THE GEOLOGICAL SOCIETY OF AUSTRALIA (Victoria Division)
Next General Meeting Thursday 21st March at 6:15 p.m.
tectonic activity and landscape evolution over the last 10 million years
Assoc Prof John Webb, Environmental Geoscience, La Trobe University
Fritz Loewe Theatre, School of Earth Sciences, University of Melbourne
(corner of Swanston & Elgin Sts Carlton)
Talks will be preceded by drinks from 5:30pm in the 4th floor tearoom, cost $2
Despite the public perception that Australia is a tectonically stable continent with ancient landscapes (boring!), the southeastern part of the continent (particularly Victoria) has experienced extensive tectonic and volcanic activity over the last few million years, extending virtually to the present. In the Late Miocene – Early Pliocene (~8 Ma to ~4 Ma), a period of relatively intense NW-SE oriented compression occurred, known as the Kosciusko Uplift, due to a change in relative motion between the Australian and Pacific plates associated with formation of the Southern Alps of New Zealand. This completely changed the Victorian landscape. The Southern Uplands (Otway, Strezelecki and Hoddle Ranges) were uplifted by up to 600 m at ~150 m/Ma (a substantial uplift rate on a passive margin), and the Southeastern Highlands were uplifted by up to 700-1000 m at the same time. The resulting increased gradients caused a pulse of high energy sedimentation along the southern margin of the highlands, depositing an extensive apron of alluvial fan and braided stream sediments. In western Victoria the Kosciusko Uplift displaced Late Miocene strandlines by up to 180 m, with a total uplift of up to 300 m, but did not affect Early Pliocene basalts, which partially infilled the landscape around uplifted blocks. North of the divide the effects of the Kosciusko Uplift were less, but uplifts of 30 m or more occurred along several N-S faults. Following this intense phase of activity, tectonism has been ongoing at a lower intensity through to the present, and the region is still under strong NW-SE oriented compression, with a significant amount of seismic activity over the last 150 years. At Cape Liptrap, displacement of Late Pliocene – Pleistocene marine terraces along the Waratah Fault gives uplift rates of 10-40 m/Ma, and the most recent movement on this fault kinked dune limestones dated at 70-80 ka. At Bushrangers Bay on the Mornington Peninsula, beach sediments dated at 90 ka have been uplifted at least 8 m. There is a suggestion that intervals between major seismic events are of the order of 100 ka, and the youngest substantial movement could have occurred perhaps ~20,000 years ago. Thus Victoria’s landscape has changed greatly over the last few million years, and tectonic activity has not ceased. Although the earthquake risk for Victoria (and Melbourne) is regarded as low, it is not non-existent, a fact that needs to be kept in mind for building codes