Gravity exploration was conducted to determine the distribution of igneous complex related to lithium pegmatite in the Boam deposit of Uljin, Gyeongsangbuk-do, and the spatial relationship with the regional geology and ore bodies were studied.The gravity exploration result shows that the Boam deposit area is characterized by relatively low gravity anomaly that surrounds the deposit.The Boam deposit is located near the southwest-northeast directional boundary of gravity anomalies where igneous complex (granite gneiss) contacts with the Yuli and Wonnam groups in the southeast, Janggun limestone layers in the east-west direction, and Dongsugok metasedimentary rocks.While the western boundary in the southwest-northeast direction is relatively clear, there may also be unknown igneous complex that are not exposed on the surface at the eastern and southern boundaries because a relatively low gravity anomaly surrounds the deposit.The distribution characteristics of these hidden igneous complex will be used as useful data for predicting the distribution of the lithium pegmatite in the future.
The Moho structure and its deformation in the southern part of the Korean Peninsula were estimated using gravity and topography data. Gravity signals from the upper and lower crust were separated using a filter that was computed from isostacy and elastic thickness. The result of this study shows three characteristic features of the Moho deformation. First, the Moho folding structure is parallel to SKTL (the South Korean Tectonic Line), which indicates positive association with the collision of the Yeongnam and Gyeonggi Massifs and repeated compression afterwards. In contrast, noticeable deformation of the Moho was not observed along the Imjingang Belt, which is interpreted as another continental collisional belt in the Korean Peninsula. Second, the Moho beneath the Gyeongsang Basin has remarkably risen; this seems to be the result from both the collisional compression and buoyancy caused by magmatic underplating. Third, the Moho deformation is shallowest in the east of the Taebaek Mountains and deepens toward the west, consistent with the topographic characteristic of the Korean Peninsula of high east and low west. It can be interpreted as the results of the opening of the East Sea and Ulleung Basin. A tectonic explanation for this could be the ascent of the mantle induced by continental rifting and horizontal extension at the early stage of the opening of the East Sea. The Moho deformation model computed in this study correlates well with the earthquake distribution and crustal movement measured by GPS. We suggest that the compression along the SKTL is still exerted, consequently, the Moho deformation is active, although it may be weak.
NMC 몰랜드 광산은 고생대 조선누층군을 관입한 화성암류에 의해 형성된 접촉교대 또는 스카른 광상으로 공간적으로 제천화강암과 인접해 있어 이 화강암을 관계 화성암으로 간주하여 대보 화성 활동과 관련하여 형성된 광상으로 해석하였으나, 최근에는 백악기 천부 반화강암질 암체에서 기원된 광화유체로부터 스카른화작용과 더불어 Mo 광화작용이 진행된 것이라는 해석이 제시되었다. 본 연구에서는 광산 일원에 대한 중력탐사를 통해 지하구조를 해석함으로써, 지표 지질에서 제천화강암이 광산에 훨씬 인접한 것과는 달리 지하에서는 남쪽의 백악기 무암사화강암이 광산 하부와 주변으로까지 뻗어있을 것으로 여겨지며, 결과적으로 광상 형성의 관계 화성암으로 작용하였을 것임을 제시한다. NMC Moland mine, which is classified as a contact replacement or skarn deposit, has been interpreted to have been formed by Daebo igneous activity which intruded into the Joseon Supergroup, because it is quite closely located to Jecheon granite. However, an alternative interpretation was recently suggested that the mine could be related with the hydrothermal fluid originated from Cretaceous granitic rocks, bringing about skarnization and Mo mineralization. Here we present an interpretation on the source granite of the mine based on the gravity exploration: the gravity anomaly, unlike the surface geology, shows that the Muamsa granite could be the related granite of the mine, because its hidden subsurface structure is expected to be more widely extended to surrounding area of the mine and deeper than the Jecheon granite.
Although the prevailing wavelength of the Moho fold has been estimated from the spectral analysis of gravity and topography, there has not been a suggested method developed to reveal its structure. Here we present a three‐dimensional (3D) Moho fold structure beneath Tibet which clearly reflects the continental collision. For the structure estimation a new method has been introduced based on the gravity inversion and flexural model. The estimated direction and wavelength of the Moho fold are consistent with the velocities calculated from Global Positioning System (GPS) and with an elastic plate model under horizontal compression. The prevailing wavelength of the Moho fold is estimated to be 300 to 420 km, which corresponds to an elastic plate with effective elastic thickness (EET) of about 35 km, and much smaller than the prior estimates of 500 to 700 km.
An area exists on the South Sea of Korea where the magnetic compass is said to turn round and round not giving a fixed direction to the navigator.We newly assembled a three axis magnetometer system composed of a three axis fluxgate magnetometer, a GNSS compass, a two axis clinometer, and a multi-channel data logger, to investigate if there really exists such an area.We supposed that the horizontal component of the vectorially measured magnetic field on such an area will be too small to maintain the magnetic compass’s needle to one fixed direction. We processed the measured data mainly through two steps. Firstly we transformed the random coordinate system into a fixed coordinate system, i. e., into the geographical coordinate system. Secondly we performed an inversion to eliminate the effect of the ship itself from the measured data.On the finally achieved anomaly map we could not find such an area where the horizontal component is so small that it could not maintain the magnetic compass’s needle to a fixed direction.We conclude that such statement about the existence of the so-called magnetic north immeasurable area on the South Sea of Korea does not have a strict scientific base.
Knowledge of the variation of crustal thickness is essential in many applications, such as forward dynamic modelling, numerical heat flow calculations, seismologic applications and geohistory reconstructions. We present a 3-D model of the Moho undulations over the entire Tibetan plateau derived from gravity inversion. The gravity field has been obtained by using the Gravity Recovery and Climate Experiment (GRACE) potential field development which has been integrated with terrestrial data, and is presently the best available in the studied area. For the effective use of the global geopotential model that has no height information of observation stations, upward continuation is applied. The Moho model is characterized by a sequence of troughs and ridges with a semi-regular pattern, which could reflect the continent—continent collision between the Indian and Eurasian plates. The three deep Moho belts (troughs) and shallow Moho belts (ridges) between them are clearly found to have an E—W directional trend parallel to the border of the plateau and tectonic lines, while variation of the directionality is observed in central to southeast Tibet. To describe the distinctive shape of the Moho troughs beneath Tibet, we introduce the term, 'Moho ranges'. The most interesting aspects of the Moho ranges are (1) that they run in parallel with the border and tectonic sutures of the plateau, (2) that the distances between ranges are found at regular distances of about 330 km except in northeast Tibet and (3) that the splitting of the ranges into two branches is found as the distance between them is increasing. From our study, we conclude that the distinctive undulations of the Tibetan Moho have been formed by buckling in a compressional environment, superimposed on the regional increase in crustal thickness. According to our analysis, the GRACE satellite-only data turns out to have good enough resolution for being used to determine the very deep Moho beneath Tibet. Our Moho model is the first one that covers the entire plateau.
Abstract The growth mechanism of the Tibetan Plateau, postulated by a number of hypotheses, remains under intense debate. Our analysis of recent satellite‐based gravity model reveals that Tibetan lithosphere has been decoupled and folded. It is further evidenced by the existence of crustal melts and channel flow that have been observed by seismic and magnetotelluric explorations. Based on 3D geodynamic simulations, we elucidate the exact buckling structures in the upper crust and lithospheric mantle: at mixed wavelengths between ∼240 and ∼400 km, the lower crustal viscosity is smaller than ∼10 19 Pa·s, implicating weak lower crustal flow beneath the Plateau. This mixed wavelength is consistent with the result of our inverse gravity modeling. Our results facilitate a new plausible hypothesis that the decoupled lithospheric folding mechanism can explain the growth mechanism of the anomalously thick and wide Tibetan Plateau by conflating our idea and contemporary hypothesized scientific findings.
To analyze the tectonic movements of the Geumwang Fault and its association with development of the Pungam Basin, the distributions of the gravity field and aeromagnetic field were interpreted.The low gravity zone (LGZ) around the Geumwang Fault shows an asymmetrical distribution, indicating sinistral (left-lateral) movement with the left side of the fault moving southeastward.The observed gravity anomaly suggests a displacement of approximately 9.3 km.The aeromagnetic distribution supports this horizontal displacement with very distinct magnetic characteristics.Using Euler deconvolution, the average depth of the Geumwang Fault was calculated to be about 1,000 meters, and it is estimated that the southwest side of the Pungam Basin is approximately 700 meters deeper than the northeast side.This strongly suggests that the Geumwang Fault has moved not only in a strike-slip but also in a dip direction.Such fault movement is characteristic of a hinge fault and has contributed to the formation of the basin through fault margin sag.
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