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Monuments Locality: Transposition and the Role of Melt in Strain Distribution
Broken Hill Block, Curnamona, Australia

Roberto Weinberg, Monash University, Australia

M-GiG: Barney Stevens, Bill Collins and Mark Pawley



Copyright 2004-2011 by Roberto Weinberg. All rights reserved. Unlimited permission to copy or use is hereby granted for non-profit driven enterprise, subject to inclusion of this copyright notice and acknowledgment of the source URL:


I would very much appreciate an email stating how this material will be used: Roberto Weinberg, Monash University, Australia. Thanks, RW.


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truncation of fold, transposition, migmatite

Unmelted, gently folded pelitic layers, truncated E and W by transposed , high strain, melt-rich, heterogeneous layers (south is towards the top of the figure). Details of this panorama are shown below.


The Monuments Locality, NE of Broken Hill, in Sundown Group rocks, records the heterogeneous behaviour of pelites and psammites. The Na-poor pelites do not melt at the peak metamorphic conditions in this locality, but the psammites do. The melt-rich psammite-dominated regions become disrupted by granitic melt flow. The original tectonic foliation and bedding become transposed into the axial planar orientation of F2 and more refractory layers become disrupted. These high strain zones loose some or most of their melts, producing heterogeneous bands rich in refractory material, and cushioning the more competent pelite-rich areas from the impact of deformation. The result is that in between these melt-rich high strain zones,weakly folded pelite boudins are preserved.


This outcrop is included in the excursion because it demonstrates the role of melt in deformation and because, similar to the issue of permeability of rocks, it shows that it is not the small scale viscosity of partially molten rocks as measured in the laboratory that controls the behaviour of rock masses, but it is their ability to take up strain which is fundamentally controlled by the dynamically changing geometrical distribution of channelways through the rock mass.


A) West side of the open fold in the pelite, showing truncation and fragments of pelites in melt-rich zone transposed to the axial planar orientation of F2 (N30E).
truncation of fold, transposition, migmatite


B) West side of the open fold in the pelite, and breccia-like distribution of fragments.
truncation of fold, transposition, migmatite


C) East side of the open fold in the pelite, showing truncation.
truncation of fold, transposition, migmatite