mineral assemblage anomalies in the slip zone of the 1999 taiwan chi chi earthquake ultrafine particles preserved only in the latest slip zone we determined mineral assemblages of samples from the taiwan chelungpu fault and from milling and heating experiments by using x ray diffraction and scanning and transmission electron microscopy the fault system contains three dominant fault zones the shallowest of which slipped during the 1999 chi chi earthquake the quartz and clay mineral contents of the primary slip zone were low and it contained partly amorphous ultrafine particles several tens of nanometers up to 30 weight percent of materials in that zone could not be fit to standard diffraction patterns whereas nearly 100 weight percent of those in surrounding samples could be the unfitted component could be attributed to the observed ultrafine particles produced by comminution during the earthquake because weak diffraction intensities are caused from mineral lattice distortion granulation and amorphous coatings such particles are a potential proxy for identifying the slip zone of the most recent earthquake along a fault

production of nanoparticles during experimental deformation of smectite and implications for seismic slip abstract nanoparticles and amorphous materials are common constituents of the shallow sections of active faults understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets here we present the results of 59 rotary shear experiments conducted at room humidity conditions on gouge consisting of mixtures of smectite ca montmorillonite and quartz experiments with 60 50 25 0 wt ca montmorillonite were performed to investigate the influence of variable clay content on nanoparticle production and their influence on frictional processes all experiments were performed at a normal stress of 5 mpa slip rate of 0 0003 v 1 5 ms 1 and at a displacement of 3 m to monitor the development of fabric and the mineralogical changes during the experiments we investigated the deformed gouges using scanning and transmission electron microscopy combined with x ray powder diffraction quantitative phase analysis this integrated analytical approach reveals that at all slip rates and compositions the nanoparticles grain size of 10 50 nm are partly amorphous and result from cataclasis wear and mechanical solid state amorphization of smectite the maximum production of amorphous nanoparticle occurs in the intermediate slip rate range 0 0003 v 0 1 ms 1 at the highest frictional work and is associated to diffuse deformation and slip strengthening behavior instead the lowest production of amorphous nanoparticles occurs at co seismic slip rates v 1 3 ms 1 at the highest frictional power and is associated with strain and heat localization and slip weakening behavior our findings suggest that independently of the amount of smectite nanoparticles they produce fault weakening only when typical co seismic slip rates 0 1 ms 1 are achieved this implies that estimates of the fracture surface energy dissipated during earthquakes in natural faults might be extremely difficult to constrain

quantifying rates of syndepositional marine cementation in deeper platform environments new insight into a fundamental process abstract syndepositional marine cementation on carbonate platforms is a fundamental process that results in physical stabilization and lithification of sediments as well as reduction in primary porosity and permeability which may affect the migration of later diagenetic fluids rapid submarine cementation is commonly invoked to explain various aspects of carbonate platform geometry especially related to the preservation of steep slopes but the rates typically reported for cementation in deeper parts of the platform are mostly conjectural in an attempt to quantify the rates of syndepositional cementation in marginal platform environments substrates containing samples of carbonate sand ooids suspended above the sea floor were emplaced at various depths across a modern platform in the bahamas to evaluate the rates of cement growth through direct observation initial results indicate that cementation may lead to partial lithification of carbonate sands within a minimum of 8 months in water depths of up to 60 m these results provide some of the first direct evidence for rates at which cementation can occur in deeper marginal environments and confirms earlier hypotheses that very rapid syndepositional marine cementation may not be limited to very shallow subtidal and intertidal environments

diagenesis and sea level change lessons from moruroa french polynesia an inclined borehole through the margin of moruroa atoll french polynesia in which the ages of units are constrained by radiometric dates provides a unique opportunity to examine the relationship between sea level change and diagenetic history the oldest rocks encountered are dated at 355 840 yr bp and the overlying succession records five cycles of deposition and emergence culminating in the deposition of the holocene sequence successive intervals of emergence provided the driving force for diagenetic change promoting the through flow of chemically aggressive fresh waters that were at times either pervasive or confined within relatively isolated pathways on a millimetre to metre scale these locally varying flows and flow rates defined restricted zones characterized by differences in water chemistry and diagenetic signatures in contrast to some sequences elsewhere the upper part of the moruroa atoll succession is typified by alternations of intervals in which either several generations of cement growth are represented or only one or two cements are present three distinct diagenetic signatures can be recognized as follows i fresh water phreatic zones are reflected in neomorphism and the deposition of cements that occluded porosity thus preventing the deposition of later cements and conferring relative stability ii vadose zones are characterized by more complex diagenetic sequences reflecting multiple phases of dissolution and cement growth attesting to the relative duration of exposure and the conditions of ground water flow and iii deep in the core sequences reflect lowstand deposits that suffered sub sea diagenesis generating pores in which multi layered marine cements apparently formed at more frequent intervals than those subject to subaerial processes all cements appear to be of narrowly constrained ages these results highlight the contentious issue of whether the distributions of former diagenetic environments can be related to specific sea level stands and erosion surfaces it seems that in the moruroa core some cycles of sea level change are reflected in distinctive packages of diagenetic features that have not been overprinted by cement growths during later sea level stands

carbonate slopes and gravity deposits in this special volume a series of papers are presented that concentrate on sedimentation patterns observed on the slopes and within the basins surrounding shallow water carbonate depositional systems four papers discuss depositional patterns on the slope to basin transect of the cainozoic sedimentary system of the bahamas four other papers examine gravity deposits of jurassic and cretaceous carbonate depositional systems

characterization and interpretation of discontinuity surfaces in a jurassic ramp setting high atlas morocco discontinuity surfaces are widely recognized but often poorly understood features of epeiric carbonate settings in sedimentary systems these features often represent hiatus surfaces below biostratigraphic resolution and may represent a considerable portion of the time contained in the sediment record from an applied perspective discontinuities may represent horizontal flow barriers and result in reservoir compartmentalization here a total of 80 condensed surfaces s1 firmgrounds s2 and hardgrounds s3 from a jurassic middle and upper bajocian assoul formation ramp setting of the high atlas in morocco are carefully documented with respect to their morphology their secondary impregnation by fe and mn oxides and phosphates and their palaeoecological record a statistical frequency distribution of two surfaces of the s1 type 1 1 surfaces of the s2 type and 0 4 surfaces of the s3 type per 10 section metres is observed along a 220 m long carbonate succession based on two stratigraphically and spatially separated study windows and correlative sections the stratigraphic frequency distribution the lateral extent and the nature of facies change across discontinuities are documented in a quantitative manner specific features of the study site include the considerable stratigraphic thickness of the assoul formation and the conspicuous absence of subaerial exposure related features based on the data presented here firmground and hardground surfaces are best interpreted as maximum regression related features relative sea level lowstand results in a lowered wave base and wave orbitals and currents result in sea floor omission and lithification care must be taken to avoid overly simplistic interpretations as differences in bathymetry and carbonate facies result in marked changes in discontinuity characteristics in proximal distal transects the data shown here are of significance for those concerned with the interpretation of shoal water carbonate environments and are instrumental in the building of more realistic carbonate reservoir flow models

effect of carbonate platform morphology on syndepositional deformation insights from numerical modeling abstract we use finite element numerical modeling to show that carbonate platform morphology is a control on syndepositional deformation in steep walled carbonate platforms we simulate gravity application on three end member carbonate platform margin morphologies 1 a mixed planar concave up shaped shelf margin from tobacco cay belize 2 a concave up shaped system representing the capitan profile guadalupe mountains west texas and 3 a sigmoidal jurassic amellago ramp we model the platform material with a brittle failure criteria that captures tensile and shear failure we show that the presence of a vertical reef wall and thereby lack of lateral confining stress seaward leads to a tensile stress state in the middle of the shelf and the shelf edge promoting the development of opening mode mode i tensile fractures fractures occur in the absence of additional loading or burial indicating that their formation is consistent with a syndepositional setting overall our results demonstrate that carbonate platforms with a near vertical reef wall are routinely modified by syndepositional deformation and failure in the absence of compaction we show that zones of high tensile stress can result in brittle tensile failure and confirm that tensile failure is a critical element of building and may contribute to maintaining steep walled carbonate platform systems

quaternary slope development of the western leeward margin of the great bahama bank abstract to date work on the great bahama bank s western leeward margin has centred chiefly on seismic scale expressions of carbonate sequences and systems tracts however periplatform slope sediments also exhibit very well developed cyclicity on scales of decimetres to several metres it is these small scale high frequency cycles within the larger scale facies successions of the quaternary which form the main topic of this paper previous studies have shown that the small scale cycles correlate to the orbitally forced high frequency sea level changes therefore these cycles should indicate how sea level has affected the slope development and thus platform margin evolution during this period through detailed high resolution sequence stratigraphy of the great bahama bank s leeward margin obtained via 18o isotope and mineralogical xrd analyses confined by u th dating and nannofossil bioevents a greater understanding of the bedding geometries within the pleistocene holocene seismic sequences and clues as to the nature of the slope development has been achieved the high resolution seismic profiles indicate that since the plio pleistocene change in geometry in which the great bahama bank developed into a rimmed platform continued steepening and subsequent progradation of the leeward margin has typified slope development during the quaternary which is described as an accretionary slope however on the basis of our observations we conclude that only the early to lower middle pleistocene section isotope stages 45 20 and the holocene isotope stage 1 of the leeward margin is accretionary this indicates that a degree of erosion and or by passing has occurred on the leeward margin since the lower middle pleistocene isotope stage 19 during the first part of this period isotope stages 19 12 erosion and or by passing occurred in the middle to lower slope regions and toe of slope by the end of the upper middle to late pleistocene phase isotope stages 11 2 erosion also occurred on the upper slope this erosion by currents at the toe of slope and oversteepening of the upper and middle slopes have led to back cutting upslope and resulted in the progressive retreat of the toe of slope towards the platform to the east however the rise in sea level since the last glacial maximum to its present day level has allowed high productivity on the platform top during the holocene and the deposition of a thick sediment wedge on the slope and sedimentation across the entire leeward flanks this has led to the redevelopment of an accretionary slope and continued westward progradation of the great bahama bank s western leeward margin

forward modelling of carbonate platforms sedimentological and diagenetic constraints from an application to a flat topped greenhouse platform triassic southern alps italy abstract quantitative analysis of depositional systems using numerical models can provide insights into the interaction of sedimentological and diagenetic processes difficult to interpret from observation of the geological products alone numerical models can highlight the interactions between controlling parameters and generate best estimate simulations of case studies from the geological record different data sources e g outcrop data uniformitarian constraints paleogeography need to be considered in the process of defining the parameters required to produce models that honour individual case studies we present a best estimate forward model of a high relief triassic carbonate platform southern alps italy generated using carb3d a 3d process based forward modelling package able to simulate sedimentary facies geometries and early diagenesis of isolated carbonate platforms the parameters used for the presented forward model have been deduced by the study of architectural sedimentological and diagenetic features of the platform and by data from modern and ancient analogues sensitivity analysis is used to tune values of the parameters within ranges defined from field laboratory and uniformitarian constraints critical evaluation of the model allowed 1 verification of the validity of the used parameters 2 evaluation of the interaction between controlling parameters 3 development of a numerical model that can be used for quantitative elaborations furthermore the process of generating the numerical model required a detailed interpretation of field and laboratory data crucial to define the required input parameters modelling the studied carbonate platform also enabled evaluation of the effects of the interaction between productivity environmental energy and creation of accommodation space promoting the understanding of their role on the architecture of carbonate systems forward modelling also allows the integration of data from conventional data sets with data from present day observations and seismic geometries promoting a transition from a qualitative to a semi quantitative quantitative sedimentological approach forward models can further be used to parameterise petrophysical properties of complex geobodies with potential application for the management of georesources

clast cortex aggregates in experimental and natural calcite bearing fault zones abstract we investigated the formation mechanisms of rounded clast cortex aggregates a composite grain found in the slipping zones of faults hosted in calcite and clay rich rocks the natural aggregates contain a central clast commonly made of host rock fragments or reworked cataclasite from the slipping zone the central clasts are surrounded by an outer cortex of calcite or clay grains a few m or less in size in laboratory experiments on calcite gouges using two rotary shear apparatus we investigated the dependence of clast cortex aggregate formation on the applied slip rate normal stress total displacement and ambient humidity clast cortex aggregates formed at all investigated slip rates 100 m s to 1 m s but only at relatively low normal stresses 5 mpa the aggregates were better developed with increasing displacement up to 5 m and did not form in experiments with water dampened gouges in the experiments aggregates formed in low strain regions within the gouge layers adjacent to the highest strain slip zones we propose that clast cortex aggregates in calcite bearing slip zones form in the shallow portions of faults during shearing in relatively dry conditions but our experiments suggest that they cannot be used as indicators of seismic slip formation involves clast rotation due to granular flow accompanied by accretion of fine matrix material possibly facilitated by electrostatic forces

geomorphology of carbonate platform marginal uppermost slopes insights from a holocene analogue little bahama bank bahamas as the product of a variety of sediment sources and sedimentation and re sedimentation and erosion processes the geomorphology and sedimentology of carbonate slopes are highly variable the purpose of this study is to describe sub bottom profiles and side scan sonar multibeam and optical data acquired by an autonomous underwater vehicle to explore variability in geomorphological and sedimentological character of the present day platform marginal uppermost slope environments 240 m water depth on the north open ocean facing flank of little bahama bank bahamas although at time scales of greater than 100 ka this margin is progradational the data illustrate a complex juxtaposition of erosional and depositional processes and features erosion is evidenced by two prominent escarpments 70 m and 120 m that expose eroded bedded rocky outcrops these escarpments are interpreted to represent relict features related to past sea level positions although they still may be shedding debris aside from erosional remnants sedimentation and active transport is indicated by several features including active bedforms especially above the 70 m escarpment but ripples occur to depths of ca 200 m several mass transport complexes that overlie and cover the lower escarpment gravity flow deposits and rare slump features similarly a thick up to 20 m onlapping sediment wedge interpreted to be holocene in age suggests lateral accretion of the slope by more than 75 m in this period data illustrate that this open ocean margin is distinct from windward margins in the bahamas which typically include near vertical walls of erosion or bypass flanked downdip by rubble and talus and leeward margins which have onlapping muddy wedges but that lack marked terraces or escarpments collectively the results provide perspectives into the nature and controls on complex geomorphological patterns of erosion and deposition in holocene uppermost slope systems concepts potentially applicable to ancient analogues

can grain size sensitive flow lubricate faults during the initial stages of earthquake propagation recent friction experiments carried out under upper crustal p t conditions have shown that microstructures typical of high temperature creep develop in the slip zone of experimental faults these mechanisms are more commonly thought to control aseismic viscous flow and shear zone strength in the lower crust upper mantle in this study displacement controlled experiments have been performed on carbonate gouges at seismic slip rates 1 m s 1 to investigate whether they may also control the frictional strength of seismic faults at the higher strain rates attained in the brittle crust at relatively low displacements 900 c grain boundary sliding gbs deformation mechanisms the observed micro textures are strikingly similar to those predicted by theoretical studies and those observed during experiments on metals and fine grained carbonates where superplastic behaviour has been inferred to a first approximation the measured drop in strength is in agreement with our flow stress calculations suggesting that strain could be accommodated more efficiently by these mechanisms within the weaker bulk slip zone rather than by frictional sliding along the main slip surfaces in the slip zone frictionally induced grainsize sensitive gbs deformation mechanisms can thus account for the self lubrication and dynamic weakening of carbonate faults during earthquake propagation in nature

contour current imprints and contourite drifts in the bahamian archipelago new data collected along the slopes of little and great bahama bank and the abyssal plain of the bahama escarpment provides new insights about contour current related erosive structures and associated deposits the bahamian slope shows abundant evidence of bottom current activity such as furrows comet like structures sediment waves and drifts at a seismic scale large erosion surfaces and main periods of drift growth resulted from current acceleration related to plate tectonic processes and progressive opening and closure of gateways and long term palaeoclimate evolution at present day erosion features and contourite drifts are either related to relatively shallow currents 2500 m water depth it appears that the carbonate nature of the drifts does not impact the drift morphology at the resolution addressed in the present study classical drift morphologies defined in siliciclastic environments are found such as mounded plastered and separated drifts in core contourite sequences show a bi gradational trend that resembles classical contourite sequences in siliciclastic deposits showing a direct relationship with a change in current velocity at the sea floor however in a carbonate system the peak in grain size is associated with increased winnowing rather than increased sediment supply as in siliciclastic environments in addition the carbonate contourite sequence is usually thinner than in siliciclastics because of lower sediment supply rates little bahama bank and great bahama bank contourites contain open ocean input and slope derived debris from glacial episodes inner platform platform edge and open ocean pelagic input characterize the classical periplatform ooze during interglacials in all studied examples the drift composition depends on the sea floor topography surrounding the drift location and the type of sediment supply carbonate particles are derived from either the slope or the platform in slope and toe of slope drifts very deep contourites have distant siliciclastic sources of sediment supply the recent discovery of the importance of a large downslope gravitary system along bahamian slopes suggests frequent interactions between downslope and along slope contour currents processes the interlayering of mass flow deposits and contourites at a seismic scale or the presence of surface structures associated with both contour currents and mass flow processes shows that both processes act at the same location finally contour currents have an important impact on the repartition of deep water coral mounds currents can actively interact with mounds as a nutrient and oxygen supplier or have a passive interaction with mounds solely being obstacles orienting erosion and deposition

controls on carbonate platform architecture and reef recovery across the palaeozoic to mesozoic transition a high resolution analysis of the great bank of guizhou

the influence of variable progradation to aggradation ratio and facies partitioning on the development of syndepositional deformation in steep walled carbonate platforms abstract one of the remaining questions in carbonate geology and reservoir studies is the origin of steep walled carbonate platform margins and the role of gravity versus other structural processes in controlling the distribution of fracture networks and failure surfaces this study is an attempt to document the role of gravity interacting with stratigraphic architecture in controlling fracture patterns and margin collapse we employ finite discrete element models to illustrate that progradation aggradation p a ratio and facies tract partitioning in steep walled carbonate platforms affects the development distribution and intensity of syndepositional deformation all models are under the influence of only gravitational loading where pore pressure is held constant we utilize a modified mohr coulomb constitutive law with a rankine rotating crack tensile corner to capture both tensile and shear brittle failure our results illustrate that p a ratio affects the distribution and intensity of discrete fractures that form in steep walled carbonate platforms our results suggest that deformation is more extensive in a progradational carbonate platform where shelf edge angles are greatest and the clinoform is thickest alternatively aggradational carbonate platforms experience localized deformation in front of the antecedent shelf edge where the clinoform is thickest and steepest the introduction of mechanical heterogeneity associated with facies tract partitioning affects the intensity of fracturing in both progradational and aggradational models with the greatest number of fractures developing in reef facies in both scenarios development of brittle deformation under the sole application of gravity and lack of seaward lithostatic confinement is consistent with the syndepositional nature of deformation in these settings this work illustrates the interplay between carbonate platform geometry facies distribution and the resulting syndepositional deformation

controls on syndepositional fracture patterns devonian reef complexes canning basin western australia abstract syndepositional fractures are an important feature of high relief carbonate systems and exert a profound control on many facets of platform evolution and reservoir development based on data collected from the canning basin s devonian reef complexes this study characterizes syndepositional fracture patterns as a function of variations in lithofacies depositional position stratigraphic architecture and mechanical stratigraphy fracture parameters such as extension and fracture intensity are documented to vary strongly as a function of lithofacies the highest syndepositional extension values occurring in the microbial facies of the famennian platform margin with extension values three times higher than observed in equivalent frasnian strata position along the depositional profile exerts a strong control on fracture patterns with an approximate two fold increase in syndepositional extension and fracture intensity typically observed from the platform interior to the platform margin syndepositional fracture intensity is shown to vary systematically with changes in platform margin trajectory with high fracture intensities observed in strongly progradational platforms and decreased fracture development in aggradational and retrogradational platforms evidence for the temporal evolution of the mechanical stratigraphy of the devonian reef complexes is presented with early lithified strata effectively behaving as a single large scale 50 150 m mechanical unit during syndepositional fracture development while secondary fractures become increasingly affected by bed scale 0 25 5 m mechanical heterogeneity introduced by progressive diagenesis the results presented here potentially provide a tool for predicting fracture characteristics e g intensity orientation location and vertical extent from limited subsurface data and provide a method for characterizing syndepositional deformation in other systems

morphometric analysis of plunge pools and sediment wave fields along western great bahama bank abstract a systematic assessment of morphometric parameters of over 200 plunge pools along the steep escarpment base of western great bahama bank and 2000km2 of sediment wave fields of upslope migrating cyclic steps documents the importance of cascading density currents for sedimentation on the slope the abrupt downslope termination of the holocene mud wedge with the cyclic steps shows that strike parallel bottom currents interacting with the off bank currents are responsible for slope sediment distribution the plunge pools and sediment waves were generated by density cascading and both are related to a hydraulic jump caused by two gradient changes along the slope profile the bedforms highlight both the erosional and depositional nature of this current regime which is controlled by slope gradient and the directional confinement by either grooves on the steep escarpment or gullies on the middle to lower slope the 20 m deep plunge pools are ellipsoidal with an average width of 211 m and length of 283 m but variations between 80 360 m and 100 650 m for width and length occur they are caused by an average gradient break of 16 on the cemented upper slope gradient breaks of as low as 2 are reported to be sufficient to scour a plunge pool like the ones in the study area but higher gradient breaks between 10 and 20 are quite common for consistent plunge pool development a second gradient break further down slope changes flow conditions for the generation of cyclic steps with asymmetric waveforms of 4 m high average in large wave fields in the entire the study area orientation is mainly orthogonal to the main slope direction but wave lengths and sinuosity vary depending on the morphologic location of the wave field the comparison with sediment wave fields with similar geometries or in similar environments shows that flow reconstruction based on the wave height and wave length alone along with a characterization of sediment grain size however yields non unique results and is site specific

petrography and environmental controls on the formation of phanerozoic marine carbonate hardgrounds early marine seafloor lithification of carbonate sediments leads to the formation of hardgrounds and is known from rocks as old as the proterozoic hardground surfaces however are more commonly recorded in the phanerozoic sedimentological archive while ecological studies of hardground biota abound the environmental and physico chemical parameters leading to the development of seafloor lithification remain in many cases poorly understood and documented this paper reviews published evidence on the petrography mineralogy and geochemistry of phanerozoic carbonate hardgrounds within a process oriented context of their environmental controls hardgrounds typically develop in warm and shallow agitated tropical waters that are saturated with respect to caco3 but are also reported from hemi pelagic to bathyal realms and from cool water temperate settings a range of early marine cement types are documented from present day hardgrounds whereas preserved cement fabrics and related mineralogies are less diverse in early phanerozoic hardgrounds carbonate hardgrounds are widespread during calcite sea periods e g ordovician and cretaceous as opposed to some preservation bias ancient aragonite seas e g permian and triassic obviously the relation between seawater chemistry and hardground abundance and attributes is not an easy one here the concept of aragonite versus calcite seas serves as a first order template for the sake of a structured discussion whereas modern aragonite mode oceans document a significant diversity in spatial and bathymetric seawater properties this spatio temporal complexity is perhaps reflected in the scarce record of calcite sea hardgrounds from the devonian or paleogene rock record contrasted by the abundance in aragonite holocene and recent seas holocene hardgrounds allow for the direct assessment of rates of and reasons for early subaquatic lithification and in most cases escaped subsequent non marine diagenetic overprint conversely studies of ancient hardgrounds are often biased by early biological and mechanical erosion the degree of diagenetic overprint and tectonic to orogenic processes the formation distribution and physical properties of hardgrounds depend on the global climatic context greenhouse icehouse modes on sea level changes on ocean stratification on the spatial extent of epicontinental seas and carbonate depositional environments and on the spatially different mineralogies and rates of carbonate production the fabrics and mineralogies of early hardgrounds cement seem to substantially hinge on variations in atmospheric co2 thus the application of lessons from modern to ancient hardground case examples is not straightforward a holistic model explaining the full variability of controls on submarine hardground formation and their cement petrography throughout the phanerozoic is as yet lacking a more critical view on secular changes in hardground formation patterns is clearly needed future work should rely on the combination of sedimentological stratigraphic palaeoecological petrographic and geochemical approaches specific emphasis should be given to time intervals triassic carboniferous cambrian etc where the bulk of reported case studies is at best scarce

importance of mechanochemical effects on fault slip behavior during earthquakes 1 recent analyses of natural faults and laboratory rock friction experiments have revealed that physical transformations and chemical reactions of minerals and fluids occur in faults during earthquakes however exactly how these physicochemical processes affect fault weakening and earthquake energetics is not well understood we present quantitative experimental evidence of mechanochemical changes in a clay mineral illite during frictional slip and evaluate the effect of those changes on slip behavior friction distorts the crystal structure of illite and dramatically decreases the activation energy ea of the dehydroxylation reaction numerical modeling shows that the lower ea allows the dehydroxylation to occur at relatively low temperatures 200 300 c which in turn drastically reduces fault strength by pressurization of released water from the mineral thus dynamic thermo chemo mechanical processes can strongly affect earthquake instability especially along clay rich faults such as those in the shallow part of subduction zone plate boundaries

phase transformation and nanometric flow cause extreme weakening during fault slip faults weaken during earthquakes laboratory simulations of earthquake rupture show that the nanometric scale fault gouge created during slip is inherently weak and flows by grain boundary sliding providing a mechanism to weaken faults

syndepositional fractures and architecture of the lastoni di formin carbonate platform insights from virtual outcrop models and field studies abstract the recent rapid improvement of unmanned aerial vehicles together with advances in photogrammetry and structure from motion techniques have enhanced the role of digital outcrop models in many field of geology due to the possibility to obtain quantitative information from large and inaccessible areas in this study we integrated digital outcrop modeling techniques and field survey to investigate the architecture of the middle triassic platform of lastoni di formin italian dolomites the research highlighted the presence of two superimposed carbonate bodies the lower unit cassian i is dominated by low angle clinoforms dipping north northeastward and prograding over the basinal san cassiano fm the upper unit cassian ii is characterized by a thick sequence of peritidal cycles connected northward to another generation of clinoforms the inner platform beds of the upper unit display a lateral thickening that is particularly evident near the shelf break and that has been interpreted as due to the increased subsidence and the consequent down to basin tilting of the outermost part of the platform moreover the structural analysis performed on the digital outcrop models and supported by field observations highlighted the presence of an early generation of faults and joints that indicate an early gravitational deformation of the buildup possibly caused by the platform progradation and compaction induced subsidence of the san cassiano basinal deposits these wnw ese synsedimentary structures are formed by normal faults and extensional joints that are oriented nearly perpendicular to the direction of progradation of the carbonate platform

opaque minerals in chondrules and fine grained chondrule rims in the bishunpur ll3 1 chondrite we present a detailed petrographic and electron microprobe study of metal grains and related opaque minerals in the chondrule interiors and rims of the bishunpur ll3 1 ordinary chondrite there are distinct differences between metal grains that are completely encased in chondrule interiors and those that have some portion of their surface exposed outside of the chondrule boundary even though the two types of metal grains can be separated by only a few microns metal grains in chondrule interiors exhibit minor alteration in the form of oxidized p cr and si bearing minerals metal grains at chondrule boundaries and in chondrule rims are extensively altered into troilite and fayalite the results of this study suggest that many metal grains in bishunpur reacted with a type i chondrule melt and incorporated significant amounts of p cr and si as the system cooled some metal oxidation occurred in the chondrule interior producing metal associated phosphate chromite and silica metal that migrated to chondrule boundaries experienced extensive corrosion as a result of exposure to the external atmosphere present during chondrule formation it appears that chondrule derived metal and its corrosion products were incorporated into the fine grained rims that surround many type i chondrules contributing to their fe rich compositions we propose that these fine grained rims formed by a combination of corrosion of metal expelled from the chondrule interior and accretion of fine grained mineral fragments and microchondrules

microfacies and diagenesis of older pleistocene pre last glacial maximum reef deposits great barrier reef australia iodp expedition 325 a quantitative approach during integrated ocean drilling program expedition 325 34 holes were drilled along five transects in front of the great barrier reef of australia penetrating some 700 m of late pleistocene reef deposits post glacial largely 20 to 10 kyr bp in water depths of 42 to 127 m in seven holes drilled in water depths of 42 to 92 m on three transects older pleistocene older than last glacial maximum 20 kyr bp reef deposits were recovered from lower core sections in this study facies diagenetic features mineralogy and stable isotope geochemistry of 100 samples from six of the latter holes were investigated and quantified lithologies are dominated by grain supported textures and were to a large part deposited in high energy reef or reef slope environments quantitative analyses allow 11 microfacies to be defined including mixed skeletal packstone and grainstone mudstone wackestone coral packstone coral grainstone coralline algal grainstone coral algal packstone coralline algal packstone halimeda grainstone microbialite and caliche microbialites that are common in cavities of younger post glacial deposits are rare in pre last glacial maximum core sections possibly due to a lack of open framework suitable for colonization by microbes in pre last glacial maximum deposits of holes m0032a and m0033a 20 kyr bp marine diagenetic features are dominant samples consist largely of aragonite and high magnesium calcite holes m0042a and m0057a which contain the oldest rocks 169 kyr bp are characterized by meteoric diagenesis and samples mostly consist of low magnesium calcite holes m0042a m0055a and m0056a 30 kyr bp and a horizon in the upper part of hole m0057a contain both marine and meteoric diagenetic features however only one change from marine to meteoric pore water is recorded in contrast with the changes in diagenetic environment that might be inferred from the sea level history values of stable isotopes of oxygen and carbon are consistent with these findings samples from holes m0032a and m0033a reflect largely positive values 18o 1 to 1 and 13c 1 to 4 whereas those from holes m0042a and m0057a are negative 18o 4 to 2 and 13c 8 to 2 holes m0055a and m0056a provide intermediate values with slightly positive 13c and negative 18o values the type and intensity of meteroric diagenesis appears to have been controlled both by age and depth i e the time available for diagenetic alteration and reflects the relation between reef deposition and sea level change

origin of atoll lagoons a database of 301 atolls from the pacific indian and atlantic oceans has been analyzed with respect to factors governing maximum atoll lagoon depth statistically significant correlations between maximum atoll lagoon depth and both atoll area and present day rainfall are viewed as the combined effect of paleorainfall precipitation and catchment area in contributing to overall atoll morphology this interpretation is supported by the gross saucer shaped morphology of several of the lau group of the fiji islands and the subsurface cretaceous golden lane atoll of mexico where evidence of reef rim construction is lacking but evidence for significant solution relief is compelling the contribution of reefs to atoll rim construction appears to be limited generally to 10 m leaving more than 20 m of relief to be explained at most atolls at a number of these the last interglacial highstand surface is 15 20 m beneath holocene rim sediments subsidence rates of even 5 cm k y do not suffice to explain the subsea depth of this unconformity suggesting the dominating influence of solution on relief expression calculations of solution rates relative to the residence time of sea level below given depths during the past 700 k y suggest that the observed atoll relief is in part inherited from more than one pleistocene or perhaps earlier glacial stage whatever the precise time of origin the data available strongly suggest that atoll morphology is solution determined rather than growth predicated

facies stacking patterns in high frequency sequences influenced by long term sea level change on a permian panthalassan oceanic atoll an example from the akiyoshi limestone sw japan abstract detailed core examination of middle permian guadalupian lagoonal successions of mid panthalassan atoll carbonates akiyoshi limestone revealed distinct stratigraphic variation of facies and sequence stacking patterns the long term sea level change reconstructed from the atoll stratigraphy of the akiyoshi includes a bolorian to midian gentle sea level fall punctuated by a short lived murgabian sea level rise these sea levels can be regarded as eustasy judging from the monotonous subsidence of the akiyoshi atoll and its good correspondence with the sea level curves of other areas two sequence types are defined sequence type a consisting of shoal to lagoon grainstone occurred in low accommodation during the sea level highstand to falling stages of the late murgabian to midian this sequence type is bounded by two successive subaerial exposure surfaces and exhibits the uneven but generally low 13c values throughout the section in contrast sequence type b occurred in increasing accommodation during the sea level rise of the early murgabian this sequence type represents a composite sequence comprising parasequences and consists of various types of facies such as peritidal lime mudstone lagoonal packstone and boundstone and shoal grainstone sequence type b is characterized by consistently high 13c values except for the negative shift immediately below the sequence top surface these lines of evidence demonstrated that long term eustatic sea level change obviously controls the variations of sequence types in atoll carbonates

amorphous material in safod core samples san andreas fault evidence for crush origin pseudotachylytes 1 several types of amorphous material in ultracataclastic core samples recovered from 3194 m and 3294 m depth of the main bore hole of the san andreas fault observatory at depth are identified and described with transmission electron microscopy and scanning electron microscopy we observed 1 amorphous material on a slickenside surface 2 glassy bands contained in an ultracataclastic matrix and 3 amorphous rims surrounding quartz or feldspar clasts chemical analyses of the amorphous material reveal that silica content is slightly enriched or similar as in the adjacent matrix we suggest that all amorphous material was formed by comminution of clasts crush origin pseudotachylytes rather than by melting melt origin pseudotachylytes the observed amorphous phases may act as lubricating layers that reduce friction in the san andreas fault

the great barrier reef a 700 000 year diagenetic history a deep borehole through ribbon reef 5 in the great barrier reef off north eastern australia has identified a variety of cements including epitaxial radial prismatic and spherular aragonite together with blocky prismatic and fibrous calcite these cements are discontinuously arranged within the sequence that consists predominantly of grainstones but locally includes clotted muddy and filamentous textures that may be of microbial origin calcite cements vary in morphology with groups of crystals that include acute scalenohedral rhombohedral and flattened concordant terminations these show varying densities of inclusions that locally define growth zones and in some terminations divide in the manner of split crystals to form fibrous fringes morphological changes in calcite are inferred to reflect changes in water chemistry and crystal growth rates at the time of growth allied to their relationship to the palaeo water table and linked in turn to changes in sea level neomorphism and dissolution are widespread and variations in the severity of both imply response to the degree of undersaturation of pore waters that at times were probably balanced within very narrow limits a total of 10 depositional units are identified those units at the base of the borehole reflect deposition and diagenesis within a marine environment the influence of meteoric waters indicated by stable isotopes is first apparent at the top of unit 1 and in unit 2 184 to 155 m below sea floor petrographic evidence of vadose conditions appears at the tops of unit 3 131 to 99 m below sea floor units 4 to 8 all deposited under marine conditions provide isotopic evidence of meteoric or mixing zone waters and petrographic indicators of vadose conditions typically at the top of the units evidence indicates that in unit 5 the water table was mobile and units 6a 6b 7 and 8 all characterized by ultraviolet fluorescent cements are capped by sub aerial erosion surfaces unit 9 the holocene reflects the recent re establishment of marine conditions the extent of alteration of the entire sequence reflects the substantial and pervasive influence of meteoric waters this effect is interpreted as a result of a greater rainfall and river flow from the mainland during early and late stages of interstadial periods the study reflects progress in the ability to recognize the diagenetic signal generated by sea level change however whereas the isotopic results reflect the changing relationships between vadose and phreatic zones in groundwater systems beneath successive emergent surfaces their correspondence with petrographic features is expressed only weakly and commonly lacks the systematic sequential overprinting implied by the distribution of cathodoluminescent zones of cements in many ancient limestones