Flexural uplift of a lithospheric slab near the Vema Transform (Central Atlantic): timing and mechanisms

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 118 N in the Atlantic. It rises abruptly about 140 km from the axis of theMid-Atlantic Ridge (MAR) in ~10 Myr old crust and runs continuously up to ~25 Myr old crust. It reaches over 3 km above the predicted lithospheric thermal contraction level. It is absent in crust younger than 10 Myr; thus, the uplift of the VTR must haveended roughly 10 Ma. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that wasgenerated at an 80 km long MAR segment. Based on satellite gravimetry imagery this MAR segment was born roughly 50 Ma andincreased its length at an average rate of 1.6 mm/yr. Multibeam data show that the MAR-parallel sea floor fabric south of the VTRshifts its orientation by 58 to 108 clockwise in ~11–12 Myr old crust, indicating a change at that time of the orientation of the MARaxis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and 10 Ma 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 lithosphericsection (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagiccarbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that theentire VTR rose vertically as a single block within the active transform offset. A 50 km long portion of the crest of the VTR roseabove sea level, subsided, was truncated at sea level and covered by a carbonate platform. Subaerial and submarine erosion hasgradually removed material from the top of the VTR and has modified its slopes. Spreading half rate of the crust south of thetransform decreased from 17.2 mm/yr between 26 and 19 Ma to ~16.9 mm/yr between 19 and ~10 Ma, to ~13.6 mm/yr from 10 Ma to present. The slowing down of spreading occurred close in time to the change in ridge/transform geometry, suggesting that the two events are related. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that theextent of the uplift depends on the thickness of the brittle layer, consistent with the observed greater uplift of the older lithospherealong the VTR.