Geophysical evidence for 'blind' magmatism associated with Devonian rifting, Lachlan Orogen, New South Wales

Abstract

The Silurian?Middle Devonian history of the Lachlan Orogen is characterised by the formation of rift basins and the emplacement of large amounts of granite. Many rift basins contain felsic or mixed felsic and mafic volcanic rocks, indicative of crustal as well as mantle melting being involved in lithospheric extension. However, there are several large rift basins that are filled by siliciclastic sedimentary rocks in which volcanics occupy < 1 % of the basin fill and may be buried. For the latter basins the question is: was rifting amagmatic, or are products of melting present at depth below the surface, either in deep basin sediments or in basement below the basin. In this paper, we attempt to address this question for sedimentary basins in the Cobar?Louth region of western New South Wales. For the Late Silurian?Early Devonian Cobar Basin, we use 1989 explosion?generated seismic reflection data that have been reprocessed using a new semblance filtering technique to improve the data quality. For the Nelyambo Trough, part of the Devonian Darling Basin in western NSW, we used Vibroseis deep seismic reflection data recently acquired in cooperation with Geoscience Australia and the Predictive Mineral Discovery Cooperative Research Centre. Gravity profiles were acquired along the Cobar lines. For the Nelyambo Basin, gravity data were extracted from a statewide dataset to match the seismic lines. The combined seismic and gravity data sets suggest that bright reflectors in the seismic sections represent mafic volcanics. These reflectors lie within inferred rift fill near the base of the Nelyambo Trough, but also occur in basement under the southwestern margin of the Cobar Basin.