Permo-Triassic high-pressure metamorphism in the central western Korean Peninsula, and its link to Paleo-Tethyan Ocean closure: Key issues revisited

Permo-Triassic high-pressure(HP) mafic granulites, together with the Bibong retrogressed eclogite,preserved along the central western Korean Peninsula provide important insights into the Late Permian to Triassic collisional orogeny in northeast Asia. The metamorphic pressureetemperatureetime(P-T-t)paths of these rocks, however, remain poorly constrained and even overestimated, owing to outdated geothermobarometers and inaccurate isopleth techniques. Here we evaluate the metamorphic Pe T conditions of Triassic HP mafic granulites including those in Baekdong, Sinri and Daepan and the Bibong Triassic retrogressed eclogite in the Hongseong area, and the Permo-Triassic Samgot mafic granulite in the Imjingang Belt of the central western Korean Peninsula through the application of modern phase equilibria techniques. The Baekdong and Samgot mafic granulites and the Bibong retrogressed eclogite yield a range of 12.0 -16.0 kbar and 800 -900℃, representing HP granulite facies conditions. The Sinri and Daepan granulites from the Hongseong area show relatively lower grade metamorphic conditions between HP granulite and normal granulite facies, and are characterized by sub-isothermal decompression during exhumation. The similarities in the metamorphic ages and the post-collisional igneous activity from the central western Korean Peninsula indicate that the Triassic ages represent the retrograde stage of the metamorphic Pe T paths. In contrast, the Late Permian metamorphic ages, which are older than protolith ages of the post-collisional igneous rocks, correspond to the possible prograde stage of metamorphism. The P-T-t paths presented in this paper, together with the metamorphic ages and post-orogenic igneous events reported from these areas suggest trace of the subduction, accretion and exhumation history, and indicate a tectonic linkage among the northeast Asian continents during the Paleo-Tethyan Ocean closure.

High Ba-Sr adakitic charnockite suite from the Nagercoil Block,southern India:Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane(SGT)in India and is mainly composed of charnockitic rocks and felsic gneisses(charnockite suite).In this study,we present petrologic,geochemical,zircon U-Pb,REE,and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block.Based on field investigations and petrologic studies,the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene.The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks,with enrichment in LREEs(light rare earth elements)and LILEs(large ion lithophile elements),and depletion in HREEs(heavy rare earth elements)and HFSEs(high field strength elements).Some of the charnockites and the leucogneiss show typical HSA(high silica adakite)characters,(high SiO_(2),Al_(2)O_(3),Ba-Sr,La/Yb,and Sr/Y).The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge.The high Ba-Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian.The magmas were underplated and subjected to fractional crystallization.Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims.Magmatic zircon grains from the charnockites show ages ranging from 1983±8.8 Ma to 2046±14 Ma,and the metamorphic domains show an age range of 502±14 Ma to 547±8.7 Ma.Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860±20 Ma and 575.6±8.8 Ma.Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma.The REE data of the zircon grains show LREE depletion and HREE enrichment,with the metamorphic grains showing more depletion in HREE.Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yieldedε_(Hf)(t)values from-1.17 to 0.46 with T_(DM)and T_(DM)~C and age peaks at 2392 Ma and 2638 Ma,suggesting Neoarchean to Paleoproterozoic juvenile sources.We suggest that the high Ba-Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent,and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic-Cambrian.Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

The role of mechanical stratigraphy in the lateral variations of thrust development along the central Alberta Foothills, Canada

Fold-thrust belts generally exhibit significant variations in structural styles such as differences in thrust geometries and frequencies in imbrication. A natural laboratory of this pattern is preserved in the central Alberta Foothills of the Canadian Rockies, where differences in thrust geometries are represented by the existence vs. non-existence of triangle zones. To seek the factors that make this difference in these regions in terms of structural geometry, stratigraphic thickness variations and mechanical stratigraphy of the sedimentary layers, structural interpretation is conducted based on admissible cross-sections and well log interpretations. In northern region, a backthrust is detached from an incompetent layer(viz.Nomad Unit of the Wapiabi Formation), which gets thinner from the Foothills to the Plains, indicating that it is developed where the shale layers are pinched out where triangle zone is developed. Backthrust is also developed in the southern region, where mechanical strengths of strata(viz. Bearpaw Formation)increase toward the foreland. In the central region, however, only forethrusts are developed along the weak continuous decollement layers(viz. Turner Valley and Brazeau formations), forming an imbricate fan without development of the triangle zone. Incompetent layers such as the top Wapiabi(Nomad),Brazeau(Bearpaw), Coalspur and Paskapoo formations are also pinched out laterally, forming fault glide horizons in different stratigraphic levels in each region. These results indicate that, along the transport direction, triangle zone is developed in relation to the stratigraphic pinch out of the Nomad Unit in the northern region, and is formed associated with the variations in strengths of the layers constituting the Bearpaw Formation in the southern region. It is notable that all the glide horizons are developed along incompetent layers. However, triangle zones are not developed in the areas of continuous stratigraphy of the Nomad Unit, which does not serve as a glide horizon in the central region. This suggests that factors such as stratigraphic thickness changes of incompetent layers and mechanical stratigraphy of the sedimentary layers play an important role in the development of lateral variations in thrust system evolution in terms of triangle zone vs. imbricate fan in the central Alberta Foothills.

Inversion of two-phase extensional basin systems during subduction of the Paleo-Pacific Plate in the SW Korean Peninsula:Implication for the Mesozoic “Laramide-style” orogeny along East Asian continental margin

During subduction, continental margins experience shortening along with inversion of extensional sedimentary basins. Here we explore a tectonic scenario for the inversion of two-phase extensional basin systems, where the Early-Middle Jurassic intra-arc volcano-sedimentary Oseosan Volcanic Complex was developed on top of the Late Triassic-Early Jurassic post-collisional sequences, namely the Chungnam Basin. The basin shortening was accommodated mostly by contractional faults and related folds. In the basement, regional high-angle reverse faults as well as low-angle thrusts accommodate the overall shortening, and are compatible with those preserved in the cover. This suggests that their spatial and temporal development is strongly dependent on the initial basin geometry and inherited structures.Changes in transport direction observed along the basement-sedimentary cover interface is a characteristic structural feature, reflecting sequential kinematic evolution during basin inversion. Propagation of basement faults also enhanced shortening of the overlying sedimentary cover sequences. We constrain timing of the Late Jurassic-Early Cretaceous(ca. 158-110 Ma) inversion from altered K-feldspar 40 Ar/39 Ar ages in stacked thrust sheets and K-Ar illite ages of fault gouges, along with previously reported geochronological data from the area. This "non-magmatic phase" of the Daebo Orogeny is contemporaneous with the timing of magmatic quiescence across the Korean Peninsula. We propose the role of flat/low-angle subduction of the Paleo-Pacific Plate for the development of the "Laramide-style" basement-involved orogenic event along East Asian continental margin.

Coastal landscape evolution tracked from late Holocene wave-cut benches in the Sinan Archipelago,Southwest Korea

Glacio-isostatic adjustment(GIA)and tectonic activity are important factors in the formation of marine terraces.Late Holocene wave-cut benches in the eastern part of the West Sea of Korea,also called the Yellow Sea,can be divided into two steps:531 cm above sea level(ASL)for the upper bench(T2)and 464-481 cm ASL for the lower bench(T1).Sediments on the benches a re classified into four units,and are interpreted to be beach deposits acco rding to gravel shape,textu re,and seaward inclination.The ove rlying sediment indicates that T2 was formed at approximately 530 cm ASL before 2900 yr BP,and T1 at approximately 460-480 cm ASL before 1520 yr BP.Late Holocene(4000-2000 yr BP)relative sea level(RSL)curves based on GIA models are inconsistent with the wave-cut bench elevations.Comparing T1 and T2 benches to the RSL curves of the West Sea,the upper and the lower benches were uplifted by approximately 5-8 m and 4-7 m,respectively.Although the area is several hundred kilometers away from plate boundaries,the high frequency of earthquakes in the West Sea may have induced the uplift of wave-cut benches during the last 2000 years.These indicate that the west coast of the Korean Peninsula(KP)should no longer be considered an area of subsidence,but be assigned to a regime of uplift during the late Holocene.

An alternative interpretation for the map expression of "abrupt" changes in lateral stratigraphic level near transverse zones in fold-thrust belts

The map expression of ＂abrupt＂ changes in lateral stratigraphic level of a thrust fault has been traditionally interpreted to be a result of the presence of （1） a lateral （or oblique） thrust-ramp, or （2） a frontal ramp with displacement gradient, and/or （3） a combination of these geometries. These geometries have been used to interpret the structures near transverse zones in fold-thrust belts （FTB）. This contribution outlines an alternative explanation that can result in the same map pattern by lateral variations in stratigraphy along the strike of a low angle thrust fault. We describe the natural example of the Leamington transverse zone, which marks the southern margin of the Pennsylvanian-Permian Oquirrh basin with genetically related lateral stratigraphic variations in the North American Sevier FTB. Thus, the observed map pattern at this zone is closely related to lateral stratigraphic variations along the strike of a horizontal fault. Even though the present-day erosional level shows the map pattern that could be interpreted as a lateral ramp, the observed structures along the Leamington zone most likely share the effects of the presence of a lateral （or oblique） ramp, lateral stratigraphic variations along the fault trace, and the displacement gradient.

Paleozoic tectonic evolution of the proto-Korean Peninsula along the East Asian continental margin from detrital zircon U-Pb geochronology and Hf isotope geochemistry

Detrital zircon geochronology and Hf isotope analysis can be used for inferring provenance characteristics,and to evaluate the tectonic evolution of sedimentary basins and their link with regional orogenesis.The Paleozoic sequences of the Okcheon Belt consist of the Lower Paleozoic Joseon and the Upper Paleozoic Pyeongan supergroups with Middle Paleozoic hiatus locally on top of the Neoproterozoic bimodal volcanic rocks,reflecting an intracontinental rift setting between the two basements(viz.Gyeonggi and Yeongnam massifs)at southern part of the Korean Peninsula.Our detrital zircon U-Pb ages and Lu-Hf isotope results show that all these Paleozoic strata commonly have Paleoproterozoic and Paleozoic zircon ages with rare Meso-to Neoproterozoic ages.The individual zircon populations display following features,allowing estimation of their sedimentary provenances:(i)The Paleoproterozoic zircons(ca.1.85 Ga and 2.50 Ga)with similar ranges ofεHf(t)values are most common in the basement rocks of the Korean Peninsula,and were sourced from both the Gyeonggi and Yeongnam massifs.(ii)The Meso-to Neoproterozoic zircons,preserved only in the Middle to Late Cambrian clastic sedimentary rocks within the carbonate sequences probably reflect proximal provenance.(iii)The youngest Paleozoic zircons of each formation,almost coincident with their deposition ages,suggest presence of syndepositional magmatism,indicating proximal magmatic sources during their deposition.(iv)The Cambrian-Ordovician zircons,from the Lower Paleozoic sequences,but rare in the successive Upper Paleozoic sequences,suggest a provenance change after the hiatus between the two sedimentary successions.(v)The Permian zircons showing differentεHf(t)values indicate that detrital sources were varied at that time.The integrated results in our study suggest provenance variability linked to diverse tectonic environments,reflecting prolonged subduction-related crustal evolution of the proto-Korean Peninsula during the Paleozoic.