Cooling and Denudation History of the Rokko Area, Southwest Japan, Based on Fission-track Thermochronology

Abstract

Quantitatively estimating denudation is generally difficult because it essentially involves the removal and loss of materials in situ. The denudation rate of mountainous areas in Japan has commonly been studied from the volume of sediment in a basin or catchment. Nonetheless, the availability of these methods is constrained spatially by upstream area and temporally by depositional age. In the last few decades, thermochronometric methods that evaluate thermal history using radiometric-dating methods have been used to evaluate the denudation and tectonic history of orogenic belts around the world. The advantages of thermochronometric methods are that we can calculate the denudation rate at each sampling point and that combining multiple methods and/or target minerals enables us to calculate denudation rates in multiple periods. However, thermochronometric methods have been applied to areas with extraordinarily high denudation rates in island arc areas such as Japan. Thus, the effectiveness of thermochronometric methods for estimating denudation rates in island arc areas has not been demonstrated. We applied apatite and zircon fission-track thermochronometry to granitic rock samples collected from outcrops and a borehole to estimate the tectonic history of the Rokko area, southwest Japan. Previous studies suggested that the Rokko Mountains have been uplifted by active faulting along their northern and southern margins during the Rokko movements, a Quaternary tectonic movement in the Kinki district. However, the tectonic history of the Rokko area prior to 1 Ma has not been revealed due to a lack of prevalent geologic markers. We estimated average denudation rates in various periods based on apatite and zircon FT ages and previously reported radiometric ages. We obtained denudation rates at about 0.04-0.10 mm/yr after about 30 Ma,

Spatial distribution of the apatite fission‐track ages in the Toki granite, central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin

“…The Rokko granite has been dated at 77.8 ±1.1 Ma (n = 1: whole rock Rb–Sr; Terakado & Nohda, ) and at 72.9 ±3.9 Ma and 71.6 ±3.6 Ma (n = 2: biotite K–Ar; Huzita & Maeda, ), which are emplacement ages similar to that of the Toki granite. The relation between the AFT age and closure depth gives an exhumation rate of about 0.04 mm/year for the Rokko granite at 30.8–19.5 Ma (n = 8: Sueoka et al, ), which is an order of magnitude smaller than that of the Toki granite at 50–40 Ma. The Toki granite underwent continual slow exhumation at less than 0.16 ±0.04 mm/year after 40 Ma, which can be similar to such a slow exhumation obtained from the Rokko granite (0.04 mm/year).…”

“…Such a cooling pattern is attributable to the exhumation related to the regional tectonic history (e.g. Lim & Lee, ; Nakajima et al, ; Sueoka et al, ). Thus, the predominant constraint on the cooling of the Toki granitic pluton under the conditions at the AFT PAZ was exhumation in the East Asian continental margin.…”

“…A granitic pluton under high‐temperature conditions in the post‐intrusion/emplacement event is dominantly cooled by thermal release into the neighborhood (country) rocks because of the thermal difference between these rocks (Okudaira et al, ; Yuguchi, Miyazoe, & Nishiyama, ). The thermal difference between a granitic pluton under low‐temperature conditions around the AFT PAZ and country rocks is very small; this shows that the cooling of the granitic pluton is closely related to its regional exhumation history (Lim & Lee, ; Nakajima, Shirahase, & Shibata, ; Sueoka et al, ). The temperature decrement is constrained by the geothermal gradient with the elevation of the pluton; therefore, the low‐temperature cooling process of the granitic body, deduced from the AFT thermochronology, indicates a method to evaluate the exhumation characteristics of the pluton.…”

Denudation history of the Kiso Range, central Japan, and its tectonic implications: Constraints from low‐temperature thermochronology

“…1a), such as 40–30 Ma in the southern Shiga and Rokko areas (Nishimura & Mogi 1986); a mean age of 54.2 Ma in the Ina, Ena, and Ichishi areas (Tagami et al . 1988a); 59.2–39.9 Ma in the Sakai‐touge Fault area, including sandstone samples (Ito & Takahashi 2008); and 65.7–40.9 Ma in the Rokko area (Sueoka et al . 2010).…”

“…With respect to the Kinki and Chubu regions where the Hida area and Kiso Range are located, Goto (2001) ( Fig. 1a), such as 40-30 Ma in the southern Shiga and Rokko areas (Nishimura & Mogi 1986); a mean age of 54.2 Ma in the Ina, Ena, and Ichishi areas (Tagami et al 1988a);59.2-39.9 Ma in the Sakai-touge Fault area, including sandstone samples (Ito & Takahashi 2008); and 65.7-40.9 Ma in the Rokko area (Sueoka et al 2010). Most of the older AFT ages obtained in this study are also equivalent to these Paleogene age ranges.…”