evolution of mica fish in mylonitic rocks abstract mineral fish are lozenge shaped porphyroclasts single crystals in a finer grained matrix which occur in ductile shear zones and which are commonly used as shear sense indicators mineral fish of biotite tourmaline k feldspar garnet hypersthene and quartz occur in mylonites but most common are white mica fish these mica fish can be subdivided into six morphological groups that develop by different mechanisms determined by different initial shapes and orientations the principal mechanisms of formation are intracrystalline deformation combined with rigid body rotation concomitant selective grain size reduction occurs by recrystallisation cataclastic separation pressure solution and diffusional mass transfer microboudinage has been proposed for the breakdown of large mica fish into smaller ones but many mica fish undergo shortening rather than extension along their long axes evidence is presented for an alternative process in which the tips of mica fish are isoclinally folded and then break off along the hinge of these microfolds all the presented fish shaped porphyroclasts or mineral fish have a specific shape preferred orientation with their long axis at a small antithetic angle with respect to the foliation they represent a special group of objects with a stable orientation due to flow partitioning into narrow shear bands

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

tectonic feedback intraplate orogeny and the geochemical structure of the crust a central australian perspective the geological record of intraplate deformation in central australia implies that past tectonic activity basin formation deformation and erosion has modulated the response of the lithosphere during subsequent tectonic activity in particular there is a correspondence between the localization of deformation during intraplate orogeny and the presence of thick sedimentary successions in the preserved remnants of a formerly widespread intracratonic basin this behaviour can be understood as a kind of tectonic feedback effected by the longterm thermal and mechanical consequences of changes in the distribution of heat producing elements induced by earlier tectonism from a geochemical point of view one of the most dramatic effects of intraplate orogeny in central australia has been the exposure in the cores of the orogens of deep crustal rocks largely depleted in the heat producing elements the geochemical structuring of the crust associated with the erosion of the heat producing upper crust resulted in long term cooling of the deep crust and upper mantle with associated lithospheric strengthening this is illustrated here by mapping the consequences of deformation and associated tectonic responses onto the h qc plane where h is the characteristic length scale for heat production distribution and qc is the total crustal heat production because rates of intraplate deformation in central australia appear to be much slower than that typical of plate margin orogens it is possible that the ongoing geochemical structuring of the crust has played an important role in terminating intraplate orogeny in central australia by providing a thermal lock the diagnostic geophysical signature of this lock may be the extraordinary gravity anomalies of the central australian intraplate orogens central australia provides an intriguing record of intraplate deformation resulting in the development of a number of extraordinary orogens that expose deep crustal rocks in their cores forman 1971 teyssier 1985 goleby et al 1989 the spatial distribution of intraplate deformation during the alice springs and petermann orogenies suggests that the mechanical response of this region has varied through time consequently this response is important for our understanding of the processes controlling reactivation in intraplate settings in previous contributions it has been argued that the distribution of deformation in central australia has been strongly influenced by prior tectonic processes such as basin formation and erosion sandiford hand 1998a hand sandiford 1999 this paper focusses on the way in which intraplate deformation modifies the geochemical structure of the crust in this region in particular the distribution of heat sources recognizing that the distribution of heat sources provides a first order control on the thermal structure of the crust the consequences of intraplate deformation for the longterm thermal and mechanical evolution of the crust is then explored our aim is to demonstrate the relevance of a novel style of tectonic feedback which seems to have modulated the longterm tectonic behaviour of the central australian region and which may be relevant to continental interiors in general the notion of tectonic feedback is compatible with a highly temperature dependent lithospheric rheology as suggested by many experimental studies of natural rocks such feedback has profound implications for the longterm tectonic evolution of continental interiors particularly their potential for reactivation the record of intraplate deformation in

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

impact of the lateral blast on the spatial pattern and grain size characteristics of the 18 may 1980 mount st helens fallout deposit the 18 may 1980 eruption of mount st helens started with a lateral blast that fed a pyroclastic surge which then uplifted to form a co blast plume thirty minutes later plinian activity started at the vent and fluctuated in intensity for 9 h the resulting fallout deposit documented to 600 km from vent presents some striking features 1 displacement of the overall deposit to the north of the vent 2 a secondary thickness and mass maximum at 300 km from vent 3 a total grain size distribution dominated by fine ash 62 wt of the deposit 63 m and 4 individual grain size distributions that vary dramatically in the crosswind direction from strongly bimodal in the south to skewed unimodal in the north results from a new deconvolution of the individual grain size distributions show that they are a combination of a coarse subpopulation that decreases in size with distance from vent and a constant fine subpopulation with a mean of 15 m relative proportions of each subpopulation vary asymmetrically in the crosswind directions with the fine subpopulation preponderant toward the north and the coarse one dominating the south of the deposit both reach their absolute maxima in mass on the deposit axis componentry analyses of selected samples show that blast derived material is greatly enriched toward the north of the deposit these results indicate that the co blast plume dispersed fine grained material over great distances and dominated the fine subpopulation comparison with reanalysis data of atmospheric wind fields and satellite images of the spreading ash cloud suggests contrasting ash transport and depositional processes for the early co blast plume and the later vent derived plinian plumes the co blast plume is displaced to the north it had a high overshoot height and eastward dispersion via strong winds low in the stratosphere 10 15 km the plinian plumes were lower and dispersed most of the material to the southeast as the direction of high velocity winds shifted just before the late climactic plinian eruptive phase fine ash fine subpopulation was deposited continuously throughout the deposit with an increase of sedimentation rate 300 km from the vent where there is a secondary maximum in the deposit mass and thickness fine ash probably settled by a combination of enhanced sedimentation mechanisms including not only aggregation but also gravitational convective instabilities of cloud base hydrometeor formation and destruction and entrainment of small particles by larger ones finally we show that half of the deposit by mass in the medial area was deposited by the co blast plume and that a significant proportion of the mount st helens fallout deposit is nonjuvenile which has implications for the magmatic budget of this eruption

mechanics of basin inversion abstract the long term consequences of rifting for the thermal state of the deep crust and upper mantle reflect in part cooling induced by the reduction in heat production in the attenuated lithosphere and heating due to the burial of this heat production beneath the basin provided that the heat production is largely concentrated in the upper half of the crust these factors result in significant increases in temperature at deep crustal and upper mantle levels because the moho depth is likely to be reduced in the long term limit of an isostatically balanced basin these same factors may lead to slight cooling or slight heating of the moho depending on the nature of the basin fill for a brace goetze lithospheric rheology i e a rheology governed by a combination of frictional sliding and power law creep significant long term lithospheric weakening up to 5 per kilometre of basin fill accompanies basin formation when the lower crust is initially relatively strong and the basin fill is characterised by appreciable heat production and low thermal conductivity in contrast initially weak lower crustal rheologies may result in long term lithospheric strengthening evidence for basin inversion in the geological record may therefore imply that heat production is strongly concentrated in the upper half of the crust and under normal continental thermal regimes the lower crust is strong

researching the earth and a few of its neighbors during my career our knowledge of erupting geysers and volcanoes in the solar system has exploded in this prefatory i tell how i became fascinated with high speed processes through studying meteorite impact dynamics and then how my initial idea of studying old faithful geyser as a volcanic analog led me to work not only on the dynamics of eruption of mount st helens in 1980 but also on geysers erupting on io a fiery satellite of jupiter triton a frigid satellite of neptune and enceladus an active satellite of saturn unforeseeably the study of these events also led to work on mineral thermodynamics and the hydraulics and geomorphic evolution of rapids in the grand canyon this is a narrative not a formal review article but the reader can find references in the related resources section to explore topics in more detail

inter and intratill boulder pavements in the carboniferous hoyada verde formation of west argentina an insight on glacial advance retreat fluctuations in southwestern gondwana abstract two glacially abraded surfaces have been identified in the glaciomarine section of the carboniferous hoyada verde formation calingasta uspallata basin in west argentina the glacial erosion surfaces ges found in the hoyada verde section are boulder pavements one is an intertill boulder pavement developed as a single boulder layer resting on massive diamctites dmm and overlain by thick to thin bedded diamictites with ice rafted debris ird the second surface identified in recent field work can be described as an intratill boulder pavement it is also a single boulder layer but under and overlain by massive diamcitites dmm the intratill boulder pavement consists of a laterally discontinuous planar concentration of large clasts with faceted and striated upper surfaces it lies approximately 10 m below the intertill boulder pavement and is exposed on the southern closure of the hoyada verde anticline in both pavements the upper surfaces of the clasts are polished and striated the striae are parallel and subparallel to the a axes of the boulders and indicate similar paleoflow ice directions toward the s and s sw both pavements were probably originally laid down as residual gravel lag surfaces formed by the erosion of underlying diamict when wave and or tidal reworking was strong during a low stand and were subsequently abraded by advancing glaciers although both boulder pavements can be considered to be equivalent to glacial erosional surfaces ges caused by abrasion during a glacial advance and hence sequence boundaries form the sequence stratigraphic viewpoint their hierarchies are different the intertill boulder pavement demarks the demise of the mid carboniferous glacial episode in west argentina whereas the underlying intratill boulder pavement can be interpreted as the product of a minor fluctuation during the same glacial episode

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

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

a multidisciplinary study on the emplacement mechanism of the qingyang jiuhua massif in southeast china and its tectonic bearings part i structural geology ams and paleomagnetism during the cretaceous the south china block scb experienced a widely distributed extensional event including numerous plutons emplacement and basin opening investigations on the tectonic regime coeval with pluton emplacement and emplacement mechanism of the pluton remain relatively rare in the scb in order to address these questions a multidisciplinary approach including field structural and petrographic observations anisotropy magnetic susceptibility ams and paleomagnetic analyses was carried out on the qingyang jiuhua granitic massif which intrudes into the lower yangtze fold belt in the northeastern part of scb the qingyang jiuhua massif is composed of the granodioritic qingyang and monzogranitic jiuhua plutons dated by zircon u pb method at ca 142 ma and ca 131 ma respectively our structural observations show that the intrusion of the qingyang jiuhua massif does not modify the fold strike a weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism in the contact aureole of the massif the foliation follows the pluton contour and the mineral lineation is rare when present it exhibits a down dip attitude field and microstructural observations indicate isotropic magmatic textures in most parts of the massif the ams analysis of 93 sites reveals weak values for the anisotropy degree pj 0 for most of the measured samples two principal foliation patterns are identified horizontal foliations in the center of the plutons and vertical foliations on the boundaries magnetic lineation strike is largely scattered and weakly inclined at the scale of the entire massif the paleomagnetic investigations indicate that a the younger jiuhua pluton did not produce a remagnetization in the older qingyang pluton b no relative movement can be observed between these two plutons c the entire massif did not experience any important relative movement with respect to south china considering the paleomagnetic uncertainties integrating the newly obtained results with previous observations our study favors a permissive emplacement mechanism for the two plutons i e vertical magma intrusion into an opening space controlled by the nw se brittle stretching of the upper crust which is in agreement with a weak extensional regional tectonic framework of the scb

crustal structure of the southern dabie ultrahigh pressure orogen and yangtze foreland from deep seismic reflection profiling a new 140 km long seismic reflection profile provides a high resolution crustal scale image of the southern dabieshan high pressure hp metamorphic belt and the yangtze foreland fold and thrust belt the seismic image of the stacked section shows that the southern dabieshan metamorphic terrane and yangtze foreland belt are separated by a large north dipping fault in the foreland the upper crust is dominated by a series of folds and thrusts formed during the collisional stage in the mid triassic it was reworked by crustal extension resulting in the formation of a late jurassic and cretaceous red bed basin the southern dabieshan profile shows stacked crustal slabs developed along the margin of the collisional orogenic belt the moho reflectors at 10 11 s 30 33 km are seismically prominent and segmented by a number of south verging thrusts that were probably developed by foreland directed thrusting of the deeply subducted continental crust during exhumation the seismic reflection profile suggests that structures related to the triassic jurassic subduction and exhumation of the yangtze plate are preserved despite the severe crustal extension superimposed during the late mesozoic and cenozoic

experiments on a turbulent plume shape analyses 1 we report the results of laboratory experiments on a turbulent plume a simplified model of a positively buoyant volcanic plume with a small content of fine ash to study how the shape of the plume changes as a function of time to model such plumes we continuously inject a dense fluid downward at a constant exit velocity and study how the plume shape changes with time as vortices develop from flow instability and entrainment here a series of experiments are conducted with exit velocity and the density difference as the changeable parameters and the jet plume transition is realized in the laboratory initially the plume has a finger like shape which becomes unstable and forms a plume head and later transforms into a cone like self similar shape we also find that when the buoyancy becomes sufficiently large compared to inertia there is a temporary deviation from the cone like shape to form a headed cone we devise new methods to quantitatively characterize these changes of shape and define four regimes as a function of time we find that the onset times of the flow instability and the regimes have negative power law dependence on the initial re or the exit velocity and that buoyancy causes the regime transitions to become earlier our experiments suggest that monitoring the change of the shape of the rising volcanic plumes and analyzing the regime onset times can be used as a measure to constrain their buoyancy

phanerozoic polyorogenic deformation in southern jiuling massif northern south china block constraints from structural analysis and geochronology abstract the structure of the jiuling massif has been investigated in order to delineate the polyorogenic deformation and discuss its geodynamic evolution and orogenic mechanisms detailed structural analysis indicates that the d1 event is characterized by top to the nnw ductile shearing with pervasive foliation and mineral and stretching lineation developed in the entire region compared with the d1 deformation d2 structures are localized in ductile shear zones with subvertical foliation and subhorizontal e w trending lineation indicating a dextral ductile shearing the d3 event marked by folds and thrusts mainly in a brittle domain modified the d1 structures by asymmetrical folds the dominant d4 structures are gravitational folds and normal faults corresponding to a later extension our new geochronological data suggest that the d1 event occurred between 465 and 380 ma with d2 dextral shearing at the end of this early paleozoic orogen and the d3 event has been constrained at 245 215 ma the final uplift of the jiuling massif by the d4 event can be correlated with the late mesozoic extension across the eastern south china block along with previous studies in the south china block the structural pattern of the jiuling massif elucidates the influence of the early paleozoic and early mesozoic intracontinental belts triggered by repeated reactivation of the jiangshan shaoxing fault combined with deformation to the south the early paleozoic belt shows a positive flower pattern with opposing kinematics rooted in the jiangshan shaoxing fault during the early mesozoic a general intracontinental belt was developed with uniform kinematics in both the jiuling massif and the xuefengshan belt possibly resulted from the far field effect of the triassic nw directed paleo pacific subduction

an 40ar 39ar geochronology on a mid eocene igneous event on the barton and weaver peninsulas implications for the dynamic setting of the antarctic peninsula the genesis of basaltic to andesitic lavas mafic dikes and granitoid plutons composing the subaerial cover on the barton and weaver peninsulas antarctica is related to arc formation and subduction processes precise dating of these polar rocks using conventional 40ar 39ar techniques is compromised by the high degree of alteration with loss on ignition as high as 8 in order to minimize the alteration effects we have followed a sample preparation process that includes repeated acid leaching acetone washing and hand picking followed by an overnight bake at 250 c after this procedure groundmass samples can yield accurate age plateaus consisting of 70 100 of the total 39ark released using high resolution heating schedules the different rock types studied on the barton and weaver peninsulas yielded almost coeval ages suggesting a giant igneous event in the weaver and barton peninsulas at 44 5 ma a compilation of newly published ages indicate that this event took place throughout the whole south shetland islands suggesting a dynamic incident occurred at this stage during the arc evolution history we related this igneous event to a mantle delamination mechanism during eocene times the delamination process began at 52 ma and the resultant upwelling of asthenosphere baffled the subduction of phoenix plate causing an abrupt decrease in convergence rate then multiple magmatic sources were triggered resulting in a culminating igneous activity during 50 40 ma with a peak at 45 ma along the archipelago the delamination also caused the extension regime indicated by the dike swarm plugs and sills all over the archipelago and the uplift of smith metamorphic complex and livingston island delamination process may have finished at some time during 40 30 ma leaving a weak igneous activity at that stage and thereafter the convergence rate then recovered gradually as indicated by the magnetic anomaly identifications this model is supported by seismic observation of deep velocity anomalies beneath the antarctic peninsula

tropical stone lines and podzolized sand plains as paleoclimatic indicators for weathered cratons abstract stone lines and podzolized white sand plains are characteristic features of the regolith in many low latitude cratonic landscapes but nevertheless often seem to be out of phase with the present pedogenic and geomorphic regimes of the humid tropics and subtropics consideration of contemporary environments where the genetic processes related to these features can be observed implies several conditions for prior events in landscape evolution 1 tropical stone lines may be interpreted as winnowed mudflows or lag deposits that reflect morphoclimatic events of brief duration torrential rains within an arid or semi arid phase of pronounced global cooling 2 within the present zone of the prevailing westerlies stone lines may reflect periglacial activity during brief interstade fluctuations 3 in lower latitudes the principal stone line which commonly truncates saprolites appears to be late wisconsinan wurmian in age coinciding more or less with the pleniglacial phase 4 the podzolized sands represent leached residuals and accordingly reflect extended intervals of leaching during the pleistocene and 5 the nature and distribution of podzolized sand plains indicates a pedogenic history of considerable variety and no brief episodicity as implied with stone lines

climatic control of sedimentation in bays and fjords of the northern antarctic peninsula abstract the northern antarctic peninsula is dominated by strong climatic gradients which are locally enhanced by subsidiary orographic and topographic effects these gradients are reflected in the sediments of the bays and fjords of the region the climate of the south shetland islands is relatively warm with high levels of precipitation this leads to a temperate glacial setting in which many glaciers terminate on land and bay waters which are turbid with terrigenous debris sediments which consist of terrigenous muds and sands with a low ice rafted debris ird content are relatively thick 70 m and have now filled in and almost completely mask the underlying basement topography the palmer archipelago is colder than the south shetland islands and is the wettest area of the antarctic it is heavily glaciated and all major glaciers are grounded below sea level only rarely have sediment plumes been seen the sediments which are primarily terrigenous muds and sands with a variable ird component are thin maximum 40 m but are ponding in every available bathymetric depression the pristine nature of the ice visible above sea level indicates that terrigenous debris is entering the bays from below sea level sediment gravity flows are important in redistributing sediment the danco coast lies within the precipitation shadow of the graham land plateau its climate is slightly colder and is the driest in the study area with less than half of the precipitation of the palmer archipelago nevertheless it is also heavily glaciated with few exceptions sediments here are diatomaceous muds and oozes they are very thin and drape across the rugged basement topography rather than ponding in lows sediment gravity flow deposits are rare these observations indicate a lower rate of production of terrigenous sediment here a consequence of the drier climatic setting

ice formed intertidal boulder pavements in the sub antarctic abstract boulder pavements are flat tightly packed mosaics of ice smoothed and striated boulders that occur in low gradient intertidal and lacustrine areas in polar regions in the sub antarctic intertidal boulder pavements are located in three morphogenetic environments spanning l0 of latitude some boulder pavements are composed of ice pushed or ice rafted boulders which have been forced into the substrate by the pressure of grounding ice blocks other pavements may result from wave winnowing of glacial till at sea level which has been subsequently modified by grounding ice shallow depressions in the pavement surface may correspond to the loci of large grounded ice blocks and this interpretation is supported by striation and boulder orientations on the ridges between the depressions the occurrence of the pavement appears to depend on the following factors a a boulder supply b frequent onshore movement of floating ice and c a low gradient intertidal zone given such conditions the degree of development of the pavement seems to be controlled by the frequency of onshore ice movement because the best formed pavements occur in areas subject to the highest frequencies of freely moving floating ice the occurrence of raised pavements in the south shetland islands suggests that little morphogenetic change has occurred in the area since 9 000 years ago radiocarbon dating indicates that where the frequency of ice grounding is relatively low periods in excess of 200 years are necessary for pavement development whereas at locations where the ice grounding frequency is high pavements are well formed within 300 years

volcan reventador s unusual umbrella 1 photographs of a volcanic column in a recent eruption of reventador show a prominently scalloped umbrella that is unlike any umbrella previously documented on a volcanic column we propose that the scallops in this umbrella are the result of a turbulent rayleigh taylor rt instability a type of fluid instability with no precedents in volcanology negative buoyancy drives this instability and we ascribe the unusual negative buoyancy of the reventador umbrella to the fact that the reventador column fed on a cool co ignimbrite cloud from the wavelength of the scallops we estimate a value for the eddy viscosity of the umbrella d 4 000 m2 s the first such value to be inferred directly from an observation in the field collapse of the umbrella back to the ground could result in a previously unrecognized hazardous flow we hope this work will elicit new reports on scalloped umbrellas and further study of the characteristics and evolution of such umbrellas

intertidal boulder pavements in the northeastern gulf of alaska and their geological significance abstract striated boulder pavements consisting of planar concentrations of clasts having striated upper surfaces are a common feature of glacigenic deposits but their origin is not well understood laterally extensive pavements are currently forming in the intertidal zone west of icy bay in the gulf of alaska pavements comprise armoured layers of interlocking boulders one clast thick that have been eroded from underlying outcrops of late cenozoic glaciomarine diamictites they originate essentially as lag surfaces along a high energy storm dominated mesotidal shoreline boulder pavements are either flat or show a nucleated plan form where successively smaller boulders have been accreted around a large core boulder nucleation imparts a hummocky surface topography to the pavements and suggests that some form of size sorting of clasts has occurred packing is promoted by repeated tamping of the clast lag by floating masses of glacier ice which become grounded across the intertidal zone at low tide repeated abrasion of the pavement surface by debris contained within ice blocks produces smooth flattened clast upper surfaces and short randomly oriented striations data from icy bay can be used to constrain the origin of laterally extensive boulder pavements exposed in late cenozoic glaciomarine sediments on middleton island the significance of such pavements in the geologic record is that they form along erosional unconformities and may identify sequence boundaries

sedimentology and palaeoecology of lonestone bearing mixed clastic rocks and cold water carbonates of the lower permian basal beds at fossil cliffs maria island tasmania australia insight into the initial decline of the late palaeozoic ice age

on the causes of pulsing in continuous turbidity currents velocity pulsing has previously been observed in continuous turbidity currents in lakes and reservoirs even though the input flow is steady several different mechanisms have been ascribed to the generation of these fluctuations including rayleigh taylor rt instabilities that are related to surface lobes along the plunge line where the river enters the receiving water body and interfacial waves such as kelvin helmholtz instabilities however the understanding of velocity pulsing in turbidity currents remains limited herein we undertake a stability analysis for inclined flows and compare it against laboratory experiments direct numerical simulations and field data from lillooet lake canada and xiaolangdi reservoir china thus enabling an improved understanding of the formative mechanisms for velocity pulsing both rt and kelvin helmholtz instabilities are shown to be prevalent in turbidity currents depending on initial conditions and topography with plunge line lobes and higher bulk richardson numbers favoring rt instabilities other interfacial wave instabilities holmboe and taylor caulfield may also be present while this is the most detailed analysis of velocity pulsing conducted to date the differences in spatial scales between field direct numerical simulations and experiments and the potential complexity of multiple processes acting in field examples indicate that further work is required in particular there is a need for simultaneous field measurements at multiple locations within a given system to quantify the spatiotemporal evolution of such pulsing

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

a new view of the dynamics stability and longevity of volcanic clouds abstract powerful explosive volcanic eruptions inject ash high into the atmosphere which spreads to form umbrella clouds identifying key physical processes governing the dynamics stability and longevity of umbrella clouds is central to assessing volcanic hazards as well as the nature of volcanic forcings on climate here we present a series of laboratory experiments producing turbulent particle laden jets and subsequent axisymmetric intrusive gravity currents into a stratified environment our experiments reproduce many of the main dynamical regimes observed during the formation of an explosive volcanic column and highlight new dynamics for the umbrella cloud theoretical predictions of column collapse from a simple model of a turbulent jet are in good agreement with experimental observations as well as previous studies depending on the flow intensity the strength of the initial environmental density stratification and the particle concentration at the source resulting umbrella clouds can however evolve through a series of new regimes as a result of the dynamics of particle sedimentation within these flows as well as from their bases using scaling theory we show that during cloud spreading internal sedimentation drives the growth and intermittent overturn of thin gravitationally unstable particle boundary layers pbls as particle rich plumes this pbl driven convection can have remarkable effects ranging from progressive dilution of clouds to their catastrophic overturn and collapse in natural eruptions whether the dynamics of pbls play a major role in particle sedimentation depends on the grain size distribution inside the cloud and on eruption column height in general particles larger than 60 m 1 mm are expected to settle individually whereas finer particles will accumulate pbls resulting in the formation of armless mammatus clouds or dangerous gravity currents at much larger distances from the volcanic vent than ever considered before such dynamics is apparent in observations of numerous modern eruptions and is inferred from the deposits of historic and prehistoric eruptions for where there exist appropriate data consideration of the consequences of these phenomena for problems such as volcanic hazards to humans and climate change may thus be very important in the assessment of future eruptions

intraplate deformation in central australia the link between subsidence and fault reactivation abstract central australia has experienced two intraplate orogenic events involving significant north south shortening the late neoproterozoic to early cambrian petermann orogeny and the devonian to carboniferous alice springs orogeny in each event pre existing structures inherited from mesoproterozoic terrain amalgamation were reactivated and basement rocks exhumed from beneath thick sedimentary successions accumulated in the centralian superbasin the pattern of fault reactivation during these events shows a striking similarity to the pattern of subsidence in the overlying basin immediately prior to the petermann orogeny the centralian superbasin was thickest in the vicinity of the musgrave block the region in which deformation was subsequently localised at the same time crustal scale faults elsewhere in central australia that were covered by a relatively thin sheet of sediment remained inactive despite being favourably oriented to accommodate the north south shortening between the petermann and alice springs orogenies subsidence patterns shifted such that fault systems in the arunta block and also those in the southern musgrave block were buried by significant thicknesses of sediment whereas the major structures that were exhumed during the petermann orogeny were not significantly buried during the alice springs orogeny reactivation once again occurred along the most deeply buried faults even in the instances where those faults had remained inactive during the earlier petermann orogeny importantly the major petermann aged structures that were not buried during renewed subsidence remained inactive during the alice springs orogeny the record of reactivation implies that the presence of pre existing crustal scale faults alone was insufficient to localise deformation rather fault reactivation appears to have required a priming process that modulated the strength of the lithosphere on a regional scale the correspondence between the distribution of basement fault reactivation and subsidence patterns during both the petermann and alice springs orogenies implies a link between relatively thick sedimentation and long term lithospheric weakening we show that this link is compatible with the thermal effects of a thick sedimentary blanket in the context of central australia the mechanical impact of basin formation is likely to be enhanced by the presence of regionally elevated heat production in the proterozoic basement

structural style and tectonic significance of the jianglang dome in the eastern margin of the tibetan plateau china abstract the jianglang tectonic dome jtd lies on the eastern margin of the tibetan plateau and consists of three tectono stratigraphic units separated by two extensional detachment faults a schist complex a middle ductile slab and an overlying sedimentary unit the schist complex is composed of quartz mica schists and amphibolites of possible mesoproterozoic age and exhibits two early foliations s 1 and s 2 overprinted by two later foliations s 3 and s 4 the ductilely deformed middle slab consists of paleozoic quartzite phyllite marble amphibolite and meta basalt it is characterized by pervasive intrafolial and intraformational recumbent folds with a penetrative axial foliation s 1 an equivalent of s 3 in the basement rocks the cover unit the triassic xikang group of flysch sedimentary rocks experienced shallow level buckling and flattening deformation with an axial cleavage defining s 1 an equivalent of s 4 in the middle slab and basement rocks the jtd appears to have experienced three stages of extension immediately after orogeny the earliest d 1 recorded by an axial surface s 1 of s 0 intrafolial folds and sub parallel to s 0 in the schist complex probably occurred prior to the collision between the south china and north china blocks this extensional event was followed by subduction of the yangtze block to the west towards the eastern margin of the tibetan plateau the two detachment faults are characterized by structures formed by ductile brittle to brittle shear deformation which are sub parallel to the layer of s 2 in the lower detachment and to s 0 in the upper detachment these faults mark the second stage of extension and may have caused thinning and selective removal of the sinian and paleozoic strata as well as exhumation of the schist complex and the middle slab in the middle jurassic 177 ma the final stage of extension d 5 shown by brittle deformation and thermo metamorphism from the lower detachment to the xikang group was associated with the indian eurasia collision

extensional tectonics within a subduction type orogen the case study of the wugongshan dome jiangxi province southeastern china abstract in southeastern china the wugongshan massif consists of metamorphic rocks and granites generally considered as basement rocks formed during an early paleozoic orogeny our structural and radiometric reappraisal of this massif shows that it results from a mesozoic doming in the metamorphic rocks the foliation defines an e w elliptical shape and bears a n s stretching lineation the axial zone of the dome is occupied by oriented plutons the outer part of the dome consists of devonian quartzite carboniferous permian limestone and triassic jurassic sandstone ductile deformation observed in the permian carbonates and older units develops contrasting kinematics the northern and southern flanks present top to the north and top to the south motions respectively the same divergent pattern of motion is recorded from brittle structures up to the jurassic sandstone the late cretaceous red beds are undeformed in the axial zone coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates along the n and s flanks of the dome post folial recumbent folds are symmetrically overturned to the n and s respectively but at the two extremities of the dome the fold axes are parallel to the stretching lineation 40 ar 39 ar ages of biotite and muscovite from mylonite and granite suggest a late triassic age for the ductile deformation and an early cretaceous age for the final doming the wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime a model which emphasizes plutonism is discussed from permian to early cretaceous southeastern china was a subduction type orogen the crust of which was thickened by accretion of mantle derived magmas the emplacement of a large amount of mesozoic granitoids in southeastern china promoted thermal softening and gravitational instability that allowed the crust to extend the formation of the wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology

isotopic and paleomagnetic constraints on the mesozoic tectonic evolution of south china in order to better constrain the paleogeographic evolution of south china we measured sm nd and rb sr isotopic compositions for 23 mesozoic granites that crop out throughout the area tightly grouped neodymium depleted mantle model ages 1 4 0 3 ga suggest the region is underlain by relatively homogeneous proterozoic crust and fail to define crustal provinces neither the isotopic nor geologic data suggest that a mesozoic suture exists however granites possessing anomalously high sm 8 ppm and nd 45 ppm concentrations relatively high initial epsilon neodymium 4 to 8 and high but variable initial 87sr 86sr 0 759 to 0 713 form a northeast trending zone that coincides with two prominent mesozoic basins southeast of the zone lie the majority of mesozoic intrusives and upper triassic to lower cretaceous extensional basins found in south china mesozoic paleomagnetic poles are well clustered northwest of the zone pre cretaceous poles southeast of it are discordant with respect to those from the northwest the only recognized tectonostratigraphic terrane in south china lies southeast of the zone the terrane is bordered by a northeast trending sinistral fault that was active in the mesozoic other faults in south china have similar attitudes ages and sense of shear together the observations suggest that the mesozoic tectonic regime in south china consisted of strike slip activity plus concomitant rifting as terranes or fragments of similar crust were transported north along sinistral faults the zone defined by the granites enriched in nd and sm demarcates displaced terranes to the southeast from relatively stable land to the northwest

geochronology and isotope analysis of the late paleozoic to mesozoic granitoids from northeastern vietnam and implications for the evolution of the south china block abstract in northeastern vietnam late paleozoic and permo triassic granitic plutons are widespread but their tectonic significance is controversial in order to understand the regional magmatism and crustal evolution processes of the south china block scb this study reports integrated in situ u pb hf o and sr nd isotope analyses of granitic rocks from five plutons in northeastern vietnam zircon sims u pb ages of six granitic samples cluster around in two groups 255 228 ma and 90 ma bulk rock end t ranges from 11 to 9 7 suggesting that continental crust materials were involved in their granitic genesis in situ zircon hf o isotopic measurements for the granitic samples yield a mixing trend between the mantle and supracrustal derived melts it is suggested that the granitic rocks were formed by re melting of the continental crust these new data are compared with the paleozoic and mesozoic granitic rocks of south china we argue that northeastern vietnam belongs to the south china block though still speculated an ophiolitic suture between ne vietnam and south china so called babu ophiolite appears unlikely the late paleozoic to mesozoic magmatism in the research area provides new insights for the magmatic evolution of the south china block

structural signatures and 40ar 39ar geochronology of the indosinian xuefengshan tectonic belt south china block abstract to advance our understanding of the kinematics of the xuefengshan tectonic belt xfstb and the mesozoic tectonics of the south china block scb this paper presents new data and interpretations from our structural studies and thermochronological dating the xfstb is characterized by greenschist facies metamorphism and development of s and s l tectonic fabrics an asymmetric positive flower structural pattern is composed of nw wnw and se ese dipping cleavages faults and shear zones kinematic indicators indicate a dominant top to wnw thrusting with a sinistral strike slip component on the ese dipping shear zones and top to ese back thrusting on the wnw dipping shear zones the quartz c axis orientations of mylonitic rocks exhibit monoclinic point maximum asymmetry indicative of a sinistral shear sense under the a basal gliding condition the timing of the major deformation event has been constrained to the middle triassic to early jurassic 244 195 ma on the basis of 40ar 39ar geochronology and other geological observations the structural characteristics of the xfstb are probably related to an indosinian oblique convergent zone in combination with tectonic wedging and associated back thrusting all above a low angle se dipping basal detachment oblique northwestward and upward movement along the basal detachment was partitioned into the nw wnw directed thrusting se ese directed back thrusting and subsidiary strike slip movement along the ne and nne trending faults and shear zones the xfstb may represent part of a huge structural fan between the yangtze and cathaysian blocks and can be interpreted as a product of the indosinian intracontinental collision involving a weak zone

glacioisostasy and lake level change at moosehead lake maine abstract reconstructions of glacioisostatic rebound based on relative sea level in maine and adjacent canada do not agree well with existing geophysical models in order to understand these discrepancies better we investigated the lake level history of 40 km long moosehead lake in northwestern maine glacioisostasy has affected the level of moosehead lake since deglaciation ca 12 50014c yr b p lowstand features at the southeastern end and an abandoned outlet at the northwestern end of the lake indicate that the lake basin was tilted down to the northwest toward the retreating ice sheet by 0 7 m km at 10 00014c yr b p water level then rose rapidly in the southeastern end of the lake and the northwestern outlet was abandoned indicating rapid relaxation of landscape tilt lowstand features at the northwestern end of the lake suggest that the lake basin was tilted to the southeast at ca 875014c yr b p possibly as the result of a migrating isostatic forebulge after 800014c yr b p water level at the southeastern end was again below present lake level and rose gradually thereafter we found no evidence suggesting that postglacial climate change significantly affected lake level the rebound history inferred from lake level data is consistent with previous interpretations of nearby relative sea level data which indicate a significantly steeper and faster moving ice proximal depression and ice distal forebulge than geophysical models predict

compression to extension switch during the middle triassic orogeny of eastern china the case study of the jiulingshan massif in the southern foreland of the dabieshan abstract the jiulingshan massif is an e w trending anticlinorium in the south china block scb that forms the southern foreland of the qinling dabie orogen the jiulingshan consists of middle proterozoic ca 1 ga 800 ma metamorphic and plutonic rocks unconformably overlain by sinian to triassic rocks in these cover rocks two episodes of deformation of pre late triassic age are recognized namely a n s compressional phase characterized by south verging thrusts upright folds and vertical cleavage followed by an extensional phase characterized by layer parallel slip collapse folds and extensional allochthons of proterozoic slate on top of carboniferous to middle triassic carbonates the extensional tectonics appears to be a mechanical consequence of the compressional tectonics on a regional scale the jiulingshan massif forms one of the domed crustal scale shear zones of the scb that accommodated the continuing convergence that immediately followed the qinling dabie collision

intracontinental deformation in a frontier of super convergence a perspective on the tectonic milieu of the south china block abstract since mesozoic the south china region has been located at the center of a triangular area surrounded by westward subduction of the pacific plate northward subduction of the india plate beneath the eurasia plate and collision of the north and south china blocks along the central china orogen this region thus marks the frontier of a super convergent regime within the super convergence domain the compressional structures in the center of the south china block are mainly characterized by shortening thrusting and decollement the block underwent inhomogeneous rejuvenation of the pre existing crust and lithospheric structures through reactivation of late stage activities the surface deformation of the xuefeng intracontinental tectonic system within the south china block is possibly derived from intraplate tectonics particularly the distinct magmatism between the west and east limbs of the xuefeng precambrian uplift is a possible response of the rheological structure of the lithosphere the deep structures as revealed from tomographic studies show marked difference of lithosphere architecture between the eastern and western sub blocks of the south china block these data illustrate the long term mosaic of multi block convergence which led to present day inhomogeneity in the continental lithosphere in south china a comparison of the intensive contraction of the south china block with the distinct features of rifting of the north china block brings out the contrasting structural and tectonic signatures developed in the same frontier of one of the largest super convergent systems on the globe during the mesozoic to cenozoic

microtectonics and 39ar 40ar dating of high pressure metamorphic rocks of the south ryukyu arc and their bearings on the pre eocene geodynamic evolution of eastern asia abstract the pre eocene metamorphic rocks found in the yaeyama islands the southern part of the ryukyu arc have disputed relationships with the metamorphic formations of japan and taiwan they are divided into the tomuru and fusaki formations 1 1 the tomuru formation is composed of high pressure hp metamorphic rocks derived from oceanic sediments and the uppermost oceanic crust the first deformation phase is characterized by a synmetamorphic nw se trending stretching lineation l1 kinematic analysis shows that the deformation corresponds to a subhorizontal ductile shear directed along l1 from the northwest to the southeast 2 2 the fusaki formation or chaotic formation is an olistostrome of triassic or lower jurassic radiolarite olistoliths the age of the matrix is assumed to be late jurassic it is slightly schistose and only a s vergent deformation phase is observed it is assumed that the hp schists overthrust the mesozoic olistostrome 39ar 40ar step heating experiments have been carried out on phengite and na amphibole from the tomuru formation phengite and zoned barroisite crossite give well defined plateaus at 225 4 8 ma and 237 6 3 ma respectively they are interpreted as the age of the ductile deformation crossite yields an integrated data of 104 8 8 3 ma interpreted as the age of the thrusting of the hp schists above the chaotic formation the lithostratigraphy petrology microtectonics and 39ar 40ar ages of the hp schists and the chaotic formation are similar to the permian sangun hp schists and the jurassic tanba olistostrome of the inner belt of southwestern japan respectively there the sangun schists also overthrust the tanba olistostrome during the mesozoic orogeny the same tanba like mesozoic olistostrome is also found in taiwan but was intensely deformed and metamorphosed in miocene times the geodynamic collision model proposed to account for the late permian to early triassic orogen of southwestern japan is extended to the south ryukyu and taiwan conversely the mesozoic sanbagawa hp schists and the upper jurassic to lower miocene turbidites and olistostrome sanbosan shimanto zones of southwestern japan are neither exposed in the south ryukyu arc nor in taiwan hypotheses are discussed that call upon tectonic erosion or non deposition to account for this absence

subglacial imprints associated with the isolation and decay of an ice mass in the lower lough erne basin co fermanagh nw ireland high resolution chirp sub bottom profiler and side scan sonar data record the final phases of ice margin activity in the lower lough erne basin at the end of the last cycle of quaternary glaciations in ireland relative to the terrestrial glacial landforms the features in the lough are smaller in scale and are considered to represent local ice mass dynamics that followed regional scale events four phases are identified 1 after the last ice sheet wide readvance associated with the killard point stadial between 15 0 and 14 1 14 c ka bp stagnation zone retreat resulted in isolation of a residual ice block in the lower lough erne basin 2 proglacial waters developed coeval with retreat of the western margin of the ice block drawdown induced localized surging and the generation of push features and lineations 3 squeeze up features reflecting a heavily crevassed ice margin mark the quiescent phase of the local surge cycle 4 iceberg grounding pits and keel marks record calving and rapid disintegration of the lough erne ice margin the well preserved glacigenic features observed in the lake basin suggest formation by a succession of mechanisms that were too short lived to obliterate the underlying evidence of ice margin dynamics

phanerozoic tectonothermal events of the xuefengshan belt central south china implications from upb age and luhf determinations of granites the xuefengshan belt characterized by large scale fold and thrust structures and widespread granites is a key area to decipher the tectonic evolution of the south china block in this belt two magmatic episodes are recorded by early paleozoic and early mesozoic granites in this paper we carried out precise sims zircon usingle bondpb dating and in situ lusingle bondhf isotopes measurements on these granitic plutons our study indicates that the early paleozoic and the early mesozoic granites are late orogenic products of the early paleozoic orogen and the middle triassic xuefengshan orogen respectively in the xuefengshan belt the early paleozoic event is poorly registered since this area corresponds to the outer zone of the early paleozoic orogen the silurian early devonian granites are late orogenic plutons emplaced after the main tectonic and metamorphic stage in the wuyi baiyun yunkai belt coeval with a widespread subsequent extensional event in eastern and southern south china on the other hand triassic granites are formed in an intracontinental environment with weakly to strongly peraluminous signatures zircon usingle bondpb ages presented here associated with a summary of newly acquired data in the same region suggest that the emplacement of anatectic granites occurred around 225 215 ma not in a rather wide range of ca 245 200 ma in situ zircon ehf t values indicate a crust derived source without a mantle derived input for the two generations of granites combining our data with recent studies we infer that the central area of the south china block has experienced two tectonothermal events the early paleozoic magmatism developed as a result of the collapse of the early paleozoic orogen while these early mesozoic granites can be the late orogenic products of the intracontinental xuefengshan orogen most likely manifesting the far field effect by the

intertidal forms produced by floating ice in vestfirdir iceland forms produced by the action of intertidal ice in the fjord of kaldalon vestfirdir iceland are described and interpreted in terms of their morphogenetic environment isolated boulders resting on top of undisturbed fine sediments are interpreted as dropstones from floating intertidal ice following rockfall events boulder pavements and ridges which occur in the upper intertidal zone are related to the onshore movement of ice packing and bulldozing boulders together the occurrence of such ice formed features in kaldalon is attributed to their preservation within a low energy environment in spite of a limited ice climate the paucity of such forms preserved in the sedimentary record suggests that opportunities for burial and preservation are limited except where local infill is rapid

evolution of the hongzhen metamorphic core complex evidence for early cretaceous extension in the eastern yangtze craton eastern china we conducted structural and geochronological studies on the hongzhen metamorphic core complex in the eastern yangtze craton the hanging wall of the hongzhen metamorphic core complex consists of neoproterozoic to permian unmetamorphosed strata which were folded during nw se shortening in the middle triassic and cretaceous terrestrial basins the exposed paleoproterozoic dongling complex in the footwall is widely overprinted by a detachment ductile shear zone which has consistent sw plunging mineral elongation lineations and top to the sw shear indicators analysis of the mesostructures microstructures and quartz c axis fabrics indicates that they formed within a low angled sw dipping extensional shear zone at midcrustal levels four muscovite grains separated from mylonites within the shear zone yielded 40 ar 39 ar plateau ages ranging from 128 5 1 0 ma to 126 1 1 1 ma they are interpreted as cooling ages of the shear zone associated with the hongzhen metamorphic core complex we propose that the hongzhen metamorphic core complex was initiated as a midcrustal low angle extensional shear zone with top to the sw shear sense at ca 145 ma and the shear zone was then warped and uplifted by the emplacement of the hongzhen granite at 122 ma brittle normal faulting and basin rifting in the hanging wall also developed during the formation of the hongzhen metamorphic core complex but these processes continued until the late cretaceous this study of the hongzhen metamorphic core complex demonstrates that the eastern yangtze craton was also involved in the early cretaceous extension widely occurring in the eastern china continent a ne sw extensional direction during the early cretaceous is indicated by the hongzhen metamorphic core complex

shore platform development in the south shetland islands antarctica shore platforms in the south shetland islands antarctica display morphological differences from mid latitude platforms that are explicable in terms of the morphogenetic environment these differences include marked horizontality of the platform surface terminated abruptly by low tide and high tide gradient changes since the processes of ice freezing on quarrying by impact and abrasion are responsible for such horizontality the platforms are widest in areas of restricted fetch where ice persists longest in exposed locations wave quarrying and abrasion limit the development of horizontality stratigraphic evidence suggests that the shore platforms of the south shetland islands are inherited features that have been rapidly retrimmed by polar shore processes some suggestions for future research are made