magnetic susceptibility anomalies of lithosphere beneath eastern europe and the middle east standard inversion methods for inverting aeromagnetic anomalies into magnetic susceptibility contrast in the crust are based on the assumption that the core field is constant over the region considered this assumption however does not hold over regions of a few thousand kilometer extent an inversion technique is developed which takes into account the variations in the core field both in direction and in intensity it can also take into account the surface topography of a magnetic layer within the lithosphere the technique is applied to a scalar magnetic anomaly map of eastern europe and the middle east derived from the magnetometer satellite magsat data the magnetic susceptibility contrast map thus obtained delineates the major tectonic features of the region small scale precambrian shields are characterized by high magnetic susceptibility the alpine himalayan belt has a distinct magnetic signature the collision zones are low susceptibility regions whereas the continental plates within the b

crust beneath the northwestern deccan volcanic province india evidence for uplift and magmatic underplating the northwestern deccan volcanic province of india and its pericratonic rift basins were reactivated during different stages after the breakup of india from the gondwanaland and collision with the asian plate in this study we present results of crustal thickness and average crustal vp vs ratios beneath this plume affected region using common conversion point imaging and h k stacking analysis of 6893 receiver functions using data from a network comprising 58 broadband seismic stations sited on diverse tectonic terrains we find large variations in crustal thickness with the moho depths varying from 28 to 43 km in the kachchh rift 28 to 38 km in the cambay rift 39 5 41 5 km in the north and eastern parts of the cambay rift and 29 to 39 km in the saurashtra region and south gujarat a moho upwarp of 6 to 7 km in the saurashtra region can be attributed to positive buoyancy and uplift due to thermal influx affected by the reunion plume high crustal vp vs ratios beneath the kachchh rift 1 8 to 2 05 coastal areas of saurashtra 1 75 to 2 06 and north gujarat 1 81 to 1 85 indicate dominance of a mafic ultramafic crust high regional heat flow high electrical conductivity large intracrustal s wave velocity reduction and high average crustal poisson s ratios are consistent with partial melt related to the process of magmatic underplating in the lower crust at other stations the crust appears to be felsic with vp vs ratios in the range of 1 57 to 1 76

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 galapagos spreading center at 86 w a detailed geothermal field study appendix is available with entire article on microfiche orderfrom american geophysical union 2000 florida avenue n w washington d c 20009 document j80 013 01 00 payment mustaccompany order we report here measurements of the heat flow field of the galapagos spreading center on crust of age less than 1 0 m y the 443 measurements in an area of about 570 km2 reveal the general planform of the geothermal flux and permit the first truly areal estimate of the near axis conductive heat flux the intrusion process and associated hydrothermal circulation dominate the surface heat flow pattern with circulation apparently continuing beyond the limits of our survey the areal average of the conductive heat flux is 7 1 0 8 hfu 295 33 mw m2 about one third the heat flux predicted by plate models the remaining heat is apparently removed by venting of hydrothermal waters at the spreading axis and through basalt outcrops and hydrothermal mounds off axis the pattern of surface heat flux is lineated parallel to the axis and the strongly lineated topography sharp lateral gradients in the heat flow greater than 10 hfu km near escarpments and commonly expressed as high heat flow at the tops of the scarps and lower heat flow in the valleys may indicate a local concentration of the circulation by surface fault systems and or variable sediment thickness

satellite magnetic anomalies over subduction zones the aleutian arc anomaly positive magnetic anomalies seen in magsat average scalar anomaly data overlying some subduction zones can be explained in terms of the magnetization contrast between the cold subducted oceanic slab and the surrounding hotter nonmagnetic mantle three dimensional modeling studies show that peak anomaly amplitude and location depend on slab length and dip a model for the aleutian arc anomaly matches the general trend of the observed magsat anomaly if a slab thickness of 7 km and a relatively high induced plus viscous magnetization contrast of 4 a m are used a second source body along the present day continental margin is required to match the observed anomaly in detail and may be modeled as a relic slab from subduction prior to 60 m y ago

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

remanent magnetization and coercivity of rocks under hydrostatic pressure up to 1 4 gpa 1 we designed an isothermal remanent magnetization irm acquisition system based on permanent magnets and sized to accommodate an amagnetic hydrostatic pressure cell this pressure cell fits in a superconducting rock magnetometer allowing for the measurement of remanent magnetization of pressurized samples with this system we determined the coercivity of remanence bcr at different hydrostatic pressures up to 1 4 gpa for rock and dispersed mineral samples with various magnetic mineralogy and domain state irm and bcr are nearly identical before compression and after decompression indicating no permanent changes in the magnetic properties during pressure cycling hydrostatic pressure up to 1 4 gpa does not significantly increases irm under pressure except for multidomain pyrrhotite and magnetite which show an increase of about 40 relative increase of bcr under pressure is mild except for a near single domain titanomagnetite where bcr doubles

the global continent ocean magnetization contrast abstract a schematic global model for the distribution of magnetization in the lithosphere is devised this model is characterized by a uniform stepwise change of radially integrated susceptibility at continent ocean boundaries and an axial dipolar inducing field a spherical harmonic expansion of the magnetic field produced by this source distribution is computed then the standard method of truncation at degree n 13 is used to determine the respective contribution of this field to the main field and to the lithospheric anomaly field the latter is shown to display significant anomalies near continental margins as well as within continents and oceanic basins the energy density spectrum of this field is shown to be very similar to the spectrum of the magsat derived lithospheric anomaly field furthermore several features of this field are shown to account for unexplained anomalies of the magsat field this supports the hypothesis that such an abrupt magnetization variation may occur at several continental margins and contribute to the signal in the magsat derived long wavelength anomaly maps this hypothesis provides an order of magnitude estimate for the magnetization difference between oceanic and continental lithospheres finally it is argued that the synthetic anomaly field produced by this model provides an upper bound to the energy density spectrum of the lithospheric field at low harmonic degrees and concluded that the lithospheric field does not contribute significantly to the earth s field below degree 13

dsdp hole 504b the first reference section over 1 km through layer 2 of the oceanic crust hole 504b provides a reference section for correlation of in situ petrological and geophysical studies with regional crustal models based on dredged samples and remote surveys and at least for the medium spreading rate costa rica rift hole 504b confirms that the main features of the upper part of an idealized ophiolite sequence occurs in the oceanic crust

permeability underpressures and convection in the oceanic crust near the costa rica rift eastern equatorial pacific in situ permeability and pore pressures were measured 200 m deep beneath the top of the oceanic crust at dsdp site 504b these measurements have relevance for the transition from convective to conductive heat flow on the south flank of the costa rica rift conventional slug and constant rate injection tests were made below a hydraulic packer set at various depths in the hole the packer was first set in a massive flow unit 37 m below the sediment basement interface the bulk permeability of the 172 5 m of pillow basalts and basaltic flows below the packer was found to be about 40 millidarcys 4 10 10 cm2 measurements over 3 and 15 m intervals at the bottom of the hole in an altered pillow zone indicated a bulk permeability of 2 4 millidarcys these values are thought to be accurate to 30 formation pore pressures were found to be approximately 8 bars 2 below hydrostatic interpretation of the data with respect to simple numerical convection models suggests that the transition from convective to conductive heat flow is controlled by the cessation of convective heat transport through the sedimentary layer rather than the cessation of convection in the sediment furthermore the agreement between observed and modeled underpressures implies that hole 504b penetrated an active ocean crustal convection system the thick sedimentary layer layers of basal chert and massive flow basalts above the layer 2a pillow flows apparently form an impermeable lid effectively isolating the convection system from the ocean

seismicity and active faulting of the kanab fredonia area of the southern colorado plateau 1 detailed study of the historic seismicity of the kanab fredonia area adds information to the location of the colorado plateau tectonic boundary in this area and to the nature of contemporary seismicity and faulting on this part of the colorado plateau historic seismicity over 45 years 1959 2004 defines a band of activity which extends through fredonia arizona and into southern utah around kanab the area is dominated by north trending high angle normal faults the spatial distribution of epicenters consists of clusters associated with the west kaibab big springs and toroweap sevier faults indicating that these faults may be active four focal mechanism solutions show basin range style high angle normal faulting on north to north northwest trending surfaces that are parallel to subparallel in most cases to mapped quaternary faults this suggests a present east west extension occurring on the southern colorado plateau for the kanab fredonia area and an expansion of the adjacent basin range province eastward into the plateau as far as the west kaibab big springs faults the west kaibab big springs faults are proposed to be the tectonic boundary of the southern colorado plateau in this area since they mark the limit of significant seismic activity that is combined with basin range type faulting

small scale features in the earth s magnetic field observed by magsat a spherical harmonic expansion to degree and order 29 is derived using a selected magnetically quiet sample of magsat data global maps representing the contribution due to terms of the expansion above n 13 at 400 km altitude are compared with previously published residual anomaly maps and shown to be similar even in polar regions an expansion with such a high degree and order displays all but the sharpest features seen by the satellite and gives a more consistent picture of the high order field structure at a constant altitude than do component maps derived independently the appendix is available with the entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b83 008 2 50 payment must accompany order

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

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

life of an oceanic plate cooling time and assimilation time an oceanic plate undergoes two different thermal stages in its life from its creation at the spreading center to its assimilation under the subduction zone first the cooling stage and secondly the heating stage introducing the terms cooling time and assimilation time as measures of intervals of the former and latter stages we discussed the quantitative correlation between the two the cooling time ac is defined as the time required for the oceanic plate to move from a spreading center to a trench and is equivalent to the age of the oceanic plate when it just reached the trench the assimilation time tm is defined as the time required for the subducting plate to lose the ability to generate earthquakes since it began to subduct we estimated the values tm and ac at 50 sites in major subduction zones of the world the result indicates a positive linear relation between ac and tm simply expressed by tm ac9 2 ma while mckenzie s subduction model and the thickening plate model theoretically expect the relationship to be expressed as tm ac the discrepancy between our result and the theoretical model suggests the delay of effective heating of the subducting plate under the subduction zone and or the rapid increase of the plate thickness around the spreading center

an interpretation of magnetic signatures of aulacogens and cratons in africa and south america abstract magnetic susceptibility anomalies associated with the lithosphere in africa and south america show that the lithosphere underlying sedimentary basins within aulacogens which originated during the rupture of the gondwana continent have lower magnetic susceptibility than the lithosphere underlying the cratons this can be explained by the demagnetization effects of high temperature asthenospheric material possibly intruded into the aulacogens during the rupture thermal evolution of different intrusion models implies that a thermal demagnetization may last for about 300 ma b there is no close relationship between the early subsidence rate of a basin and the magnetization of the underlying lithosphere c surface heat flow anomalies do not necessarily correlate with the magnetic susceptibility anomalies and d once an intrusion of more magnetic material cools to the ambient temperature the total magnetic susceptibility of the lithosphere may increase high magnetic susceptibilities associated with sedimentary basins over old crustal rifts can be explained by this phenomenon

the subduction of young lithosphere studies using a variety of techniques of the subduction beneath western south america and the southwest honshu kyushu region of japan indicate that volcano free segments occur where the subducted and continental lithospheres remain in contact without intervening asthenosphere the subduction is initiated at normal dip angles but the plate deforms at some depth 100 km under central peru to travel horizontally immediately beneath continental lithosphere the most plausible reason for this geometry is that the subducted plate is buoyant a model is developed constrained by age of the plate bathymetry and heat flow estimates of the density of oceanic plates as a function of age show that younger ocean floor may be less dense than the asthenosphere into which it subducts if the high density tranformation of crustal basalt to eclogite is retarded by low temperatures the plate can remain buoyant to considerable depth for long periods of time heat flow data from western south america are consistent with this model

evidence for excess pore pressures in southwest indian ocean sediments brown clay cores from the madagascar and crozet basins show the following evidence of excess pore pressures large amounts of flow in increasing average sedimentation rate with age and nonlinear temperature gradients additionally many hilltops in these basins have no visible sediment cover the bare hilltops may result from periodic slumping caused by excess pore pressures calculated excess pore pressures which equal or exceed the overburden pressure were inferred from water fluxes predicted by nonlinear temperature gradients and laboratory permeability measurements by using darcy s law since pore pressures which exceed the overburden pressure are unreasonable we attribute this discrepancy to laboratory measures which underestimate the in situ permeability the widespread presence of overpressured sediments in areas of irregular topography provides a process for resuspension of clay sized particles this mechanism does not require high current velocities for the erosion of clay and therefore can be applied to many areas where no strong currents are evident carbonate rich sediments from the madagascar ridge the mozambique ridge and the agulhas plateau had almost no flow in and occurred in areas where all topography was thickly draped with sediment since the age and tectonic location of the ridges and plateaus preclude water circulation in the basement wemore attribute these differences between the brown clay and the carbonate rich material to an absence of significant excess pore pressures in the plateau and ridge sediments less

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