Stratigraphic and structural relations of the southern dun mountain ophiolite belt and enclosing strata, Northwestern Southland, New Zealand

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Abstract

Three lithostratigraphic groupings are recognised to the northwest and southeast of the Five Rivers Plain in northwestern Southland: the lower Maitai Group consisting of the Greville, Little Ben, Tramway, and Upukerora Formations; the conformably underlying Dun Mountain Ophiolite Belt; and the Caples Group which lies to the east of the ophiolite and is separated from it by the Livingstone-Macpherson Fault System. On the northern side of the plain, the Maitai Group and ophiolite belt are deformed into a series of sinistral-verging steeply plunging folds which become extensively disrupted by left-lateral strike-slip faults in the east. The deformational pattern has resulted in acrossstrike repetition of these units and accounts for their apparent offset between West Dome and Lintley and the apparent thickening of the ophiolite belt between West Dome and Black Ridge. Sandstones of the Greville and Little Ben Formations are quartz poor and were derived from an undissected basic-intermediate volcanic arc. Tramway and Caples sandstones contain up to 20% quartz and show a mixed silicic volcanic and lowgrade metamorphic provenance. The ophiolite belt at Lintley is divided into a northern mafic igneous association, composed largely of spilite, and a southern silicic igneous association of quartz keratophyre and plagiogranite. The latter association is not recognised to the north of Five Rivers Plain where the mafic igneous association occurs in conjunction with an ultramafic melange of schistose serpentinite. The Livingstone-Macpherson Fault System is up to 600 m wide at Lintley and contains blocks of enclosing rock units together with a unit of limestone-bearing conglomerate of unknown stratigraphic affinities. Elsewhere along the fault, this conglomerate occurs in association with mafic volcanics (on Five Rivers Plain) and ultramafic melange (Cromel Stream). Asymmetric folds similar to those mapped on the northern side of Five Rivers Plain also occur elsewhere in Southland at North Peak, Mt Misery, and Conical Hill and reflect the effects of WNWESE regionally directed compression which probably originated in response to late Cenozoic transpression along the India-Pacific plate boundary. Secondary mineral assembages show changes in metamorphic facies both across and along strike. The change in metamorphic grade of the Maitai Group from zeolite facies at Lintley to prehnitepumpellyite facies east of West Dome marks the change from metamorphism under a simple hydrostatic load in the south to the start of compressive regional metamorphism in the north. The ophiolite belt is metamorphosed to pumpellyite-actinolite facies as a result of sea-floor metamorphism prior to accumulation of the Maitai Group. Mineral assemblages in the Caples Group are of prehnitepumpellyite facies. Zeolite facies assemblages within the Maitai Group at Lintley indicate that this region cannot be in stratigraphic continuity with similar grade strata outcropping in the Hokonui Hills, some 9.5 km across strike to the southeast. The two regions must be separated by a major fault running under or along the margin of the intervening Waimea Plains.

Original languageEnglish
Pages (from-to)179-203
Number of pages25
JournalNew Zealand Journal of Geology and Geophysics
Volume29
Issue number2
DOIs
Publication statusPublished - 1 Jan 1986
Externally publishedYes

Keywords

  • Caples group
  • Dun mountain ophiolite belt
  • Lowgrade metamorphism
  • Maitai group
  • Strike-slip faults

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