Seismic imaging of oceanic detachment faulting

Opara-Nestor, Chisomaga AAzubike (2022). Seismic imaging of oceanic detachment faulting. University of Birmingham. Ph.D.

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Abstract

We understand plate tectonics by understanding relative plate motion at the plate boundaries, and the processes occurring there. Diverging plate boundaries are classified according to their spreading rate, with two broad categories as fast or slow-spreading. While fast-spreading is considered dominantly a magmatic process, faulting is much more important during slow spreading, leading to a far rougher, more rugged and more three-dimensional seafloor morphology which together with the large acoustic impedance contrast between basement and seawater, presents a severe seismic imaging challenge, with scattering of energy by the rough seafloor, velocity distortions from the rugged seafloor and strong side-coming events from the three-dimensional topography. Overcoming these challenges is critical to determine the geometry, extent, and mechanics of the faults, including large offset normal faults, called oceanic detachment faults (ODFs). As their footwalls - known as oceanic core complexes (OCCs) - consist of plutonic gabbros and mantle rocks, the fault must root beneath the crust. Their dimensions in the spreading direction indicate large offset, together suggesting that ODFs locally take up much of the plate divergence, but key questions remain about their geometry, mechanics, and lateral extent. This thesis addresses these issues and the challenge of seismic imaging of slow-spread crust in the 13° N area of the Mid-Atlantic ridge through a study of two oceanic detachment faults (13°20’ N and 13°30’ N, shortened hereafter to 1320 and 1330). A processing scheme – consisting of downward continuation to collapse side-swipe diffractions, followed by amplitude muting to remove them, deconvolution and velocity filtering processes - was developed to allow 2D seismic data to resolve the fine structure of the detachments, to suppress side-coming events, to reveal the geometry of ODFs in depth, and to determine the lateral extent of the detachments and the interaction between neighbouring detachments. The fine structure of the ODF was resolved to be anastomosing subsurface features, consistent with the latest ideas for the origin of the corrugated surfaces. Depth imaging shows that ODF steepen smoothly downwards from the low-angle of the exposed OCC to dips of ~60° at depths of ~5km, projecting to the bands of micro-earthquakes observed, a geometry everywhere consistent with slip-angle allowed by rock mechanics. Finally, Imaging on four intersecting profiles outline the extent of the two ODFs (1320 and 1330) in the slip and isochron direction showing the 1320 ODF cuts across and deeper than the 1330, in agreement with micro-earthquake data that show it is active while 1330 is not but suggesting they could have been linked in the past.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):