Jökull - 01.12.1999, Page 32

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Figure 5. Tectonic maps around Langavatn (a), Hreðavatn (b) and Hallarmúli (c), showing normal and dextral
movements along both WNW and NNE striking faults. 1) Holocene sediments; 2) Quaternary and Holocene
volcanics younger than 0.78 Ma; 3) Tertiary lava pile older than 0.78 Ma; 4) Ignimbrite of Hallarmúli, dated
from 6.7 Ma; 5) Normal faults (ticks on downthrown block); 6) Dextral strike-slip faults; 7) Basaltic dykes
(thickness exaggerated); 8) Undifferentiated fractures (joints, faults, dykes); 9) Strike and dip direction of
the lava pile. The age of the volcanic products are from Jóhannesson (1975, 1994). — Höggunarkort af
svæðinu umhverfis Langavatn (a), vestan Hreðavatns (b) og við Hallarmúla (c). Siggengi og hægri handar
sniðgengi með VNV og NNA stefnur einkenna þessi svœði. Önnur jarðfrœðileg fyrirbœri eru; 1) setmyndanir
frá síðjökultíma; bergmyndanir yngri (2) og eldri (3) en 0.78 milljón ára; 4) flikruberg við Hallarmúla; 5)
siggengi, smástrikin snúa að sighliðinni; 6) hœgri handar sniðgengi; 7) berggangur úr basalti (þykktin erýkt);
8) aðrar sprungur (joints), misgengi og berggangar sem ekki voru rannsökuð sérstaklega; 9) stefna og halli
jarðlaga. Aldursákvarðanir eru byggðar á rannsóknum Hauks Jóhannessonar (1975, 1994).
mal fault bends at its intersection with the NNE fault.
To the south, another WNW normal fault cuts through
the Hallarmúli ignimbrite and is offset 27 m by the
right-lateral movement of this same NNE fault. Jó-
hannesson (1975) suggested the NNE fault also has
major vertical displacement, which is likely, but in
the present study this could not be determined in the
field. Above observations indicate that WNW dykes
and normal faults are older than the dextral motion of
the NNE faults.
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