Phanerozoic cratonization by plume welding

Deformation-resistant cratons comprise > 60% of the continental landmass on Earth. Because they were formed mostly in the Archean to Mesoproterozoic, it remains unclear if cratonization was a process unique to early Earth. We address this question by presenting an integrated geological-geophysical data set from the Tarim region of central Asia. This data set shows that the Tarim region was a deformable domain from the Proterozoic to early Paleozoic, but deformation ceased after the emplacement of a Permian plume despite the fact that deformation continued to the north and south due to the closure of the Paleo-Asian and Tethyan Oceans. We interpret this spatiotemporal correlation to indicate plume-driven welding of the earlier deformable continents and the formation of Tarim’s stable cratonic lithosphere. Our work highlights the Phanerozoic plume-driven cratonization process and implies that mantle plumes may have significantly contributed to the development of cratons on early Earth.

The role of mantle plumes in the evolution of the cratonic mantle lithosphere is conten-tious. Mantle plumes may (re)grow the continental mantle keel via basal accretion (Lee et al., 2011;Liu et al., 2021) or destroy the mantle root by convective removal or thermally induced delamination (Hu et al., 2018;Celli et al., 2020). Plume-lithosphere interactions may drive mantle melt depletion to generate thick, buoyant continental mantle capable of resisting future continental deformation (Xu et al., 2021). If plume processes can build stable cratonic mantle keels (>150 km) via mass and heat exchange between the crust and mantle (Jordan, 1978;Pearson et al., 2021), cratonization should have occurred throughout Earth history. Furthermore, the probability of craton generation in a particular geologic period should depend on the frequency and spatial abundance of mantle plumes.
The Tarim region of central Asia (Fig. 1A) exhibits a unique geologic history best explained by Permian plume-induced welding of Precambrian lithosphere. We detail this mechanism by presenting new magnetic data, three-dimensional (3-D) inverse modeling results, and high-resolution seismic profiles that elucidate the extent and consequence of the plume emplacement below the Tarim region. We demonstrate that Tarim's lithospheric thickness increased and deformation ceased after the plume impingement.

GEOLOGIC SETTING
The Tarim domain contains the North and South Tarim terranes, with Precambrian basement rocks separated by a Neoproterozoic suture that was reactivated in the early Paleozoic (Figs. 1B and 1C;Fig. S1 in the Supplemental Material 1 ; Guo et al., 2005;Zuza and Yin, 2017;Zhao et al., 2021). Despite earlier Neoproterozoic-early Paleozoic deformation, the region has remained undeformed since the Carboniferous, as indicated by a regional Carboniferous unconformity over deformed early Paleozoic strata (Guo et al., 2005). The post-Carboniferous stability of the Tarim region could be attributed to the lack of plate boundary-induced deformation, but the closure of the Tethyan oceans in the Mesozoic should have renewed deformation across Tarim since the late Permian, which is not observed (e.g., Yin and Harrison, 2000). The transition from the earlier deformable to the later nondeformable state correlates with the emplacement of a mantle plume at ca. 300-270 Ma, which may have strengthened Tarim's mantle lithosphere (Xu et al., 2021).

NEW GEOPHYSICAL CONSTRAINTS
Extensive Cenozoic sedimentary cover makes it difficult to estimate the extent of the region affected by the Permian plume. To overcome this issue, we used aeromagnetic data that are sensitive to iron-rich mafic rocks (Grant, 1985;Frost and Shive, 1986;Saltus et al., 1999). We determined the distribution of magnetic bodies across the Tarim region by performing a 3-D regularization inversion of the aeromagnetic data (Figs. S2 and S3 and the Supplemental text ; e.g., Hu et al., 2019). The model consisted of 130 × 74 × 50 grid cells at a resolution of 10 × 10 × 1 km. Our results reveal a ring-like pattern of magnetic anomalies with increasing magnitudes from the upper (∼10 km) to lower (∼45 km) crust (Figs. S3B and S4). A circular magnetic low was observed in the center of the ring-like structure (H in Fig. 2; Fig. S4), and its diameter decreases from ∼150 to ∼25 km from depths of 45 to 10 km, respectively. A zone of 170 × 45 km west-trending, highly magnetic anomalies (HMAs) at depths of ∼25-40 km traverses across most of the Tarim domain ( Fig.  S4); the same zone also intersects the ring-like anomaly ( Fig. 2; Fig. S4). Two northeast-trending, mid-to lower-crustal anomalies (T1 and T2-T3 in Fig. 2) are transcrustal features cutting upward from 50 to 25 km (Fig. S4). An isolated anomaly (T4 in Fig. 2) in westernmost Tarim occurs at depths of 30-45 km, with an increasing magnetic intensity and extent with depth ( Fig.  S4), whereas an anomaly in northernmost Tarim (M in Fig. 2; Fig. S4) is less extensive with depth.
We interpreted two north-south-oriented seismic profiles (Fig. 3). They both traversed the Central Tarim suture zone and imaged to ∼15 km depth, with age assignments from industry drill core (Fig. S5). A regional unconformity is present at the base of Carboniferous strata at ∼6 km depth across the basin (Fig. 3). Above the unconformity, flat-lying reflectors suggest Mesozoic-Cenozoic strata are largely undeformed, consistent with the present-day Tarim region's low relief. Toward its southern end, post-Carboniferous reflectors are sharply truncated against the Cenozoic left-slip Cherchen fault (Fig. 3). South of this structure, reflectors that represent the basal Carboniferous unconformity are folded and truncated, suggesting relatively strong deformation in this southern domain. Below the Carboniferous unconformity, early Paleozoic strata and Proterozoic basement are penetratively folded and faulted with series of thrust faults, with the highest frequency of deformation correlating with the suture zone (Fig. 3).

DISCUSSION
The prominent high-amplitude, longwavelength, crust-scale magnetic anomalies observed in this study are interpreted as thick mafic intrusions (Figs. S2B and S4;Grant, 1985;Frost and Shive, 1986;Saltus et al., 1999). The northeast-trending transcrustal anomalies (T1 and T2-T3 in Fig. 3) cut across the suture zone and the early Paleozoic orogenic belt in central Tarim (Fig. 1), requiring their formation to postdate both tectonic features. We interpret these anomalies to have formed by the impingement of the Perm- ian plume because its emplacement was the only regional Phanerozoic igneous activity in this area (Xu et al., 2014). We suggest the ring-like magnetic patterns represent mafic intrusions emanating from the plume head (Fig. 4). The ring-like anomaly truncates the HMA zone, thus requiring the zone to predate the Permian. The HMA zone may represent an Archean nucleus with characteristic long-wavelength magnetic highs, similar to HMAs distributed across other cratons within China (e.g., the Ordos and Sichuan cratons; Fig.  S6) and globally ( Fig. S7; Maus et al., 2009). Early Paleozoic reactivation of the Neoproterozoic suture between North and South Tarim (Fig. 1) led to deformation of early Paleozoic and older rocks, which were covered by Carboniferous-to-younger strata with an angular unconformity (Fig. 3). Boreholes show subtle domal uplifting and regional unconformities that correspond to the initial ca. 300 Ma impingement of the Tarim mantle plume (Li et al., 2014), but Mesozoic-Cenozoic strata are mostly flat lying, demonstrating negligible deformation since the late Paleozoic. The profiles across the central Tarim Basin reveal deformational patterns before and after the Carboniferous (Fig. 3), including the correspondence between strain and crustal composition interpreted from the magnetic inversion (Fig. 2). Parts of the profiles interpreted to be underlain by large mafic bodies show no post-Carboniferous strain, whereas those in the southeast without magnetically imaged mafic bodies experienced some deformation (Fig. 2). The absence of post-Permian deformation persists today, despite the closure of the Paleo-Asian and Tethyan ocean systems, which resulted in pervasive Mesozoic-Cenozoic deformation to the north and south (Windley et al., 1990;Yin and Harrison, 2000). These observations require the Tarim region to have become a deformation-resistant craton-like continent since Permian plume emplacement.
In summary, voluminous mafic intrusions overprint and postdate Neoproterozoic suturezone structures. The observed magnetic features radiating outward from the center of the plume head are absent in the southeast Tarim domain, which was more strongly deformed than centralwestern Tarim (Fig. 2). These observations suggest that the mid-to lower-crust mafic intrusions derived from the Permian plume stitched and welded the suture zone, making the Tarim region stronger than its surrounding areas. The distribution of crustal mafic bodies across Tarim overlaps the regions with higher crustal velocity ( Fig. S8; Bao et al., 2015) and density ( Fig. S9A; Deng et al., 2017), which supports our interpretations.
Crustal enrichment of mafic materials is compositionally related to the extraction and depletion of the underlying mantle (Jordan, 1978), which could generate a cratonic mantle keel (Jordan, 1978;Pearson et al., 2021). We interpret that the Permian mantle plume may have initially impinged upon the relatively thin, metasomatized lithosphere along the Tarim suture zone, potentially causing further local thinning via thermal erosion, which thus allowed the generation of a large volume of melts and crustal magmatism by ca. 290 Ma ( Fig. 4A; Xu et al., 2014). Assimilation and fractional crystallization of the melts may have, in turn, led to the formation of the central magma conduit of the plume composed mostly of high-Nb-Ta rhyolites ( Fig. 2; Xu et al., 2014). The plume-derived mantle melts modified the crust via lower-crust underplating (Xu and He, 2007;Chen et al., 2015) and the emplacement of mafic intrusions across the entire crust. This heat and mass transfer process built a dehydrated, depleted, and thickened mantle root that coherently welded the sutured Tarim lithosphere (Fig. 4A). The patchwork, relatively thin Permian lithosphere (<150 km; Xu et al., 2014) was subsequently thickened to >200 km today (Xu et al., 2002).
The region most modified by the mantle plume, as tracked via magnetic signals, corresponds to an inferred mantle keel with high velocity, reduced density, and high resistivity (Figs. S8 and S9B;Bao et al., 2015;Deng et al., 2017;Zhang et al., 2020). In our model, plumederived, melt-depleted mantle residue accreted to the base of the Tarim lithosphere to form a structurally intact cratonic lithosphere (Fig. 4B), which explains the present-day >200 km mantle root (Xu et al., 2002;Bao et al., 2015) and Cenozoic tectonic quiescence (Guo et al., 2005).
Although geophysical observations suggest lithospheric modification correlative with the plume-head location (Figs. S8 and S9B), eastern Tarim shows more variability of relative lithospheric density and resistivity (Deng et al., 2017;Zhang et al., 2020). These differences likely reflect heterogeneous inherited mantle lithosphere characteristics from the Precambrian continental terranes, early Paleozoic lithospheric thickening during orogeny (McKenzie and Priestley, 2016;Pearson et al., 2021), and spatially variable plume melt depletion . The Precambrian North and South Tarim terranes have Archean nuclei, consistent with our HMA interpretations, which may have consisted of initial, variably thick continental mantle. The Permian plume would have impinged, welded, and cratonized this initially heterogeneous continental mantle ( Fig. 4; Pearson et al., 2021).
Although mantle plumes may erode mantle lithosphere and even drive their delamination (Hu et al., 2018;Celli et al., 2020), plume impingement can be important for building (Lee et al., 2011) or healing  a strong mantle root. Plume-driven cratonization can strengthen preexisting weaknesses in the lithosphere, such as suture zones (Heron et al., 2016). Conversely, the other main suture zones in Tibet were strongly deformed by Cenozoic orogeny (Yin and Harrison, 2000;Zuza et al., 2018). Mountain building is an alternative mechanism to thicken the mantle lithosphere (McKenzie and Priestley, 2016) and potentially generate a cratonic keel . However, we can rule out the possibility that Tarim was cratonized solely by early Paleozoic orogeny because the coeval fold-andthrust belt outside the plume-impacted region was reactivated by Cenozoic deformation in northern Tibet (Zuza et al., 2018). Thus, the protracted his-tory of early Paleozoic orogeny and lithospheric thickening followed by plume impingement and welding possibly represents a protracted cycle of cratonization .
Given billions of years of plume activity in Earth history (Condie, 2001), this documented process may have been important for developing cratonic mantle throughout Earth history. Plume-related continental mantle development may be associated with a crustal record consisting of iron-rich mid-to lower crust that is detectable via magnetic imaging. Many cratons nested within global continents have highamplitude, long-wavelength magnetic highs that may reflect voluminous mafic material in the mid-to lower crust ( Fig. S7; Frost and Shive, 1986;Maus et al., 2009), which was ultimately extracted from the melt-depleted mantle as originally envisioned by Jordan (1978).
Higher mantle potential temperatures in early Earth require more rigorous, frequent, and regionally extensive plume activities, which may have affected the evolution of the continental lithosphere (Herzberg and Rudnick, 2012;Gerya et al., 2015;Bedle et al., 2021). In contrast, reduced Phanerozoic plume activity (Tomlinson and Condie, 2001) means that cratonization, as suggested by the Phanerozoic evolution of the Tarim lithosphere, was rare. Taken together, the plume-dominated first half of Earth's history would have favored the formation of stable continental regions that we refer to today as cratons.