Deep in the ice lands of East Antarctica, specifically in the Akebono Rock formation, a rare geological phenomenon has been uncovered, providing new insights into the history of ancient crustal movements. At this shear zone location, amphibolite rocks containing large garnet crystals with an internally curved structure resembling the letter “S” (sigmoidal) were discovered. The discovery of these curved-pattern crystals is the first detailed report of rotated phenocrysts in the entire Lützow-Holm Complex (LHC) of Antarctica. This unique pattern in the garnet acts as a natural recorder of past conditions; the crystal’s core records an “S”-shaped growth trail, while its outer rim records a “Z”-shaped trail. This double curved shape indicates that the rock did not merely grow, but also rotated simultaneously due to the incredibly strong friction and pressure of tectonic plates during its formation.
Figure: Geological map of the Lützow-Holm Complex region on the coast of East Antarctica.
Through the reading of its chemical composition to measure ancient temperature and pressure, the extraordinary journey of this crystal rock was revealed. The rock harboring this sigmoidal garnet initially formed very deep in the Earth’s crust under extreme pressure conditions of around 8 kilobars and temperatures reaching 650–700 degrees Celsius. Due to intense tectonic faulting or shear zone activity, the rock blocks from this depth were then pushed and lifted vertically toward a shallower crustal level. This uplift process, accompanied by a rapid drop in pressure, is what changed the direction of the frictional rotation, thus triggering the formation of the “Z”-shaped trails on the outer rim of the garnet crystal.
Figure: Microscopic view (photomicrograph) of a sigmoidal garnet crystal found in Akebono Rock.
For the general public and the scientific community, this record of vertical crustal uplift helps solve one of the major puzzles regarding the origin of landmasses in the region, particularly concerning the existence of the mysterious ancient rock block at Cape Hinode, located not far from there. For decades, experts suspected that the anomalous rocks at Cape Hinode were a foreign land block that moved over great horizontal distances across the ancient ocean due to continental drift. However, the evidence of the vertical uplift fault at Akebono Rock provides a new, more plausible perspective: the high-grade ancient rocks in the area likely never moved that far horizontally, but were directly pushed up from the deepest parts of the Earth’s crust to the surface due to local tectonic fault movements.
ARTICLE SOURCE:
- Title: Tectono-metamorphic evolution and significance of shear-zone lithologies in Akebono Rock, Lützow-Holm Complex, East Antarctica.
- Journal: Antarctic Science 33(1), 52–72 (2021)
- Authors: Sotaro Baba, Tomokazu Hokada, Atsushi Kamei, Ippei Kitano, Yoichi Motoyoshi, Prayath Nantasin, Nugroho Imam Setiawan, and Davaa-Ochir Dashbaatar.
- Afiliation: University of the Ryukyus (Jepang), National Institute of Polar Research (Jepang), SOKENDAI (Jepang), Shimane University (Jepang), Kyushu University (Jepang), Kasetsart University (Thailand), Universitas Gadjah Mada (Indonesia), and Mongolian University of Science and Technology (Mongolia).
- DOI: https://doi.org/10.1017/S0954102020000450