Magnitude, Architecture, Timing and Rate of Regional Deformation and Metamorphism in the Himalayan Fold-Thrust Belt of Nepal

Abstract

Orogenic systems such as the Himalayan fold-thrust belt accommodate crustal scale deformation that occur at convergent plate margins. Thus, understanding magnitude, architecture, timing and rate of deformation in fold-thrust belts reveal how, when and why the lithosphere is reshaped during convergence in a classic continent-continent collisional plate boundary. This study examines three sectors of the Himalayan fold-thrust belt: the Okhaldhunga region of eastern Nepal, the Annapurna region of central Nepal and the Dadeldhura klippe in western Nepal to investigate collisional orogenesis.In eastern Nepal, new detrital and igneous zircon geochronology integrated with regional mapping reveals that the Okhaldhunga window exposes the Ramgarh thrust sheet, composed of Paleoproterozoic Lesser Himalayan Sequence. New balanced cross-sections presented herein quantify minimum shortening of ~561-544 km suggesting an eastward decline in shortening rates. A novel approach of combining monazite petrochronology with regional balanced structural cross-sections documents the metamorphic chronology of Himalayan thrust belt development. Amphibolite facies Greater Himalayan Sequence rocks in the Main Central Thrust hanging-wall record Eocene-early Miocene prograde metamorphism initiating at approximately ca. 47 Ma, followed by Miocene retrograde metamorphism. In contrast, greenschist facies Lesser Himalayan Sequence rocks in the Main Central Thrust footwall document Early Miocene prograde metamorphism and Middle-Late Miocene retrograde metamorphism. Finally, low-temperature thermochronology and thermal history models reveal a northward-propagating cooling pattern within the predominantly southward-propagating thrust belt of eastern Nepal, suggesting that climate-tectonic interactions control long-term erosion rates and tectonomorphic development. This study also introduces an application of a novel laser ablation inductively coupled mass spectrometry method for mapping U-Th-Pb and trace and rare earth elements in entire zircon crystals. Through comprehensive single-grain imaging, we demonstrate that individual zircon crystals preserve the complete tectonometamorphic evolution of the western Nepal sector of the Himalayan thrust belt, providing new insights into regional deformation accommodation patterns. Finally, a multi-proxy investigation of the Annapurna region in central Nepal Himalaya corroborates a discrete rather than gradual eastward decline in shortening magnitude in the Nepal Himalaya. Detrital zircon U-Pb geochronology, Nd isotope geochemistry, monazite petrochronology, and low-temperature thermochronology integrated with balanced cross sections constrain the timing, magnitude, and architectural development of collisional orogenesis in this critical sector of the Himalayan orogen. Altogether, this dissertation combines regional structural geology (mapping, balanced cross sections and kinematic reconstructions) with geochronology, petrochronology and thermochronology to investigate the tectonic evolution of the Himalayan orogen and provides new insights into how the mid-upper crust responds to continental collision.