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Earthquake fault slip accompanied by surface ruptures may occur not only along main fault cores but also along subsidiary faults in damage zones of major (or mature) faults. Nevertheless, most previous studies of fault and earthquake geology have focused on geological observations of main core zones rather than subsidiary faults. We conducted microstructural and mineralogical analyses of fault rock materials from two subsidiary faults (F1 and F2) of the NNE-SSW-striking Yangsan Fault, which is a major strike-slip fault in southeastern Korea (F1 at Pohang Bogyeongsa and F2 at Ulsan Eonyang-Bangok), to understand their possible slip zone processes and slip behaviors. The fault cores of the subsidiary faults are up to 20 cm thick and are composed of clay-rich gouge bands measuring a few millimeters in thickness and enclosed fractured lenses. Microscopic observations reveal that linear, and narrow micro-scale principal slip zones (micro-PSZs; < 20 µm thick), which are characterized by strong preferred orientation of clay minerals, occur not only at the boundaries between the gouge band and adjacent fault rocks but also in the central part of the gouge band. Along the micro-PSZs, microstructures such as clasts truncated by rapid slip localization and gouge injections by thermal pressurization of wet gouge materials during rapid slip are observed. Thus, the structures together may indicate the occurrence of seismic slip on the subsidiary faults. Mineralogical analyses reveal that the total clay fractions (consisting mainly of illite, chlorite, and kaolin) of the gouge materials of F1 and F2 are 60.1 and 59.7 wt%, respectively. The gouge band of F2 is enriched with kaolin (59.7 wt%), which is regarded as a gouge material that can trigger dynamic weakening by dehydration-induced thermal pressurization during seismic slip. Therefore, these results imply that the kaolin-rich gouge band in F2 may be dynamically weakened when seismic reactivation occurs along F2. This study shows that a comprehensive investigation of slip behaviors of subsidiary faults as well as main fault cores is necessary to improve our understanding of the seismic faulting mechanisms of major tectonic fault zones.

Caldera fault zones, identified in many modern and ancient volcanoes, have been the subject of geological and geophysical observations and of analog and numerical modeling. However, the physicochemical processes in fault zones during a caldera collapse are still poorly understood. Here, we present field observations from a caldera fault zone in the Cretaceous Jangsan Caldera, SE Korea. The fault zone is ∼30 m wide and juxtaposes an intracaldera rhyolitic volcanic complex against older dacitic rocks; it consists of a minor fault, a main fault, and a series of fault-related intrusions (rhyolite and tuffisite). The main fault dips 90°–88°NW (vertically to steeply inward) and strikes N30°–40°E. A layer of pseudotachylyte (less than ∼10 cm thick) with some injection veins occurs along the main fault. The pseudotachylyte contains rounded to sub-rounded clasts, commonly displaying fuzzy or embayed boundaries, as well as euhedral feldspar microlites (

Quartz minerals in Earth’s crust are thought to melt at high temperatures. Laboratory friction experiments, however, show that metastable melting of quartz on a fault surface can occur at lower temperatures, and could lead to large earthquakes.

Graphite is a well-known solid lubricant and can be as important as clay minerals in reducing the frictional strength of faults. Some natural fault zones contain carbonaceous material (CM) even where host rocks do not contain it, and seismic fault motion can promote the graphitization of low-grade CM. Thus, the origin of CM in fault zones is an important issue in fault mechanics. Previous high-velocity friction experiments have revealed various chemical reactions in fault zones during seismic fault motion, but most experiments have been conducted in an atmosphere under oxic conditions. Here we report experimental results on Carrara marble (free of CM), conducted under N2 or H2 atmospheres at a slip rate of 1.3 m/s and normal stresses of 2.0–3.1 MPa. A small amount of blackish material formed in generated gouge only under reducing conditions with the H2 atmosphere, and Raman spectroscopic analysis revealed the presence of CM (amorphous carbon) in the material. The CM is attributable to (1) the generation and pyrolitic dissociation of CH4, and/or (2) a reduction reaction of emitted CO2 due to calcite decomposition. We confirmed the formation of CH4 using gas chromatography. The CM produced in experiments resembles CM in the Nojima fault (Japan) gouge in terms of Raman spectra. The granitic host rock of this fault is free of CM, and calcite is precipitated close to the CM; therefore, the CM probably formed through processes similar to those simulated in our experiments. Future research should investigate the amount and origin of CM in natural fault zones.

Clayey gouges are common in fault slip zones at shallow depths. Thus, the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths. We conducted rotary shear tests on clayey gouge (~41 wt % clay minerals) at a seismic slip rate of 1.3 m/s. Here we report that the gouge was melted at 5 MPa of normal stress and room humidity conditions. The initial local melting was followed by melt layer formation. Clay minerals (e.g., smectite and illite) and plagioclase were melted and quenched to glass with numerous vesicles. Both flash heating and bulk temperature increases appear to be responsible for the melting. This observation of clayey gouge melting is comparable to that of natural faults (e.g., Chelungpu fault, Taiwan). Due to heterogeneous fault zone properties (e.g., permeability), frictional melting may be one of the important processes in clayey slip zones at shallow depths.

The Permo–Triassic collision of the North and South China blocks caused the development of the Dabie–Sulu Orogen in China and Songrim Orogen in the Korean Peninsula. Extension after this collision is known from the Dabie–Sulu Orogen, but post-orogenic extension is not well defined in the Korean Peninsula. Extensional deformation along the southern boundary of the Gyeonggi Massif in Korea is characterized by top–down-to-the-south ductile shearing and subsequent brittle normal faulting, and was predated by regional metamorphism and north-vergent contractional deformation. Extension occurred between ~220 and 185 Ma based on the ages of pre-extensional regional metamorphism and post-extensional pluton emplacement. 40Ar/39Ar dating of syn-extensional muscovite in quartz–mica mylonite yields an age of 187.8 ± 5.6 (2σ) Ma, in agreement with constraints from structural relationships. Together with the extensional deformation identified along the northern boundary of the Gyeonggi Massif (~226 Ma), the extension along the southern boundary is probably related to the exhumation of the massif during late-orogenic or post-orogenic extension associated with the Songrim Orogeny of the Korean Peninsula and forms an important event in the Phanerozoic crustal evolution of East Asia.

[1] Slide-hold-slide tests were conducted on gray blackish gouge (GBG) and yellowish gouge (YG) from Pingxi fault zone to see how rapidly the strength of Longmenshan fault recovers after the 2008 Wenchuan earthquake. Twenty dry runs were made at a normal stress of 0.8 MPa, at a seismic slip rate of 1.4 m/s, and with hold time th ranging 0.3–105 s. Results exhibit very rapid healing by more than 0.4 in friction coefficient μ in less than 5–10 s, followed by gradual healing in proportion to log(th). Healing rates, (increase in μ)/(log (th)), during rapid and slow healing are 0.188 and 0.015 for GBG and 0.154 and 0.016 for YG, respectively. The average temperature in the outer half of a 5 µm thick slip zone decreases from 260–300°C to 110–170°C in 5–10 s, and hence, temperature drop appears to be correlated with the rapid healing. Previously reported rapid healing at subseismic slip rates (85–90 mm/s) begins to occur in 10–300 s after the stop of sliding, and this cannot be explained by the cooling of gouge. The difference in healing between subseismic slip rates (delayed and rapid healing) and seismic slip rates (immediate and rapid healing) suggests that the dominant weakening mechanism shifts from tribochemical processes at subseismic slip rates to frictional heating at seismic slip rates. Slip-zone structures are too complex and variable from run to run to reveal microscopic mechanisms for the strength recovery. Rapid healing following seismic slip can be a cause for reduced aftershocks along major coseismic faults.

The Songrim and Daebo orogenies represent two major Phanerozoic tectonic events that are well-preserved across much of the present-day Korean Peninsula. The Songrim orogeny corresponds to the Late Permian–Triassic collision of the North and South China cratons whereas the Jurassic Daebo tectonic event represents a thin-skinned contractional deformation in a continental arc setting. It is well-established that the Songrim orogeny left a strong imprint on the geological record preserved in the middle and northern parts of the Korean Peninsula (e.g., the Pyeongnam Basin and the Imjingang belt) with only a minor impact on the geology of southern Korea (e.g., the Okcheon and Taebaeksan basins). It was the Daebo tectonic event, however, which generated most of the deformational structures observed within the Okcheon and Taebaeksan basins. The Deokpori thrust in the Taebaeksan Basin is a significant fault structure that formed during the Daebo tectonic event; no other regional structures related to the Songrim orogeny have been found in the Taebaeksan Basin. In the vicinity of the Deokpori thrust, we have identified in this study a previously undocumented reverse-slip shear zone, which is named the Geounri shear zone. Microfabrics observed in phyllonite and marble mylonite samples suggest that this shear zone was developed in a plastic deformation regime at ∼400 °C, in contrast with the nearby Deokpori thrust, which formed in a brittle deformation regime. A geochronological analysis of muscovite isolated from phyllonite samples of the Geounri shear zone, yielded two 40 Ar/ 39 Ar age spectra with a combined weighted mean age of 209 ± 5 (2 σ ) Ma. Thermal modeling of the Ar data, combined with structural interpretations, suggests that this muscovite 40 Ar/ 39 Ar age represents a robust lower age limit for the timing of the Geounri shearing event, therefore linking formation of the shear zone with deformational events caused by the Songrim orogeny. These results imply that structural features caused by ‘Songrim deformation’ were developed at higher temperatures than structures caused by ‘Daebo deformation’ as found within the Taebaeksan Basin.

Nanoparticles are known to form in narrow slip zones in natural and experimental fault zones, but their possible role in dynamic weakening of faults during seismic slip remains almost unexplored. We conducted friction experiments on periclase (MgO) nanoparticles (50 nm in average size) at rates as high as 1.3 m s −1 , a typical speed of seismic slip. The nanoparticles were used as the initial gouge to avoid complexities arising from comminution and fluid release. The dynamic friction decreased with increasing slip rate and the steady-state frictional coefficient reduced to as low as 0.1 at a slip rate of 1.3 m s −1 . Flash heating is not effective for nanoparticles, and we propose that development of slickensides and dominant operation of nanoparticle rolling cause such a weakening. Nanoparticle lubrication appears to be as effective as melt lubrication and thermal pressurization, and the formation of nanoparticles in slip zones may be an important fault lubrication process.

Anomalously low heat flow around active faults has been a recurrent subject of debate over past decades. We present a series of high-velocity friction experiments on gypsum rock cylinders showing that the temperature of the simulated fault plane is efficiently buffered due to large-scale endothermic dehydration reaction. The tests were performed at 1 MPa normal stress and a velocity of 1.3 m s −1 , while measuring the temperature close to the sliding surface and the relative humidity around the sample. The temperature close to the sliding surface is remarkably stable at ∼100 °C during the dehydration reaction of gypsum. Microstructural and X-ray diffraction investigations show that dehydration occurs at the very beginning of the test, and progresses into the bulk as slip increases. In the hottest parts of the sample, anhydrite crystal growth is observed. The half-thickness of the dehydrated layer ranges from 160 μm at 2 m slip to 5 mm at 68 m slip. Thermodynamic estimates of the energy needed for the dehydration to occur yield values ranging from 10% to 50% of the total mechanical work input. The temperature plateau is thus well explained by the energy sink due to the dehydration reaction and the phase change from liquid water into steam. We suggest that similar endothermic reactions can efficiently buffer the temperature of fault zones during an earthquake. This is a way to explain the low heat flow around active faults and the apparent scarcity of frictional melts in nature.

The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two well-known orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3 ± 1.5 and 187.2 ± 1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic. Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages. The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergence developed in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks, but an “intracontinental” thrust that developed after their juxtaposition.

[1] The Deokpori Thrust, juxtaposing the early Paleozoic Yeongwol-type Joseon Supergroup (Yeongwol Group) against the Duwibong-type Joseon Supergroup (Taebaek Group) in the Okcheon belt, is an important regional structure in the Phanerozoic tectonic evolution of Korea. Although the initiation of the thrust has recently been conjectured to occur during the Middle Jurassic Daebo tectonic event in a continental magmatic arc setting, its movement history has never been studied. We report timing of brittle reactivation of the thrust obtained from K-Ar dating of fault gouges in the core of the Deokpori Thrust zone. K-Ar illite age data of fault gouges suggest reactivation of the thrust at ca. 72 Ma after a contact metamorphism around 94 Ma. Our result is the first documentation of the reactivation of a major Jurassic ‘Daebo’ structure and the reactivation may represent a significant contractional tectonic event during the Late Cretaceous in the Korean Peninsula.

Pseudotachylyte formed by frictional melting has been the only unequivocal evidence of past seismogenic fault slip. We report from high-velocity friction experiments on siderite-bearing gouge that mineral decomposition due to frictional heating also can leave evidence of paleoseismic events along shallow crustal faults other than pseudotachylyte. Experiments were conducted room dry on simulated gouge composed of siderite or mixture of siderite, calcite, and quartz, initially at room temperature, under normal stresses of 0.6–1.3 MPa and at seismic slip rates of 1.3–2.0 m/s. In all cases, gouge exhibited dramatic slip weakening and siderite was decomposed into nanocrystalline magnetite and CO 2 gas, as confirmed by CO 2 measurement, X-ray diffraction analyses, and transmission electron microscopy. The weakening was caused by the low frictional strength of ultrafine decomposition products at seismic slip rates. Magnetite formation during shearing changed gouge color to black and increased magnetic susceptibility by a few orders of magnitude. Those changes can be recognized in natural fault zones, and black gouge in the Chelungpu fault zone in Taiwan is perhaps such an example. Thus our results suggest that thermal decomposition in shallow crustal faults can be an important co-seismic process not only for dynamic fault weakening, but also for leaving seismic slip records.