Marine Magnetic Profiles Research Articles

Structure, age, and origin of the Caribbean Plate unraveled

Our compilation of marine magnetics of the Caribbean Plate allows us to identify magnetic anomalies from the Cretaceous Normal Superchron (CNS) in the Colombia and Venezuela basins, from which we propose a model for the age and origin of the Caribbean Plate and its Large Igneous Province (CLIP). The comparison of selected marine magnetic profiles across these basins with high resolution profiles from other areas across the CNS indicates that the Colombia Basin formed 73-92 million years ago (Ma) and the Venezuela Basin 92-108 Ma. The two basins and the older part of the CLIP formed at a ridge axis in the Pacific Ocean, near the Central American Seaway. These results strongly constrain regional plate tectonic models for America and the Pacific Ocean.

Normal polarity magnetosubchrons in 24r and the age of the Paleocene–Eocene boundary

A late Paleocene to early Eocene sequence of flat-lying continental strata occurs in an area known as Obed Mountain, west-central Alberta. The upper 110 m consist of interbedded fluvial channel sandstones and overbank mudrocks containing five back swamp coal seams. Two coreholes, 3.5 km apart, that extend through the entire coal zone were sampled for magnetostratigraphy and 13C isotope analysis. Bentonites in the No. 1 (lowest) and No. 5 coal seams and a tuff in the No. 3 coal seam were sampled for U–Pb and (or) Rb–Sr dating of zircon and biotite, respectively. Magnetostratigraphic analysis of 520 samples identified the younger part of chron 25r, the whole of chron 25n and the older half of chron 24r. We find six normal polarity subzones in this part of chron 24r, which we correlate to tiny wiggles 6 to11 in marine magnetic profiles. Carbon isotope analysis of 14 samples from two cores revealed a negative shift of about 2‰ peaking near the base of 24r.8r. We interpret this as the carbon isotope excursion (CIE), the base of which is now accepted as defining the Paleocene–Eocene boundary. The thickness of the CIE in the Obed Mountain section implies that it lasted between 210 000 and 254 000 years. Radiometric dates of 58.4 ± 0.2, 57.7 ± 0.3, and 56.9 ± 0.8 Ma are obtained for the No. 1, No. 3, and No. 5 coals, respectively. Combining these with magnetostratigraphy and cyclostratigraphy yields an age of 57.1 ± 0.1 Ma for the Paleocene–Eocene boundary.

Motion of the Scotia Sea plates

Earthquake data from the Scotia Arc to early 2002 are reviewed in the light of satellite gravity and other data in order to derive a model for the motion of plates in the Scotia Sea region. Events with magnitude greater than or equal to5, which occurred on or near the boundaries of the Scotia and Sandwich plates, and for which Centroid Moment Tensor (CMT) solutions are available, are examined. The newer data fill some of the previous sampling gaps along the boundaries of the Scotia and Sandwich plates, and provide tighter constraints on relative motions. Variations in the width of the Brunhes anomaly on evenly spaced marine magnetic profiles over the East Scotia Ridge provide new estimates of Scotia-Sandwich plate spreading rates. Since there are no stable fracture zones in the east Scotia Sea, the mean azimuth of sea floor fabric mapped by sidescan is used to constrain the direction of spreading. 18 newrate estimates and four azimuths from the East Scotia Ridge are combined with 68 selected earthquake slip vectors from the boundaries of the Scotia Sea in a least-squares inversion for the best-fitting set of Euler poles and angular rotation rates describing the 'present-day' motions of the Scotia and Sandwich plates relative to South America and Antarctica. Our preferred model (TLP2003) gives poles that are similar to previous estimates, except for Scotia Plate motion with respect to South America, which is significantly different from earlier estimates; predicted rates of motion also differ slightly. Our results are much more robust than earlier work. We examine the implications of the model for motion and deformation along the various plate boundaries, with particular reference to the North and South Scotia Ridges, where rates are obtained by closure.

Structure and tectonic evolution of the South Sandwich arc

Abstract Detailed analysis of marine magnetic profiles from the western part of the East Scotia Sea confirms continuous, organized back-arc spreading since at least 15 Ma ago. In the eastern part of the East Scotia Sea, the South Sandwich arc lies on crust that formed at the back-arc spreading centre since 10 Ma ago, so older back-arc crust forms the basement of the present inner forearc. Interpretations of two multichannel seismic reflection profiles reveal the main structural components of the arc at shallow depth, including evidence of trench-normal extension in the mid-forearc, and other features consistent with ongoing subduction erosion. The seismic profile interpretations have been used to constrain simple two-dimensional gravity models. The models were designed to provide constraints on the maximum possible thickness of the arc crust, and it is concluded that this is 20 and 19.2 km on the northern and southern lines, respectively. On the northern line the models indicate that the forearc crust cannot be much thicker than normal oceanic crust. Even with such thin crust, however, the magmatic growth rate implied by the cross-section of the arc crust is within the range recently estimated for two other arcs that have been built over a much longer interval.

Geophysical correlations in the Ungava Bay area

We used gravity and magnetic data to extend geological features across Ungava Bay and Hudson Strait, from northern Quebec and Labrador to Baffin Island. Tectonic domains identified on land were extended offshore by visual inspection of the potential fields. The boundaries identified on colour shaded relief maps were drawn on the original marine magnetic profiles to verify the robustness of the conclusions. Our interpretation in Ungava Bay remains speculative but gives some insight into the architecture of the northeastern Canadian Shield. Onshore, where trends are well marked, the magnetic fabric of the Superior Province reappears east of the New Quebec Orogen in the western core zone. Geochronology and geochemical data also suggest the affinity of the western core zone with the Superior Province. On the gravity map, the New Quebec Orogen appears to vanish offshore. A major trend in magnetic and gravity gradients can be followed offshore possibly as far north as Hudson Strait. It has been identified as the extension of the George River and Moonbase shear zones. The De Pas Batholith is associated with a linear trend west of this shear zone. To the east, several features appear distinctly in the magnetic map. A narrow northwest-trending anomaly can be followed from Quebec all the way to Baffin Island. On land, this feature marks the western limit of the Lake Harbour Group. Two wide north-trending bands that can be followed from the Torngat Orogen all the way to Baffin Island are interpreted as the extensions of the Lac Lomier Complex and Tasiuyak Domain.

Early Tertiary seafloor spreading magnetic anomalies and paleo-propagators in the northern Arabian Sea

Study of closely spaced new marine magnetic profiles, in conjunction with published magnetic data, provide an updated identification of linear magnetic anomalies in the northern Arabian Sea from the Owen Fracture Zone in the west to the Laxmi and Chagos–Laccadive Ridges in the east. For the first time the entire magnetic anomaly sequence 28 through 20 is mapped eastwards from 65°E up to about 70°E. Further eastward the post-anomaly 28 oceanic crust appears to have been overlain by the volcanic material forming the Chagos–Laccadive Ridge. Contrary to earlier interpretations, the identified magnetic lineations are observed to have been segmented mainly by oblique offsets representing the pseudofaults associated with 11 major paleo-propagating ridges. These ridge propagation events commenced around anomaly 28 time (∼63 Ma) and continued at least until anomaly 23 time (∼52 Ma) and during the process a large amount of crust was transferred from the African plate to the Indian plate. The pre-anomaly 24 period was characterized by ridge reorientation, whereas such a phenomenon was not apparent during the post-anomaly 24 period. Almost all these propagating ridges appear to have been propagating westward, probably to migrate away from the proximity of the Reunion hotspot. The accretionary process of the Early Tertiary oceanic crust in the Arabian Sea thus appears to have been greatly influenced by the propagating ridges and the nearby hotspot.

Geophysical Evidence of a Relict Oceanic Crust in the Southwestern Scotia Sea

The southwestern part of the Scotia Sea, at the corner of the Shackleton Fracture Zone with the South Scotia Ridge has been investigated, combining marine magnetic profiles, multichannel seismic reflection data, and satellite-derived gravity anomaly data. From the integrated analysis of data, we identified the presence of the oldest part of the crust in this sector, which tentative age is older than anomaly C10 (28.7 Ma). The area is surrounded by structural features clearly imaged by seismic data, which correspond to gravity lows in the satellite-derived map, and presents a rhomboid-shaped geometry. Along its southern boundary, structural features related to convergence and possible incipient subduction beneath the continental South Scotia Ridge have been evidenced from the seismic profile. We interpret this area, now located at the edge of the south-western Scotia Sea, as a relict of ocean-like crust formed during an earlier, possibly diffuse and disorganized episode of spreading at the first onset of the Drake Passage opening. The successive episode of organized seafloor spreading responsible for the opening of the Drake Passage that definitively separated southern South America from the Antarctic Peninsula, instigated ridge-push forces that can account for the subduction-related structures found along the western part of the South Scotia Ridge. This seafloor accretion phase occurred from 27 to about 10 Ma, when spreading stopped in the western Scotia Sea Ridge, as resulted from the identification of the marine magnetic anomalies.

Seafloor spreading magnetic anomalies in the southeastern Arabian Sea

Study of closely spaced new marine magnetic profiles in the southeastern Arabian Sea has enabled fairly confident identification of E-W trending linear magnetic anomalies 24 through 20. These magnetic lineations are consistently right laterally offset along numerous fracture zones and extend eastwards up to the base of the western slopes of the Laccadive Ridge. Obliteration of magnetic lineations further eastward is attributed to the emplacement of volcanic material of the Laccadive Ridge over the pre-existing oceanic crust. DSDP Site 221 is observed to be located on the reversely magnetized oceanic crust between anomalies 22 and 21 which provide a reliable age constraint (48 Myr) for the oceanic crust at this site. The presence of additional unidentifiable normal magnetic anomalies suggests a relatively complex spreading pattern (ridge jump?) in the vicinity of the Laccadive Ridge during the interval of anomalies 24 and 23.

Updated interpretation of magnetic anomalies and seafloor spreading stages in the south China Sea: Implications for the Tertiary tectonics of Southeast Asia

We present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ≈50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ≈16 Ma) and the average spreading rate was slower, ≈35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b‐7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E‐W to NE‐SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons, we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo‐Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left‐lateral motion on the Red River‐Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large‐scale “domino” models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left‐lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India‐Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion along the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike‐slip fault of SE Asia.

A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic

We have constructed a magnetic polarity time scale for the Late Cretaceous and Cenozoic based on an analysis of marine magnetic profiles from the world's ocean basins. This is the first time, since Heirtzler et al. (1968) published their time scale, that the relative widths of the magnetic polarity intervals for the entire Late Cretaceous and Cenozoic have been systematically determined from magnetic profiles. A composite geomagnetic polarity sequence was derived based primarily on data from the South Atlantic. Anomaly spacings in the South Atlantic were constrained by a combination of finite rotation poles and averages of stacked profiles. Fine‐scale information was derived from magnetic profiles on faster spreading ridges in the Pacific and Indian Oceans and inserted into the South Atlantic sequence. Based on the assumption that spreading rates in the South Atlantic were smoothly varying but not necessarily constant, a time scale was generated by using a spline function to fit a set of nine age calibration points plus the zero‐age ridge axis to the composite polarity sequence. The derived spreading history of the South Atlantic shows a regular variation in spreading rate, decreasing in the Late Cretaceous from a high of almost 70 mm/yr (full rate) at around anomaly 33–34 time to a low of about 30 mm/yr by anomaly 27 time in the early Paleocene, increasing to about 55 mm/yr by about anomaly 15 time in the late Eocene, and then gradually decreasing over the Oligocene and the Neogene to the recent rate of about 32 mm/yr. The new time scale has several significant differences from previous time scales. For example, chron C5n is ∼0.5 m.y. older and chrons C9 through C24 are 2–3 m.y. younger than in the chronologies of Berggren et al. (1985b) and Harland et al. (1990). Additional small‐scale anomalies (tiny wiggles) that represent either very short polarity intervals or intensity fluctuations of the dipole field have been identified from several intervals in the Cenozoic including a large number of tiny wiggles between anomalies 24 and 27. Spreading rates on several other ridges, including the Southeast Indian Ridge, the East Pacific Rise, the Pacific‐Antarctic Ridge, the Chile Ridge, the North Pacific, and the Central Atlantic, were analyzed in order to evaluate the accuracy of the new time scale. Globally synchronous variations in spreading rate that were previously observed around anomalies 20, 6C, and in the late Neogene have been eliminated. The new time scale helps to resolve events at the times of major plate reorganizations. For example, anomaly 3A (5.6 Ma) is now seen to be a time of sudden spreading rate changes in the Southeast Indian, Pacific‐Antarctic, and Chile ridges and may correspond to the time of the change in Pacific absolute plate motion proposed by others. Spreading rates in the North Pacific became increasingly irregular in the Oligocene, culminating in a precipitous drop at anomaly 6C time.

Effects of ridge crest‐trench interaction on Antarctic‐Phoenix Spreading: Forces on a young subducting plate

Precise measurements of spreading rates on marine magnetic profiles collected to the west of the Antarctic Peninsula have enabled some consideration of the forces governing plate motion, since Antarctic—Phoenix motion has been controlled by the local rather than global force balance over the last 35 m.y. The total effective driving force per unit length of trench is calculated to have ranged between 2.6 and 3.6×1012 N/m, which is much less than is commonly thought necessary to support subduction. Conventional calculations may overestimate slab pull for old slabs because they neglect the effect of extensional disruption in limiting the contribution to the balance of forces at the trench. The low estimate of driving forces obtained here implies that resistive forces are also smaller than is generally assumed. Driving forces show a strong correlation with observed spreading rates, which indicates that resistive forces were largely velocity dependent. Fluid migration up the subduction zone may elevate temperatures in and around the shear zone, reducing resistive forces below the levels required by purely conductive models. Changes in convergence rate may affect the depths of both the brittle/ductile deformation boundary and the basalt/eclogite phase change, causing a negative feedback which would appear as a velocity‐dependent resistive force. The different driving forces acting on the NE and SW parts of the Phoenix plate, as a consequence of older oceanic lithosphere at the trench in the NE, caused Antarctic–Phoenix spreading to take place about a near pole to the SW since 21 m.y. ago, and ultimately resulted in disruption of the Phoenix plate about 9 m.y. ago. Spreading rates decreased abruptly about 6 m.y. ago, probably because of E‐W compression across the long transform faults bounding the Phoenix plate. However, spreading on the last three segments of the Antarctic–Phoenix Ridge continued at least until 4 m.y. ago. Either spreading stopped progressively from SW to NE, or the final stage took place about a very near pole to the SW. A magnetic quiet zone extends up to 95 km from the margin between the Tula Fracture Zone and the North Anvers Fracture Zone, and is thought to indicate that the ridge crest became buried by terrigenous sediment prior to collision. The absence of a magnetic quiet zone associated with the most recent ridge crest–trench collisions suggests a change in sedimentary regime during the late Miocene. Anomalously fast apparent spreading rates between 23 and 21 m.y. ago are thought to indicate an error in this part of the magnetic reversal time scale.

Magnetic crust in the Bay of Bengal

Four marine magnetic profiles across the Bay of Bengal are analyzed and the magnetic nature of the upper part of the crust is investigated by inversion in the frequency domain. The magnetization distribution at the average depth of the basement below the sediments is computed and shows high magnetization with alternating polarity. The spectral decay provides a maximum estimate of the depth to basement of 8 and 10 km along profiles approximately parallel to 10° and 14°N respectively. This suggests a thick pile of sediments in this region, with thickness increasing towards the north. The inversion of the recorded magnetic field along the 10°N profile at an average depth of 8 km provides alternating magnetization of approximately 1–2 × 10 −3 cgs units, corresponding to basic rocks. This indicates a magnetic basement in the Bay of Bengal with alternating polarities typical of oceanic crust. Greater magnetization (3 × 10 −3 cgs units) over the 85° East Ridge and the pattern of computed magnetization distribution on either side of this ridge indicate that it might have been a spreading ridge at sometime in the geological past which subsequently aborted.

A revised estimate of Pacific‐North America motion and implications for Western North America Plate boundary zone tectonics

For the past 20 years, much effort has been directed to determining the present‐day relative motion of the Pacific and North American plates using two independent approaches. One uses geologic observations and geodetic measurements along the San Andreas Fault and other faults in the plate boundary zone. The other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies, transform azimuths, and earthquake slip vectors. Geologic and geodetic studies find two principal elements of deformation: slip along the San Andreas of ∼34 mm/yr directed N41°W, and extension across the Basin and Range province of about 10 mm/yr directed N56°W. In contrast, plate motion studies find 56‐60 mm/yr directed N35°W. The discrepancy between these estimates, a vector of about 15 mm/yr oriented nearly due north, is often attributed to a combination of slip along faults parallel to the San Andreas and shortening normal to it. Here we revise the estimate of Pacific‐North America motion by analyzing marine magnetic profiles from the Gulf of California. Since 3 Ma, spreading has averaged 48 mm/yr, 10 mm/yr slower than estimated before, and consistent with the 49 mm/yr spreading predicted by a new global plate motion model derived without any data along the Pacific‐North America boundary. The discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm/yr parallel to the San Andreas, 60% less than estimated before, and 7 mm/yr of shortening across the San Andreas, similar to prior estimates. These results suggest that strike‐slip motion on faults west of the San Andreas is less than thought before, a conclusion consistent with geodetic, seismological, and other geologic observations.

Marine magnetic anomaly timescales for the Cenozoic and Late Cretaceous: A précis, critique, and synthesis

Anomaly timescales for the last 90 million years, derived from marine magnetic profiles and published prior to mid‐1979, are summarized, illustrated for comparison, and critically reviewed. A revised timescale is constructed using calibration points which fix the ages of anomalies 2.3′, 5.5, 24, and 29. An equation is presented for converting K‐Ar dates that is consistent with the recent adoption of new decay and abundance constants. The calibration points used in the revised timescale are so converted, as are the boundary ages of geologic epochs within the range of the timescale.

Mesozoic magnetic lineations in the Bering Sea Marginal Basin

A compilation of marine magnetic profiles from the Bering Sea marginal basin has revealed the existence of north-south trending magnetic lineations over the deep abyssal basins. The anomalies in the Aleutian and Bowers basins have maximum peak to peak amplitudes of 350 γ and wavelengths characteristic of sea floor spreading anomalies (25–100 km). These lineations are as much as 600 km long. In contrast, linear trends within the Komandorsky Basin are less distinct and have anomalies with variable amplitudes. The magnetic anomalies along an 850-km profile crossing the Aleutian Basin have been correlated with the M1-M13 (117–132 m.y.) sequence of Larson's (1974) Mesozoic time scale. These anomalies increase in age from east to west and yield a half spreading rate of 5.1 cm/yr. North-south trending anomalies within Bowers Basin exhibit shapes and spacings similar to those from the Aleutian Basin, suggesting that these trends may have been connected at one time. Detailed model studies of the anomalies in the Aleutian Basin yield results which support the hypothesis that the igneous crust in the eastern Bering Sea is trapped oceanic plate which may have been part of the Mesozoic Pacific Basin. The model studies have indicated that (1) normalized magnetization intensities are comparable to those estimated for the Mesozoic Hawaiian Lineations in the Pacific, (2) the approximate phase angle of the observed anomalies (−10°±20°) would allow the magnetic lineations either to be formed at their present location or else to be formed at a more southerly site and then moved to their present location, and (3) the width of transitions between adjacent polarity intervals is relatively large (W ≥ 16 km) and may be related to a redistribution of the remanent magnetization within a deeply buried layer 2. The corrected depth to layer 2 is within 5% of the depth predicted for Mesozoic crust (120 m.y.) on the Sclater (1972) depth versus age curves. Although the north-south trending anomalies in the Aleutian and Bowers basins are interpreted as sea floor spreading anomalies, the origin of the anomaly pattern in the Komandorsky Basin is not understood.

Short-wavelength magnetic anomalies in a region of rapid seafloor spreading

THE main features of the geomagnetic time scale for 0–3 Myr BP are well defined1–3, but the existence and/or number of short events (polarity intervals lasting less than 50,000 yr) is still under discussion. Interest in correlating these short events with features in marine magnetic profiles has focused on three short-wavelength anomalies: anomaly X4,5 which appears at approximately 2.3 Myr BP; anomaly W5–7 at 2.0 Myr BP; and a short-wavelength feature in the centre of the central anomaly3,6,8,9. These anomalies appear rarely, and then usually only in profiles from regions of rapid spreading. We present here an analysis of a remarkable new set of data from just such a region. Our results provide evidence of the existence of two, or possibly three, short events in (the interval 0–3 Myr BP, and of the existence of a magnetised ridge at the precise centre of seafloor spreading.

Vector magnetic data for detecting short polarity intervals in marine magnetic profiles

To identify short polarity intervals recorded during the early Cenozoic in the oceanic crust, a three-component magnetometer was flown across the magnetic stripes north of the Murray fracture zone. The vector magnetometer was flown at a low altitude (335 meters) in order to reduce attenuation of short-wavelength anomalies. The conventional means used to distinguish between magnetic stripes and anomalies due to three-dimensional sources is to correlate wiggles visually over a number of widely separated total-field profiles. The single profile of three orthogonal components was found to provide the distinction directly. The component parallel to the magnetic stripes hy recorded only anomalies due to three-dimensional sources, whereas the horizontal hx and vertical hz components perpendicular to hy were found to record both magnetic stripes and three-dimensional anomalies. By comparing profiles, it was possible to distinguish anomalies due to seamounts or other local sources from anomalies due to short polarity events. Analysis of this single profile from the north Pacific supports the existence of two short polarity events previously identified by stacking profiles of the total field recorded at the sea surface.

Identification of short polarity events by transforming marine magnetic profiles to the pole

A method is presented that permits marine magnetic anomaly profiles from all parts of the world to be stacked for the purpose of enhancing coherent small anomalies due to short geomagnetic polarity events. In general, the shape of a profile depends not only on the sequence of normally and reversely magnetized magnetic prisms that constitute the source, but also on the following five angles: the declination and inclination of the regional field; the declination and inclination of the magnetization; and the azimuth of the magnetic stripes. Our method transforms a measured anomaly to a second anomaly which would have been produced by an identical source with vertical magnetization and vertical regional field. The method uses Fourier transform and linear systems theory and employs filters with a flat spectral response, so that no information is lost at short wavelengths. No assumptions are made about the true depth or shape of the source: this may be any two-dimensional body or bodies in which the direction of magnetization is constant and the intensity varies arbitrarily. A preliminary application of the method to marine profiles from the East Pacific rise has yielded encouraging results.

Plate Tectonics, Paleomagnetism, and the Opening of the Atlantic

Finite rotations of the African, European, and North and South American plates about the poles of Bullard and others (1965) have been employed to reconstruct the ancient configuration of the Atlantic Ocean. The rates and duration-times for the motions have been estimated from sea-floor spreading interpretations of marine magnetic profiles and deep-sea drilling results across the Mid-Atlantic Ridge. The central Atlantic, Caribbean, and Gulf of Mexico began to form in Triassic time about 200 m.y. ago, with the drift of Africa and South America away from North America. The South Atlantic first opened about 150 m.y. ago as South America separated from Africa. The formation of the North Atlantic was somewhat more complex. Initial opening began in the Rockall Trough between 200 and 150 m.y. ago. The Bay of Biscay formed during the period 150 to 80 m.y. ago. The Labrador Sea opened between 80 and 45 m.y. ago. The present spreading regime along the Reykjanes Ridge and Mid-Atlantic Ridge north of the Azores began about 65 m.y. ago. Paleomagnetic poles for the respective continental plates for various geologic time intervals have also been subjected to the same finite rotations as the continents. A world mean polar wander path reconstructed from the rotated paleomagnetic poles reveals that the entire Atlantic plate system has gradually shifted northward toward the present spin axis as opening progressed.

A discussion on the structure and evolution of the Red Sea and the nature of the Red Sea, Gulf of Aden and Ethiopia rift junction - Magnetic anomalies in the Red Sea

Marine magnetic profiles over the Red Sea between 18° N and 25° N latitudes confirm previous hypotheses that strongly magnetic rocks underlie the axial trough. The symmetrical nature of the anomalies and their close correspondence to seafloor spreading magnetic models support a rifting origin for the trough. The dominant magnetic anomaly trends strike about N 35° W in the northern and southern parts of the trough. In the central region between 20° and 22° N the trend direction is about N 60° E. Geometrical considerations of possible spreading mechanisms suggest that the true separation direction of Africa away from Arabia near 21° N latitude is in either a N 10° E or N 60° E direction. The separation rates then are 3.2 cm a -1 and 2.0 cm a -1 , respectively.