mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

origin of tar mats in petroleum reservoirs part ii formation mechanisms for tar mats abstract tar mats and oil legs from several marine source rock derived oil fields including the ula and oseberg fields in the north sea and an unidentified north american example have been analysed geochemically in all cases the tar mats were derived from the same petroleum system as the oil leg oils and were found in the most porous and permeable portions of the reservoir comparing case history data with published data on the subsurface solubility characteristics of asphaltenes in petroleums it is possible to eliminate some potential tar mat formation mechanisms and suggest the most likely causative mechanisms thus the adsorption of asphaltenes on reservoir clays and light to moderate levels of biodegradation are considered unlikely to contribute to tar mat formation in the ula field thermal degradation of the oil in the carrier and reservoir causing asphaltene precipitation may be a major contributing factor to the tar mats there in the oseberg field increased gas solution in the oil leg may have contributed to deasphaltation tentative mass balance calculations the extreme pressure dependence of asphaltene solubility in subsurface crude oils the occurrence of tar mats in zones of elevated permeability plus a simple transport model suggest that during secondary migration an asphaltene rich petroleum phase may be produced in dipping carrier beds and may be carried into the reservoir contributing in some instances to tar mat formation

seismicity and active tectonic processes in the ultra slow spreading lena trough arctic ocean summary with its remote location in the ice covered fram strait lena trough is a poorly known segment of the global mid ocean ridge system it is a prominent member of the ultra slow spreading mid ocean ridges but its spreading mechanisms are not well understood we relocalized teleseismically recorded earthquakes from the past five decades to identify tectonic processes in lena trough and the adjacent spitsbergen fracture zone fz during two cruises with rv polarstern in 2008 and 2009 we deployed seismic arrays on ice floes to record the local seismicity of lena trough we could identify and localize microseismic events which we assume to be present in the entire rift valley in contrast our relocalization of teleseismically recorded earthquakes shows an asymmetric epicentre distribution along lena trough with earthquakes occurring predominately along the western valley flanks of lena trough in 2009 february march several high magnitude earthquakes peaking in an mb 6 6 event occurred in an outside corner setting of the spitsbergen fz this is the strongest earthquake which has ever been recorded in fram strait and its location at the outside corner high of the ultra slow spreading ridge is exceptional comparing the seismicity with the magnetic anomalies and high resolution multibeam bathymetry we divide lena trough in a symmetrically spreading northern part and an asymmetrically spreading southern part south of the south lena fz we propose that a complex interaction between the former de geer megashear zone which separated greenland from svalbard starting at late mesozoic early cenozoic times and the developing rift in the southern lena trough resulted an increasing eastward dislocation towards the spitsbergen fz between older spreading axes and the recent active spreading axis which we believe to be located west of the bathymetric rift valley flanks in a wide extensional plain

geomorphic response to seasonal variations in rainfall in the southwest united states the interaction of the north american monsoon with watershed hydrology and landscape response is evaluated by observing geomorphic characteristics of hillslopes hydrology and stream channels in two mountain ranges with contrasting intensity of precipitation the study compares watersheds in the hualapai and santa catalina mountains in arizona which are similar in lithology elevation tectonic setting vegetation and annual precipitation but differ in the proportion of precipitation received in summer thunderstorms the hualapai mountains receive most of their precipitation in winter whereas rainfall in the santa catalina mountains occurs mostly in summer drainages in the santa catalinas are more variable in local relief and exhibit much more exposed bedrock and higher drainage density the trunk channel of a major drainage in the santa catalina mountains has measured discharges several orders of magnitude greater than a channel draining a comparable area in the hualapai mountains the alluvial segment of the channel in the santa catalina piedmont additionally displays greater concavity smaller width to depth ratios and a larger caliber of bed material in contrast to conventional interpretation summer monsoonal precipitation is not primarily responsible for generating most of the discharges that modify channels in large scale drainages in the monsoonal climate regime of the santa catalinas small basins flood most often in summer whereas larger drainages exhibit peak discharges in response to low intensity winter precipitation we attribute the paradoxical discharge response of larger drainages to the small spatial scale of summer thunderstorms which fail to deliver enough precipitation to generate floods in these larger basins but do prime hillslopes by stripping colluvium and lowering hillslope infiltration rates making the large drainages more responsive to areally extensive winter storms

influence of late quaternary climatic changes on geomorphic and pedogenic processes on a desert piedmont eastern mojave desert california radiocarbon dating of late quaternary deposits and shorelines of lake mojave and cation ratio numerical age dating of stone pavements dorn 1984 on the adjacent soda mountains piedmont provide age constraints for alluvial and eolian deposits these deposits are associated with climatically controlled stands of lake mojave during the past 15 000 yr six alluvial fan units and three eolian stratigraphic units were assigned ages based on field relations with dated shorelines and piedmont surfaces as well as on soil geomorphic data all but one of these stratigraphic units were deposited in response to time transgressive climatic changes beginning approximately 10 000 yr ago increased eolian flux rates occurred in response to the lowering of lake mojave and a consequent increase in fine sediment availability increased rates of deposition of eolian fines and associated salts influenced pedogenesis stone pavement development and runoff infiltration relations by 1 enhancing mechanical weathering of fan surfaces and hillslopes and 2 forming clay and silt rich surface horizons which decrease infiltration changes in alluvial fan source areas from hillslopes to piedmonts during the holocene reflect runoff reduction on hillslopes caused by colluvial mantle development and runoff enhancement on piedmonts caused by the development of less permeable soils inferred increased in early to middle holocene monsoonal activity resulted in high magnitude paleo sheetflood events on older fan pavements this runoff triggered piedmont dissection which in turn caused increased sediment availability along channel walls thus runoff infiltration changes during the late quaternary have occurred in response to eolian deposition of fines pedogenesis increased sheetflood activity in the holocene and vegetational changes which are related to many complicated linkages among climatic change lake fluctuations and eolian hillslope and alluvial fan processes

origin of tar mats in petroleum reservoirs part i introduction and case studies abstract three case studies of petroleum reservoirs containing tar mats oseberg field and ula field both from the north sea and a north american field have been studied using a variety of organic geochemical techniques with special emphasis on the geochemistry of the asphaltenes the results were evaluated in combination with the available geological data and with numerical simulations to evaluate possible mechanisms of tar mat formation part ii wilhelms and larter 1994 in part i the petroleum geochemistry of three sets of tar mats is described the tar mat extracts were characterized by a sharp increase in the asphaltene content 20 60 wt compared with the oil leg extracts 1 5 wt the extract yields kg eom t rock in the tar mats ca 4 46 for the oseberg field 8 18 for the ula field and 8 43 for the north american case study are significantly higher than in the corresponding oil leg extracts which have extract yields of ca 1 14 kg t rock the tar mat extracts are geochemically related to the pooled oil as revealed by geochemical fingerprinting including biomarker analysis the tar mats are situated in high porosity and high horizontal permeability sandstone layers above permeability barriers several zones of low extract yield and immature petroleum geochemical characteristics were observed within the oil legs and between tar mats which might represent minor amounts of petroleum generated locally from indigenous organic matter

intraplate seismicity and stresses in young oceanic lithosphere we have conducted a systematic study of oceanic intraplate seismicity in the period 1964 1983 occurring in lithosphere younger than 35 million years focal mechanisms determined for 26 events using first motions p wave modeling and surface wave amplitude radiation patterns along with 13 previously determined mechanisms indicate a diversity of faulting styles and stress orientations in young oceanic lithosphere there is no evidence for a worldwide transition from extension to compression at a specific lithospheric age tensional axes of normal faulting events are oriented oblique to the spreading direction and do not show extension in the spreading direction congressional axes of thrust events show a weak preferred orientation in the spreading direction presumably due to ridge push stresses however stress axes of thrust events in very young lithosphere age 9 ma do not show a correlation with the spreading direction suggesting that the ridge push force is small near the ridge these observations indicate that tensional stress in the spreading direction is concentrated in a very narrow zone at the ridge axis in agreement with models which assume that the axial region has very little tensile strength normal faulting events occur at greater depths and temperatures than thrust events possibly because rocks under tensional stress have strength maxima at higher temperatures alternatively the depths of the normal faulting events may be a result of the higher thermal gradients at depth in young lithosphere if thermoelastic stresses are important in generating this seismicity differential thermal contraction along fracture zones does not seem to be a significant source of near ridge intraplate seismicity high seismicity rates in young lithosphere noted previously on a global scale seem characteristic of all three major ocean basins additionally most of the total oceanic intraplate moment release takes place in young lithosphere however most of the events and more than 96 of the total moment release of the near ridge intraplate seismicity are contained in five areas two zones in the indian ocean one near chagos bank and the other in the amsterdam st paul region indicate tension parallel to the trend of the plate boundary a region of high seismicity near the east pacific rise between 3 and 7 n shows both normal and thrust faulting while another region on the cocos plate west of the panama fracture zone is characterized by strike slip faulting thrust strike slip and normal faulting events are found in a small seismically active zone of the south central pacific the extremely uneven distribution of seismicity if not a result of the relatively short time period studied 20 years indicates local perturbations strongly affect near ridge seismicity

geochemical characterization of solid bitumen in the chesterian mississippian sandstone reservoir of the hitch field southwest kansas solid bitumen was identified within the mississippian chester sandstone reservoir in the hitch field southwest kansas the adjacent etzold field has similar reservoir properties but lacks solid bitumen although the hitch and etzold fields were thought to be in pressure communication and have common source rocks hitch and etzold crude oils and core extracts were characterized geochemically to gain a better understanding of the reservoir filling history and geological and geochemical controls on solid bitumen formation in the hitch reservoir based on a variety of geochemical characteristics we propose that the hitch and etzold oils are mixtures derived from ordovician and late devonian early mississippian woodford shale source rocks the solid bitumen in the hitch reservoir probably results from mixing of oils having different geochemical compositions which filled the reservoir over an extended period no evidence of severe biodegradation or thermal alteration is observed a reservoir filling scenario is proposed in an effort to explain why the hitch oils are geochemically more heterogeneous than etzold oils furthermore gas de asphalting and regional pressure drops as a result of post laramide orogeny may have contributed to a phase change in the reservoir fluid and deposition of solid materials by disturbance of thermodynamic equilibrium the major significance of this study is related to improving our knowledge on the occurrence of the presence of solid bitumen in reservoirs that can introduce barriers into the reservoir complicating waterflood operations and leading to erroneous oil in place calculations and subsequently lower than expected oil recovery

coevolution of nonlinear trends in vegetation soils and topography with elevation and slope aspect a case study in the sky islands of southern arizona 1 feedbacks among vegetation dynamics pedogenesis and topographic development affect the critical zone the living filter for earth s hydrologic biogeochemical and rock sediment cycles assessing the importance of such feedbacks which may be particularly pronounced in water limited systems remains a fundamental interdisciplinary challenge the sky islands of southern arizona offer an unusually well defined natural experiment involving such feedbacks because mean annual precipitation varies by a factor of five over distances of approximately 10 km in areas of similar rock type granite and tectonic history here we compile high resolution spatially distributed data for effective energy and mass transfer eemt the energy available to drive bedrock weathering above ground biomass soil thickness hillslope scale topographic relief and drainage density in two such mountain ranges santa catalina scm pinaleno pm strong correlations exist among vegetation soil topography variables which vary nonlinearly with elevation such that warm dry low elevation portions of these ranges are characterized by relatively low above ground biomass thin soils minimal soil organic matter steep slopes and high drainage densities conversely cooler wetter higher elevations have systematically higher biomass thicker organic rich soils gentler slopes and lower drainage densities to test if eco pedo geomorphic feedbacks drive this pattern we developed a landscape evolution model that couples pedogenesis and topographic development over geologic time scales with rates explicitly dependent on vegetation density the model self organizes into states similar to those observed in scm and pm our results highlight the potential importance of eco pedo geomorphic feedbacks mediated by soil thickness in water limited systems

southeast indian ocean ridge earthquake sequences from cross correlation analysis of hydroacoustic data summary parameters of earthquake sequences for instance location and timing of foreshocks and aftershocks are critical for understanding dynamics of mid ocean ridge and transform faults whole sequences including small earthquakes in the ocean cannot be well recorded by land based seismometers due to large epicentral distances recent hydroacoustic studies have demonstrated that t waves are very effective in detecting small submarine earthquakes because of little energy loss during propagation in sound fixing and ranging channel for example an mw 6 2 2006 march 6 40 11 s 78 49 e transform fault earthquake occurred at the southeastern indian ocean ridge but national earthquake information center only reported three aftershocks in the first following week we applied cross correlation method to hydroacoustic data from the international monitoring system arrays in the indian ocean to examine the whole earthquake sequence we detected 14 aftershocks and none foreshock for the earthquake and locations of these aftershocks show an irregular pattern from the observation we suggest that the feature could be caused by complicated transcurrent plate boundary dynamics between two overlapped spreading ridges that is possibly explained by the bookshelf faulting model

location of the africa australia india triple junction and motion between the australian and indian plates results from an aeromagnetic investigation of the central indian and carlsberg ridges accepted 1994 june 7 received 1994 may 23 in original form 1993 may 24 summary prior studies have proposed and examined the hypothesis that india and australia are separate rigid plates separated by a wide near equatorial e w striking plate boundary attempts to place narrow limits on the location of the africa australiaindia triple junction have been hindered however by the lack of useful magnetic profiles crossing the eastern carlsberg ridge and northern central indian ridge herein we present near ridge portions of new profiles from an aeromagnetic survey of the carlsberg ridge east of 66 e and of the central indian ridge north of 19 s these new data are used to estimate 35 new spreading rates averaged from the middle of chron 2a 3 03 ma to the present all other plate motion data along the central indian and carlsberg ridges are also analysed to investigate the present kinematics of the indian ocean especially the motion and boundary between the indian and australian plates unlike prior efforts we objectively estimate uncertainties in the strikes of transform faults along the carlsberg and central indian ridges

segmentation of transform systems on the east pacific rise implications for earthquake processes at fast slipping oceanic transform faults seven of the eight transform systems along the equatorial east pacific rise from 12 n to 15 s have undergone extension due to reorientation of plate motions and have been segmented into two or more strike slip fault strands offset by intratransform spreading centers itscs earthquakes recorded along these transform systems both teleseismically and hydroacoustically suggest that segmentation geometry plays an important role in how slip is accommodated at oceanic transforms results of thermal calculations suggest that the thickness of the brittle layer of a segmented transform fault could be significantly reduced by the thermal effect of itscs consequently the potential rupture area and thus maximum seismic moment is decreased using coulomb static stress models we illustrate that long itscs will prohibit static stress interaction between transform segments and limit the maximum possible magnitude of earthquakes on a given transform system furthermore transform earthquakes may have the potential to trigger seismicity on normal faults flanking itscs

magnetic and bathymetric investigations over the vema region of the central indian ridge tectonic implications abstract magnetic and bathymetric data were collected along n50 e profiles in the vema region 7 12 s 64 70 e across five ridge segments of the central indian ridge cir anomalies up to a5 10 ma identified by generating synthetics for each profile show asymmetric spreading though inter segmental variations in spreading rate have been observed the mean half spreading rate increases from 1 8 cm yr in the region north of vema transform to 2 1 cm yr south of the vema transform the average spreading rate for the last 10 ma has not always been symmetric the interpretation of magnetic and bathymetric data shows the complex nature of this part of the cir which could probably be attributed to i the short ridge segment ii interaction with fracture zone and iii interaction between the cir and the wide boundary zone wbz separating the indian and australian plates

a study of the holzener asphaltkalk northern germany observations regarding the distribution composition and origin of organic matter in an exhumed petroleum reservoir abstract the holzener asphalkalk deposit is located on the southwestern flank of the hils syncline northern germany it consists of a series of kimmeridgian portlandian bitumen impregnated limestones cumulative thickness of nearly 200 m overlain unconformably by neocomian claystones and underlain by older jurassic and triassic sediments the most bitumen rich limestone layers have been extensively mined with the result that there is three dimensional access to what is probably an exhumed series of stacked petroleum reservoirs geochemical petrological and petrophysical techniques were used to gain some insight into the distribution and the source of the organic matter in two of the 13 layers of the deposit most disseminated organic matter was found to be soluble in organic solvents and rich in non hydrocarbons its distribution appears to be controlled by ooid versus carbonate matrix abundances the occurrence of fractures and degree of impregnation small meso to micro pores have high bitumen saturations whereas large pores meso to macro have much lower saturations mature petroleum is thought to have once filled all pores but alteration has probably removed as much as 85 of it mainly from the large pores regional geochemical considerations pyrolysis and stable carbon isotope data indicate that the original petroleum was most probably derived from the underlying posidonia shale toarcian source rock asphaltene pyrolysis data indicate that an algal lacustrine co source might have also contributed to the deposit

geochemical characterization of solid bitumen migrabitumen in the jurassic sandstone reservoir of the tut field shushan basin northern western desert of egypt abstract asphaltene rich solid bitumen was identified within the pore spaces of the middle jurassic khatatba sandstone reservoir in the tut field shoushan basin egypt despite good reservoir properties have been detected the solid bitumen forms barriers in the reservoir which prevents economic petroleum production petrographical and organic geochemical techniques were applied in order to characterize the source rock of the bitumen and its formation mechanisms organic geochemical results confirm the presence of actively migrating non biodegraded hydrocarbons within the khatatba sandstones biomarker data suggest that the solid bitumen originated from a marine shale source rock that was deposited in mildly anoxic to suboxic conditions biomarker oil source rock correlation with the organic rich sediments of the khatatba formation indicates that the shale and coaly shale within the khatatba formation are an effective source rock in the area the natural deasphalting is the predominant mechanism for the formation of the asphaltene rich solid bitumen in the khatatba sandstones adsorption of asphaltenes onto authigenic kaolinite played a major role in the fixation of bitumen furthermore gas deasphalting caused by gas injection may have contributed to a phase change in the reservoir fluid and deposition of solid bitumen the injected gas is generated from khatatba type iii kerogen or older source rocks in the shoushan basin during the mature stage of hydrocarbon generation

micromorphological study of irrigation canal sediments landscape evolution and hydraulic management in the middle gila river valley phoenix basin arizona during the hohokam occupation the hohokam were a long lived 300 1450 ad agrarian society in the deserts of southern arizona with an efficient irrigation system and a complex socio political organization dependent on environmental conditions they coped with hydrological and geomorphic changes which potentially had an impact on their cultural evolution irrigation canals were regularly maintained and or abandoned and their fills record secondary signatures of fluvial dynamics landscape change and land use micromorphological studies allow for the creation of a typology of canal fills based on semi quantitative measurements a synchronous and diachronic analysis of canal systems can then be undertaken by means of pedosedimentary correlation as part of cultural resource management projects on the gila river indian reservation south of phoenix studies have been conducted on the prehistoric santan irrigation system in use from the 10th to the 15th century ad phases of low flow were identified followed by alluvial fan flooding and seasonal deposition between the 10th 14th century ad sedentary early classic period during the 14th 15th century ad late classic period evidence of active erosion is initially recorded in the sediments followed by seasonal water supply and ultimately canal abandonment despite probable efficient irrigation

the june 2000 mw 7 9 earthquakes south of sumatra deformation in the india australia plate 1 two large mw 7 9 earthquakes occurred on 4 and 18 june 2000 south of sumatra beneath the indian ocean both earthquakes were predominantly left lateral strike slip on vertical n s trending faults that we interpret to be reactivated fracture zones the 4 june enggano earthquake occurred at the edge of the rupture area of the 1833 subduction earthquake the first strike slip subevent within the subducting plate triggered a thrust subevent on the plate interface which comprised at least 35 of the total moment and ruptured se away from the 1833 earthquake the 18 june earthquake in the wharton basin is one of the largest shallow strike slip faulting earthquakes ever recorded a small second subevent with reverse slip is required to fit the body waves the orientation of both subevents in our preferred model is consistent with the current stress field in the region both the june 2000 earthquakes are consistent with recent models of distributed deformation in the india australia composite plate the occurrence of the enggano earthquake implies that the stress field within the indian plate continues to a depth of 50 km in the subducting slab the purely strike slip source model of the wharton basin earthquake obtained by robinson et al 2001 matches the p waves very poorly and fits the s waves no better than our preferred model the strike slip subevents of both earthquakes had few aftershocks and higher stress drops than the subduction thrust subevent of the enggano earthquake this difference is consistent with previous observations of oceanic and subduction earthquakes

degradation and maturity controls on oil field petroleum column heterogeneity in the gullfaks field norwegian north sea abstract a geochemical mapping of the gullfaks reservoir petroleum column based on 226 reservoir core extracts and 23 drill stem test dst and repeat formation test rft oil samples indicates the presence of statistically testable lateral compositional heterogeneities in the petroleum column these different petroleum populations can be related to lateral filling of the field from two source basins and a more pervasive subsequent episode in reservoir biodegradation petroleum maturity gradients within the field were detected using conventional biomarker gc ms methods on saturated hydrocarbon fractions the degree of biodegradation was monitored by quantitative gc of n alkanes in whole oil samples using a squalane internal standard the observed compositional differences in the gullfaks petroleum column are consistent with diffusion being the primary mixing process thermal convection being eliminated on both compositional and modeling grounds compositional differences between core extracts and dst samples from the same intervals suggest that fractionation of petroleum might occur during testing and that interpretation of reservoir petroleum columns from mixed sample sets should be treated cautiously

tectonic footprints and landscape evaluation along kulur river valley kumaun lesser himalaya india abstract tectonic reactivation along the major boundary thrusts and subsidiary thrust faults in the himalaya during the quaternary period has resulted in the rejuvenation of the landforms considering the role of neotectonic activity in the hazardous kumaun region we aim to study the neotectonically formed landscape along the kulur river valley a non glacier fed tributary of the saryu river in the zone of the berinag thrust bt the field evidences encompassing a variety of geomorphic indicators e g unpaired fluvial terraces palaeo river channels alluvial fans active and stabilized landslides lineaments waterfalls alignment of springs deeply dissected hills straight meandering course of rivers streams v shaped valleys triangular fault facets and water logged river ponding along with integrated values of eight morphometric parameters and stream steepness index ks indices characterize the modification of landscape in the kulur river valley the computed morphometric parameters in the kulur river drainage and its tributaries show that the basin has been tilted and uplifted forming v shaped valleys along with deformed and disequilibrium longitudinal profile thereof suggesting it to be controlled by lithology and tectonics based on the response of active tectonics in form of geomorphic features and quantitative measurements of river channels the entire basin has been divided into three segments in which the northern and southern segments are active compared to the middle sector a possible nne ssw trending lineament fault running parallel to the kulur river has been observed in the upper catchment and is characterized by fluvial terraces river ponding and deflection and offsetting of drainage with rectangular pattern etc this may have given rise to the widening and straightening of river courses which provide the favorable room for river ponding resulting in deposition of about 2 m thick lacustrine mud deposit

a rare great earthquake on an oceanic fossil fracture zone first motion solution found in this study and has a strike consistent with the fracture zone north of the bend the southern fault plane with strike 178 d ip 54 and rake 65 has a strike consistent with the portion of the fracture zone to the south of the bend and has a dip which can be explained by the style of deformation that the region is undergoing the centroid moment tensor solution of the broad band model is calculated and found to be consistent with the region of low misfit in mantle wave solution space the broad band solution has a moment of 1 53 10 21 nm mw 8 1 again consistent with the mantle wave data slip propagated bilaterally with an approximate rupture velocity of 3k ms 1 80 per cent of local shear wave speed the rupture front is less well resolved to the south of the epicentre than to the north the majority 75 per cent of moment originated from slip on the northern fault the broad band data requires significant slip below the oceanic moho with as much as 70 per cent of moment due to slip in the brittle uppermost mantle in the preferred model

recognition and quantification of the effects of primary migration in a jurassic clastic source rock from the norwegian continental shelf a detailed quantitative geochemical study was made of a mature jurassic clastic source rock sequence in order to attempt to recognize and quantify petroleum migration effects two different quantitative approaches were followed to monitor primary migration effects on both the bitumen 1 e g whole rock extract and the bitumen 2 e g isolated kerogen extract fractions these methods were a comparisons of toc normalized extract yields and quantified molecular compositions of petroleums from both tightly carbonate cemented zones and those from less cemented zones and b comparison of petroleums from shale laminae vs those from interbedded siltstone laminae on this basis it was possible to assess apparent primary migration efficiency relative expulsion efficiencies in certain shale samples and relative import efficiencies in siltstone bands ranged from 11 total extract to 85 saturated hydrocarbons and individual n alkanes no carbon number dependent fractionation effects were observed for n alkanes of bitumen 1 in the c15 to c35 range i e expulsion efficiencies were independent of carbon number suggesting bulk oil flow with chromatographic modification as the predominant transport mechanism

fractionation of azaarenes during oil migration abstract a comparative study of alkylbenzoquinolines in crude oils and rock extracts bitumens from japan and sumatra shows that the ratios of nitrogen masked isomers nmis to nitrogen exposed isomers neis and the ratio of higher homologs to lower homologs is higher in crude oils than in corresponding bitumens also a regular increase in the ratios of nmis to neis with increasing migration distance is observed for a series of crude oils from the sarukawa oil field northeastern japan these results are attributed to the preferential migration of nmis caused by their weak adsorption on clay minerals and or their low solubility in interstitial water and the selective removal of lower honologs caused by their irreversible adsorption onto clays during oil migration which are regarded as geochromatographic phenomena it is suggested that those ratios can be used to estimate the degree of fractionation of oils during primary and secondary migration

historical seismicity near chagos a complex deformation zone in the equatorial indian ocean abstract over the last twenty years chagos bank has a seismicity rate disproportionate to its supposed intraplate location earthquake relocation also shows a high seismicity rate in pre wwssn time 1912 1963 with seven events located off of the central indian ridge including large events in 1912 m 6 8 and 1944 m 7 2 this study uses the moment variance technique a systematic search for the mechanism which best fits p pp sh love and rayleigh amplitudes to determine the focal mechanisms of two pre wwssn earthquakes a test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available moment variance analysis shows a thrust faulting mechanism for the 1944 event northeast of chagos bank near the chagos laccadive ridge and a strike slip focal mechanism for a smaller 1957 event west of chagos bank the 1944 event one of the largest oceanic intraplate earthquakes known moment 1 4 1027 dyne cm indicates that the chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought but rather regional tectonic deformation extending northeast of chagos bank and including thrust normal and strike slip events this seismicity and previously studied seismicity near the ninetyeast ridge and central indian basin suggest a broad zone of deformation stretching across the equatorial indian ocean this zone contains all known magnitude seven oceanic intraplate earthquakes not associated with subduction zones or continental margins suggesting that elsewhere such extensive deformation occurs only along plate boundaries this study proposes that a slow diffuse plate boundary extends east from the central indian ridge to the ninetyeast ridge and north to the sumatra trench a recent plate motion study confirms this boundary and suggests that it separates the australian plate from a single indo arabian plate

tectonic reconstruction of the clipperton and siqueiros fracture zones evidence and consequences of plate motion change for the last 3 myr bathymetry side looking sonar and magnetics data from the northern east pacific rise have been analyzed to determine the tectonic history of the clipperton fracture zone cfz and the siqueiros fracture zone sfz over the last 2 3 m y results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at 2 5 ma 1 2 1 5 ma 1 2 and 0 5 ma 4 5 along the right stepping clipperton transform the most recent change in spreading direction has resulted in fracture zone normal compression which we propose has created a median ridge and transform parallel troughs along the active transform fault volcanic intersection highs located near the ridge transform intersections rti of the cfz are interpreted to be the result of fracture zone normal extension that has created pathways for magma emplacement into onto the older lithosphere across from the rti along the left stepping sfz we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges tectonic reconstructions indicate a single euler pole is unable to describe the motion of the pacific cocos plate boundary between the cfz and sfz during periods of spreading direction change since 3 ma transtensional transforms will adjust to a new spreading direction almost immediately while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform

heterogeneous distribution of pyrobitumen attributable to oil cracking and its effect on carbonate reservoirs feixianguan formation in the jiannan gas field china few previous studies have focused on solid bitumen distribution and its effect on gas reservoir quality during oil cracking solid bitumen is commonly found in both gas and nongas reservoirs of the triassic feixianguan formation in the jiannan gas field the natural gases are mainly secondary cracking gases of oil generated from source rock of the permian wujiaping formation and the reservoir experienced temperatures above 150 c 302 f for about 35 m y a relatively narrow range of values and a wide range of values and widespread solid bitumen indicate that oil cracking took place in the gas field low concentrations of commonly 8 0 can be high quality gas reservoirs after oil cracking and should be favorable targets for future gas exploration in the northeastern sichuan basin and adjacent areas

isotopic analysis of wetland development in the american southwest the analysis of stable isotope and elemental fractions of organic material collected from san bernardino cienega was used to understand the history of vegetation composition and climate change within this desert wetland a 4000 yr record of sediment buildup based on four 14c measurements provides unique opportunities for the study of environmental conditions within an arid landscape and documents climate shifts from drier to wetter conditions in the late holocene 13c 15n and c n values were measured from a 3 8 m deep sedimentary section to understand the dynamics of vegetation and hydrology in desert wetlands through this section we observe 13c and c n values indicating a shift in the dominant source of organic matter within the section prior to 850 cal yr bp below 60 cm aquatic vascular plants and occasionally terrestrial vegetation were the primary organic sources whereas freshwater algae were the dominant organic matter source above this level these values indicate that while condition

a holocene climatic record for the sonoran desert from pollen analysis of montezuma well arizona usa abstract pollen and macrofossil analyses of a radiocarbon dated core 1125 cm long from montezuma well elev 1125 m yavapai county arizona provide a record of summer precipitation similar to that of other monsoonal regions of the world precipitation was least from 4000 to 5000 yr b p and generally was greater than today before 8400 yr b p during the early holocene moist period oak and grass pollen are abundant and the best modern analogs are within the arizona monsoon boundary so increased summer precipitation is indicated climatic parameters are estimated with the technique of best modern analogs for fossil samples temperature was highest ca 3400 and 5700 yr b p and generally was cooler than today before 6800 yr b p brief cold intervals in the holocene match periods of global cooling and of positive 14 c and 10 be anomalies

characterisation of non producible oil in eocene reservoir sandstones bad hall nord field alpine foreland basin austria abstract an asphaltene rich non producible petroleum tar fills the pore spaces of an eocene sandstone reservoir in the alpine foreland basin near bad hall austria despite good reservoir properties the solid bitumen prevents economic petroleum exploitation petrographical and geochemical techniques were applied in order to characterise the source rock of the bitumen and the formation mechanisms of the tar biomarker data argue for a marine shaly to marly source and indicate that the schoneck formation the main source rock in the area also sourced the bad hall nord structure the methylphenanthrene index implies a source rock maturity equivalent to 0 7 rr indicating a lateral migration distance in the order of 15 20 km natural deasphalting is the predominant mechanism for the formation of the tar adsorption of asphaltenes onto authigenic kaolinite played a major role during deasphalting and in the fixation of asphaltenes gas injection and migration related asphaltene precipitation are further feasible mechanisms

late pleistocene holocene alluvial stratigraphy of southern baja california mexico abstract a late pleistocene to holocene alluvial stratigraphy has been established for the basins of la paz and san jose del cabo in the southern tip of the baja california peninsula mexico six discrete alluvial units qt1 through qt6 were differentiated across the region using a combination of geomorphologic mapping sedimentological analysis and soil development these criteria were supported using radiocarbon optically stimulated luminescence and cosmogenic depth profile geochronology major aggradation started shortly after 70 ka qt2 and buildup of the main depositional units ended at 10 ka qt4 after deposition of qt4 increasing regional incision of older units and the progressive development of a channelized alluvial landscape coincide with deposition of qt5 and qt6 units in a second incisional phase all units consist of multiple 1 3 m thick alluvial packages deposited as upper flow stage beds that represent individual storms main aggradational units qt2 qt4 occurred across broad 2 km channels in the form of sheetflood deposition while incisional stage deposits are confined to channels of 0 5 2 km width continuous deposition inside the thicker 10 m pre qt5 units is demonstrated by closely spaced dates in vertical profiles in a few places disconformities between these major units are nevertheless evident and indicated by partly eroded buried soils the described units feature sedimentological traits similar to historical deposits formed by large tropical cyclone events but also include characteristics of upper regime flow sedimentation not shown by historical sediments like long 10 m wavelength antidunes and transverse ribs we interpret the whole sequence as indicating discrete periods during the late pleistocene and holocene when climatic conditions allowed larger and more frequent tropical cyclone events than those observed historically these discrete periods are associated with times when insolation at the tropics was higher than the present day conditions determined by precessional cycles and modulated by the presence of el nino like conditions along the tropical and northeastern pacific the southern baja california alluvial record is the first to document a precession driven alluvial chronology for the region and it constitutes a strong benchmark for discrimination of direct tropical influence on any other alluvial record in southwestern north america

analysis of petroleum fractions by tlc fid applications to petroleum reservoir description abstract thin layer chromatography and flame ionization detection iatroscan tlc fid offers a rapid and accurate method for quantifying saturated and aromatic hydrocarbons and resin asphaltene fractions in solvent extracts of petroleum source rocks reservoir rocks and crude oils while the instrument is superficially simple special precautions need to be taken in calibration and parameter setting due to complexities arising from the operational characteristics of the detector system nevertheless the technique offers superior speed and precision to conventional mplc analysis of petroleum fractions this paper describes the choice of operational parameters provides comparison with the traditional group type analysis technique mplc and presents some geochemical applications

evidence for younger dryas global climate oscillation and human response in the american southwest whether or not abrupt younger dryas climate change affected regional paleoenvironments and late pleistocene hunter gatherer populations is an important topic in the archaeology of the american southwest this paper reviews multiple age resolved proxy evidence to gauge the magnitude and direction of younger dryas chronozone ydc environmental changes in different settings and systems there is no record of ydc pluvial lake highstands in arizona or new mexico but there are impressive records of vegetation faunal stable isotope and geomorphological change coincident with the ydc these correlate with important adaptive changes in human hunting and land use as revealed in the analysis of the spatiotemporal distribution of late pleistocene hunting technologies clovis and folsom projectile point distributions do not support extant models of paleoenvironmental conditions in these interpretations significant cultural changes that coincide with the ydc include the clovis to folsom transition the demise of mammoth hunting and the development of a highly successful emphasis on bison increased regionalization and the abandonment of the northwestern chihuahuan and the sonoran deserts by mobile big game hunters

relationships between hydrocarbon evolution and the geochemistry of solid bitumen in the guanwushan formation nw sichuan basin abstract recently the devonian guanwushan formation in the northwestern sichuan basin gained industrial attention when solid bitumen was discovered in reservoirs and outcrops this discovery initiated the exploration and development of the devonian natural gas devonian in the northwestern sichuan basin is a new stratum for the exploration and development of paleozoic marine natural gas in sichuan basin however the unclear recognition on the genesis the sources and the hydrocarbon evolution history of solid bitumen and natural gas seriously restricted further exploration and development based on the solid bitumen reflectance carbon isotopes saturated hydrocarbon chromatography and solvent extract bitumen biomarkers combined with the component and carbon isotopes of natural gas comprehensive analyses have been performed on the origin of the solid bitumen and the accumulation and evolution of the natural gases the results showed that the outcrop solid bitumen is derived from biodegraded oil from the lower cambrian source rocks and is characterised by low bitumen reflectance low saturated aromatic hydrocarbon ratios seriously damaged normal alkanes and severe migration of the gas chromatographic baseline the reservoir bitumen formed through thermal cracking of oil from lower cambrian and lower permian source rocks and is characterised by high bitumen reflectance high saturated aromatic hydrocarbons and complete normal alkanes the differences of tectonic evolution take control of the two different genesis bitumen the longmen mountain strata in the northwestern sichuan basin were uplifted sharply after the jurassic and the early generated crude oil was biodegraded during uplifting and formed the outcrop bitumen however strata in well shuangtan 3 st3 continued to subside and the paleo reservoir thermal cracked during deep burial into dry gas and heavy components gathered to form reservoir bitumen revealing the devonian hydrocarbon evolution in sichuan basin is of highly significance and is meaningful to guide further researches on the gas source correlation of natural from paleo strata it is of great significance to reveal the devonian hydrocarbon evolution in sichuan basin and to guide further researches on the gas source correlation of natural gas from paleo strata

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea i gross composition of c15 soluble organic matter and molecular composition of c15 saturated hydrocarbons in mature oil prone source rocks of the upper jurassic kimmeridge clay in the brae field area north sea the yield and the gross composition of solvent extractabte pctroleum likc compounds as well as the molecular proportions of c15 saturated hydrocarbons are primarily controlled by the effects of hydrocarbon expulsion during primary migration within sample series extending from shale centres towards adjacent reservoir sandstones regular and pronounced yield decreases have been documented as well as marked compositional changes for c15 soluble organic matter and its compound class fractions saturated hydrocarbons aromatic hydrocarbons nso compounds and for normal and isoprenoid alkanes relative migration losses indicating relative expulsion efficiencies were determined for the c15 alkanes throughout the outer five meters of thick shales relative expulsion efficiencies increase regularly and drastically for all compounds considered e g exceeding 90 for the c15 n alkanes in the outermost shale samples samples from thin 5 50 cm shale layers interbedded between reservoir sandstones reveal similarly high expulsion efficiencies for n alkanes the degree of depletion remains uniform throughout the molecular range c15 to c30i e no compositional fractionation occurred during primary migration which is interpreted to reflect expulsion of oil as a single phase fluid a simple conceptual model is proposed to explain the mechanisms and relationships of generation and expulsion processes in time and space a geochemically relevant conclusion is that the c15 soluble organic matter recovered by solvent extraction of mature source rocks represents only a portion of the total non volatile petroleum like material originally generated

paleohydraulic reconstruction of flash flood peaks from boulder deposits in the colorado front range nine watersheds in the colorado front range with steep bedrock channels were used to test the accuracy of paleohydraulic reconstruction of large flash floods using boulder deposits the nine basins consist of eight small ungauged basins ranging in size from 1 6 to 29 km2 and the big thompson river at the mouth of the big thompson canyon draining 790 km2 between 1923 and 1976 all nine basins had had one catastrophic flash flood the magnitude of which has been estimated by the conventional slope area method in each basin coarse boulder deposits of the large flash floods were identified and three axes of the five largest boulders were measured along with at least two profiles of the valley cross section a simple arithmetic average of two theoretical and two empirical relationships was used to estimate average flood velocity using boulder size and shape average depth was estimated as the arithmetic average of four values computed from the manning equation a regression equation for boulder size and unit stream power a relative smoothness equation and a modified shields relationship the appropriate flood width for the estimated average depth was found by iteration using the valley cross sections the paleohydraulic discharges thus computed generally underestimate conventional slope area discharge estimates on small streams by as much as 75 although the average amount is only 28 too low and the reconstructed discharge in one stream was 31 too large the big thompson river flood of 1976 was overestimated by 76 reasons for discrepancy in reconstructed peaks could include 1 the possibility that floods may have been able to move boulders larger than those available to be moved 2 overestimation of the slope area discharge because high water marks were set prior to erosion of the channel 3 underestimation of original roughness coefficients and 4 macroturbulent effects during fast deep flows the paleohydraulic technique is applied to two other streams in colorado with sedimentological evidence of large flash floods but no conventional indirect discharge estimates a small tributary to the big thompson river draining 1 8 km2 has a paleohydraulic reconstructed flood peak of about 60 m3 s from a flood in 1976 using boulders excavated from a foundation site in holocene alluvium along boulder creek in boulder colorado a paleohydraulic reconstructed flood peak of between 860 and 1 512 m3 s is calculated this is 1 4 to 2 4 times the magnitude of the estimated 500 yr flood

heterogeneity of oil composition within a reservoir as a reflection of accumulation history abstract the unaltered petroleums of a small oil field in the f r germany reveal pronounced spatial heterogeneities in bulk and molecular composition over most of the field area the oil in high porosity high permeability sand intervals bears a higher proportion of saturated hydrocarbons and has a higher maturity chemical signature as compared to oils occupying lower porosity lower permeability zones for all sands with less than about 13 porosity a regular relationship was observed between oil composition and porosity permeability with a decrease of the latter petrophysical properties increasingly lower chemical maturities and increasingly different bulk chemical compositions lower proportions of saturated hydrocarbons and higher proportions of nso compounds were recorded maximum contrasts measured for chemical maturities are e g 0 63 versus 0 82 vitrinite reflectance equivalents rc values calculated on the basis of the methylphenanthrene index according to radke and welte 1983 0 31 versus 0 84 for the ratio of triaromatic to mono plus triaromatic steroid hydrocarbons and 0 38 versus 0 47 for the 20s 20s 20r ratio of c29 5 h 14 h 17 h steranes these spatial heterogeneities in the composition of the oils are interpreted to reflect 1 i successive stages of trap filling due to episodes of increasing burial and maturation of the relevant source rocks in combination with 2 ii low rates of in reservoir mixing of these individual oil charges by convection or diffusion recognition of compositional heterogeneities within the oil column of individual fields may not only provide a basis for the reconstruction of reservoir filling history but may also have important implications for improving petroleum production

in situ evidence for dextral active motion at the arabia india plate boundary the arabia india plate boundary also called theowen fracture zone is perhaps the least known boundary among large tectonic plates1 6 although it was identified early on as an example of a transform fault converting the divergent motion along the carlsberg ridge to convergent motion in the himalayas7 its structure and rate of motion remains poorly constrained here we present the first direct evidence for active dextral strike slip motion along this fault based on seafloor multibeam mapping of the arabia india somalia triple junction in the northwest indian ocean there is evidence for 12km of apparent strike slip motion along the mapped segment of the owen fracture zone which is terminated to the south by a 50 km wide pull apart basin bounded by active faults by evaluating these new constraints within the context of geodetic models of global plate motions we determine a robust angular velocity for the arabian plate relative to the indian plate that predicts 2 4mmyr 1 dextral motion along the owen fracture zone this transformfault was probably initiated around 8 million years ago in response to a regional reorganization of plate velocities and directions8 11 which induced a change in configuration of the triple junction infrequent earthquakes of magnitude 7 and greater may occur along the arabia india plate boundary unless deformation is in the formof aseismic creep

middle and late wisconsin paleobotanic and paleoclimatic records from the southern colorado plateau usa the colorado plateau is a distinct physiographic province in western north america which presently straddles the transition between summer wet and summer dry climatic regimes to the south and northwest respectively in addition to climate the diversity of environments and plant communities on the colorado plateau has resulted from extreme topographic diversity desert lowlands as low as 360 m elevation are surrounded by forested plateaus and even higher peaks greater than 3800 m elevation this environmental diversity provides a unique opportunity to study the history of biotic communities in an arid region of north america although the colorado plateau harbours numerous potential sites the paleoecological record of the plateau is poorly known potential deposits for analysis include packrat middens alluvial and cave sites at lower elevations and lake bog and wetland sites at higher elevations forty six sites have been analysed across the nearly 337 000 km2 region of which 27 contain records that span marine oxygen isotope stage is 2 data with is 3 information coming from only 12 sites most is 2 and 3 sites are clustered along the lowland regions of the colorado river corridor and the uplands of the mogollon rim area we compiled selected data from long paleoecological records to examine patterns of vegetation and climate change across the southern colorado plateau for the middle and late wisconsin during the middle wisconsin mixed conifers covered middle elevations presently dominated by ponderosa pine pinus ponderosa and juniper juniperus woodland grew at elevations today covered by blackbrush coleogyne and sagebrush artemisia desert during the late wisconsin boreal conifers primarily engelmann spruce picea engelmannii replaced the mixed conifer association estimates of mean annual temperatures mat during is 3 were at least 3 4 c cooler than today whereas is 2 mat estimates are at least 5 c colder our investigation of millennial scale climatic variability within the region provided equivocal results the packrat midden sequence could not distinguish vegetation changes that might be associated with heinrich events in the north atlantic from the lake records however many heinrich events were associated with generally drier intervals often with elevated sagebrush pollen concentrations future paleoecological investigations should concentrate on the northern colorado plateau as well as the eastern and western margins additional sites along with closer spaced sampling in regions already studied will be important in determining the history of important climatic phenomena such as the timing of the arizona monsoon

temporal frequency distributions of alluvium in the american southwest taphonomic paleohydraulic and demographic implications abstract the use of radiocarbon frequency distributions to reconstruct prehistoric human and animal populations must account for taphonomic loss and other factors that affect the archaeological and paleontological records surovell et al jas 36 1715 1724 have recently proposed a correction for taphonomic bias that is based on the radiocarbon frequency of a global sample of volcanic deposits analysis of 717 radiocarbon dates sampled from the alluvium of the san pedro and santa cruz rivers and their tributaries in southeastern arizona shows that discovery and scientific biases also play an important role in the creation of radiocarbon frequency distributions and that the rate of taphonomic bias in prehistory is not predicted by the radiocarbon frequency of volcanic deposits the latter principle is further argued using a sample of 123 pliocene to clovis age proboscideans from the san pedro valley we propose an alternative model that is based on the nature of the stratigraphic record with discovery bias scientific bias taphonomic loss and the shape of the calibration curve all operating to influence the temporal frequency distribution of radiocarbon dated prehistoric phenomena

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

fast rift propagation at a slow spreading ridge bathymetric magnetic gravity and morphologic data from the flank of the slow spreading mid atlantic ridge reveal obliquely oriented features that offset magnetic isochrons and morphological patterns within individual ridge segments these features form angles of 10 40 with the isochrons and are inferred to result from rift propagation at rates several times the spreading rate representing the fastest propagators yet observed at a slow spreading ridge these fast propagators appear to have formed as a result of tectonic extension migrating along ridge segments as the segments change from more magmatic to less magmatic periods of spreading

the factors controlling the abundance and migration of heavy vs light oils as constrained by data from the gulf of suez part ii the significance of reservoir mass transport processes abstract reservoir flow processes have a major impact on the oil quality and distribution of heavy vs light oils both within reservoirs and on the petroleum system scale in the gulf of suez gos gravitational segregation of black oil columns is a common phenomenon an understanding of the mass transport processes in these reservoirs is important for evaluating their impact on the composition of migrating petroleum 2d simulation results show that pressure diffusion is a too slow process to be the main cause for the vertical compositional gradients a reservoir filling model which is a refined version of the geological model of gussow linked to the composition of the gos expelled petroleum fluids and their pvt behaviour and constrained by the gos oil field data is presented this model gives viable explanations for a formation of gravitationally segregated oil columns b tar mat occurrences and their affinity to low maturity heavy oils and c wide spread occurrences of heavy oils and their relationship with light oils in this model continuous density overturning by compositionally controlled bulk flow is the prime cause for the formation of gravitationally segregated columns occurring when a the charge to the reservoir displays a maturity related decrease in density with time and b the reservoir volume relative to the charge volume is large enough for the reservoir to retain a wide maturity range lateral compositional variability in many reservoirs is today interpreted to image the initial filling of the reservoirs it is demonstrated that such lateral gradients can form effectively from re orientation of vertical gradients via ordinary diffusion these gradients have symmetry elements relative to the petroleum mass distribution in the reservoir fill and spill from gravitationally segregated columns enhance the efficiency of the migration of heavy oils

aftershock sequences in the mid ocean ridge environment an analysis using hydroacoustic data abstract hydroacoustic data from autonomous arrays and the u s navy s sound surveillance system sosus provide an opportunity to examine the temporal and spatial properties of seismicity along portions of the slow spreading mid atlantic ridge mar intermediate spreading juan de fuca ridge jdfr and fast spreading east pacific rise epr aftershock and foreshock events are selected from the hydroacoustic earthquake catalog using single link cluster slc analysis with a combined space time metric in the regions examined hydroacoustic data improve the completeness level of the earthquake catalog by 1 5 2 0 orders of magnitude allowing the decay constant p of the modified omori law mol to be determined for individual sequences a non parametric goodness of fit test indicates six of the seven sequences examined are described well by a mol model the p values obtained for individual ridge and transform sequences using hydroacoustic data are larger than that previously estimated from the analysis of a stacked sequence generated from teleseismic data for three sequences along the siqueiros discovery and western blanco transforms p values are estimated to be 0 94 1 29 the spatial distribution of aftershocks suggests that the mainshock rupture is constrained by intra transform spreading centers at these locations an aftershock sequence following a 7 1 m s thrust event near the northern edge of the easter microplate exhibits p 1 02 0 11 within the sequence aftershocks are located to the north of a large topographic ridge which may represent the surface expression of the shallow dipping fault that ruptured during the mainshock two aftershock sequences near 24 25 n and 16 35 n on the mar exhibit higher p values 1 74 0 23 and 2 37 1 65 although the latter estimate is not well constrained because of the small number of aftershocks larger p values along the ridge crest might reflect a hotter thermal regime in this setting additional monitoring however will be needed to determine if p value differences between the ridge and transform sequences are robust a 1999 sequence on the endeavour segment of the jdfr which has been correlated with changes in the hydrothermal system is described poorly by the mol model the failure of the mol model the anomalously large number of earthquakes within the sequence and absence of a clearly dominant mainshock are inconsistent with aftershock activity and the simple tectonic origin that has been proposed previously for this sequence

macrofracturing of oceanic lithosphere in complex large earthquake sequences major earthquakes in oceanic lithosphere seaward of the subduction zone outer trench slope are relatively uncommon but several recent occurrences have involved very complex sequences rupturing multiple nonaligned faults and or having high aftershock productivity with diffuse distribution this includes the 21 december 2010 m w 7 4 ogasawara bonin 11 april 2012 m w 8 6 indo australia 23 january 2018 m w 7 9 off kodiak island and 20 december 2018 m w 7 3 nikol skoye northwest pacific earthquakes major oceanic intraplate event sequences farther from plate boundaries do not tend to be as complex in faulting or aftershocks outer trench slope extensional faulting can involve complex distributed sequences particularly when activated by great megathrust ruptures such as 11 march 2011 m w 9 1 tohoku and 15 november 2006 m w 8 3 kuril islands intense faulting sequences also occur near subduction zone corners with many fault geometries being activated including some in nearby oceanic lithosphere as for the 29 september 2009 m w 8 1 samoa 6 february 2013 m w 8 0 santa cruz islands and 16 november 2000 m w 8 0 new ireland earthquakes the laterally varying plate boundary stresses from heterogeneous locking recent earthquakes or boundary geometry influence the specific faulting geometries activated in nearby major intraplate ruptures in oceanic lithosphere preexisting lithospheric structures and fabrics exert secondary influences on the faulting intraplate stress release in oceanic lithosphere near subduction zones favors distributed macrofracturing of near critical fault systems rather than localized single fault failures both under transient loading induced by plate boundary ruptures and under slow loading by tectonic motions and slab pull

the relationship between seismicity and fault structure on the discovery transform fault east pacific rise there is a global seismic moment deficit on mid ocean ridge transform faults and the largest earthquakes on these faults do not rupture the full fault area we explore the influence of physical fault structure including step overs in the fault trace on the seismic behavior of the discovery transform fault 4s on the east pacific rise one year of microseismicity recorded during a 2008 ocean bottom seismograph deployment 24 377 0 ml 4 6 earthquakes and 24 years of mw 5 4 earthquakes obtained from the global centroid moment tensor catalog are correlated with surface fault structure delineated from high resolution multibeam bathymetry each of the 15 5 4 mw 6 0 earthquakes that occurred on discovery between 1 january 1990 and 1 april 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross correlation technique microseismicity was relocated using the hypodd relocation algorithm the western fault segment of discovery dw is composed of three zones of varying structure and seismic behavior a zone with no large events and abundant microseismicity a fully coupled zone with large earthquakes and a complex zone with multiple fault strands and abundant seismicity in general microseismicity is reduced within the patches defined by the large repeating earthquakes while the extent of the large rupture patches on dw correlates with physical features in the bathymetry step overs in the primary fault trace are not observed at patch boundaries suggesting along strike heterogeneity in fault zone properties controls the size and location of the large events

millennial scale dynamics of valley fills over the past 12 000 14c yr in northeastern new mexico usa we studied the alluvial histories of eight small 2 watersheds in the uplands of northeastern new mexico the data come from radiocarbon dated sections exposed in the banks of arroyos and permanent channel streams results show that nine periods of valley aggradation separated by incision episodes occurred over the past 12 000 14 c yr these millennial scale cycles occurred in rough synchrony within different drainages parts of some cycles coincided with well known climatic fluctuations for example aggradation occurred during the younger dryas chronozone 11 000 10 000 14 c yr b p and valley fills persisted in incised states during the early holocene peak in milanko vitch summer insolation incision occurred early in the medieval warm period ca a d 1000 1300 aggradation during the little ice age a d 1300 1880 and incision during the last century changes in precipitation tied to the strength of the north american monsoon system probably drive these cycles of aggradation and incision when the system is strong more frequent summer thunderstorms cause increased flooding in these small watersheds wetter summers over periods of decades to centuries allow forest vegetation to spread which reduces sediment input from hillslopes at the same time floods are eroding valley fills aggradation of valley fills probably occurs when summer thunderstorms are less frequent and large floods are correspondingly rare prolonged summer drought reduces forest cover which increases erosion from slopes and causes sediment to accumulate in valleys in turn the strength of the north american monsoon system is probably controlled by fluctuating sea surface temperatures the geomorphic cycles we describe may reflect a previously unrecognized millennial scale climate oscillation that is important in shaping the landscapes of the southern great plains

late quaternary eolian and alluvial response to paleoclimate canyonlands southeastern utah began at ca 6 ka and ended by ca 3 2 ka followed by a shift to drier modern conditions localized mobilization of dune sand has persisted to the present these interpretations are similar to those of studies at the chaco dune fi eld new mexico and the tusayan dune fi eld arizona and are consistent with paleoclimate interpretations of pollen and packrat middens in the region a period of rapid deposition and infi ltration of eolian dust derived from distant igneous source terranes occurred between ca 12 and 8 ka before ca 17 ka and apparently back to at least 45 ka paleosols contain little or no such infi ltrated dust after ca 8 ka either the supply of dust was reduced or the more arid climate inhibited translocation of dust into the soils

tectonic and climatic controls on holocene channel migration incision and terrace formation by the rio grande in the palomas half graben southern rio grande rift usa in order to evaluate potential effects of tectonics and climate change on the behaviour of the axial rio grande in the rio grande rift a 16 5 km stretch of modern floodplain and holocene terraces were mapped in the tectonically active palomas half graben south central new mexico usa in addition 51 cores and natural exposures were logged and 20 radiocarbon dates were obtained from charcoal bulk organic matter mollusc shells and pedogenic calcite the holocene alluvium comprises four terraces above the modern floodplain each of which formed by a period of river incision followed by stability and renewed floodplain construction to a level below that of the previous terraces estimated times of incision between terraces i and ii ii and iii and iii and iv are after 12 400 8040 to 5310 and 760 to 550 yr bp respectively whereas the incision between terrace iv and the modern floodplain occurred within the last 260 years although there is some evidence for tectonic control on river behaviour in the southern part of the basin terrace formation is interpreted as being related to climate change with periods of incision corresponding to times of increased aridity and low sediment water discharge ratio in the rio grande this process may have resulted from a reduction in intensity and magnitude of summer storms which supply sediment to the axial river coupled with an increase in spring discharge peak caused by snowmelt in upstream mountain catchments

the motion and boundary between the capricorn and australian plates the motions between the somalian antarctic and australian plates the three plates believed to meet at the rodrigues triple junction in the indian ocean are inconsistent with the assumption that all three plates are rigid the discrepancy is best explained if the australian plate contains two component plates thus the traditionally defined indo australian plate consists of three component plates and multiple diffuse plate boundaries the pattern of present deformation indicates that the boundaries between the three component plates are two unconnected zones accommodating divergence and a larger zone which we interpret as three diffuse convergent plate boundaries and a diffuse triple junction

tropical squall lines of the arizona monsoon abstract squall lines possessing nearly all the characteristics of tropical squall lines occasionally develop during the summer monsoon over southern arizona and northwestern mexico initial thunderstorm formation is over the continental divide in the late afternoon and the systems become organized within a few hours satellite imagery cloud to ground lightning strike data and surface observations indicate the squall lines move from east to west or northeast to southwest by discrete propagation faster than the environmental flow at all levels below 20 kpa so that most of the anvil clouds lag behind the synoptic scale circulation is anomalous with a strong ridge located over the western united states and a deep trough located over the eastern united states west to northwest winds are found in the boundary layer over southern arizona and northwest mexico while a deep layer of east winds are observed above as a result most of the environmental wind shear is confined to the lowest 2 5 km above the ground

compound rupture of the great 1998 antarctic plate earthquake the march 25 1998 antarctic plate earthquake ruptured a portion of the antarctic plate more than 200 km west of its boundary with the australian plate the harvard centroid moment tensor solution indicates that the earthquake was primarily a strike slip event but the large non double couple component of the moment tensor suggests considerable complexity in the rupture process we use a finite fault inversion method to determine details of the rupture process from teleseismic body waves recorded by the global seismic network the p waves are poorly fit by one or more subevents having only a strike slip mechanism we find that the presence of a large oblique normal faulting subevent located to the east of the hypocenter is necessary to improve the fit this subevent combines with a larger strike slip subevent to the west to comprise the main moment release in the earthquake and is the cause of the large non double couple component in the long period focal mechanism the earthquake exhibited very high slip and high stress drop compared with most interplate strike slip events and the rupture was largely confined to the upper 15 20 km of the lithosphere both constituent focal mechanisms indicate that this part of the antarctic plate is under nw se oriented tension although the origin of these stresses is unknown

near ridge intraplate earthquakes in the indian ocean abstract moment release computation based on a detailed catalog of near ridge earthquakes between 1912 and 1993 pertaining to the younger lithosphere in the indian ocean suggest that the overall seismicity level in the central indian ocean is not significantly higher as compared to the seismicity pattern near the southwest indian ridge swir the characteristic seismicity pattern along the southeast indian ridge seir and central indian ridge cir in the central indian ocean over a much wider zone represents the plate wide stress distribution though the seismicity near chagos bank appears to be similar to that observed near the atlantis ii fracture zone along the southwest indian ridge swir several lines of evidence such as more frequent occurrence geographically wide spread seismicity consistent faulting pattern and plate motion inversion results suggest that the chagos bank seismicity forms part of the plate wide stress distribution in the central indian ocean the present study also demonstrates two examples of highly energetic sequences of events characterized by thrust faulting while the thrust faulting events near the southern part of the central indian ridge can be inferred due to thermoelastic stresses related to cooling of the lithosphere the events near the indomed fracture zone on the african plate occur due to slow relative motion along the boundary between nubia and somalia which connects to the swir along a diffuse compressional segment the available mechanisms confirm that the near ridge seismicity is characterized by dominantly normal faulting in the indian ocean the moment release computed for different ages suggests that a greater fraction of moment release takes place in the 15 to 35 my old lithosphere

episodic sediment delivery and landscape connectivity in the mancos shale badlands and fremont river system utah usa abstract the fremont river drains about 1000 km 2 of mancos shale badlands which provide a large percentage of the total sediment load of its middle and lower reaches factors controlling sediment movement include weathering that produces thin paralithic soils mass movement events that move the soil onto locations susceptible to fluvial transport intense precipitation events that move the sediment along rills and across local pediments and finally fremont river floods that move the sediment to the main stem colorado river a forty year erosion pin study has shown that down slope creep moves the weathered shale crust an average of 5 9 cm yr weather records and our monitoring show that wet winters add large slab failures and mudflows recent sediment trap studies show that about 95 of sediment movement across pediments is accomplished by high intensity summer convective storms between 1890 and 1910 a series of large autumn floods swept down the fremont river eroding its floodplain and transforming it from a narrow and meandering channel to a broad braided one beginning about 1940 the fremont s channel began to narrow sequential aerial photos and cross sections suggest that floodplain construction since about 1966 has stored about 4000 to 8000 m 3 of sediment per kilometer per year these data suggest that it will take two centuries to restore the floodplain to its pre 1890 condition which is in line with geologic studies elsewhere on the colorado plateau the various landscape elements of slope pediment and floodplain are semi independent actors in sediment delivery each with its own style accelerated mass movement on the slopes has an approximate 20 year recurrence sediment movement from slope across pediments to master stream is episodic and recurs more frequently the slope to pediment portion of the system appears well connected however sediment transport through the floodplain is not well connected in the decadal time scale but increases in the century and millennial time scales and changes over time depending on the cycle of arroyo cutting and filling

late quaternary weathering erosion and deposition in nahal yael israel an impact of climatic change on an arid watershed in their seminal paper in 1979 bull and schick proposed a conceptual model for the geomorphic response to pleistocene to holocene climate change based on the hyperarid nahal yael watershed in the southern negev desert in this model the change from semiarid late pleistocene to hyperarid early holocene climates reduced vegetation cover increased the yield of sediment from slopes and accelerated aggradation of terraces and alluvial fans the model is now over 30 yr old and during this time chronologic paleoenvironmental and hydrogeomorphic research has advanced here we reevaluate the model using data acquired in nahal yael over the 30yr since the original model was proposed recent studies indicate that the late pleistocene climate was hyperarid and a transition from semiarid to hyperarid climates did not occur the revised chronology reveals a major 35 20 ka episode of accelerated late pleistocene sediment production on slopes with lower rates probably already at ca 50ka due to increased frequency of wetting drying cycles caused by frequent extreme storms and floods between 35 and 27 ka without lag time these sediments were transported and aggraded in depositional landscape components fluvial terraces and alluvial fans this intensified sediment production and delivery phase is unrelated to the pleistocene holocene transition the depositional landforms were rapidly incised between 20 and 18 ka since and or soon after this last glacial maximum lgm incision most material leaving the basin originated from sediments stored in depositional landforms and was not produced from bedrock using these new data we propose a revision to the bull and schick model in this hyperarid environment our revision suggests that the model should include the frequent storms and floods responsible for a late pleistocene pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans we also discuss the common use and pitfalls of using the bull and schick conceptual model to explain observations in diverse arid environments usually without sufficient data on basin specific stratigraphic chronologic paleoenvironmental and paleoclimatic information

breaking into the plate a 7 6 mw fracture zone earthquake adjacent to the central indian ridge 1 on 15 july 2003 an extremely large 7 6 mw strike slip earthquake initiated on or near the end of an active transform along the northern central indian ridge the event propagated away from the plate boundary along the typically inactive fracture zone with a sense of slip that opposes the active transform slip direction seismically and hydroacoustically determined aftershock locations delineate a 210 25 km long mainshock rupture the seismic moment and rupture dimensions imply a stress drop of 4 5 6 5 mpa and a mean slip of 3 0 0 5 m the largest aftershock 5 6 nw occurred on the active portion of a neighboring transform at a distance of 160 km where mainshock induced static stress changes are predicted to promote failure near axis fracture zone earthquakes may promote and inhibit ridge parallel diking along different spreading segments perhaps contributing to inter segment variability in the rate and asymmetry of spreading

an enhanced role for the tropical pacific on the humid pleistocene holocene transition in southwestern north america abstract climate effects on landscape evolution during the late pleistocene holocene transition 14 6 8 ka in southwestern north america traditionally are linked to the activity of the north american monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity we performed an integrated analysis of regional alluvial fan lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies our focus was on the potential role the tropical pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region our analysis indicates that the onset of alluvial fan aggradation in most of the region at 13 5 ka could have been a response to semi permanent el nino like conditions in the tropical pacific which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region the north american monsoon was restricted in extent and intensity until 7 ka and probably was not a major factor in alluvial fan aggradation a second stage of alluvial fan aggradation from 11 5 to 9 ka was dominated by hyper concentrated flows and sheet flood sedimentation along with deposition in fluvial settings storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra tropical cyclones with moisture advected directly from the tropical pacific at 8 ka favorable conditions for the occurrence of these storms waned and storm tracks shifted northward analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the tropical pacific can affect storm properties enough to explain the observed geomorphic effects regardless of other factors traditionally considered of large impact like vegetation change our results suggest that the tropical pacific plays a larger role than currently thought in landscape evolution of the region

a diffuse plate boundary model for indian ocean tectonics it is suggested that motion along the virtually aseismic owen fracture zone is negligible so that arabia and india are contained within a single indo arabian plate divided from the australian plate by a diffuse boundary the boundary is a zone of concentrated seismicity and deformation commonly characterized as intraplate the rotation vector of australia relative to indo arabia is consistent with the seismologically observed 2 cm yr of left lateral strike slip along the ninetyeast ridge north south compression in the central indian ocean and the north south extension near chagos

migration of hydrocarbons in the tampen spur area norwegian north sea a reservoir geochemical evaluation abstract this paper describes the application of reservoir geochemical protocols to a study of oil field and regional petroleum compositional variations in the major fields in the tampen spur area the tampen spur high is located in the middle of several oil mature to gas mature source rock kitchens and most of the structures might receive petroleum from several different directions based on the definition of several different petroleum populations within the tampen spur area we define five major petroleum charging systems correlating the petroleum population variations with geological information allows us to derive likely migration fill scenarios for many of the oil fields and requires us to invoke a new upper jurassic carrier system in parts of the area connecting brent group reservoirs across major faults based on this hypothesis two exploration wells were drilled on structures to test this carrier and petroleum was found in one of the structures an understanding of the migration pathways and petroleum drainage efficiency of the basins is of great economic interest because most of the smaller substructures are marginally economic thus it is important to provide an effective risk analysis and ranking of the satellite prospects prior to drilling this study has demonstrated that filling and migration in a geological province are controlled by a series of complex processes and that the filling history of the structures within the tampen spur is more complicated than suggested earlier

geochemical characterization of a tar mat in the oseberg field norwegian sector north sea abstract concentrations of heavy oil usually at or near the oil water contact in reservoirs are called tar mats this phenomenon can arise from reservoir oil alteration processes such as deasphalting biodegradation or maturation effects of tar mats include reduction in water drive efficiency and lower oil quality in the bottom section of a reservoir the study of tar mats can be greatly improved by the systematic application of petroleum geochemical techniques and a new classification scheme is proposed based on these methods application of these general concepts to a tar mat occurrence in the oseberg field showed that although some biodegradation probably occurred in the early stages of field formation there was little evidence to indicate that this process was a significant factor in the formation of the main tar mat deasphalting was considered to be the major if not the only process leading to tar mat formation in this field