fault death a perspective from actively deforming regions abstract patterns of activity on large faults in regions of distributed continental deformation can change quite rapidly in less than 1 ma such changes include faults becoming permanently or temporarily inactive spatial migration of activity between different faults or faults sets within a deforming region and faults apparently going through episodes of intense activity separated by long quiescent periods greater knowledge of these changes combined with our increasingly detailed understanding of the geometry of large faults have implications for a variety of structural and tectonic issues such as 1 the nature of the interaction between neighbouring faults 2 the link between the discontinuous deformation on faults in the seismogenic upper crust and the more distributed deformation in the aseismic lithosphere beneath and 3 the relation between the instantaneous kinematics and the cumulative finite deformation examples from the active extensional province of central greece are used to illustrate the nature and possible significance of changing fault patterns

carbonate platform evolution from a bioconstructed platform margin to a sand shoal system devonian guilin south china the depositional organization and architecture of the middle late devonian yangdi rimmed carbonate platform margin in the guilin area of south china were related to oblique extensional faulting in a strike slip setting the platform margin shows two main stages of construction in the late givetian to frasnian with a bioconstructed margin evolving into a sand shoal system in the late givetian the platform margin was rimmed with microbial buildups composed mainly of cyanobacterial colonies mostly renalcis and epiphyton these grew upwards and produced an aggradational locally slightly retrogradational architecture with steep foreslope clinoforms three depositional sequences s3 s5 are recognized in the upper givetian strata which are dominated by extensive microbialites metre scale depositional cyclicity occurs in most facies associations except in the platform margin buildups and upper foreslope facies in the latest givetian at the top of sequence s5 relative platform uplift subaerial exposure and associated rapid basin subsidence probably a block tilting effect caused large scale platform collapse and slope erosion to give local scalloped embayments along the platform margin and the synchronous demise of microbial buildups subsequently sand shoals and banks composed of ooids and peloids and a little later stromatoporoid buildups on the palaeohighs developed along the platform margin from which abundant loose sediment was transported downslope to form gravity flow deposits another strong tectonic episode caused further platform collapse in the early frasnian at the top of sequence s6 leading to large scale breccia release and the death of the stromatoporoid buildups siliceous facies banded cherts and siliceous shales were then deposited extensively in the basin centre as a result of the influx of hydrothermal fluids the platform margin sand shoal bank system possibly with gullies on the slope persisted into the latest frasnian until the restoration of microbial buildups four sequences s6 s9 characterized by abundant sand shoal deposits on the margin and gravity flow and hemipelagic deposits on the slope are distinguished in the frasnian strata smaller scale depositional cyclicity is evident in all facies associations across the platform slope basin transect the distinctive depositional architecture and evolution of this yangdi platform are interpreted as having been controlled mainly by regional tectonics with contributions from eustasy environmental factors oceanographic setting biotic and sedimentary fabrics

the macolod corridor a rift crossing the philippine island arc abstract the macolod corridor in southwestern luzon is an approximately 40 km wide zone of still active intense quarternary volcanism which perpendicularly crosses the island in a ne sw direction the tectonic setting of the corridor is still somewhat questionable due to several not yet well understood major structural elements affecting southwestern luzon based on the presently available information we believe that the corridor is a pull apart zone formed by a diffuse system of nw se oriented shearing

uplift deformation and fluid involvement within an active normal fault zone in the gulf of corinth greece the active pisia fault zone is exposed close to the crest of a rotated carbonate fault block that forms part of the southern margin to the gulf of corinth half graben in central greece the crest of the fault block lies above sea level whilst in the hanging wall a marine basin exists as a result the diagenetic and structural characteristics of the fault zone record complex fluid involvement both early phreatic carbonate syn kinematic cements characterized by drusy fabrics baroque dolomites and crack seal textures and late vadose carbonate syn kinematic cements characterized by flowstones containing cave collapse debris exist in the uplifted footwall to the fault downward percolation of vadose meteoric waters and the upwelling of pore waters with elevated temperatures produced the diagenetic features observed within the fault zone the co existence of early phreatic and late vadose cements within the fault zone is related to footwall uplift across the water table and subsequent erosional unroofing the present day elevation of phreatic cements to c 650m above current sea level provides a minimum estimate of footwall uplift along the fault the wider implication is that temporal changes in deformation and fluid flow may typify fault block evolution

recognition of contrasting magmatic processes using sb systematics an example from the western central luzon arc the philippines abstract the western central luzon volcanic system is associated with eastward subduction along the manila trench two arcs the bataan ba and the mindoro ma arcs have resulted from this subduction and are separated by a ne sw oriented rift related zone of volcanism called the macolod corridor mc the ba consists of volcanism related to partial melting of the mantle wedge initiated by fluids from the subducted slab the ma volcanics are also related to the subduction process but lile and radiogenic sr isotope data compared with the western part of the ba are significantly higher the mc also has high lile data compared with the western section of the ba but lower radiogenic sr isotope values than the ma trace element modelling has been used to explain sb systematics sr ca vs ba ca observed in geochemical data from volcanoes worldwide these trends and their sense of curvature are most probably the result of variations in plagioclase and clinopyroxene crystal fractionation sb systematics have been examined from four volcanoes mt natib and mt mariveles within the western section of the ba mt maquiling within the mc and the mt san cristobal banahaw complex within the ma the results suggest that crystal fractionation was a dominant process during the evolution of the magma chambers below the various volcanoes in addition initial melt compositions suggest that the source of the ma magmas has been enriched in a crustal component the original magma composition s from the mc volcano mt maquiling appears to have been generated by smaller degrees of partial melting compared with the ba volcanoes studied

multiple large earthquakes in the past 1500 years on a fault in metropolitan manila the philippines the first 14 c based paleoseismic study of an active fault in the philippines shows that a right lateral fault on the northeast edge of metropolitan manila poses a greater seismic hazard than previously thought faulted hillslope colluvium stream channel alluvium and debris flow deposits exposed in trenches across the northern part of the west marikina valley fault record two or three surface faulting events three eroded clay rich soil b horizons suggest thousands of years between surface faulting events whereas 14 c ages on detrital charcoal constrain the entire stratigraphic sequence to the past 1300 1700 years we rely on the 14 c ages to infer faulting recurrence of hundreds rather than thousands of years minimal soil development and modern 14 c ages from colluvium overlying a faulted debris flow deposit in a nearby stream exposure point to a historic age for a probable third or fourth most recent faulting event

composite transform convergent plate boundaries description and discussion abstract the leading edge of the overriding plate at an obliquely convergent boundary is commonly sliced by a system of strike slip faults this fault system is often structurally complex and may show correspondingly uneven strain effects with great vertical and translational shifts of the component blocks of the fault system the stress pattern and strain effects vary along the length of the system and change through time these margins are considered to be composite transform convergent ctc plate boundaries examples are given of structures formed along three ctc boundaries the aleutian ridge the solomon islands and the philippines the dynamism of the fault system along a ctc boundary can enhance vertical tectonism and basin formation this concept provides a framework for the evaluation of petroleum resources related to basin formation and mineral exploration related to igneous activity associated with transtensional processes

the significance of normal faults in the exploration and production of north sea hydrocarbons abstract the mesozoic geological history of the northern north sea was dominated by extensional faulting in addition to forming the most important hydrocarbon traps such faults can be shown to exert a significant degree of control over the distribution of mature source rocks and the deposition and erosion of reservoir rocks as exploration and production have progressed the importance of small normal faults has been increasingly recognized although they do not totally offset reservoir formations such small faults may provide lateral seals thus disrupting hydrocarbon migration routes forming subtle traps and causing technical problems during production by unanticipated compartmentalization of fields consequently as the commercial size of fields is reduced by improving production technology and the north sea matures as a hydrocarbon province a thorough understanding of the origins and characteristics of normal faults as well as a command of the increasingly sophisticated techniques for analysing the behaviour of these faults will be vital to successful exploration programmes

geochemistry of silicic magmas in the macolod corridor sw luzon philippines evidence of distinct mantle derived crustal sources for silicic magmas silicic volcanic deposits 65 wt sio2 which occur as domes lavas and pyroclastic deposits are relatively abundant in the macolod corridor sw luzon philippines at makiling stratovolcano silicic domes occur along the margins of the volcano and are chemically similar to the silicic lavas that comprise part of the volcano pyroclastic flows are associated with the laguna de bay caldera and these are chemically distinct from the domes and lavas at makiling stratovolcano as a whole samples from the laguna de bay caldera contain lower concentrations of mgo and higher concentrations of fe2o3 t than the samples from domes and lavas the laguna de bay samples are more enriched in incompatible trace elements the silicic rocks from the domes makiling volcano and laguna de bay caldera all contain high alkalis and high k2o na2o ratios melting experiments of primitive basalts and andesites demonstrate that it is difficult to produce high k2o na2o silicic magmas by fractional crystallization or partial melting of a low k2o na2o source however recent melting experiments sisson et al contrib mineral petrol 148 635 661 2005 demonstrate that extreme fractional crystallization or partial melting of k rich basalts can produce these silicic magmas our model for the generation of the silicic magmas in the macolod corridor requires partial melting of mantle derived evolved moderate to k rich crystallized calc alkaline magmas that ponded and crystallized in the mid crust major and trace element variations along with oxygen isotopes and ages of the deposits are consistent with this model

estimating sio2 content of lava deposits in the humid tropics using remotely sensed imagery abstract remote sensing methods used to determine the rheology and sio 2 composition of lava flows on mars were utilized to estimate the composition of lava deposits in the philippines test cases were conducted on two lava domes and two lava flow deposits to determine whether remote sensing methods can be applied as a rapid and economical means to assess hazards associated with volcanoes in the humid tropics our study shows that dimensional parameters derived from digital elevation models dems generated from airborne sensors are effective in determining the sio 2 content of lava deposits the sio 2 values computed from the rheological properties of lava are found to be comparable to geochemically analyzed field samples these results suggest that remote sensing methods to estimate the composition of lava deposits is viable and can serve as a potentially useful tool for rapid and economic hazards assessment of volcanoes in tropical regions with the growing number of high resolution satellite sensors that routinely image the earth s surface such a technique can be widely utilized

footwall uplift associated with late jurassic normal faulting in the northern north sea tilted fault blocks in the north sea show varying degrees of erosion such footwall erosion is predicted by the domino model of extension in which rigid fault blocks rotate as extension proceeds profiles of three north sea fields snorre brent and heather are analysed to estimate the amount of uplift that accompanied late jurassic normal faulting and these estimates are compared with the footwall uplift predicted by the domino model agreement between observed and predicted uplift is excellent varying from 1 km at snorre to c 100 m at heather for a given amount of extension large fault blocks tend to be bounded by large faults and to show a greater degree of erosion the brent province of the northern north sea is an area of prolific hydrocarbon discoveries almost all of the major oil fields occur in tilted fault blocks which were formed by late jurassic extension and subsequently buried by cretaceous tertiary thermal subsidence badley et al 1988 marsden et al 1990 the crests of the tilted blocks are typically eroded or degraded with varying amounts of middle lower jurassic and upper triassic section removed this footwall erosion is commonly ascribed in the literature to pre rifting doming ziegler 1982 heterogeneous stretching coward 1986 local transpression beach 1985 or even regional compression frost 1987 the purpose of this paper is to show that the uplift and degradation is a direct consequence of the late jurassic normal faulting and fault block rotation the degree of uplift is related in a simple way to the size across strike of

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

plate convergence measured by gps across the sundaland philippine sea plate deformed boundary the philippines and eastern indonesia summary the western boundary of the philippine sea ph plate in the philippines and eastern indonesia corresponds to a wide deformation zone that includes the stretched continental margin of sundaland the philippine mobile belt pmb extending from luzon to the molucca sea and a mosaic of continental blocks around the ph australia sunda triple junction the gps geodyssea data are used to decipher the present kinematics of this complex area in the philippines the overall scheme is quite simple two opposing rotations on either side of the left lateral philippine fault clockwise to the southwest and counterclockwise to the northeast transfer 55 per cent of the ph sundaland convergence from the manila trench to the northwest to the philippine trench to the southeast further south 80 per cent of the ph sunda convergence is absorbed in the double subduction system of the molucca sea and less than 20 per cent along both continental margins of northern borneo finally within the triple junction area between the sundaland ph and australia plates from sulawesi to irian jaya preferential subduction of the celebes sea induces clockwise rotation of the sulu block which is escaping toward the diminishing celebes sea oceanic space from the eastwardadvancing ph plate to the south we identify an undeformed banda block that rotates counterclockwise with respect to australia and clockwise with respect to sundaland the kinematics of this block can be defined and enable us to compute the rates of southward subduction of the banda block within the flores trench and of eastward convergence of the makassar straits with the banda block the analysis made in this paper confirms that this deformation is compatible with the eastward motion of sundaland with respect to eurasia determined by the geodyssea programme but is not compatible with the assumption that sundaland belongs to eurasia as was often assumed prior to this study

a three dimensional 3d structural model for an oil producing basin of the brazilian equatorial margin abstract hydrocarbon discoveries in regions such as the brazilian equatorial margin equatorial africa and french guiana have recently confirmed their importance as new exploration frontiers the mundau sub basin located on the brazilian equatorial margin is an oil and gas producing region with four producing fields in shallow water xareu atum espada e curima in order to understand the structural and seismic stratigraphic frameworks of an oil producing area of the brazilian equatorial margin this work addresses the 3d geometry and spatial distribution of main faults in the curima and espada fields the occurrence of hydrocarbons in the mundau sub basin is compared with fields in other parts of the brazilian equatorial basin and in equatorial africa data from 12 wells and a 3d post stack time migrated multichannel seismic volume are used to define nine 9 main seismic stratigraphic units the syn rift mundau formation units 1 2 3 and 4 the transitional paracuru formation unit 5 and the drift ubarana uruburetama and itapage members units 6 and 7 tibau and guamare formations units 8 and 9 the study area is dominated by nw se planar normal faults basinward dipping that formed multiple half grabens and tilted blocks with small anticlines and synclines genetically related to a transtensional system three types of plays are recognised in the mundau sub basins structural combined structural stratigraphic and stratigraphic turbiditic in the eastern part of the region where the basement is shallow no oil was found conversely oil was discovered in an anticlinal trap formed on a hanging wall block analogous to fields in cote d ivoire ghana transform margin this work shows that combined traps on footwall blocks are successful plays near the shelf break of the mundau sub basin in similarity with the espoir and baobab fields in ivory coast basin furthermore turbiditic reservoirs in drift units are analogous to the stabroek block in guyana and prospects in the gulf of guinea the structural and petroleum play analyses in this work are therefore crucial to understand the multiple geological processes leading to the trapping of hydrocarbons in the larger equatorial atlantic ocean

drainage systems and the development of normal faults an example from pleasant valley nevada abstract drainage systems in regions of active extension contain information about fault zone structure and development and particularly about the lateral growth of individual fault segments that is difficult to obtain by more conventional means such as the dating of sedimentary material we investigate drainage fault interaction in pleasant valley nevada where a large normal faulting earthquake occurred in 1915 we find geomorphological evidence for a propagation increase in length of several km at both ends of one of the 1915 fault segments at one end we estimate the propagation rate to be about 50 m per earthquake this estimate is uncertain but is within the range predicted by theoretical models of fault growth drainage along the axes of half graben in this region may be influenced by the spacing of tilted fault blocks the highest valleys are also the narrowest whether the axial streams succeed in reaching lower neighbouring sinks depends on whether sedimentation rates are able to keep basins filled to the levels of the basement highs that separate half graben of opposite polarity a knowledge of modern drainage fault interactions may help predict drainage systems and the position of potential syn rift reservoir sediments in older extensional terrains where fault configurations but not facies distributions are known from seismic reflection surveys

migration of activity within normal fault systems examples from the quaternary of mainland greece abstract we examine five areas of mainland greece where active extension occurs on sub parallel systems of normal faults and where geomorphological and stratigraphic evidence indicates that the faulting has migrated basinwards into the original hanging walls in several cases within the late quaternary by comparing fault slip rates estimated from geomorphological data with current extension rates known from geodetic measurements it appears that the newest faults can account for effectively all the present day motions fault migration of this sort is easy to recognize in young systems close to sea level because vertical movements of footwalls and hanging walls are obvious and reveal which faults are currently most active but is less easy to confirm away from reliable reference levels or in older terrains with poorer time resolution it is probably more common than is appreciated and has a profound effect on syn rift sedimentation and erosion patterns fault migration is probably an inevitable consequence of the interplay between stresses generated by the fault related topography and the ultimate strength of major faults it is likely to be further encouraged in places where lower crustal flow or rotations about a vertical axis are important however it is not clear why migration should preferentially occur into the hanging walls as observed in central greece

palaeoecology of syn rift topography a late jurassic footwall island on the josephine ridge central graben north sea abstract understanding rift topography is essential for determining source areas sediment pathways and the type of sediment delivered to a rift basin factors essential for interpreting petroleum systems in ancient rifts here we investigate upper jurassic sediments from the josephine ridge region of the central graben north sea by integrating geophysical palynological petrophysical and sedimentological datasets to analyse the palaeoenvironments of the jade and judy horsts the tops of which are not preserved interpretation of geophysical and petrophysical data together with core descriptions shows study wells to step progressively away from the josephine ridge into adjacent grabens one hundred and five palynological samples from six wells range from the oxfordian to the lower tithonian spanning the syn rift period of the central graben samples from the adjacent grabens and the jade horst are rich in dinoflagellate cysts and possess

a synthesis and review of historical eruptions at taal volcano southern luzon philippines abstract the philippines is an area of persistent volcanism being located in one of the most tectonically active regions in the world taal volcano in southern luzon is the second most frequently erupting volcano of the 24 active volcanoes in the philippines a comprehensive and critical review of published and unpublished references describing the 33 known historical eruptions of taal may provide answers to knowledge gaps on past eruptive behavior processes and products that could be utilized for hazard and risk assessment of future eruptions data on the prehistoric eruptions and evolution of taal caldera and subsequent deposits are limited only four caldera forming events were identified based on four mapped ignimbrite deposits from oldest to youngest these are the silicic alitagtag ali and caloocan cal pumice flow deposits the dacitic sambong ignimbrite sam and the basaltic andesitic taal scoria flow renamed scoria pyroclastic flow sfl except for sfl with 14c dating yielding 5380 70 to 6830 80 ky there are no age constraints or estimates of extent for the three older deposits a comprehensive review of the historical eruptions of taal volcano is the central element of this paper and includes all eruptions from ad1572 the first known historic event to ad1977 eruption styles and the interplay between processes and products for each eruption are reinterpreted based on the narrative descriptions from all available accounts a change of classification of eruption styles and eruptive products is undertaken for some events at least nine reported eruptions were deemed uncertain including the ad1605 ad1611 event more likely seismic swarms the ad1634 ad1635 and ad1645 may simply be solfataric or hydrothermal activity events and the ad1790 ad1825 ad1842 ad1873 and ad1903 events that were listed in recent published and unpublished documents but do not provide any details to describe and confirm the eruptions except for listing a default vei of 2 pyroclastic density currents brought devastating impacts to the communities around taal during the ad1749 ad1754 ad1911 and ad1965 eruptions and remain the biggest threat in the case of renewed volcanic activity significant implications for aviation are implied by the narrative of tephra fall dispersal towards manila the central gateway of international aviation operation in the philippines during the ad1754 eruptions the dispersal of tephra in the event of an explosive eruption at taal towards metro manila would have catastrophic effects to transport utilities and business activity potentially generating enormous economic losses hazards from earthquake events associated with future volcanic activity may also have localized impacts occurrences of liquefaction phenomena as a consequence of severe ground shaking are interpreted during the ad1749 ad1754 and ad1911 eruptions more work needs to be done to develop a comprehensive understanding of the hazards and risks associated with an explosive eruption at taal volcano especially related to the older quaternary caldera forming eruptions that produced large volume pyroclastic deposits that are extensively distributed and exposed we acknowledge that there may be additional prehistoric eruptions where the eruptive products have not been preserved recognized or reported events that cannot be verified or do not have sufficient details to confirm the eruption have been downgraded to uncertain eruptions that are confirmed with identified dispersal and emplacement of tephra fall and other eruptive deposits as interpreted from narrated records could provide crucial information that may be utilized in hazard assessment

numerical modelling of the response to variable stretching rate of a domino fault block system abstract using a numerical model of domino fault block development the effects of a variable stretching rate during rifting were investigated it is shown that a varable stretching rate can have an important influence on synrift sequence geometries this can lead to complex synrift sequences composed of wedge shaped units formed during periods of high stretching rate and more symmetrical units formed during periods of relative tectonic quiescence at the fault block crest increases in stretching rate can produce offlapping synrift sequences whereas decreases in stretching rate allow onlap onto the fault block crest if the rifting period contains multiple pulses of stretching complex intra synrift unconformities can be formed the initial bathymetry is also shown to have an important influence on the modelled synrift stratigraphy the modelled stratigraphies are similar to those thought to result from variations in absolute sea level pseudo facies markers based on the combined initial depth and slope of deposition of the modelled sediments are shown to be a useful tool and graphically illustrate the response of the system to pulses of tectonic activity

significance of slab derived partial melts and aqueous fluids for the genesis of tholeiitic and calc alkaline island arc basalts evidence from mt arayat philippines abstract mt arayat an inactive stratovolcano is located behind the volcanic front of the bataan segment of the taiwan luzon arc in the central valley of luzon the philippines volcanism in this arc segment results from subduction of south china sea basin crust along the manila trench eruption products of mt arayat comprise pyroclastics and porphyritic basalt and andesite lavas of high k calc alkaline affinity compared to typical oceanic island arc volcanics the basalts display slightly elevated abundances of ta and nb relative to zr and hf and of ta nb zr hf and light rare earth elements lree s relative to y and heavy rare earths hree s their ree distribution suggests mixing between high hree low lree hree and low hree high lree hree components the latter suggesting the presence of residual garnet in the source region of mt arayat basalts enrichment of lree s relative to hree s is typical of calc alkaline and high k calc alkaline island arc basalts but is usually missing or small in island arc tholeiites lree enrichment is often attributed to a metasomatizing aqueous fluid generated during dehydration of the subducted slab for this model to be applicable extremely low abundances in the fluid of agents like f oh and or co2 3 which form hard complexes with the ree s are a prerequisite due to the temperature dependence of the eu 3 eu 2 redox potential divalent eu predominates in aqueous fluids of low complex forming capacity in the subduction zone environment whereas the other ree s with the possible exception of yb are still trivalent a lree enriched aqueous fluid generated by slab dehydration should therfore display a positive eu anomaly however lree enriched island arc basalts generally do not show positive eu anomalies implying that this lree enrichment is unlikely to result from ree transfer by an aqueous fluid instead a model is favoured which assumes contributions from both slab derived aqueous fluids and partial melts in the source region for island arc tholeiites the mantle wedge is only affected by a lile enriched aqueous fluid whereas the source region for calc alkaline and high k calc alkaline basalts is additionally metasomatized by small volumes of melt generated by small degrees of partial melting of the subducted slab with garnet as a residual phase these lree enriched partial melts may react with the depleted peridotite of the mantle wedge to produce domains of refertilized garnet peridotite subsequent partial melting produces calc alkaline melts whose trace element compositions display signatures of both depleted peridotite and more fertile garnet peridotite

tsr versus non tsr processes and their impact on gas geochemistry and carbon stable isotopes in carboniferous permian and lower triassic marine carbonate gas reservoirs in the eastern sichuan basin china abstract the palaeozoic and lowermost mesozoic marine carbonate reservoirs of the sichuan basin in china contain variably sour and very dry gas the source of the gas in the carboniferous permian and lower triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking and tsr related processes for these gases sixty three gas samples were collected and analysed for their composition and carbon stable isotope values the gases are all typically very dry alkane gases being 97 5 methane with low 2 s and co 2 carboniferous gas is negligibly sour while the lower triassic gas tends to be most sour the elevated h 2 s up to 62 is due to thermochemical sulphate reduction with the most sour triassic and permian reservoirs being deeper than 4800 m the non tsr affected carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen derived oil and primary gas and is highly mature carboniferous and non sour triassic and permian gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane a reversal of typical low to moderate maturity patterns the gas in the non sour triassic and permian reservoirs has the same geochemical and isotopic characteristics and therefore the same source as the carboniferous gas tsr in the deepest triassic reservoirs altered the gas composition reaching 100 dryness in the deepest most sour reservoirs showing that ethane and propane react faster than methane during tsr ethane evolves to heavier carbon isotope values than methane during tsr leading to removal of the reversed alkane gas isotope trend found in the carboniferous and non sour triassic and permian reservoirs however methane was directly involved in tsr as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded co 2 increased in concentration as gas souring proceeded but typical co 2 carbon isotope ratios in sour gases remained about 4 v pdb showing that it was not solely derived from the oxidation of alkanes instead co 2 may partly result from reaction of sour gas with carbonate reservoir minerals such as fe rich dolomite or calcite resulting in pyrite growth as well as co 2 generation

the flexural isostatic response of the lithosphere to extensional tectonics abstract the tectonic loading resulting from the extension of the lithosphere is examined loads resulting from crustal thinning thermal perturbations and basin infill are recognised and the resulting flexural isostatic deflection of the lithosphere is modelled in response to each of them in the presence of a listric fault during rifting regional uplift or rebound occurs as a result of both crustal thinning due to normal faulting simple shear and the elevation of the geothermal gradient while subsidence is generated by the downward acting loads created by both basin infill and thinning of the lower crust by stretching pure shear following rifting further flexural subsidence occurs in response to the cooling of the temperature field and associated sedimentary infill one of the major tectonic effects resulting dominantly from crustal thinning by normal faulting is the generation of footwall uplift the magnitude of this uplift along with basin depth and underlying crustal structure are strongly determined by the flexural rigidity of the lithosphere the amount of extension and the dip of the major basement fault along which extension is taking place depending upon the values of these parameters model calculations predict between a few tens of metres and 1 5 km of footwall uplift these values correlate closely with the magnitude of footwall uplift estimated from examples within the geological record the erosion of this rift induced topography is also proposed as a possible driving mechanism for the generation of major basin unconformities

syn rift evolution and resulting play models in the snorre h area northern north sea abstract an upper jurassic wedge shaped syn rift succession comprising the heather and draupne formations is present in the hangingwall trough of the snorre fault block the succession is bounded to the west by the statfjord east fault whereas it onlaps the snorre fault block to the east it consists of a two fold coarsening upward sequence from shale to sandstone of shallow marine shoreline origin active fault block rotation and subsidence in the snorre h area commenced in the mid bathonian and lasted through the ryazanian the heather formation was deposited during the early rift stage mid bathonian early oxfordian 3 cumulative tilt whereas the draupne formation late oxfordian ryazanian 9 cumulative tilt accumulated during the main and late rift stages the lower part of the heather formation was likely deposited across a submerged tilted fault block terrain with a predominant extra basinal sediment supply deposition of the upper heather formation however was governed by gradually emerging footwall islands albeit yet without significant local erosion as a result of increased fault block rotation during deposition of the draupne formation shale sequences i ii late early oxfordian early mid volgian footwall islands became firmly established providing a predominant local sediment source to the snorre h sub basin clay and silt were supplied from erosion of the heather formation on the snorre h hangingwall with subordinate input of sand from the statfjord east footwall subsequent deposition of the draupne formation sequences iii v late mid volgian ryazanian was governed by significant relief on the footwall islands causing deep erosion into the brent group on the snorre h hangingwall dip slope and leading to progradation of the upper draupne sandstone shoreline complex across the snorre h area deposition of the draupne formation and temporal shoreline position were likely partly controlled by northwards fault tip propagation of the statfjord east fault various syn rift play models and depositional reservoir facies are present within the snorre h hangingwall basin they include dip slope shallow marine shoreline sands basin floor gravity transported sands and likely footwall talus sands enveloped in organic rich shales of the draupne formation the distribution of reservoir facies is intimately linked to exposure and erosion of the middle jurassic brent group below the syn rift unconformity

recent enrichment in the source region of arc magmas from luzon island philippines sr and nd isotopic evidence luzon the philippines is wedged between east and west directed subducting slabs westward subduction has resulted in pre miocene igneous rocks from northern luzon characterized by 87 sr 86 sr and 143 nd 144 nd ratios typical of intra oceanic arcs where sediment involvement is minor in addition pliocene to holocene volcanic eruptions of sr and nd isotopic signatures similar to those of the pre miocene rocks have developed along the bicol arc in southern luzon these rocks are also associated with the westward subduction in contrast pliocene to holocene volcanoes have been generated by eastward subduction along the western coast of central luzon the bataan arc in the north and the mindoro arc in the south and are significantly higher in radiogenic sr and lower in radiogenic nd than the rocks discussed above the pre miocene and bicol arc isotopic characteristics appear to reflect the mixing of small amounts of products from the philippine sea crust with the mantle wedge magmas erupted along the bataan and mindoro arcs may reflect that larger amounts of sediment are available for subduction in the south china sea rocks containing the highest radiogenic sr and lowest radiogenic nd values so far noted in philippine igneous rocks occur in the mindoro arc and may have been contaminated by slivers of the continental mindoro north palawan terrane which apparently had been carried down to mantle depths after the collision of this terrane and the arc in the miocene these interpretations imply that nd and by inference other rare earth elements can migrate from the subducted slab to the mantle wedge thus variable nd isotopic ratios in island arc magmas e g sunda and lesser antilles arcs may not be a primary feature of the mantle wedge but a consequence of variations in the release of nd from the subducted slab

implications of fault scarp degradation for brent group prospectivity ninian field northern north sea although the majority of the oil reserves contained in the ninian field are being produced from tilted middle jurassic sandstones of the brent group significant volumes of oil have also been discovered within a poorly documented subtle structural trap that lies in the immediate footwall to the field s controlling eastern boundary fault integration of well data and the interpretation of seismic surveys allow the definition of an area characterized by major footwall degradation the data demonstrate that the 1 25 mi 2 km wide area affected lies to the east of the zone of brent group truncation along the whole 15 mi 25 km length of the field a combination of sedimentary and stratigraphic information shows that the degradation and products obtain a thickness that locally exceeds 450 ft 126 m the degradation process appears to have been triggered by accelerated extension during the callovian early oxfordian deposition of the heather formation the development of pronounced fault terraces due to footwall collapse copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received february 26 1996 revised manuscript received august 12 1996 final acceptance december 24 1996 2department of geology and geophysics the university of edinburgh kings buildings west mains road edinburgh eh9 3jw scotland e mail jru glg ed ac uk 3chevron uk ltd ninian house crawpeel road aberdeen ab12 3lg scotland 4department of earth sciences university of manchester oxford road manchester m13 9pl england we would like to thank the ninian partners chevron uk ltd murphy petroleum ltd oryx uk energy company sands petroleum formerly neste north sea ltd sun oil britain ltd and ranger oil for permission to publish this article however the views expressed here are ours and are not necessarily those of chevron or the ninian partnership help was received from rick davis robert heming bryan man bill higgs andrew harding and john connor of chevron gerry white is thanked for drafting the diagrams interpretations carried out using geoquest ies and iesx software mounted on edinburgh s seismic workstation facility were supported by the petroleum science and technology institute psti norsk hydro shell expro and esso exploration and production uk ltd the study forms part of a wider research program studying the occurrence and effects of fault scarp degradation and subtle stratigraphic trap formation in the brent province currently being supported at the university of edinburgh by a natural environmental research council nerc realising our potential award ropa grant gr3 r9521 finally we would like to acknowledge aapg reviewers alan roberts and j brint for helpful and constructive comments

miocene to recent calc alkalic volcanism in eastern taiwan k ar ages and petrography abstract k ar radiometric datings are presented for volcanic rocks from the coastal range of taiwan and from lanhsu and lutao islands the samples involved are basalts andesites and dacites which show the main petrological and geochemical characteristics of island arc magmas the k ar data show that volcanic activity occurred from early miocene to early pliocene times in the coastal range and in lanhsu island and during pliocene times ca 1 3 4 3 ma in lutao island the geological significance of early and middle miocene ages is discussed with respect to hydrothermal metasomatic alterations which have affected most of the samples a significant increase in incompatible elements e g k and sr is shown to have occurred during pliocene to recent times and is exemplified by the compositions of the lanhsu and lutao volcanic rocks the origin of these chemical variations is related to the magmatic effects of crustal thickening linked to the transition from subduction to collision regimes

growth and linkage of a segmented normal fault zone the late jurassic murchison statfjord north fault northern north sea abstract the structure of the 25 km long northeastern portion of the murchison statfjord north fault zone and adjacent syn rift stratigraphy are integrated to reconstruct the temporal and spatial evolution during c 30 5 myr of late jurassic north sea rifting based on a structural analysis d d data alone approximately 14 precursor fault strands are identified incorporation of stratigraphic data shows that only six of these strands were important in controlling stratal architecture and distribution three main stages in the evolution of the fault zone are recorded in the syn rift stratigraphy and are biostratigraphically constrained these are 1 following initiation of rifting six isolated fault strands developed each 9 km long fault segments separated by a 2 km wide relay ramp that controlled the stratigraphy for at least the following c 10 5 myr and 3 the two fault segments hard linked forming a single continuous fault trace during the final c 7 myr of rifting the results of this study reveal the necessity to adopt an integrated structural and stratigraphic approach when reconstructing the evolution of normal fault zones the results may also help to further constrain models of fault evolution

impacts of small scale faults on continental to coastal plain deposition evidence from the realgrunnen subgroup in the goliat field southwest barents sea norway abstract in this study we synthesise sedimentological fault and amplitude versus angle ava analysis and propose that the fruholmen and tubaen formations realgrunnen subgroup are syn kinematic deposits that record a previously undocumented early phase of mesozoic rifting on the troms finnmark fault complex and within the hammerfest basin the realgrunnen subgroup hosts one of two triassic reservoirs currently being produced in the goliat field here the subgroup sits unconformably on top of the storfjorden subgroup carnian snadd formation away from the goliat field which is characterised by a periclinal anticline the realgrunnen subgroup also comprises the lower middle jurassic nordmela and sto formations sedimentological analysis of six exploration wells reveals that the fruholmen formation was deposited in a prodelta to delta plain environment where tide influenced and fluvial dominated distributary channels are represented by clay siltstones and very fine grained sandstones the overlying tubaen formation is characterised by medium to very coarse grained deposits locally conglomeratic and represents a widespread braid plain with localised alluvial fans displacement profiles of faults and along fault thickness variations demonstrate that an immature fault system was active during deposition of the realgrunnen subgroup a series of unconnected fault segments hosted isolated sub basins and erosional catchment areas in their hanging and footwalls respectively an ava attribute map generated from a 10 ms interval of the uppermost part of the subgroup reveals gross sand prone depositional bodies i e individual and amalgamated channels some of which show meandering geometries ox bow lakes and alluvial fans sand bodies frequently show elongate geometries parallel to faults indicative of syn depositional fault related subsidence driving mechanisms responsible for the norian to rhaetian event may relate to contemporaneous rejuvenation of the fennoscandian hinterland development of the novaya zemlya fold and thrust belt and or the early cimmerian tectonic phase in northern europe

the influence of fault array evolution on synrift sedimentation patterns controls on deposition in the strathspey brent statfjord half graben northern north sea the dispersal and deposition of sediments in a rift basin are controlled by the sediment supply and the generation of accommodation space hence for the facies mosaic and depositional architecture of synrift sediments to be understood both these variables must be constrained subsurface data sets comprising three dimensional 3 d seismic and well data provide the opportunity to quantify the rates of sediment supply and accommodation generation for the duration of the extensional event and over an area of regional extent in this article we address the controls on synrift sedimentation through detailed analysis of a high resolution subsurface data set from the late jurassic northern north sea rift basin the sedimentation history in the study basin comprises four discrete stages intimately linked to the growth of the normal fault population the earliest stage of rifting is characterized by a distributed fault population comprising a large number of faults with low slip rates sediment supply outpaced tectonic subsidence at this time and rising eustatic sea level was the primary control on sedimentation as rifting progressed strain was localized onto a smaller number of active structures with higher displacement rates the basin developed a grabenlike geometry and the lateral propagation and linkage of fault strands controlled accommodation generation although the basin was flooded the rate of sediment supply remained high and largely kept pace with the rate of tectonic subsidence during the final two stages the fully linked half graben bounding fault was the only active structure in the basin the rate of sediment supply at this time was greatly exceeded by the rate of tectonic subsidence and the basin became underfilled significantly the final stage of the sedimentation history is characterized by large scale fault interactions that changed the fault controlled basin floor topography hence modified sediment dispersal and deposition we conclude that sediment dynamics and facies distribution in a rift can only be understood in the context of the coevally active fault population as the faults active at the close of a rift event are very different in location and character to those active during the initiation of rifting this further emphasizes the need to integrate structural sedimentary and stratigraphic studies in rift basins

faulting and graben formation in western and central europe rifts and rift related basins play a pre eminent role among the sedimentary basins of western and central europe through time grabens developed in a number of different megatectonic settings whereby the principal mechanisms governing their subsidence was crustal stretching and subcrustal erosion the level of volcanic activity associated with rifting is highly variable and can change significantly during the development history of a rift some rifts are totally non volcanic the development of rift domes is generally associated with volcanic activity uplifting of a rift dome can induce a reversal in the subsidence pattern of a rift intracontinental rifts with their thinned crust are prone to inversion when the respective craton is subjected to tangential stresses in the process of inversion the crust of rifts is mechanically thickened again

heterogeneous stretching simple shear and basin development abstract the models of basin development which involve either homogeneous stretching of the whole lithosphere or displacement on a lithospheric scale shear zone are but two end members of a range of possible extensional models the homogeneous extension model thins the lower lithosphere beneath the thinned upper crust and superimposes a thermal subsidence basin on the earlier fault bounded basin the shear zone model offsets the zone of lower lithospheric stretching and thermal subsidence it is more likely that the zones of upper and lower lithospheric stretching will be heterogeneous and patchy but will often overlap in plan view this will produce localised uplift and subsequent thermal subsidence within the faulted basin and may explain many of the anomalies between the various stretching estimates made using different structural stratigraphic and geophysical techniques the model which combines heterogeneous lithospheric stretching and associated simple shear may explain 1 variations in dip of the major detachment zones in the large basins 2 variations in types of strain on or beneath the detachment zones 3 regional uplift of part of a basin to erode the earlier fault blocks and 4 the development of volcanism in basins with only low values of upper crustal extension the zone of stretched lower crust and lithospheric mantle may lie beneath the centre of the zone of upper crustal stretching or to one side it may be linked to the upper crustal zone by faults which dip consistently in one direction or by extensional systems which change their dominant dip direction with depth pulling out the mid crust as one or more wedges possible examples are discussed from the basin and range province and northwest european continental shelf

internal structure and deformation of an accommodation zone in the northern part of the suez rift abstract detailed structural study of the eastern onshore part of the gharandal accommodation zone that separates the northern sw dipping and central ne dipping half grabens of the suez rift helped decipher the internal structure and deformation of accommodation zones of continental rifts this 60 km wide zone is affected by pure normal faulting the ne dipping faults of the northern half graben extend southward into the accommodation zone where they interfinger with sw dipping faults extending from the central half graben these two sets of rift parallel faults form several horsts and grabens in the accommodation zone areas dipping parallel to the northern or southern half grabens form several intermixing dip domains in the accommodation zone smaller scale accommodation of dip between these dip domains proceeds by the development of rift parallel folds twist zones in contrast to the southern accommodation zone of the suez rift the internal structure of the gharandal accommodation zone is believed to be representative of accommodation zones in regions a unaffected by prerift structures lying at high angles to the rift and b experiencing relatively small extension accommodation zones in areas having pre rift structures lying at high angle to the rift have relatively narrow width and are characterized by transverse strike slip faults strike slip movement on these faults is related to the torsional strain resulting from the opposite tilt directions and transport of fault blocks of adjacent half grabens

seismic imaging of late palaeozoic early mesozoic extensional and contractional structures in the bowen and surat basins eastern australia abstract the bowen and surat basins in eastern australia form part of a sedimentary basin system that was initiated in the latest carboniferous or earliest permian the basin system developed in a back arc plate tectonic setting and has had a complex evolutionary history with early phases of normal and oblique extension being followed by later phases of contraction and transpression the geometries of the phases are best observed in selected industry seismic data and in long regional bmr deep seismic reflection profiles recorded across the basins in 1984 and 1989 the denison trough along the western margin of the bowen basin was initiated as a series of extensional half grabens bounded by north to north northwest striking faults the polarity of the half grabens are relatively consistent along strike with the bounding faults being mainly on the western side of the haif grabens farther east the taroom trough is the main depositional centre in and around seismic line bmr 84 14 the taroom trough also displays a half graben geometry but only a very limited amount of horizontal extension 3 3 km maximum can be accommodated on the steep bounding fault at its eastern margin this trough developed by oblique extension with the bounding fault having a significant strike slip component reactivation of the bounding faults in the denison trough in the late permian to middle triassic led to tectonic inversion and the development of contractional structures at the same time to the east of the denison trough in the central bowen basin major shortening was accommodated by thin skinned thrusting on a series of listric thrust faults that dip to the east these thrusts root in a major east dipping detachment that appears to flatten in the middle crust in and around seismic line bmr 84 14 the zone of deformation is extremely localised confined to near the basin bounding faults and the structures are interpreted to result from transpression resulting from reactivation of the strike slip faults thus seismic data from the bowen and surat basins provide excellent images of early normal and oblique extensional structures and also of the later contractional structures in the denison trough and central bowen basin and the transpressional structures associated with the major bounding faults in the east

the structural evolution of the northern viking graben and its bearing upon extensional modes of basin formation recent advances in the understanding of rift basin formation coupled with the increasing public availability of seismic and well data across the northern viking graben 60 61 n have enabled a detailed analysis of its development two rifting episodes can be recognized late permian early triassic and bathonian ryazanian major regional unconformities thought to be primarily tectonic rather than eustatic in origin separate and subdivide the rifting episodes the earlier episode involved extension about a n s axis ensuing triassic mid jurassic thermal subsidence was accommodated on steep faults during the later episode a new ne sw fault trend was superimposed on pre existing patterns major block rotation marking active rifting ceased at the end of the ryazanian during the second post rift episode there was a progressive migration of active faulting towards the basin margins and as a result a widening with time of the area undergoing subsidence asymmetric subsidence of the central part of the basin was hinged at the western margin of the horda platform and accommodated to the nw on major faults within the tampen spur factors for the second rifting episode were calculated both by relating subsidence to extension and by measuring observed extension values calculated by both methods increase consistently towards the basin axis for both rifting and thermal subsidence phases but are greater for the latter phase subsidence patterns are similar for both rifting and thermal subsidence episodes so that there is vertical stacking of relatively thick sequences in the axis of the northern viking graben these factors preclude the application of models involving uniform and non uniform stretching and also preclude oblique extension depth dependent stretching is preferred

characteristics and consequences of flow in the lower crust in some places there is strong evidence that the lower continental crust has flowed so as to smooth out variations in crustal thickness caused by differential crustal extension or shortening in order to better understand the processes involved we investigate the behavior of a fluid layer over a fluid half space to see how such a system responds to the deformation of its upper and lower boundaries this simple system can be used to study both the decay of crustal thickness contrasts and the behavior of a thin lithospheric sheet the changing response of the system to variations in density and viscosity contrasts and to different boundary conditions imposed on the fluid interface can easily be studied analytically the most important results are that variations in crustal thickness on a wavelength of a few times the thickness of the flowing channel will decay quickest and that large lateral variations in crustal thickness cause the fluid to develop a steep front which may cause a topographic step above it at the earth s surface deformation within the channel will be principally by simple shear the clear association of lower crustal flow with regions of thickened crust and magmatic activity suggests that both can reduce the viscosity of the lower crust to levels at which flow can occur the smoothing of crustal thickness contrasts leads to differential vertical motions and is thus a method by which substantial tilting can occur without faulting this differential uplift may be responsible for rotating and exhuming some of the detachment faults in metamorphic core complexes in the basin and range province of the western united states it is also a method of causing structural inversion in basins that does not require the reactivation of normal faults as thrusts or reverse faults

geodynamic evolution of the taiwan luzon mindoro belt since the late eocene abstract the structural framework of the taiwan luzon mindoro belt or festoon is described following three major transects the luzon transect with active subduction and active island arc the taiwan transect with active collision the mindoro transect with active subduction and inactive collision based on this geological study and on available geophysical data a model for the geodynamic evolution of this portion of the philippine sea and eurasia plates boundary is proposed in a succession of reconstructions between the late eocene and the present the major geodynamic events are 1 1 beginning of the opening of the south china sea s c s in lower oligocene times contemporaneous with obduction of the zambales and angat ophiolites on luzon 2 2 subduction of a mesozoic oceanic basin along the proto manila trench from the upper oligocene to the lower miocene 3 3 obduction of the south china sea oceanic crust onto the chinese and reed bank calamian passive margins in middle miocene time 14 15 ma related to a major kinematic reorganization end of opening of the s c s 4 4 beginning of collision between the luzon microblock and the two margins of the s c s in the upper miocene 7 ma collision is still active in taiwan whereas it stopped in mindoro during the pliocene

the influence of tectonics on early cretaceous sedimentation in bedfordshire england the lower cretaceous sediments of the bedfordshire area are aptian and albian in age during this time the area was situated within an extensional tectonic regime and the pattern of sedimentation in terms of lateral variation both in sediment type and thickness reflects the influence of normal faulting jurassic topography created by block faulting and erosion controlled the distribution of later facies largely by continued tectonic processes especially the reactivation of the jurassic fault block system which produced syn sedimentary faulting of the overlying sequence thus the traditional view of the infill of an erosional basin by the lower greensand is challenged

geochemistry and tectonic setting of the luzon arc philippines miocene 10 ma to recent volcanism is associated with eastward subduction along the manila trench for some 1 200 km from the coastal range in taiwan south to min doro we suggest calling this the luzon arc there are five distinct segments along this arc mindoro bataan northern luzon babuyan and taiwan the bataan and mindoro segments in the south are separated by a northeast southwest zone of volcanoes the northern luzon segment consists of pliocene to pleistocene necks and plugs that run along the entire western section of northern luzon the babuyan segment 10 ma to recent is made up of a larger number of active volcanoes than are other segments of the luzon arc the taiwan segment is composed entirely of extinct volcanoes in the coastal range and on the islands of lanhsu and lutao late miocene to 1 8 ma the nature of volcanism throughout the luzon are is clearly subduction related with calc alkaline affinities high th la ratios and negative nb ta and ti anomalies the earliest phase of volcanism so far recorded 10 ma however is tholeiitic island of calayan in the babuyan segment with mid oceanic ridge basalt like morb like tight rare earth element and large ion lithophile element concentrations and low th u ratios there are clear differences in sr and nd isotopic ratios among the samples from the luzon are samples from the southern segments mindoro through bataan fall within the mantle array with higher sr and lower nd isotopic values than those of morb pb isotope data indicate sediment involvement the isotopic values have been interpreted to result from source contamination by slab derived fluids bataan segment and by subducted crustal material from collision of the mindoro palawan terrane mindoro segment with the manila trench in contrast babuyan segment samples north have unusual island arc nd and sr isotopic ratios that fall below the mantle array they appear to be the result of sediment source contaminants or a unique mantle composition incorporation of sediments analyzed from eastern china and taiwan into a morb type mantle can explain the nd and sr isotopic ratios of the babuyan segment

relations between normal fault geometry tilting and vertical motions in extensional terrains an example from the southern gulf of suez abstract earthquakes and data from subsurface oil exploration suggest that large active normal faults in the southern gulf of suez are approximately planar with dips of 30 40 from the surface to around 10 km depth these faults and the blocks they bound appear to rotate about a horizontal axis as they move causing tilting this tilting is seen both in young vertical movements of the coastline such as raised beaches and marshlands and in the distribution of middle miocene marine rocks which are uplifted to elevations of 400 500 m in footwalls of faults and found at depths of around 3500 m in the adjacent grabens the absolute amplitude of the observed vertical motions can be approximately modelled by planar rotating normal faults that impose a saw tooth topography on a regional subsidence caused by crustal and lithospheric thinning the observations required for this simple model are the present day fault dip the amount of tilting and the width of the rotating blocks the virtues of the model are its simplicity and its compatibility with our knowledge of how large active normal faults move elsewhere on the continents

seabeam survey at the southern end of the manila trench transition between subduction and collision processes offshore mindoro island philippines abstract the morphological and structural study conducted at the southern tip of the manila trench reveals that convergence between the south china sea basin and luzon is accommodated differently depending on the nature of the subducted slab when the oceanic crust is subducted a simple accretionary prism fore arc basin pattern is developed conversely where the continental margin of this basin is subducted intraplate deformation is randomly distributed across the major part of the fore arc area which is fragmented into various crustal microblocks results of seabeam mapping and detailed geophysical surveying conducted at this subduction collision transition zone during the pop2 cruise with r v jean charcot are presented and discussed here and allow us a new insight into the mechanism of such a subduction collision transition zone

degradation of a footwall fault block with hanging wall fault propagation in a continental lacustrine setting how a new structural model impacted field development plans the sirikit field thailand the sirikit field is a large field developed in a complex tilted fault block the relatively simple stratigraphy on the tilted fault block flank becomes difficult to trace onto the tilted fault block crest a recent reinterpretation of the reprocessed three dimensional seismic reflection data revealed that the difficulties of correlating reservoirs toward the crest are caused by features associated with degradation of the footwall particularly 1 the prevalence of low angle detachment faulting and 2 the development of discordant stratigraphic sections resulting from multiple episodes of erosion at the tilted fault block crest followed by the eastward onlap of the section onto the eroded footwall surface the main characteristic of this structure is the preservation of the onlapping sections through periodic propagation of the main bounding fault toward the hanging wall which resulted in the accretion of former hanging wall rocks to the footwall this propagation sequence appears to be atypical for most degraded footwalls described in the literature the degraded footwall model replaced earlier splaying bounding fault models used for development planning implementing this new structural model opened up new appraisal opportunities as the compartmentalization and reservoir distribution pictures changed appraisal of the reservoir now present above the hydrocarbon water contacts following the new structural model led to a 15 increase in recoverable hydrocarbon volumes

structural styles and depositional architecture in the triassic of the ninian and alwyn north fields implications for basin development and prospectivity in the northern north sea abstract interpretation of well calibrated three dimensional seismic volumes sedimentological analysis and electrical well log correlations from the ninian and alwyn north fields challenge the long held view that mid late jurassic extensional faults in the east shetland basin represent a simple reactivation of older triassic fault systems restoration for the effects of the younger predominantly eastward dipping mid late jurassic structures clearly demonstrates that triassic precursors had a steep westerly dip in contrast to the eastern flank of the viking graben e g troll and oseberg areas where the west dipping triassic structures are reutilised in the mid late jurassic those of the east shetland basin have largely been dissected and rotated during the later event those west dipping faults that did see later movement appear to have simply acted as minor antithetic structures to the throughgoing east dipping ones the triassic normal fault patterns actively controlled sediment thicknesses and facies distribution within the lunde and teist formations in the basin use of seismic stratigraphic surfaces calibrated by biostratigraphy and chemostratigraphic markers provides strong evidence that the triassic depocentres are spatially offset from their mid late jurassic counterparts the combination of structural stratigraphic and sedimentary effects reveal the existence of an emergent deeper triassic play opportunity in footwall locations to the mid late jurassic normal faults which has the potential to extend the life of what is otherwise mature acreage

source controlled carbonates in a small eocene half graben lake basin shulu sag in central hebei province north china carbonate deposits which unconformably overlie the palaeozoic bedrocks extensively occur in the base of the tertiary lake succession in the half graben shulu sag central hebei province north china this study focuses on the basal carbonate successions on the hinged western slope based on seismic borehole and core data nine facies are identified in the carbonate successions and are further grouped into five facies associations mid proximal alluvial fan distal alluvial fan fan fringe moderately deep lake and deep lake the first two facies associations constitute alluvial fans formed by debrisflows at the edge of lake and are dominated by mounded to lobate shaped matrix to clastsupported carbonate rudstones with minor calcretes in the lowermost rudstone units and basinward increase in interfingering with lacustrine carbonate facies the fan fringe moderately deep lake and deep lake associations are dominated by pebbly carbonate arenites or rare carbonate arenites calcisiltitecalcilutites and varve like calcilutites calcareous shales and oil shales respectively widespread occurrences of fine grained limestone packages containing varve like organic rich laminations minor authigenic glauconite and pyrite and planktonic and plant fossils suggest a meromictic anoxic deep lake under a semi humid to humid climate probably with a connection to marine basins similarities in lithology and fossil assemblages e g trilobites of lithoclasts with those of the mid upper cambro ordovician bedrock carbonates suggest that the clastic and dissolved carbonate loads were sourced from this lower palaeozoic catchment and shed off the surrounding highlands into the basin these carbonate facies associations represent the lake lowstand and transgressive deposits of the basal third order sequence ia in which the highstand deposits are composed of lacustrine siliciclastics during the lake lowstand stage or initiation of basin filling under an intermediate climate carbonate alluvial fans occurred mostly subaerially at the bottom of the hinged slope with a narrow shallow lake zone basinwards and locally were perched within the palaeovalley on the mid upper slope during the transgressive deepening stage under a semi humid to humid climate carbonate alluvial fans became smaller in size and episodically stepped backwards upon the slope with greatly expanded and deepened lake nevertheless the carbonate system was switched to an exclusively siliciclastic system during the highstand stage the exhumation and erosion of the mid lower cambrian bedrock dominated by siliciclastics was probably the cause due to further uplift of the drainage basin all these facts indicate

the role of fault interaction and linkage in controlling synrift stratigraphic sequences late jurassic statfjord east area northern north sea examination of well constrained three dimensional seismic data demonstrates the role of fault interaction and linkage in controlling the nature of synrift sequences on the hanging wall of the statfjord east fault a typical late jurassic structure in the northern north sea brent province al though now a single fault the statfjord east fault originally consisted of several en echelon segments each of which defined individual subbasins structural and stratigraphic evidence both along and across fault strike indicates that the fault resulted from segment propagation interaction and linkage facies architecture thickness variations and the internal character of synrift formations are temporally and spatially related to the subbasin geometry variations in displacement along the fault segments exhibit characteristics of interacting en echelon faults including anomalous displacement gradients in regions of segment overlap we attribute the observed shifts in depocenters to local enhancement of displacement rates resulting from the interaction of neighboring fault segments the results have far reaching consequences for synrift plays in the northern north sea because they imply that only from the perspective of fault growth and linkage can the late jurassic structure and stratigraphy be fully understood

a 2 ma record of explosive volcanism in southwestern luzon implications for the timing of subducted slab steepening 1 new chemical and isotopic analyses of the tephra layers plus deep sea tephrostratigraphic record from two cores from either side of luzon island philippines have allowed the identification of two periods of explosive volcanic activity originating from the macolod corridor in the southwestern part of the luzon the first period extended from prior to 1355 ka to 1977 ka and the second period extended from 478 ka to the present separated by a period of relative quiescence the time intervals between large explosive eruption events in each period were 31 15 ka and 156 52 ka respectively combined with published chronological and geochemical data from onshore volcanic deposits the tephrostratigraphic record shows that the locus of large explosive eruptions has migrated southwestward from the northeastern section to the middle and southwestern sections of the macolod corridor the period of relative quiescence is characterized by monogenetic volcanism in the central section of the corridor the migration of active volcanism across the southwestern part of luzon during the quaternary is used to infer the evolution of the subducting south china sea crust the period of relative quiescence represents a period of adjustment of the subducted slab by steepening which began around 1355 ka or shortly thereafter and finished at around 478 ka

the western central luzon volcanic arc the philippines two arcs divided by rifting abstract the western central luzon arc is apparently a complex zone of volcanism and tectonism the volcanoes of the northern segment of the arc the bataan arc ba fall along two semi parallel lineaments the western bataan lineament wbl and the eastern bataan lineament ebl this northern segment is cut off from the southern extension of the arc by a ne sw cross arc zone of volcanism the macolod corridor mc the volcanoes of the southern segment of the arc mindoro arc ma fall along two parallel lineaments the western mindoro lineament wml and the eastern mindoro lineament eml samples collected from the mc ebl and ma are alkali and lil enriched compared with the wbl the enrichment of the ebl is believed to be the result of the release of smaller amounts of hydrous rich fluids within the mantle below the ebl than below the wbl this process generated smaller degrees of partial melting below the ebl as compared to the wbl which resulted in the higher concentrations of lil elements in the ebl magmas as compared to the wbl magmas the alkali and lil enriched nature of the mc is believed to be the result of small degrees of partial melting during rifting the enriched nature of the ma is thought to be due to slivers of crustal material from the palawan mindoro block being incorporated into the mantle during subduction this region has the highest radiogenic sr values found in the philippines the mc may represent a pull apart rift zone between the philippine and west luzon shear zones and the transform between the manila and philippine trenches

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

dynamics of extensional basin formation and inversion introduction this volume contains a cohection of papers on the dynamics of extensional basin formation and inversion presented at the 3rd annual workshop of the international lithosphere program task force origin of sedimentary basins in sundvollen norway 18 23 august 1992 previous workshops of the ilp task force had been in rueil malmaison france 1990 and in matrahaza hungary 19911 on the rim of the pannonian basin the task force also organized a number of sessions during the 1992 european geophysical society and the 1993 european union of geosciences conferences reports of these meetings have been published in a number of special volumes and papers cloetingh et al 1993a b stephenson 1993 cloetingh sassi and task force team 1994 the 4th task force meeting focusing on the interplay of extension and compression was held in benevento southern italy in october 1993 a selection of papers presented at that task force workshop will be published in a forthcoming special volume of tectonophysics the emphasis of the 5th workshop of the task force taking place in the dead sea rift israel is on rifting and strike slip tectonics the 1995 workshop will be organized in the southern pyrenean foreland basin with the aim of examining the relationships between tectonics and eustasy in foreland fold and thrust belts the emphasis of the sundvollen workshop was on modelling studies as well as observational approaches to the dynamics of basin extension and inversion important elements of the meeting were the discussion of extensional tectonics rifting in passive margin settings and volcanism and magmatism in rifts and basins following the task force strategy to link subsurface data to field analogues an important part of the meeting was devoted to a thematic excursion in the oslo rift field guides have been published separately larsen et al 1992a

analysis of crustal deformation in luzon philippines using geodetic observations and earthquake focal mechanisms we utilize regional gps velocities from luzon philippines with focal mechanism data from the harvard centroid moment tensor cmt catalog to constrain tectonic deformation in the complex plate boundary zone between the philippine sea plate and eurasia the sundaland block processed satellite imagery and digital elevation models are used with existing gravity anomaly seismicity and geologic maps to define a suite of six elastic blocks geodetic and focal mechanism data are inverted simultaneously to estimate plate rotations and fault locking parameters for each of the tectonic blocks and faults comprising luzon major tectonic structures that were found to absorb the plate convergence include the manila trench 20 100 mm yr 1 and east luzon trough 9 15 mm yr 1 philippine trench 29 34 mm yr 1 which accommodate eastward and westward subduction beneath luzon respectively the left lateral strike slip philippine fault 20 40 mm yr 1 and its northward extensions the northern cordillera fault 17 37 mm yr 1 transtension and the digdig fault 17 27 mm yr 1 transpression the macolod corridor a zone of active volcanism crustal thinning extension and extensive normal and strike slip faulting in southwestern luzon is associated with left lateral transtensional slip of 5 10 mm yr 1 the marikina fault which separates the central luzon block from the southwestern luzon block reveals 10 12 mm yr 1 of left lateral transpression our analysis suggests that much of the philippine fault and associated splays are locked to partly coupled while the manila and philippine trenches appear to be poorly coupled luzon is best characterized as a tectonically active plate boundary zone comprising six mobile elastic tectonic blocks between two active subduction zones the philippine fault and associated intra arc faults accommodate much of the trenchparallel component of relative plate motion 2006 elsevier b v all rights reserved