zircon u pb chronology and elemental and sr nd hf isotope geochemistry of two triassic a type granites in south china implication for petrogenesis and indosinian transtensional tectonism abstract a detailed study utilizing zircon u pb dating major and trace element geochemistry and sr nd hf isotope geochemistry has been carried out for the caijiang granite in jiangxi province and the gaoxi granite in fujian province south china the new data indicate that the caijiang and gaoxi granites are triassic 228 230 ma and have the petrographic and geochemical characteristics of a type granites in both granites biotite occurs along the boundary of euhedral plagioclase and quartz which implies that the primary magma could have been anhydrous the two granites show high contents of total alkalis na 2 o k 2 o 7 81 12 15 high field strength elements e g zr 240 458 ppm y 16 8 38 0 ppm nb 13 5 33 8 ppm and zr nb ce y 382 604 ppm and rare earth elements total ree 211 373 ppm as well as high ga al ratios 10000 ga al 2 41 3 53 the lowest magmatic temperatures estimated from zircon saturation thermometer were 800 840 c for the caijiang granite and 820 850 c for the gaoxi granite respectively the caijiang granite has relatively high 87 sr 86 sr i ratios of 0 71288 to 0 72009 low e nd t values of 9 9 to 9 3 and low zircon e hf t values peak value of 7 5 whole rock nd isotopic model ages and zircon hf isotopic model ages mostly vary from 1 65 ga to 1 80 ga the gaoxi granite has also high 87 sr 86 sr i ratios of 0 71252 to 0 71356 low e nd t value of 13 8 and low zircon e hf t values peak value of 12 0 whole rock nd isotopic model ages and zircon hf isotopic model ages mostly vary from 1 95 ga to 2 10 ga according to these data we suggest that the two granites might have been derived from partial melting of precambrian crustal rocks that had been granulitized during an earlier thermal event our study of the caijiang and gaoxi granites together with previous studies on two triassic alkaline syenites tieshan and yangfang in fujian province and one a type granite wengshan in zhejiang province in south china indicate a wide transtensional tectonic environment in the cathaysia block that lasted at least from 254 ma to 225 ma combined with extant data for the indosinian granites and tectonic evolution in south china we suggest that the formation of a type granites was related to the local ne trending extensional faults probably caused by collision between the south china block and the indochina block or the north china block

base level buffers and buttresses a model for upstream versus downstream control on fluvial geometry and architecture within sequences abstract the effects of downstream base level control on fluvial architecture and geometry are well explored in several broadly similar sequence stratigraphic models cretaceous dakota group strata u s western interior have characteristics reflecting combined downstream and upstream base level controls that these models cannot address particularly three layers of amalgamated channel belt sandstone within this group thicken and are continuous for distances 300 km along dip that stretch the reasonable lengths for which these models are intended to apply as well architecture in up dip reaches records repeated valley scale cut and fill cycles this contrasts with equivalent strata down dip which record channel scale lateral migration with no such valley scale cycles apparent we here introduce the concept of buffers and buttresses to address these observations we assume that river longitudinal profiles are each anchored down dip to some physical barrier e g the sea strand etc that we refer to as a buttress buttress shift is considered the primary downstream control on base level profiles extrapolated up dip from the buttress over any modeled duration of buttress shift can range widely because of high frequency variability in upstream base level controls e g discharge etc all these potential profiles however are bounded above by the profile of highest possible aggradation and below by the profile of maximum possible incision these upper and lower profiles are buffers and they envelop the available fluvial preservation space thickness of the buffer zone is determined by variability in upstream controls and should increase up dip to the limit of downstream profile dominance dakota valley scale surfaces record repeated cut and fill cycles driven by up dip controls and are confined between thick stable buffers equivalent strata down dip record lateral reworking within a thinner channel scale buffer zone that was positioned by downstream controls regression exposed slopes similar to the buffer zone thus buffers were stable for long distances and durations this prompted dip extensive lateral reworking of strata into upstream valley scale and downstream channel scale sheets buffers and buttresses provide a broadly applicable model for fluvial preservation that captures upstream vs downstream base level controls on geometry and architecture the model lends general insights into dip oriented variations in fluvial architecture production of sheet vs lens geometry total preservation volumes for fluvial systems and variations in these factors related to contrasting climatic conditions and basin physiography the model can be amended to existing sequence stratigraphic approaches in order to capture dip oriented variations in sequence architecture

the emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and ph front here we argue that life emerged on earth from a redox and ph front at c 4 2 ga this front occurred where hot c 150 c extremely reduced alkaline bisulphide bearing submarine seepage waters interfaced with the acid warm c 90 c iron bearing hadean ocean the low ph of the ocean was imparted by the ten bars of co 2 considered to dominate the hadean atmosphere hydrosphere disequilibrium between the two solutions was maintained by the spontaneous precipitation of a colloidal fes membrane iron monosulphide bubbles comprising this membrane were inflated by the hydrothermal solution upon sulphide mounds at the seepage sites our hypothesis is that the fes membrane laced with nickel acted as a semipermeable catalytic boundary between the two fluids encouraging synthesis of organic anions by hydrogenation and carboxylation of hydrothermal organic primers the ocean provided carbonate phosphate iron nickel and protons the hydrothermal solution was the source of ammonia acetate hs h 2 and tungsten as well as minor concentrations of organic sulphides and perhaps cyanide and acetaldehyde the mean redox potential eh across the membrane with the energy to drive synthesis would have approximated to 300 millivolts the generation of organic anions would have led to an increase in osmotic pressure within the fes bubbles thus osmotic pressure could take over from hydraulic pressure as the driving force for distension budding and reproduction of the bubbles condensation of the organic molecules to polymers particularly organic sulphides was driven by pyrophosphate hydrolysis regeneration of pyrophosphate from the monophosphate in the membrane was facilitated by protons contributed from the hadean ocean this was the first use by a metabolizing system of protonmotive force driven by natural aph which also would have amounted to c 300 millivolts protonmotive force is the universal energy transduction mechanism of life taken together with the redox potential across the membrane the total electrochemical and chemical energy available for protometabolism amounted to a continuous supply at more than half a volt the role of the iron sulphide membrane in keeping the two solutions separated was appropriated by the newly synthesized organic sulphide polymers this organic take over of the membrane material led to the miniaturization of the metabolizing system information systems to govern replication could have developed penecontemporaneously in this same milieu but iron sulphur and phosphate inorganic components of earliest life continued to be involved in metabolism

p t t evolution of ultrahigh temperature granulites from the saxon granulite massif germany part ii geochronology the granulites of the saxon granulite massif equilibrated at high pressure and ultrahigh temperature and were exhumed in large part under near isothermal decompression this raises the question of whether p t t data on the peak metamorphism may still be retrieved with confidence felsic and mafic granulites with geochronologically useful major and accessory phases have provided a basis to relate p t estimates with isotopic ages presented in a companion paper the assemblage garnet clinopyroxene in mafic granulite records peak temperatures of 1010 1060 c consistent with minimum estimates of around 967 c and 22 3 kbar obtained from the assemblage garnet kyanite ternary feldspar quartz in felsic granulite multiple partial overprint of these assemblages reflects a clockwise p t evolution garnet and kyanite in the felsic granulite were successively overgrown by plagioclase spinel plagioclase sapphirine plagioclase and biotite plagioclase most of this overprinting occurred within the stability field of sillimanite garnet clinopyroxene in the mafic granulite were replaced by clinopyroxene amphibole plagioclase magnetite the high p t conditions and the absence of thermal relaxation features in these granulites require a short lived metamorphism with rapid exhumation the ages of peak metamorphism 342 ma and shallow level granitoid intrusions 333 ma constrain the time span for the exhumation of the saxon granulites to about 9 ma

geochemistry of champagne hot springs shallow hydrothermal vent field and associated sediments dominica lesser antilles the champagne hot springs chs shallow submarine hydrothermal system is located along the submerged flank of the plat pays volcanic system on the southwest section of the island of dominica lesser antilles we have conducted a detailed geochemical study of the hydrothermal system with the objectives to investigate the source of the hydrothermal fluids and gases their effect on sediment and precipitate chemistry as well as comparing the submarine vent chemistry with nearby on land hydrothermal vents finally we compare our findings to previous preliminary data from chs and compare sediment chemistry to that of average caribbean sediments we also report on a newly discovered area of submarine hydrothermal venting located approximately 40 m to the north of chs this area consists of hydrothermally altered areas of sand that contain abundant coatings of hydrous ferric oxides hfo on sediment grains geochemical and mineralogical analyses of vent waters pore waters gases sediments and precipitates reveal that the vent fluids consist of a mixture of entrained seawater and meteorically derived hydrothermal fluid in varying proportions vent fluids are depleted in br s o4 c l n a k and sr 2 relative to ambient seawater these species are all positively correlated with mg 2 which is also depleted relative to seawater boron fe as sb mn si and li are all enriched relative to ambient seawater pore waters in the hydrothermally altered sand patches have essentially the same chemistry mixing between fe 2 rich vent fluids and seawater causes rapid oxidation of fe 2 to insoluble fe 3 and leads to precipitation of hfo at the vent site and subsequent formation of hydrothermally altered sand patches the elevated concentrations of as and sb in the precipitates and sediments relative to average caribbean seafloor sediments reflect adsorption by hfo gas samples from the vent site are typical arc type gases and have both meteoric and magmatic signatures d 2005 elsevier b v all rights reserved

chemical weathering of basalts southwest iceland effect of rock crystallinity and secondary minerals on chemical fluxes to the ocean river ground and peat water from a basaltic catchment area laxa in kjos southwest iceland were sampled and analyzed for major element concentrations to define the effect of glassy versus crystalline basalt the formation of secondary minerals and runoff on fluxes of dissolved elements to the ocean the proportions of the dissolved solids in the water samples derived from precipitation air and rock weathering were estimated the amounts of na mg ca k and s originating from precipitation were calculated from the respective solute cl marine ratios and the estimated aqueous cl originated from precipitation which was calculated using measured b content of the waters and b cl molal marine and rock ratios the rock contribution from weathering ranged from 22 to 46 percent of the total dissolved solids of the waters iron and al showed low mobility compared to na this was primarily due to consumption of these elements by ferrihydroxides allophane imogolite and clay minerals also si and to a lesser degree ca show apparent slightly lower mobility than na these results are in good agreement with these phases being dominant in the soils of the study area wada and others 1992 and the main secondary minerals of basaltic glass in iceland crovisier and others 1992 reaction progress calculations support these findings according to a model interaction of basaltic glass and meteoric water al fe and si are consumed by amorphous fe and al hydroxides and imogolite and allophane at low reaction progress with increasing reaction progress ca fe mg smectite became the primary secondary minerals limiting the mobility of ca fe mg si and al these findings suggest that the weathering in the laxa in the kjos catchment area has evolved into the beginning of the smectite weathering stage and that it is dominated by a relatively low reaction progress and a high water to rock ratio in the present study the effect of runoff and rock crystallinity was quantified for dissolved elemental fluxes temperature lithology other than rock crystallinity and rock age were similar in all of the catchment areas in this study silica ca f s al k mg and b had a very similar dependence on runoff on the other hand na fluxes were found to be less dependent on runoff and fe fluxes were found to be independent of runoff basaltic glass dissolves faster than fully crystallized basaltic rocks gislason and eugster 1987a therefore increased glass content of the primary rocks is thought to increase the leaching and elemental fluxes at constant runoff however because some of the solutes particularly al and fe were highly influenced or even controlled by the formation of secondary minerals this increasing dissolution with increasing glass content was hidden on the other hand basaltic glass was observed to enhance the fluxes of more mobile elements like na si ca f and s by a factor of 2 to 5 at a constant runoff of 200 cm yr constant vegetative cover and 0 to 100 percent glass content of the rocks the k and mg fluxes were found to be independent of rock crystallinity however runoff alone cannot explain the fluxes of these elements and it seems that vegetative cover and seasonal variations in biomass activities influence the k and mg fluxes

understanding mesozoic accretion in southeast asia significance of triassic thermotectonism indosinian orogeny in vietnam results from a zircon u pb study of the metamorphic basement of vietnam reveal that a large part of the continental crust was affected by a short lived episode of ductile deformation and high temperature metamorphism between 258 6 ma and 243 5 ma although coincident with final stages of north south china collision qinling orogenesis the thermotectonism in vietnam was caused by accretion of sibumasu to indochina south china this accretion event indosinian orogeny has regional significance because it contributed to the final stages of north south china collision an aspect not explicitly recognized in qinling orogenic models

structural petrological and thermal evolution of a tertiary ductile strike slip shear zone diancang shan yunnan the diancang shan a horst massif within the red river fault zone in yunnan people s republic of china preserves a structural petrological and thermal record of two distinct phases of tectonic activity a left lateral ductile shear that terminated between 20 and 17 ma and a ductile to brittle phase of normal faulting which began at 4 7 ma and remains active mylonitic rocks in the core of the range display an early steep high temperature ht schistosity and a horizontal stretching lineation that are both parallel to the trend of the belt kinematic indicators indicate that shear was left lateral the complex shape of the ht schistosity at the southern termination of the massif likely results from a large scale oblique left lateral c shear plane that dismembered the shear zone and separated the ailao shan and the diancang shan as left lateral deformation terminated thermochronological and thermobarometric results suggest that the gneisses were partially unroofed during this event along the eastern edge of the diancang shan the ht fabrics were overprinted by low temperature structures during activation of east dipping normal faults cooling associated with this normal right lateral faulting along the diancang shan and perhaps activation of the right lateral normal movement on the range front fault farther south along the ailao shan began at 4 7 0 1 ma these results tend to support the view that extrusion of indochina occurred along the left lateral red river shear zone between 35 and 19 17 ma initiation of right lateral normal slip during the late miocene may relate to eastward extensional collapse of the thickened tibetan crust or more probably to initiation of the second phase of extrusion

no change in fluvial style across a sequence boundary cretaceous blackhawk and castlegate formations of central utah u s a nonmarine sequence stratigraphic models are based largely on studies of fluvial units in the cretaceous western interior of north america including the blackhawk and castlegate formations of central utah these models suggest that fluvial units should show a transition from mudstone prone isolated meander belt deposits of the transgressive and highstand systems tract into amalgamated braided stream sandstones of the lowstand systems tract these mudstone rich fluvial strata are largely assumed rather than documented to represent the deposits of isolated meander belt deposits despite recent work that suggests that there is no simple relationship between the proportion of preserved floodplain mud and fluvial style there are numerous studies documenting the braided character of the sandstone rich castlegate formation but there are no corresponding studies that document the internal fluvial facies architecture within the blackhawk formation to support the interpretation that the blackhawk comprises the deposits of a meandering stream system we test the validity of the sequence stratigraphic model by comparing dimensions and styles of associated bar and channel deposits as well as cross stratal thicknesses between the blackhawk and castlegate formations along salina canyon in utah data compiled from 24 measured sections and 6 photomosaics show that the blackhawk formation comprises isolated sandy channel belt sheet sandstones 5 8 m thick contained within thick floodplain mudstones whereas the castlegate formation consists of about 80 m of amalgamated sandy channel belt sheets 4 7 m thick with only minor mudstone the average height of cross sets is 13 cm in the castlegate formation and 14 cm in the blackhawk formation and formative dune height is estimated to be 38 cm for the castlegate formation and 45 cm for the blackhawk formation average bar accretion thicknesses are 2 m in both the blackhawk and castlegate formations corresponding water depths are estimated to be 2 5 4 1 m for the blackhawk and 2 3 3 8 m for the castlegate the correlation of master bedding planes in the blackhawk formation shows overlapping lens shaped bar deposits and channel fills bar tops appear to dip in several directions indicating both lateral and downstream accretion of mid channel braid bars while there is some indication that castlegate rivers were slightly shallower the differences do not suggest a major change in fluvial style across the sequence boundary both are braided the lack of change in scale of channels also is not compatible with a hypothesized increase in aridity between the blackhawk and the castlegate the difference in fluvial architecture between the two formations is interpreted to reflect changes in accommodation that were likely tectonic in origin this supports that idea that preservation of thick successions of fine grained overbank material is not a function of the plan view channel geometry

direct measurement of strain rates in ductile shear zones a new method based on syntectonic dikes received 17 january 2008 revised 31 august 2008 accepted 29 october 2008 published 23 january 2009 1 we describe a new method to estimate directly ductile strain rates at an outcrop scale from the deformation of dikes emplaced within a shear zone the method is tested in a well constrained shear zone the ailao shan red river shear zone for which global strain rates can be calculated from published fault rates the strain rate was determined by measuring independently the shear strain g recorded by the dikes and the age t of dikes emplacement the shear strain was quantified by three different methods that take into account either the stretching of the dikes or their angle variations during deformation or both of them the values of minimum shear strains range between 0 2 and 9 7 for the less to the most deformed dikes respectively the ages of dike emplacement were obtained by th pb sensitive high resolution ion microprobe shrimp dating of monazites we obtained three groups of ages the younger age is 22 55 0 25 ma the intermediate age is 26 81 0 66 ma and the oldest ages are 29 89 0 46 ma and 29 93 0 38 ma the geochronological data are in agreement with the structural data the most deformed dikes being the oldest the minimum strain rates deduced from these measurements are 3 to 4 10 14 s 1 which is consistent with previous estimates of geological strain rates in ductile shear zones

structural evolution of the day nui con voi metamorphic complex implications on the development of the red river shear zone northern vietnam the day nui con voi dncv metamorphic complex in north vietnam is the southernmost high grade metamorphic zone along the nw se trending red river shear zone rrsz in indochina the rrsz was considered as a classical large scale continental strike slip fault that had played a significant role in the continental extrusion of southeast asia since the collision of india and eurasia earlier ideas determined the rrsz as a steep shear zone that penetrated the entire lithosphere both metamorphism and structures within rocks along the dncv metamorphic complex have been previously thought to be formed syn tectonically by left lateral shearing of the rrsz during the oligocene miocene continental escape tectonics however our meso and microstructural re examination of this region shows that these metamorphic rocks were formed during earlier tectonic episodes unrelated to strike slip shearing high angle to near orthogonal overprinting fabrics indicated that this region recorded three episodes of ductile deformation followed by brittle faulting events with different intensity spanning from the triassic to the tertiary d1 is preserved as nw se striking upright folds under garnet grade regional metamorphism during the triassic indosinian orogeny as south china block amalgamated with the indochina block the large scale horizontal d2 folds with a dominant top to n nw bottom to s sw sense of shear and sub horizontal fold axial planes suggest that the dncv metamorphic complex remained at midcrustal depths since the indosinian orogeny the youngest ductile deformation event d3 refolded d2 recumbent folds into a dome and uplifted the dncv as lower temperature fabrics s3 indicated steep mylonite zones with left lateral kinematic indicators and brittle faulting were developed on both limbs of the dome along the steep song hong and song chay faults during left lateral movement of the rrsz our new spatial temporal and kinematic correlations of metamorphic fabrics within the dncv metamorphic complex support the suggestion that the rrsz developed after regional metamorphism and remained purely a crustal fault the complicated deformation history recorded within the dncv metamorphic complex provides an alternative interpretation and suggests that crustal scale strike slip faults such as the rrsz need not root from the mantle

direct dating of left lateral deformation along the red river shear zone china and vietnam 1 exposures of high grade midcrustal rocks within the red river shear zone rrsz which separates the indochina and south china blocks exhibit clear evidence of left lateral ductile deformation assuming that the south china sea represents a pull apart basin formed at the southeastern termination of the rrsz it has been argued that seafloor magnetic anomalies constrain the timing of sinistral slip accommodated by the rrsz between 32 and 17 ma at a rate of 4 cm yr while 40ar 39ar thermochronometry indicates that left lateral slip occurred along the rrsz between 25 and 17 ma the timing of earlier high temperature deformation has not been directly constrained in situ th pb ion microprobe dating of monazite inclusions in garnets allows direct assessment of the timing of amphibolite grade metamorphism and synchronous left lateral shearing results from northern segments of the rrsz in yunnan china indicate that synkinematic garnet growth occurred between 34 and 21 ma and are the first to document late oligocene metamorphism and left lateral shearing data from the southern rrsz within vietnam are complicated by tertiary overprinting of rocks that experienced amphibolite facies metamorphism during the indosinian orogeny 220 ma the period during which sinistral deformation is now constrained to have occurred along the rrsz i e 34 17 ma is essentially coincident with spreading of the south china seafloor 32 17 ma this temporal and kinematic link between left lateral shearing along the rrsz and opening of the south china sea supports the view that indochina was extruded from asia as a block along lithospheric scale strike slip faults

enriched h2 ch4 and 3he concentrations in hydrothermal plumes associated with the 1996 gorda ridge eruptive event abstract the rapid field response in march and april 1996 to diking eruptive events on the northern gorda ridge allowed the first detailed co measurement of 3 he h 2 and ch 4 in plumes associated with a submarine eruption and examination of the chemical transformation of these volatile species during aging of the plumes the diking eruptive event generated extensive event plumes with a low 3 he heat ratio of 0 4 10 12 cm 3 stp gm 1 c 1 while chronic plumes had 3 he heat ratios of 2 1 6 2 10 12 cm 3 stp gm 1 c 1 hydrogen concentrations in the initial event plume ep96a reached a maximum value of 47 nm 235 times that of ocean background concentration 0 2 nm in contrast roughly one month later h 2 concentrations although still elevated had drastically dropped to 40 times background values methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments reaching a maximum value of 7 nm the intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high localized concentrations of h 2 the data indicate however that maximum rates of h 2 production during event plume formation could have lasted a few days at most

the carnian pluvial event in western europe new data from iberia and correlation with the western neotethys and eastern north america nw africa regions abstract at the beginning of the late triassic a period of increased precipitation occurred over large tracts of central pangea the carnian pluvial event it was a short lived event 1 ma coeval with a marked sea level fall and active rifting new and classical sections of carnian age in the iberian peninsula have been studied and a new correlation scheme is proposed for the iberian domain integrating field data and subsurface electric log data this is the first time that the portuguese and spanish late triassic carnian sediments are correlated the carnian pluvial event is identified in the iberian peninsula as a continental clastic wedge manuel fm and coeval sediments in between marine evaporite formations during a marked sea level drop event its age is considered to be julian early carnian by palynological and paleogeographic considerations a new correlation of the late triassic sediments of the iberian peninsula with coeval sediments of western europe nw africa and eastern north america is proposed and a new paleogeographical map of central eastern pangea for this period is presented in order to demonstrate that the geographical extent of the siliciclastic sediments related to the carnian pluvial event was much larger than accepted before

a chronostratigraphic framework for the upper jurassic morrison formation western u s a the attached document is the authors final accepted submitted version of the journal article you are advised to consult the publisher s version if you wish to cite from it

stability of andalusite and sillimanite and the al_ 2 sio_ 5 triple point constraints from the ballachulish aureole scotland andalusite and and sillimanite sil occur in well defined zones in the ballachulish aureole in most of the aureole s sillimanite bearing rocks sillimanite grain diameters range from 2 m to 10 m the boundary between prograde andalusite and prograde sillimanite is constrained to a zone of max 30 m width upgrade of the muscovite quartz andalusite k feldspar h 2 o reaction isograd this boundary is interpreted to correspond to the and sil equilibrium the common occurrence of andalusite well upgrade of the first occurrence of sillimanite is ascribed to sluggish kinetics of the polymorphic inversion the p t position of the and sil equilibrium in the aureole is estimated as 645 20 c and 3 0 0 4 kbar based on a combination of 1 location of the ms qtz and kfs h 2 o reaction corrected for non end member fluid and mineral compositions and 2 independent pressure estimates obtained from equilibria in metapelites marbles calcsilcates and granitoids all in exce

exhumation of the diancang shan metamorphic complex along the ailao shan red river belt southwestern yunnan china evidence from 40ar 39ar thermochronology abstract exhumation of the high grade metamorphic rocks from deep to shallow crustal levels in the diancang shan dcs massif is evident by the temporal transition from shearing and mylonitization at the amphibolite facies in the lower to middle crust through retrograde ductile brittle faulting at greenschist facies in the middle crust and brittle faulting near the surface new 40ar 39ar ages of amphibole muscovite biotite and k feldspar mostly from mylonitic rocks help constrain the thermal evolution of the massif during its exhumation the thermochronologic studies have shown at least three stages of exhumation and cooling from late oligocene to pliocene e g 28 13 ma 13 4 ma and 4 0 ma respectively the initiation of the unroofing history of the dcs massif resulted from ductile left lateral shearing along the ailao shan red river shear zone asrr since the late oligocene diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history of the massif the first stage of exhumation might have stopped due to the cessation of the left lateral shearing at ca 21 ma although cooling continued until 13 ma ago to relatively low temperature conditions the second and third stages of cooling started from 13 ma and lasted until the recent active faulting the diversity of cooling rates during the second stage from 13 to 4 ma suggests differences in exhumation rates at various localities ductile to brittle shearing along the eastern flank retrograde normal shear zone is the best explanation for the exhumation during this stage the inhomogeneous cooling and exhumation of the lower plate of the normal fault zone possibly resulted from the tilting of the lower plate or differential exhumation of slabs in the lower plate during the third stage cooling and exhumation of the dcs massif since 4 ma ago is recognized when higher temperature cooling paths are extended and plotted at a lower temperature of about 16 c the present day average surface temperature therefore the diachronous cooling and exhumation continuously spanned to the surface the tectonic exhumation of the dcs massif constitutes a part of the tectono thermal evolution and diachronous exhumation of the metamorphic complexes in the fan shaped area bounded broadly in se asia by the red river fault zone and sagaing fault zone since 36 ma extrusion induced strike slip shearing and rotation associated normal faulting played important roles in the diachronous cooling and exhumation of the dcs massif and other metamorphic complexes in se asia

valleys that never were time surfaces versus stratigraphic surfaces discussion strong and paola 2008 have written a seminal paper rich with insights that contribute to our understanding of the origin and time significance of sequence boundaries these insights derive from the animated proxy for natural depositional systems offered by the experimental flume approach their perspective differs from the finished product vantage provided by the rock and sediment record and the work in progress view granted by the recent stratigraphy of extant depositional systems still these rock and recent perspectives provide critical angles to the full view of sequences and through their differing datal slant have already rendered comparatively supportable versions of the strong and paola 2008 conclusions their paper does not well consider the testimony of these other witnesses this discussion provides a few examples where conclusions of strong and paola have more converged with existing field insights and further comments on areas where the flume has enhanced the recent and rock perspectives a central point of strong and paola s paper is that sequence boundaries form as composite surfaces that are never synchronously exposed but instead are cut and reshaped diachronously by sequential scour and burial beneath locally active topography of a larger and laterally migrating fluvial system this process is observed

cracking mechanisms during galena mineralization in a sandstone hosted lead zinc ore deposit case study of the jinding giant sulfide deposit yunnan sw china there are two types of lead zinc ore bodies i e sandstone hosted ores sho and limestone hosted ores lho in the jinding giant sulfide deposit yunnan sw china structural analysis suggests that thrust faults and dome structures are the major structural elements controlling lead zinc mineralization the two types of ore bodies are preserved in two thrust sheets in a three layered structural profile in the framework of the jinding dome structure the sho forms the cap of the dome and lho bodies are concentrated beneath the sho cap in the central part of the dome quartz feldspar and calcite and sphalerite pyrite and galena are the dominant mineral components in the sandstone hosted lead zinc ores quartz and feldspar occur as detrital clasts and are cemented by diagenetic calcite and epigenetic sulfides the sulfide paragenetic sequence during sho mineralization is from early pyrite to galena and late sphalerite galena occurs mostly in two types of cracks i e crescent style grain boundary cracks along quartz pyrite or rarely along pyrite pyrite boundaries and intragranular radial cracks in early pyrite grains surrounding quartz clasts the radial cracks are more or less perpendicular to the quartz pyrite grain boundaries and do not show any overall whole rock orientation pattern their distribution morphological characteristics and geometrical relationships with quartz and pyrite grains suggest the predominant role of grain scale cracking thermal expansion cracking is one of the most important mechanisms for the generation of open spaces during galena mineralization cracking due to heating or cooling by infiltrating fluids resulted from upwelling fluid phases through fluid passes connecting the sho and lho bodies provided significant spaces for crystallization of galena the differences in coefficients of thermal expansion between pyrite and quartz led to a difference in volume changes between quartz grains and pyrite grains surrounding them and contributed to cracking of the pyrite grains when temperature changed combined thermal expansion and elastic mismatch due to heating and subsequent cooling resulted in the radial and crescent cracking in the pyrite grains and along the quartz pyrite grain boundaries

nonmarine sequence stratigraphy updip expression of sequence boundaries and systems tracts in a high resolution framework cenomanian dunvegan formation alberta foreland basin canada ten allomembers labeled j a have been mapped in the deltaic dunvegan formation using 2340 wells and 63 outcrop sections each allomember spans about 200 k y is bounded by regional transgressive surfaces and consists of 3 10 component shingles apparently of autogenic origin allomembers h e are discussed in this article their internal stacking patterns and bounding surfaces permit interpretation as depositional sequences sequences h f have a sigmoidal prismatic geometry downlap to the southeast and thin northwest onto the coastal plain no differential thickening exists toward the orogen in the southwest in contrast sequence e thickens markedly toward the northwest the tabular geometry of sequences h f suggests accommodation was controlled by sea level change whereas the wedge geometry of sequence e suggests significant tectonic tilting subaerial unconformities that bound valleys and extend onto adjacent interfluve paleosols can be traced 120 330 km updip into outcrop the base of each sequence is characterized by shoreline backstep of 80 150 km transgressive and highstand shingles in each sequence comprise marine deltaic deposits that pass landward into coastal plain strata falling stage shingles offlap seaward with little or no nonmarine equivalents updip whereas lowstand shingles show renewed coastal onlap in outcrop three nonmarine systems tracts are recognized 1 a channel dominated low accommodation systems tract fills valleys up to 35 m deep and is equivalent to the early transgressive systems tract begin page 1968 2 a lacustrine dominated high accommodation systems tract comprises mudstone enclosing ribbon channel sandstones blankets interfluves and forms the bulk of each sequence it is equivalent to late transgressive and early highstand systems tracts 3 a paleosol dominated low accommodation systems tract occupies the upper part of each sequence and is capped by the sequence boundary it is equivalent to the late highstand systems tract sequence bounding interfluve paleosols represent tens of thousands of years of pedogenesis related to slow aggradation followed by fluvial entrenchment and clastic bypass and represent the late highstand falling stage lowstand and early transgressive systems tracts coastal plain deposits record only part perhaps

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

the nature of the ailao shan red river asrr shear zone constraints from structural microstructural and fabric analyses of metamorphic rocks from the diancang shan ailao shan and day nui con voi massifs abstract the structural geology timing of shearing and tectonic implications of the asrr shear zone one of the most striking lineaments in southeast asia have been the topics of extensive studies over the past few decades the xuelong shan xls diancang shan dcs ailao shan als and day nui con voi dncv metamorphic massifs along the shear zone have preserved important information on its structural and tectonic evolution our field structural analysis detailed microstructural and fabric analysis as well as the quartz sillimanite and garnet fabric studies of the sheared rocks from the massifs demonstrate the dominant roles of three deformation episodes during cenozoic tectonic evolution in the shear zone among the contrasting structural and microstructural associations in the shear zone d2 structures which were formed at the brittle to ductile transition during large scale left lateral shearing in the second deformation episode predominate over the structural styles of the other two deformation episodes discrete micro shear zones with intensive grain size reduction compose the characteristic structural style of d2 deformation in addition several types of folds early shearing folds f 2 1 and late shearing folds f 2 2 were formed in the sheared rocks including discrete to distributed mylonitic foliation stretching lineation and shear fabrics e g mica fish domino structures as well as sigma and delta fabrics a sequence of microstructures from syn kinematic magmatic flow high temperature solid state deformation to brittle ductile shearing is well preserved in the syn kinematic leucocratic intrusions deformation structures from the first episode d1 are characterized by f1 folds and distributed foliations s1 in rocks due to pure shearing at high temperatures they are preserved in weakly sheared d2 rocks along the eastern margin of the als belt or in certain low strain tectonic enclaves within the shear zone furthermore semi brittle deformation structures such as hot striae and discrete retrogression zones are attributed to normal slip shearing in the third deformation episode d3 which was probably locally active along the eastern flank of the dcs range for example there are four quartz c axis fabric patterns in the mylonitic rocks including type a point maxima type b y point maxima with crossed girdles superimposition type c quadrant maxima as well as type d point and quadrant maxima combination they are consistent with microscopic observations of microstructures of high temperature pure shearing low temperature simple shearing and their superimposition integrated microstructural analysis and fabric thermometer studies provide information on both high temperature up to 750 c and dominant low temperature 300 600 c deformations of quartz grains in different rock types sillimanite and garnet fabrics especially the latter were primarily formed at the peak metamorphism during high temperature pure shearing the above structural microstructural and fabric associations were generated in the tectonic framework of the indian eurasian collision the low temperature microstructures and fabrics are attributed to left lateral shearing along the asrr shear zone from 27 to 21 ma during the southeastward extrusion of the indochina block which postdated high temperature deformation at the peak metamorphism during the collision

late triassic post collisional slab break off along the ailaoshan suture insights from oib like amphibolites and associated felsic rocks what dynamic processes the south china indochina collision had gone through is still a pending problem our recent investigations identified a late triassic gabbroic intrusion in mengdong village and several granitic plutons near wana village yunnan province sw china both have underwent strong metamorphism and been altered to amphibolite and granitic gneiss respectively we carried out sims and la icpms zircon u pb dating for the amphibolites and granitic gneisses respectively dating results yield weighted mean 206pb 238u ages of 221 5 5 3 ma for the amphibolites and 224 0 1 8 and 235 4 0 6 ma for the granitic gneisses the amphibolite samples have low mg 41 7 42 8 high tio2 3 7 wt and na2o na2o k2o 1 89 2 68 contents and depleted sr nd isotopic compositions 87sr 86sr i 0 707647 and e nd t 1 17 they exhibit oib like ree patterns and spidergrams with strongly enriched lree contents insignificant eu anomalies eu 0 99 1 03 and moderate negative sr anomalies sr 0 46 0 56 the protolith gabbro of the mengdong amphibolites is derived from low degree partial melting of a homogeneous oib type mantle source in the garnet stability field and experienced significant fractionation of olivine clinopyroxene and plagioclase during magma evolution the wana samples are strongly peraluminous a cnk 1 1 and k enriched k2o na2o 2 93 3 63 they show enriched sr nd isotopic compositions with 87sr 86sr i 0 718589 0 719754 and e nd t 11 34 to 10 92 the wana plutons are typical s type granite and product of the dehydration melting of meta sedimentary rocks we summarized late triassic age data in the literature along the south china indochina suture zone and recognized a significant late triassic magmatic flare up in combination with previous studies on the late triassic hp uhp metamorphic rocks we proposed that during the late triassic the south china indochina suture zone had transformed into post collisional setting and accompanied slab break off triggered the oib type asthenospheric mantle upwelling and provided the heat source for the late triassic magmatic flare up

reappraisal of the sequence boundary in time and space case and considerations for an su subaerial unconformity that is not a sediment bypass surface a time barrier or an unconformity abstract the sequence bounding unconformity bears the key defining traits of being a surface separating younger from older strata along which there is evidence of subaerial erosional truncation or subaerial exposure with a significant hiatus indicated van wagoner et al 1988 this subaerial component of sequence boundaries subaerial unconformity su is also broadly considered to form as a topographic surface of sediment bypass carved during relative sea level fall and buried by backfilling during relative sea level rise accordingly the su is commonly presumed to record an approximate time barrier which separates older from younger strata along its full length in this paper we show that regional composite scour rcs surfaces that are traditionally mapped as an integral component of the su were never a single subaerial topographic surface characterized by sediment bypass are not unconformities do not record an effective time barrier and form diachronously at the channel belt scale over the entire fall to rise of a base level cycle these rcs surfaces and by inference the su surfaces they comprise thus do not fully fit key defining characteristics embodied in the conceptual sequence boundary flume observations and field data show that the rcs is buried by fluvial sediment simultaneously as it is scoured accordingly the rcs is perennially covered with stored sediment during formation is only exposed as a subaerial topographic surface at the local place and time where it is undergoing active growth and forms over the duration of local marine drainage during a relative sea level cycle this cut and cover model differs greatly from more established bypass models which assume that the rcs was roughly sediment free and subaerially exposed for long durations of incision during regression and thus preserves a significant depositional hiatus upon later burial instead the rcs may commonly and locally record a hiatus more typical of a facies bounding diastem without a lacuna significantly greater than that of surfaces within the strata it binds fragments of fluvial strata may commonly and sporadically be preserved above the rcs that are older than underlying marine units overrun by this surface consequently the rcs is not an effective time barrier lateral planation by migrating and avulsing channels as the rcs expands laterally after nucleation can place younger fluvial strata over much older units which means that this surface is also composite and highly diachronous laterally at the scale of channel belts the cut and cover model has additional implications not captured by the bypass model first significant sediments may be stored within fluvial strata above the rcs during regression that are not available for contemporary falling stage and lowstand marine shorelines this can result in marine sediment starvation particularly of the sand fraction and in extreme cases can result in sand autodetachment and an absence of regressive marine reservoir sandstones second cutting of the rcs co generates a suprafluvial surface above the covering fluvial strata during regression that may be used as a mappable proxy for the conceptual maximum regressive surface mrs the mrs may be raised above this surface locally by low accommodation aggradation during lowstand normal regression but in either case preserves an approximate time line where not reworked during later transgression third valley development across the rcs does not exclusively form by landward knickpoint growth and may include complexly formed and potentially cross cutting buffer valleys su valley incision can be divided into four modes which include denudation structural buttress and buffer valley components which may work together locally and tend to have variable importance along the shore to hinterland profile although the rcs is not a good rock proxy for the conceptual sequence boundary it remains a very mappable surface which may separate facies of potentially very different origin and reservoir quality the rcs is also inseparable from the su and typically the only terrestrial erosional surface of extent in most short duration sequences its nullification as a sequence boundary would mean abandonment of depositional sequence stratigraphy as a correlation and interpretive tool within these sections an alternative to abandonment of the su as a sequence boundary is to loosen the definition of a sequence boundary to a discrete surface of erosional truncation carved landward of contemporary shorelines that is traceable beyond the scale of a single valley or comparable local system and its correlative surfaces of conformity and or non deposition and continue its use as before

onset timing of left lateral movement along the ailao shan red river shear zone 40ar 39ar dating constraint from the nam dinh area northeastern vietnam left lateral motion along the ailao shan red river asrr shear zone has been widely advocated to be the result of the collision between the indian and eurasian plates and to account for sea floor spreading in the south china sea our new 40 ar 39 ar data on the south easternmost outcrop of the day nui con voi metamorphic massif northern vietnam suggest that the exhumation of metamorphic massif by shearing along the asrr zone began 027 ma and lasted until 022 ma a perfect correlation between location and cooling path for the samples along the shear zone suggests that the transtensional deformation may have propagated northwestward at a rate of 0 6c m y 1 such a good correlation also indicates that the onset of the leftlateral movement of the shear zone may have occurred later than 027 5 ma this conclusion is consistent with our previous interpretation that collision induced southeastward extrusion of indochina along the asrr shear zone postdates the opening of the south china sea and that extrusion tectonics in se china may not be responsible for the opening of the south china sea 2000 elsevier science ltd all rights reserved

on the emergence of life via catalytic iron sulphide membranes we propose that the precipitation of a gelatinous iron sulphide membrane is the necessary first step towards life membrane vesicles were inflated with alkaline sulphide bearing hydrothermal 200 c solution and grew on a submarine sulphide mound in acid iron bearing hadean seawater once a critical size had been reached 0 1 1 mm vesicles would have budded contiguous self similar daughters we assume that the membrane was rendered insulating by the adsorption and or oxidative precipitation of organic and organosulphur compounds as a consequence of the naturally induced proton motive chemiosmotic force and the activity of the iron monosulphide redox catalysts within the membrane organic compounds would have accumulated within the vesicle osmotically driven growth therefore became more significant with time the geochemical environment envisaged as responsible for this first step towards life is consistent with that widely accepted for the early earth

an early miocene transition in deformation regime within the red river fault zone yunnan and its significance for indo asian tectonics recent work has tended to support the view that lateral extrusion of indochina along the red river fault zone was a significant factor in accommodating the convergence of india with asia to better understand the nature and timing of this feature 40ar 39 ar age spectrum analyses were undertaken on minerals from the ailao shan red river metamorphic belt in southern yunnan china interpreted using diffusion domain theory these results yield thermal histories in the temperature range 550 150 c discontinuities in cooling rates observed from these results suggest a transition in deformation style in the shear zone during the early miocene at least in the vicinity of our samples left lateral strike slip ductile deformation appears to have ceased by about 20 ma the mylonitic gneisses in the shear zone were subsequently obliquely unroofed probably due to a component of normal faulting along the eastern edge of the shear zone causing a brief locally 100 c m y cooling episode this transition may have resulted from a component of northeast southwest extension attributable to greater clockwise rotation of indochina relative to south china after 20 ma than that consistent with pure strike slip along the fault k ar ages on clay minerals from gouge within the normal faults bounding the ailao shan and diancang shan yield a range of ages between 20 and 185 ma but show a significant clustering at 20 25 ma these results suggest that early miocene movement on a precursor to the range front normal fault in the ailao shan caused oblique unroofing of the shear zone this early miocene event corresponds in time with a transition from low to very high denudation rates throughout the himalaya and southern tibet this coincidence may reflect that once left lateral strike slip motion on the red river fault zone slowed or was terminated accommodation of much of the continued northward convergence of india occurred on the main central thrust resulting in rapid uplift in the tethyan himalaya and gangdese belt

intraplate tectonics in asia a precise age for large scale miocene movement along the ailao shan red river shear zone china tertiary left lateral movement along the 1000 km long ailao shan red river shear zone appears to have played an important role in absorbing post collisional northward penetration of india into asia crustal strike slip shear along this zone caused the formation of a gneiss belt metamorphosed to amphibolite grade including anatectic melting both metamorphism and melting were induced by ductile deformation yielding the possibility to date the major tectonometamorphic event that shaped the ailao shan red river belt 17 u pb isotope analyses were performed on small size fractions of zircon monazite and xenotime extracted from two different leucogranitic layers the two samples are located about 50 km apart in the central segment of ailao shan in structurally well controlled settings where the melts crystallized within the strongly foliated gneisses during late stages of deformation all mineral u pb analyses had to be corrected for excess or deficit amounts of radiogenic 206 pb originating from initial 230 th disequilibrium in the 238 u decay series in monazite such disequilibrium 206 pb reaches 20 of total radiogenic 206 pb the corrected u pb ages of monazite and xenotime lie between 22 1 and 23 9 ma whereas zircon yields significantly discordant u pb ages between 30 5 and 33 9 ma pointing to precambrian material in the magma source region inherited components could also be detected in monazite the set of u pb data shows that monazite and xenotime formed simultaneously in both localities substantiating an early miocene age of 23 0 0 2ma for late kinematic crystallization of anatectic melts in the metamorphic belt of the ailao shan red river shear zone

duration of strike slip movements in large shear zones the red river belt china abstract sixty four size fractions of zircon monazite xenotime and titanite were dated by the u pb method to temporally and spatially constrain left lateral movements in the red river shear zone between the depths of 15 and 20 km the shearing led to the formation of the high grade metamorphic gneisses that form the bulk of the 4200 m high diancang shan and 3100 m high ailao shan mountain ranges the omnipresence of melt layers and pockets in these gneisses shows that temperatures associated with tectonometamorphism lie at and above the solidus of continental crust and deformation patterns are very homogeneous over the entire length of the belt north of the gneisses subvolcanic bodies intrude unmetamorphosed cover rocks in particular eocene red beds along the projected trend of the red river shear zone for a leucogranitic layer in the diancang shan gneisses an age of 24 2 0 2 ma 2 is obtained for monazite and 22 4 0 2 ma is obtained for xenotime whereas zircon formed through two growth stages at 24 23 ma these ages record solidification of the anatectic melts over a period of at least 1 8 0 4 ma during which left lateral shear was continuously active in the ailao shan gneisses zircon and titanite ages are 26 3 0 3 and 26 1 0 2 ma for two monzonitic bodies and 24 1 0 2 and 22 4 0 2 ma for two pegmatitic layers that occur within the monzonitic intrusions these results show that zircon monazite titanite and probably also xenotime are closed systems for u and pb under melting conditions 650 c therefore the ages date mineral formation in the melt rather than cooling below certain closure temperatures a significantly older age of 35 0 0 1 ma is obtained for a monzonitic subvolcanic intrusion north of the diancang shan gneisses because these intrusions follow the trend of the shear zone they most likely represent the surface expression of early activity along that zone indicating that strike slip movement had already occurred in latest eocene time the total set of u pb ages shows that magmatic and metamorphic activity occurred from 35 to 20 ma strengthening the idea that the red river shear zone absorbed most of the intraplate movements required to compensate the opening of the south china sea between 32 and 16 ma the existence of monzonitic syenitic intrusions along the red river zone implies that melting affected source rocks other than the diancang shan and ailao shan gneiss protoliths potential sources to produce such rocks lie in the deep crust possibly including melts from the mantle consequently the red river shear zone must be a deep rooted structure along which melts migrated to the surface moreover the contemporaneity of shear motion and magma generation suggests that melting occurred in direct relation to strike slip activity

40ar 39ar single grain dating and mineral chemistry of hornblendes south of the red river shear zone vietnam new evidence for early proterozoic tectonothermal event abstract the pre mesozoic metamorphic belt runs parallel to the day nui con voi red river shear zone in vietnam to the south the belt is mainly composed of hornblende gneisses amphibolite lenses and mica schists five hornblendes from a gneiss and an amphibolite were analyzed chemically and chronologically by electron probe micro analysis epma and 40ar 39ar methods epma analyses show that hornblendes in the gneiss and the amphibolite have significant amount of edenite component and similar average composition however the recalculated fe3 content is significantly heterogeneous in a thin section while total fe is nearly the same among the analyses the rim of each crystal is higher in fe3 fe3 fe2 than the core these chemical and petrological features suggest that the hornblendes have suffered significant oxidation in particular largely in the gneiss 40ar 39ar analyses showed that the gneiss has a significant variation of plateau ages 2089 14 1977 19 and 1873 13 ma among three hornblende grains whereas the amphibolite gives the same plateau ages 2056 14 and 2044 21 ma for two grains all grains of both samples have excess ages in the first few fractions at low temperatures and partial loss ages between the excess and plateau spectra the ca k ratios indicate some disturbed phases for the lower temperature spectra but the partial loss ages are also derived from hornblende phase these facts suggest that hornblende in the gneiss has experienced partial argon loss by oxidation and or thermally activated argon diffusion process however the gneiss and the associated amphibolite have preserved the early proterozoic tectono metamorphic event in the hornblende crystals except for their rims giving new evidence for the early proterozoic event within the pre mesozoic metamorphic belt northern vietnam south of the red river shear zone in indochina

abiotic methane on earth 1 over the last 30 years geochemical research has demonstrated that abiotic methane ch4 formed by chemical reactions which do not directly involve organic matter occurs on earth in several specific geologic environments it can be produced by either high temperature magmatic processes in volcanic and geothermal areas or via low temperature 100 c gas water rock reactions in continental settings even at shallow depths the isotopic composition of c and h is a first step in distinguishing abiotic from biotic including either microbial or thermogenic ch4 herein we demonstrate that integrated geochemical diagnostic techniques based on molecular composition of associated gases noble gas isotopes mixing models and a detailed knowledge of the geologic and hydrogeologic context are necessary to confirm the occurrence of abiotic ch4 in natural gases which are frequently mixtures of multiple sources although it has been traditionally assumed that abiotic ch4 is mainly related to mantle derived or magmatic processes a new generation of data is showing that low temperature synthesis related to gas water rock reactions is more common than previously thought this paper reviews the major sources of abiotic ch4 and the primary approaches for differentiating abiotic from biotic ch4 including novel potential tools such as clumped isotope geochemistry a diagnostic approach for differentiation is proposed

eustatic control on alluvial sequence stratigraphy a possible example from the cretaceous tertiary transition of the tornillo basin big bend national park west texas u s a paleosol bearing alluvial strata of latest cretaceous and earliest tertiary age are continuously exposed along dawson creek in big bend national park west texas u s a and exhibit a three tier hierarchy of depositional cyclicity meter scale fluvial aggradational cycles facs occur as fining upward successions that are gradation ally overlain by paleosols or are sharply overlain by the coarser grained base of the succeeding fac without an intervening paleosol facs stack into decameter scale fluvial aggradational cycle sets fac sets that also fine upward and from base to top contain either a grad ual upsection increase in soil maturity and soil drainage or a somewhat symmetrical pattern of increasing and decreasing paleosol maturity longer period trends of fac thickness lithologic proportions paleosol maturity and paleosol drainage indicate that two complete and two partial hectometer scale fluvial sequences occur within the study in terval from base to top each sequence is characterized by an asym metric increase and decrease in fac thickness a decrease in the pro portion of sand prone fluvial facies an increase in paleosol maturity and better paleosol drainage whereas facs and fac sets are interpreted to record cyclic epi sodes of channel avulsion and stability and longer term avulsive chan nel drift within the alluvial valley respectively fluvial sequences may coincide with third order sea level changes within the north american western interior seaway as such the cretaceous tertiary k t transition within the tornillo basin may provide an example of me gascale stratigraphic cyclicity that is controlled by eustatic sea level within a fully fluvial succession thickening and thinning successions of facs record a third order period of accelerating transgressive equivalent and decelerating highstand equivalent base level rise and subsequent base level fall falling stage to lowstand equivalent se quence boundaries are placed at the sharp inflection between thinning and thickening facs sand prone facies and immature more poorly drained paleosols are associated with the transgressive equivalent por tion of each sequence and mudrock dominated overbank facies and their associated mature well drained paleosols are associated with the highstand and falling stage equivalent

growth of self assembling tubular structures of magnesium oxy hydroxide and silicate related with seafloor hydrothermal systems driven by serpentinization

two new geobarometers for garnet amphibolites with applications to southeastern vermont two new geobarometers for the assemblage garnet hornblende plagioclase quartz have been calibrated on the basis of the equilibrium 6 anorthite 3 tremolite 2 grossular 1 pyrope 3 tschermakite 6 quartz and its fe end member equivalent data representing 37 natural samples that equilibrated at conditions of 2 5 to 13 kbar and 500 to 800 c were fitted to the general equation rt in keq a bt p i c by using a weighted least squares method multiple correlation coefficients are high 0 98 and 0 97 lines of constant keqhave extremely shallow slopes in p t space 2 to 8 bars per degree suggesting that pressures may be deduced precisely even where temperatures are only poorly constrained typical analytical errors and temperature imprecisions propagate to very small errors in pressure about 500 bars caution is advised in applying the barometers outside the range of calibrant mineral compositions application of the calibrations to samples from southeastern vermont near the strafford chester and athens domes documents relatively high pressure metamorphism 7 to 10 kbar for these structures

a review and technical summary of the aapg hedberg research conference variations in fluvial deltaic and coastal reservoirs deposited in tropical environments this conference addressed the question of whether fluvial deltaic and coastal reservoirs deposited in tropical environments exhibit different facies relationships than those of temperate systems although this conference concluded that there is nothing distinctly unique about clastic depositional systems in tropical environments the magnitude of the processes that operate in these environments differs from nontropical environments because these processes influence the distribution of reservoir and nonreservoir facies geologists need a fundamental understanding of the contrasts between tropical and nontropical environments when interpreting subsurface data and building depositional models we highlight the following features that seem to be more favorably developed in tropical environments 1 many tropical systems exhibit complexly stacked channel and overbank deposits although some fluvial deposits have the classic meandering pattern seismic attributes indicate that many tropical channel belts are anastomosing systems that intersect each other at large angles subsurface correlation without the benefit of three dimensional seismic data is all but impossible 2 in contrast to deltaic deposits in temperate climates which exhibit high sand shale ratios near the delta with sand content decreasing basinward and laterally away from the delta tropical deltas tend to be muddier near the river mouth and have cleaner well sorted sand bodies better developed laterally in the shoreface and shallow marine environments 3 modern tropical coasts are home to large estuarine environments dominated by mangroves estuarine deposits interpreted in the subsurface and observed in outcrop are sand rich yet little sand is observed in modern mangrove estuaries mangrove estuarine deposits also contain abundant organic rich layers thin coals and dispersed organic matter that may favor the development of a self sourced petroleum system the flushing of these systems during relative low stands may be an important mechanism not only for reservoir development but also for dispersing organic matter into deep water systems this conference was dominated by discussions of depositional systems in southeast asia which have a strong tectonic overprint further research on tropical systems from the south american and african conjugate margin could provide important tests for the ideas highlighted above

sequence architecture within a low accommodation setting an example from the permian of the galilee and bowen basins queensland australia the coal bearing upper permian betts creek beds bcb of the northeastern intracratonic galilee basin in north central queensland australia record sedimentation within an alluvial coastal plain in a low accommodation basin margin setting six third order depositional sequences are preserved within the bcb and are characterized by a sequence architecture that is overall relatively thin 15 m 49 ft condensed and top truncated sequences have a lowstand systems tract composed of extensive braided fluvial sheet sandstones a transgressive systems tract characterized by tidally influenced fluvial sheet sandstones and bioturbated estuarine mudrocks and a highstand systems tract composed of coastal plain mudrocks and coals sequences can be correlated throughout the entire galilee basin and related to the lithostratigraphy established in the adjacent bowen basin a foreland basin to the east the overall late permian fill is considered a second order sequence the stacking patterns show an overall retrogradational to progradational architecture suggestive of relatively low accommodation space sequences show an extremely laterally continuous geometry 1000 km 600 mi and are relatively uniform in thickness sequences do not show evidence of onlap toplap or erosional truncation but instead are thin in areas of low accommodation and expand into areas of higher accommodation that these third order sequences can be traced across basins with differing tectonic regimes together with the persistent sheetlike nature of the sequences suggests that tectonic effects on the sequence architecture were minimal the basin fill covers a time interval during which two glacial events occurred in the final stages of the late paleozoic ice age suggesting that changes in glacioeustasy may have been the dominant controls on the sequence architecture of the late permian succession in the galilee basin of northeastern australia

a petrogenetic grid for pelitic schists in the system sio 2 al 2 o 3 feo mgo k 2 o h 2 o a quantitative petrogenetic grid for pelitic schists in the system kfmash that includes the phases garnet chlorite biotite chloritoid cordierite staurolite talc kyanite andalusite sillimanite and pyrophyllite with quartz h2o and muscovite or k feldspar in excess is presented the grid is based on thermodynamic data of berman et al 1985 and berman 1988 for endmember kfash and kmash equilibria and natural fe mg partitioning for the kfmash system calculation of p t slopes and the change in fe fe mg along reactions in the kfmash system were made using the gibbs method in addition the effect on the grid of mno and cao is evaluated quantitatively the resulting grid is consistent with typical buchan and barrovian parageneses at medium to high grades at low grades the grid predicts an extensive stability field for the paragenesis chloritoid biotite which arises because of the unusual facing of the reaction chloritoid biotite quartz h2o garnet chlorite muscovite which proceeds to the right with increasing t in the kfmash system however the reaction proceeds to the left with increasing t in the mnkfash system so the assemblage chloritoid biotite is restricted to bulk compositions with high fe fe mg mn typical metapelites will therefore contain garnet chlorite at low grades rather than chloritoid biotite

p t t paths and post metamorphic exhumation of the day nui con voi shear zone in vietnam abstract the day nui con voi belt in vietnam is the southeasternmost part of the red river shear zone in asia it is a narrow high grade metamorphic core complex consisting of garnet sillimanite biotite gneisses mylonite bands amphibolite layers and migmatites geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite facies conditions of 690 60 30 c and 0 65 0 15 gpa and the subsequent mylonitization occurred under greenschist facies conditions of 480 c and under 0 3 gpa fifteen synkinematic hornblende and biotite separates from gneisses amphibolites and mylonites were dated with the k ar method hornblende separates from the day nui con voi give k ar ages of 26 4 28 5 ma and the biotite separates do give 24 5 24 7 ma combination of thermobarometric and geochronological data yields the cooling history of 500 c at 28 ma and 300 c at 24 ma with a cooling rate of 70 110 c ma 1 and 23 km post metamorphic exhumation of the core complex the first 16 km exhumation from the peak of metamorphism at probably 31 ma to 28 ma was triggered by the left lateral strike slip displacement of the red river shear zone

muscovite k ar ages of the sanbagawa schists japan and argon depletion during cooling and deformation seventy muscovites from schists in the sanbagawa terrain in central shikoku were dated by the k ar method the muscovite ages are consistently older with increasing metamorphic grade within the same zone the ages are significantly younger in schists which have been more severely deformed these k ar age variations could be due to systematic argon depletion during deformation i e to the dynamic recrystallization of muscovites during ductile deformation that formed a large scale recumbent fold during the uplift and cooling argon loss was greater in schists that were more extensively deformed and in the lower grade zone that experienced a longer period of low temperature deformation than the higher grade zone the relationships between age and grain size in a pelitic schist suggest that coarse grained muscovites lost more argon than the finegrained ones there was no significant resetting of ages in the vicinity of major strike slip faults such as the median tecotonic line or near thrust faults the combination of geochronological and geological data constrains the cooling rate of the sanbagawa schists to 9 12 c ma in the oligoclase biotite zone in central shikoku japan

thermochronologic constraints on the tectonic evolution of active metamorphic core complexes d entrecasteaux islands papua new guinea metamorphic core complexes in the d entrecasteaux islands papua new guinea formed as the result of active extension at the western end of the propagating woodlark basin spreading center domes of high grade metamorphic rocks i e amphibolites eclogites and migmatites intruded by large granodiorite bodies comprise the lower plate of the d entrecasteaux metamorphic core complexes the domes are transected by kilometer scale shear zones a thermochronologic study of the d entrecasteaux islands utilizing k ar 40ar 39ar and fission track techniques has documented the unroofing history of these active metamorphic core complexes gneisses in the cores of the domes cooled rapidly 100 c m y as indicated by hornblende and biotite 40ar 39ar apparent ages of 2 7 to 3 0 ma and 1 6 to 1 7 ma respectively and apatite fission track ages of 0 4 to 0 9 ma 40ar 39ar apparent ages on white mica biotite and potassium feldspar and fission track ages on apatites from shear zone gneisses indicate extremely rapid cooling in some cases 500 c m y and suggest shear zones were active from 4 0 to 3 5 ma and 1 9 to 1 4 ma in general 40ar 39ar mineral ages for retrogressed core zone gneisses shear zone gneisses and granodiorites are 2 0 to 3 0 ma amphiboles 1 5 to 1 7 ma muscovites and 1 4 to 1 8 ma biotites and 1 0 to 2 0 ma k feldspars apatite fission track ages from core zone gneisses shear zone gneisses and granodiorites range from 0 4 to 1 0 ma thermochronologic results indicate that emplacement of granodiorites closely coincided with retrogression of the metamorphic basement and movement on the outer shear zones bounding the gneiss domes the granodiorite bodies associated with the d entrecasteaux islands domes represent syn kinematically emplaced granitoids intruded into an area of active continental extension

triassic high strain shear zones in hainan island south china and their implications on the amalgamation of the indochina and south china blocks kinematic and 40ar 39ar geochronological constraints abstract a kinematic and geochronological study has been carried out on the triassic high strain shear zones in hainan island the southern south china block there are wnw and ne trending high strain shear zones with greenschist to amphibolite facies metamorphism in this island kinematic indicators suggest a dextral top to the nne thrust shearing for the wnw trending high strain shear zones and a sinistral top to the se thrust shearing for the ne trending shear zones the quartz c axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system the timing of shearing for these shear zones has been constrained by the 40ar 39ar dating analyses of synkinematic minerals middle triassic 242 250 ma and late triassic early jurassic 190 230 ma have been identified for the wnw and ne trending shear zones respectively a synthesis of these kinematic and thermogeochronological data points to a two stage tectonic model for hainan island that is top to the nne oblique thrusting at 240 250 ma followed by top to the se oblique thrusting at 190 230 ma in combination with the available data from the southern south china and indochina blocks it is inferred that south hainan and north hainan have affinity to the indochina and south china blocks respectively the tectonic boundary between south hainan and north hainan lies roughly along the wnw trending changjiang qionghai tectonic zone probably linking to the song ma and ailaoshan zones the middle triassic structural pattern of hainan island is spatially and temporally compatible with those of the south china and indochina blocks and thus might be a derivation from the amalgamation of the indochina with south china blocks in response to the closure of the paleotethys ocean and subsequent subduction collision

effects of progressive mylonitization on ar retention in biotites from the santa rosa mylonite zone california and thermochronologic implications the santa rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the peninsular ranges batholith a wide variation in k ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation induced reduction in grain size we suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes previous workers have shown that deformation in the santa rosa mylonite zone involved the formation of mylonites and an imbricate series of low angle faults which divide the area into structural units field work petrographic studies and tem analysis of deformation mechanisms in biotite show that the granodiorite mylonite the lowermost structural unit formed below the granodiorite solidus the granodiorite mylonite varies from protomylonite to ultramylonite with regions of high strain distributed heterogeneously within the zone previously reported biotite and hornblende k ar dates from the granodiorite protolith below 82 89 ma and the asbestos mountain granodiorite above 61 68 ma the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units other workers fission track dates on sphene zircon and apatite from the granodiorite mylonite and the asbestos mountain granodiorite suggest thermal homogenization and rapid cooling to 100 c by ca 60 ma within and adjacent to the mylonite zone k ar dates on 5 samples of biotite from variably deformed granodiorite range from 62 76 ma dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction 40ar 39ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum 40ar 39ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39ar recoil loss k analysis of samples allowing k ar dates to be calculated is therefore recommended as an adjunct to 40ar 39ar step heating analysis in rocks that have experienced similar deformation during mylonitization biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0 05 m locally within porphyroclasts as large as 1 mm because biotite compositions are relatively uniform fe fe mg mn ti alvi 0 47 0 52 and show no systematic variation with grain size compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in ar retention ar closure temperatures calculated with appropriate diffusion parameters for the observed grain sizes are in the range 220 280 c and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the asbestos mountain granodiorite above the mylonite zone changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330 c 80 ma above the closure temperature for thermally activated diffusion of argon in biotite and continued to a minimum of ca 220 260 c 62 ma

discussion on the role of the red river shear zone yunnan and vietnam in the continental extrusion of se asia journal vol 163 2006 1025 1036 p h leloup p tapponnier r lacassin write in his recent paper searle 2006 acknowledges that the 1000 km long ailao shan red river shear zone is a large miocene left lateral shear zone but speculates that left lateral slip started after 21 ma and claims that the total finite offset remains unknown from this he concludes that continental extrusion was only a relatively minor tectonic factor during the india asia collision as long argued by other workers e g england houseman 1986 cobbold davy 1988 dewey et al 1989 houseman england 1993 we summarize below the field and geochronological evidence that makes us maintain a viewpoint in better accordance with facts timing of left lateral shear along the ailao shan red river shear zone the ailao shan red river shear zone is composed mostly of high grade metamorphic rocks and deformed granitoids with ubiquitous evidence for left lateral shear parallel to the belt e g tapponnier et al 1986 1990 leloup et al 1993 1995 2001 the crystallization of the granitoids has been dated between 22 and 35 ma fig 1d f for example by scharer et al 1990 1994 and zhang scharer 1999 leading those workers to propose that left lateral shear started at least c 35 ma ago in contrast searle 2006 claims that all the deformed granitoids found within the shear zone predate left lateral shear and that their crystallization ages should thus be interpreted to provide an upper limit for the onset of deformation rather than a lower limit thus suggesting a maximum age of 21 ma for this deformation a clear understanding of the p t history and in situ deformation history of the intrusions as well as of their relationships with surrounding paragneisses is fundamental for interpreting correctly the geochronological

possible climate variability in the lower old red sandstone conigar pit sandstone member early devonian south wales uk abstract the early devonian conigar pit sandstone member cpsm of the freshwater west formation lower old red sandstone lochkovian age at freshwater west in south wales comprises a heterolithic predominantly alluvial suite mudstones fine to coarse grained sandstones conglomerates with varying depositional architecture a number of discrete facies associations are observed the fine grained inclined and non inclined heterolithic association is dominant within the cpsm it represents dryland sinuous channelized flow ihs bedsets and unconfined flow non inclined bedsets at terminal and intermediate floodouts deposited under semi arid conditions mudstones were deposited in two distinct environments associated with this semi arid river system laminated and burrowed reddish brown mudstones were deposited in shallow semi permanent lakes or pools on the floodplain massive mudstones were deposited as within channel muddy braid bars with sedimentary structures being lost during subsequent compaction the mudstones exhibit vertisol pedogenesis reflecting the seasonal wetting and drying associated with the semi arid climate periods of intense desiccation are indicated by deep desiccation cracks and associated rubbly surfaces possible though unproven tidal conditions influenced a small percentage of the heterolithic channels at freshwater west the extent of these channels is uncertain the coarser grained multi storey sandstone association was deposited by low sinuosity rivers with a fluctuating but perennial discharge associated with these laterally extensive sandbodies was a high water table with surface ponding wetlands that enabled the preservation of plant detritus during high discharge events flow expanded over the contemporaneous floodplain depositing the sheet sandstone association within splay complexes the interpretation that perennial discharge may have been triggered by basin wide climate change challenges the long held view that the lower ors climate was continuously semi arid in nature the spatial temporal extent of this climate change is uncertain as it is possible that the multi storey sandbodies were deposited by allogenic rivers draining a distant possibly uplifted source area a decrease in the observed frequency and maturity of vertisol profiles and a corresponding increase in multi storey sandbodies upward through the succession may reflect a long term episodic pattern of climate change

fluvial sequence stratigraphy the wapiti formation west central alberta canada abstract outcrop exposures and high resolution subcrop data allow the description and mapping of four stratigraphic discontinuities in the fully nonmarine strata of the campanian maastrichtian wapiti formation western interior foreland basin alberta canada this framework defines four unconformity bounded depositional sequences a d based on sequence stratigraphic methodology well log patterns and facies analysis in ascending order sequence a records the transition from the underlying marine facies of the puskwaskau formation into the wapiti fluvial facies and consists of strongly progradational and aggradational stacking patterns sequences b and c show a similar pattern of basal amalgamated channel fill deposits that grade upwards into floodplain dominated strata finally sequence d is dominated by channelized sediments and extensive overbank facies an aggradational stacking pattern suggests deposition under high accommodation conditions maximum flooding surfaces are interpreted within fine grained deposits in the upper portions of sequences c and d they are tied to regionally extensive coals that accumulated more than 250 km away from the coeval shoreline this study provides new evidence that major coal seams may represent the extension of marine maximum flooding surfaces into the downstream controlled fluvial realm furthermore the new stratigraphic framework of subaerial unconformities and maximum flooding surfaces provides the means of subdividing the previously undifferentiated fully nonmarine wapiti formation into depositional sequences and component systems tracts

geological development of the central and south vietnamese margin implications for the establishment of the south china sea indochinese escape tectonics and cenozoic volcanism abstract the vietnamese margin forms a key region to the understanding of escape tectonics and the development of the south china sea scs the existing geological reconstructions of the region are restricted to studies of single basins are based on limited amounts of geophysical data or analysis of onshore geological features these models are critically assessed on the basis of interpretation of the most comprehensive 2 d digital seismic database published to date within the area combined with a thorough analysis of existing literature and a new model is presented the vietnamese margin is underlain by a series of paleogene rift basins established through southeastward extrusion of indochina the east vietnam boundary fault evbf forms the almost 1000 km long seaward continuation of the left lateral ailao shan red river shear zone asrrsz toward the southern half of the phu khanh basin the evbf breaks up into discrete segments and splays into the se directed tua hoa fault zone paleogene faults splayed from the evbf and the mae ping shear zone and accommodated the coeval motion of these two major left lateral structural lineaments during the late oligocene basin inversions offshore occurred contemporaneously with initial right lateral inversion along the mae ping shear zone and the onset of major uplift of the metamorphic core complexes along the asrrsz it is suggested that a dramatic change of the regional stress pattern occurred in response to the northward movement of india and the effective coupling of the west burma block and india the later resulting in broadening of the indenting continental mass after the mid oligocene left lateral movements across the offshore evbf decreased and eventually ceased later onshore sinistral movements were accommodated by internal shortening and local clockwise block rotations within the shan thai terrain renewed rifting offshore south vietnam resulted from the jump of the scs spreading axis and subsequent neogene southwestward propagation of continental break up the neogene opening of the scs is viewed as a consequence of a slab pull associated with subduction of the proto scs underneath borneo as offshore neogene escape tectonism had virtually ceased during this phase of rifting right lateral transtension is inferred along n to nw oriented fault zones in the phu khanh and the nam con son basins rifting and by inference seafloor spreading continued until the end of middle miocene time although at significantly reduced rates during the final 5 10 ma termination of seafloor spreading is marked by a distinct latest middle miocene unconformity in the nam con son and the southern phu khanh basins neogene volcanism resulted in the build up of several magmatic structures across the central and south vietnamese margin and induced reactivation of older faults in the phu khanh basin the offshore magmatism heralded widespread volcanism onshore southern indochina associated with a latest neogene uplift and denudation event the uplift led to significantly increased siliciclastic accumulation rates offshore which repressed widespread carbonate deposition

inorganic chemistry gas compositions and dissolved organic carbon in fluids from sedimented young basaltic crust on the juan de fuca ridge flanks abstract the permeable upper oceanic basement serves as a plausible habitat for a variety of microbial communities there is growing evidence suggesting a substantial subseafloor biosphere here new time series data are presented on key inorganic species methane hydrogen and dissolved organic carbon doc in ridge flank fluids obtained from subseafloor observatory corks circulation obviation retrofit kits at integrated ocean drilling program iodp boreholes 1301a and 1026b these data show that the new sampling methods cowen et al 2012 employed at 1301a result in lower contamination than earlier studies furthermore sample collection methods permitted most chemical analyses to be performed from aliquots of single large volume samples thereby allowing more direct comparison of the data the low phosphate concentrations 0 06 0 2 m suggest that relative to carbon and nitrogen phosphorus could be a limiting nutrient in the basement biosphere coexisting sulfate 17 18 mm hydrogen sulfide 0 1 m hydrogen 0 3 0 7 m and methane 1 5 2 m indicates that the basement aquifer at 1301a either draws fluids from multiple flow paths with different redox histories or is a complex environment that is not thermodynamically controlled and may allow co occurring metabolic pathways including sulfate reduction and methanogenesis the low doc concentrations 11 18 m confirm that ridge flank basement is a net doc sink and ultimately a net carbon sink based on the net amounts of doc oxygen nitrate and sulfate removed 30 m 80 m 40 m and 10 mm respectively from entrained bottom seawater organic carbon may be aerobically or anaerobically oxidized in biotic and or abiotic processes

sedimentary architecture during carboniferous rift initiation the arid billefjorden trough svalbard analysis of the sedimentary architecture of the early billefjorden trough an exceptionally well exposed carboniferous half graben located in svalbard arctic norway allows detailed reconstruction of fault development and basin geometry sedimentary facies distribution reveals facies variability in an early synrift fill when an array of meso scale intra basinal faults segmented the basin into rotated blocks with differential subsidence growth of these faults into larger systems impacted the distribution of accommodation space and the drainage pattern expressed in fluvial and deltaic sandstone dominated units an axially back stepping character of the early synrift facies associations stages 1 3 is reflected in aggrading homogenous facies types of continental depositional stage 1 which evolved into back stepping facies belts of deltaic to nearshore depositional environment stages 2 and 3 due to the opening of a connection to the sea the drainage pattern and accommodation distribution in the early synrift deposits reflect a relatively symmetrical basin that predates a later prominent half graben configuration of the basin supplementary material figures presenting lithostratigraphic columns of the well data supplementary material 1 location of logs in ragnardalen supplementary material 2 and geological map used in figure 2b supplementary material 3 are available at https doi org 10 6084 m9 figshare c 4293077

high temperature metamorphism in a major strike slip shear zone the ailao shan red river people s republic of china abstract petrographic and thermobarometric analysis provides constraints on thept path of the mylonitic gneisses of the left lateral ailao shan red river shear zone which has accommodated the lateral extrusion of indochina during the tertiary two different paragenesis p1 and p2 are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies microprobe analysis reveals that p1 garnets bear a chemical zonation from core to rim this zonation indicates a temperature increase during garnet growth conditions of formation of garnet rims p1b which are estimated using biotite garnet and plagioclase garnet thermobarometers are close to the granitic solidus 710 70 c and4 5 1 5 kbar p2 conditions are estimated to be approximately 500 c and

bounds on strain in large tertiary shear zones of se asia from boudinage restoration we have used surface balanced restoration of stretched boudinaged layers to estimate minimum amounts of finite strain in the mylonitic gneisses of the oligo miocene red river ailao shan shear zone yunnan china and of the wang chao shear zone thailand the layer parallel extension values thus obtained range between 250 and 870 we discuss how to use such extension values to place bounds on amounts of finite shear strain in these large crustal shear zones assuming simple shear these values imply minimum total and late shear strains of respectively 33 6 and 7 3 at several sites along the red river ailao shan shear zone for the wang chao shear zone a minimum shear strain of 7 4 is deduced assuming homogeneous shear would imply that minimum strike slip displacements along these two left lateral shear zones which have been interpreted to result from the india asia collision have been of the order of 330 60 km red river ailao shan and 35 20 km wang chao

processes controlling the distribution of boron and chlorine in natural waters in iceland abstract the concentrations of b and cl in natural waters in iceland lie in the range 0 001 10 and 1 20 000 ppm respectively lowest concentrations occur in surface and groundwaters in the central highlands highest cl concentrations are found in high temperature waters on the reykjanes peninsula and highest b concentrations in well waters from high temperature geothermal systems in the axial zones of the active volcanic belts the b and cl contents of icelandic basalts are 0 1 6 6 and 75 750 ppm respectively these large variations are considered to result from variable degassing during consolidation as well as concentration variations in the initial magma tholeftes are lowest in both elements av 1 2 ppm b and 170 ppm ci transitional basalts intermediate av 2 1 ppm b and 240 ppm ci and alkali basalts highest av 3 8 ppm b and 340 ppm cl cl b ratios in tholeiites are most often within 25 50 but somewhat lower in alkali basalts most geothermal waters contain b and cl within 0 05 1 and 10 100 ppm respectively these rather low values are attributed to the low content of b and cl in the basaltic rock the b and cl distribution in the icelandic natural waters indicate that these elements act as essentially incompatible their sources of supply include 1 the atmosphere seawater spray and aerosols 2 the rock with which the water interacts 3 seawater groundwater and 4 magma intrusions in surface water and nonthermal groundwater the dominant source of supply of cl is seawater spray and aerosols but the soil and rock contribute often significantly to the b in these waters their cl b ratio is similar to that of seawater 1330 or somewhat lower boron and cl concentrations generally increase with water temperature warm waters cl b ratios intermediate between those of seawater and basalt with increasing temperature this ratio decreases and gradually approaches that of the rock some high temperature geothermal waters have as low cl b ratios as 1 and b concentrations as high as 10 ppm the cause is either b degassing of the magmatic intrusive heat source or phase separation in the producing aquifer some low temperature waters

comment on onset timing of left lateral movement along the ailao shan red river shear zone 40ar 39ar dating constraint from the nam dinh area northeastern vietnam by wang et al 2000 journal of asian earth sciences 18 281 292

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

constraints on the left lateral shearing and crustal melting of the ailaoshan massif yunnan province southwest china abstract the ailaoshan red river shear zone asrr in se tibet accommodated about 20 convergence of the cenozoic india asia continental collision which resulted in complicated thermal history during paleocene to miocene the ailaoshan massif in the middle part of asrr is characterized by extensive mylonitization and migmatization but the exact timing of the large scale left lateral shearing and whether the crustal partial melting occurred prior to or coeval with the left lateral shearing remain controversial the detailed structural observation suggests the pre kinematic nature of the ailaoshan corundum with respect to the left lateral shearing la icp ms u pb dating of metamorphic zircon and monazite inclusions in the pre kinematic ailaoshan corundum obtained a weighted 206pb 238u ages of 28 0 0 5 ma mswd 3 3 n 27 and 28 3 0 3 ma mswd 1 8 n 18 respectively which gave an upper limit on the initiation of the left lateral shearing in the ailaoshan massif moreover the presence of protogenetic siliceous melt inclusions in the pre kinematic corundum indicates that the cenozoic crustal anatexis of the ailaoshan massif has occurred before the left lateral shearing along with available geochronological data the crustal partial melting of the ailaoshan massif was attributed to joint results which at least include the neoproterozoic convergence the triassic indosinian orogeny and the cenozoic evolution as a response to the india asia continental collision

complex fluvial response to low gradients at maximum regression a genetic link between smooth sequence boundary morphology and architecture of overlying sheet sandstone abstract the mesa rica sandstone is an extensive fluvial sandstone sheet that is separated from underlying marine strata throughout northeastern new mexico by a regionally smooth and laterally continuous sequence bounding unconformity the mesa rica sandstone consists mostly of channel fill elements with lesser proportions of lateral accretion elements and almost everywhere is of the same thickness as the largest channel fills this broad sandstone sheet reflects deposition on a coastal plain by frequent avulsion and minor lateral migration of straight and low sinuosity streams under nearly stable base level conditions which are here attributed to rapid progradation of the adjacent mesa rica deltaic shoreline relative to change in relative sea level during maximum kiowa skull creek regressi n rapid deltaic shoreline progradation under such conditions caused seaward extension and gradient reduction in the adjacent mesa rica coastal plain overextension and resultant loss of power in mesa rica rivers crossing this coastal plain forced both stream straightening and storage of nontransportable coarser sediment in coastal plain channels because base level was stable mesa rica channels could aggrade only minimally and were forced to spread excess sediments mostly laterally by avulsion frequent avulsion and minor lateral migration of mesa rica channels under these conditions caused both regional scouring of a smooth sequence bounding unconformity and deposition of a sheet sandstone with minimal channel stacking the mesa rica sandstone and the sequence boundary that underlies it represent a distinctive genetic association of single story fluvial sheet sandstone and smooth sequence boundary morphology that is probably not unique to the lower cretaceous of northeastern new mexico such unincised sequence boundaries are also implied in conceptual models and flume experiments of previous authors

coalesced delta front sheet like sandstone bodies from highly avulsive distributary channels the low accommodation mesa rica sandstone dakota group new mexico u s a