Nature and Significance of the West Baram Line, NW Borneo

Cullen, Andrew¹; vanVleet, Arthur²
¹Chesapeake Energy, OKC, OK.
²Petronas PMU, Kuala Lumpur, Malaysia.

The boundary between the Luconia-Balingian (SW) and Baram (NE) basins is placed along the West Baram Line (WBL, offshore) and Tinjar Line (TL, onshore). These "lines" are enigmatic features of Borneo's geological framework; their tectonic significance is unclear. Most reconstructions composite the two lines into a single entity that marks the NE edge of the Luconia microplate. Some tectonic reconstructions link the WBL with the East Vietnam-Red River fault systems to help accommodate extrusion of the Indo-China Block and opening of the South China Sea, ca. 32-16Ma (SCS); other reconstructions depict the WBL-TL as a transform fault separating Luconia from the Dangerous Grounds as the latter was subducted beneath NW Borneo.

Our interpretation of 2D seismic data across the WBL indicates the following:

1. The WBL is a fault zone, 50 km wide, composed of several large normal faults that dip steeply NE.
2. The WBL does not project directly into the TL.
3. Seismic sequences correlated across the WBL include a sequence that ties to Late Oligocene to Early Miocene (Cycle I-Cycle II) clastics in the Luconia-Balingian Province.
4. The Oligo-Miocene sequence (OMS) isochron map shows no evidence for large-scale differential displacement across the WBL. Thus, it is unlikely that WBL was a major Early Miocene strike slip fault.
5. The OMS is a post-rift sag sequence. It exceeds 3 km thickness NE of the WBL, but thins abruptly to the NW, onlapping rifted crust of Dangerous Grounds. Therefore, a major phase of extension on the Dangerous Ground (Late Eocene?) pre-dates opening of the SCS.

When we consider our observations in light of regional heat flow, gravity, and geological data, we favor interpreting the WBL and TL as an ancient suture along which deep structures have been reactivated. The Dangerous Grounds and the Luconia-Balingian Province are linked by the OMS and share an episode of Late (?) Eocene rifting. NE of the WBL the percent of crustal thinning during that rifting appears to have been much larger than in the Luconia-Balingian Province; a circumstance that may be related to rheological contrasts across the old suture. We see no evidence to support large-scale Oligo-Miocene strike slip faulting along the WBL, which implies less Neogene subduction beneath NW Borneo than is shown in most proposed tectonic models.

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