IEG 환경지질연구정보센터

10월 25일(목)․퇴적학

218 2007 추계지질과학연합학술대회 초록집

Clastic dykes in the Late Cretaceous Non-marine Deposits, Southeastern Gyeongsang basin, Korea: Occurences and origin

Hee Cheol Kang․In Sung Paik

․Sook Ju Kim․Hyun Joo Kim Dept. of Environmental Geosciences, Pukyong National University, Busan 608-737, Korea

[email protected]

Clastic dykes in the Late Cretaceous Jindong, Goseong, and Dadaepo formations of south- eastern Gyeongsang Basin of South Korea were examined on their structural, geometric and compositional characteristics. More than 60 clastic dykes cutting across depositional bedding or/and igneous dykes at a high angle were observed. Most of the dykes are filled with sandstone matrix and clasts of host rocks. The dykes range from about 3 to 30 cm in thickness and from 1.2 to 20 m in length. The morphology of the dykes indicate that the infilling materials were rapidly and forcefully injected from below, and some dykes show downward injection. These clastic dykes are grouped into three types based on the mode of occurrence and mechanism of origin: (1) Depositional or Neptunian clastic dykes. They are not injection structures, and are interpreted as tension cracks resulting from local deformation as the gravity of overlying sedimentary beds or earthquake disturbances. The cracks or fissures are about 1~1.5 m in length, with a downward drop-shaped termination. (2) Injection clastic dykes showing rapid upward injection of liquefied sand. They provide the evidence for past earthquake. (3) Clastic dykes associated with igneous intrusions, such as brecciate pipes, which is likely to be involved in the injection of hydrothermal fluids along pre-existing fractures parallel to adjacent igneous dykes. The infilling materials show the gradual change from clast-magma mixtures on the lower part to clast-rich on the upper part. Although these clastic dykes have variable strikes, there are strong preferred orientations at about N35oW, N05oE and N85oE. From cross-cutting relations between the clastic dykes, the sequence of development is N05oE - N35oW - N85oE.

The paleostress analysis using the orientation of these clastic dykes and adjacent fault-slip data shows that the tectonic events recorded in these deposits are divided into N-S com- pression and E-W extension, NW-SE compression and NE-SW extension, and E-W compression and N-S extension in chronological order. These kinematics are concordant with the paleo- stress field as well as with the regional tectonic setting around the Korean Peninsula during the Cretaceous in which the relative motion of the Izanagi Plate subduction changed from north to northwest.

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