Journal of the Geological Society, London, Vol. 154, 1997, pp. 369–372. Printed in Great Britain
Discussion on structural development and stratigraphy of the Kyokpo Pull-Apart Basin, South Korea and tectonic implications for inverted extensional basins
Special Publications, No. 88, 1995, pp. 457–471
S. B. Kim, S. S. Chun & S. K. Chough write: Lambiase & These problems suggest an alternative hypothesis for the Bosworth (1995) suggested that the Kyokpo Basin, southwest tectonic origin of the Kyokpo Basin and other late Mesozoic Korea opened as a pull-apart basin in the Late Cretaceous and basins in eastern Asia. It is notable that these basins are underwent inversion (transpression) and uplift in the late intimately associated with the Tan-Lu fault system that under- Miocene. They attributed both the basin opening and inver- went sinistral transcurrent movements in the late Jurassic to sion to the India–Asia collision, assuming that the tectonic Cretaceous (Xu et al. 1987; Xu & Zhu 1994). The scale, history of the basin was analogous to that of neighbouring rift orientation, shear sense and offset of the Tan-Lu fault suggest basins in eastern China. This led them to the conclusion that that this transcurrent movement probably resulted from con- the inversion tectonics of the basin resulted from local changes tinental shearing due to oblique motion of the proto-Pacific in stress along a strike-slip fault system. They stated ‘All the oceanic plate relative to the Asian continent (Klimetz 1983; Xu structural features observed in the Kyokpo Basin actually are & Zhu 1994). The timing of movement on the Tan-Lu fault the product of a complex, but single, regional tectonic event correlates well with the reconstructed timing of motion of the (India–Asia collision)’ and thus ‘It is neither necessary nor palaeo-oceanic plate from palaeomagnetic data on the Pacific desirable to invoke major changes in regional structural seafloor (Engebretson et al. 1985; Otsuki 1992). A proto- geometries to explain the evolution of this basin’ (p. 469) (our Pacific oceanic plate (Izanagi plate) began to subduct obliquely italics). against the Asian continent in the latest Jurassic (150–145 Ma), It is, however, unrealistic to attribute the formation of the coinciding with the initiation of transcurrent movements of the Kyokpo Basin to the India–Asia collision. The collision oc- Tan-Lu fault (Xu & Zhu 1994). At about 135 Ma, the rate of curred after 70–65 Ma (Klootwijk et al. 1992, 1994), whereas subduction of the Izanagi plate rapidly increased, which ac- sediment filling of the Kyokpo Basin had already commenced counts well for the maximum offset of the Tan-Lu fault during prior to 97–75 Ma, as indicated by the ages of andesite and the Hauterivian (Xu & Zhu 1994). We therefore suggest that basalt flows interlayered within the lower part of the sedimen- opening of the Kyokpo Basin and other late Mesozoic basins tary sequence of the basin (Table 1). Moreover, the age of in eastern Asia originated from transtension associated with opening of other Cretaceous strike-slip basins (akin to the subduction-related intracontinental wrench tectonics (Chun & Kyokpo Basin) in Korea is known to be as early as Chough 1992; Kim et al. 1995b). Widespread acidic volcanism Valanginian–Hauterivian (Seo 1985; Choi et al. 1995). Also, and emplacement of granotoid and calc-alkaline magma in and the neighbouring basins in eastern China, that are regarded near these basins also indicate the influence of subduction of as resulting from the India–Asia collision by Lambiase & an oceanic plate on basin evolution (Maruyama & Seno 1989). Bosworth (1995), were in fact formed by enlarging and co- Lambiase & Bosworth suggested that the Kyokpo Basin alescing of small-scale strike-slip basins of late Jurassic to continued to subside until the onset of Miocene inversion, Cretaceous age (Li 1984; Liu 1986). Besides the discrepancies based on a seismic section from the South Yellow Sea Basin in timing, the India–Asia collision cannot explain the structure (their fig. 10) which is interpreted to record continuous thick- of these basins. The Kyokpo Basin and other late Mesozoic ening of the Cretaceous to Miocene strata into normal faults. basins in eastern Asia lie within the domain of a sinistral However, in other cross sections from the basin, most Creta- strike-slip fault system (Xu et al. 1987; Chun & Kim 1995), ceous strata do not show any obvious thickening into normal whereas India-indentation-linked faults exhibit dextral sense in faults, unlike the overlying Paleogene strata (see Chun & the Far East (Molnar & Tapponnier 1975). Chough 1992). Even in Lambiase and Bosworth’s example
Table 1. K–Ar ages of igneous rocks of the Kyokpo Basin
Lithology Occurrence Age* (Ma) Age† (Ma)
Mafic dike Intrusion into basement granite 66.1&2.6 Porphyritic rhyolite Intrusion into sedimentary sequence 62.6&2.7 76.1&2.2‡ Rhyolitic tuff Overlying the sedimentary sequence 62.6&2.4 83.5&2.4 Green tuff Interlayered with sedimentary sequence 64.3&2.6 Basalt flow Interlayered with sedimentary sequence 74.8&3.1 Volcanic breccia Interlayered with sedimentary sequence 85.7&3.4 Andesite flow Interlayered with sedimentary sequence 97.4&3.1 Granite Basement 85.9&3.9 128.0&3.0‡ (quartz monozonite)
*From KIER (1981) †From Lambiase & Bosworth (1995). ‡Data obtained from K-feldspar; the rest from whole-rock dating.
369
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/369/4887395/gsjgs.154.2.0369.pdf by guest on 27 September 2021 370 DISCUSSION
(their fig. 10), thickening of Cretaceous strata is not obvious. ripple cross-laminated division C (Chun & Kim 1995). This Together with this, the ‘ponded’ occurrence of the Cretaceous most probably reflects a shallower (<100 m) lake, because strata occupying only local depressions within the South shallow water does not exhibit the necessary space in which Yellow Sea Basin may suggest that Cretaceous small-scale turbidity currents can fully dilate and sustain to the develop- strike-slip basins transformed and coalesced into a large-scale ment of ripple-bed conditions. Furthermore, the turbidites extensional basin in the Cenozoic. Like these subbasins, the occasionally include wave-ripple marks on top of the bed (Kim Kyokpo Basin seems to have ceased opening in the late 1993). Their interpretation of the sedimentary features shown Cretaceous rather than to have continued opening into the in their fig. 4a and 4b is also questionable. (Both figures are Miocene, because the sedimentary fill of the basin is overlain misprinted right side to the left.) We suggest that the channel- by a rhyolitic tuff body of 83.5 or 62.6 Ma (Table 1) and the like geometry of the turbiditic sandstone (not conglomerate) in subsequent volcaniclastic succession overlaps the basin bound- fig. 4a is a scour feature formed at the site of flow reflection of ary. This inference is consistent with the tectonic history of a density current and that the conglomerates in fig. 4b are not other Cretaceous strike-slip basins in Korea that were closed debrites but channel-bar complexes on a delta plain (Chun & and underwent compressional deformation in the late Creta- Kim 1995). ceous (Kim et al. 1994). Meso-Cenozoic basins in eastern In conclusion, we believe that Lambiase & Bosworth’s China and southwestern Japan were also subject to a tempor- hypothesis linking evolution of the Kyokpo Basin to the ary cessation of basin subsidence and extension in the late India–Asia collision is rooted in a misinterpretation of the Cretaceous. For example, Cretaceous sequences of the basin history and a misconception of the regional tectonics. It Songliao and Bohai basins in China are deformed and uncon- should be abandoned. The geological structures and regional formably overlain by Tertiary deposits (Liu 1986). The Creta- tectonic setting of late Mesozoic strike-slip basins in eastern ceous Kanmon and Himenoura groups in Japan are both Asia, including the Kyokpo Basin, suggest an alternative unconformably overlain by Paleogene strata that show differ- origin for the basins, from intracontinental wrench tectonics ent structural trends from the Cretaceous strata (Okada & due to oblique motion of the proto-Pacific plate (Izanagi and Sakai 1993). then Kula plates) relative to the Asian continent. These basins These interruptions of basin subsidence, whether permanent seem to have stopped subsiding in the late Cretaceous when or temporary, might have been produced only by local changes the subduction became trench-normal and thus continental from transtension to transpression along the strike-slip fault shearing ceased. This tectonic scenario indicates that the system; nonetheless, their regional contemporaneity suggests evolution of these basins should be considered in the light of that they may have been associated with a plate tectonic event. plate tectonics rather than local tectonics along a strike-slip It is interesting to note that the obliquity of motion of a fault system. The stratigraphy and sedimentology of the proto-Pacific oceanic plate (Kula plate) relative to the Asian Kyokpo Basin are also erroneously described and interpreted continent changed to near normal at about 85–75 Ma by Lambiase & Bosworth, and may need to be reappraised in (Engebretson et al. 1985). This event is thought to have been an alternative manner. responsible for the cessation of strike-slip movement on Meso- zoic faults in Japan (Otsuki 1992). The Tan-Lu fault also 27 June 1996. stopped transcurrent movement in the late Cretaceous, chang- ing to extensional motion (Xu & Zhu 1994). We therefore J. J. Lambiase writes: Many of the points raised by Kim et al. envisage that this event would have affected the whole eastern are centred around the tectonic origin of the Kyokpo Basin. margin of Asia, terminating the opening of the transtensional Generally, their points on this issue are well taken; however, basins thereon. that entire discussion forms but a minor part of the original Lambiase & Bosworth also made erroneous descriptions of paper. The main points of Lambiase & Bosworth (1995) are the stratigraphy of the Kyokpo Basin (their fig. 2). Among mostly concerned with the structural and stratigraphic evolu- others, the main errors are: (1) conglomerates (breccias) di- tion of the basin, rather than with the ultimate tectonic cause rectly resting on basement granite (quartz monzonite) are of its development; therefore, this reply will address only those almost monomictites of granitic clasts, not of volcanics; (2) topics related to the structural and stratigraphic evolution of other conglomerates are usually polymictic, not including the Kyokpo Basin. monomictites of volcanic clast; even the conglomerates with The additional age data (Table 1), which confirm a tuffaceous matrices contain granitic clasts, comprising more Cretaceous age for the origin of the basin are very welcome. than 20–40% of the total clast population; (3) the section south However, their contention that isolated, small strike-slip basins of Kyokpo comprises two fining-upward successions; however, formed during the Cretaceous, yielding ‘ponded’ Cretaceous this trend is missed or not depicted. They invoked dominantly strata without significant thickening into faults is questionable. westward-directed paleocurrents, but we find overwhelming Firstly, it is difficult to envisage strike-slip basins, of any scale, evidence for paleocurrents of the opposite direction, although forming without the significant fault-controlled subsidence the direction locally varies from northeast to southeast (Kim that has been well-documented in numerous basins. Secondly, 1993; Chun & Kim 1995; Kim et al. 1995a). Lambiase & significant thickening of the Cretaceous against a fault is Bosworth inferred that the sedimentary sequence had been clearly shown near the middle of the seismic line in fig. 10 in formed in a steep-sloped subaqueous fan-to-lacustrine en- Lambiase & Bosworth (1995). The same geometry is observed vironment. Only in a broad sense, do we agree with this throughout the area covered by the seismic grid and is related inference (see Chun & Kim 1995; Kim et al. 1995a). Their to the active faulting that was associated with the opening of contention that the lake was deep seems conjectural because the basin. the coarse-grained clastics only indicate steep slopes. Besides The seismic line in Lambiase & Bosworth (1995, fig. 10) also the siltstone succession (sandstone/mudstone couplets) consists clearly shows significant thickening against faults of the of thin turbidite units of partial Bouma sequences, with Palaeogene and Miocene sections, indicating that subsidence variable combinations of divisions A to E, but typically lacking continued well into the Tertiary, without an episode of uplift.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/369/4887395/gsjgs.154.2.0369.pdf by guest on 27 September 2021 DISCUSSION 371
Kim et al. prefer to consider the Kyokpo Basin as analogous to deposition had occured in shallow water from turbidity cur- the Puyeo Basin which ceased to subside in the late Cretaceous rents which were not fully dilated, as proposed by Kim et al., (Kim et al. 1994) and their conclusion is based on the presence there would have been higher flow densities and consequent of an early Tertiary volcaniclastic succession which overlaps increases in densiometric Froude numbers and flow velocities. the basin boundary. However, the volcaniclastics are derived This would have produced higher flow regime bedforms and from a rhyolitic tuff which overlies the sedimentary section of resultant sedimentary structures, among which, almost cer- the Kyokpo Basin and they present no evidence that the tainly, would have been Bouma sequence division C beds. volcaniclastics could not have been deposited during subse- The feature depicted in fig. 4a is simply a channel with a quent unroofing of the rhyolitic tuff. Their analysis also fails to typically erosional base, which is filled with turbidite conglom- explain the petrographic evidence, presented in the original eratic sandstones; a more complex interpretation, such as paper, for mechanical compaction of the Kyokpo strata. The density current reflection, is unwarranted. Another problem is compaction required a burial depth of approximately 3 km, the interpretation by Kim et al. of the conglomerates shown in burial that had to have occurred subsequent to the youngest fig. 4b as channel-bar complexes on a delta plain. The stacked dated syn-sedimentary volcanics (62.6&2.4 Ma, Table 1). beds have little or no erosion at their bases which would be The first of three major objections with the stratigraphic extremely unusual for a traction deposit with such coarse descriptions listed by Kim et al. can be reconciled by correcting clasts. Also, most of the large clasts are matrix-supported as in a drafting error in fig. 2a; the basal conglomerate should be debris flows without any of the imbrication, clast orientation shown as containing granitic rather than volcanic clasts. or cross-bedding that would be expected in a traction deposit. However, their assertion of granite instead of quartz mon- This clearly points to a debris-flow origin for the conglomer- zonite is untenable based on petrographic point-counting (R. ates. Finally, there are no associated subareal or transitional Wegrzyn pers. comm.), as is their usage of breccia for sedimen- facies as could be expected of a delta plain setting; all evidence tary conglomerate. Similarly, their second point refers to one suggests a deep water environment. line in the text that possibly should have repeated the state- The sediments which crop out in the Kyokpo Basin succes- ment of the previous line that the discussion of clast composi- sion are unequivocably deep water deposits and the facies tion referred to the larger clasts in each flow. The third associations overwhelmingly support a subaqueous fan com- objection is irrelevant as the figures in the original paper are plex with associated channels and distal turbidites in a basinal intended to illustrate representative stratigraphic sections on setting. Similar sedimentary systems are well-documented and off the subaqueous fan complex and to depict the facies along basin-bounding faults in many extensional and trans- present in the basin. It is assumed that the fining-upwards tensional basins (e.g. East Greenland, the North Sea Brae successions to which Kim et al. refer are those observed south complex, Newark Basin, USA, amongst many others). Apart of Kyokpo within the facies described. The boulder cluster from modern fans, most examples remain in the subsurface or method used by Kim et al. (1995a), and referred to here, for are poorly exposed. The Kyokpo Basin succession is an determining palaeocurrent direction is of questionable accu- outstanding outcrop example of this type of fan system; it is racy. It apparently yields internally inconsistent results as fig. 7 most unfortunate that Kim et al. have failed to recognize it as in Kim et al. (1995a) displays several boulder clusters of such. significantly different palaeocurrent direction from that they In conclusion, the extensive arguments presented by Kim et propose. al. regarding the regional tectonic history are persuasive. With regard to the sedimentology, Kim et al. have made However, their analysis of the timing and geometry of the fundamental errors in interpretation most of which result from subsiding Cretaceous Basin does not agree with observations their failure to recognize that all the Kyokpo strata were on offshore seismic data nor does it account for the estimated deposited in deep water environments. This misconception is 3 km of burial, prior to uplift, of the Kyokpo Basin strata; caused primarily by their interpretation of the siltstone succes- these issues should be investigated further. The sedimentologi- sion. Whilst Kim et al. recognize that the sandstones within the cal interpretations and conclusions presented by the authors in siltstone succession are indeed thin-bedded turbidites, they Chun & Kim (1995), Kim et al. (1995a) and here contain many assign them to a shallow lake environment. Apart from occa- serious errors mostly emanating from their failure to recognize sional to rare small wave ripples, all the associated sedimentary the Kyokpo Basin as a deep-water environment. The delta structures indicate deposition below wave base (i.e. deep wa- plain and shallow-water interpretations should be abandoned. ter). To describe the succession as sandstone/mudstone cou- 3 October 1996. plets (Chun & Kim 1995) is to imply a genetic relationship between the sandstone and siltstone. This is clearly not the case; the depositional environment was a low energy setting References which experienced intermittent sand input via turbidity C, S.-J., K, B.C., C,H.Y.&K, Y.B. 1995. Charophytes from the currents. Chopyeong Formation (Cretaceous) of the Eumsung Basin, Korea. Journal The turbidites themselves are relatively low-energy deposits, of the Geological Society of Korea, 31, 523–528 [in Korean with English resulting either from overbank deposition produced by large abstract]. flows in adjacent channels or as distal deposits of sediments C,S.S.&C, S.K. 1992. Tectonic history of Cretaceous sedimentary transported down-slope across an asymmetric extensional ba- basins in the southwestern Korean Peninsula and Yellow Sea. In:C, S.K. (ed.) Sedimentary Basins in the Korean Peninsula and Adjacent sin or along its axis. The turbidites are thin (0.1 m or less), Seas. Korean Sedimentology Research Group, Special Publications, 60– composed of fine sand and there was no erosion of the 76. underlying siltstone by each succeeding flow. Most are graded ——&K, S.B. 1995. The Cretaceous Kyokpori Formation, SW Korea: with minor parallel lamination, suggesting relatively rapid sublacustrine steep-sloped delta facies. Journal of the Geological Society of deposition from suspension during waning flow conditions in a Korea, 31, 215–236. E, D.C., C,A.&G, R.G. 1985. Relative Motions between basinal setting. This process would produce the observed Oceanic and Continental Plates in the Pacific Basin. Geological Society of absence of Bouma sequence division C beds. In contrast, if America, Special Papers, 206.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/369/4887395/gsjgs.154.2.0369.pdf by guest on 27 September 2021 372 DISCUSSION
K 1981. [Report on Stability of Proposed Site for a Nuclear Power Plant (Buan L, D. 1984. Geological evolution of petroliferous basins of continental shelf of site)]. Korea Institute of Energy and Resources [in Korean]. China. American Association of Petroleum Geologists Bulletin, 68, 993– K, J.H., L,J.Y.&K, W.S. 1994. Structural evolution of the Cretaceous 1003. Puyeo Basin, Korea. Journal of the Geological Society of Korea, 30, 182–192 L, H. 1986. Geodynamic scenario and structural styles of Mesozoic and [in Korean with English abstract]. Cenozoic basins in China. American Association of Petroleum Geologists K, S.B. 1993. Bouldery Deposits in the Lowermost Part of Cretaceous Kyokpori Bulletin, 70, 377–395. Formation, SW Korea: Subaqueous Cohesionless Debris Flows and Gravity M,S.&S, T. 1989. A review of paleogeographic reconstructions of Falls on Steep Slope. MSc Thesis, Seoul National University. Asia and its environs back to the Permian—toward the understanding of ——, C,S.K.&C, S.S. 1995a. Bouldery deposits in the lowermost the Wilson cycle. Report of Institute Kuroshio Sphere, Kochi University, 4, part of the Cretaceous Kyokpori Formation, SW Korea: cohesionless 135–177. debris flows and debris falls on a steep-gradient delta slope. Sedimentary M,P.&T, P. 1975. Cenozoic tectonics of Asia: effects of a Geology, 98, 97–119. continental collision. Science, 189, 419–426. ——, L, S.H., R,W.H.&C, S.K. 1995b. Cretaceous nonmarine O,H.&S, T. 1993. Nature and development of Late Mesozoic and basins in the Korean Peninsula: implications for sedimentation and tecton- Early Cenozoic sedimentary basins in southwest Japan. Palaeogeography, ics in the Yellow Sea Basin. Third International Conference on Asian Marine Palaeoclimatology, Palaeoecology, 105, 3–16. Geology – Evolution and Dynamics of the Asian Seas, Abstract, 93–94. O, K. 1992. Oblique subduction, collision of microcontinents and subduc- K, M.P. 1983. Speculations on the Mesozoic plate tectonic evolution of tion of oceanic ridge: their implications on the Cretaceous tectonics of eastern China. Tectonics, 2, 139–166. Japan. The Island Arc, 1, 51–63. K, C.T., C, P.J., N,R.& J, K.A. 1994. Further palaeomagnetic data from Chitral (Eastern Hindukush): evidence S, S.J. 1985. Lower Cretaceous Geology and Paleontology (Charophyta) of for an early India–Asia contact. Tectonophysics, 237, 1–25. Central Kyongsang Basin, Korea. PhD Thesis, Kyungpook National Uni- versity [in Korean with English abstract]. ——, G, J.S., P, J.W., S,G.M.&MF, P.L. 1992. An early India–Asia contact: paleomagnetic constraints from Ninetyeast Ridge, X,J.&Z, G. 1994. Tectonic models of the Tan-lu fault zone, eastern China. ODP Leg 121. Geology, 20, 395–398. International Geology Review, 36, 771–784. L,J.J.&B, W.P. 1995. Structural development and stratigra- ——, Z, G., T, W., C,K.&L, Q. 1987. Formation and evolution of phy of the Kyokpo Pull-Apart Basin, South Korea and tectonic impli- the Tancheng-Lujiang wrench fault system: a major shear system to the cations for inverted extensional basins. In:B,J.G.&B, northwest of the Pacific Ocean. Tectonophysics, 237, 273–310. P.G. (eds) Basin Inversion. Geological Society, London, Special Publi- cations, 88, 457–471.
Scientific editing by Alan Roberts.
S. B. K, Department of Oceanography, Seoul National University, Seoul 151-742, Korea (e-mail: [email protected]). S. S. C, Department of Geology, Chonnam National University, Kwangju 500-757, Korea (e-mail: [email protected]). S. K. C, Department of Oceanography, Seoul National University, Seoul 151-742, Korea (e-mail: [email protected]). J. J. L, Department of Petroleum of Geoscience, Universiti Brunei, Darussalam, Bandar Seri, Begawan 2028, Brunei (e-mail: [email protected]).
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/154/2/369/4887395/gsjgs.154.2.0369.pdf by guest on 27 September 2021