Abstract:
Late Cretaceous to Miocene sedimentary and igneous rocks are preserved within outliers throughout the Clarence Valley, Marlborough, New Zealand. Such an outlier occurs between Limestone Hill and Muzzle Stream, the area in which this study was undertaken. The structure of the area was investigated by geological mapping. Four fault bounded structural blocks are distinguished. This study proposes that reverse movement on the faults bounding these structural blocks has caused folding of the Late Cretaceous to Miocene cover rocks. The initiation of this fault propagation folding probably occurred in the early Pliocene. Evidence for this is based upon the age of initiation of oblique strike slip motion on the Clarence fault and the recent age for uplift of the Kaikoura Ranges.
Alkaline igneous rocks of Eocene age occur within the core of the Limestone Hill - Bluff Hill synclinal outlier. Geochemically these rocks have OIB type enriched mantle characteristics, typical of alkaline intraplate basanites and basalts associated with lithospheric extension and continental rifting. Classification of samples collected from localities between Limestone Hill and Muzzle stream define a suite of rocks from olivine melanephelinite to nephelinite. The olivine melanephelinites have geochemical characteristics approaching mantle type conditions. The geochemical characteristics suggest that the alkaline igneous rocks were derived by small volume (< 5%) partial a melting of an enriched mantle plume. The primitive nature of these rocks allows an insight into the nature of the source from which these rocks were derived.
Geochemical comparison of the Eocene igneous rocks with locally occuring Cretaceous basaltic lavas within the synclinal oudier, indicate that the Eocene rocks originated from a more enriched source. This source variation appears to be restricted to the local Clarence Valley area. Geochemical evidence from other Cretaceous and Eocene-Oligocene igneous rocks from Marlborough and the Chatham Islands suggests that the source region for these rocks and the Eocene rocks from this study is similar.
The occurrence of the Eocene alkaline rocks in the study area has implications on the timing of extensional tectonic episodes across what is at present a convergent plate margin between the Pacific and Australian plates. Seismic interpretations from the western Chatham Rise suggest three extensional episodes in this region; Late Cretaceous; mid Eocene; and Neogene. The first two periods can be correlated to the Late Cretaceous and Eocene magmatic events recorded in the study area. The Neogene extensional event recorded on the Chatham Rise is absent from Marlborough. This is due to the onset of compression across the converging Pacific-Australian Plates in the early Miocene.
