探討碰撞帶的熱事件: 以熱定年方式探討西北喜馬拉雅和菲律賓中部之構造事件

Abstract

Abstract The work presented in this dissertation is the outcome of research carried out in the Himalaya and the Central Philippine. Rock samples were collected from the studied areas and a combination of radiometric dating techniques was applied in an effort to get better insight into various aspects in respective areas. New fission track data document the cooling and exhumation history of Kulu-Beas Valley in NW Himalaya, India. There is a large set of existing thermochronological data from the eastern and northern parts of NW Himalaya, while, there is very little thermochronological data from the southern part. For this study, (between lattitude 31°30’-32°20’ and longitude 76°30’-77°15’), the cooling history through 250°C-110°C has been investigated by FT chronology on zircon and apatite. The young Pliocene-Pleistocene cooling ages obtained in this study are in good harmony with the ages obtained from neighboring Sutlej Valley and these ages show very good correlation with the precipitation in this part. It is suggested, based on our data that the “zone of high erosion” that lies in the Sutlej Valley extends up to the Beas Valley. In addition, the pattern of ages suggests non-uniform exhumation for different units of NW Himalaya. Late Miocene cooling ages from the fault zone are related to the activity on the thrusts present in the area. Tectonic reconstructions of the Philippines are severely hampered by a paucity of geochronologic and exhumation age constraints. First time reported apatite and zircon fission track, 40Ar/39Ar and U/Pb data from Central Philippines place time constraints on the collision of Palawan Continental Terrane (PCT) with Philippine Mobile Belt (PMB). In Mindoro, the geochronological data from different parts of the metamorphic complex show different age patterns. From the north coast, we obtained Permian U/Pb ages while from the southern exposure of the metamorphics, the ages are very young. Moreover, old inherited zircons are reported from the west and also from the southern part but such old zircons are not observed in the samples from the north coast. U/Pb ages from the metamorphics of Palawan are distinctly different in ages. Results are compared with the age obtained from Tablas Island where Romblon Metamorphics are exposed. The results reveal diverse protolith ages for metamorphics in these islands. In addition, the data provide clues for the provenance of detrital zircons from Panay Island, Mindoro Island and Palawan. The very young cooling ages, as constrained by low-temperature fission track dating, indicate recent and non-uniform exhumation. Layout of the Thesis This dissertation is divided into eight chapters. The different chapters are designed to address problems pertaining to individual areas. Chapter 1 provides an introduction about the areas and the potential problems I have tried to deal with in this study. Here, I also discuss, in short, about the existing problems that need to be addressed. Chapter 2 deals with the methodology used in this study. I provide detailed description of each method used and their principles. Chapter 3 describes the regional geology of the areas in general. Various units of Himalaya and tectonic setting of the Philippine is discussed here. Chapter 4 gives the details of work done in NW-Himalaya and the results obtained using FT dating. The results have revealed very young exhumation in this part of the NW Himalaya. In Chapter 5, I discuss about the samples collected from Panay Island, which were analyzed to constrain various issues related to collision of Palawan Continental Terrane with Philippine Mobile Belt. These samples were dated using a combination of Fission Track Dating, 40Ar/39Ar Dating and U/Pb Dating methods. North-east of the Panay lies the island of Mindoro. NW Mindoro and later East Mindoro was targeted in an attempt to use thermochronology in revealing its affinity to PCT and the relation of detrital zircons with South China during Mesozoic (Chapter 6). The results reveal diverse protolith ages for the Mindoro Metamorphics. In Chapter 7, the results of U/Pb dating from Central Palawan are discussed. Chapter 8 summarizes the results and interpretations.