Late Eocene Myanmar tectonics constrained by magnetostratigraphy ofthe Yaw Formation, Chidwin Basin, Kalewa

International audienceSedimentary basins in Myanmar have recorded key events of the India-Asia collision including associated geody-namic movements and paleoclimatic records. In particular, Paleogene deposits provide invaluable insight on theaccretion of the Burma terrane, its rotation associated with the alleged extrusion of Indochina and the formation ofthe Indo-Burman ranges. They also yield unique records of monsoonal intensity during the growth of the TibetanPlateau and a rich paleontological assemblage including some of the earliest primates. However, understanding thepotential relations between these recorded events is strongly hindered by insufficient age control on these deposits.As part of the Myanmar Geodynamic & Paleoclimate Initiative and the ERC “MAGIC” project, our initial focusis to date Paleogene deposits of Myanmar with better accuracy using magnetostratigraphy. We present prelimi-nary results from the Chindwin Basin where we sampled a 400-meter section of the top of the Yaw formationrecording a major sedimentological facies transition previously estimated roughly as Eocene to Oligocene in age.Detailed rock magnetic analyses enabled to identify and isolate primary Characteristic Remanent Magnetizationsof normal and reversed polarities carried by iron sulfides, iron carbonates and/or iron oxides. A correlation tothe Geomagnetic Polarity Time Scale can be proposed suggesting deposition between the base of chrons C16n2nand the base of C13r (36.3 and 34.8 Ma). This age suggests the facies transition may be more likely associatedwith regional tectonics such as the Indo-Burman uplift rather than sea-level drop linked to ice-sheet formation atthe Eocene-Oligocene Transition at 33.9 Ma. In addition, the mean observed paleomagnetic declination (13.3+/-3.7) is statistically indistinguishable from declinations expected by geodynamic models with limited vertical-axisrotations of the Burma terrane and therefore supports little to no rotational extrusion since 35 M

Burma Terrane part of the Trans-Tethyan arc during collision with India according to palaeomagnetic data

International audienceConvergence between the Indian and Asian plates has reshaped large parts of Asia, changing regional climate and biodiversity, yet geodynamic models fundamentally diverge on how convergence was accommodated since the India–Asia collision. Here we report palaeomagnetic data from the Burma Terrane, which is at the eastern edge of the collision zone and is famous for its Cretaceous amber biota, to better determine the evolution of the India–Asia collision. The Burma Terrane was part of a Trans-Tethyan island arc and stood at a near-equatorial southern latitude at ~95 Ma, suggesting island endemism for the Burmese amber biota. The Burma Terrane underwent significant clockwise rotation between ~80 and 50 Ma, causing its subduction margin to become hyper-oblique. Subsequently, it was translated northward on the Indian Plate by an exceptional distance of at least 2,000 km along a dextral strike-slip fault system in the east. Our reconstructions are only compatible with geodynamic models involving an initial collision of India with a near-equatorial Trans-Tethyan subduction system at ~60 Ma, followed by a later collision with the Asian margin

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

International audienceThe Burma Terrane (Myanmar) played an important role in the India-Asia collision and moved over 2000 km northward on the Indian Plate during the Cenozoic, before colliding with the Asian margin. However, the timing of this collision and its correlation to regional uplift phases, sedimentary provenance and basin development, remain poorly constrained. We report sedimentological, paleomagnetic and geochronological data from the late Eocene to early Miocene strata of the Chindwin Basin in the Burmese forearc, constraining the paleogeographic evolution of the Burma Terrane and the Eastern Himalayan orogen. Our results highlight two unconformities of late Eocene-middle Oligocene and latest Oligocene-early Miocene age, revealing a two stage interaction of the Burma Terrane with the Asian margin during its northward translation. The first unconformity follows rapid ~0.6 m/k.y. subsidence in the Burmese forearc, as shown by magnetostratigraphy. The transition to a fluvial depositional environment and the occurrence of reworked sediments at this first unconformity likely records the commencing collision of India and the northern extent of the Burma Terrane with the Asian margin. The second unconformity shows drastic changes in magnetic properties, mineralogy and provenance, with high-grade metamorphic grains and early Miocene apatite U-Pb and fission track ages indicating that it is coeval to a major deformation phase in Myanmar and the Eastern Himalayan orogen. It likely records the indentation of the Burma Terrane into the Eastern Himalayan collision zone, forming the modern Eastern Himalayan Syntaxis