西北太平洋岩石圈有效弹性厚度及其构造意义

本文引入滑动窗口导纳技术(MWAT),计算西北太平洋岩石圈有效弹性厚度(Te).首先,基于SIO V15.1海底地形模型,模拟研究了MWAT法计算Te的精度,表明当Te5km时,误差在±1km以内,当Te≥5km时,相对误差在10%以内.分别采用GEBCO、SIO V15.1和BAT_VGG海底地形模型,构建了西北太平洋Te,通过对获得的洋壳密度参数和实测导纳与模型导纳之差的均方根进行分析,结果表明,BAT_VGG模型更适用于Te计算.西北太平洋Te均值为13.2km,标准差为6.9km,以板块冷却模型为参考,主要分布在150℃~450℃等温线深度范围内.白垩纪和侏罗纪时期岩石圈Te分布在150℃~300℃等温线深度范围内,且未随海山加载时岩石圈年龄增大而增大,说明海山加载时岩石圈年龄不是影响其强度的唯一因素.南太平洋超级海隆活动,以及研究区域广泛存在的断裂带构造,都曾对本区域岩石圈演化产生过重要影响,可能是本地区岩石圈Te较小的构造原因.

The lithosphere effective elastic thickness and its tectonic implications in the Northwestern Pacific

A variety of methods have been applied to estimate lithospheric effective elastic thickness (Te). Scientists have calculated Te of the lithosphere under various features, but there are contradictory results among them. In this paper, accuracy of Te calculated by MWAT (Moving Window Admittance Technique) was analyzed synthetically based on SIO (Scripps Institute of Oceanography) V15.1 bathymetry model and a simple lithospheric flexure isostatic model. The Te model of the Northwestern Pacific (15°S-45°N, 145°E-215°E) is re-evaluated. We discussed the tectonic setting of the lithosphere in the studied area based on Te and age samples of the seafloor. The theoretical basis for Te estimating is the flexural isostatic model. Te can be established by minimizing the RMS misfit between the observed and theoretical admittance. The theoretical admittance is calculated based on flexural isostatic model. The observed admittance is calculated based on cross-spectral analysis of bathymetry and gravity model. We can calculate Te in two steps. First, at 20~50 km wave bands, the uncompensated theoretical admittance is calculated with different ρ_c (2300~2900 kg·cm³) and d (mean model depth ±500 m). The value of ρ_c and d can be recovered area by area by fitting the theoretical and observed admittance. Secondly, at wave lengths longer than 50 km, with the recovered ρ_c and d, the theoretical admittance can be computed for different Te. We can obtain an optimal Te when the RMS misfit between the theoretical and observed admittance is minimized. With the MWAT method, six windows range from 400 km×400 km to 1400 km×1400 km are used to estimate Te, using 3D spectral analysis technique. The final Te is weighted mean of these six results. For comparison, three kinds of bathymetry model are used. They are GEBCO, SIO V15.1 and BAT_VGG. BAT_VGG is a bathymetry model formed with ship soundings and vertical gravity gradient anomalies. In our simulations, Te centered on (21°N, 157°E) is recovered for different inputted Te (1~50 km). The effects of crustal density, Young's modulus and high order terms are evaluated. The simulated results shown that the accuracy of Te calculated with MWAT method is ±1 km for Te<5 km and the relative accuracy can be better than 10% for Te≥5 km.The spatial variations of Te in the Northwestern Pacific are evaluated based on BAT_VGG and altimetric gravity anomaly data from SIO V20.1, with MWAT method. In the Northwestern Pacific, the results show that Te is in the range of 0~50 km with a mean of 13.2 km and a standard deviation of 6.9 km. High Te (20~40 km) distributes around the Hawaiian Islands and along the trench outer rise. The trench outer rise has the oldest lithosphere in the studied region, and their rigidity may have been modified by the plates' collision too. Hawaiian-Emperor is an intraplate original Seamount Chain. High Te around Hawaiian Islands is consistent with their formation on old, strong oceanic lithosphere. Te decreases monotonically along the Hawaiian Islands from the south-east end to the north-west end. Te along the northern Emperor Seamounts varies less systematically. Lithosphere around Shatsky Rise has the lowest Te (<5 km). Shatsky Rise formed at the Pacific-Farallon-Izanagi triple junction during the Late Jurassic to Early Cretaceous. Magnetic lineations indicate that the lithosphere is young at the time of Shatsky Rise loading. Low Te is consistent with the age of lithosphere at time of loading indicated by magnetic lineations. A wide oceanic basin spread in the south of Hawaiian-Emperor Seamount Chain. The Jurassic lithosphere locates in the west of the basin and has moderate Te (10~15 km). In the east of the basin, the lithosphere is formed during Cretaceous and has lower Te (<10 km). In the studied area, the dependence of Te on the age of oceanic lithosphere at the time of loading is given mostly by the depth to the 150℃~450℃ oceanic isotherm based on a cooling plate model, slightly lower than the depth given by Watts (2001) which is 300℃~600℃. Te of the Cretaceous and Jurassic lithosphere distributes in the 150℃~300℃ isotherm depth. Te of the Cretaceous lithosphere is less than the Jurassic lithosphere. Te of the lithosphere does not increase with the age of lithosphere at the time of loading. The result indicates that Te does not controlled only by the age of lithosphere at the time of loading. The reheating of the lithosphere by thermal activities inner the earth, such as the "super swells" in the South Pacific Ocean, and modification of the lithosphere by fracture zones will decrease the strength of the lithosphere. We can draw the following conclusions from this paper: ① The gravity isostatic admittance in 20~50 km wavelengths band will not change significantly with Te. The regional crustal density and mean water depth can be recovered based on the admittance at 20~50 km wavebands. ② The simulated results shown that the accuracy of Te calculated with MWAT method is ±1 km for Te<5 km and the relative accuracy can be better than 10% for Te≥5 km. ③ The bathymetry model predicted from VGG and ship soundings (BAT_VGG) is superior to GEBCO and SIO V15.1 in Te estimating. With BAT_VGG, the mean recovered crustal density is consistent with CRUST2.0. The mean of standard deviation of the misfits between observed and theoretical admittance is 5.233 mGal/km, and about 56.869% of the grids have the standard deviation which lower than 5 mGal/km. ④ The results show that Te is in range of 0~50 km with a mean of 13.2 km and a standard deviation of 6.9 km over the Northwestern Pacific. The lithosphere of Hawaiian-Emperor Chain show relatively high Te. ⑤ Over the Northwestern Pacific, the dependence of Te on the age of oceanic lithosphere at the time of loading is given mostly by the depth to the 150℃~450℃ oceanic isotherm based on a cooling plate model, slightly lower than the depth given by Watts (2001), which is 300℃~600℃.