long term fore arc basin evolution in response to changing subduction styles in southern alaska detrital zircon u pb and fission track double dating and hf isotopes from the mesozoic and cenozoic strata in the southern alaska forearc basin system reveal the effects of two different modes of flat slab subduction on the evolution of the overriding plate the southern margin of alaska has experienced subduction of a spreading ridge ca 62 50 ma and an oceanic plateau ca 40 0 ma when a subducting spreading ridge drives slab flattening our data suggest that after the ridge has moved along strike retro arc sediment sources to the forearc become more predominant over more proximal arc sources spreading ridge subduction also results in thermal resetting of rocks in the upper plate that is revealed by thermochronologic data that record the presence of young age peaks found in subsequent thin sedimentary strata in the forearc basin when a subducting oceanic plateau drives slab flattening our data suggest that basin catchments get smaller and local sediment sources become more predominant crustal thickening due to plateau subduction drives widespread surface uplift and significant vertical uplift in rheologically weak zones that combined create topography and increase rock exhumation rates consequently the thermochronologic signature of plateau subduction has generally young age peaks that generate short lag times indicating rapid exhumation the cessation of volcanism associated with plateau subduction limits the number of syndepositional volcanic grains that produce identical geochronologic and thermochronologic ages this study demonstrates the merit of double dating techniques integrated with stratigraphic studies to expose exhumational age signatures diagnostic of large scale tectonic processes in magmatic regions

provenance of xigaze fore arc basin clastic rocks cretaceous south tibet the xigaze group of south tibet was deposited in a fore arc basin produced by northward subduction of the neotethys under eurasia during middle cretaceous time clasts of andesite from the gangdese magmatic belt to the north and limestone clasts from a shelf rimming the basin to the north are the near exclusive detrital components in the xigaze group in the eastern part of the studied region and in older albian deposits in the west younger cenomanian coniacian deposits from western and central parts of the xigaze group show increasing amounts of tonalite pebbles in conglomerates and quartz sand these trends reflect differential arc uplift and erosional dissection with higher rates in the west some arenites show evidence for sediment contribution from an additional source probably from an orogen to the north of the lhasa block the presence of this orogen indicates that crustal thickening of tibet is not restricted to cenozoic times the middle cretaceous gangdese belt apparently lacked acid volcanic rocks this suggests a low degree of crustal contamination and or differentiation of primitive magma probably because the crust beneath the arc area was thin a thin crust is in accordance with a low elevation of the arc deduced from continuous carbonate production on and marine excursions beyond the arc magma eruption rates of the arc estimated from the xigaze basin sediment volume were probably low 7 5 km 3 km arc segment per million years but within the range of modern arcs the varying sediment compositions of the xigaze group may be typical of fore arc basin deposits in front of continental magmatic arcs the lack of a correlation of trends in sediment composition with fining upward sedimentary supercycles in the xigaze group is taken as evidence in favor of a eustatic cycle control

intermediate to felsic middle crust in the accreted talkeetna arc the alaska peninsula and kodiak island alaska an analogue for low velocity middle crust in modern arcs 1 seismic profiles of several modern arcs have identified thick low velocity midcrustal layers vp 6 0 6 5 km s that are interpreted to represent intermediate to felsic plutonic crust the presence of this silicic crust is surprising given the mafic composition of most primitive mantle melts and could have important implications for the chemical evolution and bulk composition of arcs however direct studies of the middle crust are limited by the restricted plutonic exposures in modern arcs the accreted talkeetna arc south central alaska exposes a faulted crustal section from residual subarc mantle to subaerial volcanic rocks of a jurassic intraoceanic arc and is an ideal place to study the intrusive middle crust previous research on the arc which has provided insight into a range of arc processes has principally focused on western exposures of the arc in the chugach mountains we present new u pb zircon dates radiogenic isotope data and whole rock geochemical analyses that provide the first high precision data on large intermediate to felsic plutonic exposures on kodiak island and the alaska peninsula a single chemical abrasion thermal ionization mass spectrometry analysis from the afognak pluton yielded an age of 212 87 0 19 ma indicating that the plutonic exposures on kodiak island represent the earliest preserved record of talkeetna arc magmatism nine new dates from the extensive jurassic batholith on the alaska peninsula range from 183 5 to 164 1 ma and require a northward shift in the talkeetna arc magmatic axis following initial emplacement of the kodiak plutons paralleling the development of arc magmatism in the chugach and talkeetna mountains radiogenic isotope data from the alaska peninsula and the kodiak archipelago range from nd t 5 2 to 9 0 and 87sr 86srint 0 703515 to 0 703947 and are similar to age corrected data from modern intraoceanic arcs suggesting that the evolved alaska peninsula plutons formed by extensive differentiation of arc basalts with little or no involvement of preexisting crustal material the whole rock geochemical data and calculated seismic velocities suggest that the alaska peninsula represents an analogue for the low velocity middle crust observed in modern arcs the continuous temporal record and extensive exposure of intermediate to felsic plutonic rocks in the talkeetna arc indicate that evolved magmas are generated by repetitive or steady state processes and play a fundamental role in the growth and evolution of intraoceanic arcs

geochemical and stratigraphic analysis of the chisana formation wrangellia terrane eastern alaska insights into early cretaceous magmatism and tectonics along the northern cordilleran margin

crystal size distribution in magmatic rocks proposition of a synthetic theoretical model the crystal size distribution csd corresponds to the number of crystals of a mineral per unit volume within a series of defined size intervals many crystal size distributions in igneous systems are straight lines when plotted on the classical diagram of ln population density versus size other common csds are concave down on such graphs for the low size values the effect of growth rate on csds seems to be small and nucleation apparently increases exponentially with time although magmatic systems are always multiphase only a few csd studies deal with several phases on the basis of the few available published examples it seems that the parallelism defined as y spacing constancy of the normalized csds i e where the crystal lengths are normalized to the greatest length for each phase is a common feature in igneous systems the aim of our modelling is to propose a mathematical framework to unify the csd typology to explain the common occurrence of straight lines on the classical csd diagram and the y spacing constancy of the normalized csds for multiphase rocks and also to provide a convenient tool to easily test petrogenetic scenarios through efficient computer simulations the bulk balance modelling is based on a square matrix having for coefficients the number of nuclei of one phase by the crystallized volume of another phase this interaction matrix is the mainspring of our csd modelling which requires straightforward assumptions about the uniformity of crystal shape and growth 1 at each time every crystal of a phase has the same shape and 2 the one dimensional growth rate of a crystal is independent of its size consequently the values of the normalized csd are linked together by means of an integral equation for the most part the behaviour of the csds theoretically obtained with a constant matrix may also be observed in computer simulations of more general cases this extended approach makes feasible the application of the model to natural cases concave down shapes for the lowest size are classically interpreted as a consequence of the ending of crystallization by roughly taking into account crystal interactions during the growth process through the concept of mean clutter we successfully simulate such concave down csds

mesozoic cenozoic deformation in the canadian cordillera the record of a continental bulldozer abstract a possible mesozoic cenozoic trajectory for the north american craton is outlined from latitude changes of the craton derived from a revised apparent polar wander path and from westward movement of the craton based on the assumption that africa has been the least mobile continent geographically since the latest paleozoic during each of five time intervals that span 220 million years the craton trajectory had a different vector each vector appears to be reflected in the canadian cordillera by the dominant style and orientation of structures formed during that interval for much of the past 220 million years the meridionally oriented western margin of the pangea laurasia north america plate has been the site of arc magmatism where weak arc back arc lithosphere sandwiched between strong ocean floor and craton lithospheres at times coupled across convergent or transform plate boundaries focused and recorded strain when the craton apparently moved due westward between 180 160 ma and 120 60 ma the dominant structures formed then record orogen normal compression and were accompanied by orogeny structures formed during the earlier episode are mainly in the eastern and interior cordillera and their formation shortly followed or coincided with accretion of most terranes to the craton margin in the later episode compressional structures span the entire cordillera which emerged as a tectonic and physiographic entity before and between these intervals when the craton apparently moved mostly northwestward mainly geological and paleomagnetic considerations indicate some terranes moved southward sinistrally relative to the craton after 60 ma southwestward movement of the craton was concurrent with large northward dextral strike slip faults that disrupted the newly established ancestral cordillera the coincidence between the age of structures that record dominant orogen normal compression at times when the craton apparently moved due westward and orogen parallel displacements when the craton had either northward or southward components of motion suggests the craton acting as a continental bulldozer was the primary driver of cordilleran deformation and orogenesis

mt etna a model for the 1989 eruption abstract during the 1989 eruption of mt etna one of the most important in the last 20 years in terms of effusion rate a pair of fractures formed on the slopes of the southeast crater cone and propagated in a few days trending ca northeast and southeast with large quantities of magma being drained from the northern fracture in contrast the southern fracture after a short initial eruptive phase grew by 5 km in five days without eruptive or seismic activity when the fracture system crossed the southern scarp of the valle del bove wall some 6 km se of the active crater an intense seismic swarm started in the same area this lasted about four days and consisted of several hundred events clustering in a small focal volume although such a sequence of failure events contains elements of compatibility with a shallow dyke intrusion it did not lead to the expected flank eruption which would have threatened settlements just a few kilometres from the distal end of the fracture the question arises therefore whether or not the fracture system was dyke related and if so to what extent we attempt here to answer such a question by integration and interpretation of the available field observation and geophysical data indeed field observation constraints on the southern fracture system and the typical depth 2 km of the swarm do not contain evidence for shallow dyke emplacement beneath the lowermost part of the sse fracture conversely the inferred direction and shape of the tremor source the location and extent of a gravity anomaly observed a few months before the eruption the near horizontal migration of the seismic focii during the swarm and the change with time of related focal mechanisms lead us to hypothesize that the extreme development of the fracture system at its south was due to a deeper intrusion beneath the southern slope of the volcano

the xigaze forearc basin evolution and facies architecture cretaceous tibet abstract the mid cretaceous to eocene flysch deposits of the xigaze forearc basin in southern tibet were investigated in a 120 km segment along the indus yarlung suture zone the basin evolved south of the magmatic arc gangdise belt of the lhasa block on top of trapped oceanic or transitional crust remnants of shelf carbonates are preserved along the northern edge of the basin the mapped segment was shortened by about 65 metamorphism reached low grade conditions along the northern margin of the basin the flysch sequence reaches a thickness of at least 5 km and consists to a large degree of volcaniclastic andesitic to dacitic material shed from the magmatic arc gangdise belt particularly in the western part of the study area plutonic and sedimentary rocks from deep erosion levels and or more distal sources contributed to the basin fill rivers from the lhasa block acted as point sources and fed five major deep sea channel systems turbidity currents in the channels were directed towards the growing accretionary wedge of the subduction zone thus indicating that the basin was continuously filled up to outer ridge level and gradually shallowing the forearc flysch is subdivided into at least three megasequences which begin with wide up to several km incised 10 to 50 m coarse grained channel fills and their associated fan deposits the upper parts of the megasequences contain hemipelagic dark shales and marls deposited above the calcite compensation depth lateral channel migration channel lobe switching but also volcanic pulses generated a predominantly fining upward high frequency cyclicity after continental collision the marine sedimentation in the forearc basin was replaced by fluvial deposits of the eocene oligocene qiuwu formation which is time equivalent to the kailas and indus molasses farther west and rich in coarse gravel derived from the gangdise belt both forearc flysch and qiuwu formation were deformed simultaneously during the miocene we assume that the molasse type qiuwu formation represents the final continental facies of the forearc basin filling

rapid change in drift of the australian plate records collision with ontong java plateau earth s largest and thickest oceanic plateau the ontong java plateau is currently colliding with the australian plate but it has been difficult to date the start of this momentous event with much accuracy now a team from the university of queensland presents strong evidence for a collision starting about 26 million years ago the dating comes from geochronological data on hotspot volcanoes in the tasman sea east of australia which reveal a link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain suggesting that immense oceanic plateaus like ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale geochronological data on hotspot volcanoes in eastern australia are presented which reveal a strong link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain and thus provide strong evidence that immense oceanic plateaus can contribute to initiating rapid change in plate boundaries and motions on a global scale the subduction of oceanic plateaux which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood basalt provinces is an important factor in many current models of plate motion1 2 3 4 and provides a potential mechanism for triggering plate reorganization5 to evaluate such models it is essential to decipher the history of the collision between the largest and thickest of the world s oceanic plateaux the ontong java plateau and the australian plate but this has been hindered by poor constraints for the arrival of the plateau at the melanesian trench here we present 40ar 39ar geochronological data on hotspot volcanoes in eastern australian that reveal a strong link between collision of the greenland sized ontong java plateau with the melanesian arc and motion of the australian plate the new ages define a short lived period of reduced northward plate motion between 26 and 23 myr ago coincident with an eastward offset in the contemporaneous tracks of seamount chains in the tasman sea east of australia these features record a brief westward deflection of the australian plate as the plateau entered and choked the melanesian trench 26 myr ago from 23 myr ago australia returned to a rapid northerly trajectory at roughly the same time that southwest directed subduction began along the trobriand trough6 the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the pacific plate including the archetypal hawaiian chain7 and thus provide strong evidence that immense oceanic plateaux like the ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale

paleoseismology at high latitudes seismic disturbance of upper quaternary deposits along the castle mountain fault near houston alaska most paleoseismic studies are at low to moderate latitudes here we present results from a high latitude 61 30 n trenching study of the castle mountain fault in south central alaska this fault is the only one known in the greater anchorage alaska area with historical seismicity and a holocene fault scarp it strikes east northeast and cuts glacial and postglacial sediments in an area of boreal spruce birch forest shrub tundra and sphagnum bog the fault has a prominent vegetation lineament on the upthrown north side of the fault nine trenches were logged across the fault in glacial and postglacial deposits seven along the main trace and two along a splay in addition to thrust and strike slip faulting important controls on observed relationships in the trenches are the season in which faulting occurred the physical properties of the sediments liquefaction a shallow water table soil forming processes the strength of the modern root mat and freeze thaw processes some of these processes and physical properties are unique to northern latitude areas and result in seismic disturbance effects not observed at lower latitudes the two trenches across the castle mountain fault splay exposed a thrust fault and few liquefaction features radiocarbon ages of soil organic matter and charcoal within and overlying the fault indicate movement on the fault at ca 2735 cal calendar yr b p and no subsequent movement in the remaining seven trenches surface faulting was accompanied by extensive liquefaction and a zone of disruption 3 m or more wide the presence of numerous liquefaction features at depths of 0 5 1 0 m indicates faulting when the ground was not frozen i e from about april to october sandy matrix till sand silt gravel and pebbly peat were injected up to the base of the modern soil but did not penetrate the interlocking spruce birch root mat the strength of the root mat prohibited development of a nonvegetated scarp face and colluvial wedge in only one trench did we observe a discrete fault plane with measurable offset it lay beneath a 2 m thick carapace of liquefied sand and silt and displayed a total of 0 9 1 85 m of thrust motion since deposition of the oldest deposits in the trenches at ca 13 500 yr b p we found liquefaction ejecta on paleosols at only one other trench where there were bluejoint calamagrostis canadensis tussocks that lacked an extensive root mat from crosscutting relationships we interpret three paleoliquefaction events on the main trace of the castle mountain fault 2145 1870 1375 1070 and 730 610 cal yr b p these four earthquakes on the castle mountain fault in the past 2700 yr indicate an average recurrence interval of 700 yr as it has been 600 700 yr since the last significant earthquake a significant magnitude 6 7 earthquake in the near future may be likely paleoseismic data indicate that the timing and recurrence interval of megathrust earthquakes is similar to the timing and recurrence interval of castle mountain fault earthquakes suggesting a possible link between faulting on the megathrust and on crustal structures

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

composition of primary kimberlite magma constraints from melting and diamond dissolution experiments experiments are applied to constrain the composition of primary kimberlitic magmas which were in equilibrium with lithospheric peridotite and could resorb the entrained diamond to form typical dissolution features the experiments are run on samples of a model carbonatite and a melt of the udachnaya kimberlite at 6 3 gpa and 1400 c and at unbuffered or re reo2 buffered oxygen fugacity 1 2 log units above ni nio near liquidus dry fe3 free carbonatitic melt derived from carbonated harzburgite is saturated with the ol grt opx mgs assemblage and is almost inert to diamond carbonatitic melts that bear 4 6 6 8 wt fe2o3 or 1 5 wt h2o are in equilibrium only with mgs ol near the liquidus dissolution of diamond by these melts produces surface textures uncommon corrosion sculptures or common negative oriented trigons shield shaped laminae and elongate hillocks to kimberlitic diamonds the near liquidus melt of the udachnaya kimberlite yakutia with 10 12 wt h2o is saturated with the ol grt cpx assemblage and may result from melting of carbonated garnet bearing wehrlite hydrous kimberlitic melt likewise resorbs diamonds forming typical negative oriented trigons shield shaped laminae and elongate hillocks on their surfaces therefore the melts that could originate in the thermal conditions of subcratonic lithosphere entrain diamond and dissolve it to produce dissolution features on crystal surfaces were compositionally close to kimberlite 16 19 wt sio2 and rich in h2o dry fe3 bearing carbonatites with fo2 controlled by the ferric ferrous equilibrium slightly above the ni nio buffer cannot be diamond carriers

latest cretaceous forearc basin development along an accretionary convergent margin south central alaska upper cretaceous sedimentary strata exposed in south central alaska provide insight on tectonic processes that shaped the northern pacific margin following accretion of the wrangellia composite terrane the largest addition of crust to north america over the past 100 m y sedimentologic geochronologic biostratigraphic and petrographic data from the matanuska formation permit reconstruction of the tectono sedimentary history of strata in a forearc basin constructed upon accreted oceanic arc crust the matanuska formation consists of 3 km of sedimentary strata exposed in the northern chugach mountains matanuska valley and southern talkeetna mountains of interior south central alaska measured stratigraphic sections and lithofacies analyses demonstrate that mass slumps and slides debris flows and turbidity currents deposited campanian maastrichtian sandstone conglomerate and mudstone on a gullied trenchward dipping submarine ramp benthic foraminifera inoceramid bivalves and nereites ichnogenera indicate deposition mainly at bathyal water depths sandstone and conglomerate petrofacies are characterized by monocrystalline quartz plagioclase feldspar and volcanic lithic fragments q39f40l21 qm29f40lt32 lm25lv42ls32 and qm42p54k4 jurassic cretaceous arc plutons exposed north of the basin were an important sediment source based on u pb zircon ages of granitoid clasts from conglomerate and detrital zircons from sandstone coeval arc plutons were unroofed relatively quickly judging by the presence of 77 71 ma detrital zircons in sandstone and 79 77 ma granitic clasts in conglomerate together with maastrichtian 71 65 ma ammonite and foraminifera fossils sparse paleozoic triassic detrital zircons indicate minor sediment contribution from inboard sources including the yukon tanana composite terrane and recycled jurassic cretaceous sedimentary strata kahiltna assemblage new data from the upper matanuska formation together with recent studies from age equivalent strata exposed in the alaska range and wrangell mountains provide an exceptional example of basin development along a subduction margin shortly following accretion of an oceanic arc forearc basin development was dominated by subsidence and sediment gravity flow deposits enriched in plutonic and volcanic clasts eroded from both remnant and coeval arc plutons within the arc newly recognized conglomerate in the northern talkeetna mountains records erosion of coeval and remnant arc source terranes to the south and precambrian paleozoic sources to the north farther inboard syndepositional shortening prompted thrust top basin development and accumulation of alluvial lacustrine strata derived from both the former continental margin to the north and accreted oceanic rocks to the south regional subsidence and basin development terminated during late maastrichtian early paleocene time coincident with subduction of progressively younger oceanic lithosphere inboard of an oceanic spreading center

seismic heterogeneity of mt etna structure and activity a dense seismic array maintained temporarily for 7 weeks provides an approach to deducing the 3 d heterogeneity of the structure of the upper crust beneath mt etna volcano from the inversion of p and s wave arrivals of local earthquakes independent data from a different source array geometry using waves generated by distant shots at sea confirm a main lateral variation a high velocity body underlies part of the volcano its contrast with the surrounding thick sedimentary pile and its location extending from the present summit to the southeast suggests that it may have been built by the development and migration of past eruptive centres followed by caldera collapse local low velocity anomalies near its sharp western edge may be associated with presently active features

olivine growth rates in a tholeiitic basalt an experimental study of melt inclusions in plagioclase abstract basaltic glass inclusions trapped in plagioclase phenocrysts an 84 are remnant of their parent magmatic liquid they can be used as natural reactors for the experimental investigation of olivine growth rate as a function of temperature the growth of one olivine nucleus can be observed at constant temperature supercooling from 15 to 150 c have been investigated growth habits vary from equant to feather in qualitative agreement with previous studies growth rates vary from 10 m s 1 to 6 10 7 m s 1 they vary with the direction the growth process planar or dendritic and the degree of supercooling chemical analysis of crystal overgrowth and the remaining liquid composition enables a mass balance calculation which confirms the rates determined optically the small number of results obtained so far does not permit to draw inferences on the growth mechanisms

geochronology of augen gneiss and related rocks yukon tanana terrane east central alaska using several isotopic techniques we have determined the ages of selected metamorphic rocks in the yukon tanana terrane ytt of east central alaska u pb zircon data from an augen gneiss body in the big delta quadrangle indicate that the granitoid protolith of the gneiss was intruded 341 3 m y ago lower intercept age an upper intercept age of 2 136 31 m y indicates an inherited early proterozoic component in these zircons this inheritance age is substantiated by a sm nd whole rock model age of 2 09 0 08 b y from the big delta augen gneiss body detrital zircons from quartzitic wall rocks to this body were also derived from an early proterozoic 2 1 to 2 3 b y old crustal source s zircons from three other augen gneisses occurring in an east west belt which extends into the southern yukon territory canada have similar mississippian and early proterozoic intercept ages a rb sr whole rock isochron from widely separated bodies of augen gneiss has an age of 333 26 m y and an initial 87 sr 86 sr ratio of 0 728 0 002 confirming the mississippian intrusive age for the protolith the high initial 87 sr 86 sr ratio further indicates an old crustal component in these rocks a rb sr mineral isochron 115 4 m y k ar data from hornblende and micas 128 to 107 m y and u pb data from sphene 134 m y from augen gneiss and related rocks are similar to many k ar ages in this region and confirm the occurrence of an early cretaceous thermal event u pb ages of zircons from three metavolcanic units in the ytt suggest that extrusion of the protoliths of these rocks occurred 360 380 m y ago scatter in the data is caused by ubiquitous inheritance and multiple lead loss events significant devonian mississippian igneous activity 380 340 m y took place in the ytt followed by one or possibly two metamorphic episodes mississippian and cretaceous as yet no early proterozoic source rocks for the paleozoic magmas have been identified in the ytt but rocks of similar early proterozoic ages occur to the east and south in the northwest territories and british columbia canada

an approach to problems on energy sources at mount etna based on seismological and volcanological data based on seismograms from the most continuously operating station of the seismic network at mt etna monte vetore 1665 m a s l the energy release patterns through time have been obtained for volcanic tremors and earthquakes which occurred between 1978 1982 both energies range between 1011 1013 j a but their release patterns are not strictly correlated to each other considering only the eruptions which occurred during the same time span the amount of thermal energy released and the associated potential energy needed to raise the magma to the surface from a 2 km deep hydrostatic equilibrium level were estimated to be about 1017 and 1015 j a respectively the computed energies for earthquakes and tremors are at least one order of magnitude less than what is needed to match a model of magma transport based solely on the jerky propagation of melt filled cracks the energy needed to raise the magma to the surface could be supplied by expanding gases in the upper levels of the magma column and or by tectonic stresses acting on shallow batches of magma

andean forearc dynamics as recorded by detrital zircon from the eocene talara basin northwest peru abstract eocene strata of the talara forearc basin of northwest peru contain detrital zircon dz populations that when integrated with other provenance indicators record paleogeographic shifts in the andean forearc that can be linked to larger scale tectonic drivers such as subduction erosion plate convergence rate and regional accretion events because forearc basins along the western margin of south america are particularly difficult to preserve the new dz results provide rare insight into the trench arc dynamics associated with the andean orogenic cycle the talara basin tapped into both the andean volcanic arc and older basement and the dz populations subsequently reflect long lived plate interactions and progressive crustal growth along the northwestern edge of amazonia gondwana and south america dominant u pb zircon age populations include 36 110 ma andes convergence 215 285 ma western gondwana extension 460 625 ma rodinia extension and western gondwana convergence 950 1250 ma grenville orogeny and 1250 ma amazonia assembly the data corroborate previous geochronology on pre andean basement and support the existence of a famatinian arc 480 ma and carboniferous triassic magmatism along northwestern gondwana pre andean and amazonian dz likely was recycled from paleozoic metasedimentary rocks of the andean cordillera and amotape terrane although the data do not preclude delivery of amazonian detritus directly into the forearc via a large scale westward drainage the andean dz population constrains the timing of arc activity and quiescence with two peaks at 75 ma and 50 ma and a gap at 70 ma linked to accretion of the caribbean oceanic plateau against northwest south america arc volcanism was concurrent with eocene forearc deposition and syndepositional dz is used to revise maximum depositional age for the upper eocene helico member and verdun formation progressive subsidence and arc shut down in the late eocene coincide with arc collision to the north uplift of the amotape block subduction erosion and slab shallowing

reply to comment on u pb and hf isotope analysis of detrital zircons from mesozoic strata of the gravina belt southeast alaska by yokelson et al 2015

jurassic cretaceous basins along the canadian coast belt their bearing on pre mid cretaceous sinistral displacements the dominantly plutonic canadian coast belt is flanked on the northwest by a late jurassic early cretaceous intra or back arc basin in the south it contains strata of the same age deposited in an accretionary fore arc complex east of a coeval arc this distribution is ascribed to pre mid cretaceous sinistral movement of part of the jurassic cretaceous north american plate margin arc to a position west of the accretionary fore arc complex

detrital zircon record of neogene exhumation of the central alaska range a far field upper plate response to flat slab subduction far field deformation in the upper plate of many ancient and modern convergent margins is commonly attributed to flat slab subduction processes recent studies have suggested that the high topography of the central alaska range alaska usa for example is the far field deformational response to neogene shallow subduction processes in southern alaska in this study we analyze six stratigraphically controlled detrital zircon samples from miocene pliocene sandstone of the tanana foreland basin to evaluate the temporal and spatial links between exhumation of the central alaska range and shallow subduction the tanana basin located north of the central alaska range contains 2 km of neogene strata lower miocene strata from the basin have detrital zircon ages with dominant precambrian populations these grain ages are attributed to sediment derivation from the yukon composite terrane source area located at the northern edge and north of the alaska range during the early middle miocene the provenance of the tanana basin switched to a distinctly mesozoic source signal these mesozoic age grains are attributed to sediment derived from source areas within and south of the alaska range the early middle miocene change in provenance from predominantly precambrian to mesozoic sediment sources in the context of previously published basin analysis studies is interpreted to represent the onset of neogene uplift of the central alaska range the timing of exhumation of the central alaska range is important not only for understanding the tectonics of the highest mountain range in north america but additionally because recent modeling studies predict that this range is one of the first expected products of neogene flat slab subduction along the southern alaska convergent plate boundary results of our study provide a needed chronologic framework to fully evaluate the temporal and spatial upper plate responses to shallow subduction processes

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

the 1989 1990 activity of etna magma mingling and ascent of h2oclsrich basaltic magma evidence from melt inclusions abstract the melt inclusions in olivine phenocrysts from basaltic tephra erupted by etna volcano during the 1989 1990 eruptive period were investigated for major elements cl and s by electron microprobe fluorine by pigme particule induced gamma ray emission and h 2 o by sims the results indicate the ascent of basaltic magmas with different k 2 o concentrations possibly related to different feeding dykes lk basaltic melt inclusions rich in cl s and h 2 o are trapped in olivine fo 84 from the 1990 tephra and represent the early crystallization stage their composition in major elements is quite similar to those of most of the historic and even pre etnean alkali basalts but significantly lower in k 2 o compared with the 1989 1990 lavas only scarce hk basaltic melt inclusions were observed in the outer rim fo 79 6 of reversely zoned olivine phenocrysts and correspond to late crystallization stage at low pressure high sulfur concentration s 0 3 wt is a general characteristic of the etnean basaltic magmas with h 2 o s ratio as high as 7 the volatile abundances of the etna primitive basalts were computed at 1 45 wt for h 2 o 0 17 wt for cl and 0 047 wt for f with a high cl f ratio close to 3 6 the 1989 lava samples with cl at 660 90 ppm and f at 770 30 ppm have suffered a significant cl loss the possible behaviour of the volatile phase as co 2 h 2 o even cl is discussed with h 2 o and chlorine late degassing and an active role of h 2 o during the lava fountaining activity

detrital zircon geochronology and regional correlation of metasedimentary rocks in the coast mountains southeastern alaska u pb ages have been determined for 55 detrital zircon grains from a metasedimentary sequence along the west flank of the coast mountains in southeastern alaska these rocks belong to the port houghton assemblage which consists of upper paleozoic pelitic and psammitic schist metaconglomerate metabasalt and marble the port houghton assemblage rests unconformably on metamorphosed and deformed mid paleozoic arc type volcanics endicott arm assemblage which gradationally overlie upper proterozoic lower paleozoic continental margin strata tracy arm assemblage three main clusters of ages are present 330 365 ma 19 grains 1710 2000 ma 27 grains and 2450 2680 ma 6 grains additional grains are approximately 2334 2364 and 3324 ma comparison of these ages with detrital zircon ages in other cordilleran assemblages supports previous interpretations that metasedimentary rocks in the coast mountains i form a southwestern continuation of the yukon tanana terrane of eastern alaska and yukon

examination of the deep levels of an island arc evidence from the tonsina ultramafic mafic assemblage tonsina alaska the tonsina ultramafic and mafic assemblage located in the northern chugach mountains along the trans alaska crustal transect tact is interpreted to be the remains of a magma chamber that crystallized at the base of a mature interoceanic island arc it is one of a number of isolated masses of peridotite and gabbro that comprise the basal part of the peninsular terrane along the northern fringe of the chugach mountains in southern alaska these masses the roots of a mid jurassic island arc have been uplifted along the north dipping border ranges fault system and crop out sporadically for 700 km from kodiak island to eastern alaska the tonsina ultramafic mafic assemblage is a layered sequence whose basal rocks are dunite and harzburgite 1 km thick overlain by a narrow zone of websterite and clinopyroxenite above these ultramafic rocks is a 5 to 6 km thick layered gabbro that is garnet bearing at its base textural relationships and chemical trends throughout the series of dunite pyroxenite garnet bearing gabbro and spinel gabbronorite point to an origin by igneous accumulation from a hydrous tholeiitic parent magma at deep levels i e at the base of a mature island arc before being emplaced along the border ranges fault the rocks were crystallized from a melt at pressures between 9 5 and 11 kbar 28 to 33 km depth and equilibrated at temperatures of 800 to 90o c based on phase relationships in the cumulus assemblages pyroxene thermobarometry and olivine spinel thermometry transitional sequences from ultramafic to mafic rocks are characterized by increasingly al rich pyroxenes and spinels or garnet and the absence of olivine plagioclase assemblages deep level fractionation of al poor phases such as olivine and pyroxene at moderately high pressures and temperatures resulted in concentration of al in the evolving magma as crystallization continued the phases recorded this alumina concentration and the resultant gabbros are al rich this observed trend supports the theory of high alumina basalt generation by fractional crystallization of olivine and pyroxene comparison of the tonsina rocks with aleutian lower crustal xenoliths as well as other exposures of arc related lower crustal magma chambers in japan pakistan and western north america indicates that this process is a common mechanism for island arc magma evolution at depth

integration of zircon color and zircon fission track zonation patterns in orogenic belts application to the southern alps new zealand an exhumed crustal section of the mesozoic torlesse terrane underlies the southern alps collision zone in new zealand since the late miocene oblique horizontal shortening has formed the northeastern southwestern trending orogen and exhumed the crustal section within it on the eastern side rocks are zeolite to prehnite pumpellyite grade greywacke on the western side rocks they have the same protolith but are greenschist to amphibolite facies of the alpine schist zircon crystals from sediments in east flowing rivers hinterland have pre orogenic fission track ages 80 ma and are dominated by pink radiation damaged grains up to 60 these zircons are derived from the upper f10 km crustal section unreset ft color zone that includes the late cenozoic zircon partial annealing zone both fission tracks and color remain intact and unaffected by orogenesis many zircon crystals from sediments in west flowing rivers foreland have synorogenic ftages and about 80 are colorless due to thermal annealing they have been derived from rocks that originally lay in the reset ftcolor zone and the underlying reset ft colorless zone the reset ft color zone occurs between f250 and 400 jc in this zone zircon crystals have color but reset ft ages that reflect the timing of orogenesis d 2002 elsevier science b v all rights reserved

the southern kahiltna terrane implications for the tectonic evolution of southwestern alaska regionally extensive and highly deformed jurassic to cretaceous basinal turbidite deposits occur along most of the inboard boundary of the talkeetna superterrane in southern alaska and western canada the southern kahiltna terrane of southwestern alaska consists largely of such basinal deposits regional and detailed studies in and adjacent to this terrane suggest several new conclusions regarding the origin of the basin and its role in the tectonic evolution of southwestern alaska stratigraphic sedimentologic compositional and structural evidence suggests that the upper jurassic to lower cretaceous strata of the southern kahiltna terrane were derived from and deposited on rocks of the adjacent talkeetna superterrane deposition of these clastic rocks apparently postdated arc magmatism in the talkeetna superterrane suggesting deposition in a basin developed on the suture between the talkeetna superterrane and north america if true this observation implies that collision of the talkeetna superterrane with north america began prior to latest jurassic time probably at a more southerly location the superterrane has since been translated northward probably by coast parallel strike slip displacement to its present location where it was structurally juxtaposed with middle to upper creteceous rocks of the kuskokwim group a magmatic overprint indicates that the talkeetna superterrane was in approximately its present location by latest cretaceous to paleocene time

origin and age of the border ranges fault of southern alaska and its bearing on the late mesozoic tectonic evolution of alaska local geologic relationships in the western chugach mountains together with regional considerations suggest that in the early cretaceous a subduction zone was formed along the southern edge of the wrangellia peninsular alexander composite terrane talkeetna superterrane of csejtey et al 1982 of southern alaska the effects of this event include 1 a shattering of older pre cretaceous crystalline rocks along a complex fault system 2 emplacement of a tectonic melange beneath the shattered crystalline rocks and 3 a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks this event created a regionally mappable structural contact between broken crystalline rocks and the melange the border ranges fault of mackevett and plafker 1974 regional stratigraphy as well as radiometric ages of rocks predating and postdating the border ranges fault appear to bracket the age of the border ranges fault and its associated deformational effects to the interval between about 135 and 120 ma further regional tectonostratigraphic associations suggest that the deformation along the border ranges fault represents the nucleation of a north and or east dipping subduction zone beneath the talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the gravina nutzotin belt of southeast alaska mismatches in the distribution of different elements of this early cretaceous arc trench system are probably a result of late cretaceous or early tertiary strike slip motion but early cretaceous ridge trench interaction suggested by the occurrence of early cretaceous near trench plutons may have played a role as well the talkeetna superterrane is generally thought to be an exotic block that collided with the cordillera in the middle cretaceous coney et al 1980 the age data commonly cited as evidence for a middle cretaceous age for the collision are however misleading a model is proposed here in which the initiation of convergence along the trailing edge of the talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle cretaceous deformation recognized along the leading edge of the talkeetna superterrane csejtey et al 1982 records destruction of a syncollisional flysch basin during the final phase of the collision

petrology and tectonic significance of gabbros tonalites shoshonites and anorthosites in a late paleozoic arc root complex in the wrangellia terrane southern alaska plutonic rocks intrusive into the late paleozoic tetelna formation of southern alaska are the underpinnings of the late paleozoic skolai arc of the wrangellia terrane there are four groups of intrusive rocks within the skolai arc 1 gabbro diorite plutons that contain gabbroic to anorthositic cumulates along with a differentiated series of gabbros and diorites of basaltic to andesitic composition 2 silicic intrusions including tonalite granodiorite and granite 3 monzonitic to syenitic plutonic rocks of the ahtell complex and related dikes and sills 4 fault bounded bytownite anorthosite of uncertain age and association these anorthosites may be related to post skolai nikolai greenstone magmatism the silicic rocks yield discordant u pb zircon ages of 290 320 ma early to late pennsylvanian relative age relations suggest that the oldest intrusive rocks are the gabbro diorite plutons the youngest are the monzonitic rocks and that the silicic rocks span this range the gabbro diorite pluto

the border ranges ultramafic and mafic complex south central alaska cumulate fractionates of island arc volcanics a discontinuous elongate zone of mafic and ultramafic plutonic rock crops out in south central alaska for a distance of more than 1000 km intermediate and detailed scale geologic mapping petrographic study and compositional data suggest that the plutonic rocks are compositionally petrologically and mineralogically distinct from rocks in mid ocean ridge and back arc basin ophiolites the mafic and ultramafic rocks instead represent part of the plutonic core of an intraoceanic island arc the mafic ultramafic zone referred to as the border ranges ultramafic and mafic complex brumc is composed of ultramafic cumulates gabbronorite cumulates and massive gabbronorites a very minor amount of tectonized ultramafic rock of mantle origin is present in the southern part of the brumc a thick sequence of andesitic volcanic rocks the talkeetna formation of early jurassic age lies to the north of and structurally above the mafic ultramafic zone voluminous calcalkaline plutons composed of quartz diorite

a model for predicting the volumes of vesicles in submarine basalts the total volume percentage of vesicles in submarine pillow basalt rinds can be modelled by the exsolution of h o and co vapor using available solubility determinations for these components in basalt in accordance with observations on natural samples co is predicted to be the major vapor component in vesicles at eruption depths 1 2 km only small initial contents of co 0 1 0 2 wt are consistent with the vesicle volumes observed in most deep sea pillow lavas h o is important only at shallow depths crystallization of pillow interiors can lead to more extensive vesiculation inside of pillow lavas than in the vesicle poor outer rinds

effects of magma storage and ascent on the kinetics of crystal growth the size distributions of crystals of olivine plagioclase and oxides of the 1991 93 eruption at mt etna italy are analyzed the simultaneous collection of this information for different minerals gives precious insight into the cooling history of lavas three distinct episodes are detectable a storage of the magma in a deep reservoir characterized by nearly constant and low nucleation and growth rates near to equilibrium an ascent phase with an ever increasing nucleation rate related to volatile exsolution and finally a quenching phase in addition to geochemical and geophysical evidence the similarity of the crystal size distributions of the present eruption with those of previous ones of this century makes it possible to exclude that crystal size distributions of etnean lavas are due to mixing of different populations this strongly suggests that the main features of the volcano feeding system have not changed despite observed variations in the magma output rates

arc parallel flow within the mantle wedge evidence from the accreted talkeetna arc south central alaska 1 residual mantle exposures in the accreted talkeetna arc alaska provide the first rock analog for the arc parallel flow that is inferred from seismic anisotropy at several modern arcs the peridotites exposed at the base of the jurassic talkeetna arc have a moho parallel foliation and indicate dislocation creep of olivine at temperatures of 1000 to 1100 c slip occurred chiefly on the 001 100 slip system which has only rarely been observed to be the dominant slip system in olivine stretching lineations and olivine 100 slip directions are subparallel to the long axis of the talkeetna arc for over 200 km indicating that mantle flow was parallel to the arc axis the alignment of the olivine 100 axes yields a calculated s wave anisotropy with the fast polarization direction parallel to the arc thus 1 the fast polarization directions observed parallel to some modern arcs now have an exposed geological analog 2 arc parallel fast polarization directions can be caused by anisotropic peridotites and do not require the presence of fracture zones fluid filled pockets or glide on the 010 001 h2o induced slip system 3 seismic anisotropy beneath modern arcs may be due to slip on 001 100 with a horizontal foliation rather than slip on 010 100 with a vertical foliation and 4 the observed dominance of the 001 100 slip system may be due to high h2o concentrations suggesting that strain in the oceanic upper mantle may be accommodated dominantly by 001 100 olivine slip

a middle jurassic to early tertiary andean sierran arc model for the coast belt of british columbia regional tectonic features of the western canadian cordillera can be interpreted in terms of middle jurassic accretion of a single composite superterrane stikinia wrangellia alexander to ancestral north america closure of the intervening cache creek bridge river ocean moved the continental edge to a new position west of the accreted superterrane the coast belt is primarily a succession of superimposed middle jurassic to early tertiary magmatic arcs related to prolonged subduction of pacific ocean lithosphere beneath the new north american margin discrete magmatic pulses separated by lulls or periods of relative quiescence successively overprinted the new margin the insular and intermontane superterranes previously viewed as widely separate entities prior to mid cretaceous time were already amalgamated before an initial middle jurassic overprint produced an ancestral coast belt late jurassic to early cretaceous rift related marine basins mid to late cretaceous compressional structures and early tertiary extensional features coincident with the coast belt are subsidiary intraplate attributes that reflect external adjustments in plate motions within a primary subduction related andean magmatic arc

cluster analysis of soil co2 data from mt etna italy reveals volcanic influences on temporal and spatial patterns of degassing soil co2 concentration data were collected periodically from july 2001 to june 2005 from sampling site grids in two areas located on the lower flanks of mt etna volcano paterno and zafferana etnea santa venerina cluster analysis was performed on the acquired data in order to identify possible groups of sites where soil degassing could be fed by different sources in both areas three clusters were recognised whose average co2 concentration values throughout the whole study period remained significantly different from one another the clusters with the lowest co2 concentrations showed time averaged values ranging from 980 to 1 170 ppm vol whereas those with intermediate co2 concentrations showed time averaged values ranging from 1 400 to 2 320 ppm vol and those with the highest concentrations showed time averaged values between 1 960 and 55 430 ppm vol we attribute the lowest co2 concentrations largely to a biogenic source of co2 conversely the highest co2 concentrations are attributed to a magmatic source whereas the intermediate values are due to a variable mixing of the two sources described above the spatial distribution of the co2 values related to the magmatic source define a clear direction of anomalous degassing in the zafferana etnea santa venerina area which we attribute to the presence of a hidden fault whereas in the paterno area no such oriented anomalies were observed probably because of the lower permeability of local soil time series analysis shows that most of the variations observed in the soil co2 data from both areas were related to changes in the volcanic activity of mt etna seasonal influences were only observed in the time patterns of the clusters characterised by low co2 concentrations and no significant interdependence was found between soil co2 concentrations and meteorological parameters the largest observed temporal anomalies are interpreted as release of co2 from magma batches that migrated from deeper to shallower portions of etna s feeder system the pattern of occurrence of such episodes of anomalous gas release during the observation period was quite different between the two studied areas this pattern highlighted an evident change in the mechanism of magma transport and storage within the volcano s feeder system after june 2003 interpreted as magma accumulation into a shallow 8 km depth reservoir

sedimentary processes in modern and ancient oceanic arc settings evidence from the jurassic talkeetna formation of alaska and the mariana and tonga arcs western pacific abstract sediment deposited around oceanic volcanic ares potentially provides the most complete record of the tectonic and geochemical evolution of active margins the use of such tectonic and geochemical records requires an accurate understanding of sedimentary dynamics in an arc setting processes of deposition and reworking that affect the degree to which sediments represent the contemporaneous volcanism at the time of their deposition we review evidence from the modern mariana and tonga arcs and the ancient arc crustal section in the lower jurassic talkeetna formation of south central alaska and introduce new data from the mariana arc to produce a conceptual model of volcaniclastic sedimentation processes in oceanic arc settings all three arcs are interpreted to have formed in tectonically erosive margin settings resulting in long term extension and subsidence debris aprons composed of turbidites and debris flow deposits occur in the immediate vicinity of arc volcanoes forming relatively continuous mass wasted volcaniclastic records in abundant accommodation space there is little erosion or reworking of old volcanic materials near the arc volcanic front tectonically generated topography in the forearc effectively blocks sediment flow from the volcanic front to the trench although some canyons deliver sediment to the trench slope most volcaniclastic sedimentation is limited to the area immediately around volcanic centers arc sedimentary sections in erosive plate margins can provide comprehensive records of volcanism and tectonism spanning 10 my the chemical evolution of a limited section of an oceanic arc may be best reconstructed from sediments of the debris aprons for intervals up to 20 my but no longer because subduction erosion causes migration of the forearc basin crust and its sedimentary cover toward the trench where there is little volcaniclastic sedimentation and where older sediments are dissected and reworked along the trench slope

seismic patterns and fluid dynamic features preceding and accompanying the january 15 1990 eruptive paroxysm on mt etna italy abstract on the basis of data collected during a seismological investigation carried out on mt etna italy during a phase of the volcanic activity marked by progressive transition from strombolian bursts to effusive paroxysms at the se crater january 1990 three types of seismic regime have been recognized they are interpreted to be linked to modifications in the regimes of volcanic gases and vapours within the upper levels of the magma column as suggested by recent studies on the dynamics of magmatic fluids our analysis also reveals that the use of usual seismic parameters such as the temporal patterns associated with the occurrence of discrete low frequency events spindles and the coefficient m of the ishimoto iida law can furnish information about variations in the behaviour of the system before the irreversible evolution of the volcanic activity toward eruptive paroxysms

crystal size distribution systematics and the determination of magma storage times the 1959 eruption of kilauea volcano hawaii abstract the theory of crystal size distribution csd theory is based on a steady state population balance that monitors the flux of crystals growing into and out of specificed size categories in precipitating solutions the conservation equation describing this balance permits crystal growth and nucleation rates to be determined directly from crystal size distribution data in this investigation it is shown that csd analysis can alternatively be used to calculate the residence time of crystals in the system or in volcanologic terms the magma storage time prior to eruption if the characteristic crystal growth rate can be independently determined or estimated the crystal size data needed for storage time determinations are easily obtained from thin sections of glassy eruption samples and the mathematical calculations are relatively simple analysis of the errors inherent in this new technique predicts storage estimates accurate to within an order of magnitude

plutonic rocks of jurassic age in the alaska aleutian range batholith chemical variations and polarity plutonic rocks of jurassic age exposed on the eastern or pacific side of the alaska aleutian range batholith represent the roots of a magmatic arc generally considered to have been generated in response to northwest directed subduction these rocks form a compositionally continuous calc alkaline suite that ranges from hornblende gabbro through quartz monzonite tonalite and quartz diorite are the dominant rock types trend surface analysis was used to examine the geographic variation of major oxides and a few simple oxide ratios for 102 samples from widely separated localities statistical tests indicate that most of the trends although weak are real the direction of slope of the trends is approximately normal to the jurassic magmatic arc k 2 o and sio 2 increase toward the east southeast the other oxides increase toward the west northwest the k 2 o trend accounts for about 19 of the variance in the data and is significant at 99 9 level of confidence if the chemical trends reflect the approximate geometry of a paleosubduction zone the polarity of the jurassic magmatic arc is to the northwest that is the paleosubduction zone was on the northwest side of the arc and subduction was directed toward the southeast the paleosubduction zone is on the opposite side of the arc from the position that has generally been assumed indicating that the jurassic plutonic rocks were not generated in response to classical andean type convergent plate margins the magmatic arc may have formed in an intra ocean environment and subsequently has been rafted northward and accreted to this part of the northern pacific rim during the late mesozoic middle and upper jurassic clastic sediments underlying cook inlet to the southeast and derived from the magmatic arc are classified as backarc deposits rather than as an arc trench gap sequence

the 1983 mount etna eruption thermochemical and dynamical inferences thermochemical calculations and laboratory phase equilibration experiments on lavas of the 131 day 1983 mt etna flank eruption of 0 1 km3 were undertaken to investigate possible systematic variations in inferred melt phenocryst equilibration conditions as a function of time the 1983 mt etna lavas are multiply saturated plagioclase clinopyroxene and olivine the dominant phenocrysts occur in the ratio 1 1 2 1 4 melts glasses plot close to the plagioclase saturated olivine clinopyroxene low pressure cotectic on a walker o hara diopside forsterite silica diagram suggesting equilibration of melt and phenocrysts in a high level magma reservoir total pressures temperatures and dissolved h2o concentrations were calculated using the isoactivity method of carmichael and coworkers based on about 300 elelctron microprobe analyses of coexisting olivine clinopyroxene and plagioclase phenocrysts microphenocrysts and groundmass microlites for samples collected 6 46 and 125 days after the start of the eruption total pressures pt temperatures and h2o contents based on representative olivine clinopyroxene pairs are 140 mpa 1105 c 2 4 wt h2o 255 mpa 1112 c 1 0 wt h2o and 85 mpa 1096 c 1 8 wt h2o respectively for the early 283 middle i83 and late l83 samples corresponding equilibration depths are in the range 3 to 10 kilometers plagioclase feldspar phenocrysts while showing more evidence of disequilibrium provide compatible estimates of pt and t when analysis is restricted to the low anorthite mode of the plagioclase frequency composition histograms 133 mpa and 1115 c 260 mpa and 1117 c and 103 mpa and 1104 c repectively for 283 i83 and l83 the pre eruptive i e in situ temperature pressure gradient calculated from olivine clinopyroxene equilibria is 10 6 k kbar this compares well with independent estimates of the temperature pressure derivative of the pseudo invariant point composition 10 to 12 k kbar in both model e g diopside forsterite anorthite presnall et al 1978 and natural e g walker et al 1979 grove et al 1982 systems apparently magma within the etna reservoir was in a quasiequilibrium state buffered by its multiply saturated character immediately preceding eruption the temporal variation of computed pt t and h2o concentrations for melt phenocryst equilibrium agrees well with predictions based on simulations of the withdrawal of magma from a body zoned with respect to dissolved h2o provided the temporal record of magma discharge is taken into account discharge varied by a factor of about 100 during the sample collection interval the intermediate pt but high h2o content inferred for sample 283 reflects the withdrawal of h2o enriched magma during an early phase of high average discharge of about 3 50 m3 s before evaculation isochrons became quasistationary the high pt and relatively dry i83 magma reflects the deepening of the evacuation isochrons after 50 days of intermediate discharge with the development of quasi stationary isochrons in time and space sample l83 from day 125 near the end of the eruption reflects the shoaling of evacuation isochrons hence low pt and relatively high h2o content associated with the observed low 0 5 m3 s discharge our results show that thermochemical modeling efforts provide important opportunities for testing the predictions of magma with drawal simulations

tectonic controls on greenhouse gas flux to the paleogene atmosphere from the gulf of alaska accretionary prism the late paleocene to early eocene ca 61 56 ma was a period of long term global warming perhaps the warmest in the cenozoic recent modeling suggests that methane loading of the atmosphere and related development ofpolar stratospheric clouds could have been an important forcing mechanism for this period of warm climate the gulf of alaska accretionary prism contained 6 x 10 6 km 3 of siliciclastic sediments deposited in trench and slope settings along alaska s maastrichtian and paleogene continental margin these sediments underwent complex deformation accretion and unusual high heat flow soon after deposition accretion processes thermally overmatured the sediments during a time that overlaps the 61 56 ma period of long term global warming assuming a modest average organic carbon content of 0 3 wt in these sediments an estimated 8 35 x 10 1 5 kg of methane were generated in the accretionary prism over an 5 m y period this methane was not effectively trapped and migration pathways to the atmosphere were developed through complexly deformed and emergent continental borderlands the gulf of alaska accretionary prism is a possible source of the atmospheric methane needed to force paleocene and early eocene global warming and an example of how tectonic processes can significantly recycle carbon from the geosphere

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

integrated ammonite biochronology and u pb geochronometry from a basal jurassic section in alaska new results from integrated biochronologic and geochronometric studies on the basal jurassic section at puale bay alaska peninsula improve the calibration of the early jurassic time scale previously the interval around the triassic jurassic system boundary was poorly dated which hampered our understanding of geologic and biotic events e g the end triassic mass extinction and subsequent recovery published suggestions for the presence of the earliest hettangian planorbis zone and a continuous boundary section at puale bay are not substantiated although the kamishak formation is likely to contain an uninterrupted sedimentary record pre middle hettangian strata are locally faulted resulting in an apparent rhaetian to early hettangian gap in the fossil record the hettangian ammonite zonal schemes developed locally for nevada and the queen charlotte islands permit reliable correlation with alaska but have limited applicability the faunal succession recorded at puale bay is useful in the development of a regional zonation for north america we obtained three u pb zircon dates that are tied into an ammonite biochronology at the zonal level a middle hettangian tuff layer from near the top of the kamishak formation is dated at 200 8 2 7 2 8 ma tuffs from the overlying talkeetna formation are bracketed by middle and late hettangian ammonites and yield crystallization ages of 197 8 1 2 0 4 and 197 8 1 0 ma these new calibration points require that the hettangian sinemurian boundary be younger than 199 ma the triassic jurassic boundary is likely to fall between 200 and 205 ma similar studies are needed for the uppermost triassic to obtain tighter constraints zircon u pb systematics of two samples revealed strong proterozoic and late archean inheritance patterns that require revised tectonic models to account for the proximity of the talkeetna arc to evolved crustal blocks

petrology and sr nd isotope geochemistry of recent lavas from mt etna bearing on the volcano feeding system abstract the volcanic activity of mt etna during the period 1971 1987 has been investigated based on new volcanological petrographic geochemical and isotopic data the magma output rate since 1971 is much higher than that of the last 200 years marking a significant change in the feeding regime of mt etna the highest output rate has been observed for the 1981 eruption sub aphyric lavas emitted during the excentric activity of 1974 are the most primitive of those studied mg 57 55 the major and trace element chemistry of the lavas indicates limited differentiation by crystal fractionation mainly involving mafic phases nevertheless the observed variability of some ratios between highly incompatible elements suggests possible source heterogeneities the sr nd isotopic compositions range between 0 70362 0 70377 and 0 512829 0 512892 respectively with the lowest and highest 87sr 86sr ratios measured in the 1981 and 1974 lavas the observed isotopic variability is not due to late contamination processes but probably to the mantle source variations in sr and nd isotopic composition could indicate the lack of large magma chambers at any level in mt etna feeding system suggesting a model in which magma reaches the surface through a network of dikes where complete homogenization of different batches cannot be attained

one two and three phase viscosity treatments for basaltic lava flows 1 lava flows comprise three phase mixtures of melt crystals and bubbles while existing one phase treatments allow melt phase viscosity to be assessed on the basis of composition water content and or temperature two phase treatments constrain the effects of crystallinity or vesicularity on mixture viscosity however three phase treatments allowing for the effects of coexisting crystallinity and vesicularity are not well understood we investigate existing one and two phase treatments using lava flow case studies from mauna loa hawaii and mount etna italy and compare these with a three phase treatment that has not been applied previously to basaltic mixtures at etna melt viscosities of 425 30 pa s are expected for well degassed 0 1 w h2o and 135 10 pa s for less well degassed 0 4 wt h2o melt at 1080 c application of a three phase model yields mixture viscosities 45 crystals 25 35 vesicles in the range 5600 12 500 pa s this compares with a measured value for etnean lava of 9400 1500 pa s at mauna loa the three phase treatment provides a fit with the full range of field measured viscosities giving three phase mixture viscosities upon eruption of 110 140 pa s 5 crystals no bubble effect due to sheared vesicles to 850 1400 pa s 25 30 crystals 40 60 spherical vesicles the ability of the three phase treatment to characterize the full range of melt crystal bubble mixture viscosities in both settings indicates the potential of this method in characterizing basaltic lava mixture viscosity

pore volume and surface area of the carboniferous coals from the zonguldak basin nw turkey and their variations with rank and maceral composition abstract the macropore volume micropore volume and surface area of carboniferous bituminous coals from the zonguldak basin in northwestern turkey have been determined furthermore variations of these coal properties with rank and maceral composition have been investigated helium density mercury porosimetry and low pressure co 2 gas adsorption measurements were performed in order to determine the pore volumes and the surface area of coals helium and mercury densities of the coals vary between 1 28 1 41 and 1 11 1 32 g cm 3 respectively the macropore volume determined by mercury porosimetry varies between 0 0065 and 0 0687 cm 3 g for the interpretation of the adsorption mechanism and for the determination of the respective pore volumes and surface areas langmuir and dubinin radushkevich d r equations were applied the micropore volume and respective micropore surface area values are ranging from 0 0139 to 0 0524 cm 3 g and from 42 to 160 m 2 g respectively the langmuir monolayer volume which can be considered as an estimate of the gas adsorption capacity varies between 14 and 37 cm 3 g the corresponding specific surface area values range from 80 to 216 m 2 g a comparison of pore volumes with rank and maceral composition of coals indicated that micropore volume decreases with increasing maturity up to a value of 1 0 r o and then increases with further increase of coal rank a similar trend is also observed with langmuir monolayer volume and specific surface area in general an indicative correlation could not be observed between maceral composition and pore volume and surface area however a weak correlation does exist between langmuir monolayer volume and vitrinite content

u pb zircon and geochemical evidence for bimodal mid paleozoic magmatism and syngenetic base metal mineralization in the yukon tanana terrane alaska new shrimp sensitive high resolution ion microprobe u pb zircon ages and trace element geochemical data for mafic and felsic metaigneous rocks of the pericratonic yukon tanana terrane in east central alaska help define the tectonic setting of mid paleo zoic magmatism and syngenetic hydrothermal zn pb ag mineralization along the ancient pacific margin of north america we compare data from similar greenschist facies sequences of bimodal volcanic and subvolcanic rocks associated with carbonaceous and siliciclastic marine sedimentary rocks in the wood river area of the alaska range and the salcha river area of the yukon tanana upland and from amphibolite facies augen gneiss and mafic gneiss amphibolite in the goodpaster river area of the upland allowing for analytical uncertainties igneous crystallization age ranges of 376 353 ma 378 346 ma and 374 358 ma are indicated by 13 new shrimp u pb dates for the wood river salcha river and goodpaster river areas respectively bimodal magmatism is indicated by late devonian crystallization ages for both augen gneiss 371 3 and 362 4 ma and associated orthoamphibolite 369 3 ma in the upland and by stratigraphic interleaving of mafic and felsic rocks in the alaska range metabasites in all three study areas have elevated hfse high field strength element and ree rare earth element contents indicative of generation in a within plate extensional tectonic setting within plate trace element signatures also are indicated for peralkaline metarhyolites that host the largest volcanogenic massive sulfide deposits of the bonnifield district in the wood river area and for metarhyolite tuff interlayered with the carbonaceous nasina assemblage which hosts sedimentary exhalative sulfide occurrences in the salcha river area most of the other felsic metaigneous samples from the alaska range and the yukon tanana upland have geochemical signatures that are similar to those of both average upper continental crust and continental margin arc rocks generated in thick continental crust given the absence in our study areas of intermediate composition magmatic products generally found in most arcs and the presence of bimodal magmatism the alkalic within plate chemistry of the mafic rocks and some of the felsic rocks and the widespread occurrence of interlayered carbonaceous sedimentary rocks indicative of deposition within a restricted marine basin or submerged continental margin we consider it most likely that this prolonged late devonian to early mississippian magmatic episode resulted from attenuation of the ancient continental margin of western north america rather than development of an arc as proposed by many others

crystal size distribution csd in rocks and the kinetics and dynamics of crystallization ii makaopuhi lava lake crystal size distribution csd theory has been applied to drill core samples from makaopuhi lava lake kilauea volcano hawaii plagioclase and fe ti oxide size distribution spectra were measured and population densities n were calculated and analyzed using a steady state crystal population balance equation n n0 exp l g slopes on ln n versus crystal size l plots determine the parameter g a product of average crystal growth rate g and average crystal growth time the intercept is j g where j is nucleation rate known temperature depth distributions for the lava lake provide an estimate of effective growth time allowing nucleation and growth rates to be determined that are independent of any kinetic model plagioclase growth rates decrease with increasing crystallinity 9 9 5 4 10 11 cm s as do plagioclase nucleation rates 33 9 1 6 10 3 cm3 s ilmenite growth and nucleation rates also decrease with increasing crystallinity 4 9 3 4 10 10 cm s and 15 2 2 10 3 cm3 s respectively magnetite growth and nucleation rates are also estimated from the one sample collected below the magnetite liquidus g 2 9 10 10 cm s j 7 6 10 2 cm3 s moments of the population density function were used to examine the change in crystallization rates with time preliminary results suggest that total crystal volume increases approximately linearly with time after 50 crystallization a more complete set of samples is needed for material with 50 crystals to define the entire crystallization history comparisons of calculated crystallization rates with experimental data suggests that crystallization in the lava lake occurred at very small values of undercooling this interpretation is consistent with proposed thermal models of magmatic cooling where heat loss is balanced by latent heat production to maintain equilibrium cooling

population density and zoning of olivine phenocrysts in tholeiites from kauai hawaii the population density of olivine phenocrysts of the tholeiites display an exponential variation which is typical of igneous as well as contact metamorphic rocks the exponential variation is explained by a new growth probability model which is consistent with experimental work the forsterite content of the olivine phenocrysts decreases with decreasing size various phenocryst features suggest that the tholeiites first crystallized slowly in a magma chamber after which they underwent crystallization for a short period of time in a feeder dyke before eruption took place

paleocene eocene record of ophiolite obduction and initial india asia collision south central tibet 1 uppermost cretaceous to eocene marine sedimentary sequences occur both to the south and north of the yarlung zangbo suture in south central tibet they consist of indian margin strata of the northern tethyan himalaya and asian margin strata of the gangdese forearc both assemblages are characterized by major changes in depositional environment and sedimentary provenance at 65 ma and an appearance of detrital chromium rich spinel of ophiolite affinity tio2 generally 0 1 wt during the paleocene ophiolitic melange exposed along the suture could have provided a source for detrital spinel the melange occurs in the hanging wall of a north dipping south directed mylonitic shear zone which includes a tectonic sliver of mafic schist amphibole from the schist yields 40ar 39ar ages of 63 ma which we attribute to cooling during slip along the shear zone and southward obduction of the melange melange obduction was coeval with the development of an angular unconformity within the gangdese forearc basin to the north between late maastrichtian time and 62 ma upper paleocene to middle eocene sandstones in the northern tethyan himalaya yield 200 120 ma u pb detrital zircon ages and 190 170 ma 40ar 39ar detrital mica ages these detrital grains were most likely sourced from regions north of the yarlung zangbo suture suggesting that onset of india asia collision in south central tibet is middle eocene or older in age collectively our results support previous suggestions that oceanic rocks were obducted onto the northern margin of india during latest cretaceous earliest tertiary time coeval changes in gangdese forearc sedimentation raise the possibility that this obduction event marks onset of tectonic interaction between india and asia at 65 ma alternatively in concert with the conventional view of eocene collision initiation the obducted oceanic rocks may be of intraoceanic origin while coeval changes in gangdese forearc sedimentation may be a consequence of an increase in the rate of ocean continent convergence following the demise of the intraoceanic subduction zone

cooperation among tectonic and surface processes in the st elias range earth s highest coastal mountains investigations of tectonic and surface processes have shown a clear relationship between climate influenced erosion and long term exhumation of rocks numerical models suggest that most orogens are in a transient state but observational evidence of a spatial shift in mountain building processes due to tectonic climate interaction is missing new thermochronology data synthesized with geophysical and surface process data elucidate the evolving interplay of erosion and tectonics of the colliding yakutat microplate with north america focused deformation and rock exhumation occurred in the apex of the colliding plate corner from 4 to 2 ma and shifted southward after the 2 6 ma climate change the present exhumation maximum coincides with the largest modern shortening rates highest concentration of seismicity and the greatest erosive potential we infer that the high sedimentation caused rheological modification and the emergence of the southern st elias intercepting orographic precipitation and shifting focused erosion and exhumation to the south

igneous and metaigneous age constraints for the aishihik metamorphic suite southwest yukon the pericratonic aishihik metamorphic suite occurs outboard of accreted oceanic and oceanic arc terranes in southwest yukon it consists of a structurally lower unit of feldspathic quartz mica schi

four cordilleran paleorivers that connected sevier thrust zones in idaho to depocenters in california washington wyoming and indirectly alaska upper cretaceous sandstones from 17 localities from california to southeastern alaska united states contain unexpectedly large populations of detrital zircons with proterozoic u pb ages with age peaks at 1800 1650 and 1380 ma these peaks are indicative of a sediment source region in the southern part of the proterozoic belt supergoup basin in central idaho which hosts 1800 1650 ma detrital zircons and which was intruded by rift related 1380 ma bimodal plutons and sills belt rocks were strongly uplifted and eroded during late cretaceous sevier shortening and fed four paleoriver systems the lemhi pass hawley creek river system flowed east and sourced the beaverhead harebell pinyon nonmarine megafan in the cordilleran foreland basin the kione river flowed southwest to northern california where it sourced a very large ca 82 80 ma 600 m thick delta and submarine fan complex within the northern great valley forearc basin considerable kione detritus also transited the forearc basin to reach the franciscan trench sourcing a pulse of deposition and subduction accretion in central california and even part of southern california the swakane river flowed northwest out of idaho into washington sourcing the protolith for the high grade swakane gneiss more speculatively a yakutat river may have flowed northwest and deposited yakutat strata in a trench off washington or british columbia before those rocks were translated north to southeastern alaska recognition of a major source area in central idaho for zircons with an uncommon age of 1380 ma helps constrain the ca 85 65 ma paleogeography and paleotectonics of major sectors of the north american convergent margin orogen

regionally extensive mid cretaceous west vergent thrust system in the northwestern cordillera implications for continent margin tectonism the intermontane insular superterrane boundary zone represents a fundamental crustal boundary separating the two largest allochthonous crustal fragments in the north american cordillera structural stratigraphic and geochronologic relations along this boundary indicate that substantial west vergent compression and concomitant crustal thickening occurred there in mid cretaceous time this orogenic zone extends for more than 1200 km along strike length between southern southeast alaska and northern washington in southern southeast alaska and northwest british columbia rocks of the insular superterrane were imbricated along a series of west to southwest vergent thrust faults in northern washington and southwestern british columbia a wide zone encompassing the margins of the two superterranes as well as numerous intervening smaller fragments was shortened principally along west vergent thrusts known geologic relations do not discriminate among existing tectonic models that explain the origin of the mid cretaceous thrust system

detrital zircon record of a mesozoic collisional forearc basin in south central alaska the tectonic transition from an oceanic to continental arc

textural constraints on the kinetics of crystallization of igneous rocks natural systems provide time controlled experiments on scales unreachable in the laboratory and with complexities unapproachable by computer experiments since temperature undercooling cannot be directly scaled with time in situ crystallization experiments provide information unavailable by other means the kinetics of crystallization in simple silicate systems have been studied extensively for glass ceramic systems e g uhlmann 1982 james 1982 but predictive models for nucleation behavior in particular remain inadequate part of the problem in creating models of nucleation lies in the difficulty of experimentally isolating the nucleation process crystal growth measurements are more accessible experimentally and for this reason crystal growth is better understood e g dowty 1980 kirkpatrick 1981 james 1982 baronnet 1984 however predictive models of crystal growth e g lasaga 1982 require accurate models for the temperature and compositional dependence of both the thermodynamic and kinetic driving forces for crystallization information that is currently limited the cse parameterization of the position of ymax as a function of t tl and t suggested by dearnley 1983 provides a potentially useful model that requires further testing similarly application of the adiabatic nucleation model of meyer 1986 to simple systems suggests that the position of tmax relative to tl has an approximately constant value of 0 55 0 60 exceptions are low dsf minerals such as albite and quartz additionally while experimental crystallization rate data is a necessary link between processes in natural melts and numerical models experimental growth rate data is confined to simple systems and conditions of moderately large undercooling while growth rate estimates in complex natural systems suggest that under most conditions undercoolings remain very small quantitative textural studies of dikes suggest a pronounced dependence of growth rate on total crystallization time e g ikeda 1977 which is in turn related to effective undercooling a function of both total dike width and distance from the dike margin by analogy minimum crystallization rates may be estimated from programmed cooling experiments if a temperature interval of crystallization is assumed growth rates determined in this way show the same relationship between growth rate and total crystallization time seen in the dike studies reflecting a systematic relationship between growth rate and cooling rate effective undercooling for plagioclase in basaltic systems while the data are not available for other mineral systems there is no reason to believe that similar patterns would not exist for other silicate minerals i e the similarity in dendritic olivine growth rates estimated by fowler et al 1989 using fractal analysis to the growth rates of plagioclase under conditions of rapid cooling estimated from programmed cooling rate experiments analysis of measured crystal size frequencies provides a means of extracting effective bulk kinetic information from geologic systems that can help to constrain the dynamic feedback between physical models of magmatic systems and rates of chemical change

cyclical processes in the north american cordilleran orogenic system to understand linkages among processes that combined to build the north american cordillera we synthesize its tectono magmatic history from the california accretionary wedge to the retroarc foreland of wyoming at this latitude the cordilleran magmatic arc experienced high flux events hfes at ca 160 150 ma and ca 105 90 ma retroarc shortening provided the main source of hfe magmas which in turn created eclogitic arc roots that later foundered into the mantle and cleared the sub arc region of excess mass and provided space to accommodate renewed retroarc shortening the forearc arc and retroarc regions all responded variably to this cycle of tectono magmatic processes and laramide flat slab subduction may have both disrupted and been enhanced by events within this cycle