It is probable that a major fault separates the schists of the Dunstan Range from the less metamorphosed rocks to the east in the valley of Dunstan Creek. The evidence for the existence of such a fault is somewhat indefinite, and is based upon the present topography, the contorted state of the rocks in this neighbourhood, and the rapid change in metamorphic grade between Dunstan Peak and Dunstan Creek.
Throughout the area between Dunstan Creek and the Waitaki River the faults fell into an older compressional series and a younger tensional series (block faults).
(a) The Compressional Series.
The most obvious member of this series is the Waitaki Thrust (T) which crosses the Awahokomo Valley about four miles above its junction with the Waitaki. The movement probably took place at the same time as the folding of the greywackes, under the influence of pressure from the north-east in late Jurassic or-early Cretaceous times. It may be considered as a result of shearing along an overturned aniclinal fold, probably not confined to any single plane, but rather the result of movements distributed through a zone. The greywackes lying immediately along the south-west of this shear-zone, and to some extent also those to the north-east, were so strongly affected by the movements that they developed a schistosity which almost obliterated their original clastic structure (“Kurow Schists” of earlier writers). The intensity of this purely local dynamic metamorphism decreases rapidly in a south-westerly direction.
A somewhat similar though less powerful movement probably took place along the line now marked by the Otematata River (above its junction with the Otamatakau River). Here, again, the rocks locally becomes schistose, the amount of reconstitution increasing suddenly as the fault-line is approached from the north-east, and dying away on the south-westerly side of the river. A band of conglomerate in which the pebbles have been sheared to lensoid shapes outcrops on the north-east side of the river, but is absent on the other side, where, in the absence of a fault, it would be expected to appear.
A notable feature in the Otematata Valley when viewed from the Parson's Rock Spur is the large bend which occurs in the river gorge just above the Chimney Creek confluence (Text-fig. 2). From such evidence as fairly intense corrugation, jointing, and shattering of the surrounding rocks, and topographic features, it appears probable that a fault crosses the major Otematata Fault here. Its continuations are traceable both where it crosses Parson's Rock Creek and in the north-west branch of Chimney Creek (which follows
its general direction). Other movements in a roughly parallel direction may account for the courses taken by portions of the Otamatakau River, Long Gully, an other creeks. From the somewhat complex nature of the structure near the large bend in the Otamatakau River east of Turnagain, it is possible that this spot marks the intersection of three faults: (i) The Otamatakau Fault—i.e., that determining the lower reaches of this river, and having an approximate bearing of 40°; (ii) a fracture-zone bearing about 110°–120°, and determining the course of the Otamatakau River south of Turnagain (a parallel movement is indicated by the topography to the north of Turnagain; (iii) a minor block-fault running almost north and south, belonging to the younger fault-series. The rocks near this point have suffered some reconstitution, while the direction of strike has been locally disturbed and is highly variable.
Text-Fig. 2.—The fault running from A to B cuts across the narrow saddle directly above C, on the ridge between the Otematata River (seen in the middle foreground) and Chimney Creek.
The Otematata Fault is probably one of the same age as the Waitaki Thrust. The other fractures, with the exception of the minor block-fault, may also be of this age, but the evidence in support of the correlation is not so striking.
(b) The Tensional Series.
The block-faults (probably a tensional series) whose general direction in this district appears to be almost north and south may be summarised thus:—
A possible renewal of movement long the Otematata Fault.
A minor fault about two miles to the east of Turnagain.
A major fault forming the western scarp of the Hawkdun Mts. as described by Dr C. A. Cotton.
A major fault forming the western scarp of the St. Bathans Mts.
All these block-faults belong to a later (Tertiary) tectonic movement, and hence have had a considerable effect upon the evolution of the modern topography. The amounts of the movements involved can be gauged roughly by the break in the old peneplain level across the lines of the faults.
Attention is drawn to the fact that the three main directions of joints and veins (see below) coincide approximately with the three principal fault directions, indicating some connection (see circular figure, Plate 18).