Miocene Geodynamic Evolution of the Central NW Borneo Wedge: Subduction of a Crustal Salient

Ringenbach, Jean Claude¹; Sapin, François¹; Pubellier, Manuel²
¹Structural Geology, TOTAL, PAU, France.
²Petronas Univ, IPOH, Malaysia.

Subduction of oceanic highs has been described in offshore accretionary wedges thanks to bathymetric and seismic data, and simulated in sand-box models. Where documented, it is limited to oceanic subduction of seamounts.The hydrocarbon rich NW Borneo Margin is a complex structural domain where deformation style is a mix of compression and extension features associated with transverse structures. The study is based on field work and industrial data. The structural system consists of several geomorphological domains:

- The Rajang-Crocker Wedge, mainly onshore, is oriented N-S in Sabah, curving to E-W in Sarawak. It follows a strong bend and is characterized by short wavelength thrusted folds. The age of deformation is Mid-Eocene to Early Miocene with sequence of thrusting propagating NWestward.

The central-frontal area of the Rajang-Crocker wedge present a ‘hummocky' topography inside the bend with slightly metamorphosed shales and an absence of dominant trends.

In the same domain, (Melinau area-N Sarawak), the Rajang-Crocker Wedge is affected by large out-of-sequence thrust and back-thrust filled with the first deltaic sediments in the area during the Early Miocene.

- The Frontal NW Borneo Wedge, oriented NE-SW from North Sarawak to Palawan Island, is disturbed by the emplacement of the large Champion-Baram deltaic province in front of the bend. It can be divided into four structurally different domains, two compressive and two extensive basins above the Mid-Miocene Unconformity (MMU). This complexity is due to a gravity tectonics overprint. The formation of this second wedge is contemporaneous with post Mid. Miocene uplifts in the Rajang-Crocker Wedge.).

Moreover, the central area records thermal anomalies indentified thanks to several methods of low-temperature analysis.

Two sections (northern Sarawak-Brunei vs. Sabah) and a new structural map illustrate the geometries.

It is proposed that the anomalous bend and the emplacement of the Champion-Baram deltaic province result from the subduction of a crustal salient of the South China Sea margin named ‘Mulu Block' into the NW Borneo subduction, which induced:

- Tremendous uplifts of the bend area, modifying the zone of sediment source,

- Associated deep basin collecting the sediments and allowing the development of a deltaic province with gravity tectonics (The Champion-Baram Basin).

This new geodynamic model shed a new light of the evolution of structural and petroleum systems.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012