Three-dimensional SeaMARC II,gravity, and magnetics study of large-offset rift propagation at the Pito Rift,Easter microplate

We present results from a SeaMARC II bathymetry, gravity, and magnetics survey of the northern end of the large-offset propagating East Rift of the Easter microplate. The East Rift is offset by more than 300 km from the East Pacific Rise and its northern end has rifted into approximately 3 Ma lithosphere of the Nazca Plate forming a broad (70–100 km) zone of high (up to 4 km) relief referred to as the Pito Rift. This region appears to have undergone distributed and asymmetric extension that has been primarily accommodated tectonically, by block faulting and tilting, and to a lesser degree by seafloor spreading on a more recently developed magmatic accretionary axis. The larger fault blocks have dimensions of 10–15 km and have up to several km of throw between adjacent blocks suggesting that isostatic adjustments occur on the scale of the individual blocks. Three-dimensional terrain corrected Bouguer anomalies, a three-dimensional magnetic inversion, and SeaMARC II backscatter data locate the recently developed magmatic axis in an asymmetric position in the western part of the rift. The zone of magmatic accretion is characterized by an axis of negative Bouguer gravity anomalies, a band of positive magnetizations, and a high amplitude magnetization zone locating its tip approximately 10 km south of the Pito Deep, the deepest point in the rift area. Positive Bouguer gravity anomalies and negative magnetizations characterize the faulted area to the east of the spreading axis supporting the interpretation that this area consists primarily of pre-existing Nazca plate that has been block faulted and stretched, and that no substantial new accretion has occurred there. The wide zone of deformation in the Pito Rift area and the changing trend of the fault blocks from nearly N-S in the east to NW-SE in the west may be a result of the rapidly changing kinematics of the Easter microplate and/or may result from ridge-transform like shear stresses developed at the termination of the East Rift against the Nazca plate. The broad zone of deformation developed at the Pito Rift and its apparent continuation some distance south along the East Rift has important implications for microplate mechanics and kinematic reconstructions since it suggests that initial microplate boundaries may consist in part of broad zones of deformation characterized by the formation of lithospheric scale fault blocks, and that what appear to be pseudofaults may actually be the outer boundaries of tectonized zones enclosing significant amounts of stretched pre-existing lithosphere.     

Devising methods for geomagnetic studies in the World Ocean

The importance of geomagnetic studies in the World Ocean for deep structure research and fore-casting of mineral resources is noted. A combined method for development of a marine nuclear magnetometer is adduced. The physical background of operation of nuclear magnetometers is analyzed in order to optimize the measurements of the magnetic field. The results of the experiments on detecting nuclear precession signals against the background of ship noises are considered and the elaboration of an MM-1 nuclear magnetometer at the Shirshov Institute of Oceanology is described. A technique for magnetic survey and comprehensive geological interpretation of the anomalies and Z and H field components are presented. Examples of geomagnetic studies performed in the World Ocean with the MM-Inuclear magnetometer are assessed: for the first time in Russia, linear magnetic anomalies were identified and sea-floor spreading rates were calculated (the northern part of the Indian Ocean); a regional geomagnetic survey was carried out in the region of Iceland, which proved the spreading origin of the seafloor in this vast region. A systematic analysis of geomagnetic data obtained with the MM-1 magnetometer in the World Ocean provided the creation and development of the methodology for their tectonic and geodynamical interpretation. On the basis of the geomagnetic data obtained, new fundamental conclusions about the deep structure, kinematics and paleogeodynanics of the World Ocean floor were made.     

南桑威奇俯冲带大地水准面异常及其地幔对流特征分析

以南桑威奇俯冲带为例,根据EGM2008超高阶地球重力场模型、卫星重力数据为基础,利用移去-恢复原理计算了研究区大地水准面,实现了研究区不同场源深度大地水准面异常信息的分离,根据Runcorn模型计算了研究区小尺度地幔对流应力场,并结合天然地震空间展布和前人研究成果,对俯冲带结构特征与地幔对流模式进行了探讨。结果表明:南桑威奇俯冲带具有俯冲倾角较大、地震震级较低、弧前侵蚀明显等典型的马里亚纳型俯冲带特征,俯冲带南北部俯冲深度存在明显差异,中段偏北俯冲深度可达500 km;受到软流圈与上地幔上部物质密度差异的控制,东斯科舍海脊下存在沿海脊轴向南流动强地幔流;俯冲带结构与小尺度地幔对流应力场具有很强的相关性。本研究对于搞清南桑威奇俯冲带深部构造特征,理解俯冲运动、地幔对流方向及其动力控制机制提供了新的研究思路和方法。     

A topographic surface reduction of aeromagnetic anomaly field over the Tyrrhenian sea area (Italy)

This paper shows the results of a detailed reprocessing of aeromagnetic data, obtained by the downward projection to the seabed. The area of interest is centered over the Tyrrhenian Basin, whose bathymetric–topographic lay-out is characterized by a somewhat irregular trend. The origin of the intense depth variations depends on the Tyrrhenian structural setting, that is associated with the presence of several tectonic lineaments, seamounts or volcanic islands. The data were characterized by good quality and dense sampling, but they have been reprocessed in order either to solve some problems in the original compilation, and to reduce the distortion of the geomagnetic anomaly field caused by the difference of distance between the survey level and the magnetic source. The reprocessed magnetic map is proposed as an effective analysis tool for the Tyrrhenian area that is characterized by high susceptibility lithotypes. Downward projection of the aeromagnetic data by BTM algorithm increases the definition of the anomalous magnetic signal without distortions in the geometric pattern of the field, thus showing a more stable and effective association between the magnetic anomalies and their geological sources. This effect is particularly true for high frequency anomalies that are directly comparable after the topographic projection because the depth filtering effect is attenuated. Moreover, the BTM method has been applied for the first time to a regional scale survey that shows substantial advantages because no fictitious anomalies in the high frequency sector of the spectrum were generated. This has been a typical effect of the traditional downward projection methods widely used before. The final result is a BTM anomaly map that is able to show the structural connections between the geological magnetic sources of the Tyrrhenian Sea area.     

Characteristics of gravity anomalies and tectonic analysis of Enderby Land in East Antarctica and its adjacent areas

Enderby Land in East Antarctica and its adjacent areas, which are closely related to the Indian Plate in their geological evolution, have become one of the key zones for studies on how the Antarctic continent evolves. Based on the isostasy and flexure theories of the lithosphere and using the CRUST1.0 model as the depth constraint, this paper uses the gravity field model EIGEN-6C4 and topographic data to calculate the isostatic gravity anomalies of Enderby Land and its adjacent areas. Then, the ...     

First manned submersible dives on the East Pacific Rise at 21°N (project RITA): General results

A submersible study has been conducted in February–March 1978 at the axis of the East Pacific Rise near 21°N. The expedition CYAMEX, the first submersible program to be conducted on the East Pacific Rise, is part of the French-American-Mexican project RITA (Rivera-Tamayo), a 3-year study devoted to detailed geological and geophysical investigations of the East Pacific Rise Crest. On the basis of the 15 dives made by CYANA in the axial area of the Rise, a morphological and tectonic zonation can be established for this moderately-fast spreading center. A narrow, 0.6 to 1.2 km wide zone of extrusion (zone 1), dominated by young lava flows, is flanked by a highly fissured and faulted zone of extension (zone 2) with a width of 1 to 2 km. Further out, zone 3 is dominated by outward tilted blocks bounded by inward-facing fault scarps. Active or recent faults extend up to 12 km from the axis of extrusion of the East Pacific Rise. This represents the first determination from direct field evidence of the width of active tectonism associated with an accreting plate boundary. Massive sulfide deposits, made principally of zinc, copper and iron, were found close to the axis of the Rise. Other signs of the intense hydrothermal activity included the discovery of benthic fauna of gian size similar to that found at the axis of the Galapagos Rift. We emphasize the cyclic character of the volcanicity. The main characteristics of the geology of this segment of the East Pacific Rise can be explained by the thermal structure at depth below this moderately-fast spreading center. The geological observations are compatible with the existence of a shallow magma reservoir centered at the axis of the Rise with a half-width of the order of 10 km.     

南海热流分布特征

南海地热异常明显与主要构造断裂带和水热/岩浆活动有关。东部平行于马尼拉海沟的一条SN向低热流异常带起因于南海洋壳对吕宋岛的俯冲。南沙海槽及其南部陆缘的地温场比较复杂。南部的曾母盆地是一个显著的高地热异常区,它起因于年轻的构造拉张,其地幔热流高达中央海盆洋壳的地幔热流值。西南次海盆也是一个高地热异常区,虽然该次海盆形成较早,但与年轻的构造拉张有关。热流资料的分析结果表明,南海中央海盆西缘断裂带、西南次海盆和曾母盆地构成的NE向高热流异常带可能是一个大型的现代构造拉张带。     

Magnetic anomalies over the Central Levant continental margin

The magnetic field over the central Levant continental margin, off northern Israel and southern Lebanon, and the adjacent Levant Basin has two distinct trends. Mount Carmel and its offshore continuation (Carmel Nose), which are the surface expression of a large subbottom structure that extends from the land area across the continental shelf to the continental slope, form a dividing zone between the two magnetic trends. South of the Carmel structure the magnetic field trends east-west, while north of the Carmel structure it trends northeast and north-northeast.Several pronounced magnetic anomalies exist mainly north of the Carmel structure, the majority of which trend north-northeast and northeast, parallel and sub-parallel to the trend of the magnetic field in this area. Some also trend northwest, perpendicular to the trend of the magnetic field. In several cases the magnetic anomalies indicate large lithological elements which continue from land to sea.Gravity and seismic refraction data show that the two magnetic domains north and south of the Carmel structure are associated with areas of different crustal structure. South of the Carmel structure the continetal-oceanic crustal transition zone is located beyond the continental margin at the base of the continental slope, while north of the Carmel structure it is located under the continental shelf, near the shore. On land, there are also differences in the structure of the crust north and south of the Carmel structure, the crust being much thinner north of the structure than south of it.We suggest that some of the large magnetic anomalies off the Central Levant were formed during the rifting phase of the eastern Mediterranean.     

Deep structure of the crust beneath the Sea of Okhotsk inferred from 3D seismic density modeling

Potential field anomalies of the Sea of Okhotsk region are analyzed for compiling a map of the basement’s tectonic structures. A 3D density model of the Earth’s crust is constructed using seismogeological and experimental-petrophysical data, which made it possible to obtain a visual image of main structures of the region reflecting the observable geophysical anomalies. The obtained data allow a domain located in the central part of the Sea of Okhotsk beyond the limits of the exclusive economic zone of the Russian Federation to be considered as a natural continuation of the continental shelf since the latter is structurally similar to western Kamchatka. The deep structural boundaries rise beneath the large sedimentary Deryugin and Tinro basins, which is characteristic of petroliferous basins.     

Large-scale oblique features in an active transform fault,the Wilkes fracture zone near 9°S on the East Pacific Rise

The Wilkes fracture zone offsets the East Pacific Rise about 200 km right-laterally near 9°S. The bathymetric expression of the fracture zone ranges from a simple slope or step along its inactive extension to a 100 km wide zone of oblique structural features in the active portion. A low ridge 200 to 300 m high, 5 to 15 km wide and 185 km long is the dominant oblique structure; it trends 23° north of the main transform trend. A high-amplitude magnetic anomaly trends 097° along the southern part of the active portion and apparently marks the main transform direction. The structurally simple, inactive portions of the Wilkes fracture zone trend 105°. Plots of epicenter locations reveal two groupings of earthquakes, one along an 082° trend in the central part of the fracture zone, and a cluster near the southwestern fracture zone — spreading center intersection.Taken together the data suggest that some event, other than a shift in the Nazca-Pacific pole of rotation, occurred 0.9 m.y. ago to change the Wilkes fracture zone from a simple fault to a complex zone of shearing. Since that time the long oblique ridge, probably the surface expression of a Riedel shear, was formed. At present the entire 200 km long, 100 km wide region between the offset axes is seismically active, but transform motion may be largely confined to the southern margin of the active zone, coincident with the high-amplitude magnetic anomaly there.     

Extension of the Narmada — Son lineament on the continental margin off Saurashtra,Western India as obtained from magnetic measurements

Magnetic total intensity values and bathymetric data collected on the continental margin off Saurashtra were, used to prepare magnetic anomalies and bathymetric contour maps. The magnetic anomalies are considered to have been caused by the Deccan Trap flood basalts which underlie the Tertiary sediments. Interpretation of the magnetic data using two-dimensional modelling method suggests that the magnetic basement is block faulted and deepens in steps from less than 1.0 km in the north to about 8.0 km towards the southern portion of the study area. The WNW-ESE trending faults identified in the present study extend across the Saurashtra continental margin between Porbandar and Veraval and appear to represent a major linear tectonic feature. The relationship of these fault lineaments with the regional tectonic framework have been discussed to indicate that they conform better as the northern boundary faults of the Narmada rift graben on the continental margin off Saurashtra.     

An investigation of tsunami source mechanism off the coast of central Peru

The source mechanism of the tsunami generated by the earthquake of 17 October, 1966 off the coast of central Peru was inferred by studying the seismic and oceanic phenomena associated with this event. The seismic mechanism was deduced from geologic structure, seismic intensities, energy releases, spatial distribution of aftershocks, and fault-plane solutions. Using this information and empirical relationships of seismic parameters, the fault length, azimuthal orientation of the tsunamigenic area, and initial tsunami height, were obtained. From the tsunami arrival times at selected stations and from a reverse wave-refraction technique, the limits of the tsunami-generating area were estimated. Using these source dimensions, an estimate of the tsunami energy was obtained. The spatial distribution of aftershocks associated with the main earthquake and the earthquake strain-release pattern correlated well with known seismotectonic trends and the seismic-velocity structure anomalies which are characteristic of thrust fault systems at continent-ocean boundaries. The investigation revealed that the tsunamigenic area was on the continental shelf off Peru, northwest of Lima, in the western part of an active seismic belt between the Andean Mountain block and the Peru-Chile trench. This area is considered to be one of three distinct seismic zones in the Peruvian upper mantle and has been responsible for a number of tsunamigenic earthquakes within recorded history. The aftershock distribution and strain-release patterns suggest that the earthquake fault was a seaward extension of a fault system which has a pronounced surface expression in the Tertiary formations of the area near Ancon, Peru. The limits of the tectonic displacements and the tsunami-generating area were determined by a reverse wave-refraction method, refracting waves from Chimbote, Callao-Lima, San Juan, and Honolulu. The investigation revealed that the tsunami was generated by displacements of crustal blocks with a total area of 13,000 sq. km. Seismic and water motion data indicated that the uplifted portion of the crustal block was on the continental side of the rift. The energy of the main earthquake was estimated to be 1.122·10²³ ergs. The energy of the aftershocks was estimated to be 2.357·10²⁰ ergs. The tsunami energy was calculated to be 6.8·10¹⁹ ergs, or $\text{1}\text{1,650}$ of the earthquake energy.     

Crustal thickness variations in the Eastern Mediterranean and southern Aegean region

In this paper, regional analog gravity anomaly map obtained from the General Directorate of Mineral Research and Exploration (MTA) was digitized and used for the calculation of the crustal thickness (Moho depth) variations in the Eastern Mediterranean and the southern part of the Aegean Region. In the gravity anomaly map, there are mainly E–W trending apparent gravity anomalies represented by the contours up to 150 mGal. They are generally parallel to the shorelines of Africa, Turkey and Crete. Crustal thickness variations were calculated from the gravity anomalies, using an empirical equation in this study. Obtained thicknesses (Moho depths) were mapped and correlated with the previous investigations and seismological findings. According to the estimations, crustal thicknesses are about 25–30 km along the coastal regions and more than 30 km on the onshore part of Turkey increasing up to 42 km through the eastern Anatolia. However, there are thin crustal zones around 17 km in the offshore Egypt, to the NW part of Cyprus and about 19 km to the north of Crete. They may be related with the main tectonic trends in this region except the circular thinning to the south of Kas (southwestern part of Turkey). In order to determine the locations and boundaries of prominent tectonic elements, Analytic Signal (AS) and maxspots maps of the gravity anomalies were also prepared in this study. All produced maps are generally consistent to each other and the boundaries of main tectonic units were apparently illustrated in the maxspots map from the horizontal gradient of Bouguer anomalies.     

Fault inference and boundary recognition based on near-bottom magnetic data in the Longqi hydrothermal field

Near-bottom magnetic prospecting, which provides useful information to study shallow geological structures, is an efficient method for investigating active and inactive hydrothermal fields and researching the structure of hydrothermal systems. We collected near-bottom magnetic data in the Longqi hydrothermal area on the Southwest Indian Ridge using the Autonomous Benthic Explorer in 2007 and set up a processing system for magnetic data calibration. By removing the influence of terrain on magnetic anomalies and using the intensity of the spatial differential vector (ISDV) method, we inferred the presence of an N–S-trending fault and estimated its crush zone to be about 120 m wide and >2 km long along the known hydrothermal vents. This inferred fault is consistent with the precise topography mapped during the ABE 201 dive. The fault may be connected to a known detachment fault and form part of a hydrothermal channel. We delineated the hydrothermal alteration zone using the ISDV method and conclude that demagnetization was induced by hydrothermal alteration.     

Extension of active fault zones on Nisyros volcano across the Yali-Nisyros Channel based on onshore and offshore data

Nisyros island is a volcano at the eastern edge of the Aegean volcanic arc within the Hellenic arc and trench system along the convergence zone of the Eurasian and African plates. Several fault zones have been mapped and analyzed on the island with fault displacements reaching 100?C150 m as deduced from the morphology and the offset of the stratigraphic formations of the volcano. Seismic activity during 1995?C1998 affected the island with damage along the western edge of the Mandraki town, related to the Mandraki fault. The geological, tectonic and morphological data on land show that the Mandraki fault throw is 80?C100 m and its length about 2 km. Its continuation northwards under the sea was studied within a systematic survey of the broader area of the Kos-Nisyros-Tilos islands; and the bathymetric and lithoseismic data showed the existence of some active tectonic structures. In the area of the Yali-Nisyros Channel the prolongation of the Mandraki fault has a 100 m high submarine scarp between the two sides of the fault. Morphological slopes along the fault are high between 20 and 50% in contrast to slopes of 1?C5% observed on top of the two adjacent tectonic blocks. The general structure both on land and offshore shows a westward tilt contemporaneous to the extension in the E-W direction observed in this area. Observations of the submarine fault during a dive with submersible Thetis showed spectacular landslides and loose rocks along the fault scarp and very abrupt linear topographic change along the strike of the fault. The synthesis of the onshore and offshore data on a digital topographic map shows that the Mandraki fault is a secondary structure of the major F3 fault zone of Nisyros which separates the neotectonic block/horst of Prophitis Ilias in the west from the Emborio/Nikia block in the east. The GPS data from the period 1997?C2001 show excellent agreement with the neotectonic block structure of Nisyros. The seismic hazard of the F3/Mandraki fault zone is discussed together with the volcanic hazard of Yali-Nisyros area with the general conclusion that the expected seismic magnitude of 6.1?C6.3 is significantly higher than that observed in 1995?C1998. The ascent of magma from a chamber 7.5?C8.5 km deep between the Yali and Nisyros islands may trigger tectonovolcanic activity similar to that observed at the end of the 19th century.     

Definition of the plate boundary along the East Pacific Rise off Mexico

Tectonic and volcanic activity along the East Pacific Rise near Lat. 21°N is generally restricted to a 3–4‐km‐wide area centered over the rise axis. The East Pacific Rise is a medium‐rate (60 mm/yr) spreading center characterized by modest (100–200 m) relief of hills and seapeaks across the crestal region that is typical for such spreading centers. Few tectonic features appear in an axial volcanic zone 600–1, 200 m wide characterized by fresh, glassy pillow basalt and little or no sediment cover. This volcanic terrain is commonly flanked by tectonic zones where older lavas are cut by numerous normal faults bounding horst and graben systems and open fissures; these tectonic zones are commonly of unequal width on each side of the central volcanic zone and locally may be absent on one side. Bottom photographs and visual observations from a manned submersible indicate that most faults and fissures in the tectonic zones are young. Farther than 2 or 3 km from the axial volcanic zone, recent tectonic activity appears limited to a few faults that bound linear abyssal hills with total offset, suggesting relatively minor extension, so that instrumentation to measure the rate of plate separation along the rise crest will have to span both the volcanic and tectonic zones. The total width of the active plate boundary is at least 20 km, although less than 10% of the separation of the oceanic plates is accounted for by fault displacement and open fissures observed in the tectonic zones and on adjacent rise flanks. The asymmetric widths of the extensional tectonic zones result from migration of the volcanic extrusive zone over time.     

Crustal features of the northeastern South China Sea: insights from seismic and magnetic interpretations

We interpret seven two-dimensional deep-penetration and long-offset multi-channel seismic profiles in the northernmost South China Sea area, which were collected by R/V Marcus G. Langseth during the TAIwan GEodynamics Research (TAIGER) project in 2009. To constrain the crustal characteristics, magnetic inversion and forward magnetic modeling were also performed. The seismic results clearly show tilted faulting blocks in the upper crust and most of the fault plane connects downward to a quasi-horizontal detachment as its bottom in the south of the Luzon-Ryukyu transform plate boundary. North of the plate boundary, a small-scale failed rifted basin (minimum 5 km in crustal thickness) with negative magnetization probably indicates an extended continental origin. Significant lower crustal material (LCM) was imaged under a crustal fracture area which indicated a continent and ocean transition origin. The thickest LCM (up to 6.5 km) is located at magnetic isochron C15 that is probably caused by the magma supply composite of a Miocene syn-rift volcanic event and Pliocene Dongsha volcanic activity for submarine volcanoes and sills in the surrounding area. The LCM also caused Miocene crustal blocks to be uplifted reversely as 17 km crustal thickness especially in the area of magnetic isochron C15 and C16. In addition, the wide fault blocks and LCM co-existed on the magnetic striped area (i.e. C15–C17) in the south of the Luzon-Ryukyu transform plate boundary. Magnetic forward modeling suggests that the whole thick crustal thickness (>12 km thick) needs to be magnetized in striped way as oceanic crust. However, the result also shows that the misfit between observed and synthetic magnetic anomaly is about 40 nT, north of isochron C16. The interval velocity derived from pre-stack time migration suggests that the crust is composed of basaltic intrusive upper crust and lower crustal material. The crustal nature should refer to a transition between continent and ocean. Thus, the magnetic reversals may be produced in two possible ways: basaltic magma injected along the crustal weak zone across magnetic reversal epoch and because some undiscovered ancient piece of oceanic crust existed. The crustal structure discrimination still needs to be confirmed by future studies.     

Two-dimensionality of magnetic anomalies over Iceland and Reykjanes Ridge

The relationship between the magnetic anomalies over Iceland and those over Reykjanes Ridge is investigated using the data of the 1965 Dominion Observatory survey. A method is developed for determining the two-dimensionality of the anomalies from the component data measured in this survey. This method is based on testing the first and the second derivative of the magnetic potential with respect to the direction of two-dimensionality, using the component data along a single flight line. Testing the first derivative also yields the direction of two-dimensionality. The outcomes of the two tests (based on a single line) are compared with the observed two-dimensionality (established by narrowly spaced earlier surveys) of Reykjanes Ridge, showing good agreement. As the outcomes of the two tests provide complementary information they are combined into a single factor: A. This factor of two-dimensionality is very low for the anomalies over the shelf of Iceland indicating that the anomalies over Iceland cannot be continued directly into those over Reykjanes Ridge. Over Iceland A is generally low. Over the neovolcanic zone in eastern Iceland twodimensionality is associated with long wavelengths that are not present in the spectrum of the anomalies over Reykjanes Ridge. Thus, Reykjanes Ridge-type anomalies are absent with the exception of the central anomaly. This may not be used as evidence against crustal spreading since the kinematic model proposed by Pálmason for Iceland has a wide transition zone between rock of opposite polarity. The same model if computed for a mid-ocean ridge has narrow transition zones. The larger width of the transition zone blurs the anomalies related to the reversals of the earth magnetic field.     

CHARACTERISTICS OF MAGNETIC LINEATIONS AND TECTONIC EVOLUTION OF THE SOUTH CHINA SEA BASIN

Based on the analysis of new data of a total of 60 000 km, the authors propose a new spreading model of the tectonic evolution of the South China Sea Basin and suggest that magnetic anomalies in the Basin are the magnetic anomalies of a new type in marginal basins, that the tectonic development of the Basin can be divided into three seafloor spreading phases and that the southward migration of spreading ridge is related to the asymmetric spreading.     

Crustal structure and tectonic provinces of the Riiser-Larsen Sea area (East Antarctica): results of geophysical studies

About 16,000 km of multichannel seismic (MCS), gravity and magnetic data and 28 sonobuoys were acquired in the Riiser-Larsen Sea Basin and across the Gunnerus and Astrid Ridges, to study their crustal structure. The study area has contrasting basement morphologies and crustal thicknesses. The crust ranges in thickness from about 35 km under the Riiser-Larsen Sea shelf, 26–28 km under the Gunnerus Ridge, 12–17 km under the Astrid Ridge, and 9.5–10 km under the deep-water basin. A 50-km-wide block with increased density and magnetization is modeled from potential field data in the upper crust of the inshore zone and is interpreted as associated with emplacement of mafic intrusions into the continental margin of the southern Riiser-Larsen Sea. In addition to previously mapped seafloor spreading magnetic anomalies in the western Riiser-Larsen Sea, a linear succession from M2 to M16 is identified in the eastern Riiser-Larsen Sea. In the southwestern Riiser-Larsen Sea, a symmetric succession from M24B to 24n with the central anomaly M23 is recognized. This succession is obliquely truncated by younger lineation M22–M22n. It is proposed that seafloor spreading stopped at about M23 time and reoriented to the M22 opening direction. The seismic stratigraphy model of the Riiser-Larsen Sea includes five reflecting horizons that bound six seismic units. Ages of seismic units are determined from onlap geometry to magnetically dated oceanic basement and from tracing horizons to other parts of the southern Indian Ocean. The seaward edge of stretched and attenuated continental crust in the southern Riiser-Larsen Sea and the landward edge of unequivocal oceanic crust are mapped based on structural and geophysical characteristics. In the eastern Riiser-Larsen Sea the boundary between oceanic and stretched continental crust is better defined and is interpreted as a strike-slip fault lying along a sheared margin.