秦岭─大别造山带及其南北缘地震层析成像

SEISMIC TOMOGRAPHY BENEATH THE QINLINGDABIE OROGENIC BELTS AND BOTH THE NORTHERN AND SOUTHERN FRINGES

利用秦岭─大别造山带及其毗邻地区３１０个地震台站记录到的区域地震２３６００条Ｐ波到时数据，重建了该区地壳和上地幔三维速度图像。结果表明：１．秦岭─大别造山带及其毗邻地区地壳和上地幔存在显著的横向不均匀性，直至１１０ｋｍ深度处依然明显。２．地壳上部的速度图像与地表地质构造密切相关：造山带隆起区显著高速；盆地及坳陷区明显低速。由速度鲜明对比勾勒出的秦岭─大别造山带南界基本上位于扬子北缘主边断裂带上。３．中地壳的速度图像表明，造山带内部的一些低速区对应于一些大型推覆构造。４．４０＋０ｋｍ深度处的速度图像反映了该区莫霍界面深度的起伏。大致以１０７°Ｅ为界，以东地区地壳厚度小于４０ｋｍ，以西地区大于４０ｋｍ，且呈现出往西地壳逐渐加厚的趋势。５．位于滦川、商县、丹凤的北秦岭构造带，上地幔顶部出现低速异常，异常速度值约为７．３９—７．５５ｋｍ／ｓ。结合地球物理测深的结果，可能是由下地壳、上地幔顶部的热过程所致。

In this paper 3-D velocity images of the crust and upper mantle beneath the Qinling-Dabie Oregenic belts and both the northern and southern fringes are reconstructed using 23600 P-wave arrivals of regional earthquakes recorded at 310 stations in the study area.The image results are summarized as follows:1.The velocity images given in this paper indicate a significant lateral heterogeneity in the crust and upper mantle beneath the Qinling-Dabie Orogenic belts and adjacent regions,which persists down to a depth of 110km.2.The velocity images of the upper crust are closely related to surface geological features:the velocities of upfold zones in the orogenic belts are high,and of basin.and depression zones are low.The southern boundary of the Qinling-Dabie Orogenic belts have been drawn by the velocity contrast on both sides of them.3.The image of the mid-crust shows some of low-velocity areas within the orogenic belts,corresponding to some large-scale nappe structures.4.The velocity image at a depth of 40+0km shows clearly fluctuation of Mohointerface.5.The low-velocity abnormity appears in the Northern Qinling tectonic belt located at Luanchuan,Shangxian,Danfeng,and it is probably due to thermal processes in the depths of the upper mantle.