Rejuvenation Of The Awatere Fault Cicatrice
The long fault cicatrice on the Awatere Fault consists of a reverse scarplet and a trench closely parallel to it. The feature has been formed by intermittent fault movement of the kind that may be described as in reverse as compared with that which long ago made the tectonic Awatere Valley. At one place after destruction of the cicatrice by lateral river corrasion the latest small displacement on the fault has renewed it along a line that runs obliquely down a river-cut bluff.
The term “earthquake rent,” which apparently Alexander McKay found already in popular use in Marlborough, was applied by him not only to open fissures ranged in lines or grouped en échelon, but also to continuous mountainside trenches, of which that along the north-west side of the Awatere Valley is the most conspicuous example (Plate 12). McKay did not succeed in producing a clear and intelligible account of these features; but he refers to them in one place as “a sudden drop, producing a kind of sunken wall.” Hector describes part of the cicatrice of the Awatere Fault as a “terrace-like depression.” Most readers of the old geological reports have, however, failed to form a clear picture of these so-called earthquake rents, and for the most part their tectonic significance has been overlooked. Hector, indeed, and even McKay (1892) seem to have become obsessed by a theory that the latest movements must have been of such a nature as to rejuvenate the major scarps, and so they minimized or forgot the evidence of reversal.
It seems best to call this kind of feature a “reverse scarplet,” thus directing attention not so much to the accompanying trench as to the recently made infacing scarplet that encloses the trench. In parenthesis it may be observed that the scarplets of fault cicatrices are in some cases “rejuvenating” and in other cases reverse. One such feature, with a reverse scarplet, was described and figured by the writer in 1913. In most of the accounts of fault cicatrices that have appeared more recently there has been little reference to the difference between rejuvenating and reverse scarplets; but in that published by Waghorn (1927) of scarplets in the north-eastern part of the Ruahine Range the fact is recorded that the recent fault movements manifested by the scarplets reverse those which upheaved the range. It should be noted that no evidence of lateral (transcurrent) movement on the faults has been observed in the case of any of the cicatrices mentioned. It is as well to bear in mind also that renewed movement along the base of a major scarp may be at one place reversal and at another rejuvenation of the earlier upheaval. (Reversal is replaced by rejuvenation, indeed, as the south-western end of the long Awatere scarp and cicatrice is approached.)
That movement took place on the Awatere Fault in 1848 seems evident from the reports of contemporary observers, though actually these were so impressed by the opening of rents, or gaping cracks, that they failed to observe what differential movement of the walls of the fault had taken place. As reported by Lyell in 1808 from information supplied to him by eye-witnesses: A great rent was… caused.… This fissure of 1848 was not more than 18in. in average width, but was remarkable for its length, for it had been… traced… for sixty miles.” According to Suess, however, a downthrow towards the north-west was placed on record by Lyell.
The reverse scarplet and nearly continuous trench along the line of the fault are so conspicuous that it is clear the cicatrice as a whole can have been formed in no other way than by a renewal in reverse in comparatively recent times of fault movement on or very closely parallel to the old fault plane of major dislocation which was a boundary between differentially moving blocks when the ranges of Marlborough were upheaved. The large dimensions of the trench and scarplet (seen in Plate 12) are such, however, as to show that this movement in reverse has been in progress for some considerable time (presumably many thousands of years). No doubt it has been intermittent, and the latest displacement, which perhaps took place in 1848, must have been one of a long series.
A separation of the latest small dislocation from earlier displacement may be seen at a place (between Richmond Brook and Altimarlock) where the Awatere River has, when flowing at the level of what is now the tread of a low terrace, undercut the valley side so recently as to destroy the whole previously
existing fault cicatrice, so that its trace comes to an end where intersected by a river-side bluff that borders a high terrace. Such truncation of a cicatrice is shown in Fig. 1, stage B. Later, perhaps in 1848, when a renewal of movement has taken place, and when also probably the rest of the cicatrice has been refreshed as its enclosing scarplet has grown in height, the reverse scarplet has been renewed across the river-cut bluff with a displacement of three or four feet (compare Fig. 1, stage C). The new cicatrice runs obliquely down the bluff from the high to the low terrace level, as illustrated in the photographs reproduced as Plate 13, Figs. 1 and 2.
Cotton, C. A., 1913. The Physiography of the Middle Clarence Valley. New Zealand. Geographical Journal, 42, pp. 225–246.
—— (1947). Revival of Major Faulting in New Zealand. Geological Magazine, 84, pp. 79–88.
Hector, J., 1890. Progress Report. Rep. Geol. Explor., 20, pp. xiv–lvii, see p. xli.
Lyell, C., 1868. Principles of Geology, 10th ed., Vol 2, p. 78.
McKay, A., 1886. On the Eastern Part of the Marlborough Provincial District. Rep. Geol. Explor., 17, pp. 27–136.
—— 1892. On the Geology of Marlborough and South-East Nelson. Rep. Geol. Explor., 21, pp. 1–28.
Suess, E., 1906. The Face of the Earth, Vol. 2, p. 28, Oxford.
Waghorn, R. J., 1927. Earthquake Rents as Evidence of Surface Faulting in Hawke's Bay. N.Z. Journal of Science and Technology, 9, pp. 22–26.