late archaean to early proterozoic granitoid magmatism and high grade metamorphism in the central limpopo belt south africa the central zone of the limpopo belt in southern africa has previously been interpreted as a segment of archaean crust which experienced its main deformation and metamorphism around 2 7 ga ago we report new single zircon u pb and pb pb ages for granitoid gneisses supracrustal rocks and anatectically derived granitic melt patches in the area around messina south africa the sand river gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2 6 and 3 2 ga the singelele gneiss a heterogeneous granodioritic to quartz monzonitic rock has protolith ages between 2 55 and 2 58 ga since both the sand river and singelele gneisses experienced polyphase high strain ductile deformation this must have occurred later than 2 55 ga ago granulite facies pelitic gneisses of the beit bridge complex contain abundant spherical multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism these zircons provide ages with a mean at 2026 5 6 3 ma which is interpreted as reflecting a high p t event 10 kbar 825 25 c granitic melt patches in the metapelites as well as in the sand river gneiss and anatectic granites are probably related to rapid near isothermal decompression to below 3 5 kbar and 600 750 c these rocks contain new magmatic zircons which yielded a mean age of 2005 6 4 4 ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage our zircon data support previous suggestions for only one single granulite facies event in the central zone and we suggest that this event occurred c 2027 ma ago since most of the deformation seen in the gneisses of the messina area must have occurred later than 2 55 ga ago it is likely that the limpopo orogeny at least in the central zone is not an archaean event but took place in the early proterozoic

coesite bearing eclogites from the dabie mountains central china petrogenesis p t paths and implications for regional tectonics coesite and kyanite bearing eclogites are abundant in the southern part of the dabie mountains southern dabie terrane sdt two types of eclogites from the sdt types iii and iv were selected for detailed paragenetic study type iii eclogites most abundant in the northern part of the sdt occur as blocks in gneisses and marble and contain eclogitic assemblages of omphacite garnet phengite epidote coesite kyanite carbonate rutile ilmenite these minerals exhibit weak compositional zoning and contain few mineral inclusions type iv eclogites mostly in the southern part of the sdt occur as coherent layers interbedded with gneisses and amphibolites and have assemblages of omphacite garnet glaucophane kyanite epidote phengite quartz rutile ilmenite garnets of type iv eclogites exhibit a prograde compositional zoning and have mineral inclusions of paragonite phengite epidote quartz and rutile in the core and omphacite barroisite and mg katophorite in the rim prograd

numerical simulation of shallow water dam break flows in open channels using smoothed particle hydrodynamics summary a meshless numerical model is proposed to investigate shallow water dam break flows in 1d open channels the numerical model is to solve the shallow water equations swe based on smoothed particle hydrodynamics sph the concept of slice water particles swp is adopted in the sph swe formulation the numerical sensitivity analysis is first performed to study the appropriate swp number and variable smoothing length through dam break flows in an idealized 1d channel with dry wet beds extensive validation by comparison with laboratory and field data is next conducted for four benchmark problems including dam break flows through a rough flat channel a rough bumpy channel with various downstream boundary conditions a nonprismatic channel and a realistic scale model of the toce river in italy the simulated results indicate that accurate performance is reached in the presence of shock discontinuities shock front motion hydraulic jumps dry wet bed flow supercritical subcritical transcritical flow reverse flow contraction flow overtopping flow partial reflections and multiple wave interaction without special numerical treatment all of the simulations are carried out by pc within 10 min thus the proposed numerical model has proved its efficiency and reliability for dam break flow computations in open channels

upwind conservative scheme for the saint venant equations an upwind conservative scheme with a weighted average water surface gradient approach is proposed to compute one dimensional open channel flows the numerical scheme is based on the control volume method the intercell flux is computed by the one sided upwind method the water surface gradient is evaluated by the weighted average of both upwind and downwind gradients the scheme is tested with various examples including dam break problems in channels with rectangular and triangular cross sections hydraulic jump partial dam break problem overtopping flow a steady flow over bump with hydraulic jump and a dam break flood case in a natural river valley comparisons between numerical and exact solutions or experimental data demonstrated that the proposed scheme is capable of accurately reproducing various open channel flows including subcritical supercritical and transcritical flows the scheme is inherently robust stable and monotone the scheme does not require any special treatment such as artificial viscosity or front tracking technique to capture steep gradients or discontinuities in the solution

structural and tectonic interpretation of geophysical data along the eastern continental margin of india with special reference to the deep water petroliferous basins abstract the study area encompasses the eastern continental margin of india ecmi and the adjoining deep water areas of bay of bengal the region has evolved through multiple phases of tectonic activity and fed by abundant supply of sediments brought by prominent river systems of the indian shield detailed analysis of total field magnetic and satellite derived gravity data along with multi channel seismic reflection sections is carried out to decipher major tectonic features basement structure and the results have been interpreted in terms of basin configuration and play types for different deep water basins along the ecmi interpretation of various image enhanced gravity and magnetic anomaly maps suggest that in general the ene wsw trending faults dominate the structural configuration at the margin these maps also exhibit a clear density transition from the region of attenuated continental crust proto oceanic crust to oceanic crust based on which the continent ocean boundary cob has been demarcated along the margin basement depths estimated from magnetic data indicate that the values range from 1 to 12 km below sea level and deepen towards the bengal fan in the north and reveal horst graben features related to rifting a comparison of basement depths derived from seismic data indicates that in general the basement trends and depths are comparable in cauvery and krishna godavari basins whereas in the mahanadi basin basement structure over the 85 e ridge is clearly revealed in seismic data further eight multichannel seismic sections across different basins of the margin presented here reveal fault pattern rift geometries and depositional trends related to canyon fills and channel levee systems and provide a basic framework for future petroleum in this under explored frontier

temporal and spatial correlations between changes in plate motions and the evolution of rifted basins in africa we present the results of a backstripping study incorporating over 200 wells in 14 different basins or groups of basins located on the african interior or continental margins these results presented as mean tectonic subsidence rates averaged over appropriate periods are compared with the opening histories and changes in plate motions of the atlantic and indian oceans and the tethys mediterranean area for seven time steps from the late jurassic until present the results indicate a temporal and spatial correlation between phases of rapid subsidence in african basins and changes in plate motions and therefore are promising in terms of applicability of large scale plate motions in the studies of paleo stresses in sedimentary basins especially at times of initial rifting and formation of subduction zones reactivation of the basins appears to be primarily controlled by the orientation of the basin and underlying basement structures with respect to the stress direction the basins are preferably formed within weaker zones mostly pan african shear zones and the distance from the active plate boundary to the weak zone plays a minor role in whether or not it will be reactivated the presence of three important hot spots which were active during the early stages of break up of gondwana may have been important in preweakening the lithosphere at their sites this may have played an additional role in the localization of the basin deformation

age and tectonic evolution of neoproterozoic ductile shear zones in the southern granulite terrain of india with implications for gondwana studies 1 the high grade rocks of the southern granulite terrain sgt of peninsular india are bounded to the north by the archean dharwar craton another high grade terrane the mesoproterozoic eastern ghats occurs to the northeast of the sgt the tectonic relationship between these crustal domains is complex we present new geochronological and structural data that indicate a continuation of the dharwar craton into the southern granulite terrain as far south as a newly identified neoproterozoic shear zone here named the karur kamban painavu trichur shear zone kkptsz south of the kkptsz mesoproterozoic dates of the sgt are similar to those recorded in the eastern ghats and the two domains may have been conterminous thirty three new u pb th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the sgt has experienced at least seven thermo tectonic events at 2 5 ga 2 0 ga 1 6 ga 1 0 ga 800 ma 600 ma and 550 ma and two distinct episodes of metasomatism charnockitization between 2 50 2 53 and between 0 55 0 53 ga deformation along a number of major shear zones in the sgt is neoproterozoic to earliest paleozoic in age with an early phase d2 concentrated between 700 800 ma and a later phase d3 between 550 and 600 ma major charnockitization 530 550 ma post dates d3 and is in turn overprinted by granitization retrogression and uplift between 525 and 480 ma the kkptsz active between 560 and 570 ma is either a terrane boundary or a tectonized decollement between cover and archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north respectively other regional neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of nd model ages and rb sr biotite whole rock dates the neoproterozoic cambrian tectonothermal history of the sgt and eastern ghats is similar to that recorded in parts of madagascar east africa and antarctica and is used to reconstruct parts of central gondwana here named the deccan continent with more robust confidence

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

eye to eye with a mega sheath fold a case study from wadi mayh northern oman mountains sheath folds are highly curvilinear folds typically considered to develop by hinge rotation toward the transport direction during intense deformation most sheath folds are recognized on the centi meter to meter scale with very few detailed studies of larger kilometer scale structures we here present evidence for a megascale sheath fold superbly exposed in the northern oman mountains the saih hatat culmination consists of an exhumed highpressure subduction zone with deep level eclogites and garnet blueschists as sifah unit epidote blueschists hulw unit and carpholite bearing metasediments part of the upper plate sheath folds are present at a variety of scales throughout the as sifah hulw and upper plate tectonic packages curvilinear fold hinges along a 15 km long profi le outline the wadi mayh mega sheath fold which closes to the ssw and faces to the nne it consists of at least four secondary sheath folds with cross sectional y z eye folds measur ing 200 m stacked on top of one another within permian and triassic shelf car bonates these eye folds display highly elliptical geometries ryz 5 and cat s eye fold patterns consistent with simple general shear the sheath fold is enveloped by ordovician and older rocks that do not show equal amounts of internal deformation the upper bounding envelope is a detachment that has been subsequently sheath folded the lower boundary of the sheath fold is the upper plate lower plate detachment which forms a major high strain shear zone showing little vertical displacement but signifi cant horizontal displacement the sheath fold displays a ssw elongation x axis of at least 15 km and possibly up to 25 km coupled with signifi cant amounts of vertical fl attening and wnw ese shortening

pan african granulite formation in the kabye massif of northern togo west africa pb pb zircon ages metamorphic zircons from seven granulite facies orthogneisses of tonalitic composition enderbites collected from four different nappes of the kabye massif in the dahomeyide belt of northern togo were dated by the pb pb evaporation method they yielded consistent neoproterozoic ages with a mean of 612 5 0 8 ma interpreted to reflect the peak of regional granulite facies metamorphism following pan african continental collision between the west african and benin nigerian plates these results support previous ages obtained by various chronometers on high grade rocks from the same suture zone and from surrounding units in togo benin and ghana they are also similar to zircon ages from granulites in the mozambique belt of souteastern africa these pan african metamorphic ages reflect continental amalgamation resulting in the formation of the gondwana supercontinent towards the end of the neoproterozoic

the kokchetav massif kazakhstan type locality of diamond bearing uhp metamorphic rocks abstract after the discovery of metamorphic coesite in crustal rocks from the western alps italy and the western gneiss region norway in the mid 1980s of the last century metamorphic diamond was observed only a few years later in situ in the kokchetav massif kazakhstan findings of such coesite and diamond bearing ultrahigh pressure metamorphic uhp rocks with protoliths formed or embedded in crustal levels and subsequently experienced pt conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes these occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface recent studies even suggest continental subduction to depths exceeding 300 km these rocks have been extensively studied and many new and important observations have been made thus far more than 350 papers have been published on various aspects of kokchetav uhp rocks the kokchetav massif of northern kazakhstan is part of one of the largest suture zones in central asia and contains slices of hp and uhp metamorphic rocks classical uhp rocks mainly occur in the kumdy kol barchi kol and kulet areas and include a large variety of lithologies such as calcsilicate rocks eclogite gneisses schists marbles of various compositions garnet pyroxene quartz rocks and garnet peridotite most of them contain microdiamonds some of which reach a grain size of 200 m most diamond grains show cuboid shapes but in rare cases diamonds within clinozoisite gneiss from barchi kol occur as octahhedral form microdiamonds contain highly potassic fluid inclusions as well as solid inclusions like carbonates silicates and metal sulfides which favour the idea of diamond formation from a c o h bearing fluid nitrogen isotope data and negative 13c values of kokchetav diamonds indicate a metasedimentary origin pt estimates of kokchetav uhp rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950 1000 c some zircon separates show inherited proterozoic cores and 537 530 ma uhp metamorphic mantle zones several ar ar ages on micas scatter around 529 528 and 521 517 ma and reflect different stages of the exhumation history migmatization occurred during exhumation at about 526 520 ma isotopic studies on calcsilicate rocks confirm a metasedimentary origin 18o values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith similar studies on eclogites indicate their basaltic protolith having experienced water rock interaction prior to uhp metamorphism a number of unique mineralogical findings have been made on kokchetav uhp rocks k feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx structure under upper mantle pressures other significant observations are coesite exsolutions in titanite quartz rods in cpx the discovery of k tourmaline as well as new minerals like kokchetavite a hexagonal polymorph of k feldspar and kumdykolite an orthorhombic polymorph of albite the kokchetav uhp rocks represent a unique and challenging stomping ground for geoscientists of various disciplines from crystallography petrology and geochemistry to geophysics and geodynamics geotectonics it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions

the afro arabian rift system an overview abstract the afro arabian rift system is reviewed beginning with the dead sea transform and gulf of suez in the north followed by the gulf of aden and red sea and ending with a brief mention of rifting through eastern and southern africa a consistent interpretation is obtained for geophysical data from the gulf of aden and red sea and geological data for the dead sea transform suez graben and red sea and gulf of aden margins geophysical data are used to estimate the locations of the ocean continent boundaries in the northern red sea these are found to coincide when the total 107 km shear along the dead sea is restored the red sea is therefore reconstructed in stages beginning with the 45 km post miocene shear and 62 km early miocene shear along the dead sea transform followed by estimates for extension in the early red sea suez graben when this is done there is a remarkable alignment of various features revealed on landsat tm imagery for both sides of the northern red sea north south differences in the red sea and west east differences hi the gulf of aden are discussed and found to be best explained by a propagating rift model

numerical modeling of dambreak induced flood and inundation using smoothed particle hydrodynamics summary smoothed particle hydrodynamics sph is used to solve the two dimensional shallow water equations 2d swes for modeling dambreak induced flood and inundation the lagrangian concept of cylindrical water particles cwps is adopted to generate horizontal flows in rivers and floodplains parameter sensitivity analysis and model validation are described to demonstrate the benefits and limits of the 2d sph swe modeling particular attention is devoted to the numerical performance of dealing with free surface discontinuities different upstream downstream boundary conditions wetting drying moving interfaces river floodplain roughness sensitivity and complex topography variations the simulated results indicate that many complicated flow phenomena occurred in dam break flows such as transcritical mixed flows shock front propagation overtopping flows partial reflections hydraulic jumps hydraulic drops and multiple wave interaction can be faithfully simulated thus the proposed sph approach has proved its robustness and reliability for 2d flood and inundation simulations and can provide an alternative to investigate practical hydraulic engineering problems

the diamond bearing kokchetav uhp massif in northern kazakhstan exhumation structure in the kokchetav ultrahigh pressure uhp massif in northern kazakhstan diamond bearing uhp rocks occur exclusively in a western rhomb shaped domain that differs from an eastern transpressional domain with coesite bearing remnants indicating highest uhp conditions different mechanisms may have contributed to the early ascent of the uhp kokchetav massif the geometry and structure of the diamond bearing domain are interpreted as a sheath like fold coeval with early stage melting in contrast the coesite bearing domain has a sheet like geometry at mid crustal level this early difference in the ascending uhp wedge is reflected in a western rhomb horst and an eastern transpression structure respectively the latter extends to the east borovoye where the uppermost sequences of the uhp massif were defined by others sheath folding is postulated as a suitable mechanism contributing to the early buoyancy driven ascent of the subducted rocks and explains the selective spatial distribution of diamond and coesite bearing sequences preserved in the wedge

raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh pressure metamorphic rocks from diamonds in sample preparation materials natural diamond from three ultrahigh pressure metamorphic uhpm terranes erzgebirge massif germany kokchetav massif northern kazakhstan rhodope metamorphic province greece and synthetic diamond from cutting and polishing materials paste spray saw blade were studied by means of optical microscopy and raman microspectroscopy to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones i e diamond as residual particles contaminants from the cutting and polishing material such as the diamond size the presence of inclusions coatings or coexistent phases and two diamond raman band parameters i e the raman shift and the full width at half maximum height fwhm

polyphase zircon in ultrahigh temperature granulites rogaland sw norway constraints for pb diffusion in zircon shrimp u pb ages have been obtained for zircon in granitic gneisses from the aureole of the rogaland anorthosite norite intrusive complex both from the ultrahigh temperature uht 900 c pigeonite in zone and from outside the hypersthene in isograd magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis a sample from outside the uht zone has magmatic cores with an age of 1034 7 ma 2 n 8 and 1052 5 ma 1 n 1 overgrown by m1 metamorphic rims giving ages between 1020 7 and 1007 5 ma in contrast samples from the uht zone exhibit four major age groups 1 magmatic cores yielding ages over 1500 ma 2 magmatic cores giving ages of 1034 13 ma 2 n 4 and 1056 10 ma 1 n 1 3 metamorphic overgrowths ranging in age between 1017 6 ma and 992 7 ma 1 corresponding to the regional m1 sveconorwegian granulite facies metamorphism and 4 overgrowths corresponding to m2 uht contact metamorphism giving values of 922 14 ma 2 n 6 recrystallized areas in zircon from both areas define a further age group at 974 13 ma 2 n 4 this study presents the first evidence from rogaland for new growth of zircon resulting from uht contact metamorphism more importantly it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c 950 c for at least 1 myr magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information a result inconsistent with some experimental data on pb diffusion in zircon which predict measurable pb diffusion under such conditions the implication is that resetting of zircon ages by diffusion during m2 was negligible in these dry granulite facies rocks imaging and th u y systematics indicate that the main processes affecting zircon were dissolution reprecipitation in a closed system and solid state recrystallization during and soon after m1

geochronology and isotopic character of ultrahigh pressure metamorphism with implications for collision of the sino korean and yangtze cratons central china the collision zone between the sino korean and yangtze cratons in central china is marked by subduction related metamorphic rocks composed of crustal protoliths these range in metamorphic grade from ultrahigh pressure eclogite to low greenschist facies the paragenesis of the ultrahigh pressure rocks coesite and diamond bearing indicates subduction to depths greater than 120 km as a result of continental collision despite extreme metamorphic conditions zircons have not undergone pb loss but rather illustrate two periods of growth crystallization between 700 and 800 ma and ultrahigh pressure metamorphism at 218 2 5 ma previous studies have predicted a time transgressive collision but age data presented in this study indicate that the collision was approximately coeval along the length of the suture u pb zircon ages of these ultrahigh pressure metamorphic rocks are 218 4 2 5 ma and 217 1 8 7 ma for the dabie mountains and shandong peninsula respectively the similar ages imply that the tan lu fault which offsets the ultrahigh pressure metamorphic rocks is a secondary feature not related to collision or subduction whole rock isotopic data show that both rb sr and u pb were open systems during metamorphism but that sm nd probably remained closed nd initial isotopes display crustal signatures indicating that the crust subducted as a coherent slab rather than becoming intercalated with mantle material at depth sm nd data in conjunction with u pb zircon dating indicate that the ultrahigh pressure rocks are not typical yangtze craton basement 2 9 ga but that they originally crystallized in a rift environment between 700 and 800 ma discrepancies between 40ar 39ar plateaus sm nd isochrons and u pb zircon ages render calculation of a p t t path unrealistic

a model for stress controlled pipe growth abstract the rock mechanics theory for deformation of underground mining excavations under high stress conditions can be used to explain the growth and geometry of volcanic pipes in an underground excavation stress concentrates greatest on the sides of an excavation perpendicular to the principal vector of compression if the stress is high enough fractures will develop causing scaling of the tunnel sidewalls and tunnel growth perpendicular to the principal vector of compression pre existing structures aid the physical mechanisms of pipe growth such as gravitational collapse explosive fragmentation and turbulent erosion and by reducing the strength of the rock mass should also aid stress induced scaling universal distinct element code numerical modelling demonstrated in this study reproduces the stress conditions around a circular pipe under uniaxial compression and simulates pipe growth as wedges bounded by failed pre existing joints form around the pipe and are assimilated into the pipe the results show how a volcanic pipe will tend to grow perpendicular to the principal vector of compression if the internal magma pressure is low or absent the orientations of the pre existing joints affect the exact direction of pipe growth in a predictable manner examples from other publications demonstrate that the model is consistent even in extensional tectonic environments case studies from kimberlite occurrences in the limpopo belt at finsch mine and the gross brukkaros volcanic complex demonstrate that dykes and magmatic bodies of kimberlite with high overpressures during emplacement normally have geometries trending near parallel to the principal vector of compression yet pipes or parts of pipes that underwent strong underpressures during emplacement often ending up comprising fragmental volcaniclastic infill have an elongation near perpendicular to the same vector thus the stress induced pipe growth model is demonstrated to be important for some pipes the study of volcanic pipe and dyke shapes can therefore be used to determine the stress regime at the time of emplacement and to distinguish between kimberlite occurrences formed at different times within different stress tensors

rift basins of interior sudan petroleum exploration and discovery in early 1975 chevron overseas petroleum inc commenced what became a major petroleum exploration effort in previously unexplored interior sudan with the cooperation of the sudanese government chevron has acquired a vast amount of geological and geophysical data during the past 12 years these data include extensive aeromagnetic and gravity surveys 36 040 mi 58 000 km of seismic data and 86 wells this information has defined several large rift basins now recognized as a major part of the central african rift system the sedimentary basins of interior sudan are characterized by thick nonmarine clastic sequences of jurassic cretaceous and tertiary age over 45 000 ft 13 716 m of sediment was deposited in the deepest trough and extensive basinal areas are underlain by more than 20 000 ft 6 096 m of sedimentary rocks the depositional sequences include thick lacustrine shales and claystones floodplain claystones and lacustrine fluvial and alluvial sandstones and conglomerates those lacustrine claystones deposited in a suboxic environment provide good oil prone source rocks reservoir sandstones have been found in a wide variety of nonmarine sandstone facies the extensional tectonism that formed these basins began in the jurassic early cretaceous movement along major fault trends continued intermittently into the miocene this deformation resulted in a complex structural history that led to the formation of several deep fault bounded troughs major interbasinal highs and complex basin flanks this tectonism has created a wide variety of structures many of which have become effective hydrocarbon traps during the past eight years several important oil discoveries have been made significant accumulations have been delineated in the heglig and unity areas where estimated recoverable reserves are 250 300 million bbl of oil

making ends meet in gondwana retracing the transforms of the indian ocean and reconnecting continental shear zones interpretation of the detailed patterns of ocean floor transforms revealed by satellite altimetry enables the creation of the indian ocean to be described quantitatively as four consecutive plate tectonic regimes separated at 200 136 89 and 43 ma each regime is reversed in turn by keeping transform termini coincident and colinear until conjugate points on the margins of pre existing plates regain their pre regime integrity progressive elimination of the indian ocean demonstrable as a smooth computer animation http www kartoweb itc nl gondwana leads to a refined re assembly of the continental fragments of central gondwana that is substantiated by new geological data a sequence of euler interval poles that describes the dispersal of the gondwana fragments time calibrated against available magnetic anomaly data is given the model requires a mid cretaceous position for india s southern tip about 1000 km south of madagascar prior to india s rapid northward migration

dispersive and nonhydrostatic pressure effects at the front of surge undular bores and shocks generated by dam break flows or tsunamis are examined considering nonhydrostatic pressure and dispersive effects in one and two horizontal dimensional space the fully nonlinear boussinesq type equations based on a weakly nonhydrostatic pressure assumption are chosen as the governing equations the equation set is solved by a fourth order accurate finite volume method with an approximate riemann solver several typical benchmark problems such as dam break flows and tsunami wave fission are tested in one and two horizontal dimensional space the computed results by the boussinesq type model are at least as accurate as the results by the hydrostatic shallow water equations this is particularly evident near the steep front of the wave where frequency dispersion can play an important role the magnitude of this nonhydrostatic pressure and dispersive effect near the front is quantified and the engineering implications of neglecting these physics as would be done through the use of a hydrostatic model are discussed

thermochronological dating of brittle structures in basement rocks a case study from the onshore passive margin of sw sri lanka 1 the brittle structural inventory of southern and southwestern sri lanka has been studied by kinematic mineralogical and thermochronological techniques thermochronological analyses of faults comprise apatite and zircon fission track ft data and apatite u th sm he ages from fault planes and undisturbed host rocks and range between 65 and 230 ma the ages of both settings are undistinguishable for topographic altitudes below 100 m while fault planes from higher elevations are significantly younger than the corresponding host rocks thermal history modeling and qualitative interpretation of the thermochronological data identify at least five episodes of thermal overprint associated with faulting activity occurring at 159 18 144 14 120 10 94 8 and 70 10 ma the kinematic mineralogical and thermochronological data collectively show that sri lanka was subjected to major n s oriented extension subsequent to the gondwana breakup the resulting brittle structures exert primary control on the island s geomorphology especially on the southern escarpment of the sri lankan highlands

the kaoko belt namibia part of a late neoproterozoic continental scale strike slip system the neoproterozoic kaoko belt of northwestern namibia to the west of the congo craton consists of basement gneisses and a sequence of highly deformed metasedimentary and metavolcanic rocks the metasedimentary rocks are interpreted as the infill of a narrow north trending basin floored by attenuated continental crust similar basins are represented by the western continuation of the kaoko belt the dom feliciano and ribeira belts of eastern brazil on the basis of consideration of data from these belts the kaoko belt rocks indicate the absence of a large neoproterozoic oceanic basin between the congo and southwestern american cratons deformation structures in the kaoko belt show transpressional kinematics with strain partitioning at around 550 ma north northwest south southeast sinistral transcurrent shear in the west passes eastward into east southeast directed thrusting the transcurrent shear component dominates temporally and in strain intensity dextral transcurrent motion at 550 ma is recorded in the east northeast trending schlesien mwembeshi shear zone bordering the congo craton to the south accordingly the congo craton extruded northeastward along the kaoko belt and the schlesien mwembeshi shear zone relative to the kalahari craton in the southeast and the south american cratons to the west this extrusion probably resulted from a final phase of convergence between east and west proto gondwana

mechanics of continental rift architecture continental rifts are long narrow features bounded on one or both sides by normal faults along which several kilometers of displacement has occurred resulting in deep rift valleys that form sedimentary basins continental rifts all share the same geometrical features they have widths that are characteristic of the region in which they form they are predominantly asymmetric and they are segmented along axes at a length scale that is also characteristic of the region we describe a very simple mechanical model to explain these three features the rift initiates with the formation of a conjugate set of normal faults that intersect at the center of the seismogenic layer one of these becomes the boundary fault system the other the failed conjugate fault being offset by the former becomes locked early in the rifting history hence an asymmetric graben is formed which is the basic building block of rifts the failed conjugate fault has distinctive properties that allow it to be easily distinguished from other faults examples of these faults from the tanganyika and malawi rifts show excellent agreement with predictions about their positions with respect to the boundary fault system and their net displacement and slip history the width of the asymmetric graben is a simple function of seismogenic thickness the net displacement on the boundary fault system is limited by the resistive friction and the flexural restoring force we calculate this limiting boundary fault system displacement and using the displacement length scaling law for normal faults suggest that this controls the along axis segmentation length both maximum fault slip and segmentation length are functions of the seismogenic thickness and the effective elastic thickness of the crust these calculations agree very well with observations of segmentation from the african rifts and the maximum length of range fronting normal faults in nevada

late palaeozoic deformation in central africa a result of distant collision seismic reflection profiles from the cuvette centrale central basin of zaire show that a major contractional deformation affected a part of central africa during the late permian and early triassic periods at the same time other regions of central africa experienced extensional and strike slip deformation and associated rift formation the coexistence of such varied styles of tectonic activity over a large continental area resembles the cenozoic tectonics of central asia and northwest europe the intracontinental deformation seen in these latter areas is attributed to collisional processes at a distant continental margin1 3 by analogy we argue here that deformation of central africa in the late palaeozoic period was a direct result of collisional processes some distance away at the southern margin of gondwana

cretaceous reactivation and intensified erosion in the archean proterozoic limpopo belt demonstrated by apatite fission track thermochronology abstract cratons are generally assumed to be regions of long lasting tectonic stability in particular the study of the phanerozoic exhumation history of cratons has been largely hampered by the scarcity of suitable stratigraphic controls onshore this fact is even more pronounced in terranes lacking mesozoic or younger penetrative structural fabrics and metamorphic overprinting our study in the limpopo belt shows that modern apatite fission track thermochronology provides a hitherto unavailable perspective in the study of these rocks and has profound implications for the crustal evolution of the zimbabwe craton apatite fission track data from 35 samples taken along two transects in the southern edge of the zimbabwe craton and in the central zone of the limpopo belt suggest that extensive regions experienced kilometer scale exhumation in two discrete events as recently as the cretaceous the first occurred at around 130 ma and the second at around 90 ma basin subsidence and sedimentation loads on the mozambique margin support the timing of these events and provide strong indications of the source and pathways for the eroded material further the results indicate that young and old surfaces in a geomorphological sense may be structurally juxtaposed in regions of high elevation in zimbabwe this is contrary to early ideas of surface chronologies based on summit accordances or invoking pediplanation

dating of zircon from ti clinohumite bearing garnet peridotite implication for timing of mantle metasomatism garnet peridotites from the kokchetav ultrahigh pressure uhp massif contain abundant volatile and high field strength element hfse bearing minerals such as ti clinohumite and zircon these characteristics are interpreted to be related to fluid infiltrated mantle metasomatism from the oceanic lithosphere that had been subducted below the area the zircons from the peridotites were dated by using sensitive high resolution ion microprobe shrimp and yielded apparent u pb ages of 554 494 ma weighted mean age 528 ma that are mostly consistent with the timing of uhp metamorphism deduced from diamond bearing country rocks in this massif these zircons have an almost flat rare earth element ree pattern and very low ree concentrations these characteristics are similar to those observed in kimberlitic zircons inherited zircon cores although only rarely preserved yielded apparently proterozoic ages and have different trace element characteristics compared to the overgrowth rims these features indicate that the mantle metasomatic events and the recrystallization of ti clinohumite and zircon were due to hfse enriched fluid infiltration during the uhp metamorphism at great depths the metasomatized mantle may have been transported farther into the deep upper mantle and contributed to the source of intraplate magmas such as kimberlites and alkali basalts because these rocks have characteristically high volatile and hfse concentrations such as those of the ti clinohumite bearing garnet peridotites

geochemistry and age of ultrahigh pressure metamorphic rocks from the kokchetav massif northern kazakhstan isotopic and geochemical data of the zerenda series metamorphic rocks from the kokchetav massif are reported some of these rocks contain microdiamond inclusions in garnets and other indicators of ultrahigh pressure metamorphism p 40 kbar t 900 1000 c the diamond bearing rocks exhibit distinctive geochemical characteristics compared to typical crustal rocks the ree patterns range from lree depleted to slightly lree enriched chondrite normalized la yb n 0 1 5 4 with a negative eu anomaly they are depleted in incompatible elements e g sr ba u th with respect to the upper crust in contrast non diamondiferous rocks of the zerenda series exhibit normal crustal geochemistry all rocks of the zerenda series have very radiogenic lead isotopes the measured values 238u 204pb compared with those calculated for the interval between crust formation and ultrahigh pressure uhp metamorphism suggest a decrease by factors of up to 200 during the uhp metamorphism the sm nd mineral isochrons from the diamond bearing rocks and other rock types of the zerenda series give a middle cambrian 524 535 ma age of metamorphism the nd model ages show that crust formation occurred about 2 3 ga ago significant fractionation of sm and nd and loss of incompatible elements may be due to partial melting of the protoliths the ar ar age determinations of secondary biotite and muscovite from the diamond bearing rocks yield an age of 517 5 ma this cooling age requires a short time interval between uhp metamorphism and uplift to a crustal level ultrahigh pressure metamorphism might be a significant source of pb for the mantle we propose that the radiogenic pb of the oceanic array is the contamination traces of numerous uhp events beside the geological aspect we demonstrate a method of dating a high grade metamorphic terrain using nd isotopes we compare whole rock isochrons and mineral isochrons and in this way get some insight into the behaviour of the sm nd system during very high grade metamorphic events

fluid evolution and mineralization mechanism of the east kounrad porphyry mo w deposit in the balkhash metallogenic belt central kazakhstan abstract the east kounrad mo w deposit is located in the eastern part of the balkhash metallogenic belt central kazakhstan the intrusion in the east kounrad deposit is a permain complex that includes medium and fine grained granites both the primary magma and hydrothermal fluids of the east kounrad deposit are characterized by high fluorine and high oxygen fugacity which would be important for the formation of high grade large mo deposit four mineralization stages are identified in this study stage i quartz wolframite molybdenite fluorite veins which were associated with intensively greisen alteration were formed from fluids with high temperature 340 460 c high pressure 250 400 bars high low salinity 54 5 wt and 3 4 10 2 wt respectively and high oxygen fugacity with a fluid system of h2o nacl co2 stage ii quartz molybdenite pyrite veins which were accompanied by k feldspars muscovite fluorite alteration were formed from fluids with medium high temperature 260 400 c medium pressure 100 250 bars high low salinity 39 1 58 4 wt and 2 4 11 7 wt respectively and relatively low oxygen fugacity after the main stage of w and mo mineralization stage iii quartz pyrite molybdenite veins associated with quartz sericite alteration were formed from fluids with medium low temperature 160 340 c low pressure less than 100 bars various salinity 4 5 46 2 wt and high oxygen fugacity stage iv quartz polymetallic sulfide veins related to argillic alteration were formed under temperature of 120 220 c and salinity of 1 7 8 1 wt remarkably fluid boiling is the most important precipitation mechanism of molybdenite and wolframite in addition the decreasing oxygen fugacity and temperature also play a vital role in triggering deposition of molybdenite the pb isotopic composition of sulfides shows that the ore forming materials at east kounrad are dominantly derived from the lower crust

experimental investigations of partial breach dam break flows abstractthis paper presents experimental results on partial breach dam break flows three dimensional 3d flood waves resulting from an instantaneous breach of a dam are experimentally investigated by combining different measurement techniques the dam break flow is initiated by quickly lifting a gate separating a reservoir and a downstream dry basin three different initial upstream water depths are considered the study utilizes the digital particle tracking velocimetry dptv technique for measuring the free surface velocity and water depth stereoscopic configuration of two high definition video cameras recording the movement of hollow plastic balls enabled measurement of the 3d velocity field and the water surface elevation in the near field the pressure head is measured by pressure transducers at various locations ultrasonic velocity profilers uvp are used to obtain velocity profiles in the emptying reservoir a simple nondimensional relationship for the variation of water surface elevation in

ultrarapid exhumation of ultrahigh pressure diamond bearing metasedimentary rocks of the kokchetav massif kazakhstan the diamond bearing ultrahigh pressure kokchetav massif recrystallized at eclogite facies conditions deep in the mantle at 180 km depth at 535f3 ma and yet new 40 ar 39 ar ages suggest that it may have been exhumed to crustal depths as indicated by closure of mica to ar loss by f529 ma these data indicate a possible exhumation rate of tens of kilometers per million years i e a rate that is comparable to rates of horizontal plate motion and subduction d 2003 elsevier b v all rights reserved

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

coupled fluid dynamics sediment transport modelling of a crater lake break out lahar mt ruapehu new zealand summary lahars and other sudden onset floods are highly dynamic with temporally and spatially evolving sediment loads numerical models of sudden onset flood behaviour particularly those applied to real world events have tended to neglect sediment transport processes this is a serious though perhaps understandable shortcoming given a lack of field measurements on flow bed interactions the extra computational complexity required and the problem of iteratively updating plastic channel boundaries this paper applies a fluid dynamics model with suspended sediment and bedload transport to calculate flow conditions and rapid landscape change due to an initially dilute lahar from mt ruapehu new zealand we take advantage of an unprecedented dataset of net topographic changes determined from pre and post lahar airborne lidar surveys and time series of hydrological measurements to run and validate the model a sensitivity analysis of key controls on water surface elevation and final sediment thickness illustrates how the flow was supply limited in the upper reaches it was capable of transporting a greater volume and calibre of material than that available modelled patterns of net erosion and deposition can replicate those observed r 2 0 79 and are dominantly controlled by channel gradient and topographic

nitrogen aggregation in metamorphic diamonds from kazakhstan microdiamonds that occur in high grade metamorphic rocks of the kokchetav massif in kazakhstan have been interpreted as part of the metamorphic mineral assemblage implying previously unrecorded pressures for the metamorphism of crustal rocks analysis of the infrared and visible absorption spectra from kokchetav microdiamonds extracted from recent stream sediments draining the metamorphic rocks shows roughly 48 63 aggregation of single n substitution to a centres but no measurable further aggregation calculated residence times assuming peak metamorphic temperatures of 800 900 c are from 0 5 ma to 40 ma and support the petrographic evidence that these diamonds formed during the high pressure metamorphism rather than being inherited from premetamorphic sediments

tight reassembly of gondwana exposes phanerozoic shears in africa as global tectonic players abstract aeromagnetic surveys help reveal the geometry of precambrian terranes through extending the mapping of structures and lithologies from well exposed areas into areas of younger cover continent wide aeromagnetic compilations therefore help extend geological mapping beyond the scale of a single country and in turn help link regional geology with processes of global tectonics in africa india and related smaller fragments of gondwana the margins of precambrian crustal blocks that have escaped or successfully resisted fracture or extension in phanerozoic time can often be identified from their aeromagnetic expression we differentiate between these rigid pieces of precambrian crust and the intervening lithosphere that has been subjected to deformation usually a combination of extension and strike slip in one or more of three rifting episodes affecting africa during the phanerozoic karoo early cretaceous and post miocene modest relative movements between adjacent fragments in the african mosaic commensurate with the observed rifting and transcurrent faulting lead to small adjustments in the position of sub saharan africa with respect to north africa and arabia the tight reassembly of precambrian sub saharan africa with madagascar india sri lanka and antarctica see animation in http kartoweb itc nl gondwana can then be extended north between nw india and somalia once the early cretaceous movements in north africa have been undone the seychelles and smaller continental fragments that stayed with india may be accommodated north of madagascar the reassembly includes an attempt to undo strike slip on the southern trans africa shear system this cryptic tectonic transcontinental corridor which first formed as a pan african shear belt 700 500 ma also displays demonstrable dextral and sinistral movement between 300 and 200 ma not only evident in the alignment of the unsuccessful karoo rifts now mapped from tanzania to namibia but also having an effect on many of the eventually successful rifts between africa arabia and east gondwana we postulate its continuation into the tethys ocean as a major transform or megashear allowing minor independence of movements between west gondwana partnered across the tethys ocean with europe and east gondwana partnered with asia europe and asia being independent before the 250 ma consolidation of the urals suture the relative importance of primary driving forces such as subduction pull and jostling forces experienced between adjacent rigid fragments could be related to plate size the larger plates being relatively closely coupled to the convecting mantle in the global scheme while the smaller ones may experience a preponderance of jostling forces from their rigid neighbours

unusual hf contents in metamorphic zircon from coesite bearing eclogites of the dabie mountains east central china implications for the dating of ultrahigh pressure metamorphism metamorphic zircon from coesite bearing eclogites in the dabie mountains encloses high p phases and may have formed at the peak of ultrahigh pressure uhp metamorphism morphologically the metamorphic zircon typically occurs as small multi faceted near spherical grains with homogeneous internal structure and weak backscattered electron bse luminescence geochemically it is characterized by extremely high and relatively constant contents of hafnium hf and very low contents of y u and th reflecting the contraction of the zircon lattice under the uhp conditions high contents of hf may be characteristic of zircon formed during uhp metamorphism which has important consequences for interpretation of geochronological results we propose that the metamorphic zircon extremely enriched in hf may be used to date the peak of uhp metamorphism that produced the coesite bearing eclogites in the dabie mountains and potentially in other uhp terranes

stable numerical model for transcritical flow and sediment transport on uneven bathymetry a stable and well balanced finite volume method was proposed to solve the shallow water equations coupled with a sediment transport equation an approximate riemann solver with a monotone upstream centered schemes for conservation laws muscl reconstruction was used for the computation of the flux terms a special but relatively simple treatment of source terms made it possible to ensure numerically and physically balanced computational results on very steep sloped beds various numerical simulations conducted on rigid beds have demonstrated that the proposed numerical scheme could provide stable computational results in the cases of flat and nonflat bottoms from the comparisons of experiments on erodible beds in one and two dimensional channels good agreement was obtained when proper parameters for turbulent flow and sediment transport were provided lastly the effects of the parameters in relation to hydraulic characteristics included in the flow and the sediment transport models were investigated

zircon response to diamond pressure metamorphism in the kokchetav massif ussr ultrahigh pressure metamorphism of the diamond bearing gneisses in the kokchetav massif ussr has been dated at 530 7 ma 2 sgr by shrimp ion microprobe analysis of metamorphic zircons zircon xenocrysts as old as 2000 ma have retained original isotopic compositions through the extreme metamorphic conditions and indicate that the protolith that was buried to diamond forming pressures included early proterozoic components highly variable reset ting of u pb isotopic systems in zircon xenocrysts at the time of metamorphism suggests that diffusive lead loss had commenced but was incomplete a disequilibrium observation consistent with relatively transient exposure of the rocks to extreme crustal depths

south atlantic fits and intraplate boundaries in africa and south america abstract the misfit problems raised by the pre drift reconstructions of the south and equatorial atlantic compel us to resort to intraplate deformations it is shown that acceptable deformations of the african plate in line with the benue trough and the transafrican rifts and shear zones provide a far from satisfactory solution to the geometrical misfit problems in the south atlantic these problems can instead be alleviated by considering a degree of deformation of the south american plate along an intraplate boundary in line with the parana province the rio grande rise and the walvis ridge in spite of the scarcity of field observations in this area of south america we show that such a deformation cannot be ruled out but it will have to be associated with deformations in the equatorial atlantic and in other areas of the african and south american plates

dam failure and a catastrophic flood in the mekong basin bolaven plateau southern laos 2018 abstract sudden breaching of a saddle dam on july 23 2018 on the perimeter of the xe nammoy hydroelectric power reservoir recently constructed in the mekong basin southern laos caused catastrophic flooding that resulted in fatalities and displaced thousands of individuals this study aims to 1 assess the cause of the dam breach and 2 understand the hydrologic and geomorphic impact of the flood our analysis shows that the collapse of the dam occurred as the reservoir was nearly full and the water level was rising against the inner slope of the earthen dam as the water discharged through the breach we calculate that the reservoir fell 22 m and lost 350 million m3 more than a third of the total stored volume on july 23 inspection of publicly available photographs of the failed dam confirms our interpretation that the failure was structural involving both piping and rotational slumping and not due to overtopping of the dam failure to properly process the local materials before construction of the earth core of the dam is the likely cause of failure our analysis of satellite imagery bolstered by field observations reveals that the flooding inundated an area of 46 km2 of villages and rural land in the vang ngao river a tributary of the mekong river basin by using 1d and 2d hydrological modeling we calculate a peak flood discharge of 8500 m3 s about half the mean discharge of the mississippi river the dam failure and concomitant losses are consistent with common inadequacies in this region in assessing potential hazards challenges of geomorphological geotechnical engineering when using tropical materials for construction and emergency planning and warning thus this analysis has important implications for the safer design and construction of the myriad of other dams that are either operating or planned within the multi national mekong river basin

dividing the south american continent to fit a gondwana reconstruction a model based on continental geology abstract the south american continental plate has been affected by intraplate deformation since the start of west gondwana disruption in the lower cretaceous about 140 ma that the present shape of south america is not precisely the same as its shape in reassembled gondwana partly explains the imperfect fits of the conjugate margins of the south atlantic proposed since the first reconstruction models of the early 20th century several attempts at defining subplates within south america have been published but not all take account of existing knowledge of its continental geology here a subdivision into eight rigid subplates is proposed based primarily on geological and tectonic evidence our model is tested against three published models of a multi subplate africa as re shaped to the pre breakup outline of that continent by visual fitting and the use of piercing points the south america blocks were rotated and the euler poles calculated interactively in reconstruction software all three proposed fits had overlapping block margins within south america indicating post breakup rifting except for the transbrasiliano lineament this nne ssw crustal scale shear zone was used as a boundary for seven of the eight blocks it is clearly the main intraplate accommodation zone in south america and an important piercing point in relation to the kandi lineament in west africa the other block boundaries correspond to narrow zones with sedimentary basins and or dyke swarms that have developed since south atlantic opening each fit required a different configuration of the south america subplates since the pre rift disposition of the african subplates also differ from each other contributing to the uncertainty in any detailed reassembly

experimental study of the impact factors of natural dam failure introduced by a landslide surge a large amount of loose deposit between the sides of a landslide lake may slide into a reservoir at any time because of influences such as earthquakes heavy rainfall or other triggering factors these factors can cause the formation of a huge surge and result in the instantaneous failure of a landslide dam the failure of a landslide dam can threaten people s lives and property downstream this paper analyzes the destruction mechanism of landslide dams because of a surge and the impact of the contact area the landslide height and the distance between the entry point and the dam site for a dam break through physical experiment and compares the experimental results with the calculated values experimental results show that the contact area and the landslide height have a significant impact on the dam break surge a measure based on the previously mentioned factors can be made to reduce the dam failure risk caused by a landslide surge two empirical methods pan s method and miller s method are used to predict the landslide surge wave and compared with the experimental results the simulated results show that the miller s method used for predicting the landslide surge wave is more reasonable

on the occurrence and boron isotopic composition of tourmaline in ultra high pressure metamorphic rocks abstract the extensive p t stability and the high chemical variability of tourmaline tur together with its common occurrence in metasediments proves its high potential for petrological and isotope geochemical studies on fluid rock interaction in subduction and collision related rocks this paper reviews the occurrence major element chemistry and boron isotopic composition of tur in high and ultrahigh pressure metamorphic uhpm rocks in addition it presents a new discovery of coesite bearing tur schorl from the erzgebirge germany as well as tur dravite related to the retrograde history of coesite and diamond bearing rocks from the erzgebirge and the kokchetav massif kazakhstan the scarce data on worldwide occurrences of u hpm tur reveal a high occupation of the crystallographic x site dominated by na and the possible presence of excess b with little further distinctiveness in its major element composition when compared with tur from medium grade rocks high k 2 o contents in tur are probably not related to uhp growth or equilibration the b isotopic composition of u hpm tur ranges in 11 b from 16 to 1 with many samples in or below the range of continental crust in contrast tur formed during retrograde fluid influx typically shows high 11 b values up to 28 suggesting heavy b fluids infiltrating the exhuming u hpm units coesite inclusions in tur characterized by raman spectroscopy are regarded as the best indicator for its uhp stability supplementary material analytical methods tourmaline compositions and boron isotope values are available at http www geolsoc org uk sup18354

experiments on flow at a 90 open channel junction although open channel junctions are common in many hydraulic structures no comprehensive data set has been compiled that describes the 3d flow field within the junction itself this physical model study examined a 90 sharp edged open channel junction for channels of equal width depth measurements were made using a point gauge while velocity measurements were taken using an acoustic doppler velocimeter over a grid defined throughout the junction region the average velocity and turbulence intensity were calculated from a time series of velocities that was recorded at each location in addition a 2d mapping of the water surface was performed on a 76 2 mm square grid throughout the channel junction this paper presents the details of the experimental procedure and the general flow characteristics observed the full data set generated during this experimental work is available for downloading on the internet using a small portion of the data recorded an evaluation of several previously proposed theori

40ar 39ar u pb and sm nd constraints on the timing of metamorphic events in the maksyutov complex southern ural mountains new 40 ar 39 ar u pb and sm nd data from 13 metamorphic samples constrain the formation and exhumation of the high pressure maksyutov complex in the southern uralide orogen the maksyutov complex records the highest pressure metamorphism in the southern uralides and is part of the thrust stack that composes the footwall beneath the main ural fault the new isotopic data from eclogite in the lower unit of the maksyutov complex include 40 ar 39 ar data from phengite u pb data from rutile and apatite and sm nd data from garnet clinopyroxene rutile and apatite these data indicate that eclogite facies metamorphism occurred in the mid devonian epoch c 380 ma corresponding with eastward subduction of the east european craton beneath the magnitogorsk island arc after partial exhumation the lower unit was juxtaposed c 360 ma based on 40 ar 39 ar white mica data with the middle and upper units at mid crustal levels during the development of a d 3 shear zone within the subduction zone by the late carboniferous epoch the maksyutov complex was exhumed to upper crustal levels compatible with sedimentologic evidence that at this time the foreland thrust and fold belt began to form and material was shed off it into the foreland basin

petroleum geology of cretaceous tertiary rift basins in niger chad and central african republic this overview of the petroleum geology of rift basins in niger chad and central african republic car is based on exploration work by exxon and partners in the years 1969 1991 the work included 50 000 km of modern reflection seismic 53 exploration wells 1 000 000 km2 of aeromagnetic coverage and about 10 500 km of gravity profiles the results outline ten cretaceous and tertiary rift basins which constitute a major part of the west and central african rift system wcars the rift basins derive from a multiphased geologic history dating from the pan african approximately 750 550 ma to the holocene main rifting was probably caused by the stretching and subsidence of african crustal blocks during the breakup of gondwana in the early cretaceous and by r activated plate movements in the early tertiary wcars in the study area is divided into the west african rift subsystem was and the central african rift subsystem cas was basins in niger and chad are chiefly extensional and are filled by up to 13 000 m of lower cretaceous to holocene continental and marine clastics the basins contain five oil 19 43 api and two oil and gas accumulations in upper cretaceous and eocene sandstone reservoirs the hydrocarbons are sourced and sealed by upper cretaceous and eocene marine and lacustrine shales the most common structural styles and hydrocarbon traps usually are associated with normal fault blocks cas rift basins in chad and car are extensional and transtensional and are filled by up to 7500 m of chiefly lower cretaceous continental clastics the basins contain eight oil 15 39 api and one oil and gas discovery in lower and upper cretaceous sandstone reservoirs the hydrocarbons are sourced by lower cretaceous shales and sealed by interbedded lacustrine and flood plain shales structural styles range from simple fault blocks through complex flower structures the main hydrocarbon traps are in contractional anticlines geological conditions favor the discovery of potentially commercial volumes of oil in wcars basins of niger chad and car

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

geochemistry of garnetiferous ti clinohumite rock and talc kyanite phengite almandine schist from the kokchetav uhp terrane kazakhstan an insight to possible origins of some chemically unusual uhp rocks abstract major element trace element and o isotope analyses were carried out for garnetiferous ti clinohumite rocks and eclogites from the kumdy kol area as well as talc kyanite phengite almandine schists previously named as whiteschists and eclogites from the kulet area of the kokchetav ultrahigh pressure uhp terrane to explore their possible origins both garnetiferous ti clinohumite rocks and talc kyanite phengite almandine schists are possibly genetically related to eclogites as they occur in close association with eclogites enclosed within gneiss metapelite in these two areas most eclogites from the kumdy kol and the kulet areas exhibit chemical characteristics similar to that of ocean floor basalt although a few show arc affinity with slight ta and nb depletions garnetiferous ti clinohumite rocks from the kumdy kol region are characterized by very high mg low si ca na and k contents compared with eclogites they also exhibit lree depleted patterns with negative eu anomalies their o isotope compositions are less than 5 6 on the other hand the rocks show comparable al ni cr co and hree contents as eclogites the mg enrichment of these garnetiferous ti clinohumite rocks is suggested to have originated from their mafic protoliths that had been subjected to mafic rock ultramafic rock fluid interactions talc kyanite phengite almandine schists from the kulet area are typically low in ca and na but high in mg k rb ba cs th u and o isotope compositions 6 to 10 relative to eclogites this peculiar chemical composition can be accounted for if the basaltic protoliths had been subjected to sequential high temperature enrichment of mg and low temperature enrichment of k rb ba cs th u and o isotope compositions seawater basalt interactions one talc kyanite phengite almandine schist sample klw 7 is distinctly high in si and k but low in mg and fe which can be attributed to more severe low temperature hydrothermal alterations however except for mg the enrichment of rb ba cs th u and o isotope compositions in these schist samples could also be enhanced by or otherwise be interpreted as a result of fluid influx sourced from enclosing metapelite during subduction

metamorphic evolution of diamond bearing and associated rocks from the kokchetav massif northern kazakhstan representative diamond bearing gneisses and dolomitic marble eclogite and ti clinohumite bearing garnet peridotite from unit i at kumdy kol and whiteschist from unit ii at kulet eastern kokchetav massif northern kazakhstan were studied diamond bearing gneisses contain variable assemblages including grt bt qtz pl kfs zo chl tur cal and minor ap rt and zrn abundant inclusions of diamond graphite chlorite or calcite phengite clinopyroxene k feldspar biotite rutile titanite calcite and zircon occur in garnet diamond bearing dolomitic marbles consist of dol di grt phl inclusions of diamond dolomite graphite biotite and clinopyroxene were identified in garnet whiteschists carry the assemblage ky tlc grt rt garnet shows compositional zoning and contains abundant inclusions of talc kyanite and rutile with minor phlogopite chlorite margarite and zoisite inclusions and zoning patterns of garnet delineate the prograde p t path inclusions of quartz pseudomorphs after coesite were identified in garnet from both eclogite and gneiss other ultrahigh pressure uhp indicators include na bearing garnet up to 0 14 wt na2o with omphacitic cpx in eclogite occurrence of high k diopside up to 1 56 wt k2o and phlogopite in diamond bearing dolomitic marble and cr bearing kyanite in whiteschist these uhp rocks exhibit at least three stages of metamorphic recrystallization the fe mg partitioning between clinopyroxene and garnet yields a peak temperature of 800 1000 c at p 40 kbar for diamond bearing rocks and about 740 780 c at 28 35 kbar for eclogite whiteschist and ti bearing garnet peridotite the formation of symplectitic plagioclase amphibole after clinopyroxene and replacement of garnet by biotite amphibole or plagioclase mark retrograde amphibolite facies recrystallization at 650 680 c and pressure less than about 10 kbar the exsolution of calcite from dolomite and development of matrix chlorite and actinolite imply an even lower grade greenschist facies overprint at c 420 c and 2 3 kbar a clockwise p t path suggests that supracrustal sediments together with basaltic and ultramafic lenses apparently were subjected to uhp subduction zone metamorphism within the diamond stability field tectonic mixing may have occurred prior to uhp metamorphism at mantle depths during subsequent exhumation and juxtaposition of many other tectonic units intense deformation chaotically mixed and mylonitized these lithotectonic assemblages

a customised gis to aid gondwana research geographical information systems gis provide tools for manipulating and analysing the large thematic datasets associated with gondwana research we have used a customised version of environmental systems research institute s arcgis version 8 2 as the basis of our gondwana and southern ocean computer model gscm the additional functionality necessary for paleogeographic work was written in visual basic for applications vba and includes routines for assigning data to the relevant plates plate sieving and transformation from present to paleo coordinates rotate data the sieving function employs a user supplied plate network so that the same data can be assigned to different plate systems at different scales paleo coordinate transformations are handled entirely within arcgis preserving the links to data attributes and thereby permitting spatial analysis with continents in their former configuration we give a detailed description of these functions together with some simple examples of the system s capabilities using data to large igneous provinces

the three dimensional shape and localisation of deformation within multilayer sheath folds abstract sheath folds are widely believed to develop by the passive geometric amplification of folds in which layering has no mechanical influence during non coaxial deformation where layering becomes rheologically significant then active sheath folds may form in which the inner nose of the sheath decouples along detachments and undergoes a translation relative to the outer folded layers that surround and envelope the nose we present a systematic 3 d analysis of multiple folded layer geometries in a serially sectioned natural sheath fold weak layers which are reactivated as detachments define different shaped sheath folds relative to other layers with the sense of cut off along detachments reversing across the axial surface as the inner fold nose has protruded into the surrounding sheath envelope detachment layers are more tightly folded meaning that such active sheath folds are non similar shapes the obliquely oriented bifurcating en echelon nature of the fold hinges developed in adjacent layers suggest that pre cursor folds formed with pronounced 3 d obliquity relative to the subsequent shear plane mineral lineations folded around sheath closures display asymmetric star burst patterns consistent with recrystallisation during active folding and hinge rotation we show that the eye fold shapes exposed in any 2 d y z slice can be used to predict the geometry of marker horizons back along the x axis in the third dimension this self similarity may be of value when tracing stratiform mineralised horizons in large scale sheath folds

buoyancy driven rapid exhumation of ultrahigh pressure metamorphosed continental crust abstract preservation of ultrahigh pressure uhp minerals formed at depths of 90 125 km require unusual conditions our subduction model involves underflow of a salient 250 150 km wide 90 125 km long of continental crust embedded in cold largely oceanic crust capped lithosphere loss of leading portions of the high density oceanic lithosphere by slab break off as increasing volumes of microcontinental material enter the subduction zone buoyancy driven return toward midcrustal levels of a thin 2 15 km thick low density slice finally uplift backfolding normal faulting and exposure of the uhp terrane sustained over 20 million years rapid 5 mm year exhumation of the thin aspect ratio uhp sialic sheet caught between cooler hanging wall plate and refrigerating downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces the intracratonal position of most uhp complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone uhp metamorphic terranes consist chiefly of transformed yet relatively low density continental crust compared with displaced mantle material otherwise such complexes could not return to shallow depths relatively rare metabasaltic metagabbroic and metacherty lithologies retain traces of phases characteristic of uhp conditions because they are massive virtually impervious to fluids and nearly anhydrous in contrast h2o rich quartzofeldspathic gneissose schistose more permeable metasedimentary and metagranitic units have backreacted thoroughly so coesite and other uhp silicates are exceedingly rare because of the initial presence of biogenic carbon and its especially sluggish transformation rate uhp paragneisses contain the most abundantly preserved crustal diamonds

evidence of fluid inclusions in metamorphic microdiamonds from the kokchetav massif northern kazakhstan abstract microdiamonds from garnet clinopyroxenites of the kokchetav massif northern kazakhstan and associated alluvial diamonds have been investigated using infrared spectroscopy all diamonds have high nitrogen contents from 747 up to 4488 20 at ppm and are of type ib iaa nitrogen data are consistent with a diamond formation over a narrow temperature range microdiamonds from garnet clinopyroxenites are characterized by water and carbonate inclusions similar to those ocurring in fibrous coated diamonds from kimberlite which suggest that diamonds grew from a c h o fluid in contrast alluvial microdiamonds do not contain any carbonate inclusions and h2o is absent or present in only minor amounts alluvial diamonds are considerably more n rich but have a similar range of nitrogen aggregation states it is suggested that alluvial diamonds belong to a separate diamond population compared with the microdiamonds from garnet clinopyroxenites the kokchetav microdiamonds are distinct from kimberlitic fibrous diamonds in that they contain abundant ib centres have high nitrogen contents and show different populations depending on host rock type these characteristics support a metamorphic origin for the microdiamonds from the uhpm rocks of the kokchetav massif

dam break in channels with 90 bend in practice dam break modeling is generally performed using a one dimensional 1d approach for its limited requirements in data and computation however for valleys with multiple sharp bends such a 1d model may fail for predicting as well the maximum water level as the wave arrival time this paper presents an experimental study of a dam break flow in an initially dry channel with a 90 bend with refined measurements of water level and velocity field the measured data are compared to some numerical results computed with finite volume schemes associated with roe type flux calculation the 1d approach reveals the expected limits while a full two dimensional 2d approach provides fine level prediction and rather satisfactory information about the arrival time a hybrid approach is now proposed mixing the 1d model for the straight reaches and local 2d models for the bends the compatibility of the roe fluxes at the interfaces requires a careful formulation but the resulting scheme seems able to capture reflection and diffraction processes in such a way that the results are really good in what concerns the water level

dam break outburst flood propagation and transient hydraulics a geosciences perspective summary dam break outburst floods are sudden short lived and high magnitude discharges of water and sediment propagating over an initially dry terrain both field observations by geoscientists and experimental measurements by engineers have produced understanding of outburst floods but neither approach has satisfactorily supplied data that is directly of use to hazard managers this study draws together those two literature groups and provides new experimental data to test hypotheses concerning the effect of a series of external controls on resultant longitudinal flow propagation and transitory hydraulics these external controls form experimental treatments and are i the height of the lock gate raise as a surrogate for dam breach evolution and initial hydrograph ii bed roughness and iii suspended sediment concentration these control both horizontal and vertical fluid motion the latter of which is a key property for controlling aeration bed shear stress and turbulence intensity the experimental treatments show that outburst floods on horizontal beds can be identified to possess three longitudinal flow regimes firstly there is a short acceleration due to the reservoir pressure level i e the depth of impounded water secondly channel flow quickly converges to an inertial regime the third flow regime is viscous and dominated by channel bed friction flow depth increases in the inertial regime then decreases in the viscous regime experiments herein show that the timing of the peaks at a point down channel of each hydraulic quantity varies most notably peak bed shear stress precedes flow velocity which precedes peak flow depth a point of inflexion occurs with distance down channel that is a manifestation of the transition from accelerating to decelerating flows these external controls and the effects of them on longitudinal and vertical motion are analysed statistically and quantitative relationships are derived to underpin the resultant conceptual model this conceptual model should serve as a platform for future probabilistic and process based modelling of outburst floods

widespread remagnetization of late proterozoic sedimentary units of uruguay and the apparent polar wander path for the rio de la plata craton summary a palaeomagnetic study was performed on three neoproterozoic sedimentary successions exposed in central and eastern uruguay in order to better constrain the palaeogeographic evolution of the rio de la plata craton by the latest proterozoic these units comprise the latest ediacaran to early cambrian calcareous cerro victoria fm exposed in the central nico perez terrane the late ediacaran clastic yerbal fm and the calcareous polanco fm exposed in the dom feliciano belt and the ediacaran clastic rocha fm exposed in the easternmost punta del este suspect terrane the study showed that none of the units carry the original remanence and that they were affected by remagnetizations the most widespread remagnetization is shown by the cerro victoria yerbal and rocha formations a dual polarity post folding remanence carried by hematite was isolated in all these units mean directions from them are cv cerro victoria dec 179 1 inc 59 0 95 2 9 n 79 samples y yerbal dec 182 4 inc 61 9 95 4 5 n 38 samples r rocha dec 4 2 inc 64 9 95 2 7 n 42 samples their corresponding pole positions are virtually identical cv 82 6 s 309 3 e a95 3 9 y 77 0 s 298 4 e a95 5 9 r 76 6 s 291 0 e a95 4 2 suggesting the same magnetization age and process comparison with the south american path suggests the late permian early triassic or the latest cretaceous palaeogene as the most likely times for its acquisition these poles are also identical to the pole of the late proterozoic la tinta fm which is interpreted as affected by the same remagnetization and therefore invalid for palaeogeographic reconstructions a different magnetic component also dual polarity and post folding was defined at two sites of the cerro victoria fm and other two of the polanco fm possibly carried by magnetite the mean geomagnetic poles from both units cvc 4 2 n 343 2 e a95 13 8 and p 3 2 n 325 8 e a95 15 2 fall on the middle cambrian to early ordovician segment of the gondwana apwp suggesting that age for the remagnetization this permits to establish the age of the tectonic event affecting the neoproterozoic units of the rio de la plata craton as early cambrian ca 525 ma the available palaeomagnetic poles for this craton permit to constrain its palaeogeographic evolution only since approximately 600 ma

formation of the kokchetav subduction collision zone northern kazakhstan insights from zircon u pb and lu hf isotope systematics abstract the kokchetav subduction collision zone is located in the western part of the ancestral central asian orogenic belt this zone is built up by the precambrian kokchetav microcontinent which includes a hp uhp metamorphic belt and the north kokchetav tectonic zone nktz which represents an accretionary complex between the kokchetav microcontinent and the adjacent stepnyak island arc the entire region is widely intruded by palaeozoic granitoids which were emplaced after the collision of the stepnyak island arc with the kokchetav microcontinent we present zircon u pb ages and lu hf systematics in zircon to better characterize the tectonic evolution of the kokchetav subduction collision zone the lu hf results indicate that the kokchetav basement rocks are derived from late neoarchaean early palaeoproterozoic 2 5 ga crust for the granite gneiss basement of the kokchetav microcontinent early mesoproterozoic grenville age 1 17 1 14 ga zircon u pb crystallization ages were obtained for the nktz two main age components were recognized 1 an oldest mesoproterozoic age component 1 20 1 05 ga similar as for the kokchetav microcontinental zircons and 2 a younger early cambrian 540 520 ma or late cambrian early ordovician 490 480 ma age component th u ratios 0 1 and are typical of a magmatic protolith the distinction between both zircon types is supported by characteristic cathodoluminescence images these results confirm previous observations indicating early palaeozoic high grade metamorphism 540 520 ma and collisional deformation 490 480 ma of the mesoproterozoic kokchetav zone as a result of subduction accretion and collision of the stepnyak island arc to the kokchetav microcontinent for two additional samples of the balkashin granitic complex early devonian 415 395 ma zircon crystallization ages magmatic th u ratios were obtained the occurrence of a mesozoic xenocryst within these leucogranites may indicate that they were emplaced in a continental arc setting

experimental study and numerical verification of silted up dam break abstract in this study multiphase flood waves caused by failure of silted up dams have been scrutinized experimentally and efficacy of the public domain cfd software openfoam numerical models to represent such phenomenon is evaluated experimental observations are sparse on dam break for reservoirs with high sediment depths called silted up reservoirs sediment layer in this case may behave as a viscous fluid and produce a complex multi layer flow of air and water overtopping a sediment layer massive sediment quantities can have a substantial impact on the wave propagation pattern caused by dam break this study contributes to an experiment based understanding of dam break phenomenon with silted up reservoir and elaborates the propagation of multi layer waves herein different hydraulic conditions were experimentally observed including 8 different upstream sediment depths in the range of 0 0 24 m and 4 downstream initial water levels of 0 0 02 0 04 and 0 05 m which collectively created 32 scenarios the resulted waves were filmed by high speed cameras and data were extracted through image processing an open source cfd software namely openfoam was employed to simulate the experiments using two distinct numerical approaches volume of fluid vof and eulerian results of both numerical approaches were in good agreement with the experimental measurements with mae and rmse error values varying from 0 006 m to 0 031 m and 0 008 m to 0 043 m respectively 2 to 10 and 2 6 to 14 with respect to the reservoir height of 0 3 m the eulerian approach showed marginally superior performance to vof especially in cases with high sediment depths which is likely due to accommodating phase mixing a wide collection of high quality data is made available online in the appendix data and could be utilized in future studies

k2o prograde zoning pattern in clinopyroxene from the kokchetav diamond grade metamorphic rocks missing part of metamorphic history and location of second critical end point for calc silicate system abstract despite extensive studies of calc silicate rocks of the kokchetav massif there is no satisfactory explanation of the origin of potassium bearing clinopyroxene in alkali poor metamorphic rocks in this paper we report the finding of potassium bearing clinopyroxene with prograde zonation k 2 o increases from core to rim from diamond grade but diamond free uhp calc silicate rocks of the kokchetav massif we believe that the crystallization of potassium bearing clinopyroxene started on the prograde stage and slightly prior to the peak of uhp metamorphism thus prograde metamorphic history is only traceable in diamond free uhp calc silicate rocks while in diamond bearing uhpm rocks it is completely reset fluid and polyphase solid inclusions originally representing melt inclusions occur in the core of potassium bearing clinopyroxene and imply that melt and fluid may coexist in calc silicate rocks even at 1000 1100 c and 6 7 gpa

constraints on extension of continental lithosphere the extension of the continental lithosphere is considered in terms of the deformation of a thin viscous sheet with a temperature dependent rheology which is governed primarily by the strength of the upper mantle the lithosphere is subjected either to a constant strain rate or to a strain rate decreasing with time corresponding to the stretching of a region of fixed volume with a constant velocity boundary condition the progress of the extension is influenced by the decrease in strength of the lithosphere owing to its attenuation and by an increase in strength resulting from cooling the relative importance of these two effects depends on two parameters a dimensionless strain rate and the total strain for a given strain rate the extending lithosphere goes through two phases as the strain increases in the first the effect of thinning the lithosphere is predominant and the average strength of the lithosphere remains close to and perhaps slightly below its original strength in the second the effect of cooling of the lithosphere leads to very rapid increase in strength as a function of strain resulting presumably in an effective limit on the degree of stretching that the lithosphere experiences for the case of strain rate decreasing with time when the initial strain rates are of order 10 15s 1 this maximum extension is between 100 and 150 for a strain rate of 10 l4 s 1 the maximum lies around 400 to 500 at higher strain rates than this very large extension may occur before cooling significantly affects the strength of the lithosphere this model accords well with the constraints available from the continental margins of the north atlantic

a plate tectonic setting for mesozoic rifts of west and central africa abstract africa by virtue of its central position within gondwana has recorded much of the complex history of plate interactions which have progressively fragmented this supercontinent since the early mesozoic continental reconstructions reveal both a temporal and spatial relationship between the development of the continental margins of africa and the formation of rifted sedimentary basins deep within the african continent the multi stage opening of the atlantic ocean and associated rifting in west and central africa provide one of the most impressive examples of ocean continent tectonic interactions during the early cretaceous the onset of rifting along the future margins of the south atlantic is contemporaneous with intra continental rifting generating both strike slip and extensional basins within west and central africa the syn rift phase of intra continental basin development continued until the santonian by which time africa and south america had been physically separated for approximately 10 ma except for minor rifting during the senonian and tertiary the short lived phase of deformation at about 80 ma marks the transition into the post rift or sag phase of basin development this deformational event can be correlated with a period of plate motion changes recognised from fracture zone geometries seen in both the central and south atlantic oceans using present day stress analogies cretaceous rifting in africa can provide a means of indicating the regional palaeostress directions within africa at this time

closure to treatment of natural geometry in finite volume river flow computations by h capart t i eldho s y huang d l young and y zech

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it

diamond inclusions in garnets from metamorphic rocks a new environment for diamond formation diamonds commonly occur in kimberlites lamproites and alluvial sediments derived from these rocks more recently diamonds or their graphite pseudomorphs have been discovered in ultramafic massifs1 and picrites2 here we report the occurrence of diamonds in situ in crustal rocks highly retrograded high pressure metamorphic garnet pyroxene and pyroxene carbonate garnet rocks biotite gneisses and schists from the kokchetav massif northern kazakhstan ussr the diamonds are cubo octahedral averaging 12 m in size and occur in zircons and with euhedral graphite as inclusions in unzoned garnets we believe that the zircon and garnet matrices protected these diamonds from retrogressive transformation to graphite mica rutile titanite clinopyroxene kyanite and zircon also occur as inclusions in garnet often intergrown with the diamonds equilibration relations of inclusions and host garnets indicate that both diamonds and graphite crystallized from a fluid phase under static conditions at pressures of 40 kbar and temperatures 900 1 000 c

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

exhumation of high pressure rocks of the kokchetav massif facts and models abstract the exhumation of ultrahigh pressure uhp metamorphic units from depths more than 100 120 km is one of the most intriguing questions in modern petrology and geodynamics we use the diamondiferous kumdy kol domain in the kokchetav massif to show that exhumation models should take into consideration initially high uplift velocities from 20 down to 6 cm year and the absence of the deformation of uhp assemblages the high rate of exhumation are indicated by ion microprobe shrimp dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds diamondiferous rocks in the kumdy kol domain occur as steeply dipping 60 80 thin slices few hundred metres within granite gneiss using geological petrological and isotopic geochemical data we show that partial melting of diamondiferous metamorphic rocks occurred a very important factor which has not been taken into account in previous models deformation of diamondiferous rocks at kumdy kol is insignificant diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation all these facts could be explained by partial melting of metapelites and granitic rocks in the kumdy kol domain the presence of melt is responsible for an essential reduction of viscosity and a density difference between crustal rocks and mantle material and reduced friction between the upwelling crustal block the subducting and overriding plates besides the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle we construct different models for the three stages of exhumation a model similar to corner flow for the first superfast exhumation stage an intermediate stage of extension most important from structural point of view and a very low rate of exhumation in final diapir erosional uplift