Denudation history constrained by thermochronological multi-inversion; An Example from the eastern Nepalese Himalaya

Nakajima, Toru; Kawakami, Tetsuo*; Iwano, Hideki*; Danhara, Toru*; Sakai, Harutaka*

no journal, , 

Understanding the processes of continental crustal denudation in a collision zone has advanced significantly with the development of thermochronological methods. Currently, the most common technique for reconstructing denudation history is to investigate the distribution pattern of a cooling age on elevations or transects. However, this technique is considered to be insufficient mainly in terms of temporal resolution in a collision zone with spatiotemporal changes in the denudation rate. In this study, we attempted to improve the temporal resolution of the reconstructed thermal history of Himalaya by performing the thermochronological inversion, and to quantitatively reconstruct the spatiotemporal variation of the denudation rate by thermokinematic modeling. Thermochronological inversion was undertaken for new results of fission-track (FT) age and FT length data of zircon and apatite in order to reconstruct the time-temperature (t-T) paths in eastern Nepal. Eight t-T paths calculated along the across-strike section show that the cooling process of the upper crust in this study area is characterized by: 1) gradual cooling followed by rapid cooling and subsequent gradual cooling, 2) northward-younging of the timing of the rapid cooling. The observed FT ages and t-T paths were then compared with those predicted by forwarding thermokinematic modeling. The result of the thermokinematic modeling indicates that:1) the cooling pattern observed in the field mainly occurred when the upper crust overthrusted accompanied with the activity of the plate boundary fault showing the flat-ramp-flat geometry, 2) and its geometry has been stable since the underplating of the Indian continental plate before 9 Ma.

Termination criterion of step-etching in monazite fission-track thermochronometry; Toward the realization of etching of various characteristic monazites

Fukuda, Shoma; Nakajima, Toru; Suzuki, Ayase*; Sueoka, Shigeru; Tagami, Takahiro*

no journal, , 

The monazite fission-track method has been expected to be a powerful tool as an ultra-low temperature (below 50 degrees Celcius) thermochronometers. Jones et al., (2019) proposed the optimum for the MFT etching (6 M HCl at 90 degrees of Celcius for 60-90 min) by using 252Cf-derived semi-tracks of the Devonian monazite. In our preliminary experiments, spontaneous tracks in the Cretaceous monazites of Japan were successfully etched within 90-120 min as FT densities attain a plateau. However, Quaternary monazites in Japan yielded few and very thin (i.e., under-etched) spontaneous tracks even after step-etching for 600 min. Our results from Raman spectrometry and EPMA analyses suggested that less-damaged monazites tend to require a longer etching time. Therefore, we proposed a termination criterion for MFT step-etching: to etch until the thinnest track width reaches approximately 1.0 0.5 micron, similar to zircon FT step-etching. This 1-micon width criterion is valid regardless of monazite characteristics, e.g., radiation damage and chemistry.

Thermochronological constraints on the thermal evolution of magmatic rocks associated with Chelopech Au-Cu deposit, Bulgaria

Balkanska, E.*; Georgiev, S.*; Marton, I.*; Kounov, A.*; Peytcheva, I.*; Anti, M.*; Tagami, Takahiro*; Sueoka, Shigeru; Wijbrans, J.*

no journal, ,