. Tectonic reconstructions of Southeast Asia have given rise to numerous controversies that include the accretionary history of Sundaland and the enigmatic tectonic origin of the proto-South China Sea. We assimilate a diversity of geological and geophysical observations into a new regional plate model, coupled to a global model, to address these debates. Our approach takes into account terrane suturing and accretion histories, the location of subducted slabs imaged in mantle tomography in order to constrain the evolution of regional subduction zones, as well as plausible absolute and relative plate velocities and tectonic driving mechanisms. We propose a scenario of rifting from northern Gondwana in the latest Jurassic, driven by northward slab pull from north-dipping subduction of Tethyan crust beneath Eurasia, to detach East Java, Mangkalihat, southeast Borneo and West Sulawesi blocks that collided with a Tethyan intra-oceanic subduction zone in the mid-Cretaceous and subsequently accreted to the Sunda margin (i.e., southwest Borneo core) in the Late Cretaceous. In accounting for the evolution of plate boundaries, we propose that the Philippine Sea plate originated on the periphery of Tethyan crust forming this northward conveyor. We implement a revised model for the Tethyan intra-oceanic subduction zones to reconcile convergence rates, changes in volcanism and the obduction of ophiolites. In our model the northward margin of Greater India collides with the Kohistan–Ladakh intra-oceanic arc at ∼53 Ma, followed by continent–continent collision closing the Shyok and Indus–Tsangpo suture zones between ∼42 and 34 Ma. We also account for the back-arc opening of the proto-South China Sea from ∼65 Ma, consistent with extension along east Asia and the formation of supra-subduction zone ophiolites presently found on the island of Mindoro. The related rifting likely detached the Semitau continental fragment from South China, which accreted to northern Borneo in the mid-Eocene, to account for the Sarawak Orogeny. Rifting then re-initiated along southeast China by 37 Ma to open the South China Sea, resulting in the complete consumption of proto-South China Sea by ∼17 Ma when the collision of the Dangerous Grounds and northern Palawan blocks with northern Borneo choked the subduction zone to result in the Sabah Orogeny and the obduction of ophiolites in Palawan and Mindoro. We conclude that the counterclockwise rotation of Borneo was accommodated by oroclinal bending consistent with paleomagnetic constraints, the curved lithospheric lineaments observed in gravity anomalies of the Java Sea and the curvature of the Cretaceous Natuna paleo-subduction zone. We complete our model by constructing a time-dependent network of topological plate boundaries and gridded paleo-ages of oceanic basins, allowing us to compare our plate model evolution to seismic tomography. In particular, slabs observed at depths shallower than ∼1000 km beneath northern Borneo and the South China Sea are likely to be remnants of the proto-South China Sea basin.

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The Cretaceous and Cenozoic tectonic evolution of Southeast Asia

[1] Based on geological and geophysical data collected from the West Philippine Basin and its boundaries, we propose a comprehensive Cenozoic history of the basin. Our model shows that it is a back arc basin that developed between two opposed subduction zones. Rifting started around 55 Ma and spreading ended at 33/30 Ma. The initial spreading axis was parallel to the paleo-Philippine Arc but became inactive when a new spreading ridge propagated from the eastern part of the basin, reaching the former one at an R-R-R triple junction. Spreading occurred mainly from this second axis, with a quasi-continuous counter-clockwise rotation of the spreading direction. The Gagua and Palau-Kyushu ridges acted as transform margins accommodating the opening. Arc volcanism occurred along the Palau-Kyushu Ridge (eastern margin) during the whole opening of the basin, whereas the paleo-Philippine Arc decreased its activity between 43 and 36 Ma. The western margin underwent a compressive event in late Eocene-early Oligocene time, leading to the rising of the Gagua Ridge and to a short subduction episode along Eastern Luzon. In the western part of the basin, the spreading system was highly disorganized due to the presence of a mantle plume. Overlapping spreading centers and ridge jumps occurred toward the hot region and a microplate developed. Shortly after the end of the spreading, a late stage of amagmatic extension occurred between 30 and 26 Ma in the central part of the basin, being responsible for the deep rift valley that cut across the older spreading fabric.

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The West Philippine Basin: An Eocene to early Oligocene back arc basin opened between two opposed subduction zones

Rates of processes in magmatic arcs : implications for the timing and nature of pluton emplacement and wall rock deformation

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Tectonic evolution and deep mantle structure of the eastern Tethys since the latest Jurassic

The transition from magmatic to high-temperature solid-state deformation: implications from the Mount Stuart batholith, Washington

In the Foothills terrane, western metamorphic belt, California, the Bear Mountains fault zone (BMFZ)—a zone of complex, multiple deformation—has deformed composite maficfelsic granitoids of the Guadalupe igneous complex (GIC) and the Hornitos pluton at magmatic to upper amphibolite facies temperatures. Evidence for high-temperature deformation includes the occurrence of orthopyroxene, clinopyroxene, and brown hornblende in syndeformationally recrystallized aggregates in deformed rocks of the GIC and the occurrence of melt leucosomes in the axial surfaces of folds in cordierite—K-feldspar migmatites in contact metamorphic country rocks in the BMFZ. Away from the BMFZ, composite granitoids of the GIC show no evidence of magmatic flow, so that deformation of the GIC granitoids in the BMFZ was entirely by solid-state flow. This suggests that the GIC was intruded before this mylonitic deformation in the BMFZ. However, other evidence suggests some deformation in the BMFZ before intrusion of the pluton. In the Hornitos pluton components of composite granitoids showing relatively weak solid-state deformation are strongly elongate, suggesting a component of magmatic flow, although later subsolidus deformation also occurred in most places. Supporting evidence of magmatic flow in the Hornitos pluton also includes dimensional alignment of plagioclase laths and prismatic hornblende grains, as well as preferred orientation of twins in recrystallized aggregates of elongate grains of plagioclase. Away from the BMFZ, the regional metamorphic grade is low, suggesting that heat for the high-temperature deformation and metamorphism was supplied by the intrusions. U/Pb zircon ages are consistent with emplacement of the GIC and the Hornitos pluton, as well as their deformation in the BMFZ, around 150 Ma. 40 Ar/ 39 Ar ages of hornblende in foliations in the deformed granitoids and of biotite in foliations in the contact aureole rocks suggest that the plutons and the country rocks in the BMFZ remained hot enough for amphibolite to upper greenschist facies conditions to persist for 5 to 15 Ma after emplacement of the plutons.

Evidence for syntectonic intrusion of plutons in the Bear Mountains fault zone, California

Geological field investigations and 40Ar/39Ar ages from two basement complexes in southeast Luzon document the first known occurrences of pre-Late Cretaceous age rocks in the eastern Philippines. However, individual components within the two complexes vary in age from Late Jurassic (Caramoan basement complex) to Early Cretaceous and early Miocene (Camarines Norte-Calaguas Islands basement complex). These and other data show that southeast Luzon basement complexes are genetically diverse, and they indicate that the concept of an old, autochthonous basement in the Philippines is open to question. This supports the hypothesis that the Philippine Archipelago is an amalgamation of allochthonous Mesozoic and Cenozoic island-arc, ocean-basin, and continental fragments that were assembled during the Tertiary.

E. E. Geary

Papers

Evidence for syntectonic intrusion of plutons in the Bear Mountains fault zone, California

Diverse ages and origins of basement complexes, Luzon, Philippines