cessation of plastic deformation during exhumation of metamorphic tectonites revealed by microboudinage structures abstract microboudinage structures are analysed to examine how quartzose metamorphic tectonites cease plastic deformation during retrograde metamorphism in exhumation the microboudinage of columnar mineral grains embedded in the quartz matrix record whether plastic deformation ceased as result of the relaxation of differential stress or the plastic brittle transition of the matrix the fundamental strain data for microboudinage structures reveal four possible cases in terms of the timing of the plastic brittle transition in the differential stress history the classification is closely related to the degree of post tectonic annealing which obliterates the microstructures produced during earlier plastic deformation the approach is applied to three metamorphic tectonites from high pressure temperature metamorphic belts and all are demonstrated to have ceased plastic deformation due to plastic brittle transition of the matrix suggesting that the rocks experienced increasing differential stress during exhumation in retrograde metamorphism

development of structures under the influence of heterogeneous flow field around rigid inclusions insights from theoretical and numerical models rocks that are mechanically heterogeneous due to the presence of stiff or rigid inclusions floating in a ductile matrix commonly show a variety of micro to macro scale structures developing under the influence of heterogeneous flow field in the neighbourhood of the inclusions it is of fundamental importance to apprehend the nature of strain heterogeneity around inclusions to understand progressive development of structures associated with rigid inclusions such as strain shadow foliation drag porphyroclast mantle porphyroblast inclusion trails intragranular fractures etc the development of these diverse types of structures can be analyzed with the help of a suitable hydrodynamic theory in this paper we review different continuum models that have been proposed to characterize the heterogeneous flow field around rigid inclusions focusing on recent developments recent studies reveal that jeffery s proc r soc lond a 120 1922 161 theory dealing with the motion of ellipsoidal rigid bodies in an infinitely extended viscous medium is more general in nature and applicable for modeling the heterogeneous flow around both equant and inequant shapes of inclusions and ideal or non ideal shear deformation of the matrix the application of this theory therefore has advantages over other models based on lamb s lamb h 1932 hydrodynamics cambridge university press cambridge theory dealing with spherical inclusions the review finally illustrates numerical simulations based on hydrodynamic theories highlighting the controls of physical and kinematic factors on the progressive development of the structures mentioned above

inferring surface water equilibrium calcite δ18o during the last deglacial period from benthic foraminiferal records implications for ocean circulation the ocean circulation modifies mixed layer ml tracer signals as they are communicated to the deep ocean by advection and mixing we develop and apply a procedure for using tracer signals observed upstream by planktonic foraminifera and downstream by benthic foraminifera to constrain how tracer signals are modified by the intervening circulation and by extension to constrain properties of that circulation a history of ml equilibrium calcite 18o 18oc spanning the last deglaciation is inferred from a least squares fit of eight benthic foraminiferal 18oc records to green s function estimated for the modern ocean circulation disagreements between this history and the ml history implied by planktonic records would indicate deglacial circulation changes no deviations from the modern circulation are diagnosed because the two estimates of ml 18oc agree within their uncertainties over the deglaciation but diagnostics suggest data collection and modeling procedures useful for inferring circulation changes uncertainties of benthic derived ml 18oc are lowest in the high latitude regions chiefly responsible for ventilating the deep ocean additional high resolution planktonic records constraining these regions are of particular utility benthic records from the southern ocean where data are sparse appear to have the most power to reduce uncertainties in benthic derived ml 18oc understanding the spatiotemporal covariance of deglacial ml 18oc will also improve abilities of 18oc records to constrain deglacial circulation

fracturing of garnet crystals in anisotropic metamorphic rocks during uplift discussion ji rt al 1997 proposed a shear lag model for the development of the ordered fractures in garnet crystals from the morin shear zone in the grenville province quebec such fractures are also very commonly observed but poorly understood elsewhere such as in the east athabasca mylonite triangle of the snowbird tectonic zone hanmer et al 1994 snoeyenbos et ai 1995 jiang 1996 hanmer and williams unpublished data major points of their argument may be summarized as follows

timing and structure of mega sacz events during heinrich stadial 1 a substantial strengthening of the south american monsoon system sams during heinrich stadials hs points toward decreased cross equatorial heat transport as the main driver of monsoonal hydroclimate variability at millennial time scales in order to better constrain the exact timing and internal structure of hs1 over tropical south america we assessed two precisely dated speleothem records from central eastern and northeastern brazil in combination with two marine records of terrestrial organic and inorganic matter input into the western equatorial atlantic during hs1 we recognize at least two events of widespread intensification of the sams across the entire region influenced by the south atlantic convergence zone sacz at 16 11 14 69 kyr b p and 18 1 16 66 kyr b p labeled as hs1a and hs1c respectively separated by a dry excursion from 16 66 to 16 11 kyr b p hs1b in view of the spatial structure of precipitation anomalies the widespread increase of monsoon precipitation over the sacz domain was termed mega sacz

on microboudin paleopiezometers and their applications to constrain stress variations in tectonites abstract accounting for interfacial shear induced near end fracturing we propose a new microboudin piezometer e c 0 25 l w where is the shear flow stress of the ductile matrix e g polycrystalline quartz c is the tensile fracture strength of a columnar mineral e g tourmaline which ranges generally between e 1000 to e 250 e is young s modulus e is a dimensionless parameter named equivalent shear strength and l w is the mean length width ratio of the microboudins formed by near end fracturing this provides a straightforward method of estimating the magnitude of shear flow stresses responsible for ductile deformation of natural rocks the piezometer is easy to apply because it does not require the use of tem sem ebsd or the identification of prevailing deformation mechanism e g dislocation creep diffusion creep hardening or weakening the samples from the gaoligong metamorphic belt yunnan china yield the equivalent shear flow stresses of 0 145 0 424 36 106 mpa if taking c e 800 or 250 mpa for felsic gneisses mylonites and phyllonites the earlier formed microboudins had consistently a larger mean length width ratio than the later formed ones suggesting that the rocks recorded a tectonic history of continuous hardening prior to the cessation of quartz plasticity

rainfall infiltration performance and its relation to mesoscopic structural properties of a gravelly soil slope abstract soil infiltration performances under rainfall affect shear strengths and play a crucial role in landslide problems in this paper the infiltration performances of gravelly soils under different rainfall conditions were investigated by 1d and 2d seepage tests computed tomography ct scans were adopted to identify the mesoscopic structural properties of gravelly soils the significances of mesoscopic properties on the saturated coefficient of permeability k s were subsequently analyzed the results showed a temporal and spatial variation of infiltration performances of a gravelly soil slope the infiltration performances measured by 1d infiltration tests were different from those obtained from 2d tests an exponential relationship between 1d and 2d infiltration rates was observed the results further highlighted that void area ratio uniformity coefficient and gravel content were three most significant factors to k s three permeability formulas that respectively expressed by gravel content initial dry density and two mesoscopic properties were proposed in this paper

cenozoic evolution of asian climate and sources of pacific seawater pb and nd derived from eolian dust of sediment core ll44 gpc3 1 the large diameter piston core ll44 gpc3 from the central north pacific ocean records continuous sedimentation of eolian dust since the late cretaceous two intervals resolved by nd and pb isotopic data relate to dust coming from america prior to 40 ma and dust coming from asia since 40 ma the intertropical convergence zone itcz separates these depositional regimes today and may have been at a paleolatitude of 23 n prior to 40 ma such a northerly location of the itcz is consistent with sluggish atmospheric circulation and warm climate for the northern hemisphere of the early to middle eocene since 40 ma correlations between nd 7 55 end t 10 81 and pb 18 625 206 4pb 18 879 15 624 207 4pb 15 666 38 611 208 4pb 38 960 0 8294 207 6pb 0 8389 2 0539 208 6pb 2 0743 isotopes reflect the progressive drying of central asia triggered by the westward retreat of the paleo tethys comparisons between the changes with time in the isotopically well defined dust flux and nd and pb isotopic compositions of pacific deep water allow one to draw two major conclusions 1 dust bound nd became a resolvable contribution to pacific seawater only after the one order of magnitude increase in dust flux starting at 3 5 ma therefore eolian nd was unimportant for pacific seawater nd prior to 3 5 ma 2 the lack of a response of pacific deep water pb to this huge flux increase suggests that dust bound pb has never been important instead mobile pb associated with island arc volcanic exhalatives probably consists of a significant contribution to pacific deep water pb and possibly to seawater elsewhere far away from landmasses

subgrade reaction and load settlement characteristics of gravelly cobble deposits by plate load tests this paper presents the result of plate load tests conducted on a gravelly cobble deposit in taichung basin taiwan the geologic formation of the gravelly cobble deposit makes it very difficult to

can a newtonian viscous matrix model be applied to microboudinage of columnar grains in quartzose tectonites a newtonian viscous rheology was assumed for the matrix around the microboudins of columnar mineral grains in metamorphic tectonites and a new expression was derived for predicting the proportion of boudinaged grains as a function of the aspect ratio of the columnar grains and the magnitude of far field differential stress the predicted proportion was compared with the measured proportions of tourmaline piemontite and sodic amphibole in seven samples from various orogenic belts the comparison revealed that the predictions were remarkably different from the measured values this suggests that the viscous matrix model cannot be applied to the stress state of the matrix during microboudinage

the african humid period and the record of marine upwelling from excess 230th in ocean drilling program hole 658c using a high resolution 230 th normalized record of sediment flux we document the deglacial and holocene history of north african aridity and coastal upwelling at ocean drilling program hole 658c at both the end of the younger dryas and after the 8 2 ka event there are significant drops in terrigenous accumulation at our site indicating an increase in the monsoon moisture flux over africa at this time at 5 5 ka there is an abrupt end to the african humid period and a return to stronger upwelling conditions for carbonate and opal fluxes the 230 th normalization completely changes the shape of each record based on percentage variations alone this site is a clear example of how variations in one sediment component can obscure changes in the others and it demonstrates the need for radionuclide measurements more generally in paleoceanography by taking our new records and a large amount of previous data from this site we conclude that increases in african moisture are tightly coupled to decreases in coastal upwelling intensity

strain stress and uplift strain distribution across the high pressure type sambagawa regional metamorphic belt was measured by studies of shape change of initially spherical radiolaria strains change from 5 to 2000 with increasing metamorphic temperatures aburpt strain increase apparently occurs in the intermediate metamorphic grades of the terrane deformation stresses during metamorphism were inferred using the grain sized quartz piezometer method which is applicable to dynamically recrystallized quartzose rocks stress distribution across the terrane was composed of two levels the stress level in the lower grade zones was about 130 mpa but in the higher grade zones was about 50 mpa combining stress distribution strain distribution and temperatures of the studied metamorphic terrane the activation energy of the deformation of these quartzose rocks is estimated to have been about 80 90 kj mol the huge amount of deformation of the deep seated sambagawa metamorphic terrane which was formed probably at the lower crustal to uppermost mantle levels over the subducting lithosphere was sufficient to bring about the uplift of parts of the terrane to the earth s surface

northeastern pacific oxygen minimum zone variability over the past 70 kyr impact of biological production and oceanic ventilation 1 during the last glacial period the oxygen minimum zone omz within the northeastern pacific ocean strengthened and weakened on a millennial time scale demonstrating a tight linkage with northern high latitude climate although the precise mechanisms responsible remain unknown core md02 2508 retrieved off baja california was analyzed for major and trace elements br ca ti fe mn and sr using a xrf scanner and redox sensitive trace elements cu ni cd as v cr mo and u using the icp ms the trace element content the fe ti ratio and br based organic carbon exhibit higher values during the holocene and during warm dansgaard oeschger events than during the last glacial maximum lgm stadials and heinrich h events a principal component analysis of the element al ratio indicated that the following two main factors controlled the chemical composition of the sediments 1 export production as represented by organic carbon that was lower during cold periods and 2 regional intermediate water oxygenation as represented by u and mo variability that was not supported by a change in export production the latter suggests that intermediate water oxygenation improved during h events but slightly deteriorated during late marine isotope stage mis 3 and mis 2 a local biogeochemical effect forced by atmospheric processes impacted the lgm and h events in the same manner whereas regional intermediate oceanic circulation varied in an opposite manner during the lgm and h events possibly as a result of the global reorganization of intermediate water circulation during the lgm

submillennial to millennial variability of diatom production off mauritania nw africa during the last glacial cycle 1 the low latitude upwelling regime off the mauritanian coast in the subtropical ne atlantic accounts for a significant part of global export production although productivity variations in coastal upwelling areas are usually attributed to changes in wind stress and upwelling intensity productivity dynamics off mauritania are less straightforward because of the complex atmospheric and hydrographic setting here we integrate micropaleontological diatoms and geochemical bulk biogenic sediment components x ray fluorescence and alkenones proxies to examine on submillennial to millennial changes in diatom production that occurred off mauritania nw africa for the last 25 ka during the last glacial maximum lgm 19 0 23 0 ka b p moderate silicate content of upwelled waters coupled with weakened ne trade winds determined moderate diatom productivity no significant cooling is observed during the lgm suggesting that our alkenone based sst reconstruction represents a local upwelling related signal rather than a global insolation related one extraordinary increases in diatom and opal concentrations during heinrich event 1 h1 15 5 18 0 ka b p and the younger dryas yd 13 5 11 5 ka b p are attributed to enhanced upwelling of silica rich waters and an enlarged upwelling filament due to more intense ne trade winds the synchronous increase of caco3 and k intensity and the decreased opal and diatoms values mark the occurrence of the bolling allerod ba 13 5 15 5 ka b p due to weakened eolian input and more humid conditions on land although the high export of diatoms is inextricably linked to upwelling intensity off mauritania variability in the nutrient content of the thermocline also plays a decisive role

the flickering switch of late pleistocene climate change polar ice contains a unique record of past climate variations previous greenland ice cores have documented relatively warm interstadial periods during the last glaciation and short centuryscale returns to colder conditions during the glacial to interglacial warming see for example ref 1 these climate features have also been observed to varying degrees in ocean sediment cores2 4 and terrestrial pollen and insect records5 7 here we report electrical conductivity measurements from a new greenland ice core which confirm these previous observations and also reveal a hitherto unrecognized mode of rapid climate variation fluctuations in ice conductivity on the scales of 5 20 years reflect rapid oscillations in the dust content of the atmosphere this flickering between two preferred states would seem to require extremely rapid reorganizations in atmospheric circulation

abrupt increase in greenland snow accumulation at the end of the younger dryas event the warming at the end of the last glaciation was characterized by a series of abrupt returns to glacial climate the best known of which is the younger dryas event1 despite much study of the causes of this event and the mechanisms by which it ended many questions remain unresolved1 oxygen isotope data from greenland ice cores2 4 suggest that the younger dryas ended abruptly over a period of about 50 years dust concentrations2 4 in these cores show an even more rapid transition 20 years this extremely short timescale places severe constraints on the mechanisms underlying the transition but dust concentrations can reflect subtle changes in atmospheric circulation which need not be associated with a large change in climate here we present results from a new greenland ice core gisp2 showing that snow accumulation doubled rapidly from the younger dryas event to the subsequent preboreal interval possibly in one to three years we also find that the accumulation rate change from the oldest dryas to the bo11ing allerod warm period was large and abrupt the extreme rapidity of these changes in a variable that directly represents regional climate implies that the events at the end of the last glaciation may have been responses to some kind of threshold or trigger in the north atlantic climate system

millennial scale precipitation changes in southern brazil over the past 90 000 years 1 a u th dated 90 000 year long speleothem oxygen isotope record from southern brazil anti correlates remarkably with the cave calcite records from eastern china but positively correlates with the speleothem record from northeastern brazil suggesting an interhemispheric anti phasing of rainfall on both millennial and orbital timescales likely related to displacement in the mean position of the intertropical convergence zone and associated asymmetry in hadley circulation the phase relationships among these records are consistent with the hypothesis that abrupt climate events during the last glacial period are triggered by oceanic circulation changes in the high latitudes and enhanced by tropical feedbacks

development of different types of pull apart microstructures in mylonites an experimental investigation abstract intragranular fracturing of feldspar porphyroclasts in granite mylonites of a ductile shear zone has led to development of diverse types of pull apart and bookshelf structures porphyroclasts with centrally located fractures show pull aparts with parallel walls type 1 whereas those with off centered fractures display pull aparts with both parallel type 1 and non parallel type 2 geometry analog model experiments performed by embedding segmented rigid elliptical objects within a viscous matrix indicate that the orientation and location of the fracture and the aspect ratio of the porphyroclast are the principal parameters in controlling the development of bookshelf structures and pull aparts in the case of centrally located fractures porphyroclasts of moderate aspect ratios develop either type 1 pull apart or bookshelf depending upon the fracture orientations whereas those of large aspect ratios 3 form only type 1 pull aparts irrespective of the fracture orientation off centered fracturing of porphyroclasts gives rise to fragments of unequal size which rotate independently at equal or contrasting velocities forming type 1 or type 2 v pull aparts respectively in the latter case depending upon the orientation of fracture the smaller fragment rotates faster or slower than the larger fragment showing relative tilt synthetic type 2a or antithetic type 2b to the bulk shear sense respectively type 2b geometry generally develops when the fracture angle with respect to the short axis of porphyroclasts is low antithetic to the shear direction and the long axis of the porphyroclast is at a high angle to the shear plane

boudinage in multilayered rocks under layer normal compression a theoretical analysis abstract this paper presents a dynamic analysis of boudinage in multilayers of alternate brittle and ductile layers under layer normal compression based on the mode of fracturing of individual brittle layers boudinage is classified into three types tensile fracture boudinage type 1 shear fracture boudinage type 2a and extensional shear fracture boudinage type 2b the layer thickness ratio t r t b t d and the strength ratio f t 2 e between the brittle and the ductile units are the principal physical factors determining the type of boudinage type 1 boudinage develops rectangular boudins and occurs when t r is low f is high 0 8 in contrast type 2a boudinage takes place when t r is high 8 5 or f is low a r of all the types of boudins is inversely proportional to layer thickness ratio t r however type 1 and type 2 boudins have contrasting aspect ratios which are generally greater and less than 1 respectively

distribution and provenance of wind blown se pacific surface sediments article i nfo the reconstruction of low latitude ocean atmosphere interactions is one of the major issues of paleo environmental studies the trade winds extending over 20 to 30 of latitude in both hemispheres between the subtropical highs and the intertropical convergence zone are major components of the atmospheric circulation and little is known about their long term variability on geological time scales in particular in the pacific sector we present the modern spatial pattern of eolian derived marine sediments in the eastern equatorial and subtropical pacific 10 n to 25 s as a reference data set for the interpretation of se pacific paleo dust records the terrigenous silt and clay fractions of 75 surface sediment samples have been investigated for their grain size distribution and clay mineral compositions respectively to identify their provenances and transport agents dust delivered to the southeast pacific from the semi to hyper arid areas of peru and chile is rather fine grained 4 8 m due to low level transport within the southeast trade winds nevertheless wind is the dominant transport agent and eolian material is the dominant terrigenous component west of the peru chile trench south of 5 s grain size distributions alone are insufficient to identify the eolian signal in marine sediments due to authigenic particle formation on the sub oceanic ridges and abundant volcanic glass around the galapagos islands together with the clay mineral compositions of the clay fraction we have identified the dust lobe extending from the coasts of peru and chile onto galapagos rise as well as across the equator into the doldrums illite is a very useful parameter to identify source areas of dust in this smectite dominated study area

selective organic matter preservation in burn down turbidites on the madeira abyssal plain 1 oxidized intervals of five organic rich madeira abyssal plain map turbidites deposited during the miocene pliocene and pleistocene all displayed comparable major loss of total organic carbon toc 84 3 1 accompanied by a negative isotopic 13c shift ranging from 0 3 to 2 9 major but significantly lower loss of total nitrogen ntot 61 7 1 also occurred leading to a decrease in toc relative to ntot c ntot and a 1 3 to 2 7 ntot isotopic 15n shift compound specific isotopic measurements on plant wax n alkanes indicate the terrestrial organic component in the unoxidized deposits is 13c enriched owing to significant c4 contribution selective preservation of terrestrial relative to marine organic carbon could account for the 13c behavior of toc upon oxidation but only if a 13c depleted component of the bulk terrestrial signal is selectively preserved in the process although the c ntot decrease and positive 15n shift seems inconsistent with selective terrestrial organic preservation results from analysis of a modern eolian dust sample collected in the vicinity indicate these observations are compatible regardless of the specific explanation for these isotopic observations however our findings provide evidence that paleoreconstruction of properties such as pco2 using the 13c of toc is a goal fraught with uncertainty whether or not the marine sedimentary record considered is contaminated with significant terrestrial input nonetheless despite major and selective loss of both marine and terrestrial components as a consequence of postdepositional oxidation intensive organic geochemical proxies such as the alkenone unsaturation index u37k appear resistant to change and thereby retain their paleoceanographic promise

nature and causes of quaternary climate variation of tropical south america abstract this selective review of the quaternary paleoclimate of the south american summer monsoon sasm domain presents viewpoints regarding a range of key issues in the field many of which are unresolved and some of which are controversial 1 el nino southern oscillation variability while the most important global scale mode of interannual climate variation is insufficient to explain most of the variation of tropical south american climate observed in both the instrumental and the paleoclimate records 2 significant climate variation in tropical south america occurs on seasonal to orbital i e multi millennial time scales as a result of sea surface temperature sst variation and ocean atmosphere interactions of the tropical atlantic 3 decadal scale climate variability linked with this tropical atlantic variability has been a persistent characteristic of climate in tropical south america for at least the past half millennium and likely far beyond 4 centennial to millennial climate events in tropical south america were of longer duration and perhaps larger amplitude than any observed in the instrumental period which is little more than a century long in tropical south america these were superimposed upon both precession paced insolation changes that caused significant variation in sasm precipitation and eccentricity paced global glacial boundary conditions that caused significant changes in the tropical south american moisture balance as a result river sediment and water discharge increased and decreased across tropical south america lake levels rose and fell paleolakes arose and disappeared on the altiplano glaciers waxed and waned in the tropical andes and the tropical rainforest underwent significant changes in composition and extent to further evaluate climate forcing over the last glacial cycle 125 ka we developed a climate forcing model that combines summer insolation forcing and a proxy for north atlantic sst forcing to reconstruct long term precipitation variation in the sasm domain the success of this model reinforces our confidence in assigning causation to observed reconstructions of precipitation in addition we propose a critical correction for speleothem stable oxygen isotopic ratios which are among the most significant of paleoclimate proxies in tropical south america for reconstruction of variation of paleo precipitation or sasm intensity however it is already well known that any particular 18o value observed in speleothem carbonate is affected by two processes that have nothing to do with changes in precipitation amount the influence of temperature on carbonate water isotopic fractionation in the cave and the influence of changing 18o of seawater quantitatively accounting for both artifacts can significantly alter the interpretations of speleothem records in tropical south america both adjustments act in the same direction and have the tendency to increase the true amplitude of the paleo hydrologic signal but by different amounts in glacial and inter glacial stages these corrections have even graver implications for the interpretation of tropical northern hemisphere speleothem records e g chinese speleothems where the combined adjustments tend to decrease or even eliminate the true signal amplitude

experimental and numerical investigations of the seismic response of a rock soil mixture deposit slope inertial instability induced slope failure is one of the most destructive seismic hazards encountered in china depositional slopes are widely distributed across the south western part of china to study the seismic performance of rock soil mixture deposit rsmd slopes in detail physical and mechanical tests of air dried rsmd and a 1 50 geometric scale generalised rsmd slope centrifugal shaking table modelling tests were conducted the acceleration response slope deformation and instability mode of the air dried rsmd slope under the continuous action of four earthquakes with different intensities are analysed besides the dynamic finite difference numerical simulations were performed with flac to simulate the centrifuge model test the numerical simulation results were compared with the test results in terms of the horizontal acceleration response time history of crest settlements and the slope failure characteristics the results show that the horizontal peak ground acceleration pga amplification factors of the rsmd slope reflect typical elevation and surface amplification effects in addition the spectral distribution of the seismic wave has changed significantly with propagation along the slope elevation the energy of input seismic wave in a frequency range close to the natural frequency of an rsmd slope is significantly amplified during the test it was observed that the failure of an air dried rsmd slope occurred when the pga reached 0 216 g and the settlement at the crest of the slope was obvious the failure mode was dominated by shallow collapse

development of microboudinage an estimate of changing differential stress with increasing strain microboudin structure has so far been used in structural analysis as a reliable measure of plastic strain and recently it has been proposed to use it as a measure of relative differential stress this paper demonstrates that stress and strain can be simultaneously deduced if the stress and strain analyses are carried out in combination the analysed samples are quartz schists from four localities of different high p low t type metamorphic belts in japan the samples include microboudinaged competent piedmontite grains within a ductile quartz matrix the analyses strongly suggest that the estimated differential stresses tend to increase with increasing strain and that at the final stage of the microboudinage they reach a maximum

study on the shear strength of soil rock mixture by large scale direct shear test abstract soil rock mixtures s rm which formed in the quaternary period are a type of extremely inhomogeneous and loose geomaterial with a certain percentage of rock blocks they are composed of rock blocks with various sizes and high strength fine grained soil and pores the meso failure mechanism and macro physical and mechanical characteristics of s rm are largely controlled by its rock block proportion and the granular distribution as we know when the rock blocks in the s rm are larger it is difficult to take an in situ sample for an on site test in addition it is difficult to obtain the granular distribution of rock blocks in s rm by traditional sieving tests this paper uses a new method called digital image processing dip in which the rock blocks in s rm samples are separated from the soil matrix and the proportion and distribution of the rock blocks is obtained quantitatively the results are used for the sample preparation of the large scale direct shear tests which provide a new method for the test study of s rm according to the results of large scale direct shear tests the rock block size proportion controls the deformation and fracture mechanism of the s rm the shape of the shear stress vs horizontal displacement curve and the vertical displacement vs horizontal displacement curve of the s rm samples are different from that of general soil and rock with the increment of the rock block proportion the shear band of the s rm increases when the rock block proportion lies in the range of 25 70 the increment of the internal friction angle linearly increases with the increment of the rock block proportion the cohesion of the s rm decreases compared with that of the soil when the rock block proportion is larger than 30 however there is only a little decrease in the cohesion with the increment of the rock block proportion

comment on catastrophic ice shelf breakup as the source of heinrich event icebergs by c l hulbe et al hulbe et al 2004 argue that the original binge purge model of their coauthor macayeal 1993a 1993b is not appropriate for heinrich events but that a new ice shelf collapse mechanism may work we believe that thenew collapse mechanism disagrees with important dataand that macayeal s original model remains viable afterappropriate modifications we have enjoyed a fascinatingdiscussion with hulbe et al on this topic which isstimulating our thinking and research and we presentsome of the arguments here for a wider audience

modeling tropical convergence based on the moist static energy budget abstract the vertically integrated moist static energy equation provides a convenient starting point for the construction of simple models of the time mean low level convergence in the tropics a vertically integrated measure of the moist static stability the gross moist stability proves to be of central importance minima in this quantity mark the positions of the tropical convergence zones we argue that the positions of these minima are determined by the time mean moisture field which is in turn closely tied to the time mean surface temperature

alkenone producers during late oligocene early miocene revisited this study investigates ancient alkenone producers among the late oligocene early miocene coccolithophores recorded at deep sea drilling project dsdp site 516 contrary to common assumptions reticulofenestra was not the most important alkenone producer throughout the studied time interval the comparison between coccolith species specific absolute abundances and alkenone contents in the same sedimentary samples shows that cyclicargolithus abundances explain 40 of the total variance of alkenone concentration and that the species cyclicargolithus floridanus was a major alkenone producer although other related taxa may have also contributed to the alkenone production at dsdp site 516 the distribution of the different alkenone isomers mec37 2 etc38 2 and mec38 2 remained unchanged across distinct changes in species composition suggesting similar diunsaturated alkenone compositions within the noelaerhabdaceae family during the late oligocene early miocene however the overall larger cell size of cyclicargolithus may have implications for the alkenone based reconstruction of past partial pressure of co2 our results underscore the importance of a careful evaluation of the most likely alkenone producers for periods 1 85 ma predating the first occurrence of contemporary alkenone producers i e emiliania huxleyi and gephyrocapsa oceanica

growth history of garnet in pelitic schists of the sanbagawa metamorphic terrain in central shikoku abstract during the prograde metamorphism of the sanbagawa metamorphic terrain in central shikoku garnet has grown in the pelitic schists whilst maintaining surface equilibrium with coexisting minerals such as chlorite and epidote etc in the quartz albite layer it has grown euhedrally in the micaceous layer its crystal morphology varies according to the different minerals in contact and their different orientations garnet enclosed in or sandwiched between micas is always ellipsoidal extended parallel to the basal planes of the micas however when the edge of a garnet has intruded at an acute angle into the micas euhedral garnet has grown it is concluded that the growth of garnet at the interface with micas ceased when the 110 surface of garnet came into contact with the basal plane of the micas in the tectonic c axis direction other models such as chloritization resorption pressure solution and mechanical erosion or grinding do not adequately explain the origin of the flattened garnet during retrograde metamorphism chloritization and resorption of garnets took place in the matrix resulting in the rough and irregular surface of garnet garnets included in albite porphyroblasts have been largely unaffected by the retrograde metamorphism

computational homogenization for mechanical properties of sand cobble stratum based on fractal theory abstract sandy cobble strata is one of the most commonly encountered engineering geological body a further research into the stability of ground and underground structure demands for precise modeling of actual formation in this paper a meso scale numerical method for modelling sandy cobble strata is presented which considers rock blocks and soil matrix as separate constituents in the approach the particle size distribution and spatial distribution of rock blocks are two crucial points for the first point i e the particle size distribution of rocks the fractal scaling theory is included in particle size distribution description and design in the log log plots 60 sets of particle size distributions of sandy cobble soil in beijing are examined for fractal behavior for the second point i e the spatial distribution of rocks the monte carlo principle is used to generate random spatial distribution of rocks in soil matrix while the rock shape is assumed to be circular in two dimension or sphere in three dimension based on the compassion of rock volume content between the numerical model and theoretical calculation value it is validated that the meso scale method could basically be used to establish the meso model of sandy cobble strata then the present sandy cobble strata model is extended to investigate tunnel excavation in such a formation based on the simulation results the deformation characteristics on transverse section and longitudinal profile are explored respectively and their variations with fractal dimension or the maximum rock size are discussed subsequently

geometrical statistical modelling of systems of fracture zones along oceanic ridges summary a mechanical statistical model is presented that aims to help to understand the history and geometry of the process of formation of fracture zones along oceanic ridges it uses ideas of statistical fracture theory used in engineering namely the weibull fracture model the approximate parallelism of the fracture zones along ridges makes it possible to use a onedimensional point process model with points along the ridge axes which represent the transform faults the ratios of the lengths of the corresponding fracture zones to the ocean width are used to obtain a rough estimate of the weibull modulus which is an important material parameter in fracture theory the theory is refined by introducing a hard core point process model the corresponding positive minimum distance between subsequent fracture zones results from stress relaxation in the vicinity of a given fracture zone

controls on the failure mode of brittle inclusions hosted in a ductile matrix plane strain deformation experiments were performed on elliptical inclusions of cohesive sand embedded within a slab of pitch with the aim of investigating the mode of fracturing of brittle inclusions within a ductile matrix under pure and simple shear the inclusions failed in three different modes tensile fracturing mode 1 shear fracturing mode 2a and extensional shear fracturing mode 2b jeffrey s 1922 theory of the flow of a viscous medium around an ellipsoidal body was applied to the experimental results to determine the principal tensile and compressive stresses within an inclusion and analyze the failure modes using griffith s criterion the analysis reveals that the aspect ratio r and the orientation u of the inclusion control the principal tensile and compressive stresses within it and in turn govern the mode of brittle deformation at a particular inclusion orientation the tensile stress increases whereas the compressive stress decreases monotonically with increasing aspect ratio of the inclusion the principal stresses also vary with inclusion orientation for a given aspect ratio but not monotonically the analysis delimits the fields of each mode of brittle deformation of inclusions in r u space under pure shear and simple shear q 2001 elsevier science ltd all rights reserved

variations in tropical convection as an amplifier of global climate change at the millennial scale the global expression of millennial scale climatic change during the glacial period and the persistence of this signal in holocene records point to atmospheric teleconnections as the mechanism propagating rapid climate variations we suggest rearrangements in the tropical convection system globally affected the concentration and location of atmospheric water vapour and modulated terrestrial and marine emissions of ch4 and n2o providing a tropical mechanism of amplifying and perpetuating millennial scale climate changes a multi proxy reconstruction reflecting various aspects of the intensity of the arabian sea summer monsoon shows strong millennial scale variability over the past 90 kyr in which low intensity is associated with a southern shift of the intertropical convergence zone itcz and an eastward shift in the equatorial convergence zone the monsoon reconstruction which is based on new data from a somali margin sediment core is supported by previously reported tropical paleoclimatic records and suggests that global scale millennial climatic variability is in part driven by modulations in the tropical hydrological cycle and tropical emissions of the greenhouse gases ch4 and n2o

theoretical analysis of cross joint geometries and their classification one commonly observed bedrock joint pattern consists of cross joints that terminate against an older set of longer systematic joints dyer 1988 showed that unique cross joint geometries may develop in response to a combination of remote stresses and local perturbations of the stress field in the vicinity of preexisting joints we expand upon dyer s analysis by providing the general solutions for cross joint geometry as a function of remote principal stress orientations and ratios specific cross joint geometries are determined by solving for the local cross joint angle as a function of distance from the preexisting joint on the basis of theoretical derivations cross joint geometries are grouped into five main categories curving parallel curving perpendicular quasi curving parallel quasi curving perpendicular and noncurving geometries furthermore by discussing the stress state in the vicinity of the preexisting joint a general expression for the dimension of compressive zone is derived and a more detailed classification of cross joint geometry is provided results from this study provide a method to estimate the relative magnitudes of remote principal stresses simply based on the observed cross joint geometry which may help constrain the tectonic history of a region in addition knowledge of precise cross joint curvature and intersection geometries may provide important insights into their connectivity and fluid flow characteristics

a core top assessment of proxies for the ocean carbonate system in surface dwelling foraminifers we have assessed the reliability of several foraminifer hosted proxies of the ocean carbonate system 11b b ca and u ca using holocene samples from the atlantic and pacific oceans we examined chemical variability over a range of test sizes for two surface dwelling foraminifers globigerinoides sacculifer and globigerinoides ruber measurements of 11b in g ruber show no significant relationship with test size in either atlantic or pacific sites and appear to provide a robust proxy of surface seawater ph likewise there is no significant variability in the 11b of our atlantic core top g sacculifer but we find that 11b increases with increasing test size for g sacculifer in the pacific these systematic differences in 11b are inferred to be a consequence of isotopically light gametogenic calcite in g sacculifer and its preferential preservation during postdepositional dissolution the trace element ratio proxies of ocean carbonate equilibria u ca and b ca show systematic increases in both g ruber and g sacculifer with increasing test size possibly as a result of changing growth rates this behavior complicates their use in paleoceanographic reconstructions in keeping with several previous studies we find that mg ca ratios increase with increasing size fraction in our well preserved atlantic g sacculifer but not in g ruber in contrast to previous interpretations we suggest that these observations reflect a proportionally larger influence of compositionally distinct gametogenic calcite in small individuals compared to larger ones as with 11b this influences g sacculifer but not g ruber which has negligible gametogenic calcite

a stress transfer model for the development of extension fracture boudinage abstract the term boudinage is used to describe a wide variety of extensional structures in deformed rocks this paper is mainly concerned with boudinage resulting from through layer extension fractures followed by separation of the layer segments thus forming boudins with more or less rectangular cross sections in principle this process is similar to the break up of fibres in fibre reinforced composite materials extended parallel to the fibre direction both processes are controlled by the transfer of stress from the matrix to the fibre or layer and a mathematical model for fibre matrix stress transfer the fibre loading model is well established we have used this as a basis for developing a stress transfer model for boudinage the only difference in the basic mathematical formulation results from geometric differences between the two systems the geometric expressions in the fibre loading model have therefore been rederived for the layer matrix case stress transfer theory predicts that the tensile stress in a layer segment rises from a minimum at the end of a segment to a maximum at the centre this behaviour which is clearly shown by finite element models of boudinage structure suggests that extension fracture boudinage develops by successive mid point fracturing according to stress transfer theory the process will continue until a layer is reduced to segments boudins all of which are shorter than some critical length for which the tensile fracture strength of the layer is equal to the tensile stress at the mid point in practice successive fracturing will be influenced by two other factors 1 in nature the controlling material properties tensile fracture strength elastic moduli will not be single valued but will have a distribution reflecting local variations in lithology and microstructure and 2 major pre deformation flaws may be present in a layer which will control the starting length of layer segments these factors are incorporated with the stress transfer theory into a statistical monte carlo model for extension fracture boudinage which results in a prediction of boudin aspect ratios the predicted distribution compares very closely with the observed distribution of 91 quartzite boudins within lower carboniferous slates at tintagel cornwall the stress transfer model implies that boudin defining fractures occur sequentially so that inter boudin gap lengths will be unequal strain estimates based on boudinage structure will vary according to which part and how much of a layer is sampled a much improved strain estimate is possible based on sequentially closing the inter boudin gaps the stress transfer theory also leads to the possibility of estimating palaeostress from boudinage structure and is the only model available which predicts an aspect ratio distribution of boudins formed by extension fracture other than our own we know of no published data on boudin aspect ratio distributions hence further elaboration of the model is not possible until more field data is available we hope that our work will encourage the systematic measurement of boudinage as well as the development of alternative models

comparative rheological behaviour of albite and quartz in siliceous schists revealed by the microboudinage of piedmontite abstract this paper presents a detailed comparison of the microboudinage of piedmontite in two different mineralogical hosts a quartz matrix and albite porphyroblasts in a siliceous schist with the aim of clarifying the rheological properties of albite in relation to those of quartz stress and strain analyses of the microboudinage confirm that the boudinage took place in the retrograde stage of metamorphism during decreasing temperature and reveal that albite deformed at the same strain rate as quartz above the plastic brittle transition temperature of albite

molecular stratigraphy of cores off northwest africa sea surface temperature history over the last 80 ka the molecular stratigraphy of biogeochemical oceanic flux study core 31k 19 n 20 10 w and ocean drilling program hole 658c 20 45 n 18 35 w has been studied for c37 alkenone abundances over the past 80 ka at high resolution circa 200 500 years the derived uk37 sea surface temperature record for both cores shows a range of temperatures from about 18 c during the last glacial to 21 5 c during the early holocene both records also reveal changes in sea surface temperature as much as 2 4 c over a few hundred years which correlate well with similar abrupt climatic changes observed in cores from elsewhere in the ne atlantic associated with heinrich events our results indicate that meltwater produced by these ice rafting events was transmitted southward by the canary current where it had considerable impact on sea surface temperatures in the subtropical eastern atlantic

interhemispheric comparison of deglacial sea surface temperature patterns in atlantic eastern boundary currents abstract sediment core geob 1023 5 from the eastern south atlantic was investigated at high temporal resolution for variations of sea surface temperature sst during the past 22 kyr using the alkenone u k 37 method ssts increased by 3 5 c from about 18 c during the last ice age 21 2 cal kyr bp to about 21 5 c at 14 5 cal kyr bp this warming trend associated with the deglaciation phase was followed by a cooling event with lowest ssts near 20 c persisting for about 1000 years between 13 and 12 cal kyr bp the ssts then continued to increase to about 22 5 c at the holocene climatic optimum at 7 cal kyr bp and decreased again during the late holocene to a core top value of 19 8 c that is comparable to modern annual mean sst values when compared with alkenone sst records from the eastern north atlantic our sst record indicates continuous warming throughout the deglaciation phase in the benguela current while its northern counterpart the canary current experienced prominent cooling during heinrich event 1 h1 on the other hand for the time period corresponding to the younger dryas yd cooling event the benguela sst record exhibits a cold temperature interval that corresponds to that observed in the eastern north atlantic sst records this observation suggests that interhemispheric climate response in atlantic eastern boundary current systems was different with respect to the two abrupt climate events associated with termination i for the h1 the eastern south atlantic sst record strongly supports the hypothesis that an anti phase thermal behavior in south atlantic surface waters was forced by the slowdown of the north atlantic deep water formation during cold spells in the north atlantic in contrast the abrupt cooling in the eastern south atlantic coincident with the yd period was probably induced by more vigorous global atmospheric circulation enhancing the upwelling intensity in both eastern boundary current systems this atmospheric control may have overridden any effect caused by changes in thermohaline circulation on the south atlantic ssts during the yd which leads to the assumption that the thermohaline circulation was already much closer to its interglacial mode during the yd than during the h1

mineralogical and geochemical compositions of the eolian dust from the northeast equatorial pacific and their implications on paleolocation of the intertropical convergence zone 1 eolian components of a 328 cm long piston core collected from the northeast equatorial pacific at 16 12 n and 125 59 w were investigated for mineral and geochemical compositions in order to constrain the sources of dust and determine the latitudinal position of the intertropical convergence zone itcz recorded in the core the eolian components below 250 cm are characterized by smectite and phillipsite rich mineral composition depleted rare earth elements rees and high eu sm ratios indicative of volcanic rich composition these characteristics are found in equatorial and south pacific surface sediments of which eolian particles are supplied from central and south america in contrast eolian components above 250 cm are characterized by quartz and illite rich mineralogy and more shale like ree and trace element compositions which are common in surface sediments of the central pacific north of the itcz where eolian particles are sourced from the asia and north america the observed changes are attributed to the shifting of its eolian sources from the central and south america to the china and north america across the hemispheric dust barrier of the itcz this result suggests that smectite illite transition a phenomenon that smectite amount increases over illite at a depth can be used as a tracking tool for the paleolocation of the itcz in the northeast and central pacific backtrack path construction of pacific plate indicates paleolocation of the itcz north of 12 n 2 prior to late miocene

reconstructing the phytoplankton community of the cariaco basin during the younger dryas cold event using chlorin steryl esters 1 a record of the downcore distribution of chlorin steryl esters cses through the younger dryas was produced from cariaco basin sediments in order to assess the potential use of cses as recorders of the structure of phytoplankton communities through time using an improved high performance liquid chromatography method for the separation of cses we find significant changes in the distribution of cses during the younger dryas in the cariaco basin during the younger dryas enhanced upwelling in the cariaco basin caused an increase in the diatom population and therefore an increase in the relative abundance of cses derived from diatoms in contrast the dinoflagellate population and therefore cses derived from dinoflagellates decreased in response to the climate change during the younger dryas these community shifts agree well with the shifts observed in the present day on a seasonal basis that result from the north south migration of the intertropical convergence zone over the cariaco basin we also identify changes in the abundance of several cses that seem to reflect rapid warming and cooling events this study suggests that cses are useful proxies for reconstructing phytoplankton communities and paleoenvironments index terms 1055 geochemistry organic geochemistry 4267 oceanography general paleoceanography 4855 oceanography biological and chemical plankton keywords younger dryas cariaco basin chlorin steryl esters

large fluctuations of dissolved oxygen in the indian and pacific oceans during dansgaard oeschger oscillations caused by variations of north atlantic deep water subduction a s and s h were supported by the paleoclimate program of the national science foundation as part of the paleovar project atm 0602395

microboudin structures of piemontite along the sambagawa metamorphic belt japan implications for lateral variation of differential stress during regional metamorphism evidence is presented of a lateral variation in differential stress during metamorphism along a regional metamorphic belt on the basis of the proportion of microboudinaged piemontite grains p in a quartz matrix in metacherts it is proposed that p is a practical indicator of relative differential stress analysis of 123 metacherts from the 800 km long sambagawa metamorphic belt japan reveals that p values range from 0 01 to 0 7 in this region most samples from wakayama in the mid belt area have p values of 0 4 0 6 whereas those from western shikoku have p values of 0 1 this difference cannot be explained by variations in metamorphic temperature and is instead attributed to a regional lateral variation in differential stress during metamorphism

hemispherically asymmetric trade wind changes as signatures of past itcz shifts abstract the atmospheric hadley cells which meet at the intertropical convergence zone itcz play critical roles in transporting heat driving ocean circulation and supplying precipitation to the most heavily populated regions of the globe paleo reconstructions can provide concrete evidence of how these major features of the atmospheric circulation can change in response to climate perturbations while most such reconstructions have focused on itcz related rainfall here we show that trade wind proxies can document dynamical aspects of meridional itcz shifts theoretical expectations based on angular momentum constraints and results from freshwater hosing simulations with two different climate models predict that itcz shifts due to anomalous cooling of one hemisphere would be accompanied by a strengthening of the hadley cell and trade winds in the colder hemisphere with an opposite response in the warmer hemisphere this expectation of hemispherically asymmetric trade wind changes is confirmed by proxy data of coastal upwelling and windblown dust from the atlantic basin during heinrich stadials showing trade wind strengthening in the northern hemisphere and weakening in the southern hemisphere subtropics in concert with southward itcz shifts data from other basins show broadly similar patterns though improved constraints on past trade wind changes are needed outside the atlantic basin the asymmetric trade wind changes identified here suggest that itcz shifts are also marked by intensification of the ocean s wind driven subtropical cells in the cooler hemisphere and a weakening in the warmer hemisphere which induces cross equatorial oceanic heat transport into the colder hemisphere this response would be expected to prevent extreme meridional itcz shifts in response to asymmetric heating or cooling understanding trade wind changes and their coupling to cross equatorial ocean cells is key to better constraining itcz shifts and ocean and atmosphere dynamical changes in the past especially for regions and time periods for which few paleodata exist and also improves our understanding of what changes may occur in the future

sensitivity of the atlantic intertropical convergence zone to last glacial maximum boundary conditions 1 recent paleoproxy records suggest that the mean latitude of the atlantic intertropical convergence zone itcz varied synchronously with north atlantic climate over a range of timescales throughout the holocene and last glacial maximum we show that the present day meridional mode of atmosphere ocean variability in the tropical atlantic is a potentially useful model for understanding these paleoclimate changes the tropical atlantic in a coupled atmospheric general circulation and slab ocean model responds to last glacial maximum conditions with a southward displacement of the itcz this response arises primarily through the land ice sheet that forces increased north atlantic trades analogous to the forcing on the present day meridional mode changes to sea ice coverage and to ocean heat transport associated with a weakened atlantic thermohaline circulation also cause a meridional mode response though through different mechanisms our results highlight the potential for tropical atlantic paleoclimate to be driven from the high latitude influences in particular land ice on glacial interglacial timescales

geometry of torn boudin an indicator of relative viscosity abstract the present study determines the role of viscosity on the development of rectangular torn boudin and its various types defined by the curvature of their exterior and face margins numerical modeling was performed with the help of finite element method considering maxwell visco elastic materials in commercial code ansys seven different viscosities were used and interchanged among the boudin inter boudin and matrix materials to understand the effect of viscosity ratios specifically of relative viscosity of inter boudin material results show that the viscosity of inter boudin material has significant control on the shape of torn boudins apart from the viscosity ratio of boudin to matrix material bone shaped boudin develops only when the inter boudin is more competent than boudin and it becomes more prominent when matrix is also competent than boudin but incompetent than inter boudin when boudins are stiffer than inter boudin barrel shaped and fish head boudins with concave faces develop exterior or face margins remain almost straight when boudin is relatively rigid compared to its surrounding matrix materials or when there is no or very little viscosity contrast between boudin and inter boudin material even in case of large boudin matrix viscosity contrast therefore the relative viscosity among the boudin inter boudin and matrix materials can be estimated qualitatively by studying the shape of boudin in the field

on stress and strain in a continuous discontinuous shear zone undergoing simple shear and volume loss i summarise observations within a continuous discontinuous shear zone to discuss the local stress and strain conditions experienced within a mixed rheology shear zone undergoing volume loss and deformation approximating simple shear the chrystalls beach complex new zealand comprises phacoids formed from dismembered beds by layer parallel extension enclosed within a relatively incompetent matrix local extension is generally subparallel to the regional direction of shortening and overall it appears that layer parallel extension is a geometrical necessity in low angle shear zones where significant flattening occurs in response to simple shear accompanied by volume loss preferential stress loading of phacoids is predicted by fibre loading theory and the failure of phacoids by brittle fracture is thereby governed by fibre stresses transferred from the matrix the principal stress orientations in a phacoid are likely rotated relative to the matrix and either parallel or perpendicular to the phacoid matrix interface as preferential loading of phacoids decreases the stress level in the matrix an increased volume fraction of phacoids increases the strength of the shear zone as a whole however only small matrix volume fractions are required for the composite to act nearly as weak as the matrix 2012 elsevier ltd all rights reserved

the effect of orientation and shape distribution of gravel on slope angles in central taiwan abstract we compare gravel orientation and slope angle distribution to highlight their possible relationship with various gravel shapes in the ninety nine peaks area in central taiwan in the study area four gravel shapes disk equant bladed and roller are collected and studied disk and equant shapes comprise more than 65 of the gravel eighty five percent weight of gravel particles is composed of quartz sandstone sand particles only make up 14 of the matrix the matrix strength ranges from 100 to 250 kg cm2 the slope angle ranges from 60 to 70 and more than 50 of the study area consists of such slopes linear regression analysis shows that when the proportion of disk bladed and roller shapes in the gravel increases the slope angle increases although the linear relationship remains indistinct an important inverse relationship is that as the proportion of equant shapes in the gravel increases the slope angle clearly performs differently by decreasing the results of the investigation also show that the slope angle is lower when the imbrication direction of the gravel is parallel to the slope surface

fracturing of garnet crystals in anisotropic metamorphic rocks during uplift garnet crystals in granulite facies mylonites and gneisses from the morin shear zone in the grenville province quebec are characterized by pervasive closely spaced relatively straight tensile fractures aligned systematically normal to the mylonitic foliation and lineation the fractures developed preferentially in coarse grains 0 2 mm or in grains with a large aspect ratio 2 fractured segments of each garnet crystal have not been separated or filled with matrix minerals quartz and feldspar or with retrograde minerals biotite muscovite and chlorite this suggests that the garnets were fractured at shallow crustal depths ti300 c or 15 km for a thermal gradient zo c km we propose that the garnet fractures were formed by the response of anisotropic metamorphic rocks to a horizontal extension during uplift and cooling of the metamorphic terrane within the upper crust using a modified shear lag theory we can explain why tensile fracturing took place preferentially in stiff garnet rather than in felsic material and why the fractures are unequally spaced in a garnet grain our data suggest that the seouential tensile fractures of garnet grains record a progressive uplift process of metamorphic rocks within the upper crust 0 1997 elsevier science ltd

belemnites strain analysis and regional tectonics a critical appraisal abstract deformed belemnites offer great potential for estimating strain in deformed rocks there are two fundamental approaches 1 fragmentation and 2 reorientation of the belemnite guard the fragmentation approach is based on measurements of the original fragment and final fragment plus intervening gap belemnite lengths three variants exist standard ramsay strain reversal ferguson and modified standard hossain methods of which the second is the most accurate but also the slowest the third method is both simple and relatively accurate strain ellipse estimates are conventionally determined from the best fit envelope to plots of belemnite elongation 1 e against orientation badoux beach method but this demands a large number of belemnites per locality we describe an improved strain ellipse construction which requires only few belemnites the reorientation approach is based on strain induced shape modification of belemnite orientation distributions two variants exist the standard sanderson and modified standard lloyd methods but they are essentially similar we describe an adaption of the lloyd method which involves constructing distribution moment ratio plots contoured in terms of strain ratio and initial standard deviation the relative merits of the two approaches are considered using data from the rivet thrust sheet exposed in the la grave region of the external french alps we find that whereas the fragmentation approach yields relatively simple strain estimates true extensions and a strain ellipse the reorientation approach only provides an estimate of the in plane strain ratio if the exact shape of the belemnite orientation distribution before deformation is known the implications of these results are discussed in terms of local belemnite deformation histories and the evolution of the la grave thrust belt and the alpine orogeny

mechanisms for millennial scale global synchronization during the last glacial period global climate during the last glacial period was punctuated by abrupt warmings and occasional pulses of freshwater into the north atlantic that disrupted deepwater production these massive freshwater pulses known as heinrich events arose in part from instabilities within the laurentide ice sheet paleoevidence from the north atlantic suggests that these events altered the production of deep water and changed downstream climate throughout the northern hemisphere in the tropical western pacific sea surface temperatures and salinity varied together with ocean and climate changes at high latitudes here we present results from coupled modeling experiments that shed light on a possible dynamical link between the north atlantic ocean and the western tropical pacific this link involves a global oceanic standing wave pattern brought about by millennial scale glacial density variations in the north atlantic atmospheric teleconnections triggered by meridional sea surface temperature gradients and local air sea interactions furthermore our modeling results are compared with hydrological records from the cariaco basin the indian ocean the sulu sea and northern australia

loading configurations and driving mechanisms for joints based on the griffith energy balance concept abstract using the griffith energy balance concept to model joint propagation in the brittle crust two laboratory loading configurations serve as appropriate analogs for in situ conditions the dead weight load and the fixed grips load the distinction between these loading configurations is based largely on whether or not a loaded boundary moves as a joint grows during displacement of a loaded boundary the energy necessary for joint propagation comes from work by the dead weight i e a remote stress when the loaded boundary remains stationary as if held by rigid grips the energy for joint propagation develops upon release of elastic strain energy within the rock mass these two generic loading configurations serve as models for four common natural loading configurations a joint normal load a thermoelastic load a fluid load and an axial load each loading configuration triggers a different joint driving mechanism each of which is the release of energy through elastic strain and or work the four mechanisms for energy release are joint normal stretching elastic contraction poroelastic contraction under either a constant fluid drive or fluid decompression and axial shortening respectively geological circumstances favoring each of the joint driving mechanisms are as follows the release of work under joint normal stretching occurs whenever layer parallel extension keeps pace with slow or subcritical joint propagation under fixed grips a substantial crack normal tensile stress can accumulate by thermoelastic contraction until joint propagation is driven by the release of elastic strain energy within the earth the rate of joint propagation dictates which of these two driving mechanisms operates with faster propagation driven by release of strain energy like a dead weight load acting to separate the joint walls pore fluid exerts a traction on the interior of some joints joint propagation under fluid loading may be driven by a release of elastic strain energy and or work by the pore fluid during poroelastic contraction again propagation velocity dictates the driving mechanism with critical to supercritical joint propagation accompanying fluid decompression and subcritical crack propagation taking place under a constant fluid drive during axial shortening a jointing process called axial splitting may occur following wing crack growth from an initial crack tilted relative to the maximum compressive stress here the joint driving mechanism may be either the work of a remote stress i e a dead weight or the release of elastic strain energy i e fixed grips

millennial variability and long term changes of the diatom production in the eastern equatorial pacific during the last glacial cycle 1 the modern eastern equatorial pacific eep is a major natural source for atmospheric carbon dioxide and is thought to be connected to high latitude ocean dynamics by oceanic teleconnections on glacial interglacial timescales a wealth of sedimentary records aiming at reconstructing last quaternary changes in primary productivity and nutrient utilization have been devoted to understanding those linkages between the eep and other distant oceanic areas most of these records are however clustered in the pelagic eep cold tongue with comparatively little attention devoted to coastal areas here we present downcore measurements of the composition and concentration of the diatom assemblage together with opal biogenic silica concentration at site md02 2529 recovered in the coastal panama basin piston core md02 2529 collected in an area affected by a multitude of processes provides evidence for strong variations in diatom production at the millennial timescale during the last glacial cycle the maxima in total diatom concentration occurred during the early marine isotopic stage mis 4 as well as during the mis 4 3 transition and mis 3 rapid changes in diatom concentrations during the mis 3 mimics bond cycles as independently recorded by the sea surface salinity estimation derived from planktonic foraminifera from the same core such patterns indicate a clear linkage between diatom production in the coastal eep and rapid climate changes in the high latitude north atlantic in parallel the long term succession of the diatom community from coastal diatoms predominantly thriving during mis 5 and 4 toward pelagic diatoms dominant during mis 3 and 2 points to a long term change in the surface hydrology during heinrich events diatoms strongly reduced their production probably because of enhanced stratification in the upper water column after the last glacial maximum diatom production and valve preservation strongly decreased in response to the advection of nutrient depleted h2sio4 warmer water masses our high resolution record highlights how regional climatic processes can modulate rapid changes in siliceous primary production as triggered by wind induced local upwelling indicating that millennial climatic variability can overtake other prominent hydrological processes such as those related to silicic acid leakage

genesis of the qingyijiang river on the northern fringe of mt huangshan china based on a combined analysis of gravel fabrics and esr dates abstract the qingyijiang river which originates from the northern fringe of mt huangshan is an important tributary of the lower yangtze river a field investigation focused on the natural outcrops of the qingyijiang in the jingxian basin where a well preserved sequence of three gravel beds of fluvial terraces is present based on gravel fabric analysis and esr dating this paper discusses the genesis of the qingyijiang on the northern fringe of mt huangshan the main results show that 1 the diameter of the gravels is mainly medium and coarse showing that the hydrodynamic conditions were strong in the formation periods of the terrace gravel beds especially for the t3 terrace gravel bed 2 the gravel orientations in the gravels of the t3 and t2 terraces are 191 and 250 which are to the ssw and sww respectively and are similar to the present flow which is from southwest to northeast 3 the roundness of the gravels is mainly subrounded or rounded implying that these gravels were transported significant distances from their provenances 4 the terrace gravels contain mainly quartz sandstone sandstone vein quartz and quartzite which account for 90 of the rudaceous clasts in each terrace gravel which shows that there was no significant provenance change 5 the gravels of the t2 and t1 terraces are typical fluvial sediments whereas that of the t3 terrace is a product of superimposition of debris flow and fluvial deposits and 6 the qingyijiang originated from a braided river on an alluvial or pluvial fan and the timing of the qingyijiang can be traced back at least to 905 4 272 4 kyr ago when the yangtze cut through the three gorges

north atlantic thermohaline circulation during the past 20 000 years linked to high latitude surface temperature during a surface cooling event 10 000 to 12 000 years ago higher cd ca and lower 13c 12c ratios are observed in benthic foraminifera shells from rapidly accumulating western north atlantic sediments data from sediment cores show that marked nutrient depletion of intermediate waters occurs in association with reduced glacial north atlantic deep water flux it is proposed that cold high latitude sea surface temperatures enhance intermediate water formation at the expense of deep water formation

an empirical approach for estimating shear strength of decomposed granites in galicia spain decomposed granites are often found in galicia spain where they are referred to as jabres similar decomposed granites are also found in many other parts of the world granite weathering rarely produces an homogeneous altered rock mass with all material decomposed to the same degree or even a simple weathering profile where the degree of alteration decreases progressively with depth this paper presents an empirically based approach that allows estimation of the shear strength of this type of materials in terms of mohr coulomb cohesion and internal friction angle values the approach is based on back analysis of failed slopes in decomposed granites and it is implemented through simple in situ testing and classification of the weathered granite treating it as a soil material an advantage of the used approach is that it allows prediction of shear strength values at different locations within the deposit in a quick and economical way from which average values of shear strength parameters can be computed this is particularly important when considering the heterogeneous nature of decomposed granite formations in relation to the scale of the engineering work involved analysis of three slopes instability cases in jabres in galicia illustrating the approach are described in detail these correspond to a road cut a natural slope and the wall of an open pit mine

simulated changes in atmospheric dust in response to a heinrich stadial 1 heinrich stadials hs thought to be triggered by increased ice discharge in the high latitude north atlantic at glacial periods resulted in large freshwater forcing that weakened the atlantic meridional overturning circulation amoc these events are strongly expressed in paleoclimate records throughout the atlantic basin and much of the tropics compared to the last glacial maximum recent proxy data suggest hs were much drier and dustier in large parts of the tropics we use a global climate model coupled to a prognostic dust model to examine the response of dust to changes in climate during hs despite some significant changes in regional precipitation patterns in response to the simulated shift in the intertropical convergence zone we find that changes in winds have a larger effect on dust mobilization and deposition patterns than soil moisture or vegetation changes associated with a weaker amoc although europe was colder and drier during hs the annual mean glaciogenic dust emission rate is lower because a southward shift in the northern hemisphere jet stream leads to weaker winds and less dust mobilization the proximity of the westerly wind anomaly associated with the jet stream shift increases gustiness and dust mobilization over northwestern africa and shifts the african easterly jet southward resulting in less african dust transport and deposition over the north tropical atlantic drier conditions over the sahel region of north africa however do not lead to increased dust mobilization in our model when we perturb sahelian soil characteristics our results are in better agreement with proxies

a strain reversal method for estimating extension from fragmented rigid inclusions abstract in the standard method of strain analysis using fragmented rigid inclusions such as stretched belemnites extension is estimated by comparing the initial length of the inclusion with its final stretched length this approach fails to recognise that not all of the total extension is recorded by separation of the fragments even when cross fractures are present at the onset of deformation this paper presents an alternative approach in which the extensional strain is reversed the inter fragment gaps being incrementally closed until the inclusion is restored to its original state this allows an estimate of the unrecorded extension accommodated by inhomogeneous matrix flow near the inclusion as well as that recorded by fragment separation model experiments in which the fragmented inclusion is represented by wooden blocks set in a ductile matrix allow the strain reversal method to be compared with the standard method the mean percentage relative error is much smaller in the former 10 2 than in the latter 37 2 the main disadvantage of the strain reversal method is that it requires the collection of more data fragment lengths and gap lengths in correct sequence which must be processed by computer a short basic program is provided although there is no one to one correspondence between strain estimates derived from the two methods an approximate numerical transformation from one to the other may be possible one such transformation based on a study of natural boudinage has been applied to the experimental data and reduces the mean percentage relative error in the standard method estimates to 11 5

location of tensile fracture within rigid brittle inclusions in a ductile flowing matrix abstract the fundamental assumption in the fibre loading theory proposed by cox was used to develop a new theoretical model that predicts the location of tensile fracture within rigid brittle inclusions embedded in a continuous ductile flowing matrix both theoretical prediction and microstructural observations show that fracture location depends strongly on inclusion shape successive mid point fracturing occurs within cylindrical grains while the circular truncated conical grains sequentially fragment into two segments of unequal length the longer of which has the greater diameter

high resolution migration history of the subtropical high trade wind system of the northeastern pacific during the last 55 years implications for glacial atmospheric reorganization 1 guaymas basin gulf of california is a restricted basin located under the present day wet dry subtropical divide 27 n and is ideally circumstanced for detecting variations in the north pacific subtropical high npsh trade wind system controlled by climate cell boundary displacement npsh midwinter location was the primary influence on timing and intensity of upwelling seasons in guaymas basin analysis of high resolution x ray fluoresence data and sediment fabric log from 75 laminated core md02 2517 2515 western guaymas basin reveals systematic changes in npsh behavior over the last 55 kyr bp southward displacement of the wet dry subtropical divide controlled upwelling related diatom productivity while sea level and regional rainfall controlled terrigenous supply the basin was oxic during the glacial and preservation was ensured by high burial rate due to the increased deposition of terrigenous sediment sediment fabric style number and or thickness of laminae plus color banding and homogeneous intervals changes systematically through the core and gives insights into the number of seasons occurring in guaymas basin and the occurrence and intensity of the upwelling season five millennial scale low flux events with close timing to heinrich events and ten decadal centennial scale low biogenic silica events occurring in the interval 33 16 5000 years before present kyr bp are interpreted as times of extreme aridity at 16 5 kyr bp a regime shift from terrigenous dominated oxic to evenly balanced biogenic terrigenous dysoxic conditions occurred although there was a further extreme arid event at 11 5 kyr bp 16 5 kyr bp was essentially the beginning of the interglacial

asymmetrical pull aparts and foliation fish as kinematic indicators abstract from field observations asymmetrical boudins pinch and swell structures and ellipsoidal volumes of oblique foliation segments in anisotropic rocks lacking competent layers are potentially useful kinematic indicators in shear zones they are derived by the modification of initially symmetrical pull aparts and or layering by superposed layer parallel shearing the response of boudins to such shear is a synthetic flow of those corner volumes lying within the extensional quadrants of the kinematic framework of the deformation into the inter boudin gaps type 1 asymmetrical pull aparts the response of pinch and swell structure is a back rotation of the swells to form type 2 asymmetrical pull aparts discrete extensional shears in the pinches may be either absent type 2a or present type 2b analogues of types 2a and 2b asymmetrical pull aparts occur in anisotropic materials with no compositionally distinct competent members where ellipsoidal volumes contain back rotated foliation segments foliation fish the field based kinematic interpretation was successfully tested by laboratory experiment wherein initially symmetrical trains of pre formed competent internally anisotropic boudins and pinch and swell structures were deformed in bulk simple shear a mechanical model is presented in terms of two material properties of the inclusions competence viscosity and internal rheological anisotropy in an attempt to account for the observed flow in the experiments and the deduced flow and kinematic significance of the natural examples in the presence of angular corners and associated geometrical stress concentrations competence viscosity controls the response to shear in the boudins in the absence of angular corners internal anisotropy controls the response to shear in the pinch and swell structures and in the foliation fish

strength of two phase rocks a model based on fiber loading theory abstract in order to predict the strength of two phase rocks in terms of the component volume fraction the aspect ratio of the strong phase and the strength contrast between the strong and weak phases a simple model is developed according to a fiber loading theory modified for short fiber composites the analytical solution derived from elastic theory is found to hold in good approximation for rocks consisting of power law viscous minerals there is a good agreement between the predicted results and the experiments for two phase aggregates anhydrite halite calcite halite fe ag and clinopyroxene plagioclase the model is compared with several previous different models

contrast in stress strain history during exhumation between high and ultrahigh pressure metamorphic units in the western alps microboudinage analysis of piemontite in metacherts abstract our analyses of microboudinage structures of piemontite grains embedded within six samples of metachert one collected from an ultrahigh pressure uhp metamorphic unit at lago di cignana in italy of the western alps and the other five from surrounding high pressure hp metamorphic units in italy and france have revealed that the structures are all symmetrical in type and were presumably produced in coaxial strain fields stress strain analyses of the microboudinaged grains revealed significant contrasts in the stress and strain histories of the uhp and hp metamorphic units with the differential stress recorded by the uhp sample being unequivocally lower than that recorded by the five hp samples in addition our analyses showed that the uhp sample underwent stress relaxation during microboudinage whereas the five hp samples did not on the basis of these observations and analyses we discuss the mechanical decoupling of the uhp and hp units that led to different histories in differential stress between the units during exhumation of the western alps

triaxial stress state deep in orogenic belts an example from turkey abstract a rare metachert pebble containing amphibole grains with microboudin structures in a wide range of orientations provides an opportunity to perform stress analysis in two orthogonal orientations on the foliation surface the sample was analysed by the microboudin method to infer the triaxial stress state during microboudinage stress parameters proportional to the far field differential stress were determined for sodic amphibole grains in the two orientations the ratio of the stresses in the two orthogonal orientations 1 2 1 3 was calculated to be 0 64 indicating that 2 lies closer to the midpoint between 1 and 3 than to 3

refinements of shear lag model and its applications abstract since it was developed by cox cox h l 1952 the elasticity and strength of paper and other fibrous materials br j appl phys 3 72 79 the shear lag model slm or fiber loading theory has been widely used by material scientists and structural geologists as a powerful analytical method for analyzing mechanical interactions between hard and soft phases layers in composites and two phase rocks we found that several approximations in previous mechanical derivations of the model can be improved in terms of mechanical equilibrium and boundary conditions our refinements make the model more rigorous and elaborate applications of the improved model include 1 prediction of the axial tensile stress in strong inclusions embedded in a continuous weak matrix and consequently interpretation of extension fracture boundinage 2 modeling of the distribution of shear stresses in the weak matrix surrounding the strong inclusions and accordingly interpretation of the variations of dislocation density and of recrystallized grain size in composites and polyphase rocks and 3 prediction of mechanical properties of industrial composites and two phase rocks the analytical results of the improved model agree approximately with those obtained from both experimental investigations and numerical calculations

microboudin structure of piedmontite in quartz schists a proposal for a new indicator of relative palaeodifferential stress abstract this paper proposes a new method to infer the relative differential stress for the rock body bulk differential stress from microboundinage the method is applied to four piedmontite bearing quartz schists collected from the high pressure low temperature metamorphic belts in japan nuporomaporo yamagami asemi and matsunosako in the samples brittle piedmontite grains are broken and subsequently pulled apart within the flowing quartz matrix the method consists of 1 statistical measurement of the fraction of microboudinaged grains 2 theoretical prediction of this fraction as a function of bulk differential stress and 3 inference of bulk differential stress by comparing the measured with the theoretical fraction the theoretical prediction has two aspects one is the model connecting bulk differential stress acting on a whole rock quartz schist with the tensile stress generated in the competent material piedmontite and the other is the model for the fracture strength of the competent material we use a fibre loading theory for the stress and a weakest link theory for the fracture strength the theoretical analysis is found to explain successfully the measured data by a chi squared test and we can obtain relative estimates of bulk differential stress the b value they are 1 0 0 7 0 4 and 0 2 for nuporomaporo yamagami matsunosako and asemi respectively

a constitutive model for gravelly soils considering shear induced volumetric deformation this study elucidates the deformational behavior of gravelly soils by analyzing how hydrostatic pressure and pure shearing affect deformational behavior a series of drained triaxial compression tests have been performed using large specimens made of gravelly soils where the grain size distribution curve was based on the field condition the volumetric and shear deformations of gravelly soils have been determined by performing experiments with controlled stress paths hydrostatic pressure was applied first followed by pure shearing a simple and innovative constitutive model is also proposed the proposed model is characterized by the following features of gravelly soils i significant shear induced volumetric deformation prior to failure ii modulus stiffening under hydrostatic loading and degradation under shearing and iii stress induced anisotropy in the proposed model deformational moduli k and g vary according to the stress state the stiffening and degradation of these moduli result in d

interactions between environmental change and shallow water carbonate buildup along the northern tethyan margin and their impact on the early cretaceous carbon isotope record 1 the evolution of the early cretaceous northern tethyan carbonate platform was not only influenced by changes in sea level detrital influx and surface water temperature but also by changes in trophic levels we distinguish between phases of carbonate production dominated by oligotrophic photozoan communities and by mesotrophic and eventually colder water heterozoan communities superimposed on this bimodal trend in platform evolution were phases of platform demise for which we provide improved age control based on ammonite biostratigraphy the initial phase of these episodes of platform demise corresponds in time to episodes of oceanic anoxic events and environmental change in general on the basis of a comparison between the temporal changes in an early cretaceous ammonite calibrated d 13 c record from southeastern france and coeval changes in the platform record we suggest that the history of carbon fractionation along the northern tethyan margin was not only influenced by changes in the oceanic carbon cycle such as in the rate of production and preservation of organic and carbonate carbon and in the size of the oceanic dissolved inorganic carbon reservoir but it was also influenced by the above mentioned changes in the ecology and geometry of the adjacent carbonate platform phases of photozoan carbonate production induced positive trends in the hemipelagic carbonate d 13 c record phases of heterozoan carbonate production pushed the d 13 c system toward more negative values platform drowning episodes implied an initial increase in d 13 c values followed by a longerterm decrease in d 13 c values

successive mid point fracturing during microboudinage an estimate of the stress strain relation during a natural deformation abstract this paper describes a piedmontite microboudinage during plastic flow of surrounding quartz in a quartz schist collected from the high pressure low temperature sambagawa metamorphic belt in southwest japan nine microboudins originated from a single grain by successive mid point fracturing and subsequent pull apart such features strongly indicate a fibre loading mechanism for the microboudinage the strain history inferred by successive use of the strain reversal method and stress data quantitatively calculated by a formula of the fibre loading theory provide us with an estimate of the stress strain relation during the microboudinage it apparently proceeded with increasing bulk stress during the last stage of metamorphism

estimation of the shear strength of gravel deposits based on field investigated geological factors abstract a gravel deposit is a composite material composed of gravel and soil matrix the largest size of gravel in the study areas generally ranges from 100 mm to 300 mm the estimation of the shear strength of gravel deposits often requires large scale in situ direct shear tests or large scale triaxial tests in laboratory both kinds of the large scale tests are expensive and the triaxial tests using remolded specimens with reduced size gravel may lower estimate the shear strength of gravel deposits this study first estimates the shear strength of gravel deposits employing their topographic characteristics which are the envelopes of slope inclinations and slope heights each envelope corresponds to a strength parameter of a gravel deposit then field investigation is conducted to obtain geological factors such as the diameter in the grain size distribution curve corresponding to 50 finer d 50 gravel content and the unconfined compressive strength of matrix the strength parameter and the field investigated geological factors are correlated through linear regression the geological factor that most influences the shear strength of the studied gravel deposits is the unconfined compressive strength of matrix the regression model is tested using field investigated data obtained from other locations and the derived cohesion and friction angles are compared to the results in literature the study establishes an economic way to estimate the shear strength of gravel deposits in central taiwan however the idea is applicable to gravel deposits bimrock alluvium or colluvium elsewhere in the world in which large gravels or cobbles exist and shear strength cannot be estimated by conventional methods