tectonothermal evolution of the mayuan assemblage in the cathaysia block implications for neoproterozoic collision related assembly of the south china craton the mayuan assemblage in the cathaysia block southeast china consists of felsic paragneiss pelitic schist greenschist amphibolite marble calcsilicate and quartzite that underwent three episodes of deformation d1 d3 and four episodes of metamorphism m1 m4 in the early neoproterozoic the m1 assemblage consists of mineral inclusions defining an early foliation s1 within porphyroblasts represented by chlorite muscovite biotite plagioclase quartz in pelitic schist and actinolite chlorite epidote albite quartz enclosed in amphibolite m2 coincides with the development of the regional schistosity s2 and represents the formation of the porphyroblasts and growth of matrix minerals resulting in development of prograde metamorphic zones chlorite biotite garnet staurolite and kyanite zones m3 is simultaneous with the third phase of deformation d3 and produced sillimanite bearing mineral assemblages in pelitic schist and hornblende bearing assemblages in amphibo lite the last metamorphic episode m4 gave rise to the retrogressive assemblage chlorite muscovite in pelitic rock and actinolite chlorite epidote in amphibolite the sequence of mineral assemblages and history of metamorphic reactions built from the petrogenetic grid of pelites suggest a near isothermal decompression clockwise p t path for the mayuan pelitic schists using the tweequ software program the garnet biotite thermometer and garnet muscovite biotite plagioclase barometer yield p t conditions for m1 of 5 5 to 6 0 kb and 450 to 500 c and conditions for the garnet staurolite and kyanite zones of m2 of 6 0 to 7 0 kb and 550 to 600 c 6 0 to 7 5 kb and 600 c and 11 0 to 11 5 kb and 600 c the p t conditions of m3 were estimated at 570 to 625 c and 4 0 to 4 5 kb using the muscovite biotite thermometer and hornblende plagioclase geothermo barometer the garnet chlorite thermometer yields temperatures of 300 to 400 c for m4 but the pressures of m4 cannot be quantitatively estimated because of the lack of a suitable geobarometer these p t estimates also define an near isothermal decompression clockwise p t path which is involved in initial crustal thickening followed by rapid exhumation and final cooling and is related to amalgamation of the cathaysia and yangtze blocks to form the south china craton

magmatic history of red sea rifting perspective from the central saudi arabian coastal plain a newly recognized tertiary dike complex and comagmatic volcanic rocks exposed on the central saudi arabian coastal plain record early stages of magmatism related to red sea rifting intrusive and stratigraphic relationships and new potassium argon dating indicate episodic magmatism from about 30 ma to the present additional stratigraphic and radiometric evidence suggests that limited rift related magmatism began as early as about 50 ma an early phase of crustal extension in the region was accompanied by faulting and graben formation and by dike swarm intrusion the style of extension and intrusion changed at 20 ma localized volcanism and sheeted dike injection ceased and were replaced by the intrusion of thick gabbro dikes this change may mark the onset of sea floor spreading in the central red sea the dikes and volcanic rocks consist of a bimodal mafic felsic suite with transitional subalkaline to alkaline chemistry although no unique petrogenetic model can be developed for the suite the following observations and conclusions have been drawn from the available reconnaissance geochemistry 1 strontium isotopic initial ratios overlap in the range 0 7031 to 0 7047 and are not clearly related to alkalinity or silica content the bimodality and alkalinity of the suite are therefore not products of contamination by the radiogenic precambrian upper crustal granitic rocks exposed in the region 2 several of the evolved rocks rhyolites and comendite show large to extreme negative europium anomalies and heavy rare earth element hree enrichment features that cannot be explained by low pressure crystal fractionation of potential parent magmas an intriguing possibility is that hree enrichments and large negative eu anomalies of some peraluminous rhyolites and granites may result from the complete melting of garnet from middle or lower crustal rocks 3 most of the basalts are light rare earth element lree enriched consistent with derivation from fertile mantle in a continental rift setting however interaction of the mafic melts with lower continental crust cannot be ruled out a model of poly baric mantle melt derivation producing several alkaline subalkaline cycles best explains magmatism in the red sea region differences in the depths and dynamics of mantle melt extraction and transport brought about through changes in crust and mantle structure as the rift and paar developed may account for the transition from mixed alkaline subalkaline bimodal magmatism of the pre 20 ma rift basin to exclusively subalkaline tholeiitic magmatism at the red sea spreading axis and to predominantly alkali basalt volcanism inland within the arabian shield

magma systems related to the red sea opening abstract four distinct magma environments can be recognized in association with the red sea basin 1 1 mid ocean ridge basalts within the present actively spreading axial trough 2 2 basaltic dikes and layered gabbros within the extended precambrian crust on the arabian margin 3 3 granites and rhyolites developed in the extended precambrian crust and 4 4 plateau basalts asymmetrically distributed on the arabian flanks of the red sea these magmas developed as the result of the initial break up of africa and the establishment of an oceanic spreading center within the red sea original asymmetric continental extension occurred on east dipping detachment faults in the oligocene and caused morb like magmas to invade an extending precambrian crust contemporaneous small scale mantle convection initiated limited partial melting under the arabian plate to form the undersaturated alkali olivine plateau basalts slow extension through the miocene allowed an evaporite basin to develop in the red sea while small volumes of rift and plateau basalts formed on the arabian plate a small change in plate motion in the pliocene initiated the seafloor spreading in the axial trough where morb magmas formed oceanic crust rise of the asthenosphere produced small scale thermal perturbations under the arabian plate controlled by an east dipping master detachment fault small scale mantle convection extended partial melting 2 5 eastward under the arabian plate producing more undersaturated diverse alkali olivine plateau basalts distinct chemical and isotopic signatures of these four magma types lead to obvious actualistic correlations with the geodynamic setting

making geological sense of big data in sedimentary provenance analysis abstract sedimentary provenance studies increasingly apply multiple chemical mineralogical and isotopic proxies to many samples the resulting datasets are often so large containing thousands of numerical values and complex comprising multiple dimensions that it is warranted to use the internet era term big data to describe them this paper introduces multidimensional scaling mds generalised procrustes analysis gpa and individual differences scaling indscal a type of 3 way mds algorithm as simple yet powerful tools to extract geological insights from big data in a provenance context using a dataset from the namib sand sea as a test case we show how mds can be used to visualise the similarities and differences between 16 fluvial and aeolian sand samples for five different provenance proxies resulting in five different configurations these configurations can be fed into a gpa algorithm which translates rotates scales and reflects them to extract a consensus view for all the data considered together alternatively the five proxies can be jointly analysed by indscal which fits the data with not one but two sets of coordinates the group configuration which strongly resembles the graphical output produced by gpa and the source weights which can be used to attach geological meaning to the group configuration for the namib study the three methods paint a detailed and self consistent picture of a sediment routing system in which sand composition is determined by the combination of provenance and hydraulic sorting effects

gas geochemistry of the hot spring in the litang fault zone southeast tibetan plateau abstract the southeast tibetan plateau is a region with high level seismic activity and strong hydrothermal activity several large 7 5 m 7 historical earthquakes have occurred in the litang fault zone lfz eastern tibetan plateau since 1700 litang ms 5 1 earthquake occurred on sept 23 2016 indicating the reactivation of the lfz this study was undertaken to elucidate spatial temporal variations of the hot spring gas geochemistry along the lfz from jun 2010 to april 2016 the chemical components he ne and c isotropic ratios of bubbling gas samples taken from 18 hot springs along lfz were investigated helium isotope ratios 3he 4he measured in hot springs varied from 0 06 to 0 93 ra ra air 3he 4he 1 39 10 6 with mantle derivd he up to 11 1 in the lfz assuming r ra 8 for mantle indicated the fault was a crustal scale feature that acts as a conduit for deep fluid from the mantle co2 concentrations of the majority of hot spring gas samples were 80 vol co2 3he ratios varied from 1 4 to 929 5 1010 and 13cco2 values varied from 19 2 to 2 3 vs pdb the proportions of mantle derived co2 varied from 0 to 1 8 crustal marine limestone was the major contributor 75 to the carbon inventory of the majority of hot spring gas samples before litang ms 5 1 earthquake the 3he 4he ratios obviously increased in the heni spring from may 2013 to apr 2016 the geographical distribution of the mantle derivd he decreased from east to west along 30 n in the southeast tibetan plateau relative to a corresponding increase in the radiogenic component the gas geochemical data suggested that the upwelling mantle fluids into the crust play an important role in seismic activity in the strike slip faults along 30 n in the southeast tibetan plateau

the 1997 umbria marche italy earthquake sequence a first look at the main shocks and aftershocks a long sequence of earthquakes six with magnitudes between 5 and 6 struck central italy starting on september 26 1997 causing severe damages and loss of human lives the seismogenic structure consists of a nw se elongated fault zone extending for about 40 km the focal mechanisms of the largest shocks reveal normal faulting with ne sw extension perpendicular to the trend of the apennines consistently with the quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions preliminary data on the main shocks and aftershocks show that extension in this region of the apennines is accomplished by normal faults dipping at low angle 40 to the southwest and confined in the upper 8 km of the crust these normal faults might have reactivated thrust planes of the pliocene compressional tectonics the aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km long fault segments adjacent and slightly offset from one another

a new model for the indochina and south china collision during the late permian to the middle triassic article i nfo a comparative analysis of indochina and south china during the early paleozoic indicates the former may be extended to include north vietnam part of the qinzhou tectonic zone and southern hainan island these three regions were traditionally regarded as parts of south china separated from indochina by the song ma suture in central north vietnam a new suture called the dian qiong suture is proposed here approximately along the southern margin of the present nanpanjiang basin this suture is linked to its eastern counterpart in hainan island through a nne trending dextral transform fault zone along the eastern margin of the nanpanjiang basin the conventional song ma suture originally constitutes its western extension and was translated to its present location by sinistral displacement along the red river fault zone during the tertiary upper paleozoic deep water turbidites and associated mid oceanic ridge basalts along the suture in the nanpanjiang basin illustrate a coeval ocean between indochina and south china the ocean was consumed by south directed subduction beneath indochina during the late permian to the middle triassic this suture zone was finally re shaped by the indentation between the active margin of indochina and the irregular passive continental margin of south china during the late triassic as well as displacement along the red river fault zone during the cenozoic

timing and composition of continental volcanism at harrat hutaymah western saudi arabia abstract harrat hutaymah is an alkali basalt volcanic field in north central saudi arabia at the eastern margin of a large neogene continental intraplate magmatic province lava flow tephra and spatter cone compositions in the field include alkali olivine basalts and basanites these compositions contrast with the predominantly tholeiitic fissure fed basalts found along the eastern margin of the red sea the hutaymah lava flows were erupted through proterozoic arc associated plutonic and meta sedimentary rocks of the arabian shield and commonly contain a range of sub continental lithospheric xenoliths although the lavas themselves show little indication of crustal contamination previous radiometric dating of this volcanic field a single published k ar age 1 8 ma is suspiciously old given the field measurement of normal magnetic polarity only i e brunhes interval 780 ka we report new age determinations on 14 lava flows by the 40ar 39ar laser step heating method all younger than 850 ka to better constrain the time frame of volcanism and major trace and rare earth element compositions to describe the chemical variation of volcanic activity at harrat hutaymah crystal fractionation was dominated by olivine clinopyroxene at a range of upper mantle and crustal pressures rapid ascent and eruption of magma is indicated by the array of lower crustal and lithospheric xenoliths observed in lava flows and tephra modeling suggests 1 7 melting of an enriched asthenospheric mantle source occurred beneath harrat hutaymah under a relatively thick lithospheric cap 60 80 km

north directed triassic nappes in northeastern vietnam east bac bo abstract a careful re examination of the previous tectonic descriptions and the acquisition of new structural and kinematic data have been undertaken in northeastern vietnam the structure of the area consists of a system of slightly metamorphosed but ductilely deformed nappes including recumbent folds formed during the triassic prior to the unconformable deposition of the upper triassic terrigenous sediments these results confirm the previous interpretation of preyunnanaises nappes represented by middle upper paleozoic foliated limestone resting through a flat mylonitic contact over an intermediate song mien unit deprat 1915 the latter made essentially of deformed lower triassic sedimentary and volcanic formations which previously filled the song hien rift bourret 1922a bourret 1922b in the external part of the belt the triassic strata conformably overlie slightly deformed and unmetamorphosed upper paleozoic rocks forming an autochthonous domain alternatively the same triassic succession can be seen resting tectonically through decollement zones directly over the middle paleozoic marbles to form a distinct but subsidiary allochthonous unit our data show that the song chay orthogneiss and its lower paleozoic sedimentary country rocks belong to the main nappe they also demonstrate that this nappe was transported to the n ne a newly discovered ophiolitic melange of supposed oceanic nature and triassic age lying along the song chay fault north of the nui con voi could hypothetically represent the rooted zone of the nappe a tertiary overprinting event in particular accommodated by strike slip movements likely accounts for the present orocline of ne vietnam

lhasa terrane in southern tibet came from australia the u pb age and hf isotope data on detrital zircons from paleozoic metasedimentary rocks in the lhasa terrane tibet defi ne a distinctive age population of ca 1170 ma with e hf t values identical to the coeval detrital zircons from western australia but those from the western qiangtang and tethyan himalaya terranes defi ne an age population of ca 950 ma with a similar e hf t range the ca 1170 ma detrital zircons in the lhasa terrane were most likely derived from the albany fraser belt in southwest australia whereas the ca 950 ma detrital zircons from both the western qiangtang and tethyan himalaya terranes might have been sourced from the high himalaya to the south such detrital zircon connections enable us to propose that the lhasa terrane is exotic to the tibetan plateau system and should no longer be considered as part of the qiangtang greater india tethyan himalaya continental margin system in the paleozoic reconstruction of the indian plate as current models show rather it should be placed at the northwestern margin of australia these results provide new constraints on the paleogeographic reconstruction and tectonic evolution of southern tibet and indicate that the lhasa terrane evolved as part of the late precambrian early paleozoic evolution as part of australia in a different paleogeographical setting than that of the qiangtang greater india tethyan himalaya system

neoproterozoic to early triassic tectono stratigraphic evolution of indochina and adjacent areas a review with new data the neoproterozoic to early triassic tectono stratigraphic evolution of indochina and adjacent areas can be divided into six mega episodes 1 the neoproterozoic early cambrian mega episode was characterized by shallow marine environment in the sino vietnam composite terrane meanwhile the different blocks of the indochina composite terranes were still separated by branches of the proto tethys sino vietnam and part of indochina amalgamated along the proto song ma suture by the end of the early cambrian 2 during the middle cambrian early ordovician mega episode shallow shelf carbonate deposition prevailed north of the proto song ma suture with associated strata containing a rich benthic fauna this contrasts with deep marine and volcanic arc setting in the south of the proto song ma suture 3 the middle ordovician silurian mega episode was characterized by subduction of the tam ky phuoc son ocean beneath the viet lao terrane in the north and the kon tum terrane in the south this was followed by a collisional orogeny that sutured the kon tum terrane with the viet lao and viet cambodia terranes along the tam ky phuoc son suture the hypothetic po ko and chu sinh sutures are discussed the collision led to an intra continental orogeny in the sino vietnam composite terrane by the end of the mega episode a unified sino vietnam indochina super terrane was established during the latest silurian early devonian benthic fauna and flora of the sino vietnam and indochina composite terranes shared cathaysian affinity 4 the devonian tournaisian mega episode was characterized by an early middle devonian transgression and a late devonian earliest carboniferous regression except for parts of northern indochina where deep marine conditions remained 5 during the visean middle permian the song ma ocean reopenned in the north of indochina meanwhile in the west a volcano plutonic belt formed due to eastward subduction of the paleo tethys under the indochina composite terrane 6 the late permian early triassic mega episode was characterized by emeishan mantle plume related continental rifting and the indosinian orogeny resulting from resuturing of the indochina and sino vietnam composite terranes along the song ma suture

analysis of seismological and geological observations formoderate size earthquakes the colfiorito fault system central apennines italy to contribute to the understanding of the relationships between moderate earthquakes and the faults that are recognisable in the geological record we analysed seismological and geological data related to the 1997 98 umbria marche central italy earthquake swarm the seismological recordings collected by local networks allowed accurate location of about 1000 events whereas the geological field observations provided a picture of the structural features and the ground surface deformations we also re examined and utilised some already published data and results mostly about the fault plane solutions and the geology on the basis of earthquake locations fault plane solutions and geological mapping we explored the possible correlation between the earthquake causative fault planes and the normal faults exposed in the area our results show that the two mainshocks that occurred on september 26 1997 mw 5 7 and mw 6 0 originated on the same structure reactivating at depth the colfiorito normal faults both ruptures did not propagate up to the ground surface but triggered gravitational sliding that occurred along pre existing fault scarps the earthquake that occurred on october 14 1997 mw 5 6 originated on another fault branch at a much shallower depth in spite of the lower magnitude this earthquake produced tectonic ruptures where the fault plane projects to the surface in an area where no faults were previously mapped by comparing the paleostress reconstruction based on slickenside lineations analysis and the focal mechanism solutions we also suggest a possible correlation between the long term early middle pleistocene cumulative effects of the colfiorito fault system and the short term behaviour of the fault planes observed during this earthquake swarm favouring the idea of a seismogenic source producing clustered moderate size earthquakes rather than large events scattered in time our results suggest that the integration of geological and seismological observations can constrain the seismotectonic interpretations in regions of low level seismicity and that this integration can pose a reliable basis in the seismic hazard assessment

early paleozoic orogenesis along gondwana s northern margin constrained by provenance data from south china abstract the cambrian ordovician boundary unconformity in the southern part of the south china craton is spatially and temporally related to coeval orogenic activity along the indian margin of east gondwana detrital zircon age spectra from strata above and below the unconformity range in age from 3580 450 ma with peaks in the late mesoproterozoic and neoproterozoic the patterns are similar to time equivalent sequences elsewhere in south china and together with regional facies relationships and paleocurrent data indicate derivation from a gondwana source the disconformity at the base of the ordovician succession forms part of a regional break that has also been documented in the himalaya qiangtang lhasa sibumasu and western australia all these successions have similar detrital zircon age spectra suggesting derivation from common source s in south china the effects of this tectonic event are relatively mild and are represented by a local disconformity at the base of the ordovician succession but elsewhere in north gondwana this event is marked by an angular unconformity with metamorphism of older units and relatively widespread magmatic activity south china was likely located in a distal location to the northeast of the pulse of tectonic activity which was focused in the himalaya region and was close to the continent ocean boundary between northern gondwana and the proto tethys

discovery of middle late devonian and early permian magmatic events in east asia and their implication for the indosinian orogeny in south china insights from the sedimentary record

fluid geochemical transect in the northern apennines central northern italy fluid genesis and migration and tectonic implications chemical and isotopic characteristics of natural gas and thermal water discharges from the western back arc tyrrhenian sea across the apennine thrust belt to the po valley and adriatic coast foredeep basins in the northern apennines central northern italy reveal a large scale fluid motion in the upper crust both vertically and horizontally on the basis of gas compositions two different domains of rising fluids have been distinguished 1 co2 rich he poor 3he 4he high domain in the western peri tyrrhenian extensional sector 2 ch4 rich he rich 3he 4he low domain in the eastern adriatic compressional sector such gases rising from various depths are crossed by a huge lateral n2 rich water flow in the peri tyrrhenian sector of ca so4 hco3 meteoric derived waters that move in a regional wide aquifer hosted in a quite thick mesozoic carbonate series morphologically the co2 vents consist of mud basins with high gas rate emission where the rising fluids move upwards through diatremes on the other hand ch4 emissions seep out from typical mud volcanoes with a reduced gas flow rate where the fluid motion is likely related to saline diapir extrusions and the methane is mostly carried to the surface by the associated mud the two rising mechanisms described locate southwest and northeast of the apennine watershed respectively from a seismic point of view the ch4 domain in the thrust belt and foredeep areas is characterized by a large number of earthquakes indirectly pointing to a different rheological behavior of the terrains with respect to the more internal peri tyrrhenian area owing to the quite high thermal gradient of the latter the boundary of the brittle ductile crust in the peri tyrrhenian sector can be located at a 10 km shallow depth although the presence of several post orogenic basins would suggest widespread extensional tectonics in all areas west of the apennine watershed those located in the easternmost part display gas vents with typical crustal 3he 4he ratios as this ratio is very sensitive to deep fluids rising from the mantle we hypothesize that such basins at the foot of the apennines are not due to tensive stress as suggested by their morphological shape they are piggy back thrust top basins developed and evolved in a still acting compressive tectonic regime

viscoelastic relaxation and long lasting after slip following the 1997 umbria marche central italy earthquakes summary we combine global positioning system gps measurements with forward modelling of vis coelastic relaxation and after slip to study the post seismic deformation of the 1997 umbria marche central apennines moderate shallow earthquake sequence campaign gps mea surements spanning the time period 1999 2003 are depicting a clear post seismic deformation signal our results favour a normal faulting rupture model where most of the slip is located in the lower part of the seismogenic upper crust consistent with the rupture models obtained from the inversion of strong motion data the preferred rheological model obtained from viscoelastic relaxation modelling consists of an elastic upper crust underlain by a transition zone with a viscosity of 10 18 pa s while the rheology of deeper layers is not relevant for the observed time span shallow fault creep and after slip at the base of the seismogenic upper crust are the first order processes behind the observed post seismic deformation the deep after slip below the fault zone at about 8 km depth acting as a basal shear through localized time dependent deformation identifies a rheological discontinuity decoupling the seismogenic upper crust from the low viscosity transition zone

carbon dioxide earth degassing and seismogenesis in central and southern italy 1 we present the first regional map of co2 earth degassing from a large area most of central and south italy derived from the carbon of deep provenance dissolved in the main springs of the region the investigation shows that a globally significant amount of deeply derived co2 10 of the estimated global co2 emitted from subaerial volcanoes is released by two large areas located in western italy the anomalous flux of co2 suddenly disappears in the apennine in correspondence to a narrow band where most of seismicity concentrates here at depth the gas accumulates in crustal traps generating co2 overpressurized reservoirs which induce seismicity

the boundary between the simao and yangtze blocks and their locations in gondwana and rodinia constraints from detrital and inherited zircons abstract the simao block which is the northern extension of the indochina block and the yangtze block were amalgamated by the closure of a paleo tethys branch ocean commonly referred to as the ailaoshan ocean the exact location of the suture between these two continental blocks has been contested some researchers believed that it is located within the cenozoic ailaoshan shear zone whereas other researchers thought that it is present 50 km farther to the west the detrital and inherited xenocrystic zircons from paleozoic sedimentary rocks and a triassic granite dyke respectively in the laowangzhai mojiang suspect terrane which occurs between the two suggested locations for the same suture show that neither of these previously suggested locations is accurate the age distribution pattern of detrital and inherited xenocrystic zircons from the laowangzhai mojiang suspect terrane is similar to that of detrital zircons from the simao indochina block but different from that of those from the jianshui area in the western margin of the yangtze block this indicates that the laowangzhai mojiang suspect terrane belongs to the simao indochina block the actual location of the suture between the simao indochina and yangtze blocks is suggested to superimpose the ailaoshan late devonian to early carboniferous ophiolite belt within the suspect terrane the new interpretation is also supported by hf isotopes of detrital zircons from the related continental blocks the age distribution pattern of precambrian detrital zircons from the western part of the yangtze block is different from that of those from australia or tethyan himalaya the northern part of the indian continent indicating that the yangtze block was not an integral part of australia or india in rodinia the age distribution pattern of pre carboniferous detrital zircons from the simao indochina block is remarkably similar to that of those from the tethyan himalaya indicating that the simao indochina block was derived from indian gondwana

large southward motion and clockwise rotation of indochina throughout the mesozoic paleomagnetic and detrital zircon u pb geochronological constraints abstract we report a combined paleomagnetic and u pb geochronologic study of sedimentary rocks from the huai hin lat and nam phong formations of mesozoic age in ne thailand in order to provide independent constraints on the tectonic movement of the indochina block during convergence of the major blocks now comprising east asia the maximum allowable depositional age of the two formations is estimated to be 227 ma and 215 ma respectively from detrital zircon u pb geochronologic analysis which also indicates a sediment source transition in the khorat plateau basin during the middle late jurassic a formation mean paleomagnetic direction of d g i g 21 4 38 1 k g 19 5 95 9 6 before and d s i s 43 0 48 0 k s 47 4 95 6 1 n 13 after tilt correction is derived from samples with different lithologies bedding attitudes magnetic carriers and polarities and yields a positive fold test hence the magnetization is likely primary the revised mesozoic apwp of the indochina block yields paleolatitudes for a reference site of 22 n 102 e of 33 4 7 2 n during the norian late triassic 25 9 5 9 n during the late triassic to early jurassic 23 9 8 n during the late jurassic to early cretaceous 27 5 3 2 n during the early cretaceous and 24 5 4 9 n by the late cretaceous corresponding declinations are 45 2 8 6 38 0 6 6 36 3 8 8 29 6 3 6 and 24 9 5 4 respectively these data indicate a significantly southward displacement accompanied by clockwise rotation during the mesozoic a reconstruction of the indochina block within the now well studied merging process of south china and north china indicates that the indochina block was located at a higher latitude than the south china block during the norian stage of late triassic times whilst no significant relative poleward displacement apparently occurred during the early jurassic to early cretaceous interval our study supports a post cretaceous tectonic extrusion model with a southeastward displacement of indochina with respect to the south china block estimated to be 1000 850 km since the late cretaceous

the isotopic composition of radiogenic helium and its use to study groundwater movement in confined aquifers abstract the 3 he 4 he ratio for radiogenic he is shown to be primarily determined by the abundance of li and neutron absorbing elements in the host formations and is almost independent of the radioelement abundance the 3 he 4 he ratios have been computed for various li contents in rock matrices with average abundances of other elements the 3 he 4 he ratio is specific for each formation and may therefore be used to study crustal diffusion of he it is shown however that both the 3he and 4he contents of continental basements are likely to exceed those of the mantle so that diffusion of mantle he through continental crust is not likely to occur changes in the 3 he 4 he ratio for dissolved he have potential for studying groundwater movement and mixing processes atmospheric he which was dissolved at recharge is progressively diluted by radiogenic he with an isotopic ratio characteristic of the aquifer as the groundwater migrates down dip mixing proportions for formation he and atmospheric he in aquifers for which there is admixture of fracture borne water and interstitially stored water can also be estimated from isotopic measurements these processes are discussed in relation to two aquifers in eastern england

the south china block indochina collision where when and how this study uses new field observations and existing studies to shed new light on the origin and significance of two nw se striking orogens in nw and ne vietnam we conclude that the architecture of each belt is a stack of ne directed nappes formed either under deep ductile synmetamorphic conditions or under shallow depth in the sw and ne parts respectively the song ma zone and song chay ophiolitic melange represent two ophiolitic sutures however the late permian song da and babu mafic rocks are not ophiolites but intraplate basalts related to the emeishan plume a late triassic unconformity the 225 205 ma postorogenic plutonism and the 250 230 ma syntectonic metamorphism support an early to middle triassic age for these tectonic events both nw and ne belts are due to sw directed subduction with arc magmatism ocean closure and continental collision though two contemporary s dipping subduction zones might explain the structural evolution of the two belts a single convergent system offset by the tertiary red river fault is preferred as this s directed subduction better accounts for the late permian intraplate magmatism this scenario is discussed in the general geodynamic framework of se asia

geochemical precursors to seismic activity studies of earthquake precursory phenomena during the last several decades have found that significant geophysical and geochemical changes can occur prior to intermediate and large earthquakes among the more intensely investigated geochemical phenomena have been 1 changes in the concentrations of dissolved ions and gases in groundwaters and 2 variations in the concentrations of crustal and mantle volatiles in ground gases the concentration changes have typically showed no conanomalies trend either increasing or decreasing and the spatial and temporal distribution of the observed anomalies have been highly variable as a result there is little agreement on the physical or chemical processes responsible for the observed anomalies mechanisms proposed to account for precursory groundwater anomalies include ultrasonic vibration pressure sensitive solubility pore volume collapse fracture induced increases in reactive surfaces and aquifer breaching fluid mixing precursory changes in soil gas composition have been suggested to result from pore volume collapse micro fracture induced exposure of fresh reactive silicate surfaces and breaching of buried gas rich horizons an analysis of the available field and laboratory data suggests that the aquifer breaching fluid mixing ab fm model can best account for many of the reported changes in temperature dissolved ion and dissolved gas concentrations in groundwater ultrasonic vibration and pressure sensitive solubility models cannot reasonably account for the geochemical variations observed and although the pore collapse model could explain some of the observed chemical changes in groundwater and ground gas uncertainties remain regarding its ability to generate anomalies of the magnitude observed other geochemical anomalies in particular those associated with hydrogen and radon seem best accounted for by increases in reactive surface areas irsa model that may accompany precursory deformation around the epicenter of an impending earthquake analysis of the probable response of these models to the earthquake preparation process as well as to other environmental factors suggests that geochemical monitoring programs can provide information that may be valuable in forecasting the probability of an earthquake however because of the complexity of the earthquake preparation process the absolute prediction of seismic events using geochemical methods alone does not presently appear to be feasible

geodetic and seismologic evidence for slip variability along a blind normal fault in the umbria marche 1997 1998 earthquakes central italy summary we analysed elevation changes induced by the 1997 1998 umbria marche central apennines italy earthquakes we employed data from a first order geodetic levelling line measured in 1951 1992 and 1998 the line bears a record of pre seismic and coseismic strains associated with the causative fault of the 1997 september 26 09 40 mainshock mw 6 0 a first level analysis performed under the assumption of slip homogeneity of coseismic slip shows misfits that cannot be reduced simply by altering the fault size and geometry a more detailed analysis based on a distribution of coseismic slip obtained from broad band seismograms provides a better fit and is in agreement with 1951 1992 elevation changes interpreted as precursory slip by previous investigators the levelling data sets new constraints on the location extent dip and depth of the fault in full agreement with seismological evidence and images from sar interferometry the data show no evidence for slip in the uppermost 3 km of the crust suggesting that a major and widely recognized normal fault that exists in the area is no longer active and showing a tendency of present tectonic strains to revert the current topographic setting

structural development of the lower paleozoic belt of south china genesis of an intracontinental orogen abstract an extensive structural study of wuyishan and surrounding areas south china brings data on the structures formed prior to the devonian unconformity building the lower paleozoic belt an ordovician tectonic event was responsible for i south directed structures in the wuyishan proper and its southern border related to both thin and thick skinned tectonics accompanied by metamorphism and crustal thickening ii north directed structures to the west of ganjiang fault and north of jiangshan shaoxing fault where only thin skinned tectonics is visible the southward deformation accommodating the shortening includes emplacement of thrust sheets involving deep crustal material and some mantle peridotite and likely repetition of continental material both responsible for crustal thickening the orogeny was due to the underthrusting of the southern part of the south china block beneath the northern part of this block closing the pre existing nanhua rift created at around 850 800 ma and involving the rift sedimentary fill in the southward thrusting the syn metamorphic tectonic piling was followed by anatexis and granite emplacement at around 440 390 ma our new u th pb epma monazite dating gives ages of 433 9 ma and 437 5 ma early silurian for the main anatectic event and a younger age of 412 5 ma late silurian early devonian for a late re heating process the lower paleozoic belt is an intracontinental orogen without oceanic suture record it shows some similarities with the cenozoic european pyrenees chain with a strong asymmetry

imaging the complexity of an active normal fault system the 1997 colfiorito central italy case study 1 six moderate magnitude earthquakes 5 mw 6 ruptured normal fault segments of the southern sector of the north apennine belt central italy in the 1997 colfiorito earthquake sequence we study the progressive activation of adjacent and nearby parallel faults of this complex normal fault system using 1650 earthquake locations obtained by applying a double difference location method using travel time picks and waveform cross correlation measurements the lateral extent of the fault segments range from 5 to 10 km and make up a broad 45 km long nw trending fault system the geometry of each segment is quite simple and consists of planar faults gently dipping toward sw with an average dip of 40 45 the fault planes are not listric but maintain a constant dip through the entire seismogenic volume down to 8 km depth we observe the activation of faults on the hanging wall and the absence of seismicity in the footwall of the structure the observed fault segmentation appears to be due to the lateral heterogeneity of the upper crust preexisting thrusts inherited from neogene s compressional tectonic intersect the active normal faults and control their maximum length the stress tensor obtained by inverting the six main shock focal mechanisms of the sequence is in agreement with the tectonic stress active in the inner chain of the apennine revealing a clear ne trending extension direction aftershock focal mechanisms show a consistent extensional kinematics 70 of which are mechanically consistent with the main shock stress field

coseismic and postseismic displacements related with the 1997 earthquake sequence in umbria marche central italy we study the coseismic and postseismic displacements related with the 1997 umbria marche earthquake sequence by means of leveling lines along a deformed aqueduct located in the epicentral area comparing the 1960 and 10 1997 measurements we obtain 0 49 0 10 m of coseismic displacement distributed along 3 km across the normal fault zone modeling of the coseismic surface dislocation is obtained from a combination of low angle 38 faults at depth and high angle 80 upper fault branches the best fit model indicates that the upper branches stop at 0 4 km below the ground surface and have 60 of slip with respect to the lower faults the postseismic displacement measured during 1998 is 0 18 m and represents 36 of the apparent coseismic deformation moderate earthquakes in the apennines and related surface deformation may thus result from curved faults that reflect the brittle elastic properties of the uppermost crustal structures

paleozoic tectonic setting and paleogeographic evolution of the qin fang region southern south china block detrital zircon u pb geochronological and hf isotopic constraints

cambrian ordovician orogenesis in himalayan equatorial gondwana an early paleozoic tectonic event the kurgiakh orogeny has long been known from the western tethyan himalaya and it is conspicuously recorded by an angular unconformity between cambrian marine shelf deposits and coarse ordovician conglomerate as well as widespread granitic plutons although the lateral extent of this event is poorly known two regions in the central and eastern himalaya the kumaon of india and klippen of bhutan contain conglomerate units that may be correlative with this event in the kumaon the ralam formation which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca 512 ma is overlain by sandstone containing the arthropod walking trace diplichnites gouldi an ichnospecies known from ordovician and younger strata the late early cambrian trilobite redlichia sp recovered from the underlying martoli group indicates that the conglomerate is post early cambrian and based on stratigraphic analysis also likely records the kurgiakh orogeny in central bhutan the lower paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix these units have also been assigned to a variety of ages but rocks that apparently underlie these strata contain the furongian late cambrian zonal trilobite kaolishania granulosa here we show that detrital zircon ages of a conglomerate clast no grains younger than ca 781 ma are distinctly older than those of the matrix which contains a large peak at 490 500 ma thus these conglomerate units are also potential lateral correlatives of the ordovician conglomerate deposits of the western himalaya our paleontological and geochronologic data suggest that the kumaon and bhutan conglomerate units and possible equivalents in tibet correlate with those of the western himalaya this increases their known along strike extent by over 1000 km bringing the total extent to 1200 km thus the kurgiakh orogeny spanned much of the himalayan orogen along the northern gondwanan margin and reflected a widespread tectonic reorganization of the peri gondwanan realm to andean type margins recent work indicates that it also likely extended into the lhasa block and both the north and south china blocks this event was associated with widespread ca 500 ma arc related plutonism recorded in the himalaya and with abundant detrital zircon grains of that age in younger himalaya strata based on current fossil data the duration of the hiatus associated with the cambrian ordovician boundary which separates pre and postdeformational strata ranges from 22 to 36 m y available geochronological data do not provide greater constraint if the event was close in age to the depositional age of the ordovician molasse then the main phase of tectonic uplift erosion and deposition may have been much later than the presumed cambrian ordovician boundary interval age of ca 485 ma

geochemical evidence of seismogenic induced anomalies in the dissolved gases of thermal waters a case study of umbria central apennines italy both during and after the 1997 1998 seismic swarm 1 in this paper we present the first geochemical data set regarding long term monitoring of dissolved gases in thermal waters from a seismic area three sites in umbria central apennines italy were studied both for the chemical and for the helium isotopic composition of the dissolved gases data were collected during and after the seismic crisis that struck the region in 1997 1998 the chemical composition of the dissolved gases revealed that a co2 rich gas phase was always mixed with an atmospheric derived component dominated by n2 a normal faulting marked the beginning of the seismic activity enhancing the release of co2 on a regional scale variations in both the chemical and isotopic compositions of the dissolved gases were also observed as preseismic synseismic and postseismic phenomena related to the seismic shock of march 1998 those geochemical modifications were interpreted as being the consequence of a drop in the co2 degassing rate in good agreement with the compressive focal mechanism of that seismic event furthermore this interpretation was also consistent with the geologic and tectonic setting of the study area and induced us to postulate that changes in the local rock permeability due to crustal deformations i e coseismic deformation and postseismic release were responsible for the geochemical modifications observed on the basis of the foregoing we have concluded that the geochemistry of dissolved gases in groundwaters represents a useful tool for the investigation of the relationships between circulating fluids and seismic activity

drivers for late paleozoic to early mesozoic orogenesis in south china constraints from the sedimentary record abstract unconformity bound permo triassic successions in the qinfang basin south china craton reflect tectonic processes associated with tectonothermal activity along the margins of the craton the provenance record of these successions based on modal analysis geochemical composition detrital zircon u pb ages and paleocurrent data indicate derivation from the adjoining precambrian yunkai massif and surrounding paleozoic sedimentary units the quartz dominated nature of the siliciclastic units along with their high th sc and zr sc ratios indicate a recycled orogen source detrital zircons from late permian to early triassic samples display major age groups at 1100 800 ma 650 500 ma 480 420 ma with subordinate age groups at 2800 2400 ma 1900 1600 ma and 1400 1200 ma late triassic sandstones show a similar detrital zircon age pattern as well as a 300 250 ma age group integration of this provenance data with regional geological information suggests that the unconformity at the base of the permian succession is related to subduction of the paleo tethys ocean to the south of the craton whereas the major driver for triassic tectonic activity corresponds with terrane accretion and the termination of paleo tethys ocean subduction along the southwest margin of the craton

the oldest known rocks in south western china shrimp u pb magmatic crystallisation age and detrital provenance analysis of the paleoproterozoic dahongshan group abstract the paleoproterozoic dahongshan group outcrops along the present day southwest margin of the yangtze craton it is the oldest rock found in the region and forms the basement for the late mesoproterozoic sedimentary successions the dahongshan group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist lower amphibolite facies some of the original sedimentary features such as unimodal cross bedding in quartzite and rounded conglomerate clasts are still preserved suggesting deposition in a fluvial environment abundant carbonate rocks also found within the sedimentary sequence indicate periods of shallow marine or lagoonal conditions a weighted mean shrimp 206pb 207pb age of 1675 8 ma for a tuffaceous schist unit confirms the paleoproterozoic formation age of the group shrimp and lam icp ms u pb analysis of detrital zircon grains from metasedimentary units indicate a late archean to paleoproterozoic ca 2780 1860 ma source region the age spectrum of the detrital zircons is similar to that of the late archean and early paleoproterozoic basement rocks previously identified in the yangtze craton but includes a population of paleoproterozoic grains ca 2400 2100 ma which have no known source region in the exposed yangtze craton

intracontinental subduction a possible mechanism for the early palaeozoic orogen of se china the early palaeozoic orogen of se china consists of three lithotectonic elements from top to bottom a sedimentary upper unit a metamorphic lower unit and a gneissic basement the boundaries between these units are flat lying south directed ductile decollements the lower one is coeval with an amphibolite facies metamorphism m1 the belt is reworked by migmatite granite domes high temperature metamorphism m2 and granitic plutons related to post orogenic crustal melting we date here the syn m1 ductile shearing at 453 7 ma by u th pb method on monazite previous ages and our new 40 ar 39 ar ages of biotites and muscovites show that the metamorphic rocks experienced syn m2 exhumation from 440 to 400 ma the early palaeozoic orogen of se china is an intracontinental belt in which decollements accommodated the north directed subduction of the cathaysian continent this orogen is an example of intracontinental subduction that was not preceded by oceanic subduction

active tectonics in the central apennines and possible implications for seismic hazard analysis in peninsular italy abstract the central apennines fault system cafs of peninsular italy overprints earlier structures of a neogene fold and thrust belt and includes segments characterized by diffuse seismicity distributed within a nnw sse trending zone 50 60 km wide the system has been analysed by means of morphotectonic and structural analysis of exposed active fault segments the resulting fault structure consists of an interconnecting network of roughly n s trending left lateral strike slip segments and mostly nw se oriented transtensional to normal faults evidence for recent activity of cafs structures is provided by faulted middle pleistocene holocene deposits including 30 40 ka old pyroclastites and 40 ka old palustrine sediments fresh scarps in both bedrock and late quaternary continental deposits and decametric lateral offsets locally affecting the post wurmian drainage pattern of the area the regional stress field responsible for the development and evolution of the cafs as inferred from fault slip data is characterized by a nw se compression and by a ne sw extension the cafs pattern and its present day kinematics have been related to left lateral strike slip motion on north south trending crustal faults the existence of deep seated strike slip faults in the central apennines has implications for seismic hazard analysis motion along these structures suggests in fact that coseismic surface faulting is distributed and that cumulative displacements include normal transtensional and strike slip components the seismogenic potential of cafs structures can therefore be best described by multiple rupture models and be better analysed in terms of partial contributions of lower rank features constituting congruent structural associations within the system

complex normal faulting in the apennines thrust and fold belt the 1997 seismic sequence in central italy a long sequence of moderate magnitude earthquakes 5 m 6 struck central italy in september and october 1997 at the end of the sequence a year later the seismogenic area extends for about 60 km along the apennines the analysis of historical seismicity suggests that this seismic sequence filled a 700 year gap in this portion of the chain other historical sequences in the same area are characterized by prolonged seismic release on adjacent fault segments probably due to the in volvement of shallow and complex structures inherited by the compressive tectonics the distribution of seismicity and the fault plane solutions show that the extension in this region is accomplished by normal faults dipping at relatively low angles 40 to the southwest the focal mechanisms of the largest shocks reveal normal faulting with extension perpendicular to the apenninic chain northeast southwest consistently with the quaternary tectonics of the internal sector of the northern apen nine belt and with previous earthquakes in adjacent regions three mainshocks oc curred on distinct 5 to 10 km long fault segments adjacent and slightly offset be tween each other high quality aftershock locations show that seismicity is confined within the sedimentary mesozoic cover in the upper 8 km of the crust and that most of the aftershocks are shallower than the largest shocks which nucleated at 6 km depth faults evidenced by aftershock locations have a planar geometry and show increased complexity toward the surface most of the aftershock focal mechanisms are dominated by normal faulting several strike slip events occurred at shallow depths reactivating portions of pre existing thrust planes that segment the normal fault system the spatiotemporal evolution of seismicity shows a peculiar migration of hypocenters along the strike of the main faults with multiple ruptures and the activation of fault segments before the occurrence of the main rupture episodes

seismological geological and geophysical constraints for the gualdo tadino fault umbria marche apennines central italy abstract the april 3 1998 mw 5 1 gualdo tadino earthquake central italy was the last significant event in the 6 month long umbria marche seismic crisis this event and its aftershocks occurred in an area where active faulting produces no striking geological and geomorphological effects in this study we investigated the ruptured fault using detailed seismological data and a re processed and re interpreted seismic reflection profile aftershock location and focal mechanisms were used to constrain the geometry and kinematics of the ruptured fault and a comparison was made with the subsurface image provided by the seismic profile we found that the 1998 gualdo tadino earthquake occurred on a wsw dipping normal fault with a length of about 8 km and a relatively gentle dip 30 40 confined between 3 5 and 7 km in depth kinematics of the mainshock and aftershocks revealed a ne trending extension in agreement with the regional stress field active in the northern apennines belt the mw 5 1 earthquake originated above the top of the basement and ruptured within the sedimentary cover which consists of an evaporites carbonates multilayer we hypothesised that the active fault does not reach the surface blind normal fault

long term 1997 2007 geochemical monitoring of gases from the umbria marche region abstract the paper presents the results of ten year long geochemical monitoring carried out in the area hit by the 1997 98 umbria marche seismic sequence northern apennines italy showing the advancement in our understanding of the reciprocal relationships between fluids and tectonic structures in this area long term geochemical monitoring has allowed us to define the geochemical features of the gas phase and to model its origins and interactions all the released gases including dissolved and venting gases have predominantly crustal origin and have suffered from substantial modifications in their geochemical features the whole seismogenic process is considered to be responsible for the modifications recorded over a wide time interval including the production of co 2 from mechanical stress on carbonate rocks we interpret the modifications occurred to the gas phase during the 1997 98 period as related to faulting activity while those recorded after the end of the seismic crisis are interpreted as being a consequence of the crustal relaxation the data show that it is almost impossible to interpret the temporal variation without coupling long term geochemical monitoring and knowledge of the genetic features of the fluids therefore the information coming from the circulating fluids is a powerful tool to better understand the physical processes governing the faulting activity

early crustal evolution in the western yangtze block evidence from u pb and lu hf isotopes on detrital zircons from sedimentary rocks abstract in situ u pb and lu hf data on detrital zircons from paleo to neoproterozoic sediments have been used to gain a clearer picture of the regional tectonic background and crustal evolution in the western part of the yangtze block the youngest concordant zircon ages for sedimentary units suggest maximum depositional ages of 1014 ma for the kunyang group 750 649 ma for the sinian sequences and 525 ma for the cambrian sediments the yinmin formation previously assigned to the middle part of the kunyang group was actually deposited after 1667 ma and contains zircons whose ages are dominantly from late archean to paleoproterozoic 2 8 2 7 ga 2 5 2 3 ga and 1 85 ga the heishantou formation in the lower part of the kunyang group has two major age populations of 1 0 ga and 1 8 1 6 ga the sinian and cambrian sedimentary rocks are dominated by neoproterozoic zircons with age peaks at 760 ma and 825 ma consistent with the ages of widespread igneous rocks around the yangtze block the lu hf isotope data suggest that a significant juvenile input took place during archean and neoproterozoic times respectively while crustal reworking was dominant during the paleoproterozoic time the archean paleoproterozoic detritus is isotopically distinct from the archean paleoproterozoic basement exposed in the northern part of the yangtze block suggesting that a subarea of old crust lies beneath the young sediments covering the craton the presence of abundant 1 85 ga zircons suggests that the yangtze block was probably part of the columbia supercontinent during paleoproterozoic time comparisons to the other parts of columbia suggest that the yangtze block could have been adjacent to the north china craton and or australia

gas geochemistry and seismotectonics a review abstract publications on soil and spring gases have been examined regarding their relationships with both tectonic and seismic activities the main sources behaviours and uses of species detected in soils and springs are displayed and their mode of sampling and analysing briefly described the main patterns of degassing in soils are described and we outline the wide range of geochemical signatures as the result of both permeability and mineralogical contrasts because thermomineral waters have been in contact with great volumes of crustal rocks at various depths spring gases might be more representative of the local environment than soil gases moreover gas signature comparisons show that spring gases are much more enriched with deep gases and slightly contaminated by atmospheric gases therefore they can be considered as better samples for identifying precursors of earthquakes environmental perturbations are examined and it is shown from divergent cases that pressure temperature soil moisture or earth tides may generate very high perturbations of the degassing process such effects demonstrate that no systematic correction law can be proposed and that removing external contributions from gas concentrations must be performed case by case this demonstrates therefore the need for the simultaneous measurement of external parameters during gas monitoring a qualitative examination of about 150 claimed precursors proposed in the literature has been reviewed as noted by previous authors anomalies appear at distances sometimes much greater than typical source dimensions and occur in the field of strain higher than 10 9 most of them being in the field of strain higher than 10 8 taking into account the very high heterogeneity of such a set of data we can suggest that amplitudes of gas anomalies are independent of both magnitudes and epicentral distances of related earthquakes suggesting local conditions to control amplitudes on the contrary precursory time and duration of anomalies seem to increase both with magnitudes and epicentral distances abundant evidence demonstrates the major role of crustal fluids in the earthquake cycle many works have outlined the fact that crustal instabilities can appear as the result of low stress strain perturbations during loading it has been suggested that motion of fluids may occur at various scales from microcrack fluid transfer up to changes of hydraulic levels of water tables the study of subsequent anomalies is expected to supply a tool for earthquake prediction following previous authors we outline the need for further methodological improvements including the setting up of multiparameter station networks and the simultaneous recording of the main external parameters atmospheric pressure water and air temperature soil moisture for signal processing

earthquake induced chemical changes in the thermal waters of the umbria region during the 1997 1998 seismic swarm the compositional data collected at three thermal springs in the umbria region over a sampling period lasting two and a half hydrological years included the entire seismic sequence striking the central apennines italy in 1997 1998 wide temporal variations in the chemical composition and temperature of thermal waters were observed at all the sampling sites the widest were recorded at the triponzo sampling site where a temperature drop of 2 c and 20 c preceded by 4 days and 2 days respectively the occurrence of the event characterized by the deepest hypocentre of the entire seismic sequence despite such a macroscopic preseismic anomaly the recorded variations were not related to single seismic events or to the release rate of the seismic energy they actually seem to have been induced by permeability variations related to crustal deformation in the absence of elastic energy release and were thus linked to the whole seismogenic process

anomalies of mantle derived helium during the 1997 1998 seismic swarm of umbria marche italy more than 100 he 4he isotope ratios were measured in co2 dominated gas samples collected during the 1997 1998 seismic crisis that shook the umbria marche central apennines italy although the gas emissions were located in a typical crustal environment the measured values delineate variable mixing proportions between mantle derived and crustal helium during the monitoring period anomalous he 4he ratios were observed together with a significant increase of fluid outflows the observed he 4he anomalies appear to be driven by stress induced crustal deformations affecting micro fracturing and bulk permeability and in turn by a seismogenic induced modification of the crustal permeability of rocks since crustal deformations can also occur aseismically the geochemical anomalies must be considered indicators of processes occurring below the surface which could eventually help to estimate the probability increase of an impending earthquake instead of pure empirical precursors

preliminary stratigraphic and structural architecture of bhutan implications for the along strike architecture of the himalayan system article i nfo preliminary mapping and stratigraphic correlation of lesser himalayan rock in eastern bhutan using field characteristics u pb detrital zircon dating and nd geochemistry define the first order stratigraphic architecture of the indian passive margin sequence in the eastern himalaya we use this new image of the lateral and vertical relationships of the original stratigraphy to determine the structural framework of the eastern himalayan fold thrust belt in bhutan we propose that lesser himalayan rock in bhutan can be divided into lower lesser himalayan rocks with a paleoproterozoic detrital zircon signal and upper lesser himalayan rocks with detrital zircon signals of 1000 500 ma the 500 ma detrital grains are from rocks in the frontal portions of the fold thrust belt north of the main boundary thrust as well as directly in the footwall of the main central thrust in the hinterland our preliminary stratigraphic study coupled with mapping allows us to construct a composite balanced cross section through the kuru chu valley in eastern bhutan which provides the first image of the geometry and amount of shortening through bhutan the main frontal thrust tilts a 6 km of thick section of neogene foreland basin deposits these deposits are separated from folded and faulted upper lesser himalayan rocks by the main boundary thrust north of the main boundary thrust we propose two duplex systems the southernmost duplex contains 9 repeated sections of upper lesser himalayan units including the permian gondwana sequence and the cambrian or younger baxa group the northernmost duplex is located in the footwall of the main central thrust and is comprised of two repeated sections of the proterozoic shumar and daling formations the southern boundary of this northern duplex is the shumar thrust which acts as a roof thrust for the southern duplex system and may correlate to the ramgarh thrust in nepal and india we propose that the development of the duplex systems passively folded overlying tethyan and greater himalayan rocks minimum shortening for this part of the himalayan fold thrust belt is 359 km all of which has occurred from 22 ma to present defining a long term shortening rate of 16 mm yr

linking the indochina block and gondwana during the early paleozoic evidence from u pb ages and hf isotopes of detrital zircons abstract to constrain the paleoposition of indochina within gondwana during the early paleozoic we performed in situ u pb and hf isotope analyses on detrital zircons from three river sediment samples in the truong son belt of the indochina block the age distributions yield dominant neoarchean 2 5 ga mesoproterozoic 1 7 1 4 ga grenvillian 0 95 ga and pan african 0 65 0 5 ga age groups and minor paleo to meso archean zircons hf isotope compositions of zircons for each age group exhibit large ranges of ehf t suggesting that the zircon host rocks have diverse sources the oldest hf model ages for zircons of neoarchean grenvillian and pan african age group yield 3 7 ga or older while those of mesoproterozoic age group show 3 3 ga the remarkable similarity of age distribution and hf isotope compositions among detrital zircons of indochina and those of tethyan himalaya western cathaysia and qiangtang suggests that indochina was located outboard of qiangtang and south of south china in the indian margin of gondwana during the early paleozoic our results are consistent with the paleontological correlations of east gondwana margin during the early paleozoic

40ar 39ar geochronology of the neogene quaternary harrat al madinah intercontinental volcanic field saudi arabia implications for duration and migration of volcanic activity abstract new 40 ar 39 ar ages based on incremental heating techniques for groundmass separates of 25 samples are presented for the harrat al madinah volcanic field part of harrat rahat in the north western part of the arabian plate this area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 ad field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions in addition to local more explosive eruptions the lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite benmoreite and trachyte that occur mainly as domes tuff cones and occasionally as lava flows previous k ar dating shows that the harrat al madinah lava flows and associated domes comprise seven units spanning an age range of ca 1 7 ma recent the new 40 ar 39 ar age determinations confirm to a great extent the previously obtained k ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units however the 40 ar 39 ar plateau ages show that volcanism in this area began in the neogene 10 ma and continued to recent with the most voluminous eruptions occurring in the quaternary neogene volcanism occurred in at least three pulses around 10 5 and 2 ma whereas quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1 90 ma recent thus the whole duration of volcanic activity in the harrat al madinah 10 ma recent appears much longer than that previously identified the longevity of volcanism in the same part of the moving arabian plate and absence of evidence for uni directional migration of volcanic activity indicate that there is no fixed plume beneath this region the nnw trending distribution of the volcanic vents is parallel to the red sea and suggests their origin is related to periodic extensional episodes along the reactivated red sea fault system

detrital zircon u pb ages and hf isotopes of lower middle devonian to middle jurassic sandstones in the qinfang basin southern south china block constraints on provenance and tectonic setting abstract the affinity and tectonic setting of the qinfang basin have long been an enigma here we conduct detrital zircon u pb geochronology and hf isotope analysis of lower middle devonian to middle jurassic samples from the basin along with published age data and paleontology from the basin and its east and west sides this study constrains the sediment provenance and tectonic setting of the basin sandstones of devonian jurassic exhibit an age distribution with major age groups at 2700 2300 ma absence for jurassic 1200 800 ma 600 400 ma and 334 246 ma for jurassic alone zircons peak 2500 ma were sourced from the yangtze block while 1200 800 ma grains may originate from the yunkai massif and jiangnan orogen 600 400 ma zircons came from yunkai massif and or cambrian to silurian sediments in the basin carboniferous early triassic 334 246 ma zircons were probably supplied by an older sequence in the great youjiang basin and the darongshan shiwandashan igneous rocks the qinfang basin and its two sides are considered to share a common east gondwana source in silurian the similarity in age spectra between silurian devonian samples in the basin and ailaoshan belt suggests that the basin is a remnant of the paleo tethyan ailaoshan ocean however the distinct age distribution between the basin and indochina block shows that the basin is not a part of the indochina block by middle jurassic following the final closure of the ailaoshan ocean in the late triassic the adjacent western area may be still tectonically active thereby feeding the basin with detritus of early permian

helium isotopes in alpine regions abstract the isotopic composition of helium has been determined for a variety of groundwaters from springs and boreholes in the alps and adjacent regions the main alpine block in france italy switzerland and austria is characterized by a virtual absence of mantle derived 3he in near surface groundwaters in contrast adjacent areas where crust has been thinned by extension to the west in the rhone valley and ardeche to the north in the rhine valley and to the east in the vienna and pannonian basins mantle derived helium is clearly identified helium and undoubtedly other gases are introduced by melts whose generation is associated with crustal thinning however the surface distribution of mantle helium covers a much larger than that of associated surface volcanics this probably results from the dispersion of helium following its solution in groundwaters the po and the molasse neogene sedimentary basins developing by loading on the south and the north sides of the alps respectively have helium with typical crustal composition and a mantle component cannot be resolved these results clearly indicate the control of tectonics on the distribution of helium isotopes in the alpine regions

the 1997 98 umbria marche earthquake sequence geological vs seismological faults abstract the 1997 98 umbria marche earthquake provided a huge amount of good quality seismological data the geology of the region has been extensively mapped for over a century by geologists from all over the world the subsurface geology is constrained by a set of seismic reflection profiles and deep boreholes acquired for oil exploration the earthquake stimulated the acquisition of many other geological and geophysical data sets surface ruptures structural analysis geodetic data geoeletrical data etc as a consequence the 1997 98 earthquake is one of the best case studies in the world for extensional earthquakes driving the possibility of building up an integrated model of an extensional system we use this favourable test site to discuss the possibility of integrating and connecting the seismological faults i e the short term instantaneous ruptures generated during the earthquakes representing the effect of a single event and the geological faults i e the long term active faults exposed at the surface that are the effects of multiple repeated events of faulting the review of this complex data set points to an integrated model where the moderate seismicity of the region is driven by sw dipping normal faults whose dip changes with depth from 60 70 the fault surfaces mapped at the surface to 35 45 the fault segments ruptured during the earthquake at depths of 2 8 km to less than 20 at depth 8 km these normal faults are not inherited from previous tectonic events however the pre existing arc shaped compressional structures act as barriers segmenting the extensional fault system the depth and thickness of the seismogenic layer is lithologically controlled since the seismicity is confined within the sedimentary cover and do not penetrate the underlying basement the mainshocks are located close to the bottom of the seismogenic layer within the triassic evaporites

rn he and co2 soil gas geochemistry for the study of active and inactive faults abstract two italian areas characterized by different seismological histories were investigated to enhance the basic knowledge of gas migration mechanisms during earthquakes sharp variations occur in the movement and concentration of some gaseous species due to the evolution of the local stress regime the first area colpasquale is located in the central italian region of marche and provided a good location to study gas migration in a seismically active region the area was devastated by a sequence of shallow earthquakes over a 3 month long period september december 1997 the occurrence of this catastrophic event as well as the long duration of the seismic sequence presented a unique opportunity to study gas migration in a zone undergoing active displacement soil gas surveys were performed 1 day 1 week 1 year and 2 years after the main shock ms 5 6 in the colpasquale area in particular results highlight a change in the rn distribution during the three monitoring years indicating a variation of gas migration that may be linked to the evolution of the stress regime the second study area is located in the campidano graben southern part of sardinia island this area is characterized by seismic quiescence displaying an almost complete lack of historical earthquakes and instrumentally recorded seismicity the consistently low values observed for all analyzed gases suggest that the studied area is likely characterized by sealed non active faults that prevent significant gas migration the comparison of data from both studied areas indicate that soil gas geochemistry is useful to locate tectonic discontinuities even when they intersect non cohesive clastic rocks near the surface and thus are not visible i e blind faults

pore fluid in experimental calcite bearing faults abrupt weakening and geochemical signature of co seismic processes while it is widely recognized that fluids influence fault strength and earthquake nucleation propagation and arrest their effects on co seismic sliding friction are only conjectured to shed light on these effects 55 high velocity 1ms 1 friction experiments were conducted at room temperature on carrara marble samples in the presence of pore fluid up to 15mpa pore pressure room humidity and vacuum 10 4mbar conditions in all the experiments the friction coefficient evolved from a peak value of 0 6 0 8 to a steady state value of 0 1 in about 1 1 5m of slip however experiments performed in the presence of pore fluid had a large and more abrupt decrease in friction at the initiation of sliding 65 after 20mm of slip whereas experiments performed under vacuum and room humidity conditions showed initial velocity strengthening behavior followed by a more gradual reduction in friction this indicates that calcite bearing rocks are more prone to slip in the presence of water under room humidity conditions co2 was detected during the entire duration of the experiment in the presence of pore fluid hco3 and ca2 were detected for slips 0 1m the lack of decarbonation products hco3 and ca2 in pore fluid experiments for slip 0 1m implies that the abrupt weakening is not related to decarbonation or that the abundance of the reaction products is below the resolution of the analytical methods given the modest thermal expansion of water the estimated thermal pressurization during the abrupt weakening appears to be negligible instead we suggest that abrupt weakening is due to subcritical crack growth hydrolytic weakening and brittle failure of the asperities on the sliding surfaces modeling shows that the occurrence in nature of co seismic water present decarbonation reactions similar to those triggered in the laboratory could yield sufficient reaction product to be detected in aquifers located in the proximity of active faults 2012 elsevier b v

crustal shortening and duplication of the moho in the northern apennines a view from seismic refraction data abstract a reappraisal of the dss refraction seismic campaigns of 1978 and 1974 in the northern apennines and the northern tyrrhenian sea after digitization of the original analog data and a new interpretation of crustal structures are presented in this paper the layering of the adria crust consists of a low velocity lower crust topped by a 6 7 km s horizon and an upper crust again formed by a low velocity layer capped by a faster one in the tuscany sector the crust mantle boundary and lower crust show attenuated velocities and a reduced thickness with respect to the adriatic counterpart these differences are due to the extensional tectonics restricted to tuscany and occurred in a high heat flow regime related to an uplifting astenosphere it has been confirmed that a deep thrust is responsible for moho doubling in correspondence with the zero bouguer anomaly line the tuscan moho overlaps the umbro marchean one for a length of about 30 km it is hypothesized that another shallower thrust involves upper crust in the mid tuscany range mt cetona and perugia massifs had their roots in the mantle and that subsequent extensional tectonics obliterated every trace of thrusting in lower crust and mantle rocks no conclusive proof of the existence of a european moho below a tuscan one in the area underneath elba has yet been found revision of dss data excludes the presence of a refractor reflector 60 km deep the same data however are compatible with the existence of a crust mantle boundary at 30 35 km depth

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

indian derived sediments deposited in australia during gondwana assembly abstract sedimentary exchanges across continents during the collisional assembly and lifespan of supercontinents provide a powerful way of testing the assembling process and configuration of supercontinents the ord basin in north western australia contains mid cambrian shallow marine and deltaic sedimentary successions that post date the ca 650 520 ma paterson petermann orogeny ppo but are synchronous with the ca 530 470 ma kurgiakh orogeny in north india we report new u pb ages and hf isotopes of detrital zircons from the mid cambrian ord basin that suggest indian derived sedimentary input to australian cambrian basins compilations of published ediacaran ordovician samples from the centralian superbasin indicate that during the ca 650 520 ma ppo the 1200 1100 ma musgrave province in central australia was exhumed and fed sediments to the syn ppo centralian superbasin soon after the ppo termination the musgrave province likely remained a topographic high and continued to supply detritus to the amadeus and officer basins that were fragmented from the centralian superbasin supplemented by the 630 ma granites of the paterson orogen conversely we conclude that the mid cambrian ord basin did not record similar sedimentary provenance from the musgrave province rather indian derived detritus travelled along the northeastern gondwanan continental margin via longshore currents and in turn incorporated paterson granitic detritus during the assembly of gondwana including the kurgiakh orogeny in north india a broad provenance comparison between potential eastern gondwanan source regions and relevant asian neighbours suggests that south china and south qiangtang are affiliated with northern india but lhasa appears to have had a mixed affiliation between india and australia

gas geochemistry applied to earthquake prediction an overview terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas especially in ussr china japan and united states in search of premonitory changes that might be useful for earthquake prediction concentrations of radon helium hydrogen mercury carbon dioxide and several other volatiles have been found generally to be anomalously high along active faults suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers these sensitive stations are generally located along active faults especially at intersections or bends of faults or some other structurally weak zones possibly because of tectonic strain concentration at such places however gas concentrations especially those measured at or near the ground surface may also be significantly affected by meteorological hydrological and other nontectonic changes in the environment the nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake related anomalies

origin and interactions of fluids circulating over the amik basin hatay turkey and relationships with the hydrologic geologic and tectonic settings abstract we investigated the geochemical features of the fluids circulating over the amik basin se turkey syria border which is crossed by the northern extension of the dsf dead sea fault and represents the boundary area of three tectonic plates anatolian arabian and african plates we collected 34 water samples thermal and cold from natural springs and boreholes as well as 8 gas samples bubbling and gas seepage besides the gases dissolved in the sampled waters the results show that the dissolved gas phase is a mixture of shallow atmospheric and deep components either of mantle and crustal origin coherently the sampled waters are variable mixtures of shallow and deep ground waters the latter being characterised by higher salinity and longer residence times the deep groundwaters from boreholes deeper than 1000 m have a ch 4 dominated dissolved gas phase related to the presence of hydrocarbon reservoirs the very unique tectonic setting of the area includes the presence of an ophiolitic block outcropping in the westernmost area on the african plate as well as basalts located to the north and east on the arabic plate the diffuse presence of co 2 enriched gases although diluted by the huge groundwater circulation testifies a regional degassing activity fluids circulating over the ophiolitic block are marked by h 2 dominated gases with abiogenic methane and high ph waters the measured 3 he 4 he isotopic ratios display contributions from both crustal and mantle derived sources over both sides of the dsf although the serpentinization process is generally independent from mantle type contribution the recorded helium isotopic ratios highlight variable contents of mantle derived fluids due to the absence of recent volcanism over the western side of the basin african plate we argue that co 2 rich volatiles carrying mantle type helium and enriched in heavy carbon are degassed by deep rooted regional faults rather than from volcanic sources

kwangsian crustal anatexis within the eastern south china block geochemical zircon u pb geochronological and hf isotopic fingerprints from the gneissoid granites of wugong and wuyi yunkai domains abstract gneissoid granites were traditionally thought to be the components of the precambrian basement in the eastern south china block but twenty four gneissoid granite samples from the wugong northern wuyi southern wuyi and yunkai domains gave zircon u pb ages of 424 456 ma 410 457 ma 426 438 ma and 415 450 ma respectively the existence of the abundant kwangsian gneissoid granites which have incorrectly been assigned to the precambrian basement indicates that the so called precambrian stratigraphic sequence of the cathaysia block should be termed complex rather than group these gneissoid granites are peraluminous granites with relatively high al 2 o 3 mgo tio 2 feot and cao na 2 o but low cao al 2 o 3 mgo feot and rb sr ratios they exhibit strongly negative ba sr nb p and ti and positive pb anomalies in primitive mantle normalized diagrams and have initial 87 sr 86 sr ratios ranging from 0 70924 to 0 72935 and negative end t values from 6 4 to 11 4 similar to those of paragneissic enclaves and precambrian sedimentary rocks in the eastern south china block the zircons crystallized in the kwangsian gneissoid granites gave ehf t values ranging from 2 4 to 19 4 with the peak at 4 almost all grains clustering at 1 to 12 and hf model ages of 1 1 2 1 ga the synthesis of these geochemical and in situ zircon hf isotopic data indicates that the kwangsian gneissoid granites dominantly originated from proterozoic metapelitic and meta igneous components with insignificant input of juvenile mantle derived magmas a model of two stage crustal anatexis is proposed for the kwangsian granitic magma in the eastern south china block with 1 the formation of 460 430 ma granite through the breakdown of hydrous minerals under the condition of the doubly thickened crust and 2 the generation of the 430 400 ma granite accompanying promoted melting along a path of isothermal decompression due to the increasing thermal weakening for the collapse of the thickened crust the crustal anatexis has probably occurred in an intracontinental tectonic regime that was geodynamically linked to the far field response to the assembly of the australian indian plate with the cathaysia block during middle paleozoic kwangsian time

components and episodic growth of precambrian crust in the cathaysia block south china evidence from u pb ages and hf isotopes of zircons in neoproterozoic sediments abstract detailed lam icpms u pb dating and lam mc icpms lu hf isotope analysis were carried out on zircons from nine samples of basement metamorphic rocks in the southern cathaysia block south china the chemical compositions of these metamorphic rocks and zircon morphology indicate that their protoliths were sedimentary zircon u pb dating results show they mainly formed during the late neoproterozoic the precambrian crust of the cathaysia block can be divided into two distinct tectonic domains the wuyishan terrane to the northeast and the nanling yunkai terrane to the southwest the nearly e w boundary between these two terranes is also supported by geophysical petrological and geochemical evidence and the basement rocks of the two terranes are comprised of different components the wuyishan terrane is characterized by dominant paleoproterozoic 1 86 ga and lesser neoarchean magmatism and the southern central part of the terrane suffered strong neoproterozoic and early paleozoic tectonothermal reworking the zircon data suggest that there were five important episodes of juvenile crust generation 3 6 ga 2 8 ga 2 6 2 4 ga 1 85 ga and 0 8 0 7 ga in the wuyishan terrane strong paleoproterozoic 1 85 ga magmatism in northern wuyishan and neoproterozoic 0 8 0 7 ga magmatism in central and southern wuyishan both principally involved the remelting of old crust with minor input of juvenile material in contrast the nanling yunkai terrane contains abundant neoarchean and grenvillian zircons some evidence of mesoproterozoic activity and rare paleo to meso archean and neoproterozoic zircons u pb ages and hf isotopic compositions of the detrital zircons from the late neoproterozoic to early paleozoic meta sedimentary rocks in the nanling yunkai area suggest that the growth of juvenile crust mainly occurred at 3 6 ga 3 3 ga 2 5 2 6 ga 1 6 ga 1 0 ga and 0 8 0 7 ga neoarchaean 2 6 2 5 ga grenvillian 1 0 ga and neoproterozoic 0 8 0 7 ga magmatism mainly involved the recycling of old crust minor remnants of 4 1 ga crust may remain in the terrane this crustal history is distinct from that of the wuyishan terrane and analogous to those recorded for the eastern india east antarctic domain of northern gondwanaland suggesting that they probably were once linked

early palaeozoic orogenesis along the indian margin of gondwana tectonic response to gondwana assembly abstract shrimp u pb dating of zircons from a peralkaline s type lesser himalayan granite from the kathmandu region nepal indicate an age of emplacement of 475 ma the granites along with metasedimentary xenoliths show a similar signature of inherited detrital zircon ages ranging from archaean to early palaeozoic with prominent late mesoproterozoic grenvillian and neoproterozoic pan african age peaks and have a maximum age of 500 ma based on the youngest detrital grains deformation structures in xenoliths are truncated by the granite and along with the granites are assigned to a cambro ordovician orogenic event herein termed the bhimphedian orogeny that can be traced across the himalaya from pakistan to the eastern himalaya and possibly extends west into afghanistan we interpret the orogeny as being related to andean type orogenic activity on the northern margin of the indian continent following gondwana assembly the magmatic arc was associated with andesitic and basaltic volcanism and was active from c 530 to 490 ma the arc activity overlaps with and is succeeded by regional deformation crustal melting and s type granite emplacement that extended to 470 ma orogenic activity was driven by coupling across the plate margin either during on going subduction or through accretion of microcontinental ribbons possibly represented by the lhasa and qiangtang blocks it represents the termination of an orogenic cycle termed the north indian orogen that commenced with neoproterozoic rifting and passive margin development and terminated with the bhimphedian orogeny this was succeeded by a return to passive margin setting along the northern indian margin of gondwana which continued until the cenozoic himalayan orogeny

rate of diffuse carbon dioxide earth degassing estimated from carbon balance of regional aquifers the case of central apennine italy central italy is characterized by an anomalous flux of deeply derived co2 in the western tyrrhenian sector of central italy co2 degassing occurs mainly from focused emissions vents and strong diffuse degassing and thermal springs whereas in the eastern apennine area deep co2 is dissolved in cold groundwaters of regional aquifers hosted by mesozoic carbonate evaporite formations influx of deep co2 into 12 carbonate aquifers 12 500 km2 of the central apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data mass balance calculations estimate that 6 5 1010 mol yr 1 of inorganic carbon are dissolved in the studied aquifers approximately 23 of this amount derives from biological sources active during the infiltration of the recharge waters 36 comes from carbonate dissolution while 41 is representative of deep carbon sources characterized by a common isotopic signature 13c 3 the calculated deep co2 influx rate ranges from 105 to 107 mol yr 1 km 2 increasing regionally from east to west in the study area

imaging ruptured lithosphere beneath the red sea and arabian peninsula abstract the red sea rift an archetype of a newly formed ocean basin is an ideal environment in which to study the controversial processes associated with continental rifting different models have been proposed to explain how rifting in the red sea evolved however accurate constraints on lithospheric structure have not been available to discriminate rifting models we use the s wave receiver function technique to produce the first images of the lithosphere asthenosphere boundary lab structure along the red sea and throughout the arabian peninsula lithospheric thickness varies considerably with thin lithosphere centered on the rift axis thickening toward the arabian interior gravity data are well fit by our structural model and indicate that high surface topography along the rift flank is not in isostatic equilibrium requiring dynamic compensation for its support while our derived structure is consistent with active rifting processes previous studies demonstrated that the red sea initiated as a passive rift therefore our results suggest a two stage rifting history where extension and erosion by flow in the underlying asthenosphere are responsible for variations in lab depth lab topography guides asthenospheric flow beneath western arabia and the red sea demonstrating the important role lithospheric variations play in the thermal modification of tectonic environments

moment tensor analysis of the central italy earthquake sequence of september october 1997 the larger earthquakes in the umbria marche central italy seismic sequence of september october 1997 are analyzed using long period seismograms from the mediterranean seismographic network mednet and additional data from the global seismographic network gsn we modify the harvard centroid moment tensor cmt algorithm to allow moment tensor inversion of long period waveforms primarily rayleigh and love waves for small earthquakes at local to regional distances 5 5 in the sequence moment tensors have previously been determined using teleseismic waveforms and standard methods of analysis our results agree well with those of earlier studies we determine additional moment tensors for the largest foreshock and 10 aftershocks with mw 4 2 the earthquakes are characterized by normal faulting mechanisms with a ne sw tension axis and the presumed fault plane dips towards the sw only one of the fourteen events studied has a different faulting geometry indicating instead right lateral strike slip faulting on a plane oriented approximately e w or left lateral faulting on a plane oriented n s the september 26 mainshock 09 40 ut accounts for only approximately 50 of the total moment release in the sequence

triassic tectonics of the ailaoshan belt sw china early triassic collision between the south china and indochina blocks and middle triassic intracontinental shearing in se yunnan the ailaoshan belt has been extensively studied for the ductile shearing coeval with the left lateral cenozoic activity of the red river fault however the late triassic unconformity of the continental red beds upon metamorphic and ductilely deformed rocks demonstrates that the ailaoshan belt was already built up by early mesozoic tectonics from west to east the belt is subdivided into western central eastern ailaoshan and jinping zones the western ailaoshan and central ailaoshan zones correspond to a carboniferous permian magmatic arc and an ophiolitic melange respectively the eastern ailaoshan and the jinping zones consist of deformed proterozoic basement and paleozoic to early triassic sedimentary cover series both belonging to the south china block this litho tectonic zonation indicates that the ailaoshan belt developed through a sw directed subduction followed by the collision between indochina and south china blocks crustal thickening triggered per aluminous magmatism dated at ca 247 240 ma field and microscope scale top to the ne ductile shearing observed only in the pre late triassic formations but never in late triassic or younger formations complies with this geodynamic polarity furthermore the late collisional two mica granitoids and felsic per aluminous volcanites record a ductile deformation that argues for a continuing crustal shearing deformation after the early triassic collision up to the middle triassic therefore a two stage tectonic evolution accounts well for the documented structural and magmatic features the triassic architecture of the ailaoshan belt and its geodynamic evolution correlate well to the south and north with the north vietnam orogens and the jinshajiang belt respectively

the isotopic composition of yb and the precise and accurate determination of lu concentrations and lu hf ratios by isotope dilution using mc icpms 1 we report yb isotopic data for the purpose of accurately correcting measurements of lu and lu hf ratios during multicollector inductively coupled plasma mass spectrometer mc icpms analysis the yb isotopic ratios agree with recent results of chu et al 2002 and amelin and davis 2004 when corrected for mass fractionation to the same reference values protocols for lu measurement involving simultaneous subtraction of yb interference and application of a fractionation correction based on yb are described it is shown that lu concentrations and lu hf ratios determined by mc icpms using this methodology are more precise and potentially more accurate than results based on analysis of lu by thermal ionization mass spectrometry tims a number of mc icpms lu hf isotopic analyses of standard rocks and other geological samples previously analyzed by tims are presented these document reproducibility of lu hf within 0 2 where most discrepancies can be assigned to the unknown lu mass fractionation during tims analysis

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

lithospheric structure of the arabian shield from the joint inversion of receiver functions and surface wave group velocities abstract we estimate lithospheric velocity structure for the arabian shield by jointly modeling receiver functions and fundamental mode group velocities from events recorded by the 1995 1997 saudi arabian portable broadband deployment receiver functions are primarily sensitive to shear wave velocity contrasts and vertical travel times and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages so that their combination bridge resolution gaps associated with each individual data set our resulting models correlate well with the observed surface geology the asir terrane to the west consists of a 10 km thick upper crust of 3 3 km s overlying a lower crust of 3 7 3 8 km s in the afif terrane to the east the upper crust is 20 km thick and has an average velocity of 3 6 km s and the lower crust is about 3 8 km s separating the terranes the nabitah mobile belt is made of a gradational upper crust up to 3 6 km s at 15 km overlying an also gradational lower crust up to 4 0 km s the crust mantle transition is found to be sharp in terranes of continental affinity east and gradual in terranes of oceanic affinity west the upper mantle shear velocities range from 4 3 to 4 6 km s temperatures around 1000 c are obtained from our velocity models for a thin upper mantle lid observed beneath station taif and suggest that the lithosphere could be as thin as 50 60 km under this station

co2 degassing over seismic areas the role of mechanochemical production at the study case of central apennines field observations coupled with experimental results show that co2 can be produced by mechanical energy applied to carbonate rocks becoming an unexpected additional gas source besides that degassed from the mantle or produced by thermometamorphism the evidence that a large amount of carbon dioxide associated with radiogenic type helium r ra as low as 0 01 0 08 is released through continental areas denotes the absence of a contribution from the mantle or from mantle derived fluids data collected during the seismic crisis which struck the central apennines in 1997 98 have shown an enhanced co2 flux not associated with the presence of mantle or thermometamorphic derived fluids on the other hand new experimental results highlight the possibility of producing co2 by mechanical energy that acts on the calcite crystalline lattice while the co2 released over the geothermal areas e g larderello geothermal field is obviously derived by mantlederived activities this is not the case of the huge amount of co2 released over the seismically active areas where the presence mantle derived products is ruled out we propose that mechanical energy e g released during seismic events microseismicity or creeping processes is a possible additional energy source able to produce co2 and thus could explain the presence of co2 degassing over tectonic areas where the influence of the mantle is low