Tectonic Evolution of the Western Margin of the Burma Microplate Based on New Fossil and Radiometric Age Constraints

Abstract

Results of biostratigraphic and geochronological investigations in eastern Nagaland and Manipur, NE India, provide new constraints on the tectonic evolution of the western margin of the Burma microplate. U/Pb zircon ages indicate that the Naga Hills ophiolite developed in a suprasubduction zone setting as part of an intraoceanic island arc developed during late Early Cretaceous (mid‐Aptian) time and is younger than similar rocks exposed along the Indus‐Yarlung Tsangpo suture zone. Radiolarian microfossils provide Jurassic and Cretaceous age constraints for Tethyan ocean floor sediments that were subducted beneath the forming ophiolite. Timing of the emplacement of these rocks onto the passive margin of eastern India is constrained by Paleocene/Eocene radiolarians in sediments over which the ophiolitic assemblage has been thrust. Previously undated schists and gneisses in the Naga Metamorphics are of Early Ordovician age, and their sedimentary protolith was most likely derived from sources in the south of Western Australian and East Antarctica. After Barrovian‐style metamorphism, these rocks were uplifted and eroded becoming an important source of detritus shed into the Eocene Phokphur Formation. This unit also contains abundant clasts sourced from the disrupted basement of the Naga Hills ophiolite, which it overlies. It also contains Permo‐Triassic‐aged detritus eroded off an enigmatic source that was possibly a continental convergent margin arc system somewhere along the northern margin of Gondwana.

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

“…Furthermore, plate reconstructions suggest that deformation associated with this collision would have propagated from north to south starting in the late Eocene (Figure 11a; Morley et al, 2020; Westerweel et al, 2019). This fits with the late Eocene setting of the IBR with ongoing continental collision in the northern IBR (Aitchison et al, 2019), incipient emergence of the central IBR (Licht et al, 2019; this study), and no or only localized uplift in the southern IBR (Gough et al, 2020). It also fits with the prevailing marine conditions and ongoing subsidence in the Minbu Basin (Gough et al, 2020; Zhang et al, 2019), in contrast to the latest Eocene‐middle Oligocene unconformity to the north (this study).…”

“…For these reasons, we propose that the exhuming northernmost perpetuation of the WPA, forming a segment of the Trans-Tethyan Arc in the Greater Burma region, should have significantly contributed to the late Eocene sediment supply of central Myanmar alongside the Wuntho Ranges (Figure 11a). Similarly, pre-Cretaceous zircons in the Yaw Formation could have been supplied by exhuming basement rocks in the Greater Burma region to the north, because the Eocene Phokphur conglomerates in the northern IBR (Aitchison et al, 2019;Cai et al, 2019) and Eocene sediments in the northernmost Chindwin Basin (Arboit et al, 2020) have comparable pre-Cretaceous age peaks as the Yaw Formation suggesting similar sourcing (Figure 8). Yaw Formation sourcing from north of the BT could have occurred in the Burmese backarc where sparse paleocurrent measurements suggest southward directed drainage systems since the late Eocene (Figure 11a; Thein & Maung, 2017).…”

Andaman Ophiolite: An Overview

“…95 Ma). In case this age is assigned to the age of Andaman Ophiolite it is difficult to fit in the regional geodynamic setting of this region because two neighbouring ophiolites from Nagaland-Manipur and Myanmar, both lying farther north along the same ophiolite belt date~117 Ma (Singh et al 2017, Aitchison et al 2019 and~127 Ma (Liu et al 2016) respectively. Field and geochemical studies of Andaman plagiogranites indicate that the~95 Ma age should be assigned to arc maturity.…”

“…So, what does this 95 Ma age really indicate? Does it represent the true age of the ophiolite and if it does, then how does it fit to the regional geodynamic setting where the zircons of plagiogranites from Nagaland-Manipur Ophiolite and Kalaymyo Ophiolite (Myanmar), located farther north along the same ophiolite belt gave ages of 116.4 ± 2.2 to 118.8 ± 1.2 Ma (Singh et al 2017); 117.55 ± 0.55 Ma and 116.63 ± 0.30 Ma (Aitchison et al 2019) and 127 Ma (Liu et al 2016)…”

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

“…In the Chin Hills of Myanmar, the Western Belt Ophiolites are overlain by fossiliferous flysch and conglomerate of the Late Cretaceous‐Early Paleogene Kabaw Formation (Socquet et al, 2002). In the Naga Hills of northeastern India, limestone and metasedimentary rocks of the early Paleozoic‐Mesozoic Naga Metamorphics are thrust to the northwest atop the Early Cretaceous‐Eocene Nagaland Ophiolitic Complex and late Eocene‐Oligocene Jopi‐Phokphur Formation (Aitchison et al, 2019; Brunnschweiler, 1966; Chatterjee & Ghose, 2010). The Nagaland Ophiolitic Complex is notable for containing blueschist and eclogite blocks with Early Jurassic peak metamorphic ages, which have been interpreted to reflect the earliest phase of Neo‐Tethys subduction beneath the West Burma block (Rajkakati et al, 2019).…”