Deformation of the Qinling belt revealed by P-wave velocity and azimuthal anisotropy tomography

The Qinling belt is a transitional zone lying among three units: the North China block (NCB), the South China block (SCB) and the northeastern Tibetan Plateau (NETP). Owing to the interaction of these units, complex deformation has occurred in the Qinling belt. Although many studies have been conducted to understand the deformation mechanism in the Qinling belt, some key issues are still under debate, such as whether middle-lower crustal flow exists beneath the western Qinling belt (WQB). High-resolution images of subsurface structures are essential to shed light on the deformation mechanism. In this article, high-resolution images of the velocity structure and azimuthal anisotropy beneath the Qinling belt are obtained by using an eikonal equation-based traveltime tomography method. Our seismic tomography inverts 38,719 high-quality P-wave first arrivals from 1,697 regional earthquakes recorded by 387 broadband seismic stations. In the WQB, our tomography results show low-velocity anomalies but relatively weak anisotropy in the middle-lower crust. These features suggest that middle-lower crustal flow may not exist in this area. In the central Qinling belt (CQB), we find low-velocity anomalies in the middle-lower crust; however, the fast velocity directions (FVDs) no longer trend E–W but vary from NNE–SSW to N–S. These characteristics can be ascribed to the convergence and collision between the NCB and the SCB. In addition, we find strong low-velocity anomalies in the uppermost mantle beneath the CQB, which may indicate delamination of the lower crust. In the southern Qinling belt (SQB), we observe significant high-velocity anomalies in the upper crust beneath the Hannan–Micang (HNMC) and Shennong–Huangling (SNHL) domes. These high-velocity anomalies indicate a mechanically strong upper crust, which is responsible for the arc-shaped deformation process of the Dabashan (DBS) fold. Based on the P-wave velocity and azimuthal anisotropic structures revealed by the inversion of high-quality seismic data, the deformation of the Qinling belt is affected mainly by the convergence between the NCB and the SCB rather than by the middle-lower crustal flow from the Tibetan Plateau.

秦岭造山带是夹持于华北地块（North China block, NCB）、华南地块（South China block, SCB）与青藏高原东北部（northeastern Tibetan Plateau, NETP）三大构造单元之间的过渡带。受上述构造单元相互作用的影响，秦岭造山带发生了复杂的变形作用。尽管已有诸多研究旨在阐明秦岭造山带的变形机制，但仍有若干关键问题尚存争议，例如西秦岭造山带（western Qinling belt, WQB）下方是否存在中下地壳流。高分辨率的地下结构图像对于揭示该区域变形机制至关重要。
本文采用基于程函方程的走时层析成像方法，获取了秦岭造山带下方速度结构与方位各向异性的高分辨率图像。本次地震层析成像反演了387个宽频带地震台站记录的、来自1697个区域地震的38719条高质量P波初至数据。
在西秦岭造山带，层析成像结果显示中下地壳存在低速异常，但各向异性强度相对较弱。该特征表明本区可能不存在中下地壳流。在中秦岭造山带（central Qinling belt, CQB），中下地壳同样观测到低速异常，但快波方向（fast velocity directions, FVDs）不再呈东西向展布，而是由北北东-南南西向转变为南北向。这些特征可归因于华北地块与华南地块的汇聚与碰撞作用。此外，中秦岭造山带下方的上地幔顶部存在显著低速异常，这或许指示了下地壳拆沉作用。在南秦岭造山带（southern Qinling belt, SQB），汉南-米仓（HNMC）与神农架-黄陵（SNHL）穹窿下方的上地壳存在显著高速异常。此类高速异常反映了上地壳具有较强的力学强度，是大巴山（Dabashan, DBS）褶皱带弧形变形过程的主控因素。
基于高质量地震数据反演得到的P波速度与方位各向异性结构，秦岭造山带的变形主要受华北地块与华南地块的汇聚作用控制，而非来自青藏高原的中下地壳流。