Jökull - 01.12.1999, Blaðsíða 24

Figure la. Main volcano-tectonic elements of the Borgarfjörður area. 1) Central volcano; 2) Acidic intrusion;
3) Snæfellsnes syncline (extinct volcanic rift zone); 4) Borgarnes anticline; 5) Hreðavatn unconformity; 6)
Pleistocene-Holocene swarms of volcanic craters in the Snæfellsnes volcanic zone. Modified from Jóhannesson
(1980,1982). S: Snæfellsjökull; Ly: Lýsuskarð; Lj: Ljósufjöll; in the small frame: R: Reykjanes-Langjökullrift
zone; H: Húnaflói bay. — Einfaldað jarðfrœðikort af rannsóknarsvœðinu. 1) Megineldstöð; 2) súrt innskot; 3)
ás Snœfellsnes samhverfunnar; 4) ás Borgarnes andhverfunnar; 5) lega Hreðavatnsmislœgisins; 6) Utbreiðsla
eldvarpa Snœfellsnesgosbeltisins frá ísöld og nútíma. Kortið er byggt á gögnum Hauks Jóhannessonarfrá 1980
og 1982.
active and that normal faulting is currently occurring.
The SVZ is complicated, in particular its relation to
the Tertiary rifting and rift jump. A transform ori-
gin has been suggested for this fracture zone, going
back to Tertiary time (Sigurðsson, 1967; Jóhannesson,
1975; Sæmundsson, 1978; Jancin etal, 1985).
Tertiary and Quatemary lava in Borgarfjörður are
cut by numerous dykes and faults of different types
and with variable strike. The lack of common marker
horizons and exposures of cross-cutting striae make it
difficult to establish the chronology of events in the
area. This paper reports descriptive structural data on
normal faults, strike-slip faults, striated planes, and
dykes, measured during a preliminary field campaign
in 1995 (Figure lb). Some features of the tectonic
pattern and the observed structures will be discussed
in relation to the tectonic evolution of the area, and
compared with previous studies. Preliminary results
obtained here contribute to the understanding of an
area between overlapping rift zones.
Definitions
Large scale, small scale, major and minor are used
as a descriptive scale for the faults. Large scale or
major refer to faults with cartographic scale, as drawn
on maps and aerial photographs. Small scale or minor
(order of a metre or decimetre) refer to small planes on
which the pitch of the striae were generally measured,
22 JÖKULL, No. 47