HR: 13:40h
AN: V12G-01 INVITED     [PDF]
TI: Effects of Mechanical Layering on Dike Emplacement, Faulting, and Surface Deformation in Rift Zones
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Department of Structural Geology and Geodynamics, Geoscience Centre, University of Gottingen, Goldschmidtstrasse 3, Gottingen, D-37077 Germany
AU: Fjeldskaar, I
EM: Ingrid.Fjeldskaar@geo.uib.no
AF: Department of Earth Science, University of Bergen, Allegaten 41, Bergen, N-5007 Norway
AB: All rift zones in volcanic islands contain normal faults and dikes as their main structural elements. During rifting episodes there is normally fault slip, graben development and dike emplacement. Field and geodetic studies, however, show that most dikes emplaced in rift zones do not reach the surface but rather become arrested at certain crustal depths. Geodetic measurements of the surface deformation during rifting episodes are routinely used to model the depth to the tips, as well as the geometries, of arrested dikes. Most inverse models to infer the geometries of arrested dikes assume the rift zone to be a homogeneous, isotropic, elastic half space. All rift zones in volcanic islands, however, contain normal faults and other discontinuities, and consist of rock layers that often have contrasting mechanical properties, such as soft pyroclastic rocks and stiff basaltic lava flows. To explore the effects of mechanical layering on fault slip and dike emplacement during a rifting episode, many boundary-element models were run. In these models, the rift zone already has a graben, represented by two (boundary) normal faults, dipping at 70$\deg$ toward the center of the rift zone. The rift zone is taken to be 10 km thick. In all the models the lower tips of the boundary faults are at the depth of 4 km below the surface; in some models the upper fault tips extend to a depth of 1.5 km below the surface, in others all the way to the surface. The rift zone is composed of alternating stiff (high Young's modulus) and soft (low Young's modulus) layers. In most models, the only loading is the internal magmatic overpressure that drives the dike. The first models indicate that the stresses generated by a dike propagating vertically up toward the bottom part of a graben tend to open up the boundary faults of the graben. However, when the upper tip of the dyke reaches the same crustal level as the bottom tips of the boundary faults, the magmatic overpressure associated with the dike forces the faults to close and subsequently, as the dike tip continues its propagation up into the graben, encourages reverse slip on these normal faults. The faults remain closed until the magmatic overpressure is relaxed. For the faults extending to the surface, the reverse slip generates a horst. The second models show that soft layers and a weak contact at shallow depths suppress dike-tip tensile stresses and encourage dike arrest. Soft layers and a weak contact also suppress the surface stresses and deformation induced by arrested dikes and encourage transfer of the surface tensile stresses to the regions above the lateral ends of the weak contact. In these models, the tensile stress at the rift-zone surface in a large area above the arrested dike itself is very small, but rises above the lateral ends of the weak contact, many kilometers from the dike tip. Thus, for a dike arrested in a layered rift zone, straightforward inversion of surface-deformation data may yield geometric results that have little relation to the actual geometry of the arrested dike. The predictions of the numerical models are generally supported by field results from the volcanic rift zones of Tenerife (Canary Islands) and Iceland.
DE: 8010 Fractures and faults
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8164 Stresses--crust and lithosphere
DE: 8434 Magma migration
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting