提川 西南部 沃川帶 地域에 對한 重力探査硏究

Gravity Survey on the Southwestern Area of Jechŏn in the Okchŏn Zone

The gravity measurement has been conducted at 61 stations with an interval of about 500 to 1,000 m along two survey lines of about 47 Km between Chungju-Jechŏn and Salmi-Dŏcksanmyŏn in order to study on the subsurface geologic structure and structural relation between Okchŏn Group and Great Limestone Group of Chosŏn Supergroup. The Bouger gravity anomalies were obtained from the reduction of the field observations, and the distribution patterns of the basement and subsurface geologic structure were interpreted by means of the Fourier-Series and Talwani method for two-dimensional body. The depth of Conrad discontinuity varies from 12.7 Km to 15.7 Km, and vertical displacements along the Osanri and Bonghwajae faults are 1.0 Km and 1.5 Km, respectively between Chungju and Jechŏn. The depth of Conrad discontinuity varies from 13.8 Km to 15.4 Km, and vertical displacement along the Bonghwajae fault is 0.5 Km between Salmi and Dŏcksanmyon. The basement is widely exposed at several places between Chungju and Jechŏn. In the unexposed area between Osanri and Wŏlgulri, its depth is from 1.5 Km to 2.1 Km. It is displaced downward along the Osanri and Bonghwajae faults by 0.8 Km and 0.6 Km, respectively, and is displaced upward along the Dangdusan fault by 1.6 Km. On the other hand, the depth of the basement varies abruptly by the Sindangri, Jungwon, Kounri, and Bonghwajae faults between Salmi and Dŏcksanmyŏn, and it is from 2.8 Km to 3.2 Km around Salmimyŏn, from 1.6 Km to 2.5 Km between the Sindangri and Bonghwajae faults, 3.0 Km near Koburangjae, and 2.5 Km at Dojŏnri. The high Bouguer gravity anomalies are due to the accumulation of Okchŏn Group and Jangsŏnri Metamorphic Complex whose density is higher than the basement exposed between Sondong and Osanri, and imply the existance of Bonghwajae Metabasite or hornblende gabbro of high density distributed along the Bonghwajae fault in the vicinity of Koburangjae. The low Bouguer gravity anomalies resulted form the fracture zone associated with fault or rock of low density imply the existance of the Osanri, Bonghwajae, Dangdusan faults and Daedŏcksan thrust between Chungju and Jechŏn, the uplift of the basement by the Sindangri, Jungwon, Kounri, and Bonghwajae faults, and extensive distribution of Cretaceous biotite granites between Salmi and Docksanmyŏn. The thickness of Okchŏn metasediments varies from 1.5 Km to 3.2 Km, and that of Great Limestone Group of Chosŏn Supergroup from 200 m to 700 m. It is interpreted that Okchŏn Group is in contact with Great Limestone Group of Chosŏn Supergroup by the fault zones of the Bonghwajae and Daedŏcksan faults, and the Bongwhajae fault is a thrust of high angle, by which the east of the basement is displaced downward 0.5 Km between Chungju and lechon, and 1.0 Km between Salmi and Dŏcksanmyŏn.