The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 11ø N in the Atlantic. It rises abruptly about 137 km from the axis of the Mid Atlantic Ridge (MAR) in ~10 My old crust and runs continuously up to ~ 25 My old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 My; thus, the uplift of the VTR must have ended roughly 10 My. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere. Multibeam data show that the MAR-parallel seafloor fabric south of the VTR shifts its orientation by 5ø to 10ø clockwise in ~ 11-12 My old crust, indicating a change at that time of the orientation of the MAR axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 My ago by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the entire VTR rose vertically as a single block within the active transform offset. Erosion has gradually removed material from the top of the VTR and has modified its slopes. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the extent of the uplift depends on the thickness of the brittle layer, consistent with the observed topography of the VTR. Spreading half rate of the crust south of the transform decreased from 17.2 mm/yr between 26 and 19 My ago to ~16.9 mm/yr between 19 and ~10 My ago, to ~13.6 mm/yr from 10 My ago to present. The slowing down of spreading occurred close in time to the change in ridge/transfo
Flexural uplift of a lithospheric slab near the Vema transform (Central Atlantic): timing and mechanism / Ligi, M.; Bonatti, E.; Brunelli, Daniele; Buck, R. W.; Cipriani, Anna. - In: EOS. - ISSN 0096-3941. - ELETTRONICO. - 85 (47):(2004), pp. Abstract T44A-06-Abstract T44A-06. (Intervento presentato al convegno AGU Fall Meeting tenutosi a S. Francisco CA, USA nel Dec. 2004).
Flexural uplift of a lithospheric slab near the Vema transform (Central Atlantic): timing and mechanism
BRUNELLI, Daniele;CIPRIANI, Anna
2004
Abstract
The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the Vema transform at 11ø N in the Atlantic. It rises abruptly about 137 km from the axis of the Mid Atlantic Ridge (MAR) in ~10 My old crust and runs continuously up to ~ 25 My old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 My; thus, the uplift of the VTR must have ended roughly 10 My. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere. Multibeam data show that the MAR-parallel seafloor fabric south of the VTR shifts its orientation by 5ø to 10ø clockwise in ~ 11-12 My old crust, indicating a change at that time of the orientation of the MAR axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 My ago by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the entire VTR rose vertically as a single block within the active transform offset. Erosion has gradually removed material from the top of the VTR and has modified its slopes. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the extent of the uplift depends on the thickness of the brittle layer, consistent with the observed topography of the VTR. Spreading half rate of the crust south of the transform decreased from 17.2 mm/yr between 26 and 19 My ago to ~16.9 mm/yr between 19 and ~10 My ago, to ~13.6 mm/yr from 10 My ago to present. The slowing down of spreading occurred close in time to the change in ridge/transfo
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