Art. XXX.—Rough Ridge, Otago, and its Splintered Fault-scarp.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 51, 1919, Page 282

Art. XXX.—Rough Ridge, Otago, and its Splintered Fault-scarp. By C. A. Cotton. [Read before the New Zealand Institute, at Christchurch, 4th–8th February, 1919; received by Editor, 11th February, 1919; issued separately, 16th July, 1919.] Introduction. In another publication* C. A. Cotton, Block Mountains in New Zealand, Am. Journ. Sci., vol. 44, pp. 249–93, 1917. I have referred to Rough Ridge as one of the upland blocks of the Central Otago system of block mountains and associated depressions. It is there described as follows:— “Its crest is very even for many miles with a height of about 3,200 ft. above the sea. On its north-western and north-eastern sides this block is similar to the Raggedy—Blackstone block. Farther south, however, it is complex and relatively wide, two broad splinters descending towards the north-east and forming offsets on the lowland level between the north-east-trending fault-scarp portions of the boundary-line between the Rough Ridge block and the next depression to the east.”† Loc. cit., pp. 274–75. The map and diagram, figs. 11 and 13 of the paper referred to, show the relation of the Rough Ridge block to the structure and relief of the district as a whole, and fig. 3 is a sketch of its back slope. This block is of interest both on account of its remarkably even and little-dissected back slope (a fossil plain) on the north-western side, and on account of the faults en échelon bounding it on the eastern side, between which two well-defined splinters descend from the upland to the level of the adjacent depression (see fig. 1). Splintered Fault-scarps. So far as I am aware, the term “splinter” was first used in a similar connection by Davis, who figured a “rock-splinter on the Hurricane fault” and referred to it in these words: “The view northward showed at a distance of ten or fifteen miles a curious offset in the fault whereby a splinter of upper Aubrey at the edge of the Uinkaret bends down and descends southward to the Shivwits level.”‡ W. M. Davis, An Excursion to the Plateau Province of Utah and Arizona, Bull. Mus. Comp. Zool. Harvard, vol. 42, pp. 1–50 (see p. 30 and fig. 9), 1903. In New Zealand another splintered fault-scarp, in the Waitaki Valley, has been described and figured.§ C. A. Cotton, The Fossil Plains of North Otago, Trans. N.Z. Inst., vol. 42, pp. 429–32 (see p. 432 and pl. xxx, fig. 2), 1917. A splintered fault differs from a distributed fault and from a branching fault in that, while the displacement on the whole fault-system remains constant throughout its length or varies constantly in one direction or the other (as might the displacement on a single simple fault), dwindling displacement on one line (such as AB, fig. 2) is compensated by the development parallel to it of another line of fault (such as CD) with increasing displacement, and this may occur more than once (EF); so that discontinuous faults en échelon separating successive splinters form the complex boundary between adjacent high- and low-lying blocks. It is as though faulting had followed pre-existing lines of weakness — lines of least resistance—running diagonally across the boundary between the tectonic blocks.

Rough Ridge. The back slope of Rough Ridge is probably the most perfectly preserved inclined fossil plain in Otago. The surface, as shown in fig. 3 of the paper referred to above, is in the stage of erosion when the overmass has been stripped off and the ancient planed floor is undergoing dissection by numerous parallel consequent streams none of which is master. While a few feet of rock have been removed from the stripped floor since the removal of the cover, as is shown by the presence of abundant residual tors of schist, the form of the ancient eroded plain, now tilted with an inclination Fig. 1.—Northern end of Rough Ridge (from maps by Department of Lands). Fig. 2.—Diagram of a splintered fault dislocating a plane surface. of 10°, is preserved almost perfectly by the existing surface of the inter-fluves. Though these are narrow, they slope towards the numerous stream-lines only near their edges, which are rounded off. The profiles of the little valleys of the dissecting streams are roughly graded, though incised to but a small depth below the general slope, this depth being, as would be expected, greatest about the middle of the slope. Thus, if the former presence of a cover be disregarded, the cycle of erosion for the surface is still in the stage of youth; and it may remain, and probably has remained, at this stage for a long period, for the conditions are here extremely favourable for long persistence of such pseudo-youthful topography.* C. A. Cotton, loc. cit., p. 259. In this way only can the frequent recurrence of similar back slopes be explained.

Fig. 3.—The first splinter from the Rough Ridge fault-scarp. View looking north-west. Fig. 4.—The first splinter from the Rough Ridge fault-scarp. The second splinter is seen on the left. View looking south-westward up along the splinter from a point on Little Rough Ridge to the right of fig. 3.

Though the back slope of the Rough Ridge block is remarkably uniform, the stripped surface arches over near the crest-line, becomes horizontal, and then slopes down towards the south-eastern side, or front, of the block. Around the north-eastern end this little-dissected surface has the form of a plunging anticline, and, close to the north-eastern end, even on the front, the strongly warped fossil plain seems to form the slope, though farther to the south it undoubtedly passes into a fault-scarp. The eastward-sloping stripped surface, or fold-scarp, has an inclination of 20°, and is more deeply and maturely dissected than the back slope. Tors survive on the interfluves. The structure of the schist forming the Rough Ridge block is a broad, open anticline with its axis transverse to the elongation of the block. On the crown of this anticline, which is near the northern end of the block, the ancient eroded surface truncating the anticlinal structure is practically parallel with the foliation of the schist. The deformation to which the uplift of Rough Ridge is due is here shown not only by the form of the surface, but also by the dip of the schist foliation, which arches over the top of the ridge accordant with the surface. Such an agreement between the surface-slope and the dip of the foliation may occasionally be noted in other parts of Central Otago where the schist is flat-lying; but, on the other hand, there are frequent changes of dip which are quite discordant with the general slopes of the surface, and are obviously of more ancient origin. The same is true of the broader structures of the schist—as, for example, the broad anticline referred to above in the Rough Ridge block. As the eastern front of Rough Ridge is followed southward the foldscarp is found to be replaced by a fault-scarp. Here, however, near the base of the scarp lies a strip of the schist undermass, the north-eastern part of which is but little above the general level of the neighbouring portion of the Maniototo depression and is separated from the scarp by a narrow strip of planed undermass (see fig. 4). This low, flat outcrop of schist is named Little Rough Ridge, on account of its tor-covered surface. Followed south-westward its surface rises—the displacement on the fault which separates it from the main block decreasing in that direction—so that it is seen to be a splinter with its stripped surface inclined towards the main block (see figs. 3 and 4). This first splinter is bounded on the front, or southeastern side, by a fault-scarp similar to that of the main block. At the base of this scarp is another low-lying area of schist, which rises south-westward to form a second splinter similar to the first. At the base of the fault-scarp of the second splinter lies a schist surface which emerges from beneath the overmass in the Maniototo depression and rises southward to form part of the plateau into which the Central Otago block mountains and depressions merge when followed southward. In the fault-angle behind the higher portion of the first splinter a remnant of the cover is preserved.* A. McKay, Report on the Older Auriferous Drifts of Central Otago, Wellington, 1897 (see map). A glance at figs. 3 and 4 shows that the drainage of the depression behind the splinter, after following what appears to be the normal, longitudinal, consequent course in the fault-angle for some distance, turns south-eastward and crosses the splinter in gorges. This points to some bending-down of the initial surface adjacent to the crest of the fault-scarp, for there is nothing to suggest that capture has taken place, and the stream-courses are probably consequent throughout their length.