Marine Magnetic Anomalies Research Articles

Reinterpretation of Marine Magnetic Anomalies in the Bay of Bengal: Implications for Plate Tectonics During the Mid‐Cretaceous

AbstractThe breakup of India from Antarctica in the Early Cretaceous and subsequent seafloor spreading in the NW‐SE direction initiated the formation of the Bay of Bengal and its conjugate Enderby Basin. Two hypotheses are proposed to explain the evolutionary tectonics between India and Antarctica. One involves a single breakup, while the second invokes another breakup episode. Further, a major reorganization of the plate boundaries in the mid‐Cretaceous caused the spreading center to align E‐W by Late Cretaceous. The tectonics involved in this spreading direction change is unknown and difficult to understand as it occurred in the mid‐Cretaceous, coincident with the Cretaceous Normal Superchron. In the present study, we have generated a revised magnetic isochron map of the Bay of Bengal and used the modulus of its analytic signal to understand the evolutionary tectonics between the two plates, India and Antarctica. Mesozoic magnetic anomalies M12n to M0, and mid‐Cretaceous incursion Q2 (108 Ma) are identified in the western basin of the Bay of Bengal, while Q2, Q1 (92 Ma) and Late Cretaceous isochron A34 are inferred in its eastern basin. Gravity grid reconstructions demonstrate asymmetrical crustal accretion in the conjugate corridors. Fan‐shaped spreading and a southward ridge jump between 108 and 92 Ma in the eastern basin, aided the ∼40° clockwise rotation of the spreading center. The presence of volcanic basement beneath the additional crust suggests on‐axis mode of formation for the Ninetyeast Ridge by 100 Ma. The present study favors the two‐phase breakup model for the evolution of these plates.

Salient Changes of Earth's Magnetic Field Toward the End of Cretaceous Normal Superchron (CNS)

AbstractChanges in Earth's magnetic field during the Cretaceous Normal Superchron (CNS) spanning ∼121 Ma to ∼84 Ma hold important clues about the geodynamo evolution. Canonical models predict a persistently strong geomagnetic field with low variability during CNS, which, however, has not been observed in the available absolute paleointensity data and seafloor marine magnetic anomaly (MMA) records. The lack of relative paleointensity (RPI) data across CNS further impedes tests of model predictions. Here, we present a ∼9‐Myr (∼94–∼85 Ma) RPI record from a Turonian to Santonian hemipelagic succession from IODP Site U1512 offshore southern Australia. Detailed paleomagnetic and rock magnetic analyses demonstrate that the ratio of natural remanent magnetization (NRM) demagnetized at 20 mT over magnetic susceptibility (MS), that is, NRM20mT/MS, as a reliable proxy for the RPI of the Upper Cretaceous succession. The new RPI record shows marked changes in both intensity and variability at ∼90.8 Ma. Also, the 6 Myr‐long (∼94–∼88 Ma), near‐continuous, ∼1.2 kyr‐resolution RPI record exhibits a strong positive correlation between field intensity and variability. Assuming this correlation holds for the entire CNS, an extrapolated RPI curve for the entire CNS is obtained by integrating the positive correlation with field variability estimates from the MMA data. The extrapolated RPI curve shows a strong and highly variable field in the middle CNS but a weak and stable field at its beginning and ending. These features imply a much more dynamic geodynamo than previously thought, and provide crucial benchmarks for unraveling the geodynamo evolution during CNS.

Impact of Kerguelen, Marion, and Reunion plumes on the ophiolitic obduction at the northern margin of Indian plate

Geochronological data compiled for 29 major ophiolitic occurrences from the northern margin of the Indian plate precisely coincide with the three major mantle plume interactions over Indian plate (viz., Kerguelen at 130–110 Ma, Marion at 90–84 Ma, and Reunion at 65–70 Ma). We analysed i) the geological juxtapositions of these ophiolitic belts, ii) seismic tomographic imaging of the slab breakoffs, iii) equivalent deflections in the marine magnetic anomalies (MMA), iv) stratigraphic records of the Cretaceous formations and v) the paleomagnetic inclination anomalies; and propose significant influence of plume-driven forces over the subduction and obduction processes in Himalayas. The impingement of Kerguelen plume strongly influenced the ∼ 2500 km long subduction of Neo-Tethys, leading to Indus-Tsangpo-Suture-Zone (ITSZ) ophiolites with their metamorphic soles. The impact of Marion plume further led to repeated tilting of the Indian plate reinvigorating Tethyan subduction at 90 ± 5 Ma. The Reunion event at ∼ 65 Ma, led to the mega-obduction along the Tethyan margin of the Indian subcontinent. The Kerguelen plume thus played a key role in initiating the intraoceanic subductions in equatorial Neo-Tethys, and the combined slab pull and plume induced rifting separated Greater India microcontinent from the northern margin of Indian subcontinent at ∼ 120 Ma. The impact of the Reunion plume resulted in the obduction of ITSZ ophiolites at the northern margin and the Pakistan ophiolites at the northwestern margin. The mantle plume impingements caused plate tilting and triggered the subduction/obduction that is well preserved by sharp inflexions in MMA and the spreading rates. We therefore infer that the three mantle plumes have a greater role in the subduction, obduction and tilting of the Indian plate throughout the Late Mesozoic.

A critical reappraisal of paleomagnetic evidence for Philippine Sea Plate rotation

The kinematic history of the Philippine Sea Plate (PSP) is crucial for interpreting its geological record related to subduction initiation processes and the paleogeography of the junction between the Paleo-Pacific and Tethyan oceanic realms. However, reconstructing PSP's kinematic history is difficult because the plate has been surrounded by subduction zones for most of its history. In absence of marine magnetic anomalies to constrain PSP's motion relative to its neighboring plates, paleomagnetic data may be used as quantitative constraints on its motion. Previous paleomagnetic studies interpreted easterly deflected declinations to infer clockwise rotations of up to 90° since the Eocene. However, rotations inferred from these datasets may also reflect local block rotations related to plate margin deformation. We here re-evaluate to what extent paleomagnetic data from the PSP unequivocally demonstrate plate motion rather than local rotation. To this end, we provide new data from Guam, in the Mariana forearc, and reassess published paleomagnetic data. Our new data from Guam come from two localities in the Eocene, two in the Oligocene, and two in the Miocene. Our compilation assesses data quality against recently defined criteria. Our new results demonstrate that in Guam, declination differences of up to 35° exist in rocks of Eocene age, indicating local rotations. Our compilation identifies both clockwise and counterclockwise rotations from the plate margins, with little confidence which of these would reflect plate-wide rotation. We compiled paleolatitude data from igneous rocks, which we correct for microplate rotation constrained by intra-PSP marine magnetic anomalies and show a northward drift of the PSP of ∼15° since the Eocene, but without a paleomagnetic necessity for major vertical axis rotation. Hence, with the currently available data, rotations of the PSP may be permitted, but are not required. Plate motion is currently better reconstructed from geological constraints contained in circum-PSP orogenic belts.

Comparing the spatial domain and frequency domain algorithms for upward continuation of marine magnetic anomaly

Space potential field continuation is a method frequently used for the spatial transformation of magnetic anomaly in the sea area, and is the key technology for constructing three-dimensional marine magnetic field model and the application of marine magnetic survey data. In this study, we analyze the principle of upward continuation of potential field from plane to plane, demonstrate the spatial domain algorithm and frequency domain algorithm, and propose the standards for evaluating the accuracy of the two algorithms. Using the theoretical data of the multi-dipole sphere model, we employ the two algorithms to continue the magnetic anomaly of the observation plane at the height 0 m to 100 m, 200 m, and 300 m, for noisy and noise-free conditions. The results show that the continuation error of the two methods is extremely small at the plane center, but large on the edge; the spatial domain algorithm has obviously better accuracy than the frequency domain algorithm, so it should be preferentially selected in high-precision applications, such as magnetic anomaly detection in the sea area.

Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: Contribution of marine magnetics

The heterogenous magma supply at ultraslow spreading ridges creates diverse seafloor morphologies and lithospheric structures, which in turn generate a large variability in marine magnetic anomalies. The variability brings difficulties to interpret the evolution of oceanic lithosphere. On the other hand, different magnetic signatures of different seafloors provide an opportunity to identify the modes of seafloor spreading on the ultraslow spreading ridges. Here, we modeled several across-axis magnetic profiles selected from the Gakkel Ridge, Southwest Indian Ridge and Mid-Cayman Spreading Center to explore the lithospheric structure and seafloor spreading processes. Considering conjugate flanks, we observed three modes of seafloor spreading, Magmatic vs Magmatic, Magmatic vs Tectonic, and Tectonic vs Tectonic, on the three ultraslow spreading ridges. These three spreading modes reflect a strong, intermediate, and starved magma supply, respectively. Furthermore, four alternances of the different spreading modes were identified including the Magmatic vs Magmatic to Magmatic vs Tectonic, Magmatic vs Tectonic to Tectonic vs Magmatic, Tectonic vs Tectonic to Magmatic vs Magmatic, and Mixed. These alternances of spreading modes in across-axis direction suggest oscillations of the magma supply at different levels. Variation in the modes of seafloor spreading on four nearby profiles over the Mid-Cayman Spreading Center reveals the evolution of magma supply along the axis of this short ridge.

Influence of the oceanic crust structure on marine magnetic anomalies: Review and forward modelling

Marine magnetic anomaly strips are important for plate tectonic studies, thereby promoting the development of the geoscience revolution. Most previous studies typically consider a lava layer with vertical polarity boundaries as a simplified magnetic source layer. However, it cannot accurately explain the observed magnetic anomaly data; thus, complex oceanic crusts with multiple magnetic source layers, such as lava, dike, and gabbro layers, and various polarity boundary dip angles should be considered. To date, there is a lack of systematic understanding about how the dike and gabbro layers affect the total marine magnetic anomalies, particularly the effect of lava layers with different boundary inclinations on the intensity and polarity boundary of the magnetic anomaly. This study aimed to confirm the significance of these factors. Hence, we systematically simulated and analysed the magnetic anomaly characteristics with different inclination and magnetization settings for each layer using the forward modelling method and determined the impact of the complexity of the magnetic structure of the oceanic crust on the magnetic anomaly. Our results showed that the contributions of the dike and gabbro layers to the magnetic anomalies was up to 10%–30% and cannot be ignored. Moreover, the polarity transition point may shift with the inclination of the lava, dike, and gabbro layers but is insensitive to the magnetization of each layer. This study emphasizes the significance of the dike and gabbro layers and the inclination variations of different layers, which should be thoroughly studied. This study facilitates a reasonable interpretation of the magnetic structure of the oceanic crust and further refines the boundary of the magnetization zone with alternative polarity.

A small Greater India restored by Himalayan crustal mass balance calculations

Greater India, a postulated northern extension of the modern Indian continent that has been subducted beneath Tibetan Plateau, is needed to constrain the process of the greatest collision event between India and Asia. However, how far Greater India extends northward remains controversial, with proposed estimates from 100 to 2400 km. Here, we calculate Greater India’s size by estimating the crustal mass balance of the Himalaya, as Himalaya was created by the peeling-off of Greater India’s crust during the convergence. Our results show that Greater India only extends northward ∼ 620 km, further indicating that the Greater India continental subduction started at ∼ 24 Ma with the velocity of India from marine magnetic anomalies. This timing corresponds to the Himalayan uplift age. This result leaves open the possibility of both one-stage and two-stage India-Asia collision assuming that the massive Indian continental subduction started only long after the initial collision.

MARINE GEOMAGNETIC ANOMALY BELT AND ITS RELATIONSHIP TO THE REMNANT ARCS IN THE NORTHWESTERN JAVA SEA, INDONESIA

The continuous marine geomagneticsurvey within a timeintervalof1-secondsampling and a precision of 0.1 nT was conducted in the northwestern Java Sea to identify and interpret the general trend of total marine magnetic anomalies and the possibility related to thegeological resourcepotential. These magnetic data were then processed according to the formula corrected and applied to marine magnetic data. The total marine magnetic anomalies of the northwestern Java Sea indicate a well-defined lateral trend belt of anomaly contours. Anomalies are divided into four delineation zones: Zones I, II, III, and IV. A preliminary analysis of these anomalies led to the interpretation, reflecting the residual of a slightly east-west trending geological body underneath.Examination of magnetic anomalies suggests Zone I and IV characterize a basinalarea, Zone II depicts a granitic belt, and Zone III describes a Cretaceousmagmatic arc system in the east that extends from Middle Java across the Java SeathroughSouthern Kalimantan. These magnetic anomalies seem to coincide with the free air gravity anomalies data derived from TOPEX satellite data.

An automatic identification method of marine magnetic anomalies based on the sliding window correlation coefficient method

The identification of marine magnetic anomalies has fundamental importance to the study of plate tectonics and geodynamics. The traditional approach to identifying marine magnetic anomalies is by visual method and much depends on the experience of experts. Though the identification results are generally reasonable, they lack quantitative evaluation basis. Therefore, we proposed the sliding window correlation coefficient (SWCC) method to automatically identify marine magnetic anomalies and provide a quantitative and objective evaluation for the identification results. The Pearson correlation coefficient (PCC), Spearman rank correlation coefficient (SRCC) and Kendall rank correlation coefficient (KRCC) are compared in the SWCC method. The different skewness, spreading rates and random noises to the identification results are tested. The results show that the SWCC method is most optimal for identifying fast-spreading magnetic anomalies. The absolute values of the correlation coefficients at the same sliding steps are usually PCC > SRCC>KRCC, but the resolving abilities for different polarity chrons are usually KRCC>SRCC>PCC. Applications in the southwest Pacific verified the feasibility and effectiveness of the SWCC method, and suggest that short theoretical windows usually have limited feature information; therefore, combined neighbouring polarity chrons can improve the identification results of the SWCC method.

Compiling ship and airborne measurements for the Antarctic's second-generation magnetic anomaly map

In 2001, the Antarctic Digital Magnetic Anomaly Project produced the ADMAP-1 compilation that included the first magnetic anomaly map of the region south of 60◦S. To help fill ADMAP-1’s regional coverage gaps, the international geomagnetic community from 2001 through 2014 acquired an additional 2.0+ million line-km of airborne and marine magnetic anomaly data. These new data together with surveys that were not previously in the public domain significantly upgraded the ADMAP compilation for Antarctic crustal studies. The merger of the additional data with ADMAP-1’s roughly 1.5 million line-km of survey data produced the second-generation ADMAP-2 compilation. The present study comprehensively reviews the problems and progress in merging the airborne and ship magnetic measurements obtained in the harsh Antarctic environment since the first International Geophysical Year (IGY 1957–58) by international campaigns with disparate survey parameters. For ADMAP-2, the newly acquired data were corrected for the diurnal and International Geomagnetic Reference Field effects, edited for high-frequency errors, and levelled to minimize line-correlated noise. ADMAP-2 provides important new constraints on the enigmatic geology of the Gamburtsev Subglacial Mountains, Prince Charles Mountains, Dronning Maud Land, and other poorly explored Antarctic areas. It links widely separated outcrops to help unify disparate geologic and geophysical studies for new insights on the global tectonic processes and crustal properties of the Antarctic. It also supports studies of the Antarctic ice sheet’s geological controls, the crustal transitions between Antarctica and adjacent oceans, and the geodynamic evolution of the Antarctic crust in the assembly and break-up of the Gondwana and Rodinia supercontinents.

Analysis of Pacific Hotspot Chains

AbstractSeamount trails created by mantle plumes are often used to establish absolute reference frames for plate motion. When plume drift is considered, changes in seamount trail direction and age progression cannot be attributed to plate motion change alone. Here, improvements to age‐progressive models of eleven Pacific hotspot chains are made independently of plate motion models. Our approach involves bathymetry processing to robustly predict a smooth, continuous hotspot path by connecting maxima in filtered seamount bathymetry, with across‐track uncertainties from seamount trail width and amplitude. Published ages from seamount samples are projected orthogonally onto these paths. We determine best‐fit models of age as functions of along‐track distance, giving continuous age‐progression models for each seamount chain with uncertainties in age and geometry. Different sources of paleolatitudes are examined by incorporating data from the magnetization of seamount drill‐core samples, paleo‐poles from marine magnetic anomaly skewness inversions, and paleo‐spin‐axes inferred from shifts of equatorial sediments. Improved paleolatitude models for the Hawaiian‐Emperor and Louisville chains are determined by combining these different types of data. Paleolatitude models are predicted for other chains during periods when sufficient amounts of paleo‐pole or paleo‐spin‐axis data are available. Analysis of the eleven Pacific seamount chains provide constraints for future plate and plume motion models. We analyze the temporal change in distance between coeval seamounts, reflecting relative drifts between the hotspots at different times in the past. The observed changes imply systematic relative hotspot drifts compatible with paleolatitude trends.

Extraction of Marine Magnetic Anomalies Under Magmatic Disturbances Based on an Anisotropic Elliptical Directional Filter

Extraction of Marine Magnetic Anomalies Under Magmatic Disturbances Based on an Anisotropic Elliptical Directional Filter

New Magnetic Anomaly Constraints on the Antarctic Crust

AbstractThe Antarctic Digital Magnetic Anomaly Project’s first‐generation magnetic anomaly map (ADMAP‐1) was produced for the region south of 60°S from about 1.5 million line‐kms of airborne and marine magnetic anomaly data (Golynsky et al., 2001; https://www.bas.ac.uk/data/our‐data/maps/thematic‐maps/admap‐magnetic‐anomaly‐map‐of‐the‐antarctic/). The second‐generation ADMAP‐2 compilation (Golynsky et al., 2017; https://doi.org/10.22663/ADMAP.V2) incorporated an additional roughly 2.0 million line‐kms of airborne and marine magnetic anomaly data from international mapping through 2015. The present study integrates satellite magnetic observations from the Swarm mission with the near‐surface data of ADMAP‐2 to help fill the regional coverage gaps and better define the altitude behavior of the Antarctic's magnetic anomalies for enhanced geological analysis. The resulting satellite magnetic data‐supplemented compilation, ADMAP‐2s, yields further constraints on the enigmatic geology of the Gamburtsev Subglacial Mountains, Prince Charles Mountains, Wilkes Land, Dronning Maud Land, and other poorly explored Antarctic areas. It offers insights on the global tectonic processes and crustal properties of the Antarctic and helps to unify disparate geologic and geophysical studies by linking widely separated outcrops. It also supports studies on the geological controls of the Antarctic ice sheet, the crustal transitions between Antarctica and the adjacent oceans, and the geodynamic evolution of the Gondwana and Rodinia supercontinents.

The Cretaceous Normal Superchron: A Mini-Review of Its Discovery, Short Reversal Events, Paleointensity, Paleosecular Variations, Paleoenvironment, Volcanism, and Mechanism

The Cretaceous Normal Superchron (CNS) was first defined in the 1960s to explain the Cretaceous Quiet Zone in marine magnetic anomaly profiles, which includes no or fewer geomagnetic reversals. This ∼37 million years period is considered the most unique and extreme geomagnetic feature for the last 160 Myr. Superchrons may be caused by the geodynamo operating at peak efficiency with a unique heat flux at the core-mantle boundary (CMB). Previous studies suggest that the CNS is a sign of the connection between Earth’s interior and surface. During the CNS, the geomagnetic intensity may have fluctuated significantly, and the average may have changed with time, and the paleosecular variations had unique features. The warm climate around the CNS may have been caused by volcanic activity associated with active mantle convection. Such mantle convection increases heat flux at the CMB during the CNS, but geodynamo simulations predict small heat flux, which are inconsistent. This discrepancy may be resolved by the growth and collapse of a superplume or by an increase and decrease in the subduction flux.

The process of oceanic peridotite serpentinization: From seafloor hydration to subduction dehydration

蛇纹石化是海底最重要的水岩相互作用之一，指基性岩和超基性岩中的橄榄石和辉石等镁铁质矿物在相对低温条件下发生水热蚀变产生蛇纹石等矿物的热液变质作用。蛇纹石族矿物主要有三种，分别是利蛇纹石、纤蛇纹石和叶蛇纹石。低温状态蛇纹石族矿物主要以利蛇纹石和纤蛇纹石的形式存在，高温状态下主要以叶蛇纹石的形式存在。影响大洋蛇纹石化过程的因素不容忽视，温度、氧化还原程度、pH值、水岩比（W/R）等都在其中扮演着重要的角色。总的来说，地幔物质易出露在地壳减薄区域和断裂构造处，这有利于与流体充分接触反应，从而决定了大洋蛇纹石化作用发生的可能位置。对蛇纹石化程度的描述，当前人们大多通过岩石微观结构、地球化学指标来定性指示，磁学指标有望实现对蛇纹石化程度的定量解释。蛇纹石化作用对海底磁异常、地球生命演化进程、成矿作用等都有一定的贡献。此外，俯冲带脱水及弧岩浆的形成都与之有联系。总之，基性与超基性岩石蛇纹石化与俯冲带蛇纹岩脱水过程是地球水循环过程的重要机制，但未来揭示蛇纹岩的磁学性质和俯冲变质过程，仍需进一步探索。;As one of the most important water-rock interactions on earth, serpentinization refers to the hydrothermal alteration of mafic and ultramafic rocks under medium-low temperature conditions. Serpentinization is mainly the water-rock metasomatism of olivine, orthopyroxene and amphibole and other magnesia end-members. There are three kinds of serpentine group minerals, including 1izardite, chrysotile and antigorite. They reside at different temperatures, e.g., 1izardite and chrysotile at low temperature, while antigorite at high temperature. Serpentinization process is controlled by temperature, the degree of redox, pH value, water-rock ratio (W/R), etc. In general, serpentinization is prone to occur at the areas with the thinning of the crust or the existence of faulted structures where mantle materials are easy to expose to contact and react with fluids. The serpentinization degree is generally quantified by the rock microstructure and geochemical methods, however, magnetic parameter is a prospective proxy for this index. Serpentinization makes a certain contribution to the marine magnetic anomalies, the biological evolution process on the earth, and the mineralization, in particular to the dehydration at the subduction zone and the formation of arc magma. However, much more investigations on the detailed mechanism are needed in the future work.

Geomagnetic Field Paleointensity Spanning the Past 11 Myr From Marine Magnetic Anomalies in the Southern Hemisphere

AbstractGlobal variations in geomagnetic intensity can provide dating markers for sedimentary records, which is particularly helpful in precisely reconstructing climate changes. However, there are few continuous relative paleointensity (RPI) records derived from sediments spanning the past 11 Myr, and those that exist generally bear a high degree of uncertainty. Therefore, independent evidence from other continuous paleointensity records is needed to verify the existing information. Here, we analyzed magnetic anomaly profiles from the East Pacific Rise and the Southeast Indian Ridge and discovered correlative tiny wiggles in the stacked profiles from three different areas. A comparison with a previous magnetic study on the South Atlantic Ridge and the synthetic profile from RPI records generated for the past ∼8 Myr confirmed that the small wavelength anomalies result from global geomagnetic intensity fluctuations. The newly calibrated timescales from selected marine magnetic anomalies could provide a potential dating reference for sedimentary strata.

Reconstructing lost plates of the Panthalassa Ocean through paleomagnetic data from circum-Pacific accretionary orogens

The Panthalassa Ocean, which surrounded the late Paleozoic-early Mesozoic Pangea supercontinent, was underlain by multiple tectonic plates that have since been lost to subduction. In this study, we develop an approach to reconstruct plate motions of this subducted lithosphere utilizing paleomagnetic data from accreted Ocean Plate Stratigraphy (OPS). We first establish the boundaries of the Panthalassa domain by using available Indo-Atlantic plate reconstructions and restorations of complex plate boundary deformation at circum-Panthalassa trenches. We reconstruct the Pacific Plate and its conjugates, the Farallon, Phoenix, and Izanagi plates, back to 190 Ma using marine magnetic anomaly records of the modern Pacific. Then, we present new and review published paleomagnetic data from OPS exposed in the accretionary complexes of Cedros Island (Mexico), the Santa Elena Peninsula (Costa Rica), the North Island of New Zealand, and Japan. These data provide paleolatitudinal plate motion components of the Farallon, Phoenix and Izanagi plates, and constrain the trajectories of these plates from their spreading ridges towards the trenches in which they subducted. For 83 to 150 Ma, we use two independent mantle frames to connect the Panthalassa plate system to the Indo-Atlantic plate system and test the feasibility of this approach with the paleomagnetic data. For times prior to 150 Ma, and as far back as Permian time, we reconstruct relative and absolute Panthalassa plate motions such that divergence is maintained between the Izanagi, Farallon and Phoenix plates, convergence is maintained with Pangean continental margins in Japan, Mexico and New Zealand, and paleomagnetic constraints are met. The reconstruction approach developed here enables data-based reconstruction of oceanic plates and plate boundaries in the absence of marine magnetic anomaly data or mantle reference frames, using Ocean Plate Stratigraphy, paleo-magnetism, and constraints on the nature of circum-oceanic plate boundaries. Such an approach is a crucial next step towards quantitative reconstruction of the currently largely unknown tectonic evolution of the Earth9s oceanic domains in deep geological time.

Magnetic Mineral Populations in Lower Oceanic Crustal Gabbros (Atlantis Bank, SW Indian Ridge): Implications for Marine Magnetic Anomalies

AbstractTo learn more about magnetic properties of the lower ocean crust and its contributions to marine magnetic anomalies, gabbro samples were collected from International Ocean Discovery Program Hole U1473A at Atlantis Bank on the Southwest Indian Ridge. Detailed magnetic property work links certain magnetic behaviors and domain states to specific magnetic mineral populations. Measurements on whole rocks and mineral separates included magnetic hysteresis, first‐order reversal curves, low‐temperature remanence measurements, thermomagnetic analysis, and magnetic force microscopy. Characteristics of the thermomagnetic data indicate that the upper ~500 m of the hole has undergone hydrothermal alteration. The thermomagnetic and natural remanent magnetization data are consistent with earlier observations from Hole 735B that show remanence arises from low‐Ti magnetite and that natural remanent magnetizations are up to 25 A m−1 in evolved Fe‐Ti oxide gabbros, but are mostly <1 A m−1. Magnetite is present in at least three forms. Primary magnetite is associated with coarse‐grained oxides that are more frequent in the upper part of the hole. This magnetic population is linked to dominantly “pseudo‐single‐domain” behavior that arises from fine‐scale lamellar intergrowths within the large oxides. Deeper in the hole the magnetic signal is more commonly dominated by an interacting single‐domain assemblage most likely found along crystal discontinuities in olivine and/or pyroxene. A third contribution is from noninteracting single‐domain inclusions within plagioclase. Because the concentration of the highly magnetic, oxide‐rich gabbros is greatest toward the surface, the signal from coarse oxides will likely dominate the near‐bottom magnetic anomaly signal at Atlantis Bank.

Northern segment of the North Anatolian Fault in the Gulf of Izmit inferred from marine magnetic anomalies

The Gulf of Izmit is a critical basin in assessment of the kinematics of the North Anatolian Fault (NAF) at the eastern margin of the Sea of Marmara. The available seismic reflection data do not provide sufficient and clear evidences to outline the deeper structures because of limited penetration. In agreement with the known main tectonic elements from available seismic data and multi-beam bathymetry, new magnetic data gathered within the scope of this study show E–W trending anomalies and permit us to outline satisfactorily the deeper tectonic setting of the gulf region. The northern strand of the NAF can be evidently correlated with the maxima on the magnetic profiles, implying a significant thinning in the crust along the weakness zone placed between the Istanbul Zone and Karakaya Complex. Such a tectonic structure must have been formed as parallel transform strike-slip pressure, which is different from the characteristic negative-flower structure observed at the East Marmara trough. This pattern occurs in a relatively narrow zone, because it has been superimposed on a paleo-tectonic lineament.