high efficiency of natural lamellar remanent magnetisation in single grains of ilmeno hematite calculated using mossbauer spectroscopy rocks from large remanent magnetic anomalies on earth have been found to contain exsolved rhombohedral oxides which also have been proposed as earth analogues for the rocks creating the observed large remanent anomalies on mars theoretical considerations and previous case studies of natural rocks have shown that the natural magnetisation is carried by lamellar magnetism due to uncompensated moments at the interface between antiferromagnetic hematite and paramagnetic ilmenite here single grains 250 m of titanohematite with ferri ilmenite exsolution lamellae from mesoproterozoic metamorphic rock samples in southwest sweden and the adirondack mountains usa are studied using room temperature mossbauer spectroscopy to identify possible characteristic magnetic signatures of lamellar magnetism mossbauer spectra of synthetic samples of titanohematite fe0 95ti0 05o3 and fe0 9ti0 1o3 were collected for comparison and showed a dominant six line magnetic spectrum due to fe3 in titanohematite with a weak sextet due to fe2 fe3 charge transfer mossbauer spectra of the natural ilmeno hematite grains are similar to those for synthetic titanohematite but contain additionally two paramagnetic doublets corresponding to fe2 and fe3 in ilmenite however several grains also contain an additional weak broad magnetic component that we assign to iron in contact layers according to the lamellar magnetism model similar to the previous mossbauer results for natural hemo ilmenite there is no evidence for superparamagnetic behaviour of the nanoscale titanohematite lamellae contained within coarser ferri ilmenite lamellae and no evidence for single domain or superparamagnetic magnetite the compositions of titanohematite and ferri ilmenite in the individual ilmeno hematite grains calculated from the mossbauer area ratios show that compositions are closer to end member values than the compositions inferred from electron microprobe and transmission electron microscopy on the same grains consistent with observations that lamella thicknesses are as small as a few nm bulk compositions of ilmeno hematite grains calculated from the mossbauer data confirm that the swedish samples are significantly more ilmenite rich than the adirondack samples and agree with an estimation of bulk composition through point counting of electron backscatter images the mossbauer data allow a quantitative estimation of contact layer abundance which is used to determine the efficiency of lamellar natural remanent magnetisation acquisition because minerals with a high natural remanent magnetisation efficiency are expected to create larger remanent magnetic anomalies contact layer abundance determination by mossbauer spectroscopy provides a valuable new tool for mineral based magnetic anomaly interpretation on earth and other planetary bodies

the anaerobic oxidation of methane and sulfate reduction in sediments from gulf of mexico cold seeps we determined the geochemical characteristics of sediments and measured rates of the anaerobic oxidation of methane aom and sulfate reduction sr in samples collected near thermogenic structure ii gas hydrate mounds and in areas lacking hydrates along the continental slope in the gulf of mexico we used radiotracer c 14 and s 35 techniques to determine rates of aom and sr over depth in sediment cores abundant mats of white and orange beggiatoa spp were common in areas of active seepage and these sediments were enriched in hydrogen sulfide and methane in cores collected from areas without beggiatoa or hydrate concentrations of redox metabolites showed little variation over depth and these sites were inferred to be areas of low seepage integrated aom rates were low in beggiatoa free cores 0 05 mmol m 2 day 1 and averaged 2 8 4 6 mmol m 2 day 1 in seep cores that contained beggiatoa or gas hydrate integrated sr rates were also low in beggiatoa free cores 1 mmol m 2 day 1 and averaged 54 94 mmol m 2 day 1 in cores with beggiatoa or hydrate rates of sr generally exceeded rates of aom and the two processes were loosely coupled suggesting that the majority of sr at gulf of mexico hydrocarbon seep sites is likely fueled by the oxidation of other organic matter possibly other hydrocarbons and oil rather than by aom

mud volcano growth by radial expansion examples from onshore azerbaijan abstract we investigated the mode of growth of flat topped mud volcanoes through the study of three active edifices onshore azerbaijan they are the bozdag guzdek the ayazakhtarma and the akhtarma pashaly mud volcanoes the three edifices are up to 80 m high and 3 km wide and have eruptions with recurrent times from a few months to a few years surface changes during and between eruptions were documented by a combination of mapping from satellite pictures repeated direct observation over five years and structural analysis in addition resistivity profiles and microgravity measurements were used to decipher their subsurface geometries we interpret the flat topped character of the mud volcanoes as the result of rapid isostatic readjustment of a brittle surface crust

recent sedimentation and organic carbon burial in a shelf sea the north sea to obtain a carbon budget of the north sea recent sedimentation rates and organic carbon and nitrogen concentrations were determined on 27 box cores from the shallow north sea with a water depth 200 m and a surface area of 500 103 km2 the sedimentation rates in these box cores range from 5 to 35 cm 100 year 1 corg concentrations are 0 05 1 based on these numbers and published data the total amount of corg buried in the shallow north sea is calculated as 1 105 tons year 1 in large parts of the shallow north sea recent sedimentation is nearly absent reworking of relict deposits and partial mixing of these deposits with small amounts of recent fine grained sediments by biological and physical processes and human activities beam trawl fishery result in apparent sedimentation rates which are likely to exceed the real net sedimentation rates considerably the results of this study suggest that the majority of the organic matter is exported from the shelf over the shelf edge into the norwegian sea and into inner shelf deeps skagerrak and norwegian channel coarse grained shelf sediments are only of minor significance for the preservation of organic matter

pore water geochemistry in shallow sediments from the northeastern continental slope of the south china sea abstract the northeastern continental slope of the south china sea is one of the promising areas for gas hydrates which is characterized by its variable topography and complex tectonics in this paper two gravity piston cores were sampled from the shallow sediments in the vicinity of the region where various morphologies of gas hydrate was recovered during the second gas hydrate expedition of guangzhou marine geological survey conventionally the identification of gas hydrate was primarily dependent on the bottom simulating reflector bsr apart from bsr the anomalies in pore water geochemistry which are highly related to various microbially mediated processes and features of fluid from deep hydrocarbon reservoir are also helpful for the gas hydrate exploration in this study upward methane flux may be responsible for distinct characteristics of pore waters observed at the two sample sites a reactive transport model was accordingly established to compare contrastive fluid features and to estimate the upward methane flux since fluid migration is typically associated with complex tectonic settings of the study area it is proposed that heterogeneous distribution of gas hydrates along the continental margin should be well considered

magnetic phases in hemo ilmenite insight from low velocity and high field mossbauer spectroscopy 1 we have studied natural samples of hemo ilmenite ilmenite with hematite exsolution lamellae by use of mossbauer spectroscopy the samples are from the allard lake intrusion in quebec the stoichiometric compositions of the ilmenite host and the hematite lamellae are approximately fe2 0 84mg2 0 14ti4 0 98fe3 0 04o3 and fe2 0 17mg2 0 01ti4 0 18fe3 1 64o3 respectively from low velocity mossbauer measurements the fe2 and fe3 components of the fe2 substituted hematite and fe3 substituted ilmenite formed by incomplete exsolution of hematite and ilmenite have been suggested identified high field mossbauer measurements obtained at low temperature indicate the presence of a minor ferrimagnetic component

large pockmarks gas charged sediments and possible clay diapirs in the skagerrak abstract extensive patches of shallow gas charged sediments have previously been documented in the eastern skagerrak the strait between norway and denmark recently obtained high resolution side scan sonar and sub bottom profiler data presented here document several surface and sub surface features at the border and inside zones of such gas charged shallow sediments four examples are presented and discussed 1 a low profile 3 m deep pockmark and gas charged sediments 2 a low profile 6 m deep pockmark associated with a rugged sub surface reflector 3 large up to 24 m deep and 500 m wide pockmarks and 4 patches of exposed material from the rugged sub surface reflector from similarities with previously described features and also with features found in the kattegat further south it is concluded that the pockmarks are formed by gas eruption and seepage and that the rugged sub surface reflector could possibly represent the upper surface of a diapirically deformed unit of gas charged plastic clays

sea bed pockmarks related to fluid migration from mesozoic bedrock strata in the skagerrak offshore norway abstract in the deepest part of the skagerrak the sea floor morphology and the subsurface geology have been mapped from multibeam echo sounder data seismic data and bedrock cores an area of high pockmark density occurs along the shallowest part of a gentle ridge in the central norwegian trench where a middle jurassic sandstone unit subcrops below 30 40 m of late weichselian sediments in contrast few pockmarks occur at the seabed where lower and upper jurassic shales and clayey siltstones subcrop the concentration of pockmarks above the middle jurassic sandstone unit is thus attributed to seepage of gas or liquids migrating up dip from deeper parts of the sedimentary basin in the skagerrak more than 90 of the observed pockmarks occur above mesozoic sedimentary rocks while less than 10 occur in areas of palaeozoic metasediments or crystalline rocks a linear concentration of pockmarks is observed above a fault that terminates at the base of the quaternary succession and this is therefore attributed to seepage of gas or liquids along the fault plane the density of pockmarks is low in areas with thick successions of quaternary sediments indicating that the pockmarks are not related to seepage of biogenic gas from these deposits pockmarks in the skagerrak are thus caused by seepage of thermogenic gas and or other pore fluids from deeper sources and their distribution may help to improve our understanding of the fluid system and migration regime of this part of the north sea

patterns of folding and fold interference in oblique contraction of layered rocks of the inverted cobar basin australia abstract the inverted cobar basin within the lachlan fold belt of new south wales australia comprises a mid palaeozoic cover sequence originally deposited in a nnw trending basin the pattern of f1 folding in the layered cover rocks changes from east to west from tight well cleaved folds parallel to the nnw trending basin margin on the east to open poorly cleaved en echelon folds at about 35 to the margin further to the west the change in fold trend and strain intensity has been repeatedly ascribed to the differing behaviour of discrete zones decoupled across a north trending strike slip fault boundary new field data show that the changes in orientation and strain intensity of f1 structures are progressively developed that an abrupt boundary between discrete zones cannot be substantiated and that interpretations involving decoupled blocks are not supported by the evidence conversely the data require coherent behaviour across the basin such that the overall pattern of f1 folding must be explained by strain compatible processes this new interpretation of the f1 deformation pattern has been modelled and quantitatively analysed theoretical predictions of the orientation of structures in unlayered isotropic material undergoing oblique contraction are inapplicable to layered anisotropic material the style of deformation in layered material will reflect the interaction of the bulk strain pattern due to convergence together with the influence of the layering anisotropy the orientations of the finite strain axes inferred from the folding need not match those of the bulk deformation the amount of strain recorded by folding may be unrepresentative of that developed in the deformed tract oblique contraction at a range of convergence angles was simulated by models employing layers of wet tissue paper quantitative analysis of the strain patterns in this layered anisotropic material showed consistent departures from the theoretical predictions for isotropic material the orientations of the principal finite horizontal extension proximal to the margin yielded higher convergence angles than those which were imposed the orientations distal from the margin yielded substantially lower apparent convergence angles this is because the layering anisotropy results in tight folds dissipating the normal component of the oblique convergence vector close to the margin whereas more open structures further from the margin show orientations controlled by the progressively more dominant shear component of the vergence vector modelling of d1 the cobar basin shows that the f1 pattern is consistent with dextral oblique convergence at 60 to the eastern margin of the basin the deformation patterns in both the model and the cobar basin yield higher proximal and substantially lower distal apparent convergence angles this is as expected from theoretical considerations and quantitative analysis of oblique contraction over a range of convergence angles the rheological anisotropy of the cover sequence of the basin is replicated by that of the layered wet tissue paper wet tissue modelling of the superposition of the second period of deformation d2 on f1 demonstrates the way in which the tightness and orientation of early folds influence the type of fold interference pattern at the eastern margin of the cobar basin where d1 was most intense this resulted in major swings of the strike of bedding and cleavage and of the trend of f1 folds further west open basin and dome patterns developed where d1 was least intense principles developed in relation to the inversion of the cobar basin are equally applicable to other basins in which layered cover rocks have undergone inversion by oblique contraction many basins in the lachlan fold belt and in general would fall within this category

magnetic properties of anorthosites a forgotten source for planetary magnetic anomalies 1 anorthosites igneous rocks very rich in plagioclase rarely considered to be strongly magnetic are common on earth and the moon and inferred to be on other planets magnetic properties of anorthosites could be important in investigating associated mineral deposits and in studying magnetic anomalies especially on mars here we investigate three late proterozoic anorthosites in rogaland norway for magnetic and petrographic properties two of the anorthosites have large natural remanent magnetization nrm with intensities comparable to tertiary basalts susceptibility nrm and hysteresis properties provide information about the magnetic minerals present and their response to inducing fields microscopic observations show ubiquitous hemo ilmenite in the anorthosites whereas magnetite is common in the haland helleren but rare in the ana sira and egersund ogna bodies this study illustrates that anorthosites can be important sources of magnetic anomalies and can retain a remanent field over geologic time it also supports the recently described property of lamellar magnetization

anaerobic oxidation of methane and sulfate reduction along the chilean continental margin anaerobic oxidation of methane aom and sulfate reduction sr were investigated in sediments of the chilean upwelling region at three stations between 800 and 3000 m water depth major goals of this study were to quantify and evaluate rates of aom and sr in a coastal marine upwelling system with high organic input to analyze the impact of aom on the methane budget and to determine the contribution of aom to sr within the sulfate methane transition zone smt furthermore we investigated the formation of authigenic carbonates correlated with aom we determined the vertical distribution of aom and sr activity methane sulfate sulfide ph total chlorins and a variety of other geochemical parameters depth integrated rates of aom within the smt were between 7 and 1124 mmol m 2 a 1 effectively removing methane below the sediment water interface single measurements revealed aom peaks of 2 to 51 nmol cm 3 d 1 with highest rates at the shallowest station 800 m the methane turnover was higher than in other diffusive systems of similar ocean depth this higher turnover was most likely due to elevated organic matter input in this upwelling region offering significant amounts of substrates for methanogenesis sr within the smt was mostly fuelled by methane aom led to the formation of isotopically light dic 13c 24 6 vpdb and of distinct layers of authigenic carbonates 13c 14 6 vpdb

anaerobic methane oxidation and a deep h2s sink generate isotopically heavy sulfides in black sea sediments the main terminal processes of organic matter mineralization in anoxic black sea sediments underlying the sulfidic water column are sulfate reduction in the upper 2 4 m and methanogenesis below the sulfate zone the modern marine deposits comprise a ca 1 m deep layer of coccolith ooze and underlying sapropel below which sea water ions penetrate deep down into the limnic pleistocene deposits from 9000 years bp sulfate reduction rates have a subsurface maximum at the so42 ch4 transition where h2s reaches maximum concentration because of an excess of reactive iron in the deep limnic deposits most of the methane derived h2s is drawn downward to a sulfidization front where it reacts with fe iii and with fe2 diffusing up from below the h2s fe2 transition is marked by a black band of amorphous iron sulfide above which distinct horizons of greigite and pyrite formation occur the pore water gradients respond dynamically to environmental changes in the black sea with relatively short time constants of ca 500 yr for so42 and 10 yr for h2s whereas the fes in the black band has taken ca 3000 yr to accumulate the dual diffusion interfaces of so42 ch4 and h2s fe2 cause the trapping of isotopically heavy iron sulfide with 34s 15 to 33 at the sulfidization front a diffusion model for sulfur isotopes shows that the so42 diffusing downward into the so42 ch4 transition has an isotopic composition of 19 close to the 23 of h2s diffusing upward these isotopic compositions are however very different from the porewater so42 43 and h2s 15 at the same depth the model explains how methane driven sulfate reduction combined with a deep h2s sink leads to isotopically heavy pyrite in a sediment open to diffusion these results have general implications for the marine sulfur cycle and for the interpretation of sulfur isotopic data in modern sediments and in sedimentary rocks throughout earth s history

multidomain hematite a source of planetary magnetic anomalies thermoremanent magnetization trm in hematite is larger than trm in magnetite for grain sizes 10 m we show that hematite s weak spontaneous magnetization m s causes its strong trm since the self demagnetizing field h d opposing large domain wall displacements is proportional to m s in hematite h d is comparable to the earth s magnetic field but in magnetite h d is 1000 times larger as a result earth s field trm of md hematite 0 3 am 2 kg outweighs trm and induced magnetization of md magnetite 0 01 0 02 am 2 kg and rivals trm of single domain and psd magnetite as a source of magnetic anomalies on earth and perhaps on mars

the relationship between exsolution and magnetic properties in hemo ilmenite insights from mossbauer spectroscopy with implications for planetary magnetic anomalies 1 remanence properties of ilmenites with exsolved hematite have recently been the object of much study because their high natural remanent magnetization nrm is greater than predicted for these two minerals x ray diffraction xrd and mossbauer spectroscopy in conjunction with electron microprobe and nrm measurements were used to explore the possibility that single domain sd hematite lamellae in the ilmenite host could explain the observed remanence and to determine if magnetite is present xrd results show the presence of ilmenite and hematite mossbauer data show that only species assigned to hematite ilmenite and a small amount of tetrahedral fe3 are present magnetic properties at high and low t also indicate that the only magnetic minerals are ilmenite and hematite magnetic data suggest that ultra fine hematite lamellae are magnetically ordered and their resultant remanent magnetic anomalies may contribute significantly to magnetism on terrestrial planets even those without present day magnetic fields

lamellar magnetism and exchange bias in billion year old titanohematite with nanoscale ilmenite exsolution lamellae i mineral and magnetic characterization

control of sulfate pore water profiles by sedimentary events and the significance of anaerobic oxidation of methane for the burial of sulfur in marine sediments abstract gravity driven mass flow deposits proven by sedimentary and digital echosounder data are indicative for prevailing dynamic sedimentary conditions along the continental margin of the western argentine basin in this study we present geochemical data from a total of 23 gravity cores pore water so 4 is generally depleted within a few meters below the sediment surface by anaerobic oxidation of methane aom the different shapes of so 4 profiles concave kink and s type can be consistently explained by sedimentary slides possibly in combination with changes in the ch 4 flux from below thus mostly representing transient pore water conditions since slides may keep their original sedimentary signature a combined analysis and numerical modeling of geochemical physical properties and hydro acoustic data could be applied in order to reconstruct the sedimentary history we present first order estimates of the dating of sedimentary events for an area where conventional stratigraphic methods failed to this day the results of the investigated sites suggest that present day conditions are the result of events that occurred decades to thousands of years ago and promote a persisting mass transport from the shelf into the deep sea depositing high amounts of reactive compounds the high abundance of reactive iron phases in this region maintains low hydrogen sulfide levels in the sediments by a nearly quantitative precipitation of all reduced sulfate by aom for the total region we estimate a so 4 or ch 4 flux of 6 6 10 10 moles per year into the zone of aom projected to the global continental slope and rise area this may sum up to about 2 6 10 12 moles per year provided that the sulfur is completely fixed in the sediments it is about twice the global value of the recent global sulfur burial in marine sediments of 1 2 10 12 moles per year as previously estimated thus aom obviously contributes very significantly to the regulation of global sulfur reservoirs which is hitherto not sufficiently recognized this finding may have implications for global geochemical models as sulfur burial is an important control factor in the development of atmospheric oxygen levels over time

the acoustic turbid layer in muddy sediments of eckernfoerde bay western baltic methane concentration saturation and bubble characteristics in eckernfoerde bay in the western baltic an enclosed basin with organic rich mud acoustic turbidity has been observed since 1952 to investigate the relationships between acoustic properties and free gas we developed a method for rapid sub sampling of gravity cores to determine the total methane content from temperature measurements determinations of the salinity of the pore water and data from the literature saturation limits were calculated for each core it is demonstrated that there is a certain amount of free gas in the mud of eckernfoerde bay but subbottom depths of the oversaturation and of the acoustic turbidity do not correspond the existence of free gas implies the existence of bubbles in the sediment which are regarded to be responsible for the acoustic turbidity bubble sizes observed by x ray computed tomography while still under in situ pressure and temperature vary between 1 and 10 mm equivalent diameter the distribution of bubbles is discontinuous occurring in depth zones from 2 to 20 cm thickness

marine pore water sulfate profiles indicate in situ methane flux from underlying gas hydrate marine pore water sulfate profiles measured in piston cores are used to estimate methane flux toward the sea floor and to detect anomalous methane gradients within sediments overlying a major gas hydrate deposit at the carolina rise and blake ridge u s atlantic continental margin here sulfate gradients are linear implying that sulfate depletion is driven by methane flux from below rather than by the flux of sedimentary organic matter from above thus these linear sulfate gradients can be used to quantify and assess in situ methane flux which is a function of the methane inventory below

diagenetic relationships of methanogenesis nutrients acoustic turbidity pockmarks and freshwater seepages in eckernforde bay abstract high organic loading 4 5 wt and sedimentation rates 1 4 mm yr 1 in eckernforde bay sediments lead to anaerobic conditions within the uppermost 15 cm intense bacterial sulphate reduction 0 011 0 15 mm so 4 red 2 yr 1 exhausts dissolved sulphate around 150 cm sediment depth resulting in methanogenesis at greater sediment depth by carbonate reduction 1 8 8 5 m ch 4 yr 1 extensive regions of eckernforde bay are characterized by acoustically turbid sediments 300 cm sediment depth resulting from deeper sediments supersaturated in methane forming free gas accumulation zones methane migrating upward into the sulphate reduction zone is effectively consumed by sulphate reducing bacteria at rates 3 10 times greater than methanogenesis i e 46 95 m ch 4 ox yr 1 the pathways of methane formation and anaerobic oxidation are confirmed by stable carbon and hydrogen isotope evidence pockmarks i e shallow surface depressions in some eckernforde bay sediments are not associated with gas ebullition rather these physical features result from the expulsion of freshwater these episodic springs or seepages of groundwater from the underlying holocene glacial lags and sands into the basin at pockmark sites have characteristic low chloride and methane concentrations the geochemical evidence indicates that the commercial accumulation of petroleum in the lower cretaceous dogger beta hauptsandstein schwedeneck field in eckernforde bay 1500 mbsf has no surface manifestation and does not influence either the occurrence of acoustic turbidity or pockmarks

a closer look at remanence dominated aeromagnetic anomalies rock magnetic properties and magnetic mineralogy of the russell belt microcline sillimanite gneiss northwest adirondack mountains new york a large distinct negative aeromagnetic anomaly of over 2000 nt associated with microcline sillimanite quartz gneisses in the russell area northwest adirondack mountains was previously shown to be remanence dominated although the carriers of remanence were not well documented russell belt gneisses have a strong natural remanent magnetization with steep remanence directions d 263 i 58 an average intensity of 3 6 a m and typical susceptibilities of 10 4 si the remanence is thermochemical in origin acquired during cooling from peak metamorphic conditions of 650 750 c during the ottawan orogen 1050 1080 ma the reversed polarity of remanence reflects a reversed paleofield rather than self reversed contrary to earlier suggestions the gneisses contain up to 3 oxide predominantly metamorphic titanohematite which accounts for the low susceptibility values and highly stable remanence optical observations show titanohematite grains with multiple generations of ilmenite pyrophanite rutile and spinel exsolution lamellae microprobe analyses confirm titanohematite compositions ranging from 72 to 97 fe2o3 with hematite83 being most typical in rare samples inclusions of magnetite were identified the ubiquitous presence of titanohematite and the rare occurrence of magnetite is supported by thermal and alternating field demagnetization studies saturation magnetization measurements hysteresis properties temperature hysteresis studies and low temperature remanence measurements numerous crustal granulites have titanohematite as part of the oxide assemblage and this may contribute a strong remanent component to what have previously been considered to be solely induced anomalies

nonlinear preisach maps detecting and characterizing separate remanent magnetic fractions in complex natural samples natural remanent magnetization carriers in rocks can contain mixtures of magnetic minerals that interact in complex ways and are challenging to characterize by current measurement techniques here a non linear mapping scheme is described that efficiently enhances sensitivity and the resolution power of remanent preisach maps using this scheme a large dynamic range of magnetic moments and coercivities can be reliably resolved the method is applied to synthetic and natural standard samples containing magnetite and hematite as well as to natural samples from remanent magnetic anomalies where complex microstructures are observed it is shown that certain offset high coercivity patterns in remanent preisach maps may serve as fingerprints for exsolution structures of ilmenite in hematite or hematite in ilmenite and that in some magnetite bearing remanent anomalies the magnetite coercivity is increased beyond its intrinsic coercivity range experimental results and theoretical considerations indicate a minimal coercivity of about 10 mt for sd magnetite such that observation of lower coercivities implies psd or md grain sizes a diagnostic hematite pattern with a peak downwards offset of 17 2 of the intrinsic coercivity is found that is stable over a large range of intrinsic coercivities and may be related to shielding of internal defect or lamellar moments by a spin canting response to the internal field

strain partitioning in transpression zones abstract transprcssional strain acting upon structurally anisotropic rocks can be partitioned into separate deformational domains of pure shear and simple shear this contrasts with homogeneous transpression in which both the pure shear and the simple shear strain components are uniformly distributed across the zone of deformation structural weaknesses capable of partially or fully accommodating one component of deformation include lithological contacts rheological heterogeneities and faults or shear zones situated within the deformation zone or lying along its boundaries partitioning of transprcssional strain can occur when stress is applied oblique to pre existing structural weaknesses or can occur during later stages of progressive strain when the early deformation of isotropic rocks imparts sufficient anisotropy to allow subsequent strain to be partitioned partitioning of transpressional strain into domains lying parallel to the deformation zone boundaries can be distinguished from fault stepped transpression in which strain is partitioned along the length of a segmented fault zone mesofracture analysis of rocks affected by mid devonian deformation on both sides of the highland boundary fault zone hbfz in central scotland shows that strain was not homogeneous the mesofracture data suggest that regional north south compression orientated oblique to the pre existing ne sw trending hbfz was partitioned into separate deformational domains the hbfz accommodated most of the simple shear component whilst the rocks flanking the zone were deformed predominantly by pure shear a contemporary example is the san andreas fault in central california where analyses of neo tectonic stresses show that the direction of principal compression is perpendicular to the fault zone many examples of the partitioning of transpressional strain have also been recognised at destructive plate margins where the direction of plate motion is oblique to the edge of the over riding plate

experimental kinematic analysis of en echelon structures in relation to the cobar basin lachlan fold belt abstract en echelon arrays of structures are common in zones of oblique deformation such as transtension and transpression in such zones the incremental and finite strain axes are not orthogonal to the boundary displacement direction clay modelling indicates that en echelon extensional faults in the cobar basin of the lachlan fold belt formed due to sinistral divergent boundary displacement at approximately 25 to the basin margin modelling with wet tissue paper indicates that the complex pattern of en echelon and basin parallel f 1 folds formed due to dextral convergent boundary displacement at approximately 60 to the basin margin the model also explains the sharp transition from high to low strain away from the eastern margin of the basin

partitioned transtension an alternative to basin inversion models inversion structures e g folds reverse faults spatially associated with basin bounding faults are very widely recognised in rift basins in both onshore and offshore settings worldwide the great majority of such structures are attributed to local or regional crustal shortening events there is however an alternative which is investigated in this paper inversion could reflect a horizontal shortening component of deformation formed during progressive and partitioned transtension a case study from the carboniferous northumberland basin shows that shortening structures can also form in obliquely divergent rifts if the bulk strain undergoes kinematic partitioning into distinct regions of wrench and extension dominated transtension such strain partitioning appears to be particularly favoured in basins where fault localisation is strongly influenced by pre existing basement structures this may occur because the pre existing anisotropies are zones of long lived weakness that lie in an orientation particularly favourable to the preferential accommodation of either strike slip or dip slip displacements our strain analysis applied to the northumberland basin traditionally considered as a classic example of a variscan inverted basin reduces the deformation history to a single kinematically partitioned phase of dextral transtension during the late carboniferous early permian our findings have profound implications for the interpretation of inversion structures in any rift basin where the direction of extension may be significantly oblique to the basin margins

influence of p t conditions on the style of normal fault initiation and growth in limestones from the se basin france abstract a conventional and cathodoluminescence microscopy stable isotope and fluid inclusion study was undertaken on normal fault zones exposed on vertical cliffs in tithonian limestones of the french se basin the aim was to determine the chronology of deformation and factors controlling normal fault nucleation and evolution in carbonate rocks two models are proposed for fault zone development depending on the depths at which deformation takes place at depths of 3 3 5 km incipient shear zones consist of en echelon veins filled by fibrous calcite and confined to limestone layers bounded by thin clayey interlayers as deformation increases and the layers rotate slip occurs along the clay rich interlayers a system of pull aparts forms allowing displacement to increase pull aparts are commonly filled by fibrous calcite crystals at depths of 1 5 2 km faults initiate as small slip surfaces early nucleating in some limestone layers their propagation is impeded by thin clayey interlayers which enhances fracturing and bending of the limestone layers as slip surfaces propagate they overlap and can connect with one another by means of dilational jogs that are filled by large euhedral calcite crystals in the first case pull aparts are coupled with the solution seams that reuse preexisting rotated clay rich interlayers as they formed they progressively filled and the fluid circulation remained local in the second case longer slip planes connect by dilation jogs that are not immediately filled which enhance the circulation of fluids which show chemical variations it appears that p t conditions strongly control the relative role of brittle processes and solution crystallization mechanisms and the style of the early stages of normal faulting in the same limestone rocks

shear thickening dilatancy in crystal rich flows abstract rheological models of molten rock are mostly based on behaviours in which viscosity is constant with respect to strain rate e g newtonian bingham plastic or viscosity decreases with increasing strain rate pseudo plastic or shear thinning rheological models in which viscosity increases with increasing strain rate shear thickening or dilatant models have rarely been used yet such behaviour is common in particulate systems whereas magmas with low crystal content may be appropriately modelled by the former behaviours greater crystal content may lead to particle interactions resulting in shear thickening behaviour the effect is illustrated using a powdered rhyolitic rock mixed with water which exhibits extreme shear thickening behaviour caused by particles moving apart as they move past each other the expansion of the grain fabric during flow is opposed by the applied loads such that dilatancy is invariably associated with shear thickening rheology crystal rich lava may exhibit shear thickening dilatancy a proposition supported by observations of glass rich microscopic shear zones disrupting crystal fabrics plutonic crystal mush may also exhibit shear thickening dilatancy which can explain synplutonic intrusion involving brittle fracture followed by ductile disruption in general strain rate is an important contributor to the transition from ductile to brittle behaviour in crystal rich igneous rocks

methane fluxes in marine sediments quantified through core analyses and seismo acoustic mapping bornholm basin baltic sea abstract methane is a terminal product of anaerobic degradation of organic matter in subsurface marine sediments depleted of reactive oxidants the depth and age of the sediment where sulfate is depleted determines the extent of methane production relative to the burial of organic carbon we aimed to understand how this methane production is controlled and distributed in an apparently uniform sediment basin we combined seismo acoustic surveys and geochemical analyses of sediment cores to explore the distribution of methane fluxes in brackish marine mud deposits of the bornholm basin southern baltic sea geophysical mapping revealed the depth distribution of a the thickness of the holocene organic rich mud layer overlaying organic poor postglacial clay and b the upper boundary of methane gas bubbles trapped in the holocene sediment by correlating these two parameters with the methane distributions in sediment cores from 44 stations we developed algorithms to estimate and map at high spatial resolution the diffusive methane fluxes in the 75 95 m deep and 4000 km2 large bornholm basin the two approaches termed the fgd free gas depth model and the hml holocene mud layer model yielded similar budgets for the total upwards methane flux through the sediment column in the bornholm basin about 17 ton methane c day 1 complete sulfate depletion at depth and thus onset of methane production required a minimum threshold thickness of the holocene mud layer of 4 m although the bornholm basin has an even depth contour and uniform surface sediments methane production was strongly focused in hotspots where the hml has greatest thickness this heterogeneity could not be predicted from bathymetry or from properties of the surface sediment but was related to the topography of the glacial landscape buried underneath the holocene mud blanket this demonstrates the importance of including seismo acoustic mapping of subsurface stratigraphy for the interpretation and geographic extrapolation of sediment core data for biogeochemical processes such as methane production and methane flux the two approaches which were here combined for the first time may thereby be applied to map methane production also in other coastal and shelf sediments with shallow gas and distinct holocene deposits

simple constructions for deformation in transpression transtension zones abstract a simple geometrical construction is demonstrated for determining the orientations of the principal axes of the infinitesimal strain ellipsoid in tectonic zones where boundaries obliquely converge or diverge the construction may also be used to determine the relative magnitudes of the principal axes and the ellipsoid shape for infinitesimal and finite transpressional transtensional strains the analysis leads to a tightening of the definitions of the loosely used terms compression wrench and extension applications of the simple construction and convenient graphical solution are illustrated by brief examples

sulfate reduction in surface sediments of the southeast atlantic continental margin between 15 38 s and 27 57 s angola and namibia sulfate reduction rates in the surface sediments from 17 stations from an along slope transect 1 300 m and from a cross slope transect 855 4 766 m were determined in the continental margin sediments of the benguela upwelling system profiles at all sites in the upwelling area showed increasing sulfate reduction rates from near zero at the surface to a peak at 2 5 cm up to 29 nmol cm 23 d 21 and then decreasing exponentially with depth to near background rates at 10 20 cm depth 2 nmol cm 23 d 21 depth integrated sulfate reduction rates were greatest at 1 300 m and decreased exponentially with water depth along the transect following the 1 300 m isobath depthintegrated sulfate reduction rates were highest in the north cape basin 1 16 6 0 23 mmol m 22 d 21 decreased over the walvis ridge 0 67 6 0 02 mmol m 22 d 21 and were lowest in the south angola basin 0 31 6 0 23 mmol m 22 d 21 depth integrated sulfate reduction rates were consistent with the known pattern of coastal upwelling intensities and were also strongly correlated with surface organic carbon concentrations sulfate reduction rates both as a function of depth and in comparison with sediment trap data indicated that lateral downslope transport of organic carbon occurs sulfate reduction was estimated to account for 20 90 of the published rates of total oxygen consumption for the sediments at 1 300 m depth and 3 16 of sediments from 2 000 to 3 000 m depth comparison of the sulfate reduction rate profiles with the published diffusive oxygen uptake rates showed that the kinetics of oxygen utilization in the surface sediments are much faster than those for anaerobic organic carbon remineralization although the underlying cause of the difference was not clear

analysis of spatial order in sandstones ii grain clusters packing flaws and the small scale structure of sandstones abstract sandstone microstructure at scales of 5 5000 m is a domainal structure consisting of juxtaposed clusters of efficiently packed grains bounded by packing flaws the flaws represent zones of loose packing arising from grain to grain mismatches between well packed clusters flaws tend to propagate throughout the sediment and are marked in thin section by zones of oversized pores known to be associated with enhanced fluid flow using a two dimensional fourier transform it is possible to identify these flawed zones and examine not only their spatial distribution but also the relative contribution of close packed and flawed porosity to the pore network as a whole flaws represent zones of disorder formed through a variety of depositional and postdepositional processes two flaw producing mechanisms have been recognized in random aggregates of spheres created in a laboratory the first arises from grain to grain mismatches at the boundaries of concurrently growing grain clusters the second type is associated with the growth of well packed clusters over a preexisting cohesive substrate in the laboratory this is represented by the walls of the container holding the aggregate the distribution of loose packed and well packed grain domains in natural sands is governed by depositional environment the size and shape of the constituent grains define the fundamental spatial frequency of porosity but factors such as grain orientation and size sorti g affect the orientation of clusters and flaws as well as the scale at which they exist for example flaws in poorly sorted sands arise from the misfit of the finer size classes in a grain matrix controlled by the larger grains the flaws generated by this misfit propagate throughout the fine grained sections of the sediment with a spatial period characteristic of the coarser size classes grain clusters and flaws can be recognized as major textural elements in all sands and sandstones even those with macroporosity as low as 5 not only is the depositional signal preserved as porosity decreases its existence affords the opportunity to closely examine the effects of diagenesis upon the pore network as porosity decreases the relative proportion of flawed porosity increases by the time total porosity has been reduced to 5 flawed porosity has increased to the point that it constitutes 80 of the remainder the increase in relative proportion of loose packed porosity is due to preferential cementation within well packed clusters as well us an increase in loose packed porosity due to dissolution of unstable mineral grains in all the diagenetically altered sandston s examined the only grain molds or partially dissolved grains present were found to be located either within or immediately adjacent to packing flaws

the obliquely convergent plate boundary in the south island of new zealand implications for ancient collision zones abstract the alpine fault of new zealand forms the western boundary of a zone of distributed deformation formed by the oblique convergence of continental crust belonging to the pacific and australian plates structural and geodetic data from the alpine fault show that a large proportion of the total plate displacement is accommodated by rapid oblique slip on the fault the remaining displacement is distributed over a 200 km wide zone to the east the collision may be modelled as a two sided deforming orogen with the partitioning of deformation being controlled by erosional differences between the narrow high strain inboard and broad low strain outboard sides in ancient collision zones little evidence may remain of the nature and amount of displacement on the inboard side partitioning of deformation among pre existing structures complicates interpretation of the outboard zone radiometric ages may post date collision by several tens of millions of years and indicate slow isostatic uplift and unroofing collision ages if preserved may be recognized by high uplift rates calculated from muscovite biotite pairs

pathways of organic carbon oxidation in three continental margin sediments we have combined several different methodologies to quantify rates of organic carbon mineralization by the various electron acceptors in sediments from the coast of denmark and norway rates of nh4 and sigma co2 liberation sediment incubations were used with o2 penetration depths to conclude that o2 respiration accounted for only between 3 6 17 4 of the total organic carbon oxidation dentrification was limited to a narrow zone just below the depth of o2 penetration and was not a major carbon oxidation pathway the processes of fe reduction mn reduction and sulfate reduction dominated organic carbon mineralization but their relative significance varied depending on the sediment where high concentrations of mn oxide were found 3 4 wt mn only mn reduction occurred with lower mn oxide concentrations more typical of coastal sediments fe reduction and sulfate reduction were most important and of a similar magnitude overall most of the measured o2 flux into the sediment was used to oxidized reduced inorganic species and not organic carbon we suspect that the importance of o2 respiration in many coastal sediments has been overestimated whereas metal oxide reduction both fe and mn reduction has probably been well underestimated

what is magnetic in the lower crust long wavelength and satellite magnetic anomalies require that some regions of the earth s crust contain minerals that are magnetic at lower crustal conditions studies of high grade metamorphic rocks emphased multi domain magnetite as a likely source members of the hematite ilmenite series were not considered because hematite commonly has a relatively weak magnetization and though ilmenite compositions in the range ilm 50 70 can be strong ferrimagnets their low curie t 260 c makes them an unlikely source however recent investigations have described hematite ilmenite magnetism and coercivity in terms of a new ferrimagnetic substructure related to interfaces between ilmenite and hematite in exsolution lamellae down to unit cell thickness in this lamellar magnetism ferrimagnetism is coupled to principal af moments of hematite and retains many properties of titanohematite including high coercivity and thermal stability if exsolution lamellae are the source of the magnetism a temperature above that predicted for lamellar resorption should destroy it to look at behaviour of lamellae at mid to lower crustal conditions we selected a hemo ilmenite sample ilm 84 on which detailed studies of microstructures mineral chemistry and magnetic properties had been made we designed experiments at 10 kbar as an analog for lower crustal conditions after initial 5 day runs at 540 685 and 950 c a temperature of 580 c was chosen for 8 and 21 day runs at one atmosphere this t for ilm 84 is above that predicted for complete resorption of all lamellae to produce a homogeneous paramagnetic r3 ilmenite in these experiments exsolution lamellae coarsened if these features were reproduced in the lower crust hemo ilmenite or ilmeno hematite would retain a magnetic moment and higher than expected coercivity due to the microstructures emp and tem analyses were used to understand the compositional differences between the natural sample products of the experiments and one atmosphere phase diagrams if the experimental changes are interpreted as moving the compositions closer to equilibrium at 580 c 10 kbar then a considerable widening of miscibility gaps as a function of pressure and the 20 geikielite component in the ilmenite as well as an increase in neel temperature of hematite is predicted an alternate is to consider the response as ostwald steps short of equilibrium experimental results indicate that exsolution microstructures play a key role in mineral magnetism and that such microstructures as well as coarse hematite may be preserved at pressures and temperatures of the lower crust

restoration of transpression transtension by generating the three dimensional segmented helical loci of deformed lines across structure contour maps abstract in transpression transtension zones the strain is three dimensional and rotational this causes material to move through the plane of cross section often invalidating balancing and restoration within this plane methods are presented which allow the three dimensional segmented irregular helical locus of an originally straight line to be constructed in any direction on a structure contour map of a folded and faulted surface this construction depends on a knowledge of the kinematics of folding and faulting and can be modified to suit local conditions the ratio of the length of the cylindrical envelope bounding this helical locus to the sum of the lengths of the helical fragments between faults gives the true stretch in the direction of the envelope when the traces of the segmented helices are constructed in different directions on a deformed surface the sectional finite strain ellipse can be found for that surface knowledge of the dimensions of this ellipse and its orientation with respect to the kinematic axes of the transpression zone allows the tensor components to be constrained this permits the three dimensional boundary conditions to be determined and thus restored the methods are applied to the ardross fault zone in central scotland the solutions suggest this fault zone underwent a phase of dextral transpression along a nw zone boundary during hercynian e w compression in the scottish midland valley contemporaneous e w dyke swarms and n s regional flexures support these kinematics

magnetic properties of ultrahigh pressure eclogites controlled by retrograde metamorphism a case study from the zk703 drillhole in donghai eastern china parallel studies on rock magnetic properties petrology and mineralogy were conducted on 16 eclogite samples from the zk703 hole and magnetic susceptibilities and densities of 41 eclogite samples with different degree of retrograde metamorphism from fresh eclogite to fully retrograded eclogite from the chinese continental scientific drilling ccsd near the zk703 hole located at donghai southern sulu ultrahigh pressure metamorphic belt eastern china results show 1 that the high field slopes obtained from the hysteresis loops the paramagnetic fraction para and density have a positive correlation with the volume concentration of garnet omphacite a typical mineral assemblage used to semi quantify the degree of retrograde metamorphism the low field slopes obtained from hysteresis loops the ferrimagnetic susceptibility fraction ferri saturation isothermal remanent magnetization mrs and saturation magnetization ms have a positive correlation with the volume concentration of symplectite a mineral related to retrograded metamorphism therefore they could be potential indicators for quickly semi quantifying the degree of retrograde metamorphism of the eclogite units 2 the dominant magnetic carriers in retrograded eclogites are magnetite particles in pseudo single domain grain size region 3 the p t conditions during the retrograde decompressional process could first increase the concentration of magnetite which can reach up to 3 for extensively retrograded eclogite and then was dissolved for fully retrograded eclogite therefore change in the magnetite contents during the retrograde process is the major factor controlling the magnetism of retrograde eclogites

aeromagnetic anomalies magnetic petrology and rock magnetism of hemo ilmenite and magnetite rich cumulate rocks from the sokndal region south rogaland norway aeromagnetic maps of the egersund mid proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nt the positive magnetic anomalies are over magnetite norites and overlying mangerites and quartz mangerites of the bjerkreim sokndal layered intrusion these rocks are dominated by multi domain md magnetite the negative magnetic anomalies are over ilmenite rich norites of the bjerkreim sokndal layered intrusion the tellnes ilmenite norite ore deposit and massif anorthosites these rocks are dominated by hemo ilmenite and or by silicates containing fine grained oxide exsolution lamellae electron microprobe analyzes of coexisting fe ti oxides in the layered intrusion confirm earlier observations that oxides in early magmatic rocks are dominated by hemo ilmenite with minor end member magnetite followed by more reduced oxides dominated by titanomagnetite with minor near end member ilmenite what is not fully understood is the property of ilmenite with hematite exsolution lamellae or even more striking hematite with ilmenite lamellae to produce strong remanent magnetization of high coercivity and with a neel temperature equal to or above the curie temperature of magnetite this property makes the rhombohedral oxides an important candidate to explain some high amplitude deep crustal anomalies on earth or strong remanent magnetization on other planets a remarkable feature in the egersund province is that primitive magmas produced rocks rich in hemo ilmenite causing negative magnetic anomalies related to magnetic remanence and more evolved magmas produced rocks rich in magnetite related to positive induced magnetic anomalies all in the course of crystallization differentiation

complex strain patterns developed at the frontal and lateral tips to shear zones and thrust zones abstract many of the complex strain patterns seen in shear zones and thrust zones such as variable fabric orientations refolded folds and fabrics together with folds with hinges almost parallel to the main transport direction can be explained in terms of differential movement within the shear zones these strains are developed at the frontal and lateral tips of the zones as they propagate examples are taken from the moine thrust zone of scotland which show variations in strains particularly at the lateral tips the form of differential movement described here may lead to complex strain paths and non plane strain ellipsoids and the spatial variations in finite strain may be used to delineate zones of extensional and compressional flow and differential movement in the shear zones or thrusts

complex fault patterns transtension and structural segmentation of the lofoten ridge norwegian margin using digital mapping to link onshore and offshore geology an integrated onshore offshore study involving regional to outcrop scale fault analysis is used to develop a self consistent structural model for transtension along the lofoten ridge the lofoten vesteralen archipelago lva is a segmented basement high showing distinct lateral variations in trend deformational style and structural complexity this study investigates whether segmentation can be linked to differences in the obliquity of preexisting structures relative to plate movement vectors regional analysis of fault lineament patterns using geographic information systems gis reveals that the lva can be subdivided into a series of distinct lineament domains these domains are closely coincident with changes in ridge trend and variations in structure within offshore models derived from seismic reflection studies digital field mapping and spatial analysis of faulting in the north lofoten reveal that multimodal faulting is dominated by transtensional dip slip and oblique slip movements which are comparable to analogue models where the ridge axis is 30 oblique to regional extension the overall change in fault orientation fault geometry and deformation style are consistent with models for transtension where the ridge bounding structure becomes increasingly oblique to regional extension previously identified transfer zones simply reflect segment domain boundaries and are not reactivating basement structures this model is a possible analogue for other orthogonal and oblique rift structures on the norwegian and other margins

holocene sedimentation in the skagerrak a review abstract stratigraphic sedimentological geochemical and micropaleontological aspects of holocene sedimentation processes in the skagerrak are reviewed published data show the skagerrak to be the main depositional basin of the north sea with additional inputs from the north atlantic the baltic and the scandinavian mainland as this area is a highly dynamic and complex environment it is difficult to define overall controlling factors and processes from a consideration of available and lacking approaches and data suggestions are made for future research

kinematics of secondary synthetic p faults in wrench systems abstract natural and experimental wrench fault systems commonly develop two distinct synthetic minor fault orientations termed primary and secondary synthetic faults experiments show that secondary synthetic faults are restricted to cases where overall simple shear kinematics are attained or closely approximated the secondary synthetic faults serve to link the primary faults the formation of secondary synthetic faults is explained in terms of infinitesimal kinematics primary synthetic faults alone produce a kinematic pattern incompatible with the requirement of simple shear that there be no longitudinal strain parallel to the principal deformation zone pdz when combined with secondary synthetic faults the resultant kinematic pattern approximates simple shear thus preserving compatibility primary and secondary synthetic faults form to accommodate the fixed displacements imposed on the pdz in a simple shear wrench system

dilatancy fabrics in conglomerates in the awatere fault zone new zealand abstract analysis of fabrics of faulted and unfaulted conglomerate within the awatere fault zone of new zealand shows that movement on individual faults was accompanied by dilatancy of the clast fabric the fabric of unfaulted conglomerate is clast supported comprising 78 clasts by area individual fault zones studied range in width from 4 to 21 cm and have a clay matrix supported fabric comprising 33 clasts by area the abundance of clay within fault zones indicates a highly plastic matrix intruded from the wall rocks into fault zones during localised dilation of the clast fabric the shear thickening shear stress strain rate relationship typical of dilatant materials suggests that the observed fabrics are more compatible with faults developing slowly as accommodation structures rather than forming rapidly during seismic movements a rudimentary analysis of the transfer of the minimum lithostatic load through the conglomerate suggests that contact loads between clasts were significantly less than the strength of clasts determined by point load testing the results provide a mechanism to support a previous interpretation of the fault pattern in terms of dilatant faulting within an oblique divergent tectonic setting

deep sulfate reduction completely mediated by anaerobic methane oxidation in sediments of the upwelling area off namibia abstract porewater concentrations of sulfate methane and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off namibia which were recovered from the continental slope at water depths of 1300 and 2000 m at all four stations a distinct sulfate methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100 of deep sulfate reduction within the sulfate methane transition zone and consumes the total net diffusive sulfate flux a significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could therefore be excluded we state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction

geochemical signature related to lipid biomarkers of anmes in gas hydrate bearing sediments in the ulleung basin east sea korea abstract the emission of methane as a greenhouse gas is controlled by the anaerobic oxidation of methane aom which plays an important role in the biogeochemical methane cycle during the second ulleung basin gas hydrate drilling expedition ubgh2 the distribution of lipid biomarkers and their compound specific stable carbon isotope ratios related to methane were investigated in venting and non venting sites ubgh2 3 ubgh2 10 of gas hydrate bearing sediments in the ulleung basin the objective of this study was to understand the microbial signatures related to methane cycling in organic rich sediment in a marginal sea east sea japan sea of the western north pacific the concentrations of methane related specific biomarkers archaeol and sn 2 hydroxyarchaeol at the sulphate methane transition zone smtz sediment depth in ubgh2 3 1 2 mbsf in ubgh2 10 6 8 mbsf are typically higher than in other sediment sections and their 13 c valuesare apparently depleted 73 3 to 102 7 in the ubgh2 3 and ubgh2 10 study sites however the 13 c values of archaeol and sn 2 hydroxyarchaeol between 59 6 and 66 5 are not depleted with the increased methane concentration in the sediments below the smtz in ubgh2 3 compared to the 13 c values about 60 of in situ methane this suggests that methane production processes should be dominant in the deeper sediment sections 2 7 3 8 mbsf rather than methane consumption by anaerobic methanotrophs anmes at the corresponding sediment depths there were also higher 13 c values 47 to 32 for archaeol and sn 2 hydroxyarchaeol in the 3 6 mbsf sections at ubgh2 10 suggesting the prevalence of methanogenic activities however the 13 c values 89 0 to 92 2 of archaeol and sn 2 hydroxyarchaeol were unexpectedly depleted in the deeper sediment section 5 2 mbsf of the venting site ubgh2 3 indicating that the past aom occurred under low sulphate concentrations in the corresponding pore water this study used the biomarker ratio sn 2 hydroxyarchaeol archaeol of archaea as a tool to demonstrate the different anme communities which was supported by 16s rrna analysis in the sediments of venting and non venting sites ubgh2 3 ubgh2 10 consequently the biochemical signatures of methanotrophic and methanogenic activity were found at varying sediment depths at both sites

gas hydrate destabilization enhanced dewatering benthic material turnover and large methane plumes at the cascadia convergent margin mixed methane sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the cascadia convergent margin vent fields from which methane charged low salinity fluids containing sulfide ammonia 4he and isotopically light co2 escape are associated with these exposures they characterize a newly recognized mechanism of dewatering at convergent margins where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward this process augments the convergence generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths the enhanced turnover results from the oxidation of methane hydrogen sulfide and ammonia by vent biota the injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide a large fraction of the methane is oxidized within the water column and generates 13c anomalies of the dissolved inorganic carbon pool

expulsion of shallow gas in the skagerrak evidence from sub bottom profiling seismic hydroacoustical and geochemical data a combined high resolution seismic sub bottom profiling and multi beam echo sounding survey in the skagerrak danish sector of the north sea together with gas analyses at a station along the profile exhibit the expulsion of gas mainly methane and the presence of gas charged sediments at shallow depth the echo soundings yield detailed insight into the distribution and shape of typical sea floor features associated with gas seepage such as pockmarks the pockmarks reach dimensions of 800 m in length 300 m in width and 15 m in depth with the long axis running parallel to the slope of the norwegian trench processing of the multi channel high resolution seismic data and the digitally recorded sub bottom profiler signals indicate an internal compressional velocity of about 1050 m s 1 within the gas charged sediments reaching from the sea floor to a sub bottom depth of about 23 m using the lateral distribution and thickness of the gas charged sediments in conjunction with a mean concentration of gas of 3000 ppb the present amount of trapped gas is estimated to be 6 45 1011 g ch4 the flux of methane through the sea bed into the water column appears to be 7 2 1010 g ch4 per year to explain the small difference in size between the methane pool in near surface sediments and the annual flux through the sea bed a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active

transtensional origin of the ne sw simitli basin along the strouma strymon lineament sw bulgaria several depocentres have developed along the nnw sse striking strouma strymon lineament since the miocene forming the strouma strymon graben system some of these basins including the dzherman and simitli strike at high angles to the lineament the rhomboid shaped simitli basin is limited in the south by the ne sw striking seismically active kroupnik fault and contains terrestrial clastic sediments which have been deposited since the sarmatian its formation is attributed to wrench dominated transtension d b event which activated the basin9s ne sw striking boundary faults as left lateral strike slip structures during the early middle miocene this deformation was associated with nne ssw contraction and wnw ese extension and succeeded the nne ssw contraction caused by pure shear dominated transpression d a event which governed the region in late oligocene early miocene times during the middle late miocene the d b event was followed by wnw ese pure shear dominated extension d 1 event which caused further widening of the basin the transtensional origin of the simitli basin is explained by lateral extrusion of the crustal mass squeezed between the apulian adriatic microplate and the european foreland towards the north aegean sea the subduction retreat of the hellenic orogen possibly enhanced this lateral extrusion and from the late miocene onwards the ne sw oriented back arc extension balanced the continuing retreat

seismicity and stress rotation in a granular model of the brittle crust the basic mechanical process responsible for earthquakes and faulting is not known the intuitive notion of frictional slip on faults between elastic crustal blocks cannot be reconciled with laboratory measurements of the strength of rocks1 field observations of heat flow2 3 and stress orientation4 around the san andreas fault in california and seismological estimates of the energy radiated by earthquakes5 the weakening of large faults by elevated pore fluid pressure6 has been suggested as one solution to this paradox but places severe constraints on the hydrological conditions of the faults concerned here i propose an alternative model for earthquake mechanics in which the crust is treated as a system of many interlocking blocks divided by many faults7 the model combines this random granular structure with simple deterministic mechanical interactions numerical simulations of the deformation of an aggregate of rough grains under compressive stress show earthquake like elastodynamic failures without frictional heat production and substantial rotation of stresses across shear zones which mimics field observations there remain problems of scale in comparing these simulations with nature but a seismological test of the model may be possible

linkage and evolution of conjugate strike slip fault zones in limestones of somerset and northumbria conjugate strike slip fault zones that cover metre scale areas at beadnell northumbria and kilve somerset were initiated as conjugate vein arrays early conjugate faults are linked by the propagation of one fault that eventually by passes the other fault a model for the development of strike slip faults is presented using fault and vein geometries and the position of damage zones with respect to the master faults as an indication of the propagation direction this model includes the evolution of networks from 1 the initial random development of vein arrays to 2 the isolated development of several unconnected conjugate fault segments that pass into vein arrays through 3 the intersection of a conjugate set of master faults and linkage with minor antithetic faults and the formation of new vein arrays with extensional geometries after a linked network of faults is established to 4 breaching of intersection points by dominant faults and finally 5 the propagation towards oversteps that are breached to form a through going fault the geometry of the active structures simplifies with time as strain is localised along the master fault but the complexities are preserved in the fault walls

experimental formation of brittle structural assemblages in oblique divergence abstract a series of experiments is reported in which brittle minor structures are initiated in narrow deformation zones in clay under conditions of kinematically controlled oblique divergent displacement nineteen settings of boundary displacement angle were used from pure wrench to pure divergence under conditions favouring either faults dry experiments or extension fractures wet experiments pure wrench produced the well known assemblage of riedel strike slip faults whereas experiments in pure divergence produced conjugate arrays of normal faults and extension fractures with a dihedral angle of 30 bisected by the direction of the zone as has been described in rift zones experiments with boundary displacements at intermediate settings show a continuum of structural orientations and dihedral angles between these two extremes a boundary between assemblages dominated by strike slip faults and extensional faults was found at a displacement angle of 45 from the deformation zone these results are interpreted kinematically in terms of 1 principal axes of infinitesimal incremental strain 2 material dilatancy control on shear structure dihedral angles and 3 whether the vertical strain in divergent wrench settings is a thickening strike slip assemblage or a thinning normal fault assemblage

using ammonium pore water profiles to assess stoichiometry of deep remineralization processes in methanogenic continental margin sediments 1 in many continental margin sediments a deep reaction zone exists which is separated from remineralization processes near the sediment surface here methane diffuses upward to a depth where it is oxidized by downwardly diffusing sulfate however the methane sources that drive this anaerobic oxidation of methane aom in the sulfate methane transition zone smt may vary among sites in particular these sources can be thought of as either i internal sources from in situ methanogenesis regardless of where it occurs in the sediment column that are ultimately coupled to organic matter deposition and burial or ii external sources such as hydrocarbon reservoirs derived from ancient source rocks or deeply buried gas hydrates both of which are decoupled from contemporaneous organic carbon deposition at the sediment surface using a modeling approach we examine the relationship between different methane sources and pore water sulfate methane dissolved inorganic carbon dic and ammonium profiles we show that pore water ammonium profiles through the smt represent an independent tracer of remineralization processes occurring in deep sediments that complement information obtained from profiles of solutes directly associated with aom and carbonate precipitation i e dic methane and sulfate pore water dic profiles also show an inflection point in the smt based on the type of deep methane source and the presence absence of accompanying upward dic fluxes with these results we present a conceptual framework which illustrates how shallow pore water profiles from continental margin settings can be used to obtain important information about remineralization processes and methane sources in deep sediments

nanoscale haematite ilmenite lamellae in massive ilmenite rock an example of lamellar magnetism with implications for planetary magnetic anomalies summary massive nearly pure haemo ilmenite layers from historic ore deposits in rogaland norway contain very few silicates or other oxides and typically produce remanence dominated magnetic anomalies these rocks are ideal for evaluating the magnetic properties of fine exsolution intergrowths and the larger titanohaematite lamellae in the host ilmenite grains a typical bulk composition ilm 84 exsolved at high temperature to produce host ilmenite ilm 94 and micron sized haematite lamellae ilm 23 as measured by electron microprobe emp subsequent undercooling of the ilmenite and the micron scale haematite lamellae led to metastable nucleation of nanoscale lamellae down to unit cell scale leaving depleted hosts between lamellae with compositions of ilm 98 and ilm 15 13 as measured by tem edx samples have high coercivities and average nrm values of 25 a m 1 which typically show 2 per cent saturation in the nrm state the amount of magnetization in these samples is too high to be solely accounted for by a spin canted af moment in the haematite based on monte carlo simulations of haematite ilmenite interfaces at the atomic scale and on measured rock magnetic properties we predict that the magnetization is carried by a ferrimagnetic substructure produced at the contacts of the very fine scale titanohaematite and ilmenite exsolution lamellae

acetate in recent anoxic sediments direct and indirect measurements of concentration and turnover rates abstract while acetate is generally regarded as an important intermediate in the mineralization of organic matter in anoxic sediment systems some quantitative studies in marine systems including our own have measured acetate oxidation rates in excess of sulphate reduction rates where sulphate is known to be the principal electron acceptor we revisited skan bay alaska where we had previously made such observations for a reexamination of acetate turnover measurements of acetate concentrations production rate oxidation rate and sulphate reduction rate as well as bioenergetic considerations led to the conclusion that acetate oxidation rate in 15 18 cm deep sediment is 1 1 1 5 m h 1 the possibility that previous measurements were high because of a non citric acid cycle pathway of acetate oxidation suggested by recent laboratory studies was excluded it appears that our previous turnover measurements were high mainly because of high acetate concentrations procedures used for the isolation of porewater for acetate determination may influence results to an extent not previously recognized

sulfate reduction and methane oxidation in continental margin sediments influenced by irrigation south east atlantic off namibia abstract sulfate reduction rates srr and concentrations of so 4 2 h 2 s pyrite sulfur total sulfur ch 4 and organic carbon were measured with high depth resolution through the entire length of the so 4 2 zone and well into the ch 4 zone at two continental slope stations in the eastern south atlantic benguela upwelling area the sediments were characterized by a high organic carbon content of approx 7 5 at geob 3703 and 3 7 at geob 3714 at geob 3703 so 4 2 concentrations decreased linearly with depth to about 40 m at the sulfate methane transition zone smt at 3 5 m while at geob 3714 so 4 2 remained at sea water concentration in the top 2 m of the sediment and then decreased linearly to about 70 m at the smt at 6 m direct rate measurements of srr 35 so 4 2 showed that the highest srr occurred within the surface 3 5 cm with peak rates of up to 20 and 7 nmol so 4 2 cm 3 day 1 at geob 3703 and geob 3714 respectively srr decreased quasi exponentially with depth at geob 3703 and the cumulative srr over the length of the so 4 2 zone resulted in an areal srr srr area of 1114 3493 mol m 2 day 1 median value 2221 mol m 2 day 1 at geob 3703 with more than 80 of the total sulfate reduction proceeding in the top 30 cm sediment at geob 3714 srr exhibited more scatter with a cumulative srr area of 398 1983 mol m 2 day 1 median value 1251 mol m 2 day 1 and with 60 of the total sulfate reduction occurring below a depth of 30 cm due partially to a deeply buried zone of sulfate reduction located between 3 and 5 m depths srr peaks were also observed in smt of both cores ostensibly associated with methane oxidation but with rates about 10 times lower than at the surface modeled srr balanced both methane oxidation rates and measured srr within the smt but severely underestimated by up to 89 the total srr area that were obtained from direct measurements modeled and measured srr were reconciled by including solute transport by irrigation described by a non local pore water exchange function which had values of up to 0 3 year 1 in the top sediment and decreased exponentially to zero i e no irrigation at 2 3 meters i e above smt these results suggested that co existing sulfate reduction processes and linear so 4 2 gradients can be maintained by a non local transport mechanism such as irrigation by which pore water in tubes or burrows is exchanged with bottom waters by activities of tube dwelling animals or some similar physical transport phenomenon i e bubble ebullition further support for an irrigation mechanism was found in the observations of open tubes of up to 8 mm id at depths down to 6 m which also contained fecal pellets indicating that these tubes were or had been inhabited

anaerobic oxidation of methane aom in marine sediments from the skagerrak denmark i geochemical and microbiological analyses abstract the organic rich sediments of the skagerrak contain high quantities of shallow gas of mostly biogenic origin that is transported to the sediment surface by diffusion the sulfate methane transition zone smtz where anaerobic oxidation of methane aom and sulfate reduction occur functions as a methane barrier for this upward diffusing methane to investigate the regulation of aom and sulfate reduction rates srr and the controls on the efficiency of methane consumption pore water concentrations and microbial rates of aom sulfate reduction and methanogenesis were determined in three gravity cores collected along the slope of the norwegian trench in the skagerrak srr occurred in two distinct peaks at the sediment surface and the smtz the latter often exceeding the peak aom rates that occurred at the bottom of the smtz highest rates of both aom and srr were observed in a core from a pockmark where advective methane transport occurred generating high methane and sulfate fluxes but even at this site with a shallow smtz the entire flux of methane was oxidized below the sediment surface aom srr and methanogenesis seem to be closely associated and strongly regulated by sulfate concentrations which were in turn regulated by the methane flux rate measurements of srr aom and methanogenesis revealed a tight coupling of these processes bicarbonate based methanogenesis occurred at moderate sulfate concentrations 5 mm above the aom zone but seemed to be inhibited in the depth where aom occurred the unbalanced stoichiometry of aom and srr in the smtz was more pronounced in rate measurements than in methane and sulfate fluxes and seemed more likely be related to enhanced srr in this zone than an underestimation of methane fluxes

origin distribution and accumulation of organic carbon in the skagerrak abstract organic carbon contents have been determined in grain size fractions of sediments from the skagerrak southern slope and basin c org concentrations were generally inversely related to grain size the 62 63 m fraction in surface sediments from deeper waters had higher contents attributable to agglutination of fine material by tube forming benthic organisms especially in the clay and fine silt fractions c org increased with increasing fraction content presumably as a result of higher grazing pressure on fine material in coarse sediments a comparison of high resolution age 210 pb porosity and grain size composition profiles indicates that deviations in the 210 pb curves are due both to bioturbation and sedimentation events porosity profiles also show high regional variability on a small scale when primary production c org vertical fluxes and accumulation rates are compared an excess of organic carbon stored in skagerrak sediments becomes apparent potential sources for this carbon such as river input dumping or sea floor erosion are examined

life at the edge of methane ice microbial cycling of carbon and sulfur in gulf of mexico gas hydrates the processes of methane oxidation and sulfate reduction were examined in subsamples of gas hydrate associated materials collected along the gulf of mexico continental slope standard radiotracer techniques were used to determine rates of microbial activity in different layers of the hydrate environment including outer sediment os interface sediment is worm burrow sediment wb interior hydrate in and a mixture of hydrate and sediment mix the anaerobic oxidation of methane aom and sulfate reduction sr were observed in all hydrate samples examined and the rates of these processes showed similar spatial trends between different hydrate layers highest rates of both aom and sr were observed at interface between the sediment and hydrate aom rates were about 3 11 nmol cm 3 day 1 in worm burrow and interface sediments as compared to 1 nmol cm 3 day 1 in other hydrate material types rates of sr ranged from 59 to 490 nmol cm 3 day 1 in worm burrow and interface sediments while rates in interior hydrate samples were an order of magnitude lower these rates observed in hydrate materials are lower than rates from nearby methane rich sediments at ambient temperatures nevertheless our data show that active microbial populations inhabit all layers of the hydrate environment and suggest their activity may impact biogeochemical methane and sulfur cycling in this unique niche d 2004 elsevier b v all rights reserved

elongate depressions on the southern slope of the norwegian trench skagerrak morphology and evolution abstract multibeam echosounder surveys conducted on the southern slope of the norwegian trench between norway and denmark show that elongate depressions interpreted as current modified pockmarks are widespread in water depths of 100 500 m in the holocene recent succession the long axes of the depressions are oriented at an angle of about 15 downslope relative to the depth contours the long axes appear to almost exactly parallel the effective bottom currents in this part of the skagerrak the present current directions were probably established about 4000 years ago following a major shift in the oceanographic circulation pattern the most characteristic feature of the depressions is their large size with maximum depths of 45 m widths of 400 m and lengths of 2 km strong bottom currents continuously transport particles thrown into suspension by the seepage process away from the pockmarks and prevent further deposition continuous seepage of shallow gas at fixed pockmark sites over long time periods combined with generally high sedimentation rates are thought to be responsible for the exceptional size of many depressions the shallow gas probably has a mixed biogenic thermogenic origin however the thermogenic component is considered to be most important for the formation of pockmarks seepage from permeable layers in the jurassic cretaceous bedrock is evidenced by linear rows of depressions above these horizons while pockmarks above sub surface faults indicate gas seeps also from there in shallow water areas the shape and size of pockmarks are locally dependent on their location with respect to giant sand waves which trend sub parallel to the depth contours and are interpreted to be of late weichselian to early holocene age in deeper water areas the pockmarks have been further modified by gravity slides caused by collapse of steep pockmark walls

effect of fine scale microstructures in titanohematite on the acquisition and stability of natural remanent magnetization in granulite facies metamorphic rocks southwest sweden implications for crustal magnetism mid proterozoic granulites in sw sweden having opaque minerals hematiteilmenite with minor magnetite and occurring in an area with negative aeromagnetic anomalies have strong and stable reversed natural remanent magnetization 9 2 a m with 100 remaining after demagnetization to 100 mt samples were characterized by optical microscopy electron microprobe emp transmission electron microscopy tem and rock magnetic measurements earliest oxide equilibrium was between grains of titanohematite and ferri ilmenite at 650 600 c initial contacts were modified by many exsolution cycles hematite and ilmenite ilm hosts and lamellae by emp are ilm 24 25 ilm 88 93 like titanohematite and ilmenite above 520 c on burton s diagram 1991 finer hosts and lamellae by tem are ilm16 3 and ilm 88 4 like coexisting antiferromagnetically ordered af hematite and ilmenite below 520 c on burton s diagram this may be the first example of analytical identification in one sample of former hematite now finely exsolved and af hematite tem microstructures consist of gently curving semicoherent ilmenite lamellae within hematite flanked by precipitate free zones and abundant ilmenite disks down to unit cell scale 1 2 nm strain contrast of disks suggests full coherence with the host and probable formation at the reaction titanohematite af hematite ilmenite at 520 c magnetic properties are a consequence of chemical and magnetic evolution of hematite and ilmenite with bulk compositions ilmenite richer than ilm 28 that apparently exsolved without becoming magnetized down to 520 c where hematite broke down to af hematite plus ilmenite producing abundant af hematite below its neel temperature intensity of magnetization is greater than possible with hematite alone and tem work suggests that ultrafine ilmenite disks in af hematite are associated with a ferrimagnetic moment due to local imbalance of up and down spins at coherent interfaces

magnetism at depth a view from an ancient continental subduction and collision zone recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle two approaches to constraining magnetizations are developed providing minimum 0 01 si and maximum 0 04 si susceptibility estimates given some assumed thickness 15 km here these values are higher than are found in many continental rocks are there rocks deeper in the crust or upper mantle that are more magnetic than expected or are the model assumptions incomplete what is the magnetic behavior of deep crustal and upper mantle rocks when slightly cooler than the curie or neel temperatures of their magnetic minerals after being exhumed from locations of high grade metamorphism at greater depth different sets of equilibrium metamorphic minerals can be considered that would form under different conditions results on 1501 samples from the western gneiss region wgr norway mainly from mafic and ultramafic bodies subducted to depths of 60 200 km and temperatures of 750 up to 950 c at the very highest pressures show that rocks did not fully equilibrate to the dominant metamorphic facies conditions there is a large variation in petrophysical properties oxide minerals and mineral assemblages in wgr samples though they cannot explain the broad high amplitude deep seated anomalies measured in this region the presence of magnetite and exsolved titanohematite and hemo ilmenite in samples shows those magnetic phases are preserved even at eclogite facies conditions in part because complete eclogite facies equilibrium was rarely achieved

magnetic petrology of high fe ti eclogites from the ccsd main hole implications for subduction zone magnetism 1 the high fe ti eclogites with exsolved lamellar in 530 600 m depths from the chinese continental scientific drilling ccsd main hole in the sulu ultra high pressure uhp metamorphic belt eastern china record an anomalously high susceptibility natural remanent magnetization nrm and koenigsberger ratio q nrm ji ji is induced magnetization this provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks in this paper we have measured systematically magnetic properties and mineral assemblage and structure for three special samples no 83 no 86 and no 89 results show that these fe ti rich eclogites are the result of the fine grained titano magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo ilmenite we found that the dominant fe bearing oxide minerals in samples studied are ilmenite hematite hem ilm up to 25 little titano magnetite and pyrite the ferromagnetic susceptibility is mainly controlled by fine grained titano magnetite and nrm is closely related to the exsolved lamellar structure we hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno hematite during cooling and decompression processes these fe ti rich eclogites might be one of the sources of high magnetic anomalies observed in the sulu subduction zone eastern china

gas hydrate destabilization and methane release events in the krishna godavari basin bay of bengal abstract methane release events have been linked to global warming alteration of the carbon cycle and influence on biota however unequivocal evidence of paleomethane release events are limited we report several negative carbon stable isotope excursions in planktic and benthic foraminifera in a core md161 8 from the krishna godavari k g basin bay of bengal the most negative 13 c spikes are recorded during the marine isotope stages mis 4 and at the transition of mis 5 to 4 occurrence of highly 13 c depleted average 13 c 48 2 4 vpdb authigenic high magnesian calcite are also reported within this time window from the core md161 8 in the present work an unequivocal explanation for the observed 13 c depletion in the marine planktic and benthic foraminifera is difficult to achieve solely from the optical electron microscopy or c o stable isotope ratio analyses due to possible influence of diagenetic alteration we attribute the observed episodic methane expulsion events as inferred from the negative 13 c excursions and earlier reports on the occurrence chemosynthetic bivalves and mo concentration anomaly to the destabilization of the base of gas hydrate stability zone bghsz sea level drop and shale tectonics induced focused fluid flow are the two possible causes of hydrate destabilization discussed here shale tectonics were possibly responsible for creating fault systems which acted as the conduit for gas flow through the sediment column and subsequent seepage shale and salt tectonics in the passive continental margins being a globally observed phenomenon its role as an important driving force for enhanced methane emission needs detailed investigation to understand the climatic perturbations through geologic time additional evidence of methane emission from site md161 15 further supports the link between shale tectonics and methane emission

magnetic properties of rocks of the kapuskasing uplift ontario canada and origin of long wavelength magnetic anomalies summary the sources of long wavelength magnetic anomalies lwa in the crust are poorly understood we have measured remanent and induced magnetizations of 210 samples of anorthosite tonalite and mafic gneiss from the kapuskasing uplift an exposed partial crustal crosssection in northwestern ontario canada anorthosites have generally high q ratios of remanent induced magnetization in the range 0 3 60 and their natural remanent magnetization nrm is resistant to both thermal and alternating field af demagnetization however anorthositenrms 0 001 0 3am 1 aretooweaktoexplainlwaamplitudes maficgneisses also have relatively high q ratios peaking in the range 1 10 and the nrm is resistant to thermal and af demagnetization nrm and induced magnetization im intensities are in the ranges 0 01 2 and 0 01 0 6 a m 1 respectively tonalites have a bimodal distribution of magnetization the more strongly magnetic group has both nrm and im intensities in the range 0 1 5 a m 1 and wide ranging q values from 0 1 to 10 approximately some tonalites could be an lwa source although the long term stability of their nrms at high temperature in the crust is questionable because unblocking temperatures are broadly distributed from 100 to 600 c in general q values measured at surface temperatures overestimate remanence at depth in the deep crust im remains more or less constant but remanence decreases both reversibly and irreversibly leading to q ratios of 0 2 0 3 for multidomain grains and 1 3 for single domain grains near their blocking temperatures thermoviscous magnetization over the brunhes chron could add substantially to the effective induced magnetization typically induced thermoviscous magnetization in the direction of the present earth s field will outweigh remanence in the direction of an ancient field as a source of lwa originating in the deepest crust remanence may play a larger role for mid crustal sources where single domain grains are well below their blocking temperatures

a 400 km scale strike slip zone near the boundary of thetis regio venus we present a structural analysis of an area of venus surface centred on 3 s 116 e the area is dominated by a wide zone of deformation striking 050 n distinguished from the terrain on either side of it by markedly different structural and morphological characteristics the deformation zone contains two principal fault sets defined by form position and orientation members of the first set are present along the length of the deformation zone they are discontinuous consistently right stepping and are interpreted as riedel shears in a sinistral strike slip regime members of the second fault set are interpreted as normal faults and are often seen in pairs forming extensional grabens these faults are most prominent in the central region of the deformation zone and coincide with a deflection in strike of both the first fault set and the boundaries of the deformation zone a detailed kinematic analysis of all fault orientations and the boundaries of the deformation zone fit the predictions of transtensional theory supporting the hypothesis that this sinistral strike slip zone contains an extensional jog this interpretation is further supported by analysis of the interaction between faults belonging to the two sets variation in intercept geometry kinematic linking of the two fault sets and rotation of fault strike between the two fault sets all strongly suggest simultaneous development of these structures and therefore a common tectonic origin a driving mechanism for such large scale horizontal deformation on venus has yet to be established