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Jökull - 01.12.1982, Qupperneq 112

Jökull - 01.12.1982, Qupperneq 112
0.2 Fig. 2. Seismic profiles to the west of Flatey show- ing the steep contact between basement rocks (shaded) and sediments. Shading on profile G indicates submarine extension of Flatev volcanics. Vertical scale in seconds of two-way travel (left). and basaltic-rhyolitic rocks of the Flateyjardalur central volcano. The age of the sequence on land ranges from 12 to less than 7 million years (Aronson and Sœmundsson 1975). Sparker measurements were made in the area between Eyjafjörður and Tjörnes in order to map this basement/sediment contact. The profiles (Fig. 1) were positioned so as to cross the gravity con- tours. The survey was carriedout on vessels belong- ing to the Marine Research fnstitute, Reykjavík. Navigation was based on radar and Loran C. Some of the profiles overlap lines surveyed by Mc- Master et al. (1977). RESULTS The FlateyJault Several profiles measured to the west of Flatey island (profiles A-F, Figure 2) cross the boundary between crystalline rocks and sediments. This boundary is found 2 nautical miles off Gjögurtá and appears to merge with the coast near Flatey. The profiles show the contact to be near vertical and its straightness and orientation suggest that this is indeed a fault in direct continuation of the Húsavík faults, as predicted by Semundsson (1974) and Mc- Masteretal. (1977). This feature will be referred toas the Flatey fault in the following discussion. The Flatey fault is very strongly reflected in the topography: the sea-floor is seen in profde A to drop from 60 milliseconds (44 meters) to 280 ms (204 meters) across the fault. Reflectors in the sediments were recorded at 420 ms depth (minimum of 310 meters) and these are not thought to represent bas- altic basement. The coastline in this area is mount- ainous and rises steeply ffom sea-level to elevations above 800 meters. These figures give an indication of the magnitude of the discontinuity, i.e. a min- imum vertical fault component of 1100 m. A narrow platform has been cut in the basement between the Flatey fault and shore. The edge of this Approximate scale in metres also shown. Horizontal scale indicated with divisions of one nautical mile. Mynd 2. Endurvarpssnið vestan Flateyjar, sem syna skil milli berggrunns úr storkubergi og setlaga. A sniði G kemur fram storkuberg tengt hraunlögunum í Flatey. Lóð- re'ttur kvarði er syndur sem endurvarpstími í sekúndum, en einnig er sýnt dýpi t metrum (ónákvœmt). Láréttur kvarði táknaður með strikum á einnar sjómílu bili. 108 JÖKULL 32. ÁR
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