the current state of knowledge about shatter cones introduction to the special issue shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments the occurrence of shatter cones is the only accepted meso to macroscopic recognition criterion for impact structures shatter cones exhibit a number of geometric characteristics orientation apical angles striation angles sizes that can be best described as varied from case to case possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them a database of shatter cone occurrences has been produced for this introduction paper to the special issue of meteoritics and planetary science on shatter cones distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter with a few currently questionable exceptions all pertinent hypotheses of formation are presented and discussed several may be discarded in light of the most recent observations the branching fracture mechanism and the interference models proposed respectively by sagy et al 2002 and baratoux and melosh 2003 require further evaluation new observations experiments or theoretical considerations presented in this special issue promise an important step forward based on a renewed effort to resolve the enigmatic origin of these important features

chemical weathering river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on luzon island the philippines abstract we investigated rates of chemical weathering of volcanic and ophiolitic rocks on luzon island the philippines luzon has a tropical climate and is volcanically and tectonically very active all factors that should enhance chemical weathering seventy five rivers and streams 10 draining ophiolites 65 draining volcanic bedrock and two volcanic hot springs were sampled and analyzed for major elements alkalinity and 87sr 86sr cationic fluxes from the volcanic basins are dominated by ca2 and mg2 and dissolved silica concentrations are high 500 1900 m silica concentrations in streams draining ophiolites are lower 400 900 m and the cationic charge is mostly mg2 the areally weighted average co2 export flux from our study area is 3 89 0 21 106 mol km2 yr or 5 99 0 64 106 mol km2 yr from ophiolites and 3 58 0 23 106 mol km2 yr from volcanic areas uncertainty given as 1 standard error s e this is 6 10 times higher than the current best estimate of areally averaged global co2 export by basalt chemical weathering and 2 3 times higher than the current best estimate of co2 export by basalt chemical weathering in the tropics extrapolating our findings to all tropical arcs we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments which amount to 1 of the global exorheic drainage area chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long term carbon cycle

a strain based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets numerical modelling of impact cratering has reached a high degree of sophistication however the treatment of porous materials still poses a large problem in hydrocode calculations we present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation in contrast to previous attempts e g p alpha model snowplow model our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure our new e alpha model requires only four input parameters and each has a physical meaning the model is simple and intuitive and shows good agreement with a wide variety of experimental data ranging from static compaction tests to highly dynamic impact experiments our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation we present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity scaled source size

magnetic signature of river sediments drained into the southern and eastern part of the south china sea malay peninsula sumatra borneo luzon and taiwan abstract magnetic properties of 22 river samples collected in the malay peninsula sumatra borneo luzon and taiwan have been investigated in order to magnetically characterize the sediments drained and deposited into the south china sea the geological formations as well as the present climatic conditions are different from one region to another laboratory analyses include low field magnetic susceptibility anhysteretic arm and isothermal irm remanent magnetizations acquisition and decay back field acquisition thermal demagnetization of three axes irm hysteresis cycles and low temperature magnetic measurements the magnetic properties indicate that the sediments are a mixture of hematite magnetite and pyrrhotite in different proportions depending on the region combined with results previously reported for the three main asian rivers pearl red and mekong rivers the new data indicate that in general hematite rich sediments are delivered to the southern basin of the south china sea while the northern basin is fed with magnetite and pyrrhotite rich sediments in addition to this general picture some variability is observed at smaller geographic scales indeed the magnetic assemblages are closely related to the geology of the various catchments while clay minerals previously reported for the same samples are more representative of the climatic conditions under which the parent rocks have evolved within each catchment the magnetic fraction now well characterized in the main river sediments drained into the south china sea can be used as a tracer for changes in precipitation on land and in oceanic water mass transport and exchange

a geophysical study of the manila trench luzon philippines 2 fore arc basin structural and stratigraphic evolution the pattern of sediment distribution within the manila trench subduction zone west of luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic hemipelagic sediment layer and oceanic basement in the northern part of the manila trench these turbidites were transported from the uplifted collision zone of taiwan southward along the manila trench by gravity controlled processes similarly thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the north palawan block and the central philippines sedimentation within the manila trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis in contrast the fore arc basin system of the manila trench receives sediment through submarine canyons incised into the west luzon continental slope the rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench the size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench a thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region this generalization may apply to other convergent plate margins as well the uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins this variability results in the formation of several different types of fore arc morphology along the manila trench the overall seismic stratigraphy of the west luzon trough fore arc basin is remarkably uniform with seismic sequences continuous for up to 110 km along strike the unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long term approximately 10 ma uplift of the accretionary prism and relatively short term approximately 1 2 ma variations in sediment supply to the fore arc basin modulated by eustatic sea level changes

the mjolnir marine impact crater porosity anomaly abstract we have developed a quantitative model for the porosity change induced by the mjolnir impact using density and seismic traveltime distributions and postimpact sediment deformation compared with the surrounding undisturbed platform sediments the porosity increased immediately after impact up to 6 3 in the collapsed and brecciated crater periphery whereas porosity decreased by 1 in the dense central uplift following impact the crater was progressively buried by a 2 2 5 km thick overburden which caused radially varying differential compaction among the disrupted sedimentary rocks this compaction has significantly affected the postimpact crater evolution and decreased the porosity anomaly to values of 2 5 in the periphery and 2 2 in the center high resolution seismic profiles reveal possible gas related seismic amplitude anomalies spatially related to the 13 33 km3 excess pore space in impact deformed strata along the crater periphery potential plays may include lower to middle jurassic sands structurally trapped by impact generated tilted fault blocks and the synimpact extensively fractured allochthonous breccia however the crater is a very high risk hydrocarbon prospect because reservoir properties may have been destroyed by the deep mesozoic burial followed by large scale cenozoic uplift and erosion nonetheless the mjolnir porosity anomaly provides new data about the postimpact crater deformation similar data are almost entirely absent from the terrestrial impact record

arc volcanic rocks characterize the landward slope of the philippine trench off northeastern mindanao five dredge hauls from the landward flanks 9300 3600 m of the philippine trench at 10 30 n northeast of mindanao recovered densely plagioclase clinopyroxene phyric basalt basaltic andesite and andesite with small amounts of diabase these rocks are all transitional from an arc tholeiitic to arc calcalkaline volcanic series and have low ni and cr 10 30 and 30 70 ppm respectively low tio2 0 7 1 2 and high incompatible element concentrations sr 260 530 ppm ba 170 410 ppm a single dredge on the offshore slope of the trench included aphyric tholeiitic basalt similar to n type ocean ridge basalts sampled from other locations in the west philippine basin tio2 1 0 1 2 ni 70 80 ppm cr 130 ppm ba 2 10 ppm sr 70 85 ppm the landward samples are distinct from the offshore samples and are identical in texture and composition to arc volcanic rocks from southeast and northeast mindanao the mindanao rocks are interpreted to be part of the samar east mindanao arc which collided with the central mindanao sangihe island arc in the late oligocene early miocene the volcanic rocks exposed on the landward slope of the philippine trench are apparently part of the eastern margin of the samar east mindanao arc after the collision the philippine trench was initiated immediately adjacent to the arc massif there is no evidence of any accretion of material from the subducted philippine sea crust the polarity of the samar east mindanao arc is unknown but there is no evidence of either a deformed volcanoclastic apron or an older forearc terrane along the trench such materials must have been limited in extent prior to collision have foundered and been subducted as the trench was initiated or have been faulted or rotated so that they are not locally exposed

paleomagnetism of the zambales ophiolite luzon northern philippines abstract paleomagnetic results are reported from 52 sites in the zambales ophiolite the sites provide vertical sampling from the overlying sediments in the north and south to layered gabbros and dunites in the acoje and coto mines in the west they extend over an area of approximately 100 by 50 km a late miocene pliocene intrusion in the tarlac region gives a direction marginally distinguishable from the present field indicating a small clockwise rotation this is consistent with other indications of clockwise rotation since late miocene in luzon early and mid miocene sediments from barlo in the north and tarlac in the south indicate a period of rapid counterclockwise rotation during the miocene again consistent with other results from luzon eocene oligocene sediments overlying the ophiolite in the north and south indicate further counterclockwise rotation with northerly translation the interpretation of the directions in the ophiolite itself is more problematical directions similar to those in the eocene oligocene and in mid miocene sediments probably reflect secondary magnetizations and magnetization associated with the unroofing of the ophiolite we interpret the most strongly counterclockwise rotated magnetizations represented by a 44 ma sill in the barlo area and of the 46 ma dykes at coto as the primary direction of the ophiolite the counterclockwise rotation and northward translation interpreted from our results is consistent with the holloway model of the origin of the philippines however this model neglects the role of the philippine sea plate in the tectonic history of the philippines the counterclockwise rotation is also consistent with rotation of luzon in the left lateral shear between the philippine sea plate and asia as suggested by sarewitz and karig our data do not distinguish between counterclockwise rotation of a single block or of a number of smaller blocks the data suggest post late miocene clockwise rotation for luzon when it was coupled to the philippine sea plate our data are not consistent with a philippine sea plate origin for zambales

the first description and confirmation of the vista alegre impact structure in the parana flood basalts of southern brazil abstract the vista alegre structure centered at 25 57 s and 52 41 w has been recently proposed as a meteorite impact structure the 9 5 km diameter structure is located in the parana state of southern brazil within the parana basin which contains one of the largest and most extensive flood basalt provinces on earth the parana flood basalts belong to the serra geral formation and are temporally related to the opening of the south atlantic ocean having been dated at about 133 132 ma tholeiitic basalts dominate the western portion of parana state with some minor rhyodacites morphologically vista alegre has a prominent circular outline in the form of an incomplete ring of escarpments and an inner depression the presence of a central uplift is not obvious but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure these sandstones are possibly related to the triassic piramboia formation and or to the cretaceous botucatu formation these units are normally at stratigraphic depths of about 700 800 m below the present surface in this portion of the parana basin the structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick extensively used for agriculture as a result there are limited outcrops in the interior of the structure all of polymict breccias some of them melt bearing we report the extensive occurrence of shatter cones in the form of fine grained rock clasts within the polymict breccias the shatter cone bearing breccias occur at different locations within the structure separated by several kilometers the nested shatter cones range in size from about 0 5 to 20 cm for individual cones and up to half a meter for complete assemblages the shatter cones formed in fine grained parana flood basalt and might be the first examples of shatter cones in such a rock type in addition planar deformation features pdfs were found in quartz grains within sedimentary rock clasts of the polymict breccia these findings confirm the impact origin of the vista alegre structure

numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence discussions on the evolution of the tainan basin existing evidence shows that an oligocene erosion event occurred on the northern continental margin of the south china sea and the tainan basin area might be at the center of this event followed by a rapid tectonic subsidence in the late oligocene and early miocene period the rapid tectonic subsidence is mainly thermal controlled and the effect of the yichu fault on the tainan basin is limited to the basin s eastern part we develop a 2 d thermal mechanical kinematic numerical model to explore the relationship between thermal uplift and subsequent rapid subsidence in the tainan basin our modeling indicates that the oligocene uplift erosion and subsequent rapid subsidence could be caused by a thermal event and the differential subsidence of the basement caused by thermal contraction can initiate the development of small faults however it also suggests that other mechanisms might be needed to jointly account for the observed erosions citation shi x h xu x qiu k xia x yang and y li 2008 numerical modeling on the relationship between thermal uplift and subsequent rapid subsidence discussions on the evolution of the tainan basin tectonics 27 tc6003 doi 10 1029 2007tc002163

a geophysical study of the manila trench luzon philippines 1 crustal structure gravity and regional tectonic evolution the manila trench subduction zone is an active convergent plate margin between the south china sea and the northern philippines the trench trends northerly and is associated with a volcanic arc an east dipping benioff zone beneath luzon and a well developed fore arc basin system the luzon trough fore arc basins lie landward of the manila trench and contain up to 4 5 km of cenozoic sediments offscraping and accretion of turbidite sediments deposited in the manila trench have resulted in the upbuilding and outbuilding of an accretionary prism which forms the seaward margin of the fore arc basin system the uplifted zambales ophiolite and its offshore extension form the landward side of the fore arc basin multichannel seismic reflection profiles reveal that folding and thrust faulting of trench strata occur at the base of the trench slope the major structural decollement at the manila trench usually forms near the unconformity separating the hemipelagic sediments from the turbidite sediments subduction and perhaps underplating of the basal hemipelagic trench sediments accompany the deformation and accretion of the overlying clastic deposits faulting and uplift occur within a narrow zone of the fore arc where seamounts associated with the relict spreading center of the south china sea basin have presumably been subducted north of lingayan gulf the fore arc is disrupted by active fault systems that trend north and northwest offshore from the luzon central valley and from the southern cordillera central based on regional geological and tectonic observations we infer that subduction probably began at the manila trench in late oligocene to middle miocene time the long term average convergence rate at the manila trench is estimated to be between 10 and 20 mm yr and may be slowing in the north owing to the collision of taiwan with eurasia

evolution of a forearc basin luzon central valley philippines the cenozoic history of the 14 km thick luzon central valley sequence illustrates the development of a forearc basin forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations the luzon basin is floored by oceanic crust on the seaward western side and older accreted terranes on the arc eastern side initial sedimentation on this oceanic crust occurred during early tertiary northward translation and emplacement of the crust as an ophiolite along a strike slip or oblique slip zone the basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite a sequence of arc derived sediments at least 26 000 ft 8 km thick was shed into the astern arc side of the basin during late paleogene to quaternary convergence along the western margin of luzon by the middle miocene the central valley became a continuous elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths nonmarine deposition began in central portions of the basin in the pliocene and migrated with time both north and south along the basin axis late miocene to holocene movement along the philippine fault zone caused uplift and folding of adjacent parts of the basin exploration models for the central valley predict gas prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred the hydrocarbon potential of the central valley has not been determined adequately

sr isotopic and trace element variations in oligocene to recent igneous rocks from the philippine island arc evidence for recent enrichment in the sub philippine mantle oligocene miocene plutonic rocks from the philippine island arc are characterized by uniform87sr 86sr isotopic ratios of 0 7035 0 7039 in contrast volcanics erupted in the past 5 ma in central luzon marinduque and mindoro have higher radiogenic sr 0 7042 0 7054 and show regional variations in their sr isotopic compositions volcanics of the pliocene recent bicol volcanic chain of southern luzon have87sr 86sr in the same range 0 7034 0 7040 as the mid tertiary plutonics increases in radiogenic sr are coupled with increases in rb sr ba and th abundances in the central luzon mindoro marinduque volcanic belt while no such correlation exists for the mid tertiary plutonics we consider the low values observed for mid tertiary plutonics and bicol volcanics to be controlled by fluids released from the subducting philippine sea plate basalt that have87sr 86sr of the same range sr isotopic compositions of the volcanics in the northern part of the central luzon volcanic belt may be controlled by fluids generated from the subducted south china sea crust the highest values of radiogenic sr 0 7051 0 7054 and highest lil abundances which are observed in the southern part of the central luzon volcanic arc appear to be the result of source contamination by slivers of the continental crust of the north palawan mindoro terrane that collided with the philippine arc in the miocene

cenozoic plate interaction of the australia and philippine sea plates hit and run tectonics abstract recent studies in northwest new guinea have shown the presence of at least two marginal basins of different age both of which formed in back arc settings the older basin opened between the middle jurassic and early cretaceous a remnant of which is now preserved as the new guinea ophiolite its obduction started at 40 ma and it was finally emplaced on the australian margin at 30 ma the younger basin was active during the oligocene to middle miocene and was obducted in the early pliocene studies of the western edge of the philippine sea also reveal an important deformation of the philippine arc in the oligocene which hitherto has remained unexplained using information from these systems paleomagnetic results kinematic reconstructions and geochemistry of the supra subduction ophiolite we present a plate model to explain the region s eo oligocene development we suggest that an extensive portion of oceanic crust extended the australian plate a considerable distance north of the australian craton as australia began its steady 7 8 cm year northward drift in the early eocene this lithosphere was subducted thus the portion of the philippine sea plate carrying the taiwan philippine arc to its present site may have actually been in contact with the ophiolite now in new guinea and obduction led to deformation of the philippine sea plate itself neogene plate kinematics transported the deformed belt in contact with the sunda block in the late miocene and pliocene this interpretation has implications for the origin for the philippine sea plate and the potential incorporation of continental fragments against its boundaries

forearc basin evolution in the pan african damara belt central namibia the hureb formation of the khomas zone abstract the hureb formation in central namibia is one of the uppermost stratigraphic units of the precambrian pan african damara sequence the hureb formation occurs only in a part of the khomas zone a forearc basin which evolved complexly with time the formation is composed of the well graded upward fining schists of the quartzitic keises member the graphitic and calcitic waterval member and the altered scapolite bearing dagbreek member these units accumulated in an elongate turbidite fan system longitudinally deposited on the northern slopes of a wide forearc basin the active continental margin initiated when the kalahari plate subducted oceanic crust underneath the passive continental margin of the congo craton the development and deposition of the turbiditic hureb formation in the foredeep occurred during subduction of oceanic crust simultaneous with d 1 deformation and tholeiitic to andesitic arc volcanism on the continental margin the basin was bound to the north by the rising volcanically active continental margin and to the south by the accretionary prism of obducted slivers of oceanic crust and pelagic sediments above the subduction zone with continental collision the sedimentation in the forearc and continental margin arc volcanism ceased the forearc basin was uplifted and shortened horizontally by synclinoria and anticlinoria which propagated northwards and died out at the edge of the forearc basin the deformation slightly affected the underlying foliated fahlwater schist from the passive margin setting further uplift of the basin and re adjustment between the oblique sutured cratons simultaneous with the d 3 doming in the congo continental margin arc occurred with the underplating of the thin passive margin of the kalahari craton as no more crustal shortening could be absorbed by the upright folding dk 1 reactivation of the thrusts in the accretionary prism leads to the producing of southeast verging with a right lateral sense of movement thrusts dk 2 in the folded forearc basin the dk 2 thrusts propagated and died out northwards and extended towards the south the stacking of the d k2 thrusts southwards thickened the crust creating a low heatflow environment with higher pressure conditions it leads to the preservation of chlorite and production of sillimanite garnet staurolite and kyanite to the south of the accretionary prism collision related deformation created a peripheral foreland thrust zone outlasting orogenic activities in the overriding congo continental margin where isostatic rebound already had occurred

hydrocode modeling of the sierra madera impact structure we present the first hydrocode simulations of the formation of the sierra madera structure west texas usa which was caused by an impact into a thick sedimentary target sequence we modeled sierra madera using the isale hydrocode and here we present two best fit models 1 a crater with a rim final crater diameter of 12 km in agreement with previous authors interpretations of the original structure and 2 a crater 16 km in diameter with increased postimpact erosion both models fit some of the geologic observational data but discrepancies with estimates of peak shock pressure extent of deformation and stratigraphic displacement remain this study suggests that sierra madera may be a larger crater than previously reported and illustrates some of the challenges in simulating impact deformation of sedimentary lithologies as many terrestrial craters possess some amount of sedimentary rocks in the target sequence numerical models of impacts into sedimentary targets are essential to our understanding of target rock deformation and the mechanics of crater formation

shock deformation confirms the impact origin for the cerro do jarau rio grande do sul brazil structure cerro do jarau is a conspicuous circular morpho structural feature in rio grande do sul state brazil with a central elevated core in the otherwise flat pampas terrain typical for the border regions between brazil and uruguay the structure has a diameter of approximately 13 5 km it is centered at 30o12 s and 56o32 w and was formed on basaltic flows of the cretaceous serra geral formation which is part of the parana etendeka large igneous province lip and in sandstones of the botucatu and guara formations the structure was first spotted on aerial photographs in the 1960s ever since its origin has been debated sometimes in terms of an endogenous igneous origin sometimes as the result of an exogenous meteorite impact event in recent years a number of studies have been conducted in order to investigate its nature and origin although the results have indicated a possible impact origin no conclusive evidence could be produced the interpretation of an impact origin was mostly based on the morphological characteristics of the structure geophysical data as well as the occurrence of different breccia types extensive deformation silicification of the rocks within the structure in particular the sandstones and also on the widespread occurrence of low pressure deformation features including some planar fractures pfs a detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with pfs and feather features ffs as well as a number of quartz grains exhibiting planar deformation features pdfs while most of these latter grains only carry a single set of pdfs we have observed several with two sets and one grain with three sets of pdfs consequently we here propose cerro do jarau as the seventh confirmed impact structure in brazil cerro do jarau together with vargeao dome santa catalina state and vista alegre parana state is one of very few impact structures on earth formed in basaltic rocks

sedimentation history and biostratigraphy of ophiolite related tertiary sediments luzon philippines pelagic and hemipelagic sediments deposited on the zambales ophiolite contain a nearly continuous depositional record of the original setting and emplacement history of this large ophiolite from the late eocene through the miocene pelagic limestone with thin ash layers the aksitero formation caps the volcanic complex of the ophiolite along its east flank calcareous nannofossil biostratigraphy of this limestone gives sedimentation rates of 3 5 m m y from the late eocene through the early oligocene rates increase to 10 m m y or more in the upper aksitero formation where sandy turbidites appear in the middle and upper oligocene sections lower miocene mudstone sandstone and conglomerate of the moriones formation were deposited at much higher rates this formation includes channels and debris flow deposits characteristic of deep sea fans oligocene sandstones are predominantly volcaniclastic whereas sandstones in the lower miocene section contain serpentine and other components derived from the ultramafic complex of the ophiolite sedimentary facies and sandstone composition show that the zambales was deeply eroded by the early miocene and probably first emerged above sea level in the middle or late oligocene only 10 to 15 m y after it formed as new ocean crust a comparison of the aksitero formation with deep sea drilling project dsdp sites in the western pacific suggests that the zambales ophiolite was originally part of a marginal basin and not an island arc

application of a roughness length representation to parameterize energy loss in 3 d numerical simulations of large rivers 1 recent technological advances in remote sensing have enabled investigation of the morphodynamics and hydrodynamics of large rivers however measuring topography and flow in these very large rivers is time consuming and thus often constrains the spatial resolution and reach length scales that can be monitored similar constraints exist for computational fluid dynamics cfd studies of large rivers requiring maximization of mesh or grid cell dimensions and implying a reduction in the representation of bedform roughness elements that are of the order of a model grid cell or less even if they are represented in available topographic data these subgrid elements must be parameterized and this paper applies and considers the impact of roughness length treatments that include the effect of bed roughness due to unmeasured topography cfd predictions were found to be sensitive to the roughness length specification model optimization was based on acoustic doppler current profiler measurements and estimates of the water surface slope for a variety of roughness lengths this proved difficult as the metrics used to assess optimal model performance diverged due to the effects of large bedforms that are not well parameterized in roughness length treatments however the general spatial flow patterns are effectively predicted by the model changes in roughness length were shown to have a major impact upon flow routing at the channel scale the results also indicate an absence of secondary flow circulation cells in the reached studied and suggest simpler two dimensional models may have great utility in the investigation of flow within large rivers

past environmental and circulation changes in the south china sea input from the magnetic properties of deep sea sediments abstract the south china sea located at the transition between the pacific and the indian ocean receives every year mainly during the rain season enormous amounts of river sediments originating from the erosion weathering of rocks in the catchment basins at sea these sediments are carried by different water masses to their deposition site and they constitute a unique archive for past environmental studies in this region the magnetic fraction of deep sea sediments though forming a minority in volume provides incredibly valuable information for paleoceanographic reconstructions as long as its provenance and source to sink processes are well constrained after a brief description of the climatic sedimentological and oceanographic context of the south china sea scs a review of the information available so far in the literature about the magnetic properties of scs sediments is presented it shows a large variety of interpretations conclusions that finally results in a rather unclear picture because in such a context the characterization of the sediment at the source is critical the magnetic properties recently obtained from a set of samples from rivers and marine surface sediments are summarized to describe the present day situation they are then used to interpret paleorecords from a set of seven marine cores distributed from the southern to the northern basins at different water depths and all covering at least the last climatic cycle the results reported here for the first time suggest that the magnetic mineralogy remains rather stable in time on land and that its time and spatial distribution at sea is an interplay of changes in sea level and deep sea circulation during low sea level periods bottom deep sea circulation is weak and the deposited sediment originates from the proximal rivers on the contrary during high sea level the circulation is enhanced transporting more sediment most likely from taiwan and luzon to the northwestern part of the scs and also in smaller proportion to the southern basin where it mixes with the local river borne sediment by comparing the two longest records we observe that this pattern is repeated over the last 900 ka superimposed to the 100 kyr cyclicity we also observe a longer term evolution with a maximum in the bottom current strength around 500 ka coinciding with global changes in the deep ocean circulation and carbon cycle these new results based on a wide spectrum of magnetic properties of numerous marine sedimentary cores from the scs show that the magnetic fraction yields important insights into past changes of the sedimentary pathways in particular the dynamic of the deep sea circulation depending on the global climatic context

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

tracing tectonic deformation using the sedimentary record an overview tectonic activity on a range of scales is a fundamental control on sedimentary activity the range of structural deformation within a region extends from the plate tectonic scale governing for example rift initiation to the basin scale with the formation of basin bounding faults internal basin configuration is also strongly influenced by tectonic activity however the relationship between tectonic activity and sedimentation is a complex one given the many additional factors which can also influence sedimentary activity including erosion sediment transport source area lithology groundwater chemistry range of depositional environments climate eustasy and the relative location of an area and its distality to marine influences in this paper we provide a selective overview of the issues associated with the interlinked themes of tectonics and sedimentation examining the main basin types forming in both extensional and compressional plate settings we then review the various models of sedimentation in the selected basins both on a local and a basinal scale finally we look to the future providing a series of possible research areas almost exclusively multidisciplinary which would help to improve existing models of interlinked sediment tectonics systems

discrimination of alluvial and mixed bedrock alluvial multichannel river networks this paper explores the use of planview morphological metrics to quantitatively describe and distinguish mixed bedrock alluvial multichannel networks from alluvial multichannel networks the geometries of the channel planforms of two bedrock constrained networks mekong and orange rivers are compared with the classic alluvial anastomosed upper columbia river and the wandering ganga river widely recognised indices utilized include channel link count and channel sinuosity with additional emphasis being given to the less common metrics network bifurcation angles and island shape characteristics i e aspect ratio compactness roundness and convexity link count data with one notable exception conform to theoretical expectations bifurcation angles for all four multichannel rivers are significantly greater than angles reported for braiding rivers island convexity clearly discriminates the two alluvial rivers from the two bedrock influenced rivers the width of the macrochannel in which each network develops has a positive influence on the number of channel links and is further related to channel slope variations which in turn are influenced by terrain structure revealed using trend surface analysis the geometry of multichannel networks are often laterally constrained such that the values of channel bifurcation angles and link sinuosity values reduce as the network intensifies and channel links are shortened these latter observations go some way to explain the oft noted relatively straight links seen within multichannel networks which are a necessary adjustment to space filling constraints placed on a network

insights into the morphology of the serra da cangalha impact structure from geophysical modeling forward modeling is commonly applied to gravity field data of impact structures to determine the main gravity anomaly sources in this context we have developed 2 5 d gravity models of the serra da cangalha impact structure for the purpose of investigating geological bodies structures underneath the crater interpretation of the models was supported by ground magnetic data acquired along profiles as well as by high resolution aeromagnetic data ground magnetic data reveal the presence of short wavelength anomalies probably related to shallow magnetic sources that could have been emplaced during the cratering process aeromagnetic data show that the basement underneath the crater occurs at an average depth of about 1 9 km whereas in the region beneath the central uplift it is raised to 0 5 1 km below the current surface these depths are also supported by 2 5 d gravity models showing a gentle relief for the basement beneath the central uplift area geophysical data were used to provide further constraints for numeral modeling of crater formation that provided important information on the structural modification that affected the rocks underneath the crater as well as on shock induced modifications of target rocks the results showed that the morphology is consistent with the current observations of the crater and that serra da cangalha was formed by a meteorite of approximately 1 4 km diameter striking at 12 km s 1

length scale of braided river morphology 1 pool bar topography in single channel rivers has a length scale proportional to channel width in braided rivers confluence bifurcation units are analogous to pool bar morphology and in some cases develop from initial alternate bars consequently confluence bifurcation units are expected to have length that scales with the central anabranch width and that constitutes a basic length scale braided channel morphology this idea was tested using measurements from a physical model of a gravel bed braided river and from aerial photographs of braided rivers length distance from confluence to bifurcation anabranch width and confluence angle of confluence bifurcation units were measured a simple length scaling is evident across the range of scales confluence bifurcation length is 4 5 times the channel width this scaling is a fundamental element of braided river morphology and suggests that braided patterns are created by processes and have morphological regularity similar to pool bar units of low sinuosity single thread rivers

zircon u pb geochronology of the zambales and angat ophiolites luzon philippines evidence for an eocene arc back arc pair two basement terranes the zambales ophiolite in the west and the angat ophiolite in the east are exposed on the island of luzon separated by a circa 10 km thick and circa 100 km wide sedimentary basin the structural and age relationships between the two ophiolitic blocks are central to understanding the geologic and tectonic development of the northern philippines and evaluating models of terrane evolution proposed for this area of the western pacific we analyzed zircons from the zambales and angat terranes to better constrain their origin two zircon fractions from tonalite in the acoje block of the zambales ophiolite give concordant u pb ages at 44 2 0 9 ma two zircon fractions from plagiogranite and one fraction from diorite in the coto block of the zambales ophiolite give concordant u pb ages of 45 1 0 6 ma these results provide a middle eocene age for the zambales ophiolite in agreement with the minimum late eocene age of the overlying aksitero formation no age difference is discerned between the arc like acoje block and morb like coto block of the zambales ophiolite four zircon fractions from two sample sites in the angat ophiolite give concordant ages of 48 1 0 5 ma this age is considerably younger than the late cretaceous age based on radiolarian fauna derived from a sheeted dike pillow lava sediment sequence south southeast of the main ophiolite the small age difference between the zambales and angat ophiolites suggests a common origin and obviates the need for a major structural discontinuity west of the southern sierra madre beneath the central valley of luzon the cretaceous biostratigraphic ages of the ophiolitic rocks southeast of the eocene angat ophiolite implies that there are two ophiolitic basements exposed in the southern sierra madre the relationship between the two ophiolites is constrained by the overlying stratigraphic relations which indicate that an eocene volcanic arc and associated volcaniclastic apron was built on both the eocene and cretaceous ophiolitic basement this suggests that the zambales angat ophiolite represents a preserved eocene back arc basin that opened behind an eocene arc that developed within cretaceous oceanic basement in this model the zambales angat ophiolites are therefore not allochthonous terranes but part of a single plate generated in situ forming part of the autochthonous basement of luzon

submarine fan deposystems and tectonics of a late cretaceous forearc basin along an accretionary convergent plate boundary maccoll ridge formation wrangell mountains alaska analysis of upper cretaceous sedimentary and volcanic strata in the wrangell mountains of south central alaska provides an opportunity to study the tectonics depositional systems and provenance of a forearc basin that developed along an accretionary convergent plate boundary new data from the 1150 m thick maccoll ridge formation indicate that deposition occurred during the campanian on a coarse grained submarine fan that was derived from an uplifted allochthonous terrane exposed in the hanging wall of a fault system that separated the forearc basin from the subduction complex new age controls include palynoflora indicative of a late middle to late campanian age and compatible radiometric age determinations of volcanic vitric crystal tuffs near the top of the formation which have 40ar 39ar isochron ages of 79 4 0 7 and 77 9 2 1 ma sedimentological and paleontological data show that sedimentation occurred on the inner portions of a sand and gravel rich submarine fan system evidence for this inte

analogue models for the transpressional docking of volcanic arcs in the western pacific abstract many convergent zones involve oblique subduction of oceanic crust the subduction often ends with oblique collision of a microcontinent island arc or oceanic plateau against a larger continental area we have conducted a series of experiments in which scaled multilayered models of the lithosphere made of sand and silicone were subjected to horizontal shortening oblique to lithospheric boundaries the aim was to investigate tectonic phenomena occurring at such boundaries for example subduction reversal partitioning of dip slip and strike slip motions lateral translation of continental fragments block rotations and entrapment of oceanic material the physical models ranged in complexity from a simple uniform oceanic lithosphere to configurations involving a volcanic arc against a continental margin our models explore the importance of rheology and width of continental indenters rather than the obliquity of convergence the models confirm that intra oceanic deformation is likely prior to complete subduction beneath both the arc and the continental margin they further explain the presence of extensional features in a convergent setting subduction zones resulting from oblique convergence may be associated with large strike slip faults by partitioning of the convergence vector when a model volcanic arc collides against a model continent a subduction reversal occurs after suturing and the strike slip fault which accompanies the subduction reversal initiates with a horse tail pattern the resulting plate boundary is somewhat different from the one that existed prior to the docking of the allochthonous belt these experiments tested many of the mechanisms proposed for terrane formation and accretion the models have been used to interpret the late neogene geodynamic setting of the western pacific with emphasis on the southern philippines and the island of new guinea

shallow burial and exhumation of the peloritani mountains ne sicily italy insight from paleothermal and structural indicators we used vitrinite refl ectance and mixedlayered clay minerals to investigate levels of diagenesis of the oligocene miocene basin developed on the nappes of the alpine orogen exposed in the peloritani mountains ne sicily paleothermal indicators were integrated with stratigraphic and structural analyses and published apatite fi ssion track and u th sm he ages to defi ne the late evolutionary stages of the peloritani mountains this multimethod approach allowed us to reconstruct the paleogeothermal gradient of the basin in oligocene miocene times to constrain its burial evolution and discriminate between areas where it has been affected by sedimentary and or tectonic load in the southern area of the basin organic and inorganic thermal parameters increase as function of depth suggesting that their evolution was ruled by sedimentary burial they record a decrease in paleogeothermal gradient values marking the evolution of the basin from a forearc to a thrust top setting during the convergence collision process between the calabria peloritani arc and the african plate on the other hand in the northern edge of the basin vitrinite refl ectance values 0 46 0 58 indicate that the thermal evolution of this area was controlled by tectonic burial related to late langhian early serravallian out of sequence thrust tec tonics the tectonic overburden has been totally removed by extensional tectonics and or erosion since the late miocene the short time span at maximum temperature 2 m y elapsing between thrust stack emplacement and the beginning of tectonic overburden removal has allowed only vitrinite refl ectance and thermochronological indicators to record this compressive reactivation

the terrestrial impact cratering record approximately 130 terrestrial hypervelocity impact craters are currently known due to variations in preservation and in geologic knowledge this sample is biased towards young x 20 km craters on the cratons of australia europe including the former u s s r and north america the rate of discovery of new craters is 3 5 craters per year although modified by erosion terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies such as the moon terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters which can be used to constrain interpretations of lunar samples and models of crater formation due to erosion and its effects on form terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects these include shatter cones microscopic planar deformation features solid state and fusion glasses high pressure polymorphs and whole rock melting and vaporization shock recovery experiments indicate that these features occur over shock pressures of 5 gpa to x 100 gpa terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact induced fracturing they include gravity and magnetic lows and reductions in seismic velocity the gravity anomalies are seldom greater than 30 mgal due to the limiting effects of lithostatic pressure on fracturing at large complex craters the gravity signature may include a central relative gravity high due to uplift and short wavelength central magnetic anomalies due to a variety of processes much current work is focused on the effects of impact on earth evolution previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from cretaceous tertiary boundary sites as resulting from a major impact suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record by analogy with the lunar record and modelling of the effects of very large impacts it has been proposed that biological and atmospheric evolution of the earth could not stabilize before the end of the late heavy bombardment 3 8 ga ago the present terrestrial cratering rate is 5 4 2 7 10 15 km 2a 1 for a diameter 20 km this represents a local threat on historic time scales on a global scale a major impact sufficient to cripple human civilization severely will occur on time scales of 106 a

geology of lonar crater india lonar crater india is one of the youngest and best preserved impact structures on earth the 1 88 km diameter simple crater formed entirely within the deccan traps making it a useful analogue for small craters on the basaltic surfaces of the other terrestrial planets and the moon in this study we present a meter scale resolution digital elevation model geological map of lonar crater and the surrounding area and radiocarbon ages for histosols beneath the distal ejecta impact related deformation of the target rock consists of upturned basalt fl ows in the upper crater walls and recumbent folding around rim concentric subhorizontal noncylindrical fold axes at the crater rim the rim fold hinge is preserved around 10 15 of the crater although tearing in the rim fold is inferred from fi eld and paleomagnetic observations no tear faults are identifi ed indicating that large displacements in the crater walls are not characteristic of small craters in basalt one signifi cant normal fault structure is observed in the crater wall that offsets slightly older layer parallel slip faults there is little fl uvial erosion of the continuous ejecta blanket portions of the ejecta blanket are overlain by aerodynamically and rotationally sculpted glassy impact spherules in particular in the eastern and western rim as well as in the depression north of the crater known as little lonar the emplacement of the continuous ejecta blanket can be likened to a radial groundhugging debris fl ow based on the preserved thickness distribution of the ejecta the effi cient exchange of clasts between the ejecta fl ow and the underlying histosol and the lack of sorting and stratifi cation in the bulk of the ejecta the ejecta profi le is thickened at the distal edge and similar to fl ejecta structures observed on mars

geology and impact features of vargeao dome southern brazil abstract vargeao dome southern brazil is a circular feature formed in lava flows of the lower cretaceous serra geral formation and in sandstones of the parana basin even though its impact origin was already proposed in the 1980s little information about its geological and impact features is available in the literature the structure has a rim rim diameter of approximately 12 km and comprises several ring like concentric features with multiple concentric lineaments the presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the botucatu and piramboia formations we present the morphological structural characteristics of vargeao dome characterize the different rock types that occur in its interior mainly brecciated volcanic rocks bvr of the serra geral formation and discuss the deformation and shock features in the volcanic rocks and in sandstones these features comprise shatter cones in sandstone and basalt as well as planar microstructures in quartz a geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the bvrs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies no traces of meteoritic material have been found so far the results confirm the impact origin of vargeao dome making it one of the largest among the rare impact craters in basaltic targets known on earth

relationship between sedimentation rate and total organic carbon content in ancient marine sediments an analysis of deep sea drilling project dsdp cores indicates quantitative relations between sedimentation rate and total organic carbon content in fine grained ancient marine sediments of jurassic cretaceous and cenozoic ages these relations are largely independent of geographic setting geologic age and differential compaction but are strongly affected by lithology for any given sedimentation rate the total organic carbon content toc by weight percent increases from calcareous sediments to calcareous siliceous sediments to siliceous sediments to black shale the total organic carbon content also varies with the sedimentation rate for each lithology starting at low rates of sedimentation toc first increases with sedimentation rate owing to a more rapid passage of the sediment through the near surface zone of intense organic degradation above a critical sedimentation rate toc typically decreases with increasing sedimentation rate owing to clastic dilution of the organic input near surface organic degradation however continues to be less efficient at higher burial rates therefore the oil generation potential of oil prone organic constit ents should continue to increase with increasing sedimentation rate similar relations were also established between total organic carbon accumulation rate and grain accumulation rate and support both reduced near surface organic degradation and the clastic dilution effect by using the toc cutoff for a petroleum source rock these relations can be used to identify the presence of petroleum source rock formations in marine sedimentary basins the results of this study may have important implications for petroleum exploration in frontier regions sedimentation rate can be determined from seismic reflection records or seismic isopachs in basins where only limited well data are generally available seismic records can thus be used to identify potential petroleum source rocks and to estimate the relative oil generation potential of source rocks in various frontier sedimentary basins prior to and during the early phases of exploratory drilling

integrated 3 d model from gravity and petrophysical data at the bosumtwi impact structure ghana the bosumtwi impact structure of central ghana was drilled in 2004 as part of the international continental scientific drilling program icdp a vast amount of geoscience data is available from the pre site surveys and the actual drilling phase a 3 d gravity model was constructed and calibrated with the available data from the two icdp boreholes lb 07a and lb 08a the 3 d gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data and with the petrophysical data from the lb 08a and lb 07a core materials and the two borehole logs furthermore the model exhibits lateral density variations across the structure and refines the results from previous 2 5 d modeling an important new element of the 3 d model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling

paleo oceanographic and tectonic settings of early miocene reefs and associated carbonates of offshore southeast asia the late oligocene to early middle miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast asia a generally rising eustatic sea level during this period allowed the accumulation of thick reefal units an expansion of the coral algal regional facies belt north to japan and south to new zealand occurred during the early miocene as the result of a reduced oceanic latitudinal temperature gradient an episode of enhanced organic carbon burial is reflected in a positive dgr13c excursion from 17 5 to 13 5 ma increasing paleo oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen minimum zone marine oil prone or anic carbon was deposited in silled anoxic basins both bounded by and within island arc systems a combination of tectonic and paleo oceanographic factors therefore created conditions that favored contemporaneous deposition of organic carbon rich source beds and reef reservoirs during the latest early to early middle miocene paleogeographic and facies reconstructions for an 18 m y interval indicate that deposition occurred in three tectonic settings 1 passive continental margins 2 convergent plate boundaries and 3 obliquely convergent plate boundaries in all these settings the vertical motion histories of fault bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups their scal and morphology duration diagenesis and relationship to siliciclastics

the complex impact structure serra da cangalha tocantins state brazil abstract serra da cangalha is a complex impact structure with a crater diameter of 13 700 m and a central uplift diameter of 5800 m new findings of shatter cones planar fractures feather features and possible planar deformation features are presented several ring like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata the target at serra da cangalha is composed of devonian to permian sedimentary rocks mainly sandstones that are interlayered with siltstone and claystones nne ssw and wnw ese striking joint sets were present prior to the impact and also overprinted the structure after its formation as preferred zones of weakness these joint sets partly controlled the shape of the outer perimeter of the structure and in particular affected the deformation within the central uplift joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift was formed these planes were used as major displacement zones the asymmetry of the central uplift with preferred overturning of strata in the northern to western sector may suggest a moderately oblique impact from a southerly direction buckle folding of tilted strata as well as strata overturning indicates that the central uplift became gravitationally unstable at the end of crater formation

rim uplift and crater shape in meteor crater effects of target heterogeneities and trajectory obliquity 1 we have analyzed the rim structure of meteor crater arizona in order to understand the mechanism of rim uplift in simple craters and the causes of the shape of polygonal impact craters for this purpose we systematically determined bedding orientation of the autochthonous crater wall and overturned flap and analyzed the kinematics of major radial faults we found that rim uplift correlates with the crater shape and increases in the corners of the crater the two main mechanisms of differential uplift are the formation of horizontal interthrust wedges leading to the doubling of strata in the rim and radial corner faults or tear faults that vertically displace bedrock the development of meteor crater s quadrangular shape is caused by more effective crater excavation flow parallel to major joint sets additionally we infer the impact direction with a newly developed technique the two corners model and review the arguments in favor of an oblique trajectory while the data set is ambiguous several indicators suggest an impact direction from the nnw we conclude that oblique impacts should have an effect on early cratering and excavation flow whereas target heterogeneities like joints start to play a prominent role in later stages when the stresses induced by the excavation flow are in the same order of strength as the material involved

seismic stratigraphy and cenozoic evolution of west sumatra forearc basin late tertiary neogene sediments deposited in the sunda forearc reveal the stratigraphic and structural evolution of this active plate margin a dense grid of multichannel seismic reflection data and information from 17 exploratory wells were used to establish a detailed seismic stratigraphic framework of the forearc region from 0 to 6 n several important tectonic cycles are recognized paleogene orogeny neogene subsidence and late tertiary tectonism superimposed on these regional tectonic events are three major transgressive regressive cycles of sedimentation related to changes in sea level and provenance paleogene and older metasedimentary and metamorphic rocks comprise basement beneath the landward inner margin of the forearc basin both basement rocks and lower tertiary sedimentary rocks were deformed and eroded approximately 25 to 30 ma the continental shelf was exposed to subaerial erosion and basin deposits were restricted offshore coincident with a worldwide lowstand of sea level in the oligocene the paleogene orogeny probably occurred prior to the erosional event that cut the prominent angular unconformity on the shelf the neogene history of forearc basin development is characterized by subsidence and nearly continuous sedimentation a basal transgression began in latest oligocene time and culminated in the middle miocene alternating sequences of limestone and shale comprise two second order cycles of sedimentation that are superimposed on an overall transgressive event a major regressive sequence followed in the pliocene owing to an influx of siliciclastic clay silt and sand derived from sumatra sedimentation rates were high and large volumes of terrigenous material were deposited in deltaic systems on the shelf the shelf slope break prograded basinward nearly 10 km 6 mi through lateral accretion and aggradation during a relative highstand or stillstand of sea level

planform evolution of two anabranching structures in the upper peruvian amazon river we present a study to relate the sinuosity of the main channel and its effect on the dynamics of the secondary channels of anabranching structures for this purpose two locations of the peruvian amazon river were selected 1 a site with a medium to high sinuosity main channel ms site muyuy peru and 2 a site with a low sinuosity main channel ls site at the triple boundary between brazil colombia and peru the main channels for both the ms and ls anabranching structures have freedom to migrate in the lateral direction while at least one of their secondary channels is adjacent to the geological valley for ms and ls sites temporal analysis of planform evolution was carried out using 30 years of satellite imagery from which metrics such as width sinuosity and annual maximum migration rates of main and secondary channels were calculated additionally detailed hydrodynamic and bed morphology field measurements were carried out and a two dimensional shallow water numerical model was developed for a medium to high sinuosity main channel anabranching structure the secondary channels present a dominant mechanism for reworking the floodplain while for the low sinuosity anabranching structure the main channel planform dynamics is dominant flow velocities along the main and secondary channels for low transition and high flow discharges describe that for ms ls site the velocities are much higher along the secondary main channels

target rocks impact glasses and melt rocks from the lonar impact crater india petrography and geochemistry the lonar crater india is the only well preserved simple crater on earth in continental flood basalts it is excavated in the deccan trap basalts of cretaceous tertiary age a representative set of target basalts including the basalt flows excavated by the crater and a variety of impact breccias and impact glasses were analyzed for their major and trace element compositions impact glasses and breccias were found inside and outside the crater rim in a variety of morphological forms and shapes comparable geochemical patterns of immobile elements e g rees for glass melt rock and basalt indicates minimal fractionation between the target rocks and the impactites we found only little indication of post impact hydrothermal alteration in terms of volatile trace element changes no clear indication of an extraterrestrial component was found in any of our breccias and impact glasses indicating either a low level of contamination or a non chondritic or otherwise iridium poor impactor

tertiary arc rifting in northern luzon philippines the north luzon terrane nlt comprising the section of luzon north of the philippine fault is one of the largest arc terranes in the philippine archipelago numerous features suggest that the nlt is a late oligocene to early miocene analogue for the processes in the modern intra arc rift zone at the northern end of the mariana trough first the nlt has bifurcating magmatic arcs sharing similar magmatic histories these include the northern sierra madre nsm and cordillera central cc magmatic arcs which are separated by the cagayan basin but which are linked in the caraballo range to the south the rock record indicates that the nsm cc and caraballo ranges were active arcs in late eocene to late oligocene time second seismic reflection and well data indicate that the cagayan basin formed by extensional faulting in late oligocene to early miocene time third alkalic arc magmatism recognized to be a precursor of intra arc rifting in modern settings occurred at the juncture of the nsm and cc arcs in late oligocene to early miocene time fourth oceanic crust represented by the itogon ophiolite formed at the southwestern end of the cagayan basin in late oligocene to early miocene time major and trace element chemistry show that the itogon sheeted dikes have tholeiitic arc and backarc basin basalt affinities the rock record and geophysical offshore data suggest that the nlt was developing in an island arc system above the subducting west philippine plate in late eocene time rifting occurred in the island arc from late oligocene to early miocene time but did not mature into backarc spreading most likely because of the collision of the benham rise a basaltic rise in the west philippine basin with the nlt the arc rifting in the nlt may be another manifestation of the extensional tectonism that affected most of southeast asia in late oligocene to early miocene time during which the south china and southeast sulu basins formed subsequent to arc rifting the history of the nlt has been linked to the subduction of the south china plate along the manila trench the structural history of the cagayan basin and magmatic history of the southern cc suggest that the subduction in the manila trench at the latitude of the nlt began about 15 ma

geology of the zambales ophiolite luzon philippines abstract the zambales ophiolite of western luzon philippines exposes a typical succession of basalt flows diabasic dikes gabbro and tectonized harzburgite the age established by limiting strata is late eocene lack of evidence of thrust faulting and the general domal disposition of the lithologie units indicate that the ophiolitic rocks are exposed by uplift highly complex internal layered structures within the complex are related to processes developed during formation of the ophiolite and the zambales ophiolite may be one of the least disturbed by emplacement ophiolitic masses known the exposed mass trends north and the upper surface plunges at low angles a few degrees to the north and south the chemistry and composition of the rocks in the northwest part of the zambales area acoje block is distinct from that in the southeastern segment coto block the acoje block according to evans 1983 and hawkins and evans 1983 resembles on a chemical basis arc tholeiite series rocks from intra island arcs and the rocks in the coto block are typical back arc basin rock series the present writer believes that the ophiolite composes a single genetic unit and that the changes in composition are the result of changes that took place during the initial formation the gabbro probably formed below a spreading center in an elongate in cross section v shaped magma chamber the gabbro is estimated by the writer to be less than 2 km thick and may be less than 1 km in places numerous erosional windows through the gabbro in the northern and eastern side of the zambales area show that the gabbro remaining in those areas is likely to be only a few hundred meters thick harzburgite is exposed to a depth of about 800 m in the bagsit river area and this may be the deepest part of the ophiolite accessible for study on which there is any control on depth a transitional zone about 200 m thick lying between the gabbro and harzburgite is composed of serpentinized dunite commonly the dunite contains disseminated sulfide minerals and at the acoje mines platinum group elements a compositional layering within the gabbro is in places cumulate in the lower part of the unit but may have formed by nucleation higher up on the relatively steep sides of the magma chamber a widespread gneissic banding in the gabbro forms large mappable structures which are many times more complex than is the disposition of the major rock units these structures are believed to be the result of extensive slumping in the magma chamber the structure produced by the cumulate layering merges with the gneissic banding commonly without discernible change in attitude this tectonic layered structure crosses the gabbro peridotite boundary at any angle without seeming to disturb the original rock distribution at greater depths below the boundary ca 800 m the harzburgite contains low dipping banding which probably reflects the result of differential movement within the mantle chromite occurs almost exclusively in a zone that generally lies no more than 200 300 m below the gabbro peridotite boundary refractory grade chromite is found in this zone below the olivine gabbro in the goto block and as low grade metallurgical grade chromite below norite in the acoje block at acoje mines the chromite is present in layers in dunite which the writer interprets as being distributed in a zone along the gently dipping ca 25 gabbro peridotite boundary the steeply dipping ca 60 80 individual layers lie en echelon along the boundary at an angle ca 50 to the contact at coto the chromite forms large discontinuous masses in the lowest dunite and in the uppermost harzburgite except for the chromite present as layers at acoje the regional tectonic layering crosses the chromite deposits without structural deviation the chromite deposits and associated peridotite may be cumulate in origin but have been modified to such an extent that cumulate textures are generally obliterated the angle of repose of cumulate layers in the acoje area and in the coto block dip towards each other raising the possibility that the zambales area may contain the relics of a spreading center initial emplacement of the zambales ophiolite took place by uplift and the ultramafic portion was exposed to erosion in the earliest miocene or late oligocene submergence of some of the ophiolite followed the previous uplift and on the west side of the zambales range submergence of several kilometers is indicated final emergence appears to have taken place in pliocene or pleistocene time by block uplift and areas of greatest uplift closely conform to the present topographic surface

investigating the lake bosumtwi impact structure insight from numerical modeling 1 the current topography of the lake bosumtwi crater and some of its structural dimensions have been determined by geophysical methods we combine these data with sophisticated numerical models to evaluate the cratering process itself for example melt and tektite generation as well as to test the modeling code the geophysical maps show some asymmetry in plan view with the main anomaly north of the crater center the simulations of the early stage show asymmetric patterns only in ejecta and tektite distributions while the late stage is modeled for the vertical impact without any asymmetry we estimate the projectile size from scaling laws and then varying material properties reproduce a crater which is similar to the bosumtwi but too deep bulking allows us to reconcile differences between the model results and the observed topography shock melt estimates are in good agreement with the bosumtwi magnetic signature modeled distribution of tektites assumes an impact angle of 30 45 and an impact direction from the n ne the combination of numerical models and field evidence not only provides necessary information for upcoming scientific drilling of the structure but also suggests interesting and well suited drill sites besides the central uplift and the annular moat with a suggested thick breccia cover drilling at the location of the geophysical anomalies and comparison of downrange and transversal locations will provide new insight into preimpact and impact induced asymmetries

modeling damage and deformation in impact simulations numerical modeling is a powerful tool for investigating the formation of large impact craters but is one that must be validated with observational evidence quantitative analysis of damage and deformation in the target surrounding an impact event provides a promising means of validation for numerical models of terrestrial impact craters particularly in cases where the final pristine crater morphology is ambiguous or unknown in this paper we discuss the aspects of the behavior of brittle materials important for the accurate simulation of damage and deformation surrounding an impact event and the care required to interpret the results we demonstrate this with an example simulation of an impact into a terrestrial granite target that produces a 10 km diameter transient crater the results of the simulation are shown in terms of damage a scalar quantity that reflects the totality of fragmentation and plastic strain both total plastic strain the accumulated amount of permanent shear deformation regardless of the sense of shear and net plastic strain the amount of permanent shear deformation where the sense of shear is accounted for damage and plastic strain are both greatest close to the impact site and decline with radial distance however the reversal in flow patterns from the downward and outward excavation flow to the inward and upward collapse flow implies that net plastic strains may be significantly lower than total plastic strains plastic strain in brittle rocks is very heterogeneous however continuum modeling requires that the deformation of the target during an impact event be described in terms of an average strain that applies over a large volume of rock large compared to the spacing between individual zones of sliding this paper demonstrates that model predictions of smooth average strain are entirely consistent with an actual strain concentrated along very narrow zones furthermore we suggest that model predictions of total accumulated strain should correlate with observable variations in bulk density and seismic velocity

el gygytgyn impact crater chukotka arctic russia impact cratering aspects of the 2009 icdp drilling project the el gygytgyn impact structure in chukutka arctic russia is the only impact crater currently known on earth that was formed in mostly acid volcanic rocks mainly of rhyolitic with some andesitic and dacitic compositions in addition because of its depth it has provided an excellent sediment trap that records paleoclimatic information for the 3 6 myr since its formation for these two main reasons because of the importance for impact and paleoclimate research el gygytgyn was the subject of an international continental scientific drilling program icdp drilling project in 2009 during this project which due to its logistical and financial challenges took almost a decade to come to fruition a total of 642 3 m of drill core was recovered at two sites from four holes the obtained material included sedimentary and impactite rocks in terms of impactites which were recovered from 316 08 to 517 30 m depth below lake bottom mblb three main parts of that core segment were identified from 316 to 390 mblb polymict lithic impact breccia mostly suevite with volcanic and impact melt clasts that locally contain shocked minerals in a fine grained clastic matrix from 385 to 423 mblb a brecciated sequence of volcanic rocks including both felsic and mafic basalt members and from 423 to 517 mblb a greenish rhyodacitic ignimbrite mostly monomict breccia the uppermost impactite 316 328 mblb contains lacustrine sediment mixed with impact affected components over the whole length of the impactite core the abundance of shock features decreases rapidly from the top to the bottom of the studied core section the distinction between original volcanic melt fragments and those that formed later as the result of the impact event posed major problems in the study of these rocks the sequence that contains fairly unambiguous evidence of impact melt which is not very abundant anyway usually less than a few volume is only about 75 m thick the reason for this rather thin fallback impactite sequence may be the location of the drill core on an elevated part of the central uplift a general lack of large coherent melt bodies is evident similar to that found at the similarly sized bosumtwi impact crater in ghana that however was formed in a target composed of a thin layer of sediment above crystalline rocks

multiple ophiolite generation preserved in the northern philippines and the growth of an island arc complex abstract although all oceanic arcs grow through the addition of subduction generated magmas the geology of the northern philippines demonstrates that a major contribution to arc crustal growth can come from repeated episodic intra arc back arc and or fore arc oceanic crust generation with subsequent preservation of the basic ultrabasic units in the arc complex at least five episodes of oceanic crust generation are represented in the northern philippines by preserved ophiolitic sequences and recent intra arc seafloor spreading each episode is distinct in age as confirmed by modern dating techniques with the ages ranging from pre jurassic to quaternary although the philippines is widely regarded as an amalgamation of allochthonous terranes a review of the available data shows that there is currently no compelling evidence that these ophiolites are of exotic origin and that they have been tectonically accreted to the philippine arc complex rather the evidence suggests that most and possibly all of the ophiolites were generated as back arc fore arc or intra arc crust within the philippine arc complex hence there is a close spatial association of several ophiolitic terranes of diverse ages spanning 150 myr that formed as part of the arc complex such an association may have arisen from episodic generation of oceanic crust during periods of local extension in a suprasubduction zone setting which has experienced changing and possibly overlapping subduction from the east and west sides in the current reference frame disruption of the ophiolitic basement terranes has been and continues to be effected primarily by wrench faulting this style of arc growth has implications for the paleotectonic interpretation of ancient ophiolite arc terranes in continents and the petrologic evolution of island arcs

geochemical complexities preserved in the volcanic rocks of the zambales ophiolite philippines the geochemical characteristics of the volcanic rocks from the zambales ophiolite luzon philippines indicate that the oceanic crust now preserved in the ophiolite must have been generated in an oceanic subduction zone environment the volcanic rocks and dikes range in composition from magnesian basalt including mid ocean ridge basalt morb island arc tholeiite and boninite like lavas to andesite and dacite and have trace element abundances which suggest derivation from a mantle source which was heterogeneous on both regional and local scales the sources are mixtures of components variably depleted with respect to a morb source differences exist between respective exposures of the ophiolite from north to south the following geochemical patterns in the basaltic rocks emerge sm nd decreases with corresponding decreases in 143nd 144nd rare earth element ree patterns range from extremely light ree lree depleted with generally low abundances of all ree 1 5x chondritic to patterns which are slightly lree depleted similar to morb patterns throughout the range sr isotopic abundances are relatively high 87sr 86sr 0 7035 0 7040 perhaps more significant than the regional geochemical trends however is that all volcanic provinces display ranges of rock compositions and the spatial distribution of basalts with such variable geochemical affinities indicate superposition interfingering and possible mixing of different lavas we suggest that the array of geochemical data from the zambales ophiolite can be explained in terms of processes observed in present day convergent plate margins such as the marianas or lau basin in the western pacific complicated plate boundaries which have existed for long periods of time including closely opposing and changing subduction systems the rifting of arcs and the formation of backarc basins may result in the superposition of one lava type on another or may produce many small domains in the upper mantle sources for subduction related lavas some of which become extremely depleted or secondarily enriched magmas derived from such a heterogeneous mantle will display ranges in geochemical characteristics possibly similar to those observed in the zambales ophiolite indeed recent studies from present day backarc systems suggest that young arc volcanism is superimposed on morb like crust and that basalt compositions of certain backarc basins are zoned inward from arc like to morb like these studies indicate that relationships between arc and morb like crust in young subduction complexes is extremely complex hawkins and melchior 1985 volpe et al 1987 1990 we extrapolate arguments from studies of zoned backarc systems to the zambales ophiolite and in turn use data from the zambales ophiolite as a ruler for defining the scale of variation in these complex supra subduction zone systems

structural variety and tectonic evolution of strike slip basins related to the philippine fault system northern luzon philippines the northern part of the philippine fault zone in luzon corresponds to a complex braided system of left lateral strike slip faults the nw oriented main active branch which emerges from the philippine sea splits into an array of north striking splays responsible for the tectonic evolution of the central cordillera this complex fault pattern has favored local stress field variations strike slip basins have evolved in this framework along or in the vicinity of the main splays of the philippine fault apart from the classical pull apart tectonics on a releasing fault termination overlap or bend we describe other mechanisms such as the strike slip tilting or the warping of a strip limited by two strike slip faults the strike slip basins are good recorders of the evolution of the philippine fault system those located along the north striking cordilleran faults individualized in late early pliocene to pleistocene time when the present day philippine fault initiated but their main control is the fault shape acquired in upper middle miocene time the recent tectonic evolution of the fault system is best recorded in central luzon where the active basins trace an asymmetrical v shape with the longest branch trending ne parallel to the east luzon trough and the shortest one trending nw related to the philippine fault both the fault pattern and the basin distribution demonstrate the influence of the benham rise in the tectonic evolution of luzon the structural setting is interpreted as the result of an early miocene collision event between the benham rise and the eastern margin of luzon and subsequent inception of the nw striking strand of the philippine fault the present locations of the basins result from the interaction between the structural heritage and the present day regional plate motion