The origin of mélanges: Cautionary tales from Indonesia
Highlights
► Mélange on Timor interpreted as olistostromes: no olistostromes in Timor Trough. ► Diapirs formed in Australian Shelf sediments before entering accretionary complex. ► Mélange on Nias attributed to debris flows and thrusting in accretionary complex. ► Mélange on Nias shown to be diapiric within basins on the accretionary complex. ► Why are diapirs common on present day margins, but scarce in older orogenic belts?
Introduction
When Greenly (1919) described a chaotic unit in the Gwna Group of Anglesey as ‘mélange’, containing a variety of blocks of ocean floor and continental shelf materials, such as spilitic pillow lava and tuff, red jasper chert, manganiferous limestone, shale, quartzite and grey limestone in a fine-grained matrix, it seemed obvious that this mélange had been disrupted and sheared during a process of tectonic deformation. Later Beneo (1955) described extensive deposits of mélange in Sicily in the western Mediterranean, which he interpreted as having been formed in a sedimentary environment as the product of large-scale submarine landslides. This interpretation has been used frequently to account for the large-scale mélanges of the Argille Scagliose throughout Italy. In the discussion following Beneo’s (1955) paper, Flores introduced the terms olistostrome for these deposits, and olistoliths for the included blocks. Although Hsü, 1968, Hsü, 1974 recognised tectonic and olistostromal mélanges he suggested that the term ‘mélange’ should be restricted to those formed tectonically, in practice, due to the difficulty of determining their origins, the term has been used to describe all ‘block in matrix’ mélange deposits worldwide, however they may have originated.
Subsequently both tectonic and sedimentary interpretations, sometimes in combination, have been offered for many other occurrences, especially where mélanges are found in association with accretionary complexes, formed in subduction trenches from oceanic material scraped off downgoing oceanic plates (e.g. Aalto, 1981, Cowan, 1985, Collins and Robertson, 1997, Ogawa, 1998, Robertson and Pickett, 2000, Robertson and Ustaömer, 2011). Meanwhile, geologists using seismic reflection profiling and drilling for oil in deltaic deposits, frequently encountered salt diapirs and shale diapirs containing block-in-matrix deposits in deltas on passive margins, attributed to the mobilisation of salt and shale units due to overpressure generated in areas with very high sedimentation rates, e.g. Nile Delta (Loncke et al., 2006), Niger Delta (Morley and Guerin, 1996) Baram Delta (Morley et al., 1998, Morley, 2003). Shale diapirs are also encountered on active continental margins, where the overpressuring and mobilisation of shales is attributed to the stacking of thrust slices, e.g. Barbados (Brown and Westbrook, 1988, Stride et al., 1982, Westbrook and Smith, 1983), the Makran (Stöcklin, 1990, Grando and McClay, 2007), Arakan, Burma (Maurin and Rangin, 2009). Much effort has been expended in defining criteria by which tectonic, olistostromal and diapiric mélanges might be distinguished (e.g. Barber et al., 1986, Camerlenghi and Pini, 2009, Codegone et al., 2012b, Cowan, 1985, Dilek et al., 2012, Festa et al., 2010, Festa, 2011, Lash, 1987, Orange, 1985; Pini, 1999, Raymond, 1984, Silver and Beutner, 1980, Sengör, 2003, Wakabayashi, 2011).
Experience gained during study of mélanges regions of present day active tectonics in the Outer Banda Arc and the Outer Sunda Arc in Indonesia, to which both tectonic and olistostromal origins have been attributed, but for which substantive evidence of diapirism has been subsequently obtained, suggests that diapiric mélange is much more common than many structural and tectonic geologists have appreciated: Diapirism should be considered as an origin for mélanges, before tectonism or submarine slumping are automatically assumed.
Section snippets
Mélange in the Outer Banda Arc
The island of Timor in eastern Indonesia, just to north of Australia, forms part of the Outer Banda Arc (Fig. 1). The island was formed from Australian Precambrian basement and continental margin sediments, ranging in age from the Permian to the Pliocene, imbricated in accretionary complexes and thrust beneath an ophiolite nappe, during the collision between the northern continental margin of Australia and the Banda Volcanic Arc (Barber et al., 1977, Charlton et al., 1991). The eastern part of
Mélange in the Outer Sunda Arc
Outcrops of mélange were mapped in the outer arc islands of Nias, Simeulue, Siberut, Sipora and Pagai, associated with subduction of the Indian Ocean floor beneath Sumatra. (Andi Mangga and Burhan, 1994, Budhitrisna and Andi Mangga, 1990, Endharto and Sukido, 1994). The mélange contains blocks of serpentinite, gabbro, basalt, chert, calcilutite, Eocene limestone with foraminifera, granitic and metamorphic rocks together with abundant greywacke shale and claystone in a scaly clay matrix.
In the
Conclusions
Contrary to the hypothesis of Audley-Charles (1965), evidence from Sumba and Timor indicates that there are no débris flows containing continental shelf blocks, associated with the development of the accretionary complex forming at the present day, which could account for the Bobonaro Scaly Clay Mélange on Timor. Evidence from Sumba indicates that large volumes of mélange, in the form of diapirs, were generated by overpressured fluids in the Australian continental marginal sediments, before
Acknowledgements
The author is indebted to Dr. Tim Charlton for his companionship and support over many years of work on the geology of Timor. He is also thanked for reinterpreting the side-scan sonar imagery across the Timor Trough, shown as Fig. 5 in this paper, to clarify some ambiguities in the original interpretation prepared for his Ph.D. Thesis in 1987.
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