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Depth Variation of the Conrad Discontinuity in the Qaidam Basin, Northwestern China, and Its Crustal Dynamic Implications

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Depth Variation of the Conrad Discontinuity in the Qaidam Basin, Northwestern China, and Its Crustal Dynamic Implications Earth and Planetary Physics RESEARCH ARTICLE 5: 296–304, 2021 SOLID EARTH: SEISMOLOGY doi: 10.26464/epp2021030 Depth variation of the Conrad discontinuity in the Qaidam Basin, northwestern China, and its crustal dynamic implications Biao Yang1, YanBin Wang1*, Li Zhao1, LiMing Yang2, and ChengNing Sha2 1Department of Geophysics, School of Earth and Space Sciences, Peking University, Beijing 100871, China; 2Earthquake Administration of Qinghai Province, Xining 810001, China Key Points: ● Broadband records from a dense seismic network were used to analyze the crustal phases and investigate the depth of the Conrad and Moho discontinuities in the Qaidam Basin. ● The depth of the Conrad discontinuity under the eastern part of the basin is shallower than the western part, which can be attributed to different crustal shortening mechanisms. ● The depth of the Moho discontinuity under the Qilian orogenic belt and Bayan Har block is deeper than under the Qaidam Basin, which is consistent with the previous results. Citation: Yang, B., Wang, Y. B., Zhao, L., Yang, L. M. and Sha, C. N. (2021). Depth variation of the Conrad discontinuity in the Qaidam Basin, northwestern China, and its crustal dynamic implications. Earth Planet. Phys., 5(3), 296–304. http://doi.org/10.26464/epp2021030 Abstract: We use broadband records from a dense seismic network deployed in and around the Qaidam Basin in northwestern China to analyze the crustal phases and investigate the depth of the Conrad and Moho discontinuities as well as the P-wave velocity. Waveform cross-correlation is used to assist in the identification of the crustal phases and in determining their arrival times. Depth of the Conrad discontinuity is determined by fitting the travel times of Conrad-diffracted P-waves using a two-layer model. The depth of the Conrad discontinuity under the eastern part of the basin is shallower than the western part, which can be attributed to different crustal shortening mechanisms. The upper crust shortening in the western part of the basin leads to thickening of the upper crust, while multiple thrust faults result in the rise of the Conrad discontinuity in the east. These two different mechanisms determine the depth change of the Conrad discontinuity in the basin from the west to the east, which is supported by the results in this study. Keywords: Conrad discontinuity; regional seismic data; crustal structure; Qaidam Basin; Tibetan Plateau 1. Introduction discontinuity at a depth of ~30 kilometers. Teng JW et al. (1995) The Qaidam Basin is located in the northern margin of the Tibetan revealed from Rayleigh-wave dispersion that the crustal thickness Plateau (Figure 1) in northwestern China. It is surrounded by the of the Qaidam Basin is 42−58 kilometers. The velocity and Poisson Tarim block in the northwest, the Qilian block in the north and the ratio obtained by wide-angle seismic reflection/refraction data Bayan Har block in the south. The Altyn Tagh Range in the north- show that there is an obvious low-speed layer in the depth range west, the Qiman Tagh Thrust in the southwest, the North Kunlun of 12−20 kilometers in the north and south margins of the Thrust in the south, and the Qilian Mountains in the north and Qaidam Basin (Zhao JM et al., 2006). east form the boundaries of the Qaidam Basin. The concentration Structural complexity in the mid-crust as a global phenomenon of tectonic stress in the Tibetan Plateau results in strong tectonic has been recognized since the very beginning of seismology. Con- activities in and around the basin (Huang Y et al., 2017). The in- rad (1925) observed an arrival he named P* when he examined re- ternal geological structure of the basin is very complex as a result cords from an earthquake in Tauern, Austria. He interpreted P* ar- of multiple cycles of sedimentation (Meng QR, 2009). rival as a wave propagating immediately under a mid-crustal in- Many geophysical investigations have been conducted on the terface, analogous to Pn. Jeffreys (1926) subsequently determ- crustal structure of the Qaidam Basin. Teng JW et al. (1974) used ined the P* velocity as 6.5 km/s. Litak and Brown (1989) inter- deep crustal reflections recorded at distances of 40–90 km from preted the discontinuity as the boundary between a granitic up- deep seismic profiling in eastern Qaidam basin to infer the de- per crust and a basaltic lower crust and the arrival traveling in the tailed crustal structure and confirmed the existence of the Conrad upper crustal layer was named Pg (for granitic). On seismic re- cords, the discontinuity is manifested as a prominent P-wave velo- Correspondence to: Y. B. Wang, [email protected] city (Vp) that jumps from 6.1 km/s to 6.7 km/s (Merriam, 2006). The Received 31 DEC 2020; Accepted 16 MAR 2021. Conrad discontinuity has been identified on most continents (Nel- Accepted article online 14 APR 2021. son et al., 1985; Merriam, 2006), but is absent in oceanic crust and ©2021 by Earth and Planetary Physics. in continental crust with strong tectonic activity because it is Earth and Planetary Physics doi: 10.26464/epp2021030 297 89°E 90°E 91°E 92°E 93°E 94°E 95°E 96°E 97°E 98°E 99°E 100°E 101°E 41°N 41°N Tarim Block 40°N 40°N H22 H21 H23 39°N 39°N ATR Qilian Block H20 MQS M SQS w6.3 38°N H19 38°N H17 M QTT w4.9 H18 H10 H16 M w5.1 H12 37°N Qaidam Basin 37°N H13 NKT SQS A04 A01 H06 36°N H01 H02 36°N H04 EKS H03 Bayan Har Block H05 35°N D09 D04 35°N JS D08 D07 KF D10 D19 D01 34°N Qiangtang Block F01 D11 34°N 33°N 33°N 89°E 90°E 91°E 92°E 93°E 94°E 95°E 96°E 97°E 98°E 99°E 100°E 101°E Figure 1. Tectonic setting and distributions of earthquakes and stations in the Qaidam Basin region. The yellow triangle represents the station used to constrain the velocity of the crust, while red triangles represent the stations used to constrain the velocity of the upper mantle and the depth of the Moho discontinuity. JS, EKS, SQS, MQS represent Jinshajiang Suture, East Kunlun Suture, South Qilian Suture and Middle Qilian Suture, respectively. KF, NKT, QTT and ATR represent Kunlun Fault, North Kunlun Thrust, Qiman Tagh Thrust and Altyn Tagh Range, repectively. Tectonic blocks and sutures refer to Zhang GM et al. (2005). closely related to the evolution of the crust (Litak and Brown, 0 Station 1989; Kerr, 1989). Pg, Sg Source The Kola Superdeep Borehole drilling program of the former So- 20 viet Union (Clarke et al., 1986) did not provide supportive evid- PdP, SdS P*, S* Conrad discontinuity ence for the existence of the lithologic discontinuity. Zha- 40 maletdinov (2014) interpreted the Conrad discontinuity as a Depth/km PmP, SmS brittle-ductile transition at mid-crustal depth, based on the res- 60 Pn, Sn ults from the deepest borehole Kola SG-3 (12262-meters depth) Moho discontinuity and seismic sounding. 0 50 100 150 200 250 300 Density profiles in the crust have also been used to infer the exist- Distance (km) ence of mid-crustal discontinuity. Zhang YQ et al. (2017) obtained the density distribution in the eastern margin of the Tibetan Plat- Figure 2. A schematic illustration of the ray paths of crustal phases in eau by gravity inversion. The density changes abruptly at a depth a two-layer crustal model. They include reflected phases PdP and SdS of ~28 kilometers, which may be considered as the Conrad dis- and refracted phases P*and S* from the Conrad discontinuity, continuity. Earthquakes in the study area of Zhang YQ et al. (2017) reflected phases PmP and SmS, and refracted phases Pn and Sn from are all distributed above the discontinuity, providing plausible the Moho discontinuity, and the direct arrivals Pg and Sg. evidence for the Conrad discontinuity as a brittle-ductile trans- ition. Conrad discontinuity in the Xilong Plateau, northeast India is 18 kilometers. Jiao YY et al. (2017) inferred the existence of the Con- Ray tracing has been the most common method for studying the rad discontinuity in the northeast margin of the Tibet Plateau. Conrad discontinuity. Figure 2 shows the basic phases in a two- layered crust model. He XB and Hong TK (2010) observed well-de- Based on data from dense seismic arrays, Horiuchi et al. (1982), veloped Conrad-refracted phases P* and S*. Using the travel times Oda and Ushio (2007), and Abdelwahed et al. (2013) obtained the of direct and reflected phases from 75 shallow crustal earth- lateral variations in the depths of the Conrad discontinuity in quakes, Bora and Baruah (2012) estimated that the depth of the northeastern Japan, in Kyushu region and in Egypt. Yang B and Wang YB et al.: Depth variation of the Conrad discontinuity in the Qaidam Basin 298 Earth and Planetary Physics doi: 10.26464/epp2021030 Studies on different sections of the Qaidam Basin all have found a phases can be better distinguished. Figure 3 shows that in the discontinuity where the P-wave velocity jumps from ~5.8 km/s to range of 100−240 km, the refracted waves from the Conrad dis- ~6.6 km/s at a depth of ~30 km (Teng JW, 1974; Cui ZZ et al., 1995; continuity are the first arrived seismic waves. When the epicentral Xia WC et al., 2001; Wang YX et al., 2005; Guo B et al., 2019).
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