the oceanic crust in 3d paleomagnetic reconstruction in the troodos ophiolite gabbro the troodos complex cyprus provides an opportunity to study the structural configuration along a fossil intersection of a spreading axis and a transform fault we complement studies at troodos that have reconstructed the brittle deformation of the upper crust by new paleomagnetic data from the gabbro suite the gabbro suite is exposed at the extinct spreading axis continuing the solea graben toward the intersection with the fossil arakapas oceanic transform this is a unique exposure of deep crustal rocks formed at both an inside corner and an outside corner of a ridge transform intersection remanence directions from gabbros 23 sites were used as indicators for rigid body rotation the spatial distribution of rotation axes allow recognition of three regions to which deformation is partitioned 1 a western region outside corner that experienced primarily tilt about horizontal axis 2 a central region with minor rotation and 3 an eastern area inside corner where vertical axis rotations are dominant the absence of significant rotation in the 6 km wide central domain together with its location between the inside and the outside corner uncover the root of a fossil axial volcanic zone a zone sufficiently hot so the upper crust can decouple from the substrate clockwise rotation in the gabbro increases from the axial zone eastward similar to that in the overlying dikes indicating coupling of the lower crust with the brittle upper oceanic crust the transition from the decoupled layers of sheeted dikes and gabbro in the axial zone to the dikes gabbro coupling in the inside corner is in keeping with deepening of the brittle ductile transition from the dike gabbro boundary into the lower crust away from the axial zone our conclusions are consistent with one of the previous reconstructions in which the solea spreading axis was orthogonal to the arakapas trunsform fault and with recent studies of the present day lower oceanic crust however the newly inferred surface trace of the solea spreading axis is further to the east probably reflecting the tilt of axial upper crust rotated blocks c 2006 elsevier b v all rights reserved

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

microearthquakes beneath median valley of mid atlantic ridge near 23 n tomography and tectonics data from a microearthquake experiment in the median valley of the mid atlantic ridge near 23 n in 1982 are used to measure earthquake source parameters to determine the laterally heterogeneous seismic velocity structure across the inner floor and to develop a kinematic tectonic model for this portion of the median valley fifty three microearthquakes occurred over a 10 day period beneath the median valley inner floor and eastern rift mountains twenty of 23 well located inner floor epicenters define a line of activity about 17 km long having a strike of n25 e and located near an along axis depression some 300 400 m deeper than surrounding regions earthquakes with well resolved hypocenters generally have focal depths of 4 8 km beneath the seafloor of both the inner floor and the rift mountains the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure composite fault plane solutions for inner floor events indicate normal faulting on planes dipping at angles near 45 normal faulting mechanisms although poorly constrained are also indicated for the rift mountain microearthquakes seismic moments approximate fault dimensions and average stress drops for the largest events recorded are 1019 1020 dyn cm 200 400 m and 1 70 bars respectively a twodimensional tomographic inversion of p wave travel time residuals from microearthquakes and local shots indicates a well resolved lateral heterogeneity in crustal velocity structure across the median valley inner floor p wave velocities at 1 5 km depth within a zone less than 10 km wide beneath the central inner floor are lower by several percent than in surrounding regions the most likely explanation for the low velocities is that the region is the site of the most recent local magmatic injection and remains pervasively fractured as a result of rapid hydrothermal quenching of the newly emplaced crustal column by this view the seismic velocity structure at the ridge axis evolves probably by the sealing of cracks and pores within the first few hundred thousand years of crustal accretion consideration of the detailed sea beam bathymetry in this region of the inner floor the characteristics of large earthquakes that the region has experienced during the past 25 years and the results of the microearthquake and tomography analysis suggests that this section of the median valley has been undergoing continued horizontal extension and block faulting without significant crustal injection of magma for at least the past 104 years

the accretion of oceanic crust by episodic sill intrusion seismic refraction data from mature oceanic regions show remarkable consistency in crustal thickness between sites yet the characteristics of different spreading centers show great variation this leads to the paradox that the similarity in mature oceanic crust suggests that the igneous processes in crustal accretion are the same across a range of spreading rates but that the differences in the spreading centers themselves imply the opposite we have developed a simple two dimensional model for crustal accretion at oceanic spreading centers by the episodic addition of sills at a high level within the crust as implied by recent geophysical studies of spreading centers using our model we have calculated velocity and temperature fields in the crust including the effects of hydrothermal cooling we show that oscillations in the temperature field due to episodicities of 200 20000 years have an effect localized to the region within 50 500 m of the intrusion by assuming that all latent heat and excess specific heat introduced by the injection of sills is convected away by hydrothermal circulation we estimate that the maximum heat transfer associated with hydrothermal cooling in the upper crust is of the order of 10 times greater than the conductive flux we have calculated seismic velocities associated with the model temperature field in the crust and we show that these are consistent with results from both seismic refraction and reflection experiments the model may be applied to both fast and slow spreading ridges and it accounts for the apparent increases in both the mean crustal temperature and the frequency of presence of melt with increasing spreading rate hence resolving the paradox we also show that the implications of this model are consistent with geological observations of layering in ophiolites

spreading structure of the troodos ophiolite cyprus orientations of dikes within the sheeted dike complex of the troodos ophiolite reveal primary spreading structure produced at a complex ridge transform intersection three structural grabens are defined by listric and planar normal faults and rotated dikes that dip symmetrically toward graben axes faults flatten at depth into a detachment within the upper parts of the plutonic complex large exhalative massive sulfide deposits occur within the pillow sections of two of the grabens and appear to be associated with underlying altered and mineralized normal fault zones that channeled hydrothermal fluids we suggest that the grabens are fossil axial valleys produced by successive eastward jumps of an approximately north trending present coordinates slow spreading ridge crest a simple model for ridge migration indicates eastward jumps of 8 13 km changes in ridge orientation are suggested by changes in trends of dikes and graben axes comparison of the pattern of dikes near the arakapas fault zone with the structure of active ridge transform intersections suggests that the fault is a right offset sinistral transform in contrast to earlier models in which a ridge to the west of the exposed troodos complex was proposed

surface hydrology and soil movement in an arid karst the nullarbor plain australia the nullarbor plain is the largest karst area in australia 220 000 km2 and one of the largest in the world its climate is arid koppen bwk and bwh and the surface relief is less than 10m the landscape is divided into extensive closed karstic depressions separated by low rocky ridges and the dominant vegetation is chenopod shrubland the extent and severity of soil degradation has been assessed using remote sensing gps rectified images from 1972 1973 1979 1983 1988 and 1991 have been compared for two sites on the nullarbor over the 19 years the total extent of bare soil has reduced significantly but some areas around water points have degraded and there is some disturbance due to fossorial wombats sheet flow processes occur during intense rainfall events which happen two or three times per decade runoff only occurs after the 10 or 50 year return frequency events and at these times turbid water ponds in depressions and enters caves surface soil sorptivity and hydraulic conductivity differ markedly between ridges and depressions the ridges are clearly zones of groundwater recharge while ponding is evident in most depressions sorptivity is influenced by the extent and nature of ground cover and cryptogam crusts on the soil the landscape has been divided into classes on the basis of vegetation type and percentage of foliage cover in each class the activities of fallout radionuclides will be determined by high resolution spectroscopy preliminary results suggest that in undisturbed sites little sediment movement has occurred over the time scale of cesium 137 the last 35 years but that the landscape has been well sorted over a much longer time scale future work will investigate disturbed sites to estimate relative soil loss during the pastoral period

magnetic anisotropy of serpentinized peridotites from the mark area implications for the orientation of mesoscopic structures and major fault zones 1 mantle derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from ocean drilling program odp site 920 mid atlantic ridge south of kane mark to provide insights into the tectonic processes responsible for the exposure of these deep seated rocks at the seafloor paleomagnetic data from 214 samples from site 920 document a remarkably consistent inclination 36 1 0 8 1 4 that is shallower than either the expected geocentric axial dipole inclination 40 7 or present day inclination 41 9 at the site we show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence possibly augmented by viscous processes at moderate temperatures these properties were acquired during cooling from the relatively high temperatures 350 c at which serpentinization occurred the remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif however interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric which presumably resulted in a deflection of the remanence we estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy the corrected remanent inclinations mean 39 5 more closely approximates the time averaged inclination at the site indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350 c accounting for the anisotropy related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation after this reorientation the dominant mesoscopic foliation in these rocks defined by the preferred orientation of orthopyroxene and subparallel serpentine veins has an average orientation that closely parallels the regional scale fault zones on the western median valley wall

characteristics and evolution of the segmentation of the mid atlantic ridge between 20 n and 24 n during the last 10 million years abstract high resolution bathymetry and geophysical data collected along the slow spreading axis and flanks of the mid atlantic ridge between 20 n and 24 n reveal the characteristics and history of different wavelengths of segmentation during the last 10 m y the bathymetric data exhibit a morphotectonic pattern dominated by ridge normal and oblique bathymetric lows that partition the ridge flanks into rhomb shaped areas of relatively high elevation at least four different types of oblique bathymetric lows have been identified which represent the off axis traces of axial discontinuities and suggest a complex and ongoing evolution of ridge axis segmentation one group of oblique structures is represented by two deep ridge normal depressions with typical fracture zone characteristics that are connected to the present active transform by oblique depressions near the ridge axis these oblique traces correspond to the southward shift of axial discontinuities associated with the propagation of the ridge axis while maintaining a constant offset of the latter two other types of oblique structures correspond to elongate bathymetric lows and oblique alignments of ridge parallel bathymetric lows symmetric about the ridge axis both types of oblique structures frequently change their orientation from normal to subparallel to the ridge axis and appear to merge and diverge off axis these oblique depressions are characterized by positive filtered mantle bouguer anomalies high magnetizations complex magnetic anomaly patterns and possible exposure of mantle lithologies the ridge segments defined by these oblique depressions lengthen or shorten along the ridge axis with propagation rates varying from 0 to 25 km m y 1 the last and smallest discontinuities observed in this area correspond to small ridge axis offsets and off axis traces identified by alignments of the terminations of abutting abyssal hills the ridge flank morphotectonic patterns produced by the evolution of these elementary segments of accretion may represent temporally variable upwelling volumes of melt the centres of the rhomb shaped areas correspond to maximum crust production and thin lithosphere and the discontinuities correspond to a thick lithosphere with very thin crust and possible outcrops of peridotites we propose a model which accounts for the punctuated injection of magma and the evolution of elementary segments of accretion over periods of several million years

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

dike orientations fault block rotations and the construction of slow spreading oceanic crust at 22 40 n on the mid atlantic ridge the first paleomagnetic results from oriented dike samples collected on the mid atlantic ridge shed new light on the complex interplay between magmatic accretion and mechanical extension at a slow spreading ridge segment an upper crustal section about 1 5 km thick is exposed along a west dipping normal fault zone that defines the eastern median valley wall of the southern segment of the mid atlantic ridge south of the kane fracture zone mark area two distinct groups of dikes are differentiated on the basis of orientation and paleomagnetic characteristics one group on the basis of the paleomagnetic data appears to be in its original intrusion orientation this group includes both ridge parallel vertical dikes as well as dikes in other orientations calling into question assumptions about uniform dike orientations at oceanic spreading centers the second group consists of dikes that have paleomagnetic directions that are distinct from the predicted dipole direction and we interpret them to have been tectonically rotated these also occur in many orientations the spatial relations between rotated and nonrotated dikes indicate that intrusion faulting and block rotation were contemporaneous beneath the median valley floor nonrotated dikes exposed on the eastern median valley wall indicate that there has been no net rotation of this upper crustal assemblage since magmatic construction ceased hence slip and associated uplift probably occurred in the fault zones present orientation these results provide the basis for a general model of mechanical extension and dike intrusion for this segment of the mid atlantic ridge initially a portion of crust forms beneath the median valley by synkinematic dike intrusion into laterally discontinuous fault blocks slip and associated uplift along a cataclastic normal fault zone later exposes this crustal section on the valley margin as spreading continues this valley bounding cataclastic normal fault zone is abandoned in favor of a new fault system thus passively moving the exposed crustal section away from the median valley

evidence for the protracted construction of slow spread oceanic crust by small magmatic injections abstract gabbroic cumulates drilled south of the kane transform fault on the slow spread mid atlantic ridge preserve up to three discrete magnetization components here we use absolute age constraints derived from the paleomagnetic data to develop a model for the magmatic construction of this section of the lower oceanic crust by comparing the paleomagnetic data with mineral compositions and based on thermal models of local reheating we infer that magmas that began crystallizing in the upper mantle intruded into the lower oceanic crust and formed meter scale sills some of these magmas were crystal laden and the subsequent expulsion of interstitial liquid from them produced cumulus sills these small scale magmatic injections took place over at least 210 000 years and at distances of 3 km from the ridge axis and may have formed much of the lower crust this model explains many of the complexities described in this area and can be used to help understand the general formation of oceanic crust at slow spread ridges

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

magnetite rods in plagioclase as the primary carrier of stable nrm in ocean floor gabbros abstract the ubiquitous occurrence of magnetite rods in plagioclase may account for the stable intense magnetization of ocean floor gabbros these single domain grains are formed by the subsolidus exsolution of fe 2 and fe 3 incorporated into the plagioclase at higher temperatures we expect that this process will be particularly important in the upper portions of the oceanic gabbro unit because of the relative paucity of femic minerals at these levels the geometry of accretion prohibits a constructive contribution to marine magnetic anomalies by the lower olivine rich gabbros as a result of the presence of these rods ocean floor gabbros may constitute an important source for marine magnetic anomalies

flexural rotation of normal faults a conceptual model is proposed for the generation of low angle normal faults in metamorphic core complexes the model is based on three assumptions 1 the isostatic response to normal fault motion is of regional extent 2 when a fault segment is significantly rotated from the optimum angle of slip relative to the crustal stress field it is replaced by a new planar fault oriented in the optimum direction and 3 the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust the stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations active low angle normal faults are difficult to reconcile with rock mechanics theories earthquake focal mechanism studies and geochronologic results indicating rapid cooling of core complex rocks the model does not require active fault slip on low angle faults the flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation the anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust this can explain the observed short wavelength of topographic response to normal fault loads the model results in 1 a nearly flat lying abandoned normal fault or detachment below slices of upper plate rocks and sedimentary infill 2 a strong contrast in metamorphic grade across the abandoned detachment 3 rapid movement of lower plate rocks from midcrustal depths to shallower depths these results are qualitatively in agreement with geologic observations for core complexes

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

magnetization of axial lavas from the southern east pacific rise 14 23 s geochemical controls on magnetic properties although the spatial association of iron rich lavas and high amplitude magnetic anomalies is well documented a causal link between enhanced iron content and high remanent magnetization has been difficult to establish here we report magnetic data from approximately 250 samples with 8 16 feo total iron as feo from the southern east pacific rise epr that provide strong support for the presumed geochemical dependence of remanent intensity the limited age range 0 6 ka of axial lavas from this ultrafast spreading ridge 150 mm yr full rate effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low temperature alteration systematic sampling relative to the chilled margin illustrates that substantial grain size related variations in magnetic properties occur on a centimeter scale both microprobe data and curie temperatures suggest that the average groundmass titanomagnetite composition in the southern epr samples is approximately constant modal modified ulv6spinel content 0 67 over a wide range of lava compositions saturation magnetization and saturation remanence are highly correlated with feo r 0 73 and 0 83 respectively indicating that more iron rich lavas have higher abundances of otherwise similar titanomagnetite we show that there is a good correlation between natural remanent magnetization nrm and feo provided that sufficient specimens are used to determine the average nrm of a sample r 0 63 because the range of iron contents in mid ocean ridge basalts is limited the best fit slope 4 44 a m per feo in an ambient field of 0 030 mt should provide reasonable bounds on the equatorial magnetization of submarine lavas 10 a m at 8 5 feo and 50 a m at 16 feo finally we demonstrate that along axis variations in nrm closely parallel geochemical changes along the southern epr where magnetization values deviate significantly from those predicted from the range of measured feo contents these discrepancies may reflect additional unrecognized geochemical variability

along axis variations in seafloor spreading in the mark area recent investigations with the manned submersible alvin and the angus deep towed camera sled greatly extended the known range of variations in the style of seafloor spreading along the axis of the mid atlantic ridge five transects of the spreading centre at intervals of10 20 km south of the kane fracture zone at 24 n latitude demonstrate dramatic changes in the style and magnitude of tectonic extension development of the neovolcanic zone expression of hydrothermal venting types oflithologic exposures and morphology of the median valley

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

contribution of oceanic gabbros to sea floor spreading magnetic anomalies the contribution of oceanic gabbros representative rocks for layer 3 of the oceanic crust to sea floor spreading magnetic anomalies has been controversial because of the large variation in magnetic properties ocean drilling program odp leg 118 contains a continuous 500 7 meter section of oceanic gabbro that allows the relations between magnetization and petrologic characteristics such as the degree of metamorphism and the magmatic evolution to be clarified the data suggest that oceanic gabbros together with the effects of metamorphism and of magmatic evolution account for a significant part of the marine magnetic anomalies

magnetization of the la palma seamount series implications for seamount paleopoles paleopoles determined from seamount magnetic anomalies constitute the major data source for the pacific apparent polar wander path but relatively little is known about the processes of remanence acquisition in seamounts since magnetic anomalies reflect both natural remanence nrm and the induced field it is important first to assess whether the nrm is likely to represent an original field direction and second to constrain the magnitude of the induced component to this end we present paleomagnetic data from an uplifted subaerially exposed section through a seamount on la palma canary islands the pliocene seamount series of la palma comprises a 6 km sequence of alkalic extrusives and intrusives which includes all lithologies likely to be volumetrically important in seamounts the structural tilt of the seamount series allows separation of early thermal or chemical remanence from magnetization components acquired after tilting e g viscous remanence the nrm provides a poor indication of the original magnetization direction although the characteristic magnetization of many la palma samples is compatible with the original pretilt direction hydrothermal alteration has resulted in the production of ti poor magnetite and an increasing contribution of hematite with increasing degree of alteration more importantly well defined magnetization directions which deviate from any reasonable geomagnetic direction at la palma can be attributed to hydrothermal alteration in a different polarity than prevalent during the original magnetization based on a comparison of the magnitude of low stability components of magnetization and laboratory acquisition of viscous remanence and previous estimates of the induced magnetization we conclude that viscous and induced magnetization probably account for 15 25 of the total magnetization of seamounts the resulting paleopole bias is a function of the polarity and paleolatitude of the seamount and ranges from 4o to 16 o for cretaceous seamounts in the pacific

fracture systems and petrophysical properties of tight sandstone undergoing regional folding a case study of the cretaceous reservoirs in the kuqa foreland thrust belt tarim basin abstract the kuqa foreland thrust belt of the tarim basin hosts a prolific hydrocarbon system the main reservoirs are characterized by tight sandstone with low porosity and ultra low permeability buried presently at more than 5000 m depth natural fracture systems have a significant impact on reservoir performance and it may form paths for fluid flow in tight sandstone reservoirs and therefore plays an essential role in controlling high production rates with the aim of analyzing the distribution of favorable reservoir we studied subsurface fracture patterns and petrophysical properties of the bashijiqike formation with data collected from dabei structure and keshen structure we document the presence of four fracture systems whose distribution is related to far field stress thrusting and folding in the keshen anticlines higher fracture intensities are observed in the limbs with principal east west hinge parallel fracture system direction the hinge parallel fracture system includes a moderate dipping fracture set and a vertical fracture set crosscutting relationships stress field inversion and fracture surface textures suggest that the moderate dipping fracture set consists of shear fractures and can be explained by intra layer shear caused by thrusting the vertical fracture set may be formed due to the local outer arc extension during folding the oblique fracture system shows scattering in strike and is supposed to be related to a shear event of locally left lateral strike slip whereas in the dabei anticlines the major system is hinge perpendicular fracture system nw se showing a trending of increasing fracture intensity from limbs to hinge the hinge perpendicular fractures system in the dabei anticlines consist of shear fractures and tension fractures stress inversion result shows it corresponds to northwest southeast orientation compression stage besides we analyzed petrophysical properties include porosity permeability young s module poisson ratio and compressive strength of the bashijiqike formation concerning their structural position within folds it indicates that the highest porosity and permeability are associated with dissolution and fracturing the compressive strength of different structural positions also restrict the variations of mechanical properties of rocks the best reservoir quality is suggested in the hinge zone and areas close to thrust the findings have given maybe helpful for providing favorable reservoir targets for prospecting in the compressional basin

rare earth and trace elements in igneous and high temperature metamorphic minerals of oceanic gabbros mark area mid atlantic ridge sims analyses have been carried out on clinopyroxenes plagioclases and amphiboles of six gabbroic samples from holes 921 924 of the ocean drilling program leg 153 sited in the mark area of the mid atlantic ridge at the ridge transform intersection to investigate the rare earth trace and volatile element distribution in the lower ocean crust during igneous crystallization and higher grade metamorphic conditions the metagabbros underwent granulite to subgreenschist facies conditions through three main tectono metamorphic phases 1 ductile regime 750 t 1000 c and p 0 3 gpa 2 transitional regime 600 t 700 c and p 0 2 gpa 3 brittle regime 350 t 600 c and p 0 2 gpa igneous clinopyroxenes show c1 chondrite normalized patterns depleted in lree and nearly flat for hree the rare earth and trace element distributions in igneous clinopyroxenes and plagioclases indicate that these minerals act as ree reservoirs and comprise the main contribution to the overall rock content the abundances in igneous minerals reflect the degree of fractionation of the parent liquids in metamorphic clinopyroxenes recrystallized in anhydrous assemblages the ree and trace elements patterns mimic those of the primary ones conversely clinopyroxene re equilibrated in amphibole bearing assemblages shows a significant increase in ree ti zr y and v a negative eu anomaly and slight decreases in sr and ba an overall increase of ree and some trace elements is evident in hydrous assemblages with preferential partitioning in the amphibole it shows high ti 18196 22844 ppm lree depleted patterns and lan smn 0 10 0 33 lan ybn 0 10 0 30 amphiboles from granoblastic assemblages show homogeneous patterns with no or a positive anomaly for tin and negative anomalies for srn and zrn volatiles in amphibole are low with cl f 1 h2o is significantly lower than the stoichiometric ratio 1 33 1 53 the composition of the clinopyroxene and amphibole recrystallized in low strain domains records evidence of incomplete re equilibration and element diffusion and partitioning is in part controlled by the textural site the possible origins of the fluids involved in the metamorphic recrystallization are discussed 1 remobilization from igneous amphibole 2 exsolution from evolved melts 3 introduction of seawater derived fluids modified in rock dominated systems 4 injection of highly evolved hydrous melts during the metamorphic process

crustal structure of the mid atlantic ridge at 23 n from seismic refraction studies an explosive seismic refraction experiment using four ocean bottom hydrophone receivers was carried out along a 120 km long section of the mid atlantic ridge mar median valley at latitude 23 n immediately south of its intersection with the kane fracture zone the data are interpreted in terms of laterally homogeneous horizontally layered structures using travel time inversion and synthetic seismogram modeling techniques these solutions are combined using two dimensional ray tracing to produce a single model defining the major along axis structural changes occurring beneath this section of the median valley with the exception of slightly low layer 3 velocities the crust beneath several tens of kilometers of the mar median valley has all the characteristics of simple mature ocean crust including a well defined moho transition zone 8 km s upper mantle velocities and a total crustal thickness of 6 7 km no evidence is found for the presence of a steady state axial magma chamber in the crust or upper mantle and the excellent propagation characteristics and orderly amplitude versus range relationships imply the existence of a considerable degree of lateral homogeneity beneath the median valley over distances of several tens of kilometers along axis we infer from these observations that the basic seismic characteristics of the ocean crust are frozen in in a time period less than that separating the major volcanic injection events 10 000 50 000 years by hydrothermal circulation penetrating to the base of the crust along the 100 km long ridge segment studied here two major along axis structural changes occur the first is a 10 to 15 km wide along axis zone of lowered velocities in the lower crust centered beneath a major along axis topographic high this is interpreted to be the remnants of the most recent phase of injection that has temporarily left behind it a region of elevated temperatures and pervasive cracking and thus reduced velocities the second occurs abruptly at latitude 23 15 n and is accompanied by both a major change in rift valley and crestal mountain morphology and the apparent noncoincidence of the median valley and central magnetic anomaly north of this boundary the 30 km long ridge segment that abuts the kane fracture zone has a crustal thickness of 4 5 km and no distinctive layering to produce the characteristic amplitude patterns that typify mature oceanic crust interpretation of the cause of this structural change is uncertain but may be related to local tectonic events perhaps a recent 10 to 20 km ridge jump to the east

emplacement of deep crustal and mantle rocks on the west median valley wall of the mark area mar 23 n abstract exposures of deep crustal and mantle rocks within the axial rift valley characterize accretion processes in the slow spreading mid atlantic ridge nine dives of the submersible nautile explored the western axial valley wall in the northern cell of the mark area mid atlantic ridge kane fracture zone where peridotite and gabbro outcrops had been previously reported in order to constrain the structure and determine emplacement mechanisms the ridge transform intersection massif on the western wall shows a section of gabbros from 6000 to 2500 mbsl locally overlain by metabasalts and metadolerites capped by slightly weathered basalts the morphology is controlled by ridge parallel faults dipping moderately 40 65 to the east transform parallel scarps present in the northernmost dives become rare toward the south brittle deformation along moderately dipping fault scarps produced a dense microcrack network filled with greenschist facies minerals chlorite actinolite epidote quartz locally overprinting high temperature ductile deformation fabrics the small hill located at 23 20 in the western wall shows good exposures of serpentinized peridotite between 3700 and 3100 mbsl above 3100 mbsl the summit of the hill is composed of pillow basalts and sediments the peridotite outerops display a strong schistosity dipping 20 40 to the east parallel to striated normal fault planes some steeper east facing fault scarps truncate the lower dipping fault surfaces the serpentinites deriving from a harzburgitic protolith are cut by rare dikelets of highly differentiated gabbro these new data combined with previous results of alvin dives are used to draw a generalized geological map of the western axial valley wall this map suggests a variation in the thickness of crustal units and composition along strike in the northern cell of the mark area the gabbro body disappears toward the south where basalts appear to directly overlie the mantle peridotites which are cut by isolated gabbroic dikelets low angle stretching affecting this heterogeneous lithosphere is probably responsible for the exposure of gabbros in the intersection massif and of peridotites further south

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

models of ocean ridge lithospheric deformation dependence on crustal thickness spreading rate and segmentation we use three dimensional 3 d temperature and rheology models to investigate the effects of crustal thickness and ridge segmentation on mid ocean ridge lithospheric structure we find that crustal thickness variations associated with focused magma accretion at a slow spreading ridge segment can cause significant along axis gradients in crustal temperature and a pita pocket shaped weak zone in the lower crust that decouples brittle upper crust from upper mantle in contrast fast spreading ridge segments with little crustal thickness variation are found to be uniformly weak along axis the overthickened crust produced by ridge hotspot interaction alters the heat balance between magma emplacement and hydrothermal cooling creating an extremely weak lithosphere in a steady state model we apply a simple two dimensional 2 d cyclic faulting model to across axis sections of temperature and rheological structure slow spreading segments are predicted to have relatively large along axis variations in fault height and spacing in contrast fast spreading and hotspot affected segments are predicted to have much smaller variations in faulting styles except in the immediate vicinity of a major transform fault the presence or lack of a steady state axial rift valley is also predicted to depend on crustal thickness spreading rate and the ability of rift bounding normal faults to propagate along axis for fast spreading and hotspot affected segments the sizes of axial rift valleys are predicted to be small and may not be distinguishable from those of neovolcanic or magma chamber supported isostatic features this is in contrast to most slow spreading segments where axial rift valleys are predicted to be a dominant morphological feature for a slow spreading segment of great length however the axial rift valley size is predicted to diminish toward the segment center because large faults formed at segment distal ends cannot propagate through a large decoupling zone created by locally thickened crust at the segment center

redbeds and thermoviscous magnetization theory thermal demagnetization character istics of a brunhes age viscous overprint in appalachian redbeds and a thermoviscous component acquired in the laboratory at moderate tempera ture agree well with relaxation time blocking temperature relations for hematite proposed by pullaiah et al 1975 supporting evidence was obtained from redbeds associated with an igneous dike intrusion although the interpretation of these data are complicated by magnetochemical alterations paradoxically experimental data for magnetite in some limestones were shown to agree more closely with an alternative theory of walton 1980 the hematite in redbeds is predominantly singledomain sd whereas magnetite in the lime stones studied extends well into the multidomain md range thus experiment and theory could be reconciled if it is assumed that pullaiah eta applies strictly to sd material whereas waltoh s theory somehow describes thermoviscous effects dominated by larger md grains

slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the mid atlantic ridge south of the kane fracture zone mark area 1 the remanent magnetization of gabbroic material of the mid atlantic ridge south of the kane fracture zone mark area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment the remanence of these gabbroic samples is often complex with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales tens of centimeters to a few meters moreover several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components given the nominal age 1 ma of the crust we suggest that this pattern of normal reversed normal polarity most plausibly reflects emplacement and or cooling through three successive polarity intervals jaramillo normal polarity interval 1 07 0 99 ma a portion of the matuyama reversed polarity interval 0 99 0 78 ma and the brunhes normal polarity interval 0 78 ma to present a small number of samples with three well defined magnetization components have magnetic characteristics compatible with a remanence carried by fine grained possibly single domain magnetite laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the jaramillo matuyama 0 99 ma and matuyama brunhes 0 78 ma polarity transitions together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as 350 475 c for a minimum of 0 21 m y after emplacement we suggest that continued injection of liquid in the form of sills or small magma bodies over a broad region half width of 3 km is responsible for this slow cooling in addition inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole suggesting that little if any resolvable tilt occurred during uplift of these rocks to the seafloor

petrology and palynology of select coal samples from the permian waterberg coalfield south africa abstract the petrology and palynology of coals from the permian aged waterberg coalfield situated in the limpopo province of south africa are considered the waterberg coalfield contains a significant portion of the country s coal reserves but is still comparatively underexplored detailed maceral analyses combined maceral microlithotype analyses mean random and maximum vitrinite reflectance analyses etching and palynology studies were undertaken on coals from four productive horizons there is a general upward trend in vitrinite content through the sequence 15 59 vol mineral matter free dominated by collotelinite along with a slight decrease in coal rank the lower vryheid formation coal has a comparatively low mineral matter and high inertinite content dominated by inertodetrinite the upper grootegeluk formation samples contain above 35 vol mineral matter and liptinite contents ranging from 14 to 21 vol the use of blue light fluorescence in conjunction with incident light single scan method revealed that liptinite is intimately associated with inertodetrinite and mineral matter specifically clays and occasionally pyrite it is evident that the liptinite maceral content has previously been under reported in the waterberg coals the palynofloras correlate with the well studied witbank coalfield the petrography along with the palynology essentially confirm the palaeoenvironment of the karoo basin as progressing from a cooler more oxidising environment to a wetter seasonal environment

a view into the cretaceous geomagnetic field from analysis of gabbros and submarine glasses the nature of the geomagnetic field during the cretaceous normal polarity superchron cns has been a matter of debate for several decades numerical geodynamo simulations predict higher intensities but comparable variability during times of few reversals than times with frequent reversals published geomagnetic paleointensity data from the cns are highly scattered suggesting that additional studies are required here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the troodos ophiolite cyprus 92 1 ma old together with recently published data from the troodos upper crust we obtain three independent paleointensity time series these sequences reveal quasi cyclic variations of intensities about a mean value of 54 20 z am 2 providing insight into the fluctuating nature of the cretaceous magnetic field our data suggest the cns field was both weaker and more variable than predicted by geodynamo simulations the large amplitudes of these variations may explain the wide range of dipole moments previously determined from the cns 2007 elsevier b v all rights reserved

three dimensional analysis of gravity anomalies in the mark area mid atlantlc ridge 23 n we have carried out a three dimensional analysis of the gravity field over the mid atlantic ridge at kane mark area a well studied portion of the mid atlantic ridge south of the kane fracture zone between 23 40 n and 22 30 n the mid atlantic ridge in this area is composed of two distinct spreading segments separated by a small nontransform discontinuity at 23 15 n the mantle bouguer anomalies in the mark area are dominated by v shaped bands of anomalous mass and small amplitude gravity lows and highs located both on and off axis which indicate that the anomaly distribution is more complex than the simple bulls eye pattern described along some other parts of the mid atlantic ridge within the kane fracture zone and in the ridge transform intersection area mantle bouguer anomalies do not accurately reflect the extent of crustal thinning that has been documented seismically this is attributed to lower average crustal and mantle densities due to fracturing and alteration within the fracture zone and the inability of gravity data to resolve the abrupt change to thinner fracture zone crust which is confined to a narrow zone along the fracture zone trough and beneath the nodal basin at the ridge transform intersection outside of the kane fracture zone the magnitude of the crustal thickness variations predicted solely from the residual gravity anomalies compare reasonably well with the crustal structure derived from seismic refraction studies the largest residual gravity anomalies implying significant reductions in crustal thickness are observed over the small nontransform offset at 23 15 n the anomalous mass associated with this feature forms a v shape that can be traced well off axis suggesting that these discontinuities are fundamental ridge segmentation boundaries that can persist for millions of years the distinctive v shaped pattern of gravity anomalies in the mark area reflects systematic crustal thickness variations related to the northward migration of this nontransform offset over the past 3 4 m y