Crustal structure across the Jiangnan orogen from teleseismic virtual-source reflection method
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摘要: 江南造山带是华南地区扬子地块与华夏地块碰撞的产物，其地壳构造记录了两地块的碰撞过程，研究江南造山带的地壳构造有助于重建扬子地块与华夏地块的碰撞过程。本研究在江南造山带上布设了两条流动地震台阵，利用虚震源反射法提取其所记录的远震事件初至P波在地表的反射波（PPdp）波形，重构了研究区内两条测线下方的上地壳结构。结果显示：江绍断裂两侧上地壳沉积层的厚度变化明显，推断该断裂是扬子地块与华夏地块的东边界；相较华夏地块，江南造山带与扬子地块的层位连续性更强，符合江南造山带先与扬子地块合为整体后再与华夏地块碰撞的多期构造过程及其对应产生的亲扬子地块属性；江绍断裂西北侧的地层不整合以及赣东北断裂区域的断陷构造，可为了解古华夏洋向扬子陆块俯冲及碰撞和随后的构造运动过程提供参考依据。Abstract: The Jiangnan orogen belt （JOB） was formed by the collision between the Yangtze block （YB） and the Cathaysia block （CB）. The crustal structure of the JOB recorded the collisional process between the two blocks. Hence, studying crust of the JOB can illuminate the process between the YB and the CB, and thus better understand the tectonic evolution of the South China Plate （SCP）. In this study, we applied the teleseismic virtual reflection method to derive the phase of the reflection waves （PPdp） of large teleseismic earthquakes recorded by two temporary seismic arrays that were deployed around the JOB and obtained the upper crust structures. The results suggest that the thickness of the upper crust sediments on two sides of the Jiangshao fault changes obviously. It is also inferred that the Jiangshao fault is the eastern boundary between the Yangtze block and the Cathaysia block. The JOB and the YB have more apparent stratigraphic continuity than that of the CB. This result agrees with a multi-stage tectonic model which suggests that the JOB first bounded the YB and then collided with the CB. Hence the characteristics of the JOB are closer to that of the YB. The stratigraphic unconformities in the northwest of the Jiangshao fault and the fault structure in the northeast Jiangxi region provides a basic profile for understanding the subducting and colliding processes between the ancient Cathaysian ocean and the YB as well.
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图 1 江南造山带及其邻近地区地质图
Figure 1. Geological map of the Jiangnan orogen and its surrounding region
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图 2 远震地震射线PPdp与PPmp传播示意图
Figure 2. A diagrammatic view of the ray path of the teleseismic PPdp and PPmp phases
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图 3 正演模型与反演结果
（a） 正演使用的两层水平模型；（b） 运用虚震源反射方法反演得到的时间-距离剖面
Figure 3. Forward model and inversion results
（a） The two-layer horizontal model；（b） Time-distance profile obtained by the teleseismic virtual-source reflection （TVR） inversion
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图 4 事件②波形在AA′剖面台站的处理结果
（a） 去仪器响应并通过0.1—2 Hz的带通滤波后的波形，右图为平均各道所得到的震源子波，背景中细灰色线代表用于叠加的各道波形，倒三角形标出了用于图6的台站位置；（b） 事件②虚震源反射成像剖面；（c） 事件①，②，③的虚震源反射叠加剖面；（d） 叠加后对各道进行三道圆滑的结果
Figure 4. Waveform processing results of event ② along the AA′ profile
（a） Waveform data after removing the instrument response from the raw data and it was filtered by a 0.1−2 Hz band-pass filter。The averaged seismic trace is shown on the right，and the gray background curves are the overlapped traces of all channels。The inverse triangle marks the location where the station is taken for Fig. 6；（b） The original TVR profile from event ②；（c） The stacked TVR profile for teleseismic events ①，② and ③；（d） The smoothed TVR profile
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图 5 事件⑤波形在BB′剖面台站的处理结果
（a） 去仪器响应并通过0.1—2 Hz的带通滤波后的波形，右图为平均各道所得到的震源子波，背景中细灰色线代表用于叠加的各道波形，倒三角形标出了用于图6的台站位置；（b） 事件⑤虚震源反射成像剖面；（c） 叠加事件④—⑧的虚震源反射剖面；（d） 叠加后对各道进行三道圆滑的结果
Figure 5. Waveform processing results of event ⑤ at the BB′ profile
（a） Waveform data after removing the instrument response from the raw data, it was filtered by a 0.1−2 Hz band-pass filter. The averaged seismic trace is shown on the right，and the gray background curves are the overlapped traces of all channels. The inverse triangle marks the locations where the station is taken for Fig. 6；（b） The original TVR profile from event ⑤；（c） The stacked TVR profile for teleseismic events ④−⑧；（d） The smoothed TVR profile
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图 6 远震虚源反射法与传统接收函数结果对比
（a） 事件②在台站 （图4a中倒三角形）处的虚源反射剖面（上图）与合成信号（下图）的对比，箭头代表识别的震相，合成信号所用参数为vP为5.2 km/s，vS为3.1 km/s，反射层深度为6.5 km；（b） 事件②在台站（图4a中倒三角形）处Ps转换波接收函数（箭头）（上图）与人工合成信号（下图）的对比，合成信号所用参数与图（a）相同；（c） 与图（a）相似，事件⑤在台站（图5a中倒三角形）处的对比，合成信号所用参数为vP为5.2 km/s，vS为3.1 km/s，反射层深度为5 km；（d） 与图（b）相似，事件⑤在台站（图5a中倒三角形）处的对比，合成信号所用参数与图（c）相同
Figure 6. Example of TVR profile （top） for events ② and ⑤ at the marked station （inverse triangle in Figs. 4a and 5a） versus synthetic seismogram （bottom）
（a） Example of TVR profile （top） for the event ② at the marked station (inverse triangle in Fig. 4a) versus synthetic seismogram （bottom）. The arrow represents the recognized seismic phase. The synthetics were calculated to fit the data using vP of 5.2 km/s and vS of 3.1 km/s and the depth of the seismic layer generating reflection phase is 6.5 km. （b） Example of conventional Ps receiver function （top） at the marked station （inverse triangle in Fig. 4a） compared with the synthetic seismogram （bottom）. The parameters used to synthesize the signal are the same as those in Fig. （a）. （c） Example of TVR profile （top） for the event ⑤ at the marked station （inverse triangle in Fig. 5a） versus synthetic seismogram （bottom）. The arrow represents the recognized seismic phase. The synthetics were calculated to fit the data using vP of 5.2 km/s and vS of 3.1 km/s and the depth of the seismic layer generating reflection phase is 5 km. （d） Example of conventional Ps receiver function （top） at the marked station （inverse triangle in Fig. 5a） compared with the synthetic seismogram （bottom）. The parameters used to synthesize the signal are the same as those in Fig. （c）
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图 7 结合已知的地层信息与断层对成像剖面AA′ （a）和 BB′ （b）的解释
两剖面以纵波速度5.2 km/s进行时深转换结果。绿色线表示沉积层，蓝色和红色线代表上地壳内两段反射面
Figure 7. Interpretation of profiles AA′ （a） and BB′ （b） from the interfaces and known faults
The two profiles are converted using average P-wave velocity of 5.2 km/s. The green lines represent sedimentary layers，and the blue and red lines represent two reflections in the shallow crust
表 1 本研究所用地震事件的参数
事件
编号发震日期 MS 地理位置 震源深度
/km震中距
/°年-月-日 时:分:秒 ① 2019-04-03 05:35:33 6.4 （52.25°N，178.00°E） 20 50.711 ② 2019-04-07 05:55:00 6.1 （6.90°S，125.07°E） 540 36.746 ③ 2019-04-12 22:51:32 5.8 （6.43°S，148.57°E） 20 47.157 ④ 2019-06-03 14:04:36 5.5 （0.35°N，97.72°E） 20 35.244 ⑤ 2019-06-15 04:10:52 5.6 （5.86°S，130.75°E） 120 37.696 ⑥ 2019-06-16 06:55:00 7.2 （30.80°S，178.10°W） 20 85.704 ⑦ 2019-06-16 13:17:15 6.2 （30.97°S，178.06°W） 30 85.836 ⑧ 2019-06-17 14:02:04 5.9 （30.85°S，177.60°W） 20 86.071 注：震源参数引自中国地震台网中心 
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