There are, then, two elements in the geography of west Nelson—the peneplains and the rift-valleys—and the limits of both of these are generally determined by powerful faults. There are two groups of normal down-throw faults, separated by the great alpine overthrust, those lying to the west of this overthrust belonging to west Nelson, and those to the east to the alpine range.
The main structural fault of west Nelson, and, indeed, of the South Island, is the great alpine overthrust† which runs from Foveaux Strait to D'Urville Island. This overthrust follows the Gregory Valley of Morgan,‡ which may be traced from Lake Kanieri to Lake Rotoiti.§ Its course has been indicated by Hector‖ and Mackay.¶ On the western side of this great fault have been intruded igneous magmas, which abut at intervals against the overthrust from South Otago to Lake Rotoiti, in Nelson. On both sides of the fault occur basic and ultrabasic intrusions of later age than the granitic intrusions already mentioned. On the east side they occur from D'Urville Island to Otago, forming in part the Pounamu formation of the Geological Survey. On the west side these rocks are represented by lavas at Koiterangi and Paringa, and by dykes at many points.
The faults of the alpine range are exceedingly numerous, and tend to run either north and south or east and west. ** They are often indicated by hot hepatic springs, as in valleys of the Maruia, Upper Grey, Hurunui, Trent, and Taipo Rivers. North-and-south faults occur in the Upper Taipo, Otira, Waikite, and Trent. East-and-west lines of faulting—perhaps blatten—are followed by the Upper Taramakau and Hurunui, the Waiheke, and Doubtful, the Marchant, Upper Grey, Maruia, and Henry Rivers. The Wairau† fault is different from the faults of the alpine peneplain, hitherto considered: it has a more easterly trend than either the main alpine overthrust or the great faults of the Kaikouras and North Island ranges—it is, in fact, a connecting-link between these two great parallel systems of breaks.
The faults to the west of the alpine overthrust tend to run in two directions—north and south, and north-east and south-west.‡ The first to be considered, the Motueka fault, probably skirts the coast of Tasman Bay, running south from Separation Point to the mouth of the Motueka,
[Footnote] † Mackay: Geol. Surv., No. 21, p. 20. Morgan: N.Z. Geol. Surv. Bull. No. 6 (n.s.), p. 71 et seq.
[Footnote] ‡ Morgan: N.Z. Geol. Surv. Bull. No. 6 (n.s.), p. 38.
[Footnote] § Haast: Geol. Explor. of West Nelson, p. 94.
[Footnote] ‖ Hector: Geol. Surv., No. 4, p. 29.
[Footnote] ¶ Mackay: Mines Statement and Goldfields Reports, 1893, pp. 136, 174.
[Footnote] ** Cf. Morgan: N.Z. Geol. Surv. Bull. No. 6 (n.s.), p. 70.
[Footnote] †† Mackay: Geol. Surv., No. 21, p. 19.
[Footnote] ‡‡ Park: Geol. surv., No. 19, p. 83. Mackay: Geol. Surv., No. 21, pp. 20, 21. Mackay: Geol. Surv., No. 12, pp. 124, 125, 127 (in this and in many following references faulting is not definitely stated, but may be inferred).
where it turns south-west and follows the flanks of the Mount Arthur Range, contorting the strata in the Graham, Baton, and Wangapeka Rivers. One branch runs south-east along the Sherry River,* and probably reaches the alpine overthrust near Lake Rotoiti. The main fault is continued into the basin of the Owen,† which river it follows to the junction of the Mangles, thence crossing to the Matakitaki. The Tutaki† fault leaves the Motueka fault at the Owen, flanks the Murchison peneplain on the west, into the head of the Tutaki, thence by the Glenroy junction along the Warbeck and Warwick Rivers to the Maruia. The Matiri‡ fault, parallel to the Motueka fault, shows itself in the Upper Wangapeka and probably the Upper Crow, follows the Upper Matiri to the Maruia junction, thence by Deepdale into Larry's Creek, and flanks the Victoria peneplain on the west, reaching the alpine overthrust near Lake Haupiri.
The Takaka fault§ follows the Takaka River for twenty miles, thence crosses the range joining the Motueka fault near the Baton. The Karamea fault‖ crosses the Mount Arthur tableland from the Takaka to the Leslie, thence into the Upper Karamea; it determines the course of the north and south branches of the Mokihinui, thence to the Buller by way of New Creek. The Mawhera fault,¶ a direct continuation, flanks the eastern base of the Paparoas, running to sea south of the Taramakau. A subsidiary parallel fault to the west determines the course of the Orikak and Blackwater Rivers. The Inangahua fault ** leaves the Karamea fault at New Creek, flanks the western base of the Brunner Range and the foothills of Victoria Range, reaching as far south as the Big Grey, where its southern continuation is overlain by Old Man gravels. The Ross fault† is a south-west fault, flanking the western of the granite foothills from the Grey River to Ross; it manifests itself at Koiterangi and Mount Greenland.
The Aorere fault ‡ runs along the western shores of Golden Bay and follows the Aorere, and thence by way of Brown's Creek across the Gouland Downs. The Wanganui fault runs south-west through West Wanganui Inlet, and a probable parallel fault determines the trend of the coast from Cape Farewell to Rocks Point. The Lower Buller fault § follows the coast-line south-west from north of the Mokihmui, and skirts the higher land from the Ngakawhau to the entrance of the Buller Gorge, thence to the coast at the mouth of Fox's River. The Mount William fault ‖ flanks
[Footnote] * Mackay: Geol. Surv., No. 12, p. 130.
[Footnote] † Haast: Geol. Explor. of West Nelson, pp. 95, 96. Cox: Geol. Surv., No. 16, pp. 5, 6, 8. Park: Geol. Surv., No. 19, pp. 79, 82, 84.
[Footnote] ‡ Hector: Geol. Surv., No. 4, p. 27. Cox: No. 16, p. 6. Park: No. 19, p. 80. Mackay: Geol. S.W. Nelson, pp. 11, 12, 57.
[Footnote] § Park: Geol. Surv., No. 20, p. 192. Mackay: No. 21, p. 21. Bell: N.Z. Geol. Surv. Bull. No. 3 (n.s.), p. 50.
[Footnote] ‖ Hector: Geol. Surv., No. 4, p. 27. Cox: No. 9, p. 117. Mackay: No. 21, p. 21: Mackay here ascribes the dislocation of the rock at the base of the Lyell Mountains and at New Creek to his Motueka fault. Mackay: Geol. S. W. Nelson, p. 12.
[Footnote] ¶ Mackay: Geol. Surv., No. 8, pp. 103, 104, 105; No. 21, p. 21 (Motueka fault); Geol. S.W. Nelson, p. 58.
[Footnote] ** Mackay: Geol. Surv., No. 15, pp. 145, 146, 147.
[Footnote] †† Bell: N.Z. Geol. Surv. Bull. No. 1 (n.s.), p. 81. Morgan: N.Z. Geol. Surv. Bull. No. 6 (n.s.), pp. 70, 71.
[Footnote] ‡‡ Hector: Geol. Surv., No. 4, p. 19. Cox: No. 15, pp. 71, 72. Park: No. 20, p. 202. Bell: N.Z. Geol. Surv. Bull. No. 3 (n.s.), p. 50. Mackay: Papers and Reports, 1896, C.-11, p. 9.
[Footnote] §§ Mackay: Geol. Surv., No. 8, pp. 107, 109, 112; No. 21, pp. 22, 78.
[Footnote] ‖‖ Mackay: Geol. Surv., No. 8, pp. 76, 110, 113; No. 21, p. 78; Geol. S.W. Nelson, p. 58. Webb: Geol. Surv., 2nd Ann. Rep. (n.s.), p. 26.
the granite ranges south from Mount Domett, enters the Mokihinui basin by Rough-and-Tumble Creek, thence along the Upper Ngakawhau and Waimangaroa, past Mount William, and across the Buller by way of Cascade Creek; thence it flanks the Paparoa peneplain to Point Elizabeth: a branch probably crosses the Grey between the first and second gorges.
The faults of west Nelson, as far as the writer can tell, are all distributed faults, often many chains wide.