climatic controls on late pleistocene alluvial fans cyprus alluvial fans are commonly associated with tectonically active mountain ranges and tectonism is frequently held responsible for abrupt coarsening and cyclical sedimentation of alluvial fan sequences whilst it is accepted that tectonism provides the opportunity for alluvial fan development through the creation of topography increasing gradients of fluvial systems supplying sediments and creating accommodation for the storage of sediment flux the role of climate in fan development is frequently neglected the hypothesis that climatically controlled events can produce recognisable sedimentary signatures in alluvial fan deposits is tested in the active supra subduction zone setting of the late pleistocene of southern cyprus this study demonstrates through architectural analysis and the reconstruction of palaeoflood hydrology a recorded pattern of increasing and decreasing palaeoflow dynamics with switches from a wetter to drier mode clearly exhibited by changes in the sedimentology of the fan at the present day cyprus has a semi arid climate and is influenced by a strongly seasonal rainfall pattern largely restricted to the winter months plus rare occurrences of summer cyclones however at precession minima increased activity of western mediterranean depressions produces wetter summers using inference we propose that longer term increases in rainfall increased river discharge as recorded in the fan palaeoflood hydrology and occurred at minima in the precession these periods correlate with the deposition of conglomeratic channels and open framework gravels drier periods are exhibited by sandier braided fluvial deposits shorter term or seasonal change is recorded in the form of 2nd and 3rd low order bounding surfaces this increased activity of mediterranean summer depressions increased precipitation to the wider levantine area and was coincident with increased intensity of the north african and indian ocean sw monsoons the resultant increase in river discharges at precession minima from both the nile and the wider levant resulted in the formation of sapropels in the eastern mediterranean and is recorded as wet periods in speleothem deposits in the soreq and peqiin caves of israel the predominant control of sedimentation on the late pleistocene alluvial fans of southern cyprus was climate

identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

cause of dominance by sheetflood vs debris flow processes on two adjoining alluvial fans death valley california two large adjoining alluvial fans of the panamint range piedmont death valley california are composed of different facies assemblages deposited by contrasting sedimentary processes the anvil spring fan was built solely by water flow processes incised channel floods and sheetfloods whereas the neighbouring warm spring fan has been constructed principally by debris flows the boundary between these fans delineates a sharp provincial piedmont contact between sheetflood dominated fans to the south and debris flow dominated fans to the north factors such as climate catchment area fan area catchment relief aspect vegetation types and density and neotectonic setting are essentially identical for these two fans the key difference between them is that their catchments are underlain by dissimilar bedrock types which weather to yield distinctive sediment suites weathering of the granite and andesite of the anvil fan catchment produces significant volumes of medium to very coarse sand granules pebbles cobbles and boulders but minimal silt and clay in contrast the shale quartzite and dolomite that dominate bedrock in the warm spring catchment weather to yield a wide suite of sedimentary particles spanning from clay to boulders the abundance of mud and the unsorted character of the yielded sediment cause precipitation induced slope failures in the warm spring catchment to transform readily into debris flows this propensity is due to the low permeability of the colluvial sediment which causes added water to become trapped quickly and pore pressure to rise rapidly promoting transformations to debris flows in contrast the limited volume of sediment finer than medium sand yielded from the anvil fan catchment causes the colluvium to have high permeability this factor prevents the transformation of wet colluvium to a debris flow during hydrologically triggered slope failures instead maintaining sediment transport as entrained bed load or suspended load in a water flow

metamorphic consequences of thrust emplacement fall mountain new hampshire metamorphic pressure temperature p t paths from the upper and lower plates of the fall mountain nappe southwest new hampshire reveal different thermal histories in the two structural levels upper plate rocks experienced early low p high t contact metamorphism to peak p t conditions of 700 750 c 3 4 5 kbar these p t conditions were followed by loading to approximately 5 6 kbar and then by nearly isobaric cooling to approximately 500 c 4 5 kbar upper plate rocks then experienced minor heating with unloading and finally cooling the lower plate rocks also experienced early low p high t contact metamorphism but only to p t conditions of 480 510 c 2 3 5 kbar nappe emplacement resulted in nearly isothermal loading to 500 c 5 6 kbar which was followed by either heating or cooling depending on the proximity of the sample to the upper plate the early pressures recorded in both the upper and lower plate rocks suggest a depth of approximately 10 km for origination of the nappe the post nappe pressures 5 6 kbar imply a total tectonic thickness of approximately 20 km or a doubling of the overburden thickness diffusion calculations suggest that the emplacement of the nappe took less than approximately 10 m y and u pb chronology on zircon suggests that it occurred between 400 and 410 ma 4o ar 39 ar cooling ages on micas indicate that the terrane was cooled to approximately 300 c by 340 ma

constraining the relative strengths of high grade metamorphic rocks using foliation refraction angles an example from the northern new england appalachians foliation refraction angles are used to estimate effective viscosity contrasts between metapelitic and metapsammitic layers in amphibolite facies metaturbidites in the presidential range of the white mountains in eastern new hampshire an early formed foliation developed during km scale nappe folding consistently displays larger bedding foliation angles in metapelitic units than metapsammitic units suggesting that the metapelitic units had higher effective viscosities during this deformation we collected bedding foliation angle measurements from a combination of outcrops and locally derived boulders to test the hypothesis that foliation refraction angles could be used to estimate the effective viscosity ratio between different rock types at high metamorphic grades our results show that the metapelitic layers were between 2 and 3 times more viscous than the metaspsammitic layers which we attribute to the presence of large up to 15 cm long abundant up to 30 vol effectively rigid andalusite porphyroblasts in the metapelitic layers we present a methodology for using foliation refraction angles to determine the effective viscosity ratios of different rock types address the practical limitations we encountered and suggest this method is an easy way of estimating the strength contrast between different rock types finally we hypothesize that metamorphism resulting in porphyroblast growth can strengthen large crustal volumes during orogenesis

geologic thermobarometry of retrograded metamorphic rocks an indication of the uplift trajectory of a portion of the northern scandinavian caledonides kyanite grade pelitic schists from the efjord area 68 15 n 16 45 e northern norway show abundant textural evidence of retrograde reequilibration during late orogenic uplift and cooling of this part of the scandinavian caledonides graphical analysis of mineral compositions in seven samples that show a range of retrograde textures indicates that they reached final chemical equilibrium under nonuniform pressure temperature conditions three element partitioning geothermometers and geobarometers yield consistent pt estimates in individual samples which together define a trend in pt space the sample showing the fewest signs of retrogression yields the highest pressure and temperature estimates whereas samples exhibiting abundant reequilibration textures give lower pt estimates we suggest that the trend indicated by these samples reflects a portion of the pt trajectory assumed by the efjord area during uplift thus the judicious use of element partitioning geothermometers and geobarometers on samples showing signs of retrograde metamorphism may yield important new information concerning the late tectonic evolution of metamorphic terrains

u pb isotopic dating of fluid infiltration and metasomatism during dalradian regional metamorphism in connemara western ireland a metasomatic diopside rock occurs at the top of the dolomitic connemara marble formation of western ireland and contains titanite and k feldspar in addition to around 90 diopside xmg 0 90 0 97 u pb isotopic measurements on this mineral assemblage show that the titanite is both unusually uranium rich and isotopically concordant with the result that a precise u pb age of 478 2 5 ma can be determined the age is identical within error to a less precise rb sr age of diopside k feldspar of 483 6 ma petrological evidence indicates that the assemblage crystallized at c 620 c close to or below the closure temperature of titanite the age thus provides a precise estimate of the time of metamorphism this age is 11 3 ma younger than the 490 ma age for nearby gabbroic plutons which has previously been used to constrain the peak metamorphic age this difference accords well with geological evidence that the gabbros were emplaced prior to the metamorphic peak analysis of minerals with high closure temperature from assemblages whose crystallization is unambiguously associated with a specific episode of fluid infiltration at the peak of metamorphism provides the basis for a new approach to dating metamorphism the success of this approach is demonstrated by the results from connemara

a framework of holocene and late pleistocene environmental change in eastern iran inferred from the dating of periods of alluvial fan abandonment river terracing and lake deposition abstract we review studies of the holocene and late pleistocene stratigraphy of eastern iran to infer past changes in the environment within this presently arid region we build a scenario of widespread and presumably climatically driven evolution of the landscape through the holocene six sites covering a 10 range in latitude indicate a regional abandonment of alluvial fan surfaces at 10 3 ka with the younger 9 ka end of this age range supported by several of the best constrained studies incision of rivers into the fan surfaces has occurred in discrete stages in the early to mid holocene 9 7 ka leading to the formation of flights of river terraces detailed records of lakebed deposition in the presently arid interior of iran are rare though the available data indicate lake highstand conditions at

regional stratigraphy of the zagros fold thrust belt of iran and its proforeland evolution the latest neoproterozoic through phanerozoic stratigraphy of the zagros fold thrust belt of iran has been revised in the light of recent investigations the revised stratigraphy consists of four groups of rocks each composed of a number of unconformity bounded megasequences representing various tectonosedimentary settings in the lowest group ranging in age from latest precambrian to devonian the uppermost neoproterozoic to middle cambrian rocks constitute a megasequence of evaporites siliciclastic deposits and interlayered carbonates which were deposited in pull apart basins that developed by the najd strike slip fault system this megasequence is overlain by a second one middle to late cambrian in age which consists of shallow marine siliciclastic and carbonate rocks representing deposition in an epicontinental platform the overlying shales siltstones and partly volcanogenic sandstones of ordovician silurian and devonian age are local remnants of stratigraphic units that were extensively eroded during development of several major unconformities the second group consists of two megasequences one permian and the other triassic composed of widespread transgressive basal siliciclastic rocks and overlying evaporitic carbonates of an equatorial epi pangean very shallow platformal sea the third group is composed of four megasequences formed of shallow and deep water carbonates with some siliciclastic and evaporite deposits which accumulated on a neo tethyan continental shelf during earliest jurassic through late turonian time the fourth group comprises siliciclastic and carbonate deposits of a largely underfilled nw to se trending forward and backward migrating late cretaceous to recent proforeland basin which has evolved as an integral part of the zagros orogen this last group consists of three megasequences ix x and xi with distinctive lateral and vertical facies variations which reflect specific tectonic events megasequence ix comprises uppermost turonian to middle maastrichtian prograding and retrograding siliciclastic and carbonate deposits whose accumulations reflect emplacement obduction of ophiolite slivers and subsequent collisional events in the zagros orogen megasequence x consists of uppermost maastrichtian to upper eocene siliciclastic and carbonate rocks which deposited first progradationally in front of the zagros orogenic wedge with reduced contractional tectonic activity and then retrogradationally due to intensified thrust stacking in the interior parts of the orogen megasequence xi consists of oligocene and lower miocene carbonate strata deposited retrogradationally shortly after a period of intensified late eocene thrust faulting in the deformational wedge and an overlying succession of upward coarsening northeasterly derived siliciclastic deposits of lower miocene to recent age which are composed of erosional byproducts of the southwest vergent zagros thrust sheets

response of the ganges dispersal system to climate change a source to sink view since the last interstade the forcing of sediment dispersal systems by climate is widely accepted but there remains disagreement on the general responses to climate change such as how resulting signals propagate downstream and ultimately how they are preserved in the stratigraphic record towards a better understanding of these issues this paper presents a comprehensive overview of ganges dispersal system behavior since the last interstade which reveals major coeval responses to changes in the south asian monsoon at 3000 km from its mountain headwaters to deep sea fan the ganges dispersal system is immense and yet shows tight coupling between source area catchment basin and coastal and marine depocenters furthermore system wide responses and downstream transfer of the signal appear to occur at least as fast as current temporal resolution of 1 2 ky this tight linkage of source to sink components is considered a function of the southwest summer monsoon s overwhelming control on regional hydrology about 80 of the ganges discharge and 95 of its sediment load are delivered to the margin during only 4 months making the system extremely sensitive to this seasonal forcing in addition the regional scale and distribution of the monsoon weather system means that changes in atmospheric circulation affect the entire drainage basin rather than local subcomponents finally despite its present intensity strength of the summer monsoon has varied significantly over the past 150 ky under orbitally driven changes in insolation and global glacial boundary conditions the resulting changes in precipitation both well above and below modern values have forced system wide responses ranging from glacier advance retreat to sediment fluxes to the margin and deep sea fan the overall conclusions are 1 that this immense dispersal system responds to multimillennial scale 104 years climate change in a system wide and largely contemporaneous manner and 2 that major sedimentary signals can be transferred rapidly from source to sink with little apparent attenuation furthermore these acute responses to climate change have produced sedimentary stratigraphic features that diverge from traditional sequence models in their nature and timing

on the interpretation of peak metamorphic temperatures in light of garnet diffusion during cooling abstract finite difference models of fe mg diffusion in garnet undergoing cooling from metamorphic peak conditions are used to infer the significance of temperatures calculated using garnet biotite fe mg exchange thermometry for rocks cooled from high grades where the garnet was initially homogeneous the calculated temperature tcalc using garnet core and matrix biotite depends on the size of the garnet the ratio of garnet to biotite in the rock vgarnet vbiotite and the cooling rate for garnets with radii of 1 mm and vgarnet vbiotite 1 tcalc is 633 700 and 777 c for cooling rates of 1 10 and 100 c ma for vgarnet vbiotite 1 and 4 and a cooling rate of 10 c ma tcalc is approximately 660 and 610 c respectively smaller and larger garnets have lower and higher tcalc respectively these results suggest that peak metamorphic temperatures may be reliably attained from rocks crystallized at conditions below tcalc of the garnet core provided that vgarnet vbiotite is sufficiently small 0 1 and that the composition of the biotite at the metamorphic peak has not been altered during cooling numerical experiments on amphibolite facies garnets with nominal peak temperatures of 550 600 c generate a well in fe fe mg near the rim during cooling maximum calculated temperatures for the assemblage garnet chlorite biotite muscovite plagioclase quartz using the fe fe mg at the bottom of the well with matrix biotite range from 23 43 c to 5 12 c below the peak metamorphic temperature for cooling rates of 1 and 100 c ma respectively maximum calculated temperatures for the assemblage garnet staurolite biotite muscovite plagioclase quartz are approximately 70 c below the peak metamorphic temperature and are not strongly dependent on cooling rate the results of this study indicate that it may be very difficult to calculate peak metamorphic temperatures using garnet biotite fe mg exchange thermometry on amphibolite facies rocks tmax 550 c because the rim composition of the garnet which is required to calculate the peak temperature is that most easily destroyed by diffusion

timing of intrusion of the narragansett pier granite relative to deformation in the southwestern narragansett basin rhode island the pennsylvanian sediments of the narragansett basin rhode island were deformed during the al leghanian orogeny and record the effects of alleghanian deformation in southeast new england without the complication of earlier structures during this deformation the basin was intruded at its southwestern corner by the narragansett pier granite which has a crystallization age of 273 ma mapping of basin metasediments along the narragansett bay shore in the southwestern portion of the basin reveals features formed by several episodes of deformation two generations of isoclinal folds along north trending axes f1a and f1b are the earliest structures in the area followed by a set of small closed folds f2 also on north trending axes the fla f1b and f2 folds are the result of east west closure of the basin minor sinistral and more significant dextral motion on faults within the beaverhead shear zone produced local east trending crenulations s3 and open folds with north trending axes f4 in the study

magnetostratigraphy of miocene pliocene zagros foreland deposits in the front of the push e kush arc lurestan province iran abstract the timing of the deformation in the zagros simply folded belt is poorly constrained because of the lack of an accurate absolute chronology of the syntectonic sedimentary sequences the foreland basin infill at the front of the push e kush arc is composed of fine grained fluvial plain deposits agha jari fm and coarse conglomerates at the top of the section bakhtyari fm a magnetostratigraphic study was carried out in a composite section spanning about 2800 m in order to date growth strata and to constrain the timing of the deformation in the mountain front flexure mff magnetostratigraphic correlation of the base of the agha jari fm with chron c5a yields an age of 12 8 to 12 3 ma for this base the transition to the conglomerates of the bakhtyari fm correlates with the upper gauss chron c2an at approximately 3 ma the deposition age of the top of the preserved bakhtyari fm is extrapolated around the pliocene pleistocene boundary the base of the agha jari fm growth strata and thus the beginning of the deformation in the front of the push e kush arc is dated at 8 1 7 2 ma the topmost preserved bakhtyari is folded in the ne flank of the changuleh anticline and is unconformably overlying the sw flank of the anaran anticline this indicates that the tectonic deformation in the front of the push e kush arc was active at least during 5 my the mff is a relatively long lived structure active from 8 1 to 7 2 ma to about the pliocene pleistocene boundary partly synchronous with the changuleh anticline to the foreland after mff tectonic cessation only the changuleh anticline remained active

supplemental material zircon and monazite geochronology in the palmer zone of transpression south central new england usa constraints on timing of deformation high grade metamorphism and lithospheric foundering during late paleozoic oblique collision in the northern appalachian orogen analytical methods for zircon u pb and monazite th pb sims geochronology zircon u pb la icp ms geochronology data tables and supporting previous work

climatic and tectonic controls on fluvial incision and aggradation in the spanish pyrenees the influences of tectonic and climatic changes on upland river systems are investigated using data from plio pleistocene terraces of the rio cinca river in the southern pyrenees this river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the ebro basin detailed field mapping combined with long profile and palaeohydraulic reconstructions yields a comprehensive picture of changes in palaeoriver character during the plio pleistocene as the area is over 150 km from the basin outlet in the mediterranean sea changes in base level are unlikely to have influenced terrace development although tectonic activity has exerted a strong control on the position of the river the main period of thrust propagation pre dates the terraces and activity has waned from the pliocene through to the present it is concluded that the main control on incision in this area is climate through its influence on sediment supply rivers which are starved of sediment by climate change will have the power to incise whereas aggradational phases are linked to periods of increased sediment flux

u pb isotopic and geologic evidence for late paleozoic anatexis deformation and accretion of the late proterozoic avalon terrane southcentral connecticut

active and passive tectonic controls for transverse drainage and river gorge development in a collisional mountain belt dades gorges high atlas mountains morocco abstract transverse drainages are discordant river patterns whose flow directions cut across geological structures such as faults folds and the regional tectonic fabric strike of mountain belts such drainages can form spectacular geomorphological features with reaches often occupying narrow and deeply dissected canyons gorges that cut through prominent topographic barriers within collisional plate margins e g foreland basin systems studies have frequently examined transverse drainage development in response to active tectonics e g thrusting here transverse drainage will develop via incision or become deflected and fixed in response to oblique fault fold growth structures particularly within wedge top thrust front and foredeep basin settings within this paper the development of transverse drainage within the little studied fold thrust component of an orogen is examined using a spectacular series of river gorges along the river dades from the high atlas mountains of morocco integrated field derived geological and geomorphological data are used to explore the interplay between active tectonics lithology and structural configuration passive tectonics of bedrock for the creation of transverse drainage and associated river gorge reaches at the southern edge of the high atlas fold thrust belt the river dades passes through the deeply dissected tarhia n dades and main dades gorges before emerging into the ait seddratt wedge top basin fluvial incision is principally controlled by plio quaternary uplift of the high atlas mountains the larger main dades gorge has formed a deeply dissected antecedent course via drainage inheritance into structurally thickened jurassic bedrock via uplift between two key structural components of the foreland basin system the fold thrust belt and the wedge top basin the smaller tarhia n dades gorge involves a more complex interplay between active tectonics and the passive structural arrangement of an asymmetric plunging syncline combined with the stratigraphic arrangement and strength of the jurassic bedrock the drainage network is configured to the fold and associated fault joint structures to form respective nw se and ne sw orientated dip slope and strike drainage progressive incision by tributary dip slope streams cutting down through resistant limestone bedrock into underlying weaker mudstone bedrock has resulted in accelerated headward erosion and drainage network expansion by river captures this has resulted in a minor re routing of the river dades as a transverse drainage through a former dip slope tributary stream to form the tarhia n dades gorge

melt in the mantle beneath the amagmatic zone southern nevada surface wave tomography of the western united states reveals the presence of an unusually slow shear wave velocity anomaly beneath a region in the basin and range province sometimes termed the amagmatic zone which is renowned for its lack of volcanic activity we pre sent a model of the three dimensional shear wave velocity structure beneath the southwestern united states constructed from the inversion of fundamental mode rayleigh wave dispersion the abundance of data from the deployment of the usarray a component of the earthscope project provides unprecedented resolution there is an excellent correlation between the location of slow shear wave velocity anomalies and recent younger than 1 ma volcanism this correlation and the unusually low values of absolute shear velocity beneath the magmatic gap lead us to conclude that there is melt present beneath this region previous studies have proposed that the preservation of cold continental lithosphere beneath the magmatic gap accounted for the lack of volcanism however this hypothesis is inconsistent with anomalously slow shear wave velocities in the 50 100 km depth range perhaps the prevalence of low angle normal faulting in this region has made it difficult for the melt to escape the mantle beneath an immediately adjacent region the southwestern nevada volcanic field where volcanism was abundant 10 15 m y ago but has since waned significantly higher shear velocities suggest that the melt has been extracted from the mantle

deformation induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses tectonic and metamorphic implications a sequence of at least three al2sio5 bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the willimantic window connecticut the ductile deformation localized at and near the boundary between the putnam nashoba terrane and underlying avalon terrane is characterized by a network of anastomozing shear zones that outline metre scale tectonic blocks of migmatitic kfs sil gt bi pg qtz ilm ru gneiss these assemblages record acadian or older metamorphic conditions of 6 kbar 700d c mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss the reconstituted rocks exhibit relict ky st grt pl bt qtz rt ilm assemblages and require a minimum pressure for the ky str grade metamorphism of 8 5 kbar kyanite in block margins is widely replaced by sillimanite and locally by andalusite during a period of post alleghanian ductile deformation the interiors of blocks do not record this sequence of polymorphs the pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane bounding fault zone strain provided the activation energy for recrystallization and retrograde mineral reactions the p t conditions of post alleghanian ductile deformation evolved from 600d c and 6 kbar to 550d c and 3 kbar the occurrence of ky str bearing assemblages overprinting acadian kfs sil bearing assemblages and subsequently overprinted by alleghanian sillimanite and andalusite bearing assemblages along with reset hornblende 40ar 39 ar mineral ages from mississippian to permian requires a prograde alleghanian metamorphism of rocks previously metamorphosed during the acadian thus mineral assemblages from gneisses in the willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high grade acadian orogenesis tectonic loading resulting from alleghanian collision of avalon with north america and tectonic exhumation in permo triassic time the c 3 kbar pressure decrease between prograde and retrograde alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the putnam nashoba terrane cover sequence following alleghanian orogenesis in south eastern new england

40ar 39ar ages from the narragansett basin and southern rhode island basement terrane their bearing on the extent and timing of alleghenian tectonothermal events in new england pennsylvanian sedimentary rocks record the effects of progressive barrovian metamorphism and polydeformation in the narragansett basin of rhode island whole rock phyllite samples from low grade portions of this terrane display internally concordant 40 ar 39 ar age spectra with plateau ages of 248 to 258 m y four samples biotite from higher grade rocks records plateau ages of 240 to 248 m y eight samples similar 40 ar 39 ar biotite ages are displayed westward across the southern rhode island basement terrane 239 to 250 m y nine samples although coexisting hornblende records slightly older plateau ages of 255 m y two samples the 40 ar 39 ar plateau ages are interpreted to date times of post metamorphic cooling through temperatures required for argon retention in the various mineral phases examined the ages suggest that a distinct late paleozoic alleghenian thermal event affected much of southern rhode island together with the depositional ages of the variably metamorphosed pennsylvanian rocks westphalian b stephanian a or b the 40 ar 39 ar data permit calibration of the sequence of alleghenian tectonothermal events recorded in the narragansett basin the controls suggest a permian event lasting about 40 to 50 m y between initial barrovian regional metamorphism and synchronous d 1 d 2 ductile deformation between 295 and 275 m y and post d 3 cooling below argon retention temperatures between 250 and 240 m y

development and environmental significance of an eolian sand ramp of last glacial age central iran abstract a 25 m thick section of mostly eolian sediment is exposed in the stream cut flank of a sand ramp accumulated in a mountain saddle near ardakan playa central iran the well sorted eolian sediments of the ramp contain talus beds and incipient paleosols morphology and bedding structures imply that southeasterly winds were primarily responsible for deposition of the eolian sand optical dating shows that the sand was deposited near the last glacial maximum within about 5000 yr surface stabilization lack of surface scree and the development of a stream cut between the mountain and the ramp show that sediment accumulation ceased after this time the siberian high pressure system may have been established over central iran during the last glacial maximum causing cooler temperatures and a prevalence of southeasterly winds these colder windier conditions would have led to frost shattering and eolian transport both of which are not significant processes today but which are evident as past processes from the ramp sediments

the effects of porphyroblast growth on the effective viscosity of metapelitic rocks implications for the strength of the middle crust numerical models are used to examine the effects of porphyroblast growth on the rheology of compositionally layered rocks metapelites and metapsammites and by extension the middle crust during prograde metamorphism as porphyroblast abundance increases during prograde metamorphism metapelitic layers will strengthen relative to porphyroblast free metapelitic units and potentially relative to quartzofeldspathic metapsammitic units as metapelitic layers become stronger the integrated strength of compositionally layered successions increases potentially causing large volumes of mid crustal rock to strengthen altering the strain rate distribution in the middle crust and affecting the geodynamic evolution of an orogenic belt the growth of effectively rigid porphyroblasts creates strength heterogeneities in the layer undergoing porphyroblast growth which leads to complex strain rate distributions within the layer at the orogen scale the strengthening of large crustal volumes on the order of thousands of cubic kilometres changes the strain rate distribution which may change exhumation rates of high grade metamorphic rocks the geothermal structure and the topography of the orogen the presence of a strong zone in the middle crust causes strain rate partitioning around the zone suppressed uplift rates within and above the zone and leads to the development of a basin on the surface

a late pleistocene long pollen record from lake urmia nw iran abstract a palynological study based on two 100 m long cores from lake urmia in northwestern iran provides a vegetation record spanning 200 ka the longest pollen record for the continental interior of the near east during both penultimate and last glaciations a steppe of artemisia and poaceae dominated the upland vegetation with a high proportion of chenopodiaceae in both upland and lowland saline ecosystems while juniperus and deciduous quercus trees were extremely rare and restricted to some refugia hippophae rhamnoides constituted an important phanerophyte particularly during the late last glacial period a pronounced expansion in ephedra shrub steppe occurred at the end of the penultimate late glacial period but was followed by extreme aridity that favoured an artemisia steppe very high lake levels registered by both pollen and sedimentary markers occurred during the middle of the last glaciation and late part of the penultimate glaciation the late glacial to early holocene transition is represented by a succession of hippophae ephedra betula pistacia and finally juniperus and quercus the last interglacial period eemian slightly warmer and moister than the holocene was followed by two interstadial phases similar in pattern to those recorded in the marine isotope record and southern european pollen sequences

tectonic controls on drainage evolution and development of terminal alluvial fans southern pyrenees spain an understanding of how drainage patterns respond to tectonics can provide an insight into past deformational events within mountain belts and where sediment flux is supplied to depositional basins the transverse rivers draining the spanish pyrenees show sudden diversions to axial courses and the capture of lateral systems producing large trunk rivers that break through the thrust front at structurally controlled points the drainage reorganization from the initial regularly spaced pattern in the late eocene caused by the growth of thrust controlled topography influenced the location of outlets into the ebro basin the headward capture and merger of rivers as a result of structural diversion formed two large terminal fan systems during the oligo miocene along the pyrenean thrust front the early structural topographic controls on drainage evolution will have long term effects on sedimentation and stratigraphic architecture of foreland basins this will only be maintained as long as there is tectonic uplift and the river systems strive to re attain a regular drainage spacing across the orogenic belt as partly achieved in the pyrenees

strike slip kinematics in central and eastern iran estimating fault slip rates averaged over the holocene 1 according to gps measurements the right lateral shear between central iran and afghan blocks amounts to 16 mm yr a model based on very long term estimates of fault rates suggests the current shear originated about 5 ma ago and has been accommodated by strike slip faulting limited to the western 2 mm yr gowk nayband fault and eastern 14 mm yr sistan system fault edges of the lut block we have used high resolution spot5 pixel size 2 5 m images to measure recent cumulative offsets and estimate slip rates over shorter time periods that average several seismic cycles only recent offsets a few tens of meters have been found along the anar fault inside the central iran plateau and along the sistan faults east of the lut the offset morphologies postdate the last incision of the network and are most probably of holocene age 12 2 ka the corresponding slip rates range between 0 5 0 75 mm yr 1 75 2 5 mm yr 1 5 mm yr 1 2 5 mm yr for the anar east neh west neh and asagie fault respectively these estimates suggest the gps shear rate across the lut may not extrapolate over the holocene they also indicate strike slip faulting is not confined to the lut edges but also occurs in central iran suggesting the ongoing strike slip tectonics might have originated between 8 and 22 ma ago earlier than considered previously and consistent with observations in ne iran

tectonic and climatic controls on fan systems the kohrud mountain belt central iran abstract late pleistocene to holocene fans of the kohrud mountain belt central iran illustrate the problems of differentiating tectonic and climatic drivers for the sedimentary signatures of alluvial fan successions it is widely recognised that tectonic processes create the topography that causes fan development the existence and position of fans along the kohrud mountain belt ne of esfahan are controlled by faulting along the qom zefreh fault system and associated fault zones these faults display moderate amounts of historical and instrumental seismicity and so may be considered to be tectonically active however fluvial systems on the fans are currently incising in response to low gavkhoni playa lake levels since the mid holocene producing incised gullies on the fans up to 30 m deep these gullies expose an interdigitation of lake deposits dominated by fine grained silts and clays with evaporites and coarse gravels that characterise the alluvial fan sediments the boundaries of each facies are mostly sharp with fan sediments superimposed on lake sediments with little to no evidence of reworking in turn anhydrite glauberite mirabilite and halite crusts drape over the gravels recording a rapid return to still water shallow ephemeral saline lake sedimentation neither transition can be explained by adjustment of the hinterland drainage system after tectonic uplift the potential influence in central iran of enhanced monsoons the northward drift of the intertopical convergence zone itcz and mediterranean climates for the early holocene 6 10 ka point to episodic rainfall during winter months associated with discrete high magnitude floods on the fan surfaces the fan sediments were deposited under the general influence of a highstand playa lake whose level was fluctuating in response to climate this study demonstrates that although tectonism can induce fan development it is the sensitive balance between aridity and humidity resulting from changes in the climate regime of central iran that influences the nature of fan sequences and how they interrelate to associated facies

calcite graphite isotope thermometry a test for polymetamorphism in marble tudor gabbro aureole ontario canada graphitization and coarsening of organic material in carbonate bearing metasedimentary rocks is accompanied by carbon isotope exchange which is the basis of a refractory pressure independent geothermometer comparison of observed isotopic fractionations between calcite and graphite a13ccal gr with independent petrological thermometers provides the following empirical calibration over the range 400 800 c a13ccal gr 5 81 x lo6 x t k 2 61 this system has its greatest potential in marbles where calcite graphite is a common assemblage and other geothermometers are often unavailable the temperature dependency of this empirical calibration differs from theoretical calibrations reasons for this are unclear but the new empirical calibration yields temperature estimates in better agreement with independent thermometry from several terranes and is preferred for geological applications both calcite graphite isotopic thermometry and calcite dolomite solvus thermometry are applied to marble adjacent to the tudor gabbro in the grenville province of ontario canada the marble has undergone two metamorphic episodes early contact metamorphism and later regional metamorphism values of a13cca gr decrease regularly from c 8 0 in samples over 2 km from the pluton to values of 3 4 within 200m of the contact these samples appear to preserve fractionations from the early thermal aureole with the empirical geothermometer and indicate temperatures of 450 500 c away from the intrusion and 700 750 c near the gabbro this thermal profile around the gabbro is consistent with conductive heat flow models in contrast the distribution of mg between calcite and dolomite has been completely reset during later regional metamorphism and yields uniform temperatures of c 500 c even at the contact graphite textures are important for interpreting the results of the calcite graphite thermometer coarsening of graphite approaching the tudor gabbro correlates with the decrease in isotopic fractionations and provides textural evidence that graphite crystallization took place at the time of intrusion in contrast to isotopic exchange during prograde metamorphism which is facilitated by graphitization retrogressive carbon isotopic exchange appears to require recrystallization of graphite which is sluggish and easily recognized texturally resistance of the calcite graphite system to resetting permits thermometry in polymetamorphic settings to see through later events that have disturbed other systems

microstructure and crystallographic preferred orientation of polycrystalline microgarnet aggregates developed during progressive creep recovery and grain boundary sliding abstract optical microscopy electron probe microanalysis and electron backscatter diffraction methods have been used to examine a broad range of garnet microstructures within a high strain zone that marks the western margin of a major transpression zone in the southern new england appalachians garnet accommodated variable states of finite strain expressed as low strain porphyroclasts type 1 high strain polycrystalline aggregates type 2 and transitional morphologies type 3 that range between these end members type 1 behaved as rigid porphyroclasts and is characterized by four concentric ca growth zones type 2 help define foliation and lineation are characterized by three ca zones and possess a consistent bulk crystallographic preferred orientation of 100 symmetrical to the tectonic fabric type 3 show variable degrees of porphyroclast associated with aggregate where porphyroclasts display complex compositional zoning that corresponds to lattice distortion low angle boundaries and subgrains and aggregate cpo mimics porphyroclast orientation all aggregates accommodated a significant proportion of greenschist facies deformation through grain boundary sliding grain rotation and impingement and pressure solution which lead to a cohesive behavior and overall strain hardening of the aggregates the characteristic cpo could not have been developed in this manner and was the result of an older phase of partitioned amphibolite facies dislocation creep recovery including chemical segregation and recrystallization of porphyroclasts this study demonstrates the significance of strain accommodation within garnet and its affect on composition under a range of pt conditions and emphasizes the importance of utilizing ebsd methods with studies that rely upon a sound understanding of garnet

an experimental investigation of autogenic behaviour during alluvial fan evolution the findings from a series of laboratory experiments designed to examine the role of autogenic process form interactions as controls on alluvial fan evolution are presented the experimental approach follows the similarity of processes concept and is therefore not scaled to a real world prototype the experiments were conducted in the university of exeter sediment research facility using a 3 3 m fixed bed experimental table with controlled sediment and water supply fed to the fan apex and evacuation of all sediment that passes the distal boundary the evolution of the fan was recorded using high resolution time lapse overhead photography to permit analysis of fan topography flow patterns channel migration and avulsion on the fan surface over the course of the experimental run the qualitative observations from three experimental scenarios are discussed and evaluated in the context of current understanding of alluvial fan evolution these demonstrate that in the absence of any change in the extrinsic variables there is an autogenic transition from sheetflow to channelised flow as the experimental fans evolve this is consistent with theory that predicts this change in response to a decline in aggradation rate through time

10be dating of alluvial deposits from southeastern iran the hormoz strait area abstract a key area for the understanding of the southeastern iran geodynamics the hormoz strait area was investigated tectonically active it provides useful sequences of quaternary deposits whose deposition may have ended either when the climate dried or when the tectonic activity uplifted the riverbed in situ produced cosmogenic 10be provides surface abandonment ages at 5 6 0 6 8 4 1 0 12 8 1 0 20 1 1 5 and 44 0 3 4 10be kyr located at the transition between the mediterranean winter rain and the indian monsoon domains and south of the northernmost position of the itcz intertropical convergence zone the studied area is very sensitive to climate changes some of the calculated 10be abandonment ages are within the entire investigated area coincidental with large scale climate shifts the offset of the mid holocene humid period 5 6 0 6 10be kyr the onset of the dry younger dryas cold and dry episode 12 8 1 010be kyr and the end of the last glacial maximum lgm 21 2 kyr at 20 1 1 5 10be kyr these events correspond to large scale changes in atmospheric circulation linked to global climate changes and or to sea level fluctuations in the persian gulf which was totally dry during the lgm locally observed abandonment ages at 8 4 1 0 and 44 0 3 4 10be kyr may likely be related to enhanced tectonic activity without regional impact

lateral erosion toe cutting of alluvial fans by axial rivers implications for basin analysis and architecture abstract we document the neglected phenomenon of lateral erosion toe cutting of alluvial fans by non incising axial river channels field examples from the holocene of the big lost river basin idaho and the plio pleistocene of the rio grande rift new mexico help to establish architectural models with more general application to basin analysis the process of toe cutting can lead to complete fan destruction and may be a response to climate change tectonic tilting fault propagation or a combination of these variables it gives rise to near horizontal erosion surfaces cut in fan sediment steep fan margin scarps progressive up fan incision from the scarp by a network of channels soil formation up fan away from the incised channel network a deposit of axial alluvium that overlies the erosion surface and onlaps the scarp once avulsion occurs to take the axial channel away from the bajada margin distinctive healing wedges of fan alluvium prograde across abandoned axial river channel and floodplain deposits gradually onlapping the eroded scarp and its upstream network of incised channels toe cutting has important stratigraphic basin analysis and economic consequences bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments

modeling alluvial landform change in the absence of external environmental forcing alluvial fan evolution and morphology are often considered to respond primarily to external forcing e g tectonics climate and base level change here we present a numerical model of alluvial fan evolution that shows that dramatic and persistent fan entrenchment may occur in the absence of such forcing this process is driven by positive autogenic feedbacks between flow width sediment transport and rate of fan aggradation entrenchment is initiated where sediment accommodation space limits continued fan growth our results highlight a need to rethink both the representation of fluvial width adjustment in landscape evolution models and the established framework for the interpretation of fluvial landforms as archives of environmental change

isotopic dating in metamorphic belts isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions of these temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed the central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated in order to illustrate the factors which control closure temperatures and limit their application in practice the possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined taking examples involving k ar rb sr sm nd and u pb analyses the indirect dating of metamorphism via structurally dated plutons is briefly considered and the problems of interpreting both discordant u pb zircon ages and rb sr whole rock isochrons in metamorphosed granites are discussed the value of near concordant zircon ages is emphasized the extent of isotopic equilibration during metamorphism specifically of sr isotopes in metasediments is shown not to be a simple function of metamorphic grade the flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently the concluding discussion briefly examines existing data for the dalradian and indicates the potential for further progress using both well established and novel approaches

source area and tectonic control on alluvial fan development in the miocene fohnsdorf intramontane basin austria middle miocene alluvial fans in the intramontane fohnsdorf basin of the eastern alps originated along normal faults and linked strike slip faults in a continental half graben setting the fans display considerable facies differences debris flows of the rachau fan are characterized by a sandy matrix and large boulders whereas debris flows of the apfelberg fan are characterized by higher silt and clay content and smaller clasts key control of debris flow facies is the lithology contrast in the fan source areas sand pebbles and large outsized boulders originated predominantly from the resistant augengneissand amphibolite dominated hinterland of the rachau fan whereas a significant higher proportion of mud and silt and smaller boulders have been derived from the apfelberg fan catchment which was dominated by mica schists and marble from harvey a m mather a e stokes m eds 2005 alluvial fans geomorphology sedimentology dynamics geological society london special publications 251 207 216 0305 8719 05 15 the geological society of

palaeolimnology of lake zeribar iran and its climatic implications lake zeribar sediments covering the time period of the last 25 000 years were examined for the contents of seeds fruits characeae diatoms and molluscs reconstructions of the variations in the lake water level salinity and trophy suggest a sequence of climatic changes three pronounced stages of low and varying lake water level occurred ca 17 700 15 400 12 600 12 000 and 10 000 6000 cal yr bp some water level changes were correlated with variations in salinity the most pronounced increase of salinity occurred 17 700 15 700 and 12 600 12 000 cal yr bp and less distinct ones occurred about 6400 5900 and 2500 cal yr bp diatom assemblages indicated a strong increase of lake trophy ca 20 200 cal yr bp between 6000 and 5000 cal yr bp diatoms characteristic of eutrophy increased in core 63j and at about 3200 cal yr bp a distinct increase in mesotrophic forms occurred in core 70b the changes in the occurrence of various organisms indicate increased temperatures about 21 000 cal yr bp between 15 400 and 12 600 about 12 000 and about 11 700 cal yr bp the reduced occurrence or disappearance of some of them suggest temperature decreases about 17 700 15 400 and 12 600 12 000 cal yr bp

partitioning of deformation along an orogen and its effects on porphyroblast growth during orogenesis an identical succession of foliation inflection intersection axis trends in porphyroblasts fias is present in three separate areas affected by acadian deformation and metamorphism along 130 kms of the appalachians from east central vermont to north central massachusetts the fias trend successively nw se sw ne w e nnw sse and ssw nne and appear to have formed during the same succession of deformations which have been dated in se vermont as follows not yet dated pre 425 ma 425 to 404 ma 404 to 385 ma and 385 to 360 ma respectively the two areas that are furthest apart in east central vermont and north central massachusetts have strikingly similar total distributions of fias in garnet porphyroblasts that show a somewhat inverse relationship to those observed in the chester athens dome area of se vermont that separates them that is the number of samples containing the first formed fia set is greatest for the two regions lying furthest apart and decreases in younger sets whereas the number of samples containing the first fia set is the least in the central region and increases for more recently developed sets partitioning of deformation into zones of progressive shearing and shortening has been demonstrated to control the sites of development of differentiated crenulation cleavage versus porphyroblast growth and sites of porphyroblast growth on one limb of a regional fold versus the other we argue that partitioning of the deformation around competent feldspathic gneiss bodies at an orogen scale produced the variation in the number of samples containing the various fia sets between the central and outer areas described above porphyroblasts most commonly grew during sw ne directed shortening to the north and south of the chester athens dome region of the appalachians but only rarely grew in the latter region this suggests that deformation at this time partitioned around an outlier of competent basement feldspathic gneiss that underlies the chester athens dome region and this protected the rocks above and in the strain shadow to the nw and se from the effects of the great bulk of the deformation that occurred over the period of time during which nw se trending fias formed we attribute the increase in the number of samples containing progressively younger fias in the chester athens dome area during subsequent periods of nw se n s wsw ene and wnw ese directed shortening to the rise in temperature and pressure recorded in the rocks above the chester athens dome with peak conditions occurring during the development of the nnw sse oriented fia set between 404 and 385 ma as the temperature and pressure increased the gneiss below would have become less competent allowing the deformation associated with the succession of fia sets to partition more pervasively through the dome core this provided more sites for porphyroblast nucleation and growth and hence the number of samples recording these fia sets increased the concomitant decrease in the number of samples in which porphyroblasts grew in the rock within the areas to the north and south with successive fias possibly resulted from a progressive increase in competency of these rocks as the number of porphyroblasts increased

coupled caal nasi diffusion in plagioclase feldspar experiments and applications to cooling rate speedometry abstract the rate of caal nasi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400 to 1000 c in calcic plagioclase an80 81 by homogenizing coherent exsolution lamellae the dependence of the average interdiffusion coefficient on temperature is given by the expression d 10 99 cm 2 sec exp 123 4 kcal mol rt t in k this value is for diffusion perpendicular to the 03 1 interface of the lamellae caal nasi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400 to 1200 c and possibly 10 orders of magnitude slower at subsolidus temperatures the large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions e g the skaergaard which retain delicate ca na compositional zoning profiles on the micron scale but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks caal nasi diffusion is slow the closure temperature is high within the solidus liquidus interval and ca na zoning is preserved oxygen diffusion is faster the closure temperature is lower 350 400 c and the feldspars exchange oxygen with the low temperature hydrothermal fluids the complex micron scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase cooling histories typical of large mafic intrusions e g the stillwater are slow begin at high initial temperatures 1200 c and result in homogenization of oscillatory zones on the scale of 10 microns the oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature 1000 c limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion

combining compositional zoning and foliation intersection axes fias in garnet to quantitatively determine early p t t paths in multiply deformed and metamorphosed schists north central massachusetts usa quantitative compositional and microstructural analysis of garnet porphyroblasts in kyanite staurolite garnet grade rocks from the northeastern flank of the pelham dome north central massachusetts distinguishes the effects of acadian deformation and metamorphism from extensive overprinting alleghanian shearing the p t conditions and the metamorphic path during the acadian were determined using samples preserving well defined stages in a lengthy tectonic history revealed by a succession of five foliation intersection axis trends preserved within porphyroblasts fias this acadian succession extends at least 120 km to the north into rocks where no evidence has been found of an alleghanian overprint for each sample where garnet first nucleated during one of these stages in the tectonic history the pt of core growth was determined by plotting the intersection of the mn fe and ca isopleths calculated for the core composition on a p t pseudosection for that sample using thermocalc combining the pt data from all these samples indicates that the temperature and pressure increased throughout acadian orogenesis causing episodic garnet growth during the alleghanian locally intense shearing especially against the margin of the pelham dome formed the dominant schistosity which truncated all foliations defined by inclusion trails in porphyroblasts and obliterated all remains of acadian deformation and metamorphism in the rock matrix shearing was accompanied by near complete homogenization of the compositional zoning in garnet porphyroblasts and an associated apparent increase in the temperature of the matrix to 700 c in those rocks lying directly adjacent to the pelham dome and resulted from the rocks of the northfield syncline being thrust a large distance southwards over the gneisses in the dome

active tectonics and late cenozoic strain distribution in central and eastern iran 1 right lateral shear between central iran and afghanistan is accommodated on n s right lateral strike slip faults surrounding the aseismic dasht e lut north of 34 n the right lateral shear is accommodated on left lateral faults that rotate clockwise about vertical axes little is known of the late tertiary and younger offsets and slip rates on the active fault systems results that are important for understanding the regional tectonics we use observations from satellite imagery to identify displaced geological and geomorphological markers which we use in conjunction with the overall morphology and orientation of the active fault systems to estimate the total cumulative right lateral shear estimates of cumulative fault movements from offset features and inferred vertical axis rotation of fault bounded blocks suggest that the late cenozoic strain is concentrated toward the eastern margin of iran along the sistan shear zone where bedrock offsets of at least 70 km are observed across the active faults the geomorphology of the deh shir anar and great kavir strike slip faults in central iran suggest that although little shortening is accommodated across this region they might still be active and hence capable of producing earthquakes present day activity on these faults in central iran would not be expected from distributions of instrumental and historical earthquakes although speculative the late tertiary strain distribution described in this paper is consistent with what we know of the present day rates of shear in eastern iran and provides a framework to which later more detailed work can be added

braided stream and flood plain architecture the rio vero formation spanish pyrenees early to middle miocene fluvial sandstones of the rio vero formation were studied in an area around the town of barbastro south central pyrenees spain the outstanding quality of outcrops in this area allows a three dimensional study of architectural elements six architectural elements are recognised described in detail and interpreted from three key localities seven main lithofacies were identified and sub divided into gravelly sandy and fine grained lithofacies the architectural elements and lithofacies have been combined with a hierarchy of depositional bounding surfaces to fully interpret the evolution of the depositional system at the meso and macro scale not only the different architectural elements and lithofacies of the complete braided fluvial system but also the lateral variation of the architectural elements were emphasised in this study differential tectonic movements seasonal climate change and their effect on vertical and lateral evolution of the area were the main control on basin sedimentation channel interconnection palaeocurrent patterns and consequently the fluvial architecture the presence of lateral ramp anticlines caused the fluvial system to be laterally restricted with the main channel belts being located in the areas of highest subsidence and lowest topography intervening topographic highs acted as both flood plains and lateral barriers between the main channel systems the proposed depositional model comprises broad low sinuosity perennial but seasonal moderate energy streams the sandstone architecture is dominated by channel fill and sheet sands and associated simple and more complex bars adjacent to the main channel belts fine grained sandstones siltstones and immature paleosols occur the along strike relationship between major fluvial systems and their outlets into a foreland basin has important implications for the infill of the basin and the modelling of fluvial systems along mountain belt fronts

evidence for extension at the willimantic dome connecticut implications for the late paleozoic tectonic evolution of the new england appalachians

fracturing of garnet crystals in anisotropic metamorphic rocks during uplift garnet crystals in granulite facies mylonites and gneisses from the morin shear zone in the grenville province quebec are characterized by pervasive closely spaced relatively straight tensile fractures aligned systematically normal to the mylonitic foliation and lineation the fractures developed preferentially in coarse grains 0 2 mm or in grains with a large aspect ratio 2 fractured segments of each garnet crystal have not been separated or filled with matrix minerals quartz and feldspar or with retrograde minerals biotite muscovite and chlorite this suggests that the garnets were fractured at shallow crustal depths ti300 c or 15 km for a thermal gradient zo c km we propose that the garnet fractures were formed by the response of anisotropic metamorphic rocks to a horizontal extension during uplift and cooling of the metamorphic terrane within the upper crust using a modified shear lag theory we can explain why tensile fracturing took place preferentially in stiff garnet rather than in felsic material and why the fractures are unequally spaced in a garnet grain our data suggest that the seouential tensile fractures of garnet grains record a progressive uplift process of metamorphic rocks within the upper crust 0 1997 elsevier science ltd

contrasting styles of taconian eastern acadian and western acadian metamorphism central and western new england the two major early to middle palaeozoic tectonic metamorphic events in the northern appalachians were the taconian middle to late ordovician in central to western areas and the acadian late silurian to early middle devonian in eastern to west central areas this paper presents a model for the acadian orogenic event which separates the acadian metamorphic realm into eastern and western belts based on distinctively different styles we propose that the acadian metamorphism in the east was the delayed consequence of taconian back arc lithospheric modification east of the taconian island arc thick accumulations of late ordovician and silurian sediments coupled with plutons rising along a magmatic arc produced crustal thermal conditions appropriate for anomalously high t low p metamorphism accompanied by major crustal anatexis in this zone upward melt migration was coupled with subsequent e w crustal shortening possibly due to outboard collision with the avalon terrane to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west and probably to the east as well the basis for the distinction between the eastern and western acadian events lies in the contrasting styles of metamorphism accompanying each evidence for contrasting metamorphic styles consists of 1 estimated metamorphic field gradients mfgs based on thermobarometric studies and 2 petrological evidence for contrasting p t trajectories west of the acadian metamorphic front the taconian zone has an mfg in which peak temperatures of 400 600 c were reached at pressures of about 4 6 kbar with both p and t increasing to the east near its western edge the western acadian metamorphic overprint has a similar mfg to the taconian and is mainly discriminated by 40ar 39ar dating and microtextural evidence east of this narrow zone the western acadian overprint is characterized by progressively higher temperatures 600 725 c and pressures 6 5 10 kbar or more to the east yielding an overall mfg that lies along or slightly above the kyanite sillimanite boundary on a p t diagram there is little or no plutonism accompanying western acadian metamorphism in contrast thermobarometry in the eastern acadian east of the bronson hill belt yields high t intermediate p conditions for the highest grade rocks known in new england t 650 750 c p 4 5 6 5 kbar for granulite facies assemblages which apparently formed along an anticlockwise p t path the bronson hill belt lies geographically between the eastern and western acadian zones and shows transitional petrological behaviour anomalously high temperatures at intermediate pressures but a clockwise path with decompression cooling radiometric dating indicates peak taconian conditions may have been achieved as early as 475 ma in the taconian hinterland and as late as 445 ma in the taconian foreland including the taconic allochthons eastern acadian magmatism may have started as early as 425 ma and most nappe stage deformation and metamorphism in the eastern acadian zone appears to have ended by about 410 ma tectonic thickening in the western acadian including the western counterparts of the nappe stage deformation documented in the eastern acadian must pre date attainment of peak metamorphic conditions dated at 395 385 ma dome stage deformation clearly post dates peak metamorphism and deforms metamorphic isograds the end of western acadian deformation is well constrained by 370 375 ma radiometric ages of late pegmatites and granitoids which cross cut all structures

p t t paths and differential alleghanian loading and uplift of the bronson hill terrane south central new england late paleozoic u pb ages of sphene and 4 0 ar 3 9 ar cooling ages of amphibole and muscovite from rocks of the bronson hill terrane in connecticut and central massachusetts reflect a late paleozoic alleghanian overprinton acadian metamorphic rocks prograde alleghanian sphenes crystallized during the late pennsylvanian and eliminate the possibility that amphibole ages reflect delayed permian cooling from devonian acadian metamorphism fourteen new amphibole ages from connecticut form a north to south trend of decreasing age from 294 to 245 ma while in massachusetts four new amphibole ages together with three others from the literature produce a random carboniferous pattern seven new muscovite ages support existing data indicating uniform cooling throughout the bronson hill terrane through 350 c in the early triassic the rate of permian cooling defined by amphibole muscovite pairs increases from 4 c my in northern connecticut to 50 c my near long island sound hinged loading and hinged but delayed exhumation in the southern part of the bronson hill terrane with the hinge in central connecticut explain these ages and cooling rates as well as a southerly increasing metamorphic field gradient one dimensional thermal modeling indicates that loading of bronson hill rocks must have begun by the late mississippian the time of peak alleghanian metamorphic temperature decreases southward from early permian in northern connecticut to late permian to the south these results demonstrate that the metamorphic effects of the alleghanian orogeny are not restricted to the avalon terrane of southeastern new england on the contrary the alleghanian orogeny reset 4 0 ar 3 9 ar mineral ages recrystallized minerals partially melted felsic rocks and transposed fabrics at least as far west as the bronson hill terrane in south central new england

a new tectonic scenario for the sanandaj sirjan zone iran abstract recent geochemical studies of volcanic rocks forming part of the ophiolites within the zagros and naien baft orogen indicate that most of them were developed as supra subduction ophiolites in intra oceanic island arc environments intra oceanic island arcs and ophiolites now forming the naien baft zone were emplaced southwestward onto the northeastern margin of the south sanandaj sirjan zone while those now in the high zagros were emplaced southwestward onto the northern margin of arabia thereafter subduction continued on opposite sides of the remnant oceans the floor of neo tethys ocean was subducted at a low angle beneath the entire sanandaj sirjan zone and the floor of the naien baft ocean was subducted beneath the central iranian micro continent the naien baft ocean extended into north west iran only temporarily this failed ocean arm between the urumieh dokhtar magmatic assemblage and the main zagros thrust was filled by thick upper triassic upper jurassic sediments the naien baft ocean finally closed in the paleocene and neo tethys closed in the early to middle eocene after arabia was sutured to iran the urumieh dokhtar magmatic assemblage recorded slab break off in the middle eocene

holocene settlement shifts and palaeoenvironments on the central iranian plateau investigating linked systems for thousands of years humans have inhabited locations that are highly vulnerable to the impacts of climate change earthquakes and floods in order to investigate the extent to which holocene environmental changes may have impacted on cultural evolution we present new geologic geomorphic and chronologic data from the qazvin plain in northwest iran that provides a backdrop of natural environmental changes for the simultaneous cultural dynamics observed on the central iranian plateau well resolved archaeological data from the neighbouring settlements of zagheh 7170 6300 yr bp ghabristan 6215 4950 yr bp and sagzabad 4050 2350 yr bp indicate that holocene occupation of the hajiarab alluvial fan was interrupted by a 900 year settlement hiatus multi proxy climate data from nearby lakes in northwest iran suggest a transition from arid early holocene conditions to more humid middle holocene conditions from ca 7550 6750 yr bp coinciding with the settlement of zagheh and a peak in aridity at ca 4550 yr bp during the settlement hiatus palaeoseismic investigations indicate that large active fault systems in close proximity to the tell sites incurred a series of large mw 7 1 earthquakes with return periods of 500 1000 years during human occupation of the tells mapping and optically stimulated luminescence osl chronology of the alluvial sequences reveals changes in depositional style from coarse grained unconfined sheet flow deposits to proximal channel flow and distally prograding alluvial deposits sometime after ca 8830 yr bp possibly reflecting an increase in moisture following the early holocene arid phase the coincidence of major climate changes earthquake activity and varying sedimentation styles with changing patterns of human occupation on the hajiarab fan indicate links between environmental and anthropogenic systems however temporal coincidence does not necessitate a fundamental causative dependency

contrasting p t t paths thermochronologic evidence for a late paleozoic final assembly of the avalon composite terrane in the new england appalachians strongly contrasting pressure temperature time paths for the avalon composite terrane and the structurally overlying putnam nashoba zone in eastern new england obtained from thermochronologic and thermobarometric data are best explained by a late paleozoic underthrusting of cover rocks by the avalon composite terrane we present new ar and u pb thermochronologic data that show that in the southern hope valley zone permian 280 ma anatectic metamorphic conditions of 700 c and 6 kbar were quenched by relatively rapid cooling 12 c m y and exhumation 0 5 km m y for 40 m y in contrast peak metamorphic conditions in the putnam nashoba zone predate silurian intrusions and slower cooling 3 5 c m y began at about 400 ma one dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the permian metamorphism of the avalon composite terrane because of the absence of high grade alleghanian metamorphism in rocks overlying the avalon terrane we conclude that high grade alleghanian metamorphism in the avalon terrane occurred east of rocks now overlying it and that significant motion between avalon and this cover occurred after peak alleghanian metamorphism similarly contrasting metamorphic histories between avalon inliers willimantic window massabesic complex gneiss pelham dome and their cover rocks reveals the regional significance of this boundary the core rocks all show permian cooling but the cover rocks show post acadian cooling ages decreasing from east to west to the pelham area where hornblende ages in avalon and cover differ by only 35 rather than 80 m y model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years consequently underthrusting of avalon is constrained to be middle mississippian or younger because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover core rocks of the pelham dome must have been relatively close to this leading edge thus carboniferous to permian underplating from a generally eastward direction best explains these thermochronologic relationships

timing of atmospheric precipitation in the zagros mountains inferred from a multi proxy record from lake mirabad iran abstract a sediment core 7 2 m long from lake mirabad iran was examined for loss on ignition mineralogy oxygen isotopic composition of authigenic calcite and trace element composition of ostracodes to complement earlier pollen and ostracode assemblage studies pollen ostracode inferred lake level and high sr ca ratios indicate that the early holocene 10000 to 6500 cal yr bp was drier than the late holocene low 18 o values during this interval are interpreted as resulting from winter dominated precipitation characteristic of a mediterranean climate increasing 18 o values after 6500 cal yr bp signal a gradual increase in spring rains which are present today a severe 600 yr drought occurred at ca 5500 cal yr bp shortly after the transition from pistachio almond to oak forest during the late holocene two milder droughts occurred at about 1500 and 500 cal yr bp within the resolution of the record no drought is evident during the collapse of the akkadian empire 4200 3900 cal yr bp rather a decrease in 18 o values to early holocene levels may indicate the return to a mediterranean precipitation regime

the influence of bedrock weathering on the response of drainage basins and associated alluvial fans to holocene climates san bernardino mountains california usa the primary factors that control alluvial fan evolution still remain in question particularly for the holocene holocene centennial and millennial scale climate fluctuations are relatively subtle and more frequent than those of glacial interglacial transitions therefore intrinsic factors such as rock type or basin size are hypothesized to moderate significantly the influence of holocene climate and climate change on alluvial fan processes here we examine variability in styles and rates of alluvial fan aggradation along a single mountain front that is characterized by basins of varying size and rock type carbonate versus granite basin rock type is more closely correlated to variability in the episodic nature and magnitude of alluvial fan aggradation than is basin area bedrock physical and chemical weathering properties control sediment delivery to the piedmont and thus influence alluvial fan aggradation we suggest that the particle size of grus produced by weathering of granitic rocks fosters sedi

comparison of conventional and garnet aluminosilicate quartz o isotope thermometry insights for mineral equilibration in metamorphic rocks oxygen isotope thermometry based on fractionations among quartz garnet kyanite sillimanite and potassium feldspar in amphibolite to granulite facies schists and gneisses was compared to thermometry based on mineral equilibria conventional thermometry the former approach takes advantage of the exchange between quartz and more refractory minerals such as garnet and the aluminosilicates and their potential for preserving peak oxygen isotopic fractionations although isotopic thermometry using these phases is potentially an approach for circumventing the problem in slowly cooled rocks of retrograde oxygen isotopic exchange with micas and feldspars and of fe and mg exchange between garnet and biotite the results are accurate or geologically meaningful in only certain settings in most cases the results of isotopic thermometry must be evaluated with the aid of quantitative forward models for calculating the amount of retrograde diffusive exchange among micas feldspars and quartz two models used here predict quartz isotopic compositions that differ by only 0 1 per mil and in most cases reproduce to within 0 2 per mil the measured isotopic composition of quartz excellent agreement between isotopic and conventional thermometry is obtained for mid to upper amphibolite facies rocks that have experienced one period of recrystallization and mineral growth and if they are coarse grained or cooled in the absence of a hydrous fluid phase isotopic temperatures of 850 degrees c are in excellent agreement with conventional thermometry in migmatitic granulite facies metapelites if cooling occurred under anhydrous conditions in other cases isotopic temperatures in granulite facies rocks are higher than conventional temperatures a reasonable result in view of the tendency for retrograde cation exchange between garnet and biotite in samples that have undergone multiple periods of recrystallization or metamorphism isotopic temperatures from coexisting quartz garnet and aluminosilicates may be modified by diffusional exchange of quartz with micas and feldspars aided by recrystallization by retrograde reactions involving the latter phases or by open system interaction with externally derived fluids external fluid infiltration combined with recrystallization of quartz and aluminosilicate may result in isotopic reversals between garnet and matrix phases conventional thermobarometry in such complex rocks based in large part on mineral equilibria involving garnet may be erroneously assigned geological significance in the absence of insights provided by oxygen isotope analysis

identification of the scales of differential element mobility in a ductile fault zone two fundamentally different scales of element mobility have been identified in the upper amphibolite facies hunts brook fault zone of south central connecticut usa field relations mineral chemistry and 80 bulk rock analyses provide overwhelming evidence that the blastomylonitic schists and gneisses of the fault zone were derived from the enclosing granodioritic rope ferry orthogneiss two sample mass balance calculations between the rope ferry gneiss and fault rocks strongly suggest that the rope ferry gneiss was segregated into biotite rich schists and quartz plagioclase rich gneisses through the centimetre scale transfer of si al ca na ba and zr from schists to gneisses three and four sample mass balance calculations provide convincing evidence that al ca na and ba were lost and si and k gained by the entire fault zone both scales of metasomatism have been identified at two localities separated by 20 km implying that the metasomatic processes were pervasive throughout the fault zone from the mass balance calculations an apparent disparity in k mobility arises k appears to be gained by the entire fault zone but is immobile during the centimetre scale segregation this is because mass balance calculations can only detect differential element mobility differential element mobility requires that the mechanism of metasomatism produces chemical potential gradients therefore the identification of differentially mobile elements provides insight into the mechanisms of metasomatism the immobility of k during segregation of schists and gneisses implies that the segregation was not achieved through the extraction of partial melt however the element mobility is reconcilable with deformation driven metamorphic differentiation the gains of si and k and losses of ca na al and ba for the entire fault zone are also inconsistent with the intrusion or extraction of partial melt but may reflect the infiltration of a metasomatizing aqueous fluid

the coupling status of alluvial fans and debris cones a review and synthesis alluvial fans and debris cones link two zones of the fluvial system e g hillslope gully systems to stream channels mountain catchment sediment source areas to main river systems or to sedimentary basins and therefore have important coupling or buffering roles these roles may be both functional and preservational the functional role includes debris cone coupling which controls sediment supply from hillslope gully systems to stream channels influencing channel morphology coupling through larger alluvial fans expressed by fanhead trenching causes a distal shift in sedimentation zones or when expressed by through fan trenching causes complete sediment by pass the preservational role stems from the fact that fans and cones are temporary sediment storage zones and may preserve a record of source area environmental change more sensitively than would sediments preserved further downsystem fan coupling mechanisms include distally induced coupling basal scour toe cutting marginal incision and proximally induced coupling fanhead and midfan trenching these mechanisms lead initially to partial coupling either extending the immediate sediment source area to the stream system or shifting the focus of sedimentation distally complete coupling involves transmission of sediment from the feeder catchment through the fan environment into the downstream drainage or a sedimentary basin the implications of coupling relate to downstream channel response fan morphology sedimentation patterns and vertical sedimentary sequences temporal and spatial scales of coupling are related and with increasing scales the dominant controls shift from storm events to land cover to climatic and base level change and ultimately to the relationships between tectonics and accommodation space finally future research challenges are identified modern dating techniques and sophisticated analysis of remotely sensed data can greatly improve our understanding of fan dynamics and should lead to better cross scale integration between short term process based approaches and long term sedimentological applications while maintaining high quality field based observations copyright 2011 john wiley sons ltd

rheological weakening of high grade mylonites during low temperature retrogression the exhumed continental ailao shan red river fault zone se asia abstract we present a detailed case study of an exhumed continental strike slip fault zone the ailao shan red river asrr strike slip fault zone to investigate how deformation promotes strain localization and how the weak second phases and fluids trigger rheological weakening during retrogression near the ductile to brittle transition during exhumation along the asrr strike slip fault zone in the diancang shan dcs metamorphic massif high temperature ductile deformation d 1 pervasively occurred during shearing and exhumation since late oligocene the high temperature microstructures and textures are in part or entirely altered by subsequent low temperature shearing d 2 since late miocene which is under the conditions of frictional viscous transition of k feldspar ca 450 c during further exhumation to the upper crustal levels the formation of d 2 microstructures and shear bands overprinted high temperature intracrystalline plasticity phases d 1 in mylonitic rocks depending on the main rock forming minerals the results also demonstrate that the brittle ductile transition involves a combination of different deformation mechanisms and possible rheological paths in quartz rich rocks quartz was deformed in the dislocation creep regime and records transition of microfabrics and slip systems during decreasing temperature which lasted until retrogression related to exhumation as a result grain size reduction associated with fluids circulating within the asrr strike slip fault zone at brittle ductile transition leads to reaction and texture weakening rheological weakening is the consequence of the syntectonic deformation fluid flow reaction softening reaction creep and textural softening the hydrous fluids resulted in hydration of silicates decompression occurred during shearing and as a result of tectonic exhumation all these results demonstrate that the exhumation through the ductile to ductile brittle transition involves a combination of different deformation mechanisms rheological transition features and feedbacks between deformation decreasing temperature and fluids in the asrr fault zone

loess chronology of the caspian lowland in northern iran the loess palaeosol sequences of the caspian lowland in northern iran provide detailed archives of climate and environmental change showing a close relationship to global cooling and warming trends for the quaternary period the magnitude of these changes is under discussion owing to uncertainties in the chronologies for individual sites a chronological frame was set up for the last glacial loess record for three upper pleistocene key sections by infrared optically stimulated luminescence irsl in the caspian lowland irsl dating gives reliable ages for sediments covering at least the past 60 ka however for sediments older than 60 ka a significant age underestimation is likely the first strongly developed buried bt bwt horizons or pedocomplexes at the three sections under study correlate most likely with the last interglacial mis 5 at least four periods of increased sediment accumulation were determined for the last glacial period intercalated by periods of weak soil formation the loess palaeosol sequences in northern iran recorded coeval and similar major climatic changes as in south eastern central europe and central asia

differential effects of base level tectonic setting and climatic change on quaternary alluvial fans in the northern great basin nevada usa

differential features of alluvial fans controlled by tectonic or eustatic accommodation space examples from the betic cordillera spain abstract the creation of accommodation space in subaerial sedimentary environments is closely linked to uplift the source area along basin margin faults or to a rise in base level depending amongst others on the relative importance of these two factors the margins of terrestrial sedimentary basins where alluvial fans develop produce aggradational progradational or retrogradational stratigraphic architecture we studied a total of 43 quaternary alluvial fans in the granada and bajo segura basins betic cordillera spain the analysis of their stratigraphic architecture and sedimentary facies has revealed the existence of three groups characterised by distinct stratal stacking patterns which is the result of differences in eustasy and tectonics the qualitative and quantitative analysis of these alluvial fans and their drainage basins has allowed us to define certain characteristic geomorphological patterns related in each case to the main mechanism operative in the creation of accommodation space these patterns are described as follows 1 alluvial aggradation is the main result of high tectonic subsidence with the development of quite open but small fans with a constant steep slope lacking incised channels and headward eroding gullies 2 in contrast when the principal mechanism in creating accommodation space is low tectonic subsidence progradation occurs and the fans are narrower and larger moreover the slopes of such fans are less steep in relation to the drainage area than in the above case being constant or even concave in longitudinal profile they also have short deep incised channels culminating downfan in a depositional lobe in inactive zones long deep headward eroding gullies develop 3 areas of extremely low tectonic subsidence and base level rise produce very open fans in the sedimentary basin although with a pronounced mountain embayment which gives them a characteristic mushroom shape in plan view in the latter case retrogradation is the typical stratigraphic stacking pattern and the fans are quite extensive with respect to their drainage areas usually showing a segmented longitudinal profile with a low gradient the feeder channels do not surpass stream order 3 and various extremely long and shallow incised channels can appear 4 contrary to the case of fans developing under a regime of subsidence when the accommodation is due to base level rise the fan slopes bear no relation to the extent of the catchment basins in such a case the overall fan slope is strongly influenced by the extent of the mountain embayment because it determines the distributions of debris and sheet flows over the alluvial surface

a model of alluvial megafan sedimentation ganga megafan abstract ganga megafan is one of several megafans developed in the northern and central part of the ganga plain that formed during the middle late pleistocene under conditions of higher sediment water discharge and higher regional slopes than today incision by the ganga river exposes 15 30 m thick megafan deposits in the valley walls and cliffs ganga megafan deposits can be separated into four zones zone i gravelly braided streams zone ii sandy braid plain zone iii anastomosing channel plain and zone iv meandering channels with broad interfluve areas deposits of each zone show distinct facies associations palaeocurrents fan from sw to se and e while the grain size decreases from proximal to distal fan at present fan building activity is highly subdued and is restricted only to the northernmost 5 10 km wide belt of piedmont plain