Detrital Mineral Thermochronology of the Indus Molasse, Northwestern Indian Himalaya

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This project will use detrital mineral thermochronology to greatly improve our understanding of the age range, depositional history, and thermal evolution of an important sedimentary succession that constrains the timing of the continent-continent collision between Eurasia and India. Continental sediments of the Indus Group of northwest India provide a critical constraint on the age of the collisional event. The generally accepted beginning of deposition of the Indus molasse in Lutetian-Ypresian time is not explicitly supported by paleontological or geochronological data. Other than the fact that most of the molasse must be younger than Ypresian limestones, age-diagnostic fossils or volcanic horizons have not been identified throughout most of the section, and the stratigraphic position of the only well-documented fossil assemblage from the Indus Group (Upper Oligocene ostracods) is debatable. There are no firm constraints on the ages of any deposits in this important sequence. The principal tools of this work will be laser fusion 40Ar/39Ar biotite thermochronology and both laser heating and laser ablation (U-Th)/He zircon thermochronology. The first method capitalizes on the wide distribution of biotite in Indus Group sandstones and on the fact that post-depositional metamorphism of the sequence was at a low enough temperature to eliminate the possibility of thermal resetting of the 40Ar/39Ar biotite chronometer. Thus, the youngest detrital biotites in each sample provide an upper boundary of the sample's depositional age. The (UTh)/He zircon chronometer, on the other hand, has a closure temperature range low enough that it should have been reset by post-depositional metamorphism in the highest-grade portions of the Indus Group sequence. The zircon ages from these parts of the sequence will provide a lower boundary on depositional ages of all parts of the sequence, since the entire depositional basin experienced epizonal-anchizonal metamorphism at that time. In addition to these important brackets on the age of sedimentation, the results of this study will provide better insights into the provenance of Indus Group sediments, the likely developmental age of the modern Indus River drainage system, and the age of regionally important deformation that accompanied epizonal-anchizonal metamorphism.Broader ImpactsThe project will involve both graduate-level and undergraduate-level education through research participation. A female graduate student will make this research the principal focus of her Ph.D. dissertation. With regard to research and education infrastructure, the proposed work reinforces international research relationships among Arizona State and foreign research institutions in India and the United Kingdom.