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Permian plume-strengthened Tarim lithosphere controls the Cenozoic deformation pattern of the Himalayan-Tibetan orogen
Geology ( IF 4.8 ) Pub Date : 2020-09-11 , DOI: 10.1130/g47961.1 Xi Xu 1, 2, 3 , Andrew V. Zuza 4 , An Yin 3 , Xiubin Lin 1 , Hanlin Chen 1 , Shufeng Yang 1
Geology ( IF 4.8 ) Pub Date : 2020-09-11 , DOI: 10.1130/g47961.1 Xi Xu 1, 2, 3 , Andrew V. Zuza 4 , An Yin 3 , Xiubin Lin 1 , Hanlin Chen 1 , Shufeng Yang 1
Affiliation
The high strength of the Tarim Basin (northwestern China) lithosphere, widely regarded as a Precambrian craton, is evidenced by its resistance to Cenozoic deformation in the Himalayan-Tibetan orogen. However, Neoproterozoic suturing and early Paleozoic shortening within the Tarim Basin suggest that its rigidity is a relatively recent phenomenon with unknown cause. We reprocessed high-resolution magnetic data that show a 300–400-km-diameter radial pattern of linear anomalies emanating from a central region characterized by mixed positivenegative anomalies. We suggest that this pattern was generated by the previously hypothesized Permian (ca. 300–270 Ma) plume beneath the Tarim Basin. Constrained by published geochemical and geochronological data from plume-related igneous rocks, we propose that the ∼30 m.y. Permian plume activity resulted in a more viscous, depleted, thicker, dehydrated, and low-density mantle lithosphere. The resulting stronger lithosphere deflected strain from the Cenozoic India-Asia convergence around Tarim Basin, including Pamir overthrusting to the northwest and Altyn Tagh left-slip displacement to the northeast, thus shaping the geometry of the Himalayan-Tibetan orogen. INTRODUCTION The interior of the Tarim Basin, northwestern China, has remained undeformed during the development of the Cenozoic HimalayanTibetan orogen, expressed by flat-lying Cenozoic strata (e.g., Jia, 1997). Although its rigidity has been attributed to the presence of Precambrian basement (e.g., Neil and Houseman, 1997), this is inconsistent with the observation of an early Paleozoic thrust belt developed across the Tarim Basin (e.g., Yin and Nie, 1996; Jia, 1997; Carroll et al., 2001), which requires the acquisition of greater strength to resist Cenozoic deformation between the early Paleozoic and end of the Mesozoic. Alternatively, the rigidity of the Tarim lithosphere could have developed via the presence of a Precambrian oceanic plateau that was strengthened from below by a Permian plume (e.g., Deng et al., 2017). This model implies that Tarim was already strong prior to the Phanerozoic, inconsistent with aforementioned Paleozoic shortening across the basin. Because Early Permian (ca. 300– 270 Ma) mafic magmatism, interpreted to have been derived from a plume (Yang et al., 1996; Chen et al., 2006; Xu et al., 2014), was the only regionally extensive event affecting the Tarim Basin after the early Paleozoic (e.g., Jia, 1997; Carroll et al., 2001; Guo et al., 2005), it is possible that plume-induced processes profoundly strengthened the earlier Tarim mantle lithosphere. Assessing this possibility requires a sound knowledge of the root zone of the plume, which is obscured by ∼10 km of overlying Mesozoic–Cenozoic strata (Jia, 1997; Guo et al., 2005). Aeromagnetic anomalies have proven to be effective in locating and quantifying the geometry and extent of mafic intrusions associated with a mantle plume (e.g., Finn and Morgan, 2002). The success of this approach inspired us to reexamine the aeromagnetic data across Tarim. The extent of Permian intrusive rocks beneath the Tarim cover sequence has never been systematically determined before with aeromagnetic data sets (e.g., Xiong et al., 2016). Our reprocessed data reveal a 300–400-kmdiameter radial pattern of magnetic lineaments that emanate from a central region characterized by mixed positive and negative anomalies. Constrained by the existing geochemical and geochronological data from the plumerelated igneous rocks, we argue that Tarim’s mantle lithosphere was thickened, depleted, and dehydrated by the Permian plume to transform Tarim into a rigid craton-like continent in situ through the development of a thick mantle keel and input of mafic intrusions into the crust (e.g., Lee et al., 2011). Tarim’s stronger lithosphere deflected strain from Cenozoic India-Asia convergence around the Tarim Basin to shape the deformation pattern of the Himalayan-Tibetan orogen. GEOLOGICAL SETTING The Precambrian basement of the Tarim Basin consists of continental blocks that amalgamated along the Neoproterozoic central Tarim suture zone (Guo et al., 2005; Yang et al., 2018). The margins of the Tarim Basin were affected episodically by arc development and arc-continent collision throughout the Paleozoic during the development and closure of the Paleo-Asian oceans to the north (i.e., present southern Tian Shan; e.g., Yin and Nie, 1996; Xiao et al., 2005), and the Tethyan oceans to the south (i.e., present Tibetan Plateau; e.g., Yin and Harrison, 2000; Wu et al., 2016). The subduction polarity of the Carboniferous–Permian arc systems is enigmatic due to poor exposure and Cenozoic modification, but based on correlative subduction systems to the east (e.g., Xiao et al., 2003; Wu et al., 2016), there is a strong possibility that both subduction systems dipped beneath the Tarim continent. *E-mail: hlchen@zju.edu.cn Published online 11 September 2020 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/1/96/5204680/96.pdf by UCLA Digital Collect Serv user on 12 January 2021 Geological Society of America | GEOLOGY | Volume 49 | Number 1 | www.gsapubs.org 97 The Tarim Basin’s interior was affected by two regional events in the Phanerozoic. The first occurred in the early Paleozoic, expressed by reactivation of the Neoproterozoic suture zone as evidenced by thrusting and folding (Fig. 1; Jia, 1997; Carroll et al., 2001; Guo et al., 2005), which was coeval with the closure of the Qilian and Kunlun oceans in northern Tibet (Wu et al., 2016). The second event occurred during the Early Permian (300–270 Ma), expressed as widespread igneous activity, defining a large igneous province (LIP), possibly induced by a mantle plume from below (Chen et al., 2006; Xu et al., 2014). The full extent of the LIP is obscured by a >∼10 km section of sedimentary cover, but its wide distribution is indicated by surface exposures, subsurface samples, and extrapolation of these findings by analysis and interpretation of available seismic reflection profiles (Fig. 2A; e.g., Xu et al., 2014). The Permian plume-head location has been variably assigned to the Tian Shan and southwestern or northern Tarim (Chen et al., 2006; Li et al., 2014; Liu et al., 2016; Cheng et al., 2018). Subsequent Cenozoic India-Asia convergence resulted in concentrated shortening along the margins of Tarim Basin, including thin-skinned thrust belts in the north and west and strike-slip faulting in the southeast (Fig. 1; Yin et al., 1998; Guo et al., 2005). PERMIAN PLUME HEAD Data used in this study are mainly from the aeromagnetic data set of China (Xiong et al., 2016) with a 1 km × 1 km grid spacing and 1 km datum, supplemented by the Earth Magnetic Anomaly Grid Version 2 (EMAG2; Maus et al., 2009; see also Fig. S1A in the Supplemental Material1). The combined data set was first processed by reduction to pole (Fig. S2; e.g., Arkani-Hamed, 1988) and next by analytical signal transformation (Fig. S3; see details in the Supplemental Material; Roest et al., 1992). A series of high-amplitude, long-wavelength anomalies are superimposed by four variably oriented, short-wavelength, positive linear anomalies across the northwestern Tarim Basin (e.g., the Bachu Uplift; i.e., L1–L4 in Fig. S2). Highamplitude, long-wavelength magnetic anomalies have previously been correlated to signals derived from the Tarim Precambrian crystalline basement (e.g., Xiong et al., 2016; Yang et al., 2018). The final reprocessed result (Fig. 2A) reveals a radial pattern of linear anomalies: L1 lineaments trend southeast, L2 lineaments trend east and southeast, L3 lineaments trend east, and L4 lineaments trend northeast (Fig. 2A; Fig. S3), converging near Bachu City (∼39.8°N, ∼78.6°E). The radial linear anomalies overlap the Permian volcanic field (Fig. 2A). They converge beneath the observed ca. 300 Ma kimberlite complex, which are the oldest Permian intrusive rocks in Tarim Basin that are proposed to be associated with, and derived from, the plume head (Fig. 2A; e.g., Chen et al., 2006; Xu et al., 2014). This spatial correlation led us to interpret the linear magnetic anomalies to represent a giant dike swarm with arms radiating ∼150–200 km outward from the center interpreted as the Permian plume head. Additionally, isolated magnetic highs dispersed across Tarim Basin may have been related to plume-induced mafic intrusions (Fig. 2A). To assess whether the linear anomalies are related to Permian intrusions, we compiled 25,643 detailed rock-magnetic susceptibility measurements from within and around the Tarim Basin (Table S1). The measured susceptibilities concentrated in the Permian igneous suites range from ∼27 × 10−5 to ∼11,700 × 10−5 SI, with high mean values (>5300 × 10−5 SI; Fig. S1B). The high values of Permian intrusions show that the radial pattern of the positive linear anomalies can indeed be explained by Permian mafic intrusions, corroborated by surface exposures and drill core (Fig. S4). Although radially trending dikes should be distributed symmetrically around a plume center (Ernst and Buchan, 2003), our observed radial magnetic lineaments appear to occur only on the east side of the interpreted plume head. We note that the interpreted plume head is close to the southern Cenozoic Tian Shan thrust belt (Figs. 2B and 2C), which could have obscured the projected radial dikes northwest of the inferred plume head. To test this possibility, we schematically restored 20%–30% crustal shortening across the southern Tian Shan thrust belt and 10% shortening across the Tarim Basin (Fig. S5; e.g., Yin et al., 1998). This simple exercise yielded a magnetic anomaly pattern consistent with a radial dike swarm around the interpreted central plume head (Figs. 2C and 2D). The impact and extent of the interpreted Permian plume have been constrained by the Carboniferous-Permian unconformity in the Tarim Basin (Figs. 3A and 3B; Chen et al., 2006; Li et al., 2014;
更新日期：2020-09-11
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