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Article

Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts

by
Jong Kyu Park
1,
Ji Young Shin
2,
Seungwon Shin
3,* and
Yong-Hee Park
3,*
1
Department of Geophysics, Kangwon National University, Chuncheon 24341, Republic of Korea
2
Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China
3
Division of Geology and Geophysics, Kangwon National University, Chuncheon 24341, Republic of Korea
*
Authors to whom correspondence should be addressed.
Minerals 2023, 13(8), 1011; https://doi.org/10.3390/min13081011
Submission received: 9 July 2023 / Revised: 27 July 2023 / Accepted: 27 July 2023 / Published: 29 July 2023
(This article belongs to the Special Issue Application of Geophysical Data Interpretation in Geological and Mineral Potential Mapping)
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Abstract

:
This paper presents a comprehensive analysis of the anisotropy of magnetic susceptibility (AMS) and paleomagnetic data from Quaternary basalt outcrops along the Hantangang River, Korea. A total of 554 samples were collected from 20 sites, representing three distinct units, Unit I, Unit II, and Unit III. Paleomagnetic data reveal a difference in the timing of eruptions between Units I and II, suggesting distinct periods by volcanic episodes. The mineral magnetic analysis identified titanomagnetite as the dominant magnetic carrier in the samples. AMS results indicated weak anisotropy and scattered AMS directions, indicating a low degree of preferred orientation of grains within the basalt rocks. The inverse AMS fabrics observed at specific sites are attributed to single-domain (SD) grains. Comparing the AMS data with the anisotropy of anhysteretic remanent magnetization (AARM) data, three distinct types of magnetic fabrics (normal, intermediate, and inverse) were discerned. The magnetic fabric was utilized to ascertain the flow direction based on the findings obtained from the AMS results. The findings suggest that the Quaternary basalts in this study’s area were primarily confined to the Hantangang River channel and its immediate vicinity during lava flow. However, distinct flow patterns are observed in the southwestern region, implying the presence of unknown volcanic sources.

1. Introduction

The Hantangang River, located in the central part of the Korean Peninsula, is unique in South Korea and known for its distinct geologic and geomorphologic features resulting from basalt erosion. Despite extensive research, the exact eruptive origin of the Quaternary Hantangang River Volcanic Field (HRVF) [1] remains uncertain, with two proposed possibilities [2,3,4,5,6,7,8]. Ori Mountain (38°23’25”N, 127°16’01”E) and Upland (680 m) in North Korea have been identified as potential sources, suggesting that low-viscosity basaltic lava may have erupted multiple times and flowed through ancient river channels, resulting in the formation of the HRVF, which extends for 110 km and varies in thickness from 3 to 40 m [2,3,4,6,7]. The erupted lava undergoes cooling processes, resulting in the formation of columnar jointed structures and pillow lavas. Subsequently, these formations undergo erosion by reformed river systems, giving rise to waterfalls and other distinct features along the channel walls. These geologic valuations led to the establishment of the Hantangang River National Geopark in 2015, later certified as a UNESCO Global Geopark in 2020.
The HRVF has been the subject of ongoing research to determine the origin of the lava [2,6,7,9]. However, accurate research on the origin has been challenging due to the presumed location in North Korea. In such cases, one of the methods employed to infer the origin of lava flows is the analysis of anisotropy of magnetic susceptibility (AMS). AMS is a technique that examines the preferred orientation of minute magnetic minerals in volcanic or sedimentary rocks, allowing for the estimation of the direction of lava flow or paleocurrents. This technique has been proven valuable in geologic studies [10,11] and has been extensively utilized in many research effects [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26].
Although several previous studies have examined the paleomagnetic characteristics of the Hantangang River [3,27,28,29], none have utilized AMS analysis. In this study, we conducted a rigorous AMS analysis of Quaternary basalt outcrops along the Hantangang River to determine the source of the lava and gain insights into the paleo-river channels in this study’s area.

2. Geological Setting

This study’s area, where the Hantangang River is situated within the Chugaryeong rift valley, also known as the Chugaryeong Fault Zone, is a geological feature in the central part of the Korean Peninsula. It consists of several parallel fault lines trending from northeast to southwest (Figure 1). The Chugaryeong rift valley formed approximately 25–30 million years ago during the late Cenozoic era as a result of the movement of the Eurasian and Philippine Sea plates [30,31,32]. Stretching for about 250 km in length and varying in width from 20 to 60 km, the Chugaryeong rift valley holds significant geologic and tectonic importance for the Korean Peninsula. It has profoundly influenced the region’s landscape, geology, and natural resources. The valley is also an active seismic zone, occasionally experiencing earthquakes along the fault lines.
The basement rock in this study’s area consists of a Precambrian metamorphic complex intruded by Jurassic and Cretaceous granite, where the fault zones have intersected [4]. The Quaternary alkali olivine basalts in this study’s area erupted during the latest volcanic activity in the Chugaryeong rift valley, following the volcanic eruptions of acidic volcanisms during the Cretaceous period [3]. The Cretaceous silicic volcanic rocks occupy an oval-shaped region bound by the Dongducheon and Dongsong faults. At the same time, the tholeiitic basalt outcrops are sparsely distributed in the lower part of the acidic volcanic rock outcrops. The Quaternary basalts exhibit a narrow and elongated distribution along the Hantangang River, covering the process of the Dongducheon and Dongsong faults.
In this study, the term “Hantangang basalts” will be used to refer to all the basalts distributed around the Hantangang River, even though these Quaternary basalts are commonly known as Jeongok basalts in the Jeongok region. The Hantangang basalts are characterized by short eruption intervals, as inferred from their chemical composition. However, their precise eruption history remains unknown [2,3].
Based on the volcanic stratigraphy analyzed in this study’s area, the Hantangang basalt has been classified into three units. Unit I represents the lowermost part of the basalt outcrop along the Hantangang River, with an observed thickness of about 3 to 4 m. This unit displays either weakly developed or poorly observed columnar fractures. Pillow lava is also a typical feature of this unit. Evidence suggests that the lava unit experienced a relatively slow cooling process, indicated by the development of vertical and horizontal segregation textures within the lava flow. The lava flow appears massive and void-free exposures, with a K-Ar dating estimate of 0.51 ± 0.07 Ma [33] (Danhara et al., 2002 [33]).
Unit II is characterized by prominently exposed lava flows along the Hantangang River, exhibiting well-developed vertical columnar fractures. This unit can be further classified into lower, middle, and upper parts. The upper part of Unit II is distinguished by the presence of multiple layers of lava, with thicknesses ranging from 20 to 30 cm and a total thickness from 1 to 1.5 m.
During a volcanic eruption, the lower part of the lava flow is in contact with the Earth’s surface, while the upper part is exposed to the atmosphere and gradually cools. As a result, it is likely that the relatively dense lower part of Unit II primarily cooled through heat conduction in contact with the Earth’s surface. In contrast, the upper part of Unit II represents the surface portion of the lava flow that cooled upon contact with the atmosphere. This cooling process, combined with fluctuations in the thickness of the lava flow, led to the development of multiple layers of lava. The absence of distinct features, such as paleosols, sedimentary layers, or erosion surfaces at the upper part of Unit II, indicates a temporal gap, making it challenging to differentiate between the upper part of Unit II and Unit III in the field.
Several age estimates have been reported for this basalt unit, including 1.08 ± 0.158 Ma [34], 0.40 Ma [35], and 0.51 ± 0.01 Ma [36]. The lower part of Unit II was estimated to have a burial age of 0.48 Ma [37] based on cosmic-origin isotopes of the Baekui-ri layer, while the upper part was estimated to be 0.09 ± 0.03~0.18 ± 0.03 Ma [36] based on K-Ar dating.
Unit III is a lava flow unit that typically rests atop a scarp. Although thinner than Unit II, its thickness varies considerably between outcrops and locations, measuring up to 5 to 6 m in some areas. The lava flows in Unit III generally lack columnar fractures but display well-defined pores in the upper and lower parts, along with clear segregation structures. Lee et al. [38] estimated that this unit formed approximately 40,000 years ago, likely representing the most recent eruption of Hantangang basalt.

3. Methods

A total of 554 samples were collected from 20 sites in this study’s area, five sites from Unit I, thirteen sites from Unit II, and three sites from Unit III (Figure 1). All samples were cored with a gasoline-powered portable rock drill and oriented with a Brunton compass in the field. In the laboratory, the samples were subsequently sliced into 25 mm diameter and 22 mm long cylinders and stored in mu-metal shield boxes to prevent the acquisition of viscous remanence caused by the external magnetic field, such as the Earth’s field.
AMS data for each sample were measured in fifteen positions [39] using a Bartington magnetic susceptibility meter (Model MS-2) connected with an MS-2B susceptibility bridge. Eigen parameters for each AMS data were calculated using the Hext statistic [40]. The data were visualized by an ellipsoid with the principal susceptibility axes labeled Kmax > Kint > Kmin, which were calculated using the PmagPy program [41]. The mean magnetic susceptibility (Km) for a single sample is calculated by Km = (Kmax + Kint + Kmin)/3. The magnitude and shape of the susceptibility ellipsoid are expressed by the corrected degree of anisotropy (P’) and the symmetry of shape on the vertical axis (T), respectively, proposed by Jelinek [42].
The anisotropy of anhysteretic remanent magnetization (AARM) of pilot samples from each site was measured in fifteen different orientations on a Molspin spinner magnetometer. This was carried out to determine the orientation distribution of remanence-bearing or ferromagnetic (sensu lato) minerals and to test the possibility of an inverse fabric influenced by single-domain magnetite (SD effect; [43]). The ARM was imparted using a Molspin AF demagnetizer and an ARM attachment with a 0.05 m TDC bias field and a peak alternating field of 90 mT. The tensor of the AARM was then calculated using the Hext statistic [40].
Magnetic remanence measurements for paleomagnetic analysis were performed using a Molspin spinner magnetometer. Natural remanent magnetization (NRM) was measured and demagnetized stepwise up to a peak field of 90 mT in 5~10 mT intervals. The palaeomagnetic data from all specimens were projected onto the orthogonal vector diagram [44]. The characteristic remanent magnetization (ChRM) direction of each specimen was determined using principal component analysis with the anchored line fit method [45] from at least three or more points.
Low-field thermomagnetic measurements (K-T curves) at low and high temperatures were conducted on bulk rock samples using a Bartington MS2 susceptibility meter equipped with an MS2WF furnace. Twenty cylindrical subsamples of 15 mm in height were made from residual fragments of core samples. Low-temperature measurements were carried out by heating the samples cooled to about −170 °C in liquid nitrogen to room temperature. Then, the samples underwent a gradual heating process, reaching temperatures of up to 700 °C at a rate of 20 °C/min, followed by cooling to room temperature at the same rate. Magnetic susceptibility measurements were taken at intervals of 1 °C during the process.
To determine the grain size of magnetic minerals, hysteresis parameters were measured on 80 cylindrical subsamples (8 mm in diameter and 10 mm in height) using a Molspin vibrating sample magnetometer (Model VSM Nuvo) calibrated by a paramagnetic standard sample containing 0.6 g of FeSO4.

4. Results

4.1. Paleomagnetic Results

The NRMs of the basalt samples accepted by the selection criteria [46] vary from 140.8 to 5386.2 mA/m in intensity, with predominantly northerly positive directions. Most samples display a simple decay of a northerly and moderately steep down component toward the origin above 15 mT or 20 mT, indicating the effective isolation of the ChRM component through the AF demagnetization method (Figure 2). This observation suggests that a low-coercivity ferrimagnetic mineral, such as titanomagnetite, is the predominant magnetic mineral. Field observations reveal that the lava flows in this study’s area exhibit very gentle dips from 1° to 6° toward the flow direction, indicating a topographic origin rather than a tectonic origin. Previous petrographic investigations also support these findings, reporting no evidence of tectonic movement following lava eruptions [3]. Therefore, in this study, we did not correct for tilting in the paleomagnetic directions.
The site-mean paleomagnetic directions were calculated from 427 samples, excluding 35 samples that showed anomalous directions likely due to the acquisition of isothermal remanence or chemical alteration (Table 1). The ChRM directions within each site cluster demonstrate high precision parameters (k ≥ 118.5) and narrow 95% confidence limits around the mean direction (α95 ≤ 3.2°). The unit-mean directions are as follows: Unit I, D/I: 6.8°/60.4° (α95 = 1.8°); Unit II, D/I: 2.3°/58.7° (α95 = 1.5°); Unit III, D/I: 0.2°/60.8° (α95 = 5.9°) (Figure 3). F-test results indicate that the directions of Unit I and Unit II are statistically distinguished from each other at a 5% confidence level, suggesting different timing of remanence acquisition for these two units. The direction of Unit III is not statistically distinguishable from those of Units I and II at a 5% significance level due to the large 95% confidence circle of Unit III. However, it is interpreted that the acquisition of remanent magnetization in Units I and III could not have occurred simultaneously.

4.2. Mineral Magnetic Results

4.2.1. Thermomagnetic (K-T) Curves

Figure 4 displays representative thermomagnetic curves (K-T curves). The low-temperature K-T curves exhibit a gradual increase in susceptibility as the temperature rises from −200 °C, with a sharp increase occurring at the inflection point ranging from −42 °C to −74 °C. This behavior agrees with the findings of [47] using synthetic titanomagnetite (Fe3-xTixO4) samples with a Ti-rich composition (x > 0.5). The high-temperature K-T curves of all samples reveal irreversible heating and cooling curves, indicating a magnetic phase alteration during the thermal treatment. In the heating curve, the susceptibility peaks around 0 °C, followed by a rapid decrease within the range of 64 °C to 88 °C, suggesting a relatively low Curie temperature (Figure 4). These observations strongly support that Ti-rich titanomagnetites (x ≈ 0.7) are major magnetic carriers in the samples [48]. Furthermore, the heating curves exhibit a gradual rise above ~300 °C, followed by two subsequent decreases at 467~505 °C and 553~565 °C, indicating the possible presence of Ti-poor titanomagnetite or metastable titanomaghemite resulting from the low-temperature oxidation of titanomagnetite during the heating process [19,49]. During the cooling cycle, a significant increase in susceptibility is observed at approximately 550 °C (Figure 4), suggesting the formation of new magnetic minerals such as Ti-poor titanomagnetite through the transformation of Ti-rich magnetite during the heating process [48,50].

4.2.2. Hysteresis Parameters

Magnetic hysteresis loops were measured for 80 samples, and paramagnetic corrections were applied using the best-fitting high field slope (KHF). Hysteresis parameters, such as coercivity of remanence (Bcr), coercive force (Bc), saturation remanence (Mrs), and saturation magnetization (Ms), were obtained (Table S1). For the magnetic grain size analysis, the hysteresis ratio of Mrs/Ms versus Bcr/Bc was plotted on a Day diagram, as shown in Figure 5 [51,52].
Day diagrams can be ambiguous due to the presence of various variables [53]; nonetheless, we have chosen to adopt them for quantitative rock magnetism interpretation. The distribution of these hysteresis ratios ranges from single domain (SD) to pseudo single domain (PSD) particle size. More than half of the data points fall in the PSD region, indicating a predominance of PSD particles in samples.
On the other hand, about a quarter of the data points fall in the SD region. The rest of the points are close to the SD/PSD boundary and the theoretical curve for a mixture of SD and 10 nm superparamagnetic (SP) particles proposed by [52] (Figure 5c). These results suggest that the magnetic particles in most samples are predominantly composed of SD or PSD grains.
In contrast, a subset of samples contains a dominant population of SD grains with some presence of SP grains. Hysteresis loops in these samples are slightly wasp-waisted (Figure 5a,b), supporting the presence of SP grains [54]. Because the data distribution is far from the multi-domain region or the theoretical curve for the mixture of SD-MD grains [52], the content of MD grains in samples is negligible.

4.3. Anisotropy of Magnetic Susceptibility (AMS) Results

The site-mean magnetic susceptibility (Km) ranges from 0.97 × 10−3 SI to 6.53 × 10−3 SI, with an overall average of 2.36 × 10−3 SI (Table 2). These values are relatively lower than the expected range for basaltic rocks (> 5 × 10−3 SI). The lower Km values can be attributed to two factors, the small grain size falling within the SD-PSD range and the mineralogy of the magnetic grains, primarily composed of Ti-rich titanomagnetite [10,55]. Sites dominated by SD grains (KH01, KH02, KH03, KH04, KH8, KH11, KH12, KH18, KH20) generally exhibit lower Km values compared to other sites (Table 2), providing further evidence of the influence of grain size on the observed lower Km values.
Lava flows typically exhibit a low degree of anisotropy and a scattered distribution of AMS directions due to the weak preferred orientation of grains [56,57]. Table 2 presents the mean values of the degree of anisotropy (P’), which range from 1.005 to 1.017, indicating very low anisotropy. When projected on a stereonet, the AMS principal directions display significant scattering (Figure 6). Despite the weak anisotropy and scattered directions, the axial clusters of AMS ellipsoids can provide valuable information on the flow direction [58]. The mean directions for the principal eigenvectors (Kmax, Kint, and Kmin) calculated using bootstrap statistics are listed in Table 2. In certain sites (KH15, KH17, and KH20), the Kmax axes are nearly horizontal, while the Kmin axes are predominantly vertical (Figure 6). Although the mean Kmax and Kmin directions exhibit slight deviations (less than ~20 degrees) from the horizontal and vertical orientations, respectively (Table 2), these deviations do not appear statistically significant, considering the relatively large scatter in the principal AMS directions. For these sites, the flow directions of lava flows can be inferred from the magnetic lineation.
On the other hand, the remaining sites show mean Kmin axes plunging at angles of 50 to 70 degrees from the horizontal axis (Table 2), indicating magnetic imbrications. The flow directions for these sites can be deduced from the imbricated sense of elongated grains. However, some of these sites (KH01, KH02, KH03, KH04, KH08, KH11, and KH12) exhibit anomalously horizontal to sub-horizontal Kmin axes (Figure 6 and Table 2). We attributed this phenomenon to the interchanged principal axes resulting from inverse components in the AMS fabrics, as discussed further in Section 5.2.
The site-mean shape parameters (T) are plotted on the Jelinek diagram [42] in Figure 7. Some sites (KH01, KH02, KH06, KH07, KH08, KH09, and KH16) are predominantly prolate (T < 0), indicating an elongated ellipsoidal shape. Only KH05 exhibits a predominantly oblate shape (T > 0), indicating a flattened ellipsoidal shape. The remaining sites exhibit a minimal site-mean T value due to a wide range of T values. This result reflects the significant scatter in the AMS ellipsoid axes and the measurement uncertainties resulting from the low anisotropy.

4.4. Anisotropy of Anhysteretic Remanent Magnetization (AARM) Results

Four representative samples from each of the 20 sites were subjected to AARM measurements, and the results are shown in Table 3. The site-mean degree of anisotropy (P’) of the AARM ellipsoids ranges from 1.072 to 1.230 (Table 3), which is higher than that of the AMS ellipsoids (from 1.005 to 1.017). This result supports the general observation that remanence is generally more anisotropic than susceptibility [43,59,60]
As with the AMS data, the principal eigenvectors (AARMmax, AARMint, and AARMmin) for all sites were plotted on a stereonet (Figure 8), and the mean directions were calculated (Table 3). The fabric types of AMS were determined by comparing the directional data between AMS and AARM. Most sites (KH05, KH06, KH07, KH08, KH09, KH10, KH13, KH14, KH15, KH16, KH17, KH20) were classified as having normal AMS fabrics, as the mean principal axes of the AMS ellipsoid were nearly coaxial with those of the AARM ellipsoid. Four sites (KH02, KH03, KH8, and KH11) were identified as having inverse AMS fabrics, as the mean Kmax and Kmin orientations were sub-parallel to those of AARMmin and AARMmax, respectively. These inverse components of AMS fabrics can mix with normal components, resulting in an “intermediate” AMS fabric [61,62]. In our samples, intermediate AMS fabrics are observed in five sites (KH01, KH04, KH12, KH18, KH19). Two sites (KH01, KH12) exhibit an interchange of all three principal axes of the AMS ellipsoid compared to the AARM fabrics, while three sites (KH04, KH18, KH19) show an interchange between two axes (Kint and Kmax). When considering both AMS and AARM results, anomalous AMS fabrics (KH01, KH02, KH03, KH8, KH11, KH12) with highly tilted Kmin axes from the vertical orientation were found to be associated with the inverse components of the AMS ellipsoid.
In most sites, the site-mean T values indicate a tendency toward oblate AARM ellipsoid shapes (Table 3). Some sites exhibit prolate AARM ellipsoid shapes (KH05, KH13, KH14, KH15, KH17, KH18, KH19, KH20). However, when comparing the T values between AMS and AARM, no consistent relationship was observed with the AMS fabric types. It is likely due to the weak preferred orientation of the magnetic grains and the interference from the inverse components of the AMS ellipsoids.

5. Discussion

5.1. Paleomagnetic Implications for the Age of Eruption

The paleomagnetic data from each site met the data selection criteria proposed by McElhinny and McFadden [63]. These criteria included obtaining mean ChRM components from more than five samples per site, maintaining a sufficiently small radius of the 95% confidence circle (α95 ≤ 3.2°), and ensuring that the virtual geomagnetic pole (VGP) latitude for each site exceeded 50°.
As discussed in Section 4.1, the mean directions of Unit I and Unit II were statistically distinguished. At the same time, Unit III did not pass the significance test with the other units due to the limited number of sites and a larger α95 value. However, the F-test results comparing Unit I and Unit II indicate different lava eruption times for these two units.
The VGP positions for each unit are as follows: Unit I, 83.8°N/183.0°E (A95 = 2.5°); Unit II, 87.7°N/179.0°E (A95 = 1.7°). These VGPs are located close to the geographic North Pole, suggesting that any significant tectonic movement has not influenced the basalts in this study’s area since the time of the lava eruption. Furthermore, the primary flow fabrics (which will be discussed in Section 5.2) support that tectonic correction is not necessary for determining the ChRM directions.
Previous studies on eruption ages for Quaternary basalts in this study’s area reported the following: Unit I, 0.51 Ma [33]; Unit II, 1.08 Ma to 0.09 Ma [34,35,36,37]; Unit III, 0.04 Ma [38]. Notably, these basalts exhibit only normal polarity, suggesting eruption during the Brunches Epoch, namely, after 0.78 Ma. Additionally, the significantly large k-value (700.1) and small α95 value (1.5°) of the mean direction of Unit II indicate that the sampled basalts erupted over a relatively short geologic time. The paleomagnetic directional data derived from this study are expected to contribute to a global geomagnetic field database as more detailed and precise dating data on basalts in this study’s area are accumulated in the future.

5.2. Single Domain Effect on Magnetic Fabrics

The occurrence of inverse AMS fabrics, characterized by the interchange of Kmax and Kmin axes, can be attributed to mainly two causes: (1) secondary processes such as hydrothermal alteration [64] and tectonic overprinting [65]; (2) the presence of SD grains [66,67]. Our samples exhibit predominantly sub-horizontal AARMmax axes and sub-vertical AARMmin axes (Figure 8 and Table 3), indicating that the original fabrics are associated with the primary lava flow and have no significant subsequent alteration. Although some sites show inclined AARMmin axes of up to 50° from the horizontal axis (Table 3), this can be attributed to the imbrication of the magnetic foliation plane.
Our mineral magnetic results suggest that SD grains are responsible for the observed inverse AMS fabrics. Figure 5c presents the grain sizes of the dominant AMS carriers for each site, classified according to the AMS fabric type. Sites exhibiting inverse AMS fabrics display hysteresis parameters (white circles) within the SD range (Figure 5c), providing evidence of SD grains’ influence on the inverse fabric. Conversely, sites with normal AMS fabrics predominantly exhibit hysteresis ratios (grey circles) within the PSD range, indicating that SD grains did not significantly contribute to the normal fabric.
The hysteresis ratios vary across the SD to PSD range for sites displaying intermediate AMS fabrics (white and grey triangles). They are distributed near the boundary between the SD and PSD regions (Figure 5c). These data points align closely with the SD-SP mixing line proposed by Dunlop [52] (Figure 5c), indicating the presence of SD grains mixed with some SP grains, as indicated by slightly wasp-waisted hysteresis loops (Figure 5a,b). Thus, the intermediate fabrics observed in our samples might arise from a combination of inverse components resulting from the SD effect mixed with the normal components.

5.3. Inferred Lava Flow Direction

Flow directions were determined for sites with normal AMS fabric. For these sites, the flow direction is inferred from the magnetic lineation. One of them, site KH20, exhibits a mean Kmin axis nearly vertical with a slight tilt of 2.3° (Table 2), indicating minimal imbrication. In contrast, the remaining sites with normal AMS fabrics show an oblique orientation of the mean Kmin axes, ranging from 14.8° to 42.3° from the vertical (Table 2). Most sites have mean Kmin axes plunging approximately at 20° (14.8°~24.6°) from the vertical axis, while a few sites (KH06, KH10, KH14) exhibit highly inclined mean Kmin axes, deviating by around 40° (34.4°~42.3°).
In this study, mean Kmin axes plunging at 70° are considered nearly vertical based on the significant scattering of the principal AMS directions and the poor grouping of Kmin axes. However, mean Kmin axes with high obliquity (20°~45°) from the vertical indicate magnetic imbrication of elongated grains. This imbrication is likely caused by a parabolic velocity profile within the lava flow between the ground surface and a solidified roof [13,14]. As the roof solidifies, the flow velocity in the upper and lower parts of the lava becomes lower compared to the central part, resulting in the opposite imbrication of elongated grains [13,14,68,69]. In opposite imbrications, the plunge direction of the magnetic foliation pole (Kmin azimuth) indicates the flow direction [14,70]. Given that our samples were collected from the lower half of the lava flow, the azimuth of the Kmin axes could be parallel to the flow vector [13,17,70].
The determined lava flow directions were shown on the geologic map (Figure 9). Three sites (KH15, KH17, KH20) used bidirectional lineation as the flow direction indicator. For the remaining sites, flow directions were determined using magnetic imbrication (Figure 9). The inferred flow directions, pointing toward the south–southwest, are consistent with previous work that utilized vesicle structure in the field [3]. This agreement is observed at sites KH09 and KH20.
The lava flow directions were determined for four sites (KH02, KH03, KH08, KH11) exhibiting an inverse fabric (Kmax-Kmin interchange). The direction of Kmax axes, which deviates from the vertical axis, was taken as the direction of imbrication. For three sites (KH04, KH18, KH19) showing intermediate fabric (group 1; Kmax-Kint interchange), the direction of the Kmin axes deviated from the vertical axis by from 22.2° to 44.9°. Therefore, this deviation was considered magnetic imbrication. In the case of two sites (KH01, KH12) with intermediate fabric (group 2), where all three axes interchanged, the axes’ distribution was considered. In the case of site KH01, the direction of Kmax axes was estimated as the magnetic lineation (Figure 9).
In this study, lava flow directions were determined from a total of 20 sites. However, due to the limited number of sampling sites, it has not been possible to identify specific lava sources definitively. Nonetheless, the lava flow directions obtained for each Unit provide valuable information about the past topography during the lava eruption.
Unit I, the oldest basalt in this study’s area, is exclusively found in the upstream region of the Hantangang River (the northeastern part). The lava flow directions observed at five sites (KH01, KH06, KH07, KH09, and KH10) consistently point toward the southwest or southeast, indicating a downstream flow direction. These observations suggest that the Unit I lavas originated from a previously proposed source within North Korea, such as Mt. Ori and the 680 m Upland [2]. Similarly, both Unit II (KH02 and KH03) and Unit III (KH05) demonstrate a strong alignment with the current channel of the Hantangang River (Figure 9). These findings imply that the paleo-channel and surrounding topography of the Hantangang River during the time of lava flows were largely consistent with the present-day configuration, providing further support for the work by Hakim et al. [9].
However, the Jeongok region in the southwestern part of this study’s area shows a different pattern compared to the northeastern part. Although most lava flows from Units II and III follow the Hantangang River channel, the flow directions suggest that they may have originated from other directions to the north or northwest rather than upstream in the northeastern region (e.g., KH14, KH15, KH16, KH19). Therefore, the possibility of potentially unknown volcanic sources in the surrounding area cannot be ruled out.
In summary, the distribution of Quaternary basalts in this study’s area is predominantly confined to the channel of the Hantangang River and its immediate vicinity during lava flow. The sampling sites are limited to the outcrops located within the current channel of the Hantangang River, as much of the larger area has been eroded or buried. Furthermore, the complete stratigraphic relationship of the lava flows has yet to be fully elucidated, emphasizing the need for a more robust eruption history related to the source regions. Given several faults with a north–south orientation in this study’s area (as shown in Figure 9), further comprehensive investigations are required to identify potential volcanic sources throughout the area.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min13081011/s1, Table S1: Magnetic hysteresis parameter of a representative sample of the site.

Author Contributions

Conceptualization, J.K.P., J.Y.S., S.S. and Y.-H.P.; methodology, J.Y.S. and Y.-H.P.; software, J.K.P. and J.Y.S.; validation, J.K.P. and Y.-H.P.; formal analysis, J.K.P. and J.Y.S.; investigation, J.K.P., J.Y.S., S.S. and Y.-H.P.; resources, S.S. and Y.-H.P.; data curation, J.Y.S., J.K.P. and Y.-H.P.; writing—original draft preparation J.K.P., J.Y.S. and Y.-H.P.; writing—review and editing, J.K.P., J.Y.S., S.S. and Y.-H.P.; visualization, J.K.P., J.Y.S. and Y.-H.P.; supervision, Y.-H.P.; project administration, S.S. and Y.-H.P.; funding acquisition, J.K.P., J.Y.S., S.S. and Y.-H.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Research Foundation of Korea, grant number 2019R1A6A1A03033167 and 2022R1F1A1073012(Y.-H.P.), and RS-2023-00211265 (S.S.) and by Kangwon National University in 2019 (Y.-H.P.).

Acknowledgments

The authors would like to thank the referees for their constructive reviews. The authors also sincerely thank Seong-Jae Doh at Korea University, Daekyo Cheong at Kangwon National University and Yongjae Yu at Chungnam National University for their valuable advice on this study This research was also supported by the 2021 Research Project for the UNESO Hantangang River Global Geopark and the Gyeonggi Provincial Office.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of this study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

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Minerals 13 01011 g001 550
Figure 1. (a) A map of Korean Peninsula with its vicinity showing the location of the Hantangang River Volcanic Field (HRVF) [1]. (b) A satellite map showing the distribution of the HRVF (inside of the red closed curve) with Mt. Ori and 680 uplands. MDL: Military Demarcation Line between South and North Korea. (c,d) Satellite maps of this study’s area with sampling site locations.
Figure 1. (a) A map of Korean Peninsula with its vicinity showing the location of the Hantangang River Volcanic Field (HRVF) [1]. (b) A satellite map showing the distribution of the HRVF (inside of the red closed curve) with Mt. Ori and 680 uplands. MDL: Military Demarcation Line between South and North Korea. (c,d) Satellite maps of this study’s area with sampling site locations.
Minerals 13 01011 g001
Minerals 13 01011 g002 550
Figure 2. Demagnetization behaviors of representative samples. Orthogonal vector diagrams with normalized intensity are shown in geographic coordinates. Steps of AF demagnetization are labeled with M below the sample number. The horizontal and vertical projections are plotted with cross (red) and solid (blue) symbols, respectively.
Figure 2. Demagnetization behaviors of representative samples. Orthogonal vector diagrams with normalized intensity are shown in geographic coordinates. Steps of AF demagnetization are labeled with M below the sample number. The horizontal and vertical projections are plotted with cross (red) and solid (blue) symbols, respectively.
Minerals 13 01011 g002
Minerals 13 01011 g003 550
Figure 3. Unit-mean paleomagnetic directions with 95% confidence circle on an equal area projection; D/I: declination/inclination; k: precision parameter; α 95 : 95% confidence level. Red, blue and green symbols are Unit I, II and III, respectively.
Figure 3. Unit-mean paleomagnetic directions with 95% confidence circle on an equal area projection; D/I: declination/inclination; k: precision parameter; α 95 : 95% confidence level. Red, blue and green symbols are Unit I, II and III, respectively.
Minerals 13 01011 g003
Minerals 13 01011 g004 550
Figure 4. Thermomagnetic curves at low and high temperatures for representative samples. Heating curve and cooling curve are in red and blue, respectively.
Figure 4. Thermomagnetic curves at low and high temperatures for representative samples. Heating curve and cooling curve are in red and blue, respectively.
Minerals 13 01011 g004
Minerals 13 01011 g005 550
Figure 5. (a,b) Examples of slight wasp-wasted hysteresis loop. A blue solid line is paramagnetic corrected curve and a red dotted line is uncorrected curve. Samples KH04-04b and KH18-10a correspond to the Intermediate Group I of AMS fabric type. (c) Hysteresis ratios (Mrs/Ms versus Bcr/Bc) for 20 sites (four samples for each site) classified by AMS fabric type are plotted on the modified Day diagram [51,52]. The theoretical curves indicated by the solid lines represent the mixing lines of single-domain (SD), multi-domain (MD), and SD- superparamagnetic (SP) mixtures [52].
Figure 5. (a,b) Examples of slight wasp-wasted hysteresis loop. A blue solid line is paramagnetic corrected curve and a red dotted line is uncorrected curve. Samples KH04-04b and KH18-10a correspond to the Intermediate Group I of AMS fabric type. (c) Hysteresis ratios (Mrs/Ms versus Bcr/Bc) for 20 sites (four samples for each site) classified by AMS fabric type are plotted on the modified Day diagram [51,52]. The theoretical curves indicated by the solid lines represent the mixing lines of single-domain (SD), multi-domain (MD), and SD- superparamagnetic (SP) mixtures [52].
Minerals 13 01011 g005
Minerals 13 01011 g006a 550Minerals 13 01011 g006b 550
Figure 6. Equal area, lower-hemisphere projection of principal directions of AMS fabric in geographic coordinates. Squares (red), triangles (green), and circles (blue) represent maximum, intermediate, and minimum eigenvectors, respectively. The site-mean directions of three eigenvectors and their corresponding error ovals, calculated using the bootstrap method, are shown on the right side.
Figure 6. Equal area, lower-hemisphere projection of principal directions of AMS fabric in geographic coordinates. Squares (red), triangles (green), and circles (blue) represent maximum, intermediate, and minimum eigenvectors, respectively. The site-mean directions of three eigenvectors and their corresponding error ovals, calculated using the bootstrap method, are shown on the right side.
Minerals 13 01011 g006aMinerals 13 01011 g006b
Minerals 13 01011 g007 550
Figure 7. Jelinek diagram showing the site-mean values of degree of anisotropy (P’) and shape parameter (T). Closed and open symbols are oblate (T > 0) and prolate (T < 0) shapes, respectively.
Figure 7. Jelinek diagram showing the site-mean values of degree of anisotropy (P’) and shape parameter (T). Closed and open symbols are oblate (T > 0) and prolate (T < 0) shapes, respectively.
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Minerals 13 01011 g008 550
Figure 8. Equal area, lower-hemisphere projection of principal directions of AARM data for four representative samples from each site. Squares (red), triangles (green), and circles (blue) represent maximum, intermediate, and minimum eigenvectors, respectively.
Figure 8. Equal area, lower-hemisphere projection of principal directions of AARM data for four representative samples from each site. Squares (red), triangles (green), and circles (blue) represent maximum, intermediate, and minimum eigenvectors, respectively.
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Minerals 13 01011 g009 550
Figure 9. (upper left) A satellite map showing this study’s area. MDL: Military Demarcation Line between South and North Korea. (lower left/right) Inferred lava flow directions with 95% confidence level on the map in the downstream/upstream regions of Hantangang River. DF: Dongduchen Fault, DSF: Dongsong Fault, JF: Jeongok Fault, KF: Kimhwa Fault, DTF: Damteo Fault, SF: Singo Fault, and PF: Pochen Fault.
Figure 9. (upper left) A satellite map showing this study’s area. MDL: Military Demarcation Line between South and North Korea. (lower left/right) Inferred lava flow directions with 95% confidence level on the map in the downstream/upstream regions of Hantangang River. DF: Dongduchen Fault, DSF: Dongsong Fault, JF: Jeongok Fault, KF: Kimhwa Fault, DTF: Damteo Fault, SF: Singo Fault, and PF: Pochen Fault.
Minerals 13 01011 g009
Table
Table 1. Paleomagnetic results in this study’s area.
Table 1. Paleomagnetic results in this study’s area.
UnitSiten/NDec
(°)		Inc
(°)		kα95
(°)		Sampling Site Location
Lat
(°N)		Long
(°E)		Elev
(m)
ⅠKH0117/186.059.6162.22.638.2466127.2864188.1
KH0633/3310.860.8640.61.038.0803127.2236106.0
KH0722/248.562.4227.72.038.0789127.2178113.0
KH0920/211.260.5118.52.938.0628127.2069143.0
KH1026/267.458.5761.11.038.0617127.2003143.0
Unit Mean6.860.41262.21.8
ⅡKH0218/200.159.2277.52.038.2105127.2658150.2
KH0311/12352.857.4584.31.838.2105127.2658150.2
KH0417/186.658.2246.22.238.1907127.2887125.3
KH0814/16356.961.2350.42.038.0655127.204796.7
KH1115/163.861.1131.13.238.0241127.137561.6
KH128/106.960.0249.03.138.0256127.137261.8
KH135/174.658.1741.02.338.0405127.113450.7
KH1528/318.559.1497.21.238.0614127.116649.3
KH1625/25357.355.7373.11.538.0449127.057729.1
KH1819/231.856.7476.11.538.0112127.072739.1
KH1924/248.158.3713.71.138.0300127.060243.4
KH2032/321.158.5131.12.238.0283127.048154.3
Unit Mean2.358.7700.11.5
ⅢHK0535/359.562.4671.20.938.1072127.2688124.6
KH1430/30347.459.5694.81.038.0625127.120180.5
KH1728/314.459.5317.11.538.0455127.057563.1
Unit Mean0.260.8187.05.9
n/N: number of accepted/measured samples; Dec: declination; Inc: inclination; α95: 95% confidence level; Lat: latitude; Long: longitude; Elev: elevation.
Table
Table 2. AMS results in this study’s area.
Table 2. AMS results in this study’s area.
Site
(Unit)		NKm
(10−3 SI)		Mean AMS
parameters		KmaxKintKmin
P’TDec (°)Inc (°)Dec (°)Inc (°)Dec (°)Inc (°)
KH01(I)301.2821.016−0.4366207.238.514.650.8112.36.2
KH02(II)241.4371.015−0.2760133.439.3248.927.73.338.2
KH03(II)221.2961.010−0.0893182.052.757.723.2314.827.4
KH04(II)181.4931.0090.0885198.135.1108.10.117.954.9
KH05(III)353.3001.0100.2804337.122.769.04.4169.466.9
KH06(I)333.1471.009−0.1903312.334.052.214.2161.452.3
KH07(I)241.9031.011−0.3391151.212.556.819.0272.667.0
KH08(II)270.9741.012−0.6346234.643.3355.228.5106.233.4
KH09(I)233.7831.010−0.305465.223.8155.61.0247.966.2
KH10(I)336.5321.006−0.161615.633.6281.26.6181.655.6
KH11(II)301.2931.012−0.2887158.347.8336.442.267.21.0
KH12(II)221.1121.009−0.1227173.447.4353.342.683.40.1
KH13(II)221.8811.008−0.0523284.518.117.810.1135.769.1
KH14(III)302.3151.007−0.0719318.842.0225.24.1130.747.7
KH15(II)312.0601.0060.1137295.413.4203.38.881.173.9
KH16(II)254.8561.017−0.2017342.417.9249.39.7132.269.5
KH17(III)312.3151.007−0.0924186.212.194.48.4330.475.2
KH18(II)232.0751.0090.052811.019.5117.338.4260.145.1
KH19(II)242.1301.0050.0696170.39.7263.819.755.467.8
KH20(II)322.1401.011−0.309684.22.2174.20.6279.787.7
N: number of samples measured; Km: mean magnetic susceptibility; P’/T: degree of magnetic anisotropy/shape parameter; Kmax/Kint/Kmin: mean AMS eigenvectors corresponding to the maximum/intermediate/minimum susceptibility; Dec/Inc: declination/Inclination.
Table
Table 3. AARM results in this study’s area.
Table 3. AARM results in this study’s area.
Site
(Unit)		NMean AARM Parameters A A R M m a x A A R M i n t A A R M m i n AMS
Fabric
Type
P’TDec (°)Inc (°)Dec (°)Inc (°)Dec (°)Inc (°)
KH01(I)41.1170.0419351.629.495.222.8216.951.4Int (Group 2)
KH02(II)41.0780.22340.726.9262.415.7145.458.2Inverse
KH03(II)41.1440.0351296.832.045.527.2167.245.6Inverse
KH04(II)41.1170.1781108.33.1199.521.310.468.4Int (Group 1)
KH05(III)41.119−0.1809322.326.1227.49.9118.361.8Normal
KH06(I)41.0780.0130336.414.5240.521.897.563.4Normal
KH07(I)41.1340.1216161.74.569.822.6262.366.9Normal
KH08(II)41.2040.4252108.618.010.324.1231.759.2Inverse
KH09(I)41.1570.256050.85.7141.56.8281.481.1Normal
KH10(I)41.1540.1694342.921.6251.14.5149.967.9Normal
KH11(II)41.1990.08200.838.7261.211.7157.548.9Inverse
KH12(II)41.1340.01022.930.1272.01.6179.359.9Int (Group 2)
KH13(II)41.095−0.2983121.74.529.822.6222.366.9Normal
KH14(III)41.142−0.4803321.017.753.57.9166.770.5Normal
KH15(II)41.112−0.3199279.44.6188.115.625.673.7Normal
KH16(II)41.1240.1711191.85.1283.619.888.169.5Normal
KH17(III)41.079−0.3168165.414.673.38.1315.173.2Normal
KH18(II)41.230−0.1969113.517.011.035.3224.649.6Int (Group 1)
KH19(II)41.072−0.0241138.29.645.913.5262.673.3Int (Group 1)
KH20(II)41.154−0.058774.022.3164.82.0259.767.6Normal
N: number of samples measured; P’/T: degree of magnetic anisotropy/shape parameter; AARMmax/AARMint/AARMmin: mean AARM eigenvectors corresponding to the maximum/intermediate/minimum intensity; Dec/Inc: declination/Inclination; Int: Intermediate.
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Park, J.K.; Shin, J.Y.; Shin, S.; Park, Y.-H. Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts. Minerals 2023, 13, 1011. https://doi.org/10.3390/min13081011

AMA Style

Park JK, Shin JY, Shin S, Park Y-H. Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts. Minerals. 2023; 13(8):1011. https://doi.org/10.3390/min13081011

Chicago/Turabian Style

Park, Jong Kyu, Ji Young Shin, Seungwon Shin, and Yong-Hee Park. 2023. "Preferred Orientations of Magnetic Minerals Inferred from Magnetic Fabrics of Hantangang Quaternary Basalts" Minerals 13, no. 8: 1011. https://doi.org/10.3390/min13081011

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