Eustatic change modulates exhumation in the Japanese Alps

The exhumation of bedrock is controlled by the interplay between tectonics, surface processes and climate. The highest exhumation rates of cm/yr are recorded in zones of highly active tectonic convergence such as the southern Alps of New Zealand or Himalayan syntaxes, where high rock uplift rates combine with very active surface processes. Here, we use a combination of different thermochronometric systems, and notably trapped-charge thermochronometery, to show that such rates also occur in the Hida Range, Japanese Alps. Our results imply that cm/yr rates of exhumation may be more common than previously thought.

The Hida Range is the most northern and most extensive of the Japanese Alps, and reaches elevations of up to 3000 m a.s.l. The Hida Range is thought to have uplifted in the last 3 Myr in response to E-W compression and magmatism. Our study focuses on samples from the Kurobe gorge, which is one of the steepest gorges in Japan. Previous work has shown that exhumation rates in this region are exceptionally high, as documented by the exposure of the ~0.8 Ma Kurobe granite (Ito et al., 2013) in the gorge. We combined 12 new zircon (U-Th/He) ages and 11 new OSL-thermochronometry ages together with existing thermochronometric data to investigate the late Pleistocene exhumation of this region.

We found that exhumation rates increased to ~10 mm/yr within the past 300 kyr, likely in response to river base-level fall that increased channel steepness due to climatically controlled eustatic changes. Our thermochronometry data allow the development of time-series of exhumation rate changes at the timescale of glacial-interglacial cycles and show a four-fold increase in baseline rates over the past ~65 kyr. This increase in exhumation rate is likely explained by knickpoint propagation due to a combination of very high precipitation rates, climatic change, sea-level fall, range-front faulting and moderate rock uplift. Our data show that in regions with horizontal convergence, coupling between climate, surface processes and tectonics can exert a significant effect on rates of exhumation.

References

Ito, H., Yamada, R., Tamura, A., Arai, S., Horie, K., Hokada T., 2013. Earth’s youngest exposed granite and its tectonic implications: the 10-0.8 Ma Kurobegawa Granite. Scientific Reports 3: 1306.