Durney, David W1
1Earth & Environmental Sciences, Macquarie University, Australia
A particular style of convergent deformation in upper crustal sedimentary basins in many parts of Australia is gentle to close upright folding on multiple trends, typified by dome-basin interference patterns. This style occurs widely in Siluro-Devonian continental back-arc sedimentary and volcanic strata of the Lachlan Orogen (LO) in New South Wales (NSW). Several hypotheses have been suggested for folding in that region, most notably up to four successive extensional and ‘orogenic’ events.
As conventional methods of structural analysis have limited applicability, the present talk aims to outline the basis of the approach used by Hood et al. at this conference in their analysis of this style in the mildly deformed Quidong Basin in NSW.
The basis is both geological and mechanical. It is geological in the sense that:
- the area is one of very-low grade metamorphism where the geologically observable solution-transfer/pressure-solution deformation mechanism is known to be prominent in many sedimentary rocks, and
- extensional deformation is acknowledged in the fromation of the basin.
It is mechanical the sense that:
- the theoretical rheology of solution-transfer deformation is approximately linear-viscous, meaning slow deformation can occur under very low stress,
- because of that, ductile rather than brittle deformation is prominent and
- viscous buckling theory applies to layered strata under horizontal compression.
- As a separate matter, equilibrium of horizontal forces is recognised as a constraint on likely structural combinations at any given time over broad regions of continental upper crust.
It is suggested that these points also limit the range structural hypotheses that are applicable to such regions.
Briefly, the methods have been selected to emphasise short-period incremental deformations: (1)identification of separate fold directions, (2) application of buckling theory to infer associated shortening directions and their relative timing in upright Ramsay Type 2 or ‘banana’ refolds, (3) correlation of incrementally sensitive mesoscopic spaced cleavages in limestones with the folds, (4) use of outcrop-scale kinematic indicators to establish the 3D character of the deformations, (5) use of buckling theory to explain “fault-related minor folds” and (6) application of force equilibrium to infer regional distribution.
David Durney began geology at London under John Ramsay (BSc 1965, MSc 1968, PhD 1972), has taught structural geology and mapping (Macquarie University 1973–92, 1993–2005; Barcelona 1992–93; UNSW 2008) and maintains an interest in geology of the Lachlan Orogen.