Crustal deformation studies in subaerial parts of the world oceanic rift system: Iceland and Afar.

摘要

Crustal deformation in Iceland and Afar, both subaerial parts of the oceanic rift system, is studied for a better understanding of tectonic processes and rheology in these areas. This thesis evaluates the mechanics of the 1991 Hekla eruption in Iceland, rheology and glacio-isostatic crustal movements around the Vatnajokull ice cap in Iceland, and strain accommodation between overlapping spreading centers in Afar.; Between January 17 and March 11, 1991, 0.15 km{dollar}sp3{dollar} of lava erupted initially from several radial fissures and subsequently from a single fissure on the SE flank of the Hekla volcano in Iceland. Hekla is within an array of geodetic control points that were measured in 1989 using the Global Positioning System and re-measured during and after the eruption. Changes in inter-station vectors indicate the eruption was associated with a surface deflation of 0.1{dollar}sbsp{lcub}-0.04{rcub}{lcub}+0.08{rcub}{dollar} km{dollar}sp3{dollar} centered on the volcano (63.995{dollar}spcirc{dollar}N{dollar}sbsp{lcub}-3{rcub}{lcub}+4{rcub}{dollar} km, 19.69{dollar}spcirc{dollar}W{dollar}sbsp{lcub}-2{rcub}{lcub}+1.5{rcub}{dollar} km). The depth to the magma reservoir is 9{dollar}sbsp{lcub}-7{rcub}{lcub}+6{rcub}{dollar} km, but is poorly constrained due to the absence of GPS control points close to the volcano.; Measurements of the lake level of Lake Langisjor at the SW edge of the Vatnajokull ice cap in Iceland indicate a tilt of 0.26 {dollar}pm{dollar} 0.06 {dollar}mu{dollar}rad/year away from the ice cap between 1959-1991. The tilt is too large to be explained as an elastic response of the earth to ice retreat during this century or caused by change in the gravitational pull of the ice cap. Rather, it can be explained by sub-lithospheric viscous adjustment when the sub-lithospheric viscosity is 1 {dollar}times{dollar} 10{dollar}sp{lcub}18{rcub}{dollar}-5 {dollar}times{dollar} 10{dollar}sp{lcub}19{rcub}{dollar} Pa s. Regional subsidence in historical times in SE Iceland can similarly be attributed to viscous adjustment resulting from the increased load of Vatnajokull during the Little Ice Age. A GPS geodetic network was installed in the area to monitor further elevation changes.; In August 1989 a sequence of m{dollar}sb{lcub}rm b{rcub}{dollar} 5-6 normal faulting earthquakes occurred in the area between the Ghoubbet Asal-Manda Inakir and Manda Haroro-Abhe Bad rifts in central Afar. Relocation of the events suggest they occurred on major faults in the Dobi graben located midway between the principal rift zones. The earthquakes are evidence that extensional strain is still occurring in the area. A previously proposed "bookshelf" tectonic model for central and southern Afar has been modified to account for ongoing extension in the area between the principal rifts as well as distributed shear.