MAGNETOCHRONOLOGY OF THE YUMEN CONGLOMERATES AND MULTI-PULSED FOLDING AND THRUSTING IN THE NORTHERN QILIANSHAN
祁连山北缘玉门砾岩的磁性地层年代与褶皱过程*

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.