A Triassic to Cretaceous Sundaland–Pacific subduction margin in West Sarawak, Borneo

Abstract

Metamorphic rocks in West Sarawak are poorly exposed and studied. They were previously assumed to be pre-Carboniferous basement but had never been dated. New 40 Ar/ 39 Ar ages from white mica in quartz-mica schists reveal metamorphism between c. 216 to 220 Ma. The metamorphic rocks are associated with Triassic acid and basic igneous rocks, which indicate widespread magmatism. New U-Pb dating of zircons from the Jagoi Granodiorite indicate Triassic magmatism at c. 208 Ma and c. 240 Ma. U-Pb dating of zircons from volcaniclastic sediments of the Sadong and Kuching Formations confirm contemporaneous volcanism. The magmatic activity is interpreted to represent a Triassic subduction margin in westernmost West Sarawak with sediments deposited in a forearc basin derived from the magmatic arc at the Sundaland-Pacific margin. West Sarawak and NW Kalimantan are underlain by continental crust that was already part of Sundaland or accreted to Sundaland in the Triassic. One metabasite sample, also previously assumed to be pre-Carboniferous basement, yielded Early Cretaceous 40 Ar/ 39 Ar ages. They are interpreted to indicate resumption of subduction which led to deposition of volcaniclastic sediments and widespread magmatism. U-Pb ages from detrital zircons in the Cretaceous Pedawan Formation are similar to those from the Schwaner granites of NW Kalimantan, and the Pedawan Formation is interpreted as part of a Cretaceous forearc basin containing material eroded from a magmatic arc that extended from Vietnam to west Borneo. The youngest U-Pb ages from zircons in a tuff layer from the uppermost part of the Pedawan Formation indicate volcanic activity continued until c. 86 to 88 Ma when subduction terminated.

Cenozoic Rotation History of Borneo and Sundaland, SE Asia Revealed by Paleomagnetism, Seismic Tomography, and Kinematic Reconstruction

“…In the east, Sundaland is truncated by the Billiton Depression, a N‐S trending transform fault running south of Natuna (Ben‐Avraham & Emery, ; Ben‐Avraham & Uyeda, ; Figure ). Hall, Clements, and Smyth () and Hall () proposed that the Billiton Depression is a suture separating SW Borneo from Sundaland, but recent studies suggest that the suture between Sundaland and Borneo may be located onshore west Borneo (Breitfeld et al, ). N‐S trending normal faults of Late Eocene‐Early Oligocene age in the western Java Sea are associated with the Billiton depression and indicate a component of E‐W extension (Barber & Crow, ; Cole & Crittenden, ).…”

“…Borneo is characterized by multiple continental and arc fragments separated by Mesozoic‐Cenozoic ophiolites, mélange complexes, sutures, and fault zones, which are unconformably overlain by Cenozoic basins (Breitfeld et al, ; Haile, ; Hutchison, ; Metcalfe, ; Figure ). Fragments include the Kuching Zone (Haile, ; Metcalfe, , ) and Gondwana‐derived fragments in SW Borneo and eastern Borneo that accreted to Sundaland in the Late Cretaceous (Hall, ).…”

“…The Kuching Zone (Haile, ) has an ESE‐WNW trending structural grain (Figure b). It comprises an amalgamation of Paleozoic‐Mesozoic basement rocks and sediments and the Upper Cretaceous Boyan Mélange Complex (Breitfeld et al, ; Metcalfe, ; Williams et al, ). Upper Cretaceous‐Upper Eocene basins and younger basins unconformably cover the Paleozoic‐Mesozoic rocks of the Kuching Zone (Williams et al, , ).…”

Cretaceous–Palaeogene sedimentary evolution of the South China Sea region: A preliminary synthesis

“…The pre‐Cenozoic rifting episode before the Proto‐South China Sea opening has been recognized by Ye et al () as ~100–72 Ma (though uncertain) when the largely ENE‐striking extensional fault systems and associated basins were gendered along the South China margin. During this period, the termination of the Izanagi subduction has been foreseen by the youngest emplacement of subduction‐related magmatism found in areas like Borneo (Breitfeld et al, ; Breitfeld & Hall, ), Palawan Continental Terrane (Knittel, ; Padrones, Tani, Tsutsumi, & Imai, ), Hainan Island (Jiang & Li, ), northern South China Sea (F. C. Li, Sun, & Yang, ; Q. Yan et al, ), and Taiwan (Chen et al, ; Yui et al, ). Subsequent Proto‐South China Sea spreading could probably be responsible for the development of compressional structure, represented by the ENE‐striking thrust system observed in the northern South China Sea by means of ridge push (Ye et al, ) or even for the Palaeocene regional unconformity (Morley, ).…”

Evolution and Geological Significance of Larger Benthic Foraminifera

“…By the late 1920s, the larger foraminifera had become the preferred fossil group for biostratigraphy in the Indonesian area (Sharaf et al, 2014;Li et al, 2015;Gold et al, 2017a, b;Breitfeld et al, 2016;Advocaat et al, 2017;White et al, 2017). They had the advantage that they were more abundant than molluscs, and also a scheme was developed that utilised assemblage zones rather than percentages of extant forms.…”