Integrated geophysical appraisal of crustal architecture in the eastern Lachlan Orogen

Abstract

Forward modelling of potential field data, combined with new geological mapping and deep seismic reflection transects acquired by the Australian Geodynamics Cooperative Research Centre (AGCRC) and New South Wales Department of Mineral Resources, has led to iterative testing of models of crustal architecture of the eastern Lachlan Orogen in New South Wales. This integrated analysis has led to new conclusions about the subsurface that are unlikely to be deduced solely from any of the individual data sets used. Conclusions supported by the consideration of these data include: · Presence of lower crust in the eastern Lachlan Orogen, characterised by higher than average crustal density, high P-wave velocities, and repeated, stacked bands of strong reflectivity. This crust is interpreted to be a stacked pile of metaturbidites and modified oceanic crust (greenstones). · Presence of large volumes of Ordovician volcanic rocks underlying many areas of Silurian-Devonian basin rocks. · Evidence for extensive, deep-cutting blind thrust faults and detachments throughout the crustal section. Major movements on these faults during the early Silurian appear to have significantly thickened the whole crust. · Evidence for many high-level upper crustal slivers, mostly formed during the Carboniferous. · Differences between the western Ordovician Junee-Narromine Volcanic Belt and the eastern Ordovician Molong Volcanic Belt. The former is quite dense, and is inferred to have a large volume of lavas and intrusive rocks. Its structural style is predominantly that of an imbricate stack around a deeper-rooted core. The latter has lower bulk density, and a higher volume of volcaniclastic material. It is now entirely composed of thin, imbricate slices. These differences suggest that the eastern belt may be the rifted off forearc or apron of the western belt which may be the original magmatic centre. · Evidence for different styles of granite intrusion, reflected in different intrusive geometry of Silurian, Devonian and Carboniferous granites.