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Tectonics is broadly accepted as one of the main factors controlling long-term landscape evolution. The impact of tectonics on short timescales is most often observed through earthquake rupturings that produce localized, metric-scale deformations. Although these deformations significantly affect the landscape, it remains challenging to precisely correlate major landscape changes with these localized earthquake deformations. Therefore, linking instantaneous deformation to long-term morphological changes often involves a thought experiment with potentially limited temporal resolution. At a paleoseismological site along the slow-moving strike-slip Yangsan Fault in South Korea, we employed optically stimulated luminescence (OSL) dating and detrital zircon analysis on all exposed unconsolidated layers in trench walls. The results reveal a significant provenance shift in the sediments accumulating in the trench, indicating a major reorganization of the drainage network. Based on our OSL and detrital zircon data, we estimate that this change occurred around 70 ka. We propose that this drastic drainage reorganization was caused by a combination of very slow, yet continuous, earthquake activity and a temporary reduction in river erosion during the onset of the cold, dry spell characteristic of the MIS4 stage across the Korean Peninsula.
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Dating drainage reversal using
mineral provenance along the
Yangsan Fault, South Korea
Tae-Ho Lee1, Jin-Hyuck Choi1,2, Youngbeom Cheon1, Shinae Lee3 & Yann Klinger4
Tectonics is broadly accepted as one of the main factors controlling long-term landscape evolution.
The impact of tectonics on short timescales is most often observed through earthquake rupturings that
produce localized, metric-scale deformations. Although these deformations signicantly aect the
landscape, it remains challenging to precisely correlate major landscape changes with these localized
earthquake deformations. Therefore, linking instantaneous deformation to long-term morphological
changes often involves a thought experiment with potentially limited temporal resolution. At
a paleoseismological site along the slow-moving strike-slip Yangsan Fault in South Korea, we
employed optically stimulated luminescence (OSL) dating and detrital zircon analysis on all exposed
unconsolidated layers in trench walls. The results reveal a signicant provenance shift in the sediments
accumulating in the trench, indicating a major reorganization of the drainage network. Based on our
OSL and detrital zircon data, we estimate that this change occurred around 70 ka. We propose that this
drastic drainage reorganization was caused by a combination of very slow, yet continuous, earthquake
activity and a temporary reduction in river erosion during the onset of the cold, dry spell characteristic
of the MIS4 stage across the Korean Peninsula.
Landform changes tell us history of interaction between tectonic processes, mostly orogenic processes, and
surcial geologic processes, such as erosion and sedimentation, over long periods of time. One of the main
tectonic processes participating to landform changes is large continental earthquake-induced faulting. ese
events can oset vertically and/or horizontally landforms by several meters, over several tens to hundreds
of kilometers13. Repetition of such earthquakes through successive earthquake cycles leads to cumulative
deformation that eventually participates in reshaping the landscape.
Along major active fault systems corresponding to dierent tectonic plate boundaries, the deformation rate
can vary between a few cm/yr and a few mm/yr; Accordingly, the return time for large earthquakes along such
fault systems typically does not exceed a few thousands of years at maximum. Hence, it is usually possible to
document several earthquake cycles using classical paleoseismological approaches, relying most oen on 14C
dating techniques to constrain the chronological framework. In the case of slow deforming areas, however, rate
of deformation is classically lower than 1mm/yr and the recurrence interval for signicant earthquakes can
be as long as a few tens of thousands of years47, beyond the duration of the Holocene. us, to decipher the
detail of the earthquake cycle and the impact of successive earthquakes on the landscape, it may be necessary to
investigate a longer temporal window, possibly beyond the range of the 14C dating. In such cases other dating
methods with wider age ranges, such as OSL and cosmogenic nuclides (e.g., 10Be, 26Al), should be used711.
In addition to limitations introduced by dating techniques, when considering earthquake deformation and
landscape evolution over long time windows, it also becomes crucial to properly account for changes in climatic
conditions, which directly impact the sediment supply and erosion capacities of uvial and aeolian systems.
e Korean Peninsula is considered as a slow-deforming region located far away from any major plate
boundary. No destructive earthquake involving surface ruptures has been documented in the peninsula since
the beginning of seismological observations, nor during the 2,000 years period covered by historical archives. To
date, the 2016 ML 5.8 Gyeongju earthquake is the largest earthquake instrumentally recorded in South Korea12.
Another earthquake, resulting in about 100 casualties13, occurred in CE779 in the same area, although the exact
location remains unknown. us, geomorphic and stratigraphic records of surface ruptures associated with
prehistoric earthquakes are the only existing evidence that supports the fact that large earthquakes may have
1Geologic Hazards Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, South Korea.
2Geological Science, Korea University of Science and Technology, Daejeon 34113, Republic of Korea. 3Division
of Scientic Instrumentation and Management, Basic Science Institute Ochang Center, Cheongju 28119,
Republic of Korea. 4Université de Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France. email:
cjh9521@kigam.re.kr
OPEN
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occurred in the Korean Peninsula. Such evidence is oen limited or ambiguous, as local climatic conditions
associated with rugged topography are prone to signicant erosion/deposition processes that tend to erase
any remain of past ruptures. In recent times, anthropogenic activities have also signicantly contributed to
the reshaping of landforms, further complicating the documentation of geomorphic evidence of fault activity.
Recently, through a combination of a systematic airborne LiDAR (Light Detection and Ranging) surveys
targeting potential active structures and paleoseismological trenching, there has been progress in documenting
geomorphic eects of prehistoric earthquakes, particularly in the southeastern part of the Korean Peninsula5,14,15.
In most cases, however, the available information has been limited to determining the timing of the most recent
earthquakes and assessing the visible vertical oset in the excavation walls.
Zircon (ZrSiO4) is an ubiquitous accessory mineral in many rocks. It is physically and chemically extremely
resistant and thus it is widely utilized in geochronological studies16,17. Nevertheless, since zircon age dating
relies on the U‒Pb isotope system, which involves a long half-life, only a few studies have used this technique
for paleogeographic reconstruction associated with recent active-tectonic systems, or to investigate landform
changes18,19. Here, we conduct both OSL and detrital zircon U‒Pb age dating for unconsolidated sediments
exposed in trench walls up to 8m deep in SE Korea, previously reported5, to investigate the detail of landform
changes history. OSL dating provides information on the depositional timing of unconsolidated sediments, and
detrital zircon U-Pb dating identies the sources of the sediments in the study area. Combining those results
with evidence for earthquake related deformations, it allows to bring forward the interplay between variation
of climatic conditions and cumulative tectonic deformation through successive earthquakes, that eventually
controls the evolution of the landscape.
Geological background
Seismotectonic and geologic settings
e Yangsan Fault is one of the major structures across South Korea. It is approximately 200km long on land,
and has a minimum nite right-lateral displacement of 21.3km over the last about 53 Ma20 (Fig.1a and b). e
Yangsan Fault crosses along the eastern part of the Gyeongsang Basin (Fig.1b), which is the largest terrestrial
Cretaceous basin in the Korean Peninsula21. e Gyeongsang Basin consists of three geologic groups that dier
in their relative contents of volcanic material: the lower Sindong Group (predominantly sedimentary rocks), the
middle Hayang Group (sedimentary and partially volcanic rocks), and the upper Yucheon Group (dominantly
volcanic and volcanoclastic rocks)21. e Sindong Group represents alluvial fan, uvial plain, and lacustrine
system depositional environments, and the maximum depositional ages of these sedimentary rocks reportedly
range from ca. 127 to 109 Ma22,23. e Hayang Group is composed mainly of uvial and lacustrine sedimentary
rocks, and their maximum depositional age inferred to be approximately 109 to 100Ma, based on detrital zircon
U‒Pb ages22. e Yucheon Group, which is the uppermost group of the Gyeongsang Basin, corresponds to an
intra-arc to back-arc tectonic setting. It results from intense volcanic activity during the Late Cretaceous, and is
mainly composed of rhyolitic to andesitic lava and tu21 (Fig.1b).
e Yangsan fault morphology is expressed as a fault zone valley. Consistently with the present stress eld,
which has a maximum horizontal stress in the ENE‒WSW direction24, the Yangsan Fault accommodates mostly
horizontal right-lateral motion with a subsidiary reverse slip component. Paleoseismological investigations
indicate that this fault was active during the Holocene and has records of experiencing earthquakes aer around
3 ka25,26 (Fig.1b). Additional potential events have been identied based on paleoseismological trenches studies
conducted along various segments of the fault27,28, although the precise chronology of these paleoearthquakes
remain uncertain due to limited availability of high-resolution age data at those specic sites. It is worth noting
that, the long recurrence interval of earthquakes in South Korea, compared to the rapid erosion/sedimentation
cycle associated with the monsoon climate, generally creates unfavorable conditions for recording earthquake
time series in trenches.
The Inbo paleoseismic site
e Southern Yangsan Fault (SYF) is generally characterized by a narrow (< 1km) fault zone valley. In this work
we targeted the Inbo site located along the northern part of the SYF (Fig.1b and c). At this site, the SYF delineates
the Hayang Group to the east from the Yucheon Group to the west. Due to variations in terrain elevation and
rugged topography in this region, rivers and streams typically originate from the western mountains and
predominantly ow eastward, oen following the north-south-oriented valley formed by the fault zone (Fig.1c).
In a paleoseismological trench at the Inbo site, Cheon et al.5 reported 2.5m of vertical oset in the clay-silt
layer of unit 3 (Fig.2). Cheon et al.5 pointed out that the upli of the eastern fault block by earthquakes resulted
in landform and drainage changes, as well as the deposition of the clay-silt layer of unit 3. In the following
paragraph, we summarize the stratigraphic observation and dating results reported by Cheon et al.5.
Based on four stratigraphic sections in two trench excavations, the unconsolidated sediments were classied
into four sedimentary units (units 4 to 1, base to top of the trenches) according to the sediment composition5
(Fig.2). e lowermost unit 4, which unconformably overlies Cretaceous sedimentary rock (the Hayang Group),
consists mainly of channel deposits containing boulder- to pebble-sized clasts. Unit 3 is composed of silt and clay
deposited in a lacustrine environment. is unit displays diverse so sediment deformation structures. Unit 2 is
a light brown sand layer containing cobbles to pebbles. e upper most unit, unit 1, is a medium to dark brown
layer with pebbles to granules. In the Inbo trenches, Cheon et al.5 could identify at least three fault splays that
cut unconsolidated units (Fig. S1). Few ages from OSL and 14C datings, are available to constrain the timing of
the past earthquakes. e most recent ruptures occurred aer 29 ± 1ka, while the earlier ruptures were between
ca. 70 and 29ka.
Here, we have dated thirteen additional OSL samples to bring age constraints for the entire sedimentary
stratigraphic section and further constrain the overall timing of fault activity. In addition, we processed 300
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grains for U‒Pb age dating of detrital zircons that were collected from units 1, 3, and 4, to study the sediment
provenance (Fig.3). Results and interpretation of these new data are presented in the following sections.
Materials and methods
OSL
To precisely determine the age of all unconsolidated units identied in the two Inbo trenches, we collected
thirteen OSL samples from the lowermost unit 4 to the uppermost unit 1 using metal tubes (30cm long and
5cm in diameter). e thirteen samples collected for OSL analysis are distributed as follows: one from unit 4
(AF-1), six from unit 3 (AH-1, AF-2, AF-3, AF-4, BH-1, and BH-2), three from unit 2 (AF-5, BF-1, and BF-2),
and three from unit 1 (AH-2, AF-6, and BF-3) (Fig.3). e samples were handled under subdued red light in the
luminescence laboratory at the Korea Institute of Geoscience and Mineral Resources (KIGAM). e sediment
at each end of the tubes that may have been exposed to light was removed and subjected to water content and
dose rate measurements. e remaining bulk samples were wet-sieved to obtain the (coarse-grained) 90–250μm
fraction of particles. e extracted fractions were then chemically treated with 10% HCl (3h) and 33% H2O2
(1h) to dissolve carbonates and any organic materials. Aer acid treatment, quartz grains were recovered by
heavy liquid separation using sodium polytungstate (SPT) with a density of 2.62g cm-3. Subsequently, separated
Fig. 1. Geological map of the study area. (a) Index map of East Asia. e position of the Gyeongsang Basin is
marked in green. (b) Geological map of the Gyeongsang Basin showing the Yangsan and Ulsan faults (modied
from Lee et al.22). (c) Geological map and LiDAR image of the study area (modied from Lee46). Figures were
drawn using Adobe Illustrator 2024 (https://www.adobe.com/).
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quartz concentrates were treated with 48% HF (1h) to eliminate remnant feldspar grains and the outer alpha-
irradiated rim on quartz grains29. e remaining quartz grains underwent additional treatment with 35% HCl
for 1h to remove any contaminating uorides. Fine-grained (4–11μm) polyminerals were also separated, using
the Stokes’ law method, to be compared with the OSL results of the coarse-grained (90–250μm fraction) quartz.
en, routine acid treatments, including 10% HCl (2 d), 33% H2O2 (1 d) and 40% H2SiF6 (14 d), were carried out
to recover the ne quartz fraction from the collected polyminerals30. In addition, further analysis was performed
aer separating ne quartz grains from three previously reported samples from the Inbo trenches5. e OSL
measurements were conducted using a Risø TL/OSL automatic reader (TL/OSLDA-20) at the KIGAM. is
reader is equipped with a 90Sr/90Y beta source delivering 0.096Gy s-1 (for coarse-grained quartz) and 0.091Gy
s-1 (for ne-grained quartz) to the sample position. Quartz OSL signals were obtained by stimulation with blue
LEDs (470 ± 20nm) and were detected with an EMI-9635Q photomultiplier tube through a 7.5mm-thick Hoya
glass lter. e separated coarse quartz grains were mounted on stainless steel sample discs with a diameter of
9.7mm using silicone oil, and ne quartz extracts were mounted on aluminum sample discs of the same size.
Quartz equivalent dose (De) values were analyzed using the single-aliquot regenerative (SAR) dose protocol31,32.
Before De analysis, feldspar contamination tests (IR depletion test), preheat plateau tests, and dose recovery
tests were carried out on both the coarse and ne quartz concentrates33,34. De evaluation was conducted using at
least 12 aliquots of each sample. e dose response curve was tted with an exponential function, and aliquots
were accepted when the recycling ratio and recuperation were less than 10% and 5%, respectively. When the
De value was greater than the 2D0 value (De > 2D0), more than 86.4% of the traps in the quartz lattice defects
were considered to be saturated33, and we calculated only the minimum depositional ages of unconsolidated
sediments. e De values of all samples were calculated using the central age model (CAM)35, and all uncertainties
are shown as 1σ standard errors. e dose rates for the samples were determined by a gamma-ray spectrometer
system (Broad Energy Germanium Detector, BE6530) installed at the KIGAM. e dose rate conversion factors
and cosmic ray contributions were adopted from references36,37, respectively. is study adopted a mean ɑ-value
for ne-grained quartz of 0.04 ± 0.0238 and a beta attenuation factor for coarse-grained quartz of 0.88 ± 0.04, as
suggested by Mejdahl39.
Fig. 2. Photographs of the southern faces of Inbo trenches A (a) and B (b), including OSL sample points
from Cheon et al.5 (yellow) and this study (white). e red lines represent the positions of representative
stratigraphic logs shown in Fig.3.
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Detrital zircon U-Pb dating
U–Pb age determination was conducted on detrital zircons from three samples collected in the two Inbo
trenches (Fig.3). Detrital zircons were extracted from ca. 5kg of unconsolidated sediments using ring mill,
dry sieving, panning, magnetic and heavy liquid (diiodomethane) separation techniques. Approximately 200
detrital zircon grains were randomly selected from each of the three samples, and they were prepared as epoxy
mounts, along with the FC-1 standard zircon40. To reveal the internal textures of the detrital zircons, we obtained
backscattered electron (BSE) and cathodoluminescence (CL) images with a scanning electron microscope
(JEOL JSM-6610LV) at the Korea Basic Science Institute (KBSI). e BSE and CL images were evaluated for
the occurrence of micromineral inclusions, inherited cores, metamorphic overgrowth zones and fractures to
determine the analytical points (Fig. S2). e U–Pb age determination of the detrital zircons was performed
with a sensitive high-resolution ion microprobe (SHRIMP) IIe/MC at the KBSI. For SHRIMP U–Pb dating, the
current of the O2- primary ion beam was approximately 4.5 nA, and the beam diameter was 25μm. Standard
zircons FC-1 (1,099.0 ± 0.6Ma) and SL13 (U of 238 ppm) were used for calibration of the age and uranium
concentration, respectively40,41. Data reduction was conducted with the SQUID 2.5 program42. Tera–Wasserburg
concordia diagrams and age histograms were drawn using Isoplot 3.7143. e 207Pb correction method was used
for 206Pb/238U ages younger than 1,000Ma, and the 204Pb correction method was adopted for 207Pb/206Pb zircon
ages greater than 1,000Ma. e ages of detrital zircons with a discordancy greater than 10% were excluded, and
all uncertainties for each data point are given at the 1-sigma level.
Results
OSL dating
OSL analysis was conducted on both coarse-grained (90–250μm grain size window) and ne-grained (4–11μm
grain size window) quartz to compare OSL ages obtained from two dierent particle size distributions. OSL
signals from both coarse and ne-grained quartz drop to about 10% of their initial intensities aer 2s, indicating
that the OSL decay curves of samples are dominated by the fast component (Fig. S3). In addition, the distribution
of De measured from most samples shows narrow Gaussian distributions (Fig. S4). All OSL data are given in
Table S1. e results of the OSL age determination range from saturation age to approximately 30ka. With the
Fig. 3. Schematic stratigraphic logs measured from the two trenches at the Inbo site, including OSL ages
from Cheon et al.5 (yellow) and this study (white). e OSL ages of the ne-grained fraction are indicated in
parentheses. H and F in the section names represent the hanging wall and foot wall, respectively. e OSL ages
summarizing of each unit, MIS stages, and paleoclimate were illustrated in the insert gure. (modied from
Jouzel et al.54). e bold red lines indicate faults.
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exception of saturation ages, the coarse-grained quartz OSL age is 70 ± 4ka for unit 4, while it ranges respectively
from 72 ± 4ka to 66 ± 6ka for unit 3, from 75 ± 4ka to 68 ± 3ka for unit 2, and from 68 ± 4ka to 29 ± 1ka for
unit 1 (Fig.3). Similarly, the OSL ages of the ne-grained quartz fraction are respectively 76 ± 6ka, from 80 ± 6
to 68 ± 6ka, from 73 ± 5ka to 67 ± 5ka, and from 76 ± 5ka to 33 ± 3ka, for units 4 to 1 (Fig.3). Hence, within
uncertainties, the two age distributions for the dierent grain size fractions are mutually consistent, and both
distributions align with the stratigraphic positions of the sampled units (Fig.3).
Detrital zircon U–Pb dating
e U‒Pb ages of 300 detrital zircon grains (100 each from three samples; Fig.3), were analyzed by SHRIMP.
Although the age of the zircon samples cannot be used to derive the depositional age, they bear information about
the age of the sediment source, which can be used to trace sediment provenance. In our samples, detrital zircons
with euhedral morphology and well-preserved edges, which are characteristics indicative of a short transport
distance, dominate (Fig. S2). Hence, it suggests that the majority of the sampled sediments is originating from
nearby drainages. All U‒Pb age data for the detrital zircons are given in Table S2, and Tera–Wasserburg diagrams
and age distribution graphs are shown in Fig.4. Sample Z1 from unit 4 yields U‒Pb detrital zircon from 1,918Ma
to 71Ma, with approximately 89% of the ages falling within the Late Cretaceous period (ca. 80 to 70Ma), and
only about 11% of the ages are much older (> 100Ma). Sample Z2 from unit 3 yields ages ranging from 2,632
Fig. 4. (a), (c) and (e) Tera–Wasserburg diagrams and pie charts of detrital zircon U‒Pb ages from three
unconsolidated units. (b), (d) and (f) Cumulative probability density diagrams of detrital zircon ages together
with age ranges of the Hayang and Yucheon groups. Pink: Late Cretaceous ages (western origin); green: the rest
of the ages (eastern origin). Sample locations are shown in Fig.3.
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to 68Ma, corresponding to: Archaean to Proterozoic (9%), Paleozoic (4%), Triassic (12%), Jurassic (6%), Early
Cretaceous (31%) and Late Cretaceous (38%). Sample Z3 from unit 1 yields ages from 2,554 to 73Ma but, unlike
samples Z1 and Z2, it features an age distribution that is only 2% Late Cretaceous and 98% older detrital zircon
ages (> 100Ma) corresponding to the Proterozoic (9%), Paleozoic (14%), Triassic (9%), Jurassic (4%) and Early
Cretaceous (62%).
Discussion
Rate of sedimentation accumulation
Our OSL results indicate that all units found in the Inbo trench, which form a few meter-thick deposit that
includes dierent sedimentary environments, were deposited in a short period (at approximately 70ka). us, we
paid specic attention to all the prescreening tests, including infrared (IR) test, preheat plateau (PHP) test, and
dose recovery test (DRT) (Figs. S5, S6 and S7) to ensure that the clustering of our ages is not an artifact due to an
oversaturated dose level33,44. is approach ensures that the determined ages truly reect the depositional ages
of the sediments. Condence in the validity of our OSL ages is reinforced by the good consistency between ages
for the dierent grain-size fractions, as the ne-grained quartz has relatively high dose saturation levels. Among
all the samples processed, one result for the sample AF-2 in unit 3 yields an age of 80 ± 6ka, which is in reverse
order relative to its stratigraphic position. We attribute this inconsistency in position to upward transportations
of sediments from unit 4 into unit 3, due to ground shaking associated with paleoearthquakes occurring aer the
deposition of unit 3. is interpretation is supported by the presence of numerous so-sediment deformation
structures (SSDS; e.g., convolute and ball-and-pillow structures) which were recorded in unit 35.
erefore, considering all the available OSL ages and despite the chronological challenges in distinguishing
the three units solely based on deposition times, we propose that the emplacement of sediments in the two Inbo
trenches can be divided into three stages. is dierentiation is possible due to their distinct sedimentological
characteristics, allowing us to identify each stage more clearly as follows: Stage (1) deposition of unit 4 occurred
before approximately 70ka. Stage (2) deposition of the overlying units 3 and 2 took place at approximately
70ka, and Stage (3) deposition of the uppermost unit 1 began around 70ka and continued until aer 30ka. If
this interpretation is correct, it suggests abrupt changes in the sedimentary condition and rapid sedimentation
around 70ka.
Changes in provenances
Our detrital zircon U‒Pb dating results are dominated by two primary age groups: the Late Cretaceous (ca. 80
to 70Ma) and Early Cretaceous (ca. 120 to 100Ma), alongside with four other age groups in lower proportion:
the Jurassic, Triassic, Paleozoic and Archean to Proterozoic. Interestingly, these two major age groups exhibit
distinct proportion in their respective distribution for the three dierent sedimentary units. Indeed, the age
distribution of the detrital zircons between unit 4 and unit 1 is nearly completely opposite, while unit 3 presents
a more mix age distribution, albeit slightly closer to unit 1.
erefore, when considered together, the distinctive euhedral morphology and well-preserved edges of
the zircon samples, which are characteristics indicative of a short transport distance, along with the varied age
distributions within the dierent sedimentary units indicate that at the Inbo site we have distinct sedimentary
units originating from dierent sources that have distinct ages, albeit all located relatively close to the trench
outcrop.
To the west of the fault zone valley of the SYF, the Yucheon Group is extensively distributed, with reported
ages of its volcanic units ranging from ca. 80 to 7045,46. erefore, these volcanic units are likely the main source
of the Late Cretaceous detrital zircons. On the eastern side of the valley, numerous Early Cretaceous detrital
zircon ages have been documented in the Hayang Group46,47. Additionally, detrital zircons associated with the
four other minor age groups have been found in basin sediments constituting the Hayang Group. As a result,
the eastern Hayang Group is likely the source area for the Early Cretaceous detrital zircons and those of the four
other minor age groups.
In summary, the deposition history of the sediments observed in the trenches can be outlined as follows:
Unit 4 characterized by 89% of detrital zircons with Late Cretaceous ages, is strongly inferred to originate from
the volcanic rocks of the Yucheon Group, predominantly sourced from the west of the fault. Cheon et al.5 using
vintage aerial photography from the 1960s and airborne LiDAR data from 2017, reported that the paleo-channel
traces started in the northwest, crossed the inferred fault line, and owed southeast. Based on the paleo-channel
traces and sedimentary characteristics, the sediments of unit 4 were primarily supplied by a uvial system. is
interpretation aligns with the regional drainage system that ows from northwest to southeast. In unit 3, there
is a notable shi in zircon age distribution, indicating a change in sediment source. e proportion of Late
Cretaceous zircons originating from the west decreases to 38%, while the proportion of Early Cretaceous zircons
from the east increases to 62%, along with the other minor age groups. is suggests a transitional period, where
sediment sources become more diverse, originating from both the eastern and the western sides of the fault
zone. In the uppermost unit 1, only approximately 2% of the detrital zircons are Late Cretaceous indicating a
signicant shi in provenance. During the deposition of unit 1, the majority of sediments at the Inbo site are
inferred to have originated from the east. Consequently, the detrital zircon U–Pb ages and inferred provenances
imply changes in source from west to east.
Changes in landform and deposition: integrated eects of climate and tectonics
At the Inbo site, along the SYF, the stratigraphic sequence exposed in paleoseismological trenches indicates that
active deformation is accommodated through earthquakes. e return time for those earthquakes, although
only loosely constrained, is of the order of a few tens of thousands of years. In this study, we provide a detailed
history of sedimentation at that site for the late Pleistocene and Holocene. e series of OSL ages indicates
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that the sedimentation rate has not been steady over time. Based on the dated stratigraphic sequence, we
can demonstrate a major change with a peak of sedimentation around 70ka. Aer this period, although the
sedimentation rate remained higher than during the period before 70ka, it decreased compared to the peak
period. Interestingly, analyses of the provenance of zircon minerals also indicate a major change in sediment
source around 70ka. e age distribution of zircon according to potential source, indicates that the change of
source for the sediment was quite rapid and denitive, with no return to the initial situation up to the present
time. Taken together, these dierent observations suggest that around 70ka, the Inbo area experienced a major
reorganization of the drainage system that led to a change in the amount and origin of sediments deposited at the
Inbo site. Careful examination of the local topography derived from an airborne LIDAR survey shows that east
of the fault, at the Inbo site, there is a perched drainage forming a wind gap, with its inlet currently standing 5m
above the level of the active river owing from the west (Fig.5d). We propose that this abandoned drainage was
indeed active during deposition of unit 4 and earlier. Although the SYF was already active, the erosional power
of the main drainage owing from west to east was strong enough to overcome occasional topographic changes
that occur whenever an earthquake breaks the Inbo section of the SYF (Fig.5a, stage 1 of Fig.5e). As evidenced
by Antarctic temperature records and growth patterns of Korean speleothems, around 70ka, the general cooling
of the area associated with the onset of the MIS4 glaciation period resulted in reduced precipitation in the
region4851 (Fig.5f). is decrease in precipitation, in turn, diminished the capacity of the drainages to keep
up with topographic changes caused by the SYF activity. During that period, the persistent activity of the SYF
resulted in the accumulation of horizontal osets of about a few tens of meters and vertical motions, which were
partially due to apparent vertical displacements caused by topographic mismatches between the two sides of
the fault. Additionally, the river’s limited capacity to erode the topography led to the disconnection of the river
outlet from its feeding catchment. Assessing the precise amount of horizontal oset is challenging due to recent
human-made modications of the landscape. is disconnection, in turn, led to an increase in sedimentation at
the toe of the scarp, west of the fault, as water accumulated against the scarp. Simultaneously, the western end
of the abandoned drainage began to experience a reversal in drainage pattern, with small local gullies draining
the local Hayang Group westward and transporting Early Cretaceous zircon to the Inbo site (Fig.5b, stage 2 of
Fig.5e). Eventually, with the return of warmer more humid conditions, the rivers owing from west regained
sucient energy to readjust their course. However, the cumulated tectonic deformation at the Inbo site was
already too large and rivers could not reuse the past drainages, having to nd a new outlet southward of the Inbo
site. is resulted in the abandonment of any signicant sedimentation originating from west at the Inbo site. At
this stage, corresponding to the deposition of unit 1, all sediments deposited at the Inbo site were only derived
from small local drainages originating from the east (Fig.5c, stage 3 of Fig.5e).
Recurrent behaviors of slow-deforming earthquake faults
With the additional OSL dating conducted in the Inbo trench, we aim to provide a more precise understanding
of the earthquake history at this site. In the following section, we attempt to better constrain how earthquakes
may have occurred over time along the SYF.
In their early work, Cheon et al.5 identied three fault strands traceable up to unit 1, although two of these
splays do not intersect subdivided unit 1.3 (Fig. S1). Cheon et al.5 performed OSL age determination on three
samples collected from units 1, 2 and 4, revealing that the most recent earthquake (MRE) that aected the Inbo site
occurred aer 29ka, with the penultimate earthquake (PE) occurring between ca. 70ka and 29ka. In this study,
we obtained additional OSL ages from unit 1.3 (BF-3), yielding ages of 52 ± 3ka and 49 ± 3ka for the coarse- and
ne-grained fraction of quartz ages, respectively. Consequently, the timing of the PE that impacted the Inbo site
was further constrained to be between ca. 70ka and 50ka. In addition to the MRE and PE, as initially identied
by Cheon et al.5 and further rened in this study, the signicant changes in the distributions of the detrital zircon
U‒Pb ages suggest the occurrence of multiple additional events with signicant surface displacement around
70ka, and certainly before the deposition of unit 1. is observation implies that earthquake activity along the
SYF may not follow a regular pattern. Conversely, if our assumption is correct, it suggests that the SYF may
exhibit earthquake clustering behavior. is would be consistent with what has been observed along other large
strike slip fault, such as the North Anatolian Fault52 or the Dead Sea Fault53, albeit occurring at a slower pace
commensurate with the slow slip rate of the SYF, typical of slow deforming intraplate fault systems.
Conclusions
e depositional ages of the Inbo trench layers are delineated into distinct periods, with unit 4 before 70ka,
units 3 and 2 around 70ka, and unit 1 spanning from 68ka to 29ka. Our OSL dating suggests that the most
recent earthquake activity occurred aer 29ka, while the penultimate event falls between 70 and 50ka. A crucial
observation from our study is the eastward shi in sediment provenance over time, primarily driven by fault
activities that signicantly restructured the local drainage systems. is shi was particularly predominated
around 70ka, when fault movements coupled with climatic changes spurred a major reorganization in drainage
patterns, accelerating sediment deposition and altering sediment sources. e integration of OSL and detrital
zircon U–Pb dating not only claries the depositional ages and sediment provenance but also chronicles the
timing of these fault movements. is study emphasizes the complex interplay between tectonic activities
and climatic conditions, shaping the landscape through ongoing deformation and varied sedimentation rates
throughout the late Pleistocene. ese insights highlight the dynamic and interconnected processes inuencing
regional geomorphology and stress the importance of an integrated approach to understanding geological and
environmental changes.
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Fig. 5. Schematic showing channel ow direction, fault activity and sedimentary provenance in aerial
photograph of the study area. (a) Before 70ka, most sediments originated from the west, supported by the
paleocurrent direction, and unit 4 was deposited. (b) At approximately 70ka, right-lateral oset changed the
direction of the paleocurrent and reduced the input of sediments originating from the west. As the eastern part
uplied, a sag pond formed, and units 3 and 2 were deposited as colluvial slope deposits from the east along
with reduced western-origin materials. (c) Aer 70ka, another palaeoearthquake completely changed the
direction of the paleocurrent to the south. us, unit 1 contains sediments mostly from the eastern slope. (d)
Topographic prole along A-A’ across the fault on (c). (e) Schematic evolution model of the study area driven
by the interaction with palaeogeographical features and faulting events. (f) Comparison of the paleoclimatic
record with the growth frequency record from Korean speleothems over the past 190ka and MIS stages. e
Blue line represents Antarctic temperature record (Jouzel et al.54), while the red curve indicates the growth
frequency record of Korean speleothems (Jo et al.49). e grey bars represent glacial and stadial periods
(modied aer Fu et al.55). e green bar represents cluster of unit ages in the Inbo trench site, approximately
70ka.
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Data availability
Data is provided within the manuscript or supplementary information les. e datasets used and/or analysed
during the current study available from the corresponding author on reasonable request.
Received: 26 April 2024; Accepted: 16 September 2024
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Acknowledgements
We thank Constantin Athanassas, Daniel Tentori and one anonymous journal reviewer for their constructive
comments and suggestions. is work was supported by a grant from the Basic Research Project (GP2020-014)
of the KIGAM funded by the Korean Ministry of Science and ICT.
Author contributions
T.L. and J.C. conceived of the presented idea. J.C., Y.C., and Y.K. conducted eldwork. T.L. and S.L. carried out
the experiments of age dating. T.L. and J.C. wrote the manuscript incorporating the coauthors’ comments. All
authors discussed the results and contributed to the nal manuscript.
Declarations
Competing interests
e authors declare no competing interests.
Additional information
Supplementary Information e online version contains supplementary material available at https://doi.
org/10.1038/s41598-024-73242-w.
Correspondence and requests for materials should be addressed to J.-H.C.
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Article
Full-text available
Some parts of the Yangsan Fault, a prominent mature intraplate fault on the Korean Peninsula, are still active. However, structural and paleoseismic investigations are limited because a large portion of the fault zone is covered by Quaternary sediments. To characterize the northern Yangsan Fault (NYF) and its paleoseismic features, we conducted topographic analyses, geological mapping, electrical resistivity surveys, borehole drilling, SHRIMP U–Pb age dating, a trench survey, and optically-stimulated-luminescence age dating (OSL). This multidisciplinary approach shows that the NYF is expressed as a nearly straight incised valley and a ridge-disrupting topographic lineament with anastomosing multiple core zones at outcrops. The NYF in Yeonghae area exhibits a NNE-striking eastern strand and a NNW- to N–S-striking western strand. Between these, a Jurassic granite (> 300-m-wide) is distributed as an enclosed lens, bounded by Precambrian metamorphic rocks to the east and Cretaceous sedimentary rocks to the west. The eastern strand likely passes S–N through the offshore area (East Sea) to onshore Pyeonghae area. A trench survey identified faults transecting Quaternary strata, providing the first paleoseismic record along the NYF. Stratigraphic features and OSL ages show that the most recent rupture occurred after 97 ± 7 ka, with the rupture along the western boundary of the mature fault core. Although older structures are prominent, paleoseismic records are few—a limitation for our onshore investigations. To reveal YF neotectonic activities under the East Sea, we need further information about off-fault damage (landslide, turbidite, tsunami records) within marine deposits as well as on-fault damage.
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The spatial distribution of large earthquakes in slowly deforming continental regions (SDCR) is poorly documented and, thus, has often been deemed to be random. Unlike in high strain regions, where seismic activity concentrates along major active faults, earthquakes in SDCR may seem to occur more erratically in space and time. This questions classical fault behavior models, posing paramount issues for seismic hazard assessment. Here, we investigate the M7, 1967, Mogod earthquake in Mongolia, a region recognized as a SDCR. Despite the absence of visible cumulative deformation at the ground surface, we found evidence for at least 3 surface rupturing earthquakes during the last 50,000 years, associated with a slip-rate of 0.06 ± 0.01 mm/year. These results show that in SDCR, like in faster deforming regions, deformation localizes on specific structures. However, the excessive length of return time for large earthquakes along these structures makes it more difficult to recognize earthquake series, and could conversely lead to the misconception that in SDCR earthquakes would be randomly located. Thus, our result emphasizes the need for systematic appraisal of the potential seismogenic structures in SDCR in order to lower the uncertainties associated with the seismogenic sources in seismic hazard models.
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Sedimentary rocks provide key information on source‐to‐sink systems and the regional expression of plate tectonics. The abruptly changing provenance signature of sedimentary rocks is commonly ascribed to changes in the source regions, such as continental collision events, the rise of mountain chains, reorganization of river systems and climatic change. However, moving the positions of the sinks (depocenters) geographically can also change the provenance record, but this is rarely considered. Typically, large‐scale strike‐slip faults can transport depocenters for long distances, thus altering the provenance information. In this study, we present a systematic investigation of the provenance changes in the Sikhote‐Alin orogenic belt in the Russian Far East to evaluate this process. The Central Sikhote‐Alin Fault is a margin‐parallel strike‐slip fault that developed along the NE Asian continental margin from the late Mesozoic. Our new detrital zircon U‐Pb data, together with a compilation of geochronological data, indicate two different types of provenance signatures available: the sediments to the west of the fault were mainly derived from the adjacent Central Asian Orogenic Belt, but to the east of the fault, they were mainly supplied from the Korean Peninsula, which is several hundred kilometers away. It is proposed that these different sedimentary rocks were juxtaposed by syn‐ and post‐subduction sinistral displacements along the fault, and were not related to local variations in the source regions. Thus, understanding the shifting positions of depocenters is important when decoding provenance change because lateral displacement is critical for reconstructing regional paleogeography along oblique convergent plate margins.
Book
Optical dating is a rapidly developing technique, used primarily in the dating of sediments deposited in the last 500,000 or more years. As such increasing numbers of Quaternary geologists, physical geographers, archaeologists, and anthropologists are now relying on the results produced. Written by one of the foremost experts on optical dating, this book aims to bring together in a coherent whole the various strands of research that are ongoing in the area. It gives beginners an introduction to the technique as well as acting as a valuable source of up to date references. The text is divided into three parts; main text, technical notes and appendices. In this way the main text is accessible by those researchers with a limited knowledge of physics, with the technical notes providing depth of understanding for those who require it. The first part of the book is concerned with basic notions and an introduction to the standard techniques, as well as several illustrative case histories. It goes on to then discuss the limitations of the technique and factors affecting reliability.
Article
Quartz and Feldspar are ubiquitous minerals, widely used in Optically Stimulated Luminescence (OSL) dating. Quartz is widely used in Cosmogenic Radionuclide (CRN) dating as well. In order to utilize them for dating purpose, it is important to extract pure quartz or feldspar. Quartz and feldspar have different luminescence properties, therefore it is essential to get pure quartz before the measurements. Similarly, in the CRN dating the production rate of ¹⁰Be and ²⁶Al is different for different minerals, and is well understood in the case of quartz. Therefore, in order to get reliable ages from CRN, extraction of pure quartz from the sediment becomes necessary. Protocols have been developed to extract pure quartz from the sediment, e.g., the separation of quartz from feldspar and other heavy minerals can be achieved by applying 2-step density separation. This is followed by HF etching which usually removes the feldspar contamination from the quartz. However, in some cases, this method is not successful, especially in the case of feldspar rich samples or in the samples with intergrowth of feldspar in quartz crystals. In this paper, we report a method that is effective in separating quartz and feldspar. The heavy minerals were removed using isodynamic magnetic separation. A small amount of fine grained iron powder was sprinkled on the sample treated with dilute HF and mixed, iron powder settles into the pits created on the surface of the feldspar grain. Feldspar grains mixed with iron powder behaves as a magnetic mineral and are separated from the quartz effectively. The proposed methodology have been checked with different analytical instruments such as scanning electron microscope (SEM), OSL/TL reader and inductively coupled plasma mass spectrometry (ICP-MS). The suggested methodology is able to eliminate ∼95% of feldspar.
Article
Seismic hazard assessment and geochronology are closely linked disciplines. The quantity and quality of the geochronological data used for fault-source characterization is crucial in seismic hazard estimates, which may have significant socio-economic implications. The characterization of Quaternary faults in the central sector of the Iberian Chain (NE Spain) has traditionally been based on ages provided by a now closed commercial luminescence laboratory. In this work, we compare new geochronological data obtained by a multi-method dating approach from pediment and terraces (Electron Spin Resonance: ESR, Optically Stimulated Luminescence: OSL; U-series: U/Th) and short-transport colluvial facies (radiocarbon), with ages provided by the commercial luminescence lab from the same units. The thirteen new numerical ages from terraces and pediments associated with Quaternary faults are systematically 6–3 times older than the previous ones, strongly suggesting that they lead to significant overestimates of fault activity and seismic hazard in the region. These new ESR, OSL and U/Th ages and the lack of information about the methodology applied by the closed luminescence laboratory seriously question the reliability of the previous ages. It also highlights the need of revisiting the Quaternary and geomorphological studies carried out in Spain using non-reliable ages.
Article
We re-evaluate the stress state across the Korean Peninsula by analyzing the stress tensor inversion results, strike–rake relationships, and faulting-type indicator–magnitude relationships of published earthquake focal mechanism data. Previous studies have shown that the stress state is characterized by a strike-slip faulting stress regime with ENE–WSW maximum compression, NNW–SSE minimum compression, and vertical intermediate stress. The differential stress magnitudes are estimated to be on the order of 100 MPa under the assumption of the strong-fault earthquake with a high Coulomb friction coefficient of 0.85. Conversely, other studies have shown that the weak-fault model is applicable across the Japanese Archipelago, where the differential stress level is only a few tens of MPa. We, therefore, review previous studies, evaluate the focal mechanism data across the Korean Peninsula, and infer the absolute stress level. Our comparison of the geological and tectonic histories of the Korean Peninsula and Japanese Archipelago suggests that the NNW–SSE tensional stresses in the Korean Peninsula are caused by slab rollback of the Philippine Sea Plate at around 5 Ma, west of Kyushu Island, southwestern Japan. We also find that the earthquake faulting types across the Korean Peninsula are closely related to their fault strikes, confirming a spatially uniform distribution of the strike-slip stress regime with nearly equal tensional and compressional tectonic stresses. However, this situation leads to the apparent paradox of strike-dependent fault strength. We eliminate this paradox by proposing a model whereby reverse- and normal-faulting events are caused by local stress heterogeneities around the edges of strike-slip faults under a uniform low-strength strike-slip stress state for all faulting types. We also highlight the usefulness of strike–rake and faulting-type indicator–magnitude diagrams in considering stress states.
Article
The NNE–SSW-striking Yangsan Fault in southeastern Korea has been regarded as one of the most prominent seismogenic structures in the Korean Peninsula on the basis of instrumental and historical seismicity, and paleoseismic records along the fault zone. Its seismic behavior is, however, still uncertain due to long recurrence intervals of strong earthquakes and insufficient historical and geologic records. We conducted a detailed paleoseismic investigation, including an 8 m-deep excavation, in order to understand recent earthquake-faulting events along the Southern Yangsan Fault. Our observations indicate that at least two and possibly four dominantly strike-slip surface-faulting events have occurred along a subsidiary fault in the eastern boundary of the fault valley since the late Pleistocene. Stratigraphic features and OSL ages of the stratigraphy exposed in the trenches indicate that the most recent rupture(s) occurred after 29.2 ± 1.4 ka and the timing of earlier ruptures are constrained to between 70.0 ± 3.7 ka and 29.2 ± 1.4 ka. Using a contact of the oldest dated unit that has at least 7.5 m of vertical separation, we calculate a minimum vertical slip rate during the late Quaternary of 0.11 mm/yr. The minimum horizontal slip rate is presumed to be two times that of the vertical slip rate based on striations observed on clasts next to the fault core. We also propose that the late Quaternary earthquake-faulting kinematics along the Yangsan Fault, expressed as contractional dextral slip with east-side-up geometry, is strongly dependent on a pre-existing fault zone architecture with a strike of N10–20°E that dips to the east, and the direction of neotectonic maximum horizontal stress (ENE–WSW to E–W).