magnetic stability of oceanic gabbros from odp hole 735b11measurements were performed while at the bundesanstalt fur geowissenschaften and rohstoffe hannover germany abstract ocean drilling program odp hole 735b was drilled to a depth of 1 5 km in a tectonic window of gabbroic lower oceanic crust created at the southwest indian ridge the gabbros have a very stable natural remanent magnetization nrm of reversed polarity with most unblocking temperatures slightly below the curie temperature of magnetite the nrm includes a drilling induced overprint but its intensity decays strongly towards the interior of the drill core the demagnetization data yield no or only a very small secondary magnetization component acquired during the present brunhes chron or an earlier normal chron suggesting cooling through most of the blocking temperature range during chron c5r and a strong resistance against the acquisition of thermoviscous magnetization a novel furnace has been designed to measure magnetizations and their time dependences at high temperatures up to 580 c inside a commercial squid magnetometer magnetic viscosity experiments have been conducted on the gabbros at temperatures up to 550 c to determine the time and temperature stability of remanent magnetization viscosities are generally small and increase little with temperature below the main blocking temperature where the increase becomes almost an order of magnitude extrapolations to geological times infer viscous acquisitions that would be 5 25 of a thermoremanence in 100 kyr and at temperatures of 200 500 c at ocean bottom temperature the predicted magnetization of one sample acquired in the present brunhes chron should be 10 of the nrm however this is not recognized during nrm demagnetization and partial thermoremanent magnetization ptrm acquisitions at 250 c are also much smaller than predicted it thus appears that the nrms are generally magnetically harder than magnetizations acquired after heating to 570 c in the laboratory susceptibility changes during heating are small

a new shipboard three component magnetometer the three components of the geomagnetic field were measured at sea by stcm shipboard three component magnetometer with a relative accuracy of 50 25 nt the stcm was controlled by a microcomputer and the three geomagnetic components were calculated by reducing the magnetic field due to the ship s remanent and induced magnetic moments the only assumption being that the magnetic field is linearly related to the magnetic moments although stcm has no particular magnetic compensation device it can be set at any place on board of any kind of ship in the kh82 5 cruise the three component measurements were conducted along latitude 42 n north of the mendocino fracture zone where there are well defined magnetic anomaly lineations trending north and south stcm measured the north component anomalies with very small amplitudes compared to the east and downward component anomalies which shows that lineations trend north and south the three measured component anomalies are almost identical to the anomalies

stress tensor analysis of the 1998 1999 tectonic swarm of northern quito related to the volcanic swarm of guagua pichincha volcano ecuador the phreatic activity and the subsequent dacitic dome growth in 1998 1999 at guagua pichincha volcano ecuador were associated with two seismic swarms one located in the northern part of quito population 1 500 000 and another one just below the active volcano about 15 20 km sw from the first one quito swarm tectonic events have high frequencies from 1 to 10 15 hz we registered more than 3200 events among which 2354 events of 1 4 m l 4 2 between june 1998 and december 1999 at the 2 and 17 km depth the volcanic events below the guagua pichincha caldera have high from 1 to 10 15 hz and low less than 3 hz frequencies approximately 130 000 events were registered between september 1998 and december 1999 at the 2 4 and 3 5 km depth here we study the stress tensors of these two swarms deduced from the polarities of p first motions and compare them to the regional stress tensor deduced from cmt harvard focal mechanisms the quito swarm stress tensor is relatively close to the regional stress tensor the 1 axis was oriented n117 e close to the n102 e direction of the plate motion found by the gps measurement and 3 is nearly vertical the difference may be due to the action of the closely active guagua pichincha volcano the guagua pichincha stress tensor is very different from the regional tectonic one the 1 axis of the volcano is oriented n214 e almost perpendicular to the 1 of the swarm of quito and 3 is almost horizontal even if these two tensors are different they can be explained in a more general tectonic scheme the almost horizontal direction of 3 just below the volcano is compatible with an extensional horizontal direction that may be expected in the shallow extrados part of a compressional region and consistent with an opening of the top of the guagua pichincha volcano the movement of the fluids magma gas and or groundwater produced by the closely active guagua pichincha volcano seems to have an influence in the acceleration of the generation of seismic events

three dimensional inversion of marine magnetic anomalies on the equatorial atlantic ridge st paul fracture zone delayed magnetization in a magmatically starved spreading center the st paul fracture zone fz in the equatorial atlantic is interrupted by three intratransform ridge itr spreading centers a detailed magnetic survey corrected for the diurnal variations using a moored magnetic station six submersible dives and three bottom towed video camera tracks provide data on the most eastern itr 0degrees37 n 25degrees27 w visual observations and submersible sampling displayed a high ultramafic volcanic ratio supporting the assumption that the itr is in a magmatically starved state volcanics were mainly found on the rift valley floor from 4700 to 4000 m and as a thin cap 160 m on the top of the eastern rift crest 2700 m most of the rift walls consist essentially of serpentinized peridotites and gabbros the magnetic data show a well defined ridge centered anomaly a generalized inversion method was applied to the field data to calculate the crustal equivalent magnetization assuming that the seafloor is broken down into elementary cells of 1x1x0 5 km 3 which fit the topography the average of absolute value of equivalent magnetization is 2 7 a m 1 the width of the central normal polarity brunhes epoch is wider at least 34 km than that indicated by the nuvel 1 kinematic model 24 5 km this 40 excess is believed to be significant and is thought to be the result of prolonged chemical remanent magnetization acquired during the serpentinization of peridotites in a magmatically starved accretion segment we suggest that peridotites could continue to acquire magnetization as long as tectonic activities facilitate the circulation of seawater in the upper mantle

modes of faulting at mid ocean ridges abyssal hill bounding faults that pervade the oceanic crust are the most common tectonic feature on the surface of the earth the recognition that these faults form at plate spreading centres came with the plate tectonic revolution recent observations reveal a large range of fault sizes and orientations numerical models of plate separation dyke intrusion and faulting require at least two distinct mechanisms of fault formation at ridges to explain these observations plate unbending with distance from the top of an axial high reproduces the observed dip directions and offsets of faults formed at fast spreading centres conversely plate stretching with differing amounts of constant rate magmatic dyke intrusion can explain the great variety of fault offset seen at slow spreading ridges very large offset normal faults only form when about half the plate separation at a ridge is accommodated by dyke intrusion

deformation in the diffuse india capricorn somalia triple junction from a multibeam and magnetic survey of the northern central indian ridge 3 s 10 s 1 we use new multibeam and magnetic observations from 3 s 10 s along the central indian ridge to describe morphotectonic features and spreading fabrics within the study area and quantify deformation across the diffuse india capricorn somalia triple junction a megamullion adjacent to the vityaz transform fault is the first reported for the northern central indian ridge the principal transform displacement zone ptdz of the vema transform fault consists of several closely spaced faults that can be traced for more than 150 km serpentinite or volcanic intrusions that are offset by the vema pdtz have inferred ages of 30 000 45 000 years testifying to the youth of active features on the valley floor abyssal hill orientations throughout the study area have remained constant since at least 4 ma implying little or no change in the opening direction during this period abyssal hills from opposite sides of the ridge have the same orientations within our 1 3 errors implying that slip along reactivated fracture zones in the diffuse triple junction neither significantly rotates nor shears the intervening seafloor transform fault azimuths in the survey area agree with the nuvel 1 africa australia slip direction but are 5 9 clockwise from the africa india direction in contrast seafloor spreading rates in the study area agree better with the nuvel 1 africa india predictions these seemingly conflicting observations are reconciled by a model in which seafloor within the diffuse triple junction rotates about the india capricorn pole at angular rotation rates that increase monotonically from zero to the full rotation rate across the deforming zone

comprehensive analysis of marine magnetic vector anomalies we present a comprehensive description of the analysis of marine magnetic vector anomalies the marine magnetic vector field can be measured with a shipboard three component magnetometer stcm the stcm has the advantage of measuring three components but it suffers from several types of noise sources because the principles of its measurement are more complex than that of a proton precession magnetometer we have estimated the quality of stcm data constructed an optimal filter and evaluated the reliability of the filtered data then we have organized an analytical procedure composed of four steps 1 the discrimination between two and three dimensional magnetic sources 2 the determination of magnetic boundaries 3 the calculation of the boundary strikes and 4 the estimation of the boundary magnetization contrasts we show examples of these procedures using the data near the east pacific rise 28 s 31 s collected during leg 5 of the gloria expedition all the results of the above processing are merged in order to illustrate the distinct pattern of the seafloor magnetization in the surveyed area

asymmetric generation of oceanic crust at the ultra slow spreading southwest indian ridge 64 e we describe topographic gravity magnetic and sonar data from a southwest indian ridge spreading segment near 64 e 28 s we interpret these to reveal crustal structure spreading history and volcanic and tectonic processes over the last 12 myr we confirm that the crust is some 2 km thicker north of the ridge axis though it varies along and across axis on scales of 10 km and 4 myr the plate separation rate remained approximately constant at 13 1 km myr 1 but half spreading rates were up to 40 asymmetric varying between faster to the north and faster to the south on a 4 myr timescale topography shows a dominant e w lineation normal to the n s spreading direction this is superficially similar to faulted abyssal hill terrain of the mid atlantic ridge mar but inferred fault scarps are 3 4 times more widely spaced and have greater offsets conjugate pairs of massifs on either plate are interpreted as volcanic constructions similar to the large volcano currently filling the median valley at the segment center they have temporal spacings of 4 myr and are thought to reflect episodic melt focusing along an otherwise melt poor ridge additionally there are places mainly on the southern plate where lineated topography is replaced by a much blockier topography and embryonic ocean core complexes similar to those recently reported on the mar near 13 n there is generally more extrusive volcanism on the northern plate and more tectonism on the southern one extrusive volcanism has propagated westward from the segment center since 2 ma the fuji dome core complex and adjacent seafloor to its east and west appear to be part of a single coherent block capped by extrusive rock near the segment center exposing gabbro via a detachment fault over the dome and probably exposing deeper crust or upper mantle farther west near the segment end magnetic anomalies are continuous along this block we suggest that at its eastern boundary the detachment is simply welded onto magmatically emplaced crust to the east in a similar way to young crust being welded to the old plate at ridge transform intersections

crustal evolution of the mid atlantic ridge near the fifteen twenty fracture zone in the last 5 ma author posting american geophysical union 2003 this article is posted here by permission of american geophysical union for personal use not for redistribution the definitive version was published in geochemistry geophysics geosystems 4 2003 1024 doi 10 1029 2002gc000364

constraints on deformation conditions and the origin of oceanic detachments the mid atlantic ridge core complex at 15 45 n 1 deformed rocks sampled from a corrugated detachment fault surface near the mid atlantic ridge 15 45 n constrain the conditions of deformation and strain localization samples recovered in situ record deformation restricted to the cold shallow lithosphere greenschist facies with no evidence for significant high temperature deformation either at the fault zone or in the footwall near it high temperature deformation 720 750 c is observed only at two sites and cannot be directly linked to the detachment detachment faulting was coeval with dyke intrusions that cross cut it as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones and by the presence of chill margins in diabase against fault rock basalts are very scarce and restricted to clasts in breccias with no evidence of pillows or extrusive structures gabbros crop out along mass wasted and fault scarps structurally below the detachment footwall rocks show little or no deformation due to strain localization along a narrow shear zone 200 m with fluid flow as required to form talc and amphibole schists after an ultramafic protolith we speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold shallow lithosphere is applicable to other corrugated surfaces along slow and intermediate spreading ridges these observations preclude detachment models rooting in melt rich zones i e atlantis bank southwest indian ridge or recording high temperature deformation we infer that oceanic detachment faults 1 localize strain at t 500 300 c 2 persist during active magmatism and 3 root at shallow rheological boundaries such as a melt rich zone or magma chamber hot detachments or an alteration front cold detachments

detachment faulting at ocean core complexes intracrustal reflections from a cretaceous inside corner are interpreted as a detachment fault that dips gently within the basement and can be followed up the flank of a domed high the fault heave is sufficient for ridge normal extension to have exhumed deep crustal and probably mantle rocks as sampled at some inside corner domed massifs the morphology of these massifs closely resembles that of the cretaceous high implying that the structures are analogous and supporting the notion that modern inside corner massifs are oceanic core complexes formed by unroofing the footwalls of similar detachment faults partially exposed as corrugated slip surfaces this is the first demonstration of the overall geometry of an oceanic core complex detachment fault

seafloor geomagnetic vector anomaly of the intersection of the mid atlantic ridge and the kane transform fault implications for magnetization of the oceanic crust we investigated the fine scale magnetic structure of the oceanic crust and upper mantle around the western intersection of the mid atlantic ridge and the kane transform fault using the geomagnetic vector field measured at the seafloor the strikes of magnetic boundaries were found to reflect of the local tectonics represented by fault strikes and dips the ridge transform intersection is characterized by magnetic boundary strikes rotated eastward the strike directions reflect the direction of recent magmatic activity these indicate that a few traces of faults extending obliquely from the ridge axis to the transform fault are associated with neovolcanism magnetic boundary strikes in the lower part of the transform valley wall and near a linear trace of the transform fault indicate deformation or tilting of the crust associated with transform faulting it is also indicated that the crustal block around the median ridge is tilted by uplift accompanied by injection or emplacement and serpentinization of peridotites the nodal basin consists of extrusive basalts in the e w direction and n s trending magnetic strikes submersible observation of in situ magnetization of oceanic crust shows that extrusive basalts are typically strongly magnetized the most recent neovolcanic ridge has a mean magnetization of 50 a m the mean magnetization of the ridge axis is 20 a m the magnetization of the 3 ma ridge flank in the inside corner high is about 10 a m the gabbroic layer in the transform valley has a magnetization less than 2 a m the serpentinized peridotites in the median ridge have magnetization less than i a m this magnetization records a reversal in polarity which is important because the serpentinized peridotites contribute to the marine geomagnetic anomaly pattern

seismic reflection imaging of an oceanic detachment fault atlantis megamullion mid atlantic ridge 30 10 n abstract we present multichannel seismic reflection data collected over the atlantis megamullion at the eastern ridge transform intersection of atlantis fracture zone on the northern mid atlantic ridge and over its conjugate crust these data image for the first time the internal structure of a young well developed megamullion dome formed by tectonic extension across a long lived oceanic detachment fault the exposed corrugated detachment fault surface exhibits a sharp coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor at the termination of the megamullion the fault is imaged 13 km along strike beneath a volcanic hanging wall block at a sub seafloor depth of 0 2 0 5 s two way travel time reaching north as far as 30 19 n the eastward dipping of the fault beneath the hanging wall block is estimated to be 6 14 the corrugated fault surface is underlain by a continuous strong and relatively smooth reflection d at 0 2 0 25 s sub bottom below the central axis of the dome this reflection deepens up to 0 6 s sub bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope we suggest that atlantis massif formed by sequential slip on two different detachment faults that merged at depth with breakaways as little as 2 km apart the initial detachment is represented by reflection d and the second corresponds to the presently exposed fault surface in this interpretation much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity it lies in contact with less altered higher velocity mantle below the first detachment resulting in the strong smooth character of reflection d mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism in conjugate crust corresponding to termination of the megamullion observed sub bottom reflections are interpreted as base of seismic layer 2a this layer is as thick as or thicker 570 900 m than layer 2a in normal atlantic crust and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming

do layer 3 rocks make a significant contribution to marine magnetic anomalies in situ magnetization of gabbros at ocean drilling program hole 735b we estimate the average magnetization of a vertical section of oceanic gabbros using paleomagnetic and downhole magnetic logging techniques and evaluate the ability of these crustal rocks to contribute to marine magnetic anomalies results from ocean drilling program hole 735b show that the drilled crustal section has a mean effective remanent magnetization of 2 5 a m olivine gabbros which make up 60 of the section have an average effective magnetization between 1 and 2 a m we used two dimensional forward modeling and the average effective magnetization of olivine gabbros to predict the magnitude of a typical layer 3 anomaly observable at sea surface these results indicate that if polarity boundaries within oceanic layer 3 are near vertical crustal sections similar to 735b would contribute from 25 to 75 of the overlying marine magnetic anomalies at the sea surface the shape of the polarity boundaries in lower crust will be controlled by its thermal evolution and potentially could be constrained by systematic analyses of the amplitude and skewness of marine magnetic anomalies at individual spreading centers

relationship of the central indian ridge segmentation with the evolution of the rodrigues triple junction for the past 8 myr located near 25 33 s 70 00 e the rodrigues triple junction is the joining point of the intermediate spreading southeast indian and central indian ridges with the ultraslow spreading southwest indian ridge bathymetric data and magnetic anomalies are used to analyze the relationship between the evolution of the central indian ridge segmentation and the evolution of the rodrigues triple junction for the past 8 myr the central indian ridge domain exhibits a complex morphotectonic pattern dominated by ridge normal and oblique bathymetric lows interpreted as the off axis traces of axial discontinuities the short lived nontransform discontinuities as well as the segments that lengthen or shorten along the ridge axis reveal that the central indian ridge segmentation is unstable near the rodrigues triple junction the combined study of the central indian ridge and southeast indian ridge domains shows that the triple junction evolves between two modes a continuous mode where the central indian ridge and southeast indian ridge axes are joined and a discontinuous mode where the two ridge axes are offset owing to spreading asymmetry and differences in axis direction or in lengthening rates of the central indian and southeast indian ridges the continuous mode is unstable and evolves rapidly 2 myr into a discontinuous mode this last one is more stable and can evolve into a continuous mode only through the formation of a new central indian ridge segment which takes place facing the northern southeast indian ridge segment the evolution of the rodrigues triple junction configuration and the evolution of the central indian ridge segmentation are thus closely related

submersible study of an oceanic megamullion in the central north atlantic recently discovered megamullions on the seafloor have been interpreted to be the exhumed footwalls of long lived detachment faults operating near the ends of spreading segments in slow spreading crust we conducted five submersible dives on one of these features just east of the rift valley in the mid atlantic ridge at 26 35 n and obtained visual rock sample gravity and heat flow data along a transect from the breakaway zone where the fault is interpreted to have first nucleated in 2 0 2 2 ma crust westward to near the termination 0 7 ma our observations are consistent with the detachment fault hypothesis and show the following features in the breakaway zone faulted and steeply backtilted basaltic blocks suggest rotation above a deeper shear zone the youngest normal faults in this sequence are interpreted to have evolved into the long lived detachment fault in younger crust the interpreted detachment surface rises as monotonously flat seafloor in a pair of broad gently sloping domes that formed simultaneously along isochrons and are now thinly covered by sediment the detachment surface is locally littered with basaltic debris that may have been clipped from the hanging wall the domes coincide with a gravity high that continues along isochrons within the spreading segment modeling of on bottom gravity measurements and recovery of serpentinites imply that mantle rises steeply and is exposed within 7 km west of the breakaway but that rocks with intermediate densities prevail farther west within 5 km of the termination small volcanic cones appear on the detachment surface indicating melt input into the footwall we interpret the megamullion to have developed during a phase of limited magmatism in the spreading segment with mantle being exhumed by the detachment fault 0 5 m y after its initiation increasing magmatism may eventually have weakened the lithosphere and facilitated propagation of a rift that terminated slip on the detachment fault progressively between 1 3 m y and 0 7 m y identifiable but low amplitude magnetic anomalies over the megamullion indicate that it incorporates a magmatic component we infer that much of the footwall is composed of variably serpentinized peridotite intruded by plutons and dikes

widespread active detachment faulting and core complex formation near 13 n on the mid atlantic ridge oceanic core complexes are massifs in which lower crustal and upper mantle rocks are exposed at the sea floor1 2 3 they form at mid ocean ridges through slip on detachment faults rooted below the spreading axis2 3 4 5 6 to date most studies of core complexes have been based on isolated inactive massifs that have spread away from ridge axes here we present a survey of the mid atlantic ridge near 13 n containing a segment in which a number of linked detachment faults extend for 75 km along one flank of the spreading axis the detachment faults are apparently all currently active and at various stages of development a field of extinct core complexes extends away from the axis for at least 100 km our observations reveal the topographic characteristics of actively forming core complexes and their evolution from initiation within the axial valley floor to maturity and eventual inactivity within the surrounding region there is a strong correlation between detachment fault morphology at the ridge axis and high rates of hydroacoustically recorded earthquake seismicity preliminary examination of seismicity and seafloor morphology farther north along the mid atlantic ridge suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow spreading ridge than previously suspected

paleomagnetic evidence of large footwall rotations associated with low angle faults at the mid atlantic ridge exposures of gabbros and mantle derived peridotites at slow spreading oceanic ridges have been attributed to extension on long lived low angle detachment faults similar to those described in continental metamorphic core complexes in continental settings such detachments have been interpreted as having originated and remained active at shallow dips alternatively currently shallow dipping fault surfaces may have originated at moderate to steep dips and been fl attened by subsequent fl exure and isostatic uplift while the latter interpretation would be more con sistent with andersonian faulting theory it predicts large footwall tilts that have not been observed in continental detachment faults here we use the magnetization of oceanic gabbro and peridotite samples exposed near the fifteen twenty fracture zone on the mid atlantic ridge to demonstrate that substantial footwall rotations have occurred widespread rotations ranging from 50 to 80 indicate that original fault orientations dipped steeply toward the spreading axis

the 10th generation international geomagnetic reference field abstract the international association of geomagnetism and aeronomy iaga on 12 december 2004 released the 10th generation international geomagnetic reference field igrf the latest version of a standard mathematical description of the earth s main magnetic field and used widely in studies of the earth s deep interior its crust ionosphere and magnetosphere the coefficients were finalised by a task force of iaga the igrf is the product of a large collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and observatories around the world

direct geological evidence for oceanic detachment faulting the mid atlantic ridge 15 45 n from a detailed survey and sampling study of corrugated massifs north of the fifteen twenty fracture zone on the mid atlantic ridge we demonstrate that their surfaces are low angle detachment fault planes as proposed but not previously verified spreading direction parallel striations on the massifs occur at wavelengths from kilometers to centimeters oriented drill core samples from the striated surfaces are dominated by fault rocks with low angle shear planes and highly deformed greenschist facies assemblages that include talc chlorite tremolite and serpentine deformation was very localized and occurred in the brittle regime no evidence is seen for ductile deformation of the footwall synkinematic emplacement of diabase dikes into the fault zone from an immediately subjacent gabbro pluton implies that the detachment must have been active as a low angle fault surface at very shallow levels directly beneath the ridge axis strain localization occurred in response to the weakening of a range of hydrous secondary minerals at a very early stage and was highly efficient

evidence for temporally varying sticky spots at the base of trapridge glacier yukon territory canada during the 1992 summer field season we installed arrays of plough meters and water pressure transducers beneath trapridge glacier yukon territory canada to study hydromechanical coupling at the ice bed interface diurnal signals recorded with two of these ploughmeters appear to correlate with fluctuations in subglacial water pressure these diurnal variations can be explained by changes in basal resistance to sliding as mechanical conditions at the bed vary temporally in response to changes in the subglacial hydrological system we propose that a lubricating water film associated with high water pressures promotes glacier sliding whereas low pressures cause increased basal drag resulting in sticky areas using a theoretical model we analyze the sliding motion of glacier ice over a flat surface having variable basal drag and show that a consistent explanation can be developed results from our model calculations provide strong support for the existence of time varying sticky spots which are associated with fluctuations in subglacial water pressure

magnetic structure of an oceanic core complex at the southernmost central indian ridge analysis of shipboard and deep sea three component magnetometer data 1 we investigated the magnetic structure of an oceanic core complex occ at the southernmost central indian ridge cir near 25 s on the basis of newly collected magnetic and bathymetric data and submersible dives the occ is located off axis on the western flank of the cir and is characterized by typical megamullion morphology with flow line parallel corrugations the occ is similar in size to those found along the mid atlantic ridge mar but the cir occ formed in an estimated 0 8 ma which is slightly shorter than the average time for the formation of occs along the mar the magnetization intensity over the cir occ is quite weak compared to that over adjacent seafloor of the same age mostly gabbroic rocks were recovered from outcrops during submersible dives fault rocks were also recovered from the corrugated surface suggesting that the occ surface represents the plane of a detachment fault although the magnetization intensity is rather low we were able to identify the brunhes matuyama boundary and the jaramillo and olduvai subchrons on and around the occ using vector magnetic analysis over the occ the jaramillo subchron which cannot be recognized from magnetic data acquired at the sea surface was identified using data from a magnetometer attached to a submersible the locus of this subchron is consistent with that observed over the seafloor adjacent to the occ combining the results of magnetic survey and rock sampling we would like to point out the possibility that the gabbroic rocks constituted the occ and preserved seafloor spreading history although there are several interpretations of the low 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

magnetic properties of variably serpentinized abyssal peridotites we have compiled new and published data on the magnetic properties of 245 serpentinized abyssal peridotites from seven deep sea drilling project and ocean drilling program sites the magnetic susceptibility k of these samples does not increase linearly with the degree of serpentinization s instead k remains modest in partially serpentinized samples s 75 peridotites in the oceanic crust moderately serpentinized peridotites s 75 have nrm values 5 a m and k 0 05 si low temperature oxidation of magnetite is found to lower the nrm and k values of seafloor samples and of cataclastically deformed drilled samples