HR: 0800h
AN: G11A-0767    [Abstracts]
TI: Extensional Strain Accommodation by Diking in the Transitional Main Ethiopian Rift
AU: * Keir, D
EM: d.keir@gl.rhul.ac.uk
AF: Geology Department, Royal Holloway University of London, Egham, TW20 0EX United Kingdom
AU: Ebinger, C
EM: c.ebinger@gl.rhul.ac.uk
AF: Geology Department, Royal Holloway University of London, Egham, TW20 0EX United Kingdom
AU: Stuart, G
EM: graham@earth.leeds.ac.uk
AF: School of Earth Sciences School of Earth Sciences School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Casey, M
EM: m.casey@earth.leeds.ac.uk
AF: School of Earth Sciences School of Earth Sciences School of Earth Sciences, University of Leeds, Leeds, LS2 9JT United Kingdom
AU: Daly, E
EM: e.daly@nuigalway.ie
AF: Department of Earth & Ocean Sciences, National University of Ireland , Galway, IRL Ireland
AU: Ayele, A
EM: atawon@yahoo.com
AF: Geophysical Observatory, Addis Ababa University, Addis Ababa, P.O.B 1176 Ethiopia
AB: Strain localizes as rifting proceeds to continental breakup, but the partitioning of strain between faults and magmatic intrusion remains controversial. We integrate new local seismicity data with fault slip, numerical models and geodetic data from the volcanically active Main Ethiopian Rift (MER) to evaluate strain partitioning in a rift transitional between continental and oceanic in style. Geodetic data show that strain has localized to 20 km-wide, 60 km-long 'magmatic segments' marked by aligned eruptive centers, dikes, and small offset normal faults within the central MER (Bilham et al, Geophys. Res. Lett., 27, 1999). From October 2001 to February 2003 over 2000 local earthquakes were recorded on 170 broadband seismic instruments deployed within the Northern MER and its uplifted rift flanks. Earthquake locations are estimated with the best 1-D and 3-D velocity models obtained from local earthquake tomography. The catalogue of earthquakes is complete above Ml - 2. Earthquakes cluster within the magmatic segments and the majority of events occur at depths of 6-10 km. Epicentres of small magnitude events (Ml $<$ 4) parallel faults that cut Quaternary - Recent lavas and aligned eruptive centres within the magmatic segments. Large offset border faults bounding the MER are inactive, excluding a dense cluster of events concentrated at the intersection of the 29 My Red Sea Rift and the 11 My MER, where the older Red Sea Rift flank is flexing into the younger MER. Historical events follow the same spatial patterns. Fault plane solutions and fault slip data show N100 extension, with some local variations for events beneath shield volcanoes. Thus, the distribution and mechanism of local and historical seismicity, as well as existing geodetic data show that strain is concentrated within the narrow magmatic segments, and not along border fault detachments. Local earthquake tomography shows 20 km-wide high velocity zones extending to the base of the seismogenic layer beneath magmatic segments, which we interpret as basaltic intrusions that feed dikes, fissures, and shallow magma chambers. We test these interpretations with elastic dislocation models of deformation above dikes, and compare results with seismicity, fault, and dike distributions. This integrated study shows that strain is accommodated by magmatic intrusion and small displacement faults. Current geodetic data is restricted to 1 profile and thus a dense GPS survey is needed to quantify and simulate strain partitioning.
DE: 9305 Africa
DE: 7230 Seismicity and seismotectonics
DE: 8109 Continental tectonics--extensional (0905)
DE: 1206 Crustal movements--interplate (8155)
DE: 1242 Seismic deformations (7205)
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting