The Gundahl Complex of the New England fold belt,Eastern Australia: a tectonic mélange formed in a palaeozoic subduction complex

The New England Fold Belt of eastern Australia preserves a Palaeozoic fore-arc terrain with a magmatic arc, fore-arc basin and a subduction complex. The Gundahl Complex is a tectonic mélange of regional extent in the subduction complex. The matrix and slabs of the Gundahl Complex have six mappable lithofacies: argillite, greywacke-argillite, greywacke, argillite-tuff, bedded chert and greenstone. The argillite matrix is pervasively sheared with many slickensided shear fractures. Locally the matrix is formed by highly sheared greenstone. Greywacke and greenstone blocks are affected by internal shear zones and the blocks themselves pinch and swell. Folds, in places with axial-surface spaced cleavage, are common within those slabs comprised of well-bedded sequences. Bedding-plane shear and faulting at a high angle to bedding also occur in these slabs. On a map-scale much of the Gundahl Complex comprises slabs up to 10 km long in imbricate fault-bounded slices which repeat the disrupted pre-mélange stratigraphic sequence. Elsewhere there are lithologically distinctive blocks containing thick coherent sequences which are structurally incorporated into the Gundahl Complex. The unit is believed to have formed by accretion, imbrication and subsequent tectonic disruption of arc-derived sediments and less abundant pelagic sediment and greenstone in an ancient subduction complex.