gas hydrate reservoirs and gas migration mechanisms in the terrebonne basin gulf of mexico highlights identify new fine grained hydrate filled fracture units in the terrebonne basin identify new hydrate bearing thin sands mostly within fractured muds present detailed seismic amplitude maps of the new hydrate bearing units discuss methane migration mechanisms and hydrate formation in thin sands identify and discuss source reservoir relationships between thick muds and thin sands abstract the interactions of microbial methane generation in fine grained clay rich sediments methane migration and gas hydrate accumulation in coarse grained sand rich sediments are not yet fully understood the terrebonne basin in the northern gulf of mexico provides an ideal setting to investigate the migration of methane resulting in the formation of hydrate in thin sand units interbedded with fractured muds using 3d seismic and well log data we have identified several previously unidentified hydrate bearing units in the terrebonne basin two units are 100 m thick fine grained clay rich units where gas hydrate occurs in near vertical fractures in some locations these fine grained units lack fracture features and they contain 1 4 m thick hydrate bearing sands in addition several other thin sand units were identified that contain gas hydrate including one sand that was intersected by a well at the location of a discontinuous bottom simulating reflector using correlation of well log data to seismic data we have mapped and described these new units in detail across the extent of the available data allowing us to determine the variation of seismic amplitudes and investigate the distribution of free gas and or hydrate we present several potential source reservoir scenarios between the thick fractured mud units and thin hydrate bearing sands we observe that hydrate preferentially forms within thin sand layers rather than fractures when sands are present in larger marine mud units based on regional mapping showing the patchy lateral extent of the thin sand layers we propose that diffusive methane migration or short migration of microbially generated methane from the marine mud units led to the formation of hydrate in these thin sands as discontinuous sands would not be conducive to long range migration of methane from deeper reservoirs

interpretation of the gravity and magnetic anomalies over the mull tertiary intrusive complex nw scotland a 50 mgal local positive gravity anomaly over the mull intrusive complex is interpreted as caused by an underlying dense body of mafic basic and or ultrabasic rocks extending to a depth of between 6 5 and 13 km and occupying a volume of between 2000 and 3600 km 3 depending on density contrast the granites produce smaller superimposed negative gravity anomalies which indicate that the granites of the late loch ba centre extend to a depth of about 2 km and those of the early glen more centre occupy a significantly smaller volume the granites occupy between 5 and 9 of the total volume consistent with a derivative origin the complicated aeromagnetic anomalies have been simplified by applying the pseudogravimetric transformation the most conspicuous positive pseudogravimetric anomaly occurs over the rocks of centres 1 and 2 this is of much smaller areal extent than the positive gravity anomaly and it is attributed to highly magnetic rocks extending to a depth of about 2 km beneath centres 1 and 2 including cone sheets the magnetization of the deeper parts of the complex is much weaker and has not been resolved a partial ring of linear or arcuate negative magnetic anomalies occurs along the se s and w margins of the deep seated intrusive complex and may mark an unexposed early ring dyke with strong reverse magnetization it is estimated that up to a quarter of the volume occupied by the intrusive complex may have been provided by various types of forceful intrusion but that most of the upper crustal space has been made available by ring subsidence and other types of stoping this is feasible because the magma was demonstrably lower in density than the moine and lewisian the mantle derived magma probably rose through the more ductile lower crust by diapirism thereby causing a slight thickening of the underlying crust which may provide isostatic support for the excess mass of the intrusive complex and its associated topography

high resolution p cable 3d seismic imaging of gas chimney structures in gas hydrated sediments of an arctic sediment drift the newly developed p cable 3d seismic system allows for high resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions gas chimneys shallow gas and gas hydrate reservoirs seismic imaging of a geofluid system of an arctic sediment drift at the vestnesa ridge offshore western svalbard provides significantly improved details of internal chimney structures from the seafloor to not vert similar500 m bsf below seafloor the chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments the pockmarks are not buried and align at the ridge crest pointing to recent topography controlled fluid discharge chimneys are fuelled by sources beneath the base of gas hydrate stability zone ghsz that is evident at not vert similar160 170 m bsf as indicated by a bottom simulating reflector bsr conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments episodically active pressure driven focused fluid flow could explain the hydro fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the vestnesa ridge high amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near surface gas hydrates and or authigenic carbonate precipitation acoustic anomalies expressed as high amplitudes and amplitude blanking are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and or carbonate formation in space and time

contact metamorphism and anatexis of torridonian arkose by minor intrusions of the rum igneous complex inner hebrides scotland

vertical evolution of fluid venting structures in relation to gas flux in the neogene quaternary of the lower congo basin offshore angola abstract the processes preceding the genesis of pockmarks and their subsequent evolution has been rarely studied since pockmarks were discovered in 1970 a high resolution 3 d seismic survey from the continental slope of the lower congo basin allows us to study in detail the structures that developed before the formation of pockmarks as well as their subsequent geometric evolution this paper aims at reporting new type of fluid related structures that relate to pockmarks the objective of this research is to understand 1 the evolution of these structures and their genetic relationship with pockmarks and 2 how their morphologies may be influenced by changes in venting intensity over time pockmark and chimney structures in this survey are hosted within late miocene to quaternary hemipelagites above clastic reservoirs in a petroliferous basin they are all associated with positive high amplitude anomalies phaas on seismic data phaas are interpreted from their dramatically higher acoustic impedance relative to the background sedimentary values of the host units and by their association with seismic pull up effects from their planform and cross sectional geometry phaas are primarily interpreted as being due to the development of methane related carbonates but they may be associated with gas hydrates in one or two examples methanogenic carbonates form either by authigenic accumulation or by anaerobic methane oxidation amo once upwardly migrating methane enters the sulfate methane transition zone smtz typically within the first few tens of meters below the seabed therefore vertical accumulations of phaas are interpreted to reflect the temporal evolution of methane flux migrating upward to reach the paleo seabed or smtz timing of local scale fluid leakage can therefore be diagnosed by analyzing phaas at seep locations that occur in association with pockmark chimney structures a conceptual model for the evolution of different fluid venting structures is proposed here and is based on the analysis of venting structures that we expressed by 3 d seismic reflections in this model the main features are linked into a process based sequence culminating in the infill and termination of the pockmarks as fluid venting sites our model summarizes different rhythms of venting which are expressed by different types of phaas along with venting structures linear phaas and conduits inferred slow seeps they can be succeeded by sub circular phaas and shallow depressions which inferred slow to moderate seeps then the seep can become either a fast vent and terminated into a pockmark crater or it can become less active and be sealed afterwards subsurface sediment fluidizations and remobilizations are likely to happen in the last case and are probably caused by hydrate dissociations

thermochronology of isotopically zoned micas insights from the southwestern usa proterozoic orogen abstract we have used three different 40 ar 39 ar laser microprobe methods to explore the distribution of radiogenic 40 ar in 1 0 1 5 mm biotite crystals from the ca 1680 ma horse mountain monzogranite of central arizona incremental heating of two single crystals with a defocused laser beam produced flat age spectra with near plateau ages of 1190 ma showing no indication of intracrystalline 40 ar inhomogeneity in contrast total fusion of twenty five biotite fragments 100 pm yielded apparent ages ranging from 1006 7 to 1212 0 ma detailed age mapping in the 001 plane of two crystals with the laser focused to a minimum spot size confirms that the age dispersion in the fragment data reflects the existence of 200 m y age gradients in single crystals the two mapped crystals display very different age distribution patterns that suggest radiogenic 40 ar loss through two mechanisms volume diffusion on a scale comparable to that of the grain radius and more rapid diffusion along discrete zones of high crystal defect density simple inverse modeling of the overall age dispersion in the two mapped crystals and the fragment population is consistent with the development of the observed age gradients by slow cooling at an average rate of 0 5 k m y the horse mountain results as well as previously published studies indicate that conventional incremental heating of hydrous phases can homogenize intracrystalline gradients in 40 ar thereby masking important details of the thermal history of analyzed samples in contrast detailed isotopic mapping studies offer a wealth of information and will become more powerful with continued improvement in the spatial resolution of 40 ar 39 ar laser microprobes total fusion studies of crystal fragment populations can be readily automated making them less labor intensive than mapping studies our preliminary experiment on a limited horse mountain fragment population suggests that this procedure has great potential as a reconnaissance tool for thermal history research

direct measurement of ar diffusion profiles in a gem quality madagascar k feldspar using the ultra violet laser ablation microprobe uvlamp abstract controversy surrounding the mechanisms and controls on argon diffusion in k feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures over a range of temperatures measurements of argon diffusion profiles in a gem quality iron rich orthoclase heated in a cold seal apparatus have been undertaken in situ using an ultra violet laser ablation microprobe uvlamp technique the results agree very closely with the previously determined bulk values for benson mines orthoclase activation energy e 43 8 1 kcal mol 1 and vacuum furnace cycle heating studies of k feldspars e 46 6 kcal mol 1 however instead of defining a single activation energy e and diffusion coefficient do the data yield two sets of parameters a low temperature 550 720 c array with an e of 47 2 2 5 kcal mol 1 198 2 10 5 kj mol 1 and a do of 0 0374 0 1123 0 0281 cm2 s 1 and a high temperature 725 1019 c array with an e of 63 8 3 4 kcal mol 1 268 0 14 3 kj mol 1 and a do of 55 0 225 5 44 2 cm2 s 1 the new results closely reproduce two sets of apparent activation energies previously measured in cycle heating studies of madagascar k feldspar 40 3 and 57 3 kcal mol 1 previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion however the uv depth profile analyses indicate simple diffusion to the grain surface and importantly diffusion radii calculated by combining the uvlamp and cycle heating data are the same as the physical grain sizes used in the experiments around 1 mm vacuum furnace stepped heating experiments on slowly cooled k feldspars have been interpreted as showing diffusion radii of around 6 m and indicate complex populations of sub grains this study indicates that madagascar k feldspar and thus probably all gem quality k feldspars act as single diffusion domains and that short circuit or pipe diffusion was not an important loss mechanism an apparent diffusion compensation relationship in the stepped heating data for madagascar k feldspar implies that similar relationships seen in other k feldspars are a result of a range of diffusion mechanisms

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

two contrasting styles of interaction between basic magmas and continental crust in the british tertiary volcanic province some of the magmas that were extruded to form the 59 ma early plateau forming basalt dominated floods in the british tertiary volcanic province mixed with small amounts of sialic melt during their uprise through the continental crust while others underwent concomitant fractional crystallization and crustal assimilation magma batches of the first group originated in the uppermost asthenosphere and mostly ponded at the moho where they crystallized fractionally to form a basalt benmoreite suite the major element compositions of members of this series show that they underwent negligible further fractional crystallization between the moho and the surface nevertheless they paused long enough during their uprise to dissolve their high pressure phenocrysts and in many cases to mix with small amounts of acid melt from lower crustal granulite facies archaean lewisian leucogneisses basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in mull and skye their major element compositions show that they equilibrated within the upper third of the crust under conditions approximating to anhydrous 1 atm cotectic equilibria most of these lavas are in sw mull where proterozoic moine metasediments form the uppermost crust the major and trace element and isotopic compositions of the sw mull basal lavas show that they assimilated substantial amounts of moine metasediments progressively as they fractionated within the upper crust pb isotope data reveal that before their final upper crust evolution some of the sw mull basal lavas had already mixed with small amounts of lower crust acid melts

40ar 39ar laser probe dating by step heating and spot fusion of phengites from the dora maira nappe of the western alps italy 40ar 39ar laser probe dating of phengitic micas has been carried out by step heating and spot fusion procedures these micas represent successive deformation stages in the structural evolution of the internal dora maira nappe western alps single phengites from a gneiss affected by a single ductile strain under retrogressive conditions sample 99 1 display complete isotopic resetting with nearly homogeneous intracrystalline ar distribution and yield plateau ages of about 40 ma small clusters of phengites from an earlier foliation were selected from a polydeformed mica schist sample ptx3 they show a partial isotopic resetting in response to overprinting during the retrogressive deformation stage with a concentric age zoning from 68 ma on the rim to 87 ma in the core on one cleavage plane this zonation is fully consistent with the laser derived discordant age spectrum which ranges from 40 to 90 ma from low to high temperatures according to the deformation history of both samples these preliminary data suggest a deformation control on ar migration during recrystallization processes and they are consistent with the timing of the collisional evolution previously reported for southern dora maira units this study shows that the 40ar 39 ar continuous laser probe dating technique produces data accurate enough to discriminate several tectonometamorphic episodes recorded in single hand samples

distribution and origin of seismic chimneys associated with gas hydrate using 2d multi channel seismic reflection and well log data in the ulleung basin east sea abstract analysis of multi channel seismic reflection and well log data from the ulleung basin east sea reveals that two distinct types of seismic chimneys have been identified in the basin based on their geometries seismic reflection patterns well log response and lithology type i is a mound like feature with a transparent or chaotic reflection pattern well log interpretation and lithology of this type indicate that type i consists of mobilized homogeneous mud including fracture filling gas hydrate this type is mainly located on the structural highs and is mostly restricted to the pliocene sedimentary succession in contrast type ii is a pipe like feature with vertically stacked distorted reflectors based on well log data and lithology type ii filled with fracture filling gas hydrate without breaking up primary sedimentary structure this type appears along the deep seated fault and mainly developed within the quaternary deposit our primary conclusion is that type i resulted from the subsurface sedimentary remobilization in response to regional uplift in the pliocene whereas type ii resulted from the focused vertical fluid flow through fractures triggered by reactivation of deep seated fault during the quaternary

paleomagnetic estimates of temperatures reached in contact metamorphism a paleomagnetic study of some silurian devonian lavas from the lorne plateau scotland locally reheated by the intrusion of small tertiary dikes has shown that a laboratory equivalent of the actual peak temperature reached in the reheating can be accurately determined theoretically derived corrections are applied to the experimentally determined temperatures to obtain an estimate of the actual peak temperatures reached in a series of samples across the contact zones these corrected experimental profiles can be compared with peak temperature profiles calculated from heat conduction models the two profiles fit well when the temperatures of the lavas immediately prior to the intrusion are taken into account ambient temperature estimates of 75 25 c and 150 25 c have been obtained for the lavas before intrusion by a normally magnetized dike and a reversely magnetized dike respectively

flow localization in fissure eruptions during a basaltic fissure eruption heat transfer from the hot magma to the surrounding rock causes a dramatic increase in the magmatic viscosity and solidification at the margins both viscosity contrast and solidification can amplify initial variations in the flow rate and lead to localization of the flow along the strike of the fissure however for typical parameters amplification driven by solidification is slower and significantly weaker than amplification driven by viscosity variations in fact for the parameters examined the amplification due to solidification is so weak that its effect is almost insignificant whereas viscosity variation provides a strong active mechanism for flow localization laboratory experiments illustrate viscous localization and suggest that this mechanism is robust the dependence of viscosity on temperature can cause a small change in the pressure of the magma chamber to lead to a large jump in the flow rate of magma through the fissure

dating the cooling of exhumed central uplifts of impact structures by the u th he method a case study at manicouagan abstract forty titanite grain fragments from 9 central uplift samples of metamorphosed anorthosite from the manicouagan impact structure were dated by the u th he technique a u th he central age of 207 1 6 4 ma 2 standard error se n 40 has been determined with 4 outlier ages removed the central age is refined to 208 9 5 1 ma 2 se both of these ages are within error of the previously determined u pb zircon age of 214 1 ma 2 derived from the impact melt manicouagan s central uplift formed due to rapid elevation from 7 10 km depth as part of the modification stage of the impact process which has facilitated the dating of its emplacement due to resulting rapid exhumation and cooling and closure of the u th he system in titanite correlation with the previous u pb zircon 214 1 ma impact melt crystallization age indicates that the u th he titanite dating technique offers a new approach to dating complex impact structures in the absence of viable melt sheets or other melt products the youngest ca 195 ma u th he dates preserved in some titanite fragments are synchronous with early jurassic rift induced lithospheric thinning and associated igneous activity that defines the central atlantic magmatic province camp the u th he titanite data from manicouagan indicate that the influence of this regional event may extend west of the previously proposed limit of camp activity

the loch scridain xenolithic sill complex isle of mull scotland fractional crystallization assimilation magma mixing and crustal anatexis in subvolcanic conduits

40ar 39ar age gradients in micas from a high temperature low pressure metamorphic terrain evidence for very slow cooling and implications for the interpretation of age spectra estimates of the time temperature history of geologic samples are often based on 40 ar 39 ar ages obtained through incremental heating experiments on minerals with different ar retentivities laser incre mental heating of single crystals of muscovite and biotite from an 1700 ma monzogranite from a low pressure high temperature metamorphic terrain in central arizona suggested an essentially uniform distribution of radiogenic 40 ar in each crystal and yielded ages of 1412 5 ma for the muscovite and 1410 10 ma for the biotite the similarity in these ages implies rapid 25 k m y cooling of the monzogranite after a reheating episode slightly before 1412 ma in contrast detailed age mapping of single crystals from the same samples with the laser microprobe revealed large radiogenic 40 ar gradients indicative of very slow 400 m y evidence for such low cooling rates has important implications for the longevity of high thermal gradients in the early middle proterozoic lithosphere in addition inconsistency between the laser mapping and incremental heating results for these micas suggests caution when interpreting incremental heating data for slowly cooled samples

an 40 ar 39 ar investigation of the contact effects of a dyke intrusion kapuskasing structural zone ontario an 40ar 39ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14 m wide kapuskasing dyke in the chapleau block of the kapuskasing structural zone employed resistance furnace step heating of bulk mineral separates single grain laser step heating and laser spot dating of individual grains in thick section all three methods yielded concordant results for the biotite indicating that it has been uniformly overprinted at ca 2 05 ga this date is probably a close estimate of the age of the dyke intrusion the argon data from the hornblende are more difficult to interpret all three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment laser spot dates record an apparent age gradient extending radially outward from 2 25 ga in the grain cores to 2 4 2 7 ga in the grain rims corresponding 37ar 39ar values show no systematic variation across the hornblende although 38ar 39ar ratios display a similar pattern to the age distribution with lower values in the cores than in the rims the bulk separate and single grain step heating spectra mimic a volume diffusion ar loss profile that would appear to be inconsistent with the laser spot dates three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties an effective radius of diffusion which is much smaller than the actual grain radius or an effective radius of diffusion that is of the order of the grain radius it is clear that for the 40ar 39ar dating of samples that have experienced complex geologic histories it is important to utilize both resistance furnace and laser techniques

geological hydrodynamic and biogeochemical variability of a new zealand deep water methane cold seep during an integrated three year time series study abstract cold seeps are widely distributed on active and passive margins and display considerable temporal variability in terms of gas and fluid expulsion rates and volume over scales of hours to days to constrain this variability two cold seeps the north and south tower located at the wairarapa seep site on opouawe bank cook strait new zealand were monitored in an integrated geological and biogeochemical study during eight surveys between 2005 and 2008 to ensure sampling of the water column within the flare overlying the seeps the vessel was manoeuvred in response to the backscatter data from a hull mounted 38 khz single beam echo sounder high methane concentrations 100 nmol l 1 were found within 400 450 m high flares at both sites and within an 825 m high flare at a newly identified seep southeast of pahaua bank elevated bottom water methane was also found over newly identified mud volcanoes on the campbell bank cook strait the locations of the seeps appear to be structurally controlled with active methane vents occurring on anticlinal ridges and associated with a 30 m high normal fault scarp on opouawe bank fluid flow is facilitated by sedimentary layer permeability properties structural focusing and dilational fracturing on the crest of a large anticlinal ridge cored by a seaward verging reverse fault the south tower cold seep did not significantly influence bulk sediment and water column particulates and nutrients although high bacterial biomass productivity and methane oxidation rates were associated with elevated methane in the flare the seep flares exhibited depleted 13c ch4 values 63 to 70 characteristic of shallow biogenic gas sources incorporation of the 13c ch4 and ch4 concentration data in a mixing model and comparison of the water residence time with methane oxidation rates indicated that the primary fate of methane in the flare was dilution the dissolved methane originating from the seeps was retained in the water column at depths 700 m with minimal contribution to atmospheric emissions as confirmed by surface methane mapping nevertheless the south tower seep was a significant source of methane with a column integrated burden below 700 m of 5 6 mmol m 2 and an estimated annual emission of 0 5 1 106 mol ch4 the structural and tectonic setting of the wairarapa seep provides a useful analogue model for other convergent margin cold seeps on the hikurangi margin and elsewhere this unique three year integrated study provided valuable insights into the linkage between geological structure and overlying water column biogeochemistry and confirmed the value of this cold seep site for long term monitoring

three dimensional seismic investigations of the sevastopol mud volcano in correlation to gas fluid migration pathways and indications for gas hydrate occurrences in the sorokin trough black sea new 3 d seismic investigations carried out across the sevastopol mud volcano in the sorokin trough present 3 d seismic data of a mud volcano in the black sea for the first time the studies allow us to image the complex three dimensional morphology of a collapse structured mud volcano and to propose an evolution model the sevastopol mud volcano is located above a buried diapiric structure with two ridges and controlled by fluid migration along a deep fault system which developed during the growth of the diapirs in a compressional tectonic system overpressured fluids initiated an explosive eruption generating the collapse depression of the sevastopol mud volcano several cones were formed within the depression by subsequent quiet mud extrusions although gas hydrates have been recovered at various mud volcanoes in the sorokin trough no gas hydrates were sampled at the sevastopol mud volcano a bsr bottom simulating reflector is missing in the seismic data however high amplitude reflections bright spots observed above the diapiric ridge near the mud volcano at a relatively constant depth correspond to the approximate depth of the base of the gas hydrate stability zone bghsz thus we suggest that gas hydrates are present locally where gas fluid flow occurs related to mud volcanism i e above the diapir and close to the feeder channel of the mud volcano depth variations of the bright spots of up to 200 ms twt might be caused by temperature variations produced by variable fluid flow

a coral reef in the telkwa range british columbia the earliest jurassic example an end triassic mass extinction profoundly affected reef ecosystems that flourished in the late triassic tethys seaway the collapse of late triassic coral sponge reefs was followed by an early jurassic hettangian sinemurian perturbation interval with a near global absence of reefs and sharp reductions in diversity a jurassic sinemurian reef in the hazelton group of central british columbia appears to fill the gap its paleoecology and composition show it to be the first large framework example in north america and perhaps the world it demonstrates that the reef building triassic coral phacelostylophyllum survived the extinction event and was constructing reefs in early jurassic time during a global reef eclipse the reef is a 48 m thick bioherm that grew within the island arc complex of stikinia following a decrease in volcanism reef building began with bivalves growing upon water lain tuffs the reef was dominated by large dendroid phaceloid corals primarily phacelostylophyllum rugosum a spe

a margin wide bsr gas hydrate assessment canada s atlantic margin abstract it is the intent of this paper to explore a significant extent of an entire passive continental margin for hydrate occurrence to understand hydrate modes of occurrence preferred geologic settings and estimate potential volumes of methane the presence of gas hydrates offshore of eastern canada has long been inferred from estimated stability zone calculations but little physical evidence has been offered an extensive set of 2 d and 3 d single and multi channel seismic reflection data comprising in excess of 140 000 line km was analyzed bottom simulating reflections bsr were unequivocally identified at seven sites ranging between 250 and 445 m below the seafloor and in water depths of 620 2850 m the combined area of the bsrs is 9311 km 2 which comprises a small proportion of the entire theoretical stability zone along the canadian atlantic margin 715 165 km 2 the bsr within at least six of these sites lies in a sedimentary drift deposit or sediment wave field indicating the likelihood of grain sorting and potential porosity and permeability reservoir development although there are a variety of conditions required to generate and recognize a bsr one might assume that these sites offer the most potential for highest hydrate concentration and exploitation total hydrate in formation at the sites of recognized bsr s is estimated at 17 to 190 10 9 m 3 or 0 28 to 3 12 10 13 m 3 of methane gas at stp although it has been shown that hydrate can exist without a bsr the results from this regional study argue that conservative estimates of the global reserve of hydrate along continental margins are necessary

constraints on melting and magma production in the crust major intrusions of granitic rocks are found in several tectonic settings and in all cases crustal melts may contribute to the volumes of granitic magma high grade metamorphism and partial melting of the crust take place predominantly under fluid absent conditions we present a model for calculating the amounts of melt that may be formed by fluid absent breakdown of micas and amphiboles in common crustal rock types pelitic quartzofeldspathic intermediate and mafic melt proportions depend mainly on the kind of source rock the pressure at which melting takes place the temperature and the hydrous mineral h2o content of the source as a consequence of the pressure dependence of water solubility in silicate melts any given source rock will produce more melt by a given fluid absent reaction at lower pressure at a given pressure higher temperature reactions can produce more melt from a given source rock based on a survey of the compositions of common rock types we show that the amounts of melt can vary from 50vol thus crustal rocks vary widely in their fertility as magma sources depending on the types and amounts of hydrous phases they contain in general muscovite breakdown in pelites will yield only small quantities of melt and lead to migmatite formation biotite breakdown in pelites occurs at higher temperature and because most high grade pelites below granulite grade are biotite rich can yield up to about 50 vol of granitoid melt rocks of intermediate composition and hornblende rich mafic rocks are potentially highly fertile magma sources also provided that the high temperatures necessary for biotite and hornblende breakdown are realized pyroxene rich mafic rocks and quartzofeldspathic rocks are much less potentially fertile data suggest that mechanisms exist for the efficient segregation of melt and restite in systems with 30and probably 20vol melt the pressure temperature history of a region can greatly influence crustal source fertility through its control over the occurrence of subsolidus dehydration and melting equilibria

seismic indicators of gas hydrate and associated gas in the ulleung basin east sea japan sea and implications of heat flows derived from depths of the bottom simulating reflector abstract analysis of multi channel seismic reflection data from the ulleung basin east sea japan sea reveals various seismic indicators of gas hydrate and associated gas including the bottom simulating reflector bsr enhanced reflections below the bsr and seismic chimneys the recent recovery of massive gas hydrate by drilling together with these seismic indicators strongly suggests favorable conditions for the formation of gas hydrate in the area the bsr is most common and of a wide range of amplitude and continuity seismic chimneys characterized by columnar zones of amplitude reduction terminate mostly within the gas hydrate stability zone ghsz but a few extend to pockmarks or mounds on the seafloor seismic chimneys with seafloor expressions probably represent vertical vents for gas originating from below the ghsz the increase in the degree of amplitude reduction in seismic chimneys with increasing seismic frequency may be due to seismic attenuation by gas bubbles trapped in hydrate filled coated fractures in the ghsz bsr derived heat flows range from about 65 to over 115 mw m2 comparable to those from direct measurements heat flows in the northern part of the study area are very high for the age of the basin suggesting hotter than normal mantle temperature the area of high 105 mw m2 heat flows also coincides largely with the distribution of the presumed incipient oceanic crust formed during the earliest stage of seafloor spreading

determination of high spatial resolution argon isotope variations in metamorphic biotites the circumstances of preservation of relict metamorphic minerals and fabrics which have subsequently been involved in later metamorphic cycles remain enigmatic because the kinetic parameters are not well understood textures and chemistry cannot always give direct information on the conditions attained in the successive events we show in this study that 40ar39ar laser studies can in contrast provide thermal history determinations and successfully discriminate between geological models two samples have been examined from the seconda zona diorito kinzigitica iidk of the sesia zone italian alps a unit which preserves relict hercynian metamorphic assemblages and is now located structurally above rocks which preserve alpine eclogite and greenschist facies assemblages the circumstances of this preservation remain controversial with proposed models falling into two main categories one hypothesis advocates that the iidk was juxtaposed into its present position late in the alpine orogeny and experienced no significant heating event alternatively the iidk has been in its present position for the duration of the alpine orogenic cycle and has been exposed to high temperatures but due to sluggish reaction kinetics it failed to reequilibrate and develop alpine metamorphic minerals a high spatial resolution 40ar39ar study using the ultraviolet laser ablation microprobe uvlamp technique was undertaken with the aims of reassessing the metamorphic history of the iidk and investigating the behavior of argon within the unit during its thermal evolution 128 analyses have been obtained from ten profiles between 60 and 275 m long across biotite grains which show apparent ages ranging from 514 to 161 ma within individual crystals these apparent age variations are systematic according to location in the grain with the oldest ages at the grain edges and the youngest in the cores these apparent age variations can not be explained by chemical alteration and require diffusion of excess argon into the grains the age profiles show a remarkable fit to theoretically derived curves for volume diffusion of argon time integrated diffusion values obtained for the profiles fall between 2 10 6 cm 6 and 5 10 6 cm2 this variation is interpreted as being due to spatial and temporal fluctuations in the availability of argon from the grain boundary network diffusion and hence profile development initiated at different times within the biotites consequently producing the range of time integrated diffusion values the temperatures at which the profiles developed have been estimated using published biotite diffusion parameters to model possible thermal histories using the assumptions of either a short thermal pulse event or continuous cooling from high temperatures the models suggest that the profiles developed at temperatures less than 300 c possible geological models are proposed which document the thermal and excess argon history of the iidk from the hercynian through to the alpine orogeny the study shows the key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages

geochemical evolution of the tertiary mull volcano western scotland the early tertiary mull volcano western scotland is one of the most dissected and best exposed igneous complexes of the north atlantic province the new and published geochemical data enable us to chart the magmatic evolution of the mull volcano from the oldest lavas through the intrusive rocks of three overlapping igneous centres to the youngest dykes in this study we identify four successive magma types within the remnant volcano the earliest type the mull plateau group comprises mildly alkaline basaltic rocks with steep chondrite normalized rare earth element ree patterns this type is succeeded within the lava succession and dyke swarm by the coire gorm magma type with essentially flat chondrite normalized ree patterns a third magma type represented within the lava and dykes the central mull tholeiites is more depleted in incompatible trace elements than the preceding types and has flat to lree depleted chondrite normalized patterns the major intrusions and cone sheets of mull centre 1 and early centre 2 belong to this magma type midway through the igneous activity associated with centre 2 the magma type changed to become more alkalic and more enriched in incompatible trace elements this magma type the late mull type is found to persist through the cone sheets and major intrusions of centre 3 to the youngest dykes these changes in magma composition were related to variations in the mantle source and depth of partial melting beneath mull and or differences in the efficiency of melt pooling before ascent through the lithosphere with the exception of the early staffa magma sub type part of the mull plateau group the location of magma chambers in which the bulk of contamination occurred changed with time from deep lower crustal lewisian gneiss to shallow upper crustal moine schist intermediate members of the plateau group and the late mull magma type are enriched in fe ti and p relative to the central mull tholeiites we attribute this difference to the more alkalic nature of these suites lower fo2 and the formation of fe3 p complexes in the magma the intermediate rocks were important in magma mixing processes with two types of mixing identified on mull 1 cryptic mixing between basalts and low fe intermediate magmas typified by lavas and early basic cone sheets of the central mull tholeiite magma type 2 observable mingling between rhyolitic magmas and high fe intermediate magmas of the late mull type shown by the mixed magma bodies of the glen more and loch ba ring dykes the main factor in determining which type of mixing occurred appears to have been the density contrast between the various magmas it may safely be maintained that mull includes the most complicated igneous centre as yet accorded detailed examination anywhere in the world bailey et al 1924 memoir of the geological survey of great britain scotland hmso edinburgh 1924

laser probe40ar39ar measurements of loss profiles within individual hornblende grains from the giants range granite northern minnesota usa abstract a nd glass laser has been used as a probe mapping natural argon diffusion in partially outgassed hornblende grains at a resolution of around 50 m the hornblendes are from the giants range granite 2700 ma minnesota taken approximately 2 km from the contact with the duluth gabbro complex 1100 ma the full range of ages can be discerned within individual hornblende grains using the laser extraction technique but more importantly diffusion profiles up to 500 m in length have been detected the profiles reflect large variations in domain size within single grains diffusive argon loss was controlled not only by lattice dominated volume diffusion but a network of fast diffusion paths now defined by biotite inclusions and also indirectly by mineral reactions associated with the contact metamorphism three natural argon loss profiles from subgrains of different sizes yield good fits to theoretical volume diffusion loss curves and a mean time integrated diffusion parameter d t dt of 7 1 0 9 10 5 cm 2 this parameter constrains the possible heat loss models for the duluth gabbro

the difference between oceanic and continental tholeiites a fluid dynamic explanation continental tholeiites have higher sio2 k2o and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts these differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface although there is little doubt that most continental tholeiites have assimilated crustal material the lcoation and mechanism of assimilation remain uncertain longhi 1981 has argued that magmas derived directly from the mantle should crystallize little orthopyroxene the abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust since orthopyroxene is an early crystallizing phase in layered intrusions this assimilation must occur early before the magma enters the chamber assimilation can occur at the margins of the dykes which feed magma chambers depending on the nature of the flow if the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls if the flow is laminar a conductive profile develops at the margin and the flowing magma chills against the walls protecting them from thermal erosion the nature of flow in a dyke can be predicted from the reynolds number the criteria for turbulence reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and if this is the case assimilation of low melting point components in the walls of the dyke is inevitable it is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust

multiple episodes of fluid flow in the sw barents sea loppa high evidenced by gas flares pockmarks and gas hydrate accumulation abstract gas and gas hydrates have gained considerable attention in the last 20 yr due to their possible impact on global climate change from methane escalations our study area the sw barents sea is unique in its evolution due to the effect of glaciations which removed thick layers of sediment from the seabed unloading due to erosion and deglaciation resulted in opening of pre existing faults and creation of new ones facilitating fluid migration and eventually escape into the water from the subsurface expressions of hydrocarbon gas accumulation and fluid flow such as gas hydrates and pockmarks are widely distributed in the barents sea several gas flares some of them 200 m high in echograms occur along a segment of the ringvassoy loppa fault complex indicating open fractures and active fluid flow observation of gas flares along regional fault complexes outside pockmark regions indicates that present gas escape occurs along faults only the pockmark formation and gas flares belong to different episodes of fluid flow the pockmarks are absent outside soft sediment depocentres indicating that their existence is related to the presence of recording medium the thin sediment cover in pockmarks and their penetration down into glaciomarine sediments indicate that they formed after deposition of these sediments and that fluid escape was active to the very recent past methane hydrate stability zone mhsz modelling shows that after the last glacial maximum 18 000 20 000 14c years ago the mhsz has thinned from 600 m to zero in most parts of the sw barents sea the fluid expulsion probably happened after retreat of the glaciers causing release of methane from dissociation of methane hydrates through fluid escape processes which lasted until recently bottom simulating reflectors bsr observed are probably due to structure ii hydrates formed from gas containing higher order hydrocarbon components such as ethane propane etc along with methane the study area falls close to the new gigantic oil discoveries skrugard and havis and north of the snohvit hydrocarbon field

magma rheology variations in sheet intrusions of the ardnamurchan central complex scotland inferred from gabbro inclusion characteristics entrainment of xenoliths and their consequent assimilation are key processes in modifying the crystallization kinetics and magma dynamics of conduit systems here an integrated textural and mineral chemical study of the evolution of a suite of gabbroic inclusions within a set of sheet intrusions from the ardnamurchan central complex nw scotland is presented the key findings are as follows 1 the host magma sheets and inclusions are not cognate 2 there are microstructural and mineral chemical similarities between the gabbroic inclusions and the textures and mineralogy of the major hypersthene gabbro on ardnamurchan 3 orientations of magnetic fabrics within the host sheet groundmass and within the gabbroic inclusions are virtually identical field observations suggest that the inclusions were derived from the hypersthene gabbro and were entrained in a few laterally restricted magma segments that subsequently coalesced with inclusion free segments into continuous sheets using stokes law and adaptations thereof we calculate that the magma within the inclusion free segments behaved as a newtonian fluid with a potential settling velocity of50 028 m s in contrast the presence of gabbro inclusions probably modified the magma dynamics to bingham like behaviour we infer that this variation in the magma rheology of separate segments continued after coalescence and internally partitioned the magma sheet preventing lateral mixing and inclusion transport

seismic velocities from the yaquina forearc basin off peru evidence for free gas within the gas hydrate stability zone multichannel seismic mcs data from the yaquina forearc basin off peru reveal a complex distribution of gas and gas hydrate related reflections lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity amplitude and signal attenuation underneath are observed as well as the possible occurrence of paleo bottom simulating reflectors bsrs phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone ghsz to constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the mcs line from high resolution ocean bottom hydrophone recordings with two independent methods heat flux values estimated on the basis of the velocity depth functions increase with decreasing amplitude of the bsr and peak near chemoherms these results suggest a model of the yaquina basin where free gas is trapped under parts of the bsr and within the ghsz particularly under the seafloor and under an erosional unconformity the hypothesis of a paleo bsr that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values

a 40ar 39ar laser probe study of micas from the sesia zone italian alps implications for metamorphic and deformation histories two samples from the eclogite micaschist complex emc and the seconda zona diorito kinzigitica iidk of the sesia zone have been studied using a high spatial resolution laser probe 40ar 39ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories data from a single large phengite grain from the emc show a range of ages from mid jurassic to upper cretaceous these apparent age variations are spatially related to both location within the grain and to intragrain microstructure modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain parallel to the mica cleavage profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places the temperatures of the initiation of deformation and the possible time scales for the deformation can be calculated as a function of cooling rate all estimates suggest deformation at greenschist facies in accord with the observed retrograde mineral assemblage absolute temperature estimates for deformation vary by less than 22 c for different cooling rates of 10 and 30 c ma 1 but vary by 80 c with different estimates of diffusion parameters the duration of deformation was for at least 2 ma at 10 c ma 1 or 0 7 ma at 30 c ma 1 biotites from the iidk sample record a permian to upper cretaceous age range that correlates with grain size the smallest grain sizes yielding the youngest ages this relationship is best explained by a partial resetting of biotites during an alpine thermal event initiated not more than 70 ma ago modelling of these data suggest that the sample never exceeded 300 c during the alpine the profoundly different thermal histories of the two units the emc recrystallized at 550 c whilst the iidk remained below 300 c suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism

magma flow regimes in sills deduced from ar isotope systematics of host rocks the best place to seek evidence of the style of past magma flow through a conduit is in the country rock heat flow has been studied in country rock adjacent to two tertiary dolerite sills intruding the caledonian schists and quartzites on the isle of mull scotland radiogenic 40ar loss within mica grains in the thermal aureoles of the intrusions has been measured at high spatial resolution using the ultra violet laser ablation micro probe to discriminate between a history of prolonged magma flow a history of conductive cooling following laminar flow and instantaneous emplacement of the intrusions the 40ar 39ar mica data and thermal modeling suggest that a prolonged period of magma flow of 3 5 months resulted in extensive argon loss from the micas country rock melting and mineral breakdown adjacent to a 6 m sill these features were absent from the wall rocks of a smaller 2 7 m thick sill which exhibited even less argon loss than might have been predicted for an instantaneous intrusion if the heat loss from the 6 m sill observed in one locality had been repeated along its length it would have formed an important magma conduit to the mull volcano but dolerite is not a common flow composition on mull if on the other hand the heat loss from the sill varies along strike it constitutes strong evidence for channelling and heterogeneous flow within the sill

the character and amplitude of discontinuous bottom simulating reflections in marine seismic data abstract bottom simulating reflections bsrs identified in seismic data are well documented and are commonly interpreted to indicate the presence of gas hydrates along continental margins as well as to estimate regional volumes of gas hydrate a bsr is defined as a reflection that sub parallels the seafloor but is opposite in polarity and cross cuts dipping sedimentary strata bsrs form as a result of a strong negative acoustic impedance contrast bsrs however are a diverse seismic phenomena that manifest in strikingly contrasting ways in different geological settings and in different seismic data types we investigate the characteristics of bsrs using conventional and high resolution 2d and 3d seismic data sets in three locations the terrebonne and orca basins in the gulf of mexico and blake ridge on the us atlantic margin the acquisition geometry and frequency content of the seismic data significantly impact the resultant character of bsrs as observed with depth and amplitude maps of the bsrs furthermore our amplitude maps reinforce the concept that the bsr represents a zone over which the transition from hydrate to free gas occurs as opposed to the conventional model of the bsr occurring at a single interface our results show that a bsr can be mapped in three dimensions but it is not spatially continuous at least not at the basin scale rather a bsr manifests itself as a discontinuous or patchy reflection and only at local scales is it continuous we suggest the discontinuous nature of bsrs is the result of variable saturation and distribution of free gas and hydrate acquisition geometry and frequency content of the recorded seismic data the commonly accepted definition of a bsr should be broadened with careful consideration of these factors to represent the uppermost extent of enhanced amplitude at the shallowest occurrence of free gas trapped by overlying hydrate bearing sediments

geochemical transitions in the ancestral iceland plume evidence from the isle of mull tertiary volcano scotland the mull tertiary volcano lies on the eastern edge of the north atlantic igneous province and was active soon after the initiation of the iceland plume detailed sampling of the mull lava pile has allowed temporal changes in the mantle source to be determined a significant change at 700 m above sea level from n morb like to icelandic marks the first arrival beneath the region of mantle from the core of the ancestral plume the earlier basalts represent an outer plume sheath of heated upper mantle from the initiation of magmatism in mull it took 1 9 0 4 ma for this transition to occur

the morphologies and genesis of mega pockmarks near the xisha uplift south china sea pockmarks are normally regarded to be manifestations of fluids escape through the seabed kilometer wide depressions here called mega pockmarks occur as widespread features near the xisha uplift northern south china sea most of the pockmarks observed in this area are multi kilometers and much larger than normal pockmarks reported worldwide the maximum diameter observed is 3210 m and the maximum depth observed is 165 2 m based on multibeam data and 2d seismic data the pockmarks are circular elliptical and crescentic in plan view seismic profiles show that the genesis of pockmarks is related to fluid gas and or pore water escape according to the fluids pathways the pockmarks fall into four types 1 gas chimney related 2 depositional boundary related 3 gas chimney and inclined structure fault related 4 inclined structure related bottom currents are strong and complex in the study area the multibeam data and seismic profiles indicate that they may play an important role on extension maintenance and shaping of pockmarks the research of the study area is in its initial stage and the identification of these features as indicators of fluid flow is probably useful for hydrocarbon exploration c 2011 elsevier ltd all rights reserved

thermal diffusivity of igneous rocks at elevated pressure and temperature thermal diffusivity measurements of seven igneous rocks were made to temperatures of 400 sup 0 c and pressures of 200 mpa the measuring method was based on the concept of cylindrical symmetry and periodic heat pulses the seven rocks measured were westerly rhode island granite climax stock nevada quartz monzonite pomona washington basalt atikokan ontario canada granite creighton ontario canada gabbro east bull lake ontario canada gabbro and stripa sweden granite the diffusivity of all the rocks showed a positive linear dependence on inverse temperature and excluding the east bull lake gabbro showed a linear dependence on quartz content quartz content varied from 0 to 31 by volume diffusivity in all cases rose or remained steady with increasing confining pressure the pressure effect was strongest at lowest pressures and vanished by levels between 10 and 100 mpa depending on rock type the pressure effect measured as a percentage change in diffusivity is stronger in the four rocks of granite composition than in the three of basaltic composition our results agree well with existing thermal diffusivity measurements at atmospheric pressure

flow duration of a dike constrained by palaeomagnetic data summary we have conducted a palaeomagnetic study of a permian red sandstone layer intruded by a 70cm wide plio pleistocene basaltic dike with the ultimate goal of estimating the flow duration of the magma before its solidification 31 samples were collected at various distances from the dike sandstone interface and were thermally demagnetized in order to separate the overprinted pliopleistocene magnetization component from the primary permian one and to determine for each sample the unblocking temperature tub necessary to erase totally the overprint this temperature can be used to determine the maximum palaeotemperature reached at any distance from the contact provided the secondary magnetization is a p trm and the thermal fluctuation effects and actual cooling rates are taken into account examination of various magnetic parameters nrm intensity 15 days vrm intensity initial magnetic susceptibility and hysteresis curves allowed us to establish the nature of the overprinted magnetization and consequently to exclude samples carrying a crm overprint an apparent palaeotemperature profile and two corrected ones were thus deduced from the samples in which the dike emplacement produced no crystallization of new magnetic minerals these profiles were compared with the calculations of a new conductive thermal model which includes the basalt s latent heat of crystallization the flow duration of the dike the width of the circulating magma and the temperature dependence of the thermal conductivity k in the basalt and in the sandstone or alternatively convection simulated by an increase of the k values good fits with the experimental data are obtained for a flowing width consistent with field observations and when the latent heat is taken into account in the two following cases first when k is temperature dependent in the basalt and in the sandstone which implies a magma flow duration of 5 days second when a limited convection process is assumed in the sandstone only here a flowing time of only 1 day is required the corrected palaeotemperature profiles argue in favour of the second hypothesis suggesting that when a dike is emplaced heat transfer by fluids may play an important role in the surrounding rock

seismic detection of marine methane hydrate as offshore petroleum exploration and development move into deeper water industry must contend increasingly with gas hydrate a solid compound that binds water and a low molecular weight gas usually methane gas hydrate has been long studied in industry from an engineering viewpoint due to its tendency to clog gas pipelines however hydrate also occurs naturally wherever there are high pressures low temperatures and sufficient concentrations of gas and water these conditions prevail in two natural environments both of which are sites of active exploration permafrost regions and marine sediments on continental slopes in this article we discuss seismic detection of gas hydrate in marine sediments gas hydrate in deepwater sediments poses both new opportunities and new hazards an enormous quantity of natural gas likely far exceeding the global inventory of conventional fossil fuels is locked up worldwide in hydrates ex traction of this unconventional resource presents unique exploration engineering and economic challenges and several countries including the united states japan canada india and korea have initiated joint industry academic governmental programs to begin studying those challenges hydrates also constitute a potential drilling hazard because hydrates are only stable in a restricted range of pressure and temperature any activity that sufficiently raises temperature or lowers pressure could destabilize them releasing potentially large volumes of gas and decreasing the shear strength of the host sediments assessment of the opportunities and hazards associated with hydrates requires reliable methods of detecting hydrate and accurate maps of their distribution and concentration hydrate may occur only within the upper few hundred meters of deepwater sediment at any depth between the seafloor and the base of the stability zone which is controlled by local pressure and temperature hydrate is occasionally exposed at the seafloor where it can be detected either visually or acoustically by strong seismic reflection amplitudes or high backscatter

polarity stratigraphy and duration of the tertiary igneous activity of mull scotland previous work left unresolved whether the tertiary igneous activity had spanned a magnetic polarity succession r n r n r or if overprinting had occurred only r n r new work is reported that demonstrates that thermal overprinting has occurred in a narrow zone c 600 m wide bordering the outermost major intrusions of centre 2 criteria for the recognition of overprinting are examined the polarity sequence is now believed to be r n r in conjunction with radiometric dates which show that activity occurred from c 60 ma to c 57 ma our data show that the sequence corresponds to chrons 26r 26 25r of the polarity time scale of harland et al 1982 or 25r 25 and 24r of breggren et al 1985 upper and lower limits to the duration of the activity are respectively 4 67 ma and 0 67 ma or 4 07 ma and 0 60 ma a duration of about 3 5 ma is thought probable the magnetic evidence supports the view that the loch scridain sills correspond to the early acid and intermediate cone sheets of centre 1