quantification of tertiary erosion in the inner moray firth using sonic velocity data from the chalk and the kimmeridge clay abstract sonic velocities for the chalk and kimmeridge clay were analysed to determine the importance and magnitude of tertiary erosion for 26 wells in the inner moray firth apparent erosion height above maximum burial depth was derived by the displacement along the depth axis of a given velocity depth trend from the normal undisturbed trend the similarity in apparent erosion results derived from the cretaceous chalk and late jurassic kimmeridge clay successions of the inner moray firth suggests that burial beyond present depths caused the observed overcompaction rather than any sedimentological and or diagenetic process resultant apparent erosion estimates are variable across the inner moray firth such values reach approximately 1 km in the western part of the basin and decrease progressively to zero in the eastern inner moray firth however the actual magnitude of erosion equal to the sum of apparent erosion and post erosional tertiary burial was approximately 1 km throughout the whole inner moray firth area indicating that early tertiary erosion was regional and not simply restricted to the inverted basin margins however a well defined mechanism sufficient to have generated the regional event is still sought

the continental margin of the mesozoic tethys in the western alps abstract most of the structural units of the western alps were derived from the european continental margin of the ligurian ocean a segment of the mesozoic tethys ocean their mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages 1 deposition of the triassic platform carbonates essentially prerift but nevertheless bearing the imprint of some extensional movements 2 the liassic middle jurassic rifting corresponds to the creation of a horst and graben system with especially tilted blocks and hence a shoal and basin palaeogeography but without a preliminary doming event 3 the late jurassic early cretaceous opening and spreading of the ligurian ocean began with a general collapse of the continental margin thermal subsidence latest middle jurassic and earliest late jurassic in this paper emphasis is given to the refting derived structures more especially to the major tilted crustal blocks a few tens of kilometres wide which can be either reconstructed or directly observed the rifting stage lasted roughly 40 m y with the alternation of extensional tectonic phases and of relatively quiet periods during the extensional phases movements along fault planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere the tectonic evolution of the margin continued but decreased during the late jurassic early cretaceous oceanspreading stage tectonic activity resumed in the late cretaceous this probably resulting from the beginning of contraction of both the ocean and the continental margin leading progressively to the continental collision in the tertiary

revised sequence stratigraphic interpretation of the eocene oligocene boundary interval mississippi and alabama gulf coast basin u s a abstract examination of regional stratigraphic relationships and graphic correlation of biostratigraphic data from two important localities has resulted in a new interpretation of the sequence stratigraphy of the eocene oligocene boundary interval in mississippi and alabama the much studied section at st stephens quarry in southwestern alabama does not contain all of the components of the depositional sequence at st stephens the highstand systems tract is missing and is represented by a marine hiatus condensed section at the top of the upper eocene shubuta clay as a consequence the surface of maximum starvation and the overlying sequence boundary are merged lowermost oligocene sediments which were deposited in the lowstand systems tract rest on this maximum flooding surface seque ce boundary these findings agree with those from other parts of the gulf coast where the eocene oligocene boundary has been recognized as a sequence boundary associated with a subaerial unconformity and a downward shift in coastal onlap revision of the sequence stratigraphic interpretation at st stephens quarry alleviates much of the confusion concerning the relationship between the eocene oligocene boundary and the sequence boundary this new interpretation shows that not all sections contain all components of a particular depositional sequence and that caution is warranted when sequence stratigraphic interpretations are used to evaluate observations the sequence stratigraphic interpretation of the section at st stephens has undergone revision as new observations have become available

the petroleum geology of the unst basin north sea abstract the unst basin is situated in the northern north sea between the east shetland basin and the shetland isles the basin is essentially a three armed permo triassic fault controlled basin containing up to 3600 m of red beds this is overlain by a westerly thickening jurassic and early cretaceous sequence the stratigraphy of which is very similar to that of the east shetland basin in particular the brent group 140 m humber group 685 m and cromer knoll group 300 m are well represented as a result of laramide uplift of the area the thick upper cretaceous and palaeocene strata of the east shetland basin are absent from the unst basin this uplift resulted in substantial erosion within the unst basin providing the major source for palaeocene sands in the viking graben and the faeroes basin late palaeocene and younger tertiary strata transgress westwards across this erosion surface petroleum exploration within the basin culminated in the drilling of two exploration wells these wells encountered potential reservoir and source rocks in the jurassic section however geochemical analyses indicate these source rocks are immature for hydrocarbon generation within the unst basin it is concluded that the unst basin has a low petroleum potential

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

aspects of basin inversion in southern britain the wessex basin of southern england a permian to cretaceous extensional basin can be structurally divided into a set of constituent asymmetrical graben bounded by major e w trending zones of en echelon syn depositional normal faults the graben were inverted in late cretaceous and tertiary times by compressive stresses oriented roughly north south inversion structures fall into two related categories regional upwarps overlie earlier graben depocentres and were formed by bulk shortening of the graben fill superimposed upon and geographically delimiting the regional upwarps are roughly east west trending linear zones of en echelon inversion structures these coincide with the earlier graben bounding faults and typically have the form of monoclinal or periclinal flexures each underlain by a partially reversed normal fault the linear reverse fault monocline inversion structures were a relatively inefficient method of basin shortening because upper crustal faults in the region steepen upward markedly reversal of these faults under compression resulted in a shortening discrepancy at shallow depths locally particularly in south dorset this was overcome by bucklefolding and low angle reverse faulting which increased the amount of shortening attainable across the linear inversion structures elsewhere however the shortening discrepancy was accommodated by bulk shortening and regional upwarp of the graben sedimentary fill this occurred preferentially in those graben containing young poorly lithified and therefore weak sediments thus the early cretaceous depocentres of the weald and channel basins were strongly upwarped with axial uplifts of over 1000 m conversely graben having older more lithified sequences suffered little regional upwarp shortening primarily by fault reversal along the linear inversion structures the amount of crustal shortening which accompanied inversion was considerably less than the earlier crustal extension

rock magnetic proxies of climate change in the loess palaeosol sequences of the western loess plateau of china summary results of the first detailed study of the climate proxy record in the loess palaeosol sequence at xining one of the few palaeoclimate sites in the currently arid western loess plateau of china illustrate the importance of making many types of rockmagnetic measurements other than susceptibility a multiparameter approach yielded confirmation that here as elsewhere in the loess plateau the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite nevertheless magnetic enhancement was caused not exclusively by changes in relative grain size but also by variations in concentration and mineralogy of the magnetic fraction the effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence respectively the resulting signal was ascribed more confidently to variation in magnetic grain size which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility variations in magnetic mineralogy were also determined to constrain interpretations further the data were then used to discuss climate history at xining finally results from xining were compared with other western sites and contrasted with eastern sites in summary 1 data is presented from a new loess plateau site which also appears to yield a global climate signal 2 a demonstration is made of a more rock magnetically robust way to separate concentration composition and grain size controls on susceptibility and other magnetic parameters and 3 models are provided for inter regional comparisons of palaeoclimate proxy records

the petrology and origin of the tertiary lundy granite bristol channel uk the british tertiary volcanic province btvp comprises within plate central igneous complexes associated with plateau lavas and regional dyke swarms lundy is the southernmost complex of the btvp and comprises granite small tilde90 emplaced into deformed devonian sedimentary rocks within the hercynian cornubian granite province of southwest england the complex is intruded by a northwest southeast trending dyke swarm in common with other btvp igneous complexes lundy is associated with positive gravity and magnetic anomalies which are interpreted in terms of the presena of an underlying basic intrusion at shallow depth with a volume exceeding that of the overlying granite the lundy intrusion is a coarse grained megacrystic granite containing up to 20 alkali feldspar megacrysts in a coarse grained groundmass composed of alkali feldspar quartz lithium bearing muscovite and biotite lithian siderophyllite with a range of aaxssory minerals the main granite has a coarse grained locally miarolitic pegmatitic facies and is intruded by thin sheets and veins of fine grained aplite and microgranite the mineralogy indicates crystallization of the lundy granite from a highly fractionated h2o and halogen rich magma at a relatively shallow crustal level the main lundy granite is a peraluminous leucogranite with na2o 3 4 k2osmall tilde5 low tio2 meo cao zr and sr and high rb and rb sr in comparison with many other peralurninous granites including those from the cornubian batholith and the btvp a new rb sr whole rock isochron for the granite yields an age of 58 7 1 6 ma with an initial 87sr 86sr of 0 715 0 006 nd values for the granite i 0 9 to 1 9 plot between contemporaneous mantle positive nd and cornubian granites nd ca i 11 the trace element data rb y nb show affinities with syn collision and within plate granites as the sr isotope data indicate a major crustal component and the nd isotope data suggest both mantle and crustal components we propose that the lundy granite is derived from a parental magma comprising crustal components derived from a similar source to that of the cornubian granite batholith and a mantle derived component derived from a differentiate of contemporaneous basaltic magma this magma experienced fractional crystallization of plagioclase alkai feldspar fe mg minerals and ree bearing accessory minerals before emplacement and the lundy granite experienced further in situ fractional crystallization associateded with crustal contamination by the devonian shale after emplacement

comparison of modern and geological observations of dynamic support from mantle convection the topographies of africa and antarctica form patterns of interlocking swells the relationship between topography and gravity indicates that these swells are dynamically supported by mantle convection with swell diameters of 1850 450 km and full heights between 800 and 1800 m the implication is that mantle convection not only supports swells surrounding hotspots but also influences topography across the entire surface areas of africa and antarctica we investigate whether dynamically supported swells are also observed throughout the geological record focusing on intensively studied mesozoic cenozoic sedimentary rocks around britain and ireland vertical motions of britain and ireland a typical piece of continental lithosphere far from a destructive plate boundary have been demonstrably affected by dynamic support for over half of the past 200 ma period the diameters and maximum heights of the mesozoic british swells and the modern african and antarctic swells are similar the ancient british swells grew in 5 10 ma and decayed over 20 30 ma suggesting vertical motion rates comparable with those estimated from geomorphological studies of africa igneous production rate and swell height are not correlated in the modern and the geological records mantle convection should be considered as a common control on regional sea level

lithospheric extension and magmatism in the porcupine basin west of ireland the porcupine seabight basin is a north south trending extensional sedimentary basin situated on the continental shelf west of ireland it is filled with sedimentary rocks which range in age from devonian to present day a large amount of geological and geophysical data is now available for this basin mainly as a result of oil industry exploration in this paper information from 23 well logs and from regional seismic reflection surveys is used to determine the lithospheric stretching history of the basin subsidence analyses indicate that there was one main phase of stretching which began in late liassic times 180 ma ending in earliest cretaceous 145 ma stretching factors at the northern end of the basin are relatively small 1 1 1 7 but increase rapidly southwards along the axis of the main porcupine basin in the southern part of the region the seabight basin is greater than 6 the simplest way to accommodate this rapid variation in stretching is by clockwise rotation of the porcupine ridge through approximately 20 away from the irish shelf subsidence derived stretching values are in reasonable agreement with those determined previously from crustal thicknesses based on gravity models and deep seismic data normal incidence and wide angle the existence location and inferred age of the porcupine median volcanic ridge are also consistent with the subsidence derived stretching factors after intermittent paleocene igneous activity an anomalous increase in the rate of subsidence occurred in the eocene between 55 and 42 ma there is little evidence that this rapid increase is caused by lithospheric stretching and we conclude that it must be associated in some unknown way with melt generated by development of the iceland hotspot

a history of basin inversion scarp retreat and shallow denudation the araripe basin as a keystone for understanding long term landscape evolution in ne brazil at the border between the states of ceara and pernambuco northeast brazil the chapada do araripe is a high plateau 800 1000 m a s l formed by a slab of cenomanian fluvial sandstone this caprock is underlain by lacustrine or marine albian layers and older rift deposits during the cretaceous the araripe basin lay at a palaeoelevation close to sea level through a presentation and discussion of original field and cartographic data we analyse the mechanisms of topographic inversion in this sedimentary basin in relation to local or regional crustal upwarp the contrast between the plateau a weakly dissected structural surface and the surrounding lowlands is explained through a study of the erosional scarps cuesta like landforms and their outliers that fringe the chapada no evidence of local tectonic inversion is found river incision spring sapping landslides and other forms of mass movement are listed as efficient processes of topographic inversion and scarp retreat the rates and patterns of which appear to be controlled by lithological contrasts and conditions of exhumation of the basement geometric relationships with regional stepped surfaces e g the low elevation sertaneja surface exhumed palaeosurfaces and regional drainage systems are analysed our estimation of the amplitude of denudation and topographic inversion 0 6 0 7 km differs significantly from apatite fission track derived estimates reported in recent literature which would imply burial by considerable thicknesses of younger sediments followed by 1 5 km or more of post rift denudation not just in the study area but also in the tucano jatoba basin to the south the exhumation and reworking of surrounding basement surfaces probably began during the early cenozoic as shown to the northwest of the chapada by the presence of widespread laterites of probable palaeogene age a second stage of topographic inversion occurred during the oligocene or later this would correspond to the major stages of river incision partial planation and basin inversion

asthenospheric and lower lithospheric mantle contributions to continental extensional magmatism an example from the british tertiary province abstract the compositions of palaeocene basic magmas in skye and mull british tertiary igneous province btip shed light on the general problem of the relative importance of lithospheric and asthenospheric mantle as sources of continental magmatism during extension after the effects of crustal contamination in some magmas have been detected and the samples showing it eliminated from consideration the remainder show a significant diversity of trace element compositions basic lavas from skye and mull which lack crustal contamination have contents of compatible and moderately compatible elements e g ni cr y yb and also of moderately incompatible elements e g ti p zr hf sr mree which are within the range shown by the tholeiitic and mildly alkalic ocean island basalts oib as from iceland in contrast their contents of strongly and very strongly incompatible elements e g k rb ba u th nb ta lree are within the range shown by normal mid ocean ridge basalts these btip magmas are interpreted as second stage melts from mantle which had already lost a very small melt fraction the latter is thought to be represented by the strongly alkalic permo carboniferous dyke swarms associated with rifting at that time in nw scotland residual uppermost asthenosphere from the late palaeozoic magma genesis event is postulated to have become attached to the thickening lithosphere beneath nw scotland and thus to have remained coupled to it throughout the mesozoic until reactivated by palaeocene stretching occasional hawaiites in the top third of the skye lava pile remnant and also basalt dykes of the swarm cutting the granites of the central complex the last significant magmatic event at this centre have the elemental characteristics of typical north atlantic oib notably la ta 10 12 these compositions are thought to demonstrate input from asthenospheric source basic magmas throughout the lithospheric source dominated skye and mull magmatism it is suggested that all the basic magmas in this province are ultimately derived from asthenospheric sources but that the larger magma batches became contaminated during uprise by melting and assimilating the most fusible parts of overlying lithospheric mantle followed by lower crust in many cases smaller magma batches traverse the overlying lithosphere with little or no contamination perhaps the difference between the two lies in their flow behaviour laminar or turbulent during uprise

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

northern scotland as an atlantic north sea divide president9s anniversary address 1984 from the end of the caledonian orogeny until the present northern scotland has acted as a structural high separating the north sea and atlantic basins or their forerunners the geological history of this positive massif is discussed in relation to that of the adjacent cratonic basins with the objective of determining the extent to which it adapted to regional tectonic developments and the manner in which its responses were modified by the pre existing basement structure from devonian to early palaeocene times the massif was generally low lying and its behaviour with respect to vertical movements was essentially neutral the land surface carved in the crystalline basement during the devonian period was preserved with relatively small modifications and little metamorphic detritus was supplied to the adjacent basins tectonic events of major importance in the basins such as the formation of the north sea and hebridean rift systems produced little change of conditions in the intervening massif it is inferred that the underlying keel of lithospheric mantle was protected from the effects of lithospheric stretching the uneventful history of the massif was interrupted by two phases of mafic igneous activity in late carboniferous and palaeocene times which record responses to regional plate motions the formation of the palaeocene igneous province was followed by a unique phase of uplift as a result of which the ancient sub devonian land surface was elevated and dissected and abundant crystalline detritus was supplied to the north sea the style of these events and the anomalous character of the continental margin off nw britain may have been determined by the refractory nature of the precambrian basement which inhibited stretching prior to sea floor spreading

low temperature effects of the skye tertiary intrusions on mesozoic sediments in the sea of hebrides basin fission track analysis of samples from the sea of hebrides basin and surrounding regions demonstrates two very distinct age groups around 50 ma and 300 ma regardless of the stratigraphic age apatite fission track results from sediments within 8 km of the tertiary igneous complex on skye were totally reset by temperatures 110 c during intrusion of the centres but now yield ages younger than the granites zircon fission track results from the granites also demonstrate ages significantly younger than their hosts mean track lengths in apatites of 13 5 m and reduced apatite and zircon ages suggest that temperatures remained elevated throughout the 6 ma during which intrusive activity occurred but were hotter within the granite bodies than the surrounding sediments prior to intrusion of the complex temperatures in the basin sediments presently at outcrop were unlikely to have been higher than 50 c and beyond the effects of the tertiary intrusions age and track length distributions from northern skye the isle of lewis and the west coast of scotland illustrate that the area is unlikely to have been buried beneath more than 2 km of sediment at any one time since the end of the devonian north of the highland boundary fault tertiary uplift and erosion in scotland is considered to have been between 1 1 5 km this is consistent with a regional pattern of tertiary erosion identified across the british isles but considerably less than that recognised in northern england the geological society 1992

compressive episodes and faunal isolation during rifting southwest iberia abstract evidence for three short lived compressive episodes of late carixian late callovian early oxfordian and tithonian berriasian ages that lasted 5 m yr and occurred during the process of rifting of the algarve basin is presented these tectonic inversion episodes are described at outcrop and cartographic scales and have been dated with the accuracy provided by the ammonoid scale an uplift event of late toarcian aalenian age of undetermined tectonic origin is also described we show that these four tectonic episodes coincide in time with important ecological events such as the onset of migration and or the segregation of boreal and tethyan ammonite species and the confinement of the algarve basin stratigraphic and paleoecological data from the algarve and lusitanian basins are compared and discussed together with eustatic and tectonic information we propose that the tectonic inversion episodes that caused uplift are the origin of the mesozoic sedimentary gaps and the intermittent opening and

the deep structure of northern england and the iapetus suture zone from birps deep seismic reflection profiles the nec deep seismic reflection profile images structures in the lower crust which represent the iapetus suture zone beneath southern scotland and northern england on the basis of reflectivity characteristics of the lower crust and reflection moho we recognize four different crustal zones which correspond to four crustal terrane types midland valley zone a a sub continental subduction complex zone b lake district zone c and midland platform zone d the junctions between each of these zones are interpreted as being tectonic a and b originate from the northern continental margin of the iapetus ocean and are separated from c and d which are derived from a southern continent by strong northerly dipping reflections between approximately 15 and 30 km deep in the crust the structure responsible for these reflections does not displace the reflection moho north of the ab cd junction the base of zone b and base of the reflective crust is marked by a pair of parallel reflectors persistent for a horizontal length of about 55 km these reflectors are interpreted as the top and bottom of a slice of remnant oceanic crust at their northerly limit the deepest of the two parallel reflectors transects the moho to a depth of 6 7 km into the uppermost mantle we suggest that their termination delimits the northerly extent of a decollement or shear zone at the present continental moho the shear zone was active during the late stages of caledonian collision when mantle originally from beneath the southern continent underthrust old and newly created crust of the collision zone

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

paleoecological patterns in molluscan extinctions and recoveries comparison of the cretaceous paleogene and eocene oligocene extinctions in north america abstract exposures across the cretaceous tertiary k t and eocene oligocene e o boundaries in texas and mississippi respectively probably represent the most complete and best preserved fossil molluscan sequences across these boundary intervals in the world outcrops from both boundaries contain pristine aragonitic and calcitic molluscan shells which were deposited in fine grained sediments from open marine environments the k t and the e o extinctions exhibit very different recovery patterns probably reflecting very different causes as well as magnitudes of extinction the k t sequence contains a molluscan fossil record that is consistent with an abrupt extinction event at the k t boundary and a prolonged initial recovery in hostile oceanographic conditions the uppermost 10 m of upper cretaceous sediments contain a diverse approximately 40 species molluscan fauna dominated by suspension feeders the earliest paleocene sediments immediately above the tsunami bed contain an impoverished fauna dominated by deposit feeders the paleocene fauna slowly climbs in diversity but remains relatively impoverished and dominated by deposit feeders for several hundred thousand years after the extinction in conjunction with anomalous 13c values that suggest prolonged suppression of marine primary productivity diverse suspension feeder dominated molluscan assemblages reappear with the resumption of normal conditions of primary production in the long term early to middle paleocene gamma diversity includes evolutionary bloom taxa families that exhibit unusual speciation bursts that subside in the eocene total diversity for the gulf coast does not approach cretaceous levels until the late eocene representing a total recovery interval of nearly 25 million years while the e o event also reflects a molluscan extinction rate of over 90 in the gulf of mexico there are no signs of hostile environmental conditions in the recovery fauna early oligocene molluscan assemblages are diverse and dominated by suspension feeders characteristic of normal marine conditions the hiatus at the e o boundary however could have obscured a short term recovery fauna there is also no sign of long term perturbation by the e o extinction there are no bloom taxa and gamma diversity approaches pre extinction levels within a few million years the overall pattern of the e o extinction is consistent with extinction and or migration associated with long term cooling

tectonic evolution of the north sea basin crustal stretching and subsidence summary the lithospheric stretching model for the formation of sedimentary basins was tested in the central north sea by a combined study of crustal thinning and basement subsidence patterns a profile of crustal structure was obtained by shooting a long range seismic experiment across the central graben the main axis of subsidence a seabed array of 12 seismometers in the graben was used to record shots fired in a line 530 km long across the basin the data collected during the experiment were interpreted by modelling synthetic seismograms from a laterally varying structure and the final model showed substantial crustal thinning beneath the graben subsidence data from 19 exploration wells were analysed to obtain subsidence patterns in the central north sea since jurassic times changes in water depth were quantified using foraminiferal assemblages where possible and observed basement subsidence paths were corrected for sediment loading compaction and changes in water depth through time the seismic model is shown to be compatible with the observed gravity field and the small size of observed gravity anomalies is used to argue that the basin is in local isostatic equilibrium both crustal thinning and basement subsidence studies indicate about 70 km of stretching across the central graben during the mid jurassic to early cretaceous extensional event this extension appears to have occurred over crust already slightly thinned beneath the graben and the seismic data suggest that total extension since the early permian may have been more than 100km the data presented here may all be explained using a simple model of uniform extension of the lithosphere

symmetric conjugation of continent ocean boundary structures along the norwegian and east greenland margins abstract a characteristic structural feature of the norwegian continental margin that has been described from multichannel seismic data as well as dsdp and odp drilling is a major buildup of layered extrusive basaltic rocks forming wedges that dip seaward along the entire margin models for their origin describe them as either products of a late stage intra continental rift phenomenon with the volcanic rocks overlying attenuated continental crust or as an integral component of the oceanic crust formed during the first few million years of seafloor spreading in the norwegian greenland sea from studies of the norwegian margin alone it was not possible to unequivocally distinguish between the two proposed models so an investigation of the conjugate portions of the east greenland margin was undertaken which used two ship multichannel seismic acquisition as its focus we found that the wedges were not only present on the east greenland margin but that details of their along strike variability are mirrored on both margins together with their spatial relationship to the oldest magnetic lineation pattern the wedges are relatively small in the northern part of the margin where they occur almost entirely landward of lineation 24 moving south we see that the wedges broaden encroach into the pattern of lineation 24 then develop into two separate wedges the seaward of which is associated with lineation 23 lineations 24 and 23 on both margins are distorted when associated with wedges we believe that these data lend strength to a tectonic association of the wedges with nascent plate boundary processes their observation in many margins throughout the world suggest that a prolific volcanic phase may be commonly associated with the inception of a divergent plate boundary and we believe that the most likely mechanism to generate the excess volcanism is the convective partial melting scheme recently proposed by mutter and buck 1986 and mutter et al in press

a unified view of craton evolution motivated by recent deep seismic reflection and refraction results summary an attempt is made to integrate recent continental seismic reflection refraction and geologic observations into a unified scheme of craton evolution 1 nascent continental crust of basaltic bulk composition is produced in island arcs as with oceanic crust moho in this setting lies within the magmatic edifice and corresponds to a downward transition from dominantly mafic cumulate rocks above to dominantly ultramafic cumulates below the petrologic crust mantle boundary lying 25 per cent deeper in the lithosphere has no seismic expression and corresponds to a depositional magmatic contact between cumulate ultramafic rocks above and residual mantle ultramafic tectonite below both contacts are presumably extensively disrupted by magmatic injection 2 island arcs are amalgamated into continents at collision zones delamination of the lower mafic ultramafic portion of the crust along with the mantle lithosphere during hard collisions acts as a mechanical refining process that pushes the bulk crust toward intermediate composition 3 subsequent extensional collapse of the overthickened crust together with thermal re equilibration of the lithosphere results in youthful stable continental crust which is 30 km thick with its top surface awash at sea level and has the bulk composition of andesite this crust characteristically exhibits extensional features half grabens in the upper portion and is prominently laminated in the lower portion due to the injection of modest amounts of basaltic magma in the form of sills during delamination the moho and petrologic base of the crust coincide in this setting and can correspond to either a ductile high strain zone or intrusive contact separating mafic intermediate composition material above from fertile mantle lherzolite below 4 subsequent cratonization is the cumulative effect of episodic injection of the crust by mafic magma at intervals of hundreds of myr as manifest by the ubiquitous overlapping dike swarms of the precambrian shields this results in net long term thickening of the crust by underplating shifts the bulk crust back toward mafic composition periodically produces a new deeper moho and petrologic crust mantle boundary that do not coincide and through repeated dike injection disrupts pre existing laminated reflectors the first magmatic injection event sweats out the light melting fraction in the pre existing andesitic crust producing voluminous granitic magmatism typified by the great proterozoic anorogenic granite rhyolite provinces subsequent injection events into refractory crust produce uplift with attendant erosional denudation of supracrustal sequences the cumulative result is shield type crust in which intermediate crustal levels are exposed over wide areas crustal thickness commonly exceeds 40 km average crustal velocity is somewhat higher than in younger stable crust and the crust is commonly complexly reflective diffractive throughout

thermal history reconstruction using apatite fission track analysis and vitrinite reflectance a case study from the uk east midlands and southern north sea apatite fission track analysis afta is a method of obtaining ther mal history information in sediments in sections that have been hotter in the past afta provides estimates of maximum palaeotemperatures and the time at which a sedimentary section began cooling from maximum palaeotempera tures it is useful for studying the thermal history of sequences containing source rocks and can provide information critical to the understanding of the timing of oil generation integration of afta and vitrinite reflectance vr data allows a coherent thermal history framework to be established and a formal method of analysing palaeogeothermal gradients has been developed by combining afta and vr data this approach allows definition of the major facets of thermal history based on directly measurable parameters rather than on assumed values of palaeo heatflow and provides rigorous estimates of the amount of uplift and erosion the case study demonstrates the application of this approach to the east midlands shelf region here afta data have revealed an episode of maximum palaeotemperatures prior to cooling commencing at around 60 ma combined afta and vr data show that this cooling was due to kilometre scale early ter tiary uplift and erosion under near normal geothermal gradients our estimates of uplift and erosion are significantly greater than those of pervious workers and show that uplift and erosion was not restricted to the recognized axes of inversion but was regional in extent this has implications for the evolution of source rock maturity in this region and in other basins in around the uk where afta has indicated early tertiary heating and where the extent and amount of tertiary uplift and erosion may have also been underestimated by previous workers apatite fission track analysis afta tm is a method of obtaining thermal history information in sediments and is particularly useful for studying the thermal history of sequences containing hydrocarbon source rocks as well as providing estimates of maximum palaeotemperatures afta provides a direct estimate of the time at which a sedimentary section began cooling from its maximum palaeotemperature a key advantage over traditional geochemical methods in many situations this knowledge is critical to the understanding of the timimg of oil generation and migration in relation to trap formation information on the timing of maximum palaeotempera ture can also be of significant value in the interpretation of vitrinite reflectance vr data and combination of afta and vr data allows a consistent integrated thermal history interpretation the afta technique is applied most usefully in sedimentary sections which have been hotter in the past than they are at the present day it is therefore applicable to large regions of britain and the uk continental shelf that have been subject to deeper burial in the past or affected by other sources of heating the basis of the technique has

sequence stratigraphic interpretations from palynofacies dinocyst and lithological data of upper eocene lower oligocene strata in southern mississippi and alabama u s gulf coast abstract this paper presents new information on the sequence stratigraphy of the upper eocene lower oligocene strata in southern mississippi and alabama based on palynological and lithological data from five sections by integrating lithological characteristics with palynofacies assemblages several dinocyst paleoecological groups the deflandrea group and the abundance of reworked dinocysts in samples paleobathymetric curves were reconstructed and used to revise the sequence stratigraphy of the area we identified a maximum flooding surface in the middle of the shubuta clay formation in southern mississippi and dated it as latest eocene in southern alabama the eocene oligocene boundary was placed within a condensed section representing about 0 19 ma and it coincided with the shubuta clay vicksburg group contact this condensed section is equivalent to the accumulation of the upper shubuta and red bluff formations in southern mississippi the forest hill mint spring contact which has been interpreted as a sequence boundary that merged with a transgressive surface does not correlate with the tejas a ta 4 3 4 4 sequence boundary of haq et al 1988 the pachuta marl and lower shubuta clay formations are constituents of a late eocene transgressive systems tract in southern mississippi but this systems tract extended to the shubuta clay vicksburg group contact in southern alabama the overlying early oligocene highstand systems tract consisted of the upper shubuta in mississippi and the red bluff bumpnose forest hill formations a thin sandstone in the mint spring was interpreted as a deposit of a possible lowstand systems tract above the sequence boundary in one southern mississippi section in general the mint spring marl and marianna limestone constituted a transgressive systems tract

basalts water or shear zones in the lower continental crust abstract bright sub horizontal laterally extensive multi cyclic layered reflections have now been recorded in the lower continental crust from many areas in contrast this reflection signature is rare in the continental upper crystalline crust and upper mantle and in the oceanic lithosphere with the present state of knowledge it is sensible to seek a single quantitative explanation for all such layering qualitative explanations for lower crustal layering abound and include igneous metamorphic structural and fluid related models measurements of reflection coefficients from the lower crust place strong quantitative constraints on possible models structurally induced anisotropy metamorphic layering or free fluids cannot realistically produce sufficiently bright reflections igneous layering or the juxtaposition of pre existing heterogeneities by shear zones can produce strong reflections results from crustal xenoliths helium isotope ratios palaeo geothermal gradients and chemistry of flood basalts together with numerical modelling of lithospheric extension and asthenospheric melting suggest that underplating and intrusion of the lower crust with mantle derived mafic igneous rocks is common the apparent spatial relationship between lower crustal layering and continental extension and the strength and continuity of the reflections suggest that underplating related to extension and or elevated mantle temperatures is a good explanation for lower crustal layering

shallow marine ostracode turnover during the eocene oligocene transition in mississippi the gulf coast plain usa abstract the eocene oligocene transition eot is associated with a major eustatic sea level fall sea surface cooling change in ocean stratification and enhanced seasonality while these changes are well known to have significantly restructured open ocean ecosystems comparatively little is known about how these multiple environmental changes affected coastal biotas here we describe ostracode faunas in shelf deposits in the gulf coast of mexico through the late eocene to early oligocene analyzing the biostratigraphy and paleoecology of ostracodes from the mossy grove core mississippi mossy grove ostracode faunas suffered gradual extinction and diversity loss in the 150 kyr between 33 96 and 33 81 ma during the turnover 10 of 21 species disappeared seven of the 12 disappearing species became extinct while others are lazarus taxa that later re appeared in the vicksburgian 33 7 ma during the sea level fall associated with the eocene oligocene e o boundary 33 90 ma six of seven outer shelf taxa disappeared while three of four inner shelf taxa increased in abundance the major changes in ostracode assemblages occur during the eot 1 and eot 2 events prior to the oi1 isotope excursion and appear to reflect a combination of impacts from eustatic sea level fall and local enhanced surface ocean seasonality shown in stable isotope records from the st stephen s quarry core alabama

seismic profiling the continental lithosphere deep seismic reflection profiling is successfully being used worldwide to study the structure of the continental crust1 7 the moist8 profile recorded by birps british institutions reflection profiling syndicate was the first to show that the structure of the upper mantle could also be imaged given a powerful source and long recording times birps has recently acquired an ultra deep seismic reflection line drum deep reflections from the upper mantle the drum line was designed specifically to investigate the reflectivity of the lower continental lithosphere and was therefore recorded to a two way time twt of 30 s giving a depth of penetration into the lithosphere of 110 km this profile has now revealed strong reflections which are unequivocally from the lower lithosphere well below the base of the crust and which are as far as we are aware the deepest and most continuous structures imaged in the upper mantle by multi channel seismic reflection profiling

evidence for kilometre scale neogene exhumation driven by compressional deformation in the irish sea basin system simon p holford paul f green jonathan p turner gareth a williams richard r hillis david r tappin ian r duddy

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

glaucony and carbonate grains as indicators of the condensed section omma formation japan the fifth order depositional sequences of the early pleistocene omma formation exposed along the japan sea coast of central japan were formed by glacial eustasy during oxygen isotope stages 50 to 28 in each depositional sequence two ecostratigraphic datums are always present the appearance and disappearance datums of warm water molluscan species these datums are independent of sequence stratigraphic concepts because the establishment of them is based on immigration events of molluscan species associated with glacio eustatic sea level changes determination of time planes shows that the appearance datum seems to occur near the midpoint of sea level rise on the glacial to interglacial shift in deep sea d 18 o records in order to evaluate the significance of the condensed section in sequence stratigraphy and also to facilitate its recognition this study examines the stratigraphic relationship of the condensed section indicators glaucony and carbonate grains with respect to the position of the ecostratigraphic datums in depositional sequences of the omma formation the results show that the maximum concentration of carbonate grains is a more reliable maximum flooding surface indicator than the concentration of glaucony the combination of indicators of condensed section and ecostratigraphic datums represented by incursion epiboles enables the boundary between transgressive and highstand systems tracts to be recognized in the inner shelf parts of depositional sequences moreover truncation of ecostratigraphic datums during sea level falls demonstrates significant erosion at the sequence boundaries integration of climatic palaeoecology and sequence stratigraphy permits a level of correlational precision of the order of a few thousands of years 1998 elsevier science b v all rights reserved

a possible mechanism for epeirogenic uplift continental geology is dominated by vertical movements some of these are the result of crustal shortening and extension associated with large horizontal displacements involving processes which are now understood at least in outline however both uplift and subsidence can occur without associated shortening or extension of the continental crust most investigations of such epeirogenic movements have been concerned with subsidence partly because of the economic importance of the resulting sedimentary basins and partly because of the existence of a simple model for the process1 which can account for the development of several basins in some detail2 4 however uplift has been relatively neglected and cannot be produced by the same mechanism the purpose of this paper is to examine whether epeirogenic uplift can result from the intrusion of large thicknesses of basic magma into the lower part of continental crust this suggestion was considered as a possible mechanism for the regional uplift of the colorado plateau by gilbert5 more than 100 years ago and also by holmes6 though both rejected the idea however a recent discussion of no less than 14 different mechanisms for generating uplift7 did not include this process even though it seems able to produce the observed vertical motions and also to account for some of the features of the evolution of sedimentary basins that cannot be explained by the uniform stretching model3 8 9 although the suggestion is not new the argument used here to support it is and depends on an understanding of the relationship between ocean island volcanoes and mantle circulation

thermal and tectonic history of the east midlands shelf onshore uk and surrounding regions assessed by apatite fission track analysis apatite fission track analysis has been used to study the thermal and tectonic history of the east midlands shelf and surrounding areas of onshore uk apatites from outcrops of palaeozoic crystalline basement give fission track ages between 300 29 and 420 31 ma which are the oldest yet reported in the uk region and reflect the relative stability of this cratonic region mean track lengths in these samples are between 12 and 13 m namurian and triassic sandstones from outcrop give apatite fission track ages between c 100 and c 390 ma with mean track lengths generally between 10 and 12 5 m distributions of single grain ages and track lengths suggest that all samples have been annealed implying exposure to higher temperatures since deposition with maximum palaeotemperatures reached some time in the 50 80 ma range prior to the onset of cooling samples from five exploration wells show consistent patterns in which apatite fission track ages decrease sharply from around or just less than the stratigraphic age at near surface to values in the 50 80 ma range at depths of c 1 2 km present temperatures of c 40 70 c showing that the samples studied have all been exposed to higher temperatures single grain ages and track length data suggest that maximum palaeotemperatures were attained at or before c 60 ma subsequent cooling was protracted and the data are compatible with two phases in the early and mid tertiary palaeotemperature analysis suggests that palaeo geothermal gradients were similar to present values and that elevated palaeotemperatures were due to greater depth of burial with cooling due to subsequent uplift and erosion estimates of uplift and erosion vary from c 1 1 5 km on the east coast around the humber estuary to c 2 2 km in the east midlands and southern pennines compilation of palaeotemperature estimates from all samples analysed from the uk onshore region shows that mesozoic burial and tertiary uplift has affected a wide area and that uplift was due to broad scale regional warping with recognized inversion axes being areas where uplift and erosion were particularly pronounced or where younger sediments and or inverted basinal structures allow the identification of uplift and erosion the recognition of late cretaceous early tertiary unconformities across wide areas of northwest europe both onshore and offshore suggests that the regional uplift and erosion may have been far more widespread than previously recognized this has important implications for hydrocarbon exploration in northwest europe

evidence for post early eocene tectonic activity in southeastern ireland the role played by cenozoic deformation in denudation and landscape development in ireland has historically been difficult to assess because of the lack of widespread pre glacial cenozoic deposits onshore here we combine analysis of apatite fission track data and geomorphic observations to place constraints on the timing kinematics and magnitude of onshore deformation in southeastern ireland relationships between apatite fission track central age and elevation for samples from the wicklow and blackstairs mountains and tullow lowland suggest that these rocks record an exhumed apatite partial annealing zone which after cooling was dismembered by differential vertical displacements of up to several hundred metres we use inverted models of sample thermal history to show that samples across the region experienced very similar thermal histories up to and including a cooling event in late paleocene or early eocene time this effectively rules out strongly spatially heterogeneous denudation and implies that differential rock uplift occurred in post early eocene time the central age elevation relationships define at least three spatial domains with internally consistent apatite fission track data separated by known faults or topographic escarpments geomorphic analysis of these structures shows that patterns of catchment incision and sinuosity as well as the presence of antecedent drainage are best explained by differential vertical displacements at or near the domain boundaries the kinematics and magnitudes of these displacements are consistent with those implied by the apatite fission track results and are compatible with other examples of known cenozoic deformation from ireland and the adjacent continental margin

timing and mechanisms of north atlantic cenozoic uplift evidence for mantle upwelling postrift domal uplift patterns are a distinct feauture of northern north atlantic margins on the basis of apatite fission track data offshore seismic stratigraphy geomorphology gravity and seismic tomography we argue that southern norway is characterized by predominantly neogene domal uplift currently available data suggest similar processes for other uplifted regions such as spitsbergen northern norway the british isles aad parts of east greenland

lispb revisited a new look under the caledonides of northern britain summary the lithospheric seismic profile in britain lispb shot in 1974 included a 700 km long north south profile of the crust and upper mantle of northern britain the wide angle seismic data from lispb was of exceptional quality and coverage making a reinterpretation using current modelling programs worthwhile this reinterpretation shows that the original crustal model published by bamford et al 1978 satisfies the observed first arrival traveltimes but that further detail of crustal velocity structure may be obtained by model shows strong variations in crustal velocity structure that appear to be correlated with geological and tectonic structures mapped at the surface the new lithospheric model is compatible with the observed gravity field when converted to appropriate densities and the variations in loading along the base of the density model show that the lithosphere is close to local isostatic equilibrium profiles of the variation of velocity and density with depth from the new model are compared with measurements of lewisian rocks made at confining pressures up to 8 kbar allowing the seismic model to be interpreted geologically a synthetic normal incidence profile of the new model is superimposed on the nearby nec deep seismic reflection profile and the features imaged by the two techniques are compared