the deep water fold and thrust belt offshore nw borneo gravity driven versus basement driven shortening the deep water region offshore nw borneo is an active fold and thrust belt that hosts a signifi cant number of proven hydrocarbon accumulations in the past two mechanisms have been discussed as primary control for neogene to holocene folding and thrusting in this deep water province 1 basement driven crustal shortening and 2 gravity related delta tectonics in this study new balanced interpretations of regional crustal scale depth migrated twodimensional 2 d multichannel seismicrefl ection profi les are presented that provide for the fi rst time quantitative data on tectonic shortening throughout the entire deep water fold and thrust belt of nw borneo we use our tectonic restorations to compare the amount of deep water shortening on the nw borneo slope to the amount of extension across the nw borneo shelf a key result of this balancing study is the observation that pliocene to holocene gravity driven shortening decreases from south to north while the total amount of shortening increases slightly to the north consequently the amount of purely basement driven compression along nw borneo is strongly inferred to increase toward the north because most of the shortening is late pliocene and younger we interpret the tectonic shortening to be ongoing

stratigraphic development of synkinematic deposits in a large growth fault system onshore brunei darussalam kilometre scale synsedimentary faults associated with well preserved sedimentary successions on both hanging wall and footwall sides are rarely seen in outcrop however one such example is found in a middle miocene deltaic sand shale sequence exposed along the jerudong anticline in onshore brunei darussalam integration of detailed outcrop information with regional geophysical subsurface data documents that 1 initiation of the jalan tutong growth fault was related to delta front instability and failure 2 sediment loading under shallow marine conditions maintained syndepositional faulting 3 growth fault abandonment coincided with a significant basinward shift of deposition and subaerial exposure high resolution analysis of facies characteristics stratal expansion and throw development within the studied growth sequence indicates that differential fault movement strongly influenced facies composition sedimentary geometry and bedding characteristics sandstone poor shelf to lower shoreface successions developed during periods of rapidly increasing fault movement whereas sandstone rich amalgamated upper shoreface deposits preferentially formed during intervals of constant or decreasing throw development the data and interpretations presented may help to focus attention on the complex interplay between tectonics and sedimentation associated with the growth and decay of deltaic faults by providing for the first time an outcrop based view on a feature documented in similar scale only by geophysical subsurface data

metamorphic evolution of the attic cycladic metamorphic belt on naxos cyclades greece utilizing 40ar 39ar age spectrum measurements 40ar 39ar age spectrum analysis of phengite separates from naxos part of the attic cycladic metamorphic belt in greece indicates that cooling following high pressure low to medium temperature metamorphism m1 occurred about 50 ma ago phengite has 40ar gradients that suggest that part of the scatter observed in conventional k ar ages was caused by diffusion of radiogenic argon from the minerals during a younger metamorphism m2 in central naxos this metamorphism m2 has overprinted the original mineral assemblages completely and is associated with development of a thermal dome excellent 40ar 39ar plateaus at 15 0 0 1 ma 11 8 0 1 ma and 11 4 0 1 ma obtained on hornblende muscovite and biotite respectively from the migmatite zone indicate that relatively rapid cooling followed the m2 event and that no significant thermal overprinting occurred subsequent to m2 toward lower m2 metamorphic grade 40ar 39ar plateau ages of hornblendes increase to 19 8 0 1 ma concomitantly the proportion of excess 40ar in the spectra increases as well we propose that the peak of m2 metamorphism occurred beween 15 0 and 19 8 ma ago k ar ages of biotites from a granodiorite on the west coast are indistinguishable from those found in the metamorphic complex and hornblende k ar ages from the same samples are in the range 12 1 13 6 ma as the latter ages are somewhat younger than most ages obtained from the metamorphic complex intrusion of the granodiorite most likely followed the peak of the m2 metamorphism the metamorphic evolution of naxos is consistent with rapid crustal thickening during the cretaceous or early tertiary causing conditions at which supracrustal rocks experienced pressures in the range 900 1500 mpa transition to normal crustal thicknesses ended the m1 metamorphism about 50 ma ago the m2 metamorphism and granodiorite intrusion occurred during a period of heat input into the crust possibly related to the migration of the hellenic volcanic ar c in a southerly direction through the area

modeling of rainfall triggered shallow landslide by integrating hydrological modeling with the infinite slope stability analysis a rainfall triggered shallow landslide model was developed by iverson water resour res 36 1897 1910 2000 in iverson s model the infiltration capacity is assumed to be equivalent to the saturated hydraulic conductivity for finding pressure heads analytically however for general infiltration process the infiltration capacity should vary with time during the period of rain and the infiltration rate is significantly related to the variable infiltration capacity to avoid the unrealistically high pressure heads iverson employed the beta line correction by specifying that the simulated pressure heads cannot exceed those given by the beta line in this study the suitability of constant infiltration capacity together with the beta line correction for hydrological modeling and landslide modeling of hillslope subjected to a rainfall is examined by amending the boundary condition at ground surface of hillslope in iverson s model the modified iverson s model with considering general infiltration process is developed to conduct this examination the results show that the unrealistically high pressure heads from iverson s model occur due to the overestimation of infiltration rate induced from the assumption that the infiltration capacity is identical to the saturated hydraulic conductivity considering with the general infiltration process the modified iverson s model gives acceptable results in addition even though the beta line correction is applied the iverson s model still produces greater simulated pressure heads and overestimates soil failure potential as compared with the modified iverson s model therefore for assessing rainfall triggered shallow landslide the use of constant infiltration capacity together with the beta line correction needs to be replaced by the consideration of general infiltration process

tectonic and metasomatic mixing in a high t subduction zone melange insights into the geochemical evolution of the slab mantle interface abstract the catalina schist california contains an amphibolite grade 0 8 1 1 gpa 640 750 c melange unit consisting of mafic and ultramafic blocks in high mg schistose melange matrix with varying modal proportions of talc chlorite anthophyllite calcic amphibole enstatite and minor phases including zircon rutile apatite spinel and fe ni sulfides this melange unit is interpreted as a kilometer scale zone of tectonic and metasomatic mixing formed within a juvenile subduction zone the study of which may yield insight into chemical mixing processes at greater depths in subduction zones relationships among the major and trace element compositions of the mafic and ultramafic blocks in the melange the rinds developed at the margins of these blocks and the surrounding melange matrix are compatible with the evolution of the melange matrix through a complex combination of infiltrative and diffusional metasomatism and a process resembling mechanical mixing simple linear mixing models are compatible with the development of the melange matrix primarily through simple mixture of the ultramafic and mafic rocks with cr al ratios serving as indicators of the approximate proportions of the two lithologies this conclusion regarding mafic ultramafic mixing is consistent with the field observations and chemical trends indicating strong resemblance of large parts of the melange matrix with rinds developed at the margins of mafic and ultramafic blocks the overall process involved development of metasomatic assemblages through complex fluid mediated mixing of the blocks and matrix concurrent with deformation of these relatively weak rind materials which are rich in layer silicates and amphibole this deformation was sufficiently intense to transpose fabrics progressively disaggregate more rigid block derived materials in weaker chorite and talc rich melange and in some particularly weak lithologies e g chlorite talc and amphibole rich materials intimately juxtapose adjacent lithologies at the sub cm scale approaching grain scale sampled by the whole rock geochemical analyses chemical systematics of various elements in the melange matrix can be delineated based on the cr al based mixing model simple mixing relationships exhibited by al cr mg ni fe and zr provide a geochemical reference frame for considerations of mass and volume loss and gain within the melange matrix the compositional patterns of many other elements are explained by either redistribution local stripping or enrichment at varying scales within the melange ca na k ba and sr or massive addition from external sources si and h2o the latter probably in infiltrating h2o rich fluids that produced the dramatic o and h isotopic shifts in the melange melange formation resulting in the production of high variance ultramafic assemblages with high volatile contents may aid retention of volatiles in this case h2o to greater depths in subduction zones than in original subducted mafic and sedimentary materials the presence of such assemblages i e containing minerals such as talc chlorite and mg rich amphiboles would impact the rheology of the slab mantle interface and perhaps contribute to the low velocity seismic structure observed at near the slab mantle interface in some subduction zones if operative along the slab mantle interface complex mixing processes such as these involving the interplay between fluid mediated metasomatism and deformation also could impact slab incompatible trace element and isotopic signatures ultimately observed in arc magmas producing fluids with geochemical signatures inherited from interactions with hybridized rock compositions

a geothermobarometric investigation of garnet peridotites in the western gneiss region of norway in the western gneiss region of norway are found numerous peridotite lenses which have been extensively recrystallized under amphibolite fades conditions during the caledonian orogeny however evidence for an earlier caledonian high pressure metamorphism has been recorded by abundant eclogite and granulite relicts within gneiss and by the presence of at least ten garnet perioditite bodies preserved within chlorite peridotites two garnet bearing ultramafic assemblages have been recognized olivine orthopyroxene clinopyroxene garnet and olivine ortho pyroxene pargasitic amphibole garnet

stratigraphy and age estimate of neogene shallow marine fossiliferous deposits in brunei darussalam ambug hill tutong district abstract the ambug hill in brunei is an exceptional geological site where a series of siliciclastic rocks crops out with some layers extremely rich in marine fossils such fossiliferous outcrops are extremely rare in the northern part of borneo and their description is of primary importance as their fossil content can be used to correlate the regional depositional sequences with global biostratigraphic zonations in this work we present for the first time a detailed sedimentary profile completed with sr isotopes and biostratigraphic dating the succession is divided into four sedimentary units the first unit comprises bioturbated sandstone followed by a second unit of clay silt rich levels whose first 9 5 m contain rich marine fossil assemblages calcareous nannoplankton data indicate a late tortonian early messinian nn11 age which is confirmed by sr isotope dating derived from bivalves giving a numerical age range from 8 3 to 6 2 my after a major emersion surface the third sedimentary unit of sand and siltstone lacks suitable fossils for bio and isotope stratigraphy the age of emersion and the related sedimentary gap can be either correlated with the me1 7 25 my or the me2 5 73 my sequence boundary a fourth thin sedimentary unit is recognized on the top of the profile with silt and claystone beds without age diagnostic remains and calcareous fossils

high temperature hydration of ultramafic granulites from the southern marginal zone of the limpopo belt by infiltration of co2 rich fluid abstract the peak metamorphic assemblage of olivine orthopyroxene calcic amphibole spinel is replaced by a variety of high temperature retrograde assemblages in ultramafic granulites from the southern marginal zone of the limpopo belt the observed replacement of olivine according to the reaction forsterite co2 enstatite magnesite is restricted to the zone of rehydration which bounds the granulite zone in the south this observation is consistent with the post peak metamorphic infiltration of an externally derived co2 rich fluid during the high temperature post peak metamorphic history of the limpopo belt this event established a retrograde orthoamphibole isograd conditions of the retrogression are estimated to be near 670 c 6 kbar and xco2 fluid 0 67 c isotopic compositions of co2 extracted from magnesite produced by the high temperature breakdown of olivine supports a deep seated mantle source for the co2 rich retrograde fluid

sediment supply systems of the champion delta of nw borneo implications for deepwater reservoir sandstones abstract middle miocene to pliocene sedimentation on the nw borneo margin has been interpreted as the product of one relatively large deltaic system the champion delta however several lines of evidence indicate that the champion system was not a simple large delta its drainage basin was too small fluvial outcrops indicate multiple relatively small rivers and outcrop studies indicate the same facies associations as the diverse modern depositional systems the number and location of rivers reaching the shoreline changed as rapidly subsiding footwall synclines episodically active inversion anticlines and growth faults created an evolving structurally generated topography that not only controlled drainage pathways but also segregated champion strata into thick wave dominant and tide dominant successions although the principal rivers within the champion system the limbang padas and trusan rivers transport significant loads of coarse sediment the intermittent proximal ponding of sand in local basins as is currently occurring in brunei bay resulted in a variable delivery of sand to the shelf edge the number and distribution of shelf edge canyons also changed with time consequently the spatial and temporal distribution of deepwater sand accumulations sourced from the champion system are not solely related to relative sea level fluctuations such accumulations should be smaller and more scattered than those sourced from a large shelf edge delta because the catchments of the champion system s principal rivers represent different provenances the system s deepwater sands may carry the signal of specific rivers for example mineralogical contrasts between in the main reservoir sands of the deepwater gumusut and kikeh fields suggest that the relative contributions of the principal rivers shifted with time with the trusan and limbang rivers dominating sand supply for the youngest reservoirs at gumusut

shallow landslide hazard assessment using a physically based model and digital elevation data a model for the analysis of topographic influence on shallow landslide initiation is applied to an experimental mountain basin where high resolution digital elevation data are available the cordon catchment 5 km2 located in northern italy the model delineates those areas most prone to shallow landsliding due to surface topographic effects on hydrologic response the model is composed of two parts a steady state model for shallow sub surface runoff and an infinite slope coulomb failure model which assumes that the soil is cohesionless at failure an inventory of landslide scars is used to document sites of instability and to provide a test of model performance by comparing observed landslide locations with model predictions the model reproduces the observed distribution of landslide locations in a consistent way although spatial variations in soil strength and transmissivity which are not accounted for in the model influence specific distribution of landslide areas within regions of similar topographic control

characteristics of repeated detached miocene pliocene tectonic inversion events in a large delta province on an active margin brunei darussalam borneo abstract the baram delta province evolved during the middle miocene to present day from a foreland basin to a shelf margin episodic folding events affected the region causing uplift of the hinterland delta progradation and inversion of gravity related faults existing models place the n s and ne sw trending folds in a strike slip transpressional setting new geological mapping and re interpretation of existing data suggest the region is better understood as the development of a west verging thrust belt in a middle miocene foreland basin filled by sandstones and shales of the belait and setap formations with key major folds jerudong and belait anticlines belait syncline forming during the middle miocene as fault bend and fault propagation folds the deformation style is complicated by the predominantly shaley middle miocene pliocene setap formation becoming thicker and more overpressured from south to north onshore where the setap formation is thin or absent the belait formation is attached to the underlying lower miocene and older sequences offshore and in a narrow onshore strip the overpressured setap formation causes deformation in the beltait formation to be detached from the underlying cretaceous tertiary accretionary prism basement the detached style exhibits considerable structural complexity including lift off folds growth faults shale diapirs and pipes onshore thrust and inversion features are dominantly n s trending and began activity in the middle miocene deformation is probably associated with an e w maximum horizontal stress direction in the late miocene around 7 5 ma watters et al 1999 ne sw striking inversion folds developed located mostly over early counter regional faults and associated reactive diapirs folds verge towards the nw when underlain by counter regional faults and towards the s or se when underlain by regional faults folds and diapirs along n s trends were also reactivated continuation of this deformation into the pliocene is largely confined to the offshore area onshore the n s structures no detachment were not reactivated during the pliocene

p t paths from high temperature shear zones beneath ophiolites mctamorphic rocks of the st anthony complex of north western newfoundland are best interpreted in terms of a high temperature shear zone formed between down going continental margin rocks and overriding oceanic lithosphere in a subduction zone high grade rocks immediately beneath the oceanic lithosphere peridotite display retrograde meta morphism in high strain zones whereas lower grade rocks near the base of the metamorphic complex display prograde metamorphism in high strain zones mylonite zones in meta basitcs at all levels in the complex contain the assemblage epidote hornblende albite sodic oligoclase these observations suggest that the inverted metamorphic gradient within the st anthony complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone the degree of re equilibration is strongly dependent on the degree of strain and is best achieved in synmetamorphic mylonite zones this interpretation of the st anthony complex can be extended to other sub ophiolite metamorphic sheets which show very similar relationships it is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones the p t paths of which preserve records of burial and exhumation in subduction zones

compression versus extension in the exhumation of the dora maira coesite bearing unit western alps italy abstract several recent papers suggest that uplift and exhumation of high pressure low temperature hp lt metamorphic rocks are due to crustal extension coeval with and or post dating the contraction responsible for the metamorphism a detailed survey of the coesite bearing unit of the southern dora maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra high pressure uhp rocks this uhp unit consists of pre alpine basement rocks which equilibrated at a depth of 100 km probably 120 100 ma ago it is overlain by two units with a similar lithology and eoalpine metamorphic age and high p t gradients which equilibrated at 60 and 50 km respectively this eoalpine basement complex is intercalated between blueschist facies units the whole pile was affected by a mesoalpine greenschist facies mylonitic deformation during which the early contacts were reactivated as low angle shallow dipping shear zones extension is documented only during the mesoalpine collisional period and is referred to the gravity collapse of a thickened orogenic wedge with an exhumation rate of about 1 mm yr the most important part of the exhumation of the uhp rocks occurred earlier at a higher rate close to 4 mm yr a forced return flow or extrusion tectonics of imbricate slices in an on going subduction setting seems appropriate to account for this early stage the fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed

relationship between structural style overpressures and modern stress baram delta province northwest borneo christopher k morley mark tingay and richard hillis australian school of petroleum university of adelaide adelaide australia rosalind king

potential for landsliding dependence on hyetograph characteristics 1 this paper examines the effect of hyetograph shape on the potential for landsliding in soil mantled landscapes an existing pore pressure response model iverson 2000 is used to study the effects of unsteady rainfall infiltration in hillslopes and the effect of slope and convergent topography is expressed using a steady state model of slope parallel subsurface flow slope stability is assessed using an infinite slope analysis this theoretical framework is coupled with simple hyetograph models and to intensity duration frequency functions to determine the return period of landslide triggering rainfall results also show that hyetographs with a peak at the end of a rainfall event have a stronger destabilizing effect than hyetographs with a constant rainfall or with a peak at the beginning of a storm thus the variability of hyetograph shapes adds uncertainty to the assessment of landsliding triggered by rainfall

rainfall characteristics for anisotropic conductivity of unsaturated soil slopes the purpose of this study was to investigate the effects of the anisotropic ratio on the stability of slopes using the reliability index approach a numerical analysis of the relationship between the three rainfall patterns advanced normal and delayed and the anisotropic ratios was designed this study also considered three different soil properties sand silt and clay to simulate rain infiltration in this study probability analysis was used to evaluate the stability of unsaturated soil slopes the finite element computer program geo studio was used to simulate the process of rainwater infiltrating a slope the pore water pressure results evaluated from seepage analysis seep w were imported into the slope stability program slope w results for the anisotropic ratio of hydraulic conductivity indicate that when the anisotropic ratios become higher the reduction in the reliability index is insignificant in addition the simulation results indicated that when saturated hydraulic conductivity ks was less than rainfall intensities i the percentage probability of the occurrence of a landslide was larger than when ks was greater than i finally in the cases of anisotropic ks stability of the high ratio soil slopes was not found to be sensitive to the reliability index variation during the simulation period moreover when ks was greater than i slope stability decreased earlier than was the case in the opposite situation

a physically based model for the topographic control on shallow landsliding a model for the topographic influence on shallow landslide initiation is developed by coupling digital terrain data with near surface through flow and slope stability models the hydrologic model topog o loughlin 1986 predicts the degree of soil saturation in response to a steady state rainfall for topographic elements defined by the intersection of contours and flow tube boundaries the slope stability component uses this relative soil saturation to analyze the stability of each topographic element for the case of cohesionless soils of spatially constant thickness and saturated conductivity the steady state rainfall predicted to cause instability in each topographic element provides a measure of the relative potential for shallow landsliding the spatial distribution of critical rainfall values is compared with landslide locations mapped from aerial photographs and in the field for three study basins where high resolution digital elevation data are available tennessee valley in marin county california mettman ridge in the oregon coast range and split creek on the olympic peninsula washington model predictions in each of these areas are consistent with spatial patterns of observed landslide scars although hydrologic complexities not accounted for in the model e g spatial variability of soil properties and bedrock flow control specific sites and timing of debris flow initiation within areas of similar topographic control

shale tectonics and deformation associated with active diapirism the jerudong anticline brunei darussalam the jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock the n s trending anticline overlies a high angled basement fault zone that was episodically active moving predominantly sinistrally in response to transpressive stresses the earliest stage of deformation in the study area was mid miocene e w to ne sw trending growth faulting and shale diapir growth early normal faults exerted an important influence on the large and small scale bedding geometries and facies changes of syn tectonic shallow marine shoreface and tidal strata in particular there are laterally rapid facies changes and differential rotation of strata across growth faults diapir activity is indicated by the presence of shale dykes which commonly intrude normal fault planes and concordant shale intrusions shale intrusions associated minor thrust faults and normal faults were rotated during a late miocene early pliocene phase of folding related to transpression few shale dykes were intruded during this phase probably because the horizontal principal stresses were relatively large compared with the pore fluid pressure at the end of the transpressive phase shale dykes were intruded into steeply dipping beds as a result of stress relaxation continued uplift and erosion elevated overpressured horizons to a point where hydraulic fracturing reached the surface and holocene age mud volcanoes were developed

petrology of marble and peridotite in the seiad ultramafic complex northern california usa peridotite and infolded marble of the seiad ultramafic complex were recrystallized in the upper amphibolite facies as part of the regional progressive metamorphism of the rattlesnake creek terrane field relations including the occurrence of metarodingites and metasomatic zones between dissimilar rock types demonstrate that the metasediments and serpentinized ultramafic rocks were juxtaposed prior to regional barrovian metamorphism temperatures are estimated to have reached 760 800 c at pressures of 7 8 kbar during the peak of metamorphism four low variance parageneses have been identified within a small 3 km2 area of the complex which may reasonably be assumed to have formed under the same p and t conditions isobaric t xco2 diagrams of appropriate equilibria are presented for three different internally consistent sets of thermodynamic data despite the seemingly small numerical differences between the standard state thermodynamic properties of the data sets only one diagram allows the four observed assemblages to coexist within a reasonable temperature range all three phase diagrams require differences in fluid composition on the scale of a thin section strong evidence for effective control of pore fluid composition by local mineral reactions during metamorphism

effect of h2o released by dehydration of serpentine and chlorite on compressional wave velocities of peridotites at 1 gpa and up to 1000 c abstract compressional wave velocities v p of serpentine bearing wehrlite 0 2 wt h 2 o serpentinized dunite 4 1 wt h 2 o chlorite bearing websterite 1 3 wt h 2 o and chlorite bearing dunite 0 6 wt h 2 o were measured at 1 gpa and up to 1000 c during heating to and cooling from 1000 c during heating the serpentine bearing rocks showed a sudden decrease in v p at 600 700 c when serpentine dehydration took place similarly the chlorite bearing rocks exhibited a sharp change in v p at 800 900 c due to chlorite dehydration during cooling experiments on both serpentine and chlorite bearing rocks v p showed a linear increase down to room temperature suggesting that no rehydration reaction took place with decreasing temperature the decrease in v p due to dehydration reactions was relatively small 1 5 in rocks with low h 2 o content 0 2 1 3 wt whereas the dehydration reaction of serpentinized dunite with higher 4 1 wt h 2 o led to a marked decrease in v p 5 1 which is comparable to the degree of v p reduction reported from the low velocity anomaly regions in subducting slabs the effect of h 2 o fluid on the v p of peridotite is evaluated by assessing the ratio of v p values v p v p 0 under both wet and dry conditions the v p v p 0 ratio was 0 97 0 99 in rocks with 0 2 1 3 wt h 2 o but was markedly lower in rocks with 4 1 wt h 2 o 0 87 the run products showed variable geometries of fluid filled pores which clearly depended on the h 2 o content of the rock studied the run products with 0 2 1 3 wt h 2 o had relatively small and isolated fluid filled pores whereas run products with 4 1 wt h 2 o contained relatively large fluid filled pores that were interconnected our results suggest that connected fluid filled pores cause a significant decrease in v p but that isolated fluid pores have little effect on v p

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

shallow landslides in pyroclastic soils a distributed modelling approach for hazard assessment an effective assessment of shallow landslide hazard requires spatially distributed modelling of triggering processes this is possible by using physically based models that allow us to simulate the transient hydrological and geotechnical processes responsible for slope instability some simplifications are needed to address the lack of data and the difficulty of calibration over complex terrain at the catchment s scale we applied two simple hydrological models coupled with the infinite slope stability analysis to the may 1998 landslide event in sarno southern italy a quasi dynamic model barling et al 1994 was used to model the contribution to instability of lateral flow by simulating the time dependent formation of a groundwater table in response to rainfall a diffusion model water resour res 36 2000 1897 was used to model the role of vertical flux by simulating groundwater pressures that develop in response to heavy rainstorms the quasi dynamic model overestimated the slope instability over the whole area more than 16 but was able to predict correctly slope instability within zero order basins where landslides occurred and developed into large debris flows the diffusion model simulated correctly the triggering time of more than 70 of landslides within an unstable area amounting to 7 3 of the study area these results support the hypothesis that both vertical and lateral fluxes were responsible for landslide triggering during the sarno event and confirm the utility of such models as tools for hazard planning and land management

temperature time history of subducted continental crust mount olympos region greece in the mt olympos region of northeastern greece continental margin strata and basement rocks were subducted and metamorphosed under blueschist facies conditions and thrust over carbonate platform strata during alpine orogenesis subsequent exposure of the subducted basement rocks by normal faulting has allowed an integrated study of the timing of metamorphism its relationship to deformation and the thermal history of the subducted terrane alpine low grade metamorphic assemblages occur at four structural levels three thrust sheets composed of paleozoic granitic basement and mesozoic metasedimentary cover were thrust over mesozoic carbonate rocks and eocene flysch thrusting and metamorphism occurred first in the highest thrust sheets and progressed downward as units were imbricated from ne to sw 40ar 39ar spectra from hornblende white mica and biotite samples indicate that the upper two units preserve evidence of four distinct thermal events 1 293 302 ma crystallization of granites with cooling from 550 c to 20 km extension in the olympos region was synchronous with extension in the mesohellenic trough and the aegean back arc and concurrent with westward progressing shortening in the external hellenides

modeling of layered infinite slope failure triggered by rainfall the infinite slope is typically regarded as composed of a single layered soil with a uniform property in various physical based models used for modeling rainfall induced shallow landslides this study extends the physical based model to consider the layered infinite slope to examine the importance of soil layer distribution for rainfall induced shallow landslides hypothetical scenarios of infinite slope composed of soil layers with different thicknesses and parameters are employed to conduct this examination the results show that pressure heads caused by rainfall infiltration are strongly related to soil layer distribution this shows the significant influence of soil layer distribution in assessing infinite slope stability failure of a layered infinite slope does not necessarily occur at the impervious bottom of the hillslope soil but may also occur at the interface between two soil layers this result shows that a neglect of soil layer distribution could misestimate failure depth hence soil layer distribution must be considered to reliably analyze infinite slope failure induced by rainfall

multiple generations of extensional detachments in the rhodope mountains northern greece evidence of episodic exhumation of high pressure rocks abstract an integrated structural and petrological study shows that exhumation of high rocks of the rhodope occurred in several pulses the structurally uppermost kimi complex recording an early cretaceous high p metamorphism was exhumed between about 65 and 42 ma the sidironero kardamos and kechros complexes that record early tertiary high metamorphism at least 19 kbar at 700 c were exhumed at 42 30 ma exhumation occurred with isothermal decompression strain episodes depict thrusting of medium high grade above lower grade high p rocks syn thrusting extension and post thrusting extension polyphase extension created several generations of detachment zones which in sum excise about 20 km of material within the crustal profile this reduction in crustal thickness is consistent with a reduction of the present crustal thickness from more than 40 km to less than 30 km in the eastern rhodope we mapped the post thrusting xanthi low angle detachment system over 100km from its break up zone above the sidironero complex central rhodope into the eastern rhodope this detachment shows an overall ese dip with a ramp and flat geometry cutting across the earlier thrust structures the kimi complex is the hanging wall of all synand post thrusting extensional systems on top of the kimi complex marine basins were formed from the lutetian c 48 43 ma through the oligocene during extension successively at c 26 to 8 ma the thasos pangeon metamorphic core complexes were exhumed in these times representing the early stages of aegean back arc extension the strymon and thasos detachment systems caused crustal thinning in the western rhodope renewed heating of the lithosphere associated with magmatism and exhumation of hot middle crust from beneath the sidironero complex occurred we focusoxn the geometry timing and kinematics of extension and contraction structures related to the 42 30 ma interval and how these exhumed high p rocks we interpret high rocks exhumed in this interval as a window of the apulian plate beneath the earlier in the cretaceous accreted kimi complex

phase relations to 3 kbar in the systems edenite h2o and edenite excess quartz h2o abstract amphiboles approached edenite naca 2 mg 5 si 7 alo 22 oh 2 richterite na 2 camg 5 si 8 o 22 oh 2 tremolite ca 2 mg 5 si 8 o 22 oh 2 solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite and edenite additional quartz all with excess h 2 o for the stoichiometric edenite bulk composition excess h 2 o the equilibrium phase assemblage is diopside na phlogopite forsterite fluid at and just above the amphibole high temperature limit at 850 5 c 500 bar and 880 5 c 1000 bar the breakdown temperature of sodic phlogopite is 855 3 c at 500 bar and 890 5 c at 700 bar producing nepheline plagioclase or melt additional forsterite and fluid diopside and na phlogopite solid solution coexist over a broad p fluid t region even within the amphibole field where they are associated with an edenite richterite tremolite solid solution of approximate composition ed 35 rc 50 tr 15 in the system edenite 4 quartz excess h 2 o nearly pure tremolite and albite coexist stably between 670 and 830 c at 1000 bar and give way to the possibly metastable assemblage diopside talc albite below 670 c in the presence of albite tremolite reacts to produce diopside quartz enstatite fluid above 830 c at 1000 bar for the investigated silica rich bulk composition amphibole p fluid t stability is divided by the albite melting curve into a tremolite albite field and a tremolite aqueous melt field substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica excess bulk compositions metastable edenite rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free sio 2 edenitic amphibole is stable only over a very limited temperature range in silica undersaturated environments thus accounting for its rarity in nature na phlogopite solid solutions are also disfavored by high a sio 2 even for nepheline normative lithologies a hypothesized rapid low temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks

amphibole in a spinel lherzolite xenolith evidence for volatiles and partial melting in the upper mantle beneath southern british columbia pargasitic amphibole has been observed for the first time in an ultramafic xenolith from british columbia the xenolith is a chrome diopside bearing spinel lherzolite trapped within an alkali basaltic lava flow at lightning peak near vernon british columbia amphibole 5 occurs within the xenolith as small grains interstitial between other xenolith mineral phases and always shows evidence of melting microprobe analyses of the amphibole reveal that it is a pargasite rich in mgo mgo 17 1 17 7 wt mg mg fe2 0 89 and cao 10 4 10 7 wt textural and chemical evidence suggests that the pargasite is in equilibrium with the other phases in spinel lherzolite the pargasite probably crystallized within the spinel stability field of the upper mantle from a volatile rich metasomatic fluid that was produced by dehydration of subducted material melting in the amphibole may have been caused by one of three processes superheating by the host alkali basalt decompression as the magma ascended or

stability and composition relations of calcic amphiboles in ultramafic rocks calcic amphiboles are observed in ultramafic rocks that have equilibrated under a broad span of geological conditions and might prove to be good indicators of metamorphic grade if their stabilities could be determined as a function of their compositions experiments were performed on the stability of tremolite plus forsterite in the system h2o cao mgo sio2 from 5 to 20 kbar a univariant curve was fitted to the experimental brackets using volume water fugacity and heat capacity data the results indicate that the maximum stability of tremolite in the presence of forsterite is about 825 c at 5 kbar addition of al2o3 to this system increases the stability of tremolitic amphibole by only 20 40 c and induces solubility of 5 7 wt al2o3 in the amphibole as determined from quantitative sem analyses of individual amphibole crystals thus substitution of the tschermakite component ca2 mg3al2 si6al2 o22 oh 2 alone cannot lead to the greatly enhanced al2o3 contents or thermal stability of natural calcic amphiboles

geochronology of blueschists from west central baja california and the timing of uplift in subduction complexes a geochronologic study of blueschists from west central baja california has provided constraints on the timing of high pressure low temperature metamorphism and the subsequent uplift of this subduction complex mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by 40 ar 39 ar fission track and rb sr methods 40 ar 39 ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite matrix melange show slow cooling gradients between 115 95 ma rb sr apparent ages for one block are concordant with these results 40 ar 39 ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below 100 c at 22 ma and 32 ma respectively subduction related metamorphism of the coherent blueschists occurred in la

zambales ophiolite philippines i geology and petrology of the critical zone of the acoje massif abstract the acoje massif constitutes the northernmost massif of the zambales ophiolite complex an assemblage of relatively intact fragments of a mesozoic oceanic lithosphere the critical zone of the massif consists of interlayered dunite harzburgite lherzolite wehrlite pyroxenite and feldspathic units which grade upsection into gabrronorite and gabbro cumulates this sequence occurs between highly deformed residual harzburgite and a 4 to 5 5 k thick cumulate and isotropic gabbro sequence the critical zone can be subdivided into a 540 m thick ultramafic zone and a 390 m thick mafic feldspathic zone the boundary between these zones is marked by the first appearance of cumulus plagioclase the paragenetic sequence that best describes the cumulus mineralogy is chromite chromite olivine chromite olivine orthopyroxene chromite olivine clinopyroxene orthopyroxene olivine clinopyroxene plagioclase orthopyroxene amphibole the lithologie and chemical including o 2 variation across the critical zone reflects multiple magma influx assimilation of tectonized harzburgite magma mixing fractional crystallization and intercumulus melt crystal interactions in a sub oceanic magma chamber therefore this critical zone represents the basal cumulate sequence of the acoje massif and not the uppermost portion of the residual mantle there is a general trend of decreasing of deformation from the bottom to the top of the critical zone this is attributed to the rheological inhomogeneity of the partially crystallized basal cumulate sequence during deformation geothermometric estimates based on the mg fe 2 exchange between olivine and spinel indicate equilibration to subsolidus temperatures

synthetic and natural tremolite in equilibrium with forsterite enstatite diopside and fluid the following equilibrium among tremolite forsterite diopside and orthorhombic enstatite has been investigated using either synthetic tremolite or natural amphibole in the starting materials ca2mg5si8o22 oh 2 mg2sio4 2 camgsi2o6 5mgsio3 h2o a significant increase in the stability of the reactants was observed with natural rather than synthetic tremolite for example in nearly pure h2o with the h2 content of the fluid buffered by nickel bunsenite at one kilobar 108 pascals the breakdown of the assemblage with synthetic amphibole occurs at 708 20 c the breakdown of the assemblage with natural amphibole ca2 16mg4 94fe0 03si7 92 al0 01o22 oh 2f0 03 occurs at 841 47 c the shift in the breakdown curve is attributed to variation in the properties of the amphiboles since all other factors were common in the experiments the reactions have also been investigated with hydrogen fugacity defined by the methane buffer and the nb oh xg coh buffer analysis of the experimental data by linear programming indicates that the enthalpy of reaction is tightly constrained when the calorimetrically determined entropy of 160 92 joules degree is used the resulting enthalpy of reaction is 113 96 1 82 kilojoules with the natural amphibole and 104 83 0 12 kilojoules with synthetic tremolite deviation of the natural amphibole from the ideal tremolite formula as well as a greater number of defects and dislocations in the synthetic amphibole may have contributed to the change in stability

discontinuous change in temperature derivative of vp in lower crustal rocks 1 compressional wave velocities vp of gabbronorite pyroxene granulite wehrlite and websterite were measured up to 1000 c at 1 gpa during heating and cooling the wehrlite and websterite which contain secondary hydrous minerals show irreversible change in temperature derivative of vp vp t during heating these rocks show sudden decrease in vp when dehydration reactions took place during cooling the rocks show a linear increase in vp in contrast the gabbronorite and pyroxene granulite which are nearly dry crystalline assemblage show reversible and discontinuous change in vp t at around 400 c it changes from 2 4 and 2 1 10 4 km s 1 c 1 to 4 7 and 4 5 10 4 km s 1 c 1 this reversible and discontinuous change in vp t is difficult to explain by the previously proposed mechanisms such as thermal cracking dehydration reaction and or partial melting the data suggest that this reversible and discontinuous change would be associated with phase transition in plagioclase

slab derived origin of tremolite antigorite veins in a supra subduction ophiolite the peridotite nappe new caledonia as a case study hydration of mantle peridotites provides information on the exhumation history and on the fluid regime accompanying exhumation of these rocks reservoirs involved fluid rock ratios temperature of interaction highly depleted harzburgites and dunites of the peridotite nappe of new caledonia are crosscut by fractures which have been pervasively serpentinized producing lizardite brucite magnetite and minor chrysotile in a near static environment probably by sea water circulating in cooling joints this event is generally referred to as primary serpentinization in a next step already serpentinized joints were re opened to produce tension and shear cracks sealed by higher temperature synkinematic fibrous minerals locally tremolite bearing veins and pockets which do not display evidence for void infill were generated by metasomatic replacement of the wall rock peridotite most veins only contain fibrous antigorite but some display tremolite antigorite intergrowths or even pure tremolite the latter rocks yield high contents of ca and incompatible elements which contrast with the overall depletion of the peridotite host rock and suggest contribution of an external source whole rock geochemical and isotopic features 87sr 86sr 18o and h suggest that antigorite veins which bear highly radiogenic sr isotope signatures were strongly influenced by fluids emitted by the subducted slab and associated sediments in contrast the geochemical and isotopic signatures of tremolite bearing rocks suggest a genetic link with early eocene supra subduction dykes and fluids that leached them calcium strontium and ree bearing oxidized fluids reacted with already serpentinized fracture wall rock as shown by fibre nucleation chromite alteration and high cr contents in tremolite the bulk of syn tectonic fluid rock interaction associated with shear and tensile fracture development probably occurred at the onset of eocene intra oceanic subduction when the buoyant lower plate the south loyalty basin obliquely forced its way beneath the nascent loyalty fore arc mobile elements extracted from supra subduction dykes temporarily enriched the circulating fluids and generated tremolite antigorite mg chlorite and magnetite instead of antigorite and magnetite solely tremolite crystallization probably ceased due to the exhaustion of ca rich fluids in a globally cooling fore arc environment

the structural evolution of folds in a deepwater fold and thrust belt a case study from the sabah continental margin offshore nw borneo se asia abstract the deepwater fold and thrust belt offshore sabah nw borneo is located on a continental margin that hosts a number of proven hydrocarbon accumulations reprocessed and depth migrated regional 2d seismic data reveal the occurrence of a wide range of compressional syn depositional deformation features including large scale thrust faults and prominent thrust hangingwall anticlines this paper focuses on the documentation and detailed description of the style of deepwater folds and the associated stratal patterns within a study area of ca 3000 km 2 offshore sabah nw borneo this region exhibits a variety of fold systems including 1 anticlines with large interlimb angles that exhibit no seafloor expression 2 anticlines with medium interlimb angles seafloor relief and a faulted anticline crest 3 anticlines with small to medium interlimb angles a seafloor expression and flanks associated with slides and slumps and finally 4 anticlines with small interlimb angles that are commonly buried by a thick sedimentary overburden and in places modified by superimposed syn sedimentary deltaic normal faults if analysed section per section perpendicular to the strike of the fold and thrust belt the widest anticlines occur in the most seaward parts of the fold belt near the present day thrust front whereas the narrower anticlines are located further landward however if viewed along the strike of the deepwater fold belt there are significant variations in the geometry of laterally corresponding contemporaneously developing fold systems to explain these differences factors such as rock inhomogeneities in the deformation wedge variable surface slope irregular basement topography differential sediment input or the presence or absence of subsurface gas hydrates need to be taken into account these factors vary significantly within the studied system and are therefore difficult to generalise and transfer to other submarine fold and thrust belts

an improved approach for evaluating the time dependent stability of colluvial landslides during intense rainfall the paper presents an improved analytical approach for evaluating the time dependent stability of colluvial landslides during intense rainfall infiltration the approach comprises two steps first to simulate rainfall infiltration and then to compute the safety factor in terms of the irregularity of the natural landslide surface in implementation of the method the landslide is divided into several soil slices with approximately straight sides each single slice is regarded as a finite slope so that the infiltration formula and safety factor can be deduced the infiltration formula is derived with combination of the green ampt model and mass conservation law considering seepage perpendicular and parallel to slope surface simultaneously in the modified infiltration model both size effect and angle effect are vividly observed in the process of rainfall infiltration into the finite slope while the former of which is not presented in the original green ampt model the safety factor is computed using the limit equilibrium method with the influence of infiltrating water on the shear strength gravity and seepage force of soil slices considered by case study of the shuping landslide in three gorges decline in the safety factor and the decrease in the tendency are definite specifically under the rainfall intensity of 50 mm h the failure of shuping landslide is most likely to occur at the time of 102 h in addition the results highlight shallow failure along the wetting front under intense rainfall

chlorite stability in mantle peridotite the reaction clinochlore enstatite forsterite pyrope h2o chlorite crystallising in hydrated mantle peridotite is a potential source of water for subduction zone volcanism the reaction clinochlore enstatite forsterite pyrope h2o is the important reaction for defining the stability of chlorite in these rocks it has been investigated in phase equilibrium experiments in piston cylinder and multi anvil apparatus between 2 and 5 gpa the reaction has a steep negative p t slope at low pressures with brackets at 860 880 c at 2 5 gpa 840 860 c at 3 0 gpa and 820 840 c at 3 5 gpa the reaction flattens out with increasing pressure to 5 0 gpa at 650 c chlorite compositions close to the reaction were determined from unit cell parameters measured using high resolution synchrotron powder diffraction these showed a decrease in al content with increasing pressure along the reaction consistent with calculations using a previously published thermodynamic data set the experimental results are consistent with the results of earlier studies on the reaction defining the maximum thermal stability of clinochlore clinochlore forsterite pyrope spinel h2o they have been used in the derivation of revised thermodynamic data for clinochlore in the latest version of the previously published data set the new results show that chlorite in subducting slab or overlying mantle wedge could dehydrate below subduction zone volcanoes and be a major source of the water required for melting however the dehydration reaction is not sufficiently pressure dependent to be responsible for the narrow range in depth from the volcanoes to the slab and chlorite will not be the only hydrous mineral involved

a distributed slope stability model for steep forested basins a distributed physically based slope stability model dslam based on an infinite slope model a kinematic wave groundwater model and a continuous change vegetation root strength model is presented it is integrated with a contour line based topographic analysis and a geographic information system gis for spatial data extraction and display the model can be run with either individual rainfall events or long term sequences of storms these inputs can be either actual storm records or synthesized random events based on monte carlo simulation the model is designed to analyze rapid shallow landslides and the spatial distribution of safety factor fs in steep forested areas it can investigate the slope stability problem in both temporal and spatial dimensions for example the impact of timber harvesting on slope stability either at a given time or through an extended management period the probability of landslide occurrence for a given year and the delivery of landslide sediments to headwater streams the dslam model was applied in a steep forested drainage of cedar creek in the oregon coast ranges using actual spatial patterns of timber harvesting and measured rainfall during a major storm which triggered widespread landslides in that area in 1975 simulated volume and number of failures were 733 m3 and 4 respectively these values agreed closely with field measurements following the 1975 storm however the effect of parameter uncertainty may complicate this comparison for example when soil cohesion values of 2 0 and 3 0 kpa were used the failure volume changed by factors of 2 04 and 0 41 respectively compared with the average condition of 2 5 kpa used in the simulation for soil depths 30 higher and lower than the standard condition the failure volume changed by factors of 2 0 and 0 27 respectively when maximum root cohesion changed from 12 5 kpa average condition to 10 kpa the failure volume increased 1 73 fold for the case of 15 kpa the failure volume changed by a factor of 0 55 the simulated failures caused by the storm were mostly in hollows the simulations show that the spatial distribution of fs is controlled mainly by topography and timber harvesting patterns and is greatly affected by groundwater flow patterns during major rainstorms most areas with fs 3 0 corresponded with the distribution of blocks clear cut in 1968 and all elements with fs 2 0 were in areas clear cut in 1968 areas with low fs 1 0 1 6 expanded dramatically during the rainstorm and decreased at a slow rate after the storm factors of safety in hollows declined sharply during the storm

analysis of infiltration processes into fractured and swelling soils as triggering factors of landslides the influence of rainfall in triggering landslides is a widely discussed topic in scientific literature the slope stability of fractured surface soils is often influenced by the soil suction rainfall infiltrating into soil fractures causes the decrease in soil suction and shear strength which can trigger the collapse of surface soil horizons water flow through fractured soils can also be affected by soil swelling and by capillary barrier effects in the case of low permeable soil overlying a more permeable one

field based evidence for devolatilization in subduction zones implications for arc magmatism metamorphic rocks on santa catalina island california afford examination of fluid related processes at depths of 15 to 45 kilometers in an early cretaceous subduction zone a combination of field stable isotope and volatile content data for the catalina schist indicates kilometer scale transport of large amounts of water rich fluid with uniform oxygen and hydrogen isotope compositions the fluids were liberated in devolatilizing relatively low temperature 400 degrees to 600 degrees c parts of the subduction zone primarily by chlorite breakdown reactions an evaluation of pertinent phase equilibria indicates that chlorite in mafic and sedimentary rocks and melange may stabilize a large volatile component to great depths perhaps 100 kilometers depending on the thermal structure of the subduction zone this evidence for deep volatile subduction and large scale flow of slab derived water rich fluids lends credence to models that invoke fluid addition to sites of arc magma genesis

biopyribole evolution during tremolite synthesis from dolomite and quartz in co2 h2o fluid a series of experiments with and without sample retreatment was performed on a starting mixture of dolomite plus quartz at 0 5 gpa and 600 c in a co 2 h 2 o fluid with a mole fraction of co 2 of 0 2 for durations up to 582 h the initial reaction of dolomite and quartz led to rapid formation of talc and calcite instead of tremolite and calcite by the intended reaction 5 dolomite 8 quartz h 2 o tremolite 3 calcite 7 co 2 with continued treatment the talc calcite quartz assemblage gradually reacts to form increasing amounts of tremolite and diopside with the eventual loss of quartz and nearly complete loss of talc the detailed structure of the amphibole and the nature of the mg enrichment were revealed using aem analysis of individual amphibole crystals the most abundant defects are triple chain chain multiplicity faults cmfs which appear as isolated lamellae with single periodicity in short duration experiments and increasingly as groups of lamellae with variable multiplicity and periodicity in longer duration experiments and especially in a long duration experiment without retreatment in this latter experiment a calcian clinojimthompsonite domain extending 15 unit cells along the b axis was observed the sample after 582 h and five retreatments shows on average 3 5 true solid solution with mg expressed as the mg cummingtonite mc component after correction for the presence of cmfs this sample is thought to most closely approach the equilibrium composition for the amphibole the sample after 250 h with three retreatments has about 10 whereas that after 582 h without retreatment has 0 6 mc component the latter having a relatively high density of cmfs this study affirms the importance that precursor non amphibole biopyriboles play in the formation of tremolitic amphibole

estimating hinterland exhumation from late orogenic basin volume nw borneo a new approach for restoring sediment volumes onto the sediment source area to estimate uplift timing and magnitude is discussed and used in the context of late tertiary basin and topographic development in nw borneo sediment volumes for the baram deltaic province were estimated for four time periods latest early middle and late miocene and pliocene recent using 2d and 3d seismic horizons wells and outcrops volume restoration onto the palaeo sediment source area determined exhumation amount at the drainage divide c 5 km from 17 ma to recent and provided a reasonable match with other denudation estimation methods cooling ages and an older regional sediment restoration study using 1979 vintage sediment isopach maps restoration took into account 1 changing sediment densities during the erosion deposition cycle 2 changing area of the sediment source province with time e g changing shoreline location as a result of eustacy uplift and delta progradation tectonic shortening 3 uplift and partial erosion of baram deltaic province sediment denudation at the drainage divide for the middle miocene late miocene and pliocene recent has proceeded at a similar rate for each period initial uplift of central borneo has been attributed to buoyancy of thinned continental crust that jammed the subduction zone under nw borneo in the early miocene however the absence of decay in the erosion rates with time from the middle miocene to recent suggests operation of an additional uplift mechanism that may be related to delamination of mantle lithosphere slab detachment is the favoured explanation

anatomy of rapid margin progradation three dimensional geometries of miocene clinoforms new jersey margin documentation of along strike variations in the morphologies of continental margin clinoforms is essential for understanding mechanisms of progradation one of the fundamental relationships between depositional processes and preserved stratigraphy maps based on a grid of commercial multichannel seismic data offshore new jersey extending more than 70 km along strike and approximately 50 km downdip reveal the three dimensional morphology and evolution of four buried surfaces correlated with middle upper miocene sequence boundaries calibrated by drilling on the adjacent continental slope miocene clinoform breakpoints are not depositional analogs of the modern shelf edge they are linear to gently arcuate breakpoint and slope trends indicate a systematic southward displacement of depocenters over about 5 6 m y progradation responded to point fluvial sediment sources but efficient along strike sediment dispersal muted their influence canyons are absent on three of four clinoform slopes the fourth slope has one v shaped canyon and a broad erosional area possible slope failure planar floored canyons also occur albeit rarely seaward of clinoform toes apparently v shaped and planar floored canyons previously ascribed to downslope erosion vs slope failure headward erosion respectively can coexist the accretionary northern slope of little bahama bank is a possible morphologic analog by analogy with pleistocene shelf slope geometries an absence of canyons breaching clinoform breakpoints suggests that rivers did not discharge at paleoshelf edges indicating that sea level lowstands postulated for the middle upper miocene did not expose breakpoints reconstruction of breakpoint paleoelevations supports this conclusion for three of the four mapped surfaces suggesting that elevations of some miocene lowstands on the global sea level curve are too high by up to 60 m

stability analyses of rainfall induced landslides the slope stability issues concerning rainfall induced slope failures are investigated and presented specifically the effect of both negative and positive pore water pressures on the stability of initially unsaturated slopes are carefully explained and coupled with infinite slope analysis methods in order to present a predictive formulation of slope failures that occur as a result of rainfall events the formulation serves as a baseline analysis method for evaluating potentially unstable soil slopes that are subject to surface infiltration and explains the various triggering mechanisms that may occur based on individual combinations of the slope geometry soil strength and infiltration parameters a procedural method is outlined for utilizing the analytical formulation to predict the change in the factor of safety for a slope subject to infiltration and a detailed analysis of a case study is presented to verify the method quantitative statements are made concerning the time and depth of failure in relationship to the soil slope and rainfall parameters

large rotaliid foraminifera as biostratigraphic and palaeoenvironmental indicators in northwest borneo an example from a late miocene section in brunei darussalam abstract palaeonvironmental interpretations and biostratigraphic zonations in northwest borneo are still a matter of debate but are important for petroleum related applications the outcrop of ambug hill a late miocene marine siliciclastic sequence in brunei darussalam has been investigated for its foraminifera assemblage from a 9 5 m clay rich interval 50 taxa have been recognized and classified whenever possible to species level the studied fauna is characterized by mostly hyaline foraminifera dominated by two rotaliid species cavarotalia annectens and heterolepa dutemplei additionally a number of miliolids lageniids bolivinids and also planktonic foraminifera are present but in much lower abundance the dominance of the rotaliids points to a sea floor rich in organic carbon characterized by muddy substrates with very low light penetration and the possible growth of patchy sea grass or macroalgae in the system as indicated by epiphytic foraminifera in the samples the results obtained give additional insights on how the environmental conditions might have been during the deposition of the deltaic sequences very widespread in the region and how the fauna coped with such settings throughout the investigated samples the dominance of the assemblages clearly shows alternating stress conditions typical of pro deltaic environments the large abundance of rotaliid foraminifera points toward possible biostratigraphic applications as several taxa within this group have short biozones in the region even though have strong facies dependency if their role as biostratigraphic indicators could be finally proven they might be used to date the large depositional sequences occurring in the region since planktonic index fossils are scarce due to the very proximal settings of the sediments

the limoeiro deposit ni cu pge sulfide mineralization hosted within an ultramafic tubular magma conduit in the borborema province northeastern brazil the limoeiro deposit located in a high grade mobile belt of the brasiliano orogenic cycle 650 500 ma represents one of the few magmatic ni cu pge sulfide discoveries in recent years the deposit is hosted by an ultramafic chonolith consisting in a partly deformed and metamorphosed tube like body affected by brittle faults within high grade gneiss and schist the absolute age of the ultramafic intrusion that hosts the limoeiro deposit is not yet known but regional geologic constraints suggest that both intrusion and host rocks belong to a terrane older than the orogenic cycle peak metamorphic parageneses of the ultramafic intrusion and country rocks indicate granulite facies of metamorphism interpreted as the result of the neoproterozoic brasiliano orogenic cycle the limoeiro deposit is the first significant discovery of ni cu sulfide in the borborema province opening a new window for exploration in this large orogenic belt despite the high grade metamorphism and the tectonic deformation the primary magmatic structure of the intrusion hosting the limoeiro deposit is remarkably well preserved this intrusion forms a tube like conduit chonolith with cross sections a few hundreds of meters in diameter and with a considerable length of more than 4 km the conduit consists of two distinct sequences of ultramafic rocks denominated upper and lower sequences each one consisting of a core of harzburgite enveloped by orthopyroxenite with an irregular and discontinuous outer shell of amphibolite these ultramafic sequences characterized by similar composition and structure have distinctively different s contents resulting from the presence of disseminated ni cu sulfide mineralization only in the upper sequence the magmatic system of the limoeiro deposit is interpreted to result from the input of two major pulses of magma with similar composition but sulfide saturation and segregation is restricted to the second magma pulse upper sequence nickel cu pge sulfide mineralization is broadly concordant with the upper sequence and different orebodies resulted from tectonic slicing of the originally continuous tube like structure the bulk of the mineralization in all orebodies consists of thick up to 150 m and elongated up to 1 km masses of rock containing disseminated sulfides broadly concordant with the chonolithic structure thin layers of massive ore occur in the lower portions of some of the orebodies and represent about 3 vol of the limoeiro deposit the intrusion hosting the limoeiro deposit is probably the best preserved chonolith that has been overprinted by granulite facies metamorphism within an orogenic terrane this study indicates that reconstruction of small magma conduits an important tool for exploration for ni cu sulfide deposits may be successfully achieved within high grade orogenic terranes

metamorphic history of serpentinite mylonites from the happo ultramafic complex central japan serpentinite mylonites from the happo ultramafic complex show evidence of two stages of mylonitization at different temperature conditions peridotite mylonites exhibit two types of olivine porphyroclasts and neoblasts produced at the earlier stage the olivine neoblasts have a stretching lineation with a fabric suggesting plastic deformation along 0 1 0 0 0 1 in addition to the olivine fabric the stable association of olivine orthopyroxene and tremolite in the peridotites that survived later serpentinization and the si and na contents of tremolite suggest that the earlier mylonitization took place at temperatures between 700 and 800 c later mylonitization was associated with high temperature serpentinization to form serpentinite mylonites in contrast to a common type of serpentinite in orogenic belts the serpentinite mylonites are cohesively foliated rich in olivine and diopside and poor in antigorite the diopside has low al cr and na contents typical of a retrograde origin and the olivine has a homogeneous composition except in areas subjected to contact metamorphism at a later stage modal composition and mineral chemistry suggest that the serpentinite mylonites were formed by a hydration reaction of tremolite and olivine to produce diopside and antigorite under stable conditions of olivine at temperatures between 400 and 600 c later stage mylonitization has preferentially been superimposed on the earlier stage mylonite zone with a common direction of foliation the difference in temperature between the two mylonitization stages suggests that the shear zone was episodically active during the emplacement of the happo complex conditions of relatively high temperature for serpentinization at a convergent plate boundary and high permeability caused by the early mylonitization favoured the formation of the serpentinite mylonites

new contributions to the understanding of kiruna type iron oxide apatite deposits revealed by magnetite ore and gangue mineral geochemistry at the el romeral deposit chile abstract iron oxide apatite ioa or kiruna type deposits are an important source of iron and other elements including ree u ag and co the genesis of these deposits remains controversial with models that range from a purely magmatic origin to others that involve variable degrees of hydrothermal fluid involvement to elucidate the formation processes of this deposit type we focused on the chilean iron belt of cretaceous age and performed geochemical analyses on samples from el romeral one of the largest ioa deposits in northern chile we present a comprehensive field emission electron microprobe analysis fe empa dataset of magnetite apatite actinolite pyroxene biotite pyrite and chalcopyrite obtained from representative drill core samples two different types of magnetite grains constitute the massive magnetite bodies an early inclusion rich magnetite type i and a pristine inclusion poor magnetite type ii that usually appears as an overgrowth around type i magnetite high v 2500 2800 ppm and ti concentrations 80 3000 ppm and the presence of high temperature silicate mineral inclusions e g pargasite 800 1020 c determined by micro raman analysis indicate a magmatic origin for type i magnetite on the other hand high v 2300 2700 ppm and lower ti 50 400 ppm concentrations of pristine inclusion poor type ii magnetite indicate a shift from magmatic to hydrothermal conditions for this mineralization event furthermore the composition of primary actinolite ca and mg rich cores within type ii magnetite and the presence at depth of fluorapatite and high co ni ratios 1 10 of pyrite mineralization are consistent with a high temperature up to 840 c genesis for the deposit at shallow depths of the deposit the presence of pyrite with low co ni ratios

zambales ophiolite philippines the acoje massif is part of a mafic ultramafic complex the zambales ophiolite and is a fragment of mesozoic oceanic crust this paper documents the occurrence and phase relations of sulfides and associated phases in the critical zone of the acoje massif the acoje critical zone acz forms the basal cumulate sequence of the massif and consists of a variably serpentinized lower ultramafic zone and a relatively less altered upper mafic zone two distinct sulfide associations have been identified 1 a troilite pyrrhotite dominated group hosted by the mafic zone and 2 a pentlandite dominated group hosted by the ultramafic zone troilite dominated assemblages represent the original mineralogy of magmatically precipitated sulfides in the entire cumulate sequence the pentlandite dominated group appears to have evolved from the primary magmatic sulfides during low temperature re equilibration the paragenetic evolution from the magmatic assemblage to the low temperature assemblage appears to have proceeded as follows 1 s rich hexagonal pyrrhotite pentlandite chalcopyrite or cubanite magnetite 2 s poor hexagonal pyrrhotite pentlandite intermediate solid solution iss phase and or cubanite magnetite 3 troilite or mackinawite pentlandite iss magnetite 4 troilite or mackinawite pentlandite iss native cu magnetite 5 pentlandite native cu magnetite and 6 pentlandite native cu fe ni alloy magnetite this evolutionary trend in conjunction with the observed textural chemical and sulfur isotopic relations indicates that the native metal and alloy phases in the acz were produced by low temperature reduction of the primary magmatic sulfides correlations between sulfide assemblages and coexisting silicate hydrosilicate oxide assemblages further indicate that this alteration occurred during retrograde serpentinization of the acoje massif two end member models that could explain the inferred low temperature mineralogic evolution of the acz sulfides are described 1 an isothermal reduction model and 2 a non isothermal equilibration model both isothermal and non isothermal effects apparently were involved in the development of variably reduced sulfide oxide metal assemblages from the initial magmatic sulfides

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

depositional environment and sequence stratigraphy of miocene deltaic cycles exposed along the jerudong anticline brunei darussalam abstract kilometer scale prograding clinoforms associated with deltas are rarely seen in outcrop however one such example is found in a miocene sand shale sequence exposed along the jerudong anticline in brunei darussalam regional sequence stratigraphic interpretation shows that large clinoforms at the base of the miocene belait delta represent a succession of at least three major sand shale sequences the stratigraphically highest and best exposed sequence exhibits large slumps and sharp based detached sand bodies at its base accumulation of these units most likely occurred during a relative sea level lowstand an overlying 1 1 5 km thick shale unit is interpreted to have developed during subsequent transgressive and early highstand conditions rapid progradation of thick sand dominated shoreface deposits characterizes the late highstand systems tract the clinoforms below show similar depositional geometries slumps and thin blankets of shallow marine sandstones mark the individual bases shales and mudstones succeed and progradational shoreface and tidal deposits form the top of each clinoform new sedimentological and micropaleontological data document that all sediments regardless of whether sand or shale dominated formed in a shoreface to shelfal setting in front of a mud rich delta this differs from previous studies interpreting a continental slope to deep marine depositional environment for all shale dominated units and fig 1 a landsat satellite composite image of ne brunei darussalam after sandal 1996 and b geological map after wilford 1961 the belait formation is dominated by shallow marine shoreface and tidal sandstones the setap shale formation setap shales is mainly composed of shelfal mudstones and shales note pronounced progradational geometries at the base of the belait formation on the eastern margin of the belait syncline black boxes indicate locations of figures 2 and 4 end page 913 indicates that kilometer scale clinoforms can develop entirely on the continental shelf in water depths less than 200 m

pressure temperature evolution of lawsonite bearing eclogites pinchi lake british columbia blueschists and retrogressed eclogites are located along the pinchi fault zone near 54 30 n and 124 w in central british columbia the pinchi fault separates rocks of contrasting geological histories and the blueschists and eclogites occur with ultramafic rocks as fault bounded blocks the retrogressed eclogites occur as tectonic blocks now in glacial debris a few metres across blueschists occur in coherent kilometre sized tracts eclogites contain garnet omphacite rutile quartz with glaucophane lawsonite and titanite some of the lawsonite appears to be stable with omphacite and garnet tectonic blocks of eclogite from two different localities have recorded different p t histories in one tectonic block p t estimates for garnet inclusions in clinopyroxene and lawsonite c 565 c 13 1 kbar suggest that garnet and omphacite initially equilibrated outside the stability field of lawsonite a decrease in temperature as recorded in garnet rims and matrix clinopyroxene resulted in crystallization of lawsonite and other retrogressive minerals later crystallization of stilpnomelane locally pseudomorphing garnet howieite winchite and actinolite was triggered by an influx of fluid under p t conditions outside the stability field of garnet lawsonite appears to have been stable suggesting a minimum pressure of about 3 kbar in the second tectonic block clinopyroxene inclusions in garnet suggest temperatures near 350 c p 10 kbar and garnet rims equilibrated with matrix clinopyroxene suggest temperatures near 450 c at pressures above c 12 kbar

coal bearing strata of labuan mode of occurrences organic petrographic characteristics and stratigraphic associations the current study describes the mode of occurrences of labuan cenozoic coals as observed in the field and identifies the coal macerals based on their organic petrographic characteristics as observed under reflected white light and blue light excitation in this study some sedimentological aspects such as the depositional environment were correlated with the organic petrological characteristics of the coals based on the organic petrographic features and the thermal maturity as determined by vitrinite reflectance some new stratigraphic associations have been identified similarities in sedimentological and organic petrological characteristics suggest that the shaly strata associated with carbonaceous sandstones exposed on the north western part of labuan island and the outcrop exposed in the south within the east kiam sam peninsula belong to the same sequence within the setap shale formation based on petrographic characteristics described and vitrinite reflectance values obtained from this study at least four distinct units that are associated with coal bearing sediments can be recognized strata with vitrinite reflectance vr ro in the range of approximately 0 4 0 5 is the youngest and belongs to the belait formation which include the tg layang layangan unit the oldest sequence outcropping at tg punei near the labuan crude oil terminal shell terminal belongs to the west crocker formation possesses vitrinite reflectance in the range of 0 8 0 9 whereas the intermediate unit is the setap shale formation which possesses vr of 0 55 0 6 including the east kiam sam sandstone unit and temburong formation which possesses vr in the range of 0 65 0 75 it ought to be noted that the differences in the thermal maturity variations within the cenozoic sediments of labuan reported here does not form the fundamental basis of the stratigraphic subdivision of labuan but an attempt to associate it with what appear to be the still unresolved stratigraphic issues of labuan these vr values suggest the belait formation is immature setap shale formation is early mature while the temburong and west crocker formations are at peak maturity for liquid hydrocarbon generation the analysed coals are predominantly mangrove derived and considered to be oil prone as suggested by the common occurrences of oil haze suberinite bituminite exsudatinite and perhydrous vitrinite

crust mantle relationships in the french variscan chain the example of the southern monts du lyonnais unit eastern french massif central garnet lherzolite from the lyonnais area eastern french massif central occurs as several lenses elongated within the regional foliation of garnet biotite sillimanite gneisses within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses petrological and thermobarometric studies emphasize a tectonometamorphic re equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution during the burial stage interpreted as lithospheric subduction the peridotites underwent their mylonitic deformation under high pressure conditions 23 30 kbar in contrast the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions 6 8 kbar 700 c our thermobarometric investigations allow us to interpret the granulitic ultramafic association from the monts du lyonnais area as a lithospheric section buried into a palaeozoic subduction zone laminated during continental collision and uplifted by erosion processes

experimental phase relations of hydrous peridotites modelled in the system h 2 o cao mgo al 2 o 3 sio 2 the hydration of peridotites modelled by the system h2o cao mgo al2o3 sio2 has been treated theoretically after the method of schreinemakers and has been investigated experimentally in the temperature range 700 900 c and in the pressure range of 8 14 kbar in the presence of excess forsterite and water the garnet to spinel peridotite transition boundary intersects the chlorite dehydration boundary at an invariant point situated at 865 5 c and 15 2 0 3 kbar at lower pressures a model spinel lherzolite hydrates to both chlorite and amphibole bearing assemblages at an invariant point located at 825 10 c and 9 3 0 5 kbar at even lower pressures the spinel to plagioclase peridotite transition boundary intersects the dehydration curve for amphibole forsterite at an invariant point estimated to lie at 855 10 c and 6 5 0 5 kbar both chlorite and amphibole were characterized along their respective dehydration curves chlorite was found to shift continuously from clinochlore with increasing temperature to more aluminous compositions amphibole was found to be tremolitic with a maximmum of 6 wt al2o3 the experimentally determined curves in this study were combined with the determined or estimated stability curves for hydrous melting plagioclase talc anthophyllite and antigorite to obtain a petrogenetic grid applicable to peridotites modelled by the system h2o cao mgo al2o3 sio2 that covers a wide range of geological conditions direct applications of this grid although quite limited can be made for ultramafic assemblages that have been extensively re equilibrated at greenschist to amphibolite facies metamorphism and for some highgrade ultramafic assemblages that display clear signs of retrogressive metamorphism

a review of magnetite geochemistry of chilean iron oxide apatite ioa deposits and its implications for ore forming processes abstract magnetite is the most important iron ore in iron oxide apatite ioa deposits which represent the cu poor end member of the iron oxide copper gold iocg clan magnetite chemistry has been used as a petrogenetic indicator to identify the geological environment of ore formation and as a fingerprint of the source reservoir of iron in this study we present new textural and microanalytical epma and la icp ms data of magnetite from carmen fresia mariela and el romeral ioa deposits located in the cretaceous coastal cordillera of northern chile we also provide a comprehensive summary and discussion of magnetite geochemistry from andean ioas including los colorados cerro negro norte el romeral chilean iron belt and the pliocene el laco ioa deposit located in the central volcanic zone of the chilean andes microtextures coupled with geochemical data were used to define and characterize the occurrence of different magnetite types magnetite exhibits a variety of textural features including oscillatory zoning colloform banding re equilibration textures exsolution lamellae and symplectites the magmatic vs hydrothermal origin of the different magnetite types and the evolution of ioa deposits can be assessed using diagrams based on compatible trace elements however magnetite is very susceptible to hydrothermal alteration and to both textural and compositional re equilibration during magmatic and superimposed hydrothermal events based on the data presented here we conclude that v and ga are possibly the most reliable compatible elements in magnetite to trace ore forming processes in the andean ioa deposits magnetite chemistry reveals different conditions events of formation for each ioa deposit ranging from high temperature low oxygen fugacity o2 purely magmatic 600 c conditions to lower temperature and higher o2 magmatic hydrothermal 300 600 c to low temperature hydrothermal

evidence of phase nucleation during olivine diffusion creep a new perspective for mantle strain localisation for the past decades grain size reduction leading to diffusion creep in olivine is believed to be a very important process for strain localisation in the lithospheric mantle however the mechanisms of grain size reduction in this regime are still poorly understood e g platt 2015 here we show new experimental results that document grain size reduction and material weakening during wet olivine diffusion creep while occurring for both mono phase and two phase aggregates grain size reduction is coeval with strain localisation and local phase mixing in olivine pyroxene aggregates based on evidence of fluid inclusions and cracks filled with a fine grained phase mixture we conclude that grain size reduces as a result of fluid assisted nucleation that takes place in the presence of an aqueous fluid during diffusion creep cavitation induced by grain boundary sliding creep cavitation can be inferred and may play a critical role for olivine grain size reduction amongst their implications for rock rheology in general our findings highlight a key process for strain localisation in the ductile uppermost mantle

some remarks on the stability of blueschists and related high p low t assemblages in continental orogens the thermal evolution of the crust in continental orogens is sensitive to the deformation in the mantle part of the lithosphere a point obscured in models that apply a boundary condition of constant basal heat flux within the mantle lithosphere we show that for part of the plausible range of conductivities and heat source distributions within the continental lithosphere the maximum or potential temperatures in thickened continental crust typical of collision zones do not necessarily exceed the stability field of the high p low t facies such as blueschists provided the mantle lithosphere is also thickened during the deformation the appropriate range of parameters includes conductivities towards the upper limit of the plausible range k 2 2 w m i k l together with low crustal heat production rates with distributions concentrating heat production in the upper crust and relatively thin initial lithosphere these observations suggest that at least some of the high p low t assemblages found within continental orogens for example in the dalradian of the scottish sw highlands may reflect the thermal response to whole lithospheric thickening and do not necessarily reflect thermal regimes attendent with subduction or unusually rapid excavation

regional progressive high pressure metamorphism seward peninsula alaska blueschist facies rocks on the seward peninsula constitute a structurally coherent terrane measuring at least 100 150 km radiometric age data indicate that high pressure metamorphism probably occurred in jurassic rather than in palaeozoic or precambrian time as previously suggested protolith sediments nome group are of intracontinental basin or continental margin type and of lower palaeozoic and possibly late precambrian age thus predating the high pressure metamorphism by more than 200 m y blueschist facies mineral assemblages were developed in almost all lithologies of the nome group and are best preserved in feti rich metabasites glaucophane almandine epidote and pelites glaucophane chloritoid phengite a lawsonite crossite subfacies was developed in possible nome group rocks on the east flank of the darby mountains albite epidote amphibolite facies assemblages characterize nome group rocks in the southwestern part of the peninsula metamorphism in the central zone of the terrane passed from early lawsonitic to subsequent epidote almandine glaucophane schist subfacies with the local development east of the nome river of eclogitic assemblages the high pressure metamorphic minerals were synkinematic with the development of mesoscopic scale intrafolial isoclinal folds and a flattening foliation of consistent orientation initiation of uplift probably corresponded to the growth of barroisite rims on earlier sodic and actinolitic amphiboles and partial post kinematic greenschist facies replacements record later stages of decompression ophiolites and melange are not associated with the seward peninsula blueschists the high pressure metamorphism was caused by tectonic loading of a continental plate by an allochthon of indeterminate origin the pt conditions of high pressure metamorphism were approximately 9 11 kbar 400 450 c thus falling between the pt paths of the shuksan and franciscan terranes

the role of fluids in the metamorphism of komatiites agnew nickel deposit western australia the agnew nickel sulfide deposit is spatially associated with a lenticular body of ultramafic rocks which shows a concentric zonation in metamorphic mineralogy olivine tremolite chlorite cummingtonite enstatite assemblages occur at the margin of the ultramafic lens giving way to olivine anthophyllite olivine talc and olivine antigorite assemblages successively inwards these rocks are interpreted as having crystallized from komatiitic lavas and exhibit a spectrum of compositions from those of original flow tops to pure olivine adcumulates the relative modal abundances of metamorphic olivine tremolite and chlorite reflect original proportions of cumulus olivine and komatiite liquid in the protolith peak metamorphic conditions are estimated at 550 c based on garnet biotite thermometry at a maximum pressure of 3 kb this temperature falls within the narrow range over which metamorphic olivine may co exist with enstatite anthophyllite talc or antigorite depending upon the fugacity of water in the metamorphic fluid the observed mineralogical zonation is therefore attributed to infiltration by co2 rich fluids generated by decarbonation of talc carbonate rocks formed during pre metamorphic marginal alteration of the ultramafic lens metamorphic fluids were essentially binary mixtures of water and co2 with minor h2s having a maximum partial pressure less than 1 percent of total pressure enstatite bearing assemblages formed in the presence of co2 rich fluids at fluid rock volume ratios close to one while anthophyllite talc and antigorite bearing assemblages formed in the presence of progressively more water rich fluids at progressively lower fluid rock ratios

the errors in surface runoff prediction by neglecting the relationship between infiltration rate and overland flow depth abstract many simplifications are used in modeling surface runoff over a uniform slope a very common simplification is to determine the infiltration rate independent of the overland flow depth and to combine it afterward with the kinematic wave equation to determine the overland flow depth another simplication is to replace the spatially variable infiltration rates along the slope i x t due to the water depth variations h x t with an infiltration rate that is determined at a certain location along the slope the aim of this study is to evaluate the errors induced by these simplications on predicted infiltration rates overland flow depths and total runoff volume the error analysis is accomplished by comparing a simplified model with a model where the interaction between the overland flow depth and infiltration rate is counted in this model the infiltration rate is assumed to vary along the slope with the overland flow depth even for homogeneous soil profiles the kinematic wave equation with interactive infiltration rate calculated along the slopy by richard s equation are then solved by a finite difference scheme for a 100 m long uniform slope in the first error analysis we study the effect of combining an exact and approximate one dimensional infiltration rate with the kinematic wave equation for three different soil surface roughness coefficients the terms exact and approximate stand for the solution of richard s equation with and without using the overland flow depth in the boundary condition respectively the simulations showed that higher infiltration rates and lower overland flow depths are obtained during the rising stage of the hydrograph when overland flow depth is used in the upper boundary condition of the one dimensional richard s equation during the recession period the simplified model predicts lower infiltration rates and higher overland flow depths the absolute relative errors between the exact and approximate solutions are positively correlated to the overland flow depths which increase with the soil surface roughness coefficient for this error analysis the relative errors in surface runoff volume per unit slope width throughout the storm are much smaller than the relative errors in momentary overland flow depths and discharges due to the alternate signs of the deviations along the rising and falling stages in the second error analysis when the spatially variable infiltration rate along the slope i x t is replaced in the kinematic wave equation by i t calculated at the slope outlet the overland flow depth is underestimated during the rising stage of the hydrograph and overestimated during the falling stage the deviations during the rising stage are much smaller than the deviations during the falling stage but they are of a longer duration this occurs because the solution with i x t recognizes that part of the slope becomes dry after rainfall stops while overland flow still exists with i t determined at the slope outlet as obtained for the first error analysis the relative errors in surface runoff volume per unit slope width are also much smaller than the relative errors in momentary overland flow depths and discharges the relation between the errors in overland flow depth and discharge to different mathematical simplifications enables to evaluate whether certain simplifications are justified or more computational efforts should be used

spatial variations in antigorite fabric across a serpentinite subduction channel insights from the ohmachi seamount izu bonin frontal arc abstract we conducted a microstructural study of samples from a natural serpentinite shear zone in the ohmachi seamount izu bonin frontal arc the serpentinite samples consist mainly of columnar antigorite grains that show marked variations in texture from two approximately orthogonal sets of grains interpenetrating to aligned schistose forms because the two types of grains have similar compositions these textural differences are interpreted to reflect the existence of a strain gradient toward a plate interface in a subduction zone the crystal preferred orientation cpo of antigorite with interpenetrating texture is almost randomly oriented whereas in the case of schistose texture the cpo shows a typical 010 001 pattern we also found that with increasing intensity of schistosity the polarization plane of v s 1 for antigorite grains becomes aligned parallel to the flow plane consistent with a plane oriented normal to the maximum concentration of slow antigorite c axes this configuration results in seismic anisotropy that is approximately five times higher than that for olivine grains these findings indicate that if a serpentinite layer on the plate interface attains large bulk shear strains 2 the resultant alignment of antigorite grains within the layer strongly influences the orientation and magnitude of seismic anisotropy in the mantle wedge depending on the dip angle of the subducting slab

alteration of oceanic crust and geologic cycling of chlorine and water we report new estimates of transport rates for h2o and cl between the mantle and surface reservoirs our estimates take into consideration alteration of oceanic crust especially that of plutonic rocks and possible subduction of sediments the effect of hydrothermal alteration on the cl budget seems to be negligible but the effect on the h2o budget is significant altered oceanic crust excluding sediments contains about 10 times as much h2o as the unaltered crust and its subduction may result in a net transport of h2o to the upper mantle in subduction zones however the rate of expulsion of h2o from the mantle by subduction zone magmatism is comparable to the amount released by ridge magmatism and is only about 10 of the amount subducted therefore about 90 of the subducted h2o must be returned to the mantle or returned to the crust by other processes in addition subduction of oceanic sediments to mantle depths will result in 1 1 a further increase in the return rate of h2o to the mantle reservoir 2 2 possible net transfer of cl to the mantle depending on the rate of pore water expulsion

pyribole evolution during tremolite synthesis from oxides the formation mechanisms of tremolite in a time series of hydrothermal experiments at 850 c and 6 1 kbar were studied using transmission electron microscopy tem analytical tem and x ray diffraction the starting materials were unseeded mixtures of reagent grade oxides after the first 8 min of reaction the synthesis products were diopside enstatite clinoenstatite quartz and minor c centered and primitive monoclinic amphibole 13 wt total the compositions of the first formed amphiboles vary considerably from essentially pure cummingtonite ca mg 0 to nearly ideal tremolite ca mg 0 4 three groupings of amphiboles with successively higher ca mg ratios were identified respectively 0 1 0 22 and 0 35 over a period of 97 h the proportions of mg rich amphibole gradually decrease and the average ca mg ratios of all amphiboles steadily increase from about 0 22 to 0 35 other changes include 1 a decrease in the aspect ratio of amphibole crystals with increasing time 2 a decrease in the number of non amphibole chain multiplicity faults cmfs from early a 2 values as low as 0 81 to a final value of 0 94 and 3 a distinct change in the rate of amphibole growth after 1 h of synthesis these observations suggest that two mechanisms of amphibole formation are dominant 1 formation of pyroxene modules with subsequent hydration and arrangement into the double chain amphibole structure and 2 direct nucleation and growth of amphibole from hydrothermal solution a third mechanism involving formation of talc like modules followed by their breakdown into double chain modules incorporation of ca and ordering into the amphibole structure may also occur but probably only at the lamellar or unit cell scale because these syntheses were performed outside the stability field of talc

analysis of physical testing of rainfall induced soil slope failures rainfall is a significant factor that triggers slope failures around the world this paper reports a series of physical tests which were conducted to simulate rain induced slope failures the experiments dealt with two scenarios including 1 rainwater infiltration into the slope and 2 slope failures induced by artificial rainfall with different initial conditions slope deformation and slope failures were observed and possible mechanisms were interpreted based on the experimental results the results confirm the hypothesis that pore water pressure and water content in a loose soil slope change rapidly and that water infiltration into cracks in the slope has a great impact on landslide development the observed slope failures can be divided into three types overall sliding failure partial sliding failure and flow slide the effect of slope gradient rainfall intensity and distribution of initial suction on the slope deformation and failure process are also summarized for possible applications under the similar conditions

metamorphism of the scourian complex nw scotland the rocks of the scourian complex have been intensively studied but there is still no consensus as to the conditions of the granulite facies metamorphism preserved in these rocks recent estimates of these conditions fall into two groups one at 820 920 c and ca 11 kbar and the second at ca 1000 c and 12 kbar investigation of a variety of rocks shows that the recorded conditions vary with grain size with higher grade conditions recorded by the cores of coarser ca 10 mm crystals and lower grade conditions recorded by the rims of coarser grains and by finer grains this observation suggests that re equilibration during recovery of these rocks to the surface has been important which may account for the discrepancy in estimated p t conditions revised estimates of the equilibration conditions of the scourian complex of t 1000 c and p 8 5 kbar are presented the conditions suggested for the peak of metamorphism mean that the role of anatexis in the genesis of these rocks must be considered and the nature of the fluid phase thoroughly investigated

cold moho boundary beneath island arc systems an example from the north kamchatka arc a suite of amphibole bearing ultramafic and mafic xenoliths representing the crust mantle boundary was sampled in the northern segment of the kamchatka arc these nodules exhibit low equilibration temperatures at elevated pressures suggesting relatively cold conditions at moho depths which are uncommon in modern island arcs a model including successive stages of subduction initiation subduction cessation resulting in cooling of the lithospheric wedge followed by intra arc rifting due to upwelling of astenospheric diapirs is proposed to explain the observed textural petrologic and chemical characteristics of sampled rock assemblage

mapping susceptibility of rainfall triggered shallow landslides using a probabilistic approach to prepare a landslide susceptibility map is essential to identify hazardous regions construct appropriate mitigation facilities and plan emergency measures for a region prone to landslides triggered by rainfall the conventional mapping methods require much information about past landslides records and contributing terrace and rainfall they also rely heavily on the quantity and quality of accessible information and subjectively of the map builder this paper contributes to a systematic and quantitative assessment of mapping landslide hazards over a region geographical information system is implemented to retrieve relevant parameters from data layers including the spatial distribution of transient fluid pressures which is estimated using the trigrs program the factor of safety of each pixel in the study region is calculated analytically monte carlo simulation of random variables is conducted to process the estimation of fluid pressure and factor of safety for multiple times the failure probability of each pixel is thus estimated these procedures of mapping landslide potential are demonstrated in a case history the analysis results reveal a positive correlation between landslide probability and accumulated rainfall this approach gives simulation results compared to field records the location and size of actual landslide are well predicted an explanation for some of the inconsistencies is also provided to emphasize the importance of site information on the accuracy of mapping results

pressure temperature and tectonic evolution of triassic lawsonite aragonite blueschists from pinchi lake british columbia a blueschist and eclogite terrane is associated with one of the largest faults in the canadian cordilleran orogen the pinchi fault blueschists in situ and retrogressed eclogite blocks occur along the pinchi fault zone near 54 30 n and 124 w critical blueschist facies mineral assemblages include lawsonite glaucophane jadeite lawsonite glaucophane quartz and aragonite white mica 40ar 39ar spectra on blueschist and eclogite yield ages in the range 221 8 1 9 to 223 5 1 7 ma establishing a direct link between the blueschists and eclogites preservation of aragonite sets rigid constraints on the pressure temperature fluid time conditions of unroofing k ar dates indicate that this is some of the oldest documented metamorphic aragonite comparison with computed petrogenetic grids suggests that metamorphic temperatures were in the range 200 300 c with pressures greater than 8 10 kbar 1 kbar 100 mpa unroofing likely occurred during collision of the cache creek terrane with quesnellia in the lat

serpentinization and infiltration metasomatism in the trinity peridotite klamath province northern california implications for subduction zones the trinity peridotite was emplaced over metabasalts and metasedimentary rocks of the central metamorphic belt along the devonian trinity thrust zone three metamorphic events can be recognized in the trinity peridotite 1 antigorite d 63 to 65 formation related to regional underthrusting of the central metamorphic belt 2 contact metamorphism associated with mesozoic dioritic plutons and 3 late stage formation of lizardite brucite and chrysotile d 127 to 175 due to infiltration of meteoric waters abundant relict phases indicate incomplete reactions and strongly suggest that the availability of h2o was a controlling factor during serpentinization antigorite event 1 formed as a result of infiltration into the trinity peridotite of mixed h2o co2 fluids derived from the underlying central metamorphic belt foliation defined by magnetite veins and shear zones within antigorite serpentinites are subparallel to the trinity thrust the assemblage fo atg chl mag tr carb reflects partial hydration of peridotite at 425 570 c talc rich serpentinite formed along the thrust as a result of the infiltration of silica bearing fluids metasomatic mass balance calculations based on silica solubilities and the extent of antigorite serpentinization suggest that 80 175 volumes of fluid have passed through a given volume of original peridotite at the trinity thrust the trinity thrust probably represents a devonian subduction zone thermodynamic calculations suggest that hydration reactions account for 30 35 of the total heat released by the cooling trinity peridotite by analogy similar hydration reactions are to be expected in the overlying mantle wedge of a subduction zone which act to retard cooling of the hanging wall just as dehydration reactions delay heating of the downgoing slab metasomatic zones formed in peridotite at the trinity thrust may reflect similar metasomatic processes to those proposed to occur in the mantle wedge above a subducting slab

modeling the stability of terraced slopes an approach from valtellina northern italy this study focuses on the relationships between hydrological processes and the stability of terraced slopes emphasis is given to the presence of dry stone retaining walls and how to explicitly consider them when performing a distributed stability analysis valtellina an alpine valley of northern italy is selected as study area in detail the slope uphill the village of tresenda affected by several superficial landslides since 1983 is studied dry stone walls heavily influence the hydrological and geotechnical processes at the slope scale and it is therefore crucial to consider them when performing a stability analysis on mountainous terraced areas walls are usually 0 60 1 00 m wide and therefore base maps digital elevation models and soil depth map with a horizontal resolution of 1 m have been appositely derived working at this horizontal resolution leads to the failure of some basic assumptions of the infinite slope method usually applied for distributed stability analysis in this study we propose an alternative approach based on the global method of equilibrium applied to sections of single terraces one cell wide a specific sensitivity analysis allowed to fix some constrains to the model in order to improve the reliability of the results under different mechanical and hydrological conditions the proposed stability model was calibrated and evaluated on recorded past rainfall events it resulted to be a good instrument for the identification of the most unstable areas when stressed with high pore water pressure conditions

the effect of tschermak s substitution on assemblages in aluminous dolomites a calculated petrogenetic grid for the system cao mgo al2o3 sio2 co2 h2o camasch incorporating tschermak s substitutions in amphibole chlorite talc and clinopyroxene is used to examine phase relationships in aluminous marbles a series of diagrams illustrating the effect upon stable mineral assemblages of increasing the aluminium content of a bulk composition is used to show the way aluminous minerals enter mineral assemblages in progressively more aluminous rocks the effects of changing pressure and the incorporation of fe into the bulk composition on the stable mineral assemblages are also examined the calculated equilibria are shown to be in reasonable agreement with natural assemblages and the incorporation of new experimental data on amphiboles into the existing dataset is shown to improve the agreement between observed and natural amphibole compositions

unsaturated and saturated flow through a thin porous layer on a hillslope the subsurface stormflow in a layer of porous earth overlying an impervious hill is investigated in the case when the thickness of the layer is small compared to a typical hill dimension if the infiltration rate at the surface is low and varies slowly with time the head is approximately constant along normals to the hillslope and the flow in both the saturated and unsaturated regions is described by a simple kinematic wave equation solutions of this equation describe the spatial and temporal development of the water table as well as the flow in the saturated and unsaturated regions a numerical example is given