Earthquake Traces Near Waverley And Their Tectonic Setting
Although neither Henderson (1944) nor Ongley (1944) reported recent earthquake traces from Taranaki Peninsula, unpublished and unconfirmed reports
Fig. 1.—Fault creature of recent development running obliquely down the Luce of a river-cut bluff truncating a [ unclear: ] in the lower Awatere Valley.
of their ocurrence have been made. During the geological survey of Wanganui Subdivision, an extensive series of fault scarplets has recently been recognised and mapped by the writer. At Oturi Lake, one and a-half miles south of Waverley, a flight of scarplets has a total throw of over 30 ft. in less than 20 chains and strikes north-eastward through the south-east corner of the town of Waverley. A single scarplet, with mean throw of about five feet, continues for a mile further north-east, and before it dies out a parallel trace develops 60 chains to the north-west and continues to a point some two and a-half miles north-west of Waverley. Another line of fractures strikes north-east from Moumahaki Station and has been traced intermittently for three miles along the foot of a 100 ft. scarp interpreted as due to Pleistocene activity of the same fault.
The downthrow of all scarplets seen is to the south-east. Individual scarplets may exceed 10 ft. in height, but are usually less, and a splintered arrangement of echelon faults with varying throw is not uncommon. “Earthquake rents” with opposing scarps were not observed. Most of the scarplets are developed on an undulating surface of Pleistocene consolidated dunes, thickly veneered by andesitic ash from the Taranaki volcanoes, the Egmont Showers of Grange and Taylor (1933), but their formation postdates the erosion of stream valleys below this surface down to (approximately) their present beds. The earthquakes that formed the scarplets are believed to be older than the ponding and aggradation of small streams by the advance of Recent dune-sands. These, again, are older than the thin deposit of subaerial ash that coats them at the mouth of the Manawapou River. The last soil-forming ash from Mount Egmont, the Burrell Shower, is believed by Grange and Taylor to be no older than 500 years, so that this gives a minimum age to the Waverley earthquake traces.
Both sets of traces lie on the line of older faults in the underlying Lower Pliocene (Waitotaran) sediments. All important faults in the coastal area between Wanganui and Patea are gravity faults dipping steeply southeastwards. The Nukumaru Fault-zone, striking north-east from the coast 12 miles west of Wanganui, is a belt of such fractures marking the axis of greatest flexure of a monoclinal warp separating the low anticlines of central Taranaki from the Late Pliocene-Pleistocene Palmerston-Wanganui geosynclinal basin (Fleming, 1947, p. 308). There is evidence that this monoclinal structure dates from Nukumaruan (middle Pliocene) times, and that it operated to delimit Nukumaruan and Castlecliffian shorelines in the west.
The structure of the Cook Strait area may be considered as consisting of interdigitating plunging anticlinal and pitching synclinal fractured folds, in general alternating on either shore. Thus, the main anticlinal axis of the South Island plunges northward as the Marlborough Sounds block towards the restricted Late Pliocene geosyncline of Wanganui. On the east, these structures are bounded by the Wairau Fault, with which the Kapiti Fault is continuous, and Kapiti may be considered co-axial with the line of basement “highs” extending north to the plunging nose of the Kaimanawa anticline. On the west the Waimea Fault (or monocline) is in spatial continuity with the Nukumaru faulted monocline, their opposite downthrow suggesting a “scissors” relationship. West of the Waimea Fault, the Nelson (Moutere) Hills are a Pliocene-Pleistocene synclinal segment continuous in space with the south-plunging central Taranaki anticlines. A hypothesis which accepts the “staggered” relationship of the structural “highs” and “lows” is, it may be submitted, more in accord with field evidence than one which accounts for that relationship by invoking low-angle thrusting and dislocation at a late geologic date (King, 1939).
Fleming, C. A., 1947. Trans. Roy. Soc. N.Z., vol. 76, part 3, pp. 300–326.
Grange, L. I., and Taylor, N. H., 1933. Dept. S.I.R., 7th Annual Report, pp. 33–35.
Henderson, J., 1944. N.Z. Journ. Sci. Tech., vol. 24, no. 5B, pp. 195B–219B.
King, L. C., 1939. Trans. Roy. Soc. N.Z., vol. 68, pp. 544–569.
Ongley, M., 1944. N.Z. Journ. Sci. Tech., vol. 25, Sec. B, no. 2, pp. 67–78.