Nuclear Tracks And Radiation Measurements最新文献

Fission-track (FT) dates of apatate and zircon recording, respectively, paleo-temperatures of 100–120°C and ∼230°C provide a useful tool for understanding the thermotectonic history of the Himalaya, which is the world's loftiest and most dynamic orogenic belt, brought about by the continent-to-continent collision of the Indian and Asian plates. Systematic FT ages are now available for the crystalline rocks of the Higher Himalaya and Trans-Himalaya in Pakistan and N.W. India, and some FT ages have been reported from southern Tibet, the Karakorum and the Kunlum. These FT data are synthesized and discussed here in order to assess the contribution of FT thermochronology to Himalayan geology. FT zircon and apatite ages from the Higher Himalayan Crystalline rocks are confined to the Neogene. For example, in the Zanskar region of N.W. India and the Babusar-Kaghan region of Pakistan, zircon ages are 13–17 Ma and apatate ages range from 4–10 Ma (mostly ∼6 Ma) indicating an exhumation of at least 8 km for these rocks since the Middle Miocene and average unroofing rates of ∼ 6 mm yr⁻¹. In the northwestern Himalaya, overall the FT ages become younger towards the Nanga Parbat Massif, which is the promontory of the Indian plate and has experienced a rapid Quaternary uplift and exhumation. Other areas of young uplift in the Himalayan include antiformal domes in Zanskar, such as the Kishtwar Window. The bulk of the Cretaceous-Paleocene Trans-Himalayan crystalline rocks, which are situated to the north of the Indus-Tsangpo Suture Zone and were generated from the consumption of the Tethyan ocean beneath Asia, seem to have cooled rapidly through paleo-temperatures of 200–500°C in the Eocene as a result of uplift and erosion that affected the Trans-Himalayan Batholith after the India-Asia collision. Apatite ages from these rocks in western Kohistan, southern Ladakh, and Lhasa are Early-Middle Miocene. In the Karakorum (western Tibet) and the Kunlun (northern Tibet), Miocene apatite ages demonstrate young tectonics of the ranges beyond the Himalaya related to the India-Asia convergence.

Apatite fission-track analysis of samples collected along the length of theSouth Mountains metamorphic core complex and in a vertical profile from the adjacent Sierra Estrella reveal rapid cooling during regional crustal extension. Fission-track ages of 17 samples from South Mountains overlap at the 2σ level and have a weighted mean of 17.5 ± 1.0 Ma. Mean fission-track lengths are all greater than 14 μm, indicating rapid cooling at about this time. Integrating these data with K-Ar and ⁴⁰Ar/³⁹Ar cooling ages of hornblende and biotite yields an average cooling rate of approximately 190°;C/my between 21 and 17 Ma for lower-plate rocks in the South Mountains. Four samples collected over 600 m of relief from the adjacent Sierra Estrella yield apatite ages with a weighted mean of 24.7 ± 0.4 Ma and mean track lengths greater than 14 μm, which also reflect rapid cooling. Geologic constraints suggest that the Sierra Estrella is most likely lower-plate with respect to the South Mountains detachment fault and that its uplift/cooling history is linked to the detachment-style denudation of the South Mountains core complex. If cooling simply reflects tectonic denudation of upper-plate rocks, the calculated rate of extension for the core complex is ≈ 0.3 cm/yr, a rate comparable to those estimated for other core complexes.