Date of Final Oral Examination (Defense)
Type of Culminating Activity
Master of Science in Geology
Matthew J. Kohn, Ph.D.
Sean P. Long, Ph.D.
C. J. Northrup, Ph.D.
V. Dorsey Wanless, Ph.D.
Temperature-time (T-t) paths derived from coupled Zr-in-titanite thermometry and U-Pb geochronology are useful for examining middle-crustal tectonic processes. T-t data collected from the titanite-bearing calcsilicates of the upper Greater Himalayan Sequence (GHS) along the Marsyandi River, Annapurna Himalaya, illustrate a significantly different metamorphic history than previously recorded in central Nepal. Combined with traditional thermobarometry, textural relationships, and field mapping, titanite T-t data reveal a protracted history of mid-crustal tectonics. Peak Zr-in-titanite temperatures are 750-850 °C at 10-14 kbar. T-t paths are divided into two groups, which are separated by the Chame Shear Zone (CSZ). Group I (upper) rocks show cooling at rates of 1-10 °C/Ma between ca. 25 and ca. 10 Ma, while Group II (lower) rocks show heating at rates of 1-10 °C/Ma over the same time interval. Between ca. 25 and ca. 15 Ma, a thermal inversion occurs structurally upwards through Group I, but by ca. 10 Ma temperatures decrease structurally upwards. In opposition, temperatures decrease upwards in Group II rocks at all times and are as high as ~830 °C at ca. 9 Ma. While Group II rocks are interpreted as an intact, structurally-upright, section, the thermometric inversion within Group I is interpreted as thrusting of hotter rocks over colder from ca. 25 to ca. 15 Ma. The thrusting interpretation is also supported by simultaneous cooling and heating of Group I and II rocks, lithologic repetition of orthogneiss at the CSZ, and retrograde water-rich fluid flux restricted to Group I rocks. The CSZ is structurally higher than previously-identified intra-GHS thrusts in central Nepal, and thrusting duration was ~3-6 Ma longer than proposed for other identified intra-GHS thrusts in this region. Thrusting was followed by cooling at ~ 60 °C/Ma from ca. 10 Ma to Ar closure in muscovite at ca. 3.8 Ma; however, ~ 55 km to the west, cooling rates were ~ 20 °C/Ma and muscovite 40Ar/39Ar cooling ages ca. 10-15 Ma. These data suggest considerable along-strike variation in the position and duration of thrusting, as well as subsequent cooling, in the Himalaya. Formation of the Lesser Himalayan duplex in the late Miocene may have played a more important role in cooling and unroofing the GHS in the Marsyandi sector than at other locations. Additionally, exhumation from ~ 40 km depth requires erosion rates of ≥ 4.0 mm/yr, comparable to modern rates, for the past 10 Ma. Sector zoning of Zr in titanite suggests that diffusion rates are slower than determined experimentally, preserving thermometric constraints for metamorphism into the lower granulite facies.
Walters, Jesse Bennett, "Protracted Thrusting Followed by Late Rapid Cooling of the Greater Himalayan Sequence, Annapurna Himalaya, Central Nepal: Insights from Titanite Petrochronology" (2016). Boise State University Theses and Dissertations. 1148.