Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 144

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mal faults. I think these are local features of minor signi-
ficance for the entire structure which undouhtedly is better
comparable to rift systems such as the Rhine Rift or the
basin and range province of westem North America than
e.g. the San Andreas fault.
Near Steamhoat, Nevada (near the western border of the
basin and range province) Thompson & White (Thompson,
G. A. & White, D. E., 1964: Regional geology of the Steam-
boat Springs area Wanhoe County, Nevada. — U.S. Geol.
Surv. Prof P. 458A: Al—A51) report swarms of en echelon
faults, much more prominent than our Icelandic ones, which
they interpret as normal faults. It seems that en echelon
features do not necessarily mean strike slip movements.
I want to stress that in the Hengill area, which is a part
of the western branch of the central Icelandic grahen I
have nowhere observed horizontal displacements nor any-
thing pointing to the existence of such displacements, al-
though slickensides occasionally are exposed.
Trausti Einarsson: The stepped fissures are significant and
occur widely. I don’t think they can be so easily dismissed.
I have pointed out a reason why they are not always found.
S. Björnsson: For comparison, I would like to ask Prof.
T. Einarsson to sketch the surface pattern of fissures result-
ing from tensional stress in the crust.
Trausti Einarsson: Shearing stress always occurs when
the two normal stresses are unequal (speaking of 2-axial
fields). Pure tension is such a case. It then depends on the
material, whether brittle or plastic, whether it yields to the
normal stress (the tension) or to the maximum shearing
stress at an angle of about 45° to the normal stress. Tension
across the principal fractures in Iceland might lead to shear-
ing movement but not along the principal fractures. I may
emphasize that to get shearing stress and shearing cracks
there is only the requirement of unequal normal stresses,
which both may be pressures, whereas pure tension is a
highly special state and probably quite unrealistic in tec-