南秦岭勉略地区“三河口群”的解体及地质意义

Disintegration of the “Sanhekou Group” in the MianxianLueyangArea,South Qinling and Its Geological Significance 

“三河口群”分布于南秦岭西段、陕甘川交界的勉县—略阳—康县—文县临江—南坪塔藏一带，其南北均为断裂带限制。过去，“三河口群”的研究在史密斯地层学的理论指导下，整体时代认为是下、中泥盆统。在非史密斯地层学理论的指导下，对勉略带“三河口群”进行了解体，建立了该带的原地和异地地层系统。新的地层系统包括了从太古界到石炭系不同时代的地层，该地层系统揭示了南秦岭古海洋和大地构造演化过程：南秦岭晚震旦世—早寒武世为扬子板块的一部分，奥陶纪—志留纪时期沿南秦岭勉县—略阳—安康—随县（南）形成裂陷槽盆地，该盆地于中晚志留世有所萎缩但尚未完全关闭。泥盆纪时期该裂陷槽进一步开裂并形成有限洋盆，石炭纪洋盆出现向南的俯冲。该洋盆可能在二叠纪仍然存在，于印支期最终闭合。

The “Sanhekou Group” is mainly distributed in the western sector of south Qinling,at the junction of Shaanxi,Gansu and Sichuan provinces.It is limited by faults in the north and south,and is bulit up of a series of structural sheet.The “Sanhekou Group” was formerly divided into the Lower and Middle Devonian according to the thought of Smith stratigraphy.In this paper,a new autochthonous and allochthonous stratigraphic system is built according to the theory of NonSmith stratigraphy.The new stratigraphic system includes the strata ranging in age from the Archean to the Carboniferous.The evolutionary history of MianxianLueyang palaeocean reflected by the NonSmith stratigraphy is also discussed in this paper.We suggest that MianxianLueyang Zone existed in a continental margin rift in the Early Palaeozoic,formed a little ocean in the Late Palaeozoic.The MianxianLueyang little ocean was a inheritable basin.In the period from the Late Sinian to the Early Cambrian,South Qinling was a part of northern edge of the Yangtze plate.Rift trough was formed from the Ordovician to the Early Silurian.The rift trough shrank but was not closed in the Middle to Late Silurian.This relict rift trough was further opened and gradually developed into a little ocean in the Devonian and was transformed into an active continental margin in the Carboniferous.It is possible that this ocean existed in the Permian and was closed in the Triassic affected by the collision between the Yangtze and North China plates.