Jökull


Jökull - 01.12.1980, Side 14

Jökull - 01.12.1980, Side 14
ation of the latter, fault breccias and intensive secondary mineralization at the faults, and a juxtaposition at the faults of rock suites of varying age and dips. An age gap of 5 m. y was inferred across the Húsavík faults. Sœmundsson suggested that the Tjörnes beds were probably formed in a subsiding trough initiated approximately north of Eyjafjöröur as the Eastern zone became established. A new spreading axis across the Tjörnes Fracture Zone, the Grímsey shoal, then poured lavas over the sediment filled trough during the upper Gauss epoch. Finally, opening up of the Axarfjörður spreading zone produced lava flows of late Matuyama epoch age, covering the Breiðavík beds, which had been deposited in a minor tectonic depression, a forerunner of the Axarfjörður trough. An analysis of earthquakes and general features of the Tjörnes Fracture Zone was published by Ward (1970, 1971). Scháfer (1972) inferred a right lateral transform fault in the area. Active volcanism north of Tjörnes has been reported by Thoroddsen (1902, 1925), Wolff (1931), and Thorarinsson (1965). P. Ein- arsson (1976) analyzed the seismicity of the Tjörnes Fracture Zone and demonstrated that the transform motion between the Kolbeinsey Ridge and the Eastern zone occurs along three or more parallel strike slip faults. Walker (1975) did not accept the view that sectors of the rift zone had been offset by northwesterly trending faults and postulated an en echelon arrangement of spreading sectors from the Eastern zone to Tjörnes. McMaster et al. (1977) studied the insular shelf off North Iceland. They interpreted the arrangement of volcano-tectonic features within the Tjörnes Fracture Zone as a transient or leaky type of transform fault and stated that Walker’s en echelon rift zone configuration was confirmed by their studies. Recent contributions to the geological history of the Eastern zone (Aðal- steinsson 1974, Watkins et al. 1975, Alberlsson 1976, Watkins and Walker 1977) indicate that volcanism in the Eastern zone has been virtu- ally continuous since about 13.5 Ma. Radiometric dating and lithological sludies Aronson and Sœmúndsson (1975) studied the plateau basalts beneath the Tjörnes beds at Kaldakvísl and the sections at Héðinshöfði, and reported radiometric ages from this part of the Tjörnes sequence. The Kaldakvísl ages confirmed a Tertiary age for the rocks in question, but the low ages obtained from the Héðinshöfði locality indicated a possible cor- relation with the Furuvík beds. The first radiometric ages within the Tjör- nes sequence above the basal basalts were presented by Albertsson (1976, 1977, 1978), who initiated an exhaustive sampling programme to this effect in 1972. Albertsson correlated the sequence with the geomagnetic polarity time scale and concluded that the alternative con- sidered less likely by Th. Einarsson et al. (1967) was partly confirmed. According to Albertsson (1976, 1978) the Mactra/Serripes Zone boun- dary dates from ca. 3 Ma., and the oldest tillite bed of the sequence (in Furuvík) is ca. 2 Ma. old. The Tjörnes sequence was reviewed by Th. Einarsson (1977, 1978) in the light of the new radiometric ages, and palynological results from the Tjörnes sequence were also presented. Eiríksson (1979) examined the stratigraphy and origin of the sedimentary rocks of the upper part of the Tjörnes sequence. Lithos- tratigraphical holostratotypes were defined for the Breiðavík Group, which includes the Furuvik beds and all higher strata of the Tjörnes sequence. Informal lithozones were defined for the lower part of the Tjörnes sequence. According to Eiríksson the Breiðavík Group is characterized by at least eleven or twelve lithological cycles deposited during as many climatic glacial — interglacial cycles. DISCUSSION Past geological research on Tjörnes has answered many questions about the origin, age, and tectonic evolution of the area. How- ever, the answers are probably outnumbered 12 JÖKULL 30. ÁR

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