TIMING OF GEODYNAMIC PROCESSES IN ARUNACHAL HIMALAYA USING Rb-Sr AND FISSION TRACK AGES

Abstract

Recent thermochronological works in different sectors ofthe Himalayan orogenic belt highlighted the positive feedback between tectonics and exhumation, while high precipitation and climate-induced erosion control the exhumation, and still remain controversial. The present work focuses this aspect in parts ofthe Arunachal Himalaya in the east with the following objectives (Chapter 1-Introduction). (i) Cooling and exhumation histories of different tectonic units of the Arunachal Himalaya in relation to major faults in the Subansiri and Siyom catchment areas, using Rb-Sr mica and Fission Track - zircon and apatite ages, (ii) Source rock characterization from present-day river detrital sediments, using Apatite Fission Track ages, (iii) To investigate the factors controlling the exhumation, i.e. the tectonics, the erosion or a combination of the both in Arunachal, which receives almost twice the rainfall during the Indian Monsoon in contrast to the NW Himalaya. The Arunachal Himalaya is characterized by the Sub-Himalayan Siwalik Belt, the Lesser Himalayan Sedimentary Belt, the Himalayan Metamorphic Belt (HMB), the Tethyan Sedimentary Zone, the Indus Tsangpo Suture Zone (ITSZ) and the Trans- Himalayan Batholiths between 26° 30':29 ° 30' Nand 91° 30':99° 30'E (Chapter 2- Geological Framework). The main core of the Eastern Himalayan Syntaxis lies just beyond this region in the northeast as a major antiform. All these tectonic units trend ENE-WSW in the west and swing to the NE-SW before bending to NW-SE along the Siang gorge. Anewly-compiled geological map of the Arunachal Himalaya and the adjoining regions by Singh and Jain (2007) is the base for the mutual geological relationships and thermochronological works in the present study. In the Subansiri Valley, the Siwalik Group ofthe Cenozoic Himalayan foreland basin rises abruptly over the Brahmaputra Alluvium along the Main Frontal Thrust (MFT). Further north, the Permian Gondwana Belt overrides this belt along the Main Boundary Thrust (MBT). The Lesser Himalayan metasediments are separated from the Himalayan Metamorphic Belt (HMB) along the folded Main Central Thrust (MCT). The southernmost exposure of the MCT is locally named as the Tamen Thrust. In the upper reaches ofthe Subansiri River, the folded HMB gets eroded of, leading to the exposure of the underlying Lesser Himalayan sequence in two windows. The MCT is again exposed near Taliha and Nacho, and separates the high grade metamorphics of the Higher Himalayan Crystalline (HHC) belt from the Lesser Himalayan (LH) sedimentary belt. Further north, the HHC unit is separated from the Tethyan Sedimentary Sequence by the South Tibetan Detachment Zone (STDZ). Cooling and exhumation histories of different tectonic units of the Arunachal Himalaya have been investigated in the Subansiri and Siyom catchment areas, using Rb- Sr mica and Fission Track (FT) - zircon and apatite ages so as to cover the cooling temperature range between about 500 to 120° C (Chapters 3 and 4). For the Rb-Sr dating of muscovite and biotite, 19 gneiss and gneissose granite samples were selected from the Himalayan Metamorphic Belt (HMB) and Lesser Himalaya (LS) of the Arunachal Himalaya; out ofwhich 18 biotite and 4muscovite mineral ages were obtained against the respective whole-rock isotopic ratios These samples were analyzed by the Thermal Ionization Mass Spectrometer (TIMS, TRITON Tl) after standard sample preparation, isotopic dilution, ion chromatography etc. (Chapter 3-Rb-Sr Dating). The biotite ages spans from 6.85±0.07 to 18.73±0.02 Ma, whereas the muscovite ages vary from 10.32±0.17 Ma to as old 24.90±0.02 Ma. Out ofthese, the oldest mica ages were obtained from the Lesser Himalayan gneissose granite, which intruded the Khetabari IV x Formation'. The Daporijo Gneiss ofthe Lesser Himalayan metamorphic belt yielded ages between 11.1±0.02 and 14.80±0.02 Ma (5 samples from western Kimin-Koluriang Section), while these are somewhat older as 15.18±0.30 and 15.77±0.16 Ma in the Subansiri River Section (3 samples). Muscovite could be dated only from one Daporijo Gneiss sample, yielding an age of 22.68±0.02 Ma in the extreme west. In contrast to the Lesser Himalayan Metamorphic Belt, the HHC metamorphics revealed adistinct drop in biotite and muscovite ages to 9.19±0.02 Ma and 20.36±0.08 Ma, respectively on the hanging wall of the MCT in the western section. In the middle section, biotite ages were between 7.24±0.09 and 8.24±0.09 (3 samples), while these range between 6.85±0.07 and 10.53±0.21 Ma (4 samples) with an exception 17.83±0.21 Ma at the base ofthe HHC in the eastern Siyom Valley. Chapter 4-Fission Track Dating deals with the FT dating of bedrock across most of the tectonic units of the Arunachal Himalaya, where atotal of 22 AFT (Apatite Fission Track) and 16 ZFT (Zircon Fission Track) ages were generated from 26 samples along three traverses, using the EDM method and the Zeta calibration approach. The AFT ages from the HHC belt range between 2.2±0.3 and 6.0±0.6 Ma with a northward younging. The ZFT ages fall between 3.3±0.3 and 7.9±0.4 Ma. Within the Lesser Himalayan windows, the AFT and ZFT ages are 2.0±0.3 and 5.6±0.5 Ma, respectively. These are in contrast to the AFT data between 5.1±0.6 and 12.1±1.2 Ma from the overthrust Lesser Himalayan Daporijo Gneiss, which reveals astring-shaped pattern. A ZFT age of 13.2±0.7 Ma from this belt is the oldest from the Arunachal Himalaya samples under consideration. The Lesser Himalayan intrusive granite gave an AFT age of 4.7±0.4 Ma and a quartzite sample an age of 11.0±0.6 Ma. Azircon FT age from the quartzite sample yielded an age of 12*0.8 Ma. APermian gritty sandstone sample from within the Gondwana Belt yielded areset AFT age of8.5*1.1 Ma. Apatite FT detrital thermochronology of the Subansiri and Siyom rivers sands as well as the Upper Siwalik Group (Pliocene-Pleistocene) on 436 apatite grains (Chapter 5-Detrital AFT Thermochronology) from the 10 samples have been undertaken with the objective to decipher the source rocks ofthese modern sands. Individual AFT grain ages range from 0.4 to 39.0 Ma and reveal the time elapsed since it attained its closure temperature of 135° C. These mixed population of different ages were reduced to individual peaks between 1.9 to 19.5 Ma, using the BINOMFIT software (Brandon, 2002), and have been assigned to different sources in the Higher and Lesser Himalayas. Thermochronological mineral ages are the proxies for cooling and exhumation rates since these attained their respective closure temperatures (Chapter 6-Discussions and Conclusions). These rates have been calculated for the FT mineral ages, described in Chapter 4, using AGE2EDOT programme for ID modelling. Cooling rates, calculated from the closure temperatures of AFT, ZFT, Rb-Sr biotite and muscovite thermochronometers, indicate spatio-temporal variations in cooling patterns across the Himalayan orogen in Arunachal. The cooling rates vary between 204.9° C/Ma and 6.8° C/Ma. The exhumation rates vary between 0.25 mm/yr to 1.2 mm/yr, using ID numerical modelling and confirm their correlation with the structural layout. The fastest exhumation rate is observed in the LH windows and has been followed by the HHC. The LH sedimentary sequences and LH crystallines sequences experienced slower exhumation rates. Hence, cooling and exhumation of different lithotectonic units in the Arunachal Himalaya are mainly controlled by the folded MCT and simultaneous doming, thus providing evidences for tectonics forcing on exhumation. Detrital AFT thermochronology of river sediments of the Subansiri and Siyom Rivers along with the Upper Siwalik Group highlighted the source characteristics of the vi sediments carried by these rivers. Correlation of different AFT peak ages with bedrock AFT ages from this region showed the contribution from all the lithounits of this area. Thermochronological FT age distribution in different parts of the NW Himalaya reveals that young ages are mainly concentrated inthe HHC rocks, separated from the LH sedimentary belts by the MCT. Besides this, young ages are also observed within the HHC domes and the LH windows, which are exposed due to the folding ofthe MCT and simultaneous erosion. These regions consistently experienced high exhumation rates with a strong east-west similarity. A strong thermochronological evidence for tectonic control on the exhumation ofHimalayan orogen is thus indicated in the present work.