HR: 15:25h
AN: S52G-07    [PDF]
TI: Faults, Fissures and Flexures: Upward Propagating Normal Faults in the Koka Magmatic Segment, Main Ethiopian Rift System.
AU: * Rowland, J V
EM: j.rowland@auckland.ac.nz
AF: Department of Geology The University of Auckland, Private Bag 92019, Auckland, 1701 New Zealand
AB: Where do normal faults nucleate? The intra-rift structural fabric of the Main Ethiopian Rift System (MER) provides constraints on normal fault growth models, with a recent interpretation arguing for nucleation in the near-surface consequent on the downward propagation of surface fissures. Examples of genetically related fissures and normal faults occur within the Koka Magmatic Segment near its southern termination against the Asela Border Fault. Considerable along-strike variations in displacement are evident, resulting in flexure of the rift floor. Ramps, breached ramps, hanging wall anticlines and footwall synclines indicate that normal faults have grown by segment linkage. Fault spacing, throw, and back tilting decrease toward the intra-rift axial zone, which comprises subparallel grabens separated by narrow, occasionally synclinal, horsts. Fissures on the order of 1 to several meters wide occur within these grabens and have similar length scales to primary segment lengths revealed by linkage structures along the normal faults. Commonly, fissures are located at the top of narrow monoclinal flexures trending parallel to the axial graben. Where a throw ($<$ 1-2m) occurs across the fissure, the associated flexure is more subdued. Monoclinal flexures continue for some distance (generally $<$ a few 10s of meters) beyond the lateral tips of the associated fissure, though their curvature reduces significantly. Partially filled fissures of similar width occur at the foot of steep normal fault scarps with throws of $>$ 20m. Thus, there appears to be an evolution from small-offset fissure to large-offset fault. The top-down fault nucleation model invokes fault drag as a consequence of higher friction at the fissure-fault transformation depth to explain the observed surface flexure. However, examples of monoclines without fissures or with small poorly connected fissures occur both along the graben floor and within hanging walls of large normal fault splays, suggesting that in some cases flexure development precedes fissure development. This may arise as a consequence of upward propagation of a blind normal fault nucleated at depth, which induces warping and associated fissuring in cover materials; or alternatively, as a consequence of localised sub-surface mass transport into or out of the axial zone.
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
DE: 8109 Continental tectonics--extensional (0905)
SC: Seismology [S]
MN: 2003 Fall Meeting