The Deep Lithospheric Structure of the Junggar Terrane, NW China: Implications for Its Origin and Tectonic Evolution

Abstract

The Central Asian Orogenic Belt (CAOB) is one of the world's largest and oldest accretionary orogens. 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. Here we use a probabilistic inverse method to jointly invert newly processed Rayleigh wave dispersion data, surface heat flow, geoid anomalies, and absolute elevation. The output is a 3-D model of the seismic, temperature, bulk density, and compositional structure of the whole lithosphere beneath the western part of the Junggar Terrane. Our results reveal a subduction-shaped mantle anomaly beneath the western flank of the Junggar Basin that resembles closely the structure of a fossil subduction zone. We also confirm the presence of a thick lower crust beneath the basin with properties similar to those of basaltic rocks. On account of these results, we further conduct a hypothesis test and 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. Based on all these observations, we propose an evolutionary model for the western Junggar Terrane that reconciles a number of previously conflicting interpretations.

Thermochemical State of the Upper Mantle Beneath South China From Multi‐Observable Probabilistic Inversion

“…Although we will use the terms “fertile” and “depleted” in our discussions to refer to mantle domains with low and high Mg#, respectively, we emphasize that our main interest is to highlight the first‐order compositional heterogeneities/anomalies inside the lithospheric mantle regardless of their actual nature (e.g., melt extraction). We also note that composition is the least well‐constrained field (Afonso et al., 2010; Afonso, Fullea, Yang, et al., 2013; A. Zhang et al., 2019), and therefore we focus only on the largest contrast within the lithospheric mantle. Additional compositional information on the full CFMAS system can be found in Figure S2.…”

“…The means represent the input used in the inversion and the variances are utilized as a measure of the natural variability/uncertainty of the field inside each cell. To account for measurement variability as well as theoretical modeling errors (those related to the solution of the forward problems), we assign minimum uncertainties to elevation and geoid height of 150 and 1.5 m, respectively (see e.g., Afonso, Fullea, Yang, et al., 2013; Guo et al., 2016; Shan et al., 2014; A. Zhang et al., 2019). Since SHF is the least robust data set in terms of both spatial coverage and reliability of the measurements, we assign a relatively large, yet realistic, uncertainty of 20%.…”

A Partial Molten Low‐Velocity Layer Atop the Mantle Transition Zone Beneath the Western Junggar: Implication for the Formation of Subduction‐Induced Sub‐Slab Mantle Plume

“…Various geological and geophysical evidences indicate that slab window opening, plate melting and break-off once occurred in the WJ during the late Carboniferous to early Permian (e.g., Geng et al, 2009;Tang et al, 2010Tang et al, , 2012Wu et al, 2018;Xu et al, 2020;Yin et al, 2013;A. Q. Zhang et al, 2019).…”

“…It is characterized by widespread Paleozoic high‐temperature magmatism and crustal deformation, and is a window to unravel accretionary orogenesis and continental growth of the CAOB (e.g., Xiao & Santosh, 2014; G. Yang et al., 2015). Recent magnetotelluric and seismic evidences (e.g., Wu et al., 2018; Xu et al., 2020; A. Q. Zhang et al., 2019) suggest that there are remnants of a rare fossil intraoceanic plate northwesterly subducted and preserved at depths from 120 to 220 km beneath the WJ (Figure 1b), which further supports a young spreading mid‐oceanic ridge subduction accompanied with slab window opening, slab melting and break‐off during the late Carboniferous to early Permian (∼320–290 Ma) from various geochronological, petrological and geochemical evidences (e.g., Geng et al., 2009; Tang et al., 2010, 2012; Yin et al., 2013). Meanwhile, extensive emerging A‐type granitoids with high positive ε Nd( t ) values (e.g., R. Gao et al., 2014), Baiyanghe dolerites associated with ocean island basalts (OIB)‐like magmatism (e.g., Miao et al., 2018), Hatu tholeiites (e.g., Tang et al., 2012) and Yeyagou and Maliya OIB‐like alkaline basalts (e.g., J. N. Zhang, 2009) are suggested to be results of mantle plume or enriched upwelling asthenospheric mantle source in the same geological period.…”

“…Yang et al, 2015). Recent magnetotelluric and seismic evidences (e.g., Wu et al, 2018;Xu et al, 2020;A. Q. Zhang et al, 2019) suggest that there are remnants of a rare fossil intraoceanic plate northwesterly subducted and preserved at depths from 120 to 220 km beneath the WJ (Figure 1b), which further supports a young spreading mid-oceanic ridge subduction accompanied with slab window opening, slab melting and break-off during the late Carboniferous to early Permian (∼320-290 Ma) from various geochronological, petrological and geochemical evidences (e.g., Geng et al, 2009;Tang et al, 2010Tang et al, , 2012Yin et al, 2013).…”