Palaeozoic crustal thickness in the southern part of the Lachlan orogen deduced from volcano and pluton‐spacing geometry

Abstract

The concept that spacing geometry of volcanoes reflects crustal or lithospheric thickness can be extended to granite plutons. Various factors may control pluton spacing, and the influence of faulting and erosion level is discussed. In southern New South Wales pluton spacing appears to have been largely controlled by a 6 to 10 km thick ‘brittle’ upper layer, but pluton clusters and batholiths are spaced wider, suggesting that the magma was derived from 12 to 25 km within the crust—a figure in accord with available mineral barometric data. S‐ and I‐type, granites have similar spacing geometries; the boundary between their source layers cannot be specifically determined, but is inferred to have been at about 20 km. A possible crustal thickness of 35 km is inferred from the spacing of granites in Victoria and Silurian volcanic centres.

These crustal layers, inferred from spacing geometery compare remarkably wel! with the present‐day seismic profile. A tentative model is proposed for the Palaeozoic crust, which closely resembles the present‐day crust in SE Australia. By inference, post Silurian erosion cannot have removed more than 5 km of crust.