There are two sides to every trough – the story of young plate collision and its shadow

Keep, M1., Haig, D. W.2

1School of Earth Sciences, The University of Western Australia, 2Oceans Institute, University of Western Australia

As the orogenic product of the collision between the Australian Plate and Banda Arc, the Timor Orogen provides a unique insight into the processes and dynamics of the early stages of plate collision. These early stages, overprinted during ongoing orogenesis, provide information and constraints as to the timing of elements of subduction, the episodic nature of collision, and the resulting regional geodynamics.  Investigating the tectonic, kinematic and geodynamic history of an orogen requires integration of a broad range of field and analytical techniques, the most important element of which must be that any models are consistent with the field geology. That requires a robust understanding of the age and nature of the exposures.

The collisional front of the Timor Orogen includes a severely dismembered sequence of stratigraphic units, which include a number of different pre-collision stratigraphic associations, as well as a structural melange and a synorogenic phase. Juxtaposition of different elements along young, high-angle strike-slip faults means that stratigraphic associations are often juxtaposed across vertical boundaries, commonly eroded. Most original fold-and-thrust geometries are largely dismembered. Biostratigraphy, used in the field as the main determinant of stratigraphic age and association, is perhaps the most important and robust tool in the geodynamic toolbox, although largely overlooked due a lack of familiarity. Painstaking and robust stratigraphic analysis of thousands of samples from across East Timor over the last two decades has facilitated reconstruction of the stratigraphy of the island, which is not easily recognised through lithostratigraphic approaches, allowing interpretations of original formation and subsequent deformation at the collisional front.

In addition, broad regional interpretation of extensive 2D and 3D seismic data from Australia’s North West Shelf over the last 25 years, tied closely to well data (biostratigraphic ages), has allowed correlation of regional events from the Timor Sea westwards to the Carnarvon Basin. These events, marked by uplifts, unconformities, inversion episodes and stratigraphic responses, relate closely to events unfolding at the collisional front. With strain partitioned more strongly at the leading edge, more subtle responses in the strain shadow are not overprinted, and less prone to misidentification. These events corroborate the timing of events seen at the collisional front, occur over a wide area, and tell a different part of the collisional story.

Reconstructing this young and complex orogen requires data from both sides of the Timor Trough.


Myra Keep and David Haig have been working in East Timor since early 2003, making many dozens of visits and collecting thousands of samples for dating. Together they have over 65 years of work history on the Australian NWS, from PNG to the Carnarvon and Perth basins.

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