multi parametric climatological analysis associated with global significant volcanic eruptions during 2002 2017 in this work we search for physical and chemical climatological anomalies preceding major volcanic explosive eruptions mostly vei volcanic explosivity index 4 occurred from 2002 to 2017 by applying two specific algorithms i e capri and means the former algorithm has been already used for a multi climatological analysis of the amatrice norcia 2016 2017 earthquake preparatory phase piscini et al in pure appl geophys 174 3673 3688 2017 here we analyse some climatological parameters for a three month period before each volcanic explosive eruption then we compare the behavior with the typical one of the past the analysis is applied to an area with dimensions comparable to the volcano crater because it is the increase of the magmatic camera activity that in turn can cause a temperature increase whose evidence could be detected at the surface slezin in j volcanol geotherm res 122 1 2 7 50 2003 while the use of a larger area would provide a greater probability of occurrence of other events e g other volcano eruption meteorological storms etcetera therefore a smaller area of study reduces the risk to get false alarms in particular we considered thermal skin temperature skt and total water vapour content tcwv from ecmwf european centre and aerosol optical thickness aot sulphur dioxide so2 and atmospheric dimethylsulphide dms are obtained from nasa merra 2 global modeling and assimilation data archive the latter compound was added in the analysis to check the validity of the method since we did not expect significant anomalies from this parameter the models above described are used for their temporal spatial completeness allowing performing time series analyses and for a real time monitoring on a global scale by simultaneous analysis we found for almost all volcanic eruptions some anomalies in about all analyzed parameters that precede by 75 days to 20 days the explosion these anomalies are not always simultaneous but we find an interesting synchronicity that probably reveals a correlation among the different datasets in addition the agung volcano which has recently started an eruptive activity 25 november 2017 without reaching a vei4 explosion has also been investigated the data related to this volcano present a small number of anomalies much lower than all attributed to the other analyzed explosive volcanoes and this is in high agreement with the low explosive nature of the agung volcano eruption we find that the occurrence of thermal anomalies typically preceding stratovolcano caldera eruptions seems to take place some days after so2 emissions the climatological anomalies that precede eruptions at high latitudes usually surround the volcano in a wider area outside the volcanic edifice we verify that the number of positive anomalies is systematically greater in the year that precedes the investigated eruption with respect to a quiet year of comparison with accuracy from 91 to 100 suggesting that the applied methods detected climatological anomalies likely related to imminent volcanic eruptions

a deforming plate tectonic model of the south china block since the jurassic abstract the south china block has experienced multiple phases of tectonic deformation since the middle jurassic understanding this deformation history helps reveal the driving mechanisms responsible and sheds light on the associated magmatism and mineralization here we construct deformable plate tectonic reconstructions for the south china block from 170 ma to the present using gplates the deformation starts with a nw se oriented shortening phase from 170 to 136 ma subsequently south china experienced two episodes of extension in the cretaceous during 136 118 ma and 100 80 ma respectively the cenozoic extension occurred mostly along the southeast continental margin including the east china sea shelf basin and rift basins in the northern south china sea margin and the taiwan margin segment the rifting was initiated nearly simultaneously along the continental margin at 65 ma and ceased at 34 30 ma the northern portion of the okinawa trough started rifting at 15 ma while the southern portion rifted in the last few million years the luzon volcanic arc thrusting onto the southeasternmost south china block since 6 5 ma caused the still active taiwan orogeny the predicted accumulated deformation at present day is characterized by overall shortening onshore with contraction factors mostly between 0 8 and 1 and extension offshore with stretching factors generally between 1 and 2 the central china orogen and taiwan orogen experienced more intense shortening relative to the interior south china by factors down to 0 6 the okinawa trough experienced intense stretching by factors up to 2 5 in its central part the intraplate deformation in the south china block is closely related to the subduction of the izanagi and pacific plates the jurassic and earliest cretaceous shortening was likely induced by trench advance and the extension during the cretaceous and the cenozoic was mostly driven by subduction retreat

the characteristics and formation mechanism of a regional fault in shale strata insights from the middle upper yangtze china abstract shale is a type of fracture prone rock but few regional faults are observed in shale strata here we investigate a regional detachment fault in the middle upper yangtze muy to understand the formation mechanism of a special regional fault in shale strata based on a lithological mineralogical and mechanical study as well as semi quantitative model of displacement we found that the formation of this detachment fault is closely related to the lithological association and flexural slip processes the lithological association of this detachment fault is characterized by two thick competent layers separated by a thin weak interlayer at both the macroscopic and microscopic scales when these layers are folded the detachment fault zone is commonly present in a thin weak interlayer namely the wufeng formation at the macroscopic scale and primarily along interfaces in laminar siliceous shales lss and secondarily in k bentonite interlayers within the lss at the microscopic scale the regional fault zone is characterized by parallel to strata surfaces and abrupt contacts with the protolith the fabric of the fault core is obviously controlled by the flexural slip intensity and the number of slip events as well as their direction and structural position the natural shale gas obtained from well hd 1 has proven that this regional detachment fault in shale strata is not only a migration pathway for hydrocarbons but also acts as an adsorption state reservoir for free gas from other areas this regional detachment fault in shale strata is likely to advance the shale gas industry and to expand the exploration of conventional gas

sinoprobe deep reflection profile reveals a neo proterozoic subduction zone beneath sichuan basin abstract a new multichannel seismic reflection profile collected across the sichuan basin in southern china by sinoprobe images prominent reflectors that originate within the lower crust and penetrate well into the underlying mantle the geometry of these mantle reflectors is very similar to those observed on other deep reflection profiles that have been interpreted as relicts of ancient subduction considering the geological history of the basement beneath and surrounding sichuan basin and ages of granites encountered in nearby wells we propose that these newly revealed reflectors are the remnants of neo proterozoic subduction that occurred along the nw margin of the yangtze craton this interpretation is consistent with geochemical studies from a gneissic complex at the west margin of the sichuan basin moreover preservation of these reflectors supports the idea that the sichuan lithosphere served as a consolidated tectonic buttress against which the tibetan plateau has impinged to produce the longmenshan orogenic belt

seismic evidence of mylonite reflectivity and deep structure in the kettle dome metamorphic core complex washington the first direct test of the seismic reflectivity of mylonites is provided by a seismic reflection profile over a mylonite zone exposed in the kettle dome metamorphic core complex northeastern washington the mylonite zone developed at mid crustal levels and has undergone a minimum of retrogression and cataclasis during uplift downdip projection of the exposed mylonite zone into the seismic profile indicates that the compositionally layered mylonite zone is reflective and traceable to at least 5 km depth as a complex zone of multicyclic reflections the seismic data also show that the crust beneath the kettle dome is characterized by abundant strongly reflective moderately dipping layers down to at least 18 km these layers can be interpreted in terms of compressional or extensional orogenic events or a combination of both models for the structure and genesis of metamorphic core complexes must take these data into account

postrifting anelastic deformation around the spreading plate boundary north iceland 2 implications of the model derived from the 1987 1992 deformation field a decade long rifting episode that began in the krafla volcanic system north iceland in 1975 caused substantial regional postevent anelastic deformation this was modeled as viscous relaxation in an elastic viscoelastic structure by hofton and foulger this issue the results from modeling the deformation detected in north iceland have far reaching implications both for local and regional processes and for the fundamental behavior of deformation around spreading plate boundaries in general tilt in the vicinity of the krafla volcano fits the model well after 1988 1989 which suggests that the volcano magma chamber stopped inflating deflating in 1988 a viscosity of 0 8 1018 pa s was required to match the local tilt data less than that predicted for north iceland as a whole vertical motion measured using the global positioning system gps 1987 1992 around the ice cap vatnajokull is inconsistent with isostatic uplift using the elastic viscoelastic model to predict motion in other regions of iceland suggests that the deformation effects of the krafla episode are significant in many parts of iceland and should be taken into account when modeling deformation there though not a realistic plate boundary model interesting complexities of the elastic viscoelastic model are highlighted by deformation modeling of an infinitely long dike this predicts that the amount of horizontal displacement close to the dike may exceed the amount of initial dike opening early in the spreading cycle a more realistic approximation to the plate boundary in north iceland involving five overlapping segments that experience dike emplacements at discrete intervals suggests that the width of the zone within which transient time dependent deformation occurs may be several hundred kilometers wide considerably wider than the neovolcanic zone a kinematic approach to describing plate motions is not appropriate close to spreading plate boundaries and elsewhere where the viscosity of the earth is low

boundary current eddies and their role in the restratification of the labrador sea an idealized model is used to study the restratification of the labrador sea after deep convection with emphasis on the role of boundary current eddies shed near the west coast of greenland the boundary current eddies carry warm buoyant irminger current water into the labrador sea interior for a realistic end of winter state it is shown that these irminger current eddies are efficient in restratifying the convected water mass in the interior of the labrador sea in addition it is demonstrated that irminger current eddies can balance a significant portion of the atmospheric heat loss and thus play an important role for the watermass transformation in the labrador sea

eastward tectonic migration and transition of the jurassic cretaceous andean type continental margin along southeast china abstract a mesozoic andean type active continental margin is believed to have developed along the southeast china continental margin which has been investigated in detail however the architecture and evolution of its retroarc arc forearc system which is important for us to understand the interaction or connection between continental and oceanic lithospheres are ambiguous using multiple disciplinary data comprising seismic profiles field observations and numerical paleo topographies this study synthetically focused on the jurassic cretaceous evolution and geodynamics of the andean type southeast china continental margin subduction of the paleo pacific plate was initiated during the early jurassic 200 ma generating a nne striking jurassic magmatic arc along the east china sea bounded by imbricate thrust faults on both sides and coinciding with a paleo topographic coastal mountain range while the jurassic arc erupted along an ene striking triassic suture zone in the northern south china sea dominated by the tethyan tectonic domain and overprinted by the pacific tectonic domain the jurassic coastal mountain range began to collapse and transited into a rift basin at 135 ma and then the andean type continental margin gradually switched to the western pacific type during the late cretaceous 100 72 ma accompanied by jurassic thrust faults inverting into cretaceous detachment faults and magmatic arc migrated eastward to the taiwan rukyu areas such eastward tectonic migration and transition is associated with the eastward retreat of the subducted paleo pacific slab then a regional latest cretaceous 72 66 ma compression prevailed southeast china probably related to ridge subduction when the pacific plate replaced the paleo pacific plate to subduct beneath east asia

gravity and elevation changes at askja iceland ground tilt measurements demonstrate that askja is in a state of unrest and that in the period 1988 1991 a maximum 48 3 rad tilt occurred down towards the centre of the caldera this is consistent with 126 mm of deflation at the centre of the caldera with a 2 5 3 0 km depth to the source of deformation the volume of the subsidence bowl is 6 2x106 m3 when combined with high precision microgravity measurements the overall change in sub surface mass may be quantified after correction for the observed elevation change using the free air gradient of gravity measured for each station the total decrease in mass is estimated to be less than 109 kg a small residual ground inflation and net gravity increase in the southeastern part of the caldera may be caused by dyke intrusion in this region the minimum dimensions of such an intrusion or complex of intrusions are 1 m width up to 100 m deep and up to several hundred metres thick

the cordilleran ribbon continent of north america the north american cordilleran orogen is the result of a two stage process a triassic jurassic accretion within panthalassa forming saybia a composite ribbon continent and b late cretaceous collision of saybia with north america this model requires that a large portion of the continental foreland of the orogen is exotic the exotic continental component of saybia cassiar platform is distinguished from the autochthon on the basis of its a triassic eurasian fauna b involvement in a major late triassic early jurassic orogenic event and c young in part grenvillian basement and mantle a mid cretaceous magmatic arc records west dipping subduction beneath the east margin of saybia the related accretionary prism consists of imbricated shale chert and deep water limestones the medial basin and overlies an isotopically juvenile mantle domain carbonatite complexes delineate the cryptic suture separating saybia and the autochthon paleomagnetic and paleobotanical data place saybia 2000 km to the south relative to the autochthon at 80 ma late cretaceous thrust belt development records transpression between the north moving ribbon continent and the autochthon pinning against the okhotsk chukotka arc in siberia buckled saybia giving rise to the alaskan promontory

petrogenesis of late jurassic two mica granites and associated diorites and syenite porphyries in guangzhou se china abstract the south china block scb is considered to have undergone extensive reworking of continental crust however the mechanisms of this crustal reworking are unclear here we report on late jurassic 162 155 ma two mica granites and associated diorites and syenite porphyries from the huolushan longyandong area of guangzhou city southeastern china the two mica granites have high sio2 contents 69 6 73 6 wt and weakly peraluminous to strongly peraluminous compositions these rocks have enriched sr nd hf isotopic compositions 87sr 86sr i 0 7094 0 7145 end t 10 5 to 9 3 ehf t 12 7 to 4 2 the huolushan two mica granites have variable zircon o isotopic compositions 18o 6 8 10 4 whereas the longyandong two mica granites have homogeneous compositions 18o 8 6 9 9 the huolushan syenite porphyries have moderate sio2 59 6 60 1 wt and high alkaline k2o na2o 9 4 9 8 wt contents their sr nd hf o isotopic compositions 87sr 86sr i 0 7105 0 7119 end t 9 9 to 9 8 ehf t 14 1 to 5 9 18o 6 5 10 6 are similar to those of the huolushan two mica granites the huolushan diorites have moderate sio2 contents 59 9 60 4 wt and are high k calc alkaline and metaluminous with mg values of 43 2 43 8 their sr nd isotopic compositions 87sr 86sr i 0 7106 0 7109 end t 8 8 to 8 6 are less enriched than those of the two mica granites and syenite porphyries in addition the diorites have high zircon 18o values 7 8 8 9 and a wide range of zircon ehf t values 13 4 to 5 6 we propose that the huolushan two mica granites were derived from a metagreywacke dominated hybridized source with minor juvenile crustal rocks and that the longyandong two mica granites were generated by partial melting of metagreywackes the huolushan syenite porphyries were formed by the mixing of metasedimentary rock derived felsic magmas and minor enriched mantle derived alkaline magmas the huolushan diorites were most likely derived by partial melting of mafic lower crust with the dioritic magmas undergoing assimilation and fractional crystallization afc during ascent all of these intrusive rocks were likely formed in an extensional intraplate setting the mantle derived magmas which could have been triggered by lithospheric extension underplated the middle lower crust and provided heat for crustal reworking and materials for late mesozoic crustal growth

evolution and provenance of the xuefeng intracontinental tectonic system in south china constraints from detrital zircon fission track thermochronology abstract the xuefeng intracontinental tectonic system xits is a large scale compressional tectonic system in the central south china block it is characterized by northeast trending fold thrust belts involving the xuefeng zone the chevron syncline fold thrust zone the chevron anticline fold thrust zone and the central sichuan basin this paper reports detrital zircon fission track zft analyses of the sandstones sampled from neoproterozoic to middle jurassic sequences and further discusses their relation with low temperature tectonic evolution and sediment provenance the totally annealed zft data of 452 377 ma from the neoproterozoic and devonian sandstones were related to an early paleozoic low temperature exhumation of the jiangnan orogen due to intracontinental orogeny in the south china block those annealed zft components from triassic to jurassic sandstones grouped 236 189 ma 189 142 ma and 142 124 ma decreasingly from east to west structurally imply a northwestward progressively squeezing by the xuefeng intracontinental orogeny as a result of the paleo pacific subduction the unannealed age components 411 ma 362 377 ma 270 216 ma and 209 183 ma of the upper triassic to middle jurassic sandstones are comparable to the multistage tectonic evolution of the qinling orogen through subduction accretion and collision and are therefore indicative of sample potential provenance

tectonic transition from subduction to retreat of the palaeo pacific plate new geochemical constraints from the late mesozoic volcanic sequence in eastern fujian province se china

present day crustal deformation in china constrained by global positioning system measurements global positioning system gps measurements in china indicate that crustal shortening accommodates most of india s penetration into eurasia deformation within the tibetan plateau and its margins the himalaya the altyn tagh and the qilian shan absorbs more than 90 of the relative motion between the indian and eurasian plates internal shortening of the tibetan plateau itself accounts for more than one third of the total convergence however the tibetan plateau south of the kunlun and ganzi mani faults is moving eastward relative to both india and eurasia this movement is accommodated through rotation of material around the eastern syntaxis the north china and south china blocks east of the tibetan plateau move coherently east southeastward at rates of 2 to 8 millimeters per year and 6 to 11 millimeters per year respectively with respect to the stable eurasia

mesozoic tectonic evolution of the daba shan thrust belt in the southern qinling orogen central china constraints from surface geology and reflection seismology the daba shan thrust belt is located along the southern margin of the qinling orogen that separates the north china block in the north from the south china block in the south despite decades of research the total magnitude of shortening accommodated by continent continent convergence across the qinling orogen after triassic ocean closure between north and south china remains poorly constrained the lack of knowledge on the shortening magnitude in turn limits our ability to test a wide array of tectonic models for the development of the qinling orogen and thus the convergence history between north and south china in order to address this issue we construct a balanced cross section and develop a new kinematic model for the evolution of the daba shan thrust belt this work was accomplished by integrating 1 surface geologic mapping 2 detailed kinematic analysis of key structures 3 existing geochronologic and thermochronological data and 4 a recently obtained lithospheric scale seismic reflection profile restoration of the cross section indicates that the minimum shortening strain increases northward from 10 in the foreland to 45 in the thrust belt interior the estimated amount of upper crustal shortening across the daba shan thrust belt is 130 km which is sufficient to allow the inferred mafic lower crust of the subducted south china lithosphere to have experienced eclogite phase transition thus our work supports that the development of the daba shan thrust belt may have been driven by slab pull of the subducted mafic lower crust at the leading edge of the down plunging south china continental lithosphere

thermo mechanical controls of flat subduction insights from numerical modeling this study was supported by national basic research program of china 2014cb440901 the strategic priority research program b of cas xdb18020104 national science foundation of china 41190073 41372198 and 41304071 and nerc grant ne j021822 1

refining the spatio temporal distributions of mesozoic granitoids and volcanic rocks in se china abstract combining published age data and geological maps we provide most updated information about the spatio temporal distributions of mesozoic granitoids and volcanic rocks in se china the outcrop areas of the mesozoic felsic magmatic rocks with an interval of 10 myrs are estimated allowing us to put forward by far the finest evolution pattern of the spatio temporal distributions of the mesozoic magmatic rocks which were not available before the early mesozoic indosinian felsic magmatic rocks originally occurred sporadically in the south china block and then showed a ne trending distribution followed by an outward in particularly northeastward migration subsequently there was a relative magmatic quiescence from 210 ma to 180 ma which was followed by a tectonic transition from tethys regime to paleo pacific regime at 180 170 ma probably very close to 170 ma the late mesozoic yanshanian magmatic activities also showed an outward expansion and migration the 170 150 ma magmatic activities migrated from present inland to the northeastern and coastal areas of se china with outcrop area increasing gradually while the 150 120 ma felsic magmatic activities showed dominantly a northeastward migration with outcrop area decreasing gradually till the period 120 80 ma the felsic magmatic activities migrated to the present southeast coastal areas with increasing intensity of volcanic activity based on the detailed spatio temporal evolution we suggest that the formation of mesozoic granitoids and volcanic rocks in se china was related to multiple stages of subduction and roll back of oceanic slab on the southeast margin of the south china block

building southeast china in the late mesozoic insights from alternating episodes of shortening and extension along the lianhuashan fault zone abstract understanding the mesozoic intraplate structural response of south china inboard of the paleo pacific south china convergent margin is a key issue in unravelling the continental tectonics of se asia the lianhuashan fault zone of south china represents a high strain structure pivotal for constraining the multi phased intraplate deformation strain localization and partitioning in response to the subduction of the paleo pacific plate new data coupled with published work describes the geometry kinematics and age of this fault zone establishing a multi stage tectono thermal history the first stage late jurassic shortening involved top to the w thrust sense shear along ne striking shear zones the associated regional e w shortening is interpreted to have been related to the advancing nw ward subduction of the paleo pacific plate the second stage early cretaceous deformation is poorly documented in the lianhuashan fault zone but is associated regionally with magmatism and early cretaceous basins formed under nw se extension this extension has been correlated with the rollback of the paleo pacific plate the third stage late early cretaceous nw se shortening produced top to the se thrust sense shear zones the shortening inverted the extensional basins and caused a lull in the magmatic activity it possibly originated from the re initiation of low angle paleo pacific subduction the fourth stage deformation generated the top to the w wuhua detachment normal sense ductile fabrics and distributed shear zones in its footwall and widespread hanging wall basins in the late cretaceous this regional e w extension has been associated with the re initiation of slab rollback our work predicts episodes of slab steepening and shallowing in retreating and advancing settings changes in the slab geodynamics may have caused the alternating contractional and extensional episodes as well as the flare up lull and resumption of magmatism during the late mesozoic evolution of south china

a possible buried paleoproterozoic collisional orogen beneath central south china evidence from seismic reflection profiling abstract the south china craton consisting of the yangtze and cathaysia blocks has figured importantly in the neoproterozoic supercontinent of rodinia however its lack of grenville age structures and metamorphism and the presence of paleoproterozoic high grade metamorphism and s type magmatism raise questions about the timing and nature of the yangtze cathaysia collision here we present results of a 400 km long high resolution seismic reflection profile across the purported suture between the yangtze and cathaysia blocks the seismic profile reveals folded and thrust imbricated seismic reflectors which we interpret to represent the relics of the yangtze cathaysia collisional orogen the inferred orogen was extended in the neoproterozoic resulting in its burial by neoproterozoic flysch strata geochronological data suggest that this buried orogen was formed in the paleoproterozoic 2 0 1 9 ga likely associated with the assembling of the columbia supercontinent these results call for major revision of the models for the formation of the south china craton and its role in the assembling of the rodinia and the columbia supercontinents

the oscillation model of hydrothermal dynamics beneath aso volcano southwest japan after small eruption on may 2011 a new understanding model using repeated absolute and relative gravity measurement abstract at the end of 2010 the seismic activity in aso volcano intensely increased and water level in the nakadake crater decreased until early in 2011 then was followed by a small eruption in may 2011 after the eruption and heavy rain the volcanic activity subsided to calm period crater bottom was refilled with water and water level increased in the nakadake crater the next tremor reappeared in 2014 and tracked to eruption in november 2014 this eruptive pattern and water level variation in the crater repeatedly appeared on the surface and it should be related to the hydrothermal dynamics beneath aso volcano we initiated the gravity measurements in relation to hydrothermal dynamics in the subsurface of aso volcano using scintrex cg 5 549 and lacoste romberg type g 1016 relative gravimeter at 28 benchmarks in april 2011 one month before the eruption the repeated gravity measurements continue to monitor aso volcano with a series of the measurement after the eruption in every three months to a half year we analyze the gravity variation from 2011 to 2014 between the time of the phreatic and strombolian eruption the measurements covered the area more than 60 km 2 in the west side of aso caldera a new gravity network was also installed in may 2010 at seven benchmarks using a10 017 absolute gravimeter which re occupied in october 2010 june 2011 and two benchmarks in june 2014 as a result the gravity changes distinguish hydrothermal dynamic in the subsurface which has a direct correlation to water level fluctuation in the crater after the first eruption and before the second discharge the monitoring data notice large gravity changes between the surveys at benchmarks around nakadake crater and kusasenri area the simple 3d inversion models of the 4 d gravity data deduce the density contrast distribution beneath aso volcano the inversion and mass change result generate the oscillation typical as a new understanding model the variation of the mass shows a similar trend with the hydrothermal input rate to the crater of past research the third year monitoring from april 2013 displays a large gravity and mass variation while precipitation data in this period is smaller than the previous season the largest increased mass about 43 million tons by gaussian method occurred between may 2013 and september 2013 according to the three year gravity monitoring the calm period in aso volcano happens after may 2011 eruption until september 2013 which is followed by the active period before the november 2014 eruption this result will contribute to understand the process of eruption

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

widening of the krafla fissure swarm during the 1975 1981 volcano tectonic episode frequent distance measurements across the krafla fissure swarm north iceland recorded the extension accompanying the sequence of rifting events which started in december 1975 and lasted for 6 years an 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events maximum widening of about 8 m occurred 10 to 12 km north of leirhnjukur which is located above the center of the krafla magma reservoir from that location the amount of widening decreased north wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting off the north coast about 70 km north of leirhnjukur the amount of widening also decreased southwards and approached zero at 15 to 20 km south of leirhnjukur the ground deformation associated with these rifting events can be summarised as a narrow strip 1 to 2 km wide along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm this fractured strip has subsided 2 to 3 m relative to its flanks the flanks of the fractured zone have been uplifted relative to regions farther away the uplift is not well constrained but tilt observations at several locations indicate about 1 m uplift the flanks of the rift zone have contracted perpendicular to the fissure swarm the maximum contractional strain exceeds 300 mm per km the amount of areal expansion windening of the fissure swarm times the length of the fractured zone associated with these rifting events is estimated to be about 0 30 km2 for individual events the area of expansion has been roungly proportional to the volume of subsidence above the krafla magma reservoir if the width of a new dike is equal widening of surface fissures the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2 4 to 2 8 km this ratio is significantly less for events of large lava production but even during these events the majority of magma leaving the krafla reservoir was apparently emplaced in subsurface fissures

origin and tectonic significance of a mesozoic multi layer over thrust system within the yangtze block south china abstract in the yangtze block south china a well developed mesozoic thrust system extends through the xuefeng and wuling mountains in the southeast to the sichuan basin in the northwest the system comprises both thin and thick skinned thrust units separated by a boundary detachment fault the dayin fault to the northwest the thin skinned belt is characterized by either chevron anticlines and box synclines to the northwest or chevron synclines to the southeast the former structural style displays narrow exposures for the cores of anticlines and wider exposures for the cores of synclines thrust detachments occur along silurian fs and lower cambrian fc strata and are dominantly associated with the anticlines to the southeast this style of deformation passes gradually into one characterized by chevron synclines with associated principal detachment faults along silurian fs cambrian fc and lower sinian fz strata there are however numerous secondary back thrusts therefore the thin skinned belt is like the valley and ridge province of the north american applachian mountains the thick skinned belt structurally overlies the thin skinned belt and is characterized by a number of klippen including the xuefeng and wuling nappes it is thus comparable to the blue ridge province of appalachia the structural pattern of this thrust system in south china can be explained by a model involving detachment faulting along various stratigraphic layers at different stages of its evolution the system was developed through a northwest stepwise progression of deformation with the earliest delamination along lower sinian strata fz analyses of balanced geological cross sections yield about 18 1 21 total 88 km shortening for the thin skinned unit and at least this amount of shortening for the thick skinned unit the compressional deformation from southeast to northwest during late jurassic to cretaceous time occurred after the westward progressive collision of the yangtze block with the north china block and suggests that the orogenic event was intracontinental in nature

magma movements in etna volcano associated with the major 1991 1993 lava eruption evidence from gravity and deformation the 1991 1993 eruption was probably the largest on mt etna for 300 years since then the volcano has entered an unusually quiescent period a comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before during and after the eruption the gravity and deformation changes observed before the eruption 1990 1991 record the intrusion of magma into the summit feeder and the sse trending fracture system which had recently been active in 1978 1979 1983 and 1989 creating the feeder dyke for the 1991 1993 eruption in the summit region gravity changes between 1992 and 1993 spanning the end of the eruption reflect the withdrawal of magma from the conduit followed more recently 1993 1994 by the re filling of magma in the conduit up to pre eruption levels in contrast in the vicinity of the fracture zone gravity has remained at the 1991 1992 level indicating that no withdrawal has occurred here rather magma has solidified in the fracture system and sealed it such that the 1993 1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano

surface deformation at the krafla volcano north iceland 1982 1992 extensive measurements of ground deformation at the krafla volcano iceland have been made since the beginning in 1975 of a series of eruptions and intrusions into the fissure system that extends north and south of the volcano i concentrate on measurements before and after the eruption of september 1984 the last event of this series when the largest volume of lava was erupted the patterns of ground deformation associated with the 1984 eruption determined by precision levelling electronic distance measurements and lake level observations were similar to earlier intrusions and eruptions in that the surface of the volcano subsided and the fissure system widened as magma moved laterally from a shallow central reservoir into the fissure system the shallow magma reservoir of krafla continued to expand for about five years after the eruption but a slow subsidence of the central area began in 1989 besides the presence of an inflating and deflating shallow magma reservoir at a depth of 2 5 km beneath the krafla caldera another inflating magma reservoir may exist at much greater depth below krafla the accumulation of compressive strain by numerous rift intrusions and eruptions since 1975 along the flanks of the north south krafla fissure swarm is being released slowly and will probably be reflected in the results of deformation measurements near krafla for the next several decades the total horizontal extension of the krafla rift system in 1975 1984 was about 9 m equal to about 500 years of constant plate divergence the extension is twice the accumulated divergence since previous rifting events and eruptions in 1724 1729

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

post eruptive gravity changes from 1990 to 1996 at krafla volcano iceland the 1975 1984 krafla rifting episode was a major lava and dyke producing event associated with the release of extensional strain accumulated over more than 200 years at the divergent plate boundary in north iceland the present work provides a unique example of gravity decreases and increases sustained over a long period following a major eruptive episode at a rift volcano after height correction persistent net gravity decreases over the source of observed mogi type deflation occur with gravity increases occurring further away from this centre of deformation gravity decreases are interpreted in terms of drainage from a shallow magma chamber the net gravity decreases require that at least 4 1010 kg of magma must have been drained during the 6 year observation period assuming a density of 2700 kg m 3 this magma would occupy 1 5 107 m3 and by analogy with results obtained for kilauea this implies a magma chamber volume change of 4 1 106 m3 this is consistent with the chamber volume change deduced from ground deformation data assuming a poisson s ratio of 0 25 and a mogi source net gravity increases are more spatially extensive and are most likely caused by migration of the steam water interface and or closure of micro fractures in lavas above the magma chamber during post eruptive cooling and contraction we present a model for the krafla magma chamber in which a cooling contracting and draining magma body causes subsidence at the surface these results contrast with observations from the askja caldera iceland where post eruptive deflation has been shown to be accompanied by negligible net gravity changes above the mogi type source in the caldera long term post eruptive deflation and magma drainage have not been observed at subduction related volcanoes this may be a function of a difference in magma viscosity

tectonics of the middle triassic intracontinental xuefengshan belt south china new insights from structural and chronological constraints on the basal decollement zone in orogenic belts a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover in the early mesozoic intracontinental xuefengshan belt of south china such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late orogenic granitic plutons our detailed multi scale structural analysis documents a synmetamorphic ductile deformation in the basal decollement the neoproterozoic pelite and sandstone and the intruding early paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss respectively the metamorphic foliation contains a nw se stretching lineation associated with top to the nw kinematic indicators the ductile shearing of these high strained rocks can be correlated with nw verging folds and thrusts recognized in the neoproterozoic to early triassic sedimentary cover monazite u th pbtot chemical dating and zircon sims u pb dating provide age constraints of the ductile shearing between 243 and 226 ma and late orogenic granite emplacement around 235 215 ma in agreement with recent geochronological data these new results show that the xuefengshan belt is an early mesozoic orogen dominated by the nw directed shearing and thrusting at the southeastern boundary of the xuefengshan belt the chenzhou linwu fault separates the early mesozoic domain to the nw from the early paleozoic domain to the se the tectonic architecture of this belt was possibly originated from the continental underthrusting to the se of the south china block in response to northwest directed subduction of the paleo pacific plate

an andean type retro arc foreland system beneath northwest south china revealed by sinoprobe profiling abstract in the mesozoic south china was situated along the convergent margin between the asian and pacific plates providing an excellent laboratory to understand the interactions between deformation sedimentation and magmatism in a retroarc environment the crustal architecture of northwest south china is displayed along the 600 km long sinoprobe deep seismic reflection profiles and reveals from east to west 1 highly folded and truncated reflectors in the upper crust of the yangtze fold zone which correspond to thin and thick skinned thrust systems and document large scale intraplate structural imbrication and shortening 2 a crustal scale flat ramp flat structure termed the main yangtze decollement which forms a weak viscous layer to accommodate strain decoupling and material transport in the thin and thick skinned systems and 3 nearly flat lying reflectors in the sichuan basin which support interpretation of the basin as a weakly deformed depocentre the yangtze fold zone and the sichuan basin represent a retro arc foreland basin system that is 800 km away from the continental margin magmatic arc we suggest that tectonic processes across the arc and retro arc systems including arc magma flare up basin sedimentation retroarc thrust propagation lithosphere underthrusting root foundering and extension related magmatism were interrelated and governed mass transfer age data and geological relations link the tectonic processes to evolving geodynamics of the subducting paleo pacific plate

longmen shan fold thrust belt and its relation to the western sichuan basin in central china new insights from hydrocarbon exploration the longmen shan fold thrust belt is one of the key regions of demonstrable mesozoic cenozoic tectonic evolution in china and the sichuan basin was the first natural gas producing area in china in this article the structural features of the longmen shan belt are presented using both seismic profiles and field data the complex structures of the northeast trending longmen shan fold thrust belt and its foreland in the western sichuan basin are formed by southeast directed thrusting several eastward verging rootless thrust sheets and imbricates of cambrian triassic rocks have been recognized in the northern longmen shan belt evidence suggests that the northern longmen shan belt experienced at least two major periods of deformation in the late triassic and cenozoic however the southern longmen shan belt is represented by the basement involved thrust structures and klippen and its major periods of deformation were in the latest cretaceous early cenozoic sedimentary features in the western sichuan basin reflect a two phase flexural loading history and illustrate that the late triassic foreland basin extends along the foredeep of the entire length of the longmen shan belt but the uppermost cretaceous paleogene rejuvenated foreland basin is restricted in the southern part of the western sichuan basin structural geometries suggest that prospective traps are mainly developed in the frontal zone of the longmen shan fold thrust belt and in the southern part of the western sichuan basin one of the major contributions of this article is finding preexisting paleozoic rift basins under the cenozoic thin skinned thrust belts which represent a new potential hydrocarbon play

cenozoic and mesozoic structure of the eastern basin and range province utah from cocorp seismic reflection data cocorp seismic reflection data collected from the eastern basin and range in west central utah provide information on cenozoic extensional tectonics mesozoic thrusting and their interrelationships those data show a series of remarkably continuous low angle reflectors that extend more than 120 km perpendicular to strike and can be traced as deep as 15 20 km over that distance none of these events are significantly cut by any high angle normal faults a major detachment beneath the sevier desert can be traced from a surface zone of normal faulting to a depth of 12 15 km with a regional apparent westward dip of 12 tentative correlation of upper and lower plate events suggests 30 60 km of extensional displacement on this detachment whether this structure is a reactivated mesozoic thrust is uncertain west steepening splays off the end of the detachment reach depths of 20 km and may represent a major mesozoic ramp or zones of distributed ductile shearing during extension some events are interpreted to be mesozoic thrusts of which at least one beneath the house range has been reactivated during the cenozoic the snake range decollement dips gently east and has a sense of cenozoic displacement opposite to that of other cenozoic detachments farther east deep events are most numerous beneath the east side of the sevier desert where they occur to depths of 30 km at the top of or perhaps partly within the anomalously low velocity upper mantle

multiple emplacement and exhumation history of the late mesozoic dayunshan mufushan batholith in southeast china and its tectonic significance 1 structural analysis and geochronological constraints the south china block scb experienced a polyphase reworking by the phanerozoic tectono thermal events to better understand its late mesozoic tectonics an integrated multidisciplinary investigation has been conducted on the dayunshan mufushan composite batholith in the north central scb this batholith consists of two major intrusions that recorded distinct emplacement features according to our structural analysis two deformation events in relation to batholith emplacement and subsequent exhumation are identified the early one d1 was observed mostly at the southern border of the batholith characterized by a top to the sw ductile shearing in the early stage intrusion and along its contact zone this deformation chiefly associated with the pluton emplacement at ca 150 ma was probably assisted by far field compression from the yangtze foreland belt the second but main event d2 involved two phases 1 ductile shearing d2a prominently expressed along the dayunshan detachment fault at the western border of the batholith where the syntectonic late stage intrusion and minor metasedimentary basement in the footwall suffered mylonitization with top to the nw kinematics 2 subsequent brittle faulting d2b further exhumed the entire batholith that behaved as rift shoulder with half graben basins developed on its both sides geochronological constraints show that the crustal ductile extension occurred during 132 95 ma such a cretaceous nw se extensional tectonic regime as indicated by the d2 event has been recognized in a vast area of east asia this tectonism was responsible not only for the destruction of the north china craton but also for the formation of the so called southeast china basin and range tectonics

faroe iceland ridge experiment 2 crustal structure of the krafla central volcano the seismic velocity structure of the krafla central volcano is characterized by large variations in compressional velocity a 40 km wide high velocity dome extends from the lower crust 11 14 km depth beneath the volcano narrowing upward a magma chamber sits at its top near 3 km depth it is defined by both 0 2 0 3 s compressional wave delays and shear wave shadowing to be 2 3 km n s 8 10 km e w and 0 7 1 8 km thick the near surface structure uppermost 2 5 km of the krafla caldera is approximately flat lying with only minor lateral heterogeneities the crust beneath the magma chamber has low shear wave attenuation and anomalously high compressional and shear wave velocities shear waves reflected from a 19 km deep moho are clearly visible for some paths through the crustal volume below the magma chamber even though the more shallow diving s waves are severely attenuated the midcrust beneath the shallow magma chamber cannot contain partial melt or even be at near solidus temperatures the krafla central volcano plays a major role in crustal genesis along the plate boundary the high velocity dome in our view represents crust generated in and around the magma chamber which has subsequently been advected to greater depths

rise of the andes the surface uplift of mountain belts is generally assumed to reflect progressive shortening and crustal thickening leading to their gradual rise recent studies of the andes indicate that their elevation remained relatively stable for long periods tens of millions of years separated by rapid 1 to 4 million years changes of 1 5 kilometers or more periodic punctuated surface uplift of mountain belts probably reflects the rapid removal of unstable dense lower lithosphere after long term thickening of the crust and lithospheric mantle

geology geochemistry and genesis of the longhua low temperature hydrothermal ni co arsenide deposit in sedimentary rocks guangxi south china abstract the longhua ni co deposit hosted in cambrian carbonaceous sandstone and siltstone in south china consists of disseminated niccolite mineralization and high grade ni co arsenides quartz carbonate vein mineralization the disseminated mineralization is characterized by niccolite hosted in the center of quartz fragments and in argillaceous concretions in cambrian banded pisolitic carbonaceous silty mudstone and by parallel niccolite grains in cambrian pisolitic pelite these characteristics indicate that the disseminated niccolite originated through redeposition the ni co vein mineralization forms high grade ores and is composed mainly of ni co arsenides the homogenization temperatures of fluid inclusions in the mineralized vein quartz range from 118 to 219 c with corresponding salinities of 1 2 to 8 8 wt nacl equiv the ore forming fluids yield dv smow and 18oh2o values of 54 to 66 and 3 0 to 7 7 respectively indicating that the source was circulating meteoric water the longhua ni co vein mineralization gives a niccolite re os isochron age of 462 6 8 5 ma mswd 1 03 the geological and geochemical characteristics of the longhua ni co deposit indicate that it formed as follows 1 precambrian quartz niccolite mineralization was eroded and transported to the longhua sediment trap forming low grade disseminated niccolite mineralization or ni co rich source beds during the cambrian 2 high grade ni co arsenide vein mineralization was formed by further enrichment of ni co from the ni rich source beds by circulated meteoric water during the ordovician caledonian orogeny the formation of the longhua low temperature high grade ni co vein ore indicates that ni can be transported in low temperature fluids that are rich in arsenic and hco3 or co32 the longhua ni co deposit might represent a new type of sedimentary hosted low temperature hydrothermal ni deposit

cretaceous exhumation of the triassic intracontinental xuefengshan belt delayed unroofing of an orogenic plateau across the south china block abstract a large plateau can be produced by crustal thickening in convergent zones such as continental collision belts and andean type subduction zones but the life cycles of such plateaux are not well understood in particular it is not clear how long they persist after construction before other tectonic processes or erosion reduce crustal thickness and elevation to near normal levels triassic subduction and collision tectonics produced intense deformation magmatism and metamorphism across the entire south china block this large scale crustal shortening created a broad orogenic belt uplifted most parts of the south china block and probably initiated the growth of an orogenic plateau our study presents low temperature thermochronology data from the xuefengshan belt in the interior of the south china block there was along strike variation in exhumation the north orogenic core was subjected to triassic 245 210 ma and late cretaceous 100 80 ma exhumation whereas the cooling path of the south orogenic core reflects a two stage cretaceous evolution the variable exhumation pattern reflects non uniform tectonics in different regions but both regions were subject to late cretaceous extension we tentatively reconstruct the original plateau paleo elevation to be 1 5 km above sea level based on the amount of exhumation 10 km and the present crustal thickness 35 km the t t trajectories of the xuefengshan belt and other triassic belts highlight the significance of cretaceous extension and exhumation in shaping the tectonic configuration of the south china block large scale extension was probably triggered by rollback of the paleo pacific subduction zone

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

tracing an early jurassic magmatic arc from south to east china seas drilling has revealed suites of magnesian granite and diorite emplaced in early jurassic time 198 195 ma and an arc related low temperature 678 to 696 c magmatism in ne south china sea these rocks have 87sr 86sri 0 705494 to 0 706623 and endt 0 9 to 2 2 as evidence of evolved mantle derived magmas coupled with enriched fluid mobile elements cs to k and pb implying involvement of subduction zone fluids another early jurassic granodiorite zircon u pb 187 ma drilled from the sw east china sea is a magnesian high k calc alkaline is comparably confined to a range of low temperature 675 c arc related granite characterized by enrichment of fluid mobile elements and nb ta depletion its sr nd isotopes 87sr 86sri 0 705200 endt 1 1 suggest a product of evolved mantle derived melts together with detrital igneous zircons from paleocene sequences these observations reveal an early jurassic arc related low temperature 600 to 740 c magmatism in the sw east china sea these arc related granitoids along with those from se taiwan could define an early jurassic ne sw trending dongsha talun yandang magmatic arc zone along the east asian continental margin paired with jurassic accretionary complexes from sw japan e taiwan to the w philippines this arc subduction complex assembly was associated with oblique subduction of the paleo pacific slab beneath eurasia presumably responsible for early jurassic lithospheric extension in south china block

development of contrasting folding styles in the western yangtze block south china insights from numerical modeling abstract two contrasting folding styles are well developed along both sides of the qiyueshan fault south china the western part is characterized by chevron anticlines and box synclines in the eastern sichuan domain while the eastern part features typical chevron synclines and box anticlines in the western hunan hubei domain these contrasting structures are important in better understanding the intracontinental orogeny around the eastern sichuan basin in this study we conducted a series of 2 d numerical simulations using the finite difference method the model results indicate that the key parameters include stratigraphic competence number and distribution of intermediate weak layers and thickness of basal weak layer among them the thickness of basal weak layer plays the most important role in shaping the two distinct structural styles in the models with thin basal weak layer 8 km a series of coherent chevron synclines are developed and mainly form decollement folds these characteristics are similar to the structural styles revealed by seismic profiles in the western hunan hubei domain this weak layer may represent the accumulation of mechanically weak rocks e g slate schist and phyllite in neoproterozoic lengjiaxi and tieshajie groups in the western hunan hubei domain which is also consistent with the geophysical observations e g weak seismic reflection high electrical conductivity and bouguer gravity low anomaly both the differential thickness of basal weak layer along the flat ramp flat decollement and the overlying multiple intermediate weak layers contribute to the formation of the contrasting folding styles around the eastern sichuan basin

jurassic intracontinental deformation of the central north china plate insights from syn tectonic sedimentation structural geology and upb geochronology of the yungang basin north china abstract jurassic tectonism resulted in late mesozoic intracontinental deformation of the north china plate a fold and thrust belt related growth strata and volcanic materials have been identified in the jurassic yungang basin which can be used to constrain the timing and scale of faulting and sedimentation the age of volcanic materials in the growth strata and the sedimentary cycle of jurassic strata indicate that the late middle jurassic yungang formation was the earliest sedimentary response to the onset of jurassic deformation i e the yanshanian orogeny from 170 to 160 ma deformation involved detachment and fault propagation folding which produced a marginal thrust belt and limb rotation of anticline restored cross sections show that the northern basin experienced more intense shortening than the southern basin consistent with field observations possibly controlled by the qifengshan strike slip fault in the center of the fold and thrust belt fault striations conjugate joints and asymmetric folds show that the yungang basin underwent nw se trending compression during the middle late jurassic therefore we propose that the eastern ordos block was in a compressional setting during the middle late jurassic due to flat subduction of the paleo pacific plate beneath the east asian continent

post rifting stress relaxation at the divergent plate boundary in northeast iceland interaction of the elastic lithosphere with the underlying anelastic asthenosphere causes strain to propagate along the earth s surface in a diffusion like manner following tectonism at plate boundaries this process transfers stress between adjacent tectonic segments and influences the temporal tectonic pattern along a plate boundary observations of such strain transients have been rare and have hitherto been confined to strike slip and underthrusting plate boundaries1 here we report the observation of a strain transient at the divergent spreading plate boundary in iceland a global positioning system survey undertaken a decade after an episode of dyke intrusion accompanying several metres of crustal spreading reveals a spatially varying strain field with the expected diffusion pulse shape and an amplitude three times greater than the 5 7 cm that would be expected from the average spreading rate2 a simple one dimensional model with a thin elastic layer overlying a viscous layer fits the data well and yields a stress diffusivity of 1 1 a 0 3 m2s 1 combined with struc tural information from magnetotelluric measurements this implies a viscosity of 0 3 2 1019 pa s a value comparable to that derived for iceland from post glacial rebound23 but low compared with estimates for mantle viscosity obtained elsewhere3

andean tectonics related to geometry of subducted nazca plate seismological and geological data show that tectonic segmentation of the andes coincides with segmentation of the subducted nazca plate which has nearly horizontal segments and 30 east dipping segments andean tectonics above a flat subducting segment between 28 s to 33 s are characterized by from west to east 1 a steady topographic rise from the coast to the crest of the andes 2 no significant quaternary and possibly neogene magmatism 3 a narrow belt of eastward migrating apparently thin skinned neogene to quaternary shortening of the andes and 4 plio pleistocene uplift of the crystalline basement on reverse faults in the pampeanas ranges from about 15 to 24 s over a 30 dipping subducted plate a west to east andes cross section includes 1 a longitudinal valley east of coastal mountains 2 an active neogene and holocene andesitic volcanic axis 3 the altiplano puna high plateau 4 a high neogene but inactive thrust belt eastern cordillera and 5 an active eastward migrating subandean thin skinned thrust belt tectonics above a steeply subducting segment south of 33 s are similar west of the volcanic axis but quite different to the east early cenozoic tectonics of western north america were quite similar to the neogene andes however duration of segmentation was longer and the width of deformation was greater in the western united states patterns of crustal seismicity are systematically related to plio quaternary structural provinces implying that current deformational processes have persisted since at least the pliocene horizontal compression parallel to the plate convergence direction is indicated to a distance of 800 km from the trench above flat subducting segments crustal seismicity occurs over a broad region whereas over steep segments it is confined to the narrow thrust belt strain patterns in the forearc region are complex and perhaps extensional and a broad region of the altiplano puna and eastern cordillera appears to be aseismic

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

permo triassic structural evolution of the shiwandashan and youjiang structural belts south china abstract we conducted field mapping coupled with radiometric dating across the shiwandashan and youjiang structural belts swsb and yjsb to investigate how southwest south china evolved and to better understand its links to plate boundary dynamics during the late permian to middle triassic our results reveal an episodic tectono magmatic evolutionary history of the swsb and yjsb the swsb underwent significant nw se shortening punctuated by 250 240 ma s type pluton emplacement during the late permian to middle triassic the shortening was expressed by thin skinned nw verging thrusts and folds and conjugate sets of n trending sinistral and e trending dextral faults the nw se shortening overlapped with and was succeeded by triassic ne sw shortening in the yjsb the ne sw shortening was expressed by ne verging thrusts and folds which documented a northeastward propagation of foreland deformation the ne verging folds overprinted older nw verging folds forming superimposed folds at the juncture of the yjsb and swsb in the long an area our results combined with regional considerations support a model of the nw se shortening as an andean type orogeny that developed in response to westward subduction of the paleo pacific plate and the ne sw shortening as a product of the indochina south china collision the subduction of the paleo pacific plate assisted in westward motion of the south china oceanic lithosphere which may have facilitated the closure of the paleo tethys ocean and subsequent collisions of south china with north china and indochina

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

the origins of the intraplate stress field in continental australia the ridge push force acting on the indo australian plate exerts a significant torque 8 5 1025n m about a pole at 30 3 n 34 5 e the angular difference between this torque pole and the observed pole of rotation for the plate 19 2 n 35 6 e is less than 12 and suggests that the ridge push force plays an important role in the dynamics of the indo australian plate we have used an elastic finite element analysis to study the predicted intraplate stress field in continental australia for four models which employ different boundary conditions to balance the ridge push torque acting on the plate the modeling indicates that a number of important features of the observed stress field within the australian continent can be explained in terms of balancing the ridge push torque with resistance imposed along the himalaya papua new guinea and new zealand collisional boundaries segments these features include n s to ne sw oriented compression in the northern australia and e w oriented compression in southern australia our analysis also shows that subduction processes along the northern and eastern boundaries provide only second order controls on the intraplate stress field in continental australia

evolution of meso cenozoic subduction zones in the ocean continent connection zone of the eastern south china block insights from gravity and magnetic anomalies abstract the evolution of unified east asian continental margin was accompanied by large scale magmatic and tectonic events here we investigate the tectonic evolution history of the ocean continent connection zone in the eastern south china block scb during the mesozoic cenozoic subduction of the paleo pacific plates based on gravity and magnetic methods that provide a window to the deep crustal structure the decomposed multi scale gravity wavelets reveal subduction related intra crustal structures and the deep structure of the mesozoic greater south china block gscb the depth of moho discontinuity was inverted by gravity data constrained by the latest global crustal model of crust 1 0 and the inverted depth is closely consistent with and slightly larger than the depth on seismic profiles the undulation of the moho discontinuity beneath the south china block and the deep supra crustal faults along the continental margin are considered to be associated with the subduction of the paleo pacific plates in general the moho depth in the eastern scb gets shallower from west to east the oblique subduction related strike slip faults can be traced on the 7 4 km upward continued field of the magnetic anomaly the geologically indistinct jurassic magmatic arc along the scb continental margin is verified by residual crustal field of the bouguer gravity anomaly the southern ryukyu taroko line is speculated as the inland segment of the early jurassic paleo subduction zone and represents the southeastern paleo plate boundary of the gscb in the jurassic as the backarc arc forearc paleo system migrated eastwards since the early cretaceous the subduction zone retreated to the current taiwan okinawa trough and the jurassic magmatic arc forearc was transformed to a backarc indicating an eastward migration or jumping of the yanshanian subduction along the continental margin of the scb the tectonic boundary of the mesozoic gscb was also identified through the magnetic anomaly field and the high order gravity field

oblique closure of the northeastern paleo tethys in central china a branch of the paleo tethys ocean once separated the north china plate from the south china plate however the mode of closure of the northeastern paleo tethys ocean during the late paleozoic to early mesozoic has been debated one reason for this debate is that the collisional suture zone was later buried by large scale thrust faults in the southern qinling dabieshan orogen which made it difficult to reconstruct the amalgamation of the supercontinent in central china new regional geologic mapping provides stratigraphic and structural constraints on the mechanism of this ocean closure our results indicate that dextral transpressional suturing in the southern qinling dabieshan foreland fold thrust belt resulted in the formation of the northern yangtze foreland basin where the stratigraphy precisely shows the time transgressive closure of the ocean and the orogenic sediments shed over 1000 km westward from eastern china to the closing paleo tethys therefore we propose an oblique subduction model to describe the closure of the paleo tethys ocean our findings suggest that prolonged slab pull during the oblique subduction of the oceanic plate continued to drive deep continental subduction thereby forming high and ultrahigh pressure metamorphic rocks and leading to sustained ocean closure

jurassic intraplate magmatism in southern hunan eastern guangxi 40ar 39ar dating geochemistry sr nd isotopes and implications for the tectonic evolution of se china the mesozoic geology of se china is characterized by intensive and widespread magmatism however the tectonic regime that accounted for the mesozoic magmatism has been an issue with little consensus a comprehensive study of ar 40 ar 39 dating geochemistry and sr nd isotopes has been conducted on basalts from southern hunan and syenite intrusions from eastern guangxi three episodes of jurassic magmatism i e alkaline basalts of c 175 ma in age syenitic intrusions of c 160 ma and high mg basalts of c 150 ma are identified the older c 175 ma alkaline basalts are characterized by low sr i sr 0 7035 0 7040 and high nd epsilon nd t 5 to 6 isotopic compositions and oib like trace element patterns e g nb la 1 in contrast the younger c 150 ma high mg basalts have high sr i sr c 0 7054 and low nd epsilon nd t c 2 isotopic compositions and incompatible trace element patterns of arc affinity the c 160 ma syenitic intrusions display a relatively large range of sr and nd isotopic compositions i sr 0 7032 0 7082 epsilon nd t 5 5 to 4 1 with the qinghu syenites having the lowest i sr highest epsilon nd t and oib type incompatible trace element patterns analogous to the c 175 ma alkaline basalts such a secular variation in rock types and geochemical and isotopic characteristics reveals changes in melt segregation depth and mantle sources which are inferred to have resulted from the post indosinian orogenic lithosphere extension and thinning the c 175 ma alkaline basalts are suggested to have formed by small degrees of decompression melting of the astheno sphere or an enriched lithospheric mantle source accreted by asthenosphere derived melts during the initial extension the c 160 ma syenitic and c 150 ma high mg basaltic rocks mainly originated from the enriched lithospheric mantle that melted owing to a raised geotherm caused by lithosphere thinning this interpretation is at odds with the active continental margin related to the subduction of palaeo pacific plate but consistent with continental rifting and extension for the mesozoic of se china

cretaceous granitic intrusions in fujian province cathaysia block implications for slab rollback and break off of the paleo pacific plate abstract the geodynamic transformation of the cathaysia block and its relationship with the cretaceous granitic intrusions have been controversial in recent decades in this study petrology zircon u pb ages lu hf isotopic composition whole rock geochemistry and sr nd isotopes were carried on cretaceous granites that were collected from the fujian province se china combining previously published ages with the new ages obtained in this study indicates that the magmatism in the study area occurred in three distinct stages at 145 137 ma 136 118 ma and 107 86 ma the first stage i type granites were intruded at 137 1 ma and 145 1 ma which are characterized by low end t values of 7 3 to 6 8 and ehf t values of 12 5 to 2 5 it was formed by partial melting of mesoproterozoic medium to high k basaltic rocks in the thickened crust of 40 km without significant hybridization of mantle derived magmas and fractional crystallization the second stage highly differentiated i type granites yield zircon u pb ages of 125 1 ma and 128 1 ma which contain slightly negative end t values of 5 4 to 5 3 and ehf t values of 8 4 to 3 2 it was generated by partial melting of the mesoproterozoic basement at the depth of 30 40 km triggered by underplating of a small amount of mantle derived magma with primary crustal melts undergoing plagioclase dominated fractionation crystallization the third stage i type granites yield zircon u pb ages of 104 1 ma and 105 1 ma which contain end t values of 1 7 to 7 3 and ehf t values of 6 0 to 0 5 it was derived from partial melting of mesoproterozoic basaltic source at a depth of 30 km besides a mass of mantle derived magma was mixed into the crustal melts accompanied with plagioclase dominated fractionation crystallization combining previously published data and those obtained in this study provides significant information for the tectonic evolution of paleo pacific in stage 1 145 137 ma the existence of i type granites and adakitic rocks suggests that the dip angle subduction of the paleo pacific plate led to the partial melting of the thickened lower crust under the compressional environment in stage 2 136 118 ma the generation of i type and a type granitic magmas imply the thinning of the lithospheric crust under the extensional environment and the upwelling of the asthenospheric mantle which are caused by slab rollback of paleo pacific in stage 3 107 86 ma the appearance of much more i type and a type granites indicates that the break off of the paleo pacific plate led to much stronger crust mantle interactions under the extensional environment

integration of micro gravity and geodetic data to constrain shallow system mass changes at krafla volcano n iceland new and previously published micro gravity data are combined with insar data precise levelling and gps measurements to produce a model for the processes operating at krafla volcano 20 years after its most recent eruption the data have been divided into two periods from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at krafla is decaying exponentially the net micro gravity change at the centre of the caldera is shown using the measured free air gradient to be 85 gal for the first and 100 gal for the second period after consideration of the effects of water extraction by the geothermal power station within the caldera the net gravity decreases are 73 17 gal for the first and 65 17 gal for the second period these decreases are interpreted in terms of magma drainage following a mogi point source model we calculate the mass decrease to be 2 1010 kg year reflecting a drainage rate of 0 23 m3 s similar to the 0 13 m3 s drainage rate previously found at askja volcano n iceland based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems we suggest a pressure link between askja and krafla at deeper levels at the lower crust or the crust mantle boundary after the krafla fires co rifting pressure decrease of a deep source at krafla stimulated the subsequent inflow of magma eventually affecting conditions along the plate boundary in n iceland as far away as askja we anticipate that the pressure of the deeper reservoir at krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber possibly initiating a new rifting episode we have demonstrated that by examining micro gravity and geodetic data our knowledge of active volcanic systems can be significantly improved

petrogenesis and oxidation state of granodiorite porphyry in the jurassic chuankeng skarn cu deposit south china implications for the cu fertility and mineralization potential abstract large scale jurassic porphyry skarn cu mineralization in the qin hang metallogenic belt of south china has attracted much attention but comparative studies of porphyries in large and small scale deposits are lacking in this paper we present new zircon u pb ages trace element composition and hf isotopic composition as well as whole rock geochemical and sr nd isotopic compositions for the porphyries associated with mineralization in the small scale chuankeng cu deposit drawing on our data for the chuankeng deposit and existing data for the nearby giant dexing cu deposit we aim to identify factors controlling the mineralization potential of both deposits granodiorite porphyries of chuankeng deposit were emplaced at ca 161 158 ma they have moderate sio2 contents of 61 9 66 1 wt and high sr y 59 135 and la yb n 4 14 ratios their high mgo contents 0 2 2 9 wt lack of negative eu anomaly depletion in y and yb relatively high initial 86sr 87sr ratios 0 7076 0 7078 and low end t values 4 5 to 4 9 indicating that the chuankeng porphyries were derived from partial melting of delaminated lower crust the middle late jurassic chuankeng porphyries are considered to have been formed in a localized intra continental extension environment along the jiangshao fault in response to far field stress from paleo pacific plate subduction although no middle late jurassic magmatic arc rocks have been found in the study area the chuankeng porphyries display similar characteristics to arc rocks on the basis of their zircon ehf t values 2 6 to 0 9 two stage hf model ages 1 4 1 1 ga and regional geological history we infer that magma source of chuankeng porphyries were mainly neoproterozoic juvenile crust with the involvement of paleoproterozoic ancient crust thus their arc like features were inherited from the neoproterozoic juvenile crust that formed by subduction of oceanic crust during the jiangnan orogeny chuankeng porphyries have lower zircon ce4 ce3 and higher ti in zircon temperatures and lower whole rock end and zircon ehf values than the dexing porphyries indicating they are less oxidized and less hydrous and with more involvement of the ancient crustal material in the magma source which could explain the relatively small scale cu mineralization in chuankeng this study highlights the values of integrated studies of whole rock nd and zircon trace element and hf isotopic compositions in assessing the potential for cu mineralization

age and granite association of skarn w mineralization at niutangjie district south china block abstract numerous w sn deposits are developed in the south china block with the majority related to the yanshanian jurassic to cretaceous granites a smaller number of deposits have been found associated with kwangsian middle paleozoic granites the largest of which is the niutangjie skarn w deposit the niutangjie deposit is spatially associated with two suites of granites the unmineralized yuechengling biotite and two mica granites have zircon u pb ages of 427 1 2 9 ma and 427 5 3 5 ma respectively whereas the mineralized niutangjie muscovite granite yields a younger age of 421 0 1 5 ma u pb dating of cassiterite coexisting with scheelite from the niutangjie muscovite granite indicates a mineralization age of 420 8 8 0 ma consistent with the age of the host rock the unmineralized yuechengling biotite granites have a cnk ratios of 0 97 1 16 and show characteristics of both i type and s type granites in contrast the mineralized niutangjie muscovite granites are s type granites with a cnk ratios of 1 12 1 72 and have higher sio2 contents and rb sr ratios and lower nb ta zr hf and k rb ratios than the unmineralized yuechengling biotite granites geochemical differences together with zircon hf isotopic compositions indicate that the relatively fractionated mineralized niutangjie muscovite granites were generated by partial melting of proterozoic meta sediments whereas the unmineralized yuechengling biotite granites were derived from partial melting of proterozoic meta sediments with subordinate meta igneous rocks the unmineralized yuechengling two mica granites have similar geochemical characteristics to the mineralized niutangjie muscovite granites but their relatively small volumes and lack of exploration may explain why the yuechengling two mica granites have not so far been found to be mineralized similarly the larger volume and more highly fractionated nature of the mesozoic yanshanian mineralized granites may explain the greater abundance of w sn mineralization associated with these rocks compared to the paleozoic kwangsian granites

mirror image thrusting in the south china orogenic belt tectonic evidence from western fujian southeastern china abstract the mesozoic orogenic belt in south china is readily divided into two fold and thrust belts and a central multiply deformed zone of ductile deformation dominated by folds with steeply dipping axial planes southeast directed thrusting in the eastern belt and northwest directed thrusting in the western belt resulted in a mirror image of one another during the mesozoic the eastern fold and thrust belt can be further divided into three domains that overlap one another and exhibit a southeast verging wedge like geometry and kinematics deviatoric stress has been calculated from quartz dislocation density at 91 5 mpa 74 5 mpa and 73 8 mpa decreasing toward the southeast upper devonian and lower carboniferous sandstones have been used to constrain bed length for cross section balancing to estimate crustal shortening from three typical areas these indicate 45 41 6 and 37 3 shortening and suggest that 110 km of contraction has been achieved as a result of thrusting this orogen correlates with the ogcheon belt in south korea to the northeast to the southwest it appears to correlate with a thrust belt in western guangdong this southeast verging thrusting may be at least partially controlled by cathaysia to the east