The structure of mature oceanic crust: Tectonic features revealed in superfast-spread Cocos plate by multichannel seismic grids and swath bathymetry

Multi-channel seismic (MCS) investigations have revealed much concerning the structure of seafloor spreading centers, mainly along the fast-spreading East Pacific Rise (EPR), but the emphasis on young fast-spread seafloor represents a significant experimental bias, as there exist relatively few modern MCS reflection studies of “normal” oceanic plate distant from spreading centers, transform faults and known hotspots. This dissertation presents an analysis and interpretation of MCS data collected in March–April 1999 in three grids spaced along a 15–17 Ma flowline of the central Cocos plate and constitutes part of a renewed effort to investigate accretion and evolution processes across a wide range of plate spreading rates. The crust studied in this experiment accreted during an episode of superfast spreading at the EPR, and the westernmost area (15 Ma age) was selected by the Ocean Drilling Program (ODP) for a major drilling operation (site 1256, Leg 206). MCS data contain an abundance of strong seismic reflectors with a predominance of isochron dip orientation, unprecedented in view of numerous reflection studies near the EPR where no similar structures have been observed. The analysis suggests upper crustal reflectors are small-slip thrust faults/fractures, possibly driven by differential thermoelastic stress. These fractures may have important implications for hydrothermal flow, alteration and advective heat transport in the oceanic lithosphere.; Chapter 1 of the dissertation provides an overview of crustal reflection seismology and related faulting studies. Chapter 2 covers the experiment, methods and an overview of the acquired seafloor bathymetry and MCS data. Chapter 3 is a detailed study of the westernmost grid area, which was the highest drilling priority and contained several exceptionally bright reflectors. This chapter synthesizes bathymetry, MCS and synthetic seismogram modeling to argue a small-slip faulting/fracturing origin for kilometer-scale dipping structures. Chapter 4 is an integrated study of each grid (Areas 1–3) along the central flowline. A consistent reflection pattern is demonstrated along the central flowline with the aid of a statistical description. Chapter 5 is an application of pre-stack imaging techniques to several prominent dipping reflectors.