late mesozoic bimodal volcanic rocks in the jinniu basin middle lower yangtze river belt yrb east china age petrogenesis and tectonic implications abstract late mesozoic intrusive and volcanic rocks are widespread in the southeast hubei province middle lower yangtze river belt yrb east china detailed in situ zircon u pb and hf isotope elemental and sr nd pb isotopic data are presented in this paper for late mesozoic volcanic rocks from the jinniu basin yrb aiming to constrain their age petrogenesis and tectonic implications the jinniu volcanic rocks show a bimodal distribution in composition with dominant rhyolite and dacite and subordinate basalt and basaltic andesite new shrimp and la icpms zircon u pb ages indicate that the volcanic rocks of three formations in the jinniu basin were erupted at quite a short age range of about 5 ma during the early cretaceous 130 125 ma the mafic rocks are moderately enriched in large ion lithophile elements lile e g ba th u and pb and light rare earth elements lree and are characterized by negative nb ta and ti anomalies and relatively high tio2 0 72 2 06 and nb 9 20 26 5 ppm contents these analyses indicate that the geochemical characteristics of the mafic rocks in the jinniu basin are similar to worldwide phanerozoic nb enriched basalt and andesites neba new in situ zircon u pb ages and field geological relationships demonstrate that neba in the southeast hubei province are not spatially or temporally associated with high silica adakitic rocks but were most likely derived from an enriched lithospheric mantle with assimilation of minor crustal materials and then fractional crystallization during the evolution of the magma overall the felsic rocks in the jinniu basin have geochemical characteristics and sr nd pb signatures and in situ zircon hf isotopic compositions similar to those of the mafic rocks compared with the mafic rocks the felsic rocks are characterized by enriched and variable concentrations of lile and ree e g ba 33 3 1372 ppm y 11 4 33 6 ppm ybc 5 07 18 7 and negative eu anomalies eu 0 22 0 98 as well as a wide range of radiogenic nd pb isotopic values with end t 10 2 to 2 4 206pb 204pb i 17 659 18 705 207pb 204pb i 15 478 15 663 and 208pb 204pb i 37 654 38 935 and in situ zircon hf compositions of ehf t 12 7 to 1 8 these features indicate that the genesis of felsic magma in the jinniu basin is consistent with extensive fractional crystallization and large amounts of crustal contamination from an evolved mafic magma sio2 55 the bimodal volcanic rocks in this study provide convincing evidence that early cretaceous volcanic rocks in the yrb developed in a back arc extensional tectonic regime

transition of subduction related magmatism from slab melting to dehydration at 2 5 ga abstract the archean plate tectonics and subduction related magmatism have been concerned for several decades here we investigate the archean plate subduction from a new perspective using the secular records of global scale non ttg tonalite trondhjemite granodiorite felsic rocks with arc like geochemical affinities the statistical results show that the la yb and sm yb of non ttg arc felsic rocks evolved at high values in the archean and decreased sharply at 2 5 ga this abrupt transition indicates a significant change of the petrogenesis during the archean the oceanic crust subducted into the mantle along a high geothermal gradient and reached the conditions of partial melting with garnet stable in the residue resulting in high la yb and sm yb in the arc felsic rocks as a consequence of the secular cooling of the mantle the geothermal gradient of a post archean subducting cold oceanic crust intersected with the stability limits of hydrous minerals before it reached the wet solidus of basalts so that the dehydration occurred preferentially and resulted in a drier crust with a much higher temperature of solidus as such the post archean arc felsic rocks were mostly originated from melting of the hydrated mantle wedge overlying the subducted oceanic slab the magma source of the arc felsic rocks thereafter became much shallower corresponding to the low la yb and sm yb of those rocks as a consequence of the less stable garnet in the residue we thus conclude that the subduction related magmatism changed from slab melting to dehydration model at 2 5 ga

active thrusting and folding along the northern tien shan and late cenozoic rotation of the tarim relative to dzungaria and kazakhstan we have studied geometries and rates of late cenozoic thrust faulting and folding along the northern piedmont of the tien shan mountain belt west of urumqi where the m 8 3 manas earthquake occurred on december 23 1906 the northern range of the tien shan rising above 5000 m overthrusts a flexural foredeep filled with up to 11 000 m of sediment of the dzungarian basement our fieldwork reveals that the active thrust reaches the surface 30 km north of the range front within a 200 km long zone of neogene quaternary anticlines fault scarps are clearest across inset terraces within narrow valleys incised through the anticlines by large rivers flowing down from the range in all the valleys the scarps offset vertically the highest terrace surface by the same amount 10 2 0 7 m inferring an early holocene age 10 2 kyr for this terrace which is continuous with the largest recent fans of the piedmont yields a rate of vertical throw of 1 0 0 3mm yr on the main active thrust at the surface a quantitative morphological analysis of the degradation of terrace edges that are offset by the thrust corroborates such a rate and yields a mass diffusivity of 5 5 2 5 m 2 kyr a rather fresh surface scarp 0 8 0 15 m high that is unlikely to result from shallow earthquakes with 6 m 7 in the last 230 years is visible at the extremities of the main fold zone we associate this scarp with the 1906 manas earthquake and infer that a structure comprising a deep basement ramp under the range gently dipping flats in the foreland and shallow ramps responsible for the formation of the active fault propagation anticlines could have been activated by that earthquake if so the return period of a 1906 type event would be 850 380 years the small size of the scarp for an earthquake of this magnitude suggests that a large fraction of the slip at depth 2 3 is taken up by incremental folding near the surface comparable earthquakes might activate flat detachments and ramp anticlines at a distance from the front of other rising quaternary ranges such as the san gabriel mountains in california or the mont blanc aar massifs in the alps we estimate the finite cenozoic shortening of the folded dzungarian sediments to be of the order of 30 km and the cenozoic shortening rate to have been 3 1 5 mm yr assuming comparable shortening along the tarim piedmont and minor additional active thrusting within the mountain belt we infer the rate of shortening across the tien shan to be at least 6 3 mm yr at the longitude of manas 85 5 e a total shortening of 125 30 km is estimated from crustal thickening assuming local airy isostatic equilibrium under the same assumption serial n s sections imply that cenozoic shortening across the belt increases westwards to 203 50 km at the longitude of kashgar 76 e as reflected by the westward increase of the width of the belt this strain gradient implies a clockwise rotation of tarim relative to dzungaria and kazakhstan of 7 2 5 around a pole located near the eastern extremity of the tien shan west of hami 96 e 43 5 n comparable to that revealed by paleomagnetism between tarim and dzungaria 8 6 8 7 a 6 mm yr rate of shortening at the longitude of manas would imply a rate of rotation of 0 45 m y and would be consistent with a shortening rate of 12 mm yr north of kashgar taking such values to be representative of late cenozoic rates would place the onset of reactivation of the tien shan by the india asia collision in the early to middle miocene 16 22 9 m y in accord with the existence of particularly thick late neogene and quaternary deposits such reactivation would thus have started much later than the collision roughly at the time of the great mid miocene changes in tectonic regimes denudation and sedimentation rates observed in southeast asia the himalayas and the bay of bengal and of the correlative rapid change in seawater sr isotopic ratio 20 to 15 ma like these other changes the rise of the tien shan might be a distant consequence of the end of indochina s escape

mechanics of fold and thrust belts and accretionary wedges cohesive coulomb theory a critically tapered fold and thrust belt or submarine accretionary wedge is one that is on the verge of coulomb failure everywhere including its base where frictional sliding along a decollement is assumed to be occurring cohesion within a wedge can add significantly to the overall strength near the toe the effect of this is to decrease the near toe taper leading to a critical topographic profile that is concave upward if the decollement is planar we obtain an approximate self consistent solution for the state of stress within a thin skinned cohesive critical coulomb wedge and determine the relationship between the wedge taper and its strength and basal friction the theory is then applied to the presently deforming fold and thrust belt of western taiwan fitting of theoretical critical wedge shapes to topographic profiles and measurements of the step up angles of thrust faults from the basal decollement are used to constrain the taiwan wedge strength parameters an attractive assertion fully consistent with all the observations is that the mechanics of fold and thrust belts and accretionary wedges is governed by normal frictional and fracture strengths of rocks measured in the laboratory in particular if byerlee s law b 0 85 is adopted as the coefficient of sliding friction on the base we find a coefficient of internal friction 0 9 1 0 in the wedge and a wedge cohesion so 5 20 mpa other solutions having strengths and ambient stresses up to 4 times lower than this can also however satisfy the data

the generation of oceanic rhyolites by crystal fractionation the basalt rhyolite association at volcan alcedo galapagos archipelago

cenozoic tectonics in the urumgi korla region of the chinese tien shan cenozoic deformation within the tien shan of central asia has accommodated part of the post collisional indentation of the indian plate into asia within the urumgi korla region of the chinese tien shan this occurred dominantly on thrusts with secondary strike slip faulting the gross pattern of deformation is of moderate to steeply dipping thrusts that have overthrust foreland basins to the north and south of the range the junggar and tarim basins respectively smaller foreland basins lie within the margins of the range itself turfan chai wo pu korla and qumishi basins these lie in the footwalls of local thrust systems both the turfan and the korla basins contain major thrusts within them they are complex foreland basins deformation has progressively affected regions further into the interior of the junggar basin and propagated into the interiors of the intermontane basins no unidirectional deformation front has passed across the tien shan in the neogene and quaternary an oligocene unconformity may indicate the time of the onset of the cenozoic deformation but most of the cenozoic molasse has been deposited after the palaeogene the rate of deposition in basins next to the uplifted ranges has increased since the onset of deformation there has been at least about 80 km of cenozoic shortening across this part of the tien shan cenozoic shortening is greater in sections of the range further west these are nearer to the northern margin of the indian indenter cenozoic compression has reactivated structures created by the two late palaeozoic collisions that created the ancestral tien shan these palaeozoic structures have exerted a strong control over the style and location of the cenozoic deformation

characteristic slip for five great earthquakes along the fuyun fault in china the seismic hazard associated with an individual fault can be assessed from the distributions of slip and recurrence times of earthquakes however seismic cycle models 1 that aim to predict rupture lengths and fault displacements of successive earthquakes on one fault remain poorly validated it is therefore unknown whether individual fault segments rupture independently producing earthquakes with a diverse range of magnitudes and recurrence times or slip by characteristic amounts with characteristic magnitudes here we use highresolution satellite data to document the horizontal offsets of stream channels and terraces created by strike slip motion on the fuyun fault xinjiang china during five historical earthquakes we find that the ms 7 9 11 august 1931 earthquake produced a surface rupture with a length of 160 km dispersed over three different fault segments the 290 measured stream channel and terrace offsets record an average slip of 6 3 m we use the degree of preservation of geomorphological markers to assign relative ages to individual fault offsets and identify at least four distinct older earthquakes we find that these older earthquakes also produced fault offsets with a similar distribution to the 1931 earthquake as the slip distributions during five successive earthquakes were so similar we conclude that ruptures on the fuyun fault obey a characteristic slip model very few large earthquakes are known to have ruptured the same fault segment twice in historical or instrumental time 1 the

zircon u pb ages and lu hf isotope systematics from late tectonic granites geita greenstone belt implications for crustal growth of the tanzania craton granite plutons that intruded south of lake victoria and north of the geita greenstone belt have geochemical characteristics similar to high k granites when compared to the late high k granites from the musoma mara region they have lower sio2 content and higher tio2 al2o3 cao na2o p2o5 mgo and feot they also show higher values for v sr zr ba and hf concentrations all samples display high total ree abundances ree 139 393 and weak to moderate eu depletion eu eu 0 25 0 77 their chondrite normalized pattern indicates that the light ree are moderately fractionated 3 03 lan smn 6 24 whereas the heavy ree are weakly fractionated 1 63 gdn ybn 2 80 the granite plutons are generally undeformed with 207pb 206pb ages between 2660 and 2620 ma and intruded after the main ductile deformation that affected the geita greenstone belt the 176hf 177hf ratios for zircons from all samples are identical within error suggesting evolution from an isotopically uniform reservoir the hf values plot along the chur evolution line indicating that the granite represents juvenile crustal melts derived from greenstone material available zircon ages from the tanzania craton suggest that crustal growth occurred in three distinct periods 2850 2800 ma 2770 2730 ma and 2700 2620 ma with the dominant period of crustal growth around 2700 ma zircon ages from the sukumaland and musoma mara greenstone belts indicate that the greenstone sequences formed during the same three periods of crustal growth

petrological and geochemical characteristics of cenozoic high k calc alkaline volcanism in konya central anatolia turkey abstract late miocene to pliocene volcanic rocks outcrop west northwest and southwest of the konya area in central anatolia turkey volcanic products are lava domes nuee ardentes and ignimbrite deposits predominantly andesitic to dasitic in composition together with rare basalt basaltic andesite basaltic trachyandesite and trachyandesite 50 35 69 39 sio2 the serie exhibits high k calc alkaline affinities fractional crystallization of pyroxene plagioclase and fe ti oxides is the main process in the magmatic evolution of konya volcanic rocks volcanic units exhibit typical high k calc alkaline character their geochemical characteristics e g enrichments in lil elements such as k rb ba sr depletion in hfse such as ti nb and high ba nb and low nb y ratios are consistent with those of active continental margin regions high 87 sr 86 sr 0 704841 0 707340 and low 143 nd 144 nd 0 512390 0 512618 ratios suggest crustal involvement in their petrogenesis correlations between 86 sr 87 sr isotope with rb rb nb rb ba and rb sr also emphasize the effect of crustal contamination on the andesitic and dacitic magmas as a consequence konya volcanic rocks are products of assimilation and fractional crystallization afc processes of a magma which seems to be linked to the subduction of the african plate underneath the anatolian plate during miocene

geochronology and geochemistry of the triassic bimodal volcanic rocks and coeval a type granites of the olzit area middle mongolia implications for the tectonic evolution of mongol okhotsk ocean abstract the olzit volcanism in middle mongolia comprises a bimodal suite of basalts and peralkaline rhyolites adjacent to the main mongolia lineament the basalts are characterized by enrichment in lile and lree and depletion in hfse with typical sr nd isotopic signatures end t 2 50 to 0 38 and 87sr 86sr i 0 7058 0 7063 indicating they were likely derived from partial melting of an enriched lithospheric mantle modified by subducted slab derived fluids the rhyolites show a close affinity to a type granites with enrichment in lile and lree and depletion in nb ta and ti they also show a significant negative eu anomaly and have end t values ranging from 0 50 to 1 38 and initial 87sr 86sr ratios ranging from 0 7022 to 0 7200 suggesting the rhyolites stem from partial melting of crustal rocks rather than fractional crystallization of the basaltic melt the rhyolite porphyry yields a shrimp zircon u pb age of 207 2 ma mswd 1 42 indicating the bimodal volcanic suite formed in the late triassic the miarolitic per alkaline granite and biotite bearing granite which are associated with the bimodal volcanic rocks show typical a type granitic geochemical affinity with end t 0 89 0 91 and 87sr 86sr i 0 7021 0 7043 indicating they are likely generated by partial melting of crustal rocks similar to the rhyolitic end member of bimodal suite the miarolitic per alkaline granite and biotite bearing granite yielded shrimp zircon u pb ages of 209 2 ma mswd 0 91 and 213 3 ma mswd 1 65 respectively which are nearly coeval with the age of the bimodal volcanic suites in view of the new geochemical and chronological data in this study we suggest the olzit late triassic bimodal volcanic rocks together with the coeval a type granites represent a back arc basin extensional environment which probably related to the roll back of mongol okhotsk oceanic plate during the southward subduction under the central mongolia microcontinent

a geologically constrained monte carlo approach to modeling exposure ages from profiles of cosmogenic nuclides an example from lees ferry arizona we present a user friendly and versatile monte carlo simulator for modeling profiles of in situ terrestrial cosmogenic nuclides tcns our program available online at http geochronology earthsciences dal ca downloads models html permits the incorporation of site specific geologic knowledge to calculate most probable values for exposure age erosion rate and inherited nuclide concentration while providing a rigorous treatment of their uncertainties the simulator is demonstrated with 10be data from a fluvial terrace at lees ferry arizona interpreted constraints on erosion based on local soil properties and terrace morphology yield a most probable exposure age and inheritance of 83 9 14 1 19 1 ka and 9 49 2 52 1 21 104 atoms g 1 respectively 2 without the ability to apply some constraint to either erosion rate or age shallow depth profiles of any cosmogenic nuclide except for nuclides produced via thermal and epithermal neutron capture e g 36cl cannot be optimized to resolve either parameter contrasting simulations of 10be data from both sand and pebble sized clasts within the same deposit indicate grain size can significantly affect the ability to model ages with tcn depth profiles and when possible sand not pebbles should be used for depth profile exposure dating

3 d inversions of magnetic gradiometer data in archeological prospecting possibilities and limitations a vertical gradient magnetic system based on optically pumped cesium sensors has been used to map subtle magnetic anomalies across infilled pit houses and ditches at a medieval archeological site in northern switzerland for estimating the locations and dimensions of these features from the recorded data we have designed and implemented an appropriate inversion scheme tests of this scheme on realistic synthetic data sets suggested that suitable minimum magnetic susceptibility contrasts and smoothing parameters for the inversion may be directly extracted from the data inversions with minimum magnetic susceptibility contrasts generated causative bodies with maximum plausible sizes by using higher magnetic susceptibility contrasts a complete suite of models that matched the data equally well was produced to constrain better the magnetic susceptibility constrast within a selected area of the archeological site shallow samples of topsoil and sediment were analyzed in the laboratory an inversion based o

late cenozoic tectonic evolution of the southern chinese tian shan structural sedimentological magnetostratigraphic and 40ar 39ar thermochronological investigations were conducted in the southern chinese tian shan on the basis of our own mapping and earlier investigations in the area the late cenozoic southern tian shan thrust belt may be divided into four segments based on their style of deformation from west to east they are 1 kashi aksu imbricate thrust system 2 the baicheng kuche fold and thrust system 3 the korla right slip transfer system and 4 the lop nor thrust system the westernmost kashi aksu system is characterized by the occurrence of evenly spaced 12 15 km imbricate thrusts the baicheng kuche and korla systems are expressed by a major north dipping thrust the kuche thrust that changes its strike eastward to become a nw striking oblique thrust ramp the korla transfer zone the lop nor system in the eastern most part of the southern chinese tian shan consists of widely spaced thrusts all involved with basement rocks geologic mapping and cross section construction suggest that at least 20 40 km of crustal shortening with a horizontal shortening strain of 20 30 has occurred in the southern chinese tian shan during the late cenozoic these estimates are minimum because of both conservative extrapolation of the thrust geometries and partial coverage of the thrust belt by the cross sections the timing of initial thrusting is best constrained in the kuche basin where crustal shortening may have occurred at 21 24 ma the time of a major facies transition between lacustrine and braided fluvial sequences constrained in general by biostratigraphy and in detail by magnetostratigraphy this estimate represents only a minimum age as development of thrusts in the southern chinese tian shan may have propagated southward toward the foreland thus the sedimentary record only represents the southernmost and therefore youngest phase of thrusting if our estimate of timing for the thrust initiation 21 24 ma is correct using the estimated magnitude of shortening 20 40 km and shortening strain 20 30 the averaged rates of late cenozoic horizontal slip and shortening strain are 1 1 9 mm yr 1 and 2 9 4 5 10 16 s 1 respectively our reconnaissance 40ar 39ar thermochronological analysis in conjunction with earlier published results of apatite fission track analysis by other workers in the chinese tian shan suggests that the magnitude of cenozoic denudation is no more than 10 km most likely less than 5 km we demonstrate via a simple airy isostasy model that when the thermal effect on changes in surface elevation is negligible determination of the spatial distribution and temporal variation of both horizontal shortening strain and denudation becomes a key to reconstructing the elevation history of the tian shan using this simple model the loosely constrained magnitude of crustal shortening strain and denudation in the southern chinese tian shan implies that it may have been elevated 1 0 2 0 km since the onset of cenozoic thrusting

different sources involved in generation of continental arc volcanism the carboniferous permian volcanic rocks in the northern margin of the north china block abstract new zircon u pb dating results on the carboniferous permian volcanic rocks in the northern margin of the north china block ncb indicate their eruption during the early carboniferous to late permian from 347 3 ma to 258 1 ma and a slight decrease of the upper limits of the volcanic sequences from west to east they have a main rock association of basalt basaltic andesite andesite dacite rhyolite tuff and tufaceous sandstone most of them have calc alkaline compositions and exhibit variable sio2 contents from 48 2 wt to 77 1 wt there is no significant gap between the mafic and felsic volcanic rocks in major and trace element classification diagrams indicating that they are not bimodal in composition the carboniferous permian volcanic rocks exhibit subduction related geochemical features such as negative nb and ta anomalies of mafic to intermediate rocks on primitive mantle normalized diagrams indicating they were formed in an andean type continental arc during southward subduction of the paleo asian oceanic plate beneath the northern ncb however their wide range of whole rock sr nd and zircon hf isotopic compositions indicate that their source areas are very complex and different sources were involved in generation of these volcanic rocks geochemical and sr nd hf isotopic results show that the basalt and some andesite were produced by fractional crystallization of mafic magma derived from partial melting of mantle wedge and subducted oceanic crust however most of the intermediate to felsic volcanic rocks were derived from partial melting of lower continental crust there is an increasing input of crustal materials from the carboniferous to permian as indicated by increasing volumes of felsic volcanic rocks in the volcanic sequences the results show that origin of the continental arc volcanism is very complex and both materials from the subducted oceanic crust and sediments mantle wedge and arc continental crust could be involved in their generation for the permian andesite dacite and rhyolitic volcanic rocks there is a striking correlation between their sr nd hf isotopic compositions and those of the underlying basement sr nd hf isotopic differences between the permian intermediate to felsic volcanic rocks from either side of the northern boundary fault of the north china craton suggest that mixture and homogenization of the magmas in the melting assimilation storage hybridization mash zone within this continental arc was incomplete and incomplete homogenization of magmas is likely a common feature of many mash zones within continental arcs our results on the volcanic rocks also show that the final closure of the paleo asian ocean along the solonker suture zone has not been finished prior to the middle permian and the decease of the paleo asian oceanic subduction beneath the northern ncb in its middle western parts is a little earlier than that in its eastern part

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

episodic bimodal magmatism at an active continental margin due to paleo pacific plate subduction a case study from the southern segment of the tan lu fault zone eastern china abstract volcanic rocks in the southern part of the tan lu fault zone tlfz eastern china provide key evidence for the connection between spatially and temporally related pulses of cretaceous magmatism activity on the tlfz and the dynamics of the paleo pacific plate we identify two stages of bimodal volcanism in the chaohu lujiang segment southern tlfz based on lithological assemblages and precise isotopic ages the early cretaceous 125 121 ma volcanic rocks are fractionated aluminous a type rhyolites and basalts whereas the late cretaceous volcanic rocks 97 83 ma range in composition from porphyritic andesites and trachydacites to rhyolites the early cretaceous rhyolites display strongly negative nd hf isotopic compositions end t 16 3 to 16 1 ehf t 24 1 to 17 9 the most likely source rocks of the fractionated aluminous a type rhyolites are metaigneous rocks of the feidong complex the early cretaceous potassic basalts k2o na2o 1 5 3 4 end t 9 0 exhibit more enriched isotopic compositions than the late cretaceous sodic porphyritic andesites k2o na2o 0 6 0 8 end t 5 6 ehf t 15 1 to 6 8 indicating derivation from heterogeneously enriched lithospheric mantle this heterogeneity is ascribed to derivation from more fertile and isotopically enriched mantle sources with lower solidus temperatures modeling of selected elements reveals that the late cretaceous trachydacites end t 12 2 to 8 2 ehf t 16 5 to 6 4 originated by mixing between coeval mafic and felsic magmas the most negative end t 18 8 to 17 1 ehf t 20 5 to 4 7 isotopic compositions along with neoproterozoic inherited zircons within the late cretaceous rhyolites indicate derivation from lower crust of the south china block we suggest that the early cretaceous bimodal volcanic rocks formed in an extensional backarc setting induced by retreat of the subducting izanagi slab however the tlfz became a focus of late cretaceous bimodal volcanism due to weak far field extension caused by the slow northward motion of the paleo pacific plate moreover the high velocity and low angle of subduction of the izanagi slab were the main driving mechanisms behind an early cretaceous transpressional event that separated the two stages of bimodal volcanism

complex calc alkaline volcanism recorded in mesoarchaean supracrustal belts north of frederikshab isblink southern west greenland implications for subduction zone processes in the early earth we present new geochemical data for three mesoarchaean supracrustal belts ravns storo bjornesund and perserajoorsuaq situated north of frederikshab isblink in southern west greenland for which we propose the collective name the ikkattup nunaa supracrustal association they comprise mainly amphibolites of tholeiitic basalt composition and leucoamphibolites of calc alkaline andesite composition both lithological units are cut by aplite sheets of tonalite trondhjemite granodiorite ttg composition with u pb zircon ages of c 2900 ma lu hf and sm nd isochrons based on whole rock amphibolite and leucoamphibolite samples yield ages of 2990 41 ma and 3020 78 ma respectively which are within error of the age of the fiskenaesset complex situated 5 25 km to the north leucoamphibolites from the three supracrustal belts show apparent geochemical mixing trends between tholeiitic amphibolites and ttg gneisses as the end members by assimilation fractional crystallisation afc modelling we can show that one group of leucoamphibolites can indeed be explained by contamination of the parental melts by a ttg like end member and another group of high p2o5 la and nb leucoamphibolites can be explained by contamination involving a hypothetical low silica adakite end member however the leucoamphibolites are juvenile with epsilon nd 2970 ma from 2 1 to 3 5 and epsilon hf 2970 ma of 3 5 to 4 3 thus the mafic source of the felsic contaminant melts must have been derived from a depleted mantle source more or less at the same time 60 ma as the volcanism took place our preferred interpretation of the geochemical and isotope data is that the protoliths of the supracrustal rocks formed in an island arc setting where early tholeiitic volcanism gave way to calc alkaline volcanism in a maturing arc the apparent afc trends are thus explained by in situ partial melting of basaltic arc crust to form juvenile ttg and adakite melts that mixed with mafic magmas or contaminated their mantle source to produce the calc alkaline leucoamphibolite protolith this model has important implications for the general interpretation of other archaean supracrustal belts because afc and geochemical mixing trends towards a ttg end member are not uniquely diagnostic of crustal contamination but may rather reflect processes operating at lower crustal or source levels in arcs such as melting assimilation storage homogenisation mash or slab melt low silica adakite metasomatism of the mantle source respectively c 2012 elsevier b v all rights reserved less

co seismic ruptures of the 12 may 2008 ms 8 0 wenchuan earthquake sichuan east west crustal shortening on oblique parallel thrusts along the eastern edge of tibet abstract the m s 8 0 wenchuan earthquake which devastated the mountainous western rim of the sichuan basin in central china produced a surface rupture over 200 km long with oblique thrust dextral slip and maximum scarp heights of 10 m it thus ranks as one of the world s largest continental mega thrust events in the last 150 yrs field investigation shows clear surface breaks along two of the main branches of the ne trending longmen shan thrust fault system the principal rupture on the nw dipping beichuan fault displays nearly equal amounts of thrust and right lateral slip basin ward of this rupture another continuous surface break is observed for over 70 km on the parallel more shallowly nw dipping pengguan fault slip on this latter fault was pure thrusting with a maximum scarp height of 3 5 m this is one of the very few reported instances of crustal scale co seismic slip partitioning on parallel thrusts this out of sequence event with distributed surface breaks on crustal mega thrusts highlights regional ew directed present day crustal shortening oblique to the longmen shan margin of tibet the long rupture and large offsets with strong horizontal shortening that characterize the wenchuan earthquake herald a re evaluation of tectonic models anticipating little or no active shortening of the upper crust along this edge of the plateau and require a re assessment of seismic hazard along potentially under rated active faults across the densely populated western sichuan basin and mountains

basement controlled structure and evolution of the queen charlotte basin west coast of canada abstract fault block tectonics usually associated with intracontinental areas characterizes the western part of the north america plate immediately adjacent to the pacific plate the long lived basement supported mosaic of fault bounded blocks controlled the evolution of this part of the west coast of canada since at least the late jurassic manifesting itself strongly in the tertiary this conclusion is based on recently collected geological and geophysical data gravity and magnetic maps and seismic profiles reveal orthogonal fault networks across the region in the tertiary queen charlotte sedimentary basin and on its pre tertiary rim the faults were reactivated and block movements sometimes inverted in the cenozoic similarity of fault networks across the region suggests inheritance of many structures in the basin from earlier time passage of the region over a mantle hot spot in the miocene may have been a partial cause of renewed block movements crosscutting relationships of large faults and their displacement histories onshore preclude regional strike slip faulting in the tertiary tectonic extension within the basin was small maximum 10 the previous model of formation of the queen charlotte basin by continental rifting sensu mckenzie 1978 is negated evolution of the region was not affected fundamentally by plate interactions at the adjacent continental margin in particular by the change from subduction to strike slip regime at 42 ma a similar structural style existed in the area before and after that event

large earthquake rupturing and slipping behavior along the range front maidan fault in the southern tian shan northwestern china abstract slipping behavior of active thrust faults are not well explored because of their variable vertical displacements governed by dip angle changes along the fault striking the range front maidan fault mdf is a thrust fault in the southern tian shan and exhibits clear paleoearthquake rupturing during the late quaternary in this study based on field investigation trench exposing detailed terrace surveying with differential gps and optically stimulated luminescence osl dating we revealed that the latest paleoearthquake on the mdf occurred at 1 76 ka and generated a vertical displacement of 3 0 m then we analyzed the vertical displacements on different terraces of the yushanguxi river versus the corresponding terrace 10be exposure ages we suggest that the large earthquake on the mdf is an irregular recurrence the mdf still has a high seismic risk in spite of only 1 76 ka elapsed time for the latest paleoearthquake we propose that the growth and uplift of the western tian shan orogen is caused by fault thrusting and crustal shortening of a series of block like patterns

critical taper model of fold and thrust belts and accretionary wedges the fold and thrust belts and submarine accretionary wedges that lie along compressive plate boundaries are one of the best understood deformational features of the earth s upper crust although there is considerable natural variation among the many fold and thrust belts and accretionary wedges that have been recognized and explored several features appear to be universal in cross section fold and thrust belts and accretionary wedges occupy a wedge shaped deformed region overlying a basal detachment or decollement fault the rocks or sediments beneath this fault show very little deformation the decollement fault characteristically dips toward the interior of the mountain belt or in the case of a submarine wedge toward the island arc the topography in contrast slopes toward the toe or deformation front of the wedge deformation within the wedge is generally dominated by imbricate thrust faults verging toward the toe and related fault bend folding two north american fold and thrust belts that exhibit these features are shown in figure 1 neither of these two examples is tectonically active today the southern canadian fold and thrust belt was active during the late jurassic and cretaceous 150 100 ma whereas the southern appa lachians were deformed during the late carboniferous to permian alle ghenian orogeny 300 250 ma figure 2 shows two examples that are currently active the taiwan fold and thrust belt produced by the sub duction of the eurasian plate beneath the philippine sea plate suppe 1981 1987 and the barbados accretionary wedge produced by the sub

fault slip and earthquake recurrence along strike slip faults contributions of high resolution geomorphic data article i nfo understanding earthquake eq recurrence relies on information about the timing and size of past eq ruptures along a given fault knowledge of a fault s rupture history provides valuable information on its potential future behavior enabling seismic hazard estimates and loss mitigation stratigraphic and geomorphic evidence of faulting is used to constrain the recurrence of surface rupturing eqs analysis of the latter data sets culminated during the mid 1980s in the formulation of now classical eq recurrence models now routinely used to assess seismic hazard within the last decade light detection and ranging lidar surveying technology and other high resolution data sets became increasingly available to tectono geomorphic studies promising to contribute to better informed models of eq recurrence and slip accumulation patterns after reviewing motivation and background we outline requirements to successfully reconstruct a fault s offset accumulation pattern from geomorphic evidence we address sources of uncertainty affecting offset measure ment and advocate approaches to minimize them a number of recent studies focus on single eq slip distribu tions and along fault slip accumulation patterns we put them in context with paleoseismic studies along the respective faults by comparing coefficients of variation cv for eq inter event time and slip per event and find that a single event offsets vary over a wide range of length scales and the sources for offset variability differ with length scale b at fault segment length scales single event offsets are essentially constant c along fault offset accumulation as resolved in the geomorphic record is dominated by essentially same size large offset in crements and d there is generally no one to one correlation between the offset accumulation pattern constrained in the geomorphic record and eq occurrence as identified in the stratigraphic record revealing the higher resolution and preservation potential of the latter while slip accumulation along a fault segment may be dominated by repetition of large nearly constant offset increments timing of surface rupture is less regular

the chi chi earthquake sequence active out of sequence thrust faulting in taiwan we combined precise focal depths and fault plane solutions of more than 40 events from the 20 september 1999 chi chi earthquake sequence with a synthesis of subsurface geology to show that the dominant structure for generating earthquakes in central taiwan is a moderately dipping 20 to 30 thrust fault away from the deformation front a second subparallel seismic zone lies about 15 kilometers below the main thrust these seismic zones differ from previous models indicating that both the basal decollement and relic normal faults are aseismic

spatial and temporal distributions of the late mesozoic volcanic successions and their controlling effects on the changling fault depression of the songliao basin northeast china volcanic successions including their spatial temporal tectonic and geochemical attributes were identified based on well data 2d and 3d seismic data u pb isotopic ages and major and trace element data from the changling fault depression of the songliao basin in northeastern china three eruption cycles developed in the changling fault depression k1h huoshiling formation 124 118 ma k1yc1 first member of the yingcheng formation 115 106 ma and k1yc3 third member of the yingcheng formation 106 102 ma these three eruption cycles comprised seven eruption stages the volcanic successions in every eruption cycle were bimodal and evolved from basic to acidic the magma of the basic and intermediate rocks in these cycles was derived from partial melting of the asthenosphere in k1yc1 and k1yc3 partial melting of the newly formed basic rocks in the lower crust formed the high sio2 acidic magma in addition fractional crystallization of basic magma formed the small scale intermediate acidic m

cenozoic deformation and exhumation history of the central kyrgyz tien shan new low temperature thermochronological data from 80 samples in eastern kyrgyzstan are combined with previously published data from 61 samples to constrain exhumation in a number of mountain ranges in the central kyrgyz tien shan all sampled ranges are found to have a broadly consistent cenozoic exhumation history characterized by initially low cooling rates 1 c myr followed by a series of increases in exhumation that occurred diachronously across the region in the late cenozoic that are interpreted to record the onset of deformation in different mountain ranges combined with geological estimates for the onset of proximal deformation our data suggest that the central kyrgyz tien shan started deforming in the late oligocene early miocene leading to the development of several widely spaced mountain ranges separated by large intermontane basins subsequently more ranges have been constructed in response to significant shortening increases across the central kyrgyz tien shan notably in the late miocene the order of range construction is interpreted to reflect variations in the susceptibility of inherited structures to reactivation reactivated structures are also shown to have significance along strike variations in fault vergence and displacement which have influenced the development and growth of individual mountain ranges moreover the timing of deformation allows the former extent of many intermontane basins that have since been partitioned to be inferred this can be linked to the highly time transgressive onset of late cenozoic coarse clastic sedimentation

late carboniferous bimodal volcanic rocks and coeval a type granite in the suman khad area southwest mongolia implications for the tectonic evolution abstract the volcanic rocks in the suman khad area in southwest mongolia form a bimodal suite consisting mainly of peralkaline rhyolites with subordinate basalts the rhyolite sample collected from the bimodal suite yielded a shrimp zircon u pb age of 314 5 ma mswd 1 41 n 12 which was interpreted to represent formation time of the bimodal volcanic suite the basalts were characterized by enrichment in lile and lree and depletion in hfse indicating their formation was related to subduction processes these features together with their positive end t values 6 3 6 7 suggest that the basalts were likely derived from a depleted mantle source metasomatized by subduction related fluids in various tectonic discrimination diagrams the basalts exhibited a transition from true arc basalts to intraplate basalts and thus were suggested to from in a back arc tectonic setting the rhyolites show a close affinity to a type granites with enrichment in lile and lree depletion in nb ta and ti and positive end t values 6 0 6 4 considering the observed distinct compositional gap between the endmembers of the bimodal suite the rhyolites are proposed to originate from partial melting of juvenile basaltic crustal rocks rather than fractional crystallization of basaltic melt the granite associated with the bimodal volcanic rocks yielded a shrimp zircon u pb ages of 312 5 ma mswd 0 75 n 13 indicating that the granite is contemporaneous with the bimodal volcanic suite the granite samples showed typical a type granitic geochemical affinities and are considered to have been formed by partial melting of crustal rocks in a within plate tectonic setting based on a combination of the available data we suggest that the late carboniferous bimodal volcanic suite together with the coeval a type granites in the suman khad region probably document a back arc basin extensional environment which probably related to the roll back of the paleo asian oceanic plate during the northward subduction under the central mongolia microcontinent

irregular earthquake cycle along the southern tianshan front aksu area china 1 a long deformation record 16 5 ka showing nonsteady earthquake recurrence is established for the aksu thrust tianshan china on the basis of cosmogenic 10be dating of faulted surfaces topographic leveling across the aksu fault scarp indicates up to 10 m uplift across a moraine abandoned 16 5 ka and similar uplift of 6 m across two inset surfaces yielding ages of 12 5 and 5 ka successively smaller uplifts of terraces younger than 5 ka indicate that three or more major earthquakes occurred during this period these data show that the aksu thrust fault was quiet for at least 7500 years and active in the last 5000 years and probably in the interval 16 5 12 5 ka the seismic cycle along the aksu thrust fault over the last 16 500 years was thus strongly irregular showing long quiescence and clustering which is a challenge for paleoseismology and hazard assessment this irregularity also makes it more difficult to estimate long term shortening rates nevertheless we are able to constrain a minimum total shortening rate 7 mm yr across the southern tianshan front for the last 12 500 years that is about a third of the total geodetic rate for the western tianshan 20 mm yr

seismic moments of major earthquakes and the rate of shortening across the tien shan three twentieth century earthquakes that richter assigned m 8 and two of comparable magnitude in the nineteenth century imply rapid deformation within the tien shan seismic moment tensors of major earthquakes in this century suggest an average shortening rate of 7 2 mm yr 1 across the tien shan in the western part however where three of the five largest earthquakes occurred the calculated rate is consistent with the value of 20 mm yr 1 measured using gps in that area by abdrakhmatov et al as avouac et al suggested the high rate in the western part apparently is a manifestation of counter clockwise rotation of the tarim basin relative to eurasia about an axis near the east end of the tien shan

slip in the 1857 and earlier large earthquakes along the carrizo plain san andreas fault the moment magnitude mw 7 9 fort tejon earthquake of 1857 with a 350 kilometer long surface rupture was the most recent major earthquake along the south central san andreas fault california based on previous measurements of its surface slip distribution rupture along the 60 kilometer long carrizo segment was thought to control the recurrence of 1857 like earthquakes new high resolution topographic data show that the average slip along the carrizo segment during the 1857 event was 5 3 1 4 meters eliminating the core assumption for a linkage between carrizo segment rupture and recurrence of major earthquakes along the south central san andreas fault earthquake slip along the carrizo segment may recur in earthquake clusters with cumulative slip of 5 meters

the petrology and geochemistry of the aniakchak caldera forming ignimbrite aleutian arc alaska aniakchak caldera alaska produced a compositionally heterogeneous ignimbrite 3400 years ago which changes from rhyodacitic at the base to andesitic at the top of the eruptive sequence interpretations of compositionally heterogeneous ignimbrites typically include either in situ fractional crystallization of mafic magma and generation of a stratified magma body or replenishment of a silicic magma chamber by mafic inputs another possibility silicic replenishment of a more mafic chamber exists geochemical characteristics of the caldera forming rhyodacite and several late precaldera rhyodacites indicate independent origins for each within a maximum of 5000 years prior to caldera formation isotopic considerations preclude derivation of the caldera forming rhyodacite from the caldera forming andesite however the caldera forming rhyodacite can be explained as the residual liquid of a mostly crystallized basalt with addition of crustal material the aniakchak andesite probably formed in a shallow chamber by successive mixing events involving small volumes of basalt and rhyodacite together with contamination the pre caldera rhyodacites represent erupted portions of intruding silicic magma whereas another portion homogenized with the resident mafic magma the caldera forming event reflects a large influx of rhyodacite which erupted before significant mixing occurred and also triggered draining of much of the andesitic magma from the chamber

late cenozoic tectonics of the kepingtage thrust zone interactions of the tien shan and tarim basin northwest china the kepingtage kalpin thrust zone northwest china is an actively deforming part of the india asia collision system it lies south of the tien shan has an area of 16 000 km2 and consists of arcuate emergent imbricates overall vergence is toward the interior of the tarim basin to the south thrust sheets typically expose upper cambrian to permian platformal strata thrusting is largely thin skinned thin upper cambrian evaporites are likely to be the main decollement horizon the kepingtage thrust zone is the only margin of the tarim basin to deform in this style in the cenozoic it is replaced to the east and west by thrust zones which have propagated shorter distances into the interior of the tarim basin thick up to 10 km mesozoic and cenozoic clastic successions are present and deformed in these regions such successions are not present in the kepingtage thrust zone or the adjacent bachu uplift within the tarim basin because of episodic activity on steep northwest southeast trending thrusts which define the margins of the bachu uplift these mesozoic cenozoic strata may have suppressed the ability of regions along strike from the kepingtage thrust zone to deform by thin skinned thrusting utilizing the upper cambrian decollement the along strike variation in active thrusting at the southern margin of the tien shan is an example of syntectonic sedimentation controlling thrust belt deformation style a balanced section across the thrust zone indicates 28 shortening equivalent to 35 km this is equivalent to an average slip rate of 1 8 mm yr 1 and a strain rate of 4 4 10 16 s 1 assuming deformation began at circa 20 ma active deformation is focused along the frontal thrust the kepingtage fault the northern boundary of the kepingtage thrust zone is formed by the south tien shan fault this major north dipping thrust is seismically active with published earthquake focal depths at midcrustal levels 14 and 18 km it juxtaposes upper carboniferous sedimentary rocks of different facies and different paleozoic deformation histories speculatively it represents a reactivation of a late paleozoic thrust which originated at the boundary between the platformal interior of the tarim block and a deeper water foreland basin to its north

inherited strike slip faults as an origin for basement cored uplifts example of the kungey and zailiskey ranges northern tian shan 1 basement cored uplifts bounded by steeply dipping reverse faults are mechanically difficult to explain reactivation of strike slip faults that aid the formation of new high angle reverse faults in the surrounding crust may provide one origin for these structures this hypothesis is explored by examining the late miocene to quaternary evolution of the kungey and zailiskey ranges in the northern tian shan these ranges are cored by the kemin chilik fault kcf an inherited paleozoic structure with sinistral separation of basement terranes range growth in response to northward propagation of the tian shan has taken place along a network of steeply dipping reverse and oblique slip faults surrounding the kcf deformation of a low relief unconformity separating neogene strata from paleozoic basement records structural growth in response to fault slip deformed river terraces surrounding the ranges are correlated to a well preserved chronosequence in the southern kungey range cosmogenic10be dating of this chronosequence combined with offset measurements yields slip rates ranging from 0 07 to 0 37 mm yr for dip slip faults and 1 1 to 1 5 mm yr for strike slip faults late quaternary activity in the kungey zailiskey ranges is consistent with the longer term outward stepping pattern of range growth based on cross sections constrained from the folded unconformity surface deformed neogene strata and quaternary terraces faults building the kungey range are inferred to steepen at depth and emanate from a shear zone co located with the reactivated kcf this geometry is consistent with a slip partitioned system developed by an obliquely slipping reactivated fault at depth

petrogenesis and emplacement of the ttg and k rich granites at the buzwagi gold mine northern tanzania implications for the timing of gold mineralization abstract the buzwagi gold mine found in the neoarchaean nzega greenstone belt of northern tanzania is underlain by neoarchaean mafic volcanic rocks which are intruded by a massive body of ultramafic rocks that are cross cut by grey colored ttg and pink microcline k rich granites geochemical alteration studies for the sheared and hydrothermally altered k granites show that the lfse and ree were significantly mobilized during the shearing and subsequent injection of hydrothermal fluid events whereas the hfse remained virtually unchanged the buzwagi mine ttg exhibit geochemical characteristics of other worldwide known ttg al2o3 15 0 wt na2o k2o ratios of 1 19 5 16 low concentration of heavy ree with y contents of 3 7 ppm and yb 0 3 0 5 ppm leading to high sr y ratios 61 152 and la yb 32 140 which are comparable to those of phanerozoic adakites the buzwagi mine ttg are characterized by strongly fractionated ree patterns la ybcn 23 100 with slightly negative to no eu anomalies eu eu 0 77 1 05 negative anomalies of nb ta and ti and end 2713 values of 1 19 to 1 77 these geochemical and isotopic characteristics are interpreted as formation of the ttg by partial melting of the hydrous basaltic crust at pressures and depths where garnet and amphibole were stable phases in the late archaean subduction zone the buzwagi mine k rich granites differ from their ttg counterparts in having elevated concentrations of incompatible elements like k zr th hf and ree they however share similar nd isotopic compositions end 2674 ma 1 19 to 1 22 and some geochemical features including fractionated ree patterns la ybcn 13 204 with slightly negative to no eu anomalies eu eu 0 78 0 89 and negative anomalies of nb ta and ti these geochemical features and isotopic signatures have been interpreted as formation of the k granites by anhydrous partial melting of the ttg and greenstones the buzwagi mine k granites were emplaced at 2674 18 ma some 40 ma years after the emplacement of ttg at 2713 8 ma the timing of the buzwagi mine granitic magmatism falls within the peak crustal growth period in the northern part of the archaean tanzania craton gold mineralization at the buzwagi mine is hosted in quartz veins that are within strongly sheared and hydrothermally altered k granites in which silica and sericite alterations and sulphides mainly pyrite and chalcopyrite although chalcocite is also present are important the buzwagi gold deposit is thus a typical archaean orogenic deposit whose mineralization is hydrothermally and structurally controlled and the abundance of sulphides silica and sericite alteration assemblages suggests that the deposition conditions were reducing and acidic in nature owing to its epigenetic nature it can be inferred that the maximum age of deposition of gold at the buzwagi mine is 2674 ma which is more or less the same age inferred for the golden pride mine 2680 ma both of which are found in the nzega greenstone belt

geochemical variation within the northern ryukyu arc magma source compositions and geodynamic implications the major and trace element and pb sr nd isotopic compositions of quaternary mafic lavas from the northern ryukyu arc provide insights into the nature of the mantle wedge and its tectonic evolution beneath the volcanic front in the northern part of the arc the subducted slab of the philippine sea plate bends sharply and steepens at a depth of 80 km lavas from the volcanic front have high abundances of large ion lithophile elements and light rare earth elements relative to the high field strength elements consistent with the result of fluid enrichment processes related to dehydration of the subducting slab new pb isotopic data identify two distinct asthenospheric domains in the mantle wedge beneath the south kyushu and northern ryukyu arc which in a parallel with data from the lau basin appear to reflect mantle with affinities to indian and pacific type mid ocean ridge basalt morb indian ocean morb type mantle contaminated with subducted ryukyu sediments can account for the variation of lavas erupted on south kyushu and probably in the middle okinawa trough in contrast magmas of the northern ryukyu volcanic front appear to be derived from sources of pacific morb type mantle contaminated with a sedimentary component along arc variation in the northern ryukyus reflects increasing involvement of a sedimentary component to the south compositions of alkalic basalts from the south kyushu back arc resemble intraplate type basalts erupted in nw kyushu since 12 ma we propose that the bending of the subducted slab was either caused by or resulted in lateral migration of asthenospheric mantle yielding indian ocean type characteristics from a mantle upwelling zone beneath nw kyushu and the east china sea this model also accounts for 1 extensional counter clockwise crustal rotation 4 2 ma 2 voluminous andesite volcanism 2 ma and 3 the recent distinctive felsic magmatism in the south kyushu region

intracrustal controls on the coexistence of tholeiitic and calc alkaline magma series at aso volcano sw japan aso caldera sw japan was formed incrementally over a period of 300 kyr by the eruption of four basaltic to dacitic pyroclastic flow and fall deposits aso 1 4 these units are collectively referred to as the aso pyroclastic flow deposit apfd in which tholeiitic and calc alkaline magmas have been erupted in close temporal and spatial proximity detailed petrological and geochemical studies indicate that a major shift in magmatic composition occurred between aso 1 and 4 as the caldera forming stage evolved changing from a predominantly tholeiitic to a calc alkaline signature the mineralogical and geochemical characteristics of aso 1 4 suggest that magmatic evolution within each unit is controlled by fractional crystallization associated with magma mixing crustal assimilation by altering the fractional crystallization mineral assemblage as well as the relative amounts of fractional crystallization magma mixing and crustal assimilation the systematic shifts in differentiation trends and magmatic series observed between the four eruptive units can be reproduced this suggests that the coexistence of tholeiitic and calc alkaline magma series can be attributed to shallow level intracrustal processes

presentation of magnetic anomaly map data by stereo projection of magnetic shadowgrams coloured magnetic maps published at the 1 1 000 000 scale by the geological survey of canada have been found to be useful aids in support of regional geological mapping and compilation projects however because of the quantization into colour intervals some of the fine detail contained in the original magnetic data is lost these smaller features can be brought out quite effectively with the shaded relief technique although there is an attendant loss of amplitude information which can be recovered by means of stereo shadowgrams a second shadowgram is produced by offsetting the original shadowgram by an amount proportional to the magnetic intensity of each data point and then the offset and original shadowgrams are viewed as a stereo pair an example of the effectiveness of these techniques is the detail that can be seen on the shadowgrams and stereo shadowgrams for the lockhart river imw np 12 13 and the thelon river imw nq 12 13 14 map sheets

the relationship between adakitic calc alkaline volcanic rocks and ttgs implications for the tectonic setting of the karelian greenstone belts baltic shield abstract two types of coeval acid intermediate rocks with different petrological geochemical and isotopic features have been discovered among volcanic rocks and surrounding synkinematic tonalite trondhjemite granodiorite ttg plutons of late archaean greenstone belts in the karelian granite greenstone terrane type 1 rocks comprise trondhjemites and sub volcanic occasionally volcanic dacite rhyolite rocks they are characterized by high sr low y and hree contents high sr y ratios and strongly fractionated ree patterns with no significant positive or negative eu anomaly initial nd is positive indicating a generation from juvenile source with little or no contribution of ancient continental crust type 2 is represented by diorite granodiorites and calc alkaline basalt andesite dacite rhyolite badr series as compared to type 1 these rocks differ by their lower sr higher y and hree contents lower sr y ratios and less fractionated hree patterns with negative eu anomalies initial nd varies from negative to positive values thus indicating a variable contribution of sialic crust geochemistry of the two magmatic series suggests their formation in a convergent plate margin setting the type 1 rocks resemble phanerozoic adakites which represent slab derived melts contaminated by overlying mantle wedge the type 2 rocks resemble badr series which were derived from a mantle wedge metasomatized by slab derived fluids and melts with subsequent variable crustal contamination the spatial distribution of these two types of magmatic series defines the asymmetry of the studied granite greenstone structures which presumably reflects the primary lateral zoning of island arc formed under specific thermal conditions in the archaean mantle adakite melts upraised to the surface in the frontal part of the island arc where mantle wedge was thin showing no or little interaction with metasomatized mantle and formed adakite type plutonic and sub volcanic rocks at greater depths adakitic melts and fluids interacted with the overlying mantle wedge and caused its partial melting with generation of calc alkaline badr volcanic rocks and diorite granodiorite plutons in the rear part of the island arc our data suggest that greenstone belt volcano plutonic arcs were initiated on different types of crust which presumably determined the petrogenetic and isotope variations of the studied badr and adakite type island arc complexes

erosion as a driving mechanism of intracontinental mountain growth in nature mountains can grow and remain as localized tectonic features over long periods of time 10 m y by contrast according to current knowledge of lithospheric rheology and neglecting surface processes any intracontinental range with a width that exceeds that which can be supported by the strength of the lithosphere should collapse within a few tens of millions of years for example assuming a quartz dominated crustal rheology the relief of a range initially 3 km high and 300 400 km wide is reduced by half in about 15 m y as a result of lateral spreading of its crustal root we suggest that surface processes might actually prevent such a subsurface collapse removal of material from topographic heights and deposition in the foreland oppose spreading of the crustal root and could eventually drive a net influx of material toward the orogeny we performed a set of numerical experiments in order to validate this hypothesis a section of a lithosphere with a brittle elasto ductile rheology initially loaded by a mountain range is submitted to horizontal shortening and to surface processes if erosion is intense material is removed more rapidly than it can be supplied by crustal thickening below the range and the topography is rapidly smoothed for example a feature 3 km high and 300 400 km wide is halved in height in about 15 m y for an erosion coefficient k 10 3 m 2 yr the erosion rate is of the order of a few 0 1 mm yr this regime might be called erosional collapse if erosion is not active enough the crustal root spreads out laterally and subsurface collapse occurs in the third intermediate regime removal of the material by erosion is dynamically compensated by isostatic rebound and inward flow in the lower crust so that the range can grow in this mountain growth regime the range evolves toward a characteristic graded shape that primarily depends on the erosion law the erosion rate may be high e g 0 5 0 9 mm yr close to the rate of tectonic uplift e g 0 7 1 1 mm yr and few times higher than the rate of topographic uplift 0 15 0 2 mm yr these experiments show that surface processes can favor localized crustal shortening and participate in the development of an intracontinental mountain surface processes must therefore be taken into account in the interpretation and modeling of long term deformation of continental lithosphere conversely the mechanical response of the lithosphere must be accounted for when large scale topographic features are interpreted and modeled in terms of geomorphologic processes

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

paleoearthquakes at frazier mountain california delimit extent and frequency of past san andreas fault ruptures along 1857 trace large earthquakes are infrequent along a single fault and therefore historic well characterized earthquakes exert a strong influence on fault behavior models this is true of the 1857 fort tejon earthquake estimated m7 7 7 9 on the southern san andreas fault ssaf but an outstanding question is whether the 330 km long rupture was typical new paleoseismic data for six to seven ground rupturing earthquakes on the big bend of the ssaf restrict the pattern of possible ruptures on the 1857 stretch of the fault in conjunction with existing sites we show that over the last 650 years at least 75 of the surface ruptures are shorter than the 1857 earthquake with estimated rupture lengths of 100 to 300 km these results suggest that the 1857 rupture was unusual perhaps leading to the long open interval and that a return to pre 1857 behavior would increase the rate of m7 3 m7 7 earthquakes

thermochronologic insight into late cenozoic deformation in the basement cored terskey range kyrgyz tien shan 1 basement cored ranges formed by reverse faulting within intracontinental mountain belts are often composed of poly deformed lithologies geological data capable of constraining the timing magnitude and distribution of the most recent deformational phase are usually missing in such ranges in this paper we present new low temperature thermochronological and geological data from a transect through the basement cored terskey range located in the kyrgyz tien shan using these data we are able to investigate the range s late cenozoic deformation for the first time displacements on reactivated faults are constrained and deformation of thermochronologically derived structural markers is assessed these structural markers postdate the earlier deformational phases providing the only record of cenozoic deformation and of the reactivation of structures within the terskey range overall these structural markers have a southern inclination interpreted to reflect the decreasing inclination of the reverse fault bounding the terskey range our thermochronological data are also used to investigate spatial and temporal variations in the exhumation of the terskey range identifying a three stage cenozoic exhumation history 1 virtually no exhumation in the paleogene 2 increase to slightly higher exhumation rates at 26 20 ma and 3 significant increase in exhumation starting at 10 ma

mafic to felsic magmatism and crustal recycling in the obonga lake greenstone belt western superior province evidence from geochemistry nd isotopes and u pb geochronology abstract the central wabigoon subprovince of the western superior province contains mesoarchean granitoid and supracrustal rocks 3 01 2 83 ga and 2 78 2 69 ga granitoid plutons and supracrustal sequences it is a key area for understanding the relationship between greenstone belts and surrounding granitoid rocks that may have acted as basement the obonga lake greenstone belt contains two distinct assemblages 1 a ta nb th they have end values of 0 7 to 2 4 a dacitic unit in the northern assemblage has high la yb high sr y low nb y and heavy ree and is interpreted to represent a mantle modified slab melt similar to adakites in cenozoic arcs the associated enriched basalts may represent melts from the mantle wedge modified by slab melt adakitic metasomatism or they may represent an enriched oib like asthenospheric source rocks of the southern assemblage mostly show th and light ree enrichment and negative nb and ta anomalies low end values down to 0 9 can be modelled through contamination of a mafic liquid by 3 3 3 2 ga sialic crust although rocks of that age have not been observed suggested mesoarchean basement to the southern assemblage may therefore have had a complex history spanning several hundred million years the southern assemblage volcanism may have occurred in a continental arc to continental back arc system where crustal recycling played an important role

the petrology and geochemistry of calc alkaline andesites on shodo shima island sw japan petrographical and geochemical characteristics of calc alkaline mclennan 1995 that are produced at convergent plate boundaries and are characterized by lower feo mgo andesites on shodo shima island sw japan having bulk comratios than tholeiitic andesites miyashiro 1974 an positions largely identical to the continental crust are presented understanding of the origin of such orogenic calc alkaline the following petrographic observations suggest a role for magma andesites is thus essential for comprehending not only mixing in producing such andesite magmas 1 two types of olivine the magma flux in the modern earth but also the role phenocrysts and spinel inclusions one with compositions identical of subduction related magmatism in the evolution of the to those in high mg andesites and the other identical to those in solid earth since the pioneering work of eichelberger basalts are recognized in terms of ni mg and cr al fe 1975 1978 anderson 1976 sakuyama 1979 1981 relations respectively 2 the presence of orthopyroxene phenocrysts 1984 and luhr carmichael 1980 the majority of with mg number 90 suggests the contribution of an orthopyroxenepetrologists have considered mixing of mafic and felsic bearing high mg andesite magma to production of calc alkaline magmas to be one of the major mechanisms of generation andesites 3 reversely zoned pyroxene phenocrysts may not be in of calc alkaline andesites continental crust formation equilibrium with mg rich olivine suggesting the involvement of a however may not be elucidated solely by such magma differentiated andesite magma as an endmember component 4 the mixing processes the reasons for this are twofold first presence of very fe rich orthopyroxene phenocrysts indicates the calc alkaline andesite magmatism typifies continental arcs association of an orthopyroxene bearing rhyolitic magma conat least on the modern earth suggesting that the contributions from the above at least five endmember magmas to the calctinental crust itself plays an important role in such andesite alkaline andesite genesis can also provide a reasonable explanation of formation possibly as the source of the felsic endmember the pb sr nd isotope compositions of such andesites component second there is a slight but significant difference in mgo contents between the bulk continental crust and the typical calc alkaline andesite e g kelemen

a new osl chronology for dust accumulation in the last 130 000 yr for the chinese loess plateau abstract a sensitivity corrected multiple aliquot regenerative dose protocol has been developed for fine grained quartz osl dating of chinese loess its reliability has been assessed on the basis of the methodology and by dating reference samples of known age close to the transition from the last interglacial paleosol s 1 to the last glacial loess l 1 which corresponds to the marine oxygen isotope stage mis 5 4 transition on the basis of the fine grained quartz osl age estimates for 33 loess samples from the upper part of the luochuan profile a detailed chronostratigraphy of continuous dust accumulation in the past 130 ka has been proposed changes in the accumulation rate occurred during the last glacial period mis 4 to mis 2 unexpectedly high accumulation rates were found in the weakly developed l 1 2 s paleosol of the last interstadial mis 3 rather than in the classic l 1 1 and l 1 3 loess of the cold dry glacial condition mis 2 and 4 the osl ages show some disagreement with the previous numerical chronology for the loess paleosol sequence based on correlation of variations in grain size with sedimentation rate the latter method resulted in an almost constant accumulation rate from 72 to 12 ka

geochemistry and geochronology of the bimodal volcanic rocks of the suguti area in the southern part of the musoma mara greenstone belt northern tanzania abstract the suguti volcanic rocks of the southern musoma mara greenstone belt in northern tanzania comprise mainly of a bimodal suite of tholeiitic basalts basaltic andesites and calc alkaline rhyolites with a subordinate amount of intermediate rocks zircon u pb and whole rock sm nd geochronology suggests that the two suites are cogenetic and were emplaced at 2755 1 ma with a common initial nd value of 2 1 the tholeiitic basalts are characterised by relatively flat chondrite normalised ree patterns with la ybcn ratios of 0 8 1 6 mean 1 0 the basalts also exhibit negative ti and nb anomalies in primitive mantle normalised multi element diagrams the flat ree patterns the presence of prominent negative nb anomalies and the positive initial nd value of 2 1 suggest that the basalts were formed by low pressure melting of a mantle wedge in an active continental margin setting compared to the tholeiitic basalts the calc alkaline rhyolites are characterised by low abundances of the transition elements cr

relatively recent construction of the tien shan inferred from gps measurements of present day crustal deformation rates the tien shan a high seismically active intracontinental mountain belt 1 000 2 000 km north of the himalaya has grown as a result of india s collision with asia1 the crustal shortening 200 50 km refs 2 3 and thickening that gave rise to the tien shan accommodates only a small fraction of india s total penetration into asia 2 000 3 000km and the temporal relationship of deformation in this belt to the india asia collision remains unclear here we report geodetic measurements of the tien shan using the global positioning system gps that indicate that the current crustal shortening rate is nearly half of india s convergence rate with eurasia in this area4 we infer a total shortening rate for the tien shan of 20 mm yr 1 which is approximately twice that inferred previously from the extrapolation of slip rates in the holocene3 and earthquake induced displacements during this century5 suggesting that the rate of mountain building in this region has accelerated several fold since the onset of collision 50 55 myr ago6 7 if as we argue the current shortening rate can be extrapolated to geological timescales then our results suggest that most of the tien shan has been constructed during the past 10 myr perhaps in response to an increased horizontal force following an abrupt rise of the tibetan plateau8 9

composition gaps critical crystallinity and fractional crystallization in orogenic calc alkaline magmatic systems the recognition of a three way correlation between magmatic sio2 content critical crystallinity and the size magnitude of crystal fractionation generated composition gaps in calc alkaline magmatic systems suggests an important control of magmatic critical crystallinity on the formation of such composition gaps to explain this correlation it is proposed that fractionation generated composition gaps are caused by 1 simultaneous interior i e non substrate crystallization and vigorous chamberwide convection which leads to progessive crystal suspension 2 cessation of convection when the percentage of suspended crystals reaches the critical crystallinity of the magma and 3 eventual buoyancy driven crystal liquid segregation producing a discrete body of fractionated magma which is separated from the initial magma by a composition gap this mechanism implies that many if not most magma bodies are characterized by interior crystallization and vigorous convection conditions which are not universally agreed upon at present given that such conditions characterize natural magma bodies fractional crystallization through crystal settling in low velocity boundary layers should be an important mechanism of fractional crystallization in a crystallizing and convecting body of magma composition gap formation should represent one endmember of a complete spectrum of possible evolutionary paths governed by the relative rates of crystal settling and crystal retention as a given volcanic plumbing system matures with time average settling retention ratios within individual magma bodies should increase due to higher average wall rock temperatures it follows that within a given volcanic center early stage volcanism should be more likely to display fractionation generated composition gaps than later stage volcanism such a temporal evolution has been documented at at least two aleutian calc alkaline volcanic centers

mineral disequilibrium in lavas explained by convective self mixing in open magma chambers characteristic features of many porphyritic andesite and dacite lavas are that they are rich in crystals and display a range of disequilibrium features including reversely zoned crystals resorption surfaces wide ranges of mineral compositions and minerals which are not in equilibrium with the surrounding rock matrix these features are often interpreted as evidence of the mixing of magmas of contrasting composition temperature and origin1 2 here however we propose that such features can also be caused by convection within a magma body with a single composition that is heated from below and cooled from above we describe petrological observations of andesite lava erupted at the soufriere hills volcano montserrat which indicate a heating event and the intermingling of crystals that have very different thermal histories we present experimental data on a representative groundmass composition of this lava which indicate that it is difficult to explain the calcic compositions of plagioclase overgrowth rims and microphenocrysts unless parts of the magma were at temperatures much higher than the inferred average temperature the concept of convective self mixing allows us to explain the occurrence of compositions of minerals that apparently cannot coexist under equilibrium conditions

optical dating of sediments a new method for dating sediments is proposed which determines the time since the sediment was last exposed to sunlight an argon ion laser is used to excite electrons from thermally stable light sensitive traps and the subsequent luminescence used as a measure of the past radiation dose two sample sequences spanning the periods 0 700 kyr and 0 6 kyr show steadily increasing luminescence with age an age of 62 8 kyr is obtained for a silt radiocarbon dated at 58 8 0 3 kyr problems were found with two much younger samples

dating basaltic volcanism in the neoarchaean sukumaland greenstone belt of the tanzania craton using the sm nd method implications for the geological evolution of the tanzania craton abstract metabasalts from the sukumaland greenstone belt of north western tanzania yield a whole rock sm nd isochron age of 2823 44 ma initial e nd 2 7 mswd 1 24 this age which is interpreted as dating the eruption of the oldest mafic volcanics in the belt is at the 95 confidence level indistinguishable from 2780 3 and 2808 3 ma single zircon u pb ages previously reported from stratigraphically higher rhyolitic pyroclastic rocks from the southern margin of the belt the age equivalency suggests that the entire 5 7 km thick greenstone sequence that has been traditionally classified into a predominantly mafic lower part overlain by an upper part in which felsic volcanics and bif predominate was emplaced within a relatively short time interval not exceeding 44 ma the sm nd age of the metabasalts is significantly older than a published zircon u pb age of 2680 3 ma obtained from a migmatitic gneiss on the southernmost fringe of the sukumaland greenstone belt this corroborates previous evidence that high grade metamorphism in the tanzania craton postdates emplacement of the greenstones and is most likely associated with the regional emplacement of the large granitic bodies that intrude and flank the greenstones

the repetition of large earthquake ruptures abstract this survey of well documented repeated fault rupture confirms that some faults have exhibited a characteristic behavior during repeated large earthquakes that is the magnitude distribution and style of slip on the fault has repeated during two or more consecutive events in two cases faults exhibit slip functions that vary little from earthquake to earthquake in one other well documented case however fault lengths contrast markedly for two consecutive ruptures but the amount of offset at individual sites was similar adjacent individual patches 10 km or more in length failed singly during one event and in tandem during the other more complex cases of repetition may also represent the failure of several distinct patches the faults of the 1992 landers earthquake provide an instructive example of such complexity together these examples suggest that large earthquakes commonly result from the failure of one or more patches each characterized by a slip function that is roughly invariant through consecutive earthquake cycles the persistence of these slip patches through two or more large earthquakes indicates that some quasi invariant physical property controls the pattern and magnitude of slip these data seem incompatible with theoretical models that produce slip distributions that are highly variable in consecutive large events

the origin of composition gaps at south sister volcano central oregon implications for fractional crystallization processes beneath active calc alkaline volcanoes abstract south sister is an active calc alkaline strato volcano located in west central oregon in the high cascades south sister contains lavas that range from basalt 50 sio 2 to rhyolite 75 sio 2 within this compositional spectrum the lavas are restricted to three distinct populations basalt basaltic andesite andesite and rhyolite compositional textural and mineralogic criteria indicate that the basaltic andesites are hybrid mixtures of basalt and andesite magma this implies that the system is characterized by basalt andesite and rhyolite magma populations that are separated from one another by composition gaps major minor and trace element modeling suggests that the three magma populations are related to one another primarily through fractional crystallization we propose a standard fractionation model for generating compositional diversity amongst erupted lavas wherein in situ fractional crystallization occurs within a downward descending solidification front extraction and eruption of the interstitial fractionated liquid now separated from the parental liquid by a composition gap occurs only after the percentage of crystallization within the solidification front has exceeded the appropriate critical crystallinity value for the given parental magma we envision a magmatic plumbing system characterized by multiple independently solidifying bodies of magma thus at any given time discrete bodies of basalt andesite and rhyolite magma may exist interaction and mixing among these bodies could generate the continuous range of lava compositions that characterize most calc alkaline volcanic complexes

quaternary folding of the eastern tian shan northwest china abstract the tian shan east west trending more than 2000 km is one of most active intracontinental mountain building belts that resulted from india eurasia collision during cenozoic in this study quaternary folding related to intracontinental mountain building of the tian shan orogenic belt is documented based on geologic interpretation and analyses of the satellite remote sensing images landsat thematic mapper tm enhanced thematic mapper etm and india remote sensing irs pan combined with field geologic and geomorphic observations and seismic reflection profiles analyses of spatial temporal features of quaternary folded structure indicate that the early quaternary folds are widely distributed in both piedmont and intermontane basins whereas the late quaternary active folds are mainly concentrated on the northern range fronts field observations indicate that quaternary folds are mainly characterized by fault related folding the formation and migration of quaternary folding are likely related to decollement surfaces beneath the fold and fault zone as revealed by seismic reflection profiles moreover analysis of growth strata indicates that the quaternary folding began in late stage of early pleistocene 2 1 1 2 ma finally tectonic evolution model of the quaternary deformation in the tian shan is presented this model shows that the quaternary folding and faulting gradually migrate toward the range fronts due to the continuous compression related to india eurasia collision during quaternary time as a result the high topographic relief of the tian shan was formed