on the british siluro devonian palaeomagnetic field problem reply to comments by t h torsvik et al we appreciate this opportunity to clarify our position on a few central issues of this debate by our rejection of a number of palaeozoic palaeomagnetic data from scotland storetvedt et al 1990 torsvik et al accuse us of disregarding all classical field tests we have never disregarded any classical field test but would argue that such tests have sometimes been misapplied also we obviously have not rejected all earlier british siluro devonian data including scottish results e g table 3 storetvedt et al 1990

space weathered rims found on the surfaces of the itokawa dust particles on the basis of observations using cs corrected stem we identified three types of surface modification probably formed by space weathering on the surfaces of itokawa particles they are 1 redeposition rims 2 3 nm 2 composite rims 30 60 nm and 3 composite vesicular rims 60 80 nm these rims are characterized by a combination of three zones zone i occupies the outermost part of the surface modification which contains elements that are not included in the unchanged substrate minerals suggesting that this zone is composed of sputter deposits and or impact vapor deposits originating from the surrounding minerals redeposition rims are composed only of zone i and directly attaches to the unchanged minerals zone iii zone i of composite and composite vesicular rims often contains nanophase fe mg s the composite rims and the composite vesicular rims have a two layered structure a combination of zone i and zone ii below which zone iii exists zone ii is the partially amorphized zone zone ii of ferromagnesian silicates contains abundant nanophase fe radiation induced segregation and in situ reduction are the most plausible mechanisms to form nanophase fe in zone ii their lattice fringes indicate that they contain metallic iron which probably causes the reddening of the reflectance spectra of itokawa zone ii of the composite vesicular rims contains vesicles the vesicles in zone ii were probably formed by segregation of solar wind he implanted in this zone the textures strongly suggest that solar wind irradiation damage and implantation are the major causes of surface modification and space weathering on itokawa

palaeomagnetism of nw scotland syenites in relation to local and regional tectonics summary a detailed thermal demagnetization study has been carried out on a number of syenitic intrusions from north of the great glen fault in scotland multicomponent remanences of normal and reversed polarity have been recognized these define a series of palaeomagnetic poles from which an ordovician to silurian apparent polar wander path for northern scotland is deduced comparison of this apparent polar wander path with the established path for britain throws new light on the sequence of thrusting and magmatic activity in the nw highlands and precludes large scale 1500 km movement along the great glen fault later than the ordovician the data emphasize previously published discrepancies between the apparent polar wander paths of britain baltica and laurentia and are consistent with the suggestion that britain was among a set of intracratonic microplates which underwent complicated distinct movements throughout the caledonian orogeny

size frequency distributions of chondrules and chondrule fragments in ll3 chondrites implications for parent body fragmentation of chondrules we measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of semarkona ll3 0 bishunpur ll3 1 krymka ll3 1 piancaldoli ll3 4 and lewis cliff 88175 ll3 8 the mean apparent diameter of chondrules in these ll3 chondrites is 0 80 units or 570 um much smaller than the previous rough estimate of 900 m chondrule fragments in the five ll3 chondrites have a mean apparent cross section of 1 60 phi units or 330 m the smallest fragments are isolated olivine and pyroxene grains these are probably phenocrysts liberated from disrupted porphyritic chondrules all five ll3 chondrites have fragment chondrule number ratios exceeding unity suggesting that substantial numbers of the chondrules in these rocks were shattered most fragmentation probably occurred on the parent asteroid porphyritic chondrules porphyritic olivine porphyritic pyroxene porphyritic olivine pyroxene are more readily broken than droplet chondrules barred olivine radial pyroxene cryptocrystalline the porphyritic fragment chondrule number ratio 2 0 appreciably exceeds that of droplet textured objects 0 9 intact droplet chondrules have a larger mean size than intact porphyritic chondrules implying that large porphyritic chondrules are fragmented preferentially this is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules 57 compared to that within the set of the smallest chondrules 81 differences in mean size among chondrule textural types may be due mainly to parent body chondrule fragmentation events and not to chondrule formation processes in the solar nebula

the santorini volcanic complex and its relation to the stratigraphy and structure of the aegean arc greece abstract the santorini complex is the most active volcano of the southern aegean volcanic arc its relationship to the surrounding area was examined by using a dense set of seismic reflection profiles three seismic units were recognized in the surrounding area unit a consists of generally strong subparallel continuous reflectors into which two subunits a1 and a2 were distinguished that are unconformable in the upper slopes and the shelves and conformable at lower sectors the age of unit a which consists mainly of volcaniclastic sediments is considered quaternary unit b consists of faulted and slightly folded strong continuous reflectors that pinch out at lower basin margin sectors than unit a its age is mainly pliocene its sediments are probably terrigenous and it was deposited in the basins at the early and middle stages of their formation finally unit c represents the acoustic mainly metamorphic basement the dominant tectonic structure in the area is the ne sw trending normal faults superimposed on earlier e w normal faults the santorini complex includes one caldera at santorini and one crater at columbus about 7 km to the northeast the caldera of santorini consists of four deep basins depth from 290 to 390 m the columbus volcano has a well defined crater with a single basin depth 500 m and was formed in 1650 a d by an underwater explosion the start of volcanic activity of santorini which initiated in the early pleistocene and continues up until today may be related to the change in the normal fault orientation from e w to ne sw and to the unconformity between units a and b

submarine volcanoes along the aegean volcanic arc abstract the aegean volcanic arc has been investigated along its offshore areas and several submarine volcanic outcrops have been discovered in the last 25 years of research the basic data including swath bathymetric maps air gun profiles underwater photos and samples analysis have been presented along the four main volcanic groups of the arc the description concerns i paphsanias submarine volcano in the methana group ii three volcanic domes to the east of antimilos volcano and hydrothermal activity in southeast milos in the milos group iii three volcanic domes east of christiana and a chain of about twenty volcanic domes and craters in the kolumbo zone northeast of santorini in the santorini group and iv several volcanic domes and a volcanic caldera together with very deep slopes of several volcanic islands in the nisyros group the tectonic structure of the volcanic centers is described and related to the geometry of the arc and the neotectonic graben structures that usually host them the ne sw direction is dominant in the santorini and nisyros volcanic groups located at the eastern part of the arc where strike slip is also present whereas nw se direction dominates in milos and methana at the western part where co existence of e w disrupting normal faults is observed the volcanic relief reaches 1100 1200 m in most cases this is produced from the outcrops of the volcanic centers emerging usually at 400 600 m depth and ending either below sea level or at high altitudes of 600 700 m on the islands hydrothermal activity at relatively high temperatures observed in kolumbo is remarkable whereas low temperature phenomena have been detected in the santorini caldera around kameni islands and in the area southeast of milos in methana and nisyros hydrothermal activity seems to be limited in the coastal areas without other offshore manifestations

polyphase magnetization in lower carboniferous rocks of s scotland palaeomagnetic and tectonic considerations abstract lower carboniferous lavas and intrusives from the southern margin of the midland valley scotland have yielded a principal magnetization termed the b component with d i 192 12 95 5 9 this magnetization is thought to have formed essentially during the original cooling but evidence of low temperature mineralogical changes as well as the presence of dual polarity remanence structures suggest that the magnetization processes continued beyond the deuteric stage covering at least the time span of geomagnetic field inversions an overprinted magnetization the a1 component of inferred lower tertiary age has a mean direction of d i 188 49 95 3 5 this secondary magnetization is found principally in three sites two from silica veins and one from a basaltic neck located within the southern uplands fault zone at dunbar microscopic evidence suggests that the a1 overprint was acquired thermochemically through alpine tectonic re activation of the fault

development of basins in the inner moray firth and the north sea by crustal extension and dextral displacement of the great glen fault abstract reflection seismic data provide evidence that mesozoic dextral movements along the great glen fault line have had an important influence on the development of the inner moray firth basin geophysical evidence further indicates that deep structure beneath the inner basin is dissimilar to that beneath the outer part and viking and central grabens in the north sea tectonic development of the inner basin can nevertheless be fitted into a pattern of north sea extensional movements which led to the formation of the graben system with which the major north sea hydrocarbon resources are associated

multicomponent magnetization in the helmsdale granite north scotland geotectonic implication abstract a palaeomagnetic study of the helmsdale granite u pb 420 m y k ar 400 m y northeast scotland has revealed a multicomponent remanence dominated by two characteristic axes of magnetization the suggested oldest of these magnetizations the direction of which is nearly horizontal and directed n s is thought to have been acquired in upper silurian lower devonian times the existence of this shallow direction of magnetization discounts a recent hypothesis of a ca 2000 km sinistral offset along the great glen fault the second component of magnetization appears to be partly carried by haematite that apparently formed through disintegration of biotite and or plagioclase this secondary magnetization has a direction that can be associated with a permian early mesozoic age similar overprinted magnetizations are characteristic features also in the devonian sedimentary sequences north of helmsdale

palaeozoic palaeomagnetic results from scotland and their bearing on the british apparent polar wander path abstract palaeomagnetic studies of the lower and upper old red sandstone ors western midland valley indicate the presence of two principal remanence components ib and hb both components can be demonstrated to be of secondary post upper ors origin component ib compares with palaeomagnetic results from upper carboniferous permian dykes and sills in the midland valley whereas hb conforms to palaeomagnetic results from the overlying lower carboniferous lavas a combination of hb components from the lower upper ors and lower carboniferous lavas western midland valley yields a relative pole position near s14 and e322 which compares well with lower carboniferous data from the kinghorn and burntisland lavas eastern midland valley s14 and e332 from kinghorn the palaeomagnetic data can be demonstrated to be of primary lower carboniferous origin seen in conjunction with the british polar wander path the lower carboniferous poles reported here seemingly dissociate middle upper devonian and upper silurian lower devonian poles owing to the lack of magnetic age control in the middle upper devonian data base it is argued that the majority of middle upper devonian data from the british isles should be reconsidered as carboniferous magnetic overprints this carries the implication that any tectonic models based on middle upper devonian palaeomagnetic results should be considered highly suspicious

palaeozoic reconfigurations of north scotland based on palaeomagnetic results from orkney dykes abstract the camptonite and monchiquite dykes of the orkney islands intruding middle devonian sedimentary strata constitute two significantly different axes of magnetization the inferred older of these remanences has an inclination corresponding to an upper devonian to early carboniferous age while the inclination of the secondary magnetization is in line with the kiaman overprint repeatedly recorded in orcadian basin rocks however the unique palaeomagnetic property of these dykes is that both magnetizations have mean declinations of 170 i e the palaeomagnetic axes are rotated 20 anticlockwise with respect to the corresponding nw european reference fields the data represent vital new information for the discussion of transcurrent motions along the great glen fault it is concluded that the orkney dykes originated at a late stage of an acadian age sinistral megashear 600 km at a time when the northern highlands block was positioned to the south of the central highlands partial remagnetization of the dykes took place at an early stage of hercynian fault reactivation the present declination discrepancy of both primary and overprinted magnetizations was brought about by a hercynian dextral displacement on the great glen fault 300 km leading essentially to the present juxtaposition of the northern and central highlands

macroporosity and grain density of rubble pile asteroid 162173 ryugu rubble pile asteroids such as 162173 ryugu have large bulk porosities which are believed to result from void spaces in between the constituent boulders macroporosity as well as void spaces wit

the british siluro devonian palaeofield the great glen fault and analytical methods in palaeomagnetism comments on paper by k me storetvedt et al summary late silurian early devonian palaeomagnetic poles throughout the british isles lie in a coherent group about 1 s and 314 e a95 9 6 the clustering of these poles which are derived from 11 individual studies of siluro devonian rocks of all the major tectonic elements of britain carries two important tectonic implications 1 the british sector of the iapetus ocean recognizable from ordovician poles had closed by late silurian times and 2 any postulated mega shear whether related to acadian or hercynian deformation is below the limit of palaeomagnetic resolution the collective rejection of all siluro devonian results by storetvedt et al 1990a b is demonstrated to be unfounded

volcano tectonic evolution of the polygenetic kolumbo submarine volcano santorini aegean sea abstract here we show for the first time the 3d structural evolution of an explosive submarine volcano by means of reflection seismic interpretation four to five vertically stacked circular and cone shaped units consisting mainly of volcaniclastics build the kolumbo underwater volcano which experienced its first eruption 70 ka ago and its last explosive eruption 1650 ad 7 km ne of santorini volcano southern aegean sea the summed volume of volcaniclastics is estimated to range between 13 22 km3 the entire kolumbo volcanic complex has a height of 1 km and a diameter of 11 km all volcaniclastic units reveal the same transparent reflection pattern strongly suggesting that explosive underwater volcanism was the prevalent process growth faults terminate upwards at the base of volcaniclastic units thus representing a predictor to an eruption phase similarities in seismic reflection pattern between kolumbo and near by volcanic cones imply that the smaller cones evolved through explosive eruptions as well hence the central aegean sea experienced several more explosive eruptions 23 than previously assumed thus justifying further risk assessment however the eruption columns from the smaller volcanic cones did not reach the air and consequently no sub aerial pyroclastic surge was created the anydros basin that hosts kolumbo volcanic field opened incrementally nw to se and parallel to the pliny and strabo trends during four major tectonic pulses prior to the onset of underwater volcanism

tectono stratigraphic evolution through successive extensional events of the anydros basin hosting kolumbo volcanic field at the aegean sea greece abstract the structural evolution of the south aegean sea is little explored due to the lack of marine seismic data our present day understanding is mainly based on some island outcrops and gps measurements in this study we discuss the rather incremental opening of the anydros basin in the pliocene during six major tectonic pulses and the subsequent basin fill processes by interpreting seismic data and derived time isochore maps between the active pulses basin floor tilting persisted on a much lower rate seismic data illustrate the depositional processes in the emerging anydros basin the observation of onlap fill strata divergent reflection pattern moat channels and contourite drifts imply that deposition was controlled by turbidity and contour currents as well as the tilting basin floor the metamorphic attico cycladic basement shows a rise that aligns along an nw se directed axis crossing anydros island this axis marks a structural change of the santorini amorgos ridge and thus represents a major structural boundary dip angles of ne sw trending major faults like the santorini amorgos fault indicate normal faulting to be the superior mechanism forming the present horst and graben environment hence the area is likely to be in a state of nw se directed extensional stresses forming the asymmetric graben structure of anydros secondary fault clusters strike the same direction but show much steeper dip angles possibly indicating strike slip movement or resulting from deformational stresses along the hinge zones of the normal faults the majority of the faults we discovered are located in the area of earthquake clusters which is another indication of recent faulting ring faults around kolumbo submarine volcano result from caldera collapse and mark the diameter of the magma chamber approximately to 20 km

seismic monitoring of diffraction images for detection of local heterogeneities diffracted waves contain valuable information regarding both the structure and composition of the media they are in in seismic data processing however these waves are usually regarded as noise in this paper we present an attempt to use scattered diffracted waves for the detection of local heterogeneities the method is based on the detection of diffracted waves by concentrating the signal amplitudes from diffracting points on the seismic section this is done using a correlation procedure that enhances the amplitude of the seismic signal at the location of the diffractors on the common diffraction point section d section the new local time correction for diffraction traveltime curve parameterization is based on the radius of curvature of the diffracted wavefront and near surface velocity we use the idea of seismic monitoring for detection and delineating local objects which may occur within the subsurface resulting from human activity or fast geological processes the method consists of continuou

regolith migration and sorting on asteroid itokawa high resolution images of the surface of asteroid itokawa from the hayabusa mission reveal it to be covered with unconsolidated millimeter sized and larger gravels locations and morphologic characteristics of this gravel indicate that itokawa has experienced considerable vibrations which have triggered global scale granular processes in its dry vacuum microgravity environment these processes likely include granular convection landslide like granular migrations and particle sorting resulting in the segregation of the fine gravels into areas of potential lows granular processes become major resurfacing processes because of itokawa s small size implying that they can occur on other small asteroids should those have regolith

poststack velocity analysis by separation and imaging of seismic diffractions small geologic features manifest themselves in seismic data in the form of diffracted waves which are fundamentally different from seismic reflections using two field data examples and one synthetic example we demonstrate the possibility of separating seismic diffractions in the data and imaging them with optimally chosen migration velocities our criteria for separating reflection and diffraction events are the smoothness and continuity of local event slopes that correspond to reflection events for optimal focusing we develop the local varimax measure the objectives of this work are velocity analysis implemented in the poststack domain and high resolution imaging of small scale heterogeneities our examples demonstrate the effectiveness of the proposed method for high resolution imaging of such geologic features as faults channels and salt boundaries

poststack diffraction imaging using reverse time migration abstract we propose a method for imaging small scale diffraction objects in complex environments in which kirchhoff based approaches may fail the proposed method is based on a separation between the specular reflection and diffraction components of the total wavefield in the migrated surface angle domain reverse time migration was utilized to produce the common image gathers this approach provides stable and robust results in cases of complex velocity models the separation is based on the fact that in surface angle common image gathers reflection events are focused at positions that correspond to the apparent dip angle of the reflectors whereas diffracted events are distributed over a wide range of angles the high resolution radon based procedure is used to efficiently separate the reflection and diffraction wavefields in this study we consider poststack diffraction imaging the advantages of working in the poststack domain are its numerical efficiency and the reduced computational time the numerical results show that the proposed method is able to image diffraction objects in complex environments the application of the method to a real seismic dataset illustrates the capability of the approach to extract diffractions

palaeomagnetism of caledonian intrusive suites in the northern highlands of scotland constraints to tectonic movements within the caledonian orogenic belt abstract to evaluate the scale of tectonic movements within the northern sector of the 500 400 ma caledonian orogenic belt and its precambrian foreland zone between the great glen fault ggf zone to the southeast and the laurentian block to the northwest we have studied the palaeomagnetism of minor intrusive rocks within the northern highlands terrain these rocks include 1 1 amphibolites and other metamorphic rocks predating deformation 2 2 microdiorities dolentes and related suites emplaced and probably magnetised between 450 and 420 ma and 3 3 lower middle devonian lamprophyres a range of predominantly nne negative and ssw positive components are resolved by cleaning treatment with a dispersion of declinations towards a minority wnw ese axis isolated southerly negative directed hematite held components suggests limited but no widespread remagnetisation in devonian carboniferous times comparison is made with data from other tectonic divisions in the caledonian orogenic belt and the bordering forelands palaeopoles from the northern highlands closely conform in part with north american ordovician poles and in part with the post ordovician palaeopoles from britain south of the ggf the definitive motions of the british caledonides to emerge from the palaeomagnetic analysis are an anticlockwise rotation of the caledonian terrain in early ordovician times small relative motions during the remainder of ordovician times followed by large clockwise and then anticlockwise rotations during late ordovician to early silurian times contemporary with the last major movements on the moine thrust ca 430 ma late silurian devonian movements along the ggf were probably below the limits of palaeomagnetic detectability the collective data require that apparent polar wander movements and concomitant continental movements have currently been incompletely recovered by north american studies and the path for lower palaeozoic times is more complex than recognised hitherto

3 d seismic attributes using a semblance based coherency algorithm seismic coherency is a measure of lateral changes in the seismic response caused by variation in structure stratigraphy lithology porosity and the presence of hydrocarbons unlike shaded relief maps that allow 3 d visualization of faults and channels from horizon picks seismic coherency operates on the seismic data itself and is therefore unencumbered by interpreter or automatic picker biases we present a more robust multitrace semblance based coherency algorithm that allows us to analyze data of lesser quality than our original three trace cross correlation based algorithm this second generation semblance based coherency algorithm provides improved vertical resolution over our original zero mean crosscorrelation algorithm resulting in reduced mixing of overlying or underlying stratigraphic features in general we analyze stratigraphic features using as narrow a temporal analysis window as possible typically determined by the highest usable frequency in the input seismic data in the limit one may confidently apply our new semblance based algorithm to a one sample thick seismic volume extracted along a conventionally picked stratigraphic horizon corresponding to a peak or trough whose amplitudes lie sufficiently above the ambient seismic noise in contrast near vertical structural features such as faults are better enhanced when using a longer temporal analysis window corresponding to the lowest usable frequency in the input data the calculation of reflector dip azimuth throughout the data volume allows us to generalize the calculation of conventional complex trace attributes including envelope phase frequency and bandwidth to the calculation of complex reflector attributes generated by slant stacking the input data along the reflector dip within the coherency analysis window these more robust complex reflector attribute cubes can be combined with coherency and dip azimuth cubes using conventional geostatistical clustering and segmentation algorithms to provide an integrated multiattribute analysis

orthogonal joint systems in the bergen area southwest norway and their regional significance abstract joint orientation measurements are reported from the bergen district southwest norway the prevailing joint systems form two near vertical near orthogonal sets striking preferentially nw se and ne sw the orientation of these fissure systems is consistent with fissure systems dissecting the surface formations precambrian to quaternary throughout britain and northwest france they also show general agreement with lineaments in southern norway as revealed by landsat imagery it is suggested that the orthogonal joint network represents a fundamental tectonic structure originally implanted into the mobile outer layer of the earth in precambrian time the network has been repeatedly reactivated throughout the earth s dynamic tectonic history it was probably formed in a shear regime caused by torsional deformation associated with rotation of large scale lithospheric units which is still operating today

common reflection surface stack image and attributes the common reflection surface stack provides a zero offset simulation from seismic multicoverage reflection data whereas conventional reflection imaging methods e g the nmo dip moveout stack or prestack migration require a sufficiently accurate macrovelocity model to yield appropriate results the common reflection surface crs stack does not depend on a macrovelocity model we apply the crs stack to a 2 d synthetic seismic multicoverage dataset we show that it not only provides a high quality simulated zero offset section but also three important kinematic wavefield attribute sections which can be used to derive the 2 d macrovelocity model we compare the multicoverage data derived attributes with the model derived attributes computed by forward modeling we thus confirm the validity of the theory and of the data derived attributes for 2 d acquisition the crs stack leads to a stacking surface depending on three search parameters the optimum stacking surface needs to be determined for each point

palaeomagnetism of the loch doon granite complex southern uplands of scotland the late caledonian palaeomagnetic record and an early devonian episode of true polar wander abstract the southern uplands terrane is an ordovician silurian back arc foreland basin emplaced at the northern margin of the iapetus ocean and intruded by granite complexes including loch doon 408 3 1 5 ma during early devonian times protracted cooling of this 130 km 3 intrusion recorded magnetic remanence comprising a predominant a magnetisation linked to initial cooling with dual polarity and mean direction d i 237 64 95 4 palaeopole at 316 e 21 n subsidiary magnetisations include mesozoic remanence correlating with extensional tectonism in the adjoining irish sea basin b d i 234 59 and minority populations c d i 106 2 and d d i 199 1 recording emplacement of younger 395 ma granites in adjoining terranes and the variscan orogenic event the a directions have an arcuate distribution identifying anticlockwise rotation during cooling a comparable rotation is identified in the orthotectonic caledonides to the north and the paratectonic caledonides to the south following closure of iapetus continental motion from midsoutherly latitudes 40 s at 408 ma to equatorial palaeolatitudes by 395 ma is identified and implies minimum rates of continental movement between 430 and 390 ma of 30 70 cm year more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander silurian devonian palaeomagnetic data from the british scandinavian caledonides define a 430 385 ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from gondwana they reconcile pre 430 ma and post 380 ma apw from this supercontinent and show that laurentia baltica avalonia lay to the west of south america with a relict rheic ocean opening to the north which closed to produce variscan orogeny by a combination of pivotal closure and right lateral transpression

3d constraints on a possible deep 2 5 km massive sulphide mineralization from 2d crooked line seismic reflection data in the kristineberg mining area northern sweden 2d crooked line seismic reflection surveys in crystalline environments are often considered challenging in their processing and interpretation these challenges are more evident when complex dif

identification of wrench faults using subsurface structural data criteria and pitfalls 1 many structures imaged with reflection seismic can resemble a wrench fault system because of the great structural diversity of strike slip faults and the wide range of contractional and extensional features associated with these faults because of these resemblances numerous faults are misidentified as wrench faults by workers using subsurface structural data credible identification requires combinations of profile and map criteria 1 narrow long straight throughgoing solitary master fault or zone of deformation 2 steep to moderate dip of the master fault at depth 3 offset of the top of basement 4 changes in relative upthrown side separation sense and or fault dip direction at depth or along the strike of the master fault 5 narrow fault slices within the master fault zone that steepen and join at depth negative and positive flower structures 6 different separation sense and orientation of relative upthrown side at these fault slices and 7 coeval en echelon flanking structures the identification should be confirmed with other criteria that refute alternative interpretations the main pitfalls to a reliable identification are not considering alternative structural styles and the misinterpretation of features or patterns that superficially resemble wrench systems the profile geometries of basement involved reverse or normal faults particularly those with structural inversion can resemble the single strand of convergent or divergent wrench faults profile geometries of flower structures can be mimicked by narrow contractional horsts or narrow extensional grabens complexly faulted anticlines or synclines splay faults fault zones intruded by salt or shale and overprinted structures pitfalls in interpreting map relationships are possible when structural patterns combine several tectonic influences are controlled by rock anisotropy or include elements hat are not coeval and if the patterns characteristic of other styles are misinterpreted many erroneous identifications of wrench faults can be eliminated by demonstrating that the investigated fault or feature lacks a straight trace lateral length or throughgoing continuity

palaeomagnetism of the foyers and strontian granites scotland abstract palaeomagnetic data from the newer granites of foyers and strontian show similar directions of magnetization with southerly declinations and horizontal to intermediate mostly downward dipping inclinations these palaeomagnetic directions which accord with previous data from the helmsdale granite are thought to be of siluro devonian age the present results are not sufficiently precise to justify discussion of possible lateral displacements along the great glen fault of the order of a few hundred kilometres but it is clear that the recent idea of a c 15 20 latitudinal offset along the fault in the carboniferous has to be disregarded

diffraction imaging by multifocusing correct identification of geologic discontinuities such as faults pinch outs and small size scattering objects is a primary challenge of the seismic method seismic response from these objects is encoded in diffractions our method images local heterogeneities of the subsurface using diffracted seismic events the method is based on coherent summation of diffracted waves arising in media that include interface discontinuities and local velocity heterogeneities this is done using a correlation procedure that coherently focuses diffraction energy on a seismic section by flattening diffraction events using a new local time correction formula to parameterize diffraction traveltime curves this time correction which is based on the multifocusing method depends on two parameters the emergent angle and the radius of curvature of the diffracted wavefront these parameters are estimated directly from prestack seismic traces the diffraction multifocusing stack dmfs can separate diffracted and reflected energy on a stacked section by focusing diffractions to the diffraction location and defocusing the reflection energy over a large area

analysis of ordinary chondrites using powder x ray diffraction 1 modal mineral abundances abstract powder x ray diffraction xrd is used to quantify the modal abundances in wt of 18 h 17 l and 13 ll unbrecciated ordinary chondrite falls which represents the complete petrologic range of equilibrated ordinary chondrites types 4 6 the xrd technique presents an effective alternative to traditional methods for determining modal abundances such as optical point counting and electron microprobe phase emp mapping the majority of chondrite powders in this study were previously prepared for chemical characterization from 8 to 20 g of material which is consistent with the suggested mass 10 g necessary to provide representative sampling of ordinary chondrites olivine and low ca pyroxene are the most abundant phases present comprising one half to three fourths of total abundances while plagioclase high ca pyroxene troilite and metal comprise the remaining xrd measured mineralogy pigeonite may also be present in some samples but it is fitted using a high ca pyroxene standard so exact abundances cannot be measured directly using xrd comparison of xrd measured abundances with calculated cross iddings pirsson washington cipw normative abundances indicates that systematic discrepancies exist between these two data sets particularly in olivine and high ca pyroxene this discrepancy is attributed to the absence of pigeonite as a possible phase in the cipw normative mineralogy oxides associated with pigeonite are improperly allocated resulting in overestimated normative olivine abundances and underestimated normative high ca pyroxene abundances this suggests that the cipw norm is poorly suited for determining mineralogical modal abundances of ordinary chondrites

mapping olivine abundance on asteroid 25143 itokawa from hayabusa nirs data abstract olivine is one of the main abundant mineral in the solar system and the determination of its abundance on a surface may give fundamental information about its evolution the study of surface distribution of olivine on asteroid 25143 itokawa through near infrared reflectance spectroscopy is a difficult goal because olivine and pyroxene bands centred at 1 m and 2 m are not entirely included in hayabusa nirs spectral range in this work the retrieval of olivine abundance has been performed by applying two different methods the first one uses some spectral indices to retrieve olivine abundance whilst the second one consists of the application of the hapke s theory in order to create synthetic spectra aimed at fitting a selection of nirs spectra the analysis performed with the first method brought to an approximately homogeneous distribution of olivine content 60 15 on average on itokawa s surface with the exception of sagamihara region which has a slightly up to 10 lower olivine content the second method brought to an average 60 7 5 olivine content within 5 selected spectra with the same reduction found in the spectrum from the sagamihara region all these values are in agreement with literature values on this topic especially with the ones retrieved from particles sampled in muses sea by the hayabusa probe

palaeomagnetism of the middle upper devonian esha ness ignimbrite w shetland abstract the ignimbrite division of the esha ness volcanics of w shetland has been studied palaeomagnetically the major blocking temperature spectrum ranges between 650 and 680 c implying that haematite is the principal remanence carrier the magnetization build up is simple a minor low stability magnetization aligned along the direction of the present earth s field is superimposed on a well defined and highly stable palaeomagnetic component with dec 220 and inc 3 the esha ness volcanic sequence is folded into a gentle syncline trending nne the rock collection concerned covers 8 sites 7 from the western flank and 1 from the eastern flank of the syncline the 7 sites on the west flank are consistently reversely magnetized while the site on the east flank has a normally directed magnetization after structural unfolding the normal and reverse directions are almost perfectly antiparallel i e the stable magnetization most likely predates the tilting which is probably of devonian age esha ness is located some 12 km to the west of the walls boundary fault wbf which is considered as the northward continuation of the great glen fault ggf the shallow palaeomagnetic inclination found in old red sandstone rocks to the west of ggf wbf corresponds to that encountered in most other european old red formations discounting therefore megascale 1000 km late post devonian movements along this dislocation

mineral chemistry of muses c regio inferred from analysis of dust particles collected from the first and second touchdown sites on asteroid itokawa the mineralogy and mineral chemistry of itokawa dust particles captured during the first and second touchdowns on the muses c regio were characterized by synchrotron radiation x ray diffraction and field emission electron microprobe analysis olivine and low and high ca pyroxene plagioclase and merrillite compositions of the first touchdown particles are similar to those of the second touchdown particles the two touchdown sites are separated by approximately 100 meters and therefore the similarity suggests that muses c regio is covered with dust particles of uniform mineral chemistry of ll chondrites quantitative compositional properties of 48 dust particles including both first and second touchdown samples indicate that dust particles of muses c regio have experienced prolonged thermal metamorphism but they are not fully equilibrated in terms of chemical composition this suggests that muses c particles were heated in a single asteroid at different temperatures during slow cooling from a peak temperature of approximately 800 c chemical compositions of plagioclase and k feldspar seem to have been modified ab and or contents changed during cooling but an did not this compositional modification is reproduced by a numerical simulation that modeled the cooling process of a 50 km sized itokawa parent asteroid after cooling some particles have been heavily impacted and heated which resulted in heterogeneous distributions of na and k within plagioclase crystals impact induced chemical modification of plagioclase was verified by a comparison to a shock vein in the kilabo ll6 ordinary chondrite where na k distributions of plagioclase have been disturbed

caldera formation on santorini and the physiography of the islands in the late bronze age the caldera of santorini is a composite structure with a subsidence history extending over 100 ka or more geomorphological mapping shows that the present day caldera wall is a complex assemblage of cliff surfaces of different ages and that collapse at santorini has repeatedly exhumed earlier caldera cliffs and unconformities cliffs bounding the southern southeastern and northwestern rims of the caldera are morphologically fresh and probably formed during or soon after the minoan eruption in the late bronze age the well scalloped shape of these cliffs is attributed to large scale rotational landslip around the margins of the minoan caldera the deposit from one landslip is preserved subaerially minoan landslips in southeast santorini detached along the basement unconformity exposing a cliff of the prevolcanic island the caldera wall in the north northeast and east preserves evidence for three generations of cliff those of minoan age and two earlier generations of caldera wall the two early calderas can be dated relative to a well established statigraphy of lavas and tuffs the presence of in situ minoan tephra plastered onto the present day caldera wall provides evidence that these ancient caldera cliffs had already been exhumed prior to the minoan eruption field relationships permit reconstruction of the physiography of bronze age santorini immediately before the minoan eruption the reconstruction differs from some previously published versions and is believed to be the most accurate to date bronze age sa ntorini had a large flooded caldera formed 21 ka ago this caldera must have acted as an excellent harbour for the bronze age inhabitants of the island the 3 6 ka minoan eruption deepened and widened the extant caldera the volume of minoan collapse 25 km3 is in good agreement with published estimates for the volume of discharged magma if between 5 and 8 km3 of minoan ignimbrite ponded as intracaldera tuff

british paleomagnetism iapetus ocean and the great glen fault new paleomagnetic data for britain establish the width of the iapetus ocean as 1000 km in mid ordovician time but do not yet allow precise estimates of the rate of its subsequent closure the new lower paleozoic data from northern scotland are similar to data from the rest of britain but are not compatible with north america data on the bullard et al reassembly if this discrepancy is correct lines of major transcurrent dislocation have yet to be identified either in the north atlantic continental shelves and margins or in eastern north america and the great glen fault was not the main locus of offset between britain and north america the tectonic complexities associated with this configuration require further evaluation

anisotropic 3d full waveform inversion abstractwe have developed and implemented a robust and practical scheme for anisotropic 3d acoustic full waveform inversion fwi we demonstrate this scheme on a field data set applying it to a 4c ocean bottom survey over the tommeliten alpha field in the north sea this shallow water data set provides good azimuthal coverage to offsets of 7 km with reduced coverage to a maximum offset of about 11 km the reservoir lies at the crest of a high velocity antiformal chalk section overlain by about 3000 m of clastics within which a low velocity gas cloud produces a seismic obscured area we inverted only the hydrophone data and we retained free surface multiples and ghosts within the field data we invert in six narrow frequency bands in the range 3 to 6 5 hz at each iteration we selected only a subset of sources using a different subset at each iteration this strategy is more efficient than inverting all the data every iteration our starting velocity model was obtained using standard psdm model bui

short paper palaeomagnetic constraints on the geological terrane models of the scottish caledonides a compilation of new and published palaeomagnetic data from the scottish caledonides allows construction of apparent polar wander paths apwps for the grampian highlands midland valley and southern uplands terranes geological incompatibilities between terranes are the result of predominantly palaeolongitudinal movements prior to late silurian times a common late silurian early devonian corner in each apwp suggests the terranes to have docked to within the errors of the individual determinations by this time

palaeomagnetic re examination of the basal caithness old red sandstone aspects of local and regional tectonics abstract a palaeomagnetic re examination of the basal strata of the caithness old red sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region after structural correction the dominant remanence component has d 205 i 3 95 6 4 n 27 the existence of this shallow inclined magnetization in the middle devonian strata of caithness invalidates the model proposed by van der voo and scotese 1981 involving a ca 2000 km sinistral offset along the great glen fault in the carboniferous however the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone however the possibility of there having been a much larger transcurrent shift between europe and north america in late post devonian times accumulated along various fracture zones within the caledonian fold belt is discussed on the basis of an inferred overprinted magnetization it is tentatively concluded that the tectonic deformation of the old red sandstone of caithness has a mid jurassic or younger age

diffraction imaging in depth high resolution imaging is of great value to an interpreter for instance to enable identification of small scale faults and to locate formation pinch out positions standard approaches to obtain high resolution information such as coherency analysis and structure oriented filters derive attributes from stacked migrated images since they are image driven these techniques are sensitive to artifacts due to an inadequate migration velocity in fact the attribute derivation is not based on the physics of wave propagation diffracted waves on the other hand have been recognized as physically reliable carriers of high or even super resolution structural information however high resolution information encoded in diffractions is generally lost during the conventional processing sequence indeed migration kernels in current migration algorithms are biased against diffractions we propose here methods for a diffraction based data oriented approach to image resolution we also demonstrate the different behaviour of diffractions compared to specular reflections and how this can be leveraged to assess characteristics of subsurface features in this way a rough surface such as a fault plane or unconformity may be distinguishable on a diffraction image and not on a traditional reflection image we outline some characteristic properties of diffractions and diffraction imaging and present two novel approaches to diffraction imaging in the depth domain the first technique is based on reflection focusing in the depth domain and subsequent filtering of reflections from prestack data the second technique modifies the migration kernel and consists of a reverse application of stationary phase migration to suppress contributions from specular reflections to the diffraction image both techniques are proposed as a complement to conventional full wave pre stack depth migration and both assume the existence of an accurate migration velocity

palaeomagnetic study of the cairnsmoor of fleet granite and criffel dalbeattie granodiorite contact aureoles caledonian tectonics of the southern uplands of scotland and devonian palaeogeography the plutons of cairnsmoor of fleet 392 2 ma and criffel dalbeattie 397 2 ma both mineral isochron ages comprise two of four major post tectonic granitic complexes emplaced into the southern uplands an ordovician silurian back arc and foreland basin complex formed at the northern margin of the iapetus suture to expand the palaeomagnetic record of the southern uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks a uniform anisotropy of magnetic susceptibility ams fabric across the cairnsmoor of fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late acadian regional stress operative during or soon after emplacement of the pluton in middle devonian times magnetization during slow cooling recorded a dual polarity a remanence in granite and hornfels with mean direction d i 92 2 95 6 5 yielding a palaeopole q 6 at 2 n 265 e linked to cooling at c 392 ma subsidiary magnetizations are overprints imparted during variscan tectonism b d i 194 6 and jurassic rifting within the adjoining irish sea basin c c 160 140 ma d i 172 52 the criffel dalbeattie pluton has more complex ams fabrics recording both deformation and emplacement effects hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole which is otherwise dominated by paramagnetism the granodiorite is more strongly magnetized than the country rocks accounting for a positive aeromagnetic anomaly a fairly dispersed dual polarity remanence mean d i 115 55 95 18 in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of britain into southerly palaeolatitudes at c 410 ma and indicates an early devonian emplacement age the palaeofield at c 397 ma the currently accepted isotopic age is recorded by a minority overprinted remanence mean d i 272 2 95 12 similar to the record in the cairnsmoor of fleet pluton and granites from the adjoining lake district terrane granite complexes of the southern uplands block collectively record regional rotation and excursion of britain into southerly latitudes between c 410 and 390 ma comparable silurian devonian palaeomagnetic poles identify common apparent polar wander apw in paratectonic and orthotectonic terranes from the variscan front in the south to the laurentian foreland in the north following climactic acadian deformation apw between 430 and 390 ma embracing the post closure history of the caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander the 110 arc length is identical to polar shift identified between mid silurian and lower middle devonian poles from gondwana the two paths superimpose to show that the western margin of gondwana was in proximity to the se margin of laurentia during acadian deformation in early middle devonian times and remote from the caledonides the residual rheic ocean subsequently closed by a combination of pivotal and left lateral strike slip motions

hypervelocity impact fragmentation of basalt and shale projectiles abstract results are presented for the fragmentation of projectiles in laboratory experiments 1 5 mm cubes and spheres of basalt and shale were impacted onto water at normal incidence and speeds from 0 39 to 6 13 km s 1 corresponding to peak shock pressures 0 7 32 gpa projectile fragments were collected and measured over 100 000 fragments in some impacts at sizes down to 10 m power laws were fitted to the cumulative fragment size distributions and the evolution of the exponent vs impact speed and peak shock pressure found the gradient of each of these power laws increased with increasing impact speed peak shock pressure the percentage of the projectiles recovered in the impacts was found and used to estimate projectile remnant survival in different solar system impact scenarios at the mean impact speed appropriate to that scenario for pluto the moon and in the asteroid belt approximately 55 40 and 15 respectively of an impactor could survive and be recovered at an impact site finally the catastrophic disruption energy densities of basalt and shale were measured and found to be 24 104 j kg 1 and 9 104 j kg 1 respectively a factor of 2 5 difference these corresponded to peak shock pressures of 1 to 1 5 gpa basalt and 0 8 gpa shale this is for near normal incidence impacts where tensile strength is dominant for shallow angle impacts we suggest shear effects dominate resulting in lower critical energy densities and peak shock pressures we also determine a method to ascertain information about fragment sizes in solar system impact events using a known size of impactor the results are used to predict projectile fragments sizes for the veneneia and rheasilvia crater forming impacts on vesta and similar impacts on ceres

multicomponent magnetizations in the foyers old red sandstone northern scotland and their bearing on lateral displacements along the great glen fault summary the foyers sandstone a constituent of the old red sandstone of northern scotland located on the eastern flank of the great glen fault has yielded a multicomponent palaeomagnetic structure the principal magnetization called the b component with in situ mean direction d i 185 8 ag5 2 7 post dates the local mid devonian acadian tectonic event but this direction corresponds well with characteristic directions displayed by devonian and lower carboniferous rocks of scotland the most significant overprinted component the a2 magnetization with in situ mean direction d i 194 27 ag5 4 lo has a more patchy occurrence than the b remanence the strongest development of a2 being observed at sites that are situated close to the great glen fault zone the a2 direction is consistent with a hercynian age a third magnetization the a1 component of suggested lower tertiary age has been found at one of the sites close to the fault zone the a2 and b remanences support previously proposed declination discordances in corresponding magnetizations across the great glen fault it is concluded that the foyers sandstone developed its multicomponent magnetization through repeated tectonic events on the great glen fault in acadian hercynian and alpine times respectively

palaeozoic palaeogeography of laurentia and its margins a reassessment of palaeomagnetic data summary the recognition of wide spread remagnetization events during the palaeozoic has greatly added to the problems of reconstructing the palaeozoic drift history of laurentia and in recent years increasing emphasis has been placed on evaluating the relative reliability of palaeomagnetic data in an attempt to overcome some of these difficulties in the light of these developments a revised apparent polar wander apw path has been constructed following a rigorous evaluation of all published palaeozoic palaeomagnetic data from laurentia and its palaeozoic margins i e north america northern britain northern ireland and greenland the use of data from the laurentian margins when corrected for the opening of the north atlantic has resulted in a revision of previously published palaeozoic palaeogeographic reconstructions and these new palaeogeographic reconstructions for laurentia are in good agreement with faunal and lithological data a comparison of this new apw path with other apw paths from avalonia and baltica reveals a very close agreement between all three paths from mid silurian times onwards indicating that the closure of the iapetus ocean and the tornquist sea was essentially complete by this time laurentia occupied equatorial latitudes from cambrian to early silurian times and its drift history was dominated by anti clockwise rotation up to 0 7 ma 1 and low latitudinal drift rates up to 3 cm yr 1 late silurian and early devonian times were characterized by increased rates of anti clockwise rotation up to 1 3 ma 1 and southerly directed latitudinal drift rates up to 13 cm yr 1 of the continent which place laurentia in moderate southerly latitudes 30 40 s by the early devonian these palaeolatitudes are comparable with those from avalonia and baltica for this time indicating that the amalgamation of eurmerica had already taken place by late devonian time laurentia had begun to drift north rates up to 5 cm yr 1 towards equatorial latitudes by mid carboniferous time

carbide metal assemblages in a sample returned from asteroid 25143 itokawa evidence for methane rich fluids during metamorphism abstract we found that the particle ra qd02 0115 returned by the hayabusa spacecraft from near earth asteroid 25143 itokawa contains the iron carbide haxonite fe 21 9 22 7 co 0 2 0 3 ni 0 2 0 8 c 6 and several fe ni alloys including multi domain tetrataenite and spinodally decomposed taenite ellipsoidal to nearly spherical voids occur throughout the particle and suggest the presence of a fluid phase during textural and chemical equilibration of the host rock within the parent asteroid of 25143 itokawa the calculated solubility of carbon in fe ni metal indicates that the carbide formed at temperatures larger than 600 c during thermal metamorphism of the ll chondritic mineral assemblage haxonite formed metastably with respect to graphite and cohenite probably due to its high degree of lattice match with neighboring taenite a low cooling rate at peak metamorphic temperatures and the hindered nucleation of graphite thermodynamic equilibrium calculations indicate that the fluid present was dry h 2 o poor and dominated by methane the reactive fluid most plausibly had an atomic h c ratio of 4 5 and was derived from the reduction of macromolecular insoluble organic matter iom that initially co accreted with water ice the initial presence of water is a necessary assumption to provide sufficient hydrogen for the formation of methane from hydrolyzed iom metallic iron was in turn partially oxidized and incorporated into the ferromagnesian silicates during the high temperature stage of metamorphism an exemplary bulk reaction from unequilibrated material on the left to an equilibrated assemblage on the right may be written as 330 ch 0 8 o 0 2 iom 500 h 2 o ice g 681 fe in alloy 566 fesio 3 in opx 300 ch 4 g 32 h 2 g 5 fe 23 c 6 in hx 566 fe 2 sio 4 in ol opx orthopyroxene hx haxonite ol olivine g fluid species the best estimate of the fluid rock ratio in the region of the ll parent body where ra qd02 0115 formed is about 3 10 3 and corresponds to an initial ice rock ratio of about 7 10 3 both by mass

3 d seismic discontinuity for faults and stratigraphic features the coherence cube seismic data are usually acquired and processed for imaging reflections this paper describes a method of processing seismic data for imaging discontinuities e g faults and stratigraphic features one application of this nontraditional process is a 3 d volume or cube of coherence coefficients within which faults are revealed as numerically separated surfaces figure 1 compares a traditional 3 d reflection amplitude time slice with the results of the new method to our knowledge this is the first published method of revealing fault surfaces within a 3 d volume for which no fault reflections have been recorded

geophysical lineaments and deep continental structure lineaments may be detected remotely by imaging electromagnetic radiation both inside and outside the visible spectrum and by examining the natural force fields particularly the magnetic and gravitational most electromagnetic imagery shows shallow structure which is readily correlated with surface geology gravimetry and magnetometry may show much deeper structure which is unrelated to surface structure or incompatible with its most obvious downward extrapolation seismology then offers scope for the high resolution study of buried lineaments revealed in this way from such examinations it is clear than many lineaments do not traverse the entire vertical extent of the lithosphere internal horizontal detachment may cause once through going structures to become segmented a number of examples will be given including local scottish examples the great glen fault and the cruachan lineament

the tethys sea and the alpine himalayan orogenic belt mega elements in a new global tectonic system abstract analysis of meso cainozoic palaeomagnetic data for africa india and eurasia has led to the development of a new mobilistic alpine plate tectonic model characterized by a hierarchical system of plates in relative rotation the new model which discounts seafloor spreading implies that there have been no significant palaeogeographic changes in the overall distribution of continental and oceanic regions the mid oceanic ridges are interpreted as transpressive tectonic features caused by rotation of megaplates containing both continental and oceanic crust the isostatic uplift due to crustal lithospheric thickening giving rise to the general ridge topography as well as to the ridge parallel structural grain the new plate tectonic theory gains strong support from a variety of geophysical geological and palaeoclimatological evidence and several observations that have remained enigmatic or awkward within the context of the orthodox model can be readily accounted for in the new tectonic framework the model maintains the tethys as a relatively narrow epicontinental sea which during its maximum extent stretched latitudinally from the caribbean across the central atlantic to se asia the alpine himalayan orogenic belt developed along the boundary of two megaplates in relative rotation which provided a transpressive tectonic regime the location of the plate boundary to the north of the mediterranean has important implications for discussion of mediterranean microplates for example it now seems that italy has been subjected to 10 15 of clockwise microplate rotation previous conclusions in favour of 30 40 of anticlockwise rotation are regarded as artefacts which arise from incorrectly linking the mediterranean region to the european palaeomagnetic frame instead of to the african one the model suggests further that the indo pakistani plate was closely tied to eurasia this challenges the conventional view that the peninsula was part of an alleged gondwanaland the new pre drift configuration implies that the indo pakistani plate rotated 135 clockwise at around the cretaceous tertiary boundary before redocking with asia in approximately its present relative orientation

structure of remanent magnetization and magnetic mineralogy of the cheviot lavas lower devonian northeast england abstract demagnetization data and magnetomineralogical results from the lower devonian cheviot lavas have revealed a complex remanence structure associated with extensive low temperature alteration of the primary magnetic oxides the original cooling magnetization has apparently been replaced by chemical remanences acquired in one or more dual polarity fields superposition of normal and reverse field components appears to have created fairly stable but what may be palaeomagnetically irrelevant directions which if not recognized or experimentally resolved may easily lead to incorrect conclusions evaluation of the total data base suggests that the characteristic palaeomagnetic axis is sub horizontal with an approximately n s declination the age range of this magnetization may vary from lower devonian to lower carboniferous the new experimental results throw into question some previous palaeomagnetic conclusions regarding the british lower old red sandstone lavas further critical study of the fossil magnetization of lower devonian rocks from britain should be on intrusive masses as such rocks are likely to have escaped at least the extensive surface oxidation and associated remagnetization complexities that have apparently developed in the extrusive suites

strike slip terranes and a model for the evolution of the british and irish caledonides evidence is presented that many of the major strike faults in the british and irish caledonides were active as sinistral strike slip zones in the end silurian to pre mid devonian period some such as the highland boundary fault moved in this way at an earlier stage in the ordovician these data allow the caledonian rocks lying between the laurentian miogeocline whose basement is represented by the lewisian moine and possibly the dalradian and the gondwanaland miogeocline midland platform and welsh basin to be re analysed as a group of disorganized terranes which originated to the southwest in north america and southwest europe africa prior to the silurian the highland border terrane and northern belt terrane are interpreted as duplicated pieces of a mid ordovician sequence which was a back are to northwest subduction the midland valley terrane is interpreted as a slice of laurentian foreland onto which ophiolites were obducted in the lower ordovician but which became the basement of a continental margin arc to northwest subduction in the mid ordovician the cockburnland terrane is inferred to be part of the same arc repeated and then broken up and dispersed by continuing strike slip the connemara terrane is regarded as an allochthonous piece of the dalradian miogeocline and the south mayo terrane as a remnant of an early ordovician arc and fore arc which in mid ordovician times became a back arc marginal basin to northwest subduction the lake district wexford terrane is part of an arc to southeast subduction under gondwanaland whose activity climaxed in the mid ordovician the central terrane is interpreted as a silurian overstep assemblage which blankets the junction between laurentian and gondwanaland derived oceanic terranes and therefore iapetus is regarded as an ordovician ocean which closed prior to the silurian the model suggests that at the end of the silurian a clockwise rotating gondwanaland having carried laurentia into collision with baltica broke free and created a major sinistral strike slip zone which disrupted the ordovician palaeogeography in the british isles north american sector of iapetus

oxygen three isotope ratios of silicate particles returned from asteroid itokawa by the hayabusa spacecraft a strong link with equilibrated ll chondrites abstract oxygen three isotope ratios of four mineral phases olivine low ca and high ca pyroxene and plagioclase in seven silicate particles from asteroid itokawa were measured to investigate oxygen isotope systematics of asteroidal materials in order to obtain highly precise and accurate oxygen isotope ratios 0 3 in 18 o we used a new indium mounting method that minimizes potential instrumental mass fractionation due to surface topography the oxygen isotope data of the seven itokawa particles are distributed above the terrestrial fractionation line with indistinguishable o 17 o 17 0 52 o 18 values of 1 34 0 36 2sd n 22 which is similar to those of itokawa particles analyzed by yurimoto et al 2011 and nakamura et al 2012 however 18 o values of the seven itokawa particles are tightly clustered 3 9 to 5 9 vsmow compared to previous analyses of the itokawa particles 1 4 to 8 8 the average 17 o values and 2sd of the two equilibrated ordinary chondrites eocs which were analyzed for comparison are 1 31 0 49 for st severin ll6 and 0 83 0 64 for guarena h6 the 17 o values of the seven itokawa particles are similar to those of st severin the average bulk 18 o value of the seven itokawa particles is estimated as 4 8 which is within the 18 o range of bulk ll5 and ll6 chondrites it is suggested that itokawa particles are similar to those of equilibrated ll chondrites which strengthens a link between asteroid itokawa and equilibrated ll chondrites that fell to earth the seven itokawa particles and two eocs have 18 o values of high ca pyroxene that are lower than those of olivine indicating that oxygen isotopes between the two mineral phases are not in equilibrium apparent oxygen isotope equilibrium temperatures estimated from a pair of low ca pyroxene and olivine are excessively higher than those from a pair of plagioclase and high ca pyroxene 800 c and peak metamorphic temperatures oxygen isotopes in olivine and low ca pyroxene do not represent equilibrium on the other hand oxygen isotope ratios in high ca pyroxene and plagioclase were mass dependently fractionated accompanied by recrystallization and possibly by diffusion during thermal metamorphism values of 18 o in high ca pyroxene were lowered by exchange which may have resulted in the reverse 18 o fractionation between olivine and high ca pyroxene

paleomagnetic results from the peterhead granite scotland implication for regional late caledonian magnetic overprinting abstract palaeomagnetic results from the peterhead granite silurian ne grampian highlands define a characteristic magnetization with overall parameters of dec 180 4 inc 22 3 and a 95 5 2 these results agree with other newer granite data from north scotland an east west trending dyke cutting the peterhead granite shows an extremely well defined paleomagnetic direction with dec 192 5 inc 19 1 and a 95 1 3 suggesting a permo carboniferous age the peterhead granite results are seen in conjunction with other palaeozoic palaeomagnetic data from scotland it is concluded that the observed directional spread of characteristic remanence directions previously reported from the aberdeenshire gabbros are not related to slow post orogenic cooling watts and briden but is rather seen as the result of late caledonian magnetic overprinting

three dimensional microstructure of samples recovered from asteroid 25143 itokawa comparison with ll5 and ll6 chondrite particles in this study the three dimensional 3 d microstructure of 48 itokawa regolith particles was examined by synchrotron microtomography at spring 8 during the preliminary examination of hayabusa samples moreover the 3 d microstructure of particles collected from two ll6 chondrites ensisheim and kilabo meteorites and an ll5 chondrite tuxtuac meteorite was investigated by the same method for comparison the modal abundances of minerals especially olivine bulk density porosity and grain size are similar in all samples including voids and cracks these results show that the itokawa particles which are surface materials from the s type asteroid itokawa are consistent with the ll chondrite materials in terms of not only elemental and isotopic composition of the minerals but also 3 d microstructure however we could not determine whether the itokawa particles are purely ll5 ll6 or a mixture of the two no difference between the particles collected from rooms a and b of the sample chamber corresponding to the sampling sequence of the spacecraft s second and first touchdowns respectively was detected because of the statistically small amount of particles from room b

structural restoration of seismic and geologic sections in extensional regimes seismic sections from extensional basins can be structurally transformed to produce restored seismic sections showing subsurface geology of the past restoration is achieved using a simple and effective algorithm which is suited for use in an interactive seismic interpretation environment seismic data are resampled along paleovertical trajectories which are the deformed equivalents of what were originally closely spaced vertical lines paleoverticals are generated within the present day section as follows 1 origin points are equally spaced along a reference horizon that corresponds to an original depositional surface 2 paleovertical segments are vertical within fault blocks 3 paleoverticals are offset across faults by the reference horizon slip and 4 restor d depth is equated to distance down a paleovertical these rules can be modified to conform with such styles of extensional deformation as vertical simple shear inclined shear or rigid rotation and to account for compaction structures with complex fault patterns readily are restored several examples from the gulf of mexico and offshore west africa show that seismic restoration is useful for 1 unravelling structural and stratigraphic history 2 revealing tectonically masked features and 3 checking interpretations across faults for seismic correlation and structural balance geologic sections can be restored in similar fashion

muses c target asteroid 25143 1998 sf36 a reddened ordinary chondrite abstract near earth asteroid 25143 1998 sf36 is a planned target for the japanese muses c sample return mission high signal to noise and relatively high resolution 50 a visible and near infrared spectroscopic measurements obtained during this asteroid s favorable 2001 apparition reveal it to have a red sloped s iv type spectrum with strong 1 and 2 m absorption bands analogous to those measured for ordinary chondrite meteorites this red slope which is the primary spectral difference between 25143 1998 sf36 and ordinary chondrite meteorites is well modeled by the spectrum of 0 05 nanophase iron npfe0 proposed as a weathering mechanism by pieters et al 2000 asteroid 1998 sf36 appears to have a surface composition corresponding to that of ordinary chondrite meteorites and is most similar in spectral characteristics and modeled olivine pyroxene content to the ll chondrite class

nanomorphology of itokawa regolith particles application to space weathering processes affecting the itokawa asteroid abstract the morphological properties of 26 regolith particles from asteroid itokawa were observed using scanning electron microscopes in combination with an investigation of their three dimensional shapes obtained through x ray microtomography surface observations of a cross section of the ll5 chondrite and of crystals of olivine and pyroxene were also performed for comparison some itokawa particles have surfaces corresponding to walls of microdruses in the ll chondrite where concentric polygonal steps develop and euhedral or subhedral grains exist these formed through vapor growth owing to thermal annealing which might have been caused by thermal metamorphism or shock induced heating in itokawa s parent body most of the itokawa particles have more or less fractured surfaces indicating that they were formed by disaggregation probably caused by impacts itokawa particles with angular and rounded edges observed in computed tomography images are associated with surfaces exhibiting clear and faint structures respectively these surfaces can be interpreted by invoking different degrees of abrasion after regolith formation a possible mechanism for the abrasion process is grain migration caused by impact driven seismic waves space weathered rims with blisters are distributed heterogeneously across the itokawa regolith particles this heterogeneous distribution can be explained by particle motion and fracturing combined with solar wind irradiation of the particle surfaces the regolith activity including grain motion fracturing and abrasion might effectively act as refreshing process of itokawa particles against space weathered rim formation the space weathering processes affecting itokawa would have developed simultaneously with space weathered rim formation and regolith particle refreshment

magnetization history of the upper silurian dingle group sw ireland summary the dingle group a continental upper silurian redbed succession in sw ireland has yielded a multicomponent palaeomagnetic record the oldest magnetization retrieved the b component with in situ mean direction of d i 191 9 k 40 95 3 6 n 41 post dates the local late silurian mid devonian acadian tectonic deformation and is in very good agreement with the prevailing remanences in other caledonian rocks of western ireland an overprinted a component with in situ mean direction d i 194 16 k 53 95 3 1 n 41 is about as prevalent as the b component and was probably acquired in late palaeozoic kiaman times by comparison with other published data including a positive fold test for corresponding magnetization of upper silurian redbeds in clare island it is concluded that the flat lying b field relative to britain and ireland was in existence at least from the late silurian to late middle carboniferous i e covering a minimum time span of 100 m yr

microseismic activity in the hellenic volcanic arc greece with emphasis on the seismotectonic setting of the santorini amorgos zone abstract the volcanic arc of the hellenic subduction zone with its four volcanic centers is of major relevance when evaluating the seismovolcanic hazard for the aegean region we present results from a 22 station temporary seismic network cycnet in the central hellenic volcanic arc hva cycnet recordings allow to analyze the level and spatio temporal evolution of microseismic activity in this region for the first time a total of 2175 events recorded between september 2002 and july 2004 are analyzed using statistical methods cluster analysis and relative relocation techniques we identify distinct regions with significantly varying spatio temporal behavior of microseismicity a large portion of the seismic activity within the upper crust is associated with the presence of islands representing horst structures that were generated during the major oligocene extensional phase in contrast the central part of the cyclades metamorphic core complex remains aseismic considering our magnitude threshold of 1 8 except one spot where events occur swarm like and with highly similar waveforms the highest activity in the study area was identified along the sw ne striking santorini amorgos zone within this zone the submarine columbo volcano exhibits strong temporal variations of seismic activity on a high background level this activity is interpreted to be directly linked to the magma reservoir and therein the migration of magma and fluids towards the surface ne of columbo where no volcanic activity has yet been reported we observe a similar seismicity pattern with small scaled activity spots that might represent local pathways of upward migrating fluids or even developing volcanic activity within this zone of crustal weakness in contrast the santorini and milos volcanic complexes do not show significant temporal variations and low to moderate background activity respectively relating our results to the distribution of historical earthquakes and the gps derived horizontal velocity field we conclude that the santorini amorgos zone is presently in the state of right lateral transtension reflecting a major structural boundary of the volcanic arc subdividing it into a seismically and volcanically quiet western and an active eastern part

the miocene to present kinematic evolution of the eastern mediterranean and middle east and its implications for dynamics the present kinematics in the eastern mediterranean and middle east is now well constrained by gps measurements and dominated by a circular counterclockwise motion while arabia rotates rigidly the rotational velocities increase from the levant to aegea causing extension in western anatolia we place the present pattern in light of the post middle miocene geological history and propose that in addition to the effects of the hellenic subduction and zagros collision subduction zones on driving the surface motion an underlying asthenospheric flow is also required this counterclockwise toroidal flow is expected to exist around the eastern edge of the african slab owing to subduction rollback and it helps explain the initiation of the north anatolia fault the extrusion and uplift of anatolia and the mid miocene to recent volcanism in eastern anatolia the afar plume had a similar effect on the acceleration of arabia at 40 mya