compression of albite naalsi3o8 the structure and equation of state of low albite naalsi 3 o 8 has been determined using high pressure single crystal x ray diffraction to a maximum pressure of 9 43 gpa low albite remains triclinic in space group c 1 over the entire pressure range and no phase transitions were observed although the evolution of the unit cell parameters and volume exhibit some unusual features at pressures above 4 gpa a third order birch murnaghan equation of state fit to 6 p v data points up to 3 4 gpa has k 0 54 2 7 gpa and k 5 3 6 at higher pressures the volume pressure curve exhibits d 2 v d p 2 p v data set can be approximated with a fourth order birch murnaghan equation of state with k 0 52 3 9 gpa k 0 8 8 6 and k 0 2 8 2 gpa 1 the anisotropy of the compression of albite is typical of that of feldspars with 65 of the volume compression accounted for by the compression of the 100 plane normal this is due to the closing up of the crankshaft chains of tetrahedra that are characteristic of the feldspar structure single crystal x ray intensity data sets collected at 6 4 8 4 and 9 4 gpa also show that the four membered rings of tetrahedra within the structure undergo significant shear at high pressures changes in the rate of shear of the four membered rings with pressure are associated with changes in the variation of the unit cell angles with pressure

al si order in albite and its effect on albite dissolution processes a monte carlo study the role of al si disorder in the kinetics of albite dissolution is explored through parameterized monte carlo methods two dissolution mechanisms multilayer leaching and single layer retreat were tested on 48 albite test configurations in simulations involving multilayer leaching dissolution rates increased with decreasing long range order lro we observed a fivefold maximum increase in dissolution rate tied to al si disordering roughly equivalent to that accompanying a decrease of 1 4 ph units at ph 1 5 this increase in rate due to disordering in albite is similar to that observed from compositional variation in plagioclase feldspars an47 vs an at low ph in contrast al si disordering had no discernible effect during the simulations involving the single layer retreat mechanism these results suggest that the effect of al si disorder on albite dissolution rate is mechanism dependent

nannobacterial alteration of pyroxenes in martian meteorite allan hills 84001 in martian meteorite allan hills alh 84001 this scanning electron microscope study was focused on the ferromagnesian minerals which are extensively covered with nanometer size bodies mainly 30 100 nm in diameter these bodies range from spheres to ovoids to caterpillar shapes and resemble both in size and shape nannobacteria that attack weathered rocks on earth and that can be cultured dense colonies alternate with clean smooth cleavage surfaces possibly formed later statistical study shows that the distribution of presumed nannobacteria is very clustered in addition to the small bodies there are a few occurrences of ellipsoidal 200 400 nm objects that are within the lower size range of normal earthly bacteria we conclude that the nanobodies so abundant in alh 84001 are indeed nannobacteria confirming the initial assertion of mckay et al 1996 however whether these bodies originated on mars or are antarctic contamination remains a valid question

slab portal beneath the western aleutians tomographic images of the distribution of shear wave speed beneath the northwestern pacific delineate the configuration of the subducted oceanic lithosphere beneath the western aleutian arc at 100 km depth a fast shear wave speed anomaly is beneath the aleutian arc everywhere east of 1738e between 1648e and 1738e however seismic velocities at this depth are slow relative to the surrounding mantle the lateral termination of the fast shear wave speed anomaly at depth coincides with a gap in deep seismicity beneath the aleutians the absence of these two distinctive traits of subducting slabs leads us to conclude that this segment of the aleutian arc overlies a very large window in the otherwise continuous lithospheric slab that we term a slab portal this portal is likely to facilitate the production of distinctive volcanic rocks adakites or high mg andesites by partially melting the adjacent edges of the slab the miocene age of most adakites in the westernmost aleutians where no slab is present may indicate that the portal formed relatively recently the chemistry of western aleutian adakites approximates that of typical continental crust so their genesis and subsequent lateral transport toward kamchatka is a plausible mechanism for new continent formation

an experimental approach to the sequence of the stability of rock forming minerals towards chemical weathering abstract the dissolution rates of milligrams per square metre per day for some main rock forming minerals were measured in experiments over prolonged periods of time rectangularly cut pieces of single crystals or monomineralic polycrystalline specimens were used the experimental set up provided for the rapid removal of all dissolved cations from the solution in order to maintain conditions far from chemical equilibrium experiments were performed at ph 5 5 and 4 0 at 20 c the stability towards chemical weathering decreases in the order muscovite alkali feldspar biotite amphibole pyroxene plagioclase nepheline olivine calcite a decrease of the ph from 5 5 to 4 0 accelerates the weathering process considerably

compositional suites of terrigenous deep sea sands of the present continental margins the compositional variability shown by sands of present continental margins was determined through conventional microscopic techniques this allowed the establishment of a descriptive classification of deep sea terrigenous sands based on a restricted number of commonly used parameters the fundamental sand types include 1 qf suite sands in which quartzose and or feldspathic grains predominate carbonate l bearing sands on average richer in lithics occur within this suite 2 ql suite in which quartzose and or lithic grains predominate little compositional diversification is apparent within this suite 3 fl suite in which lithic and or feldspathic grains predominate two compositional members can be identified based on the k feldspar plagioclase ratio 4 lv suite consists of sands in which volcanic grains largely dominate the lithic types vitric and serpentine sands occur within this suite the suites are related to distinct source area types defined as follows 1 cratonic block 2 sedimentary orogenic belt 3 volcanic orogenic belt 4 volcanic arc some sands cannot be easily assigned to a single compositional suite because petrologic parameters overlap with an adjacent suite such critical compositions are commonly arkosic using roundness of the framework components it is possible to define a category identified as matured sands which are greatly enriched in resistates

the komandorsky basin as a product of spreading behind a transform plate boundary abstract a long term study of the komandorsky basin in particular by the cruises of r vvulkanolog and r vdmitry mendeleev in the summer of 1988 made it possible to describe in detail the main structural features of this marginal basin several ne and n s trending spreading axes were detected in the basin in the rear of the transform sector of the aleutian arc spreading axes are shifted relative to each other along nw striking transform faults there is variation in style from north to south along the strike of the spreading axes and a less well defined symmetry compared to that typical of mid oceanic ridges the tectonic pattern and heat flow reflect several stages starting from the eocene in the history of the opening of the basin caused by the changes in the velocity and direction of the relative motion between the pacific and north american plates

high resolution solid state sodium 23 aluminum 27 and silicon 29 nuclear magnetic resonance spectroscopic reconnaissance of alkali and plagioclase feldspars

dissolution rates of actinolite and chlorite from a whole rock experimental study of metabasalt dissolution from 2 ph 12 at 25 c abstract the dissolution rates of the minerals actinolite and chlorite were determined from metabasalt element release rates measured at 25 c and 2 11 86 mol m 2 s mineral dissolution rates obtained in this study are within the range of corresponding values reported in the literature this observation suggests that the dissolution rates of major constituent minerals in a multi phase rock are not affected by the presence of the other minerals this conclusion validates the common assumption that the dissolution rate of an individual mineral is equal to that of the same mineral in a dissolving multi mineralogic rock at least for major constituents

terrane accretion production and continental growth a perspective based on the origin and tectonic fate of the aleutian bering sea region orogenesis in the aleutian bering sea region would create an expansive new area of pacific rim mountain belts the region itself formed about 55 ma as a consequence of the suturing of a single exotic fragment of oceanic crust aleutia to the pacific s alaskan siberian margin a massive overlap assemblage of the igneous crust of the aleutian arc and the thick sedimentary masses of the aleutian basin have since accumulated above the captured basement terrane of aleutia future closure of the aleutian bering sea region either northward toward the continent or southward toward the aleutian arc would structurally mold new continental crust to the north american plate the resulting beringian orogen would be constructed of a collage of suspect terranes although some terranes would include exotic crustal rocks formed as far as 5000 km away most terranes would be kindred or cotetonic blocks composed of the overlap assemblage and of relatively local 100 1000 km derivation the aleutian bering sea perspective bolsters the common supposition that although disrupted and smeared by transcurrent faulting examples of kindred assemblages should exist and perhaps commonly in ocean rim mountain belts

detrital modes of recent deep sea sands and their relation to tectonic setting a first approximation complete petrographic analyses of modern deep sea sands are surprisingly rare but are needed if problems such as the influence of tectonics on petrology or changes in petrology due to diagenesis are to be understood in ancient sandstones this paper is a first attempt using data from our own study of 80 sands plus data from the literature to establish a baseline for modern deep sea sands with which ancient sandstones can be compared we assigned sands to five plate tectonic categories each of which has a distinct average petrologic composition 1 basins associated with trailing edge continental margins q62f26l12 2 with leading edge continental margins either subduction q16f53l31 or 3 strike slip q34f39l27 4 back arc basins q20f29l51 5 fore arc basins q8f17l75 the amount and type of lithic fragments and the type of feldspar are particularly important discriminators the amount of quartz less so trailing edge and fore arc basins have quite distinctive compositions but back arc basins and basins near the two types of leading edge margins show some overlap and thus may be hard to differentiate in ancient sandstones

experimental determination of plagioclase dissolution rates as a function of its composition and ph at 22 c abstract the steady state far from equilibrium dissolution rates of nine distinct plagioclases ranging in composition from an2 to an89 were measured in mixed flow reactors at 22 2 c and ph from 2 to 11 the dissolution rates of all plagioclases based on silica release show a common u shaped behaviour as a function of ph where rates decrease with increasing ph at acid condition but rise with increasing ph at alkaline conditions consistent with literature findings constant ph plagioclase dissolution rates increase with increasing anorthite content at acidic conditions measured anorthite dissolution rates are 2 5 orders of magnitude faster than those of albite at ph 2 perhaps more significantly rates are independent of plagioclase composition at alkaline conditions interpretation and data fitting suggests that plagioclase dissolution rates are consistent with their control by the detachment of si rich activated complexes formed by the removal of al from the mineral framework taking account of this mechanism and transition state theory yields equations describing plagioclase dissolution rates r as a function of both the mineral and aqueous fluid compositions found in natural earth surface systems for ph 6 rates are consistent with log r mol cm 2 s 0 35 log a h 3 a al 3 11 53 and for ph log r mol cm 2 s n acid log a h 3 a al 3 0 033 an 14 77 where an represents the percent anorthite in the plagioclase solid solution ai corresponds to the activity of the ith aqueous species and nacid is given by n acid 0 004 an 0 05

paleogene history of the kula plate offshore evidence and onshore implications paleocene to middle eocene magnetic anomalies were mapped over oceanic crust that accreted at the kula pacific spreading center and is now obliquely entering the western aleutian trench between 179 e and 168 e the strike of anomalies and the pattern of abyssal hills and fracture zones changed abruptly during 56 55 ma when north south spreading veered to northwest southeast 310 130 kula pacific spreading ceased in 43 ma a 75 km long section of the fossil kula rift axis has avoided subduction although it now intersects the trench axis almost orthogonally near 171 5 e a narrow remnant of the former kula plate northwest of this fossil spreading center is bounded by a fossil kula pacific transform with a high transverse ridge alongside a sediment filled transform valley anomalies on this remnant show that eocene kula pacific spreading was highly asymmetric 2 1 the 56 55 ma change in kula plate rotation inferred from the change in spreading direction coincided with birth of the aleutian subduction zone and was probably a consequence of the resulting change in slab pull stresses on the oceanic lithosphere the change in direction of kula north american motion is a plausible explanation for the detachment of continental terranes from the pacific northwest and their migration around the gulf of alaska and for the early eocene demise of alaska range are volcanism the cessation of kula pacific spreading coincides with a major change in pacific plate rotation and the subsequent direction of convergence of the pacific plate with the aleutian arc was similar to the 55 43 ma direction of kula plate convergence

the role of carbonates in the evolution of early martian oceans central to the question of life on mars is whether there has been liquid water on the martian surface and how the planet could have evolved from possible initial warm and wet conditions to the cold and dry present state virtually all models for this climatic evolution rely strongly on the removal of an initial thick carbon dioxide atmosphere by precipitation of carbonate minerals from surface waters that may have been quite similar to those of hadean eon earth s oceans in order for this to occur a hydrologic cycle would be necessary in which chemical weathering of silicate rocks consumes co2 that precipitates as carbonates in an acidic martian ocean which probably had a very high alkalinity the consumption of atmospheric co2 by this process would result in a gradual decrease of the atmospheric greenhouse influence and cooling of the climate once the surface of mars became cold enough so that freezing conditions prevailed the hydrologic cycle would largely cease and the uptake of co2 by silicate rock weathering would greatly diminish the alkalinity of the freezing seawater would probably be sufficient to result in the removal of all calcium as calcium carbonate some magnesium and sodium would also likely be removed as carbonates as well the removal of these cations as carbonates has a major influence on the final temperature at which liquid brines would be able to persist on the surface of mars during the period of freezing the oceans would act as a source of co2 rather than a sink further slowing the rate of climate change on mars

the durability of rocks developing a test of rock resistance to chemical weathering dissolution rates of 39 rock forming minerals were determined at 20 c and 100 c and different ph values using experimental set ups which achieve near zero cation concentrations weathering rates in nature for rock above surface were estimated from data in this study and data in the literature extraction in a soxhlet extractor with boiling 2 5 m acetic acid causes an acceleration of such estimated natural weathering rates by a factor of 0 25 103 to 7 104 for nearly all important rock forming minerals including the carbonates the applicability of such a comparative test for rock resistance to chemical weathering was corroborated by testing 32 different rocks and building stones run times of only one to four days were needed for a reliable result

large scale deformation related to the collision of the aleutian arc with kamchatka the far western aleutian island arc is actively colliding with kamchatka westward motion of the aleutian arc is brought about by the tangential relative motion of the pacific plate transferred to major right lateral shear zones north and south of the arc early geologic mapping of cape kamchatka a promontory of kamchatka along strike with the aleutian arc revealed many similarities to the geology of the aleutian islands later studies support the notion that cape kamchatka is the farthest west aleutian island and that il has been accreted to kamchatka by the process of arc continent collision deformation associated with the collision onshore kamchatka includes gravimetrically determined crustal thickening and formation of a narrow thrust belt of intensely deformed rucks directly west of cape kamchatka the trend of the thrust faults is concave toward the collision zone indicating a radial distribution of maximum horizontal compressive stress offshore major crustal faults trend either oblique to the kamchatka margin or parallel to major aleutian shear zones these offshore faults are complex accommodating both strike slip and thrust displacements as documented by focal mechtmisms and seismic reflection data earthquake activity is much higher in the offshore region within a zone bounded to the north by the northernmost aleutian shear zone and to the west by an apparent aseismic front analysis of focal mechanisms in the region indicate that the present day arc continent contact zone is located directly east of cape kamchatka in modeling the dynamics of the coiiision zone using thin viscous sheet theory the rhealogical parameters are only partially constrained to values of n the effective power law exponent 2 3 and ar the argand number s 30 these values are consistent with a fnrcarc thermal profile of kamchatka previously determined from heat flow modeling the thin viscous sheet modeling also indicates that onshore thrust faulting is a consequence not only of compressive stresses resulting from the west directed collision but also of sediment induced coupling of the subducting pacific plate

epitaxial growth of nanophase magnetite in martian meteorite allan hills 84001 implications for biogenic mineralization crystallographic relationships between magnetite sulfides and carbonate rosettes in fracture zones of the allan hills alh 84001 martian meteorite have been studied using analytical electron microscopy we have focused on those magnetite grains whose growth mechanisms can be rigorously established from their crystallographic properties individual magnetite nanocrystals on the surfaces of carbonates are epitaxially intergrown with one another in stacks of single domain crystals other magnetite nanocrystals are epitaxially intergrown with the surfaces of the carbonate substrates the observed magnetite carbonate hkl miller indices orientation relationships are 1 1 3 m 1 1 0 c and 1 1 1 m 0 0 3 c with lattice mismatches of approximately 13 and approximately 11 respectively epitaxy is a common mode of vapor phase growth of refractory oxides like magnetite as is the spiral growth about axial screw dislocations previously observed in other magnetite nanocrystals in alh 84001 epitaxy rules out intracellular precipitation of these magnetites by martian organisms provides further evidence of the high temperature 120 degrees c inorganic origins of magnetite in alh 84001 and indicates that the carbonates also have been exposed to elevated temperatures

maskelynite formation via solid state transformation evidence of infrared and x ray anisotropy we present the results of a combined study of shocked labradorite from the lonar crater india using optical microscopy micro raman spectroscopy nuclear magnetic resonance nmr spectroscopy high energy x ray total scattering experiments and micro fourier transform infrared micro ftir spectroscopy we show that maskelynite of shock class 2 is structurally more similar to fused glass than to crystalline plagioclase however there are slight but significant differences preservation of original preimpact igneous zoning anisotropy at infrared wavelengths x ray anisotropy and preservation of some intermediate range order which are all consistent with a solid state transformation from plagioclase to maskelynite

provenance of the palaeocene eocene clastic rocks of the komandorsky islands the palaeocene eocene sequence on the komandorsky islands is represented by sediments and minor volcanics with a total thickness of more than 3000 m sandstone siltstone mudstone siliceous rocks and conglomerate which have accumulated in the upper and middle parts of a deep water fan prevail all the debris was delivered from the north east from the side of the contemporary komandorsky basin in the bering sea products of volcanic denudation and fresh tephra dominate its composition however at some levels the sandstone contains fragments of jasper quartzite slate schist gabbro and granite as well as non volcanic quartz along with a volcanic component the heavy minerals fraction of the sandstone contains considerable quantities of apatite zircon garnet and chromite as well as pyroxenes amphiboles and magnetite the chemical composition of the garnet is specific to metamorphic rocks a significant admixture of chromite is indicative of denudation from an ophiolite complex the folded and partly metamorphosed pre palaeogenic rocks were a source of these fragments their most likely source terrane was the shirshov bowers chain cretaceous palaeoarc the komandorsky block originally was the forearc part of the aleutian arc where the latter was superimposed on the shirshov bauers chain a subsequent north west displacement of forearc structures in response to the transform fault at the boundary between the pacific and north american plates could have resulted in the separation of the komandorsky block from its early palaeogene debris source area

putative martian microbes called microscopy artifacts life on marsin the heated debate over the putative microfossils in martian meteorite alh84001 most scientists have focused on arcane arguments about mineral isotopic compositions and formation temperatures but for many they simply look lifelike looks can be deceptive however and three meteoriticists now argue that there s nothing lifelike about the martian bugs rather they re simply a trick of the eye abetted by the peculiarities of the powerful microscopes used to image them in a short paper in this week s nature this group presents its own nanometer scale images of alh84001 they argue that most of the microfossils are nothing more than narrow ledges of mineral protruding from the underlying rock that under certain viewing conditions can masquerade as fossil bacteria

density measurements on melts at high pressure using the sink float method limitations and possibilities abstract we investigated the use of the sink float method to determine the density of silicate melts with a range of compositions intended to provide a general understanding of density changes as functions of composition and pressure for each composition estimates of both the melting temperature and the melt viscosity which determines the required experimental run time at the pressures of interest are required prior to the density experiments we constrained the melting relationships for two sodium silicate melts and a melt of albite composition and were able to calculate the maximum possible viscosities of the sodium silicate melts the sink float method is limited by the absence of high pressure high temperature density data for minerals that may be used as buoyancy markers in the experiments these data need to be sufficiently precise to justify their use in the determination of melt densities at high pressure at present high quality data are available for only a few minerals such as spinel forsterite quartz and some garnets another limitation is that the markers should not react with the melt reaction between the buoyancy markers melt and capsule material changes the density of both melt and markers errors on the order of 2 on the determined densities may be introduced by the changing density of the mineral markers through reaction with the melt although the sink float method is very elegant it has significant limitations that prohibit the precise determination of the density of an arbitrary silicate melt at high pressure in most cases only loose constraints on the density and compressibility of melts can be obtained as a function of pressure

formation of carbonates in the tatahouine meteorite the tatahouine meteorite in southern tunisia shows terrestrial contamination that developed during 63 years of exposure on earth s surface samples collected on the day of the fall in 1931 contained fractures with no secondary minerals whereas samples collected in 1994 contain calcite aggregates 70 to 150 micrometers and rod shaped forms 100 to 600 nanometers in length and 70 to 80 nanometers in diameter on the fractures carbon isotope analysis of the carbonates within the tatahouine meteorite delta13c 2 0 per mil pee dee belemnite standard pdb and the underlying ground delta13c 3 2 per mil pdb confirm their terrestrial origin

theoretical approach to evaluating plagioclase dissolution mechanisms abstract the more rapid dissolution of ca rich feldspars relative to na k rich feldspars has been attributed to the preferential leaching of al deep within the feldspar structure evidence from surface microanalysis e g hellmann et al 2003 however shows that preferential dissolution of al is confined to the top layers of the feldspar lattice and that the amorphous surface layer most likely results from precipitation versus dissolution it is thus critical to examine the extent of preferential al removal here we present a theoretical study of plagioclase dissolution behavior using parameterized monte carlo simulations two different dissolution mechanisms a mechanism involving preferential leaching of al and an interfacial dissolution reprecipitation mechanism are tested using compositions representing the entire plagioclase solid solution series our modeling results indicate that under the control of the preferential al leaching mechanism the influence of al si disorder on the dissolution rate is significant at a fixed composition an increase in the degree of al si disorder yields an increased dissolution rate with an 8 fold increase in dissolution rate observed for highly disordered albite an 0 compared to low albite increasing anorthite content tends to decrease the variation in the dissolution rate due to disorder the difference in the dissolution rate of 293 tested oligoclase configurations with a composition of an 20 is 3 fold and the difference is reduced to 2 fold among 107 andesine configurations of an 30 furthermore feldspar configurations with completely disordered al si distributions yield a consistent log linear dependence of dissolution rate on the anorthite content an while other feldspar configurations with modest degrees of al si disorder exhibit rates less than this trend in contrast when al removal is confined to the top surface layers a variety of feldspar configurations with different al si disorder but a single fixed composition have similar dissolution rates and the dissolution rate of ca rich feldspars departs positively from its log linear relationship with anorthite content this departure occurs around an 80 and is in good agreement with previous experimental studies subsequent modeling results of aluminum inhibition g dependence and formation of altered surface layers in the framework of the interfacial dissolution reprecipitation mechanism are all comparable with experimental investigations and these results suggest that an interfacial dissolution reprecipitation mechanism governs the dissolution of plagioclase feldspars

paleomagnetism of paleogene rocks of the central east kamchatka and komanorsky islands tectonic implications abstract paleomagnetic investigation of paleocene eocene to miocene rocks has been carried out at five localities in the eastern kamchatka and komandorsky islands after thermal and af cleanings consistent paleomagnetic directions were isolated for a number of formations data from the kronotsky peninsula and medny island passed both the fold and reversal tests and thus can be considered as reliable other results are of lower quality all the results were derived from sedimentary rocks and thus the corresponding paleolatitudes may be biased due to inclination errors a limited collection of basalts yielded inclination values systematically lower than those in sediments this anomaly was tentatively explained by shape anisotropy the correctness of paleolatitude estimations was thus left unproved several models of the northern pacific tectonic evolution were suggested and analyzed but the authors failed to find a model that was fully compatible with paleomagnetic kinematic and geological data

origin transport and emplacement of an exotic island arc terrane exposed in eastern kamchatka russia the regional stratigraphy of eastern kamchatka includes an exotic early late cretaceous ophiolite and late cretaceous island arc volcanic sequence integrating the existing geologic and geophysical data we examine the origin transport emplacement and postemplacement deformation of the island arc terrane which is named the olyutorsky island arc results from several paleomagnetic studies consistently indicate that the island arc terrane originated 1000 km to the south of where it is presently exposed although the formative paleolatitudes of the island arc rocks approximately correspond to the location of the izanagi farallon subduction zone the age of the volcanic rocks postdates the cessation of izanagi farallon convergence thus indicating that an unnamed plate or back arc basin existed in the northwest pacific during late cretaceous time we examine two possible models for northward transport of the island arc terrane to kamchatka 1 infra oceanic transport with the pacific or kula plates and 2 coastwise translation of the island arc terrane after accretion to the eurasian margin far to the south of kamchatka for both models the dominant eocene and miocene deformation ages observed in eastern kamchatka are used as two possible age limits for the cessation of northward transport although the observed paleolatitudes from paleomagnetic data correspond best with the infra oceanic transport model the provenance of the paleogene transport stratigraphy indicates a near shore sediment supply our preferred interpretation is that the island arc terrane 1 accreted onto the eurasian margin concurrent with cessation of island arc volcanism maastrichtian danian and 2 underwent northward coastwise translation along a major strike slip fault zone ending by middle late eocene time 43 50 ma it is unclear whether the ophiolite was exposed during arc continent collision or whether the ophiolite was obducted onto the island arc prior to collision a consequence of either infra oceanic transport or coastwise translation is that an open corridor between the western terminus of the aleutian arc and kamchatka must have existed until middle to late eocene time spreading within the komandorsky basin subduction of sea mounts and collision of the aleutian arc with kamchatka are proposed to have instigated the second miocene phase of deformation which uplifted and reexposed the island arc terrane

reconstruction of subduction and back arc spreading in the nw pacific and aleutian basin clues to causes of cretaceous and eocene plate reorganizations the eocene 50 45 ma major absolute plate motion change of the pacific plate forming the hawaii emperor bend is thought to result from inception of pacific plate subduction along one of its modern western trenches subduction is suggested to have started either spontaneously or result from subduction of the izanagi pacific mid ocean ridge or from subduction polarity reversal after collision of the olyutorsky arc that was built on the pacific plate with ne asia here we provide a detailed plate kinematic reconstruction of back arc basins and accreted terranes in the northwest pacific region from japan to the bering sea since the late cretaceous we present a new tectonic reconstruction of the intraoceanic olyutorsky and kronotsky arcs which formed above two adjacent oppositely dipping subduction zones at 85 ma within the north pacific region during another pacific wide plate reorganization we use our reconstruction to explain the formation of the submarine shirshov and bowers ridges and show that if marine magnetic anomalies reported from the aleutian basin represent magnetic polarity reversals its crust most likely formed in an 85 to 60 ma back arc basin behind the olyutorsky arc the olyutorsky arc was then separated from the pacific plate by a spreading ridge so that the 55 to 50 ma subduction polarity reversal that followed upon olyutorsky ne asia collision initiated subduction of a plate that was not the pacific hence this polarity reversal may not be a straightforward driver of the eocene pacific plate motion change whose causes remain enigmatic

revised age of aleutian island arc formation implies high rate of magma production radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the aleutian island arc provides insight into the timing of arc formation in the middle eocene twenty eight 40 ar 39 ar ages constrain the duration of arc magmatism to the last 46 m y basaltic lavas from the finger bay volcanics the oldest exposed rocks in the arc gave an isochron age of 37 4 0 6 ma which is 12 17 m y younger than a widely cited age of 55 50 ma three main pulses of arc wide magmatism occurred at 38 29 16 11 and 6 0 ma which coincide with periods of intense magmatism in other western pacific island arcs using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m y we calculate a time averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude