长江中下游成矿带中段岩石圈电性结构研究

长江中下游成矿带位于大别造山带、长江中下游凹陷、江南隆起带等大地构造单元结合部位,通过在研究区内布设两条首尾相接共计150km长的大地电磁剖面,获得了50km以浅岩石圈尺度的电性分布.长江中下游地区中段地下电性结构显示出在地下10km和30km处分别存在明显的圈层结构,以此认为现今横向稳定的"电莫霍"反映了研究区经历燕山期陆内构造-岩浆活动后已基本上完成壳幔重新平衡;而分隔大地构造单元的郯庐断裂带、长江断裂带以及江南断裂带在电性上具有特征的梯度显现,在印支造山期后的引张背景下,断裂带成为强伸展活动带与控制了燕山期大范围的陆内岩浆活动;高导地幔的局域性存在以及从北向南地幔导电性的变化反映了在经受深部动力学过程中处于不同大地构造部位的地幔所遭受的不同类型的改造以及地幔深部的构造极性.

Lithospheric electrical structure in the middle and lower reach of Yangtze River metallogenic belt inferred from magnetotelluric sounding

The Middle and Lower Reach of Yangtze River(MLYR) metallogenic belt is a juncture among the Dabie orogen, the MLYR depression and the Jiangnan uplift, and an important Cu-Fe-Mo-Au polymetallic metallogenic belt in eastern China. The study of the lithospheric structure in this region is important to reconstruct the geodynamic processes controlling metallogenesis and understand the genetic mechanism of the metallogenic belt. With advantages of high lateral resolution and deep investigation depth, magnetotelluric sounding can provide electrical constraints for layered structure of continental lithosphere and location of tectonic boundaries as well as properties of continental lithospheric mantle.#br#Broad-band magnetotelluric (BBMT) data at a total of 150 sites were acquired along two approximately northwest-southeast trending 300-km-long profiles across the middle corridor of the MLYR metallogenic belt. Modern processing techniques were applied to these data to ensure that the accurate and realistic MT response curves were produced to the longest period possible for each site. These techniques included robust estimate, processing using robust remote referencing codes for the BBMT data, and analysis of geoelectric strike direction as well as dimensionality by phase tensor decomposition. The electrical structure down-to 50 km depths was finally imaged by two-dimensional TE+TM mode continuum medium inversion.#br#The deep conductivity structure revealed mainly includes the crust-mantle transition, fault zones which separate tectonic units, as well as conductibility of the mantle. (1) The electric crust-mantle boundary is at the depth of 30 km, separating the conductive lower crustal granulite above from the more resistive underlying lithospheric mantle, except the conductive mantle beneath the Yangtze River. Another boundary at the depth of 10 km separates the resistive upper crustal granitoids from the lower crust. (2) The lateral electrical gradients at the Tan-Lu fault zone and Jiangnan fault zone separate the resistive crust of Dabie orogen, the relatively conductive crust of MLYR depression and the resistive crust of Jiangnan uplift, respectively. (3) The mantle conductivity model shows three kinds of mantle with a conductive mantle beneath the Yangtze River in the middle of the profile, and both resistive mantles in the north-western and south-eastern, respectively.#br#It can be inferred from the present stable electric crust-mantle boundary that MLYR metallogenic belt experienced a lithospheric re-balance (an adjustment of lithosphere) after the Yanshanian intracontinental tectono-magmatic and metallogenic processes. The distinct electrical gradients at the boundaries of deep fault zones (e.g. Tan-Lu fault zone, concealed Yangtze River fault zone and Jiangnan fault zone) imply their extensional property in post-Indosinian and playing a key role of controlling the Yanshanian tectonic-magmatic activity. The high conductive mantle's localized existence reflects mantle beneath different geotectonic units that experienced different kinds of transformation(deformation and alteration) during the geodynamic process and formed the geological polarity at depth.