塔里木西缘晚新生代造山过程的记录——磨拉石建造及生长地层和生长不整合

RECORDS OF LATE CENOZOIC MOUNTAIN BUILDING IN WESTERN TARIM BASIN:MOLASSES,GROWTH STRATA AND GROWTH UNCONFORMITY

塔里木西缘帕米尔 -西昆仑山北麓和西南天山南麓前陆盆地中巨厚的晚新生代磨拉石建造在岩石地层上包括阿图什组、西域组、乌苏组和戈壁组。通过对这套磨拉石建造及其中生长地层和生长不整合的野外观测 ,结合磁性地层年代学 ,初步研究了西域砾岩 (即西域组 )和西域运动的起始年代以及西域运动可能的演化阶段 ,得到如下初步认识 :1 )研究区西域砾岩开始沉积于 3 .5～ 4.6MaB .P .之前 ,其底界具有穿时性 ,其年代从山体向前陆盆地一般逐渐变新 ;2 )在上述前陆盆地分布着 4～ 6排由晚新生代磨拉石建造组成的逆断层 -褶皱带。各排褶皱带均发育有生长地层及生长不整合 ,生长地层的开始沉积指示了该排褶皱和相应生长不整合的开始形成。从最靠山体一排褶皱带生长地层开始沉积的年代 ,初步判定西域运动的起始年代在西昆仑山叶城附近约为 3 .6MaB .P .,在西南天山喀什附近显然早于 2 .4MaB .P .。此后该运动可能经历了 4～ 6个活动阶段 ,每一阶段又由若干个相对活跃期和相对稳定期组成。它们可通过各排褶皱带内生长地层的产状、组成和年代测定来确定。本文还探讨了生长地层和生长不整合形成过程中构造变形与侵蚀 -剥蚀和沉积作用间的相互关系。

In two foreland basins along the north foot of Pamirs-West Kunlun Shan and the South foot of Tian Shan,very thick late Cenozoic molasses include four formations:Atushi,Xiyu,Wusu,and Gebi. The commonly assigned age of this sequence is based on lithostratigraphic correlations with few biostratigraphically constrained sections in western Tarim,resulting in an erroneous inference:it should be the synchronous deposition of the Xiyu Conglomerate around the Tarim basin,e.g.,the Xiyu Conglomerate=the Xiyu Formation=Pleistocene. The bottom of the conglomeratic Xiyu Formation is acutely diachronous. Late Cenozoic deformation of this sequence in these two foreland basins appeared as 4～6 separate fold-thrust zones. In each of such zones there are growth strata and growth unconformities. The growth strata indicate that folding and thrusting of this sequence began at 3.6Ma B.P. near Yecheng and earlier than 2.4Ma B.P. on the southern margin of Tian Shan near Kashi. There are 4～6 folding and thrusting episodes to be identified by using growth strata and growth unconformities. The relative rates of deformation,denudation,and sedimentation were not constant during each episode.