HR: 17:45h
AN: T34B-08    [Abstracts]
TI: Incipient seafloor spreading in the Ethiopian rifts: Analogue for crustal formation at slow-spreading ridges?
AU: * Ebinger, C
EM: cindy@gl.rhul.ac.uk
AF: Royal Holloway University of London, Department of Geology, Egham, TW20 0EX United Kingdom
AU: Keir, D
EM: d.keir@gl.rhul.ac.uk
AF: Royal Holloway University of London, Department of Geology, Egham, TW20 0EX United Kingdom
AU: Kidane, T
EM: tesfakida@geol.aau.edu.et
AF: Addis Ababa University, Department of Geology and Geophysics, PO Box 1176, Addis Ababa, AA0 Ethiopia
AU: Rowland, J
EM: j.rowland@auckland.ac.nz
AF: University of Auckland, Private Bag 92019, Auckland, 1004 New Zealand
AU: Casey, M
EM: m.casey@earth.leeds.ac.uk
AF: University of Leeds, School of Earth Sciences, Leeds, LS2 9JT United Kingdom
AB: New seismic, gravity, magnetotelluric, structural, remote sensing, petrological, paleomagnetic, and geodetic studies in the highly evolved Main Ethiopian rift, East Africa, reveal a continental rift at the transition to seafloor spreading (Project EAGLE). Plate kinematic data predict an opening velocity of ~6 mm/yr between Nubia and Somalia, or, ultra-slow seafloor spreading. The objectives of our study are to examine the initiation of along-axis segmentation and the process of crustal accretion within a sub-aerial setting, using this active rift as an analogue for slow-spreading ridge processes. Clearly, some aspects will be unique to the passive margin setting. Set within the rift valley defined by largely aseismic Miocene border faults lie ca. 60 km-long, 20 km-wide zones of magma intrusion, extrusion, and normal faulting. These localised zones of faulting and magmatism (magmatic segments) show a segmentation independent of the border fault segmentation, and are interpreted as incipient oceanic spreading centres. Extension is accommodated by magma intrusion into the lower crust, and faulting and diking in the brittle upper crust. Shields with ephemeral shallow crustal magma reservoirs occur near the tips of magmatic segments; these reservoirs and extracted felsic melts may help maintain the along-axis segmentation. We model new and existing gravity and topography profiles across these magmatic segments, using crustal tomography and magneto-telluric models to constrain the distribution of surface and subsurface loads on the plate. Seismic anisotropy, seismicity, and magnetotelluric data provide snapshots of the distribution of melt in the system. Observations from the Main Ethiopian and older, more evolved southern Red Sea rifts indicate that seaward-dipping volcanic sequences initiate in moderately stretched continental crust above a narrow zone of persistent magma intrusion; they mark the onset of slow-spreading along passive continental margins.
DE: 8109 Continental tectonics: extensional (0905)
SC: Tectonophysics [T]
MN: Fall Meeting 2005