Because of their widespread occurrence and ability to carry stable remanence, continental redbeds in central Asia are frequently used in paleomagnetic studies. However, the paleomagnetic inclinations recorded by redbeds are much shallower than the expected values, as redbeds are usually subjected to inclination shallowing. To recognize and correct the inclinations recorded by the Cenozoic redbeds, the paleomagnetic data that were used for magnetostratigraphic studies in the Kuqa Depression, northern Tarim Basin, are reanalyzed in this study. The mean inclinations of the four groups of samples (Eocene, Oligocene, Miocene, and Pliocene) are systematically ~20° shallower than the expected values calculated from the apparent polar wander paths (APWPs) of Eurasia, indicating the presence of inclination shallowing. We apply the elongation/inclination (E/I) method to correct the inclination shallowing. The mean inclinations of the Eocene, Oligocene, Miocene, and Pliocene sediments are corrected from 40.5° to 63.1°, 41.0° to 63.8°, 42.0° to 63.8°, and 44.7° to 63.2°, within 95% confidence limits between 55.1° and 71.6°, 53.7° and 70.4°, 51.5° and 72.7°, and 52.2° and 71.3°, respectively, which are indistinguishable from the expected inclination values. Our results suggest that inclination shallowing in the redbeds of central Asia can be reasonably corrected using the E/I method, and sedimentary processes such as compaction and/or imbrication in the very early stage of burial are important causes for inclination shallowing. Paleolatitudes calculated from the E/I-corrected inclinations show that the Tarim Basin should have reached or been at least close to its current latitude since the Cretaceous. The Cenozoic crustal shortening estimate of the northern Tarim Basin is not detectible for paleomagnetic study.

Accurate measurements of the paleomagnetic inclinations are crucial to determine paleolatitude, because paleomagnetic inclination is a function of the paleolatitude of deposition. Therefore, inaccurate inclinations can yield spurious paleogeographic reconstructions and faulty tectonic interpretations.

Terrigenous redbeds are frequently used in paleomagnetic studies because of their widespread distribution and capability to carry stable remanences. However, a controversy still remains regarding the accuracy of paleomagnetic inclinations recorded by sedimentary rocks because they are particularly prone to inclination shallowing (such as Garcés et al., 1996; Gilder et al., 2001; Tan and Kodama, 2003; Tan et al., 2002, 2003, 2010; Tauxe and Kent, 2004). Shallow biases in paleomagnetic inclinations have been noted and studied for decades (e.g., Anson and Kodama, 1987; Arason and Levi, 1990; Chauvin et al., 1996; Chen et al., 1992, 1993; Garcés et al., 1996; Hailwood, 1977; Huang et al., 2013; Jackson et al., 1991; Kodama, 2009; Krijgsman and Tauxe, 2004; Li et al., 2013; Tan and Kodama, 2003; Tan et al., 2002, 2003, 2010; Tauxe and Kent, 2004; Sun et al., 2006; Tauxe, 2005; Yan et al., 2005).

Several explanations have been proposed, such as non-dipolar geomagnetic field geometry (Chauvin et al., 1996; Dupont-Nivet et al., 2002; Si and Voo, 2001; Torsvik and Voo, 2002; Van der Voo and Torsvik, 2001; Westphal, 1993), undetected deep fractures that accommodate the northward displacement of different blocks (Halim et al., 1998), non-rigidity of the Eurasian plate (Chauvin et al., 1996; Tauxe, 2005; Thomas et al., 1993), and flexural subsidence of blocks (Sato et al., 2011). Additionally, a variety of processes occurring during or after deposition can also lead to shallow inclinations (e.g., Anson and Kodama, 1987; Arason and Levi, 1990; Bilardello and Kodama, 2009; Domeier et al., 2011; Fang et al., 2000; Hamano, 1980; Krijgsman and Tauxe, 2004; Tan et al., 2002, 2007; Tauxe and Kent, 1984; Yan et al., 2005). Depositional processes, such as rolling of magnetic particles after they have settled or flattening of platy particles due to gravity, have also been proposed as a cause of shallowing inclinations (Verosub, 1977). However, the exact causes of the shallowing bias are still uncertain. Calculating paleolatitudes from paleomagnetic inclinations in redbeds is therefore hindered by the major challenge of recognizing and correcting the inclinations.

Corrections of shallowing inclinations are based on the relationship between the magnitude of inclination error and field inclination (Griffiths et al., 1960; King, 1955; King and Rees, 1966):


where Im is the measured inclination, I0 is the corrected inclination, and f is the flattening factor, which ranges between 0.4 and 0.83 in hematite-bearing rocks and between 0.54 and 0.79 in magnetite-bearing rocks, respectively (Bilardello et al., 2011; Kodama, 2012).

An anisotropy-based inclination correction method has been used successfully in sedimentary rocks to detect and correct paleomagnetic inclinations shallowed by depositional and postdepositional processes (e.g., Jackson et al., 1991; Kodama, 1997; Kodama and Davi, 1995; Tan and Kodama, 1998, 2003). In this method, both the remanence anisotropy and/or individual magnetic particle anisotropy of the sample must be measured. Anisotropy of anhysteretic remanent (AAR) was found to be correlated with shallowing inclinations and has been used to correct the low inclination of magnetite-bearing sediments (Huang et al., 2013; Jackson et al., 1991; Kim and Kodama, 2004; Kodama, 1997; Kodama and Davi, 1995). However, the measurement of AAR is difficult for the hematite-bearing sediments (Kodama, 2009). Although a high-field (up to 5 T) anisotropy of isothermal remanence magnetization (hf-AIR) technique was applied to measure the anisotropy of hematite particle directly (Bilardello and Kodama, 2009), it is not widely used because it is particularly time-consuming and requires specialized laboratory equipment. A simplified anisotropy-based, inclination-correction technique for magnetite- and hematite-bearing rock using a curve-fitting method was proven effective in determining the individual grain anisotropy, which is critical for the correction of inclination shallowing (Kodama, 2009; Tan et al., 2007).

Assuming that the Earth's magnetic field is a geocentric axial dipole, the elongation/inclination (E/I) of the distribution of magnetic directions based on the statistical model (TK03.GAD) was developed (Tauxe and Kent, 2004). The E/I method determines the plausible value of ƒ that simultaneously makes the values for the average inclination and elongation of the unflattened distribution consistent with TK03.GAD (Krijgsman and Tauxe, 2004, 2006). The main advantage of this technique is that it does not require measurements of the magnetic fabric and individual particle anisotropy, thereby bypassing the time-consuming laboratory procedures. However, observations that are large enough (usually >100) are required to yield statistically significant results for the inclination correction in sedimentary rocks (Tauxe and Kent, 2004). Therefore, this method works well for studies that have been designed specifically for magnetostratigraphic studies and has been proven to be valid in correcting inclination shallowing in different areas (Huang et al., 2013; Krijgsman and Tauxe, 2004, 2006; Tan et al., 2010; Yan et al., 2005).

To better recognize and correct the Cenozoic inclinations recorded by the redbeds in central Asia, the E/I method is applied to the paleomagnetic data set that was used to construct the polarity sequences of the Kelasu Section, NE Tarim Basin (Zhang et al., 2015, 2016). The corrected inclinations are compared to the coeval expected values of this area to quantify the inclination shallowing. Our results provide a basic understanding and methodology of the correction of the shallowing inclinations in the Cenozoic redbeds in the Tarim Basin and shed light on the possible causes of this phenomenon.

The Tarim Basin, which is bounded by the Tian Shan Ranges to the north, the Pamir Plateau and West Kunlun Mountains to the west and southwest, and the Altyn Tagh Mountains to the south (Fig. 1), is one of the major ancient plates of China. It plays an essential role in accommodating and transferring tectonic stress caused by the Indian-Eurasian collision to the vast area to the north. Due to the intensive rejuvenation of the adjacent orogens in the Cenozoic, thick terrigenous sediments accumulated in the forelands (e.g., Kuqa Depression, Kashgar Depression, and SW Tarim Basin), providing great potential for studying the tectonic evolution and paleogeographic reconstruction of the Tarim Basin and adjacent orogens.

The Kuqa Depression, located on the NE margin of the Tarim Basin (Fig. 1), is a typical rejuvenated foreland basin in response to the far-field effect of the Indian-Eurasian collision (Windley et al., 1990). As one of the two Cenozoic depocenters along the northern margin of Tarim Basin (He et al., 2004), more than 5 km of the terrigenous sediments accumulated on the Paleozoic basement, which can be assigned to the Cenozoic. The Cenozoic succession comprises, in ascending order, the Kumugeliemu, Suweiyi, Jidike, Kangcun, Kuqa, and Xiyu formations (Figs. 2 and 3), with no obvious unconformities between them (Geological and Mineral Resources Bureau of Xinjiang Uygur Autonomous Region, 1993). Several magnetostratigraphic studies have provided the chronological framework for these sediments (e.g., Charreau et al., 2006; Huang et al., 2006; Sun et al., 2009; Zhang et al., 2015, 2016, 2018, 2019). In this study, our published magnetostratigraphic data set from the Kelasu Section is used; the data set constrains the basal ages of the Suweiyi, Jidike, Kangcun, and Kuqa formations at ca. 46 Ma, ca. 34 Ma, ca. 9.7 Ma, and ca. 5.3 Ma, respectively (Zhang et al., 2015, 2016).

In this study, the sampling strategy was designed for magnetostratigraphy of the Cenozoic sediments on the northern flank of the Kasangtuokai Anticline (Fig. 2), Kelasu Section, northern Tarim Basin were used. The paleomagnetic directions were divided into four groups (Eocene, Oligocene, Miocene, and Pliocene) to calculate the mean directions according to the magnetostratigraphic results. To ensure the correction accuracy, the paleomagnetic directions with the maximum angular deviation (MAD) larger than 10° were rejected. Moreover, outliers and transitional directions lying over 45° from the mean were systematically discarded (Deenen et al., 2011). Then, the E/I method was applied for correcting the shallowing inclinations recorded by redbeds. Finally, several magnetic parameters across the section were measured and used to discuss the possible causes of inclination shallowing.

3.1 Rock Magnetism Analyses

Based on the temperature-dependent magnetic susceptibility (k-T curves) experiments on representative redbeds samples, a major decrease at ~580 °C and a minor decrease at ~700 °C can be observed (Zhang et al., 2015, 2016); these decreases in temperature correspond to the unblocking temperature of magnetite and hematite, respectively. Additionally, the acquisition of isothermal remanent magnetization (IRM) curves shows rapid increases from 0 to ~200 mT and more than 70% of the maximum magnetization was acquired at ~300 mT, indicating the presence of low-coercivity magnetic minerals, such as magnetite. The magnetization was unsaturated in fields up to 1.5 T. This implies the existence of high-coercivity magnetic minerals. Moreover, the thermal demagnetization results showed a decrease in magnetization intensities near the Curie temperature of magnetite of ~580 °C and more frequently of hematite of ~680 °C. All of the results for rock magnetism together support that both magnetite and hematite are generally present as remanence carriers in the sediments. For more details on the rock magnetism analyses, readers are referred to Zhang et al. (2015, 2016).

3.2 Anisotropy of Magnetic Susceptibility (AMS) Analyses

The AMS of the samples was measured using an AGICO KLY-4S susceptibility meter. Magnetic fabrics are quantified based on the following parameters: the lineation L, the foliation F, the corrected degree of anisotropy Pj, and the shape parameter T; these parameters are calculated from the three main axes of the magnetic ellipsoids (Kmax, Kint, and Kmin) (Jelinek, 1981). The equal-area projections of the AMS of the four groups are shown in Figure 4, the Kmax scatters on the bedding plane and perpendicular to the main strain associated with the regional north-south–directed shortening, whereas Kmin is nearly perpendicular to the bedding after tilt-correction. On the T-Pj plots, the majority of samples were located in the area with 0<T<1, suggesting oblate ellipsoids were caused by compaction normal to the bedding. These characteristics indicate incipient deformation magnetic fabrics across the whole section. The other AMS parameters F, L, Pj, T and the inclination of Kmin, of the four groups are shown in Table 1.

The AMS in weakly deformed mud rocks can undergo a series of changes as the intensity of deformation increases (e.g., Parés, 2004; Saint-Bezar et al., 2002). The first stage is the cluster of the Kmax perpendicular to the shortening direction, whereas the Kmin is still normal to the bedding (Borradaile and Tarling, 1981; Parés et al., 1999). With further shortening, the Kmin becomes distributed along a girdle that is parallel to the tectonic shortening direction, and the magnetic ellipsoid is prolate. The angles between magnetic foliation and the bedding plane have also been used as a way to determine the intensity of tectonic deformation experienced by sediments (Robion et al., 2007; Sagnotti and Speranza, 1993). In this study, Kmax axes distribute parallel to the shortening direction, whereas Kmin axes are roughly perpendicular to the bedding (Fig. 4). Additionally, the magnetic foliations are roughly parallel to the bedding, and the angles between magnetic foliations and the bedding are <15°. Therefore, we suggest that the fabric can be assigned to embryonic fabrics in weakly deformed sedimentary rocks, implying that the sediments in this section have been subjected to incipient deformation. However, the magnetic fabrics are still dominated by depositional processes, rather than tectonic origin.

3.3 Characteristic Remanent Magnetization (ChRM) Analyses

Step-wise thermal demagnetization was performed to isolate the ChRM carried by hematite. Four successive points at least were used for the calculation of characteristic direction through principal component analysis (Kirschvink, 1980). Most of the samples have three components of magnetization isolated by step-wise thermal demagnetization: the low-temperature components were unblocked before ~350 °C, whereas the intermediate components that were unblocked at ~580 °C (corresponding to the unblocking temperature of magnetite) were coaxial with the ChRMs carried by hematite. All the ChRMs were acquired at high temperature (mostly between 650 °C and 680 °C). To ensure the accuracy of the correction, only directions with MAD angles smaller than 10° were chosen for this study. In general, 848 directions were collected for further analysis (Eocene, 356; Oligocene, 148; Miocene, 226; Pliocene, 118). The mean inclinations of the Eocene, Oligocene, Miocene and Pliocene samples are 40.5°, 41.0°, 42.0°, and 44.7°, respectively, which are ~20° shallower than the expected values for the region calculated from the APWP of the Eurasian Plate with the same ages (Table 2; Torsvik et al., 2012). The uncorrected inclinations are in accordance with previous results (Charreau et al., 2006; Chen et al., 1991, 1993; Fang et al., 2000; Huang et al., 2008; Wu et al., 2002; Zhu et al., 1998). This suggests that the paleomagnetic inclinations recorded by the Cenozoic redbeds from the NE Tarim Basin have suffered from obvious shallow bias. The projections of ChRMs after tilt-correction showed that they were elongated in E-W direction (Figs. 5A, 5D, 5G, and 5J), which was caused by inclination shallowing (Tauxe and Kent, 2004).

3.4 Reliability Analyses

To assess the nature of the acquired ChRM directions, both reversal and fold tests were applied (Zhang et al., 2015, 2016). The positive fold test result and the classification-B reversals test indicate a primary magnetization without secondary bias in directions, thus providing reliable results for the correction of inclination shallowing in this study.

The E/I method based on the statistical paleosecular variation models of the geocentric axial dipole magnetic field suggested by Tauxe and Kent (2004) was applied to the four groups, respectively (Fig. 5). The value of ƒ ranges from unity to 0.3. One thousand bootstrapped data points were also used to estimate the uncertainty.

The E/I corrected inclination of the Eocene sediments is 63.1° with 95% confidence limits between 55.1° and 75.6°, while its mean inclination before correction is 40.5°. The corrected inclinations of the Oligocene, Miocene, and Pliocene are 63.8°, 63.8°, and 63.2°, with 95% confidence within the ranges of 53.7–70.4°, 51.5–72.7°, and 52.2–71.3°, respectively (Fig. 5 and Table 2). The flattening parameters of the four groups are the same value (ƒ = 0.4). We recalculate the inclinations of all the samples based on Equation (1) and replot the equal-area projections (Fig. 6), which suggest that the inclinations are elongated in the N-S direction, whereas the initial distributions of directions (uncorrected directions) are elongated E-W (Fig. 5). The E/I corrected results suggest that paleomagnetic inclinations of the Cenozoic redbeds have indeed been affected by shallowing bias. Furthermore, the E/I correction implies that the ChRM directions were acquired during or shortly after deposition and were subsequently flattened.

5.1 The Reliability of E/I Correction

The E/I correction method assumes that the scatter in paleomagnetic directions is caused by either variation in the geomagnetic field or sedimentary flattening following Equation (1), and the directional dispersion can be predicted. It is a relatively easy technique for detecting and correcting shallow inclinations totally independently of anisotropy measurements. The E/I method determines the optimum value of f by adjusting it until the corrected directions have an elongation and inclination consistent with that predicted by the paleosecular variation field model. To obtain a robust correction, a data set large enough (usually >100) that has sampled secular variation of the geomagnetic field (Tauxe and Kent, 2004; Tauxe et al., 2008) is needed. For this reason, the best E/I corrections come from the analysis of magnetostratigraphic studies that typically collect many sites (Kodama, 2012). In sedimentary rocks, it can be assumed that each sample represents a geological instant in time; therefore, it is reasonable to use the E/I method for individual sedimentary rock directions (Bilardello et al., 2011). This method has been proven by numerous published studies to be effective in detecting and correcting for both magnetite-bearing and hematite-bearing sedimentary rocks and resulted in corrected inclinations that give geologically or tectonically reasonable interpretations (Channell et al., 2010; Dupont-Nivet et al., 2010; Hillhouse, 2010; Huang et al., 2013; Kent and Olsen, 2008; Kent and Tauxe, 2005; Krijgsman and Tauxe, 2004, 2006; Liu et al., 2010; Meijers et al., 2010; Schmidt et al., 2009; Tan et al., 2010; Tauxe et al., 2008; Wang and Yang, 2007; Yan et al., 2005). Moreover, a comparison of the E/I and anisotropy-based correction techniques for two case studies presented by Tauxe et al. (2008) indicated that both the anisotropy and E/I corrections are statistically indistinguishable from each other.

In this study, our previously published paleomagnetic directions, which were designed for magnetostratigraphic study from the Kelasu section, northern Tarim Basin, were used to correct the inclination by applying the E/I method. The paleomagnetic directions were subdivided into four groups according to the published magnetostratigraphic results (Zhang et al., 2015, 2016), all of which have a sufficiently number of paleomagnetic directions (>100, Table 2). The E/I correction gave best-fit inclinations of 63.1° (95% confidence bounds range from 55.1° to 71.6°), 63.8° (95% confidence bounds range from 53.7° to 70.4°), 63.8° (95% confidence bounds range from 51.5° to 72.7°), and 63.2° (95% confidence bounds range from 52.2° to 71.3°) for the Eocene, Oligocene, Miocene, and Pliocene, respectively; these values are consistent with the predicted values calculated from the APWPs of the stable Eurasia (Table 2).

Kodama (2012) proposed that one limitation of the E/I technique is that it is assumed that each site is an independent spot measure in time of geomagnetic field secular variation. Because each sedimentary rock sample integrates its recording of the field over the time of its deposition, paleomagnetic samples of finite thickness average some secular variation. Furthermore, unless all the samples from a paleomagnetic site are collected from exactly the same horizon, there will be some averaging of secular variation within a site. This could lead to an underestimate of the initial corrected inclination.

The paleomagnetic directions used in this study were designed for analysis of the magnetostratigraphy of the Kelasu section, northern Tarim Basin. We collected only one or two samples from the same horizon, and therefore we inevitably average the secular variation of geomagnetism. It is worth noting that the E/I corrected inclinations of the four age groups in this study are consistent with the expected values calculated from the APWPs of the stable Eurasia, indicating that the E/I corrections using the paleomagnetic directions are valid.

Tan et al. (2003) studied the shallow inclinations recorded by the Cretaceous redbeds in the northern Tarim Basin, illustrating an approach that uses both chemical demagnetization and the measurement of AMS to correct the shallow inclinations in hematite-bearing sedimentary rocks. In this technique, the individual particle anisotropy was determined by finding the best fit between the corrected inclinations and the theoretical correction curves. Finally, Tan et al. (2003) successfully corrected the paleomagnetic inclinations from 29.0° to 61.5°, using the magnetic anisotropy-based method. This was the first result to show that the anomalously shallow inclinations recorded by the Cretaceous redbeds from central Asia were due to inclination shallowing rather than standing non-dipole fields or undetected continental-scale shear zones, as had been previously proposed. Moreover, the result is roughly consistent with our E/I-corrected inclinations, together suggesting that paleomagnetic inclination shallowing is the cause of low inclinations recorded by the redbeds in the Tarim Basin. This result and our E/I correction suggest that the anisotropy-based correction and E/I method are effective in correcting the shallowing inclination in the northern Tarim Basin.

In this study, the AMS carried by the ChRM-carrying hematite particles was not isolated, because the samples were initially designed for magnetostratigraphic study of the section. Therefore, the anisotropy-based correction method was not used here. However, a comparative study on the E/I and anisotropy-based corrections for a variety of lithologies, including redbeds, suggested that both methods are valid (Tauxe et al., 2008).

5.2 Potential Causes for Inclination Shallowing

Although the shallow bias in redbeds of central Asia has been widely noted and studied for several decades, its accurate causes are still debated. For the purpose of testing whether depositional processes can affect the sedimentary inclinations, redeposition and compaction experiments have been carried out in the laboratory (Anson and Kodama, 1987; Hamano, 1980; Tan et al., 2002). These results suggested that compaction after deposition can indeed cause inclination shallowing. Numerical depositional experiments have been carried out in the lab to test the effects of several factors, such as compaction, grain size, and sedimentation rates, during and/or after the processes of sedimentation, suggesting that faster sedimentation rates, coarser grain size, and more intensive compaction can produce more inclination shallowing (Borradaile, 1987; Griffiths et al., 1962; Hamilton and King, 1964; Jezek and Gilder, 2002; King, 1955; Rees and Woodall, 1975). Based on observations, Gilder et al. (2003) suggested that the Cretaceous redbeds in the Tarim Basin have much shallower inclinations than those in North and South China because the sedimentation rates around the Tarim Basin at the same time are much higher. Similarly, Jezek and Gilder (2006) suggested that the shallower inclinations observed in Neogene sedimentary rocks from the southern flank of Tian Shan compared to the inclinations measured in coeval sedimentary units from the northern flank can be partly attributed to higher mean sedimentation rates.

In order to better understand the potential causes of inclination shallowing in the Cenozoic redbeds, NE Tarim Basin, we also analyzed the AMS parameters and ChRM inclinations of the studied section. The lineation L appears to generally decrease upward throughout the whole section (Fig. 7A), indicating that magnetic minerals in the lower part are more orientated than in the upper part. The general up-section increases in F and T (Fig. 7B and 7C) suggest that either compaction is related to higher clay content or finer grain size, because finer sediments generally experience more compaction than coarser sediments. This can be further evidenced by the changes of lithology across the whole section based on our field observations. The bottom of the section (0–~400 m) is dominated by reddish mudstones, occasionally interbedded with thin layers of conglomerates. There is a coarsening trend from the bottom to the top of the section, especially in the Pliocene sediments, as syntectonic deposits implied that the late Cenozoic deformation initiated at ca. 5.3 Ma (Zhang et al., 2016). The mean values of Kmin inclinations are deflected ~20° southward/southeastward from the poles of the bedding plane (Fig. 4), which are nearly parallel to the regional north-south–directed shortening and paleocurrent directions determined from the imbrication of gravels throughout the section. Therefore, the southerly and/or southeasterly tilted inclinations of Kmin were caused by paleocurrents, implying an upward decrease in the degree of sedimentary imbrication. We conclude that the AMS is not a function of tectonic strain origin. Although compaction and/or imbrication can induce inclination shallowing, as reported from laboratory experiments (Kodama and Sun, 1992), the plot of ChRM inclinations versus thickness (Fig. 7E) suggests no obvious dependence between the inclination error and burial depth. This can be explained by the fact that compaction-caused inclination errors only occur at the very early stage of burial, during dewatering and rearrangement of platy magnetic particles (Deamer and Kodama, 1990), while no more significant shallowing is induced by further increases of sediment load. Although higher sedimentation rates can also lead to more inclination shallowing than lower rates (Jezek and Gilder, 2002), there is no significant increase in the magnitude of shallowing inclinations since ca. 5.3 Ma, indicating that sedimentation rate is not the dominant reason for it in this study. Because the sediments coarsen upsection, it is expected that the foliation and oblateness would decrease and the more compacted sediments (with stronger foliation and oblateness) would be at the bottom of the section, since finer-grained sediments would compact more easily. However, the inclinations across the whole section remain roughly constant. Therefore, we suggest that the degree of shallowing inclinations is not related to the subsequent compaction after deposition.

These results demonstrate that depositional processes (such as compaction and imbrication) play important roles in affecting inclination recorded by sedimentary redbeds at the very early stage of burial, as documented by previous studies (Hrouda, 1991; Jackson et al., 1991; King and Rees, 1966; Tan and Kodama, 1998). However, to clarify this problem, the magnetic remanence acquisition mechanism of minerals, especially the platy hematite in redbeds, should be further studied. For geological studies, it is more important to reasonably test and correct the inclination shallowing.

5.3 Implications

As one of the major plates forming the continent of China, the Tarim Block plays an important role in paleogeographic reconstructions. Although the paleolatitude of the Tarim Block in the Paleozoic is fairly exact, there is still no agreement about the accurate positions of it in the Mesozoic (Cretaceous, in particular) and Cenozoic due to the shallowing inclinations recorded by sedimentary rocks. According to geological and paleogeographic data, the Tarim Block was in the low latitude of the Southern Hemisphere (~19°S), attached to the Gondwana in the early Ordovician. Then, it broke away from Gondwana in the late Paleozoic, drifting across the Paleo-Tethys Ocean (Burrett et al., 1990; Fang and Shen, 2001; Nie, 1991; Smith et al., 1997). The southern Tian Shan Ocean basin began to subduct beneath the Yili micro-continent from the late Silurian to Devonian (Huang et al., 2008). At the same time, the Tarim Block moved northward quickly with clockwise rotations (~15°) and reached the low latitude in the Northern Hemisphere (~14°N) in the middle Silurian. The collision between the eastern part of the Tarim Block and Kazakhstan initiated in Late Devonian near 27°N (Huang et al., 2008), forming the west-facing remnant oceanic basin. Oblique collision between the Tarim Block and the Yili micro-continent finally closed the Southern Tian Shan oceanic basin in the Late Permian (Chen et al., 1999). There was no obvious change in paleolatitude from the late Carboniferous to the late Permian, indicating that there was no apparent northward movement in this period. The abrupt decrease in northward movement since the late Carboniferous could be due to the obstruction caused by the blocks to the north (Fang and Shen, 2001; Huang et al., 2008). During the Permian–Jurassic, the paleolatitude of the Tarim Block remained constant, which indicates that it no longer significantly moved to the north. However, the mean paleolatitudes of the Tarim Block in the Jurassic and Cretaceous were suggested to be at ~32.6°N and ~21.0°N, respectively (Fang and Shen, 2001; Feng et al., 2011). Assuming the results are correct, the Tarim Block must have drifted southward for more than 1000 km in the Cretaceous, as suggested by Fang and Shen (2001). Using the AMS parameters and individual particle magnetic susceptibility anisotropy, Tan et al. (2003) corrected the inclinations of the Early Cretaceous redbeds (the Kapushaliang Formation) of Kuqa Depression from 29.0° to 61.5° (the corresponding paleolatitude is 42.6°). The results indicated that the paleolatitude of the Tarim Block had already reached or was close to its current position in the Cretaceous. In this study, the E/I-corrected inclinations of the Eocene, Oligocene, Miocene, and Pliocene are 63.1°, 63.8°, 63.8°, and 64.1°, respectively (Table 2). These results are consistent with the expected values, which suggest that the paleolatitudes of the NE Tarim Block from the Eocene to Pliocene were roughly the same (~44°, regardless of the error). Therefore, the previously proposed large-scale southward movement of the Tarim Block (e.g., Fang and Shen, 2001; Feng et al., 2011) in the Cretaceous and the inferred northward drift in the Cenozoic were caused by the inclination shallowing. Previous balanced cross-section restoration suggests that the total crustal shortening of the Kuqa thrust-fold system is between 10 and 30 km (Burchfiel et al., 1999; Liu et al., 1994; Yu and Wang, 2009; Zhang and Chen, 1998; Zhang et al., 2014). The result of balanced cross-section restoration in the Kelasu section is evidently comparable to these results (Tian et al., 2016). Therefore, we suggest that the Cenozoic crustal shortening of the northern Tarim Basin is not detectible given the uncertainties in paleomagnetic measurements.

The ~20° discrepancy between the measured inclinations recorded by the Cenozoic sedimentary redbeds from the northern Tarim Basin and the expected values calculated from coeval APWPs of the stable Eurasia suggests that paleomagnetic inclination has been subjected to significant shallowing bias. The E/I method, based on the paleosecular variation of geomagnetic field models (Tauxe and Kent 2004), is used to correct the shallowing inclinations recorded by redbeds from the Tarim Basin. The inclination shallowing can be successfully corrected using the E/I method. The corrected inclinations imply that the northern Tarim Basin has reached or at least been close to its current position since the early Cretaceous, and the Cenozoic crustal shortening is not detectible by paleomagnetic measurements. Our results suggest that there is no obvious dependence between inclination and burial depth, and sedimentary processes (e.g., compaction and imbrication) at the very earliest stage of burial are important reasons for the inclination error. The E/I method, which can provide reliable shallowing-corrected paleomagnetic inclinations for paleogeographic reconstruction, is effective in testing and correcting the shallowing inclinations recorded by redbeds in central Asia.

This study is jointly supported by the National Science and Technology Basic Resources Investigation Program of China (2021FY100102), the Basic Scientific Research Fund, Institute of Geology, China Earthquake Administration (IGCEA2113), the Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070202), and the National Nature Science Foundation of China (41888101, 41702209, and 41672168). We are grateful to Pengcheng Tang, Yangli Yu, Bo Zhao, and Zhonghua Tian for their help in the field sampling. The constructive suggestions on our manuscript from Prof. K.P. Kodama and another anonymous reviewer are sincerely appreciated.

Science Editor: David E. Fastovsky
Associate Editor: Jose M. Hurtado
Gold Open Access: This paper is published under the terms of the CC-BY-NC license.