Simultaneous or diachronous mountain building in the Taiwan orogenic belt:
Massive conglomerate strata with thickness up to several hundred meters are widely distributed in the western foothills of Taiwan. These thick conglomerate deposits were resulted from rapid erosion and uplift of the Taiwan orogen during the Pleistocene. Thus, understanding their precise depositional ages has important implications in Taiwan orogeny. In the past three years, a detailed magneto-biostratigraphic study was carried out on the conglomerate strata in the northern and central Taiwan areas. The results reveal that deposition ages are 1.3-1.1 Ma and 1.0-0.8 Ma respectively for the northern and central Taiwan conglomerates. In addition, deposition of the central Taiwan conglomerate migrated westward after 0.8 Ma. This indicates that the rapid uplift events of the Taiwan orogen from northern to central Taiwan is diachronous, rather than simultaneous proposed by Lee et al. (2015).
Unusual magnetic properties of of sedimentary pyrrhotite forming in methane-seepage sediments:
Diagenetic pyrrhotite and greigite, forming through complex biogeochemical processes in reducing environments, are the two most important magnetic iron sulfides in methane or gas hydrate sediments. Compared to well-studied greigite, diagenetic pyrrhotite has rarely been reported in literature. To better understand magnetic characteristics of diagenetic pyrrhotite, a mineral magnetic study was performed on a marine sediment core (MD10-3276) offshore southwestern Taiwan where high methane concentration in sediments has been detected. In this 25.58-m long sediment sequence, numerous magnetic nodules were found in black speckles and veins. X-ray diffraction analyses on these nodules indicate that pyrrhotite has hexagonal structure. Scanning electron microscope observations reveal that pyrrhotite and pyrite developed in the cavities of nodules at different stages, indicating that they were from by authigenesis and the cavities resulted from outgassing under a high methane flux environment. In contrast to monoclinic pyrrhotite commonly found in ore deposits and metamorphic rocks, the hexagonal pyrrhotite forming through diagenetic processes has rather high coercivities (Bcr: 125-200 mT), low S-ratios (0.50-0.80), low susceptibility (c: 1-15×10-6 m3/kg), but no Besnus transition near 34 K. These magnetic properties are unusual and quite different from those of monoclinic pyrrhotite.
Magnetic properties of sediment cores offshore of SW Taiwan:
Studies of rock magnetism and magnetic mineralogy have been conducted on sediment cores recovered from gas hydrate-bearing potential area off southwestern Taiwan. The results indicate that intervals in each core may have different magnetic mineral components (e.g., magnetite, greigite, pyrrhotite) and rock magnetic properties (susceptibility, Ms, Mr, Bc, Bcr), revealing that they had undergone various degrees of reducing processes during early diagenesis in methane anoxic environment. In general, the intervals can be categorized into one of four types: (1) non-reduced, (2) slightly-reduced, (3) moderately-reduced, and (4) intensively-reduced. The results show that moderately- and intensively-reduced sediment intervals usually occur at very shallow depths for cores recovered from submarine topographic highs, such as ridges and mud volcanoes, while non-reduced sediment interval can persist down to a deeper depth for cores recovered from topographic lows, like basins and deep-sea plains. This discrepancy is resulted from different methane fluxes which are controlled by tectonic structures. Another factor that controls methane flux is sedimentation processes, particularly through turbidity currents, which greatly affect sediment grain size distribution and porosity.
Several joint studies with foreign and domestic collaborators have been carried out in the past four years, which include: (1) using paleomagnetism to interpret the arcuate shape and tectonic evolution of northern Taiwan; (2) using rock magnetism to understand sediment dispersal systems of the southwest end of Ryukyu Trench; (3) using a high-quality paleointensity record of marine core to investigate cosmogenic signature of geomagnetic reversals and excursions from the Reunion event to the Matuyama-Brunhes transition; (4) using magnetic iron sulfides found in Siberian sediments across the Paleocene-Eocene Thermal Maximum (PETM) to provide direct evidence for methane venting during the PETM; (5) using a sediment core from the South China with well-dated C-14 ages and clumped isotope (Δ47) thermometry to reconstruct the sea surface temperatures of the South China Sea from the Last Glacial Maximum to the Holocene; (6) using magnetic force microscopy and rock magnetism to investigate magnetic domain structures of magnetic minerals in a pseudotachylyte sample and to discuss possible correlation between the magnetic field direction and magnetic domain structures.