Skew of mantle upwelling beneath the East Pacific Rise governs segmentation

Abstract

Mantle upwelling is essential to the generation of new oceanic crust at mid-ocean ridges, and it is generally assumed that such upwelling is symmetric beneath active ridges. Here, however, we use seismic imaging to show that the isotropic and anisotropic structure of the mantle is rotated beneath the East Pacific Rise. The isotropic structure defines the pattern of magma delivery from the mantle to the crust. We find that the segmentation of the rise crest between transform faults correlates well with the distribution of mantle melt. The azimuth of seismic anisotropy constrains the direction of mantle flow, which is rotated nearly 10 degrees anticlockwise from the plate-spreading direction. The mismatch between the locus of mantle melt delivery and the morphologic ridge axis results in systematic differences between areas of on-axis and off-axis melt supply. We conclude that the skew of asthenospheric upwelling and transport governs segmentation of the East Pacific Rise and variations in the intensity of ridge crest processes.

Sill to surface: Linking young off-axis volcanism with subsurface melt at the overlapping spreading center at 9°03′N East Pacific Rise

“…The overlapping spreading center (OSC) at 9103′N EPR presents an ideal opportunity for studying the relationships between volcanism and magmatism because the subsurface melt distribution and seafloor morphology have recently been well-characterized in a number of geophysical and geological studies (e.g., Bazin et al,a low velocity anomaly interpreted as melt in the uppermost mantle extending across the OSC, with the lowest velocities centered beneath the eastern limb (Dunn et al, 2000;Toomey et al, 2007), melt accumulation in the overlying crust (Singh et al, 2006), and a mid-crustal mush zone at the northern end of the overlap basin at~9108′N (Bazin et al, 2003;Crawford and Webb, 2002). In addition, a 3-D seismic reflection study reveals a~4 km wide, upper-crustal melt sill beneath the eastern limb that extends westward into the northern portion of the overlap basin (Kent et al, 2000).…”

Thickening of young Pacific lithosphere from high-resolution Rayleigh wave tomography: A test of the conductive cooling model

“…Third, seismic velocity anomalies extend to greater depth and change more rapidly with distance from the East Pacific Rise axis than expected if simple conductive cooling from above is the primary control on structure (Dunn and Forsyth, 2003;Forsyth, 1977;Hammond and Toomey, 2003). Finally, the presence of pronounced Earth and Planetary Science Letters 278 (2009) [96][97][98][99][100][101][102][103][104][105][106] asymmetries across spreading centers (Evans et al, 1999;Forsyth et al, 1998;Key and Constable, 2008;Toomey et al, 2007) suggests that flow in the mantle is more complex than is assumed in the simple models. As pointed out by many of these authors and others, the presence of water and/or melt in the asthenosphere and a dehydrated lithosphere following extraction of melt are probably necessary to explain the pattern of velocity and conductivity anomalies.…”

Geochemical variability in MORB controlled by concurrent mixing and crystallisation

“…Lonsdale, 1989;Toomey et al, 2007), it remains unclear how surface tectonics might drive or respond to the mantle based processes mentioned above and so contribute to creating coherent geochemical structure in suites of basalts on >100 km scales. The discussion above focuses on understanding the onset of resolvable CMC at ∼300 km.…”