祁连山北缘玉门砾岩的磁性地层年代与褶皱过程*

在我国西部致使玉门砾岩强烈褶皱变形的构造运动是喜马拉雅运动中重要的一幕。祁连山北缘老君庙逆断裂-褶皱带前翼发育的生长地层为认识玉门砾岩褶皱的过程提供了重要证据。通过对这套生长地层及其内部渐进不整合几何形态和结构的详细填图以及磁性地层年代学研究,认为玉门砾岩的底界具有穿时性,其年龄可能由东南向西北变小,其底界年龄在牛胳套剖面约4Ma,在青草湾西剖面约3.55Ma。玉门砾岩的褶皱变形在牛胳套剖面起始于约3Ma,沉积速率由约260m/Ma变为170m/Ma,在青草湾西剖面起始于约1.2Ma。结合构造变形的样式、强度及侵蚀程度推断老君庙逆断裂-褶皱带是以约7.8km/Ma的速率由东南向西北侧向扩展生长的。发育在背斜前翼的多个退覆-超覆生长楔表明玉门砾岩的褶皱变形由3~4个活跃期和平静期组成,这反映了祁连山及其北部前陆盆地内褶皱逆冲席体向前陆方向的不断扩展生长过程, 但并不能简单地以此作为整个祁连山或青藏高原多次垂直隆升的证据。

MAGNETOCHRONOLOGY OF THE YUMEN CONGLOMERATES AND MULTI-PULSED FOLDING AND THRUSTING IN THE NORTHERN QILIANSHAN

Nearly continuous exposure of deformed Plio-Pleistocene sediments reveal that the Laojunmiao fold-and-thrust belt is particularly favorable for determining the geometry,style,timing,and rate of active faulting and folding within a foreland basin along the northern margin of the Qilianshan.Paleomagnetic investigations of two Plio-Pleistocene terrestrial successions provide detailed magnetostratigraphy for the Upper Cenozoic strata in this region.Results show that the conglomeratic Yumen Formation is diachronous with the basal age ranging from ca.4Ma in the Niugetao section to ca.3.55Ma in the Qingcaowan west section ca.14km to the northwest on the northeast limb of the anticline.Northwestward lateral propagation and growth of the Laojunmiao fold-and-thrust belt initiated at ca.3Ma in the Niugetao,and ca.1.2Ma in the Qingcaowan west with a rate of ca.7.8km/Ma.The cross section in the Niugetao provides a conservative estimate of shortening rate of 1.2mm/a and uplift rate of 1.1mm/a since ca.3Ma.Detailed mapping of the rotational offlap onlap geometrical growth strata at the forelimb indicates multi-pulsed folding and thrusting,which suggests that the Laojunmiao fold-and-thrust belt developed as a result of northeastward migration and propagation of thrusting and folding of the northeastern edge of the Qinghai-Tibetan Plateau.