expansion of ice marginal lakes at the vatnajokull ice cap iceland from 1999 to 2009 abstract outlet glaciers from the vatnajokull ice cap in iceland europe s largest ice cap have been retreating since the end of the little ice age only interrupted by short intervals of advances the net mass balance has been negative since 1995 and ice marginal lakes in front of the outlets have been expanding since then the switch from land based glacier termini to lake terminating glaciers is an eye catching environmental change the heat capacity of the ice marginal lakes and the change into calving glacier fronts accelerates ice loss from vatnajokull landsat 7etm satellite imagery from 1999 to 2009 reveals high resolution snapshots of the lake development during the last decade ice marginal lakes are growing at many outlet glaciers draining the southern sector of vatnajokull this is most pronounced at the large brei amerkurjokull glacier where the jokulsarlon lake expanded from 15 km 2 in 2000 to more than 21 km 2 in 2009 the lakes act as traps for sediment that was earlier transported across outwash plains toward the atlantic coast hence the glacier retreat and sediment trapping enhance coastline retreat and river incision in the outwash plains similar changes although at a much larger scale occurred in many past glaciated regions during the last glacial termination thus the lakes in front of vatnajokull provide important modern analogues for processes that likely impacted global climate in the past

franciscan subduction off to a slow start evidence from high precision lu hf garnet ages on high grade blocks lu hf analyses of garnet from metabasic amphibolite glaucophane schist and eclogite facies blocks from the franciscan complex give highly precise ages that allow us to place new constraints on the early thermal history of the franciscan subduction zone garnets yield 176 lu 177 hf ratios ranging from 1 5 to 28 with the highest ratios from garnets with high spessartine pyrope ratio sulphuric acid leaching sal of garnets revealed the presence of inclusions with significantly higher lu hf ratios than those of garnet itself most likely apatite their removal by sal brings the 176 lu 177 hf ratios in garnets down by as much as 40 this suggests that 176 lu 177 hf ratios of apparently pure garnets can be greatly overestimated due to the presence of such inclusions sm nd garnet analyses were dominated by inclusions mainly sphene and failed to provide precise and accurate age information the oldest lu hf ages are 168 7f0 8 and 162 5f0 5 ma on plagioclase bearing garnet amphibolite from panoche pass and the berkeley hills respectively which suggests initiation of the subduction zone at about 169 ma coeval with the formation of the tectonically overlying coast range ophiolite relatively high temperature conditions persisted for about 14 ma as indicated by 153 4f0 8 ma garnet growth recorded in epidote amphibolite and 157 9f0 7 in eclogite from ring mountain and jenner respectively a 146 7f0 7 ma age was obtained from garnet glacuophane schist metamorphosed at around 400 jc the sequence of ages from central and northern california shows a younging trend with decreasing metamorphic grade which supports previous suggestions that the high grade metamorphic blocks and slices resulted from progressive underthrusting and underplating in a cooling subduction system combining geothermometry with geochronological data allow us to estimate cooling rate along the subduction zone interface from amphibolite to blueschist facies conditions as ca 15 jc ma the thermal history requires high initial geothermal gradients within both the footwall and the hangingwall of the subduction zone and a relatively slow subduction rate of the order of 10 km ma during the initial stages of franciscan subduction such conditions are consistent with initiation of the subduction zone at or close to an oceanic spreading centre the data also suggest slow exhumation rates and significant residence time at depth of the earliest franciscan rocks

a markov model for beach changes on the holderness coast of england markov models offer an objective and quantitative method of assessing beach changes for a stretch of the holderness coast a beach classification scheme was devised and a probabilistic first order matrix model based on surveyed profile data was produced this could describe and predict transitions between beach types and between different time periods different profile types dominated different coastal locations and seasonal variations were seen in order to improve the accuracy of prediction throughout the year a second winter model was added to the original summer one although the models had been prepared independently of wave conditions a comparison of the wave record and beach transitions revealed that waves under 0 3 0 5 m high produced fairly static beaches when waves were between 0 5 and 1 0 m the beach was more dynamic and variable while waves over 1 0 m led to the depletion of the upper beach this was broadly in accordance with published theory markov models have the advantage that they can be adjusted periodically if conditions change and are thus useful for prediction on coasts for which no wave records exist

subduction dynamics as revealed by trench migration 1 new estimates of trench migration rates allow us to address the dynamics of trench migration and back arc strain we show that trench migration is primarily controlled by the subducting plate velocity vsub which largely depends on its age at the trench using the hot and weak arc to back arc region as a strain sensor we define neutral arcs characterized by the absence of significant strain meaning places where the forces slab pull bending and anchoring almost balance along the interface between the plates we show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions vt 0 5vsub 2 3 in cm a 1 in the hs3 reference frame deformation occurs when the velocity combination deviates from kinematic equilibrium balancing the torque components of the forces acting at the trench indicates that stiff old subducting plates facilitate trench advance by resisting bending

a field based classification scheme for gravel beaches a tripartite classification of gravel beaches based upon morphodynamic properties is proposed and demonstrated for 42 new zealand beaches the main advantage of this scheme is that it is based on simple visual classification that can be applied globally in the field and is underpinned by morphodynamic differences between the beach types the three types identified from the results are 1 pure gravel beach 2 mixed sand and gravel beach 3 composite gravel beach pure gravel beaches have steep slopes tan 0 08 0 24 and gravels extending from the storm berm to below mean low water spring tide level mixed sand and gravel beaches have moderate slopes tan 0 04 0 13 with sand and gravel entirely mixed both cross shore and at depth composite gravel beaches have a steep gravel berm fronted by a low angle intertidal terrace with overall beach slopes of tan 0 05 0 14 on composite beaches there is distinct hydrodynamic cross shore sorting of the sand and gravel component these broad types are tested using discriminant analysis and the three classes are shown to be highly significant f 16 24 df 8 p 0 00005 the key discriminating variables are iribarren number beach width average grain size and storm berm height

thermal evolution of convergent plate margins thermal modeling and reevaluation of isotopic ar ages for blueschists in the franciscan complex of california modeling of heat flow through a wedge shaped accretionary complex emplaced beneath the overriding plate at a convergent plate margin provides insight into the tectonic significance of ar isotopic ages and metamorphic progressions across blueschist belts if convergence rates are maintained at rates of a few centimeters per year or more material recrystallized at depth where the accretionary wedge is narrow can be rapidly heated to high temperatures and then rapidly cooled to the low temperatures where ar loss by diffusion and recrystallization effectively ceases maximum temperatures are lower and the periods of heating and cooling are longer at shallower levels where the wedge is wider because of this difference in thermal history k ar and 40ar 39ar ages from blueschists recrystallized in the upper part of the accretionary wedge can be younger by several tens of million years than ages from material initially recrystallized at the same time but at greater depth application of this thermal modeling to the franciscan subduction complex of northern california indicates that the coherent blueschists in the eastern belt typically with 115 to 125 m y ar isotopic ages and the high grade blueschist blocks in the mud matrix melanges of the western portion of the central belt typically with 140 to 150 m y ar isotopic ages could both have formed during a single prolonged period of continuous convergence

identifying the behavioural characteristics of clay cliffs using intensive monitoring and geotechnical numerical modelling abstract coastal monitoring is routinely undertaken to provide an archival record of cliff line movement that can be used in the development and validation of predictive coast retreat and evolution models however coastal monitoring is often purely quantitative in nature and financial necessity requires deployment over extensive coastal sections as a result for local site conditions in particular only limited geomorphological data are available or included during the development of such predictive models this has resulted in many current models incorporating a simplistic or generalised representation of cliff behaviour an approach that progressively loses local credibility when deployed over extensive heterogeneous coastlines this study addresses this situation at a site of extreme coastline retreat holderness uk through the application of intensive monitoring of six representative cliff sections nested within a general geomorphological appraisal of the wider coastline as a whole the data from these surveys have been used to validate a finite difference based geotechnical modelling assessment of clay cliff stability once validated the geotechnical model was used to simulate a range of scenarios that were sufficient to represent the range of topographic hydrogeological geological and littoral conditions exhibited throughout the region our assessment identified that the cliff retreat occurs through the combined influence of direct marine erosion of the cliff with shallow structurally controlled failures or substantial mass failures critically the predisposition to any one of these failure mechanisms arises principally as a result of initial cliff height the results of the numerical modelling have been combined into an empirical slope model that derives the rate of landslide induced retreat that would arise from mass failures under various future scenarios results of this study can be used in the selection and development of retreat models at coastlines of similar physiographic setting to that found at holderness the results represent a key step in linking material deformation properties to the processes of cliff change and the subsequent range of landforms found on clay cliffs as such the results could also be used more generally to illustrate the likely cliff behaviour of other soft rock coastlines

mineralogy and phase relations in the blueschist facies of the black butte and ball rock areas northern california coast ranges microprobe study of minerals in rocks of the blueschist facies from the black butte and ball rock areas of the south fork mountain schist northern california coast ranges is the basis in this report for analysis of reactions and phase relations among lawsonite pumpellyite epidote and associated minerals observed assemblages are intermediate in a progression from those observed elsewhere in high p low t schists of the franciscan e 9 panoche pass where lawsonite is present in nearly all lithologies to the relatively low phigh t assemblages of the shuksan blueschist washington where epidote is ubiquitous two successive reactions in this sequence in order of increasing temperature are l lawsonite na amphibole epidote quartz albite chlorite hzo 2 na amphibole pumpellyite quartz epidote ca amphibole albite chlorite h2o the pressure of metamorphism for the black butte and ball rock areas is estimated from the jadeite content of pyroxene and from the stability of aragonite to have been approximately 7 kbar temperature estimates are not tightly constrained by mineral equilibria but are suggested to be in the range of 250 to 300 c

counterclockwise p t t paths from amphibolites franciscan complex california relics from the early stages of subduction zone metamorphism overprinting mineral relationships in amphibolites from the franciscan complex california suggest metamorphic evolution from high temperature amphibolite facies to the blueschist facies with increasing p t ratio a counterclockwise p t t path early formed pargasitic amphiboles are both crosscut and rimmed by subcalcic hornblendes that are in turn cut or rimmed by sodic amphiboles some amphibolites are overprinted successively by eclogite and blueschist facies assemblages clinopyroxenes are zoned with increasing jadeite component from core to rim multiple generations of phengites are present in the same sample and are more si rich with each successive generation of growth garnets in eclogite overprinted amphibolites are zoned with an initial mgo increase away from the core to a maximum mgo zone followed by mgo decrease toward the rim garnets in other amphibolites show continually increasing mgo from core to rim several samples representative of the variety of franciscan amphibolites are describ

nappes tectonics of oblique plate convergence and metamorphic evolution related to 140 million years of continuous subduction franciscan complex california this paper presents a new synthesis of franciscan complex tectonics with the emphasis on the pre san andreas fault history of these rocks field relations suggest that the franciscan is characterized by nappe structures that formed during sequential accretion at the trench the presence of these structures along with other field relations including the lack of evidence for large offset of conglomerate suites indicates that strike slip fault systems of large displacement 500 km did not cut the franciscan complex during subduction regional geology and comparisons to modern arc trench systems suggest that strike slip faulting associated with oblique subduction took place inboard east of the franciscan in the vicinity of the magmatic arc the franciscan varies along strike because individual accreted elements packets of trench sediment seamounts etc did not extend the full length of the trench different depths of underplating distribution of post metamorphic faulting and level of erosion pro

the mass balance of circum arctic glaciers and recent climate change the sum of winter accumulation and summer losses of mass from glaciers and ice sheets net surface mass balance varies with changing climate in the arctic glaciers and ice caps excluding the greenland ice sheet cover about 275 000 km2of both the widely glacierized archipelagos of the canadian norwegian and russian high arctic and the area north of about 60 n in alaska iceland and scandinavia since the 1940s surface mass balance time series of varying length have been acquired from more than 40 arctic ice caps and glaciers most arctic glaciers have experienced predominantly negative net surface mass balance over the past few decades there is no uniform recent trend in mass balance for the entire arctic although some regional trends occur examples are the increasingly negative mass balances for northern alaska due to higher summer temperatures and increasingly positive mass balances for maritime scandinavia and iceland due to increased winter precipitation the negative mass balance of most arctic glaciers may be a response to a step like warming in the early twentieth century at the termination of the cold little ice age arctic ice masses outside greenland are at present contributing about 0 13 mm yr 1to global sea level rise

the effects of sub blocking temperature metamorphism on the k ar systematics of hornblendes 40ar 39ar dating of polymetamorphic garnet amphibolite from the franciscan complex california the effects of sub blocking temperature metamorphism on the k ar system in hornblende as revealed by 40ar 39ar release spectra have been studied in a polymetamorphic knocker from the franciscan complex fc california a primary amphibolite assemblage of horn blende rutile epidote apatite garnet sphene is variably overprinted by a blueschist facies assemblage of blue amphibole lawsonite chlorite white mica pumpellyite sphene the secondary assemblage formed at a temperature of 370 c below that at which rapid ar diffusion is expected in hornblende hornblendes from three variably altered samples of garnet amphibolite yield total gas ages of 147 to 161 ma but the corresponding plateau ages of 163 0 2 8 160 6 2 2 and 161 8 2 2 ma are identical within error hornblende separates with lower total gas ages come from more highly overprinted rocks have excess k compared to that expected on the basis of electron probe analyses and exhibit anomalously high k ca ratios in the low temperature fractions of their incremental heating spectra the reduced total gas ages result from the presence of thin 2 m sheets of younger white mica in hornblendes from the moderately and highly altered amphibolites the secondary micas are difficult to detect because of their small size and low abundance 2 but because their k content is 50 to 100 times that of the host hornblende they contribute significantly to the k and ar budgets of the sample the mica intergrowths are not removed by normal sample preparation but because the mica inclusions degas at lower temperatures than hornblende during vacuum extraction incremental heating analyses can provide precise cooling ages for the hornblendes as well as useful estimates of the age of the mica inclusions the hornblende separate from the most altered sample contained 20 10 younger blue amphibole replacing hornblende but its plateau age was not significantly affected this is consistent with a replacement process in which k and ar loss from the hornblende are coupled leaving the k ar system undisturbed in relict primary grains the k and ar budgets of the sample are not strongly affected by the blue amphibole because of its very low k content because partial replacement of primary amphibole by high k phyllosilicates occurs in many geological environments effects like those described here could be widespread

catastrophic ice shelf breakup as the source of heinrich event icebergs 1 heinrich layers of the glacial north atlantic record abrupt widespread iceberg rafting of detrital carbonate and other lithic material at the extreme cold culminations of bond climate cycles both internal glaciologic and external climate forcings have been proposed here we suggest an explanation for the iceberg release that encompasses external climate forcing on the basis of a new glaciological process recently witnessed along the antarctic peninsula rapid disintegrations of fringing ice shelves induced by climate controlled meltwater infilling of surface crevasses we postulate that peripheral ice shelves formed along the eastern canadian seaboard during extreme cold conditions would be vulnerable to sudden climate driven disintegration during any climate amelioration ice shelf disintegration then would be the source of heinrich event icebergs

petrology of amphibolite facies mafic and ultramafic rocks from the catalina schist southern california metasomatism and migmatization in a subduction zone metamorphic setting the catalina schist of southern california is a subduction zone metamorphic terrane it consists of three tectonic units of amphibolite high p greenschist and blueschist facies rocks that are structurally juxtaposed across faults forming an apparent inverted metamorphic gradient migmatitic and non migmatitic metabasite blocks surrounded by a meta ultramafic matrix comprise the upper part of the catalina amphibolite unit fluid rock interaction at high p high t conditions caused partial melting of migmatitic blocks metasomatic exchange between metabasite blocks and ultramafic rocks infiltration of silica into ultramafic rocks and loss of an albitic component from nonmigmatitic clinopyroxene bearing metabasite blocks partial melting took place at an estimated p 8 11 kbar and t 640 750 c at high h2o activity the melting reaction probably involved plagioclase quartz trondhjemitic melts were produced and are preserved as leucocratic regions in migmatitic blocks and as pegmatitic dikes that cut ultramafic rocks the metasomatic and melting processes reflected in these rocks could be analogous to those proposed for fluid and melt transfer of components from a subducting slab to the mantle wedge aqueous fluids rather than melts seem to have accomplished the bulk of mass transfer within the mafic and ultramafic complex

a simple model for the influence of push morainal banks on the calving and stability of glacial tidewater termini push morainal banks at the grounding lines of tidewater termini of temperate glaciers are the source of two types of restraining forces operating at the glacier terminus horizontal normal forces derive from the lateral support and transport of the bank of sediment at the terminus whereas a horizontal shear force operates along the base of a bank pushed in front of an advancing glacier the simple model we present suggests that bank related restraining forces are significantly larger than the restraining force derived from the hydrostatic pressure of water adjacent to the submerged terminus of a glacier during glacier advance restraining forces continually increase resulting in decreasing flow rates glacier thickening and the eventual cessation of advance during retreat restraining forces continually decrease resulting in increasing flow rates glacier thinning and the potential for unstable rapid sustained retreat the normal seasonal oscillatory advance retreat cycle of a glacier is moderated by restraining forces associated with push moraines unstable retreat is likely initiated when bank related restraining forces fall below some threshold value during the seasonal retreat cycle calving is not a primary cause of glacier retreat but is more likely a short term response to increased flow rates increased flow rates result in glacier thinning and an approach toward buoyancy both of which fluctuate seasonally in accordance with bank related restraining forces

large pressure gap between the coastal and central franciscan belts northern and central california abstract the nature of the metamorphism of the central belt of the franciscan complex has long been debated did the whole central belt or its melange matrix undergo high p t metamorphism to solve this problem we have investigated the regional distribution of aragonite in these rocks in northern and central california we have discovered more than 300 new localities of aragonite using the staining method by friedman 1959 aragonite occurs as network veins or pods in both metagraywacke and matrix forming metashale and is invariably partially replaced by calcite on its rim thus we conclude that aragonite is stable in all parts of the franciscan central belt from the lower boundary between the coastal and central belts to the topmost part of the central belt and that both the matrix forming melange shale and exotic blocks of the central belt suffered high p t metamorphism we traced the boundary between the central and coastal belts in northern california using the staining method along the gently eastward dipping thrust plane between the underlying coastal and overlying central belts more than 2 5 kbar pressure break is obtained through the lawsonite laumontite aragonite quartz relations such an observed large pressure gap was formed during the exhumation process of the central belt of the franciscan complex

storminess and high tide beach change stanwell park australia 1943 1978 abstract coastal storms have been considered significant agents in transporting sediment modifying morphology and causing recent beach erosion along the new south wales coast the concomitance of storms warmer sea surface temperatures and poleward movement of the hadley cell was linked to beach erosion on stanwell park beach between 1943 and 1978 this result was defined using an accurately constructed compilation of coastal storms and a precisely measured time series of high tide positions taken from 105 oblique photographs the two data sets are amongst the best of their kind in the world indices of storm magnitude representing cumulative significant wave height were constructed at quarterly intervals and for the year prior to each photograph the quarterly and cumulative storm indices explain 9 9 and 16 6 respectively of the high tide variance at stanwell park each unit increase in the latter index equivalent to an additional 1 m in storm wave height shifts the high tide position shoreward by 0 47 m this is equivalent to the effect produced by a 1 cm rise in sea level while storminess is a crucial variable controlling high tide position on this beach its role cannot be separated easily from that produced by rainfall sea levels sea surface temperatures and atmospheric circulation along the east australian coast and across the pacific ocean over 40 of the high tide data variance on stanwell park beach can be accounted for in decreasing order of importance by rainfall storminess tropical pacific ocean air circulation and sea level these results have implications for other beaches in that single factors such as increased storminess or rising sea levels may not necessarily be the best factors for explaining recent worldwide beach erosion

storm generated variations in nearshore beach topography abstract a series of nearshore beach profile measurements from the outer banks of north carolina spanning a four month period have been examined for temporal variations in nearshore topography principal component analysis of the profile data indicates that most of the variation in nearshore topography occurs in four principal modes two quasiseasonal and two subseasonal the first principal component or eigenvector corresponds to a bar berm function the second to a terrace function combined the first two vectors explain 76 3 of the total variance the third and fourth components representing subseasonal modes are a ridge and runnel and a storm bar function respectively both occur in direct response to storm wave activity although the bar berm and terrace modes of profile variation have been previously identified using principal component analysis techniques the subsequent modes have not the ridge and runnel function accounts for 10 6 of total profile variability and the storm bar function accounts for 5 0

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

sediment deformation beneath glaciers rheology and geological consequences experiments beneath breidamerkurjokull in iceland have led to development of flow laws for the subglacial till relating strain rate to shear stress and effective pressure and assuming either bingham fluid or nonlinearly viscous fluid behavior water pressures in the till are less than ice pressures and it is suggested that this may lead to infiltration of ice into the sediment which inhibits sliding at the ice sediment interface where water pressures are equal or near to ice pressures infiltration does not occur and sliding may result a one dimensional theory of subglacial deformation is developed in which the empirical flow law is coupled with a model of subglacial hydrology and consolidation this predicts stable states in which subglacial sediment either does not deform or a dilatant deforming horizon forms with positive effective pressures at the ice bed interface or unstable states where zero or negative effective pressures are predicted time dependent analyses show that response times following perturbations of the system may be of the order of 103 years and thus that unsteady behavior may be normal on glaciers flowing over unlithified sediment beds it is suggested that the natural variability of material properties in subglacial sediment beds leads to the development of drumlins on the glacier bed it is suggested that unstable deformation at zero or negative effective stress leads to piping in subglacial sediments at the glacier terminus and the growth of sediment floored subglacial tunnels their frequency is that which is sufficient to draw down subglacial water pressures so as to prevent unstable deformation where they discharge large water volumes subglacial sediments flow laterally toward them producing tunnel valleys this sediment is then removed by water flowing along the axial tunnel tunnel valleys can be regarded as the equivalent in soft sediment areas of eskers in bedrock areas

morphodynamics of a macrotidal beach abstract an intensive field investigation of hydrodynamic processes processes of sediment entrainment and suspension and morphologic change was carried out on an unprotected macrotidal beach near broome in northwestern australia the spring tide range was 9 5 m waves had heights of 0 5 1 2 m and periods of 9 13 s the beach had an overall concave upward profile with low gradient and dissipative subtidal and low tidal zones and steeper more reflective mid tidal and high tidal zones direct measurement of energy flux dissipation over the intertidal profile showed dissipation rates on the order of 1 2 w m 2 of bed and indicated an approximate balance between shoaling and dissipation of unbroken waves so as to maintain a constant wave height time averaged predictive estimates of wave work over the lunar half cycle for different points on the intertidal profile show similar dissipation rates and reveal a relatively uniform distribution of work over most of the profile but with maxima in the middle of the low tidal zone and over the lower part of the high tidal zone most of the work over the low and mid tidal zones was performed by unbroken shoaling waves rather than by surf zone processes surf zone processes only dominate over the high tidal zone the nature of the surf zone processes varied across the profile as local gradient and degree of reflectivity changed with changing tide level the growth of standing waves and infragravity surf beat oscillations as identified from spectra and cross spectra of surface elevation and currents u and v was inhibited over most of the profile however well developed secondary standing wave energy particularly at infragravity frequencies was observed over the high tidal zone at spring high tide and over the mid tidal zone at neap high tide over the low tidal and subtidal zones strong shore parallel tidal currents were subordinate only to the orbital velocities of unbroken incident waves over the subtidal zone asymmetrical tidal currents skewed toward the north attained maximum speeds of 0 5 m s 1 just after high water field measurements of suspended sediment concentration profiles under broken and unbroken waves showed very good fit to a diffusion model for wave induced sediment suspension and suggested that sediment suspension was probably attributable largely to waves northerly advection of wave suspended sediment by asymmetrical tidal currents over the subtidal and low tidal zones accounted for a net northerly longshore transport the greatest morphologic mobility of the intertidal profile occurred over the lower high tidal and upper mid tidal zones corresponding to the position of the coarsest material and secondary maximum of time averaged wave work and to a beach state intermediate between the reflective and dissipative extremes temporally the greatest mobility of the profile as a whole was observed on the short within tidal cycle time scale net changes over the longest time scale of a lunar half cycle were negligible

characteristics of recessional moraines at a temperate glacier in se iceland insights into patterns rates and drivers of glacier retreat icelandic glaciers are sensitive to climate variability on short term timescales owing to their north atlantic maritime setting and have been undergoing ice marginal retreat since the mid 1990s recent patterns rates and drivers of ice frontal retreat at skalafellsjokull se iceland are examined using small scale recessional moraines as a geomorphological proxy these small scale recessional moraines exhibit distinctive sawtooth planform geometries and are constructed by a range of genetic processes associated with minor ice margin re advance including i combined push squeeze mechanisms ii bulldozing of pre existing proglacial material and iii submarginal freeze on remote sensing investigations and lichenometric dating highlight sequences of annually formed recessional moraines on the northern and central parts of the foreland conversely moraines are forming on a sub annual timescale at the southeastern skalafellsjokull margin using annual moraine spacing as a proxy for annual ice margin retreat rates imrrs we demonstrate that prominent periods of glacier retreat at skalafellsjokull are coincident with those at other icelandic outlet glaciers as well as those identified at greenlandic outlet glaciers analysis of imrrs and climate data suggests summer air temperature sea surface temperature and the north atlantic oscillation have an influence on imrrs at skalafellsjokull with the glacier appearing to be most sensitive to summer air temperature on the basis of further climate data analyses we hypothesise that sea surface temperature may drive air temperature changes in the north atlantic region which in turn forces imrrs the increase in sea surface temperature over recent decades may link to atmospheric driven variations in north atlantic subpolar gyre dynamics

calving processes and the dynamics of calving glaciers abstract calving of icebergs is an important component of mass loss from the polar ice sheets and glaciers in many parts of the world calving rates can increase dramatically in response to increases in velocity and or retreat of the glacier margin with important implications for sea level change despite their importance calving and related dynamic processes are poorly represented in the current generation of ice sheet models this is largely because understanding the calving problem involves several other long standing problems in glaciology combined with the difficulties and dangers of field data collection in this paper we systematically review different aspects of the calving problem and outline a new framework for representing calving processes in ice sheet models we define a hierarchy of calving processes to distinguish those that exert a fundamental control on the position of the ice margin from more localised processes responsible for individual calving events the first order control on calving is the strain rate arising from spatial variations in velocity particularly sliding speed which determines the location and depth of surface crevasses superimposed on this first order process are second order processes that can further erode the ice margin these include fracture propagation in response to local stress imbalances in the immediate vicinity of the glacier front undercutting of the glacier terminus by melting at or below the waterline and bending at the junction between grounded and buoyant parts of an ice tongue calving of projecting submerged ice feet can be regarded as a third order process because it is paced by first or second order calving above the waterline first order calving can be represented in glacier models using a calving criterion based on crevasse depth which is a function of longitudinal strain rate modelling changes in terminus position and calving rates thus reduces to the problem of determining the ice geometry and velocity distribution realistic solutions to the problem of modelling ice flow therefore depend critically on an appropriate choice of sliding law models that assume that basal velocities are controlled by basal drag can replicate much of the observed behaviour of calving glaciers with grounded termini but an important limitation is that they cannot be used to model floating glacier termini or ice shelves alternative sliding laws that parameterise drag from the glacier margins provide more flexible and robust ways of representing calving in ice sheet models such models can explain a remarkable range of observed phenomena within a simple unifying framework including downglacier increases in velocity and strain rates where basal and or lateral drag diminishes flow acceleration in response to thinning through time the tendency for glaciers to stabilise at pinning points in relatively shallow water or fjord narrowings the constraints on ice shelf stability and the contrasts in calving rates between tidewater and freshwater calving glaciers many unresolved issues remain however including the role played by the removal of backstress in the acceleration of retreating calving glaciers and the controls on melting at and below the waterline

zagros blueschists episodic underplating and long lived cooling of a subduction zone pressure temperature time p t tp t t trajectories of high pressure rocks provide important constraints to understand the tectonic evolution of convergent margins new field observations and p t tp t t constraints for the evolution of the only known blueschist facies exposure along the zagros suture zone in southern iran are reported here these blueschists now exposed in tectonic windows under the sanandaj sirjan zone upper plate constitute accreted fragments of the tethyan domain during n directed cretaceous subduction three units were identified in the field from top to bottom the ashin unit mafic and felsic gneisses the seghin complex mafic tuffs and ultramafics and the siah kuh massif coherent volcanic edifice microstructural observations p tp t estimates and rb sr deformation ages indicate that the ashin unit possibly underwent burial down to 30 35 km and 550 c along a relatively warm p tp t gradient c 17 km and was ultimately deformed between 85 and 100 ma the seghin complex exhibits remarkably well preserved hp lt assemblages comprising lawsonite glaucophane aragonite omphacite and garnet p t tp t t reconstruction indicates that this slice was subducted down to c 50 km at temperatures of c 500 c along a very cold subduction gradient c 7 km deformation in the seghin complex stopped at around 65 ma close to peak metamorphic conditions field relationships and estimates of the p tp t trajectory followed by the siah kuh volcanic edifice indicate that this massif was lately subducted down to 15 km depth along the same very cold gradient this slice stack represents a well preserved field example i highlighting the existence of transient underplating processes juxtaposing pluri kilometric tectonic slices along the subduction channel and ii imaging the discontinuous down stepping of the active main subduction thrust with ongoing accretion the zagros blueschists also record an apparent cooling of the zagros subduction zone between 90 and 65 ma most likely as a thermal response of a geodynamic perturbation toward the end of the cretaceous

short term flow dynamics of a retreating tidewater glacier leconte glacier alaska u s a an analysis of motion in the terminus region of leconte glacier alaska u s a delineates mechanisms that are important to tidewater glacier stability this glacier is undergoing rapid retreat since 1995 it has retreated 2 km and thinned 125 m at the present location of the terminus ice velocities in this region are quite high at the terminus they exceed 27 m d 1 our analysis reveals that fluctuations in speed and surface elevation are forced by ocean tides surface melt and precipitation the near terminus ice speed is 180 out of phase with the tide such that high tide corresponds to low speed smaller melt forced diurnal variations in speed are present throughout the lowermost 7 km speed ups in excess of 10 of the mean often occur after rainstorms as a result of increased basal water pressure and storage but the relation is not simple the velocity does not vary over the spring and summer seasons

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

sedimentation beneath ice shelves the view from ice stream b abstract ice shelf development is favored by rapid flow of cold ice from outlet glaciers or ice streams into protected embayments with localized high spots basal melting of ice shelves is rapid near the ice front and may occur near the grounding line ice from outlet glaciers may contain significant englacial debris that is deposited as a dropstone diamicton in regions of basal melting englacial debris is sparse or absent in ice streams evidence from ice stream b draining into the ross ice shelf of west antarctica suggests that the rapid ice velocity arises from deformation of a several meter thick water saturated basal till layer that is eroding an unconformity on sediments beneath and that has deposited a till delta tens of meters thick and tens of kilometers long at the grounding line sea level fall would cause conveyor belt recycling of this till delta and grounding line advance across the ross sea to the edge of the continental shelf forming an ice sheet with a low ice stream profile resting on a several meter thick deforming till layer eroding an unconformity the modern ross sea is characterized by a regional unconformity overlain by a diamicton of probable latest pliocene pleistocene age measuring several meters to tens of meters thick we hypothesize that this diamicton is a deformed glacial till and that the ross sea sediments record one or more expansions of the till lubricated west antarctic ice sheet to the edge of the continental shelf

relationship between tidewater glacier calving velocity and water depth at the calving front an analysis of the relationship between iceberg calving rates and water depth has been completed for 22 tidewater glaciers a linear relationship provides reasonable accuracy with a correlation coefficient of 0 85 for all tidewater glaciers examined whether they be polar or temperate the polar glaciers have a slightly lower calving rate for a given water depth this relationship indicates a lower calving rate for water depths over 50 m than determined by brown and others 1982 it is based only on glaciers or ice streams and cannot be applied to ice shelves

short term changes in the morphodynamic states of beaches and surf zones an empirical predictive model a time series of 6 5 years of daily observations of beach states and wave characteristics from southeastern australia was analyzed with the aim of determining the degree to which time varying beach state can be explained and predicted the six commonly occurring beach states dissipative longshore bar trough rhythmic bar and beach transverse bar and rip low tide terrace and reflective discussed earlier were related by means of discrete discriminant analyses to the parameter hb wst where hb is breaker height ws is the fall velocity of the beach sand and t is peak breaker period using a total of 1545 cases two values were used in the analyses the immediate value occurring on the day the particular state was observed and a weighted mean value expressing recently antecedent conditions the immediate value made a negligible contribution to explaining day to day beach state observations however the antecedent conditions showed a strong relationship and provide a successful means of prediction by examining cases where the time derivatives of both state and were near zero it was possible to define the equilibrium conditions associated with each state directions of change erosional or accretionary are predicted in terms of departures of from the equilibrium value appropriate to the beach state prevailing at the time change begins

provenance and thermal history of the franciscan accretionary complex constraints from zircon fission track thermochronology the franciscan accretionary complex of california contains large belts of sediments that were subducted to depths of 10 to 30 km metamorphosed under high pressure and low temperature conditions and later returned to the surface reconnaissance zircon fission track ft data 21 samples place constraints on the sedimentary provenance thermal history and or exhumation history of various franciscan units in a transect across the northern coast ranges through mendocino pass metagraywacke samples from the valantine springs formation of the franciscan eastern belt experienced maximum temperatures tmax ranging from 250 to 310 c zircon ft and previously reported apatite ft data indicate that parts of the western eastern and eastern central belts cooled at about 100 70 ma from 300 c to 100 c considering the low paleogeothermal gradient of roughly 9 c km during franciscan subduction these rocks were apparently uplifted and exhumed from 30 to 10 km depth at this time one sample from the eyler mountain sequence in the northwestern diablo range similarly shows tmax 310 c and major cooling at 100 70 ma this uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the franciscan wedge accompanied by extensional thinning at shallower levels as a result of massive sediment influx into the franciscan trench from the sierra nevada magmatic arc in the northern coast ranges the western part of the central belt and the coastal belt experienced lower tmax probably 200 c and retain source area detrital zircon ft ages burnt hills terrane samples in the diablo range also experienced tmax 230 c and retain detrital ages the detrital single grain age patterns from these areas are compatible with those expected from the sierra nevada magmatic arc and its country rocks this is consistent with derivation of franciscan sediments from arc terranes in california and provides no particular suggestion that these parts of the franciscan are highly allochthonous paleogene coastal belt sandstones also yield detrital ages compatible with derivation from the sierra nevada but also include early tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the idaho batholith

thermo mechanical controls of flat subduction insights from numerical modeling this study was supported by national basic research program of china 2014cb440901 the strategic priority research program b of cas xdb18020104 national science foundation of china 41190073 41372198 and 41304071 and nerc grant ne j021822 1

surges of glaciers in iceland surges are common in all the major ice caps in iceland and historical reports of surge occurrence go back several centuries data collection and regular obser vationover thelast several decadeshave permitteda detailed descriptionof several surges fromwhich it is possible to generalize onthe nature of surging in icelandic glaciers com bining the historical records of glacier front variations and recent field research we sum marize the geographic distribution of surging glaciers their subglacial topography and geology the frequency andduration of surges changes in glacier surface geometry during the surge cycle and measured velocity changes comparedto calculatedbalance velocities wenotetheindicatorsofsurgeonsetanddescribechangesinice waterandsedimentfluxes duringa surge surgesaccomplishasignificantfractionofthetotalmasstransportthrough the main outlet glaciers of ice caps in iceland and have important implications for their hydrology ouranalysisof the data suggeststhat surge type glaciersin icelandarecharac terizedbygentlysloping surfacesandthatthey movetoo slowlytoremain inbalance given their accumulation rate surge frequency is neither regular nor clearly related to glacier size or massbalance steeply sloping glaciers whetherhard or soft bedded seemto move sufficiently rapidly to keep inbalance with the annual accumulation

scales and rates of glacial sediment removal a 20 km long 300 m deep trench created beneath breiðamerkurjokull during the little ice age

excess argon in k ar and ar ar geochronology the k ar and ar ar dating techniques occasionally produce anomalously old ages attributed to excess argon and such data is often rejected as not offering any insight into the age thermal history or geochemistry of the rock however improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems solubility data for argon in hydrous fluids melts and emerging data for minerals can be used to understand the behaviour of excess argon and provide valuable insights into the environment in which the samples cooled to their argon retention or closure temperature treating excess argon as a trace element also throws light on its behaviour in minerals above the closure temperature in deeply buried dry systems such as eclogites blueschists granulites and even in the lithospheric mantle extremely low partition coefficients between k feldspar and hydrous fluid phases predict lower excess argon susceptibility than micas and this is observed in fluid poor systems variation of partition coefficients can lead to excess argon in fluids being introduced into minerals or removed from minerals as grain boundary fluids change during flow through a rock however excess argon can also be introduced or removed from minerals by varying temperature without the need for fluid flow high mineral melt and mineral fluid partition coefficients are also the reason why excess argon is often concentrated in inclusions within minerals partition coefficients between minerals and hydrous fluids as low as 10 6 lead fluid inclusions to dominate the radiogenic argon budget particularly in low potassium minerals melt inclusions are less dominant but become critical in dating younger samples

creation and preservation of subduction related inverted metamorphic gradients peak metamorphic temperatures and recrystallization increase structurally upward in the lower plate of several paleosubduction zones in the westernmost u s cordillera inverted metamorphic gradients are preserved in the pelona orocopia schist southern california the catalina schist california borderland the central metamorphic belt klamath province the south fork mountain schist northern california coast ranges and possibly the shuksan suite northern washington with the exception of the south fork mountain schist peak metamorphic temperatures increase structurally upward from 400 c to 650 c over 1 2 km indicating metamorphic gradients in excess of 100 c km 1 estimates of metamorphic gradients in some terranes such as the catalina schist depend critically on the effects of postmetamorphic faulting the tectonic settings and high metamorphic pressures 800 mpa suggest that the inverted metamorphic gradients formed in paleosubduction zones four out of five inverted metamorphic gradients lie structurally beneath ultramafic hanging walls whereas the pelona orocopia schist formed beneath granitic crystalline rocks the high temperatures recorded by the metamorphic rocks strongly suggest that inverted metamorphic gradients form during the early stages of subduction and thus provide insight into the early thermal history of subduction zones two separate two dimensional numerical models were constructed in order to constrain the thermal conditions under which inverted metamorphic gradients are created and preserved rapid subduction 10 cm yr 1 beneath young oceanic lithosphere 10 ma results in rapid heat conduction downward from the hanging wall and the creation of inverted thermal gradients in excess of 100 c km 1 in the top portion of the downgoing slab in order to be preserved in the geologic record the inverted metamorphic gradient must be accreted to the base of the hanging wall accretion of the upper portion of the subducting slab may occur because of declining temperatures that cause downward migration of the viscous slip zone and or greenschist facies devolatilization reactions that weaken the descending slab alternatively inverted metamorphic gradients may represent the continuous accretion of material at rates of 1 10 mm yr 1 under conditions of declining temperature continuously accreted inverted metamorphic gradients could form in oceanic lithosphere as old as 30 ma continued subduction of 1000 km of oceanic lithosphere effectively refrigerates the early accreted inverted metamorphic gradient synsubduction uplift and or moderately fast postsubduction uplift rates of 1 mm yr 1 are required to preserve the inverted metamorphic gradient the models predict that 1 the high grade metamorphic rocks in inverted metamorphic gradients form early in the subduction process and therefore effectively date the initiation of subduction and 2 inverted metamorphic gradients mark sites of major plate convergence in excess of 1000 km

crystallographic preferred orientation of talc determined by an improved ebsd procedure for sheet silicates implications for anisotropy at the slab mantle interface due to si metasomatism

graphitization in a high pressure low temperature metamorphic gradient a raman microspectroscopy and hrtem study the graphitization of carbonaceous material cm in a high pressure metamorphic gradient is characterized along a cross section in the schistes lustres formation western alps along this 25 km cross section both the cm precursor and the host rock lithology are homogeneous and the prograde evolution of the pressure temperature metamorphic conditions from the lower blueschist facies 13 kbar 330 c to the eclogite facies 20 kbar 500 c is tightly constrained by literature data raman microspectroscopy shows that at the micrometre scale this process is progressive and continuous with increasing metamorphic grade and that the structure of cm is very sensitive to temperature variations at the nanometre scale hrtem the cm is composed of a mixture of a microporous phase and an onion ring like phase both known as non graphitizing under the effect of temperature at ambient pressure the hp lt graphitization produces structurally and microtexturally heterogeneous cm with increasing metamorphic grade the graphitization of the two types of cm proceeds up to the triperiodic graphite stage because of microtextural and structural changes that are specific to each type of cm the microporous material is progressively transformed into graphite through a macroporous transitional stage in this case graphitization mainly occurs on the pore walls as a result of pore growth in the case of concentric onion ring like material graphitization occurs in the regions with the largest radius of curvature i e on the outer part of the ring in comparison with 1 bar experiments pressure seems to induce microtextural changes which allows the subsequent structural modifications of the starting material

a melange of subduction temperatures evidence from zr in rutile thermometry for strengthening of the subduction interface abstract the catalina schist contains a spectacular km scale amphibolite facies melange zone thought to be part of a cretaceous convergent margin plate interface in this setting blocks ranging from centimeters up to 100 m in diameter are surrounded by finer grained matrix that is derived from the blocks blocks throughout the melange represent a diversity of protoliths derived from basalts cherts and other sediments and hydrated mantle but all contain assemblages consistent with upper amphibolite facies conditions suggesting a relatively restricted range of depths and temperatures over which material within the melange was metamorphosed this apparent uniformity of metamorphic grade contrasts with other melanges such as the franciscan complex where coexisting rocks with highly variable peak metamorphic grade suggest extensive mixing of materials along the subduction interface this mixing has been ascribed to flow of material within relatively low viscosity matrix the zr content of rutile in samples from across the amphibolite facies melange of the catalina schist was measured to determine peak metamorphic temperatures identify whether these temperatures were different among blocks and whether the spatial distribution of temperatures throughout the melange was systematic or random resolvably different zr contents between 290 and 720 10 40 ppm are found among the blocks corresponding to different peak metamorphic temperatures of 650 to 730 2 16 c at an assumed pressure of 1 gpa these results are broadly consistent with previous thermobarometric estimates no systematic distribution of temperatures was found however like other melange zones material flow within the catalina schist melange was likely chaotic but appears to have occurred on a more restricted scale compared to some other localities progressive metamorphism of melange matrix is expected to produce rheologically stiffer matrix minerals such as amphiboles and pyroxenes at the expense of weaker matrix minerals sheet silicates affecting the overall rheological behavior of the melange and dictating the scale of flow the catalina schist amphibolite facies melange matrix appears to provide a snapshot of hotter stiffer portions of a subduction interface perhaps more representative of rheological behavior at depths approaching the subarc than is found in some other exhumed melange zones

the morphodynamic effects of incident wave groupiness and tide range on an energetic beach abstract the roles played by daily tide range and incident wave groupiness in affecting the short term morphodynamic responses of an energetic beach are examined demodulation of original tide and wave time series from the moderate energy narrabeen beach australia resulted in new time series of daily tide range and a daily grouping factor which expresses the relative amplitude of the alternation between groups of high and low waves using these new time series together with time series of daily beach state and changing breaker conditions statistical analyses were performed aimed at determining the contributions made by tide and groupiness in explaining time varying beach state additional to those made by time varying wave conditions eigenvector analyses of monthly surveyed beach and surf zone profiles provided a set of time series on modes of profile configuration change which were also analyzed to determine the relative effects of waves and tide range on profile shape both tide range and groupiness are significant determinants of beach state the six beach states are reflective low tide terrace transverse bar and rip rhythmic bar and beach longshore bar trough and dissipative wright and short 1984 provided that wave conditions are within the appropriate range to favor beach states at the reflective end of the sequence spring tides will tend to favor the low tide terrace over the transverse bar and rip state higher tide ranges are also associated with more subdued bar trough topography the more accentuated bar trough profiles are developed when tide range is minimal overall higher relative incident wave groupiness favors the more dissipative states incident wave groupiness is the dominant factor determining whether the low tide terrace state or transverse bar and rip state prevails provided incident wave steepnesses permit one or the other of these states groupiness is also important in discriminating between the transverse bar and rip and rhythmic bar and beach states

water pressure coupling of sliding and bed deformation ii velocity depth profilesr b alley 130 139water pressure coupling of sliding and bed deformation iii application to ice stream b antarctica

dynamics of tidewater glaciers comparison of three models a minimal model of a tidewater glacier based solely on mass conservation is compared with two one dimensional numerical flowline models one with the calving rate proportional to water depth and the other with the flotation criterion as a boundary condition at the glacier terminus the models were run with two simplified bed geometries and two mass balance formulations the models simulate the full cycle of length variations and the equilibrium states for a tidewater glacier this study shows that the branching of the equilibrium states depends significantly on the bed geometry the similarity between the results of the three models indicates that if there is a submarine undulation at the terminus of a tidewater glacier any model in which the frontal ice loss is related to the water depth yields qualitatively the same non linear behaviour for large glaciers extending into deep water the flotation model causes unrealistic behaviour

metamorphic evolution of two different eclogites in the franciscan complex california u s a the eclogites in california u s a have been regarded as tectonic blocks which formed 150 160 ma ago and have been tectonically incorporated into younger rocks of the franciscan complex of 90 130 ma however in ward creek northern california two different types of eclogites were identified type i is a typical tectonic block and type ii is a possible in situ eclogite formed during the franciscan metamorphism most eclogites in the franciscan complex are type i eclogites type i eclogites in ward creek consist of garnet omphacite epidote glaucophane phengite rutile barroisite quartz and have coarse grained gneissic texture they have precursors of the epidote amphibolite facies and are overprinted in various degrees by blueschist facies retrograde metamorphism remnants of metamorphic rinds which consist of actinolite talc chlorite and formed during metasomatic interaction with serpentinizing peridotite occur at the margins of type i eclogite blocks type ii eclogites are medium grained and consist of garnet chloromelaniticclinopyroxene epidote glaucophane paragonite phengite rutile quartz petrochemical characters of type ii eclogites are similar to those of in situ eclogitic schists in the ward creek metabasite sequence epidote amphibolite f facies precursors metamorphic rinds and overprint of retrogressive blueschist facies metamorphism are not found in type ii eclogites garnets in type ii eclogites are medium grained unaltered and idioblastic and have higher spessartine contents sp14 25 whereas garnets in type i eclogites are coarse grained and chloritized and have lower spessartine contents sp2 7 the pt conditions of peak metamorphism for type ii eclogites are 380 10 11 5 kb type i eclogites follow a counter clockwise pt path from the epidote amphibolite through eclogite to blueschist facies whereas the presumable in situ type ii eclogites follow a clockwise pt path from the blueschist to eclogite facies

the active temperate glacial landsystem a model based on breiðamerkurjokull and fjallsjokull iceland accurate interpretations of ancient glaciated terrains rely heavily on our knowledge of process form relationships in contemporary glacierized basins a landsystems model for temperate actively receding glaciers is presented based upon brei amerkurjokull and fjallsjokull iceland historical documentation maps and or aerial photography documenting recession since 1903 provide a unique series of snapshots of the evolving glacial geomorphology at these snouts this documentation is employed in association with sedimentological investigations to assess the evolution of sediment landform assemblages at active temperate glacier margins using the wealth of geomorphological and sedimentological information produced during the recent recession of brei amerkurjokull and fjallsjokull three depositional domains are recognized 1 areas of extensive low amplitude marginal dump push and squeeze moraines derived largely from material on the glacier foreland and often recording annual recession of active ice 2 incised and terraced glacifluvial forms such as recessional ice contact fans and hochsandur fans and simple and complex anabranched eskers and small areas of pitted outwash 3 subglacial landform assemblages of flutes drumlins and overridden push moraines located between ice marginal glacifluvial depo centres the lack of supraglacial sediment in active temperate glaciers like brei amerkurjokull and fjallsjokull generally precludes the widespread development of chaotic hummocky moraine the hummocky terrain previously termed kame and kettle topography has mostly evolved by melt out into a complex network of anabranched eskers over the period 1945 1998 or actually comprises pitted or kettled outwash sandar the tills across the foreland were emplaced by subglacial deformation and lodgement and comprise materials derived from pre existing stratified sediments in addition to localized abrasion of rock surfaces and patches of lake sediments till sequences thicker than 2 m have been constructed by the sequential plastering of till layers onto stratified sediments and bedrock because this stacking is a sub marginal process it is suggested that complex till sequences similar to those observed at brei amerkurjokull fjallsjokull may be employed in the reconstruction of ancient glacier margins additionally the geomorphology of the active temperate landsystem at east brei amerkurjokull may contain subtle surge signatures verifying the historical record of small surges by this part of the glacier this illustrates the danger of employing landform sediment associations from restricted study areas e g parts of landsystems as representative process form models for glaciated terrains

polyphase and anticlockwise p t evolution for franciscan eclogites and blueschists from jenner california usa high grade exotic blocks in the franciscan complex at jenner california show evidence for polydeformation metamorphism with eight distinct stages two parallel sets of mineral assemblages e eclogite and bs laminated blueschist representing different bulk chemistry were identified stage 1 recorded by parallel aligned inclusions s1 of crossite omphacite epidote ilmenite titanite quartz e and glaucophane actinolite epidote titanite bs in the central parts of zoned garnets represents the epidote blueschist facies the onset of a second stage stage 2 is represented by a weak crenulation of s1 and growth of garnet this stage develops a well defined s2 foliation of orientated barroisite epidote titanite e or subcalcic actinolite epidote titanite bs at c 90d to s1 with syntectonic growth of garnet defining the albite epidote amphibolite facies a third stage with aligned inclusions of glaucophane subcalcic actinolite phengite parallel to s2 in the outermost rims of large garnet grains is assigned to the transitional albite epidote amphibolite garnet bearing epidote blueschist facies the fourth stage represents the peak metamorphism and was identified by unorientated matrix minerals in the least retrograded samples in this stage the mineral assemblages garnet omphacite glaucophane phengite e and garnet winchite phengite epidote bs both represent the eclogite facies stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite glaucophane garnet omphacite epidote phengite e and glaucophane actinolite epidote phengite bs with the development of an s5 foliation subparallel to s2 stage 6 represents a crenulation of s5 with the development of a well defined s6 crenulation cleavage wrapping around relics of the eclogite facies assemblages this crenulation cleavage is further weakly crenulated during a d7 event post d7 stage 8 is recorded by the growth of lawsonite chlorite actinolite replacing garnet and by veins of lawsonite pumpellyite aragonite and phengite apatite the different yet coeval mineral parageneses observed in rock types e and bs are probably due to differences in bulk chemistry the metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous following an anticlockwise p tpath 1 epidote blueschist garnet free to 2 albite epidote amphibolite to 3 transitional epidote blueschist garnet bearing albite epidote amphibolite to 4 eclogite to 5 epidote blueschist garnet bearing to 6 7 epidote blueschist garnet free facies to 8 lawsonite pumpellyite aragonite bearing assemblages this anticlockwise p t path may have resulted from a decreasing geothermal gradient with time in the mesozoic subduction zone of california at early or pre franciscan metamorphism

a model for the origin of himalayan anatexis and inverted metamorphism the origin of the paired granite belts and inverted metamorphic sequences of the himalaya has generally been ascribed to development of the main central thrust mct although a variety of models have been proposed that link early miocene anatexis with inverted metamorphism recent dating studies indicate that recrystallization of elements of the mct footwall occurred in the central himalaya as recently as 6 ma the recognition that hanging wall magmatism and footwall metamorphism are not spatially and temporally related renders unnecessary the need for exceptional physical conditions to explain generation of the high himalayan leucogranites and north himalayan granites which differ in age petrogenesis and emplacement style we suggest that their origin is linked to shear heating on a continuously active thrust that cuts through indian supracrustal rocks that had previously experienced low degrees of partial melting numerical simulations assuming a shear stress of 30 mpa indicate that continuous slip on the himalayan decollement beginning at 25 ma could trigger partial melting reactions leading to formation of the high himalayan granite chain between 25 and 20 ma and the north himalayan belt between 17 and 8 ma the ramp flat geometry we apply to model the himalayan thrust system requires that the presently exposed rocks of the hanging wall resided at middle crustal levels above the decollement throughout the early and middle miocene late miocene out of sequence thrusting within the broad shear zone beneath the mct provides a mechanism to bring these rocks to the surface in their present location i e well to the north of the present tectonic front and has the additional benefit of explaining how the inverted metamorphic sequences formed beneath the mct we envision that formation of the mct zone involved successive accretion of tectonic slivers of the lesser himalayan formations to the hanging wall and incorporate these effects into the model the model predicts continued anatexis up to 400 km north of the himalayan range consistent with the timing and geochemistry of leucogranites exhumed on the flank of a south tibetan rift

early miocene continental subduction and rapid exhumation in the western mediterranean new lu hf garnet growth ages from high pressure rocks from the structurally lowest terrane nevado filabride complex in the betic cordillera of southern spain indicate early miocene subduction of continental crust requiring significant revision of the timing and processes of orogeny in the western part of the alpine orogenic system mafic eclogite and pelitic schist give lu hf ages in the range 18 14 ma and fission track data show that the terrane was exhumed by 12 8 ma the overlying terrane alpujarride complex shows early tertiary high pressure temperature metamorphism this was followed by a thermal event that accompanied rapid crustal extension at 22 18 ma leading to the formation of the alboran sea backarc basin the evidence therefore suggests two phases of continental subduction separated by a phase of large scale intraorogenic extension the two terranes are now separated by a mid late miocene low angle extensional fault the timing and paleogeographic setting suggest that the nevado filabride terrane originally was close to the rifted south iberian margin and was subducted during westward motion of the extending upper terrane between the converging african and iberian plates

glacimarine processes and inductive lithofacies modelling of ice shelf and tidewater glacier sediments based on quaternary examples abstract three variables control the type of glacial debris source and where and when debris is introduced to an ocean 1 type of glacial ice source ice sheet or valley glacier 2 condition of a grounded glacier melting freezing or frozen base and cold or warm internal ice and 3 type of glacier front ice shelf or tidewater using these variables in different combinations eight glacimarine regimes are postulated to exist by integrating information from holocene sediment and pleistocene glacimarine sequences with the eight inductive regimes lithofacies relationships are predicted and modelled the models show a complexity of glacimarine lithofacies the importance of recognizing this complexity lies in realistic interpretations of ancient sequences ice shelf sediment may be limited in the geologic record because of tectonic and climatic constraints required for ice shelf formation alternatively in some tidewater glacier regimes larger volumes of glacimarine sediment are produced than in ice shelf regimes ice proximal sediment is distinctive especially near subglacial meltwater stream effluxes ice shelves and tidewater glaciers may produce similar ice proximal lithofacies e g morainal bank associations and laminites in certain regimes however details of lithofacies may aid in distinguishing ice shelf from tidewater regimes ice shelf and tidewater iceberg zone sediment can be distinguished from each other if basal and high level englacial debris can be distinguished ice shelf zone sediment out from the grounding line has a basal debris source high level englacial debris is deposited in more distal ice shelf zones resulting in sediment by pass in corresponding positions of a tidewater glacier regime iceberg rafted debris comprises mixed basal and high level englacial debris deeper water zones receive large volumes of sediment when a melting freezing base grounding line is at the edge of a continental shelf when the grounding line retreats sediment supply may be drastically reduced because of a back sloping continental shelf and trapping by morainal banks cool water bioclastic carbonates may accumulate once a grounding line has retreated although ice shelf iceberg zones are more extensive than those in tidewater regimes ice shelf icebergs have low debris content and resulting ice rafted debris may be scarce the smallest volume of sediment is produced by a frozen base ice sheet ending as a tidewater front and the largest volume by a melting freezing base valley tidewater glacier glacimarine sediment has a relatively high preservation potential near continental interiors or margins however at continental margins mass flow olistostrome and tectonic melange deposits may be preserved with glacimarine diamictites these deposits may be difficult to distinguish from each other especially if metamorphosed such as at a convergent margin the models presented do not fully represent the glacimarine environment because many other variables are omitted however the principal concepts of generating glacimarine lithofacies associations are considered continued research will verify if each regime exists and will provide data on those regimes that exist today to compare and modify these models

metastability of granulites and processes of eclogitisation in the uhp region of western norway the flatraket complex a granulite facies low strain enclave within the western gneiss region provides an excellent example of metastability of plagioclase bearing assemblages under eclogite facies conditions coesite eclogites are found 28 kbar unprecedented metastability of plagioclase and quartz is implied alternatively a tectonic break exists between the flatraket complex and uhp eclogites supporting the concept of a tectonic boundary to the uhp zone of the western gneiss region the distribution of eclogite and amphibolite facies metamorphic overprints demonstrates that the reactivity of the crust during deep burial and exhumation is strongly controlled by fluid availability and is a function of the protolith

the association of poorly ordered graphite coke and bitumens in greenschist facies rocks of the ponikla group lugicum czech republic the result of graphitization of various types of carbonaceous matter greenschist facies rocks of the ponikla group ordovician silurian czech republic contain several types of carbonaceous matter that differ in their morphology texture reflectance and raman characteristics the first type consists of large up to 3 mm irregularly bound particles of low reflectance romin 0 9 romax 5 6 the area ratio of the 1585 cm 1 to 1350 cm 1 raman peaks 1 08 1 17 indicates an intermediate degree of graphitization the formation of this type of highly porous particle displaying a texture reminiscent of regular or needle coke is attributed to the thermal alteration of the amorphous structureless kerogen of the precursor sediments the second type consists of lamellar particles up to 30 m thick which can be associated with the latter or can occur independently in white mica rich laminae this type is characterized by high bireflectance romin 0 6 romax 11 9 and by lower ratios 0 70 0 82 of the raman peak areas these particles are interpreted as the product of solid state diffusion controlled graphitization of a chemically homogeneous organic material e g of graptolite periderms the third type consists of isometric up to 2 mm large commonly fractured grains and fragments which mainly occur in quartz rich laminae in reflected light the texture is either homogeneous or consists of various types of anisotropic mozaics the raman peak area ratios 0 75 1 14 indicate a highly variable degree of structural ordering these particles are considered as the remains of metamorphosed bitumens accumulated in the sandy laminae of the original sediments the fourth type consists of small particles of carbonaceous matter maximum length 25 m thickness 1 2 m which occur adjacent to crystal faces of white micas this type is probably the product of epitaxial growth of graphite from the gaseous phase the results of this work indicate that the differences in the degree of graphitization of the carbonaceous matter in low grade metamorphosed rocks can be mainly related to the initial nature of the sedimentary organic matter and to its premetamorphic history

flow dynamics of tidewater glaciers a numerical modelling approach the dynamics of grounded tidewater glaciers is investigated with a time dependent numerical flow model which solves the full equations for the stress and velocity fields and includes a water pressure dependent sliding law the calving criterion implemented in the model shifts the calving front at each time step to the position where the frontal ice thickness exceeds flotation height by a prescribed value with this model the linear relation between calving rate and water depth proposed on empirical grounds is qualitatively reproduced for the situation of a slowly retreating or advancing terminus but not for situations of rapid changes length changes of tidewater glaciers i e especially rapid changes are dominantly controlled by the bed topography and are to a minor degree a direct reaction to a mass balance change thus accurate information on the near terminus bed topography is required for reliable prediction of the terminus changes due to climate changes the results also confirm the suggested cycles of slow advance and rapid retreat through a basal depression rapid changes in terminus positions preferably occur in places where the bed slopes upwards in the ice flow direction

contrasting early and late mesozoic petrotectonic evolution of northern california devonian middle jurassic terrane assemblies in the klamath mountains and sierra nevada foothills consist chiefly of ophiolite chert argillite sequences mafic ultramafic complexes are oceanic whereas associated fine grained deep water terrigenous sediments were derived mainly from adjacent previously docked klamath sierran terranes coeval calc alkaline arc rocks are volumetrically rare geologic and petrochemical relations suggest a rifted arc origin for klamath mafic metavolcanic units inter layered with distal turbidites in the 170 200 ma north fork terrane detrital zircon u pb ages indicate that the clastic debris had a regional eastern klamath source the eastern hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby eastern klamath antelope mountain quartzite the seaward 200 ma rattlesnake creek terrane is an ophiolitic melange with north fork petrotectonic affinities the north fork eastern hayfork rattlesnake creek amalgam correlates with the calaveras complex and the outboard jura triassic arc belt in the sierran foothills geochemical bulk rock and zircon u pb age data support interpretation of the 200 ma jura triassic arc as an adjacent offshore mafic belt overlying a 300 ma ophiolitic basement these oceanic complexes were sutured against the central metamorphic belt eastern klamath feather river northern sierra terrane backstop before deposition and deformation of the outboard upper jurassic galice and mariposa formations klamath sierran terrane assemblies reflect 230 m y of transpression transtension involving only minor episodes of subduction producing ubiquitous ophiolite chert argillite lithologies and rare felsic arc rocks in contrast the late jurassic to largely cretaceous klamath sierra nevada quartzo feldspathic volcanic plutonic arc attests to massive calc alkaline magmatism attending a strong eastward component of underflow by the farallon plate the coeval galice mariposa formations followed by the cretaceous great valley forearc and franciscan trench deposits are first cycle felsic debris shed mainly from the klamath sierran arc these units record 70 m y of rapid sialic crustal growth attending major periods of approximately margin normal convergence this profound transition in northern california included devonian middle jurassic rifting drifting and stranding of ophiolite chert argillite terranes along an adjacent curvilinear continental margin then nearly head on cretaceous subduction that resulted in massive calc alkaline igneous activity the erosion of which generated the felsic great valley group forearc basin and franciscan complex trench clastic sedimentary units

structural styles in mesozoic and cenozoic melanges in the western cordillera of north america the term melange is currently used to describe several different kinds of mudstone rich rocks that are broadly characterized by an obscure stratigraphy stratal disruption or a chaotic block in matrix fabric four types of melange which can be defined in outcrop on the basis of mesoscopic fabric and lithologic composition are particularly widespread and distinctive type i includes sequences of originally interbedded sandstone and mudstone that record incipient to thorough disruption and fragmentation of strata accomplished largely by layer parallel extension type ii consists of similarly deformed thin layers of green tuff radiolarian ribbon chert and minor sandstone originally interbedded with black mudstone disruption in both types i and ii which probably occurred while the sediments were incompletely consolidated has been ascribed to either imbricate faulting in accretionary wedges or gravitationally driven deformation type iii comprises inclusions of diverse shapes sizes and compositions enveloped in a locally scaly pelitic matrix the ultimate source of fragments is obscure because the majority were not derived by either the progressive disruption of interbedded sediments or in situ tectonic plucking and abrasion of adjacent rocks although some type iii melanges may have originated deep within accretionary prisms final emplacement as olistostromes muddy debris flow deposits or mud diapirs seems likely type iii melanges are mechanically analogous to scaly sheared serpentinites many probably have been tectonically remobilized or even intruded into shallow level fault zones type iv consists of lenticular inclusions bounded by an anastomosing network of subparallel faults their fabric records progressive slicing in brittle fault zones each of the four types of melange described here could in theory have formed in a variety of settings on or within an accretionary wedge at an active convergent margin none can yet be singled out as a uniquely diagnostic subduction melange

metamorphism of franciscan tectonostratigraphic assemblage pacheco pass area east central diablo range california coast ranges the pacheco pass area well known for the relatively high pressure low temperature mineral assemblages developed in a well ordered section of chiefly metaclastic strata was restudied in order to elucidate the geologic paragenetic and textural relationships among the various lithologic units the mapped 7 minute quadrangle extends from 4 km west of the range crest eastward to the ortigalita fault tectonic contact of the franciscan assemblage with coeval great valley strata semicontinuous metachert beds underlain by scraps of na cpx and or na amphibole bearing metabasaltic rock mark the base of terrigenous tectonostratigraphic units evidently decoupled from the downgoing oceanic crust capped plate underplating along newly recognized bedding plane faults has juxtaposed members of an apparently conformable franciscan sedimentary sequence four depositional units each consisting of monotonous metagraywacke and interlayered metashale are stacked within the accretionary section from top down these are tectonostratigraphic members a b c and d unit d differs from overlying members in possessing a greater abundance of bluestone greenstone pods minor serpentinite bodies and rare metaconglomerate lenses varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation the ortigalita fault is nearly vertical strikes north northwest and exhibits apparent dextral offset it truncates the east west striking gonzaga fault and other franciscan structures the topographic low occupied by san luis reservoir is a cenozoic tectonic depression sited at a releasing bend on the ortigalita fault newly acquired textural chemical mineralogic and areal relations of albitic and jadeitic pyroxene bearing franciscan metagraywacke near pacheco pass demonstrate that 1 na cpx and na amphibole are metamorphic minerals not clastic materials 2 pumpellyite is a minor neoblastic phase in albite bearing metagraywacke west of pacheco pass and in the north western corner of the quadrangle but it does not occur in jadeitic pyroxene bearing metaclastic rocks 3 higher textural grade metagraywacke specimens generally contain higher modal proportions of jd 4 abundance textural grade and chemistry of na cpx are not related to tectonostratigraphic units and 5 because of sluggish reaction rates albite persisted metastably into the higher pressure jadeitic pyroxene quartz p t field production of na cpx resulted from a reaction of the sort ab chl jd iws qtz gln h 2 o irregular oscillatory zoning within single jadeitic pyroxene prisms and variable jd ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a p t transition zone high pressure overstepping of the phase boundary explains variable modal proportions of ab and jd as well as chemical heterogeneity of the jd elevated p attended metamorphism of na cpx bearing rocks exposed in the pacheco pass area as indicated by the wide spread coexistence of quartz and naalsi 2 o 6 rich pyroxene inferred physical conditions of prograde metamorphism were 150 50 c at 7 8 kbar or more franciscan rocks in the east central diablo range underwent subduction zone metamorphism accompanying paleo pacific lithospheric plate descent during mid cretaceous convergence underplating contraction and sequestering of the tectonostratigraphic assembly at depths of 25 30 km during underflow followed by gradual rise took place under conditions of refrigeration due to protracted descent of the oceanic slab paleogene diapiric uplift and extension followed indicating more complete decoupling of the buoyant trench complex from the downgoing plate finally passage of the east pacific rise triple junction in neogene time initiated the present dextral slip regime in the california coast ranges

morphodynamics of a bar trough surf zone abstract a field study was made of the distinguishing morphodynamic processes operating in a surf zone which perennially exhibits accentuated bar trough topography the longshore bar trough and rhytmic bar and beach states as described by wright and short 1984 characteristic features of the morphology include a shallow bar with a steep shoreward face a deep trough and a steep beach face this morphology which is favored by moderate breaker heights and small tidal ranges strongly controls the coupled suite of hydrodynamic processes in contrast to fully dissipative surf zones the bar trough surf zone is not at all saturated and oscillations at incident wave frequency remain dominant from the break point to the subaerial beach the degree of incident wave groupiness does not change appreciably across the surf zone infragravity standing waves which in dissipative surf zones dominate the inshore energy remain energetically secondary and occur at higher frequencies in the bar trough surf zone analyses of the field data combined with numerical simulations of leaky mode and edge wave nodal antinodal positions over observed surf zone profiles indicate that the frequencies which prevail are favored by the resonant condition of antinodes over the bar and nodes in the trough standing waves which would have nodes over the bar are suppressed sediment resuspension in the surf zone appears to be largely attributable to the incident waves which are the main source of bed shear stress in addition the extra near bottom eddy viscosity provided by the reformed non breaking waves traversing the trough significantly affects the vertical velocity profile of the longshore current whereas the bar is highly mobile in terms of onshore offshore migration rates the beach face and inner regions of the trough are remarkably stable over time

high and low stress subduction zones recognized in the rock record abstract it is generally accepted that the main strength of subduction boundaries occurs in the shallower region where frictional deformation is dominant however estimates of absolute values commonly expressed as apparent coefficient of friction show great variation frictional shear heating is closely related to and estimates of the extra thermal energy supplied by shearing can in principle be used to estimate subduction zone strength one such approach is based on surface heat flow measurements however heat flow in convergent margins shows large local scatter and even in the same area different studies using this method show large variations in estimates of indicating large uncertainties the thermal record of subduction conditions preserved in subduction type metamorphic rocks is developed over geological time scales that average out local complexities in heat flow and therefore has good potential as an alternative indicator of the amount of shear heating and hence shear strength along subduction boundaries thermal models that incorporate shear heating were developed for two contrasting and well known subduction type metamorphic belts the relatively warm sanbagawa belt of sw japan and the relatively cold franciscan belt of western usa high grade rocks of the sanbagawa belt show strongly curved p t paths that display increasing p t to about 2 gpa information on the rate of plate movement and the age of the subducting slab at the time of metamorphism can be combined with modelling results to show that relatively high shear stresses equivalent to 0 13 are required to account for the observed curved p t paths in contrast the high grade rocks of the franciscan belt show relatively cool p t conditions that do not allow for strong shear heating with an appropriate upper bound for of 0 03 modelling suggests subduction rate and lithology are potentially important controls on the development of high versus low stress subduction zones high stress subduction zones are likely to be associated with high aseismic seismic slip ratios possibly related to slab roughness

radioisotopic and biostratigraphic age relations in the coast range ophiolite northern california implications for the tectonic evolution of the western cordillera the coast range ophiolite cro in northern california includes two distinct remnants the elder creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites layered gabbro wehrlite pyroxenite quartz diorite a mafi c to felsic dike complex and mafi c felsic volcanic rocks the entire suite is cut by late mid oceanic ridge basalt morb dikes and overlain by ophiolitic breccia the stonyford volcanic complex sfvc comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite structurally below this seamount are melange blocks of cro similar to elder creek u pb zircon ages from plagiogranite and quartz diorites at elder creek range in age from 165 ma to 172 ma u pb zircon ages obtained from cro melange blocks below the sfvc are similar 166 172 ma 40 ar 39 ar ages of alkali basalt glass in the upper sfvc are all younger at 164 ma radiolarians extracted from chert lenses intercalated with basalt in the sfvc indicate that the sedimentary strata range in age from bathonian unitary association zone 6 6 of baumgartner et al 1995a near the base of the complex to late callovian to early kimmeridgian unitary association zones 8 10 in the upper part the sfvc sedimentary record preserves evidence of a major faunal change wherein relatively small sized polytaxic radiolarian faunas were replaced by very robust oligotaxic nassellarian dominated faunas that included praeparvicingula spp we suggest that cro formation began after the early middle jurassic 172 180 ma collision of an exotic or fringing arc with north america and initiation of a new or reconfi gured east dipping subduction zone the data show that the cro formed prior to the late jurassic nevadan orogeny probably by rapid forearc extension above a nascent subduction zone we infer that cro spreading ended with the collision of an oceanic spreading center ca 164 ma coincident with the oldest high grade blocks in the structurally underlying franciscan assemblage we further suggest that the classic nevadan orogeny represents a response to spreading center collision with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in north american plate motion to rapid northward drift j2 cusp causing sinistral transpression and transtension in the sierra foothills these data are not consistent with models for late jurassic arc collision in the sierra foothills or a back arc origin for the cro

effects of subduction parameters on geothermal gradients in forearcs with an application to franciscan subduction in california geothermal gradients in forearcs are often suppressed below normal values because of the cooling effect of the relatively cold downgoing plate in this paper finite difference thermal modeling is used to evaluate the influence on forearc gradients of variations in six potentially important subduction zone parameters variations in forearc radiogenic heat production over the general range likely in forearc rocks 0 to 1 5 10 6 w m 3 cause only minor changes in temperatures at the base of the forearc about 2 to 35 depending on other parameters and thus have relatively little effect on pressure temperature pt conditions of metamorphism however their effect on gradients at shallow levels and thus on surface heat flows may be greater up to about 100 likely variations in forearc thermal conductivity 1 5 to 3 5 w m 1 c 1 similarly change temperatures at the base of the forearc only modestly 10 25 but change surface heat flows more substantially 1 45 1 75 times decreasing the subduction angle from 27 to 3 6 increases gradients by about a factor of 2 at both deep and shallow levels decreasing the subduction rate by a factor of 8 160 to 20 km m y increases gradients by about a factor of 2 when frictional heating at the plate contact is excluded and less when modest amounts of frictional heating are included forearc gradients increase virtually linearly with the presubduction geothermal gradient in the subducting slab and thus are higher above a younger slab when frictional heating is excluded forearc gradients are 2 0 2 8 times higher above a 10 m y old slab than above a 150 m y old slab again the relative effect is lessened when frictional heating is incorporated the amount of frictional heating at the plate contact potentially has the greatest influence on forearc gradients the frictional heat production h is given by h vs where is the shear stress along the plate contact and vs is the subduction rate the modeling shows that forearc gradients increase linearly with unless plate contact shear stresses are only a small fraction of those expected with frictional sliding of dry rocks outer parts of forearcs would be the sites of pervasive low p t metamorphism greenschist facies rather than high p t metamorphism blueschist facies very high fluid pressures along the plate contact are probably the way shear stresses are reduced pressure temperature conditions of blueschist facies metamorphism in the franciscan subduction complex of california are easily explained with typical subduction rates and slab ages with plate contact shear stresses of the order of 10 mpa equivalent to around 3 of lithostatic pressure but stresses within the range zero to a few tens of megapascals are probably permitted by the thermal constraints speculative application of the modeling results assuming a shear stress of 4 of lithostatic pressure to plate motion reconstructions for the franciscan forearc suggests that forearc gradients were about 8 c km around 85 ma when the subducting slab was perhaps 145 m y old and the subduction rate was perhaps 95 km m y gradients increased moderately through the latest cretaceous to middle tertiary as subduction became slower and the subducting slab became younger reaching about 16 c km at 28 ma when the slab age was about 11 m y and the subduction rate was about 48 km m y the slab age subduction rate and forearc gradient then remained fairly constant until 5 ma when subduction slowed to about 32 km m y and the slab age decreased to about 8 m y causing gradients to rise to about 20 c km

temperatures heat flux and frictional stress near major thrust faults we examine the two dimensional advective and conductive transport of heat in a region of thrust faulting both simple theoretical considerations and numerical experiments show that the steady state temperatures near the fault are reduced by a divisor s 1 b z vsin below what they would be with the same heat sources but in the absence of advection in this expression z is the depth to the fault v is the slip rate is the component of the dip of the fault in the direction of underthrusting is thermal diffusivity and b is a dimensionless factor that is essentially equal to one for most forms of heating initial changes in temperatures near the fault are given by t z t t z 0 12vsin t z 0 z these simple formulae are successful because of the neglible influence of lateral conduction of heat at least for slip rates of a few mm yr or more two time constants govern the transition from the initial change in temperature to steady state t1 u v where u is the distance along the fault in the direction of underthrusting from the surface to the depth in question and t2 z 2 2 where z is the depth to the fault when elapsed times exceed the sum of these two time constants temperatures differ from their steady state values by only about 10 percent the numerical experiments indicate that the simple formulae are sufficiently accurate that sophisticiated numerical modeling of temperatures in specific regions is unwarranted putain an application of these simple formulae to measurements of conductive heat flow at island arcs implies that shear stresses at island arcs approach 100 mpa and are greater than 30 mpa calculations of temperatures appropriate for the himalaya suggest that shear stresses of 100 mpa on the main central thrust probably are required to account for the tertiary granites of the region if melting took place after slip began on the thrust similarly the cut off in seismicity at a depth of about 15 km in the himalaya if due to temperatures exceeding 350 to 450 c implies a deviatoric stress close to 100 mpa

simulation of the evolution of breidamerkurjokull in the late holocene 1 we have simulated the advance and retreat of breidamerkurjokull a tidewater glacier in iceland by means of a one dimensional numerical ice flow model the calculations were done with two different schemes for iceberg calving the flotation model and the water depth model the model simulates the glacier history by an advance into a lake filled by unlithified sediments without the occurrence of calving followed by a retreat with calving into the excavated lake the results indicate that the flotation model is not capable of producing the observed glacier retreat into deep water well whereas the water depth model provides a glacier evolution similar to the observed retreat the water depth model predicts that breidamerkurjokull will retreat out of the lake after approximately 100 years under the present climatic conditions

subducted sedimentary serpentinite melanges record of multiple burial exhumation cycles and subduction erosion abstract serpentinite matrix melanges give insight into large scale convergent plate margin processes particularly because of the derivation of the serpentinite from oceanic mantle similar to shale matrix melanges a field geologist may easily recognize the sedimentary origins of little deformed serpentinite matrix melanges but melanges within accretionary prisms have undergone significant deformation and recrystallization of matrix serpentinite melanges of the franciscan subduction complex of california have a seemingly intact and foliated matrix such exposures contrast sharply with the granular undeformed sedimentary serpentinite melanges of the coeval great valley group gvg forearc basin deposits that depositionally overlie coast range ophiolite that structurally overlies the franciscan nonetheless franciscan serpentinite melanges display evidence of sedimentary origins including sedimentary breccia composed of exotic block material tolay ridge sedimentary serpentinite breccia panoche pass road basal serpentinite conglomerate with exotic clasts sunol regional wilderness and serpentinite sandstones and conglomerates including a basal conglomerate overlying coherent metagraywacke tiburon peninsula these examples record two burial exhumation cycles to blueschist facies depths in addition a melange breccia in the panoche pass area may have components that record three burial exhumation cycles to blueschist or greater depth exhumation rates for various cycles ranged from about 1 2 to 10 mm year the tiburon peninsula serpentinite melange occupies the structurally highest horizon in the franciscan of the san francisco bay area and regional field relationships indicate deposition at ca 100 ma apparently about 65 ma of subduction erosion non accretion followed initiation of franciscan subduction in this region the oldest franciscan serpentinite melanges are at least 35 ma younger than sedimentary serpentinites of the gvg subduction erosion may have facilitated reworking of forearc sedimentary serpentinite deposits into the trench evidence of multiple burial exhumation cycles indicates reworking of subduction complex material consistent with existence of clastic units that have fossils indicative of significantly older ages than detrital zircon ages

dating subduction zone metamorphism with combined garnet and lawsonite lu hf geochronology studies of ancient and active subduction zones are critically important to understanding processes of interplate coupling crust mantle recycling and arc magmatism dating subduction metamorphism along prograde and retrograde paths in order to constrain such processes however has proved to be extremely difficult the complex thermal history of subduction systems poses significant challenges to low t geochronometers and subduction zone assemblages commonly lack suitable minerals for higher temperature geochronology garnet and lawsonite however are critical index minerals of high and low t subduction zone metamorphism and are well suited to lu hf geochronology in addition the closure temperature for lu hf in garnet and lawsonite should be sufficiently high that an age will date mineral crystallization and therefore the timing of subduction zone metamorphism the relative stability and timing of garnet and lawsonite formation will be controlled by the bulk composition peak metamorphic conditions and shape of the p t path experienced by a particular sample to test the influence of metamorphic conditions and p t path on lu hf geochronology garnet and lawsonite bearing samples were dated from rocks where lawsonite formed after garnet along a retrograde path stabilized prior to garnet along a prograde path and formed contemporaneously with garnet in the franciscan complex the ages of garnet epidote amphibolite garnet epidote blueschist garnet lawsonite blueschist and lawsonite blueschist range from c 166 130 ma and generally decrease with decreasing metamorphic grade consistent with previous studies garnet lawsonite blueschist eclogite formed along an apparent prograde path at ward creek records an apparent age of c 152 ma lower temperature lawsonite blueschist at ward creek however failed to provide a geologically significant date and likely reflects isotopic disequilibrium at low temperatures the apparent temperature time history from franciscan complex lu hf ages most likely reflects samples derived from various portions of the subduction zone or that were subducted and metamorphosed at different times in the thermal evolution of the subduction zone in the sivrihisar massif lawsonite eclogite and garnet lawsonite blueschist record distinctly different ages of 91 1 1 3 ma and 83 3 1 8 ma the different ages date the timing of high p metamorphism within each protolith and suggest that garnet lawsonite eclogite metamorphism pre dated garnet lawsonite blueschist metamorphism in these samples by c 8 ma the age of lawsonite eclogite metamorphism extends the timing of high p metamorphism and requires subduction initiation prior to 91 ma the results indicate that the lu hf system provides a reliable tool for dating the wide range in p t conditions of subduction zone metamorphism lawsonite dating in particular provides a reliable method by which to date low t retrograde and prograde metamorphism in the absence of garnet lawsonite may not be ideal for geochronology if sufficient garnet coexists in the mineral assemblage the lawsonite has very low t stability or if extremely fine grained hf phases such as zircon are present in lawsonite in poly metamorphic assemblages where the pressure and temperature can be estimated for separate garnet and lawsonite sub assemblages the age discrepancy between garnet and lawsonite may provide the ability to quantify the rates of heating or cooling and subduction or exhumation

bar morphology of dissipative beaches an empirical model this study of 150 examples of nearshore bar morphology along the highly dissipative beaches 33 of the southeast mediterranean shoreline employs 32 years of aerial photography and wave data aided by detailed short term field studies three major bar types were delineated non rhythmic parallel meandering bars inner single crescentic bars and double crescentic bars each of these bar families includes variations the bars were related to daily ship and shore wave data including wave spectra from the day of observation back to 60 days prior to the observation the increase in total bar occurrence during summer is related to crossing of a major wave energy threshold in the spring when significant wave heights 1 m sharply increase to 70 85 in april may the bar morphology wave comparisons further indicate that as the significant wave heights decrease and remain below 1 m non rhythmic bars form within 7 10 days single crescentic bars require 15 days and double crescentic bars require 20 30 days this adjustment period of the bars to wave power causes a delayed response which accounts for lack of coincidence between wave energy and bar occurrence the formation of the initial double crescentic bar and its transformation to the mature double crescentic type requires a short pulse of wave energy 0 5 h13 1 5 m some bar families occur throughout the year the aseasonal occurrence is best shown by the mature double crescentic type which apparently is the final stage in the crescenticbar development sequence however other bar families show a tendency for a seasonal distribution which reflects their sensitivity to wave energy inner single crescentic and initial double crescentic bars are largely restricted to the calmest wave months of may april to october november there is an antiphase relationship between the frequencies of non rhythmic and crescentic bars

low grade blueschist facies metamorphism of metagreywackes franciscan complex northern california metagreywackes in the eastern belt of the franciscan complex contain the assemblage qtz ab lws chl ph pmp fgl hem cal arg or compatible subassemblages blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part the compositions of the coexisting minerals and the nature of the continuous reactions in these low grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the eastern belt of the franciscan complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism in rocks lacking pumpellyite white mica may be essential to the growth of blue amphibole but carbonate plays only a limited role the continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form pmp chl ab qtz hem h2o femg 1 fgl lws this reaction requires significant hydration as pressure increases in order to produce blue amphibole most of the eastern belt of the franciscan complex formed in limited ranges of temperature and pressure which are estimated to be 240 280 c 6 5 7 5 kbar pressures in the westernmost part of the area were about 1 kbar lower than in the east pressures of about 8 5 10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene

early cretaceous transition from nonaccretionary behavior to strongly accretionary behavior within the franciscan subduction complex 1 during subduction at the franciscan trench beginning at 170 160 ma and continuing to the present marine sedimentary and lesser volcanic rocks have been underthrust accreted and metamorphosed to form the franciscan accretionary wedge the south fork mountain schist sfms forms the eastern margin and structural top of the wedge and so was apparently the first unit of substantial size to accrete into the franciscan new u pb ages of detrital zircons and 40ar 39ar ages of metamorphic white micas indicate that the sfms accreted surprisingly late in the time span of franciscan subduction probably at ca 123 ma at least parts of the valentine spring formation and yolla bolly terrane accreted soon thereafter therefore accretion of the sfms apparently marks a fundamental transition from virtually nonaccretionary behavior to strongly accretionary behavior in the franciscan forearc about 75 of modern forearcs are nonaccretionary so recognition of an early nonaccretionary period is consistent with modern examples the franciscan mafic high grade exotic blocks and related very small slabs which yield ca 132 to 169 ma metamorphic ages are the only well documented rock record of the early 40 m y long nonaccretionary period and represent very sparse accretion during that interval the transition into an accretionary regime at ca 123 ma was essentially synchronous with the end of the late jurassic early cretaceous magmatic lull in the sierra nevada magmatic arc with the waning of early cretaceous faulting and warping within the great valley forearc basin and possibly with changes in motion vectors of the pacific and farallon plates

constraints on early franciscan subduction rates from 2 d thermal modeling abstract mesozoic subduction along the western margin of north america in california jumped west in mid jurassic time to form the franciscan accretionary wedge outboard of the coast range ophiolite and the great valley forearc basin based on mesozoic plate motion analysis for the pacific region overall rates of subduction are likely to have been on the order of 50 km m y new and previously published lu hf garnet growth ages on high grade blocks in the franciscan however suggest that early franciscan metamorphism took place in a slowly cooling environment characteristic of slow subduction rates in order to assess rates of early franciscan subduction 2 d thermal modeling was carried out based on the following assumptions 1 subduction was initiated at 170 ma based on the age of the oldest tectonic block and on the age of the coast range ophiolite believed to have formed in a forearc setting behind the newly initiated subduction zone 2 at the start of subduction the hanging wall mantle wedge was at asthenospheric temperatures consistent with initiation of subduction at the ophiolite spreading center 3 the earliest subducted lithosphere was 10 m y old at the start of subduction 4 tectonic blocks were metamorphosed at an average pressure of 1 4 gpa corresponding to a depth of 43 km beneath mantle rocks assuming 3 3 g cm 3 subduction rates were investigated between 1 and 300 km m y and the best fit between the ages and estimated metamorphic temperatures of the blocks was found to be 10 5 km m y this is five times slower than the 50 km m y determined from plate motion analysis for that time and suggests that some reappraisal of the configuration and velocities of the oceanic plates may be needed

geochemical evidence for the tectonic setting of the coast range ophiolite a composite island arc oceanic crust terrane in western california the middle to late jurassic age coast range ophiolite cro of california contains two geochemically distinct volcanic rock associations that formed in different tectonic settings volcanic rocks from the southern cro point sal cuesta ridge stanley mountain llanada quinto creek and del puerto and parts of the northern cro healdsburg elder creek are similar to low k tholeiites and calc alkaline rocks of the island arc suite the thin volcanic sections of these ophiolite remnants suggest formation by intra arc rifting in contrast volcanic rocks from stonyford seamount and paskenta in the northern cro are transitional subalkaline metabasalts with geochemical characteristics similar to enriched mid ocean ridge basalts or ocean island tholeiites these rocks are associated with tithonian radiolarian cherts and may be part of the franciscan complex alternatively they may represent a change in tectonic setting within the cro during the late jurassic regardless the cro as currently conceived cannot be considered a single terrane with one mode of origin

biostratigraphic constraints on formation and timing of accretion in a subduction complex an example from the franciscan complex of northern california the determination of the total age coverage of pelagic bedded chert is particularly important in studies of ancient accretionary complexes because the time span represents the minimum travel time of an oceanic plate before accretion at an island arc or continental margin the yolla bolly terrane of the franciscan complex consists of rare metabasalt overlain by bedded radiolarian chert which in turn is overlain by metagraywacke and subordinate slaty mudstone and conglomerate the bedded chert ranges in age from aaleian early middle jurassic to tithonian latest jurassic a lithologic gradation from pelagic bedded chert through hemipelagic siliceous mudstone occurred during the tithonian and thus marks the arrival of the oceanic sediments at the continental margin the tithonian age also agrees with previously reported tithonian to valanginian early cretaceous fossils in the overlying terrigenous metagraywacke both chert and metagraywacke are intruded by gabbroic sills suggesting off ridge volcanism

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

fast tidewater glaciers some iceberg calving outlet glaciers flow continuously at speeds normally associated with surging glaciers arid exhibit dramatic instability scenarios related to those suggested for marine ice sheets no temperate tidewater glaciers are known to have floating termini but many polar and subpolar tidewater glaciers do the fast flow of temperate calving glaciers is almost entirely due to basal sliding and appears to be a function of the effective pressure on the bed which may approach zero and the longitudinal back stress on the terminus the terminus boundary condition the calving relation is imperfectly known yet is vital to the dynamics of these glaciers calving relations for grounded tidewater glaciers have been suggested on empirical grounds but have not been rigorously tested the calving relations for floating termini are virtually unknown this together with the imperfect understanding of basal sliding inhibits confidence in our understanding of the stability of these glaciers columbia glacier alaska is an instructive example because observations have been made on the major changes in its geometry calving rate and dynamics that have occurred in less than 10 years the calving flux has increased more rapidly than the glacier flux causing thinning and retreat as a result the ice velocity has increased markedly the short term velocity changes relate to changes in back pressure ice recession tidal changes and the flux of water injected to the bed these results have relevance to the mechanisms of basal sliding glacier surges and the stability of marine ice streams

the quantitative description of beach cycles abstract a quantitative method is developed to describe sequential changes in beach profile morphology dominant cycles in temporal eigenfunctions computed by empirical eigenfunction analysis and rotary component analysis are defined and used to construct a sequence of profiles representing the energetic portions of beach cycles five sets of beach profile data from torrey pines beach in southern california were analyzed with these techniques because the first and second eigenfunctions account for between 65 and 90 of the variability of the data the beach profiles are accurately described with only these two variables and the mean profile using rotary component analysis beach changes represented by the two dominant eigenfunctions can be separated into many elliptical components each representing a particular frequency associated with cycles in beach state profiles are reconstructed using points from the perimeter latera recta of these beach state ellipses a frequency of 1 cpy is the most energetic frequency in the first and second eigenfunctions corresponding to the cyclic onshore offshore sediment movement associated with bar and berm profiles profiles constructed from points on the elliptical path associated with annual cycles graphically depict the sequential development of bar and berm during the beach cycle beach cycles corresponding to a frequency of 2 cpy are also significant primarily due to the energy of the second eigenfunction no other frequencies are consistently found to be as significant as the annual and semi annual cycles at this beach in particular monthly beach cycles have little energy associated with them the method described provides a uniform way to objectively discriminate energetic beach cycles and yields a concise representation for beach modeling and prediction it should be a valuable tool for uniform quantitative intercomparison of beaches and beach cycles

holderness coast erosion and the significance of ords a sample ord a low section of beach characteristic of the holderness coast is examined as it moved southwards between 1977 and 1983 and its significant role in coast erosion is demonstrated the reduction in beach level at the cliff foot by up to 3 9 m enabled most hwn tides to reach it as compared with only some hws tides along the inter ord beach and the volume of till eroded from the cliffs increased by eight times to an annual mean of 72m3m 1 the holderness tills are shown to be composed of 31 per cent sand and coarser sediment which is the sediment range of the beaches where an ord is sited the massive injection of beach sediment goes to form the ord s most prominent constructional feature the lower beach ridge which extends southwards from the centre of the ord analysis of 12 months observer wave data collected at withernsea in 1969 70 indicates that a net southward sand movement of 144 000 m3 occurred comparison of this rate with beach sediment input rates along the whole coast backed by till cliffs suggests a sediment deficit at the northern end and a surplus towards the southern end this conclusion is supported by an overall increase in beach sediment volume southwards from barmston within this longshore sediment transport system the ords migrate southwards from their point of origin in the barmston skipsea area without losing their identity until reaching the tip of spurn head

holocene history of hubbard glacier in yakutat bay and russell fiord southern alaska stratigraphic and geomorphic data defined by radiocarbon ages tree ring dates and historical observations provide evidence of three major holocene expansions of hubbard glacier early in each advance the hubbard glacier margin blocked russell fiord to create russell lake raising base level and causing stream beds and fan deltas throughout the russell drainage basin to aggrade each hubbard glacier expansion continued with an ice lobe advancing through disenchantment and yakutat bays in the west and an eastern lobe advancing into russell fiord the earlier two holocene expansions were respectively under way at 7690 and 5600 calibrated yr b p and each advance culminated more than 1 k y later the late holocene advance was under way by 3100 yr ago and reached 13 km farther south in russell fiord than the preceding two expansions late holocene deglaciation of yakutat and disenchantment bays was complete before a d 1791 nunatak glacier flowing from neves east of russell fiord became the primary ice source to the russell fiord lobe at or before this date ice retreat from the southern end of russell fiord began in the late eighteenth century and the penultimate russell lake drained ca a d 1860 the relatively slow advances and more rapid retreats of hubbard glacier are consistent with the model of the iceberg calving glacier cycle hubbard glacier is currently advancing and will likely reestablish russell lake in the near future affecting local fisheries however glacier lobes are unlikely to reach the area of the town of yakutat built on late holocene glacial deposits in the next 1 k y