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 ~150m 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 subsides from 150-200 m to 250-450 m within 1-2 km of the rise axis, and then remains at a relatively constant depth beneath the seafloor. 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 subseafloor conduits. Off-axis sill emplacement also contributes to the thickening of layer 2A. According to this model, the shallow crustal architecture is in place within 1-2 km of the rise axis. We suggest that the process of dike subsidence is controlled by the axial magma chamber (AMC), which we define as the melt lens and underlying mush zone. Within the neovolcanic zone, buoyancy forces associated with the AMC are supporting the extrusive layer and sheeted dikes. With distance from the rise axis, the AMC solidifies, the crust cools, the buoyancy forces are reduced, and the sheeted dike complex subsides. Concurrently, the extrusive layer thickens resulting in significantly less subsidence of the seafloor. The primary implication of this model is that dikes will subside to a greater depth for a robust magma chamber than for a weak magma chamber. A prominent deval is located at latitude 9°35'N, and this is coincident with our observations of a 50% decrease in dike subsidence as determined from MCS, WAP, conventional airgun refraction, and tomography data. Our subsidence model would predict that a relatively weak magma chamber is located at 9°35'N. A low magma supply at the devallocation is compatible with tomography and MCS seismic data. The decrease in layer 2A thickness suggests that the localized region of low magma supply has persisted for 175,000-275,000 years. Knowledge of shallow crustal structure is the key to understanding emplacement processes at mid-ocean ridges. Seismic studies at the fast-spreading East Pacific Rise indicate that each technique has advantages and disadvantages. On-bottom seismic refraction experiments can provide high-resolution determinations of upper crustal velocities, but only for limited areas. Conventional airgun refraction studies can extend the velocity structure to a larger region, at the expense of resolution. Multi-channel seismic and wide-angle profile data can map horizons in the shallow crust over large areas, but require good velocity information to be properly interpreted. Future work can ground truth seismic observations with observed lithology, and expand our knowledge of emplacement processes to intermediate-spreading and slow-spreading ridges.