The Deep Lithospheric Thermochemical Structure Of The Junggar Terrane,NW China:Implications For Its Origin And Tectonic Evolution

The Central Asian Orogenic Belt(CAOB)is one of the world's largest accretionary orogens.It was formed by subduction-accretion processes starting in the early Neoproterozoic(? 1.0 Ga)and culminating in the late Paleozoic(? 250 Ma).Given its long tectonic history and the preservation of multiple tectonic events,the CAOB is considered an ideal natural laboratory to study accretionary deformation processes,postcollision dynamics and continental growth mechanisms in general.The Junggar terrane,located in the southwestern CAOB,is considered a key tectonic element to understand the evolution of the whole CAOB.However,given the lack of xenoliths and limited geophysical data with sensitivities to mantle depths,the nature of its basement and its deep lithospheric structure are still highly debated.Some earlier works considered the basement of JB to be a small Precambrian continental block,while more recent studies interpreted it as fragments of a Paleozoic oceanic crust.Although the “oceanic” hypothesis currently is the preferred explanation for the origin of the JB,there are a number of open questions that need to be addressed.Foremost among these are i)why is the crystalline crust so thick and with characteristics of mafic rocks?,ii)what is the nature and structure of the lithospheric mantle beneath the JB? and iii)if it is a remnant of an old oceanic plate,why hasn't it been subducted?This work provides new insights into these questions,with particular emphasis on the nature and structure of the lithosphere beneath the JB.In doing so,a probabilistic inverse method is used to jointly invert newly processed Rayleigh wave dispersion data,surface heat flow,geoid anomalies and absolute elevation.This method is based on a thermodynamically-constrained formulation and driven by powerful Markov Chain Monte Carlo algorithms.The output is a 3D model of the seismic,temperature,bulk density and compositional structure of the whole lithosphere beneath the western part of the Junggar terrane.The presence of a “mafic-like” high-velocity,high-density lower crust is confirmed beneath the JB,consistent with previous seismic studies.The compositional result reveals a lower magnesium number of the lithospheric mantle beneath the JB while higher magnesium number in surroundings.The inverted LAB depth(lithosphere-asthenosphere boundary)shows a thick lithosphere(?240 km)beneath the western flanks of the JB and a sharp and large step in lithospheric thickness towards the WJ,where the LAB reaches depths of about 200 km.The results reveal a subductionshaped low-temperature,high-density,and high-velocity anomaly at mantle depths,dipping northwestward towards the WJ.These images resemble closely those obtained in thermomechanical simulations of remnant subduction zones after slab break-off.On account of these results,this work further investigates the ‘continental' vs ‘oceanic' question by creating refined/constrained end-member models.The results demonstrate that an “oceanic model” for the origin of the basement of the Junggar Basin explains the available geophysical and geochemical information better than other alternative models.In addition,this work combines the temperature and composition results inverted from multi-observable probabilistic inverse with the electrical conductivity model from a recent magnetotelluric(MT)data to estimate the water content of the lithospheric mantle beneath the Junggar.This approach isolates the water content effect on electrical conductivity by stripping off the background effects of temperature and bulk composition from the MT data.The estimated water content is substantially larger than the water storage capacity of nominally anhydrous minerals and the temperature profile is lower than the water-saturated solidus.Therefore,the sub-continental lithospheric mantle conductor is not attributed to water content or partial melting,and it could be ascribed to the volatile-bearing metasomatic minerals.Based on all these observations,this work supports an oceanic model for the origin of the western Junggar terrane and the fossil northwestward subducted Paleo-Asian oceanic plate that has been well preserved.Finally,this work proposes an evolutionary model that reconciles a number of previously conflicting interpretations.In this model,the oceanic plate trapped beneath the JB was originally created either in a ‘hot ridge' environment(e.g.under the influence of a hot spot)or modified by the later action of a plume impinging at the base of the oceanic plate.The result was a plate with an anomalously thick crust and a highly depleted uppermost mantle.The intrinsic buoyancy of this plate(strictly,a region of the original oceanic plate)prevented its subduction and contributed to its preservation.Based on a simple 1D transient heat conduction model,this work suggests that the present-day lithospheric thickness is the result of ? 300 Ma of cooling and an associated thickening of ? 120 km.This work not only provides unprecedented information on the deep structure of the Junggar terrane and sheds new light on the nature of the lithosphere beneath this region,but also motivate new ideas and the development of the new surveys to improve our understanding of overall accretionary evolution of the whole CAOB.