Passive margin

Abstract: We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's 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.

Abstract: It is contended that the Late Triassic to present-day gross evolution of the Alpine system in the Mediterranean region has been the result of activity along an evolving network of accreting, transform, and subducting plate boundaries between the large stable cratons of Europe and Africa. A refined assembly of the outlines of the continents around the North and central Atlantic, before the initial dispersion of Gondwanaland in Early Jurassic times, is presented. By considering geologic facies, structural fabric, and paleomagnetic criteria, the smaller continental fragments now found within the Alpine system are restored to their proposed initial positions relative to each other in the reconstruction offered. The motion of the major plate of Africa relative to Europe, commencing with the initial continental fragmentation, is documented by analysis of the sea-floor spreading history of the Atlantic Ocean, with the assumption that plate accretion there has occurred between torsionally rigid lithospheric plates. By the computerized fitting of well-defined and well-dated key pairs of symmetric magnetic anomaly lineations back together by a series of finite rotations, the relative position of North America to both Europe and Africa has been determined for the following times: 180 m.y. (Toarcian Stage, Early Jurassic); 148 m.y. (Kimmeridgian Stage, Late Jurassic); 80 m.y. (Santonian Stage, Late Cretaceous); 63 m.y. (Danian Stage, Paleocene); 53 m.y. (Ypresian Stage, Eocene); and 9 m.y. (Tortonian Stage, Miocene). From these positions, a series of rotation poles presumed to describe the stepwise motion of Africa relative to Europe were computed. The motions of the smaller intervening microplates have been inferred from the style of tectonic deformation on their borders, and these motions have been constrained to satisfy both changes in paleo-latitude with time and progressive rotations relative to the large macroplates that can be deduced from paleomagnetic measurements. The evolution of Tethys does not involve a single simple plate boundary between Europe and Africa, as has been envisioned previously, but, instead, a constantly evolving mosaic of subsiding continental margins, migrating mid-oceanic ridges, transform faults, trenches, island arcs, and marginal seas (back-arc basins). The periods of passive-continental margin development are recognized by a transgressive facies of platform carbonate rocks and thick prisms of continental-rise type sedimentation; accreting ridges by ultramafic rocks, gabbro, pillow basalt, deep-sea pelagic ooze, and abyssal red clay of the ophiolite suite; trenches by a migrating series of progressively younger linear flysch troughs whose immature mineral composition reflects nearby andesitic and metamorphic source terrains; the arcs themselves by calc-alkaline volcanism and the intrusion of silicic to intermediate plutons; the polarities of these arcs by the direction of overthrust nappe sheets and gradients in the ratio of potash to silica in the extrusives; their orientation by paired belts of high T and P and high P-T metamorphics; and finally the spreading back-arc basins by outpourings of basaltic magmas and evidence of flipping Benioff planes. A compilation of eight phases or chapters in Atlantic spreading history are outlined, which are based on the recognition of discrete differences and (or) relative motion between the continents bordering the Atlantic. All of these changes are reflected in the Tethys by reorganizations of the intervening plate boundaries and, we believe, are most explicitly recorded in the deformational history of the subducting zones. A montage of geometrically assembled plate-boundary interpretations are pictorially displayed as time-lapse frames of the evolving Alpine system. The montage begins with the Late Triassic (pre-Atlantic) setting of the Tethys 1 Ocean and extends to the present through nine phases of Tethyan history. Each phase is recognized on the basis of the age of intrusion and extrusion of basic lavas in ophiolite complexes, which mark the creation of new oceanic areas by both axial accretion in rift valleys of mid-oceanic ridges between rigid plates or by a more uncertain type of spreading in basins behind active island arcs. All the schemes presented are best estimates of the gross geometrical arrangements at discrete time intervals and should be treated as merely educated guesses. Despite the fact that we only have rigorous constraints for the relative positions of the nondeformed forelands of Europe and Africa, our models nevertheless imply that the motions of the larger plates will, by and large, dictate the general behavior of the smaller microplates through the particular styles of deformation set up along the adjoining plate boundaries. The Tethys 1 Ocean, located between Africa and Europe in Triassic times, has been almost entirely swallowed up in subduction zones of the Major Caucasus Mountains along its former northern margin and in similar zones of the Pontides and Minor Caucasus along its southern margin. The only remnants of Tethys 1 are the areas of oceanic crust in the Black and South Caspian Seas. There is considerable evidence to suggest that the Tethys 1 Ocean had an actively spreading ridge. Some tens of millions of years prior to the opening of the central North Atlantic, a branch of this ridge system entered into the Vardar Zone of eastern Greece and broke off fragments of northeast North Africa to initiate the development of the present-day Ionian and Levantine Basins of the eastern Mediterranean. Additional fragments (the Moroccan and Oranaise Meseta) were ruptured from northwest Africa following its separation from North America. The intervening Jurassic Atlas, seaway developed along an accreting plate boundary extending from the eastern Tethys to the crest of the embryonic Mid-Atlantic Ridge where it formed a migrating triple junction whose trace, we believe, follows the trend of the New England seamount chain. The western Mediterranean basins of the Alboran, Balearic, and Tyrrhenian Seas are very much younger, being initially opened in the early Miocene as a string of back-arc marginal seas behind the developing Apennine, Tel Atlas, and Rif suture zone that today marks the sites of subduction of Jurassic and Lower Cretaceous oceanic crust. The contemporary Alpine system displays a spectrum of stages in the building of mountain belts. Embryonic nappes within the Mediterranean Ridge in proximity to melange zones of the inner wall of the Hellenic Trench are, perhaps, signs of the initial deformation of sedimentary passengers on oceanic crust arriving at a subduction zone. Total closure of an ocean followed by the partial consumption of a passive continental margin leads to events such as the tectonic emplacement of crystalline basement nappes of the European “chaine calcaire” onto northwest Africa. Arc-continent collisions of this type which have then been succeeded by total destruction of marginal back-arc basins are recognizable in the Hellenides and Pontides. There are, as well, collisions that have not involved the disappearance of large oceanic areas; these are most apparent in the particular tectonic style of the Pyrenees and High Atlas Mountains.

Abstract: Large igneous provinces (LIPs) are a continuum of voluminous iron and magnesium rich rock emplacements which include continental flood basalts and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts Such provinces do not originate at “normal” seafloor spreading centers We compile all known in situ LIPs younger than 250 Ma and analyze dimensions, crustal structures, ages, and emplacement rates of representatives of the three major LIP categories: Ontong Java and Kerguelen-Broken Ridge oceanic plateaus, North Atlantic volcanic passive margins, and Deccan and Columbia River continental flood basalts Crustal thicknesses range from 20 to 40 km, and the lower crust is characterized by high (70-76 km s?1) compressional wave velocities Volumes and emplacement rates derived for the two giant oceanic plateaus, Ontong Java and Kerguelen, reveal short-lived pulses of increased global production; Ontong Java’s rate of emplacement may have exceeded the contemporaneous global production rate of the entire mid-ocean ridge system The major part of the North Atlantic volcanic province lies offshore and demonstrates that volcanic passive margins belong in the global LIP inventory Deep crustal intrusive companions to continental flood volcanism represent volumetrically significant contributions to the crust We envision a complex mantle circulation which must account for a variety of LIP sizes, the largest originating in the lower mantle and smaller ones developing in the upper mantle This circulation coexists with convection associated with plate tectonics, a complicated thermal structure, and at least four distinct geochemical/isotopic reservoirs LIPs episodically alter ocean basin, continental margin, and continental geometries and affect the chemistry and physics of the oceans and atmosphere with enormous potential environmental impact Despite the importance of LIPs in studies of mantle dynamics and global environment, scarce age and deep crustal data necessitate intensified efforts in seismic imaging and scientific drilling in a range of such features