The geological history of the Burmese subduction margin, where India obliquely subducts below Indochina, remains poorly documented although it is key to deciphering geodynamic models for the evolution of the broader Tibetan-Himalayan orogen. Various scenarios for the evolution of the orogen have been proposed, including a collision of India with Myanmar in the Paleogene, a significant extrusion of Myanmar and Indochina from the India-Asia collision zone, or very little change in paleogeography and subduction regime since the India-Asia collision. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene-Oligocene times to reconstruct the evolution of the Burmese margin during the early stages of the India-Asia collision. We report on sedimentological, geochemical, petrographical, and geochronological data from the Chindwin Basin-the northern part of the Burmese forearc-and integrate these results with previous data from other basins in central Myanmar. Our results show that the Burmese margin acted as a regular Andean-type subduction margin until the late middle Eocene, with a forearc basin that was open to the trench and fed by the denudation of the Andean volcanic arc to the east. We show that the modern tectonic configuration of central Myanmar formed 39-37 million years ago, when the Burmese margin shifted from an Andean-type margin to a hyper-oblique margin. The forearc basin was quickly partitioned into individual pull-apart basins, bounded to the west by a quickly emerged accretionary prism, and to the east by synchronously exhumed basement rocks, including coeval high-grade metamorphics. We interpret this shift as resulting from the onset of strike-slip deformation on the subduction margin leading to the formation of a paleo-sliver plate, with a paleo fault system in the accretionary prism, pull-apart basins in the forearc, and another paleo fault system in the backarc. This evolution implies that hyper-oblique convergence below the Burmese margin is at least twice older than previously thought. Our results reject any India-Asia convergence scenario involving an early Paleogene collision of India with Myanmar. In contrast, our results validate conservative geodynamic models arguing for a close-to-modern pre-collisional paleogeometry for the Indochina Peninsula, and indicate that any post-collisional rotation of Indochina, if it occurred at all, must have been achieved by the late middle Eocene. Asia collision, extrusion models propose that the Burma Terrane was located either along the collision front (e.g., Replumaz and Tapponnier, 2003) or farther east, away from the collision zone (e.g., Royden et al., 2008). The sedimentary deposits of the forearc basin of central Myanmar provide a unique opportunity to document the paleogeography and deformation history of the Burmese subduction margin and decipher these different models. This article examines the history of the Burmese forearc basin, with a particular focus on Eocene-Oligocene times to reconstruct the evolutio...

Convergence 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 paleomagnetic data from the Burma Terrane, at the eastern edge of the collision zone and 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 ~80-50 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 2000 km, along a dextral strike-slip fault system in the east. Our reconstructions are only compatible with geodynamic models involving a first collision of India with a near-equatorial Trans-Tethyan subduction system at ~60 Ma, followed by a later collision with the Asian margin. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

Abstract:Among the large rivers rising on the Tibetan Plateau and adjacent high mountains, the discharge and suspended sediment load of the Ayeyarwady (Irrawaddy) River are the least well known. Data collected between 1969 and 1996 at Pyay (Prome) are analysed to provide the best available modern estimate of discharge (379 š 47 ð 10 9 m 3 /year) and suspended sediment load (325 š 57 ð 10 6 t/year) for the river upstream of the delta head. A statistical comparison with data collected in the nineteenth century (1871 to 1879) shows discharge has significantly decreased in the last ¾100 years. Regression and correlation analyses between discharge in the modern period and indices of El Niño-Southern Oscillation (ENSO) show a relationship.

The Burma Terrane (Myanmar) played an important role in the India-Asia collision and moved over 2,000 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/ky 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.

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