Three‐dimensional seismic structure and physical properties of the crust and shallow mantle beneath the East Pacific Rise at 9°30'N

Abstract

Abstract. The seismic structure of the crust and shallow mantle beneath the East Pacific Rise near 9ø30'N is imaged by inverting P wave travel time data. Our tomographic results constrain for the first time the three-dimensional structure of the lower crust in this region and allow us to compare it to shallow crustal and mantle structure. The seismic structure is characterized by a low-velocity volume (LVV) that extends from 1.2 km depth below the seafloor into the mantle. The cross-axis width of the LVV is narrow in the crust (5-7 km) and broad in the mantle (-18 km). Although the width of the top of the LVV is similar to previous estimates, its narrow shape at lower crustal depths and its significant widening in the mantle are previously unknown features of the rise velocity structure. In the rise-parallel direction the LVV varies in magnitude such that the lowest velocities are located between two minor rise axis discontinuities near 9ø28'N and 9ø35'N. From the seismic results we estimate the thermal structure and melt distribution beneath the rise. The thermal structure suggests that heat removal is relatively efficient throughout the crust yet inefficient at Moho and mantle depths. Estimates of the melt distribution indicate that magma accumulates at two levels in the magmatic system. One is at the top of the magmatic system and is capped by the shallow melt lens detected by seismic reflection surveys; the other is within the Moho transition zone and topmost portion of the mantle. The highest melt fractions occur within the upper reservoir, whereas the lower reservoir contains a lower melt fraction distributed over a broader area. By volume, however, there may be up to 40% more melt in the lower reservoir than in the upper reservoir. Along-axis variations in crustal melt content are similar to those in the mantle, supporting the hypothesis that the mantle, midway between the 9ø28'N and 9ø35'N devals, is presently delivering greater amounts of melt to the lower crust than to regions immediately to the north or south. We see no evidence (from seismic anisotropy) for diapiric mantle flow, suggesting that solid-state flow and melt migration are decoupled in the shallow mantle. Our results are not compatible with models that require a large, segment-scale redistribution of melt within the crust. Instead, our results imply that crustal magma chambers are replenished at closely spaced intervals along the rise.

Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

“…6d). We note that Maclennan et al's hydrothermal models employed adjustable heat extraction rates to match the thermal structures derived from the 2-D seismic tomography in Dunn et al (2000). Instead, using permeability to explicitly regulate porous hydrothermal flows in the thermal modeling, Cherkaoui et al (2003) suggested that a permeability of ∼4 × 10 −14 m 2 or greater is required to reproduce the thermal structure of Dunn et al (2000).…”

“…A schematic illustration of crustal formation beneath fast-spreading mid-ocean ridges. Thin white curves are isotherms from Dunn et al (2000) showing the thermal structure derived from seismic tomography assuming the effects of anharmonicity and anelasticity. Blue curves with arrows indicate hydrothermal circulations near the ridge axis for efficiently removing heat across the entire crust.…”

“…However, high-porosity melt channels also likely appear in the mush zone for delivering relatively primitive melts to the ridge axis (e.g., Natland and Dick, 2009). Assuming a termination temperature of 1000 • C, the mush zone may extend to ∼1.5-3.5 km away from the ridge axis according to the thermal structure of Dunn et al (2000; white curves in Fig. 7).…”

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…An axial magma chamber (AMC) has been detected -1.2-2 km beneath the axis in the surveyed area and the continuity of the AMC reflector is disrupted at several locations along-axis (e.g., at 90 17'N and 90 53'N), suggesting possible individual magma systems [Detrick et al, 1987;Kent et al, 1993; Vera and Diebold, 1994; Toomey et al, 1994] or locally enhanced mantle melt supply [Dunn et al, 2000]. Above the AMC, in the upper crust, the reflection from the base of seismic Layer 2A is observed to increase in depth from -0.2 km at the axis to -0.5 km at 1-2 km off axis [Christeson et al, 1992;Harding et al, 1993;.…”

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).…”