The Narooma terrane (also referred to as the Narooma Accretionary Complex) is a geological structural region on the south coast of New South Wales, Australia, interpreted as the remains of an oceanic terrane or subduction complex. It is exposed at the surface around Narooma, Batemans Bay, and extends south into Victoria near Mallacoota. It forms part of the outboard (eastern) portion of the Lachlan Orogen (also called the Lachlan Fold Belt), which is part of the larger Tasman Orogenic System of eastern Australia.
Geological Setting
The Narooma terrane is considered a tectonostratigraphic terrane that was accreted to the eastern margin of Gondwana during the Early Silurian Benambran Orogeny. It has been interpreted either as an exotic terrane transported by the Pacific Plate approximately 2,500 km westward before colliding with the Gondwana margin, or as a component of the Lachlan Fold Belt itself. According to the classification of Willman et al. (2002), it lies within the Mallacoota Zone of the Eastern Lachlan Fold Belt, which is part of the broader Benambra Terrane.
Stratigraphy
The Narooma terrane comprises the Wagonga Group, which consists of two main formations:
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Narooma Chert: A sequence of bedded chert (ribbon chert) and interbedded siliceous mudstone, with a lower massive chert part and an upper part containing shale interbedded with chert. Conodont biostratigraphy indicates the lower part ranges from mid-Late Cambrian to Darriwilian–Gisbornian (late Middle to early Late Ordovician), while the upper part contains Eastonian (Late Ordovician) graptolites.
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Bogolo Formation: An argillaceous unit consisting of slate, phyllite, mudstone, and mélange, conformably overlying the Narooma Chert where not disrupted by faulting. It contains subordinate arenite beds, boulder beds, and chert lenses.
The Wagonga Group is also associated with the Kianga Basalt, which includes pillow lavas and mafic breccia/conglomerate.
Structure
The Narooma terrane consists of a stack of imbricate thrust slices caught between two thrust faults. The structural sequence from top to bottom is:
- Turbidite sequence (Early Ordovician) at the top
- High-strain zone containing broken fragments, pressure solution textures, dilational veins, boudinage, and mylonite
- Chert (Late Cambrian to Late Ordovician)
- Block-in-matrix mélange at the base, containing blocks of turbidite, chert, and pillow basalt in various orientations and sizes
The Narooma anticlinorium is a north-plunging, apparent antiformal structure developed within the imbricated sequence. Deformation involved superposition of early structures (related to subduction underplating) by later structures (wedge shortening). The cleavage strike direction is approximately 330°. Inland zones display chevron folding with reverse faults.
Metamorphism
Metamorphic studies using Kübler Index values indicate lower epizonal (greenschist facies) conditions. White mica b₀ lattice parameter values are characteristic of intermediate-pressure facies series, suggesting pressures of approximately 4 kbar and temperatures around 300 °C, corresponding to a depth of burial of approximately 15 km and a geothermal gradient of about 20 °C/km during subduction.
Geochronology
40Ar/39Ar and K–Ar dating of white micas from the complex indicates that the cleavage-forming event occurred at approximately 460–440 Ma (Late Ordovician to Early Silurian). Younger ages (~413 Ma) in the finest grain fractions may record a later thermal event. K–Ar ages from the Adaminaby Group west of Batemans Bay (384.6 ± 7.9 Ma and 395.8 ± 8.1 Ma) are interpreted to reflect the thermal influence of Middle to Late Devonian granitoid intrusions.
Tectonic Interpretation
The Narooma terrane is interpreted as an oceanic terrane that accumulated in a remote ocean basin for approximately 50 million years. The upward increase in terrigenous components (shale, argillite, siltstone, sandstone) records the approach of the terrane to the Australian sector of the Gondwana margin. Blocks of chert, argillite, and sandstone reflect extensional/strike-slip disruption as the terrane approached a transform trench along the Gondwana–proto-Pacific plate boundary. Blocks of basalt and basalt breccia represent detritus from a seamount entering the trench.
There is ongoing debate regarding the tectonic setting. Some researchers interpret the Narooma terrane as an accretionary prism/subduction complex formed during the Benambran Orogeny, while others (e.g., Glen et al., 2004) argue there is no evidence that it formed in an accretionary prism/subduction complex, instead interpreting it as an oceanic terrane that was thrust onto the continental margin. Alternative models (Packham & Hubble, 2016) propose that the boundary between the Narooma and Albury-Bega terranes represents a thrusted passive margin accumulation rather than a subduction complex.
Key References
- Glen, R.A., Stewart, I., & Percival, I.G. (2004). The Narooma Terrane: implications for the construction of the outboard part of the Lachlan Orogen. Australian Journal of Earth Sciences, 51(6), 859–884.
- Miller, J.M., & Gray, D.R. (1997). Subduction-related deformation and the Narooma anticlinorium, Eastern Lachlan Fold Belt, Southeastern New South Wales. Australian Journal of Earth Sciences, 44(2), 237–251.
- Packham, G., & Hubble, T. (2016). The Narooma Terrane offshore: a new model for the southeastern Lachlan Orogen. Australian Journal of Earth Sciences, 63(1), 23–61.
- Prendergast, E., Offler, R., Phillips, D., & Zwingmann, H. (2011). 40Ar/39Ar and K–Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW. Australian Journal of Earth Sciences, 58(1), 21–32.
- Wilson, C.J.L. (1969). Geology of the Narooma area, N.S.W. Journal and Proceedings of the Royal Society of New South Wales, 101, 147–157.
