A 20-Myr record of creation of oceanic lithosphere at a segment of the central Mid-Atlantic-Ridge is exposed along an upliftedsliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry ofthe exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of ,3–4 Myrsuperimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustalthickness indicates that the solid mantle is upwelling at an average rate of ,25mmyr21, but this appears to vary through time.Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axissegmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadilyhotter over the past 20 Myr, possibly owing to north–south mantle flow.The oceanic lithosphere covers two-thirds of our planet: understanding how it forms and evolves is a major challenge in the Earth sciences. It is generally agreed that the oceanic lithosphere forms along mid-ocean ridges, where mantle material upwells and undergoes decompression and partial melting. The melt rises rapidly and freezes, producing the crust, while the peridotitic residue forms the lithospheric mantle. Mid-ocean-ridge topography, structure and composition indicate that near zero age processes of lithosphere formation vary along ridge axis1–3. Less is known, however, of how these processes vary through time, a question important for our understanding of how ocean basins evolve. Variations through time of the thermal regime and/or composition of a mid-ocean ridge should be recorded in lithosphere lying at increasing distances from the ridge axis along sea-floor spreading flow lines. However, older lithosphere is normally covered by sediment and not easily accessibleto high-resolution observation and sampling.An uplifted sliver of oceanic lithosphere (Fig. 1), exposing in thecentral Atlantic an ,20-Myr-long record of creation of lithosphereat a segment of the Mid-Atlantic Ridge (MAR), gave us theopportunity to investigate the temporal variability in the formationof lithosphere, to estimate the upwelling velocity of the mantlebelow the ridge, and to determine whether passive or dynamicmodels of creation of oceanic lithosphere prevail at slow-spreadingridges. A sliver of exposed oceanic lithosphere A major topographic anomaly, the Vema transverse ridge, runs onthe southern side of the Vema transform, which offsets the MAR by 310 km at ,118N (Fig. 1). Its exposed northern scarp (that is, the southern wall of the transform valley) reaches in height up to 4 km above the valley floor (Fig. 2). Submersible4, multibeam5 and seismic reflection data, and bottom samples (Fig. 1) revealed that the northern side of the Vema transverse ridge exposes a ,3–4-km thick, relatively complete and tectonically undeformed upperlithospheric

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere / Bonatti, E; Ligi, M; Brunelli, Daniele; Cipriani, Anna; Fabretti, P; Ferrante, V; Gasperini, L; Ottolini, L.. - In: NATURE. - ISSN 0028-0836. - STAMPA. - 423:(2003), pp. 499-505.

Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere.

BRUNELLI, Daniele;CIPRIANI, Anna;
2003

Abstract

A 20-Myr record of creation of oceanic lithosphere at a segment of the central Mid-Atlantic-Ridge is exposed along an upliftedsliver of lithosphere. The degree of melting of the mantle that is upwelling below the ridge, estimated from the chemistry ofthe exposed mantle rocks, as well as crustal thickness inferred from gravity measurements, show oscillations of ,3–4 Myrsuperimposed on a longer-term steady increase with time. The time lag between oscillations of mantle melting and crustalthickness indicates that the solid mantle is upwelling at an average rate of ,25mmyr21, but this appears to vary through time.Slow-spreading lithosphere seems to form through dynamic pulses of mantle upwelling and melting, leading not only to along-axissegmentation but also to across-axis structural variability. Also, the central Mid-Atlantic Ridge appears to have become steadilyhotter over the past 20 Myr, possibly owing to north–south mantle flow.The oceanic lithosphere covers two-thirds of our planet: understanding how it forms and evolves is a major challenge in the Earth sciences. It is generally agreed that the oceanic lithosphere forms along mid-ocean ridges, where mantle material upwells and undergoes decompression and partial melting. The melt rises rapidly and freezes, producing the crust, while the peridotitic residue forms the lithospheric mantle. Mid-ocean-ridge topography, structure and composition indicate that near zero age processes of lithosphere formation vary along ridge axis1–3. Less is known, however, of how these processes vary through time, a question important for our understanding of how ocean basins evolve. Variations through time of the thermal regime and/or composition of a mid-ocean ridge should be recorded in lithosphere lying at increasing distances from the ridge axis along sea-floor spreading flow lines. However, older lithosphere is normally covered by sediment and not easily accessibleto high-resolution observation and sampling.An uplifted sliver of oceanic lithosphere (Fig. 1), exposing in thecentral Atlantic an ,20-Myr-long record of creation of lithosphereat a segment of the Mid-Atlantic Ridge (MAR), gave us theopportunity to investigate the temporal variability in the formationof lithosphere, to estimate the upwelling velocity of the mantlebelow the ridge, and to determine whether passive or dynamicmodels of creation of oceanic lithosphere prevail at slow-spreadingridges. A sliver of exposed oceanic lithosphere A major topographic anomaly, the Vema transverse ridge, runs onthe southern side of the Vema transform, which offsets the MAR by 310 km at ,118N (Fig. 1). Its exposed northern scarp (that is, the southern wall of the transform valley) reaches in height up to 4 km above the valley floor (Fig. 2). Submersible4, multibeam5 and seismic reflection data, and bottom samples (Fig. 1) revealed that the northern side of the Vema transverse ridge exposes a ,3–4-km thick, relatively complete and tectonically undeformed upperlithospheric
423
499
505
Mantle thermal pulses below the Mid-Atlantic Ridge and temporal variations in the formation of oceanic lithosphere / Bonatti, E; Ligi, M; Brunelli, Daniele; Cipriani, Anna; Fabretti, P; Ferrante, V; Gasperini, L; Ottolini, L.. - In: NATURE. - ISSN 0028-0836. - STAMPA. - 423:(2003), pp. 499-505.
Bonatti, E; Ligi, M; Brunelli, Daniele; Cipriani, Anna; Fabretti, P; Ferrante, V; Gasperini, L; Ottolini, L.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11380/583637
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