[S-IT29] [EE] New perspectives on East Asia geodynamics from the crust to the mantle

[SIT29-P07] Dissolved helium isotopes in groundwater: Implication for subduction of continental crust in an active arc-continent collision

*Ai-ti Chen¹, Chuan-chou Shen¹, Timothy B Byrne², Yuji Sano³, Naoto Takahata³, Tsanyao Frank Yang⁴, Cheng-Hong Chen⁴, Tsung-Kwei Liu⁴, Kuan-Yu Chen⁵, Yunshuen Wang⁶ (1.High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, Taipei, Taiwan, R.O.C., 2.Center for Integrative Geosciences, University of Connecticut, Storrs, CT 06269-1045, United States, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8564, Japan, 4.Department of Geosciences, National Taiwan University, Taipei, Taiwan, R.O.C., 5.Industrial Technology Research Institute, Taipei, Taiwan, R.O.C., 6.Central Geological Survey, MOEA, Taipei, Taiwan, R.O.C.)

Keywords:Chauchou fault, noble gas, active tectonics, mantle-derived fluids

Here we use helium isotopes in groundwater from along the Chauchou/Tulungwan fault system in southern Taiwan to evaluate the involvement of the upper mantle in an active arc-continental collision. Helium isotope ratios of sixteen groundwater samples, four bedrock samples, and one hot spring sample, collected along 10 km on the fault, were measured and reported as R_A (relative to an air helium isotope ratio (³He/⁴He) of 1.39 × 10^-6). Measured groundwater helium isotope ratios, ranging from 0.07 to 1.00 R_A, express a clear mixing model with three endmembers of air (1.00 R_A), crust (0.06 R_A) and upper mantle (8 R_A). Samples from southern part of the fault show normal distribution of crustal signal. By correcting the helium from air-contamination, samples from northern part of the fault with 0.30-0.78 R_A reveal significant upper mantle signal. The Chauchou/Tulungwan fault system, in the area of detected mantle-derived fluids, projects down dip to a zone of ambient tremors and a nearby zone of high conductivity; both features extend to nearly 40 km, which is close to the crust-mantle boundary in this area. These observations suggest that mantle-derived fluids penetrate the crust through the zone of the tremor activity, reaching the surface long the Tulugwan fault zone. This study shows that non-volcanic, mantle-derived fluids can be involved in tectonic processes associated with an active arc-continent collision zone.