The proposed rotation of Borneo

This topic is essential background for tectonic models, but the data is equivocal. As Hutchison (2005) noted:

“A considerable amount of palaeomagnetic research has been carried out on rocks collected from Borneo (Fuller et al., 1999), and in particular from Sarawak. The Sarawak results were first comprehensively reported by Sclimidtke et al. (1990). The most complete and coherent work comes from western Sarawak, but with specimens collected from contiguous Kalimantan. This study has become the standard for the whole of Borneo. The results are relatively straightforward. The Miocene high-level intrusions are increasingly anti-clockwise clockwise rotated with age, reaching a maximum of 51° in an intrusion dated by K:Ar to be 25.8 ± 1.9 Ma. The Late Eocene Silantek Formation gives 41° of anti-clockwise clockwise rotation. The Mesozoic samples from the Bau Limestone, Kedadom and Pedawan formations in Sarawak, and from the Jurassic-Cretaceous and Triassic sites across the border in Kalimantan, all indicate a strong rotation of about 90° that took place sometime after 93 Ma, the age of the Cretaceous dyke. These results indicate that anti-clockwise rotation, at least of northwest Borneo (Sarawak), and perhaps of a greater part of Borneo, has been taking place and was completed by about 10 Ma ago, when Borneo achieved its present orientation. The mechanism for anti-clockwise wise rotation remains enigmatic and many geologists continue to be sceptical because it is difficult to satisfactorily incorporate into regional tectonics.“

Precision accuracy and trueness

As with so much in geology, the topics of precision, accuracy and trueness are critical but glossed-over in the summary above. The “51° in an intrusion dated by K:Ar to be 25.8 ± 1.9 Ma” is within a few kilometres of multiple other samples of similar ages with much lower amounts of rotation including several dated 23.8 Ma and with zero rotation (see second map below). This reflects low accuracy. The “Late Eocene Silantek Formation” is actually dated within a broad age range of later Cretaceous through Early Eocene (Haile 1954, 1957). This Formation, which is locally shallow marine and Paleocene or Eocene (on sparse larger foraminifera), is stratigraphically below the non-marine Plateau Sandstone, which is dated by Morley (1998) as mostly Paleocene, possibly late Maastrichtian at the base.

The importance of timing of orogenies

We know there were orogenies in Borneo that caused much deformation at specific times and places. Each of these events ended, and then there was a pause before the next compressive phase. These are the times of deformation when plate rotation is most likely to have occurred. There was late Mesozoic accretion (with the sheared mélanges in the Kucing Zone. Then the Early? to Middle Eocene Sibu Compression in the Sibu Zone that ended at about 40 to 38 Ma, and the Sabah Orogeny in the north from about roughly 17 or 16 Ma to 3 Ma. There was also the latest Oligocene compression of north Borneo, and the effects of this deformation are restricted to NW Borneo (see Lunt 2021, and Lunt and Said in press)

The map above from Advocaat et al. (2018) does indeed seem to show western Borneo (esp. the Kucing Zone) underwent substantial rotation that did not affect the young samples from eastern Borneo. 

If we re-colour the map of Advocaat et al. by age relative to these periods of deformation, rather than self-evident location, a different story appears

On the re-coloured map above the ages of the dark blue data (34-16 Ma) is contemporaneous with the time of sea-floor spreading in the South China Sea, which is the time the slab-pull tectonic model assumes Borneo rotated counter-clockwise (the top image, from Hall et al, 2008). The data coloured brown and pink were deformed by the Mesozoic (Kucing Zone) and early Cenozoic (Sibu Zone) compressions. These were severe deformations with sheared mélanges in the older event and near vertically folded beds in the Sibu Zone from the latter.

There is a gap in data from the start of the extension and the drift in the South China Sea (the Oligocene, roughly 34 to 24 Ma) which would include the samples most strong rotated by this proposed plate motion (roll-back over subduction). These should contrast from the ages after the end of South China Sea drift (light blue – none in Borneo, and green). Just two samples in SW Sarawak dated 25.8 Ma (mentioned above) and 16.4 Ma show evidence for rotation but these are close to multiple other samples with contradictory zero rotation. The location of the Early Miocene dark blue data points (five samples at 23.8, 19.6 in two locations, 19, 18 and 17.1 Ma) have palaeomagnetic directions indistinguishable from the samples younger than 12 Ma.

The work of Andrew Cullen et al.

Cullen et al. (2012) noted the gap in paleomagnetic data in north Borneo, from sediments that would be affected most by rotation driven by movement of the SCS. Eleven new locations were sampled in NW Borneo for characteristic remanent magnetization (ChRM) directions and integrated with regional data. These authors interpreted an early episode of strong regional counterclockwise rotation, which was the Eocene Sarawak Orogeny extending north into Sabah. This was overprinted with a very mixed set of results. In their Abstract the authors suggested the Eocene deformation was later “overprinted not only by differential clockwise rotation of crustal blocks during opening of the South China Sea (32–23 Ma), but also locally by a younger (after 10 Ma) counterclockwise rotation“.

The data, which is heavily affected by remagnetisation, is not as confident in supporting this simple historical conclusion. In their own conclusion the authors just stated “the younger CW and CCW rotations are too complex to reconcile with models that show the entire island of Borneo and south Palawan rotating CCW as a rigid block between 30 and 10 Ma. Instead the data imply that strain has been partitioned in a complex manner with several overprinting episodes of deformation.”

Conclusion

In conclusion the palaeomagentic data is still highly equivocal. However the data in west Sarawak shows a majority of samples dated as close to the Oligo-Miocene boundary, when the main ridge-jump and onset of drift and subduction orthogonal to Sabah was just starting (Barckhausen et al. 2014), have an orientation almost identical to the present orientation. This suggests negligible net rotation of Borneo, and an indication that Borneo, especially in its northern part, was not always a single rigid plate

References

Advokaat, E.L., Marshall, N.T., Li, S., Spakman, W., Krijgsman, W., van Hinsbergen, D.J.J., 2018. Cenozoic Rotation History of Borneo and Sundaland, SE Asia Revealed by Paleomagnetism, Seismic Tomography, and Kinematic Reconstruction. Tectonics 37(8), 2486-2512

Barckhausen, U., Engels, M., Franke, D., Ladage, S., Pubellier, M., 2014. Evolution of the South China Sea: Revised ages for breakup and seafloor spreading. Marine and Petroleum Geology 58, 599-611

Cullen, A., Zechmeister, M.S., Elmore, R.D., Pannalal, S.J., 2012. Paleomagnetism of the Crocker Formation, northwest Borneo: Implications for late Cenozoic tectonics. Geosphere 8(5), 1146-1169

Fuller, M., Ali, J.R., Moss, S.J., Frost, G.M., Richter, B. & Mahfi, A. (1999) Paleomagnetism of Borneo. J. Asian Earth Sci., 17, 3-24.

Haile, N.S. (1954) The geology and mineral resources of the Strap and Sadong valleys. West Sarawak, including the Klingkang Range coal. Geological Survey Department, British Territories in Borneo, Memoir 1.

Haile, N.S., 1957. The geology and mineral resources of the Lupar and Saribas valleys. West Sarawak. Geological Survey Department British Territories in Borneo, Memoir 7.

Hall, R., van Hattum, M.W.A., Spakman, W., 2008. Impact of India–Asia collision on SE Asia: The record in Borneo. Tectonophysics 451, 366-389 

Hutchison, C.S., 2005. Geology of North-West Borneo: Sarawak, Brunei, and Sabah. Elsevier Science.

Lunt, P., 2021. Re-examination of the Base Miocene Unconformity in West Sabah and its part in the tectono-stratigraphic development of the region. Journal of Asian Earth Sciences, 

Lunt, P., Said, Y., in press. A review of the Kudat Formation in north Sabah and implications for palaeogeographic and plate tectonic reconstructions.

Morley, R.J. 1998. Palynological evidence for Tertiary plant dispersals in the SE Asian region in relation to plate tectonics and climate. In Biogeography and geological evolution of SE Asia, eds. R. Hall, and J.D. Holloway, 211-234. Leiden, The Netherlands: Backhuys Publishers.