Carbonate U‐Pb Ages Constrain Paleocene Motion Along the Altyn Tagh Fault in Response to the India‐Asia Collision

The kinematics and deformation pattern along the Altyn Tagh fault (ATF), one of the largest strike‐slip faults on Earth is of great significance for understanding the growth of the Tibetan Plateau. However, the initial rupture along the ATF remains debated given the limited constraints on the depositional age of associated Cenozoic syntectonic strata. Here we investigated the syntectonic Cenozoic strata in the Xorkol Basin, associated with the strike‐slip faulting along the ATF. New uranium‐lead analyses of the carbonate deposits in the Paleogene strata yield dates of 58.9 ± 1.29 Ma, representing the initial rupture of the ATF. This first documented radioisotopic age coincides with the ca. 60 Ma onset timing of India‐Asia collision, highlighting its far‐field effect at the northern edge of the Tibetan Plateau. We infer that the deformation of the entire Tibetan Plateau started synchronously with the India‐Asia collision.


Introduction
The formation of the Tibetan Plateau as a result of the Cenozoic India-Asia collision had a profound impact on the Asian tectonics configuration and climate dynamics (e.g., An et al., 2001;Ding et al., 2022).However, the geodynamic mechanisms that built the plateau remain disputed.Two main end-member models have been proposed: (a) The Tibetan Plateau has grown progressively and the northward-propagating deformation reached its present-day northeastern edge no earlier than the Neogene (e.g., England & Houseman, 1985;Tapponnier et al., 2001); (b) The northern and southern Tibetan Plateau underwent Paleogene deformation related to the incipient India-Asia collision (e.g., Dupont-Nivet et al., 2004;Jolivet et al., 2001;Yin et al., 2002).Determining the onset timing of Cenozoic deformation along the tectonic structures in the northern Tibetan Plateau is critical in resolving this dispute, as it will provide insights into how the northern Tibetan Plateau responded to the collision.
The Altyn Tagh fault (ATF) is a lithospheric left-lateral strike-slip fault that marks the northwestern boundary of the Tibetan Plateau (Yin et al., 2002) (Figure 1).Despite a great achievement on the timing of deformation in the northern Tibetan Plateau, the initiation age and configuration of the ATF remain debated given its long-lasting growth history.Yin et al. (2002) proposed a ca.49 Ma initiation of strike-slip faulting along the present ATF based on field investigation, magnetostratigraphy and subsurface reflection data.However, others have proposed different initiation timing and configurations.For instance, Tapponnier et al. (2001) proposed that the ATF gradually propagated from west to east during the Cenozoic.Yue and Liou (1999) proposed an early Oligocene initiation of strike-slip activity by analyzing the sedimentary basins along the fault.Inspired by the strike-slip duplex model raised by Cowgill et al. (2000), L. Wu et al. (2019) proposed that the ATF fault system initiated at ca. 53.5 Ma, featured by a large restraining bend consisting of the North Altyn fault and Jinyanshan fault, with the central ATF forming in the Miocene.
By far, low-temperature thermochronology on basement rocks that outcrop along the ATF is widely used to date the initial rupture along the ATF.However, the limited exhumation generated by strike-slip motion makes it challenging to capture the initiation of such motion through recorded ages.In addition, some published data are based on single-sample inverse thermal history modeling (e.g., Qi et al., 2016;Shi et al., 2018), the resilience and soundness of which remain questionable (Green & Duddy, 2021).Therefore, the exhumation history of the basement rocks along the ATF remains elusive, highlighting the need for a comprehensive understanding that integrates various methods.
Cenozoic syntectonic strata are well-developed along the northern and southern sides of the ATF.These terrestrial strata are associated with the ATF, providing a direct proxy to address the kinematics and timing of faulting.However, due to the lack of tephra layers and index fossils, the exact depositional age of these strata remains poorly constrained.Recent advancements in calcite U-Pb dating offer a promising tool to establish the absolute depositional age of the carbonate strata (e.g., Parrish et al., 2019;Rembe et al., 2022;Roberts et al., 2020).Here we report the first U-Pb radioisotopic age of lacustrine carbonates in the syntectonic strata that outcrop along the ATF.The obtained age of ca.60 Ma suggests that the Cenozoic strike-slip motion along the ATF initiated during the Paleocene, highlighting the synchronous deformation at both the northern and southern margins of the Tibetan Plateau in the early Cenozoic.

Methods
Field investigations were conducted in the East Xorkol Basin.The Xorkol Basin is an intermontane basin within the Altyn Tagh Range.Previous studies have reached an agreement that the formation of the Xorkol Basin has been governed by the ATF.Evidence includes the narrow and elongated geometry, the finer-grained deposits in the center compared to the edges, a series of stepped faults at the boundaries, and the presence of syntectonic conglomerates along the boundaries (e.g., Z. L. Chen et al., 2004;E. Wang et al., 2008).
The Xorkol Basin is subdivided into three parts: the Xorkol Valley Basin covered by Quaternary deposits, the North Xorkol Basin which accumulates Paleogene-Neogene sediments, and the eastern end named East Xorkol Basin (Figure 1b).The bedrock surrounding the East Xorkol Basin is attributed to the Ordovician Lapeiquan Group, which is composed of basaltic-andesitic volcanic rocks, slightly-metamorphosed clastic rocks, and carbonate rocks (QBGMR, 1986).Recent studies assigned a meso-Proterozoic depositional age on these carbonate rocks based on detailed mapping and structural analysis (e.g., B. Chen, 2018).
A total of 25 carbonate samples were collected from these Cenozoic strata, and 19 carbonate thick sections were dated using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing.We followed the analytical and data processing routine described by S. T. Wu et al. (2022).Specific details of sampling, sample preparation, and dating are given in Text S1 of Supporting Information S1.A total of 40 laser ablation spots were analyzed for each sample.Calcite veins, probably indicating late fluid circulations, were avoided during analysis to minimize potential contamination.

The Syntectonic Strata in the East Xorkol Basin
Here we provide additional evidence demonstrating that the North and East Xorkol basins are pull-apart basins of the strike-slip ATF.First, satellite images show that the both North and East Xorkol basins are characterized by serrated boundaries and rhomb grabens (Figures 1c and 1d); distinct topographic features resulting from the evolution of a pull-apart basin (Aydin & Nur, 1982).Second, the Paleogene strata in the East Xorkol Basin exhibit typical syntectonic features associated with the strike-slip faulting in a pull-apart basin (Christie-Blick & Biddle, 1985), as characterized by the following observations.The strata in the basin consist of brownish conglomerates along the margins and the carbonate rocks are interlayered with reddish mudstone and greenish sandstone at the center (Figure 2; Figures S1a and S1b in Supporting Information S1).The strata are in fault contact with the bedrock on both the northern and southern sides of the basin, evidenced by brecciated sediments series and fault gouges (Figures 2b, 2c, and 2e).Along the northern boundary, brecciated conglomerates display the tectonic cleavage that strongly imprints the original bedding (Figure 2c).Fault scarps and fault gouges with a strike of approximately N80°were also observed between the strata and the bedrock.Slickensides on the fault surface indicated a left-lateral strike-slip movement compatible with that of the ATF (Figure S1c in Supporting Information S1).The evolution of the depositional environment from alluvial fan to lacustrine carbonate deposits, coarse lateral sediment inputs, as well as the mirror-like symmetry on both the northern and southern sides of the basin, indicates syn-strike-slip fault sedimentation in the East Xorkol basin along the ATF.
Therefore, we suggest that the onset of Cenozoic sedimentation in the North and East Xorkol basins was associated with the initial tectonic activities of the ATF, and the initial depositional age of strata within the basin should be coincident with the initiation of strike-slip faulting along the ATF.The carbonate crops are either adjacent to or interbedded with the clastic rocks (Figure S1 in Supporting Information S1), indicating that the depositional age of the carbonate rocks might be slightly younger than the initial sedimentation of the strata.It is worth noting that the timing of deposition of these strata is coincident with the onset of basin formation.This precluded the possibility of these strata being basement rocks that were re-exposed due to the rejuvenated fault activities.This inference is further evidenced by the distinct contrast between the basin's strata and the surrounding basement rock, which contains gray phyllite and light gray dolomite.

Carbonate U-Pb Dating of Syntectonic Strata
The exposed carbonate outcrops in the Cenozoic strata in the East Xorkol Basin are about 60 cm thick at the ANB site and about 30 m thick at the KS site (Figures 3a and 3b).They are exposed with bedding parallel to the long axis of the basin.The carbonate is relatively homogeneous in appearance and exhibits a white and pink mottled coloration.It has been partially broken, most probably by tectonic activity, with tilted sedimentary layers that are not always clearly visible.The deposits are mainly characterized by primary micritic limestone displaying smallscale microbialite features.Evidence of alteration on hand specimen is scarce and the absence of post-depositional re-crystallization is confirmed by cathodoluminescence (CL) analysis (especially marked by evenly colored micrite) (Figure 3).This feature further affirms that the carbonate belongs to the Cenozoic syntectonic strata instead of the bedrock.Additional sample photos can be found in Figure S1 of Supporting Information S1.
We collected a total of 19 carbonate samples from two outcrops for U-Pb dating, of which 11 yielded coherent ages (Table S2 in Supporting Information S1).The complete data is available in Data Set S1, and the Tera-Wasserburg plots for each sample are present in Figure S2 of Supporting Information S1.These ages span from 43.0 Ma to 76.6 Ma.This relatively large uncertainty might be attributed to high common-Pb content in the individual samples.However, the carbonates from these 11 samples display homogeneous features in the microscope and cathodoluminescence images (Figure 3), thus the entire data set obtained from samples defines a single age population.We calculated two ages to describe the carbonate: a weighted mean age of 55.7 ± 1.96 Ma (based on 11 samples), and a regressed lower intercept age of 58.9 ± 1.29 Ma (based on a total of 432 spot analysis).The regressed age is slightly older than the weighted mean age, which is sensitive to large error margins carried by some of the data.We thus consider the lower intercept U-Pb age of 58.9 ± 1.29 Ma as a robust estimate of the age of the carbonate deposits in the East Xorkol Basin.

Paleocene Deformation Along the ATF
We report a new radioisotopic time constraint of 58.9 ± 1.29 Ma for the depositional age of carbonate strata in the East Xorkol Basin.This depositional age demonstrates that the sedimentation in the East Xorkol Basin initiated during the late Paleocene.Given the direct correlation between basin formation and ATF deformation, and considering the syntectonic nature of the strata, we propose that the initiation of strike-slip motion along the ATF occurred no later than 58.9 Ma, leading to the formation of the East Xorkol Basin as a composite pull-apart basin (Figure 4).This result further contributes to our understanding of the initiation timing and configuration of the ATF.First, we provide the first direct dating of syn-tectonic strata along the ATF, yielding the earliest deformation time, slightly predating previous research based on stratigraphy and thermochronology (Cheng et al., 2015(Cheng et al., , 2016;;H. Xie et al., 2022;Yin et al., 2002).Second, we reveal that the initial rupture along the ATF occurred in its central segment of the modern ATF system.This challenges the notion of a gradual northeastward propagation of the ATF (Jiao et al., 2023;Tapponnier et al., 2001) or the proposal that the ATF system initiated at the North Altyn fault (Gao et al., 2022;L. Wu et al., 2019) (Figure 1).Instead, our results show that the modern configuration of the ATF might have already been established since its initial stage in the early Cenozoic.

Insights on Paleocene-Eocene Strata and Deformation in the Northern Tibetan Plateau
The fine-grained deposits in the East Xorkol Basin exhibit similarities with the red clay layers in the North Xorkol Basin (Figure 5c), whose depositional age has been constrained to >51 Ma by recent magnetostratigraphy studies (Li et al., 2018).Furthermore, the deposits share a similar deposition pattern with the strata in the south of the ATF, including (a) distal fine-grained facies exhibiting massive reddish mudstone interbedded with greenish-gray sandstone that contains high levels of carbonate and gypsum and (b) proximal syntectonic conglomerate facies, mostly composed of poorly sorted matrix-supported pebbles-boulders (e.g., Lu et al., 2019) (Figures 5b and 5g).Therefore, we further suggest that the onset of deposition in the northern Tibetan Plateau is earlier than or equal to ca. 58.9 Ma.
While sedimentation onset has been considered to be closely associated with the initial deformation (e.g., W. Wang et al., 2017;Yin et al., 2002), we suggest that the relief building in the northern Tibetan Plateau initiated in the early Cenozoic.While mountain building in the Neogene, especially Miocene, is better recorded by the low-temperature thermochronology studies, rapid exhumation from Paleocene to Oligocene has also been observed in the Altyn Tagh Range, Qilian Shan and Kunlun Shan (e.g., He et al., 2021;Jolivet et al., 2001;Sobel et al., 2001) (Figure 1b).By comparing the timing of rapid basement exhumation with the onset timing of growth strata, Cheng et al. (2023) further indicates Paleogene tectonic activity in the northern Tibetan Plateau.
The initial continental collision between India and Asia plates is indicated to be 65-60 Ma ago (e.g., Cai et al., 2011;Ding et al., 2022;Hu et al., 2015;Yin & Harrison, 2000).We demonstrate that the deformation of the northern Tibetan Plateau initiated near-simultaneously with the collision.This implies that the collisionoriginated stress swiftly transferred to the north and instigated the tectonics activity along the ATF and in the northern Tibetan Plateau, even far into the Tian Shan (e.g., Jolivet et al., 2010).Several numerical simulations of  Aydin and Nur (1982).Note that the left-lateral faulting would induce the development of small grabens, then coalescing into composite structures, and finally forming a large basin with a serrated basin boundary.
the tectonic growth of the Tibetan Plateau have been established to explain this instant far-field deformation, highlighting the importance of the strong Tarim basement (e.g., Dayem et al., 2009;Xu et al., 2021), the relatively strong Qaidam basement (Cheng et al., 2017;Huangfu et al., 2022;R. Xie et al., 2023), and the lithospheric sutures zones forming pre-existing weaknesses (Cheng et al., 2021;Kelly et al., 2020;Kong et al., 1997;Mouthereau et al., 2013;Zuza et al., 2020) during this process.The contrast in lithospheric strength between the strong Tarim block and the weaker Tibet is a prerequisite for the instant distal orogenesis (Yin & Harrison, 2000).Meanwhile, the Qaidam crust is relatively strong and has the capacity to resist internal deformation (Cheng et al., 2017).Thus, the crustal deformation is concentrated on the pre-existing weaknesses such as sutures and faults, including the ATF (e.g., L. Chen et al., 2017;Cheng et al., 2021).

Conclusion
In this study, we provide the first carbonate U-Pb dating, which yields 58.9 ± 1.29 Ma from the syntectonic sediments in the pull apart East Xorkol Basin.This result provides compelling evidence that the ATF initiated its strike-slip motion during the Paleocene-early Eocene.This age estimate is also indicative of the depositional age of the syntectonic strata in the adjacent area, suggesting that the syntectonic sedimentation in the northern Tibetan Plateau initiated during the Paleogene.This result highlights the synchronized onset of deformation of the entire Tibetan Plateau with the initial India-Asia collision.S3 of Supporting Information S1.(b) Fine-grained strata in the East Xorkol Basin.(c and d) Fine-grained Unit 1 from the Caihonggou section (section CHG in Figure 1c) in the North Xorkol Basin and the Hongsanhan section (section E in Figure 1b) in the south of the ATF.(e) Syntectonic conglomerate in the East Xorkol Basin.(f and g) Syntectonic conglomerate in Unit 1 from the Lulehe and Hongshan sections (sections A and B in Figure 1b) in the south of the ATF.The strata exhibit similar characteristics across the described sections.

Geophysical Research Letters
10.1029/2023GL107716 YI ET AL.This paper is dedicated to the memory of Prof.An Yin (1959-2023), an outstanding geologist who devoted his life to Tibetan Plateau research.Financial support for this study was provided by the Key Program of the National Natural Science Foundation of China (Grant 41930213).Yi expresses gratitude for the financial support received from the China Scholarship Council for sponsoring her visit to the Université de Rennes.We gratefully acknowledge the assistance of Haiyan Cao during the fieldwork and Shitou Wu during measurements in the laboratory.We thank Editor Quentin Williams, Ryan Leary and Alex Webb for the outstanding review and suggestions.

Figure 1 .
Figure 1.Maps of the study area.(a) Tectonic location of Altyn Tagh fault defining the northern boundary of the Tibetan Plateau.Major strike-slip faults are shown in black lines.(b) SRTM-based digital topographic map of the northern Tibetan Plateau and the study area with distribution of the major timing of deformation.References are shown in Table S1 of Supporting Information S1.Locations of sedimentary sections in Figure 5 are denoted.(c) Simplified geological map of the North and East Xorkol basins modified from XBGMR (1981) and QBGMR (1986).Unit 1-5 are lithostratigraphic units corresponding to Paleogene-Eocene, Oligocene, lower Miocene, upper Miocene, and Quaternary strata respectively.CHG: Caihonggou section.(d) Google Earth satellite image of East Xorkol Basin with sampling locations.Note the serrated basin boundary and carbonate rock strata.The location of the geological profile in Figure 2f is indicated.

Figure 2 .
Figure 2. Field features of the East Xorkol Basin.(a) Panoramic photograph of the East Xorkol Basin.(b) Strike-slip fault surface with fault gouge and slickensides at the northern boundary of the basin.(c) Outcropped brecciated conglomerates within which a strong imprint of the tectonic cleavage replaces the original bedding.(d) Deformed fine-grained strata in the East Xorkol Basin.(e) Thrust fault with fault gouge at the basin's southern boundary.(f) Geological profile of the East Xorkol Basin.

Figure 3 .
Figure 3. Outcrop, samples, and dating results of carbonate rocks.(a and b) ANB and KS outcrops of carbonate rocks exhibiting fracturing, with a relatively fresh appearance and moderate weathering.(c and d) Photomicrograph and cathodoluminescence (CL) image of sample KS-14, revealing microbial features and primary sedimentary structures as a representative example.The uniformity of color observed in the CL image suggests that the sample has undergone little post-depositional alteration.(e) Weighted mean U-Pb ages of carbonate samples.(f) Tera-Wasserburg plot for the whole 432 data.The red lines are Concordia curves.Ellipses are 95% confidence level.

Figure 4 .
Figure 4. Block diagram illustrating the formation of the North and East Xorkol basins.The left-lateral motion of the Altyn Tagh fault formed en-enchelon, pull-apart basins and led to the early Cenozoic syntectonic sedimentation, no later than 59 Ma.(b) Model showing formation of a composite pull-apart basin associated with en enchelon strike-slip faults, modified afterAydin and Nur (1982).Note that the left-lateral faulting would induce the development of small grabens, then coalescing into composite structures, and finally forming a large basin with a serrated basin boundary.

Figure 5 .
Figure 5. Paleogene strata surrounding the Altyn Tagh fault (ATF).(a) Compiled chronostratigraphic studies of the Paleogene strata Unit 1 and 2, modified from Hu et al. (2022).Section locations are indicated in Figure 1b, and references are supplied in TableS3of Supporting Information S1.(b) Fine-grained strata in the East Xorkol Basin.(c and d) Fine-grained Unit 1 from the Caihonggou section (section CHG in Figure1c) in the North Xorkol Basin and the Hongsanhan section (section E in Figure1b) in the south of the ATF.(e) Syntectonic conglomerate in the East Xorkol Basin.(f and g) Syntectonic conglomerate in Unit 1 from the Lulehe and Hongshan sections (sections A and B in Figure1b) in the south of the ATF.The strata exhibit similar characteristics across the described sections.