To show at a glance the main features of the lithospheric structure along the Altai-Tianshan profile, in Figure 6 we show both the average Vp and the thickness of crust and lithosphere based on the final P-wave velocity model, while in Figure 7 we include the velocity-depth curves extracted for each of the blocks crossed by the reference profile for comparison with the curves deduced for other regions.

Our model reveals a deeper LAB below the southern Altaids (Figure 6D), a crust-mantle transition layer (∼50–60 km) with high Vp values of 7.2–7.4 km/s, and also two apparent HVAs located within the middle-crust and the top mantle (Figures 5B, C). The velocity-depth curve for this region (Figure 7A) also shows high Vp values for the crust (6.65 km/s) and the rest of the lithosphere (7.25 km/s). All of this evidence is consistent with mantle-derived rocks and minerals, such as the alkaline iron-rich high titanium basalt series related to ilmenite deposits, the tholeiitic basalt–calc-alkaline series related to copper-ickel deposits, and the ophiolite type rocks with rich Mg and poor Fe characteristic related to chromite deposits (Wang et al., 2013).

He et al. (1994) proposed that Altaids contains a continental margin from the late Precambrian to the early Paleozoic, and that subduction of oceanic crust could have occurred in the Paleozoic (Windley et al., 2002; Xiao et al. al., 2004). The mid-lower part (depth >10 km) of the average Vp curve (Figure 7A) is close to that of the continental arcs and global continent average (Christensen and Mooney, 1995). This suggests that, despite the strong Paleozoic subduction and orogenic process, it still retains traces of the ancient components of the pre-Early Paleozoic continental crust, confirming that it is a Japanese-type arc (Xiao et al., 2015; Xiao et al., 2018).

The slab window caused by the subduction of mid-ocean ridges was used to explain the origin of adakite, Nb-rich basalt, high-Mg andesite and Devonian high-grade metamorphic complexes around southern Altai (Zhao et al., 2006; Windley et al., 2007; Sun et al., 2008; Shen et al., 2010; Xiao et al., 2018), and it is believed that there were two oceanic ridge subductions in the late Paleozoic in the southern margin of Altai. Also, there are orogenic gold deposits such as the Kalatongke, Tokuzibayi and Donbastau copper-nickel deposits in the Erqis shear zone in the southern margin of Altai. Chronological studies (e.g., Han et al., 2007; Li et al., 2007) show that the above deposits were formed in the Carboniferous-Permian. The ore magma originated from the upper mantle and is the product of upward intrusion or tectonic-magmatic-fluid activity of mafic magma in a post-collisional extensional environment.

The Ulungur Depression is the northeastern part of the Junggar Basin (Figure 1), which is a late Paleozoic-Mesozoic and Cenozoic multi-cycle superimposed basin, with rich oil and gas resources (Zhu, 2009; Chen et al., 2010). Our velocity model (Figures 5B, C) shows that two LVAs exist within the shallow and lower crust. The former corresponds to the thick sedimentary layer of this place. The latter could be interpreted to mean that the top mantle is quite hot and that there is probably a strong thermal interaction across the Moho boundary, which could be related to the high heat flow observed in the Ulungur depression (Jiang et al., 2019) compared to other values of the eastern Junggar area.

Figure 6 shows relatively low average Vp values for the crust (6.59 km/s) and lithosphere (7.16 km/s), respectively. The velocity-depth curve for this region (Figure 7B) also shows that the Vp value of the deepest part of this block (depth>25 km) is clearly less than that of the global continent average (Christensen and Mooney, 1995), and it is also close to the curve of orogens.

The Yemaquan Arc is located between the Almantai and Kalamaili ophiolite belts and is a part of the eastern Junggar orogenic belt (Figure 1). The folded basement of the Yemaquan continental block is a flysch volcanic basin, and its southern continental margin volcanic arc is covered with Devonian acid volcanic and pyroclastic rocks, neritic terrigenous clastic rocks from the late Early Devonian to the Early Carboniferous, and post-collisional alkaline granite bedrock (Li et al., 1990).

This arc is located in the larger Kalamaili area, where gold deposits represented by the Jinshuiquan, Shuangquan, Nanmingshui and Sujiquandong deposits are developed, forming a set of metallogenic gold systems related to the late Paleozoic collisional orogeny.

Han et al. (2006) showed that the Kalamaili granite belt is a huge NW-trending batholith, which is caused by magmatic events in the post-collision extensional environment of late Paleozoic 330–280 Ma. Zhang et al. (2015) believed that the Late Carboniferous–Permian post-collisional tectonics dominated the major metallogenic systems in this area. These authors proposed that extensional strike-slip structure produced by continental lithospheric delamination and asthenospheric mantle upwelling, crust-mantle magma interaction, and mixed-fluid action is the geodynamic mechanism of metal mineralization in the Karamaili area.

Our model clearly shows that a LVA exists within the middle-lower crust (Figure 5B and especially Figure 5C). This can be attributed to marine sediments or rocks brought to this depth due to Paleozoic subduction. Our model also reveals two apparent HVAs, one shallow near the surface and the another one within the upper mantle (Figure 5C), and a significant Moho uplift (Figure 6C). Based on above geological evidences, we suggest that the LVA, HVAs and Moho uplift of this area could be correlate with Paleozoic post-collision delamination after subduction in this region, and moreover these evidences were probably preserved in the lithosphere due to the lack of strong tectonics during the Mesozoic-Cenozoic period.

Above disclosed clear Moho uplift matches with the imaging results given by Yang et al. (2022), and this block has thinnest crust with a minimum thickness of ∼43.0 km (Figure 6C), a thicker lithosphere with a maximum thickness of up to 90.0 km (Figure 6D), while it has the lowest local Bouger anomaly value of −178 mgcal (Figure 6E). This low gravity anomaly agrees with the apparent LVA within the middle-lower crust (Figure 5C), and the low average Vp value of the crust whose minimum is only 6.5 km/s (Figure 6A). The corresponding velocity-depth curve (Figure 7C) shows that the Vp value for the deeper part of this block (depth>25 km) is clearly similar to that of the orogens curve, suggesting a powerful Paleozoic subduction and orogeny around this area.

This area has a high average Vp value for the lithosphere, whose maximum reaches 7.30 km/s (Figure 6B), and a high local Bouguer anomaly value whose maximum is −142 mgcal (Figure 6E). Our velocity model shows that, except for the LVA corresponding to the surface sedimentary cover, there are two HVAs below the surface LVA and within the upper mantle (Figures 5B, C).

The upper segment of the Junggar Basin velocity curve (depth <30 km) is similar to those of the Costa Rican volcanic front (Gazel et al., 2015) and the British Columbia accreted terrain (Morozov et al., 1998), while the deeper part (depth >30 km) of the velocity curve is close to those of the continental arcs or global continental average (green and blue curves in Figure 7D). This implicates that the shallow structure of the ancient continental crust below the Junggar Basin has been modified enormously due to the strong Paleozoic subduction and orogeny in this area.

Previous studies (Guo et al., 2003; Zhang et al., 2015) have revealed that Bogda Mountain has successively experienced the back-arc basin splitting in the Early Carboniferous, subduction during the Late Carboniferous, with extension in the Early Permian (Xiao et al., 2004), until reaching stability in the Middle-Late Permian and finally reactivating due to the long-range effect of the India-Tibet collision since the Neogene.

Our velocity model shows that although there is a HVA near the surface, a significant LVA is found within the middle-lower crust and uppermost mantle (Figures 5B, C). This LVA is consistent with the velocity pattern provided by Kong et al. (2021), both of which support the idea that the Paleo-Asian Ocean had subducted southward below the Bogda block (Xiao et al., 2004; 2018; Li et al., 2022). Likewise, this block shows a local low average Vp velocity of 6.57 km/s for the crust (Figure 6A) together with the lowest velocity of 7.0 km/s for the mantle Figure 6B). Furthermore, the upper segment (10–30 km) of the velocity-depth curve is similar to that of a continental arc, while the velocity for the deeper segment (depth >30 km) is clearly less than that of the rifts and orogens (Figure 7E). We suppose it indicates that the original deep structure of the lithosphere has been completely modified since the Paleozoic.

The Turpan-Hami Basin southeast of Bogda Mountains (Figure 1) is a continental intermountain basin formed by Mesozoic-Cenozoic tectonic movements, especially the northward extrusion of the Indian Plate since the Miocene. It began to be formed from the Late Permian, and experienced complicated stages of tectonic evolution, such as marginal fault depression, fault-depression transition, expansion depression, etc., which caused it to develop thick sedimentary strata (Yu and Fu, 1993).

Our velocity model shows that in addition to a prominent LVA in the shallow crust, corresponding to a thick sedimentary layer whose maximum thickness reaches 8.0 km, there are also two apparent HVAs within the lower crust and the upper mantle (Figure 5C). This block is characterized by having the minimum value of the Bouger anomaly of −202 mgal (Figure 6E), average Vp velocity of 6.56 km/s for the crust (Figure 6A), crustal thickness of 48.4 km (Figure 6C) and lithospheric thickness of 77.5 km (Figure 6D). The upper segment (5–30 km) of the velocity-depth curve is similar to that of the continental arcs, while the velocity at deeper depth (>30 km) is close to that of the Costa Rica volcanic front (Figure 7F). Therefore, we deduce that the powerful orogeny and volcanism caused by the subduction of the Tianshan Paleozoic Ocean slab completely modified the original middle-lower crust in this basin.

The Dananhu Arc is a part of the East Tianshan orogenic belt (Figure 1). It is located tectonically at the intersection of the Siberian plate, the Junggar terrane and the Tarim Basin (Windley et al., 1990; Xiao et al., 2004; Li et al., 2006), so it is an important window to study the convergence of ancient plates and the accretion of crustal collages. A series of igneous rocks, intrusive rocks and basic-ultrabasic complexes are widely developed in this island-arc belt, whose ages start from the Middle Ordovician (Xiao et al., 2004) and last until around the Permian (Li et al., 2006; Wang et al., 2019). Studies have shown that during the evolution of this block, it has experienced multiple stages of plate tectonic activity, such as plate subduction and collision, plate convergence-extension, and crustal ocean-land transition (Qin et al., 2002; Qin et al., 20011; Chen et al., 2016).

Our velocity model (Figure 5C) shows HVAs near the surface (depth <7.0 km) and in the middle-lower crust (depth >29.0 km) that can be correlated with the southern Kangurge structural belt, along of which there is ophiolite of the Cambrian-Silurian (416–494 Ma) widely exposed (Li et al., 2020). We assume that these HVAs suggest the strong orogeny and volcanism due to the northward subduction of the Paleozoic Tianshan Ocean slab.

This block has the maximum value of the Bouger anomaly of −137 mgcal (Figure 6E) and high average Vp velocity values of 6.69 km/s (Figure 6A) and 7.22 km/s (Figure 6B) for the crust and lithosphere, respectively. The middle segment (10–31.5 km) of the velocity-depth curve is close to that of the continental arcs or rifts, while the Vp value at deeper depth (>31.5 km) is much greater than that of the Costa Rica volcanic front (Figure 7G). These results support the idea that mantle-derived magma was powerfully introduced into the shallow crust and lower crust due to the Paleozoic subduction and inter-slab collage around the eastern Tianshan area.

The formation of Altaids is the product of the long-term subduction of the Paleo-Asian Ocean through a complex process dating back to ∼1.0 Ga (Khain et al., 2002) and that continued until ∼250 Ma (Windley et al., 2007; Xiao et al., 2018). Figure 5A illustrates the Nd isotopic model age (T_DM) profile along the study region (revised from Wang et al., 2020), which shows that although most blocks have developed a juvenile crust whose age is generally less than 500 Ma, the southern Altai has preserved the oldest crust. This is consistent with the respective velocity-depth curves (Figure 7), which shows that the deeper part velocity-depth curve (depth >25 km) is close to that of the global average of continent (in blue). On the other hand, those depart part curves (depth >25 km and in black) of the blocks Yemaquan arc, Bogda Mountain, Tuha basin and Dananhu arc are far from the blue curve, and it suggests the mid-lower crust within these younger blocks has been modified enormously due to the Paleozoic crust-mantle interaction related to subduction and orogeny.

Altaids has a complex paleogeography with multiple islands and seas (Hsü et al., 1991; Xiao et al., 2008; Pan et al., 2009); its western segment is a multi-oceanic basin, multiple subduction zone, complex multidirectional accretionary orogeny (Xiao et al., 2008), and large-scale oroclinal bending during accretionary orogeny (Sengör et al., 1993; van der Voo, 2004; Xiao et al., 2015; 2018).

According to recent results given by Yang et al. (2022), a large number of Paleo-Asian oceanic basins may be trapped during supercontinent formation, and hence make up a large proportion of the juvenile continental crust. They proposed that when the Tarim collage joined the amalgamating Kazakhstan and Tuva-Mongol oroclines in the late Permian-middle Triassic, the Junggar ocean have been protected by both the curvilinear oroclinal belt of the Kazakhstan orocline and the rectilinear lines of the northern Tarim (Xiao et al., 2015; Xiao et al., 2018), thus the remnant ocean basins in eastern and western Junggar were preserved in the complicated multiple convergence process (Yang et al., 2022).

Figure 5C shows that beneath the Yemaquan Arc, ocean slabs subducted northward of the Paleo-Asian Ocean were preserved within the lower crust, which is consistent with the results given by Yang et al. (2022); while another southward subducted ocean slab locates beneath Bogda Mountain, according to the geological results given by Xiao el a. (2004) and Li et al. (2022). Moreover, we have found significant Moho uplift beneath the Yemaquan Arc (Figure 6C), which could be attributed to uplift of mantle material from the depths due to Paleozoic subduction or delamination.

Orogenic deposits exist along the Erqis shear belt on the southern margin of the Altai (Figure 1, north segment of the profile), such as the Kalatongke copper-nickel deposit and the Tokuzibayi and Donbastao gold deposits. The early Permian Kalatongke copper-nickel sulfide deposit (around the shot point S02, Figure 1) is the largest copper-nickel mine in Xinjiang, and its mineralization is related to mafic-ultramafic rocks. Previous petrological and geochemical studies (Wang et al., 2000; Han et al., 2006; Han et al., 2011; Li et al., 2007) revealed that the cited deposits were formed in the Carboniferous-Permian and that the ore-forming magma originated from the upper mantle due to ascending intrusion of mafic magma or magmatic-tectonic fluid activity in a post-collision extensional environment.

Our velocity model shows an HVA in the middle crust between 20 and 30 km depth, a crust-mantle transition layer, and another HVA in the top mantle (Figure 5C). Also, the Vp value from 7.1 to 7.6 km/s corresponding to the deepest section (depth >40 km) of the velocity-depth curve (thick red line in Figure 7H) is greater than the global average for the continent (thin blue line in Figure 7H). These geophysical data support the earlier mantle-derived metallogenic mechanism, and we hypothesize that the strong convergence and continued northward compression of the Paleoasiatic oceanic plate caused delamination of the lithosphere in this region, leading to upwelling of mantle material from of the asthenosphere and subsequent intrusion into the lower crust and thus forming the crust-mantle transition zone. Hence, ore-bearing magma and hydrothermal fluid outcropped along deep faults and fissures to eventually form widespread copper and nickel deposits on the surface.

In the eastern Tianshan area of Xinjiang (Figure 1, south end of the profile), a large number of Permian mafic-ultramafic plutons developed, forming multiple magmatic copper-nickel sulfide deposits and vanadium-titanium magnetite deposits (Mao et al., 2006). Previous studies (Mao et al., 2010; Lü et al., 2019; Zhou et al., 2019) revealed that a series of mafic intrusions (Early Permian) originated in the mantle environment in the Kalatage mining area near the shot point S09 (Figure 1), and also in the Red Sea massive Cu-Zn deposit (Early Devonian) and the Yawan copper-nickel sulphide deposit (Early Permian). It is speculated that deep crustal faults and fractures provided the magma upwelling channel.

The Vp value corresponding to the deepest section (depth >30 km) of the velocity-depth curve for the Kalatage mineral area (thick blue line in Figure 7H) is clearly higher than any other for continental arcs, rifts and Costa Rica volcanic front. Considering the HVAs near the surface and in the lower crust and top mantle (Figure 5C), and the aforementioned ore-forming mechanism for the Kalatage area (Xiao et al., 2004; Li et al., 2006; Mao et al., 2010; Lü et al., 2019; Zhou et al., 2019), we understand that the original deep crust and top mantle undergone a strong modification due to the northward subduction of the Paleozoic Tianshan Ocean slab and the associated orogenic process. In the course of this process, mantle-derived magma and mineral-bearing hydrothermal fluids emerged along deep faults and fractures and then formed mineral deposits around the eastern Tianshan area, such as the Kalatage and Huangshan-Jingerquan copper-nickel mine.