intermediate depth earthquakes facilitated by eclogitization related stresses eclogitization of the basaltic and gabbroic layer in the oceanic crust involves a volume reduction of 10 15 one consequence of the negative volume change is the formation of a paired stress field as a result of strain compatibility across the reaction front here we use waveform analysis of a tiny seismic cluster in the lower crust of the downgoing pacific plate and reveal new evidence in favor of this mechanism tensional earthquakes lying 1 km above compressional earthquakes and earthquakes with highly similar waveforms lying on well defined planes with complementary rupture areas the tensional stress is interpreted to be caused by the dimensional mismatch between crust transformed to eclogite and underlying untransformed crust and the earthquakes are probably facilitated by reactivation of fossil faults extant in the subducting plate these observations provide seismic evidence for the role of volume change related stresses and possibly fluid related embrittlement as viable processes for nucleating earthquakes in downgoing oceanic lithosphere

palaeomagnetism of devonian ring complexes from the bayuda desert sudan new constraints on the apparent polar wander path for gondwanaland summary the approach of gondwana and or microplates derived from its northern margins towards laurentia and the closure of the intervening palaeo tethys is a controlling factor in the geodynamic evolution of the atlantic bordering continents during the early and mid palaeozoic however the currently available palaeomagnetic data for gondwana especially for the geodynamically critical silurian to devonian interval are still rather sparse and are compatible with a number of palaeogeographic scenarios in order to address this issue 114 samples 18 sites have been collected from a series of high level acidic intrusive rocks rhyolitic lavas and tuffs of mid devonian age 377 5 ma rb sr along the southern escarpment of the gilif hills sudan 17 83 n 32 67 e detailed demagnetization experiments revealed the presence of three directionally distinct components of magnetization labelled a b and c the overall mean direction of component a dec 0 9 inc 29 8 95 4 6 k 91 1 n 12 sites is indistinguishable from either the direction of the present field or the geocentric axial dipole field in northern sudan mixed polarities suggest a recent or pleistocene age of this component component b also displays mixed polarities the site mean directions for this component average to 6 5 declination 40 2 inclination with 95 7 0 and k 48 1 based on 10 sites and yield a palaeomagnetic pole at 48 8 s 23 5 e dp 5 1 1 dm 8 4 maghaemite and haematite have been identified to be the predominant magnetic phases in samples which carry component a and or b both a and b are the probable results of low temperature oxidation of magnetite although the age of component b can not be directly determined it is inferred by correlation with published data to reflect a late carboniferous overprint component c identified in 11 sites 39 samples is characterized by a very steep downward magnetization dec 296 2 inc 79 3 95 10 8 k 18 7 n 11 sites which corresponds to a palaeomagnetic pole position in the southwestern corner of libya 25 9 n 11 6 e dp 19 6 dm 20 6 maximum blocking temperatures of component c generally below 580 c are indicative of magnetite as carrier of this remanence the results of thermal remanent magnetization experiments demonstrate that component c is a thermo remanent magnetization probably primary and suggest this component to be representative for the direction of the earth s magnetic field during emplacement of the gilif hills volcanic rocks in mid devonian times the position of the resulting palaeomagnetic pole lends new support to geodynamic scenarios advocating the existence of a wide approx 5000 km pre hercynian ocean during devonian times the simplest reconciliation of component b with the reference apparent polar wander path is achieved by assuming it to be of carboniferous age and by invoking a westward cusp in the carboniferous part of the apparent polar wander path taken at face value the age of this cusp coincides with large scale post orogenic right lateral strike slip movements in hercynian europe and north africa this interpretation does not require the apparent polar wander apw loop and very high rates of plate motion implied by some alternative models however if it is valid it will require reinterpretation of the remanence age of one west african intrusive suite and of some eastern australian palaeozoic rocks

morphology of the subducting slab boundary in the northeastern japan arc abstract the detection and location of the subducting slab boundary is crucial for a better understanding of the mechanical coupling between the subducting oceanic plate and the overlying continental plate and of the occurrence of large and destructive earthquakes on the main thrust zone in the forearc region of subduction zones it is also important for the understanding of the dehydration of the subducting slab and the genesis of island arc magmatism and volcanism in this study we have estimated the morphology of the upper surface of the subducting pacific plate under northeastern ne japan by using 1372 sp converted waves at the slab boundary and 666 earthquakes beneath the pacific ocean that are well located with sp depth phases we found that the pacific slab beneath ne japan subducts at a small dip angle of less than 10 for the first descent to about 20 km depth and has a steeper dip angle of 15 25 at 30 50 km depth and about 30 at deeper areas the slab depth is approximately 50 km under the pacific coast 150 km under the japan sea coast and nearly 100 km beneath the volcanic front at 60 150 km depths earthquakes in the lower plane of the double seismic zone occur in the central part of the slab while the upper plane events occur within 10 15 km beneath the slab boundary there is a change in the slab surface geometry around 38 n which corresponds to the landward extension of an oceanic fracture zone the strike of the slab is nne in the north and becomes ne in the south the southern part of the slab has a steeper dip angle than the northern part we believe that these features are related to the different patterns of interplate seismicity and mechanical coupling between the northern and southern parts of ne japan many great m8 0 8 5 interplate earthquakes have occured in northern ne japan during recorded history 830 1995 and microearthquakes show a wide distribution in the forearc region from the japan trench to the pacific coast suggesting strong interplate seismic coupling in that region in contrast only a few m7 0 7 5 class earthquakes have occurred in southern ne japan in the same period and microearthquakes tend to cluster in many small areas suggesting weaker interplate coupling there the subducted slab may be segmented by the oceanic fracture zones and the degree of seismic coupling changes from segment to segment the behavior of the segment in southern ne japan might be affected by the subducted seamounts so that the slab subducts at a steeper dip angle this interpretation is supported by geomagnetic topographic and geological studies which show that many small sized seamounts were subducted in the past from the southern part of the japan trench but almost no seamounts were subducted beneath the northern part

competitive hydration and dehydration at olivine quartz boundary revealed by hydrothermal experiments implications for silica metasomatism at the crust mantle boundary abstract serpentinization occurs via interactions between mantle peridotite and water that commonly passes through the crust given that such a fluid has a high silica activity compared with mantle peridotite it is thought that serpentinization and silica metasomatism occur simultaneously at the crust mantle boundary in this study we conducted hydrothermal experiments in the olivine ol quartz qtz h 2 o system at 250 c and vapor saturated pressure under highly alkaline conditions naoh aq ph 13 8 at 25 c to clarify the mechanism of silica metasomatism at the crust mantle boundary composite powders consisting of a qtz layer and an ol layer were set in tube in tube vessels after the experiments the extents of serpentinization and metasomatic reactions were evaluated as a function of distance from the ol qtz boundary the mineralogy of the reaction products in the ol hosted region changed with increasing distance from the ol qtz boundary from smectite serpentine smc zone to serpentine brucite magnetite brc zone olivine hydration proceeded in both zones but the total h 2 o content in the products was greater in the brc zone than in the smc zone mass balance calculations revealed that olivine hydration occurred without any supply of silica in the brucite zone in contrast the smc zone was formed by silica metasomatism via competitive hydration and dehydration reactions in the smc zone smectite formed via the simultaneous progress of olivine hydration and serpentine dehydration and around the boundary of the smc and brc zones serpentine formation occurred by olivine hydration and brucite dehydration the relative extent of hydration and dehydration reactions controlled the along tube variation in the rate of h 2 o production consumption and the rate of volume increase our findings suggest that the competitive progress of serpentinization and silica metasomatic reactions would cause fluctuations in pore fluid pressure possibly affecting the mechanical behavior of the crust mantle boundary

monitoring velocity variations in the crust using earthquake doublets an application to the calaveras fault california we present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates we compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal a moving window analysis of the entire seismograms including the coda gives t the difference in arrival times versus running time along the seismogram the time resolution of the method is an order of magnitude better than the digitization interval the t technique is illustrated with a pair of microearthquakes m 1 7 and 2 0 that occurred before and after the coyote lake california earthquake m 5 9 of august 6 1979 and on the same segment of the calaveras fault that ruptured during the earthquake the coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet so that its seismogram appears as a stretched version of the earlier event we interpret this systematic variation in t along the coda as a change in the average s velocity in the upper crust in the time interval between the two doublets s wave velocities appear to have decreased by 0 2 in an oblong region 5 10 km in radius at the south end of the aftershock zone

paleozoic and mesozoic geological history of northeastern africa based upon new interpretation of nubian strata 1 the nubia terrain in southern egypt and northern sudan is the type area for strata of mainly cretaceous age because nubian sandstone or nubian formation or similar terms were used for strata in other areas and of other stratigraphic positions the name nubian has become meaningless this paper summarizes results of field work done in egypt and sudan since 1976 reconstruction of the tectonics and paleogeography of northeast africa since the early paleozoic permits division of all strata related broadly to the nubian into three cycles the paleozoic cycle cambrian to early carboniferous is mainly marine this cycle was interrupted by the collision of gondwana with northern continents the resultant karoo cycle representing purely continental sedimentation exte ded from the late cretaceous until the early jurassic the disintegration of pangea led to a third cycle that began in the late jurassic and resulted in the marginal marine to continental strata of the nubian cycle that ranges in age from late jurassic to latest cretaceous

physical characterization of explosive volcanic eruptions based on tephra deposits propagation of uncertainties and sensitivity analysis abstract regardless of the recent advances in geophysical monitoring and real time quantitative observations of explosive volcanic eruptions the characterization of tephra deposits remains one of the largest sources of information on eruption source parameters esps i e plume height erupted volume mass mass eruption rate mer eruption duration total grain size distribution tgsd esps are crucial for the characterization of volcanic systems and for the compilation of comprehensive hazard scenarios but are naturally associated with various degrees of uncertainties that are traditionally not well quantified recent studies have highlighted the uncertainties associated with the estimation of esps mostly related to i the intrinsic variability of the natural system ii the observational error and iii the strategies used to determine physical parameters here we review recent studies focused on the characterization of these uncertainties and we present a sensitivity analysis for the determination of esps and a systematic investigation to quantify the propagation of uncertainty applied to two case studies in particular we highlight the dependence of esps on specific observations used as input parameters i e diameter of the largest clasts thickness measurements area of isopach contours deposit density downwind and crosswind range of isopleth maps and empirical constants and wind speed for the determination of mer the highest uncertainty is associated to the estimation of mer and eruption duration and is related to the determination of crosswind range of isopleth maps and the empirical constants used in the empirical parameterization relating mer and plume height given the exponential nature of the relation between mer and plume height the propagation of uncertainty is not symmetrical and both an underestimation of the empirical constant and an overestimation of plume height have the highest impact on the final outcome a 20 uncertainty on thickness measurements area of isopach contours integration limit for the power law fit and deposit density result in esp uncertainties 20 for plume height and erupted volume mass finally a third case study has also been used to explore the sensitivity of the voronoi tessellation strategy for the determination of tgsd and the inversion on both mass area and grain size data for the determination of erupted mass and plume height results confirm the validity of the methods but also the strong dependence on the distribution and number of observations

seismic attenuation beneath northeastern japan constraints on mantle dynamics and arc magmatism we apply a three step approach to estimate three dimensional 3 d p wave attenuation qp 1 structure beneath northeastern japan first corner frequencies of earthquakes are determined using the spectral ratio method for s coda waves then whole path attenuation terms t and site amplification factors are simultaneously estimated by a joint inversion the set of t is finally inverted for 3 d attenuation structure the results show that the mantle wedge has low attenuation in the fore arc and high attenuation in the back arc a depth profile of qp 1 in the back arc mantle is explained by attenuation expected for a two dimensional 2 d thermal model with qp qs 2 and grain sizes of 1 and 3 cm however an inclined high attenuation zone observed in the back arc mantle wedge which is interpreted as an upwelling flow shows higher attenuation than that calculated from the 2 d thermal model the higher seismic attenuation is probably caused by the concentration of partial melt in the upwelling flow a combined interpretation of seismic attenuation and velocity structures further suggests that the degree of partial melt in the upwelling flow varies along the arc and that volcanoes are clustered transverse to the arc below which the upwelling flow contains a higher degree of melt these observations indicate that magmatism is controlled by a mantle wedge process that depends strongly on spatial variations in the degree of partial melt in the upwelling flow our results further imply the breakdown of hydrous minerals in a hydrous layer above the pacific plate at a depth of 120 km

geochemical characteristics of organic rich shale upper yangtze basin implications for the late ordovician early silurian orogeny in south china abstract the tectonic sedimentary environment of the upper yangtze basin in china underwent significant changes from the late ordovician to early silurian the widely distributed shale formed during this interval has recorded key information about paleo ocean conditions analysis of major trace and rare earth elements was conducted on 135 shale samples collected from five wells in the study area the covariation of the mo u enrichment factors indicates that the upper yangtze basin in the wufeng stage was characterized by sedimentary differentiation the southern basin was dominated by a reducing bottom water environment under euxinic conditions the northern and central basins were dominated by a suboxic anoxic environment up to the early longmaxi stage the upper yangtze basin formed a relatively uniform sedimentary environment with gradually increasing oxygen content in the bottom water the paleoproductivity index indicates that marine organisms flourished in the wufeng to early longmaxi stages making an important contribution to the siliceous content in organic rich shale subsequently terrestrial siliciclastics gradually dominated analysis of paleogeography rare earth elements and weathering indicates that chuanzhong and qianzhong uplifts in the wufeng stage were the provenance area of the upper yangtze basin with a relatively high weathering degree weathering decreased in the beginning of the longmaxi stage and then gradually increased with the xuefeng uplift becoming the main provenance areas after the early longmaxi stage controlled by stress transfer from the continuous syn caledonian kwangsian orogeny in eastern south china the upper yangtze basin and xiangzhong basin were squeezed and pushed up successively the uplift separated the open shallow sea and formed a relatively limited deep water shelf environment resulting in a relatively euxinic bottom water environment which was favorable for organic matter accumulation the tectonic sedimentary evolution of the upper yangtze basin in the late ordovician to early silurian proposed in this study indicates that the formation of organic enriched shale was not only related to glacio eustatic change but also strongly controlled by tectonic activities in south china

the paleozoic petroleum geology of central arabia exploratory drilling in central saudi arabia has established significant paleozoic petroleum potential in this region in recent discoveries the permian interval of the unayzah formation has been identified as the primary oil and gas reservoir the unayzah formation consists of alluvial and fluvial to shallow marine siliciclastic sandstones and siltstones that were deposited on a major pre unayzah unconformity saudi arabian paleozoic oil is a high gravity 43 53 degrees api crude with low sulfur content usually less than 0 07 that is classified as arabian super light oil gas with condensate has also been discovered in the unayzah formation the identified paleozoic traps are moderate relief fault generated structures with generally 30 100 m of closure unayzah structural traps developed primarily during the triassic to early jurassic however an earlier phase of significant pre unayzah structural development is also indicated between the devonian and late carboniferous and is related to a hercynian structural event post triassic structure in central arabia is mainly regional and results principally from late cretaceous to tertiary development of the central arabian arch the established source rock is the basal qusaiba shale a widespread organic rich shale that was deposited following regional deglaciation in the early silurian migration occurred regionally updip through unayzah formation reservoirs from the underlying qusaiba shale subcrop and or vertically along fault bounded zones on the flanks of structures from the underlying qusaiba shale interbedded upper permian shales and evaporites provide a top seal for the unayzah and form the basal sequence of the overlying transgressive khuff carbonate pre qusaiba reservoir targets include the cambrian ordovician saq sandstone the ordovician qasim formation and upper ordovician to lower silurian periglacial clastics of the zarqa and sarah formations high gravity low sulfur oil has also been discovered in this pre qusaiba clastic section the reservoirs of the pre qusaiba structures are fault bounded and sources laterally by the down faulted qusaiba shale

deep fluid release in warm subduction zones from a breached slab seal abstract petrological models and seismic data from subduction zones with geotherms of 7 k km 1 or higher suggest that slabs in these systems dehydrate effectively in the forearc a large fluid flux is nevertheless released from these slabs at and beyond subarc depth suggesting that large amounts of h2o can remain slab bound to much greater depth than expected we propose that this is due to a transient sealing effect exerted by the subducting lower crust to test this concept the petrological and geochemical evolution of such gabbroic crust is investigated through a textural petrological and li chronometric analysis of eclogitized gabbros from an exhumed ultrahigh pressure terrane the samples record pristine transitions from dry rigid gabbro to hydrated eclogite and eclogite mylonite which occurred when these rocks resided at 90 110 km depth the observations characterize step by step the deformation and overstepped mineral reactions that following the influx of external fluids along a developing network of permeable shear zones lithium chronometry indicates that the gabbroic rocks were breached and permeated within a few weeks at a very specific depth within the 90 110 km interval depths where in warm subduction zones large fluid filled channel system emanate from the slab the data support a model in which fluids produced in the deserpentinizing slab mantle are trapped at high pore pressure beneath the slab moho and are ultimately released at subarc depth where the lower crust fails and develops highly permeable fluid vents the subducting lower crust thus may play an important role in regulating h2o and element budgets and controlling slab rheology in warm subduction zones

compressional metamorphic core complexes low angle normal faults and extensional fabrics in compressional tectonic settingscompressional core complexesm searle t lamont metamorphic core complexes mccs are interpreted as domal structures exposing ductile deformed high grade metamorphic rocks in the core underlying a ductile to brittle high strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension extension is supposedly the driving force that has governed exhumation however numerous core complexes notably himalayan karakoram and pamir domes occur in wholly compressional environments and are not related to lithospheric extension we suggest that many mccs previously thought to form during extension are instead related to compressional tectonics pressures of kyanite and sillimanite grade rocks in the cores of many of these domes are c 10 14 kbar approximating to exhumation from depths of c 35 45 km too great to be accounted for solely by isostatic uplift the evolution of high grade metamorphic rocks is driven by crustal thickening shortening regional barrovian metamorphism isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel channel flow following peak metamorphism beneath a passive roof stretching fault in naxos low angle normal faults associated with regional aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening the fact that low angle normal faults exist in both extensional and compressional tectonic settings and can actively slip at low angles c 60 is needed

deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions abstract we conducted triaxial deformation experiments using a griggs type solid pressure medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the sambagawa metamorphic belt a greenschist and an epidote amphibole schist at a confining pressure of 1 gpa 400 c and strain rate of 10 5 1 s differential stresses for all mafic schists were higher than 1 gpa the greenschist samples were weaker than epidote amphibolite samples under all experimental conditions microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi brittle deformation samples of the epidote amphibole schist deformed at 500 c show shear localization along a single fault oriented parallel to the foliation while mechanical data shows no rapid stress drop on the surface of the fault we observe the formation of slickensides with little gouge all sample types exhibit a high stress exponent n 15 and strain rate strengthening i e positive a b values frictional behavior that inhibits earthquake nucleation differential stress increased with increasing confining pressure while the friction coefficient decreased with increasing confining pressure and temperature greenschist and epidote amphibolite facies metamorphism typically occurs at hot subduction zones such as cascadia and southwest japan the lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi brittle deformation or frictional sliding with a positive a b during deformation of these types of metamorphic rocks weak and aseismic fault zones in subducting slabs might promote slab mantle decoupling at the corner of the mantle wedge and detachment and underplating of oceanic crust from the subducting slab to forearc crust

upper mantle velocity structure estimated from ps converted wave beneath the north eastern japan arc summary the upper boundary of the descending oceanic plate is located by using ps waves converted from p to s at the boundary in the tohoku district the north eastern part of honshu japan the observed ps p time data are well explained by a two layered oceanic plate model composed of a thin low velocity upper layer whose thickness is less than 10 km and a thick high velocity lower layer the upper and lower layers respectively have 6 per cent lower and 6 per cent higher velocity than the overriding mantle the estimated location of the upper boundary is just above the upper seismic plane of the double planed deep seismic zone this result indicates that events in the upper seismic plane at least in the depth range from 60 to 150 km occur within the thin low velocity layer on the surface of the oceanic plate

seismic low velocity layer at the top of subducting slabs observations predictions and systematics abstract layered structure persists at the tops of subducting plates to depths in excess of 150 km a low velocity channel several km thick produces guided seismic guided waves which indicate that the top of the downgoing plate remains coherent and probably has not eclogitized to 150 km depth the surrounding medium has seismic velocities 3 6 faster than standard earth models as inferred from direct travel times presumably because the waveguide lies atop cold descending lithosphere however guided waves travel more slowly several tests with full waveform synthetics show that the guided waves can be observed only if excited by sources that lie near or within the low velocity channel thus the frequent observation of dispersed body waves indicates that intermediate depth earthquakes tend to lie in waveguides also receivers must be favorably placed observations for seven circum pacific subduction zones show such signals which can be explained as low velocity channels 2 8 km thick velocity anomalies within low velocity channels are largest at shallowest depths up to 14 slower than surroundings at depths less than 150 km diminishing to

unbending combined with dehydration embrittlement as a cause for double and triple seismic zones 1 using a model of elastic perfectly plastic plate i demonstrate that plate unbending combined with dehydration embrittlement can account for the morphology and stresses of double or triple seismic zones dsz s or tsz s observed in many subducting slabs densifying metamorphic reactions may induce tensile stresses as well as high pore fluid pressures in the slab crust and thus generate the upper seismicity band of the tsz limited dehydration may moderately elevate the fluid pressure in the slab mantle but unbending usually dominates mantle stresses and results in another two seismicity bands dsz s are underdeveloped tsz s

an intraslab seismic sequence activated by the 2011 tohoku oki earthquake evidence for fluid related embrittlement 1 using quantitative analysis of waveforms we investigate a seismic cluster that occurred at a depth of 67 km in the philippine sea slab 8 months after the 11 march 2011 megathrust tohoku oki earthquake mw 9 0 the sequence started with an m 4 1 normal fault event on 14 november in the deepest part of the cluster and subsequent earthquakes migrated upward by 6 km along a narrow conduit like zone the earthquakes have stress drops of 0 5 40 mpa and groups of earthquakes with coherent waveforms are observed we explain these observations in terms of fluid related embrittlement and the migration of overpressured fluids the low permeability plate interface of the pacific plate may have been broken by the coseismic or postseismic slips of the tohoku oki earthquake with the fluids subsequently being liberated from the underlying crust of the pacific slab and migrating into the philippine sea slab due to the pore pressure gradient the tensional stresses generated by the coseismic slip promoted the efficient upward migration of fluids and also acted to enhance the deviatoric stress around the source area the heightened pore pressures and the resulting reduced effective normal stress lowered the strength of the faults sufficiently to bring the system into the brittle regime under the enhanced deviatoric stress the lag of 8 months may represent the time needed for the unsealing of the plate interface the upward migration of fluids and the increase in pore pressure to become sufficient to overcome the lithostatic pressure

a unifying basis for the interplay of stress and chemical processes in the earth support from diverse experiments the interplay between stress and chemical processes is a fundamental aspect of how rocks evolve relevant for understanding fracturing due to metamorphic volume change deformation by pressure solution and diffusion creep and the effects of stress on mineral reactions in crust and mantle there is no agreed microscale theory for how stress and chemistry interact so here i review support from eight different types of the experiment for a relationship between stress and chemistry which is specific to individual interfaces chemical potential helmholtz free energy normal stress at interface molar volume the experiments encompass temperatures from 100 to 1300 degrees c and pressures from 1 bar to 1 8 gpa the equation applies to boundaries with fluid and to incoherent solid solid boundaries it is broadly in accord with experiments that describe the behaviours of free and stressed crystal faces next to solutions that document flow laws for pressure solution and diffusion creep that address polymorphic transformations under stress and that investigate volume changes in solid state reactions the accord is not in all cases quantitative but the equation is still used to assist the explanation an implication is that the chemical potential varies depending on the interface so there is no unique driving force for reaction in stressed systems instead the overall evolution will be determined by combinations of reaction pathways and kinetic factors the equation described here should be a foundation for grain scale models which are a prerequisite for predicting larger scale earth behaviour when stress and chemical processes interact it is relevant for all depths in the earth from the uppermost crust pressure solution in basin compaction creep on faults reactive fluid flow systems serpentinisation the deeper crust orogenic metamorphism the upper mantle diffusion creep the transition zone phase changes in stressed subducting slabs to the lower mantle and core mantle boundary diffusion creep

lower palaeozoic unconformities in an intracratonic platform setting glacial erosion versus tectonics in the eastern murzuq basin southern libya the stratigraphic record of the eastern murzuq basin has been importantly influenced by deformation resulting in angular and or deeply erosional unconformities though the overall context is intracratonic major transgressive events and the ordovician glaciation are nevertheless documented allowing the delineation of tectonic eustasy or climate driven unconformities lower palaeozoic key events and related unconformities that characterize the north gondwana platform have therefore a signature in the eastern murzuq basin the basement cover unconformity also known as the infra tassilian surface truncates all the deformed and metamorphosed lower cambrian and older rocks above is a middle cambrian to lower ordovician megasequence murizidie and hasawnah fms which is in turn truncated by an intra ordovician angular unconformity this megasequence is unconformably overlain by a middle ordovician hawaz fm to silurian tanzzuft and akakus fms megasequence which includes the upper ordovician glaciogenic unit mamuniyat fm bounded at the base by a polygenic glacial erosion surface showing corrugated glacial lineations tillites and glaciotectonic structures the middle ordovician to silurian megasequence is finally truncated by a base devonian angular unconformity overlain by fluvial sandstones regarding the possibility that those fluvial deposits may be as younger as late devonian in the eastern murzuq basin based on palaeoflora the so called caledonian unconformity might be here a much younger mid eifelian surface and the occurrence of the lower devonian tadrart fm is questioned the upper ordovician glacial erosion surface which is sometimes referred to as the taconic unconformity usually truncates middle ordovician strata in the murzuq basin but reaches significantly deeper stratigraphic levels in places that have been previously involved in the intra ordovician deformation event in the murizidie southeastern murzuq basin the infra tassilian surface the intra ordovician unconformity and the upper ordovician glacial erosion surface amalgamate together here an estimate of the glacial erosion depth cannot be derived from the stratigraphic hiatus beneath the glacial incision the main part of which relate to the intra ordovician tectonic event the upper ordovician climate related glacial erosion surface is not a valid unconformity for a sequence hierarchy framework of the lower palaeozoic although it presents most of the physical attributes of tectonic driven unconformities

mafic granulite rheology implications for a weak continental lower crust abstract the jelly sandwich model of continental rheology long the consensus model has recently been challenged the controversy is based on the rheology of mafic granulite that is thought to make up most of the lower continental crust however because of technical issues in experimental rock deformation there are very few data on this problem here we report the rheology and fabrics of a reconstituted fine grained nominally dry i e no hydrous mineral mafic granulite 57 plagioclase 24 clinopyroxene 14 orthopyroxene 5 opaque minerals 0 16 0 28 wt h 2 o deformed at 1 1 1 2 gpa pressure in a modified griggs type deformation apparatus the rheology of this mafic granulite can be described by the constitutive equation of e a 3 2 0 4 exp 244 35 kj mol r t where e is in s 1 in mpa t in kelvin and a 10 2 0 1 6 mpa 3 2 s 1 our results provide new experimental evidence in support of the jelly sandwich lithosphere strength model and imply that mafic granulite with a moderate amount of water 0 05 0 08 wt h 2 o is likely to be a weak layer in the lithosphere both pyroxenes and plagioclase develop pronounced fabrics in responding to axial deformation the deformation mechanism is dislocation creep with 100 001 and 001 100 010 100 being the dominant slip systems for pyroxenes and plagioclase respectively the low strength of mafic granulite is ascribed largely to the significant weakening effect of dissolved water in pyroxenes and plagioclase a weak continental lower crust has many important implications for geodynamics of crust mantle interactions such as lower crust channel flow and delamination

early paleozoic paleogeography of the northern gondwana margin new evidence for ordovician silurian glaciation during the early paleozoic transgressions and the distribution of sedimentary facies on the northern gondwana margin were controlled by a regional nnw sse to almost north south striking structural relief in early silurian times a eustatic highstand enabled the sea to reach its maximum southward extent

existence of a seismic belt in the upper plane of the double seismic zone extending in the along arc direction at depths of 70 100 km beneath ne japan 1 we relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the pacific slab beneath ne japan we found a seismic belt parallel to the iso depth contour of the plate interface that is beneath the forearc area at depths of 70 100 km indicating that the distribution of the upper plane earthquakes is non uniform the location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where h2o contents change in the slab crust events in the upper seismic plane have mainly down dip compression type focal mechanisms but several events have normal fault type nf type versions whose spatial distribution appears to correspond to these boundaries these nf events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions

contrasting fracture patterns induced by volume increasing and decreasing reactions implications for the progress of metamorphic reactions abstract hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock and some of these reactions proceed with fracturing for this paper we have developed a model for coupled hydraulic chemical mechanical processes using the distinct element method in order to understand the relationships among chemical reactions fluid flow and fracturing during metamorphism the model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure with the help of the model volume decreasing dehydration reactions and volume increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions respectively both types of reaction proceed inwards from drained boundaries or fractures but they show contrasting fracture patterns a volume decreasing dehydration reaction produces tree type fractures with the new fractures generated as branches of a pre existing fracture a volume increasing hydration reaction produces a polygonal network of fractures where new fractures nucleate at sites far from the pre existing fracture and extend to form t junctions these contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient in the case of the tree type fractures the fractures are continuously generated at the reaction front and the reaction proceeds smoothly by positive feedbacks between reaction fracturing and fluid flow in contrast in a volume increasing hydration reaction fracturing initially occurs in response to the irregular boundary shape but as the reaction progresses a compressive stress field is generated which inhibits further fracture generation the compressive stress field also prevents fluid flow by closing the pre existing fractures which slows down the reaction these contrasting feedback systems between volume decreasing dehydration and volume increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved

the lower silurian qalibah formation of saudi arabia an important hydrocarbon source rock 1 the lower silurian qalibah formation represents a distinct lithostratigraphic time transgressive sequence it is disconformably bounded below by the upper ordovician to possibly lower silurian sarah formation and above by the continental to marginal marine lower devonian and possibly upper silurian tawil sandstone formation this paper redefines the qalibah formation within these boundaries in its reference section near the town of al qalibah in northwest saudi arabia the qalibah is 499 m thick whereas in a subsurface composite section it reaches 955 m the qalibah formation is a coarsening upward progradational sequence consisting of a lower qusaiba shale member and an upper sharawra sandstone member the basal hot shale of the qusaiba is a black euxinic bottomset shale up to 70 m thick which was deposited during the early silurian sea level rise following the deglaciation of gondwana this paleozoic shale is organic rich and the most likely source rock of the low sulfur high gravity oil condensate and gas discovered in recent years in the paleozoic rocks of central and eastern saudi arabia lithologically similar silurian sequences were deposited over most of the broad stable shelf of gondwana from the middle east to the african sahara

eclogite formation and dynamics of crustal roots under continental collision zones field observations on deep parts of the root zone of the caledonian orogen are combined with the geophysical data available for recent continental collision zones such as the alps to provide a model for dynamics of crustal root zones rocks descending into the root zone are locally infiltrated by fluids which cause eclogitization along shear zones and fluid pathways the eclogite facies rocks vary in density from 3 1 to 3 6 g cm 3 reflecting the compositional variation in their protoliths where the amount of highly ductile eclogitic material reaches about 40 the descending crustal segment loses its coherence and disintegrates into a breccia where blocks of granulite facies rocks float in a matrix of eclogite material a density stratified root zone develops where light unreacted or partly eclogitized material floats on an ocean dominated by eclogite facies rocks the top of which may correspond to the moho catastrophic decent of this eclogitic layer may 1 transport blocks of light material to depths of 90 km which upon further disintegration due to eclogitization are released and return by buoyancy forces to the crust 2 result in extension and uplift of the partly eclogitized lighter material

evidence from high frequency seismic waves for the basalt eclogite transition in the pacific slab under northeastern japan abstract seismic multi pathing effects attributed to a contrast in seismic attenuation between the back arc mantle wedge and subducted crust are detected in central honshu northeastern japan we observe an initial broadened p wave which is followed by a delayed higher frequency p wave signal their discrepant frequencies are best explained by attenuation effects delayed p wave signals travel in the low attenuation oceanic crust and therefore contain more high frequency components the time separation between the initial broadened p waves and the delayed p wave signals are affected by the seismic velocity in the subducted oceanic crust we observe systematic variation in the delay times of the later waves indicating an increase in seismic velocity in the oceanic crust relative to the mantle wedge at 130 150 km depth high frequency seismic simulations incorporating mineral physics derived models show that a 4 vp increase due to the blueschist decomposition and a 9 vp increase associated with the lawsonite talc eclogite transition replicate the observed delay time variation the blueschist breakdown may occur at a depth of 100 km and the lawsonite talc eclogite transition might be linked with the reduced seismicity level at depths greater than 150 km distinct from traditional guided waves the multi pathing effects in this study are mainly controlled by attenuation contrast and therefore may not require the oceanic crust to have low velocity and any special decoupling mechanism the multi pathing effects offer us another important tool to image subducted oceanic crust below back arc mantle wedges especially where guided waves are not observable in this study we demonstrate the value of observing and simulating high frequency seismic waves 20 hz in advancing our understanding of subduction zones