Talk: A 240-to-86-million-year-old subduction margin in West Sarawak

Dear members and friends,

We are inviting you for talk about the geology of West Sarawak. We will look at how, more than 80 million years ago, the crust of the Pacific Ocean dipped beneath the crust of the Asian continent. Among other things, this caused volcanoes to erupt and today we can see these volcanic rocks around Serian. 

Speaker:  Hans Hazebroek

Date:       Tuesday 26 February 2019

Time:       7.30pm -9.30 pm Please be on time.

Venue:    Islamic Information Centre (Lower Baruk)

                Jalan Ong Tiang Swee (behind Swinburne University)

• Please register by sending email to mnskuching@gmail.com latest by Sunday 24 February.

'Melange’ rock: a jumble of rock fragments of many sizes and compositions embedded in a slaty matrix. Such rocks suggest submarine sliding and slumping.

A 240-to-86-million-year-old subduction margin in West Sarawak

It has long been suspected that there is an ancient Plate Tectonic Boundary in Sarawak approximately along the valley of the Lupar River. This ‘Lupar Line’ geologically separates West Sarawak from North Sarawak. There are huge differences of the geological make-up between these two areas. Many geologists have regarded this Lupar Line as a ‘suture’ or scar, marking a former ‘subduction margin’ (where ocean crust was thrust beneath continental crust) that was thought to have been active between about 80 and 60 million years ago (Late Cretataceous to Early Eocene). However, new data show that there was insufficient nearby magmatic activity for the Lupar Line to be interpreted as a subduction margin. In other words, a volcanic arc was absent at this time. Therefore the Lupar Line is now considered to be a large fault zone. 

Much new radiometric age data*) derived from West Sarawak and West Kalimantan rock samples has led to a drastic revision of the tectonic model: A much older subduction margin existed in West Sarawak about 240-86 million years ago (Triassic to Cretaceous). Subduction was directed westward (at an angle to the Lupar Line) with Pacific ocean crust being thrust beneath the continental crust of Sundaland. Sundaland is an extension of the Asian continental crust that includes parts of Borneo, Malaya, Java and Sumatra, as well as the shallow seas (the Sunda Shelf) in between. West Sarawak was part of the easternmost extension of Triassic Sundaland. Pacific ocean crust was subducted in two pulses along the long-lived subduction margin: 

• An initial subduction pulse between about 240-200 million-years-ago (Late Triassic). The Serian Volcanic Formation and the Jagoi Granodiorite represent the volcanic arc (a string of volcanoes) at this time, accompanied by deposition of sediments that contain fragments of volcanic rock (‘volcaniclastics’ of the Kuching and Sadong Formations) in the forearc basin (the region between the oceanic trench and the associated volcanic arc). 
• A later subduction pulse between about 140-86 million-years-ago (Early Late Cretaceous). At this time the Schwaner Mountains form the volcanic arc in SW Borneo and the volcaniclastic Pedawan Formation is deposited in the forearc basin. 

*) obtained and interpreted by the SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London.

About the speaker:

Drs Hans P. Hazebroek — Geologist, Nature Photographer and Writer

Books and book chapters written and photographed:

in Sarawak  – 

• National Parks of Sarawak (2000) (with co-author Abang Kashim bin Abang Morshidi)
• A Guide to Gunung Mulu National Park (2002) (with co-author Abang Kashim bin Abang Morshidi)
• A Guide to Bako National Park (2006) (with co-author Abang Kashim bin Abang Morshidi)
• Geology and Geomorphology: chapter in Tanjung Datu National Park— Where Borneo Begins (2015)
• Geology and Geomorphology: chapter in Gunung Penrissen — The Roof of Western Borneo (2017)
• Geology andGeomorphology: chapter in Gunung Santubong — Where Nature meets Culture (in press) 

in Sabah –

• Maliau Basin – Sabah’s Lost World (2004) (with co-authors Tengku Zainal Adlin and Dr. Waidi Sinun)
• Danum Valley — The Rain Forest (2012) (with co-authors Tengku Zainal Adlin and Dr. Waidi Sinun)
• Tertiary tectonic evolution of the NW Sabah Continental margin (1993) (with co-author Dennis N. K. Tan)

Principal photographer for:

On the Forests of Tropical Asia – Lest the memory fade (2014) by Peter S. Ashton  

Contributing photographer for:

Phillipps Field Guide to Mammals of Borneo (2018) by Quentin Phillipps and Karen Phillipps

Orchids of Sarawak (2001) by Beaman, T.E., Wood, J.J., Beaman, R.S., Beaman, J.H. 

Regards,

Cynthia Lobato

MNSKB Secretariat