Late Palaeogene emplacement and late Neogene-Quaternary exhumation of the Kuril island-arc root (Kunashir island) constrained by multi-method thermochronometry

The Kuril islands constitute a volcanic island arc-trench system, stretching from eastern Hokkaido （Japan） to Kamchatka （Russia） along the northwestern Pacific subduction system, The current arc consists of several volcanic islands mainly with Neogene basement and capped by several, predominantly andesitic, active subduction stratovolcanoes, Kunashir Island is the southwestern-most island of the arc, just off the Hokkaido coast and represents the study area in this paper. The island is composed of a Lower Complex of mainly late Miocene to Pliocene volcanic rocks, covered by an Upper Complex of younger （basaltic） andesitic lava flows and tuffs on which currently four active volcanic edifices are built. In the Lower Complex sub-volcanic and deeper-seated intrusives of the so-called Prasolov and Dokuchaev magmatic complexes are found, More differentiated, tonalitic-granodioritic rocks were collected from these small intrusive bodies. An early Oligocene zircon LA-ICP-MS U/Pb age of 31 Ma for the Prasolov Complex was obtained, showing that the basement of Kunashir Island is older than previously thought. Thermochronometry （apatite fission-track and U-Th-Sm/He and zircon U-ThJHe analyses） further shows that the magmatic basement of the island was rapidly exhumed in the Pleistocene to present levels in a differential pattern, with He-ages ranging from 1.9 to 0.8 Ma. It is shown that the northern section of the island was hereby exhumed more intensely.

Progress in Methodology of Low Temperature Thermochronometry

Objective Thermochronometry is based on the temperaturedependent radiogenic/fissionogenic products（e.g.,fission track or a particle）or the radiation effects of a radioactive isotopic system for a single mineral.The characteristics of a thermochronological system can be described by its closure temperature.

Tectono-thermal Evolution in the Bachu Uplift,Tarim Basin,China

The thermal evolution of source rocks in the Paleozoic has long been a problem to petroleum exploration in the Bachu uplift, Tarim basin, since the thermal history in the Paleozoic could not be rebuilt objectively due to lack of effective thermal indicators in the Lower Paleozoic successions. The apatite and zircon （U-Th）/He thermochronometry can be used as a new kind of technique to study the thermal history and tectonic uplift of sedimentary basins. Based on the measured apatite and zircon （U-Th）/He ages, apatite fission track data and equivalence vitrinite reflectance （%EVRo）, the tectonothermal histories in 5 wells of the Bachu uplift were modeled. The modeling results show that there was relatively high gradient at the Early Paleozoic in the Bachu uplift and it decreased gradually during the entire Paleozoic： 33-35℃/km in the Cambrian- Ordovician, 32-33℃/km in the Silurian-Devonian, 30-32℃/km at the end of Carboniferous and 27.5- 31℃/km at the end of Permian. Therefore, the thermal history can be modeled by combining multiple thermal indicators of AFT, （U-Th）/He ages and EVRo data. Especially, this provides a new method to rebuild the thermal history for the Low Paleozoic carbonate successions in the Tarim Basin.

Spatio-temporal variation in rock exhumation linked to large-scale shear zones in the southeastern Tibetan Plateau

Crustal-scale shear zones are believed to have played an important role in the tectonic and landscape evolution of orogens. However, the variation of long-term rock exhumation between the interior of shear zones and adjacent regions has not been documented in detail. In this study, we obtained new zircon U-Pb, biotite ^(40)Ar/^(39)Ar, zircon and apatite(U-Th)/He data, and conducted inverse thermal history modeling from two age-elevation profiles(the Pianma and Tu'er profiles) in the southeastern Tibetan Plateau. Our goal is to constrain the exhumation history of the Gaoligong and Chongshan shear zones and adjacent regions, so as to explore the effect of the shear zones on exhumation and their thermal effect on cooling that should not be ignored. Our results suggest that during the interval of 18–11 Ma the exhumation rates of rocks within the Gaoligong shear zone are anomalously high compared with those outside of. The rapid cooling during 18–11 Ma appears to be restricted to the shear zone, likely due to localized thermal effects of shearing and exhumation. After 11 Ma, both the areas within and outside of the shear zones experienced a similar two-stage exhumation history: slower cooling until the early Pliocene, and then a rapid increase in cooling rate since the early Pliocene. Our results indicate a synchronized exhumation but with spatially varied exhumation rates. Our study also highlights the important role of large-scale shear zones in exposing rocks, and thus the importance of the structural context when interpreting thermochronological data in the southeastern margin of the Tibetan Plateau.

Diffusion of helium in FCT zircon

To gain a better quantitative understanding of zircon(U-Th)/He ages and evaluate the applicability of zircon(U-Th)/He dating, the diffusion characteristics, He diffusion kinetics, helium partial retention zone(HePRZ), closure temperature, and(U-Th)/He ages were investigated using high-precision laboratory step heating experiments based on the thermally activated diffusion process. The ln(D/a^2) in Fish Canyon Tuff(FCT) zircons determined from laboratory step heating experiments was negatively correlated with reciprocal temperature, as expected for thermally activated volume diffusion. The zircon activation energies ranged from 144 to 184 kJ mol^(-1) with a mean of 169±12 kJ mol^(-1). The closure temperatures ranged from 144 to 216°C(a cooling rate of 10°C Ma^(-1) and an effective grain radius of 38–60 μm) with an average of 176±18°C. The calculated closure temperature increased with increasing cooling rate, yielding an average zircon He closure temperature of ～136°C at a slow cooling rate of 0.1°C Ma^(-1), whereas the closure value was ～199°C at a cooling rate of 100°C Ma^(-1). The closure temperature increased with the equivalent spherical radius assuming a constant cooling rate. The He ages from FCT zircons were negligibly affected by grain size because of the rapid cooling. He preserved in the zircon was sensitive to temperature and holding time, and the temperature range for zircon HePRZ gradually decreased with increasing holding time. The(U-Th)/He ages from 26 FCT zircons yielded an algorithmic mean of 28.3±0.3 Ma(S.E.) and a geometric mean of 28.4±0.3 Ma(S.E.), consistent with the ages of 28.4±1.9 Ma reported by other laboratories. The FCT zircons were characterized by rapid cooling, young(U-Th)/He ages with good reproducibility, and low alpha doses. Weak correlations between the He ages and effective uranium(eU) concentrations from the FCT zircons indicated radiation damage did not significantly affect He diffusivity.