内蒙古东部古生代块体碰撞区的大地电磁测深研究——Ⅱ.二维解释

本文对内蒙古东部地区赤峰-东乌旗剖面上12个测点的MT资料进行了二维解释.针对测区具体情况,在计算中主要强调拟合有效视电阻率ρeff,ρaz和有效相位Φeff,Φaz.二维模型给出,地壳上地幔在西乌旗以北和翁牛特旗以南地区有5层结构,壳内高导层埋深20-25km,上地幔软流层顶面埋深约100km.大兴安岭火山岩区有6层或7层结构,上地壳10km左右和下地壳30-40km深度处分别有一高导层,软流层顶面埋深约75km.结合地质等资料得出:1.翁牛特旗以南地区至少在深部仍然属于中朝块体北缘;2.翁牛特旗-巴林右旗之间是中朝块体北缘古生代以前的一个增生体,西拉木伦河断裂是由于后期的构造活动在增生体内产生的一个逆冲断层;3.上地幔隆起和下地壳高导层（岩浆房?）的存在是产生大兴安岭火山岩的深部原因;4.大兴安岭地区可能存在推覆构造,推覆面即为上地壳高导低速层;5.二维模型没有反映出贺根山地区地下有岩浆通道或深断裂;6.中朝和西伯利亚块体早古生代和晚古生代的缝合带分别位于林西一带和西乌旗一带.

A MAGNETOTELLURIC STUDY OF THE PALAEOZOIC COLLISION ZONE IN THE EAST OF INNER-MONGOLIA——Ⅱ. TWO-DIMENSIONAL MODELLING

Magnetotelluric data for 12 stations on a profile from Chifeng to Dong Ujimqin Qi in the East of Inner-Mongolia are interpreted using two-dimensional modelling. According to the actual conditions of the study area the effective apparent resistivities （ρcff,ρaz） and effective impedance phases （φeff,φaz） are emphasized in the interpretations. The resulted 2-D model shows 5-layered structures for the crust and upper mantle in the northernmost and southernmost of the profile. The mid-crustal conductive layer and the top boundary of the upper mantle conductive layer lie at depths of about 20-25 and 100km respectively. In Great Hinggan volcanic regions 6 or 7-layered structure are preferred with two conductive layers in the crust, one at depth of about 10km, the other at 30-40km, and the top of the upper mantle conductive layer rising to 75km. From the 2D model combining with the geological data, some inferences can be deduced: 1. The south of Ongniud Qi, at least the depth, belongs to the north margin of the Sino-Korean block. 2. Between Ongniud Qi and Bairin Youqi is an accreted body to the north margin of the Sino-Korean block before the Paleozoic time, and the Xar Moron River fault seems to be a thrust fracture within the body resulted from the later tectonization. 3. The deep background of the Great Hinggan volcanics may be the rise of the upper mantle and the presence of the conductive materials （magmatic province?） near the Moho discontinuity. 4. There is probably a nappe structure in the Great Hinggan regions whose sliding surface would be the upper crustal conductive layer. 5. No obvious magmatic channels or deep faults heve been found beneath the Hegenshan this time. 6. The Early and Late Paleozoic suture zones of the Sino-Korean and Siberian blocks would be nearby Linxi and Xi Ujimqin Qi, respectively.