Evolution Of Oceanic Lithosphere Research Articles

What Lies Beneath: The Formation and Evolution of Oceanic Lithosphere

What Lies Beneath: The Formation and Evolution of Oceanic Lithosphere

Accurate focal depth determination of oceanic earthquakes using water-column reverberation and some implications for the shrinking plate hypothesis

Investigation of oceanic earthquakes is useful for constraining the lateral and depth variations of the stress and strain-rate fields in oceanic lithosphere, and the thickness of the seismogenic layer as a function of lithosphere age, thereby providing us with critical insight into thermal and dynamic processes associated with the cooling and evolution of oceanic lithosphere. With the goal of estimating hypocentral depths more accurately, we observe clear water reverberations after the direct P wave on teleseismic records of oceanic earthquakes and develop a technique to estimate earthquake depths by using these reverberations. The Z–H grid search method allows the simultaneous determination of the sea floor depth (H) and earthquake depth (Z) with an uncertainty less than 1 km, which compares favorably with alternative approaches. We apply this method to two closely located earthquakes beneath the eastern Pacific. These earthquakes occurred in ∼25 Ma-old lithosphere and were previously estimated to have similar depths of ∼10–12 km. We find that the two events actually occurred at dissimilar depths of 2.5 km and 16.8 km beneath the seafloor, respectively, within the oceanic crust and lithospheric mantle. The shallow and deep events are determined to be a thrust and normal earthquake, respectively, indicating that the stress field within the oceanic lithosphere changes from horizontal deviatoric compression to horizontal deviatoric tension as depth increases, which is consistent with the prediction of lithospheric cooling models. Furthermore, we show that the P-axis of the newly investigated thrust-faulting earthquake is perpendicular to that of the previously studied thrust event, consistent with the predictions of the shrinking-plate hypothesis.

Imaging the seismic lithosphere-asthenosphere boundary of the oceanic plate

[1] The seismic lithosphere-asthenosphere boundary (LAB) or G discontinuity, a seismologically characterized abrupt drop in wave speed in the uppermost mantle, is one of the key issues in current geodynamics. Although plate tectonics started as a theory for the ocean, reports on LAB for normal oceanic regions are scarce due to paucity of seismic data, and whether or not the oceanic LAB grows with age is the key issue to be resolved. We conduct a systematic survey for the oceanic LAB using S-to-p converted seismic waves along three margins of oceanic plates whose crustal age ranges from ∼10 Myr to ∼130 Myr, and we observe laterally continuous oceanic LAB images. The thickness of the oceanic plate estimated from LABs increases with the plate age, though scattered, suggesting that the evolution of oceanic lithosphere is predominantly governed by temperature and that the oceanic seismic LAB represents a boundary that grows with age.

Mission Moho: Formation and evolution of oceanic lithosphere

The formation and evolution of the oceanic lithosphere is the dominant process in the chemical differentiation and physical evolution of our planet. Plate tectonic processes completely repave the ocean basins every 100–200 million years. Lithosphere formation encompasses the transfer and transformation of material and energy from Earth's mantle to the crust and from the crust to the ocean and atmosphere. Independent of sunlight, the evolving ocean crust supports life in unique seafloor and subseafloor habitats that may resemble Earth's earliest ecosystems. From its formation until its return to the mantle by subduction, the evolving oceanic lithosphere interacts with seawater, sequesters water and other materials, and ultimately recycles them back into the mantle.

Chondritic osmium isotopic composition of late Archean convecting upper mantle: Evidence from Zunhua podiform chromitites, Hebei, North China

Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os contents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sections of ophiolites. The podiform chromite is one of the robust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic melange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the187Os/l88Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (> 300 ng/g), and their187Os/188Os ratios range from 0.11021 to 0.11030, averaging 0.11026±0.00005 (σ), equivalent to a γOs=∼-0.12 ±0.06 at 2.6 Ga, which means that the Os isotopic composition of convecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.

Constraints on lithospheric thermal structure for the Indian Ocean from depth and heat flow data

Models for the thermal evolution of oceanic lithosphere are primarily constrained by variations in seafloor depth and heat flow with age. These models have been largely based on data from the Pacific and Atlantic Ocean basins. We construct seafloor age relations for the Indian Ocean which we combine with bathymetric, sediment isopach and heat flow data to derive curves for depth and heat flow versus age. Comparison of these curves with predictions from three thermal models shows that they are better fit by the shallower depths and higher heat flow for the GDH1 model, which is characterized by a thinner and hotter lithosphere than previous models.

Symposium on ophiolite genesis and evolution of oceanic lithosphere

The recently published Special Publication No 49 of the Geological Society of London The Geology and Tectorlics of the Oman Region edited by A. H. F. Robertson, M. P. Searle and A. C. Ries (Feb. 1990) resulted largely from a six year AMOCO-funded project concentrating mainly on the structure, strat atigraphy and sedimentology of the platform margin and basin sediments beneath the Semail ophiolite. This study, and more particularly the BRGM-French project (1982–1990) formed the basis of the ophiolite conference. It was held at the Sultan Qaboos University, Sultanate of Oman, 7–18 January 1990, and was the first, major, international geological conference to be held in Oman. Over 300participants from 27 countries attended. Since the classic work of Glennie et al. (1974) and the Shell team, the Semail ophiolite in the Oman Mountains has been widely regarded as the largest, best exposed and most spectacular example of an obducted ophiolite complex anywhere in the world. Subsequent field projects, including the Open University Oman Ophiolite project (1975–1985) summarized by Lippard et al. (1986) and the University of California-USGS project (1977–1979) published in a special issue of Journal of Geophysical Research (1981) vol. 86 , concentrated mainly on the Semail ophiolite complex. Since 1982, the Bureau de Recherches Geologiques et Minieres (BRGM) have remapped large areas of the central and southern mountains and 13 maps at 1: 100,OOO scale have been published. Together with new detailed studies of the Semail ophiolite, some of the French teams’ results have been published in a

Oman: the nex Skaergaard?

Terra NovaVolume 2, Issue 1 p. 5-7 Oman: the nex Skaergaard? Conference report: Symposium on ophiolite genesis and evolution of oceanic lithosphere. Muscat, Sultanate of Oman, 7–18 January 1990. Paul Browning, Paul BrowningSearch for more papers by this author Paul Browning, Paul BrowningSearch for more papers by this author First published: January 1990 https://doi.org/10.1111/j.1365-3121.1990.tb00030.xAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Volume2, Issue1January 1990Pages 5-7 RelatedInformation

Shear wave travel time residuals from oceanic earthquakes and the evolution of oceanic lithosphere

Shear wave travel time residuals have been measured for 25 oceanic earthquakes, including normal faulting events on ridge crests and intraplate events. When the residuals are plotted against the age of the lithosphere in which the event occurred, they show a decrease of 6±1 s from ridge crest to oceanic lithosphere 100 m.y. old. This decrease is caused principally by an increase in shear velocity within the upper mantle as a function of lithospheric age. Study of depth phases from oceanic earthquakes indicates that all such events are shallower than 10 km below the sea floor, so focal depth variations are not an important contributor to the travel time residual versus age relationship. Modeling of seismic velocities in the upper mantle, using a standard thermal model for the spreading lithosphere and published phase diagrams for dry and wet gabbro-eclogite and peridotite, was conducted in an attempt to produce a residual curve similar to that observed. Comparison of the predicted and observed residuals indicates that the mantle cannot be dry but is unable to distinguish between the wet systems, principally because of the large number of assumptions involved in modeling velocity changes across phase boundaries. However, the wet gabbro-eclogite model is unable to match local shear velocities in the oceanic lithosphere obtained from Sn wave velocities and inversion of surface wave phase velocity, and a peridotitic upper mantle is favored. A strong azimuthal variation of residual exists for an event along the Gibbs fracture zone, in approximate agreement with the velocity versus age models. Shear wave residuals for events on the Indian plate are systematically less by 2 s than residuals for earthquakes in lithosphere of comparable age in the Atlantic or Pacific; this offset may result from the rapid northward motion of the Carlsberg and southeast Indian ridges with respect to the mesosphere.