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

kinematic setting and structural control of arc volcanism abstract the evaluation of the kinematic setting and the structural control of volcanic arcs are important in defining the tectono magmatic processes along convergent plate boundaries however our knowledge is fragmented and the available data highlight different behaviours this study analyzes the kinematic setting the structural control and the volcanic productivity of 16 arcs these arcs are characterized by predominant extensional compressional strike slip or oblique motions there is an overall coupling between the normal vs parallel motions along the arc and those of the underlying slab therefore the higher the trench normal or parallel component of the subduction rate the higher is the amount of arc normal or parallel motion this relation confirms that strain partitioning is in general feasible at many convergent settings involving also the volcanic arc portion the arc normal motion may be characterized by extension or compression as a function of the increase in the trench normal convergence or subduction velocity the lack of an evident relation between the subduction rate or convergence rate and the volcanic productivity of the arc is partly inconsistent with previous studies which highlight a relation between the subduction rate and melt production below arcs this discrepancy mainly suggests that different processes control the generation rise and eruption of magma at different depths varying the intrusive extrusive ratio along arcs the structure of the arc does not control the distribution of the volcanoes however it does control the volcanic output through different processes even though regional or local extension associated with strike slip or compressive structures is the ultimate requisite in any setting in general the higher the amount of extension the higher is the output rate along the arc

evolution of calc alkaline to alkaline magmatism through carboniferous convergence to permian transcurrent tectonics western chinese tianshan continuous magmatic activity occurred in the western chinese tianshan central asia from the carboniferous to the permian i e before and after the late carboniferous amalgamation of junggar and the yili blocks zircon u pb la icpms and ar ar data reveal a coincidence in time between regional wrench faulting and granitoid emplacement permian post collisional granitoids crop out within or at the margins of large scale dextral strike slip shear zones some of them show synkinematic fabrics the whole rock geochemical features of the early middle permian granitoids indicate an evolution from high k calc alkaline towards alkaline series in other places of the north tianshan alkaline magmatism occurred together with deep marine sedimentation within elongated troughs controlled by wrench faults therefore in contrast with previous interpretations that forwarded continental rift or mantle plume hypotheses the coexistence of diverse magmatic sources during the same tectonic episode suggests that post collisional lithosphere scale transcurrent shearing tightly controlled the magmatic activity during the transition from convergent margin to intraplate anorogenic processes

upper neogene stratigraphy and tectonics of death valley a review abstract new tephrochronologic soil stratigraphic and radiometric dating studies over the last 10 years have generated a robust numerical stratigraphy for upper neogene sedimentary deposits throughout death valley critical to this improved stratigraphy are correlated or radiometrically dated tephra beds and tuffs that range in age from 3 58 ma to

debris slides rapid earth flows in the carbonate massifs of the campania region southern italy morphological and morphometric data for evaluating triggering susceptibility abstract this paper examines the morphological and morphometric properties of debris slides rapid earth flows which have occurred in pyroclastic deposits of the campania region over the last few centuries the slopes bordering the main carbonate massif of western campania are covered by several metres of pyroclastic deposits from the somma vesuvio volcano that due to the steepness of the slopes are easily eroded by surface waters and periodic debris slides rapid earth flows these flows are sudden and unpredictable and have caused both damage to the environment and loss of life over the last few decades after consulting scientific papers and historical documents the areas most recently affected by debris slides rapid earth flows were analysed through aerial photographs and a geological survey an inventory of 172 landslides was produced and landslides were mapped at a scale of 1 2000 for further geomorphological and morphometric analysis morphological analysis was primarily concerned with predisposing factors it confirmed that 86 of the slope failurecrowns developed in response to man made cuts and tracks 44 or due to the presence of cliffs 42 two different kinds of slopes are affected by debris slides rapid earth flows slopes lacking drainage basins and slopes cut by one or several drainage basins typical triangular shaped earth flows with a landslide mass located at the foot of the slope unchannelled debris slides rapid earth flows were found along the former slopes in contrast channelled debris slides rapid earth flows formed along the latter slopes these originated in the drainage divide area developed along the main water segment and invaded the alluvial plain far from the foot of the slope an intermediate group of flows started as triangular shaped and then became channelled morphometric analysis also investigated the parameters most commonly found in the literature slope angle of the crown and sliding zone crown altitude areal extent of the landslide together with other parameters apical angle of the crown zone and landslide body slope relief energy and potential zone of triggering this analysis identified critical values for the steepness of crown zones and for the steepness of the sliding zone critical values of the apical angle of the crown zone for mixed and unchannelled landslides were also established correlations among some of the investigated parameters were investigated in particular the relationships between slope relief energy and areal extent of landslides or the height of the landslide crown zone all the factors and parameters analysed in this paper can be used to identify areas which need to be stabilized or monitored and could be important for planning defensive measures of debris slides rapid earth flows

age and progression of volcanism wrangell volcanic field alaska the wrangell volcanic field covers more than 10 000 km2 in southern alaska and extends uninterrupted into northwest yukon territory lavas in the field exhibit medium k calc alkaline affinities typical of continental volcanic arcs along convergent plate margins eleven major eruptive centers are recognized in the alaskan part of the field more than 90 k ar age determinations in the field show a northwesterly progression of eruptive activity from 26 ma near the alaska yukon border to about 0 2 ma at the northwest end of the field a few age determinations in the southeast extension of the field in yukon territory canada range from 11 to 25 ma the ages indicate that the progression of volcanism in the alaska part of the field increased from about 0 8 km ma at 25 ma to more than 20 km ma during the past 2 ma the progression of volcanic activity and its increased rate of migration with time is attributed to changes in the rate and angle of pacific plate convergence and the progressive decoupling of the yakutat terrane from north america subduction of yakutat terrane pacific plate and wrangell volcanic activity ceased about 200 000 years age when pacific plate motion was taken up by strike slip faulting and thrusting

mantle flow through the northern cordilleran slab window revealed by volcanic geochemistry the northern cordilleran slab window formed beneath western canada concurrently with the opening of the californian slab window beneath the southwestern united states beginning in late oligocene miocene time a database of 3530 analyses from miocene holocene volcanoes along a 3500 km long transect from the northern cascade arc to the aleutian arc was used to investigate mantle conditions in the northern cordilleran slab window using geochemical ratios sensitive to tectonic affi nity such as nb zr we show that typical volcanic arc compositions in the cascade and aleutian systems derived from subduction hydrated mantle are separated by an extensive volcanic fi eld with intraplate compositions derived from relatively anhydrous mantle this chemically defi ned region of intraplate volcanism is spatially coincident with a geophysical model of the northern cordilleran slab window we suggest that opening of the slab window triggered upwelling of anhydrous mantle and displacement of the hydrous mantle wedge which had developed during extensive early cenozoic arc and backarc volcanism in western canada high heat fl ow throughout the western canadian cordillera is broadly coincident with the fi eld of intraplate volcanism and is linked to slab window induced mantle upwelling

exhumation along the fairweather fault southeastern alaska based on low temperature thermochronometry 1 the southern alaskan syntaxis marks the spectacular junction between the 1000 km long pacific north america transform margin and the chugach st elias belt where subduction and terrane accretion drive rapid convergent deformation and rock uplift new low temperature thermochronometry reveals that intense orogenic deformation is not restricted to one side of the syntaxis but extends nearly 300 km south along the dextral fairweather fault apatite and zircon u th he ages as young as 0 9 and 2 0 ma suggest maximum exhumation rates of nearly 2 mm a in close proximity 0 5 mm a along the entire plate margin we estimate that long term rock uplift accommodates 3 mm a of fault normal convergence in this area this suggests that the fairweather fault is slightly transpressive and highly partitioned analogous to the central san andreas fault this convergence only accounts for 1 5 of the obliquity between pacific plate motion and the continental margin however implying the deficit is taken up by 1 2 cm a thrust sinistral motion along the offshore transition fault additionally thermochronometry shows a marked increase in bedrock cooling coincident with onset of heavy glaciation similar to what has been observed in other parts of the pacific northwest the tectonically active fairweather corridor is distinguished however by the magnitude of the acceleration and the depth of exhumation since pliocene climate change

tectonic block motion and glacial isostatic adjustment in southeast alaska and adjacent canada constrained by gps measurements 1 we use data from campaign and continuous gps sites in southeast alaska and the neighboring region of canada to constrain a regional tectonic block model that estimates block angular velocities and derives a self consistent set of fault slip rates from the block motions present day tectonics in southeast alaska is strongly influenced by the collision of the yakutat block our model predicts a velocity of 50 3 0 8 mm a toward n22 9 0 6 w for that block our results suggest that the eastern edge of the yakutat block is deforming along this edge the fairweather fault accommodates a large portion of the pacific north america relative plate motion through 42 9 0 9 mm a of dextral slip further south along the queen charlotte fault our model predicts an average of 43 9 0 6 mm a of dextral slip and a southward increasing amount of transpression strain from the yakutat collision is transferred far to the east of the strike slip system this strain transfer causes the region north of glacier bay to undergo a clockwise rotation south of glacier bay and inboard of the queen charlotte fault a smaller but clearly defined clockwise rotation is observed the heterogeneous block motion north and south of glacier bay may indicate the area is undergoing internal deformation and could explain regional patterns of diffuse seismicity the northern cordillera of canada displays a small northeasterly motion our block model suggests that the entire southeastern alaska northwestern canada margin is mobile

a plutonic brother from another magma mother disproving the eocene foraker mcgonagall pluton piercing point and implications for long term slip on the denali fault

reconciling surface plate motions with rapid three dimensional mantle flow around a slab edge a central axiom of the theory of plate tectonics is that the surface plates are coupled to and carried along by convection in the earth s mantle the situation is rather different at subduction zones however margarete jadamec and magali billen now present calculations that quantify this variance they use three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system to determine the surrounding mantle flow field they show that near a subduction zone edge mantle flow velocities can have magnitudes of greater than ten times the surface plate motions and that the complex mantle flow field is consistent with observations from seismic anisotropy these results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally implying large scale coupling between the mantle and surface plates but the fit is poor at subduction zones here three dimensional numerical models of buoyancy driven deformation for the alaska subduction transform system are used to find the origin of the regional decoupling of flow the results imply local rapid transport of geochemical signatures through subduction zones the direction of tectonic plate motion at the earth s surface and the flow field of the mantle inferred from seismic anisotropy are well correlated globally suggesting large scale coupling between the mantle and the surface plates1 2 the fit is typically poor at subduction zones however where regional observations of seismic anisotropy suggest that the direction of mantle flow is not parallel to3 4 5 6 7 and may be several times faster than6 plate motions here we present three dimensional numerical models of buoyancy driven deformation with realistic slab geometry for the alaska subduction transform system and use them to determine the origin of this regional decoupling of flow we find that near a subduction zone edge mantle flow velocities can have magnitudes of more than ten times the surface plate motions whereas surface plate velocities are consistent with plate motions8 and the complex mantle flow field is consistent with observations from seismic anisotropy5 the seismic anisotropy observations constrain the shape of the eastern slab edge and require non newtonian mantle rheology the incorporation of the non newtonian viscosity9 10 results in mantle viscosities of 1017 to 1018 pa s in regions of high strain rate 10 12 s 1 and this low viscosity enables the mantle flow field to decouple partially from the motion of the surface plates these results imply local rapid transport of geochemical signatures through subduction zones and that the internal deformation of slabs decreases the slab pull force available to drive subducting plates

subduction geometry beneath south central alaska and its relationship to volcanism the southern alaskan margin captures a transition between compression and strike slip dominated deformation accretion of the over thickened yakutat terrane termination of aleutian arc magmatism and the enigmatic wrangell volcanic field the extent of subduction and mantle structure below this region is uncertain with important implications for volcanism we present compressional and shear wave mantle velocity models below south central alaska that leverage a new seismometer deployment to produce the most complete image of the subducting pacific yakutat plate to date we image a steeply dipping slab extending below central alaska to 400 km depth which abruptly terminates east of 145 w there is no significant slab anomaly beneath the nearby wrangell volcanoes a paucity of volcanism is observed above the subducting yakutat terrane but the slab structure below 150 km depth and wadati benioff zone here are similar to those along the aleutian alaska arc features of the mantle wedge or overlying lithosphere are thus responsible for the volcanic gap

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

early miocene arc volcanism in the mexico city basin inception of the trans mexican volcanic belt abstract in central mexico remnants of the early volcanic activity in the trans mexican volcanic belt tmvb are exposed at sierra de guadalupe in the northern part of the mexico city basin and to the south in the tepoztlan malinalco tenancingo and chiltepec areas a few published studies indicate a general early to middle miocene age for some of these volcanic centers we present new geologic geochronologic and geochemical data that refine the timing of this early volcanic activity and its geochemical character shedding light on the geodynamic evolution during this period the sierra de guadalupe is a volcanic complex with effusive and explosive activity that generated andesitic lavas with plagioclase ortho and clinopyroxene plus amphibole phenocrysts as well as dacitic lava flows and domes with plagioclase orthopyroxene amphibole and or biotite phenocrysts pyroclastic and epiclastic lahars deposits are also observed in the northern slopes of this volcanic complex the chemical composition of the sierra de guadalupe rocks is calc alkaline andesitic to dacitic new 40ar 39ar and u pb zircon ages demonstrate that volcanic activity at sierra de guadalupe lasted ca 5 0 ma from ca 20 1 ma to 14 8 ma remnants of volcanic centers found in the tepoztlan malinalco tenancingo and chiltepec areas 80 km to the south show similar compositions and eruptive styles with 40ar 39ar ages ranging between 21 and 16 ma rocks with similar ages and lithologies were also described in deep wells drilled in the mexico city basin as a whole the age distribution shows that early to middle miocene volcanism making up the initial activity of the tmvb migrated toward the north from tepoztlan 310 km from the trench up to pachuca 445 km from the trench and is likely associated with a shallowing the subducting cocos slab underneath this region our results confirm that magmatism in the tmvb began in the early miocene by dehydration of the cocos slab as attested by high ba la and th yb ratios

wrangell benioff zone southern alaska the first unequivocal evidence for the existence of a benioff zone that may be related to the quaternary wrangell volcanoes comes from a set of 86 well located hypocenters for earthquakes smaller than about magnitude 4 in the region between lat 61 and 62 5 n and between long 142 and 145 5 w about half of the earthquakes occur at depths of 25 km or less below 40 km a clearly defined north northeast dipping zone of seismicity here termed the wrangell benioff zone extends to a depth of about 85 km and continues for about 115 km along strike subparallel to the volcanic trend the western end of the zone may be offset from the northern end of the much more active aleutian benioff zone where the benioff zone shoals to 30 to 40 km it becomes nearly horizontal and cannot be clearly distinguished from upper plate seismicity it is uncertain whether the subducted plate segment that contains the wrangell benioff zone is structurally part of the pacific plate or the yakutat block

multiple sulfur isotope signature of early archean oceanic crust isua sw greenland abstract the archean sulfur cycle was different from the present day cycle as the emission of volcanogenic sulfurous gases was the dominant process in the anoxic environment of the early earth this emitted sulfur exhibits mass independently fractionated sulfur isotopes mif s resulting from photochemical reactions in the atmosphere and it differs substantially from unfractionated sulfur in the mantle so far the main focus of multiple sulfur analyses 32 s 33 s 34 s and 36 s was placed on the sedimentary part of the archean sulfur cycle in order to constrain the magmatic part of the sulfur cycle we analyzed the sulfur isotopic composition of oceanic crustal rocks from the ca 3 7 3 8 ga isua supracrustal belt isb differently altered samples were taken from two units 1 the undifferentiated amphibolites ua and 2 the younger amphibolites with boninitic affinity ab the mean values are 34 s crs 0 01 0 65 values range from 0 87 to 1 37 crs chromium reducible sulfur 33 s crs 0 02 0 12 values range from 0 17 to 0 26 36 s crs 0 47 0 06 values range from 0 56 to 0 38 thus the mean isotope values support the assumption that the sulfur isotopic signature reflects the expected near zero signature of their mantle origin however differences in 33 s crs values are discernible and non zero suggesting that different sources are contributing to the isotopic signature an influence of alteration is excluded for all samples as different alteration sensitive geochemical parameters do not show any correlation with the multiple sulfur isotope signatures further it is unlikely that the small magnitudes in 33 s crs are generated by microbial mass dependent processes because of the narrow range of 34 s crs values possible sources contributing an atmospheric mif s signature include seawater sulfate negative 33 s crs values through hydrothermal circulation the assimilation of ocean floor sediments during the ascent of the melt and or a mantle source contamination by subducted oceanic slab

sedimentology of the debris flow dominated warm spring canyon alluvial fan death valley california facies analysis of widely distributed exposures of the 32 6 km2 and 8 1 km long warm spring canyon fan central death valley shows that it has been built principally by debris flow deposits these deposits were derived from a mature panamint range catchment mostly underlain by precambrian mudrock quartzite and dolomite stacked clast rich and matrix supported debris flow lobes of slightly bouldery muddy pebble cobble gravel in beds 20 150 cm thick dominate the fan from apex to toe accounting for 75 98 of most exposures interstratified with the debris flows are less abundant 2 25 of cuts thinner 5 30 cm and more discontinuous beds of clast supported and imbricated pebble cobble gravel deposited by overland flows and gully flows this facies formed by the surficial fine fraction water winnowing of the debris flows primarily during recessional flood stage of the debris flow events two other facies associations make up a small part of the fan the incised channel tract consists of a 250 m wide clast supported ribbon of irregularly to thickly bedded boulder pebble cobble gravel nested within debris flow deposits this channel fill is oriented generally perpendicular to the panamint range front it formed by extensive erosion and winnowing of debris flows deposited within the incised channel into which all water discharge from the catchment is funnelled the limited presence of this facies only straddling the present incised channel indicates that this channel overall has maintained a consistent position on the fan except for slight lateral shifts some caused by strike slip offset fault offset temporarily closed the upper incised channel causing recessional debris flow mud to be ponded behind the dam the other local facies assemblage consists of subrounded to rounded moderately sorted pebble gravel in low angle cross beds that slope both basinwards and fanwards this gravel was deposited in beachface backshore and shoreface barrier spit environments that developed where lake manly impinged on the warm spring fan during late pleistocene time these deposits straddle headcuts into and were derived from erosion of the debris flow deposits wave energy sorted finer sediment from the shore zone concentrated coarser sediment and rounded the coarse to very coarse pebble fraction by selective reworking

volcanism in an arc transform transition zone the stratigraphy of the st clare creek volcanic field wrangell volcanic belt yukon canada the st clare creek volcanic field in the southwestern yukon overlies a tectonic transition in the wrangell volcanic belt between subduction to the northwest in alaska and transform faulting along the duke river fault in the southeast two large polygenetic volcanic centres dominated the miocene landscape of the st clare creek field the 18 16 ma wolverine centre and the 16 10 ma klutlan centre the wolverine centre evolved from a small alkaline shield volcano at 18 ma from which alkaline basalts hawaiites and mugearites erupted to a larger composite volcano between 18 and 16 ma composed of transitional basalt basaltic trachyandesite trachyte and rhyolite lavas and pyroclastic rocks the youngest wolverine lavas are calc alkaline basaltic andesites andesites and hybrid lavas transitional calc alkaline this temporal progression from alkaline through transitional to calc alkaline volcanism is accompanied by a systematic increase in the degree of silica saturation and decrease in fe si nb y and

20th century variations in area of cirque glaciers and glacierets rocky mountain national park rocky mountains colorado usa comparison of historic maps and aerial and ground based photographs for the small cirque glaciers and glacierets of rocky mountain national park in the northern front range of colorado usa indicates modest change during the 20th century the glaciers retreated through the first half of the 20th century advanced slightly from the mid 1940s to the end of the century and have retreated slightly since high interannual variability in area and temporal gaps in data complicate the trends local climate records indicate a lack of systematic change between 1950 and 1975 but significant warming afterwards local topographic effects e g wind redistribution of snow and avalanching are important influences these small glaciers respond to changes in regional climate summer temperature alone is a good predictor of the mass balance of andrews glacier r 0 93 spring snowfall is also an important factor that winter precipitation is not statistically significant supports the notion that these small glaciers gain much snow from wind drift and avalanching making winter snow accumulation almost indifferent to variations in direct snowfall less than expected glacier retreat may be due to increased summer cloudiness

what we know about mars from its impact craters martian impact craters provide insights into formation ages of terrain units nearsurface properties and the planet s degradational history crater size frequency distribution analyses allow determination of terrain unit relative ages and extrapolation of the lunar crater chronology can provide estimates of absolute ages for these units such analyses indicate that mars has been geologically active throughout its history but it also retains extensive regions dating back to ca 4 ga higher cratering rates during the late heavy bombardment lhb helped to create the martian regolith and formation of large impact basins helped to erode the initially denser martian atmosphere and may have created the hemispheric dichotomy low global degradation rates since the end of lhb ca 3 8 ga resulted in well preserved impact crater morphologies the presence of layered ejecta blankets and central pits associated with fresh to moderately eroded craters suggests a volatile rich crust impact crater depth diameter relationships allow estimation of the vertical extent of crustal volatiles to depths greater than those probed by current spacecraft instrumentation temporal changes in subsurface ice reservoir depths are revealed by differences in layered ejecta onset diameter as a function of crater age an understanding of the morphology and morphometry of pristine impact craters facilitates efforts to determine the amount and types of degradation and the periods over which they operated impact craters therefore play a major role in unraveling the climatic and geologic history of mars

the age and origin of miocene pliocene fault reactivations in the upper plate of an incipient subduction zone puysegur margin new zealand

geochemical evidence for the melting of subducting oceanic lithosphere at plate edges most island arc magmatism appears to result from the lowering of the melting point of peridotite within the wedge of mantle above subducting slabs owing to the introduction of fluids from the dehydration of subducting oceanic crust1 volcanic rocks interpreted to contain a component of melt not just a fluid from the subducting slab itself are uncommon but possible examples have been recognized in the aleutian islands baja california patagonia and elsewhere2 3 4 the geochemically distinctive rocks from these areas termed adakites are often associated with subducting plates that are young and warm and therefore thought to be more prone to melting5 but the subducting lithosphere in some adakite locations such as the aleutian islands appears to be too old and hence too cold to melt6 7 this implies either that our interpretation of adakite geochemistry is incorrect or that our understanding of the tectonic context of adakites is incomplete here we present geochemical data from the kamchatka peninsula and the aleutian islands that reaffirms the slab melt interpretation of adakites2 but in the tectonic context of the exposure to mantle flow around the edge of a torn subducting plate we conclude that adakites are likely to form whenever the edge of a subducting plate is warmed or ablated by mantle flow the use of adakites as tracers for such plate geometry may improve our understanding of magma genesis and thermal structure in a variety of subduction zone environments

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

little ice age debris lobes and nivation hollows inside ubehebe crater death valley california analog for mars craters the 250 m deep ubehebe crater is the youngest 1000 years and largest 800 m wide phreatomagmatic crater of the ubehebe maar field of northern death valley california usa the crater is blanketed by loose unconsolidated ejecta debris and prominent tongue shaped steep fronted debris lobes up to 200 m long occur at the base of the crater s steep northeast facing interior slope geomorphological observations sedimentological analysis of outcrops and a ground penetrating radar survey show the largest lobe to be composed of downslope dipping progradational beds of ejecta resulting from repeated downslope avalanching grain flow and cohesive debris flow inconsistent with an origin of the lobe as a singular coherent slump or flow large oval depressions on the upper surface of lobes are interpreted as relict nivation hollows occupied by former seasonal snow patches during recent little ice age cooling c 700 year bp when numerous rock glaciers and other periglacial landforms formed at high elevations in surrounding mountains lobes are interpreted as protalus ramparts formed when loose non cohesive ejecta from the steep unstable backwall of the crater moved downslope across snow patches and accumulated around their outer margins snow patch and lobe formation is considered to be a function of the local aspect related meteorology of the crater s interior involving shading of northeast facing interior slopes radiative cooling and ponding of cold air enhanced snow accumulation in the lee of the steep backwall combined with the availability of large volumes of loose ejecta ubehebe crater and its lobes expand the geomorphic record of cool climate processes in death valley the most well known being the winter ice facilitated sliding boulders of racetrack playa having formed in an extreme environment lobes offer an accessible terrestrial analog for poorly understood debris lobes within martian impact craters

imaging the subsurface stratigraphy in the ubehebe hydrovolcanic field death valley california using ground penetrating radar abstract ground penetrating radar gpr surveys were carried out to collect subsurface images of the basaltic base surge deposits in the ubehebe hydrovolcanic field death valley national park california antennae with frequencies of 50 100 and 200 mhz were used this technique allowed the collection of useful geologic data for example the lower stratigraphic boundary of the pyroclastic deposits can be imaged and their thickness can be estimated different radar responses were also obtained from base surge deposits and underlying sedimentary rocks which enable their recognition where no outcrops are available furthermore gpr data confirmed the presence of small eroded craters which are partially filled by alluvium in this case an unconformity between the overlying horizontally bedded alluvium and the underlying bowl shaped base surge deposits can be recognized within the crater and the thickness of the alluvium estimated common mid point cmp surveys suggested subsurface velocities of the electromagnetic waves in the upper part of these deposits between 0 095 0 1 m ns

extreme southwestern margin of late quaternary glaciation in north america timing and controls well preserved latero frontal moraines in the eastern san bernardino mountains of southern california provide evidence for several glacial advances during the late quaternary and mark the southwesternmost limit of glaciation in the western cordillera using geomorphology and 10be cosmogenic radionuclide dating a succession of moraines from three glaciated valleys is dated to 18 20 ka last glacial maximum 15 16 ka heinrich event 1 12 13 ka younger dryas stade and 5 9 ka early middle holocene these ages substantiate the view that glaciation throughout the american cordilleras was synchronous during the late quaternary furthermore these data show that glacial advances in southern california occur when a significant decrease in summer temperature is coupled with an increase in moisture flux producing high winter snowfall this allows for perennial snow accumulation that may under appropriate conditions persist to form glacial ice

transition from arc to oceanic magmatism at the kamchatka aleutian junction spatial geochemical variations of quaternary lavas erupted along the northern segment of the kamchatka arc are used to trace changes in magma generation across the subducting pacific slab edge the late pleistocene holocene lavas of the northern end of the central kamchatka depression north of the pacific slab edge show strong enrichment in high field strength elements and light rare earth elements relatively unradiogenic strontium and lead but radiogenic neodymium isotope ratios and oxygen isotope compositions similar to those of mid oceanic ridge basalts these geochemical characteristics are distinct from the southern central kamchatka depression volcanoes located above the subducting pacific slab extensive fluid triggered mantle melting dominates magma genesis beneath the largest kamchatka volcanoes in the south whereas low degree decompression melting of the pacific asthenospheric mantle is the major magma generation process north of the pacific slab edge quaternary detachment of the subducted pacific plate fragment resulted in the influx of fertile mantle beneath kamchatka we propose that upwelling and southward flow of this hotter more fertile mantle is the main reason for recent magmatism in northern kamchatka and for the exceptional productivity of the central kamchatka depression volcanoes klyuchevskoy and sheveluch the most active arc volcanoes on earth

debris covered glaciers in the sierra nevada california and their implications for snowline reconstructions abstract ice walled melt ponds on the surfaces of active valley floor rock glaciers and matthes little ice age moraines in the southern sierra nevada indicate that most of these landforms consist of glacier ice under thin ca 1 10 m but continuous covers of rock fall generated debris these debris blankets effectively insulate the underlying ice and greatly reduce rates of ablation relative to that of uncovered ice such insulation explains the observations that ice cored rock glaciers in the sierra actually debris covered glaciers are apparently less sensitive to climatic warming and commonly advance to lower altitudes than do adjacent bare ice glaciers accumulation area ratios and toe to headwall altitude ratios used to estimate equilibrium line altitudes elas of former glaciers may therefore yield incorrect results for cirque glaciers subject to abundant rockfall inadvertent lumping of deposits from former debris covered and bare ice glaciers partially explains an apparently anomalous regional ela gradient reported for the pre matthes recess peak neoglacial advance distinguishing such deposits may be important to studies that rely on paleo ela estimates moreover matthes and recess peak ela gradients along the crest evidently depend strongly on local orographic effects rather than latitudinal climatic trends indicating that simple linear projections and regional climatic interpretations of ela gradients of small glaciers may be unreliable