THE ORIGIN OF PASSIVE CONTINENTAL MARGIN ESCARPMENTS: INSIGHTS FROM COMBINING MULTIPLE ANALYTICAL TECHNIQUES

Numerical surface process models predict various styles of margin development, ranging from steady, lateral escarpment retreat to escarpment formation and pinning at an inland drainage divide. Evaluation of these disparate models requires quantitative geomorphic constraints from a range of passive margins on appropriate time-scales (10⁴ -10⁸ a). Investigations of the geomorphic history of passive margins commonly focuses on large-scale escarpments as these outstanding features dominate many margins, both young and old (>60 Ma), and ideas about how they form are central to the margin debate.

Recently we demonstrated the benefits of producing an integrated, quantitative record of post-break-up denudation for the south-west African margin in central Namibia (Cockburn et al., 2000). Through the combined use of apatite fission track thermochronology and in-situ-produced cosmogenic isotopes we were able to show that escarpment behaviour in central Namibia was dominated by very slow retreat, ~10m/Ma, for a major portion of the margin’s history. Our results refute a long-standing model of rapid, steady escarpment retreat (~1 km/Ma) for this area and support recent numerical studies that suggest formation of a laterally stable escarpment pinned at an internal drainage divide.

Here we report ongoing studies in southeast Australia and Namibia where we incorporate U-Th/(He) apatite thermonchronometry to provide complementary data to fission track and cosmogenic isotope analysis. Previous research has pointed to slow erosion rates in southeast Australia through the Tertiary, but a range of denudation chronologies still coexist. Preliminary cosmogenic data along a transect across the escarpment at Brown Mt., NSW (36° 45'S), indicate bedrock erosion rates on the escarpment crest >15 m/Ma compared to lower rates both on the inland plateau and coastal plain. These new measurements combined with existing fission track data are inconsistent with a steady retreat model for the southeastern Australian escarpment at this location.

Cockburn, et al., 2000, Earth and Planetary Science Letters, 179, 429-435.