Drainage reorganization of the Irrawaddy River constrains the time and magnitude of displacement on the Sagaing Fault, Myanmar

Information on the amount and timing of displacement on the Sagaing Fault, SE Asia’s longest active strike-slip fault, is required for reconstructions of the eastern margin of Neotethys, and furthering our knowledge of crustal deformation processes associated with India-Asia convergence. However, such information is much debated and very poorly constrained.

One hypothesised approach to determining the magnitude of displacement on the Sagaing Fault is the proposal that the upper Irrawaddy used to flow into the Chindwin River before being beheaded due to strike-slip motion on the fault (Maung, 1987). However, unambiguous evidence to document this proposal has so far not been evidenced, nor the timing of proposed beheading determined. Previous provenance studies have shown the existence of Mogok metamorphic belt (MMB) detritus, characteristic of the Irrawaddy headwaters, in Neogene Chindwin Basin deposits (e.g. Arboit et al., 2021; Licht et al., 2018; Najman et al., 2022; Wang et al., 2014; Westerweel et al., 2020; Zhang et al., 2019). However, the southerly locations of these studies in the Chindwin basin allow for two palaeo-drainage options, one of which does not require drainage reorganisation (Zhang et al., 2021). Our new multi-technique provenance study from a critical northernmost location in the Chindwin Basin, allows differentiation between these two models, indicating that the upper Irrawaddy did previously flow into the upper Chindwin, and therefore the riverine offset can be used to constrain the magnitude of displacement.

Determination of when the riverine headwater beheading occurs is complicated by extensive recycling in the basin, meaning that an MMB-provenance might be retained long after the Irrawaddy ceased to flow into the Chindwin basin.  Recycling was previously only attributed to basin inversion.  However, we show from detrital mineral age data that, at this high latitude, the high topography of the eastern Indo-Burman Ranges (IBR) which make up the western margin of the Chindwin Basin, do not consist of Cretaceous-Eocene strata as commonly mapped. Instead, we validate the less well known mapping of Bannert et al (2011), which represents the eastern IBR at this latitude as Neogene strata. Our provenance data indicate that the eastern IBR at this latitude consist of thrusted Neogene Chindwin basin strata comprising MMB-derived detritus deposited by the Irrawaddy. This region is therefore in all probability the most dominant source of recycled material to the basin; thus a knowledge of the timing of the eastern IBR’s exhumation at this latitude allows us to place maximum constraints on the time after which Irrawaddy detritus in the Chindwin Basin cannot with certainty be attributed to direct deposition from the Upper Irrawaddy. We therefore undertook  an age-elevation low temperature thermochronological study of the adjacent Indo-Burman Ranges to determine the timing of its exhumation, and therefore to ascertain the earliest time that MMB-derived material in the Chindwin basin may be attributed to recycling.

 With our new constraints to the amount and time of displacement on the Sagaing Fault, we calculate its averaged motion and consider its relative importance in the accommodation of motion between India and Sundaland over time.