大别造山带地壳结构反演及其动力学意义

The Significance of Crustal Structure and Its Dynamical Implication Inferred from a Local Tomography in the Dabie Orogenic Belt

大别造山带是全球最大的碰撞造山带之一,三叠纪时期,扬子板块深俯冲至地幔的200km处,经历了超高压变质作用。白垩纪早期,该造山带发生了强烈的伸展和垮塌,以及大规模的后造山地幔源岩浆侵入和火山活动。本研究收集了大别造山带及其邻区(29°~34°N、114°~119°E)的震相资料,采用双差层析成像技术,对大别造山带地壳结构进行反演,研究地壳结构与后造山地幔源岩浆侵入和火山活动之间的关系。结果显示,大别造山带中上地壳存在低速结构,该低速结构可能是熔融的幔源侵入物质,由于俯冲板片断裂,或下地壳/岩石圈发生拆沉,导致软流圈物质上涌至地壳底部、侵入地壳中,形成大别造山带地壳中的低速结构;同时,合肥盆地显示为低速区,可能是受浅部沉积层影响。研究中横切大别山的4条剖面显示,该地区下方存在北向倾斜高速结构,该高速结构可能是襄樊-广济断层,或者是扬子板块向华北板块下方俯冲的遗迹。

The Dabie Orogenic Belt,which involves the deep subduction and continental crust collision in the Triassic,is one of the major continental collision zones on the globe. The Yangtze Block subducted to mantle depths of ca. 200 km beneath the North China Block,which resulted in the formation of UHP metamorphic terranes. In addition,the area underwent extensional tectonic collapse and post-orogenic magmatism/volcanism in the early Cretaceous. Here,we collected abundant local earthquakes and performed a double-difference tomographic study to construct the crustal velocity structure in the Dabie Orogenic Belt（ 29° ~ 34° N,114° ~ 119° E） and study the relationship between crustal structure and post-orogenic magmatism/volcanism. Results show there is a low-velocity structure（ Lv1） beneath the Dabie Orogenic Belt,which might be melting material of mantle source which might be associated with magmatism and/or volcanism in the early Cretaceous. The upwelling asthenosphere can be generated by the break-off of the subduction slab or low crustal/lithospheric delamination in the Mesozoic in the Dabie Orogenic Belt. The upwelling asthenosphere might invade into the crust and form the melting material. Finally,magmatism and volcanism were likely to be triggered by the melting material in the crust. Meanwhile,there is another large-scale low-velocity structure（ Lv2） located at the Hefei Basin,which might be generated by the sedimentary layer in the shallow. There is a dipping vestige of high velocity beneath the Dabie Orogenic Belt,which might be the Xiangfan-Guangji fault or northward subduction relic.