HR: 15:25h
AN: V23C-08    [Abstracts]
TI: The internal structure of fault zones in basaltic sequences
AU: * Holland, M
EM: m.holland@ged.rwth-aachen.de
AF: Geologie - Endogene Dynamik, RWTH Aachen, Lochnerstrasse 4-20, Aachen, D-52056 Germany
AU: Urai, J L
EM: j.urai@ged.rwth-aachen.de
AF: Geologie - Endogene Dynamik, RWTH Aachen, Lochnerstrasse 4-20, Aachen, D-52056 Germany
AU: Martel, S
EM: smartel@hawaii.edu
AF: Dept. of Geology & Geophysics, University of Hawaii, 2525 Correa Rd, Honolulu, HI 96822 United States
AB: In contrary to most sedimentary rocks that need burial for consolidation, effusive basalts solidify quickly thus imposing a different mechanical behavior at the surface. Extensional stresses due to gravitational failure, caldera collapse or general tectonic forces generate prominent morphologies and large dilatant structures with impacts for hydraulic, mechanical and also bionomic aspects. In this study we present insights of field work on the Koa`e fault zone on Kilauea volcano/Hawai`i combined with the analysis of a scaled analogue model of normal faults in cohesive sequences. The Koa`e fault zone is a 12 km long normal fault zone connecting sections of the two rift zones. Unlike the predominantly mode-I cracks of the rift zones, the Koa`e faults show up to 20 m high sub-vertical fault scarps accompanied with footwall fissures. Open fractures, broken or buckled ramp structures and sub-vertical walls are the key elements in what is considered to be a volcanic growth fault system. Our analogue model visualizes the deformation of brittle flow units on top of a buried fault. The model uses dry hemihydrate powder with a tensional strength of 33 Pa, and a curved yield envelope. Depending on the rock prototype a scaling relationship of 1:5000-40000 is apparent. The faults initiate as sub-vertical mode-I fissuring at the surface propagating downward. Some of the open fissures on the footwall are deactivated; others evolve into faults producing the morphological scarps. A shallow antithetic fracture decouples a surface slap on the hanging wall producing the morphological ramps seen in the field. Its rotation is responsible for cavities and buckling. The internal structure of the shallow faults is open and filled partly with collapsing wall fragments that are progressively milled down at deeper levels. The model implies that normal faults in basalt are largely dilatant systems with a prominent mode-I component up to several meters magnitude. If the insights of the work are not only applicable to rift zones but also to normal faulting at Mid Oceanic Ridges, local hydrothermal systems could be focused to the open voids. Mineral precipitation as well as the formation of habitats within the faults is imaginable.
DE: 0450 Hydrothermal systems (1034, 3017, 3616, 4832, 8135, 8424)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3045 Seafloor morphology, geology, and geophysics
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005