Jökull


Jökull - 01.12.1979, Side 27

Jökull - 01.12.1979, Side 27
Fig. 18. Major intrusions in SE-Iceland unrelated to central volcanoes of the lava pile may have formed in the roots of later volcanoes superimposed on the lava pile. Their relation to the axial rift zone of their day was probably similar as that of the Öraefajökull-Snaefell line of volcanoes to the cur- rently active axial rift zone. Based on information from Helgi Torfason. graben formátion is lacking, indicating that crustal separation along the volcanic axis is negligible, and the extra load of the volcanic edifices is compen- sated by isostatic subsidence on a more regional scale. The Snaefellsnes volcanic chain has a trend transverse to the axial rift zones. The fault pattern has been interpreted variously in terms of horizon- tal north-southerly extension or right lateral shear which tears up three parallel volcanic swarms (Fig. 17). A fourth subparallel fissure swarm connects the Snaefellsnes transverse chain to the north- Langjökull region. In southern Iceland the flank zone continues southwards from the intersection of the axial rift zone with the South-Iceland seismic zone. From an analysis of the tectonic fracture pattern associated with the Surtsey and Heimaey eruptions, both within the Vestmannaeyjar volcanic field, a conju- gate shear set with faults trending NNE-SSW and ENE-WSW has been inferred. The orientation of the volcanic axes of Hekla, Tindfjallajökull and Eyjafjallajökull (ENE-WSW or E-W) follows the same general trepd. The southern part of the eastern volcanic zone, here referred to as a flank zone, may thus be the result of sinistral shear movement with the axis of maximum compression oriented nearly N 45° E, and the axis of maximum tension acting NW-SE, subparallel to the spreading direction. The Öraefajökull-Snaefell line of strato-vol- canoes is located in the eastern Vatnajökull region some 40—50 km east of the main rift axis, and two volcanoes on this line are partly (Esjufjöll) or wholly (Breidabunga) buried by the ice sheet. Snaefell is ridge-shaped sitting on a short volcanic axis but Öraefajökull (2119 m), the only one active in Postglacial time, is almost circular. Both lack the fissure swarms characteristic of the axial rift zones. These volcanoes overlie an older basement that has been eroded before Öraefajökull started to form. The rocks erupted are slightly transitional as far as is known. It is possible that similar volcanoes existed in this part of Iceland in Tertiary time, also offset eastwardly relative to the main spreading axis (Fig. 18). Evidence of this is seen in the large gabbro and granophyre intrusions of southeastern Iceland of up to 20 km2 in area. They bear no obvious relation to the build up of the surrounding lava pile having been intruded during a later magmatic period, and have no dyke or sheet swarms localized about them. They most likely formed in the roots of stravo-volcanoes that grew on top of the lava pile removed from the zone of active spreading. Tectonics of segments connecting the axial rift zones The axial rift zones in Iceland connect with the submerged ocean ridges along transverse structures which were originally defined as fracture zones from earthquake distribution patterns. They dis- play a variety of tectonic features along their strike including oblique segments of the neovolcanic zones with pronounced en echelon features as well as older fractured crust. The connecting segment between the Reykjanes Peninsula and the eastern volcanic zone lies across an eroded pile of Plio-Pleistocene rocks dipping to the northwest and north. The fault pattern indi- cates a component of sinistral shear and the zone as a whole has been referred to as the South-Iceland seismic zone. It has a slightly more east-westerly trend than that branch of the neovolcanic zone which lies along the Reykjanes Peninsula. The fault pattern in its western part is also very similar, with SW-NE trending en echelon fissures. Farther east N-S and ENE-WSW trending conjugate faults predominate, some of them having lateral offset. JÖKULL 29. ÁR 25
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