09:00 〜 09:30
[SCG44-01] Origin and significance of areas of low relief perched at high elevations in an arc-continent collision, Taiwan
★Invited Papers
キーワード：low-relief topography, exhumation, extrusion, high pressure metamorphism
Areas with relatively low topographic relief occur throughout the metamorphic core of the Taiwan orogen (Lin, 1957). Recent mapping shows that these areas occur at high and low elevations and are not correlated with structural, lithologic, or tectonostratigraphic units (Vohra, 2021; Ouimet et al., 2013). Instead, they dominate the lower elevations in the northern and southern parts of the range or are perched at high elevations along the drainage divide of the range.
We integrate detailed structural mapping with compilations of low-temperature geochronological data and propose the orogenic system developed in three stages, including a middle stage of low topographic growth in the metamorphic core when low-relief surfaces formed. The early stage of the orogen is well recorded by the development of a regional-scale, overturned syncline, which defines one of the primary structures in the core. For example, along a subhorizontal transect across the overturned limb, the oldest cooling ages of totally reset detrital ZrnFT grains are ca 6 Ma and come from the youngest stratigraphic units. In contrast, the youngest reset ages, ca 2 Ma, come from the oldest stratigraphic levels, suggesting progressive cooling as the rocks were structurally overturned (see modeling in Tan et al., 2024). Tectonic extrusion of a high-pressure metamorphic belt (i.e., Yuli Belt) was also initiated during this early stage, ca 4 to 3 Ma (Byrne et al., 2024; Chen et al., 2017).
The middle stage is marked by the cessation of overturning in the regional-scale syncline ca 0.9 Ma, extension throughout much of the core, local areas of low paleo-topography (Chang et al., 2023) in the core, and, in the foreland, the progressive development of the fold-and-thrust belt. Cessation of regional-scale folding is indicated by the consistent ZrnHe ages of totally reset detrital zircon grains collected across the overturned limb, which range from 0.7 to 0.9 Ma (Lee et al., 2022). In addition, ZrnHe ages of reset detrital grains separated from sandstones just below two of the larger low-relief surfaces also yield cooling ages of ca 0.9 to 1.0 Ma, suggesting the change in tectonic activity was regional. Age-elevation profiles from these two areas, including the 0.9 and 1.0 Ma samples and two additional profiles, show relatively rapid exhumation cooling starting 2 to 1 Ma and continuing to ca 0.25 Ma, with exhumation rates varying from 2.3 to 6.5 Km/Myr (Lee et al., 2022). Extension in the metamorphic core during the middle stage is recorded by a suite of late-stage veins, locally containing adularia, and numerous normal faults. 40Ar/39Ar ages of the adularia yielded three ages, 2.4, 1.5, and 1.1 Ma (Byrne et al., 2024; Chen et al., 2022). Pseudotachylite from one of the faults also yielded a 40Ar/39Ar age of 1.6 Ma (Chen et al., 2017). Simultaneous with extension and exhumation cooling in the core, the foreland expanded westward as the Shungtung, Chelungpu, and Changhua thrust faults formed at 1.1, 0.7, and 0.5 Ma, respectively (Huang et al., 2022).
We propose that the transfer of horizontal shortening from the metamorphic core to the foreland ca 2 to 1 Ma initiated a period of extension in the core, allowing erosion to outpace rock uplift and form low relief surfaces at low elevations. This regime change may have been partly driven by a substantial increase in the convergence rate between the PSP and EUR plates (Byrne et al., 2024), which accelerated the westward extrusion of the Yuli Belt. Extrusion was accommodated by extension at higher structural levels and growth of the fold-and-thrust belt at lower levels.
Finally, the youngest stage of orogen growth was initiated less than a few hundred thousand years ago. This resurgence in growth is evidenced by the current high rates of surface uplift in the center of the range (up to 10 mm/yr, Hsu et al., 2018), an AHe cooling age of 0.25 Ma obtained along one of the age-elevation profiles, and the uplifted low-relief surfaces.
We integrate detailed structural mapping with compilations of low-temperature geochronological data and propose the orogenic system developed in three stages, including a middle stage of low topographic growth in the metamorphic core when low-relief surfaces formed. The early stage of the orogen is well recorded by the development of a regional-scale, overturned syncline, which defines one of the primary structures in the core. For example, along a subhorizontal transect across the overturned limb, the oldest cooling ages of totally reset detrital ZrnFT grains are ca 6 Ma and come from the youngest stratigraphic units. In contrast, the youngest reset ages, ca 2 Ma, come from the oldest stratigraphic levels, suggesting progressive cooling as the rocks were structurally overturned (see modeling in Tan et al., 2024). Tectonic extrusion of a high-pressure metamorphic belt (i.e., Yuli Belt) was also initiated during this early stage, ca 4 to 3 Ma (Byrne et al., 2024; Chen et al., 2017).
The middle stage is marked by the cessation of overturning in the regional-scale syncline ca 0.9 Ma, extension throughout much of the core, local areas of low paleo-topography (Chang et al., 2023) in the core, and, in the foreland, the progressive development of the fold-and-thrust belt. Cessation of regional-scale folding is indicated by the consistent ZrnHe ages of totally reset detrital zircon grains collected across the overturned limb, which range from 0.7 to 0.9 Ma (Lee et al., 2022). In addition, ZrnHe ages of reset detrital grains separated from sandstones just below two of the larger low-relief surfaces also yield cooling ages of ca 0.9 to 1.0 Ma, suggesting the change in tectonic activity was regional. Age-elevation profiles from these two areas, including the 0.9 and 1.0 Ma samples and two additional profiles, show relatively rapid exhumation cooling starting 2 to 1 Ma and continuing to ca 0.25 Ma, with exhumation rates varying from 2.3 to 6.5 Km/Myr (Lee et al., 2022). Extension in the metamorphic core during the middle stage is recorded by a suite of late-stage veins, locally containing adularia, and numerous normal faults. 40Ar/39Ar ages of the adularia yielded three ages, 2.4, 1.5, and 1.1 Ma (Byrne et al., 2024; Chen et al., 2022). Pseudotachylite from one of the faults also yielded a 40Ar/39Ar age of 1.6 Ma (Chen et al., 2017). Simultaneous with extension and exhumation cooling in the core, the foreland expanded westward as the Shungtung, Chelungpu, and Changhua thrust faults formed at 1.1, 0.7, and 0.5 Ma, respectively (Huang et al., 2022).
We propose that the transfer of horizontal shortening from the metamorphic core to the foreland ca 2 to 1 Ma initiated a period of extension in the core, allowing erosion to outpace rock uplift and form low relief surfaces at low elevations. This regime change may have been partly driven by a substantial increase in the convergence rate between the PSP and EUR plates (Byrne et al., 2024), which accelerated the westward extrusion of the Yuli Belt. Extrusion was accommodated by extension at higher structural levels and growth of the fold-and-thrust belt at lower levels.
Finally, the youngest stage of orogen growth was initiated less than a few hundred thousand years ago. This resurgence in growth is evidenced by the current high rates of surface uplift in the center of the range (up to 10 mm/yr, Hsu et al., 2018), an AHe cooling age of 0.25 Ma obtained along one of the age-elevation profiles, and the uplifted low-relief surfaces.