ar ar and u pb ages of marble hosted ruby deposits from central and southeast asia to date the formation of ruby deposits and link it to the regional metamorphism associated with tertiary himalayan orogenesis 40ar 39ar stepwise heating experiments were performed on single grains of phlogopite syngenetic with ruby and zircon inclusions in ruby and spinel were dated with the u pb method by ion probe the ar ar ages of phlogopites associated with ruby are oligocene 24 7 0 3 ma at jegdalek in afghanistan miocene at mogok in myanmar 18 7 0 2 to 17 1 0 2 ma at hunza in pakistan 10 8 0 3 to 5 4 0 3 ma and chumar in nepal 5 6 0 4 ma and pliocene 4 6 0 1 ma at ruyil in nepal in vietnam a zircon included in a ruby from the quy chau deposit yielded a 238u 206pb age of 53 8 4 6 ma whereas in the red river shear zone ruby formed at around 40 36 ma during ductile deformation under peak metamorphic conditions the ages obtained in this study are in agreement with those previously published for the ruby bearing metamorphic belts and document extensional tectonics tha

the modi taung nankwe gold district slate belt central myanmar mesothermal veins in a mesozoic orogen abstract the recently discovered quartz gold veins comprising the modi taung nankwe gold district in central myanmar are largely hosted by mudstones of the late palaeozoic slate belt mesozoic rocks of the paunglaung mawchi zone separate the slate belt from the shan scarp and shan plateau to the east at modi taung 5 km of exploration adits indicate that the veins within steeply dipping oblique reverse slip shear zones are displaced by movements along the shears intruded by late jurassic calc alkaline dykes and offset on conjugate cross faults drill intercepts show that mineralization extends vertically for more than 500 m coarse visible gold book and ribbon texture and stylolitic laminations and trace metal values are consistent with mineralization at mesothermal depths of several kilometres in the brittle ductile regime our field observations suggest that mineralization took place in the early jurassic following collision of myanmar on the passive western margin of a greater shan thai continental block with an oceanic arc on the overriding plate to the west collision generated intra continental thrusting in and east of the paunglaung mawchi zone metamorphism of plateau rocks thrust west beneath the slate belt to form the mogok metamorphics and ascent of mineralizing fluids expelled from dehydrating underthrust rocks reversal in orogenic polarity initiated late jurassic eastward subduction of oceanic crust beneath myanmar and generation of a magmatic arc with dykes cutting the quartz gold veins renewed east directed thrusting translated the slate belt over arc clastics and probably resulted in exhumation of the mogok metamorphics and offsets of the veins on conjugate faults young k ar rb 87 sr 86 and ar 40 ar 39 ages on the mogok metamorphic belt imply renewed tertiary uplift and possibly intrusion of mid tertiary or palaeogene granites

ice core paleovolcanic records from the st elias mountains yukon canada 1 we previously reported a record of regionally significant volcanic eruptions in the north pacific using an ice core from eclipse icefield st elias mountains yukon canada the acquisition of two new ice cores from eclipse icefield along with the previously available eclipse icefield and mount logan northwest col ice cores allows us to extend our record of north pacific volcanism to 550 years before present using a suite of four ice cores spanning an elevation range of 3 5 km comparison of volcanic sulfate flux records demonstrates that the results are highly reproducible especially for the largest eruptions such as katmai a d 1912 correlation of volcanic sulfate signals with historically documented eruptions indicates that at least one third of the eruptions recorded in st elias ice cores are from alaskan and kamchatkan volcanoes although there are several moderately large volcanic explosivity index vei 4 eruptions recorded in only one core from eclipse icefield the use of multiple cores provides signals in at least one core from all known vei 4 eruptions in alaska and kamchatka since a d 1829 tephrochronological evidence from the eclipse ice cores documents eruptions in alaska westdahl redoubt trident and katmai kamchatka avachinsky kliuchevoskoi and ksudach and iceland hekla several unidentified tephra bearing horizons with available geochemical evidence suggesting alaskan and kamchatkan sources were also found we present a reconstruction of annual volcanic sulfate loading for the north pacific troposphere based on our ice core data and we provide a detailed assessment of the atmospheric and climatic effects of the katmai eruption

a reconnaissance of the major holocene tephra deposits in the upper cook inlet region alaska abstract the upper cook inlet region of southcentral alaska would be significantly impacted by a major tephrafall owing to a widespread population and heavily travelled transportation corridors to evaluate the likelihood of such an occurrence the tephra deposits of the region have been inventoried approximately 90 deposits of holocene age are sufficiently thick to have been preserved for sampling the frequency of such major tephrafalls ranges from 1 every 200 years near sources on the west side of upper cook inlet to 1 every 1000 years on the more populated east side the volcanoes located on the west side of upper cook inlet are from north to south hayes spurr redoubt and iliamna hayes volcano produced the most extensive set of 6 to perhaps 8 tephra layers in the region about 3650 yr b p and produced one other less extensive tephra layer during holocene time spurr and redoubt volcanoes have produced respectively approximately 35 and 30 holocene layers which were dispersed eastward toward population centers no holocene tephra layers of iliamna have been recognized with certainty consequently several tephra layers which originated to the south of the region must have a source at augustine volcano or some more distant volcano tephra layers of hayes volcano are calc alkaline dacites most of the spurr deposits are tholeiitic basaltic andesites whereas those of redoubt volcano are calc alkaline andesites and dacites

remnant colloform pyrite at the haile gold deposit south carolina a textural key to genesis auriferous iron sulfide bearing deposits of the carolina slate belt have distinctive mineralogical and textural features traits that provide a basis to construct models of ore deposition our identification of paragenetically early types of pyrite especially remnant colloform crustiform and layered growth textures of pyrite containing electrum and pyrrhotite establishes unequivocally that gold mineralization was coeval with deposition of host rocks and not solely related to paleozoic tectonic events ore horizons at the haile deposit south carolina contain many remnants of early pyrite 1 fine grained cubic pyrite disseminated along bedding 2 fine grained spongy rounded masses of pyrite that may envelop or drape over pyrite cubes 3 fragments of botryoidally and crustiform layered pyrite and 4 pyritic infilling of vesicles and pumice detailed mineral chemistry by petrography microprobe sem and eds analysis of replaced pumice and colloform structures containing both arsenic compositional banding and electrum points to coeval deposition of gold and the volcanic host rocks and thus confirms a syngenetic origin for the gold deposits early pyrite textures are present in other major deposits of the carolina slate belt such as ridgeway and barite hill and these provide strong evidence for models whereby the sulfide ores formed prior to tectonism the role of paleozoic metamorphism was to remobilize and concentrate gold and other minerals in structurally prepared sites recognizing the significance of paragenetically early pyrite and gold textures can play an important role in distinguishing sulfide ores that form in volcanic and sedimentary environments from those formed solely by metamorphic processes exploration strategies applied to the carolina slate belt and correlative rocks in the eastern united states in the avalonian basement will benefit from using syngenetic models for gold mineralization

sharpening the tomographic image of the subducting slab below sumatra the andaman islands and burma summary taking advantage of the increased ray coverage due to seismicity following the 2004 december and 2005 march great earthquakes an improved iterative regional global tomographic method was applied to the sumatra andaman and adjacent regions to better constrain the 3 d mantle velocity heterogeneity in the region velocity and hypocentral parameters were iteratively perturbed to sharpen the image of the subducted slab several iterations were performed and the effects of source mislocation were considered in the iterative process an issue usually neglected in global tomography we find that source relocation between iterations increases the amplitudes of slab anomalies and sharpens the definition of slab geometry beyond what can be achieved by a fixed source iterative inversion alone in addition extensive restoration tests of synthetic data were conducted that emphasize enhancements obtained by our iterative process these tests show significant increases in amplitude and decreased smearing of synthetic slab features thus when applied to the real data similar enhancements are inferred in the resulting model which better illustrates the complex slab geometry in the upper mantle and transition zone regions along the sumatra andaman and burma subduction zones

a 500 year long record of tephra falls from redoubt volcano and other volcanoes in upper cook inlet alaska abstract volcanic ash layers preserved in glacial lacustrine sediments at skilak lake on the kenai peninsula of southcentral alaska constitute a record of eruptions at redoubt volcano and other alaskan volcanoes which affected the upper cook inlet area during the last 500 years high resolution magnetic susceptibility profiling delineates similar sequences of tephra layers in several 1 m long lake sediment cores electron microprobe analyses of glass shards from the tephras indicate correlation of some ash layers with known reference tephras from the source volcanoes while other ash layers record previously unknown prehistoric eruptions skilak lake cores contain ash from the historic 1912 katmai eruption the 1902 redoubt eruption and the 1883 mount st augustine eruption as well as prehistoric ash layers erupted from crater peak at mt spurr ca 250 350 years ago from redoubt volcano at ca 300 400 years ago and again at ca 350 450 years ago and a 500 year old ash from mount st augustine still older tephras from redoubt volcano and crater peak at mt spurr are found lower in the cores the cores indicate that volcanoes in the cook inlet area have erupted every 10 35 years during the 20th century and ash falls have occurred at skilak lake at least once every 50 100 years for the last 500 years with redoubt spurr and augustine volcanoes being the most important sources of tephra

rupture patterns and preshocks of large earthquakes in the southern san jacinto fault zone we relocated the large 1937 1942 and 1954 earthquakes in the san jacinto fault zone the epicenters of the main shocks aftershocks and some preshocks were determined using empirical station corrections from recent small events in the study areas the 1937 ml 5 9 earthquake has an epicenter between the surface traces of the san jacinto and buck ridge faults and aftershocks suggest about 7 km of rupture predominantly to the northwest a significant increase in small earthquake activity occurred about formula yr before this event the 1954 m l 6 2 earthquake is located at the southeast end of the mapped trace of the san jacinto fault and aftershocks suggest about 15 km of rupture further southeast into an area of folded young sediments with no surface fault trace this event was preceded by a cluster of small earthquakes which occurred within an 8 hr period 10 weeks before the main event and in the eventual rupture zone the 1942 m l 6 3 earthquake is located southwest of the southeast end of the coyote creek fault large aftershocks of this event are spread over a 15 by 18 km area southwest of the coyote creek fault and are not associated with any one fault the relation of the 1942 event to the san jacinto fault zone is not simple

u pb zircon dating of granitoid plutons from the west coast province of peninsular malaysia u pb zircon dating of granitoid plutons from the west coast province of peninsular malaysia yields precise and unambiguous ages of 198 220 ma late triassic these ages are considerably older than previously reported k ar mica ages which have been largely reset but agree with or are younger than rb sr whole rock apparent ages the u pb zircon ages are considered to represent the best estimate of the intrusive ages of the plutons some of the problems associated with the interpretations of the pre triassic rb sr whole rock apparent ages are discussed here and alternative ways to explain these ages in the light of recent work are considered reversely discordant patterns displayed by zircon populations from a number of plutons yield consistent upper intersection ages of 1500 1700 ma that approximately date source derived inherited zircon components the precambrian ages of these inherited zircon components suggest that the west coast province s type granitoids are derived from sedimentary sources recycled at least in part from mid proterozoic crystalline basement sources

tectonics of the western margin of the shan plateau central myanmar implication for the india indochina oblique convergence since the oligocene abstract we present new data and interpretations on the neogene tectonics of the shan scarp area central myanmar and its relationship with the india indochina oblique convergence we describe ductile and brittle fabrics associated with the major features in this area the mogok metamorphic belt mmb the shan scarp and the sagaing fault from these data we identify a succession of two tectonic regimes first a dominant nnw sse trending extension marked by ductile stretching that characterizes the mmb and associated n70e brittle normal faults later from middle or upper miocene to the present these structures were cross cut by brittle right lateral faults among which the most important are the n20w transpressive shan scarp fault zone and the n s sagaing fault to explain this transition from a dominant transtensive to a transpressive stress regime that occurred during miocene we place our data within a larger geodynamic context we suggest that like the intraplate deformation in the indian ocean the end of spreading in the south china sea the opening of the andaman basin or the end of subduction within the indo burma range the change in the tectonic regime in central myanmar could be in response to a major miocene regional plate kinematic reorganization

varve counting vs tephrochronology and 137cs and 210pb dating a comparative test at skilak lake alaska the age of recently deposited sediments in skilak lake has previously been estimated only by counting open quotes varves close quotes we measured the sedimentation rate of skilak lake using two radionuclides sup 137 cs and sup 210 pb and found it to be about an order of magnitude lower than previous estimates based on varve counts we also identified several tephras through a combination of visual inspection core x radiographs observation of variations of the magnetic susceptibility intensity of magnetization of the unconsolidated sediments and microprobe analyses of volcanic glass shards tephrochronologic dates using matches with the katmai 1912 tephra and an augustine tephra from 500 550 yr b p are in good agreement with the sedimentation rate estimates based on the radioisotope data and indicate that the rhythmite layers at skilak lake are not annual varves these new estimates of sedimentation rate reaffirm that care is needed in varve dating and require that earlier work on sunspot and climate changes thought to have been recorded in the skilak lake sediments be reevaluated 16 refs 6 figs 1 tab

gondwana and cathaysian blocks palaeotethys sutures and cenozoic tectonics in south east asia the triassic indosinian orogeny followed extinction of the palaeotethys ocean resulting in suturing of gondwana affinity and cathaysian blocks

jurassic to miocene magmatism and metamorphism in the mogok metamorphic belt and the india eurasia collision in myanmar situated south of the eastern himalayan syntaxis at the western margin of the shan thai terrane the highgrade mogok metamorphic belt mmb in myanmar occupies a key position in the tectonic evolution of southeast asia the first sensitive high resolution ion microprobe u pb in zircon geochronology for the mmb shows that strongly deformed granitic orthogneisses near mandalay contain jurassic 170 ma zircons that have partly recrystallized during 43 ma high grade metamorphism a hornblende syenite from mandalay hill also contains jurassic zircons with evidence of eocene metamorphic recrystallization rimmed by thin zones of 30 9 plus or minus 0 7 ma magmatic zircon the relative abundance of jurassic zircons in these rocks is consistent with suggestions that southern eurasia had an andean type margin at that time mid cretaceous to earliest eocene 120 to 50 ma i type granitoids in the mmb myeik archipelago and western myanmar confirm that prior to the collision of india an up to 200 km wide magmatic belt extended along the eurasian margin from pakistan to sumatra metamorphic overgrowths to zircons in the orthogneiss near mandalay date a period of eocene 43 ma high grade metamorphism possibly during crustal thickening related to the initial collision between india and eurasia at 65 to 55 ma this was followed by emplacement of syntectonic hornblende syenites and leucogranites between 35 and 23 ma similar syntectonic syenites and leucogranites intruded the ailao shan red river shear belt in southern china and vietnam during the eocene oligocene to miocene and the wang chao and three pagodas faults in northern thailand that most likely link with the mmb were also active at this time the complex history of eocene to early miocene metamorphism deformation and magmatism in the mmb provides evidence that it may have played a key role in the network of deformation zones that accommodated strain during the northwards movement of india and resulting extrusion or rotation of indochina

rock relationships in the mogok metamorphic belt tatkon to mandalay central myanmar abstract the mogok metamorphic belt mmb over 1450 km long and up to 40 km wide consists of regionally metamorphosed rocks including kyanite and sillimanite schists and granites lying along the western margin of the shan plateau in central myanmar and continuing northwards to the eastern himalayan syntaxis exposures in quarries allow correlation of palaeozoic meta sedimentary early mesozoic meta igneous and late mesozoic intrusive rocks within a 230 km long northerly trending segment of the mmb from tatkon to kyanigan north of mandalay and with the mogok gemstone district 100 km to the northeast relationships among the metamorphic and intrusive rocks with sparse published radiometric age controls indicate at least two metamorphic events one before and one after the intrusion of late jurassic to early cretaceous calc alkaline rocks these relationships can be explained by either of two possible tectonic histories one constrained by correlation of mid permian limestones across myanmar requires early permian and early jurassic regional metamorphic events prior to an early tertiary metamorphism in the western part of but within a shan thai western myanmar block the second not compatible with a single laterally continuous permian limestone requires pre upper jurassic regional metamorphism and orogenic gold mineralization in the mergui group and western myanmar early cretaceous collision of an east facing mergui western myanmar island arc with the shan plateau and early tertiary metamorphism in the mmb related to reversal in tectonic polarity following the arc plateau collision

machine learning classifiers for attributing tephra to source volcanoes an evaluation of methods for alaska tephras glass composition based correlations of volcanic ash tephra traditionally rely on extensive manual plotting many previous statistical methods for testing correlations are limited by using geochemical means masking diagnostic variability we suggest that machine learning classifiers can expedite correlation quickly narrowing the list of likely candidates using well trained models eruptives from alaska s aleutian arc alaska peninsula and wrangell volcanic field were used as a test environment for 11 supervised classification algorithms trained on nearly 2000 electron probe microanalysis measurements of glass major oxides representing 10 volcanic sources artificial neural networks and random forests were consistently among the top performing learners accuracy and kappa 0 96 their combination as an average ensemble effectively improves their performance using this combined model on tephras from eklutna lake south central alaska showed that predictions match traditional methods and can speed correlation although classifiers are useful tools they should aid expert analysis not replace it the eklutna lake tephras are mostly from redoubt volcano besides tephras from known holocene active sources holocene tephra geochemically consistent with pleistocene emmons lake volcanic center dawson tephra but from a yet unknown source is evident these tephras are mostly anchored by a highly resolved varved chronology and represent new important regional stratigraphic markers

volcanic impacts on peatland microbial communities a tephropalaeoecological hypothesis test abstract volcanic eruptions affect peatlands around the world depositing volcanic ash tephra and a variety of chemicals including compounds of sulphur these volcanic impacts may be important for many reasons in particular sulphur deposition has been shown to suppress peatland methane flux potentially reinforcing climatic cooling experiments have shown that sulphur deposition also forces changes in testate amoeba communities potentially relating to the reduced methane flux large volcanic eruptions in regions with extensive peatlands are relatively rare so it is difficult to assess the extent to which volcanic eruptions affect peatland microbial communities palaeoecological analyses across tephra layers provide a means to resolve this uncertainty in this study testate amoebae were analysed across multiple monoliths from a peatland in southern alaska containing two tephras probably representing the 1883 eruption of augustine volcano and a 20th century eruption of redoubt volcano results showed relatively distinct and often statistically significant changes in testate amoeba community coincident with tephra layers which largely matched the response found in experimental studies of sulphur deposition the results suggest volcanic impacts on peatland microbial communities which might relate to changes in methane flux

marine tephrochronology of the mt edgecumbe volcanic field southeast alaska usa abstract the mt edgecumbe volcanic field mevf located on kruzof island near sitka sound in southeast alaska experienced a large multiple stage eruption during the last glacial maximum lgm holocene transition that generated a regionally extensive series of compositionally similar rhyolite tephra horizons and a single well dated dacite med tephra marine sediment cores collected from adjacent basins to the mevf contain both tephra fall and pyroclastic flow deposits that consist primarily of rhyolitic tephra and a minor dacitic tephra unit the recovered dacite tephra correlates with the med tephra whereas many of the rhyolitic tephras correlate with published mevf rhyolites correlations were based on age constraints and major oxide compositions of glass shards in addition to lgm holocene macroscopic tephra units four marine cryptotephras were also identified three of these units appear to be derived from mid holocene mevf activity while the youngest cryptotephra corresponds well with the white river ash eruption at 1147 cal yr bp furthermore the sedimentology of the sitka sound marine core ew0408 40jc and high resolution swath bathymetry both suggest that extensive pyroclastic flow deposits associated with the activity that generated the med tephra underlie sitka sound and that any future mevf activity may pose significant risk to local population centers

diachronous cooling along the mogok metamorphic belt shan scarp myanmar the trace of the northward migration of the indian syntaxis abstract we present new observations and new mineral ages eight by the 40 ar 39 ar and nine by the 40 k 40 ar methods from high grade metamorphic and foliated intrusive rocks collected along the mogok metamorphic belt of central myanmar that complete preliminary results we previously published all 30 ages presented here confirm the oligocene to middle miocene age of the mogok metamorphism moreover the ages are young from the gulf of martaban in the south to the china myanmar border in the north showing a clear diachronism these results confirm a northward migration of the tectonic event responsible for the cooling of our samples at a velocity similar to the india relative to indochina velocity at the same period in addition intensive field work along a 800 km long section of the mogok metamorphic belt has revealed the numerous ductile structures that resulted from a strong nnw sse trending extension from the analogy between the present day local extrusion around the eastern indian syntaxis and the extension observed along the mogok metamorphic belt and from the diachronism we document we suggest that the central part of myanmar has recorded the northward passage of the eastern indian syntaxis with respect to indochina from oligocene to middle miocene

reply to a discussion of corundum cr muscovite rocks at o briens zimbabwe the conjunction of hydrothermal desilicification and lil element enrichment geochemical and isotopic evidence rd

intermediate term pre earthquake phenomena in california 1975 1986 and preliminary forecast of seismicity for the next decade intermediate term observations preceding earthquakes of magnitude 5 7 or greater in california from 1975 through 1986 suggest that 1 the sudden appearance of earthquakes in a previously inactive area indicates an increased likelihood of a significant earthquake in that area for a period from days to years 2 these larger earthquakes tend to occur towards the ends of creeping fault segments 3 one large earthquake in a region increases the likelihood of a subsequent significant event in the adjacent area and 4 marginal evidence for the occurrence of a regional deformation event suggests that such events increase the probability of earthquake occurrence throughout the entire area a common element in many of these observed patterns appears to be the transmission and amplification of tectonic stress changes by the mechanism of fault creep and suggests that surface fault creep is a sensitive indicator of changes in stress the preceding critieria are used to construct a preliminary forecast of the likely locations of significant earthquakes over the next decade

recurrent explosive eruptions from a high risk main ethiopian rift volcano throughout the holocene martin jones was funded by a natural environment research council nerc algorithm ph d grant ne 10983 01 field work at lake chamo was funded by the german research foundation dfg within the scope of the crc 806 our way to europe smith acknowledges funding from the natural environment research council grant ne l013932 1 brown and frank acknowledge dfg spp 1488 grant ko2870 4 1

new approach to assessing age uncertainties the 2300 year varve chronology from eklutna lake alaska usa abstract developing robust chronological frameworks of lacustrine sediment is central to reconstructing past environmental changes we present varve chronologies from five sites extending back 2300 years from eklutna lake in the chugach mountains of south central alaska the chronologies are built from image analysis of high resolution photographs and ct scans of sediment cores the age uncertainty of each record is tested by three methods we first present varve chronologies from individual sites and reconcile the difference in varve delimitation from two observers the varve chronologies from each site are then compared to each other using a series of marker beds that can be traced across the lake basin finally using a new bayesian probabilistic model we develop age models that incorporate information regarding age uncertainty from the multiple observer method and the age distribution of marker layers from multiple cores to evaluate the accuracy of the bayesian model output we used seven radiocarbon ages from terrestrial macrofossils and four tephra layers traceable across the core sites the major element geochemistry of the tephra layers and their ages are presented here for the first time the bayesian age model offers a new approach to quantifying age uncertainty in inter correlated cores of varved sediment

ore deposits of the french massif central insight into the metallogenesis of the variscan collision belt the french massif central fmc represents the whole west european variscan wev belt in terms of both the geodynamic evolution and the metallic content thus a study of the metallogenic evolution of the fmc may elucidate the conditions that allow the mineralisation of a collision belt since recent collision belts e g the himalayas or the alps show that mineralisation does not necessarily result from the collision process the palaeozoic history of the fmc is divided into three geodynamic stages unevenly involved from the metallogenic view point the eo variscan stage cambrian to silurian was not important the meso variscan stage devonian early carboniferous was of limited importance and most of the mineralisations formed during the neo variscan stage late carboniferous early permian in addition some more mineralisation was produced during the mesozoic because of the thermal reactivation linked with the alpine orogenies

metallogeny of tin in central thailand a genetic concept tin mineralization in central thailand is associated with granitic rocks of the thai burmese border range western group granitic intrusions east of bangkok and near the kampuchea border eastern group have no tin fractional crystallization is the fundamental petrogenetic process that controls the evolution of both groups of granites the tin bearing alkali feldspar aplogranites of the western group display an extreme degree of differentiation that has no petrological equivalent in the eastern group these aplogranites are the product of a combination of magmatic fractionation primary tin enrichment trend modified by fluid interaction the latter process is responsible for a secondary geochemical tin deficiency that is balanced by redistribution of tin in fracture systems i e tin mineralization the pattern of tin depletion in the aplogranites provides an indication of the tin potential in a given ore forming system

postglacial eruption history of redoubt volcano alaska abstract volcaniclastic deposits preserved in valleys on the flanks of redoubt volcano comprise a record of the volcano s postglacial eruption history the oldest and largest deposit is the harriet point debris avalanche emplaced more than 10 500 yr b p this debris avalanche travelled more than 30 km down the redoubt creek valley to cook inlet about 3600 yr b p a massive slope failure of redoubt volcano produced at least two lahars that travelled 30 km down the crescent river valley riehle et al 1981 a series of smaller eruptions between ca 3600 1800 yr b p generated additional lahars and floods that affected the upper crescent river valley a pyroclastic fan on the south flank of redoubt volcano probably also formed during this time interval sometime between 1000 and 300 yr b p hydrothermally altered debris collapsed from the summit edifice and produced a large lahar that travelled more than 30 km down the drift river valley at least 5 6 eruptions in the last 250 300 years have produced lahars and floods large enough to impact the site of the drift river terminal on the lower drift river fan if the eruptive pattern of the last several centuries continues another eruption is likely sometime in the next 25 100 years the holocene eruptions produced calc alkaline high silica andesite and dacite although quenched andesite and basaltic inclusions record the presence of more mafic magmas chemical discontinuities indicate that small chemically discrete batches of magma fed individual eruptions progressive enrichments of highly incompatible trace elements presumably reflect crustal contamination of holocene magmas

correlation of alaskan varve thickness with climatic parameters and use in paleoclimatic reconstruction the thickness of varves in the sediments of skilak lake alaska are correlated with the mean annual temperature r 0 574 inversely correlated with the mean annual cumulative snowfall r 0 794 and not correlated with the mean annual precipitation r 0 202 of the southern alaska climatological division for the years 1907 1934 a d varve thickness in skilak lake is sensitive to annual temperature and snowfall because skilak glacier the dominant source of sediment for skilak lake is sensitive to these climatic parameters trends of varve thickness are well correlated with trends of mean annual cumulative snowfall r 0 902 of the southern alaska climatological division and with trends of mean annual temperature of the southern r 0 831 and northern r 0 786 alaska climatological divisions trends of varve thickness also correlate with trends of annual temperature in seattle and north head washington r 0 632 and 0 850 respectively comparisons of trends of varve thickness with trends of annual temperature in california oregon and washington suggest no widespread regional correlation trends of annual snowfall in the southern alaska climatological division and trends of annual temperature in the southern and northern alaska climatological divisions are reconstructed for the years 1700 1906 a d climatic reconstructions on the basis of varve thickness in skilak lake utilize equations derived from the regression of series of smoothed climatological data on series of smoothed varve thickness reconstruction of trends of mean annual cunulative snowfall in the southern alaska climatological division suggests that snowfall during the 1700s and 1800s was much greater than that during the early and mid 1900s the periods 1770 1790 and 1890 1906 show marked decreases in the mean annual snowfall reconstructed trends of the annual temperature of the northern and southern alaska climatological divisions suggest that annual temperatures during the 1700s and 1800s were lower than those of the early and mid 1900s two periods of relatively high annual temperatures coincide with the periods of low annual snowfall thus determined

cretaceous cenozoic tectonic events in the western myanmar burma assam region correlation of late mesozoic mafic and ophiolitic rocks suggests that in the late lower cretaceous a northeasterly facing mafic arc was emplaced onto southeast borneo western sumatra western myanmar burma then 1100 km south of its present position the mogok belt of myanmar and the denqen bangong co ophiolite zone in tibet all then situated on the southwestern margin of asia reversal in tectonic polarity and eastward subduction generated a late cretaceous magmatic arc in western myanmar and sumatra and its continuation through the mogok belt into the gandise batholith of tibet magmatism interrupted in the latest cretaceous probably resumed in the western myanmar tibet and sumatra arc segments in the early palaeogene it was again interrupted in the mid eocene when an east vergent ophiolite nappe overrode the indo burman ranges deforming lower eocene turbidites derived from the magmatic arc to the east following renewed northward and eastward subduction of ocean floor in the latest eocene india collided with northern myanmar and with tibet along a suture south of the jurassic triassic flysch and adjacent yarlung ophiolite in the mogok segment of the late cretaceous arc crust thickened in the early cretaceous orogeny underwent extension to form a metamorphic core complex as myanmar west of the sagaing fault moved northward more than half of this northward movement preceded the well documented 450 km of post early miocene dextral displacement on the sagaing fault and related spreading in the andaman sea generation of late cretaceous to early eocene i and s type granites in and east of the mogok belt was related either to crustal shortening or more probably to subsequent extension in the mogok belt

late quaternary distal tephra fall deposits in lacustrine sediments kenai peninsula alaska tephra fall deposits from cook inlet volcanoes were detected in sediment cores from tustumena and paradox lakes kenai peninsula alaska using magnetic susceptibility and petrography the ages of tephra layers were estimated using 21 14 c ages on macrofossils tephras layers are typically fine gray ash 1 5 mm thick and composed of varying proportions of glass shards pumice and glass coated phenocrysts of the two lakes paradox lake contained a higher frequency of tephra 0 8 tephra 100 yr 109 over the 13 200 yr record the unusually large number of tephra in this lake relative to others previously studied in the area is attributed to the lake s physiography sedimentology and limnology the frequency of ash fall was not constant through the holocene in paradox lake tephra layers are absent between ca 800 2200 3800 4800 and 9000 10 300 cal yr bp despite continuously layered lacustrine sediment in contrast between 5000 and 9000 cal yr bp an average of 1 7 tephra layers are present per 100 yr the peak period of tephra fall 7000 9000 cal yr bp 2 6 tephra 100 yr in paradox lake is consistent with the increase in volcanism between 7000 and 9000 yr ago recorded in the greenland ice cores 2007 university of washingston all rights reserved

the 1883 and late prehistoric eruptions of augustine volcano alaska abstract the eruptive history of augustine volcano has been characterized by cycles of growth and destruction of the volcano repeated failure of 5 10 of the edifice has produced mobile debris avalanches that reached the sea on all sides high lava extrusion rates rapidly restore the volcano to its pre failure configuration this equilibrium between constructive and destructive processes has resulted in a relatively low lava dome complex surrounded by an apron of volcaniclastic debris three times the volume of the dome complex the most recent edifice collapse occurred in 1883 producing the 0 3 km3 burr point debris avalanche deposit three major slide blocks extended the shoreline up to 2 km and produced a tsunami that swept across cook inlet hummock morphologies change from a proximal radial orientation to a dominantly transverse alignment reflecting deceleration and compression of the avalanches as they enter the sea the breached crater formed by collapse was then largely filled by a 0 09 km3 lava dome and a 0 04 km3 lava flow travelled down the north flank lithic block and ash flows and pyroclastic surges reached the coast the burr point avalanche deposit partially overlaps the rocky point debris avalanche deposit to the west that was probably emplaced 200 400 years b p a major collapse event at ca 1540 110 a d produced the west island debris avalanche ending a period of expansion of the western side of the island an accompanying lateral blast overtopped the avalanche covering a 40 sector of the west flank the plinian tephra layer b may also have been erupted at the time of the west island eruption today augustine volcano has rebuilt itself to a size similar to that which preceded the last edifice failure in 1883 the frequency of past collapses three in the last 500 years suggests that summit collapse is a possibility during any future eruption the next major collapse is expected to involve 0 1 0 5 km3 of the summit the ensuing debris avalanche would likely reach the coast producing a tsunami that could impact populated areas of the kenai peninsula the largest tsunami would result from collapse in directions other than to the north or west tsunami magnitude is contingent on failure volume direction and timing with respect to tides

distal volcanic impacts on peatlands palaeoecological evidence from alaska abstract despite the fact that volcanic ash tephra layers are found preserved in peat deposits around the world comparatively little research has investigated the impacts of distal volcanic emissions on peatlands this study investigates the impacts of several late holocene volcanic eruptions on five peatlands in southern alaska using a palaeoecological approach testate amoebae analysis peat humification analysis and a basic analysis of plant macrofossil components were applied across 11 tephra layers changes in macrofossil and testate amoebae assemblages occur across several of the tephra layers the humification results were considered unreliable because of a methodological problem a finding which may have implications for other studies using this technique redundancy analyses on testate amoebae data show statistically significant changes associated with two tephras the most likely causes of the impacts are volcanic gases acidic precipitation or tephra derived leachates the finding that some tephras are associated with impacts whereas others are not may relate to the season of the eruption or meteorological conditions at the time of ash fall these results suggest the sensitivity of peatlands and peatland microbial communities to distal volcanic products and imply that changes in key palaeoclimatic proxies may be caused by a mechanism independent of climate change implications of the results for peat based palaeoclimatic studies are discussed as are possible directions for future research

the 1989 1990 eruptions of redoubt volcano an introduction abstract redoubt volcano located on the west side of cook inlet in south central alaska erupted explosively on over 20 separate occasions between december 14 1989 and april 21 1990 fourteen lava domes were emplaced in the summit area thirteen of which were subsequently destroyed the eruption caused economic losses estimated at over 160 000 000 making this the second most costly eruption in u s history this economic impact provided the impetus for a integrated comprehensive account of an erupting volcano using both modern and classical research and modern techniques which in turn led to advances in eruption monitoring and interpretation research on such topics as dome formation and collapse and the resulting pyroclastic flows elutriated ash lightning tephra and flooding was blended with the rapid communication of associated hazards to a large user group the seismology successes in predicting and monitoring eruption dynamics were due in part to 1 the recognition of long period seismic events as indicators of the readiness of the volcano to erupt and 2 to the development of new tools that allowed the seismicity to be assessed instantaneously integrated studies of the petrology of erupted products and volatile content over time gave clues as to the progress of the eruption towards completion

ion microprobe zircon u pb age and geochemistry of the myanmar jadeitite combined geochemistry and geochronology of the myanmar jadeitite were determined bulk rock trace element compositions display u shaped ree patterns with pronounced positive eu anomalies the total ree abundances are very low less than half chondritic and the high field strength elements and some large ion lithophile elements are moderately enriched these features indicate a metasomatic origin there are three groups of zircons with different interior characteristics cathodoluminescence mineral inclusions chemical compositions and sensitive high resolution ion microprobe u pb ages group i zircons with a mean age of 163 2 3 3 ma mostly have distinct oscillatory zoning highest u and th contents and na free mg rich mineral inclusions and thus indicate an igneous formation of oceanic crust or hydrothermal serpentinization and or rodingitization event in the middle jurassic group ii zircons with a mean age of 146 5 3 4 ma have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions suggesting that formation of the myanmar jadeitites as well as subduction of the eastern indian oceanic plate occurred in the late jurassic group iii zircons have an age of 122 2 4 8 ma which represents a later unknown thermal event discovery of the middle jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern indo burman range

spatial changes in volcanic and seismic activity prior to great earthquakes spatial relationship between volcanism and seismicity prior to the occurrence of several great interplate earthquakes in the circum pacific area has been examined in order to understand the process of underthrusting in detail the locations of the epicenters of the great earthquakes have been examined in terms of locations of concentrated volcanic and seismic activity in a short time period in order to determine if there is any relationship between these activities and whether such relationships can be used to refine the underthrusting model the study shows that in most cases 1 there is no volcanic activity in the vicinity of the epicenter of the great earthquake and 2 maximum volcanic activity is localized i e volcanoes adjacent to one another exhibit considerable activity in a short time period very little systematic spatial relationship between these three parameters is observed although in most cases there is no volcanic activity at the time of maximum earthquake activity locations of active volcanoes and earthquake activity during the five years prior to the occurrence of the great earthquake do not appear to be a guide to the epicenter of the great earthquake this study therefore suggests that although there is temporal relationship between the occurrence of maximum volcanic and seismic activity and the occurrence of great earthquake there appears to be no systematic spatial relationship between these three parameters

neo tethyan magmatism and metallogeny in myanmar an andean analogue abstract plate margin tectonism magmatic and hydrothermal processes and the genesis of mineral deposits are intricately linked in myanmar two near parallel magmatic belts that together contain a significant proportion of that country s mineral wealth have contrasting metallogenic endowments the mogok mandalay mergui belt hosts crustal melt s type granites with significant tin tungsten mineralization while the wuntho popa arc comprises i type granites and granodiorites with copper gold mineralization the spatial juxtaposition of the two belts and their distinct but consistent metallogenic endowment bears strong similarities to the metallogenic belts of the south american cordillera recent u pb age dating has shown the potential for the two belts to be near contemporary from the late cretaceous to eocene here we investigate whether an andean type setting during subduction of neo tethys could explain the observed magmatism and mineralization within these two belts

fault slip in southern california measurements of slip on major faults in southern california have been performed over the past 18 yr using principally theodolite alignment arrays and tautwire extensometers they provide geodetic control within a few hundred meters of the fault traces which complements measurements made by other techniques at larger distances approximately constant slip rates of from 0 5 to 5 mm yr over periods of several years have been found for the southwestern portion of the garlock fault the banning and san andreas faults in the coachella valley the coyote creek fault the superstition hills fault and an unnamed fault 20 km west of el centro these slip rates are typically an order of magnitude below displacement rates that have been geodetically measured between points at greater distances from the fault traces exponentially decaying postseismic slip in the horizontal and vertical directions due to the 1979 imperial valley earthquake has been measured it is similar in magnitude to the coseismic displacements analysis of seismic activity adjacent to slipping faults has shown that accumulated seismic moment is insufficient to explain either the constant or the decaying postseismic slip thus the mechanism of motion may differ from that of slipping faults in central california which move at rates close to the plate motion and are accompanied by sufficient seismic moment seismic activity removed from the slipping faults in southern california may be driving their relatively aseismic motion

the elmore ranch and superstition hills earthquakes of 24 november 1987 introduction to the special issue on 24 november 1987 two significant earthquakes occurred along the southern san jacinto fault zone and related structural elements in southern california not far from the international border these two events the elmore ranch earthquake m 6 2 at 0154 gmt and the superstition hills earthquake m 6 6 at 1315 gmt both moment magnitudes from sipkin 1989 and their aftershocks have yielded a rich harvest of geological seismological and engineering data pertinent to the cause and effect of earthquakes in this region where the southern san jacinto fault zone enters the salton depression from the peninsula ranges bordering it on the southwest fig 1 this special issue of the bulletin presents 18 geologic and seismologic investigations of these earthquakes a collection of papers born in el centro california on 8 and 9 february 1988 at a meeting attended by approximately 60 scientists interested in these earthquakes for one reason or another

destructive upper crustal earthquakes of central america since 1900 we have compiled a catalog of 51 destructive upper crustal earthquakes in central america since 1900 an event is included if it caused casualties or heavy damage of modified mercalli mm intensity vii all events larger than magnitude ms 5 7 are found to be destructive we believe the catalog is complete for events of ms 6 along the volcanic front the catalog includes estimates of epicentral coordinates depth magnitude and casualties for all events and presents mm intensity vii contours for most events data in this catalog place severe constraints on the spatial temporal and magnitude distribution of destructive upper crustal earthquakes in central america the catalog contains 30 events of ms 6 depths of well constrained mainshock hypocenters range from 5 to 15 km the most striking feature of the catalog is the spatial alignment along the volcanic front of 23 of the 30 events volcanic front earthquake source mechanisms for such events show east west tension and are compatible with a model of the volcanic front as a zone of right lateral strike slip faulting volcanic front earthquake magnitudes do not exceed ms 6 5 except in southeastern guatemala where they reach ms 6 9 maximum earthquake magnitudes are apparently related to the distance between adjacent holocene volcanic centers destructive events occur at offsets in the volcanic chain as isolated mainshock aftershock sequences however within linear segments destructive earthquakes have a strong tendency to cluster and are often preceded by significant foreshock activity although the volcanic front is the locus of both destructive earthquakes and volcanism we find no temporal correlation between the two of the seven events of ms 6 that did not originate near the volcanic front six originated near the transcurrent caribbean north american plate boundary including the ms 7 5 guatemala earthquake of 1976 along the volcanic front destructive upper crustal earthquakes have occurred at a rate of about one event every 2 1 2 years during this century much more frequently than along either the caribbean north american plate boundary or the subduction zone in central america since 1900 about 17 000 people have died from volcanic front earthquakes but only about 2000 people are known to have died from subduction zone earthquakes although subduction zone earthquakes can have larger magnitudes and produce more widespread damage destructive volcanic front earthquakes pose the greater hazard because they are more frequent and originate much closer to the bulk of central america s population which is concentrated along the volcanic front

correlation of the holocene jarvis creek tangle lakes cantwell and hayes tephras in south central and central alaska abstract the geochemistry petrography and distribution of the jarvis creek ash pewe 1965 1975a indicate that this tephra from the lower delta river area of central alaska is correlative with vol volcanic ash from sites in south central alaska near tangle lakes upper delta river area and the cantwell ash from hayes volcano found in the upper nenana river area riehle et al 1990 volcanic glass compositions of distal jarvis creek and tangle lakes tephra samples are compositionally restricted while several discrete glass populations are present in some samples are compositionally collected nearer hayes volcano these correlations extend the known distribution of hayes volcano tephras across the alaska range and into central alaska a distance of more than 650 km new geochronologic data for the jarvis creek ash suggest it was deposited ca 3660 125 yr b p consistent with previous age estimates of tephra eruptions at the hayes volcano the name jarvis creek ash has well established priority with respect to cantwell ash or other local names for this tephra layer from the hayes volcano

depth of seismicity in the imperial valley region 1977 1983 and its relationship to heat flow crustal structure and the october 15 1979 earthquake focal depths from over 1000 earthquakes occuring between 1977 and 1983 in the imperial valley southern peninsular ranges are used to study relationships between the depth of seismicity heat flow and crustal structure this study used relocated a and b quality events from the california institute of technology catalog that were carefully selected to insure focal depth precision of 2 km regional variations in focal depth appear to be related to regional heat flow variation whereas local variations in focal depth especially in the central imperial valley may be related to crustal structure these variations are studied by rheologic modeling a comparison of focal depths of earthquakes occurring before and after the october 15 1979 m 6 6 earthquake indicates that aftershocks during the first 2 months of the sequence were 2 3 km deeper than earthquakes occurring in other time periods the deepest earthquakes in imperial valley are spatially associated with a subbasement dome near the northern end of the imperial fault this dome coincides with the region where the imperial fault undergoes a transition from stick slip behavior to aseismic fault creep models of slip during the 1979 mainshock are also compared with premainshock and postmainshock seismicity a relocation of the 1940 m 7 1 mainshock suggests that this sequence began by rupturing the same portion of the fault that experienced maximum slip during the 1979 mainshock

a 100 year record of north pacific volcanism in an ice core from eclipse icefield yukon territory canada iceland and the 1989 redoubt alaska eruptions non sea salt so4 2 residuals above a robust spline and empirical orthogonal function eof analysis were used to identify volcanic so4 2 signatures volcanic sulfate values are more conservatively identified by the eof analysis as sulfate deposition from other sources is more robustly accounted for some eruptions are also recorded as peaks in non sea salt chloride the volcanic signals in the eclipse ice core are mostly attributable to alaskan aleutian or kamchatkan eruptions conversely the eclipse ice core provides a poor record of globally significant tropical eruptions these results are promising for the development of longer ice core based records of paleovolcanism in the north pacific rim index terms 0305 atmospheric composition and structure aerosols and particles 0345 4801 0370 atmospheric composition and structure volcanic effects 8409 8409 volcanology atmospheric effects 0370 keywords yukon territory ice cores volcanism atmospheric effects kamchatka alaska

the phuket terrane a late palaeozoic rift at the margin of sibumasu abstract it is widely accepted that sibumasu rifted from gondwana in the late palaeozoic but the rifts themselves have not previously been documented in southeast asia this paper identifies the pre middle permian kaeng krachan group of upper peninsular thailand as the infill of one such rift which is given the name phuket terrane indirect evidence suggests the rift infill is several kilometres thick and glacially influenced diamictites are conspicuous in the succession there are significant similarities with the 3 km thick pre middle permian rift infill of the carnarvon basin of western australia east of the khlong marui fault belt the succession is thinner and diamictites are a minor component a tectono stratigraphic model is proposed involving gondwana glaciers dropping their load at the present western margin of the phuket terrane from where it was re sedimented in the rapidly subsiding marine rift basin it is suggested that the khlong marui fault formed part of the eastern boundary of the rift system the three pagodas fault belt similarly juxtaposes different pre middle permian successions rifting ceased in the early permian and a passive margin formed as the mesotethys ocean widened the upper part of the kaeng krachan group and the overlying ratburi limestone representing the post rift sequence

tourmalinites from the stratiform peraluminous metamorphic suite of the central namaqua mobile belt south africa tourmalinites as proximal fades equivalents of stratiform peraluminous metamorphic rocks occur stratigraphically below base metal deposits and above thick metarhyolite horizons their premetamorphic protoliths are believed to have originated by tourmaline precipitation from exhalative b f and w rich brines also transporting aluminous clay colloids and dissolved silica tourmaline chemistry is used as an effective petrogenetic sensor the tourmalines are al saturated alkali deficient dravite schorl solid solutions which are in the compositional range of tourmalines originated by exhalative processes f substitution in tourmalines is governed by fe f avoidance f is relatively enriched in the tourmalines and can potentially be used as a tracer for the source of primary hydrothermal solutions ti is introduced into the tourmalines by the substitution scheme ti aliv aly si the high ti contents of the tourmalines as well as those of coexisting muscovites represent evidence of high temperature metamorphism many tourmalines exhibit continuous zoning which can partly be attributed to external fluid influx near peak metamorphic conditions

subduction of the indian lithosphere beneath the tibetan plateau and burma abstract subduction of the indian lithosphere under eurasia plays an important role in the tectonic evolution of the tibetan plateau and surrounding regions to improve our knowledge of pertinent mantle structures through tomographic imaging we combine p wave arrival time data from temporary arrays in tibet and stations of the chinese seismograph network with reprocessed data from the international seismological centre the new images reveal considerable spatial variations in mantle structure along the collision zone and the horizontal distance over which presumed continental indian lithosphere slides northward beneath the plateau decreases from west where it underlies the himalayas and the entire plateau to east where no indication is found for present day underthrusting beyond the himalayan block and indus tsangpo suture p wavespeed appears low in the shallow mantle beneath much of central and eastern tibet these observations suggest that indian lithosphere underlies only the southwestern part of the plateau and that the central and northeastern part is underlain by lithosphere of asian origin the continental parts of the indian plate that currently underthrust or subduct below the plateau appear generally detached from the oceanic slab fragments that subducted longer ago and that have been detected deeper in the mantle except perhaps in the central part of the collision zone 80 90 e this suggests that ongoing india eurasia collision must increasingly be driven by other forces such as subduction of the indian plate further west and east e g beneath indochina our images reveal structures associated with eastward subduction along the burmese arc southeast of the eastern himalayan syntaxis the tengchong volcanic complex in southwest china is marked by slow wave propagation to 150 km depth suggesting a causal link to subduction along the burmese arc the low velocities beneath the red river fault region extend to greater depth and may be related to upper mantle processes further southeast

early and late holocene glacial fluctuations and tephrostratigraphy cabin lake alaska marked changes in sediment types deposited in cabin lake near cordova alaska represent environmental shifts during the early and late holocene including fluctuations in the terminal position of sheridan glacier cabin lake is situated to receive meltwater during periods when the outwash plain of the advancing sheridan glacier had aggraded a brief early holocene advance from 11 2 to 11 0 cal ka is represented by glacial rock flour near the base of the sediment core non glacial lake conditions were restored for about 1000 years before the water level in cabin lake lowered and the core site became a fen the fen indicates drier than present conditions leading up to the holocene thermal maximum an unconformity spanning 5400 years during the mid holocene is overlain by peat until 1110 ce when meltwater from sheridan glacier returned to the basin three intervals of an advanced sheridan glacier are recorded in the cabin lake sediments during the late holocene 1110 1180 1260 1540 and 1610 1780 ce the sedimentary sequence also contains the first five reported tephra deposits from the copper river delta region and their geochemical signatures suggest that the sources are the cook inlet volcanoes redoubt augustine and crater peak and possibly mt churchill in the wrangell volcanic field copyright 2013 john wiley sons ltd

mid holocene glacier peak and mount st helens we tephra layers detected in lake sediments from southern british columbia using high resolution techniques abstract a glacier peak tephra has been found in the mid holocene sediment records of two subalpine lakes frozen lake in the southern coast mountains and mount barr cirque lake in the north cascade mountains of british columbia canada the age depth relationship for each lake suggests an age of 5000 5080 14 c yr b p 5500 5900 cal yr b p for the eruption which closely approximates the estimated age 5100 5500 14 c yr b p of the dusty creek tephra assemblage found near glacier peak the tephra layer which has not been reported previously from distal sites and was not readily visible in the sediments was located using contiguous sampling magnetic susceptibility measurements wet sieving and light microscopy the composition of the glass in pumice fragments was determined by electron microprobe analysis and used to confirm the probable source of this mid holocene tephra layer using the same methods the a d 1481 1482 mount st helens we tephra layer was identified in sediments from dog lake in southeastern british columbia suggesting the plume drifted further north than previously thought this high resolution method for identifying tephra layers in lake sediments which has worldwide application in tephrachronologic paleoenvironmental studies has furthered our knowledge of the timing and airfall distribution of holocene tephras from two important cascade volcanoes

volcanic tsunamis and prehistoric cultural transitions in cook inlet alaska abstract the 1883 eruption of augustine volcano produced a tsunami when a debris avalanche traveled into the waters of cook inlet older debris avalanches and coeval paleotsunami deposits from sites around cook inlet record several older volcanic tsunamis a debris avalanche into the sea on the west side of augustine island ca 450 years ago produced a wave that affected areas 17 m above high tide on augustine island a large volcanic tsunami was generated by a debris avalanche on the east side of augustine island ca 1600 yr bp and affected areas more than 7 m above high tide at distances of 80 km from the volcano on the kenai peninsula a tsunami deposit dated to ca 3600 yr bp is tentatively correlated with a southward directed collapse of the summit of redoubt volcano although little is known about the magnitude of the tsunami the 1600 yr bp tsunami from augustine volcano occurred about the same time as the collapse of the well developed kachemak culture in the southern cook inlet area suggesting a link between volcanic tsunamis and prehistoric cultural changes in this region of alaska

strata bound sulfide deposits wall rock alteration and associated tin bearing minerals in the carolina slate belt south carolina and georgia massive sulfide deposits gold deposits and tin bearing minerals occur in rocks of the carolina slate belt in south carolina and adjacent georgia the belt of greenschist metamorphic rocks in which the ore deposits occur is mostly cambrian in age it is divided into an upper metasedimentary unit and a lower unit of predominantly felsic metavolcanic and metavolcaniclastic rocks the ore deposits occur in the lower unit chiefly near the contact between the two units the rocks have been deformed into two maior sets of coaxial folds and at least two minor deformations coarse grained unmetamorphosed carboniferous i type granites and abundant triassic jurassic diabase dikes cut the slate belt and appear largely unrelated to the ore deposits remnants of cretaceous and younger sedimentary rocks overlie the crystalline rocks near several of the larger mineralized areas massive sulfides largely pyrite and some polymetallic sulfides occur in mines opened for gold in the nineteenth century the largest of these in south carolina are the haile brewer and dorn mines these mines and the little mountain and cedar creek blythewood areas have many similar lithologic characteristics these are hydrothermally altered wall rock consisting of quartz sericite kaolinite schist and quartz sericite schist abundant aluminous silicates such as kyanite andalusite pyrophyllite and topaz zones of iron enriched rocks and a suite of resistant heavy minerals that includes tin bearing minerals heavy mineral concentrates from alluvium of small streams showed 20 000 ppm tin cassiterite and nigerite have been identified at the brewer mine gold has been produced from altered felsic volcanic rocks that contain silicified breccia massive topaz abundant pyrite minor enargite and probably cassiterite massive pyrite and gold have been mined at the haile mine in the mccormick lincolnton area a near surface granitoid pluton is thought to be the source for volcaniclastic rocks that contain polymetallic massive sulfide deposits gold and associated deposits of kyanite barite and manganese tin was found there in heavy mineral concentrates and rutile occurring with kyanite contains 1 000 ppm tin at little mountain cassiterite and hematite are present in rocks considered to be metamorphosed hot spring deposits in the cedar creek blythewood area nigerite is present together with cassiterite chrysoberyl and seventeen other heavy minerals in concentrates panned from alluvium in streams draining an area of quartz sericite kaolinite schist the deposition of massive sulfides gold and tin minerals is considered to be part of a continuum of volcanic activity that included alteration deformation and metamorphism altered rocks which host the ore deposits result from superimposed processes beginning with alteration syngenetic with the massive sulfides and ending with fracture controlled alteration

the timing of strike slip shear along the ranong and khlong marui faults thailand 1 the timing of shear along many important strike slip faults in southeast asia such as the ailao shan red river mae ping and three pagodas faults is poorly understood we present 40ar 39ar u pb shrimp and microstructural data from the ranong and khlong marui faults of thailand to show that they experienced a major period of ductile dextral shear during the middle eocene 48 40 ma centered on 44 ma which followed two phases of dextral shear along the ranong fault before the late cretaceous 81 ma and between the late paleocene and early eocene 59 49 ma many of the sheared rocks were part of a pre kinematic crystalline basement complex which partially melted and was intruded by late cretaceous 81 71 ma and early eocene 48 ma tin bearing granites middle eocene dextral shear at temperatures of 300 500 c formed extensive mylonite belts through these rocks and was synchronous with granitoid vein emplacement dextral shear along the ranong and khlong marui faults occurred at the same time as sinistral shear along the mae ping and three pagodas faults of northern thailand a result of india burma coupling in advance of india asia collision in the late eocene 37 ma the ranong and khlong marui faults were reactivated as curved sinistral branches of the mae ping and three pagodas faults which were accommodating lateral extrusion during india asia collision and himalayan orogenesis

tectonics and history of the andaman sea region abstract the andaman sea is an active backarc basin lying above and behind the sunda subduction zone where convergence between the overriding southeast asian plate and the subducting australian plate is highly oblique the effect of the oblique convergence has been formation of a sliver plate between the subduction zone and a complex right lateral fault system the late paleocene collision of greater india and asia with approximately normal convergence started clockwise rotation and bending of the northern and western sunda arc the initial sliver fault which probably started in the eocene extended through the outer arc ridge offshore from sumatra through the present region of the andaman sea into the sagaing fault with more oblique convergence due to the rotation the rate of strike slip motion increased and a series of extensional basins opened obliquely by the combination of backarc extension and the strike slip motion these basins in sequence are the mergui basin starting at 32 ma the conjoined alcock and sewell rises starting at 23 ma east basin separating the rises from the foot of the continental slope starting at 15 ma and finally at 4 ma the present plate edge was formed alcock and sewell rises were separated by formation of the central andaman basin and the faulting moved onshore from the mentawai fault to the sumatra fault system bisecting sumatra

varve formation during the past three centuries in three large proglacial lakes in south central alaska the sediments stored in the large deep proglacial lakes of south central alaska are largely unstudied we analyzed sediments in 20 cores up to 160 cm long from eklutna kenai and skilak lakes using a combination of repeated lamination counting radionuclide dating event stratigraphy and tephrochronology we show that the characteristically rhythmic layers were deposited annually most of these glacial varves consist of one coarse grained base and a fine grained top but varves composed of multiple coarse grained turbidite pulses are common too they are likely related to successive episodes of high sediment discharge during flooding and they become more frequent in all three lakes along with increased sedimentation rates during the nineteenth century late phase of the little ice age these flood turbidites were generated by rain events and intense melting of snow and ice other mega turbidites are a result of earthquake triggered slope collapses e g a d 1964 some event layers are present in all three lakes in addition the annual time series of varve thickness normalized annual sedimentation rate are significantly correlated among the three lakes p 0 27 p 0 001 differences between the varve thickness records can be attributed partly to the dam construction at eklutna lake and outbursts from an ice dammed lake at skilak lake geomorphologic differences among the catchments result in further differences in sedimentation patterns in the three lakes

using cryptotephras to extend regional tephrochronologies an example from southeast alaska and implications for hazard assessment abstract cryptotephrochronology the use of hidden diminutive volcanic ash layers to date sediments has rarely been applied outside western europe but has the potential to improve the tephrochronology of other regions of the world here we present the first comprehensive cryptotephra study in alaska cores were extracted from five peatland sites with cryptotephras located by ashing and microscopy and their glass geochemistry examined using electron probe microanalysis glass geochemical data from nine tephras were compared between sites and with data from previous alaskan tephra studies one tephra present in all the cores is believed to represent a previously unidentified eruption of mt churchill and is named here as the lena tephra a mid holocene tephra in one site is very similar to aniakchak tephra and most likely represents a previously unidentified aniakchak eruption ca 5300 5030 cal yr bp other tephras are from the late holocene white river eruption a mid holocene mt churchill eruption and possibly eruptions of redoubt and augustine volcanoes these results show the potential of cryptotephras to expand the geographic limits of tephrochronology and demonstrate that mt churchill has been more active in the holocene than previously appreciated this finding may necessitate reassessment of volcanic hazards in the region

a 300 to 550 year history of slip on the imperial fault near the u s mexico border missing slip at the imperial fault bottleneck subsurface investigation of the imperial fault zone approximately 50 60 m south of the u s mexico international border in mexicali valley reveals that only the 1940 imperial valley earthquake m l 7 1 has produced significant surface slip in the past 300 years the 1940 earthquake produced about 5 m of slip in this vicinity as indicated by an offset tree line we excavated a buried channel adjacent to these trees and determined that it is also offset 5 m these data indicate that at least the upper meter of sediments has only been displaced by the 1940 event one other slip event of undetermined magnitude is recorded within the lacustrine highstand deposits of the last lake about 300 years ago there is no evidence of earlier faulting events in sediments that possibly may correlate to the penultimate lake approximately 550 years b p below an unconformity 4000 year old deltaic deposits are intensely faulted by previous slip along the imperial fault these data suggest that the slip rate on the imperial fault for the past 300 550 years is only about 15 20 mm yr substantially less than that expected from geodetic measurements and regional structural models this in turn suggests that either slip is accommodated by other faults in the border region or that strain release on the imperial fault is characterized by episodic clustered behavior

spatial and temporal variations in seismicity in the imperial valley 1902 1984 earthquakes with m l 4 5 that occurred between 1932 and 1973 in the imperial valley region and their related foreshocks and aftershocks are relocated using station delays derived from calibration events that were well recorded at regional distances between 1979 and 1981 the relocated seismicity shows that there was an increase in seismicity in regions northeast of the imperial fault 5 to 6 yr prior to the 1940 m s 7 1 imperial valley earthquake a study of intensity reports for felt earthquakes prior to 1932 also suggests an increase in seismicity in regions surrounding the imperial fault as early as 1917 isoseismal patterns description of the mainshock sequence and patterns of seismicity before and after the 1915 imperial valley earthquakes suggest that the earthquakes occurred off the imperial fault in this case seismicity between 1906 and 1940 would nearly encircle the portion of the imperial fault that ruptured during 1940 in both the 1940 and 1979 imperial valley earthquake sequences aftershocks of m l 4 5 occurred only for 2 months following the mainshock the aftershock sequences were followed by periods of quiescence lasting up to 10 yr in the region within 30 km of the mainshock this is in contrast to m l 6 0 earthquakes occurring near the edges of the imperial valley that had aftershocks of m l 4 5 occurring for up to 4 yr after the mainshock these differences may be related to the high heat flow within the imperial valley

geochemistry of granitic melts produced during the incongruent melting of muscovite implications for the extraction of himalayan leucogranite magmas granites formed from the incongruent melting of muscovite preserve evidence of chemical disequilibrium both between restite and melt and within individual phases during crystallization miocene leucogranites from the himalayas exemplify melt formation from a pelitic source through muscovite breakdown trace elements rb sr and ba reside almost entirely in micas and feldspars in the protolith and therefore are appropriate for quantitative modeling equilibrium between melt and restite was established for sr under fluid phase absent conditions but some disequilibrium is suggested by rb abundances consistent with known diffusion rates for these elements abundances of zr and ce in the granites which provide essential structural constituents of the accessory phases zircon and monazite indicate that the himalayan granite melts were saturated in these elements if formed at temperatures of 670 700 c but slightly undersaturated if formed at somewhat higher temperatures ion microprobe traverses of y and heavy rare earth element abundances across single garnets from himalayan leucogranites provide evidence for disequilibrium between melt and phenocryst during garnet growth thus constraining the evolution of magmatic compositions during melt formation and crystallization in some cases progressive melting of the protolith and crystallization of garnet from the melt appear to have been concomitant indicating that minimum solidus temperatures were maintained throughout the melting event this suggests that thermal buffering rather than melt extraction provided the rate determining step in magma evolution

geomorphic consequences of volcanic eruptions in alaska a review abstract eruptions of alaska volcanoes have significant and sometimes profound geomorphic consequences on surrounding landscapes and ecosystems the effects of eruptions on the landscape can range from complete burial of surface vegetation and preexisting topography to subtle short term perturbations of geomorphic and ecological systems in some cases an eruption will allow for new landscapes to form in response to the accumulation and erosion of recently deposited volcaniclastic material in other cases the geomorphic response to a major eruptive event may set in motion a series of landscape changes that could take centuries to millennia to be realized the effects of volcanic eruptions on the landscape and how these effects influence surface processes has not been a specific focus of most studies concerned with the physical volcanology of alaska volcanoes thus what is needed is a review of eruptive activity in alaska in the context of how this activity influences the geomorphology of affected areas to illustrate the relationship between geomorphology and volcanic activity in alaska several eruptions and their geomorphic impacts will be reviewed these eruptions include the 1912 novarupta katmai eruption the 1989 1990 and 2009 eruptions of redoubt volcano the 2008 eruption of kasatochi volcano and the recent historical eruptions of pavlof volcano the geomorphic consequences of eruptive activity associated with these eruptions are described and where possible information about surface processes rates of landscape change and the temporal and spatial scale of impacts are discussed a common feature of volcanoes in alaska is their extensive cover of glacier ice seasonal snow or both as a result the generation of meltwater and a variety of sediment water mass flows including debris flow lahars hyperconcentrated flow lahars and sediment laden water floods are typical outcomes of most types of eruptive activity occasionally such flows can be quite large with flow volumes in the range of 107 109 m3 a review of the lahars generated during the 2009 eruption of redoubt volcano will illustrate the geomorphic impacts of lahars on stream channels and riparian habitat although much work is needed to develop a comprehensive understanding of the geomorphic consequences of volcanic activity in alaska this review provides a synthesis of some of the best studied eruptions and perhaps will serve as a starting point for future work on this topic

postsubduction porphyry cu au and epithermal au deposits products of remelting of subduction modified lithosphere porphyry cu mo au and some epithermal au deposits are formed from hydrothermal fluids exsolved from cooling water rich calc alkaline magmas emplaced in volcanoplutonic arcs above subduction zones these magmas originate by partial melting of the metasomatized asthenospheric mantle wedge however there is increasing evidence for the existence of a suite of porphyry cu au and epithermal au deposits related to magmas generated after subduction beneath the arc has ceased associated magmas tend to be mildly alkaline relatively sulfur poor and emplaced as isolated complexes rather than in voluminous volcanoplutonic arcs they are likely formed by remelting of previously subduction modified arc lithosphere triggered by postsubduction lithospheric thickening lithospheric extension or mantle lithosphere delamination metasomatized mantle lithosphere or hydrous lower crustal cumulates residual from first stage arc magmatism contain small amounts of chalcophile and siderophile element rich sulfides and constitute a fertile source for hydrous au rich but relatively sulfur poor magmas during later remelting the recognition that porphyry cu au and related epithermal au systems can also form in postsubduction and collisional tectonic settings expands the range of geological environments and geographical terranes that are prospective for such deposits

tectonic evolution of the mogok metamorphic belt burma myanmar constrained by u th pb dating of metamorphic and magmatic rocks 1 the mogok metamorphic belt mmb extends for over 1500 km along the western margin of the shan thai block from the andaman sea north to the eastern himalayan syntaxis previous geochronology has suggested that a long lasting jurassic early cretaceous subduction related event resulted in emplacement of granodiorites and orthogneisses 171 120 ma and a poorly constrained tertiary metamorphic event on the basis of new u pb isotope dilution thermal ionization mass spectrometry and u th pb laser ablation multicollector inductively coupled plasma mass spectrometer geochronology presented here we propose two tertiary metamorphic events affected the mmb in burma the first was a paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at 59 ma a second metamorphic event spanned late eocene to oligocene at least from 37 possibly 47 to 29 ma this resulted in the growth of metamorphic monazite at sillimanite grade growth of zircon rims at 47 43 ma sillimanite muscovite replacing older andalusite and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45 5 0 6 ma and 24 5 0 7 ma these data imply high temperature sillimanite muscovite metamorphism peaking at 680 c and 4 9 kbar between 45 and 33 ma to around 606 656 c and 4 4 4 8 kbar at 29 3 0 5 ma the later metamorphic event is older than 24 5 0 3 ma the age of leucogranites that crosscut all earlier fabrics our structural and geochronological data suggest that the mmb links north to the unexposed middle or lower crust rocks of the lhasa terrane south tibet and east to high grade metamorphic core complexes in northwest thailand

permian tectonic framework and palaeogeography of se asia abstract the principal continental lithospheric terranes now comprising se asia and that existed in the permian are the south china indochina super terrane and the sibumasu simao west burma and sw borneo semitau terranes the simao terrane is here regarded a separate terrane derived from south china by back arc spreading in the lower carboniferous rather than as an extension of indochina the main palaeo tethys ocean in thailand is identified as being represented by the chiang mai and sra kaeo sutures and the eastern boundary of the sibumasu terrane is thus interpreted to lie farther west in thailand than previously inferred the nan uttaradit suture is interpreted as representing a marginal back arc basin and contiguous with the ailaoshan and jinshajiang sutures in sw china the south china indochina super terrane and the simao terrane were located within the palaeo tethys in equatorial latitudes during the permian where they together with north china exhibit cathaysian floras and faunas the sibumasu terrane was located on the nw australian margin of gondwana until the sakmarian major shifts in faunal affinities from gondwana province faunas to an independent sibumasu province and then to assimilation into the cathaysian province records the separation and rapid northward drift of sibumasu after the late sakmarian the principal permian palaeogeographic models for the region are assessed for internal consistencies and conflicts

diachronous and independent histories of plutonism and mineralization in the cornubian batholith southwest england concordant u pb dates of magmatic monazites in the dominant coarse grained biotite muscovite granites of the late variscan cornubian batholith demonstrate that the emplacement of the six main plutons was diachronous ranging from 293 1 1 3 ma carnmenellis to 274 5 1 4 ma land9s end southern lobe 40 ar 39 ar age spectra for magmatic muscovites confirm that cooling to about 320 c at the present level of exposure was accomplished within 4 to 5 ma in each intrusion moreover age spectra for hydrothermal muscovites show that development of the major sn cu lodes of the south crofty mine commenced before 286 ma only 2 to 3 ma after cooling of the hosting cam brea granite a satellite of the carnmenellis pluton the older plutons had therefore approached ambient temperatures and had even experienced intense hydrothermal activity before emplacement of the youngest

age extent and climatic significance of the c 3400 bp aniakchak tephra western alaska usa tephra deposits at whitefish lake and cape espenberg on the northernmost seward peninsula of western alaska were derived from the aniakchak caldera on the alaska peninsula more than 1500 km to the south radiocarbon dates on the climactic caldera forming aniakchak eruption and on proximal and distal tephra indicate the aniakchak tephra was deposited about 3435 40 bp 3614 3815 cal bp and forms an isochronous marker horizon across much of western alaska the aniakchak tephra eruption and several other targe eruptions during the seventeenth century bc may have had widespread effects on climate

cenozoic metamorphism along the shan scarp myanmar evidences for ductile shear along the sagaing fault or the northward migration of the eastern himalayan syntaxis the mogok metamorphic belt exposed along the n s trending shan scarp and sagaing fault in the eastern part of myanmar has been regarded as paleozoic to precambrian for a long time new observations in the shan scarp area close to the sagaing fault from thaton in the south to mandalay in the north allowed us to collect samples of high grade metamorphic and intrusive rocks that have been analyzed by 40ar 39ar step heating method the 13 samples we analyzed provide oligocene to lower miocene ages for this metamorphism oriented thin sections and field observations suggest that this metamorphism was caused by a nnw sse to n s ductile extension therefore we suggest that this metamorphism is not directly related to the sagaing fault but could be instead related to the northward migration of the eastern himalayan syntaxis characterized by crustal thinning resulting from the india asia oblique collision

timing of metamorphism of the lansang gneiss and implications for left lateral motion along the mae ping wang chao strike slip fault thailand the mae ping fault mpf western thailand exhibits dominantly left lateral strike slip motion and stretches for 600 km reportedly branching off the right lateral sagaing fault in myanmar and extending southeast towards cambodia previous studies have suggested that the fault assisted the large scale extrusion of sundaland that occurred during the late eocene early oligocene with a geological offset of 120 150 km estimated from displaced high grade gneisses and granites of the chiang mai lincang belt exposures of high grade orthogneiss in the lansang national park part of this belt locally contain strong mylonitic textures and are bounded by strike slip ductile shear zones and brittle faults geochronological analysis of monazite from a sample of sheared biotite k feldspar orthogneiss suggests two episodes of crystallization with core regions documenting th pb ages between c 123 and c 114 ma and rim regions documenting a significantly younger age range between c 45 37 ma these data are interpreted to represent possible magmatic protolith emplacement for the lansang orthogneiss during the early cretaceous with a later episode of metamorphism occurring during the eocene textural relationships provided by in situ analysis suggest that ductile shearing along the mpf occurred during the latter stages of or after this metamorphic event in addition monazite analyzed from an undeformed garnet two mica granite dyke intruding metamorphic units at bhumipol lake outside of the mae ping shear zone produced a th pb age of 66 2 1 6 ma this age is interpreted to date the timing of dyke emplacement implying that the mpf cuts through earlier formed magmatic and high grade metamorphic rocks these new data when combined with regional mapping and earlier geochronological work show that neither metamorphism nor regional cooling was directly related to strike slip motion

first evidence of the cambrian basement in upper peninsula of thailand and its implication for crustal and tectonic evolution of the sibumasu terrane abstract although the sibumasu terrane in asia was previously considered to be composed of phanerozoic rocks with cambrian crystalline basement no reliable or direct radiometric dating evidences of such crystalline basement was ever reported our new in situ zircon u pb dating of the khao tao orthogneiss yields a concordant age of 501 5 7 5 ma 2 which provides the first robust evidence for the cambrian crust in upper peninsula of thailand the zircon ehf t values range from 3 7 to 6 1 with model ages tcdm of 1244 1827 ma suggests a mixed crust mantle source the chemical similarity and spatial continuity of the khao tao orthogneiss with other pre neotethys marginal eurasian and sibumasu granitoids indicate the linear paleogeographic association under a similar magmatic arc related regime along the gondwana india australia margin as part of the pan african orogeny system

old crow tephra found at the palisades of the yukon alaska abstract a 20 cm thick volcanic ash layer at the palisades along the yukon river is geochemically correlated with the old crow tephra a buried organic and wood rich layer above the old crow tephra probably records the last interglaciation the recognition of the old crow tephra provides the first chronologic information on the age of the thick sequence of pleistocene sediments at the palisades an important ice age mammal site in central alaska and a key locakity for the reconstruction of the history of the yukon river and the last interglaciation in central alaska

surface faulting along the superstition hills fault zone and nearby faults associated with the earthquakes of 24 november 1987 the m 6 2 elmore desert ranch earthquake of 24 november 1987 was associated spatially and probably temporally with left lateral surface rupture on many northeast trending faults in and near the superstition hills in western imperial valley three curving discontinuous principal zones of rupture among these breaks extended northeastward from near the superstition hills fault zone as far as 9 km the maximum observed surface slip 12 5 cm was on the northern of the three the elmore ranch fault at a point near the epicenter twelve hours after the elmore ranch earthquake the m 6 6 superstition hills earthquake occurred near the northwest end of the right lateral superstition hills fault zone surface rupture associated with the second event occurred along three strands of the zone here named north and south strands of the superstition hills fault and the wienert fault for 27 km southeastward from the epicenter in contrast to the left lateral faulting which remained unchanged throughout the period of investigation the right lateral movement on the superstition hills fault zone continued to increase with time a behavior that was similar to other recent historical surface ruptures on northwest trending faults in the imperial valley region we measured displacements over 339 days at as many as 296 sites along the superstition hills fault zone and repeated measurements at 49 sites provided sufficient data to fit with a simple power law data for each of the 49 sites were used to compute longitudinal displacement profiles for 1 day and to estimate the final displacement that measured slips will approach asymptotically several years after the earthquakes the maximum right lateral slip at 1 day was about 50 cm near the south central part of the north strand of superstition hills fault and the predicted maximum final displacement is probably about 112 cm at imler road near the center of the south strand of the superstition hills fault the overall distributions of right lateral displacement at 1 day and the estimated final slip are nearly symmetrical about the midpoint of the surface rupture the average estimated final right lateral slip for the superstition hills fault zone is about 54 cm the average left lateral slip for the conjugate faults trending northeastward is about 23 cm the southernmost ruptured member of the superstition hills fault zone newly named the wienert fault extends the known length of the zone by about 4 km the southern half of this fault south of new river expressed only vertical displacement on a sinuous trace the maximum vertical slip by the end of the observation period there was about 25 cm but its growth had not ceased photolineaments southeast of the end of new surface rupture suggest continuation of the superstition hills fault zone in farmland toward mexico

a re examination of historic earthquakes in the san jacinto fault zone california the high level of seismic activity and the potential for large earthquakes in the san jacinto fault zone southern california make it desirable to have accurate locations and source parameters for as many previous events as possible prior to the installation of a dense seismic network in this region earthquakes were located using only a few stations with generally poor azimuthal coverage resulting in considerable uncertainty in the locations we relocate and obtain moment estimates for historic pre wwssn earthquakes in the western imperial valley by comparing the waveforms and travel times with recent earthquakes in the region all the events are in the m l 5 5 to 6 5 range the historic earthquakes of interest occurred in 1937 1942 and 1954 we use the 1968 borrego mountain 1969 coyote mountain and 1987 elmore ranch earthquakes as calibration events we employ regional and teleseismic data from continuously operating stations with pasadena debilt berkeley ottawa and st louis recording most of the events the waveforms imply that all the events are almost pure strike slip events on vertical or near vertical faults approximate values for the strikes were obtained and are within the range of observed strikes for well studied earthquakes in this region the earthquakes are relocated by comparing s p and surface wave s travel times of historic events with the presumably well located recent events the relocations require only a small change in location for the 1954 event and a larger adjustment in the 1942 epicenter it also appears that the 1969 earthquake may have been mislocated the moment estimates are obtained by direct comparison of the maximum amplitudes the moment estimates imply that the 1968 and not the 1942 earthquake is the largest to have occurred in the region this century previous magnitude estimates suggested the 1942 event was larger

slip distribution aftershocks and co seismic fault of some large persian earthquakes this investigation concentrates on slip distribution aftershocks mainshock location and related co seismic faults for seven continental large earthquakes in the persian plateau most of the earthquakes m s 5 7 7 7 1981 2005 with both measurable surface slip along a mature rupture and a precisely aftershock surveying with a local seismological network in iran are evaluated in most of the case studies we observe an acceptable correlation between maximum surface slip along the fault trace and the asperity an area which is recognized as an obvious gap or an area with lack of aftershocks on a fault plane on the other hand we conclude that the aftershocks which are associated with immeasurable or smaller surface slips mainly concentrate at the outside of the asperities also the mainshocks in most of the earthquakes are in neighboring with the big asperities and segment the co seismic faults we could see a gradual increase in the co seismic slip when the magnitude of the earthquake increases but examples with defuse aftershocks and with no specific pattern are also found

formation of the maturin foreland basin eastern venezuela thrust sheet loading or subduction dynamic topography 1 the maturin basin in eastern venezuela is considered a good example of a peripheral foreland basin earthquake and tomographic data indicate that eastern venezuela is affected by the oblique subduction of the south american plate underneath the caribbean plate new forward flexural isostatic modeling of eastern venezuela has been carried out in order to determine whether the maturin basin was generated purely by thrust sheet loading from the serrania and monagas foreland thrust belts a sequence of forward models from middle miocene to present was generated for 3 profiles across the serrania del interior thrust belt the monagas foreland thrust belt and the maturin foreland basin the predictions of these models are constrained using seismic reflection and well data the flexural isostatic modeling shows that thrust sheet loading associated with the serrania del interior and monagas foreland thrust belts is insufficient to generate the observed subsidence within the maturin basin dynamic fluid flow modeling of subduction related dynamic topography of eastern venezuela has been used to investigate the influence of south american plate subduction on the generation of the accommodation space observed in the maturin basin fluid flow modeling of subduction related dynamic topography suggests that the subduction of the south american lithospheric mantle caused downward deflection of the south american crust affecting the maturin basin and the serrania thrust belt this modeling suggests that the maturin basin subsidence has two components 55 related to thrust sheet loading and 45 driven by continental subduction

crustal structure restoration and evolution of the greater himalaya in nepal south tibet implications for channel flow and ductile extrusion of the middle crust

late pleistocene and holocene tephrostratigraphy of interior alaska and yukon key beds and chronologies over the past 30 000 years abstract the aleutian arc alaska peninsula and wrangell volcanic field are the main source areas for tephra deposits found across alaska and northern canada and increasingly tephra from these eruptions have been found further afield in north america greenland and europe however there have been no broad scale reviews of the late pleistocene and holocene tephrostratigraphy for this region since the 1980s and this lack of data is hindering progress in identifying these tephra both locally and regionally to address this gap and the variable quality of associated geochemical and chronological data we undertake a detailed review of the latest pleistocene to holocene tephra found in interior alaska and yukon this paper discusses nineteen tephra that have distributions beyond southwest alaska and that have the potential to become or already are important regional markers this includes three modern events from the 20th century ten with limited data availability but potentially broad distributions and six that are widely reported in interior alaska and yukon each tephra is assessed in terms of chronology geochemistry and distribution with new bayesian age estimates and geochemical data when possible this includes new major element geochemical data for crater peak 1992 redoubt 1989 90 and two andesitic tephra from st michael island tephra d as well as revised age estimates for dawson tephra oshetna hayes set h aniakchak cfe ii and the white river ashes northern and eastern lobes

u pb zircon geochronology of early permian to late triassic rocks from singapore and johor a plate tectonic reinterpretation article i nfo the results of u pb zircon geochronology of permian to middle triassic granitoids from singapore and neighbouring johor are presented rhyolite from pulau sibu is 276 5 ma gabbros from singapore give ages of 260 2 to 249 2 ma granites from singapore give ages from 249 2 to 230 6 ma and rhyolite from telkuma gives an age of 238 2 ma u pb detrital zircon age spectra from a late triassic conglomerate from singapore show carboniferous mid dle triassic and late triassic spikes at 350 245 and 217 ma respectively these new data when combined with published u pb zircon ages allow a revision of the plate tectonic evolution of the malaysian peninsula region a tin bearing magmatic arc evolved in the eastern malaysian peninsula part of indochina from the early permian to the middle triassic when the palaeo tethys ocean was subducted and sibumasu continental lithosphere was rifted from pangea this opened up the neo tethys ocean that the granitoid ages show a progressive decrease in age from pulau sibu 276 5 ma to singapore 230 6 ma suggests that during this andean type orogenic phase the subduction zone rolled back and steepened causing the locus of mainly i type magmatism to migrate from east to west in present day coordinates across the eastern and central belts and stretch the lithosphere as it did so in the late triassic sibumasu collided with indochina causing the western belt sibumasu to over thrust the central belt indochina along the bentong raub suture the lower half of the thickened crust formally the tin bearing indochina plate was partially melted and pre dominantly s type tin bearing granites were intruded into the upper crust formally sibumasu this late triassic collisional phase is the indosinian orogeny in peninsular malaysia

source characteristics of earthquakes along the southern san jacinto and imperial fault zones 1937 to 1954 abstract body waveform inversion techniques are used to study the source parameters of four earthquakes occurring between 1937 and 1954 along the southern san jacinto and imperial faults 1937 buck ridge 1940 imperial valley 1942 borrego mountain and 1954 salada wash events all earthquakes had simple rupture histories with the exception of the 1940 imperial valley main shock which consisted of at least four subevents whose relative locations indicate unilateral rupture toward the southeast earthquakes in regions of high heat flow 80 mw m 2 had focal depths near the base of the seismogenic zone 8 to 10 km the 1937 buck ridge earthquake located in a region of lower heat flow however appears to have occurred at a shallow 3 2 km depth the location mechanism and aftershock distribution for the 1942 borrego mountain earthquake suggest it could have occurred along the split mountain fault a recently identified northeast trending cross fault located between the elsinore and coyote creek faults or along an unnamed fault that parallels the trend of the coyote creek fault moment and rupture length estimates obtained from this study agree well with estimates obtained in previous studies that used different data sets

m 6 earthquakes in southern california during the twentieth century no evidence for a seismicity or moment deficit abstract a broadly based report on seismic hazards in southern california wgcep 1995 concluded that the predicted seismicity exceeds that observed since 1850 a subsequent independent analysis argued that infrequent huge m 8 earthquakes are needed to explain the low rate of large earthquakes jackson 1996 frequency magnitude relationships and earthquake reporting suggest that the 1903 to 1997 catalog we present here with a b value of 1 0 and a rate of m 6 shocks of 0 42 to 0 49 yr 1 is nearly complete in contrast the 1850 to 1994 catalog used by wgcep is incomplete before the turn of the century and thus its reported seismicity rate of 0 32 m 6 shocks yr 1 is too low principally because the wgcep 1995 model results in b values of up to 4 0 for regions of lesser and blind faults the rate of m 6 shocks off the san andreas system predicted by the wgcep 1995 model is three times greater than that observed in this century because they obtained b 0 4 for m b 2 2 for m 7 3 on major faults their expected rate of m 7 san andreas shocks is twice as high as observed thus part of the seismicity and moment discrepancy identified by wgcep was caused by use of an incomplete catalog and part was caused by inappropriate b values we obtain a southern california moment release rate of 8 to 12 10 18 n m yr 1 which cannot be distinguished from the moment release estimated by fault slip or the moment accumulation inferred from plate motions or geodetically measured shear strain we thus find no evidence for a moment deficit significant aseismic moment release or for rare m 8 earthquakes off the san andreas fault system finally the number of m 6 earthquakes per decade does not depart significantly from a poisson process during this century and thus we find no evidence that the rate of seismicity is increasing now or at any other time since 1900

nested strike slip duplexes and other evidence for late cretaceous palaeogene transpressional tectonics before and during india eurasia collision in thailand myanmar and malaysia the late cretaceous palaeogene structural and tectonic events in the northern region of sundaland represent a poorly known but potentially important orogenic development in se asia extensive late cretaceous s type granites extending from malaysia through thailand to myanmar have long been used to infer an episode of crustal thickening an inference supported by the presence of late cretaceous eocene ophiolites in myanmar however structural evidence for a well developed associated fold and thrust belt has been lacking apatite and zircon fission track studies of thailand indicate a broad regional uplift from about 80 ma to 40 ma suggesting an episode of crustal thickening but the uplift amount is modest and not that expected for a major fold and thrust belt left lateral motion on major nw se trending strike slip fault zones mae ping and three pagodas faults in myanmar and thailand has been attributed to himalayan tibetan escape tectonics this paper demonstrates that the fault zones are considerably more complicated than just nw se trends they are a network of branching faults with important north south trends as well as nw se trends they form large scale nested strike slip duplex structures and affect a region over 1000 km long and up to 250 km wide this diffuse branching network of strike slip fault trends may represent a substantial late cretaceous palaeogene transpressional belt himalayan escape tectonics represent later increments of deformation with both oligocene sinistral and late oligocene recent dextral reactivation

the stampede tephra a middle pleistocene marker bed in glacial and eolian deposits of central alaska a deformed block of silt incorporated in a push moraine of the lignite creek drift of the nenana river valley in central alaska contains the 10 cm thick stampede tephra the silt block and tephra are folded sheared and cut by multiple fault planes and appear to have been entrained by a glacier and deformed by glaciotectonic processes while frozen the stampede tephra is also found in a paleosol preserved within a sequence of eolian sediments near the tanana river some 175 km to the northeast allowing direct tephrochronologic correlations between the depositional record of quaternary glaciations in the alaska range and eolian sediments in unglaciated central alaska the lignite creek drift predates deposits of the late wisconsin riley creek and penultimate healy glaciations but postdates the stampede tephra indicating that it dates to the middle quaternary and is not part of the tertiary nenana gravel formation