Geological Structures and Controls on Half-Graben Inversion in the Western Gunsan Basin, Yellow Sea

Marine and Petroleum Geology 68 (2015) 480e491

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Marine and Petroleum Geology

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Research paper Geological structures and controls on half-graben inversion in the western Gunsan Basin, Yellow Sea

Young Jae Shinn

Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea article info abstract

Article history: This paper focuses on the structural styles of an inverted half-graben in the western Gunsan Basin Received 11 March 2015 (CretaceousePaleogene) in the Yellow Sea. Detailed seismic interpretation calibrated by well data in- Received in revised form dicates that the half-graben formed by the Cretaceous to Eocene extension and subsequently underwent 26 August 2015 contraction which led to the basin inversion. The inversion occurred during the Oligocene and Middle Accepted 28 September 2015 Miocene. In the Oligocene inversion phase, the hanging-wall strata underwent shortening by inverted Available online 13 October 2015 extensional faults, newly-formed reverse faults, and fault-related folds. The inversion deformation was caused by an NNE-directed regional contraction that is nearly normal to the orientation of earlier Keywords: Inverted half-graben extensional faults. The style and distribution of internal deformation was mainly governed by variations Inversion structures in the dip angle of the pre-inversion fault plane. The Early Miocene inversion phase commenced with the Gunsan Basin development of a broad asymmetric hanging-wall anticline caused by the preferential reactivation of the Northern South Yellow Sea Basin NW-trending bounding fault. This preferential reactivation suggests an orientation change of inversion stress from the Oligocene NNE-directed to the Miocene NE-directed contraction. The bounding fault continued to be reactivated during the Middle Miocene, which caused the growth of a monocline. © 2015 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

1. Introduction from the Bohai Bay, East China Sea, Subei, and Songliao basins (Ma et al., 1989; Allen et al., 1997; Liu et al., 2000; Grimmer et al., 2002; The Cretaceous to Paleogene Gunsan Basin is part of the Northern Su et al., 2009). A few studies have focused on the spatial and South Yellow Sea Basin that formed as a large intracontinental basin temporal variations of inversion structures to understand their between the Tan-Lu fault system and the subduction margin of the geological controls and inversion history, although the Oligocene proto-Pacific plate (Fig. 1). The Gunsan Basin initially formed by an inversion was widely recognized from sedimentary basins in East extension under a transtensional tectonic setting during the Creta- Asia. ceouseEocene and was subsequently deformed by the Oligocene This study further elucidates deformation styles of a low-to- contraction (Ryu et al., 2000; Yi et al., 2003; Park et al., 2005; Shinn moderately inverted half-graben based on seismic reflection data et al., 2010). The contraction was interpreted to be associated with a in the western Gunsan Basin. Restoration and balancing of cross- dramatic change in the stress regime from dextral transtension to sections are used to validate geological interpretation and to eval- sinistral transpression along the Tan-Lu fault (Shinn et al., 2010), uate the timing and amount of deformation through time (e.g., which caused tectonic inversion of the Gunsan Basin in a complex Hossack, 1979; De Paor, 1988; Buchanan, 1996; Coward, 1996; manner and formed an inverted half-graben that was particularly Bulnes and McClay, 1999; Dubey et al., 2001; Zhou et al., 2006). developed in the western Gunsan Basin. This study will help to reveal the main factors responsible for The inverted half-graben in the western Gunsan Basin repre- structural variability of a low-to-moderately inverted half-graben. sents typical inversion structures affected by reverse reactivation of extensional faults and related folds (Shinn et al., 2010), which likely 2. Geologic setting reflects multi-episode deformation with varying degree of inversion style. Regional Oligocene inversion has also been documented The Gunsan Basin is the eastern extension of the Northern South Yellow Sea Basin, located in the central Yellow Sea between the Shandong Peninsula and Korea (Fig. 1). The basin is bounded by the E-mail address: [email protected]. Qianliyan Massif to the north and by the Central Massif to the south http://dx.doi.org/10.1016/j.marpetgeo.2015.09.013 0264-8172/© 2015 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 481

Fig. 1. Distribution map of the Cretaceous to Paleogene sedimentary basins around the study area and major structures of the Northern South Yellow Sea Basin and Gunsan Basin (modiﬁed from Zhang et al., 1989; Yi et al., 2003; Shinn et al., 2010). Abbrevia- tions: GB ¼ Gunsan Basin; NSYSB ¼ Northern South Yellow Sea Basin; SSYSB ¼ Southern South Yellow Sea Basin; SB ¼ Subei Basin.

(Zhang et al., 2007; Wu et al., 2008). The southern margin of the basin is marked by a series of ENE- or NE-striking basement faults with significant throws. To the north of the basin, the Qianliyan fault continues with the north-bounding fault of the Subei Basin, which is part of a wrench fault branching from the Tan-Lu fault system (Fig. 1). The distribution and geometry of the basin were largely controlled by the eastward-fanning bounding faults and the associated NW-striking fault set (Shinn et al., 2010). The formation of the Gunsan Basin was initiated during the Early Cretaceous when sinistral wrench faulting affected the eastern margin of the Eurasian continent (Xu et al., 1987; Lee, 1999; Chough et al., 2000; Shinn et al., 2010). The regional wrench-induced, normal faulting formed fault-bounded depressions and structural highs. The depressions continued to subside until the regional uplift and erosion occurred and the following Miocene succession covered the basin. The basin was infilled by up to 6 km of non- marine successions mostly comprising CretaceouseEocene alluvial to fluvio-lacustrine deposits with subordinate volcanic rocks (Ryu et al., 2000; Yi et al., 2003). The overlying Lower Miocene successions are marked by a regional unconformity at the base and areas of local subsidence were developed to accommodate the reactivation of faults. The main phase of postrift subsidence began in the Middle Miocene (Ryu and Kim, 2007). The Middle Miocene to present-day succession is characterized by flat-lying configuration of continuous seismic reflectors and basin-wide distribution with uniform thickness. These seismic characteristics are indicative of tectonic quiescence and several events of marine transgression. The Middle Miocene to present-day succession cannot be subdivided Fig. 2. Lithologic column, biostratigraphy, and depositional environment of the due to the lack of geological information from exploration wells. Kachi-1 well (after Ryu et al., 2000; Yi et al., 2003). The basement rock of the Gunsan Basin is unconformably overlain by the Cretaceous to Eocene synrift strata. It varies in age and lithology over the basin, containing sedimentary and meta- JurassiceEarly Cretaceous, Early Cretaceous, and Late Cretaceous morphic rocks of the Paleozoic Yangtze Platform (Kim and Oh, units (Yi et al., 2003; Ryu and Kim, 2007)(Fig. 2). The Late Juras- 2007; Wu et al., 2008). The maximum depth of the basement siceEarly Cretaceous unit mainly contains poorly sorted, arkosic rock in the Gunsan Basin was estimated to range from 6 to 8 km on sandstones with tuffs, resulting from a rapid deposition in alluvial the basis of magnetic and gravity data (Park et al., 2009). environment (Yi et al., 2003). Interbedded reddish mudstones and The Cretaceous succession encountered in the Kachi-1 well sandstones become predominant in the Early Cretaceous unit, drilled in the western Gunsan Basin is up to 2079 m in thickness which reflects depositions in fluvial and floodplain environments (Fig. 2). The Kachi-1 well reached the basement rock of the Triassic (Yi et al., 2003). The overlying Late Cretaceous unit is characterized dolomite (Yi et al., 2003). Based on lithological and micropaleon- by interbeds of dark gray mudstones and limestones deposited in tological data, the Cretaceous succession was subdivided into Late fluvio-lacustrine to deltaic and marginal lacustrine environments 482 Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491

(Yi et al., 2003; Ryu and Kim, 2007). In the Kachi-1 well, the upper part of the Cretaceous unit is incomplete and the PaleoceneeEarly Miocene unit is missing due to signiﬁcant uplift and erosion. The Middle Miocene succession rests unconformably on the Cretaceous succession and consists mainly of unconsolidated sands interbedded with clays and lignites that are interpreted to be ﬂuvio- deltaic in origin (Yi et al., 2003).

3. Results

3.1. Seismic to well correlation

The available seismic reflection data for this study include composite multi-channel seismic reflection profiles acquired by the Korea National Oil Corporation (KNOC) and the Korea Institute of Geoscience and Mineral Resources (KIGAM). A grid of seismic data in the western Gunsan Basin mainly comprise SWeNE trending profiles perpendicular to the major structural trend and subordinate NWeSE and EeW trending profiles. The Kachi-1 well was drilled in an anticlinal structure for hydrocarbon exploration but did not encounter hydrocarbons. Well data were obtained from unpublished sedimentological and palynological reports of the KIGAM and KNOC, incorporated with the work of Ryu et al. (2000), Yi and Batten (2002), and Yi et al. (2003). The geological informa- Fig. 4. Time-structure map of acoustic basement showing the major fault distribution tion from the well data provides the calibration for the seismic and structural domain in the western Gunsan Basin. The inverted half-graben is reflection data. separated by the Kachi fault from the interbasinal ridge. The SW margin of the half- e Based on seismic interpretation together with well data, seven graben is bounded by a set of WNW ESE trending and NNE-dipping faults. seismic horizons are correlated over the western Gunsan Basin (Fig. 3). The regional unconformity of Middle Miocene (BaseMM) to be post-Miocene or intra-Pliocene on the basis of carbon isotope and the top of acoustic basement are confidently calibrated with analysis (Hong and Shinn, 2014). biostratigraphic boundaries (Figs. 2 and 3). Yi et al. (2003) suggested a biostratigraphic boundary between late Maastrichtian and early MaastrichtianeCenomanian stages of the Cretaceous. This 3.2. Structural domain and fault framework boundary is not tied to a mappable horizon in seismic sections. Immediately above it, the intra-Late Cretaceous horizon (IntraLK) is The present-day structure beneath the Middle Miocene un- recognized as an unconformable surface near the end of the conformity consists mainly of a NWeSE trending fault system Cretaceous. A boundary of top Early Cretaceous is most likely (Fig. 4). This fault system exerted a major control on the develop- correlated with a seismic horizon (TopEK) with very strong ment of NW-trending structural domains, consisting of a major amplitude. The age of the uppermost boundary (PostM) is inferred half-graben and tilted fault blocks separated by an interbasinal

Fig. 3. Interpretation of a regional seismic section across the Kachi-1 well showing the location and style of the inverted half-graben. For location of the seismic section, see Fig. 1. Numbers along the well trajectory indicate biostratigraphic boundaries marked in Fig. 2. Abbreviations: PostM ¼ post Miocene; BaseMM ¼ base Middle Miocene; IntraEM ¼ intra Early Miocene; TopE ¼ top Eocene; IntraLK ¼ intra Late Cretaceous; TopEK ¼ top Early Cretaceous; TopBST ¼ top basement. Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 483 ridge (Fig. 4). The development of the half-graben was controlled by a high-angle SW-dipping Kachi fault, which connected the NW- and NNW-trending fault segments, and its conjugate bounding faults, a series of WNWeESE oriented faults at the opposite margin of the half-graben (Fig. 4). The conjugate faults run slightly oblique to the Kachi fault and the half-graben gradually deepens and ex- pands to the northwest. The interbasinal ridge is deﬁned as elon- gated topographic highs bounded by oppositely-dipping major extensional faults and constitutes narrow faulted highs to the NW and broad highs to the SE. The narrow faulted highs are bounded to the NE by step faults that continue to the fault block domain and to the SW by the Kachi fault. The half-graben bounding faults partially experienced reverse- sense reactivation. The Kachi fault appears to have variable degrees of throws with both normal and reverse separation along its segments. Reverse-sense throws are observed in the SE segment of the Kachi fault, whereas there are dominant normal-sense throws in the NW segment (Fig. 5). The conjugate bounding faults exhibit normal-sense throws due to the small amounts of reverse reactivation. The amount of normal-sense throws generally increase toward the northwest. The northeast-dipping normal faults in the fault block domain have a relatively planar geometry where they cut through the latest CretaceouseEocene and Early Miocene units, and have a reduced dip angle as they cut deeper units (Fig. 3). A series of tilted fault- bounded blocks rotated during extension, and deposition occurred in triangular depressions between the rotating fault blocks. There is no clear evidence for the reverse reactivation of the block faults. Structural highs related to magmatic intrusion and uplifting are Fig. 6. Tectonostratigraphic unit and its correlation with regional tectonic event. observed in the eastern part of the fault block domain (Fig. 4).

3.3. Tectonostratigraphic units inversion. The main phases of inversion occurred during the Oligocene and Early Miocene (Fig. 6). At the same time, the syn- Four tectonostratigraphic units are identiﬁed in the western inversion I unit was deposited in local depression caused by the ﬂ Gunsan Basin: prerift basement, synrift, syninversion, and post- loading and exural bending associated with the adjacent inversion inversion units (Fig. 6). The top of the synrift unit is marked by an uplift. The top of the syninversion I unit is marked by the Middle erosional unconformity representing the Oligocene hiatus of Miocene unconformity (horizon BaseMM in Fig. 7) that exhibits regional extent (horizon TopE in Fig. 7). The synrift unit is divided erosional truncation in some places toward an inversion uplift, into the Cretaceous synrift I and latest Cretaceous to Eocene synrift whereas grades into its correlative conformity basinward. The II units. These units are separated by the Late Cretaceous uncon- overlying syninversion II unit was affected by continued growth of formity or its correlative conformity (horizon IntraLK in Fig. 7). It is an inversion anticline during the Middle Miocene to Pliocene. The not clear if this unconformity is related to regional tectonic changes post-Pliocene succession, postinversion unit, was unaffected by (e.g., pulsed extension) or to relatively local processes such as strain inversion-related fold growth. The lower boundary of the post- localization (e.g., Olsen, 1997; Gupta et al., 1998). As the basin inversion unit is marked by an erosional unconformity in the subsidence ceased, the basin underwent several phases of growth fold zone and its correlative conformity elsewhere (PostM horizon in Fig. 7).

3.3.1. Synrift unit The Cretaceous synrift I unit is subdivided into the lower and upper units by the top Early Cretaceous horizon (TopEK). The lower unit is acoustically disorganized with diffuse and chaotic reflectors, or partly stratified (Fig. 7). The top Early Cretaceous horizon is represented by high-amplitude reflectors defining the upper boundary of chaotic seismic facies. The upper unit is acoustically more reflective and better organized than the lower unit (Fig. 7). Internal reflectors represent subparallel and divergent configuration, forming wedge-shaped strata. Seismic time-thickness maps show the distribution and thickness of the lower and upper units (Fig. 8). The lower unit occurs mainly in the fault block domain, with thickness variation across the fault blocks. However, minor change in the thickness of the lower unit occurs in the half-graben and interbasinal ridge. The upper unit was mainly deposited in the half-graben during the Late Cretaceous. At the same time, topo- Fig. 5. Throw variations along the Kachi fault. Note that the throw is measured at the acoustic basement level (TopBST). Positive values on the vertical axis indicate normal graphic relief of the interbasinal ridge was probably more pro- slip. nounced and the fault block domain continued to subside. 484 Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491

Fig. 7. Representative seismic sections showing the different structural styles in the NW segment (A and B) and the SE segment (C and D) of the inverted half-graben. For location of the seismic section, see Fig. 4.

The latest CretaceouseEocene synrift II unit is well recognized Kachi fault (Fig. 7). Internal reﬂectors exhibit subparallel or parallel in the fault block domain as unconformity-bounded wedges or conﬁguration with onlap termination against the steeper lower sheet, whereas it is partially preserved in the hanging-wall of the boundary and erosional truncation against the overlying Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 485

Fig. 8. Seismic time-thickness maps of (A) lower synrift I unit (Early Cretaceous) and (B) upper synrift I unit (Late Cretaceous). Note that the location and orientation of anticlines are related to the Oligocene inversion. unconformity. The upper boundary is marked by the Oligocene Oligocene unconformity (horizon TopE). The upper unit is variable unconformity (TopE horizon) underlying the Early Miocene syn- in reflection continuity, frequency, and configuration. It contains inversion I unit. The synrift II unit is significantly eroded across the stratal packages increasing in thickness toward the Kachi fault inversion high adjacent to the Kachi fault (Fig. 7). confining the half-graben. Seismic time-thickness maps show that The lower synrift I unit consists of unsorted, arkosic sandstones the lower and upper units have different thickness variations and tuff or tuffaceous deposits, showing a wide range of ages from (Fig. 9). The lower unit appears to be thick in the SE frontal area of the Triassic to Early Cretaceous. Ryu and Kim (2007) suggested that the inverted half-graben but thin and relatively uniform in the NW the volcanic flows in the top of the lower unit may be correlated frontal area. The thickness variation of the upper unit indicates that with the Jurassic Xihengshan Formation in the Subei Basin. Inter- the subsidence occurred over the entire frontal area and was mixed occurrence of volcaniclastic and volcanic strata suggest enhanced particularly in the NW frontal area. incipient rifting-associated volcanism in the study area. The over- The onlap stratal pattern, pinch-out geometry, and thickness lying upper synrift I unit, seismically more reflective and contin- variation of the syninversion I unit indicate that the accumulation uous, consist of interstratified fluvial and lacustrine strata with of the lower Miocene growth strata was accompanied by tectonic upward-fining cycles (Ryu and Kim, 2007). The upper unit, inversion of the pre-existing half-graben. The thickness increase of deposited mainly during the Late Cretaceous, is characterized by the syninversion I unit toward the Kachi fault primarily resulted intercalation of dolomite and dark gray mudstone, suggesting a from subsidence controlled by the loading and flexural bending transition from fluvial-dominant to lacustrine-dominant associated with inversion and uplift of the half-graben. Addition- environments. ally, some newly-formed and reactivated extensional faults The distribution and thickness variation of the synrift I unit contributed to the subsidence. suggest that a series of fault-controlled deponcenters formed in the The temporal and spatial variations in the thickness of two fault block domain during the Early Cretaceous (deposition of the syninversion I units suggest that the initiation of the half-graben lower synrift I unit). Major transition of the depocenter from the inversion was diachronous along the inverted Kachi fault. The fault block domain to the half-graben occurred during the Late initial inversion occurred during the Oligocene and possibly Cretaceous (deposition of the upper synrift I unit), indicating that resulted in the internal deformation of the synrift unit and the the half-graben was initiated in the Early Cretaceous and mainly diachronous deposition of the lower syninversion I unit. The subsided in the Late Cretaceous. The bounding fault, the Kachi fault diachronous nature of the inversion most likely affected lateral between the half-graben and interbasinal ridge, played a major role thickness variation of the lower syninversion I unit (i.e., a slight in the development of the half-graben. Important fault control on thickness increase toward the Kachi fault in the SE frontal area and the synrift sedimentation is also revealed by the Upper Cretaceous less thickness variation in the NW frontal area). The following fanning stratal pattern toward the Kachi fault. inversion predominantly occurred along the Kachi bounding fault, causing the inversion and uplift of the half-graben. The upper 3.3.2. Syninversion unit syninversion I unit formed simultaneously in the frontal area of the The Early Miocene syninversion I unit overlies an angular un- inverted half-graben. conformity and pinches out against the inverted and uplifted area The Middle Miocene to Pliocene syninversion II unit is charac- where large amounts of synrift and syninversion strata were terized by slightly continuous reflectors parallel to the unconfor- eroded (Fig. 7). The syninversion I unit is subdivided by the intra mity (Fig. 7). The Middle Miocene unconformity is almost flat-lying Early Miocene horizon (IntraEM horizon) into the lower and upper and gently folded above the upper tip of the buried Kachi fault units on the basis of the thickness variation and stratal pattern. The (Fig. 7). This localized and mild folding most likely reflects renewed lower unit contains internal reflectors parallel or subparallel to the reactivation of the buried Kachi fault. The thickness increase of the 486 Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491

Fig. 9. Seismic time-thickness maps of (A) lower syninversion I unit, (B) upper syninversion I unit, and (C) syninversion II unit.

syninversion II unit toward the Kachi fault indicates the develop- graben bounded by the Kachi fault (Fig. 7). The inversion struc- ment of growth strata. Regional subsidence was likely enhanced by tures are represented by anticlineesyncline pairs affecting the the loading and flexural bending related to the inversion uplift in overall synrift units. The folded synrift strata and the overlying addition to general thermal contraction and burial compaction. syninversion strata were subsequently shortened and deformed by uplifting and folding along the Kachi fault. Consequently, two 3.3.3. Postinversion unit different phases of contractional events were superimposed in a The gently folded reflectors are truncated at a shallow depth by complex manner. Based on the variations in the geometry of folds an erosional unconformity parallel to the seafloor (horizon PostM and reactivated faults and their stratigraphic level, two different in Fig. 7). The unconformity is overlain by packages of parallel and phases of inversion-related deformation are identified: the continuous reflectors up to the seafloor. The reflection packages Oligocene and Early Miocene inversion phases. may be representative of marine sedimentation during the post- The structural style of the first (Oligocene) inversion deforma- inversion period. The postinversion marine strata seem to have tion is characterized by reactivated pre-existing normal faults and been deposited during the Pliocene time. related folds, newly-formed reverse and normal faults, and fold- accommodation faults. The anticlines show significant variations 3.4. Inversion structures in size, approximately ranging in wavelengths from 650 to 6000 m and in amplitude up to 500 ms in TWT (Fig. 7). The cross sectional Inversion-related deformation is well recognized in the half- geometries of the anticlines generally exhibit symmetrical shape Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 487 with an axial plane approximately parallel to the reactivated pre- greatest near the Kachi-1 well and slightly decreases towards both existing normal faults. The anticlines are seen to have been the southeast and the northwest. The wavelength of the anticline dissected by reverse and normal faults on crests and limbs and such gradually decreases toward the southeast, as the former half- secondary faults are linked at depth to the reactivated normal fault graben becomes narrow and shallow. Although syninversion or its antithetic fault (Fig. 7). growth patterns are not clearly observed under the present The anticlines are interpreted as fault-related folds developed seismic resolution, lateral thickness variation across the monocline by the reverse reactivation of pre-existing normal faults. According indicates the syninversion deposition (Fig. 9C). The crest of this fold to the different dip angle of the bounding fault, two types of fault- is truncated by a flat-lying unconformity at a shallow depth, which related folds are differentiated. A type 1 fold represents anti- approximately constrains the end of monoclonal growth deposition clineesyncline pairs bounded by a conjugate fault system consist- to post-Miocene time. ing of a low-angle major reactivated fault and its antithetic faults (Fig. 7A, B). The anticlines occur in a relatively shallow part of the 3.5. Restoration of inverted half-graben hanging-wall strata where secondary faults branch upward from the low-angle bounding fault or its antithetic normal fault. The The first step of structural restoration is to construct a geological antithetic faults are either newly-formed reverse faults or partially- cross-section from an interpreted time section by using an appro- reactivated normal faults, which serve to nucleate a zone of priate timeedepth relation. Depth conversion is performed using inversion anticline. This antithetic fault system is necessary to interval velocities for each seismic unit, assuming that the interval accommodate internal deformation during the reverse reactivation velocities are constant with the depth for each unit. A SWeNE of the low-angle border fault. Type 1 folds are not always involved trending seismic section (section C in Fig. 7) is selected for the in basement structures, suggesting possible detachment folds. structural restoration. Restoration and balancing methods are valid Some secondary faults are rarely involved in basement structures, for the section parallel to the direction of tectonic transport, i.e., the i.e., decoupled from the basement. They are interpreted as ‘fold- principal plane of extension or contraction. A SWeNE direction of accommodation faults’ that accommodate strain discontinuities extension is deduced in the western Gunsan Basin from the related to structural and stratigraphic position during fold evolu- orientation of basement structures. The direction of the restored tion (e.g., Mitra, 2002). section is nearly perpendicular to this structural orientation. A A type 2 fold represents narrow anticlines associated with similar direction of contractional transport is also deduced from the reverse reactivation of a high-angle bounding fault (Fig. 7C, D). The inversion structures. Thus, it is assumed that no material moved high-angle bounding fault at the SE segment of the half-graben was into or out of the plane of the cross section during the restoration. reactivated to form narrow inversion anticlines and related reverse The depth-converted section is restored sequentially back faults with small throws. Similar anticlines also occur along the through time using the 2DMove software. A conventional proce- Kachi fault linked with reverse antithetic faults, with relatively dure is that the top sedimentary layer for each stage is first large crests and slightly inclined axial planes. Type 2 folds are removed and the underlying units are decompacted to correct for restricted to a narrow zone adjacent to the steep reactivated the compaction caused by the overlying sedimentary layer. bounding fault and are involved with the basement structure. Type Compaction and decompaction of sedimentary units cause a 1 and 2 folds have axial planes that are parallel or subparallel to the change in fault geometry and unit thickness. In the region where adjacent inverted faults (Fig. 8B). lithology and thickness of sedimentary units are apparently vari- The second (Early Miocene) inversion event is revealed by the able and the present depth may not reflect the actual compaction folded Oligocene unconformity (TopE horizon) parallel to the due to significant uplift and erosion, estimates of compaction can overlying syninversion I strata (Fig. 7A). These folds have gentle be erratic. There is little geologic data in the study area to estimate anticlines up to 3500 m in wavelength and are not laterally trace- the lithology of the sequence as well as the amount and timing of able. The wavelength and amplitude of the gentle anticlines are the uplift and erosion marked by the two regional unconformities, clearly different from those of the former deformed synrift units, so the effect of sedimentary compaction by overburden is not suggesting that the Early Miocene folding was a later (second) considered in this restoration process. event. The gently folded Oligocene unconformity is progressively After the top sedimentary layer is stripped off at a given stage, elevated northeastward and truncated near the Kachi fault by the the section is restored to a paleobathymetric datum by removing Middle Miocene unconformity. The projected geometry of the the fault offset and unfolding using an appropriate algorithm, then Oligocene unconformity seems to present an asymmetric fold with generating a restored horizon and structure. In this study, the a steep forelimb dipping to the Kachi fault and a gentle backlimb section cannot be restored to a paleobathymetric datum because dipping to the southwest (Fig. 7). The axial trace of the fold may the geological data is insufficient to determine precise paleo- trend parallel to the inverted Kachi fault. The projected asymmetric bathymetric profiles to be constructed along the length of the geometry is probably more pronounced along the SE segment of section. Instead, at each stage of restoration the top sedimentary the Kachi bounding fault. layer is removed and the underlying layers are restored to an The asymmetric fold with a steep forelimb toward the Kachi arbitrary horizontal datum. Two algorithms are mainly applied to fault is interpreted as a fault-propagation fold; it formed as the restore geological structures: (1) a flexural-slip algorithm to restore Kachi fault was reactivated as a new reverse fault propagating folds formed during the periods of basin inversion; (2) an inclined- upward through the syninversion strata. During the development shear algorithm to restore extensional movement and hanging- of the inversion anticline, progressive uplifting and rotation of the wall deformation in areas of normal faulting. Fault geometries at hanging-wall produced an outward-fanning pattern of syn- each restoration stage are simplified to avoid unnecessary com- inversion growth strata in the backlimb region. The growth strata plications during the restoration. Although many assumptions are also accumulated in a foreland depression with a NWeSE trend applied to this restoration, the sequential restored sections will along the Kachi fault (Fig. 9B). visualize stratal geometries before and during two different in- The Kachi fault continued to be reactivated with mild fault versions and allow the estimation of amounts of extension and movement during the Middle Miocene to Pliocene, which caused contraction across the inverted half-graben structure. the growth of a monocline with a relatively steep forelimb along ASWeNE trending depth-converted cross section is sequen- the buried Kachi fault (Fig. 7). The amplitude of the monocline is tially restored to stratigraphic horizons (Fig. 10). After removal of 488 Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 the uppermost unit, a gentle fold was restored with respect to the strata progressively onlap to the backlimb of the asymmetric fold post-Miocene horizon and then was unfolded to illustrate the flat- and pinch-out to the crest (Fig. 10D). It is also estimated that the lying Middle Miocene unconformity (Fig. 10C). The Middle Miocene eroded lower Early Miocene unit has relatively uniform thickness horizon is a regional unconformity that truncated the crest of an and originally extends across the entire length of the section asymmetric fold. The upper Early Miocene unit is interpreted as (Fig. 10E). growth strata associated with the development of the asymmetric The formation of the asymmetric fold probably occurred after fold. Based on this interpretation, it is assumed that the growth deposition of the lower Early Miocene unit which formed prior to

Fig. 10. Sequential restoration of a depth-converted balanced cross section through the inverted half graben in the western Gunsan Basin. The total amount of shortening is approximately less than 5%. For location of the restored section (line C), see Fig. 4. Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 489 the reactivation of the Kachi fault. The Kachi fault propagated during the deposition of the upper Early Miocene unit, leading to the formation of the asymmetric fold and the related growth strata in the half-graben. In this restoration, reverse displacement along the Kachi fault is estimated to be 1100 m, with a throw of more than 600 m. To consider the Oligocene inversion, the top Eocene horizon is projected across the inversion structure (Fig. 10F). The eroded latest CretaceouseEocene unit is restored assuming that the unit thickens toward the Kachi fault as the underlying unit does (Fig. 10F). The restored Oligocene section shows strong differential uplifts in west-east direction, with the inverted half-graben structurally higher than the eastern fault blocks (Fig. 10F). The inversion deformation was concentrated in the pre-existing half- graben, indicating that the contractional strain was largely accommodated by the reactivation of the pre-existing half-graben structure. The restored Eocene section shows how the major reverse faults and related folds are restored (Fig. 10G). The southwest-dipping Kachi fault was an active growth fault with a remarkable increase in fault displacement during the Late CretaceouseEocene. The restoration to the top Early Cretaceous horizon shows that the Kachi fault was relatively insignificant in basin subsidence during the Late JurassiceEarly Cretaceous (Fig. 10H). The wedge-shaped Fig. 11. Schematic map illustrating the distribution and orientation of major basement geometry and stratal pattern thickening to the normal faults are faults and inversion anticlines with different reactivation styles. characteristics of the upper synrift I and synrift II units, whereas this stratal geometry is much less pronounced in the underlying lower synrift I unit. These observations suggest that the synrift extension most likely culminated during the depositional time of 4.2. Main factors controlling fault reactivation the upper synrift I and synrift II units. The total shortening, measured by line-length analysis, is The structural style and distribution of the inversion deforma- calculated to be less than 5%. The magnitude of inversion was tion were primarily controlled by the reactivation style of the relatively mild in the NW segment where the normal faults appear bounding faults. In general, fault reactivation is a selective process to be in net-extension, whereas it is relatively moderate in the SE being not all faults equally susceptible to inversion (Bonini et al., segment where the initial normal movement appears to be 2012). The main factors governing the preferential reactivation of removed partially by reverse reactivation (see sections in Fig. 7). normal faults include pre-inversion fault steepness, orientation of the fault with respect to the inversion stress, and frictional resis- 4. Discussion tance along the fault plane (Bonini et al., 2012). In the study area, the different reactivation styles of the half-graben bounding faults 4.1. Control of pre-inversion fault steepness on internal deformation were developed both in different times and along different fault segments. Based on the geometry of the bounding faults, the half-graben is In the NW segment of the half-graben, the low-angle south- separated into two structural segments: NW and SE segments bounding faults were mostly reactivated during the Oligocene (Fig. 11). The NW segment of the half-graben is bounded by the inversion phase (Fig. 11). The low-angle south-bounding faults are NWeSE trending, low-angle normal fault at the south and the not defined as a single fault but a set of closely spaced parallel faults relatively steep Kachi fault at the north (Fig. 7). The SE segment of that are merged at a deep level. The downward-merging nature of the half-graben is bounded by relatively steep, reactivated faults such faults may reduce frictional resistance along the fault plane along both the south and the north boundaries (Fig. 7). The south- and enhance the reactivation tendency in addition to the low-angle bounding faults are segmented with an en-echelon pattern. nature of the fault plane. During the Early Miocene inversion phase, The south-bounding faults in the NW segment are planar or most of the pre-existing faults including the Kachi fault were rarely gently curved with a low dip angle of the fault plane. Usually, the involved in internal deformation of the NW segment (Fig. 11). low-angle bounding faults are associated with several antithetic Instead of the Kachi fault, pre-inversion normal faults in the faults that are either reactivated normal faults or syninversion interbasinal ridge were reactivated with mild fault movement. The faults. The area between the low-angle bounding fault and the orientation of these inverted faults with respect to the contrac- antithetic fault defines a large bulge with internal folding and tional stress may be more susceptible to reactivation than that of reverse faulting (Fig. 12). On the other hand, the south-bounding the Kachi fault in the NW segment. Such selective reactivation of faults in the SE segment are steep, planar faults and are locally normal faults in the interbasinal ridge may be attributed to clock- associated with reverse antithetic splay faults. The area between wise change in inversion stress direction normal to the NWeSE the steep bounding fault and the antithetic fault defines a positive, orientation of the inverted faults during the Early Miocene inver- flower-like structure. The positive structure generally shows the sion phase (Fig. 11). anticline with short wavelength and high amplitude compared In the SE segment of the half-graben, basement-cored and with the anticline associated with the low-angle bounding fault narrow folds were developed along the inverted Kachi fault. It is not (Fig. 12). This observation indicates that the dip angle of pre- clear whether these folds were initiated by the Oligocene or the inversion bounding faults controls the geometry and distribution Early Miocene inversion phase due to the absence of the syn- of inversion anticlines. inversion strata. However, a slight thickness increase of the lower 490 Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491

Fig. 12. Schematic diagrams illustrating different styles of inversion anticlines. Note that the distribution and geometry of inversion anticlines are largely controlled by the dip angle of the bounding fault. syninversion I unit toward the Kachi fault in the SE segment identiﬁed in the half-graben. The pre-inversion bounding normal possibly indicates that the reactivation of the Kachi fault initially faults in the half-graben were typically reactivated during the occurred during the Oligocene inversion phase. In this regard, the Oligocene inversion, resulting in the formation of several fault- development of the Kachi fault-related folds is interpreted to have related folds in a wide area. In contrast, the Early Miocene inver- been initiated during the Oligocene inversion phase and then to sion was characterized by the selective reactivation of the Kachi have proceeded during the Early Miocene inversion phase. fault, forming of an asymmetric fold and uplifting along the Kachi Different from the continuous reactivation of the Kachi fault, the fault. The Kachi fault continued to be reactivated during the Middle south-bounding faults were reactivated mainly in the Oligocene Miocene, with only mild fault movements. The following key inversion phase (Fig. 11). This interpretation further suggests that geological controls on the half-graben inversion are interpreted: the NWeSE oriented fault segment of the Kachi fault was selec- tively reactivated due to its orientation normal to the Early Miocene inversion stress.

4.3. The amount of shortening

The reverse reactivation of the half-graben bounding fault is a typical process of inversion deformation commonly accompanied by newly-formed shortcut thrusts in footwall and/or backthrust in the hanging-wall (Eisenstadt and Withjack, 1995; McClay, 1995; Yamada and McClay, 2004). The inversion-related thrusts, however, are not common in the inverted half-graben of the study area. The development of thrusts is likely unfavorable due to the small amount of horizontal shortening (Eisenstadt and Withjack, 1995) and the planar and steep geometry of precursor border fault bounding relatively rigid footwall (Sibson, 1995). During the Oligocene inversion phase, the amount of shortening estimated by structural restoration are an order of magnitude smaller than typically suggested by analog studies of inversion structures. A small amount of shortening most likely accounts for the lack of backthrust or shortcut thrusting in the study area. In addition, the steep bounding fault with rigid basement in the footwall, probably acting as a mechanical barrier, is unfavorable for the horizontal shortening by thrusting (Glen et al., 2005). The high- angle Kachi fault and the relatively rigid basement most likely produced ‘buttress structures’ in the synrift strata during the overall inversion period (Fig. 13).

5. Conclusion

The geological structures in the western Gunsan Basin are delineated on the basis of the detailed mapping and geological interpretation of seismic reﬂection data calibrated by well data. The half-graben in the western Gunsan Basin is bounded by the Kachi fault, a connected system of NW- and WNW-trending fault seg- Fig. 13. Conceptual models showing hanging-wall deformation of the half-graben during different inversion phases. (A) Folding and internal deformation accommoda- ments, and its conjugate faults, a series of WNWeESE oriented ting the Oligocene contraction. (B) An asymmetric hanging-wall anticline caused by faults at the opposite margin. Based on the structural style and reverse reactivation and upward propagation of the Kachi fault during the Early timing of inversion, two major phases of inversion deformation are Miocene. Y.J. Shinn / Marine and Petroleum Geology 68 (2015) 480e491 491

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