Gravity gradient tensor analysis has been a powerful tool for investigating subsurface structures and recently its application to a two-dimensional fault structure has been developed. To elucidate the faulting type and spatial extent, specifically the continuity and the size, of the subsurface fault structure of an active fault through gravity gradient tensor analysis, we analyzed Bouguer anomalies, which were composed of dense gravity measurement data over the land and seafloor, and indices calculated from a gravity gradient tensor around the Togi-gawa Nangan fault (TNF), Noto Peninsula, central Japan. The features of Bouguer anomalies and their first horizontal and vertical derivatives demonstrate clearly that the TNF is a reverse fault dipping to the southeast. Furthermore, the combination of those derivatives and the dimensionality index revealed that the spatial extent of the subsurface fault structure is coincident with that of the surface fault trace and that it shows no evidence of connecting the TNF with surrounding active faults. Furthermore, the dip angle of the subsurface fault structure was estimated as 45°-60° from the minimum eigenvectors of the gravity gradient tensor. We confirmed that this result is coincident with the dip angle estimated using the two-dimensional Talwani's method. This high dip angle as a reverse fault suggests that the TNF has experienced inversion tectonics.
Non-metamorphosed, autochthonous Lesser Himalayan sediments (LHS), which are correlated to the Kuncha and Naudanda Formations, were found in a narrow belt between the Main Boundary Thrust and the Lesser Himalayan Thrust at the base of the Kuncha nappe in southeastern Nepal. The autochthonous Naudanda Formation is comprised of cross-bedded and rippled orthoquartzite and yielded a maximum depositional age of 1795.1 Ma ±5.1 Ma using detrital zircons. Low-grade metamorphosed quartzite in the Kuncha nappe yielded a maximum depositional age of 1867.4 Ma ±3.4 Ma, although it is totally recrystallized. These ages and age distribution patterns of detrital zircon grains indicate that the meta-quartzite of the nappe is originally Naudanda Formation. A zircon fission-track age of the autochthonous Naudanda Formation shows partially annealed age of 864 Ma ±56 Ma, in contrast, that of the Kuncha nappe shows a totally annealed age of 11.9 Ma ±1.6 Ma. These results suggest that the autochthonous LHS have never undergone metamorphism during the Himalayan orogeny. We also discovered a non-metamorphosed Heklang Formation that rests on the Naudanda Formation, and designated it as a sub-type section on the basis of detailed lithostratigraphic study. It is characterized by black and light green slate with dolerite sills and ill-sorted quartzose sandstone, and correlated to the metamorphosed Dandagaon Phyllites in the Kathmandu area. Nonmetamorphosed autochthonous formations distributed to the south of the nappe front suggest that they escaped from thermal metamorphism by hot nappe.
To determine local deposit features in the vein‐type gold–silver ore deposit of the historical Togi mine, Noto Peninsula, central Japan, we investigated the occurrence and chemical compositions of ore minerals, especially silver‐ and/or gold‐bearing minerals, and wall‐rock alteration in the Togo No. 3 tunnel, a major tunnel of the Urukami mining area of the Togi mine. In the tunnel, the stockwork veins which are generally 0.1–1.5 m wide runs in Neogene pyroxene andesite. In the ore‐bearing quartz vein, in addition to the sulfides such as sphalerite, pyrite and chalcopyrite, the following Au and/or Ag ore minerals were identified: electrum with <49.3 mol% Ag, uytenbogaardtite, fischesserite, silver‐rich minerals of the Ag2S–Ag2Se solid solution series, and Se‐rich pearceite. Gangue minerals in the quartz veins are quartz and adularia. The wall rock is silicified, chloritized, and sericitized pyroxene andesite. Multiple generations of adularia grown on quartz suggest that adularia has grown by repeatedly passing the hydrothermal solution through the vein. Based on the characteristic occurrence of the Ag and/or silver‐rich ore minerals such as electrum, fischesserite, and Se‐rich pearceite and a gangue mineral, adularia, and on the wall‐rock alteration characterized by chlorite, sericite, and calcite, the Togi deposit is classified as a Se‐type and low‐sulfidation epithermal gold vein deposit. The K‐Ar ages of illite/smectite mixed‐layer minerals from the altered outcrop and plagioclase from a fresh andesite sample are 17.68 ± 0.41 and 17.52 ± 0.43 Ma, respectively, in agreement with published data of the Besshodake andesite. These results indicate that the hydrothermal fluids generated by the Miocene andesitic magma activity infiltrated the fissures relating to the formation of the Togi‐gawa Nangan Fault at the northern boundary of the Togi mining area, and Au and/or Ag‐rich ore minerals were formed from the hydrothermal fluid with decreasing temperature at the shallow level.
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