Contemporary Uplift and Erosion of the Southern Alps, New Zealand: Summary

Contemporary uplift and erosion of the Southern Alps, New Zealand: Summary

JOHN ADAMS* Geology Department, Victoria University, Private Bag, Wellington, New Zealand

INTRODUCTION in place in the rivers (Schumm and Stevens, 1973). A method of analysis is presented that relates the channel long profile to an The South Island of New Zealand is crossed by the Alpine Fault, equivalent abrasion coefficient for the gravel bed load and shows the surface expression of part of the Indian-Pacific plate boundary. that for many of the short steep rivers draining west from the The Southern Alps lie east of the Alpine Fault and are presumed to Southern Alps the amount of bed load is halved in the 25 km owe their origin to the now obliquely convergent plate motion. The between the Alpine Fault and the coast. plate collision has caused an estimated 80 km of crustal shortening In the Clutha River, a comparison of catchment bed rock (chlo- (Walcott, 1978) across the South Island in the last 15 m.y., most of rite schist with 5% quartz veins) and bed load (95% quartz peb- it in the past few million years. At current rates of shortening (22 ± bles) carried to the river mouth at Balclutha shows that more than 2 mm yr-1), the increase of crustal lithosphere near the 500-km- 94% of the schist has been abraded to silt and finer sizes and is now long Alpine Fault amounts to 700 ± 160 X 109 kg yr"1. carried as suspended load. A similar high proportion of suspended Wellman (1979) has prepared a map of uplift rate in the South load is indicated for other rivers. Hence, in the high-gradient New Island which indicates that 600 ± 100 X 109 kg of crustal rock is Zealand rivers (as in most rivers elsewhere), in order to estimate raised above sea level each year. According to his scheme, the erosion, the suspended load must be well evaluated. shortening is converted to uplift of the Pacific plate along a curved fault plane, the crustal lithosphere being decoupled from the mantle lithosphere. The Southern Alps is the upturned edge of the Pacific plate and in its most rapid part is being upthrust at the shortening rate. Without intervention by some other process, the uplift would be stopped by gravity. However, as is documented in my accompany- ing article in Part II, the erosion rates in the Southern Alps are sufficiently high to remove all newly uplifted rock, making the uplift no more difficult with time. Continental crust is removed from the collision zone by river erosion and is carried away to the east and west and deposited on the converging plate margins. Sediment deposited on the Pacific plate might eventually be uplifted and again eroded. Thus the state for the past few million years could continue almost indefinitely.

BED-LOAD ABRASION AND THE IMPORTANCE OF RIVER SUSPENDED LOAD

Most New Zealand rivers have gravel beds and appear to carry large amounts of bed load. As the gravel is moved, it abrades and produces fine-grained products that are added to the river sus- EROSION RATE pended load. Field observations show that the Sternberg Law de- scribes well the wear of sound pebbles in the middle and lower 10000 reaches of rivers, but poorly that of unsound pebbles in river head- '»* «1 waters. Unsound pebbles abrade faster near their source and more 3000 slowly away from it than is predicted by the Sternberg Law (Adams, 1979). Abrasion occurs about ten times faster in the rivers than in tumbler experiments, probably because pebbles can abrade I03kg km2/' Figure 1. Map of South Island site catchments showing calcu- lated suspended-load transport rates expressed as erosion rates. Present address: 71 James St., Ottawa, Ontario KIR 5M2, Canada. Areas for which rates have been estimated have not been enclosed.

The complete article, of which this is a summary, appears in Part II of the Bulletin, v. 91, no. 1, p. 1 — 114.

Geological Society of America Bulletin, Part I, v. 91, p. 2-4, 2 figs., January 1980, Doc. no. S00101.

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NORMAL RIVER LOAD

The dissolved load of South Island rivers has been estimated from water analyses. The bed load has been estimated from a formula derived in the text from the Einstein-Brown formula; it is checked against independent estimates of bed load carried by New Zealand rivers and shown to be reasonable. To date, the sampling of river suspended load concentrations in New Zealand has been inadequate, and few high flows and even fewer rising stages have been sampled. The load carried in suspen- sion by New Zealand rivers is less than they could carry, and hence must be controlled by the availability of load. In many floods, the available load is exhausted early in the flood and causes a "flushing effect" with more load being carried during the rising than during the falling flows. The flushing effect combined with the poor sampling of rising flows leads to an underestimate of load when usual methods are applied. A method that allows for the flushing effect (Thompson and Adams, 1979) has been applied to the determination of normal river suspended loads in the South Island (Fig. 1). Rivers draining the steep, wet western face of the Southern Alps carry sediment equivalent to an annual erosion rate in excess off 10,000 metric tons per km2 (107 kg km-2), equal to a mean lowering of the land- scape by 3.8 mm or more per year. Thompson and Adams (1979) also related erosion to runoff and showed that a power relation exists, and they applied the relation to the eastern slopes of the Southern Alps for which there is a steep precipitation gradient. Their method is used to determine the spatial extent of erosion and to calculate erosion from ungauged areas. The estimated normal river load is 285 x 109 kg yr"1 from 154,000 km2. Suspended load is 93% of the total, dissolved load is 4%, and bed load is 3%. Figure 2. Map showing offshore areas in which the sediment eroded from the South Island is deposited. Inset shows the shape of ABNORMAL RIVER LOAD: the deposit in the Tasman Sea. Rates in each area are given in units 9 1 THE EFFECTS OF EARTHQUAKES of 10 kg yr .

Abnormal events are considered to be those that are large and infrequent, in contrast with the normal events considered in the previous section. With one exception, only normal loads have been sampled. Abnormal loads are supplied by earthquakes and by estimated to be 580 ± 110 x 109 kg. The estimates of river load are localized intense storms. Estimates for the load supplied by three confirmed in part by the sediment in the traps, but much more earthquakes in the northwest part of the South Island were made by work is needed to refine the estimates. measuring earthquake-caused landslips from air photographs. The amounts supplied by each earthquake were: Arthur's Pass (mag- THE SOUTHERN ALPS AS A STEADY-STATE nitude 6.9, 1929), 106 x 109 kg; Buller (7.7, 1929), 2,300 x 109 MOUNTAIN RANGE kg; and Inangahua (7.1,1968), 94 x 109 kg. Sediment load concentrations were measured on the Buller River below the slipped area Continued rapid uplift of a mountain range leads inevitably to before and after the Inangahua earthquake and show that the rapid erosion, and in the long term, erosion must equal uplift. A additional abnormal load was ten times the normal load for a short steady state is demonstrated for the Southern Alps and is also period after the earthquake. Half of the abnormal load entered the achieved in some other mountain ranges (for example, in Taiwan, sea in the first 19 months after the earthquake. An estimate of the Japan, and the European Alps). Two types of mountains are distin- abnormal load, prorated as if it were an annual increment, of 400 guished: "spiky" mountains with straight slopes and sharp tops ± 200 X 109 kg has been made from a relation between magnitude whose height represents a steady state balance between uplift and and amount slipped and from an analysis of earthquake frequencies erosion; and "flat-topped" mountains that are not in steady state. in New Zealand. The flat tops are the remains of a pre-uplift surface of low relief. The river loads were checked where possible against the mass The growth of the Southern Alps has been modeled by develop- that has accumulated in sediment traps such as lakes and the sea. ing relations between runoff and mountain height; between height, Many of the traps on land are partially open and provide minimum runoff, and erosion; and by applying constraints discussed in Part estimates only, but some (for example, deep lakes) would provide 11. According to the model, the mountains of the Southern Alps at- good rates if the amount of fill could be determined precisely. tained erosional steady state soon after uplift began, and the range Offshore sediment accumulation rates provide the final check for itself attained a steady state after about 1.5 m.y. Even though the erosion rates inferred from onshore data (Fig. 2). The annual rate is range as a whole is in a steady state, some of the individual

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mountains change with time, flat-topped mountains from the east REFERENCES CITED (for example, Lammerlaws in Otago) becoming steady-state spiky mountains as they are uplifted and moved toward the Alpine Fault Adams, J., 1979, Wear of unsound pebbles in river headwaters: Science, v. 203, p. 171-172. by the crustal shortening. Schumm, S. A., 1963, The disparity between present rates of denudation and orogeny: United States Geological Survey Professional Paper CONCLUSIONS 454-H, 13 p. Schumm, S. A., and Stevens, M. A., 1973, Abrasion in place, a mechanism Uplift and erosion are approximately in balance in the Southern for rounding and size reduction of coarse sediment in rivers: Geology, v. 1, p. 37-40. Alps, and the following rates have been determined: Thompson, S. M., and Adams, J., 1979, Suspended load in some major riv- 9 crustal shortening 700 ± 160 x 10 kg yr'' ers of New Zealand, in Murray, D. L., and Ackroyd, P., eds., Physical tectonic uplift 600 ± 100 x 109 kg yr"1 hydrology — New Zealand experience: New Zealand Hydrological river load 700 ± 200 x 109 kg yr1 Society, p. 213-229. offshore deposition 580 ± 110 x 109 kg yr"'. Walcott, R. I., 1978, Present tectonics and late Cenozoic evolution of New Zealand: Geophysical Journal of the Royal Astronomical Society, The relation here of continental plate collision and mountain v. 52, p. 137-164. formation to erosion has considerable relevance to the growth and Wellman, H. W., 1979, An uplift map for the South Island of New Zealand erosion of other mountain ranges. The balance on the obliquely and a model for the uplift of the Southern Alps, in Walcott, R. I., and convergent Indian-Pacific plate boundary suggests that in other Cresswell, M. M., eds., Origin of the Southern Alps: Royal Society of New Zealand Bulletin 18. areas of rapid crustal uplift and shortening, erosion may be sufficiently rapid to balance the uplift and that there need be no MANUSCRIPT RECEIVED BY THE SOCIETY FEBRUARY 7, 1979 "disparity between uplift and erosion" rates as considered by REVISED MANUSCRIPT RECEIVED SEPTEMBER 4, 1979 Schumm (1963). MANUSCRIPT ACCEPTED SEPTEMBER 7, 1979

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