基于瑞利面波层析成像研究太平洋板块下方小尺度地幔对流

传统的板块构造理论在解释太平洋板块内部广泛分布的非热点线性海山链和古老海洋板块中海底地形变浅、热流值偏高等方面存在一定的局限性。小尺度地幔对流模型通常被用来解释这些现象。然而，大多数关于小尺度对流的研究主要停留在理论模拟及分析阶段，缺乏来自实际观测结果的支撑。本文使用了布设在85−100 Ma西南太平洋下方存在明显的条带状自由空气异常区域的30台宽频带海底地震仪阵列记录，选取了85个高质量的远震事件，应用基于双平面波地震层析成像方法得到了20−83 s周期范围内的瑞利面波相速度图，进而获得了该区域下方岩石圈和软流圈剪切波速度结构。成像结果显示软流圈中存在平行于局部绝对运动板块方向的交替的高速和低速异常。这表明，较冷、密度较大的下降流和较暖、密度较小的上升流在100−200 km深度范围内的软流圈低粘度中心迁移，上升流在高粘度岩石圈水平遇到阻力从而剥蚀岩石圈底部（< 100 km），在板块底部形成“搓衣板”状地形，反映了岩石圈下方动态的小尺度对流系统。对重力信号的分析证实了这一对流单元产生的横向密度变化，并与自由空气异常和地幔布格重力异常变化具有良好的对应关系，进一步支持了研究区域下方的小尺度对流模型。本文通过地震瑞利波所构建的剪切波速度结构模型，为成熟海洋板块下方的小尺度对流提供了直接的观测证据，揭示了海洋岩石圈演化过程中与软流圈相互作用与转化的高度复杂的动态过程。

The traditional plate tectonic theory encounters certain limitations in explaining the formation mechanism of widely distributed non-hotspot linear seamount chains within the Pacific plate and the shallow seafloor topography and high heat flow in old ocean plates. Small-scale convection model is commonly invoked to explain these phenomena. However, the majority of researches on small-scale convection remain predominantly theoretical simulation and analysis stage, lacking of support from practical observation results. This thesis uses earthquake records from 30 broadband ocean bottom seismometers array deployed on the 85−100 Ma southwest Pacific, with prominent free air anomaly lineations beneath the study region. We select 85 high-quality teleseismic earthquake events, applying seismic tomography method based on two-plane-wave to obtain Rayleigh wave phase velocity maps at the period of 20−83 s and then determine the lithosphere and asthenosphere shear wave velocity structure. Tomographic imaging results demonstrate alternating high-velocity and low-velocity anomaly in asthenosphere, parallel to absolute plate motion. It indicates that the migration of colder, denser downwellings and warmer, less dense upwellings in 100−200 km low-viscosity central asthenosphere, while upwellings confront resistance at high-viscosity lithosphere level and erode the bottom of the lithosphere (<100 km), creating a washboard shaped topography on the underside of the plate, reflecting a dynamic small-scale convection system beneath the lithosphere. The analysis of gravity signals confirms lateral density variations generated by convection units, demonstrating a good corresponding relationship with free air anomaly and mantle Bouguer gravity anomaly and further improving small-scale convection model beneath our study region. The shear wave velocity structure model constructed by seismic Rayleigh wave propagation in this thesis provide direct observational evidence for small-scale convection beneath the mature ocean plates, revealing the highly complex dynamic process of interaction and transformation with the asthenosphere during the evolution of the ocean lithosphere.

[1] Turcotte D L, Oxburgh E R. FINITE AMPLITUDE CONVECTIVE CELLS AND CONTINENTAL DRIFT [J]. J Fluid Mech, 1967, 28: 29-&.
[2] Crosby A G, McKenzie D, Sclater J G. The relationship between depth, age and gravity in the oceans [J]. Geophys J Int, 2006, 166(2): 553-73.
[3] Parsons B, Sclater J G. An analysis of the variation of ocean floor bathymetry and heat flow with age [J]. J Geophys Res-Solid Earth, 1977, 82(5): 803–27.
[4] Ritzwoller M H, Shapiro N M, Zhong S J. Cooling history of the Pacific lithosphere [J]. Earth Planet Sci Lett, 2004, 226(1-2): 69-84.
[5] Stein C A, Stein S. A MODEL FOR THE GLOBAL VARIATION IN OCEANIC DEPTH AND HEAT-FLOW WITH LITHOSPHERIC AGE [J]. Nature, 1992, 359(6391): 123-9.
[6] MORGAN W J. Convection Plumes in the Lower Mantle [J]. Nature, 1971, 230(5288): 42-3.
[7] Morgan W J. Plate Motions and Deep Mantle Convection [J]. Studies in Earth and Space Sciences, 1972: 7-22.
[8] Ballmer M D, van Hunen J, Ito G, et al. Intraplate volcanism with complex age-distance patterns: A case for small-scale sublithospheric convection [J]. Geochem Geophys Geosyst, 2009, 10: 22.
[9] Ballmer M D, van Hunen J, Ito G, et al. Non-hotspot volcano chains originating from small-scale sublithospheric convection [J]. Geophys Res Lett, 2007, 34(23): 5.
[10] Koppers A A P, Staudigel H, Pringle M S, et al. Short-lived and discontinuous intraplate volcanism in the South Pacific: Hot spots or extensional volcanism? [J]. Geochem Geophys Geosyst, 2003, 4: 49.
[11] McNutt M K, Caress D W, Reynolds J, et al. Failure of plume theory to explain midplate volcanism in the southern Austral islands [J]. Nature, 1997, 389(6650): 479-82.
[12] Sandwell D T, Winterer E L, Mammerickx J, et al. EVIDENCE FOR DIFFUSE EXTENSION OF THE PACIFIC PLATE FROM PUKAPUKA RIDGES AND CROSS-GRAIN GRAVITY LINEATIONS [J]. J Geophys Res-Solid Earth, 1995, 100(B8): 15087-99.
[13] Buck W R, Parmentier E M. CONVECTION BENEATH YOUNG OCEANIC LITHOSPHERE - IMPLICATIONS FOR THERMAL STRUCTURE AND GRAVITY [J]. Journal of Geophysical Research-Solid Earth and Planets, 1986, 91(B2): 1961-74.
[14] Richter F M, Parsons B. On the interaction of two scales of convection in the mantle [J]. Journal of Geophysical Research, 1975, 80(17): 2529-41.
[15] Evernden J F. DIRECTION OF APPROACH OF RAYLEIGH WAVES AND RELATED PROBLEMS [J]. Geol Soc Am Bull, 1952, 63(12): 1352-.
[16] Laske G. GLOBAL OBSERVATION OF OFF-GREAT-CIRCLE PROPAGATION OF LONG-PERIOD SURFACE-WAVES [J]. Geophys J Int, 1995, 123(1): 245-59.
[17] Forsyth D W, Li A B. Array analysis of two-dimensional variations in surface wave phase velocity and azimuthal anisotropy in the presence of multipathing interference[J]. Geophysical Monograph Series, 2005: 81-97.
[18] Forsyth D W, Webb S C, Dorman L M, et al. Phase velocities of Rayleigh waves in the MELT experiment on the East Pacific Rise [J]. Science, 1998, 280(5367): 1235-8.
[19] Zhou Y, Dahlen F A, Nolet G. Three-dimensional sensitivity kernels for surface wave observables [J]. Geophys J Int, 2004, 158(1): 142-68.
[20] Yang Y J, Forsyth D W. Regional tomographic inversion of the amplitude and phase of Rayleigh waves with 2-D sensitivity kernels [J]. Geophys J Int, 2006, 166(3): 1148-60.
[21] Haxby W F, Weissel J K. EVIDENCE FOR SMALL-SCALE MANTLE CONVECTION FROM SEASAT ALTIMETER DATA [J]. Journal of Geophysical Research-Solid Earth and Planets, 1986, 91(B3): 3507-+.
[22] Richter F M. Convection and the large-scale circulation of the mantle [J]. Journal of Geophysical Research, 1973, 78(35): 8735-45.
[23] Buck W R. WHEN DOES SMALL-SCALE CONVECTION BEGIN BENEATH OCEANIC LITHOSPHERE [J]. Nature, 1985, 313(6005): 775-7.
[24] Korenaga J, Jordan T H. Linear stability analysis of Richter rolls [J]. Geophys Res Lett, 2003, 30(22): 4.
[25] van Hunen J, Huang J S, Zhong S J. The effect of shearing on the onset and vigor of small-scale convection in a Newtonian rheology [J]. Geophys Res Lett, 2003, 30(19): 4.
[26] Ballmer M D, Ito G, van Hunen J, et al. Spatial and temporal variability in Hawaiian hotspot volcanism induced by small-scale convection [J]. Nat Geosci, 2011, 4(7): 457-60.
[27] Davis A S, Gray L B, Clague D A, et al. The Line Islands revisited: New 40Ar/39Ar geochronologic evidence for episodes of volcanism due to lithospheric extension [J]. Geochem Geophys Geosyst, 2002, 3: 28.
[28] Winterer E L, Sandwell D T. EVIDENCE FROM EN-ECHELON CROSS-GRAIN RIDGES FOR TENSIONAL CRACKS IN THE PACIFIC PLATE [J]. Nature, 1987, 329(6139): 534-7.
[29] Cormier M H, Gans K D, Wilson D S. Gravity lineaments of the Cocos Plate: Evidence for a thermal contraction crack origin [J]. Geochem Geophys Geosyst, 2011, 12: 19.
[30] Gans K D, Wilson D S, Macdonald K C. Pacific Plate gravity lineaments: Diffuse extension or thermal contraction? [J]. Geochem Geophys Geosyst, 2003, 4: 17.
[31] Sandwell D, Fialko Y. Warping and cracking of the Pacific plate by thermal contraction [J]. J Geophys Res-Solid Earth, 2004, 109(B10): 12.
[32] Weeraratne D S, Forsyth D W, Yang Y J, et al. Rayleigh wave tomography beneath intraplate volcanic ridges in the South Pacific [J]. J Geophys Res-Solid Earth, 2007, 112(B6): 18.
[33] Weeraratne D S, Parmentier E M, Forsyth D W. Viscous fingering instabilities in miscible fluids and the oceanic asthenosphere [J]. EOS Trans AGU, 2003b, 84(86).
[34] Anderson D L. The thermal state of the upper mantle; no role for mantle plumes [J]. Geophys Res Lett, 2000, 27(22): 3623-6.
[35] Harmon N, Forsyth D W, Lamm R, et al. P and S wave delays beneath intraplate volcanic ridges and gravity lineations near the East Pacific Rise [J]. J Geophys Res-Solid Earth, 2007, 112(B3): 12.
[36] Harmon N, Forsyth D W, Scheirer D S. Analysis of gravity and topography in theGLIMPSE study region: Isostatic compensation and uplift of the Sojourn and Hotu Matua Ridge systems [J]. J Geophys Res-Solid Earth, 2006, 111(B11): 20.
[37] McNutt M K. Superswells [J]. Rev Geophys, 1998, 36(2): 211-44.
[38] Lin P Y P, Gaherty J B, Jin G, et al. High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere [J]. Nature, 2016, 535(7613): 538-+.
[39] Forsyth D W, Harmon N, Scheirer D S, et al. Distribution of recent volcanism and the morphology of seamounts and ridges in the GLIMPSE study area: Implications for the lithospheric cracking hypothesis for the origin of intraplate, non-hot spot volcanic chains [J]. J Geophys Res-Solid Earth, 2006, 111(B11): 19.
[40] Harmon N, Forsyth D W, Weeraratne D S, et al. Mantle heterogeneity and off axis volcanism on young Pacific lithosphere [J]. Earth Planet Sci Lett, 2011, 311(3-4): 306-15.
[41] Harmon N, Rychert C A, Kendall J M, et al. Evolution of the Oceanic Lithosphere in the Equatorial Atlantic From Rayleigh Wave Tomography, Evidence for Small-Scale Convection From the PI-LAB Experiment [J]. Geochem Geophys Geosyst, 2020, 21(9): 18.
[42] Eilon Z C, Gaherty J B, Zhang L, et al. The Pacific OBS Research into Convecting Asthenosphere (ORCA) Experiment [J]. Seismol Res Lett, 2022, 93(1): 477-93.
[43] Russell J B. Structure and Evolution of the Oceanic Lithosphere-Asthenosphere System from High-Resolution Surface-Wave Imaging, F, 2021 [C].
[44] Eilon Z C, Zhang L, Gaherty J B, et al. Sub-Lithospheric Small-Scale Convection Tomographically Imaged Beneath the Pacific Plate [J]. Geophys Res Lett, 2022, 49(18): 10.
[45] Shearer P M. Introduction to Seismology, F, 2019 [C].
[46] 万永革. 地震学导论 [M]. 地震学导论, 2016.
[47] Nishimura C E, Forsyth D W. The anisotropic structure of the upper mantle in the Pacific [J]. Geophys J Int, 1989, 96(2): 203-29.
[48] Aki K, Lee W H K. Determination of three-dimensional velocity anomalies under a seismic array using first P arrival times from local earthquakes: 1. A homogeneous initial model [J]. Journal of Geophysical Research (1896-1977), 1976, 81(23): 4381-99.
[49] McEvilly T V. Central U.S. crust—Upper mantle structure from Love and Rayleigh wave phase velocity inversion [J]. Bull Seismol Soc Amer, 1964, 54(6A): 1997-2015.
[50] Nafe J E, Brune J N. Observations of phase velocity for Rayleigh waves in the period range 100 to 400 seconds [J]. Bull Seismol Soc Amer, 1960, 50(3): 427-39.
[51] Satô Y. Attenuation, dispersion, and the wave guide of the G wave [J]. Bull Seismol Soc Amer, 1958, 48(3): 231-51.
[52] Yao H J, van der Hilst R D, de Hoop M V. Surface-wave array tomography in SE Tibet from ambient seismic noise and two-station analysis - I. Phase velocity maps [J]. Geophys J Int, 2006, 166(2): 732-44.
[53] Yao H J, Xu G M, Zhu L B, et al. Mantle structure from inter-station Rayleigh wave dispersion and its tectonic implication in western China and neighboring regions [J]. Phys Earth Planet Inter, 2005, 148(1): 39-54.
[54] Li A, Li L. Love wave tomography in southern Africa from a two-plane-wave inversion method [J]. Geophys J Int, 2015, 202(2): 1005-20.
[55] Lin F C, Ritzwoller M H, Snieder R. Eikonal tomography: surface wave tomography by phase front tracking across a regional broad-band seismic array [J]. Geophys J Int, 2009, 177(3): 1091-110.
[56] Jin G S. Surface-wave analysis and its application to determining crustal and mantle structure beneath regional arrays, F, 2015 [C].
[57] Lin F C, Ritzwoller M H. Helmholtz surface wave tomography for isotropic and azimuthally anisotropic structure [J]. Geophys J Int, 2011, 186(3): 1104-20.
[58] 李伦, 蔡晨, 付媛媛, 等.多种面波层析成像方法及其在青藏高原的应用与对比[J].地球与行星物理论评(中英文),2023,54(02):174-96.
[59] Cai C, Wiens D A, Shen W, et al. Water input into the Mariana subduction zone estimated from ocean-bottom seismic data [J]. Nature, 2018, 563(7731): 389-+.
[60] Dave R, Li A B. Destruction of the Wyoming craton: Seismic evidence and geodynamic processes [J]. Geology, 2016, 44(11): 883-6.
[61] Liu Y D, Li L, van Wijk J, et al. Surface-wave tomography of the Emeishan large igneous province (China): Magma storage system, hidden hotspot track, and its impact on the Capitanian mass extinction [J]. Geology, 2021, 49(9): 1032-7.
[62] Friederich W, Wielandt E. Interpretation of seismic surface waves in regional networks: joint estimation of wavefield geometry and local phase velocity. Method and numerical tests [J]. Geophys J Int, 1995, 120(3): 731-44.
[63] Li A B, Forsyth D W, Fischer K M. Shear velocity structure and azimuthal anisotropy beneath eastern North America from Rayleigh wave inversion [J]. J Geophys Res-Solid Earth, 2003, 108(B8): 24.
[64] Li L, Fu Y V. Surface-Wave Tomography of Eastern and Central Tibet from Two-Plane-Wave Inversion: Rayleigh-Wave and Love-Wave Phase Velocity Maps [J]. Bull Seismol Soc Amer, 2020, 110(3): 1359-71.
[65] Yang T, Liu F, Harmon N, et al. Lithospheric structure beneath Indochina block from Rayleigh wave phase velocity tomography [J]. Geophys J Int, 2015, 200(3): 1582-95.
[66] Press W H, Teukolsky S A, Vetterling W T, et al. Numerical recipes in FORTRAN: The art of scientific computing [J]. 2nd edn, 1992: 963.
[67] Tarantola A, Valette B. Generalized nonlinear inverse problems solved using the least squares criterion [J]. Rev Geophys, 1982, 20(2): 219-32.
[68] Bell S W, Forsyth D W, Ruan Y. Removing Noise from the Vertical Component Records of Ocean-Bottom Seismometers: Results from Year One of the Cascadia Initiative [J]. Bull Seismol Soc Amer, 2015, 105(1): 300-13.
[69] Crawford W C, Webb S C, Hildebrand J A. Estimating shear velocities in the oceanic crust from compliance measurements by two-dimensional finite difference modeling [J]. J Geophys Res-Solid Earth, 1998, 103(B5): 9895-916.
[70] Friedrich A, Kruger F, Klinge K. Ocean-generated microseismic noise located with the Grafenberg array [J]. J Seismol, 1998, 2(1): 47-64.
[71] Longuet-Higgins M S. A theory of the origin of microseisms [J]. Philosophical Transactions of the Royal Society of London Series A, Mathematical and Physical Sciences, 1950, 243: 1 - 35.
[72] Crawford W C, Webb S C. Identifying and removing tilt noise from low-frequency (<0.1 Hz) seafloor vertical seismic data [J]. Bull Seismol Soc Amer, 2000, 90(4): 952-63.
[73] 海底地震仪垂直分量倾斜噪声的去除——以磐鲲南海测试数据为例[J].地震学报,2023,45(03):568-78.
[74] Bendat J S, Piersol A G. Random Data: Analysis and Measurement Procedures, F, 1987 [C].
[75] Janiszewski H A, Gaherty J B, Abers G A, et al. Amphibious surface-wave phase-velocity measurements of the Cascadia subduction zone [J]. Geophys J Int, 2019, 217(3): 1929-48.
[76] Saito M. DISPER80: A subroutine package for the calculation of seismic normal mode solutions [J]. Seismological Algorithms: Computational methods and computer Programs, 1988: pp. 293–319.
[77] Laske G, Masters G, Ma Z, et al. CRUST1.0: An updated global model of Earth's crust [J]. EGUGA, 2013.
[78] Shapiro N M, Ritzwoller M H. Monte-Carlo inversion for a global shear-velocity model of the crust and upper mantle [J]. Geophys J Int, 2002, 151(1): 88-105.
[79] Le B M, Yang T, Morgan J P. Seismic Constraints on Crustal and Uppermost Mantle Structure Beneath the Hawaiian Swell: Implications for Plume-Lithosphere Interactions [J]. J Geophys Res-Solid Earth, 2022, 127(11): 25.
[80] Parker R L. RAPID CALCULATION OF POTENTIAL ANOMALIES [J]. Geophysical Journal of the Royal Astronomical Society, 1973, 31(4): 447-55.