Seismic constraints on shallow crustal emplacement processes at the fast spreading East Pacific Rise

Abstract

We present the results of nine on‐bottom seismic refraction experiments carried out over young East Pacific Rise crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high‐resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC) over “zero‐age” crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low‐velocity (<2.5 km/s) layer, a 100 to 150‐m‐thick transition zone with velocities increasing by ∼2.5 km/s, and a layer with velocities of ∼5 km/s at a depth beneath the seafloor of ∼130–190 m. The surficial low‐velocity layer and transition zone are defined as seismic layer 2A, and the ∼5 km/s layer is defined as the top of layer 2B. Both the surficial low‐velocity layer and the transition zone double in thickness within ∼1 km of the rise axis. We model layer 2A as the extrusive sequence and transition zone and the 2A/2B boundary as the top of the sheeted dikes. The primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from ∼150 m to ∼300 m within 1 km of the ASC. The thickening of the extrusive layer is interpreted to be due to lava that either overflows the ASC walls, is emplaced through eruptions outside of the ASC, or travels laterally from the ASC through conduits. The most probable cause for the thickening of the transition zone is sill emplacement outside of the ASC, either from magma that does not reach the surface in an off‐axis eruption or magma that is transported laterally during the drainage process creating the ASC. We suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2A.

Breaking new ground: Estimates of crack depth along the axial zone of the East Pacific Rise (9°12′–54′N)

“…The commonly assumed Poisson ratio for oceanic crust, based on the laboratory measurements of Christensen [19], is 0.3 (yielding VJV, = 1.9). However, the amplitude modeling of the on-bottom seismic refraction data of Christeson et al [20] near 9"30'N indicates a Poisson ratio at the seafloor of at least 0.43 (i.e., V,/V, > 3). This value is similar to other determinations of Poisson's ratio for young oceanic crust, which fall in the range 0.39-0.46 .…”

“…For this study, a Poisson ratio of 0.43 is adopted as the preferred value (Table 1). We also evaluate the consequences of using a Poisson ratio of 0.48, which is the preferred value of Christeson et al [20], based on high-resolution seismic measurements in our study area. …”

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

“…Four cross axis seismic profiles (Fig. 2-2c, 2-3b-three Common Depth Profiles (CDP) and one wide angle profile (WAP) profile [Vera and Diebold, 1994]) and several other shorter profiles (conventional airgun data [Christeson et al, 1996] and refraction study profiles [Christeson et al, 1994]) show that seismic Layer 2A thickness at the axis decreases from north to south (Fig. 2-4c The source layer thickness required to produce the along-axis variation in the peak CAMH can be calculated using magnetic moment, which is the product of magnetization intensity and source layer thickness.…”

The influence of magma supply and eruptive processes on axial morphology, crustal construction and magma chambers

“…in expanding spread profiles [20] at 9°35'N, and in on-bottom refraction studies [27] at 9°30' N. Table 1 shows the on-axis and off-axis thicknesses and the width of the 2A accretion zone -the distance from the axis over which layer 2A thickens to the mean off-axis thickness -for different seismic experiments on the northern and southern EPR. Moving off the rise crest, layer 2A is observed to approximately double in thickness over a distance of a few kilometers.…”

“…The underlying crust, with velocities > -5 km/s and a very gradual velocity gradient, corresponds to layer 2B [27]. The base of the high gradient region is imaged in multichannel seismic reflection (MCS) experiments as a shallow wide angle reflection formed by refracted energy turning in the steep gradient [24,28,29].…”