influence of the subtropical andes on baroclinic disturbances a cold front case study abstract the andes cordillera produces a significant disruption to the structure and evolution of the weather systems that cross south america in particular cold fronts tend to be channeled to the north immediately to the east of the andes fostering the advance of cold air incursions cold surges well into subtropical and sometimes tropical latitudes in contrast active cold fronts hardly reach subtropical latitudes along the western side of the andes pacific sea border instead as a cold front moves equatorward along the east side of the andes a marked low level warming tends to appear along the west side of the subtropical andes leading to the formation of a mesoscale coastal low or trough in this region to further understand the processes that lead to a contrasting evolution of the cold front at each side of the andes a typical frontal passage is studied in this work using synoptic observations and a regional model eta centro de previsao de tempo e estudos climaticos cptec simula

clay mineral provinces in tidal mud flats at germany s north sea coast with illite k ar ages potentially modified by biodegradation abstract mineralogical studies chemical analyses and k ar dating were carried out on clay fractions from tidal mud flats along the lower saxony coast and its bays to identify material sources and sedimentary processes at this dynamic interface between air land and sea from the coast into the bays sediments are enriched in fine grained smectite relative to the coarser grained illite chlorite and kaolinite due to the weakening of the tidal current energy in the bays in addition the study area can be divided into two provinces on the basis of the illite k rb ratios and mg contents to the west schiermonnikoog dollart ley bay up to norderney island longshore currents carry suspensions from the belgian and dutch coasts to the east from langeoog island jade bay to the helgoland mud area suspensions from the elbe and weser rivers are mixed with submarine reworked glacial sediments whereas the portion of longshore current suspensions from the west decreases becoming negligible in the helgoland mud area off the elbe and weser estuaries the illite k ar data vary considerably and fail as source indicators due to differential settling and mixing of the clay material and probably to ar loss from illite by biodegradation during digestive processes only further offshore outside the zone of dynamic sediment dispersion do the k ar data fit provenance patterns

latent heat effects of the major mantle phase transitions on low angle subduction abstract very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate we have investigated the effects of latent heat for this situation where mantle material is pushed through the major mantle phase transitions during shallow low angle subduction below the overriding plate the significance of the buoyancy forces arising from the latent heat effects on the dynamics of the shallowly subducting slab is examined by numerical modeling when a 32 ma old slab is overridden with 2 5 cm yr by a continent flat subduction occurs with a 4 5 cm yr convergence rate when latent heat is included in the model forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces under these circumstances the flat slab segment subducts horizontally about 350 km further and for about 11 ma longer than in the case without latent heat before it breaks through the 400 km phase transition the style of subduction strongly depends on the mantle rheology increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow similarly an overriding velocity of less than 1 cm yr leads to steep subduction which gradually changes to flat subduction when increasing the overriding velocity however these model parameters do not change the aforementioned effect of the latent heat provided that low angle subduction occurs in all models latent heat resulted in a substantial increase of the flat slab length by 300 400 km varying the olivine spinel transition clapeyron slope from 1 to 6 mpa k reveals a roughly linear relation between and the horizontal length of the slab based on these results we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low angle subduction below an overriding plate

subtropical anticyclones and summer monsoons the summer subtropical circulation in the lower troposphere is characterized by continental monsoon rains and anticyclones over the oceans in winter the subtropical circulation is more strongly dominated by the zonally averaged flow and its interactions with orography here the mechanics of the summer and winter lowertropospheric subtropical circulation are explored through the use of a primitive equation model and comparison with observations by prescribing in the model the heatings associated with several of the world s monsoons it is confirmed that the equatorward portion of each subtropical anticyclone may be viewed as the kelvin wave response to the monsoon heating over the continent to the west a poleward flowing low level jet into a monsoon such as the great plains jet is required for sverdrup vorticity balance this jet effectively closes off the subtropical anticyclone to the east and also transports moisture into the monsoon region the low level jet into north america induced by its monsoon heating is augmented by a remote response to the asian monsoon heating the rossby wave response to the west of subtropical monsoon heating interacting with the midlatitude westerlies produces a region of adiabatic descent it is demonstrated here that a local diabatic enhancement can lead to a strengthening of the descent longitudinal mountain chains act to block the westerly flow and also tend to produce descent in this region below the descent sverdrup vorticity balance implies equatorward flow that closes off the subtropical anticyclone to the west and induces cool upwelling in the ocean through ekman transport feedbacks involving for example sea surface temperatures may further enhance the descent in these regions the conclusion is that the mediterranean type climates of regions such as california and chile may be induced remotely by the monsoon to the east hence it can be argued that the subtropical circulation in summer comprises a set of weakly interacting monsoon systems each involving monsoon rains a low level poleward jet a subtropical anticyclone to the east and descent and equatorward flow to the west in winter it is demonstrated how the nonlinear interaction between the strong zonal mean circulation associated with the winter hadley cell and the mountains can define many of the large scale features of the subtropical circulation the blocking effect of the longitudinal mountain chains is shown to be very important subsequent diabatic effects such as a local diabatic enhancement would appear to be essential for producing the observed amplitude of these features

the south american rainfall dipole a complex network analysis of extreme events intraseasonal rainfall variability of the south american monsoon system is characterized by a pronounced dipole between southeastern south america and southeastern brazil here we analyze the dynamical properties of extreme rainfall events associated with this dipole by combining a nonlinear synchronization measure with complex networks we make the following main observations i our approach reveals the dominant synchronization pathways of extreme events for the two dipole phases ii while extreme rainfall synchronization in the tropics is directly driven by the trade winds and their deflection by the andes mountains extreme rainfall propagation in the subtropics is mainly dictated by frontal systems and iii the well known rainfall dipole is in fact only the most prominent mode of an oscillatory pattern that extends over the entir ec ontinent this provides further evidence that the influence of rossby waves which cause frontal systems over south america and impact large scale circulation patterns extends beyond the equator

the oldest uhp eclogites of the world age of uhp metamorphism nature of protoliths and tectonic implications coesite bearing eclogitic rocks from the pan african belt in northern mali were dated by multichronological methods rb sr sm nd and ar ar rb sr and sm nd isotope analyses on whole rock and mineral separates of an omphacite kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 ma this is interpreted as the time of the eclogitisation which represents the oldest ultrahigh pressure uhp metamorphic event so far recorded in the continental crust the nearly identical rb sr and sm nd ages suggest very rapid exhumation as observed in many phanerozoic uhp rocks 40ar 39ar dating of phengites from two uhp micaschists gave much older ages of 1045 and 760 ma phengite from a quartzite collected from the same site yielded a plateau date of 623 3 ma which is in agreement with the rb sr and sm nd dates the two 40ar 39ar ages older than 625 ma testify once more the recurrent problem of excess ar in ultrahigh pressure phengites the occurrence of the mali uhp eclogites indicates that by the late precambrian the earth has cooled enough to sustain the formation and preservation of deeply subducted uhp metamorphic rocks the new ages of ca 620 ma also constrain the amalgamation in nw gondwana to have occurred in the latest proterozoic but not in the middle cambrian this is supported by several recent chronological data for the southern segment of the major suture zone in the dahomeyide orogenic belt

kinematics strain pattern and geochronology of the salem attur shear zone tectonic implications for the multiple sheared salem namakkal blocks of the southern granulite terrane india abstract structural mapping strain analysis and a variety of geochronological studies were carried out to determine the tectonothermal evolution of the salem attur shear zone in the southern granulite terrane of south india the salem namakkal blocks containing the shear zone consisted of quartzofeldspathic gneiss charnockite and mafic granulite and had undergone multiple phases of magmatism spanning over a period of 3 2 0 5 ga the rocks were deformed by four phases of deformation d1 d4 the d1 deformation was characterized by isoclinal and recumbent ne sw trending f1 fold with a pervasive subhorizontal axial planar granulitic fabric s1 and associated quartzofeldspathic leucosomes granulite metamorphism was dated at ca 2 5 2 3 ga the f1 fold and s1 fabric were coaxially refolded by tight to isoclinal upright to steeply inclined ne sw trending f2 folds during d2 deformation the d2 deformation was associated with f2 axial planar shear zones crenulations and leucosomes s2 fabric large scale d2 shear zones characterized by high temperature ductile shear fabric with a vertical flow host syntectonic syenite pluton which was dated at ca 2 5 2 4 ga a p t condition of 7 kb 600 c was inferred for the d2 deformation the d3 deformation was characterized by nw se to e w trending f3 folds and the salem attur shear zone the shear zone was a greenschist to amphibolite facies shear zone being characterized by mylonitic foliation and dominantly down dip stretching lineation defined by quartz biotite and hornblende minerals and dated at ca 2 0 ga it indicated n nne vergence of thrusting with the mean kinematic vorticity number wm as 0 7 suggesting general simple shear strain with 50 pure shear component the d4 deformation was manifested as nne ssw striking strike slip faults and nw se striking extensional normal faults pseudotachylite veins having an age of 1 9 ga injected during strike slip faulting and granite pegmatite veins showing age of 0 8 0 5 ga intruded during normal faulting the salem namakkal blocks thus recorded a long lived shearing history we suggest that the salem attur shear zone and other shear zones such as palghat cauvery moyar bhavani karur kambam painavu trichur and achankovil shear zones were paleoproterozoic intraterrane shear zones which were overprinted by meso neoproterozoic cambrian ductile and brittle deformations

provenance of pliocene sediments and paleoenvironmental changes in the southern north sea region using samarium neodymium sm nd provenance ages and clay mineralogy abstract an 800 m thick sedimentary succession from the late pliocene north sea has been investigated on the provenance of the sediments and on changes of the source regions related to transport processes and climate changes the lower zanclean to piacenzian part of the succession from 1250 to 1050 m depth and the middle early gelasian part from 1100 to 700 m depth belong to one distributory system with at least two contributing source areas provenance ages calculated on basis of sm nd content indicate one old archean and one young

quantitative clay mineralogy as provenance indicator for recent muds in the southern north sea abstract the origin of recent mud deposits as well as the coastal turbidity maximum in the french belgian dutch nearshore area of the southern north sea is still under debate in the literature some models favor the erosion of the cretaceous chalk cliffs along the english channel and subsequent ne ward directed transport other models focus on the erosion of eocene to earliest oligocene and pleistocene to holocene clays outcropping on the seafloor off the belgian coast in order to validate these hypotheses the detailed qualitative and quantitative clay mineral composition of these sediments was used as a provenance indicator by comparing the clay mineral composition of the mud deposits and the associated suspended particulate matter spm with the composition of potential nearby and more remote sources such as the present day marine environment estuaries and rivers coastal erosion areas and the geological substratum the origin of the mud deposits and the spm could be traced results showed that only the clay composition of the scheldt estuary coincides with those of the mud deposits and the coastal turbidity maximum and that all other potential sources could be excluded our data suggest that the clay mineral composition of the mud deposits has a similar composition since at least about 100 000 years indicating that these deposits originate from a paleo scheldt river rather than from the recent river system as the present day scheldt estuary is not source of fine grained sediments the present day spm in the belgian dutch nearshore area originates mainly from the erosion and resuspension of the existing mud deposits situated in the belgian nearshore this study demonstrates the value and suitability of quantitative bulk and clay mineralogy techniques in sediment provenance studies and highlights the importance of incorporating the recent geological history in hydrodynamic studies of sedimentary basins

possible detachment zone in precambrian rocks of kanjamalai hills cauvery suture zone southern india implications to accretionary tectonics abstract existence of a possible detachment zone at elampillai region nw margin of kanjamalai hills located in the northern part of cauvery suture zone csz southern india is reported here for the first time detailed structural mapping provides anatomy of the zone which are rarely preserved in precambrian high grade terranes the detachment surface separates two distinct rock units of contrasting lithological and structural characters the upper and lower units the detachment zone is characterized by a variety of fold styles with the predominance of tight isoclinal folds with varied plunge directions limb rotations and the hinge line variations often leading to lift off fold like geometries and deformed sheath folds presence of parasitic folding and associated penetrative strains seem to be controlled by differences in mechanical stratigraphy relative thicknesses of the competent and incompetent units and the structural relief of the underlying basement our present study in conjunction with other available geological geochemical and geochronological data from the region indicates that the structures of the detachment zone are genetically related to thrust tectonics forming a part of subduction accretion collision tectonic history of the neoproterozoic gondwana suture

on the separation of crustal component from subducted oceanic lithosphere near the 660 km discontinuity the recent mineral physics observations suggest that the crustal and mantle components of subducted oceanic lithosphere have distinct densities and viscosities the deformation of such chemically stratified oceanic lithosphere as it collides with the 660 km discontinuity is examined using a simple analytical model it is shown that the separation of the crustal component can occur through a small scale folding 10 20 km if the viscosity of the crustal garnetite layer is significantly higher than the surrounding mantle 1021 pa s but is less than 1023 pa s leading to a garnet enriched transition zone behind subduction zones

influence of the frontal systems on the day to day convection variability over south america abstract cold cloud top fractions derived from international satellite cloud climatology project images and latitude time diagrams are used to study the interaction of frontal systems with tropical convection over south america sa an 11 yr climatology for three frequent types of frontal system tropical convection interaction is built and the associated day to day convection variability is described using satellite images complex principal components and wavelet transforms type 1 is frequent throughout the year especially in austral summer and is characterized by the penetration of a cold front in subtropical sa that interacts with tropical convection and moves with it into lower tropical latitudes type 2 is also more frequent in austral summer and is characterized by amazon convection and enhancement of a quasi stationary northwest southeast oriented band of convection extending from the amazon basin to subtropical sa along the passage of a cold front in the subtropics when the type 2 pattern

thermobarometric constraints on burial and exhumation of 2 billion year old eclogites and their metapelitic hosts abstract one of the first appearances of eclogite facies mineral assemblages in the geological record occurs in the c 2000 ma palaeoproterozoic usagaran belt in central tanzania where the extended margin of the tanzanian craton is interpreted to have been subducted mafic rocks are interpreted to have contained the mineral assemblage garnet omphacite rutile quartz amphibole this high pressure assemblage has been overprinted by a secondary mineral assemblage containing clinopyroxene plagioclase hornblende ilmenite orthopyroxene mineral equilibria forward modelling indicates that the eclogite facies assemblages reached minimum peak pressure temperature p t conditions of 17 kbar and 700 c inclusions in garnet document a prograde p t history consistent with burial through upper amphibolite facies conditions and possible partial melting petrological and compositional evidence from garnet suggests that following peak metamorphism the eclogite facies rocks were heated while stalled at approximate peak pressures temperature estimates derived from zr concentrations in interpreted texturally retrograde rutile support a near isothermal post peak p t evolution for the eclogite facies rocks an evolution that terminates at retrograde p t conditions of approximately 7 6 8 2 kbar and 680 790 c the relict eclogite domains form part of a larger assemblage with enclosing migmatitic metapelitic lithologies the isimani suite the metapelitic gneisses contain garnet kyanite biotite staurolite hornblende plagioclase muscovite rutile quartz and preserve minimal evidence of a high pressure history conceivably due to post peak mineralogical recrystallisation p t modelling inclusion assemblages and compositional zonation patterns in porphyroblastic garnet suggests the metapelitic gneisses similarly to the relict eclogites experienced burial to minimum peak pressures of approximately 16 5 17 kbar compositional zoning patterns in eclogitic garnet suggest the isimani system was buried reached peak metamorphic conditions and was subsequently exhumed within a timeframe of up to 20 myr a tectonic regime involving crustal thickening and subduction followed by extensional exhumation of the entire isimani suite is our preferred model for the development of the c 2000 ma usagaran belt

petrological and geochronological constraints on lower crust exhumation during paleoproterozoic eburnean orogeny nw ghana west african craton we present new petrological and geochronological data on high grade ortho and paragneisses from northwestern ghana forming part of the paleoproterozoic 2 25 2 00 ga west african craton the study area is located in the interference zone between n s and ne sw trending craton scale shear zones formed during the eburnean orogeny 2 15 2 00 ga high grade metamorphic domains are separated from low grade greenstone belts by high strain zones including early thrusts extensional detachments and late stage strike slip shear zones paragneisses sporadically preserve high pressure low temperature hp lt relics formed at the transition between the blueschist facies and the epidote amphibolite sub facies 10 0 14 0 kbar 520 600 c and represent a low 15 c km 1 apparent geothermal gradient migmatites record metamorphic conditions at the amphibolite granulite facies transition they reveal a clockwise pressure temperature time p t t path characterised by melting at pressures over 10 0 kbar followed by decompression and heating to peak temperatures of 750 c at 5 0 8 0 kbar which fit a 30 c km 1 apparent geotherm a regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise p t t path characterised by peak metamorphic conditions of 7 0 10 0 kbar at 550 680 c which match a 20 25 c km 1 apparent geotherm these p t conditions were reached after prograde burial and heating for some rock units and after decompression and heating for others the

modern style plate subduction preserved in the palaeoproterozoic west african craton the timing of onset of modern style plate tectonics on earth is debated analysis of rocks in the west african metamorphic province which is more than 2 gyr old reveals that some minerals formed under conditions similar to those in modern day subduction zones suggesting that subduction occurred on the palaeoproterozoic earth

changes in clay mineral content of tidal flat sediments resulting from dike construction along the lower saxony coast of the north sea germany the clay mineral content of the 2 m fraction in tidal flat sediments in the former harle bay on the lower saxony coast has changed considerably since the thirteenth century up to the present time the proportion of smectite has decreased from 29 to 11 wt whereas the proportion of illite has increased by about 10 wt and those of chlorite and kaolinite by about 3 wt each the chemical composition of the major clay mineral illite has also changed al2o3 has decreased by about 2 wt k2o by about 1 5 wt and sio2 by about 3 wt it can be concluded that the changes in the clay mineral content are mainly influenced by dike construction and sea level rise modifying flow pattern submarine morphology sedimentation and tidal range these processes cause smectite to settle less rapidly because of its small grain size and low density resulting from interlayer water so that the sediment becomes impoverished in this mineral

pressures and temperatures in precambrian metamorphic rocks modelp t estimates from nearly 100 precambrian rocks have been derived by comparing published descriptions of metamorphic mineral assemblages to a standard set of mineral stability data pressures temperatures and metamorphic geotherms show large variations nevertheless modelp t data reveal two distinct secular trends average metamorphic geotherms have declined through time and average metamorphic pressures have increased through time metamorphic geotherms and pressures from archean rocks average 54 c km and 4 1 kbar respectively corresponding figures for early proterozoic terranes are 47 c km and 4 3 kbar and for late proterozoic terranes 35 c km and 6 kbar these secular trends may reflect gradual cooling and thickening of the earth s crust at least in orogenic belts there are exceptions to both trends found in localities where mineral assemblages indicate high pressures 6 12 kbar and low metamorphic geotherms 20 30 c km these assemblages formed in archean crust that at least locally was thicker than 20 km during metamorphism

paleozoic hp granulite facies metamorphism and anatexis in the dulan area of the north qaidam uhp terrane western china constraints from petrology zircon u pb and amphibole ar ar geochronology abstract hp mafic and felsic adakitic granulite bodies in the dulan area of the north qaidam ultrahigh pressure uhp terrane record high pressure hp granulite facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation mafic hp granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite the hp mafic granulites comprise garnet clinopyroxene plagioclase and quartz the felsic granulite leucosome is mainly composed of k feldspar plagioclase quartz kyanite garnet detailed zircon u pb and amphibole ar ar geochronology combined with trace element geochemistry indicate peak metamorphism for the mafic hp granulite at 434 3 to 435 3 ma and peak metamorphism and partial melting for the felsic hp granulite at 433 5 to 438 4 ma which overlaps the ages of uhp metamorphism for adjacent eclogite 430 446 ma 40 ar 39 ar amphibole ages of 423 to 432 ma represent amphibolite facies retrograde metamorphism and indicate rapid cooling during exhumation of the hp granulite bodies our geochronological data combined with field relationships petrology and geochemistry suggest that hp granulite facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event in this case the felsic hp granulites leucosome formed from an adakitic melt derived from partial melting of mafic hp granulite in the overriding plate in a relatively higher geothermal gradient 15 18 c km leaving garnet cumulate and or meta ultramafic mainly garnet pyroxenite as the residual component in contrast the nearby uhp eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient 6 10 c km penecontemporaneous metamorphic ages but different geothermal gradients between hp granulites and related uhp eclogite define a possible paired metamorphic belt generated in a subduction collision setting associated with the north qaidam continental collisional orogeny during the late ordovician early silurian

ion microprobe u pb age determinations on zircon from the late archean granulite facies transition zone of southern india abstract the southern dharwar craton of karnataka and tamil nadu states in south india exposes a depth profile of late archean crust amphibolite facies felsic to intermediate peninsular gneiss of protolith ages 3 4 2 9 ga with subordinate infolded supracrustal relics and the large closepet granite body give way southward to granulite facies orthopyroxene bearing gneisses over a transition zone about 40 km wide with mineralogic pressure indicators increasing from about 4 kbar corresponding to about 15 km paleodepth to about 7 kbar corresponding to near 25 km paleodepth previous whole rock and zircon geochronology have established that amphibolite facies metamorphism migmatization and granite intrusion took place in the latest archean at 2 53 2 51 ga however some workers have considered the foliated granulites as products of an earlier metamorphism distinct from the terminal archean events this work presents whole rock geochemical studies coupled with ion microprobe 207pb 206pb ages and th u

shonkinites from salem southern india implications for cryogenian alkaline magmatism in rift related setting abstract alkaline and potassic igneous rocks although minor components of the continental crust provide important constraints on magma processes and tectonic settings here we report petrology mineral chemistry whole rock geochemistry zircon u pb and lu hf data on shonkinites from the salem block in the southern granulite terrane of india the shonkinite is composed mainly of porphyritic clinopyroxene k feldspar and olivine and intrudes into the surrounding ultramafic rocks composed of wehrlite clinopyroxene and olivine and dunite mostly olivine with minor spinel the geochemical data on shonkinites from salem show alkaline and ultrapotassic features with marked enrichment in incompatible elements and high k 2 o and k 2 o na 2 o ratios suggesting magma derivation from metasomatized lithospheric mantle their hfse depletion relative to lile compositions coupled with positive nb anomalies suggests continental rift setting the zircon grains in shonkinite show typical magmatic crystallization features and their la icp ms u pb data yield weighted mean age of 818 6 3 ma mswd 0 32 the zircon e hf values range between 11 1 and 12 7 consistent with magma derivation from an enriched mantle with depleted mantle model ages t dm in the range of between 2406 and 2513 ma indicating the involvement of neoarchean paleoproterozoic subduction related components together with similar aged alkaline plutons with comparable geochemical features we envisage a major alkaline magmatic event associated with rifting during cryogenian in the northern part of the southern granulite terrane

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

high pressure granulites at the dawn of the proterozoic abstracthigh pressure metamorphism is uncommon in the ancient geo logical record kanja malai in the salem crustal block southern india contains high pressure kyanite garnet bearing felsic granu lites that equilibrated at 14 16 kbar and 820 860 c laser abla tion inductively coupled plasma mass spectrometry u pb zircon and in situ monazite geochronology indicate that these assemblages grew ca 2490 ma these pressure temperature time constraints pro vide a rare record indicating that thickened crust and low apparent thermal gradient conditions existed during the archean proterozoic transition a period of earth history for which the rock record com monly preserves evidence for comparatively high apparent thermal gradients the thermal regimes required to generate these metamor phic conditions are typical of collisional orogenesis and suggest that the continental lithosphere was capable of supporting crustal thick ening to 45 50 km such crustal thickening provides supporting evi dence that tectonic regimes similar to modern earth style tectonics were in operation at the archean proterozoic transition introduction

hydrothermal and unexpected diagenetic alteration in permian shales of the lodeve epigenetic u deposit of southern france traced by k ar illite and k feldspar dating abstract mineralogical and chemical analyses combined with k ar dating of illite and k feldspars were carried out on bulk samples and different fractions from permian shales around the lodeve u deposit to identify diagenetic and hydrothermal alteration episodes and their timing authigenic 2m1 and 1m illite with k ar ages between 270 and 245 ma are the predominant mineral types in the shales they are of early diagenetic origin relative to deposition and formed apparently directly from pyroclastic materials in a lagoonal to deltaic environment under semi arid conditions albite and a first generation of k feldspar developed by a replacement of analcime the k feldspar growing only in the former lagoonal environment that favored also early diagenetic dolomitization at the expense of calcite the hydrothermally altered shales next to the u deposit contain authigenic quartz fe chlorite neoformed illite and a second generation k feldspar with k ar ages beginning at about 220 ma triassic although jurassic and cretaceous ages were also obtained this younger age spectrum jurassic cretaceous is close to previous u pb isotope data from the u deposit probably reflecting several episodes of u mobilization therefore the hydrothermal activity extended episodically from triassic to cretaceous time within the u deposit the hydrothermal temperatures attained locally 220 c as the k ar system of illite from early permian diagenesis is partially reset triassic hot fluids ascended along faults into the northern area of the lodeve basin during an initial rifting of the nearby western tethys ocean further fluid pulses were recorded at lodeve during the opening of the north atlantic and separation of europe from africa jurassic cretaceous which also caused the peak hydrothermal activities in central and western europe as the lodeve area is located between two independently developing oceans it could have been supplied with hot fluids for a long period of time

delamination of smectite in river borne suspensions at the fluvial marine interface an experimental study abstract fine fractions the variations in fwhm d values and peak intensity of the smectitic material are assumed to be caused by an exchange of na for ca in the interlayer spaces and subsequent delamination of the particles along the na activated interlayers only those smectite particles in which adjacent sheets are still stabilized by the remaining ca are preserved after contact with seawater the experiments show that delamination of smectite and i s mixed layer minerals is likely to occur in the brackish environment at the mouth of a river however formation of these fine laminae is expected to be a transitional stage in the development of clay particle size owing to the formation of flocs and aggregates in the higher salt concentrations of seawater

subseasonal variations of rainfall in south america in the vicinity of the low level jet east of the andes and comparison to those in the south atlantic convergence zone abstract regional and large scale circulation anomalies associated with variations in rainfall downstream of the south american low level jet are identified and compared to those in the south atlantic convergence zone sacz composites of precipitation associated with strong jets reveal an approximate doubling of the quantities one would expect from climatology with an evolution of the rainfall pattern from south to north the occurrence of extreme precipitation events follows a similar pattern meridional cross sections of composite wind reveal a distinct low level jet near 20 s and a baroclinic development farther south that appears to force the jet geopotential height temperature and large scale wind composites suggest that this developing disturbance is tied to a wave train that originates in the midlatitude pacific and turns equatorward as it crosses the andes mountains similar composites based on sacz rainfall reveal similar features but of opposite sign suggesting that the phase of the wave

does a monsoon climate exist over south america the climatology and the basic state of the summertime circulation and rainfall over south america are studied using assimilation products from the data assimilation system of goddard earth observing system 1 geos1 and satellite derived rainfall results indicate the existence of a regional summer monsoon circulation regime induced by strong diabatic heating over the subtropical south american highland centered at the altiplano plateau sensitivity of the results to the assimilation scheme is tested by comparing that with the national centers for environmental prediction ncep reanalysis and with satellite rainfall estimates results show general agreement between the model produced rainfall anomaly and the satellite estimates as well as consistency between the basic circulation features in the geos 1 and the ncep reanalyses a case study of 1989 90 south american summer monsoon sasm reveals the following characteristics 1 in late spring the onset of sasm is signaled by an abrupt merging of the upper tropospheric double westerly jets one in the subtropics and the other in the subpolar region into a single jet in the midlatitudes this is followed by the establishment of a vortex to the southeast of altiplano and occurrence of heavy precipitation over subtropical eastern brazil 2 during the mature phase of sasm the heavy rainfall zone moves over the altiplano plateau and the southernmost brazilian highland the fully established sasm features are the following a an enhancement of equatorial north atlantic trade wind which emanates from the sahara high and crosses the equator over the south american continent b a buildup of strong northwesterlies along the eastern side of the tropical andes and c development of the south atlantic convergence zone in the southernmost position with strong convective activity meanwhile the upper tropospheric return flow emerges from an anticyclone formed over the altiplano plateau crosses the equator and sinks over northwestern africa 3 the withdrawal of sasm in late summer is signaled by the resplitting of the midlatitude westerly jet at the same time the low level northwest monsoon flow diminishes reducing the moisture supply and leading to the termination of heavy precipitation over the subtropical highland results also show that the above mentioned characteristics of sasm are clearly linked to the tropospheric temperature changes over the central south american highland sensible versus latent heating over the highland are bound to play an important role in the evolution of sasm to provide further support of presence of a monsoon climate over south america sasm is compared and contrasted to the classic east asian summer monsoon easm many similar features including evolution characteristics between the two systems have been identified contrasting aspects of the sasm from the easm are also discussed it is pointed out that a number of monsoonal characteristics of the climate of south america such as the seasonal reversal of the low level wind become apparent only when the strong annual mean wind is removed based on the characteristic features and their evolution the authors conclude that a monsoon climate does exist over south america

progressive metamorphism of mafic rocks from greenschist to granulite facies in the dharwar craton of south india in the archean dharwar craton of southern india n s trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2 5 ga ago the progressive changes in the assemblages and mineral chemistry of metabasites was studied in a n s traverse covering the chitradurga and nagaman gala belts the sargur area and the nilgiri hills towards the south with increasing metamorphic grade greenschists chl act ab ep pm carb qtz give way to amphibolites hbl plag pm qtz gar cumm and mafic granulites pyx plag gar pm hbl qtz the amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic hastingsitic hornblende in the granulite zone the ti content of the amphiboles systematically increases in this direction the plagioclase composition changes from albite in the greenschist zone to oligoclase andesine in the amphibolite zone and andesine labradorite in the granulite zone almandin

mesozoic hydrothermal impact on rotliegende and bunter immature sandstones of the high rhine trough and its adjacent eastern area southern black forest germany abstract the mesozoic hydrothermal and geodynamic evolution of a complex region of the european crust is studied here on immature and highly reactive rotliegende and bunter sandstones of the intramontane high rhine trough southern black forest and its adjacent eastern area bounded by the upper rhine graben and the alpine foreland microscopic observations outline varied degrees of illitization of detrital feldspar grains muscovite flakes and illite particles xrd analysis shows that the clay fractions are dominated by illite with minor illite smectite mixed layered minerals of r3 and r1 ordering the chemical composition of these minerals is controlled by precursor minerals since the illitic material inherited its chemical components from detrital muscovite and illite towards the centre of the trough the sandstones were more strongly affected by the hydrothermal impact showing extensive replacement of feldspar grains by al rich illite k ar dating of the authigenic illite type material points to multiple hydrothermal alteration episodes a late jurassic event at 147 ma seems to have affected extensively the study area especially towards the centre of the trough at the periphery of the trough relics of an early jurassic activity are likely preserved an additional early cretaceous hydrothermal event seems to have occurred locally near schopfheim and next to the rhine graben boundary fault long before its main rifting phase the 147 ma event was obtained also nearby in the previously investigated breisgau schramberg offenburg teinach and baden baden troughs and is characteristic for the black forest and some other areas outside this region it concludes obviously a period of regional hydrothermal activities between 160 and 145 ma whereas the early jurassic and cretaceous signatures are also known throughout central and western europe suggesting repeated fluid pulses the hydrothermal flows were controlled by reactivated basement faults due to periods of extensive reorientation of the tectonic stress during the break up of pangaea illitization was triggered by hot and low saline solutions of about 180 c and eq whereas a later fluid pulse of about 80 c and 20 wt nacl eq caused quartz overgrowth fluid flows delivered by the long lasting orogeny of the nearby alps were probably limited towards the black forest becoming part of the european margin of the tethys in the mesozoic the pre alp area evidently developed a separate fluid flow system

variability in warm season atmospheric circulation and precipitation patterns over subtropical south america relationships between the south atlantic convergence zone and large scale organized convection over the la plata basin warm season precipitation variability over subtropical south america is characterized by an inverse relationship between the south atlantic convergence zone sacz and precipitation over the central and western la plata basin of southeastern south america this study extends the analysis of this south american seesaw precipitation dipole to relationships between the sacz and large long lived mesoscale convective systems llcss over the la plata basin by classifying sacz events into distinct continental and oceanic categories and building a logistic regression model that relates llcs activity across the region to continental and oceanic sacz precipitation a detailed account of spatial variability in the out of phase coupling between the sacz and large scale organized convection over the la plata basin is provided enhanced precipitation in the continental sacz is found to result in increased llcs activity over northern northeastern and western sections of the la plata basin in association with poleward atmospheric moisture flux from the amazon basin toward these regions and a decrease in the probability of llcs occurrence over the southeastern la plata basin increased oceanic sacz precipitation however was strongly related to reduced atmospheric moisture and decreased probability of llcs occurrence over nearly the entire la plata basin these results suggest that continental sacz activity and large scale organized convection over the northern and eastern sections of the la plata basin are closely tied to atmospheric moisture transport from the amazon basin while the warm coastal brazil current may also play an important role as an evaporative moisture source for llcss over the central and western la plata basin

climatology of the low level jet east of the andes as derived from the ncep ncar reanalyses characteristics and temporal variability abstract a climatology of the south american low level jet east of the andes sallj is developed using the 1950 2000 circulation and moisture fields from the ncep ncar reanalyses and available upper air observations made in bolivia and paraguay since 1998 upper and low level circulation fields were derived for seasonal means and sallj composites during the warm and cold seasons the bonner criterion 1 was applied for sites in central bolivia and downstream near northern paraguay to determine the spatial and temporal characteristics of the sallj on the circulation characteristics sallj composites during the warm season show the enhanced low level meridional moisture transport coming from equatorial south america as well as an upper level wave train emanating from the west pacific propagating toward south america the intensification of the warm season sallj follows the establishment of an upper level ridge over southern brazil and a trough over most of argentina the circulation anomalies at upper a

the south atlantic convergence zone intensity form persistence and relationships with intraseasonal to interannual activity and extreme rainfall abstract the characteristics of intensity geographical location and persistence of the south atlantic convergence zone sacz during the austral summer are investigated intensity and spatial features of the sacz are identified by performing a factor analysis of structural properties of outgoing longwave radiation olr data the first two leading factors explain 65 of the total variance of structural properties and characterize the sacz according to intensity and location oceanic versus continental an index is constructed based on the magnitude of the factor scores to identify intense weak and oceanic continental sacz the intense sacz category is associated with negative olr anomalies over a large area of tropical south america extending from the western amazon to the atlantic ocean the weak sacz category is observed with positive olr anomalies over tropical south america and negative olr anomalies over southeastern south america oceanic and continental aspects of the sacz are related to a

climatic controls on debris flow activity and sediment aggradation the del medio fan nw argentina in the central andes several studies on alluvial terraces and valley fills have linked sediment aggradation to periods of enhanced sediment supply however debate continues over whether tectonic or climatic factors are most important in triggering the enhanced supply the del medio catchment in the humahuaca basin eastern cordillera nw argentina is located within a transition zone between sub humid and arid climates and hosts the only active debris flow fan within this intermontane valley by combining 10be analyses of boulder and sediment samples within the del medio catchment with regional morphometric measurements of nearby catchments we identify the surface processes responsible for aggradation in the del medio fan and their likely triggers we find that the fan surface has been shaped by debris flows and channel avulsions during the last 400 years among potential tectonic climatic and autogenic factors that might influence deposition our analyses point to a combination of several favorable factors that drive aggradation these are in particular the impact of occasional abundant rainfall on steep slopes in rock types prone to failure located in a region characterized by relatively low rainfall amounts and limited transport capacity these characteristics are primarily associated with the climatic transition zone between the humid foreland and the arid orogen interior which creates an imbalance between sediment supply and sediment transfer the conditions and processes that drive aggradation in the del medio catchment today may provide a modern analog for the conditions and processes that drove aggradation in other nearby tributaries in the past

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

earliest paleoproterozoic high grade metamorphism and orogenesis in the gawler craton south australia the southern cousin in the rae family neoarchean earliest paleoproterozoic rocks of the gawler craton preserved within the mulgathing complex south australia record metamorphism and deformation during an earliest paleoproterozoic tectonothermal event known as the sleafordian orogeny we present metamorphic data from pelitic gneisses of the mulgathing complex from three locations challenger mt christie and blackfellow hill in order to constrain the pressure temperature time p t t evolution of this orogeny in situ u pb dating via laser ablation inductively coupled plasma mass spectrometry shows that monazites within garnet porphyroblasts have ages ca 2470 ma providing an estimate of the timing peak p t conditions monazite crystallisation from melt occurred at ca 2460 ma in the presence of peritectic garnet either late on the prograde path during biotite dehydration melting or immediately following peak t lobate and patchy rims in leucosome hosted monazite suggest a dissolution reprecipitation process occurred during cooling across the elevated dry solidus at ca 2440 ma monazite younger than ca 2430 2420 ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and or subsolidus mineral breakdown during the waning stages of the thermal event phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at 7 kbar and 850 c with apparent thermal gradients in the order of 125 135 c kbar 35 45 c km metamorphism involved a tight anticlockwise p t t path based on mineral assemblage paragenesis and indicates a slow shallow decompressive cooling path sub parallel to the apparent thermal gradient in the order of few degrees per myr a protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the mulgathing complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent transpressive deformation a significant period of post 2 4 ga quiescence in the gawler craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin the similarity in timing and style between the sleafordian orogeny in the gawler craton and the 2 5 2 28 ga arrowsmith orogeny of the rae craton supports the notion that these terranes together with the broader gawler adelie craton of the mawson continent the north australia craton elements of the north china sino korean and dharwar cratons formed part of a family of neoarchean terranes described as the rae family of cratons with a distinctive tectono metamorphic record as compared with other archean cratons we suggest the mawson continent is the southern member of this family dominated as it is by terranes now present in the northern hemisphere

intraseasonal predictions for the south american rainfall dipole the south american rainfall dipole sad is a renowned spatial structure present in the austral summer as part of the south american monsoon system sad phases have been related with extreme precip

plate tectonics on the early earth limitations imposed by strength and buoyancy of subducted lithosphere the tectonic style and viability of modern plate tectonics in the early earth is still debated field observations and theoretical arguments both in favor and against the uniformitarian view of plate tectonics back until the archean continue to accumulate here we present the first numerical modeling results that address for a hotter earth the viability of subduction one of the main requirements for plate tectonics a hotter mantle has mainly two effects 1 viscosity is lower and 2 more melt is produced which in a plate tectonic setting will lead to a thicker oceanic crust and harzburgite layer although compositional buoyancy resulting from these thick crust and harzburgite might be a serious limitation for subduction initiation our modeling results show that eclogitization significantly relaxes this limitation for a developed ongoing subduction process furthermore the lower viscosity leads to more frequent slab breakoff and sometimes to crustal separation from the mantle lithosphere unlike earlier propositions not compositional buoyancy considerations but this lithospheric weakness could be the principle limitation to the viability of plate tectonics in a hotter earth these results suggest a new explanation for the absence of ultrahigh pressure metamorphism uhpm and blueschists in most of the precambrian early slabs were not too buoyant but too weak to provide a mechanism for uhpm and exhumation

composition and origin of clay minerals in holocene sediments from the south eastern north sea the distribution of clay minerals varies systematically in sediments from holocene core material taken along a profile from the east frisian coast to the doggerbank the proportion of illite increases with distance from the coast at the expense of kaolinite whereas slight variations are seen in smectite and chlorite abundances the chemical composition changes and the k rb ratio and k ar isotopic age of illite increase seawards this trend results from progressive mixing processes of riverine detritus with pleistocene fluvioglacial material reworked during the holocene transgression however the clay fluvial flux only became dominant during the decreasing rate of sea level rise in the late holocene especially near the shore for example modern sediments in tidal flats contain 75 of river borne holocene supplied clay detritus whereas this amount is only 10 in modern marine sediments at the doggerbank

lithospheric scale gravitational flow the impact of body forces on orogenic processes from archaean to phanerozoic in the archaean the combination of warmer continental geotherm with a lighter sub continental lithospheric mantle suggests that gravitational forces played a more signifi cant role in continental lithospheric deformation to test this hypothesis we compare the evolution of the deformation and the regional state of stress in archaean like and phanerozoic like lithospheres submitted to the same boundary conditions in a triaxial stress field with imposed convergence in one direction for plausible physical parameters thickening of normal to cold phanerozoic lithospheres produces relatively weak buoyancy forces either extensional or compressional in contrast for archaean continental litho spheres or for anomalously warm phanerozoic lithospheres lateral gravitationally driven flow prevents significant thickening this conclusion is broadly consistent with 1 the rela tive homogeneity of the erosional level now exposed at the surface of archaean cratons 2 the sub aerial conditions that prevailed during the emplacement of up to 20 km of green stone cover 3 the relatively rare occurrence in the archaean record of voluminous detrital sediments 4 the near absence of significant tectonic metamorphic and magmatic age gra dients across archaean cratons 5 the relative homogeneity of strain across large areas and 6 the ubiquitous presence of crustal scale strike slip faults in many late archaean cratons

rainfall variability and trends of the past six decades 1950 2014 in the subtropical nw argentine andes the eastern flanks of the central andes are characterized by deep convection exposing them to hydrometeorological extreme events often resulting in floods and a variety of mass movements we assessed the spatiotemporal pattern of rainfall trends and the changes in the magnitude and frequency of extreme events 95th percentile along an e w traverse across the southern central andes using rain gauge and high resolution gridded datasets cpc uni and trmm 3b42 v7 we generated different climate indices and made three key observations 1 an increase of the annual rainfall has occurred at the transition between low 3 km areas respond differently to changing climate conditions and the transition zone between low and intermediate elevations is characterized by the most significant changes

imprints of diverse mantle deformational episodes in the cauvery suture zone south india abstract this study attempts to understand the upper mantle deformation pattern beneath the northern part of southern granulite terrain sgt located to the south of archean dharwar craton in south india the study region comprises the tectonic blocks nilgiri biligiri rangan shevaroy and madras blocks separated from the northern part of madurai block by the cauvery suture zone csz associated with subduction accretion collision tectonics shear wave splitting observed on high quality xk k s sks skks pks waveforms from 119 teleseismic events recorded by 19 broadband seismic stations mostly within the csz and the northern part of madurai block is utilized in this study the 122 splitting measurements obtained show an average delay time of 0 8 s between the fast and slow waves strong lateral variations in the fast polarization azimuths fpa are observed with the splitting parameters at the majority of stations showing a dependence on back azimuth importantly excluding the madras block a poor correlation between the fpa and the present day absolute plate motion apm direction is observed this dominance of non apm strain reflects complex anisotropy preserved in the lithosphere by plotting the fast polarization trends based on the ray piercing points at a depth of 150 km three distinct splitting patterns associated with the i biligiri rangan block and the eastern part of the nilgiri block ii eastern part of the csz and the northern madurai block nmb and iii madras block and its vicinity within the csz are observed the strong lateral heterogeneity in the deformation of the continental lithosphere might be either related with the shear zones that are associated with frozen magmatic intrusions two episodes of ocean closure and the collision that followed plate motion related strain or a combination of these similar deformation patterns in the northern part of madurai block might be related to remnants of subducted oceanic lithosphere

neoarchean lithospheric strengthening and the coupling of earth s geochemical reservoirs this paper explores the secular evolution of the height of an isostatically balanced collisional mountain belt in the context of ongoing convergence we show that until the neoarchean continents were unable to sustain topography 2500 m during the neoarchean the continental lithosphere evolved through a rheological threshold allowing for the development of signifi cant topography the consequence of the strengthening of the continental lithosphere is fundamental for the coupling of the earth s geochemical reservoirs the neoarchean was a period of global changes during which exogenic envelopes recorded major shifts in composition toward modern values we propose that during the neoarchean the exogenic envelopes which were until then coupled to the mantle through hydrothermal processes and volcanism also became coupled to the continental crust through reliefgenerating tectonics processes and erosion hence changing the balance between mantle versus crustal interaction with the exogenic earth

fluid inclusions in rocks from the amphibolite facies gneiss to charnockite progression in southern karnataka india direct evidence concerning the fluids of granulite metamorphism fluid inclusion studies of rocks from the late archaean amphibolite facies to granulite facies transition zone of southern india provide support for the hypothesis that co2 rich h2o poor fluids were a major factor in the origin of the high grade terrain charnockites closely associated leucogranites and quartzo feldspathic veins contain vast numbers of large co2 rich inclusions in planar arrays in quartz and feldspar whereas amphibole bearing gray gneisses of essentially the same compositions as adjacent charnockites in mixed facies quarries contain no large fluid inclusions inclusions in the northernmost incipient charnockites as at kabbal karnataka occasionally contain about 25 mol of immiscible h2o lining cavity walls whereas inclusions from the charnockite massif terrane farther south do not have visibile h2o microthermometry of co2 inclusions shows that miscible ch4 and n2 must be small probably less than 10mol combined densities of co2 increase steadily from north to south across the transitional terrane entrapment pressures calculated from the co2 equation of state range from 5 kbar in the north to 7 5 kbar in the south at the mineralogically inferred average metamorphic temperature of 750 c in quantitative agreement with mineralogic geobarometry this agreement leads to the inference that the fluid inclusions were trapped at or near peak metamorphic conditions calculations on the stability of the charnockite assemblage biotite orthopyroxene k feldspar quartz show that an associated fluid phase must have less than 0 35 h2o activity at the inferred p and t conditions which agrees with the petrographic observations high tio2 content of biotite stabilizes it to lower h2o activities and the steady increase of biotite tio2 southward in the area suggests progressive decrease of ah2o with increasing grade oxygen fugacities calculated from orthopyroxene magnetite quartz are considerably higher than the graphite co2 o2 buffer which explains the absence of graphite in the charnockites the present study quantifies the nature of the vapours in the southern india granulite metamorphism it remains to be determined whether co2 flushing of the crust can by itself create large terranes of largeion lithophile depleted granulites or whether removal of h2o bearing anatectic melts is essential

crustal thickening and clay controls on o isotope variation in global magmatism and siliciclastic sedimentary rocks new compilations of global o isotope data from zircon and siliciclastic sedimentary rocks highlight an increasing range in 18o values in both systems since the late archean this is consistent with an increased clay component in sedimentary rocks and subsequent incorporation into igneous rocks each of these factors can arguably be achieved by increased crustal thickening in the late archean resulting in greater burial and melting of supracrustal rocks and increased chemical weathering and recycling of upper crustal rocks despite the suggested change in tectonic regimes in the late archean stochastic modelling in this study demonstrates that 18o data do not provide evidence for a secular decrease in the proportion of mantle derived magmas in granitoid rocks instead best fit models indicate that juvenile input and reworking of supracrustal material vary with respect to the short term 100 200 myr tectonic cycles preserved in the continental crust hence major step changes in global tectonic regimes in the post hadean such as the initiation of subduction in the mid to late archean are not supported by global zircon o isotope datasets and instead minor progressive changes are indicated for earth s tectonic regimes

complex networks reveal global pattern of extreme rainfall teleconnections climatic observables are often correlated across long spatial distances and extreme events such as heatwaves or floods are typically assumed to be related to such teleconnections1 2 revealing atmospheric teleconnection patterns and understanding their underlying mechanisms is of great importance for weather forecasting in general and extreme event prediction in particular3 4 especially considering that the characteristics of extreme events have been suggested to change under ongoing anthropogenic climate change5 8 here we reveal the global coupling pattern of extreme rainfall events by applying complex network methodology to high resolution satellite data and introducing a technique that corrects for multiple comparison bias in functional networks we find that the distance distribution of significant connections p 0 005 around the globe decays according to a power law up to distances of about 2 500 kilometres for longer distances the probability of significant connections is much higher than expected from the scaling of the power law we attribute the shorter power law distributed connections to regional weather systems the longer super power law distributed connections form a global rainfall teleconnection pattern that is probably controlled by upper level rossby waves we show that extreme rainfall events in the monsoon systems of south central asia east asia and africa are significantly synchronized moreover we uncover concise links between south central asia and the european and north american extratropics as well as the southern hemisphere extratropics analysis of the atmospheric conditions that lead to these teleconnections confirms rossby waves as the physical mechanism underlying these global teleconnection patterns and emphasizes their crucial role in dynamical tropical extratropical couplings our results provide insights into the function of rossby waves in creating stable global scale dependencies of extreme rainfall events and into the potential predictability of associated natural hazards complex networks are used to analyse global scale teleconnections between extreme rainfall events revealing a peak in the distance distribution of statistically significant connections at around 10 000 kilometres

crustal architecture and tectonic evolution of the cauvery suture zone southern india abstract the cauvery suture zone csz in southern india has witnessed multiple deformations associated with multiple subduction collision history with incorporation of the related accretionary belts sequentially into the southern continental margin of the archaean dharwar craton since neoarchean to neoproterozoic the accreted tectonic elements include suprasubduction complexes of arc magmatic sequences high grade supracrustals thrust duplexes ophiolites and younger intrusions that are dispersed along the suture the intra oceanic neoarchean neoproterozoic arc assemblages are well exposed in the form of tectonic melanges dominantly towards the eastern sector of the csz and are typically subjected to complex and multiple deformation events multi scale analysis of structural elements with detailed geological mapping of the sub regions and their structural cross sections geochemical and geochronological data and integrated geophysical observations suggest that the csz is an important zone that preserves the imprints of multiple cycles of precambrian plate tectonic regimes

orthopyroxene sillimanite quartz assemblages distribution petrology quantitative p t x constraints and p t paths granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane quantitative pressure temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene sillimanite quartz are assessed with reference to calculated phase diagrams in nckfmash and its chemical subsystems peak assemblages form mainly in high variance fields and most mineral assemblage changes reflect multivariant equilibria the rarity of orthopyroxene sillimanite quartz bearing assemblages in granulite facies rocks reflects the need for bulk rock xmg of greater than approximately 0 60 0 65 with pressures and temperatures exceeding c 8 kbar and 850 c respectively cordierite coronas mantling peak minerals such as orthopyroxene sillimanite and quartz have historically been used to infer isothermal decompression p t paths in ultrahigh temperature granulite facies terranes however a potentially wide range of p t paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene sillimanite quartz indicating that an individual mineral reaction texture is unable to uniquely define a p t vector therefore the interpretation of p t paths in high grade rocks as isothermal decompression or isobaric cooling may be overly simplistic integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a p t path and in turn on tectonic models derived from these paths

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

high grade metamorphism during archean paleoproterozoic transition associated with microblock amalgamation in the north china craton mineral phase equilibria and zircon geochronology abstract metamorphic regimes in archean terranes provide important keys to the plate tectonic processes in early earth the north china craton ncc is one of the ancient continental nuclei in asia and recent models propose that the cratonic architecture was built through the assembly of several archean microcontinental blocks into larger crustal blocks here we investigate garnet and pyroxene bearing granulite facies rocks along the periphery of the jiaoliao microcontinental block in the ncc the garnet bearing granulites contain peak mineral assemblage of garnet clinopyroxene orthopyroxene magnetite plagioclase quartz biotite ilmenite mineral phase equilibria computations using pseudosection and geothermobarometry suggest peak p t condition of 800 830 c and 7 8 kbar for metamorphism isopleths using xmg of orthopyroxene and xca of garnet in another sample containing the peak mineral assemblage of garnet orthopyroxene quartz magnetite fluid yield peak p t conditions of 860 920 c and 11 14 kbar geochemical data show tonalitic to granodioritic composition and arc related tectonic setting for the magmatic protoliths of these rocks zircon la icp ms analyses yield well defined discordia with upper intercept ages of 2562 20 ma mswd 0 94 and 2539 21 ma mswd 0 59 which is correlated with the timing of emplacement of the magmatic protolith a younger group of zircons with upper intercept ages of 2449 41 ma mswd 0 83 n 6 as 2449 41 ma mswd 0 83 n 6 and 2480 44 ma mswd 1 2 n 9 constrains the timing of metamorphism zircon lu hf data show dominantly positive ehf t values up to 8 5 and yield crustal residence ages tdmc in the range of 2529 to 2884 ma suggesting magma sources from meso neoarchean juvenile components the high temperature and medium to high pressure metamorphism is considered to have resulted from the subduction collision tectonics associated with microblock amalgamation in the ncc at the end of archean together with the evidence for high pressure metamorphism from an adjacent locality our results correlate with models that predict paired metamorphism at the archean proterozoic transition with the onset of modern style plate tectonics

sedimentation history of the northern north sea margin during the last 150 ka the norwegian channel ice stream ncis is one the defining features of the fennoscandian icesheet still little is known of the detailed dynamics of this ice stream in relation to regional changes in ice cover paleoceanographic and climatic changes sedimentological data from core md99 2283 in combination with seismic data allow a detailed chronological reconstruction of the outbuilding of the margin and the ice extent in southern scandinavia through the last 150 ka an integrated stratigraphy of the margin is presented and compared to the glacial history changes in the regional ice cover are reflected in the accumulation rates the clay mineralogy the coarse chalk fraction and the number of ird 2 mm in core md99 2283 while the sedimentation on the north sea fan as derived from seismic data provides direct evidence for the glacial activity at the shelf edge tentative evidence was found for two early weichselian glacial advances in southern scandinavia and possibly scotland at around 110 and 80 ka bp from 42 cal ka bp the ice cover expanded in southern fennoscandia and led to increased deposition on the margin and the occurrence of local melt water outbursts significantly increased accumulation rates coarse chalk local meltwater output and smectite occur during the ice expansion in the north sea from around 34 cal ka bp the main outbuilding phase of the nsf during the last glacial cycle occurred after 30 cal ka bp from around 24 cal ka bp the ncis became highly active and advanced at least three times prior to the final retreat from the shelf edge around 19 0 cal ka bp

mesoscale convective systems over southeastern south america and their relationship with the south american low level jet abstract prior studies have shown that the low level jet is a recurrent characteristic of the environment during the initiation and mature stages of mesoscale convective systems mcss over the great plains of the united states the south american low level jet sallj over southeastern south america sesa has an analogous role advecting heat and moisture from the amazon basin southward into the central plains of southeastern south america generating ideal environmental conditions for convection initiation and growth into mcss this research has two purposes one is to describe the characteristics of a 3 yr mcs sample in south america south of the equator and its related geographical distribution of convection frequency the other is to advance the knowledge of the evolution of favorable environmental conditions for the development of large mcss specifically those that mature under sallj conditions high horizontal and temporal resolution satellite images are used to detect mcss in the area for the

orographic barriers high resolution trmm rainfall and relief variations along the eastern andes 1 quantifying the degree to which tectonic and erosive processes shape landscapes is key to understanding the evolution of tectonically active mountain belts here we explore the interplay of these two processes along the humid eastern flank of the south american andes we use high resolution tropical rainfall measurement mission trmm and srtm data to characterize elevation relief and hillslope angle of peak rainfall at orographic barriers over a distance of more than 3500 km along the eastern flanks we find that peak rainfall 3 5 m yr occurs at a mean elevation of 1 3 0 17 km a mean relief of 0 95 0 08 km and at moderate mean hillslope angles of 18 3 1 7 we suggest that topographic relief is the best first order rainfall predictor and we demonstrate how relief changes along strike south of the andean orocline alter rainfall distribution changes in climatic sedimentary and tectonic processes prevent the formation of high relief amounts at the mountain front and these areas are not characterized by pronounced rainfall peak

the relationship between south atlantic sst and sacz intensity and positioning this study explores the ocean atmosphere interaction in the formation and dynamics of the south atlantic convergence zone sacz through the analysis of the heat sources estimated through the outgoing longwave radiation the results obtained with this study show that the coupled variability between sacz and the south atlantic ocean indicates that in northern positioned sacz cases over southeastern brazil westerly anomalies are verified in the low level continental tropical circulation consistent with the active phase of the south america monsoon system sams in these cases cold anomalies in the subtropical atlantic ocean cause an increase in the continent ocean temperature gradient favoring an easterly flow in this region and blocking the sacz at a northerly position easterly anomalies in the tropical continent were verified in the low level circulation in southern positioned cases over southern brazil consistent with the sams break phase the sst anomaly patterns indicate cold anomalies in the tropics and warm anomalies in the subtropics which do not favor the development of an easterly flow at low levels over the western tropical atlantic in these cases two situations may occur the strengthening of the low level jet llj which prevails in the eastern subtropical south america and convergence with the south atlantic subtropical high at its southern position or the atmospheric unstable conditions caused by ocean warm sst anomalies in this case the llj may be weaker than its climatological intensity

mesoarchean subduction processes 2 87 ga eclogites from the kola peninsula russia the nature of tectonic processes on the early earth is still controversial the scarcity of high pressure metamorphic rocks such as eclogite the high pressure equivalent of basalt in archean cratons has been used to argue that plate tectonics did not operate until earth had cooled to a critical point perhaps around the 2 5 ga archean proterozoic transition however eclogites occur as meter to kilometer sized lenses enclosed in archean gneisses of the belomorian province of the fennoscandian shield geochemistry and internal features suggest that the protoliths of the eclogites were interlayered olivine gabbros troctolites and fe ti oxide gabbros greenschist facies mineral parageneses are enclosed in prograde zoned eclogite garnets and peak metamorphic conditions define an apparent thermal gradient 12 15 c km consistent with metamorphism in a warm archean subduction zone we show here that these eclogites represent the oldest known high pressure metamorphic rocks u pb dating and hf isotope analyses of zircons from the eclogites and a crosscutting felsic vein define a minimum age of 2 87 ga for the uzkaya salma eclogite a 2 70 ga age for the shirokaya salma eclogite is interpreted as the age of a granulite facies overprint thermal overprinting and growth of new zircon also occurred during the svecofennian 1 9 1 8 ga orogeny these new data imply that plate tectonic processes operated at least locally in late mesoarchean time the adakitic nature of the felsic vein suggests that partial melting of hydrated eclogites could produce archean tonalite trondhjemite granodiorite type magmas

crustal root of the eastern dharwar craton zircon u pb age and lu hf isotopic evolution of the east salem block southeast india abstract the salem block represents the northern part of the southern granulite terrane in se india adjacent to the southern dharwar craton new zircon u pb and lu hf results on charnockites granitoids and migmatites from the eastern salem block indicate that it mainly consists of bimodal juvenile neoarchaean early palaeoproterozoic 2 75 2 65 ga and 2 55 2 48 ga crust that underwent peak metamorphism at 2 53 ga these u pb ages and lu hf isotopic signatures characterize the salem block as part of the lower crust of the eastern dharwar craton the oldest 2 75 2 65 ga granulites can be traced along important shear zones salem attur and gangavalli shear zones which may thus expose deeper levels of eastern dharwar lower crust in a way similar to that proposed for the nilgiri granulites an additional minor contribution of recycled western dharwar craton in the salem block is suggested by the presence of limited 3 65 3 35 ga recycled crust and 2 9 2 6 ga detritus younger intrusions within the salem block include the cryogenian 820 800 ma yelagiri alkaline complex and the ediacaran 559 1 3 5 ma sankari tiruchengode granitoid complex these were sourced from recycled neoarchaean protoliths of the salem block cryogenian tonian age magmatism is widespread within the southern granulite terrane and is possibly related to initial subduction of the mozambique ocean the ediacaran a type granitoids are thought to have formed during the malagasy orogeny that is related with the final consumption of the mozambique ocean and the collision of india with azania east africa

paleomagnetism and geochronology of mafic dykes from the southern granulite terrane india expanding the dharwar craton southward abstract the southern granulite terrane sgt of india is a collage of archean to neoproterozoic crustal blocks hosting several generations of mafic dykes we report new paleomagnetic and geochronologic results from mafic dykes in the salem block of the sgt the 2370 ma bangalore dyke swarm is confirmed in this area with a u pb zircon age of 2363 6 6 ma on dykes with steep dual polarity paleomagnetic directions yielding a pole at 2 n and 73 e a95 14 these results precisely place the dharwar craton and northern sgt at high latitudes in the early paleoproterozoic the episodic paleoproterozoic magmatic record of india is also recovered from the sgt several groupings of sgt paleomagnetic directions compare favorably with the 2 21 ga 2 18 ga and 1 88 ga dharwar craton dykes we also report a grand mean pole of 81 n and 259 e a95 11 that we attribute to a widespread ediacaran age remagnetization associated with the final stages of gondwana assembly these paleomagnetic and geochronologic data permit a better resolved reconstruction of indian crustal elements from the paleoproterozoic onward

geochemistry of holocene salt marsh and tidal flat sediments on a barrier island in the southern north sea langeoog north west germany abstract the barrier islands of the southern north sea were formed during the holocenesea level rise these islands form part of a highly dynamic environment whoseevolution continues today subjected to sea level changes tides and stormevents the sedimentary record re ects processes occurring under varyingenergy conditions this article presents geochemical mineralogical anddiatom investigations carried out in the salt marsh of the east frisian barrierisland of langeoog which is re exposed to a rising sea level due tode embankment the major aim of this study is to improve the knowledge ofthe sedimentological and geochemical development of these deposits underthe in uence of sea level rise with a special focus on the geochemistry anddistribution of heavy mineral associated elements correlation diagramsbetween feo tio 2 and mno as well as ternary plots al 2 o 3 sio 2 zr ortio 2 clearly indicate the variable appearance of heavy minerals in differentlithological facies comprising marsh soil mixed and sand at and relocatedbeach sands a dominating abundance of ilmenite followed by zircon garnetsand some other heavy minerals is evidenced by scanning electron microscope energy dispersive x ray measurements the data presented here suggests thatthese geochemical proxies are useful tools for characterizing depositionalenergy conditions increasing depositional energy is evident for the lithologicalunits in the following order marsh soil mixed at sand at and relocatedbeach sand the energetic conditions during sediment deposition as well asthe sedimentary history are con rmed by diatom analyses as an additionalindependent indicator depending on source rock composition thegeochemical parameters used in this study may also help to investigatedepositional energy regimes of other siliciclastic sedimentary systems keywords depositional energy diatoms heavy minerals inorganicgeochemistry north sea salt marsh tidal at introductionthe depositional history of the barrier islands andadjacent tidal at system of the southern northsea coast was directly related to climate changesduring the holocene sea level rise the lasttransgressive cycle started in the weichselianlate glacial and continues today as evidenced bysedimentology 2012 59 337 355

varied pathways of river borne clay minerals in a near shore marine region a case study of sediments from the elbe and weser rivers and the se north sea abstract the the results outline significant systematic changes in the proportions of clay minerals in the k rb ratios the k ar data and in the heavy metal contents at the fluvial estuarine transition and towards the marine environment these changes result from a progressive mixing of suspended fluvial material with reworked pleistocene sediments at the bottom of the north sea near shore the proportion of reworked pleistocene sediment amounts only 20 30 the mixture is distributed within a 100 km wide zone mainly by currents parallel to the coast it is also transported during flood tide into the estuaries where it is mixed further with suspended fluvial material the k rb ratios and k ar dates obtained from illite prove to be particularly useful for modeling the mixing of the clay minerals as well as their distribution patterns and origin

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces