Variations in effective elastic thickness of the North American lithosphere

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.

The long-term strength of Europe and its implications for plate-forming processes

Field-based geological studies show that continental deformation preferentially occurs in young tectonic provinces rather than in old cratons. This partitioning of deformation suggests that the cratons are stronger than surrounding younger Phanerozoic provinces. However, although Archaean and Phanerozoic lithosphere differ in their thickness and composition, their relative strength is a matter of much debate. One proxy of strength is the effective elastic thickness of the lithosphere, Te. Unfortunately, spatial variations in Te are not well understood, as different methods yield different results. The differences are most apparent in cratons, where the 'Bouguer coherence' method yields large Te values (> 60 km) whereas the 'free-air admittance' method yields low values (< 25 km). Here we present estimates of the variability of Te in Europe using both methods. We show that when they are consistently formulated, both methods yield comparable Te values that correlate with geology, and that the strength of old lithosphere (> or = 1.5 Gyr old) is much larger (mean Te > 60 km) than that of younger lithosphere (mean Te < 30 km). We propose that this strength difference reflects changes in lithospheric plate structure (thickness, geothermal gradient and composition) that result from mantle temperature and volatile content decrease through Earth's history.

The Seismic 8degrees Discontinuity and Partial Melting in Continental Mantle

Strong, scattered reflections beyond 8 degrees (8degrees) offset are characteristic features of all high-resolution seismic sections from the continents. The reflections identify a low-velocity zone below approximately 100 kilometers depth beneath generally stratified mantle. This zone may be caused by partial melting, globally initiated at equal depth in the continental mantle. Solid state is again attained at the Lehmann discontinuity in cold, stable areas, whereas the zone extends to near the 400-kilometer discontinuity in hot, tectonically active areas. Thus, the depth to the Lehmann discontinuity may be an indicator of the thermal state of the continental mantle.