EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 18,753-755 (1993)

SHORT COMMUNICATION

REPLY. DENUDATIONAL ISOSTATIC REBOUND OF INTRAPLATE HIGHLANDS: THE LACHLAN RIVER VALLEY, AUSTRALIA

PAUL BISHOP

Victorian Institute of Earth and Planetary Science, Department of Geography and Environmental Science, Monash University. Clayton. Victoria 3168, Australia

AND

RODERICK BROWN

Victorian Institute of Earth and Planetary Science, Department of Geology, LuTrobe University, Bundoora, Victoria 3083. Australia

Received 4 March 1993 Revised 6 June 1993

ABSTRACT

Two main points are raised in replying to Fabel et al. (1993) in their discussion of the paper by Bishop and Brown (1992). It is argued that local variations in denudation are insufficient to invalidate the key conclusions, especially as the amount of sediment deposited in the Murray Basin is a key (and known) measure of the amount of denudation. The total evidence for rebound is now known to be regional in extent, supporting the relevance of a regional isostatic model. We note that isostatic rebound need not to be continuous, so that evidence of episodic rebound does not conflict with it, and that a lack of isostatic rebound would require a strongly opposing active tectonic regime. That is, a key general conclusion is that denudational rebound should always be assessed as an explanation for uplift before active tectonism is invoked.

KEY WORDS Erosion and deposition Isostatic rebound Tectonic activity Lachlan River

‘One of the oldest and perhaps the most commonly ignored physical principles of geology is isostasy, the simple relationship between surface elevation and buoyancy of the underlying lithosphere’

Lachenbruch and Morgan (1990, p. 39)

We thank Fabel et al. (1993) for their discussion, and welcome the opportunity to explore some general issues raised by their comments, as well as to discuss the particular points to which they draw attention. We will first address the particular, before turning to the more general.

Fabel et al. (1993) questioned the denudational data we used in modelling the unloading. Bishop et al. (1985) argued, on the basis of the equivalence between the rates of Neogene fluvial incision in the Lachlan highlands valley and the rates of Neogene sedimentation in the adjacent Murray Basin, that denudation has been essentially catchment-wide in a Hack style of landscape evolution (Bishop and Brown, 1992, p. 351), and has not been restricted to fluvial incision, as claimed by Fabel et al. (1993). The styles of landscape evolution are ultimately not critical to our argument, however, because the volume of Neogene sediment stored in the Lachlan fan and its upland valley (Bishop, 1985) provides a minimum estimate of the Neogene unloading that

01 97-9337/93/080753-03$06.5Q 0 1993 by John Wiley & Sons, Ltd.

7 54 SHORT COMMUNICATION

occurred in the Lachlan catchment upstream of the Lachlan alluviated valley and the Murray Basin; the rates so derived are consistent with those reported from neighbouring parts of the southeastern Australian highlands (Wellman, 1987; Young, 1983; Young and McDougall, 1993). Moreover, Bishop et al. (1985) explicitly noted that the headward decline in erosion highlighted by Fabel et al. (1993) is a minor element in the overall catchment-wide evolution of the landscape. Finally on this issue of the denudational model, Figures 3 and 8 of Bishop and Brown (1992) clearly demonstrate that the spatial extent of rejuvenation at the margin of the highlands as a result of displacement due to rebound is also minor in the overall denudational development of our study area. Indeed, we explicitly suggested that the rebound-induced rejuvenation may be accommodated within the narrow transition zone between the highlands proper and the interior alluviated lowlands (Bishop and Brown, 1992, p. 356).

Fabel et al. (1993) criticized the use of a regional isostatic model for explaining small-scale subaerial features. We would argue that there is no reason that small-scale subaerial features cannot be explained by regional lithospheric processes. We concentrated on the tectonic features we could identify in the Lachlan valley in the transition zone as it leaves the highlands (because this is one area in which rejuvenation should be clearly in evidence), but drew attention to offset extending up to 60 km north of Cowra (our Figure 3). Our continuing work in the area now shows that the zone of rebound also extends at least 25 km south of Cowra (Bishop and Goldrick, 1992). That is, although the vertical displacements due to rebound are minor, their regional extent indicates that they are the expression of rebound on a regional scale and that regional isostatic models are therefore appropriate. The low amplitudes of the effects of rebound are precisely what would be expected under conditions of low rates of denudation, and the offset surfaces resulting from recent rebound would be expected to be found at only low elevations above the modern river bed (cf. the 1 1 m referred to by Fabel et al., 1993).

We agree that confusion in our discussion of east-side-up and west-side-down displacement in the transition zone is possible. The legend in our Figure 3 notes a west-side-down offset, and this was only intended (ambiguously, we now realize) to indicate relative offset. We envisage that the major offset in the landscape due to denudational rebound would indeed be east-side-up, but we note that our modelling (cf. our Figure 9c and d, and text p. 355) indicates that a small amount of relative depression on the basinal side of the rebound fault (i.e. west-side-down) is possible in the case of faulting of the most rigid lithosphere.

A general point raised by the discussion concerns the magnitude-frequency characteristics of denudational isostatic rebound and stillstands in landscape evolution. Models such as that of Gilchrist and Summerfield ( 1 991) indicate that stress due to denudational unloading accumulates in an essentially continuous way over time scales greater than lo4 to lo5 years. But these models do not demand that the surface expression of such rebound be continuous. In fact, because of friction on the fault planes along which the rebound stress is accommodated, it cannot be continuous. Gilchrist and Summerfield (1991, p. 559) observed that compensation can be ‘episodic on the time scale of small, individual crustal displacements along faults in response to progressive loading’. Episodic movement in response to denudational unloading is therefore not only theoretically acceptable but to be expected. It is premature, therefore, to dismiss denudational rebound as a mechanism to account for episodic uplift in the Lachlan valley (and, incidentally, the Buchan area of the southeastern Australian highlands (Webb et al., 1992), especially as the inferred uplift in this latter case seems to have occurred over the last 1 Ma).

A more important general issue raised by our discussants is the relative magnitudes of active (dynamic) and passive (rebound) tectonics in any situation in which uplift or subsidence is identified. The current paradigm in geosciences generally places more emphasis on active tectonics (and active change in landscape evolution, cf. Bishop, 1982). This is demonstrably reasonable in many situations, but if we accept the fundamental reality of the principle of isostasy (Lachenbruch and Morgan, 1990), denudational rebound must follow as a matter of course, except in situations where such rebound is prevented by countervailing tendencies such as extreme crustal rigidity and/or active tectonic subsidence.

This formed the rationale of our Lachlan valley case study: could the tectonic offset identified by several lines of evidence be explained by passive tectonics (denudational rebound) or was it necessary to invoke active uplift? In the Lachlan valley, reliable data on rates of highlands catchment-wide denudation and sedimentation of the adjacent Murray Basin enabled modelling of denudational unloading. Of course, in

SHORT COMMUNICATION 755

modelling the response to denudational unloading, constraining the model parameters is crucial; this is why we stated, ‘It is not yet possible, if ever it will be, to demonstrate conclusively that such rebound is operating; rather, this paper presents a range of relevant field data and shows that these data are consistent with the amounts of denudational rebound reasonably predicted by appropriate models’ (Bishop and Brown, 1992, p. 347).

REFERENCES

Bishop, P. 1982. ‘Stability or change: a review of ideas on ancient drainage in eastern New South Wales’, Australian Geographer, 15,

Bishop, P. 1985. ‘Southeast Australian late Mesozoic and Cenozoic denudation rates: A test for late Tertiary increases in continental

Bishop, P. and Brown, R. 1992. ‘Denudational isostatic rebound of intraplate highlands: The Lachlan River valley, Australia’, Earth

Bishop, P. and Goldrick, G. 1992. ‘Morphology, processes and evolution of two waterfalls near Cowra, N.S.W.‘, Australian Geographer,

Bishop, P., Young, R. W. and McDougall, I. 1985. ‘Stream profile change and longterm landscape evolution: Early Miocene and modern

Fabel, D., Webb, J. and Finlayson, B. 1993. ‘Discussion. Denudational isostatic rebound of intraplate highlands: The Lachlan River

Gilchrist, A. R. and Summerfield, M. A. 1991. ‘Denudation, isostasy and landscape evolution’, Earth Surface Processes and Landforms,

Hack, J. T. 1960. ‘Interpretation of erosional topography in humid temperate regions’, American Journal of Science, 25?3-A, 80-97. Lachenbruch, A. H. and Morgan, P. 1990. ‘Continental extension, magmatism and elevation; formal relations and rules of thumb’,

Webb, J. A,, Fabel, D., Finlayson, B. L., Ellaway, M., Li Shu, and Spiertz, H.-P. 1992. ‘Denudation chronology from cave and river

Wellman, P. 1987. ‘Eastern Highlands of Australia; their uplift and erosion’, BMR Journal of Australian Geology and Geophysics, 10,

Young, R. W. 1983. ‘The tempo of geomorphologic change: evidence from southeastern Australia’, The Journal of Geology, 91,221-230. Young, R. and McDougall, I. 1993. ‘Longterm landscape evolution: Early Miocene and modern rivers in southern New South Wales,

219-230.

denudation’, Geology, 13, 479-482.

Surface Processes and Landforms, 17, 345-360.

23, 116-121.

rivers of the east Australian highland crest, central New South Wales, Australia’, The Journal of Geology, 93, 455-474.

valley, Australia’, Earth Surface Processes and Landforms, 18, 749-751.

16, 555-562.

Tectonophysics, 174, 39-62.

terrace levels: the case of the Buchan Karst, southeastern Australia’, Geological Magazine, 129, 307-317.

277-286.

Australia’, The Journal of Geology, 101, 35-49.