The Tamar Trough revisited: correlations berween sedimentary beds, basalts, their ages and valley evolution, North Tasmania

Abstract

The Tamar Trough, an Early Palaeogene fault structure, contains sedimentary beds and interleaved basaltic fl ows that infi ll the structure along its 70 km length. These infi lls represent a complex interplay between sedimentation, channel erosion, eruptive dislocations, and even 'out of trough' diyersions of the ancestral Tamar drainage. Several areas of resistant basalt fl ows remain in the south, upper, middle and lower Tamar reaches. Although some palynological control was known, radiometric dating of previously untested basalts now allows close integration and age-pegging for observed palynological biozones. The K-Ar and Ar-Ar ages of the basalt bodies indicate eruptive events at 47, 33-37 and 25 Ma, correlating with Proteacidites asperopo!us-Ma!vacipo!!is diversus, Nothofagites asperus and I'roteacidites tu bercu!atus biozone age sedimentary beds respectively. Basanite, alkali basalt and hawaiite fl ows dominate basalt lithology with lesser olivine nephelinite, transitional olivine basalt, olivine tholeiite and quartz tholeiite. Basalt geochemistry suggests derivation from different degrees of partial mantle melting (from 7 to 35%), with alkaline and tholeiitic basalts being derived from separate source regions. Most alkaline basalts have high-µ (HIMU) related trace element signatures, which are absent in the tholeiitic rocks. A basalt plug on the trough margin at Loira gave a Jurassic age and has Jura%ic dolerite-like geochemistry. The Tamar sequence suggests that the initial fl uvio-lacustrine and later channel-fi ll sedimentation from 65(?) to 24(?) Ma was then punctuated in places by periods of alkaline volcanism between 47 to 33(?) Ma, and alkaline and tholeiitic volcanism between 33 to 24(?) Ma. No Neogene fossils are known, so this later period was probably one of net erosion. These contrasting quiet sedimentary and more volcanic intervals are related here to a tectonic model that involves northerly drift of Victorian and Tasmanian lithosphere over several former Tasman metasomatised mantle plume sources.

Thermal Structure of Eastern Australia's Upper Mantle and Its Relationship to Cenozoic Volcanic Activity and Dynamic Topography

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