age of the pineo ridge system implications for behavior of the laurentide ice sheet in eastern maine u s a during the last deglaciation abstract the laurentide ice sheet was a major driver of global sea level change during the last deglaciation and may have impacted both atmospheric and oceanic circulation an understanding of past changes in the ice sheet is important for constraining its interaction with other components of the climate system here we present the geologic context and chronology for ice sheet fluctuations in eastern maine adjacent to the north atlantic ocean thought to be a key player in the termination of the last ice age retreat of the laurentide ice sheet through coastal maine first produced a series of lobate grounding line moraines followed by deposition of the prominent pineo ridge system which crosscut the earlier moraine set and which is characterized by extensive ice contact deltas closely spaced parallel moraines and association with eskers our new 10 be surface exposure ages indicate that the pineo ridge system which extends for more than 100 km in eastern maine and atlantic canada dates to 15 3 ka 800 years older than recent estimates our data are in accord with inboard minimum limiting radiocarbon ages of terrestrial materials which indicate deglaciation as early as 15 3 ka as well as of marine shells that are as old as 15 0 ka both the deglaciation that produced the lobate moraines and the short lived readvance that led to the pineo ridge system occurred during heinrich stadial 1 given that faunal and isotopic evidence indicates that the ocean remained cold during deglaciation of coastal maine we infer that ice recession was due to rising summer air temperatures that gave way briefly to cooling to allow minor readvance glacial deposits north of the pineo ridge system display evidence of ice stagnation and downwasting suggesting rapid ice retreat following deposition of the delta moraine complex coincident with the onset of the bolling

enrichment of trace elements in garnet amphibolites from a paleo subduction zone catalina schist southern california the abundance p t stability solubility and element partitioning behavior of minerals such as rutile garnet sphene apatite zircon zoisite and allanite are critical variables in models for mass transfer from the slab to the mantle wedge in deep regions of subduction zones the influence of these minerals on the composition of subduction related magmas has been inferred and disputed from inverse modelling of the geochemistry of island arc basalt or by experiment although direct samples of the dehydration partial melting region of a mature subduction zone have not been reported from subduction complexes garnet amphibolites from melanges of circumpacific and caribbean blueschist terranes reflect high t 600 c conditions in shallower regions such rocks record geochemical processes that affected deep seated high t portions of paleo subduction zones in the catalina schist a subduction zone metamorphic terrane of southern california metasomatized and migmatitic garnet amphibolites occur as blocks in a matrix of meta ultramafic rocks this mafic and ultramafic complex may represent either slab derived material accreted to the mantle wedge of a nascent subduction zone or a portion of a shear zone closely related to the slab mantle wedge contact or both the trace element geochemistry of the complex and the distribution of trace elements among the minerals of garnet amphibolites were studied by inaa xrf electron microprobe and sem in order of increasing alteration from a probable metabasalt protolith three common types of garnet amphibolite blocks in the catalina schist are 1 non migmatitic clinopyroxene bearing blocks which are compositionally similar to morb that has lost an albite component 2 garnet amphibolite blocks which have rinds that reflect local interaction between metabasite metaperidotite and fluid and 3 migmatites that are extremely enriched in th hfse lree and other trace elements these trace element enrichments are mineralogically controlled by rutile garnet sphene apatite zircon zoisite and allanite alkali and alkaline earth elements are much less enriched in the solid assemblage and thus appear to be decoupled from the other elements in the inferred metasomatic process es the compositions of migmatitic garnet amphibolite blocks seem to complement that of average island arc tholeiite trace element metasomatism reflects fluid solid rather than melt solid interaction the metasomatic effects indicate that h2o rich fluid perhaps with a significant component of na al silicate and alkalis carried th u sr ree and hfse fractionations of lree in migmatites resemble those of migmatitic metasedimentary rocks underlying the mafic and ultramafic complex exotic lree deposited in allanite in migmatites could have been derived from fluids in equilibrium with subducted sediment if the paleo subduction zone represented by the mafic and ultramafic complex of the catalina schist had continued its thermal and fluid evolution a selvage of similarly enriched rocks might have been generated along the slab mantle wedge contact between 30 and 85 km depth rocks affected by subduction zone metasomatism although rarely recognized at the surface could be volumetrically significant products of the initiation of subduction and may prove to be geochemical probes of convergent margins that approach the significance of xenoliths in the study of other magmatic environments

cave sediments constrain the latest pleistocene advance of the laurentide ice sheet in the champlain valley vermont usa laurentide ice sheet advance vermont usa

origin and 10be surface exposure dating of a coarse debris accumulation in the hrubý jesenik mountains central europe abstract coarse debris accumulations cover large areas in high latitude and mountain environments but their age and origin remain mostly unknown here we reassess the morphology of a prominent coarse debris accumulation in the sudetes mountain range in central europe and constrain its timing with 10be surface exposure data our observations indicate that the landform in the south eastern slope of the su ridge appears consistent with the morphology of a block slope the oldest minimum age of 150 1 4 8 ka obtained for the tor at the upper limit of the block slope indicates the disintegration of the ridge and surface lowering under periglacial conditions prior to the last glacial interglacial cycle four exposure ages retrieved for block samples range from 84 3 3 8 to 26 8 2 6 ka providing evidence for block emplacement during the last cold period the exposure ages extend existing chronological dataset for peri glacial landforms in the sudetes and constrain the timing of cold environments in the area prior the last glacial maximum the ages are also consistent with surface exposure data reported for block fields and block slopes in western europe and confirm the formation of similar landforms over the last glacial period in the sudetes

missing history 16 71 ma of the galapagos hotspot implications for the tectonic and biological evolution of the americas we present the results of volcanological geochemical and geochronological studies of volcanic rocks from malpelo island on the nazca plate 15 8 17 3 ma belonging to the gala pagos hotspot tracks and igneous complexes 20 8 71 3 ma along the pacific margin of costa rica and panama the igneous complexes consist of accreted portions of ocean island and seamount volcanoes and aseismic ridges representing the missing primarily subducted history of the gala pagos hotspot the age and geochemical data directly link the gala pagos hotspot tracks on the pacific ocean floor to the caribbean large igneous province ca 72 95 ma confirming a pacific origin for the caribbean oceanic plateau from the gala pagos hotspot we propose that emplacement of this oceanic plateau between the americas and interaction of the gala pagos hotspot tracks with the central american arc played a fundamental role in the formation of land bridges between the americas in late cretaceous paleocene and pliocene holocene time the land bridges allowed the exchange of terrestrial faunas e g dinosaurs mastodons saber tooth cats and ground sloths between the americas and served as barriers for the exchange of marine organisms between the central pacific ocean and the caribbean sea and the central atlantic ocean

cosmogenic nuclide ages for new england coastal moraines martha s vineyard and cape cod massachusetts usa abstract cosmogenic nuclide exposure ages for 13 glacially transported boulders atop the martha s vineyard moraine ma usa indicate that the southeastern margin of the laurentide ice sheet reached its maximum extent during the last glaciation 23 200 500 yr ago another 10 age determinations from the younger buzzards bay moraine near woods hole ma indicate that this moraine complex was formed 18 800 400 yr ago these ages correlate approximately with the terminations of cooling cycles recorded in greenland ice cores and coeval ice rafting events suggesting that the marginal position of this sector of the ice sheet was tightly coupled to north atlantic climate during the last glacial maximum

mesozoic adakitic rocks from the xuzhou suzhou area eastern china evidence for partial melting of delaminated lower continental crust abstract adakitic rocks in the xuzhou suzhou area eastern china consist of dioritic and monzodioritic porphyries and were dated at 131 132 ma by the shrimp u pb zircon method these rocks have high mgo content 1 47 5 73 high mg values 0 49 0 61 and high la yb and sr y ratios these features are similar to rocks derived from partial melting of a subducted oceanic slab however their high initial 87sr 86sr 0 7053 0 7075 and low end t values 4 43 to 13 14 are inconsistent with the origin from slab melting these rocks often contain garnet residual crystals and eclogite garnet clinopyroxenite and garnet amphibolite xenoliths petrographical characteristics and estimated p t conditions of these xenoliths indicate that they were once deeply subducted and subsequently underwent rapid exhumation in the early mesozoic garnet residual crystals from the porphyries show similar chemical compositions to garnets from garnet clinopyroxenite and garnet amphibolite xenoliths ages of the inherited zircons of the xenoliths and their host rocks likely indicate that sources for the adakitic magma and protoliths of the eclogite and garnet clinopyroxenite xenoliths in the study area were from precambrian basement of the north china craton the data also suggest that the lower continental crust in the eastern north china craton was thickened during the early mesozoic and delaminated in the early cretaceous the high mg adakitic magma resulted from partial melting of this delaminated lower continental crust and its subsequent interaction with the mantle during upward transport leaving garnet as the residual phase

age of the berlin moraine complex new hampshire usa and implications for ice sheet dynamics and climate during termination 1 at its late pleistocene maximum the laurentide ice sheet was the largest ice mass on earth and a key player in the modulation of global climate and sea level at the same time this temperate ice sheet was itself sensitive to climate and high magnitude fluctuations in ice extent reconstructed from relict glacial deposits reflect past changes in atmospheric temperature here we present a cosmogenic 10be surface exposure chronology for the berlin moraines in the white mountains of northern new hampshire usa which supports the model that deglaciation of new england was interrupted by a pronounced advance of ice during the bolling allerod together with recalculated 10be ages from the southern new england coast the expanded white mountains moraine chronology also brackets the timing of ice sheet retreat in this sector of the laurentide in conjunction with existing chronological data the moraine ages presented here suggest that deglaciation was widespread during heinrich stadial 1 event 18 14 7 ka despite apparently cold marine conditions in the adjacent north atlantic as part of the white mountains moraine system the berlin chronology also places a new terrestrial constraint on the former glacial configuration during the marine incursion of the st lawrence river valley north of the white mountains

origin of the southern okinawa trough volcanism from detailed seismic tomography 1 magmatism associated with subducting plate edges or slab tears has been suggested in the southern okinawa trough the cross back arc volcanic trail which consists of a cluster of about 70 seamounts is located above a ryukyu slab tear lying along the 123 3 e meridian in november 2003 more than 3300 earthquakes recorded in this area by 15 ocean bottom seismometers and surrounding land stations during a period of 12 days were used to determine the three dimensional vp and vs velocity structures and vp vs ratios a mantle inflow characterized by low vp and vs and high vp vs passing through the slab tear is imaged the fluid and or melt component is rising obliquely from the slab tear in the directions of the cross back arc volcanic trail the northern slope of the southern okinawa trough and to north of iriomote island the asthenospheric intake is also imaged by an inclined chip like high vp vs and low vp and vs body dipping northerly which might be linked to the slab retreat west of the slab tear most of the earthquakes are located around low vp and vs and high vp vs bodies which suggests that the seismicity is related to magmatic and or fluid activities east of it earthquakes are concentrated in an area characterized by high vp and vs velocities and low vp vs suggesting that the magma chamber is absent beneath the axial part of the trough and that normal faulting is the main factor controlling the seismicity

partial melting of subducting oceanic crust the conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two dimensional thermal models of subduction zones for porosities of 1 containing h20 the amount of partial melt generated at the wet basaltic solidus is limited to 100 mpa can be maintained by rocks close to or above their melting temperatures in the absence of high shear stresses substantial melting of the oceanic crust will only occur during subduction of very young 5 ma oceanic lithosphere partial melting of hydrated basalt amphibolites derived from the mid ocean ridge has been proposed e g 1 3 as being responsible for the generation of certain recent high al andesitic to dacitic volcanic rocks adakites three of these volcanic suites mount st helens southern chile and panama occur in volcanic arcs where oceanic crust 25 ma is being subducted at rates of 1 3 cm yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments however oceanic lithosphere is not currently being subducted beneath baja and new guinea where recent adakites are also present suggesting that some adakite magmas may form by water undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately

the geochemistry of young volcanism throughout western panama and southeastern costa rica an overview oblique aseismic subduction below western panama and southeastern costa rica has produced recent arc related volcanism the aseismicity is probably related to the subduction of relatively hot oceanic lithosphere the volcanism throughout this region over the past 2 ma has been quite distinct consisting of felsic magmas andesites to rhyolites but mainly dacites with geochemical signatures suggesting a metamorphosed basaltic source it is believed that the subduction of young oceanic crust sets up conditions under which the slab melts rather than the overlying mantle wedge rocks with slab melt geochemistries and associated with young subducted crust have been termed adakites elsewhere the young adakite melts are sometimes associated with a few rare young high nb basalts but there is no obvious genetic link between them through differentiation high nb basalts may also be derived from the partial melting of the subducted oceanic crust high nb basalt migmatites have been found with pegmatites of adakite compositions in the exposed subduction terrain of the catalina schist california alternatively the high nb basalts may be partial melts of phlogopite rich mantle that has previously reacted with adakite magmas eruption of adakites and high nb basalts was preceded by a 2 3 ma period of relative quiescence prior to this there was a 7 ma period of calc alkaline volcanism typical of the present day magmatism associated with a distinct benioff zone found throughout the central american arc the abrupt transition in volcanism with time from an early calc alkaline sequence to a later adakite high nb basalt sequence may record a change in the tectonic setting of western panama and southeastern costa rica over the past 12 ma

preglacial surface remnants and quaternary glacial regimes in northwestern sweden abstract we present a detailed map of the distribution of preglacial surface remnants in the kebnekaise region of northwestern sweden in this mountain area we discern four important large scale geomorphological units each representing a specific set of erosional agents and formative conditions these are i intact preglacial surface remnants characterized by gentle slopes round summits wide shallow valleys and an absence of rock basins ii preglacial surface remnants showing signs of minor glacial erosion and deposition iii glacially scoured surfaces including glacial troughs iv deep fluvial valleys cut into the preglacial surface the pattern of glacial erosion is explained as the result of three specific modes of glaciation known to have existed during the last 120 000 years and inferred to have repeatedly prevailed during the last 2 75 million years cirque glaciation mountain ice sheets and fennoscandian ice sheets a deep ocean oxygen isotope record of foraminifera from the north atlantic dsdp 607 was used to infer the temporal extent of these modes of glaciation during the last 2 75 million years we interpret the preglacial landscape preservation and the pattern of glacial erosion in terms of the configuration the basal thermal regime and the duration of such glaciation events the average subglacial thermal regime of both ice sheet types was frozen on the uplands and melting in the main valleys where outlet glaciers and ice streams formed the pre glacial landscape is best preserved at intermediate elevations low enough not to have been covered by cirque glaciers and apparently high enough not to have experienced melted bed conditions and subglacial erosion during ice sheet overriding events in a narrow high relief zone along the elevation axis interglacial fluvial erosion was morphologically important the absence of glacial erosion on uplands in this zone allowed fluvial erosion to commence on the same locations during each ice free interval in contrast no persistent fluvial valley pattern could develop in zones subjected to repeated glacial scouring and hence derangement of fluvial patterns

dating sediment burial with in situ produced cosmogenic nuclides theory techniques and limitations abstract dating sediment burial over million year time scales is crucial in many areas of the earth sciences and archeology but is often difficult using traditional techniques sediment burial can be dated by the radioactive decay of cosmogenic nuclides provided that the sediment was exposed to cosmic rays prior to burial dating calculations are straightforward if sediment is buried deeply and rapidly enough to prevent cosmogenic nuclide production after burial however the analysis can be complicated by postburial production if sediment is insufficiently shielded from secondary cosmic ray nucleons and muons this paper discusses how buried sediments can be dated over timescales up to 5 myr using 26 al and 10 be in quartz

landscape preservation under fennoscandian ice sheets determined from in situ produced 10be and 26al some areas within ice sheet boundaries retain pre existing landforms and thus either remained as ice free islands nunataks during glaciation or were preserved under ice differentiating between these alternatives has significant implications for paleoenvironment ice sheet surface elevation and ice volume reconstructions in the northern swedish mountains in situ cosmogenic 10be and 26al concentrations from glacial erratics on relict surfaces as well as glacially eroded bedrock adjacent to these surfaces provide consistent last deglaciation exposure ages 8 13 kyr confirming ice sheet overriding as opposed to ice free conditions however these ages contrast with exposure ages of 34 61 kyr on bedrock surfaces in these same relict areas demonstrating that relict areas were preserved with little erosion through multiple glacial cycles based on the difference in radioactive decay between 26al and 10be the measured nuclide concentration in one of these bedrock surfaces suggests that it remained largely unmodified for a minimum period of 845 418 461 kyr these results indicate that relict areas need to be accounted for as frozen bed patches in basal boundary conditions for ice sheet models and in landscape development models subglacial preservation also implies that source areas for glacial sediments in ocean cores are considerably smaller than the total area covered by ice sheets these relict areas also have significance as potential long term subglacial biologic refugia

uranium series dating of pedogenic silica and carbonate crater flat nevada a 230th 234u 238u dating study on pedogenic silica carbonate clast rinds and matrix laminae from alluvium in crater flat nevada was conducted using small sample thermal ionization mass spectrometry tims analyses on a large suite of samples though the 232th content of these soils is not particularly low mostly 0 1 9 ppm the high u content of the silica component mostly 4 26 ppm makes them particularly suitable for 230th u dating on single 10 to 200 mg totally digested samples using tims we observed that 1 both micro within rind and macro stratigraphic mappable deposit order of the 230th u ages were preserved in all cases 2 back calculated initial 234u 238u fall in a restricted range typically 1 67 0 19 so that 234u 238u ages with errors of about 100 kyr 2 could be reliably determined for the oldest 400 to 1000 ka rinds and 3 though 13 of the samples were 350 ka only three showed evidence for an open system history even though the sensitivity of such old samples to isotopic disruption is very high an attempt to use leach residue techniques to separate pedogenic from detrital u and th failed yielding corrupt 230th u ages we conclude that 230th u ages determined from totally dissolved multiple sub mm size subsamples provide more reliable estimates of soil chronology than methods employing larger samples chemical enhancement of 238u 232th or isochrons

a persistent and dynamic east greenland ice sheet over the past 7 5 million years 10be and 26al isotopic evidence in quartz sand from the seafloor shows that a dynamic east greenland ice sheet has existed for the past 7 5 million years understanding the growth and stability of the greenland ice sheet is a long standing priority in palaeoclimate research and one that is at the heart of modern discussion of sea level rise paul bierman and colleagues present marine records of cosmogenic isotopes and conclude that an ice sheet in east greenland was continuously eroding sediments over the past 7 5 million years although the method could miss transient deglaciations and cannot firmly distinguish between remnant and larger ice sheets the results are consistent with model simulations of a waxing and waning but continuously present regional ice sheet climate models show that ice sheet melt will dominate sea level rise over the coming centuries but our understanding of ice sheet variations before the last interglacial 125 000 years ago remains fragmentary this is because terrestrial deposits of ancient glacial and interglacial periods1 2 3 are overrun and eroded by more recent glacial advances and are therefore usually rare isolated and poorly dated4 in contrast material shed almost continuously from continents is preserved as marine sediment that can be analysed to infer the time varying state of major ice sheets here we show that the east greenland ice sheet existed over the past 7 5 million years as indicated by beryllium and aluminium isotopes 10be and 26al in quartz sand removed by deep ongoing glacial erosion on land and deposited offshore in the marine sedimentary record5 6 during the early pleistocene epoch ice cover in east greenland was dynamic in contrast east greenland was mostly ice covered during the mid to late pleistocene the isotope record we present is consistent with distinct signatures of changes in ice sheet behaviour coincident with major climate transitions although our data are continuous they are from low deposition rate sites and sourced only from east greenland consequently the signal of extensive deglaciation during short intense interglacials could be missed or blurred and we cannot distinguish between a remnant ice sheet in the east greenland highlands and a diminished continent wide ice sheet a clearer constraint on the behaviour of the ice sheet during past and ultimately future interglacial warmth could be produced by 10be and 26al records from a coring site with a higher deposition rate nonetheless our analysis challenges the possibility of complete and extended deglaciation over the past several million years

implications of the north and south panama thrust belts for the origin of the panama orocline high resolution imaging of the north nptb and south panama thrust belts using the seamarc ii seafloor mapping system and digital seismic reflection profiling provides an excellent data base with which to determine the kinematics of oroclinal deformation of panama remarkable continuity of structural orientations over 300 km along the nptb argues for widespread uniformity in direction of thrusting between panama and the caribbean plate westward decrease in the width and height of the accretionary wedge indicates a westward decrease in the total amount of convergence two major cross faults cut the wedge in its eastern part forming two minor blocks with different relative motions with respect to the caribbean frontal thrust orientations change across these faults major faults have been proposed to cut the isthmus of panama but simple cross isthmus faulting alone cannot explain the orientations of the major thrusts of the nptb bending of a flexible beam can explain much of the observed structure both directions and rates though the scale of the orocline precludes considering this as an elastic phenomenon plastic deformation of panama may be a useful mechanical explanation at least for eastern panama but consideration of plastic slip lines must include the northern andes as well changing from more rigid blocklike offsets on faults in western panama to a continuum of strain in eastern panama may be the best explanation for observations at present

a new extraction technique and production rate estimate for in situ cosmogenic 14c in quartz the potential of in situ cosmogenic 14c in situ 14c for surficial process studies is widely recognized yet a reliable means of isolating it has proved difficult to develop we present here a new method for extracting in situ 14c from quartz that overcomes difficulties encountered with earlier techniques yielding more reliable production rate estimates comparison of 14c thermal release patterns from surficial and deeply shielded quartz samples lifton 1997 demonstrated that contaminant 14c is released at or below 500 c and that 14c released between 500 and 1500 c is essentially all in situ produced the new technique builds on this key result using resistance heating of samples in the presence of a lithium metaborate libo2 flux and collection of all evolved carbon as co2 between 500 c and 1100 to 1200 c our improved method has four distinct advantages over other extraction methods 1 we can identify and quantitatively eliminate atmospheric organic 14c contamination 2 we can identify the in situ 14c component unambiguously without assumptions of 14co 14co2 production proportions within the rock or equilibria on extraction 3 in situ 14c is reliably extracted from quartz at lower temperatures and in less time than earlier methods and 4 blank 14c levels are consistently low 2 3 0 1 105 14c atoms 1 our new extraction procedures should thus enable researchers to use in situ 14c in diverse applications without reservation we developed our new procedures using samples of wave cut quartzite benches from the well dated bonneville 17 4 0 3 cal ky and provo 16 8 0 3 cal ky shorelines of pleistocene lake bonneville utah and from underlying deeply shielded locations in situ 14c was extracted from quartz separated from 2 bonneville shoreline samples 6 aliquots and 1 provo shoreline sample 2 aliquots results demonstrate that our new procedures can effectively isolate the in situ 14c fraction with replicate analytical precision better than 2 1 n 5 while remaining consistent with earlier results this level of precision and accuracy is comparable to or exceeds those currently obtainable with in situ cosmogenic 10be 26al 3he 21ne and 36cl resulting weighted mean in situ 14c site production rates for the bonneville and provo shorelines are 52 9 1 7 14c atoms g sio2 y and 48 7 2 8 14c atoms g sio2 y 1 respectively consistent with earlier production rate estimates current and previously published in situ 14c site production rate estimates were then scaled to sea level and high geomagnetic latitude using the latitude altitude scaling models of lal 1991 and dunai 2000 results indicate that both models yield sea level high latitude production rates consistent with independent estimates our new in situ 14c data yield integrated late quaternary production rate estimates at sea level and high latitude of 15 1 0 5 14c atoms g sio2 y using the lal 1991 model and 15 8 0 5 14c atoms g sio2 y with that of dunai 2000 until significant uncertainties in these models are addressed however we prefer the value from the widely used lal 1991 model as our best estimate of the integrated late quaternary production rate for in situ 14c

adakitic signature formed by fractional crystallization an interpretation for the neo proterozoic meta plagiogranites of the ne jiangxi ophiolitic melange belt south china article i nfo the low temperature high pressure metamorphic rocks which are exposed as exotic blocks within the serpentinite matrix of the ne jiangxi ophiolitic melange belt consist of enclave bearing and free meta plagiogranites both rock types are characterized by a metamorphic mineral assemblage consisting of ab rich plagioclase quartz clinopyroxene amphibole while rutile titanite and zircon occur as accessories the former primary magmatic mineral parageneses for the meta mafic enclaves and the meta plagiogranites are similar and consist of an rich plagioclase hornblende ilmenite monazite apatite however the ab rich plagioclase abundance of the meta mafic enclave is much lower and the hornblende and accessory mineral abundances are higher compared with those of the meta plagiogranite host rocks meta plagiogranites with mafic enclaves have a chemical composition similar to that of typical adakites with elevated al2o3 15 86 wt mg 53 5 and sr contents 485 ppm and a positive eu anomaly the rare earth element patterns are highly fractionated with la yb nn12 and low y b4 ppm and yb b0 4 ppm the mafic enclave has a basaltic composition with a negative eu anomaly and has ca 10 times higher hree and y concentrations than the plagiogranite host rocks enclave free meta plagiogranites also have high al2o3 16 08 wt na2o 8 88 wt and mg 52 3 values without obvious eu enrichment or depletion the yand yb concentrations 13 and 1 33 ppm respectively are higher than those of the enclave bearing meta plagiogranites and slightly lower than those of the mafic enclave zircon of the enclave bearing meta plagiogranites gives a weighted mean 206 pb 238 u crystallization age of 970 21 ma these meta plagiogranites have low 87

variation in glacial erosion near the southern margin of the laurentide ice sheet south central wisconsin usa implications for cosmogenic dating of glacial terrains we measured the abundance of cosmo genic 10 be and 26 al in 22 samples collected from five striated granite metarhyolite and quartzite outcrops in south central wiscon sin that were covered by the late wisconsin laurentide ice sheet in two outcrops mea sured nuclide abundances are consistent with the existing radiocarbon chronology of ice retreat in three outcrops nuclide abun dances were up to eight times higher than predicted by the radiocarbon chronology at these three sites several thousand years of ice flow eroded only centimeters to deci meters of rock allowing a significant quan tity of nuclides 10 5 two samples carry the equivalent of 150 000 yr of surface exposure even though they were covered by ice during the last glacial maximum advance samples highest on the landscape or in plucked ar eas have less inheritance than those from the lee sides of large hills or lower in the landscape three quartzite samples collect ed 10 km up ice from the margin contain three to four times the expected nuclide abundance 10 5 to 10 6 atoms per gram of quartz in contrast eight other quartzite and granite samples from two outcrops 50 km up ice from the former margin contain only 10 5 atoms of 10 be per gram of quartz consistent with late pleistocene ex posure and little if any nuclide inheri tance this relationship between glacial erosion and distance from the former ter minus is consistent with a marginal zone of minimal subglacial erosion the ice was ei ther frozen to its bed there or the ice thick ness and duration of ice cover were less near the terminus these data together with simple model ing of nuclide production by deeply pene trating muons suggest that many meters of rock must be removed to reduce inheri tance to negligible levels 1000 yr in con tinental terrains with low long term erosion rates our results indicate that cosmogenic dating of exposed bedrock surfaces near former ice margins or in areas where ice was frozen to the bed may be uncertain and in some cases impossible because nu clides are inherited from prior periods of cosmic ray exposure unfortunately striat ed glacial outcrops that can be used to dem onstrate most easily the assumption of no postglacial erosion are also the most likely to have undergone little glacial erosion this finding suggests that cosmogenic nuclide production rates based on glacially striated surfaces may include cosmogenic nuclides inherited from prior exposure

evidence for middle eocene and younger land emergence in central panama implications for isthmus closure the rise of the isthmus of panama linked to a number of climatic paleoceanographic and biological events has been studied mostly from indirect often distal geochemical and biotic evidence we have upgraded existing geologic mapping in central panama with more than 2000 field stations over 40 petrographic analyses and more than 30 new geochronological and thermochronological analyses this data set suggests that the isthmus was an uninterrupted chain above sea level from late eocene until at least late miocene times the basement complex of central panama is a folded faulted 3 km thick arc sequence intruded by granitoid bodies and onlapped by mildly deformed upper eocene and oligocene strata six u pb zircon ages in the granitoids along with published geochronological data reveal intense late paleocene to middle eocene magmatism 58 39 ma a temporary cessation of magmatic activity between 38 and 27 ma and renewed magmatism between 25 and 15 ma in a position 75 km south of the former magmatic axis thermochronological analyses in zircon eight u th he ages and in apatite crystals four u th he ages and nine fission track ages obtained from a subset of 58 54 ma granitoid bodies record a concordant lutetian age 47 42 ma cooling from 200 c to 70 c in 5 m y and cooling below 40 c between 12 and 9 ma cooling is linked to exhumation by an angular unconformity that separates the deformed basement complex below from mildly deformed upper eocene to oligocene terrestrial to shallow marine strata above exhumation and erosion of the basement complex are independently confirmed by lower miocene strata that have a detrital zircon signature that closely follows the central panama basement complex age distribution these results greatly restrict the width and depth of the strait separating southern central america from south america and challenge the widely accepted notion that the central american seaway closed in late pliocene time when the ice age began

exploring the ingredients required to successfully model the placement generation and evolution of ice streams in the british irish ice sheet abstract ice stream evolution is a major uncertainty in projections of the future of the greenland and antarctic ice sheets accurate simulation of ice stream evolution requires an understanding of a number of ingredients that control the location and behaviour of ice stream flow here we test the influence of geothermal heat flux grid resolution and bed hydrology on simulated ice streaming the palaeo record provides snapshots of ice stream evolution with a particularly well constrained ice sheet being the british irish ice sheet biis we implement a new basal sliding scheme coupled with thermo mechanics into the bisicles ice sheet model to simulate the evolution of the biis ice streams we find that the simulated location and spacing of ice streams matches well with the empirical reconstructions of ice stream flow in terms of position and direction when simple bed hydrology is included we show that the new basal sliding scheme allows the accurate simulation for the majority of biis ice streams the extensive empirical record of the biis has allowed the testing of model inputs and has helped demonstrate the skill of the ice sheet model in simulating the evolution of the location spacing and migration of ice streams through millennia simulated ice streams also prompt new empirical mapping of features indicative of streaming in the north channel region ice sheet model development has allowed accurate simulation of the palaeo record and allows for improved modelling of future ice stream behaviour

rapid late pleistocene incision of atlantic passive margin river gorges the direct and secondary effects of rapidly changing climate caused large rivers draining the atlantic passive margin to incise quickly into bedrock beginning about 35 000 years ago measured in samples from bedrock fluvial terraces 10 beryllium shows that both the susquehanna and potomac rivers incised 10 to 20 meter deep gorges along steep convex lower reaches during the last glacial cycle this short lived pulse of unusually rapid down cutting ended by 13 000 to 14 000 years ago the timing and rate of downcutting are similar on the glaciated susquehanna and unglaciated potomac rivers indicating that regional changes not simply glacial meltwater initiated incision

preservation of arctic landscapes overridden by cold based ice sheets for nearly 40 years a massive well preserved glaciomarine delta more than 54 000 years old and ancillary landforms have formed the cornerstone of models positing limited ice sheet extent in arctic canada during the late wisconsinan we present exposure ages for large boulders on the delta surface which coupled with preservation of relict landforms demonstrate that the region was covered by minimally erosive cold based ice during the late wisconsinan our data suggest that surficial features commonly used to define the pattern of late wisconsinan ice movement cannot be used on their own to constrain late wisconsinan ice sheet margins in arctic regions

late glacial fluctuations of the laurentide ice sheet in the white mountains of maine and new hampshire u s a abstract prominent moraines deposited by the laurentide ice sheet in northern new england document readvances or stillstands of the ice margin during overall deglaciation however until now the paucity of direct chronologies over much of the region has precluded meaningful assessment of the mechanisms that drove these events or of the complex relationships between ice sheet dynamics and climate as a step towards addressing this problem we present a cosmogenic 10be surface exposure chronology from the androscoggin moraine complex located in the white mountains of western maine and northern new hampshire as well as four recalculated ages from the nearby littleton bethlehem moraine seven internally consistent 10be ages from the androscoggin terminal moraines indicate that advance culminated 13 2 0 8 ka in close agreement with the mean age of the neighboring littleton bethlehem complex together these two datasets indicate stabilization or advance of the ice sheet margin in northern new england at 14 13 ka during the allerod greenland interstadial i

the new north american varve chronology a precise record of southeastern laurentide ice sheet deglaciation and climate 18 2 12 5 kyr bp and correlations with greenland ice core records new glacial varve records from long cores combined with records from key surface exposures and new radiocarbon ages have allowed the correction consolidation expansion and calibration of ernst antevs9 original new england varve chronology nevc in the connecticut valley of new england u s a the varve records have been reformulated with corrections and a new numbering system as the new north american varve chronology navc which is a continuous 5659 yr varve sequence that spans most of the last deglaciation 18 200 12 500 yr bp in the northeastern united states rates of ice recession for separate intervals terminated by abrupt glacial stillstands and readvances have been determined for western new england ice recession history is coupled to varve thickness changes that depict changes in meltwater production in the connecticut valley and show the relationship of changes in ablation rate summer climate variation to glacial readvances and periods of halted and rapid ice recession up to 300 m yr comparison of varve thickness records to greenland ice core climate records show that after 15 000 yr bp climate changes of sub century and longer scales recorded in both records appear identical and synchronous after 15 000 yr bp therefore there was a link between north atlantic climate and marginal processes of the southeastern sector of the laurentide ice sheet lis prior to 15 000 yr bp when the lis was closer to an equilibrium condition retreat rates were generally lower and changes in varve thickness and ablation were more subtle but can still be linked to ice sheet activity only weak relationships between varve thickness changes and greenland climate are evident suggesting that changes in the southeastern lis during this time may have been significantly influenced by climate patterns unique to the north american continent or ice dynamics

fracturing of the panamanian isthmus during initial collision with south america tectonic collision between south america and panama began at 23 25 ma the collision is significant because it ultimately led to development of the panamanian isthmus which in turn had wide ranging oceanic climatic biologic and tectonic implications within the panama canal zone volcanic activity transitioned from hydrous mantle wedge derived arc magmatism to localized extensional arc magmatism at 24 ma and overall marks a permanent change in arc evolution we interpret the arc geochemical change to result from fracturing of the panama block during initial collision with south america fracturing of the panama block led to localized crustal extension normal faulting sedimentary basin formation and extensional magmatism in the canal basin and bocas del toro synchronous with this change both panama and inboard south america experienced a broad episode of exhumation indicated by u th he and fission track thermochronology coupled with changing geographic patterns of sedimentary deposition in the colombian eastern cordillera and llanos basin such observations allow for construction of a new tectonic model of the south america panama collision northern andes uplift and panama orocline formation finally synchroneity of panama arc chemical changes and linked uplift indicates that onset of collision and isthmus formation began earlier than commonly assumed

a millennial perspective on arctic warming from 14c in quartz and plants emerging from beneath ice caps 1 observational records show that the area of ice caps on northern baffin island arctic canada has diminished by more than 50 since 1958 fifty 14c dates on dead vegetation emerging beneath receding ice margins document the persistence of some of these ice caps since at least 350 ad in situ cosmogenic 14c in rock surfaces and 14c in plant macrofossils from lake sediment cores demonstrate that the plateau remained ice free through the middle holocene but has supported ice caps for more than 2000 of the past 2800 years the rapid disappearance of these ice caps over the past century despite decreasing summer insolation further demonstrates the unusual character of 20th century warmth widespread ice cap expansion 1280 ad early in the little ice age and intensified expansion 1450 ad coincide with peak stratospheric volcanic aerosol loading and reduced solar luminosity suggesting that these mechanisms may have initiated ice cap growth subsequently maintained by strong positive feedbacks

glacial history and landscape evolution of southern cumberland peninsula baffin island canada constrained by cosmogenic 10be and 26al since the first application of cosmogenic nuclides to the study of glacial history and processes in 1990 increasing numbers of studies have used a variety of cosmogenic isotopes to quantify the exposure age and erosion rate of glaciated landscapes however obtaining chronological data from glaciated landscapes once covered by cold based nonerosive ice is challenging because these surfaces violate assumptions associated with simple cosmogenic exposure dating nonerosive glacial ice fails to completely remove nuclides produced during previous periods of exposure leaving behind rock surfaces with complex multigenetic nuclide inventories here we constrain the glacial history landscape evolution and efficiency of subglacial erosion in the pangnirtung fiord region of southern baffin island using over 300 paired analyses of in situ cosmogenic 10be and 26al simple exposure ages are 6 3 160 ka for 10be n 152 and 4 3 124 ka for 26al n 153 paired bedrock boulder samples have discordant ages simple exposure ages generally increase with elevation 10be and 26al ages for the same sample disagree and both boulders and bedrock yield multimodal age distributions all patterns suggestive of limited subglacial erosion measured 26al 10be ratios indicate that about one third of the samples in the data set experienced at least one period of pre holocene exposure followed by burial with limited erosion modeled two isotope minimum limiting exposure durations are as high as hundreds of thousands of years and minimum limiting burial durations range up to millions of years implying that parts of southern baffin island have been preserved beneath nonerosive glacial ice for much if not all of the quaternary a subset of the samples contains few nuclides inherited from prior periods of exposure and is thus useful for constraining the chronology of the most recent deglaciation using these samples we infer that deglaciation of most of the landscape occurred ca 11 7 ka and that the duval moraines a prominent feature of the last deglaciation formed ca 11 2 ka

origin of mesozoic adakitic intrusive rocks in the ningzhen area of east china partial melting of delaminated lower continental crust to the best of our knowledge modern adakites have not been documented in a nonarc environment we report geochemical and isotopic data for early cretaceous anjishan adakitic intrusive rocks that are in a continental setting unrelated to subduction the anjishan adakitic intrusive rocks which are exposed in the ningzhen area of east china have high sr y and la yb ratios coupled with low yb and y as well as relatively high mgo contents and mg numbers mg 0 4 0 6 similar to products from slab melting however low n d t values 6 8 to 9 7 and high 8 7 sr 8 6 sr i 0 7053 0 7066 are inconsistent with an origin by slab melting the tectonics and geochemistry lead us to conclude that adakitic magmas were most likely derived from partial melting of mafic material at the base of the continental crust high sr y and la yb ratios of the adakitic intrusive rocks suggest that garnet was stable as a residual phase during partial melting implying that the crustal thickness exceeded 40 km in the early cretaceous the present thickness of the crust in the ningzhen area is only 30 km and therefore the crust appears to have been thinned by at least 10 km since the early cretaceous the relatively high mgo contents and mg of the anjishan intrusive rocks suggest that adakitic magmas interacted with mantle rocks possibly coinciding with lower crustal delamination which would also account for the observed thinning

analysis of multiple cosmogenic nuclides constrains laurentide ice sheet history and process on mt mansfield vermont s highest peak abstract constraining glacial history and process on mt mansfield the highest peak in vermont 1339 m a s l provides insight into how the laurentide ice sheet shaped the underlying landscape when latest pleistocene ice retreated and how upland and lowland glacial histories relate here we quantify in situ cosmogenic 10be in 20 bedrock and boulder surfaces as well as in situ cosmogenic 14c in three of those surfaces to assess subglacial erosion and exposure history isotopic concentrations indicate that mt mansfield s lower elevations 400 1200 m a s l were deeply eroded by at least several meters during the last glaciation and then deglaciated rapidly 10be ages across this elevation span are indistinguishable and average 13 9 0 6 ka n 15 suggesting that 800 m of ice thinning occurred within at most about a millennium conversely the higher elevations 1200 m a s l preserve a more complex geomorphic history mt mansfield s summit surfaces contain 10be from previous periods of exposure indicating that the mountaintop landscapes were likely preserved beneath cold based weakly erosive glacial ice exposure ages from the shorter lived isotope 14c are younger 9 7 and 11 7 ka suggesting that mt mansfield s summit was covered until the early holocene perhaps by snowfields ice carapaces and or till our findings in context of previous work suggest that thinning laurentide ice flowed through the valleys for at most hundreds of years following deglaciation of the uplands but that the summit remained shielded by ice or sediment for millennia after the valleys became ice free

cosmogenic nuclide age estimate for laurentide ice sheet recession from the terminal moraine new jersey usa and constraints on latest pleistocene ice sheet history

water saturated magmas in the panama canal region a precursor to adakite like magma generation amphibole while uncommon as a phenocryst in arc lavas is increasingly recognized as a key constituent in the petrogenesis of arc magmas fractional crystallization of water saturated arc magmas in the lower crust can yield substantial volumes of amphibole cumulates that depending on the pressure of crystallization may also contain garnet fractionation of this higher pressure assemblage has been invoked as a possible mechanism in the production of magmas that contain an adakitic signature this study examines newly dated late oligocene 25 37 0 13 ma hypabyssal amphibole rich andesites from cerro patacon in the panama canal region these andesites contain nodules of amphibole cumulates that are 4 6 cm in diameter and are almost entirely composed of 5 10 mm amphibole crystals dominantly ferri tschermakite geochemical variations optical and chemical zoning of the cerro patacon amphiboles are consistent with their evolution in a crystal mush environment that had at least one recharge event prior to entrainment in the host andesite amphiboles hosted within the cumulate nodules differ from those hosted in the cerro patacon andesite and contain consistently higher values of ti we suggest these nodules represent the early stages of fractionation from a water saturated magma cerro patacon andesites have ree concentrations that plot at the most depleted end of central american arc magmas and exhibit a distinctive depletion in the middle ree these geochemical and petrographic observations strongly support significant amphibole fractionation during formation of the cerro patacon andesite consistent with the petrographic evidence fractionation of water saturated magmas is a mechanism by which adakitic compositions may be produced and the cerro patacon andesites do exhibit adakite like geochemical characteristics e g elevated sr y 28 34 however the relatively elevated concentrations of y and hree indicate garnet was not stable in the fractionating assemblage during this early stage of arc development

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

glacial trimlines to identify former ice margins and subglacial thermal boundaries a review and classification scheme for trimline expression abstract reconstructions of former ice sheets and glaciers provide important palaeoglaciological information about their behaviour in response to climate changes glacial trimlines record both the margin positions and palaeo ice thickness enabling the production of empirically constrained 3 dimensional reconstructions however the literature review into the characteristics interpretation and use of glacial trimlines here presented shows that these features have been under utilised and are poorly described in the existing literature with a confusing terminology currently in use a new classification scheme and terminology for trimline identification and interpretation is developed to better facilitate further research into these common features of glacierised and formerly glaciated landscapes

partial melting of slab window margins genesis of adakitic and non adakitic magmas abstract when a mid ocean spreading ridge subducts it typically splits apart at depth to form two tapered slab edges separated by asthenospheric mantle within a slab window we examine the fate of the slab edges using simplified slab window geometries specific thermal parameters and assumptions regarding shear stress and slab hydration six fundamental zones of slab anatexis are identified and classified according to expected melt and restite compositions non adakitic melts of granodioritic to tonalitic composition are generated along the plate edges at depths of 5 65 km whereas adakitic melts form proximal to the edges at depths of 25 90 km as anatexis proceeds the subducted crust is converted to a migmatite of slab melt and eclogitic restite much of the migmatite may shear away from the slab and become incorporated into the mantle the melts will rise and leave behind fragments of restite within mantle peridotite if the peridotite is part of the overriding plate then the restite fragments may become long term residents of the continental lithospheric mantle however if the restite becomes entrained in the asthenosphere it may undergo upwelling and melting or flow away as ductile streaks to become long term mantle heterogeneities slab windows are thereby identified as important sites for slab melting and geochemical replenishment of the mantle

tracking paraglacial sediment with cosmogenic 10 be using an example from the northwest scottish highlands abstract beryllium 10 concentrations in samples of sediment and bedrock from five study sites across the scottish highlands trace paraglacial sediment sources and define the nature of glacial erosion for the late quaternary exposure ages derived from 10 be concentrations in ridge and lower elevation bedrock range from 10 to 33 ka which suggest that polythermal ice and warm based ice were primarily responsible for producing glacial sediment comparisons of 10 be concentrations between catchment wide sediment 2 06 0 34 10 4 to 11 24 1 54 10 4 atoms g 1 sio 2 n 33 near surface deposits 2 71 0 33 10 4 to 3 48 0 49 10 4 atoms g 1 sio 2 n 6 4 m thick deep till 0 68 10 4 10 be atoms g 1 sio 2 n 1 ridge bedrock 8 93 0 47 10 4 to 34 05 1 66 10 4 atoms g 1 sio 2 n 20 and lower elevation polished bedrock 6 74 0 67 10 4 to 12 65 0 7 10 4 atoms g 1 sio 2 n 5 indicate that most sand fluxing through catchments in the scottish highlands is sourced from the remobilization and vertical mixing of near surface deposits these findings indicate that glaciogenic material continues to dominate paraglacial sediment budgets more than 11 ka after deglaciation

a new look at evidence for a wadati benioff zone and active convergence at the north panama deformed belt the nature of the northern boundary of the panama microplate often referred to as the north panama deformed belt npdb has been the subject of much speculation previous studies have used a variety of data including teleseismic gravity bathymetric marine magnetic and field studies of uplift and tsunami deposits as well as modified mercalli intensity distributions from historic earthquakes to reveal the nature of the npdb data have been collected for over 30 yrs and yet the character of the npdb remains unclear current analyses and interpretation provide a number of mutually exclusive options and much controversy in this article we examine local and regional seismicity combined with teleseismic observations and historic earthquake data to present an alternative analysis for the npdb using small earthquakes recorded by a local network we image a well defined wadati benioff zone dipping southward beneath the panama microplate these data provide new evidence regarding the nature of panama microplate caribbean plate boundary kinematics and demonstrate the existence of an active subduction zone a more complete understanding of the nature of the seismicity and plate interactions along the npdb offshore panama including a subduction zone capable of producing earthquakes of m 7 should be included in future earthquake hazard assessments

rapid thinning of the laurentide ice sheet in coastal maine usa during late heinrich stadial 1 abstract few data are available to infer the thinning rate of the laurentide ice sheet lis through the last deglaciation despite its importance for constraining past ice sheet response to climate warming we measured 31 cosmogenic 10 be exposure ages in samples collected on coastal mountainsides in acadia national park and from the slightly inland pineo ridge moraine complex a 100 km long glaciomarine delta to constrain the timing and rate of lis thinning and subsequent retreat in coastal maine samples collected along vertical transects in acadia national park have indistinguishable exposure ages over a 300 m range of elevation suggesting that rapid century scale thinning occurred at 15 2 0 7 ka similar to the timing of abrupt thinning inferred from cosmogenic exposure ages at mt katahdin in central maine davis et al 2015 this rapid ice sheet surface lowering which likely occurred during the latter part of the cold heinrich stadial 1 event 19 14 6 ka may have been due to enhanced ice shelf melt and calving in the gulf of maine perhaps related to regional oceanic warming associated with a weakened atlantic meridional overturning circulation at this time the ice margin subsequently stabilized at the pineo ridge moraine complex until 14 5 0 7 ka near the onset of bolling interstadial warming our 10 be ages are substantially younger than marine radiocarbon constraints on lis retreat in the coastal lowlands suggesting that the deglacial marine reservoir effect in this area was 1 200 14 c years perhaps also related to the sluggish atlantic meridional overturning circulation during heinrich stadial 1

melting features along the ryukyu slab tear beneath the southwestern okinawa trough 1 the present day active volcanic front associated with the ryukyu subduction zone extends from japan to the ilan plain northern taiwan and is located within the okinawa trough 80 100 km above the ryukyu slab an abnormal amount of arc volcanism which consists of basalt andesite and rhyolite occurs within the southwestern okinawa trough above a slab tear of the ryukyu subduction zone cbvt the power spectrum analysis of magnetic data shows the occurrence of a thin crust above the slab tear and a thick crust beneath this volcanic area we suggest that an excess of h2o rich fluid might occur at the slab tear and might increase the melt flux both are conveyed obliquely to the uppermost mantle and lower crust cbvt magmas after interactions basaltic magmas would rise up accounting for the contrast of magnetization between this volcanic body and the adjacent ot crust

adakites from continental collision zones melting of thickened lower crust beneath southern tibet adakites are geochemically distinct intermediate to felsic lavas found exclusively in subduction zones here we report the first example of such magmas from southern tibet in an active continental collision environment the tibetan adakites were emplaced from ca 26 to 10 ma and their overall geochemical characteristics suggest an origin by melting of eclogites and or garnet amphibolites in the lower part 50 km of thickened tibetan crust this lower crustal melting required a significantly elevated geotherm which we attribute to removal of the tectonically thickened lithospheric mantle in late oligocene time the identification of collision type adakites from southern tibet lends new constraints to not only the himalayan tibetan orogenesis how and when the indian lithosphere started underthrusting asia can be depicted but also the growth of the early continental crust on earth that consists dominantly of the tonalite trondhjemite granodiorite suites marked by adakitic geochemical affinities

generation of the early cenozoic adakitic volcanism by partial melting of mafic lower crust eastern turkey implications for crustal thickening to delamination abstract early cenozoic 48 50 ma adakitic volcanic rocks from the eastern pontides ne turkey consist of calc alkaline and high k calc alkaline andesite and dacite with sio2 contents ranging from 56 01 to 65 44 wt this is the first time that early eocene volcanism and adakites have been reported from the region the rocks are composed of plagioclase amphibole quartz and mg rich biotite they have high and low mg values ranging from 55 to 62 and 13 to 42 respectively high mg rocks have higher ni and co contents than low mg samples the rocks exhibit enrichments in large ion lithophile elements including the light rare earth elements depletions in nb ta and ti and have high la yb and sr y ratios their relative high isr 0 70474 0 70640 and low end 50 ma values 2 3 to 0 8 are inconsistent with an origin as partial melts of a subducted oceanic slab combined major and trace element and sr nd isotope data suggest that the adakitic magmas are related to the unique tectonic setting of this region where a transition from a collision to an extension stage has created thickening and delamination of the pontide mafic lower crust at 50 ma the high mg adakitic magmas resulted from partial melting of the delaminated eclogitic mafic lower crust that sank into the relatively hot subcrustal mantle and its subsequent interaction with the mantle peridotite during upward transport leaving garnet as the residual phase elevates the mgo content and mg of the magmas whereas low mg magmas formed by the melting of newly exposed lower crustal rocks caused by asthenospheric upwelling which supplies heat flux to the lower crust the data also suggest that the mafic lower continental crust beneath the region was thickened between the late cretaceous and the late paleocene and delaminated during late paleocene to early eocene time which coincides with the initial stage of crustal thinning caused by crustal extensional events in the eastern pontides and rules out the possibility of an extensional regime before 50 ma in the region during the late mesozoic to early cenozoic

evidence for active subduction below western panama widespread arc tholeiitic and calc alkaline volcanism characterized the geologic evolution of western panama and southeastern costa rica in middle to late miocene and quaternary time volcanism was and is clearly associated with subduction of nazca lithosphere geochemical data suggest that formation of mantle wedge derived melts and their differentiated products predominated during miocene time whereas slab derived melts were generated in the quaternary this difference in magma chemistry is attributed to a change in the thermal regime of the nazca lithosphere which may be related to emplacement of the galapagos plume

source and tectonic implications of tonalite trondhjemite magmatism in the klamath mountains in the klamath mountains voluminous tonalite trondhjemite magmatism was characteristic of a short period of time from about 144 to 136 ma early cretaceous it occurred about 5 to 10 m y after the 165 to 159 ma josephine ophiolite was thrust beneath older parts of the province during the nevadan orogeny thrusting from 155 to 148 ma the magmatism also corresponds to a period of slow or no subduction most of the plutons crop out in the south central klamath mountains in california but one occurs in oregon at the northern end of the province compositionally extended members of the suite consist of precursor gabbroic to dioritic rocks followed by later more voluminous tonalitic and trondhjemitic intrusions most plutons consist almost entirely of tonalite and trondhjemite poorly defined concentric zoning is common tonalitic rocks are typically of the low al type but trondhjemites are generally of the high al type even those that occur in the same pluton as low al tonalite the suite is characterized by low abundances of k2o rb zr and heavy rare earth elements sr contents are generally moderate 450 ppm by comparison with sr rich arc lavas interpreted to be slab melts up to 2000 ppm initial 87sr 86sr 18o and nd are typical of mantle derived magmas or of crustally derived magmas with a metabasic source compositional variation within plutons can be modeled by variable degrees of partial melting of a heterogeneous metabasaltic source transitional mid ocean ridge to island arc basalt but not by fractional crystallyzation of a basaltic parent melting models require a residual assemblage of clinopyroxene garnet plagioclase amphibole residual plagioclase suggests a deep crustal origin rather than melting of a subducted slab such models are consistent with the metabasic part of the josephine ophiolite as the source because the josephine ophiolite was at low t during nevadan thrusting an external heat source was probably necessary to achieve significant degrees of melting heat was probably extracted from mantle derived basaltic melts which were parental to the mafic precursors of the tonalite trondhjemite suite thus under appropriate tectonic and thermal conditions heterogeneous mafic crustal rocks can melt to form both low and high al tonalitic and trondhjemitic magmas slab melting is not necessary

new sw pacific tectonic model cyclical intraoceanic magmatic arc construction and near coeval emplacement along the australia pacific margin in the cenozoic various reconstructions of the sw pacific for the late cretaceous and cenozoic suggest that northeast dipping subduction began in the south loyalty basin slb at 55 50 ma and that subsequent closure of the slb resulted in the diachronous emplacement of cretaceous paleocene ophiolitic nappes onto the norfolk ridge in new caledonia at 40 34 ma and in northland new zealand around 24 21 ma a fundamental problem with these models is that they do not account for the fact that ne dipping subduction had already been established offshore papua new guinea by at least 65 60 ma which resulted in the emplacement of the papuan ultramafic belt pub ophiolite at 59 58 ma a second issue is that the reconstructions are based largely upon unfounded assumptions as to the age and nature of the basement beneath the loyalty arc and three kings ridge finally reconstructions of the northland region are based upon the erroneous assumption that the age of the majority of the igneous component comprising the northland allochthon is late cretaceous paleocene when in fact it is oligocene a new model is presented whereby the pub new caledonia and northland ophiolites formed and were emplaced in a cyclical fashion above an extensive ne dipping cenozoic intraoceanic arc system which diachronously propagated n s along the entire eastern margin of the australian plate these infant arc ophiolites represent fragments of suprasubduction zone lithosphere sszl generated in the earliest stages of magmatic arc formation that were emplaced shortly after 20 m y as a result of forearc australian plate collision subduction inception was the result of subsidence of older morb like lithosphere generated within an extensive back arc basin to the east of the norfolk ridge during the earliest stages of slb formation above a southwest dipping pacific plate during emplacement of each ophiolite a crustal fragment of the older lithosphere was scraped off the ne dipping slab and subsequently back thrust beneath each ophiolite during its emplacement

late miocene adakites and nb enriched basalts from vizcaino peninsula mexico indicators of east pacific rise subduction below southern baja california a typical slab melt association was emplaced from 11 to 8 ma in the santa clara volcanic field vizcaino peninsula baja california sur it includes adakitic domes and associated pyroclastic flow deposits together with lava flows of niobium enriched basalts the trace element and isotopic sr nd pb signatures of adakites are consistent with melting of altered mid ocean ridge basalts and the sources of the nb enriched basalts contain an enriched mantle wedge component such associations commonly form at depths of 70 80 km during low dip subduction of very young oceanic crust however the santa clara field is relatively close 100 km to the paleotrench which suggests that the genesis of its adakites and nb enriched basalts occurred in a very high thermal regime linked to the subduction of the then active guadalupe spreading center of the east pacific rise our data suggest that the asthenospheric window documented below northern baja california also developed beneath the south of the peninsula during the neogene this hypothesis is consistent with the spatial distribution and the ages of adakites and magnesian andesites from this region

intracrustal derivation of na rich andesitic and dacitic magmas an example from volcan ollague andean central volcanic zone andesitic and dacitic lavas erupted from quaternary stratovolcanoes in the central volcanic zone cvz of the andes have major and trace element compositions similar to proposed melts of subducted mid ocean ridge basalt and na rich trondjhemites tonalites and granodiorites ttg in early archean high grade gneiss terrains suites of lavas from individual volcanoes have average la yb of 25 45 sr y of 30 65 yb contents of 1 5 1 1 ppm and na 2 o contents 3 8 wt with lavas erupted on the volcanic front displaying the most affinity to the early archean suites because slab melting is not involved in the derivation of the cvz magmas these data indicate that rocks with early archean ttg compositions can also be produced through intracrustal differentiation processes isotopic major element and ree data from volcan ollague are consistent with a model incorporating contamination of mantle derived basaltic magmas in garnet bearing deep continental crust to produce basaltic andesite magma followed

first cosmogenic ages for glacial deposits from the plata range 33 s new inferences for quaternary landscape evolution in the central andes this paper presents the first numerical cosmogenic ages of glacial deposits from the eastern slope of the plata peak 6000 in the arid central andes 32 s the moraines are distributed along three different valleys vallecitos angostura and las mulas in the first only one moraine was identified and no blocks could be sampled for dating the other two have evidence of two glacial advances angostura i and angostura ii the advances loma de los morteritos i and ii were recognised in the las mulas valley in both cases first and second advances are preserved at a similar altitude surface boulders on the top of moraines were sampled for be10 obtained ages are all similar and late pleistocene in age they fit very well with local stratigraphy and regional paleoclimate evidence the loma de los morteritos moraine ii located at 3000 masl dated to 25 000 years ages of 8 000 and 12 000 years were found for the angostura ii deposit 3300 masl an age was obtained for the outwash deposit identified along the el salto valley 69 000 years associated with a late pleistocene drift quantitative datings presented here improve knowledge about the glacial chronology of the plata range and quaternary stratigraphy of the central andes even though they represent a maximum age for these glacial advances new findings support the occurrence of two late pleistocene glacial advances and an older one in the blanco river valley adjusted ages around 8 000 and 25 000 years for younger stage could be evidence of the last glacial maximum lgm period for this region

generation of sodium rich magmas from newly underplated basaltic crust sodium rich rocks of trondhjemite tonalite dacite ttd or granodiorite ttg suites form much of precambrian continental crust1 they are thought to have formed by partial melting of subducted oceanic crust2 3 a process that would have been much more widespread early in earth history than at present owing to the higher thermal gradients prevailing at that time4 phanerozoic ttd suites do exist however and seem also to relate to subduction zones5 defant and drummond6 proposed that these suites form where young 25 myr hot oceanic lithosphere is subducted and melts thus locally simulating the conditions that led to widespread crustal growth in the archaean here we describe plutonic and volcanic rocks from the cordillera blanca complex in peru which have characteristics of the high al ttd suite but which were produced above a subduction zone containing a 60 myr old slab we present evidence that the complex formed by partial melting of newly underplated basaltic crust and argue that this mechanism should be considered more generally as an additional way of generating sodium rich arc magmas

constraining multi stage exposure burial scenarios for boulders preserved beneath cold based glacial ice in thule northwest greenland abstract boulders and landscapes preserved beneath cold based non erosive glacial ice violate assumptions associated with simple cosmogenic exposure dating in such a setting simple single isotope exposure ages over estimate the latest period of surface exposure hence alternate approaches are required to constrain the multi stage exposure burial histories of such samples here we report 28 paired analyses of 10be and 26al in boulder samples from thule northwest greenland we use numerical models of exposure and burial as well as monte carlo simulations to constrain glacial chronology and infer process in this arctic region dominated by cold based ice we investigate three specific cases that can arise with paired nuclide data 1 exposure ages that are coeval with deglaciation and 26al 10be ratios consistent with constant exposure 2 exposure ages that pre date deglaciation and 26al 10be ratios consistent with burial following initial exposure and 3 exposure ages that pre date deglaciation and 26al 10be ratios consistent with constant exposure most glacially transported boulders in thule have complex histories some were exposed for tens of thousands of years and buried for at least hundreds of thousands of years while others underwent only limited burial these boulders were probably recycled through different generations of till over multiple glacial interglacial cycles likely experiencing partial or complete shielding during interglacial periods due to rotation or shallow burial by sediments our work demonstrates that the landscape in thule like many high latitude landscapes was shaped over long time durations and multiple glacial and interglacial periods throughout the quaternary

contrasting origins of late mesozoic adakitic granitoids from the northwestern jiaodong peninsula east china implications for crustal thickening to delamination two suites of granitoids the late jurassic 158 3 ma linglong suite and the early cretaceous 130 126 ma guojialing suite crop out in the northwestern jiaodong peninsula eastern china the linglong suite is a monzogranite comprising alkali feldspar plagioclase quartz and fe rich biotite the guojialing suite includes at least five plutonic bodies of both granodiorite and monzo granite the rocks are composed of plagioclase alkali feldspar quartz mg rich amphibole and mg rich biotite both the linglong and guojialing suites have adakitic affinity they are enriched in lree with high la yb ratios and show positive eu anomalies the rocks are also enriched in lile and depleted in hfse with high sr y ratios the linglong granite shows very uniform sr nd isotopic compositions with initial 87 sr 86 sr ratios of 0 7119 0 7126 and e nd t values of 21 3 to 21 6 which are similar to those of the local neoarchaean basement the guojialing suite has variable initial 87 sr 86 sr ratios 0 7108 0 7120 and e nd t values 10 8 to 17 2 which are distinct both from those of the neoarchaean basement and from those of the local enriched lithospheric mantle inferred from the coeval mafic dykes in the studied area detailed petrological and geochemical data indicate that the linglong suite was derived by partial melting of neoarchaean metamorphic lower crustal rocks at depth of 50 km with a eclogite residue whereas the guojialing suite was formed by the reaction of delaminated eclogitic crust derived melt with the upwelling asthenospheric mantle the petrogenesis of these two contrasting adakitic granitoids suggests intensive lower crustal delamination during early cretaceous times following a crustal thickening process from the late stage of the early jurassic to early stage of the late jurassic with crustal thickness of 50 km respectively

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

location of the eastern boundary of the cocos plate during the miocene abstract the eastern equatorial pacific composed of and bordered by the cocos nazca and south american plates is tectonically complex because of large changes in configuration in the recent past the north south trending eastern boundary of the cocos plate during the miocene was apparently located in the transisthmian area of the present panama canal several sets of data topography bathymetry seismicity hot spring locations faulting patterns and gravity indicate that there is a major trans isthmian tectonic boundary between 79 and 80 w coincident with the postulated paleo plate boundary this feature which has been recognized for some 50 years by many geologists we name the canal discontinuity

felsenmeer persistence under non erosive ice in the torngat and kaumajet mountains quebec and labrador as determined by soil weathering and cosmogenic nuclide exposure dating soil analyses and terrestrial cosmogenic nuclide exposure dating are combined and a conceptual model proposed to explain altitudinal weathering contrasts in high latitude highlands we show that summits in the torngat and kaumajet mountains were covered by ice during the last glacial maximum and that their felsenmeer cover probably survived multiple glaciation events for similar lithologies soils on felsenmeer covered summits are signigicantly more weathered than those below the felsenmeer limit displaying higher concentrations of crystalline iron amorphous aluminium and silicium extracted with oxalate secondary minerals such as gibbsite and kaolinite occur in felsenmeer soils whereas those formed in till lacked these secondary minerals 10be and 26al exposure ages for nine of ten samples from high elevation tors and autochthonous felsenmeer blocks range from 73 6 to 157 15 ka by contrast ages of 11 4 1 0 and 11 7 1 0 ka are measured for bedrock in the much lower saglek zone indicating extensive 3 m glacial erosion of this zone during late wisconsinan glaciation 26al 10be ratios demonstrate that exposure of the high elevation surfaces was interrupted during at least one cosmic ray shielding event by either ice or till cover in either case late wisconsinan glaciers could not have extensively eroded these surfaces five erratics dated above the saglek zone including one in the felsenmeer zone have exposure ages ranging from 11 6 1 0 to 13 6 0 7 ka this indicates that valley and high elevation ice persisted through the younger dryas chron and provides further evidence that the highlands were not nunataks during the late wisconsinan period

seismicity of the caribbean nazca boundary constraints on microplate tectonics of the panama region there are at least three kinds of models that have been used to explain plate motions and boundaries in the panama region these models suggest that the major caribbean nazca boundary is 1 a thrust fault along which the panama deformed belt has developed 2 a left lateral strike slip fault along the southern margin of panama or 3 a diffuse array of mostly thrust faults which cause internal deformation to occur throughout panama in this paper we use joint epicenter location methods to obtain improved locations for earthquakes that occurred in this region between 1964 and 1985 and we use waveform modeling to obtain more precise depths for five events hypocenters of 13 events beneath the panama deformed belt suggest that overthrusting of panama upon the caribbean plate causes active deformation which supports model 1 but does not rule out models 2 and 3 the nature of seismic faulting along the panama deformed belt is complex and supports the premise that slow north south convergence loads the cold thick caribbean lithosphere and causes it to fail seismically epicenters of 64 events along the panama fracture zone indicate that this north south trending rightlateral transform fault is splayed north of 6 n overall the seismicity throughout suggests that the panama arc can be treated as a microplate in accordance with models 1 or 2 similarly a possible second microplate may exist south of panama but if so it does not affect regional plate motions significantly the strongest evidence for model 3 occurs in the easternmost and westernmost portions of panama where the diffuse seismicity prevents us from resolving specific microplate triple junction boundaries

petrology geochemistry and genesis of high al tonalite and trondhjemites of the cornucopia stock blue mountains northeastern oregon tonalitic and trondhjemitic compositions span a narrow range of trondhjemite sio2 content 65 74 wt and exhibit characteristics of a highal tonalite trondhjemite granitoid ttg suite including light rare earth element lree enrichment low y nb and rb sr and high al2o3 and sr these compositions are consistent with an

deglaciation and late glacial climate change in the white mountains new hampshire usa recession of the laurentide ice sheet from northern new hampshire was interrupted by the littleton bethlehem l b readvance and deposition of the extensive white mountain moraine system wmms our mapping of this moraine belt and related glacial lake sequence has refined the deglaciation history of the region the age of the western part of the wmms is constrained to 14 0 13 8 cal ka bp by glacial lake hitchcock varves that occur beneath and above l b readvance till and were matched to a revised calibration of the north american varve chronology presented here using this age for when boulders were deposited on the moraines has enabled calibration of regional cosmogenic nuclide production rates to improve the precision of exposure dating in new england the l b readvance coincided with the older dryas od cooling documented by workers in europe and the equivalent gi 1d cooling event in the greenland ice core chronology 2005 gicc05 time scale the readvance and associated moraines provide the first well documented and dated evidence of the od event in the northeastern united states our lake sediment cores show that the younger dryas cooling was likewise prominent in the white mountains thus extending the record of this event westward from maine and maritime canada

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

cosmogenic radionuclides from fiord landscapes support differential erosion by overriding ice sheets the interpretation of differentially weathered mountainous areas along the fringes of pleistocene ice sheets is fundamental for determining ice sheet behavior and thickness during the last glaciation two existing interpretations are either that highly weathered uplands remained as nunataks while freshly eroded troughs held outlet glaciers during the last glaciation or that uplands and lowlands were equally covered by ice but that it was differentially erosive as a function of its spatially variable basal thermal regime cosmogenic radionuclide measurements from 33 bedrock samples and 27 upland erratics from differentially weathered fi ord landscapes on northeastern baffi n island shed light on laurentide ice sheet lis dynamics and thickness tors on weathered upland surfaces have minimum 10 be ages between ca 50 and ca 170 ka n 12 whereas the majority of erratics perched in the uplands range from ca 10 to ca 13 ka n 14 indicating that the whole landscape was glaciated during the last glacial maximum lgm glacially sculpted bedrock n 7 near sea level refl ects the age of deglaciation and increasing amounts of isotopic inheritance in highelevation sculpted bedrock n 3 higher elevation intermediately weathered bedrock n 11 and highest elevation intensely weathered bedrock refl ect the weakening of erosive power as the ice sheet transitioned from fi ords to interfi ord plateaus cold based glaciation of uplands was contemporaneous with warm based glaciation in fi ords suggesting the presence of ice streams 26 al and 10 be concentrations measured in 10 bedrock samples indicate that tors have experienced a complex history of alternating periods of exposure burial and limited glacial erosion the paired isotope data reveal a minimum duration of burial of 80 500 k y presumably by cold based ice and indicate that the tors have an age of at least 150 580 ka these data together with other reconstructions of ice streams in the eastern canadian arctic suggest that ice streams were a large infl uence on northeastern lis dynamics throughout the late quaternary thus the northeastern lis was sensitively tied with north atlantic thermohaline circulation and abrupt climate change

constraining processes of landscape change with combined in situ cosmogenic 14c 10be analysis abstract reconstructing quaternary landscape evolution today frequently builds upon cosmogenic nuclide surface exposure dating however the study of complex surface exposure chronologies on the 102 104 years timescale remains challenging with the commonly used long lived radionuclides 10be 26al 36cl in glacial settings key points are the inheritance of nuclides accumulated in a rock surface during a previous exposure episode and partial shielding of a rock surface after the main deglaciation event e g during phases of glacier readvance combining the short lived in situ cosmogenic 14c isotope with 10be dating provides a valuable approach to resolve and quantify complex exposure histories and burial episodes within lateglacial and holocene timescales the first studies applying the in situ 14c 10be pair have demonstrated the great benefit from in situ 14c analysis for unravelling complex glacier chronologies in various glacial environments worldwide moreover emerging research on in situ 14c in sedimentary systems highlights the capacity of combined in situ 14c 10be analysis to quantify sediment transfer times in fluvial catchments or to constrain changes in surface erosion rates nevertheless further methodological advances are needed to obtain truly routine and widely available in situ 14c analysis future development in analytical techniques has to focus on improving the analytical reproducibility reducing the background level and determining more accurate muonic production rates these improvements should allow extending the field of applications for combined in situ 14c 10be analysis in earth surface sciences and open up a number of promising applications for dating young sedimentary deposits and the quantification of recent changes in surface erosion dynamics

dubious case for slab melting in the northern volcanic zone of the andes it was first suggested by r w kay that adakites may represent melts of subducted slab since then the term adakite has become synonymous with slab melts based on their unusual geochemical signature this contribution using the northern volcanic zone of the andes as an example aims 1 to expose the weakness in simply associating a geochemical signature with a genetic mechanism and 2 to underline the importance of using several integrated lines of evidence in assessing the viability of slab melting we conclude that although slab melting probably occurs in some arcs regional geochemical trends in the northern volcanic zone and their relationship to the subduction zone architecture are not indicative of slab melting and can be accounted for by normal arc magmatic processes acting on wedge derived basaltic magmas

cold based laurentide ice covered new england s highest summits during the last glacial maximum to better understand glacial history and process in new england northeastern united states a mountainous area overrun by the laurentide ice sheet we measured three cosmogenic nuclides in nine upland samples the concentrations of 10be and 26al in some samples collected near the summits of katahdin maine and mount washington and little haystack mountain new hampshire are 2 10 times higher than expected for a single exposure period considering field evidence indicating that continental ice covered all new england peaks during the last glacial maximum in situ 14c exposure ages from the summits are much younger suggesting that high elevation sampling sites were ice covered before and during the last glacial maximum field and isotopic data are consistent with new england summits being covered in part by cold based continental ice that did not erode much rock the contrast in erosion rates between stable summits and deeply eroded valleys likely contributes to the development and maintenance of northern appalachian topography

age and origin of earliest adakitic like magmatism in panama implications for the tectonic evolution of the panamanian magmatic arc system abstract 40 20 ma marks a fundamental interval in the evolution of the 70 0 ma panamanian magmatic arc system during this period there is no evidence of panamanian magmatic arc activity to the east of the panama canal basin while to the west and in localized regions to the east of the panama canal basin a phase of intrusive only activity is recorded fundamentally geochemical and geochronological evidence presented herein indicate that this intrusive activity was predominantly adakitic like and becomes younger from west to east along an approximately w e striking lineament granodiorites of the petaquilla batholith western panama yield lam icp ms 206 pb 238 u zircon ages of 29 0 0 7 0 6 ma 28 5 0 7 0 5 ma 28 3 0 5 0 4 ma and 26 2 0 5 0 9 ma to the east of the panama canal basin zircons from a hypabyssal diorite of the mainly intermediate maje subvolcanic suite cedes a mean 206 pb 238 u age of 18 9 0 4 ma relative to other 70 5 ma panamanian magmatic arc lavas and intrusives maje and petaquilla intrusives yield adakitic like major and trace element abundances e g 15 wt al 2 o3 generally 3 5 wt na 2 o 400 ppm sr 120 and strongly fractionated hree patterns these 30 26 ma petaquilla and 19 ma maje suites are also compositionally similar to a subvolcanic suite of rare circa 25 ma adakitic like andesitic intrusives which occur within the panama canal basin midway between petaquilla and maje and at the same approximate latitude as petaquilla and maje collectively the geochemical and geochronological data for the adakitic like intrusives arc consistent with formation via partial melting of lowermost mafic crust above a sub horizontal slab tear that propagated from the west petaquilla to the east maje between 30 and 19 ma our new tectonic model postulates that collision between the panamanian magmatic arc system and an indentor e g a tract of thickened buoyant oceanic crust or plateau occurred at about 40 ma a time of which coincides with the initiation of left lateral offset of the early i e 70 40 ma arc system this collision resulted in the shutdown of the early arc system possible steepening of the subducting farallon slab and ultimately slab break off and the phase of mainly adakitic like intrusive activity subsequent to slab removal by 20 ma ne dipping subduction jumped to the south and initiated production of the later i e post 20 ma arc system soon thereafter

constraining landscape history and glacial erosivity using paired cosmogenic nuclides in upernavik northwest greenland high latitude landscape evolution processes have the potential to preserve old relict surfaces through burial by cold based nonerosive glacial ice to investigate landscape history and age in the high arctic we analyzed in situ cosmogenic 10 be and 26 al in 33 rocks from upernavik northwest greenland we sampled adjacent bedrock boulder pairs along a 100 km transect at elevations up to 1000 m above sea level bedrock samples gave significantly older apparent exposure ages than corresponding boulder samples and minimum limiting ages increased with elevation two isotope calculations 26 al 10 be on 20 of the 33 samples yielded minimum limiting exposure durations up to 112 k y minimum limiting burial durations up to 900 k y and minimum limiting total histories up to 990 k y the prevalence of 10 be and 26 al inherited from previous periods of exposure especially in bedrock samples at high elevation indicates that these areas record long and complex surface exposure histories including significant periods of burial with little subglacial erosion the long total histories suggest that these high elevation surfaces were largely preserved beneath cold based nonerosive ice or snowfields for at least the latter half of the quaternary because of high concentrations of inherited nuclides only the six youngest boulder samples appear to record the timing of ice retreat these six samples suggest deglaciation of the upernavik coast at 11 3 0 5 ka average 1 standard deviation there is no difference in deglaciation age along the 100 km sample transect indicating that the ice marginal position retreated rapidly at rates of 120 m yr 1

tectonic evolution of the caribbean and northwestern south america the case for accretion of two late cretaceous oceanic plateaus it is widely accepted that the thickened oceanic crust of the caribbean plate its basaltic accreted margins and accreted mafic terranes in northwestern south america represent the remnants of a single ca 90 ma oceanic plateau we review geologic geochemical and paleomagnetic evidence that suggests that the caribbean colombian oceanic plateau in fact represents the remnants of two different oceanic plateaus both dated as ca 90 ma the first of these plateaus the caribbean plateau formed ca 90 ma in the vicinity of the present day galapagos hotspot northeastward movement of the farallon plate meant that this plateau collided with the proto caribbean arc and northwestern south america 10 m y after the plateau s main phase of formation paleomagnetic evidence suggests that the second of these plateaus the gorgona plateau formed at 268 308s possibly at the site of the present day sala y gomez hotspot over the next 45 m y this plateau was carried progressively northeastward on the farallon plate and collided in the middle eocene with the proto andean subduction zone in northwestern south america the recognition of a second ca 90 ma pacific oceanic plateau strengthens the link between plateau formation and global oceanic anoxic events

70 m y history 139 69 ma for the caribbean large igneous province it is commonly accepted that large igneous provinces are formed through catastrophic volcanic events occurring over vast areas within a few million years at the initiation of hotspots mantle plumes new 40ar 39ar ages 111 139 ma and geochemical results from the nicoya peninsula costa rica extend the age range of volcanism in the caribbean large igneous province to 70 m y 69 139 ma our results are not consistent with the formation of this vast igneous province through a single plume head at the initiation of a mantle plume such as the galapagos instead we propose that multiple oceanic intraplate igneous structures such as plateaus and hotspot tracks were accumulated through the subduction process the igneous structures could be remnants of the earlier history of the galapagos hotspot making it one of the oldest active hotspots on earth alternatively they could have been derived from several spatially distinct mantle melting events that sampled similar source material e g oceanic lithosphere of similar age

oceanic intraplate volcanoes exposed example from seamounts accreted in panama two paleogene ocean islands are exposed in the azuero peninsula west panama within sequences accreted in the early middle eocene a multidisciplinary approach involving lithologic mapping paleontological age determinations and petrological study allows reconstruction of the stratigraphy and magmatic evolution of one of these intraplate oceanic volcanoes from base to top the volcano s structure comprises submarine basaltic lava flows locally interlayered with hemipelagic sediments basaltic breccias shallow water limestones and subaerial basaltic lava gabbros and basaltic dikes were emplaced along a rift zone of the island geochemical trends of basaltic lavas include decreased mg mg mg fe 100 and with time increased incompatible element contents thought to be representative of many poorly documented intraplate volcanoes in the pacific our results show that in addition to deep drilling the roots of oceanic islands can be explored through studies of accreted and subaerially exhumed oceanic sequences

the geology of the darien panama and the late miocene pliocene collision of the panama arc with northwestern south america the geology of the darien province of eastern panama is presented through a new geologic map and detailed biostratigraphic and paleobathymetric analysis of its upper cretaceous to upper miocene sediments the sequence of events inferred from the stratigraphic record includes the collision of the panama arc the southwestern margin of the caribbean plate and south american continent three tectonostratigraphic units underlie the darien region 1 precollisional upper cretaceous eocene crystalline basement rocks of the san blas complex form a series of structurally complex topographic massifs along the northeastern and southwestern margins of the darien province these rocks formed part of a 20 m y submarine volcanic arc developed in a pacific setting distant from the continental margin of northwestern south america the northerly basement rocks are quartz diorites granodiorites and basaltic andesites through dacites to rhyolites indicating the presence of a magmatic arc the southerly basement rocks are an accreted suite of diabase pillow basalt and radiolarian chert deposited at abyssal depths precollisional arc related rocks of eocene to lower miocene age consist of 4000 m of pillow basalts and volcaniclastics and biogenic calcareous and siliceous deep water sediments they consist of the eocene oligocene darien formation the oligocene porcona formation and the lower middle miocene clarita formation postcollisional deposits are mostly coarse to fine grained siliciclastic sedimentary rocks and turbiditic sandstone of upper middle to latest miocene age this 3000 m thick sedimentary sequence is deformed as part of a complexly folded and faulted synclinorium that forms the central chucunaque tuira basin of the darien the sedimentary package reveals general shallowing of the basin from bathyal to inner neritic depths during the 12 8 7 1 ma collision of the panama arc with south america the sediments are divided into the upper middle miocene tapaliza formation the lower upper miocene tuira and membrillo formations the middle upper miocene yaviza formation and the middle to upper miocene chucunaque formation the precollisional open marine units of late cretaceous middle miocene age are separated from the overlying postcollisional sequence of middle to late miocene age by a regional unconformity at 14 8 12 8 ma this unconformity marks the disappearance of radiolarians the changeover of predominantly silica deposition from the atlantic to the pacific the initiation of the uplift of the isthmus of panama and the onset of shallowing upward coarser clastic deposition this pattern is also recorded from the southern limon basin of caribbean costa rica to the atrato basin of northwestern colombia by the middle late miocene neritic depths were widespread throughout the darien region and a regional unconformity suggests completion of the central american arc collision with south america by 7 1 ma no pliocene deposits are recorded from either the darien or the panama canal basin and no sediments younger than 4 8 ma have been identified in the atrato basin of colombia suggesting rapid uplift and extensive emergence of the central american isthmus in the latest miocene northward movement of the eastern segment of the panama arc along a now quiescent panama canal zone fault during eocene oligocene time may have dislocated the pre collision arc since collision the portion west of this fault chorotega block has remained stable without rotation to the east in the darien region compression has been accommodated through formation of a panama microplate with convergent boundaries to the north north panama deformed belt and south south panama deformed belt and suturing with south america along the atrato valley deformation within the microplate has been accommodated in the darien province by several major left lateral strike slip faults that were active until the early pliocene since when the plate has behaved rigidly

mid pleistocene cosmogenic minimum age limits for pre wisconsinan glacial surfaces in southwestern minnesota and southern baffin island a multiple nuclide approach abstract paired 10 be and 26 al analyses n 14 indicate that pre wisconsinan glaciated bedrock surfaces near the northern baffin island and southern minnesota paleo margins of the laurentide ice sheet have long and complex histories of cosmic ray exposure including significant periods of partial or complete shielding from cosmic rays using the ratio 26 al 10 be we calculate that striated outcrops of sioux quartzite in southwestern minnesota southern margin were last overrun by ice at least 500 000 years ago weathered bedrock tors on the once glaciated uplands of baffin island northern margin are eroding no faster than 1 1 m myr 1 the equivalent of at least 450 000 years of surface and near surface exposure our data demonstrate that exposure ages and erosion rates calculated from single nuclides can underestimate surface stability dramatically because any intermittent burial and the resultant lowering of nuclide production rates and nuclide abundances will remain undetected

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

consistent slow exhumation in a late cenozoic glaciated landscape the presidential and carter ranges of the white mountains in new hampshire usa abstract to investigate the ability of the onset of quaternary glaciations to drive an acceleration in erosion rates we compare short timescale 103 104 yr erosion rates using the in situ terrestrial cosmogenic nuclide tcn 10be to longer time scale exhumation rates 106 107 yr using the apatite u th he method in the presidential and carter ranges of the white mountains in new hampshire usa our results are generally consistent with but refine previous studies using similar techniques resulting u th he ages and thermal modeling from mt washington in the presidential range n 4 91 7 to 117 3 ma suggest that exhumation rates from the late cretaceous to the present were slow

elevation changes of the fennoscandian ice sheet interior during the last deglaciation the dynamics of ice sheet interiors during the last deglaciation are poorly constrained hindering evaluation of ice sheet models we provide direct evidence of fennoscandian ice sheet fis interior behaviour during deglaciation through surface exposure dating our results demonstrate early thinning of the fis prior to the younger dryas yd 12 8 11 7 ka interior thinning in central norway was concurrent with retreat along the coastline exposing ice free mountainous tracts potentially as early as 20 15 ka the fis then formed moraines in these ice free tracts during the yd this is contrary to current hypotheses advocating a landscape fully covered by cold inactive ice during this period present empirical and model reconstructions fail to capture rapid interior down wastage increasing uncertainties in ice sheet volume estimates and sea level contributions

creation of the cocos and nazca plates by fission of the farallon plate abstract throughout the early tertiary the area of the farallon oceanic plate was episodically diminished by detachment of large and small northern regions which became independently moving plates and microplates the nature and history of farallon plate fragmentation has been inferred mainly from structural patterns on the western pacific plate flank of the east pacific rise because the fragmented eastern flank has been subducted the final episode of plate fragmentation occurred at the beginning of the miocene when the cocos plate was split off leaving the much reduced farallon plate to be renamed the nazca plate and initiating cocos nazca spreading some oligocene farallon plate with rifted margins that are a direct record of this plate splitting event has survived in the eastern tropical pacific most extensively off northern peru and ecuador small remnants of the conjugate northern rifted margin are exposed off costa rica and perhaps south of panama marine geophysical profiles bathymetric magnetic and seismic reflection and multibeam sonar swaths across these rifted oceanic margins combined with surveys of 30 20 ma crust on the western rise flank indicate that i localized lithospheric rupture to create a new plate boundary was preceded by plate stretching and fracturing in a belt several hundred km wide fissural volcanism along some of these fractures built volcanic ridges e g alvarado and sarmiento ridges that are 1 2 km high and parallel to absolute farallon plate motion they closely resemble fissural ridges described from the young western flank of the present pacific nazca rise ii for 1 2 m y prior to final rupture of the farallon plate perhaps coinciding with the period of lithospheric stretching the entire plate changed direction to a more easterly nazca like course after the split the northern cocos part reverted to a northeasterly absolute motion iii the plate splitting fracture that became the site of initial cocos nazca spreading was a linear feature that at least through the 680 km of ruptured oligocene lithosphere known to have avoided subduction did not follow any pre existing feature on the farallon plate e g a fracture zone trail of a transform fault iv the margins of surviving parts of the plate splitting fracture have narrow shoulders raised by uplift of unloaded footwalls and partially buried by fissural volcanism v cocos nazca spreading began at 23 ma reports of older cocos nazca crust in the eastern panama basin were based on misidentified magnetic anomalies there is increased evidence that the driving force for the 23 ma fission of the farallon plate was the divergence of slab pull stresses at the middle america and south america subduction zones the timing and location of the split may have been influenced by i the increasingly divergent northeast slab pull at the middle america subduction zone which lengthened and reoriented because of motion between the north america and caribbean plates ii the slightly earlier detachment of a northern part of the plate that had been entering the california subduction zone contributing a less divergent plate driving stress and iii weakening of older parts of the plate by the galapagos hotspot which had come to underlie the equatorial region midway between the risecrest and the two subduction zones by the late oligocene

cosmogenic nuclide evidence for minimal erosion across two subglacial sliding boundaries of the late glacial fennoscandian ice sheet abstract the existence of sliding and frozen bed areas under ice sheets is significant in understanding basal thermal regimes patterns of erosion and landform development and in constraining boundary conditions for the reconstructions of ice sheets recognition of subglacial boundaries between sliding and frozen bed areas for former ice sheets is typically based on distinct morphological contrasts between areas with glacial landform assemblages and relict areas showing little alteration of pre existing features some of these boundaries especially on continental shield areas however are clearly visible from air photos but have minimal topographic expression understanding the chronology and erosional development of such boundaries is important to provide insight into the pattern and persistence of basal conditions under ice sheets geomorphic evidence and cosmogenic radionuclide concentrations of bedrock outcrops on either side of two sliding boundaries on ultevis and arvestuottar low relief upland plateaus in northern sweden are consistent with negligible erosion in relict landscape frozen bed areas due to the last glaciation but also indicate insignificant erosion in the sliding areas such a pattern and magnitude of landscape modification indicates that sliding was short lived in these areas likely as a transient phase during deglaciation these sites demonstrate that short periods of sliding are in some cases sufficient to produce landscapes that are recognized as glacial from air photos thus regions of sliding identified on shield areas must be viewed as the cumulative total area that has experienced sliding at any time during a glaciation the actual extent of sliding areas during any single ice sheet phase is presumably considerably less than this cumulative total which has important implications for establishing appropriate basal boundary conditions for ice sheet reconstructions

tectonic evolution of the caribbean the development of the caribbean is discussed in terms of modern tectonic theory the nature of the site on which the caribbean formed is examined and the development of the rifted margins of the caribbean is described constraints on caribbean evolution from the relative motions of north and south america are briefly examined and the caribbean oceanic plateau is discussed the great island arc system of the caribbean is addressed in detail emphasizing the way the great arc of the caribbean was segmented the role of central america in caribbean history is briefly considered

cosmogenic exposure age evidence for rapid laurentide deglaciation of the katahdin area west central maine usa 16 to 15 ka abstract katahdin the highest peak in maine and part of the second highest mountain range in new england provides an opportunity to assess the timing and style of continental ice sheet surface lowering during deglaciation we collected 14 samples from boulders on the adjacent basin ponds moraine from bedrock and boulders on the upper part of the mountain and from boulders in the surrounding area to estimate the age at which they were exposed by deglaciation of the laurentide ice sheet measurements of in situ produced 10be which are consistent with measurements of 26al indicate that the katahdin edifice became exposed from under ice by 15 3 2 1 ka n 6 an age indistinguishable from the adjacent basin ponds moraine 16 1 1 2 ka n 5 a boulder in the lowlands several km south of the moraine dates to 14 5 0 8 ka and a boulder deposited at pineo ridge about 170 km se of katahdin dates to 17 5 1 1 ka these data show that samples collected over an elevation range of 1 6 km and a distance of 170 km all have exposure ages that are indistinguishable within uncertainties together these data suggest that the laurentide ice sheet surface dropped rapidly and the ice sheet margin retreated quickly across maine between about 16 and 15 ka perhaps influenced by calving of the marine based ice sheet in the st lawrence lowlands to the north and the penobscot basin to the south

accumulation of organic matter in the rome trough of the appalachian basin and its subsequent thermal history discussion 666 this discussion is intended to correct a major error of fact concerning the organic carbon concentration of the lower huron member of the ohio shale in eastern kentucky figure 2 of curtis and faure 1997 showed a map of total organic carbon concentrations in the lower huron shale based on data collected for the gas research institute this map is essentially identical to those previously published by curtis 1988 his plate ii and curtis 1990 his figure