Manik Talwani

Sea-Floor Spreading in the North Atlantic

The magnetic anomaly lineation pattern in the North Atlantic Ocean (between the latitudes of 15° N. and 63° N.) has been examined in light of the hypotheses of sea-floor spreading and plate tectonics. There is no evidence of significant subduction or deformation along the margins of the Atlantic since the Late Triassic, and thus the sea-floor spreading that has occurred since that time has resulted in continental drift only. The rate and direction of drift between Europe and North America and between Africa and North America have differed at all times since the Late Triassic. Although Eurasia may have been rifted from North America in the Jurassic, the major phase of drift did not begin until the Late Cretaceous. Separation varied from 5.0 to 4.0 cm/yr (at a latitude of 45° N.) from the Cretaceous until 53 m.y. ago. The rate of separation slowed about 53 m.y. ago. The average rate was slightly less than 2 cm/yr for the intervals from 53 m.y. to 38 m.y. ago and from 38 m.y. to 9 m.y. ago. The sediment discontinuity found by others at about the location of anomaly 5 on both flanks of the Mid-Atlanti.c Ridge, north of the Azores, thus cannot be explained by a discontinuity or drastic slowing in the rate of spreading. From 9 m.y. to the present, separation has been at a rate somewhat greater than 2.0 cm/yr. The initiation of rifting between Africa and North America may have occurred 200 m.y. ago. However, we have assumed that the active phase of drift did not begin until 180 m.y. ago. The separation proceeded at an average rate of 4.0 cm/yr from 180 m.y. to 81 m.y. ago; 3.4 cm/yr from 81 m.y. to 63 m.y. ago; 2.4 cm/yr from 63 m.y. to 39 m.y. ago; 2.0 cm/yr from 38 m.y. to 9 m.y. ago; and 2.8 cm/yr from 9 m.y. ago to the present (the rates are computed for a latitude of 35° N.). We have fitted together lineations of the same age but from opposite sides of the ridge axis in the same fashion that previous workers have fitted together continental margins. Each fit is described by a pole and angle of rotation about the pole. Each fit gives the paleogeographic relations of the respective continents and oceanic plates for the particular age of the lineation. We conclude from these paleogeographic reconstructions that there was probably no Late Cretaceous (81 m.y. to 63 m.y. ago) sea-floor spreading in the Arctic, but that the relative motion between Eurasia and North America in the Arctic region was compressional during this interval. This compression may have been accommodated by subduction at Bowers Ridge (which appears to be an inactive island-arc trench system) and subduction in eastern Siberia. It also may have been accommodated by compressional deformation in the Brooks Range, the Verkhoyansk Mountains, and the Sverdrup Basin (in central northern Canada). All the spreading in the Arctic region that has occurred since the Late Cretaceous has taken place in the last 63 m.y. The locus of this spreading has been the Mid-Arctic Ridge which lies between the Lomonosov Ridge and the Eurasian continental shelf. The effect of this spreading has been to separate the pre-existing Lomonosov Ridge from the Eurasian continental shelf. The Alpha Cordillera has not been the locus of sea-floor spreading in the Cenozoic. The exact pattern of the separation of Greenland from North America is not known. There may have been minor rifting in the Labrador Sea during the Jurassic. However, the major phase of drift occurred from the Late Cretaceous to the late Eocene. The final separation of Eurasia (Spitsbergen), Greenland, and North America did not occur until the middle Eocene. The pattern of magnetic lineations suggests that the well-documented counterclockwise rotation of the Iberian Peninsula occurred between the Late Triassic and the Late Cretaceous, and that there has been little, if any, counterclockwise rotation subsequent to that time. We have used the derived poles and the angular rates of rotation to compute isochrons which give the age of the basement in the North Atlantic. The basement ages agree well with other data such as those obtained as the result of JOIDES drilling. The isochrons sometimes give greater ages which can be reconciled with the drilling results by involving subsequent volcanism, but in no case do the isochrons give smaller ages. The Keathley sequence of magnetic anomalies which lie just seaward of the quiet zone and southwest of Bermuda in the western Atlantic and northwest of Dakar in the eastern Atlantic, has been given an age of about 130 to 155 m.y. Comparison of the isochrons with the magnetic lineations indicate that two important shifts of the ridge axis may have occurred. The first, in the region south of the New England Seamounts and the Canary Islands was a 200-km eastward jump or migration that took place prior to 155 m.y. ago; the second in the region north of the New England Seamounts and Canary Islands but south of the Azores was a more complex westward shift of 150 km maximum extent that occurred between 135(?) m.y. and 72 m.y. ago. We have also computed a pattern of synthetic fracture zones or flow lines. Previous workers have proposed that the South Atlas fault, the western Canary Islands, and the New England Seamounts lie along a fundamental fault or fracture zone. We note that these features are approximately parallel to one of these synthetic flow lines. The seaward escarpment bounding the southern Bahamas as well as several well-surveyed fracture zones and other bathymetric features are parallel to the synthetic fracture zones.

Evolution of the Norwegian-Greenland Sea

Geological and geophysical data collected aboard R/V Vema during five summer cruises in the period 1966 to 1973 have been used to investigate the geological history and evolution of the Norwegian-Greenland Sea. These data were combined with earlier data to establish the location of spreading axes (active as well as extinct), the age of the ocean floor from magnetic anomalies, and the locations and azimuths of fracture zones. The details of the spreading history are then established quantitatively in terms of poles and rates of rotation. Reconstructions have been made to locate the relative positions of Norway and Greenland at various times since the opening, and the implications of these reconstructions are discussed here.

RAPID COMPUTATION OF GRAVITATIONAL ATTRACTION OF THREE‐DIMENSIONAL BODIES OF ARBITRARY SHAPE

An expression is derived for the gravity anomaly at an external point caused by a horizontal lamina with the boundary of an irregular polygon. This expression is put in a form suitable for computation by a high speed digital computer. By making the number of sides of the polygon sufficiently large, any irregular outline can be closely approximated. Any three dimensional body can be represented by contours. By replacing each contour by a polygonal lamina, the anomaly caused by it can be obtained at any external point. By a system of interpolation between contours combined with a numerical integration the gravity anomaly caused by the three‐dimensional body can be calculated to a high degree of precision. This method may also be used for rapidly computing terrain corrections on a flat earth. By making a small modification it can further be adopted for computing the terrain correction as well as local isostatic compensation on the Airy system up to the external radius of Hayford zone O on a spherical earth. ...

Computation with the help of a digital computer of magnetic anomalies caused by bodies of arbitrary shape

Formulas are derived for the magnetic anomalies caused by irregular polygonal laminas. These are used to obtain the three components of the magnetic anomalies caused by a finite homogeneously magnetized body of arbitrary shape. There is no restriction to the direction of magnetization; in general, it may not be the same as that of the earth’s field. Total‐intensity anomalies are also obtained. Use of these formulas in a computer program is discussed and illustrated by computing the anomaly caused by Caryn Seamount. Simplified, formulas are presented for the anomalies caused by finite rectangular laminas. In addition to bodies of complex shape, the computer program can also be profitably used for computing the magnetic anomalies caused by bodies of relatively simple geometry. The second derivatives of the gravitational potential of a massive body, that is, quantities familiarly known as gradient and curvature in torsion‐balance work and the first vertical derivative in gravity work are also obtained by thi...

Origin of seaward-dipping reflectors in oceanic crust off the Norwegian margin by “subaerial sea-floor spreading”

A remarkable layered acoustic stratification is observed in the upper oceanic crust adjacent to the Norwegian continental margin. It comprises a seaward-dipping complex of reflectors in the form of a wedge. We suggest that it is a layered igneous sequence that results when crustal accretion occurs at a subaerial spreading axis and that this phenomenon may commonly occur during the earliest phase of ocean-basin genesis.

Continental Margin off Norway: A Geophysical Study

Geophysical investigations consisting of gravity, magnetic, depth sounding, seismic reflection and refraction measurements were made aboard R/V Vema on the continental margin off Norway. Utilizing these and earlier data in the area, maps showing bathymetry, free-air gravity, magnetic residual total intensity, seismic refraction results, total sediment thickness and thickness of Cenozoic sediments have been constructed. The data are also presented as profiles across the margin. The Voring Plateau is underlain by a buried escarpment; the basement is shallow on the seaward side and deep on the landward side. A similar marginal escarpment, the Faroe–Shetland, exists farther south. These escarpments mark the site of the Tertiary opening of the Norwegian Sea. Seaward, Tertiary sediments overlie a basement generated by sea-floor spreading. Landward, a thick sequence of sediments that may be as old as Paleozoic overlies a continental basement. The magnetic quiet zone on the landward side of the escarpment is attributed to the continental nature of the basement. A nearly continuous belt of positive gravity and magnetic anomalies that exists just landward of the edge of the shelf is attributed primarily to intrabasement density contrasts in rocks that are probably Precambrian in age. It extends from northwest Scotland to the Lofoten–Vesteralen islands. The continental margin off Norway formed an epicontinental sea continuous with the North Sea in which a large amount of sedimentation kept pace with subsidence—a phenomenon which perhaps started in the late Paleozoic. The thickness of pre-Cenozoic sediments exceeds 6 km in some areas, but has a relative minimum over the belt of high density rocks of Precambrian age, which presumably underwent the least relative subsidence. We suggest that the opening of the Norwegian Sea at the marginal escarpments is associated with subsidence of the continental crust between the escarpments and the shelf where the high-density belt acts as a hinge line and accounts for the existence of the shelf break. The subsided area is characterized by a regional free-air gravity low. The marginal Voring Plateau Escarpment formed at the opening of the Norwegian Sea served to dam the Tertiary sediments and develop the V0ring Plateau. The Norwegian Channel is shown not to be of tectonic origin. The Tertiary basin of the North Sea continues northward under the continental margin off Norway.

Extinct triple junction south of Greenland and the Tertiary motion of Greenland relative to North America

Geophysical data collected by R/V Vema during cruises from 1960 to 1973 in the area south of Greenland were used to establish details of the magnetic lineation pattern and the basement morphology associated with an extinct triple junction. The sequence of magnetic anomalies 20 to 24 (49 to 60 m.y. B.P.) is continuous from the western flank of the Reykjanes Ridge into the Labrador Sea north and south of the triple junction and defines a period of simultaneous sea-floor spreading in the Norwegian Sea, the North Atlantic, and the Labrador Sea. At the time of anomaly 20 (49 m.y. B.P.) a major slowdown in the rate of spreading of the Labrador Sea Ridge occurred, and it met with extinction prior to the time of anomaly 13 (∼40 m.y. B.P.). The magnetic lineation pattern shows the amount of opening in the Labrador Sea between the interval spanned by anomalies 13 to 21 and that spanned by anomalies 21 to 23. An independent estimate of the relative motion between Greenland and North America was obtained from the difference in the opening between Greenland and Europe and that between North America and Europe. This second estimate predicts more opening of the Labrador Sea than is observed. Bounds can nevertheless be placed on the early Tertiary relative motion between Greenland and North America, and this motion compares well with the history of tectonic deformation in the Canadian Arctic islands during the Eurekan orogeny.

Venus Tectonics: Initial Analysis from Magellan

Radar imaging and altimetry data from the Magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the Venus surface. The plains record a superposition of different episodes of deformation and volcanism; strain is both areally distributed and concentrated into zones of extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening; the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression. Venus displays two geometrical patterns of concentrated lithospheric extension: quasi-circular coronae and broad rises with linear rift zones; both are sites of significant volcanism. No long, large-offset strike-slip faults have been observed, although limited local horizontal shear is accommodated across many zones of crustal shortening. In general, tectonic features on Venus are unlike those in Earths oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide, and separated by stronger and less deformed blocks hundreds of kilometers in width, as in actively deforming continental regions on Earth.

Seismic structure of the U.S. Mid‐Atlantic continental margin

Multichannel and wide-angle seismic data collected off Virginia during the 1990 EDGE Mid-Atlantic seismic experiment provide the most detailed image to date of the continent-ocean transition on the U.S. Atlantic margin. Multichannel data were acquired using a 10,800 in3 (177 L) airgun array and 6-km-long streamer, and coincident wide-angle data were recorded by ten ocean bottom seismic instruments. A velocity model constructed by inversion of wide-angle and vertical-incidence travel times shows strong lateral changes in deep-crustal structure across the margin. Lower-crustal velocities are 6.8 km/s in rifted continental crust, increase to 7.5 km/s beneath the outer continental shelf, and decrease to 7.0 km/s in oceanic crust. Prominent seaward-dipping reflections within basement lie within layers of average velocity 6.3–6.5 km/s, consistent with their interpretation as basalts extruded during rifting. The high-velocity lower crust and seaward-dipping reflections comprise a 100-km-wide, 25-km-thick ocean-continent transition zone that consists almost entirely of mafic igneous material accreted to the margin during continental breakup. The boundary between rifted continental crust and this thick igneous crust is abrupt, occupying only about 20 km of the margin. Appalachian intracrustal reflectivity largely disappears across this boundary as velocity increases from 5.9 km/s to >7.0 km/s, implying that the reflectivity is disrupted by massive intrusion and that very little continental crust persists seaward of the reflective crust. The thick igneous crust is spatially correlated with the East Coast magnetic anomaly, implying that the basalts and underlying intrusives cause the anomaly. The details of the seismic structure and lack of independent evidence for an appropriately located hotspot in the central Atlantic imply that nonplume processes are responsible for the igneous material.

Age of the North Atlantic Ocean from magnetic anomalies

Abstract Magnetic anomaly lineations have been identified in the North Atlantic. These lineations correlate with the magnetic time scale describing magnetic polarity reversals for the past 71 my. The results indicate that approximately 70% of the total drift between Europe and North America has occurred in the past 72 my, whereas only about 35% of the total drift between Africa and North America has happened in the same period. Extrapolation using the magnetic anomaly data and the results of JOIDES drilling suggests that drift between Africa and North America was initiated about 180 mybp.