it is shown that a negative inversion related to extensional and strike slip faulting occurred in the frontal region of the cordillera at 3 ma synchronously with the emplacement of magmatic products timing of the extensional deformation in central patagonia is concomitant with the migration at depth of different segments of the south chile ridge which entered the trench at 6 and 3 ma respectively and the consequent opening of a slab window allows asthenospheric hot mantle to reach lithospheric regions finally migration of segments of the chile spreading ridge since 6 ma triggered regional uplift of the cordillera followed by limited extension in relation with increased thermal weakening of the crust

earthquake ground motion prediction equations for eastern north america new earthquake ground motion relations for hard rock and soil sites in eastern north america ena including estimates of their aleatory uncertainty vari ability have been developed based on a stochastic finite fault model the model incorporates new information obtained from ena seismographic data gathered over the past 10 years including three component broadband data that provide new in formation on ena source and path effects our new prediction equations are similar to the previous ground motion prediction equations of atkinson and boore 1995 which were based on a stochastic point source model the main difference is that high frequency amplitudes f 5 hz are less than previously predicted by about a factor of 1 6 within 100 km because of a slightly lower average stress parameter 140 bars versus 180 bars and a steeper near source attenuation at frequencies less than 5 hz the predicted ground motions from the new equations are generally within 25 of those predicted by atkinson and boore 1995 the prediction equations agree well with available ena ground motion data as evidenced by near zero average residuals within a factor of 1 2 for all frequencies and the lack of any significant residual trends with distance however there is a tendency to positive residuals for moderate events at high frequencies in the distance range from 30 to 100 km by as much as a factor of 2 this indicates epistemic uncertainty in the prediction model the positive residuals for moderate events at 100 km could be eliminated by an increased stress parameter at the cost of producing negative residuals in other magnitude distance ranges adjustment factors to the equations are provided that may be used to model this effect

high and low latitude climate control on the position of the southern peru chile current during the holocene 1 we reconstructed changes of temperature salinity and productivity within the southern peru chile current during the last 8000 years from a high resolution sediment core recovered at 41 s using alkenones isotope ratios of planktic foraminifera biogenic opal and organic carbon paleotemperatures and paleosalinities reached maximum values at 5500 years ago and thereafter declined to modern values whereas paleoproductivity continuously increased throughout the last 8000 years we ascribe these long term holocene trends primarily to latitudinal shifts of the antarctic circumpolar current acc the concurrence with shifts in the position of the southern westerlies points to a common response of atmospheric and oceanographic circulation patterns off southern chile millennial to centennial scale fluctuations of paleotemperatures and paleosalinities on the other hand lag displacements in the position of the southern westerlies but reveal a significant correlation to short term temperature changes in antarctica indicating a high latitude control of the acc at these timescales

the crustal and upper mantle structure of the interior arabian platform summary the crustal and upper mantle velocity structure of the interior arabian platform is derived using the spectral analysis of long period p wave amplitude ratios the ratio of the vertical to the horizontal component is utilized to obtain crustal transfer functions using the thomson haskell matrix formulation for horizontally layered crustal models 20 earthquakes recorded at the long period station ryd between azimuths n20 w and n150 e were selected for the analysis based on the following criteria focal depths in the range 5 to 215 km body wave magnitudes greater than 5 0 and epicentral distances in the range 7 to 97 a careful quality check of the data left us with six events out of 29 that had short epicentral distances 20 to be analysed the selection criterion for the final model in the forward modelling process was based on the correlation coeycient between observed and theoretical transfer function the model suggested that the crust consists of five distinct layers the upper crustal layer has a p wave velocity of about 5 6 km s 1 and is about 3 km thick the second layer has a velocity of about 6 3 km s 1 and is 10 km thick the third layer has a velocity of 6 6 km s 1 and is 8 km thick the fourth layer has a velocity of 6 9 km s 1 and is 15 km thick the lower layer has a velocity of about 7 6 km s 1 and is 10 km thick for the mohorovicic discontinuity a velocity of 8 3 km s 1 for the upper mantle and 46 km depth are indicated

lithospheric structure of the arabian shield and platform from complete regional waveform modelling and surface wave group velocities summary regional seismic waveforms reveal significant differences in the structure of the arabian shield and the arabian platform we estimate lithospheric velocity structure by modelling regional waveforms recorded by the 1995 1997 saudi arabian temporary broadband deployment using a grid search scheme we employ a new method whereby we narrow the waveform modelling grid search by first fitting the fundamental mode love and rayleigh wave group velocities the group velocities constrain the average crustal thickness and velocities as well as the crustal velocity gradients because the group velocity fitting is computationally much faster than the synthetic seismogram calculation this method allows us to determine good average starting models quickly waveform fits of the jon and sn body wave arrivals constrain the mantle velocities the resulting lithospheric structures indicate that the arabian platform has an average crustal thickness of 40 km with relatively low crustal velocities average crustal p and s wave velocities of 6 07 and 3 50 km s 1 respectively without a strong velocity gradient the moho is shallower 36 km and crustal velocities are 6 per cent higher with a velocity increase with depth for the arabian shield fast crustal velocities of the arabian shield result from a predominantly mafic composition in the lower crust lower velocities in the arabian platform crust indicate a bulk felsic composition consistent with orogenesis of this former active margin p and s wave velocities immediately below the moho are slower in the arabian shield than in the arabian platform 7 9 and 4 30 km s 1 and 8 10 and 4 55 km s respectively this indicates that the poisson s ratios for the uppermost mantle of the arabian shield and platform are 0 29 and 0 27 respectively the lower mantle velocities and higher poisson s ratio beneath the arabian shield probably arise from a partially molten mantle associated with red sea spreading and continental volcanism although we cannot constrain the lateral extent of a zone of partially molten mantle

ground motion in kuwait from regional and local earthquakes potential effects on tall buildings in recent years the construction of tall buildings has been increasing in many countries including kuwait and other gulf states these tall buildings are especially sensitive to ground shaking due to long period seismic surface waves although kuwait is relatively aseismic it has been affected by large mw 6 regional earthquakes in the zagros fold thrust belt zftb accurate ground motion prediction for large earthquakes is important to assess the seismic hazard to tall buildings in this study we first analyze the observed ground motions due to two earthquakes widely felt in kuwait the 08 18 2014 mw 6 2 earthquake 360 km nne of kuwait city and the 11 12 2017 mw 7 3 earthquake 642 km nne of kuwait city the peak spectral displacement periods of the ground motion from the 08 18 2014 mw 6 2 earthquake matched well with the ambient vibration spectra of the tallest building the al hamra tower we calculate the ground motions from potential regional and local earthquakes we use a velocity model obtained by matching the observed seismograms of the 2014 and 2017 earthquakes we calculate ground motions in kuwait due to a regional mw 7 5 earthquake and a local mw 5 0 earthquakes our study shows that a significant source of seismic hazard to tall buildings in kuwait comes from the regional tectonic earthquakes however local earthquakes have the potential to generate high peak ground accelerations 98 cm s2 close to their epicenters

long period ground motion in the arabian gulf from earthquakes in the zagros mountains thrust belt the arabian gulf is adjacent to the zagros mountains one of the most seismically active regions in the world we observe that broadband seismic records of zagros earthquakes recorded on the arabian side of the gulf display long duration surface waves while shorter periods 10 km of the gulf basin the long period energy is enhanced and transmitted efficiently consequently large earthquakes in the zagros could result in amplified ground motions at long periods 2 10 s relative to average behavior such ground motions are of concern for large engineered structures such as tall buildings and long bridges with resonant periods in the same period range here we present results of investigations of the characteristics of ground motions recorded on the western shore of the gulf from selected earthquakes in the zagros mountains region exceptionally long duration seismic waves as compared with standard models are shown to occur with periods of 2 10 s this may be due to waveguide effects in the deep sedimentary basin structure of the arabian platform in addition to analyzing recorded ground motion we performed 3d wave propagation simulations using a finite difference method and experimental velocity models of the gulf with different shallow sedimentary layers structures the simulation results confirm our hypothesis that long period waves with extremely long duration and relatively large amplitudes are caused by the geometry of the basin sedimentary layers and to some extent by shallow earthquake depths combined effects of basin edge geometry with sharp velocity contrasts and shallow sources 10 km on the eastern side of the arabian gulf can cause large long period ground motion on the western side of the gulf in contrast the short period content of ground motion 2 s at long distances is relatively weak this is mainly due to wave propagation scattering and attenuation in the shallow sedimentary layers of the gulf basin

crustal structure of iraq from receiver functions and surface wave dispersion implications for understanding the deformation history of the arabian eurasian collision summary we report the crustal structure for two locations in iraq estimated by joint inversion of p wave receiver functions rfs and surface rayleigh wave group velocity dispersion rfs were computed from teleseismic recordings at two temporary broad band seismic stations located in mosul msl in the zagros fold belt and baghdad bhd in the mesopotamian foredeep group velocity dispersion curves at the sites were derived from continental scale tomography the inversion results show that the crustal thicknesses are 39 km at msl and 43 km at bhd we observe a strong psmoho at bhd consistent with a sharp moho discontinuity however at msl we observe a weak psmoho suggesting a transitional moho where crustal thickening is likely to be occurring in the deep crust both sites reveal low velocity surface layers consistent with sedimentary thickness of about 3 km at station msl and 7 km at bhd and agreeing well with the previous reports ignoring the sediments the crystalline crustal velocities and thicknesses are remarkably similar at both stations the similarity of crustal structure suggests that the crust of the northeastern proto arabian platform was uniform before subsidence and deposition of the sediments in the cenozoic if crystalline crustal structure is uniform across the northern arabian platform then crustal thickness variations in the zagros fold belt and thrust zone should reveal the history of deformation and crustal shortening in the arabian eurasian collision zone and not reflect pre existing crustal thickness variations in the arabian plate

trench proximal volcanism following ridge crest trench collision along the antarctic peninsula on the basis of multichannel seismic reflection magnetics and bathymetry data a pleistocene holocene 0 1ma volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the antarctic peninsula the seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3 1 and 6 0 ma ago new trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt the basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts e g th ta 1 0 1 5 rb nb 0 5 87sr 86sr 0 70265 0 7028 143nd 144nd approximately 0 5129 and are totally lacking in any evidence for interaction with subduction enriched mantle geochemical similarities with other more trench distal postsubduction alkalic basalts along the antarctic peninsula are striking strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region the basalts were most likely to have been generated as a result of the formation of a slab window beneath the antarctic peninsula following ridge crest trench collision subduction component free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision and decompressional melting resulted this type of trench proximal volcanism following ridge crest trench collision differs from that in other locations where calc alkaline volcanism persisted or ophiolite obduction occurred

ree fractionation and nd isotope disequilibrium during crustal anatexis constraints from himalayan leucogranites abstract the processes controlling rare earth element ree behaviour during crustal anatexis have profound implications not only for ree abundances of crustally derived granites but also for their model nd ages for typical protoliths of anatectic granites such as are exposed in metapelites from the high himalayan crystalline series hhcs a high proportion 90 of bulk rock lree and mree are sited in the accessory phases apatite and monazite and of hree in garnet and apatite mass balance constraints indicate that observed ree abundances of himalayan leucogranites cannot be generated from ree deficient reactant phases in their metasedimentary protoliths muscovite biotite plagioclase and quartz but must result from the dissolution of accessory phases ree and th abundances in anatectic melts are modelled by combining empirical expressions for monazite apatite and zircon solubilities as a function of temperature melt composition and melt water content with analysed accessory phase ree abundances this approach which allows the contribution to the melt from each accessory phase to be identified successfully predicts himalayan leucogranite trace element abundances the lree and th concentrations of a metapelite derived melt are buffered by monazite stability although apatite dissolution contributes up to 10 of lree to the melt together with 50 of the mree and 90 of the hree zircon contributes a negligible proportion of ree to the melt during anatexis dissolution of monazite and apatite in the melt results in 1 increasing ree abundances and lree hree ratios with increasing temperature of melting 2 a negative eu anomaly in the melt 3 high sm nd ratios in the melt relative to its metasedimentary protolith and 4 nd isotope disequilibrium between the melt and its protolith all of these features are observed in the ree geochemistry of himalayan leucogranites the combined effects of sm nd fractionation and nd isotope disequilibrium during anatexis imply that model nd ages calculated for anatectic granites derived from apatite and monazite bearing protoliths should be treated with considerable caution

regional waveform propagation in the arabian peninsula regional waveform propagation is characterized in the arabian peninsula using data from a temporary network of broadband seismometers between november 1995 and march 1997 332 regional delta 15 events were recorded from nine stations deployed across the arabian shield regional phase propagation was analyzed in two ways by individual inspection of the waveforms and by stacking of waveforms inspection of the waveforms revealed consistent variations in individual seismograms according to the region of origin waveforms from events in the gulf of aqaba northwest of the network possess weak pn pg and sn but show a prominent lg phase in contrast clear pn sn and lg are observed for events located in the zagros a region northeast of the network events near the straits of hormuz also display pn and sn but lack a strong high frequency lg southern red sea and african earthquakes have moderate amplitude body phases with some lg for the stacks the data were high pass filtered at 1 hz rectified binned and then stacked by time distance or by time slowness the time distance stacks show clear differences between regions that correspond to the variations observed in individual seismograms the time slowness stacks allow comparison of relative phase velocities and amplitudes pn velocity under the network was estimated to be 8 0 0 2 km s consistent with data from prior refraction profiles the area of inefficient pn and sn propagation coincides with an area of holocene volcanism and suggests that anomalous upper mantle underlies much of the arabian shield

neogene patagonian plateau lavas continental magmas associated with ridge collision at the chile triple junction extensive neogene patagonian plateau lavas 46 5 to 49 5 s southeast of the modern chile triple junction can be related to opening of asthenospheric slab windows associated with collisions of chile rise segments with the chile trench at 12 ma and 6 ma support comes from 26 new total fusion whole rock 40ar 39ar ages and geochemical data from back arc plateau lavas in most localities plateau lava sequences consist of voluminous tholeiitic main plateau flows overlain by less voluminous 2 to 5 million year younger alkalic postplateau flows northeast of where the ridge collided at 12 ma most lavas are syncollisional or postcollisional in age with eruptions of both sequences migrating northeastward at 50 to 70 km ma plateau lavas have ages from 12 to 7 ma in the western back arc and from 5 to 2 ma farther to the northeast trace element and isotopic data indicate main plateau lavas formed as larger percentage melts of a garnet bearing oceanic island basalt oib like mantle than postplateau lavas the highest percentage melts erupted in the western and central plateaus in a migrating slab window model main plateau lavas can be explained as melts that formed as upwelling subslab asthenosphere which flowed around the trailing edge of the descending nazca plate and then interacted with subduction altered asthenospheric wedge and continental lithosphere alkaline postplateau lavas can be explained as melts generated by weaker upwelling of subslab asthenosphere through the open slab window thermal problems of high pressure melt generation of anhydrous mantle can be explained by volatiles h2o and co2 introduced by the subduction process into slab window source region s an oib like rather than a mid ocean ridge basalt morb like source region and the lack of magmatism northeast of where ridge collision occurred at 13 to 14 ma can be explained by entrainment of weak plume s or regional variations in an ambient oib like asthenosphere

late miocene earliest pliocene glaciation in southern argentina implications for global ice sheet history abstract in south america at latitude 47 s till was deposited sometime between 7 m y and 4 6 m y ago at a time when the local climate was colder than today s during this same interval glaciers extended to sea level in southeast alaska and widespread cooling of the ocean surface in middle latitudes worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred from the last three events major late miocene expansion of the antarctic ice sheet has been inferred on the assumption that the history of north atlantic ice rafting precludes the existence of northern hemisphere ice sheets until 3 m y ago this is disputed first because precipitation in antarctica would probably have decreased at temperatures below today s second because the antarctic ice sheet cannot expand appreciably until buildup of northern hemisphere ice sheets has lowered sea level third because virtually no late miocene sediments are present at the labrador sea dsdp sites that are critical to the reconstruction of north atlantic ice rafting history and fourth because the scale of late miocene glaciation in alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east if global ice sheet volume at the end of the miocene was greater than it is today by an amount that would have lowered sea level by several tens of meters the excess ice did not accumulate in antarctica but in the northern hemisphere chiefly in north america away from the atlantic coast

neogene uplift of central eastern patagonia dynamic response to active spreading ridge subduction the chile triple junction ctj episodically migrated northward during the past 14 ma from 54 s to its present day position at 46 30 s as different almost trench parallel spreading segments entered in subduction this migration resulted in the opening of an asthenospheric window below patagonia inducing a disturbance in the regional mantle convection on the overriding plate the middle miocene time corresponds to a major change in the central patagonian basin dynamics with a transition from subsidence to generalized uplift the detailed mapping and the morphological study of post middle miocene terraces evidence regional scale trench parallel tilt histories that differ depending on latitude south of 46 30 s the slopes of the fluvial terraces indicate a change from northward to southward tilt during late miocene terraces younger than the early pliocene do not show any trench parallel tilt north of 46 30 s in contrast only northward tilt active until the pleistocene is recorded we use a semianalytical model of dynamic topography which suggests that the northward migration of the chile triple junction should be accompanied by a dynamic uplift of the central patagonian basin uplift would cause both north directed and south directed tilt north and south of the triple junction respectively with values of 0 1 0 2 500 km east of the trench tilt measured in the southeastern part of the central patagonian basin 0 1 is comparable to values predicted using the semianalytical model the dynamic topography associated to the chile ridge subduction may have exerted a major control on the neogene dynamics of patagonia

holocene climate variability in antarctica based on 11 ice core isotopic records a comparison is made of the holocene records obtained from water isotope measurements along 11 ice cores from coastal and central sites in east antarctica vostok dome b plateau remote komsomolskaia dome c taylor dome dominion range d47 km105 and law dome and west antarctica byrd with temporal resolution from 20 to 50 yr the long term trends possibly reflect local ice sheet elevation fluctuations superimposed on common climatic fluctuations all the records confirm the widespread antarctic early holocene optimum between 11 500 and 9000 yr in the ross sea sector a secondary optimum is identified between 7000 and 5000 yr whereas all eastern antarctic sites show a late optimum between 6000 and 3000 yr superimposed on the long time trend all the records exhibit 9 aperiodic millennial scale oscillations climatic optima show a reduced pacing between warm events typically 800 yr whereas cooler periods are associated with less frequent warm events pacing 1200 yr

three dimensional simulation of the near fault ground motion for the 1995 hyogo ken nanbu kobe japan earthquake abstract the 17 january 1995 hyogo ken nanbu earthquake is a typical example showing that the ground motions along basin edge faults can be very destructive in this study we simulate the near fault ground motion from this earthquake based on a kinematic fault model and a simplified 3d velocity structure of the kobe area the kinematic earthquake rupture and the wave propagation are modeled using a 3d finite difference method fdm our simulation identifies the basin edge effect as an important factor that influenced the ground motion amplification pattern in the kobe area we found that the coupling of the source directivity and basin edge effects causes impulsive ground motions with extremely high amplitude at periods greater than 1 sec and in a narrow zone offset less than 1 km from the basin edge the combination of these effects acted to create a fairly continuous band of amplification that extends about 30 km in an elongated zone parallel to the basin edge boundary in some areas localized site effects might have been as important as the abovementioned effects but they cannot explain the continuity of the extended east west zone of damage

mechanisms of holocene palaeoenvironmental change in the antarctic peninsula region the antarctic peninsula is one of the three fastest warming regions on earth here we review holocene proxy records of marine and terrestrial palaeoclimate in the region and discuss possible forcing mechanisms underlying past change with a specific focus on past warm periods our aim is to critically evaluate the mechanisms by which palaeoclimate changes might have occurred in order to provide a longer term context for assessing the drivers of recent warming two warm events are well recorded in the holocene palaeoclimate record namely the early holocene warm period and the mid holocene hypsithermal mhh whereas there are fewer proxy data for the mediaeval warm period mwp and the recent rapid regional rrr warming we show that the early holocene warm period and mhh might be explained by relatively abrupt shifts in position of the southern westerlies superimposed on slower solar insolation changes a key finding of our synthesis is that the marine and terrestrial records in the ap appear to show markedly different behaviour during the mhh this might be partly explained by contrasts in the seasonal insolation forcing between these records circumpolar deep water cdw has been implicated in several of the prominent changes through the holocene but there are still differences in interpretation of the proxy record that make its influence difficult to assess further work is required to investigate contrasts between marine and terrestrial proxy records east west contrasts in palaeoclimate the history of cdw to retrieve a long onshore high resolution record of the holocene and determine the role of sea ice in driving or modulating palaeoclimate change along with further efforts to study the proxy record of the rrr and the mwp

the crustal structure of the western arabian platform from the spectral analysis of long period p wave amplitude ratios abstract the crustal structure of the western arabian platform is derived using the spectral analysis of long period p wave amplitude ratios the ratio of the vertical to the horizontal component is used to obtain the crustal transfer function based on thickness variations crustal velocities densities and the angle of emergence at the lower crust and upper mantle interface eleven well defined earthquakes recorded at the long period ryd station during the period from 1985 to 1994 were selected for analysis based on the following criteria focal depths with a range between 7 and 89 km body wave magnitudes greater than 4 7 epicentral distances with a range from 8 8 to 26 5 and back azimuthal coverage from 196 to 340 spectral analysis calculations were based on the comparison of the observed spectral ratios with those computed from theoretical p wave motion obtained using the thomson haskell matrix formulation for horizontally layered crustal models the selection of the most suitable model was based on the identification of the theoretical model which exhibits the highest cross correlation coefficient with the observed transfer function ratio by comparing the spectral peak positions of the observed and theoretical values the thickness and velocity can be resolved within 3 km and 1 km s respectively of the observed values the spectral analysis of long period p waves can detect a thin layer near the surface of about 1 6 km thick and a velocity contrast of about 10 with that of the underlying layer a strong velocity gradient of about 0 05 km s per km was found in the upper crust and 0 02 km s per km in the lower crust the derived crustal model is not unique due to the theoretical assumptions horizontal layering constant densities and velocities in each layer quality of the data and complexities of the crustal structure the crustal model suggests that the crust consists of five distinct layers the upper crustal layer has a p wave velocity of about 5 6 km s and is about 1 6 km thick the second layer has a velocity of about 6 2 km s and is 10 2 km thick the third layer shows a velocity of 6 6 km s and is 6 8 km thick the fourth layer has a velocity of about 6 8 km s and is 12 3 km thick the lower crustal layer has a velocity of about 7 5 km s and is 9 3 km thick the mohorovicic discontinuity beneath the western arabian platform indicates a velocity of 8 2 km s of the upper mantle and 42 km depth

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region

slab tearing following ridge trench collision evidence from miocene volcanism in baja california mexico abstract neogene magmatic activity in central baja california underwent a major change at ca 12 5 ma when the pacific farallon active oceanic ridge collided with the trench east of vizcaino peninsula the calc alkaline magmatism which built the comondu volcanic arc vanished and was replaced by unusual volcanic associations which were erupted within six late miocene to quaternary volcanic fields jaraguay san borja san ignacio santa rosalia santa clara la purisima delineating a 600 km array along the baja california peninsula new fieldwork k ar datings and geochemical analyses on jaraguay and san borja lavas combined with previous data allow us to show that these associations include 1 adakites emplaced between 12 5 and 8 2 ma in the santa clara santa rosalia and jaraguay volcanic fields 2 niobium enriched basalts neb in santa clara and santa rosalia 11 2 to 7 4 ma 3 tholeiitic basalts and basaltic andesites 11 3 to 7 2 ma displaying a very weak subduction imprint in la purisima and san ignacio 4 alkali trachybasalts in mesas san carlos and santa catarina northwest of jaraguay 9 3 to 7 5 ma and finally 5 basalts and associated magnesian basaltic andesites and andesites locally referred to as bajaites the latter lavas display very specific geochemical characteristics including very high sr and ba contents they were emplaced in all the above mentioned volcanic fields between 14 6 and 5 3 ma the origin of these late miocene volcanics has been considered linked either to the opening of an asthenospheric window through which they ascended tholeiites and alkali trachybasalts or to the melting of its edges due to thermal erosion adakites and the subsequent reaction between adakitic melts and the supraslab mantle neb and bajaites however the identification of the remnants of the pacific farallon fossil ridge south of vizcaino peninsula indicates that active ridge subduction was not responsible for the opening of the asthenospheric window we propose that the emplacement of slab window related volcanic rocks between 13 and 7 ma from ca 25 n to 30 n was due to a process of slab tearing which started when the active ridge collided with the trench east of vizcaino peninsula the oldest part of the downgoing plate sunk into the mantle leading to the opening of a tear in the slab which likely started from the southern limit of the already existing southern california slab window it propagated parallel to the trench over about 600 km and developed into an asthenospheric window

environment and climate of the last 51 000 years new insights from the potrok aike maar lake sediment archive drilling project pasado in this introductory paper we summarize the history and achievements of the potrok aike maar lake sediment archive drilling project pasado an interdisciplinary project embedded in the international continental scientific drilling program icdp the stringent multiproxy approach adopted in this research combined with radiocarbon and luminescence dating provided the opportunity to synthesize a large body of hydrologically relevant data from laguna potrok aike southern patagonia argentina at this site lake level was high from 51 ka until the early holocene when the southern hemisphere westerlies shw were located further to the north at 9 3 ka cal bp the shw moved southward and over the latitude of the study area 52 degrees s causing a pronounced negative water balance with a lake level decrease of more than 50 m two millennia later the shw diminished in intensity and lake level rose to a subsequent maximum during the little ice age since the 20th century a strengthening of the shw increased the evaporative stress resulting in a more negative water balance a comparison of our data with other hydrological fluctuations at a regional scale in south eastern patagonia provides new insights and also calls for better chronologies and high resolution records of climate variability

a multistep approach for joint modeling of surface wave dispersion and teleseismic receiver functions implications for lithospheric structure of the arabian peninsula 1 we present a multiple step procedure for joint modeling of surface wave group velocity dispersion curves and teleseismic receiver functions for lithospheric velocity structure the method relies on an initial grid search for a simple crustal structure followed by a formal iterative inversion an additional grid search for shear wave velocity in the mantle and finally forward modeling of transverse isotropy to resolve love rayleigh surface wave dispersion discrepancy it considers longer period surface wave group velocity swgv dispersion allowing for the resolution of deeper structure compared to previous joint inversions the grid search for simple crustal structure is facilitated using a library of precomputed receiver functions and swgv dispersion curves the iterative inversion improves fit to the data by increasing the number of layers in the crust when necessary in order to fit the swgv for periods greater than about 50 s we perform a grid search over mantle velocities including the mantle lid and low velocity zone keeping the crustal structure fixed to the values from the previous step in some cases a clear love rayleigh discrepancy prevents a simultaneous fit of the group velocities with an isotropic model the love rayleigh discrepancy can be resolved by allowing shear wave transverse isotropy with a vertical symmetry axis vsh vsv differences in the uppermost mantle the method is applied to 10 stations in the arabian peninsula sampling various tectonic environments including active continental rifting and stable regions the resulting shear velocity models confirm rapid crustal thinning of the arabian shield toward the red sea however we do not find strong evidence for crustal thickening toward the arabian platform our results suggest that the mantle lithosphere thickness varies regionally but that the mantle shear velocities beneath the arabian shield and red sea coast are generally anomalously low furthermore our results indicate the presence of strong polarization anisotropy up to about 10 in the lithospheric upper mantle in the vicinity of as well as farther away from the red sea our modeling yields vsv vsh in the southwestern part of the arabian peninsula consistent with vertical flow and vsh vsv in the northwestern part of the arabian peninsula and the continental interior consistent with horizontal flow indicating that the mantle flow pattern is not uniform along the axis of the red sea

lateglacial and holocene wet dry cycles in southern patagonia chronology sedimentology and geochemistry of a lacustrine record from laguna potrok aike argentina a high resolution multiproxy geochemical approach was applied to the sediments of laguna potrok aike in an attempt to reconstruct moist and dry periods during the past 16 000 years in south eastern patagonia the age depth model is inferred from ams 14 c dates and tephrochronology and suggests moist conditions during the lateglacial and early holocene 16 000 8700 cal bp interrupted by drier conditions before the beginning of the holocene 13 200 11 400 cal bp data also imply that this period was a major warm phase in southeastern patagonia and was approximately contemporane ous with the younger dryas chronozone in the northern hemisphere 12 700 11 500 cal bp after 8650 cal bp a major drought may have caused the lowest lake level of the record since 7300 cal bp the lake level rose and was variable until the little ice age which was the dominant humid period after 8650 cal bp