Thermal ages of the Huatung Basin determined from seismic waveform modeling: insights into Southeast Asia's evolution

The Huatung Basin (HB), situated on the leading part of the Philippine Sea Plate, is directly involved in oblique subduction and mountain building in the Taiwan region. However, previous studies have reported a wide range of ages for the HB, from 30 to 130 Ma, making it difficult to properly constrain regional tectonics. We analyzed teleseismic waveforms recorded on Taiwan that traveled through the slab associated with the HB. By waveform matching, we have constrained the slab dimensions to approximately 400 km in length and 150 km in width, accompanied by an enhanced P-wave velocity of 6% within the slab core and an apparent dip angle of 55°. We used age-dependent subduction zone thermal models to estimate the thermal ages or the ages since the last thermal event of the HB. The best-fit thermal model indicates thermal ages ranging from 20 to 50 Ma, which is consistent with a suite of geophysical observations and the age inferred from geomagnetic anomaly data. However, our results differ considerably from the ages obtained through radiometric dating of rocks dredged from the seafloor. The discrepancy in age may be attributed to either thermal rejuvenation of the plate or dating of allochthonous samples dredged from the border of the basin.

Metapopulation vicariance explains old endemics on young volcanic islands

Terrestrial plants and animals on oceanic islands occupy zones of volcanism found at intraplate localities and along island arcs at subduction zones. The organisms often survive as metapopulations, or populations of separate sub-populations connected by dispersal. Although the individual islands and their local subpopulations are ephemeral and unstable, the ecosystem dynamism enables metapopulations to persist in a region, more or less in situ, for periods of up to tens of millions of years. As well as surviving on systems of young volcanic islands, metapopulations can also evolve there; tectonic changes can break up widespread insular metapopulations and produce endemics restricted to fewer islands or even a single island. These processes explain the presence of old endemic clades on young islands, which is often reported in molecular clock studies, and the many distribution patterns in island life that are spatially correlated with tectonic features. Metapopulations can be ruptured by sea floor subsidence, and this occurs with volcanic loading in zones of active volcanism and with sea floor cooling following its production at mid-ocean ridges. Metapopulation vicariance will also result if an active zone of volcanism is rifted apart. This can be caused by the migration of an arc (by slab rollback) away from a continent or from another subduction zone, by the offset of an arc at transform faults and by sea floor spreading at mid-ocean ridges. These mechanisms are illustrated with examples from islands in the Caribbean and the Pacific. Endemism on oceanic islands has usually been attributed to chance, long-distance dispersal, but the processes discussed here will generate endemism on young volcanic islands by vicariance.

Comment on "Mantle flow drives the subsidence of oceanic plates"

Adam and Vidal (Reports, 2 April 2010, p. 83) reported sea-floor depth increasing as the square root of distance from the ridge along "mantle flow lines." However, their data actually support a depth-age relationship and "flattening" at older ages. We argue that no plausible physical mechanism supports their proposal that mantle flow drives subsidence.

Response to Comment on “Mantle Flow Drives the Subsidence of Oceanic Plates”

Croon et al . challenge our conclusion that sea-floor depth variations are driven by the underlying mantle convection. We point out that, contrary to their claim, our data analysis is pertinent and that the sea-floor linear trend as the square root of the distance from the ridge is a robust observation. The mechanism responsible for this trend is an asthenospheric flow, faster than the overlying plate, which shapes the lithosphere structure.