Jökull - 01.12.1984, Side 95

Fig. 18. Schematic illustration to explain the
apparent vertical displacement associated with a
dyke when the dyke (D) is not perpendicular to
the lava (L) it dissects. Below the lava is scoria.
The vertical displacement, x, is given by: x = tan
cþ • t, where 4> is the dip of the lava or, if the dyke
is not vertical, the angle between a line per-
pendicular to the dyke and the surface of a lava
that the dyke dissects, and t is the thickness of the
dyke. For instance, if the dip of the lava is 7°, and
if the dyke is 4m thick and vertical, then the verti-
cal displacement is about 0.5m.
Mynd 18. Skýringarmynd sem sýnir hvernig lóð-
rétt sýndarfœrsla á hraunlögunum kemur fram
þegar gangurinn sker þau undir minna en 90°
horni.
to decide whether the dyke had a significant
associated vertical displacement. Of these 116
dykes only 18 dykes (15.5%) had associated verti-
cal displacement, the average displacement being
about 3 m. These 18 dykes, however, did not
occupy ordinary normal faults. Ordinary normal
faults dip 60°—70° (Roberts 1982, p. 142, Price
1966, p. 59), but most of these dykes dip 82°—83°.
As the dip of these dykes is usually in the oppo-
site direction to the dip of the lavas then, assum-
ing the tilting of the lava pile to have occurred
later than the intrusion of the dykes, initially
most of these dykes must have been nearly verti-
cal. Moreover, the vertical displacement along
many of these dykes is reverse (for instance, the
downthrow on a dyke dipping to the east is to the
west). This indicates that the vertical displace-
ment along many of these dykes results only from
the fact that they dissect the lavas at angles less
than 90° (Fig. 18).
I suggest that it is the low in situ tensile
strength of the host rock that makes it easier for
the dykes to advance their own extension frac-
tures rather than to flow into the normal faults
they meet on their way towards the surface. The
low tensile strength, probably of the order of a
few MPa or less (Gudmundsson 1983b, Haimson
and Rummel 1982), can be attributed to the
numerous columnar joints in the lava flows (Gud-
mundsson 1984).
FORMATION OF DYKES
The dykes are usually composed of up to tens
of columnar rows. This can be explained if indi-
vidual dykes are formed in magma phases (Gud-
mundsson 1984). Each phase of magma is subse-
quently split in two by the next phase, and so on
(Fig. 19). There is some seismological evidence
that dykes formed by a lateral flow may also form
in phases in a vertical section, that is, the magma
does not intrude simultaneously on the whole
height (i.e. the vertical dimension) of the dyke
(Páll Einarsson, personal communication, 1983).
Dykes of up to three columnar rows may have
formed in a single phase. The columns in the
middle parts of dykes are sometimes bent
upwards, from which one can infer that there was
an upward movement of magma in the middle
part after the outer parts had already consoli-
dated. Consequently, the middle one of the three
columnar rows in dykes formed in a single
magma phase is attributable to movement in the
mobile part of the dyke after the outer parts had
consolidated.
Dykes that have more than three columnar
rows are likely to have formed in several magma
phases (Fig. 19). Between successive phases the
outer parts of the dyke cool down and form
columns, but the next magma phase splits the old
one in two, using the middle part where the
dyke-material is still liquid or at least weak.
Before the second magma phase intrudes, the
magma of the first one will have solidified and its
columns formed. Otherwise the structure of the
columns would be deformed by the pressure of
the second magma phase. The magma of the first
phase will, however, not have completely cooled
down before the intrusion of the next phase, as
then there would not be any particular tendency
for the second magma phase to intrude the mid-
dle of the first phase. This follows because the
tensile strength of a completely cooled dyke rock
JÖKULL 34. ÁR 93