Walter R. Roest

Age, spreading rates, and spreading asymmetry of the world's ocean crust

We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the worlds ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.

Digital isochrons of the world's ocean floor

We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge-push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.

Magnetic interpretation using the 3-D analytic signal

A new method for magnetic interpretation has been developed based on the generalization of the analytic signal concept to three dimensions. The absolute value of the analytic signal is defined as the square root of the squared sum of the vertical and the two horizontal derivatives of the magnetic field. This signal exhibits maxima over magnetization contrasts, independent of the ambient magnetic field and source magnetization directions. Locations of these maxima thus determine the outlines of magnetic sources. Under the assumption that the anomalies are caused by vertical contacts, the analytic signal is used to estimate depth using a simple amplitude half-width rule. Two examples are shown of the application of the method. In the first example, the analytic signal highlights a circular feature beneath Lake Huron that has been identified as a possible impact crater. The second example illustrates the continuation of terranes across the Cabot Strait between Cape Breton and Newfoundland in eastern Canada.

Motion of Iberia since the Late Jurassic : results from detailed aeromagnetic measurements in the Newfoundland Basin

Abstract A detailed aeromagnetic survey carried out in the Newfoundland Basin shows well developed seafloor spreading anomalies 24 to 34. Comparison of these anomalies with the corresponding anomalies in the Northeast Atlantic suggests asymmetric spreading from anomalies 31 to 34, with slower spreading in the Newfoundland Basin. Anomaly M0 is a very weak anomaly in the Newfoundland Basin compared to south of the Newfoundland Fracture Zone where it forms a prominent low within the large amplitude “J” anomaly. A similar behaviour of this anomaly is observed off Iberia. In the Newfoundland Basin it does not continue as far as the Flemish Cap but terminates in the vicinity of the Newfoundland Seamounts. The position of this anomaly as obtained here differs from previous identifications. The shapes of magnetic lineations in the Newfoundland Basin are significantly different from the corresponding lineations off Iberia. This has been interpreted as arising from shifts in the plate boundary between Africa and Eurasia during the time when Iberia was moving as part of the African plate. By combining the present data with other detailed survey data to the north we have been able to derive a plate kinematic solution for Iberia which shows that from the middle Cretaceous to the Late Eocene Iberia moved as part of the African plate and then as an independent plate until the Late Oligocene. Since then it has been moving as part of the Eurasian plate. During these times the boundary between Eurasia and Africa jumped successively from the Bay of Biscay accretion axis to the Kings Trough-North Spanish Trough lineament to the Azores-Gibraltar Fracture Zone. The kinematic solution for Iberia so derived, from chron M0 to the present, not only explains the formation of some prominent bathymetric features in the oceanic regions, such as Kings Trough, but equally well the formation of geological features on land, such as the Pyrenees. The difficulties in deriving a kinematic solution for Iberia for times earlier than chron M0 are discussed and a speculative position of Iberia at the time of its initial separation from the Grand Banks of Newfoundland is proposed. Furthermore, with the availability of a well-constrained model for the motion of Iberia, it should now be possible to relate more accurately the relative motions among Eurasia, Iberia and Africa to the history of the Mediterranean region.

Sea-floor spreading in the Labrador Sea: A new reconstruction

Sea-floor spreading magnetic lineations 25 (59 Ma) and older have been reidentified in the Labrador Sea by using previous magnetic compilations and some recently acquired data. The higher density of these identifications enabled the calculation of a new set of better constrained rotation poles that describes the sea-floor spreading history of the Labrador Sea and Baffin Bay in a way that is somewhat different from previously published reconstructions. The most important inference that emerges from this work is that the change in spreading direction between Greenland and North America after anomaly 25 time is larger than previously recognized. As a result, the position of Greenland at the time of initial opening (92 Ma) may have been about 100 km farther south than obtained in earlier reconstructions.

Kinematics of the plate boundaries between Eurasia, Iberia, and Africa in the North Atlantic from the Late Cretaceous to the present

The plate kinematic model for Iberia proposed recently by Srivastava et al. is extended here to demonstrate that the implied motions along the plate boundaries between Eurasia, Iberia, and Africa are consistent with geological observations. Additional sea-floor-spreading data were used to obtain a more precise timing of the jumps that took place in the plate boundary between Africa and Eurasia. We show that the model is consistent with the existence of anomaly 34 in the Bay of Biscay, and we give more exact limits for the location of boundary B, which was the plate boundary between Eurasia and Africa from chron 34 to about chron 17. The additional data provide evidence that Iberia must have acted as an independent plate from about chron 18 to chron 6c; King9s Trough was the plate boundary between Iberia and Eurasia for most of this period.

Magnetic evidence for slow seafloor spreading during the formation of the Newfoundland and Iberian margins

There is considerable debate concerning the nature and origin of the thin crust within the ocean^continent transition (OCT) zones of many passive non-volcanic continental margins, located between thinned continental and true oceanic crust. This crust is usually found to be underlain by upper mantle material of 7.2^7.4 km/s velocity at shallow depths (1^2 km). It has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading. One of the examples of occurrence of such a crust are the conjugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these regions to show that their OCT zones are underlain by crustal material formed by slow seafloor spreading (6.7 mm/yr) soon after Iberia separated from the Grand Banks of Newfoundland in the late Jurassic. Similarities in the magnetic anomalies and velocity distributions from these regions with those from the Sohm Abyssal Plain, a region lying immediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give further support to such an interpretation. The idea that these regions were formed by unroofing of upper mantle during rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which bear all the characteristics of seafloor spreading anomalies, makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the North Atlantic and that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception across those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges. fl 2000 Elsevier Science B.V. All rights reserved.

Late Cretaceous–Cenozoic deformation of northeast Asia

Abstract The plate tectonic paradigm implies rigid plates and narrow plate boundaries. In contrast, diffuse plate boundaries are common both in the oceans and continents [R.G. Gordon, Annu. Rev. Earth Planet. Sci. 26 (1998) 615–642], and their history is difficult to constrain, especially in remote, tectonically complex areas such as northeast Asia [M.E. Chapman, S.C. Solomon, J. Geophys. Res. 81 (1976) 921–930]. Here we show how extensive North Atlantic marine magnetic [R. Macnab et al., EOS 76 (1995) 449, 458] and gravity data [D.T. Sandwell, W.H.F. Smith, J. Geophys. Res. 102 (1997) 10039–10054] can be used to unravel, with tight confidence limits, successive periods of deformation over 80 million years, along the diffuse continental Eurasian–North American plate boundary. A period of compression in the Late Cretaceous (14 mm/yr in the Laptev Sea to 20 mm/yr in Kamchatka) led to thrusting in the Verkhoyansk Mountains, and was followed by extension from 68 to 40 Ma when ∼400 km of extension was accommodated by the formation of a series of grabens, including the Moma Rift system. Since 40 Ma, time-varying compression and transpression along the Moma Rift system created strike-slip faults, thrusts and folds at rates up to 6.3 mm/yr. In the Laptev Sea region, 600 km of extension from latest Late Cretaceous to present created the Laptev Sea and Lena Rift systems. The deformation predicted by our model fits most geological features formed in the Laptev Sea and central northeast Asia during Late Cretaceous–Cenozoic times. The most recent deformation (Late Miocene–Pliocene) is not very well constrained since our model lacks data younger than 11 Ma. The deformation that occurred in Kamchatka reflects a complex tectonic setting and our model’s predictions are only tentative.

Magnetic anomalies in the Canary Basin and the Mesozoic evolution of the central North Atlantic

The data from a recent magnetic compilation by Verhoefet al. (1991) off west Africa were used in combination with data in the western Atlantic to review the Mesozoic plate kinematic evolution of the central North Atlantic. The magnetic profile data were analyzed to identify the M-series sea floor spreading anomalies on the African plate. Oceanic fracture zones were identified from magnetic anomalies and seismic and gravity measurements. The identified sea floor spreading anomalies on the African plate were combined with those on the North American plate to calculate reconstruction poles for this part of the central Atlantic. The total separation poles derived in this paper describe a smooth curve, suggesting that the motion of the pole through time was continuous. Although the new sea floor spreading history differs only slightly from the one presented by Klitgord and Schouten (1986), it predicts smoother flowlines. On the other hand, the sea floor spreading history as depicted by the flowlines for the eastern central Atlantic deviates substantially from that of Sundvik and Larson (1988). A revised spreading history is also presented for the Cretaceous Magnetic Quiet Zone, where large changes in spreading direction occurred, that can not be resolved when fitting magnetic isochrons only, but which are evident from fracture zone traces and directions of sea floor spreading topography.

A recipe for microcontinent formation

Accreted slivers of continental margins are common in the geologic record, but the processes that lead to their formation are poorly understood. We observe an association of plume-related microcontinent isolation and subsequent long-term asymmetries in oceanic crustal accretion based on four recent examples: the Seychelles in the Indian Ocean, Jan Mayen in the Norwegian-Greenland Sea, and the East Tasman Plateau and the Gilbert Seamount Complex in the Tasman Sea. These microcontinents formed by rerifting of a young continental margin (<25 m.y. old) in the vicinity of a mantle-plume stem, followed by asymmetric seafloor spreading. Two-dimensional numerical stochastic basin modeling suggests that a yield-strength minimum along the landward edge of a rifted margin, thermally enhanced by heating from a mantle plume, may cause a spreading ridge to jump onto this zone of weakness. This action isolates a passive-margin segment. The association of large igneous provinces and microcontinents should be useful for identifying similar events in the geological record.