282 T ransactions. 



Art. XXX. — Rough Ridge, Otago, and i^s SjMntered Fault-scar]). 



By C. A. Cotton. 



[Read before the New Zealand Institute, at Christchurch, 4th-Sth February, 1919 ; received 

 by Editor, 11th February, 1919 ; issued separately, 16th July, 1919.] 



IXTRODUCTION. 



In another publication* I have referred to Eough Kidge as one of the 

 upland blocks of the Central Otago system of block mountains and associ- 

 ated 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."t 



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 echelon 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 fa\ilt " 

 and referred to it in these words : " The view northward showed at 

 a distance of ten or fifteen miles a curious oft'set in the fault whereby a 

 splinter of upper Aubrey at the edge of the Uinkaret bends down and 

 descends southward to the Shivwits level. "J 



In New Zealand another splintered fault-scarp, in the Waitaki Valley, 

 has been described and figured. § 



A splintered fault differs from a di'Stributed fault and from a branching 

 fault in that, while the displacement on the whole fault-system remains con- 

 stant 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 

 echelon 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 diagon- 

 ally across the boimdary between the tectonic blocks. 



* C A. CoTTOK, Block Mountains in New Zealand, Am. Jotirn. ScL, toI. 44, 

 pp. 249-93, 1917. 



t Loc. cit., pp. 274-75. 



j W. M. Davis, An Excursion to the Plateau Province of Utah and Arizona, Bull. 

 Mus. Cornp. Zool. Harvard, vol. 42, pp. 1-50 (see p. 30 and fig. 9), 1903. 



§ C. A. Cotton, The Fossil Plains of North Otago, Trans. X.Z. Inst., vol. 42, 

 pp. 429-32 (see p. 432 and pi. xxx, fig. 2), 1917. 



