The East Pacific Rise in cross section: A seismic model

Abstract

In 1982 we undertook a seismic refraction experiment, known as the MAGMA expedition, to examine the detailed structure of the East Pacific Rise near 12°50′N. This segment of the rise, where the full spreading rate is about 110 mm/yr, is near the projected trace of the O'Gorman fracture zone and is the site of an overlapping spreading center as well as “black smoker” hydrothermal activity. In this paper we describe the analysis of a subset of the travel time data collected during the MAGMA expedition, namely the data from profiles which were oriented normal to the rise axis. These profiles provide a data set roughly equivalent to those collected on other experiments and sample the cross‐sectional structure of the rise. We have modeled these data using a two‐dimensional ray‐tracing program. We have found that the seismic velocities in the young oceanic crust are rather high, with velocity gradients of 4.0–5.5 s−1 in the uppermost crust. The highest velocities at the seafloor occur beneath the rise axis itself and seem to decrease as the crust ages to 0.1 Ma. This decrease in velocity must result from an increase in porosity in the upper crust and may coincide with the development of abundant surface fissures as the crust spreads. The decrease in velocity does not appear to penetrate deeper than about 0.5 km and may reverse itself as hydrothermal alteration fills the pores and cracks. The fact that the highest velocities occur under the rise axis suggests that the hydrothermal circulation responsible for the black smokers is confined to seismically unresolved conduits, a result consistent with the high temperatures and discrete nature of the vents. Our best model includes a magma chamber some 4 km wide and extending from the Moho to within about 1.1 km of the seafloor. This magma chamber is far smaller than many models for the rise axis have predicted but larger than those inferred from seismic refraction experiments at other sites on the East Pacific Rise. These discrepancies probably arise because the magma chamber under the East Pacific Rise is not a steady state feature but changes with time because of hydrothermal cooling and perhaps because of an episodic supply of magma from the mantle or along the rise axis.

The seismic attenuation structure of the East Pacific Rise

“…Both the ESP data [Vera et al, 1990;Vera and Diebold, 1991] and delay time tomographic inversion [Toomey et al, 1991a] (Figure 1.2a) resolve high nearsurface velocities along the rise axis at 9 0 30'N. Such a feature is also indicated by similar studies at 13*N [McClain et al, 1985;Burnett et al, 1989;Harding et al, 1989;Caress et al, 1992]. On-bottom seismic experiments [Christeson et al, 1991a, b] resolve axial velocities of 5.0-5.5 km/s at depths of 100-200 m below the seafloor, values which contrast with maximum velocities of about 4.5 km/s observed in the uppermost 300 m of crust off axis.…”

“…McClain et al [1985] suggest that such a change may be a consequence of a reduction in near-surface porosity resulting from the filling of voids by the alteration products of passive off-axis hydrothermal circulation [e.g., Lister, 1981;Fisher et al, 1990]. Indeed, such a mechanism is generally accepted as the explanation for a large increase in layer 2A velocities observed in much older crust [Houtz and Ewing, 1976;Jacobson, 1992].…”

“…First, the decrease in velocities may result from an increase in porosity due to faulting [McClain et al, 1985;Burnett et al, 1989;Caress et al, 1992]. Second, the thickness of the extrusive layer may increase off-axis due to blanketing by volcanic flows that erupt from a narrow injection zone [Toomey et al, 1990a].…”

“…However, the vertical extent and velocities of the low-velocity anomaly detected by the refraction experiments were not consistent with a very magma body. Indeed later refraction experiments which included rise-crossing paths [Lewis and Garmany, 1982;Bratt and Solomon, 1984;McClain et al, 1985] placed strong limits on the maximum dimensions of axial magma bodies. More recently, a large body of seismological data including extensive multi-channel reflection profiles [Detrick et al, 1987;Mutter et al, 1988;Kent et al, 1990;Vera et al, 1990], expanding spread profiles [Harding et al, 1989;Vera et al, 1990], and two delay-time tomographic experiments [Burnett et al, 1989;Toomey et al, 1990a;Caress et al, 1992] has resulted in a much better understanding of the upper crustal structure along the axis of the East Pacific Rise between 9*N and 13*N. A thin, continuous, 1-2 km-deep magma lens is present along large segments of the rise axis [Detrick et al, 1987].…”

Variations in structure and tectonics along the Mid-Atlantic Ridge, 23 degress N and 26 degrees N

“…Evidence for systematic changes in velocity with distance from the ridge axis have been documented by a number of authors [e.g., Christensen and Salisbury, 1972;McClain et al, 1985;Toomey et al, 1988;. The magnitudes of the velocity changes were based on the results obtained within the MAR median valley at 23°N, where up to a 1 km/s increase in velocity over a 6-km horizontal distance from the ridge axis was resolved from a tomographic inversion of P wave travel time residuals [Toomey et al, 1988]; we have used their results for the variations to the west of the axis, which are larger, since we want to investigate a limiting case scenario.…”