From cross faults to lake deposits: geologic evidence of past and present hazards

Abstract

The Everest region in the eastern Nepal Himalaya is a natural laboratory for Earth sciences research as it consists of spectacular rock exposures, glaciers, lakes, rivers, and rockslide zones. While a plethora of research has been conducted in the region, a geomorphic and a tectonic feature remained unnoticed until recently. In 2017, a segment of a young (<12 Ma), NNE-striking cross fault, later named the Benkar Fault Zone (BFZ), was recognized and examined within the Everest region. Cross faults are faults that strike at a high angle to the trend of the orogen. The BFZ remained unexplored towards the south. Secondly, there are a number of layered, sandy deposits, around the Namche-Phortse areas, perched high on hillsides with no obvious connection to a modern depositional basin. The goals of my dissertation are to explore these unanswered questions in order to: 1) understand the southern continuity and genesis of the BFZ; and 2) constrain depositional ages for those sandy deposits and understand their depositional settings. To achieve my first goal, I mapped the BFZ towards the south from the Everest region and augmented the mapping results with kinematic and petrographic analysis. I find that the BFZ is a ~100 km long, segmented cross fault zone that possibly cuts across the entire width of the eastern Nepal Himalaya. The BFZ is a composite of a series of deformational events and might continue further towards the south into the foreland basin. This fault, like other Himalayan cross faults, likely contributes to seismic and landslide hazards. Through an extensive literature review, I have synthesized the occurrences and drivers of cross faults and lateral heterogeneity in several mountain belts around the world. For my second goal, I constrained the depositional ages of six sedimentary deposits by utilizing infrared stimulated luminescence analysis and integrated the ages (~13-42 ka) with their geomorphic setting. I interpret that some of them were likely deposited in localized, landslide- and/or moraine-related, ephemeral lakes/ponds. Such deposits provide invaluable agricultural land in these extreme topographies.