Structural development and stratigraphy of the Kyokpo Pull-Apart Basin, South Korea and tectonic implications for inverted extensional basins JOSEPH. J. LAMBIASE 1 & WILLIAM P. BOSWORTH 2 lAmerada Hess Limited, London Present address: Department of Petroleum Geoscience, University of Brunei Darussalam, Gadong 3186, Negara Brunei Darussalam 2Marathon Petroleum Egypt Ltd, Cairo Abstract: Field studies indicate that the Kyokpo Basin initially formed as a pull-apart during the Late Cretaceous Period. Syn-sedimentary normal and strike-slip faults suggest early transtensional movements. Fault-plane analysis indicates an approximately E-W extension direction for this early event. The basin filled with a succession of lacustrine deposits interbedded with abundant pyroclastic rocks and rare volcanic flows. Rapid initial subsidence is demonstrated by deep-water sediments that lie directly on pre-Late Cretaceous rocks. All exposed sedimentary facies were deposited in deep-water environments, indicating continued rapid subsidence during deposition of the entire sequence. Laminated grey siltstones interbedded with thin, distal, sandy turbidites are the most abundant sediments. Channels filled with conglomeratic debris flow deposits (clasts as large as 1 m in diameter) cut into the siltstone beds. Massive and graded pebbly sandstone turbidites are also interbedded with siltstone. A thick accumulation of stacked, conglomeratic, debris flow deposits interbedded with sandstone turbidites forms a subaqueous fan complex at the southern part of the outcrop area. Local compression caused up to 3 km of uplift and erosion following deposition of the Kyokpo strata. Thrust faults and small- and large-scale folds were formed by the W-E directed compression. Timing of the compression cannot be determined but probably correlates with a regional late Miocene transpressional event. The Kyokpo Basin, located on the western side of the Korean Peninsula (Fig. 1), is part of a complex of small basins exposed in Korea and extending westward across the Yellow Sea into eastern China. The tectonic origin of these basins is uncertain; one possibility is that they originated by transtensional movements associ- ated with the initial collision of India and Asia during the Late Cretaceous (He in press). Extensional half-graben and graben formed during this event continued to subside through- out the Palaeogene. Thick sedimentary se- quences with predominately lacustrine and fluvial facies accumulated in the subsiding basins (Zhao et al. 1985; Yang in press). A subsequent regional transpressional event during the late Miocene is thought to be a product of the Himalayan orogen (Ma et al. 1982; Hc in press). This event caused widespread uplift, basin inversion and erosion, all of varied local inten- sity (Shen & Liang in press). Field data were collected to analyse the detailed structural and stratigraphic history of the Kyokpo Basin. Outcrops were visited on the mainland north and south of the town of Kyokpo, and on the islands of Wido, Biando and Duksando (Fig. 1). In addition to structural and stratigraphic measurements, short cores were collected from the sedimentary rocks for sub- sequent geochemical, palynological and petro- graphic analysis. K-Ar dates were obtained from interbedded volcanic flows and underlying quartz monzonites to help constrain the timing of basin evolution. Stratigraphy and depositional setting Several hundred metres of basin-fill deposits crop out primarily as coastal exposures on the mainland and on several islands in the Kyokpo area (Fig. 1). The total thickness of strata could not be determined because of complex structural relationships (see the discussion below) and because the top of the sequence is not exposed. Measured stratigraphic sections indicate a strati- graphic thickness of at least 577 m (Fig. 2a), and From BUCHANAN, J. G. & BUCHANAN, P. G. (eds), 1995, BasinInversion, Geological Society Special Publication No. 88,457-471. 457