Tim Charlton, Banda Arc Geologist

reports

 

Structure of the Banda forearc

 The Banda Arc (Figure 1), situated in eastern Indonesia and Timor-Leste (East Timor), marks the zone of collision between the NW edge of the Australian continent and a former oceanic subduction zone. The external arc, including the islands of Timor, Seram, Tanimbar and the western Kei islands, is structurally a fold and thrust belt consisting of the imbricated outer edge of the Australian continental margin overlain at high structural levels by remnants of the pre-collisional oceanic forearc complex. This collision complex is an established petroleum province with small-scale commercial production in the island of Seram. So far, however, exploration has failed to establish commercial hydrocarbons elsewhere in the Banda forearc despite good petroleum indicators particularly in Timor island. A major disincentive to wider exploration in the region is a common perception of extreme structural complexity, but this has probably been overstated in previous studies. 

 
  banda arc hcs    

Figure 1: Regional hydrocarbon occurrences

In many fold and thrust belts worldwide there has long been debate regarding the degree to which underthrust basement is involved in the foldbelt structures. In the case of the Banda Arc and particularly Timor where most of the debate has been focused both thin-skinned and thick-skinned models have been proposed. The present writer favours a thick-skinned, basement-involved interpetation for the deeper structural levels of the Banda forearc (see Charlton 2002a, 2004; and further discussion in the 'Banda basement complexes' section). The thick-skinned structural domain developed beneath a thin-skinned fold and thrust belt that formed in the early stages of arc-continent collision (Figure 2).

 inversion cooper et al

Figure 2: Inversion anticlines developed from basement-involved tectonics. Redrawn from Cooper et al. (1989). This simple structural model provides an excellent explanation of Banda forearc structure.

A thick-skinned structural model suggests the possibility of large and structurally simple inversion anticlines directly beneath the near-surface complexity of the thin-skinned fold and thrust belt. Offshore seismic data is now beginning to successfully image inversion anticlines beneath the near-surface thin-skinned fold and thrust belt, such as the South Bena structure offshore south of West Timor (see here: their figure 9). Onshore the huge, structurally simple Aituto Anticline in central Timor-Leste (Figure 3) is an example of an inversion anticline breached by deep erosion.

aituto anticline

Figure 3: Aituto Anticline in oblique Google Earth view

Further towards the foreland (to the south in Timor) equivalent structures are interpreted in the subsurface where they may potentially form large hydrocarbon exploration targets (e.g. Timor-Leste, Figure 6; and West Timor, Figure 9).

Inversion anticlines and how to spot them in the subsurface

In the absence of onshore seismic data, a number of geological parameters can be used to guide the interpretation of inversion anticlines in the subsurface. These include:

Gravity data. Because inversion anticlines are often developed from pre-orogenic graben-fill successions, they frequently correspond to negative Bouguer gravity anomalies at the present ground surface. Others, however, such as the sub-Kolbano inversion anticline in West Timor (Charlton, 2002b), appear to be basement-cored structures and correspond to gravity highs (see gravity data in Boz et al., 2014).

Refolding of the surface fold and thrust belt. The Kolbano surface fold and thrust belt has apparently been refolded by the subsurface development of an inversion anticline (Charlton, 2002b). Our fieldwork in Timor-Leste has identified smaller-scale refolding of a fold and thrust belt succession in the Suai area of Timor-Leste which we interpret as marking the crest of another inversion anticline.

Systematic uplift of Quaternary reef terraces. Fringing reef limestones are widely developed around the coastlines of Banda forearc islands. These reef terraces are frequently domed up over late-stage inversion anticlines that were developing contemporaneously in the subsurface. The Lautem Dome at the eastern end of Timor island is a good example of this type of structure (Charlton, 2002c) while the island of Babar to the east of Timor seems to be a single huge dome some 20km across with Quaternary reef up to the crest of the dome at elevations of over 700m.

Shale diapirs/mud volcanoes over the anticlinal crest. Shale diapirs formed by remobilisation of collision complex shales often intrude into the laterally extending crestal regions over subsurface inversion anticlines. The surface expression of these shale diapirs is often active mud volcanism (Barber et al., 1986).

Off-structure oil and gas seeps. Oil and gas seeps are frequently found around the margins of interpreted inversion anticlines, possibly associated with spillpoints on subsurface structures.

 

 

 
 


 

 

   

 

 

© Tim Charlton
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