Graham Kerford

B.Sc. (Honours) Thesis

STRUCTURE OF THE MOHOROVIČIĆ DISCONTINUITY (MOHO) AT THE EAST PACIFIC RISE FROM 3D MULTICHANNEL SEISMIC REFLECTION DATA

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The purpose of this thesis is to provide a detailed interpretation of the Mohorovičić discontinuity (MOHO) at the East Pacific Rise (EPR) using 3D multichannel seismic data collected between 9°37.5’N and 9°57’N in 2008. The MOHO was picked throughout the survey area extending for ~10 km on the western flank and ~12 km on the eastern flank of the ridge. Observations of the MOHO indicate that it is comprised of multiple terraces with single reflections closer to the ridge, becoming increasingly impulsive further away from the ridge. Low amplitude, off lapping reflections (OLRs) are observed to off lap from the outer edge of the terraces extending out over the deeper terraces. These OLRs extend over large swaths of the lower crust and have been interpreted as sills that are periodically injected from the upper mantle into the lower crust near the ridge axis. It is hypothesized that the sills are formed from magma that has built up at the MOHO due to an increase in lithostatic pressure. Later, when the lithostatic pressure is reduced, the magma is injected into the lower crust forming sills. Questions still remain regarding the formation and periodic nature of the terraces and the injection of sills into the lower crust. Seven terraces were observed on the eastern flank and 6 on the western flank of the ridge with a maximum crustal age of ~260 ka and 216 ka, respectively indicating that they both have a formation frequency of ~37 ka. This suggests that sea level changes associated with the 41 ka Milancović cycle may influence the timing of the formation of the terraces and sills.

Keywords: MOHO, East Pacific Rise, 3D Multi Channel Seismic Survey, terraces, off lapping reflections
Pages: 67
Supervisor: Mladen R. Nedimović