J. H. Youngson

The structural evolution of active fault and fold systems in central Otago, New Zealand: evidence revealed by drainage patterns

Abstract Central Otago in New Zealand is an area of active continental shortening in which a peneplain surface cut into schist has been deformed by folds, which are developed above buried reverse faults. We use the drainage patterns in this region to demonstrate various processes in fold (and fault) growth and interaction that would be difficult to identify by other means. In particular we show: (1) how simple asymmetric folds can develop into box folds; (2) how apparently continuous ridges were formed by the coalescing of quite separate propagating fold (and fault) segments; (3) evidence for the relative ages (or relative uplift rates) of adjacent structures; and (4) evidence for the propagation direction of folds (or faults) as they grow. The few quantitative estimates we obtain for fault propagation rates suggest an increase in length of 10–50 m per earthquake on faults about 20 km long. These estimates are very uncertain, but are similar in magnitude to an estimate made in Nevada for a normal fault of similar size and are also similar to predicted estimates from theoretical growth models. They raise the question of whether fault growth, earthquake recurrence rates and climate change can interact to produce semi-regular discrete features in an active landscape.

GENES MEET GEOLOGY: FISH PHYLOGEOGRAPHIC PATTERN REFLECTS ANCIENT, RATHER THAN MODERN, DRAINAGE CONNECTIONS

Abstract We used DNA analysis of the freshwater Galaxias vulgaris complex (Pisces: Galaxiidae) to test a geological hypothesis of drainage evolution in South Island, New Zealand. Geological evidence suggests that the presently north‐flowing Nevis River branch of the Clutha/Kawarau River system (Otago) once flowed south into the Nokomai branch of the Mataura system (Southland). The flow reversal is thought to have resulted from fault and fold activity associated with post‐Miocene uplift. Mitochondrial DNA sequence data (control region and cytochrome b genes; 76 individuals; maximum divergence 7.1%) corroborate this geomorphological hypothesis: The Nevis River retains a freshwater fish species (Galaxias gollumoides; five sites; 10 haplotypes) that is otherwise restricted to Southland (nine sites; 15 haplotypes). There is no indication that the Nevis River lineage of G. gollumoides lives elsewhere in the Clutha/Kawarau system (> 30 sites). Likewise, two widespread Clutha lineages (G. ‘sp’ D; G. anomalus–G. pullus) are apparently absent from the Nevis (> 30 sites). In particular, G. ‘sp D’ lives throughout much of the Clutha (12 sites, 23 haplotypes), including a tributary of the Kawarau, but is absent from the Nevis itself. Conventional molecular clock calibrations (based on a minimum Nevis‐Mataura haplotype divergence of 3.0%) indicate that the Nevis flow reversal may have occurred in the early‐mid Pleistocene, which is roughly consistent with geological data. The broad phylogeographic structure evident in the Clutha system is consistent with the sedentary nature of nonmigratory galaxiids. Our study reinforces the value of combining biological and geological data for the formulation and testing of historical hypotheses.

Redefinition and interpretation of late Miocene‐Pleistocene terrestrial stratigraphy, Central Otago, New Zealand

Abstract The stratigraphic succession in eastern Central Otago consists of Eocene quartzose fluvial sediments and middle Tertiary marine strata (Onekakara Group), early‐middle Miocene quartzose fluvial sediments and lake deposits (Manuherikia Group), late Miocene‐Pliocene immature sandstones and conglomerates, and Quaternary terrace and fan gravels. Published literature contains at least 20 different approaches for subdivision of this succession. The late Miocene‐Pliocene conglomerates were formed during the rise of fault‐bounded greywacke and semischist mountain ranges. Conspicuous conglomerates in the upper part of this succession are widely referred to as Maori Bottom Formation, but that name was originally applied by miners to a locally auriferous erosion surface beneath Quaternary terrace and fan gravels in Otago. In addition, the term is culturally offensive. We propose the name Hawkdun Group for the late Miocene‐Pliocene succession of tectonically generated sediments in the Maniototo, Ida, and Manu...

Fault growth and landscape development rates in Otago, New Zealand, using in situ cosmogenic 10Be

Abstract We use in situ cosmogenic 10 Be measurements in quartzites to examine the growth and propagation rates of a Late Quaternary anticline forming above a blind reverse fault in Central Otago, New Zealand. We obtain average uplift and propagation rates of 0.10–0.15 mm yr −1 and 1.0–2.0 mm yr −1 respectively over the last 450 000 yr, though it is probable that growth occurred episodically, with more rapid development in the periods 600–400 kyr and 200–100 kyr and relative quiescence between about 400 and 200 kyr ago. These results quantify and confirm a qualitative analysis of the fault evolution based on geomorphology and drainage patterns. We obtain minimum 10 Be exposure ages of up to 750 000 yr for the quartzites, some of the oldest ages obtained by this method outside Antarctica, which are made possible by the extremely low erosion rates of less than 0.6 mm kyr −1 in the resistant quartzites. These rocks consequently provide an opportunity to study geomorphic and tectonic processes in this region over unusually long time periods.

Combining geomorphic observations with in situ cosmogenic isotope measurements to study anticline growth and fault propagation in Central Otago, New Zealand

Abstract Measurements of 10Be concentrations in quartz‐rich boulders exposed by the uplift of anticlinal ranges in Central Otago, New Zealand, are used to investigate the fault propagation styles and rates for the underlying blind reverse faults. 10Be ages along Little Rough Ridge reveal the propagation rate for this fault to be between zero (i.e., not propagating at all, if the ages reflect emergence of a fault of fixed length through a sloping surface) and a maximum of 8.1 mm/yr, if the ages reflect true lateral growth. Regardless of the propagation rate, Little Rough Ridge has established its length rapidly compared with the rate at which it has accumulated displacement. On Raggedy Range, the 10Be ages, together with structural data, imply that the ridge reached its current length by the merging of several small fault segments. This study emphasises the importance of combining 10Be measurements with geomorphic observations. Without understanding the geomorphological context, 10Be exposure ages are difficult to interpret at all; but even with geomorphic data, the interpretation of these ages can still be ambiguous.

Au‐Ag‐Hg and Au‐Ag alloys in Nokomai and Nevis valley placers, northern Southland and Central Otago, New Zealand, and their implications for placer‐source relationships

Abstract Gold in Quaternary placers in the Nevis and Nokomai valleys is dominantly a‐phase Au‐Ag‐Hg alloy (c.<10 wt% each of Ag and Hg) with subordinate Au‐Ag alloy. The α‐Au‐Ag‐Hg alloy is typically coarse grained (up to 2 cm), angular, and rarely flattened or folded. Crystalline texture, quartz intergrowths, and psuedo‐hexagonal crystal pluck cavities are common. Fluvial transport distance estimates based on maximum flatness index of Au‐Ag‐Hg alloy particles are typically <10–20 km. Coarse (up to 2 cm) crystalline cinnabar is commonly associated with the Au‐Ag‐Hg alloy, and both were probably derived from hydrothermal sources in western tributaries of the upper Nevis River and eastern tributaries of the upper Nokomai River. These sources are possibly related to strands of the Nevis‐Cardrona Fault System. Secondary Au‐Ag‐Hg alloy with up to 38 wt% Hg occurs locally in the lower Nokomai alluvial plain, where it coats or cements detrital α‐phase Au‐Ag‐Hg and Au‐Ag alloy particles. The secondary Au‐Ag‐Hg alloy has formed by diffusion between detrital gold particles and liquid Hg that is either natural or derived from local breakdown of cinnabar. Au‐Ag alloy dominates over Au‐Ag‐Hg alloy in the lower half of the lower Nokomai alluvial plain and in the gravel of a Pleistocene perched channel incised into pumpellyite‐actinolite facies semischist basement adjacent to the plain. The Au‐Ag alloy is rounded and commonly flattened and folded. Crystalline texture, quartz intergrowths, and pluck cavities are rare in the Au‐Ag alloy, and a fluvial transport distance of 25–40 km is estimated from maximum flatness index. Au‐Ag alloy, three types of garnet, magnetite, clinozoisite, and well‐foliated schist boulders in the abandoned channel gravel are allochthonous. They were derived from greenschist facies schist and sedimentary sources many tens of kilometres north of the Nokomai catchment, and transported either in Wakatipu Glacier till or fluvioglacial outwash gravel that entered the Nokomai valley via Nokomai Saddle, the confluence with the Mataura River, or both. Mercury minerals in hydrothermal deposits within the Otago Schist appear to be restricted to the Caples Terrane. A gold‐mercury association is proposed for the Caples Terrane.

Sulphur mobility and sulphur‐mineral precipitation during early Miocene‐Recent uplift and sedimentation in Central Otago, New Zealand

Abstract Since the late Oligocene ‐ early Miocene, three distinct uplift events have resulted in widespread sediment deposition in Central Otago, South Island, New Zealand. These events are marked in the sedimentary record by the early‐middle Miocene Manuherikia Group, late Miocene‐Pliocene Wedderburn and Maori Bottom Formations, and Quaternary schistose fan gravels. Each event was accompanied by some basement erosion and extensive recycling of older sediments. Early‐middle Miocene Manuherikia Group quartzose sands and conglomerates (Dunstan Formation) lying on the schist probably represent recycled early Tertiary freshwater ± marine sediments, still preserved in east and west Otago. Dunstan Formation sediments are cemented with framboidal marcasite and subordinate crystalline pyrite (both FeS2) to within a few metres of the surface at many localities. Heavy‐mineral concentrates from Quaternary alluvial fan sediments commonly contain small cubes and/or dodeca‐hedra of authigenic pyrite. Sand and conglomer...

Recycling and chemical mobility of alluvial gold in Tertiary and Quaternary sediments, Central and East Otago, New Zealand

Abstract Alluvial gold in Central and East Otago occurs in four different ages of quartz conglomerate: Cretaceous— Paleocene Taratu Formation, Eocene Hogburn Formation, early Miocene Dunstan Formation, and late Miocene‐Pliocene Wedderburn Formation. Gold has been recycled from older to younger formations. Younger placers also occur in Pliocene and Quaternary strata recycled from the quartz conglomerates, with some dilution by basement clasts. Gold is typically concentrated at the base of units, at or near unconformities. Gold grain size is small (<1 mm; c. 300 μm mode) in Miocene and older placers, reflecting the grain size of gold in source sediments. Gold grains are too small to be hydrodynamically equivalent to the maximum cobble size being transported, so streams were capable of transporting all available gold. A small but consistent increase in gold grain size occurs with decreasing age of the host formations, due to minor mobilisation of gold by groundwater, and reprecipitation on pre‐existing grain...

Formation of authigenic Fe2+‐bearing smectite‐vermiculite during terrestrial diagenesis, southern New Zealand

Abstract Late Cretaceous to Quaternary terrestrial sediments derived from fault scarps and actively growing folds in schist and greywacke basement are locally preserved in Otago and South Canterbury, New Zealand. Sediments are immature, sparsely carbonaceous, and generally poorly sorted and have undergone little oxidation during rapid erosion and deposition cycles. Mild oxidative alteration of sediment clasts in some horizons has resulted in pseudomorphous replacement of detrital biotite and metamorphic chlorite in greywacke clasts, and chlorite in schistose clasts, by a green, interlayered, ferrous iron‐bearing smectite‐vermiculite mineral of variable composition. Smectite‐vermiculite compositions form a regular trend from low Si and Al, high Fe and Mg to moderate Si and Al, to low Fe and Mg. The Fe2+ smectite‐vermiculite also occurs as pools in the matrix and small veinlets, and as a cement intimately intergrown with fine‐grained clastic matrix. The smectite‐vermiculite is a diagenetic product formed pr...

Blue Spur Conglomerate: Auriferous Late Cretaceous fluvial channel deposits adjacent to normal fault scarps, southeast Otago, New Zealand

Abstract Latest Cretaceous Blue Spur Conglomerate at Gabriels Gully was deposited in a northeast‐southwest‐trending valley incised >200 m into metamorphic basement within the Tuapeka Fault Zone. Clast imbrication unequivocally indicates paleoflow towards the southwest, across the Tuapeka Fault Zone from the uplifted block. Early schist‐and quartz‐rich fluvial sediments in the valley and transverse mass flow deposits derived from the valley sides are preserved on a strath terrace on the southeast side of the valley. These sediments were incised by the paleoriver and partially reworked into the base of a deeper channel cut into the underlying schist before deposition of greywacke‐rich fluvial conglomerates. There is a strong enrichment of gold on and just above schist at the base of the incised channel, and subordinate enrichment at and above the basal scours of the overlying fluvial conglomerate packets. Imbricated clasts at less well exposed Blue Spur Conglomerate at Wetherstons, Forsyth, and Waitahuna Gully suggest that southwest‐flowing rivers in incised paleovalleys crossing the Tuapeka Fault Zone also deposited those outliers. The four incised paleovalleys were probably fed by a single river that successively abandoned the valleys in favour of a new course around the propagating tip of the Tuapeka Fault Zone. Structural and geomorphic relationships along the Tuapeka Fault Zone suggest that it is multi‐stranded and segmented on the scale of a few kilometres, rather than being a single continuous structure. Independent and concomitant extension on collinear and overlapping strands, respectively, was the primary control on fluvial incision into intrafault zone blocks and on gold concentration within the valleys. Transverse topographic highs across the fault zone, between sediment sinks on the downthrown side, are probably the result of distributed deformation amongst and between strands. These relative highs controlled the location, size, and orientation of the depositional systems, and prevented the development of axial drainage along the Tuapeka Fault Zone itself. Valley incision and associated gold‐concentrating processes will not have extended significantly downstream from the Tuapeka Fault Zone. The best gold concentrations will lie on intrazone blocks that have undergone the most favourable combination of sediment accumulation, fluvial incision, and re‐concentration cycles, and not necessarily close to the absolute footwall.