# Subduction Evolution Controlled Himalayan Orogenesis: Implications from 3-D Subduction Modeling
## 2 Materials and Methods
Density \(\rho\) uses the global CRUST2.0 data from PREM and Stanford University (the upper crust is 2.6 \(\times\) 10\({}^{3}\) kg/m\({}^{3}\), the lower crust is 2.9 \(\times\) 10\({}^{3}\) kg/m\({}^{3}\), and the upper mantle is 3.3 \(\times\) 10\({}^{3}\) kg/m\({}^{3}\)) (Table S2). The kinematic plate subduction modeling method [12] is used to calculate subduction velocities inside a prescribed 3-D constrained volume of the oceanic lithosphere. The subduction velocity is set to vs. = 4.4-11.8 cm/year, as shown in Table S1. Current global MORVEL plate motion data have indicated a subduction velocity of 4-5 cm/year at the MBT [17; 18], which is in accordance with our assumption. Figure 2: Conceptual figure of model domain and the slab geometry (splitting from west to east). The bottom and perpendicular planes are prescribed as adiabatic and permeable, and the top surface is set to be permeable. Detailed model settings are available in the Supplemental Information. The uplifted land surface has slowly been eroded, with erosion reducing the thickness of the continental crustal surface. The erosion rates reach the highest value in the High Himalayas at 2.7 \(\pm\) 0.3 mm/a (1\(\sigma\) errors) and fall to <0.6 mm/a in the foothills to the south of the high mountains ([PERSON] et al., 2018), which suggests an average erosion rate as low as <1 mm/a for the present-day Himalayas. [PERSON] et al. (2018) further suggested that during the past six million years, the erosion rates were on average approximately 1 mm/a, which is close to the modern erosion rates in the Himalayas. During the Quaternary, there has been a marked increase in the erosion rate due to glacial erosion, especially in the High Himalaya (e.g., ([PERSON] et al., 2018)). Considering these factors, we assumed the average erosion rate to be 0.2-0.4 mm/a (Table S1) in our modeling. The composite model setting, including model configuration, initial and boundary conditions, and physical parameters are illustrated in the Supplemental Information. Slab thrusting, detachment, and subsequent possible delamination give a rise to the uplift of the overriding continental terrain surface. To simulate the steady subduction of the Indian plate, previously developed subduction models applied to various subduction zones have been utilized (SI). The evolution scenario is shown in Figure 3, and the surface uplift velocity is shown in Figure 4.  
Figure 3: The four stages of the Himalayan uplift based on 3-D subduction modeling. Red arrows denote the elevation of the Himalayas. MBT, main boundary thrust; YTSZ, Yarlung–Tsangpo suture zone. The insets are modified from [PERSON] et al. (2018). (**a**) Plate-convergence-induced thickening (60–42 Ma). (**b**) Progressive uplift of the Himalayas caused by slab detachment and rebound (42–23 Ma). (**c**) Upward asthenospheric flow causing rapid uplift of the Himalayas (23–11 Ma). (**d**) Slowed uplift of the Himalayas affected by surface erosion (11–0 Ma). The first stage of the early plate collision period (60\(-\)42 Ma) is prescribed by a subduction velocity of 11.8 cm/year. The dominant uplift force is plate-convergence-induced thickening, and the Himalayan uplift rate is moderate (insets indicating the time-height covariation) until the region reaches a height of around 2 km. From 42 to 11 Ma, the Himalayas experienced a dramatic increase in elevation from 2 km to around 4-5 km. During this stage, slab break-off occurred, resulting in slab rebound and the occurrence of upward lithospheric flows, which are the prevailing driving forces of buoyancy; this mechanical mechanism explains such rapid uplift. Since 11 Ma, the Himalayan uplift slowed, probably due to the cessation of upward buoyant forces, and the monsoon-associated surface erosion became another crucial control affecting the surface height, which led to a gradually declining growth rate for the Himalayas. The subduction velocity during this period has passively decreased to an average of 4.4 cm/year since the late Miocene.