Young Keun Jin

Origin of E-MORB in a fossil spreading center: the Antarctic-Phoenix Ridge, Drake Passage, Antarctica

The fossilized Antarctic-Phoenix Ridge (APR) with three segments (P1, P2, and P3), Drake Passage, is distant from the known hotspots, and consists of older N-MORB formed prior to the extinction of spreading and younger E-MORB after extinction. The older N-MORB (3.5–6.4 Ma) occur in the southeastern flank of the P3 segment (PR3) and the younger E-MORB (1.4–3.1 Ma) comprise a huge seamount at the former ridge axis of the P3 segment (SPR) and a big volcanic edifice at the northwestern flank of the P2 segment (PR2). The PR3 basalts have higher Mg#, K/Ba, and CaO/Al2O3 and lower Zr/Y, Sr, and Na80 (fractionation-corrected Na2O to 8.0% MgO) with slight enrichment in incompatible elements and almost flat REE patterns. The SPR and PR2 basalts are highly enriched in incompatible elements and LREE. The extinction of spreading at 3.3 Ma seems to have led to a temporary magma oversupply with E-MORB signatures. Geochemical signatures such as Ba/TiO2, Ba/La, and Sm/La suggest the heterogeneity of upper mantle and formation of E-MORB by higher contribution of enriched materials (e.g., metasomatized veins) to mantle melting than the N-MORB environment. E-MORB magmas beneath the APR seen to have been produced by low-degree melting at deeper regime, where enriched materials have preferentially participated in the melting. The occurrence of E-MORB at the APR is a good example to better understand what kinds of magmatism would occur in association with extinction of the ridge spreading.

Post-subduction margin structures along Boyd Strait, Antarctic Peninsula

Abstract The Pacific margin of the Antarctic Peninsula to the southwest of the Hero Fracture Zone (HFZ) is a former subducting margin which became inactive following the arrival of ridge crest segments of the Antarctic–Phoenix ridge at the margin during the Tertiary. In contrast, the part of the margin to the northeast of the HFZ remains active. Tertiary convergence was approximately perpendicular to the margin and ongoing motion is thought to have the same orientation. A new seismic reflection profile running along Boyd Strait, just northeast of the landward projection of the HFZ, shows major structural components similar to those typically observed along the margin to the southwest of the HFZ. In order of increasing proximity to the margin, these components are: the inner shelf, the shelf basin, the mid-shelf basement high (MSBH), and the outer shelf. The continuation of the post-subduction margin structures to the active margin suggests that the boundary between crust with passive and active margins characteristics is not sharply defined. Our postulated scenario for tectonic evolution along Boyd Strait is that: (1) before the arrival of the last ridge crest segment to the southwest of the HFZ, the inner shelf and the shelf basin were part of a Cretaceous–Tertiary arc and forearc area, (2) after the arrival, thermal effects resulting from interaction of the ridge crest with the margin just southwest of the HFZ lead to the formation of the MSBH to the northeast, but MSBH uplift in Boyd Strait did not prevent concurrent cross-shelf sediment transport contributing to development of an extensive outer shelf on the seaward flank of the MSBH, (3) Recent extension in Bransfield Strait, a marginal basin to the northeast of the landward projection of the HFZ, has caused about 10 km of seaward deflection in the strike of the part of the MSBH to the northeast of the projection of the HFZ.

Seismic and radar investigations of Fourcade Glacier on King George Island, Antarctica

To determine P- and S-wave velocities, elastic properties and subglacial topography of the polythermal Fourcade Glacier, surface seismic and radar surveys were conducted along a 470-m profile in November 2006. P- and S-wave velocity structures were determined by travel-time tomography and inversion of Rayleigh wave dispersion curves, respectively. The average P- and S-wave velocities of ice are 3466 and 1839 m s-1, respectively. Radar velocities were obtained by migration velocity analysis of 112 diffraction events. An estimate of 920 kg m-3 for the bulk density of wet ice corresponds to water contents of 5.1 and 3.2%, which were derived from the average P-wave and radar velocities, respectively. Using this density and the average P- and S-wave velocities, we estimate that the corresponding incompressibility and rigidity of the ice are 6.925 and 3.119 GPa, respectively. Synergistic interpretation of the radar profile and P- and S-wave velocities indicates the presence of a fracture zone above a subglacial high. Here, the P- and S-wave velocities are approximately 5 and 3% less than in the ice above a subglacial valley, respectively. The S-wave velocities indicate that warmer and less rigid ice underlies 10–15 m of colder ice near the surface of the glacier. Such layering is characteristic of polythermal glaciers. As a relatively simple non-invasive approach, integration of P-wave tomography, Rayleigh wave inversion and ground-towed radar is effective for various glaciological studies, including the elastic properties of englacial and subglacial materials, cold/warm ice interfaces, topography of a glacier bed and location of fracture zones.

Gravity models for the South Shetland Trench and the Shackleton Fracture Zone, Antarctica

Four deep crustal models of the South Shetland Trench (SST) and the Shackleton Fracture Zone (SFZ) off the northern Antarctic Peninsula are presented based on gravity data. The gravity models of the SST suggest that the dip of the subducting crust increases from southwest to northeast ranging from 25° in Line KSL93-5 to 30° in Line KSL93-6 as the age of crust increases along the trench axis. The gravity low observed near the island are is directly associated with the deep forearc basin bounded seaward by a large fault. In the SFZ, the thin crust is concentrated primarily beneath the ridge, where Moho shallows by 2.5 km. A low-density material (about 2.45 g/cm3) assigned to the SFZ ridge is presumably due to serpentinite intrusion. The gravity low associated with the trough is due to the relatively low-density sediments filled in the trough. The crustal thickening to the western slope of the ridge near the triple junction was probably caused by the collision of the SFZ ridge with the Shetland Platform.