Seismic constraints on shallow crustal processes at the East Pacific Rise

Abstract

This thesis is concerned with understanding how oceanic crust is emplaced at mid-ocean ridges. The emphasis is upon fast-spreading ridges, and the use of seismic techniques to image the uppermost several hundred meters of the crust. We present the results of nine on-bottom seismic refraction experiments carried out over young East Pacific Rise (EPR) 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 100-150 m thick transition zone with velocities increasing by approximately 2.5 km/s, and a layer with velocities of -5 km/s at a depth below 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 as layer 2B. Both the surficial low-velocity layer and the transition zone double in thickness within -1 km of the rise axis, with the depth to the 2A/2B boundary increasing from -150 m to 300-350 m over this range. The doubling of layer 2A thickness within 1-2 km of the rise axis is confirmed by multi-channel seismic (MCS) and wide-angle profile (WAP) data, which also indicate that there is no further systematic change in thickness with greater range from the rise axis.Inversions for attenuation structure demonstrate that the layer 2A/2B interface is not only a velocity boundary, but also an attenuation boundary, with Q increasing from 10-20 within layer 2A to >70 in layer 2B.The results of MCS and wide-angle experiments over plausible velocity structures are predicted quantitatively, based on velocity models constructed from on-bottom seismic refraction experiments and expanding spread profiles. We conclude that the accuracy of correlating the prominent shallow reflector observed in MCS and WAP data with the layer 2A/2B boundary is strongly dependent on the structure within layer 2A. If layer 2A consists of a surficial low-velocity layer overlying a steep velocity gradient (our gradient model), then there is an excellent correspondence between the two-way travel times to the shallow reflector and the base of layer 2A. However, the shallow reflector may follow structure within layer 2A if the upper crust contains more than one high-gradient region (our step model). A shallow structure similar to the step model is consistent with onbottom refraction experiments and expanding spread profiles located over zero-age EPR crust. With distance from the rise axis, this step-like structure is apparently destroyed, and is converted into a single steep gradient similar in appearance to our gradient model. Layer 2A is interpreted to be composed of the extrusive section and transition zone, with layer 2B consisting of the sheeted dike complex. This implies that the top of the dikes subs...

Tomographic imaging of the shallow crustal structure of the East Pacific Rise at 9°30′N

“…On the basis of these observations we infer that off-axis magmatism along this section of the EPR affects the thickness of the volcanic section and perhaps the thermal structure of young oceanic crust. In particular, the variations in structure witlain 600 m of the seafloor support the view that off-axis volcanism has increased the thickness of layer 2A in the south and west of the study area, an interpretation also supported by the results of Harding et al [1993] and Christeson [1994]. The lower velocities observed to the west of the rise at 1-2 km depth are attributed to elevated temperatures resulting from either increased off-axis magmatism or a decrease in the efficiency of hydrothermal cooling.…”

“…Under the premise that velocity anomalies within approximately 600 m of the seafloor are a result of variations in the thickness of the volcanic section, the tomographic results suggest that this extrusive layer thickens from north to south and from east to west. In support of this interpretation, Christeson [1994] reports that the thickness of layer 2A…”

“…The OBHs and the DOBHs recorded almost all, and the OBSs about 80%, of the explosive seismic sources. In addition, the continuously recording AOBHs provided records of nearby microearthquakes [Wilcock et al, 1992a] and several air gun profiles [Christeson, 1994]. Delay time seismic tomography relies on small differences among observed travel times to detect changes in structure.…”

Seismic structure and indicators of magma budget along the Southern East Pacific Rise

“…Locally, between 17øS and 17ø30'S, Mutter et al [1995] report that the magma sill becomes shallower and wider as the ridge axis becomes shallower and broader and use deviations from this expected relationship to suggest very recent or ongoing magmatic activity near 17ø26'S. Seismic layer 2A in young oceanic crust is usually interpreted to be the volcanic extrusive section [e.g., Christeson, 1994;Hooft et al, 1996]. It has been suggested that the thickness of the seismic layer 2A reflects the magma budget [Harding et al, 1993].…”

“…Locally, between 17øS and 17ø30'S, Mutter et al [1995] report that the magma sill becomes shallower and wider as the ridge axis becomes shallower and broader and use deviations from this expected relationship to suggest very recent or ongoing magmatic activity near 17ø26'S. Seismic layer 2A in young oceanic crust is usually interpreted to be the volcanic extrusive section [e.g., Christeson, 1994 [Detrick et al, 1993] EPR that are not related to changes in the depth or cross-sectional area of the ridge. Lonsdale [1983Lonsdale [ , 1986Lonsdale [ , 1989a has questioned the correlation between segmentation of the magmatic system with that of the axial morphology and suggests that small overlapping spreading centers (OSCs) may be underlain by a single, wide crustal magma chamber.…”