Cenozoic landscape development in the Blue Mountains (SE Australia): Lithological and tectonic controls on rifted margin morphology

Abstract

Cenozoic landscape development on the southeastern Australian rifted margin, as recorded by mid‐Tertiary basalt flows that preserve ancient landforms, is generally considered to be very slow. Eocene‐Miocene basalts of the southeastern Australian highlands flowed down paleovalleys, indicating that landscape dissection was already well under way at the time of their eruption. Within the deeply incised Blue Mountains, however, Miocene basalts cap relatively flat hilltops, suggesting that most incision postdates their emplacement. We have studied the controls on these apparent lateral variations in Cenozoic landscape development using both field observations and numerical models. We have mapped the Blue Mountains basalts in detail to reconstruct the Miocene landscape and to quantify both the amount of subbasalt relief and postbasalt incision rates. New geochemical and K‐Ar geochronological data indicate that most of the Blue Mountains basalts were derived from a common magma source and were erupted in a relatively short time span (20.1–14.5 Ma). Subbasalt relief is remarkably gentle; it does not exceed 100 m for any single basalt cap and is of the order of 200 m for the entire region. This contrasts sharply with a present‐day relief of up to 700 m in major river gorges. By comparing the reconstructed mid‐Miocene and present‐day topographies, we estimate plateau lowering and river incision rates at <14 and ≤40 m m.yr.−1, respectively. We explain the dramatic post‐Miocene increase in regional relief by migration of major knickpoints up the river gorges, with retreat rates estimated at 800–1200 m m.yr.−1. The kinematics of postbreakup knickpoint retreat thus play a fundamental role in modifying rifted‐margin morphology. The Blue Mountains are bounded on the east by the Lapstone Structural Complex (LSC), a major faulted monocline that forms the present‐day escarpment. Extrapolation of the estimated retreat rates suggests that knickpoints were initiated on this structure between 48 and 71 Ma. The Blue Mountains escarpment has been interpreted to result from either early Tertiary movement on the LSC or from passive denudation of previously tilted resistant sandstone at the monocline. We employ a numerical surface process model to explore these two hypotheses and conclude that, although lithological control cannot be excluded, early Cenozoic uplift, related to variations in intraplate stresses and/or magmatic underplating, appears to have been a major factor in shaping the anomalous morphology of the Blue Mountains region. This study reveals that significant lateral variation may exist in the morphologic development of rifted margins, and that local lithological and tectonic factors may interact in a complex manner to produce such variation.