Structure and variability of oceanic crust on the flanks of the East Pacific Rise between 11° and 13°N

Abstract

We use over 500 seismograms collected by five ocean bottom seismometers to examine the structure and variability of 0.5-m.y.-old crust in the Rivera Ocean Seismic Experiment (ROSE) area of the East Pacific Rise between 11 ø and 13øN. There are no significant differences among the first arrival travel times for 10 independent refraction line segments along the isochron. Amplitude patterns recorded on the individual lines, however, suggest that structural boundaries within the upper crust vary by hundreds of meters in depth over lateral distances of tens of kilometers within this 200-km segment of lithosphere. On the basis of the modeling of amplitude patterns at ranges of 5-10 km and of comparisons with the upper crust sampled at Deep Sea Drilling Project hole 504B and in ophiolite complexes, layer 2 can be divided into three units: 2A corresponds to extrusive volcanics with a steep velocity gradient, 2B to extrusives with a near-zero velocity gradient, and 2C to a complex region of transition from extrusives to sheeted dikes. The total thickness of layer 2 varies by about 0.7 km over the length of the line. Another amplitude peak which varies in range between 14 and 21 km may be related to the downward transition from sheeted dikes to isotropic gabbros within layer 3. The variability of these two amplitude patterns provides direct evidence that the along-strike structure of oceanic crust in the ROSE area is definably heterogeneous and that the process of crustal accretion is likewise variable on the scale resolved by amplitude and travel time information.

Geophysical characterization of mantle melting anomalies: A crustal view

“…In general, the V p /density gradient of Layer 2 largely reflects lithological variations in the uppermost crust and diminishing alteration and porosity/fracturing of igneous rocks with depth (Bratt and Purdy, 1984;Detrick et al, 1994). The physical properties of Layer 3 and those of the underplated bodies are thus considered to be a good proxy for those of the mostly unaltered, crack-free gabbroic rocks that probably predominate at lower crustal levels (e.g.…”

Seismic and magnetic constraints on the structure of upper oceanic crust at fast and slow spreading ridges

“…Since Houtz and Ewing [1976] first documented an age-dependent increase in the velocity of the uppermost crust, several studies have confirmed this relationship. In the Pacific alone, these studies include conventional refraction [Spudich and Orcutt, 1980;Purdy, 1982;Bratt and Purdy, 1984;Bratt and Solomon, 1984;McClain and Atallah, 1985;Bee and Bibee, 1989;, borehole [Stephen and Harding, 1983;Little and Stephen, 1985;Duennebier et al, 1987;Whitmarsh et al, 1987;Collins et al, 1989], single-ship Tolstoy et al, 1997] and two-ship multichannel [Harding et ai., 1989;Vera et ai., 1990], and on-bottom seismics [Christeson et ai., 1994a]. Sensitivity to velocities immediately beneath the seafloor varies between methods.…”

“…Crustal Age (Myr) [Linear] Extrusive V vs. Age (<l) [Purdy, 1982;Bratt and Purdy, 1984;Bratt and Solomon, 1984], Capricorn (I» [McClain and Atallah, 1985], Hole 504B (0) [ Stephen and Harding, 1983;Little and Stephen, 1985;Collins et al, 1989], ESP studies at the northern EPR (_) [Harding et al, 1989;Vera et al, 1990], multichannel studies at the southern EPR (T) Tolstoy et al, 1997], EXCO (.) , NOBEL (~) [Christeson et al, 1994a], and this study (*).…”

“…Since that time, numerous seismic experiments along the fast-spreading East Pacific Rise (EPR) have combined to make it the most extensively studied ridge in the world. They include refraction experiments (ROSE [Bratt and Purdy, 1984;Ewing and Purdy, 1982;Purdy, 1982;Fischer and Purdy, 1986], MAGMA [McClain et al, 1985], EXCO and tomographic studies [Burnett et al, 1989;Toomey et al, 1990;Caress et al, 1992;Toomey et al, 1994]), single-ship multichannel seismics (MCS) Harding et al, 1993;Kent et al, 1994;Carbotte et al, 1997;Hooft et al, 1997a], two-ship wide aperture profiles (WAP) [Vera and Diebold, 1994;Kappus et al, 1995], two-ship expanding spread profiles (ESP) [Harding et al, 1989;Vera et al, 1990], and, more recently, on-bottom seismics [Christeson et al, 1992[Christeson et al, , 1996. In combination, these methods have been a powerful tool in our effort to understand the structure of upper oceanic crust.…”

Modeling of the Sedimentary Interbedded Basalt Stratigraphy for the Idaho National Laboratory Probabilistic Seismic Hazard Analysis

“…Figure 5 shows the basalt velocity-depth relationship with the measured seismic velocities. For comparison, the P-wave velocities of INL basalts are more consistent with that of ocean floor basalts (e.g., Purdy 1982;Bratt and Purdy 1984;Wilkins et al 1991) Basalt and Rhyolite Density-Depth Relationship -The starting basalt density was anchored to the average density calculated for basalt measurements at the near surface. The density-depth relationship was extended from this value at the surface to a depth of 1100 m using basalt density measurements in drill holes 2-2A, INEL-1, and WO-2 (Figure 6).…”