Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

Abstract

[1] We present swath bathymetric, gravity, and magnetic data from the Mid-Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge-hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow-spreading mid-ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since $1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least $1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by $4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis-parallel seismic line than a model that includes additional along-axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.

The dynamics of oceanic transform faults : constraints from geophysical, geochemical, and geodynamical modeling

“…Here we present a systematic study of the residual mantle Bouguer gravity anomaly of 19 oceanic transform faults that reveals a strong correlation between gravity signature and spreading rate. Previous studies have shown that slow-slipping transform faults are marked by more positive gravity anomalies than their adjacent ridge segments 1,2,4,6 , but our analysis reveals that intermediate and fast-slipping transform faults exhibit more negative gravity anomalies than their adjacent ridge segments. This finding indicates that there is a mass deficit at intermediate and fast-slipping transform faults, which could reflect increased rock porosity, serpentinization of mantle peridotite, and/or crustal thickening.…”

“…Slow-spreading mid-ocean ridge segments exhibit significant crustal thinning towards transform and non-transform offsets [1][2][3][4][5][6][7][8][9][10][11][12] , which is thought to arise from a three-dimensional process of buoyant mantle upwelling and melt migration focused beneath the centres of ridge segments 1,2,[4][5][6][7]9,10,12 . In contrast, fast-spreading midocean ridges are characterized by smaller, segment-scale variations in crustal thickness, which reflect more uniform mantle upwelling beneath the ridge axis [13][14][15] .…”

Crustal structure and kinematics of the TAMMAR propagating rift system on the Mid-Atlantic Ridge from seismic refraction and satellite altimetry gravity

“…The systems are therefore fundamentally different. An example of a migrating oblique discontinuity is the northward propagating Mid-Atlantic Ridge segment between 8 • S and 9 • S, whose morphology, structure and tectonics are described in detail by Bruguier et al (2003) and Minshull et al (2003). The discontinuity has a small offset (10-15 km) and there is no sheared zone, giving strong evidence that older lithosphere is not ruptured as the discontinuity migrates.…”

Eos, Transactions, American Geophysical Union Volume 86, Number 22, 31 May 2005

“…Segment A3 is the shallowest segment, with crustal thicknesses up to 11 km [Bruguier et al, 2003]. The axial region is characterized by extensive sheet flows and the virtual absence of "hummocky terrain, " which is the surface expression of pillow volcanoes and a typical feature of slow spreading ridges.…”

“…Previous studies [Bruguier et al, 2003;Minshull et al, 1998] had divided this area into four major segments, A1-A4 (see Figure 1). The widely varying topography and crustal thicknesses suggested that rates of magma supply differ greatly from one segment to another.…”