the effect of low temperature hydrothermal alteration on the remanent magnetization of synthetic titanomagnetites a case for acquisition of chemical remanent magnetization the effect of hydrothermal alteration on the thermoremanent magnetization trm of synthetic titanomagnetite tm40 fe2 6ti0 4o4 has been studied to simulate the alteration that occurs in the oceanic crust pseudo single domain titanomagnetite grains similar in size to those often found in oceanic basalts were dispersed in a permeable but rigid glass matrix this resulted in a trm in the sample which was subsequently oxidized in acidic solutions while a magnetic field 0 1 mt was applied perpendicular to the trm direction the experiments were conducted in a non magnetic stainless steel pressure vessel at 150 c in solutions of acidity varying from ph 2 to ph 7 in addition to being time and temperature dependent the acquired chemical remanent magnetization crm was also found to be very ph dependent the degree of maghemitization increased drastically as the acidity of the hydrothermal solution was increased in accordance with a process controlled by the loss of iron ions in aqueous solutions long term storage experiments at carefully chosen temperatures demonstrated that no significant viscous remanent magnetization was acquired during heating it was found that during the alteration of tm40 to titanomaghemite the crm is along the ambient field direction from the onset and not partly or wholly along the trm direction as has been found in previous air oxidation experiments this has important implications for the possible cause of anomalous skewness of marine magnetic anomalies and for the anomalous directions of natural remanent magnetization found in some oceanic basalt samples

drainage systems associated with mid ocean channels and submarine yazoos alternative to submarine fan depositional systems submarine drainage systems associated with mid ocean channels and yazoo river type tributaries in small ocean basins represent a contrast to deep sea fan depositional systems deep sea fans are diverging sediment dispersal systems of distributary fan valleys deep sea channel submarine yazoo systems on the other hand form centripetally converging patterns of tributaries and yazoo type satellite channels that join a major basin draining mid ocean channel the facies model for such systems is characterized by randomly stacked fining upward gravelly and sandy channel fill and submarine point bar sequences of the main channel encased in fine grained overbank deposits second order channels contain sandy proximal overbank deposits whereas the levees of the main channel are predominantly composed of silt and clay second order channels may be braided and may broaden into braid plains morphology and surficial sediment distribution have been studied within the northwest atlantic mid ocean channel of the labrador sea and its associated levees and yazoo type and other tributaries

magnetic properties and ferromagnetic mineralogy of oceanic basalts summary rockmagnetic properties of a number of oceanic basalt samples collected by the dsdp odp drillings after leg 45 as well as one dredge haul were investigated in conjunction with palaeomagnetic characteristics of the same samples the electron probe micro analysis epma study indicated that chemical composition of the primary titanomagnetites has a variety particularly in minor constituent elements such as al2o3 mgo mno and v2o3 the variation in chemical composition in the primary phase of titanomagnetites seems to be controlled by changes in temperature oxygen fugacity and volatile pressure during the crystallization as well as chemistry of residual liquid hi magma general crystallization trend is found that is the later crystallized titanomagnetites have higher ulvospinel component higher titanium oxide and lower iron oxide due to decrease in temperature and oxygen fugacity it is established that minor constituent elements are good indicators of crystallization sequence of titanomagnetites in the contrast of variation in major constituent elements in the later stage al2o3 and mgo spinel component and v2o3 decrease whereas mno increases although there is a reversed crystallization trend with a few elements in several cases this reversed trend may suggest that there are discontinuous changes of environmental condition in magma to control titanomagnetite chemistry during a crystallization sequence the primary variation of chemistry shows different trends of the secondary variation due to oxidation oxidation process of titanomagnetites contained in oceanic basalts was examined in detail by means of an improved epma technique being able to directly determine the absolote contents of oxygen in materials the result of the quantitative oxygen measurement shows that the oxidation process of titanomagnetites is not a simple process by changes of ferrous ions to ferric ones with acrretion of oxygen but associated with migration of iron ions of the titanomagnetite lattice this is consistent with the results of recent studies on oxidation processes of titanomagnetites in oceanic basalts

a novel dynamic impact pressure model of debris flows and its application on reliability analysis of the rock mass surrounding tunnels abstract assessing the influence of the impact pressure exerted on surrounding rock mass by potential debris flows is one of the challenges in a mountain tunnel construction project this paper proposes an effective mathematical model for the assessment of impact pressure exerted by debris flows on obstacles the characteristics of the new model are analyzed and the parameters determination method for the new model are deduced also a new method for dynamic reliability analysis of the rock mass surrounding tunnels is proposed and is constituted into three key steps first by inputting impact pressure of debris flows generated by the aforementioned new model the flac3d is used as the dynamic analysis engine to obtain the time varying stress distribution of surrounding rock mass during the impact of debris flow second the factor of safety fos contours of surrounding rock mass are generated based on element states using the maximum tensile stress criterion and the mohr coulomb yield criterion third by using the time varying contours of the fos a failure probability and a reliability index are obtained from a reliability perspective a case study shows the complete analysis process and confirms the effectiveness of the above methodology in sum this paper provides new ideas new methods and a reference example for evaluating the influence of dynamic impact exerted by debris flows on the rock mass surrounding tunnels

an interpretation of magnetic signatures of subduction zones detected by magsat abstract the magnetic anomalies detected by magsat over subduction zones are related to the thermal state and the location with respect to the geomagnetic poles of the subducting oceanic lithosphere it is shown that an old about 100 m y subducting lithosphere produces about twice as strong a magnetic anomaly as that associated with a young about 40 m y one also a given subducting oceanic lithosphere produces a stronger magnetic anomaly at high geomagnetic latitudes than near the geomagnetic equator based on the thermal evolution models of the oceanic lithosphere and assuming a mean magnetic susceptibility of 13 10 3 si the pacific plate subducting along the kuril trench produces a magnetic anomaly similar to the observed one the nazca plate subducting along the peru chile trench produces a magnetic anomaly of less than 0 5 nt no distinct anomaly observed by magsat over this region

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

the natural magnetization of a 3 kilometer section of icelandic crust the magnetization and magnetic properties of samples from a 3 km vertical section of icelandic crust the iceland research drilling project section are described the section is located at the head of reydarfjordur eastern iceland at 65 01 n 14 21 w and extends vertically from about 0 5 km to 3 5 km depth beneath the original surface the section consists of 1 km of exposed flows with minor dikes and volcaniclastics underlain by a 2 km drill core component consisting of 60 flows and minor volcaniclastics and 40 dikes the section is largely normally magnetized corresponding in age and polarity to seafloor spreading anomaly 5 geomagnetic polarity epoch 9 flows which accumulated during anomaly 5 time extend vertically for 2145 m in contrast with 1000 m thick equivalent sections which are exposed updip this great increase in downdip thickening is consistent with observations elsewhere in eastern iceland and models for icelandic crustal formation involving the combination of volcanic activity subsidence and spreading a combination of paleomagnetic and structural evidence is used to provide an age calibration for the section and to predict the local presence of magnetic overprinting in the lower part of the section this overprinting is probably in part due to the production of secondary magnetite and is developed to a varying extent in epidote bearing flows at below 2 8 km crustal original depth where moderate secondary magnetite development has occurred flows frequently show both normal and reverse stable polarities with the latter being the original secondary magnetite development is high where dike density in the epidote bearing flows is also high here very strong well defined pervasive normal magnetization characterizes the flows this also being the dominant polarity of the dikes magnetic overprinting of this type imposed shortly after crustal formation could occur at 800 to 1200 m depth in typical ocean crust and may provide a contribution to the source for the linear magnetic anomaly patterns of the ocean basins the initial susceptibility of the flows shows the best defined depth trends over the section susceptibility increases with depth to about 2 km crustal depth and then decreases systematically to the bottom of the section the reason for the initial increase is not known but the decrease at depth can be explained by transfer of iron from primary oxides to secondary oxides and silicates the decrease is sufficiently linear to warrant extrapolation to a zero value at 4 3 km crustal depth or just below the estimated level of onset of greenschist facies metamorphic conditions in typical oceanic crust the equivalent level is estimated to be at 1 2 km depth the net magnetization of the section the combined remanent and induced contributions when expressed as average values for 500 m depth intervals is everywhere positive that is is in the sense of the present field variation in net magnetization is related to the proportions of the normal and reversely magnetized units in each interval

oceanic magnetic anomaly amplitudes variation with sea floor spreading rate and possible implications abstract a qualitative examination of oceanic magnetic anomalies suggests that their amplitudes increase with the spreading rate of the ocean floor this suggestion has been investigated quantitatively by inversion of the anomalies a variance matching technique has been used to calculate the average magnetisation of the crust with crustal thickness held constant or to calculate the thickness with the magnetisation fixed in spite of variations in the computed intensities of magnetisation and thickness when compared to spreading rates the results show an over all increase in the intensities with the rate of spreading the mechanisms responsible for this could include greater irregularity in distribution of magnetic polarities within the crust at slow spreading ridges greater intensity of magnetisation in crust produced at fast spreading due to initial chemical properties or enhanced hydrothermal alteration and a dependence of crustal thickness on spreading rate

constraints on age and construction process of the foundation chain submarine volcanoes from magnetic modeling abstract the interaction between the foundation hotspot and the pacific antarctic ridge south pacific is one of the two known cases where a ridge is approaching a hotspot this ridge hotspot relative movement results in a change in the morphology of the volcanoes along the chain as the age of the lithosphere at the time of edifice formation progressively diminishes four hundred kilometers west of the pacific antarctic ridge axis volcanism is distributed along two sub parallel lines of volcanoes separated by distances diminishing from 100 km at the west to 50 km near the axis of the ridge the magnetic anomalies mapped on this part of the chain show a pattern clearly dominated by the magnetic signature of the seamounts the anomalies were forward modeled using a remanent magnetization of normal or reversed polarity within the volcano topography the resulting pattern of normal and reversed magnetization is consistent with the intervals of the geomagnetic polarity time scale for the last 5 ma the age of the oldest studied volcano these magnetic anomalies therefore represent an independent means of dating the volcanic edifices the ages deduced from the modeling are consistent with the hypothesis of volcanoes built by a hotspot on the fast moving pacific plate and are in good agreement with the published radiometric dates although these dates tend to correspond to the late constructional stages of the edifices the differences between magnetic and radiometric ages suggest that the average time span to build a volcano of the foundation chain is about 1 million years in agreement with other intraplate edifices the difference between the magnetic ages of the seamounts and of the underlying crust both deduced from the magnetic anomaly analysis show that the ridge has approached the hotspot at a rate of 40 km ma

contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world s oceans regional studies have shown that remanent magnetization of the cretaceous quiet zones cqzs created during the long period of geomagnetic normal polarity at 118 84 ma produce well defined magnetic anomalies at satellite altitudes we investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale we consider entire oceanic areas because oceanic lithosphere formed during periods other than cqz but with a predominant polarity also have appreciable contributions to the satellite anomalies the magnetic anomalies of the world s oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map plate relative motions and the apparent polar wander path of africa three magnetization models are examined thermoremanent magnetization confined to extrusive layer 2a and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km although all models lead to rather similar anomaly distributions the amount of magnetization required suggests the need for deeper sources all models produce the satellite anomalies associated with the cqz in the north atlantic and parts of the indian and pacific oceans with slight differences in location depending on the models low amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the indian and pacific oceans are successfully modeled a major difference between models and observation is the north south elongated model anomalies associated for instance with the cqzs in the south atlantic and the fast spreading east pacific rise these elongated anomalies are systematically absent on the observed map probably because they were removed by along track filtering in the early stages of processing the satellite data a careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4 0 1 and 0 6 a m for the extrusive basalts intrusive basalts gabbros and peridotites respectively

the analysis of ground vibrations induced by bench blasting at akyol quarry and practical blasting charts ground vibrations arising from excavation with blasting is one of the fundamental problems in the mining industry therefore the prediction of ground vibration components plays an important role in the minimization of environmental complaints in this study 582 events were recorded during limestone production at a quarry akyol quarry during a period of time the blasting parameters of these shots were also carefully recorded during the statistical analysis of the collected data three predictor equations proposed by the united states bureau of mines usbm ambraseys hendron and langefors kihlstrom were used to establish a relationship between peak particle velocity and scaled distance described by these prediction equations as a result of this analysis the most powerful relationship was determined and proposed to be used in this site and also this equation was used in the derivation of the practical blasting charts specific to this site as a practical way of predicting the peak particle velocity and maximum charge amount per delay for future blasting

magnetic properties and opaque mineralogy of rocks from selected seafloor hydrothermal sites at oceanic ridges magnetic properties natural remanent magnetization nrm susceptibility curie point temperature tc saturation isothermal remanent magnetization irms and koenigsberger ratio q and opaque mineralogy were determined for basalts diabases gabbros peridotites and serpentinites collected by dredging and submersible from the rift valley at five hydrothermal sites 15 n 17 n 23 n the snake pit hydrothermal field 26 n the tag hydrothermal field on the mid atlantic ridge and 42 n the sea cliff hydrothermal field on the northern gorda ridge the magnetomineralogy is interpreted in terms of deuteric hydrothermal metasomatic and ambient seawater alteration evidence for unequivocal magnetic mineral modification by hydrothermal action is present only in a small percentage of extrusive basalts but is pervasive in diabases gabbros and ultramafic rocks this lithologiec distribution suggests a progressive increase in alteration intensity with depth in the oceanic crust and upper mantle from minor low temperature alteration in young surface basalts to more pervasive high temperature alteration in diabases gabbros and peridotites the studies reveal distinct correlations between magnetization intensity thermomagnetic behavior and magnetic mineralogy grain size style and intensity of alteration and rock type basalts have the highest nrm intensities and the lowest curie points 105 294 c basalts with nrm irms ratios less than 10 2 may have been remagnetized and have nrm intensities and q values much lower one third and one half respectively than basalts with nrm irms ratios of 10 2 or greater in the oceanic lithospheric suite examined remanent q 1 magnetization is present in basalts and induced magnetization q 1 occurs in metabasalts and the other rock types layer 2a basalts are the source of median valley magnetic anomalies the magnetic source may shift from the surface to deeper horizons with progressive seafloor aging and enhanced deuteric oxidation in layer 2b dikes and layer 3 gabbros and the formation of magnetite in serpentinized peridotites this shift is influenced by fluctuations in the curie isotherm related to the duration of active magma chambers and convective hydrothermal cells the magnetic and mineralogie properties of oceanic rocks determined show the effects of a wide range of alteration processes that are variously related to depth in oceanic lithosphere magmatic cooling history and hydrothermal circulation

rock damage control in bedrock blasting excavation for a nuclear power plant drill and blast is a major method for bed rock excavation in nuclear power plant engineering under blasting excavation blast induced vibration propagates in rock mass and induces damage to surrounding rock studying the characteristics of wave propagation and blast induced damage to rock mass is most important to restrict the damage extension in the present paper in support of the bedrock blasting excavation for the linao nuclear power plant project lnpp the site seismic survey and sonic wave test are presented to study the characteristics of wave propagation and the blast induced damage for rock mass the explosive load rock damage characteristics and peak particle velocity ppv attenuation law are also investigated by numerical simulation using commercial software it can be seen that the numerical simulation results agree well with that obtained by site monitoring based on the site monitoring and the numerical simulation the relationship between ppv at 30 m away from the charge hole and the damage depth of rock mass is established a simple method based on safety threshold of vibration velocity for rock mass to restrict the damage depth of rock mass under blasting excavation is presented according to the damage controlled method and the presented safety threshold of vibration velocity for rock mass the remained bedrock can be simply and effectively protected during blasting excavation c 2010 elsevier ltd all rights reserved

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

western davis strait a volcanic transform margin with petroliferous features abstract we present a compilation of the western davis strait region offshore southeastern baffin island nunavut canada with new subsurface geological structural details and observations regarding past hydrocarbon occurrences from scientific and commercial exploration this consists of seismic mapping with archival data correlated with a filtered marine bouguer anomaly gravity compilation and magnetic data sets covering northern saglek basin the western part of the lady franklin basin and the ungava fault zone from south of baffin bay a regional seismic horizon for mapping basin architecture comes from the top of the paleogene volcanic syn magmatic zone where pervasive volcanic flows and intrusions are intermingled with the sedimentary section the seismic depth map to the top of the regional volcanic seismic horizon shows pre rift sedimentary basins having maximum depths of approximately 4 5 km flanking the shallower ungava fault zone correlation of bouguer anomaly gravity and magnetic data interpretations with the seismic mapping indicate that over some areas true crystalline basement is deeper than can be determined by reflection seismic as the base of the syn magmatic section is not resolvable for seismic mapping extensive paleogene mafic intrusives and extrusive basalts dominate the architecture of this volcanic rifted margin as seen by the dominant high amplitude magnetic anomalies associated with many deeper structures the over 250 km long davis strait fault is recognised as the key structural element of the ungava fault zone ufz and valley complex the adjoining davis strait high maps on seismic as a continuous structure running the length of davis strait plunging southwards to a bouguer gravity high feature that terminates the western end of the extinct eocene spreading zone south of davis strait this horn shaped structural feature establishes the presence of an accommodation zone with multiple faults and localised thrust faulting as required to fit the complex mechanics for strike slip motion that occurs along the south end of this transform fault system the revised marine bouguer anomaly gravity data set also defines two new near shore grabens east of cumberland sound the southern tariut basin and the northern imaqpik basin each extending over 100 km in length with sediment thicknesses of at least 4 km these pre rift basins most likely originate from the paleozoic based on seafloor dredge and nearby shallow drill cores from previous studies the hydrocarbon charge of a previously unexplained petroliferous shallow marine drill core that is adjacent the eastern edge of the tariut basin is attributed to paleozoic source rocks that have undergone enhanced thermal maturation from sill intrusion associated with rifting the imaqpik basin shows strong evidence of a hydrocarbon system from the proximity of clustered interpreted sea surface oil slicks mapped from satellite radar images and a local zone of anomalously high dissolved methane measured within the water column that originates from the seafloor immediately east of cape dyer at the northern limit of that basin the overlying cape dyer flood basalt field extends from limited onshore exposures into the offshore and maps with magnetic data over an area of approximately 13 000 km2 this extrusive feature and it s implicit underlying intrusive components are likely the unconventional heat source for the anomalous enhanced thermal maturation of paleozoic to paleogene source rocks in these pre rift basins that host this previously unrecognised petroleum system

contribution of serpentinized ultramafics to marine magnetic anomalies at slow and intermediate spreading centres insights from the shape of the anomalies summary detailed characteristics of marine magnetic anomalies 33r and 20r suggest that the magnetization of the deeper magnetic layers including the lower crust and possibly the uppermost mantle is horizontally displaced with respect to that of the upper crust we examine the possibility that serpentinization of ultramafics in the lower crust and possibly the uppermost mantle delays the acquisition of magnetization and introduces a shift between the upper and lower crustal magnetization patterns thermal evolution models and the resulting magnetization patterns of the oceanic lithosphere are calculated for a wide range of physical parameters such as the nusselt number and the depth of hydrothermal circulation in the crust and the temperature range of serpentinization the models with moderate hydrothermal cooling of the whole crust and serpentinization temperatures ranging between 200 and 300 dc successfully explain the anomalous skewness and the hook shape of observed sea level magnetic anomalies created at slow and intermediate spreading rates

in situ monitoring of rockburst nucleation and evolution in the deeply buried tunnels of jinping ii hydropower station abstract rockbursts occurred frequently during the excavation of several parallel tunnels in the jinping ii hydropower station under a maximum overburden of 2525 m over an average length of 17 5 km in order to investigate the nucleation and evolution mechanism of rockbursts a comprehensive monitoring campaign consisting of a digital borehole camera cross hole acoustic apparatus and sliding micrometer was undertaken for in situ measurements in two specially excavated test tunnels b and f this paper presents the comprehensive monitoring methods applied and results of numerical analysis applied to a typical rockburst that fortuitously occurred during the testing period precursory characteristics preceding rockbursts are a abundant crack initiation propagation and coalescence b deformation of surrounding rock mass involving an accelerated deformation stage quiescence stage and reaccelerated deformation stage and c decrease of the characteristic elastic wave velocity of the rock mass the nucleation and evolution of rockbursts discussed consist of four stages a stress adjustment b energy accumulation c crack initiation propagation and coalescence and d fractured rock collapse and ejection the results provide a direct case history to assist the prediction and support of rockburst disasters and contribute to field excavation of deeply buried tunnels

spreading rate dependent magnetization of the oceanic lithosphere inferred from the anomalous skewness of marine magnetic anomalies summary the anomalous skewness of marine magnetic anomalies decreases with increasing spreading rate and becomes negligible above a spreading rate of about 50 km myr i previous magnetization models of the oceanic lithosphere fail to explain this observation we propose a new model in which the magnetic structure of the oceanic lithosphere is dependent on spreading rate anomalous skewness measurements for anomalies 33 reversed and 25 reversed at various spreading rates are in agreement with a magnetization model in which layer 3 and the uppermost mantle bear a similar saturation magnetization and the ratio of layer 3 over layer 2a saturation magnetizations varies linearly from about 0 4 at a spreading rate of 10kmmyr to about 0 at 55kmmyr i for spreading rates higher than 55 km myr layer 3 and uppermost mantle magnetizations are negligible recent advances concerning accretionary processes at mid ocean ridges suggest that the combined effect of variations with spreading rate of magma fractionation alteration of layer 2a and serpentinization of layer 3 and the uppermost mantle may account for the proposed variation of magnetization with spreading rate

hydrodynamics of turbid underflows ii aggradation avulsion and channelization a two dimensional depth averaged finite volume model is used to study the hydrodynamics of turbid underflows a sensitivity analysis is performed with regard to model input parameters and the initial bed properties simulations are then performed to illustrate aggradation and avulsion in an existing submarine channel the conditions under which channelization of deposits occur are also examined the effect of numerical dissipation on flow ignition is examined and the high resolution properties of the model are found to produce satisfactory results the model is applied to historical field scale events including the reserve fan in lake superior and rupert inlet in british columbia

magnetization during low temperature oxidation of seafloor basalts no large scale chemical remagnetization difficulty in removing the effects of grain size dependence have hampered interpretations of magnetic property changes during low temperature oxidation of titanomaghemite for the present study samples were collected perpendicular to cracks in seafloor basalts to produce subsamples of essentially constant grain size but varying degree of low temperature oxidation within a few centimeters coarse grained samples and those showing evidence of high temperature oxidation were excluded increasing curie temperature was used as the most reliable measure of increasing maghemitization intensity of natural remanent magnetization jnrm consistently decreased with increasing low temperature oxidation indicating that any increase in jnrm with age of oceanic crust must be due to other factors coercive force hc increased until curie temperatures reached 250 c and then in samples exhibiting single domain behavior decreased drastically because coercive force depended on degree of oxidation in this complex manner chemical remanent magnetization crm in even slightly oxidized grains should have a different coercivity than the original thermal remanent magnetization trm most deep sea drilling project samples in this study have experienced tectonic rotations or field reversals within 1 0 m y of extrusion thus field controlled crm if present should have different coercivities and a discrete direction and be discernible during alternating field af demagnetization alternating field demagnetizations showed no evidence for significant acquisition of field controlled crm viscous remanent magnetization vrm was the dominant secondary magnetization in samples with several components of remanence in very fine grained basalts vrm was difficult to remove by af demagnetization and may be the source of increased dispersion in highly oxidized oceanic basalts mechanisms which may in part be responsible for coercivity and vrm changes are 1 an increase in the critical size of domain transitions during low temperature oxidation and 2 incipient phase separation into fe rich and ti rich areas the fe rich areas being superparamagnetic the latter mechanism explains the increase in saturation magnetization js and concomitant decrease in jnrm when curie temperatures are above 360 c

alteration and self reversal in oceanic basalts 1 we report on magnetic properties and compositions of titanomaghemites from moderately to highly oxidized basalts recovered at nine drilling sites in the pacific ocean these new data together with similar data sets allow us to test compositional fields of titanomaghemite that could carry self reversed components of natural remanent magnetization above room temperature these compositions have been proposed on the basis of theoretical considerations of cation distributions during ionic reordering associated with low temperature oxidation maghemitization the data indicate that very high oxidation states z 0 9 and relatively high ti contents x 0 6 are needed to produce natural self reversed components this compositional range is much more limited than predicted the severe restrictions on compositions and oxidation state strongly suggest that self reversed magnetizations should not be a general feature of the magnetization carried by oceanic basalt instead the formation of self reversed components may signify unusual conditions such as extreme fluid flow and iron removal which could result in the required oxidation states while maintaining stability of the cation deficient titanomaghemite lattice structure

thermally induced inversion of al substituted titanomagnetite in basalts evidence for partial self reversal 1 we report properties of laboratory induced thermoremanence combined with detailed rock magnetic experiments on basaltic samples which contain aluminum substituted titanomagnetite fe2 24ti0 64al0 12o4 prior to thellier coe paleointensity experiments specimens were first demagnetized using an alternating field with a peak field of 90 mt and were then imparted an anhysteretic remanent magnetization arm to represent a natural remanent magnetization nrm results show that the partial thermal remanent magnetization ptrm gain and arm remaining are linearly correlated up to 300 c between 300 and 460 c specimens acquire thermoremanence with a direction antiparallel to the external field direction leading to intensity decreases this coincides with the onset of the mineral transformation from al titanomagnetite to a magnetic phase with higher tc our results support that phase interaction with distinct tc values in partially oxidized titanomagnetite can produce partial self reversal thermoremanence in rocks

discovery of a giant deep sea valley in the indian ocean off eastern africa the tanzania channel during the fanindien 2006 cruise of r v beautemps beaupre high resolution multibeam bathymetry sub bottom profiling and sediment coring was carried out along the east african margin offshore tanzania and mozambique indian ocean the newly acquired data reveal the presence of a giant deep sea valley the tanzania channel that is more than 10 km wide at 4000 m water depth along the continental rise the valley remains 70 m deep and 7 km wide at 800 km from the tanzania coast morphological comparison with worldwide submarine channels show that the tanzania channel is one of the largest known submarine valleys this discovery brings new light on development of submarine valleys that drain sediments originated from the east african rift system ears highlands i e the tanzania channel and its neighbor zambezi channel located 1000 km southward both of the systems have a morphology markedly different to the classical sinuous v shaped channels located at similar latitudes e g the zaire or amazon channels their submarine drainage system consists of a downslope converging tributary canyons joining a central trunk channel in the continental rise the presence of such giant deep sea drainage systems is probably linked to a strong structural control on the sediment pathway associated to a massive sediment transfer towards the indian ocean in relation with the tectonic activity of the east african rift system i e the uplift periods trough midmiocene and plio pleistocene times and its interplay with the east african equatorial climate changes

axial magnetic anomaly amplitude along the mid atlantic ridge between 20 n and 40 n after reduction to correct for the topography spreading rate latitude and lineation azimuth effects axial magnetic anomalies amplitudes are compared with bathymetric gravity seismological and geochemical data along a 2600 km long section of the mid atlantic ridge axis between 20 n and 40 n significant results are obtained at two different scales a regional one 100 km related to sublithospheric thermal perturbations such as those associated to hotspots and a local one 100 km related to ridge segmentation at a regional scale two long wavelength highs of the axial magnetic anomaly amplitudes are observed between 37 5 n and 40 n and between 27 n and 30 n the first high associated with marked bathymetric gravity geoid seismic velocity and geochemical anomalies is related to the azores hotspot and is interpreted as reflecting the existence of a thick and or fe ti enriched magnetic source layer a second long wavelength high centered at 28 5 n corresponds to a weaker bathymetric and gravity signature it is associated with a large low seismic velocity anomaly and a marked intermediate wavelength positive geoid high which suggest higher sublithospheric temperatures deeper and more extensive partial melting and a thicker and or fe ti enriched magnetic source layer at a local scale all but one of the 25 ridge segments investigated show that axial magnetic anomaly amplitude is higher at segment ends by a factor of 2 compared to segment centers a clear correlation is observed between the range of axial magnetic anomaly amplitude and the range of the mantle bouguer anomaly mba within each segment more complex relationships exist between median amplitude amplitude at segment centers and at segments ends and mba separation of segments in two groups hotter and colder based on mba shows different types of variations among segment with grouped amplitudes at segment centers and scattered amplitudes at segment ends for hotter segments and the opposite for colder segments these observations support shallow fractionation and the presence of serpentinized bodies in the vicinity of discontinuities as the major processes which control the axial magnetic anomaly amplitude variations

magnetic state of 10 40 ma old ocean basalts and its implications for natural remanent magnetization abstract the natural remanent magnetization nrm of ocean basalts giving rise to the pattern of marine magnetic anomalies is known to be of comparatively low intensity for about 20 ma old oceanic crust the aim of this study is to detect possible peculiarities in the rock magnetic properties of ocean basalts of this age and to establish a link between magnetomineralogy rock magnetic parameters and the low nrm intensity ocean basalts covering ages from 0 7 to 135 ma were selected for rock magnetic experiments and their room temperature hysteresis parameters curie temperature and temperature dependence of saturation magnetization ms t was determined and complemented by reflected light microscopy the majority of samples is magnetically dominated by titanomagnetite and titanomaghemite with increasing oxidation state with age for these a strong dependence of hysteresis parameters on the age of the samples is found the samples have a minimum in saturation magnetization and a maximum in magnetic stability in the age interval ranging from approximately 10 to 40 ma coinciding with the age interval of low nrm intensity the observed change in saturation magnetization is in the same order as that for the nrm intensity a further peculiarity of the titanomaghemites from this age interval is the shape of their ms t curves which display a maximum above room temperature neel p type and sometimes a self reversal of magnetization below room temperature neel n type these special rock magnetic properties can be explained by titanomagnetite low temperature oxidation and highly oxidized titanomaghemites in the age interval 10 40 ma a corresponding measurement of the nrm at elevated temperature allows to identify a maximum in nrm intensity above room temperature for the samples in that age interval this provides evidence that the nrm is equally carried by titanomaghemites and that the low nrm intensities for about 20 ma old ocean basalts are caused consequently by the low saturation magnetization of these titanomaghemites

sandy submarine braid plains potential deep water reservoirs sandy submarine braid plains like their fluvial counterparts on land are sand rich depositional environments that may display excellent reservoir characteristics in terms of sediment volume porosity and permeability the submarine examples may be laterally associated with potential source rocks such as the fine grained levee deposits of deep sea channels a side scan sonar study of the central labrador sea revealed the existence of a more than 700 km long and up to 120 km wide submarine sand and gravel plain that has been supplied with sediment by high density turbidity currents possibly resulting from subglacial lake outburst flooding in the hudson strait the side scan imagery of parts of the plain displays a conspicuous streaky pattern of alternating high and low backscatter intensity high resolution 3 5 khz seismic profiles and 12 khz bathymetric profiles show that the pattern represents a furrow and ridge erosional or channel and bar depositional topography similar to a braided alluvial plain the furrows or channels have low acoustic backscatter are less than 10 m deep and are separated by ridges or bars having high backscatter some channels terminate in depositional lobes individual channels and bars or furrows and ridges are less than 100 m wide and can be followed up to 40 km downcurrent on sleeve gun seismic profiles the total sand thickness appears to be between 200 m proximal and 100 m distal piston cores from the plain recovered massive sand layers up to 4 m thick buried under 1 m of holocene hemipelagic ooze texturally the sands and gravelly sands display a trend of improving sorting with increasing mean grain size some very coarse grained samples are moderately well sorted and almost matrix free the flooding events that deposited the sands might be the submarine counterpart of heinrich events but need not be restricted to such events radiocarbon ages of about 10 k y from the base of the ooze overlying the youngest sand gave a minimum age for the sand that is similar to the age of heinrich event 0 estimates for the discharge volume of individual events are poorly constrained and range from 103 to 105 km3 braided channel patterns in deep water begin page 1500 sandy depositional environments are not restricted to high latitudes but also have been identified in various submarine fan settings in the lower latitudes for example the orinoco var and monterey deep sea fans and in the santa monica basin the largest examples however are known from high latitudes suggesting that melt water discharge from continental ice sheets may favor the formation of this habitat of giant sands in the deep sea the occurrence of sandy braided deep water environments having favorable reservoir characteristics in a variety of tectonic settings makes this type of environment a potentially interesting deep water target

fluvial features in the deep sea new insights from the glacigenic submarine drainage system of the northwest atlantic mid ocean channel in the labrador sea abstract side scan sonar imagery and seismic profiles from the submarine drainage system of the northwest atlantic mid ocean channel namoc of the labrador sea reveal the presence of a variety of fluvial features in the deep sea the analogies extend from meso scale features to the basin wide drainage pattern at basin scale the namoc submarine drainage system consists of downslope converging tributary canyons on the slope and yazoo type tributary channels joining a central trunk channel on the basin floor in detail the following fluvial features have been observed in namoc and its tributaries meandering and braided channels and talwegs point bars some with meso scale bedforms terraces wash over fans submarine hanging valleys and chute pools and levee gullying and slumping some of these features are new and have not been recognised before in the deep sea others have not been documented in comparable detail previously although the similarity of deep sea turbidite depositional features with fluvial features has long been recognised in the literature fundamental differences exist between turbidite and fluvial facies associations the most noticeable of which is the lateral juxtaposition of a meandering deep sea channel and a submarine braidplain the latter has only been established for a glacigenic submarine drainage system the analogy with fluvial processes may be used in the quantification of flow processes of large scale natural turbidity currents

the central labrador sea facies and dispersal patterns of clastic sediments in a small ocean basin abstract the upper 500 m of the 2 km thick sedimentary section in the central labrador sea consists of upper pliocene pleistocene clastic sediments associated with the globally unique submarine trunk channel of the northwest atlantic mid ocean channel namoc and its system of tributaries turbidity currents in the namoc fractionate their sediment load into fine grained mostly silty clayey spill over sediments on the natural levees and coarse grained channel fill sediments the natural levees of the namoc are composite features containing shallow second order channels tributaries and satellite channels which have their own subdued levees these shallow channels are much less efficient in fractionating the sediment into coarse and fine fractions compared to the namoc the second order levees are sandy based on high resolution seismic profiles and sediment cores eight different facies have been differentiated in the surface sediments of the central labrador basin 1 deep channel fill facies on the namoc floor 2 slide and slump facies on the namoc walls 3 spill over facies on the namoc levees 4 channel fill facies of shallow second order channels 5 sandy spill over facies of shallow second order channels 6 debris flow deposits on the namoc levees 7 pelagic sediments with ice rafted debris and 8 contourites

insights into magmatic processes and hydrothermal alteration of in situ superfast spreading ocean crust at odp iodp site 1256 from a cluster analysis of rock magnetic properties we analyze magnetic properties from ocean drilling program odp integrated odp iodp hole 1256d 6 44 1 n 91 56 1 w on the cocos plate in 15 2 ma oceanic crust generated by superfast seafloor spreading the only drill hole that has sampled all three oceanic crust layers in a tectonically undisturbed setting fuzzy c means cluster analysis and nonlinear mapping are utilized to study down hole trends in the ratio of the saturation remanent magnetization and the saturation magnetization the coercive force the ratio of the remanent coercive force and coercive force the low field magnetic susceptibility and the curie temperature to evaluate the effects of magmatic and hydrothermal processes on magnetic properties a statistically robust five cluster solution separates the data predominantly into three clusters that express increasing hydrothermal alteration of the lavas which differ from two distinct clusters mainly representing the dikes and gabbros extensive alteration can obliterate magnetic property differences between lavas dikes and gabbros the imprint of thermochemical alteration on the iron titanium oxides is only partially related to the porosity of the rocks thus the analysis complements interpretation based on electrofacies analysis all clusters display rock magnetic characteristics compatible with an ability to retain a stable natural remanent magnetization suggesting that the entire sampled sequence of ocean crust can contribute to marine magnetic anomalies paleointensity determination is difficult because of the propensity of oxyexsolution during laboratory heating and or the presence of intergrowths the upper part of the extrusive sequence the granoblastic dikes and moderately altered gabbros may contain a comparatively uncontaminated thermoremanent magnetization

variations in magnetization intensity and low temperature titanomagnetite oxidation of ocean floor basalts the remanent magnetization of the oceanic crust exhibits a systematic long term variation which correlates with the amplitudes of marine magnetic anomalies after a sharp initial decrease of natural remanent magnetization intensity a minimum is reached at 20myr followed by a gradual increase up to ages of 120 myr the progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

magmatism at the west iberia non volcanic rifted continental margin evidence from analyses of magnetic anomalies we discuss the magmatic development of the west iberia non volcanic rifted continental margin in the north atlantic ocean so called non volcanic rifted continental margins are characterized by a lack of syn rift magmatism and are considered to be largely amagmatic however this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously we concentrate our attention on the recently described zone of exhumed continental mantle zecm to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the zecm leading to the onset of seafloor spreading we use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide angle seismic experiments presented elsewhere forward and inverse modelling of a sea surface magnetic anomaly chart and of surface and deep towed magnetometer profiles shows that anomalies within the zecm differ in trend amplitude and source type from those in the adjacent oceanic crust and thinned continental crust the zecm anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin we interpret such bodies as syn extensional intrusions that increase in volume oceanward they eventually merge in the vicinity of a margin parallel basement peridotite ridge to give rise to a continuous crust that records reversals in the earth s magnetic field from the time of anomaly m4 n m5 r i e to mark the onset of seafloor spreading we find no evidence for anomalies formed by seafloor spreading at either slow or ultraslow rates before m5 r 128 ma

anatomy of heinrich layer 1 and its role in the last deglaciation this research used data acquired at the xrf core scanner lab at the marum center for marine environmental sciences university of bremen germany this research used samples provided by the international ocean discovery program iodp funding for this research was provided by the uk natural environmental research council nerc to hodell the nerc radiocarbon facility supported two radiocarbon dates and wally broecker generously supported the remainder with funding from the comer family foundation research by rodriguez tovar and dorador was financed by project cgl2015 66835 p b m acknowledges support from the csic ramon y cajal postdoctoral programme ryc 2013 14073 j f e would like to acknowledge funding under erc advanced grant 320750 nanopaleomagnetism

sea valleys and related depositional systems in the gulf of lion and ebro continental margins 1 four main types of sea valleys submarine canyons gullies channels of turbidite systems and a mid ocean channel have been characterized in the ebro and the gulf of lion distal margins in the catalan sea together with three main types of depositional bodies pyrenean canyon deep sedimentary body pcdsb apron deposits and channel levee complexes related to the development of these sea valleys erosion and deposition mainly during the quaternary have formed the majority of these sea valleys although some structurally controlled sea valleys also exist as an example a messinian entrenchment has been confirmed in the petit rhone canyon the evolution of the drainage patterns is established from the detailed analysis of the sedimentary structure this analysis revea s the presence of buried sedimentary bodies and older drainage systems differing from those observed in the bathymetric charts specific factors determine the evolutionary differences between both margins including the absence or presence of underlying evaporitic messinian layers and the regional geodynamic evolution during and after the alpine orogeny we propose a dual evolutionary model from the end of the miocene to the quaternary

barrier holes and trench application to reduce blast induced vibration in seyitomer coal mine in this research experimental applications have been performed to reduce blast induced vibrations in open pit mines for this purpose artificial discontinuity zones such as barrier holes and trench were opened in a dragline panel of seyitomer lignite enterprise kutahya turkey peak particle velocities in front of and behind them were measured by seismographs in this way their effect against vibration was observed and compared barrier holes were opened 24 m deep through 3 parallel lines and aligned at 1 m spacing on the other hand the trench had 8 m depth and 4 m width during the research 209 measurements were taken belonging to 105 explosions twenty eight explosions were carried out in the barrier holes experiments and 77 explosions were carried out in the trench experiments a decrease in vibration of 14 3 18 5 was obtained behind the barrier holes while a decrease of 24 8 58 1 was provided by the trench

long term variations in magnetization intensity with crustal age in the northeast pacific atlantic and southeast indian oceans the magnetization intensity of oceanic crust in the northeast pacific atlantic and southeast indian oceans was calculated from marine magnetic anomaly data to investigate long term variations in magnetization intensity with crustal age the magnetization intensity was derived by two dimensional inversion of the upward continued marine magnetic anomalies the principal magnetization intensity variations versus age are summarized as follows 1 in all the oceans a high magnetization intensity exists at the ridge crest followed by a sharp decrease within 8 my and a minimum is reached at 8 20 ma 2 in the northeast pacific and atlantic oceans the magnetization intensity gradually increases up to at least 70 ma this variation pattern is consistent with the previous results of nrm measurements of drilled rocks and inversion of marine magnetic anomalies in the south atlantic

continental slope sedimentation adjacent to an ice margin iii the upper labrador slope abstract the upper labrador slope is the key area for sediment transfer from the northeastern margin of the pleistocene laurentide ice sheets lis to the deep basin it is subdivided from north to south into 8 sectors based on relief differences and echo character on 750 line km of continuous sleeve gun and 3 5 khz seismic profiles high to moderate relief sectors 2 and 4 and in part 6 and 8 are seismically transparent and well stratified with continuous individual high amplitude reflections and deep penetration low relief sectors 1 3 5 and 7 show strong bottom reflections and with the exception of sector 1 low seismic penetration poor stratification and low amplitude if any subbottom reflections the lateral distribution of these alternating high to moderate and low relief sectors reflects fundamental differences in the sediment transfer mechanisms through outlets from the lis onto the slope low relief slope sectors represent debrite and turbidite slopes and are located in front of ice outlets on the slope and adjacent regions to the north they originate from mass wasting on the upper slope of glacial detritus with a significant coarse component originally delivered as englacial or subglacial material and deposited in end moraines or as bedload by subglacial or supraglacial rivers high to moderate relief sectors originate from fall out of suspended sediment from turbid surface plumes tsp and preferentially occur off the southern half of outlets and south of major outlets as exemplified by sector 2 south of the hudson strait outlet this asymmetry is caused by the south flowing labrador current which entrains the buoyantly rising turbid meltwater plumes at the glacier front the present high relief topography which shows a dendritic pattern of upslope canyon branching is the result of retrograde headward canyon erosion by mass wasting processes of an originally much smoother mud blanket surface the sediment is remobilized by slumping and entrained in debris flows and turbidity currents the occurrence of tsp deposits on the high to moderate relief upper slope sectors requires a summer sea surface not frozen over during major parts of the pleistocene including glacial maxima tps deposition on the upper slope ice rafting and a plethora of ice margin depositional phenomena many of which give rise to mass wasting and mass flow phenomena are the main features that make high latitude continental slopes adjacent to continental ice sheets different from their lower latitude counterparts

problems concerning the evolution of oceanic lithosphere in the northern red sea abstract the pole of rotation for the opening of the red sea at 36 5 n 18 0 e arabia from africa implies the fastest spreading rates are in the south and are about 50 less in the north various studies show the spreading rate in the south to be about 1 cm yr over the last 4 5 ma problems arise as to the nature of the lithosphere beneath the northernmost red sea the problem is further complicated by the fact that the red sea may have evolved in three phases the first is the gulf of suez phase mainly oligocene the second corresponds to the first 62 km movement along the aqaba dead sea shear latest oligocene early miocene and the third to the second 45 km movement along the aqaba dead sea shear plio pleistocene these motions and their implications for the northernmost red sea are assessed the northern red sea differs from the central and southern red sea in having no clear magnetic lineations only a few isolated circular anomalies are observed there are two possible explanations 1 1 there is extremely thinned continental lithosphere occasionally pierced by strongly magnetised volcanic centres or 2 2 the oceanic lithosphere is present but without magnetic anomalies it is suggested that the first explanation applies to the first gulf of suez phase and the second explanation to the two later aqaba dead sea phases it is further suggested that the lack of magnetic anomalies may be due to a combination of large thicknesses of unstable sediments salt high temperatures and low sea floor spreading rates these lead to the evolution of oceanic lithosphere with slow cooling inhibiting the acquisition of strong magnetisation and giving subdued magnetic anomalies

amplitude variation of marine magnetic anomalies to determine the large scale amplitude variation of marine magnetic anomalies 16 profiles have been analysed 8 across the mid atlantic ridge 5 across the east pacific rise and 3 across the indian ocean the profiles were normalized to the age of the anomalies and finally transformed to the same latitude of 45 both ridges show qualitatively the same trend with the central anomaly over the spreading axis clearly dominating the flanking anomalies gradually decrease in amplitude until a minimum is reached over an age range from 10 to 30 ma beyond this minimum the anomaly amplitudes progressively increase again to another maximum near the onset of the mesozoic magnetic anomalies there exists however a distinct difference in the absolute value of the magnetic anomalies for the two oceans analysed the observed amplitude variation can well be explained by a corresponding variation of the natural remanent magnetization of the upper 1000 meters of oceanic basalts as measured on individual samples from the deep sea drilling project model calculations lead us to propose a 3 layer model for the sources of marine magnetic anomalies an uppermost layer of 1000 m basalt corresponding more or less to seismic layer 2 with age dependent magnetization and two layers corresponding to seismic layers 3a and 3b with age independent magnetization the most likely cause for the age dependency of magnetization of the uppermost layer of oceanic basalts is gradual low temperature oxidation of the magnetic minerals contained in the rocks

global equivalent magnetization of the oceanic lithosphere abstract as a by product of the construction of a new world digital magnetic anomaly map over oceanic areas we use an original approach based on the global forward modeling of seafloor spreading magnetic anomalies and their comparison to the available marine magnetic data to derive the first map of the equivalent magnetization over the world s ocean this map reveals consistent patterns related to the age of the oceanic lithosphere the spreading rate at which it was formed and the presence of mantle thermal anomalies which affects seafloor spreading and the resulting lithosphere as for the age the equivalent magnetization decreases significantly during the first 10 15 myr after its formation probably due to the alteration of crustal magnetic minerals under pervasive hydrothermal alteration then increases regularly between 20 and 70 ma reflecting variations in the field strength or source effects such as the acquisition of a secondary magnetization as for the spreading rate the equivalent magnetization is twice as strong in areas formed at fast rate than in those formed at slow rate with a threshold at 40 km myr in agreement with an independent global analysis of the amplitude of anomaly 25 this result combined with those from the study of the anomalous skewness of marine magnetic anomalies allows building a unified model for the magnetic structure of normal oceanic lithosphere as a function of spreading rate finally specific areas affected by thermal mantle anomalies at the time of their formation exhibit peculiar equivalent magnetization signatures such as the cold australian antarctic discordance marked by a lower magnetization and several hotspots marked by a high magnetization

numerical simulation of tensile damage and blast crater in brittle rock due to underground explosion the prediction of blast crater in brittle rock due to an underground explosion has gained importance in recent years due to the great number of accidental events that affected engineering safety this paper uses the taylor chen kuszmaul tck continuum damage model to analyze dynamic fracture behavior of rock in tension due to blast loading the tck damage model together with an erosion algorithm was implemented into the explicit fe code ls dyna as a constitutive augmentation the damage pattern around the blasthole and the formation of blast crater near a free surface were subsequently simulated using the developed numerical tool it is shown that the free surface is vitally responsible for the blast crater furthermore the size and shape of the blast craters can be reasonably predicted if the erosion criterion of critical tensile damage is well calibrated the effects of common charge modes on blast craters were also investigated numerically and the mechanisms of the coupled air decoupled and water decoupled charge mode are compared and presented

magnetism of al substituted magnetite reduced from al hematite this study was supported by national program on global change and air sea interaction gasi 04 01 02 the national natural science foundation of china grants 41504055 41430962 41374073 and 41025013 and chinese continental shelf deep drilling program gzh201100202 z x j further acknowledges support from the china postdoctoral science foundation a p r and d h acknowledge support from the australian research council grants dp110105419 and dp120103952

magnetization of the oceanic crust thermoremanent magnetization of chemical remanent magnetization we propose a model in which chemical remanent magnetization crm acquired within the first 20 m y of crustal evolution may account for 80 of the bulk natural remanent magnetization of older basalts the crm of the crust is acquired as the original thermoremanent magnetization trm is lost through low temperature alteration the crm intensity and direction are controlled by the posternplacement polarity history this model explains several independent observations concerning the magnetization of the oceanic crust the model accounts for amplitude and skewness discrepancies observed in both the intermediate wavelength satellite field and the short wavelength sea surface magnetic anomaly pattern it also explains the decay of magnetization away from the spreading axis and the enhanced magnetization of the cretaceous quiet zones while predicting other systematic variations with age in the bulk magnetization of the oceanic crust the model also explains discrepancies in the anomaly skewness parameter observed for anomalies of cretaceous age further our study indicates varying rates of trm decay in very young crust which depicts the advance of low temperature alteration through the magnetized layer

dynamic characteristics and safety criterion of deep rock mine opening under blast loading abstract the blasting vibration in deep mining affects the safety of the adjacent roadways to estimate the stability of a deep rock mass opening during blasting excavation the dynamic responses of the deep rock mass subjected to blast loading are investigated in this study the relationships between the blasting vibration velocity and the frequency over time horizontal distance and depth are analyzed and the stress distribution of the surrounding rock mass is depicted through field tests and numerical simulation the failure mechanism of deep rock mass is thus revealed and a new safety criterion regarding the blasting vibration of the deep rock mass is put forward the results indicate that the vertical vibration velocity is higher than the horizontal vibration velocity and that the vertical tension of the seismic wave of the blast is larger than that of the shearing action in the near blasting field which contrasts that of the far field both the vibration velocity and frequency decrease with an increase in time distance and depth and the range of influence of the blasting vibration in terms of depth is larger than that for the horizontal direction therefore the deep rock mass is more vulnerable to damage than the shallow rock mass under blast loading in addition the surrounding rock stress tends to transfer from the arc foot on the blasting side to the straight wall on the rear side and the shear stress is ahead of the tensile stress it was verified through a case calculation that the new criterion has practical value and can be used as a guide for evaluating the blasting vibration effects of the deep rock mass

studies on the effect of burden width on blast induced vibration in open pit mines this study concerns the correlation between burden and blast induced vibrations in open pit mines for this purpose two different mines were studied in these mines the vibrations caused by explosions at burdens having widths ranging from 3 to 14 m were measured from various distances from the results it was found for these cases that burden width has a significant impact on vibrations consequently it was proven that vibrations decrease as burden increases

a laurentide outburst flooding event during the last interglacial period episodes of ice sheet disintegration and meltwater release over glacial interglacial cycles are recorded by discrete layers of detrital sediment in the labrador sea1 2 the most prominent layers reflect the release of iceberg armadas associated with cold heinrich events3 but the detrital sediment carried by glacial outburst floods from the melting laurentide ice sheet is also preserved4 here we report an extensive layer of red detrital material in the labrador sea that was deposited during the early last interglacial period we trace the layer through sediment cores collected along the labrador and greenland margins of the labrador sea biomarker data ca sr ratios and 18o measurements link the carbonate contained in the red layer to the palaeozoic bedrock of the hudson bay we conclude that the debris was carried to the labrador sea during a glacial outburst flood through the hudson strait analogous to the final lake agassiz outburst flood about 8 400 years ago probably around the time of a last interglacial cold event in the north atlantic5 we suggest that outburst floods associated with the final collapse of the laurentide ice sheet may have been pervasive features during the early stages of late quaternary interglacial periods

modification of the magnetic mineralogy in basalts due to fluid rock interactions in a high temperature geothermal system krafla iceland summary active high temperature 150 c geothermal areas like the krafla caldera ne iceland often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity the main alteration processes in such an environment are low temperature oxidation 350 c maghemitization and fluid rock interactions we investigated the rock magnetic properties natural remanent magnetization nrm magnetic susceptibility and their temperature and field variation and the mineralogy using x ray diffraction microscopic methods and electron microprobe analyses of two drill cores kh1 and kh3 from the rim of the krafla caldera the drill cores have distinctly lower nrm values average 3 a m 1 compared to younger surface basalts average 20 a m 1 along with a large variation in magnetic susceptibility 1 3 10 7 4 9 10 5 m3 kg 1 the secondary mineral assemblage sulphides sphene rutile and chlorite indicates an alteration within the chlorite smectite zone for both cores without depth zoning optical miscroscopy in combination with the bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite 1 titanomaghemite with intermediate titanium concentration and probably high vacancy concentration and 2 titanomaghemite with low titanium concentration and low vacancy concentration the mineral assemblages textures and magnetic properties deduced from the mentioned magnetic measurements indicate two stage transformation mechanism 1 dissolution of titanium at low ph under oxidizing conditions the ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene and fe2 migrates out of the spinel lattice forming titanomaghemite 2 formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions the latter mechanism produces ghost textures all titanomaghemite is transformed and only their former grain shapes are preserved with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved this mechanism could explain the significant magnetization loss which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like iceland or hawaii the production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of krafla

dynamic stress concentration and energy evolution of deep buried tunnels under blasting loads abstract a theoretical formulation was first established to evaluate the dynamic stress concentration factor dscf around a circular opening under conditions of blasting stress wave incidence a two dimensional numerical model was then constructed by the particle flow code pfc in order to simulate the dynamic responses around an underground tunnel subjected to blasting load in the simulation a series of horizontal blasting stress waves were applied to an underground tunnel under various in situ stress states and then the dynamic responses around the tunnel were analyzed from the viewpoint of the dynamic stress concentration and energy evolution the results of theoretical analysis indicated that obvious dynamic effects occur at tunnel boundary during blasting stress wave incidence and the dscf at the roof and floor of the tunnel is much larger than that at two sidewalls when blasting stress wave was applied to left model boundary the numerical results showed that high static compressive stress concentration around the underground tunnel results in the accumulation of substantial strain energy at the same location the roof and floor of the tunnel are more prone to dynamic failures during the blasting loading process in addition the analysis of energy dissipation indicated that the strain energy reduction and the residual kinetic energy are positively related to the lateral pressure coefficient and the burial depth of the tunnel and the residual kinetic energy is much larger than the strain energy reduction under the same condition furthermore for an underground tunnel subjected to high in situ stress the blasting stress wave with lower amplitude is sufficient to trigger severe dynamic failures

variations in oceanic crustal structure and implications for the fine scale magnetic anomaly signal marine magnetic anomalies are traditionally modelled using a constant thickness source layer usually defined as the extrusive lavas associated with seismic layer 2a recent seismic studies reveal that the thickness of layer 2a can vary by more than a factor of two an observation which has important implications for the magnetic anomaly source layer we model deep tow magnetometer data from the endeavour segment of the northern juan de fuca ridge in combination with nearby seismic data and find a positive correlation between the thickness of the seismically defined layer 2a and the magnetic anomaly source layer this suggests that short wavelength magnetic anomalies observed within individual magnetic polarity units can arise from variations in the thickness of the extrusives rather than geomagnetic field variations or secondary magnetization

crustal magnetization and accretion at the southwest indian ridge near the atlantis ii fracture zone 0 25 ma 1 we analyze geophysical data that extend from 0 to 25 myr old seafloor on both flanks of the southwest indian ridge swir lineated marine magnetic anomalies are consistent and identifiable within the study area even over seafloor lacking a basaltic upper crust the full spreading rate of 14 km myr has remained nearly constant since at least 20 ma but crustal accretion has been highly asymmetric with half rates of 8 5 and 5 5 km myr on the antarctic and african flanks respectively this asymmetry may be unique to a 400 km wide corridor between large offset fracture zones of the swir in contrast to the mid atlantic ridge crustal magnetization amplitudes correlate directly with seafloor topography along the present day rift valleys this pattern appears to be primarily a function of along axis variations in crustal thickness rather than magnetic mineralogy off axis magnetization amplitudes at paleo segment ends are more positive than at paleo segment midpoints suggesting the presence of an induced component of magnetization within the lower crust or serpentinized upper mantle alteration of the magnetic source layer at paleo segment midpoints reduces magnetization amplitudes by 70 80 within 20 myr of accretion magnetic and ocean drilling program odp hole 735b data suggest that the lower crust cooled quickly enough to lock in a primary thermoremanent magnetization that is in phase with that of the overlying upper crust thus magnetic polarity boundaries within the intrusive lower crust may be steeper than envisioned in prior models of ocean crustal magnetization as the crust ages the lower crust becomes increasingly important in preserving marine magnetic stripes

thermoremanent magnetization of the oceanic lithosphere inferred from a thermal evolution model implications for the source of marine magnetic anomalies abstract thermoremanent magnetization models of the oceanic crust and the uppermost part of the oceanic mantle are derived on the basis of a typical thermal evolution model of the oceanic lithosphere using different magnetic blocking temperature ranges the magnetization of the basaltic layer 2a has a similar pattern but with a lower intensity to that of the standard sea floor spreading magnetic model however the magnetization patterns of the lower crust and upper mantle are skewed so that in a given polarity period the total magnetization of a lithospheric column which is created early in the period is greater than that of a column which is created later in the period magnetic anomalies of the magnetization models are calculated at sea level and at magsat elevation for slow moderate and fast sea floor spreading velocities although the magnetization models are derived based on purely physical reasoning without using any a priori information about the observed marine and magsat magnetic anomalies they do yield magnetic anomalies which are similar to the observed anomalies the models explain several features of the marine and magsat anomalies the anomalous skewness of marine magnetic anomalies is explained in terms of the magnetization of the oceanic lower crust the magnetization of the oceanic upper mantle may account for the anomalous skewness observed over the cretaceous quiet zones in the atlantic ocean the skewness of the magnetic anomalies calculated at sea level decreases with the increase in the sea floor spreading velocity

age dependent variation in the magnetization of seamounts the magnetization of pacific seamounts shows a surprisingly large variation when viewed as a function of time similar to normal upper oceanic crust the basalts that form the external shell of volcanic seamounts have a high average magnetization immediately after formation this high magnetization decays rapidly with time resulting in a factor of 5 reduction in intensity in less than 500 000 years because submarine volcanoes appear to consist of the same basalts that form the upper part of the seafloor low temperature oxidation of the magnetic mineral titanomagnetite is assumed to be responsible for the reduction in magnetization for both types of crust some large oceanic volcanoes are of sufficient volume that the upper capping rocks erupt subaerially and submerge completely due to thermal subsidence only after several million years these capping rocks can be locally abundant i e hawaii are exposed to a different post formation environment and can have a magnetic mineralogy that is different from submarine volcanoes in spite of these differences the magnetization of subaerially erupted rocks of the hawaii emperor chain also may show a similar rapid reduction in intensity with age finally seamounts of cretaceous age show a factor of 2 higher average magnetization when compared to tertiary seamounts similar to the previously observed increased magnetization of cretaceous seafloor basalts recovered from deep sea drilling project ocean drilling project cores this increase in cretaceous magnetization appears to be reflected in the increased amplitude of magnetic anomalies in some but not all of the ocean basins prior and subsequent to the cretaceous normal superchron the processes potentially responsible for the elevated cretaceous magnetization values include a high geomagnetic field intensity large scale variations in whole rock major element geochemistry of the extrusive rocks or a systematic acquisition of a secondary magnetization during the cretaceous normal superchron

the role of deep water sedimentary processes in shaping a continental margin the northwest atlantic abstract the tectonic history of a margin dictates its general shape however its geomorphology is generally transformed by deep sea sedimentary processes the objective of this study is to show the influences of turbidity currents contour currents and sediment mass failures on the geomorphology of the deep water northwestern atlantic margin nwam between blake ridge and hudson trough spanning about 32 of latitude and the shelf edge to the abyssal plain this assessment is based on new multibeam echosounder data global bathymetric models and sub surface geophysical information the deep water nwam is divided into four broad geomorphologic classifications based on their bathymetric shape graded above grade stepped and out of grade these shapes were created as a function of the balance between sediment accumulation and removal that in turn were related to sedimentary processes and slope accommodation this descriptive method of classifying continental margins while being non interpretative is more informative than the conventional continental shelf slope and rise classification and better facilitates interpretation concerning dominant sedimentary processes areas of the margin dominated by turbidity currents and slope by pass developed graded slopes if sediments did not by pass the slope due to accommodation then an above grade or stepped slope resulted geostrophic currents created sedimentary bodies of a variety of forms and positions along the nwam detached drifts form linear above grade slopes along their crests from the shelf edge to the deep basin plastered drifts formed stepped slope profiles sediment mass failure has had a variety of consequences on the margin morphology large mass failures created out of grade profiles whereas smaller mass failures tended to remain on the slope and formed above grade profiles at trough mouth fans or nearly graded profiles such as offshore cape fear