scholarly journals The Rheology and Morphology of Oceanic Lithosphere and Mid-Ocean Ridges

2013 ◽  
pp. 63-93 ◽  
Author(s):  
R. C. Searle ◽  
J. Escartín
Keyword(s):  
Oceanic Lithosphere ◽  
Ocean Ridges

Radial Anisotropy and it's Relation to Fast and Slow Moving Tectonic Plates

2021 ◽  
Author(s):  
Tak Ho ◽  
Keith Priestley ◽  
Eric Debayle
Keyword(s):  
Upper Mantle ◽  
Large Scale ◽  
Oceanic Lithosphere ◽  
Azimuthal Anisotropy ◽  
Plate Motion ◽  
Moho Depth ◽  
Dispersion Characteristics ◽  
Anisotropic Models ◽  
Ocean Ridges ◽  
Average Dispersion

<p>We present a new radially anisotropic (<strong>ξ)</strong> tomographic model for the upper mantle to transition zone depths derived from a large Rayleigh (~4.5 x 10<sup>6 </sup>paths) and Love (~0.7 x 10<sup>6</sup> paths) wave path average dispersion curves with periods of 50-250 s and up to the fifth overtone. We first extract the path average dispersion characteristics from the waveforms. Dispersion characteristics for common paths (~0.3 x 10<sup>6</sup> paths) are taken from the Love and Rayleigh datasets and jointly inverted for isotropic V<sub>s </sub>and <strong>ξ</strong>. CRUST1.0 is used for crustal corrections and a model similar to PREM is used as a starting model. V<sub>s</sub> and <strong>ξ</strong> are regionalised for a 3D model. The effects of azimuthal anisotropy are accounted for during the regionalisation. Our model confirms large-scale upper mantle features seen in previously published models, but a number of these features are better resolved because of the increased data density of the fundamental and higher modes coverage from which our <strong>ξ</strong>(z) was derived. Synthetic tests show structures with radii of 400 km can be distinguished easily. Crustal perturbations of +/-10% to V<sub>p</sub>, V<sub>s</sub> and density, or perturbations to Moho depth of +/-10 km over regions of 400 km do not significantly change the model. The global average decreases from <strong>ξ~</strong>1.06 below the Moho to <strong>ξ</strong>~1 at ~275 km depth. At shallow depths beneath the oceans <strong>ξ</strong>>1 as is seen in previously published global mantle radially anisotropic models. The thickness of this layer increases slightly with the increasing age of the oceanic lithosphere. At ~200 km and deeper depths below the fast-spreading East Pacific Rise and starting at somewhat greater depths beneath the slower spreading ridges, <strong>ξ</strong><1. At depths ≥200 km and deeper depths below most of the backarc basins of the western Pacific <strong>ξ</strong><1. The signature of mid-ocean ridges vanishes at about 150 km depth in V<sub>s</sub> while it extends much deeper in the <strong>ξ</strong> model suggesting that upwelling beneath mid-ocean ridges could be more deeply rooted than previously believed. The pattern of radially anisotropy we observe, when compared with the pattern of azimuthal anisotropy determined from Rayleigh waves, suggests that the shearing at the bottom of the plates is only sufficiently strong to cause large-scale preferential alignment of the crystals when the plate motion exceeds some critical value which Debayle and Ricard (2013) suggest is about 4 cm/yr.</p>

scholarly journals MAGNETIC CHARACTERISTICS OF METAMORPHIZED ULTRABASITES OF THE OCEANIC LITHOSPHERE

2019 ◽  
Vol 47 (4) ◽  
pp. 106-127
Author(s):  
K. V. Popov ◽  
A. M. Gorodnitskiy ◽  
N. A. Shishkina
Keyword(s):  
World Ocean ◽  
Oceanic Lithosphere ◽  
Structural Features ◽  
Residual Magnetization ◽  
Magnetic Characteristics ◽  
Transform Faults ◽  
Ocean Ridges ◽  
Submarine Ridge ◽  
Deep Layers ◽  
The Stability

As part of the study of the nature of magnetic anomalies associated with the deep layers of the oceanic crust, a comparative analysis was made of the petromagnetic characteristics of serpentinized mantle ultrabasic samples taken from oceanographic expeditions of the Institute of Oceanology and the Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences in various morphotectonic regions of the World Ocean. The purpose of the work is to obtain information on the composition, concentration, crystallization temperature and structural features of ferromagnetic minerals, which are formed in different conditions of the post-magmatic metamorphism of ultrabasites. Sample collections are divided into three groups. 1. Oceanic peridotites from the rift zones of the mid-ocean ridges and transform faults. 2. Peridotites of the submarine ridge Gorringe, located within the Azoro-Gibraltar zone of faults. 3. Dunites of the Pekulney complex (Chukotka) formed in the island arc system. It has been established that in all selected regions, samples of serpentinized hyperbasites have high values of natural residual magnetization, magnetic susceptibility and saturation magnetization. The highest values of magnetic parameters are the dunites of the Pekulney complex. Estimation of the dependence of the concentration of ferrimagnetic materials C% of the degree of serpentinization of the SS%. showed that it is practically of little significance. The main factors contributing to the increase in the concentration of magnetite are the increased iron content of olivine in ultrabasites and the temperature of metamorphism. The question of the period of formation of magnetites and the stability of their primary residual magnetization requires further study.

Plate Tectonics Now and in the Past

2004 ◽  
Author(s):  
John J. W. Rogers ◽  
M. Santosh
Keyword(s):  
Plate Tectonics ◽  
Oceanic Lithosphere ◽  
Continental Margins ◽  
Island Arcs ◽  
Low Velocity ◽  
S Waves ◽  
Ocean Ridges ◽  
1960S And 1970S ◽  
Tectonic Contact ◽  
The 1960S

The concepts known as plate tectonics that began to develop in the 1960s built on a foundation of information that included: • The earth’s mantle is rigid enough to transmit seismic P and S waves, but it is mobile to long-term stresses. • The earth’s temperature gradient is so high that convective overturn must occur in the mantle. • The top of the mobile part of the mantle is a zone of relatively low velocity at depths of about 100 to 200 km. This zone separates an underlying asthenosphere from a rigid lithosphere, which includes rigid upper mantle and crust. • Seismic activity, commonly accompanied by volcanism, occurs along narrow, relatively linear, zones in oceans and along some continental margins. • The zones of instability surround large areas of comparative stability. • Ocean lithosphere is continually generated along mid-ocean ridges and destroyed where it descends under the margins of continents and island arcs. This causes oceans to become larger, but shrinkage of oceans can occur where lithosphere is destroyed around ocean margins faster than it is formed within the basin. • Some of the belts of instability are faults with lateral offsets of hundreds of kilometers. • Some continental margins are unstable (Pacific type), but others are attached to oceanic lithosphere without any apparent tectonic contact (Atlantic type). • Different areas containing continents and attached oceanic lithosphere move around the earth independently of each other. Most of this chapter consists of a summary of plate tectonics in the present earth, including processes along plate margins and the types of rocks formed there (readers who want more detailed information are referred to Rogers, 1993a; Kearey, 1996; and Condie, 1999). We also briefly discuss plumes and then finish with a word of caution about interpreting the history of the ancient and hotter earth with the principles of modern plate tectonics. Starting from the body of continually expanding information summarized above, numerous earth scientists in the 1960s and 1970s began to establish a conceptual framework that would organize scientific thinking about the earth’s tectonic processes. This required a new terminology, and it arrived rapidly (Oreskes, 2002). Geologists decided to call the stable areas “plates” and the unstable zones around them “plate margins.” Thus, the concept became known as “plate tectonics.” Plates are essentially broad regions of lithosphere, although the failure to detect low-velocity zones under many continents leaves unresolved questions.

A magnesium budget for serpentinisation of abyssal peridotite during the Cenozoic

2020 ◽  
Author(s):  
Andrew Merdith ◽  
Muriel Andreani ◽  
Isabelle Daniel ◽  
Thomas Gernon
Keyword(s):  
Isotopic Composition ◽  
Earth Science ◽  
Oceanic Lithosphere ◽  
Relative Magnitude ◽  
Spreading Rate ◽  
Hydrothermal Systems ◽  
Iron Hydroxides ◽  
Geological Time ◽  
Ocean Ridges ◽  
Magnesium Loss

<p>The marked increase in seawater Mg/Ca during the Cenozoic is poorly understood, due to the limited availability of proxy data and uncertainty in elucidating the respective contributions of Mg sources and sinks through geological time<sup>1</sup>. Though established as a potentially large source of dissolved Mg over twenty years ago, the weathering of abyssal peridotites<sup>2</sup> is a largely unexplored potential source of Mg to oceanic budgets. The release of magnesium from peridotite weathering can occur in high temperature environments, during serpentinisation near the ridge axis<sup>3</sup>, as well as low temperature off-axis environments where peridotite and serpentinite are altered to clays, carbonates and silicates<sup>4</sup>. The relative magnitude of Mg fluxes from these sources are poorly constrained. Recent studies, however, now provide a general method for estimating bulk crustal lithologies of mid-ocean ridges based on spreading rate (i.e. proportion and mass of basalts, gabbros, peridotites and serpentinised peridotite) through time<sup>5</sup>—enabling us to quantitatively assess potential Mg contributions from these different environments.</p><p>We constructed a model for oceanic crustal weathering (proportional to depth below the seafloor) to develop estimates of the mass and isotopic composition of magnesium loss from peridotite during alteration in both high- and low-T environments. As Mg fractionation occurs predominantly in low-T reactions, the primary serpentinisation reaction in near-ridge environments is unlikely to result in isotopic differentiation. Comparably, the secondary low-T alterations, of both remaining peridotites (to clays and iron hydroxides) and serpentinite (e.g. to talc and dolomite) are likely to result in the fractionation of Mg. We extend our analysis to incorporate the fractionation of these systems<sup>4</sup> and their release of Mg into the ocean. We completed our analysis by presenting a compilation of fluid data for magnesium concentrations in ultramafic bodies from hydrothermal systems, in order to evaluate our model.</p><p><strong>References</strong></p><p>(1) Staudigel, H. "Chemical fluxes from hydrothermal alteration of the oceanic crust." (2014): 583-606.</p><p>(2) Snow, J.E. and Dick, H.J., 1995. Pervasive magnesium loss by marine weathering of peridotite. Geochimica et Cosmochimica Acta, 59(20), pp.4219-4235.</p><p>(3) Seyfried Jr, W.E., Pester, N.J., Ding, K. and Rough, M., 2011. Vent fluid chemistry of the Rainbow hydrothermal system (36 N, MAR): Phase equilibria and in situ pH controls on subseafloor alteration processes. Geochimica et Cosmochimica Acta, 75(6), pp.1574-1593.</p><p>(4) Liu, P.P., Teng, F.Z., Dick, H.J., Zhou, M.F. and Chung, S.L., 2017. Magnesium isotopic composition of the oceanic mantle and oceanic Mg cycling. Geochimica et Cosmochimica Acta, 206, pp.151-165.</p><p>(5) Merdith, A.S., Atkins, S.E. and Tetley, M.G., 2019. Tectonic controls on carbon and serpentinite storage in subducted upper oceanic lithosphere for the past 320 Ma. Frontiers in Earth Science, 7, p.332.</p>

Tracing the global consumption of carbon at subduction zones over the last 230 million years

2020 ◽  
Author(s):  
Ben Mather ◽  
Dietmar Müller ◽  
Tobias Keller
Keyword(s):  
Oceanic Crust ◽  
Volcanic Activity ◽  
Subduction Zones ◽  
Oceanic Lithosphere ◽  
Conveyor Belt ◽  
Tectonic Plates ◽  
The Past ◽  
Ocean Ridges ◽  
Plate Reconstruction

<p><span>Chemical heterogeneities in the mantle are typically introduced by recycling oceanic lithosphere through subduction, which transports volatiles into the mantle. The provenance of volatiles, such as carbon, with the down-going plate is varied; here we show how the </span><span>spatial </span><span>distribution of carbon evolves through time </span><span>with the motion of the tectonic plates</span><span>. Carbon is sequestered at mid-ocean ridges, as new oceanic crust forms, and is transported similar to a conveyor belt until it is recycled at subduction zones. We budget the amount of carbon that has been recycled at subduction zones over the past 230 million years using a global plate reconstruction. The present-day distribution of in-plate carbon,</span><span> taking into consideration the last 230 million years of plate influx, is predominantly distributed in the Atlantic. </span><span>In contrast, most of the carbon that was sequestered in Pacific seafloor from 230 Ma has since been subducted. Therefore, it is likely that the carbon stored in Pacific seafloor</span> <span>has played an important role in stimulating volcanic activity along the Ring of Fire.</span></p>

scholarly journals Kinematics and dynamics of the East Pacific Rise linked to a stable, deep-mantle upwelling

2016 ◽  
Vol 2 (12) ◽  
pp. e1601107 ◽  
Author(s):  
David B. Rowley ◽  
Alessandro M. Forte ◽  
Christopher J. Rowan ◽  
Petar Glišović ◽  
Robert Moucha ◽  
...  
Keyword(s):  
East Pacific Rise ◽  
Oceanic Lithosphere ◽  
Plate Motion ◽  
Mantle Flow ◽  
Plate Boundaries ◽  
Tectonic Plates ◽  
Ocean Ridges ◽  
East Pacific ◽  
The Pacific ◽  
Negative Buoyancy

Earth’s tectonic plates are generally considered to be driven largely by negative buoyancy associated with subduction of oceanic lithosphere. In this context, mid-ocean ridges (MORs) are passive plate boundaries whose divergence accommodates flow driven by subduction of oceanic slabs at trenches. We show that over the past 80 million years (My), the East Pacific Rise (EPR), Earth’s dominant MOR, has been characterized by limited ridge-perpendicular migration and persistent, asymmetric ridge accretion that are anomalous relative to other MORs. We reconstruct the subduction-related buoyancy fluxes of plates on either side of the EPR. The general expectation is that greater slab pull should correlate with faster plate motion and faster spreading at the EPR. Moreover, asymmetry in slab pull on either side of the EPR should correlate with either ridge migration or enhanced plate velocity in the direction of greater slab pull. Based on our analysis, none of the expected correlations are evident. This implies that other forces significantly contribute to EPR behavior. We explain these observations using mantle flow calculations based on globally integrated buoyancy distributions that require core-mantle boundary heat flux of up to 20 TW. The time-dependent mantle flow predictions yield a long-lived deep-seated upwelling that has its highest radial velocity under the EPR and is inferred to control its observed kinematics. The mantle-wide upwelling beneath the EPR drives horizontal components of asthenospheric flows beneath the plates that are similarly asymmetric but faster than the overlying surface plates, thereby contributing to plate motions through viscous tractions in the Pacific region.

scholarly journals Microseismicity and lithosphere thickness at a nearly amagmatic mid-ocean ridge

2021 ◽  
Author(s):  
Jie Chen ◽  
Wayne Crawford ◽  
Mathilde Cannat
Keyword(s):  
Hanging Wall ◽  
Oceanic Lithosphere ◽  
Spreading Rate ◽  
Southwest Indian Ridge ◽  
Flip Flop ◽  
Detachment Faults ◽  
Ocean Ridges ◽  
Tectonic Earthquakes ◽  
Mid Atlantic Ridge ◽  
Ocean Ridge

Abstract Successive flip-flop detachment faults in a nearly-amagmatic region of the ultraslow-spreading Southwest Indian Ridge (SWIR) at 64°30'E accommodate ~100% of plate divergence, with mostly ultramafic seafloor. As magma is the main heat carrier to the oceanic lithosphere, the nearly-amagmatic SWIR 64°30'E is expected to have a very thick lithosphere. Here, our microseismicity data shows a 15-km thick seismogenic lithosphere, actually thinner than the more magmatic SWIR Dragon Flag detachment with the same spreading rate. This challenges current models of how spreading rate and melt supply control the thermal regime of mid-ocean ridges. Microearthquakes with normal focal mechanisms are colocated with seismically imaged damage zones of the detachment and reveal hanging-wall normal faulting, which is not seen at more magmatic detachments at the SWIR or the Mid-Atlantic Ridge. We also document a two-day seismic swarm, interpret as caused by an upward-migrating melt intrusion in the detachment footwall (6-11 km), triggering a sequence of shallower (~1.5 km) tectonic earthquakes in the detachment fault plane. This points to a possible link between sparse magmatism and tectonic failure at melt-poor ultraslow ridges.

The structure of the oceanic lithosphere

1971 ◽  
Vol 268 (1192) ◽  
pp. 619-619
Keyword(s):  
Oceanic Crust ◽  
Upper Mantle ◽  
Oceanic Lithosphere ◽  
Ocean Floor ◽  
Mass Motion ◽  
Sea Floor ◽  
Ocean Ridges ◽  
Velocity Gradients ◽  
The Earth ◽  
Sea Floor Spreading

Sea-floor spreading requires that new ocean floor be generated at mid-ocean ridges and that along with the underlying oceanic crust it move laterally away from its site of generation. In so far as it is unlikely that the 5 km thick oceanic crust moves independently of the underlying upper mantle, the horizontal mass motion associated with spreading extends at least some way into the mantle. The lithosphere is the crust and that part of the upper mantle to which it is mechanically coupled; together they form the brittle and relatively ‘strong’ outermost part of the Earth; velocity gradients within the lithosphere are negligible.

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