10:15 〜 10:30
[SCG44-04] Can normalized channel steepness index (ksn) be utilized to identify the uplift pattern of young mountains along with thermochronology?
キーワード：地形学、熱年代、山地、急勾配度、地形発達モデル
Normalized channel steepness index (ksn) is a geomorphic index that correlates with the rock uplift rate and the bedrock erodibility along steady-state rivers [1]. ksn has been extensively employed as a proxy for the exhumation rate and its spatial distribution across mountain ranges, primarily due to the accessibility of digital elevation models (DEM). Nevertheless, the applicability of the ksn mapping for young island arc mountains remains unverified by sufficient empirical evaluations. In this study, we mapped ksn for Japanese mountains that have previously been the subject of thermochronological research, and for numerically simulated mountains using a landscape evolution model (LEM) in order to examine the question of whether it is possible to determine the uplift pattern of mountains from the spatial distribution of ksn.
Recent advancements in thermochronological studies have indicated two end-members of rock uplift in island arcs: 1) pop-up uplift of fault blocks and 2) domal uplift. With the aim of confirming the validity of ksn mapping, we used the LEM to reproduce the building of a mountain topography with these types of uplift, and to investigate a diachronic change in ksn. TTLEM [6] was employed to generate pop-up and domal uplift velocities in a model space of 30 km on each side, and the spatial distribution of ksn was examined over a period of 2.6 Myr. The models demonstrated a ksn distribution that exhibited a strong correspondence with the uplift velocity distribution in both models, with this distribution being obtained after 0.5 Ma. Next, to verify the applicability in the young mountains, the ksn mapping was carried out in the Kiso and Iide mountains, where thermochronological research had previously identified typical examples of pop-up and domal uplift, respectively [2, 3]. The 30 m mesh DEM of AW3D30 [4] was employed for each mountain range, with TopoToolbox [5] utilized for the extraction of river networks and slope data, and the calculation of ksn. In the Kiso mountains, ksn increased sharply at both edges of the mountain range towards the core, with the maximum value being observed at a point that deviated from the main ridge line. Conversely, a gradual increase in ksn was observed towards the central part of the Iide mountains. In both ranges, the spatial distribution of ksn was approximately consistent with the distribution of denudation rates indicated by thermochronological studies.
It is anticipated that the ksn mapping has the potential to complement of the quantitative exhumation rate of specific points estimated by thermochronological method and facilitate constraints on regional uplift rate distribution. Conversely, in these mountainous regions, there were local ksn abrupt changes in inactive faults, lithological boundaries, landslide topography, and dam lakes. Additionally, prior to the attainment of dynamic equilibrium in the mountainous terrain, the spatial distribution of ksn is likely to be noisy, thereby making it challenging to estimate the uplift form. When estimating the uplift form from the spatial distribution of ksn, it is necessary to carefully consider the geological and topographical background of individual mountains.
[1] Wobus et al. (2006), Special Paper of the Geological Society of America, 398, 55–74.
[2] Sueoka et al. (2012), Island Arc, 21, 32–52.
[3] Fukuda et al. (2024), Fission Track Research Group in Japan, Abstract.
[4] ALOS World 3D-30 m Ver. 4. 1. (2024), Japan Aerospace Exploration Agency.
[5] Schwanghart & Scherler (2014), Earth Surface Dynamics, 2(1), 1–7.
[6] Campforts et al. (2017), Earth Surface Dynamics, 5, 47–66.
Recent advancements in thermochronological studies have indicated two end-members of rock uplift in island arcs: 1) pop-up uplift of fault blocks and 2) domal uplift. With the aim of confirming the validity of ksn mapping, we used the LEM to reproduce the building of a mountain topography with these types of uplift, and to investigate a diachronic change in ksn. TTLEM [6] was employed to generate pop-up and domal uplift velocities in a model space of 30 km on each side, and the spatial distribution of ksn was examined over a period of 2.6 Myr. The models demonstrated a ksn distribution that exhibited a strong correspondence with the uplift velocity distribution in both models, with this distribution being obtained after 0.5 Ma. Next, to verify the applicability in the young mountains, the ksn mapping was carried out in the Kiso and Iide mountains, where thermochronological research had previously identified typical examples of pop-up and domal uplift, respectively [2, 3]. The 30 m mesh DEM of AW3D30 [4] was employed for each mountain range, with TopoToolbox [5] utilized for the extraction of river networks and slope data, and the calculation of ksn. In the Kiso mountains, ksn increased sharply at both edges of the mountain range towards the core, with the maximum value being observed at a point that deviated from the main ridge line. Conversely, a gradual increase in ksn was observed towards the central part of the Iide mountains. In both ranges, the spatial distribution of ksn was approximately consistent with the distribution of denudation rates indicated by thermochronological studies.
It is anticipated that the ksn mapping has the potential to complement of the quantitative exhumation rate of specific points estimated by thermochronological method and facilitate constraints on regional uplift rate distribution. Conversely, in these mountainous regions, there were local ksn abrupt changes in inactive faults, lithological boundaries, landslide topography, and dam lakes. Additionally, prior to the attainment of dynamic equilibrium in the mountainous terrain, the spatial distribution of ksn is likely to be noisy, thereby making it challenging to estimate the uplift form. When estimating the uplift form from the spatial distribution of ksn, it is necessary to carefully consider the geological and topographical background of individual mountains.
[1] Wobus et al. (2006), Special Paper of the Geological Society of America, 398, 55–74.
[2] Sueoka et al. (2012), Island Arc, 21, 32–52.
[3] Fukuda et al. (2024), Fission Track Research Group in Japan, Abstract.
[4] ALOS World 3D-30 m Ver. 4. 1. (2024), Japan Aerospace Exploration Agency.
[5] Schwanghart & Scherler (2014), Earth Surface Dynamics, 2(1), 1–7.
[6] Campforts et al. (2017), Earth Surface Dynamics, 5, 47–66.