Structural and Thermochronologic Constraints on the Cenozoic Tectonic Development of the Northern Indo‐Burma Ranges

Abstract

The~1,500-km-long, north trending Eastern Flanking Belt of the Himalayan-Tibetan orogenic system is located along the eastern margin of the Indian subcontinent. Although the belt is a key element of the Cenozoic India-Asia collisional zone, its tectonic evolution remains poorly understood. This lack of knowledge has impacted our ability to differentiate between competing hypotheses for the evolution of the India-Asia collision. To address this problem, we integrate constraints on the structural framework and magnitude of Cenozoic shortening strain with thermochronology of the northernmost segment of the belt located directly southeast of the eastern Himalayan syntaxis (i.e., the northern Indo-Burma Ranges). The study area exposes a southwest directed thrust belt that is bounded by the Indian craton in the west and the right-slip Jiali fault zone in the east. New and existing (U-Th)/He and 40 Ar/ 39 Ar thermochronologic data indicate that thrust-related cooling occurred from~36 Ma in the northeast to~5.6 Ma in the southwest. Episodes of out-of-sequence thrusting occurred at~30-20,~14-12, and~11-6 Ma within the thrust belt. Restoration of the thrust belt yields a minimum horizontal shortening of~280 km (~86%). These results combined with (1) the recorded local absence of several major Himalayan-Tibetan lithologic units (i.e., Tethyan Himalayan Sequence, Greater Himalayan Sequence, and southern Gangdese batholith) and (2) the southward decrease in the thrust-belt width (33-5 km) suggest a complex history of thrusting in the northern Indo-Burma Ranges and an spatial increase in Cenozoic crustal shortening and/or continental underthrusting from west to east across the eastern Himalayan syntaxis.

Burma Terrane Collision and Northward Indentation in the Eastern Himalayas Recorded in the Eocene‐Miocene Chindwin Basin (Myanmar)

“…Tectonics late Eocene thrusting in the Eastern Himalayan collision zone north of the IBR (Haproff et al, 2020), which these studies also attributed to the India-Asia collision. This together leads us to propose that the latest Eocene-middle Oligocene unconformity from this study was induced by the onset of the collision of India and Greater Burma with Asia in the Eastern Himalayas.…”

“…To the north, the transition from the BT to the Eastern Himalayan orogen is defined by the intersection of the Sagaing Fault and the IBR (Lin et al, 2019; Mitchell, 2017; Mitchell et al, 2012). However, it is unclear how much of the northern extension of the BT (i.e., Greater Burma) was shortened, subducted, and underthrust in the Eastern Himalayan Syntaxis during its northward motion (Haproff et al, 2019, 2020; Morley et al, 2020; Westerweel et al, 2019). At present, the orogen bends from ~E‐W to ~NW‐SE at the Eastern Himalayan Syntaxis (Figure 1a).…”

“…However, the timing of this emplacement as suggested by Aitchison et al (2019) is likely earlier in the Eocene, which is more in agreement with an initial early Paleogene (Paleocene‐early Eocene) collision of India with the Trans‐Tethyan Arc shown by plate reconstructions (Westerweel et al, 2019). According to these reconstructions, this first collision is followed by the collision of India, including the Greater Burma region, with the Asian margin during the late Eocene (Figure 11a; Haproff et al, 2020; Morley et al, 2020; Westerweel et al, 2019). This is supported by the coeval late Eocene onset of high temperature metamorphism along the MMMB (Searle et al, 2007, 2017, 2020) and late Eocene thrusting in the Eastern Himalayan collision zone north of the IBR (Haproff et al, 2020), which these studies also attributed to the India‐Asia collision.…”

“…According to these reconstructions, this first collision is followed by the collision of India, including the Greater Burma region, with the Asian margin during the late Eocene (Figure 11a; Haproff et al, 2020; Morley et al, 2020; Westerweel et al, 2019). This is supported by the coeval late Eocene onset of high temperature metamorphism along the MMMB (Searle et al, 2007, 2017, 2020) and late Eocene thrusting in the Eastern Himalayan collision zone north of the IBR (Haproff et al, 2020), which these studies also attributed to the India‐Asia collision. This together leads us to propose that the latest Eocene‐middle Oligocene unconformity from this study was induced by the onset of the collision of India and Greater Burma with Asia in the Eastern Himalayas.…”

Tectonothermal evolution of the Lohit Valley, Eastern Himalaya: New low‐temperature thermochronological constraints

“…Red rectangle showing the area of (b). (b) Regional geological map of the easternmost part of the Himalayan orogen after Haproff, Odlum, Zuza, Yin, and Stockli (2020). Dibang valley is shown in no.1 red oval box, while our present study area in Lohit Valley is shown in no.…”