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Jökull - 01.12.1984, Qupperneq 95

Jökull - 01.12.1984, Qupperneq 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
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