依据古地磁资料探讨华北和华南块体运动及其对秦岭造山带构造演化的影响

本文在收集和整理国内外近年来古地磁研究结果的基础上,按照一定的规则选取了部分数据,经统计分析计算,得到华南和华北块体的古生代至中生代古地磁极移曲线和古纬度变化曲线,由此导出两块体的运动形式和北向运动速度分量,估算了两块体间的南北向距离,即秦岭海域的宽度,指出两块体拼合的时限,并根据华北、华南块体的相对位置改变,探讨了秦岭造山带的构造演化历史,对秦岭研究中一些争议较大的问题提出了新的解释。

EVOLUTION OF THE QINLING FOLD BELT AND THE MOVEMENT OF THE NORTH AND SOUTH CHINA BLOCKS: THE EVIDENCE OF GEOLOGY AND PALEOMAGNETISM

Many researches have been done for elucidating the evolution of the Qinling Belt which may be a key to explaining the tectonic development of China, but recent tectonic syntheses in China based on geologic and paleomagnetic evidence obtained this decade have led to widely differing interpretations in terms of its geological history because of lack of unambiguous evidence. The key point of debate is how and when the North China Block(NCB) and South China Block (SCB) joined together. This paper is to present an explanation concerning this point with a systematic analysis and synthesis of evidence of geology and paleomagnetism newly obtained by the author and others.This study has been carried out in three aspects: 1) Geological investigation in Qinling Belt: the petrochemical and petrogenous analysis of the main tectonostratigraphic units and the structural analysis of the deformed rocks in these units are made; 2) Paleomagnetical observations in Qinling Belt and North China Block: results are acquired from the two most important tectonostratigraphic units——Danfeng Formation and Liuling Formation——and from the rocksof the Carboniferous to Triassic in NCB; and 3) Paleomagnetic results analysis: combining a re-evaluation and recalculation of the paleomagnetic pole positions recently published by Chinese and foreign workers with newly acquired paleomagnetic data obtained by the author, the Apparent Polar Wander paths (APWP) of North and South China Blocks are revised and the paleolatitude distributions of these two blocks are given. The conclusions drawn from these researches are as follows:(1) The Qinling Belt is an assemblage of various geologic units, which are characterized by various sedimentary and igneous rocks with distinct features of deformation and accampanied metamorphism and magmatism, with differing isotopic ages. According to their characters, three zones are divided in the Qinling Belt, representing the three stages of evolation of the Belt: The north-zone of Qinling was the pre-Cambrian Andes-type mountain-arc-trench system of the NCB and was composed of Taihua, Xionger (arc-basalt complex), Taowan and Kuanping (Flysch and Molasse) Formations; the middle-zone was the remains of Qinling-paleoplatform, which is composed of Erlongping Formation (ophiolite complex of Early Paleozoic), Qinling Formation (Si-Al terrane of pre-Ordovician) and Danfong Formation (ophiolite complex of Late Paleozoic); The south-zone was the continental margin of SCB, which was composed of the Paleozoic and Triassic shallow-water diposits; bettween the middle-zone and south-zone is the Liuling Formation that was the complex of melenge of the shallow-water diposit of Devonian age with flysch and molasse diposits of trench type spanning in age from Carboniforious to middle Triassic.(2) The clockwise rotation of the SCB began as early as the Early Paleozoic and ended in the late Triassic but the counter-clockwise rotation of the NCB was one of the most important movement-types during the same period. After the Cambrian the NCB progressively moved northward from the middle-latitude of fhe south sphere to north sphere and the northward-movement never stopped until the late Mesozoic, the changes of latitude being as much as 80℃. Based on this distance of displacement, the velocity of NCB in this period was about 1.4 cm/y. In the most of the Paleozoic the SCB wandered about near the equator but at the late Permian SCB began moving northward at greate velocity than that of NCB and from the late Permian to late Triassic the latitude-changes of SCB were about 25?and the component of northward displacement of it was about 2700 km, which means the velocity of SCB during this period was about 8 cm/y. The velocity of SCB was so greater than that of the NCB that the SCB caught up with the NCB which caused the collision of the two blocks at the end of the Triassic. In the late Paleozoic a considerable latitude-difference of the two blocks appeared and the greatest one (about 20°) was in the Permian, showing existence of a paleo-ocean or sea along