Megathrust Heterogeneity, Crustal Accretion, and a Topographic Embayment in the Western Nepal Himalaya: Insights From the Inversion of Thermochronological Data. Issue 7 (15th July 2022)

• Record Type: Journal Article
• Authors: Fan, Suoya; Murphy, Michael A.; Whipp, David M.; Saylor, Joel E.; Copeland, Peter; Hoxey, Andrew K.; Taylor, Michael H.; Stockli, Daniel F.
• Is Part Of: Tectonics. Volume 41:Issue 7(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-07-15
• Subjects: megathrust -- thermochronology -- ramp geometry -- crustal accretion -- landscape -- drainage; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2021TC007071
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: Between 81°30ʹE and 83°E, the Himalayan range's "perfect" arcuate shape is interrupted by an embayment. We hypothesize that thrust geometry and duplexing along the megathrust at midlower‐crustal depths play a leading role in growth of the embayment as well the southern margin of the Tibetan plateau. To test this hypothesis, we conducted thermokinematic modeling of published thermochronologic data from the topographic and structural embayment in the western Nepal Himalaya to investigate the three‐dimensional geometry and kinematics of the megathrust at midlower‐crustal depths. Models that can best reproduce observed cooling ages suggest that the megathrust in the western Nepal Himalaya is best described as two ramps connected by a long flat that extends further north than in segments to the east and west. These models suggest that the high‐slope zone along the embayment lies above the foreland limb of an antiformal crustal accretion zone on the megathrust with lateral and oblique ramps at midlower‐crustal depths. The lateral and oblique ramps may have initiated by ca. 10 Ma. This process may have controlled along‐strike variation in Himalayan‐plateau growth and therefore development of the topographic embayment. Finally, we analyze geological and morphologic features and propose an evolution model in which landscape and drainage systems across the central‐western Himalaya evolve in response to crustal accretion at depth and the three‐dimensional geometry of the megathrust. Our work highlights the importance of crustal accretion at different depths in orogenic‐wedge growth and that the midlower crustal accretion determines the location of plateau edge. Key Points: Thermochronologic data and thermokinematic modeling results suggest a complex 3D geometry of the megathrust in the western Nepal Himalaya Crustal accretion along the midlower crustal ramp in the megathrust is a mechanism of maintaining the edge of a plateau‐like landscape The development of the 3D megathrust geometry may have controlled the first‐order evolution of an embayment and regional drainage system