HR: 13:30h
AN: G23C-01    [Abstracts]
TI: How extensional structures at divergent plate boundaries become oblique to the geodetic spreading vector
AU: * Gudmundsson, A
EM: Agust.Gudmundsson@gwdg.de
AF: Geoscience Centre, University of Gottingen, Goldschmidtstr. 3, Gottingen, 37077 Germany
AB: Since the establishment of the regional plate-tectonic spreading vectors using geodetic measurements, it has become apparent that many extensional structures at divergent plate boundaries are oblique to the associated spreading vectors. In fact, entire ridges, such as the Mohns Ridge and the Reykjanes Ridge, are oblique to their spreading vectors. The spreading vector normally coincides with the regional direction of sigma 3, the minimum principal compressive (maximum tensile) stress. Since extensional structures should, as a rule, strike perpendicular to sigma 3, the fact that most are oblique to a particular spreading vector implies that the local direction of sigma 3, which largely controls the strike of extensional structures, cannot be parallel to the spreading vector. None of the volcanic zones of Iceland are perpendicular to the regional geodetic spreading vector. Most of the extensional structures within these zones are also oblique to the vector. For example, the general geodetic spreading vector in the Iceland area is about N105° E. By contrast, the general trend of the North Volcanic Zone is about N02° E, that of the West Volcanic Zone N37° E, and that of the East Volcanic Zone N45° E. Thus, the minimum angle between the spreading vector and the volcanic zones is about 77° for the North Volcanic Zone, about 68° for the West Volcanic Zone, and as little as 60° for the East Volcanic Zone. Here I do not consider the Snaefellsnes Volcanic Zone, trending roughly E-W and thus making an angle of about 15° to the spreading vector. The main extensional structures associated with the volcanic zones, particularly the rift zone, are tension fractures, normal faults, dikes, and feeder dikes (volcanic fissures at the surface). Many of these extensional structures occur in swarms, referred to as volcanic systems (fissure swarms being parts thereof), many of which are non-parallel to axes of the plate-boundary segments within in which they occur. To explain the oblique extensional structures and volcanic systems, many boundary-element and finite-element models have been made. The numerical models indicate, first, that the principal stresses, hence the direction of sigma 3, commonly rotate at contacts between the volcanic zones and the surrounding Pleistocence and Tertiary rocks. This rotation is primarily due to two factors. One factor is the difference in mechanical properties (particularly Young's moduli) between the rocks that constitute the volcanic zones and those that form the Pleistocene and Tertiary lava pile. The other factor is the type of loading: here, the spreading vector which is oblique with respect to the trends of the volcanic zones. Second, the numerical models also show that the local stresses within volcanic systems and associated composite volcanoes may be very different from those of the volcanic zones themselves. These local stresses are partly attributable to shallow chambers and deeper reservoirs that are associated with the volcanic systems. Partly, however, the local stresses are due to differences in the mechanical properties of the rocks that constitute the active volcanic systems and those that constitute the other parts of the volcanic zones. The magma chambers as well as the young lava flows and pyroclastic rocks, particularly in the composite volcanoes, act as soft inclusions that generate local stress fields which may be very different from the associated regional stress fields.
DE: 1206 Crustal movements--interplate (8155)
DE: 3210 Modeling
DE: 8010 Fractures and faults
DE: 8020 Mechanics
DE: 8164 Stresses--crust and lithosphere
SC: Geodesy [G]
MN: 2005 Joint Assembly